Wilson and Gisvold's Textbook of
ANIC MEDICINAL AND PHARMAC ICAL CHEMIS TRY E
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Wilson and Gisvold's Textbook of
ORGANIC MEDICINAL AND PHARMACEUTICAL CHEMISTRY
ELEVENTH EDITION Edited by
John H. Block, Ph.D., R.Ph. Professor of Medicinal Chemistry Department of Pharmaceutical Sciences College of Pharmacy Oregon State University Corvallis. Oregon
John M. Beale, Jr., Ph.D. Associate Professor of Medicinal Chemistry and Director of Pharmaceutical Sciences St. Louis College of Pharmacy St. Louis, Missouri
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First Editton, 1949 Second Edition. 1954 Third Edition. 1956
Filth Edition. 1966 Sixth Edition. 1971 Seventh Edition, 1977
Eighth Edition. 1982 Ninth Edition, 1991 Tenth Edition, 1998
rswrtli Edition, (962
Llbrnry or Congrnas Cataloglng.In.Publkatloit Data Wilson and Gisvold's textbook of organic medicinal and phartnaccutical chemistry.— 11th ed. / edited by John H. Block. John M. Beale Jr. p.
cm,
Includes bibliographical references attd index. ISBN 11-7817-34111-9
I. Pharmaceutical chemistry. 2. Chemistry. Organic. I. Title: Textbook of organic medicinal and pharmaceutical chemistry. II. Wilson. Charles Owens. 1911—2002 10. Gisvold. Ole.
l904- IV. Block. John H. V. Ileak. John Marlowe. IDNLM: I. Chemistry. Pharmaceutical. 2. Chemistry. Organic. QV 744 W754 2ll(9J RS403. 143 2111)4
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2 3 4 5 6 7 8 9 10
l'he Fkrenth Edüion of Wilson and Gisvold's Texibook of Organic and Medicinal Pharmaceutical Charles 0. Wilson q( Jaiine N. !)elgado
Chem i stry is' (kYiica:ed Iv the
Jaime N Delgado 1932—2001
Delgado served as coeditor for the ninth and tenth editions and was continuing
Juime N. this role before his death on October 5, 200 1 . Dr. Dclgado studied with Ole Gisvold, one of the P rofessor in
two founding editors of this textbook, and he was dedicated to maintaining the standards of excellence
established by Gisvold and his coeditor Charles Wilson. He loved teaching medicinal chemistry to students, and this textbook was a powerful aid to him. A graduate of the University of Texas at Austin and the University of Minnesota. Jaime Delgado began his teaching career as an assistant professor at the University of Texas College of Pharmacy in 959. He rose through the academic ranks to become professor and head of the Division of Medicinal Chemistry and a leader in research and graduate education. He essentially built both the graduate program and the Division from scratch, and his publication of research and scholarly works brought national recognition to the department. Although Jaime Delgado became known for his research and scholarship. his first love and his greatest legacy were in teaching and advising undergraduate and graduate students. The University of Texas at Austin awarded him five major teaching awards. and recognized him two times as one of its "best" professors. In 1997. he was elected to the Academy of Distinguished Teachers at the university and was honored as a Distinguished Teaching Professor, a permanent academic title. Former dean James Doluisio described Dr. Delgado's teaching style as "owning the classroom" because of his knowledge. communication skills, and deep conviction that pharmacy is a science-based profession. His enthusiasm and extemporaneous use of the chalkboard were legendary. In addition to his contributions to teaching at the University of Texas, Dr. Delgado traveled extensively in Mexico and South America to present lectures on pharmaceutical education. Jaime Delgado's first contributions 10 the Textbook of Organic Medicinal (111(1 Phannaceutical Chemistry were made as a chapter author in the seventh and eighth editions. Much of the material he presented came from his lecture notes Although he was proud of these contributions, which were expanded in the ninth and tenth editions, he considered his role as coeditor in the latter editions one of the highlights of his distinguished career. Jaime was a true gentleman and a pleasure to have as a collaborator. He will he greatly missed by the editors, authors4 and professional staff for the Textbook. 1
William A. Reiners
Charles 0. Wilson 1911—2002
A
s the chapters for the eleventh edition were being sent to the publisher. I was notified that my
colleague and friend. Charles Wilson. had died shortly Christmas. I-Ic was a product of the Pacific Northwest having received all of his degrees from the University of Washington. His first teaching job was at the now discontinued pharmacy school at George Washington University and then he moved to the University of Minnesota. Charles. along with other medicinal chemistry faculty at the University of Minnesota. saw the need for textbooks that presented modern medicinal chemistry. In 1949. he and Professor Ole Gisvold edited Organic chemistry in Pharmacy, which became the first edition of the Textbook of Medicinal and Pharmaceutical che,nix:rv. Continuing in this tradition. Charles and Professor Tailo Some assumed the authorship of Roger'.c Inorganic Pharmaceutical Chemistry, which included eight editions before its discontinuance. Finally. Charles and Professor Tony Jones started the American Drug Index series. Charles continued his publishing activities after moving to the University of Texas and then assumed the position of Dean of Oregon State University's School ol Pharmacy, where he oversaw a major expansion of its faculty and physical plant. Although a medicinal chemist. Charles devoted considerable time to his chosen pharmacy profession. students, and communily. Charles was an active member of the American Pharmaceutical Association as well as the pharmacy associations in each state where he lived. In addition, he was a registered pharmacist in each state where he taught: Washington. Minnesota, Texas. Oregon, and the District or Columbia. Charles chaired national committees and sections of the American Pharmaceutical Association and the American Association of Colleges of Pharmacy. Related to these, his loyalty to students included organizing student branches of the American Pharmaceutical Association al George Washington University. the University of Minnesota. and the University of Texas. He was actively involved in the local American Red Cross blood program and took the lead in developing the hugely successful student centered blood drives at Oregon State University. In 1960, Charles and his wife, Vaughn. helped launch the AFS (American Field Service) in Corvallis, an international high-school exchange program. He volunteered for Meals on Wheels for over 30 years after his retirement. We certainly miss this fine gentleman and leader of pharmacy education and the pharmacy profession.
John H. Block
PREFACE
For almost six decades, Wilson and Gisvo!d s Textbook of Organic Medicinal and Pharmaceutical chemistry has been a standard in the literature of medicinal chemistry. Generations of students and faculty have depended on this textbook not only for undergraduate courses in medicinal chemistry but also as a supplement for graduate studies. Moreover, students in other health sciences have found certain chapters useful at one time or another. The current editors and authors worked on the eleventh edition with the objective of continuing the tradition of a modem textbook for undergraduate studerns and also for graduate students who need a general review of medicinal chemistry. Because the chapters include
a blend of chemical and pharmacological principles necessary for understanding structure—activity relationships and molecular mechanisms of drug action, the book should be useful in supporting courses in medicinal chemistry and in complementing pharmacology courses.
II is our goal that the eleventh edition follow in the footsteps of the tenth edition and reflect the dynamic changes occurring in medicinal chemistry. Recognizing that the search for new drugs involves both synthesis and screening of large numbers of compounds, there is a new chapter on combinatorial chemistry that includes a discussion on how the process is automated. The power of mainframe computing now is on the medicinal chemist's desk. A new chapter describes techniques of molecular modeling and computational chemistry. With a significant percentage of the general population purchasing altemativc medicines, there is a new chapter on herbal medicines that describes the chemical content of many of these products. The previous edition had new chapters on drug latentiation and prodrugs, immunizing biologicals. diagnostic imaging agents, and biotechnology. Expansion of chapters from the tenth edition includes the antiviral chapter that contains the newest drugs that have changed the way HIV is treated. Dramatic progress in the application of molecular biology to the production of pharmaceutical agents has produced such important molecules as modified human insulins, granulocyte colony-stimulating factors, erythropoietins, and interferons. all products of cloned and, sometimes, modified human genes. The chapter on biotechnology describes these exciting applications. Recent advances in understanding the immune system at the molecular level have led to new agents that suppress or modify the immune response, producing new treatments for autoimmune diseases including rheumatoid arthritis, Crohn's disease, and multiple sclerosis. Techniques of genetic engineering now allow the preparation of pure surface antigens as vaccines while totally eliminating the pathogenic organisms from which they are derived.
The editors welcome the new contributors to the eleventh edition: Doug Henry. Phillip Bowen, Stephen i. Cutler. 1. Kent Walsh, Philip Proteau. and Michael J. Deimling. The editors extend thanks to all of the authors who have cooperated in the preparation of the current edition. Collectively, the authors represent many years of teaching and research experience in medicinal chemistry. Their chapters include summaries of current research trends that lead the reader to the original literature. Documentation and references continue to be an important feature of the book.
We continuc to be indebted to Professors Charles 0. Wilson and Ole Gisvold. the originators of the book and editors of five editions. Professor Robert Doerge. who joined Professors Wilson and Gisvold for the sixth and seventh editions and single-hundedly edited the eighth edition, and Professors
Jaime Dclgado and William Remers who edited the ninth and tenth editions. They and the authors have contributed significantly to the education of countless pharmacists, medicinal chemists, and other pharmaceutical scientists. John H. Block John M. Beale. Jr. 1st
2nd 3rd 4th 5th
1949 1954 1956 1962 1966
Wilson and Gisvold (Organic C'he,,,istrv in Pharmacy) Wilson and Gisvold Wilson Wilson and Gisvold Wilson
6th 7th 8th 9th
1977 1982
10th
1998
1971
1991
Wilson. Gisvold, and Doerge Wilson. Gisvold. and Doerge Doerge Delgado and Remers Delgado and Remers
VI,
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A
———1
CONTRIBUTORS
JOHN M. BEALE, JR.,
STEPHEN J. CUTLER,
PH.D.
PH.D.
EUGENE I. ISAACSON, PH.D.
Associate Professor of Medicinal Chemistry and Director of
Professor of Medicinal Chemistry
Professor Emeritus of Medicinal
School of Pharmacy Mercer University Atlanta, Georgia
Chemistry Department of Pharmaceutical
Pharmaceutical Sciences St. Louis College of Pharmacy St. Louis, Missouri
JOHN R.PH.
H. BLOCK, PH.D.,
Professor of Medicinal Chemistry Department ol Pharmaceutical Sciences
College of Pharmacy Oregon State University Corvallis. Oregon .1.
PHILLIP BOWEN, PH.D.
Professor of Chemistry and
Director. Center for Biomolecular Structure and Dynamics Computational Chemistry Building Cedar Street
University of Georgia Athens. Georgia
C.
RANDALL CLARK,
PH.D. Professor of Medicinal Chemistry Department of Pharmacal Sciences School of Pharmacy Auburn University Auburn. Alabama
GEORGE PH.D.
H. COCOLAS,
Professor of Medicinal Chemistry and Dean School of Pharmacy
University of North Carolina at Chapel Hill Chapel Hill. North Carolina
HORACE
G. CUTLER,
PH.D.
MICHAEL J. DEIMLING, R.PH., PH.D. Professor of Pharmacology and Chair Department of Pharmaceutical Sciences
School of Pharmacy Southwestern Oklahoma State University Weatherford, Oklahoma
JACK DERUITER, PH.D.
Atlanta. Georgia
RODNEY L. JOHNSON, PH.D. Professor of Medicinal Chemistry Department of Medicinal Chemistry University of Minnesota Minneapolis. Minnesota
Professor of Medicinal Chemistry
Department of Pharmacal Sciences School of Pharmacy
Auburn University Auburn. Alabama
JACK N. HALL, M.S., R.PH., BCNP Clinical Lecturer Department of Radiology/Nuclear Medicine
College of Medicine. University of Arizona University of Arizona Health Sciences Center Tucson. Arizona
DOUGLAS R. HENRY Advisory Scientist MDL Information Systems. Inc. San Leandro, California
THOMAS J. HOLMES, JR., PH.D. Associate Professor School of Pharmacy Campbell University Buies Creek, North Carolina
Senior Research Professor
Director of the Nutuml Products Discovery Group Southern School of Pharmacy \lcrccr University
Sciences
College of Pharmacy Idaho State University Pocatello. Idaho
TIM B. HUNTER, M.D.
DANIEL A. KOECHEL, PH.D. Professor Emeritus—Pharmacology Department of Pharmacology Medical College of Ohio Toledo. Ohio
GUSTAVO R. ORTEGA,
R.PH., PH.D. Professor of Medicinal Chemistry Department of Pharmaceutical Sciences
School of Pharmacy Southwestern Oklahoma State University Weatherford. Oklahoma
PHILIP J. PROTEAU, PH.D. Associate Professor of Medicinal Chemistry College of Pharmacy Oregon State University Corvallis. Oregon
WILLIAM A. REMERS, PH.D.
Vice-Chairman and Professor
Professor Emeritus
Department of Radiology University of Arizona Tucson. Arizona
Pharmacology and Toxicology University of Arizona Tucson. Arizona
ix
X
Coniri/nuors
GARETH THOMAS, PH.D.
ROBERT E. WILLETTI
Associate Senior I.ecturer The School of Pharmacy and
PH.D.
Auburn University Auburn. Alabama
Biomedical Sciences University of Portsmouth Portsmouth, England
Duo Research. Inc. Denver. Colorado
FORREST T. SMITH, PH.D.
T. KENT WALSH, D.O.
Associate Professor
Director
Department of Pharmacal Sciences School of Pharmacy
Nuclear Medicine Program Southern Arizona V.A. Health Care
Auburn University Auburn. Alabama
Tucson, Arizona
THOMAS N. RILEY, PH.D. Professor of Medicinal Chemistry Department of Pharmacal Sciences School of Pharmacy
System
President
A
s—a —-—4
CONTENTS
vu
Preface
Contributors
CHAPTER 1 Introduction fist,,: H. Block
a,,d Jo/u: ti!. lie::!,'. Jr.
Role of Cytochrome P-450 Monooxygenases in Oxidative Biotransformations Oxidative Reactions Reductive Reactions Hydrolytic Reactions Phase II or Conjugation Reactions Factors Affecting Drug Metabolism
67 69 103 109 111
126
CHAPTER 5 Prodrugs and Drug Latentiation
CHAP I ER 2 Physicochemical Properties Biological Action
in Relation to 3
Joh,: H. Block
Overview Drug Distribution Acid—Base Properties
Statistical Prediction of Pharmacological Activity Combinatorial Chemistry Molecular Modeling (Computer-Aided Drug Design)
Selected Web Pages
3 3
9
C HAPIE R
26
Biotechnology and Drug Discovery
41
142 142 144 152 155
Prodrugs of Functional Groups Bioprecursor Prodrugs Chemical Delivery Systems
17
27
142
/-'orrest T. Smith and C. Randall C/ask History Basic Concepts
6
160
Jo!:,: M. lfrale. Jr.
Biotechnology An Overview Biotechnology and Pharmaceutical Care Literature of Biotechnology Biotechnology and New Drug Development The Biotechnology of Recombinant DNA IrDNA) . Some Types of Cloning Expression of Cloned DNA . Manipulation of DNA Sequence Information New Biological Targets for Drug Development Novel Drug-Screening Strategies Processing of the Recombinant Protein Pharmaceutics of Recombinant DNA (rDNA)Produced Agents Delivery and Pharmacokinetics of Biotechnology .
.
CHAPTER 3 Combinatorial Chemistry
43
Daii,ç'la.c I?. Hrs:rv
.
.
.
.
.
.
.
How It Began: Peptides and Other Linear Structures Drug-Like Molecules Supports and Linkers
Solution-Phase Combinatorial Chemistry Pooling Strategies
Detection, Purification, and Analysis Encoding Combinatorial Libraries High-Throughput Screening (HIS) Virtual (in Silico) Screening Chemical Diversity and Library Design Report Card on Combinatorial Chemistry: Has It Worked' Resources for Combinatorial Chemistry Combinatorial Chemistry Terminology
.
43 46 48 49 50 51
52
53 54 55 58
60 60
Products
Recombinant Drug Products The Interleukins Enzymes Vaccines
Preparation of Antibodies Genomics Antisense Technology Gene Therapy
Afterword
CHAPTER 4 Metabolic Changes of Drugs and Related Organic compounds
173 175 175 182 183 186 187 191
193 194 194
CHAPTER 7 Immunobiologicals 65
Ste-ph:,: J. C':i:ler and Jo!::: H. Block
General Pathways of Drug Metabolism Sites of Drug Biotransformation
.
160 160 160 160 162 166 167 168 169 170 172
65
66
197
Jo/ui M. //eale. Jr.
Cells of the Immune System Immunity Acquisition of Immunity
197
200 206
xi
Xii
Contents
CHAPTER 8
CHAPT
Anti-infective Agents
217
John M. Beak. Jr. Evaluation of the Effectiveness of a Sterilant Alcohols and Related Compounds Phenols and Their Derivatives Oxidizing Agents Halogen-Containing Compounds Cationic Surfactants
.
.
219 219 221
223 223 224 226 228 228 230 247 259 264 268 268 279 279
Dyes
Mercury Compounds (Mercurials) Preservatives
Antifungal Agents Synthetic Antibacterial Agents Antiprotozoal Agents Anthelmintics Antiscabious and Antipedicular Agents Antibacterial Sulfonamides Dihydrofolate Reductase Inhibitors Sulfones
CHAPTER 9 Antimalarials Jo/rn H. Block
Stimulation of Antimalarial Research by War
.
Drug Therapy Cinchona Alkaloids
CHAPTER
283 285
0
Antibacterial Antibiotics
299
Jo/ni M. Beak. Jr.
Historical Background Current Status Commercial Production Spectrum of Activity Mechanisms of Action Chemical Classification Microbial Resistance Antibiotics The Penicillins 13-Lactamase Inhibitors Cephalosporins
Monobactams Aminoglycosides Tetracyclines Macrolides Lincomycins Polypeptides Unclassified Antibiotics
299 299 300 300 300 301 301 301
302 314 318 334 334 341
349 353 355 360
CHAPTER 11 Antiviral Agents
367
R
1
2
390
William A. Remers Tumor Cell Properties Alkylating Agents Antimetabolites Antibiotics Plant Products Miscellaneous Compounds Hormones Signal Transduction Inhibitors Immunotherapy Monoclonal Antibodies Radiotherapeutic Agents Cytoprotective Agents Future Antineoplastic Agents Potential Future Developments
390 394 402 414 424 428 433
438 440 442 444 445 446 448
.
.
CHAPTER 13 Agents for Diagnostic Imaging
454
Tin, Ii. Hunter, T. Kent Walsh, Jack N. Hall Introduction to Radiation Characteristics of Decay Biological Effects of Radiation Radionuclides and Radiopharmaceuticals for Organ Imaging Radionuclide Production Technetium Radiochemistry Fluorine Radiochemistry Gallium Radiochemistry Iodine Radiochemistry Indium Radiochemistry Thallium Radiochemistry Xenon Radiochemistry Radiological Contrast Agents Paramagnetic Compounds Ultrasound Contrast Agents Radiological Procedures
454 456 457
458 461
463 468 468 468 469 472 472 472 475 477 478
C HAPTER 14 Central Nervous System Depressants
485
Eugene I. lsaacson General Anesthetics
485 488 496 503
Anxiolytic. Sedative, and Hypnotic Agents Antipsychotics Anticonvulsant or Antiepiloptic Drugs
CHAPTER 15 central Nervous System Stimulants
510
Eugene I. lsaacson Analeptics
510
Methyixanthines Central Sympathomimetic Agents (Psychomotor Stimulants) Antidepressants Miscellaneous CNS-Acting Drugs
511
512 514 520
CHAPTER 16 Adrenergic Agents
Jo/ru M. Beak, Jr. Classification of Viruses Targets for the Prevention of Viral Infections—Chemoprophylaxis The Infectious Process for a Virus Nucleoside Antimetabolites Newer Agent5 for the Treatment of HIV Infection
E
Antineoplastic Agents
367 367 370 375 382
Rot/tier L Johnson Adrenergic Neurotransmitters Adrenergic Receptors Drugs Affecting Adrenergic Neurotransmission Sympathomimetic Agents Adrenergic Receptor Antagonists
524
.
.
524 527 528 530 539
(tnate,lts
CHAPTER 17
Inhibition of Histamine Release Mast Cell
Cholinergic Drugs and Related
Agents ...
George II. Combs and Stephen J. Cutler Cholinergic Receptors Cholinergic Neurochemistry Cholinergic Agonists Cholinergic Receptor Antagonists Cholinergic Blocking Agents Parasympathetic Postganglionic Blocking Agents Solanaceous Alkaloids and Analogues Synthetic Cholinergic Blocking Agents Ganglionic Blocking Agents Neuromuscular Blocking Agents
.
.
548 548 553 553 558 572 573 574 579 586 589
CHAPTER 18 Diuretics
596
l.)aniel it. At,i'chel
Anatomy and Physiology of the Nephron
596 596
Function
Introduction to the Diuretics Site 1 Diuretics: Carbonic Anhydrase Inhibitors Site 3 Diuretics: Thiazide and Thiazide-Like
601 .
.
Diuretics
Site 2 Diuretics. High-Ceiling or Loop Diuretics Site 4 Diuretics: Potassium-Sparing Diuretics .
.
.
.
.
Miscellaneous Diuretics Emerging Developments in the Use of Diuretics
.
Agents
622 634 642 657 663 668 673 673
Antiarrhyhmic Drugs Antihypertensive Agents Antihyperlipidemic Agents Anticoagulants Synthetic Hypoglycemic Agents Thyroid Hormones Antithyroid Drugs
CHAPTER 20 Local Anesthetic Agents
676
Gureth Thomas Historical Development
676 679 685 687
The Nervous System
Mechanism of Action Administration Factors Influencing the Effectiveness of the Anesthetic Action Rate of Onset and Duration of Anesthesia Secondary Pharmacological Action Structure Action
.
.
.
687 688 689 690
Pain Morphine and Related Compounds Antitussive Agents Anti-inflammatory Analgesics
I)eRuiter
Histamine Histamine Life Cycle Histamine
Antagonists (Antihistaminic Agents)
696 696 700
731
732 752 753
CHAPTER 23 Steroids and Therapeutically Related
Compounds
767
.
.
.
767 768 770
Changes to Modify Pharmacokinetic Properties of Steroids Steroid Hormone Receptors GnRH and Gonadotropins Sex Hormones Chemical Contraceptive Agents Androgens Adrenal Cortex Hormones
770 770 773 775 789 797 803
C H A PT ER 24 Prostaglandins, Leukotrienes, and Other
Eicosanoids Thomnas
818
J. Hohues, Jr.
History of Discovery Eicosanoid Biosynthesis Drug Action Mediated by Eicosanoids COX-2 Inhibitors Design of Eicosanoid Drugs Development of Prostacyclin-Derived Products Eicosanoid Receptors Eicosanoids Approved for Human Clinical Use Prostaglandins for Ophthalmic Use Veterinary Uses of Prostanoids Eicosanoids in Clinical Development for Human Treatment
818 818 822 822 823 823
825 827 828 828 829
CHAPTER 25 Proteins, Enzymes, and Peptide
Hormones Stephen
CHAPTER 21 Histamine and Antihistaminic Agents .... 696
731
Robert E. Willene
Numbering Steroid Biosynthesis Chemical and Physical Properties of Steroids
622
Stephen J. ('iufrr and George H. Cocola.c Antianginal Agents and Vasodilators
717 718 727
CHAPTER 22
605 610 616 618
CHAPTER 19
715
Analgesic Agents
Philip J. Proteau Steroid Nomenclature. Stereochemistry, and
618 619 619
Failure Summary Diuretic Preparations
Stabilizers
Recent Antihistamine Developments: The "DualActing" Antihistamines Histamine H2 Antagonists Histamine H3-Receptor Ligands
603
to Treat Hypertension and Congestive Heart
Tliouius N. Rilm.'v and Jack
XIII
830
J. Cutler and Horace G. Cutler
Protein Hydrolysates Amino Acid Solutions Proteins and Protein-Like Compounds
830 830
Enzymes
835 840 857
Hormones Blood Proteins
Impact of Biotechnology on the Development
831
xiv
Coiue,izs
and Commercial Production of Proteins and Peptides as Pharmaceutical Products Biotechnology-Derived Pharmaceutical Products
CHPTER 28 .
.
858 860
C HAPTER 26 Vitamins and Related Compounds Guslai,, R. Oriega. Michael J. Dei,nling. and Jaime N. !)elgado Lipid-Soluble Vitamins Water-Soluble Vitamins Miscellaneous Considerations
866 867 885 900
CHAPTER 27 An Introduction to the Medicinal
Chemistry of Herbs John M. Beak. Jr. What is an Herb? Herbal Purity and Standardization An Herb Is a Drug Types of Herbs
904 905 905 905 906
Computational Chemistry and ComputerAssisted Drug Design J. Phillip Ilunen Computer Graphics and Molecular Visualization Computational Chemistry Overview
919 .
Force Field Methods Geometry Optimization Conformational Searching Molecular Dynamics Simulations Quantum Mechanics Structure-Based Drug Design arid Pharmacophore Perception Predictive ADME
.
920 922 923 929 930 933 935
939 944
Appendix ('akulated Log P, Log D, and
948
Index
957
CHAPTER 1 Introduction JOHN H. BLOCK AND JOHN M. BEALE, JR.
The discipline of medicinal chemistry is devoted to the discovery and development of new agents for treating diseases.
bacterial drugs with better therapeutic profiles. With the
activity is directed to new natural or synthetic
ment for "nutriceutical," the public increasingly is using so-called nontraditional or alternative medicinals that are
MOSt ol this
organic compounds. Inorganic compounds continue to be important in therapy. e.g.. trace elements in nutritional therapy. antacids, and radiopharmaceuticals. but organic molewith increasingly specific pharmacological activities are clearly dominant. Development of organic compounds has grown beyond traditional synthetic methods. It flow ineludes the exciting new held of biotechnology using the cell'. biochemistry to synthesii.e new compounds. Techniques
ranging l'rom recombinant DNA and site-directed
mutugenesis to fusion of cell lines have greatly broadened the possibilities for new entities that treat disease. The pharmacist now dispenses modified human insulins that provide more convenient dosing schedules, cell-stimulating factors that have changed the dosing regimens for chemotherapy. humaniicd monoclonal antibodies that target specific tissues, and lused receptors that intercept immune cell—generated cytokines.
This hook treats many aspects of organic niedicinals: how they are discovered, how they act, and how they developed into clinical agents. The process of establishing a new pharmaceutical is exceedingly complex and involves the talents ut people from a variety of disciplines. including chemistry.
hiochetnistry. molecular biology, physiology, pharmacology. pharmaceutics, and medicine. Medicinal chemistry, itscif. is concerned mainly with the organic, analytical, and biochemical aspects of this process, hut the chemist must interact productively with those in other disciplines. Thus. medicinal chemistry occupies a strategic position at the interface of chemistry and biology. To provide an understanding of the principles of medicinal
chemistry, it is necessary to consider the physicochemical properties used to develop new pharmacologically active compounds and their mechanisms of action, the drug's mejabolisni including possible biological activities of the metaholites. the importance of stereochemistry in drug design, and the methods used to determine what "space' a drug occupies. All of the principles discussed in this book are based on fundamental organic chemistry. physical chemistry'. and biochemistry.
The earliest drug discoveries were made by random sampling of higher plants. Some of this sampling, although based
on anecdotal evidence, led to the use of such crude plant drugs as opium. belladonna, and ephedrine that have been
important for centuries. With the accidental discovery of penicillin came the screening of microorganisms and the
large number of antibiotics from bacterial and fungal sources. Many of these antibiotics provided the prototypical structure that the medicinal chemist modified to obtain anti-
changes in federal legislation reducing the efficacy require-
sold over the counter, many outside of traditional pharmacy distribution channels. It is important for the pharmacist and
the public to understand the rigor that is required for prescription-only and FDA-approved nonprescription products to be approved relative to the nontraditional products. It also
is important for all people in the health care field and the public to realize that whether these nontraditional products are effective as claimed or not, many of the alternate medicines contain pharmacologically active agents that can potentiate or interfere with physician-prescribed therapy. Hundreds of thousands of new organic chemicals arc prepared annually throughout the world, and many of them are entered into pharmacological screens to determine whether they have useful biological activity. This process of random screening has been considered inefficient, but it has resulted in the identification of new lead compounds whose structures have been optimized to produce clinical agents. Sometimes. a lead develops by careful observation of the pharmacological behavior of an existing drug. The discovery thaL amantadine protects and treats curly influenza A came from a general screen for antiviral agents. The use of amantadine in long-term care facilities showed that it also could he used to treat parkinsonian disorders. More recently. automated high-throughput screening systems utilizing cell culture systems with linked enzyme assays and receptor molecules derived from gene cloning have greatly increased the efficiency of random screening. It is now practical to screen enormous libraries of peptides and nucleic acids obtained from combinatorial chemistry procedures. Rational design, the opposite approach to high-volume screening, is also flourishing. Significant advances in x-ray crystallography and nuclear magnetic resonance have made it possible to obtain detailed representations of enzymes and other drug receptors. The techniques of molecular graphics and computational chemistry have provided novel chemical structures that have led to new drugs with potent medicinal activities. Development of HIV protease inhibitors and an-
giotensin-convcrting enzyme (ACE) inhibitors came from an understanding of the geometry and chemical character of the respective enzyme's active site. Even if the receptor structure is not known in detail, rational approaches based on the physicochemical properties of lead compounds can provide new drugs. For example, the development of cimetidine as an antinuclear drug involved a careful study of the changes in antagonism of H2-histamine receptors induced by varying the physical properties of structures based on 1
2
IViIu,,, and Gi.o'ohlx Textbook of Orga,:ic Medicinal and Pharmaceutical Chen,i.strv
histamine. Statistical methods based on the correlation of physicochcmical properties with biological potency are used
to explain and optimize biological activity. As you proceed through the chapters, think of what prob1cm the medicinal chemist is trying to solve. Why were certain structures selected? What modilications were made to
produce more focused activity or reduce adverse reactiooor produce better pharmaceutical propenics? Was the prototypical molecule discovered from random screcns, or did the medicinal chemist have a structural concept of the or an understanding of the disease process that must be interrupted?
CHAPTER 2 Physicochemical Properties in Relation to Biological Action JOHN H. BLOCK
synthesize a new structure and see what happens—contin— ucs to evolve rapidly as an approach to solving a drug design problem. The combination of increasing power and decreas-
17), suicide inhibitors of monoamine oxidase (see Chapter 14), and the aromatase inhibitors 4-hydroxyandrostenedione and exemestane (see Chapter 23). These pharmacological agents form covalent bonds with the receptor, usually an enxyme's active site. In these cases, the cell must destroy
ing cost of desktop computing has had a major impact on solving drug design problems. While drug design increas-
the receptor or enzynse, or. in the case of the alkylating agents, the cell would be replaced, ideally with a normal
Modem drug design. compared with the classical apa c/lange on an existing compound or proach—k: 's
ingly is bawd on modern computational chemical techniques. it also uses sophisticated knowledge of disease mechanisms and receptor properties. A good understanding
(if how the drug is transported into the body, distributed throughout the body compartments, metabolically altered by
the liver and other organs. and excreted from the patient is required along with the structural characteristics of the receptor. Acid—base chemistry is used to aid in formulation hiodistribution. Structural attributes and substituent patterns w.sponsiblc for optimum pharmacological activity can he predicted by statistical techniques such as regression analysis. Computerized conformational analysis permits the medicinal chemist to predict the drug's three-dimensional shape that is seen by the receptor. With the isolation and structural determination of specific receptors and the availability of computer software that can estimate the three-dimensional shape of the receptor, it is possible to design mole-
cuks that will show an optimum lit to the receptor.
ment calls for the drug's effect to last for a finite period of time. Then, if it is to be repeated, the drug will be administered again, lithe patient does not tolerate the drug well, it is even more important that the agent dissociate from the receptor and be excreted from the body.
DRUG DISTRIBUTION
Oral An examination of the obstacle course (Fig. 2-I) faced by the drug will give a better understanding of what is involved in developing a commercially feasible product. Assume that the drug is administered orally. The drug must go into solution to pass through the gastrointestinal mucosa. Even drugs administered as true solutions may not remain in solution as they enter the acidic stomach and then pass into the alkaline
OVERVIEW
A drug is a chemical molecule. Following introduction into lie body, a drug must pass through many barriers, survive alternate sites of attachment and storage. and avoid significunt metabolic destruction before it reaches the site of action.
usually a receptor on or in a cell (Fig. 2-I). At the receptor. the following equilibrium (Rx. 2-I) usually holds: Drug + Receptor
cell. In other words, the usual use of drugs in medical treat-
Drug-Receptor Complex Pharmacologic Response
(Rx. 2-I) The ideal drug molecule will show favorable binding characienstics to the receptor, and the equilibrium will lie to the right. At the same time, the drug will be expected to dissociate (toni the receptor and reenter the systemic circulation
to he excreted. Major exceptions include the alkylating agents used itt cancer chemotherapy (see Chapter 12). a few inhibitors of the enzyme acetylcholinesterase (see Chapter
intestinal tract. (This is explained further in the discussion on acid—base chemistry.) The ability of the drug to dissolve is governed by several factors, including its chemical structure, variation in particle size and particle surface area, na-
ture of the crystal form, type of tablet coating, and type of tablet matrix. By varying the dosage form and physical characteristics of the drug, it is possible to have a drug dis-
solve quickly or slowly, with the latter being the situation for many of the sustained-action products. An example is orally administered sodium phenytoin. with which variation of both the crystal form and tablet adjuvants can significantly alter the bioavailability of this drug widely used in the treatment of epilepsy. Chemical modification is also used to a limited extent to facilitate a drug reaching its desired target (see Chapter 5). An example is olsalazine, used in the treatment of ulcerative colitis. This drug is a dimcr of the pharmacologically active mesalamine (5-aminosalicylic acid). The latter is not effec-
tive orally because it is metabolized to inactive forms 3
4
Wilson and Gisvolds Textbook of Organic Medicinal and Plwrvnaceuiical Che,ni.urs
Intramuscular or
Subcutaneous Injection
Intravenous Injection
Tissue Depots
DRUG
DRUG
DRUG METAOOLffi
SYSTEMIC CIRCULATION
Serum Albumin
DRUG
DRUG
I
DRUG
DRUG METABOLITES
I
4
I Liver: site of most drug metabolism
1
DRUG METABOLITES
DRUG METABOLITES
I,
bile I
duct
DRUG METABOLITES
j
Intestinal Tract
to,
+
Undesired Etlects
Excretion ot DRUG.DRUG
Feces
Drug must pass through membranes.
Receptors
Kidney
I
METTABOLITES
Drug administered directly Into systemic circulation
Figure 2—1 • Summary of drug distribution.
before reaching the colon. The dimeric form passes through a significant portion of the intestinal tract before being cleaved by the intestinal bacteria to two equivalents of mesalamine. COOH
In contrast, these same digestive enzymes can be usell.
advantage. Chloramphenicol is water soluble enough mg/mL) to come in contact with the taste receptors auth tongue, producing an unpalatable bitterness. To mask ih; intense bitter taste, the palmitic acid moiety is added as ester of chloramphenicol' s primary alcohol. This reduce.' Ihi
0I sal az no
parent drug's water soluhility (1.05 mglmL) enough so iLl it can be formulated as a suspension that passes over bitter taste receptors on the tongue. Once in the inlectjit.. tract, the ester linkage is hydrolyzed by the digestive ases to the active antibiotic chloramphenicol and the set
common dietary fatty acid palmitic acid. NHCCI4C 2 Mesa lwni ne
02N
H—CH-CH2OR
—O—cOH
As illustrated by olsalazine. any compound passing through the gastrointestinal tract will encounter a large number and variety of digestive and bacterial enzymes, which. in theory, can degrade the drug molecule. In practice, a new
drug entity under investigation will likely be dropped from further consideration if it cannot survive in the intestinal tract or its oral bioavailability is low, necessitating parenteral dosage forms only. An exception would be a drug for which
there is no effective alternative or which is more effective than existing products and can be administered by an alternate route, including parenteral, buccal. or transdennal.
R = H Chioramphenicol Palmitate: Olsalazinc
R
and chloramphenicol palntitale are examphi
of prodrugs. Most prodrugs are compounds that are inaLliir
in their native form but are easily metabolized to the agent. Olsalazine and chloramphenicol palmitate are exan pIes of prodrugs that are cleaved to smaller compounds. 0th
of which is the active drug. Others arc metabolic to the active form. An example of this ype of prodru;
Chapter 2 • Physicoehernical Properties iii Rela:io,, to Biological Action
menadionc. a simple naphthoquinone that is converted in lie liver to phytonadione (vitamin
S
passages. The latter, many times, pass into the patient's circulatory system by passive diffusion.
Parenteral Adminisbatlon
Menad lane
Phytonadions (Vitamin 1(2(20)) Occasionally, the prodrug approach is used to enhance the absorption of a drug that is poorly absorbed from the gastrointestinal tract. Enalapril is the ethyl ester of enala. prilic acid, an active inhibitor of angiotensin-converting enzyme (ACE). The ester prodrug is much more readily absorbed orally than the pharmacologically active carboxylic
Many times there will be therapeutic advantages to bypassing the intestinal barrier by using parenteral (injectable) dosage forms. This is common in patients who, because of illness, cannot tolerate or are incapable of accepting drugs orally. Some drugs are so rapidly and completely metabolized to inactive products in the liver (first-pass effect) that oral administration is precluded. But that does not mean that the drug administered by injection is not confronted by obstacles (Fig. 2-I). Intravenous administration places the drug directly into the circulatory system, where it will be rapidly distributed throughout the body. including tissue depots and the liver, where most biotransformations occur (see below), in addition to the receptors. Subcutaneous and intramuscular injections slow distribution of the drug because it must diffuse from the site of injection into systemic circulation. It is possible to inject the drug directly into specific organs or areas of the body. Intraspinal and intracerebral routes will place the drug directly into the spinal fluid or brain, respec-
tively. This bypasses a specialized epithelial tissue, the blood—brain barrier, which protects the brain from exposure
add.
to a large number of metabolites and chemicals. The
CH3
Enalapril: R = C2H5 Enalaprilic Acid: R = H
blood—brain barrier is composed of membranes of tightly joined epithelial cells lining the cerebral capillaries. The net result is that the brain is not exposed to the same variety of compounds that other organs are. Local anesthetics are examples of administration of a drug directly onto the desired nerve. A spinal block is a form of anesthesia performed by injecting a local anesthetic directly into the spinal cord at a specific location to block transmission along specific neurons.
Unless the drag is intended to act locally in the gustrointcstinal tract, it will have to pass through the gastrointestinal mucosal barrier into venous circulation to reach the site of the receptor. The drug's route involves distribution or partihoning between the aqueous environment of the ga.strointes-
tinal tract, the lipid bilayer cell membrane of the mucosal cells. possibly the aqueous interior of the mucosal cells, the lipid bilayer membranes on the venous side of the gastroin(estinal tract, and the aqueous environment of venous circulation. Some very lipid-soluble drugs may follow the route
of dietary lipids by becoming part of the mixed micelles. incorporating into the chylomicrons in the mucosal cells into the lymph ducts, servicing the intestines, and finally entering venous circulation via the thoracic duct. The drug's passage through the mucosal cells can be pa.s-
sive or active. As is discussed below in this chapter. the lipid membranes are very complex with a highly ordered structure. Part of this membrane is a series of channels or tunnels that form, disappear. and reform. There are receptors that move compounds into the cell by a process called pino-
niosis. Drugs that resemble a normal metabolic precursor or intermediate may be actively transported into the cell by the same system that transports the endogenous compound.
On the other hand, most drug molecules are too large to enter the cell by an active transport mechanism through the
Most of the injections a patient will experience in a lifetime will be subcutaneous or intramuscular. These parenteral routes produce a depot in the tissues (Fig. 2-I), from which
the drug must reach the blood or lymph. Once in systemic circulation, the drug will undergo the same distributive phenomena as orally and intravenously administered agents before reaching the target receptor. In general, the same factors
that control the drug's passage through the gastrointestinal mucosa will also determine the rate of movement out of the tissue depot. The prodrug approach described above also can be used
to alter the solubility characteristics, which, in turn, can in. crease the flexibility in formulating dosage forms. The solubility of methyiprednisolone can be altered from essentially water-insoluble methylprednisolone acetate to slightly water-insoluble methylprednisolone to water-soluble mehhylprednisolone sodium succinate. The water-soluble sodium hemisuccinate salt is used in oral, intravenous, and intramus-
cular dosage forms. Methylprednisolone itself is normally found in tablets. The acetate ester is found in topical ointments and sterile aqueous suspensions for intramuscular injection. Both the succinate and acetate esters are hydrolyzed
to the active methylprednisolone by the patient's own systemic hydrolytic enzymes (esterases).
6
Wilson and Gisvold's Textbook of Organi Medicinal and Pharmaceutical Chemi.sirv
Protein Binding Once the drug enters the systemic circulation (Fig. 2-I). it can undergo several events, It may stay in solution, but many
drugs will be bound to the serum proteins, usually albumin tRx. 2-2). Thus a new equilibrium must be considered. Depending on the equilibrium constant, the drug can remain in systemic circulation bound to albumin for a considerable period and riot be available to the sites of the pharmacological receptors, and excretion. Drug + Albumin
Methyiprednisolone: R H Meth)lprednisolone Acetate: R C(=O}CH3 Methyiprednisolone Sodium Succinate: R = C(0)CH2CH2COO' Na'
Another example of how prodrug design can significantly alter biodistribution and biological half-life is illustr,tted by
Drug-Albumin Complex
Protein binding can have a profound effect on the drug's effective soluhility. biodistribution. half-life in the body. and interaction with other drugs. A drug with such poor water solubility that therapeutic concentrations of the unbound (active) drug normally cannot be maintained still can be a very effective agent. The albumin—drug complex acts as a reservoir by providing large enough concentrations of free drug to cause a pharmacological response at the receptor. Protein binding may also limit access to certain body compartments. The placenta is able to block passage of proteins from maternal to fetal circulation. Thus, drugs that normally would be expected to cross the placental harrier and possibly harm the fetus are retained in the maternal circulation, bound to the mother's serum proteins.
two drugs based on the retinoic acid structure used systemically to treat psoriasis. a nonmalignant hyperplasia. Etreti-
nate has a 120-day "terminal" half-life after 6 months of therapy. In contrast, the active metabolite. acitretin. has a 33-
to 96-hour "terminal" half-life. Both drugs are potentially teratogenic. Female patients of childbearing age must sign statements that they are aware of the risks and usually are
Protein binding also can prolong the drug's duration of action. The drug—protein complex is too large to pass
administered a pregnancy test before a prescription is issued.
through the renal glomerular membranes, preventing rapid excretion of the drug. Protein binding limits the amount of
Acitretin, with its shorter half-life, is recommended for a female patient who would like to become pregnant, because it can clear her body within a reasonable time frame. When effective. etretinate can keep a patient clear of psoriasis lesions for several months.
drug available for biotransformation (see below and Chapter 4) and for interaction with specific receptor sites. For example, the large. polar trypanocide suramin remains in the body
0
Etretinate
Esterase CH3CH2OH
0
Acitretin
IRs. 2-2)
Chapter 2
• Phvsicochemical
Properties it, Relation to Riolugical Action
7
Na
Sodium in the protein-bound liwni Iir as long months (11,2 = 51) days). The maintenance dose tbr this drug is based on weekly administration. At first, this might seem to be an advantage to the patient. It can be. but ii also means that, ¼hould the patient have serious adverse reactions, a significam length of tune will be required before the concentration of drug falls below toxic levels. The drug—protein binding phenomenon can lead to some clinically significant drug—drag interactions resulting when one drug displaces another from the binding site on albumin, A large number of drugs can displace the anticoagulant warfarm from its albumin-binding sites. This increases the effective concentration of wurfarin at the receptor, leading to an increased prothrombin time (increased time for clot formatioll) and potential hemorrhage.
Tissue The
Depots
drug can also be stored in tissue depots. Neutral fat
constitutes some 20 to 50% of body weight and constitutes a depot of considerable importance. The more lipophilic the drug, the more likely it will concentrate in these pharmacologically inert depots. The ultra-short-acting, lipophilic barbiturate ihiopental's concentration rapidly decreases below its effective concentration following administration. It "disappears" into tissue protein, redistributes into body fat, and
then slowly diffuses hack out of the tissue depots but in concentrations too low for a pharmacological response.
molecules absorbed from the gastrointestinal tract enter the portal vein and are initially transported to the liver. A signifi-
cant proportion of a drug will partition or be transported into the hepatocyte, where it may be metabolized by hepatic enzymes to inactive chemicals during the initial trip through the liver, by what is known as the first-pass effect (see Chap-
ter4). Lidocaine is a classic example of the significance of the first-pass effect. Over 60% of this local anesthetic antiarrhythmic agent is metabolized during its initial passage through the liver, resulting in it being impractical to administer orally. When used for cardiac arrhythmia.s, it is administered intravenously. This rapid metabolism of lidocaine is used to advantage when stabilizing a patient with cardiac arrhythmias. Should too much lidocaine be administered intravenously, toxic responses will tend to decrease because of rapid biotransformation to inactive metabolites. An understanding of the metabolic labile site on lidocainc led to the development of the primary amine analogue tocainide. In
contrast to lidocaine's half-life of less than 2 hours, tocainide's half-life is approximately IS hours, with 40% of the drug excreted unchanged. The development of orally active antiarrhythmic agents is discussed in more detail in Chapter 19.
CH3
ci'
—
Thus, only the initially administered thiopental is present in high enough concentrations to combine with its receptors. The remaining thiopenlal diffuses out of the tis.sue depots into systemic circulation in concentrations too small to be
C2H5
CH3 Li doca in.
CH3
effective (Fig. 2-I). is metabolized in the liver, and is excreted.
In general. structural changes in the barbiturate series (see Chapter 14) that favor partitioning into the lipid tissue stores decrease duration of action but increase central nervous system (CNS) depression. Conversely, the barbiturates with the slowest onset of action and longest duration of action contain the more polar side chains. This latter group of barbiturates both enters and leaves the CNS more slowly than the more lipophilic thiopental.
Drug Metabolism All substances in the circulatory system, including drugs, inciabolites. and nutrients, will pass through the liver. Most
R
—
,NH3' Ct.
H-C—C) CH3
CH3 Tocoin ide
A study of the metabolic fate of a drug is required for all new drug products. Often it is found that the metabolites are also active. Indeed, sometimes the metabolite is the pharmacologically active molecule. These drug metabolites can pro-
vide leads for additional investigations of potentially new products. Examples of an inactive parent drug that is converted to an active metabolite include the nonsteroidal anti-
8
Wilson and Giscolds Textbook of Organic Medicinal and Pharmaceutical Chemistry
inflammatory agent sulinduc being reduced to the active sultide metabolite: the immunosuppressant azathioprine being cleaved to the purinc antimetabolite 6-mercaptopunne; and purine and pyrimidinc antimetabolites and antiviral agents
being conjugated to their nucleotide form (acyclovir phosphorylated to acyclovir triphosphate). Often both the parent drug and its metabolite are active, which has ted to additional
commercial products, instead of just one being marketed. About 75 to 80% of phenacetin (now withdrawn from the U. S. market) is converted to acetaminophen. In the tricyclic antidepressant series (see Chapter 14). imipramine and ami-
triptyline are N-deniethylated to desipramine and nortriptyline, respectively. All four compounds have been marketed in the United States. Drug metabolism is discussed more fully in Chapter 4.
Although a drug's metabolism can be a source of frustration for the medicinal chemist, pharmacist. and physician and lead to inconvenience and compliance problems with the patient, it is fortunate that the body has the ability to metabolize foreign molecules (xenobiotics). Otherwise. many of these substances could remain in the body for years. This has been the complaint against certain lipophilic chemi-
cal pollutants, including the once very popular insecticide DDT. After entering the body, these chemicals reside in body
tissues, slowly diffusing out of the depots and potentially harming the individual on a chronic basis for several years. They can also reside in tissues of commercial food animals that have been slaughtered before the drug has "washed out" of the body.
The main route of excretion of a drug and its metabolites is through the kidney. For some drugs. enterohepatic circulation (Fig. 2-I). in which the drug reenters the intestinal tract from the liver through the bile duct, can be an important part of the agent's distribution in the body and route of excretion. Either the drug or drug nietabolite can reenter systemic circulation by passing once again through the intestinal mucosa. A portion of either also may be excreted in the feces. Nursing mothers must be concerned because drugs and their metabolites can be excreted in human milk and be ingested by the nursing infant.
CH3CO3H
R = CH3S(O)
Sulinduc:
Active Sulfide Mstibolite:
R • CH3S
One should keep a sense of perspective when learning about drug metabolism. As explained in Chapter 4. drug
Azathtoprine
6-Marcaptopur
metabolism can be conceptualized as occurring in two stages or phases. Intermediate metabolites that are pharmacologically active usually are produced by phase I reactions. The products from the phase I chemistry are converted into inactive, usually water-soluble end products by phase II reac-
toe
(ions. The latter, commonly called conjugation reactions. can be thought of as synthetic reactions that involve addition
of water-soluble substiiucnts. In human drug metabolism. the main conjugation reactions add glucuronic acid, sulfate. or glutathione. Obviously, drugs that are bound to serum protein or show favorable partitioning into (issue depots are going to be metabolized and excreted more slowly for the H
reasons discussed above. This does not mean that drugs that remain in the body for longer periods of time can be administered in lower doses or be taken fewer times per day by the patient. Several variables determine dosing regimens, of which the affinity of the drug for the receptor is crucial. Reexamine Reaction 2-I and Fig-
R
ure 2-I. If the equilibrium does not favor formation of the drug—receptor complex, higher and usually more frequent doses must be administered. Further, if partitioning into tissue stores or metabolic degradation and/or excretion is favored, it will take more of the drug and usually more frequent administration to maintain therapeutic concentrations at the
R
R R =
01-i
cico
receptor.
Receptor pH3
CHCH2CH2N5 R
a
Aintiriptyilne:
R
Nortrlptyiln.:
R — H
Cit3
Imipramln.: Desipremine:
R • Cit3 R • H
With the possible exception of general anesthetics (see Chapter 14). the working model for a pharmacological response consists of a drug binding to a specific receptor. Many drug receptors are the same as those used by endoge-
nously produced ligands. Cholincrgic agents interact with
Chapter 2 • the same receptors as the neurotransrnitter acetylcholine. Synthetic corticosteroids bind to the same receptors as corti-
sone and hydrocortisone. Often, receptors for the same Iigand are found in a variety of tissues throughout the body. The nonsteroidal anti-inflammatory agents (see Chapter 22) inhibit the prostaglandin-fomiing enzyme cyclooxygenuse. which is found in nearly every tissue. This class of drugs has a long list of side effects with many patient complaints. Note in Figure 2-I that, depending on which receptors contain bound drug. there may be desired or undesired effects. This is because a variety of receptors with similar structural requirements are found in several organs and tissues. Thus. the nonsteroidal anti-inflammatory drugs combine with the desired cyclooxygenase receptors at the site of the inflamma-
tion and the undesired cyclooxygenase receptors in the gastroiinestinal mucosa. causing severe discomfort and sometimes ulceration. One of the "second-generation" is claimed to cause less sedation because it does not readily penetrate the blood—brain untihistamines.
barrier. The rationale is that less of this antihistamine is available for the receptors in the CNS. which are responsible for the sedation response eharactenstic of anlihistamines. In contrast, some antihistamines are used for their CNS depres. sam activity because a significant proportion of the adminis-
tered dose is crossing the blood—brain barrier relative to binding to the histamine H1 receptors in the periphery. Although ii is normal to think of side effects as undesirable, they sometimes can he beneficial and lead to new prod-
ucts. The successful development of oral hypoglycemic agents used in the treatment of diabetes began when it was found that certain sulfonamides had a hypoglycemic effect. Nevertheless, a real problem in drug therapy is patient compliance in taking the drug as directed. Drugs that cause serious problems and discomfort tend to be avoided by patients.
Swnmary One of the goals is to design drugs that will interact with receptors at specific tissues. There are several ways to do this, including (a) altering the molecule, which, in turn, can change the hiodistribution; (b) searching for structures that show increased specificity for the target receptor that will produce the desired pharmacological response while decreasing the affinity for undesired receptors that produce adverse responses: and (c) the still experimental approach of attaching the drug to a monoclonal antibody (see Chapter 7) that will bind to a specific tissue antigenic for the antibody. Biodistribulion can be altered by changing the drug's solubility. enhancing its ability to resist being metabolized
usually in the liver), altering the fortnulation or physical characteristics of the drug, and changing the route of administration. If a drug molecule can be designed so that its binding to the desired receptor is enhanced relative to the undesired receptor and biodistribution remains favorable, smaller doses of the drug can be administered. This, in turn, reduces the amount of drug available for binding to those receptors responsible for its adverse effects.
The medicinal chemist is confronted with several challenges in designing a bioactive molecule. A good fit to a specific receptor is desirable, but the drug would normally be expected to dissociate from the receptor eventually. The specificity for the receptor would minimize side effects. The drug would be expected to clear the body within a reasonable
Propertie.s in Relation to Biological Action
9
time. Its rate of metabolic degradation should allow reasonable dosing schedules and, ideally, oral administration. Many times, the drug chosen for commercial sales has been selected from hundreds of compounds that have been screened. It usually is a compromise product that meets a medical need while demonstrating good patient acceptance.
ACID—BASE PROPERTIES Most drugs used today can be classified as acids or bases. As is noted shortly. a large number of drugs can behave as
either acids or bases as they begin their journey into the patient in different dosage forms and end up in systemic circulation. A drug's acid—base properties can greatly iniluence its biodistribution and partitioning characteristics. Over the years. at least four major definitions of acids and bases have been developed. The model commonly used in pharmacy and biochemistry was developed independently by Lowry and Brønsted. In their definition, an acid is defined as a proton donor and a base is defined as a proton acceptor. Notice that for a base, there is no mention of the hydroxide ion.
Acid-Conjugate Base Representative examples of pharmaceutically important acidic drugs are listed in Table 2-1. Each acid, or proton donor, yields a conjugate base. The latter is the product after the proton is lost from the acid. Conjugate bases range from the chloride ion (reaction a), which does not accept a proton
in aqueous media, to cphedrine (reaction h), which is an excellent proton acceptor. Notice the diversity in structure of these proton donors. They include the classical hydrochloric acid (reaction a). the weakly acidic dihydrogen phosphate anion (reaction b), the ammonium cation as is found in ammonium chloride (reac-
tion c), the carboxylic acetic acid (reaction d). the enolic form of phenobarbital (reaction e), the carboxylic acid moiety of indomethacin (reaction J), the imide of saccharin (reaction g), and the protonated amine of ephedrine (reaction It). Because all are proton donors, they must be treated as acids when calculating the pH of a solution or percent ionization of the drug. At the same time, as noted below, there are important differences in the pharmaceutical properties of ephedrine hydrochloride (an acid salt of an amine) and those of indomethacin. phenobarbital. or saccharin.
Base-Conjugate Add The Brønsted-Lowry theory defines a base as a molecule that accepts a proton. The product resulting from the addition of a proton to the base is the c'onjugate acid. Pharmaceutically important bases are listed in Table 2-2. Again, there are a variety of structures, including the easily recognizable base sodium hydroxide (reaction a): the basic component of an important physiological buffer, sodium monohydrogen phosphate (reaction b), which is also the conjugate base of dihydrogen phosphate (reaction b in Table 2-I); ammonia (reaction c), which is also the conjugate base of the ammonium cation (reaction c in Table 2-I); sodium acetate (reaction d), which is also the conjugate base of acetic acid (reac-
tion d in Table 2-I); the enolate form of phenobarbital
10
Wilson and Gisvold's Textbook of Organic Med
TABLE 2—1
Examples of Adds
—.
Acid (a)
Hy&ochlonc acid
H•
—.
MCI
+ 4
Conjugate Base
Cl-I
phosphate (monobasic sodium phosphate)
0)
Sodium
(c)
Aninonium chlondn NH4CI (NH44. CIiI
(d)
Acetic acid CH3COOH Phenobarbitat
(e)
an,J Pharmaceutical Chemicu-v
—H
+
—.
+
H4
—H
+
— H'
+
NaHPO02
N
NH
(I)
tndon,ethacin
0
O
// C
\OH
— H4
(9)
+
Saccharin
"p
// +
00 (hi
Ephedrine hydrochiotide ,CH3
CR3
(Clia
—.
+
The Midiwu muon and chlonde anion do not mike paul In
(reaction e), which is also the conjugate base of phenobarbital (reaction e in Table 2-I the carboxylate form of indo-
in Table 2.1 are the reactant bases in Table 2-2. Also, notice that whereas phenobarbital, indomethacin, and saccharin are
methacin (reaction ft. which is also the conjugate base of indomethacin (reaction fin Table 2-I); the imidate form of saccharin (reaction g). which is also the conjugate base of
un-ionized in the protonated form, the protonated (acidic)
saccharin (reaction g in Table 2-I); and the amine ephedrine (reaction I,), which is also the conjugate base of ephedrine hydrochloride (reaction h in Table 2-I). Notice that the con-
forms of ammonia and ephedrine are ionized salts (Table 2I ). The opposite is true for the basic (proton acceptors) forms of these drugs. The basic forms of phenobarbital. indomethacm, and saccharin are anions, whereas ammonia and ephedrine are electronically neutral (Table 2-2). Remember that
jugate acid products in Table 2-2 are the reactant acids in Table 2-I. Conversely, most of the conjugate base products
each of the chemical examples in Tables 2-I and 2-2 can function as either a proton donor (acid) or proton acceptor
Chapter 2 • Phv,ci,-oclu',,,ical Properties in Relation a, Biological Action
Examples of Bases
TABLE 2—2
Base (a)
WaOH (Na". 0H1 Sodium monohydrogen
Ic)
Ammonia
tel
H'
+
H' —
acetate
CH3COONa Phenobarbial sodium H..C
Conjugate Add H.,0
+
Na"
+
2Na"
(dibasic sodium phosphate)
(2Na'",HP04
Sodium
—.
+
Sodium hydroxide
(Or
dl
11
I
Na")
+
Ii'
+
H'
+
H' — CI-(3COOH
+
Na"
+
H'
+
Na'"
+
Na'"
+
Na"'
—.
NH4
0 N
)—O (Na')" NI-i
(I)
Indocnelhaciri sodium
0
0
I,
I, C
/
"
C
0(Na)" +
H'
0 (gt
\=!
Saccharin sodium
0
0
//
Ii
C
(Na)"
+
0EJH 00
H'
S.
//\\
00 (hi
\OH
Ephedrine CH3
cl-i.'
/
+
H'
—
H2N I'
OH The sontinin caIu,,n is preseni only to mainhui,i
b.iInncc-. S plays no direel acid -base title.
(base). This can best be understood by emphasizing the conof conjugate acid—conjugate base pairing. Complicated as ii may seem at lit-st. conjugate acids and conjugate bases are nothing more than the products of an acid—base reaction.
In other words, they appear to the right of the reaction arrows. Examples from Tables 2-I and 2-2 are rewritten in Table 2-3 as complete acid—base reactions. careful study of Table 2-3 shows water functioning as a proton acceptor (base) in reactions a. c-. e. g. i. k. and in and a proton donor (base) in reactions I,, d.f 11.1, I. and a, Hence.
water is known as an ti,nphotes-,e substance. Water can be either a weak base accepting a proton to form the strongly acidic hydrated proton or hydroniuni ion 1-1.10 (reactions a. c. x'. i, k. and in). or a weak acid donating a proton to form the strongly basic (proton accepting) hydroxide anion OH- (reactions b, d. I.,), I. and a).
Acid Strength While any acid—base reaction can be wrttten as an equilib-
rium reaction, an attempt has been made in Table 2-3 to
12
Wilson and Gixr'old.c 7'exthook
of Organic Medici,uzl and Pharmaccuth'al C'he,nis:rr
the Exception of Hydrochloric Acid, Whose Conjugate Base (C1) Has No Basic Properties in Water. and Sodium Hydroxide. Which Generates Hydroxide, the Reaction of the Conjugate Base in Water is Shown for Each Acid) TABLE 2—3 Examples of Acid—Base Reactions (With
Add
+
Base
Hydrochlonc acid (a)
HCI
Conjugate Acid
+
Conjugate Base
+
H2O
— H30
+
Cl -
+
NaOH
—.
Sodium hydroxide
H20
=
+
OH -(Na
Sodium dihydrogen phosphate and its conjugate base, sodium monotiydrogen phosphate + H30 H30' (c) H2pO4.(Na)a + HPO42_(2Naja (ci) H20
+
HP042 - (Na
+
OH1N8)a
Asnmonlum chloride and Its conjugate base. ammonIa + H20 (o) + NH1 H20 (9
+
NH3
+
OH -
+
CH3COO
+
OH1Na')'
(b)
=
H30'(cI-r
Acetic acid and Its conjugate base, sodium acetate + H20 (g) CH3COOH (h)
+
H3O
H30
CH3COO1NaIa
_ CH3COOH
Indornelhacan and its conjugate base. Indomethacin sodium, show the Identical acid—base chemistry as aceticactd and sodium acetate, respectively. Pttenobarbatal and its conjugate base. phenobarbital sodium
CH30
CH10
+
H30
H20
H3C
Ii)
+
o
+
H30
+
OHiNar
Saccharin and its conjugate base, saccharin sodium
0
'7 +
On)
+
00 0
,,0
// (I)
(Nala
+
00
+
cPo
Ephedrlne HCI and itS conjugate base, ephedrnse CH3
CH3
/
(Cue +
(m)
H2O
+ ,,Cl-13
1CH3
HN (',)
The
H20
+
OH1NaI°
CH3
anion and ..odinni canon are parsern aillili to autainlain charge balance. Thc,.c anions piay no other acid—baserole.
+
Chapter 2 • Phvsicothe,nical !'ropcr,ies in Relation to Biological Action tndicatc which sequences are unidirectional or show only a small reversal. For hydrochloric acid, the conjugate base. Cl. is such a weak base that it essentially does not function as a proton acceptor. That is why the chloride anion was not included as a base in Table 2-2. In a similar manner, water is such a weak conjugate acid that there is little reverse reaction involving water donating a proton to the hydroxide anion of sodium hydroxide.
Two logical questions to ask at this point are how one in which direction an acid—base reaction lies and to what extent the reaction goes to completion. The common physical chemical measurement that contains this information is known as the pK,. The pK, is the negative logarithm of the modified equilibrium constant. K,. for an acid—base reaction written so that water is the base or proton acceptor. It can he derived u.s follows: Assunie that a sveak acid. HA. reacts with water. Acid
Conj.
Conj. Acid
Base
HA + U.O = H,0
Base
+
IRs. 23)
A
The equilibrium constant. K01. for Reaction 2-3 is K
1H50 llA
—
ucidl[conj. hasel lacidlihasel
—
I
—
—
Equation 2-7 ix more commonly called the HendersonHasselbalch equation and is the basis for most calculations involving weak acids and bases. It is used to calculate the pH of solutions of weak acids, weak bases, and buffers consisting of weak acids and their conjugate bases or weak bases
and their conjugate acids. Because the pK, is a modified equilibrium constant, it corrects for the fact that weak acids do not completely react with water. A very similar set of equations is obtained from the reaction of a protonated amine. BH . in water. The reaction is Conj.
Acid
Conj.
Acid
Base
BH' + H20
+
Base B
Wcightl1()
H101 =
MW110
—
—
—
18 g
K
—
— —
Iconi. acid llconi. bawl lacidlibasel
= K01(55.5) =
IH5OIIBI
= lconj. acidllconj. basel
lacidi (Eq. 2-9)
2-10).
pH = pK, + tog
181
pK, + log
Icrnii. basel
Thu.'.. with [H201 = 55.5, Equation 2-I can be simplified
(Eq. 2-10)
(Eq. 2-2)
pH = pK, + log
By definition. pK., = —log K,
(Eq. 2-3)
and
pH = —log
(Eq. 2-4)
The modified equilibrium constant. K,. is customarily converted to pK, (the negative logarithm) to use on the same scale as pH. Therefore, rewriting Equation 2-2 in logarithmic fonn produces
Substituting Equations 2-3 and 2-4 into Equation 2-6 produces
log
1Al
pK, + log
lconi. base]
lacidl
lacidi
(Eq. 2-lI)
What about weak bases such as amnines? In aqueous solu-
tions. water functions as the proton donor or acid (Rx. 2-5). producing the familiar hydroxide anion (conjugate base). Base Acid 1-1.0 + B
Rearranging Equation 2-5 gives
—log 1110' I = —log K,, + log lA1 — log (HAl (Eq. 2-6) = —log K, + log lconj. hasel — log laeidl
lconj. base]
With this version of the equation, there is no need to remember whether the species in the numerator/denominator is ionized (A/HA) or un-ionized (B/BH The molar concentration of the proton acceptor is the term in the numerator. and the molar concentration of the proton donor is the denominator term.
log K, = log IH,0'l + log IA1 — log IHAI (Eq. 2-5) = log H,0 I + log lconj. busel — log lacidl
p11 = pK,
lacidl
Rather than trying to remember the specific form of the Henderson-Hasselbalch equation for an HA or BH acid, it is simpler to use the general form of the equation (Eq. 2II) expressed in both Equations 2-7 and 2-10.
=
lucidl
8
Rearranging Equation 2-9 into logarithmic form and substituting the relationships expressed in Equations 2-3 and 2-4 yields the same Henderson-Hasselbalch equation (Eq.
55.5 M
= Iconi. acid Jlconj. bascl
(Eq.
Notice that Equation 2-8 is identical to Equation 2-I when the general [conj. acid Jlconj. base] representation is used. Therefore, using the same simplifying assumption that svater remains at a constant concentration of 55.5 M in dilute solutions. Equation 2-8 can be rewritten as
—
to
(Rx. 2-4)
The equilibrium constant.
(Eq. 2.1)
In a dilute solution of a weak acid, the molar concentration of water can be treated as a constant, 55.5 M. This number is based on the density of water equaling I. Therefore. I L of water weighs 1000g. With a molecular weight of 18, the molar concentration of water in I L of water is
13
Conj. Acid BH
Conj. Base
+ 0H
(Rx. 2-5)
Originally, a modified equilibrium constant, the pK5, was derived following the same steps that produced Equation 22. it is now more common to express the basicity of a chemical in terms of the pK,. using the relationship in Equation 2-12.
(Eq. 2-7)
pK,, = pKh — 14
(Eq. 2-121
14
'leribiiok of' ()rganir Medit'inal giiid P/,ar,naeeiuical ClwnrLs:rt
Wilson and
TABLE 2—4 Examples of Calculations Requiring the 1
in the
What Is the ratio of eptiedulne to ephedrune l'lCt Intestinal tract at pH 8.0? Lisa Equation 2-11.
80—9.6+tog
[ephedrune) —1.6
[pheliHOl
0025
The number whose tog is —1.6 Ia 0025, meaning that there are
25 parts ephedrine for every 1000 pails ephedrine Nd in the intestinal tract whose environment is pH 8.0. 2 What Is the pH ota buffer containing 0.t M acetic acid (p1C—R
R4N'—°l
—OH--0=
1-7
\/ —OH--fl /\
NR3
b8 vanderWaals
0.5-1
ally exists. Thermodynamic arguments on the gain in en-
surface of the receptor have been proposed to validate a hydrophobic bonding nmdel. There are two problems with this concept. First, the term /zvdrop/wbk implies repulsion. The term for attraction is hsdrophiliciiv. Second. and perhaps more important. there is no truly water-free region on
1-7
0C
the concept of hydrophobic bonds has developed. There has been considerable controversy over whether the bond actu-
tropy (decrease in ordered state) when hydrophobic groups cause a partial collapse of the ordered waler structure on the
C
1-7
valine. isolcucine. and leucine" arc commonly used to explain why a nonpolar substituent at a particular position on the drug molecule is important for activity. Over the years.
\l/CC\1/
the receptor. This is true even in the areas populated by the nonpolar amino acid side chains. An alternate approach is to consider only the concept of hydrophilicity and lipophilicity. The predominating water molecules solvate polar moieties,
effectively squeezing the nonpolar residues toward each other.
1
1,0.5 .. l.rI'lo fl
See text
AIi'cfl. A Sololiso Toso.il>. Now York. loho Wik) &
956; 181.
as would thc corresponding groups on a biological receptor. Relatively little net change in tree energy would be expectcd in exchanging a hydrogen bond with a water molecule for one between drug and receptor. However, in a drug—receptor combination, several forces could be involved, including the
hydrogen bond, which would contribute to the stability of the interaction. Where multiple hydrogen bonds may be formed, the total effect may be sizable, such as that demon-
strated by the stability of the protein o helix and by the stabilizing influence of hydrogen bonds between specific base pairs in the douhle.helical structure of' DNA. Van der WooLs forces are attractive forces created by the polanzahility of molecules and are exerted when any two uncharged atoms approach each other very closely. Their
Steric Feateres of Drugs Regardless of the ultimate mechanism by which the drug and the receptor interact, the drug must approach the receptor
and fit closely to its surface. Steric factors determined by the stereochemistry of the receptor site surface and that of the drug molecules are, therefore, of primary importance in determining the nature and the efficiency of the drug—recep-
tor interaction. With the possible exception of the general anesthetics, such drugs must possess a high structural speci-
ficity to initiate a response at a particular receptor. Some structural features contribute a high structural rigidity to the molecule. For example. aromatic rings are planar. and the atoms attached directly to these rings are held in the plane of the aromatic ring. Hence, the quaternary nitrogen and carbamate oxygen attached directly to the benzene ring in the cholinesterase inhibitor neostigminc are restricted to the plane of the ring, and consequently, the spatial arrangement of at least these atoms is established.
is inversely proportional to the seventh power of the distance. Although individually weak, the summation of their forces provides a significant bonding factor in highermolecular-weight compounds. For example. ii is not possible to distill normal alkanes with more than 8() carbon atonls. because the energy of —80 kcal/mol required to separate the molecules is approximately equal to the energy required to break a carbon—carbon covalent bond. Rat structures, such as aromatic rings. pennut close approach of atoms. With van ikr Wuals' forces of —0.5 to I .() kcal/mol for each atom. about six carbons (a benzene ring) would he necessary to match the strength of a hydrogen bond. The aromatic ring
c, p—o H3C'—N CR3
N•est igm!nø
The relative positions of atoms attached directly to multiand tran,s ple bonds are also Fixed. For the double bond.
isomers result. For example. diethylslilbestrol exists in two fixed stercoisomeric forms: irans-diethylstilbestrol is estro-
32
tt'ilsøn and
Textbook of Organic Medicinal and Pharmaceutical Chemistry
genic. whereas the cix isomer is only 7% as active. In trailsdiethyistilbestrol. resonance interactions and minimal steric interference tend to hold the two aromatic rings and connecting ethylene carbon atoms in the same plane.
for interacting with a biological receptor in a structurally specilic manner. The United Stales Pharmacopeia recognizes that there are drugs with vinyl groups whose commercial form contains both their E and Z isomers. Figure 2-14 provides four examples of these mixtures. More subtle differences exist for conformasional isomers,
Like geometric isomers, these exist as different arrangements in space for the atoms or groups in a single classic structure. Rotation about bonds allows interconversion of conformational isomers. However, an energy barrier between isomers is often high enough for their independent
trens.Dl.SliySslI lb.strol
H5C2
existence and reaction. Differences in reactivity of functional groups or interaction with biological receptors may be due to differences in steric requirements of the receptors. In certain semirigid ring isomers show significant differences in biological activities. Methods for calculating these energy harriers are discussed in Chapter 28.
Open chains of atoms. which form an important part of many dnig molecules, are not equally free to assume all possible conformations; sonic are sterically preferred. Energy barriers to free rotation of the chains are present, be-
C2H5
cls-Di.thylstllb.strol Geometric ,so,,wrs, such as the cix and the lran.s isomers, hold structural features at different relative positions in space. These isomers also have signiticantly different physical and chemical properties. Therefore, their distributions in the biological medium are different, as arc their capabilities
cause of interactions of nonbonded atoms. For exumple. the atoms tend to position themselves in space so that they occupy staggered positions, with no two atoms directly facing each other(eclipsed). Nonhonded interactions in polymethyIene chains tend to favor the most cxtendcd anti conformations, although sonic of the partially extended gauche conformations also exist. Intramolecular bonding between
C2H5
_,CH2 C2H5
Z-Clomlphene
Z-Doxepln: R1 E-Doxepln: R, H: R2 •
CH2
E-Clomiphene
R2 • H
Z-Cefprozil: R, E-Cefprozll: R1
H; R2 = Gil3
Figure 2—14 • Examples of E arid Z isomers,
R2
H
Chapter 2 •
/
;cçi
H
H
CH3
n.bulane anti conformation
/
ides, a planar configuration is favored in which minimal
H
steric interference of bulky substituents occurs. Hence, an ester may exist mainly in the anti, rather than the gauche. form. For the same reason, the amide linkage is essentially planar, with the more bulky substituents occupying the anti
3-amlno-n-propanol eclipsed conformatIon
0
0
"0 resonance stabilized
anti
gauche
Stabilized planar structure of esters
0 II ,_152
N'
/
R2
H
H
anti
33
substituent groups can make what might first appear to be an unfavorable conformation favorable. The introduction of atoms other than carbon into a chain strongly influences the conformation of the chain (Fig. 215). Because of resonance contributions of forms in which a double bond occupies the central bonds of esters and am-
H H3Q
H
Properties in Relation to !luilogwal Action
resonance stabilized
gauche
position. Therefore, ester and amide linkages in a chain tend to hold bulky groups in a plane and to separate them as far as possible. As components of the side chains of drugs. ester and amide groups favor fully extended chains and also add polar character to that segment of the chain. In some cases, dipole—dipole inleracflons appear to influence structure in solution. Methadone may exist partially in a cyclic fonn in solution because of dipolar attractive tbrces between the basic nitrogen and carbonyl group or because of hydrogen bonding between the hydrogen on the nitrogen and the carbonyl oxygen (Fig. 2-16). In either conformation. methadone may resemble the conformationally more rigid potent analgesics including morphine. meperidine. and their
analogues (see Chapter 23). and it may be this form that interacts with the analgesic receptor. Once the interaction between the drug and its receptor begins, a flexible drug molecule may assume a different conformation than that pre-
Stabilized planar structure of amides
Figure 2—15 • Effect of noncarbon atoms on a molecule's configuration
dicied from solution chemistry. An intramolecular hydrogen bond, usually formed between donor hydroxy and amino groups and acceptor oxygen and nitrogen atoms, might he expected to add stability to a
particular conformation of a drug in solution. However, in aqueous solution, donor and acceptor groups tend to be
Methadone CU3
CU,
j
— H,
2''
08 'H
/\H,
I
4 5NH ""CH,
H3c
Figure 2—16 • Stabilization of conformations by secondary bonding forces.
Methadone stabilized by hydrogen bonding
Methadone stabilized by dlpolar interaclions
34
and Gi,crohl'.c Texthaok of Organic Mrdicinal and Phurniaceuticu! ('hesnlsUv
bonded to water, and little gain in free energy would be achieved by the formation of an intramolecular hydrogen bond, particularly if unfavorable steric fuctors involving nonbonded interactions were introduced in the process. Therefore, internal hydrogen bonds likely play only a secondary role to steric factors in determining the conformutional distribution of flexible drug molecules.
R1
H
effects (Fig. 2-17). (+ )-trans-2-Acetoxycyclopropyl it)methylammoniurn iodide, in which the quaternary nitrogen
Hydrogon.bonding donor groups
0=0: R3
Hydrogen-bonding acceptor groups
Conformatlonal Flexibility and Multiple Modes of Action It has been proposed that the conlormational flexibility of most open-chain neurohormones. such as acetylcholine. epi-
nephrine. scrotonin. histamine, and related physiologically active biomolecuics. permits multiple biological effects to be produced by each molecule, by virtue of their ability to.
0
OH3
interact in a different and unique conformation with different biological receptors. Thus, it has been suggested that acetylcholine may interact with the muscarinic receptor of postganglionic parasympathetic nerves and with acetylcholinesterase in the fully extended conformation and, in a different, more Iblded structure, with the nicotinic receptors at ganglia and at neuromuscular junctions (Fig. 2-17). Conformationally rigid acetylcholine-like molecules have been used to study the relationships between these various possible conformations of acetylcholine and their biological
atom and aectoxyl groups are held apart in a conformation approximating that of the extended conformation of acetylcholine, was about 5 times more active than acetylcholine in iLs muscarinic effect on dog blood pressure and was as active as acetylcholine in its muscarinic effect on the guinea pig The (+ I-trans isomer was hydrolyzed by acetylcholincsterase at a rate equal to the rate of hydrolysis of acetylcholine. It was inactive as a nicotinic agonist. In COfltrust, the (—)-tran.s isomer and the mixed ( ± )—cis isomers were, respectively. 1/500 and 1/10.00() as active as acetylcholine in muscarinic tests on guinea pig ileum and were
inactive as nicotinic agonists. Similarly. the trans diaxial relationship between the quaternary nitrogen and acetoxyl group led to maximal tnuscarinic response and rate of hydrolysis by true acetylcholinesterase in a series of isomeric
CH3 H3C
H2
Extended
Quasi-ring
Acetytthotlne
A
AH.11 OH3
trans
0 H
CH3
2-Acotoxycyctopropyl trimethylammontum Iodide
trans
cis
3-Trlmethylammonlum-2-acetoxydecatins
Figure 2—17 a Acelylcholine conformations (only one each ol the two possible trans and cis isomers is
represented).
_A Chapter 2 • Plivsieoche,nieai Properties in Re!aiio,i in Iiiologii'a! ,tc:ion
3-trimethylarnmonium-2-ucetoxydecalins."' These results could be interpreted as either that acetykholinc was acting in a trans conformation at the muscarinic receptor and not acting in a cisnid conformation at the nicotinic receptor or that the nicotinic response is highly sensitive to steric effects of substitucnts being used to orient the molecule. This approach in studying the cholinergic receptor is covered in more detail in Chapter 17.
35
hihits l2to IS times more vasoconstrictor activity than( + )epinephrine. This is the classical three-point attachment model. For epincphrine. the ben,ene ring. benzylic hydroxyl. and protonated amine must have the stereochemistry seen with the (—) isomer to match up with the hydrophobic or aromatic region, anionic site, and a hydrogen-bonding center on the receptor. The 1 +) isomer (the mirror image) will not
align properly on the receptor.
Optical isomerism and Biological Activity The widespread occurrence of differences in biological activities for optical ai1sv,tn's has been of particular importance in
the development of theories on the nature of
drug—receptor interactions. Most commercial drugs are asymmetric, meaning that they Cannot be divided into symmetrical halves. While o and L isomers have the same physical properties. a large number of drugs are dir,szereo,nerie. meaning that they have two or more asymmetric centers. Diasicreomers have different physical properties. Examples are the diastereomers ephedrine and pseudoephedrinc. The
Pseudoeph.dr in.
Eph.dr In.
(Erythro conflgur.tlon)
(Thr.o configuration)
CH3.
—oil
former has a melting point of 790 and is soluble in water. whereas pseudoephedrin&s melting point is 118°, and it is only sparingly soluble in water. Keep in mind that receptors
DH
will be asymmetric because they are mostly protein, meaning that they are constructed from L-amino acids. A ligand fitting
the hypothetical receptor shown in Figure 2-18 will have to have a positively charged moiety in the upper left corner and a hydrophobic region in the upper right. Therefore, one would predict that optical isomers will also have different biological properties. Well-known examples of this phenomenon include (—1-hyoscyamine, which exhibits 15 to 20 times more mydriatic activity than (+ 1-hyoscyamine. and —)-ephedrine. which shows 3 times more pressor activity than ( + )-ephedrine. 5 times more pressor activity than + )pseudoephedrine. and 36 times more pressor activity than I—)-pseudoephedrinc. All of ascorbic acid's antiscorbutic properties reside in the (+) isomer. A postulated fit to epincphrine's receptor can explain why (—)-epinephrinc cx-
Phe 147
Anionic Site
Receptor (— ) .Epinephrine — more active
Frequently, the generic name indicates a specific stereoisomer. Examples include levodopa. dextroamphetamine. dexromethorphan. levantisole. dexmelhylphenidatc. and levothyroxine. Sometimes the difference in pharmacological activity between stercoisolners is dramatic. The dextrorotatory isomers in the morphine series are cough suppressants with less risk of substance abuse, whereas the levorotatory isomers (Fig. 2-19) contain the analgesic activity and significant risk of substance abuse. While the direction of optical rotation is opposite to that of the morphine series. dextropropoxyphene contains the analgesic activity, and the lern isomer contains antitussive activity. Figure 2-19 contains examples ol drugs with asymmetric
carbons. Some were originally approved as racemic mixtures, and later a specific isomer was marketed with claims of having fewer adverse reactions in patients. An example of the latter is the local anesthetic levohupivacainc. which is the S isomer of hupivacainc. Both the R and S isomers have good local anesthetic activity. hut the R isomer may cause depression of the myocardium leading to decreased cardiac output. heart block hypotension. bradycardia. and
B
25
Ic A
73
ventricular ari-hythmias. In contrast, the S isomer shows less cardiotoxic responses but still good local anesthetic activity. Escialopram is the S isomer of the antidepressant citalopram. There is some evidence that the R isomer, which contains little of the desired selective serotonin reuptake inhibition. contributes more to the adverse reactions than tines the
Lye
S isomer.
102
As dramatic as the above examples of stereoselectivity may be, sometimes it may not be cost-effective to resolve the drug into its stereoisomcrs. An example is the calcium
Figure 2—18 • Diagram of a hypothetical receptor site. show-
ing distances between functional groups.
36
WiL'.on
and Gj.wuld's 1 cxl book of Organic Medicinal and Phannacewical Chemistry
52) Dextromethorphan
Levomothorplian
cM3
$
Levopropoxypt3ene
IR
D.xtropropoxyphene
r "CM2
cM3
..CH2
CH2 R.S
NC
H2C1
CM,
CM2
/LM3 R.S-Buplvacalne Esdialopram
channel antagonist ver.ipamil, which illustrates why it is diflicult to conclude that one isomer is superior to the other. S-Verapamil is a more active pharmacological stereoisomer than R-verapamil, but the former is more rapidly metabolized by the first-pass effect. (First-pass refers to orally administered drugs that are extensively metabolized as they pass through the liver. Sec Chapter 4. S- and R-warfarin are metabolized by two different cytochrome P-450 isozymes. Drugs that either inhibit or induce these enzymes can significantly affect warfarin's anticoagulation activity. Because of biotransformations after the drug is administered, it sometimes makes little difference whether a racemic mixture or one isomer is administered. The popular nonsteroidal anti-inflammatory drug (NSAID) ibuprofen is sold as
Figure 2—19 • Examples of drug stereoisomers.
the racemic mixture. The S enantiomer contains the antiinflammatory activity by inhibiting cyclooxygenase. The R isomer does have centrally acting analgesic activity, but it is converted to the S form in viva (Fig. 2-20). In addition to the fact that most receptors are asymmetric.
there are other reasons why stereoisomers show different biological responses. Active transport mechanisms involve asymmetric carrier molecules, which means that there will be preferential binding of one stereoisomcr over others. When differences in physical properties exist, the distzibution of isomers between body fluids and tissues where the receptors are located will differ. The enzymes responsible for drug metabolism are asymmetric, which means that biological half-lives will differ among possible stereoisomers
Chapter 2 • Pliv.cko.hemii'aI Propertir.c in Relation to Uioiugital ,tction
0
"Cii
AH3
II
S-Ibuprolen
Metabolic Interconversion
CM3
CM3
t
_.CH,
R'Ibuprofen
Figure 2—20 • Metabolic interconversion of R- and S-ibuprofen
of the same molccule. The latter may be a very important sariable because the metabolite may actually be the active molecule.
Calculated Conformations It should now be obvious that medicinal chemists must obtaut an accurate understanding of the active conformation
of the drug molecule. Originally, molecular models were constructed from kits containing a variety of atoms of different valence and oxidation states. Thus, ihere would be carbons suitable fur carbon—carbon single. double, and triple bonds; carbon—oxygen bonds for alcohols or ethers and the carbonyl moiety: carbon—nitrogen bonds for amines. amides. imines, and nitrites: and carbons for three-, four-, five-, and larger-member rings. More complete sets include a variety of hcteroatoms including nitrogen. oxygen. and sulfur
37
crgy diagram is shown in Figure 2-21. Notice that some of the minima are nearly equivalent, and it is easy to move from one minimum to another. From energy diagrams. ii is difficult to answer the question. which of the ligand's low or moderately low conformations fits Onto the receptor? This question can he answered partially by assuming that lower energy conformations are more highly populated and thus more likely to interact with the receptor. Nevertheless, spe-
cific interactions like hydrogen bond formation and dipole—dipole interactions can affect the energy levels of different conformations. Therefore, the bound conformation of a drug is seldom its lowest energy conformation. Numberofeonlormers =
tangle Increment
(Eq. 2.30)
There are three common quantitative ways to obtain estimations of preferred molecular shapes required for a good fit at the receptor. The first, which is the oldest and considered the most acctir,uc, is x-ray crystallography. When propcr1)' done, resolution down to a few angstrom units can be obtained. This permits an accurate mathematical description
of the molecule, providing atomic coordinates in three-dimensional space that can be drawn by using a chemical graphics program. A serious limitation of this technique is the requirement for a carefully grown crystal. Some chemicals will not form crystals. Others form crystals with mixed symmetries. Nevertheless, with the newer computational techniques, including high-speed computers. large databases of x-ray crystallographic data are now available. These databases can be searched for structures. including substructures. similar to the molecule of interest. Depending on how close
match, it is possible to obtain a pretty good idea of the low-energy conformation of the drug molecule. This is a common procedure for proteins and nucleic acids after is
oht:tining the amino acid and nucleotide sequences, respec-
tively. Obtaining these sequences is now largely an automated process.
'rhere also is the "debate" that asks if the conl'ormation
in vartous oxidation states. These kits might be ball and stick, stick or wire only, or space filling. The latter contained attempts at realistically visualiting the effect of a larger atom such as sulfur relative to the smaller oxygen. The diameters
of the atoms in these kits are proportional to the van der radii, usually corrected fur overlap eflècts. In contr,ist. the wire models usually depict accurate intraatomic distances between atoms, A skilled chemist using these kits usually can obtain a reasonably accurate three-dimensional representation. This is particularly true if it is a moderately simple molecule with considerable rigidity. An extreme ex-
ample is a steroid with the relatively inflexible fused-ring
0
I
system. In contrast, molecules with chains consisting of sevcml atoms can assume many shapes. Yet, only one shape or confonsiation can be expected to lit onto the receptor. The
itumber of conformers can be estimated from EluatiOn 230. Calculating the global minimum, the losvest energy conformation, can be a difficult computational problem. Assume that there are three carbon—carbon freely rotatable single bondsthatare rotated in 10" increments, Equation 2-3fistates that there are 46.656 different conformations. A typical en-
120
180
240
Toelon Angie Figure 2—21 • Diagram showing the energy maxima and ima as two substituted carbons connected by a single bond are rotated 360° relative to each other.
38
Wilson and Gixvold.s Textbook of Organic Medicinal awl Phar,naceu,ical Che,nistry
found in the crystal represents the conformation "seen" by the receptor. For rigid molecules, it probably is. The question is very difficult to answer for flexible molecules. A common technique is to determine the crystal structure of a protein accurately and then soak the crystal in a nonaqueous solution of the drug. This allows the drug molecules to diffuse into the active site. The resulting crystal is reanalyzed using different techniques, and the bound conformation of the drug can be determined rapidly without redoing the entire protein. Often. the structure of a bound drug can be determined in a day or less.
Hecause of the drawbacks to x-ray crystallography, two purely computational methods that require only a knowledge of the molecular structure arc used, The two approaches are known as quai,nun nieehwucs and molecular mechanics. I3oth are based on assuniptions that (a) a molecule's threedimensional geometry is a function of the forces acting on the molecule and (F') these forces can be expressed by a set of equations that pertain to all molecules. For the most part, both computational techniques assume that the molecule is
in an isolated system. Solvation effects from water, which are common to any biological system, tend to be ignored, although this is changing with increased computational power. Calculations now can include limited numbers of water molecules, where the number depends on the amount
of available computer time. Interestingly, many crystals grown for x-ray analysis can contain water in the crystal lattice. High-resolution nuclear magnetic resonance (NMR) provides another means of obtaining the structures of macromolecules and drugs in solution. There are fundamental differences between the quantum and molecular mechanics approaches. They illustrate the dilemma that can confront the medicinal chemist, Quantum mechanics is derived from basic theoretical principles at the
atomic level. The niodel itself is exact, but the equations used in the technique are only approximate. The molecular properties are derived from the electronic structure of the molecule. The assumption is made that the distribution of electrons within a molecule can be described by a linear sum of functions that represent an atomic orbital. (For carbon. this would be s.p,,p,. etc.) Quantum mechanics is computa-
tion intensive, with the calculation time for obtaining an approximate solution increasing by approximately times. where N is the number of such functions. Until the advent of the high-speed supercomputers. quantum mechanics in its pare form was restricted to small molecules. In other words. it was not practical to conduct a quantum mechanical analysis of a drug molecule. To make this technique more practical, simplifying techniques have been developed. While the computing time is decreased, the accuracy of the outcome is also lessened. In general, use of calculations of the quantum mechanics type
in medicinal chemistry is a method that is still waiting to happen. It is being used by laboratories with access to large-
scale computing, but there is considerable debate about its utility because so many simplifying approximations must be made for larger molecules. In contrast, medicinal chemists are embracing molecular mechanics. This approach is derived from empirical observations. In contrast to quantum mechanics, the equations in molecular mechanics have exact solutions. At the same time. the parameters that are used in these equations are adjusted
to ensure that the outcome fits experimental observations. In place of the fundamental electronic structure used in quantum mechanics. molecular mechanics uses a model consist-
ing of balls (the atoms) connected by springs (the bonds). The total energy of a molecule consists of the sum of the following energy terms: stretching and compren,ing of he bonds (springs) bending about a central atom E: rotation about bonds van tier Waals' interactions electrostatic inter.Ictions
Each atom is defined (parameterized) in terms of these energy terms. What this means is that the validity of molecular mechanics depends on the accuracy of the pararneferitalion process. Historically, saturated hydrocarbons have proved easy to parameteri,e, followed by selective hetcroaioms such as ether oxygens and amines. Unsaturated systems. including aromalicity. caused problems because 01' the delo-
ealization of the electrons, but this seems to have been solved. Charged atoms such as the carhoxylate anion and protonated amine can prove to he a real problem, particularly
if the charge is delocalized. Nevertheless, molecular mechanics is being used increasingly by medicinal chemists to gain a better understanding of the preferred conformation of drug molecules and the macromolecules that compose a receptor. The computer programs are readily available and run on relatively inexpensive. but powerful, desktop computers.
In summary. quantum mechanics attempts to model the position or distribution of the electrons or bonds, while molecular mechanics attempts to model the positions of the nuclei or atoms. Quantum mechanics calculations are used commonly to generate or verify molecular mechanics parameters. Larger structures can be studied by use of molecular mechanics, and with simulation techniques such as molecular dynamics, the behavior of drugs in solution or even in passage through hilayer membranes can he studied.
The only way to test the validity of the outcome from either quantum or molecular mechanics calculations is to compare the calculated structure or property with actual experimental data. Obviously, crystallographic data provide a reliable measure of the accuracy of at least one of the lowenergy conformers. Since that is not always feasible, other physical chemical measurements are used for comparison. These include comparing calculated vibrational energies, heats of formation, dipole momnems. and relative conformational energies with measured values. When results are inconsistent, the parameter alues are adjusted. This readjustment of the parameters is analogous to the fragment approach for calculating oclanol/wamer partition coefficients. The values for the fragments and the accomnpanying correction factors are determined by comparing calculated partition coefficients with a large population of experimentally determined partition coefficients.
Three-Dimensional Quantitative Structure-Activity Relationships With molecular modeling becoming more common, the QSAR paradigm that traditionally used physicochemical descriptors on a two-dimensional molecule can be adapted to
Chapter 2 • Phvsjroche,,,kul Proper: k's in Relation
three-dimensional space. Essentially, the method requires knowledge of the three-dimensional shape of the molecule. Indeed, accurate modeling of the molecule is crucial. A reference (possibly the prototype niolecule or shape is selected against which all other molecules are compared. The original method called for overlapping the test molecules with the rek'rcnce molecule and minimizing the differences in overlap. Then distances were calculated between arbitrary locationson molecule. These distances were used as variables in QSAR regression equations. While overlapping rigid ring systems such as tetracyclines. steroids, and penicillins are relatively easy. flexible molecules can prove challenging. Examine the following hypothetical molecule. Depending
fir
liiologic'al Act ion
39
genetic code to determine the amino acid sequence. The parts of the receptor that hind the drug (ligand) can be determined
by site-directed mutagenesis. This alters the nucleotide Sequence at specific points on the gene and, therefore, changes specific amino acids. Also, keep in mind that many enzymes
become receptors when the goal is to alter their activity.
represented by X, a family of compounds represented by this molecule could have a variety of conlonrtations. Even when
Examples of the latter include acetylcholinesterase, monoamine oxidase. HIV protease. rennin. ACE, and tetrahydrofolate reductase. The starting point is a database of chemical structures. They may belong to large pharmaceutical or agrochensical tirms that literally have synthesized the compounds in the database and have them "sitting on the shelf." Alternatively. the database may be constructed so that several different chemical classes and substituent patterns are represented. (See discussion of isosterism in the next section.) The first
the conformations might he known with reasonable cer-
step is to convert the traditional or historical two-dimen-
tainty. the reference points crucial for activity must he identilied. Is the overlap involving the tetrahedral carbon important for activity! Or should the live-membered ring provide the reference points? And which way should it be rotated? Assuming that R5 is an important part of the pharmacophore.
sional molecules into three-dimensional structures whose intramolecular distances are known. Keeping in mind the prob-
on the sii.e of the various R groups and the type of atom
should the live-nternbered ring be rotated so that R5 is pointed down or up? These are not trivial questions. and successful 3D-QSAR studies have depended on just how the
investigator positions the molecules relative to each other. There are several instances in which apparently very similar structures have been shown to bind to a given receptor in differettt orientations.
Cl
R8
NH ¼'
(I Rd
There are a variety of algorithms for measuring the degree of confurmational and shave similarities, including molecuar shape analysis (MSA )I distance gcotnelry,'8 and molec-
ular similarity matrices.'9'20 Many of the algorithms use graph theory. in which the bonds that connect the atoms of a nmlecule can he thought of as paths between specific points on the molecule. Molecular connectivity is a commonly used application of graph theory.21 23
Besides comparing how well a family of molecules overlaps with a reference molecule, there are sophisticated software packages that determine the physicochemical parameters located at specific distances from (he surface of the tiolecule. An example of this approach is comparative molectilar field analysis (CoMFA). This technique is described in more detail in Chapter 3.
Database Searching and Mining As pointed out above, receptors are being isolated and cloned. This means that it is possible to determitie their struc-
tures. Most are proteins, which means determining their amino acid sequence. This can he done either by degrading the pmtein or by obtaining the nucleotide sequence of the structural gene coding for the receptor and using the triplet
lems of finding the . 'correct" conformation l'or flexible molecule, false hits and misses might result from the search. Next, the dimensions of the active site must he determined. Ideally. the receptor has been crystallized. ttnd from the coordinates, the intramolecular distances between what are assumcd to be key locations are ohtttincd. If the receptor cannot be crystallized, there arc methods for estimating the threedimensional shape based on searching crystallographic databases and matching amino acid sequences of proteins whose tertiary structure has been determined.
Fortunately, the crystal Structures of literally thousands of proteins have been determined, and their structures have been stored in the Brookhaven Protein Databank. It is now
known that proteins with similar functions have similar amino acid sequences in various regions of the protein. These sequences tend to show the same shapes in terms of ra helix, parallel and antiparallel forms. urns in the chain. etc. Using this information plus molecular mechanics parameters, the shape of the protein and the dimensions of
the active site can he estimated. Figure 2-18 contains the significant components of a hypothetical active site. Notice that tour amino acid residues at positions 25. 73. 102. and 147 have been identified as important either for binding the ligand 10 the site or for the receptor's intrinsic activity. Keep in mind that Figure 2-18 ix a two-ditnensional representation of a three-dimensional image. Therefore, the distances between amino acid residues must take into account the fact that each residue is above or below the planes of the other
three residues. For an artificial ligand to "dock." or lit into the site, six distances must be considered: A. Lys—Glu: B, Glu—Phe: C'. Phe—Ser: I). Ser—Lys: E. Glu—Phe: and F, Lys—Phe. In reality, not all six distances may be important.
In selecting potential ligands. candidates might include a positively charged residue (protonated amine), aromatic ring, hydrogen bond donor or acceptor (hydroxy. phenol, amine. nitro). and hydrogen bond acceptor or a negatively charged residue (carboxylate) that will interact with the aspartate, phenylalanine. scrine. and lysinc residues, respectively. A template is constructed containing the appropriate residues at the proper distances with correct geometries, and the chemical database is searched for molecules that fit the template. A degree of lii or match is obtained for each "hit."
Their biological responses arc obtained, and the tuodel for
40
Wilson and Gistold.c Textbook of Organic Medicinal and Pharmaceutical Chemistry
the receptor is further refined. New. better-defined ligands
sites. Robotic devices are available for this testing. Based on the results, the search for viable structures is narrowed,
may be synthesized. In addition to the interatomic distances, the chemical databases will contain important physicochemical values includ-
and new compounds are synthesized. The criteria for activity
ing partition coefficients, electronic terms, molar refractivity. pK4s. solubilities. and steric values. Arrangements of atoms may be coded by molecular connectivity or other to-
pological descriptors. The resull is a "flood of data" that requires interpretation, large amounts of data storage, and rapid means of analysis. Compounds usually must fit within defined limits that estimate absorption. distiibution, metabolism, and excretion (ADME). Chemical databases can contain hundreds of thousands of molecules that could be suitable ligands for a receptor. But, no matter how good the fit is to the receptor, the candidate molecule is of no use if the absorption is poor or if the drug is excreted too slowly from the body. An analysis of 2,245 drugs has led to a set of "rules" called the Lipinski Rule of A candidate molecule is more likely to have poor absorption or permeability if I. The molecular weight cxcecds 5(X) 2. The calculated octanol/water partition coefficient exceeds 5 3. There are more than 5 H-bond donors cxpre.ssed as the sum of 0—H and N—H groups 4. There are more than 10 H.hond acceptors expressed us the sum of N and 0 atoms
The rapid evaluation of large numbers of molecules is sometimes called high-throughput screening (Fig. 2-22). The screening can be in vitro, often measuring how well the tested molecules bind In cloned receptors or enzyme active
CheiukoJ tesled
1,, so in i'iIro
I
will be based on structure and physicochemical values. QSAR models can be developed to aid in designing new active ligands.
Alternatively, the search may be virtual. Again starting with the same type of database and the dimensions of the active site, the ability of the compounds in the database to
fit or bind is estimated. The virtual receptor will include both its dimensions and physicochemical characteristic. Keeping in mind that the receptor is a protein, there will be hydrogen bond acceptors and donors (serine, threonine. lyrosine), positively and negatively charged side chains (lysine, histidine, glutamic acid, aspartic acid), nonpolar or hydrophobic side chains (leucine, isoleucine, valine, alanine), and induced dipoles (phenylalanine, tyrosine). The type of groups that will be attracted or repulsed by the type of amino acid side chain is coded into the chemical database. The virtual screening will lead to development of a refined model for good binding, and the search is repeated. When the model is considered valid, it must be tested by actual screening in biological test systems and by synthesizing new compounds
to test its validity.
The term isoslerLsni has been used widely to describe the selection of structural components—the steric. electronic, and solubility characteristics that make them interchangeable in drugs of the same pharmacological class. The concept
Target Receptor
Eveiaad.a does
or ActiVe Sit.
ealfr4'by
Chemical Structure
I Chemical Structure I Database
I Database (Includes descriptors)
(Includes descriptors)
RsflnsModel
I Virtual Screening
I
—
1 I
Results
FIgure 2—22 a High-throughput screening.
j
Chapler 2 • l'hv.sjciohesnjea!
in Rehnir,n to
.4ilioii
41
of IsOstenslil has evolved and changed
in the years since its introduction by Langmuir in 1919/' Lang-
similar electronically, are sufficiently alike in their steric nature to be frequently interchangeable in designing new
muir. while seeking a cunctation that would explain similarities in physical properties for nonisomenc molecules, defined is issleres as compounds or groups of atoms having the same number and arrangement of electrons. Isosteres that were isoelectric (i.e.. with the same total charge as well as the Sante number of electrons) would possess similar physical properties. Forexample. the molecules N2 and CO both pox-
drugs.
54155 (4 total electrons and no charge and show similar physi-
propellics. Related examples described by Langmuir N4 . and NCO (Table 2-14).
were CO2.
With immcreaxed understanding of the structures of molecuks. less emphasis has been placed on the number of elec-
Compounds may he altered by isosteric replacements of atoms or groups, to develop analogues with select biological effects or to act as antagonists to normal metaholitcs. Each
series of compounds showing a specific biological effect must be considered separately, for there are no general rules
that predict whether biological activity will be increased or decreased. Some examples of this type follow. When a group is present in ti part of a molecule in which it may be involved in an essential interaction or may influence the reactions of neighboring groups. isosteric replace. ment sometimes produces analogues that act as antagonists.
trons involved, because variations in hybridization during bond formation may lead to considerable differences in the angles. lengths, and polarities of bonds formed by atoms
The 6-NH2 and 6-OH groups appear to play essential roles in the hydrogen-bonding interactions of base pairs during nucleic acid replication in cells. The substitution of the sig-
with the same number nt peripheral electrons. Even the same atom may samy widely in its structural and electronic charac-
nificantly weaker hydrogen-bonding isosteric sulfhydryl
t41nstics when it forms part of a different functional group. Thus, nitrogen is part of a planar structure in the nitro group hut forms the apex of a pyramidal structure in ammonia and
groups results in a partial blockage of this interaction and a decrease in the rate of cellular synthesis.
Similarly, replacement of the hydroxyl group of l)terolglutamic acid (folic acid) by the amino group leads to arni-
amine_s.
nopterin, a folate antimetabolite. Addition of the methyl
Groups of atoms that impart similar physical or chemical properties to a molecule because of similarities in size. dcclrotlegativity. or stereochemistry are now frequently referred to by the general term of iso.cwre. The early recognition that hen,.ene and thiophene were alike in many of their properties
group to the p-aminohcnzoate nitrogen produced methotrexate, which is used in cancer chemotherapy. for psoriasis. and as an onmunosuppressant in rheumatoid arthritis. As a better understanding of the nature of the interactions between drug-metabolizing enzymes and biological recep-
lcd to the tenim ring equim'aie;us for the vinylene group i—CH=CH—) and divalent sulfur (—S—). This concept has led to replacement of the sulfur atom in the phenothia,ine ring system of tranquilizing agents with the vinylene
tors develops, selection of isnsteric groups with particular electronic. solubility, and steric properties should permit the rational preparation of drugs that act more selectively, At
group to produce the dibenzodiazepine class of antidepressaul drugs (see Chapter 14). The vinylenc group in an aromatic ring system may be replaced by other atoms isosteric to sulfur, such as oxygen (luran) or NH (pyrrole): however. in stich cases, aromatic character is significantly decreased. Examples of isosteric pairs that possess similar steric and electronic configurations are the carboxylate (COO-) and
of the principles of isosteric replacement are aiding in the understanding of the nature of these receptors.
sulfonamide (SO.NRJ ions, ketone (C=O) and sulfone 0 = S = O groups. chloride (Cl and trifluoromethyl (CF3) groups. Divalent ether (—0—). sulfide (—S—f. amine
The field of drug design. particularly those aspects that are computer intensive. is increasingly being featured on Web pages. Faculty and students might hod it instructive to search the Web at regular intervals. Many university chemistry departments have organized Web pages that provide excellent linkages. Listed below are a small number of representative sites that feature drug design linkages. Some have excellent illustrations. These listings should not be considered any type of endorsement by the author, editors, or publisher. Indeed, some of these sites may disappear.
i—NH—). and methylene (—CU2—) groups, although dis-
TABLE 2-14 Commonly Used Alicyclic Chemical Isosteres 5
1!ni%aknm uton%s and groups
—OH
—('H 2)
—Hr
ti
—-i-—Pr
21.015 snd groups tm
12
(SI C
—F —Ct
—SH
—CH1—
—0--- -('ONI4R
--NIl—
—('OCH2R
{'02R
—S—
---COSK
Tnsalcrmt at&)rn.s und
SELECTED WEB PAGES
htlp://www.nih.gov/ (Search menns: QSAR: molecular modeling) hmtp://www.pharma.etht.ch/qsar/ tittpJ/www.scamag.eomllinks/deiaull.htmnt http://www.inih-jentm.de/IMAGE.htmt http://www.coopcr.edu/engiimcenngIchcmechcmn/monte.hmniI hltp:lltrimon.ps.toyaku.uc.jp/—dohashi/damabase/indexc.htmnl
http://www.clunel.edu/BioDcv/ommIgatlery.htm http://www.mmetsci.org/Science/Comimpchcnm/featurcI9.hmml
http://clogp.pomona.cdu/meiichcm/chenilqsar-tlh/index.html
ti V.ok. A
the same time, results obtained by the systematic application
5. B knlk Prcss,
Organic ('twmu54r) or l)nig lk.agn and 0mg AdioS..
htmp://www.mima.ss.edulmnicrobio/rasnmohlmndex2.htm
'191.
http:I/www.wcbmo.ncti
42
VII con
wtd Gixvold'.c l'extbrsok of Organie Medicinal a,ul Pharmaceutical Chentislrv
REFERENCES I. Cvum'ltrrmn, A..
and
Fraser,
T.: R.
Soc.
Edinburgh
25:151.
11(68- 869. 2. Hansch. C..
Leo, A.. and Hockman. D.: Exploring QSAR: Hydropho-
and Steric Constants. Washington. DC. American Chemical Socicly, 1995. 3. Ilansch. C.. and Lien. E. 3.: 3. Mcd. Chem. 14:6,33. 1971. -I. Dearden. S.C.. and George, E.: 3. Phann. Phamtacol. 31:S45P, 1979. 5. Kuhinyi, H.: The bilinear model. In Kuchar. M. (ed). QSAR In Design of Iiioactive Molecules. Barcelona. 1. K. Protr,. 1984. Ii. Kut'inyi, H.: 3. Med. Chcm, 20:625. 1971. 7. Free, S. M.. and Wilson, 3. W.: .1. Med. Chem. 7:395. 1964. M.. and Ccladnfk. M.: The use of Free-Wilson 8. Waio.er, K.. model on investigating the relationship between the chemical structure and selectivity of drags. In Kttchar. M. led.). QSAR in Design at Bioaclive Molecules, Itarcelona. J. R. Prous. 1984. 9. Krusowski. M.D.. Hung. X.. Hoplingcr. A. J.. and Harrison. N. l,.:J. Med. Chem, 45:32 It), 2002. Ill. Vcdani. A.. and DottIer. M.: 3. Med. Chem. 45:2139. 2002. II. Stuper. A. 3.. Hrtlggcr, W. E.. and Jurs, P.C.: computer Assisted Studies of Chemical Structure and Biological Function. New York. John Wiley & Sons.. 1979. 12. Baum. R.. and Borman. S.: Client. Eng. News 74:28, 996. 13. Gordon, F. M., Barrett. K, W., Dower. W. 3.. ci al.: 3. Med. Chcm. 37: bic.
1385.
994,
14. Baker. B. K.: 3. Pharm. Sd. 53:347. 19M. 15. Chinu. C. Y.. Long. 3. P.. Cannon. J. G.. and Armstrong. P. D.: 3. Phunnacol. tap. 'flier. 166:243. 1969. $6. Smisstnan. E.. Nelson. W.. Day. 3.. and LaPidus. 3.: 3. Med. Chem. 9: 45)4. 1966.
$7. Hopltnger, A. 1.. and Burke. 0.3.: Molecular shape analysis: a formalism to quantitatively estuhlislt spatial molecular similnrily. In Johnson. M. A.. Maggioru, G.M. (eds.). Cotlcepts and Applications of Molecular
Similarity. New York. Johit Wiley & Sons. 1990. 18. Srivastava. S.. Richardson. W. W.. Bradely, M. P.. and Crippen. 0. M.: Three-dimensional receptor modeling using distance geometry and
Voronoi polyhydra. In Kubinyi. H. (ed). 3D-QSAR in I)rug Design: Theory. Methods and Applications. Leiden, The Netherlands. ESCOM. 1993.
19. Good, A. C.. Peterson, 5. 3.. and Richards. W. G.: 3. Med. Chetti. 36: 2929.
24. Lipinski. C. A.: J. Pharmucol. Toxicol. Methods 44:235. 2000. 25. Lipinski.C. A.. Lombardo. F.. I)omiity. It. W.. and Feeney. P.3.: Ads. Drug Dcliv. Rev. 46:3, 20(11. 26. Langmuir. I.: 3. Ant. Cherts. Soc. 41:1543. 1919.
993.
20. Grunt, A. C.. and Richards. W. G.. l)rug mt. J. 30:371. 1996. 21. Kier, I.. B.. and Hall. L. H.: Molecular Connectivity in Chemislry and Drug Research. New York. Academic Press, 1976. 22. Kier. L. B..and Hall. L. H.: Molecular Connectivity itt Structure-Activity Analysis. New York. Research Stctdies Press (Wiley). 1986. 23. Bonchev. D.: Information Theoretic Indices 6r Characterication of Chemical Structures. New York. Research Studies Press (Wiley). 1983.
SELECTED READING Abraham. 0. lcd.): Burgers Medicinal Chemistry and Drug Discovery. 6th ed. New York. Wiley.mntersciettce, 2003. Albert. A.: Selective Toxicity, 7th ml. New York. Chapman & Hall. 1985. Dean, P. M. (ed): Molecular Similarity in Drug Design. New York, Chaptnan & Hull. 1995. Devillers. 3. and Balaban. A. T.. teds.): Topological Indices and Related Descriptors in QSAR and QSPR. Amsterdam. Gordon and Breach. 1999.
Frunke. K.: Theoretical drug design methods. In Nauta. W. T.. and Rekker. R. F. (cdx.). Pharmacochemisu'y Library. vol. 7. New York. F.lxevier. 19144.
GOner. 0. F. led.): Pharmacophore Perception. Development, and Use in Drug Design. Lu Jolla. CA. International University Line. 2000. Hanach. C.. and Leo. A.: Explonng OSAR. vol. I. Fundamentals and Applications in ('heniisrry and Biology. Washington. DC. American Chemical Society. 1995.
Keverling Buisman. J. A.: Biological activity and chemical structure. In Nautu, W. 1.. Rekker. R. F. teds,). Pharmacoclsemistry Library. s'ol 2. New York. Elscvier. 1977. Kier. I.. B.. and Hall. L. H.: Molecular Structure Description, the Electrutopological Stale. New York. Academic Press. 1999. Leach, A. R.: Molecular Modeling Principles and Applications. Essex. England, Longmun 1996. Leo, A.. Hanuch, C.. and Hoekman. D.: Exploring QSAR. vol.2. Hydropho.
bic. Electronic, und Steric Constants. Washington, DC. Antericun Chemical Society. 1995. Martin, Y. C.: Quantitalive drug design. In Grunewuld. G. (edt. Medicinal Research. vol. 8. New York. Dekker, 1978. Mutschler. F.. and E. cds.l. Trends in Medicinal Chemistry. Berlin. VCH Publishers. 1987. Olson. E. C., and Chrisnollersen. R. E.: Coutputer assisted drug design. In Cunistock. M. J. (ed.l. ACS Symposium Serier. vol. 112. Washington. DC, American Chemical Society, 1979. Rappd, A. K.. and Cusewit. C. 3.: Molecular Mechanics Across Chemistry. Sauxalito. CA. 1997. Silverman, K. B.: The Organic Chctnisrry of Drug Design and Drug Action. New York. Academic Press, 1992. Topliss. 3. G.: Quantitative Structure.Activity Relationships of Drugs. Medicinal Chemistry, A Series of Monrogruphs. vol. 19. New York. Academic Press. 1983.
Young, 0.: Computational Chemistry. A Practical Guide for Applying Techniques to Rcal World Prohletnt. New' York. Wiley-lntcrscicncc. 200$.
CHAPTER 3 Combinatorial Chemistiy DOUGLAS R. HENRY
The term f)arath/,'m s/si/i is an overused one. hut in the mid1980s a true paradigm shift occurred in the way new drugs are synthesized and screened for activity. Prior to then, most
drug compounds were synthesized in milligram quantities in a serial one-at-a-time fashion. After synthesis, the compound was sent to a biologist, who tested it in several in vitro assays and returned the results to the chemist. Based on the assay results, the chemist would apply sonic structure—activity relationship (SARI or use chemical intuition to decide what changes to make in future versions of the molecule to improve activity. Using this iterative process, a
use. In 1963. Merrifield introduced the efficient synthesis of peptides on a solid support or resin (Fig. 3-2).' This made
the rapid, automated synthesis of peptides possible, and earned Merrilield a Nobel Prize in 1984. A key feature of his approach is the attachment of a growing peptide chain loan inert polymer bead. tisually about 100 4um in diameter. composed of polystyrene cross-linked with divinyl bcnzcne. Such beads were originally designed for size exclusion chromatography. The beads can be immersed in solvents. washed, heated. etc.. and when the synthesis is complete.
tures per week. Since the yield of marketable drugs from
the beads can be filtered l'rorn solution, and the reaction products can be cleaved front the polymer. yielding pure products. A Hungarian chemist. Arpad Furka, realized that
compounds synthesized and tested is only about I in 10.0(X). the road to success has been a long and expensive one, taking 6 to 12 years and costing S5(X) to $800 million per drug. In the mid- 1980s. this approach to drug synthesis changed
Merrifield's approach could be extended to allow the sytitheciv of all possible combinations of a given set of amino acids in a limited number of steps. He accomplished this by splitting and remixing portions of the peptide-bound resin at each
dramatically with the introduction ol combinatorial chemistry. The drug discovery process becanic a highly parallel
step in the synthesis (Fig. 3-3). His description of the use of combinatorial chemistry to synthesize polypeptides appeared in the Hungarian patent literature in 1982. Apparently, it is the lirst literature reference to a combinatorial
chemist would be able to synthesize only a handful of struc-
one, in which hundreds or even thousands of structures could be synthesized at one time. Interestingly, biologists had for
some lime been using high-throughput screening HTS) to perform their in vitro assays, running assays in 96-well microtiter plates and even using laboratory robotics for pipetting and analysis. The bottleneck had become the synthesis of the compounds to test. Chemists realized that syntheses could also be conducted by using a parallel approach. The term conrb,,satoru,l chemistry was coined to refer to the par— aDd generation of all possible co,ubinaiions of substituents uc components in a synthetic experiment. Whereas the yield fmm a serial synthesis is a single compound. the yield from a
chemistry experiment.2 As seen in Figure 3-3. the advantage of split-and-mix syn-
thesis is that all 27 tripeptides can be synthesized in just three steps, instead of 27 steps. The disadvantage of this approach is that in the end, one obtains three mixtures of beads with tripcptides attached, rather than the pure compounds themselves. If activity is detected in one of the mix-
combinatorial synthesis is a chemical liltrar. Figure 3-I
tures, it becomes necessary to go back and resynthesize some or all of the structures in that mixture, to see which tripeptide is responsible for the activity. As we shall see, various methods for tagging and deconvoluting combinatorial libraries
shows two common types of chemical libraries—a generic
have been devised that reduce or eliminate the need for re-
library, based on a single parent or scaffold structure and multiplesubstituentsorresidues. and a mixture library, containing a variety of structure types. The total number of structures in alibrary iseitherthe product of the various nunibersofsubstitnents (for a generic library) or the total number of structures in a mixture. The goal of conihinatonal chemistry is to be able
synthesis.
to synthesize, purify, chemically analyze. and biologically test all the structures in the library, using a.s few synthetic cxperimenisas possible. This chapterdescrihes how combinatorial chemistry and HTS are being used in drug design and discosery to find new lead structures in a sluwter time.
HOW IT BEGAN: PEPTIDES AND OTHER LINEAR STRUCTURES Combinatorial chemistry was first applied to the synthesis of peptides, since a convenient method for the automated already in svidesprcad synthesis of these compounds
The first combinatorial chemistry experiments were applied to the study of epitopes—the short sequences of amino
acids responsible for antibody recognition and binding to proteins. Early researchers used solid-phase resin beads in vials. microtiter plates. colutnns, and porous plastic mesh "tea bags" They also used brush-like arrays of plastic pins. at the ends of which compounds could be synthesized. Other media that have been used include paper and polymer sheets and glass chips—basically anything that can immobilize a structure for the purpose of exposing it to reagents and solvents (Fig. 3-4). Peptides. of course, make poor oral drug molecules because they hydrolyze in the acidity of the stomach. As combinatorial methods were applied to the synthesis of drugs, a need developed for methods of generating small (molecular weight. . These are common resins I used in site exclusion chromatography. bcnzcnc ZibOLII
• TentaGel resins. Polystyrene in which some of the phenyl groups have polyethylene glycol mPEG) groups attached in position. Thc free OH groups of the PEG allow the attachment of compounds to be synthesited. • Polyacrylamide resins. Like "simper glue.' these resins swell betmer in polar solvents and, since they contain amide bonds. more closely resemble biological materials.
49
described above, solution-phase combinatorial chemistry often leads to a mixture of products. Imagine reacting a set of 10 amincs with 10 acid chlorides, all in one flask, and with the reactants and conditions chosen so that no reaction
of amines with amines or chlorides with chlorides occurs. only reactions between amines and chlorides. The result would be a mixture of 1(X) amides, one for each possible combination of amine and acid chloride. The resulting mixture could then be tested for activity, under the assumption that the inactive amides did not interfere with binding of active molecules (not always a valid assumption). If activity is found, smaller subsets of amines and chlorides can he tested to eventually find the structure(s) responsible fur ac-
tivity. Researchers have gone one step further by reacting multiplc kinds of reactants together to produce some rather aniaz-
ing mixtures. Figure 3-Il shows an example of a four-com-
• Glass and ceramic heads. Not a type of organic resin hut
ponent Ugi reaction that yields, alter appropriate further
sometimes used when high-temperature or high-pressure reactions are needed.
transformation of the intermediate product, a mixture of carboxylic acids, esters and thioesters. pyrroles. I .4-benzodiazepine-2.5-diones. and even a monosaccharide.'4 Despite the diversity of the chemistry. the yields of products in such mixture-based experiments are often linind to he about 90%
To support the attachment of a synthetic target, the poly-
mer is usually modified by equipping it with a linker or anchor group. Such groups must be stable under the reaction
conditions, but they must be susceptible to a "cleavage" reaction that allows removal of the product. Sonic common linkers arc shown in Figure 3-la. along with the reagents that cleave the prraiuct from the resin. Some specialized linkers have been developed to meet particular reaction or product conditions. So-called Iraceless linkers can be cleaved from the resin with no residual func-
tionality left. This allows the attachment of aryl and alkyl products that do not have OH or NH functionality. These linkers usually include a silyl group
that is sen-
sitive to acids amid can be cleaved to give unsubstituted
or better. Although this is an extreme example of a multicomponent reaction, it illustrates the utility of solution-phase chemistry forgenerating great diversity in chemical libraries.
An approach that is intermediate between solid-phase chemistry and solution-phase chemistry is to use soluble polymers as a support for the product. PEG is a common vehicle in many pharmaceutical preparations. Depending on the degree of polymerization. PEG can he liquid or solid at room temperature and show varying degrees of soluhility in aqueous and organic solvents. Each molecule of PEG has an OH group at either end:
phenyl or alkyl products. A class of linkers known as "safety-catch'' linkers are inert to the synthesis conditions hut have to be chemically transformed to allow final liberalion of the product fromn the resin. Typically, two reactions are required to break the linker (hence the name). A rather elegant approach to linker chemistry is to use linkers that
are sensitive to ultraviolet WV) light, The Affymux group has used these in the synthesis of carboxylic acid and carbox-
amide products,L Finally, some groups have used linkers that can only he cleaved by cnzymues)'
SOLUTION-PHASE COMBINATORIAL CHEMISTRY Most ordinary synthetic chemistry takes place in solution. When a reaction must be modified to accommodate a solid support, it takes time and resources to develop and optimize the reaction conditions. Indeed, a combinatorial chemist may spend months designing a solid-phase reaction and gathering the necessary materials but then conduct the entire synthesis in a matter of hours or days! Many reactions cannot ever be run on solid supports because of poor yields or failed reac-
tions. For these reasons, there has been much interest in using solution-phase chemistry for the preparation of combinatorial libraries. Unlike one-bead one-compound synthesis
By converting one OH group to a methyl ether (MeO—PEG—OH), it is possible to attach a carboxylic acid functionality to the free OH and use solution-phase comuhinatonal chemistry to synthesize, for example. iV-aryl-sulfonamide structures.° The resulting mixture of PEG-bound sulfonamides can be separated by use of chromnatography. Another type of soluble support is dendnimers. These are large, highly branched molecttles with terminal amino groups that can be used like the OH groups of PEG fur the attachment of products. Finally, a class of molecules known as fluorous phases are a form of "liquid Teflon," consisting mainly of long chains of (—CF2-) groups attached to a silicon atom. When these phases are used as a soluble support for synthesis, the resulting product can be readily separated from any organic solvents and reaction by-products by extracting the reaction mixture with fluorocarbon solvents." A unique application using complementary DNA as a "support" has been reported by Harvard To "encourage" pairs of molecules in solution to react under mild conditions, they attach short strands of complementary DNA
or RNA to the structures to "zip" the structures together and promote reaction. The DNA is then removed, yielding product that would not otherwise be synthesized. Using this method makes it possible to prevent reaction of certain pairs of structures ax well.
50
Wilson wul
Mediiinul and
Textbook of.
R1—COOH + R2—NH2
0
t
-
NC
R3
H
0
/ 0
R2
R2
0
/ H
0
R3
0 Figure 3—11 u Four-component Ugi reaction reported by Keating and Armstrong."1 A combinatorial mixture of the intermediate cyclohexenyl amide can be split into several portions, and each can be further reacted to give a variety of products, all of which will be combinatorial. (Reaction redrawn from description from Keating, T. A., and Armstrong. R. W: Molecular diversity via a convertible isocyanide in the Ugi fourcomponent condensation J. Am. Chem. Soc. 117:7842—7843, 1995.)
POOLING STRATEGIES Although some solid-phase combinatorial chemistry is con-
ducted by use of the one-bead one-compound strategy. chemists have devised numerous other approaches to pooling reactants and inlemsediales to getlerate libraries. The goal is generally to achieve a balance between the simplicity
of mixing everything together in one step but then having to 'deconvolute" the resulting mixture and working with more, but smaller, mixtures. II has been likened to someone
giving you a rake and a magnet and telling you to go find and describe a needle in a field of hay. You can make one big haystack you know contains the needle, then have to deal with ever-smaller "subhaystacks." or you can use more clever approaches, such as dividing the field into regions. using overlapping regions. etc. The major approaches that have been used include the following: • One-bead one-compound strategy. With this strategy, a specific quantity of beads is allocated for cacti possible structure in the library: those beads Contain only molecules of the given library member. The beads may be tagged in various ways Isec the next section) to help identify the synthetic compound. The advantage of one-bead one-compound strategy is the violplicity of analysis and screening. The disadvantage is keeping the beads separate and having to deal with a large number of syntheses in parallel. As advances were made in robotics and automation. the problems were reduced, and today, probably most combinatorial experiments involve a one-head one-cornpound strategy.
he several) in the third group. Since it is in the third group. know aC in position 3 iv needed for activity. We synthesii.e
a smaller library of the structures, in three groups: AAC +
RAC + ('AC. ARC + BI3C ± CRC. and ACC + BCC + CCC. We do this by skipping the last set of pooling shown in Figure 3-3. Now when we screen these we find activity in the middle group of beads. This tells us that a It in position 2 is required activity. The final step is to synthesise ABC. BI3C. and ('BC. keeping iheiti separate, and screen each, to find ABC a.s active structure.
• SubtractIve deconvolution. This is similar to iterative deconsolution but uses negative logic, namely, leave out a functional
group, and if activity is absent, the functional group that is missing must be needed for activity. This is particularly useful for quantitative structure—activity relationship (QSAR)-type studies in which. say. a —Cl group is placed at several posi.
tions on a phenyl ring. The entire library is screened as a mixture to get the activity level. If activity is detected. a set of sublibraries is prepared, with each missing one building block (subtr.ictiott ofa Functional group). Sublibraries that
are missing functional groups front the active compound(s) will be less active thait the parent library. The least active subhibraries the most important functional groups. A reduced library containing only these luncticinal groups is then
prepared, and the most active compounds are identified by either one.compcnind synthesis or iterative dcconvolution.
• Bogus-coin detection. This begins with generating and screening the ctitire library as a single mixture. If activity is
20 years ago when combinatorial chemistry was started. Reex-
detected. the building blocks are divided into three group,. (a, fI. atid yt, and additional sublibrarics arc prepared. In these subsets. the nuttther of building blocks From the a group is decreased, the tiumber front (./ group is increased, and the number front the y group is unchanged. The resulting effect on acttvcty tup. down. ccnehanged suggests which group of
amine Figure 3-3 and imagine starting at the bottom nI the
building blocks was conlributing most to activity. This ap.
Iigam with three groups of beads. Each group has beads bearins a variety olcompounds. but a given structure only appears in one of the groups. Suppose lhc active structure is ABC (we
proach is applied iteratively to zoom in on the groups that are most active. • Orthogonal pxling. The tenhl orthrim,.o,tal means perpendicti-
• Iterative deconvolution. This is the strategy ftrst described
pretend here there is only one—in reality there will probably
lar or uncorrelatcd. In this type of pooling, we
the
Chapter 3 • Combinatorial Clwmis;n functional groups to be considered into sets of libraries. A. B. C. etc.. which can contain mixtures of the same compounds. However, the functional groups are distributed such that any subset in A and B shares only one functional group. For example, if we have a very small library of structures—au. ab. and
ac—we might put na and ab into group A, an and ac into group B, and ab and ac into group C. If ab is the active sttuctare, screening A. B. and C would show activity in A and C, but not in B. telling us that ub (the only structure in common) is the active one. • PositIonal scanning. This is a noniterative screening strategy
in which a subset library is created with a single building block fixed at one position and all building blocks in the other
positions. In principle, by selecting the lunctional group from the most active subset at each position. the most active compound overall is discovered. This ignores interactions between building blocks, which complicate the results.
Certain problems with mixtures must be considered when
pooling. Complex mixtures with only one or a few active stniclures can have solubility problems, especially if the compounds are poorly soluble. The inactive compounds contribute to the total ionic concentration but not to the activity.
Sometimes, compounds that have a common scaffold will have many active species, arising from the scaffold and not the substiluents. Thus, many poorly active structures may show additivity of activity, leading us to think the mixture contains a single active structure (false-positive results). Finally. partial binding of inactive structures can sometimes prevent an active structure from showing full activity (falsenegative results).
DETECTION. PURIFICATION.
AND ANALYSIS
Detection, analysis, and purification of combinatorial libraries places high demands on existing analytical techniques because (a) the quantities to be analyzed are very small, sometimes picomoks of material, (b) the analysis should
be nondestructive, to allow recovery of the compound if possible, and (c) the methods must be suitable for rapid, parallel analysis—analysis cannot be the rate-limiting step in the procedure. No single analytical technique can lit all lIre requirements. so usually some "hyphenated" analytical techniques are used, for example. high-performance liquid chromatography with a mass spectrometer detection system IHPLC-MS). We describe this and other techniques in this section,
Chromatography is usually the first step in the analysis
of a combinatorial mixture. If we start with solid-phase chemistry, we chemically cleave the compounds from the support and filter off the beads, giving a solution containing the compounds we synthesized. If the solution contains just a single compound. we might use a spectrophotometer, to measure infrared (IR) and ultraviolet (UV) absorbance or fluorescence directly, or even nuclear magnetic resonance INMR) spectroscopy. to determine the structure of the compound in solution. If the solution contains a mixture of compounds, one must separate them before determining their structures. HPLC is a standard approach. A sample of the mixture is injected into the flow of solvent entering a chromatographic column. The components in the mixture travel down the column at different rates, depending on their affin-
51
ity for the stationary phase in the column, and they exit or elate from the column at difl'erent times. They are detected by some optical method (UV absorption. fluorescence, refractive index. etc.) that gives rise to peaks on a graphical readout. Sometimes, the output from the column is passed into a spectrophototneter or mass spectrometer to generate a spectrum for each fraction of the output. These spectra can
be interpreted to determine the structure of the compound that caused a given peak. It is also possible to use much larger chromatographic columns and run preparative HPLC to separate up to several milligrams of material for further analysis or biological assay. Chromatographic separations and analyses can be fully automated. Thus, a chemist can place all the reaction vessels.
microtiter plates, etc. from a combinatorial experiment into racks and use a robotic system to draw samples. inject them into the HPLC, and collect the data output into computer file.s or databases—all without further intervention from the chemist (except to wash the dishes!). For this reason, speed and solvent handling are special concerns with combinatorial experiments. One approach that has been adopted to speed up analyses and reduce the amount of solvent that must he consumed is .supercrilical fluid (SFC), Here. the solvent is not a common organic solvent such as acetone or ethanol. Instead, it is a pressurized gas like CO2 that evaporates from the output. leaving pure compound behind. Another advantage of SFC is speed: since the solvent molecules are small, diffusion is rapid, and separations take
place in about half the time of ordinary HPLC separations or less. Finally, the amount of "solvent" that is consumed is significantly lower with SFC. A disadvantage is that certain compounds may not separate as well under SFC as under i-IPLC.'8 JR spectroscopy is often applied in combinatorial chemistry. Since IR light can be reflected from materials, one can analyze resin beads directly, without cleaving the products from them. Since the loading of product on any given bead is very small, usually computer-enhanced methods like Fourier transform JR (FTIR) are needed to enhance the very small
spectral signal from one or a few beads. Interestingly, the shape of the beads has been found to affect the IR spectra
results, and flattened rather than spherical beads give stronger IR signals)9 NMR spectroscopy gives more struc-
tural information than IR or UV spectroscopy. but it has traditionally not been nearly as sensitive. Compounds arc normally cleaved from solid support before analysis by NMR, since NMR on solid resin or on resin swollen by solvent gives broadened peaks and low resolution. A type of NMR called magic angle spinning NMR, in which the
sample is inserted into the magnetic field at an angle of about 550, reduces the peak broadening and has been used to analyze swollen polymer beads directly. Recent improve-
ments and the use of "nanoprobcs" have allowed NMR analysis of I00-mjz beads bearing less than 80() pniol of compound. Other NMR techniques that have been used to
analyze combinatorial mixtures include various "two-dimensional" (2D) NMR techniques that use multiple bagnetic fields. HPLC-NMR, capillary ekctrophoresis coupled to NMR (CE-NMR), and even NMR to detect the binding of drugs to receptors to identify active agents. This latter technique has been termed SAR mm'iI!m NMR.24t
Mass spectrometry (MS) is the technique most widely
52
tViIxo,i alul Gi.cvold s jeribr,ok of
!ile'diri,,aI and PI,ar,nace,aical ('he,njsiry
used for combinatorial library analysis. The measurements can he made on resin beads directly, a wide range of compounds can be analyzed, and MS analyses can be highly automated. Included among a number of MS techniques in
0 C)— Linker —
H
N"-'
Peplide library
H
use arc
Primer-DNA tag.Primer
a
Electrospray ionization. A solution containing the cornpounds to be analy,.ed is passed into a macs spcctronteter through an electrically charged capillaty. The droplets that emerge from lime capillary hear strong electric charges theftselves, and they literally "exphxlc" into smaller and smaller droplets and eventually into singly charged ions that are de-
Linker2 — N — Peptide library H
Linker, — Primer-DNA lag.Primer
tected by the mass spcctromemer.
linker2 — N — Peptide library
• Matrlx.as,sisted laser desorptionllonlzation tlmc.of-flight
Linker2— N — Peplicte library
is embedded in some solid matris (e.g.. 2.5.dihydroxybenzoic acid) and then bonibanied with a laser. Sample ntokcules axe
vaporized and ionized in
a
'gentle' fashion that allows
whole-molecule ions of the sample to be analyzed. The analysis is dune with use of a time-of-flight analyzer, in which ions of different mass travel different distances in a given amount
of lime. • Other less-used MS techniques. These include secondary. ion MS (SIMS) in which the sample is hit by a metal ion rather than tIme electron beam itself, and Fourier translorm MS.
H
(
(MALI)l-TOF). Quite a mouthful, ii simply means the sample
H
Figure 3—12 • Two ways of attaching DNA tags to solid supports. a. A DNA tag is attached with each peptide molecule via a bifunctional serine residue. b. The DNA and peptide groups have separate linkers
to beads on which peptides or pcptoids were being built.2' Since there are 20 possible amino acids and only four nuelco-
A very important use of MS in combinatorial chemistry is in quality control of combinatorial libraries. As much as possible. we would like to have pure compounds generated
tide bases, enough bases must be attached at each amino acid addition step to identify properly the amino acid being attached. Although three bases are used in the DNA genetic
in high yield, with no side reactions or by-products. We
code, it is customary to use up to six bases for library tagging.
also need to verify that every component actually exists in a library (i.e.. that no reactions failed). Only MS provides the sensitivity and versatility to perform this checking with both solid-phase and solution-phase libraries
For decoding. the DNA tag is amplilied by use of the poly-
,nerase ihau, reunion (PCR). the same reaction that is used in forensic DNA analysis. For this reason, the chemical tag must also bear PCR primer sequences. TWO types of anchors have been used to connect the DNA tags to the solid support (Fig. 3-12). (none type, the growing
ENCODING COMBINATORIAL LIBRARIES Once we have found a mixture or sublibrary that shows biological activity, how do we determine exactly which structure or structures are responsible for the activity? We can purify and analyze as described in the previous section. but if no direct analysis is available, we need to encode or tag the support or the molecules themselves. using physical or
molecular "barcodes." An obvious approach that can be used only with small libraries is to physically label each vial of one-bead one-compound resin. This may be practical for
a few lens of compounds, hut what if we have a library of 32.(X)0 compounds, or even I .000,000 compounds. in a mixture? Clearly. there is a need for a more automated means of identifying the structures that are in the library. The most common approach to encoding solid-phase libraries is to attach a chemical tag to the resin beads as the target molecule gets synthesized. 'rypically. at each step in the reaction, a tag is attached that is unique for the given
step. For example, if we are creating a tripeptide and we have 10 possible amino acids at each position, we need to tripeptide on this attach either a single tag that says bead has amino acid Ala at position I, Phc al position 2, and Gly at position 3." or we need to attach three different tags, one for each position. One of the earliest types of chemical encoding was the attachment of oligonucleotides (usually single-strand DNA)
DNA chain is attached to the a carbon of a serine group that is anchored to the solid support by a linker molecule. The growing peptide chain is attached to the serine atnino group, possibly through a spacer. In the second type of anchor, the DNA chain has its own anchor to the solid support. in this case, fewer DNA tags are attached than the number of polypeptide molecules.22 If DNA tags cannot be used, one can label beads by using a "binary" approach. Suppose we are bttilding a tripeplide with four possible amino acids at each position. We can use binary digits to encode which amino acid is at a given posi-
tion us follows (each binary "number" is read from the right) (Table 3-I): Thus, using 1K different tags (3 x 6) we can encode br any of the (4' = 64) members of the library. For example, if the product is Ala-Gly-Lys. the encoding
TABLE 3-1 Binary Encoding of a Tripeptide. UsIng 18 PossIble Tags
Amino Add
Position I
PosItion 2
Position 3
Ala
000000
(N) 1*) OIl
IX) (10 (Ni
Plie
000001
1)00100
010000
Cay
000010
11010(5)
100015)
l.ys
(1000 II
00 11(51
110000
Chapter 3 • C€nnbinumria! Oiemisirv
would be 00000000100011 0000, and 3 of the 18 possible tags would be attached to the support, along with the tripeptide. It is common to use polyhalogenated aromatic compounds as lags, such as
53
higher-density screening platforms. The standard layout for HTS has been a 96-well microtiter plate (12 x 8). Denser formats, up to 1,536 wells per plate, are increasingly being used. This requires advances in liquid handling, precision of detection, and laboratory automation. One of the first activities in developing a HTS assay is selecting the target. About 500 targets are currently being used by drug companies. Of these, cell membrane receptors. mostly G-protein—coupled receptors, make up the largest group (about 45% of the total). Enzymes make up the next
largest group (28%), followed by hormones (lIck), unwhere X represents some combination of halogen atoms. The
halogens make the tags show up clearly in MS analysis of the mixture, and by varying the chain length a, the tag can be made flexible enough not to interfere with attachment of the product. Other chemical tags that have been used include isotopically labeled peptides and dyes.
When it is not possible to use chemical tags, one must physically label the solid or liquid support itself. One alterna-
tive is to use radiofrequency encoding, in which tiny microchips are added to the resin or to the solution phase. As various reactions are conducted to generate the products, at each step a radiofrequency signal is stored in the microchip. This signal can be recalled to identify the sequence of reactions that generated the product (a similar principle is used when your dog or cat gets a small identification (ID) pellet implanted under the skin of the neck). Laser optical encoding is yet another approach, in which the solid support consists of a ceramic chip covered with a polypropylene—polystyrene polymer solid phase. The barcode pattern is actually burned into the ceramic at each step in the reaction and is decoded visually with use of a microscope. Finally, one can embed semiconductor particles into the solid phase that fluoresce at different wavelengths. These are called "quantum dots" by their manufacturer.23
HIGH-THROUGHPUT SCREENING (HTS)
knowns (7%). ion channels (5%), nuclear receptors (2%). and finally DNA (2%).25 It is expected that the annotation of the human genome will add additional targets. although the rate of this addition is not known. New targets must he part of some regulatory pathway in the cell and should be sensitive to some disease state, not be expressed all the time
and everywhere in the cell. The next concern in I-IFS is the library to be screened. Throughout our discussion, we have perhaps offered the impression that a given library for a particular project was the only set of compounds that were ever screened for activity. In fact, much HTS involves screening compounds that are part of the corporate storehouse of compounds synthesized in the past, or they may be a library purchased from a vendor. Such libraries usually Consist of microtiter plates containing frozen or dried samples of compound—perhaps only micrograms per well. The size of such libraries may range from a few thousand compounds to nearly a million. The cost of completely screening such a library against just a single assay may amount to over $300,000, so such largescale screens are conducted rather infrequently, compared with routine day-to-day screens. It has been estimated that one must screen at least 120,000 "quality" compounds (i.e.. diverse drug-like structures) to discover a single-lead for a therapeutically sound target.25' As discussed above in the section on pooling strategies. one can reduce the screening effort by pooling groups of structures and running assays on mixtures of compounds. This also conserves reagents and biological material, has smaller storage requirements, and requires fewer personnel.
Without the ability to screen libraries rapidly for activity,
There are potential problems with pooling. A number of
there would be no combinatorial chemistry. Fortunately, the biologists are just as adept at developing rapid high-throughpat assays as the chemists are at generating structures. HTS is an extremely broad topic. encompassing enzymes. organelks, cells, various tissues, whole organs, and even wholeanimal testing, via cassette dosing. This section briefly discusses only a part of the role of HTS in drug discovery, with
factors limit the number of different compounds we can test in a given well, including ionization, reactivity, and solubility. Compounds can enter a screening program in a nonrandom order, such that a given assay plate may have com-
pounds that are highly similar structurally. This may give rise to false-positive hits. False-negative hits are less likely to arise from pooling. Another concern is the use of repli-
emphasis on a few recent
cates—compounds from the same series—in a given assay.
Successful HTS programs integrate several activities, including target identification (genomics and molecular biology groups), reagent preparation (protein expression and Pu-
rification groups), compound management (information
If only one representative of a given series is present. the chance of missing that series as a possible lead series is greater than if multiple members are present. Therefore, it is common to include several members of each series in a
management group), assay development (biologist and phar-
given assay when possible.
macologist), and high-throughput library screening (biologists and chemists). Formerly, these activities were handled separately, and multiple handoffs of samples were involved. It is becoming more common to integrate the activities and share expertise. This increases efficiency of the screening process. Another route to increasing efficiency is a move to
To be effective, a given compound must dissolve completely in the assay medium. It is common to add a small amount (1%) of dimethyl sulfoxide (DMSO) to the assay to assist solvation. The best concentration of compound to use
is somewhat debatable. High concentrations (10 jaM and above) often lead to more false positives than screening at
54
Wilson and Gi.;wld's
of Organic Medicinal and P/,arnzaceutieal C/wn,is:rv
a low concentration (3 suM). The reason for this may be
.cc-in:illant—a compound that fluoresces in the near presence
nonspecific binding at the higher concentration. Just as there are several ways to dctcct and identify members of a combinatorial library, there are many ways so measure activity in I-ITS assays. Any such method must be accurate. reproducible. and have a high signal.to-noise ratio (SI
of the radioactive substrate (near being about 20 zrn)—is
N). Typically, the result of FITS is a qualitative (yes/no) or semiquantitative one (high-medium-low), rather than a precise value (e.g.. K180 or
The methods for detection in FITS fall into the categories of nonradiometric and radio. metric.
Nonradiometric methods include absorbance, fluorescence, and luminescence spectroscopy. Enzyme assays are
a common example. The assay is usually run at or below the value of substrate. svith only about 5% of the substrate consumed during the assay, and multiple enzyme turnovers occur during the assay. Sometimes enzyme reactions arc coupled, especially if the target reaction does not produce a product that can be detected directly in the assay.
added to the mixture. The lysed and unlysed substrate binds to the beads, and if the radioactive part of the substrate is still attached, the bead will fluoresce. If not, the radioactive parts of the substrate floating in the solution will be too far from the beads to cause any fluorescence. The presence of
fluorescence implies that the test compound inhibited the enzyme. The advantage of SPA over filtration is that no filtering of the solution is needed, so beads can be added directly to the assay mixture in wells or test tubes. Also. special scintillation fluid is not needed. The beads for SPA can be engineered to attach a variety of substrate types. Other HTS assay advances include the use of microorganisms such as bacteria and yeast, the cloning and expression
of mammalian receptors in microorganisms, probing protein—protein interactions, and very importantly. DNA and protein arrays. These are too involved to discuss here, but excellent reviews exist.24 V The increasing use of HTS to
An example is carboxypeptidase. which is coupled to the reduction of NADP to NADPH. giving rise to absorbance
screen for a molecule's absorption, distribution, metabolism.
at 340 nm.
ered as well.28
excretion, and toxicity (ADMET) properties has been cov-
Radiometric methods include filtration and scintillation proximity assay (SPA). These assays use radioisotopes. so safe storage and handling are of concern. In filtration assay. a radioactive substrate bound to a capture group is cleaved by its enzyme. removing the radioactivity from the capture
group. The mixture is filtered through special filter paper that the capture group sticks to. but everything else passes through. A scintillation fluid is added, and the radioactivity of the filter is measured. The degree to which the radioactivity is retained measures the strength of the inhibition (Fig. 3-l3a).
SPA is a newer, simpler method (Fig. 3-I3b). We start with the same radioactive substrate, which may not necessarily need a capture group. The enzyme and potential drug are added, causing the cleavage of the substrate to some degree.
Now, instead of filtering, a special resin bead coated with a
Enzyme
CO—Substrate,
VIRTUAL (IN SILICO) SCREENING Virtual, or in silico, screening refers to the use of computers to predict whether a compound will show desired properties or activity on the basis of its two-dimensional (2D) or threedimensional (3D) chemical structure or its physicochemical properties. The motivation for using virtual screening arises from the flood of new structures coming from combinatorial chemistry. the expense and time required to run conventional HTS. the ethical concerns about using animal tissue instead of predictive models, and an increasing failure rate for structures coming out of combinatorial programs. In general, a virtual screening program attempts to answer one or both of these questions:
Substrate(i
).—__ CC—Substrate,
CO—Substrate, * Substrate(') Enzyme
inhibitor
CO—Substrate, — SubsIrate(')
CO—Substrate, - Substrate(') FILTER
a Enzyme
CG—Substiale,
Subslrate(')
CO—Substrate,
Substrate(')
CO—Substrate,
Enzyme I nhlbltor
CO—Substrate, — Substrate(')
CG—Substrale, — Substrate(')
SPA Effect b Figure 3—13 • Comparison of filtration and scintiUation proximity assays. a. In filtration assay, the enzyme. substrate, and inhibitor are mixed; the uninhibited enzyme splits the radioactive portion (*) off the substrate,
and filtering the mixture, followed by measuring the radioactivity of the filter, tells how much inhibition has occurred. b. In SPA, the same mixture is treated with resin beads containing a scintillant that fluoresces only in close proximity to the radioactive source. Any radioactivity that was split off by the enzyme does not need to be filtered in SPA.
Chapter 3 U Cwnbi,wrorial CIw,nistrv I.
2
55
Will i particulur compound show sufficient binding iou known
drugs. This led Lipinski to enumerate some rules for the
rL'ccpisr?
rejection of structures. He proposed rejecting any structures
Will a particular compound possess any undesirable ADMET properties?
that fail two or more of the following criteria:
To answer these questions, we must build computer
• Molecular weight should be 1 aflatoxin B1 has been clearly linked to its metabolic oxidation to the corresponding 2.3-oxide, which is extremely reacExtensive in vitro and in vivo metabolic studies indicate that this 2.3-oxide binds covalently to DNA. RNA, and proteins. A major DNA adduct has been isolated and
0
HO H3C
-j
Styrene
Styrene Oxide
Covalent binding to proteins, nucleic acids Mercapturic Acid (major)
Dlethylslllbeslrol
Dlethyistilbestrol Epoxtde
Possible covalent binding to proteins and/or nucleic acids
Mercapturic Acid Derrvative (minof)
Chapter 4 • M':abolir Changes of Drugs wul Related Organic Compounds as 2.3.dihydro-2-(N7-guanyl )-3-hydroxyatla-
77
mide (Orinase) is oxidized extensively to the corresponding
,sin B,°
alcohol and carboxylic acid. Both titetabolites have been
Other ulefinic compoundn. such as vinyl chloride,'°" stiltvne.'°' and the carcinogenic estmgenic agent diethylstilbesaol DESI.'°2 undergo metabolic epoxidation. The coneqxinding epoxide metabolites may be the reactive species
isolated from human urine.''8 Similarly. the "benzylic"
for the cellular toxicity seen with these comAn interesting group of olefin-containing compounds Corncauses the destruction of cytochrorne P-450.'°' helonging to this group include allylisopropylacetamidc.'°6 "p' and the volatile anesthetic
fluroxene)'° It is believed that the olefinic moiety present in these compounds is activated metabolically by cytochrome P-450 to form a very reactive intermediate that
cocalemly binds to the heme portion of cytochrorne P40iUIO The abnormal heme derivatives, or "green pigments." that result from this covalent interaction have been
as N.alkylated protoporphyrins in which the N.alkyl moiety is derived directly from the olef,n adminisII 113 Long-term administration of the above-
01
three agents is expected to lead to inhibition of ,sidativc drug metabolism, potential drug interactions, and prolonged pharmacological effects.
Ozidatisa Carbon
at Benayflc Carbon Atnis
atoms attached to aromatic rings (henzylic position)
are susceptible to oxidation, thereby forming the corresponding alcohol (or carbinol) metabolite.' '' "f' Primary alcohol metaholites are often oxidized further to aldehydes and carbosylic acids (CH2OH —. CHO —. COOH). and secondary
alcohols are converted to ketones by soluble alcohol and ildehyde dehydrogena.ses.' Alternatively, the alcohol may k conjugated directly with glucuronic acid.'17 The benzylic carbon atom present in the oral hypoglycemic agent tolbuta-
methyl group in the anti-inflammatory agent tolmetin (Tolectin) undergoes oxidation to yield the dicarhoxylic acid product as the major metabolite in humans.' " 1211
Oxidation at Allylic Carbon Atoms Microsomal hydroxylation at allylic carbon atoms is commonly observed in drug metabolism. An illustrative example of allylic oxidation is given by the psychoactive component
of marijuana. J'-tetruhydrocannabinol J'-THC. This molecule contains three allylic carbon centers (C-7. C-6, and C3). Allylic hydroxylation occurs extensively at C-7 to yield 7-hydroxy- Ll'-THC as the major plasma metaholite in huPharmacological studies show that this 7-hydroxy metabolite is as active as, or even more active than. J'-THC per se and may contribute significantly to the overall central nervous system (CNS) psychotomimetic effects of the parent compound.'24 as Hydroxylation also occurs to a minor extent at the allylic C-6 position to both the epirncric 6a- and 613-hydroxy metabolites. '° Metabolism does not occur at C-3, presumably because of stcric hindrance. The antiarrhythmic agent quinidine is metabolized by al-
lylic hydroxylation to 3-hydroxyquinidinc. the principal plasma metabolite found in humans.'26 27 This metabolite shows significant antiarrhythmic activity in animals and possibly in humans.'28
C ..sTP" 1 -(2.5.Dimethoxy.4.methytphenyl) •2.aminopropane (DOM)
Arnilriptytlne
Imipramine
CR3
OH
CR2
I
CH3
Debnsoquin
3-Methytcholanthrene
Figure 4—8 • Examples of drugs and xenobiotics undergoing benzylic hydroxylation. Arrow indicates site of hydroxylation.
selec-
tive COX-2 anti-inflammatory agent celecoxib undergoes benzylic oxidation at its C-5 methyl group to give hydroxycelecoxib as a major metabolite)2' Significant benzylic hydroxylation occurs in the metabolism of the blocker metoprolol (Lopressor) to yield co-hydroxymetopro101.122 Additional examples of drugs and xenohiotics undergoing benzylic oxidation arc shown in Figure 4-8.
jUIU I
.'
I
JVU'UI(ffUhI 111111 IIIUIIIUIL (11111 III
01
__,
ro I
/\
H
[o2&] 2.3-Dihydro-2-(N'-guanyl)3-hydroxyallaloxin B1
At laloxin B1
H
H
I
H
H Vinyl Chloride
Slilbene
CH2CH3
Dielhylstilbestrol (DES)
H C—--
Fluroxene
CH3
COOH
CH2OH
—,-, SO2NHCNHC4H9 Tolbulamide
SO2NHCNHC4H9 Alcohol Molabolile
I
0
SO2NHCNHC4H9 Acid Melabolite
Chapter 4 • Metabolic Changes of Drugs and Related Organic Corn
0
0 —p HOOC
H3C
CH3
CH3
Acid Metabolite
Tolmetin
H
0
CH3 1
-yI).heiwenesultonamide
CH3OCH2CH2
CH3OCH2CH CH
OH CH3
OH3
Metroprotol
a-Ftydroxymelroprolol
CI-t2OH
CH3
H3C CH3
C5H11
C5H11
CH3 7-Hydroxy-A'-THC
CH3
H3C CH3
.THC
80
WjLcu,, and Gi.cvolgl's ii'xthoak of Organic Medicinal iiiid Pharmaceutical Chemistry
HO_C CH3O.
3-Hydroxyquirildine
Other examples of allylic oxidation include the sedative
two enantiomeric forms. Studies in humans indicate that the pharmacologically less active (R)(—) enantiomcr is metubo-
For the hepatocarcinogenic agent safrole. allylic hydrox. ylation is involved in a bioactivation pathway leading to the formation of chemically reactive This process involves initial hydroxylation at the C-I' carbon of salrole, which is both allylic and benzylic. The hydroxylated metabolite then undergoes further conjugation to form a sulfate ester. This chemically reactive ester intermediate presumably undergoes nucleophilic displacement reactions
more rapidly than its (S)( +) isomer.'3' Pentazocine undergoes allylic hydroxylation at the two terminal methyl groups of its N-butcnyl side chain to yield either the ci.s or
with DNA or RNA in vitro to form covalently bound adAs shown in the scheme. nucleophilic attack by DNA, RNA. or other nucleophiles is facilitated by a good
Ira,,.s alcohol metabolites shown in the more of the tran,c alcohol is
leaving group (e.g.. at the C-I' position. The leaving group tendency of the alcohol 01-I group itself is not enough
hypnotic hexobarbital (Sombulex) and the analgesic pentazocmc (l'alwin). The 3'-hydroxylated metabolite formed from
hexobarhital is susceptible to glucumnide conjugation as well as further oxidation to the 3'-oxo Hexobarbital is a chiral barbiturate derivative that exists in
In humans. 1.3
O-Glucuronide Conjugate
— OH CH3
CH3
3'-Oxohexobarb,tal
Hexobarbitat
N
+
Irans-Alcobof Metabohite
Pentazocine
c,s-Alcobol Metabobte
H7C
<J1.CH2
Nu"DNA.
—
RNA
' Covatenhly Bound Adduct to DNA. RNA
1 '.Hydroxysalrole R = H 0-Sulfate Ester. R = SO;
Chapter 4 e Metabolic
of l)rugs and Related Organic con:potvnois
81
N-demethylxhoii
Cl
Oxazepam
(3S)
Diazepam
or
(CH3CH2)2NCH2CH2
Nimotazepam
Fturazepam
to facilitate displacement reactions. Importantly. allylic hydmxylation generally does not lead to the generation of mactise intermediates, Its involvement in the biotoxification of safrole appears tO be an exception.
Oxidation at Carbon Atoms a to Carbonyls and lmlnes The
mixed.function oxidase system also oxidizes carbon
atoms adjacent (i.e.. a) to carhonyl and imino (C = N) ftinctionalities. An important class of drugs undergoing this type of oxidation is the benzodiazepines. For example. diazepam Valium). flurazepam (Dalmane). and nimetazepam are oxidiied to their corresponding 3-hydroxy The C-3 carbon atom undergoing hydroxylalion is a to both a lactam carbonyl and an imino functionality. For diazepam. the hydroxylation reaction proceeds with renurkable stereoselectivity to form primarily (90%) 3.hydroxydiazepam (also called N-methyloxa7.epam). with the Si absolute configuration at C-3.'3° Further N-demethylanon of the latter metabolite gives rise to the pharmacologiactive 3(S)( + )-oxazepam.
CH2CH3
CH2CH3
—0
Hydroxylation of the carbon atom a to carbonyl functionalities generally occurs only to a limited extent in drug metabolism. An illustrative example involves the hydroxylation of the sedative hypnotic glutethimide (Doriden) to 4-hydroxyglutethimide.'40
Oxidation at Aliphatic and AHcycIIc Carbon Atoms Alkyl or aliphatic carbon centers are subject to mixed-function oxidation. Metabolic oxidation at the terminal methyl group often is refeffed to as w oxidation, and oxidation of the penultimate carbon atom (i.e., next-to-the-last carbon) is
called w— I oxidation)'4' '5The initial alcohol metabolites formed from these enzymatic w and w — I oxidations are susceptible to further oxidation to yield aldehyde. kelones. or carboxylic acids. Alternatively, the alcohol metabolites may undergo glucuronide conjugation. Aliphatic w and w — I hydroxylations commonly take place in drug molecules with straight or branched alkyl chains. Thus, the antiepileptic agent valproic acid (Depakene) undergoes both wand w— I oxidation to the 5-hydroxy "° Further oxiand 4-hydroxy metabolites, respectively. dation of the 5-hydroxy metabolite yields 2-n-propylglutaric acid. Numerous barbiturates and oral hypoglycemic sulfonylureas also have aliphatic side chains that arc susceptible to
oxidation. Note that the sedative hypnotic amobarbital Gk,tethimide
4.Hydroxyglutethimrje
'I,
wOxrdat,on
— 1 Oxidation
OH
(Amytal) undergoes extensive w — I oxidation to the corresponding 3'-hydrnxylated metabolite." Other barbiturate.s. such as pentobarbital,"'° "thiumylal)47 and secobarbital.92 reportedly are metabolized by way of wand or — I oxidation. The n-propyl side chain attached to the oral hypoglycemic agent chiorpropamide (Diabinese) undergoes extensive or — I hydroxylation to yield the secondary alcohol 2'-hydroxychiorpropamide as a major urinary metabolite in humans."'° Omega and or — I oxidation of the isobutyl moiety present in the anti-inflammatory agent ibuprofen (Motrin) yields the
82
WiLson and
Gis,'old's Textbook of Organic Medicinal and Pharaweenikal C'he,nisirv nC3H7
nC3H,
HOCH2CH2CH2CHCOOH —' HOOCCHCH7CHCOOH Acid
Acid
nC3H7
CH3CH2CH2CHCOOH
Oxijnon
OH
nC3H7
CH3CHCH7CHCOOH
Vaiproic Acid
4-Hydrosyvaiproic Acid
CH3
0
OH
III
H Amobarbital
corresponding carboxylie acid and tertiary alcohol metabolites)49 Additional examples of drugs reported to undergo aliphatic hydroxylution include meprobamate)5° glutethimide)40 '" and phenylbutuzone.W The cyclohexyl group is commonly found in many medicinal agents, and is also susceptible to mixed-function oxidation (alicyclic hydroxylatk)n).' 4 Enzymatic introduction of a hydroxyl group into a monosubstituted cyclohexane ring generally occurs at C-3 or C-4 and can lead to cis and traits conformational stercoisomers. as shown in the diagrammed scheme.
An example of this hydroxylation pathway is seen in the metabolism of the oral hypoglycemic agent acetohexanude
o
(Dymelor). In humans, the :ran.s-4-hydroxycyclohexyl product is reportedly a major inetab Small amounts of the other possible stereoisomers (namely, the cis-4-. cix-3-. and :raiis-3-hydroxycyclohexyl derivatives) also have been
detected. Another related oral hypoglycemic agent. glipizide. is oxidized in humans to the trans-4- and cis-3-hydroxylcyclohexyl metabolites in about a 6:1 ratio)54 Two human urinary metabolitcs of phencyclidine PCP) have been identilied as the 4-hydroxypiperidyl and 4-hydroxycyclohexyl derivatives of the parent compound)55 Thus, from these results, it appears that "alicyclic" hydroxylotion of the six-membered piperidyl moiety may parallel closely the hydroxylation pattern of the cyclohexyl moiety.
CH2—CH=CH2
CH2CH3
/ C — OH2 H2
\
OH3
CH3 Pentobarbital
Thiamylal X = S Secobarbital X = 0
0 2
NHCH2CHCH3 OH Chtorproparnide
2'-Hydroxychlorpropamide
Chapter 4 • Metabolic Changes of Drug3 and Related Organic Compou
OH3
CH3
—i H000 —
CH3— ?HCH2 CH3
CH3 bupqoren
Catboxylic Acid Melabolite
OH
CH3
+ CH3 Tertiary Alcohol Metabolile
CH3CH2
C6H5
CkH5
CH3
0" H
OH3
CH2CH2CH2CH3
I Ptienylbutazone
Ethosuximide
Glutethimide
Meprobamale
+ trans
C$S
3-Hydroxylahon
OH
H
OH
H
+ H
H trans
o
Acetohexamide
css
0
trans-4-Hydioxyacetohexamide
H
84
WiLson and Gisyold's Textbook of Organk Medicinal and Pharmaceutical
The stereochemistry of the hydroxylated centens in the two metabolites has not been clearly established. Biotransformation of the antihypertensive agent minoxidil (Loniten) yields the 4'-hydroxypiperidyl metabolite. In the dog, this product is a major urinary metabolite (29 to 47%), whereas in tiu155 mans it is detected in small amounts
H
9
—.A—XH+ Where X
N0S
/
Usually Unslable
Oxidation Involving Carbon-Ileteroatoin Systems Nitrogen and oxygen functionalities are commonly found in most drugs and foreign compounds; sulfur lunctionalities occur only occasionally. Metabolic oxidation of carbon— nitrogen, carbon—oxygen, and carbon—sulfur systems principally involves two basic types of biotransformation
Oxidative N-. 0-. and S-dealkylation as well as oxidative deamination reactions fall under this mechanistic pathway. 2. Hydroxylation or oxidation of the heteroatom (N. S only, e.g.. N-hydroxylation. N-oxide formation. sulfoxide. and sulfonc formation).
processes:
I. Hydroxylation of the a-carbon atom anached directly to the heteroatom (N. 0. S). 'The resulting intermediate is often unstable and decomposes with the cleavage of the carbon—heteroatom bond:
Several structural features frequently determine which pathway will predominate, especially in carbon—nitrogen systems. Metabolism of some nitrogen-containing compounds is complicated by the fact that carbon- or nitrogen-
0
0 —
SO2NH H
G(pizide
Phencyclidine Metabolite
4-Hydroxypipenctyl
Metatollte
NH2
NH2
—' 0 —'
—
NH2
4'
Minoxidil
NH2 4'-Hydrosyrninoxidil
Chapter 4 U Metabolic Changes of Drugs and Related Organic compounds products may undergo secondary reactions to fomi other, more complex metabolic products (e.g.. oxime, nitrate, nitroso. imino). Other oxidativc processes that do not fall under these two basic categories are discussed mdisidually in the appropriate carbon—heteroatom section. The metabolism of carbon—nitrogen systems will be discussed first. followed by the metabolism of carbon—oxygen and earbon.-sulfur systems.
OXIDATION INVOLVING CARBON-NITROGEN SYSTEMS
Metabolism of nitrogen functionalitics (e.g.. amines. amides) is important because such functional groups are found in many natural products (e.g.. morphine, cocaine, nicotine) and in numemus important drugs (e.g., phenothiazines. antihislamines. tricyclic antidepressants. fl-adrenergic agents, phenylethylamines. benzodiazepincs).'59 The following discussion divides nitrogen-containing compounds into three basic classes: I.
Atipluitic (primary, secondary, and tertiary) and alicyclic (5cc-
ondaiy and tertiary) amincs 2. Aromatic and heterocyclic nitrogen compounds
In general, small alkyl groups, such hyde or as methyl. ethyl, and isopropyl. are removed rapidly.'67 Ndealkylation of the r-butyl group is not possible by the carbinolamine pathway because a-carbon hydroxylation cannot occur. The first alkyl group from a tertiary amine is removed more rapidly than the second alkyl group. In some instances. bisdealkylation of the tertiary aliphatic amine to the correFor sponding primary aliphatic amine occurs very example, the tertiary amine imipraminc (Tofranil) is monodemethylated to desmethylimipramine (desipramine)." This major plasma metabolite is pharmacologically active in humans and contributes substantially to the antidepressant Very little of the bisdemethyactivity of the parent lated metabolite of imipramine is detected. In contract, the local anesthetic and antiarrhythmic agent lidocaine is metabolized extensively by N-deethylation to both monoethylgly-
cylxylidine and glycyl-2.6-xylidine in humans.In rio Numerous other tertiary aliphatic amine drugs are metabolized principally by oxidativc N-dealkylation. Some of these include the antiarrhythmic disopyramide (Norpace).'71 the antiestrogenic agent tamoxifen (Nolvadex).'73 diphenhydraminc(Bcnadryl),'74 '75chlorpromazine(Thorazine),'76 177
and (+ )-a-propoxyphene (Darvon)."8 When the tertiary amine contains several different substituents capable of
3. Amides
The susceptibility of each class of these nitrogen compounds to either a-carbon hydroxylation or N-oxidation and he metabolic products that are formed are discussed. The hepatic enzymes responsible for carrying out a-car-
bon hydroxylation reactions arc the cytochrome P-450 mised-lunction oxidases. The N-hydroxylation or N-oxidation reactions, however, appear to be catalyzed not only by cylochrome P450 but also by a second class of hepatic mised-function oxidases called amine' ox.'dase.s (someThese enzymes are NADPHümes called dependent tiavoproteins and do not contain cytochromc They require NADPH and molecular oxygen p.45015 to carry out N-oxidation.
The oxidaAliphatic and Alicydic Amines. ire removal of alkyl groups (particularly methyl groups)
Tertiary
from tertiary aliphatic and alicyclic amines is carried out by hepatic cytochrome P450 mixed-function oxidasc enzymes. This reaction is commonly referred to as oxidariu'e N-dealThe initial step involves a-carbon hydroxylation In form a carbinolamine intermediate, which is unstable and undergoes spontaneous heterolytic cleavage of the C—N bond to give a secondary amine and a carbonyl moiety (aIde-
undergoing dealkylation. the smaller alkyl group is removed preferentially and more rapidly. For example, in benzphetamine (Didrex). the methyl group is removed much more rapidly than the benzyl moiety.'7" An interesting cyclization reaction occurs with methadone on N-demethylation. The demethylated metabolite normethadonc undergoes spontaneous cyclization to form the enamine metabolite 2-ethylidene- I .5-dimethyl-3.3-diphenyl-
pyrrolidine (EDDP)."° Subsequent N-dernethylation of EDDP and isomerization of the double bond leads to 2-ethyl-
5-methyl-3,3-diphenyl- I -pyrroline (EMDP). Many times. bisdealkylarion of a tertiary amine leads to the corresponding primary aliphatic amine metabolite, which is susceptible to further oxidation. For example, the bisdesmethyl metabolite of the H1-histamine antagonist brompheniramine (Dimetane) undergoes oxidative deamination and further oxidation to the corresponding propionic acid metabolite)8' Oxidative deamination is discussed in greater detail when we examine the metabolic reactions of secondary and primary amines. Like their aliphatic counterparts. alicyclic tertiary amines are susceptible to oxidative N-dealkylation reactions. For example, the analgesic meperidine (Demerol) is metabolized
0
H
—p
—, R,—NH + A2
A2 Tertiary Amino
85
Carbinolamine
Secondary Amine
Carbonyl Moety (atdahyde or ketone)
86
Wil.wn and Gisvold's Textbook
c'o
Organic Medicinal and Pharmaceutical Claenuszrv
0
0
HCH
/
CH3
Cl-I3
CH2CH2CH2N
CH2CH2CH2N\
Dssmethylimipramine (desipratflrne)
Imipramine
CH2CI-I2CH2NH2 H
CH3
CH3
Bisdesmethylirnipramine
CH3
CH3
CH
OH3
CONH2
OH3
—OH3
OH3
OH3
Tarnoxilen
Disopyrarnide
0
Chtorproma2ine
(+ )-u-Propoxyptiene
Benzphetamlne and N-debenzytation)
principally by this pathway to yield normeperidine as a major plasma metabolice in humans."'2 Morphine. N-ethylnormorphinc. and dextromethorphan also undergo some N-dealkylation.'53
chlorocyclizine is. indeed, metabolized to significant amounts of norchlorocyclizine. whereby the i-butyl group is Careful studies showed that the t-butyl group is
Direct N-dealkylation of :-butyl groups, as discussed
of the :-butyl moiety to the carbinol or alcohol Further oxidation generates the corresponding carboxylic
above, is not possible by the a-carbon hydroxylation pathway. In vitro studies indicate, however, that N-:-butyinor-
removed by initial hydroxylation of one of the methyl groups
acid that, on decarboxylation. forms the N-isopropyl deriva-
Chapter 4 • Metabolic Cl,a,,ge.s of Drugs and
H5C6
H2C—C
/ C
\CH2—CH3
H
I
Orga,,ic Cornpou,uis
87
H5C6
/
H
C=O
H5C6 _jC6H5
H20
"CH2—CH3 Cl-I3
CH3
CH3
Noaneihadone
Methadone
H5C6
2-Ethylidene-1 .5-dimethyt. 3.3-cliplienyipyrrolidine (EDDP)
C6H5
2-Ethyt-5-methyl-pyrroltne (EMDP)
a
Bisdesmethyl Metabolito
Brompheniramine
3.( p.Bomophenyt).3pyridyl. acid
live. The N-isopropyl intermediate is dealkyluted by (he nor-
mal a-carbon hydroxylation (i.e.. carbinolaminc) pathway to give norchlorocycluzine and acetone. Whether this is a general method for the toss of t-butyl groups from amines is still unclear. Indirect N-dealkylation of :-buiyl groups is not observed significantly. The N-:-butyl group present in many $-adrenergic antagonists, such as terbutaline and salbutamol. remains intact and does not appear to undergo any nigniticant
H5C6 COOCH2CH3
H5C6 COOCH2CH3
CH3 Normeperidine
Meperidine
Alicyclic tertiary amines often generate lactam metabolites by a-carbon hydroxylation reactions. For example. the tobacco alkaloid nicotine is hydroxylated initially at the ring carbon atom a to the nitrogen to yield a carhinolamine intennediate. Furthermore. enzymatic oxidation of this cyclic carbinolamine generates the lactam metabolite Cotinine)57 Formation of lactam metabolites also has been reported to occur to a minor extent for the antihistamine cyproheptadine and the antiemetic diphenidol N-oxidation of tertiary amines occurs with several The true extent of N-oxide lormation often is complicated by the susceptibility of N-oxides to undergo in vivo reduction back to the parent tertiary amine. Tertiary amines such as H1-histamine antagonists (e.g.. orphenadrine. tripe-
lenamine). phenothiazines (e.g.. chlorpromazine). tricyclic antidepressants (e.g.. imipramine). and narcotic analgesics (e.g.. morphine, codeine, and meperidine) reportedly form N-oxide products. In some instances. N-oxides possess pharmacological A comparison of imipramine Noxide with irnipramine indicates that the N-oxide itself possesses antidepressant and cardiovascular activity similar to that of the parent
Secondary and Primary Amines.
CH3O Mo'pline
R — CH3 R
CH2CH3
Dextromethorphan
Secondary amities (either parent compounds or metabolites) are susceptible to oxidative N-dealkylation. oxidative deamination. and N-oxidation reactions)53 195 As in tertiary amines. N-dealkylation of secondary amines proceeds by the carbinolamine path-
Wilson and Gisvolds Texthook of Organic Medicinal and Pharmaceutical Chemistry
CH
C6H5
CH
+ CH3
Nord,Iorocyclizine
/
I
I
COOH
CH 2OH
—Co
N—C—CH3 —p N—C—CH3 —s'
J
acartDOfl hyToxylalion
/ e corbmo1amioe
J
I
CH3
I
CH3
N.Isopropyl Metabolite
Carboxylic Acid
Aicoflol or Carbinol
J
OH
T
NH
CH3 Salbutamol
Terbutaline
Cotinine
Caibinolamine
Nicotine
_,
Lactam Metabolite
Cyproheptadine
C6H5
CH2CH2CH2—
'
CH2CH2CH2— C6H5
C6H5 Diplienidol
2Oxoduphenidol
o
Chapter 4 • Metabolic Changes of Drugs and Related Organic compounds
I
phetamine'90 wand
0
H
way. Dealkylation of secondary amines gives rise to the corresponding primary amine metabolite. For example, the aadrenergic blockers propranolol46'47 and undergo N-deisopropylation to the corresponding primary amities. N-dealkylation appears to be a significant biou-anslormation pathway for the secondary amine drugs metham-
I
NH2
[
—' —c— +
] Carbanyl
Primary Amine
yielding amphetamine
89
Hi
and norketamine. respectively.
The primary amine metabolites formed from oxidative dealkylation are susceptible to oxidazive deanzination. This
Some secondary alicyclic amines. like their tertiary amine analogues, are metabolized to their corresponding laclum
process is similar to N-dealkylation, in that it involves an initial a-carbon hydroxylation reaction to form a carbino-
derivatives. For example, the anorectic agent phenmetrazine
amine intermediate, which then undergoes subsequent carbon-nitrogen cleavage to the carbonyl metabolite and ammonia. If a-carbon hydroxylation cannot occur, then osidative deamination is not possible. For example. deamination does not occur for norketamine because a-carbon hy201 With methamphetadrosylation cannot take place. mine. oxidative deamination of primary amine metabolite
(Preludin) is metabolized principally to the lactam product 3-oxophenmetrazine.203 In humans, this lactam metabolite is a major urinary product. Methylphenidate (Ritalin) also reportedly yields a lactam metabolite, 6-oxoritalinic acid, by oxidation of its hydrolyzed metabolite, ritalinic acid, in humans.204
Metabolic N-oxidation of secondary aliphatic and alicydie amines leads to several N-oxygenated Nhydroxylation of secondary amines generates the corresponding N-hydroxylammne metabolites. Often, these hydroxylamine products are susceptible to further oxidation (either spontaneous or enzymatic) to the corresponding ni-
amphetamine produces
In general, dealkylotion of secondary amines is believed to occur before oxidative deainination. Some evidence mdicares, however, that this may not always be true. Direct deamination of the secondary amine also has occurred. For esample. in addition to undergoing deamination through its desisopropyl primary amine metabolite. propranolol can undergo a direct oxidative deamination reaction (also by acarbon hydroxylation) to yield the aldehyde metabolite and isopropylamine (Fig. How much direct oxidative de-
trone derivatives. N-benzylatnphetamine undergoes metabo-
lism to both the corresponding N.hydroxylamine and the nitrone
metabolite the urine, is believed
to be formed by further oxidation of the N-hydroxylamine intermediate N-hydroxyphenmetrazine.203 Importantly,
ainination contributes to the metabolism of secondary amities remains unclear.
OH
o
C H
CH3 Propranolol
Oxprenotol
0 NHa
NH
NH2 Phenylacetone
Melbampvietamine
"=1 NHCH3
Ketamine
Norketamine
90
Textbook of Organic Medicinal and Pharmaceutical Chemistry
Wilson and
OH Direci Oxidativo
H2N—
druxylamine ntetabolite. One such case is aniline, which nietabolized to the corresponding N-hydroxy product.223
Oxidation ni the hydroxylamine derivative to the nhtroso detivative also can occur, When one considers primary aroStatic amine drugs or metabolites. N-oxidation constitutes only a minor pathway in comparison with other biotransformation pathways. such as N-acetylation and aromatic hydroxylation, in humans. Some N-oxygenated metabolites have been reported, however. For example, the antileprotic agent dapsone and its N-acetylated metabolite are metaboheed significantly to their corresponding N-hydroxylamine derivatives225 The N-hydroxy metabolites are further conjuwith glucuronic acid.
Methenioglohinemia toxicity is caused by several uroitalic amines, including aniline and dapsone. and is a result ol the bioconversion of the aromatic amine to its N-hydroxy
derivative. Apparently, the N-hydroxylarnine oxidizes the Fe2 form of hemoglobin to its Fe3 form. This oxidized (Fe3 ) state of hemoglobin (called snethemoglobin orferrihemoglobin) can no longer transport oxygen, which leads to serious hypoxia or anemia, a unique type of chemical suffocation.221
Diverse aromatic amines (especially azoamino dyes) are known to be carcinogenic. N-oxidation plays an important role in bioactivating these aromatic amines to potentially reactive electrophilic species that covalently bind to cellular protein. DNA, or RNA. A well-studied example is the carcinogenic agent N-methyi-4-aminoazobenzene.228' idation of this compound leads to the corresponding hydrox-
ylamine. which undergoes sulfate conjugation. Because of the good leaving-group ability of' the sulfate (S042) anion. this conjugate can ionize spontaneously to form a highly reactive, resonance-stabilized nitrenium species. Covalent adducts between this spccies and DNA. RNA. and proteins have been The sulfate ester is believed to be the ultimate carcinogenic species. Thus, the example indicates that certain aromatic amines can be bioactivated to reactive intermediates by N-hydroxylation and 0-sulfate conjugation. Whether primary hydroxylamines can be binac-
tivated similarly is unclear. In addition, it is not known if this biotoxification pathway plays any substantial role in the toxicity of aromatic amine drugs. N-oxidation of the nitrogen atoms present in aromatic heterocyclic moieties of many drugs occurs to a minor extent. Clearly, in humans. N-oxidation of the folic acid antagonist
trimethoprim (Proloprim. Trimpex) has yielded approxiniately equal amounts of the isomeric I-N-oxide and 3-Noxide as minor metabolites.232 The pyndinyl nitrogen atom
present in nicotinine (the major metabolite of nicotine) undergoes, oxidation to yield the corresponding N-oxide metabolite.233 Another therapeutic agent that has been observed
to undergo formation of an N-oxide metabolite is metronida-
NH2
NH2 Phenterrnine
Chioqphenlecmcne
Amantadine
CH3
Ci
NO2
NH2 Chlorphentermime
N.Hydroxychtorphontermlfle
Nstroso Metabolite
RCH2NHOH —i.RCH2—N=O Hydroxylarnine
Nitroso
93
Nitro
Nitro Metabolite
rn and Giseold 's Textbook of Organic Medicinal and Pharmaceutical chemistry
CH3
N-Oxide
CH3
Carbon
Tartary Aromatic Amine
H""H {
Cathinolamine
/=\
/=\
OxIdat.on
1420
Hydroxytamine
Secondary Aromatic Aromas
Hydroxytarnlne (pnmary)
Nftrone
(secondary)
I-. AnUine
N=0
NHOH
NH2
= Nitroso
Hydroxylamine
(praTmary
aromatic amine)
RNH
NH2 Dapsone N-Acetyldapsone
R-H R-
0
__i,RNH_()_S0s_()_NHOH N-Hydroxydapsone
R
-It
N.AcetyI-N-I-mydroxydapsone
A
-
a
H
0
N I
CH3 I
Cotinmne
/
Amides. Amide functionalities are susceptible live carbon—nitrogen bond cleavage (via a-carbon hydrosyl ation) and N-hydroxylation reactions. Oxidative dealkyl ation of many N-substituted amide drugs and xcnobietic has been reported. Mechanistically. oxidative dcalkylaiia proceeds via an initially formed carbinolamide, which issU stable and fragments to form the N.dealkylated product Fa example. diazepam undergoes extensive N-demetliylatiaric
the pharmacologically
active
metabolite
desmeth)l&•
N
02N
,.AN
Various other N-alkyl substituents present in benzodiazeand in barbiturates (e.g.. IsU obarbital and mephobarbital)'28 are similarly dealkylated. Alkyl groups attached to the amide moiety d some sulfonylureas. such as the oral hypoglycemic clr(or pines (e.g.,
N
CH3
I
CH2CH2OH
Metronidazole 2-(2-Mathyi-5-nitro-imidazoi-1-yI)-ethanol
also are subject to dealkylation to a mist ex ent. In the cyclic amides or lactams. hydroxylation of thcalsy-
Chapter 4 . Metabolic Changes of Drug.c and Related Organic Con,pouads
95
CH3
C6H5N=N Sulfate Conjugate
1_s0. 2 CH3
CH3
DNA, RNA
adducts
and protoin
[C6H5N
=
C6H5N=N Nitrenrum Ion
OCH3
H2N
OCH3
clic carbon a to the nitrogen atom also leads to carbinolamides. An example of this pathway is the conversion of cotiInterestingly, the latter nine to 5-hydroxycotinine. caibinolamide intennediate is in tautomeric equilibrium with the ring-opened mecabolite
3-N-Oxide
1-N-Oxide
Tnmelhopnn,
y-(3-pyridyl)-y-oxo-N-methyl-
Metabolism of the important cancer chcmotherapeuuc agent cyclophosphamide (Cytoxan) follows a hydroxylation pathway similar to that just described for cyclic amides. This drug is a cyclic phosphoramide derivative and, for the most part, is the phosphorous counterpart of a cyclic amide. Because cyclophosphamide itself is pharmacologically mac-
aCH3
0
C6H5
C6H5 Diazepam
Desmethyldiazepam
Catbinolanude
I
0
(CH3CH2)2NCH2CH2
O
R1
HN
CH3
SO2NHCNHCH2CH2CH3 T
Flurazepam
A1
A2 = CH2CH3
Chlopwpamlde
96
Wi/ron and Gis,oldx Textl,aok of Organic Medicinal and Pharmaceutical Chemistry
— CH3
N
CH3 5-Hydroxycotinine
Cotinine
methylbutyramide
metabolic binactivation is required for the drug to mediate its antitumorigenic or cytotoxic effects. The key biotransformation pathway leading to the active metabolite involves an initial carbon hydroxylation reaction at C-4 to form the carbinolamide 4-Hydroxycyclophosphamide is in equilibrium with the ring-opened dealkylated metabolite aldophosphamide. Although it has potent cytotoxic properties. aldophosphainide undergoes a further elimination reaction (reverse Michael reaction) to generate acrolein and the phosphoramide mustard N.N-bis(2-chloro-ethyl)phosphorodiamidic acid. The latter is the principal species responsible for cyclophosphamide's antitumorigenic properties and chemotherapeutic effect. enzymatic oxidation of 4-hydroxycyclophosphamidc and aldophosphamide leads to the relatively nontoxic metabolites 4-ketocyclophosphamide and carboxycyclophosphamide, respectively.
N-hydroxylation of aromatic a,nide.s. which occurs to a minor extent, is of some toxicological interest, since this biotransformation pathway may lead to the formation of chemically reactive intermediates. Several examples of cytotoxicity or carcinogenicity associated with metabolic N-bydroxylation of the parent aromatic amide have been reported. For example. the well-known hepatocarcinogenic 2-acetyl-
aminofluorene (AAF) undergoes an N-hydroxylution reaction catalyzed by cytochrome P.450 to form the corresponding N-hydroxy inerabolite (also called a hydroxamic
acid).24' Further conjugation of this hydroxamic acid produces the corresponding 0-sulfate ester, which ionizes to generate the electrophilic nitrenium species. Covalent binding of this reactive intermediate to DNA is known to occur and is likely to be the initial event that ultimately leads to malignant tumor formation.242 Sulfate conjugation plays an important role in this biotoxificalion pathway (see "Sulfate Conjugation,'• for further discussion). Acetaminophen is a relatively safe and nontoxic analgesic agent if used at therapeutic doses. Its metabolism illustrates the fact that a xcnobiotic commonly produces more than one metabolite. Its metabolism also illustrates the effect of age,
since infants and young children carry out sulfation rather than glucuronidation (see discussion at the end of this chapter). New pharmacists must realize that at one time acetanilide and phenacetin were more widely used than acetaminophen. even though both are considered more toxic because
they produce aniline derivatives. Besides producing toxic aniline and p-phenetidin. these two analgesics also produce acetaminophen. When large doses of the latter drug are ingested, extensive liver necrosis is produced in humans and animals.2'" 244 Considerable evidence argues that this hepatotoxicity depends on the formation of a metabolically generated reactive intermediate.245 Until recently.24t' 247 the ac-
cepted bioactivation pathway was believed to involve an initial N-hydroxylation reaction to form N-hydroxyacetaminophcn.248 Spontaneous dehydration of this N-hydrox-
H H
OH
— CH2CH2CI
—' II
II
II
0
0
0
4-Hydrosycyclophosphamide
Cyclophosphamide
4-Ketocyclophospharnide
ii.
OH
0 NH2
CHO
I
HO—P—N II
Phosphoramide Mustard N.N-bis(2-Chloroethyt). phosphorodiamidic Acid
+
\OH'—
CH2 Acroteir,
OH CI N
0 Pidophosphansde
0 Carboxyphosphamide
Chapter 4 • Metabolic Changes of Drugs and Related Organic C'
SUNo
/c=o
/c=o
CH3
/
CH3
2Acel5tarrrnOtixcCvo
N.
CH3 O-Svtato Ester
bdrovy W
N:HydroOy
or
W
Nu /
j
[ Ni, - M.oeopnte
op
NOmnlivrr Species
DNA
0
0
0
II
II
C NH
CH3
HN
Cit3
NH
GYP 450
01,
CVP45O
CH3CHO
Acetandid
OCH3CH3 H
Phenacetin
AceloaminoØsen NH2
Aniline
(mothemaglobinanemia. hemotytic anemia OCH2CH,
0
0 II
II
C
HN
Direct renal excretion
C CHx
NH
CH2
p-Fhenotlian (methemoglobrnernla; homotytic anemia; neplitapathy)
Pathway when 70% at liver
OGlucuronide
Major route In d,llthpn
I Urine
Majoe rnute In adelts
Covalent beratng to bepahc toe, cell sinicture
1 UrIno
HN
OH
Glutathione
CHn
Hepatic necrosis;
98
Wi/con and Gi.cvold'.c Textbook of Organic Medicinal and Pharinaceulkal Chemistry
yamide produces N-acetylimidoquinone. the proposed reac-
groups, such as indomethacin
tive metabolite. Usually, the GSH present in the liver
and metoprolol have reportedly undergone significant 0-demethylation to their corresponding phenolic or alcoholic metabolites. which are further conjugated. In many drugs that have several nonequivalent methoxy groups, one particular methoxy group often appears to be 0-demethylated selectively or preferentially.
combines with this reactive mewbolite to form the corresponding GSH conjugate. If GSH levels are sufficiently depleted by large doses of acelaminophen. covalent binding of the reactive intermediate occurs with macromolecules present in the liver, thereby leading to cellular necrosis. Studies indicate, however, that the reactive N-ocetylimidoquinone intermediate is not formed from N-hydroxyacetaminophen.245247 It probably arises through some other oxidative process. Therefore, the mechanistic formation of the reactive metabolite of acetaminophen remains unclear.
For example. the 3,4,5-trimethoxyphenyl moiety in both mescaline-55 and trimethopriin232 undergoes 0-demechylation to yield predominantly the corresponding 3-0-demeth-
ylated metabolites. 4-0-demethylation also occurs to a minor extent for both drugs. The phenolic and alcoholic me-
tabolitcs formed from oxidative 0-demethylation are susceptible to conjugation, particularly glucuronidation.
OXIDATION INVOLVING CARBON-OXYGEN SYSTEMS
Oxidative 0-dealkylation of carbon—oxygen systems (principally ethers) is catalyzed by microsomal mixed function oxidases.'63 Mechanistically, the biotransformation involves an initial a-carbon hydroxylation to form either a hemiacetal or a heiniketal. which undergoes spontaneous carbon—oxygen bond cleavage to yield the dealkylated oxygen species (phenol or alcohol) and a carbon moiety (aldehyde or ketone). Small alkyl groups (e.g., methyl or ethyl) attached to oxygen are 0-dealkylaced rapidly. For example, morphine is the metabolic product of 0-demethylation of codeine.249 The antipyretic and analgesic activities of phenacetin (see drawing of acetaminophen metabolism) in humans appear to be a consequence of 0-deethylation to the active metabolite acetaminophen.25° Several other drugs containing ether
OXIDATION INVOLVING CARBON-SULFUR SYSTEMS
Carbon—sulfur lunctional grotips are susceptible to metabolic S-dealkylation. desulfuration. and S-oxidation reactions. The first two processes involve oxidative carbon—sulfur bond cleavage. S-dealkylation is analogous to 0- and N-dealkylation mechanistically (i.e.. it involves acarbon hydroxylation) and has been observed for various sulfur xenobiotics.' 256 For example. 6-(methylchio)purine is demethylated oxidatively in rats to 6-mercaptopurinc.257
S-demethylation of methitural259 and S-debenzy-
lation of 2-benzylthio-5-trifluoromethylben7.oic acid also have been reported. In contrast to 0- and N-dcalkylation. examples of drugs undergoing S-dealkylation in humans are
0 R—OH +
Hemiacetal or Hemiketal
Ether
/\
Phenol or Alcohol
\
+
Morphine
0
Phenacetin
0
H
H Codeine
Carbonyl Moiety (aldehyde or ketone)
N'3
H
CH3O
prazosin
Acetaminophen
Chapter 4 • Meiabollc Changes of Drugs and Related Organic C'oinpounds
99
Prazosin
Metoprotol
Indomethacin
I
I
OCH3
OCH3
>' NH2 OCH3
OCH3
H2N
Mescaiine
because of the small number of sulfur-containing medicinals and the competing metabolic S-oxidation prolimited
cesses (see diagram).
Oxidative conversion of carbon—sulfur double bonds (C = li) (thiono) to the corresponding carbon—oxygen double
bond (C=O) is called desulfurazion. A well-known drug example of this metabolic process is the biotransformation oIthiopental to us corresponding oxygen analogue pentobarbitaL28026' An analogous desulfuration reaction also occurs with the P= S moiety present in a number of organophosphase insecticides, such as parathion.262263 Desulfurdtion of
parathion leads to the formation of paraoxon. which is the active metabolite responsible for the aruicholinesterase activity of the parent drug. The mechanistic details of desul(oration arc poorly understood, but it appears to involve micmsomal oxidation of the C = S or P = S double Organosulfur xenobiotics commonly undergo S-oxidation to yield sulfoxide derivatives. Several phenothiazine derivatives are metabolized by this pathway. For example. both wilfur atoms present in ihioridazine (Mellaril)265266 are Susceptible to S-oxidation. Oxidation of the 2-methyllhio group yields the active sulfoxide nietabolite mesoridazinc. Interestingly, mesoridazine is twice as potent an antipsychotic agent us ihioridazine in humans and has been introduced into clinical use as
S-oxidation constitutes an important pathway in the metabolism of the H2-histamine antagonists cimetidine (Tagamet)268 and metiamide.26° The corresponding sulfoxide derivatives are the major human urinary mctabolites. Sulfoxide drugs and metabolites may be further oxidized to sulfones (-SO2-). The sulfoxide group present in the immunosuppressive agent oxisuran is metabolized to a sulfone moiety.270 In humans. dimethylsulfoxide (DMSO) is found primarily in the urine as the oxidized product dimethylsulfone. Sulfoxide metabolitcs. such as those of thioridazine. reportedly undergo further oxidation to their sulfone -SO2266
Oxidation of Alcohols and Aldehydes Many oxidative processes (e.g., benzylic. allylic. alicyclic. or aliphatic hydroxylation) generate alcohol or carbinol metabolites as intermediate products. If not conjugated, these alcohol products are further oxidized to aldehydes (if primary alcohols) or to ketones (if secondary alcohols). AIdehyde meusbolites resulting from oxidation of primary alco-
hols or from oxidative deamination of primary aliphatic amincs often undergo facile oxidation to generate polar carboxylic acid derivatives.' As a general rule, primary alcoholic groups and aldehyde functionalitics are quite vulnera-
SH I
N
LN
0
N
6-Mercaptopunne purine
100
WiLcon and Gisvold .c Textbook of Organic Medicinal and Pharmaceutical Clwmistre
0
CH2CH2S—CH3
COOH
)L1LCHCH2CH2CH3
HN
CR3 CF3 2-BeflZylIhio-5-
MeIhiIuraI
triiluoromethylbenzoic Acid
0
0
CH2CH3
II
H
0
S
CH3CH2O Paraoxon
Parathion
00
0 II
SCH3 I
I
R
R
Ring Suitone
Ring Sufloxide
N
2 S—CH3
CH2CH2 N
:
S—CH3
cx: N
\CH3
\ CH3 Mesondaznle
Sulloodazlne
00
Chapter 4 • Metabolic Changes of Drugs and Related Organic Compounds
H
101
—
HN ,, N x Cineticilne X..N—CazN X—S
0
0
II
II
Sulfoxide Metabolite
000 II
0 CH3—S—CH3
We to oxidation. Several drug examples in which primary alcohol metabolites and aldehyde metabolites are oxidized to carboxylic acid products are cited in sections above. Although secondary alcohols are susceptible to oxidation. this reaction is not often important because the reverse reaction. namely,
reduction of the ketone back to the secondary
alcohol, occurs quite readily. In addition, the secondary alcohol group, being polar and functionalized, is more likely to be conjugated than the ketone moiety. NAD
NAD'
NADH
RCH2OH
RCHO
Pnnwy Aketel
Aldehyde
—i
Dimethyl Sutfoxide
Sulfone Metaboi,te
Ox,suran
Dimethyl Suit one
other Qaddative Blofransformatlon Pathways
In addition to the many oxidative biotransfonnations discussed above, oxidative aromatization or dehydrogenation
and oxidative dehalogenation reactions also occur. Metabolic aromatization has been reported for norgestrel. Aroma-
tization or dehydrogenation of the A ring present in this steroid leads to the corresponding phenolic product I 7aethinyl-l8-homocstradiol as a minor metabolite in worn-
In mice, the terpene ring of i'-THC or
NADH
Acid
The bioconversion of alcohols to aldehydes and ketones is catalyzed by soluble alcohol dehydrogenases present in the liver and other tissues. NAD ' is required as a coenzyme. although NADP also may serve as a coenzyme. The reaction catalyzed by alcohol dehydrogenase is reversible but often proceeds to the right because the aldehyde formed is further oxidized to the acid. Several aldehyde dehydrogena.ses. including aldehyde oxidase and xanthine oxidase. carry
undergoes aromatization to give cannabinol.276 277 Many halogen-containing drugs and xenobiotics are metabolized by oxidative dehalogenation. For example. the vol-
atile anesthetic agent halothane is metabolized principally to trifluoroacetic acid in humans.278 It has been postulated that this metabolite arises from cytochrome P450-mediated hydroxylation of halothane to form an initial carbinol intermediate that spontaneously eliminates hydrogen bromide (dehalogenation) to yield trifluoroacetyl chloride. The latter acyl chloride is chemically reactive and reacts rapidly with water to form trifluoroacetic acid. Alternatively, it can acylate tissue nucleophiles. Indeed, in vitro studies indicate
out the oxidation of uldehydes to their corresponding
that halothane is metabolized to a reactive intermediate (pre-
acids.''6 171—273
sumably trifluoroacetyl chloride), which covalently binds to liver microsomal proteins.280- 281 Chloroform also appears to be metabolized oxidatively by a similar dehalogenation pathway to yield the chemically reactive species phosgene.
Metabolism of cyclic ainines to their lactam metabolites has been observed for various drugs (e.g., nicotine. phenmetrazine. and methylphenidate). It appears that soluble or mi-
cmsomal dehydrogenase and oxidases are involved in oxidizing the carbinol group of the intermediate carbinolamine to a carbonyl moiety.273 For example, in the metabolism of medazepam to diazepam, the intermediate carbinolamine (2hydroxyniedazepam) undergoes oxidation of its 2-hydroxy group to a carbonyl moiety. A microsomal dehydrogenase carries out this
CH3
C6H5 Medazepam
Phosgene maybe responsible for the hepato- and nephrotoxicity associated with chloroform.282 A final example of oxidative dehalogenation concerns the antibiotic chloramphcnicol. In vitro studies have shown that the dichloroacetamide portion of the molecule undergoes oxidative dechlorination to yield a chemically reactive oxamyl chloride intermediate that can react with water to form
CH3
C6H5 2-Hydroxymedazepam
CH3
C6H5 Diazepam
Wilson and Giuvold,c Textbook of Organic Medicinal and Pharmaceutical Cl,emi.c:rs'
CH
CH.
-t Nogestret
1 7a.Ethlnyl-1 8-homoestradiol
CH3
OH3
C5H11
CH3
CH3 Cannab4nol
H —HBr
I
0
0
II
II
—s F3C—C—Cl —,
F30—C—Br —i
+ HCI
CI Halothane
Trifluoroacetyl Chloride
Carbinol
Intermediate
H
HCI
I
II
Cl—C—Cl
I
/
H7C03 + HCI
H
I
H
Truftuoroacetic Acid
I
Cl
ci
ICl—C—CI]—L L
Chloroform
Phosgene
Covalent Binding
Dichloroacelamido portion
00
OH
OH
II
CH2OH Chtoraniphenicol
II
—HCI
I
II
—p R—NH—C—C— —
CI
00 II
II
R—NH—C—C OH Oxarruc Acid Derivative
Oxarnyl Chloride Derivative
TI
NucleophIes
Covalent Binding (toxicity?)
Chapter 4 • Metabolic Changes of Drugs and Related Organic Compounds
103
he corresponding oxamic acid metabolite or can acylate mi-
NADPH. however, the same enzyme system can reduce car-
Thus, it appears that in several instances. oxidative dehalogenation can lead to the formation of toxic and reactive acyl halide intermediates.
bonyl derivatives to their corresponding alcohols.' 6 Few aldehydes undergo bioreduction because of the relative ease of oxidation of uldehydes to carboxylic acids. One frequently cited example of a parent aldehyde drug undergoing extensive enzymatic reduction, however, is the sedative—hypnotic chloral hydrate. Bioreduction of this hydrated aldehyde yields trichloroethanol as the major metabolite in humans.288 Interestingly, this alcohol metabolite is pharmacologically active. Further glucuronidation of the alcohol leads to an inactive conjugated product that is readily excreted in the urine.
crosomal
REDUCI'IVE REACTIONS Reductive processes play an imponant role in the metaboIicm of many compounds containing carbonyl. nitro. and azo groups. Bioreduction of carbonyl compounds generates whereas nitro and azo reductions alcohol lead to amino derivatives.86 The hydroxyl and amino moicties of the metabolites arc much more susceptible to conjuga-
lion than the functional groups of the parent compounds. Hence, reductive processes, as such, facilitate drug elimination.
Reductive pathways that are encountered less frequently in drug metabolism include reduction of N-oxides to their corresponding tertiary amines and reduction of sulfoxides to sulfides. Reductive cleavage of disultide linkages and reduction of carbon—carbon double bonds also occur, but constitute only minor pathways in drug metabolism.
Reduction of Aldehyd. and katone The carbonyl moiety. particularly the ketone group, is encountered frequently in many drugs. In addition. metabolites containing ketone and aldehyde functionalities often arise from oxidative deaminalion of xenobiotics (e.g., propranouI. chlorphenirarninc. amphetamine). Because of their ease ofoxidation. aldehydes are metabolized mainly to carboxylic acids. Occasionally. aldehydes are reduced to primary alcohols. Ketones. however. are generally resistant to oxidation and are reduced mainly to secondary alcohols. Alcohol meabolites arising from reduction of carbonyl compounds genemily undergo further conjugation (e.g.. glucuronidation).
0
OH
—+
Kelone
0 II
I
._ø Cl3C—CH2OH
CI3C—C—OH Chioral Hydrate
Chioral
Tr,cflioroethanol
Aldehyde metabolites resulting from oxidativc deaminalion of drugs also undergo reduction to a minor extent. For example, in humans the f3-adrenergic blocker propranolol is converted to an intermediate aldehyde by N.dealkylation and
oxidative deamination. Although the aldehyde is oxidized primarily to the corresponding carboxylic acid (naphthoxylactic acid), a small fraction also is reduced to the alcohol derivative (propranolol Two major polar urinary metaholites of the histamine H1 antagonist chiorpheniramine have been identified in dogs as
the alcohol and carboxylic acid products (conjugated) derived, respectively, by reduction and oxidation of an aIdehyde metabolite. The aldehydc precursor arises from bis-Ndcmethylation and oxidative deamination of chlorpheniramine.2°°
Bioreduction of ketones often leads to the creation of an asymmetric center and, thereby, two possible stercoisomeric alcohols.' lot For example, reduction of acelophenone by a soluble rabbit kidney reducta.se leads to the enantiomeric alcohols (S)(—)- and (R)( + with the (S)(—) isomer predominating (3:1 ratio).-92 The preferential formation of one stereoisomer over the other is termed product s:ereoselec:ivitv in drug metabolism.2°' Mechanistically.
ketone reduction involves a "hydride" transfer from the reduced nicotinamide moiety of the cofactor NADPH or Primary Aicohols
0
OH
NADH to the carbonyl carbon atom of the ketone. It is generally agreed that this step proceeds with considerable stereo-
HO
sdectiviry)'6 291 Consequently, it is not surprising to find
Secondary Alcohols (stereoisomeric products possible)
Diverse soluble enzymes, called aldo-keto reductases. cany out bioreduction of aldehydes and ketones.'
281 They
ate found in the liver and other tissues (e.g.. kidney). As a general class, these soluble enzymes have similar physiochcmical properties and broad substrate specificities and require NADPH as a cofactor. Oxidoreductase enzymes that cany out both oxidation and reduction reactions also can
many reports of xenobiotic ketoncs that arc reduced preferentially to a predominant stereoisomer. Often. ketone reduction yields alcohol metabolites that are pharmacologically active. Although many ketone-containing drugs undergo significant reduction, only a few selected examples are presented in detail here. The xenobiotics that are not discussed in the text have been structurally tabulated in Figure 4-10. The keto
group undergoing reduction is designated with an arrow. Ketones lacking asymmetric centers in their molecules, such as acetophenone or the oral hypoglycemic acetohexaniide, usually give rise to predominantly one enantiomer
reduce aldehydes and ketones.281 For example, the important
on reduction. In humans, acetohexamide is metabolized rap-
liver alcohol dehydrogenase is an NAD -dependent oxidoreductase that oxidizes ethanol and other aliphatic alcohols
idly in the liver to give principally (S)(—)-hydroxyhexa-
to aldehydes and ketones. In the presence of NADH or
mide.291 294 This metabolite is as active a hypoglycemic agent as its parent compound and is eliminated through the
104
Textbook of Organic Medicinal and Pham,aceuiical Ow,nistrv
Wilson and
OH
o
OH
CH
0
CR
I
ttJH 2
CR CR3
Propranolol
Propranolol
Aidehyde Intermediate
OH
o
CR2 COOH
OH Acid
Propranolol Glycol
(conjugated)
bls.Ndemelflylaflon
CH3
,CHCH2CH 1
'
2)
Deomnation
j
Chlorplieniramine Ajdehyde Melabolite
\oxKiahon
CHCH2CH2OI-$ CI 3-(p-Chlorobenzyt)-3-(2-pyridyl)propan- 1-ct
HO
+
AcetoØtenone
Si — (-Methyl Phenyl Carbmo) (75%)
\ ,.OH
RI + (-Methyl Ptienyl
Carbinol (25%)
t\
—\J—
CHCH2C—OH
/ CH2
3-(p-Oilorobenzyl)-3-(2-pyr;dyO-
propanoic Acid
Chapter 4 • Metabolic Changes of Drugs and Related Organic C'onipounds
0
105
0
OH
OH
0
CH
o
IL1H
0
H5C
Diethyiproplon
Bunolol
Daunomycin
OH
0 CH3
C6H5
N(CH3)2
H2
0 CH3
S( + ).Methadone
Naloxone
Metryapone
Figure 4—10 • Additional examples of xenobiotics that undergo extensive ketone reduction, not covered in the text. Arrow indicates the keto group undergoing reduction.
Acetohexamide usually is not recommended in diabetic patients with renal failure, because of the possible accumulation of its active metabolitc. hydroxyhexamide. V/ben chiral ketones are reduced, they yield two possible
diastereomeric or epimeric alcohols. For example, the
6/3-nalirexol, whereas in chickens, reduction yields only 6a-naltrexol. In monkeys and guinea pigs, however. both epimeric alcohols are formed (predominantly 648-naltrexol).3®301 Apparently, in the latter two species, reduction of naltrexone to the epimeric 6a- and 6/3-alcohols is carried
(R)( + ) enantiomer of the oral anticoagulant warfarin undergoes extensive reduction of its side chain keto group to gen-
out by two distinctly different reductases found in the
erate the (R,S)( +)
Reduction of oxisuran appears not to be an important pathway by which the parent drug mediates its immunosuppressive effects. Studies indicate that oxisuran has its greatest immunosuppressive effects in those species that form alcohols as their major metabolic products (e.g., human. In species in which reduction is a minor pathway (e.g.. dog). oxisuran shows little immunosuppressive activThese findings indicate that the oxisuran alcohols (oxisuranols) are pharmacologically active and contribute substantially to the overall immunosuppressive effect of the
alcohol as the major plasma metabolite Small amounts of the (R.R)( +) dia,stereonet also are formed. In contrast, the (S)(—) enantiomer
in humans,56'
undergoes little ketone reduction and is primarily 7-hydroxylated (i.e., aromatic hydroxylation) in humans.
Reduction of the 6-keto functionality in the narcotic antagonist nahrexone can lead to either the epimeric 6o- or ofl-hydroxy metabolitcs, depending on the animal naltrexone
In humans and rabbits. bioreduction of is highly stereoselective and gener.utes only
®
Acelophenone
Reduced Nicotamide Moiety of NADPH or NADH
0
Acelohexamide
S( — )-Melhyl Phenyt Carbinot
Oxidized Nicotarnide Moiety of NADP • Os NAD
0
Sf - ).Hydroiiytiexaniide
106
Wilson and Giscolds Textbook of Organic Medicinal and Pharmaceutical Chemistry
0
HO
H
V
OH
OH
+
—.
RR-( +
R,S-( + )-AIcohol
R( +
Major Diastereorner
Minor Diaslereomer
and / or
6p-Naltrexol
NatIrexone
parent drug. The sulfoxide group in oxisuran is chiral. by virtue of the lone pair of electrons on sulfur. Therefore, re-
Reduction of ketones results in reduction not only of the ketone group but of the carbon—carbon dou-
duction of oxisuran leads 10 diasiereomeric alcohols.
ble bond as well. Steroidal drugs often fall into this class. including norethindrone. a synthetic progestin found in many oral contraceptive drug combinations. In women, the
00.
HOHO
Oxisuranols (diastereomeric mixture)
Oxisuran
CH3
major plasma and urinary metabolite of norethindrone is the 3/J.5/3-tetrahydro derivative.307 Ketones resulting from metabolic oxidative deamination processes are also susceptible to reduction. For instance, rabbit liver microsomal preparations metabolize amphetamine
to phenylacetone. which is reduced subsequently to I-
OH
H-
45 Norethindrone 3p.5g3-Tetratrydr000relhuxitone
NH2 I-Phenyl-2-propanol
Phenylacelone
Anrphetarnine
OH
OH H
and Ox,dat,ve Oaan,,nalion
Reduction
0 I -Nydroxy-1 -phenyl-
propan-2-one
1 -Ptrenyt-1 2-propanediol (as gLicuronide conjugate)
Chapter 4 e Metabolic phenyt.2-propanol.308 In humans, a minor urinary metabolite
01 (-)-ephedrine has been identified as the diol derivative formed from keto reduction of the oxidatively deaminated product I -hydroxy- I -phenylpropan-2-one.309
The reduction of aromatic nitro and azo xenobiotics leads Aromatic nitro to aromatic primary amine compounds are reduced initially to the nitroso and hydroxylamine intermediates, as shown in the following metabolic sequence:
— Ar—N=O —. Ar—NHQH —. Ar—NH2 0 Amine
Hydroxylainine
Nittoso
Azo reduction, however, is believed to proceed via a hydnizo intermediate (-NH-NH-) that subsequently is cleaved reductively to yield the corresponding aromatic amines:
k—N=N—k'
most of the urinary metabolites of metronidazole found in humans are either oxidation or conjugation products. Reduced metabolites of metronidazole have not been detected.319 When incubated anaerobically with guinea pig liver preparations, however, metronidazole undergoes considerable oitro reduction.32° Bacterial reductase present in the intestine also tends to
complicate in vivo interpretations of nitro reduction. For example, in rats, the antibiotic chloramphenicol is not reduced in vivo by the liver but is excreted in the bile and. subsequently, reduced by intestinal flora to form the amino metabolite.321 322
CH,OH
I
Nydrazo
CH2CH;OH
Ar—NH2 + H2N—Ar'
Mel,mi,dazole
The enzymatic reduction of azo compounds is best exem-
Ammes
Bioreduction of nitro compounds is carried out by NADPI-l-dcpendent microsomal and soluble nitro reductases present in the liver. A multicomponent hepatic microsomal reduclase system requiring NADPH appears to be responsible for a,o reduction.3 IOJt2 In addition, bacterial reductascs present in the intestine can reduce nitro and azo compounds. especially those that are absorbed poorly or excreted mainly 314
Various aromatic nitro drugs undergo enzymatic reduction to the corresponding aromatic amines. For example, the 7-aiim benzodiazepine derivatives clonazepam and nitrazepam are metabolized extensively to their respective 7-amino 316 The skeletal muscle relaxant metabolitcs in dantrolene (Dantrium) also reportedly undergoes reduction 315
to aminodantrolene in
plified by the conversion of sulfamidochrysoidine (Prontosil) to the active sulfanilamide metabolite in the This reaction has historical significance, for it led to the discovery or sulfanilamide as an antibiotic and eventually to the development of many of the therapeutic sulfonamide drugs. Bacterial reductases present in the intestine play a
significant role in reducing azo xenobiotics. particularly those that are absorbed poorly.313 314 Accordingly, the two azo dyes tartrazine324 and amaranth321' have poor oral absorption because of the many polar and ionized sulfonic acid groups present in their structures. Therefore, these two
azo compounds are metabolized primarily by bacterial reductases present in the intestine. The importance of intestinal reduction is further revealed in the metabolism of sulfasala-
zine (formerly salicylazosulfapyridine, Azulfidine). a drug used in the treatment of ulcerative colitis. The drug is ab-
02N
Clonazepam, Nitrazepam.
R=CI
7-Amino Metabotite
R..H
0
0
0 Dantrotene
107
For some nitro xenobiotics. bioreduction appears to be a minor metabolic pathway in vivo. because of competing oxidative and conjugative reactions. Under artificial anaerobic in vitro incubation conditions, however, these same nitro
k—NH—NH—Ark
Azo
in the
of Drugs and Related Organic compounds
xenobiotics are enzymatically reduced rapidly. For example.
Reduction of Nitro and Azo Compounds
Ndro
C'hange.s
o Aminodantrolene
108
Wilson and Gisvold's Textbook of Organic Medicinal and Pharmaceutical Chemistry
NH2
NH2
—i
+
Sutlarnidochsysoidine
1 2,4-Tnaminobenzene
Sullanulamide
(PrOntosil)
-
Taulrazine
cco0 —ccc
N =N
1CH3
COOH
CM3
/
CH2CH2CH2N
CH7CH2CH2N CM3
CM3
Imipam;nO
+ 5-Aminosalicylic Acid
Suit apyridine
sorbed poorly and undergoes reductive cleavage of the azo linkage to yield sulfapyridine and 5-aminosalicylic acid.327 325 The reaction occurs primarily in the colon and is carried out principally by intestinal bacteria. Studies in germ-
free rats, lacking intestinal flora, have demonstrated that sulfasalazine is not reduced to any appreciable extent.329
Miscellaneous Reductions Several minor reductive reactions also occur. Reduction of N-oxides to the corresponding tertiary amine occurs to some extent. This reductive pathway is of interest because several tertiary amines are oxidized to form polar and water-soluble N-oxide metabolites. If reduction of N-oxide metabolites occurs to a significant extent, drug elimination of the parent tertiary amine is impeded. N-Oxide reduction often is assessed by administering the pure synthetic N-oxide in vitro
or in vivo and then attempting to detect the formation of the tertiary amine. For example, imipramine N-oxide undergoes reduction in rat liver preparations.330 Reduction of sulfur-containing functional groups, such as the disul tide and sulfoxide moieties, also constitutes a minor reductive pathway. Reductive cleavage of the disulfide bond in disulfiram (Antabuse) yields N.N-diethyldithiocarbamic acid (free or glucuronidated) as a major metabolite in humans.33L332 Although sulfoxide functionalities are oxidized mainly to sulfones (-SO2-), they sometimes undergo reduction to sulfides. The importance of this reductive pathway
is seen in the metabolism of the anti-inflammatory agent sulindac (Clinoril). Studies in humans show that sulindac undergoes reduction to an active sulfide that is responsible
for the overall anti-inflammatory effect of the parent Sulindac or its sulfone metabolile exhibits little anti-inflammatory activity. Another example of sulfide fordrug.333
CH2CH3
/N—C—S—S—C—N\CH2CH3
CH3CH2
II
II
S
S
Disutfiram
/N—C—SH
CH3CH2
II
S
N.N-Duethytthlocarbamic Acid
Chapter 4 U Metabolic Changes of Drugs and Related Organic Compounds
109
i2000H
F
-t H
Sulindac Sulfide Metabotife
Sulindac
mation involves the reduction of DMSO to dimethyl sulfide. In humans. DMSO is metabolized to a minor extent by this pathway. The chaiacteristic unpleasant odor of dimethyl sulis evident on the breath of patients who use this agent.335
0 —i CH3SCH3 Dimethyl Sulfide
Dirnethyl Sulloxide
hydrolysis of cocaine to methyl ecgonine. however, also occurs in plasma and, to a minor extent, Methylphenidate (Ritalin) is biotransformed rapidly by hydrolysis to yield ritalinic acid as the major urinary metabolite in humans.342 Often, ester hydrolysis of the parent drug leads to pharmacologically active metabolites. For example. hydrolysis of diphenoxylate in humans leads to diphenoxylic acid
(difenoxin), which is, apparently, 5 times more potent an antidiarrheal agent than the parent ester.343 The rapid metab-
olism of clofibrate (Atromid-S) yields p-chlorophenoxyisobutyric acid (CPIB) as the major plasma metabolite in humans.3W Studies in rats indicate that the free acid CPIB is responsible for clofibrate's hypolipidemic
HYDROLYTIC REACTIONS
Hydrolysis of Esters and Aanldes The metabolism of ester and amide linkages in many drugs iscatalyzed by hydrolytic enzymes present in various tissues and in plasma. The metabolic products formed (carboxylic rids, alcohols, phenols, and amines) generally are polar and functionally more susceptible to conjugation and excretion than the parent ester or amide drugs. The enzymes carrying out ester hydrolysis include several nonspecific esterases found in the liver, kidney, and intestine as well as the pseudo-
Amide hydrolysis
cholinesterases present in
to be mediated by liver microsomal amidases. esteraces, and deacylases.337
Hydrolysis is a major biotransformation pathway for drugs containing an ester functionality. This is because of the relative ease of hydrolyzing the ester linkage. A classic esantple of ester hydrolysis is the metabolic conversion of aspirin acetylsalicylic acid) to salicylic acid.338 Of the two citer moieties present in cocaine, it appears that, in general. the methyl group is hydrolyzed preferentially to yield benzoylecgonine as the major human urinary metabolite.339 The
0
—CH3 + Aspirin (Acetylsalicylic acid)
Acetic Acid
Salicylic Acid
Many parent drugs have been chemically modified or derivatized to generate so-called prodrugs to overcome some undesirable property (e.g., bitter taste, poor absorption, poor solubility. irritation at site of injection). The rationale behind the prodrug concept was to develop an agent that, once inside the biological system, would be biotransforrncd to the active The presence of estenises in many tissues and parent
plasma makes ester derivatives logical prodrug candidates because hydrolysis would cause the ester prodrug to revert to the parent compound. Accordingly, antibiotics such as chloramphenicol and clindamycin have been derivatized
as their palmitate esters to minimize their bitter taste and to improve their palatability in pediatric liquid suspenAfter oral administration, intestinal esterases
0
0
II
II
C—OH
C—OCH3
,,N
COOH
COOH
— OCH3
i—H
0
0 Cocaine
Benzoylecgonine
Methylocgonine
110
Wilson and Girvold's Textbook of Organic Medicinal and Pharmaceutical chemisny
0
COOH
COOCH3
NHCCHCI2
OH
CH
O
RCH3
CH3
21 CH3CH2C N
II
OCH2CH3
H—C—Cl
N
—
H5C6
Diphenoxylate
HO L0
II
0 II
CN
O—C—(CH2),4CH3
H5C&l
Chndamycrn Patm5ate
DiphenoxyiiC Acid (Ditenoxin)
0
o CH3
C6H5CH—CNH Cl
CH3
o
00
—
II
CH3_?_C_OCH2CH3 CH3
00 I
II
CH3_?_C_OH
Carbenicftlin Indanyl Ester
CH3
Clolibrate
Acid
0 21CH20 —
and lipases hydrolyze the palmitate esters to the free antibiot-
ics. To improve the poor oral absorption of carbenicilhin, a lipophilic indanyl ester has been formulated Once orally absorbed, the ester is hydrolyzed rapidly to the paren drug. A tinal example involves derivauzation of prednisolone to its C-2 1 hemisuccinate sodium salt. This watersoluble derivative is extremely useful for parenteral
tration and is metabolized to the parent steroid drug by plasma and tissue esterases.349
Amides are hydrolyzed slowly in comparison to esters.37 Consequently, hydrolysis of the amide bond of pmcainamide is relatively slow compared with hydrolysis of the ester linkage in procaine.3M' Drugs in which amide cleavage has been reported to occur, to some extent, include lidocainc,35' indomethacin,251 252 and prazosin (Minipress).253 Amide linkages present in barbiturates (e.g., as well as in hydantoins (e.g.. 5-phenvl(phensuximide) and are also susceptible to hydrolysis.
—0
HC
HO
0 Prednisotone Nemlsuccrnate Sodiui'n Salt
0 P,ocarinmide
S ow
0
Miscellaneous Hydrolytic Reactions Hydrolysis of recombinant human peptide drugs and hormones at the N- or C-terminal amino acids by carboxy- and
0
P,ocaiow
Chapter 4 U Metabolic Changes of Drugs and Related Organic Compounds
—
ccc
0
CH3
111
cf—
H2N
CH3
Carbarnazepine
Lidocaine
CH2000H
CH3O
CH3 NH2
Prazosin
Indomethacin
C6H5
CH3 5.Pflenylhydantoin
anrmopeptidases and proteases in blood and other tissues is well-recognized hydrolytic reaction.356 Examples of
or protein hormones undergoing hydrolysis include human insulin, growth hormone (OH). prolactin. parathyroid
(PTH). and atrial natriuretic factor In addition to hydrolysis of amides and esters, hydrolytic cleavage of other moieties occurs to a minor extent in drug metabolism.5 including the hydrolysis of phosphate esters (e.g.. diethylstilbestrol diphosphate). sulfonylureas. cardiac glycosidcs, carbamate esters, and organophosphatc cornGlucuronide and sulfate conjugates also can undergo hydrolytic cleavage by $-glucuronidasc and sulfatuac enzymes. These hydrolytic reactions are discussed in hormone
following section. Finally, the hydration or hydrolytic cleavage of epoxides and arene oxides by epoxide hydrase is considered a hydrolytic reaction.
PHASE II OR CONJUGATION REACTIONS Phase I or functionalization reactions do not always produce hydrophilic or pharmacologically inactive metabolites. Varloris phase II or conjugation reactions, however, can convert these metabolites to more polar and water-soluble products. Many conjugative enzymes accomplish this objective by at-
PhensuxinlKte
taching small. polar. and ionizable endogenous molecules, such as glucuronic acid, sulfate. glycinc. and glutamine. to the phase I metabolite or parent xcnohiotic. The resulting conjugated products are relatively water soluble and readily excretable. In addition, they generally are biologically inactive and nontoxic. Other phase II reactions, such as meth-
ylation and acetylation. do not generally increase water solubility but mainly serve to terminate or attenuate pharmacological activity. The role of GSH is to combine with chemically reactive compounds to prevent damage to important biomacromolecules. such as DNA, RNA. and proteins. Thus, phase II reactions can be regarded as truly detox-
ifying pathways in drug metabolism, with a few exceptions. A distinguishing feature of most phase II reactions is that the conjugating group (glucuronic acid, sulfate, methyl. and acetyl) is activated initially in the form of a coenzyme before transfer or attachment of the group to the accepting substrate by the appropriate transferase enzyme. In other cases, such as glycine and glutamine conjugation. the substrate is activated initially. Many endogenous compounds. such as bilirubin, steroids. catecholamines, and histamine, also undergo conjugation reactions and use the same coenzymes. although they appear to be mediated by more specific transferase enzymes. The phase II conjugative pathways discussed include
those listed above in this chapter. Although other conjugative pathways exist (e.g.. conjugation with glycosides. phos-
112
Wilson and Gi.cvold's Textbook of Organic Medicinal and Pharmaceutical Chemistry
phute, and other amino acids and conversion of cyanide to thiocyanate), they are of minor importance in drug metabolism and are not covered in this chapter.
TABLE 4-3 Types of Compounds Forming Oxygen. Nitrogen, Sulfur, and Carbon Glucuronide? Oxygen Giucurooldes
Chicuronic Add Conjugation supply of u-glucuronic acid (derived from n-glucose). (b)
Hydrosyl composrnth Phenols: morphine, acciaminophen. p-hydmxyphenytoin Alcohols: Irlcholoroethanol. chlorainphenicol. propnrnolol Enols: 4-hydroxycoumarin N-Hydmxyamines: N-hydsuxydapsonc
numerous functional groups that can combine enzymatically
N-Hydmxyanudcs: N-hydmsy-2.acclylsminofluorene
with glucuronic acid, and (c) the glucuronyl moiety (with
Cirboxyl compounds Aryl adds: bcnzoic acid, salicylic acid
Glucuronidation is the most common conjugative pathway in drug metabolism for several reasons: (a) a readily available
its ionized carboxylate IpKa 321 and polar hydroxyl groups).
Aryinikyl acids: naproxen. fcnoprofcn
which, when attached to xenobiotic substrates, greatly increases the water solubility of the conjugated
Nitrogen Glucuronides
Formation of )3-glucuronides involves two steps: synthesis of an activated coenzyme. uridine-5'-diphospho-a-nglucuronic acid (UDPGA), and subsequent transfer of the
Arylamines: 7-amino.5.nitroindarole Aikylamines: deslpr.msinc Amities: meprobamale Suitonamidos: Tertiary amines: tripelennamine
glucuronyl group from LJDPGA to an appropriate sub-
strate)"
The transfer step is catalyzed by microsomal enzymes called UDP-glucuronyl:ran.sfr razes. They are found
Sulftar Giuceronider
primarily in the liver but also occur in many other tissues. including kidney. intestine, skin, lung, and brain. The sequence of events involved in glucuronidation is sum360. marized in Figure The synthesis of the coenzyme UDPGA uses a-n-glucose- I-phosphate as its initial precursor. Note that all glucuronide conjugates have the 13 configuration or 13 linkage at C-I (hence, the term 13glucuronides). In contrast, the coenzyme IJDPGA has an a linkage. In the enzymatic transfer step. it appears that nucleophitic displacement of the a-linked UDP moiety from UDPGA by the substrate RXH proceeds with complete inversion of configuration at C-I to give the /3-glucuronide. Glucuronidation of one functional group usually suffices to effect excretion of the conjugated metabolite: diglucuronide conjugates do not usually occur. The diversity of functional groups undergoing glucuronidation is illustrated in Table 4-3 and Figure 4-12. Metabolic products are classified as oxygen—, nitrogen—, sulfur—, or carbon—glucuronide. according to the heteroatom attached
SuUhydryl groups: methimazolo. propyithiouracil, diethyithiocarbamic acid
Carbon Giucurunides 3.5-Pyrazulidincdione: phcnylbuiazone. sulftnpysazone Eat unKtures and ire at
ides. Phenolic and alcoholic hydroxyls are the most conimon functional groups undergoing glucuronidation in drug
metabolism. As we have seen. phenolic and alcoholic hydroxyt groups are present in many parent compounds and arise through various phase I metabolic pathways, acetaminophen,360 and p-hydroxyphenyt-
oin (the major metabolite of phenytoin)49-
in trichloroethanol (major metabolite of chloral hydrate),2m and propranolol,'°°' 367 are also corn-
functionalities, the hydroxy and carboxy, form O-glucuron-
IJTP
are a few
examples of phenolic compoundss that undergo considerable glucuronidation. Alcoholic hydroxyls. such as those present
to the C-I atom of the glucuronyl group. Two important
a-o-Glucose- I phosphate
atuchmcnl, tee Figure 4.121.
Uridune.5'-diphospho-o-
Phospliosylase
UDPG Deliydrogenase (soluble)
p-glucose (UDPG)
2
*
2NADH+2H
COOH
COOH Acceplor
U0P H $-Glucuronide
al C-i)
l-IXR
UDP-Giucuronyllransferase (microsomal)
o—UDP Uricline-5'-diphospho'aD.glucuronrc Acid (UDPGA)
(a-linkage at C-i)
Figure 4—il . Formation of UDPGA and 13-glucuronide conjugates.
Chapter 4 • Metabolic Changes of Drugs and Related Organic
113
9 NH&H3
I
CI3C—CI-120H
NH2
NHOH— Propcano4ol
COOH
4-Hydroxycournann
N-Hydroxydapsone
CH3
CH3
acict R - H amflo(ILa,erIe
CH3
CH3 I
H
0 SuIhsoxa2oIe
N N
NCH2CH2N
/
CH3
C6H5CH2
Tnpelennamine
Piopytlhnuracil
Figure 4—12 • Structure of compounds that undergo glucuronidation. Asrows indicate sites of fl-giucuronide attachment.
CH3
114
Wilson and Gisvold's Textbook of Organic Medicinal and Pharmaceutical Chemistry R'
0
R
R—CC—OH
R—N—OH
uronic
their i
II
R—C—N—OH
In often i enous
Enol
mally
monly glucuronidated. Less frequent is glucuronidation of N-hydroxylother hydroxyl groups, such as amines,226 and N-hydmxylamides.24' For examples, refer to the list of glucuronides in Table 4-3. The carboxy group is also subject to conjugation with glucuronic acid. For example. arylaliphatic acids, such as the and fenoprofen,370'37' anti-inflammatory agents are excreted primarily as their O-gtucuronide derivatives in humans. Carboxylic acid metabolites such as those arising (see "Reducfrom and tion of Aldehyde and Ketone Carbonyls," above) form 0glucuronide conjugates. Aryl acids (e.g., benzoic acid,372 salicylic acid373' 374) also undergo conjugation with glucuronic acid, but conjugation with glycine appears to be a more important pathway for these compounds. Occasionally, N-glucuronides are formed with aromatic amines, aliphatic amines, amides. and sulfonamides. Representative examples are found in the list of glucuronides in Table 4-3. Glucuronidation of aromatic and aliphatic amines is generally a minor pathway in comparison with N-acetyla-
Lion or oxidative processes (e.g., oxidative deamination). Tertiary amines, such as the antihistaminic agents cyproheptadine (Periactin)373 and tripelennamine,376 form interesting quatemary ammonium glucuronide metabolites.
Because the thiol group (SH) does not commonly occur in xenobiotics, S-glucuronide products have been reported for only a few drugs. For instance, the thiol groups present in methimazole (Tapazole).377 and N,N-diethyldithiocarbamic acid (major reduced metabolite of disulfiram, Antabuse)38° undergo conjugation with glucuronic acid. The formation of glucuronides attached directly to a car-
O
O
0
II
II
in humans have shown that conjugation of phenylbutazone (Butazolidin)381' 382 and sulfinpyrazone (Anturane)383 yield the corresponding C-glucuronide metabolites:
H
C-Glucuronide Metabolite Phenylbutazonc, R = CH2CH2CH2CH3
Sulfinpyrazone. R = CH2CH2SC6H,
Besides xenobiotics, a number of endogenous substrates, notably bilirubin3M and steroids,385 are eliminated as glucu. ronide conjugates, which are excreted primarily in the urine, As the relative molecular mass of the conjugate exceeds 300
Da, however, biliary excretion may become an important route of Glucuronides that are excreted in the bile are susceptible to hydrolysis by enzymes present in the intestine. The hydrolyzed product may be reabsorbed in the intestine, thus leading to enterohepatic recycling.22 are also present in many
other tissues, including the liver, the endocrine system, and the reproductive organs. Although the function of these hydrolytic enzymes in drug metabolism is unclear, it appears that, in terms of hormonal and endocrine regulation.
-O—s—O—P—O
AlP
0
Mg5 II
O
bon atom is relatively novel in drug metabolism. Studies
AlP sulluryfase
Sulfate
HO
OH
Adenosine.5'-phosphosullate (APS)
Acceptor
O
"O—S—XR II
O
PAP i
HXR
O
0
II
II
-O—s—O—P—O II
O
0 Adenine
I
OH
Sulfotranslerase (soluble)
H203P0
OH
3'-Phosphoadenosine.5'-phosptrosulfate (PAPS)
Figure 4—13 • Formation of PAPS and sul• fate conjugates.
Loxicii attribc
bin wi to glu respor accuni
glucur limitec pool as ster
roxine of inot 5'-pho group variou other i events 13. SuI
lflactiv conjug chemic Pher
sulfate ties are the ant: tabolizr Th
Chapter 4 • Metabolic Changes of Drugs and Related Organic Compounds uronidases may liberate active hormones (e.g., steroids) from their inactive glucuronide conjugates.22
In neonates and children, glucuronidating processes are often not developed fully. In such subjects, drugs and endogensiusconipounds (e.g., bilirubin) that are metabolized normally by glucuronidation may accumulate and cause serious
toxicity. For example, neonatal hyperbilirubinemia may be attributable to the inability of newborns to conjugate bilirubin with glucuronic acid.381 Similarly, the inability of infants to glucuronidale chioramphenicol has been suggested to be responsible for the gray baby syndrome, which results from accumulation of toxic levels of the free
Sulfate Conjugation of xenobiotics with sulfate occurs primarily with phenols and, occasionally, with alcohols, aromatic umines. and N-hydroxy compounds.38939' In contrast to glucuronic acid, the amount of available sulfate is rather limited. The body uses a signilicant portion of the sulfate
erol)395 and terbutaline (Brethine. also undergo sulfate conjugation as Iheir principal route of metabolism in humans. For many phenols, however. sulfoconjugation may represent only a minor pathway. Glucuronidation of phenols is frequently a competing reaction and may predominate as the conjugative route for some phenolic drugs. In adults, the major urinary metabolite of the analgesic acetaminophen is the O-glucuronide conjugate, with the concomitant 0-sulfate conjugate being formed in small Interestingly. infants and young children (ages 3 to 9 years) exhibit a different urinary excretion pattern: the 0-sulfate conjugate is the main urinary product.The explanation for this reversal stems from the fact that neonates and young children have a decreased glucuronidating capacity because of undeveloped glucuronyltransferases or low levels of these enzymes. Sulfate conjugation, however, is well developed and becomes the main route of acetaminophen conjugation in this pediatnc group.
pool to conjugate numerous endogenous compounds such as steroids, heparin. chondroitin, catecholamines, and thyroxine. The sulfate conjugation process involves activation of inorganic sulfate to the coenzyme 3'-phosphoadenosine5'.phosphosulfate (PAPS). Subsequent transfer of the sulfate group from PAPS to the accepting substrate is catalyzed by varI)us soluble sulfotransferases present in the liver and other tissues (e.g.. kidney, intestine).392 The sequence of events involved in sulfoconjugation is depicted in Figure 413. Sulfate conjugation generally leads to water-soluble and inactive metabolites. It appears, however, that the 0-sulfate
9
9
NHCCH3
NHdCH3
-y
0C6H908
OH
NHCCH3
0s03-
Conjugate
Other functionalities. such as alcohols (e.g.. aliphatic C, to C5 alcohols. diethylene and aromatic amines (e.g.. aniline, 2-naphthylamine),401402 can also form sulfate conjugates. These reactions, however, have only minor im-
chemically reactive intermediates that are toxic.24' Phenols compose the main group of substrates undergoing sulfate conjugation. Thus, drugs containing phenolic moieties are often susceptible to sulfate formation. For example. the antihypertensive agent a-methyldopa (Aldomet) is metaboli,ed extensively to its 3-0-sulfate ester in humans.393
portance in drug metabolism. The sulfate conjugation of Nhydroxylamines and N-hydroxylamides takes place u.s well. occasionally. 0-Sulfate ester conjugates of N-hydroxy cornpound.s are of considerable toxicological concern because they can lead to reactive intermediates that are responsible
The f3-adrenergic bronchodilutors salbutamol (albut-
OH
CI.12/CH3
HOC,
..CH
HO
NH2
NH
CH3 I
CH3 Satbutamot (PJbutOfOI)
\
0
O-Sullate Conjugate
Acetaminophen
conjugates of some N-hydroxy compounds give rise to
H04
115
01-I
NH
CH 3
I
OH
C I
CH3 Teibutatne
116
Wilson and Giseold's Textbook of Organic Medicinal and Pharmaceutical Chemis:rs
0 9 NHCCH3
OCH2CH3
OCH2CH3 N-Hydroxyphenacetin
Phenacetin
for cellular toxicity. The carcinogenic agents N-methyl-4. aminoazobenzcne and 2-acetylaminofluorene are believed to mediate their toxicity through N-hydroxylation to the cor-
O-Sultate Conjugate of N-Hydroxyphenacetin
associated with phenacetin. Other pathways (e.g.. arene oxides) leading to reactive electrophilic intermediates are also possible.6
responding N-hydroxy compounds (see earlier section on N-
hydroxylation of amines and amides). Sulfoconjugation of the N-hydroxy metabolites yields 0-sulfate esters, which presumably are the ultimate carcinogenic species. Loss of
Conjugation With Glydne, cud Other Amino Adds
SO42- from the foregoing sulfate conjugates generates elec-
The amino acids glycine and glutarnine are used by mamma-
trophilic nitremum species, which may react with nucleophilic groups (e.g., NH2, OH, SH) present in proteins. DNA. and RNA to form covalent linkages that lead to structural and functional alteration of these crucial biomacromolecules.403 The consequences of this are cellular toxicity (tissue necrosis) or alteration of the genetic code, eventually leading to cancer. Some evidence supporting the role of sulfate conjugation in the metabolic activation of N-hydroxy compounds to reactive intermediates comes from the observation that the degree of hepatotoxicity and hepatocareinogenicity of Nhydroxy-2-acetyl-aminofluorene depends markedly on the level of sulfotransferase activity in the liver.404 The discontinued analgesic phenacetin is metabolized to N-hydroxyphenacetin and subsequently conjugated with sul-
The 0-sulfate conjugate of N-hydroxyphenacetin This pathway binds covalently to microsomal may represent one route leading to reactive intermediates that are responsible for the hepatotoxicity and nephrotoxicity
ATF
lian systems to conjugate carboxylic acids, particularly aromatic acids and arylalkyl Glycine conjugation is common to most mammals, whereas glutamine conjugation appears to be confined mainly to humans and other primates. The quantity of amino acid conjugates formed from xcnobiotics is minute because of the limited availability of amino acids in the body and competition with glucuronidation for carboxylic acid substrates. In contrast with glucuronic acid and sulfate. glycine and glutamine are not converted to activated coenaymes. Instead, the carboxylic acid substrate is activated with adenosine triphosphate (AlP) and coenzynte A (CoA) to form an acyl-CoA complex. The latter intermediate, in turn. acylates glycine orglutamine under the influence of specific glycine or glutamine N-acyltransferase enzymes. The activation and acylation steps take place in the mitochondria of liver and kidney cells. The sequence of metabolic events associated with glycine and glutamine conjuga-
PPi
CoASH AMP -
Ptsenytacetic Ac*i
IC JL An Acyl
Glycine of Glutamine
SC A
- CoA Intermediate
COOH Glycine or Glulamine
COOH R 2
H
H H
Conjugate A = CH2CH2CONH2
Figure 4—14 • Formation of glycine and glutamine conjugates of phenylacetic acid.
Chapter 4 • Metabolic Changes of Drugs and Related Organic compounds
117
lion of phenylacetic acid is summarized in Figure 4-14.
GSN or Mercapturic Add Conjugates
Amino acid conjugates, being polar and water soluble, are excreted mainly renally and sometimes in the bile.
GSH conjugation is an important pathway lbr detoxifying chemically reactive electrophilic compounds.42'428 It is now generally accepted that reactive electrophilic species
R
BenzoicAod. A =H
0
0
H
=OH
Acid, A = OH
Aromatic acids and arylalkyl acids are the major substrates undergoing glycine conjugation. The conversion of ttencoic acid to its glycine conjugate. hippuric acid, is a wellknown metabolic reaction in many mammalian systems.4m The extensive metabolism of salicylic acid (75% of dose) to salicyluric acid in humans is another illustrative examacid metabolites resulting from oxidation or hydrolysis of many drugs arc also susceptible to glycine conjugation. For example, the H,-histamine antagonist brunipheniramine is oxidized to a propionic acid metabolite that is conjugated with glycine in both human and dog.'6' Similarly. p-fluorophenylacetic acid, derived from the meabolism of the antipsychotic agent haloperidol (Haldol). is found as the glycine conjugate in the urine of rats.413 Phenylacetic acid and isonicotinic acid, resulting from the hydrolrespectively, the anticonvulsant phenacemide (Pheysis also nurune)414 and the antituberculosis agent ate conjugated with glycine to some extent.
Glulamine conjugation occurs mainly with arylacetic acids, including endogenous phenylacctie416 and 3-indolylaeClic acid.Shl A few glutamine conjugates of drug menabolites have been reported. For example. in humans, the 3.4-
dihydroxy-5.melhoxyphenylacetic acid metabolite of mescaline is found as a conjugate of glutamine.418 DiphenylmeIhoxyacctic acid, a metabolite of the antihistamine diphenhydramine (Benadryl). is biotransformed further to the comaponding glutamine derivative in the rhesus monkey.419 Several other amino acids are involved in the conjugation of carboxylic acids, but these reactions occur only occasion.illy and uppear to he highly substrate and species depen-
manifest their toxicity (e.g.. tissue necrosis. carcinogenicity, mutagenicity. teratogenicity) by combining covalently with nucleophilic groups present in vital cellular proteins and nucleic acids.4 429 Many serious drug toxicities may be explained also in terms of covalent interaction of metabolically generated electrophilic intermediates with cellular nuclcophiles.° GSFI protects vital cellular constituents against chemically reactive species by virtue of its nucleophilic sulfhydryl (SF1) group. The SF1 group reacts with electron-deficient compounds to form S-substituted GSH adducts (Fig. -426
GSH is a tripeptide (y-glutanxyl-cysteinylglycine) found in most tissues. Xenobiotics conjugated with GSH usually are not excreted as such. hut undergo further biotranslormation to give S-substituted N-acetylcystcinc products called 424426 This process involves enzymercapturic acids.76' matic cleavage of two amino acids (namely. glutamic acid and glycine) from the initially lin'med GSH adduct and subsequent N-acctylation of the remaining S-substituted cysteinc residue. The formation of GSH conjugates and their conversion to mereapturic acid derivatives are outlined in Figure 4-15. Conjugation of a wide spectrum 01 substrates with GSH is catalyzed by a family of cytoplasmic enzymes known as glutathione S-transferases.75 These enzymes are found in most tissues, particularly the liver and kidney. Degradation of OSH conjugates to niercapturic acids is carried out princi-
pally by renal and hepatic microsomal enzymes (Fig. 4Unlike other conjugative phase II reactions. GSH conjugation does not require the initial formation of an activated coenzyme or substrate. The inherent reactivity of the nuclco-
philic GSH toward an electrophilic substrate usually provides sufficient driving force. The substrates susceptible to GSH conjugation are quite varied and encompass many
'°° Ornithine (in birds), aspartic acid and serine alanine (in mouse and hamster). taurine
chemically different classes of compounds. A major prerequisite is that the substrate be sufficiently electrophilic. Compounds that react with GSH do so by two general mechanisms: (a) nuclcophilic displacement at an electron-deficient carbon or hcteroatorn or (b) nucleophilic addition to an electron-deficient double bond.42'
IH:NCH:CH2SOiH) (in mammals and pigeons), and histi-
Many aliphatic and arylalkyl halides (Cl. Br. 1. sulfates
tin rats),
sulfonates (OSO2R). nitrates (NO2). and organo-
dine (in African hats) are among these amino acids..420
Q
CH3
CHCH2CH2N( cl-I3
Br
Brornpheniramine
CHCH2C—OH —6
p
3.(p.Bromophenyl)-3(2-pyridyl). Acid
CHCH2CNHCH2COH
p
Glycurie Conjugate
118
Wil.ron and Gi.cvold's Textbook of Organic Medicinal and Pharmaceutical Chemistry
F
—[c)-- CH2(!!0H — F
Glycine Conjugate
p-Fluorophenylacetlc Acid
Halopendol
00 II
II
0
0
0
II
II
II
CH2—CNHCH2COH
CH2—COH
CH2—CNHCNH7
a
-'L)
0
0
0
0
II
H
II
C—NHCH2COH
C—OH
o
Glycine Conjugate
Phenytacetic Acid
Pttenacemide
Hydrolysis
LN)
LN) Isonicotinic Acid
Isoniazid (R = H) or N-Acetylisoniazid
Glycine Conjugate
OH
OCH3
OH
0
COOH
CH)0H —.HO
—p HO
— OCH3
OCH3
3,4-Dihydroxy.5-
Mescajine
I-i
OCR3
Glutamine Conjugate
methoxyphenylacetic Acid
CH3
C6H5
CH3
Diphenhydramme
C6H5
— —'
,CHOCH2CH2N
Diphenytmethoxyacetic Acid
atoms that react with GSH (by aliphatic nucleophilic displacement) to form OSH conjugates, as shown:
GS—CH2 + HX
X
COOH
—i C6H5
phosphates (O-P[0RJ2) possess electron-deficient carbon
A
0
Alkyl, Afyl, Benzylic, AilyliC Br, CI, I, OSO2R. OPO(OR)2
C6H5
H Glutamine Conjugate
The carbon center is rendered electrophilic as a result oF the electron-withdrawing group (e.g., halide, sulfate, phosphate) attached to it. Nucleophilic displacement often is facilitated when the carbon atom is benzylic or allylic or when X is a good leaving group (e.g., halide, sulfate). Many industrial chemicals, such as benzyl chloride allyl chloride (CH2 = CHCH2CI). and methyl iodide. are known to be toxic and carcinogenic. The reactivity of these three halides toward GSH conjugation in mammalian systems is
Chapter 4 • Miiabollc Cliwiges of Drugs and Re!awd Organic Compounds
119
NH2
NH2
C—COOH
E+
I
10
Glulathione
ElectmphWc
S.Transterase (Soluble)
0
Glutathione Adduct or Conjugate
Glutathione
Amino Acid (U)
y-Gtutamyl Transpeptidase (Microsomal)
rGkilarnyl U
Acetyl
NI-)2
h'—
Cyst044
N-Acetytase (Microsocnal)
NH2
Glycuie
E—
E—S —
Giyctnase (Microsomal)
CH2
S-Substituted
Mercapturic Acid Derivahse
Cysteare Derivative
Flgure4—15 • Formation of GSH conjugates of electrophilic xenobiotics or metaboiites (E) and their conversion to mercapturic acids.
dcmonstrated by the formation of the corresponding mercaptunc acid denvatives.424"28 Organophosphate insecticides.
such as methyl parathion, are detoxified by two different (3SH Pathway "a" involves aliphatic flu. cleophilic substitution and yields S-methylglutathione. Path-
way "b" involves aromatic nucleophilic substitution and produces S-p.nitrophenylglutathione. Aromatic or heteroaromatic nucleophilic substitution reactions with GSH occur only when the ring is rendered sufficiently electrondeficient by the presence of one or more strongly electron-
GSCH3
o7
S-Melhylglutalhione OCI-13
+
Methyl Parathion
OCH3 S.p-Nitrophenylglutathione
f
ci NO2
[ci
1
SG
No2
120
Textbook of Organic Medicinal and Pharmacetuical Che,nixlrv
WiLson and
GSH conjugation reaction. The GSH conjugate products. however, are not metabolized to mercapturic acids but in-
withdrawing substituents (e.g.. NO2. CI). For example. 2.4dichloronitrobenzenc is susceptible to nucleophilic substitution by GSH. whereas chlorobenzene is not.432 The metabolism of the immunosuppressive drug azarhioprine (Irnuran) to I -methyl-4-nitro-5-(S-glutathionyl)imidazole and 6-mercaptopurine is an example of heteroaromatic nucleophilic substitution involving Interestingly. 6-mercaptopurine formed in this reaction appears to be responsible for azathioprine's immunosuppressive tic-
stead are converted enzymatically to the corresponding alcohol derivatives and glutathione disulfide (GSSO).438
The nucleophilic addition of GSH to electron-deficient carbon—carbon double bonds occurs mainly in compounds with a,fl-unsaturated double bonds. In most instances, the double bond is rendered electron deficient by resonance or
conjugation with a carbonyl group (ketone or aldehyde). ester, nitrile. or other. Such systems undergo so-called Michael addition reactions with GSH to
tivity.431' NO2
yield the corresponding GSH For example, in rats and dogs, the diuretic agent cthacrynic acid (Edecrin) reacts with OSH to form the corresponding GSH or mercapturic acid derivatives.43i Not all compounds are conjugated with GSH. Many steroidal agents with a.flunsaturated carbonyl moieties, such as prednisone and digitoxigenin. have evinced no significant conjugation with GSH. Steric factors, decreased reactivity of the double bond, and other factors (e.g.. susceptibility to metabolic reduction of the ketone or the C = C double bond) may account for these observations. Occasionally, metabolic oxidative biotransformation reactions may generate chemically reactive systems that react with GSH. For example, metabolic oxidation of acetaminophen presumably generates the chemically reactive intermediate N-acetylimidoquinone. Michael addition of GSH to the imidoquinone leads to the corresponding mercapturic acid derivative in both animals and humans.245 248 2-Hydroxyestrogens, such as 2-hydroxy- I 7/3-estradiol.
+ CH3 1
(S-glulathoflyl) midazote
Arene oxides and aliphatic epoxides (or oxiranes) represent a very important class of substrates that are conjugated and detoxified by GSH.437 The three-membered oxygencontaining ring in these compounds is highly strained and.
therefore, reactive toward ring cleavage by nucleophiles (e.g.. GSH. H20. or nucleophilic groups present on cellular macromolecules). As discussed above. arene oxides and epoxides are intermediary products formed from cytochrome P450 oxidation of aromatic compounds (arenes) and olefins. respectively. If reactive arene oxides (e.g.. bcnzo[alpyrene4.5-oxide. 4-hrontobenzene oxide) and aliphatic epoxides (e.g.. styrene oxide) are not "neutralized" or detoxified by glutathione S-transferase. epoxide hydrase. or other pathways. they ultimately covalently bind to cellular macromolecules and cause serious cytotoxicity and carcinogenicity. The isolation of GSH or mercapturic acid adducts from pyrene. brnmobenzenc. and styrene clearly demonstrates the
undergo conjugation with GSH to yield the two isomeric mercapturic acid or GSH derivatives. Although the exact mechanism is unclear, it appears that 2-hydroxyestrogen is oxidized to a chemically reactive orthoquinone or semiquinone intermediate that reacts with OSH at either the elecu'o-
philic C-I or C-4 In most instances. OSH conjugation is regarded as a de-
toxifying pathway that protects cellular macromolecules such as protein and DNA against harmful electrophiles. In
importance of GSH in reacting with the reactive epoxide metabolites generated from these compounds.
GSH conjugation involving substitution at heteroatoms. such as oxygen, is seen often with organic nitrates. For ex-
a few cases. GSH conjugation has been implicated in causing toxicity. Often, this is because the GSH conjugates are themselves electrophilic (e.g.. vicinal dihaloethanes) or give rise to metabolic intermediates (e.g.. cysteinc metabolites of ha-
ample. nitroglycerin (Nitrostat) and isosorbide dinitrate (Isordil) are metabolized by a pathway involving an initial
CH2ONO2
02N—OCH2
CH2ONO2 GSH GSSG H—.'
GS—OCH2
2
CH2ONO2
H.-L
"—'
HOCH2
HSG Nitrogtycenn
0
0N02
H
H
GSH HNO2
GSH
GSSG
0 02N
H
OH
,O.4_.< O2Nd
H
0N02
Chapter 4 a Metabolic change.c of Drugs and Related Organic Compound.c
0
0 I
121
I
II
—c=ca— C
—C—
II
C —'—C—C—C—
Michael
+
p
I
i
I
SG
H
SG Glulathlone Adduct
a-p-Unsaturated System
CH3
CH3
CI CI
CI
CI
— 0 Etliacrynic Acid
Glutathione adduct of Ethacsynic Acid
(note np-unsaturated ketone moiety)
CH3
CI
CI
0 Mercaptunc Acid Derivatwe
0
Digitoxigenin
Prednisone
loalkenes) that arc electrophilic.424428 I .2-Dichioroethane,
two enzymes that apparently target the conjugates to the
for example, reacts with GSH to produce S-(2-chloroethyl)glutathione: the nucleophilic sulfur group in this conjugate can internally displace the chlorine group to give rise to an electrophilic three-membered ring episutfonium ion. The interaction of the episulfonium intermediate with the guanosine moiety of DNA may contribute to the
kidney.
mutagenic and carcinogenic effects observed for I .2-dichior-
The metabolic conversion of GSH conjugates to reactive cysteine metabolites is responsible for the neplltutoxicity associated with some halogenated alkanes and alkcnes.425 The activation pathway appears to involve transpeptidase and cysteine conjugate $-lyase,
Acetylation constitutes an important metabolic route for This endrugs containing primary amino groups.408 compasses primary aromatic amines (ArNH2). sulfonamides (H2NC6H4SO2NHR). hydrazines (—NHNH2), hydrazides (—CONHNH2), and primary aliphatic amines. The amide derivatives formed from acetylation of these amino function-
alities are generally inactive and nontoxic. Because water solubility is not enhanced greatly by N-acetylation. it ap-
122
Wilson and Gisvnlds Textbook of Organic Medicinal and Phannaceutical Chemistry
0 NH
0
CH3 HN
CH3
YLCH HO
Mercapturic Acid Derivative
N-Acotylimidoquinone
Acetaimnoptien
or
Orthoquinone
2.Hydroxy.1 7p-estradiol
Semiquinone
SG
pears that the primary function of acetylation is to terminate pharmacological activity and detoxification. A few reports
spleen, gastric mucosa, red blood cells, and lymphocytes. also show acetylation capability. N-Acetyltransferase enzymes display broad substrate specificity and catalyze the acetylation of several drugs and xenobiotics (Fig. 4-
amides tend to be less water soluble than their parent compounds and have the potential of crystallizing out in renal tubules (crvstalluria), thereby causing kidney damage. The frequency of crystalluria and renal toxicity is especially high with older sulfonamide derivatives, such as sulfathiazole)42° Newer sulfonamides, such as sulfisoxazole and sulfamethoxazole. however, are metabolized to relatively water-soluble acetylated derivatives, which are less likely to precipitate out. The biotransformation of hydrazine and hydrazide derivatives also proceeds by acetylation. The antihypertensive hydralazine (Apresoline)4M4" and the MAO inhibitor phencizinc (Nardil)456 are two representative hydrazine compounds that are metabolized by this pathway. The initially formed N-acctyl derivative of hydralazine is unstable and cyclizes
Aromatic compounds with a primary amino group, such as aniline,108 p-aminobenzoic acid,"8
intramolecularly to form 3-methyl-s-triazoloj3.4-alphtha. lazine as the major isolable hydralazine metabolite in hu-
indicate, however, that acetylated metabolites may be as ac-
or more toxic
tive as (e.g.. than (e.g., ent compounds.
their corresponding par-
The acetyl group used in N-acctylation of xenobiotics is supplied by acetyl-CoA.408 Transfer of the acetyl group from this cofactor to the accepting amino substrate is carried out by soluble N-acetyltransferases present in hepatic reticuloendothelial cells. Other extrahepatic tissues, such as the lung.
p-aminosalicylic acid.418 procainamide and dapsone (Avlosulfon).48° are especially susceptible to N-acecylation. Aromatic amine metabolites resulting from the reduction of aryl nitro compounds also are N-acety-
lated. For example, the anticonvulsant clonazepam (Kbnopin) undergoes nitro reduction to its 7-amino metabolite, which in turn is Another related benzodiazepam analogue. nitrazepam, follows a similar pathway.316 The metabolism of a number of sulfonamides, such as sulfanilamide,45' sulfamethoxazole (Gantanol),452 sulfisoxazolc (Gantrisin),452 sulfapyridine453 (major metabolite from azo reduction of sulfasalazine, Azulfidine), and sulfamethazinc.408 occurs mainly by acetylation at the N-4 position. With sulfanilamide, acetylation also takes place at the sul-
.,J
The antitubereulosis drug isoniazid or isonicoo-
nic acid hydrazide (INH) is metabolized extensively to NThe acetylation of some primary aliphatic amines such as histamine,457 mescaline.208 209 and the bis-N-demethylated metabolite of also has been reported.
In comparison with oxidative deamination processes. Nacetylation is only a minor pathway in the metabolism u( this class of compounds. The acetylation pattern of several drugs (e.g.. isoniazid. hydralazine, procainamide) in the human population displays a bimodal character in which the drug is conjugated either rapidly or slowly with 462 This phenomenon is termed acetvlation poh'rnorplusm. Individuals ... I..,.
123
Chapter 4 U Metabolic Changes of Drugs and Related Organic Compounds Asomalic Amines
/
/
NH2
/
NH2
/
NH2
NH2
a
Aniline
COOH
so4
p.Mnnobenzoic Acid A H Acid A OH
Procanantide
NH2 Dapsone Suit onarnides
N4 Sulfamethoxazole
R= CH3
N1
S02NH24—
CH3
Sullisoxazole
A=
Sulfapyddine
A
Sulfameuazine
N—I R=—('
Sulfanilamide
N
= CH3
) CH3
1-lydrazines and Hydrazides
0 II
LN) Hydralazine
Phenelzine
lsoniazid
AJiphatic Amines
H5C6 C6H5
CH3O
H
CH2 4-16 • Examples of types of compound undergoing N-acetylation. Arindicate sites of N-acetyla-
H
H2
OCH3 Mescaline
HO
NH2'—
H
Bisdesmethyl Metabohte
of 3S,6S-a-( -) Methadol
124
Wil.con and Giwolds Texthmrk of Organic Medicinal and Pharmaceutical Che,ni.urv
Clonazepam P = Cl
Metabolite
7.Acelamido Metabolite or
Nitrazeparn. H = H
N.Acetylated Molabolite Suffanilamide R = H H2N
Sulfamethoxazole R = N0 N, Sulfonamide Nomenclature
CH3
N4
CCH3 Sutf,soxazole R
—'c
N
CH3 N
SuIt amethaaine R N
CH3 N
Sulfapyridine H
types. This variation in acetylating ability is genetic and is caused mainly by differences in N-acetyltransIerase activity. The proportion of rapid and slow acetylators varies widely among different ethnic groups throughout the world. Oddly. a high proportion of Eskimos and Asians are rapid acetylators. whereas Egyptians and some Western European groups are mainly slow acetylators.462 Other populations are intermediate between these two extremes. Because of the bimodal distribution of the human population into rapid and slow acetylators. there appears to be signiticant individual varialion in therapeutic and toxicological responses to drugs displaying acetylation polymorphism.408 461. 462 Slow acetylatom seem more likely to develop adverse reactions, whereas rapid acetylators are more likely to show an inadequate therapeutic response to standard drug doses.
The antituberculosis drug isoniazid illustrates many of these points. The plasma half-life of isoniazid in rapid acetylators ranges from 45 to 80 minutes; in slow acetylators the half-life is about 14010 2(X) minutes.463 Thus, for a given
fixed-dosing regimen. slow acetylators tend to accumulate higher plasma concentrations of isoniazid than do rapid acetylators. Higher concentrations of isoniazid may explain the greater therapeutic response (i.e.. higher cure rate) among
slow acetylators. but they probably also account for the greater incidence of adverse effects (e.g.. peripheral neuritis and drug-induced systemic lupus erythematosus syndrome) observed among slow acetylators.'62 Slow acetylators of isoniazid apparently are also more susceptible to certain drug interactions involving drug metabolism. For example, phe-
nytoin toxicity associated with concomitant use with zid appears to be more prevalent in slow acetylators than in rapid acetylators.4M Isoniazid inhibits the metabolism of phenytoin, thereby leading to an accumulation of high and toxic plasma levels of phenytoin. Interestingly, patients who are rapid acetylators appear to be more likely to develop isoniazid-associated hepatiThis liver toxicity presumably arises from initial hydrolysis of the N-acetylated metabolite N-acetylisoniazid to acetylhydrazine. The latter metabolite is further converted (by cytochrome P-450 enzyme systems) to chemically reactive ncylating intermediates that covalently bind to hepatic tissue, causing necrosis. Pathological and biochemical stud. ies in experimental animals appear to support this hypothesis. Therefore, rapid acetylators run a greater risk of incurring liver injury by virtue of producing more acelyihydrazine. The tendency of drugs such as hydralazine and procainamide to cause lupus erythematosus syndrome and to elicit formation of antinuclear antibodies (ANAs) appears related to acetylator phenotype, with greater prevalence in slow Rapid acetylation may prevent the im-
munological triggering of ANA formation and the lupus syndrome. Interestingly, the N-acetylated metabolite of procainamide is as active an antiarrbythmic agent as the parent and has a half-life twice as long in
These findings indicate that N-acetylprocainamide may be a promising alternative to procuinamide as an antiarrhythmic agent with less lupus-inducing potential.
Chapter 4 • Metabolic Changes of Drug.u and Related Organic Compounds
0$
125
4ocN)CH
NHNH2 Hydralazine
3-Methyt-s-triazolo(3.4-a)phthatazine
N-Acetylhydralazine
COOH
o
N.acetytallon
N-A4isooiazid
lsonhxid
o
+ Acetythydrazi,e
Isonicotin,c Acid
N-oxidation Cytochrome P-450
Mechated 1
Reactive inteemediates Liver
Damage i— Covalent
[ possibly.
0
0
ii
n
I
I
CH3C+,CH3C 'j
Methylatlon
methyltransferase. and nonspecific N-methyltransferases
Mcthylation reactions play an important role in the biosynthesis of many endogenous compounds (e.g.. epinephrinc
and S-methyltransferases.358 One of these enzymes, COMT, should be familiar because it carries out 0-methylalion of such important neurotransmiuers as norepinephrine and do-
and
melatonin) and in the inactivation of numerous physioactive biogenic amines (e.g.. norepinephrine, dopa-
logically
mine, scrotonjn, and Methylation. however. constitutes only a minor pathway for conjugating drugs and
xenobiotics. Methylacion generally does not lead to polar or water-soluble metabolites, except
when it creates a quater-
ammonium derivative. Most methylated products tend to be pharmacologically inactive, although there are a few naty
inceptions. R
R
pamine and thus terminates their activity. Besides being present in the central and peripheral nerves, COMT is distributed widely in other mammalian tissues, particularly the liver and kidney. The other methyltransferases mentioned are located primarily in the liver, kidney, or lungs. Transfer-
ases that specifically methylate histamine, serotonin, and epinephrine are not usually involved in the metabolism of xenobiotics.t"5 Foreign compounds that undergo meihylation include catechols. phenols, amines. and N-heterocyclic and thiol compounds. Catechol and catecholaniine-likc drugs are metabo-
lized by COMT to inactive monomethylated catechol NI-f2
No'eclnegtnine,R-OH DoparnsteR—H
NH2 Norrnetanephrine, R = OH 3-Methoxytyvamine. R H
The coenzyme involved in methylation reactions is S-ad-
enosylmethionine (SAM). The transfer of the activated
products. Examples of drugs that undergo significant 0methylation by COMT in humans include the antihypernensive (S)(—)a-methyldopa (Aldomct),470the antiparkinsonism agent (S)(—)-dopa (levodopa).472 isoproterenol (Isuand dobutamine (Dobutrex).474 The student should note the marked structural similarities between these drugs and the endogenous catecholamines such as norepinephrine
methyl gmup from this coenzyme to the acceptor substrate scataly7.ed by various cytogasmic and microsomal methyllntnsferases (Fig. 4Methyltransferases of particu-
and dopamine. In the foregoing four drugs, COMT selectively 0-methylates only the phenolic OH at C-3. Bismethy-
in the metabolism of foreign compounds indude cauechol-O-methyluansferase (COMT). phenol-O-
matic hydroxylation of phenols (e.g., 2-liydroxylation of I " and from the arene oxide dihy-
br impoilance
lation does not occur. Catechol metabolites arising from aro-
126
Wilson
and Gisvolds Textbook of Organic Medicinal and Pharmaceutical Chemistry
COOH H2N
çooH H2N
CH2
Substrate
CH2
CH?
—S
ATP
RXH
PPi
Mothionine Adenosyl
Methyl Translerases
CH2
R—XCH3+
S.—
CH,
Transf erase
CH3 Methionine
S-Adenosylrnethionine
HO
(SAM)
S-Adenosylhomocystoine
OH
Figure 4—17 • Conjugation of exogenous and endogenous substrates (RXH) by methylation.
drodiol—catechol pathway (sec section above on oxidation of aromatic moieties. e.g., the catechol metabolite of phenytalso undergo O-methylation. Substrates undergoing 0-methylation by COMT must contain an aromatic I .2-dihydroxy group (i.e.. catechol group). Resorcinol (I .3-dihydroxybcnzene) or p-hydroquinone (I .4-dihydroxybenzene) derivatives are not substrates for COMT. This explains why isoproecrenol undergoes extensive O-methylation473 but terhutaline (which contains a resorcinol moiety) does Occasionally, phenols have been reported to undergo 0methylation hut only to a minor One interesting example involves the conversion of morphine to its 0-meth-
increases the intensity and duration of the drug action. In addition, decreased metabolic elimination may lead to accumulation of toxic levels of the drug. Conversely, an increased rate of metabolism decreases the intensity and duration of action as well as the drug's efficacy. Many factors may affect drug metabolism, and they are discussed in the following sections. These include age. species and strain, genetic or hereditary factors, sex, enzyme induction, and enzyme inhi-
ylatcd derivative, codeine, in humans. This metabolite is lonned in significant amounts in tolerant subjects and may
Age-related differences in drug metabolism are generally
account for up to 10% of the morphine dose.476 Although N-methylation of endogenous arnines (e.g., histamine. norepinephrine) occurs commonly. biotransformation of nitrogen-containing xenobiotics to N-methylated me-
tabolites occurs to only a limited extent. Some examples reported include the N-methylation of the antiviral and antiparkinsonism agent amantadine (Symmetrel) in dogs477 and
the in vitro N-tnethylation of norephedrine in rabbit lung N-methylation of nitrogen atoms present in
heterocyclic compounds (e.g.. pyridine derivatives) also takes place. For example. the pyridinyl nitrogens of nicotine'87and nicotinic acid478 are N-methylated to yield quatcrnary ammonium products.
Thiol-containing drugs. such as propylthiouracil,479 2, 3-dimercapto- I -propanol (BAL).38° and 6-mercaptopurhave been reported to undergo S-methylation.
FACTORS AFFECTING DRUG METABOLISM Drugs and xenobiotics often are metabolized by several different phase I and phase II pathways to give a number of metaholites. The relative amount of any particular metabolite is determined by the concentration and activity of the enzyme(s) responsible for the biotransformation. The rate of metabolism of a drug is particularly important for its pharma-
cological action as well as its toxicity. For example, if the rate of metabolism of a drug is decreased, this generally
Age Differences quite apparent in the newborn.487-488 In most fetal and newborn animals, undeveloped or deficient oxidative and conjugative enzymes are chiefly responsible for the reduced metu-
bolic capability seen. In general, the ability to early out metabolic reactions increases rapidly after birth and approaches adult levels in about I to 2 months. An illustration of the influence of age on drug metabolism is seen in the duration of action (sleep time) of hexobarbital in newborn and adult When given a dose of 10 mg/kg of body weight, the newborn mouse sleeps more than 6 hours. In contrast, the adult mouse sleeps for fewer than 5 minutes when given the same dose. In humans. oxidative and conjugative (e.g.. glucumnida[ion) capabilities of newborns are also low compared with those of adults. For example, the oxidative (cytochrome I'450) metabolism of tolbutamide appears to be markcdl) lower in newborns.490 Compared with the half-life of S in adults, the plasma half-life of tolbutamide in infants is more than 4.0 hours. As discussed above, infants possess poor glucuronidating ability because of a deficiency in glucumnyltransferase activity. The inability of infants to conjugate chloramphenicol with glucumnic acid appears to be responsible for the accumulation of toxic levels of this antibiotic. resulting in the so-called gray baby syndrome.388 Similarly, neonatal hyperbilirubinemia (or kernicnerus) resell' from the inability of newborn babies to glucuronidate rubin.387
The effect of old age on drug metabolism has not been as well studied. There is some evidence in animals and ha
mans that drug metabolism diminishes with old
Chapter 4 U Metabolic Changes of Drugs and Related Organic Compounds
\ HO
HO
HN
....' — CH
CH3
HO S( — )-Dopa
Dobutarnine
2-Nydroxy- 1 7a-ethinylestradiol
OH
C6H5!
II
HN
NH
OH3
HO I
H
OH3 Terbufalina (not a substrate for COMT)
Catecliol Metabolite of Ptienytoin
NH
10
0
OH
OH Amantadine
Codeine
Morphine
Norephedrine
OH3 Nicotine
0H2—OH
-
rCOOH
N
I
N
Nicotinic Acid
Trigonelline
I -
propanol (BAL)
6-Mercaptopurine
127
128
WIlson c,nd GLwold's Textbook
of Organic Medicinal and Pharmaceutical Chemistry
Much of the cvidence, however, is based on prolonged plasma hall-lives of drugs that are metabolized totally or mainly by hepatic microsomal enzymes (e.g.. antipyrinc, phenobarbital. acetaminophen). In evaluating the effect of age on drug metabolism, one must differentiate between "normal" loss of enzymatic activity with aging and the effect of a diseased liver from hepatitis, cirrhosis, etc., plus
Species variation has been observed in many oxidative biotransformation reactions. For example, metabolism of amphetamine occurs by two main pathways: oxidative deamination or aromatic hydroxylation. In the human, rabbit.
decreased renal function, because much of the water-soluble conjugation products are excreted in the liver.
drug that shows marked species differences in metabolism, In the human. phenytoin undergoes aromatic oxidation to yield primarily (S)(—)-p-hydroxyphenytoin: in the dog. oxi-
Species and Strain Differences
dation occurs to give mainly (R)( + )-ns-hydroxyphenyt.
The metabolism of many drugs and foreign compounds is often species dependent. Different animal species may biotransform a particular xenobiotic by similar or markedly different metabolic pathways. Even within the same species. individual variations (strain differences) may result in signifThis icant differences in a specific metabolic pathway.493'
(i.e., tueta or para) of aromatic hydroxylation but also in which of the two phenyl rings (at C-5 of phenytoin) under-
and guinea pig. oxidative deamination appears to be the pre-
dominant pathway: in the rat, aromatic hydroxylation ap. pears to be the more important route.495 Phenytoin is another
There is a dramatic difference not only in the position
is a problem when a new drug is under development. A new
drug application requires the developer to account for the product as it moves from the site of administration to final elimination from the body. It is difficult enough to find appropriate animal models for a disease. It is even harder to find animal models that mimic human drug metabolism.
goes aromatic oxidation. Species differences in many conjugation reactions also have been observed. Often, these differences are caused by the presence or absence of transferase enzymes involved in the conjugative process. For example, cats lack glucuronyl. transferase enzymes and, therefore, tend to conjugate pheno' lic xenobiotics by sulfation In pigs, the situation is reversed: pigs are not able to conjugate phenols with sulfate (because of lack of sulfotransfera.se enz7mes) but appear to
have good glucuronidation capability.4" The conjugation of
Phenyloin
H R( + ).m-HydroxypiienytOifl
CH3
(man, rabbit. guinea pig)
II
0
Benzosc Acid
Do
(yCH2c(CH3 AmphetamIne
2
Aroma5c
(rat)
H
N2 p-Hyckoxyamphetarnine
_______
Chapter 4 . Metabolic Changes of Drugs and Related Organic Compounds
129 OH
2
6
2'-OH-PAP Trace amounts Inhumans, rat and guinea pig
4'-OH-PAP 6% Human 3% Rat 7% Guinea 1% Mouse
5.4'-dl-OH-PAP
5-OH-PAP 50% Human 6% Rat 5% Guinea Pig
NH2
N
N
NH2
10% Human 12% Rat 5% GuInea Pig
2%Mouse
N=N
ITXO H2
N
NH2
NH2
2-Aminophenol ? Mouse
HN'
NH2
,.—
4.Amlnopl'enol
20% Human 25% Rat 40% Guinea pig 6% Mouse
OH N-Acetyt-4-amlnoptxenol
FIgure 4—18 • Phenazopyridine metabolism in humans, guinea pigs, rats and mice.
aromatic acids with amino acids (e.g.. glycine. glutamine) depends on the animal species as well as on the substrate. For example. glycine conjugation is a common conjugation pathway for benzoic acid in many animals. In certain birds e.g.. duck, goose, turkey), however. glycine is replaced by the amino acid ornithine.49° Phenylacetic acid is a substrate (or both glycine and glutamine conjugation in humans and other primates. Nonprimates, such as the rabbit and rat, excrete acid only as the glycine conjugate, how-
ev& lire metabolism of the urinary antiseptic, phenazo(Pyridium) depends strongly on the animal. The diazo linkage remains intact in over half of the metabolites in humans, whereas 40% of the metabolites in the guinea pig result from its cleavage. The metabolic product pattern in human or guinea pig does not correlate with that of either dot mouse (Fig. 4-18). °°° Strain differences in drug metabolism exist, particularly in inbred mice and rabbits. These differences apparently are caused by genetic variations in the amount of metabolizing cnzyme present among the different strains. For example, in 'ira swdieis indicate that cottontail rabbit liver microsomes metabolize hexobarbital about 10 times faster than New Zealand rabbit liver microsomes.501 Interindividual differcores in drug metabolism in humans are considered below.
itary factors are responsible for the large differences seen in the rate of metabolism of these drugs. The frequently cited
example of the biotransformation of the antituberculosis agent isoniazid is discussed above under acylation. Genetic factors also appear to influence the rate of oxidation of drugs
like phenytoin, phenylbutazone, dicumarol, and noruiptyThe rate of oxidation of these drugs varies widely among different individuals; these differences, however, do not appear to be distributed bimodally. as in acetylation. In general, individuals who tend to oxidize one drug rapidly are also likely to oxidize other drugs rapidly. Numerous studies in twins (identical and fraternal) and in families indicate that oxidation of these drugs is under genetic control.503 Many patients state that they do not respond to codeine and codeine analogues. It now is realized that their CYP 2Db isozyme does not readily 0-demethylate codeine to form morphine. This genetic polymorphism is seen in about 8% of Caucasians. 4% of African Americans, and less than 1% of Asians.50° Genetic polymorphism with CYP isozymes is well documented as evidenced by the many examples in this chapter. There is limited evidence of polymorphism involv-
ing MAO-A and MAO-B. The chemical imbalances seen with some mental diseases may be the cause.206
Hereditary or Ge.etIc Factor.
Sax Diftereaces
Marked individual differences in the metabolism of several
The rate of metabolism of xenobiotics also varies according to gender in some animal species. A marked difference is
drugs exist in
Many of these genetic or hered-
130
Wilson and Gisi'old.c Textbook of Organic Medicinal and Phannaceutical Chemistrs
observed between female and male rats. Adult male rats
dependent. Rabbits and mice, for example. do not show a
metabolize several foreign compounds at a much faster rate than female rats (e.g.. N-demethylution of aminopyrine. hexobarbital oxidation. glucumnidation of o-aminophenol). Apparently. this sex difference also depends on the substrate. because some xenobiotics are metabolized at the same rate
significant sex difference in drug metabolism.505 In humans. there have been a few reports of sex differences in metabolism. For instance, nicotine and aspirin seem to
in both female and male rats. Differences in microsomal
in terms of drug—drug interactions based on the drug's metabolism. For women, the focus is on drugs used for contraception. Note in Table 4-4 that the antibiotic rifampin, a CYP 3A4 inducer, can shorten the half-life of oral
oxidation are under the control of sex hormones, particularly androgens; the anabolic action of androgens seems to increase metabolism.505
Sex differences in drug metabolism appear to be species
be metabolized differently in women and men.506 507 On the other hand, gender differences can become significant
contraceptives.
TABLE 4-4 Clinically Significant Cytochrome P-450-Based Drug-Drug Interactions Substrates
Agent CYP 1A2
Inhibitors
inducers
Agent
AmilrIptyline
Cimctidine
Ciubatnazepinc
Imiprainine
Ctomipianiine
Ciprotloxacin
Pttenobarbital
Meperidine
Ctanthromycin
Ptienyloin
Methadone
Desiprumine
Enoxucin
Prinildone
Me,dtetine
Fluvoxam,ne
Erythromycin
Rifampin
Nonriplylinc
Fluvoxamine
CYP2C9
Substrates
Inhibitors
inducers
Carbamazcpinc
Oxycodone
lmtpnSrnilsc
lasmiuzid
Smoking
Propofenone
Ropinirole
Nalidixic acid
St. John's wort
Pt'opoxyphene
I'acflne
Norflosacln
Thionduzinc
Theopitylline
Troleandoniyciu
Tr5madol
IR).Warfiarin
Ziteuton
Diucepam
Amiodaronc
Carbamai.cpine
Phcnytoin
Ch!onlmphcnicol
Phenubarbital
Aiprazolam
(S)-Witrt'ann
Cimetidine
Phenyloin
Amtodipinc
Pnmidonc
Aiorvastatin
Amiodamne Cimetidine Cipmfloxacin Clanthromycin
Fliuvoxaminc
Busulfan
Cyclosporine
Modafinil
Isoniazid
Carbamanipine
Deluvirdine
Nevirapine
Metronidazoie
Cisapndc
Diltirizetn
Oxcarbazepine
Vunconazole
Cisritheotnycin
Efavirene
Phenobarbital
7.atiilukast
Cyctosporine
Erythromycin
Phenytoin
Tea,.odone
CYP 3A4
Allentanht
Phcnyxoin
Etuconazok
Primidone
Phenoharhital
Disopyrnmide
Fluoxetine
Rifabutin
Phenytoin
Doxorubicin
Fltavoxaii,ine
Ri(ampin
Orncpntzote
Dronabinot
Grapefruit
Riftipentine
Topiraiflate
Ergotamine
Indinavir
Sr. John's won
t'luoxetinc
l'hioridazine Modatinht
CYI' 206
Amitripsyline
Amiodamne
Carbamazepine
St. John's won
Erytheomycin unit
Cimetidine
Contraceptives
Codeine
2(5)2.1
Irracunazote Ketocunazote
Desiprumine
Pamxt'tIne
Ethinyl estradini
Meliunidazole
Drxtriirneihorpban
Quinidine
Ethosuximidc
Miconazoic
t)onepcztl
Ritonavir
Etoposide
Nolazodone
Doscpln
Senralino
Ectodipinc
Notfinuvir
Ecntanyl
Niledipine Norfioxucin
t'lccuinide
Indinavir
Hatoperulol
Isradipine Itraconazote
1-lydrocodonc
.1.
Ethostiximido Oartic
supplements
Ritampin
CVI' 2C19
Efavuene
tn,rn Lesica. T. t... and Haker. t). fi Lcnur. Dccembcr2(X)2. and Managemeni. Vancouver. WA, Applied Ihcrupeuticu.
Quinine Ritonavir Han,4en. P. D..und Hues
and Intro. 0. S. led.): Disg Interaction Facts. Si. Louis. Facts& Compansen.
Chapter 4 • Meta!,olic Changes of Drugs and Related Organic Compounds
Eizym. Induction The activity of hepatic microsomal enzymes, such as the P.450 mixed-function oxidase system, can be increased markedly by exposure to diverse drugs. pesticides. polycyclic aromatic hydrocarbons, and environmental xeno-
hiolics. The process by which the activity of these drugmetabolizing enzymes is increased is termed enzyme induc-
The increased activity is apparently caused by an increased amount of newly synthesized enzyme. Enzyme
induction often increases the rate of drug metabolism and decreases the duration of drug action. (See Table 4-4 for a list of clinically significant drug—drug interactions based on one drug inducing the metabolism of a second drug.) Inducing agents may increase the rate of their own metab-
olism as well as those of other unrelated drugs or foreign compounds (Table 4.4)32 Concomitant administration of Iwo or more drugs often may lead to serious drug interactions as a
of enzyme induction. For instance, a clinically
critical drug interaction occurs with phenobarbital and warInduction of microsomal enzymes by phenobarbital increases the metabolism of warfarin and, consequently. markedly decreases the anticoagulant effect. Therefore, if a patient is receiving warfarin anticoagulant therapy and begins taking phenobarbital. careful attention must be paid to readjustment of the warfarin dose. Dosage readjustment is also needed if a patient receiving both warfarin and phenobarbital therapy suddenly stops taking the barbiturate. The teilectiveness of oral contraceptives in women on concurrent phenobarbital or rifampin therapy has been attributed to the enhanced metabolism of estrogens (e.g.. I 7a-ethinyland rifampin5t4 inducestradiol) caused by tion.
Inducers of microsomal enzymes also may enhance the metabolism of endogenous compounds. such as steroidal hormones and bilirubin. For instance. phenobarbital can increase the metabolism of cortisol. testosterone, vitamin D, M$)The enhanced metabolism of and hilirubin in siramin induced by phenobarbital and phenytoin appears to be responsible for the osteomalacia seen in patients on long-term therapy with these two anticonvulsant drugs.°'° Interestingly. phenoburbital induces glucuronyltransferase enrymes. thereby enhancing the conjugation of bilirubin with glucuronie acid. Phenobarbital has been used occasionally to treat hyperbilirubinemia in
In addition to drugs, other chemicals, such as polycyclic aromatic hydrocarbons (e.g.. benzo[alpyrene. 3-methylcholarthrene) and environmental pollutants (e.g., pesticides. polychlorinaled biphenyls. TCDD). may induce certain oxidative pathways and, thereby, alter drug response.508 c09 Sit Cigarette smoke contains minute amounts of polycyclic aromatic hydrocarbons, such as benzolalpyrene. which are potent inducers of microsomal cytochrome P.450 enzymes. This induction increases the oxidation of some drugs in 'makers. For example, theophylline is metabolized more rapidly in smokers than in nonsmokers. This difference is reflected in the marked difference in the plasma half-life of theophylline between smokers 4.1 hours) and nonsmok7.2 Other drugs. such as phenacetin. penwocine. and propoxyphene. also reportedly undergo more rapid metabolism in smokers than in &cupational and accidental exposure to chlorinated pes-
131
ticidcs and insecticides can also stimulate drug metabolism. For instance, the half-life of antipyrine in workers occupationally exposed to the insecticides lindane and DDT is re-
portedly significantly shorter 11.7 vs. 11.7 hours) than in control subjects.52' A case was reported in which a worker exposed to chlorinated insecticides failed to respond (i.e.. decreased anticoagulant effect) to a therapeutic dose of warfarm.522
As discussed above, multiple forms (isozymes) of cytoMany chemicals selectively induce one or more distinct forms of cytochrome P-450.3' (See Table 4-4.) Enzyme induction also may affect toxicity of some drugs by enhancing the metabolic formation of chemically reactive metabolites. Particularly important is the induction of cytochrome P-450 enchrome P-450 have been demonstrated!
zymes involved in the oxidation of drugs to reactive intermediates. For example, the oxidation of acetaminophen to a reactive imidoquinone metabolite appears to be carried out by a phenobarbital-inducible form of cytochrome P-450
in rats and mice. Numerous studies in these two animals indicate that phenobarbital pretreatment increases in vivo hepatotoxicity and covalent binding as well as increases formation of reactive metabolite in microsomal incubation mixInduction of cytochrome P.448 is of toxicological concem because this particular enzyme is involved
in the metabolism of polycyclic arotuatic hydrocarbons to reactive and carcinogenic Consequently, the metabolic bioactivation of benzolalpyrene to its ultimate carcinogenic diol epoxide intermediate is carried out by cy-
tochrome P448 (see section above on aromatic oxidation for the bioactivation pathway of benzolatpyrene to its diol epoxide).523 Thus. it is becoming increasingly apparent that enzyme induction may enhance the toxicity of some xenobiotics by increasing the rate of formation of reactive metabolites.
Enzyme Inhibition Several drugs. other xenobiotics including grapefruit, and possibly other foeds can inhibit drug metabolism (Table 45)•32.
With decreased metabolism. a drug often accumulates, leading to prolonged drug action and serious ad-
TABLE 4-5 Potential Drug-Grapefruit Interactions Based on Grapefruit Inhibition of CYP 3A4 Result
Drug Amiodarone
bloavaitability
Diazepam
tncnarscd ACC
Curbtunazcpine
tncreancd ALC. pcak and trough ptaama
Cisapride Cyclosponne. wemiltitits
tncrcaLscd AUC
Aiorvasuiin. simvnsrruin
Increased atworpilon and plasma concCnirdtions
Suquinavir
Increased absorption and pla.snm concentrations
Kehnc. W Abstracted Document #15(1905. AUC. uca under the curve.
Increased AUC and scram conccntratiom
Piunnacists teller IS. September 1(812. tklail
132
Wilson aiid Gisrold's Textbook of Organk Medicinal and Pharmaceutical Ciu'n,istrv
verse effects. Enzyme inhibition can occur by diverse mech-
stances, the two enuntiomers may have totally different phar-
anisms. including substrate competition, interference with protein synthesis, inactivation of drug-metabolizing enzymes, and hepatotoxicity leading to impairment of enzyme activity. Some drug interactions resulting from enzyme inhibition have been reported in For example. phenylbutazone (limited to veterinary use) stereoselectively
macological activities. For example, (+ )-a-propoxyphene
inhibits the metabolism of the more potent (S)(—) enantiomer of warfarin. This inhibition may explain the excessive hypoprothrombincmia (increased anticoagulant effect) and many instances of hemorrhaging seen in patients Ofl both warfarin The metabolism of phenytoin and phenylbutazone
target receptors to elicit its pharmacological response. By the same token, the preferential interaction of one stereoisomer with drug-metabolizing enzymes may lead one to anticipate differences in metabolism for the two enantiomers of
is inhibited by drugs such as chioramphenicol. disulfiram. and Interestingly. phenytoin toxicity as a result of enzyme inhibition by isoniazid appears to occur primarily in slow ucetylators.4M Several drugs, such as dicumarol. chloramphenicol. and phenylbutazone,512 inhibit the biotransformation of toihutamide. which may lead to a hypogly-
mic drug often are metabolized at different rates. For instance. studies in humans indicate that the less active (+)
cemic response.
The grapefruit—drug interaction is complex. It may be caused by the bioflavonoids or the furanocoumarins. Grapefruit's main bioflavonoid, naringin. is a weak CYP inhibitor.
but the product of the intestinal flora, naringenin. does inhibit CYP 3A4. The literature is very confusing because many of the studies were done in vitro, and they cannot always be substantiated under in vivo conditions. In addition, components in grapefruit also activate P-glycoprotein. which
would activate the efflux pump in the gastric mucosa and thus interfere with oral absorption of the certain drugs. The combination of CYP enzyme inhibition and P-glycoprotein activation can lead to inconclusive The general recommendation when a drug interaction is suspected is that
the patient avoid grapefruit and its juice.
Miscellaneous Factors Affecting Drug
(Darvon) is an analgesic, wherea.s (—)-a-propoxyphene (No-
vrad) is an
Such differences in activity be-
tween stercoisomers should not he surprising, since Chapter 2 explains that stereochemical factors generally have a dramatic influence on how the drug molecule interacts with the
a racemic mixture. Indeed, individual enantiomers of a raceS
cnantiomcr of propranolol undergoes more rapid metabolism than the corresponding (—) Allylic hydroxy-
lation of hexobarbital occurs more rapidly with the R(—) enantiomer in humans."° The term substrate stereoselectiv. liv is used frequently to denote a preference for one steftoisomet as a substrate for a metabolizing enzyme or metabolic process.29'
Individual enantiomers of a racemic mixture also may be metabolized by different pathways. For instance, in dogs. the (+) cnantiomer of the sedative—hypnotic glutethimide
(Doriden) is hydroxylated primarily a to the carbonyl to yield 4-hydroxyglutethimide. whereas the (—) enantiomer undergoes aliphatic w — I hydroxylation of its C-2 ethyl group.'4" Dramatic differences in the metabolic profile of two enantiomers of warfarin also have been noted. In humans, the more active (S)(—) isomer is 7-hydroxylated (aromatic hydroxylation). whereas the (R)( +) isomer undergoes keto reduction to yield primarily the (R.S) warfarin 2% Although nualcohol as the major plasma merous other examples ol substrate stcreoselectivity or enantioselectivity in drug metabolism exist, the examples presented should suffice to emphasize the point.2" 531 CH7CH3
Other factors also may influence drug metabolism. Dietary factors, such as the protein-to-carbohydrate ratio, affect the metabolism of a few drugs. Indoles present in vegetables
-C6H5
such as Brussels sprouts, cabbage, and cauliflower, and poly-
cyclic aromatic hydrocarbons present in charcoal-broiled beef induce enzymes and stimulate the metabolism of some drugs. Vitamins, minerals, starvation, and malnutrition also apparently influence drug metabolism. Finally, physiological factors, such as the pathological state of the liver (e.g., hepatic cancer, cirrhosis, hepatitis), pregnancy, hormonal disturbances (e.g., thyroxine, steroids), and circadian rhythm. may markedly affect drug metabolism,
Stereochemical Aspects of Drug Metabolism Many drugs (e.g., warfarin, propranolol, hexobarbital, glutethimide. cyclophosphamide, ketamine. and ibuprofen) often are administered as racemic mixtures in humans. The two enantiomers present in a racemic mixture may differ in pharmacological activity. Usually, one enantiomer tends to be much more active than the other. For example, the (S)(—)
enantiomer of warfarin is 5 times more potent as an oral anticoagulant than the (R)( +) In some in-
4-Hydroxyglutethimide
CH2CH3
H
(+)-Enan5oniei
CHCH3
Glulethimido
Drug biotransformation processes often lead to the creation of a new asymmetric center in the metabolite (i.e.. ste. reoisomeric or enantiomeric products). The preferential met-
abolic formation of a stercoisomeric product
is called
product stereo relee:jt'irs'29' Thus.. bioreduction of ketonc xenobiotics. as a general rule, produces predominantly one stereoisomeric alcohol (see "Reduction of Ketone Carbonyls." above).' b The preferential formation of (S)(—)hydroxyhexamide from the hypoglycemic agent acetohcx294 and the exclusive generation of 6$-naltrexol
Chapter 4 • Metabolic Changes of Drug.c and Relaft'd Organic
133
i" (see "Reduction of Ketonc Carbonvk" for structure) are two examples of highly stereoseleehiorcdueiion processes in humans. Oxidative biotransformations display product stcreoseleclivity. too. For example, phenytoin contains two phenyl rings in its structure, both of which a priori should be susceptible III an)matic hydroxylation. In humans, however. p-hydroxylation occurs preferentially (approximately 90%) at the proSI-phenyl
ring to give primarily (S)(—)-5-(4-hydroxy-
-,
phcnyl)-5-phenylhydantoin. Although thc other phenyl ring
also is p.hydroxylated. it occurs only to a minor extent Microsomal hydroxylation of the C-3 carbon of diazepam and desmethyldiazepam (using mouse liver preparations) has been reported to proceed with remarkable stereoselectivity to yield optically active metabolites with the 3(S)
Interestingly, these two metaboare pharmacologically active and one of them. oxaze-
absolute
pam, is marketed as a drug (Serax). The allylic hydroxylation
of the N-butenyl side group of the analgesic pentazocine ITaiwin) leads to two possible alcohols (cis and trans alcohols). lit human, mouse, and monkey. pentazocine is metabolized predominantly to the trans alcohol metabolite. whereas 130 The he rat primarily tends to form the cis pnxluct stereoselectivity observed in this biotransformation involves d.c and trans geometric stereoisomers.
C6H5
C6H5
(3S) N-Methyloxazepam. A CR3 S( + ).Oxazepam, A = H
Diazepam, A = CH3 Desmeihyldiazepam, A = H
The term regioseleclivity-532 has been introduced in drug
metabolism to denote the selective metabolism of two or more similar functional groups (e.g.. OCI-13. OH. NO2) or two or more similar atoms that are positioned in different regions of a molecule. For example, of the four methoxy groups present in papaverine, the 4-OCH3 group is regioselectively 0-demethylated in several species (e.g.. rat, guinea pig, rabbit, and dog).°33 Trimethoprim (Trimpex. Proloprim) has two heterocyclic sp2 nitrogen atoms (N' and N3) in its structure. In dogs, it appears that oxidation occurs regioselectively at N3 to give the corresponding 3-N-oxide.232 Nitroreduction of the 7-nitro group in 5,7-dinitroindazole to yield the 7-amino derivative in the mouse and rat occurs with high regioselectivity.35' Substrates amenable to 0-methylasion by COMT appear to proceed with remarkable rcgioseleciivity. as typified by the cardiotonic agent dobutamine (Dobutrex),
HO
H
CH2CH3
OH
H
0
+ ).Alcohol
OH
c-ci2
OH3
Enantornei
Wart arm
C6H5
I
0
HO
0
7.Hydroxynartarlfl
OH
pro-A ring
OH pro-S ring
Phenytoin
H S( — l-5-(4-Hvdroxwiienvll-
R( + )-5.(4.Hydroxyplienyl)-
5-phenyihydantoin
134
tt'iI.%on
wul Gicrold.s lexibook of Organic Medicinal and Pharmaceutical
Che,ni.ctr%
CH2OH
CH3
CH2
C
I
Al
—
+
CH3
\ /
CH3
HO Penlazoone
cis-Alcohol
frans-Af cobol
0-methylation occurs exclusively with the phenolic hydroxy group at C-3.474
Pharmacologically Active Metabolites
CH3O
The traditional notion that drug metabolites are inactive and insignificant in drug therapy has changed dramatically in
N
recent years. Increasing evidence indicates that many drugs arc biotransformed to pharmacologically active metabolites that contribute to the therapeutic as well as toxic effects of the parent compound. Metabolites shown to have significant 535 therapeutic activity in humans are listed in Table
CH2
3 OCH3 OCH O-demethylat,on
The parent drug from which the metabolite is derived and the process involved also arc given. How significantly an active metabolite contributes to the therapeutic or toxic effects ascribed to the parent drug depend.s on its relative activity and quantitative importance
Papaverine
OCH3
N'
(e.g.. plasma concentration). In addition, whether the inetaboltte accumulates after repeated administration (e.g.. desme. thyldiazepam in geriatric patients) or in patients with renal failure is determinant. From a clinical standpoim. active metabolites are especially important in patients with decreased renal function. If
OCH3
H2N
renal excretion is the major pathway for elimination of the active metabolite. then accumulation is likely to occur in patients with renal failure. Especially with drugs such as procainaniidc. clofibrate. and digitoxin. caution should be exercised in treating patients with renal failure.2 Many
Trurnethoprim
Nitro reduction
I
of the toxic etfects seen for these drugs have been attributed to high plasma levels of their active metabolites. For example. the combination of severe muscle weakness and tenderness (myopathy) seen with clolibrate in renal failure patients is believed to be caused by high levels of the active metabolite chlorophenoxyisobutyric acid."6 Cardiovascular
02N
NO2
toxicity owing to digitoxin and procainamide in anephric
S
subjects has been attributed to high plasma levels of digoxin and N-acctylprocainamide. respectively. In such situations.
appropriate reduction in dosage and careful monitoring of plasma levels of the parent drug and its active metabolite often are recommended. The pharmacological activity of some metabolites has led many manufacturers to synthesize these metabolites and to market them as separate drug entities (Table 4-6). For example. oxyphenhutazone (Tandearil. Oxalid) is the p-hydroxylated metabolite of the anti-inflammatory agent phenylbuta-
zone (Butazolidin. Azolid). noririptyline (Aventyl) is the Ndcmethylated mctabolite of the tricyclic antidepressant atnitriptyline (Elavil). oxazepam (Serax) is the N-demcthylated
\
N-Oxide Focmalion
O.Methylation
1.
Dotutamine
Chapter 4 • Metabolic Changes of Drugs and Related Organic Compounds
135
TABLE 4-6 Pharmacolo gically Active Metabolites in Humans Parent Drug Aetohenamlde Aattylmethadol Arnitriptyline Azathioprine (arbarnazepine Clitoral hydrate Chiorpromazine Clofibrate Cortisone Diazepain
Digitoxin Diphenoxylate Imipramlne Mephobarbltal Metoprolol Phenacetin
Phenylbutazone
Metabolite
Carbamazepine-9,1O.epoxlda
Trichioroethanol 7-Hydroxychlorpromazine Chlorophenotcyisobutyric acid Hydrocortlsone Desmethyldiazepam and oxazepam Digoxin Dlphenoxyllc acid Desipramine Phenobarbital ta-Hydroxymethylmetoprolol Acetaminophen Oxybutazone Prednisolone
Procainamide
N-Acetylprocainamide 4-Hydroxypropranolol 3-Hydroxyquinidine Sulfide metabolite of sulindac Mesoridazine Warfarin alcohols
Sulindac
Thioridazine Warfarin
•ind 3-hydroxylated metabolite of diazepam (Valium). and inesoridazine (Sercntil) is the suffoxide metabolite of the inuipsycholic agent thioridazine (Mellaril). Antivirals that are used in treating herpes simplex virus. raricella-zosler virus, and/or human cytomegalovirus must
These include acyclovir, valacyclovir. famciclovir, and ganciclovir, which must be
he
phosphorylated on the pentose-like side chain to the triphos-
phile derivative to be effective in inhibiting the enzyme DNA polymerase. The antiviral cidovir is dispensed as a monophosphate and only needs to be diphosphylated for conversion to Ihe active triphosphate metabolite. The nucleoside antivirais that arc used in treating AIDS/HIV must also tuulergo a similar metabolic conversion to the triphosphate melaboiite.539 The triphosphale derivative acts as a competi-
tire inhibitor of the enzyme, reverse transcriptase, which normally uses the triphosphorylated form of nucleic acids. Esumples include zidovudinc, stavudine, zalcitabine, lamisudine.
and didanosine.
One of the more recent uses of drug metabolism in the development of a novel agent includes the example of osellamivir. a neuraminidase inhibitor used in treating influenza. Ro'64-0802, the lead drug. showed promise against both influenza A and B viruses in vitro but was not very effective
sites used in vivo. To improve the oral bioavailability, the ethyl esler, oseltamivir, was developed as a prodrug. Admin-
suation of the more lipophilic oseltamivir allowed good of the active metabolile in various tissues.. especiaHy in the lower respiratory tract. The metabolism proceeds via a simple eeter hydrolysis to yield the active free catboxylic acid.aso
Ketone reduction N-Demethylation N-Demethylatlon Glutatkiione conjugation Epoxidatlon Aldehyde reduction Aromatic hydroxylation
Nortriptyllne 6-Mercaptopurine
Piednisone Pnmidone
Propranolol Quinidine
Biotransformation Process
Hydroxyhexamide Noracetylmethadol
Ester hydrolysis
Ketone reduction N-Demethylatlon and 3-hydroxylation Ailcyclic hydroxylation Ester hydrolysis
N-Demethyiation N-Demethylation Bertzylic hydroxylation 0-Deethylation Aromatic hydroxylation Ketone reduction Hydroxylation and oxidation to ketone N-Acetylation Aromatic hydroxylatlon Allylic hydroxylation Sulfoxide reduction S-oxidation Kelone reduction
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472. Shindo. H.. cc al: Chem. Phnnn. Bull. (Tokyo) 21:1(26. 1973. 473. Morgan. C. 0.. at al.: Biochem. 3. I 14:0P. 1969. 474. WeIner. K., and Tuttle. K. K.: In Goldberg, M. E. lcd.). Phormiucologi. cal attd Biochemical Properties ol' Drug Substances. vol. I. Washing. ton, DC. American Phanuuaceutical Association. 1917. p. 109. 475. Glazko. A. J.: Drug Melab. Dispos. 1:711, 1973. 476. Bonier. U.. and Abbott. S.: Enpcricnlia 29:180. 1973. 477. Bleidner. W. B., et at.: 3. Pharrnacol. Dip. l'hcr. 50:484. 1965. 418. Komori. Y.. and Sendju. Y.: 1. Biochem. 6:163. 1926. 479. Lindsay, R. H.. cc al.: Biochem. Phanmncol. 24:463. 1915. 480. Bremer. J., and Greenberg. 13. M.: Biochim. Biophys. Ada 46:2 17. 1961.
411!. Allan. P. W.. et al.: Biochim. IIiophys. Aria 114:647, 1966. 482. Elion. 0. B.: Fed. Proc. 26:898. 1967. 483. Testa, B.. and Jenncr. P.: Drug Metabolism: Chemical and Biochemical Aspects. New York. Marcel Dckker. 1976. pp. 329—4 18. 484. Tests. B., and Jcnner. P: Drug Metab. Rev. 12:1, 1981. 485. Murray. M.. and Reidy. 0. F.: Pharnuacol. Ther. 42:85, 1990. 486. Murray. M.: Clin. Pharnuaucokinet. 23:132. 1992.
487. Wars!. K. M.. ci al.: In Avery. 0. S. (cdl. Drug Treahnuent, 2nd cii. Sydney, ALlIS Press. 1981). p. 76.
140
Wi/con cord Gisi'old's Te.rthook of Organic Medicinal and Pharmaceutical Chemistry
488. Morse)li. P. L. Drug l)isposition During Dcselopnient. Ness York. Spectrum.
977.
489. Jondorf, W. K.. ci a).: Biochem. Pharniacol. :352. 1958. 490. Niiowsky. H. M.. ci al.: J. Pediair. 69:1139. 1966. 49). Crcsik'i, I., et al.: Cm. Pharmacokinet. :280. 1976 492. Crooks. 3.. and Stevenson. I. H. teds.): Drugs and the Elderly. London, Macmillan, 1979. 493. Williams. K. 1.: Ann. N. Y. i\cad. Sci 179:141, 1971.
494. Williams. K. T.: In LaDu. B. N., etal. teds.). Fundamentals of Drug Metabolism and Disposition. Baltimore, Williams & Wilkins, 1971. p. 187.
495. Williams. R. T.. ci al.: In Snyder. 5.11.. and Usdin. E. eds.l. Frontiers in Catechsilainine Research. New York. Perganuin Press. 1973 p. 927. 496. Butler. 1'. C.. et al.: 3. Plmmtacnl. Eap. liter. 99:82. 197€. 497. Williams, K. T.: Biochenr, Soc. Trans. 2:359. 1974. 498. Itridges. 3. W.. et al.: Biiwlrem. 3. 118:47. 1970. 499. Williams, R. T.: Fed. Proc. 26: 1029, 1967. 5)8). Iinrma.s, B. H. et a), J. Pharm. Sci.. 79:321, 99)1.
Biiil. Mcd. 118:872, 965. SIll. ('rum, R. L., et at.: Proc. Soc 502. Kurt. H.. et a).: Neurology 14:542. 1962. St)3. Vesetl. B. S.: Prr,g. Med. Genet. 9:291, 1973. 504. Pelkonen,O., era).: Iii Pacilici, G. M.. and Pelkonen, 0. teds.). Interindividual Variability in Ilnntan Drug Metabolism, New York. 'I'aylor & Francis, 2)81). p.269. 5)15. Kato, K: I)rug Merab. Rev. 3:). 1974. 51)6. Becketi. A. II.. ci a).: 3. Pharm. Phannacol. 23:625. 1971. 507. Metrguy. K.. etal..Nature 239:1)12. 1972. 5)18. Couney, A. H.: Pharmaco). Rev. 19:317. 1967. 509. Snyder. K.. and Kemmer. H.: Pharrnacol. TIter. 7:203. 1979. 5)0. Parke. D. V : In Parke. I). V. led.). Enzyme )ndaction. London. Plenmu Pres.s. 975. p. 207. SI). Iistahrrxrk. K. W., and Lindenlatrh, B. teds.): The lndttction irE Drug Metabolism. Stuttgart. Schattauer Verlag. 1979. 5)2. Hansten, P. D . Drug Interactions. 4th ed. Pitiladelphia. Lea & Fehiger. 979, p. 38. 5)3 l.aenger. I).. and Detcring. K.: Lattcet 600. 1974 5)4. Skolnick, 3. L.. eta).. JAMA 236:1382, 1976. 5)5. I)enr. C. B., ci a).: Br. Med. 3.4:69. 197)1 5)6. Yeung. C Y.. and Field. C. B.: Lancet 135. 1969. 5)7. Jenne. J., eta).: Life Sci. 17:195. 1975. 518. Pantuck. B. 3.. eta).: Science 175:1248. )972. 519. Pantuck, B. J.. et a).: (')in. Pbarntaor). liter. 14:259, 973. 521). Vaughan. D. P.. ci a).: Br. 3. Clin. Pharmacol. 3:279, 1976. 521. Kolnurdin. B., et a).: C)in. Pharmaco). Ther. 10:638. 1969. 522. Jeffrey. W I).. et a). JAMA 236:288), 1976. 523. Ge)boin. H. V., and Ts'o, p. 0. P. )eds.1: Po)ycyc)ic Hydrocarbons and Cattcer: Ettvimnmettt. C)tetttistry. Mo)ecolar and Ce)) Biology. New York. Academic Press, 1978.
524. Vesel). B. S.. and Passananti. 0. T.: Drug Metab. Dispos. 1:4)12. 1973.
525. Anders. M. W.. Aitnit. Rev. Pharotaco). 11:37. 197) 526. Ke)toe. W. A.: Pharmacist's Letter. l8:# 1818)05, September 2)8)2. 527. Hewick, I)., and McEwen, 3.: 3. Pbarnt. Pharmaco). 25:458, 1973. 528. Casy. A. F.: In Barger. A. led.l. Medicinal Chetnisiry. Pan 1,3rd ed. New Yirrk, W))ey-Interscicnce. 1970. p.81. 529. George. C. F.. ci a).: Eur. 3. C)in. Pbartnaco) 4:74, 1972. 53)). Breinrer, D. D.. atid Van Rossitm, J. NI.: J. Pharm. Pharmaco). 25: 762. 1973. 53). Low. L. K.. and Castagnoli. N.. Jr.: Anna. Rep. Med. ('hem. 13:304. 1978.
532. Testa. B.. and Jcnner. P.: J. Plianti. Phartoacol. 28:73). 976. 533. Be)pairc, F. M.. ci a).: Xenohiirtica 5:4)3. 975. eta).: Xcnobiirtrca 3:511, 1973. 534. Woo)hoase, N 535. Drayer. D Ii.: US Pharot. tHosp. Bali 5:H15, 198)). 536 Pierides, A. M., eta).: I.ancet 2:1279. 1975. 537. Gabriel, K., and Pearce, 3. M Latrrct 2:906, 1976 538. Ga))ois-Monthrtin. S Schtteider. B., ('ben, Y.. ci a).: J. Bin). Chetit. 277)42 ):39953. 218)2.
539. Stein. D and Miurre, K. H.: Pharmacotherapy 21))). I). 218)1. 540. Sweeny. D. 3.. Lynch, 0.. Bidgood. eta).: Drug Meiah. I)ispos. 28)7): 737. 2188).
Anders. M. W. led.): Bioactivatton of Frrreigii Comprinnds. New York. Academic Press. 1985. Baseli. K. C.. and Crasey. K. H: Dispositiotr it) Tonic Drugs and Chemicals
in Man. Fiister City. ('A. C)rernica) i'onicri)ogv Institute. 1995. Benford, D. 3.. Bridges, 3 W., and Gibson. 0. 0. leds.t: Drug Metaho)ism—Frsirn Mii)ccu)es to Man. )ondon. Tay)nr & Fruncis. 1987. Brodie, B. B.. and Gillette. J. K. teds. I: Concepts in Biochemical Pharmaoi). ogy. Part 2. Berlin. Springer.Verlag. 1971. Caldwe)). 3.: Conjngation reactions in foreign compound nictahxilism Drug Metab. Rev. 13:745. 1982. Calulwel). 3.. and Jakirhy. W. B. )eds.l: Biological Ba.sis iif Deinnihicaiinn. New York, Academic Press. 1983. Caldwel). 3.. and Paulson. (3. 1). )eds.): Foreign Compound Metabolism. London. Taylor & Francis. 1984. Creasey. W. A.: Drug Disposition in Hanians. New York.Orshord Universit1 Press. 1979.
DeMaticis. F.. and I.ock. B. A. )eds.): Selectivity and Molecular Mechanisnis of Trixicity. London, Macmillan, 1987. Dipple. A.. Micheijda. C. 3.. and Weisburger. B K : Metabolism of chemical carciniigen.s. Phannacol. Thrr. 27:265. 1985. Drayer. D. B.: Pharmacologically active metaholites iii dnrgs and oilier )vtreign cirimmpmrunuts. Drugs 24:5)9. 1982. I)armrmn. 0.3.: (fliicirronidarion of Dnigs.'rnd Other Curmupirands. Boca Ratnn.
FL. CRC Press. 980 B'.tahrrnrk. K. W.. arid l.inden)anh. E teds.): Induction of Drug Metabolism Siaogarr. Schatratier Verlag. 1979. Ferraiolo. B. L.. Mohler. NI. A.. arid GIimtT,C. A. )eds.): Prcmtrin Pharrrracrrki-
netics and Metabolism. New Yirrk, Plenirnr Press. 1992. Gibson. 0. 0.. arid Skett. P.: hmrtordirction to Drug Metaholisrit Lurrrdrtu. Chapnrau & Hall. 1986. Gorrod. 3. W. (cdl: Drug Toxicity. Lsrndrrrr. Taylor & Francis, 1979. Gorrod, 3. W.. and Damani. 1.. A. teds.): Biir)ogical Dxidatiirn irE Nitrogen in Organic Molecules. Chiclrester, UK. E))is Horwood. 1985. Gorrod. 3. W., Oclsch)ager. Hand CaIdwell. 3. )cds.): Metubo)onr nI Xerrurhiirrics. l.ondiirr. Taylor & Francis, 1988. Granr. T. B. led.): Entrahrpatic Meraholisni iii Drugs arid Oilier Foreign ("nnrpounds. New York. SP Medical and Scientilic. 1980. (iaengerich. F. P.: Analysis and char.rcterit.arion irE drug metabolizing en zytnes. In I layes. A. W. led.;. Prirrciplcs and Methods of Trrnicolugb, 3rd rd. New York, Raven Press. 1994. Guengerich. F. P. )ed.). Mammalian Cyrocliromes P-45)t. virls. and 2 Brica Ratori. FL, CRC Press. 1987. )
Haihiway. D. B.: Meclranisrrrs of Chemical Carcinogenesis. London. Butterwonhs. 1986,
Hodgson. E.. arid Levi. P. B. teds.t A Teatlntrok 01' Modern Tonicolrrgy New Yirrk, Blvcvier. 1987. Hunrphrey. NI. I.. aird Ringrose. P. S.: Peptides arid related drugs: a rcvieu irE their ahsorptirrn. nrctahrrhisrrr arid cncretion. Drug Metah. Rev. 17: 283. 1986. Jakohy. W. B. Ied.t: Dcuoxifiearirrn and drug irretaho)isnr. .Methad.s Enry• mit). 77: 1981. Jakohy, W. B. led.): Entymatic Basis ot' Detonilication. sols. I and 2. New York. Academic Press. 19811.
Jakrrhy. W. B.. Bend, 3. K.. and Caldwell, 3. )cds.): Metabolic Ba.sis 0 Dctcmniircatiirn: Mctabolisrrm mit Functional Grrrrtps. New York. Aca' demic Press., 1982. Jeffrery. B. II.: Hrrman Drag Memabolisnr. Froiti Mirlccu)ar Bto)rrgy to Man
Boca Raron. )'t, CRC Press, 1993. Jcnncr. P., and Te.sra. B.: The intluence irE stererrcherriical factors on dues
disposition. Drug Metab. Rev. 2:1)7, 1973. Jenncr. P.. and Testa, B. tcds.t: Corrcepts in Drug Metabolism. Parts A an)
B. New York. Marcel Dekkcr. 1980. 1981. Jerina, D. M. ted.): I)rug Metabolism Concepts. Washingirmn. DC, American Ctremical Society. 1977. Jrmllow. D. 3., et a). )cds.l: Ihirmlogical Reactive Inuenrrediares: Fimnuation. Toniciry arrd Iiracrrvatirmn. New Yumrk, Plenum Press. 1977. Kaimliman. F. C. ted.I: ('on3rigariorm.-Dccrmnjngamion Reactirmns in Drug
mnbolisnr and Tonicity. Berlin. Springer-Verlag. 994. K)aasen. C. I). ted.): Casarett & I)on)l's Tonieolrmgy, 5th ml. New Yrruk.
McGruw-Hill. 1996.
SELECTED READING Aiiio. A. )ed.): ConJugation Reactions in Drug Bioiransl'ontraiion. Amsterdatn. E)sevier, 1978.
La Du. B, N.. Mandet, H. U.. and Way. B. L. )eds.): Fundanientals of Drug
Metabolism and Drug I)ispositirrri. Ba)rrmrrre. Williams & Wilkins 1971.
l.urw. L. K.. and Castagnolt. N.: Drug hiorransforrriarions. In Wolff. NI. F
Chapter 4 • Metabolic Changes of Drugs and Related Organic Conipound.c teaLt IIwgers Mcdkanal Chemistry. Part I. 4th ed. New York. Wiley-
141
Sato. R.. and Omura. 1. (ads.): Cytamchrome P.450. New York. Academic
Inirr.cwncc, 1980, p. 107. MTchrll. I IL. and Horning. M. 0. (eds.): Drug Metabolism and Drug
Press. 1978. Schenkman. i. B.. and Greim. H. (eds.): Cytochrommme P45(1. Berlin. Springer-
hl'al.
New York, Raven Press. 1984. 1) . and l,au. S. S.: Reactive intermediates and their toxicological Toticology 52:1. 1988. Maiden. (Li. lcd.): Sulfate Metabolism and Sulfate Conjugation. London.
Verlag. 1993. Scbenkman. J. B.. and Kupfer. D. teds.): Hepatic Cytochrome P.450 Mo.
& Etancis. 1985. Seisna. S 0,: Chemical and biological factors influencing drug biotransior-
Sims. 0. ted.): Drug Metabolism: Molecular Approaches and Phiarmacolog. ical Implications. Ncw York. Academic Press, 1985 Singer. B.. and Grunberger. D.: Molecular Biology oh Mutagens and Canitm-
maliemi. In Wolff. M. E. lcd.). Burgers Medicinal Chemistry, Part I. 4th cil. New York. %Viley-lnter.cicnce. 980. p. 227. Ntlnn. S. 0.: Metabolic activation and drug toxicity. J. Med. Cheni. 25:
emgens. New York. Plenum Press. 191(3. Snyder. K.. em al. teds.): Biological Reactive lttmermediates II, Pants A and K. New York. Plenum Press. 1982.
753,
Snyder. K.. em al. (edsj: Biological Reactive Intermediates HI: Animal
982.
Oitiiik Mmitellann. P.R. led.): Cytocltrome P.45th Slructurc. Mechanism. and Bindtcmistrs. New York. Plenum Press. 1986. Intcrindividual Variability in PA'iflo. 6. M.. and Pelkonen 0. Human Dntg Metabolism. New York. Taylor & Francis. 2(101. Parke. 0 V. The Biochemistry of Foreign Compounds. Ncw York, Perga. rain Press.
nmxsaygenasc Systemtt. New York. Pergamon Press. 1982.
968.
P.ukc. 0 V.. and Smith. K. I.., (isis.): Drug Metabolism: From Microbe to Man London. Taylor & Francis. 1977. Pjulsnn, 6. 0., ci iii. lcds.): Xenobiotic Conjugation Chemistry. Washing. atm. DC. American Chemical Society. 191(6. Nerd. E.. and Leppard. J. P. (cdsj: Drug Metabolite Isolation and Detection. New York, Plenum Press. 1983.
Models and Human Disease. Parts A and B. New York. Plenum Press, (91(6.
Tagashira. Y.. and Otnura. T. (eds.l: P.450 and Chemical Carcinogenesis. New York. Plenum Press. 1985.
Tests. B.. and Caldwell. J. teds.): The Metabolisni of Drugs and Other Xenobiotics. London. Academic Press. 1995. Testui. B.. and .Ienner. P.: Drug Metabolism: Chemical and Biochemical AspecLs. New York. Marcel Dckker, 1976. Timmibrell. J. A.: Principles of Biochemical Toxicology. London. Taylor & Francis, 1982.
Williams. R. T.: Detoxification Mechanisms. 2nd ed. New York, Wiley. 1959.
CHAPTER
*4
5
m..4
Prodrugs and Drug Latentiation FORREST T. SMITH AND C. RANDALL CLARK
HISTORY In 1958. Albert initially coined the term prodrug and used it to refer to a pharmacologically inactive compound that is transformed by the mammalian system into an active substance by either chemical or metabolic means.' This included both compounds that are designed to undergo a transformation to yield an active substance and those that were discovered by serendipity to do so. These two situations were distinguished by Harper. who in 1959 introduced the term drug Iatentiat,o,i to refer to drugs that were specifically designed to require bioactivation.2
These ideas led to the development of a number of currently used drugs that have advantages over their nonprodrug
counterparts. The type of prodrug to be produced depends on the specific aspect of the drug's action that requires improvement and the type of functionality that is present in the active drug. Generally. prodrug approaches are undertaken to improve patient acceptability of the agent (i.e., reduce pain associated with administration), alter absorption. alter distribution, alter metabolism, or alter elimination. The chemical nature of the prodrugs that can be prepared is some-
what limited, however, by the chemical nature of the active species.
Recently, the terms hard drugs and soft drugs were introHard drugs arc compounds that are designed to contain the structural characteristics necessary for pharma-
ing enzymes, and therefore, less interpatient variability in activation is since such compounds are chemically unstable. however, storage of these compounds may present a problem. Prodrugs can be conveniently grouped into carrier-linked prodrugs and bioprccursor prodrugs.5 Carrier-linked prodrugs are drugs that have been attached through a metabolically labile linkage to another molecule, the so-called promoiety, which is not necessary for activity but may impart some desirable property to the drug, such as increased lipid or water solubility or site-directed delivery. Several advantages may be gained by generating a prodrug: increased ab-
sorption. alleviation of pain at the site of injection if the agent is given parenterally. elimination of an unpleasant taste associated with the drug, decreased toxicity, decreased meta-
bolic inactivation, increased chemical stability, and prolonged or shortened action, whichever is desired in a par-
ticular agent. An example of such a prodrug form of chloramphenicol is provided below (Scheme 5-I Administration of a drug parenserally may cause pain at the site of injection, especially if the drug begins to precipitate Out of solution and damage the surrounding tissue. This situation can be remedied by preparing a drug with increased solubility in the administered solvent. Since chloramphenicol has low water solubility, the succinale ester was prepan.'d
to increase the water solubility of the agent and facilitate
duced.3
parenteral administration. The succinate ester usd1 is inac-
cological activity but in a form that is not susceptible to
tive as an antibacterial agent, so it must be converted to chloramphenicol for this agent to be effective. This occurs
metabolic or chemical transformation. In this way, the production of any toxic metabolite is avoided, and there is in-
in the plasma to give the active drug and succinate. The ester hydrolysis reaction can be catalyzed by esterases present in
creased efficiency of action. Since the drug is not inactivated
large amounts in the plasma. The ability to prepare estertype prodrugs depends, of course, on the presence of either a hydroxyl group or a carboxyl moiety in the drug molecule. The promoiety should be easily and completely removed after it has served its function and should be nontoxic. as is
by metabolism, it may be less readily eliminated. On the other hand, soft drugs are active compounds that after exert-
ing their desired pharmacological effect arc designed to undergo metabolic inactivation to give a nontoxic product. Thus soft drugs are considered to be the opposite of prod-
indeed the case with succinate. The selection of the appropri-
rugs.
ate promoiety depends on which properties are sought br the agent. If it is desirable to increase water solubility. then a promoiety containing an ionizable function or numerous
BASIC CONCEPTS
polar functional groups is used. If. on the other hand, the goal is to increase lipid solubilimy or decrease water solubility. a
A prodrug by definition is inactive and must be converted into an active species within the biological system. There are a variety of mechanisms by which this conversion may is most often carried out by metabolizing enzymes within
nonpolar promoiety is appropriate. A slight variation on the currier-iinked prodrug approach is seen with mutual prodrugs in which the carrier also has activity. The antineoplastic agent estramustinc, which is used in the treatment of prostatic cancer, provides an exam-
the body. Conversion to an active form may be accomplished by chemical means (e.g.. hydrolysis or decarboxylation). although this is less common. Chemical transformation does not depend on the presence or relative amounts of memaboliz-
ple of such an approach (Scheme Estramustine is composed of a phosphorylated steroid ( 17a-estradiol) linked toa nomiustard IHN(CFI2CI-l2CI)21 through acarbamate linkage. The steroid portion of the molecule helps to concentrate the
be accomplished. Generally, the conversion to an active form
142
Chapter 5 • Prnulruox and Drug Lizesniazion
143
OH H C '2
OH Cl2
H 20
H
a
Sodium Sucdnate
Scheme 5—1 • Hydrolysis of chioramphenicol succinate.
drug in the prostate, where hydrolysis occurs to give the norniustard and The normustard then acts as an alkylatHg agent and etcits a cytotoxic effect. The I 7a-estradiol
has an antiandrogcnic effect on the prostate and. hcrehy, slows the growth of the cancer cells. Since both he stcmid and the mustard possess activity. estramustine is coned a mama!
pradrug. Note that phosphorylation of the
cart be used to increase the water solubility, which jho constitutes a prodrug modification. Both types of esters earbantates and phosphates) are hydrolyzed by chemical or encymatic means.
In contrast to carrier-linked prodrugs. bioprecursor prodrugs contain no promoiety but rather rely on metabolism introduce the functionality necessary to create an active For example, the nonsteroidal anti-inflammatory drug (NSAIDt sulindric is inactive as the sulfoxide and must reduced metabolically to the active sulfide (Scheme
53)5 Sulindac is administered orally, absorbed in the small intestine, and subsequently reduced to the active species. Administration of the inactive form has the benefit of reducing the gastrointestinal (CII) irritation associated with the sulfide. This example also illustrates one of the problems associated with this approach, namely, participation of alternate metabolic paths that may inactivate the compound. In this case, after absorption of sulindac, irreversible metabolic oxidation of the sulfoxide to the sulfone can also occur to give an inactive compound.
Although seen less frequently, some prodrugs rely on chemical mechanisms for conversion of the prodrug to its active form. For example. hetacillin is a prodrug form of ampieillin in which the amide nitrogen and a-amino functionalities have been allowed to react with acetone to give an imidazolidinone ring system (Scheme This decreases the basicity of the a-amino group and reduces pro-
OPO3Na2
+
H0
lTa-abadIol H
+
Scheme 5—2 • Activation of estramustine.
+
+
2Na
144
Wilson and Gisvolsi',c Textbook of Organic Medicinal and Pharmaceutical Che,nistrt
r2COOH
F
.CH2COOH
4
CH3
Sulindac (Inactive)
Sulfide
C—
(Inadive)
Scheme 5—3 • Metabolism of sulindac.
tonation in the small intestine so that the agent is more lipo-
philic. In this manner, the absorption of the drug from the small intestine is increased after oral dosing, and chemical hydrolysis after absorption regenerates ampicillin. In such an approach. the added moiety, or promoiety. in this case acetone, must be nontoxic and easily removed after it has performed its function.
PRODRUGS OF FUNCTIONAL GROUPS
Carboxyllc Acids and Alcohols Prodrugs of agents that contain carboxylic acid or alcohol functionalities can often be prepared by conversion to an ester. This is the most common type of prodrug because of the ease with which the ester can be hydrolyzed to give the active drug. Hydrolysis is normally accomplished by estera.se enzymes present in plasma and other tissues that are capable of hydrolyzing a wide variety of ester linkage.s (Scheme Included below are a numberoithe different types of esterases that prodrugs may use:
As mentioned above, there are a variety of different types of prodrugs. and a comprehensive discussion of each individual agent is beyond the scope of this chapter. The major types
of prodrugs (grouped according to functional group) and bioprecursor drugs (grouped according to type of metabolic activation), however, are discussed briefly below.
Ester hydrulase Lipase Cholesterol ester,tsc Acetylchotinestera.se Carboxypeptidase
Cholinesterase
NH2
0 H 20
Miplclhs
COOH
+
CH
Scheme 5—4 • Hydrolysis of hetacuilin.
COOH
H3
Chapter 5 • Prodrz,ç'.t anti I)rug Lan'nnaiwn
145
0
0
Drug—C—OH
Drug—C—O—Promolety
HO —Promolety
+
or
or
Drug—O —"—Pro moiety
Drug—OH
HO—11---Promolety
+
Scheme 5—5 • Activation of ester prodrugs,
In addition to these agents, microflora present within the gut produce a wide variety of enzymes that can hydrolyze esters. Chemical hydrolysis of the ester function may also occur to some extent. An additional factor that has contributed to the popularity of esters as prodrugs is the ease with
which they can be formed, lithe drug molecule contains either an alcohol or carboxylic acid functionality, an ester prodrug may be synthesized easily. The carboxylic or alco-
hol pronlolety can be chosen to provide a wide range of lipaphilic or hydrophilic properties to the drug, depending ott what is desired. Manipulation of the scene and electronic
properties of the prontolety allows control of the rate and extent of hydrolysis. This can be an important consideration when the active drug must be revealed at the correct point in its movement through the biological system. When ills desired to decrease water solubility. a nonpolar alcohol or carboxylic acid is chosen as the prodrug moiety. Decreasing the hydrophilicity of the compound may yield a
number of benelits, including increased absorption. decreased dissolution in the aqueous environment of the stomach, and a longer duration of action. An example of increased absorption by the addition of a nonpolar carboxylic acid is
seen with dipivefrin HCI (Scheme 5-6). This is a prodrug fonu of epinephnine in which the catechol hydroxyl groups have been used in the formation of an ester linkage with pivalic acid." The agent is used in the treatment of openangle glaucoma. The increased lipophilicity relative to epi-
nephnine allows the agent, when applied. to move across the membrane of the eye easily and achieve higher intraocular concentrations. Hydrolysis of the ester functions then occurs in the cornea. conjunctiva. and aqueous humor to generate
the active form. epinephrine. Using pivalic acid as the promoiety increases the stenic bulk around the scissile ester bond, which slows the ester hydrolysis relative to less bulky groups. yet still allows this reaction to proceed after the drug has crossed the membrane harriers of the eye. In addition to this benefit, the catechol system is somewhat susceptible to oxidation, and protecting the cacechol as the die.sler prevents this oxidation and the resulting drug inactivation. Decreasing the water soluhility ola drug by the formation of a prodrug may have additional benefits beyond simply increasing absorption. A number of agents have an unpleasant taste when given orally. This results when the drug begins to dissolve in the mouth and then is capable of interact-
ing with taste receptors. This can present a significant problem, especially in pediatric patients.. and may lead to low compliance. A prodrug with reduced water solubility does not dissolve to any appreciable extent in the tnouth and, therefore, does not interact with taste receptors. This approach has been used in the case of the antibacterial chloramphenicol. which produces a bitter taste when given as the parent drug (Scheme 5-7). The hydrophobic palmitate ester does not dissolve to any appreciable extent in the mouth, so
there is little chance for interaction with taste receptors.'7
OH CH3
P'1H2
HO
CH34,..CH3 OH
CHn
C
HO
Epinephilne
cie +
C
H
CH,
Dlptvetrin HCI
Ptvaøc Add
Scheme 5—6 • Hydrolysis of d,p,vefrin HCI.
_____
146
WiI.con and Gisvojd'.c Textbook of Or,ianic Medicinal and Pharmaceutical
OH
H 1(CH C 12
.1(CHCI2 02N
Chcot
02N
OH
H3 +
Chioramphenicot Paimitate
0 CH3(CH2)14
OH
Scheme 5—7 • Hydrolysis of chloramphenicol palmitate.
The ester moiety is subsequently hydrolyzed in the GI tract. and the agent is absorbed as chloramphenicol. Listed below are a number of other agents that have been converted into ester prodrugs and other types of prodrugs to overcome an unpleasant taste: Chloramphenicol palmitate N-Acetyl sulfisoxazole N-Acetyl sulfamethoxypyridazine Erythmmycin e,stolate Clindamycin palmitate Troleandomycin
R1
I
Not all carboxylic esters are easily hydrolyzed in vivo. Stcric inhibition around the ester in some cases prevents the
prodrug from being hydrolyzed. This is seen in the $-lacturns, in which it is often desirjblc to increase the hydrophobicity of the agent to improve absorption or prevent dissolution in the stomach where acid-catalyzed decomposition may occur. Simple esters 01 the carboxylic acid moiety, however, are not hydrolyzed in vivo to the active carboxylate (Scheme 5-8). A solution to this problem was to use the so-called doubleester approach, in which an additional ester or carbonate
Penic8in Esters H
NH
\_L__s
CH3 No ReactIon
CO OR2 R2
Propyl. But)l, Ptienyt
— Esters H H
No ReactIon
Ethyl, Propyl, Butyt. Phemyl
Scheme 5—8 • Simple esters of fl-lactams with resistance to enzymatic hydrolysis.
Chapter 5 • Prodrug.c and Drug La,enria,ion (unction is incorporated into the R2 substitueni further re— ninved from the helerocyclic nucleus.'5 " Hydrolysis of such a function occurred readily, and the moiety was selected
that chemical hydrolysis of the second ester occurred quickly. This is seen in the cephalosporin celpodoxime prowheN a carbonate function was used (Scheme 5_9)20 The carbonate is also susceptible to the action of esterase cnLyines. aiid the unstable product undergoes further reac-
jun to give the active carboxylate. This approach is frequently used to improve absorption or prevent dissolution in the stomach and the subsequent acid-catalyzed decompo-
of aminopenicillins and second- and third-generation
cephalosporins (celpodoxime proxetil has been classilied as both a second- and a third-generation agent) so that these agents can be administered orally (see Scheme 5-10 for several examples). To increase the hydrophilicity of an agent, several different types of ester prodrugs have been used, including succinates. phosphates. and sulfonates. Alt are ionized at physio-
logical pH and, therefore, increase the water solubilily of the agents, making them more suitable for parenteral or oral administration when high water solubility is desirable (Scheme 5-I I). Succinate esters containing an ionizable carhoxylate are
C H3 N
H 2N __
147
2)7...JL1N H
—
H H3
,
Co2
+
H —O
H H
H
Scheme 5—9 • Hydrolysis mechanism of cefpodoxime proxetil.
H3
148
Wj150,l and
Gjcvoldx
of Organic Medicinal and Phannaci'aaieul Chenii,srr
C H3 H
H2N
CH3
II
o—(CH3
Ce
C H3
0
0
—CH—0
CH3
CH3
H2N H
0 OCH2CH3 CH3
Scheme 5—10 • Some examples of double esters of $-lactarns.
Chapter 5 a I'rodrug.s wul Drug La:ensiatiw:
Dnig—O—ll-CH2-CH2-—'I—O"N a
Drug—OH
149
+
Sucdnates
0
0 a
Drug—OH
+
OH
OH Scheme 5—11 • Succinate and phosphate esters
useful when rapid in viva hydrolysis of the ester functionally The rapid hydrolysis is related to the intramokeular attack of the carboxylate on the ester linkage, which
tide serves to increase cellular uptake by use alan amino acid
not require the participation of enzymes (Scheme
use Mannich bases as a prodrug form of the amines. Mannieh
As a result, these agents may be somewhat unstable
bases result from the reaction of two amines with an aIde-
and should he dissolved immediately prior to
hyde or ketonc. As seen with hetacillin (see Scheme 5-4), the effect of fornting the Mannich base is to tower the basicity of
in
admInistration.
Phosphate esters of alcohols offer another method of incressing the water solubility of an agent. The phosphates are completely ionized at physiological p1-I and generally hydrolyzed rapidly in vivo by phosphatase enzymes. Ionization of the phosphate function imparts high stability to these densativcs in solution, and solutions for administration can he stored for long periods of time without hydrolysis of the phosphate. Such an approach has been used to produce dinphosphate, which produces less pain at the injection site Ihan clindamycin itself (Scheme 5-13). Pain after parenicral adniinistration is associated with local irritation caused by low aqueous solubility or highly acidic or basic solutions. With chindamycin phosphate, the reduction in pain is attributed to the increased water solubility of the agent.
transporter. The amino acids are then cleaved by specific peptidase enzymes. A more common approach has been to
the amine and, thereby, increase lipophilicity and absorption. When nitrogen is present in an amide linkage, it is some-
times desirable to use the amide nitrogen as one of the amines necessary to form a Monnich base. This approach was used with the antibiotic tetracycline—the amide nitrogen was allowed to react with formaldehyde and pyrrolidine to give the Mannich base rolitetracycline (Scheme In this case, addition of the basic pyrrolidine nitrogen introduces an additional ionizable functionality and increases the water solubility of the parent drug. The Mannich base hydrolyzes completely and rapidly in aqueous media to give the active tetracycline.
Azo Unkage Dedvatization of amines to give amides has not been widely
used us a prodrug strategy because of the high chemical stability of the amide linkage and the lack of amidase enI.ymcs necessary for hydrolysis. There have been efforts at incorporating amines into peptide linkages in which the pep-
Amines have occasionally been incorporated into an azo linkage to produce a prodrug. In fact. ii was an azo dye. prontosil. that led to the discovery of the sulfonamides as the first antibacterials to be used to treat systemic infections.22
Although prontosil itself was inactive in vitro, it was active
0
Drug—OH
Succinate Pmdnig
Scheme
Succinic Mhydrlde
5-12 • Intramolecular cleavage of succinate esters.
150
Wi/so,, and Gisvolds Textbook of Organic Medicinal and P!iarn,aceutical Che,nis:rv
H
H
,.C H3
CH3
CH3CH3CH2
H
H
HO
H
HO
Ccn —
+
H3P04 Scheme 5—13 • Clindamycin activation by phosphate hydrolysis.
in vivo and was converted by aio reductase enzymes in the gut to sutfanilamide. the active species (Scheme 5-15). Although prontosil is no longer used as an antibacterial. this type of linkage appears in sulfasalazine. which is used in the treatment of ulcerative colitis. The azo linkage is broken in the gut by the action of azo reductases produced by
age and generation of aminosalicylic acid prior to absorption prevents the systemic absorption of the agent and helps concentrate the active agent at the site of action.
Carbonyl Compounds
microflora. This releases the active agent, aminosalicylic
A number of different functionalities have been evaluated
acid, which has an anti-inflammatory effect on the colon, and sulfapyridine (Scheme 5-16). The advantage of this prodrug approach is that the combination of cleavage of the azo link-
as prodrug derivatives of carbonyls (e.g., aldehydes and tones), although this approach has not found wide clinical use. These have generally involved derivatives in which the
CH3 OH
N(CH3)2
N(CH3)2
CH3 OH
IOH
4120
CO N H —C 2
o
H 20
OH
0
OH
0
Rolft*ecycflne C H2=O
Fo,makMltyde +
HNG Scheme 5—14 • Rolitetracycline synthesis and activation.
and I)rug Latenlia,ian
Chapter 5 a
Azo
=N
H2N
151
Dwg)
NH2
+
H2
H2N
NH2 Scheme 5—15 • Azo cleavage of prontosil
HO
H2
HO OC
HO
Add
H —rjj Azo Reductase
=N
HO OC
Sulfasabzlne H
Suttapyddine
Scheme 5—16 • Action of azo reductase on 5ulfasalazine.
hybridiied carbonyl carbon is converted to an sp3 hybricarbon attached to two heteroatoms. such as oxygen, nitrogeil, or sulilir. Under hydrolysis conditions, these firnetionalities are reconvened to the carbonyl compounds. An of this approach is methenaminc. shown below Methenainine releases formaldehyde in the urinc. which acts u.s an antibacterial agent by reacting with
nuckophiles present iii bacteria. The agent is administered in enteric-coated capsules to protect it 1mm premature hydrolysis in the acidic environment of the stomach. After dissolution of the enteric-coated capsules in the intestine, the agent is absorbed and moves into the bloodstream, eventually ending up in the urine, where the acidic pH catalyzes the chemical hydmlysis to give lbrmaldehyde. Use of this
H
6CH20 Formaldehyde
Methenamine
Scheme 5—17 • Methenamune hydrolysis.
+
4NH3 Ammonia
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Wilson and GisI'(,ld'.s Textbook
of Organic Medicinal and Plsam,aceu:ical Chemistry
prodrug approach prevents the systemic relea.sc of formaldehyde and reduces toxicity.
Other prodrug approaches have involved the use of oximes, imines, and enol esters, although these types of compounds have not been used clinically. A number of agents contain imine and oxime linkages, such as many of the thirdgeneration cephalosporins (e.g.. cefotaxime, ceftizoxime). but these are not prodrugs.
boxylic acid function could be eliminated from these agents:
this functional group is required for activity, however. Nahumetonc contains no acidic functionality and passes through the stomach without producing the irritation normally associated with this class of agents. Subsequent absorption occurs in the intestine, and metabolism in the liver produces the active compound as shown in Scheme 5-18. This approach, however, did not completely eliminate the gastric irritation associated with nabumetone. since it is due
only in part to a direct effect on the stomach. Inhibition of the target enzyme, cyclooxygenase. while having an antiinflammatory effect, also results in the increased release of
BIOPRECURSOR PRODRUGS
gastric acid, which irritates the stomach. So, while nabumet-
As indicated above, bioprecursor prodrugs do not contain a carrier or promoiety but rather contain a latent functionality that is metabolically or chemically transformed to the active drug molecule. The types of activation often involve oxida-
tive activation, reductive activation, phosphorylation, and in some cases chemical activation. Of these, oxidation is commonly seen, since a number of endogenous enzymes can
carry out these transformations. Phosphorylation has been widely exploited in the development of antiviral agents. and many currently available agents depend on this type of activation.
The abundance of oxidizing enzymes in the body has made this type of bioactivation a popular route. Isozymes of cytochrome P450 can oxidize a wide variety of functionalities. generally to produce more polar compounds that can be excreted directly or undergo phase 2 conjugation reactions and subsequently undergo elimination. This occurs in a fairly predictable manner and, therefore, has been successfully exploited in prodrug approaches. A good example of a prodrug that requires oxidative activation is the NSAID nabumetone (Relafen) (Scheme NSAIDs produce stomach irritation, which in patients with preexisting conditions or patients taking large amounts of NSAIDs for extended periods may be severe. This irritation is associated in part with the presence of an
acidic functionality in these agents. The carboxylic acid functionality commonly found in these agents is un-ionized in the highly acidic environment of the stomach. As a result, these agents are more lipophilic in nature and may pass into the cells of the gastric mucosa. The intracellular pH of these cells is more basic than that of the stomach lumen, and the
NSAID becomes ionized. This results in backflow of from the lumen into these cells, with concomitant cellular damage. This type of damage could be prevented if the car-
one induces less gastric irritation than other NSAIDs. this undesirable eFfect was not completely eliminated by a prodrug approach. Such an effect was also seen above with the NSAID sulindac (see Scheme 5-3). whose Gl irritation was reduced hut not completely eliminated. Reductive activation is occasionally seen as a method of prodrug activation but, because there are fewer reducing enzymes, is generally less common than oxidative activation.
One of the best known examples of reductive activation is for the antineoplastic agent mitomycin C. which is used in the treatment of bladder and lung cancer (Scheme 5-I
Mitomycin C contains a quinone functionality that under. goes reduction to give a hydroquinone. This is important because of the differential effect of the quinone and hydroquinone on the electron pair of the nitrogen. Whereas the quinone has an electron-withdrawing effect on this electron pair, the hydroquinone has an electron-releasing effect. which allows these electrons to participate in the expulsion of methoxide and the subsequent loss of the carbamate to generate a reactive species that can alkylate DNA. The cascade of events that leads to an alkylating active drug species is initiated by the reduction of the quinone func-
tionality in mitomycin C. The selectivity of mitomycin for hypoxic cells is minimal, however. The selectivity is determined in part by the reduction potential of the quinone. which can be influenced by the substituents attached to the ring. In an effort to modify the reduction potential of mitomycin C. various analogues have been prepared and tested for antineoplastic activity. It was hoped that the reduction potential could be altered so that the analogues would only be activated in hypoxic conditions, such as those found in slow-growing solid tumors that are poorly vascularized. In the.se tissues with a low oxygen Content it was thought that reductive metabolism might be more prevalent than in nor-
Adive Fonn Nabumetone
(Pmdrug)
Scheme 5—18 • Oxidative activation of nabumetone.
Chapter 5 • Pradruga and Drug Latt',t:ia:ion
153
Nuc H2N
H2N
\t=O
H2N,
H2N,
-OCH
Red
Mtomydn C
H2N
H2N
OH
__O
OH
H
H2N
CH3
IH
-OCOWH
2
N
CH3 OH
Further
IH
Scheme 5—19 • Mechanism of activation of mitomycin C.
mat Iisaues. so the agents would be selectively activated and,
selectively toxic. Although mitomycin was the first agent used clinically to he recogniaed as requiring rcductivc activation, it is only modestly selective for hypoxic cells. A much more selective therefore,
agent.
is currently undergoing phase ill clini-
cal trials.- Tirapazamine is reported to he 100 to 2(X) times more selective for hypoxic cells than for normal cells. The mechanism of activation involves a one-electron reduction that is catalyied by a number of enzymes, including cytochrome P.450 and cytochromc P-450 reductase to give a radical species (Scheme 5-20). This species, which is shown as a carbon-centered radical, can initiate breaks in the DNA chain under hypoxic conditions. Under aerobic conditions, hydroside radical is formed, which can initiate chain breaks.
Phosphorylation is a common metabolic function of' the body, which is used to produce high-energy phosphodiester bonds such as those present in ATP and GTP. The body then typically uses these molecules to phosphorylate other molecules and, in the process of doing so, activates these molecules. The type of activation achieved depends on the molecule phosphorylated. but in many cases. phosphorylaLion introduces a leaving group, which can be displaced by an incoming nucleophile. This is seen, for example. in the synthesis of DNA and RNA. in which nucleotides are added to the 3' end of a growing chain of DNA or RNA (Scheme 5-21). Phosphorylation is commonly required for the binactivaLion of antiviral agents. These agents are commonly nucleosides, which must be converted to the nucleotides to have
154
Wilsi,,, und Gjsvuhls Te,aho(;k of Organic MedEcinul and Pharmaceutical Chemist,
0
0
N
&. H
°2
02
[
• OH
Tirapazamine DNA
Scheme 5—20 • Reductive tion of tirapazamine.
Double-Strand Breaks
activa-
activity. Most often. arniviral agents disrupt the synthesis or function of DNA or RNA. which is generally accomplished by conversion to the triphosphate. Since normal cells are also involved in the synthesis of DNA and RNA. compounds have been sought that would be converted to the triphosphates. the active form, in greater amounts in infected cells than in normal cells. Therefore. nucleosides that have higher
affinity for the viral kinase enzymes than the mammalian kinases are desirable and have greater selective toxicity. This can be seen in the prodrug idoxuridiiie. which was the first agent to show clinical effectiveness against viruses (Scheme 5-22))° The nucleoside enters the cell, where it is phosphorylated. In virally infected cells, this phosphoiyla-
tion is accomplished preferentially by viral thymidine ki-
Oxidized DNA
nase, because the idoxuridine is a better substrate for the viral enzyme than for the corresponding mammalian enzyine. Therefore, the drug is activated to a greater extent in the virally infected cells and achieves some selective toxic. ity. although this selectivity is rather low, and there is significant toxicity to normal cells. Once the drug has been phos. phoiylated to the triphosphate stage, it can inhibit DNA synthesis in a number of ways, including inhibition of viral DNA polymerase and incorporation into DNA. which results in incorrect base pairing that disrupts the ability of DNA to function as a template for DNA and RNA synthesis. In addition to the selective toxicity mentioned, the prodrug approach offers the additional advantage of increased cell penetration. The prodrug can easily enter the cell via active
DNA chak'm
0 0-
DNA Polyrnerase
thymkie
0
-o Scheme 5—21 • DNA synthesis.
0
—Lo
Chapter 5 • Prodrs,gs and Drug LsUe,usar,on
155
0
HN-I
O=_I
o_J_o Wal ThdWie OH
DNA — DNA
Scheme 5—22 • Idoxuridine activation.
mechanisms, whereas the active nucleotides are unable to use this process and arc too polar to cross the iiiembrane via passive diffusion.
A good example of chemical activation is seen with the pngnn pump inhibitors such as omeprazole. In this case, chemical activation is provided by the highly acidic environneSt in and around the parietal cell of the stomach (Scheme 5.23i. This allows protonation of nitrogen on the benzimidawte flog followed by attachment of the pyridine nitrogen. Ring opening then gives the sulfenic acid that subsequently
cydiies with the loss of water. Attachment by a sulfliydryl group present on the proton pump of the parietal cell then occurs and inactivates this enzyme, preventing further release of H' into the GI tract, which is useful in treating gastric ulceration.
CHEMICAL DEUVERY SYSTEMS The knowledge gained front drug metabolism and prodrug studies may be used to target a drug to it.c site of action. Site'specifie chemical delivery systems take advantage of
higher levels of activity in a metabolic or chemical pathway at the target site. A prodrug form of the active drug is designed to serve as a substrate in that specific pathway, thus
yielding a high concentration of active drug at the target site. Site-specific chemical delivery requires that the prodrug reaches the target site and that the enzymatic or chemical process exists at the target site for conversion of the prodrug to the active drug. Many factors are involved in the relative
success of site-specilic drug delivery, including extent of target organ perfusion. rate of conversion of prodrug to active drug in both target and nontarget sites, and input/output rates of prodrug and drug from the target sites. Site-specific chemical delivery systems represent but otte approach to the selective delivery of drug molecules to their site of action for increased therapeutic effectiveness and limited side effects. Other than chemical drug delivery, many carrier systems have been evaluated lir drug delivery, including proteins. polysaccharides. liposomes. emulsions. cellular carriers (erythrocytes and leukocytes). magnetic control targeting, and implanted mechanical As thc fate of drugs in the human body has become more clearly understood, research activity to improve the delivery of active drug to the target site has increased. The basic goal
156
Wilso,, and Gisyold's Textbook of Organic Medicinal and Pharmacewical Chemistry
@ __<s=O
H-N
NH
x
x Scheme 5—23 • Mechanism of activation of proton pump inhibitors.
of these efforts is to protect the drug from the nonspecific biological environment and to protect the nonspecific bioenvironment fiom the drug to achieve some site-specific
of methenamine to formaldehyde, the active antibacterial
drug delivery. Site-specific drug delivery has been evaluated extensively for drugs with narrow therapeutic windows, such as many of the anticancer drugs. The site-specific delivery of the active drug via its prodrug counterpart requires that the prodrug be readily transported
of urinary pH-lowering agents or by diet. The pH of the
to the site of action and rapidly absorbed at the site. On arrival at the target site, the prodrug should be selectively
ture hydrolysis in the highly acidic environment of the
converted to drug relative to its rate of conversion at nontarget sites. Since high metabolic activity occurs in highly perfused tissues such as liver and kidney, delivery to these organs has a natural advantage. Unfortunately. prodrug delivery of active drug to other organs or tissues is disadvantaged for the same reasons. Furthermore, it is highly desirable to have the active drug.
agent. The rate of hydrolysis increases with increased acidity (decreased pH). and this can be promoted by administration
plasma is buffered to about 7.4. and the rate of hydrolysis is low, preventing systemic toxicity from formaldehyde. As mentioned abovc, this compound is administered in entericcoated tablets that prevent dissolution and, therefore, premastomach. A number of prodrugs for cancer chemotherapy have been designed for selective delivery o active drug to tumor tissue,
based on higher levels of activating enzyme in the tumor cell than in normal Many enzymatic systems show higher activity in tumor cells than in normal tissue because of the higher growth rates associated with tumor tissue. Peptidases and proteolytic enzymes are among those systems
once formed, migrate from the target site at a slow rate.
showing higher activity in and near tumor cells. Thus, one
On the basis of all these requirements, clearly site-specific delivery of drug to the target by a prodrug chemical delivery system is a far more complex undertaking than designing a prodrug to improve one aspect of its overall properties. Yet there are several excellent examples of site-specific chemical delivery systems in use in modem drug therapy. The target sites include cancer cells, GI tract, kidney and urinary tract, bacterial cells, viral material, ocular tissue, and the blood—brain barrier.
means of attempting to produce higher rates of drug incorporation into tumors than in surrounding normal tissue involves deriving a drug molecule with an amino acid or peptide fragment. Capecitabine is an example of a prodrug chemical delivery system that requires a series of enzymatic steps for conver-
The prodrug methenamine, described above in this chapter
(Scheme 5-li), can be considered a site-specific chemical delivery system for the urinary tract antiseptic agent formaldehyde.3' The low pH of the urine promotes the hydrolysis
sion to the active antitumor drug species. 5-Iluorouracil Tumors located in tissues with high levels of the required enzymes should respond best to treatment with capecitabine. Esterase activity occurs primarily in the liver, allowing the intact ester capecitabine to be the absorbed species following oral administration. The ester hydrolysis product itself shows some specific toxicity toward (Scheme
Chapter S •
H.
N
-iC
0
proper input/output ratios for prodrug and active drug species at the target. The relative physicochemical properties of prodrug and its phosphorylated derivative suggest an appropriate input/output ratio for site specificity. The prodrug can readily penetrate the virus, and the increased polarity of the phosphorylated derivative would serve to retain that active species inside the virus. The combination of increased polarity and viral retention of the active phosphorylated species likely reduces any human toxicity that might he associated with this active species. The amino acid drug L-dopa can be considered a sitespecific chemical delivery system that delivers the dntg do-
HO
NH7
F
157
prodrug is specific for those sites where it serves as a substrate for phosphorylation enzymes. One of the requirements for site-specific chemical delivery discussed above was the
NHCOO(CH2)4CH3 H
Pradrugs and Drug Latrn:ia:ian
0
OH
0
pamine to the brain. The brain has an active transport system
5-Fluorouracil
Scheme 5—24 • Metabolic conversion of capecitabine.
01 Llact tissue, which
prevents this molecule from serving
as an effective prodnig delivery form of 5-fluorouracil. The
other two enzymes involved in the formation of 5-fluoroura-
cii occur in high concentrations in target tissues such as cers'iv, breast. kidney, and colon.
There is considerable current interest in the general concept of tumor-activated prodrugs. and a number of strategies have been proposed for drug targeting in tumor One of the more interesting approaches is linking an exogenous nonhuman) enzyme to a tumor-specific antibody. Based on immunological response, the antibody would carry the enzyme to the tumor surface, where it would be available tbr prodrug activation. Prodrugs activated by this cxopenoin enzyme would be converted to the active species only at the tumor site. Since the activating exogenous enzyune is not normally found in human tissue, maximum accuracy in drug targeting should be achieved in this antibodydirected enzyme prodrug therapy.
The antiviral drugs, such as idoxuridine (Scheme 5-22), arc an interesting example of site-specific chemical dclivThese drugs serve as substrates for phosphorylating cneymes found in viruses, and the phosphorylated species is the active antiviral agent. The active phosphorylated spevies is incorporated into viral DNA, disrupting viral replicanon and, thus, producing the antiviral effect. These drugs do not undergo phosphorylation by mammalian cells, so the
that operates to incorporate L amino acids into the central nervous system (CNS). and L-dopa is transported into the brain in this manner. Once across the blood—brain barrier. L-dopa undergoes decarboxylation, as shown in Scheme 5-25, to yield the active metabolite, dopamine. Direct systemic administration of dopamine does not produce signiticant levels of the drug in the brain because of its high polarity and poor membrane permeability as well as its facile meta-
bolic degradation by oxidative deamination. Dopamine formed on the inside of the blood—brain barrier is held there. however, because of the poor membrane permeability of this drug. Although some specificity for brain tissue is achieved
by this delivery method, peripheral side effects of t.-dopa are the direct result of decarboxylation to dopamine in other organ systems. In this case, the enzyme activating system is not localized at the target site, and its presence in other tissues and organs leads to undesirable side effects.
Another example of the chemical delivery of a drug to the brain and CNS is the prodrug form of 2-PAM (pro-2PAM), an important antidote for the phosphate and carhamate acetylcholinesterase inhibitors used in insecticides and nerve gases.3° The polar properties of 2-PAM. a permanent cationic species, prevent this drug from being absorbed following oral administration and restrict the drug from access
to the brain, even after IV administration. Pro-2-PAM is a dihydropyridine derivative that undergoes metabolic and chemical oxidation to yield the active drug 2-PAM (Scheme 5-26). The nonionic pro-2-PAM can easily cross the
blood—brain barrier, and oxidation to 2-PAM within the brain essentially traps the active cationic drug species inside
the brain. Oxidation of the dihydropyridine ring of pro-2PAM occurs throughout the mammalian system, not just in the brain, and the levels of the resulting 2-PAM arc approximately the same in peripheral tissue as in the brain. IV ad-
HO
HO
NH2
Scheme 5—25 • Decarboxylation of v-dopa in the CNS to yield the active drug dopamine.
158
I
Medicinal and Pharmaceutical Chenii,qrv
WiLson and Giss'old'x Textbook of
CNS via passive absorption of the tertiary amine, which on oxidation restricts the resulting pyridinium amnide to the brain. Amide hydrolysis then delivers the active form of the drug at or near its site of action. The amide hydrolysis step may be slower than the dihydropyridine oxidation step, and thus a reservoir of pyridinium antide precurmr may be avail. able for conversion to the active drug species. The use of prodrug concepts has been very successful in the delivery of active drug species to the human eye following local application. Lipophilic esters of epinephrine. such u.s the dipivaloyl ester described above tsee Scheme 5-6). show better corneal penetration following direct application to the eye than the more polar parent drug epinephrine.° The esterases necessary for the hydrolysis of thc prodnig
Oxidation
II
CH=NOH
CH=N014 CH,
2-PAM
Pro-2.PAM
Scheme 5—26 • Oxidation of pro-2-PAM.
ministration of pro-2-PAM, however, yields brain levels of 2-PAM thai are approximately 10 times higher than those achieved by IV administration of the parent drug. The delivery of drugs across the blood—brain barrier has been a significant issue in the design of many therapeutic compounds. Only very lipophilic drugs can cross into the brain without the aid of some active uptake process. such as the one that operates to incorporate essential amino acids into the CNS. The facile oxidation of the dihydropyridine ring system has been extensively investigated as a general process for chemical delivery of a number of drugs to the
are readily available in the eye and skin. The more polar drug
species. epinephrine. is then localized within the lipophilic membrane barriers of the eye, and the drug remains available
at the target site to produce its antiglaucoma The local application of the prodrug species to the skin or eye allows metabolic processes to activate the drug without concern forcompetitive reactions at other tissues or sites of loss. The delivery of drugs to the colon and lower GI tract has taken advantage of the unique enzymatic processes found in colon bacteria. The glucosidase activity of these bacteria
CNS. The approach has been described as a chemical deliv-
ery system. not just a prodrug designed to penetrate the This process is a multistep procedure involving delivery of the drug—dihydropyridine derivative to (he brain via facile diffusion across the blood—brain barrier, blood—brain
allows hydrolysis of glucosidc derivatives of drugs in the colon and provides higher concentrations of active drug.32 A number of steroid drugs Schenie 5-28) demonstrate in-
followed by oxidation to the quaternary pyridine cation, which is trapped in the brain. The drug is then released from the pyridine cation by a second metabolic/chemical event. A number of functional groups can be added to the dihydropyridine to facilitate the derivatization of various functional groups found in CNS drugs. Since many CNS drugs are amines. amides of dihydropyridinc carhoxylic acids are often prepared and used to deliver the drugs across the blood—brain barrier into the brain. Additionally, these amide derivatives often serve to protect the amines from metabolic degradation before they reach the target site. Primary amines such as dopamine and norepinephrine and ninny others are readily tuetabolized and degraded by oxidative deamination before reaching the CNS. The dihydropyridinc derivative of a doparnine ester, shown in Scheme 5-27. has access to the
creased effectiveness in the lower GI tract following admninistration as their glucoside derivatives. The polar glucoside
derivatives of the steroids are not well absorbed into the bloodstream from the GI tract and remain available to serve as substrates for the bacteria that are found primarily in the human colon, The prodrug approach for the delivery of anticancer drugs to the site of action has been used in a number of cases in an effort to increa.se effectiveness and lower side effects.
Several enzyme systems that show higher activity in and near the cancer cells have been evaluated for their ability to activate the prodrug species. In most cases, the enzyme hy level is simply higher near the faster growing cancer cells,
OCOR
x
Pyridtnium Ion Intermediate
C H 2C H
H
Dopamine
Scheme 5—27 • Dihydropyridine-based drug delivery system for dopamine.
OCOR
Chapter 5 • Prodrugx too! Drug Chemistry ul Drug Design and Drug Action
HOC H2
O
O—R
Glucosidaso
OH
R —OH DrUg
4)0
Press. 1992. Chap. 8, pp. 352—41)1. (,. Glaako. A. J.. Carnes. H. F... 752—802. 1957.
Diug.Glucoslde
(Os!dase.
normal tissue prevents the of complete site specificity for these agents. This briel discussion of site-specific drug delivery shows
hal in some cases the prodrug was in use before its mechant delivery and specificity was discovered. Thus, some cenipounds were discovered to represent site-specific drug dclivcry well after they were placed into therapeutic use. An emaluahion of the properties of these agents has produced the Itamework for the design of other prodrug.c with target sites
in specific tissues. This process is really no different from the general drug discovery process in which a unique substance is observed to have desirable pharmacological effects, and studies of its properties lead to the design of better drugs.
REFERENCES
t
A.. ci at.: Antihuot Anna. 792:
10. Tsujj, A.. and Yamana. T.: Chem. Pharm, huh. 22:2434-2443. 1974. II. Jusko, W. i. and Lewis. P.: J. Ptiarm. Sri. 62:69—76. 913 12. Jusko. W. J.. Lewis, G. P.. and Schmitt. (. W.: Clin. Pharun ilmer. 14 1973.
Schwartz. MA., and Hayton. W. 1.: 3. Ptiann. Sri. 61:91)6—9(13. 1q72. 14. Bungared. H.: Ada Pharni. Succ. 13:9-26. 1976. IS. Sinkula. A. A.. and Yalkowsky. S. H.: 3. Phiurm. Sri. (v4:181. 1975. 16. Wci. C. P. Anderson. 1. A.. and Leumputld. I.: Ophlhalunuul. Vis. So. 17:315—321. 1978. I?. Sinkula, A. A.. Moro,auwich. W., and Rowe, F.. L: J. Pharm. Sri. 62: 13.
hut the presence of the enzymes in
Alhcn. A.: Nature 182:421.
York. Academic
31:897—899. 1966.
Stheme 5—28 • Activation of drug-glucoside by bacterial glu-
I
159
7. Riley. T. N.: .1. Chem. Edur. 65:947-953. 198)4. 8. Duggan. D. F..: Drug Melab. Rev. 12:725-337. 19111. 9. Hurdcasitc. (I. A.. Johnson. D. A.. Panetla. C. A.. ci al.: .1. Org. (item
OH
1
L.a:emianio,s
1958.
harper. "1. 3.: J. Mcd. Pharm. Chem. 1:467. 1959. Ariens. E. J.. and Simanin. A. M.: Optimization
of pharmacoki'
nctmrs—.mnestential aspect oIdrug development by metabolic stahiliza-
1106—1111. 1973. 18. Jan.sen. A. B. A.. and Russell. 1. 3.: .1. Client. Soc..
20. Hughes. 0. S.. Heald. 1) I... Ilurkcr. K. I).. ci al.; Cliii. Pharnuaruul. That. 46:1989, 1989. 21. Vej-Hanscn. B.. and ttuindgmuard. H.: Arch. Pharun. Clicin, Sri. Ethic. 7:65, 1979. 22. TrdIouCl, J.,
M. i.. Null. F.. and Beret. 0.: C. R. Soc. Biol.
12th756. 1935. 23. Notani. R. E.: J. Pharm. Sri. 62:863—11111, 1973
24. Mangim. F. R.. Hark. 3. 0., and Jackson. 0.: Am. 1. Mcd. $3i414l:6. 1987.
25. Moore,
H. W.. and C,.erniak, R.: Med. Res. Rev. :249. Moore. H. W,: Science 197:527, 11)77. 27. Iycr. V. N., and Sm'yhalski, W.. Science 135:55. 964.
1981.
26.
28. Renters. W. A.: Mitornycin and poruirontycin. In The Chemistry ci Antilumor Amihiotics. New York. Wiley. 1979. pp. 221—276. 29. Brown, 3. M.: Br. J. Cancer 67:1 l(,3. 1993, 30. Kaufman, H. IL: Proc. Soc. Eap. Biot. Mcd. 109:25), 1962, 3!. Friend, 0. K.: Mcd. Res. Rev. 7:33, 987. 32. Jungheim. L. N.. and Shepherd. T. A.: ('hem. Rev. 94:1553. 994. 33. Miwa, M.. Nishida. U. M.. Saoada, N., ci al : F.ur. J. Cancer 34:1274.
ion In t)evcrling Busiman. 3. A. (ad.). Strategy in Drug Research. 9112, pp. 165—178. Roint. N.: Mcd. Res. Rev. 4:449, 19114.
34. Denny. W. A.: Eur. 3. Mcd. Chew. 36:577. 2091
Silmornian. R.: Pruudrugs and drug delivery systems. In The Organic
36. Bodu,r. N.: Ade. Drug Res. 13:255. 19114.
?unisterdain. Ehccvicr,
965, 2127.
Eksroun, B., Forsgren, U.. ci at.: Antimucrumb. Ageuuts Chcmnther. 8:51%. 975.
19. Bodin. N. Ii.
1998.
35.
SicIla, V.
J.: J. Med. Client. 23:1275. 1911(1.
CHAPTER 6 Biotechnology and Drug Discovery JOHN M. BEALE, JR.
thinking about patient care. Extensive screening programs
BIOTECHNOLOGY: AN OVERVIEW Developments in biotechnology in recent times have been quite dramatic. The years between 1999 and 2001 witnessed
a tremendous increase in the number of biotechnology-related pharmaceutical products in development, and a number
of important new drugs progressed through trials and into the clinic. A grxxl reflection of the impact of biotechnology is the GenBank database. GenBank is an electronic repository of gene sequence information, specifically the nucleatide sequences of complementary DNA (cDNA), representing the messenger RNA (mRNA). and genomic clones that have been isolated and sequenced by scientists worldwide.' 2 The growth of the GenBank database has been rapid, and it has been increasing steadily since about Figures 6-I and 6-2 graphically depict these growth rates. In October 2002, the Pharmaceutical Research and Manufacturers Association (PhRMA) reported that 371 biotechnology-derived medicines were in testing at various stages and that nearly 200 diseases are being targeted by research conducted by 144 companies and the National Cancer Institute. Of these—all of which are in human trials or awaiting
Food and Drug Administration (FDA) approval— 178 are new drugs for cancer. 47 are new drugs for infectious diseases. 26 are new drugs for autoimmune diseases. 22 are new drugs for neurological disorders, and 21 are new drugs for human immumxleliciency virus (HIV) and acquired immunodeitciency syndrome (AIl)S) and related conditions.4 PhRMA also reported 194 new medicines targeted for pediatric use' Approved drugs derived from biotechnology also treat or help prevent myocardial infarction, stroke, multiple sclerosis, leukemia, hepatitis, rheumatoid arthritis, breast cancer. diabetes, congestive heart failure. lymphoma. renal cancer, cystic fibrosis, and other diseases. The number of approvals of biotechnology drugs per year has been increasing steadily. These data are shown in Figure 6-3. The Human Gcnomc Project, an international etlort to obtain complete genetic maps. including nucleotide sequences. of each of the 24 human chromosomes, has spawned much new knowledge and technology. It is awesome to consider that in the mere 30 years since 1972. the science has reached the stage of attempting genetic cures for some diseases, such as cystic fibrosis and immune deficiency disorders.
BIOTECHNOLOGY AND PHARMACEUTICAL CARE As it affects medicine and pharmaceutical care, biotechnology has forever altered the drug discovery process and the
160
once drove drug discovery on natural or synthetic compounds. Now, the recombinant DNA (rDNA)-driven drug discovery process is beginning to yield new avenues for the preparation of some old drugs. For example, insulin, once
prepared by isolation from pancreatic tissue of bovine or porcine species, can now be prepared in a pure form identical with human insulin. Likewise, human growth hormone, once isolated from the pituitaxy glands of the deceased, can now
be prepared in pure form. Recombinant systems such as these provide high-yielding, reproducible hatches of the drug
and uniform dosing for patients.
LITERATURE OF BIOTECHNOLOGY Many good literature sources on biotechnology exist for the pharmacist and medicinal chemist. These cover topics such as management issues in biotechnology." " implementation
of instruction on biotechnology in
costs of
biotechnology drugs.232" implementation in a practice setregulatory issues.'34" product evaluation and formulation.4748 patient compliance.49 and finding informs-
tion.5153 Additionally. there are a number of general review and a general resource reference eatalogue."• Any good biochemistry textbook is also a useful resource.
BIOTECHNOLOGY AND NEW DRUG DEVELOPMENT The tools of biotechnology are also being brought to bear in the search for new biological targets for presently available drugs as well as for the discovery of new biological
molecules with therapeutic utility. Molecular cloning of novel receptors can provide access to tremendous tools for the testing of drugs (e.g.. the adrenergic receptors), while ctoning of a novel growth factor might potentially provide a new therapeutic agent. Biotechnology is also being used to screen compounds for biological activity. By using cloned and expressed genes, it is possible to generate receptor pro. teifls to facilitate high-throughput screening of drugs in vitro or in cell culture systems rather than in animals or tissues. Biotechnology is being investigated in completely novel approaches to the battle against human disease, including the use of antisense oligonucleotides and gene replacement therapies for the treatment of diseases such as cystic fibrosis
Chapter 6 U Rioieehnologv and Drug Diworerv
161
18000
16000 14000
a
j
-
-
-
--
-—
-—- --—-----——-
12000
6000 4000
2000— 0
Y.ar
FIgure 6-1 • Yearly growth of Genin base pairs
md he use of monoclonal antibodies for the treatment of Riolechnology encompasses many subdisciplines includIng genomics, proteomics. gene therapy, made-to-order molcomputer-assisted drug design. and phannacogeno-
A goal of biotechnology in the early 21st century is sm eliminate the "one drug fits all" paradigm for pharmaThe drugs that are elaborated by biotechnological methods me proteins and, hence, require special handling. There are basic requirements of pharmaceutical care for the pharmacist working with biotechnologically derived products:4'
• An understanding of how the handling and stability of hiopharniaceuticuls differs from other dn,gs that phamiacists dispense • Knowledge of preparation ol the product for patient use. ineluding reeonstttulion or compounding if required • Patient cducaiion on the disease. benetits of the prescribed biophurmaceutical. potential side effects or drug interjctions to be aware of, and the techniques of self-administration • Patient counseling on reimbursement issues involving an expensive product
• Monitoring of the patient for compliance
The pharmacist must maintain an adequate knowledge of agents produced through the methods of biotechnology and
16
14 —-
rigure 6-2 • Yearly growth of Genhnk in terms of gene sequences.
——-—- -
Year
162
Wilson and Gisvold'.c Textbook of Organic Medicinal and Pharmaceutical chemistry
35
32
30
25
J20 15
I
10
7
5 0
Year
Figure 6—3 • Yearly approvals of biotechnology-derived drugs and vaccines.
remain 'in the loop" for new developments. The language of biotechnology encompasses organic chemistry, biochemistry, physiology, pharmacology, medicinal chemistry, immunology, molecular biology, and microbiology. A pharmacist has studied in all of these areas and is uniquely poised to use these skills to provide pharmaceutical care with biotechnological agents when needed. The key lechniques that unlocked the door to the biotechnology arena are those of rDNA, also known as genetic engi,zeering. rDNA techniques allow scientists to manipulate genetic programming, create new genomes, and extract genetic material (genes) from one organism and insert it into another to produce proteins.
THE BIOTECHNOLOGY OF RECOMBINANT
DNA (rDNA) Since its inception in the mid-1970s,
(genetic engineering) technology has driven much of the fundamental research and practical development of novel drug molecules and proteins. rDNA technology provides the ability to isolate
genetic material from any source and insert it into cells (plant. fungal. bacterial, animal) and even live animals and plants, where it is expressed as part of the receiving organism's genome. Before discussing techniques of genetic engineering. a review of some of the basics of cellular nucleic acid and protein chemistry is Most of the components that contribute to cellular homeostasis are proteins—so much so that more than half of the dry weight of a cell is protein. Histones. cellular enzymes, membrane transport systems, and immunoglobulins are just a few examples of the proteins that carry out the biological functions of a living human cell. Proteins are hydrated threedimensional structures, but at their most basic level, they are composed of linear sequences of amino acids that fold to create the spatial characteristics of the protein. These linear
sequences are called the pritnarv structure of the protein. and they are encoded from DNA through RNA. The information flow sequence DNA — RNA —. protein has for many years been called the biological "Central Dogma.' The specific sequence of amino acids is encoded in genes. Genes are discrete segments of linear DNA that compose the chro-
mosomes in the nucleus of a cell. The Human Genome Project has revealed that there are between 30,000 and 35,000 functional genes in a human, encompassing about 3,400,000,000 base pairs (bp).an As depicted in Figure 6-4, in the nucleus of the ccli, dou-
ble-stranded DNA undergoes a process of transcription (catalyzed by RNA polymerase) to yield a single-stranded molecule of pre-mRNA. Endonucleases then excise nonfunctional RNA sequences called intl-oils from the pre. mRNA to yield functional mRNA. In the cytoplasm, mRNA complexes with the ribosomes. and the codons are read and translated into proteins. The process of protein synthesis in Escherichia begins with the activation of amino acids
as aminoacyl-transfcr RNA (tRNA) derivatives. All 20 of the amino acids undergo this activation, an ATP-depcndent step catalyzed by aminoacyl-tRNA synthetase. Initiation involves the mRNA template. N-formylmethionyl tRNA. the initiation codon (AUG). initiation factors, and the ribosomal subunits. Elongation occurs (using several elongation factors) with the aminoacyl-tRNA5 being selected by recognition of their specific codons and forming new peptide bonds with neighboring amino acids. When biosynthesis of the specific protein is finished, a termination codon in the mRNA is recognized, and release factors disengage the protein from the elongation complex. Finally, the protein is folded and posuranslational processing occurs.51' Processes thai might be used in this step include removal of initiating residues and signaling sequences. proteolysis. modification of terminal residues, and attachment of phosphate, methyl. carboxyl, sulfate, carbohydrate, or prosthetic groups that help the protein achieve its final three-dimensional shape. Spe-
Chapter 6 •
and Drug 1)iscoi'eri
163
Transcription
DNA
Pre-mRNA
1
/ ProteIns ReplIcation
Modification
Posttranslationally modified
!3in fIgure 6-4 u Path from DNA to pro-
can also direct the three-dimenformation. Posttranslational modifications occur in cells in the endoplasvnic reticulum or the Golgi the protein is transported out of the ccli. posuranslational modifications occur only in higher
jli,ed chaperone proteins •
nun in bacteria. The three-base genetic codon sysc well known and has been conserved among all organ-
This allows rDNA procedures to work and facilitates he dcselopmcnt of a model for the amino acid sequence by correlation with the codon sequence of the ome,
Recombinant DNA (rONA) Technology fin
techniques involved in working with rDNA solaring or copying a gene; inserting the precise
gene into a transmissible vector that can be tr,rnscrihed. amplified, and propagated by a host cell's biochemical machinery; transferring it to that host cell; and facilitating the tran-
scription into mRNA and translation into proteins. Cloned DNA can also be removed or altered by using an appropriate restriction endonuclease. Since genes encode the language
of proteins, in theory it is possible to create any protein if one can obtain a copy of the corresponding gene. rDNA methods require:83 • An efficient method for cleaving and rejoinhlig pliosphodiester bonds on Iragments of DNA (genes) derived from an array ot different sourecs • Suitable vectors or carriers capable of replicating both themselves and the foreign DNA linked to them • A means of introducing the rDNA into a bacterial. yeast, plant. or mammalian cell
164
Wilco,, asul Gi.svold.c T thoak of Organic Medicinal and Pharmaceutical clwmis,rv
• Procedures for screening and selecting a clone of cells that has acquired the rDNA motceule from a large jxpulation of
mRNA Reveso Transcripisie
cells
There are two primary methods for cloning using genomic and eDNA libraries as the primary sources of DNA fragments, which, respectively, represent either the chromosomal DNA of a particular organism or the eDNA prepared from mRNA present in a given cell, tissue, or organ. In the first method, a library of DNA fragments is created from a cell's genomc. which represents all of the genes present. The library is then screened against special DNA probes. Lysing the genomic contents to generate fragments of different sizes and compositions. some of which should contain the genetic sequences that encode the specific activity that one is seekitig. creates the library. With knowledge of the protein sequence that the gene specifies. DNA probes can be synthe-
sized that should hybridize with corresponding fragments itt the library. By labeling the probes with fluorescent or radioactive tags, probe molecules that hybridize and fonn double-helical DNA can he identified and isolated electrophoretically. The DNA from the library can then be amplified by a technique such as the polymerase chain reaction (PCR). inserted into a vector, and transferred in(s) a host cell.
A comparison of these methods is given in Table 6-I The second major method for cloning DNA represents only gene.s that are being expressed54 at a given time and involves first the isolation of the mRNA that encodes the amino acid sequence of the protein of interest. Treating the tuRNA with the viral enzyme reverse transcriptase in the presence of nucleoside triphosphates (NTPs) causes a strand
of DNA to be synthesized complementary to the mRNA mains, affording a RNA—DNA hybrid, The RNA strand is
DNA Synthesis, mRNA as Matrix
RNA'DNA Hybrid I
Bsse RNAt
DNA
CONA P1151
9(OtYIO(er F55105
Figure 6—5 • Method for preparation of eDNA from a mRNA transcript.
relatively small proteins. In principle. for the preparation of a genomic library the cellular origin of the DNA is not an issue, whereas the cellular origin of mRNA is central to the preparation of a cDNA library. Therefore. genornic libraries vary from species to species but not from tissue to tissue within that species. eDNA varies with tisstte and the developmental stages of cells, tissues, or species. Another iuliportant distinction is that the fragment of DNA from eukaryotic chromosomes will contain exons (protein coding segments) and introns (noncoding segments between exons). whereas
in eDNA the introns are spliced out.
broken down in alkaline conditions, yielding a singlestranded molecule of DNA. The DNA polymerase reaction affords a complementary or copy strand of DNA (eDNA). which on lusion of a proniotor sequence can be attached to a transport vector. Figure 6-5 depicts these reactions. If the amino acid sequence of a protein (and, hence, the codon sequence) is known, automated synthesis of DNA through chemical or enzymatic means represents a third way that genes can be engineered, This method is useful only for
CharacterIstics of Genomic Versus cDNA Libraries TABLE 6-1
CharacteristIc Source of genetic material
Genomic Gcnomic DNA
cDNA Coil or tissue nsRNA
Comptesity tindependent rocumbinants)
'tOO.OaO
5000—20.000
Sue range of ,ecumbiuiaiits bp'
1.000—50.000
30—10.000
Yes
No
Yes
Muybc
Maybe
Yes
at introits Presence of regulatory elements fur iselerniogtius CSpreSSiOfl
hp. toe paü..
85
Restriction
The restriction endonuclcasc (or restriction enzyme) is probably best described as a set of "molecular scissors" in nature. Restriction endonucleases are bacterial enzymes thaL us the name implies, cleave internal phosphodiester bonds of a DNA molecule. The cleavage site on a segmetit of DNA
lies within a specific nucleotide sequence of about six to eight base pairs. More than 5(X) restriction endonucleases have been discovered, and these react with more than 100 different cleavage sites. The chemical reaction of the restriction endonuclease releases the 3' end of one base as an alco. hol and the 5' end as a monophosphate. The general reaction
is shown in Figure 6-6. The recognition sites for restriction endonucleases arc specific palindromic sequences of DNA55 not more than 8 bp long. A number of these palindromes are listed in Table 6-2. A palindrome is a sequence of letters that reads the same way forward and backward, for itistance: "A man, a plan, a canal: Panama!," "DNA-land," "Did Hannah see bees? Hannah did," Restriction endonucleases cleave DNA at palindromic sites to yield several types of cuts: 5' CCTAGG 3' 3' GGATCC 5'
—.
5' CCTAG 3' 3' GATCC 5'
The above cut yields an overhanging C/C "sticky" end that
Chapter 6 • AiowtI:,zulogs and Drug D,seoten
165
vector and insert. A total of four such bonds must be reformed, two on each strand at the 5' and 3' sites. This process is termed ligation, and the enzymes that catalyze the reaction
are named DNA I/gases. Typically. ATI' or another energy source is required to drive the ligation reaction, and linker fragments of DNA are used to facilitate coupling. There are several different types of ligation reactions that are used. depending on the type of restriction endonuclease product that was formed. The sticky-ended DNA. using complementary vector and insert ends that easily base pair at the cuts, is probably the easiest to accomplish, although methods exist
to ligate the blunt-ended varieties, using DNA ligase.
The Vectors' There are several methods available for introducing DNA into host cells. DNA molecules that can maintain themselves
FIgure 6-6 • Mechanism of a restriction endonuclease reac'Jon,
is relatively easy to
DNA molecules. c' CCIGG GGTCC
3' — 5'
5' CC GG 3' 3' GG CC 5'
HO in the field of genetic engineering. About the only caveat
is an obvious one: they must be chosen to not
rake their cut inside the gene of interest. DNA When DNA
the gene of interest has been excised from its flanking the appropriate restriction endonucleases and the
DNA has been opened (using the same restriction to break phosphodiester bonds), the two differDNA molecules are brought together by annealing. In he first step of this process, heating unwinds the doubleDNA of the vector. The insert or passenger DNA
ciitlomiclea..c
added to the heated mixture, and subsequent cooling faciliales pairing of complementary strands. Then. phosphodies-
hands are regenerated, linking the two DNA molecules.
TABLE 6-2
AM AGICI
t'.RV
elements such as plasmids or viruses that can he propagated and that have been engineered so that they can accept frag-
ments of foreign DNA. Depending on the vector, they may have many other features, including multiple cloning sites (a region containing multiple restriction enzyme sites into
ligate with complementary ends of other
A cut from an endonuclease like HaeIll:
orate complicated methods to ligate into vector DNA Palindromic cleavage sites for some selected re.stricion enoymcs are given in Table 6-2. The arrow shows the deasage site. The restriction endonucleases are a robust nmup of enzymes that form a toolbox for investigators worko their use
by replication are called rep/irons. Vectors are subsets of replicons. In genetic engineering, the vector (carrier) is the most widely used method for the insertion of loreign. or passenger, genetic material into a cell. Vectors arc genetic
which an insert can be installed or removed), selection markers, and transcriptional promoters. The passenger l)NA must integrate into the host cell's DNA or be carried into the cell
a.s part of a biologically active molecule that can replicate independently. If this result is not achieved, the inserted gene
will not be successfully transcribed. The most conimonly used biological agent for transporting genes into bacterial and yeast cells is the plasmid. such as the E. roll bacterial plasmid pBR322. A plasmid is a small, double-stranded. closed circular extrachromosomal DNA molecule. This plasmid contains 4,361 hp and can transport relatively small amounts of DNA. Plasmids occur in many species of bacteria and yeasts. Sometimes, plasmids carry their own genes. e.g..
the highly transmissible genes for antibiotic resistance in some bacterial species. An important feature of a plasmid is that it has an origin of replication (on) site that allows it to multiply independently of a host cell's DNA. Although there can be more than one copy of a plasmid in a cell, the copy number is controlled by the plasmid itself. Another type of cloning vector is the bacteriophage (Fig. 6-7). Bacteriophage A (lambda) possesses a genome of approximately 4.9 X bp and catt package large amounts
of genetic material without affecting the intèctivity of the phage. A large DNA library can be created, packaged in
PalIndromic Clea vage Sites
Mull llarttl
Ball
BamHl
BgIll
TGGJCCA
GJ.GATCC
A.kGATCT
Clat ATi.CGAT
Hhal
Hindlt
/lindltt
llpalI
A1.AGCTF .1151
Pal
l'vul
Sail
Sniat
G.L'I'CGAC
CCCJAJGG
EcoRl
GCTAC.kC Xmflt
Noit GCLGGCCGC
166
Wilso,j and Gia void's Textbook of Organic Medicinal and Pham,aieuiical Clu',nis:rv
EcoRl Resl,lctlon Stte
Left Arm
Right Arm
U A DNA 48,502 base palm
AntibIotIc ResIstance Gene BacterIophage A
OrigIn of ReplIcation EcoRl RestrIction She
E. coil plasmid pBR322
4,361 base paIrs
bacteriophage A. and when the virus infects, inserted into cells. Hybridization is then detected by screening with DNA In addition, there are special vectors called
phagemids, vaccinia and adcnovirus for cloning into mammalian cells, and yeast artificial chromosomes (YACs) that facilitate cloning in yeasts.55 Differences among these vectors concern the size of the insert that they will accept. the methods used in the selection of the clones, and the procedures for propagation. Once the passenger DNA has been created and the plasmid vector cut (both with the same restriction enzyme), the insert
is ligated into the plasmid along with a promoter (a short DNA sequence that enhances the transcription of the adjacent gene). Often, a gene imparting antibiotic resistance linked to the desired gene is inserted usa selection tool. The idea behind this is that if the gene is inserted in the proper location, the bacterial cell will grow on a medium containing
the antibiotic. Bacteria that do not contain the resistance gene and, hence, luck the required gene will not grow. This makes the tusk of screening for integration of the desired gene easier. After the molecule is ligated, the vector is finally a rDNA molecule that can be inserted into a host cell. Host cells can be bacteria (e.g.. E. co/i). eukaryotic yeast (Sacclzaromyces cerevisiae). or mammalian cell lines includ-
ing Chinese hamster ovary (CHO), African green monkey kidney (VERO). and baby hamster kidney (BHK). It is easy to grow high concentrations of bacteria and yeast cells in fermenters to yield high protein concentrations. Mammalian cell culture systems typically give poorer protein yields, but sometimes this is acceptable. especially when the product demands the key posttranslational modifications that do not occur in bacteria. Host cells containing the vector are grown
Figure 6—7 • Types of cloning vectors: a bacteriophage and a plasmid.
in small-scale cultures and screened for the desired When the clone providing the best protein yield is located. the organism is grown under carefully controlled conditions and used to inoculate pilot-scale fermentations. Parameterc such as production medium composition. pH. aeration. agita. tion. and temperature are investigated at this stage to opti. mize the The host cells divide and the plasmids in them replicate, producing the desired "new" protein. The lermentation is scaled up into larger bioreactors for large. scale isolation of the recombinant protein. Obviously, the cultures secrete their own natural proteins along with the cloned protein. Purification steps are required before the conibinant protein is suitable for testing as a new. genetically engineered pharmaceutical agent. Once the host cell line expressing the recombinant gene is isolaLed. it is essential to maintain selection pressure on it so that it does not spontaneously lose the plasmid. Typically, this pressure is applied by maintaining the cells on medium containing an antibiotic
to which they bear a resistance gene.
SOME TYPES OF CLONING A listing of some types of cloning is given in Table 6-3.
Functional Expression Cloning° Functional expression cloning focuses on obtaining a cific eDNA of known function. There are many variations on this approach, but they all rely on the ability to search for and isolate cDNAs based on some functional activity
Chapter 6 • Birizeclpwlogv and Drug Discovers'
167
TABLE 6-3 Cloning Strategies Strategy
Advantages
Disadvantages
l&,iusn.,l
Prides information underlying the genetic basis of known discases
3.3 X 0" bp. difficult to Use with diseases caused by multiple interacting alleles
Yields genetic information encoding proteins at known stntcwrc and function
Protcin purilication. especially fur low.abuitdance proteins. is exacting; availability of npprnpnate libraries. incomplete coding sequences
Yields genetic information encoding proteins of known structure and function
Involves protein puritication. unrccognited cross-reactivity. incomplete coding sequences
Vitamin I) receptor
ctplessinn
Yields genetic information encodlitg a tunctianalty uctise protein; does nor require protein purification
Function must be compatible wIth esisting lihraiy.scrcening technology
Substance K receptor
ba..e,J
Idetilllicaiton 01' related genes or gene (antilles: relatively simple
Depends on prvctisting gene sequence; can yield incomplete gene.s or genes of unknown (unction
Muscannic receptor
sequence tagging
High throughput; Identification of
lrtcotitplete coding sequences or genes of
tstscd
sequencing
Knowledge of total genome;
Cystic fibrosis gene product receptor
unknown function
novel cDNA.s
loijl
Example
bp. labor Intensise; genes of unknown function
3.3
identification oiatl potential gene
Ha,'mcplsiluv influc'nzae
products
measured. e.g.. the electrophysiological measuresent oF ion conductances following expression of cDNAs hat can be
1mg
By incrementally subdividing the cDNAs
into pools and following the activity, it is possible to obtain single cDNA clone that encodes the functionality. The
of functional expression cloning is that it does not rely on knowledge at' the primary amino acid sequence. This is a definite advantage when attempting to clone proems of low abundance.
Positional Cloning90 Positional cloning can be used to localize fragments of DNA rcpre.senling genes prior to isolating the DNA. An example it
he use of positional cloning is the cloning of the gene for cystic fibrosis (CF). By studying the patterns
ii tnherilatice of' the disease and then comparing these with known chromosotnal markers (linkage analysis), it was possihk. without knowing the function of the gene, to locate the gene on human chromosome 7. Then, by using a technique known as chrin,u,so,nc wcslkint,'. the gene was localized to
DNA ceqttcnce that encodes a protein now known as the fibrosis transmembrane conductance regulator CFTR). This prctlein. previously unknown, was shown to in CF patienls and could account for many of the of lhc disease. Like functional cloning, positional dosing has the advantage that specific knowledge of the p101cm is not required. It is also directly relevant to the understanding of human disease, and it can provide imporant new biological largels for drug development and the c)stic
ogv-hased cloning, takes advantage of the fact that nucfeotide sequences encoding important functional domains of proteins tend to be conserved during the process of evolulion. Thus. nucicotide sequences encoding regions involved with ligand binding or enzymatic activity can be used as probes that will hybridize to complementary nucleotide sequences that may be present on other genes that bind similar
ligands or have similar enzymatic activity. This approach can be combined with PCR91 Ia amplit'y the DNA sequences. The use of homology-based cloning has the advantages that it can be used to identify families of related genes. does not rely on the purification or functional aetivily of a given protein, and can provide novel targets for drug discovery. Its usefulness is offset by the possibility Ihat the isolated fragment may not encode a complete or functional protein or that in spite of knowledge of the shared sequence, the actual function of the clone may be difficult to identify.
EXPRESSION OF CLONED DNA Once cloned, there are many different possibilities for the expression and manipulation of DNA sequences. As if concerns the use of cloned genes in the process oidrug discovery and development, there are many obvious ways in which the
Another cloning slrategy involves the use of previously doted genes to guide identification and cloning of evolu-
expression of DNA sequences can be applied. One ol the most obvious is in replacement of older technologies that involve the purification of proteins for human use from eilhcr animal sources or human by-products, such as blood. An example of this is factor VIII. a clotting cascade protein used for the treatment of the genetically linked bleeding disorder hemophilia. Until recently, the only source of purified factor VIII was human blood, and tragically, before the impact of AIDS was fully appreciated, stocks of factor VIII had be-
tiumitarily related genes. This approach, referred to as hon:ol-
come contaminated with HIV-l. resulting in the infection
tIcalment of disease.
Homology-Based Cloning
168
W,Iso,, innl
Texihin;k of
Medicinal and Pharmuceu;jcaj
of as many as 757 of (he patients receiving this product. The gene encoding factor VIII has since been cloned, and recombinant factor VIII is now available as a product purified from cultured mammalian cells. Other recombinant clotting factors, including factors Vila and IX, are under devel-
opmcnt and, together with recombinant factor VIII, will eliminate the risk of exposure to human pathogens. Other examples in which the expression of cloned human genes offers alternatives to previously existing products in-
clude human insulin, which is now a viable replacement for purified bovine and porcine insulin for the treatment of diabetes, and human growth hormone, which is used for the treatment of growth hormone deficiency in children (dwarf-
ism). Unlike insulin, growth hormones from other animal species are ineffective in humans: thus until human recombinant growth hormone became available, the only source of human growth hormone was the pituitary glands of cadavers. This obviously limited the supply of human growth hormone and, like factor VIII, exposed patients to potential contamination by human pathogens. Recombinant human growth
host organism are a few. An example of the compatibility issue is that bacteria do not process proteins in exactly the same ways as do mammalian cells, so that the expression of human proteins in bacteria will not always yield an active product or any product at all. Cases like these may require expression in mammalian cell cultures. The choice of an expression system also reflects the available vectors and corresponding host organisms. A basic requirement for the bet.
erologous expression of a cloned gene is the presence of a promoter that can function in the host organism and a mechanism for introducing the cloned gene into the organ. ism. The promoter is the specific site at which DNA poly. merase binds to initiate transcription and is usually specific for the host organism. As in gene cloning, the vectors are either plasmids or viruses that have been engineered to ac-
hormone can now be produced by expression in bacterial
cept rDNA and that contain promoters that direct the expression of the rDNA. The techniques for introducing the vector into the organism vary widely and depend on whether one is interested in transient expression of the cloned gene or in stable expression. In the latter case, integration into the host genome is usually required; transient expression simply re-
cells.
quires getting the vector into the host cell.
The expression of cloned genes can be integrated into rational drug design by providing detailed information about the structure and function of the sites of drug action. With the cloning of a gene comes knowledge of the primary amino acid sequence olan encoded receptor protein. This infonnalion can be used to model its secondary structure and in an
initial attempt to define the protein's functional domains. such as its ligand-binding site. Such a model can then serve as a basis for the design of experiments that can be used to test the model and facilitate further refinement. Of particular use are mutagenesis experiments that use rDNA techniques to change a primary amino acid sequence so that the consequences can be studied. In addition, expression of a cloned target protein can be used to generate samples for various biophysical determinations, such as x-ray crystallography.
This technique, which can provide detailed infonnation about the three-dimensional molecular structure of a protein, frequently requires large amounts of protein, which in some cases is only available with the use of recombinant expression systems. Like the many strategies used to clone genes, there are
many strategies for their expression, involving the use of either bacterial or eukaryotic cells and specialized vectors compatible with expression in host cells. Since these cells do not normally express the protein of interest, this methodology is often referred to as hetcrologou.s It is also possible to prepare cRNA from rDNA. which can then be used for either in vitro expression or injection directly into cells. In the former situation, purified ribosomes are used in the test tube to convert eRNA into protein: for the latter situation, the endogenous cellular ribosomes make the protein. A relatively new development for the expression of cloned genes is the use of animals that have the cloned gene stably integrated into their genome. Such transgenic animals can potentially make very large amounts of recombinant protein. which can be harvested from the milk, blood, and ascites fluid. The choice of a particular expression system depends on a number of factors. Protein yield, requirements for biological activity, and compatibility of the expressed protein with the
MANIPULATION OF DNA SEQUENCE INFORMATiON Perhaps the greatest impact of rDNA technology lies in ability to alter a DNA sequence and create entirely new inol. ecules that, if reintroduced into the genome, can be inherited
and propagated in perpetuity. The ability to alter a DNA sequence, literally in a test tube, at the discretion of an individual. corporation. or nation, brings with it important ques-
tions about ownership, ethics, and social responsibility. There is no question, however, that potential benefits to the treatment of human disease are great. There are three principal reasons for using rDNA technology to alter DNA sequences. The first is simply to clone the DNA to facilitate subsequent manipulation. The second is to intentionally introduce mutations so that the site-specific effect on protein structure and function can be The third reason is to add or remove sequences to obtain some desired attribute in the recombinant protein. For exam-
ple, recent studies with factor VIII show that the pmlein contains a small region of amino acids that are the major determinant for the generation of anti—factor VIII a human immune system. This autoimmune response of the patient inhibits the activity of factor V ill, which is oh. viously a serious therapeutic complication for patients who are using factor VIII for the treatment of hemophilia. altering the DNA sequence encoding this determinant, however, the amino acid sequence can be changed both to reduce
the antigenicity of the factor VIII molecule and to make ii transparent to any existing anti—factor VIII antibodies (i.e.. changing the epitope eliminates the existing antibody nition sites). It is possible to combine elements of two proteins into one new recombinant protein. The resulting protein. refencd to as a chimerk protein, may then have some of
the functional properties of both of the original proteiws
Chapter 6 • Biowch,wiogv and
169
protein can then be identified by immunofluorescence or can be purified with antibodies that recognize the cpitope.
hgarid-bindrng doinams
NEW BIOLOGICAL TARGETS FOR DRUG DEVELOPMENT Receptor A
Receptor B
Receptors
Figure 6—8 • Chimeric receptors.
One of the outcomes of the progress that has been made in the identification and cloning of genes is that many proteins encoded by these genes represent entirely new targets for drug development. In some cases, the genes themselves may represent the ultimate target for the treatment of a disease in the form of gene therapy. The cloning of the cystic fibrosis gene is an example of both a new drug target and a gene that could potentially be used to treat the disease. The protein encoded by this gene. CFTR. is a previously unknown integral membrane protein that functions as a channel for chloride ions. Mutations in CFTR underlie the pathophysiology of cystic fibrosis, and in principle, replacement or coexpression of the defective gene with the healthy, nonmutated gene would cure the disease. Ii is also possible, however, that by
understanding the structure and function of the healthy This is illustrated in Figure 6-8 for two receptors labeled A B. Each receptor has functional domains that are responfor ligand binding, integration into the plasma mem-
and activation of intracellular signaling pathways. Using rDNA techniques, one can exchange these functional Jomains to create chimeric receptors that, for example, con-
tin the ligand-hinding domain of receptor B but the transand intracellular signaling domains of receptor A The application of the fusion protein strategy is discussed uniter in connection with the human growth hormone recep-
:w under the heading. Novel Drug-Screening Strategies)
CFTR, drugs could be designed to interact with the mutated CFTR and improve its function. An important outgrowth of the study of new drug targets is the recognition that many traditional targets. such us enzymes and receptors. are considerably more heterogeneous
than previously thought. Thus, instead of one enzyme or receptor. there may be several closely related subtypes. or isoforms. each with the potential of representing a separate drug target. This can be illustrated with the enzyme cyclooxygena.se (COX). which is pivotal to the formation of prostaglandins and which is the target of aspirin and the nonsteroi-
dal anti-inflammatory agents (NSAIDs). Until recently.
md with denileukin diltitox. Another reason for combining elements of two proteins
COX was considered to be a single enzyme. but phannacological and gene cloning studies have revealed that there are at least three enzyme forms, named COX- I. COX-2. and
protein is to facilitate its expression
COX-3. Interestingly, they are differentially regulated.
jiul purification. For example, recombinant glutathione Sirmuferase GST), cloned from the parasitic worm Schistos'unu japtnuicsusn, is strongly expressed in E. cvii and has a
COX-l is expressed constitutively in many tissues, whereas the expression of COX-2 is induced by inflammatory processes. Thus, the development of COX-2 selective agents can yield NSAIDs with the same efficacy u.s existing (nonselective) agents but with fewer side effects, such as those on the gastric mucosa. The elucidation of the family of adrenergic receptors is another example in which molecular cloning studies have revealed previously unknown heterogeneity, with the consequence of providing new targets for drug development. The adrenergic receptors mediate the physiological effects of the catecholanmines epinephrine and norepincphrinc. They arc also the targets for many drugs used in the treatment of such conditions as congestive heart failure, asthma, hypertension. glaucoma. and benign pmstatic hyperurophy. Prior to the molecular cloning and purification of adrenergic receptors. the pharmacological classification of this hitmily of receptors consisted of four subtypes: a1. a2. flu. and The initial cloning of the a-adrenergic receptor in 1986 and subsequent gene cloning studies revealed at least nine subtypes: flu. a8. (Chapter 16). a2A, a2n, and The evidence that there are nine subtypes of adrenergic receptors is very important in terms of understanding the
Into OUC
binding site for glutathione. Hctcrologous sequences encodIng the functional douitains from other proteins can be fused, in mime. to the carboxy terminus of GST. and the resulting fusion protein is ofhen expressed at the same levels as GST
itself. In addition, the resulting fusion protein still retains the ability to hind glutathione, which means that affinity using glutathione that has been covalently bonded to agarose. can be used for a single-step purification of he fusion protein. The functional activity of the helerolocims domains that have been fused to GST can then be studmed either as part of the fusion protein or separately following treatment of the fusion protein with specific proteases that deave at the junction between GST and the heterologous duntain. Purified fusion proteins can also he used to generate ,dtIIbodies to the heterologous domains and for other bio-
studies. Sometimes, fusion proteins are made to pauvide a recombinant protein that can be easily identitied. n esample of this is a technique called episope tagging. in mshich well-characterized antibody recognition sites are fused with recombinant proteins. The resulting recombinant
170
tViiwn and Gi.c paid's
of Organic Medicinal and Pharmaceutical Chemistry
TABLE 6-4 Selected Examples of Receptor Subtype Heterogeneity Original Subtype,
Receptor Superfamily
Present Subtypes
C..prmcin.it>upted Up,
app. 02A. a213. 02C'.
02
Dopaminc
D1.D2
D1.D2',D3.D4.D5
Prostaglandin
El'1. El'2, EP1
El'1, El'2.
Nerve growth factor receplor
TrkA.'TrkB, TTIIC
Receptor tyrosine kinase ncurolxuphiris
El'4
DNA binding Estrogen
Estrogen receptor
ERRI. ERR2
Thyroid hormone
Thyroid honnone receptor
TRa,
Retinoic acid
Retinoic acid receptor
RARa.
(ilycinc
Glycinc and/or strychnine receptor
up. a2,
GAI3A4
GABA and/or beneodiazepine receptor
Ligund.aclivatcd channels (muitisubunit)h
a2. Op.
a*. a., (rnullisobpinit)5
revnptor hetcrogencily. inKNA .p!icine creates Only the hehruyrncny Iigltnd.hrnding subunit is Ii.,Icd.u mului.ubunpt stmruufc combined with the hctceogetepty oliheother subunits emotes, vciy lunge numbcro(ps Icilitlit sUhiSl'.,..
physiology of the adrenergic receptors and of developing drugs that can selectively interact with these subtypes. For example, in the case of the a2-agonist p-aminoclonidine, an agent used to lower intraocular pressure (lOP) in the treatment of glaucoma, it may now be possible to explain some of the drug's pharmacological side effects (e.g.. bradycardia and sedation> by invoking interactions with the additional receptor subtypes. Of considerable interest is the possibility that these pharmacological effects (i.e., lowering of lOP, bradycardia, and sedation) are each mediated by
one of the three different cz2-receptor subtypes. If this is true, it might be possible to develop a subtype-selective a2agonist that lowers lop but does not cause bradycardia or sedation. Likewise, it might even be possible to take advan(age of the pharmacology and develop a2-adrenergic agents that selectively lower heart rate or produce sedation. The discovery of subtypes of receptors and enzymes by molecular cloning studies seems to be the rule rather than
the exception and is offering a plethora of potential new drug targets (Table 6-4). To note just a few: 5 dopamine receptor subtypes have been cloned, replacing 2 defined pharmacologically (Chapter 15); 7 serotonin receptor subtypes have been cloned, replacing 3; 4 genes encoding recep-
tors for prostaglandin E2 have been isolated, including 12 additional alternative mRNA splice variants; and 3 receptors for nerve growth factor have been cloned, replacing I.
from more complex native biological systems. There is a reason for this. A newly identified protein can be expressed
in isolation. Even for closely related enzyme or receptor subtypes, heterologous expression of the individual subtype can potentially provide data that are specific for the subtype
being expressed, whereas the data from native biological systems will reflect the summation of the individual subtypes that may be present. The potential advantage of heterologous expression is
lustrated in Figure 6-9 for the interaction of a drug with multiple binding sites. In panel A. which can represent 11w data obtained from a native biological system, the data air complex, and the curve reflects interactions of the drug with two populations of receptors: one with high affinity. rcpre. senting 50% of the total receptor population, and one with low affinity, representing the remaining 50%. The individual contributions of these two populations of receptors are mdicated in panel B, which could also reflect the data obtained if rDNA encoding these two receptors were expressed mdi vidually in a heterologous expression system. Although in some cases the data, as in panel A. can be analyzed with succes.s. frequently they cannot, especially if more than two subtypes are present or if any one subtype makes up less than 10% of the total receptor population or if the of the drug for the two receptor populations differ by less than 10-fold. Another important reason for integrating heterologous expression into drug-screening strategies is that data can usu-
The combination of the heterologous expression of cloned DNA, the molecular cloning of new biological targets, and
ally be obtained for the human target protein rather than an animal substitute. This does not mean that organ prepara tions or animal models will be totally replaced. For the purposes of the identification of lead compound.s and the
the ability to manipulate gene sequences has created power-
zation of selectivity, affinity. etc.. however, the use
ful new tools that can be applied to the process of drug
recombinant expression systems provides some obvious ad-
discovery and development. In its most straightforward application, the ability to simply express newly identified receptor protein targets offers a novel means of obtaining information that may be difficult, or even impossible, to obtain
vanlage.s.
NOVEL DRUG-SCREENING STRATEGIES
By combining heterologous expression with novel functional assays, it is possible to increase both specificity and throughput (the number of compounds that can be screened
Chapter 6 • Biozech,wlogy and Drug Discm'ery
100
a
171
cAMP response element (CRE). This is a specifically defined sequence of DNA that is a binding site for the cAMP response element-binding (CREB) protein. In the unstimulatcd condition, the binding of CREB to the CRE prevents
multiple binding
80
lsmntenschi
the transcription and expression of genes that follow it (Fig. 6-10). When CREB is phosphorylated by cAMP-dependent protein kinase (PKA). however, its conformation changes.
permitting the transcription and expression of the down2:
°' -14
-12
-10
-8
6
-4
log (Drug) (M)
A 100
analyzed as singe-site interactions 60
Ut
C
dent fashion if it is placed downstream of a CRE, using rDNA techniques. If the products of the expression of the
50%
20
stream gene. Thus, increases in intracellular cAMP, such as those caused by receptors that activate adenylyl cyclase (e.g., 48-adrenergic, vasopressin. and many others), will stimulate the activity of PKA. which, in turn, results in the phosphory. lation of CREB and the activation of gene transcription. In nature, there are a limited number of genes whose activity is regulated by a CRE. Biologically, however, the expression of almost any gene can be regulated in a cAMP-depen-
\
\
,.,., 42
'10
50% low aitnity
I3nM
tosidase are three examples of potential "reporter genes"
r -8
-6
-4
log [Drug)
B
downstream gene can be easily detected, they can serve as reporters for any receptor or enzyme that can modulate the formation of cAMP in the cell. The genes encoding chloramphenicol acetyl transferase (CAT), luciferase, and f3-galac-
Figure 6—9 • Convoluted data from binding to multiple recepor sobtypes versus classic mass action.
Nrunil time). For example, reporter genes have been develirpcd that ro.pond to a variety of intracellular second messengru. ssch as the activation of guanine nucleotide-binding pwtems 4G proteins), and levels of cAMP, or calcium. One to the development of novel functional assays inwIves the use of promoter regions in DNA that control the
of genes. This approach is exemplified by the
whose products can be easily detected. Sensitive enzymatic assays have been developed for all of these enzymes; thus
any changes in their transcription will be quickly reflected by changes in enzyme activity. By coexpressing the reporter gene along with the genes encoding receptors and enzymes that modulate cAMP formation, it is possible to obtain very sensitive functional measures of the activation of the coexpressed enzyme or receptor. Another example of the use of a reporter gene for highthroughput drug screening is the receptor selection and amplification technology (r-SAT) assay. This assay takes ad-
vantage of the fact that the activation of several different classes of receptors can cause cellular proliferation. If genes for such receptors are linked with a reporter gene, such as
ORE-binding Protein (CREB)
off
,7CRE
Reporter Gene
CAMP Response Element
r
Figure 6—10 • Activation of transcripb1 a CAMP response element (CRE) o phosphorylated by CAM P-depenprotein kinase.
I
CRE
Reporter Gene
172
Wilson and Gi.c void's Texthook of Oria,iic Medicinal and Pharmaceutical Che,uistr.
/3-galactosidase, the activity of the reporter will be increased as the number of cells increase as a consequence of receptor
activation. Initially, a limitation of this assay was that it only worked with receptors that normally coupled to cellular by making a mutation in one of the secondmessenger proteins involved with the proliferative response, however. it was possible to get additional receptors to work in
this assay. This second-messenger protein. Gq. was
cloned, and a recombinant chimera was made that included
part of another second messenger known as C. In native cells, receptors that activate G1 arc not known fur their stimulation of cell proliferation, but when such receptors are coexpressed in the r-SAT assay with the chimeric C5. their activity can be measured,
A similar strategy involving chimeric proteins has been used for receptors whose second-messenger signaling pathways are not clearly understood. For example, the develop-
ment of potential therapeutic agents acting on the human growth hormone receptor has been difficult because of a lack of a good signaling assay. The functional activity of other receptors that arc structurally and functionally related to the growth hormone receptor can be measured, however. in a cell prolilerauon assay. One such receptor that has been cloned is the murine receptor for granulocyte colony-stimu-
lating factor (G-CSF). By making a recombinant chimeric receptor containing the ligand-binding domain of the human growth hormone receptor with the second-messenger—coupling domain of the murcin G-CSF receptor, it was possible to stimulate cellular proliferation with human growth hormone. In addition to providing a useful pharmacological screen for human growth hormone analogues, the construction of this chimeric receptor provides considerable insight into the mechanism of agonist-induced growth hormone receptor ac-
tivation. The growth hormone—binding domain is clearly localized to the extracellular amino terminus of the receptor. while the rransmembrane and intracellular domains are implicated in the signal transduction process. It was also determined that successful signal transduction required receptor dimerization by the agonist (i.e.. simultaneous interaction of two receptor molecules with one molecule of growth hormone). On the basis of this information, a mechanism-based strategy was used for the design of potential antagonists. Thus, human growth hormone analogues were prepared that were incapable of producing receptor dimerization and were found to be potent antagonists.
crated by an infected cell line, or introduced by animal serum. Purification of a rDNA protein while maintaining the factors that keep it in its active three-dimensional conforma. tion from this mixture may be difficult because each step must be designed to ensure that the protein remains intact and pharmacologically active. Assays must be designed that allow the activity of the protein to be assessed at each purification step. Consequently, the structure and activity of the recombinant protein must be considered at all stages of puti.
fication. and assays must be conducted to measure the amount of purified, intact protein. A general scheme for purification of a rDNA protein is as follows:95
• Particulate removal. Particulates may be removed by centrifugutioil. tiltration. ultrafiltration, and tangential flow filtration. Virus particles may be inactivated by heating if the rDNA peptide can tolerate the procedure. • concentration. The volume of the mixture is reduced, which increases the concentration of the contents. Often. conccntrulion is achievable by the filtration step, especially if ultrafiltu. lion is used.
• Initial purification. l'he initial purification of the mixture is sometimes accomplished by precipitation of the proteins. using a slow. stcpwisc increase of the ionic strength of the solution (salting out). Ammonium sulfate isa typical salt that can be used in cold, aqueous solutions. Water-miscible organic solvents such as trichloroacetic acid and polyethylene glycol change the dielectric constant of the solution and also effect
precipitation of proteins. • Intermediate purification. In this stage, the proteins may be dialyzed against water to remove salts thai were used in the precipitation step. Ion exchange chromatography is used to effect a somewhat crude separation of the proteins based on their behavior in a pH or salt gradient on the resin. Anothci step that may be taken is size exclusion (gel filtration) chroma-
tography. Gels of appropriate molecular weight cutoffs can yield a somewhat low-resolution separation of proteins of desired molecular weight. If a native bacterial protein that has been corned this far is nearly the same molecular weight as
the rONA protein, no separation will occur. • Fi,,al purification. Final purification usually involves the use of high-resolution chromatography, typically high-perfw. mance liquid chromatography. An abundance of commercial stationary phases allows various types of adsorption Chromatography (normal and reversed phase), ion exchange chroma-
tography. immunoaffinity chromatography, hydrophobic interaction chromatography, and size exclusion chin. matography. The protein fractions arc simply collected when they elute from thc column and are concentrated and assayed
PROCESSING OF THE RECOMBINANT PROTEIN Processing the fermentation contents to isolate a recombinant protein is often a difficult operation, requiring as much art as science. In the fermentation broth are whole bacterial cells. lysed cells, cellular fragments. nucleotides. normal bacterial proteins, the recombinant protein, and particulate medium components. If a Gram-negative bacterium such as E. coil has been used. lipopolysaccharide endotoxins (pyrogens) may be present. When animal cell cultures are used, it is commonly assumed that virus particles may be present. Viruses can also be introduced by the culture nutrients, gen-
for activity. • Sterilization and formulation. This step can be accomplished by ultrafiltration to remove pyrogens or by heating ii the protein can withstand this. Formulation might involve reconstitution into stable solutions for administration or determining Its optimum conditions for stability when submitting for clinical trials.
Complicating factors include (a) proteins unfolding into an inactive conformation during processing (it may not be possible to refold the protein correctly) and (b) proteases that are commonly produced by bacterial, yeast. and mammalian
cells, which may partially degrade the protein.
Chapter 6 • Rioicrhno!ogv and Drug DLsrvrerv
PHARMACEUTiCS OF RECOMBINANT DNA (rDNA).PRODUCED AGENTS 'DNA ittethods have facilitated the production of very pure.
useful prolcins. The physicochemical and pli.innacetitical properties of these agents are those of prosshiclt means that pharmacists must understand the
hemistry land the chemistry of instability) of proteins to core, handle. dispense, reconstitute, and administer these drugs. Instabilities among proteins may be physical nt chentical. In the former case, the protein might stick to vessels or flocculate, altering the dose that the patient will receive. In the latter case, chemical reactions taking on the protein may alter the type or stereochemistry 1 the amino acids, change the position of disulfide bonds, dcait' the peptide chains themselves, and alter the charge disuihution of the protein. Any of these can cause unt'olding denaturation) of the protein and loss of activity, rendering lie molecule useless as a drug. Chemical instability can be a rnihkm during the purification stages of a protein, when he ntniecule might be subjected to acids or bases, but insta-
could occur at the point of administration when, for esainpk. a lyophilized protein is reconstituted. The pharma-
cit must understand a few concepts of the chemical and l!iscal instability of proteins to predict and handle potential priblents.
Chemkat Instability of Proteins'7 see Figure 6-lI.
Hydrolytic reactions of the peptide bonds can
•
breuk the polymer chain. Aspauiate residues hydrolyze 100 i;nCs faster in dilute acids than do other amino acids under the same conditions, As a general rule of pcptidc hydrolysis.
AipPru > Asp.X or X-Asp bonds. This property of Asp is due to an autocatalytic Ilinction of the Asp side chain eartmsyl group. Ann. Asp, Gin, and Glu hydrolyze exception. easily if they occur nest to Gly. Ser, Ala, and Pro. Within these groupings, Asn and Gin accelerate hydrolysis more at
tow gIl. while Asp and Glu hydrolyze mail readily at high gil. sties the side chain carboxyl groups are ionized. • Dciridwinn. Gin and Asn undergo hydrolytic reactions that deamidate their side chains. These reactions convert neutral amino acid residues into citargcd ones. GIn is converted to Glu and Ann to Asp. The amino acid type is changed. hut the chain is nut cleaved. This process is..cffcctivcly. primary isOnltntation. and it may influence biological activThe deamidution reaction of Asn residues is accelerated under neutral or alkaline pit conditions. A five-mcmbcred mite intcrmediate formed by itnramoleculur attack of ihe nitrogen atom on the carhonyl carbon of the Asn side chain
the accelerant. The cyclic imide spontaneously hydrolyzes in give a mixture of residue.s—the aspairtyl peptide and an iso tigsi.
• Raeernt:nthn. Base-catalyzed raccmizotion reactions can
•wcur iii any of the amino acids except glycine. which is aehir.d. Rucemitutions yield proteitis with mixtures of .- and n-amiD,, acid configurations. The reaction occurs following the abstraction uf the u.hydrogen from the amino acid to fonn acarhanion. As should be expected, the stability of the carban-
in controls the rate of the reaction. Asp, which undergoes rjxmioation via a cyclic innide intermediate, racemizes 105 limes laster than free Asn. By comparison, other amino acids
173
in a protein raccmize about 2 is, 4 times laster than their free counterparts.
• 8.Elirninasion. Proteins containing Cys. Ser. Thr. Phe. and Lys undergo facile n-elimination in alkaline conditions that facilitate formation of an o carbunion. • Oxidation. Oxidation can occur at the sulfur-containing amino acids Met and Cys and at the aromatic amino acids His, Trp. and Tyr. These reactions can occur during protein processing as welt as in storage. Methionine (CH,-S-R) is oxidizable at
low pH by hydrogen peroxide or molecular oxygen to yield a sulfoxide (R.SO.CH,) and a sullonc The thiol group of Cys (R-SH) can undergo successive oxidation to the corresponding sulfcnic acid (R-SOH). disulf'tdc (R.S.S. RI. suluinie acid (R-SOH). and sulfonic acid (R-SO5H). A number of factors, including pH. intluence these reactions. Free —SF1 groups. can be converted into disult"tde bonds (-S. S-i and vice versa, In the phenomenon of disulfide exchange. disulirde bonds break and rclbrm in different positions. causing incorrect folding of the protein. Major changes in the three. dimensional structure of the peptide can abolish activity. Oxidation of the aromatic rings of His. Trp. and Tyr residues is believed to occur with a variety of oxidizing enzymes.
Physical Instability of Proteins" Chemical alterations are not the only source of protein instability. A protein is a large, globular polymer that exists in some specific forms of secondary. tertiary, and quatemary structure. A protein is not a fixed, rigid structure. The molecule is in dynamic motion, and the structure samples an array of three-dimensional space. During this motion. noncovalent intramolecular bonds can break, reform, and break again. but the overall shape remains centered around an energy minimum that represents the most likely (and pharmacologically active) confonner of the molecule. Any major change in the conformation can abolish the activity of the protein. Small drug molecules do not demonstrate this problem. A
globular protein normally folds so that the hydrophobic groups are directed to the inside and the hydrophilic groups are directed to the outside. This arrangement facilitates the water solubility of the protein. If the normal protein unfolds. it can refold to yield changes in hydrogen bonding, charge. and hydrophobic effects. The protein loses its globular structure, and the hydrophobic groups can be repositioned to the outside. The unfolded protein can subsequently undergo further physical interactions. The loss of the globular structure of a protein is referred to as de,ta:urwion.
Denaluration is, by far, the most widely studied aspect of protein instability. In the process, the three-dimensional folding of the native molecule is disrupted at the tertiary and, possibly. the secondary structure level. When a protein denatures, physical structure rather than chemical composition changes. The normally globular protein unfolds, exposing hydrophobic residues and abolishing the native threedimensional structure. Factors that affect the denaturation of proteins are temperature, pH. ionic strength of the medium. inclusion of organic solutes (urea. guanidine salts. acelamide. and forniamide). and the presence of organic solvents such as alcohols or acetone. Denaturation can be reversible or irreversible. If the denatured protein can regain its native form when the denaturant is removed by dialysis, reversible denaturation will occur. Denatured proteins are generally insoluble in water, lack biological activity, and become sus-
174
Wi/so,, and
TeAtbook
of Organic Medicinal and Phannaceutical chemistry
Hydrolysis-Deamldation 0
+
Mn 0
NH2 1:!
1L
NH3
Asp
0
NH2
NH3
+ Gin
NH2
Carbanion Intermediate Planar sp2 hybridized
o-Amlno
add
(aspartate): self-catalysis
If
I R\
C
8
Base-Catalyzed
Eiinination
X= a good leaving group (Cys, Set, Phe, Tyr, Lys)
b
Enolate Intermediate
Figure 6—11 . a. Protein sition reactions. b. B-Elimination.
Chapter 6 • lSio:ec/iiiologr and Drug I)israierv
hydrolysis. The air—water interface a hYdrophobic surface that can facilitate protein 'cluluratsin Interfaces like these are commonly encounin drug delivery devices and intravenous (IV) bags. Surface adsorption of proteins is characterized by adheto cn,.ylnatic
of the protein tO surfaces, such as the walls of the conol the dosage form and drug delivery devices. ampuls. asi IV tubing. Proteins can adhere to glass, plastics. rubber.
and polyvinylchloridc. This phenomenon is to as flueeularion, The internal surfaces of intrave— nsa delivery pumps and IV delivery bags pose particular of this kind. Flocculated proteins cannot be dosed
results when protein molecules, in aqueous stiwion, seIf'assocjate to form dinners, trimers. (etramers, and large macromolecular aggregates. SeIf-assoon the pH of the medium as well as solvent ionic strength, and dielectric properties. Mod amoutits of denaturants (below the concentration that %een'gsliml
saud cause denaturalion) may also cause protein aggrega-
Ian. Partially unfolded intermediates have a tendency to Coneetitrated protein solutions, such as an immunglohiilin for injection. may aggregate with storage time The presence of particulates in the preparation is phannacist's clue that the antibody solution is defective. Precipitation usuully occurs along with denaturation. De-
insesligiaions have been conducted with insulin. finely divided precipitate on the walls of an
hrch brats a
untu,Iuuut device or its dosage form container. It is believed that insulin undergoes denaturution at the air—water interIacc. lacilitatitig the precipitation process. The concentration I .'inc ion. pH, and the presence of adjuvants such as protanise ,ilso affect the precipitation reaction of insulin.
Immunogeulcity of Blotedrnologkally P,oduced Pnncins by their very nature are antigens. A humazi protein, at
its typical physiological concentration. may
cthihlt completely different immunogenic properties when administered in the higher concentration Ihat would be used a drug. Unless a biotechnology-derived protein is engiurered to be 10(1% conipleinentaiy to the human form, it sill differ among several major epitopes. The protein may Ii:use moditicutions of its amino acid sequence (substitutions
sue amino acid fur another). There may be ttdditions or Idetions of anhino acids. N-terminal methionyl groups, infolding patterns, or oxidation of a sulfurside chain of a methionine or a cysteine. Addi!slall, shrtt a protein has been produced by using a bactenjl vector, a finite amount of imrnunoreactive material may unto the final product. All of these listed items contribute lie .rntigcnicity of a biotechnologically produced protein. \Vlucn ii is adniinistercd to a human patient, the host's imi
muse
175
DELIVERY AND PHARMACOKINETICS OF BIOTECHNOLOGY PRODUCTS99 As with any drug class, the medicinal chemist and pharmacisc must be concerned with the absorption, distribution, me-
tabolism, and excretion (ADME) parameters of protein drugs. Biotechnology-produced drugs add complexities that are not encountered with "traditional" low-molecularweight drug molecules. ADME parameters arc necessary to compute pharmacokinetic and pharmacodynainic parameters for a given protein. As for any drug. these parameters
are essential in calculating the optimum dose for a given response. determining how often to administer the drug to obtain a steady state, and adjusting the dose to obtain the best possible residence time at the receptor (phammacodynamic parameters).
Delivery of drugs with the molecular weights and properties of proteins into the human body is a complex task. The oral route cannot be used with a protein because the acidity of the stomach will catalyze its hydrolysis unless the drug is enieric coated. Peptide bonds are chemically labile, and proceolylic enzymes that are present throughottt the body can
attack and destroy protein drugs. Hydrolysis and pcptidase decomposition also occur during membrane transport through the vascular endothelium. at the site of administration, and at sites of reaction in the liver, blood, kidneys, and most tissues and fluids of the body. It is possible to circumvent these enzymes by saturating them with high concentra(ions of drug or by coadministering peptidase inhibitors. Oxidative metabolism of aromatic rings and sulfur oxidation can also occur. Proteins typically decompose into small fragments that are readily hydrolyzed. and the individual amino acids are assimilated into new peplidcs. A potentially serious hindrance to a pharmacokinetic profile is the tendency of proteins administered ax drugs to bind to plasma proteins such as serum albumin. If this happens, they enter a new biodistribution compartment from which they may slowly exit. Presently, the roulcs of administration that are available for protein drugs are largely subcutaneous and itnramuscular. Much ongoing research is targeted at making peptide drugs more bioavailable. An example of this is conjugation of interleukin-2 with polyethylene glycol (PEG). These socalled pegylated proteins tend to have a slower elimination clearance and a longer 1, than inierleukin-2 alone. Another strategy being used is the installation of a prosthetic sugar moiety onto the peptide. The sugar moiety will adjust the
partition coefficient of the drug, probably making it more water soluble.
RECOMBINANT DRUG PRODUCTS
will react to the protein just as it would to a
attack and neutralize it. This is why research has undertaken to create 100% human protein drugs, such m usulin. which patients will need to take for a long time. In .iddition. some of the most promising biotechnology prodhe monoclonal antibodies, are produced in mice by use u lucu:uni:ed genes to avoid human reaction to the mouse
Human Insulin. Recombinant, 100102 Human insulin was the first pharmacologically active biological macrotnolecule to be produced through genetic engineering. The FDA approved the drug in 1982 for the treatment of type I (insulindependent) diabetes (see Chapter 25). The insulin protein is a two-chain polypeptide containing SI amino acid residues. Chain A is composed of 21 amino acids, and chain B con-
176
II'iIson and Gi.c void's Textbook of
Medicinal and Phamiaceuticai Chemistry
tains 30. The human insulin molecule has three disulfide linkages. CysA7 to CysB7. CysA29 to CysB19, and an intrachain linkage. to CysA11. Insulin is secreted by the 46-cells of the pancreatic islets of Langerhans, initially as a single peptide chain called proinsulin. Enzymatic cleavage of the propeptide releases the insulin. Historically, insulin was isolated from bovine or porcine
terminus of the B chain. Insulin glargine. administered subcutaneously (SC), has a duration of action of 24 to 48 hours. The alteration in basicity of this agent causes it to precipitate at neutral pH, creating a depot effect. Insulin rDNA has been very successful. The only problem has arisen in patients who have been using porcine or bovine insulin for a long time. Some patients who are switched to
sources. Using these agents was not without difficulty. Both porcine and bovine insulin differ in amino acid sequence.
rDNA human insulin report difficulty in "feeling their glucose level," and these patients require extra counseling in
with Ala replacing Thr at the C terminus of the human B Bovine insulin also differs in sequence from human insulin, with Ala substituting for Thr at A8 and Val
the use of the recombinant hormone.
chain
substituting for isoleucine at A10. These differences, small though they may seem, result in immunological reactions in some patients. Adjustments to the formulation of bovine and porcine insulin led to products that differed in time of onset, time to peak reduction in glucose, and duration of action. These parameters were varied by addition of pronamine and zinc (which yielded a particulate insulin with a longer duration of action). and adjustment of the pH to neutrality, which stabilized the preparation. Insulins were characterized as reg-
ular (short-duration llctin. 4 to 12 hours). semilente (ultrashort duration). lente (intermediate II to 3 hours to peak. 24 hours durationi), and ultralente (extended duration). An o(
NPhI (,neutvat pcotamine
Hagedorn). which had an intermediate time of onset and time to peak (ito 3 and a tong duration of action (16 to 24 hours).
Producing a recombinant insulin that is chemically and physically indistinguishable from the human pancreatic hormone was a major accomplishment. The problem with immunoreactivity has been eliminated, the pyrogen content of the rDNA product is nil, the insulin is not contaminated with other peptides. and the hormone can be biosynthesized in larger quantities. Human insulin (rDNA) is available as Humulin, Novolin, and a number of analogues that differ in
their phamiacokinetic profiles. Humulin is produced by using recombinant E. cvii: Novolin is prepared by using recombinant S. cerevixiae. a yeast. There have been modifications in the production procedure since the initial success-
Glucagon.'°4
The hormone glucagon (GlucaGen) is
biosynthesized in the pancreas as a high-molecular-weight protein from which the active macromolecule is released by proteolytic cleavage. Glucagon is a single chain of 29 amino acids and generally opposes the actions of insulin. Bovine and porcine glucagons. which possess structures identical with human glucagon, have been in use for years. The rDNA form has been approved by the FDA for use in severe hypo glycemia and as a radiological diagnostic aid. Glucagon relaxes smooth muscles in the gastrointestinal (GI) tract. dc creasing GI motility and improving the quality of radiological examinations. In the treatment of severe hype glycemia in insulin-dependent diabetics, GlucaGen causo to co5wett to untreated. vere hypoglycemia (low-blood-sugar reactions) can prolonged loss of consciousness and may be fatal. The rDN\
drug has the benefit that there is no chance of bovine spongiform encephalopathy from glucagon therapy This condition, also known as mad cow disease, is causedt a prion that was suspected to infect animal pancreas tissa
Human
Growth
Hormone,
Human growth hormone (hGH) is a protein that is for normal growth and development in humans. hUH many aspects of human development and metabolism eluding longitudinal growth, regulation (increase) of pmtcr synthesis and lipotysis. and regulation (decrease) of metabolism. hUH has been used as a drug since the l95b
ful biosynthesis. Prior to 1986, Humulin was produced by creating two different vectors, one for the A chain and one for the B chain, and inserting them into E. coli. The A chain
and it has been extremely successful in the treatment olck
and the B chain would be secreted into the medium, and the
Willi syndrome. In its long history the hormone has ('cc remarkably successful and free of side effects. The primary form of hUH in the circulation is a 22.kth nonglycosylated protein produced in the anterior pituiur
two were joined chemically to form rDNA insulin. Today, the entire proinsulin gene is used to create a recombinant organism. and the connecting peptide in proinsulin is cleaved by two enzymes (an endopeptidase and a carboxypeptidase
B). yielding insulin (for details see Chapter 25). Insulin rDNA is available in severaO°3 forms. Insulin lispro (Hurnalog) has a more rapid (15 to 30 minutes) onset and a shorter duration (3 to 6.5 hours) of action than regular
human insulin (onset 30 to 60 minutes, duration 6 to tO hours). It is effective when administered 15 minutes before
a meal, unlike regular insulin, which must be injected 30 minutes before a meal. In lispro, the B-chain amino acids and B2.,Lys are exchanged. Insulin aspart (Novolog), onset IS to 30 minutes. duration 3 to 6.5 hours, with a single
amino acid substitution of Asp for Pro at B28, is effective when administered 5 to 10 minutes before a meal. The ultra-
long-acting agent insulin glargine has the Asp at A21 replaced by Gly and has two Arg residues added at the C
sic growth hormone deficiency, chronic renal in women. and Pradc
and cotnposed of 191 amino acid residues linked by bridges in two peptide loops. The structure of hUH is
lar, with four antiparallel a-helical regions. Endogenci. hGH is composed of about 85% of the 22-Wa mononict.' to 10% of a 20-kDa monomer, and 5% of a mixture of tide-linked dimers. oligomers. and other modified fonri. From the late 1950s, hUH was isolated from pituilany c tracts of cadavers, A prion associated with the was suspected to cause Creutzfeldt-Jakob disease, a degenerative neurological disorder. The first use of recombinant hGH (rhGH) was repslz. in 1982. rhGH preparations were first produced in £ c These preparations contained a terminal methionine and" amino acids. Natural sequence rhGH has since beet duced in mammalian (mouse) cell culture.
Chapter 6 • Biotechnology and Drug Discovery
n
Somauern, the first recombinant preparation, introduced 985. containc the natural 191-amino acid primary Se-
4UCOCC plus one methionyl residue on the N-terminal end. The sonrairtipin products all contain the 191-amino acid se-
177
tissue to stimulate iodine uptake into the gland, organification of iodine, and secretion of thyroglobulin. Ta. and T4. The drug is used as a tool for radioiodine imaging in the diagnosis of thyroid cancer.
qucnce and are identical with the ItCH produced by the pitui-
lam gland. The three-dimensional crystal structure shows the protein is oblate. with most of its nonpolar amino aid side chains projecting toward the interior of the molecide. This rhGH is pharmacologically identical with natural hGH.
Most current formulations of
rhGH are supplied in lyophi-
wed form and must be reconstituted prior to injection. Typi-
cally, 5 to 10 mg of protein are supplied in a powdered and/or mannitol phosphate buffer. The prepardtion
Cytokines HEMATOPOIETIC GROWTH FACTORS
Among all of the events taking place in the immune system. the bone marrow, and the bloodstream, the process of hematopoiesis is probably the most complicated. All of the cells in the blood and the immune system can trace their lineage back to a common, parental hematopoietic stem cell in the bone marrow. This cell is referred to as pluripoten: because
sill remain stable for 2 years.
under the proper stimulation it can differentiate into any other cell. The proce.sses of maturation, proliferation, and differentiation are under the strict control of a number of
niGH undergoes rapid, predictable metabolism in vivo in the kidney and the liver. Chemically, the metabolites are
cytokines (Table 6-5) that regulate a host of cellular events. Two distinct blood cell lineages exist: the lymphoid lineage
those expected for any peptide: deamidation of Asn and GIn and oxidation of Met, Tm, His, and Tyr.
that gives rise to B and T lymphocytes, and the myeloid
reconstituted with sterile water for injection, and the stabiliyof the product is quite good. If stored at 2 to 8°C, rhGH
lineage that produces granulocytes (macrophages. neutro-
phils, cosinophils, basophils, and mast cells), as well as The gonadolroFsaIIkle.Stimulating Hormone.'°''°8 pin Iollicle-stimulating hormones (FSH), follitropin alfa iGunal-FI and follitropin beta (Follistim). are produced in he anterior lobe of the pituitary gland. FSH can function in ISO ways. On the one hand, it causes increased spermatogen-
esis in males. On the other hand, in concert with estrogen and luleiniring hormone, it stimulates follicular growth and deselopment in females. Consequently, FSH may be useful in he lieatnnent of infertility. FSH is a member of a superfamily of proteins, all structur:llv related. which includes luteinizing hormone (LH), chorimc gonadoiropin, and thyroid-stimulating hormone (TSH).
platelets and erythrocytes. As many as 20 of the hematopoicsis-associated cytokines have been cloned and expressed. Some of these are listed in Table 6-5.
The cascade is shown in Figure 6-12. A further feature of the pluripolent stem cell deserves mention. Each stem cell divides into two daughter cells, one an active hematopoietic progenitor and one quiescent. The active precursor matures to give hematopoietic progenitors and then circulating blood
cells. The quiescent stem cells rejoin the stem cell pool. Hence, the number of parental cells is always the same. This process is termed self-renewal.
It is a heterodimer, the a subunit contains 92 amino acids.
PRODUCTS
and the /3 subunit contains ill amino acids. The protein
Erythropoietin Alfa.1"
rather heavily glycosylated and has a molecular mass of approximately 35 kDa. The traditional source for isolation was postmenopausal urine, which provided a preparation that was less than 5% pure and was significantly conaminaled by LH. The recombinant human FSH (rhFSH) is 'induced in a mammalian cell line, the CHO. are the same protein, but Follitropin a and follitropin hey differ with respect to the way they are formulated. Both
combinant Epoetin Alfa, Epogcn, Procrit, is a glycoprotein that stimulates red blood cell production. It is produced in
113
Eiythropoietin alfa. re-
TABLE 6-5 Cytoklnes Th at Have Been Cloned Cytokina
BiologIcal Function
a form is formulated with ucrose (as a bulk modifying agent and a lyoprotectant) and form contains ihr components of phosphate buffer. The ucnn'se. with sodium citrate as a stabilizer and polysorbate
lnterlcukin-3
Multi-stem cell factor-, controls branching from niyeloid stem ucil
as a lyoprotectant and a dispersant. The products are :aconsnituted immediately before administration. The shelf of both preparations is 2 years when they are stored in he stpplied containers at less than 30°C (not frozen) and
Intcrleukln-4
Switches B cctls from IgO to IgE
lntcrleukin-5
Activates cosinophik
lnterteukins-6 and '1
Differentiation arT lymphocytes
rrnerleukln.I2
Controls the ratio of T,11 toT112
rrulected from light.
Erythropoletin
Stimulates rest cell production
Granulocyte-macruphago
Acts with lL-3 to control myeluid
Thytotropin Alpha.'°'"°
Human thyroid-stimulating
hormone (TSH). thyrotropin alpha (Thyrogen). is a heterodascric protein of molecular mass —28,000 to 30,000 Da. The a subunit is composed of 92 amino acids, and the ubriniL 112. The specificity of the protein is controlled by he $subunil. TSH binds to TSH receptors on normal thyroid
cells or on well-differentiated cancerous thyroid
colony-stimulating factor
branch
Gr.rnulocytc colony-stimulating foctor
Neutrophil production
Macrophage cuiony.srinruiating factor
Macrophage production
Stem cell factor
Coonrots activity through myelold branch
178
tVilwn and
Medicinal and Pharnwceurieol chemistry
textbook ',f
QuIescent Daughter Cell
Plurlpotent Stem Cell SCF IL-3 OM-CSF EPO
IL-3 GM-CSF EPO
Myelold Stem Cell
Lymphold Stem Cell
SCF IL-3 GM-CSF EPO
I
SCF IL-3 GM'CSF
I I
SCF IL-3
I
4, I
/\ ® ®®® I
GM-CSF IL-3
IL-3 GM-CSF
I
$ EPO
!
GM-CSF I M-CSF I
EPO
I
IL-2 lL-6
IL-3
IL-i
GM-CSF IL-3
!1L4
I
4,
Erythcocytea
Platolol.
Neutroptilts
EOSInOpIrIIe
Basoplilte
B Lyniptiocytea
T Lymphocytes
Figure 6—12 • Cytokine-mediated cascade leading to different blood cell types. EPO. erythropotetin; GCSF. granulocyte colony-stimulating factor; GM-CSF, granulocyte-macrophage colony-stimulating factor; IL-X interleukins; M-CSF, macrophage colony-stimulating factor; SCF, stern cell factor; TPO, thrombopoietin.
the proliferation and differentialhe kidney, and it tion of specially committed erythroid progenitors in the bone marrow. Epoetin alfa (Epogen) is a 165-amino acid glycoprotein that is manufactured in mammalian cells by rDNA technology. The protein is heavily glycosylated and has a molecular mass of approximately 30,400 Da. Erythropoictin is composed of four untiparallel a helices. The rDNA protein has the same amino acid sequence as natural erythropoietin. Epoetin is indicated to treat anemia of chronic renal failure
patients, anemia in ,idovudine-treated HEY-infected patients. and in cancer patients taking chemotherapy. The results in these cases have been most patients respond with a clinically significant increase in hematocrit.
tion of granulocytes (especially neutrophils) by hematopoietic stem cells in the bone marrow. G-CSF is a glycoprotein produced by monocytes, blasts, and endothelial cells. G-CSF is a protein of 174 a molecular mass of approximately 18.800 1
The native protein is glycosylated. Filgrastim selectively stimulates proliferation and diffe entiation of neutrophil precursors in the bone marrow. flL leads to the release of mature neutrophils into the from the bone marrow. Fi Igrastim also affects mature nests phils by enhancing phagocytic activity, priming the metabolic pathways associated with the respiratory enhancing antibody-dependent killing, and increasing f:
Filgrastim.IM ItS
expression of some functions associated with cell
ulating
antigens.
Filgra.stim. granulocyte colony-stim(G-CSF). Neupogen, stimulates the prolifera-
Chapter 6 • Biotechnology and Drug Discovery
In patients receiving chemotherapy with drugs such as
179
Viius
doxorubicin. and etoposide, the mciot Itcutropenia accompanied by lever is rather high. Administration of G-CSF reduces the time of neutrophil reand duration of lever in adults with acute myelogenuns kukemia. The number of infcctions, days that antibiot-
ire required, and duration of hospitalization are also Natural Killer
reduced.
is identical with G-CSF in its amino acid scqseiwe. except that it contains an N-terminal methionine hat is necessaiy for expression of the vector in E. cpu. The protein is not glycosylated. Filgrastim is supplied in a 0.01 N sodium acetate buffer containing 5% sorbitol and 0.004% 80. It should be stored at 2 to 8°C without freez-
Infectod Host Cell
Cell
FIgure 6—13 • Antiviral mechanism of action of the inter. ferons.
Becaplermin is produced by a recombinant strain of S. cere-
Under these conditions, the shelf life is 24 months. Avoid shaking when reconstituting; although the foaming will not harm the product, it may alter the amount of drug
visiae containing the gene for the B chain of PDGF. The protein has a molecular mass of approximately 25 kDa and is a homodimer composed of two identical polypeptide
that is drawn into a syringe.
chains that are linked by disulfide bonds. It is a growth factor that activates cell proliferation, differentiation, and function, and it is released from cells involved in the healing process. Becaplermin is formulated as a gel recommended for topical use in the treatment of ulcerations of the skin secondary to diabetes.
U8 Sargramostim. granulocytemn.wniphage colony-stimulating factor (GM-CSF). Leukine.
is a glycoprolein of 127 amino acids, consisting of three molecular subunits of 19,500, 16,8(X). and 15.500 Do. The
endopenous lomi of GM-CSF is produced by T lymphotes, endothelial tibroblasts. and macrophages. Recombirout GM.CSF. produced inS. cerem'i.ciae. differs from native
human GM-CSF only by substitution of a leucine for an
The intcrfcrons arc a family of small proteins or glycoproteins of molecular masses ranging from 15,000 to 25,000
arginine at position 23. This substitution facilitates expres-
Da and 145 to 166 amino acids long. Eukaryotie cells secrete
ol the gene in the yeast. The site of glycosylation in he recombinant molecule may possibly differ from that of the native protein.
interferons in response to viral infection. Their mechanism of action is bimodal. The immediate effect is the recruitment of natural killer (NK) cells to kill the host cell harboring the
Sargramostinr binds to specific receptors on target cells and induces proliferation, activation, and maturation. Ad-
virus (Fig. 6-13). Interferons then induce a state of viral resistance in cells in the immediate vicinity, preventing
ministration to patients causes a dose-related increase in the
spread of the virus. Additionally. interferons induce a cascade of antiviral proteins from the target cell, one of which
white blood cell count. Unlike G-CSF. GM-CSF is a muhilineage hematopoietic growth factor that induces partially committed progenitor cells to proliferate and differenhiale along thin granulocyte and the macrophage pathways.
Ii also enhances the function of mature granulocytes and GM-CSF increases the chemotactiC. antifungal, and antiparasitic activities of granulocytes and ntonmsytes. It also increases the cytotoxicity of monotoward neoplastic cell lines and activates polymorpholeukocytes to inhibit the growth of tumor cells. Sargmmoslim is used to reconstitute the myeloid tissue alter aulologous bone marrow transplant and following in acute myelogenous leukemia. The prepara-
is 2',5'-oligoadenylate synthetase. This enzyme catalyzes the conversion of ATP into 2',S'-oligoadenylate, which activates ribonuclease R. hydrolyzing viral RNA. Interferons can be delined as cytokines that mediate antiviral. antiproliferative, and immunomodulatory activities. Three classes of interferon (IFN) have been characterized: a (alpha), /3 (beta), and y(gamma) (see Table 6-6). a-Interferons are glycoproteins derived from human leukocytes. /3Interferons are glycoproteins derived from fibroblasts and macrophages. They share a receptor with a-interferons. yInterferons are glycoprotcins derived from human T lympho-
inn decreases the incidence of infection, decreases the numher of days that antibiotics are required, and decreases the durjtimiu of hospital stays.
Sargr.mmostini is supplied as a solution or powder (for solutioni. Iloth forms should be stored at 2 to 8°C without mrwing. The liquid and powder have expiration dates of 24 months. Thc reconstituted lx)wder and the aqueous solution 'hould not be sha en.
TABLE 6—6
Interferon Type Alphu
lnterferons Used TherapeutIcally Endogenous Source
AvaIlable Drug Products
Lcukocytcs
ulfa-2a
Interferon utfa-2b
Becaple,min.'T' Becaplermin. Regranex Gel, an endmmgctmoas ptmlypeptide that is released from cells that are inmolsed in the healing is a recombinant human
plaleki-slerised growth factor (r-hPDGF-BB). The "BB" that hecaplermin is the hoinodimer of the B chain.
aifa-2c ttcta
Eihmhtasts, mucrophages
tllema'lim
Gainnia
TLymnphocyrcu. nuturul
Gamma-lb
lb kilter cells
180
Medicinal and Pliarnraceuiica! Ciu',njOrv
Wilson am! Girt',,iml 's Textbook of
TABLE 6—7
Summary of the x-lnterferons Interferon
Interferon
AIfa-2a
AIfa-2b
Interferon Alfa-ni
Interferon Alfa.n3
Interferon Alfacon-1
Tr.uic nwne
Roteron A
Iiitnsn A
wdtrcroit
Aileron N
Dosage form
Solution. powder
Solution, powder
Solution
Solution
Solution
Soisent
Sodium chloride. excipients
Buttered saline
ISuflered saline
Ruflered saline
phosptiate-bulfered saline
indications
Hairy cell leukemia, AIDS-rekited
hairy cell lcukeniia. AIDS-related Kuposi's sarcinna, condytonrura
Chronic hepatitis C
Condylomnia ucuminata
Chronic hepatitis C'
SC or IM
huitr.ilesionaI
SC
Human lyinphobt.tstoid
Human leuk*icytcs
E. coil
Kaposi's sarcoma. chronic hCpatiiis C Rouleir'
SC. lM. IV, infusiciri or
Siinrcc
F rob
acunuinuta, chronic hepatitis B. chronic hcpatitis C
SC. IM. IV, infusion or intrulecional iou
cell line St. oat.cuiuncou.Iv: 154.
tV. mnmr.wcnousls
cytes and NK cells. These interterons are acid labile and used to be called 'type 2 interferon." The rcceptor fir IFNy is smaller than that fir IFN-a and 90 to 95 kDa versus 95 to I 10 kDa. respectively. The three classes are not homogeneous. and each may contain several different
cells. Modulation of the host ituniune response probably plays tt role in the antitumor activity of interferon alfu-2a. The interferon is supplied as a solution or as a powder
molecular species. For example. at least 18 genetically and molecularly distinct human a-interferons have been identilied, each differing in the amino acid substitution at positions alfa-2b. and alfa-2c have been 23 and 34. lntertCrons purified and are either in clinical use or in development. A
0.33% phenol. The interlCron vials, if properly stored at 2 Ia 8°C without freezing. expire in 30 months. Prefilled syringes
listing of commercially available a-inccrferons is given in
Pegylated Interferon AIfa-2a.2'
Table 6-7. As a class, the interferons possess some common side effects. These arc lb-like symptoms, headache, fever, nutsdc aches, back pain, chills, nausea and vomiting, and diarthea. At the injection site, pain, edema, hemorrhage, and inflammation are common. Di,ziness is also commonly reported.
For the pharmacist. when predicting drug interactions with the interferomts. eytochrome P-450 metabolism should always be a key consideration. Most of the interferotis itihibit cytochrome P-450. causing drugs that are nietaboli,ed by
this route to reach higher-than-nonnal and, possibly. toxic concentrations in the blood and tissues.
for solution. The solution contains
NaCI. The powder
contains 0,9% NaCI. 0.17% human serutu ttlhuniin. and
expire itt 24 nionths. The solutions should not be shaken because the albumin will cause frothing. Pegylated interferon
alfa-2a. Pegasys. is a covaletit conjugate of recombittant terferon all'a-2a (approximate molecular mass. 20 kDa) ssilh
singly-branched bis-monomethoxypolycthylene glycal (PEG) chain (approximate molecular mass. 40 kDa). The PEG tnoiety is linked at a single site to the interferon alfa moiety by a stable antide botid to lysine. Peginterferon alfaa
2a has an approximate molecular mass of 6() kDa. Pcgasys provides sustained therapeutic serum levels for up to a fuH week (168 hours). The drug is approved for the treatment of adults with chronic hepatitis C who have compensated liver disease and who have not been previously treated wilh interferon alfa. Efficacy has also heemi demonstrated in pa tients with compensated cirrhosis.
PRODUCTS: a-INTERFERONS
Interferon AIfa-2b (Recombinant).'22
Interferon AIfa-2a (Recombinant).'20
alfa-2b. Intron A. a water-soluble protein of 165 amino acids
Interferon aI fa2a (recombinant). Roferon A. is expressed in an E. co/i sys-
tent attd purified by using high-affinity mOUSe monoclonal antibody chromatography. The protein consists of 165 amino acids with a molecular mass of approximately 19.0(X) Da. and contains lysine at position 23 and histidine at position
Interferon
and an approximate molecular mass of 19.200 Da. is cx pressed from a recombinant strain of E. coil. This interferon molecule possesses ati arginine at position 23 and a histidine
ttt position 34. Interferon alfa-2b is a broad-spectntm agent. It is mdi. cated for hairy cell leukemia. condylomna acurninata (genital
34.
is used in the treatment of hairy cell leukemia and AIDS-related Kaposi's sarconta in selected
or venereal warts). AIDS-related Kaposi's sarcoma, and chronic hepatitis B and C infections.
patients over 18 years of age. It is also used to treat chronic hepatitis C. and in patients with this disease. interferoti alfa2a can nomialiie seruni alanine antinotransierase (ALT) lev-
by infusion or by intralesional mutes. The dose is I to 35 million IU/day. depending on the application. The drug
Interferon
els. improve liver histology, and decrease viral load. The drug has a direct antiproliferative activity against tumor
Intron A can be administered by the SC. lM. or IV router.
supplied as a solution or as a powder for solutioti. attd boTh
forms contain albumin. glycine. and sodium phosphate
Chapter 6 a 'tiller. Hence, they should not be shaken. Vials of solution stiuld he stored at 2 to 8°C without freezing. The powder 'stable tar 18 months at room temperature or 7 days at
Interferon Alfa-nl.'23 interferon alfa-n I. Wellferon, uhisture of o-interfcrons isolated from a human lymphshiastoid cell line alier induction with mouse parainlluenza
Scndai strain). Each of' the subtypes of IFN-a in this product consists of 165 or 166 amino acids with an iype I
ture
molecular rna.cs of 26,000 Da. The product is a mixif each of the nine predominant subtypes of alfa-n I
in
is indicated to treat chronic hepatitis C
1$ years of age or older who have no decompenliver disease. The exact mechanism of action for inter-
alla-ul in the treatment of this disease has not been
and 1)rug Discovers
181
and assigning the tHus! COflflflOfl amino acid to each variable
position. Additionally, four amino acid changes were made to facilitate synthe.sis. The DNA sequence is also constructed
by chemical synthesis. Interferon alfacon-l differs from interferon alia-n2 at 20/166 amino acids, yielding 88% homology. The protein has a molcetilar mass of approximately 19.400 Da.
Interferon ulfacon- I is used in the treatment of chronic hepatitis C virus infection in patients 18 years of age or older
with compensated liver disease and who have anti-HCV serum antibodies or HCV RNA. The drug is administered by the subcutaneous route in a dose of 9 3 times per week, Interferon alfacon- I is supplied as 'a solution in Ishosphale-buffered saline. It should
be stored at 2 to 8°C without freezing. Avoid shaking the solution.
dacidareiL
This drag may be administered SC or EM. with a usual disc ot 3 million Hi 3 times per week. Interferon alfa-n I is 'uppllL'd as a solution containing tromethamine and buffered sline with human albumin as a stabilizer. Hence, the soluon should nor be shaken. The solution should be stored at 1n5C without freezing, and should be discarded if freezing
Properly stored solution expires in 24 months. Interferon AIfa-n3'24 Interferon alfa-n3. Aileron N. expressed from human leukocyres that are with avian Sendai virus. The Sendai virus propagated in chicken eggs. The protein consists of at least 14 molecular subtypes. The average chain length is amino acids, and molecular mass range is 16.000 to 17.1881 l)a. The polydisperse interferon alfa-n3 is extremely pare because it is processed by affinity chromatography over :i bed 01° mouse monoclonal antibodies specifically raised hit the protein.
Interferon alfu-n3 is indicated for intralesional treatment of reuractorv or recurrent condylonna acunhinata (genital tons I in patients 18 years of age or older. These xvafls are
PRODUCTS: 13-INTERFERONS
A listing of two commercially available
is given in
Table 6-8.
Beta-la (Recombinant).'2' Interferon beta-la (recombinant). Avonex. is a glycoprotein with 166 amino acids. It has a molecular mass of approximately 22.000 Da. The site of glycosylation is at the asparagine Interferon
residue at position 80. Interferon beta. I a possesses a cysteinc residue at position 17. as does the native molecule. Natural IFN-fl and interferon bela-lu are glycosylated. with each
containing a single carbohydrate moiety. The overall complex has 89% protein and II carbohydrate by weight. Recombinant interferon beta-I a is expressed in CHO cells con-
taining the recombinant gene br human IFN-/3 and is equivalent to the human fonu secreted by fibrohlasts. Interferon beta-Ia is indicated for the treatment of relapsing lbmss of multiple sclerosis. Patients treated with interferon beta-I a demonstrate a slower progression to disability and a less noticeable breakdown of the blood—brain harrier
with human papilloma virus (HPV). Interferon is especially useful in patients who haven't responded well Its other modalities (podophyllin resin. surgery. •ihla.u3
Interferon alfa-n3 is also being inves-
for the treatment of non-Hodgkin's lymphoma, herpes simplex. rhinovirus. vuccinia. and varicella zoster. A usual dose in condyloma acuminata is 250.000 lU/wart. with a 31)-gauge needle around the base of the Ic"in Interkran alfa'n3 is contraindicated in persons sensitive ho mouse immunoglobulin G. egg protein, and neomycin. .supplied as a solution with the protein Interferon alftu-n3 ii phuaphate-huffered saline with phenol as a preservative. list solution should be stored at 2 to 8°C without freezing. Properly stored solution expires at IS months.
Interferon Alfacon-1 (Recombinant). interferon alfarecombinatit). Infergen.'2° is a "consensus'' intershares structural elements of IFN—a and several 'ubtypes. The range of activity is about the same as the other ilpli.u species, but the specific activity is greater. The 166.amino acid sequence of alfacon- I is synthetic. It i.usdeseloped by comparing several natural IFN-a subtypes
TABLE 6-8
3-lnterferons Interferon Beta-la (Avonex)
Interferon Beta-lb (Betaseron)
Recombi,iani
Cl-tO cetis
Fc'hrs-ic/,iu you
Type
Maw eomplcx
NisI glycosytsied
carbohydrate
or 6 million lU/mi.
Coni,cnlraiion
31)
251) p.g or 11 uttitlion
Supplied form
Powder for rcconstituIion
Powder for rcconslituuiiu)n
Oittjcun
Sterile water—no preservative'.
NaCI 0.54% without prscrs'ativcs
Storage
2—SI
I)osage
31)
Route
tnuraunuseutar
Suticutaucous
Notable aide
In3eciion site rsac'tuiuns.
Injection site rcactioas.
cffccls
do not Irecec once a week
3%: no necrosis
lU/niL
2—S Ct do not freeze 25(1
every other day
necrosi'..
18.2
Wilson and (3isvolds Textbook of Organic Medicinal and Pharmaceutical Chemi.ctrv
as observed in gadolinium-enhanced magnetic resonance imaging (MRI).
Although the exact mechanism of action of interferon beta-la in multiple sclerosis has not been elucidated, it is known that the drug exerts its biological effects by binding to specific receptors on the surface of human cells. This binding initiates a cascade of intracellular events that lead to the expression of interferon-induced gene products. These microglobuinclude 2',5'-oligoadenylate synthetase and lin. These products have been measured in the serum and
in cellular fractions of blood collected from patients treated with interferon beta-la. The functionally specific interferoninduced proteins have not been defined for multiple sclerosis.
Adverse effects include a flu-like syndrome at the start of therapy that decreases in severity as treatment progresses. Interferon beta-la is a potential abortifacient and an inhibitor
of cytochrome P450. The dosage form is a powder for solution that is reconstituted in sterile water. Excipienis are human albumin, sodium chloride, and phosphate buffer. The solution can be stored at 2 to 8°C and should be discarded if it freezes. The lyophilized powder expires in 15 months. After reconstitution, the solution should be used within 6 hours. The solution should not be shaken because of the albumin content.
Interferon
Interferon Beta- lb (Recombinant).'27 beta-lb. Betaseron. is a protein that is expressed in a recom-
binant E. coil. It is equivalent in type to the interferon that is expressed by human fibroblasts. Interferon beta-lb possesses 165 amino acids and has an approximate molecular mass of 18.5 kDa. The native form has 166 amino acids and weighs 23 kDa. interferon beta-lb contains a serine residue
at position Il rather than the cysceine in native IFN-fl and does not contain the complex carbohydrate side chains found
in the natural molecule. In addition to its antiviral activity, interferon beta-lb possesses immunomodulating activity. Interferon beta- lb is administered SC to decrease the frequency of clinical exacerbation in ambulatory patients with relapsing—remitting multiple sclerosis (RRMS). RRMS is characterized by unpredictable attacks resulting in neurological deficits, separated by variable periods of remission. Although it is not possible to delineate the mechanisms
by which interferon beta-lb exerts its activity in MS. it is known that the interferon binds to specific receptors on cell surfaces and induces the expression of a number of interferon-induced gene products, such as 2',S'-oligoadenylate synthetase and protein kinase. Additionally, interferon beta-
lb blocks the synthesis of INF-y, which is believed to be involved in MS attacks. Interferon beta-lb is supplied as powder for solution with albumin and/or dextrose as excipients. Ii should be stored at 2 to 8°C without freezing. After reconstitution the solution can be stored in the refrigerator for 3 hours. The solution should not be shaken.
A major difference between interferon beta-la and betalb is that beta-lb causes more hemorrhage and necrosis at the injection site than does interferon beta-I a. PRODUCTS: y'INTERFERON
Interferon Gamma-lb (Recombinant).128
Interferon gamma- lb. Actimmune. is a recombinant protein expressed
in E. coil. IFN-y is the cytokine that is secreted by human T lymphocytes and NK cells. It is a single-chain glycoprotcin composed of 140 amino acids. The crystal structure of the protein reveals several helical segments arranged to approximate a tone shape.
Interferon gamma-lb is indicated for reducing the frequency and severity of serious infections associated with chronic granulomatous disease, an inherited disorder charac-
terized by deficient phagocyte oxidase activity. In this disease, macrophages try to respond to invading organisms but lack the key oxidative enzymes to dispose of them. To coin-
pensate. additional macrophagcs are recruited into the infected region and form a granulomatous structure around the site. IFN-ycan stimulate the oxidative burst in macrophages and may reverse the situation.
Interferon gamma-lb is supplied as a solution in sterile water for injection. The solution must be stored at 2 to 8°C. without freezing. The product cannot tolerate more than 12 hours at room temperature.
THE INTERLEUKINS Aldesleukin.'29 Aldesleukin. T-cell growth factor, thymocyte-stimulating factor. Proleukin. is recombinant in an engineered strain of E. coil containing an analogue of the human IL-2 gene. The
recombinant product is a highly purified protein of 133 amino acids with an approximate molecular mass of 15,300 Da. Unlike native IL-2, aldesleukin is not glycosylated. has no N-terminal alanine. and has serine substituted for Cys at site 125. Aldesleukin exists in solution as biologically active. non-covalently bound microaggregates with an average size of 27 IL-2 molecules. This contrasts with traditional solution aggregates of proteins, which often form irreversibly bound structures that are biologically inactive. Aldesleukin enhances lymphocyte mitogenesis and stimu• lates long-term growth of human lL-2-dependent cell lines. IL-2 also enhances the cytotoxicity of lymphocytes. Indac. non of NK cell and lymphocyte-activated killer (LAK) cell activity occurs, as does induction of production. In mouse and human tumor cell lines, aldesleukin activates cellular immunity in patients with profound Iymphocytosis, eosino. philia. and thrombocytopenia. Aldesleukin also activates die
production of cytokines. including tumor necrosis factor (TNF). IL-I,and IFN-y. In vivo experiments in mouse tumor models have shown inhibition of tumor growth. The media. nism of the antitumor effect of alde.sleukin is unknown. Aldesleukin is indicated for the treatment of metastalic
renal cell carcinoma in adults, It is also indicated for the treatment of metastatic melanoma in adults. Research is under way on the use of aldesleukin for the treatment of various cancers (including head and neck cancers), treatment of acute myelogenous leukemia, and adjunct therapy in the
treatment of Kaposi's sarcoma. Renal and hepatic function is typically impaired during therapy with aldesleukin, so interaction with other drugs that undergo elimination by these organs is possible. Aldesleukin is supplied as a powder for solution. After reconstitution, the solution should not be shaken. The prepa. ration is solubilized with sodium dodecyl sulfate in a phos.
Chapter 6 • l!wtec-Ii,uiloç'v mu! Drug Discovery phate buffer. Aldesleukin should be stored as nonreconstituted ;xlwder at 2 to 8°C and never frozen. Reconstituted vials
he lroocn and thawed once in 7 days without loss of .wiivitv. It expires over a period of 18 months.
Denileukin Diftitox (Recombinant.'3°
Denileukin recombinant. Ontak. is an example of a drug that
acts like a Trojan horse. One part of the molecule is involved UI recognition and binds selectively with the diseased cell.
and a highly toxic second part of the molecule effects a kill. Denileukin diltitox is a fusion protein expressed by a reconihinant str.nn of h. roll. It is a rDNA-derivcd cytotoxic protein composed of the amino acid sequences for diphtheria toxin fragments A and B (Met -Thr387)-His. followed by the
sequences for IL-2 (Alai-Thriss). The fusion protein has a molecular mass of 58.000 Da. We can think of this large salem as a molecule of diphtheria toxin in which the rcceptIlt-binding domain ha.s been replaced by IL-2 sequences. ihereb> changing its binding specificity. Cells that express the high.alTinity IL-2 receptor bind the protein tightly. The IL-2 component is used as a director to bring the
species in contact with tumor cells. The diphtheria snun inhibits cellular protein synthesis and the cells die. Malignant cells in certain leukemias and lymphonnas. includ-
ing cutaneous T-cell lymphoma. express the high-affinity Il.-2 receptor on their cell surfaces. It is these cells that tknilcukun diftitox targets.
Denileukiun diftitox is indicated for the treatment olpersisteni or recurrent cutaneotus T-cell lymphoma whose malig1:1111 cells express the CD25 component of the IL-? receptor.
Denileukin dittitox is supplied as a frozen solution in
183
thrombocylopenia. Efficacy has been demonstrated in persons who have experienced severe thrombocytopenia following a previous chemotherapy cycle. Oprelvekin causes many adverse reactions. Among these are edema. neutropenic fever, headache, nausea and/or vomiting. dyspnea. and tachycardia. Patients must be monitored closely.
Oprelvekin is supplied as a lyophilized powder for reconstitution. Excipicnts include glycine and phosphate buffer components. The powder has a shelf life of 24 months. It should be stored at 2 to 8°C. If it is frozen, thaw it before reconstitution.
Tumor Necrosis Factor (Recombinant).t33l35 The TNFs (Etanercept. Enhrel) are members of a family of cytokines that are produced pritnarily in the innate immune system by activated mononuclear phagocytes. Along with ILl. TNF is typically the first cytokine to be produced upon infection, and its reactions can be both positive and negative. On the one hand. TNF can cause cytotoxicity and inflammation, and on the other hand, it serves as a signal to the adaptive immune response. The TNFs are all endogenous pyro-
gens. and they cause chills, fever. and flu-like symptoms. There are two forms of TNF: TNF-a (eachectin) and TNF/3 (Iymphotoxin). Both bind to the same receptor and cause similar effects. Etanercept is a dimeric fusion protein consisting of the extracellular ligand-hinding portion of the human 75-kDa
(p75) TNF receptor (TNFR) linked to the Fe portion of human isotype IgGi. The Fe component of etanercept contains the domain, the CH3 domain, and the hinge region,
water for injection. It should be stored at — 10°C or colder. his suggested that the vials be thawed in a refrigerator at 2 ii for less than 24 hours or at room temperature for
hut not the Cl-I1 domaiti of IgG1. These regions are responsible for the biological effects of irnrnunoglohulins. Etanereept
a 2 hours. Prepared solutions should be used within 6 hours. The drug is administered by IV infusion from a hag
peptide chain of 934 amino acids and has a molecular mass of approximately ISO kDa. It hinds specifically to TNF and blocks its interaction with cell surface TNFRs. Each etanercept molecule hinds specifically to two TNF molecules in the
I
ii
a syringe pump.
Oprelvekin. Netusega. is recombinant human Il_-Il that is expressed in a tecaitihinunt strain of E. cob as a thiorcdoxin and/or rhiLOprelvekin
fusion protein. The fusion protein is cleaved and purilied 1 obtain the rhIL- II protein. The protein ix 177 amino acids in length and has a mass olapproximately 19.0(X) Da. Oprel-
differs front the natural I 78-amino acid IL-Il by lacking an N-terminal proline. This alteration has not resulted iii differences itt bioactivity either in vitro or in vivu. IL.l lisa thromhopoietic growth factor. It directly stimulates the proliferation of hetiiatopoietic stem cells as well as nicgakaryocyte progenitor cells. This process induces mcgasarsuicyte maturation and increased production of platelets. Die primary hettuatopoictic activity of oprelvekin is stimulation of megaknryocytopoiesis and thrombopoiesis. Primary
and mature osteoclasts express mRNAs for both
IL-Il and its receptor. IL-Il K alpha. Hence, both honeiomiing attd houue-resorhing cells are possible targets for ILOprelvekin is
indicated for the prevention of severe It reduces the need for platelet transfu-
unyclosuppressive chemotherapy itt patients with i'iiiiiiwetuid malignancies who are at high risk for severe
is produced in recombinant CHO cultures. It consists of a
synovial fluid of rheumatoid arthritis patients. It is equally efficacious at blocking TNF-a and TNF-f3. The drug is indicated for reducing signs and symptoms and inhibiting the progression of structural damage in patients with moderately
to severely active rheumatoid arthritis. Etanercept is also indicated for reducing signs and symptoms of moderately to severely active polyarticular-course juvenile rheumatoid arthritis in patients 4 years of age and older who have had an inadequate response to one or more disease-modifying antirheumatic drugs (DMARDS. Etanercept is also indicated for reducing signs and symptoms of active arthritis in patients with psorlatic arthritis.
ENZYMES
Blood-Clotting Factors The blood clotting system of the human body is typically in it carefully balanced homeostatic state. If damage occurs to a blood vessel wall, a clot will foms to wall off the damage SO that the process of regeneration can begin. Normally this process is highly localized to the damaged region. so that
184
IViLson
and Gi.vtvldx Te.vthook of Organic Medicinal and Phar,naet'uiieal chemistry
the hemostatic response does not cause thrombi to migrate to distant sites or persist longer than it is needed. Lysis of blood clots occurs through the conversion of pla.sminogen to plasmin. which causes librinolysis. converting insoluble fibrin to soluble fibrinopeptides. The plasminogen—plasmin conversion is catalyzed by several blood and tissue activators. among them urokinase. kallikrein. plasminogen activators, and some undefined inhibitors. More specifically, the conversion of plasminogen to plasniin is catalyzed by two extremely specific serine proteases: a urokinase plasmino-
gen activator (uPA) and a tissue plasminogcn activator (tPA). This section focuses on tPA. Human IPA is a serine proeasc that is synthesized in the vascular endothelial cells, It is a single-chain peplide composed of 527 amino acids and has a molecular ma.ss of approximately 64,000 Da. About 7% of the mass of the molecule consists of carbohydrate. The molecule contains 35 Cys residues. These are fully paired, giving the tPA molecule 17 disulfide bonds. There arc four N-linked glycosylation sites recognized by consensus sequences Asn-X-Ser/Thr at residues 117. 184. 218. and 448. It is suspected that
bears
an 0-fucose residue. There are two forms of tPA that differ
by the presence or absence of a carbohydrate group at Asp184. Type I tPA is glycosylated at Asn117. and while type II cPA lacks a glycosyl group at Asn215 is typically unsubstituted in both forms. Asn11-, contains a high-munnose oligosacchande. while Asn substituents 184 and 448 are complex carbohydrate substituted. Dur-
ing the process of fibrinolysis the single-chain protein is cleaved between and lIe2Th by plasmin to yield 2chain tPA. Two-chain cPA consists of a heavy chain (the A
chain, derived from the N terminus) and a light chain (B chain), linked by a single disulfide bond between and Cys395. The A chain bean. some unique structural features: the finger region (residues 6 to 36). the growth factor region (approximate residues 44 to 80). and two kringle domains. These domains are disultide-closed loops, mostly sheet in structure. The finger and kringle 2 arc responsible for tPA binding to librin and for the activation of plasminogen. The
function of kringlc I is not known. The B chain contains the serine protease domain that contains the His-Asp-Scr unit that cleaves plasminogen.
Tissue Plasminogen Activator, Recomblnant.13& 137 tPA (recombinant). alteplusc (Activase). is identical with endogenous tPA. rtPA lacks a glycosyl residue at At
one time. rIPA was produced in two-chain form in CHO cultures. Now, large-scale cultures of recombinant human melanoma cells in fermenters are used to produce a product that is about 80% single-chain rIPA. Alteplase is used to improve ventricular function following an acute myocardial infarction, including reducing the incidence of congestive heart failure and decreasing mortality. The drug is also used to treat acute ischemic stroke after computed tomography (CT) or other diagnostic imaging has ruled out intracranial hemorrhage. rtPA is also used in cases of acute pulmonary thromboembolism and is being investigated for unstable angina pectoris. Alteplase is supplied ax powder for injection, and in recon-
stituted form (normal saline or 5% dextrose in water) is intended for IV infusion only. The solution expires in 8 hours at room temperature and must be prepared just before use.
Reteplase. Reteplase (Retavase) is a deletion mutant variant of tPA that is produced in recombinant E. coli. The deletions are in domains responsible for half-life. fibrin affinity. and thrombolytic potency. It consists of the kringle2 domain and protease domain of cPA but lacks the kringle. I domain and the growth factor domain. It is considered a third-generation thrombolytic agent and has a mechanism of action similar to that of alteplase. Reteplase acts directly by catalyzing the cleavage of plasminogen and initiating thrombolysis. It has high thrombolytic potency. A comparison of alicplasc and reteplase is given in Table 6-9.
Tenecteplase.13' Tenecteplase is a iPA produced by recombinant CHO cells. The molecule is a 527-amino acid glycoprotein developed by introducing the following modifications to the eDNA construct: Thr,05 to Asp. to Gin, both within the kringle-l domain, and a tetraulanine substitution at amino acids 296 to 299 in the protease do.
main. The drug is a sterile. lyophilized powder recommended for single intravenous bolus administration after re-
constitution with sterile water. Tenecteplase should he administered immediately after reconstitution.
Factor Antihemophilic factor VIII (recomhi• nant). Recombinate. Kogenate. Biociate. Helixate. is plasma protein that functions in the normal blood-clotting a
cascade by increasing the V,rn. ftr the activation of clotting factor X by factor IXa in the presence of calcium ions and negatively charged phospholipids. Factor VIII is used in the treatment of hemophilia A. Hemophilia A is a congenital disorder characterized by bleeding. The introduction of factor VIII as a drug has improved the quality of life and the life expectancy of individuals with this disorder. Unfonunately. it ha.s been necessary to rely on an unsure soulce (human plasma) for the factor. Exposure of patients to alaantigens and viruses has been a concern. Factor VIII derived from a recombinant source will potentially eliminate man) of these problems and provide an essentially unlimited supply of the drug. Factor VIII is biosynthesized as a single-chain polypeptide of 2.332 amino acids. The protein is very heavily glycosylated. Shortly after biosynthesis. peptide cleavage occurs and plasma factor VIII circulates as an 80-kDa light dian associated with a series of heavy chains of approximate)> 210 kDa in a metal ion-stabilized complex. Factor VIII pos sesses 25 potential N-linked glycosylation sites and 22 residues. The 2l0-kDa heavy chain is further cleaved b> proteases to yield a series of proteins of molecular mass
TABLE 6—9 ComparIson of the Pharmacokinetic Parameters of Alteplase and Reteplase Phamiacokinetic Parameter Effccllvc
(minutes)
Alteplase 5
Reteplas. Is—Is
Volume of distribution (L)
8.!
5
Pb.sma clearance (mI/mitt)
360—620
250-450
Chapter 6 u ISw:e(!,,ioIogv and Drug Discovery $8 kl)a. The 90- to 188-kDa protein molecules form a reid ion-stahili,.ed complex with the light chain. Recombinant factor VIII is produced in two recombinant in hatch culture of transfccted CHO cells or in coniIwous culture of baby hamster kidney (BHK) cells. There ac four types of recombinant factor VIII available. All four ire pnduced by inserting a cDNA construct encoding the curse peptide sequence into the CHO cell or BHK cell line. c
The Cl-to cell product contains a Galaf l—.3lGal unit. cshcre.is the BilK enzyme does not. Recombinant factor VIII is pohdicperse. containing multiple peptide homologues inchiding an 80-kDa protein and various modifications of an
90-kDa subunit protein. The product contains no blood pmducts and is free of microbes and pyrogens. Recirnihinant factor VIII is indicated for the treatment of classical hemophilia (hemophilia A) and for the prevention Ireatnient of hemorrhagic episodes and perioperative management of patients with hemophilia A. The drug is also indicated for the treatment of hemophilia A in persons who psse.ss inhibitors to factor VIII. Recombinant factor VIII is supplied in sterile, single-dose slaTs. Thu product iii stabilized with human albumin and The product must be stored at 2 to 8°C. without lfceoulg. In some instances the powder may be stored at irwin lelllperature for up to 3 months without loss of biologicii activity. Shaking of the reconstituted product should be isoslud because of the presence of the albumin. The drug he administered by intravenous bolu.s or drip infusion within 3 hours of reconstitution.
Since trace amounts of mouse or hamster protein may with recombinant factor VIII, one should he cauloSs when administering the drug to individuals with known hypersensitivity to plasma-derived antihemophilic factor or with hypersensitivity to biological preparations with trace mourns of uitouse or hamster proteins.
When a percon is deficient in clotting factor IX (Christmas factor), he—
iruphilia B re.sults. Hemophilia B affects primarily males
185
Drotreco gin Alfa.'4'
About 750,000 people are diagnosed with sepsis in the United States each year. and of these, an estimated 30% will die from it. despite treatment with intravenous antibiotics and supportive care. Patients with severe sepsis often experience failures of various systems in the body. including the circulatory system, the kidneys, and clotting. Drotrecogin alfa (activated), rotrecogin alfa (activated) (Xigris). is a recombinant form of human activated protein C. Activated protein C exerts an antithrombotic effect by inhibiting factors Vu and VIlla. In vitro data indicate that activated protein C has indirect profibrinolytic activity through its ability to inhibit plasminogen activator
inhibitor-I (PAl-I) and to limit generation of activated thrombin-activutable tibrinolysis inhibitor. Additionally, in vitro data indicate that activated protein C may exert an anti-inflammatory effect by inhibiting TNF production by unonocytes, by blocking leukocyte adhesion to selectins, and
by limiting the thrombin-induced inflammatory responses within the microvascular epithelium. Vials of drotrecogin alfa should be stored at 2 to 8°C without freezing. The reconstituted solution is stable for 14 hours at 25°C.
Anticoagulant Lepirudin.
Leeches (Hirudo medicinails) have been used medicinally for centuries to treat injuries in which blood engorges the tissues. The logic behind this is solid: leeches produce an agent known as hirudin that is a potent. specific thrombin inhibitor. Leeches have been used to prevent thrombosis in the microvasculature of reattached digits. Lepirudin (Refludan) is a rDNA-dcrived protein produced in yeast. It has a molecular mass of approximately 7,000 Da. Lepirudin differs from the natural polypeptide. in that it has an N-terminal leucine instead of isoleucine and is missing a sulfate function at
Otber Eniymes
irid accounts for about 15% of all cases of hemophilia. Treat-
Recombinant Human Deoxyribonuciease I
neni iuusolves replacement of factor IX so that the blood will clot. Recombinant coagulation factor IX (BeneFix) is a highly purified protein produced in recombinant CHO cells. free of blood products. The product is a glycoprotein of nalecutar mass approximately 55.000 Da. It consists of 415 inrtno acids in a single chain. The primaty amino acid wequenccofBeneFix is identical with the Ala1.,5 allelic form of plasria-iierived factor IX. and it has structural and functional characucristies similar to those of the endogenous protein. The recombinant protein is purified by chromatography. folowed by membrane filtration. SDS-polyacrylamide gel ckcrnuphoresis shows that the product exists primarily as a
DNAse is a human endonuclease, normally present in saliva, urine, pancreatic secretions, and blood. The enzyme catalyzes the hydrolysis of cxtracellular DNA into oligonucleo-
sngte component.
Clotting factor IX. recombinant, is indicated for the conmci arid prevention of hemorrhagic episodes in persons with hemophilia B (Christmas' disease), including the control and presention of bleeding in surgical procedures.
tides. Aerosolized recombinant human DNAse (rhDNAse). dornase alfa, Pulmozyme, has been formulated into an inhalation agent for the treatment of pulmonary disease in patients with cystic fibrosis (CF). Among the clinical manifestations of CF are obstruction of the airways by viscous, dehydrated mucus. Pulmonary function is diminished, and microbes can become entrapped in the viscid matrix. A cycle of pulmonary obstruction and infection leads to progressive lung destruction and eventual death before the age of 30 most CF patients. The immune system responds by sending in neutrophils, and these accumulate and eventually degenerate, releasing large amounts of DNA. The high levels of extracellular DNA released and the mucous glycoproteins are responsible for the degenerat-
BeneFix is supplied as a sterile lyophilized powder. It
ing lung function. The DNA-rich secretions also bind to
should be stored at 2 to 8°C. The product will tolerate storage
aminoglycoside antibiotics typically used to treat the infections. In vitro studies showed that the viscosity of the secretions could be reduced by application of DNAse I. Before DNAse was purified and sequenced from human
at
temperature not above 25°C for 6 months The drug unstable following reconstitution and must be used
wIthin 3 hours.
186
Wi/con
and Gi.ccohi '.c
of ()r,c,'anie Medic'i,ial and Pharmaceutha! ChernLs,rv
sources, a partial DNA sequence from bovine t)NAse (263 amino acids) was used to create a library that could he used to screen a human pancreatic DNA library. This facilitatcd the development of the human recombinant protein. The endogenous human and recombinant protein sequences are identical. Recombinant human deoxyrihonuclease I irhDNAse) was cloned, sequenced. and expressed to examine the potential
given here. Vaccine production is a natural application of rDNA technology, aimed at achieving highly pure and efficacious products. Currently, there are four rDNA vaccines approved for human use. A number of others are in clinical trials for some rather exotic uses. It would appear that biotechnological approaches to vaccines will bring about some very useful drugs.
of DNAse I as a drug for use in CF. It has been shown that cleavage of high-molecular-weight DNA into smaller
PRODUCTS
fragments by treatment with aerosolited rhDNAse improves the clearance of mucus from the lungs and reduces the exacerbations of respiratory symptoms requiring parenteral antibiotics.
rhDNAse lisa monomeric glycoprotcin consisting of 260 amino acids produced in CHO cell culture. The molecule possesses four Cys residues and two sites that probably con-
tain N-linked glycosides. The molecular mass of the molecule is about 29 kDa, DNAse I is an endonuclease that cleaves double-stranded DNA (and to sonic extent singlestranded DNA) into 5'-phosphate-terminated polynucleotides. Activity depends on the presence of calcium and magnesiuni ions. Pulmozyme is approved (icr use in the treatment of CF patients. in conjunction with standard therapies. to reduce the frequency of respiratory infections requiring parenteral antibiotics and to improve pulmonary function. The dose is delivered at a level of 2.5 mg daily with a nchuli,er. Pulmo,.yme is not a replacement for antibiotics. bronchodilators. and daily physical therapy. Type I Gaucher's disease is a hereditary condition occurring in about 1:40.0(X) individuals. It is characterized by a functional deficiency in enzycne activity and the resulting accumulation of lipid glucocerebroside in tissue macrophages. which become engorged and are termed Gaudier s (c/is. Gaucher's cells typically accumulate in the liver, spleen. and bone marrow and. occasionally, in lung. kidney. and intestine. Secondary hematological sequelae include severe anemia and throinbocytopenia in addition to characteristic progressive hepatosple-
cerezyme.
nomegaly. Skeletal complications are common and are frequently the most debilitating and disabling feature of Gaucher's disease. Possible skeletal complications are oxteonecrosis. osteopenia with secondary pathological fractures. remodeling failure. osteosclerosis. and hone crises. Cerezyme (Imiglucerase)'44 is a recombinant, macrophage-turgeted variant 01' human $-glucocerebrosidase. punfled from CHO cells. It catalyzes the hydrolysis of the glyco-
lipid glucocerebroside to glucose and ceramide follosving the normal degradation pathway for membrane lipids. Cerezyme is supplied as a lyophilized powder for reeotistitution. The powder should be stored at 2 to 8°C until used.
The reconstituted product for IV infusion is stable for 12 hours at room temperature.
VACCINES Vaccines and immunizing hiologicals are covered thoroiivhlv in Chaoter 7 of this text, so no lengthy discussion is
Recombivax and Engerix-B.'45
Recombivax and Engerix-B are interchangeable for immunization against hepatitis 13 virus (I-IBV. serum hepatitis). I3oth contain a 226amino acid polypeptide composing 22-nm-diameter particles that possess the anhigenic epitopes of the HBV surtlict coat (S) protein. The products from two manufacturers arc expressed from recombinant S. e'erevisiae. It is recommended that patients receive 3 doses, with the secotid dose I month after the first and the third dose 6 tiionths after the first. The route and site of injection are IM in deltoid muscle or. tar infants and young children, in the anterolateral thigh.
The vaccines achieve 94 to 98% immunogenicity amont adults 20 to 39 years of age I to 2 months alter the third dose. Adults over 40 years of age reach 89% immunogenic. ity. young children, and adolescents achieve 96 to 99r4 immunogenicity. The vaccine is supplied as a suspension adsorbed to alumi-
num hydroxide. The shelf life is 36 months. The vaccine should be stored at 2 to 8°C and should be discarded if Ira.'.en. Freezing destroys potency. l.yme disease is caused by the spirochete Liorrelia burgdo,frri. The microorganism is transmitted pci. niarily by ticks and is endemic in heavily wooded areas and forests. The disease produces arthritis-like symptoms. A cine against Lyine disease was created by developing a combinant E. that contains the gene for the bacterial outer surlicce protein. This protein (OspA) is a single peptide chain of 257 amino acids with covalently bound lipids at the N terminus. The vaccine is formulated as a suspetision with alutnintttn hydroxide as an adsorption adju. vant. In testing. subjects between IS and 70 years with 3 doses of LYMErix at 0, I. and 12 months demon.
strated a 78% decrease in the likelihood of infection. LYMEr1x has a shelf life of 24 months. It should he stored
at 2 to 8°C and must he discarded if frozen, If necessar), the vaccine can tolerate 4 days at room temperature. C'omvax. Comvax is a combination of llaeinophilm in fluenzae type b conjugate and hepatitis B (recombinant). Ii was recently approved by the Advisory Committee on fin munization Practices (ACIP). Each 0.5-mL dose containc 7.5 of H. inj1sa'nzae type h polyribosylnibitol (PRP). 125 of Nejsse'rja ineningitidis outer membrane protein complex (OMPC). and 5 of hepatitis B surface antigen (HhsAg) on an aluminum hydroxide adjuvant. 'fix Committee on Infectious Diseases, the American of Pediatrics, and the Advisory Academy of Family Phyci cians recommend that all infants receive the vaccine. doses should be administered at ages 2. 4. and 12 to months. The vaccine should not be administered to infants younger than 6 weeks because of potential suppression c
Chapter 6 • Biotechnology and !)nig Discoi'erv
TABLE 6-10 Vaccines Developad
Using Biotedinology Type
Vaccine
Use
Phase of Development
Vaccine
Breast. cotorectal. lung cuncers: nielanoma; saucoma
II
Vuccjnc
Metaslalic melanono
It
Vaccine
Multiple mycloma
It
Ibsirna-densed diotvpie Ag vaccine
Vaccine
Multiple niyelisma
'tsLssn: rudinuma lherncciric,
Therapeutic vaccine
Stage 4 malignant melanoma
U \t1F- 2 I 7))-. 7))
nid.ssoma sgci.-inc
II 5)11
187
I
lit
vaccine
therapeutic vuceinc
NHl-o)24
vaccine F vaccine
ii
Hepatitis
prophylanii.
Li
Vaccine
Group A streptococci, including nccrotiting Iasciitiv, strep tiiroai. and rheumatic icvcr
it
Vacdnes In Development Qnilc a number of biotechnology-generated vaccines arc in dcclllpnicnt (Table 6-10). Some of them are in the category
These vaccines are designed to
hind to cellular receptor. endogenous molecules, and so on. pnslucing specific pharmacological effects. For example, if has a particular receptor that binds a ligand to activate ihe cell. binding an antibody raised by a specific vaccine to receptor will prevent activation. If a tumor has a requirerent for such a receptor—ligand binding. using a vaccine to delelop antibody to the receptor or the ligand should prevent 'r slow cellular proliferation.
PREPARATION OF ANT(BODIES147 149
Ilybridoma (Monodonal Antibody [Mab]) TechnIques
a humor-ji immune response. H-lymphocyte-derived cells produce antibodies with variations in chemical
Iructure. Biologically, these variations extend the utility of the secreted antibody. These variations are caused by affinity maturation, the tendency for the affinity of antibody for anti-
ecu to increase with each challenge, and mutation at the lithe of somatic recombination. These phenomena produce jniilwdies with slightly different speciticitics. Because the clones of antibody-producing cells provide more than one lnjcturjl type of antibody, they arc called polyclonal antslvdirn. Another type of antibody consists of highly homogeUcI)Us populations of hybrid proteins produced by one clone f specially prepared B lymphocytes. These antibodies, lack-
op structural variations. arc highly "focused" on their anticcthic counterparts' determinants or epitopes. and arc called 71,hhsii/)hhhul.
inkctii,n
ill
Recnntbinaiil subunit vaccine
months.
In
Gnstrocsaphageai rellun disease
Cellular vaccine
immune response to PRP-OMPC with subsequent doses ('tsmvax TM. The series should be completed by I 2 to
therapeutic vaccines
ii
I diabetes
Vaccine
Ca.'nn therapeutic vaccinc
A problem with creating MAbs is that one cannot simply prepare an antibody-producing H lymphocyte and propagate it. Such cells live only brietly in the laboratory environment. Instead, antibody-producing cells are fused with an immortal
(tumor) cell line to create hvbriduinas—long-lived. antibody-secreting cells. The trick is to select the monoclonal cells that produce the desired antibody. The hybridoma technique ha,s opened the door to new therapeutic antibodies. imaging agents, radiological diagnostic test kits, targeted radionuclide delivery agents, and home test kits.
In the hybridoma method (Fig. 6-14). a mouse or other small animal is sensitiied with an antigen. When a high enough titer of antibody against the selected antigen has been attained, the animal is sacrificed and its spleen cells are collected. The spleen cells contain a large number of B lymphocytes. and it is certain that some will he able to produce antigen-specific antibodies. Because the spleen cells are normal B lymphocytes, they have a very short lifetime in cell cultures. Therefore, a method must be used to extend their lifetime. To produce MAbs, B cells are fused with immortal myeloma cells in the presence of lusogens such as polyethylene glycol. This procedure produces genetically hall-normal and
half-myeloma cells. Since the niyelonta cells arc immortal. the longevity problem is solved. The selcction process depend.s on two different myeloma cell lines: one lacking the enzyme hypoxanthine-guanine phosphoribosyl transferase (HGPRT). a key enzyme in the nucleotide salvage pathways. and the other lacking the Th gene, a key gene in the pyrimidine biosynthetic pathways. The spleen H cells arc HGPRT
and Tk (+1, while the myeloma cells are HGPRT and Tk (—). This mycloma cell line cannot survive in a medium containing aminopterin, a thymidylate synthetasc inhibitor. because it cannot synthesice pyrimidines. The HGPRT (—) cell line cannot use the purine salvage pathways to make
nucleotides. lotting it to use thyinidylale synthetuse. With thymidylate synthetase inhibited, the cell dies. After fusion. cells arc maintained on a medium containing hypoxunthine. aminopterin. and Ihymidine (HAT). Only cells that are "cor-
188
Wilson and Gisvoids Textbook of Organic Medicinal and Pharmaceutical Chemistry
Ag sensitization of mouse
•O
2. Isolate mouse spleen cells
+
(I) I
• 0
4.Cellfuston
t:•
I
I
I
I
t:•
5, Hybridoma selection In
HAT medium
I
I
I
I
000000000000 ooooooo•oooo oooo•ooooooo 000000000000 oooooo•ooooo I
00000000
3. HPRT (—)
myeloma cells
6. Ab screenIng
00
OOS0000 96S 00
( 7. Selection of Ab(+) clones 8. Prollleratlon
+ 10. Monoclonal Abs from ascites
9. Monoclonal Abs (mm cell culture medium
Purification
Punficabon
FIgure 6—14 u General method for preparation of monodonal antibodnr, using hybridomas and HAT medium antibody; Ag. antigen.
rectly" fused between one spleen cell (HGPRT +]) and
(known as HAMA) ha.s tended to limit the use olmonoclora!
one myeloma cell (immortal). i.e.. a hybridoma. can survive
in human therapy. In developing a method for making MAbs useful in Is mans, it is necessary to remove the mouse characteristics from the MAb. The antigen-recognition is gion (Fab) of the MAb must retain its ability to bind to antigen. however, If this feature is altered, the antibody nil likely be useless. Within the light and heavy chains of b Fab portions of antibody molecules arc regions that called complementarity'de:ermining regions or CDRs.
in HAT medium. Fused myeloma cells (myeloma—rnyeloma) lack the correct genes and cannot survive. Fused spleen cells (spleen—spleen) cannot grow in culture. Thus, only the fused hybridomu (myeloma—spleen) survives. Hypoxanthine and thymidine furnish precursors for the growth suppresses eelts that of HG?RT (+ 1 ceVts. failed to fuse. Hybridomas can be isolated in a 96-well plate and transferred into larger cultures for proliferation, The cul-
ture medium will eventually contain a high concentration of MAb against the original antigen. This antibody can be purified to homogeneity. Monoclonal antibodies, being proteins, tend to be highly
chain possesses three of the.se. One of the CDRs. CDR3.
immunogenic in humans. This is especially true of the MAbs produced in mouse culture. Humans begin to develop antibodies to mouse MAbs after a single dose. This is natural. The human host is mounting an antibody response to a for-
trated there. These must be intact for specific antigcn-ans body binding. Immune responses against murine MAb r:
eign antigen. The human antimouse antibody response
located at the juncture of the variable and common domais
CDR3 is also referred to as the hypervariable region most of the variability of the antibody molecule is
directed against not only the variable regions, but constant regions. Hence, to decrease the immunogenicil) an MAb one must create antibodies that have been
Chapter 6 U Biau'chuo!ugv and Drug
ized." In MAI production. usually the
and V1 domains of a human antibody are replaced by the corresponding reglans from the mouse antibody, leaving the specificity intact. hut using human constant regions that should not be immunogenic. Antibodies like these are called numeric, and they ate less immunogenic and have a longer half-life in human patients. Examples of chimeric MAbs are abciximub. rituximab. iniliximab. and basiliximab. Methods are available for the development of MAbs with 95 to 100% human sequence. By using transgenic mice, all of the essential human antibody genes can he expressed.
Monodonal AnUbody Drugs Rituximab.'50' 151
Rituximab (Rituxan. Chimeric) is an MAb directed against the CD2O antigen expressed on the of normal and malignant B lymphocytes. The MAb k produced in mammalian (CHO) suspension culture and is achimeric (nwrine/human) MAb of the lgG1 type. The protein is composed of murine light and heavy chain variable regions and human constant regions. Rituximab is indicated fur the treatment of patients with relapsed or refractory. low-
grade or follicular. CD2O( +) B cell non-Hodgkin's lymphoma. Rituximab binds specifically to antigen CD2O human B-lymphocyte-restricted differentiation antigen, a hydrophobic transnrembrane protein expressed on pre- and mature-B lymphocytes). CD2O is a protein of 35 to 37 kDa. and it may play a role in B cell activation and regulation and may be a calcium ion channel. The antigen is also cxpressed on more than 90% of non-Hodgkin's lymphoma B cells hut is not found on hematopoietic stem cells. pro-B or other normal tissues. CD2O cells, normal plasma regulates the early steps in the activation process for cellcycle initiation and differentiation. Gemtuzumab
153
Gennuzunsab
fanricin (Mylotarg. fusion molecule) is an MAb derived from the CD33 antigen, a sialic acid-dependent adhesion protein expressed on the surface of leukemia blasts and im-
mature normal cells of myelomonocytic origin but not on
189
rivativc is released inside the lysosomes of the myeloid cells.
The released calicheamicin derivative binds to the minor groove of DNA and causes double-strand breaks and cell death. 155 Alemtuzuniah (Cumpalh) is humanized MAb (Campath- I H) that is directed against the 21-
to 28-kDa cell surface glycoprotein CD52. CDS2 is expressed (In the surface of normal and malignant B and T lymphocytes. NK cells. naonocytes. mztcrophages. and nissties of the male reproductive system. The Canipath- I H antibody is an lgG1 K form with humanized variable and constant regions and CDRs from a rat MAb. Campath-IG.
Alemtuzumah is indicated for the treatment of B-cell chronic lymphocytic leukemia in patients who have been treated with alkylating agents and who have failed on this therapy. Alemnuzumab binds to CD52. a nonmodulating antigen that is present on the surface of essentially all B and
T lymphocytes: most monocytes. niacrophages. and NK cells: and a suhpopulation of granulocytes. The proposed mechanism of action is antibody-dependent lysis of leukemic cells following cell surface binding. Ralixiniab (Simulect. Chimeric) is an MAb produced by a mouse monoclonal cell line that has been engineered to produce the busiliximah lgG1 antibody glycoprotein. The product is chimeric (murine/human). Basiliximah is indicated for prophylaxis of acute organ rejection in patients receiving renal transplantation when used as part of a regimen of immunosuppressants and corticosteroids. Basiliximab is also indicated in pediatric renal transplantation. Basiliximab specifically hinds to the lL-2 receptor achain (the CD2S antigen, part of the three-component IL-2 receptor site). These sites arc expressed on the surfaces of activated T lymphocytes. Once hound it blocks the lL-2a recep-
tor with extremely high affinity. This specific. high-affinity binding to IL-2a competitively inhibits lL-2-mcdiatcd activation of lymphocytes, a critical event in the cellular immune response in allogralt rejection.
satins! hematopoietic stem cells. CD33 binds sialic acid and
to regulate signaling in myeloid cells. The antibody sreconrhinant. humanized lgG3 K. linked with the cytotoxic
anutumor antibiotic ozogamicin ((mm the calicheamicin family). More than 98.3% of the amino acids of genituzumab are of human origin. The constant region of the MAb conrains human sequences. while the CDRs derive from a mutine antibody that binds CD33. The antibody is linked to Nrcelvl-y-calicheamicin via a bifunctional linker. Cemtuzumah ozogamicin is indicated for the treatment of patients with CD33-lxssitive acute myeloid leukemia in first elapse utliong adults 60 years of age or older who are not considered candidates for cytotoxic chemotherapy. Gemtuzuinah ozogamicin binds to the CD33 antigen cxby hcmatopoietic cells. This antigen is expressed on he surface of leukemic blasts in more than 80% of patients
Daclizumab.159' 160
Molecularly. daclizumab (Zetapax. Chimeric) is an imniunoglobulin G (lgG1) MAb that binds specifically to the a subunit of the lL-2 receptor (the complete. high-affinity activated IL-2 receptor consists of interacting a. and y subunits). IL-2 receptors are expressed on the surfaces of activated lymphocytes, where they
mediate lymphocyte clonal expansion and differentiation. Daclizumab is a chinieric proteiti (90% human and 10% mouse) IgGu. The MAb targets only recently activated T cells that have interacted with antigen and have developed from their naïve lirm into their activated form. It is at this time that the lL-2 receptors are expressed. The human amino acid sequences of daclizumab derive from constant domains
of human lgG. and the variable domains are derived from the fused Eu myelorna antibody. The murine sequences de-
rise from CDRs of a mouse anti-IL2a antibody. The indications for dacli,uniah are prophylaxis of acute
sith acute myeloid leukemia. CD33 is also expressed on nunnaf and Icukemic mycloid colony-forming cells. includng kukemic clonogenic precursors, hut it is not expressed
organ rejection in patients receiving renal transplants, as part
srplunpotetn heniatopoietic stem cells or nonhematopoietic Binding of the ant i-CD33 antibody results in a complex hat inlernali,.ed. On internalization the calicheanuicin de-
of an immunosuppressant regimen including cyclosporine and corticosteroids. The mechanism of action is the same as that of basiliximab.
190
Wi/so,, wul Gis,y,hPs Tt's,hook of Organic Medicinal and PF,ar,nace,uical ('lu'niisfn'
Muromonah-CD3 (murine. Orthoclone-OKT3) is an unmodified mouse immunoglobulin. an monoclonal. It hinds a glycoprotein on the surface of mature I lymphocytes. Mature T cells have. as pan of the signal transduction machinery of the T-cell receptor complex. a set of three glycoproteins that arc collectively called CD3. Together with the protein zeta, the CD3 molecules become phosphorylaled when the 1-cell receptor is bound to a peptide fragment and the major histocompatibility complex. The phosphorylated CD3 and zeta molecules transmit information into the cell, ultimately producing transcription factors that enter the nucleus and direct the T-cell activ-
ity. By binding to CD3. niuron,onah-CD3 prevents signal transduction into T cells. Muromonah-CD3 blocks the function of T cells that are involved in acute renal rejection. Hence. it is indicated for the treatment of acute allograft rejection in heart and liver transplant recipients resistant to standard steroid therapies.
Abciximab.'"' 165 Abciximab (ReoPro, chimeric) is an MAb engineered from the glycoprotcin lib/Illa receptor of human platelets. The preparation is fragmented, containing
only the Fab portion of the antibody molecule. This MAb is a chimeric human—mouse immunoglohulin. The Fob fragments may contain mouse variable heavy- and light-chain regions and httman constant heavy- and light-chain regions.
Abciximub is indicated as an adjunct to percutaneous transluminal coronary angioplasty or athcrectomy for the prevention of acute cardiac ischcmic complications in patients at high risk far abrupt closure of a treated coronary vessel. Abciximah appears to decrease the incidence of myo-
cordial infarction.
Abciximah hinds to the intact GPllb/GPIIIa receptor, which is a member of the integrin family of adhesion receptors and the major platelet-specific receptors involved in aggregation. The antibody prevents platelet aggregation by preventing the binding of fibrinogen. the von Willebrand factor. and other adhesion molecules on activated platelets. The inhibition of binding to the surface receptors may be due to steric hindrance or conlormauonal effects preventing large molecules from approaching the receptor. Trastuzumab.'66' 767
Trastuzumab (Herceptin. human-
ized) is an MAb engineered from the hutnan epidermal growth factor receptor type 2 (HER2) protein. This MAb is a human —niurine immunoglobulin. II contains human structural domains (framework) and the CDR of a murine antiis the body (4D5) that hinds specifically to HER2. lgG, type structure, and the antibody is monoclonal. The protein inhibits the proliferation of human tumor cells that overexpress HER2. Trastuzumab is indicated for use as a single agent far the
treatment of patients with metastatic breast cancer whose tumors overexpress the HER2 protein and who have not received chemotherapy for their metastatic disease. The HER2 proto-oncogene encodes a transmembranc receptor protein of 185 kDa that is structurally related to the epidermal growth factor receptor HER2. Overexpression of this protein is observed in 25 to of primary breast cancers. Trastuzumab binds with high affinity to the extracellular domain of HER2. It inhibits the proliferation of human
tumor cells that overexpress HER2. Trastuzumab also mediates the process of antibody-mediated cellular cytotoxicity (A DCC). This process, leading to cell death, is preferentially exerted on HER2-overexpressing cancer cells over those that do not overexpress I-IER2. 169 The MAb infliximab (Remicade. chimend is produced from cells that have been sensitized with human TNF-a. The MAb is a chimeric human—mouse immunoglobulin. The constant regions are of human peptide sequence and the variable regions are murine. The MAb is
of type lgG, K. lafliximab is indicated for the treatment of moderately to severely active Crohn's disease to decrease signs and symptoms in patients who had an inadequate response to conven-
tional treatments. Inflixiniab binds specifically to TNFa. It neutralizes the biological activity of TNFa by binding with high affinity to soluble and Lransmemhrane forms 01' the TNF. Infliximab destroys TNFa-producing cells. An additional mechanism by which inflixim-ab could work is as follows: by inhibiting TNFa. pathways leading to IL-I and IL-6 are inhibited. These interleukins are inflammatory cytokines. Inhibiting their production blocks some of the inflammation common to Crohn's disease.
Monoclonal Antibody Radlonuclide Test
Kib
Arcitumomab."°
Arcitumotnab (CEA-Scan) is a marine monoclonal Fab' fragtnent of IMMU-4, an MAb generated in murine ascites fluid. Both IMMU-4 and arcitumnomal, react with careinocmbryonic antigen (CEA). a tumor-Mood-
ated antigen whose expression is increased in a variety of carcinomas, especially those of the GI tract. The preparation is a protein, tnurinc Ig Fob fragment from lgG,. for chemical
labeling with Tc-99m. Arcitumomabfl'c-99m is for use with standard diagnostic evaluations for detecting the presence, location, and extent of recurrent or metastatic colorectal carcinoma involving the liver, extrahepatic abdomen, and pelvts. with a histologically confirmed diagnosis. IMMU-4 (and the Fab' fragments of
arcitumomah) bind to carcinoembryonic antigen (CEA). whose expression is increased in carcinoma, Arcitumomabl Tc-99m is injected, and the radionuclide scan is read 2 toS hours later.
Nofetumomab Merpentan.'7'
Nofetumomab mc,pentan (Verluma Kit) is the Fob fragment derived t'rom the
murinc MAb NR-LU-lO. The product is a protein. tnonoclonal that has been fragmented from NR-LU-I0. Nofetumomab possesses only the Fab portion. NR-LU- It) aM nofetumomab are directed against a 40-kDa protein antigen that is expressed in a variety of cancers and some normal tissues.
Nofetumomab is indicated for the detection and cvalm• tion of extensive-stage disease in patients with biopsy-con firmed, previously untreated small cell lung cancer by scan. CT scan (head, chest, abdomen) or chest x-ray. Nofetuntomab merpentan possesses a linker and a chelator that binds the technetium to the peptide. This is a phenthioac ligand. 2.3.5.6-teu-afluorophenyl-4.5-bis-S-I I -ethoxycthyll-
thioacctoamidopentanoate. hence the name
tan.
Chapter 6 • fuiozeelinulogv and I)rn,ç' Di woven Pendetide.'72 Satumomab pendetide inurine) is a kit for In-Ill. Satumonnab is pre-
Satumomab
irom a marine antibody raised to a membranc-enriched euract of human breast carcinoma hepatic metastasis. It is and monoclonal. The MAb recoglgG, ni,cs tumor-associated glycoprotein (TAG) 72. a mucin-like with a mass greater than 100.00(1 Da. Satitmoniab is indicated as a diagnostic aid in detennining the extent and location of extrahepatic malignant disease in patients with known colorectal and ovarian cancer. This aeenl is used after standard diagnostic tests are completed and when additional information is needed. The cancer must lv recurrent or previously diagnosed by other methods. Satutnoma), localizes to TAG 72. The antibody is chemi-
modified so that it links to radioactive indium-Ill. ahich is mixed with the antibody just prior to injection.
191
MAb derived from an initial sensitization with CD2O antigen. expressed on the surface of normal and malignant B cells, The antibody is a murine IgG1 K subtype, directed against CD2O antigen. It is produced in a CHO cell line. Ibritumomab ii. indicated for use as a multistage regimen to treat patients with relapsed or refractory low-grade. Ibllicular. or transformed B-cell non-Hodgkin's lymphomu. including patients with rituximab-rel'ractory follicular non-Hodgkin's lymphoma. Ibritumomab tiuxetan binds specifically to CD2O antigen
(human B-lymphocyte-restricted differentiation antigen). CD2O is expressed on pre-B and mature-B lymphocytes and
on more than 90% of B-cell non-Hodgkin's lymphoma. When the CDR of ibritumomab tiutuxan binds to the CD2O antigen. apoptosis is initiated. The tiutuxan chelate binds
Imagine techniques will reveal the localization of the satu-
indium-Ill and yttrium-90 tightly. Beta emission induces cellular damage by forming free radicals in the target
iramab as "hot spots." To link the indium-Ill to the satuniomab protein, a linker-chelator is used. This is glycylI-(N*diethylenetriacnincpentaacetic acid)-lysine hy-
cells and neighboring cells. Tiutuxan is IN-12-bis(carboxy' meihyl)aminoj-3-(p-isothiocyanatophenyl)propyll-IN-12bis(earboxymethyl)aminoj2-(methyl)-ethyl glycine.
drochloride.
frndromab Pentetate.'73
lmciromab pentetate (mu-
nrc: Myoscint Kit for the preparation of indium-Ill imcironub pentelute) is a murine iinmunoglobulin fragment raised is the heavy chain of human myosin. The drug is a protein the clans. It is monoclonal, consisting of the Fabbinding fragmrnts only, and it is bound to the linker-chelator
diethyleneiriamine pentaacetic acid for labeling with indium-Ill. lmciromah binds to the heavy chain of human the intracellular protein found in cardiac and skcle41 muscle cells.
lmciromah pentetate is indicated for detecting the prcsence and location of myocardial injury in patients after a 'uspected myocardial infarction. In normal nsyocardium. intnacchular proteins such as myosin are isolated from the cxrasascular space by the cell membrane and are inaccessible oantib4xly binding. After myocyte injury the cell membrane oss integrity and becomes permeable to macromolecules.
In-Home Test Kits"6 There are a variety of MAh-bascd in-home test kits that are designed to detect pregnancy and ovulation. For example, a pregnancy test kit targets the antigen human chorionic gonadotropin and displays a certain sign if the test is positive.
The other type of test kit predicts ovulation by targeting luteinizing hormone in the urine. Just before ovulation. luteinizing hormone surges. The test kit is designed to detect based on and signal the time of ovulation. These test the complex techniques of MAbs. are designed to be as.sirnpIe and error-free as possible for patients.
GENOMICS a term that means "a study of genes and their functions." Currently. genolnics is probably the central Genornics177 is
'stick allows lmciroinab-ln- Ill to enter the cells. where it
driving force for new drug discovery and for novel treat-
hinds to intracellular myosin. The drug localizes in infarcted t;v,ucs, where radionuclide scanning can visualize it.
nnents for disease. Gene therapy is a concept that is often discussed. The human genome project. which was largely completed in the year 2000. provided over 4 billion base pairs of data that have been deposited in public databases. Sequencing the genome itself was an enormous task, but the correlation of genomic data with disease states, sites of microbial attachment, and drug receptor sites is still in its infancy. Once these problems are solved. genomic data will be used to diagnose and treat disease and to develop new drugs specifically for disease statc.s (and possibly specific for a patient). Studying the genetics of biochemical pathways will provide an entry into enzyme-based therapies. There will undoubtedly be a host of new targets for drug therapy. Because deciphering the inlonnation that the genomic sequence provides is a complex undertaking. these benefits are probably going to occur years in the future.
Pendetide.'74 Capromab (ProstaScint Kit ii the preparation of In-I II capromab pendetide. murine) an \IAb Iniurine lgG1 id that derives from an initial sensiCapromab
'i,ation with a glycoproicin expressed by prostate epithelium Inuwn as prostate .xurface inenibrane antigen (PSMA). The
recogniics PSMA specifically and thus is specific for adenocarcinomas, The drug is used in newly dinguiocd patients with proven prostate cancer who are at high rl.k pelvic lymph metastasis, PSMA has been found in Iluny primary and mciaistatic prostate cancer lesions. The
domain marker 7El l-CS.3 reacts with more kin 9V tif adenocarcinonnas evaluated.
To join the indium-Ill to the antibody, a linker-chelator 'aced. This moiety is glycyltyrosyl-(N-ethylenelriaminepeitaicelic acid)-lysine 1-ICI.
A Therapeutic Radlonudide Monodonal Ibritumomab (Zevalin kits to ln.l II Zevalin and Y-90 Zevalin, murine) is an
Ibjitumomab Tiuxetan.
Unraveling the Genomic Code t. Determine Structure-Function Relationships: Bloinfonnatics When considering the topic 01' hioinformatics. one must rec-
ognize that thin is a broad term covering many different
292
WiIcon
and Gisio!d.s 'texibook of Organie Medicinal and Pharniaceutica! clu'nsic:rv
plasma sulfadoxine concentration occurs in 2.5 to 6 hours, and the peak plasma pyrirnethantine concentration occurs in 1.5 to 8 hours. Resistance has developed, much of ii involving mutations in either or both of the genes coding dihydrotolate reductase and thymidylate synthase.
Atovaquone and Pro guanil HCI.
Atovaquone and proguanil HCI (Fig. 9-8) are administered in combination
(Malarone) in an atovaquone-to-proguanil HCI ratio of 2.5:1. measured in milligrams (not millimoles). Proguanil. developed in 1945. is an early example of a prodrug. It is metaboliLed to cycloguanil (Fig. 9-9). primarily by CYP
2C19. The polymorphic nature of this hepatic enzyme explains why certain suhpopulations do not respond to proguanil: they cannot convert proguanil to the active cycloguanil. The basis for this combination is two distinct and unrelated mechanisms of action against the parasite. Atovaquonc is a selective inhibitor of the PIas,nodiurn's mitochondrial electron transport system. and cycloguanil is a dihydrofolate reductase inhibitor. Atovaquone's chemistry is based on it
being a naphthoquinone that participates in duclion reactions as part of its quinone—hydroqttinon tern. It is patterned after coenzyme Q. found in
electron transport chains. The drug selectively inte with mitochondrial electron transport, particularly parasite's cytochrome he1 site. This deprives the needed ATP and could cause it to become anacnibic. tance to this drug comes from a mutation in the paia cylochrome. Cycloguanil (proguanil) interferes with deoxythymid synthesis by inhibiting dihydrofolate reductase (see F and the pyrimethamine discussion). Resistance to cycloguanil is attributed to amino acid changes drofolate reductase binding site. Its elimination hnlf.lrl to 72 hours> is much shorter than that of the other anna ial dihydrofolate reductase. pyrimethamine (mean din
tion half-life of Ill hours). The combinaticit is against both erythrocytic and exoerythrocytic Plussnsj
This drug combination is indicated for malaria chloroquine, halofantrine. mefloquine. and main site is the sporozoite stage (site liii Fig. 9.li.
H—N
H
/ H3C
Proguanlt (Chioroguanide)
H—N
\\/
'C —N/ / \
H
H
C—N "CH3
shifts
/
H
H
FIgure 9—9 • Conversion of proguanil to cydoguan Cydoguanlt (actIve metaboilte)
2C 19.
Chapter 6 • Biotechnology and Drug Discovery vanous points in the array are acquired in a computer for analysis. As an
example. we can consider two cells: cell type I. a healthy cell, and cell type 2, a diseased cell. Both cell types contain an identical set of four genes: A.B.C. and D. mRNA is isolated from each cell type and used to create fluorescenttagged cDNA. In this case, red and green are used. Labeled samples are mixed and incubated with a microarray that con-
tains the immobilized genes A, B, C, and D. The tagged molecules bind to the sites on the array corresponding to the genes being expressed in each cell. A robotic scanner, also a product of silicon chip technology, excites the fluorescent lahels. and images are stored in a computer. The computer can compute the red-to-green fluorescence ratio, subtract out background noise, and so on. The computer creates a table of the intensity of red to green fluorescence for every point in the matrix. Perhaps both cells express the same levels of gene A. cell I expresses more of gene B. cell 2 (the diseased cell) expresses more of gene C, and neither cell expresses gene D. This is a simplistic explanation; experiments have been reported in which as many as 30.000 spots have been placed in the microarray. DNA microarrays can detect changes in gene expression levels, expression patterns (e.g., the cell cycle). genomic gains and losses (e.g.. lost or broken parts of chromosomes
in cancer cells), and mutations in DNA (single nucleotide polymorphism ISNPsI). SNPs are also of interest because they may provide clues about how different people respond In a single drug in different ways.
word proteoine describes protein expressed by a geanne. Proteomics is a scientific endeavor that attempts to study the sum total of all of the proteins in a cell from the print of view of their individual functions and how the interaction of specific proteins with other cellular components affects the function of these proteins. Not surprisingly, this is a very complex task. There are many more proteins than there are genes, and in biochemical pathways, a protein The
rarely acts by itself. At present, we know that the expression if multiple genes is involved for any given disease process.
"Simply" knowing the gene sequence rarely unmasks the lanction of the encoded protein or its relevance to a disease. Csiniequently, the science of pmteomics is not developed to
he point at which drug discovery can be driven by gene sequence information. There have been, however, some sig-
technology-driven approaches to the field. Highthroughput high-resolution mass spectroscopy allows the amino acid sequences of proteins to be determined very quickly. The technique of two-dimensional gel electrophoresis has likewise advanced the science of proteomics. Protcoarcs will, undoubtedly, eventually provide targets for drug discovery and the detection of disease states.
193
positively (the desired outcome). or not at all. Consequently. drugs are developed for an "average" patient. The manufac-
turer relies on clinical studies to expose potential adverse reactions and publishes them in statistical format to guide the physician. Nevertheless, when a physician prescribes a drug to a patient he or she has no way of knowing the outcome. Statistics show clearly that a single drug does not provide a positive outcome in all patients. This 'one drug does not fit all" concept has its basis in the genetics of a patient. and the science of studying these phenomena is called phammacogenomics.
A patient's response to a drug, positive or negative, is a highly complex trait that may be influenced by the activities of many different genes. Absorption, distribution, metabolism. and excretion, as well as the receptor-binding relationship, are all under the control of proteins, lipids, and carbohydrates, which are in turn under the control ol the patient's genes. When the fact that a person's genes display small variations in their DNA bane content was recognized, genetic prediction of response to drugs or infectious microbes became pos.sible. Pharmacogenomics is the science that looks at the inherited variations in genes that dictate drug response and tries to define the ways in which these variations can be used to predict if a patient will have a positive response to a drug, an adverse one, or none at all. Cataloging the genetic variations is an important phase of present research activity. Scientists look for SNPs in a person's gene sequences. SNPs are viewed as markers for slight
genomic variation. Unfortunately, traditional gene sequencing is slow and expensive, preventing for now the general use of SNPs as diagnostic tools. DNA microarrays may make it possible to identify SNPs quickly in a patient's cells. SNP screening may help to determine a response to a drug before it is prescribed. Obviously, this would be a tremendous tool for the physician.
ANTISENSE TECHNOLOGY During the process of transcription, double-stranded DNA is separated into two strands by polymerases. These strands are named the sen.se (coding or + ) strand) and the anhisense (template or f—j strand). The antisense DNA strand serves as the template for mRNA synthesis in the cell. Hence, the code for ribosomal protein synthesis is normally transmitted
through the antisense strand. Sometimes, the sense DNA strand will code for a molecule of RNA. In this case, the resulting RNA molecule is called an:isen.se RNA. Antisense RNA sequences were first reported to be naturally occurring molecules in which endogenous strands formed complemen-
abet how individual patients will respond to the agent. No wnple algorithms exist that facilitate prediction of whether
tarily to cellular mRNA, resulting in the repression of gene expression. Hence, they may be natural control molecules, Rationally designed antisense oligonucleotide interactions occur when the base pairs of a synthetic. specifically designed antisense molecule align precisely with a series of bases in a target mRNA molecule. Antisense oligonucleotides may inhibit gene expression transiently by masking the ribosome-binding site on mRNA, blocking translation and thus preventing protein synthesis. or permanently by cross-linkage between the oligonucleo-
a patient will respond negatively (an adverse drug reaction).
tide and the mRNA. Most importantly, ribonuclease H
Pliamaacogenomks18m When pharmaceutical companies develop new drugs for any iisen disease state, they are limited by a lack of knowledge
194
Wits,,,, and Gisivid's lrahovk of Organic Medicinal and Pharn,ace,,:ical ('he,nis,rv
(RNase H) can recognize the DNA—RNA duplex (antisense DNA binding to mRNA). ora RNA—RNA duplex (antisense
RNA interacting with mRNA). disrupting the base pairing interactions and digesting the RNA portion of the double helix. Inhibition of gene expression occurs because the digested mRNA is no longer competent for translation and resulting protein synthesis. Amisense technology is beginning to be used to develop drugs that might be able to control disease by blocking the
genetic code, interfering with damaged or malfunctioning genes. Among the possible therapeutic antisense agents
AFTERWORD Clearly. biotechnology has become an integral part of pharmaceutical care. Pharmacists need to become comfortable with biotechnology and its language to deliver this kind of care to their patients. This chapter has tried to present an
overview of the major biotechnotogical arenas present in the year 2003. The field is advancing rapidly. and every pharmacist must stay current with the literature on hiotech' nology.
under investigation are agents for chronic myclogenous leu-
kemia. I-IIV infection and AIDS. cytomegalovirus retinitis in AIDS patients, and some intlammatory diseases.
GENE THERAPY Gene therapy arguably represents the ultimate application
of rDNA technology to the treatment of disease. There are two ways to envision gene therapy: (a) the replacement of a defective gene with a normal gene or (b) the addition of a gene whose product can help fight a disease such as a viral
infection or cancer. In the former case, replacement of a defective gene, an actual cure can be effected instead of just
treating the symptoms. For example, in cystic fibrosis, a defective gene has been clearly identified as the cause of the disease, It is possible that replacement of the defective
gene with a corrected one could produce a cure. Similar possibilities exist for other inherited genetic disorders such as insulin-dependent diabetes, growth hormone deficiency. hemophilia, and sickle cell anemia. The ability to transfer genes into other organisms has other important applications, including the heterologous production of recombinant proteins (discussed above) and the development of animal models for the study of human diseases. Another area of exploration is the introduction of recombinant genes as biological response nioditiers. for example. in
preventing rejection following organ transplantation. If genes encoding host major histocompatibility complexes could be introduced into transplanted cells, the transplanted tissue might be recognized as "self." It might also be possible to introduce genes for substances such as transforming growth factor-a that would decrease local cell-ntediated immune responses. An opposite strategy might be considered for the treatment of cancer. whereby transplanted cells could be used to target cancer cells, increasing local cell-mediated immune responses.
The transfer of genes frotn one organism to another is termed :ransgenics. and an animal that has received such a
transgene is referred to as a :ran.vgenic animal. If the transgene is incorporated into the germ cells (eggs and
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120. Grabensicin. 1. ft led.): lmmunoIacLc: Vaccines and Immunologic Drugs. St. Louis. MO, Facts and Comparisons. 2(812. pp. 695—704. 121. Pegasys. Roche Pharmaceuticals Company Stal/Gram. Nutley. NJ, HoFfman-LaRoche, 2003. 122. Giabenstein, J. D. led.): Immunolacts: Vaccines and Immunologic Drugs, St. Louis. MO. Facts and Comparisons. 2002, pp. 705—717. 123. Grabcnsiein. J. D. led.): Immunolucis: Vaccines and Immunologic I)rugs. Si. Louis, MO, Facts and Comparisons. 2002. pp. 737-740 and references therein, 124. Grabcnstein. 3. D. (ed.): Immunofacts: Vaccines and Immunologic Drugs. St. Louis. MO. Fads and Comparisons. 2(8)2. pp. 741—745 and references therein. (25. Grabensicin. J. D. led.): Immunofacts: Vaccines and Immunologic Drugs. St. Louis, MO. Pacts and Comparisons. 2(8)2. pp. 746—752 and references therein. 126. Grahensicin. .1. I). (Cd.): Immunofacts: Vaccines and Immunologic Drugs. St. Louis, MO, Facts and Comparisons, 2002. pp. 756-764 and references therein, 127. J. I). lcd.): Immunofacts: Vaccines and Immunologic Drugs, St. Louis. MO, Facts and Comparisons. 2002, pp. 765—770 and references therein. 128. Grabenstein. J. D. led.): linmunofacts: Vaccines and Immunologic Drugs. St. Louis. MO. Facts and Comparisons. 2002. pp. 771—775 and refercncc,s therein.
Vaccines and Immunologic (29. Grabenstcin, 3. 13. led.): Immunof.u, Drugs. S, Louis, MO. Facts and Comparisons. 2(1)2. pp. 776—787 and references therein. 130. Grabcn.stein. J. 0. (ed): Immunofucts: Vaccines and Inttnunologic Drugs. St. Louis, MO. Facts and Comparisons. 2002. pp. 788-794 and references therein. 131. Graben.ctein, J. D. led.): Immunofacts: Vaccines and Immunologic Drugs. St. Louis. MO, Facts and Comparisons. 2002. pp. 795—802 and references therein. 132. Murray. K. M.. atid DahI. S. I..: Ann. Phannacoiher. 31(11): .
1335—1338, 1997.
(33. Weinh}atl. M.. et al.: Arthritis Rhcum. 40(Suppfl:S 126. 1997. 34. Moreland. 1. W.. et ul.: N. EngI. J. Med. 337:141—147. (997. 135. Veriitraate. M., Lijnen. H.. and Cullen. 0.: Drugs 50(11:29—42. 1995. 136. Facts and Comparisons. St. Louis. MO. Facts and Comparisons. 2(100. pp. 183—11(9.
137. Cannon, C. P. Gibson, C. M., ci al.: Circulation 98:2805—2814. 1998. 138. Facts and Comparisons. Si. Louis. MO. Facts and Comparisons. 2000. p. 193. 139. Shapiro. A. D.. Ragni, M. V.. Lasher. J. M.. em al.: Throinh. Haemost. 75(11:30—35. 1996.
140. Facts andComparisons. St. Louis. MO. Facts and Comparisons. 20(8), p. 195. 141. Bernard, G. R.. et al.: N. EngI, J. Med. 344:699—709. 2001. 142. Fabrialo. M.: J. Am. Soc. Extra Corporeal Tech. 331:117—125.2001. 143. Facts andComparisons. S.. Louis, MO. Facts and Comparisons. 2000. pp. 679—680. 144. Facts and Comparisons. St. Louis. MO. Facts and Comparisons. 2000. pp. 355.
145. Facts and Comparisons. St. Louis. MO. Facts and Comparisons. 2000. pp. 1529—1531. 146. Facts and ('oniparisons. St. Louis. MO. Facts and Comparisons. 2000. pp. 1505—1508. 147. Reichmamin. L., et at.: Nature 332:323—327. 1983. (48. Cohhold. S. P., and Wuldmuann, H. Nature 334:460—462. (984.
149. Muir,). K.. etal.. In Cromtnrlin, 0. J. A.. and Sindelar, R. 0. led.'.). Pharmaceutical Biotechnology: An Introduction for Pharmacists and Pharmaceutical Scientists. Amsterdam. The Netherlands. Harwood Acadentic Publishers. 1997, pp. 279—287. 151). CoiIfier. It.. et at.: N. Engl. J. Med. 346(4):280—282. 2002. 151. Maloney. D. G., ci al.: Blood 90:2188—2195. 1997. 152. Grabenstein. 3. D. (cdl: IntmunofacLs: Vaccines and ltttimtunotogic Drugs, St. Louis. MO. Facts and Comparisons, 2(8)2, pp. 44)6413 and references therein
IS). Voliotis, D.. ci ul.: Anti. Oncol. 11(41:95-100, 2000. 154. Grubenstein. 3. 0. (edt: Inimnumiofucts: Vaccines and Inmmnumiokigic Drugs. St. Louis, MO. Facts and Comparisons, 2(8)?. pp. 414—422 and rrlcrences therein.
55. McConnell. H.: Blood 00:768—773, 2002. 156. Billaud. E. M.; Therapie 55)11:177—183. 2000. 157. Ponticelli. C'., ci al.: Drugs l(1t55—60. 199'). 158. Kirkman. R. L.: Transplant. Proc. 31(1-21:1234— 1235. (999. 159. Vincenti, P.: N. Engi. 3. Mcd. 338: 101—165. (998. 160. Oberlmolzer. 3.. et at.: Transplant, lot. 14(21:169—171. 2188).
161. Chan, G. L. C., Grubcr. S. A., ci al.: ('nt. Care Clin. 6:841—892. 1990.
162. Hooks. M. A.. Wade. C. S., and Milliken, W. 3.: Pliarniacotlmrrap) 11:26—37. 1991.
63. Todd. P. A.. and Brogden. K. N.: I)rugi. 37:871—89'). 1989. (64. Tnpol, Ii. 3.. and Semsys. P. W.: Circulation 98:1802-1820. 1994. (65. Grabenstein. J. 0. led.): IntmuniiIaas: Vaccines and Imntunolugk Drugs. St. Louis. MO. Facts and Contparisons. 2(8)2, PP 455—462 and relcrencc.s therein. 166. Gelmon. K., Arnold, A.. Cl al.: Proc. Am. Soc. Clin. Oncol. 2t'l(69a1 Absrr. 271. 200). 167. Slumomi, D. J.. Lcyland—Jones. B.. ci al.: N. EngI. J. Med. 3444111 783—792. 200(1,
168. (irubenstcin, J. D. (ed): Immunofacts: Vaccines and Inimitunotogir Drugs. St. Louis. MO, Facts and Comparisons. 2(8)2. pp. 473-442 and references therein.
(69. Hanauer. S. B.: N. Engi. 3. Mcd. 334:84 I, 1996. 171). Bngunl, W. C.. Jr., et al.: Semin. Noel. Med. 19(31:202—22(1. 1919 171. A. I.: J. Noel. Mcd. 32191:1751—1753. 1991. 1991. 172. Reilly. K. M.: Ctin. Pttarmactml. 'I'her. 173, Reilly, K. M.. ci al.: Clin. Pharntacokinei. 24:126—142. 1995. 174. Grabenstein. 3. 13. (ed): Itnmunofacts: Vaccines and Imntunologv Drugs. St. Louis. MO. Fads amid Compansons. 2(8)2. pp. 535-54! and references therein. 75. Grahensiein. J. D. led.): Immunofacis: Vaccines and Immunmilogs Drugs. Si. Louis. MO. Facts and Comparisons, 21112. pp. 544-554 and references therein. 176. Quailroccltl. E., and Hove, I. 1.IS Plrann. 23(41:54—63, 1998. 177. Riot, M.: Pharm. Tech. 25(l):34—40. 2000. 178. Ramsey. Ci.: Nat. Biotcch. 16:40—44. 1998. 179. Khan. 3.. ci al: Biochim. Biophys. Acts 1423:17—28. 1999. 180. Persidis. A.: Nat. Biotech. 16:393—394, 1998. 181. Borman, S.: Chem. Eng. News 78:31—37, 2000. 182. Lau. K. F., and Sakul. H.: Annu. Rep. Med. Chetit. 36:261—269.21(4),
CHAPTER 7 Immunobiologicals ORN M BEALE, JR.
immune system constitutes the body's defense against It protects the host by identifying and elimsating or neutralizing agents that are recognized as nonscif. liv entire r.tnge of immunological responses affects essenicIly organ, tissue, and cell of the body. Immune re-
include, in part, antibody (Ab) production, allergy. stiatnination, phagocytosis. cytotoxicity. transplant and rejection, and the many signals thai regulate these At its most basic, the human immune system he described in tcnns of the cells that compose it. Every r'pcct of the immune system. whether innate and nonspea set of specialThus, this discussion of some of the fundamentals immunology begins with the cells of the immune system.
MHCs con be found on virtually all nucleated cells in the human body, while class II MHC molecules are associated only with B lymphocytes and macrophages. Class I MHCs are markers that are recognized by natural killer cells and cytotoxic T lymphocytes. When a class I MHC is coexpressed with viral antigens on virus-infected cells. cytotoxic target cells are signaled. Class II MHC molecules are markers indicating that a cooperative immune slate exists between immunocompetent cells, such as between on antigen-presenting cell and a T-helper cell during the induction of Ab formation.
Granulocytes4 If one views a granulocyte under a microscope, one can observe dense intracymoplasmic granules. The granules contain inflammatory mediators and digestive enzymes that de-
CELLS OF THE IMMUNE SYSTEM
stroy invading pathogens. coiitrol the rate and pathsvay of migration of chentotactic cells, and cause dilation of blood
All immune cells derive from pluripolent stem cells in the hoix marrow. These arc cells that can differentiate into any slier cell type, given the right kind of stimulus (Scheme 7Ii A satiety of modes of differentiation beyond the stein ctll give rise to unique cellular types, each with a specific in the immune system. The first stage of differentia1ises rise to two intermediate types of stem cells and it branch point.a These cells are the myeloid cells snyeloid lineage) and the lymphoid cells (lymphoid line-
vessels at the infected site. The increased blood tiow ensures that an ample supply of granulocytes and inflamnniatory me-
diators reaches the site of infection. There is a t'amily of granulocytic cells, each member with its own specialized function. Under microscopic examination. some granulocytes are seen to be multinuclear and some mononuclear. The configuration of the nuclear region and the staining behavior provide ways of classifying granulocytes. The group is discussed below,
.cem. Carrying the lineage further leads to additional branch-
The myeloid cells differentiate into erythrocytes and also monocyes and granulocytes. The
rIjirlets and
limphoid cell differentiates into B cells and T cells, the cells th.d ,ur am the center of adaptive immunity. The switching
or each pathway and cell type is governed by a tummher of colony-stimulating factors, stein cell flictors. and rierleukins, These control proliferation, differentiation, and scluratiott of the cells.
Major Histocompatiblilty Antigens—Self
NeubophOs' Nen:rup/mits' arc the primary innate defense against pathogenic bacteria. They make up most (50(075%) of the kukocyte fraction in the blood. Microscopically. neutrophils have multilobed nuclei. They respond to chemical motility factors such as complement mediators released from infected or inflamed tissues and migrate to a site of infection by the process of chemotaxis. There, they recognize, adhere to. and phagocytose invading microbes.
Venus Nonseif The development of most immune responses depends on hr
of what is self and what is no: se/f. This must he clear and must be done in a very
:ercral way. This recognition is achieved by the expression i specialiced surface markers on human cells. The major
soup of markers involved in this recognition consists of 'urfare proteins. These ore referred to as the major hiswcom-
cmnpler3 (MHC) or major liiswco,nparibilizv anti. Proteins expressed on the cell surfaces are class I class Il MHCs. Both classes are highly polymorrhic antI so axe highly specific to each individual. Class I
Phagocytes The phagocytic process is initiated by contact and adhesion of an invading cell with a phagocytc cell membrane. Adhesion triggers a process whereby the phagocytic cell extrudes pseudopodia that surround the adhering tnicrobe. As this process progresses, the microbe is actually surrounded by
the phagocyte cell membrane. Then. invagination of the membrane fully engulfs the particle, and the membrane is resealed, with the particle encased inside an intracellular vacuolar body called a phugosome. Lysosomes in the cyto-
plasm then fuse with the phagosome to form plwgolvso197
198
Wil.s,,,, and Gi.ssolds
of Orcrwth Med icina! and Pharrnaceuiieal ('heniisrn
Lymphocytes Cells
Natural KIller Cell Scheme 7—1 • Lineages of blood cells. All blood cells derive from a pluripotent stem cell. A variety of cytokines direct the cells into their specific populations
conies. The antimicrobial compounds in the phagosomes and
lysosornes kill the engulfed pathogen and enzymatically cleave its remains into smaller pieces.
Eoslnophlls5 Eosinophils are granulocytes that can function as phagocytes. but much less efliciently than neutrophils can. They ate present as 2 to 4% of blood leukocytes. Their name derives from the intense staining reaction of their intracellular granules with the dye eosin. Eosinophil granules contain
(IgE) receptors. Complexes ot antigen molecules with IgE receptors ott the cell surface lead to cross-linking of IgE and distortion of the cell membrane. The distort ion causes the mast cell to degranulate. releasing mediators of the allergic response. Because of its association with hypersensitivity. IgE has been called "reogin" in the allergy literature. Diagnostically. lgE levels are elevated in allergy. systemic erythematosus. and rheumatoid arthritis. Cronsolyn sodium is a drug that prevents mast cell degranulation and thus blocks the allergic response. Cromolyn is used in asthma.
inflammatory mediators such as histamine and leukotrienes.
so it makes sense that these cells are associated with the allergic response. Clues to the functions of eosinophils come
front their behavior in certain disease slates. Eosinophil counts are elevated above normal in the tissues in many different diseases, hut they are recognized primarily for their diagnostic role in parasitic infections and in a unique mode of action that lends to their extreme importance. Unlike neutrophils. eosinophils need not phagocytose a parasite to kill it. Indeed, some parasites are too large to allow phagocytosis. Eosinophils can physically
surround a large parasite. forming a cell coat around the invader. Eosinophil granules release oxidative substances capable of destroying even large. multicellular parasites. Hence, even when phagocytosis fails, a mechanism exists to destroy large parasites.
Mast Cells and Basophils
Macrophages and Monocytes4'5 Macrophages and monocyle.s are mononuclear cells that axe capable of phagocytosis. In addition to their phugocytic capabilities, they biosynthesiz.e and release soluble factors (complement. monokines) that govern the acquired immune response. The half-life of monocytes in the bloodstream about 10 hours, during which time they migrate into tissues and differentiate into macrophages. A macrophage is a terminally differentiated monocyte. Macrophages possess a true anatomical distribution because they develop in the tissues to have specialized functions. Special macrophages are found in tissues such as the liver, lungs, spleen. ga.strointesu. nal (Gil tract, lymph nodes, and brain. These specific macrophages are called either histiocvtes (generic term) or by tam specialized names (Kupffer cells in liver. Langerhwii cells in skin. ah'eo!ar ,nacrophuges in lung) tlable 7—I ). Thr
Mast cells and basophils also release the inflammatory mediators commonly associated with allergy. Mast cells are especially prevalent in the skin, lungs. and nasal mucosa: their granules contain histamine. Basophils. present at only
entire macrophage network is called the re;iculoe,:doiheliu sr.cten,. Other macrophages exist tree in the tissues. where they carry out more nonspecific functions. Macrophages kill more slowly thati neutrophils but have a much broader slrec. trum. It has been estimated that more than 1(X) soluble in
0.2% of the leukocyte fraction in the blood, also contain
tiammatory substances are produced by macrophages. These
histamine granules, but the basophile.s found circulating in the blood and not isolated in connective tissue. Both mast
substances account for macrophages' prolific abilities to di. rect. modulate, stimulate, and retard the immune response. Macrophages possess a very specialized function: they
cells and basophils have high-affinity immunoglobulin E
Chapter 7 U Inimunobiologicals
TAELE 7—i
Reticutoendothellal System Cell
Tissue Kupflcr orlLs
L.wer
Alveolar mocrophages (dust cells)
199
act as antigen-presenting cells (APCs) (Fig. 7-I). APCs are responsible for the preprocessing of antigens, amplifying the numbers of antigenic determinant units and presenting these determinant stnictures to the programming cells of the immune system. APCs internalize an organism or particle and digest II into small fragments still recognizable as antigen. The fragments are conjugated with molecules of the major
Poritoneal inacrophages
histocompatibility complex 2 (MHC-ll). These complexes
Spleen
Dcndritic eclis
5km
l,anguihuns cells
Ilialo
Microgliul cells
are responsible for self or nonself cell recognition and ascertain that cells being processed are not self. MHCs also direct the binding of the antigenic determinant with immunoreac-
tive cells. Once the antigen—MHC-IT complex forms, it undergoes transcytosis to the macrophage's cell surface. where B lymphocytes and helper T cells recognize the anti-
Antigen
1-Helper Cell
/
Clones of functionally silent memory B cells
N Antibody-producing plasma cells
—
on the metal ion, and the pH. Since such chelates are often insoluble in waler, coincidental oral administration of a quinolone with an antacid, a hematinic. or a minenil supplement can significantly reduce the oral bioavailability of the quinobone. As an example. the insoluble 2: I chelate formed between ciprolloxacin and ntagne. sium ion is shown in Figure 8-7. The presence of divalent ions (such as Mg2) in the urine may also contribute to the comparatively lower solubility of certain fluoroquinolones in urine than in plasma.
pK. of 4.2 for benzoic acid are attributed to the acid-weaken-
ing effect of hydrogen bonding of the 3-carboxyl group to the adjacent 4-carbonyl The second class of antibacterial quinobones embraces the broad-spectrum Iluoroquinolones (namely. norfioxacin. enoxacin. ciprofloxacin. olboxacin. lometloxacin. and sparfloxacm). all of which possess, in addition to the 3-carboxylic acid group. a basic piperazino functionality at the 7 position and a 6-Iluoro substituent, The pKa values for the more basic
Nalidixic Acid, USP.
I-Ethyl-I .4-dihydro-7-methyl-4oxo- I .8-naphthyridine-3-carhoxylic acid (NegGram) occur as a pale buff crystalline powder that is sparingly soluble in water and ether but soluble in most polar organic solvents.
nitrogen atom of the piperazino group tall in the range of 8.1 to 9.3 (Table At most physiologically relevant p1-1 values, significant dissociation of both the 3-carboxy lie acid and the basic 7-( I -pipera/ino) groups occurs, leading to significant fractions of zwiuerionic species. As an example, the dissociation equilibria for nortloxacin arc illustrated in Figure The tendency for certain fluoroquinobones (e.g.. norfloxacm and ciprofloxacin) in high doses to cause crystalluria in alkaline urine is. in part, due to the predominance of the comparatively less soluble zwitterionic form. Solubility data
presented for otloxacin in the 15th edition of the United States Pharmacopoeia dramatically illustrate the effect of pH on water solubility of compounds of the Iluoroquinobone
class. Thus, the solubility of ofloxacin in water is 60 mg/ nsL at pH values ranging from 2 to 5. falls to 4 mg/mL at pH 7 (near the isoelectric point. p1). and rises to 303 mg/ mL at p1-1 9.8.
The excellent chelating properties of the quinolones pro-
TABLE 8-6 DissocIation and lsoelectric Constants
for Antibacterial Qulnolone5 Qutnotone
pKj
pK1 6.03
—
—
Norfloxacin
6.39
8.56
7.47
118
6.15
8.54
7.35
2314
Ciprofloxacin
6.014
8.73
7.42
444
5.88
8.06
6.97
146
5.65
9.04
7.35
3.018
are
rum Ross. I).
I
-mit Riley. C. M.: J. Plrarm. Blomed Aria)
32). 19').).
linch value represents to, acetate of lileniluru values
USP. I-Ethyl-I .4-dihydro-4-oxol I .3Idiox. olo!4,Sgjcinnoline-3-carboxylic acid (Cinohac) is a close
those of nalidixic and oxolinic acids. —
tarosacin
Lonwilosacin
than the parent compound. Further metabolism of the actite metabolite to inactive glucuronide and 7-carhoxylic acid mc01 6 tabolites also occurs. Nalidixic acid possesses a to 7 hours. It is eliminated, in part, unchanged in the urinc and 80% as metabohites.
congener (isostere> of oxolinic acid (no longer marketed in the United States) and has antibacterial properties similar to
p1
Natidixic acid
Nalidixic acid is useful in the treatment of urinary tract infections in which Gram-negative bacteria predominate. The activity against indole-positive Proteus spp. is panics. lady noteworthy, and nalidixic acid and its congeners represent important alternatives for the treatment of tirinary tract infections caused by strains of these bacteria resistant to other agents. Nalidixic acid is rapidly absorbed. metabolized, and rapidly excreted after oral administration The 7-hydroxyniethyl metabolite is significantly more actisc
Chapter 8 • /uui-i:ifecthe Agens.s
/ /Q.
H2Q
HQ° Figure 8—6 • Ionization equilibria in the quinolone antibacterial drugs.
• A 2:1 chelate of a Mg2' ion by cip-
249
250
Wilson and Gisvold's Texthook of Organic Medicinal and Pharmaceutical Che,nisirv
It is recommended for the treatment of urinary tract infections caused by strains of Gram-negative bacteria susceptible
to these agents. Early clinical studies indicate that the drug possesses pharmacokinetic properties superior to those of either of its predecessors. Thus, following oral administration, higher urinary concentrations of cinoxacin than of nalidixic acid or oxolinic acid are achieved. Cinoxacin appears to be more completely absorbed and less protein bound than nulidixic acid.
Norfloxa c/n.
I -Ethyl-6-fluoro- 1 ,4-dihydro-4-oxo-7-( I -
pipcrazinyl)-3-quinolinecarboxylic acid (Noroxin) is a pale yellow crystalline powder that is sparingly soluble in water. This quinoline has broad-spectrum activity against Gramnegative and Gram-positive aerobic bacteria. The fluorine atom provides increased potency against Gram-positive organisms, whereas the piperazine moiety improves antipseudomonal activity. Norfloxacin is indicated for the treatment of urinary tract infections caused by E. co/i. K. pneurnoniae. Enterobacter cloacue. Proteus ,nirabili.c, indole-positive
Proteus spp. including P. vulgaris. Providencia retigeri. Morganella morganii. P. aeruginosa. S. aureus. and S. epidermidis, and group D streptococci. It is generally not effective against obligate anaerobic bacteria. Norfloxacin in a single 800-mg oral dose has also been approved for the treat-
ment of uncomplicated gonorrhea. The oral absorption of norfioxacin is about 40%. The drug is 15% protein bound and is metabolized in the liver. The is 4 to 8 hours. Approximately 30% of a dose is eliminated in the urine and feces.
Enoxacin is well absorbed following oral administration. Oral bioavailability approaches 98%. Concentrations of the drug in the kidneys, prostate, cervix, fallopian tubes, and myometrium typically exceed those in the plasma. More than 50% of the unchanged drug is excreted in the urine. Metabolism, largely catalyzed by cytochrome P.450 enzymes in the liver, accounts for IS to 20% of the orally administered dose of enoxacin. The relatively short elimination half-life of enoxacin dictates twice-a-day dosing for the treatment of urinary tract infections. Some cytochrome P.450 isozymes. such as CYP 1A2. are inhibited by enoxacin. resulting in potentially important interactions with other drugs. For example, enoxacin has been reported to decrease theophylline clearance, causing increased plasma levels and increased toxicity. Enoxacie forms insoluble chelates with divalent metal ions present in
antacids and hematinics. which reduce its oral bioavuilability.
clprofloxadn. USP. I -Cyclopropyl-6-fluoro- I .4-dihy. dro-4-oxo-7-(l-piperazinyl)-3-quinolinecarboxylicacid (Cip. ro, Cipro IV) is supplied in both oral and parenteral dosage forms. The hydrochloride salt is available in 250-. 500-. and 750-mg tablets for oral administration, Intravenous solutiuns containing 200 mg and 400 mg are provided in Lions of 0.2% in normal saline and 1% in 5% dextrose solutions.
The oral absorption of norfioxacin is rapid and reasonably efficient. Approximately 30% of an oral dose is excreted in the urine in 24 hours, along with 5 to 8% consisting of less
active metabolites. There is significant biliary excretion, with about 30% of the original drug appearing in the feces.
Enoxadn, USP. I -Ethyl-6-fluoro- I .4-dihydro-4-oxo-7(I -piperazinyl)- I ,8-naphthyridine-3-curboxyiic acid (Penetrex) is a quinolone with broad-spectrum antibacterial activity that is used primarily for the treatment of urinary tract infections and sexually transmitted diseases. Enoxacin has
The bioavailability of ciprofloxacin following oral admin. istration is good, with 70 to 80% of an oral dose being ab. sorbed. Food delays, but does not prevent, absorption. Sig-
nificant amounts (20 to 35%) of orally
administered
proved for the treatment of acute (uncomplicated) and
ciprofloxacin are excreted in the feces, in part because of biliary excretion. Biotransformation to less active metabolites accounts for about 15% of the administered drug. Approximately 40 to 50% of unchanged ciprofioxacin is creted in the urine following oral administration. This value increases to 50 to 70% when the drug is injected intravenously. Somewhat paradoxically, the elimination half-life of ciprofloxacin is shorter following oral administration (1.., 4 hours) than it is following intravenous administration (1 , 5 to 6 hours). Ciprotloxacin inhibits the P.450 species CYF
chronic (complicated) urinary tract infections.
I A2.
been approved for the treatment of uncomplicated gonococ-
cal urethritis and has also been shown to be effective in chancroid caused by Haemophilus ducre i. A single 400mg dose is used for these indications. Enoxacin is also ap-
Chapter 8 • Anti-infèclive Agesits
251
The oral dose of this quinolone is typically 25% higher than the parcnteral dose for a given indication. Probenecid significantly reduces the renal clearance of ciprofloxacin.
also widely distributed into most body tluids and tissues. In fact, higher concentrations of olloxacin are achieved in CSF than can be obtained with ciprofloxacin. The oral bioavail-
presumably by inhibiting its active tubular secretion, Ciprofloxacin is widely distributed to virtually all parts of the
ability of ofloxacin is superior (95 to 100%) to that of cipr000xacin. and metabolism is negligible (—3%). The
body, including the CSF. and is generally considered to provide the best distribution of the currently marketed quinolonc,s. This property, together with the potency and broad antibacterial spectrum of ciprofloxacin, accounts for the numemti.s therapeutic indications for the drug. Ciprofloxacin
amount of an administered dose of ofloxacin excreted in the
urine in a 24- to 48-hour period ranges from 70 to 90%. There is relatively little biliary excretion of this quinolone. Although food can slow the oral absorption of olloxacin.
exhibits higher potency against most Gram-negative
comparable. The elimination half-life of olloxacin ranges
bacterial species. including P. aeruginosa, than other quino-
4.5 to 7 hours. Ofloxacin has been approved for the treatment 01' infectiOns of the lower respiratory tract, including chronic bronchitis and pneumonia, caused by Gram-negative bacilli. It is also used for the treatment of pelvic inflammatory disease (PID) and is highly active against both gonococci and chlamydia. In common with other fluoroquinolones. ofloxacin is not effective in the treatment of syphilis. A single 400-mg oral dose of ofloxacin in combination with the tetracycline antibiotic doxycycline is recommended by the Centers for Disease Control and Prevention (CDC) for the outpatient treatment of acute gonococcal urethritis. Ofloxacin is also used for the treatment of urinary tract infections caused by Gram-negative bacilli and for prostatitis caused by E. coil. Infections of the skin and soft tissues caused by staphylococci, streptococci, and Gram-negative bacilli may also be treated with ofloxacin. Because ofloxacin has an asymitmetric carbon atom in its structure, it is obtained and supplied commercially as a race-
tones.
Ciprofloxacin is an agent of choice for the treatment of bacterial gastroenteritis caused by Gram-negative bacilli nich as enteropathogenic E. ccli. salmonella (including S. Shigella spp.. Vjbrjo app., and Aeromonas hvdrophitin, It is widely used for the treatment of respiratory tract inkctions and is particularly effective for controlling bronchitis and pneumonia caused by Gram-negative bacteria. Ciprofloxacin is also used for combating infections of the skin, soft tissues, bones, and joints. Both uncomplicated and com-
plicated urinary tract infections caused by Grain-negative bacteria can be treated effectively with ciprofloxacin. It is panicularly useful for the control of chronic infections characterized by renal tissue involvement. The drug also has important applications in controlling venereal diseases. A combination of ciprofloxacin with the cephalosporin antibiotic ceftriaxone is recommended as the treatment of choice disseminated gonorrhea, while a single-dose treatment itltcipmfloxacin plus doxycycline. a tetracycline antibiotic Chapter 10). can usually eradicate gonococcal urethritis. has also been used for chancroid. The drug as been approved for postexposure treatment of inhalational anthrax.
Injectable forms of ciprofloxacin arc incompatible with drug solutions that are alkaline because of the reduced soluhilly of the drug at pH 7. Thus, intravenous solutions should Mbe mixed with solutions of ticarcillin sodium, mezlocillin
sodium, or antinophyllinc. Ciprofloxacin may also induce aystatluria under the unusual circumstance thut urinary pH rises above 7 (e.g.. with the use of systemic alkalinizers or jcarbonic anhydrase inhibitor or through the action of ureasc daborated by certain species of Gram-negative bacilli).
9-Fl uoro-2.3-dihydro-3-methyl- 10USP. 4.tnedtyl- I -piperazin-yl)-7-oxo-7H-pyridol 1.2.3-del-I .4.benzoxa7ine.6.carboxylic acid (Floxin, Floxin IV) is a memof the quinolone class of antibacterial drugs wherein the and 8 positions are joined in the form of a 1.4-oxazinc thrg. The ring system is numbered beginning with the oxaOfloxadn,
iine osygen atom as shown below.
blood levels following oral or intravenous administration are from
mate. The racemic mixture has been resolved, and the enantiomers independently synthesized and evaluated for antibacterial The 3S(—) isomer iv substantially more
active (8 to 125 times, depending on the bacterial species) than the 3R( '4-) isomer and has recently been marketed as levofloxacin (Levaquin) for the same indications as the racemate.
Lomefloxacin, USP.
I -Ethyl-6.8-difluoro- I .4-dihydro7-(3-methyl- I -piperazinyl )-4-oxo-3-quinolinecarboxylic acid (Maxaquin) is a difluorinated quinolonc with a longer elimination half-life (7 to 8 hours) than other members of its class. It is the only quinolone for which once-daily oral dosing suflices. The oral bioavailability of lomelloxacin is estimated to be 95 to 98%. Food slows, but does not prevent. its oral absorption. The extent of biotransformation of lo-
melloxacin is only about 5%. and high concentrations of unchanged drug, ranging from 6() to 80%. are excreted in the urine. The comparatively long half-life of lomefloxacin is apparently due to its excellent tissue distribution and renal
reabsorption and not due to plasma protein binding (only —'10%) orenterohepatic recycling (biliary excretion is estimated to be —10%).
Otloxacin resembles ciprofloxacin in its antibacterial and potency. Like ciprotloxacin. this quinolone is
252
Wilson and Gjsvold's l'exthook of Organic Medicinal and Pharmaceutical Chemistry
Lornefioxacin has been approved for two primary indications. First, it is indicated for acute bacterial exacerbations of chronic bronchitis caused by H. influenzae or Moraxelia (Branhwnella) cararrhalls. but not if S:repzococcu.c pneumonroe is the causative organism. Second. it is used for pro-
pounds—nitrofurazone. furazolidone. and nitrofuran tom—have been used for the treatment of bacterial inlec
tions of various kinds for nearly 50 years. A fourtF niu'ofuran. nifurtimox. is used as an antiprotozoal agent te treat trypanosomiasis and lcishmania.sis. Another nitrohct
phylaxis of infection following transurethral surgery. Lomefloxacin also finds application in the treatment of acute cystitis and chronic urinary tract infections caused by Gram-
erocyclic of considerable importance is metronida2olc. which is an amebicide (a trichomonicide) and is used fat
negative bacilli. Lomefloxacin reportedly causes the highest incidence of
bacteria. This important drug is discussed below in
the treatment of systemic infections caused by anaerobic chapter.
phototoxicity (photosensitivity) of the currently available quinolones. The presence of a halogen atom (fluorine, in this case) at the 8 position has been correlated with an increased chance of phototoxicity in the quinolone.s.'1°
The nitrofurans are derivatives of formed on reaction with the appropriate hydrazine or aminc derivative. Antimicrobial activity is present only when the
nitro group is in the 5 position.
Sparfioxacin. Sparfloxacin. (cis)-5-amino- I -cyctopropyl-7-(3,5-dimethyl)-l-piperazinyl)-6.8-difluoro. I ,4.dihy. dro-4-oxo-3-quinolinecarboxylic acid, is a newer fluoroquinolone.
Nitrofurazone R= 2
o
"N
Furazolidone R=
0 0
This compound exhibits higher potency against Grampositive bacteria, especially staphylococci and streptococci, than the fluoroquinolones currently marketed. It is also more active against chiamydia and the anaerobe Bacieroides frog. ills. The activity of spariloxucin against Gram-negative bacteria is also very imprcs.sive. and it compares favorably with ciprofloxacin and otloxacin in potency against Mycoplasma spp.. Legionella spp.. mycobactcria. and Listeria inonocylo.
genes. Sparfioxacin has a long elimination half-life of 18 hours, which permits once-a-day dosing for most indications. The drug is widely distributed into most fluids and Effective concentrations of sparfioxacin arc achieved for the treatment of skin and soft tissue infections, lower respiratory infections (including bronchitis and bacterial pneumonias). and pelvic inflammatory disease caused by gonorrhea and chiamydia. Sparfioxacin has also been recommended for the treatment of bacterial gastroenteritis and cholecystitis. The oral bioavailability of spariloxacin is tissues.
claimed to be good, and sufficient unchanged drug is excreted to be effective for the treatment of urinary tract infections. Nearly 20% of an orally administered dose is excreted as an inactive glucuronide. The incidence of phototoxicity of sparfioxacin is the lowest of the fluoroquinolones. because of the presence of the 5-amino group, which counteracts the effect of the 8.fluorosubsticuent.
The first nitroheterocyclic compounds to be introduced into chemotherapy were the nitrofurans. Three of these corn-
Nitrofurantoin R=
The mechanism of antimicrobial action of the has been extensively studied, but it still is not fully under stood. in addition to their antimicrobial actions, the nitnifu rans are known to be mutagenic and carcinogenic under cer• tam
conditions, It is thought that DNA damage caused
metabolic reaction products may be involved in these cellu lar effects.
Nitrofurazone.
5-Nitro-2-furaldehydc semicarbazax
(Furacin) occurs as a lemon-yellow crystalline solid that a sparingly soluble in water and practically insoluble in organic solvents. Nitrofurazone is chemically stable, hut mnierately light sensitive.
It is used topically in the treatment of burns. when bacterial resistance to other agents may be a concern
It may also be used to prevent bacterial infection a broad spectrum olac-
tivity against Gram-positive and Gram-negative bacteria,
it is not active against fungi. It is bactericidal against bacteria commonly causing surface infections, including 5 aureus. Streptococcus spp.. E. co/i. closrridium perfrmnRer:
Enterobacier (Aerobacter) aerogenes, and Proteus cpp, however, P. aeruginosa strains are resistant. Nitrofurazone is marketed in solutions. ointments, a usual concentration of 0.2%.
Chapter S • Anti-infective Agents
253
3-I(5-Nitrofurylidenc)aminol-2-
Methenamine is used internally a.s a urinary antiseptic for
OJ!OtidIIl&IflC iFuroxone) occurs as a yellow crystalline with a hitter aftertaste, It is insoluble in water or Furazolidone has bactericidal activity against a relabroad range of intestinal pathogens. including S. anF. ((iii. Salmonella. Sivigella. Proteu.v spp., Enierabacand Vibrio elioh'rae. It is also active against the
the treatment of chronic urinary tract infections. The free base has practically no bacteriostatic power: formaldehyde
Furazolidone.
USP.
la,,,blia. It is recommended for the oral Ii:.arnent of bacterial or protozoal diarrhea caused by susorganisms. The usual adult dosage is 100mg 4 times a small fraction of au orally administered dose of is absorbed. Approximately 5% of the oral dose
release at the lower pH of the kidney is required. To optimize the antibacterial effect, an acidifying agent such as sodium hiphosphae or ammonium chloride generally accompanies the administration of methenamine. Certain bacterial strains are resistant to the action of methenamine because they elaborate urease. an enzyme that hydrolyzes urea to form ammonia. The resultant high urinary pH prevents the activation of methenaminc. rendering it ineffective. This problem can be overcome by the coadministration of the urease inhibitor acetohydroxamic acid (Lithostat).
in the urine in the form of several metabolites. gasinantestinal distress has been reported with its u.se. Vcihol should be avoided when lurazolidone is being used the
drug can inhibit aldehyde dehydrogenase.
Nittofurantoin, USP.
Nitrofurantoin. I -(5-nitro-2-fur(Furadantin. Macrodantin). is
nitroturan derivative that is suitable for oral use. It is rec-
Methenamine Mandelate, USP.
Hexainethylenctctramine mandelate (Mandelannine) is a white crystalline powder with a sour taste and practically no odor, It is very soluble in water and has the advantage of providing its own acidity. although in its use the custom is to carry out a preliminary acidification of the urine for 24 to 36 hours before administration.
tar the treatment of urinary tract infections by susceptible strains of E. co/i. enterococci. S. anand Alebsiellt,, E,nerobacter, and Proteus spp. The
common side effects are gastrointestinal (anorexia. and somiting); however, hypersensitivity reactions
H
pilcunionitis. raa.hes. hepatitis, and hcmolytic anemia) have
been observed. A inacrocrystalline form (Mais claimed to improve gastrointestinal tolerance ihuol interfering with oral absorption.
Z)
Methenamine Hippurate, USP. Methenamine hippurate (Hiprex) is the hippuric acid salt of methenamine. It is readily absorbed after oral administration and is concentrated in the urinary bladder, where it exerts its antibacterial activity. Its activity is increased in acid urine.
Methenamine and Its
Salts
The activity of hcxamcthylenetetinst Urotrispin. Uritone) depends on the liberation of The compound is prepared by evaporating a .ituiuni at Iorntaldehyde and strong ammonia water to Methenamine, USP.
Urinary Analgesics Pain and discomfort frequently accompany bacterial infections of the urinary tract. For this reason, certain analgesic agents, such as the salicylates or phenazopyridine. which
+ 4NH3
+
6H20
hit tree base exists as an odorless white crystalline pow-
hat sublintes at about 260°C. It dissolves in water to in alkaline solution and liberates formaldehyde when imed with mineral acids. Methenamine is a weak base fin
itk
concentrate in the urine because of their solubility properties.
are combined with a urinary anti-infective agent.
Phenazopyrldlne Hydrochloride. USP. Phenazopyridine hydrochloride. 2.6-diamino-3-(phenylazopyridine hydrochloride (Pyridium), is a brick-red fine crystalline powder. Ii is slightly soluble in alcohol, in chloroform, and in water.
o14.9.
LTD
N NH2•HCI H2N Phenazopyridine Hydrochloride
254
Wilson and
Textbook of Organic Medicinal and Pharmaceutical chemi.orv
Phenazopyridine hydrochloride was formerly used as a urinary antiseptic. Although it is active in vitro against staphylococci. streptococci. gonococci. and E. co/i. it has no useful antibacterial activity in the urine. Thus, its present utility lies in its local analgesic effect on the mucosa of the urinary tract.
Usually, phenazopyridine is given in combination with urinary antiseptics. For example, it is available as Azo-Gantrisin. a fixed-dose combination with the sulfonamide antibacterial sulfisoxazolc, and as Urobiotic, a combination with
the antibiotic oxytetracycline and the sulfonamide sulfamethizole (Chapter 10). The drug is rapidly excreted in the urine. to which it gives an orange-red color. Stains in fabrics may be removed by soaking in a 0.25% solution of sodium dithionite.
Antitubercular Agents Ever since Koch identified the tubercle bacillus. Mycohacteriu,n tuberculosis, there has been keen interest in the devel-
opment of antitubercular drugs. The first breakthrough in antitubercular chemotherapy occurred in 1938 with the observation that sulfanilamide had weak bacteriostatic properties. Later, the sulfonc derivative dapsone (4.4'-diaminodi-
phenylsulfone) was investigated clinically. Unfortunately. this drug. which is still considered one of the most effective drugs for the treatment of leprosy and which also has useful antimalarial properties, was considered too toxic because of the high dosages used. The discovery of the antitubereular
activity of the aminoglycoside antibiotic streptomycin by Waksman et al. in 1944 ushered in the modem era of tubercu-
losis treatment. This development was quickly followed by discoveries of the antitubcrcular properties of p-aminosalicylic acid (PAS) first and then, in 1952. of isoniazid. Later, the usefulness of the synthetic drug ethambulol and, eventually, of the semisynthetic antibiotic rifampin was discovered. Combination therapy. with the use of two or more antitubercular drugs. has been well documented to reduce the emergence of strains of Myeobaeteriu,n tuberculosis resistant to individual agents and has become standard medical practice. The choice of -antitubercular combination depends on a variety of factors, including the location of the disease (pulmonary, urogenital, gastrointestinal, or neural), the results of susceptibility tests and the pattern of resistance in the locality, the physical condition and age of the patient. and the toxicities of the individual agents. For some time. a combination of isoniazid and ethambutol, with or without streptomycin. was the preferred choice of treatment among
and ethambutol (or pyrazinamide). the period required for successful therapy is shortened significantly. Previous treatment schedules without rifampin required maintenance therapy for at least 2 years. whereas those based on the isoniazid—rifampin combination achieved equal or better results
in 6 to 9 months. Once considered to be on the verge of worldwide eradica. tion. as a result of aggressive public health measures and effective chemotherapy, tuberculosis has made a comeback A combination of of alarming proportions in recent factors has contributed to the observed increase in tuberculo. sis cases, including the worldwide AIDS epidemic. the gen.
eral relaxation of public health policies in many countries. the increased overcrowding and homelessness in major cities, and the increased emergence of multidrug-resistant strains of M. tube rculosis. The development of drugs useful for the treatment of lep. rosy has long been hampered. in part, by the failure of the causative organism. Mvcohacteriu,n leprae. to grow in cell culture. However, the recent availability of animal models. such us the infected mouse footpad. now permits in vii's drug evaluations. The increasing emergence of strains of M.
leprac resistant to dapsonc. long considered the for leprosy treatment, has caused public health officials to advocate combination therapy. Mycohacteria other than M. tuberculosis and M. /epru'. commonly known as "atypical" mycobacteria. were first established as etiological agents of diseases in the Atypical mycobacteria are primarily saprophytic species that are widely distributed in soil and water. Such organ. isms are not normally considered particularly virulent or infectious. Diseases attributed to atypical mycohacteria ate
on the increase, however, in large part because of the increased numbers of immunocomprornised individualt in the population resulting from the AIDS epidemic and the widespread use of immunosuppressive agents with organ transplantation. The most common disease-causing species are Mcobacferium aviu,n and Myrobacteriun, intracel/ulare. which base similar geographical distributions, are difficult to distinguish microbiologically and diagnostically, and are thus consid. ered a single complex (MAC. The initial disease attributed to MAC resembles tuberculosis, hum skin and musculoskeletal tissues may also become involved. The association or
MAC and HIV infection is dramatic. An .seminated form of the disease occurs in severely immunocompromised patients, leading In high morbidity and mortal.
clinicians in this country. However, the discovery of the
ity. Another relatively common atypical mycobacrerium.
tuberculocidal properties of rifampin resulted in its replacement of the more toxic antibiotic strcptomycin in most regimens. The synthetic drug pyrazinamide. because of its steri-
Mycobacterium kan.casii. also causes pulmonary disease and
lizing ability, is also considered a first-line agent and is frequently used in place of ethambutol in combination therapy. Second-line agents for tuberculosis include the antibiotics cycloserine. kanamycin. and capreomycin and the synthetic compounds ethionamide and p-arninosalicylic acid (PAS). A tnajor advance in the treatment of tuberculosis was signaled by the introduction of the antibiotic rifampin into therapy. Clinical studies indicated that when rifanipin is included
in the regimen, particularly in combination with isoniazid
can become disseminated in inimunocompromised Patients infected with M. kansasii can usually be treated effectively with combinations of antitubercular drugs. MAC infections, in contrast, are resistant to currently available chemotherapeutic agents.
Isoniazid, USP.
Isonicotinic acid hydraiide.
nyl hydrazide. or INH (Nydrazid) occurs as a nearly colorless crystalline solid that is very soluble in water. It is prepared by reacting the methyl ester of isonicotinic acid with hydrazine.
255
Chapter 8 • Anti-in Teethe Agents
O%,NHNH2
o
Isoniazid
is a remarkably effective agent and continues of the primary drugs (along with rifampin. pyraziand ethambutol) for the treatment of tuberculosis. is nor, however, uniformly effective against all forms of lconjaLid
Le disease. The frequent emergence of strains of the tubercie
-.eilhis resistant to isoniazid during therapy was seen as major shortcoming of the drug. This problem has been hal not entirely, overcome with the use of combina-
The activity of isoniazid is manifested on the growing bacilli and not on resting forms. Its action, which consi,kred bactericidal, is to cause the bacilli to lose lipid by a mechanism that has not been fully elucidated. most generally accepted theory suggests that the princieffect of isoniazid is to inhibit the synthesis of mycolic branched fatty esb that constitute important components of the cell walls
catalase—peroxidase enzyme complex is for the bionetivation of isoniazid!" A reactive spethrough the action of these enzymes on the is believed to attack a critical enzyme required for acid synthesis in mycobacteria.5" Resistance to Nil, estimated to range from 25 to 50% of clinical isolates NIl-resistant strains, is associated with loss of catalase .1 activities, both of which are encoded by a gene. A-oiG.7° The target for the action of INH has ::ciitiy been identified as an enzyme that catalyzes the reduction of 2-rranx-enoylacyl carrier proman essential step in fatty acid elongation.7' This enzyme
by a specific gene, in/iA. in M. tubereulosLc.72 20 to 25% or INK-resistant clinical isolates i;pL,y mutations in the inh.4 gene, leading to altered pro.as sith apparently reduced affinity for the active form of drug Interestingly, such INK-resistant strains also disresistance to ethionamide. a structurally similar antituOn the other hand. mycobacterial strains in calalase/peroxidase activity are frequently susto ethionantide.
•\kbtriigh treatment regimens generally require long-term Iculniqratuon of isoniazid, the incidence of toxic effects is .iakahly low. The principal toxic reactions are peripheral :SnhIs. gastrointestinal disturbances (e.g.. constipation, loss
and lieparotoxicity. Coadministration of pyri. irire is reported to prevent the symptoms of peripheral .anlO, suggesting that this adverse effect may result from
of a cuenzyme action of pyridoxal phosphate. does not appear to interfere with the antitubercueik'cr of isoniaiid. Severe hepatotoxicity rarely occurs iii .nniuzid alone; the incidence is much higher, however. 'err it is used in combination with rifampin. kurazid is rapidly and almost completely absorbed fol.r oral administration. It is widely distributed to all un and fluids within the body, including the CSF. Ap-
proximately 60% of an oral dose is excreted in the urine within 24 hours in the form of numerous metabolites as well as the unchanged drug. Although the metaholistn of isonia-
zid is very complex, the principal path of inactivation involves acetylation of the primary hydrazine nitrogen. In ad-
dition to acetylisoniazid. the isonicotinyl hydrazones of pyruvic and a-ketoglutaric acids. isonicotinic acid, and isonicotinuric acid have been isolated as melaboliles in humans.73 The capacity to inactivate isoniazid by acetylation is an inherited characteristic in humans. Approximately half of persons in the population are fast acctylators (plasma half-
life, 45 to 80 minutes). and the remainder slow acetylators (plasma half-life. 140 to 200 minutes).
Ethlonamide, USP.
2-Ethylthioisonicotinamide (Trecator SC) occurs as a yellow crystalline material that is sparingly soluble in water. This nicotinamide has weak bacterio-
static activity in vitro but, because of its lipid solubility. is effective in vivo. In contrast to the isoniazid series. 2substitution enhance.s activity in the thioisonicotinamide Series.
Ethlonan,lcie
Ethionamide is rapidly and completely absorbed following oral administration. It is widely distributed throughout the body and extensively metabolized to predominantly inac-
tive forms that are excreted in the urine. Less than the parent drug appears in the urine.
I
of
Ethionamide is considered a secondary drug for the treatment of tuberculosis. It is used in the treatment of isoniazidresistant tuberculosis or when the patient is intolerant to isoniazid and other drugs. Because of its low potency, the highest tolerated dose of ethionamide is usually recommended. Gastrointestinal intolerance is the most common side effect associated with its use. Visual disturbances and hepatotoxic. ity have also been reported.
Pyrazinamide, USP. Pyrazinecarboxamide (PZA) occurs as a white crystalline powder that is sparingly soluble in water and slightly soluble in polar organic solvents. Its antitubercular properties were discovered as a result of an investigation of heterocyclic analogues of fliCotUlic acid. with which it is isosteric. Pyrazinamide has recently been elevated to first-line status in short-term tuberculosis treatment regimens because of its tubereulocidal activity and comparatively low short-term toxicity. Since pyrazinamide is not active against metabolically inactive tubercle bacilli. it is not considered suitable for long-term therapy. Potential hepatotoxicity also obviates long-term use of the drug. Pyrazinamide is maximally effective in the low pH environment that exists in macrophages (monocytes). Evidence suggests bioactivation of pyrazinamide to pyrazinoic acid by an ainidase present in mycobacteria.74
256
Wilson and Gi.cvald'.c Texthe,ok of Organic Medicinal and Pharinareutica!
Aminosalicylic Acid.
Pyraztnamide
Because bacterial resistance to pyrazinamide develops rapidly. it should always be used in combination with other drugs. Cross-resistance between pyrazinamide and either isoniazid or cihionamide is relatively rare. The mechanism of action of pyrazinamide is not known. Despite its structural similarities to isoniazid and ethioiiamide. pyrazinamide apparently does not inhibit mycolic acid biosynthesis in myco-
4-Aminosalicylic acid (PAS) occurs as a white to yellowish-white crystalline solid that darkens on exposure to light or air. It is slightly soluble in water but more soluble in alcohol. Alkali metal salts and the nitric acid salt are soluble in water, but the salts of hydrochloric acid and sulfuric acid are not. The acid undergoes decarboxylation when heated. An aqueous solution has a pH of —3.2.
bacteria.
Pyrazinamide is well absorbed orally and widely distributed throughout the body. The drug penetrates inflamed meninges and, therefore, is recommended for the treatment of tuberculous meningitis. Unchanged pyrazinamide. the corre-
sponding carboxylic acid (pyrazinoic acid), and the 5-hydroxy metabolite are excreted in the urine. The elimination half-life ranges from 12 to 24 hours, which allows the drug to be administered on either once-daily oreven twice-weekly dosing schedules. Pyrazinamide and its metabolites are re-
ported to interfere with uric acid excretion. Therefore, the drug should be used with great caution in patients with hyperuricemia or gout.
Ethambutol, USP. Ethambutol. (+ )-2.2'-(ethylenediiniino)-di- I -butanol dihydrochloride, or EMB (Myambutol). is a white crystalline powder freely soluble in water and slightly soluble in alcohol.
cH20H
H
H
H
PAS is administered orally in the form of the sodium snit. usually in tablet or capsule form. Symptoms of gastroinlertinal irritation are common with both the acid and the sodium salt. A variety of enteric-coated dosage forms have been used in an attempt to overcome this disadvantage. Other forms that are claimed to improve gastrointestinal tolerance include the calcium salt, the phenyl ester, and a combinatirn with an anion exchange resin (Rezi-PAS). An antacid such as aluminum hydroxide is frequently prescribed. The oral absorption of PAS is rapid and nearly complete, and it is widely distributed into most of the body fluids and tissues, with the exception of the CSF, in which levels an significantly lower. It is excreted primarily in the urine as both unchanged drug and metabolites. The N-acetyl
live is the principal metabolite. with significant . 2HCI
H
p-Aminosalicytlc Acid
CH2OH
Ethambutol Dthydrochlortde
Ethambutol is active only against dividing mycobacteria. It has no effect on encapsulated or other nonproliferating forms. The in vitro effect may be bacteriostatic or bactericidal. depending on the conditions. Its selective toxicity toward mycohacteria appears to be related to the inhibition of the incorporation of mycolic acids into the cell walls of these organisms. This compound is remarkably stereospecific. Tests have shown that, although the toxicities of the dextro, levo. and mesa isomers are about equal, their activities vary consider-
ably. The dexiro isomer is 16 times as active as the ,nes, isomer. In addition, the length of the alkylene chain, the nature of the branching of the alkyl substituents on the nitrogens. and the extent of N-alkylation all have a pronounced effect on the activity. Ethamhutol is rapidly absorbed after oral administration. and peak serum levels occur in about 2 hours. It is rapidly excreted, mainly in the urine. Up to is excreted unchanged, with the balance being metabolized and excreted as
2.2'-(ethylenediimino)dihutyric acid and the corresponding dialdehyde. Ethambutol is not recommended for use alone, but in combinations with other antitubereular drugs in the chemotherapy of pulmonary tuberculosis.
the glycine conjugate also being formed. When with isoniazid (which also undergoes N-acetylation). PAS increases the level of free isoniazid. The biological of PAS is about 2 hours. The mechanism of antibacterial action of PAS is to that of the sulfonamides. Thus, it is believed to prewe the incorporation of p-aminobenzoic acid (PABA into tin dihydrofolic acid molecule catalyzed by the enzyme drofolate synthetase. Structure—activity studies have shose that the amino and carboxyl groups must be para to azt other and free; thus, esters and amides must readily undctp hydrolysis in vivo to be effective, The hydroxyl group be ortho or mefa to the carboxyl group, but optimal is seen in the former. For many years, PAS was considered a first-line drag the chemotherapy of tuberculosis and was generally in combination regimens with isoniazid and However, the introduction of the more effective and gena ally better tolerated agents. ethambutol and rifampin, h.r relegated it to alternative drug status.
Aminosalicylate Sodium, USP.
Sodium cylate (sodium PAS). a salt, occurs in the dihydrate a yellow-white powder or crystalline solid. It is very soirk in water in the pH range of 7.0 to 7.5. at which it is tlte mit stable. Aqueous solutions decompose readily and dankrc Two pH-dependent types of reactions ation (more rapid at low pH) and oxidation (more
Chapter 8 • Anti-infective Agents high pH). Therefore, solutions should be prepared within 24 hours of administration.
aofazimine. Clofaiimine (Lamprene) is a basic red dye that exerts a slow bactericidal effect on M. Ieprae. the bacterium that causes leprosy. It occurs as a dark red crystal-
line solid that is insoluble in water.
Clofazlmtne
Clolazimine is used in the treatment of lepromatous lepasy. including dap.sone-resistant forms of the disease. In sldition to its antibacterial action, the drug appears to posanti-inflammatory and immune-modulating effects thai an of value in controlling neuritic complications and in supwosing erythema nodosum leprosum reactions associated with leproniatous leprosy. It is frequently used in combinason other drugs, such as dapsone or rifampin. The mechanisms of antibacterial and anti-inflammatory of clofazimine are not known. The drug is known to bind to nucleic acids and concentrate in reticuloendothelial issue. It can also act as an electron acceptor and may interfere with electron transport processes. The oral absorption of clofazimine is estimated to be about
257
The chemistry of rifamycins and other ansamycins has been reviewed.76 All of the rifamycins (A. B. C. D. and E) are biologically active. Some of the semisynthetic derivatives of rifamycin B are the most potent known inhibitors of DNA-directed RNA polymerase in bacteria,7" and their action is bactericidal. They have no activity against the niammali-an enzyme. The mechanism of action of rilamycins as
inhibitors of viral replication appears to differ from that for their bactericidal action. Their net effect is to inhibit the formation of the virus particle, apparently by preventing a specific polypcptide conversion.77 Rifamycins bind to the fi subunit of bacterial DNA-dependent RNA polymerases to prevent chain initiation.78 Bacterial resistance to rifampin has been associated with mutations leading to amino acid substitution in the $ subunit.78 A high level of cross-resistance between various rifamycins has been observed. Rlfampin. USP.
Rilampin (Rifadin. Rimactane. Rifam-
picin) is the most active agent in clinical use for the treatment of tuberculosis. A dosage of as little as 5 is effective against sensitive strains of M. Rifampin is -also highly active against staphylococci and Neisseria. Hae,no-
j;/ulu.s. Legionella. and C'l,iwn din spp. Gram-negative bacilli are much less sensitive to rifampin. However, resistance to rifampin develops rapidly in most species of bacteria. including the tubercle bacillus. Consequently. rifampin is used only in combination with other antitubercular drugs. and it is ordinarily not recommended for the treatment of other bacterial infections when alternative antibacterial agents are available. CH3
It is a highly lipid-soluble drug that is distributed into lilloidal tissue and the reticuloendothelial system. Urinary ocrelion of unchanged drug and metabolites is negligible. half-life after repeated dosage is estimated to be about 70 days. Severe gastrointestinal intolerance to clofaz.imine creatively common. Skin pigmentation, ichthyosis and dry. ncss. rush, and pruritus also occur frequently. Clofaiimine has also been used to treat skin lesions caused is M. ukeran.s.
Aitftubei'cuiar Antibiotics The rifamycins area group of chemically related antibiotics
by fermentation from cultures of Szrepro:nvces •cediierranei. They belong to a class of antibiotics called ansansvcins that contain a macrocyclic ring bridged acToss two nonadjacent positions of an aromatic nucleus. The teem ansa means . 'handle," describing well thc topogracity of the structure. The rifamycins and niany of their semi-
derivatives have a broad spectrum of antimicrobial slivily. They axe most notably active against Gram-positive and M. tuberculosis. However, they are also active some Gram-negative bacteria and many viruses. Ri-
lapin, a semisynthetic derivative of rifamycin B. was recited as an antitubercular agent in the United States in 1971. cecond semisynthetic derivative. rifabutin. was approved
fl 1992 for the treatment of atypical mycobacterial infec-
Rifampin
Toxic effects associated with rifampin are relatively infrequent. [I may, however, interfere with liver function in some patients and should neither be combined with other potentially hepatotoxic drugs nor used in patients with impaired hepatic function (e.g.. chronic alcoholics). The incidence of hepatotoxicity was significantly higher when rifampin was combined with isoniasid thaji when either agent was combined with ethambutol. Allergic and sensitivity reactions to
rifampin have been reported, but they are infrequent and usually not serious. Rifampin is a powerful inducer of hepatic cytochromc P-45() oxygenases. It can markedly poten-
258
Wi/con
and Gi.rvolds Textbook of Organic Medicinal and Pharmaceutical Chemistry CH3
hate the actions of drugs that are inactivated by these enzymes. Examples include oral anticoagulants, barbiturates, benzodiazcpines, oral hypoglycemic agents. phenytoin. and theophylline.
Rifampin is also used to eradicate the carrier state in a.sympcomatic carriers of Neisseria nzeningitidis to prevent outbreaks of meningitis in high-risk areas such as military
facilities. Serotyping and sensitivity tests should be performed before its use because resistance develops rapidly.
However, a daily dose of 600 mg of rifampin for 4 days suffices to eradicate sensitive strains of N. meningitidis. Ri-
fampin has also been very effective against M. leprae in experimental animals and in humans. When it is used in the treatment of leprosy, rifampin should be combined with dapsone or some other leprostatic agent to minimize the emergence of resistant strains of M. !eprae. Other. nonlabeled uses of rifampin include the treatment of serious infections such as endocarditis and osteomyelitis caused by methicillin-resistant S. aureus or S. epidermidis. Legionnaires' disease when resistant to erythromycin, and meningitis. prophylaxis of H. Rifampin occurs as an orange to reddish brown crystalline powder that is soluble in alcohol but only sparingly soluble in water. It is unstable to moisture, and a desiccant (silica
gel) should be included with rilampin capsule containers. The expiration date for capsules stored in this way is 2 years.
Rifampin is well absorbed after oral administration to provide effective blood levels for about 8 hours. Food, however. markedly reduces its oral absorption, and rifampin should be administered on an empty stomach. The drug is distributed in effective concentrations to all body fluids and tissues except the brain, despite the fact that it is 70 to 80% protein bound in the plasma. The principal excretory route is through the bile and feces, and high concentrations of rifampin and its primary metabolite, deacetylrifampin, are found in the liver and biliary system. Deacetyirifampin is also biologically ac-
tive. Equally high concentrations of rifampin are found in the kidneys, and although substantial amounts of the drug are passively reabsorbcd in the renal tubules, its urinary excretion is significant. Patients should be made aware that rifanipin causes a reddish orange discoloration of the urine. stool, saliva, tears, and skin. It can also permanently discolor soft contact lenses.
Rifampin is also available in a parenteral dosage form consisting of a lyophilized sterile powder that, when reconstituted in 5% dextrose or normal saline, provides 600 mg of active drug in 10 mL for slow intravenous infusion. The parenteral form may be used for initial treatment of serious cases and for retreatment of patients who cannot take the drug by the oral route. Parenteral solutions of rifampin are stable for 24 hours at room temperature. Although rifampin is stable in the solid state, in solution it undergoes a variety
CH3
Rifamycin
ceeds that of rifamycin. This rifamycin derivative is not effective, however, as monotherapy for existing disseminated MAC disease. Rifabutin is a very lipophilic compound with a high aFfinity for tissues. Its elimination is distribution limited, with a half-life averaging 45 hours (range. 16 to 69 hours). Appmximately 50% of an orally administered dose of rifabutin is absorbed, but the absolute oral bioavailability is only about 20%. Extensive first-pass metabolism and significant excretion of the drug occur, with about 30 and 53% of the orally administered dose excreted, largely as metabolitcu. it the feces and urine, respectively. The 25-O-desacetyl and 31 -hydroxy metabolites of rifabutin have been identified. The parent drug is 85% bound to plasma proteins in a conS centration-independcnt manner. Despite its greater against M. tuberculosi,r in vitro. rifabutin is considered infe. rior to rifampin for the short-term therapy of tubereuloci' because of its significantly lower plasma concentrations. Although rifabutin is believed to cause less and induction of cytochrome P450 enzymes than rifampin. these properties should be borne in mind when the drug ic
used prophylactically. Rifabutin and its metabolites nrc highly colored compounds that can discolor skin, urine, tears, feces, etc.
HO,
of chemical changes whose rates and nature are pH and temperature dependent.79 At alkaline pH, it oxidizes to a quinone
in the presence of oxygen; in acidic solutions, it hydrolyzes to 3-formyl rifamycin SV. Slow hydrolysis of the ester functions also occurs, even at neutral pH.
Rlfabutin, USP.
Rifabutin, the spiroimidazopiperidyl
derivative of rifamycin B was approved in the United States for the prophylaxis of disseminated MAC in AIDS patients on the strength of clinical trials establishing its effectiveness. The activity of rifabutin against MAC organisms greatly cx-
Rlfabutln
Chapter 8 • Anzi-uifecm'e Age,,:s USP.
o-( + l-4-Amino-3-isoxazolidinone
Scrsmycin) is an antibiotic that has been isolated from the beer of three different S:reptomvees species: S. s hidaceus. S. and S. lavendulus. It occurs as shite to pale yellow crystalline material that is very soluble water. It is stable in alkaline, but unstable in acidic, solu.ts The compound slowly dimerizes to 2,5-bis(aminoxy-
in solution or standing. The stncture of cycloserine was reported simultaneously and 1-lidy ci al." Lo be o-( + )-4—amino-3Kuchl Ct It has been synthesized by Stammer et al.52 by Smail ci al.5' Cycloscrine is stereochemically related
ii scetine. However, the i-form has similar antibiotic ac-
(ON
Cycloserine is pre.sumed to exert its antibacterial action h:. preventing the synthesis of cross-linking peptide in the of bacterial cell walls?4 Rando8° has recently sug. tha it is an antitnetabolite for alanine. which acts as ucide substrate for the pyridoxal phosphate—requiring
tianine raccmasc. Irreversible inactivation of the thereby deprives the cell of the o-alanine required 'c he synthesis of the cross-linking peptide.
Although cycloserine exhibits antibiotic activity in vitro wide spectrum of both Grain-negative and Grams',itivc organisms, its relatively weak potency and frequent sc reactions limit its use to the treatment of tuberculosis. hi. icconimended for patients who fail to respond to other
drugs or who are known to be infected with toother agents. It is usually administered nesinhination with other drugs, commonly isoniazid.
259
marketed in the United States) chemically and pharmacologically, is a second-line agent used in combination with other
antitubercular drugs. In particular, it may be used in place of streptoinycin when either the patient is sensitive to. or the strain of M. tuberculosis is resistant to, streptomycin. Similar to viomycin, capreomycin is a potentially toxic drug. Damage to the eighth cranial nerve and renal damage. as with viomycin. are the more serious toxic effects associated with capreomycin therapy. There are, as yet. insufficient clinical data for a reliable comparison of the relative toxic potentials of capreomycin and streptomycin. Cross-resistance among strains of tubercle bacilli is rare between Capreomycin and streptomycin. Four capreomycins. designated IA. lB. hA, and lIB, have been isolated from cultures of S. capreolus. The clinical agent contains primarily IA and lB. The close chemical relationship between capreomycins IA and lB and viomycin was established."7 and the total synthesis and proof of structure of the capreomycins were later accomplished."" The structures of capreonnycins hA and hR correspond to those of IA and lB but lack the f3-lysyl residue. The sulfate salts are freely soluble in water.
ANTIPROTOZOAL AGENTS In the United States and other countries of the temperate zone. protozoal diseases are of minor importance. whereas bacterial and viral diseases are widespread and are the cause
of considerable concern. On the other hand. protozoal diseases are highly prevalent in tropical Third World countries. where they infect both human and animal populations, causing suffering, death, and enormous economic hardship. Protozoal diseases that are found in the United States are malaria, amebiasis. giardiasis. trichomoniasis. toxoplasmosis, and, as a direct consequence of the AIDS epidemic. Pneunwi.c:,s carinii pneumonia (PCP).
hurtle Capreomycin Sulfate, LiSP.
Capastat sulfate. a strongly basic cyclic peptide isolated uS. uprclus in 1960 by Herr ci al?" It was released
Although amehiasis is generally thought of as a tropical disease, it actually has a worldwide distribution. In some areas with temperate climates in which sanitation is poor.
'he United States in 1971 exclusively as a tubereulostatic Capreomycin. which resembles viomycin (no longer
the prevalence of amebiasis has been estimated to be as high
as 20% of the population. The causative organism. Enia-
260
Wilson and Gixvold'.s Textbook of Organic Medicinal and Phar,naceu:ical ('hen,is:rr
inoeba !u.sio/vflca, can invade the wall of the colon or other parts of the body (e.g.. liver, lungs, or skin). An ideal chemotherapeutic agent would be effective against both the intestinal and extraintestinal forms of the parasite. Amebicides that are effective against both intestinal and extraintestinal forms of the disease are limited to the somewhat toxic alkaloids emetine and dehydrocmetine. the nitro-
imidazole derivative metronidazole, and the antimalarial agent chioroquine (Chapter 9). A second group of annebicides that are effective only against intestinal forms of the disease includes the aminoglycoside antibiotic paromornycm, the 8-hydroxyquinolmne derivative iodoquinol. the arsenical compound carbarsone. and diloxanide. Other protozoal species that colonize the intestinal tract and cause enteritis and diarrhea are Balantidiurn coil and
the flagellates Giardia lanthila and Crvpwxporidium spp. Balantidiasis responds best to tetracycline. Metronidazole and iodoquinol may also be effective. Giardiasis may be treated effectively with furazolidone. metronidazole, or the antimalarial drug quinacrine (Chapter 9). Cryptosporidiosis is normally self-limiting in immunocompetent patients and is not normally treated. The illness can be a serious problem in AIDS patients because no effective therapy is currently available. Trichomoniasis. a venereal disease caused by the tiagellined protozoan Trieho,nw,as vagina/is, is common in the United States and throughout the world. Although it is not generally considered serious, this affliction can cause serious physical discomfort. Oral metronidazole provides effective treatment against all forms of the disease. It is also used to eradicate the organism from asymptomatic male carriers. Pneurnocvssis carinii is an opportunistic pathogen that may colonize the lungs of humans and other animals and. under the right conditions, can cause pneumonia. The organism has long been classified as a protozoan, hut recent RNA evidence suggests that it may be more closely related to fungi. At one time, occasional cases of P. earinu pneumonia (PCP) were known to occur in premature, undernourished infarns and in patients receiving immunosuppressant therapy. The situation changed with the onset of the AIDS epidemic. It is estimated that at least 60% and possibly as high as 85% of patients infected with HIV develop PCP during their lifetimes. The combination of the antifolate Irimethoprim and the sulfonamide sulfamethoxazole constitutes the treatment of choice for PCP. Other effective drugs include pentarnidine. atovaquone. and a new untifolate. trimetrexate. To.wpla.snia gondii is an obligate intracellular protozoan that is best known for causing blindness in neonates. Toxoplasmosis. the disseminated form of the disease in which the lymphatic system, skeletal muscles, heart, brain, eye, and placenta may be affected, has become increasingly prevalent
in association with HIV infection. A combination of the antifolate pyrimethumine (Chapter 9) and the sulfa drug sulfadiazine constitutes the most effective therapy for toxoplasmosis.
Various forms of trypanosomiasis. chronic tropical diseases caused by pathogenic metnbers of the family Trypanosomidue. occur both in humans and in livestock. The princi-
pal disease in humans, sleeping sickness, can be broadly classified into two main geographic and etiological groups: African sleeping sickness caused by Trypano.soona gain-
hiense (West African), T. rI,odesiense (East African). or T. congoleiise: and South American sleeping (Chugas' disease) caused by T. crazi. Of the various forms of trypann. somiasis. Chaga.s' disease is the most serious and generall}
the most resistant to chemotherapy. Leishmaniasis is a chronic tropical disease caused by various flagellate proto. zoa of the genus Lei.shn,ania. The more conunon visceral form caused by L donovani, called kala-azar, is similar to Chagas' disease. Although these diseases are widespread in tropical areas of Africa and South and Central America. thes are of minor importance in the United States, Europe. and Asia. Chemotherapy of trypanosomiasis and leishmaniasis remains somewhat primitive and is often less than effective. In fact, it is doubtful that these diseases can be controlled by chemotherapeutic measures alone, without successful control of the intermediate hosts and vectors that them. Heavy metal compounds, such as the arsenicals and antimonials. are sometimes effective hut frequently toxic. The old standby suramin appears to be of some value in long- and short-term prophylaxis. The nitrofuran derivative nifurtimox may be a major asset in the control of these dis'
eases, but its potential toxicity remains to he fully deter. mined.
Metronidazole,
USP. 2-Methyl-5-nitroinuidazole.l• ethanol (Flagyl. Protostat. Metro IV) is the most useful of
a group of antiprotozoal nitroimidazole derivatives that base
been synthesized in various laboratories throughout world. Metronidazole was first marketed for the topical treat ment of Trichonwnos i'aginalis vaginitis. It has since been shown to be effective orally against both the acute and carTier states of the disease. The drug also possesses useful arnebici-
dal activity and is. in fact, effective against both intestinal and hepatic amebiasis. It has also been found of use in treatment of such other protozoal diseases as giardiasis and balantidiasis.
More recently. metronidazole has been found to efficacy against obligate anaerobic bacteria, but it is itieffec. tive against facultative anaerobes or obligate aerohes. Ii particularly active against Gram-negative an-aembes. as Bac:ermdes and Fusohacterin,,, spp. It is also effective
against Grans-positive anaerobic bacilli (e.g.. spp.) and cocci (e.g.. Pepwcoccus and spp.). Because of its bactericidal action. metronidazole become an important agent for the treatment 01' serious inkc. tions (e.g.. septiccmia. pneumonia. peritonitis, pelvic mice. (ions, abscesses, meningitis) caused by anaerobic bacteria The common characteristic of nnicroorganisms (bacteria and protozoa) sensitive to mctronidazole is that they am ao. aerobic. It has been speculated that a reactive intennedias
formed in the microbial reduction of the 5-nitro group metronidazole covalently binds to the DNA ut the mierrer ganism. triggering the lethal effect.55 Potential reactive
Chapter $ • A,ni.inJreui'e Agesil.s
261
mediates include the nitroxide. nitroso. hydroxylamine. and amine. The ability of metronidasule to act as a radiosensitizmg agent is also related to its reduction potential. Meironidaa'ole is a pale yellow crystalline substance that
well known. Aqueous solutions of acid salts of oxine. particularly the sulfate (Chinosol. Quinosol). in concentrations of 1:3.000 to 1:1,0(X), have been used as topical anhiseptics.
k sparingly soluble in water. It is stable in air but is light Despite its low water solubility, metronidazole is well absorbed following oral administration. It has a large
hydroxyquinolinc.s yields compounds with broad-spectruna amebicidal properties.
The substitution of an iodine atom at the 7 position of fI-
OH
apparent volume of distribution and achieves effective con-
centrations in all body fluids and tissues. Approximately
N
of an oral dose is metabolized to oxidized or conjugated lonas. The 2-hydroxy metaholite is active; other metabolites are inactive.
Metronidazole is a weak base that possesses a of 2.5. Although it is administered parenterally only as the free base
slow intravenous infusion. metronidazole for injection is
opplied in two forms: a ready-to-inject l00-mL solution containing 5 mg of base per mL: and a hydrochloride salt 500 mg of a sterile lyophilized powder. Metronidazole hydrochloride for injection must first be reconstituted with waler to yield 5 mL of a solution having a concentrainn of 100 mg/mL and a pH ranging from 0.5 to 2.0. The resulting solution must then be diluted with either 100 mL Qlnormal saline dextrose and neutralized with 5 mEq of sodium bicarbonate to provide a final solution of nictronidaMe base with an approximate concentration of 5 mglmL a pH of 6 to 7. Solutions of metronidazole hydrochloride unsuitable for intravenous administration because of extreme acidity. Reconstituted metronidazole hydra-
5.7-Diiodo-8-quinolinol. 5.7-diiodoIodoquinol. USP. 8-hydroxyquinoline. or diiodohydroxyquin (Yodoxin. l)iodoquin, Diquinol) is a yellowish to tan microcrystalline. light-sensitive substance that is insoluble in water. It is iccommended for acute and chronic intestinal amchiasis but is not effective in extraintestinal disease. Because a relatively high incidence of topic neuropahhy has occurred with its use. iodoquinol should not be used routinely for traveler's diarrhea. OH
solutions are stable for 96 hours at 30°C. while I
r4.to-use solutions of metronidazole base are stable for hours at 30°C. Both solutions should be protected from
Furamide, or eutamide, is the 2-furoof 2.2-dichloro-4'-hydroxy-N-methylacetanilidc. It kveloped as a result of the discovery that various a.apossessed amebicidal activity in vitro.
Diloxanide, USP.
bloxanide itself and many of its esters arc also active, and
iug metabolism studies indicate that hydrolysis of the .rniik is required for the amebicidal effect. Nonpolar esters I diloxanide are more potent than polar ones. Diloxanide wuate has been used in the treatment of asymptomatic caros of E. hi,cw!ytiea. Its effectiveness against acute intesnal amehiasis or hepatic abscesses, however, has not been Diloxanide furoate is a white crystalline powder.
Emetine and Dehydroemetine.
The alkaloids emetine and dehydroemctine are obtained by separation from extracts of ipecac. They occur as Ievorotatory. light-sensitive
white powders that arc insoluble in water. The alkaloids readily form water-soluble salts. Solutions of the hydrochloride salts intended for intramuscular injection should be adjusted to pH 3.5 and stored in light-resistant containers.
is administered orally only as 500-mg tablets and may obtained in the United States from the CDC in Atlanta, Emetlne
flfydroxyquinoline. Oxine. quinophenol. or oxyquiis the parent compound from which the antiprotozoal have been derived. The antibacterial and antipopeiiies of oxine and its derivatives, which are be-
to result from the ability to chelate metal ions. are
Emetinc and dehydroemetine exert a direct antebicidal action (no various forms of E. Iusiolyuca. They are protoplasmic poisons that inhibit protein synthesis in protozoal and mammalian cells by preventing protein elongation. Because their effect in intestinal amebiusis is solely symptom-
262
WiLcon and Gi.cvold'.c Textbook of Organic Meduuial and Pliannace,uical Chen,i.ctrv
atic and the cure rate is only 10 to 15%. they should be used only in combination with other agents. The high concentrations of the alkaloids achieved in the liver and other tissues alter intramuscular injection provide the basis for their high effectiveness against hepatic abscesses and other extraintes-
tinal forms of the disease. Toxic effects limit the usefulness of emetine. It causes a high frequency of gastrointestinal distress (especially nausea and diarrhea), cardiovascular effects (hypotension and arrhythmias). and neuromuscular effects (pain and weakness). A lower incidence of cardiotoxic-
ity has been associated with the use of dehydreemetine (Mehadin). which is available from the CDC and is also amehicidal. Eme,ine and dchydroemetine have also been used to treat halantidinl dysentery and fluke infeslations. such as fascioliaxis and paragonimiasis.
0
Ct-f3
CH3
Ct-f3
Pentamidine Isethionate, USP. 4.4'-(Pentamethylenedioxy)dibenzamidine diisethionate (NebuPent. Pentam 300) is a water-soluble crystalline salt that is stable to light and air. The principal use of peniamidine is for the treatment of pneumonia caused by the opportunistic pathogenic protozoan P. eu nail, a frequent secondary invader associated with AIDS. The drug may be administered by slow intravenous
infusion or by deep intramuscular injection for PCP. An aerosol form of pentamidine is used by inhalation for the prevention of PCP in high-risk patients infected with H!V who have a previous history of PCP infection or a low pe-
Atovaquone was originally developed as an antimalarial drug, hut PIas,nodin,n fuleiparam was found to develop a rapid tolerance to its action. More recently, the effectiveness of atovaquone against P. rarinhi was discovered. It is a currently recommended alternative to trimethoprim-sulfamcthoxazole (TMP-SMX) for the treatment and prophylaxis vI PCP in patients intolerant to this combination. Atovaquone was also shown to be effective in eradicating Toxoplasma gond,i in preclinical animal studies.
ripheral CD4 lymphocyte count. Both the inhalant (aerosol) and parenteral dosage forms of pentamidine isethionate are sterile lyophilized powders that must be made up as sterile aqueous solutions prior to use. Sterile water for injection must be used to reconstitute the aerosol, to avoid precipitation of the pentamidine salt. Adverse reactions to the drug are common. These include cough and bronchospasm (inhalation) and hypertension and hypoglycemia (injection). Pentarnidine has been used for the prophylaxis and treatment of African trypanosomiasis. It also has some value for treating visceral leishmaniasis. Pentamidine rapidly disappears from the plasma after intravenous injection and is distributed to the tissues, where it is stored for a long period. This property probably contributes to the usefulness of the drug as a prophylactic agent.
do tablets. Food, especially if it has a high fat content, in
Atovaquone. USP. 3-14-(4-Chlorophenyl)-cyclohexyl I -2-hydroxy-l.4-naphthoquinone (Mepron) is a highly lipo-
Eflornithine, USP.
The oral absorption of atovaquone is slow and incomplete. in part because of the low water soluhility of the drug. Aque ous suspensions provide significantly better absorption than creases atovaquone absorption. Significant enterohepatic recycling of atovaquonc occurs, and most (nearly 95%) of the drug is excreted unchanged in the feces. In vivo, atovaquonc
is largely confined to the plasma, where it is protein hound (>99.9%). The half-life of the drug ranges from 62 to 80 hours. The primary side effect is gastrointestinal intolerance.
philic. water-insoluble analogue of ubiquinone 6. an essen-
DL-2'-Difluoromethylomithine. or DFMO (Ornidyl), an amino acid derivative, is an enzyme. activated inhibitor of ornithine decarboxylase. a
tial component of the mitochondrial electron transport
phosphate—dependent enzyme responsible for catalyzing the
chain in microorganisms. The structural similarity between atovaquone and ubiquinone suggests thai the former may act as an antimctabolitc for the latter and thereby interfere with the function of electron transport enzymes.
rate-limiting step in the biosynthesis of the diamine putres-
cine and the polyamines spermine and spermidinc. amines are essential for the regulation of DNA synthesis and
cell proliferation in animal tissues and microorganisms.
0
NH
Chapter 8 • A:ui-infrc:ive Agents H2N
F
263
is similar to that America. The effectiveness of of nifurtimox. Therapy for American trypanosomiasis with oral benznidazolc requires several weeks and is frequently accompanied by adverse effects such as peripheral neuropa. thy, bone marrow depression, and allergic-type reactions.
F
NH2
Eflomitbine is used for the treatment of West African sickness, caused by Trypanosotna brucci gainiice, h is spccitlcafly indicated for the meningoencephastage of the disease. Eflornithine is a myelosuppressivc .rsg that causes high incidences of anemia. leukopenia, and
Complete blood cell counts must be during the course of therapy. The irreversible inactivation of ornithine decarhoxylase etlornithine is accompanied by decarboxylation and reat' fluoride ion from the inhibitor.90 suggesting enre.caaly'ied activation of the inhibitor. Only the (—) iso-
related to L-ornithine. is active. Eflomithine is supplied as the hydrochloride salt, it may
siministered either intravenously or orally. Approxiof the unchanged drug is excreted in the urine. of efiornithine into the CSF is facilitated by inof the meninges. Thfurtimox. USP.
Nifurtimox is 4-I(5-nitrofurfurylidene)
13.melhylthiomorpholine- 1.1 -dioxide, or Bayer 2502 Linpill. The observation that various derivatives of 5-nitropossessed, in addition to their antibacterial and wringal properties. significant and potentially useful anti-
activity cventuaHy led to discovery of particular
MelarsoproL
2-p-(4.6-Diamino-s-tria,in-2-yI-arnino)
phenyl-4-hydroxymcthyl- I .3.2-dithiarsolinc (Mel 13. Arso-
hal) is prepared by reduction of a corresponding pentavalent arsanilate to the trivalent arsenoxide followed by reaction of the latter with 2.3-dimercapto-l-propanol (British anti-Lewisite. HAL). It has become the drug of choice for the treatment of the later stages of both forms of African trypanosomiusis. Melarsoprol has the advantage of excellent penetration into the CNS and, therefore, is effective against meningoencephalitic forms of T. gambiense and T. rijodesiense. Trivalent arsenicals tend to be more toxic to the host (as well as the parasites) than the corresponding pentavalent compounds. The bonding of arsenic with sulfur atoms tends to reduce host toxicity. increase chemical stability (to oxidation), and improve distribution of the compound to the arsenoxide. Melarsoprol shares the toxic properties of other arsenicals, however. SO its use must be monitored for signs of arsenic toxicity.
vturjns with antitrypanosomal activity.
SON H3C
Sodium Stibogluconate.
Sodium antimony gluconate (Pentostam) is a pentavalens antimonial compound intended primarily for the treatment of various forms of leishmaniasis. The
important of such compounds is nilunimox be-
It is available from the CDC as the disodium salt, which is
the
chemically stable and freely soluble itt water. The lO'/e aque-
for South American trypanosomiasis. In ne of this drug represents the only clinically proven
ous solution used for either intramuscular or intravenous Like all anlimonial drugs, this injection has a pH of drug has a low therapeutic index, and patients undergoing therapy with it should be monitored carefully for signs of heavy metal poisoning. Other organic antimonial corn-
ef its demonstrated effectiveness against T.
i!rnCnt for both acute and chronic forms of the disease. is available in the United States from the CDC.
is administered orally. Oral biouvailability is but considerable first-pass tnetabolism occurs. The
Jifc of nifurtimox is 2 to 4 hours. The drug is poorly with a high incidence of nausea, vomiting. abdomand anorexia reported. Symptoms of central and
nervous system toxicity also frequently occur nilistiimox.
USP.
N-Benzyl-2-niiroimidazole- I-ace-
.:2e Radanil. Rochagan) is a nitroimidazole derivative s for the treatment of Chagas' disease. It is not rhbtc in the United States but is used extensively in South
poutids are used primarily for the treatment of schistosomiasis and other flukes.
264
WIL,wi
and GiscoMs Textbook of Organic Medicinal and Pharmaceutical Clw,nix:rv
The antileishmanial action 01 sodium stibogluconate requires its reduction to the trivalent form, which is believed to inhibit phosphofructokinase in the parasite.
Dimercaprol. USP. 2.3-Dimcrcapto- I -propanol. BAL. or dithioglycerol is a foul-smelling, colorless liquid. It is soluble in water (1:20) and alcohol. It was developed by the
British during World War Il as an antidote for "Lewisite." hence the name British anti-Lewisite. or HAL. Dimercaprol is effective topically and systematically as an antidote for poisoning caused by arsenic, antimony, mercury, gold, and lead. It can, therefore, also be used to treat arsenic and antimony toxicity associated with overdose or accidental ingestion of organoarsenicals or organoantimonials.
BAL may be applied topically as an ointment or injected intramuscularly as a 5 or 10% solution in peanut oil.
Suramin Sodium. Sur.tmin sodium is a high-molecular-weight bisurea derivative containing six sulfonic acid groups as their sodium salts. It was developed in Germany shortly after World War I as a by-product of research efforts directed toward the development of potential antiparasitic agents front dye.stuffs. The drug has been used for more than half a century for the treatment of early cases of trypanosomia.sis. Not until several decades later, however, was .suramin discovered to be a long-term prophylactic agent whose effectiveness aM
a single intravenous injection is maintained for up to 3 months. The drug is tightly bound to plasma proteins, caus-
ing its excretion in the urine to be almost negligible.
The antidocal properties of HAL are associated with the
property of heavy metals to react with sulihydryl (SH) groups in proteins (e.g., the enzyme pyruvate oxidase) and interfere with their normal function. I .2-Dithiol compounds such as BAL compete effectively with such proteins for the metal by reversibly forming metal ring compounds of the following type:
axis. It is available from the CDC.
ANTHELMINTICS
H
S
H
Tissue penetration of the drug does not occur. apparentty because of its high molecular weight and highly ionic character. Thus, an injected dose remains in the plasma for a sery long period. Newer, more effective drugs arc now availabk for short-term treatment and prophylaxis of African sleeping sickness. Suramin is also used for prophylaxis of onchocerci-
H H
These are relatively nontoxic. metabolically conjugated (as glucuronides). and r,ipidly excreted.
Anthelniintics are drugs that have the capability of ridding the body of parasitic worms or helminths. The prevalence of human helminthic infestations is widespread throughout the globe and represents a major world health problem, par-
ticularly in Third World countries. Helminths parasitic to humans and other animals are derived from two phyla. helminthes and Nemathelminthes. Cestodes (tapewonasl
Chapter 8 • Anti 'infecine Agent.'. flukcs belong to the former, and nematodes 'trw mundwornis belong to the latter. The helminth infesol major concern on the North American continent caused by roundworms (i.e.. hookworm. pinworm. and spp.(. Human tapeworm and fluke infestations are uch seen in the United States. Screral classes of chetnjcais are used as anthelmintics :sl melanIe phenols and derivatives. piperazine and related
265
piperazine. the two anthelmintics should not be used together. Over half of the oral dose is excreted in the feces unchanged. Adverse effects associated with its use are primarily gastrointestinal.
antimalarial compounds (Chapter 9). various compounds. and natural products.
Hexahydropyrazine or diethylenediainc Arihriticine. Dispermin) occurs as colorless, volatile of tire hextihydrate that are freely soluble in water. ol'the anthelmintic properties of a derivaie.Jiethylcarbamazine. the activity ol' piperazine itself was 'uWished. Piperazine is still used as an anthelmintic for :e treatment of pinworm (Eniembius [Oxvuris/ vermicu!rnt and rotindworm (Asearis lumbricoides) infestations. asajiable in a satiety of salt forms, including the citrate tIici,tI a the USP) in syrup and tablet forms. blocks the response of the ascaris muscle to causing flaccid paralysis in the worm, which
Pipe,azine, USP.
from the intestinal wall and expelled in the
Thiabendazole, USP. 2-(4-Thiazolyl)benzimidazole (Mintezol) occurs as a white crystalline substance that is only slightly soluble in water but is soluble in strong mineral acids. Thiabendazole is a basic compound with a pK, of 4.7 that forms complexes with metal ions. Thiabendazole inhibits the helminth-specilic enzyme fumarate reductase.9' It is not known whether metal ions are involved or if the inhibition of the enzyme is related to thiabendazole's anthelmintic effect. Benzimidazoie anthelrnintic drugs such as thiahendai.ole and mebendazole also arrest nematode cell division in meraphase by interfering with mi-
I Diethylcarbamazepine Citrate, USP. N. N-Diethyl-4'.hyl.l-piperaeinecarhoxamide citrate or I -diethylcarba:14'methYlpiperazifle dihydrogen citrate (Hetrazan) is a waer.soluhle crystalline compound that has selective
activity. It is effective against various forms of including Bancrolt's. onchocerciasis. and laviasis. Jetive against ascariasis. Relatively few adverse have been associated with diethylcarbamazine.
crotubule assembly."2 They exhibit a high affinity for tubulin. the precursor protein tor tnicrotubule synthesis. H
Thiabendazole has broad-spectrum anthelmintic activity. It is used to treat enterobiasis, strongyloidiasis (thn,adworm
infection), ascariasis. uncinariasis (hookworm infection). and trichuriasis (svhipworni infectioti). It has also been used to relieve symptoms associated with cutaneous larva migrans (creeping eruption) and the invasive phase of trichinosis. In
addition to its use in human medicine. thiabendazole is widely used in veterinary practice to control intestinal helminths in livestock.
Pamoate, USP.
•
I
trans-I ,4.5.b,-TeLrahydro- I pamoate (Anti-
ts a depolariring neuromuscular blocking agent that spastic paralysis in susceptible helminths. It is used he reatniem of infestations caused by pinworms and :d.mntmrc (ascariasis). Because its action opposes that of
Mebendazole, USP.
Methyl 5-bcnzoyl-2-benzinhidazolecarbamate (Vermox) is a broad-spectrum anthelmintic that is effective against a variety of ncmatode infestations.
including whipworm. pinworm, roundworm, and hookworm. Mebendazole irreversibly blocks glucose uptake in susceptible helminths. thereby depleting glycogen stored in
266
tt'i!son and
Text/wok of Organic Medicinal and Phanniacutical
the parasite. It apparently does not affect glucose metabolism in the host. It also inhibits cell division in
from release of live ova from worm segments damaged the drug.
CH3
Mebendazolc is poorly absorbed by the oral route. Adverse reactions are uncommon and usually consist of abdom-
inal discomfort. It is teratogcnic in laboratory animals and. therefore, should not be given during pregnancy.
Albendazole, USP.
Methyl 5-(propylthio)-2-beni.imid-
azolecarhumate (Eska,.ole. Zentel) is a broad-spectrum ant-
helmintic that is not currently marketed in North America. It is available from the manufacturer on a compassionate use basis. Albendazole is widely used throughout the world for the treatment of intestinal nematode infection. It is effective as a single-dose treatment for ascariasis. New and Old World hookworm infections, and trichunasis. Multiple-dose therapy with albendazole can eradicate pinworm. threadworm. capillariasis. clonorchiasis. and hydatid disease. The effectiveness of albendazolc against tapeworms (cestodes) is generally more variable and less impressive.
Bithionol.
2.2'-Thiobis(4.6-dichlorophenol). or his(2• hydroxy-3.5-dichlorophenyl)sulfide (Lorothidol. Bithin). chlorinated bisphenol. was formerly used in soaps and metics for its antimicrobial properties but was removed from the market for topical use because of reports of contact phu
todermatitis. Bithionol has useful anthelmintic and has been used as a fasciolicide and taeniacide. It is still considered the agent of choice for the treatment of intesta tions caused by the liver fluke Fasciola hepatica and the lung fluke Paragonimu.s weswrmani. Niclosamide is believed to
be superior to it for the treatment of tapeworm
H
Albendazole occurs as a white crystalline powder that is virtually insoluble in water. The oral absorption of albendarole is enhanced by a fatty meal. The drug undergoes rapid and extensive first-pass metabolism to the sulfoxide, which is the active form in plasma. The elimination half-life of the sulioxide ranges from 10 to 15 hours. Considerable biliary excretion and enterohepatic recycling of albendazole sulfoxide occurs. Albendazole is generally well tolerated in singledose therapy for intestinal nematodes. The high-dose, prolonged therapy required for clonorchiasis or cchinococcal disease therapy can result in adverse effects such as bone marrow depression. elevation of hepatie enzymes, and alopecia.
Oxamniquine, USP. I amino)methyl]-7-nitro-6-quinolinemethanol (Vansil) is e antischistosomal agent that is indicated for the treatment of
(intestinal schistosomiasis) infection. It hus been shown to inhibit DNA. RNA. and protein synthesis in S. nzanson,
The 6-hydroxymethyl group is critical fn activity: metabolic activation of precursor 6-methyl tives is critical. The oral bioavailability of oxamniquines good: effective plasma levels are achieved in Ito 1.5 huun The plasma half-life is Ito 2.5 hours. The drug is metabolized to inactive metabolites. of which the principzl one is the 6-carboxy derivative.
Niclosamide, USP. 5-Chloro-N-(2-chloro-4.nitrophenyl).2.hydroxybenzamide or 2,5'-dichloro-4'-nitrosalicylanilide (Cestocide. Mansonil. Yomesan) occurs as a yellowish white, water-insoluble powder. it is a potent taeniacide that causes rapid disintegration of worm segments and the scoles. Penetration of the drug into various cestodes appears
to be facilitated by the digestive juices of the host, in that very little of the drug is absorbed by the worms in vitro. Niclosamide is well tolerated following oral administration. and little or no systemic absorption ot it occurs. A saline purge I to 2 hours after ingestion of the taeniacide is mended to remove the damaged scolex and worm segments. This procedure is mandatory in the treatment of pork tapeworm infestation to prevent possible cysticercosis resulting
The free base occurs as a yellow crystalline solid that slightly soluble in water but soluble in dilute aqueous mi: eral acids and soluble in most organic solvents. It is availabli
in capsules containing 250 mg of the drug. Oxamniquinci
Chapter 8 a Anti-infective Agents tolerated. Dizziness and drowsiness are corn-'i. hui transitory, side effects. Serious reactions, such as convulsions. are rare. Praziquantel, USP.
2-(Cyclohexylcarbonyl)- 1.2,3.6.7.
(Bil-
1
is a broad-spectrum agent that is effective against s.stteiv oF trematodes (flukes). It has become the agent for the treatment of infections caused by schisto-
267
Ivermectin (Cardomec. Eqvalan, lvoIvermectin, USP. mec) is a mixture of 22.23-dihydro derivatives of avermectins Bia and Bib prepared by catalytic hydrogenation. Avermectins are members of a family of structurally complex antibiotics produced by fermentation with a strain of Sireplomyces aver,ni:ilis. Their discovery resulted from an intensive
screening of cultures for anthelmintic agents from natural Ivermectin is active in low dosage against a wide variety of nematodes and arthropods that parasitize ani-
(blood flukes). The drug also provides effective for fa.sciolopsiasis (intestinal fluke), clonorchiasis
(sheep liver fluke), opisthliver fluke), ishosis (liver fluke), and paragonimiasis (lung fluke). Prazincreases cell membrane permeability of susceptible resulting in the loss of extracellular calcium. Masand ultimate paralysis of the fluke musculaix'curs. tollowed by phagocytosis of the parasite,
The structure.s of the avcrmcctins were established by a combination of spectroscopic"6 and x-ray crystallographic't7 techniques to contain pentacyclic I 6-membered-ring aglycones glycosidically linked at the 3 position to a disaccharide that comprises two oleandrose sugar residues. The side chain at the 25 position of the aglycone is sec-butyl in avermectin whereas in avermectin Bib, it is isopropyl. Ivermectin contains at least 80% of 22,23-dihydroavermectin and no more than 20% 22.23-dihydroavermectin B,5.
Ivermectin has achieved widespread use in veterinary practice in the United States and many countries throughout the world for the control of endoparasiccs and ectopara.sites
in domestic animals.95 It has been found effective for the
treatment of onchocerciasis ("river blindness") in huan important disease caused by the roundworm Oncocerca volvo/us, prevalent in West and Central Africa. the Middle East. and South and Central America. Ivermectin destroys the microfilariae. immature forms of the nematode. which create the skin and tissue nodules that are characteristic of the infestation and can lead to blindness. It also inhibits Fiillswing oral administration, about 80% of the dose is Masimal plasma concentrations are achieved in I hunts. The drug is rapidly metabolized in the liver in First pass. Ii is likely that some of the metabolites are xthe. Prai.iquantel occurs as a white crystalline solid is insoluble in water. It
is available as 600-mg film-
tablets. The drug is generally well tolerated.
H3C
the release of microfllariae by the adult worms living in the host. Studies on the mechanism of action of ivennectin indicate that it blocks interneuron—motor neuron transmission in nematodes by stimulating the release of the inhibitory neurotransmitter GABA.95 The drug has been made available by the manufacturer on a humanitarian basis to qualified treatment programs through the World Health Organization.
268
Wilson and Gis%'okI's Textbook of
Medicinal and Pharn,aceu:ical Chemistry
ANTISCABIOUS AND ANTIPEDICULAR AGENTS Scabicides (antiscabious agents) are compounds used to con-
C'rotamiton,
USP.
N-Ethyl.N-(2-methylphenyl)-2.bu.
tenamide. or N-ethyl-o-crotonotoluidide (Eurax), is a color-
less. odorless oily liquid. It is virtually insoluble in water but soluble in most organic solvents.
trol the mite Sarcoptes scabiel. an organism that thrives under conditions of poor personal hygiene. The incidence of scabies is helieved to be increasing in the United States and worldwide and has, in fact, reached pandemic propor-
Pediculicides (antipedicular agents) are used to eliminate head, body, and crab lice. Ideal scabicides and pcdiculicides must kill the adult parasites and destroy their eggs.
Benzyl Renzoate, USP. Benzyl henzoate is a naturally occurring ester obtained from Peru balsam and other resins. It is also prepared synthetically from benzyl alcohol and benzoyl chloride. The ester is a clear colorless liquid with a faint aromatic odor. It is insoluble in water but soluble in organic solvents. Benzyl bcnzoate is an effective scabicide when applied
topically. Immediate relief from itching probably results 1mm a local anesthetic effect: however, a complete cure is frequently achieved with a single application of a 25% emulsion of beozyl benzoate in oleic acid, stabilized with Uiethanolamine. This preparation has the additional advantage of being essentially odorless, nonstaining, and nonirritating to
the skin. It is applied topically as a lotion over the entire dampened body. except the lace.
Lindane is I ,2.3,4,5.6-hcxachlorocycloLindane, USP. hexane. y.benzcnc hexachloride. or benzcne hexachloride (KweIl. Scabcne. Kwildane. G-WclI). This halogenated hy-
Crotamiton is available in 10% concentration in a lotion and a cream intended for the topical treatment of Its antiprtmritic effect is probably due to a local anesthetic action.
Permethrin, USP.
Permcthrin is thenyl)-2.2-dimethylcyclopropanecarboxylic acid (3-phenoxyphenyl)methyl ester or 3-(phenoxyphenyt)methyl cix. zrans-3-(2,2-dichloroethenyl )-2.2-dimethylcyclopropanecarboxylale (Nix). This synthetic pyrethrinoid compound is more stable chemically than most natural pyrethrins and is at least as active as an insecticide. Of the four isoman present, the I(R),trans and l(R),cis isomers are responsible for the insecticidal activity. The commeirial product is a mixture consisting of 60% tran.c and 40% cii racemic isomers. It occurs as colorless to pale yellow Ionmelting crystals or as a pale yellow liquid and is insoluft in water but soluble in most organic solvents.
drocarbon is prepared by the chlorination of benzene. A mix-
ture of isomers is obtained in this process, five of which have been isolated: a, fi. y, 8. and E. The y isomer, present to JO to 13% in the mixture, is responsible for the insecticidal
activity. The y isomer may be separated by a variety of extraction and chromnatographic techniques. Permethrin exerts a lethal action against lice, ticks. milci and fleas, It acts on the nerve cell membranes of the to disrupt sodium channel conductance, It is used as a peds
ulicide for the treatment of head lice. A single applicatia of a 1% solution effects cures in more than 99% of caco ii The most frequent side effect is pruritus. which about 6% of the patients tested.
Lindane occurs as a light hull to tan powder with a persis-
tent musty odor, and it is bitter. It is insoluble in water but soluble in most organic solvents. It is stable under acidic or neutral conditions but undergoes elimination reactions under alkaline conditions.
Thc action of lindane against insects is threefold: it is a direct contact poison. it has a fumigant effect, and it acts as a stomach poison. The effect of lindane on insects is similar
ANTIBACTERIAL SULFONAMIDES The sulfonamide antimicrobial drugs were the first chemotherapeutic agents that could be used systemically the cure of bacterial infections in humans. Their introduaim led to a sharp decline in the morbidity and mortality of tious diseases. The rapid development of widespread
tance to the sulfonamides soon after their introduction
to that of DDT. Its toxicity in humans is somewhat lower than that of DDT. Because of its lipid solubility properties.
the increasing use of the broader-spectrum penicillins in th: treatment of infectious disease diminished the usefulnenir
however. lindane when ingested tends to accumulate in the body. Lindane is used locally as a cream, lotion, or shampoo for the treatment of scabies and pediculosis.
sulfonamides. Today, they occupy a rather small plac the list of therapeutic agents that can be used for infectis.disease. They are not completely outmoded. however. I 970s. the development of a combination of trimeik
Chapter 8 • An:i-injec:ne Agenis 511) and sullanietltoxa,olc and the demonstration of its use—
in lie treatment and prophylaxis of certain opportun'tic mierohial infections led to resurgence in the use of-some jltiinamides.
Fnt, Mietasch and Joseph Kiarer of the I. G. Farbeninduslahur.ituries
ystematically synthesized a series of azo
each comaining the sulfonamide functional group, as anhilmerotMal agents. Sulfonamide azo dyes were in he test series because they were readily synthesad and pos.sessed superior staining properties. The Bayer
who evaluated the new MietzschKLner dyes was a physician named Gerhard Domagk. Iii
1932. Domagk began to study a brilliant red dye, later arned Prornosif. Prontosil was found to protect against, and
269
AIDS."t7 A primary infection that is treated with the conihination is PCP. The sulfonamide-trimethoprim cotnhination can be used for treatment and prophylaxis. Additionally. cerebral toxoplasmosis can be treated in active infection or Urinary tract and burn therapy'° - round out the list of therapeutic applications. The sulfonamides are drugs of choice for a few other types of infections, but their use is quite limited in modern antimicro-
bial chemotherapy."°'1 The sulfonamides can be grouped into three classes on the basis of their use: oral absorbable agents. designed to give systemic distribution: oral ,wnabsorhable agents such as sulfasalazine: and topical agents such as sodium sulfaccta-
mide ophthalmic drops.
infections in mice)°° Interestingly. Pronsas inactive on bacterial cultures. Domagk and others nrinued to study Prontosil. and in 1933. the first of many tcs of severe bacterial infections in humans was reported luerster.'°' who treated a 10-month-old infant suffering 'win slaphylucoecal septicemia and obtained a dramatic ic. The credit for most of the discoveries relating to Pron-
belongs to Dornagk. and for his pioneering work in he was awarded the t4obel Prize in medicine ii physiology in 1938. The Gestapo prevented him from
Nomendatw'e of Sulfonamides Sulfonamide ix a generic term that denotes three different cases:
I. Antibacterials that are aniline-s,,bsiiiui'ed .cuifona,nulea (the "sulfanilamides")
iiually accepting the award. hut after the war, he received in Stockholm in 1947. 2. Prodrugs that react to generate active sulfanilarnidcs tie.. sulfasala,.inc)
Pisnosil is totally inactive in vitro but possesses excellent
in vivo. This properly of the drug attracted much _cnhIq'n and stimulated a large body of research activity into iulfonanides. In 1935. Trefouel and a structure-activity study on the sulfonamide azo and concluded that the azo linkage wa-s reductively to relca.ce the active antibacterial product. sulfanil.tide This finding was confirmed in 1937 when Fuller"0 ibid culfanilamide from the blood and urine of pabeing treated with Prontosil. Favorable clinical results
or reported with Prontosil and the active metabolite itself, 2.nilamide. in puerperal sepsis and meningococcal infec-
it' All of these findings ushered in the modern era of and the concept of the prodrug. Fliosing the dramatic success of Prontosil. a host of .Iunilamide derivatives was -synthesized and tested. By more than 4,500 compounds'°t' had been evaluated. oitiy about two dozen have been used in clinical In the late 1940s. broader experience with sulfonhad begun to demonstrate toxicity in some patients. problems brought about by indiscriminate use Iiillonamides limited their use throughout the world. The .s were escelfeni alternative-s to the sulfonamides. 1 hay largely replaced the latter in antimicrobial chemofew sulfonamides (Table 8-7) and espesulfonantidc.rrimethoprim combinations that are used
hsby. there are a
'riroely for oppertuttistic infections in patients with
3. Nonanjj iso' sulfonamides (i.e., mafenide acetate)
00 \\//
There are also other commonly used drugs that are sullbnamides orsulfanilamides. Among these are the oral hypogly-
cemic drug tolbutamide. the diuretic furosemide. and the diuretic chlorthalidone. In pharmaceutical chemistry, pKb values are not used to compare compounds that are Lewis bases. Instead, if a pK. of an amine is given. it refers to its salt acting as the conjugate acid. For example, aniline with a of 4.6 refers to
270
Wilson and G,ssoId'.c I'exthook of Organic Medicinal and Phar,naceuiical Chenjiszrv
TABLE 8-7 Therapy With Sulfonamide Antibacterlals Dlseaseflnfectlon
Sulfonamides Commonly Used
Relatively Common Use Tre4tntenl sad psophylnais of l's,ctunncysris carlnii plteumc)nja
Trmehoprim-sulramcthnaue.ole
treatment and proithylavis of cerebral toxoplasmusis
Pvrlinethamiiic-vulfadianiiie
attack of Urinary tract infection
Triincurnprint-sulf.amct)toxstzole
Bum therupy: prerritrirn and treatment o(bacutrial
Silver sUlfadianine and mafenide
Conjunclivius and related superficial ocular infeclion.s
Sodium sulfacelumide
Chloroqulne•resistant malaria (Chapter 9)
Combinations with quinine, others Sulfadoxifle Sultatene
Drugs of Choice or Alternates
Less Common infectlonsiDlseases Nocardiosic
Severe trus'elcrv diarrhea
Trirnethoprim'sutfssmethoxusule
infections
Suhlonantiden. only if proved to be sulfonamide scnsitwc; otherwise. penicillin Cs. ampicihlin. or ((or pcnicillin.nhletgic chioramphenicol shuuld hc used
Generally Not Useful Strcptucoccul infections
Most are resistant to sulfonamides.
Prophylavis of reeurnmt rheumatic lever
Most are rcsi.stanl to sulfonamides
Other bacterial untcctions
The low cOs.t of penicillin and the widespread resistance to sulfonamide. limit their use: sulfonamides arc still used in a few countries
Variulal infections
The FDA and USP-Dl rind no evidence utetTicacy
Reduction of bowel flora
not established
Corticoslemid therapy often preferred
Ulcerative calf liv
Relapses common with sulfonantides Salteylaaosuhiapyridine
Side cffects
+
H
the
sometimes mimic ulcerative colitis
are intermediates of several biosynthetic pathways that corn pose the one-carbon pool in animals, bacteria, and plants, A key reaction involving folate coenzyines is catalyzed by the
enzyme thymidylate synthase. which transfers a group from N5.N'°-tetrahydrofolic acid to deoxyuridine It does not refer to
+
H'
A negative charge on a nitrogen atom is typically not stable unless it can be delocalized by resonance. This is what
happens with the sulfanilamides. Therefore, the single pK, usually given for refers to the loss of an amide proton (Fig. 8-8).
Mechanism of Action of the Sulfonamides Folinic acid (N5.formyltctrahydrofolic acid). N5.N'°-methyienetetrahydrofolic acid, and N'°-formylletrahydrofolic acid
monophosphate to form deoxythynaidine monophosphalc.an important precursor to DNA (Fig. 8-9). Another key reaction is the generation of formyl grasps for the biosynthesis of formylmethionyl tRNA units, the pnmary building blocks in protein synthesis. The are structural analogues of PABA that competitively the action of dihydropteroate synthase. preventing the addi• tion of PAI3A to pteridine diphosphate and blocking the net biosynthesis of foiate coenzymes. This action arrests rial growth and cell division. The competitive nature of the sulfonamides' action means that the drugs do no penllancnl damage to a microorganism; hence, they are bactcriostatk. The sulfonamides must be maintained at a minimum
tive concentration to arrest the growth of bacteria long enough for the host's immune system to eradicate there. Folate coenzymcs are biosynthesized from dietary Iblk acid in humans and other animals. Bacteria and proloz&e must biosynthesize them from PABA and pteridine
Chapter 8 • Awl-infective Agenl.c
0
/
R—S—N
271
General Sulfonamide Structure
II
Sulfaniiamido
Aniline
(N1)
çH3
0
I
'I SulfanilamidoSuit amethazine:
FgureB—8 • General nomenol the sulfonamides
N'(4,6-Dlmethyl-2-pyrimidyl)sultanilamide
Microbes cannot assimilate folk acid from the growth
The reverse situation exists for the antimalarial drug pyri-
from the host. The reasons for this are poorly -jmrsind,'° hut one possibility is that bacterial cell wails
merhamine.' I? Trimethoprim does have some affinity for
idjuni or
human folate reductase. and this is the cause of some of the
toxic effects of the drug.
inipenneable to folic acid. reductase. Tnrncihoprim is an inhibitor of ih is necessary to Convert dihydrofolic acid (FAH2) into 10). Anand acid (FAIl4) in bacteria (Fig. uuiewed this biochemistry.'°2 Trirnethoprim does not
Specbum of Action of the SulfonamWes
high affinity for the malaria protoioan's folate rehut ii does have a high affinity for bacterial folate
Sulfonamides inhibit Gram-positive and Gram-negative bacteria. nocardia. C/dam villa tracho,,,aiis, and some Some enteric bacteria, such as E. co/s and Kkbsiella. SaI,noaol/a. S/iigella. and En:erohac,er spp. arc inhibited. Sulfon-
0 o
FAH4
O—P—O Thymidylate Synthetase HO'
HO'
dUMP
Other examples of folate.requiring one-carbon pool reactions: Coenzyme
Reaction Formyl.MeI-tRNA
Met-tRNA i;iue8—9 • The thymidylate syn-
and other reactions vi ins one-carbon pool
Glycine N5-Formyi-FAH2
Homocysteine
•
Serine Methionlne
dTMP
272
Wil.cin, and Gixcold',n Textby,oli
Organic Medicinal and Phar,nacesuiral
Pteridine Diphosphate
Suffonamides Suifones
Guanosine
p-Aminobenzolc Acid
Dihydropteroic Acid
N
V
Bacteria Humans
\
Folic Acid (FA) in diet
To FAIl4
amides are inlrequcntly used as single agents. Once drugs of choice for infections such as PCP, toxoplasmosis. nocardiosis, and other bacterial infections, they have been largely
replaced by the fixed drug combination TMP-SMX and many other anlimiciuhials. Many strains of once-susceptible species including tneningococci. pneumococci. streptococci.
staphylococci, and gonococci are flow resistant, Sulfon-
Figure 8—10 • Folate in humans and bacteria and sites of inhibition by suit' amides and trimethoprim.
amides are, however. useful in some urinary tract because of their high excretion fraction through the kidne
Ionization of Sulfonamides The sulfonamide group. SO2NI-12. tends to gain stahiiii) it loses a proton. because the resulting negative charge resonance stabilized.
0
-W H3C—S-——NH2
H3C—S—NH
0
o
—NH 0
Chapter 8 • Anli-infretive Agents
Dlhydrofoflc Acid (FAIl2)
Folale Reductase Tnmethoprlm
H
Folic Acid (diet): FA
Tetrahydrofolic Acid (FAIl,)
0
N H
H
H
N5-Forrnyl-FAH, (folinic acid; leucovorin)
Figure 8—10 • Continued.
273
274
Wilson and Gisvo!d's l'exthoak of Organic Medicinal and Pharmaceutical Chemistry
Since the proton-donating form of the functional group is not charged, we can characterize it as an HA acid, along with groups. phenols, and thiols. The loss of a proton can for all of the compounds in the be associated with a 5.0) indiof sultisoxazole series. For example. the
Values for Clinically Useful Sulfonamides TABLE 8-8
Sulfonamide
pK.
cates that the sulfonamide is a slightly weaker acid than
Sulfadiarine
65
acetic acid
Sutfameruzine
7.1
Sulfamethazine
7.4
4.8).
Sutilsoxuzole Sulfamethoxazole
tures are seldom used today, however, because the individuil values to be partially ionirai agents have sufficiently low and adequately soluble in the urine, providing that a: frost liz ttorinal uri,,efloii' i.r maintained. Patients must be cautioncd II
Crystailurla and the plC Despite the tremendous ability of sulfanilamide to effect cures of pathogenic bacteria, its benefits were often offset by the propensity of the drug to cause severe renal damage by crystallizing in the kidneys. Sulfanitamides and their metabolites (usually acetylated at N4) are excreted almost en-
tirely in the urine. The pK, of the sulfonamido group of sulfanilumide is 10.4. so the pH at which the drug is 50% ionized is 10.4. Obviously, unless the pH is above the little of the water-soluble salt is present. Because the urine is usually about pH 6 (and potentially lower during bacterial infections), essentially all of the sulfanilamide is in the relatively insoluble. nonionized form in the kidneys. The sulfanilanaide coming out of solution in the urine and kidneys causes crystalluria. pH
1
:
6 Urine
Atmostalllnpoorty water-soluble unionIzed'
6.1
1O.4H
pKJ
Almost all in highly water-soluble salt form
form
Early approaches to adjusting the solubility of sulfanilamide in the urine were I. Greatly increasing the urine flow. During the early years of sulfonamide use, patients taking the drugs were cautioned to
"force fluids." The idea was that if the glomerular filtration rate could be increased, there would be less opportunity for seed crystals to form in the renal tubules.
2. Increasing the pH of the urine. The closer the p1-I of the urine is to 10.4 (for sulfanilamide usd0. the more of the highly watersoluble salt form will be present. Oral sodiutn bicarbonate sometimc.s was, and occasionally still is. given to raise urine pit. The bicarbonate was administered before the initial dose of sulfanilamide and then prior to each successive dose.
3. Prepuritig derivatives of sulfanilamide that have lower pK, val. ues. closer to the p1-I ni the urine. This approach has been taken
with virtually all sulfonamides in clinical use today. Examples of the pK, values of sonic ionizable sulfonamides arc shown in Table 8-8. 4. Mixing different sulktnumide.s to achieve an appropriate total dose. The solubilities of the sulfonamides are independent of each other, and more of a mixture of sulfanilttmides can stay in water solution a! a given p1-1 than can a single sulfonamide. sulfa). contain a mixture Hence. trisulfapyrimidines. USP of sulfudiazinc, sulfameraiine. and sulfametha,ine. Such mix-
maintain a normal fluid intake: forcing fluids, however, it iv longer necessary.
The newer. semisynthetic sulfonamides possess lowcrpK. values because electron-withdrawing. heterocyclic rings art
attached to N'. providing additional stability for the sat form. Hence, the drugs donate a proton more easily, and the pK, values are lowered. Simpler electron withdruwin,t groups were extensively investigated but were found to k too toxic, poorly active, or both.
Metabolism, Protein Bifldlng, and
Distribution
Except for the poorly absorbed sulfonamides used for ulcastive colitis and reduction of bowel flora and the topical barn preparations (e.g.. mat'enidc). sulfonamides and trimelhs prim tend to be absorbed quickly and distributed well. As Mandell and Petri noted, sulfonamides can be found in bc
urine "within 30 minutes after an oral The sulfonamides vary widely in plasma protein bindiof for example. sullisoxazole, 76%; sulfamethoxazole.
sulfamethoxypyridazine. 77%: and sulfadiazine. (AnandtO2 has published an excellent table comparing Is percentage of protein binding, lipid solubility. plasma kiMlife, and N4 nietabolites.) The fraction that is protein is not active as an antibacterial, but because the binditgi reversible, free, and therefore active, sulfonamide eventnalli becomes available. Generally, the more lipid soluble a ui
fonamide is. at physiological pH. the more of it will k tcin bound. Fujita and Hansch' 3 have found that sulfonamides with similar pK, values, the lipophilicity if the N' group has the largest effect on protein binding, Acetate metatbolites of the sulfonamides are more lipid ble and, therefore, more protein bound than the stoning dnlfi themselves (which have a free 4-amino group that decrear lipid solubility). Surprisingly, the Art-acetylated merabolilti. although more strongly protein bound, are excreted ass. rapidly than the parent compounds. Currently, the relationship between plasma protein ing and biological half-life is unclear. Many competing Iators are involved, as reflected in sulfadiazine. with a nests half-life of 17 hours, which is much less protein bound tha
sulfamethoxazole. with a serum half-life of II Sulfonamides are excreted primarily as mixtures of
Chapter 8 U Anti-infective
275
aaes and glucuronides are inactive. For example. sulfisoxarole is excreted about 80% unchanged, and sulfamethoxais about oole is excreted 20% unchanged. excreted as the glucuronide. The correlation between stn,Cture and route of metabolism has not yet been defineated. though progress has been made by Fujita.' Vree et
as it would with a singly blocked pathway. The synergistic approach is used widely in antibacterial therapy with the combination of sulfamethoxaz.olc and 20 (Septra. Bactrim. Co-Trimox-azole) and in antimalarial therapy with pyrimethamine plus a sulfonamide or quinine. Additional combinations with trimethoprim have been investigated (e.g., with rifampin),'2' 22
however, have described the excretion kinetics and pK. values of N'- and N'-acctylsulfaanethoxazole and other
Toxicity and Side Effecb
parent drug.
and glucuronides.''4 The N'-ace-
,ullonamides.
About 45% of trimethoprirn and about 66% of sulfamethosazole are partially plasma protein bound. Whereas about of excreted trimethoprim and its metabolites are active a antibacterials. only 20% of sulfamethoxazole and its metabolices are active, with most of the activity coming from largely unmetabolized sulfamethoxazole. Six nietabolites of aimethoprina are known.' It is likely, therefore, that sulfonamide-tiimethopritn combinations using a sulfonamide with ahioher active urine concentration will be developed in he future for urinary tract infections. Sulfamethoxazole and tnuvthoprim have similar half-lives, about 10 to 12 hours, but
the half-life of the active fraction of sulfamethoxazole is Jtorter. about 9 hours.''' (Ranges of half-lives have been summarized by Gleckmaii et aL.°6 and a detailed summary o(phannacokinetics has been made by Hansen.''5) In pakuts with impaired renal function. concentrations of sulfarethoxazole and its metabolites may greatly increase in the plasma. A fixed combination of sulfamethoxuzole and tnnielhopriin should not be used for patients with low creatitine clearances.
A variety of serious toxicity and hypersensitivity problems have been reported with sulfonamide and sulfonamide—trinote that methopnim combinations. Mandell and Petal these problems occur in about 5% of all patients. Hypersensi-
tivity reactions include fever, rash. Stevens-Johnson syndrome, skin eruptions. allergic niyocarditis. photosensitization, and related conditions. Hematological side effects also sometimes occur, especially hemolytic anemia in individuals with a deficiency of glucose-b-phosphate dchydrogenuse. Other reported hematological side effects include agranulocytosis and aplastic anemia. Crystalluria may occur, even with the modem sulfonamides, when the patient does not maintain normal fluid intake. Nausea and related gastrointestinal side effects are so,netimes noted. Detailed sutnmaries of incidences of side effects with trimcthoprim-sulfamethoxazole have been published by Wormser and Deutsch' " and
by Gleckman ci al.'"
Sfructure-Activitv Relationships As noted above in this chapter, several thousand sulfonamides have been investigated as antihactenials (and many as
antimalarials). From hese efforts, several structure—activity
Mechanisms of Microbial Resistance to Sulfonamides noted above, indiscriminate use of sulfonamides has led is the emergence of many drug-resistant strains of bacteria. Resistance is most likely due to a compensatory increase in As
he biosynthesis of PABA by resistant" although mechanisms such as alterations in the binding strength of sulfonamides to the pathway enzymes, decreased pet-meability of the cell membrane, and active efflux of the sulfon-
As a rule, if a microbe is a,nide may play a role."2 ,esislant to one sulfonamide, it is resistant to all. Of note is the finding that sulfonamide resistance can be quickly transferred from a resistant bacterial strain to a previously ensitive one in one or two generations. This resistance propagation is most likely due to R-factor conjugation, as is the case for tetracycline resistance.
Several explanations have been reported to account for bacterial resistance to the dihydrofolace reductase inhibitor nimethoprim. including intrinsic resistance at the enzymatic level, the development of the ability by the bacteria to use he host's 5-deoxythymidine monophosphate (dTMP). and R-factor conjugation.
Synergistic Activities of Sulfonamides and Folate Reductas. Inhibitors biosynthesis of bacterial (or protozoal) folac coenzymes to blocked at more than one point in the pathway. the result will be a synergistic antimicrobial effect. This is beneficial because the microbe will not develop resistance as readily If
relationships have been proposed, as summarized by The aniline (p1') amino group is very important for activity because any modification of it other than In make prodrugs results in a loss of activity. For example. all of the melabolites of sulfonamide are inactive.
A variety of studies have shown that the active form of sulfonamide is the N-ionized salt. Thus, although many modem sulfonamides are much more active than unsubstituted sulfanilamide. they are only 2 to 6 times more active if equal amounts of N' -ionized forms are compared.' Maximal activity seems to be exhibited by sulfonamides between This reflects, in part, the need for 6.6 and enough nonionized (i.e.. more lipid soluble) drug to be present at physiological pH to be able to pass through bacterial cell walls.'27 Fujita and Hansch" also related partition coefficients, and electronic (Hammett) parameters with sulfonamide activity (Table 8-9).
4-Amino-N-(5-methyl- I .3.4-thiadiazole-2y1)benzenesulfonamide; N-IS-methyl- I .3.4-thia-
Sulfamethizole. USP.
diazol-2-yl)sulfanilamide; 5-methyl-2-sulfanilamido-l .3.4thiadiazole. Sulfumethizole's plasma half-life is 2.5 hours. This compound is a white crystalline powder soluble 1:2.000 in water.
276
Wilxnn and Gixvoid'x Textbook of Organic Medki,wI and Pluirniareulkal CFie,nia:rv
CharacteristIcs of Absorbable Shortand Intermediate-Acting Sulfonamides TABLE 8—9
Oral
Half-Life
Absorption
Sulrunamidc
Sulfisosasole
Short (6 hours)
Protupi (peak levels
SulIaznctltizole
Short (9 hours)
Prompt
SIllIadia7Inc
Intonnudiate
Slow (peak levels
in l—thuUrv)
(10—17 hoard
Sulfumcihoxazo!e
in 4—K hours)
Slow
Sulflsoxazole Diolamine, USP.
4-Amino-N-(3.5-dimethyl-5-isoxazolyl )henzenesulfonamide compound with 2.2'-iminohislelhanoll(I:l); 2,2'-iminodiethanol salt oINd• (3.4-dimethyl-5-isoxazolyl)sulfanilumide. This salt is pie. pared by adding enough diethanolamine to a solution of sul. tisoxaxole to bring the pH to about 7.5. Ii is used as a salt to make the drug more soluble in the physiological pH of 6.0 to 7.5 and is used in solution for systemic adminishna. lion of the drug by slow intravenous, intramuscular, or sub-
cutaneous injection when high enough blood levels cannot be maintained by oral administration alone. It also is used for instillation of drops or ointment in the eye for the local treatment of susceptible infections.
(10—12 hours)
Sul(ado,dne
Long (7—9 days)
Intermediate
Intermediate
Pronipt
Pynmidinc Tnmelltopr)rn
(Ii hours)
4-Amino-N-(3.4-dimethyl-5-isoxazolyl)benzenesulfonamide; N'-(3.4-dimethyl-5-isoxazolyl) sulfanilamide: 5-sulfanilamido-3.4-dimethylisoxazolc. Sultisoxazole's plasma half-life is 6 hours. This compound is a white, odorless, slightly hiner. crystalline powder. Its
Sulfisoxazole USP.
is 5.0. At pH 6 this sulfonamide has a waler solubilily of 350 mg in 100 mL. and its acetyl derivative has a solubility of 110 mg in 1(X) mL 01 water.
OH
Sulfamethazine,
USP. 4-Aniino-N-(4.6-dimethyl.2• pyrimidinyl)benarenesulfonamide; Nt-(4,6-dimethyl-2.pyn. midinyl)sulfanilamide; 2-sulfanilamido-4.6.dimethylpyrim
idine. Sulfamethazinc's plasma half-life is 7 hours. Thit compound is similar in chemical properties to sulfamcraiine and sulfadiazine but does have greater water soluhility than either. Its pK, is 7.2. Because it is more soluble in acid urine than sullamerazine is. (he possibility of kidney damage from use of the drug is decreased. The human body appears ho handle the drug unpredictably: hence, there is sonic disfavor to its use in this country except in combination sulfa thciapy (in trisulfapyrimidines. USP) and in veterinary medicine.
Sulfisoxazole possesses the action and the uses of other
sulfonamides and is used for infections involving sulfonamide-sensitive bacteria. It is claimed to be effective in the treatment of Gram-negative urinary infections.
Sulfisoxazole Acetyl, USP.
N-[(4-Aminophenyl)sulfonyfl-N-(3.4-dimethyl-5-isoxazolyl)acetamide: N-(3.4-di. methyl-5-isoxazolyl)-N-sulfanhlylacctamidc;N'-acetyl-N'-(3, 4-dimethyl-5-isoxazolyl)sullanilamide. Sullisoxazole acetyl shares the actions and uses of the parent compound. sullisoxairole. The acetyl derivative is tasteless and, therefore. suitable for oral administration, especially in liquid preparalions. The acetyl compound is split in the inlestinal tract and absorbed as sulfisoxazole; that is. it is a prodrug for sultisoxazole.
H3C
Sulfacetamide.
N- 1(4-Aminophenyl ) sulfonyl]. acea. mide; N-sulfanilylacetamide: M-acetylsulfanilamide. Suil. facetamide's plasma half-life is 7 hours. This compound is a white crystalline powder, soluble in water (1:62.5 at 37'O and in alcohol. It is very soluble in hot water, and its waler solution is acidic. It has a pK, of 5.4.
Sulfachloropyridazine.
N'-(6-Chloro-3-pyridaiinyl sulfanilamide. Sulfachloropyridazine's plasma half-life is hours.
Chapter 8 • Anti-infective
277
It is a white, odorless crystalline powder soluble in water to the extent of 1:8,100 at 37°C and 1:13,000 at 25°C, in human serum to the extent of 1:620 at 37°C. and sparingly soluble in alcohol and acetone. It is readily soluble in dilute mineral acids and bases, Its
is 6.3.
USP.
Suffapyddine,
N'-2-pyridylsulfanilamide. Sulfapyridine's hall-life is 9 hours. This compound is a white. crysHint. odorless, and tasteless substance. It is stable in air darkens on exposure to light. It is soluble in water .3 in alcohol (1:440), and in acetone (1:65) at 25°C. Ii freely soluble in dilute mineral acids and aqueous soluof sodium and potassium hydroxide. The is 8.4. is sitsianding effect in curing pneumonia was first recoged by Whitby: however, because of its relatively high it ha., been supplanted largely by sulfadiazine and Several cases of kidney damage have resulted wclylsulfapyridine crystals deposited in the kidneys. causes severe nausea in most patients. Because of its it is used only for dermatitis herpetiformis.
0
NH//
Suhapyridinc was the first drug to have an outstanding aetion on pneumonia. It gave impetus to the study whole class of N' hetcmcyclically substituted derivaof
Sulfarnethoxazole, USP. tolylbenicncsulfonamide;
4-Amino-N-(5-methyl-3-isoxN'-(5-methyl-3-isoxazolyl)
•ilanlarnide Gantanol). Sulfamethoxaaole's plasma halfII hour..
((jN Suifadiazine Sodium, USP. Soluble sulfadiazinc is an anhydrous. white, colorless, crystalline powder soluble in water (1:2) and slightly soluble in alcohol. Its water solutions
are alkaline (pH 9 to 10) and absorb carbon dioxide from the air, with precipitation of sulfadiazine. It is administered as a 5% solution in sterile water intravenously for patients requiring an immediately high blood level of the sulfonamide.
Na°
N=.alanine bond is required before peptide crosslinkage. 2-Ethoxy-l-naphthyl. penicillin
The various f3-lactam antibiotics differ in their affinities for FBI's. Penicillin G hinds preferentially to PBP 3. whereas the first-generation cephalosporins bind with higher affinity In contrast to other penicillins and to cephalospoto FBI' rins. which can bind to PBPs 1.2, and 3, amdinocillin hinds only to PBP 2.
THE PENICILUNS
Onacillin
5.Methyl.3.phenyl-4isosasolylpeniciltin
Ctoxacillin
5.MCtI,yJ-3.(2. chtorophenyt)-4-
Commercial Production and Unitage Until 1944. it was assumed that the active principle in penicillin was a single substance and that variation in activity of different products was due to the amount of inert materials
more will be added to the list in attempts to find superior products. because the penicillin first used in chemotherapy wax not
Cl
isoxazolylpenicitlin
Cl
Dictoxacillin dlchluropl%enyl)-4--
in the samples. Now we know that during the biological elaboration of the antibiotic, several closely related compounds tnny be produced. These compounds differ chemically in the acid moiety of the amide side chain. Variations in this moiety produce differences in antibiotic effect and in physicocheinical properties, including stability. Thus, one can speak of penicillins as a group of compounds and identify each penicillin specifically. As each of the different penicillins was first isolated, letter designations were used in the United States: the British used Roman numerals. Over 30 penicillins have been isolated from fermentation mixtures. Some of these occur naturally; others have been hiosynthesiscd by altering the culture medium to provide certain precursors that may be incorporated as acyl groups. Commercial production of biosynthetic penicillins today depends chiefly on various strains of Penicilliun, notazurn and P. c/,rysogenu,n. In recent years. many more penicillins have been prepared semisynthetically. and undoubtedly. many
/
isoxazolylpeniciltin
— Ampiciltia penicillin NH2
AmoxiciDin
Cyclacillin
t-A,ninvcyclohexylpenicillin
Carbenic)tlin a-Carhoxyhenzyl. penicillin CO2H
Ticarcillin
a-Cartoxy-3-thienylpc,,icilljn
cJLH
Chapter tO • Antibatierial ,tniil,iugic.s
TABLE 10-2—Continued Generic Name
Chemical Name
R Group
Pipcmcitlin
pipcraziiiytcarbonylainina)bcflzylpcnictttin
?° CH2CHS Mczlocillin
a.( t-Methanesutfonyl-2. 4)SOflflidU7.OtidiflO-
pnnicttlin
303
number I to the nitrogen atom and number 4 to the sulfur atom. Three simplified forms of penicillin nomenclature have been adopted for general ttse. One uses the name "penam" for the unsubstitutcd bicyclic system, including the umide carbonyl group. with one olihe foregoing numbering systems as just described. Thus. generally are designated according to the Cl,eniical Abstracts system as 5-ucylamino-2.2-dimethylpenam-3-curboxylic acids. The second, seen more frequently in the medical literature, uses
the name "penicillanic acid" to describe the ring system with substituents that are generally present (i.e.. 2.2-dimethyl and 3-carhoxyl). A third form, followed in this chapter. uses trivial nomenclature to name the entire 6-carbonylaminopenicillanic acid portion of the molecule penicillin and then distinguishes compounds on the basis of tIne R group of the acyl portion of the molecule. Thus, penicillin 0 is named hens.ylpenicillin. penicillin V is phenoxymethylpenicillin. meihicillin is 2.6-dimcthoxyphenylpcnicillin. and so on. For the most part. the latter two systems serve
well for naming and comparing closely similar penicillin structures, but they are too restrictive to be applied to compound.'; with unusual substituenis or to ring-modified derivaN
tives.
Stereochemistry
inhibit, in vitro, the growth of a strain of Sraphrlococcus in 50 rnL of culture medium under specified conditions. Now hat pure crystalline penicillin is available, the United States Pharmacopoeia (USP) defines unit as the antibiotic activity of USP penicillin G sodium reference standard. The weight—unit relationship of the penicillins varies with tv acyl substituent and with the salt formed of the free acid: of penicillin G sodium is equivalent to 1.667 units. I mgolpcnicillin G procaine is equivalent to 1,009 units, and lug of penicillin G potassium is equivalent to 1.530 units. The commercial production of penicillin has increased markedly since itS introduclion. As production increased. he coOl dropped correspondingly. When penicillin was first 100,000 units sold for $20. Currently, the same costs less than a penny. Fluctuations in the producson of penicillins through the years have reflected changes n the relative popularity of broad-spectrum antibiotics and miaiicillins. the development of penicillin-resistant strains of nacral pathogens, the more recent introduction of semisynhetic penicillins, the use of penicillins in animal feeds and I
fir veterinary purposes. and the increase in marketing probcmos in a highly competitive sales area. Table 10-2 shows the general structure of the penicillins
nd telates the structure of the more familiar ones to their mamious designations.
lomendature lhc nomenclature of penicillins is somewhat complex and may cumbersome. Two numbering systems for the fused heterocyclic system exist. The chemical Abstracts initiates the numbering with the sulfur atom and as'ens the ring nitrogen the 4 position. Thus. penicillins are us 4-thia-l-azabicyclo[3.2.Olhcptanes, according to system. The numbering system adopted by the USP is 'le reverse of the Chemical Abstracts procedure, assigning
The penicillin molecule contains three chiral carbon atoms (C-3. C-5. and C-6). All naturally occurring and microhiologically active synthetic and semisynthetic penicillins have the same absolute contiguratiorl about these three centers. The carbon atom bearing the acylamino group (C-6) has the t. configuration, whereas the carbon to which the curboxyl group is attached has the o configuration. Thus, the acylammo and carboxyl groups are trails to each other, with the former in the a and the latter in the orientation relative to the penam ring systetn. The atoms composing the 6aminopenicillanic acid portion of the structure are derived biosynthetically from two amino acids, t-cysteine (S-I. C5, C-6. C-7. and 6-amino) and i.-valinc (2.2-dinlethyl. C-2. C-3, N-4. and 3-carboxyl). The absolute stereochemistry of the penicillins is designated 3S:5R:6R. as shown below.
usp
Chemotcat Abstracts
0)10 Penam
H
H
I
Chenilcat Abstracts
Penicillanic Acid
304
Wilson and Gisvold's Textbook of Organic Medicinal and Pharmaceutical chemistry
Synthesis Examination
natural penicillins are strongly dextrorotatory. The solubility
of the structure of the penicillin molecule
shows that it contains a fused ring system of unusual design, the ,8-lactam thiazolidine structure. The nature of the /3-lac-
tam ring delayed elucidation of the structure of penicillin, but its determination resulted from a collaborative research program involving groups in Great Britain and the United States during the years 1943 to Attempts to synthesize these compounds resulted, at best, in only trace amounts until Sheehan and Henery-Logan'5 adapted techniques de-
veloped in peptide synthesis to the synthesis of penicillin V. This procedure is not likely to replace the established fermentation processes because the last step in the reaction series develops only 10 to 12% penicillin. It is of advantage in research because it provides a means of obtaining many new amide chains hitherto not possible to achieve by biosynthetic procedures. Two other developments have provided additional means
for making new penicillins. A group of British scientists, Batchelor et al.,'6 reported the isolation of 6-aminopenicillanic acid from a culture of P. chrysogenum. This compound can be converted to penicillins by acylation of the 6-amino group. Sheehan and Ferris'7 provided another route to synthetic penicillins by converting a natural penicillin, such as penicillin G potassium, to an intermediate (Fig. 10-1), from which the acyl side chain has been cleaved and which then can be treated to form biologically active penicillins with a variety of new side chains. By these procedures, new penicil-
lins, superior in activity and stability to those formerly in wide use, were found, and no doubt others will be produced.
The first commercial products of these research activities were phenoxyethylpenicillin (phenethicillin) (Fig. 10-2) and dimethoxyphenylpenicillin (methicillin).
and other physicochemical properties of the penicillins affected by the nature of the acyl side chain and by the cations used to make salts of the acid. Most penicillins are acids with pKa values in the range of 2.5 to 3.0, but some are amphoteric. The free acids are not suitable for oral or parenteral administration. The sodium and potassium salts of most penicillins, however, are soluble in water and readily
absorbed orally or parenterally. Salts of penicillins with ganic bases, such as benzathine, procaine, and hydrabamine. have limited water solubility and are, therefore, useful as depot forms to provide effective blood levels over a long period in the treatment of chronic infections. Some of the crystalline salts of the penicillins are hygroscopic and must be stored in sealed containers. The main cause of deterioration of penicillin is the reactivity of the strained lactam ring, particularly to hydrolysis. The
course of the hydrolysis and the nature of the degradation products are influenced by the pH of the solution.'8' '9lhus, the /3-lactam carbonyl group of penicillin readily undergoes nucleophilic attack by water or (especially) hydroxide ion to form the inactive penicilloic acid, which is reasonably stable in neutral to alkaline solutions but readily undergoes decarboxylation and further hydrolytic reactions in acidic solutions. Other nucleophiles, such as hydroxylamines, a]. kylamines, and alcohols, open the /3-lactam ring to form the corresponding hydroxamic acids, amides, and esters. It has been speculated2° that one of the causes of penicillin allergy
may be the formation of antigenic penicilloyl proteins in vivo by the reaction of nucleophilic groups (e.g., &aminoi on specific body proteins with the /3-lactam carbonyl group.
In strongly acidic solutions (pH nlhelic
Fair
Varitbk
90
Yes
Narrow
Ouxullin
Sernisynuhetic
Good
Fair (30)
85—')4
Yes
Narrow
limited usc limncd tise
Seinisyniheuic
Good
Good (50)
88—')s
Yes
Narrow
i.iiniicd use
Scmisynlhctlc
Good
Good (50)
95—OS
Yes
Narrow
L.imticd use
Mipucullin
Scmisynhiiciic
Good
Fair (40)
20—25
No
Broad
Multipurpose
Siroacillipu
Scmisyuthetic
Good
Good (75i
20—25
No
Broad
Multipurpose
Cjuhtnicilliit
Somisynthelie
Poor
50—6(1
No
t3xucutdcd
l.umikutl use
Semisynthetic
Poor
Nil Nil
45
No
Extended
l.imitcd use
\Onluuciltun
Semisynlhctie
Poor
50
No
EXIL'nded
Limited use
Pçeracullin
Sernisynlhetuc
Poor
Nil Nil
50
No
Extended
Limited use
310
Wilson and Gisvold's Textbook of Organic Medicinal and Plwmuiceutical Chemistry
sions of penicillin in peanut oil or sesame oil with white beeswax added were first used to prolong the duration of injected forms of penicillin. This dosage form was replaced by a suspension in vegetable oil, to which aluminum monostearate or aluminum distearate was added. Today, most repository forms are suspensions of high-molecular-weight amine salts of penicillin in a similar base.
Penicillin G Procaine, USP.
The first widely used
amine salt of penicillin G was made with procaine. Penicillin
G procaine (Crysticillin, Duracillin, Wycillin) can be made readily from penicillin (3 sodium by treatment with procaine hydrochloride. This salt is considerably less soluble in water than the alkali metal salts, requiring about 250 mL to dissolve I g. Free penicillin is released only as the compound dissolves and dissociates. It has an activity of 1,009 units/ rag. A large number of preparations for injection of penicillin (3 procaine are commercially available. Most of these are either suspensions in water to which a suitable dispersing or suspending agent, a buffer, and a preservative have been added or suspensions in peanut oil or sesame oil that have been gelled by the addition of 2% aluminum monostearate. Some commercial products are mixtures of penicillin (3 po-
tassium or sodium with penicillin (3 procaine; the watersoluble salt provides rapid development of a high plasma concentration of penicillin, and the insoluble salt prolongs the duration of effect.
Penicillin G Benzathine
In 1948. Bchrcns et reponed Penicillin V. USP. penicillin V. phenoxymethylpenicillin (Pen Vee. V-Cillini as a biosynthetic product. It was not until 1953, however that its clinical value was recognized by some Europear scientists. Since then, it has enjoyed wide use because of
resistance to hydrolysis by gastric juice and its ability to produce uniform concentrations in blood (when adminis. tered orally). The free acid requires about 1.200 mL of wata to dissolve I g. and it has an activity of 1,695 units/mg. Fri parerneral solutions, the potassium salt is usually used. mit salt is very soluble in water. Solutions of it are made from the dry salt at the time of administration. Oral dosage foam of the potassium salt are also available, providing rapid. ci. fective plasma concentrations of this penicillin. The salt ol phenoxymethylpenicillin with N.N'-bis(dchydroabietyl)eih.
ylenediamine (hydrabamine. Compocillin-V) provides a very long-acting form of this compound. Its high water insol ubility makes it a desirable compound for aqueous suspea sions used as liquid oral dosage forms.
H20 NH2
0
Penicillin G Procaine Penicillin V, USP
Since penicillin (3 benPenIcillin 6 Benzathine, USP. zathinc, N.N'-dibenzylethylenediamine dipenicillin (3 (Bicillin. Permapen). is the salt of a diamine, 2 moles of penicillin
are available from each molecule. It is very insoluble in water, requiring about 3.000 mL to dissolve I g. This property gives the compound great stability and prolonged dura-
tion of effect. At the pH of gastric juice it is quite stable, and food intake does not interfere with its absorption. It is available in tablet form and in a number of parcnteral preparations. The activity of penicillin G benzathine is equivalent to 1,211 units/mg. Several other amines have been used to make penicillin
Methicillin Sodium,
USP.
During 1960, methicillin to-
dium, 2.6-dimcthoxyphcnylpenicillin sodium (Staphcillio). the second penicillin produced as a result of the research thai developed synthetic analogues, was introduced for medicinal use. HSC\
/=
e int'eciitm .cnccptuui itis
Inactivated sorts Icilleacy uncennint
!udr. arri-A
Herpes sinipler 2
Genital herpes, skirt eruptions
None
Varieella ,.ostcr
Chackenpox (children). shingles (adrilla)
None
Acvelovir Acyctovir
C> tonrcgainvirua
trifeetiims in tIre iiirnnrnruiirirproinised.
None
Oancictosir.
None
None
Nitric
Podophyltin
herpes samples
I
fescarneu
neonates
Epstein-Barr siau.s
Inlecticru'. moirrinu eosir, Burkitt's
lymplionu Papovccvinca
t'cpilltrmasirus
\Varts
Polyrmasirus (IC vicrist
Pargeessive leukoetceeplicrlopathy
None
Adeciovinis Human adenovinas
Upper respiratirry tract and eye nieciioris
Nitric
None
Hcpalilis (rim> Decatur chorale)
tncrctivcitcd subunit (scry
None
Hepadcrasiru'.
Hepatitis B virus
eflettive) Pi,xvincs Varcola
Sticailpon
Vuednia Ieowtxrxi (very etiectice)
Methiscaircie
t(iytlrcma, tterncrlytic anemia
Noire
None
I'arvor lots Iticrucir trarx'osirun 1311)
induce an eft'ective antibody response. even in very young patients: the vaccine must not cause the disease that it is designed to prevent or cause some other toxic manifestation as the early killed vaccines did: and ideally, the vaccine should produce a
lasting form of immunity, with a minimum requirement for booster doses. These requirements are difficult enough to meet for viruses that cause acute untections. The chronic cases are much more complicated. It is difficult to overcome
the tendency of some viruses to undergo rapid mutation. leading to multiple antigenic epitopes: this makes development of a broadly effective vaccine much more difficult.
Biochemical Targets for Antiviral Therapy With the discovery ol antibiotics and anti-infective agents, the science of treating bacterial infections moved forward at a rapid rate. The development of useful antiviral agents (antibiotics and antiviral agents. in contrast, has historically Unlike lagged behind. There are a number of reasons for bacteria. viruses will not grow in simple synthetic culttire media. They must infect human or animal cells to propagate.5 For example. the most commonly used cell cultures
in virology derive from priniates (including humans and monkeys). rodents (including hamsters and mice), and birds (especially chickens). These culture methods are very reliaand are in widespread use the propagation of virus
particles. hut they are more difficult to perform than their bacterial counterparts. Hence, drug-screening techniques with viruses have taken longer to catch up with those in
bacteria. Another possible reason for the lag in development lies in the comparative biochemical simplicity of viruses vis-à-vis bacteria and their use of the biochctaicrl processes of a host cell. There are fewer specialized for potential attack by chemotherapeutic agents. The spectacular successes of immunization procedures ho I): prevention of certain viral diseases may have contributed I a relative lack of interest in antiviral chemotherapy. Anolirt feature of mild viral infections, such as the common cnh[ is that clinical symptoms do not appear until the iniectio3 is well established and the intmune processes of the its have begun to mount a successful challenge. Thus, formats common viral infections, chemotherapy is simply not prolar ate choice of treatment. Chemotherapeutic agertis clv needed, however, to combat viruses that cause severe r chronic infections, such as encephalitis. AIDS. and lieqv particularly in patients with compromised immune
THE INFECTIOUS PROCESS FOR A VIRUS Despite their simplicity relative to bacteria, viruses still sess a variety of biochemical targets for potential attack chemotherapeutic agents. An appropriate chemical ocr pound may interrupt each of these. Hence, a thorough standing of the specific biochemical events that occurdanny
viral infection of the host cell should guide the site-specific antiviral agents. The process of viral infcoot can be sequenced in seven stages:
Chapter II • Antiviral Agents attachment7 of the virus to specific receptors on the surface of the host cells, a specific recognition process.
I
2
Enu-i. penetration7 of the virus into the cdl.
o viral nucleic acid from the protein coat. Trwiscr,ption. production of viral inRNA from the viral ge-
3. Unroarin,n. release7 4
tame.8
Translation. synthesis8 of' viral proteins (coat protcins and en-
cynics for replication) and viral nucleic acid (i.e.. the parental genome or complimentaiy strand). This proces.c uses the host cell processes to express viral genes, resulting in a few or many viral proteins involved in tile replication process. The viral proteins modify the host cell and allow the viral genome to replicate
371
with a cyfokine
Substantial evidence indicates that viruses enter cells by endoevw.cis. a process that involves fusion of the viral envelope with the cell membrane.
intermixing of components, and dissolution of the menibranes of virus and cell. Various receptors and coreceptors facilitate this reaction.'7 Before a virus can begin a replication cycle within a host cell, its outer envelope and capsid must be retnoved to release its nucleic acid genome. For complex DNA viruses such as vaccinia (its binding receptor is the epidermal growth factor receptor), the uncoating process occurs in two
by using host and viral enzymes. The mechanisms by which his occurs are complex. This is often the stage at which the is irreversibly modified and eventually killed, ts Assembh' of the viral particle. New viral coat proteins assemble inti, capsids (the protein envelope that surrounds nucleic acid and associated molecules in the core) and viral genomes.8 Release of the mature virus from the cell by budding from the cell membrane or rupture of the cell and repeat of the process. 1mm cell to cell or individual to individual.8 Enveloped viruses typically tise budding on the plasma membrane. endoplasmic reticulutn. or Golgi membranes. Noneitveloped viruses typically ecapc by rupture of the host cell.
stagest 1:
The initial attachment of viral particles to cells probably imolves multiphasic interactions between viral attachment and host cell surface receptors. For instance, in the case of the alphaherpesviruses, internalization involves of events that involve different glycoproteins and cell surface molecules at different stages. Different surtitce proteins may be used for the initial attachment .isi entry into target cells and for cell-to-cell spread across apposed populations of cells.5 The pattern of systemic illness produced during an acute viral infection in large part &pends on the specific organs infected and in many cases in the capacity of the viruses to infect discrete populations 1 cells within these organs. This property is called tissue The tissue tropism of a virus is influenced by interaction between a variety of host and viral factors. Although the specific viral aftachmenr proteins and spe-
hut they are not selective enough to be useful us antiviral
ofic receptors on target cells are important, a variety of virus—host interactions can play an important role in the tropism of a virus. Increasing attention is king focused on corceeptors in mcdiatitig viral binding. Instance, entry of HIV-l into target cells requires the of both CD4 and a second coreceptor protein to the G-protein--coupled seven-transmembranc
fatally. including the chemokine receptor proteins CCRS and CXCR4. Cells that express CD4 but not the cisceeptor are resistant to HIV infection. Host cellular recep-
Is can he integrins. hcparans. sialic acids. gangliosides. phospholipid.s. and major histocompatibility anti(to name a few). There is substantial evidence that tile receptor for influenza viruses is the pcptidogiycan sanpenent N-acctylmuramic acid, which binds a protein hemagglutinin. projecting from the viral surface.t2 ite binding of N-acetylmurarnic acid and hemagglutinin in motion a sequence of events whereby the viral envethe host cell membrane dissolve into each other, and issiral contents enter the cell. Initiation of HIV- I infection solves the interaction of specific glycoprotein molecules
that stud the viral cell surface with an untigenic receptor molecule on helper T lymphocytes along
I. I-lost cell enzymes partially degrade the envelope and eapsid to reveal a portion of the viral DNA. which serves as a template for mRNA synthesis. 2. mRNAs code for the synthesis of viral enzymes. which complete
the degradation of the protein coat, allowing the virus to fully enter the host.
The proteins of the viral envelope and capsule are the primary targets for antibodies synthesized in response to immunization techniques. Protein synthesis inhibitors such as cyclohexinnide and purotnycin inhibit the uncoating process. agents.
In the critical fourth and liITh stages of infection, the virus usurps the energy-producing and synthetic functions of the host cell to replicate its own genome and to synthesize viral enzymes and structural proteins.-0 Simple RNA viruses conduct both replication and protein synthesis in the cytoplasm of the host cell. These contain specific RNA polymera.scs (RNA replicases) responsible for replication of the genonle. Some single-stranded RNA viruses, such as poliovirus. have a ( + )-RNA genome that serves the dual function of messenger for protein synthesis and template for the synthesis of a complementary strand of (—)-RNA. from which the (+ )RNA is replicated. In poliovirus (a picornavirus). the message is translated as a single large open reading frame whose product is cleaved enzymatically into specific viral enzymes and structural proteins)8' Other RNA viruses, such as influenza viruses, contain (—)-RNA. which serves as the tem-
plate for the synthesis of a complementary strand of N- )KNA. The (+ )-RNA strand directs viral protein synthesis and provides the template for the replication of the (—)-RNA genome. Certain antibiotics, such as the rifamycins, inhibit
viral RNA polynacrases in vitro, but none has yet proved clinically useful. Bioactivated forms of the nuckoside analogue ribavirin variously inhibit ribonucleotide synthesis. RNA synthesis. or RNA capping in RNA viruses. Rihavirin has been approved for aerosol treatment of severe lower respiratory infections caused by respiratory syncytial virus (RSV).
Retroviruses constitute a special class of RNA viruses that posses.s a RNA-dependent DNA polymerase (relerse transcripta.ve) required for viral replication. In these viruses. a single strand of complementary DNA (eDNA) is synthesized on the RNA genome (reverse :ran.vcriplion). duplica. ted, and circularized to a double-stranded proviral DNA. The proviral DNA is then integrated into the host cell chromo-
somal DNA to form the template (upovirus or virogene) required for the synthesis of mRNAs and replication of the viral RNA genome. During the process of eDNA biosyn-
372
of Organic
and Gi.cvold's
degrades the RNA strand, leaving only a DNA. Oncogenic (cancer-causing) viruses, such as the
Late stages in viral replication require important virus. specific processing of certain viral proteins by viral orcellu'
human 1-cell leukemia viruses (HTLV) and the related HIV. are retroviruses. Retroviral reverse tr.Iuscriptase is a good target for chemotherapy. being inhibited by the triphosphates of certain dideoxynucleosides. such us 2'.3'-deoxy-3'-
lar proteases. Retroviruses, such as HIV. express three genes as precursor polyproteins. Two of these gene products. designated the p55gag and p1 6Ogag-pol proteins for their Irea' tion on the genome. undergo cleavage at several sites by a virally encoded protease to form structural (viral coat) proteins (p17. p24, p8. and p7) and enzymes required for repli. cation (reverse transcriptase. integrase, and protease). The demonstration that HIV protease. a member of the aspunyl protease family of enzymes, is essential for the maturation
azidothymidine (AZT. zidovudine). 2'.3'-dideoxycytidine (ddCyd. zalcitahinc). and 2'.3'-dideoxy-2'.3'-didehydrothymidine (D4T. stavudine). all of which have been approved for the treatment of AIDS. The nomenclature of these
agents is straightforward. A 2'.3'-dideoxynucleoside is referred to as ddX. while the unsaturated 2'.3'-dideoxy-2'.3'didehydrunucleosides are named d4X. The dideoxynucleoside triphosphates arc incorporated into viral DNA in place of the corresponding 2'.deoxynucleoside (i.e.. 2'-deoxythymidine. 2'-deoxycytidine. or 2'.deoxyadenosine) triphosphate.22' This reaction terminates the viral DNA chain. since the incorporated dideoxynucleoside lacks the 3'-hydroxyl group rcqttired to form a 3'.5'-phosphodiester bond with the next 2'-deoxynucleotide triphosphate to be incorporated. The DNA viruses constitute a heterogeneous group whose genome is composed of DNA. They replicate in the nucleus of a host cell. Some al the DNA viruses are simple structures. consisting of a single DNA strand and a few enzymes surrounded by a capsule (e.g.. parvovirus) or a lipoprotein envelope (e.g.. hepatitis B virus). Others, such as the hcrpesvi-
and infectivity of HIV particles24 has stimulated major search efforts to develop effective inhibitors of this step. These efforts have led to several candidates, some that ate
on the market and many that are in clinical trials. To complete the replication cycle, the viral are assembled into the mature viral particle, or virion.
Fat
simple. nonenveloped viruses (e.g.. the picornavirus poliovi' rus). the genome and only a few enzymes are encased by capsid proteins tO complete the virion. Other, more comples viruses arc enveloped by one or more membranes containiny carbohydrate and lipoprotein components derived front the host cell membrane. Once the mature virion has been assembled, it is ready for release from the cell. The release of certain viruses (e.g.
roses and poxviruses. are large, complex structures with double-stranded DNA genonies and several enzymes en-
poliovirus) is accompanied by lysis of the host cell
cased in a capsule and surrounded by an envelope consisting
ever, are released by budding or exocvta.si.s. a process involv. ing fusion between the viral envelope and the cell membrane
of several membranes. l)NA viruses contain DNA-dependent RNA polymerases (IraItscripta.ces), DNA polymerases. and various other enzymes (depending on the complexity of the virus) that may provide targets for antiviral drugs. The most successful chernotherapeutic agents discovered thus far are directed against replication of herpesviruses. The nucleo-
side analogues idoxuridine. iritluridine. and vidarabine block replication in herpesviruses by three general mechanisms: First, as the monophosphates. they interfere with the biosynthesis of precursor nucleotides required for DNA synthesis: second, as Iriphosphates. they competitively inhibit DNA polyrnerase: and third, the triphosphates are incorpo-
rated into the growing DNA itself, resulting in DNA that is brittle and does not function normally. Acyclonucleosides (e.g., acycloguanosine) are bioactivated sequentially by viral and host cell kinases to the acyclonucleotide monophosphate and the acyclonucleoside triphosphame. respectively. The lat-
ter inhibits viral DNA polytnerase and terminates the viral DNA strand, since no 3'-hydroxyl group is available for the subsequent formation of a 3',S'-phosphodiester bond with the next nucleoside triphosphate. The structure of acyclovir with the acyclosugar chain rotated into a pentose configuration (below) shows clearly the absence of the 3'-hydroxyl group.
0
H2N
N
brane and cell death. Some of the enveloped viruses, how'
This process is nearly a reversal of the entry process: the host cell membrane remains intact under these conditions and the cell may survive. Chemoprophylaxis is an alternative to active immuni?a• tion for the prevention of viral infection. With chetnoprophy laxis. one uses a chemical agent that interferes with a in early viral infectivity. The immune system is not diredy stimulated by the drug but i.s required to respond to any active infection. It would seem that the most successful moprophylactic agents would be those that prevent peneua•
(ion of the virus into the host cell. In principle, this can k achieved by blocking any of three steps prior to the start the replication cycle: (a) attachment of the virion to the hwi cell via its receptor complex. (h) its entry into the cell endocytosis. or (c) release of the viral nucleic acid front ih: protein Coat. At present, only a single class of agents these early stages of The adamantanaininn (arnantadine and ritliantadine) have been approved forcm
trolling influenza type A infection. These drugs appeat interfere with two stages of influenza type A viral replie tion: preventing the early stage of viral uncoating and turbing the late stage of viral assembly. Clinical studies ha: shown that amantadine and rimantadine are effective in prophylaxis and treatment of active influenza type A infe' tion. Amantadire, I Amantadine, USP, and Rimanta dine. adamantanamine hydrochloride (Symmetrel). and its
tnethyl derivative rimuncudine. thylamine hydrochloride Flumadine), are unusual cyclic amines with the following structures:
i
Chupter Ii • ,tIsIi%iral AgeIus
373
lo
NH2
against type A. The drugs on influenia type B. The primary side effects are related to the central nervous system and are dopami— nergie. This is not surprising, since amantadine is used in the treatment of l'arkinson's disease. Rimuntadine has significainthy kwer side probably because of its eXtetisive biotransformation. Less than 50% of a dose of rimantadine is excreted unchanged. and more than appears in the urine as tnelaholites.25 Amantadine is excreted largely unchanged in the urine.
have no
Amantadine
Rirnantadine
Aniantadine has been used for years as a lreattucrn for Parkinsons disease. Both of these agents will specilically
inhibit replication of the influen,a type A viruses at low concentrations. Ritnantadine is generally 4 to IC) limes more
than amaniadine. The adamantanamines have two mechanisms in common: (i,) they inhibit an early step in dial replication, most likely viral and (b) in sme slrains they affect a later step that probably itivolves viral assembly, possibly by with hemaggltutinin The main hiochetnical locus of action is the intype A virus M2 protein, which is an integral nienihrane protein that functions as an ion channel. The M2 chan-
is a proton transport systeni. By interfering with the unction of the M2 protein, the adamantanamities inhibit
nel
acid-mediated dissociation of the rihonucleoprotein coniplex carly in replication. They also interfere with transinetubrane pumping, maintaining a high intracellular proton conccntraljl)n relative to the cxtracellular concetliratioti and en—
tuncing acidic pH-induced conftrmational changes in the during its intracellular transport at a later ,wgc. The conformational changes in hemagglutinin prevent
of the nascent virus particles to the cell nienibranc or exocytosis.
Resistant variants of inlluen/a type A have been recovered (torn aniantadine- and riniantudine-treated patients. Resisunce with inhibitory concentrations increased more thatt have been associated ssith single tiucleotide
that lead to amino acid substitutions in ilte trailsdomain of M2. Amantadinc and rimantadine -tore cross-susceptibility and
\matitadine and rimantadine are approved in the United Stares tbr prevetition and treatment of inlluensa type A virus
Seasonal prophylaxis with either drug is about
INTERFERONS: INTERFERON ALFA (INTRON A, ROFERON A) AND INTERFERON BETA (BETASERON)
Interterons UFNs arc extremely potent cytokines that possess antiviral. innmtunotnodntlating. and anliproliferarive actiotisH lENs are synthesiied by infected cells in response
to various itiducers (Fig. Il-I) and, in turn, elicit either an antiviral state in neighboring cells or a natural killer cell respotise that destroys tile initially inlécted cell (Fig. 11-2). There are three classes of human IFNs that possess signilicant antiviral activity. These are IFN-a (more than 20 subtypes). subtypes), and IFN-y. IFN-a is used clinically in a recombinant fonn (called interferon alfa). (Betaseront is a recombinant form marketed for the treatnient of multiple sclerosis. IFN-a and arc produced by almost all cells in response to viral challenge. Interferon production is not limited to viral stimuli, however. A variety 01 other triggers, including cytokines such as interleukin-l. interleukin-2, and tumor necrosis factor, will the production of lFNs. Both IFNa and are elicited by exposure of a cell to doublestranded viral RNA. lEN-a is produced by lymphocytes and macruphages. while IFN-fJ is biosynthesited in flhrohlasts and epithelial cells. IFN-y production is restricted to T lyniphocytes and riatttral killer cells responding to anhigetnie stimuli. milogetta. and speeilie cyhokines. IFN-a and IFN-
fi hind to the same receptor, and the genes for both are encoded on chromosome 9. The receptor for INF-yis unique. and only one subtype has been identilied. The genes for this molecule are cimeoded on chromosome 12. INF-y has less antiviretl activity than IFN-co and but more potent
Type 1 Interferons
IFN-a
Type 2 Interferons IFN.y
IFN-f1
Lymphoblasts Macrophages
Fibroblasts, Epithelial Cells
Mrtogen.slimulated T Lymphocytes Induced by Mitogens or Lectlns
Induced by Double-stranded Viral RNA: Receptors identical
Receplor Unlike Type 1
Both encoded on Chromosome 9
Encoded on Chromosome 12
Figure 11—1 • Types of interferon
374
Wilson and Gi.wold's Textbook of Organic Medicinal and Pharmaceutical Chemistr-v
Activate by IFN
Killing
Infected Cell
Natural Killer Cell
Other Cells
FIgure 11—2 • Interferon mechanisms.
immunoregulatory effects. INF-y is especially effective in activating macrophages, stimulating cell membrane expression of class II major histocompatibility complexes (MHCII). and mediating the local inflammatory responses. Most animal viruses are sensitive to the antiviral actions of IFNs. The instances in which a virus is insensitive to IFN
typically involve DNA viruses)3 On binding to the appropriate cellular receptor, the IFNs induce the synthesis of a cascade of antiviral proteins that
contribute to viral resistance. The antiviral effects of the IFNs are mediated through inhibition of ° • Viral penetration or uncoating • Synthesis of mRNA • The translation of viral proteins • Viral assembly and release
With most viruses, the lFNs predominantly inhibit protein synthesis. This takes place through the intermediacy of IFN.
4— IFN Receptor
Induction of antiviral proteIn synthesis
2'5-Otlgoadenylate synthetase ATP
Ribonuctease R Hydrolyze Viral RNA
Figure 11—3 • lFNs predominantly inhibit protein synthesi5.
Chapter II •
Age,,ts
375
NH2
N
HN N H H
'H
Figure 11—4 • Structure of 2'.5'-oligoadenylate.
induced proteins such as 2',5'-oligoadenylate (2'.5'-OA) syn-
C. chronic hepatitis B. Kaposi's sarcoma in HI V-infected
thetases (Fig. 11-3) and a protein kinase. either of which inhibit viral protein synthesis in the presence of double-
patients, other malignancies, and multiple sclerosis.
aranded RNA. 2',S'-OA activates a cellular endoribo(RNase) (Fig. 11-4) that cleaves both cellular and
nuclease
viral RNA. The protein kinase selectively phosphorylates aal inactivates eukaryotic initiation factor 2 (eIF2). preventing initiation of the mRNA—ribosome complex. IFN also isiuces a specific phosphodiesterase that cleaves a portion c4 tRNA molecules and, thereby, interferes with peptide :Iongation.3° The infection sequence for a given virus may inhibited at one or several steps. The principal inhibitory urrrctdiffers among virus families. Certain viruses can block pmduction or activity of selected IFN-inducible proteins
NUCLEOSIDE ANTIMETABOLITES
Inhibitors of DNA Polymerase Idoxuridine,
USP. Idoxuridinc. 5-iodo-2'-deoxyuridine (Stuxil. Herplex), was introduced in 1963 for the treatment of herpes simplex keratitis.32 The drug is an iodinated analogue of thymidine that inhibits replication of a number of DNA viruses in vitro. The susceptible viruses include ihe herpesviruses and poxviruses (vaccinia).
0
.nd thus counter the IFN effect. IFNs cannot be absorbed orally: to be used therapeutically
must be given intramuscularly or subcutaneously. The effects are quite long, so pharmacokinetic paramare difficult to determine. The antiviral state in periph. .ial blood mononuclear cells typically peaks 24 hours after then decreases to baseline in 6 of IFN-a and Both recombinant and natural INF-a and INF-/'J are .çproved for use in the United States for the treatment of cnndytomu acuminatum (venereal warts), chronic hepatitis
HO'
Idoxuridine
376
of Organic Medicinal and Pharmaceutical Chemistry
Wilson and Gi.c;vld's
The mechanism of action of idoxuridine has not been completely defined, but several steps arc involved in the activation of the drug. Idoxuridine enters the cell and is phosphoryluted
at 0-5 by a viral thymidylate kinase to yield a
monc)phosphate. which undergoes further biotransformation to a triphosph'ate. The triphosphate is believed to be both a substrate and an inhibitor of viral DNA polymerase. causing inhibition of viral DNA synthesis and facilitating the synthe-
sis of DNA that contains the iodinated pyrimidine. The al-
Trifluridine is approved in the United States for the treatment of primary keratoconjunctivitis and recurrent epithelini kcratitis due to HSV types I and 2. Topical trifluridine shows some efficacy in patients with acyclovir-resistant HSV cutaneous infections. Trifluridine solutions are heat sensitive and require refrigeration.
Vidarabine, USP.
tered DNA is more susceptible to strand breakage and leads
to faulty transcription. When the iodinated DNA is transcribed, the results are miscoding errors in RNA and faulty protein synthesis. The ability of idoxuridylic acid to substitute for deoxythymidylic acid in the synthesis of DNA may be due to the similar van der Waals radii of iodine (2.ISA) and the thymidine methyl group (2.OOA). In the United States. idoxuridine is approved only for the topical treatment of herpes simplex virus (HSV) keratitis. although outside the United States a solution of idoxuridine in dimethyl sulfoxide is available for the treatment of herpes labialis. genitalis. and zoster. The use of idoxuridine is lintited because the drug lacks selectivity; low. suhtherapeutic concentrations inhibit the growth of uninfccted host cells. The effective concentration of idoxuridine is at least 10 times greater than that of acyclovir. Idoxuridine occurs as a pale yellow, crystalline solid that is soluble in water and alcohol but poorly soluble in most organic solvents. The compound is a weak acid, with a pKA of 8.25. Aqueous solutions are slightly acidic, yielding a pH
of about 6.0. Idoxuridine is light and heat sensitive, It
Chemically. vidarabine (Vira-A). is The drug is the 2' epimer
of natural adenosine. Introduced in 1960 as a candidate cancer agent. vidarabine was found to have broad-spectrum activity against DNA viruses.34 The drug is active against herpesviruses, poxviruses. rhabdoviruses. hepadnavirus. and some RNA tumor viruses. Vidarahine was marketed in thc
United States in 1977 as an alternative to idoxuridine fur the treatment of HSV kerutitis and HSV encephalitis. Al. though the agent was initially prepared chemically, it is mm obtained by fermentation with strains of St reptoin vet's anti I,jtn'ic'us. NH2
is
supplied as a 0.1% ophthalmic solution and a 0.5% ophthalmic ointment. USP. Trifluridine, 5-trifluoromethyl-2'dcoxyuridine (Viroptic), is a fluorinated pyrintidine nucleoside that demonstrates in vitro inhibitory activity against HSV I and 2. CMV. vaccinia. and some adenoviruses.33 Trifluridine possesses a trifluoromcthyl group instead of an iodine atom at the 5 position of the pyrimidine ring. The van der Waals radius of the trifluoromethyl group is 2.44A. somewhat larger than that of the iodine atom. Like idoxuridine. the antiviral mechanism of trifiurkilne
Trifiuridine,
involves inhibition of viral DNA synthesis. Trifluridine monophosphate is an irreversible inhibitor of thymidylate synthetase. and the biologically generated lriphosphatc com-
petitively inhibits thymidine triphosphate incorporation into
DNA by DNA polymerase. In addition. triflundine in its triphosphate form is incorporated into viral and cellular DNA. creating fragile, poorly functioning DNA.
0 CF3
Vidarabine
The antiviral action of vidarabine is completely confined
to DNA viruses. Vidarabinc inhibits viral DNA synthesis Enzymes within the cell phosphoiylatc vidarabine to the tn phosphate. which competes with deoxyadenosine phate for viral DNA polymerase. Vidarabine triphosphares also incorporated into cellular and viral DNA. where it a chain terminator. The triphosphatc form of vidarabinc also inhibits a set of enzymes that are involved in niethyla. tion of uridine to thymidine: ribonucleoside reductase. RNA polyadenylase. and S-adenosylhomocysteinc hydrolase. At one time in the United States, intravenous vidatabin was approved for use against HSV encephalitis. neonatal herpes, and herpes or varicella zoster in immunocompw mised patients. Acyclovir has supplanted vidarahine as is drug of choice in these cases. In the treatment of viral encephalitis. vidarabine had is be administered by constant flow intravenous infusion Is cause of its poor water solubility and rapid metabolic coma
sion to a hypoxanthine derivative in vivo. These coupled with the availability of less toxic and more agents, have caused intravenous vidarabine to be withdrasn
from the U. S. market.
HO'Thj
Vidarabinc occurs as a white, crystalline monohydrareths is soluble in water to the extent of 0.45 mg/mL at 25rC. drug is still available in the United States as a 3% Trlfluridine
for the treatment of HSV keratitis.
Chapter 1
• AntiriraI Agents
377
Acyclovir. USP. Acyclovir. 9-12-(hydroxyethoxy)methyIl.911-guanine (Zovirax). is the most effective of a series of acyclic nucleosides that possess antiviral activity. In contrast
teric properties (pK. values of 2.27 and 9.25). solubility is
with true nucleosides that have a ribose or a deoxyribose sugar attached to a purine or a pyrimidine base, the group attached to the base in acyclovir is similar to an open chain sugar, albeit Jacking in hydroxyl groups. The clinically use-
equivalent to 50 mg/mL of active acyclovir dissolved in sterile water for injection. Because the solution is strongly alkaII). it must be administered by slow, constant line (pH intravenous infusion to avoid irritation and thrombophlebitir, at the injection site. Adverse reactions are few. Some patients experience occasional gastrointestinal upset, dizziness, headache, lethargy. and joint pain. An ointment composed of 5% acyclovir in a polyethylene glycol base is available for the treatment of initial, mild episodes ol herpes genitalis. The ointment is not an effective preventer of recurrent episodes.
lul antiviral spectrum of acyclovir is limited to herpesviruses. It is most active (in vitro) against HSV type I. about 2 limes less against HSV type 2. and JO times less potent against varicella-zoster virus. An advantage is that uninfected human cells are unaffected by the drug.
0
increased by both strong acids and bases. The injectable form is the sodium salt, which is supplied usa lyophilized powder.
Valacyclovir Hydrochloride.
Valacyclovir (Valtrex) is the hydrochloride salt of the t.-valyl ester of acyclovir. The compound is a water-soluble crystalline solid, and it is a prodrug intended to increase the hioavailability of acyclovir by increasing lipophilicity. Valacyclovir is hydrolyzed rapidly and almost completely to acyclovir following oral ad-
OH N
0 Acyclovir
ministration. Enzymatic hydrolysis of the prodrug is believed to occur during enterohepatic cycling. The oral
bioavailability of valacyclovir is 3 to 5 times that of The ultimate effect of acyclovir is the inhibition of vir.rl
acyclovir. or about
DNA synthesis. Transport into the cell and monophosphory-
Valacyclovir ha.s been approved for the treatment of
lation are accomplished by a thymidinc kinase that is en-
herpes zoster (shingles) in immnunocompromised patients. The side effect profile observed with valacyclovir is comparable in bioequivalent doses of acyclovir. Less than 1% of
coded by the virus
The affinity of acyclovir for the
siral thyrnidine kinase is about 200 times that of the corresponding mammalian enzyme. Hence, some selectivity is attained. Enzymes in the infected cell catalyze the conversion of the monophosphate to acyclovir triphosphate. which
an administered dose of valacyclovir is recovered in the urine. Most of the dose is eliminated as acyclovir.
is present in 40 to 1(X) Limes greater concentrations in HSVinfected than uninfected cells. Acyclovir triphosphate competes for endogenous deoxyguanosine triphosphate (dGTP);
hence, acyclovir triphosphate competitively inhibits viral DNA polymerases. The triphosphorylated drug is also incor-
into viral DNA. where it acts as a chain terminator. Because it has no 3'- hydroxyl group. no 3'.5'-phosphodies. cr can fonts. This mechanism is essentially a suicide inhibition because the terminated DNA template containing
syclovir as a ligand binds to. and irreversibly inactivates, DNA polymeruse. Resistance to acyclovir can occur, most often by deficient thymidine kinase activity in HSV isolates. Acyckwir resistance in vesicular stomatitis virus (VSV) isoLas is caused by mutations in VSV thymidine kina.se or.
often. by mutations in viral DNA polymerase. Two dosage forms of acyclovir arc available for systemic ise: oral and parenteral. Oral acyclovir is used in the initial seatment of genital herpes and to control mild recurrent episodes. It has been approved fur short-term treatment of
Vatacyclovir
Ganciclovir. Ganciclovir. 9-1(1 .3-dihydroxy-2-propoxy)methyljguanine) or DHPG (Cytovene). is an analogue of acyclovir. with an additional hydroxymethyl group on the acyclic side chain.
and chickenpox caused by varicclla-zostcr virus VZV(. Intravenous administration is indicated for initial iM recunsent infections in immunocompromised patients for the prevention and treatment of severe episodes. The drug is absorbed slowly and incompletely from the gastroin-
tract, and its oral bioavailability is only 15 to 30%. Ncoertheless. acyclovir is distributed to virtually all body Less than 30% is bound to protein. Most of he drug is excreted unchanged in the urine, about 10% cxas the .carboxy metabolite. occurs as a chemically stable, white, crystalline that is slightly soluble in water. Because of its ampho-
Ganctctovir
378
Wilson and Gisvolds Texthook of Organic Medicinal
This structural modification, while maintaining the activity against HSV and VSV possessed by acyclovir. greatly enhances the activity against CMV infection. After administration, like acyclovir. ganciclovir is phosphorylaled inside the cell by a virally encoded protein kinose to the monophosphate,'7 Host ccli enzymes catalyze the formation of the triphosphate. which reaches more than I 0-fold
higher concentrations in infected cells than in uninfected cells. This selectivity is due to the entry and monophosphorylation step. Further phosphorylation with cellular enzymes occurs, and the triphosphate that is formed selectively inhib-
Pharn,aceuiical Chemisirs,
VS V-infected cells. penciclovir is first phosphorylated by viral thymidine kinasc4' and then further elaborated to the triphosphate by host cell kinases. Penciclovir triphosphate is a competitive inhibitor of viral DNA polymerase. The pharmacokinetic parameters of penciclovir are quite different from those of acyclovir. Although penciclovir triphos. phate is about 100-fold less potent in inhibiting viral DNA polymerase than acyclovir triphosphate. ii is present in the
its viral DNA polymerase. Ganciclovir triphosphate is also incorporated into viral DNA causing strand breakage and
tissues for longer periods and in much higher concentrations than acyclovir. Because it is possible to rotate the side chain of penciclovir into a pseudo-pentose. the metabolite possesses a 3'-hydroxyl group. This relationship is shown below with guanosine. Pcnciclovir is not an obli.
cessation of elongation.*u
gate chain terminator.4' hut it does co,npetitively inhibil
The clinical usefulness of ganciclovir is limited by the toxicity of the drug. Ganciclovir causes myelosupprcssion. producing neutropenia, thrombocytopenia. and anemia. These effects are probably associated with inhibition of host cell DNA Potential central nervous system
DNA elongation. Penciclovir is excreted mostly unchanged in the urine. 0
side effects include headaches, behavioral changes. and convulsions. Ganciclovir is mutagenic. carcinogenic, and teratogenie in animals.
Toxicity limits its therapeutic usefulness to the treatment and suppression of sight-threatening CMV retinitis in immunocomproinised patients and to the prevention of life-threatening CMV infections in at-risk transplant patients.2' Oral and ptuenteral dosage forms of ganciclovir are available, hut oral bioavailahility is poor. Only 5 to of an oral dose is absorbed. Intravenous administration is preferable. More than 90% of the unchanged drug is excreted in the urine. Ganciclovir for injection is available as a lyophilized sodium salt for reconstitution in normal saline. 5% dextrose in water, or lactated Ringer's solution. These solutions are
strongly alkaline (pH — II) and must be administered by slow, constant. intravenous infusion to avoid thrombophie-
H2N
N
HO
Guanosine
/
POnCICIOVIr
Penciclovir (Denvir) has been approved for the treatment of recurrent herpes labialis (cold sores) in adults It is effective against HSV-l and HSV-2.42 It is a cream containing 10% penciclovir.
bitis.
Famclclovir and Penciclovir.
Famciclovir is a diacetyl prodrug of pcnciclovir.4° As a prodrug. it lacks antiviral activity. Penciclovir. 9-14-hydroxy-3-hydroxymethylhut- 1-yll guanine. is an acyclic guanine nucleoside analogue. The structure is similar to that of acyclovir. except in penciclovir a side chain oxygen has been replaced by a carbon atom and an extra hydroxymethyl group is present. Inhibitory concen-
trations for HSV and VSV are typically within twice that of acyclovir. Penciclovir also inhibits the growth of hepatitis B virus.
Penciclovir inhibits viral DNA synthesis. In HSV- or
Cidofovir. Cidofovir. (S)-3-hydroxy-2-phosphononieui oxypropyl cylosine (HPMPC, Vistide). is an acyclonucles. tide analogue that possesses broad-spectrum activity agains several DNA viruses. Unlike other nucleotide analogues ilul are activated to nucleoside phosphates, Cidofovir is a plios phonic acid derivative. The phosphonic acid is not hydro lyzed by phosphatases in vivo but is phosphorylated by cells. lar kinases to yield a diphosphate. The diphosphate acts s. an antimctabolite to deoxycytosine triphosphate (dCTP), 0.
dotbvir diphosphate is a competitive inhibitor of viril DNA43 polymerase and can be incorporated into the growing
viral DNA strand, causing DNA chain termination.
0
Penctclovir
Chapter II • Anlñi rat Cidofovir posses.ses a high therapeutic index against CMV
379
palierns. Cidofovir is administered by slow, constant intrave-
bolic abnormalities including increases or decreases in blood Ca2 + levels. Ncphrotoxicity is common, and this side effect precludes the use of Fosearnet in other infections caused by
nous infusion in a dose of 5 tag/kg over a I-hour period once a week for 2 weeks. This treatment is followed by a maintenance dose every 2 weeks. AbOUt of a dose of
herpesvirus or us single-agent therapy HIV infection. Foscarnet is an excellent ligand for metal ion binding, which undoubtedly contributes to the electrolyte imbalances ob-
Cidofovir is excreted unchanged in the urine, with a of 2 to 3 hours. The diphosphate antiinetabolite. in contrast. has an extremely long half-life (17 to 30 hours).
The main dose-limiting toxicity of cidofovir involves
served with the use of the drug.TM' Hypocalcemia. hypomagneseinia. hypokalemia. and hypophosphatemia and hyperphusphatemia are observed in patients treated with foscarnet. Side effects such as paresthesias. tetani. seizures, and cardiac
renal impairment. Renal function must be monitored closely. with prohenecid and prehydration with intravenous normal saline can he used to reduce the nephrotoxieity of he drug. Patients must be advised that cidofovir is not a cure for CMV retinitis. The disease may progress during or
arrhythmias may result. Since foscarnet is nephrotoxic. it may augment the toxic effects of other nephrotoxic drugs. such as ainphotericin B and pentamidine. which are frequently used to control opportunistic infections in patients with AIDS.
been approved for treating CMV retinitis in AIDS
Foscamet sodium is available usa sterile solution intended
(stInts ing treatment.
for slow intravenous infusion. The solution is compatible with normal saline and
NH2
dextrose in water but is incompat-
ible with calcium-containing buffers such as lactated Ringer's solution and total parenterril ntltrition (TPN) preparatiruls. Foscarnet reacts chemically with acid salts such as tnidazolam. vancomycin. and pentamidine. Over 80% of an injected dose of fuscamet is excreted unchanged in the urine. ° The long elimination of foscarnet is thought to result from its reversible sequestration into
Reverse
OH
Cidofovir
Inhibitors
An early event in the replication of HIV-l is reverse transcription. whereby genomic RNA from the virus is converted
into a cDNA—RNA complex. then into double-stranded Trisodium phosphonofommtc is an inorganic pymphosphate analogue that inhibits replication in herpeoviruses (CMV. HSV. and VSV) and retroviruses IIIV).4* Foscarnet (Foscavir) is taken up slowly by the cells ad does not undergo significant intracellular metabolism. is a reversible. nonconipetitive inhibitor at the pyrophosphate-binding site of the viral l)NA polynterase and transcriptase. The ultimate is inhibition of the of pyrophusphate front deoxynucleotide triphosa cessation of the incorporation of nucleoside nphosphates into DNA (with the concomitant release of Since the inhibition is noncompetitive soh respect to nucleosidc triphosphate binding. foscarnet cnacl synergistically with nucleoside triphosphate arnime(e.g., zidovudine and didanosine triphosphates) in inhibition of viral DNA synthesis. Foscarnet does not Foscarnet Sodium.
rquirchioactivation by viral or cellular enzymes and. hetice. he effective against resistant viral strains that are deli— 'cii in virally encoded nucleuside kinases.°
DNA ready for integration into the host chromosome. The enzyme that catalyzes this set of reactions is reverse trimNeriptase. Reverse transcriptase actually operates twice prior to the integration step. Its first function is the creation of the cDNA—RNA complex; reverse transcriptase acts alone in this step. In the second step, the RNA chain is digested away by RNase H while reverse transcriptase creates the doublestranded unintcgratcd DNA. All of the classical antiretroviral agents are 2'.3'-dideoxynucleoside analogues. These compounds share a common
mechanism of action in inhibiting the reverse transcriptase of WV. Because reverse transcriptase acts early in the viral inlèction sequence, inhibitors of the enzyme block acute in-
fcction of cells httt are only weakly active in chronically inlècted ones. Even though the reverse transcriptase inhibitors share a common mechanism of action, their phamiaco-
logical atid toxicological profiles differ dramatically.
Zidovudine, USP. Zidovudine. 3'-azido-3'-deoxyihymidine or AZT. is an analogue of thymidine that possesses
antiviral activity against HIV-l. HIV-2. HTLV-l. and a
Na1!,
0 Na
Trisodlum Phosphonotormate
is a second-line drug for the treatment of retini— aimed by CMV in AIDS patients. The drug causes meta-
number of other retroviruses. This nucleoside was synthesized in 1978 by Lin and Prusofl°7 u.s an intermediate in the preparation of amino acid analogues of thymidine. A screening program directed toward the identification of agents potentially effective for the treatment of AIDS patients led to the discovery of its unique antiviral properties 7 years later.4M The next year. the clittical effectiveness of
AZ1' in patients with AIDS and AIDS-related complex (ARC) was demonstrated.49 AZT is active against retrovi-
380
Wll.an and Gisyold.r Textbook of Organic Medicinal and Pharmaceutical Chemi.cuy
ruses, a group of RNA viruses responsible for AIDS and some kinds of leukemia. Retroviruses possess a reverse Iran-
larly, where it inhibits reverse Iranscriptase and is incorpo-
scriptase or a RNA-directed DNA polymerase that directs the synthesis of a DNA copy (proviral DNA) of the viral RNA genome that is duplicated, circularized, and incorporated into the DNA of an infected cell. The drug enters the host cells by diffusion and is phosphorylated by cellular thymidine kinase. Thymidylate kinase then converts the monophosphate into diphosphates and triphosphates. The rate-determining step is conversion to the diphosphate, so high levels of monophosphorylated AZ'!' accumulate in the cell. Low levels of diphosphate and triphosphate axe present. Zidovudine triphosphate competitively inhibits reverse Iranscripiase with respect to thymidine triphosphate. The 3'azido group prevents formation of a 5',3'-phosphodiester bond, so AZT causes DNA chain termination, yielding an incomplete proviral DNA.5° Zidovudine monophosphate also competitively inhibits cellular thymidylate kinase. thus decreasing intracellular levels of thymidine triphosphate. The antiviral selectivity of AZ'!' is due to its greater (bOX)5' affinity for HIV reverse traxiscriptase than for human DNA polymerases. The human y-DNA polymerase of mitochondria is more sensitive to zidovudine; this may contribute to the toxicity associated with the drug's use. Resistance is common and is due to point mutations at multiple sites in reverse transcriptase, leading to a lower affinity for
infected cells. The potency of didanosine is 10- to 100-fold less than that of AZ'!' with respect to antiviral activity and cyrotoxicity. but the drug causes less myclosuppression than AZT causes.54 Didanosine is recommended for the treatment of patienis with advanced HIV infection who have received prolonged treatment with AZT but have become intolerant to. or experienced immunosuppression from, the drug. AZT and ddl act synergistically to inhibit H1V replication in vitro, and ddl effective against some AZT-resistant strains of HI V.53 Painful peripheral neuropathy (tingling, numbness, and pain in the hands and feet) and pancreatitis (nausea, abdominal pain. elevated amylase) are the major dose-limiting toxicities a) didanosine. Didanosine is given orally in the form ol buff-
rated into viral DNA to cause chain termination in HIV.
ered chewable tablets or as a solution prepared from the powder. Both oral dosage forms are buffered to present acidic decomposition of ddl to hypoxanthine in the stomach. Despite the buffering of the dosage forms, oral bioavailuhility is quite low and highly variable. Less than 20% of a dcc is excreted in the urine, which suggests extensive mc;aholism?6 Food interferes with absorption. so the oral drug niusi be given at least I hour before or 2 hours after meals. Highdose therapy can cause hyperuricensia in some patients
cause of the increased purine load.
the drug.52
Zidovudine is recommended for the management of adult patients with symptomatic HIV infection (AIDS or ARC) who have a history of confirmed Pneumocystis carinhl pneumonia or an absolute CD4 + (T4 or TH cell) lymphocyte count below 200/mm3 before therapy. The hematological toxicity of the drug precludes its use in asymptomatic patients. Anemia and granulocytopenia are the most common toxic effects associated with AZT. For oral administration, AZT is supplied as 100-mg capsules and as a syrup containing 10 mg AZ!' per mL. The injectable form of AZ!' contains 10 mg/mL and is injected intravenously. AZ'!' is absorbed rapidly from the gastrointestinal tract and distributes well into body compartments, including the cerebrospinal fluid (CSF). It is metabolized rapidly to an inactive glucuronide in the liver. Only about 15% is excreted unchanged. Because AZT is an aliphatic azide, it is heat and light sensitive. It should be protected from light and stored at 15 to 25°C.
0
Dldanostne
Zalcitabine.
Zalcitabine. 2'.3'-didcoxycytidine e USP. ddCyd. is an analogue of cylosine that demonstrates against HIV- I and HIV-2. including strains resistant to All The potency (in peripheral blood mononuclear cells) is s,mi tar to that of AZ'!'. but the drug is more active in of monocytes and macrophages as well as in resting cells Zalcitabine enters human cells by carrier-facilitated diffs sion and undergoes initial phosphorylation by deoxycytidinc kinase. The monophosphorylated compound is further nc tabolized to the active metabolite. dideoxycytidinc.5'-ti• phosphate (ddCTP). by cellular kinases57 ddCFP reverse transcriptase by competitive inhibition with dCTF.
Most likely, ddCTP causes termination of the viral DNA chain.
HoLd
Zalcitabine inhibits host mitochondrial DNA synthesis a low concentrations. This effect may contribute to its clinical toxicity.58
The oral bioavailability of zalcitabine is over Zidovudlne
Didanosine.
Didanosine (Videx, ddl) is 2',3'-dideoxyinosine (ddl), a synthetic purine nucleoside analogue that is bioactivated to 2',3'-dideoxy-ATP (ddATP) by host cellular enzymes.53 The melabolite, ddATP, accumulates iniracellu-
it
adults and less in children.5° The major dose-limiting Jilt effect is peripheral neuropathy. characterized by pain, pars thesias, and hypesthesia. beginning in the distal lower ci tremities. These side effects are typically evident alter wi eral months of therapy with zalcitabine. A potentially lati pancreatitis is another toxic effect of treatnnenl with ddf The drug has been approved for the treatment of HIV inlet
Chapter II U Anhiviral Ageiih.s
tarn in adults with advanced disease who arc intolerant to AZT or who have disease progression while receiving AZT. ddC is combined with AZT for the treatment of advanced HIV infection. NH2
381
It is interesting that the unnatural sterenisonler (—)-(S)ddC exhibits greater antiviral activity against HIV than the
natural enantiomer ( + )-(S)-ddC.65 Both enantiomers arc bioactivated by cellular kinases to the corresponding IriphosBoth SddCTP isomers inhibit HIV reverse transcriptase and are incorporated into viral DNA to cause chain termination. (+ )-S-ddCTP inhibits cellular DNA polymeruses much more strongly than (—)-SddCTP. explaining the greater toxicity associated with (+ )-(S)-ddC. Initial metabolic comparison of SddCTP isomers has failed to explain the greater potency of the (—I-isomer against HIV. Therefore, although the intracellular accumulation of ( — }-S-ddCTP
Zalcitabine
was twice that of (+ )-S-ddCTP. the latter was I / times more potent as an inhibitor of HIV reverse transcriptase. and the two isomers were incorporated into viral DNA at comparable rates. The puzzle was solved with the discovery
Stavudine.
Stavudine. 2'3'-didehydro-2'-deoxythymi-
dine (D4T, Zerit). is an unsaturated pyrimidine nucleoside that is related to thymidinc. The drug inhibits the replication o(HIV by a mechanism similar to that of its close congener. Stavudine is bioactivaced by cellular enzymes to a tiphosphate. The triphosphate competitively inhibits the incorporation of thymidine trtphosphate (TTP) into retroviral DNA by reverse transcriptase.°1 Stavudine also causes termi-
nation of viral DNA elongation through its incorporation no DNA.
0
of a cellular 3',S'-exonuclease. which was found to cleave terminal ( + )-S-ddCMP incorporated into viral DNA 6 times faster than (—)-S-ddCMP from the viral DNA terminus. Resistance to lamivudine develops rapidly as a result of a mutation in codon 184 of the gene that encodes Ff1 V-RT when the drug is used as monotherapy for I-f IV When combined with AZT. however. lamivudinc caused substantial increases in CD4' counts. The elevated counts were sustained over the course of therapy.67 The codon mutation that causes resistance to lamivudine suppresses AZT
resistance.67 thus increasing the susceptibility of the virus to the drug combination. NH2
Stavudine
Savudine is available as capsules for oral administration. The drug is acid stable and well absorbed (about 90%) following oral administration. Stavudine has a short Ito 2 hours) in plasma and is excreted largely unchanged '15 to 90%) in the urine.62 As with ddC. the primary doseeffect is peripheral neuropathy. At the recomdosages. approximately 15 to 20% of patients expesymptoms of peripheral neuropathy. Stavudine is recamended for the treatment of adults with advanced H1V who are intolerant of other approved therapies or havc experienced clinical or immunological deteriorawhile receiving these therapies. Lantlvudine.
OH
Lamivudine
Miscellaneous Nucleoside Antimetaboiftes Ribavirin, USP.
Ribavirin is l-$.o-ribofuranosyl-l,2.4-
thiazole-3-carboxamide. The compound is a purine nucleoside analogue with a modified base and a o-nbose sugar moiety. The structure of ribavirin is shown below.
Lamivudine is (—)-2'.3'-dideoxy-3'-thia-
(2R,5S)- 1.3-oxathiolanylcytosine. 3TC. or -i(.S)-ddC. Lamivudine is a synthetic nucleoside analogue differs from 2'.3'-dideoxycytidine (ddC) by the substitusn of a sulfur atom in place of a methylene group at the 3' of the ribose ring. In early clinical trials. lamivudine 4lidine.
highly promising antiretroviral activity against
PreimiIV and low toxicity in the dosages pharmacokinetic studies indicated that it exhibited good (F = —80%) and a plasma half-life of b 4 hours.°3
0H
Ribassrin
382
Wii.cg,n
and Gi.n'old'.c Textbook of Organic Medicinal and Phannaceujica! Chemistry
Ribavirin inhibits the replication of a very wide variety of RNA and DNA including orthomyxoviruses, paramyxoviruses,
been complicated by the fact that the vaccine apparently can
modulate its antigenic structures in its chronic infectious
arenaviruses, bunyaviruscs. herpesvi-
ruses. adenoviruses. poxvirus, vaccinia. influenza virus. parainfluenza virus, and rhinovirus. In spite of the broad spectrum of activity of ribavirin. the drug has been approved for only one therapeutic indication—the treatment of severe lower respiratory infections caused by RSV in carefully selected hospitalized infants and young children.
The mechanism of action of ribavirin is not known. The broad antiviral spectrum of ribavirin. however, suggests The nucleoside is bioactivated multiple modes of by viral and host cellular kinases to give the monophosphate
(RMP) and the triphosphate (RTP). RMP inhibits inosine monophosphate (IMP) dehydrogenase. thereby preventing the conversion of IMP to santhine monophosphate (XMP). XMP is required for guanosine triphosphate (GTP) synthesis. RTP inhibits viral RNA polymerascs. It also prevents the end capping of viral mRNA by inhibiting guanyl-N'methyltransferase. Emergence of viral resistance to ribavirin has not been documented. Ribavirin occurs as a white, crystalline, polymorphic solid that is soluble in water and chemically stable. It is supplied as a powder to be reconstituted in an aqueous aerosol con-
taining 20 mg/mL of sterile water. The aerosol is administered with a small-particle aerosol generator (SPAG). Deterioration in respiratory function, bacterial pneumonia, pncumothorax. and apnca have been reported in severely ill infants and children with RSV infection. The role of ribavirin in the-se events has not been determined. Anemia, headache. abdominal pain, and lethargy have been reported in patients
receiving oral ribavirin. Unlabeled uses of ribavirin include aerosol treatment of influenza types A and B and oral treatment of hepatitis, genital herpes, and La.ssa fever. Ribavirin does not protect cells
against the cytotoxic effects of the AIDS virus.
NEWER AGENTS FOR THE TREATMENT OF HIV INFECI ION When HIV- I was characterized and identified as the causa71 scientists from all over the tive agent of AIDS in world joined in the search for a prevention or cure for the
disease. Mapping the HIV-l genome and elucidating the replication cycle of the virus have supplied key information.72 Biochemical targets, many of which arc proteins involved in the replication cycle of the virus, have been cloned and sequenced. These have been used to develop rapid, mechanism-based assays\for the virus to complement tissue culture screens for whole-virus. Several of the biochemical steps that have been characterized have served as targets for clinical candidates as well as for successfully licensed
Vacdnes.
The chronology of vaccine development and use in the 20th century is nothing short of a medical miracle. Diseases such as smallpox and polio, which once ravaged large populations, have become distant memories. The tech. nique of sensitizing a human immune system by exposure to an antigen so that an anamnestic response is generated on subsequent exposure seems quite simple on the surface. Hence, it is natural that a vaccine approach to preventing AIDS be tried. The successes achieved so far have involved
live/attenuated or killed whole-cell vaccines and, in more recent times, recombinant coat proteins. Successes with vaccines of the live/attenuated (low-viw. lence), killed whole virus or the recombinant coal protein types have primarily involved acute viral diseases in which a natural infection and recovery lead to long-term immunity.
This type of immunity is of the humoral. or antibody. mediated, type, and it is the basis for successes in
ing the human population, Causative organisms of infections do not respond to vaccines. The AiDS virus causes a chronic disease in which infection persists despite a strong
antibody response to the virus (at least initially, HIV can circumvent the humoral response to infection by attacking and killing CD4 T cells). These T cells, also known as helper cells. upregulate the immune response. By eradicating
the CD4t cells, the HIV virus effectively destroys the irn mune system. Cell-mediated immune responses are critical
to the prevention and treatment of HIV infection. To be effective, a vaccine against HIV must elicit an appropriate cellular immune response in addition to a humoral response.
In other words, the vaccine must have the potential to xt on both branches of the immune system. The initial work on vaccine development focused on typic variants of the HIV envelope glycoprotein gpI2O cditamed by recombinant DNA techniques. This target was cbs. sen because of concerns about the safety of live/attenuated vaccines. The gpl 20 glycoprotemn is a coat protein, and if great care is taken, a virus-free vaccine is obtainable. Moreover. glycoprotein gp 120 is the primary target for neumihsing antibodies associated with the first (attachment) step rn HIV infection.77 Early vaccines were so ineffective that the
National Institutes of Health suspended plans for nlascivr clinical trials in high-risk individuals.75 There arc a numbs of reasons why the vaccine failed.7" There arc multiple subtypes of the virus throughout the the virus can by means of both cell-free and cell-associated forms; dir virus has demonstrated its own immunosuppressive. immunopathologicul, and infection-enhancing properties of parof the envelope glycoprotein; and vaccines have nor beer able to stimulate and maintain high enough levels of
drugs.73'
nity to be effective. The failure of the first generation of AIDS vaccines
Despite the many advances in the understanding of the HIV virus and its treatment, there is not yet a cure for the infection. Emergent resistance75 to clinically proven drugs
to a reexamination of the whole AIDS vaccine effort.75 As. guide for research efforts, a number of criteria for an "ide-il AIDS vaccine have been developed. The "ideal" AIDSue-
such as the reverse transcripta.se inhibitors and the protease
inhibitors has complicated the picture of good therapeutic
cine should (a) be safe, (Li) elicit a protective immure a sponse in a high proportion of vaccinated individuals, fri
targets. The idea of using a vaccine as a therapeutic tool has
stimulate both cellular and humoral branches of the immire
Chapter II
U Anti viral Agents
383
system, (d) protect components against all major HIV subtypes. (e) induce long-lasting protection. (I) induce local immunity in both genital and rectal mucosa, and (g) be practical tbr worldwide delivery and administration, It is not yet known how well the second-generation AIDS vaccines will satisfy the above criteria or when one might receive approval fur widespread use in humans. A new era in the treatment of AIDS and ARC was ushered in with the advent of some clinically useful, potent inhibitors of HIV. For the first time in the history of AIDS the death nile reversed itself. There arc several different classes of drugs that can be used to treat HIV infection. These are the nucleoside reverse transcriplase inhibitors (NRTIs). the nimnucleoside reverse transcriptase inhibitors (NNRTIs), the HIV protca.se inhibitors (Pis). the HIV entry inhibitors. and the I-IIV inlegrase inhibitors (IN). Presently. at least 14
coding for the enzyme. Cross-resistance between structurally
aniretruviral agenis belonging to three distinct classes NRTh. NNRTIs. PIs) have been licensed for use in patients
Nevirapine.
in the United States. All of these agents are limited by rapid
of resistance and cross-resistance, so commonly three drugs are used at the same time, each acting at a different point in HIV replication. These drugs can effect
dmniatic reductions in viral load, but eventually, as resisunce develops, the virus reasserts itself.
different NNRTIs is more common than between NNRTIs
and NRTIs. In the future, clinical use of the NNRTIs is expected to use combinations with the nucleosides to reduce toxicity to the latter. to take advantage of additive or syner-
gistic effects, and to reduce the emergence of viral resistance."'°° The tricyclic compound ncvirapine (Viramune).52 the bis(heteroacyl)piperazine (BHAP) derivative delaviradine (Rescriptor),83 and
have been approved
for use in combination with NRTIs such as AZF for the treatment of HIV infection. Numerous others, including the quinoxaline derivative the tetrahydroimidazobenzodiazpinone (TIBO) analogue R-829 and Calanolide-A'" are in clinical trials. Nevirapine (Viramunc)°2 is more than 90% absorbed by the oral route and is widely distributed throughout the body. It distributes well into breast milk and crosses the placenta. Transplacental concentrations are about 50% those of serum. The drug is extensively transformed by cyto-
chrome P450 to inactive hydroxylated metabolites; it may undergo enteruhepatic recycling. The half-life decreases from 45 to 23 hours over a 2- to 4-week period because of autoinduction. Elimination occurs through the kidney, with less than 3% of the parent compound excreted in the urine.82 Dosage forms are supplied as a 50 mg/S mL oral suspension and a 200-mg tablet.
Nonnucleoside Reverse Transcriptase InhIbitors (NIIIRTh) Cloned HIV- I reverse transcriptase facilitates the study of he effects of a novel compound on the kinetics of the en-
Random screening of chemical inventories by the p!unnaceutical industry has led to the discovery of several NNRTIs of the enzyme. These inhibitors represent several cauclurally distinct classes. The NNRTIs share a number of rominun biochemical and pharmacological properties.7't'50'8' Unlike the nucleoside antimelabolites. the NNRTIs do not rrqtire bioactivation by kinases to yield phosphate esters. The3' are not incorporated into the growing DNA chain. Inscal. they bind to an allosteric site that is distinct from the
(nucleoside triphosphate)-binding site of reverse tauscriptase. The inhibitor can combine with either free is substrate-bound enzyme. interfering with the action of both. Such binding distorts the enzyme so that it cannot ison the enzyme—substrate complex at its normal rate. :dl once formed, the complex does not decompose at the ssmal rate to yield products. Increasing the substrate ancentration
does
not reverse
these
effects.
Hence.
exhibit a classical noncompetitive inhibition pat:m with the enzyme.
The NNRTIs are extremely potent in in vitro cell culture
and inhibit HIV4 at nanomolar concentrations. I
inhibit reverse transcriptase selectively; they do not ihbit the reverse transcriptases of other retroviruses, in-
HIV-2 and simian immunodeficiency virus (Sly). NNRTIs have high therapeutic indices (in contrast to and do not inhibit mammalian DNA polyraiscs. The NRTIs and NNRTIs are expected to exhibit a srergisdc effect on HIV. since they interact with different
on the enzyme. The chief problem with the is the rapid emergence of resistance among HIV Resistance is due to point mutations in the gene
Newraplne
Delavirdine.
Delavirdine (Rescriptor)53 must be used with at least two additional antiretroviral agents to treat HIVI infections. The oral absorption of delavirdine is rapid, and peak plasma concentrations develop in I hour. Extensive metabolism occurs in the liver by cytochrome P-45() (CYP) isozyme 3A (CYP 3A) or possibly CYP 21)6. Bioavailability
is 85%. Unlike nevirapine. which is 48% protein bound, delavirdine is more than 98% protein bound. The half-life is 2 to II hours, and elimination is 44% in feces. 51% in urine, and less than 5% unchanged in urine. Delavirdine induces its own metabolism.81 Oral dosage forms are supplied as a 200-mg capsule and a 100-mg tablet.
Efavirenz. Efavirenz (Sustiva)84 is also mandated for use with at least two other antiretmviral agents. The compound is more than 99% protein bound, and CSF concentrations exceed the free fraction in the serum. Metabolism oc-
curs in the liver. The half-life of a single dose of cfavirenz is 52 to 76 hours. and 40 to 55 after multiple doses (the drug induces its own metabolism). Peak concentration is achieved
384
Wilwn, and Gisi'old's
of Organic Medicinal and Phannacewiral
Delavirdlne
in 3 to 8 hours. Elimination is 14 to 34% in urinc (as metabofiles) and 16 to 41% in feces (primarily as efavircnz).M The oral dosage form is supplied as a capsule. H
spread of cellular infection, they should possess good oral
bioavailability and a relatively long duration of action. A long half-life is also desirable because of the known develop.
rnent of resistance by HIV under selective antiviral Resistance develops by point mutations. Most of the early protease inhibitors are high-molecular.
sure.74
weight, dipeptide. or tripeptide-like structures. generall) with low water solubility. The bioavailability of these cons. pounds is low, and the half-life of elimination is very shon because of hydrolysis or hepatic metabolism.85 Strategic' aimed at increasing water solubility and metabolic
have led to the development of several highly promisini clinical candidates. Saquinavir indinavir(Crixi van).89 ritonavir (Norvir)."° nelfinavir (Viracept).9' and anprenavir (Agenerase)92 have been approved for the treatmcnl
HIV Protease Inhibitors
of HIV-infected patients. A number of others are in clinical trials.
A unique biochemical target in the HIV- I replication cycle
itors. As a class, they cause dyslipidernia, which includo
was revealed when HIV protease was cloned and cxin Escherichia coil. HIV protca.sc is an enzyme that cleaves gag-pro propeptides to yield active enzymes that
function in the maturation and propagation of new virus. The catalytically active protcase is a symmetric dimer of two identical 99 amino acid subunits, each contributing the triad Asp-Thr-Gly to the active The homodimer is unlike monomeric aspartyl prolea.ses (renin. pepsin. cathepsin D). which also have different substrate specilicities. The designs of sonic inhibitorsus '° for HIV- I protease exploit the C2 symmetry of the enzyme. HIV- I protease has active
site specificity for the triad Tyr-Phe-Pro in the unit Ser(Thr)-Xaa-Xaa-Tyr-Phe-Pro, where Xaa is an arbitrary amino acid. HIV prolease inhibitors arc designed to mimic the transition state of hydrolysis at the active site; these compounds are called analogue inhibitors. Hydrolysis of a pcptide bond
There is an important caution for the use of prolease inhib-
elevated cholesterol and triglycerides and a redistribution of body fat centrally to cause the . 'prolease paunch." buffalo hump, facial atrophy, and breast enlargement. These also cause hyperglycemia.
Saquinavir.
Saquinavir (!nvirase)'°' is wcll following oral administration. Absorption of saquinavis poor but is increased with a fatty meal. The drug does as distribute into the CSF. and it is approximately 98% bourd to plasma proteins. Saquinavir is extensively metabolizcd by the first-pass effect. Bioavailability is 4% from a hanl capsule and 12 to 15% from a soft capsule. Saquinavir p24 antigen levels in HIV-infected palienLs. elevates CD4 counts, and exerts a synergistic antiviral effect when corn bined with reverse transcriptase inhibitors such as AZI'anJ ddC.93-95 Although H!V- I resistance to saquinavir and
proceeds through a transition state that is sp3 hybridized and, hence, tetrahedral. The analogue inhibitors possess a preexisting sp7 hybridized center that will be drawn into the
HIV protease inhibitors occurs in vivo. it is believed to lv
active site (one hopes with high affinity) but will not be
tween different HIV protease inhibitors appears to be cono mon and additive.97 suggesting that using combinations sl inhibitors from this class would not constitute rational scribing. The drug should be used in combination with least two other antiretroviral drugs to minimize resistana Dosage forms arc Invirase (hard capsule) arid capsule).
cleavable by the enzyme. This principle has been used to prepare hundreds of potentially useful transition state inhibi-
Unfortunately, very few of these are likely to be clinically successful candidates for the treatment of HIV infection. Since HIV protease inhibitors arc aimed at arresting replication of the virus at the maturation step to prevent the
less stringent and less frequent than resistance to the reversc transcriptase inhibitorsY° Nevertheless, cross-resistance lv-
Chapter 11 • Anrivira! Age,,ts
385
H/
CH3
H3C
NH2
Saqulnavir
Sndinavir.
When administered with a high fat diet, mdi-
navir (Crixivan )50 achieves a maximum serum concentration A 77sf
ol the administered dose. The drug is 60% bound in the plasma. It is extensively metabolized by CYP 3A4, and seven nietabolites have been identified. Oral bioavailability is good. with a t,,, of 0.8 ± 0.3 hour. The half-life of elimination is 1.8 hour, and the elimination products are detectable in feces and urine. Indinavir also causes dyslipide-
ruts. The available dosage forms are capsules of 200, 333, and 4(X) mg.
high fraction of hepatic metabolism. Subsequent synthesis of
nonsXmmetric derivatives DMP-850'°' (below) and DMP-
851'' yielded in vitro antiviral potency comparable with that of the already-approved Pis. These were selected as clinical candidates on the basis of their favorable pharmacokinetics in dogs. In a second approach. random screening of chemical inventories yielded the 5,6-dehydropyran-2-one—
based inhibitor'02 PD-l78390 (below). This compound, in addition to having good potency against I-IIV protease and good anti-MW activity in cell culture, exhibits high bioavailability in experimental animals. PD- 178390 appears not to share the resistance profile of the other Pis, and no virus resistant to the compound emerged, even during the prolonged in vitro selection.
H
Indinavir
Ritonavir, Amprenavir, and Nelflnavlr.
Ritonavir
Nvrsir),°° ansprenavir IAgenerase),'tm and nelfinavir (Vira(see structures on page 386) have similar properties inJ cautionary statements. All cause dyslipidemia. and they use a host of drug interactions, mainly because they inhibit ('YP 3A4. These agents must always be used with at least soother antiretroviral agents. Used properly, the protease irhibitors are an important part of H!V therapy. A number ol nonpeptide inhibitors of HIV protease have ken developed as a result of two very different approaches. Fir enaniple,
DMP-850 CH3
CH
C2 symmetry of the active site of the en-
was exploited in the structure-based design of the symreinc cyclic urea derivative DMP-323.'°' This inhibitor cxpotent activity against the protease in vitro, excellent
activity in cell culture, and promising bioavailabilin experimental animals. In phase I clinical trials, howncr. the bioavailahility of DMP-323 was poor and highly ruhlc, possibly because of its low water solubility and a
OH
P0-178390
3
Dipeptide Pis containing 2-hydroxy-3-aminn-4-arylbutanoic acid in their scaffold showed promising preclinical results. JE-2 147103 (below), containing the allophenylnorostatin
386
of ()rgonie Medicinal
a,uI
Pljarnzacenzieal Che,,li%lrv
Rftonavir
Nelllnavfr
Amprenavir
JE-2147
Chapter II U
Ageittv
387
moiety, exhibited potent in vitro anti-WV activity. JE-2 147 a variety of HIV strains resistant to multiple approved Pls and exhibits good oral hioavailabilily and a good pharmacokinetic profile in two animal species. Also. emergence of resistance was considerably delayed with JE-2 147. R=
HIV Enhy Inhibitors Entry 01 HIV into a cell is a complex process that involves several specific membrane protein interactions. Initially. viral glycoprolein gpl2O mediates the virus attachment via its binding to at least two host membrane receptors. CD4 and die chemokinc coreceptor. This bivalent interaction induces a confomiational change in the viral fusion protein Protein gp4 I acts as the anchor for gp 120 in the virus. With the conformatinnal change, the viral envelope fuses sith the host cell membrane. In addition to gpI2O—chemokine receptor interaction, the fusion activity of gp4l is currently being explored as a novel target for antiretroviral
AMD-3100
Several positively charged 9-to 14-mer peplides have been described as capable of blocking the CXCR4 coreceptor. A small molecule. exhibits high-affinity binding to the CCR5 coreceptor, specifically blocking R5 isolates.
At least one agent from each class is in clinical tNing.
thetnakine
Receptor Binders
HIV-l isolates rely on the CCR5 corcceptor for entry strains). In later stages of the disease, however, more
Most
pathogenic selection variants of the virus emerge in about of individuals, which use the CXCR4 coreceptor in sidition to CCR5 (R5X4 strains) or the CXCR4 receptor only (X4 strains). Bicyclam compound AMD-3 lOOhbM was the first compound identified as a CXCR4-specific inhibitor dot interferes with the replication of X4 but not R5 viruses. The compound is currently in phase II clinical evaluations. It is used as an injectable agent because of its limited biosailahility.
TAK-779
Inhibitors of gp4l Fusion Activity The fusion of the HIV-l viral envelope with host plasnia
membrane is mediated by gp4 I. a transmcmhrane subunit of the H!V-l glycoprotein subunit complex. Pentafuside"5' (1-20) is a 36-mer peptide that is derived from the C-termi-
H
OR
Tetrazote
OH
II 0 R=benz
II
0 Diketo
388
Wilson and Gixt'old's Textbook of Organic Medicinal arid Pharn,act'nzical Clientistr
nal repeat of' gp4 I. Pentafuside appears to inhibit the forma-
lion of the fusion-competent conformation or gp4 I by inter-
feting with the interaction between its C- and N-terminal repeal. Penrafuside is a potent inhibitor of HIV-l clinical isolates, and it is currently in phase II clinical trials.
Integras. (IN) Inhibitors Two closely related types of small molecules that block strand transfer catalyzed by recombinant integrase have been
identified. Both types show in vitro antiviral activity. The diketo acids'°7 (above) inhibit strand transfer catalyzed by MutationS that conferred resistance to the diketo acids mapped near conserved residues in the IN enzyme. This finding demonstrates that the compounds have a highly specific mecharecombinant integra.se with an
less than 0.1
nism of action. X-ray crystallography of the hound teErazole°"t derivative (above) revealed that the inhibitor was centered in the active site of IN nearacidic catalytic residues.
Acknowledgment Portions of this text were taken from Dr. Arnold Martin's chapter in the tenth edition of this book. REFERENCES I. Condit, R. C.: Principles a?' sirnlogv. lit Knipe. D. M.. and P. St. teds.). l:untk,ntental Virology. 4th ed. New York. Lippincoit Williatius & Wilkins, 2(8)1. p. 19. 2. Hurnson. S. C.: Principles iii viral siruciure. tn Kitipe. t). M.. and Howlcy. P. M. (cdv.). Fundamecital Virology. 4th cii. New York. Lip. pincoit Williams & Wilkins. 2001. p. 53. 3. Wagner. F.. K., and Hewkn. M. J. teds.): Basic Virology. Maiden. MA. Blackweil Science. 999. p. t2. 4. Wagner. F.. K.. and Hewlett. M. .1. (cdv.): Basic Virology. MaIden. MA. Blackwell Science. t999. p.61. 5. Beak. J Jr.: Iminuncibiologicals. In Block. 3. H.. and Beak, J. M.. Jr teds.). Wilson and C,isvohd's Textbook or Organic Medicinal and Pharmaceutical Chemistry. 11th cii. Baltcntore, Lippincoit Williants & Wilkins, 2004. P. 10. 6. Freshicey. K. I.: Culture 01' Animal Cells. 3rd cii. New York, Witey. l,iss. 1994. 7. Young, .1. A. T.: Virus entry and uncoaticig. In Knipc. D, M.. and Hawley. P. M. teds.). Fundamental Virology. 4th ed. New York. Lippincoti Williams & 2001, P. 1(7. 8. tInnier. Ci: Virus assembly. In Knipc. D. Macid Hawley. P. M.(eds.). Fundamental Virology. 4th cvi. New York. l.ippincott Williams & Wilkins, 2001. p. 171. 9. Lamb. R. A.. and Clioppin. K. W.: Annu. Rev. Biochem. 52:467. 91)3.
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55, Joluitson. V. A., ci al.: 3. Infect. 1)iv. 164:646. 99?. 56. Knupp. C. A.. ci al.: Clin. Pharmacol. Ther. 49:523. 1991. 57. Ynrchoan. K.. ci at.: N. EngI. 3. Med. 321:726. 989. 58. Chen, C.. M.. nit) Cheng. Y.: Mo). Pltamsacal, 625. 1991
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CHAPTER 12 Antineoplastic Agents WILLIAM A. REMERS
The chemotherapy ol neoplastic disease has become increas-
ingly important in recent years. An indication of this importance is the establishment ola medical specialty in oncology. in which the physician practices various protocols of adjuvant therapy. Most cancer patients now receive some form of chemotherapy. even though it is merely palliative in many cases. The relatively high toxicity of most anticancer drugs has fostered the development of supplementary drugs that may alleviate these toxic effects or stimulate the regrowth of depleted normal cells. The terms cancer and neoplas lie disease actually encom-
pass more than 100 different tumors, each with its own unique characteristics. Drugs active against a cancer of one tissue often are ineffective against cancers of other tissues. Even cancers of the same apparent type respond widely to a particular therapeutic protocol. Consequently, it has been
titative. Another difference is that immune mechanisms and other host defenses are very important in killing bacteria and other foreign cells, whereas they play a lesser role in killing cancer cells. cancer cells overexpress certain
antigens, and antibodies produced by recombinant DNA technology exert a selective cytotoxic effect on them. Quan.
titative differences in proteins found in signaling that control ccli proliferation, differentiation, and the induction of programmed cell death (apoptosis) also provide targets for anticancer drugs.2 Because cancer cells have over-
come the body's surveillance system. chemotherapeutic agents must kill every clonogenic malignant cell, because even one can reestablish the tumor. This kind of kill is Cxtremely difficult to effect because antineoplastic agents kill
Cancer chemotherapy has received no spectacular breakthrough of the kind that the discovery of penicillin provided for antibacterial chemotherapy. There has been substantial progress in many aspects of cancer research, however. In particular. an increased understanding of tumor biology has led to elucidation of the mechanisms of action for antineoplastic agents. It also has provided a basis for the more ra-
cells by first-order kinetics. That is. they kill a constant frac [ion of cells. Suppose that a patient had a trillion leukemia cells. This amount would cause a serious debilitation. A tent anticancer drug might reduce this population 10.000fold, in which case the symptoms would be alleviated and the patient would be in a state of remission. After cessation of therapy, however, the remaining hundred million leukemia cells could readily increase to the original number. Fur. thermore. a higher proportion of resistant cells would be present, which would mean that retreatment with the same agent would achieve a lesser response than before. For this
tional design of new agents. Recent advances in clinical tech-
reason, multiple drug regimens are used to reduce drasticafi>
niques, including large cooperative studies, are allowing
the number of neoplastic cells. Typical protocols for letikemia contain four different anticancer drugs. usually with dii. ferent modes of action.
difficult to make progress on a broad front of neoplastic diseases.
more rapid and reliable evaluation of new drugs. The combi-
nation of these advantages with improved preliminary screening systems is enhancing the emergence of newer and more potent compounds. At present, at least 10 different neoplasms can be "cured" by chemotherapy in most patients. Cure is defined here as
an expectation of normal longevity. These neoplasms are acute leukemia in children. Burkitt's lymphoma. choriocarcinoma in women. Ewing's sarcoma, Hodgkin's disease. lymphosarcoma, mycosis fungoides. rhabdomyosarcoma. retinoblastonia in children, and testicular carcinoma.1 Unfor-
tunately, only these relatively rare neoplasms are readily curable. Considerable progress is being made in the treatment of breast cancer by combination drug therapy. For carcinoma of the pancreas. colon, liver, or lung (except small cell carcinoma), however, the outlook is bleak. Short-term remissions are the best that can be expected for most patients with these diseases. There arc cogent reasons why cancer is more difficult to cure than bacterial infections. One is that there are qualitative differences between human and bacterial cells. For example, bacterial cells have distinctive cell walls, and their ribosomes differ from those of human cells. In contrast, the differences between normal and neoplastic human cells are mostly quan-
390
TUMOR CELL PROPERTIES The basic differences between cancer cells and normal cclh are uncontrolled cell proliferation, decreased cellular differ entiation, ability to invade surrounding tissue, and ability r establish new growth at ectopic sites (metastasis). Cornea) to popular belief, not all tumor cells proliferate rapidly. Pro-
liferation rates vary widely with the cell type. Thus. lym phomas and normal intestinal mucosa both proliferate faster than solid tumors. Acute leukemia cells actually proliferale
more slowly than the corresponding precursors in normal hone marrow. Development and homeostasis in multicellular organisno are controlled by processes of cell division. differentiation and death. In the adult, the steady-state number of differentiated cells is maintained by a balance between cell lion and cell death. Cell death is a complex and actixci) regulated process known as apoprosis. Apoptosis isa process of cell shrinkage, membrane blebbing. and nuclear condensation. It differs from necrosis, the cell death induced
Chapter 12 • Anhineoplastir Agents by severe cellular injury, which is characterized by swelling
391
If
and ysis.
The process of apoplosis is a complex but carefully orchestrated sequence of events. Scientists disagree on the rela-
sivc importance of factors such as mitochondrial damage. although many think that when stress factors reach a critical level, the mitochondrial membrane potential changes, and the nrjtochondria leak or rupture, resulting in their own destntction. This causes the release of factors that trigger proteolytie enzymes called caspases. Other investigators think that the primary apoptotic signals activate caspases directly and then caspases attack mitochondria along with other eeldat organdIes. Cancer can be considered a failure of cells to undergo apopiosis. In normal cells, sensors to cell abnormalities lead to withdrawal of survival signals. resulting in cell death. In contrast, cancer cells circumvent the need for survival signals by increasing their abundance of anti-apoptotic proteins. Among these anti-apoptotic proteins, members of the Bcl-2
(I)
cell cycle specific
cell cycle nonspecific 1
Drug Concentration
family, including BAX and BAK. have been identified with the initiation or progression of a variety of tumors. They block the release of cylochrome C and apoptosis.activating
FIgure 12—2 • Cell cycle specificity.
factor from mitochondria. Cells also have a variety of tumor suppressor proteins that
or transcription of nucleic acids or prevent cell division by interfering with mitotic spindles. Cells in the DNA synthesis or mitosis phases are highly susceptible to these agents. In
respond to DNA damage by shutting down cell division or by inducing apoptosis. One intensively studied protein is p53. which binds to the regulatory sequence of genes and inhibits their transcription. Many mutations produce p53 in amisiolded ftrm, resulting in a conformation unsuitable for binding to regulatory sequences. The development of half all cancers is thought to result from misfolding of p53. Recent research has produced compounds that restore p53 ro its acIivd conformation. The concept of a cell cycle is based on experiments using
'Hithymidine radiography and flow cytometry. These ax'eriments showed that DNA synthesis. as measured by inLvrporation of I'Hlthymidine. takes place at a specific pe-
nal, known as the S phase. in the life cycle of a dividing celL Periods between the S phase and cell division (niltosis tM phase) are termed G1 and G2. A circular pictorial model Fig. 12.1) was derived for the clockwise progression of the cell cycle. The duration oleach phase in the cell cycle varies considerably with the cell type and within a single tumor. Typical durations are as follows: S. 10 to 20 hours, 62. 2 u U) hours, and M. 0.5 to I hour. G1 is highly variable as l'ercsuli of another phase, G15. in which the cell is not active seeR division. Most anticancer drugs block the biosynthesis
C M Mitosis
G2 Resting
G1 R
S DNA Replication
FIgure 12—1 • The cell life cycle.
contrast, cells in the resting state are resistant to many agents.
Slow-growing tumors characteristically have many cells in the resting state.3 Antitumor agents are classified on the basis of their effects
on cell survival as a function of dose. For many drugs, including alkylating agents, cell survival is exponentially related to dose, and a plot of log cell survival against drug concentration (Fig. 12-2) gives a straight line. These drugs exert their cytotoxicity regardless of the cell cycle phase and are termed non—cell cycle phase specific. Other drugs.
including antimetabolites and mitotic inhibitors, which act at one phase of the cell cycle (cell cycle phase specific). show a plateau after an initial low-dose exponential region. The proportion of labeled cells in tissue after a specified interval (usually I hour) following injection of l3Hlthymidine or 5-bromodeoxyuridine is known as the labeling index (LI). Comparisonof the LI with the proportion of proliferat-
ing cells in DNA synthesis provides the growth fraction. Doubling times for tumor growth are calculated from the growth fraction and cell cycle times. Rarely are they as rapid
as predicted because of tumor cell loss through necrosis. metastasis, and differentiation. The cell-kill hypothesis states that the effects of antitumor drugs on tumor cell populations follow first-order kinetics. This means that the number of cells killed is proportional to the dose. Thus, chemotherapy follows an exponential or log-kill model in which a constant proportion, not a constant number, of cancer cells are killed.4 Theoretically, the fractional reductions possible with cancer chemotherapy can never reduce tumor populations to zero. Complete eradication requires another effect, such as the immune response. A modified form of the first-order log-kill hypothesis holds that tumor regressions produced by chemotherapy are descnbed by the relative growth fraction present in the tumor at the time of treatment. This idea is consistent with the finding that very small and very large tumors are less responsive than tumors of intennediate size.5
392
lVjl.wn, ciiul Gi.sI'okl.%
of
Medicinal mid Phanmueuiical
Stern cells arc the cells of origin of a cell line, which
1949. 6-mercaptopurmne iii 1952, and 5-fluorouracil in 1957.
maintain the potential to regenerate the cell population and from which the differentiated cells are derived. They are important in the chemotherapy of human tumors because they must be eradicated completely to effect a cure. Treatments that afford substantial reductions in tumor burdens
Additional alkylating agents such as mclphalan and cyclophosphuinide were developed during this period, arid the activity of natural products such as actinomycin. mitoniycin C. and the sinca alkaloids was discovered. During the progress continued in all of these areas with the discovery of cytosine urahinuside. hleornycin. doxoruhicin, and car-
can produce remissions. hut the tumor may recur if some of the stem cells remain. Their eradication is difficult because many we in the GI) phase of the cell cycle.5 Drug resistance to chemotherapy usually involve.s the selection of certain cell populations. Populations of drug-resistant cells can he produced by clonal evolution or mutation. Drug-resistant cells in tissue culture are generated at a frequency consistent with known rates rif genetic mutation. Mutagenic agents increase the frequency of generation of drugresistant cells. This effect may have clinical importance because many antitumor agents are mutagenic. Intracellular effects that cause drug resistance may he secondary to cellu-
lar adaptation or altered enzyme lcvels or properties. For example, resistance to methotrexate involves increased lev-
els of the target enzyme. dihydrofolate reductasc! Other modes of resistance to antimctabolites include reduced drug transport into cells, reduced affinity of the molecular target, stimulation of alternate biosynthetic pathways. and impaired activation or increased metabolism of the drug. A major factor in resistance to alkylating agents is the ability of tumor cells to repair DNA lesions, such as cross-links and breakage of DNA strands caused by alkytation. Cells selected for re-
sistance to one drug may show cross-resistance to other drugs, even if their chemical structures are quite different: most of these drugs are derived from natural products, how-
ever. One type of molecular explanation for this tonu of multiple drug resistance is overexpression of niemubrane gly-
coproteins termed P-g!vcoproieins. which function as drug cfflux pumps. This overexpression is associate(I with gene amplification.2 Most antineoplastic drugs are highly toxic to the patient and must be administered with extrenme caution. Some of them require a clinical setting where supportive care is available. The toxicity usually involves rapidly proliferating tissues. such as bone marrow and the intestinal epithelium.
Individual drugs produce distinctive toxic effects on the heart. lungs. kidneys. and other organs, however. Chemotherapy is seldom the initial treatment used against cancer.
If the cancer is well defined and accessible, surgery is preferred. Skin cancers and certain localized tumors are treated by radiotherapy. Generally, chemotherapy is impor-
tant when the tumor is inoperable or has metastasized. Chemotherapy is finding increasing use as an "adjuvant" after surgery to ensure that few cells remain to regenerate the parent tumor. The era of chemotherapy of malignant disease was horn in 1941. when Huggins demonstrated that the administration of estrogens produced regressions of nietastatic prostate canIn the following year. Gilmnan arid others began clinical studies on the nitrogen mustards and discovered that mech-
lorethamine was effective against Hodgkin's disease and lymphosareoma' These same two diseases were treated with cortisone acetate in 1949, and dramatic, although temporary. remissions resulted. " The next decade was marked by the
design and discovery of antituetabolites: methotrexate in
rnustine. Novel structures such as procarbazi ne. ducarbazine.
and ds-platintimu complexes were liund to be highly active. In 1965. Kennedy reported that remissions occurred in of postmenopausal women with metastatic breast cancer on treatment svith high doses of estrogen. Much of the leadership and financial support for the devetopment of anlineoplastic drugs derives from the National Cancer Institute (NCI), In 1955. this organization established the Cancer Chemotherapy National Service Center (now the Division of Cancer Treatment) to coordinate a national untary cooperative cancer chemotherapy development prothis effort had evolved into a targeted drug gram. By development program. A massive screening system was established to discover new lead compounds. and tliousanth of samples have been submitted to it. The current highly automated NCI tumor cell culture screening system achieved operational status in 199(1. It emphasizes rigorous end points such as net cell killing and tumor regression, rather than earlier growth-inhibitory end points, and it uses a wide sail ely of specific types of cancer, including runny solid tumour models, in the initial stage of screening. New drug candidates are being screened at a rate of about 20.(XX) per year. with input divided about equally between pure compounds and extracts or fractions frotni natural products. The present in vitro screening panel contains 60 human tumor cell lines arranged in seven suhpanels that represent diverse histologes: leukemia, melanoma, lung, colon, kidney. ovary, awl brain. For routine evaluation, each sample is tested in a 2-day continuous drug exposure protocol using five log 4,-spaeed M for pure compounds and concentrations staning at 100 at
for extracts. Antitumor activities are conipured
three different levels of response.
is the
concentration that produces 5Qh% inhibition in cell prolifer-
ation relative to the control. TGI (tumor growth inhibitiniti is the drug concentration at which there is no net prolifeniis the lethal concentration of drug tha tion. and produces a 501% reduction in the ntmnmher of tun,or cell' relative to the control. The primary NCI screening data are reported in a mean graph format (Fig. 12-3 in which a vertical reference ban, obtained by averaging the negative log11, Gic,, values fur all of the cell lines tested, is plotted along the drug concentrutieri axis amid then horizontal ham-s are plotted for the individual negative log ,, of each line with respect to the vertical reference bar. This graphical representation provides a chan
acteristic fingerprint for a given compound, displaying the individual cell lines that are more sensitive than average (bars to the right of the reference) or less sensitive than average (bars to the left of the referemmce. Thus. Figure I!3 shows that colon cancer cell lines are miiore sensitive than
average to 5-lluorouracil (5-FU. whereas central system (CNS) cancer cell lines are more resistant than aser age to
CD
C
0 0
C
0
CD
0
0
C
ID
CD
CD DC
I
0
CD
C
CD
0
CD
1
U
w
-4
CD
-Il
rr
9 a
r
Z
n (A.
• a 'a aSia be 'a
'a
CM
U
l.a
-C
C,
CD
394
Wilsan and Gisi'a!d's Texthaok of Organic Mediei,,aI and PI,arrnaee'nflea! C'lw,ni.cfrv
A secondary stage of preliminary screening on selected compounds is performed in vivo in xenograft models by using a subset of cell lines found to be active in the primary in vitro screen. Two xenograft models in current use are the severe combined immunodeficiency (SCUD) mouse and the
athymic nude mouse. Both of those mouse models have defi-
cient immune responses that permit transplantation of human tumor cells without rejection. Consequently, potential antitumor drugs may be tested against human tumors in an in vivo
model. These models predict human clinical tumor responses better than the older allograft models that were based
on transplanting mouse tumors such as P388 leukemia into the same strain 01 mouse (syngeneic tumors). The important antitumor drug paclitaxel was discovered by using a xenograft model. An in vitro system that is a good predictor of human clinical activity is the human-tumor-colony—forming assay (l-ITCFA). This system uses fresh human tumor tissue from it is valuable in selecting chemotheraindividual peutic agents for individual tumor types and occasionally specific patients. but its use in large-scale primary screening has not been feasible. Compounds with significant antitumor activity are subjected to prcclinical pharmacology and toxicology evaluation in mice and dogs. Clinical trials may be underwritten by the Nd. They involve three discrete phases. Phase I is the clinical pharmacology stage. The dosage schedule is developed, and toxicity parameters are established in it. Phase II involves the determination of activity against a 'signal" tumor panel, which includes both solid and hematological types. A broad-based multicenter study is usually undertaken in phase Ill. It features randomization schemes designed to statistically validate the efficacy of the new drug in comparison to alternative modalities of therapy. As might he anticipated, the design of clinical trials for antineoplastic agents is very complicated, especially in the matter of controls. Ethical considerations do not permit patients to be left untreated if any reasonable therapy is possible. A number of pharmaceutical industry laboratories and foreign institutions have made significant contributions to the development of anticancer drugs. Organizations such as the United Kingdom's Cancer Research Campaign, the European Organization for Research on the Treatment of Cancer,
and the Japanese Foundation for Cancer Research have broadened international cooperation in anticancer drug tosearch.
mechlorethamine) showed selective toxicity, especially to lymphoid tissue. This observation led to the crucial suggestion that nitrogen mustards be tested against tumors of the lymphoid system in animals. Success in this area was lot. lowed by cautious human trials that showed methchlore. thamine to be useful against Hodgkin's disease and certain lymphomas. This work was classified during World War Il but was finally published in a classical paper by Gilman and Phillips in 1946." This paper described the chemical transformation of nitrogen and sulfur mustards to cyclic "oniurn" cations and established the nucleus as the locus of their interaction with cancer cells. The now familiar pattern of toxicity to rapidly proliferating cells in hone marrow and the gastrointestinal tract was established. A!kvlasio,, is defined as the replacement of hydrogen on an atom by an alkyl group. The alkylation of nucleic acids or proteins involves a substitution reaction in which a flu. cleophilic atom (nu) of the biopolymer displaces a leaving group from the alkylating agent.
nu-H + alkyl-Y
alkyl-nu
+H
-1- Y
The reaction rate depends on the nucleophilicity of the atom (S. N. 0), which is greatly enhanced if the nucleophile is ionized. Hypothetically, the order of reactivity at physiological pH is ionized thiol. amine, ionized phosphate. and ionized carboxylic acid)6 Rate differences among various amines would depend on the degree to which they are proton. atcd and their conjugation with other groups. The N-i position of guanine in DNA (Scheme 12-5, below) is strongh nucleophilic. Reaction orders depend on the structure of the alkyluting agent. Methane sulfonates, epoxidcs, and aziridines give ond-order reactions that depend on concentrations of the a)kylating agent and nucleophile. The situation is more com(nitrogen mustards) and plex with haloalkylsulfides (sulfur mustards), because these molecuks undergo neighboring-group reactions in which the nitrogen
or sulfur atom displaces the halide to give strained. membered "onium" intennediates. These "oniuni" iota react with nucleophiles in second-order processes. The overall reaction kinetics depend on the relative rates of the two steps, however. In the case of mechlorethaminc. the aziridinium ion forms rapidly in water, but reaction with biological nucleophiles is slower. Thus, the kinetic' are second order.'7 in contrast, sulfur mustard forms the less stable episulfon
ALKYLATING AGENTS Toxic effects of sulfur mustard and ethyleneimine on ani-
mals were described in the 19th century)4 The powerful vesicant action of sulfur mustard led to its use in World War I, and medical examination of the victims revealed that tissues were damaged at sites distant from the area of conSuch systemic effects included leukopenia, bone marrow aplasia. lymphoid tissue suppression, and ulceration of
the gastrointestinal tract. Sulfur mustard was shown to be active against animal tumors, but it was too nonspecific for clinical use. A variety of nitrogen mustards were synthesized between the two world wars. Some of these compounds (e.g.,
ium ion more slowly than this ion reacts with biologic.il nucleophiles. Thus, the neighboring-group reaction is rate limiting, and the kinetics are lirst order.'5 Aryl-substituted nitrogen mustards such as chloran,budl are relatively stable to aziridinium ion formation because the aromatic ring decreases the nucleophilicity of the nitna gen atom. These mustards react according to first-order kinetics.'8 The stability of' chlorambucil allows it to be takesi
orally, whereas mechlorethamine is given by intravenou administration of freshly prepared solutions. The require ment for freshly prepared solutions is based on the gradunl decomposition of the aziridinium ion by interaction ssiti water.
_____
Chapter 12 • A,uineoplaszie Agt':,:s
CH2CH2CI verV
fast
CH
moderate
395
CH2CH2nu
N(CH2CH2CI)2 * H20
—. CH2CH2OH
Ethylene imines and epoxides arc strained ring systems, but they do not react u.s readily as aziridinium or episulfonurn ions with nucleophiles, Their reactions arc second order as! are enhanced by the presence of acid. °' Examples of antitumor agents containing ethyleneimine groups are triethylenemelaminc and thiotepa.
CH
—, HO—C—H H—C—OH
H
CH2Br Mitobomitol Triethytene Metamine
Thiotepa
Diaziquone is an investigational benzoquinonc substituted with ethyleneimine groups and carbamate groups, both of shich are cancerostatic)9 After activation by reduction of
be quinone ring to a hydroquinone. the ethyleneiminc
alkylate DNA to produce cross-links. DNA—protein cross-links also are formed. emups
Some
0
Dtanhydro.D-rnannitol
A somewhat different type of alkylating agent is the Nalkyl-N-nitrosourea. Compounds of this class are unstable in aqueous solution under physiological conditions. They produce carbonium ions (also called carbeniurn ions) that can alkylate and isocyanates that can carbamoylate. For example. methylnitrosourea decomposes initially to form isocyanic acid and methyldiazohydroxide. The latter species decomposes further to methyldiazonium ion and finally to methyl carbonium ion, the ultimate alkylating species.22
0
- NHCOC2H5
+ H20 —.
NI
C2H5OCNH
0
No
The use of epoxides as cross-linking agents in textile i?rnistry suggested that they be tried in cancer ehemotherSimple diepoxides such as I ,2:3,4-diepoxybutane
jawed clinical activity against Hodgkin's disease,2° but rue of these compounds became an established drug. Di(mitobronitol) gives the corresponding diep-
continuous titration at pH 8. This diepoxide (1,2: 5,6.dianhydroomannitol) shows potent alkylating activity esperimental tumors?' thus suggesting that dibroand related compounds such as dibromodulcitol et by way of the diepox ides.
Isocyanic Acid
I N2
Diaziquone
+ Diazohydroxide
+ 0H
Substituents on the nitrogen atoms of the nitrosourca influence the mechanism of decomposition in water, which determines the species generated and controls the biological effects. Carmustinc (BCNU) undergoes an abnormal basecatalyzed decomposition in which the urea oxygen displaces
chloride to give a cyclic intermediate (Scheme 12-I). This intermediate decomposes to vinyl diazo hydroxide. the precursor to vinyl carbonium ion, and 2-chioroethyl isocyanate. The latter species gives 2-chioroethylamine. an additional alkylating agent.22 Some clinically important alkylating agents are not active
until they have been transformed by metabolic processes. The leading example of this group is cyclophosphamide.
396
Wilxun and GLnold'.c Textbook of Organic Medicinal and Phannaceutical Chemistry
0 —H
II
CICH2C1-I2NCNHCH2CH2CI —' CICH2CH2N —C= NCH2CH2CI
0=N
O=N I
H2C=CHN=NOH + 0=CNCH2CH2CI H,O
I N2 + 0H
Scheme 12—1 • Decomposition
N
CO2 +
of carmustine (BCNU).
which is converted by hepatic cytochrome P.450 into the
1CH2CH2CI
corresponding 4-hydroxy derivative by way of the 4-hydroperoxy intermediate (Scheme 12-2). The 4-hydroxy derivajive is a carbinolamine in equilibrium with the open-chain amino aldehyde form. Nonenzymatic decomposition of the
latter form generates phosphoramide mustard and acrolein. Studies based on 31P nuclear magnetic resonance (NMR) have shown that the conjugate base of phosphoramide mustard cyclizes to an aziridinium ion,24 which is the principal cross-linking alkylator formed from cyclophosphamide. The maximal rate of cyclization occurs at pH 7.4. It was suggested that selective toxicity toward certain neoplastic cells
NHCH2CH2CI
Itostamide
Other examples of alkylating species are afforded by car-
binolamines as found in maytansine and vinylogous carbine-
lamines as found in certain pyrrolizine
might be based on their abnormally low pH. This would afford slower formation of aziridinium ions, which would
00
persist longer because of decreased inactivation by hydroxide ions.22
Cyclophosphamide has been resolved, and the enantiomers have been tested against tumors. The levorotatory form has twice the therapeutic index of the dextrorotatory form.24 Ifosfamide. an isomer of cyclophosphamide in which one of the 2-chloroethyl substitucnts is on the ring nitrogen, also
has potent antitumor activity. It requires activation by hepatic enzymes, but its metabolism is slower than that of and involves substantially more dechloroethylation, yielding a chioroacetate metabolite.
N'
0
CH2CH2CI
0
CH2CH2CI
H2N
0
0
II
0 Phosphoramide Mustard +
H2C=CHCHO Acrolam
Scheme 12—2 • Activation of cyclophosphamide.
Chapter 12 U Authwopiaxtic
0
397
example, the sesquiterpene helenalin has both of these systems.2°
OCNHCH3
CH3
/ 1NHCH3
H
0
0 CH2
Vinytogous Carbinolarnine
f4etenalin
Alkylation can also occur by free radical reactions. The
Pytrokzino Dioster
When mitomycin C is reduced enzyinatically to its semiquinone radical, disproportion and spontaneous elimination system. nf methanol afford the vinylogous Loss of the carbaninyloxy group from thIs system gives a stabilized carhoniuni ion that can alkylate DNA (Scheme 11'3). The first alkylation step results from opening of the uiridine ring, and together with the vinylogous carhinolam.
it allows mitomycin C to cross-link double-helical Molecules like mitomycin Care said to act by "bioeductive alkylation.''28 Another type of alkylating species occurs in a.$-unsatucarhonyl compounds. These compounds can alkylate nucleophiles by conjugate addition. Although there arc no
a chemical class prone to decomposilion in this manner. These compounds were tested as antitu-
inor agents in 1963. and one of them. procarbazine. was found to have a pronounced, but rather specific, effect on Hodgkin's disease.3° Procarbazinc is relatively stable at pH 7. but air oxidation to azoprocarbazine occurs readily in the
presence of metalloproteins. Isomerization of this azo compound to the corresponding hydrazone. followed by hy-
drolysis. gives methylhydr.tzine and p-formyl-N-isopropyl benzamide. The formation of methylhydrazine from procarbazine has been demonstrated in living Methylhydrazine is known to be oxidized to methyl diazine,
clinical agents of this type, many natural prod-
which can decompose to nitrogen, methyl radical, and hydrogen radical.32 The methyl group of ?rocarbazine is incorporated intact into cytoplasmic RNA: It has not been estab-
active against experimental tumors contain a-rnethylcne or a,fl-unsuturated ketone functionalitics. For
lished conclusively, however, that the methyl radical is the methylating species.
0
CH2OCONH2
CH2OCONH2
5 Dmsproporlionation
—CI-t3OH 11.1.
OH
CH2OCONH2
0
CH2OCONH2
DNA OH
NH3
N[jA kheme 12—3 • Mitomycin C activation and DNA alkylation.
398
Wilson and Gi.ci'o!ds
of Organic Medicinal and Pharmaceutical chesnis,rv
CH3N =
CONHCH(CH3)2
Aoprocarbaz,ne
Procarbazine
CH3. + H' + N2
CH3NNH
CH3NHNH2 +
Methyldiazine
Methyihydraz,ne
Dacarbazine was originally considered an antimetabolite because of its close resemblance to 5-aminoimidazolc-4-car-
helix, is slow and difficult. In contrast, if the two strands are cross-linked, they canitot separate. Hence, they renatuntle
boxamide. an intermediate in purine biosynthesis. II now
rapidly on cooling. Interstrand cross-linking occurs with
appears. however, to be an alkylating agent!4 The isolation of an N-demethyl metabolite suggested that there might be a sequence in which this metabolite was hydrolyzed to methyldiazohydroxide. a precursor to methylcarbonium ion.35 but it was found that this mecabolite was less activc than starting material against the Lewis lung tumor. An alternative mode
mechlorethamine and other "two-armed" mustards, but ac• cording to this test. husulfan appears to give intrastr-and
of action was proposed in which dacarbazine undergoes acid-catalyzed hydrolysis to a diazonium ion, which can react in this form or decompose to the corresponding carbonium ion (Scheme 12-4). Support for the latter mechanism was alforded by a correlation between the hydrolysis rates of phenyl-substituted dimethyltriazines and their antitumor activitics.3" The interaction of alkylating agents with macromolecules such as DNA and RNA has been studied extensively. No
mode of action for the lethality to cancer cells has been established conclusively, however. A good working model was developed for the alkylation of bacteria and viruses, hut there are uncertainties in extrapolating it to mammalian cells.
The present working hypothesis is that most alkylating agents produce cytotoxic. mutagenic. and carcinogenic effects by reacting with cellular DNA. They also react with RNA and proteins, but these effects are thought to be less significant!7 The most active clinical alkylating agents are
links.35
In DNA, the 7 position (nitrogen) ut guanine is especially susceptible to alkylation by mechlorethamine and other ni trogen mustards (Scheme 12-5)!" The alkylated structure has a positive charge in its imidazole ring, which renders the guanine—ribose linkage susceptible to cleavage. This cleavage results in the deletion of guanine. and the resulting "apurinic acid" ribose—phosphate link is readily hydrolyc
able. Alkylation of the imidazole ring also activates it to cleavage of the 8,9 bond.'6 Other consequences of the positively charged punne structure are facile exchange of the 8-hydrogen, which can be used as a probe for a shift to the enslized pyrimidine ring as the preferred tautonrer. The latter effect has been cited as a possible basis for abnormal base pairing in DNA replication, but this has not been ated. One example in which alkylation of guanine does lead to abnormal base pairing is the 0-6-ethylat ion produced ethyl methanesulfonate. This ethyl derivative pairs with thymine, whereas guanine normally pairs with cyto.sinc.4'
hifunctional compounds capable of cross-linking DNA. Agents such as methylnitrosourea that give simple alkylation are highly mutagenic relative to their cytotoxicity. The crosslinking process can be either intersirand or intrastrand. Interstrand links can be verified by a test based on the thermal denaturation and renaturation of DNA. When double-helical DNA is heated in water, it unwinds and the strands separate. Renaturation. in which the strands recombine in the double
NXCOI*12
CH
\CH3
H30
N2
N H
+
Dacarbazine
1.
CH
/ Scheme 12—4 • Activation of dacarbazine.
H
+ HON = NCH3 NH2
Chapter 12 • ,tn:üu'op!astir ,tgenls
399
H
-H
— P0-
0
CH3 OH
CH2CH2NCH2CH2CI
+
Scheme 12—5 • Alkylation of guanine in DNA. Other base positions of DNA attacked by alkylating agents jie N-2 and N-3 of guanine; N-3. N- I. and N-7 of adenine:
0.6 of thyminc; and N-3 of cytosine. The importance of these minor alkylation reactions is difficult to assess. The phosphate oxygens of DNA are alkylated to an appreciable extent, but the significance of this feature is unknown.42 Guanine is also implicated in the cross-linking of doubleDNA. Di(guanin-7-yl) derivatives have been identilied among the products of reaction with mechlorethamine.° Busulfan alkylation has given l'.4'-di(guanin-7-yl)-butanc.
this product is considered to have resulted from innastrand linking.35 Enzymatic hydrolysis of DNA crosslinked by mitomycin C has given fragments in which the mubiotic is covalently bound to the 2-amino groups of two liuaflOsine residues, presumably from opposite strands of the helix.40
Alkylating agents also interact with enzymes and other rrolcins. Thus, the repair enzyme DNA nucleotidyltransfer-
leukemia cells is inhibited strongly by BCNU. knuustine (CCNU). and 2-chioroethyl isocyanate. Because
was a poor inhibitor of this it was concluded that the main interaction with the azsmc was carbantoylation by the alkyl isocyanates gener. red in the decomposition of BCNU and Alkylating agents can damage tissues with low mitotic but they are most cytotoxic to rapidly proliferating oases that have large proportions of cells in cycle. Nucleic sida are especially susceptible to alkylation when their are changed or unpaired in the process of replicaalkylaling agents are most effective in the late li or S phases. Some alkylation may occur at any stage in cycle, but the resulting toxicity is usually expressed 4en cells enter the S phase (Fig. 12-I). Progression through the premitotic phase, and cell cycle is blocked at 'loxion fails.4a
If cells can repair damage to their DNA beflre the next cell division, the effects of alkylation will not be lethal. Cells have developed a complex mechanism to accomplish this repair. Initially, a recognition enzyme discovers an abnormal region in the DNA. This recognition brings about the operation of an endonucicase. which makes a single-strand break in the DNA. An exonuclease then renuwes a small segment of DNA containing the damaged buses. Finally, the DNA is restored to its original strtcture by replacing the bases and rejoining the strand.4' Thus, tumor cells with efficient repair mechanisms will be relatively resistant to alkylatiiig agents. Tumor cells outside the cell cycle, in the resting phase (Ge). will have a rather long time to repair their DNA. Thus, slow. growing tunu)rs should not respond well to alkylating agents. and this is observed clinically.
Products MechlorethMechiorethamine Hydrochloride, USP. amine hydrochloride. Mustargen. nitrogen mustard, HN.. NSC-762. 2.2-dichloro-N-methyldicthylamine hydrochloride, is prepared by treating 2.2'.unelhylimino)diethanol with thionyl chloride.47 It occurs as hygroscopic leaflets that are very soluble in water. The dry crystals are stable at tem-
peratures up to 40°C. They an.' very irritating to mucous harmful to eyes. The compound is supplied in rubber-stoppered vials containing a mixture of IC) tug of rnechlorethumine hydrochloride and 90 mg of sodium chloride. It is diluted with 10 mL of sterile water immediately before injection into a rapidly flowing intravenous infusion. Intracavity injections are sotnetimes given to control malignant etfusions.
The aziridiniuni ion tirmed from niechlorcthamine in body tluids is highly reactive. It acts on various cellular components within minutes of administration. Less than
401)
and
of
Medieinal and l'/,ar,,,aeeuiwa! Cht'n:isirv
0.01% is recovered unchanged in the urine. hut more than 50'% is excreted in urine as inactive metabolites in the first 24 hours.
Mechiorethamine is effective in l-lodgkin's disease. Current practice is to give it in combination with other agents. The combination with viucristinc (Oncovin). procarhaiine. and prednisonc, known as the MOPP regimen, was considered the treatment of choice. Other lyniphomas and mycosis fungoides can be treated with mechiorethamine. The most serious toxic reaction is hone marrow depression, which icsuits in leukopenia and thronibocytopenia. Emesis is prevalent and lasts about 8 hours. Nausea and anorexia persist longer. These gastrointestinal effects may be prevented by the antiemetic compound ondansetron. Inadvertent extravasation produces intense local reactions at the site of injection. If it occurs, the immediate application of sodium thiosulfate solution can protect the tissues thiosulfate ion reacts
very rapidly with the aiiridiitiuni ion formed from mechlor-
or under refrigeration br prolonged times. At tentper.ttures above 35°C. it liquifies and decomposition is more rapid. Ilosfamnidc usually is administered in a short infusion a 5% dextrose or normal saline. Use within 8 hours of reconsti-
tution is recommended. Pharmacokinctic studies indicate that it is handled in the same way as cyclophosphainide. except that metabolism is less extensive. There is an appareni
half-life of 7 hours and a urinary recovery of The Food and Drug Administration (FDA)—approved in dication for ifosfamide is in combination therapy for gent cell testicular Combination salvage regimens are effective against soft tissue sarcoma. ovarian and breast car-
cinonias. and leukemia. Us limiting toxicity is in the urinal) tract, especially hemorrhagic cystitis. which results fmm the excretion of ulkylating metabolites in the urinary
Vigorous hydration and/or administration of mesna arc needed to prevent bladder damage. Other toxieities inclu& nausea and vomiting. alopccia. and CNS effects.
ethaminc.
Melphalan. USP. Cyclophosphainide. CyCyclophosphamide. USP. loxan. NSC-2ô27 I. N.N-his(2-chloroethyl)Ielrahydro-2HI .3.2-oxazaphosphorine-2-amine-2-oxide. is prepared by treating his(2-chloroethyh-phosphoramide dichioride with propanolamine.45 The monohydrate is a low-melting solid that is very soluble in water. It is supplied as 25- and 50mg white tablets, as 50-mg-unit-dose cartons, and as a powder (1(X). 200. or 500 mg in sterile vials. For reconstitution. 5 mUlO() rug of Sterile Water for Injection. USP. is added. The oral dose of cyclophosphamide is 9(1% hiouvailahie. with an 8e4, first-pass loss. It must he metaboli,.ed by liver microsomes to become active. Among the melaholites. phos.
phoramide mustard has antitumor activity, and acrolein is toxic to the urinary bladder. The acrolein toxicity can be decreased by intravenous or oral administration of the sodiuin sah of 2.mercaptocthane sulfonic acid (mesnal. whose sulihydryl group gives conjugate addition to the double bond of acrolein.49 In the plasma, mesna forms a disulfide, which is converted selectively to the active sulthydryf in renal tubules.
Cyclophosphamide has advantages over other alkylating agents in that it is active orally and parenterally and can be given in fractionated doses over prolonged periods. It is active against multiple myeloma. chronic lymphocytic leukemia (CLL). and acute leukemia of children. In combination with oilier chemotherapeutic agents, it has given complete remissions and even cures in Burkeit's lymphoma and acute lymphohlastic leukemia (ALL) in The most frequently encountered toxic effects are alopecia. nausea, and vomiting. Leukopenia occurs, hut thrombocytopenia is less frequent than with other alkyluting agents. Sterile hemnorrhagic cystiis may result and even he fatal. Gonadal suppression has been reported in a number of patients.
Ifosfamide.
Ikisfamide, IFEX. Holoxan. NSC- 109724. 3-(2-chloroethyl)-21(2-chlortsrthyl)aminol-tetrahydro-2H. I.
3.2-oxazaphosphorine-2-oxide. isophospharmide. is prepared from 3-I (2—chloroethyl )aminolpropanol by treatment with
phosphorus oxychioride followed by It is supplied in I- and 3-g vials as an oil-white Iyophiliied powder. The intact vials may be stored at room temperature
Melphalan. Alkeran. mustard. NSC-8806. 4-bis(2-chloroethyl)amino-i.-phenylal. anine. is prepared by treating m..N-phthalimnido-p-aminophc.
nylalanine ethyl ester with ethylene oxide, followed by phorus oxychioride. and finally hydrolysis with hydrochknw acid.55 Scored 2-mg tablets are available for oral tion. Oral absorption is erratic and incomplete, with ahsolwc hioavailability ranging from 25 to 89%. A preparation kit provided for parcnterul formulation. It contains 100 mg ol mclphalau. which is dissolved in I niL of acid-alcohol solu
tion. and then combined with final diluent containing mg of dipotassium phosphate. 5.4 niL of propylene glyoit and Sterile Water for Injection. USP. to give 9 mL of sok tion. This preparation should be used promptly. There is no significant first-pass effect with melphalar but the drug is gr.rdually inactivated by nonensyniatic drolysis to nionohydroxy and dihydroxy Elirs inamion is hiphasic. with half-lives of 6 to 8 minutes 40 to 60 minutes. Most of the drug is cleared by nonrerui mechanisms.
Melphalan is active against multiple myeloma. It active against breast, testicular, and ovarian carcinoma.' The clinical toxicity is mainly hemanofogical. which mews that the blood count must be followed carefully. Nausea aid vomiting are infrequent, but alopecia occurs.
'NH3 Merphalan
Chlorambudil. USP.
Chloranxbucil. Leukeran. chlteaminophenc, NSC-3088. p-(di-2-chlorethyl)-aminoplrerylbutyric acid, is prepared by treating p-aminophenyihuiya
acid with ethylene oxide, followed by thionyl Chlorambucil is soluble in ether and aqueous alkali. Its absorption is efficient and reliable. Sugar-coated 2-mg aS lets are supplied. Chlor.unbucil acts most slowly and is the least toxic any nitrogen mustard derivative in use. It is indicated cially in treatment of CLL and primary macroglobuiincmu
I
Chapter 12 •
Attsüwoplas:ie Agt'n;s
401
Other indications are lymphosarcoma and Hodgkin's discaseY' Many patients develop progressive, but reversible. during treatment. Most patients also develop a dose-related and rapidly reversible ncutropcnia. For these
Carmustine. Carmustine. BiCNU. BCNU. NSC409962. I .3-bis(2-chloroethyl-l -nitrosourea. is synthesized
wacons. weekly blood counts are made to determine the total and differential leukocyte levels. The hemoglobin levels are
changes to an oily liquid at 27°C. This change is considered a sign of decomposition, and such samples should be discarded. Carmustine is most stable in petroleum ether or water at pH 4. It is administered intravenously because metabolism is very rapid. Some of the degradation products. however. have prolonged half-lives in plasma. Carmustine is supplied
also determined for monitoring both toxicity (low counts) and efficacy in CLL (raised counts). USP. Busulfan. Myleran. NSC-750. I .4-dimethnnesulfonyloxy)butane. is synthesized by treating 1.4butanediot with methanesulfonyl chloride in the presence of It is obtained as crystals that are soluble in acelone and alcohol. Although practically insoluble in water, it dissolves slowly on hydrolysis. It is. however, stable in dry (non. It is supplied as scored 2-mg tablets. Busulfan is welt absorbed orally and metabolized rapidly. Much of the drug undergoes a process known a.s sulfur
by treating l.3-bis(2-chlorocthyl)urea with sodium nitrite and formic a low-melting white powder that
stripping" in which interaction with thiol compounds such as glulathione or cysteine results in loss of two equivalents simethanesulfonic acid and formation of a cyclic sulfonium intermediate involving the sulfur atom of the thiol.6' Such intermediates arc stable in vitro, but in vivo. they
as 100-mg quantities of lyophilized powder. When it is diluted with 3 niL of the supplied sterile diluent. ethanol, and further diluted with 27 mL of sterile Water. a 10% ethanolic solution containing 3.3 mg/mI is obtained. Biotransformation of carmustine is rapid and extensive. with most of a dose recovered in urine as metubolites. The half-life has an a-phase half-life of 6.1 minutes and a /3phase half-life of 21.5 minutes.67 Because of its ability to cross the blood—brain barrier. carmustine is used against brain tumors and other tumors (e.g., leukemias) that have metastasized to the brain!" It also is used as secondary therapy in combination with other agents for Hodgkin's disease and other lymphomas. Multiple
ate readily converted into the metabolite 3-hydroxythiolaneIl-dioxide?'2 That the sulfur atom of this thiolane does not
myeloma responds to a combination of carmustine and prednisonc. Delayed myelosuppression is the most frequent and
come from a methanesulfonyl group was shown by the
serious toxicity. This condition usually develops 4 to 6
nearly quantitative isolation of labeled methanesulfonic acid in the urine when busulfan 35S is administered to
weeks after treatment. Thrombocytopenia is the most pronounced effect, followed by lettkopenia. Nausea and vomiting frequently occur about 2 hours after treatment. Carmustine is given as a single dose by intravenous injection at 1(X) to 200 mg/m2. A repeat course is not given until the blood elements retUrn to normal levels, which requires about 6 weeks.
Oral doses of husulfan are generally well tolerated. The ahoorption has zero-order kinetics, with a mean log time of 36 minutes and a 2-hour duration to the end of absorption.TM Values for mean plasma concentration X time are dose de-
with peak levels of 24 to 130 nglmL for 2- to 6mg doses. The half-life is 2.1 to 2.6 hours. The main therapeutic use of busulfan is in chronic granuheytic leukemia. Remissions are observed in 85 to 90% of patients after the first course of therapy; it is not curative. however. It is used in preparative regimens (bone marrow ablative) for bone marrow transplantation in patients with various leukemias. Toxic effects are mostly limited to myelauppression in which the depletion of thrombocytes may cad to hemorrhage. Blood counts should be done at least weekly, The rapid destruction of granulocytes can cause
which might result in kidney damage. This complication is prevented by using allopurinol. a xunthine aidase inhibitor.65
CH2—CH2
Lomustine.
Lomustine, CeeNU. CCNU. NSC-79037, I -(2-chlorethyl)I-3-cyclohexyl-l-nitrosourea. is synthesized by treating ethyl 5-(2-chloroethyl)-3-nitrosohydantoate with cyclohexylamine. followed by renitrosation of the resulting intermediate. I l-(2-chloroethyl)!-3-cyclohexyl-urea!'° It is sufficiently stable to metabolism to be administered orally. The high lipid solubility of lonnustine allows it to cross the
blood—brain barrier rapidly. Levels in the CSF are 50% higher than those in plasma. Lomustine is supplied in dose packs that contain two each of color-coded 100-. 40-. and 10-mg capsules. The total dose prescribed is obtained by appropriate combination of these capsules.
H
H
I
I
/SCH2?COR + 2CH3S03+
+HSCH2CCOR OSO2CH 3
NHR'
—.
NHR'
OH2
HO
\2O
HC — OH
/
0
402
Wilson and Gisi'old'.s Textbook of Organic Medicinal and Pharmaceutical Che,njsirv
Procarbazine has demonstrated activity against Hodgkins
disease. For this condition, it is used in combination with agents such as mechlorethamine, vincristine. and prednisonc
Lomusfine
Oral absorption of fomustine is nearly complete within 30 minutes. U is convened rapidly into cis- and trans-4-OH metabolites by liver microsomes. The half-life of the parent drug is 1.3 to 2.9 hours, and the peak concentration of metab-
olites is reached 2 to 4 hours after dosing. Lomustine is used against both primary and metastatic brain tumors and as secondary therapy in relapsed Hodgkin's disease. The most common adverse reactions are nausea and vomiting, thrombocytopenia. and leukopenia. As in the case of carmustine. the myelosuppression caused by lomustine is delayed.70 The recommended dosage of lomustine is 130 mg/rn2 orally every 6 weeks. A reduced dose is given to patients with
compromised bone marrow function,
Thiotepa, USP. Thiotepa. TSPA. NSC-6396, N,N',N"triethylene-thiophosphoramide. iris( I -aziridinyl)phosphinc
sulfide, is prepared by treating trichlorophosphine sulfide with aziridine7' and is obtained as a white powder that is water soluble. It is supplied in vials containing 15 mg of thiotepa. 80 mg of sodium chloride, and 50 mg of sodium bicarbonate. Sterile water is added to make an isotonic solu-
tion. Both the vials and solutions must be stored at 2 to 8°C, These solutions may be stored 5 days without loss of potency. Thiotepa blood levels decline in a rapid biphasic manner.
It is convened into TEPA by oxidative desulfurization, and TEPA levels exceed those of' thiotepa 2 hours after administration. Aziridine metabolism also occurs, with liberation of ethanolamine, Thiotepa has been tried against a wide variety of tumors and has given palliation in many types, although with varying frequencies. The most consistent results have been obtained in breast, ovarian, and bronchogenic carcinomas and
malignant lymphomas. It is a mainstay of high-dose regimens in treating solid tumors when followed by autologous
(MOPP program). Toxic effects, such as lcukopenia. bocytopenia. nausea, and vomiting, occur in most patienta. Neurological and dermatological effects also occur. Conciw rent intake of alcohol, certain amine drugs, and foods cow taming high tyramine levels is contraindicated. The weak monoamine oxidase-inhibiling properties of procarbaiinc may potentiate catechol amines to produce hypertension.
Dacarbazine. DTIC-Dome. DIC. DTIC Dacarbazine. NSC.45388. 5-(3,3-dimethyl-l-triazenyl)-IH-imidazok4 carboxamide, is prepared by treating the diazonium salt. piw
pared from 5.aminoimidazole-4-carboxamide, with ylamine in methanol.74 It is obtained as a colorless to colored solid that is very sensitive to light. It does not
but decomposes explosively when heated above Water solubility is good, but solutions must be from light. Dacarbazine is supplied in vials containing eithe 100 or 200 mg. When reconstituted with 9.9 and 19.7 niL respectively, of sterile water, these samples give solttion. containing 10 mglmL at pH 3.0 to 4.0. Such solutions nuy be stored at 4°C for 72 hours. injected dacarbazine disappears rapidly from plasma k cause of hepatic metabolism. The half-life is about 40 mit
utes. Excretion is by the renal tubules, and in the 6.how tic excretion fraction, 50% of the drug is intact and N-demethylated metabolite.75 Dacarbazine is indicated for the treatment of mctasiaut Combination with other antinec malignant melanoma.75' plastic drugs is superior to its use as a single agent. Anoreic nausea, and vomiting are the most frequent toxic reaction Leukopenia and thrombocytopenia. however. are the nw serious effects.75 Blood counts should be done, and 11th counts are too low, therapy should be temporarily Dacarbazine is also used in combination therapy for kin's disease. The recommended daily dosage is 2 to 4.5 mg/kg Ire
days. with repetition at 4-week intervals. Extravasation the drug during injection may result in severe pain.
bone marrow transplantation. It also is used to control intra-
cavity effusions resulting from neoplasms. Thiotepa
is
highly toxic to bone marrow, and blood counts arc necessary during therapy.
Procarbazine hyProcarbazine Hydrochloride, USP. drochloride, Matulane, MIH. NSC-77213, N-isopropyl-a(2-methylhydrazine)-p-toluamidc. is prepared from N-isopropyl.p-toluamide in a process involving condensation with
diethyl azodicarboxylate. methylation with methyl iodide and base, and acid hydrolysis.72 Although soluble in water, it is unstable in solution. Capsules containing the equivalent of 50 mg of procarbazine as its hydrochloride are supplied. Procarbazine is rapidly and completely absorbed following oral administration. It readily decomposes by chemical and metabolic routes, with a half-life of 7 to 10 minutes, to produce highly reactive species including methyl diazonium ion, methyl radicals, hydrogen peroxide. formaldehyde. and hydroxyl radicals.73
ANTIMETABOLITES Antimetabolites are compounds that prevent the biosynthe or use of normal cellular metabolites. Nearly all of the cit
cal agents are related to metabolites and cofactort, in biosynthesis of nucleic acids. They usually are lated in structure to the metabolite that is antagonized. ainimetabolites are enzyme inhibitors. They may comb with the active site as if they were the substrate or cofacin Alternatively, they may bind to an allosteric regulatory especially when they resemble the end product of a bios
thetic pathway under feedback control.11' antimetabolite must be transformed biosynthetically faa lized) into the active inhibitor. For example. tine is convened into the corresponding ribonuclcct which is a potent inhibitor of the conversion of bosylpyrophosphate into 5-phosphoribosylamine. a controlling step in the de novo synthesis of purinec5t
Chapter 12 U Antineoplast
H2O3P
H203P
HO
OH
OH
5-Phosptioribosylpyrophosptlate
Alp
5, lOMethenyl leirahydrolOlate
NH2
H2O3P
Formylgtycino
Ribonucleolide
G!utarnu,o
AlP.
H
NyM HN
JNH°
HO
OH
r
AlP
Mg.K N
HO2C
N
N
Ribonuclectide
N
H
I
H2O3P
H203P
HO
OH
Scheme 12—6 • De novo synthesis of purine nucleotides (simplified).
404
Wilson
and
of Organic Medicinal and Pharmaceutical Cliesnisirv
+ N
CH
0
LN
N
N
II
CH I
CO2H
HO
HO
HO
OH
Adenytosuccunic
IflOSifliC Acid
OH
Adenybc Acid (AMP)
Acid
0
N N
H203P
H203P
HO
OH
Guanylic Acid (GMP)
Xartthylic Acid
Scheme 12—6 • Continued.
Scheme 12-6). An auiiimctabolitc and its transformation product.s may inhibit a number of different enzymes. Thus, 6-mercaptopurine and its anabolitcs interact with more than 20 enzymes. This multiplicity of effects makes it difficult to decide which ones are crucial to the anticumor activity. The anabolites of purine and pyrimidine antagonists may be incorporated into nucleic acids. In this event, part of their
antitumor effect might result from malfunction of further macromolecular synthesis because of the abnormal nucleic acids.5'
After the formulation of the antimetabolite theory by Woods and Fildcs in 82O antimetabolites based on a variety of known nutrients were prepared. The first purine analogue to show antitumor activity in mice. 8-azaguanine. was This compound was introsynthesized by Roblin in
H3C—N.
0
S
duced into clinical trials hut was abandoned in favor ii newer and more effective agents, such as 6-mercaptopunv: and 6-Ihioguanine, developed by Hitchings and Mercaptopurine was synthesized in and was shre to be active against human leukemia in the lolkiwing To be active against neoplasms. 6-mercaptopurine mui be converted into its ribonucleotide, 6.ihioinosinate. by fr enzyme hypoxanthine-guanine Neoplasms that lack this enzyme are resistant to the 6-Thioinosinate is a potent inhibitor of the conversion phosphorihosylpyrophosphate into 5-phosphorihosybrnrni as mentioned above, It also inhibits the conversion of sinic acid to adenylic acid at two stages: (a) the reactkin ii inosinic acid with aspailate to give adenylosuccinic acid (h) the loss of fumaric acid from adcnylosuccinic acid I give adenylic acid.8' Furthermore. it inhibits the tixidali' of inosinic acid to xanthylic acid.85 The mode of artist ii 6-mercaptopurine is further complicated by the fact thz ribose diphosphate and (riphosphute anabolites are also tive enzyme inhibitors, and the triphosphate can be incotjt
rated into DNA and RNA to inhibit further chain ehsa
tion.°' Still more complex is the ability of H
to act as a substrate for a methyl tr.tnsfcrjse that tequila adenosylmethionine. which converts it into 6-niethylits'i
H
nosinate. The latter compound is responsible for certainali 8.Azaguanine
6-Mercaptopunne
Azathioprrie
metabolite activities of 6-mercaptopurine.8"
('hapter 12 • 6Thioinosinate
SR
Agenl.s
405
NH
= H) (R = CH.,)
LLT'N
P
Metabolic degradation (catabolism) of 6-mercaptopurine by guana.se gives 6-thioxanthine. which is oxidized by xanthine oxidase to yield 6-thiouric acid."° Allopurinol. an inhibitor of xanthinc oxidase. increases both the potency and toxicity of 6-mcrcaptopurinc. Its main importancc. howocr. is as an adjuvant to chemotherapy because it prevents uric acid kidney toxicity caused by the release of purines born destroyed cancer cells. Heterocyclic derivatives of 6-
such as azathioprine (Iniuran). were deto protect it from catabolic reactions.'° Although azahioprine has antitunlor activity, it is not significantly better than 6-mercuptopurine. It has an important role, however. an immunosuppressive agent in organ
P. = H Vidarabrie
A = F.
HOPO.. Fludaiabine
In contrast to the susceptibility of adenosine arahinoside to adenusinc deaminase. its 2-fluoro derivative, iludarabinc. is stable to this enzyme. Fludarahine is prepared as the 5'monophosphate. Fludarahine has good activity against CLL. Ii is converted into the corresponding triphosphate,'°1 which inhibits ribonucleotide reductasi' 2.Chloro.2'-deoxyadenosine (cladrihine) also is resistant to adenosine dcaniinusc. It is phosphorylated in cells to the triphosphate by cytidinc kinase. and the triphosphate inhibits enzymes required tbr DNA repair. Cladrihinc is highly effective against hairy cell leukemia. NH2 N
6-Thiouric Acid
Allopunnol
Thioguanine is converted into its rihonucteotide by the enzyme that acts on 6-mercaptopurine. It is converted These species inhibit the di- and nod of the same enzymes that are inhibited by 6-mercaptosiine. Thioguanine is also incorporated into RNA, and its
ntetabolite is incorporated into DNA. The signifithese "fraudulent" nucleic acids in lethality to neois uncertain.'3
Cladribine
The invention of 5.fluorouracil as an antimetaholile of uracil by Heidelberger in 1957 provided one of our toremost examples of rational drug design.'°1 Starting with the observation that in certain tumors uracil was used more than orotie acid, the major for nucleic acid pyrimidinc biosynthesis in normal hissue. he decided to synthesize an antimetaholite of uraeil with only one modification in the structure.
The 5 position was chosen fir a substituent to block the conversion of uridylate to thynsidylate (Scheme 12-7). thus diminishing DNA biosynthesis. Fluorine was chosen as the 6Thioguanine
substituent because the increased acidity caused by its induc-
tive effect was expected to cause the molecule to hind Adenine arabinoside (Vidarabine) was first prepared by bcmicat synthesis'TM and later isolated from cultures of a sugar. o-arahinosc. siqsornyces isepimeric with n-ribose at the 2' position. This strucchange makes it a competitive inhibitor of DNA polyIn addition to its anhineoplastic activity, adeninc .nbinosidc has potent antiviral action. Adeninc arabinoside
strongly to These choices were well tbunded. as 5-tluorourucil soon became one of the most widely used antineoplastic agents. H is a mainstay in the therapy of adenocarcinoma of the colon and rectum. Side arc both dose and schedule dependent. They include myelosuppression on bolos administration and mucositis on prolonged infusions. Otherwise, the drug is svcll tolerated.
of its derivatives are limited in their antitumor
5-Fluorouracil is activated by anabolism to 5-fluoro-2 dcoxyuridylic acid. This conversion may proceed by two
.d
to adenosinc deuminase. This enzyme
them into hypoxanthine arubinoside derivatives. resistance of certain tumors correlates with their levels aknminc dcaminasc."1
routes. In one route. 5-tluorouracil reacts with ribose- I phos. phale to give its riboside. which is phosphorylated by uridine kinase.'°2 The resulting compound. 5-Iluorouridylic acid, is
406
Wi/si,,, and Gisrold's ie'aI,oak of Organic Medicinal mid I'Izannaeeuzical Cl,enii.cirr
NH
Enzyme
Enzyme
2-Dooxyuridyiate
+
HN1
0 R=
CO,H R
Sdieme 12—7 • Conversion of uridylate
Thymidyiale
into thymidylate.
converted into its 2'-dcoxy derivative by ribonucleotide reductase. 5-Fluorouracil also may he iransforiiied directly into 5-tluorouridylic acid by a phosphoribosyltransferase. which is present in certain tumors. "° An alternative pharmaceutical
based on 5-fluorouracil is its 2-deoxyriboside (floxuridine).'0' This compound is phosphorylaled by 2'-dcoxyuridine kinase.
5-Fluoro-2'.deoxyuridylic acid is a powerful competitive inhibitor of thynlidylale synthetase. the enzyme that converts
2'-deoxyuridylic acid to thymidylic acid. This blockage is probably the main lethal effect of 5-fluorouracil and its melabolites. "° In the inhibiting reaction. the sultuiydryl group
IF
0
5-Fluorouracil
0
of a cysteine residue in the enzyme adds to the 6 of the fluorouracil moiety. The 5 position then hinds tei' methylene group of 5.1 O-mnethylcnetetrahydrololate. Oni
narily. this step would be followed by the transfer of thef hydrogen of uracil to the methylene group, resulting in formation of thymidylate and dihydrofolate: however. fluorine is stable to transfer, and a terminal product rad involving the enzyme. cofactor. and substrate, all bonded. Thus. 5-fl uoro-2'-deoxyuridylic acid would silied as a inhibitor."5 The rate-determining enzyme in 5-fluorouracil caiabo!hr is dihydropyrimidine dehydrogenase. Inhibition of thisn
0
0
HN)Lf F
HO
HO
OH
5-Fluorouiacil Riboside
5-Fluorodeoxyuuctylic Acid
5Fluorouracil 2-Deoxytiboside
Chapter 12 • Anhint'opia.crii Agents by 5-ethynyluracil increases the plasma concentrationcurve index IWO-
of 5-fluorouracil enough to raise its therapeutic to fourfold. NH2
NH2
407
In gemcitahinc. fluorine atoms replace the hydroxyt group and the hydrogen atom at the 2' position of After its anabolism to diphosphate and triphosphale metabolites. gemcilahine inhibits ribonucleotide reductase and competes
with 2'-deoxycytidine Iriphosphate for incorporation into DNA. These effects produce cell-cycle-specific cytotoxicity.
-
o
Gemcitabine has become a first-line treatment for locally advanced and nictastatic adenocarcinoina of the pancreas. Trifluorothymidine (Trifluridine) was designed by Heidel-
berger as an antimetaholite of thymine.'°' The rihoside is csseiilial because mammalian cells are unable to convert thymine and certain analogues into thymidinc and its analogues.
SHfl
Thymidine kinase converts trilluorothymidine into trifluorothymidylic acid, which is a potent inhibitor of thymidylate
synthetase." In contrast to the stability of most trilluoroR
Enzyme
The tetrahydrofuranyl derivative of 5-Iluorouracil. tegafur was prepared in Russia.101' It is active in clinical and less myclosuppressive than 5-fluorouracil. It has
castrointectinal and CNS toxicity, however. Tegafur is clowly metabolized to 5-fluorouracil: thus, it may he considfred a prodrug.'°7
methyl groups. that of Irifluorothynsidylic acid is extraordinarily labile. It reacts with glycinc to give an amide at neutral pH.' 0 Kinetic studies have shown that this reaction involves initial nucleophilic attack at position 6. followed by loss of HF to give the highly reactive difluoromethylenc group.°' Glycine then adds to this group and hydrolysis of the remaining two fluorine atoms follows (Scheme 12-to. The interac-
tion of trifluorothymidylic acid with thymidylate synthetase apparently follows a similar course. Thus, after preincubation. it becomes irreversibly hound to the enzyme. and the kinetics are tlonconnpetitive."°
IF 0
Cytosine arahinoside was synthesized in 1959h2 and later found as a fermentation Its structure is noteworthy in that the arabinose moiety is epimeric at the 2' position
with ribose. This modification, after anabolism to the iiiphosphate. causes it to inhibit the conversion of cytidylic acid to 2'-dcoxycytidylic acid.' '' For a number of years, this
inhibition was believed to be the main mode of action of cytosine arabinoside triphosphate: however, it was shown
Tegatur (Etorafur)
was designed rationally as a tumor-selective n,l tumor-activated prodrug of 5-fluorouracil, which would kss likely to produce severe diarrhea. It is a carbamate of 5'-deoxy.S-fluorocytidine. On oral administratie. ii is converted into 5'-deoxy-S-fluorocytidine by cytideaminase. which is in higher concentration in many urors than in most normal tissues, with the notable excep-
st liver. Activation to cytotoxic species by thymidinc occurs preferentially at tumor sites.10° Dcthis complex activation process. capecitabine still cxNbns sonic of the significant toxjcjties of 5-Iluorouracil. NHCOC5H11
recently that various deoxyrihonucleosides were just as effective as cytosine arahinoside in reducing cellular levels of 2'-deoxycylidylic acid.' Other modes of action include the inhibition of DNA-dependent DNA polymerase' II. and miscoding following incorporation into DNA and RNA."7 Cytosine arabinoside is readily transported into cells and phosphorylated by deoxycytidine kinase. It acts predominantly in the S phase of the cell cycle. Tumor cell resistance is based
on low levels of deoxycytidine kinase and the elaboration of deaniinases that convert cytosinc arabinoside into uridine
Partially purified cytidine deaminase is inhibited by tetrahydrouridine.''°
NH2
N
/ HO -
Cylarabine (Cytosine arabinoside) HO
Ancilab,ne (CyCloCyildIfle)
OH
Capecitabine
Gemcitabine
A new analogue of cytosine urabinoside is cyclocytidine (ancitubine). This analogue apparently is a prodrug that is slowly converted into cytosine arahinoside. It is reported to
408
Wilson tind
of Organic Medicinal €nijl l-'/,ar,na(-e,aital Chemistry
F
C—F
II
H2NCH7CO2H
I O
II
HN
I
C—NHCH,CD.H -
H7NCH2CO.,H
HO TritIur,dine
tlnlluorothyrnldne)
HO
Scheme 12—8 • Reaction of trifluorothymidine with glycine.
be resistant to deamination and to have a better therapeutic index than the parent compound'21 A number of pyrimidine nucleosude analogues have one more or one less nitrogen in the heterocyclic ring. They are known as azapyrimidinc or deazapyrimidine nuclcosides. 5Azacytidinc was symhesized in 1964 by Sórm in Czechoslovakia'21 and later was isolated as an antibiotic by Hanka.'22 The mode of action of this compound is complex. involving anabolism to phosphate derivatives and deamination to 5azauridinc. In certain tumor systems. it is incolVorated into nucleic acids, which may result in One of its
main effects is the inhibition of orotidylate decarboxylase
for compounds that might inhibit these deaminases. In ory, a potent dea,ninase inhibitor would produce a tic effect on the antitumor activity of the antimctabolite.eser though it might not be active itself. Two types of
inhibitors have emerged recently. One type is the analogue in which the pyrimidine ring has been expanded
to a seven-membered ring. The first example of this was 2'-deoxycoli.rntycin (pentostatin). an unusual nudcs' side produced in the same cultures as the antibiotic (a
mycin.'26 It strongly synergized the action of against organisms that produce deaminases. In trials it showed a synergistic effect on the
4
(Scheme 12-9). which prevents the new synthesis of pyrimidine nucleotides.'24 Tumor resistance is based on decreased phosphorylation of the nucleoside. decreased incorporation into nucleic acids, and increased RNA and DNA polymerase
adenine arabinoside and cytosine arabinoside. A sectni type of adenosine deaminase inhibitor has the adeiiirv
Other pyrimidine nucleoside antagonists that have received clinical study include dihydro-5-azacytidine
site of the enzyme and take advantage of strong to adjacent lipophilic regions.'27 El-INA is an cxampk ii a rationally designed inhibitor.
and
portion unchanged but is modified in the ribose Such modifications have been designed to probe the ada
NH2
NH2
H OH
N
HO—CH,
HO—Cl-I?
0 HO OH HO AzacdOno
HO'' 2'.Deoxycotormycin
Resistance to purine and pyrirnidine antimetabolites, such as adenosine arahinoside and cytosinc arabinoside. by neoplastic cells that produce deaminases has stimulated a search
C6HI3CHCHCHa OH EHNA
After the discovery of folic acid, a number of based on its structure were synthesized and tested as
Chapter 12 • .4iiIineop!a.ctir A,,'eIit.l
409
0 Aspartoto Transcarbamylase
+
H7NOH C02H
NH2 CO2H
Carbamoylaspartic Acid
Carbarnoylphosphate
J
0
0
H20,P
0 NAD Dihydroorolale
HO
Dehydrogeriaso
OH
CO2H
CO2H Orolic Add
Ofotidyfrc Acid Decarboxylase
0
0 HN
NH2 Gkjtamino
H409P3
Uridine Tripliosphate
Acid
Cytidine Triptiosphale
Scheme 12—9 • De novo synthesis of pyrimidine nucleotides (simplified),
The N'°-methyl derivative of folic acid was found activity. Antitufinally was found for the 4-amino-4-deoxy deaminopterin. and its N '°-methyl homologue. metho-
an antagonist, but it had no anlitumor
(amethopterin).' CO2H
0
Foiic Acid
Methotrexate and related compounds inhibit the enzyme dihydrofolate reductase. They bind so tightly to it that their inhibition has been termed p,seudoirresersibh'. The basis of this binding strength is in the diaminopyrimidine ring, which is protonated at physiological pH. At pH 6. methotrexate binds stoichiometrically with dihydrofolate reductase (K, I 0 '°M). hut at higher pH the binding is weaker and competitive with the substrate.'2" Folate acid antagonists kill cells by inhibiting DNA synthesis in the S phase of the cell cycle. Thus, they are most effective in the logarithmic growth phase.'3° Their effect
on DNA synthesis results partially from the inhibition of CO2H
R
NR N
dihydrofolate reductase, which depletes the poo1 of tetrahydrofolic acid. Folic acid is reduced stepwise to dihydrofolic
acid and tetrahydrofolic acid, with dihydrofolic rcductase
Aminoplerin. R
Meihoirexalo.
R
H CH3
thought to catalyze both As shown in Scheme 1210. tetrahydrofolic acid accepts the f3 carbon atom of scrine. in a reaction requiring pyridoxal phosphate. to give N5.N'0methylene tetrahydrofolic acid. The last compound transfers
410
Wilso,, aiid
Textbook
of Organic Mt'dki,ial and F'harrnacewical CI,emi.orv
Reductase
Teirahydroloiic Acid
Dihydrolohc Acid
0 Thynsdylalo
R=
Syniholase
HOCH2CHCO2H
CO2H
Pyridoxal Phosphate
[
HN HN
5.1 O-Methenyltetrahydrofohc Acid
5.1 O-Meihyionotcirahydrotolic Acid
o
CHO
H 1 O.Forrnyitetrahydrotoiic Acid
5-Formyttelrahydrotohc Acid
Scheme 12—10 • Interconversioris of bk acid derivatives.
a methyl group to 2'-dcoxyuridylate to give thymidylate in a reaction catalyzed by thymidylate synthetase. Dihydrofolic
acid is generated in this reaction, and it must be reduced back to tetrahydmfolic acid beli)re another molecule of thymidylate can be synthesized. It is partly by their effect in limiting thymidylate synthesis that folic acid analogues prevent DNA synthesis and kill cells. This effect has been termed ihymineless death.'
The inhibition of dihydmiolate reductase produces other limitations on nucleic acid biosynthesis. Thus. N5.N'°-mcthylene letrahydmfolic acid is oxidized to the corresponding methenyl derivative, which gives N"-fomiyltctrahydrofolic acid on hydrolysis (Scheme 12-10). The latter compound is a formyl donor to 5-aminoimidazole-4-carboxanside ribonucleotide in the biosynthesis of purines.'3' N-Formyltetrahydrofolic acid, also known as leucovorin and citrovorum factor, is interconveruble with the N'°-formyl analogue by way of an isomerasc-catalyzed reaction. It carries the formimino group for the biosynthesis of formiminoglycine. a precursor
of purines (Scheme 12-6). Leucovorin is used in "rescue
therapy" with methotrexate. It prevents the lethal nlethotrexate on normal cells by overcoming the of tetrahydrofolic acid production. In addition, it inhihitsth active transport of methotrexate into cells and stimulates
efflux)° Recently, it was shown that giving thymidine with trexate to mice bearing Ll210 leukemia increased vival time. This finding contradicts the idea that ilwntia deficiency is the most lethal effect of niethotrexate on it mors. It suggests that the blockade of purinc might have greater effects on tumor cells than on cells.' Consequently, the administration of might protect the normal cells relative to the tumor cdl Unfortunately, the use of such thymidine rescue in clino trials was
Numerous compounds closely related to have been prepared and tested against neoplasms. structural variations, such as alkylation of the amino gncr partial reduction, and removal or relocation of nitrogens. lead to decreased activity. Piritrexim and ma
Chapter 12 • Antineoplaslie Agents
senate are analogues of methotrexate in which one or two nitrogens in the pyridinc ring are replaced by carbons, and the benzoyl glutumic acid chain is replaced by a more lipogroup. Like methotrexate, both compounds inhibit dihydrofolate reductase; however, they do not interact with the reduced folate transport system used by methotrexate. Consequently, they arc active in vitro against some forms of cnethotrexate resistance. Their increased lipophilicity allows npid transport by simple CH3O CH2
411
nutrient for normal cells, many tumors depend on exogenous sources of it. This provides a rationale for the selective action of agents that interfere with the uptake, biosynthesis, or func-
tions of glutamine. In 1954. azaserine was isolated from a Srre,nomvc'es species.'42 It was found to antagonize many of the metabolic processes involving glutamine, with the most important effect being the conversion of formyl glycine ribonucleotide into formyglycinamidine ribonucleotide (Scheme A related compound. 6-diazo-5-oxo-i-norleucine (DON), was isolated in 1956 and found to produce similar antago-
nism)" A study involving incubation with F'4Clazaserine followed by digestion with proteolytic enzymes and acid
CH2__->\
hydrolysis produced S-f "'Clcarboxymerhylcysteine, which showed that azaserinc had reacted covalently with a sulihydryl group of cysteine on the enzyme.'45 DON is a more potent inhibitor than azaserine of this enzyme and of the enzyme that converts uridine nucleosides into cytidine nucleosides."" Although both compounds show good antitumor activity in animal models, they have been generally disappointing in clinical trials.
Piritrexim
Produce Mercaptopurine, USP.
Tnmetrexate
Although the active sites of dihydrofolic reductases from and neopla.stic cells arc identical. Baker proposed u regions adjacent to the active sites of these enzymes
differ. He designed inhibitors to take advantage of differences, thus affording species specificity. One of inhibitors, known as "Baker's antifol." shows activity iaiust experimental tumors that are resistant to methoGluramine and glutamate arc the donors of the three- and
atoms of purines and the two-amino groups They also contribute the three-nitrogen atom he amino group of cytosine'4' (Schemes 12-6 and 12Thus, they axe involved at five different Sites of nucleic biosynthesis. Although glutamine is not an essential
Mercaptopurine, Purinethol, 6mercaptopurine. 6MP. Lcukcrin, Mercalcukin, NSC-755, 6punnethiol, is prepared b; treating hypoxanthine with phosphorus pentasulfide"'7 " and is obtained as yellow crystals of the monohydrate. Solubility in water is poor. It dissolves in dilute alkali but undergoes slow decomposition. Scored 50-mg tablets are supplied. The injectable formulation is in vials containing 500mg of the sodium salt of 6-mercaptopurime, which is reconstituted with 49.8 mL of Sterile Water
for Injection, liSP. Mercaptopurine is not active until it is unabolized to the phosphorylated nucleotide. In this form, it competes with endogenous ribonucleotides for enzymes that convert mosinic acid into adenine- and xanthine-based ribonucleotides.
Furthermore, it is incorporated into RNA. where it inhibits further RNA synthesis. One of its main rnetabolites is 6mcthylmcrcaptopurine ribonucleotide. which also is a potent inhibitor of the conversion of inosinic acid into purines.'48 Despite poor absorption, low bioavailability. and firstpass metabolism by the liver. mercaptopurine has oral activity. Peak plasma levels of about 70 ng/mL are reached I to
0 U
0 .
II
HO2CHCH2OCCHN2 + HSCH2CHCO— —p HO2CHCH2OCCH2SCH2CHCO—
NH—
NH2
NH—
NH2
Azaserine Hyd:otysis
0
0 N2CHC(CH2)z?HCO2H NH2 DON
NH2
412
and
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Suite. ft. ci at.: Dt.sch. Med. Wochcnschr. (08: 190, 978. 453. Schr.ittun. C. H.: 1. Am. Chcm, Soc. 77:6231. 955. 4M. IOrocL. N., ci 81: Ear. J. Cancer Clin. Oncol. 18:1377. 1982. 415, J. A.. ci at.: J. Med. Client 12:236. 1969. D.: U.S. Patent 4.764.614. August lb. 988. Miller. 457. Mignatti, I'.. and Rilkin. I). B.: i'hysiol. Rev. 73:1. 1993. Henkin. i.: Ann,,. Rep. Med. Chew. 28:151. 993. 459. Redwood. S. M.. ci al.: Cancer 69:1212. 1992. 48). Nakajinia. M.. ci al.: J. Biol. Client. 2661 lS(:9661. 1991. 461 Fiseaslein. K.: Pharniacol. Titer. 49:1. 1991. Moses. M. A.. ci il.. Science 248:1.8)8, 1990. 463 Sakinnoto. N.. ci all.: Cancer Km. 51:903. 1991. 484 Majone. 1. F... ci at.: Science 247:77. 1990. 465 Folkinan. J., ci al.: Science 243:1490. 1989. iCe WOks. J. V.'., ciii.: Proc. Ant. Ascsw Cancer Re.s. 31:60, 1990. 467 Mitchell. M. A.. and \Vilks, J. W.: Anna. Rep. Med. Client. 27:139.
481. Venter. J. C.. ci at.: Science 291:1360. 2180. 482. Marx. J.: Science 289:1670. 2(8%) 483. MacDeath. Ci.. and Schrcibcr. S.: Science 289:1760. 21881.
(992.
469. lngber. D.. eta,).: Nature 348:555, 1990. 44)4 Ranianathan, M.. ci at.: AnI,seose Res. I)e'. .3:3. 1991 Kid>. J. S., Anna. Rep. Mcd. Chcun. 29:297. 994. ill. l.csnikowski. 7,. i.: Biorg. Chem. 21:127. 993. Vanna, R. S.: Syn. Leli. 1993:621. 1993. Fruehier. B. C.. Ct al.: Tetrahedron Len. 34:1(8)3. 1993. 474. Ranta.samy. K. S.. ci al : Tetrahedron Lint.: 35.215. 1994.
86(1. 21811.
SELECTED READING Dorr. K. I., atttd Von lioll, I), I).: Caticcr Chemotherapy Handbook. Nor walk. CT. Appleton & Lange. 1994. Fnye. W. 0. led.): Cancer Chernothcrapcutic Agents Washington. IX'. American Chemical Society. 1995. Hail. T. C. ted.): Prediction tiC Kcspttnse lit Cancer Chemotherapy. New York. Alan K. Liss. 1988. Hickman. J. A.. and Trittoit, 1. R.: Cancer Chenutther.tpy. Oxford. I)tackwell Scientific Publications. 1993. Kepplcr. B. K. lcd.): Metal Complexes in Cancer Cbetnotherampy. York, Wcinhcim. 1993. Oldhatm, R. K. led.): Principles oF Cancer Bit8herapv. New York. Raven Press. 1987.
Pined,,. II. M.. mind Giaccone. (4. teds.): Drug Resistance in die Trcalntetii olCunccr. Cantbridge. United Kittgdoitm. Caitnbridgc University Press. 1998.
474a. Lingner. J.. ci a).: Science 276:561. '477.
Powis. Ci.. and Hacker. M. leds.): The Toxicity ol Atiticancer Drugs New York. Pcrgumon Press. 1991. Schilsky. K. L.. Milano. (3. A.. and Ralain. M. J. (eds.): Princiltles ol Aittineoplastic Drug t)cveloptnent and Pharntacitlogy. New York, Marcel Dckker. 19%. Teicher. B. A. ted.): Cancer Citeinoiherapeutics. Tiatowa. N. 3.. Iluittatta
P7. Hamilton. S. Ii.. ci at.: Biochemistry 36:11873. 1977. P4.. Vilieelhouse. K. 'r.. ci a).: J. Am. Chem. Soc. 120:3621. 1998. I'9 Petty. P. J.. cliii.: 3. Med. Chew. 41:4873. 1998.
Wright. Ci. L.. Jr.: Mitnocloital Anitbodjcs and Cancer. New York. Marcel Dekker. 1984.
475. Raiajcuak. >.I. Z.. ci at.: Proc. Nail. Acad. Sci. U. S. A. 89:11823, 992.
Press. 1997,
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13
Agents for Diagnostic Imaging TIM B. HUNTER. 1. KENT WALSH, AND JACK N. HALL
Diagnostic imaging encompasses a group of techniques used in the diagnosis and treatment of disease. These techniques
often use chemical agents to improve the information pro-
vided in the imaging. This chapter is a discussion of the pharmacology, chemistry. and physics of those agents used in medical imaging. Medical imaging techniques often present less risk to patients than direct surgical visualization. Also, they often pro-
vide information or treatment methods that are simply not available by any other means. What these techniques have in common is that the information is often (but not always)
displayed as an image for interpretation by a physician trained to evaluate the meaning of the image in the context of pathophysiology. Also, all of the techniques use physical phenomena (electromagnetic radiation, ultrasonic waves) that cart pass through tissue to convey the internal infonnation necessary to create an image. From that point, the techniques of medial imaging diverge in their physical means. methods, and the information that they can provide. Medical imaging began with Roentgen's discovery of xrays in 1895, and it has been the domain of diagnostic radiology since then. In its earl iest days. the specialty of radiology used x-rays to produce images of the chest and skeleton. At the present time, diagnostic radiology uses ionizing radiation (x-rays), magnetic resonance imaging (MRI) techniques, radionuclides (nuclear medicine), and high-frequency sound
waves (ultrasound) to produce diagnostic images of the body. Today, radiologists and other physicians also use diagnostic imaging techniques to guide themselves in interventional procedures, such as organ biopsy or abscess drainage.
INTRODUCI1ON TO RADIATiON Radiation is the propagation of energy through space or mat-
ter. In chemical reactions, only the valence electrons of an atom are affected, and the nucleus remains unchanged. Nuclear reactions may result from bombardment of' a stable nucleus with high-energy particles or decomposition of an unstable nucleus. The nuclei of atoms are of two kinds: stable
and radioactive. Radioactive nuclei have more internal energy than nuclei with a stable arrangement of protons and
neutrons. They obtain stability by emitting energy in the form of particulate and electromagnetic radiation. Ionizing radiation is radiation that when interacting with matter can cause changes in the atomic or nuclear structure of matter. The first type of ionizing radiation is particulate, positron proton which includes alpha (a). beta (p). and neutron (n) particles. Radiation is energy in the form of kinetic energy and on the atomic scale is usually measured in electron volts (eV). By definition, an electron volt is the
454
energy needed to accelerate an electron across a potential difference of I volt. The second type of ionizing radiation is called electromagnetic radiation. Electromagnetic radia tion is an electric and magnetic disturbance that is propa. gated through space at the speed of light. This type of radii tion has no mass and is unaffected by either an electrical or magnetic field because it has no charge. These propeniet are shared radio waves (I0'° to l0" eV), microwaaet to 10-- eV), infrared (102 to I eV). visible light U to 2 eV), ultraviolet (2 to 100 eV). or x-rays and eV). The various forms of electromagndic rays (100 to radiation differ in their frequency and, therefore, their en ergy. The energy of electromagnetic radiation can be calcu
lated in electron volts from the following equation:
E=
=
eV-scc). where /t is Planck's constant (4.13 X the frequency (hertz). c is the speed of light (cmlscc), aid
A is the wavelength (cm). The difference between x-rays arid
y.rays is based on where they originate: x-rays come fore outside the nucleus, while y-ruys originate in the nucleusol
an atom. X-rays and y-rays can exhibit some paniculas properties, so they are sometimes called photons. Applying a very high voltage (20.000 to 150.000 to a glass vacuum tube that contains a cathode and a anode produces A-rays used in diagnostic radiology (Fig. Lt
I). The cathode is a filament that is heated to a veiy temperature, which provides a copious source of elcetroet The electrons are accelerated toward the positively chargal anode (tungsten). When the accelerated electrons strike Is anode (called the target), A-rays are produced. The tion of x-rays is a continuous spectrum, and the low-eneip
x-rays, which will not travel through the body to the a filter (aluminum). An invalualdi modification of the x-ray system is fluoroscopy. This ity allows one to visualize organs in motion, positiou ft patient for spot film exposures. instill contrast media ia hollow cavities, and, most importantly, insert catheters it arteries. Figure 13-2 shows a schematic of a system.
With conventional radiography and with computed mography (CT) (sometimes called computed axial phy ICATI) scanning, organs and tissues are made according to how well they attenuate x-rays. The attcnualia of x-rays by tissues is a complex process that depends a many factors, including the energy of the x-ray beam aid the density of the tissue. Bone has an average densit) 0 about 1.16 g/cm3, which accounts for its ability ta alan most of the radiation it encounters. CT scanning (Fig. 3) uses ordinary x-ray energies for imaging but uses cornidt
Chapter 13 • Agenca for Diag,ws:ic Imaging
455
ommendation of the International Union of Pure and Applied
Chemistry, the following notation should be used for the identification of a nuclide: Example:
where X is the symbol of the chemical element to which the nuclide belongs. A represents the atomic mass (number of neutrons plus the number of protons), and Z represents the
atomic number (number of protons). The right side of the element is reserved for the oxidation state, and N represents
the number of neutrons. For most medical applications, it suffices to indicate the element chemical symbol and the mass number (i.e.. 1311, 1-131. or iodinc-13l). The radionuclide at the beginning of' the decay sequence is referred to as the parent, and the radionuclide produced
by the decay is referred to as the daughter, which may be stable or radioactive. There are five types of radioactive decay, distinguished according to the nature of the primary radiation event. A radioactive nucleus may decay by more than one method. The dominant method at any given time depends on such factors as the size of the nucleus and the balance of protons and neutrons. The types of decay described below are in order of how commonly they are used in current diagnostic nuclear medicine practice:
-9
Film
1. Isomerlc transition (IT). Isomeric transition is a decay process involving neither the emission nor the capture of a particle. The nucleus simply changes from a higher to a lower energy level by emitting y'rays. Therefore, both mass number and atomic
rigure 13—1 • Schematic diagram of an x-ray tube producing
that pass through the patient and expose the photogiaphic film. The photographic film will not stop the x-rays. a a plastic screen coated with fluorescent particles that are by the x-rays emits light to expose the film within a
TV Monitor, V,deo System
film cassette. As x-rays pass through the body, some ci them are scattered, so a moving grid device composed of altemaling strips of lead and plastic decreases the scattered xrays that degrade the image.
reconstructions to produce images of the body it the axial and other planes. In the process, it can increase lie visibility of small differences in the radiographic dcnsi-
between tissues to a far greater extent than ordinary adlographic film can.
undergoing transformation processes. ailed radioactive decay, in most cases involve transmutaion of one element into another. A nucleus may undergo decays before reaching a stable configuration. A nu-
Optical System
Radionuclides
Image Intensifier
dearparticle, either a proton or a neutron, is called a nucleon. A igecies of atom with a specified number of neutrons and
in its nucleus is called a nuclide. Nuclides with the oar number of protons and a different number of neutrons ne called isotopes. Nuclides with same atomic mass are oiled isobars. Nuclides with the same number of protons codatomic mass but at two energy levels are called iso,ners.
The nucleus has energy levels analogous to the orbital ileciron shells but at a higher energy. The lower energy level
i.called the ground (g) state, and the highest energy level called the metasiable (m) state. Nuclides are all species if elements, of which there are about 265 stable nuclides, 331) naturally occurring radionuclides. and more than 2,500 sliticially produced radionuclides. In accordance with a rec-
Screen
X-ray Tube
Figure 13—2 • Schematic diagram of a fluoroscopic unit with the x-ray tube located behind the patient and a fluorescent screen—image intensifier system positioned on the opposite side. Amplification of the faint fluorescing image by the image intensifier increases brightness level and contrast. The real-time fluoroscopic images can be shown on a television camera for
convenient viewing during the examination and stored on videotape, video disk, or computer for later viewing without distortion or destruction of the images.
456
Wilson and Gi.si'old's Texlho,,k of Organic Medicinal and J'hannaeeutital Che,ni.t try
Original Image Reconstruction
E= where in the case of an electron. E represents energy cquivalcs:
to mass (,n = 9.109 X 10" kg) at rest, and e is the speed of light (3 x rn/see). By using the proper units it can be shown. that the mass of an electron is equivalent 100.511 MeV. called annthila,ion radiation. It is used in a specialized imaoinf technique called ,io.sirron emission U'ETI.
+ v-ray
Example:
fir + e —2y-rayslO.5Il Mcvi 4. Beta-partIcle emission fifl. The nucleus emits a negative eke. Iron when a ncuirotl changes to a proton. A y-ruy may or not accompany the emission of a fi particle.
+ ir + v-ray
Example:
5. Alpha-particle emission (a). The nucleus emits an a pattklc which consists of a helium nucleus without the clectroni. 11th: emission of the a particle leases the nucleus in an excited state, the excess energy is liberated in the tirm of a v-ray, Example:
+
+ v-ray
CHARACTERISTICS OF DECAY It is impossible to predict when an individual atom ui: radionuclide will decay. In quantitative terms, however. thi.
transformation occurs at a rate that is characteristic of specific radionuclide and is expressed as its physical life, This is the time in which one-half of the original numlst of atoms decay. The activity of radionuclides can be ci pressed in three ways: (a) in curies I Ci). tnillicuries(rnfi.
Radiation Detectors Figure 13—3 • Schematic diagram of a computerized axial tomography (CAT) system that produces thin cross-sectional im-
ages of the body. An x-ray tube rotates around the patient, and lhe transmitted s-rays are detected by a circle of moving radiation detectors. The absorptions of x-rays by tissues of dit' ferent densities are assigned numerical values (CT numbers). The computer uses complex algorithms to reconstruct an anatomical cross-sectional image on a television monitor.
or microcuries (1aCi): (h) in disintegrations per second (dpiJ and (c) in becquerels (13q: I Bq = I dps). A curie is lit
quanhity of any radionstclide that decays at a rate of 3,7'
lOin dps. This number was chosen for a hishorical rn son—this is the number of disintegrations per second in g of radium. The international systent of Units has adupini the beequerel us the official Unit of radioactivity. but lv curie is still widely used, and we will use this unit iii additsi to the official unit. A relevant conversion facior to remembr
is the lollowing: number remain unchanged. The daughter nucleus is the same chemical element as the original nucleus. The original nucleus betore the transition is said to be in a metastable (ml state. Example:
+ y-ray
2. Fiectron capture decay fEC). The nucleus captures an electron
I
millicuric lntCi) = 37 megabeqtterels (MUg)
The basic equation for radioactive decay is cxprcssedn.
follows in terms of atoms:
N, = N, e1'
from thc electron cloud of the atom (mainly the K shell), and a proton becomes a neutron. Example:
pai/i v-ray
The nucleus emits a positive electron 3. Positron emission when a proton changes to a neutron. A v-ray may or may not accompany the emission of the positron. A positron (particle of antimatter) emitted from the nucleus loses its kinetic energy. however, by interacting with surrounding atoms. It finally combines with a free electron from one of the surrounding atoms in an interaction in which the rest masses of both particles are given up as 2 v-rays of 0.511 McV emitted at 180° to each other. Einstein's theoty of relativity states that macs and energy arc equivalent and is represented by the following equation:
N, (number of atoms at time I) and N0 (number of alor at time 0) can be replaced, however, with activities: A, = .4,,
Ar
where A11 is the original activity in Ci. mCi. or PCi, the ttctivity at time :. A is the decay constant (= 0.6931i,. the physical half-life); and is the decay factor. Ancsrn pIe of a radioactive decay calculahion follows: A sample of iodide had an activity of 200 jzC' U May 14 at 12 noon C.S.T. What is the activity on May 1St E.S.T.? (Note: Calculations of elapsed time must also
Chapter 13 • Agenl.c fur Diagnasth' Imaging
457
indicate variations in time zones—elapsed time in this case is
the indirect effect, involves aqueous free radicals u.s interme-
26 hours)
diaries in the transfer of radiation energy to the biological = 13.2 hours) A =
10.693 X 26 hours' 13.2 hours
2(X)
"L
]
A = (200
fr-I
A = (2(M)
(0.255)
A = 51.0 /2Ci
BIOLOGICAL EFFECTS OF RADIATION The absorption of ionizing radiation by living cells always iwduces effects potentially harmful to the irradiated organnm. An undesirable aspect to the medical use of these types
(radiation is that a small number of the atoms in the body will have electrons removed as a result of the energies ef these photons. Radiation that does this is often called :wii:ing radiatio,, and is damaging to body tissues. Therein using ionizing radiation, as in using other pharmaagents, the risks must be balanced with the medical provided for the patient.
The amount of radiation energy absorbed by tissue is absorbed dow and is specified in rude or sillirads. A dose of I rad implies 100 ergs ol energy abper gram of any tissue. The unit of exposure for xtl)5 and y radiation in air, the roentgen, is used to specify cidbtion levels in the environment. (One roentgen is the aaunt of radiation that produces I electrostatic unit of iharge of either sign per 0,001293 g of air at STP.) The ailed radzatwn
system of Units (SI) has adopted the gray (Gy) the
(I Gy = 1(X) rads). but again we will use
)icmoretraditionul units. In the case of x-raysor yradiation medical diagnosis, the roentgen and rad turn out to be rsmerically equivalent. The major difference between eleccumagnetic radiation (x-rays or y-rays) and particulate type a particles) lies in the ability of electromagretic rays to penetrate matter. Whereas fi particles travel no a few millimeters before expending all their energy. xdistribute their energy more diffusely and travel nJ several centimeters of tissue. Therefore, particles kliver highly localized radiation doses, whereas x- and yins deliver more uniform doses in a less concentrated way 'broughoul the irradiated volume of tissue. The radiation .be of particles is more useful for a therapeutic dose of a idionuclide but not for a diagnostic dose. When cells are icidiated. damage is produced primarily by ionization and
molecules. All biological systems contain water as the most abundant molecule (70 to 90%). and radiolysis of water is the most likely event in the initiation of biological damage. The absorption of energy by a water molecule results in the ejection of an electron with the formation of a free radical ion (H20 ' ). The free radical ion dissociates to yield a hydrogen ion and a hydroxyl free radical (HO). The hydroxyl free radicals combine to form hydrogen peroxide (H202). which is an oxidizing agent. In addition, hydrogen free radicals (H can form, which can combine with oxygen (02) and form a hydroperoxy-free radical (HO2). These two reaction intermediates are very reactive chemically and can attack and alter chemical bonds. The only signiimcant "target" molecule for biological damage is DNA. Types of DNA damage include single- and double-chain breakage, and intermolecular or intramolecular cross-linking in the doublestranded DNA ntolecule. With the direct effect of radiation. the damage makes cell replication impossible, and cell death
occurs. In the indirect effect of radiation, if the damage is not lethal but changes the genetic sequence or structure, mutations occur that may lead to cancer or birth of genetically damaged offspring. Some effects of radiation may develop within a few hours: others ntay take years to become apparent. Consequently, the effects of ionizing radiation on human beings may be classified as somatic (affecting the irradiated person) or genetic (affecting progeny). Radiation dose can only be estimated and its "measurement'' is called radiation dosimetry. In the case of x—ray
exposure. most radiation "doses" in the literature are described as the entrance exposure (in rocntgcns per minute) to the patient. In diagnostic nuclear medicine procedures. patients are irntdiated by radiopharmaceuticals localized in certain organs or distributed throughout their bodies. Since the radionuclides are taken internally, there are mummy variables. and the radiation absorbed dose (r.a.d. or mdl to indi-
vidual patients cannot be measured hut only estimated by calculation. The methods of calculating the absorbed dose to patients from radiopharmaceuticals were changed in 1964 and then revised by the Medical Internal Radiation Dose
(MIRD) Committee under the auspices of the Society of Nuclear Medicinc in 1991. Although the effects of radiation arc not totally understood, the benetits associated with low doses of radiation almost always outweigh any potential risks to individual patients. A large number of scientific and advisory groups have published risk estimates for ionizing radiation, but the most widely quoted is report number 5 of the National Academy of Sciences Committee on the Hiological Effects of Ionizing
're
radicals. Particles produce damage by ionization. produce damage by free radicals. hereas x-rays and
Radiation (BEIR-V).' Under normal circumstances, no radiation worker or patient undergoing diagnostic investigation by radiopharma-
free radicals are atoms or molecules with an unpaired elec-
ceutical or radiographic procedures should ever suffer from
inn.
effects of large doses of radiation were derived from studies of the atomic bomb survivors at Hi"hima and Nagasaki. Radiohiologieal damage from large lies of ionizing radiation can be caused by two different One mechanism is the direct effect of radiation. The
rwhich damage results front absorption of radiation energy in a critical biological site or target. The other, called
any acute or long-term injury. Typical radiation doses to patients from radiophannaceuticals are similar to. or less than, those from radiographic procedures. The tim-st artificial radionuclide (phosphorus-30) was produced by the French radiochemists Frederic Joliet and Irene Curie. Nuclear medicine became a specialty in 1946 when radionuclides became available front cyclotrons and nuclear reactors. In many medical centers, nuclear medicine is con-
458
Wil.con and Gisvold.c Textbook of Organic Medicinal and Pharmaceutical Chemi.ctrv
sidered part of diagnostic radiology, although in some lo-
1V Monftor
cales it may be a freestanding discipline or reside in a pathology or intemal medicine department.
RADIONUCLIDES AND RADIOPHARMACEUTICALS FOR ORGAN IMAGING
Computer
Medical science provides a framework or paradigm in which to understand disease and to maintain health. Nuclear medicine is the branch ol medical science that contributes to medicine by the use of the radiotracer method for diagnosis and
use of in vivo unsealed radioactivity for therapy. Nuclear tnedicine involves the administration of radioactively labeled compounds to trace a biological process. This process may be mechanical (gastric emptying, blood flow. cardiac wall motion) or a variety of other physiological functions. Within the concept of a 'radiotracer" is the implication that the agent administered will not disturb the function-
PM Tubes
Nat Cyst&
ing of the process you wish to examine. In nuclear medicine, this concept is used to trace physiological processes in vivo and then compare them 10 known normal images or levels. These are then evaluated with a knowledge of pathophysiology to allow diagnosis of disease. The data can also be used
to follow the patient for improvement after treatment. In clinical practice, nuclear medicine also makes use of in vitro diagnostic methods (radioimmunoassay) and in vivo radiopharmaceutical therapy. These last two are not further addressed in this chapter, and there is minimal discussion of investigational diagnostic radiotracers, The specialty of nuclear medicine did not become available to the private hospital until the l960s. after the introduction of the molybdenum-99hechnetium-99m generator and the gamma (scintillation) camera developed by Hal Anger (Fig. 13-4). The most common use of nuclear medicine is to image the distribution of radiopharmaceuticals in specific tissues or organ systems with a scintillation (Anger) camera for diagnostic purposes. Fundamentally, these instruments or cameras allow in vivo detection and localization of radiotracers. The purpose of the gamma camera is to record the
location and intensity of the radiation within the imaging field. Radiation in the form of gamma photons (occasionally x-
rays) initially enters the camera through the collimator.
Lead Cofllmator 'v-rays
Figure 13—4 • Schematic diagram of scintillation camera (Anger) system with a multihole lead collimator attached eliminate scattered v-rays), which is used to visualize tissuer and organs after a diagnostic dose of a radiopharmaceuricaf administered.
video monitor. The images obtained with the scintillailor camera are called scintigrams. scintigraphs. or scans. No clear medicine imaging studies involve the generation ul images that represent the functional status of various argue in the body. Especially when interfaced with tems, information regarding dynamic physiological pasuim eters such as organ perfusion, metabolism, excretion, ani the presence or absence of obstruction can be obtained. Fiç ure 13-5 demonstrates a norma! dynamic function stud) ci
the liver, made by using Tc-99m mebrofenin and the lation camera. Images can be acquired of one organ or
the whole body by moving from head to toe.
made holes. It covers the detector crystal. The purpose of
Cross-sectional images of organs can be obtained by ing a position-sensitive scintillation camera detector abet the patient. This type of procedure is called single plot"
the collimator is to decrease scattered radiation and increase
emission computed tomography (SPECT). which is tlte caio
the overall resolution of the system. Photons that are not blocked by the collimator then enter a large sodium iodide (with a small amount of thallium) crystal that absorbs v-
crystal are photomultiplier (PM) tubes that convert the tight
terpart of CF or CAT scans in diagnostic radiology. Fifr: 13-6 is a schematic of a SPEC!' system. The majority i SPECF systems use one to three scintillation camera tore that rotate about the patient. SPEC!' is routinely when imaging the brain or heart to demonstrate sional distribution of radioactivity in these organs. Figm 13-7 depicts a SPECT myocardial perfusion scan of th:
flashes to electrical pulses proportional to the amount of
heart.
tight. To localize the original source of the photon (and create an image), a computer assigns x—y spatial coordinates to the various v-rays coming from the patient and stores this information in a matrix. After collection, the digital image is converted into an analogue video signal for display on a
A newer modality for imaging uses multiple dctccs.r heads to image positron-emitting
which usually is a sheet of lead with multiple small, precisely
rays. The absorbed energy in the crystal is emitted as a flash
of light (called a scintillation), which is proportional to the
energy of the v-ray. Coupled to the back of the Nal(Tl)
PET (Fig. 13-8). Many biologically important moleculestlit are physiologically identical with the nonradioactivc coc pound can be radiolabeled with positron-emitting radioc;
Chapter 13 • Agents for Diagnoslic Imaging
459
Figure 13—5 • Dynamic study of the liver and biliary system with a gamma camera. This is a normal study after injection of Tc99m mebrofenin, with each image a 3-minute time exposure. The study was done on a patient with suspected acute cholecystitis (a blockage of the duct to the gallbladder) If the patient had acute cholecystitis, the radiotracer would not have entered the gallbladder. The arrow in frame 16 shows the normal location of the gallbladder. TV Monitor
,— Computer
Figure 13-6 • Schematic diagram of a rotating triple-detector scintillation camera system for single photon emission computed tomography (SPECT) demonstrating a "cold spot lesion in the brain on the sagittal view (open arrow).
460
Wi/sc,,: and (Ji.cvold s Texthook of Organic Medicinal cnn! l3lusnnacns:ical Chemtc:rv —.,--
—
-
—a—-
9 ç) 4,
F
1
--
IF —
Ap?!
Figure 13—7 • SPECT myocardial perfusion study using thallium-201 as the radiotracer, SPECT images are
three-dimensional and are often viewed in tomographic slices. The long arrows indicate the abnormally diminished myocardial perfusion in the anterior wall of the left ventricle during stress (exercise or logical), compared with that of the same patient during rest. The stress and rest images are matched in spatial location for easier comparison The single short arrow indicates an additional abnormal area in the
inferior portion of the left ventricle. The abnormalities indicate that the patient has a high likelihood of significant coronary artery narrowing, which can be confirmed by coronary angiography.
TABLE 13-1 PET Radiotracers
Positron Emitter TV Monilor
Raóabon
Fluorine- 18 (F•IS)
Radiotracer F- i 8iialoperidol F- i S Ilunmdecixyglucocc (I•Th1
F-IS flunrodopa
F I K flurueihylspipcronc
/ 1'
F-18 flunrouracil
I L.Corp,jtor
Niirogcn-13 (N-i 3)
N-li ammonia
Carbon-I I
C-Il Acetate
C-I I curfenianil
C-il cocaine C-Il Depranyl C-li kucine Figure 13—8 • Schematic diagram of a PET imaging system with multiple scintillation detectors that localize the positron decay along a line. By using multiple position-sensitive detectors around the patient, the annihilation photons are acquired along many parallel lines and many angles simultaneously with four rings of detectors (only one ring shown). After use of reconstruction algorithms, the internal distribution of the radioactivity can be determined and displayed on a cathode ray tube.
Oxygen 15
inedlion
C-Il
niethyispipemne
C-I 1 racloprlde 0-15 waler
0-15 0.1$ Rubidiu in -1(2
lilt
C- 11
oxygen carbon
Rubidiuni.$2
dioxide
Chapter 13 • Agents for Diagnostic Imaging
461
1'
FIgure 13—9 a PET whole-body images performed to detect metastases after injection of 4 mCi (148 M8q) of fluorine f '8FJ-2 -f luoro-2 -deoxy-o-glucose (F-i 8 fDG). A. Normal whole-body PET image (coronal view) obtained on a patient with lymphoma after treatment
with chemotherapy and radiotherapy.
4
The patient fasted for 12 hours to ma intam the blood glucose level between 80
and 140 mg/i 00 ml. If the blood glucose level is not in that range, F- 18 FDG
uptake is decreased in the tumors because the mechanism of uptake is an increased rate of glycolysis. (Note increased brain and cardiac uptake because of high glycolytic rates in these organs.) B. PET whole-body image tamed with the same technique on a patient with pancreatic carcinoma. Abnormal sites of F-18 FDG uptake are seen in the upper abdomen, posterior mediastinum, and left lower neck (ar-
S
rows) consistent with neoplastic in-
S
volvement. (Note increased brain, but not cardiac, uptake of F-18 FDG in this patient who had a desirable blood glucose level for tumor imaging.)
B
dides such as carbon-Il (:in = 2 minutes), nitrogen- 13 (tic
20 minutes), oxygen-S (,a
where Cd- 1 1 2 is the stable target material, a proton (p) is
10 minutes). and fluorine-
the bombaSing particle. two neutrons (2n) are emitted from Uie nucleus, and In- 1 I 1 is the radionuclide produced. The introduction of radionuclide generators into nuclear medicine arose from the need to administer large doses of
= 110 minutes). Table 13-I shows examples of several positron radiouacers that have been investigated in iS Ku?
'it scientific literature. Figure 13-9 shows PET whole-body images from patients cancer management modality. -
PRODUCtION The radionuclides used in nuclear medicine are artificially This is accomplished when neutrons. protons. a
or other particles impinge on atomic nuclei and üntiate a process of nuclear change. The artificial production
(a indionuclide requires preparation of a target system. indiation of the target, and chemical separation of the radio:aclide produced from the target material. The radiochemiJisconvcrted to the desired radiopharmaceutical and qualassurance of the physical. chemical, and pharmaceutical
(i.e., sterility and apyrogenicity) of the final product s obtained.
The systems used for practical production of
rilionuclides are a nuclear reactor, cyclotron, or radioisogenerator.
The shorthand nuclear physics notation of a cyclotron proiicdon reaction is as follows:
(p.2n)'U
Zn
a short half-life radionuclide to obtain better statistics in imaging. in consideration of radioactive (parent and daughter) pLLIrS. we can distinguish two general cases, depending on which of two radionuclides has the longer half-life, lithe parent has a longer half-life than the daughter, a state of so-
called radioactive equilibrium is reached. Thai is. after a certain time, the ratio of the disintegration rates of parent and daughter become constant. in the second case, if the
parent half-life is shorter than that of the daughter. no equilibrium is reached at any time. Therefore. the general principle of the radionuclide generator is that the longerlived parent is bound to some adsorbent material in a chromatographic ion exchange column and the daughter is eluted from the column with some solvent or gas. There are more
than 100 possible generator systems for clinical use, but there is only one in routine use in nuclear medicine (the molybdenum-99/technetium-99m system). All of the molybdenum-99 at the present time is obtained as a fission product
of uranium-235 in a nuclear reactor. (n, fission) —4'Mo ÷ other radionuctides
By use of elegant inorganic radiochcmistry, the molybdenum-99 is separated from the other radionuclides. Molybde-
462
Wilson and GisrokFs Texthook of Organic Medicinal and Pharmaceutical Chemistry
0.9% NaO (Sterile)
3O•mL
Evacuated Vial
shield
rlc-99m pertechnalale In 09%
NaQ
---Elutlon needle
Airway needle with —% bacteriological fitter
bacterloloØc& fiRer
Injection port for loading
Mo-9øon the column by manufacturer Stainless steel fluid paths
—Mo.99 adsorbed
onaluminasnion exchange coluni '—Lead shield
Figure 13—10 • Cross section of a radionuclide generator for the production of technetium-99m fTc•99rn) by sterile 0.9% sodium chloride elution of a sterile alumina (Al,03) col-
umn that has molybdenum-99 (Mo-99) adsorbed on it. (Courtesy of Dupont—Pharma, Billerica, MA.)
num-99 = 66 hours) decays by fl-particle emission to tcchnctium-99rn = 6 hours), which dccays by isomeric transition (IT) to technctium-99 by emission of a y-ray (140 2) keV). The anionic molybdate is then loaded on a
column of alumina (Fig. 13-10). The molybdate ions adsorb firmly to the alumina, and the generator column is autoclaved to sterilize the system. Then the rest of the generator is assembled under aseptic conditions into its final form in a lead-shielded container. Each generator is eluted with sterile
1.0
normal saline (0.9% sodium chloride). The column is an inorganic ion exchange column, and the cluate contains so-
0.5
dium pertechnetatc. so the chloride ions (Cli are exchangbut not molybdate
ing for the pertechnelate ions
ions (MoOf2). The method for calculating how much daughter is present on the column at any given time is more complex. because it must consider the decay rates of the parent and daughter (Fig. 13-11). The simplified equation for any case in which both the parent and the daughter are
Mo-99 Mmdmum = S6hours
I 0.2
radioactive and in equilibrium is as follows: =
l(r"
cs
— 431
where A,, is the activity (mCi) of the parent. A1, is the activity are their respective decay conof the daughter. A1, and stants. and us the time since the last elulion of the generator. In the case of Mo-99 = 66 hours, only 87.2% of the
atoms decay to Tc-99m (in = 6 hours), and 12.8% 1)1 the atoms decay directly to Tc-99. The generator system can be
cluted several limes per day to obtain more activity (mCi) per day because the increase in Tc-99m is a logarithmic function.
Hours
Figure 13—11 • Elution graph of radioactivity versus time (linear) of the Mo-99rn/Tc-99m tor with sterile 0.9% sodium chlorcde for 2 days (actual gerew tor is useful for 12 days posicalibratcon). The upper straight br-: represents the radioactive decay of the parent (Mo-99) to iF; daughter (Tc-99m), which reaches equilibrium at four
of the daughter (6 hours x 4 = 24 hours).
Chapter 13 • Agess:s fir Diagnostic Imaging
TECHNETIUM RADIOCHEMISTRY Element 43 in the periodic table, technetium, is a transition state metal and is the only 'artificial" element with a lower
atomic number than uranium. All 22 known isotopes of technecium ant radioactive, and there are eight nuclear isomers. Because no stable isotopes of technetium exist, the chemistry has been poorly developed; however. milligram quantities years) u(Tc-99 (a weak fl-particle emitter; = 2.1 x arc now available for determination of the structures of the technetium complexes. and more than ISO structures have
characteri,ed. The chemistry of technetium is similar to that of rhenium and is dominated by forming compounds by bonding between the electron-deficient metal and electro-
groups.. which are capable of donating electron pairs. Some examples of these electronegative groups are suilbydryl. carboxylic acid, amine, phosphate. oxime. hydronyl. phosphinc. and isonitrile. Basically, all technetium radiophurmaceuticuls are metal—electron donor complexes. Compounds that contain so or more electron donor groups and bind to a metal arc ailed chelating agent.s. Technetium usa transition state element can have oxidation states from I to + 7. As a pertechion, technetium will not form many netate l1'c041 metal—donor complexes. although it can be reduced to spethat will complex with a variety of monodentate, hidenLate. or polydentate ligands. The oxidation state of techne-
rtrn in various complexes and the actual structure are ntknown for several ot the compounds. Deutsch Ct al.2 claim that the oxidation states that ant most common in the chemis-
irvoftechnetiumare + I, +3,nnd +5.Teehnetium(TcO1) an be reduced by a sttrnnous salt, ascorbic acid. sodium kmohydride. and electrolysis. The most common reducing is the sannous ion because of water solubility, stabilty. low toxicity, and effectiveness at room Resiews of the chemistry of technetium are presented by Hjclctuen4 and Schwochau,5 hut the stereochemistry of the inehnetium coordination complexes is not shown. An excelkni review by Jurisson et covers all coordination cornpsnnds used in nuclear medicine, with a special emphasis in Tc-99m complexes.
Tc-99m radiopharmaceuticals are by far the most cornused radiotracers in day-to-day diagnostic nuclear most gamma cameras are demedicine practice. In
to work most efficiently (crystal thickness and the with Tc-99m—based radiotracers. Tc-99rn radioare prepared at the hospital or local nuclear nonradioactive compo1unnacy by combining tents in a sterile serum reaction vial. The primary chemical .ubsianccs in the vial are the complexing agent (ligand) and mincing agent, usually some stannous salt (stannous chinstunnous fluoride, or stunnous tartrate). After prcparaeon of the radiopharmaceutical. tests for radiochemical puitv should be carried out to ensure that the radiotracer is in form. The analytical methods used include and thin-layer chromatography, column chromatograsolvent extraction. Likely radiochemical impurities some insoluble sodium pertechnetate
(i.e.. reduced hydrolyzed technetium tTcO2I or 4hnetium—tin colloid). and a complex different from the rather than °'5"Tcme especicd (i.e..
463
bidentate ligand). The sterile serum vials containing the Stannous salt and the ligand are lyophilized tinder a sterile inert
gas atmosphere (i.e.. nitrogen or argon). The ligand in the reaction vial determines the final chemical structure of the and the biological fate after intravenous injection of the radiopharmaccuticul.
Technethun Radiopharmaceuticals Albumin Injection. albumin for injection is a sterile, colorless to pale yellow solution containing human albumin (MW radinlubeled with Tc-99rn pertechnetate. The reducing agent is stannous tarnrate. which reduces the to an unknown oxidation state and is weakly chelated by the tartrate and possibly forms a complex with sulfisydryl groups on the albumin by ligand exchange. The precise structure of the stannous technetium—albumin complex is currently unknown. The patient Technetium
receives an intravenous injection of 25 mCi (925 Mug) of Tc-99m albumin. Multiple images of the blood in the heart are taken by electrocardiogram gating (R-R interval). The rising portion of the R wave coincides with end-diastolc. These images are stored in a computer to reconstruct a movie of the beating hean. This procedure is sometimes called a ,nul:igaied acquisition (MUGA) or a radionuclide r'enlruulogram. Information obtained by this technique includes cardiac chamber wall motion and calculation of ejection fraction. Indications for the procedure include evaluation of effects of coronary artery disease, follow-up of coronary artery bypass graft patients. heart failure, heart transplant evaluation (preoperative of cardiand postoperative). cardiomyopathies. and
otoxic drugs (i.e.. doxorubicin).
""ic-alTechnetium Aggregated. bumin aggregated is a sterile white suspension of human albumin aggregates formed by denaturing human albumin by heating at 80°C at pH 4.8 (isuelectric point of albumin). The precise structure of the stannous technetium—albumin aggregated complex is unknown at this time. The particle size and number can he estimated with a hcmocytometcr grid. The particle size of the suspension should he between This 10 and 100 gem. with no particles greater than 150 agent is used clinically to image the pulmonary microeirculation for pulmonary embolus and to assess regional pulmonary function for surgery (i.e.. lung transplants or resection). The patient receives an intravenous injection of 2 to 4 mCi (74 to 148 M13q) of the Tc-99m—albumin aggregates. which lodge in some of the small pulmonary arterioles and capillaries, and the distribution can be imaged. The number olaggre-
gates recommended for good image quality and is 100,000 to 500.0(8) particles: thus, only a small fraction of the 280 billion capillaries are occluded. Multiple images of the lung are obtained to assess lung perftision. The distribu-
lion of the particles in the lung is a function of regional blood flow; consequently. in the normal lung, the particles are distributed uniformly throughout the lung. When blood
flow is occluded because of einholi. multiple segmental "cold" (decreased radioactivity) defects are seen. This pro-
cedure is almost always combined with a xenon-l33 gas lung ventilation scan (should be normal in pulmonary ejinbo-
horn) and same-day chest radiograph x-ray (should be normal).
464
WiLton
and
Tethnetium
Textbook
of Qrxanic Medicinal and Pharmaceutical Che,nis:rv
Albumin
colloid
Injection.
"Tc—albumin colloid injection is a sterile, opalescent, colorless dispersion of colloidal human albumin labeled with Tc-99m pertechnetate after it is reduced with a slannous salt.
The precise structure of the stannous technetium—albumin colloid complex is unknown at this time. The particle size may be examined with a hemocytomcter grid. The particle size range of the colloid is 0.1 to 5.0 Alter the patient
receives an intravenous injection of 5 mCi (185 MBq) of Tc-99m —albumin colloid. the agent is cleared from the blood
by the reticuloendothelial (RE) cells. These RE cells are located principally in the liver (85%) and spleen (10%), and the remainder are in the bone marrow, kidney, and lung. An
initial dynamic flow study may be obtained to determine liver and spleen perfusion in cases of abdominal trauma. Liver and spleen imaging is useful to determine organ size. the presence of hepatic meta.stases. and the degree of hepato-
cellular dysfunction in diffuse liver disease (i.e.. cirrhosis).
Technetium Apcit!de. This new radiotracer is a synthetic peptide that binds to the GPllbIllla adhesion-
peptide with high-affinity binding to somatostatin recepton (subtypes 2, 3, and 5) present in many types of cancer. including lung cancer. Ii is approved for use in patients who are known to have, or are highly suspect for. malignanc) and exhibit pulmonary lesions on CT and/or chest x-zay. Over 170,000 new cases of lung cancer are diagnosed each year in the United States alone, and the alternative methods for determining malignancy are needle biopsy, which has as
estimated 15% complication rate, and surgery. The precise structure is cyclo-(L-homocysteinyl-N-meth. yl-L-phenylalanyl-L-tyrosyl-D-tryplophyl-L-lysyl-L-valyl). (1.1 ')-sulflde with 3-E(mercaptoacetyl)amino]-L-alanyl.L. lysyl-L-cysteinyl-L-lysinamide. A technetium Tc-99m com• plex of depreotide is formed when sterile, nonpyrogenic so. dium pertechnetate Tc-99m (15 to 20 mCi) injection. sodium chloride is added to a nonpyrogenic lyophilized mit ture of 50 of depreotide. sodium glucoheptonate dihydrate, stannous chloride dihydrate, 100 jcg of edetate disu dium dihydrate, and enough sodium hydroxide or hydrochloric acid to adjust the pH to 7.4 prior to lyophiliot tion.
molecule receptors found on activated platelets. This allows the detection of acute venous thrombosis and is Food and Drug Administration (FDA)-npproved for detection of acute
CH31
lower extremity deep venous thrombosis. A lyophilized preparation of 100 of bibapcitide in the presence of heat will split and then complex to 20 mCi Tc-99m pertechnetate. Images of area of concern are acquired at 10 and 60 minutes.
Technetium
Bicisate Injection. A sterile colorless solution of bicisate is complexed with Tc-99m pertechnetate after reduction with a stannous salt. The precise structure of the technetium complex is [N,N'-ethylene-di-Lcysteinato(3-)joxo diethyl ester. This radiopharmaceutical is a neutral and lipophilic complex that crosses the blood—brain barrier and is selectively retained in the brain. Therefore, this radiotracer is used as a brainperfusion imaging agent. After intravenous injection of 20 mCi (740 MBq) of Tc-99m bicisate, about 5% of the injected dose is localized within the brain cells 5 minutes after injec-
tion and demonstrates rapid renal excretion (74% in 24 hours). This radiotracer is used clinically to evaluate dementia, stroke, lack of brain perfusion ("brain death"), cerebral vascular reserve, or risk of stroke (acetazolamide challenge study) and to localize a seizure focus for surgical removal. 0
0
/o\ \
0
H20— CH2
so" H3C
'so'
\2 CH3
Technetium Bicisate
Technetium Dtsofenin
Technetium
Disofenin Injection.
A sictik
colorless solution of disofenin is complexed with Tc-99o pertechnetate after reduction with a stannous salt. The pit cise structure of the technetium complex is unknown at hi' time. Costello et al.7 specify, however, that an analoguesl this Tc-99m—lidofenin complex provides a single technetium (Ill) distorted octahedral (1:2) complex with a coorti nation number of 6. A newer biliary imaging agent is Te 99m mebrofenin, which is more lipophilic because it Ia. bromine on the benzene ring. In the presence of high scan bilirubin levels, there is less renal excretion because of higher lipid solubility. In addition, the product is more which makes it more cost-effective for a centralized pharmacy. The patient receives an intravenous injection of 5 nil.
(185 MBq) of Tc-99m disofenin. which is taken up by it' hepatocytes in the liver by active anionic transport. list the radiopharmaceutical is excreted in bile, via the bins canaliculus, into the bile ducts, with accumulation in I gallbladder and finally excretion via the common bile dal into the small bowel. The normal patient exhibits mulation of the radiopharmaceutical in the livcrandthegtl bladder and small bowel can be visualized within I hours after injection. An example is seen in Figure 13-5 The primary clinical indication for this study is acute cholecystitis. In acute cholecystitis, there is of the cystic duct leading to the gallbladder. The galiblailL"
Technetium (Tc-99m) Depreotide Injection. Technetium depreotide injection is a new radiolabeled synthetic
is not visualized because the radiotracer cannot enter Some other clinical conditions that can be diagnosed bylt:
Chapter 13 • Agents for Diacnoojc Imaging
i
0
images are common bile duct obstruction. biliary teak surgery. biliary atresia. and a choledochal cyst.
from
0 Exametazime Injection.
A sterkcolorless solution ol exametasime is complexed with Tc$Jm perlechnelate after reduction with a stan000s salt. The pacise structure of the technetium complex is unknown at time. Jurisson et al.° speciry. however, that analogues il this complex Tc-99m propylene amine oxime provide a (VI square pyramidal complex with a coordinaion number of 5. This radiopharmuceutical is lipid-soluble Technetium
and. therefore, crosses the blood—brain harrier and is trapped
the brain. The possible mechanisms proposed for lo-
ulization include binding to glutathione. change in ionic tale, and chctnical degradation. The patient receives an ininjection of 20 mCi (740 MBq) of Tc-99m examet-
uime in a controlled environmental state. The patient is spine, with covered eyes (20 minutes). in a quiet room and sfrh indirect lighting prior to injection. The radiopharmaceuis irreversibly bound to the brain after 10 minutes. Some
indications for this study are localization of seizure
ki, evaluation ofdcinentia. identilication of drug abuse—inbrain defects (i.e.. cocaine), and evaluation of strokc. The normal study is represented by a homogeneous and sym-
zetric distribution of radioactivity throughout the brain. Cer-
activity is usually greater than activity in the rest of frebraiit. This is the agent of choice to determine brain death
.n patients on life support systems. The major use of this at this time is the radiolubeling of nulologous leukocytes as an adjunct in the localization of rim-abdominal infection and inllammaloiy bowel disease.
Medronate Injection. A sterile, solution containing sodium medronate (methylene Jiphosphonate) and a stannous salt is complcxed with TcThu pertechnelate. A structure proposed by Libson et the technetium ntedronate complex is shown below. De Litsy et alY' specify. however, that Tc-99m bone imaging Technetium
OH
0—P
fr
Technetium Oxidronate
Technetium Mertiatide Injection. The technetiunt mertiatidc complex is a sterile, colorless solution of meiliacide complexed with Tc-99m pertechnetate after reduction with a stannous salt. The precise structure is shown below. This radiopharniaceutical is the agent of choice to provide
information about relative function of the kidneys and urine outflow because ii has a higher extraction efficiency than Tc-99m pentetate. Indications include renal artery stenosis in nonperlused kidneys, renal transplant assessment, and outflow obstruction. The patient receives a bolus intravenous injection of 10 mCi (370 MBq) of Tc-99m mertiatide. and dynamic images are obtained every 3 to 5 seconds to study blood flow to the kidneys. Sequential static images are then obtained for 20 to 30 minutes to evaluate renal cortical up. take, excretion, and tubular clearance. Delayed images may he required to evaluate patients with obstruction or renal failure. Normally, there is prompt symmetric bilateral perfusion, good cortical accumulation bilaterally with visualization of the collecting systems by 3 to 5 minutes postinjection, and rapid excretion into the bladder, with no delay to indicate
partial or complete obstruction. An older renal imaging agent, Tc-99m gluceptate (a Tc-99m hydroxy acid complex; see below for the ligand), is now used as a transchelation agent for radiolabeling monoclonal antibodies.
are mixtures of many components (polymers and that can be separated by high-performance anion chromatography. The clinical use olthis agent is for investigation of skeletal such as metastatic disease to the hones. osteomyeI;is. Paget's disease, fractures, primary hone tumors. avasnlar necrosis. metabolic bone disease, and loose or infected hp prostheses. Stress fractures can be diagnosed by bone
ruging when x-rays are completely normal. Bone radiois one of the most commonly performed nuclear diagnostic procedures because the whole-body sur-
Technetium Mertiatide
allows evaluation of the entire skeleton, which cannot us cost-ellectively by any other imaging modality. The patient receives an intravenous injection of 15 to 20 4555 to 740 MBq) of Tc-99m medronate. which localin bone according to the degree of metabolic activity. niedronate is absorbed onto hydroxyapatite crystals of the ol new bone formation with about 50 to dtse distributed throughout the skeleton within 3 hours: the rest is excreted by the kidneys.
A tewcr bone-imaging agent. Tc-99m oxidronate (a bygroup on the carbon of medronate) has a higher bindaffinity for hydroxyapatile crystals in bone: however. criteria indicate no advantage to use of this agent.
465
Technetium Gluceptale
466
Whoa,,
and
TeribooA of Organie Mediehuil and ('lu,n,,at-,'uueal Clw,niorv
Technetium Pentetate Injection. A sterile. colorless or slightly yellow solution of sodium pentetate or calcium trisodium pentetate is complexed with Tc-99m pertechnetace after reduction with a stannous salt. The precise
successful surgical splenic transplants by using heat-damS aged RBCs.
structure of Tc-99m pcntetate is unknown: howevcr. .lurisson et alP suggested the possible structure below. The primary clinical use of this agent is for renal studies and glomerulur
colorless solution of .sestamibi is synthesized by reaction with Tc-99m pertechnetute after reduction with a stannour salt. The precise structure of the technetium complex is Tc99m (MIBI)5 . where MlBl is 2-methoxyisobutyl This was the first Tc-99m—labeled agent introduced to replace thallous (201Th chloride for myocardial perfusion imaging. The shorter half-life of Tc-99m (6 hours) than of 11-
filtration rate (GFR). hut it is occasionally used for brain death and brain tumor localization. The patient receives an intravenous injection of 3 to 20 mCi (Ill to 740 MBq). and the kidneys are imaged for 20 to 30 minutes. The GFR is calculated by a quantitative method using a combination of imaging and counting the radioactivity in serum and urine samples. Normal extraction efficiency is 20% (80 to 140 mlJmin). +
Technetium
"Tc,) Sestamibi Injection.
A sterile.
201 (73 hours) allows administration of a larger dose, which
provides better image quality. Another major difference from Tl-201 is that Tc-99m sestamibj exhibits little "redistribution." or movement out of the myocardium back into the bloodstream. This cytosol binding of sestamihi more flexibility in the imaging time, although it also necescitates separate sestamibi injections at stress and at resting. which increases the expense of this method. The other radiotracers currently used to evaluate dial perfusion include Tl-20l and Tc-99m tetrofosinin.
COOH
cardial perfusion studies usually compare "stress" or creased blood flow images with resting images. The slrcs% can be brought about by physical means (treadmill. bicyclci
or by pharmacological vasodilation (with dipyridamote. COOH
HOOC
Technetium (V) Pentetate
Technetium Red Blood Cells (Autologous,). A sterile reaction vial containing stannous citrate (Ultratag RBC kit) is used to radiolabel a patient's red blood cells (RBCs) with Tc-99m pertcchnetate. Briefly, the patient's blood (Ito 3 mL) is drawn with acid citrate dextrose (ACD) or heparin (100 units) used as an anticoagulant. The blood is labeled with the patient's name and hospital number and added to the sterile reaction vial. After mixing and incubation for 5 minutes. sodium hypochlorite (6 mg) is added to the vial to oxidize excess stannous ions (Sn + 4), A citrate buffer is added to adjust the pH to about 7.4. Then. 30 mCi (1.110 MBq) of Tc-99m pertechnetate is added to the blood in the vial and mixed and incubated for 20 minutes. Without further preparation, the patient receives an intravenous injection of 25 mCi (925 MBq) of his or her own radiolaheled red blood cells. Three different studies can be perthrmed after injection of the Tc-RBC. First, the radionuclide ventriculogram study for evaluation of cardiac function can be done as with Te99m albumin (discussed above). Use of Tc-RBCs is considered the superior technique because the Tc-99m red blood cells remain in the circulating blood volume, whereas the Tc-99m albumin leaks into the extracellular spaces. This leakage increases the background radioactivity around the heart and contributes to degradation of the blood pool image. Second. the Tc-RBCs are used Ibr noninvasive localization of the preoperative site of active gastrointestinal (GI) bleeding. Patients are injected with their own Tc-RBCs. which remain within the circulating blood long enough to extravasate and accumulate within the bowel lumen at the site of bleeding. The final use of Tc-RHCs is to evaluate the spleen after trauma or to confirm an accessory spleen or to study
adenosine. or dobutamine). Myocardium with significanri) narrowed arterial supply may appear to have normal bkx+J flow at rest hut, during increased blood flow during exercice, demonstrate abnormal blood flow relative to areas with nor inal arteries. There are a variety of protocols for the rest imaging sessions, even one that combines Tl-20l and Tc-99m—sestamibi (stress). Additional indications the use of the Tc-99m—sestamibi complex now include itt preoperative localization of parathyroid adenoma and itt early diagnosis of breast cancer. CH2C(CH3)20CH3 N
H3CO(H3C)2CCH2
C\J
H3CO(H3C)2CCH'
,CH2C(CH3)20CH3
'C
N
I,,
N
CH2C(CH3)20CH3
—
Technetium Sestamibi
Technetium
Sodium Pertechnetate.
Tech':
tium sodium pcrnechncuate is a sterile, colorless solution ar taming sodium pertechnetate in normal
(0.9% NaCl. obtained by elution of the sterile Mo—9'+l'h 99m generator. The pertechnelate ion whiehh an ionic radius and charge similar to those of the (l). is concentrated in the thyroid, salivary glands. stomach, and choroid plexus in the brain. It cart Ice ed directly from the Mo—9911'c-99tn generator to image it! thyroid. Meckel'sdiverniculuin (stomach tissue tine), and salivary glands for tumors and to detect
Chapter 13 • Agen:.c for Diagnostic imaging that disrupt the blood—brain barrier (i.e.. tumors, abscesses. Unlike the iodide ion, the pertechnetate ion is not
to thyroid hormone but only trapped. Thyroid nodules can appear nonfunctional, with little or no radiotracer present. These nonfunctional nodules have about a 20% probability of being cancerous and generally require biopsy. The patient receives an intravenous injection of 5 to 10 mCi (ISS to 370 MBq) of Tc-99m pertechnetate. and images are obtained of the thyroid 0 to 20 minutes after injection. The usual dose for the other imaging procedures is the same for Meekel's diverticulum and salivary glands. and 20 mCi (740 MBq) is used for brain tumor imaging.
SuccimerInjection. Tedtnetium A sterile. colorlms solution of succimer (2.3-dimercaptosuccinic acid) is complexed with Tc-99rn pertechnetate after reduction with stannous salt at acid pH. The precise structure of Tc-99m 1111 succimer is unknown: however. Moretti et al.'' sugthe possible structure below. Tc-99m succimer is very useful (or demonstrating the functioning renal parenchyma. kcausc about 409c of the dose is bound to the renal cortex I hour after injection. The patient is injected with 5 mCi (11(5 MB4) of Tc-99m succimer. and multiple images are taken 2 to 4 hours later. This study can be useful for evaluatisp renal trauma, renal masses (e.g.. tumors. cysts). and renal Tc-99m succinser is the diagnostic agent of choice a children who have chronic urinary tract infections causing renal scarring, lithe pH is adjusted to 8.0 to 8.5. a technetium
V-succimer complex is formed, which is useful for imaging tumors.'2 Blower et al.15 have proposed the following nurture fur Tc-99m (V) succinser.
467
des. The particle size of the colloid is 0.1 to 3 After intravenous injection of 5 to 10 mCi (185 to 370 MBq) of Tc-99m sulfur colloid, the radiopharmaceutical is rapidly cleared from the blood by the RE cells of the liver, spleen. and bone marrow. Uptake of the Tc-99m—sulfur colloid depends on the relative blood perhision rate and the functional capacity of RE cells. In the normal patient. 85% of the radio-
colloid is phagocytized by Kupffer cells in the liver, 7.5% by the spleen, and the remainder by the bone marrow, lungs. and kidneys. Bone marrow imaging studies are performed I hour after injection of 10 mCi (370 MBq) of Tc-99rn—sulfur colloid. Normal bone marrow will take up the radiocolloid. but diseased bone marrow appears as "cold" defects in pa-
tients with tumor deposits in the marrow. Tc-99m—sulfur colloid is used usa secondary agent in liver and spleen imag.
ins if Tc.99un—albumin colloid is not available. It is used as the primary agent, however. for GI studies such as gastroesophageal retlux (GER) and gastric emptying of solid food. Gastroesophageal reflux imaging is performed after having
the patient swallow acidified orange juice mixed with Tc99m—sulfur colloid. Normal patients have no GER. This study reportedly has 90% sensitivity in detecting GER. Gastric emptying imaging is performed after the patient swallows solid food (i.e.. scrambled eggs or pancakes) radiola-
beled with Tc-99m sulfur colloid. In general, the normal gastric emptying half-time is less than 90 minutes for solid food.
Technetium Tetrofosmin Injection. A sterile. colorless solution of tetrofosmin is complexed with Tc-99m pertechnetate after reduction with a slannous salt. The precise structure of the technetium complex is shown below)' The formulation contains gluconate to Ibrm a weak technetium (V) chelate to keep the technetium in the (V) oxidation stale for tranuchelatiun to form the technetium (V)—tetrofosmm complex. Technetium (V)—tetrofosmin is another cationic Tc-99ns complex that thallous (201T1) chloride accumulates in viable myocardium. Myocardial uptake of this agent
in humans is about 1.2% 5 minutes afier intravenous injection and decreases to 1.0% at 2 hours. This agent was less specific for detecting ischcmia (66%) than Tl-20l chloride (77%) in a small study (252 patients). ft appears. however. to have rapid clearance through nontarget organs (liver) and thus fewer high-background imaging problems.
0
[ H,c
I1.C I
Technetium (V) Succimer
Sulfur Colloid Injection. Technccolloid injection is a sterile, opalescent colloidal
HC
\\
/
CH.
0 ic
Technetium
of sulfur, a unit of structure built up from poly-
CH.
senic molecules and ions (micellcs) radiolabeled with Tc49m pertechnetate formed by heating in dilute hydrochloric
The radiocolloid should be stahilii.ed with gelatin A inhibit clumping of the negatively charged colloidal parti-
Technetium Tetrofosmin
OC.H,
468
Wilson and Gisvolds Textbook of Organic Medicinal we! Pharmaceutical Chen,i.cgrv
FLUORINE RADIOCHEMISTRY The useful radioisotope of fluorine for organ imaging is fluo-
rine-l8. Fluorine-IS is produced in a cyclotron by the '50(pn)'8F nuclear reaction. Fluorine-IS = 109 minutes) decays by electron capture and positron emission to oxygen-IS with v-ray emissions of 511 keV (194%). Fluorine- IS can be attached to a number of physiologically active
molecules and, with the great strength of the C—F bond. appears to be a very useful label for radiopharmaceuticals.'5 Radiotracer production involves relatively complicated synthetic pathways.. however, and the preparation of high-specific-activity compounds presents many problems. The short
half-life of fluorine-IS makes it necessary to complete the synthetic and purification procedure within 3 hours. Consequently. a separate chemistry system (black box type) is needed for each compound. The chemistry of fluorine is complicated, but some compounds can be fluorinated by '8F exchange reactions and direct fluorination with elemen-
tal fluorine (°'F2): also, compounds with an aromatic ring may he fluorinated by several synthetic reactions. For exam-
ple, partially fluorinated heteroaromatics are readily obtained by the conversion of an amino group on the aromatic ring to fluoride, with use of the BaIz-Schiemman and several related reactions.
Fluorine The r8F)-2-Fluoro-2-Deoxy-o-Glucose. only F-IS radiopharmaccutical presently available is fiuorifle ("5F)-2-lluoro-2-deoxy-o-glucose (F- 18 FDG). The pre-
cise stnacture of F-IS FDG is shown below. It is the only PET agent approved by the FDA. Hamacher et al.'6 introduced the current method of synthesis of F- 18 FDG by nucleophilic fluorination. Use of this radiotracer for diagnostic imaging in oncology has increased dramatically in the last several years. It is used also as a myocardial viability agent and in evaluation of seizure disorders)1 The high glycolytic rate of many neoplasms compared with that of the surrounding tissues facilitates tumor imaging with this glucose analogue. Because of the widespread anatomical distribution of metastases. a whole-body imaging technique using a tumorspecific radiophannaceutical is very useful for tumor detection and mapping to evaluate the extent and relative metabolic activity of the disease.
capture to stable zinc-67 with principal v-ray emissions of 93 keV (38%), 185 keV (24%), and 300 keV (16%). The radiotracer is isolated by dissolution of the target in hydrochloric acid followed by isopropyl ether extraction of he gallium-67 from the zinc and other impurities. The gallium. 67 is back-extracted from the isopropyl ether ink, 0.2 N hydrochloric acid, evaporated to dryness, and dissolved a sterile, pyrogen-free 0.05 M hydrochloric acid. Gallium is an amphoteric element that acts as a metal at low p1-1 hut forms insoluble hydroxides when the pH is raised above 2.0 in the absence of chelating agents. At high pH. gallium hydroxide acts as a nonmetal and dissolves in ammonia tu form gallates. Gallium forms compounds of oxidation
+ I - + 2. and + 3: howcvcr. only the Ga
state is stable
in aqueous solutions.
Gallium
Citrate. The gallium (111)—citrate complex is formed by adding the required amount of sodium citrate (0.15 M) to gallium (Ill) chloride and adjusting the pH to 4.5 to 8.0 with sodium hydroxide. The proposed stOicture of gallium (61Ga) citrate is shown below.6 The paliem receives an intravenous injection of 5 to 10 mCi (185 to 370 MBq) of gallium (67Ga) citrate, and whole-body images are then obtained 24, 48, and 72 hours after injection. Gallium
localizes at sites of inflammation or infection as well aa variety of tumors. his used in clinical practice in the staging and evaluation of recurrence of lymphomas. Gallium localizes normally in the liver and spleen, bone, nasopharyro. lacrimal glands, and breast tissue. There is also some seaslion in the bowel; consequently, the patient may require a laxative and/or enemas to evacuate this radioactivity prior
to the 48-hour image. As more specific radiotracers been developed, the nonspecific normal localization of gal-
lium radioactivity has limited its clinical use.
HO Gallium Citrate
HO -
IODINE RADIOCHEMISTRY Fluorodeoxyglucose
GALUUM RADIOCHEMISTRY
The useful radioisotopes of iodine for organ imaging a: iodine-l31 and iodine-123 because of their desirable p1ns cal characteristics. Iodine- 131 is obtained from a reacrnrfr production of tellurium- 131. It is formed by the nuclear tion 235U(n,fission)'3tTc or 130Te(n,gamma)'"Te. Then:
lurium-l3l (1102 = 25 minutes) decays by
The only radioisotope of gallium that is presently used is
sion to iodine- 131. Iodine- 131 (11,2 = 8.04 days) translniii
gallium-67. which is produced in a cyclotron by proton bomnuclear bardment of a zinc metal target by a = 78.2 hours) decays by electron reaction. Gallium-67
by fi decay to stable xenon- 13 I. with five significant emissions of 80 to 723 key. The major v-ray of 364 ke\ (82%) provides good tissue penetration for organ
Chapter 13 • Agencc for Diagno.clk hnag!ng
469
Undesirable properties of iodine-13l are the high radiation Jose from the f3 particles, the long half-life, and the poor Iodine-I23 rage produced by the high-energy = 13.3 hours) decays by electron capture to tellurium-l23.
in the regional lymph nodes, bone, bone marrow, and soft tissues. After an initial report by Kimmig et al.2° of 1311,
principal y-ray emission of 159 keV (83%), which
makes it the ideal radioisotope of iodine for organ imaging of increased detection efficiency and reduced radia-
is so tisstte specific that it can establish the diagnosis of neurobla,stoma in a child with a tumor of unknown origin. The patient is treated with Lugol's solution (up to 40 mgI
to the patient. lodine-l23 is produced in a cyclotron
day) 24 hours before and 4 to 7 days after administration of
tombarding an antimony metal target with a particles according to the reaction or an iodine target nuclear reacsith high-energy protons by the den. The xenon- 123 decays by electron capture to iodineIt is. however, relatively expensive to produce and curvntl,v has limited availability for radiolabeling compounds. ojine is in group VIIB with the other halogens (fluorine. bromine, and astatine), in aqueous solution, corn-
the radiopharmaceutical, to block thyroid uptake of free 31I. The 131l-MIBG is administered by slow intravenous injection 0.3 to 0.5 mCi (II to 18.5 MBq). and patients are
with a
çounds of iodine are known with at least five different oxidanon states: however, in nuclear medicine, the — I and + I snidation states are the most significant. The — I oxidation
'tue represented as sodium iodide (NaI) is important for
MIBG uptake in neuroblastoma. successful use of this tracer was described by others. The increased uptake of
imaged 24, 48. and 72 hours later. Occasionally, the patient receives a renal imaging agent for better localization of the adrenal tumor.
CH2— NH — C=NH
NH3 • ½S042 1311
tobenguane Sutfate
thyroid studies and, when obtained in a reductant-free solu-
sm (no sodium thiosulfate). is the starting compound for be radiolabeling of most iodinated radiopharmuccuticals. 11w common methods for introducing radioiodine into orasic compounds are isotope exchange reactions. electrosubstitution of hydrogen in activated aromatic sysaims. nucleophilic substitution, and addition to double The replacement of aromatic hydrogen in activated systems is used for protein labeling, and clectroiodine (I') can be generated by a variety of oxidizing acnts. including (a) chloraniine-T (N-chloro-p-toluene sulcosmidel sodium. (b) enzyme oxidation of 1 (lactoperoxilad, and (c) iodogcn I .3.4.6-tetrachlora-3a-6a-diphenylThe actual iodinating molecule depends on the inidizing agent but is probably HOl or H01
Sodium Iodine ('RI) Capsules.
The major indications for thyroid imaging with sodium iodide are for evaluation of thyroid morphology, for ectopic thyroid tissue (e.g.. lingual or mediastinal), and for subslernal thyroid tissue. When thyroid nodules are being evaluated for possible thyroid cancer. 1231 has an advantage over Tc-99m pertechnetate
scans, although 1-123 is more expensive. This is because thyroid cancer cells sometimes retain the ability to trap, hut not further process, iodine to thyroid hormone. Unlike iodine. Tc-99rn pertechnetate is only trapped by the thyroid and in a nodule may give the false impression that a nodule is not cancerous. The patient fasts before receiving the oral dose of 0.4 mCi (IS MBq) of sodium iodine 0231). Images are obtained of the thyroid and surrounding area 4 to 6 hours after ingestion.
(obenguane Sulfate r311.) Injection a-131—Metaiodolobenguane sulfate is rabeflzylguanidine Sulfated.
by a Cu -catalyzed isotopic nucleophilic exlarge reaction. It is a radioiodinated arylalkylguanidine and similar to the antihypertensive drug guanethidine and to norepincphrinc. The proposed structure ciobenguane
sulfate is shown below. Iodine-123 is
used to radiolabel this tracer and may have more favora:Ic imaging properties. Functional tumors of the adrenal meMb (pheochromocytomas) and tumors of neuroendocrinc mis (neuroblastoma) can be localized on 1-131 ,neta-iodo-
enoylguanidine ('31l-MIBG) images, as abnormal tissue at takes up the radiopharmaceutical and exhibits increased tthity on the image)9 Drug intervention studies in animals. sing reserpine. have demonstrated that the 311-MIBG en-
adrencrgic neurons and chromaffin cells by an active :ansport mechanism of catecholamine uptake into adrener3C granules.
is a malignant tumor of the sympathetic mous system, which occurs most often in children. The anorisof neural crest origin and consists of cells that form nervous system, called svrnpashogonia, that to the adrenal medulla and many other parts of the Metastases may be found in the liver (stage IV) and
Sodium Iodine P37l) Oral (Solution or Capsule).
The
thyroid cancer patient receives an oral dose of 5 to 10 mCi (185 to 370 MBq) of sodium iodide (1311). which localizes
in residual thyroid tissue after "total" thyroidectomy and functioning thyroid metastasis from thyroid carcinoma. Images of the whole body are obtained 48 to 72 hours later. These mctastatic radioiodide surveys are used to detect regional or distant metasrases for large-dose 150 mCi (5.550 MBq) inpatient therapy for thyroid carcinoma. Any thyroid hormone medication should be discontinued for 2 weeks (Ti) or 4 weeks (T4). In addition, the patient should have blood
drawn for a thyroid-stimulating hormone (TSH) test to ensure that TSH is elevated before administration of the therapy dose, to permit maximum stimulation of thyroid tissue. The patient should fast before receiving tbe oral dose of radiotracer.
INDIUM RADIOCHEMISTRY The most useful radioisotope of indium is indium-Ill, which is produced in a cyclotron by proton bombardment of a cadmium metal target by a °2Cd(p.2n)' nuclear reaction.
470
Wi/eon and GLnold 's Textbook of Organic Medicinal and Pharmaceutical O,e,nistrv
Indium-Ill
= 67.4 hours) decays by electron capture to stable cadmium-Ill with principal y-ray emissions of 172 key (91%) and 247 keV (94%). The radiotracer is isolated
(MW —55.000) and two light chains (MW —20.0(X)) of gly. coproteins. held together by disulfide bonds. Many tumors
by dissolution in hydrochloric acid to form "In-chloride
tion with radiolabcled antibodies. Antibodies are produced by B lymphocytes and plasma cells sensitized to an antigen. Hybridoma technology permits the manufacture of large quantities of antibody directed against specific antigens. Diagnostic antibodies are of two types: polyclonal and mono-
and separated from cadmium and other impurities by several dissolution and extraction steps. The last extraction is done with isopropyl ether, evaporating to dryness, and dissolving in sterile. pyrogen-free 0.05 M hydrochloric acid. In aqueous solution, lower valence states of indium have been described. but they are unstable and are rapidly oxidized to the trivalent state. In acid solution, indium salts are stable at low pH but
arc hydrolyzed (above pH 3.5) to form a precipitate of indium hydroxide or tnoxide. Indium will remain in solution above pH 35. however, if it is coniptexed with a weak chelating agent such as sodium citrate and stronger chelating agents such as 8-hydroxy quinoline (oxine) or diethyleneu'iaminepemaacetic acid (DTPA). Monoclonal antibodies or peptides are radiolabeled by indium by using compounds called hiji,nctiunal chelating agents. Bifunctional chelating agents are molecules that can both hind metal ions and be attached to other molecules: one example is the cyclic anhy-
dride of DTPA.
express antigenic markers on their surfaces that permit detec-
clonal. Each chain has a variable region for antigenic binding
and a constant region for complement fixation. Polyclonal antibodies include numerous antibody species of varying at• finity for the antigen-binding surfaces. Monoclonal antibod. ies are generated from a clone of a single antibody-producing
cell and have uniform affinity for their antigenic demenninant2' Monoclonal antibodies are produced by immuni/ing a mouse with purified material from the surface of the human
tumor cell. (See Chapter 7 for additional information.) ha antigen used in Oncoscint CRJOV is a tumor-associated glycoprotein-72 (TAG-72). a high-molecular-weight glycopro. tein expressed by colorectal and ovarian The
radiolaheling of Oncoscint CRJOV monoclonal a
Indium Radiopharmaceuticals Indium ("'In) Chloride Injection.
Indium (111) chlo-
ride is a sterile, colorless solution that is radiolabeled with indium-I II in a hydrochloric acid solution ((1,05 M) and has a pH of 1.5. It is primarily used to radiolabel other compounds for use in cistemography and white blood cell labeling studies and is particularly recommended for radiolabeling nionoclonal antibodies for metastatic cancer imaging. If this agent is injected intravenously for clinical use, the patient's blood must be drawn into the syringe containing the radiopharmaceutical to buffer the agent to a higher pH to eliminate the burning sensation on injection. When the acidic
compound is mixed with blood, the indium-Ill chloride
site-specific method using
a bifunctional chelate. Briefly, carbohydrate moieties on thc monoclonal antibody (F-constant region) are oxidi,cd esith periodate. and the aldehyde groups on the antibody are aacted with a-amino groups of glycyl-Iyrosyl-lysine-N-dielh.
ylene triarninepentaacctic acid. The Schiff's base (imine) is stabilized by reduction with sodium cyanoboruh).
dride. In-Ill is chelated to a DTPA-curbohydrate attached to the constant region of the monoclonal anlibesi). The specificity of radiolaheled antibody imaging (or tumon exceeds that of gallium (°7Ga) citrate studies. Sites of non specific uptake have been reported, however, such as recenT surgical wounds, arm infianied colon, bone fracture, and nor-
mal colostomy stoma. A new nlcIlRxl of labeling with Ic99rn has recently been approved by the FI)A.
hinds quickly to transferrin. the iron-binding protein in the plasma. The localization of the indium (Ill) chloride in bone marrow is probably explained by its ability to behave metabolically like iron and yet not be incorporated into heinoglobin in the RBCs in the hone marrow. The localization of the radiotracer in tumors and abscesses is probably due to increased blood flow and capillary permeability in the area of tissue damage. Transferrin receptors have been suggested
as a means of localization but not proved at this time.
Indium
capromab Pendetide. Indium capromab pendetide is a new radiotracer for staging patients with
Fc Region
newly diagnosed prostate cancer and for those with suspected reoccurrence but a negative localization with a standard evaluation.
Indium ("'In) Oncoscint The simplified structure of indium (° 'In) satumomabpcndetide is shown below. Antibodies are a heterogeneous group of proteins isolated from human and animal serum
and are called i:nmunoglobulins. They are divided into classes on the basis of
in structure and biological
properties and are assigned to major classes called lgG (80%). 1gM (100/c). and IgA. lgD. IgE ( secondary > primary. In part.
Meprobamate is also a centrally acting skcletal muscle relaxant. The agents in this group find use in a number of conditions, such u.s strains and sprains that may produce acute muscle spasm. They have interneuronal blocking prop-
erties at the lcvel of the spinal cord, which are said to be partly responsible for skeletal muscle relaxation.27 Also, the general CNS depressant properties they possess may contribute to. or be mainly responsible for, the skeletal muscle relax-
ant activity. Dihydric compounds and their carhamate (urethane) derivatives, as described above in the discussion of meprobamate. are prominent members of the group.
CH,—CH2—CH5 0
0
H
Carisoprodo!
Chlorphenesin Carbamate.
= —CH(CH,).
Chlorphenesin carbaniate. 3-(p-chlorophenoxy)- I .2-propanediol I -carbamate (Mao-
496
Teeibook
WiLco,, and
of Organic Medicinal and
late). is the p.chloro substituted and I -carbamate derivative of the lead compound in the development of this group of agents, mephenesin. Mephenesin is weakly active and short-lived because 01' facile metabolism of the primary hydroxyl group. Carhamy-
lution of this group increases activity, p-Chlorination increases the lipid/water partition coefficient and seals off the para position from hydroxylation. Metabolism, still fairly rapid, involves glucuronidation of
C'he,ni.czrv
and formate ion. In hydroalcoholic solutions, it forms the hemiacetal with ethanol. Whether or not this compound is the basis for the notorious and potentially lethal effect ol the combination of ethanol and chloral hydrate (the "Mickey
Finn") is controversial. Synergism between two different CNS depressants also could be involved. Additionally. ethanol, by increasing the concentration of NADH. the reduction of chloral to the more active nietabolite trichiw roethanol, and chloral can inhibit the metabolism of alcohol
the secondary hydroxyl group. The biological half-life in
because it inhibits alcohol dehydrogenase. Although it
humans is 3.5 hours.
suggested that chloral hydrate per se may act as a chloral hydrate is very quickly converted to trichloroethanol, which is generally assumed to account for almost all of the
hypnotic effect. It appears to have potent barbiturate-like binding to GABAA receptors. Chiorphenosin Carbamate
Methocarbamol,
USP. Methocarbamol, 3-to-methoxyphenoxy)- I .2-propanediol I -carhamate (Robaxin). is said to be more sustained in effect than mephenesin. Likely
sites for metabolic attack include the secondary hydroxyl group and the two ring positions opposite the ether functions.
The dihydric parent compound, guaifenesin. is used as an
Tridofos Sodium.
Triclolis sodium, 2.2,2,-trichksroethanol dihydrogen phosphate monosodium salt (Triclus). irritating to the 01 mucosa. Its active nictabolite. trichlorwthanol. also has unpleasant 01 effects when given orally Triclofos is the nonirritating sodium salt of the phosphate ester of trichloroethanol and is readily converted to roethanol. Accordingly, triclofos sodium produces CNS fects similar to those of oral chloral hydrate.
expectorant.
CI3C—CH,O—P---O Na' ?F1
0— CH2 — CH — CH2OIR Gualtenesin
P
Triclolos Sodium
H
Paraldehyde, USP. Meihocarbamol R=—C—NH?
Carisoprodol,
N-isopropyl-2USP. Carisoprodol. methyl-2-propyl- I .3-propanediol dicarbamate. 2-methyl-2isopropylcarbamate propyl- I ,3-propanediol carbamate (Soma). is the mono-N-isopropyl—substituted relative of meprobamate. The structure is given in the discussion of meprobamate. It is indicated in acute skeletomuscular conditions characterized by pain, stiffness, and spasm. As can be expected, a major side effect of the drug is drowsiness.
Paraldehyde, 2.4.6-trimethyl-o-tn oxane: paraccialdehyde. is recognizable as the cyclic triton
of acetaldehyde. It is a liquid with a strong char,icteristk odor detectable in the expired air and an unpleasant taste Thcse properties limit its use almost exclusively toan in the treatment of delirium trenlens). It the past, when containers were opened and air admiued
then reclosed and allowed to stand, fatalities occuned cause of oxidation of paraldehyde to glacial acetic acid.
CH ALDEHYDES AND THEIR DERIVATIVES
For chemical reasons that are easily rationalized, few aidehydes are valuable hypnotic drugs. The aldehyde in use, chioral (as the hydrate), is thought to act principally through a metabolite. trichloroethanol. Acetaldehyde is used as the cyclic trimer derivative. paraldehyde, which could also be grouped as an ether.
CH Paraldehyde
ANTIPSYCHOTICS Chioral hydrate. trichioroacetal(Noetec), is an aIdehyde hydrate stable enough to be isolated. The relative stabil-
Chloral Hydrate, USP. dehyde monohydrate,
ity of this gem-diol
largely due to an unfavorable
Antipsychotics are drugs that ameliorate mental that are characteristic of the psychoses. The psychoses difirt
dipole—dipole repulsion between the trichloromethyl carbon and the carbonyl carbon present in the parent carbonyl corn-
from the milder behavioral disorders, such a.s the disorders, in that thinking tends to he illogical. bizane.ani loosely organized. Importantly. patients have difficulty a'
pound.25
derstanding reality and their own conditions. There are oftefl
Chioral hydrate is unstable in alkaline solutions, undergoing the last step of the haloform reaction to yield chloroform
hallucinations (usually auditory) and delusions. In the schizophrenias. in addition to these symptota.
is
Chapter 14 a
called j,osiii,e there are negative symptoms represented by apathy, social withdrawal, and anhedonia. Cognitive delicits may also be observed. Psychoses can be organic and related to a specitie toxic chemical c.g.. delirium produced by central unticholinergic Jgents). an NMDA antagonist e.g.. phencyclidinc). a definite disease process e.g.. dementia) or they can be idio-
pathic. Idiopathic psychoses may be acute or chronic. idiopathic acute psychotic reactions have been reported to follow estremely severe short-term stress. Schizophrenia is a group
of chronic idiopathic psychotic disorders with the overall described above. The term w,t,psyrhonc was slow in gaining acceptance. Now it is widely acknowledged that antipsychotics actually diminish (he underlying thought disorder that is the chief characteristic of the schizophrenia.s. The agents often have effect in agitated psychotic patients: hence, they also have been referred to as major Iran quilizers. Finally. kcause they lessen reactivity to emotional stimuli, with little dfect on consciousness, they are referred to as ne:irolepiics. The most frequent uses of these agents are in manic disor-
ders anti the schizophrenias. In the manic disorders, the agents may block DA at limbic D2 and D3 receptors, reducing euphoria, delusional thinking, and hyperactivity. In the chronic idiopathic psychoses (schizophrenias), both conven(typical) and newer (most are atypical) antipsychotics
to act to benefit positive symptoms by blocking DA and D3 limbic receptors.' The bases of the atypical activity against negLdive symptoms may be seroton,n-25 receptor (5-HT2A) block, block at receptors yet to be A Jetermined. and possibly decreased striatal D2 block.' a
classic competitive antagonism has been demonstrated at D3
D receptors. Also, in recombinantly expressed receptas. inverse agonism has been demonstrated. For this to apply in vivo. a ground state of dopaminergic activity must shown. Some preliminary signs indicate this is likely.' In the schizophrenias. which have an extremely complex multifactored etiology.31 the fundamental lesion aprears to be a defect in the brain's informational gating mechJnism. A slight abnormality in the startle response may be
in infancy. hut the disease does not emerge until in the second decade or in the third decade of life. Basithe gating system has difficulty discriminating between relesaun and irrelevant stimuli. Perception is illogical. Pro;eeding from this, thought and actions become illogical. Although the actual structural or anatonsical lesions are not known. the basic defect appears to involve overactivity of dopaminergic neurons in the mesolimbic system. Some insuggest that this is the cause of most, if not all. the common symptoms of the disease. Negative symp:nns Ie.g.. social withdrawal) may be considered secondary .vnlptoms. Others argue that all or part of the foregoing is reductionist, and that other lesions cause some all 1)1 the symptoms. A
reason for the recent interest in the negative of schizophrenia has been the introduction of as opposed to typical. antipsychotics. Typical anti-
began with the serendipitous discovery of the activity of chlorpromazine. Many compounds nrc sytuthcsi,.ed. usually with chlorprornazine as the model. an) the antipsychotic potential assessed. A clear association
the ability to block DA at mesolimbic D2 receptors
Nrn,;sec Sv.qen, Deprt'ssanrs
497
was established. During the same time, amphetamine-induced psychosis was determined to be caused by ovcractivation of mesolinibic receptors and judged to be the closest
of the various chemically induced model psychoses to the schizophrenias. The conventional typical antipsychotics are characterized by the production of EPS. roughly approximating the symptoms of Parkinson's disease. These are reversible on discontinuing or decreasing the dose of the drug and are associated with blockade of DA at D3 striatal receptors. After sustained high-dose therapy with antipsychotics, a late-appearing EPS. turdive dyskinesia. may occur. The overall symptomatology resembles the symptoms of Huntington's chorea. The condi-
tion is thought to arise from biological compensation (increased D2 activity) for the striatal D2 block of antipsycholic drugs.
Atypical antipsychotics date from the discovery of clozapine, its antipsychotic properties and its much lower production of EPS. Some investigators express concern that typical
antipsychotics. especially by producing EPS. introduce drug-induced effects that are hard to distinguish from negative symptoms. This leads to the view that diminishing EPS can account for perecived decreased negative symptoms. It is. however, reportedly also more active against negative symptoms of schizophrenia. independent of reduced EPS. and has a unique, notably expanded, receptor-blocking profile. Compounds are now under synthesis and being tested at the various CNS receptors at which clozapine acts to determine the role of these receptors in schizophrenia. Also contributing to the development of typical antipsychotics was the introduction of risperidone. It has reduced EPS. has increased activity against negative symptoms, and, in addition to its DA blocking ability, is a 5-HT2,5 antagonist. One view of the drug is that it combines structurally the features of an antidepressant and an antipsychotic. and so the two drug effects are attained. Related to this is tile V1CW that at least some negative symptoms (e.g.. depression. withdrawal) are secondary to the positive symptoms. The view has also been advanced, however, that receptors are involved in part (the negative symptoms) or wholly in schizophrenia. So far, the evidence appears to be that 5-HT2A blocking agents do not relieve positive effects of schizophre-
The view that 5-HT2A overactivity is the source of negative symptoms (pan of the basis psychosis) is not disproved at present. though sonic say it has been weakened.3" One result of the development of atypical antipsychotics has been a renewed interest in models of psychosis other than the amphetamine model. In line with possible dual involvement of 5-UT and DA. the lysergic acid diethylamide model has been cited as better fitting schizophrenias than the amphetamine model. But, this has been disputed. Interest in serotoninergic involvement is still high and involves eluci-
dating the roles of 5-UT,, and 5-HT, receptors. Interest remains in understanding the psychosis produced by several central anticholinergics. Muscarinic (M, and M4) agonists appear to offer the best approach at this tinle.-'5 The role of the M5 receptor awaits synthesis of Ms-specific Phencyclidine-induced psychosis has been proposed as a
superior model for schizophrenia because it presents both positive and negative symptoms.51' It sugges(s that deficits in glutantinergic function occur in schizophrenia. Results of
498
Wilson
and Gissolds Textbook of Organic Medicinal and
agonists of NMDA receptors overall have not been productive because of the excitatory and neurotoxic effects of the agents tested. Identification of susceptible receptor subtypes as targets, using glycine modulation or group II metabotropic receptor agonists to modulate NMDA receptors, has been proposed to circumvent the problems associated with the NMDA agonists. The ionotropic glutamic acid a-amino-3-hydroxy-5methyl-4-isoxazole propionic acid (AMPA) receptors are activated by brain-penetrating ampakines. There are suggestions that these agents exert some antipsychotic actions by increasing glutarninergic activity. The individual antipsychotic compounds are flow considered. The substituted dopamine motif is useful as an organizational device. Atypical antipsychotics are indicated when they occur. Future growth in this area should be interesting.
Phenothlaalnes Many potentially useful phenothiazine derivatives have been synthesized and evaluated pharmacologically. Consequently, the large body of information permits accurate statements about the structural features associated with activity. Many of the features were summarized and interpreted by
Gordon et al.35 The best position for substitution is the 2 position. Activity increases (with some exceptions) as electron-withdrawing ability of the substituent increases. Another possibly important structural feature in the more potent compounds is the presence of an unshared electron pair on an atom or atoms of the 2 substituent. Substitution at the 3
position can improve activity over nonsubstituted compounds but not as significantly as substitution at the 2 position. Substitution at position I has a deleterious effect on antipsychotic activity, as does (to a lesser extent) substitution at the 4 position.
——
/
—N
I
A3
\A- -i
Phenothiazine Antipsychotic Agents—General Structure
The significance of these substituent effects could be that the hydrogen atom of the protonated amino group of the side
chain H bonds with an electron pair of an atom of the 2 substituent to develop a DA-like arrangement. Horn and Snyder. from x-ray crystallography, proposed that the chlotine-substituted ring of chlorproniazine base could be superimposed on the aromatic ring of dopamine base with the
sulfur atom aligned with the p-hydmxyl of dopamine and the aliphatie amino groups of the two compounds also aligned.36 The model used here is based on the interpretation of the SARs by Gordon et al.5° and on the Horn and Snyder
but involves the protonated species rather than the free base. The effect of the substituent at the I position might be to interfere with the side chain's ability to bring the protonated amino group into proximity with the 2 substituent. In the Horn and Snyder scheme.36 the sulfur atom at
CIaemistr4
position 5 is in a position analogous to the p-hydroxyl of dopamine. and it was also assigned a receptor-binding func-
tion by Gordon et al.35 A substituent at position 4 might interfere with receptor binding by the sulfur atom. The three-atom chain between position 10 and the amino nitrogen is required. Shortening or lengthening the chain at this position drastically decreases activity. The three-atom chain length may be necessary to bring the protonated amino
nitrogen into proximity with the 2-substituent. As expected, branching with large groups (e.g.. phenyli decreases activity, as does branching with polar groups. Methyl branching on the f3 position has a variable effect on activity. More importantly. the antipsychotic potency of leos (the more active) and dexi'ro isomers differs greatly. This has long been taken to suggest that a precise lit (i.e.. receptor site occupancy) is involved in the action of these Decreases in size from a dimethylamino group (e.g., going to a monomethylamino) greatly decrease activity, as do cf fectivc size increases, such as the one that occurs with N.N diethylamino. Once the fundamental requirement of an effec. live size of about that equivalent to a dimethylamino is mainS tamed, as in fusing N.N-dicthyl substituents to generate a pyrrolidino group, activity can be enhanced with increasing chain length. as in N2-substituted piperizino compounds. The critical size of groups on the amino atom suggests
the importance of the amino group (here protonated) for receptor attachment. The effect of the added chain length. once the critical size requirement is met, could be increased affinity. It appears to have been reasonably proved that thc
protonated species of the phenotlsiazines can bind to DA receptors.37
Metabolism of the phenothiazines is complex in detail, but simple overall. A major route is hydroxylation of the tricyclic system. The usual pattern, for which there are good chemical reasons, is hydroxylation pars: to the lO-nitrogcn atom of the ring other than the ring bearing the electron. attracting substituent at the 2 position. Thus, the major metabolite is frequently the 7-hydroxy compound. This corn pound is further metabolized by conjugation with glucuronic acid, and the conjugate is excreted. Detailed reviews of the metabolites of phenothiazines (as well as SARs and pharwacokinetic factors) are available.38 PRODUCTS
The structures of the phenothiazine derivatives descñbol below are given in Table 14-3.
Promazine. Promazine, I0-(3-(dimethylamino) propyl. (phenothiazine monohydrochloride (Sparine), was intro duced into antipsychotic therapy after its 2-chloro-subste tutcd relative. The 2H substituent cis-a-vis the 20 substituent gives a milligram potency decrease as an antipsichotic, as encompassed in Gordon's rule. Tendency to EPS is also lessened, which may be significant, especially if it ci
decreased less than antipsychotic potency.
Chiorpromazine Hydrochloride. USP.
Chlorprorna-
zinc hydrochloride. 2-chioro- I 0-[3-(dimethylamino)pro. pyliphenothiazinc monohydrochloride (Thorazine), was the first phenothiazine compound introduced into therapy. II ci
Chapter 14 • ceniral Neniws
Depressants
System
499
TABLE 14-3 Phenothiazine Derivatives
Generic Name Proprietaty Name
R3
R14,
Propyl Dlatkyiamino Side Chain Proma3ino hydrochloride. USP Spanne
—(CH2)3N(CH3)2• HCI
H
—(CH2)3N(CH3)2 HCI
Ci
—(CH2)3N(CH3)2' HCI
CF3
Thioridazine hydrochioride.USP Mellani
—(CH2)2_(') HCI
SCH3
Mesoridazine besylale.USP
._.(CH2)2_C)
Chiorpromuane
USP
Tho,azirme
Triflupromacine hydrochiocide.USP Vespnn
Akyl Piperidyl Side Chain
Sero,1,1
. C6H5SO3H
SCH3
CH3
Propyl
o T
I
Side Chain
Prochiorperazine maleale,USP Compazine
i'"
—(CH2)3—NN—CH3 2C4H404
Triliuoperezine hydrochlormde. USP
2HCI
I'
Ste/azmne
Perphenazrne,USP
—ICH2)3NNCH2CH7—OI-t
Fluprienazine hydrochloride, USP Permitil. Piolixia
—(CH2)3—N
cliii useful as an antipsychotic. Other uses are in nausea vomiting and hiccough. It is the reference compound in comparisons, that is. the compound to which others compared. The drug has significant sedative and hypoproperties, possibly reflecting central and peripheral m1-noradrenergic blocking activity, respectively. Effects of xnpheral anticholinergic activity are common. As with the cher Ithenoihiazines, the effects of other CNS-deprnssant jugs. such as sedatives and anesthetics, can be potentiated.
Triflupromazine Hydrochloride. USP. Trifluproniawe hydrochloride, lO-[3-(dimethylamino)propylj-2-(triflu'mmeihyl)phenothiuzine monohydrochioride (Vesprin). has
r\
N—CH2—CH2--OH 2HCI
Cl
CF3
CI
CF3
a
an
antipsychotic. EPS
are higher. The 2-CF3 versus the 2-Cl is associated with these changes. Overall, the drug has uses analogous to those
of ehiorpromazine.
Thioridazine Hydrochloride, USP.
Thioridazine hy-
drochloride, lO-[2-( I -methyl-2-piperidyl)ethyll-2-( methylthio)phenothiazine monohydrochloride (Mellaril), is a member of the piperidine subgroup of the phenothiazines. The
drug has a relatively low tendency to produce EPS. The drug has high anticholinergic activity, and this activity in the strialum. counterbalancing a striatal DA block. may be responsible for the low EPS. it also has been suggested that there may be increased DA receptor selectivity, which may
500
Wily,,: and (li.s:'old's
of
Methr,nal and Pharn,areu:ieal Ciwmiorv
be responsible. The drug has sedative and hypotensive activ-
ity in common with chlorproinaiine and less antiemetic activity. At high doses. pigmentary retinopathy has been observed. A metabolite of the drug is mesoridazine (discussed next).
Mesoridazine besylate. Mesoridazine Besylate, USP. 2-mcthyl-2-piperidyl )ethyl I-2-(methylsulilnyl)phcnoI thiazinc monobenzencsul Innate (Serentil). shares many properties with thioridazine. No pignscntary retinopathy has been reported, however.
logical properties to the corresponding phenothiazines. Thus. thiothixene (Z.N-dimethyl-9-[3-(4-methyl- I -piperaiinyl)propylidenelthioxanthenc-2-sulfonamide (Navane). dis' plays properties similar to those of the piperazine subgroup of the phenothiazines.
Q(XJL II
H—C—CH2—CH2—N
Prothiorperazine Maleate, maleatc.
N—CH3
\—,
Prochlorperazine USP. 2-chloro- IO-(3-(4—methyl- I -piperazinyl)propyll-
phenothiazine maleate (Compazine). is in the piperazine subgroup of' the phenothiazines, characterized by high milligram antipsychotic potency. a high prevalence of EPS. and Prochiorperazine is low sedative and uutonomic more potent on a milligram basis than its alkylamino counterpart. chlorpromazine. Because of the high prevalence of EPS, however, it is used mainly fur its antiemetic effect, not for its antipsychomic effect.
A dibenzoxazepine derivative in use is loxapine succi' nate, 2-chloro- II -(4-methyl-I -piperazinyl)dibenzib. Jill. 4loxazepine succinate (Daxolin). The structural relationship to the phenothiazine antipsychotics is apparent. It is an
live antipsychotic and has side effects similar to those ic ported for the phenothiazines.
Perphenazine, USP. Perphenazine. nothiazine- IO-yl)propyl Ipiperazineethanol; 2-chloro- 10-1314-(2-hydroxycthyl )pipcrazinyl ipropyl Iphenothiai.ine (Tnlafon). is an effective antipsychotic and antiemetic.
Fluphenazine Hydrochloride. USP.
CH2COOH CH2COOH
The member of
the piperazine subgroup with a trilluoromethyl group at the 2-position ni the phenothiazine system and the most potent antipsychotic phenothiazine on a milligram basis is liuphenazine hydrochloride. 4-13-12-(trifluoromethyl)phcnazin- 10yl J propyl I-I -piperazineethanol dihydrochloride. I 013-14-(2-
hydroxycthyl)pipcrazinyll propyll- 2-tmiiluoromethylphcnothiazine dihydrochioride (Perniitil. Prolisini. It is also available as two lipid-soluble esters for depot intramuscular injection, the ei:anthate (heptanoic acid ester) and the decanoale ester. These long-acting preparations have use iii treating psychotic patients who do not take their medication or are subject to frequent relapse.
Loxapine Succinate
The dibenzodiazepine derivative is clozapine tClozacii'. It is not a potent antipsychotic on a milligram basis
the orientation of the N-methyl piperazino group relative the chlorine atom). It is effective against both positive a low to produce EPS. There are legal restrictions on us use cause of a relatively high frequency of agranulocysosis. A: a rule, two other antipsychotics are tried before recourse to therapy with clozapinc.
Ring Analogues of Phenothlazlnes: Thiozanthenes. Dlbenzoxazeplnes, and UlDenzodlazeplnes The ring analogues of phcnothiuzines are structural relatives of the phenothiazine antipsychotics. Most share many clinical properties with the phenothiazines. 'fhe dibenzodiazepinc clozapinc has some important differences, however, no-
Cl
N
\CH3
tably low production of EPS and reduction of negative symptoms. It is an important atypical antipsychotic. Clorapine
Thiothixene, USP.
The thioxanthene system differs
from the phenothiazine system by replacement of the N-H moiety with a carbon atom doubly bonded to the propylidene side chain. With the substituent in the 2 position. Z and E isomers are produced. In accordance with the concept that the presently useful antipsychotics can be superimposed on DA. the Z isomers arc the more active antipsychotic isomers. The compounds of the group arc very similar in pharmaco-
—- —r--—--—--——
The fluorobutyrophenones belong to a much-studied pin;
of compounds, many of which possess high a few of these are used in the United Suir which can be misleading about the importance of the pin' and its evolved relatives. The structural requirements hi
Chapter 14 • C'en,rul
antipsychotic activity in the group are well worked out. General features are expressed in the following structure. AR
Optimal activity is seen when AR1 is an aromatic system.
Ap.fluorn substituent aids activity. When X = C = 0. optima] activity is seen, although other groups, C(H)OH and
OU)aiyl, also give good activity. When n = 3. activity is tsptimal; longer or shorter chains decrease activity. The aliphatic amino nitrogen is required, and highest activity is
is incorporated into a cyclic form. AR2 is an jnnnatic ring aiid is needed, It should be attached directly to the 4 position or (occasionally) separated from it by one Intervening atom. The Y group can vary and assist activity. An example is the hydroxyl group of haloperidol. The empirical SARs could be construed to suggest that the 4-aryl piperidino moiety is superimposable on the 2pttcnylelhylamino moiety of dopamine and, accordingly. could promote affinity for D2 and D3 receptors. The long V.atkyl substituent could help promote receptor affinity and pmduce receptor antagonism activity and/or inverse ago's-en when it
Some members of the class are extremely potent antipsychotic agents and D2 and D3 receptor antagonists. El'S are
cxtremely marked in some members of this class, which may. in part, be due to a potent DA block in the striatum and almost no compensatory striatal anticholinergic block. Most of the compounds do not have the structural features
with effective anticholinergic activity. Haloper!dol, USP.
Svsien. Dv.prexsai.i.s
501
Risperidone.
Risperidone Risperdal has the structural features of a hybrid molecule between a butyrophenone antipsychotic and a trazodone-like antidepressant. It benefited
refractory psychotic patients, with parkinsonism controlled at one-tenth the dose of antiparkinsoniun drugs used with haloperidol.4° Coexisting anxiety and depressive syndromes were also lessened. It is reported to decrease the negative (e.g.. withdrawal, apathy) as well as the positive (e.g.. delusions. hallucinationsl symptoms of schizophrenia. This is reportedly a consequence of the compound's combination 5-HT2—D2 receptor antagonistic Overall the reasons for the decreased El'S and effectiveness against negative symptom are still under investigation. It is an important
atypical antipsychotic. N—O
0 F
CH3
The diphenylbutylpiperidinc class can be considered a modification of the fluorohutyrophenonc class. Because of their high hydrophobic character, the compounds are inherently long acting. Penfluridol has undergone clinical trials in the United States, and pimozide has been approved for
antipsychotie use. Overall, side effects for the two compounds resemble those produced by the lluorohutyrophenones. F
Haloperidol,
(Haldol). potent antipsychotic useful in schizophrenia and in psyassociated with brain damage. ft is frequently chosen as the agent to terminate mania and often used in therapy for Gilles de Ia Tourefle's syndrome.
•N
N—H
Pimozicie
Haloperidol
USP. Droperidol. I-( l-13-(p-fluorobenioyl)propylj- 1.2. 3.6-tetrahydro-4-pyridyl-2-benzimidazoliDroperidol,
none Ilnapsine). may be used alone as a preanesthetic neuroor a.s an antienietic. Its most frequent use is in
(Innovar) with the narcotic agent fentanyl Sublimaze) preanesthetically.
1!CH2CH2CH2—
Dropendol
11-Amlnoketones Several fl-aminoketones have been examined as antipsyThey evolved out of research on the alkaloid lobeline. The overall structural features associated with activity can be seen in the structure of niolindone. In addition to the
502
Wilson and (Jisro!d.s Textbook of Organic Medicinal and Plsarynoceutical
$-aminokctone group. there must he an aryl group posi-
Olanzapine and Quetiapine.
tioned as in molindone. It might be conjectured that the proton on the protonated amino group in these compounds Hbonds with the electrons of the carbonyl oxygen atom. This would produce a cationic center, two-atom distance, and an aryl group that could be superimposed on the analogous features of protonated dopamine.
and quetiapine (Seroquel) possess tricyclic systems cith greater electron density than chiorpromazine. They thus semble clozapine. The drugs are atypical antipsychotics.
Olanzapine (Zyprexai
Mollndone Hydrochloride.
Molindone hydrochloride. 3.ethyl-6,7-dihydro-2-methyl-5-morpholinomethyl)indolc4(511)-one monohydrochloride (Moban). is about as potent an antipsychotic as trifluopcrazine. Over.ill. side effccts resemble those of the phenothiazines.
0
I CH2—("O CL
CH3CH2
H
I
Otanzaplne
/ \—J
N Hydrochloride
Benzamldes The bcnzamides evolved from observations that the ga.stroprokinetic and antiemetic agent mctoclopramide has antipsy-
chotic activity related to D2 receptor block. It was hoped that the group might yield compounds with diminished EPS liability. This expectation appears to have been met. A H bond between the amido H and the unshared electrons of the methoxy group to generate a pseudo ring is considered important for antipsychotic activity in these compounds. Presumably, when the protonated amine is superimposed on that of protonated dopamine. this pseudo ring would superimpose on dopamine's aromatic ring.42 These features can be seen in sulpiride and renioxipride.
,,0
Ouetiaplne
Overall, these two compounds should hind less strnngl.i to D2 receptors and pemlit more receptor selectivity among receptor subtypes than typical antipsychotics. This cou!dz
count for decreased striatal D2-blocking activity. which would produce less discomfort in patients. It would be esting to see testing results of these drugs' activities over. broad range of receptors, as arc presently being for clozapinc.
With respect to the atypical antipsychotics. two long in the past may shed some light on the events The field of reuptake-inhibiting aittidepressants arose ehe only a very small structural change was made in an antipn.
0
25
CH3 Sulpieride
duce antipsychotics that are active against depressive
Remoxlpride Rentoxipride is a D2 receptor blocker.40 It is said to be ax effective as haloperidol with fewer EPS. Negative symptoms of schii.ophrenia arc diminished. The drug is classed as an atypical antipsychotic. The substituents on the aliphatic amino nitrogen and the substituents on the aromatic ring are interesting. C2H5
O\/NHCH2N
Remoxipride
chotic drug. and the new activity noted. (The atitiNcluls activity remained.) So. small changes in structure can
toms. Likewise, small changes in structure could selectivity among D2 receptors. Almost 40 years ago, it was noted that thioridnj.ine wasi less unpleasant for patients than its relativcs.4 Its system is far more nucleophilic than that of most otherdru?' The emphasis at the time, however, was to increase null gram potency by increasing D2 receptor affinity by tricyclic electron density. The experience of clozapine. sir increased electron density of the receptor-binding thus lower affinity, appears to validate the observation and appears to allow more selectivity D2 receptors. Lessening blocks on. for example. sifinial ll receptors. and possibly mesocortical D2 receptors as s. could produce drugs that are muich less unpleasant Its 1. patient. Additionally, a less intense 1)2 block could au. the effects of other blocks to make up more of the das total action (e.g.. 5-HT transporter block). Several
Chapter 14 • Central Nervous System Depressants
anhipsychotics have rings with enhanced nucleophility. 01 course. other structural features could be influencing receptar selectivity, for example, increasing stcric hindrance to receptor binding by the protonated amino group or to the rag binding.
Antimanic Agenb LITHIUM SALTS
The lithium salts used in the United States are the carbonate
leirahydrate) and the citrate. Lithium chloride is not used tvcause of its hygroscopic nature and because it is more nitahing than the carbonate or citrate to the GI tract. The active species in these salts is the lithium ion. The classic explanation for its antimanic activity is that it resembles the sodium ion (as well as potassium. magnesium, and calcium ions) and can occupy the sodium pump. Unlike the aiiu:n ion, it cannot maintain membrane potentials. Acflitters
it might prevent excessive release of neurotrans(e.g.. dopamine) that characterize the manic state.
Many of the actions of lithium ion have been reviewed." The indications for lithium salts are acute mania (often with neuroleptic agent for itnmediatc control, since lithurn is slow to take effect) and as a prophylactic to prevent cccurrence of the mania of bipolar manic—depressive illness.
Lithium salts are also used in severe recurrent unipolar depression. One effect of the drug that might be pertinent increase in the synthesis of presynaptic serotonin. Some speculated that simply evening Out transmission. preunhing downward mood swing. for example. could be a for antidepressant action. Because of its water solubility. the lithium ion is extensvcl> distributed in body water, It tends to become involved fl he ntany physiological processes involving sodium. poasiurn. calcium, and magnesium ions, hence, many side rtIccts and potential drug interactions exist. The margin of safety is low; therefore lithium should be used only when
503
an anmiepileptic drug is a drug used medically to control the epilepsies. not all of which are convulsive, in humans. A classification of the types of epilepsy has been widely accepted because its accuracy facilitate.s diagnosis, drug selection. and precise discussion of seizure The major classification types are (a) generalized seizures, which essentially involve the entire brain and do not have an appar-
ent local onset: (b) unilateral seizures, which involve one entire side of the body: (e) partial (or focal) seizures that have a focus (i.e., begin locally); (d) erratic seizures of the newborn: and (e) unclassified seizures (severe seizures asso-
ciated with high tnortaliiy such that time does not permit a precise categorization). Two major types of generalized seizures are the generalized tonic—clonic seizure (grand mal) and the nonconvulsive seizures or absence (petit mal) seizures. The typical general-
ized tonic—clonic seizure is often preceded by a series of bilateral muscular jerks: followed by loss of consciousness. which in turn is followed by a series of tonic and then clonic spasms. The typical absence seizure (classic petit mal) consists of a sudden brief loss of consciousness. sometimes with
no motor activity, although often some minor clonic motor activity exists. Major types of focal (partial) epilepsy are simple focal and complex focal seizures. A prototypic simple partial seizure is
jacksonian motor epilepsy in which the jacksonian mareh may be seen. As the abnormal discharge proceeds over the cortical site involved, the visible seizure progresses over the area of the body controlled by the cortical site. The complex partial seizure is represented by the psychomotor or temporal lobe seizure. There is an aura, then a confused or bizarre but seemingly purposeful behavior lasting 2 to 3 minutes.
often with no memory of the event. The seizure may be misdiagnosed as a psychotic episode. This is an extremely
difficult epilepsy to treat. Much effort has been made in recent years to develop drugs to control it.
Lith-
For broad consideration of how structure relates to antiepileptic activity, the classification of the epilepsies is traditionally further condensed (generalized tonic—clonic seizures. simple partial seizures, complex partial seizures, and absence seizures). The broad general pattern of structural features associated with antigeneralized tonic—clonic seizure activity is discernible for barbiturates. hydantoin.s, oxazolidincdiones. and succinimides. This SAR also applies to simple partial seizures. It applies with less certainty to complex partial seizures, which are relatively resistant to treatment. With fewer effective drug entities, overall structural conclusions are more tenuous. The other general seizure type for which a broad SAR pattern among the cited compounds can
uncarhoname (Eskalith. Lithane) and lithium citrate (Cihal-
be seen is the absence seizure. These features are cited under
Lb'S) are the salts commercially available in the United
the heading. SARs Among Anticonvulsants. Likewise, animal models characteristically discern three types of activity: activity against electrically induced convulsions correlates with activity against generalized tonic—clonic and partial seizures, and activity against pentylenetetrazole (PTZ)-induced seizures correlates with antiabsence activity. Of late, a fourth model, activity against pilocarpine and kainic acid seizures, is said to predict protection against temporal lobe epilepsy (a complex partial seizure). Each of the epilepsy types is characterized by a typical abnormal pattern in the EEG. The EEG indicates sudden. excessive electrical activity in the brain. Antiepileptic drugs
plasma levels can be monitored routinely. In the desired dose range. side eflécts can be adequately controlled.
Because of the toxicity of lithium, there is substantial inxrest in design of safer compounds. As more is learned about tham's specific actions, the likelihood of successful design
i ecrnpounds designed to act on specific targets is inarased. Actually. carhamazepine and valproic acid, which '.utsodiLtm channels, are proving to he effective.45 These :vn thugs are discussed in the anticonvulsant section.
Lithium Carbonate, USP, and Lithium Citrate.
Siec.
AN1'ICONVULSANT OR ANTIEPILEPTIC ORUGS
i
:e
customary. the terms antic'ans'uI.sant and a,muepik'pl:c used interchangeably in this discussion. Strictly speakhowever, an anticonvulsant is an agent that blocks cxproduced seizures in laboratory animals, and
is
504
tVilson
and
of Orçnmic Medicinal and P/iarmaceuucal Clie,nisirv
act to prevent. stop. or lessen this activity. The precise causes of the sudden. excessive electrical discharges may be many. and not all are understood. A working hypothesis is that there is a site or focus of damaged or abnormal and, consequently.
I
hyperexcitable neurons in the brain. These can fire excessively and sometimes recruit adjacent neurons that in turn induce other neurons to fire. The location and the extent of the abnormal firing determine the epilepsy. An addition to this theory is based on the kindling model.45 Experimentally. a brief and very localized electrical stimulus is applied to a site in the brain, with long intervals between applications. As the process is repeated. neuronal afterdischarges grow both longer and more intense at the original site and at new sites far from the original site. It is thought that changes occur in neurons at the discharge site, and these neurons in turn induce changes in neurons far from the site. Progressively more severe seizures can be induced, and these can arise from secondary foci that have been kindled far from the site of stimulation. A major mode of action of anticonvulsants can be positive allosteric modtilation of GABAA receptors. This is probably the mode of action of benzodiazepincs and a major mode of action of barbiturates. On the basis of the structure of barbiturates, some inorganic cation blocking action would be expected as wefl—voltage-gated sodium channel for phenobarbital and calcium I channel block for 5,5-dialkyl members. Oxazolidine-2.4-diones (only trimethadione remains> and succinimides appear to act via calcium T-type channel block. Some sodium channel block could be expected among phenyl-substitutcd succinimides. The major mode of action for phenytoin (and probably monophenyl substituted hydanacid. felbatoins). carbamazepine. oxcarbazepine. mate. topiramate. lamoirigine and zonisarnide is reported to be voltage-gated sodium channel block and is in accord with their structures. This does not exclude other expected actions in some of the examples.
Direct block of ionotropic glutamate receptors has so far not yielded clinically useful drugs. Some voltage-gated sodium channel drugs are reported to be antiglutaniate as well by blocking glutamate release. Side effects of direct ionotropic glutamic acid receptor blocking has been a serious problem, Because of this, present approaches are to use the modulatory route. That is. lessen ionotropic glutamate activity by (a) using drugs that act at the glyeine modulatory site on NMDA and (hI developing antagonists of members group II and group Ill melabotropic receptors and agonists of metahotropic group I glutamic acid receptors. These drugs would lower ionotropic glutaminergic activity. Adenosine. which may be an endogcnous anticonvulsant.
Structure common to anticonvrjtsanl drugs.
R'
0
NH
—o
An overall pattern in the foregoing is that R and R'
(grand mal) or partial seizures. If one of the hydrocartico substituents is an aryl group, activity tends to be toward generalized tonic—clonic and partial seizures and nil antiabsence activity.35 A conformational analysis of the aryl-containing antigrt eralized tonic—clonic agents indicates that the confonn tional arrangement of the hydrophobic groups is imporlarn,n
Barblthrates Although sedative—hypnotic barbiturates commonly disph:, anticonvulsant properties. only phenobarbital and meph. barbital display enough anticonvulsant selectivity for ascii
antiepileptics. For the structures of these agents. Table 14-2. and fordiscussion oichemical propeniessecthr section on barbiturates under agents. The metabolism of phenobarbital involves p.hydrm
ylation. followed by conjugation. Mcphobarbital is extensively N-demethylated in vivoni is thought to owe most of its activity to the metabolite phecs
barbital. In keeping with their structures, both agents tic effective against generalized tonic—clonic and panial c'
Hydantoins
Several major groups of drugs have the common structure shosvn below.
are loscr
alkyls. the tendency is to be active against absence seiruar (petit mal) and not active against generalized tonic—clonic
effects of agonists. has not yet yielded useful drugs. Elabo-
SARs Among Antkonvutsants
Oxazolidinediones
both be hydrocarbon radicals. If both R and
zures.
drug design.
I-tydantoins
Succinimides
continues to serve as a model hut, for reasons such as poor brain distribution and an array of cardiovascular
ration of roles of receptor subtypes may give leads lbr
Barbtturates
The hydantoins arc close structural relatives of the barhis rates. dil'Iering in lacking the 6-oxo group. They ate cyth monoacylureas rather than cyclic diacylureas. As a con' quence of losing a carbonyl group. they are weaker organ. acids than the barbiturates (e.g.. phenytoin pK,, = aqueous solutions of sodium salts, such as of phenyroin dium, generate strongly alkaline solutions.
Chapter 14 U ('eniral
TABLE 14-4 Anticonvulsant Hydantoin Derivatives
Name
conjugated. The compound is used against generalized seizures, but
R'5
?yimoUSP
-'
Mesonloin
H
hydroxyl group. The drug has a spectrum of activity similar to that al phenytoin. It may worsen absence seizures.
Ethotoin. 3-ethyl-5-phenylhydantoin (Peganone), is N.dealkylated and p-hydroxylated: the N-dealkyl inetabolite. presumably the active compound. is likewise metabolized by p-hydroxylation. TIre hydroxyl group is then
Substituents Rs
505
Ethotoin.
N—H
Generic Name
SvOens
R3
H
usually on an adjunctive basis. owing to its low potency. In general. agents that are not completely branched on the appropriate carbon have lower potency than their more completely branched counterparts.
CH3—
Oxazolidinedlones
CH3—CH2—
Replacement of the N-H group at position I of the hydantoin system with an oxygen atom yields the oxazolidine-2.4dione system. The oxazolidinedione system is sometimes equated with autiabsence activity, but this trophisni probably is more dictated by the fact that the requisite branched atom
The compounds have a trophism toward antigencralized eric—clonic rather than antiabsence activity. This is not an rnninsic activity of the hydantoin ring system. All of the dinically useful antigeneralized tonic—clonic compounds Table 14-4) possess an aryl substituent on the 5 position. to the branched atom of the general pharmacophore. Hydantoins with lower alkyl substituents reportoily have antiabsence activity.
Phenytoin and Phenytoln Sodium, USP. Phenytoin. 33-diphenyihydantoin (Dilantin). is the first anticonvulsant in which it was clearly demonstrated that anticonvulsant accould definitely be separated from sedative—hypnotic It is often cited as the prime example of an anticoncabant acting as a sodium channel blocker.'3 °' One effect of neuronal sodium channel block is to decrease presynaptic
°' acid release, giving anticonvulsant Another consequence is to reduce glutamate-induced isitiemic damage to neurons)'1 52 The drug is useful against if oeizurc types except absence. It is sometimes noted that he drug is incompletely or erratically absorbed from sites if alministration. This is due to its very low water solubility. Mesibolism proceeds by stereospecific p-hydroxylation of .n nromatic ring, followed by conjugation. Mephenytoin, USP. Mephenytoin. 5-ethyl-3-methyl'phenyl-hydantoin (Mesantoin), is metabolically N-dealkyted to 5-ethyl-5-phenylhydantoin, believed to be the active
Interestingly, 5-ethyl-5-phenylhydantoin. the hydan'iincounterpan of phenobarbital. was one of the first hydan. introduced into therapy. It was introduced as a sedaa-hypnotic and anticonvulnant under the name Nirvanol. it was withdrawii because of toxicity. Presumably. meph. ccyioin may be considered a prodrug that ameliorates some ci the toxicity—serious skin and blood disorders—of the active drug. Metabolic inactivation of mephenytoin and its demethyl is by p-hydroxylation and then conjugation of the
of these compounds is substituted with lower alkyls. Arylsubstituted Oxazo)lidine-2.4-diones have shown activity against generalized tonic—clonic seizures. The oxazolidined-
ione group of anticonvulsants used clinically has shrunk to one clinically useful member. Toxicities associated with the group may be the problem.
Trimethadione, USP. Trimethadione. 3.5.5-trimethyl. 2.4-oxctrolidinedionc. 3,5,5-trimethadionc (Tridione), was the first drug introduced specifically for treating absence seil.ures. It is important as a prototype structure for antiabsence compounds. Demiatological and hematological toxicities limit its clinical use. The drug is metabolized by N-demethylation to the putative active metabolite Dimethadione is a calcium T channel blocker. Dimethadione is a water-soluble and lowly lipophilic compound and thus is excreted as such without further metabolism.
CH3 R5 =
= CH,
Sucdnlmldes In view of the activity of antiepileptic agents sttch as the oxazolidine-2,4-diones, substituted succinimides (CU. replaces 0) were a logical choice for synthesis and evaluation. Three are now in clinical use.
Phensuximide, USP. Some trophisni toward antiabsence activity is attributed to the succinimide system. The —CH2— could be viewed as an a-alkyl branch condensed into the ring. Phensuximide. N-methyl-2-phenylsuccinimide (Milontin). is used primarily against absence seizures, hut it has low potency and is relegated to secondary status. The
Wilson 011(1
Textbook of Organic Medicinal and Phar,nacegitical Che,nis:rv
phenyl substituent confers some activity against generalized tonic—clonic and partial seizures. N-demechylation occurs to yield the putative active metabolite. Both phensuximide and the N-dcmethyl metabolite are inactivated by p-hydroxylation and conjugation.
at the (Z)cis-stilhene double bond. In humans, the epoxidc reportedly is converted largely to the lOS. I The epoxide is a suspect in the idiosyncratic reactions carba. mazepine may produce (e.g.. aplastic anemia). With this in
mind, compounds designed to avoid the epoxide such as oxcarbazepine (Trileptal) were developed.
CH2
Methsuxim4e
R=Q__R'=cH3
/
C=0
R'=CH3
R = C2515—. R' = CH3, A'
H
Methsuximide. N-demethylation and p-hydroxylation of parent and metabolite occur. Methsuximide, N.2-dimcthyl-2-phenylsuccinimide (Celontin), has some use against absence and complex partial seizures.
Ethosuximide. 2-ethyl-2-methylEthosuximide, USP. succinimide (Zarontin). conforms veiy well to the general structural pattern for antiabsence activity. The drug is more active and less toxic than trimethadione. It is a calcium T channel—blocking drug. Toxicity primarily involves the skin and blood. Some of the drug is excreted intact. The major metabolite is produced by oxidation of the ethyl group.
Wean and Monoacylureas The two chemical classes. ureas and monoacylureas. have a long history of producing compounds with anticonvulsant activity. The numerical yield of clinically useful compounds has not been great, however. Most of the simpler compounds have gone by the way. For convenience of grouping. carbamazcpine and oxcarbazepine can be considered N.N-diacyl-
Oxcarbazeplne
Oxcarbazepine is reduced to the monohydroxy compour4.
undoubtedly stercospecifically. The monohydroxy corn pound is considered the major active melabolite. The drug is used against partial seizures. The major mechanism of action is sodium channel block.
Miscellaneous Agents Primidone. Primidone. 5-ethyldihydro-5-phenyl.4,fs (IH.5H)-pyrimidinedione (Mysolinc). is sometimes scribed as a 2-dcoxybarhiturate. Ii appears to act as such
through conversion to phcnobarbital and to lonyldiarnide The efficacy is against all types ii seizures except absence. The agent has good sakty but rare serious toxic effects do occur.
/\o
ureas. H
carbamazeplne. USP. Carbamazepine. 5H-dibenzlb.fllazepine-5-carboxamide (Tegretol). for SAR discussion purposes, can be viewed either as an ethylene-bridged 1.1diphenylurea or an amido-substituted tricyclic system. The two phenyls substituted on the urea nitrogen fit the pattern
of antigeneralized tonic activity. The overall shape of the molecule suggests the mode of action, sodium channel block.
Carbamazepine is useful in generalized tonic—clonic and partial seizures.
O==C—NH2 Carbamazepine
The drug has the potential for serious hematological toxic-
ity, and it is used with caution. Metabolism proceeds largely through the epoxide formed
Pnmidone
Vaiproic Acid.
Many carhoxylic acids have anticonsU
sant activity, although often of low potency, possibly Inpi" because extensive dissociation at physiological pH prodas. poor partitioning across the blood—brain barrier. acid. 2-propylpentanoic acid (Depakene). has good and is used against several seizure types. They include cal and atypical absence seizures and absence seizure generalized tonic—clonic seizure. Mechanistically, the la is a sodium channel blocker. This is in accordance wnhq structural features. It is also reported to increase els. again in conformity with its structure. Metabolism conjugation of the carboxylic acid group and oxidatka one of the hydrocarbon chains. Many of the side effcct'rr
mild. A rare, but potentially fatal, fulminate hepatitis caused concern, however. One tends to look to the
atom a to the carboxyl acid as being labile and a toxiphore.
Chapter 14 • Central Nen'ous
Depressants
507
Lamotrigine.
Lamotrigine (Lamictal) has been found effective against refractory partial seizures. It is said to act by blocking sodium channels and preventing glutamate reIt is a member of a group of drugs that reduce gluta-
CH 3CH2CH2
mate release and thus reduce neuronal cell death in ischemia.
Vaiproic Acid
Despite the fact that gabapentin (Neuroatin) is a relative of GABA with increased hydrophobic character, its mechanism of action does not appear to involve an interaction with GABAA receptors. A binding site on calcium channels has been identified, but the mode of action of the drug is considered unclear. The drug is said to have
One trial with lamotrigine did not detect slowing of the progression of amyotrophic lateral sclerosis (ALS). Another member of the group (sodium channel blockers with antiglutamate effect), riluzole (Rilutek) (2-amino-6-(Irifuroethoxy)benzothiazole) is used to slow progression. The bottle-stopper shape of both drugs is readily apparent.
a good pharmacokinctic profile and to cross the blood—brain
barrier well, it was introduced for adjunctive therapy of refractory partial seizures and, secondarily, generalized NH2
Ionic—clonic seizures. It was studied as a single drug therapy for various
Lamotrigine
and Topiramate (Topamax).
Zonisamide H2N
OH
Gabapenrin
(Gabitril).
A glance at tiagabine's structure cuggeats an uptake inhibitor. Reportedly, it blocks GABA rcuptake as a major mode of its anticonvulsant activity. its
Zonisamide and Topiramate have, respectively, the sulfonamide and sulfate amido as the small diameter end polar group and an extensive hydrophobic group as the large diam-
eter end of the bottle stopper. Both are sodium channel blockers. Zonisamide also blocks calciuin-T channels and Topamax increases the effect of GABA and antagonizes glutamate kainic acid/AMPA receptors. Each of the drugs is employed adjunctively against partial seizures.
iw is against partial seizures. .COOH
(Na H2N
2 /\\ 0
Zonisarnide
l'iagabine Felbamate.
Felbamate (Felbalol) has been used sue-
in refractory patients with generalized tonic—clonic
and complex partial seizures. The mechanism of may involve an interdction with the strychnine-insenreceptor on the NMDA receptor.°t' It is also a sodium blocker. The drug is associated with a serious risk anemia. It is used with extreme caution after other criconvulsants have been tried and a careful risk-to-benefit has been made.
Felbamate
Topiramate
Benzodlazeplnes For details of the chemistry and SARs of the benzodiazepines, see the discussion of anxiolytk—sedative—hypnotic drugs. Among the present clinically useful drugs. the structural features associated with anticonvulsant activity are identical with those associated with anxiolylic—sedative—hypnotic activity.22 Animal models predict that benzodiazepines are modestly effective against generalized tonic—clonic and partial seizures and very highly active
508
Wilson and Gi olds Texil,ook of Organii' Medici,wI and PI,arn,aceiaical Chemistry
against absence seizures. This difference in seizure control tropism differs markedly from that of the barbiturates, hydantoins. and most other chemical compounds when they are aryl- or diaryl-subslituted. Despite the high effectiveness of benzodiazepines as a group in animal models, only a few benzodiazepines have achieved established positions in anticonvulsant therapy. Because selective anticonvulsants should be attainable among agents acting at GABAA benzodiazepine allosteric modulatory sites, the number may increase in the future. A problem with the benzodiuzepines has been decreased effectiveness over time. When physiological adaptation of this type occurs, it usually happens with sedative agents. If sedation were divorced from anticonvulsant action, possibly the latter might be sustained.
clonazepam. USP.
Clonazepam 5-(2-chlorophenyl)-3(Kionopin). dihydro-7-nitro-2H- I .4-benzodiazpin-2-one partially selective at bcnzodiazepine allosteric binding sites on GABAA receptors, is useful in absence seizures and in myoclonic seizures. Tolerance to the anticonvulsant effect
often develops, a common problem with the benzodiazepines. Metabolism involves hydroxylation of the 3 position. followed by glucuronidation and nitro group reduction, followed by acetylation.
I/I
H
0
N—C
C=N
ON 2
Clortazepam
galed nit channels as molecular sites ol alcohol and anesthelic action
Ads. Btochcm. Psychopharmaciil. 47.335, 1992.
5. Miller. K. W,: General anesthetics, in Wolff. M. D. ted.,. fluigci'
6.
Medicinal Chcotislry, part III. 4th ed. Ness York. John Wiley & Sew 1981. p. 623 (and referetices therein). F. H.. F,ynng. H.. mid i'olissar. M. 3.: The Kinetic Basiic)
Molecular Biology. New York. John Wiley & Sons. 1954. 7. Cohen, F.. N.: Br. J. Anacslh. 511:665,
978.
8. Stock. J. 0. I... and Strunin. L.: Anesthesiology 63:424. 1985. 9. Cousins, M. 3.. and Marie, R. L.: JAMA 225:1611. 1973. 10. Hiti. B et at.: J. Pliarinacul. Esp. 'flier. 2(13:193. 1977. II. Holiiday. B. A.. et at.: Anesthesiology 43:325. 975. 12. Willer.J. C.. flergeret. S.. Gaudy, J. H.. and l)authier. C.: Anesthesri ogy 63:467. 1985. 13. Takaki. K. S.. and Eppcrson. S. R.: Annu. Rep. Med. Cheni. 3441 1999.
14. Huang. i-K., and Jan. C.-R.: Life Sci. 68:611. 2(88). 15. Weinherger. B. K.: N. lingl. J. Mcd,344:l247. 2001, 6. Xue. II.. ci al.: J. Mcd. Cheni 44:1883. 20(11. 17. Xue, H.. ci al.: J. Mol. Bind. 296:739. 2000. 18. Renard. S.. et at.: J. Blot. Client. 274:13370, 1999. 19. Buhr. A.. ci at.: Mul. Pharniacol. 49:1080. 19%. 20. Buhr. A.. ci al.: Mol. Pharmacol. 52:672, 1997, 21. Bohr, A.. et at.: 3. Neurochem. 74:1310,2(88). 22. Slernbach, L. H.: In Garatlini. S., Mi,ssini, E.. and Randall, 1. 0.ieki The Benzodiazepines. New York. Raven Press. 1972. p. I. 23. Chuldress. S. J.: Antianxiety agents. In Wolff. M. B. (nil. Qarg& Medicinal Chemistry, part lIt. 4th ed New York. John Wile) and Sir 1981, p.981. 24. Grecnhlatt, 0. J.. and Shader. K. I.: Bcnzodiaeepines in Clinical Ps. 11cc. New York, Raven Press, 1974. p. 7 (and references thcreinm 25. Greenblati. D. .1., Shader. K. I.. and Abernathy. D. R.: N. EngI. 1. M:1 309:345, 410, 1983.
26. Daniels. T. C.. and Jorgensen. F. C.: Central nervous system kr sums. In Docrge, R. F. ted.). Wilson and CIissold's Tc,tlbook of Medicinal and Pharmaceutical Chemistry. 8th ed. Philadelphut, 3.
Ltppincott. 1982. p. 335. 27. Berger. F. M.: Meprobamate and other glycol derivatives. In U,ij E.. and Forrest. I. S. teds.). Psychotherapeulic Drugs, pail IL Ii:. York. Marcel Dekker. 1977. p. 1089. 28, Cram. 0. J.. and Hammond. 0. S.: Organic Chemistry. 2nd ed. 5:: York. McGruw'HiIl, 1964. p. 295. 29. Mackay. F. J.. and Cooper. J. R.: J. Pharmacol. Exp. The,. 1352' 1962.
Dlazepam.
For details on diazepam (Valium) see its
discussion under anxiolytics and sedative—hypnotic agents. The drug is mainly useful in treating generalized tonic—clonic status epilepticus. which is an ongoing and potentially fatal generalized tonic—clonic seizure.
chlorazepate.
See the detailed discussion of chlorazepate (Tranxene) in the sedative—hypnotic—anxiolytic section. U.s principal anticonvulsant use is adjunctively in complex partial seizures.
Overall, there has been progress in recent years in the intro-
duction of antiseizure drugs. Most of the progress has involved voltage-gated sodium channel blocking Good reviews arc available.57' REFERENCES I. Strange. p. (1: Phamiacol. Rev. 53:119. 2001. 2. Longoni. B.. and Olsen. K. W.: Sludie'i on the mechanism at interaction of with GABA5 receptors. Mv. Biochem. Psychophurmai• ciii. 47:365. 1992. 3. Chebib. M.. and Johnston. G. A. R.: 1. Med. Cliem. 43:1427. 2(885.
4. Weight. F. F.. Aguayo. L. 0., While. ci. ci at.: GABA- and glutamate'
30. Rowley. M.. Bristow. L. J.. atid Hutson. P. H,: J. Med. Chem. 44r 2tX)l. 31. Karlsson, H.. et il.: Proc. Natl. Acad. Set. U. S. A. 98:4634. 2151 32. Lewis. 0. A.: Proc. NatI. Acad. Sci. U. S. A. 98:4293. 2)88). 33. Felder. C. C.: Life So. 68:2605, 2001. 34. Yeotnaits. i. ci al.: Life Sd. 68:2449. 2001. Cook. L.. Tedeschi. 0. H.. and Tedcshi, R. F.: Awwn 35. Gordon hirsch. 13:318. 1963. 36. Horn. A. S.. and Snyder. S. H.: Proc. Nail. Aced. Sci. U. S. A I' 2325, 1971.
37. Miller. 0. 0.. ci al.: I. Med. Chem. 30:163. 1987. 38. Kaiser. C.. and Setler. P.: Anlipsychotic agents. Itt Wolit'. M. F::' Burger's Medicinal Chemistry, part Ill, 4th ed. Nen York. John ItI:. and Sons. 1981. p. 859. 39. Janssen. P. A. J.. and Van Bever. W. F. M.: Bttlyrnphenniccaali phenylbutylamines. In Usdin. F... and Forresl. I. S. (eds.t. Podieth.-. peutic Drugs, part II. New York. Marcel Dekker. 1977. p. 1169 41). Howard. H. R.. and Seeger. T. F.: Annu. Rep. Med. Cltetn. 2539. (and references therein).
41. Chen. X.-M.: Annu. Rep. Med. Chain. 29:331. 1994. 42. van de Watcrt,eemd. II.. and Tecta. B.: 3. Mcd. Chem. 26:24)). II" 43. Potter. W. 1. and Hisllister, L. F..: Antipsychotic agents and title
In Katiung. B. C. (cdl. Basic and Clinical
85i
New York. Lange Medical Books/McGraw.HllI. Medical PSNi'l.. Division, 2001. p. 478. 44. Entriclt. H Aldenltolf. J. B.. and Lux. Ii. I). )eds nisnis iii the Action of Lithium. Symposium Proceedings. Amerimit Excerpta Medica. 1981. 45. 1.eysen, 0.. and Pinder. R. M.: Annti. Rep. Med. Cheini. 29:1, 46. Gastatit, H.. and Broughtort. R.: In Radoaco-Thomas. C. led.): AiL' .
Chapter 14 • Ceiiiriil ,Vi'rrogg.i System Depre.v.com.c ulsani Drugs. viii. I - Inlernaijonal Etrcycliipeitvi iii l'harniacoiogy and Therapeutics. New York. Perganion. 973. p.
Ciniinussion on (Iassilicaiion and Terminology oI the lnternaiioiial
509
Med. Cheni 33:51. 1998 58. Anger. 1.. Madge. I). 3.. .MnIla, M.. and Riddall. 0.: 3. Med. Client. 57. M:nlge. I). 3: ,%nnn. Rep.
44:115. 2(8)1.
League Against Epilepsy: Epilepsia 22:489. 1981.
Wada. J. A. (cdi: Symposium: Kindling 2.
York. Raven l'ress.
1991
SELEcTED READING
Spiehnan. SI. A.: In Hanung. W. II. iedj. Medicinal I. p. Cheiniswv. vol. 5. New York. John Wiley & Sons, 1961, Wong. M. G . IX'hnu. 3 A.. and Andrew'.. P. R.: J. Med. ('hem. 29:
Chebib. M.. and Johnston. C.. A. R.: ligund gated ion channels. tnedicinai ehemistr) and molecular hiology. 3. Med. Chem.
562. 1996. Iuigge. C. F.. and Boxer. P. A.: Anna. Rep. Med. ('hem. 29:13, 1994. Knoplel, T., Knhut. R.. and Allgeier. It: J. Med. Cheun. 38:1417. 1995.
Cosliurul. N.. I). P.. McDonald. I. A.. and Schweiger. E. 3.: Recent pussgress hut anihepileptuc drug research. Annii. Rep. Med. Chem. 33:61. 1998.
(Iivse.
W. J.. and
Prey. Ft. H,. and l)rewelimer. B. II.: Arch.
Pttannacinlyn. TIter.
193:181. 1971.
Roweley, M., Ilrkliiw. L. 3.. and Hnusiin. P. H.: Cuineni and novel up. preaches to the drug treatment of schizophrenia. 3. Med. Chem. 44: 477,
Refined. (3.. Berth. (3.. ('hiappe. C., eta).: 3. Med. Chent.3(1:768, 1987. Spinks. A.. aiid Waring. W. S.: In Ellis, (3. P. and Wesi. U. B. (cdv.). Prngressin MeulicinalCiuemisury. viii. 3. Washington, IX'. llnuiersvorth. 963. p. 261 Cusfiurd. N. 1).
43:1427. 2(88).
P..
ci
al
:
Anna. Rep. Med. Client. 33:61. 1998.
21811.
Strange. P. (3.:
Aniipsyehodc dnigs: importance of dopamine receptors for uherapentic actions aiid side elTeew. Phannaeol. Ren.
niecltanisnus of
53:19. 2)811. Weinherger. D. R.: 3.
Anxiety at
Med. 344:1247. 2001.
lie loitnier iu( molecular
medicine. N. Engi.
CHAPTER 15 Central Nervous System Stimulants EUGENE I ISAACSON
This chapter discusses a broad range of agents that stimulate the central nervous system (CNS). The analeplies classically arc a group of agents witha limited range of use because
Pent ylenetetrazole. Pentylenetetrazole. 6.7.8.9-tetra. hydro-5H-tctrazolof I.5-a/azepine. I ,5-pentamethylcnecct. razole (Metrasol), has been used in conjunction with the
of the general nature of their effects. The inethylxanthines have potent stimulatory properties, mainly cortical at low doses but with more general ellects as the dose is increased. The central agents amphetamine and close relatives have alerting and antideprcs.sant properties hut medically arc used more often as anorexiants. The antidepressant drug.s are used most frequently in depres-
electroencephalograph to help locate epileptic foci. It is used
sive disorders and can be broadly grouped into the monoamine oxidase inhibitors (MAOI5). the monoamine reuptake inhibitors, and agents acting on autoreceptors. A small group of miscellaneously acting drugs. which includes a number of hallucinogens. cocaine, and cannabinoids. concludes the chapter.
sive drugs, including picroloxinin.
ANALEPTICS
Modafinil.
The traditional analeptics area group of potent and relatively nonselective CNS stimulants. The convulsive dose lies near their analeptic dose. They can be illustrated by picrotoxinin and pentylenetetrazole. Both are obsolete as drugs but remain valuable research tools in determining how drugs act. Newer agents. modafinil and doxapram, are more selective and have cisc in narcolepsy and as respiratory stimulants.
as a laboratory tool in determining potencies of anticonvulsant drugs in experimental animals. The drug acb as a convulsant by interfering with chloride conductance: It binds loan allosteric site on the GABAA receptor and act' as
a negative modulator. Overall, it appears to share
similar
effects on chloride conductance with several other consul.
Pontylenetetrazole
Modalinil (Provigil) has overall wakeful
ness-promoting properties similar to those of central thomimetics. It is considered an atypical a1-norepinephrirc (NE) receptor stimulant and is used to treat daytime clcqir ness in narcolepsy patients. Adverse reactions at therapeutk doses are reportedly not severe and may include ness, anxiety, and insomnia.
Pkrotoxin.
Picrotoxinin. the active ingredient of picrotoxin, has the following structure:
/ OH' 0
/
0 = C- -. Picroioxnrn
According to Jarhoe ci al..' the encircled hydroxylactonyl moiety is mandatory for activity, with the encircled 2-propenyl group assisting. Picrotoxinin exerts its effects by interfer-
ing with the inhibitory eflècts of y-aminobutyric acid (GABA) at the level of the GABAA receptor's chloride chun-
nd. The drug is obsolete medically. Pharmacologically, it has been useful in determining mechanisms of action of sedative—hypnotics and anticonvulsants. Butyrolactones bind to
the picrotoxinin site.
510
Modatinil
Doxapram Hydrochloride, USP.
Doxapram. l.clhy
4-(2-niorpholinoethyl)-3.3-diphenyl-2-pyrolidinone hydn. chloride hydrate (Dopram), has an obscure molecular anisni of action. Overall, it stimulates respiration by xii on peripheral carotid chemoreceptors. It has use as a respr. tory stimulant postanesthetically. after CNS depressantdft overdose, in chronic obstructive pulmonary diseases. aisi the apneas.
Chapter IS • Central Nenou.c System Simm/ants
I'
0
(
C1
'H20
CH2CH3 Doxapram Hydrothtoride
511
been little studied. At high doses, the tendency to produce convulsions is greater for theophylline than for caffeine. In addition to being conical stimulants. theophylline and caffeine are medullary stimulants, and both are used as such. Caffeine may be used in treating poisoning from CNS-depressant drugs, though it is not a preferred drug. The important use of theophytline and its preparations in bronchial asthma is discussed elsewhere. Caffeine also is reported to have valuable bronchodilating properties in asthma. Finally, because of central vasoconstrictive effects, caffeine has value in treating migraine and tension headaches
METHYLXANTHINES naturally occurring methyixanthines are caffeine, the-
The
and theobrornine. See Table 15-I for their strucsirs and occurrence and Table 15-2 for their relative potendes.
is a widely used CNS stimulant. Theophylline medical use as a CNS stimulant. hut its CNSproperties are encountered more often as some-
Caffeine some
ants severe, and potentially life-threatening, side effects of ts use in bronchial asthma therapy. Theobromine has very in)c CNS activity (probably because of poor physicochemicat properties for distribution to the CNS).
Caffeine is often used as it occurs in brewed coffee. )rewcd tea, and cola beverages, in most subjects. a dosage t185 to 250 mg of caffeine acts as a conical stimulant and clear thinking and wakefulness, promotes an abillv to concentrate on the task at hand, and lessens fatigue. Ac the dose is increased, side effects indicating excessive $mutation (e.g.. restlessness, anxiety, nervousness, and become more marked. (They may be present saying degrees at lower dose levels.) With further intirases in dosage, convulsions can occur. A review of the of caffeine in the brain with special reference to :xtors that contribute to its widespread use appears to be
and may have actual analgesic properties in the latter use. The CNS-stimulating effects of the methylxanthines were once attributed to their phosphodiesterase-inhihiting ability. This action is probably irrelevant at therapeutic doses, Evidence indicates that the overall CNS-stimulant action is related more to the ability of these compounds to antagonize adenosine at A1 and A2A receptors.3" All of the roles 01' these receptors are still under study, The adenosine receptor 1-9 subtypes and their pharmacology have been Problems with the present compounds. such as caffeine and theophylline. are lack of receptor selectivity and the ubiquitous nature of the various receptor suhtype.s. Caffeine and theophylline have pharmaceutically important chemical properties. Both are weak l3ronsted bases. The reported pK, values are t).8 and 0.6 for cat't'eine and 0.7 for
theophylline. These values represent the basicity of the imino nitrogen at position 9. As acids, caffeine has a above 14. and theophyllinc. a pK,, of 8.8. In theophylliute. a
proton can be donated from position 7 (i.e.. it can act as a Bronsted acid). Caffeine cannot donate a proton t'rom position land does not act as a Brønsted acid at pH values under 14. Caffeine does have clectrophilic sites at positions I. 3. and 7. In addition to its Brønsted acid site at 7. theophylline has clectrophilic sites at I and 3. In condensed terms, both compounds are electron-pair donors, but only theophylline is a proton donor in most pharmaceutical systems. Although both compounds arc quite soluble in hot water (e.g.. caffeine 1:6 at 80°C). neither is very soluble in water at room temperature (caffeine about 1:40. theophyllinc about
Jciinitive!
The CNS effects of theophylline at low dose levels have
1:120). Consequently. a variety of mixtures or complexes designed to increase solubility are available (e.g.. citrated caffeine, caffeine and sodium bcnzoatc. and theophylline ethylenediansine compound laminophylline I). theopliyl' Caffeine in blood is not highly protein bound. Differences in the substituent at line is about
TABLE 15-1
the 7 position may he involved. Additionally, caffeine is
Xanthme Alkaloids o
A"
Relative Pharmacological Potencies of the Xanthines TABLE 15—2 A,
Respir-
Xanthine
CNS
(A, A' & A= H) Xanthlne Compound
R CO3
CH3 H
R' CO3 CHa CH3
R" CH3 H
CH3
Common Source Colfoc. tea Tea Cocoa
CaffeIne Theophyllttie Theobromlne
atory
Stimu-
Stimu-
latlon
latlen
1' 2 3
2
t
Skeletal
nary Cardiac Muscle DlIa- Stimu- Stimurests tatlon lation latton Diu3 2
3
3
1
1
2
2
I
2 3
512
Wilson and Gisvold's Textbook of Organic Medicinal and Pharn,areu:ieal Cla'n,issrv
more lipophilic than theophylline and reputedly achieves higher brain concentrations. The half-life of caffeine is 5 to
8 hours, and that of theophylline, about 3.5 hours. About 1% of each compound is excreted unchanged. The compounds are metabolized in the liver. The major metabolite
of caffeine is I -methyluric acid, and that of theophylline. 1.3-dimethyluric acid)° Neither compound is metabolized to uric acid, and they are not contraindicated in gout.
CENTRAL SYMPATHOMIMETIC AGENTS (PSYCHOMOTOR STIMULANTS) Sympathomimetic
agents, whose effects are manifested
mainly in the periphery, arc discussed in Chapter 16. A few simple structural changes in these peripheral agents produce compounds that are more resistant to metabolism, more nonpolar. and better able to cross the blood—brain barrier. These effects increase the ratio of central to peripheral activity, and the agents are designated, somewhat arbitrarily, as central .cvnipathomirnezic agents. In addition to CNS-stimulating effects, manifested as ex-
citation and increased wakefulness, many central sympathomimetics exert an anorcxiant effect. Central sympathomi-
metic (noradrenergic) action is often the basis for these effects. Other central effects, notably dopaminergic and serotoninergic effects, can be operative, however.' l In some agents, the ratio of excitation and increased wakefulness to anorexiant effects is decreased, and the agents are marketed as anorexiants. Representative structures of this group of compounds are given in Table 15-3. The structures of the anorexiants phendimetrazine and sibuiramine and the alertins agents methylphenidate and pemoline. useful in aliention-deficient disorders, are given in the text. Structural features for many of the agents can be visualized easily by considering that within their structure they
Sympathomlmetics With SIgnIfIcant Central Stimilant ActivIty TABLE 15-3
Base Structure
Generic Name
creases activity. Mono-N subslituents larger than methyl dt-
crease excitatory properties, but many compounds anorexiant properties. Consequently. some of these ageas are used as anorexiants, reportedly with less abuse than amphetamine. There can be some departure 1mm the basic amine structure when compounds act by indirect structure. has gic mechanisms. A ever, can be visualized in such compounds. The abuse potential of the more euphoriant and stitnub tory of the amphetamines and amphetamine-like dnigs well documented. They produce an exceedingly addiction. Apparently. both a euphoric "high" lated to effects on hedonistic D2 receptors) and a ponvi phone depression (especially among amine-depleting drug-, contribute to compulsive use of these agents. Abuse
drugs (especially methamphetamine) in recent yearn reached disastrous proportions. Recognized medical indications for dextroamphetarec and some very close congeners include narcolepsy. Palan
Amphetamine
son's disease, attention-deficient disorders, and. not the preferred agents for obesity. 'appetite suppression.L some conditions, such as Parkinson's disease, for which
Mothamphetamine Phentormine Benaplietamine
H
H
H
CH.JH
H
H
Diothytpwpion
0
H
CH3 CR2C6H5 C2H,,
H
CH3
C—C—NH
Fenfluaunine CF3
Catbon5i
contain a fi-phenethylamine moiety, and this grouping can give some selectivity for presynaptic or postsynaptic drenergic systems. f3-Phenethylamine. given peripherally. lacks central activity. Facile metabolic inactivation by monoamine oxidases (MAOs) is held responsible. Branching with lower alkyl groups on the carbon atom adjacent (a) to the amino nitrogen increases CNS rather than peripheral activity (e.g.. amphetamine, presumably by retarding metabolisini. The a branching generates a chiral center. The dextrolS, isomer of amphetamine is up to 10 times as potent as the levo(R) isomer for alerting activity and about twice as a psychotomimetic agent. Hydroxylation of the ring hydroxylalion on the carbon (to the nitrogen) activity, largely by decreasing the ability to cross the blood—brain barrier. For example. with a /3-01-I. has about Il 100th the ability to cross la blood—brain harrier of its deoxy congener. amphetamine. Halogenation (F. Cl. Br) of the aromatic ring decreases sympathomimetic activity. Other activities may increase. Chloroamphetamine has strong central serotoninergic activity (and is a neurotoxin. destroying serotoninergic neumn' in experimental 'animals).'2- 13 Methoxyl or methylenedioxy substitution on the tends to produce psychotomimetic agents. pism for dopaminergic (D2) receptors. N-methylation increases activity (e.g.. compare meihan, phetamine with dextroamphetamine). Di-N-methylation
H
H
CHnCH3
main use is to decrease rigidity, the antidepressant elki of dextroamphetaminc can be beneficial. It is also an effective antidepressant in terminal malignancies. most all cases of depression. and especially in majordeprvs sive disorders of the unipolar type, however. mine has long been superseded by other agents. nolabl} IL MAOIs and the monoamine reuptake inhibiting anhidepro Saflts. The compounds and their metabolites can have
multiple utctions. In a fundamental sense, the structural for action is quite simple. The compounds and their mcuhlites resemble NE and can participate in the various neurst.
Chapter 15 • and postsynaptic processes involving NE. such as synthesis, release. reuptake. and presynaptic and postsynaptic receptor
C'e',:gruI Nervous Svstenu Ssiu,ulwns
513
been reported to be the major active metaholite involved in NE and DA release.'4
activation. Also, because dopamine (DA) and, to a lesser cxtent. serotonin (5-hydroxytryptamine 15-HTI) bear a strucurat resemblance to NE. processes in DA- and 5-HT-aeti-
systems can be atTected. To illustrate the potential complexity. the rcceptor activations that can be associated auth just one parameter, reduction in food intake, reportedly are 13a. 5HTffl. 5HT,A. 5HT,c. D1. and D2. PRODUCTS
Amphetamine Sulfate. USP. Amphetamine. (± )- I (Benzedrine), as the racemic mixture has a higher proportion of cardiovascular effects than dextro isomer. For most medical uses, the dextrorotatory corner is preferred.
Methamphetamine
Hydrochloride.
Methamphela-
mine. (+ )- I -phenyl-2-methylaminopropane hydrochloride desoxyephednnu, hydrochloride (Desoxyn). is the N-methyl analogue of dcxtroamphetamine. It has more marked central and less peripheral action than dcxtroamphctamine. It has a very high abuse potential. and by the intravenous route, its salts are known as "speed." The overall abuse problem presented by the drug is a national disaster. Medicinally accept. able uses of methamphetamine are analogous to those of dextroamphetamine.
Phentermine Ion-Exchange Resin and Phentermlne Hydrochloride, USP. The free base is a.a-dimethylphenethylamine. I -phenyl-2-methylaminopropane. In the resin
Dextroamphetamine Sulfate, USP, and Dextroamphetamine Phosphate. Dextroamphetamine. (+ )-(S)methylphenethylamine. forms salts with sulfuric acid (DexeJñne and with phosphoric acids. The phosphate is the more water-soluble salt and is preferred if parenteral administra(on is required. The dextrorotatory isomer has the (S) conflguration and fewer cardiovascular effects than the levorotakruy (R) isomer, Additionally, it may be up to 10 times as potent as the (R) isomer as an alerting agent and about twice potent a psycholomimetic agent. Although it is a more potent psychotounimetic agent than the (R) isomer, it has a better ratio of alerting to psychotomimetic effects.
The major mode of action of dextroamphetamine is rerose of NE from the mobile pool of the nerve terminal. Other mechanisms. such as inhibition of uptake. may make a mall conuibution to the overall effects. The alerting actions rtlaue to increased NE available to interact with postsynapnic sceptors (en1). Central fl-receptor activation ha,s classically tv-en considered the basis for most of the anorexiant effect.
The psychotomimetic effects are linked to release of DA iisl activation of posisynaptic receptors. D2 and mesolimbic Dc rereptors would be involved. Effects on 5-HT systems iso have been linked to some behavioral effects of dextro-
_'nphetamine. Effects via 5-HT receptors would include 9ff5 receptors and, theoretically, all additional receptors trough 5HT7.
Destroamphetamine is a strongly basic amine, with values sotu 9.77 to 9.94 reported. Absorption from the gastrointesoral tract occurs as the lipid-soluble amine. The drug is not citensively protein bound. Varying amounts of the drug are
twueted intact under ordinary conditions. The amount is under conditions of alkaline urine. Under eon:i(ons producing systemic acidosis. 60 to 70% of the drug a excreted unchanged. This fact can be used to advanin treating drug overdose. Thc n-methyl group retards, but does not terminate. mebolismby MAO. Under most conditions. the bulk ofa dose
is metabolized by N-dcalkylation to and ammonia. Phenylacetone is degraded fur-
preparation (lonamin). the base is bound with an ion-exchange resin to yield a slow-release product; the hydrochloride (Wilpowr) is a water-soluble salt.
Phentermine has a quaternary carbon atom with one methyl oriented like the methyl of(S).amphelamine and one methyl oriented like the methyl of (RI-amphetamine, and it reportedly has pharmacological properties of both the (R) and (S) isomers of amphetamine. The compound is used as an appetite suppressant and is a Schedule IV agent, indicating less abuse potential than dextroamphctamine.
Benzphetamine Hydrochloride.
Benzphetamine hydrochloride. (+ )-N-benzyl-N.a-dimethylphenethylamine hydrochloride. ( + )- I -phenyl-2-(N-methyl-N-bcnzylaminc)propane hydrochloride (Didrex). is N-benzyl-substituted
methamphetamine. The large (benzyl) N-substitucnt decreases excitatory properties, in keeping with the general structure—activity relationship (SAR) for the group. Anorexiant properties are retained. Classically, amphetamine-like
drugs with larger than N-methyl substituents are cited as anorexiant through central /3 agonism. No claims for selectivity among fl-receptor subtypes have been made in such citations. The compound shares mechanism-of-action characteristics with methylphenidate. Overall, it is said to reduce appetite with fewer CNS excitatory effects than dextroamphetamine.
Diethylpropion Hydrochloride, USP.
Because it has two large (relative to I-I or methyl) N-alkyl substituents. diethylpropion hydrochloride. I -phcnyl-2-diethylaminopropan-I-one hydrochloride (Tenuate. Tepanil), has fewer sympathomimetic, cardiovascular, and CNS-stimulatory effects than amphetamine. It is reportedly an anorexiani agent that can be used for the treatment of obesity in patients with hypertension and cardiovascular disease. According to the generalization long used for this group of drugs, increasing N-alkyl size reduces central a1 effects and increases /3 effeels, even though the effects are likely mediated principally by indirect NE release.
bet Ii) Isenwoic acid.
animals, about 5% of a dose accumulates iihc brain, especially the cerebral cortex, the thalamus, and at corpus callosum. It is first p-hydroxylaled and then (3-
to produce p-hydroxynorephedrine, which has
Fenfluramine Hydrochloride.
Fenfluramine hydrochloride. (± )N-ethyl-a-methyl-nu-(trifluoromethyl)phencthylamine hydrochloride (Pondimin), is unique in this group
of drugs, in that it tends to produce sedation rather than
514
Wilson and Gisvnki's Textbook of Organic Medicinal and Phar,nace,aica! C'hemistrv
excitation. Effects are said to be mediated principally by central serotoninergic. rather than central noradrenergic. mechanisms. In large doses in experimental animals, the drug is a serotonin It was withdrawn from human use after reports of heart valve damage and pulmo-
keted compound and is about 400 times as potent as the ervthro racemate.'7 The absolute configuration of each of the threo-methylphenidate isomers has been determined.'5 Considering that the structure is fairly complex (relative to amphetamine). it is likely that one of the two components
nary hypertension. From its structure, more apolar or hydrophobic character than amphetamine, tropism for scrotoniner-
of the ;i,reo racemate contains most of the activity. Evidence
gic neurons would be expected. Likewise, the structure
principally in the behavioral and pressor effects of the racemate.1C As is likely with many central psychomotor lants. there are multiple modes of action. Methylphenidate. probably largely via its p-hydroxy tnttabolite. blocks NE reuptake, acts as a posisynapric agonist. depletes the same NE pools as reserpine. and has effects on dopaminergic systems, such as blocking DA reupluke. Methylphenidate is an ester drug with interesting pharina. cokinetic properties arising from its structure. The pK, sal ues are 8.5 and 8.8. The protonated form in the stomach reportedly resists ester hydrolysis. Absorption of the intact drug is very good. After absorption from the gastrointestinal tract, however. 80 to 90% of the drug is hydrolyzed rapidly to inactive ritnlinic acid.aul (The extent of hydrolysis may about 5 times that for ( +) versus )Another 2 to the racemate is oxidized by liver microsomes to the macinc cyclic amide. About 4% of a dose of the racemate reportedli reaches the brain in experimental animals and there is hydroxylated to yield the putative active metabolite. Methylphenidate is a potent CNS stimulant. Indication' include narcolepsy and attention-deficit disorder. The stoic tare of the (2R,2'Ry isomer of the threo racemic mixture is shown.
Suggests an indirect mechanism. If an indirect mechanism were operative, then all postsynaptic 5-I-IT receptors could be activated. Evidence from several studies indicates that the and the 5HTw receptors are most responsible for the satiety effects of 5-H'!'. 5-HT may also intluence the type
of food selected (e.g.. lower fatter food intake).'' The (+ I isomer. dexfenfluramine (Redux). has a greater tropism for 5-HT systems than the racemic mixture. It. too, was withdrawn because of toxicity.
Phendimetrazine Tartrate, USP.
The optically pure
compound phendimetrazine tartrate. (2S.3S)-3.4-dimethyl2-phenylmorpholine-t.-( + )-tarlrale (Plegine). is considered an eftèctive anorexiant that is less abuse prone than amphetamine. The stereochemistry of (+ )phendimetrazine is as shown. II, H
0
0
indicates that the (+) -(2R,2'R)threo isomer is involval
Phendimotrazirie Tartrale
Sibutramine. Sibutramine (Meridia) is said to be an uptake inhibitor of NE and 5-HT. These mechanisms fit its structure. It is reportedly an antidepressant and an anorexiant
drug. This mechanism implies that activation of all presynaptic and postsynaplic receptors in NE and 5-HT systems is possible. The data are not completely clear, hut studies to date indicate that the receptors principally involved are a1. and
A
Methytphenldate Hydrochtoride
Il
Pemoline. The unique structure ol pemoline. 5-phcnyl-45H)-oxazolone (Cyleru), is shown below,
.CH3
Pemoline
The compound is described as having an overall
the CNS like that of methylphenidatc. Pemolinc to 4 weeks of administration, however, to take effect partial explanation for the delayed effect may be that to of the actions of the agent, as observed in rats, is to inctocs
the rate of synthesis of DA. Sibutramine
Methylphenidate
Hydrochloride,
ANTIDEPRESSANTS LiSP.
Because
methylphenidate (Ritalin) has two asymmetric centers, there
are four possible isomers. The tlireo racemate is the mar-
Oiddase Inhibitors (MAOIs) Antidepressant therapy usually implies therapy direct:J against major depressive disorders of the unipolar
Chaptor 15 • Central Nen'au.c System Stimulants
is centered around three groups of chemical agents: the
515
MAOIs. the monoamine reuptake inhibitors, and autorecepor desensitizers and antagonists. Electroshock therapy is
inhibition was almost always regarded as irreversible. From the beginning, however, it was known that it was possible to have agents that act exclusively by competitive enzyme
another option. The highest cure or remission rate
inhibition. For example. it has long been known that the
is
achieved with electroshock therapy. In some patients. especially those who are suicidal, this may be the preferred therapy. MAOIs and monoamine reuptake inhibitors have about the same response rate (—60 to 70%). In the United States. the latter group is usually chosen over MAOls for antidepres-
hannala alkaloids hurmine and harmaline act as CNS stimu-
lants by competitive inhibition of MAO. Reversible (competitive) inhibitors selective for each of the two major MAO subtypes (A and B) are reportedly forthcoming.
HO
Ill -
sant therapy.
A severe problem associated with the MAOIs that has a major factor in relegating them to second-line drug status is that the original compounds inhibit liver MAOs irreversibly in addition to brain MAOs. thereby allowing dietary pressor amines that normally would be inactivated tocscrt their effects systemically. A number of severe hyperwnsive responses, some fatal, have followed ingestion of (seds high in pressor amines. It was hoped that the developsteal of agents such as selcgiline that presumably spare liver MAO might solve this problem. The approach of using MAO selectivity did solve the hypertensive problem, but the cornwas not an antidepressant (it is useful in Parkinson's disease). Another approach using a reversible MAOI has yielded antideprnssants that lacked the hypertensive
effect. Another prominent side effect of MAOIs aorthostatic hypotension. said to arise from a block of NE released in the periphery. Actually, one MAOI, pargyline, sac used clinically for its hypotensive action. Finally, sonic the first compounds produced serious hepatotoxicity. Compounds available today reportedly are safer in this recard but suffer the stigma of association with the older corn-
The history of MAOI development illustrates the role of Isoniazid is an effective antitubereular agent hut a very polar compound. To gain better penetration into the ,tfvcobac:eriu,n tuberculosis organism, a more hydrophobic compound, isoniazid substituted with an isopropyl group on de basic nitrogen (iproniazid). was designed and synthesord. It was introduced into clinical practice as an effective arlitubercular agent. CNS stimulation was noted, however.
and the drug was withdrawn. Later, it was determined in caperimentail animats and in vitro experiments with a purified MAO that MAO inhibition, resulting in higher synaptic kwls of NE and 5-HT. could account for the CNS effecis. compound was then reintroduced into therapy as an nlidepressant agent. It stimulated an intense interest in hydnaiunes and hydrazides as anridepressants and inaugurated effective drug treatment of depression.22 It continued to be
0
C
Mociobemide
Moclohernide has received considerable attention abroad.
A reversible inhibitor of MAO-A, it is considered an effective antidepressant and permits metabolism of dietary myramine.2a Metabolites of the drug are implicated in the activity.
Reversible inhibitors of MAO-A (RIMAs) reportedly are antidepre.s.sant without producing hypertensive crises. Reversible inhibitors of MAO-B have also been studied. Pres-
ently, selective MAO-B inhibition has failed to correlate positively with antidepressant activity; selegilinc. however. has value in treating Parkinson's disease. The clinically useful MAOI antidepressunts are nonselective between inhibiting metabolism of NE and 5-HT. Agents selective for a MAO that degrades 5-HT have been under study for some time. The structures of phenelzine and tnanyl-
cyprominc are given in Table 15-4
Pheneizine Sulfate, (iSP. Phenelzine sulfate. 2-(phenyleihyl)hydrazine sulfate (Nardil). is an effective antidepressant agent. A mechanism-based inactivator. it irreversibly inactivates the enzyme or its cofactor. presumably after oxidation to the diazine, which can then break up into molecular nitrogen, a hydrogen atom, and a phenethyl free radical. The latter would be the active species in irreversible inhibi-
Tranykypromine Sulfate, (iSP.
Tranylcyprominc sul-
fate. (± )-rran.c.2-phenylcyclopropylamine sulfate (Parnate), was synthesized to be an amphetamine analogue (visu-
alize the a-methyl of amphetamine condensed onto the 13carbon It does have some amphetamine-like properties, which may be why it has more immediate CNS stimulant effects than agents that act by MAO inhibition alone. For MAO inhibition, there may be two components to thc
ned in therapy for several years but eventually was withyawn because of hepatotoxicity.
The present clinically useful irreversible inactivators can reconsidered mechanism-based inhibitors of MAO.23 They
re converted by MAO to agents that inhibit the enzyme. flay can form reactants that bond covalently with the eninc or its cofactor. A consequence of irreversible inactivais that the action of the agents may continue for up to 2 seeks after administration is discontinued. Consequently, sny drugs degraded by MAO or drugs that elevate levels if MAO substrates cannot be administered during that time. For a long time, because the agents that opened the field
i'd then dominated it were irreversible inactivator. MAO
TABLE 15-4
Monoamine Oxldase InhIbitors
Generic Name Proprietary Name
Sfructure
Phanoizine
H2S04
NatCid Tranyscypromirre nultale, USP Pamare
rj—CH_CH_-NH
516
IVi/Min and GistoisI
s
ie.sll,susk a! Orc,'a,,is Medicinal tim! Plwrn,aeeuiical
action of this agent. One is thought to arise because tranylcy-
prominc has structural feattires (the basic iiitrogeii and the quasi-IT character of the a- and
carbon
atoms) that approximate the transition state in a route of nietubolism of $-arylamines.27 As a- and $-hydrogen atoms are removed from the normal substrate of the enzyme. the quasi-ar character develops over the a.f3-carhon systeni. Duplication of the transition slate permits extremely strong, but reversible, attachment to the enzyme. Additionally, Iranylcypromine is a mechanism-based inactivator. It is metabo-
lized by MAO, with one electron of the nitrogen pair lost to liavin. This, in turn. produce.s homolytic fission of a carbon—carbon bond of cyclopropune. with one electron from the fission pairing with the remaining lone nitrogen electron to generate an imine (protonaled) and with the other residing on a methylene carbon. Thus, a free radical is formed that
reacts to form a covalent bond with the enzyme or with reduced tiavin to inactivate the enzyme.25
Monoamine Reuptake Inhibitors Originally, the monoamine reuptake inhibitors were a group of closely related agents. the tricyclic antidepressants. but
now they are quite diverse chemically. Almost all of the agents block neuronal reuptake of NE or 5-HI or both (i.e.. are selective). Reuptake inhibition by these agents is at the level of the respective monoamine transporter via competitive inhibition of binding of the monoamine to the substrate-binding compartment. Probably the same site on the protein is involved
('ls',nisirv
the substrate-binding compartment of the transporter. The overall concept of a system with addcd structural bulk, usually an aryl group, appears to be applicable to many newer compounds—selective serotonin reuptake inhibitors (SSRIs). selective norepincphrine reuptake inhibitors (SNERIs)—that do not have a tricyclic grouping. The TCAs arc structurally related to each other and, consequently, possess related biological properties that can summarized as characteristic of the group. The dimethyla. mint, compounds tend to he sedative, whereas the mono' methyl relatives tend to be stirnulatory. The dimethyl compounds tend toward higher 5-HT to NE rcuptake block ratios: in the monomethyl compounds. the proportion of NE uptakc
block tends to be higher and in some cases is selective NE reuptake. The compounds have anticholinergic properties, usually higher in the dimethylamino compounds. When treatment is begun with a dimethyl compound. a sig. nificant accumulation of the monomethyl compound desclops as N-demethylation proceeds. The TCAs are extremely lipophilic and, accordingly. sen highly tissue bound outside the CNS. Since they have anticholinergic and noradrenergic effects, both central and jv. ripheral side eliects are olien unpleasant and sometimes dam gerous. In overdose, the combination of efl'ects. as well as a quinidine-like cardiac depressant effect, can be lethal. Os. erdose is complicated because the agents are so highly pro-
tein bound that dialysis is ineffective.
for inhibitor and monoamine, but this has not yet been
PRODUCTS
proved. The mechanism of reuptake by monoamine trans-
Imipramine Hydrochloride. USP.
porters has been reviewed.39
The net effect of the drug is to increase the level of the monoamine in the synapse. Sustained high synaptic levels of 5-HI. NE, or both appear to be the basis for the antideprcs.sant effect of these agents. There is a lime lag of 2 or more weeks before antidepressant action develops. It is conSHTIA receptors and (in sidered that (in the case of the case of NE) a2 receptors undergo desensitization and transmitter release is maintained. Of course activation of postsynaptic receptors and sustained transmission is the ulti-
mate result of sustained synaptic levels of neurotransmitter.°
Tricydic Antidepressants The SARs for the TCAs are compiled in detail in the eighth edition of this text.32 The interested reader is referred to this compilation. In summary, there is a large, bulky group
encompassing two aromatic rings, preferably held in a skewed arrangement by a third central ring, and a three- or. sometimes, two-atom chain to an aliphatic amino group that is monomethyl- or dimethyl-substituted. The features can be
visualized by consulting the structures of imipramine and desiprumine as examples. The overall arrangement has features that approximate a fully extended Irons conformation of the $-arylamines. To relate these features to the mechanisna of action. reuptake block, visualize that the basic ar-
rangement is the same as that found in the plus an extra aryl bulky group that enhances affinity for
lmipramine chloride. 5-13-(dimethylamino)propyl I-It), II -dihydro-illdibenzlbjlazepine monohydrochlonde (Fofranil). is the lead compound of the TCAs. It is also a close relative of the antipsychotic phenothia-zines (replace the 10—Il bridge with sulfur, and the compound is the antipsychotic agent pram zinc). It has weaker D2 postsynaptic blocking activity thaa
proma/ine and mainly affects amines (5-HI. NE. and DAt via the transporters. As is typical of dimethylamino compounds. anticholinergic and sedative (central H5 blockcl fects tend to he marked. The compound per se has a tendency
toward a high 5-HT-to-NE uptake block ratio and can be called a serotonin transport inhibitor (SERII). Mcu. bolic inactivation proceeds mainly by oxidative hydrox>la.
tion in the 2 position, followed by conjugation with ronic acid of the conjugate. Urinary excretion predonilnalo
(about 75%), but sotne biliary excretion (up to 25'if) occur, probably because of the large nonpolar grouping. Os
dative hydroxylation is not as rapid or complete as that ii the more nucleophilic ring phenothiazine antipsyclrotic'. consequently, appreciable N-demethylation occurs. ssilb buildup of norimipranline (or desimipramine). The dcmcthylutcd mctabolitc is less anticholinergic. sedative, and more stimulatory and is a SNERI.3' quently. a patient treated with imipramine has two cm pounds that contribute to activity. Overall, the effect selective 5-HI versus NE reuptake. The activity of des-norilnipramine is terminated by 2-hydroxylation. by conjugation and excretion. A second N-denrethylarits
Chapter 15 • Central
occur. which in turn is followed by 2-hydroxylation.
line. Nortriptyline is a SNERI3: the composite action of drug and metabolite is nonselective.
and excretion. 9
517
Svstun
1
Nortriptyline Hydrochloride, USP.
Pertinent biologi-
cal and chemical properties for nortriptyline. 3-(I0.l 1-diN 6
/CH3
4
HCI R
Imipranitne Desipramine
R
= CH3
R
H
hydro-5H.dibenz.o(a.djcyclohepten-5-ylideneN-methyl- I propanantine hydrochloride. 5-(3-methyl-aminopropylidene)- 10.11 -hydro-511-dibenzola,dlcycloheptcne hydrochloride (Aventyl. Pamelor). are given above in the discus-
sion of amitriptyline. Metabolic inactivation and elimination are like those of amitriptyline. Nortriptyline is a selective NE transporter (NET) inhibitor.3'
Cesipramine Hydrochloride, USP. The structure and properties of desipramine hydrochloride. 10,11 -dihydro-N-methyl-5H-dibenz(bJlazepinc-5.propanamine monohydrochloride. 5-(3-methylaminopropyl)- 10,11 -dihydro511.dibcnzlbflazepine hydrochloride (Norpramin. Perto-
/CH3 HO
irane). are discussed under the heading. Imiprarnine. above.
Among tricyclics. desipramine would be considered when few unticholinergic effects or a low level of sedation are
R Arndriptyhne. Noririptyline
R = CH3 P=H
important. It is a SNERI.31
Hydrochloride. Clomipraminc (Anaarni) is tip to 50 times as potent as imipramine in some This does not imply clinical superiority, but it might be informativc about tricyclic and, possibly. other cuptake inhibitors. The chloro replacing the H substitueni could increase potency by increasing distribution to the Clomipramine
is unlikely that this would give the potency
UNS. but it
magnitude seen. It might be conjectured that a H bond beseen the protonated amino group (as in vivo) and the Unelectrons of the chloro substituent might stabilize a $aiylamine-like shape and give more efficient competition (or the transporter. The drug is an antidepressant. It is used a obsessive-compulsive disorder, an anxiety disorder that
Protriptyline Hydrochloride, USP.
Protriptyline hydrochloride. N-methyl-5H-dibenzo[a,d]cyclohcptenc-5-propylamine hydrochloride. 5-(3-methylaminopropyl).5H-dibenzofa.dlcycloheptene hydrochloride (Vivactil). like the other compounds under consideration, is an effective antidepressant. The basis for its chemical naming can be seen by consulting the naming and the structure of imipramine. Protriptyline is a structural isomer of noririptyline. Inactivation can be expected to involve the relatively localized double bond. Because it is a monomethyl compound, its sedative potential is low.
may have an element of depression.
CH2—CH2—CH2—N CH3 Protriplyfine
Trimipramine Maleate.
CH3
Clornipramirie
A.rnitrlptyline
Hydrochloride,
USP.
Amitriptyline. 3-
10.1
I
For details of chemical no-
menclature, consult the description of imnipramine. Replace-
.propanamine hydrochloride. 5-(3-dimethyl-
10,11 -dihydro-5H-dibenzola.djcyclohepate hydrochloride (Elavil). is one of the most anticholinerand sedative of the TCAs. Because it lacks the ring dccion-enriching nitrogen atom of imipramine, metabolic inmainly proceeds not at the analogous 2 position stat the benzylic 10 position (i.e.. toluene-like metabolism relominatcs). Because of the 5-exocyclic double bond. EZ•hydroxy isomers arc produced by oxidation metaboa Conjugation produces excretable metabolites. As is of the dimethyl compounds. N-demethylation occurs, is produced, which has a less anticholinerIeee. sedative, and more stimulant action than umitripty-
ment of hydrogen with an a-methyl substituent produces a chirai carbon, and trimipramine (Surmontil) is used as the racemic mixture. Biological properties reportedly resemble those of imipramine.
QQQ I
i
C
CH2 — N
I
CH3
CH3 Trimipramine
Doxepin Hydrochloride, USP.
Doxepin, 3-dibenz-
Ib,el-oxepin- II (6H)ylidine-N.N-dimethyl- I -propanamine
518
Wilso,, and (;isi'old c Tes:!;ook
of Organic Medici,,aI and Pharmaceutical CI,en,i.cfr.'
hydrochloride. N.N-dimethyl-3-(dihenzlh.ejoxepin- II (6H)ylidene)propylaminc (Sinequan. Adapin). is an oxa congener of arnitriptyline. as can be seen from its structure. The oxygen is interestingly placed und should influence oxidative metabolism as well as postsynaplic and presynap-
tic binding affinities. The (Z) isomer is the more active, although the drug is marketed as the mixture of isomers. The drug overall isa NE and 5-HI reupiake blocker with significant anticholinergic and sedative properties. It can be arnicipated that the nor- or des- metabolite will contribute to the
overall activity pattern. 7
4
8
10
liii
CH3 I
HC—CH,—CH,—N
\
HCI
abolishes the center ring, and one ring is moved forward from the tricyclic "all-in-a-row" arrangement.) The net effect is that the fl-arylamine-like grouping is present. as in the tricyclics. and the compounds can compete Inn the substrate-binding site of the serotonin transporter protein
(SERT). As in the tricyclics, the extra aryl group can add extra affinity and give favorable competition with the sub. stratc. serotonin. Many of the dimethylamino tricyclics are, in fact, SSRk
Since they are extensively N-demethylated in vivo to compounds. which are usually SNERIs. however, the effect is not selective. Breaking up the tricyclic system breaks up an anticholinergic pharmacophoric group and gives compounds with diminished anticholinergic efiecis Overall, this diminishes unpleasant CNS effects and 1n creases cardiovascular safety. Instead, side effects related It serotonin predominate.
CH3
Fluoxetine.
protonaled in vi,n In Iluoxetine the protonated amino group can H-bond to the ether oxygen
Donepin Hydrochloride
Maprotiline Hydrochloride, USP.
Maprotiline hydrochloride. N-methyl-9. tO-ethanoanthracene-9( I OH)-propanamine hydrochloride (Ludiomil), is sometimes described as a tetracyclic rather than a tricyclic antidepressant. The description is chemically accurate, but the compound, nonetheless. conforms to the overall TCA pharmacophore. It is
electrons, which can generate the gnwp. with the other aryl serving as the characteristic "extra" The S isomer is much more selective for SERT than Icr NET. The major metabolite is the N-dernethyl cornponrst which is as potent as the parent and more selective (SERT versus NET).
a
Therapy for 2 or more weeks is required for the aittidepre' sant effect. Somatodendritic 5HTIA nutoreceptor dcsenciti.
an
a
ethylene-bridged central ring. The compound is not
strongly anticholincrgic and has stimulant properties. It can have effects on the cardiovascular system. It is a
zation with chronic exposure to high levels of 5-HI is Ik accepted explanation for the delayed effect for this and odar
serotonin reuptake inhibitors. To illustrate a difference between selectivity for a SF.R1 and a NET, if the pare: substituent is moved to the aid position (and is less hydrophobic, typically), a NET is tamed. This and other SERTs have anxiolytic activity. Oix of several possible mechanisms would be agonism of 5HT receptors. diminishing synaptic 5-HT. Presumably. sytlaplh
levels of 5-HT might be high in an anxious state. Maprotiline Hydrochloride
Amoxaplne.
Consideration of the structure of arnoxapinc. 2-chloro- Ii -( I -piperaiinyl)dibenz-Ib.J1 II ,4loxazepine (Asendin). reinforces the fact that many antidepressants are very closely related to antipsychoties. Indeed, some, including amoxapine. have significant effects at receptors. The N-methyl-substituted relative of arnoxapine is the antipsymetabolite of chotic loxapine (Loxitane). The umoxapine is reportedly active as an antidepressant and as a Da receptor blocker.
0 NHCH3 Fluoxetine
Paroxetine. In the structure of paroxetine amino group. protonated in vivo could U-bond with —CH2—O— unshared electrons. A
strucsr:
with an extra aryl group results. The compound is a icr highly selective SERT. As expected. it is an effective pressant and anxiolytic.
Anioxapino
Selective Serotonin Reuptake Inhibitors Structurally, the SSRIs differ from the tricyclics. in that the tricyclic system has been taken apart in the center. (This
Paroneline
Chapter 15 • Central Nenvu.c System Sti,nt,lanzs Sertraline.
Inspection of seriraline (Zoloft) (lS.4S) re-
the pharnmcophore for SERT inhibition. The Cl sub-
also predict tropism for a 5-HI system. The depicted stereochemistry is important for activity. H
Most ol the activity of rehosetine resides Reboxetine. in the SS isomer (The marketed compound is RR and SS. It is claimed to he superior to Iluoxetinc in severe depression. It is marketed in Europe. At least three tricyclic compounds.
nortriptyline. and the technically tetracyclic
NHCH3
•HCI
i-i
519
maprotiline are SNERIs. They. of course. have typical characteristic TCA side effects hut lower anticholinergic and antihistaminic (sedative) effects than dimethyl compounds. SNERIs are clinically effective antidepressants. H
Sertratne
The E isomer of Iluvoxamine (Luvox) shown) can fold after protonation to the hydrophobic group is aliphatic. grouping. Here the
Reboxetine
Fluvoxamine
Citalopram (Celexa) is a racemic mixture is very SERT selective. The N-monodemethylatcd corn-
is slightly less potent hut is as selective. The aryl 'ubstiluents are important for activity. The ether function is immportant and probably interacts with the protonated amino
to give a suitable shape for SERT binding.
It would be expected that in the case of SNERIs. a5 presynaptic receptors would be desensitized. after which sustained NE transmission would be via one or more postsynaptic icreceptors are possibilities. ceptors; a1. and
Newer (Nontricyclic) Nonselective 5-HI and NE Reuptaka Inhibitors Presently. one such compound is clinically used in the United States.
The stnlcture and activity of venlafaxine Venlafaxine. (Effexor) are in accord with the general SARs for the group. As expected, it is an effective antidepressant. CH1
/N CH3
Citalopram
Selective Noreplnepbrlne Reuptake the discussion of fluoxetine opened the subject of SNERIs. flat is. movement of a porn substituent of Iluoxetine (and to an org/u, position produces a SNERI.
cl-Is
Selective Serotoninergic Reuptake Inhibitors and Antagonists The SSRIs and 5HTSA antagonists are represented by trazodone (Desyrel) and nefazodone (Serione).
\J Nisoxetine
Nisoxetine is a SNERI and is an antidepressant. Most resides in the
isomer.
The structures of these two compounds derive from those al the lluorobutyrophcnone antipsychotics. They have arylaminc-like structures that permit binding to the SERT
520
Wi/am and Gisiohl'.s
o Nefazodone
and inhibit 5-HT reuptake. In these compounds. the additional hydrophobic substituent can be viewed as being atgroup. Additached to the nitrogen of the tionally. they are antagonists. That antagonism may or may not afford antipsychotic eftéctiveness is discussed under antipsychotics. 5HT2A antagonists appear to have anti-
depressant and anxiolytic activities. They may act, at least in part, by enhancing SI-ITIA activities." Also, some of the effects may be mediated through agonism (perhaps 5-HT-acting antidepressams.) Some of the generally so side effects of SSRIs arc considered to he mediated through
Miscellaneous Antidepressants Bupropion.
The mechanism of action of
(Wellbutrin) is considered complex and reportedly involse' a block of DA reuptake via the dopamine transporter (DAT), hut the overall antidepressant action is noradrenergic. A tabolite that contributes to the overall action and its formation can be easily rationalized.
(,}
SHT2A receptors. so a 5HT2A blocker would reduce them.33 The two compounds yield the same compound on N-dealkyl-
ation. It is a serotonin reuptake inhibitor.
Agonists and Partial Agonists
Buproplon CI
Buspirone. The initial compound in this series. buspirone (BuSpar). has anxiolytic and antidepressant activities and is a partial 51-ITIA agonist. Its anxiolytic activity is reportedly due to its ability to diminish 5-HT release (via 5HTIA agonism). High short-term synaptic levels of 5-HT are characteristic of anxiety. Also, since it isa partial agonist. it can stimulate postsynaptic receptors when 5-HT levels are low in the synapse. as is the case in depression. A number of other spirones are in development as anxiolytics and antidepressants.3'
HO N H
•CH3 Metabolito
MISCELLANEOUS CNS-AcTING DRUGS Buspirone
Antagonists Mirtazapine.
Mirtazapine (Renicron) was recently introduced for clinical use in the United States; its parent mianse,-in (pyridyl N replaced with C-H) was long known to be an antidepressant. It is reported to be laster acting and more potent than certain SSRIs. The mode of action gives increased NE release via a2-NE receptor antagonism and increased 5-HI release via antagonism of NE a2 heteroreceptors located on serotoninergic neurons.33'
This section deals with a collection of drugs that do easily under other topic headings in this chapter or the ter on CNS depressants. All of the drugs are drugs of abue and could be organized under that heading. The fl-arylamino hallucinogens arose because of in the naturally occurring hallucinogens psilocin and mesci line and in modifying the amphetamines, which were popular drugs at the time. Lysergic acid diethylamide was dentally discovered during research on ergot alkaloids. flof scientific interest because it serves as one model forchincal psychosis. Phencyclidine is scientifically intemstingk cause it gives information about the ionotropic N-methyl-iasparlate glutamic acid receptor. and its CNS effects as a model for schizophrenia. Cocaine usa CNS stimulant is a pernicious drug of Research on why it is so strongly addictive and on duu measures that might mitigate its effects has been the past two decades.
.i'-Tetrahydrocannabinol and its relatives were
Mirtazapine
for many years to determine the SAks. The field was gni stimulus with the discovery of the endogenous cannahing_ receptors. Presently, the endogenous cannahinoid sysicrn under investigation.
Chapter IS N Centra! Nervous Svsfrn, Ssi,mdan:.c
1/3.Arylamlno Halluclnogens
521
CH3O
A property of the I f3-arylamino hallueinngcns is alteration of the perception of stimuli. Reality is distorted, and the user may undergo depersonalization. Literally. the effects are those of a psychosis. Additionally, the drugs can produce
CM2 — CH2 — NH2
CH3O
CH3O
anxiety, fear, panic. frank hallucinations, and additional
Mescaline
symptoms that may he found in a psychosis. Accordingly. hey are classed as hallucinogens and psychotomimetics. This group can be subgrouped into those that possess an mdolethylamine moiety, those that possess a phenylethylani-
ne moiety, and those with both. In the lust group. there is a structural resemblance to the central neurotransmitter 5I-IT, and in the second, there is a structural resemblance to NE and DA. This resemblance is suggestive, and there may he sonic selectivity of effects on the respective transmitter systems. With structures of the complexity found in many of these agents, however, a given structure may possibly affect not just the closest structurally related neurotransmit-
CH3O
CH2 CHNH2
CM3
OCH3
-Dimethoxy4-metphenyq-2-amlne (DOM. SIP)
tsr systems but other systems as well. Thus, a phenethylam-
ins system could affect not only NE and DA systems but also 54-IT systems, and an indolethylamine system could affect not only 5.HT but also NE and DA systems.
CM3
3.4-Methytenedioxyamphetamlna (MDA)
INDOLETHYLAMINES CH2CHNHCH3
Dimethylt,yptamine. Dimethyltryptamine is a very weak hallucinogen, active only by inhalation or injection.
CM3
with a short duration of action. It possesses pronounced symçsihomimetic (NE) side effects.
Psilocybln
and Psilocin.
Psilocybin is the phosphoric
acid ester of psilocin and appears to be converted to psilocin as the active species in vivo. It occurs in a mushroom. Psiloi)he montana. Both drugs are active orally, with a short suration of action.
Synthetic a-methyl-substituted relatives have a much lunger duration of action and enhanced oral potency.35 This suggests that psilocin is metabolized by MAOs. R
DMDA (ecstasy)
The presence of methoxyl or dioxymethylene (methylenedioxy) substituents on a 2-phenethylamine system is a characteristic olmany psychotomimclic compounds and strongly suggests DA involvement. AGENT POSSESSING BOTH AN INDOLETHYLAMINE AND A PHENYLETHYLAMINE MOIETY
(+).Lyserglc Acid Diethylamide.
Both an indolethylamine group and a phenylethylamine group can be seen in the structure of the extraordinarily potent hallucinogen lysergic acid diethylamide (LSD. The stereochemistry is ex-
ceedingly important. Chirality. as shown, must be mainCH3 H Dimethyllryptamine Psuiocybin
Psilocin
=H Ft4 = OPO(OH);.. Ft5 = K Ft,1 = OH, Ft5 = H Ft4 =
2•PHENYLETHYLAMINES
Mescaline. 3,4.5.trimethoxyphenethylamuc. is a much-studied hallucinogen with many complex efItcis on the CNS. It occurs in the peyote cactus. The oral required for its hallucinogenic effects is very high. as much as 5(X) mg of the sulfate salt. The low oral potency purbably results from facile metabolism by MAO. a-Methyllion increases CNS activity. Synthetic a-methyl-substituted relatives are more potent.35
tained or activity is lost; likewise, the location of the double bond, as shown, is required.37 Experimentally. LSD has marked effects on seroloninergic and dopaminergic neurons. The bases for all of its complex CNS actions are not completely understood, however. Recently. its actions have been suggested as being more typical of schizophrenic psychotic reactions than the model based on amphetamine. For more on this. see the discussion of atypical antipsychotics (Chapter 14).
0 C2H5
.-'ie
N H 2
5
Ar
The drugs DOM. MDA. and
DMDA (ecstasy) are extremely potent. dangerous drugs of Lysergic Acid Dreihylamido
522
Ttxd,ook of
Wilson wid
Medieinal and Phannact',uiral C'h,',,,iqrv
Depressant-Inti
Dissoclalive Agents
or
Phencyclidine.
Phencyclidine (PCP) was ititroduced as a dissociative anesthetic for animals. Its close structural ida-
live ketaminc is still so used and may he used in humans (Chapter 14). In humans. PCP produces a sense of intoxucalion, hallucinogenic experiences not unlike those produced by the anticholinergic hallucinogens. and often amnesia.
The drug affects many systems. including those of DA. and 5-HI. It has been proposed that PCP (and certain other psychotominnetics) produces a unique pattern of activation of ventral tegumenial area dopaminergic neurons. II blocks glutaminergic N-methyl-o-aspartate receptors. " This action is the basis for many of its CNS effects. PCP itself appears to he the active agent. The psychotic state
CH3
produced by this drug is also cited as a better model than amphetamine psychosis for the psychotic state of
CH3 10
schizophrenia.30
—
Phencychdine Hydrochloride
Cocaine. Cocaine as a euphoriant—slimulant. psychotomimetic. and drug of abuse could as well be discussed with amphetamine and methamphetamine. with which it shares many biological properties. At low doses. ii produces feelings of well-being, decreased faligue. and increased alertness. Cocaine tends to produce compulsive drug-seeking behavior. and a full-blown toxic psychosis may emerge. Many of these effects appear to be related to the effects of increased Nccss
(D1 and
receptors are pertinent). Cocaine is a potent DA
reuptake blocker, acting by competitive inhibition of the DAT. A phenethylamine moiety with added steric hulk may suffice for this action. An interaction between a hydrogen atom on the nitrogen of the proconated form of cocaine and an oxygen of the benzoyl ester group, or alternatively, an interaction between the unshared electron pair of the freebase nitrogen and the carhonyl of the heni,oyl ester group. could approximate this moiety.
o
0CH3
There are twir
conventions for numbering THC: that arising from chemistry produces J'-Tl-IC. and that based on the diks zopyran system results in a J"-THC designation. The noid convention is used here.
TetrahyQrocarlriabinol
TI-IC is a depressant with apparent stimulant sensarior, arising from depression of higher centers. Many effects. re putedly subjectively construed as pleasant. are evident low doses. The interested reader may consult a phararaurl ogy text for a detailed account. At higher doses. psychotorni metic actions, including dysphoria. hallucinations. and pin noiu, can be marked. Structural features associated with activity among cannabis-derived compounds have been c
Notably. the phenolic OH is required for Certain SAks (especially separation of potency bctwecncn antiomers) for cannahi noids suggested action at receptors.' Two receptors fur THC have been discovered. The relesan' receptor for CNS actions is C13,,44 occurs in immure tissues. The first natural ligand k,u,td for the receptor is the amide derivative of arachidonic acid, anandamide.45 Ot}ei natural cannahinoids arc urachidonic acid 2—glycerol oar and 2-arachidonyl glycerol ether." The endogenous hinoid system appears to function usa retrograde messenge system at both stimulatorv synapses and depressant syr apses. 'The synaptic transmnirter causes ses of endocannahinoids that are then transported to receptors located presynaptically where they fine-tune excitatory and inhibitory neurons.47 Because CB, rcceg turs appear to be present in all brain areas and atlect excitatory and inhibitory systems, the prospect selective cannabinoid drugs acting at receptors is not good. Designing drugs to affect the transporter is conciJ ered the most promising research route. Endocannahinoids. as regulated by leptin. are also in maintaining food intake and in other behaviors.°'51 REFERENCES I. Jartioe. C. H.. Porler. L. A.. and Buckler. R. T.: J. Mcd. Cliro 729. 1965. 2. Pcllmar. 'F. C., and WitMin. W. A Science 197:912. 1977,
A
Cocaine
Considerable research on drugs affecting the DAT has been published in recent years. A review of pharmacotherapeulic agents br cocaine abuse is available.4'
3. Fredholm. B. B.. et iii.: t'har,nacol. Res .51113. 1999. 4. Daly. J. W.: J. Med. Cl,ei,r. 25.t97. 9112. 5, SI.. and J. It Anna, Rep Mcd. Chcm. 111:1, Is' 1, Snydrr. S. It.. em al. Proc. Null. Acad Sri It. S A. 'i 7. Tuckei'. A. L.. and Limideti. J.: Re, 27:62. 8. liriun. M. D.: Annu. Rep. Mcd. Cltcm. 211:295. 993. 9 t)cN,nno. M. P.: Anna. Rep. Med. Clicin .33:111. 9911 Ill Arn,ntd. M 1.: ii,cl:iholi.,ii ,if catle,t,e. In Dews. P Caffeine. From Recen, Re,earch. Ncw York. Verlag. 9114. p. 3.
Chapter IS • Central Nen'ous Sy.s rc'nt
1
2
I), 4
I))
IIa)ionJ. 3. C. 0.. und Blundell. 1. 0.: Prop. Drug Res. 54:25. 20(X).
Filler. K W.: Ann. N. Y. Acad. Sd. 305:147, 1978. liatser, 3. A.: Ann. N.Y. Acad. Sei.305:289, 1978. Groppefli. A.. and Cosia. E.: Life Sri. 8:635, 1969. Clineschtnidl, 8. V., et ul.: Ann. N. Y, Aced. Sci. 305:222, 1987. l)voniik. U.. and Schilling, Cr.: 3. Med. Chrm. 8:466. (965. Wejs,i, I.. and Dudas, A.: Monaish. Chem. 91:840, 1960.
lUte, 0.: 3. Mcd. Chent. (2:266, (969. eta).: .1. Pharmucol. Exp. TIter. 241:152. (987. 20. ('crc), 3. M.. and Dayton. P. (3.: Mcihylphenidate. In Usdin. 13.. and Fmrest. I. S. teds.). Psycholherapcuiic Drugs, pail II. New York. Marcel Dekker. (977, p. 287. II. Srittsa.s. N. R., ci iii.: 3. Pharinacol. Eap. 'liter. 241:300, (9(17. 12. Whitclock, 0. V. led.): Ann. N. V. Acad. Sci., 8(1:1881—188), (959. 23. Richards. L. E.. and Burger. A.: Frog. Drug Rcs.30:205, (986. 23. Strupc,.ewski. 3. 0.. Ellis. D. B.. and Allen, R. C.: Annu. Rep. Med. ('hem. 26:297, 1991. 27. Green. A. L: Biochcm. l'hamiacol. (3:249, (964. 20. Burger. A.: J. Med. Pharm. Otem. 4:571. 1961. 17. Kelleau, B., and Mor.tn, 3. F.: J. Am. Chem. Soc. 82:5752. (960. 27. Bclleait, B.. and Moran, 3. F.: 3. Mcd. Phann. Client. 5:215. (962. 3) Silvennan. R, 8.: 3. Rio). Chem. 258:14766, 1983. 70. Rudnick, 0., and Clark. 3.: ISiochim. Biophys. Aria 1144:249. 1993. 'I. Ohvicr, B.: Frog. Drug Rex. 54:59. 2(88). IS. Shaffi'ee. 9. Putnck. K
11
Daniels, T. C., and Jorgensen. E. C.: Cenlral nervous system slimulanis.
In Doerge, R. F. led.). Wilson and Giavold's Textbook of Organic Medicinal and Pliarniaceutical Chemistry. lllh ed. Philadelphia, J. B. Lippiucoti. 1982. p. 383. 73. Erred, 0. A.. and Hamsoii, B. 1..: Aitnu. Rep. Mcii, Chein.34:l. 1999. 74. Olivier, B., ci al.: Prop. Drug Res. 52:103. 999. 73. Murphree. H. B.. eta).: Clin. Phurmacol. Tttcr. 2:722. 1961. 70. Shulgin. A. T.: Naiutre 201:120. 1964.
Sin)). A.. and Holmann. A.: HeIr. 0dm. Aria 38:42). 1955. 7) lowers, M. B.. Bannctn, M. 3., and Hoffman, 0. J.. Jr.: Psycltopliarnia. eulogy 93:133. 1987.
523
39. Foster. A. C.. and Fogg. 0. 0.: Nature 329:395. 1987. 40. Rowley. M.. Bristow. L. 3., aiid Hutson. P. H.: 3. Med. Chetn 44:477, 2(8)1.
41. Carroll. F. I.. Howell. L. L,and Kuhov. M. J.: I. Med. Cbcm. 42:2721. 20(X).
42, Edery, H.. ci al.: Ann. N. Y. Arad. Sci. 191:40. 1971. 43. Hol)istcr. I.. 13.. Gillespie. II. K.. and Srehnik. M.: Psydltophannacology 92:505. 1987. .14. Matsuda. L. A.. ci al.: Nature 346:561. (99(3. 45. Davannc. W. A.. ci a).: Science 258:1946, (992. 46. Mcchou(nm. R., ci al.: Proc. Nail. Acad. Sc), U.S.A. 98:3602. 2(101. 47. Egeriovu. M.. ci al.: Proc. R.Soc. London B 265:21)8. (998. 48. Wilson, R. I. and Nicoll. R. A.: Nature 4(0:588, 218(1. 49. Ohn-Shosaku. T.. Maejqma. 1.. and Kano. N.: Neuron 29:729, 2(8)1. 50. Krciizer, A. C.. and Rcgehr. W. 0.: Neuron 29:7(7, 2(101. 5). Chrisde. M. 3.. and Vaughn. C. W.: Nature 4(0:527. 2(8)1. 52. DiMar,.o, V.. eta).: Nature 410:822, 2001. 53. Mcchoulam, K., and Fride. 0.: Nature 4(0:763, 200).
SELECTED READING Currol. F. I.. Howell, F. I., and Kuliar. M. 3.: l'harnxacoihcrupies (or treatment of cocaine abuse: Preclinical aspects. 3. Mcd. ('hem. 42:2721. 2000.
Frcdholtn. B. 8.. Battig K.. Holiiieit, 3.. et al.: Actions of caffeine in the brain with special reference to (actors thitt contribute to its widespread use. Pttartnacol. Rev. 51:83. 1999. Hulford. 3. C. (1.. and Blundell. 3. 11.: Pharmacology oFappelile suppression. Prop. Drug. Rex. 54:25, 2000. Olivier. B., Soudijn, \V., and van Wujngaanden. I.: Serotonun. dopatitinc and norepinephnne transporters in the central nervous systetit and their inhibitors. Prop. Drug. Res. 54:59. 21)0(1. Xiiing. .1-N.. and Lee. 3. C.: Pharmacology of runnahinoid receptor agonists md aniugimists. Anna. Rep. Med. ('hem. 54:199. 2(8(11.
CHAPTER 16 Adrenergic Agents RODNEY L JOHNSON
Adrenergic drugs are chemical agents that exert their princi-
groups situated ortho to each other, the same arrangemern
pal pharmacological and therapeutic effects by either enhancing or reducing the activity of the various components of the sympathetic division of the autonomic nervous system. In general, substances that produce effects similar to stimulation of sympathetic nervous activity are known as svrnpaihwnimerks or udrenergk stin,,,lants. Those that de-
of hydroxyl groups as found in catechol. Aromatic corn
crease sympathetic activity arc referred to as synipatholyrie.c.
an::adrenergws. or adrenergw-hlockmg agents. Because of the important role that the sympathetic nervous system plays in the normal functioning of the body, adrenergic drugs find wide use in the treatment of a number of diseases. In addition
pounds that contain such an arrangement of hydroxyl subsiit
uents are highly susceptible to oxidation. such as epinephrine and NE. undergo oxidation in the ence of oxygen (air) or other oxidizing agents to produce ertho-quinone-like compounds. which undergo further rvac lions to give mixtures of colored products. Hence. of catecholamine drugs often are stabilized by the of an antioxidant (reducing agent) such as ascorbic acid cr sodium bisulfite.
to their effects on sympathetic nerve activity, a number of adrenergic agents produce important effects on the central nervous system (CNS). In this chapter. those agents that affect adrenergic neurotr.msmission and those that act directly on the various types of adrenergic receptors are dis-
Catethol
ortho-Quinone
cussed.
Epinephrine and NE each possess a chiral carbon atom. thus, each can exist as an enantionieric pair of isomers. enantiomer with the (R) configuration is biosynthesized
ADRENERGIC NEUROTRANSMITFERS
the body and possesses the biological activity. Catecholamines are polar substances that contain acidic (the aromatic hydroxyls) and basic (the aliphJtk amine) functional groups. For example, the pK, values the cpinephrine cation are 8.7 and 9.9 and are attributed
Structure and Physicochemlcal Norepinephrine (NE) is the neurotransmitter of the postgan-
glionic sympathetic neurons. As a result of sympathetic nerve stimulation, it is released from sympathetic nerve end-
ings into the synaptic cleft, where it interacts with specific presynaptic and posisynaptic adrcncrgic receptors. Another endogenous adrenergic receptor agonist is epinephrine. This compound is not released from peripheral sympathetic nerve endings, as is NE. Rather, it is synthesized and stored in the
adrenal medulla, from which it is released into the circulation. Thus. epinephrine is often referred to as a neurohormone. Epinephrine is also biosynthesized in certain neurons of the CNS. whcre both it and NE serve as neurotransmiters.
OH
H,
the phenolic hydroxyl group and the protonated group, respectively. Ganellin1 calculated the relative popuL
tions of the various ionized and nonionized species of and epincphrine at pH 7.4 and found that the cation loin (Fig. 16-IA) is present to an extent slightly greater than for both catecholamines. The zwittcrionic form (Fig. lb-Il, in which the aliphatic amine is prolonated and one of tic phenolic hydroxyl groups is ionized, is present to about Thus, at physiological pH. less than 2% of either cpinephtin.
or NE exist.s in the nonionized lbrm. This largely accouv' for the high water soluhility of these compounds as welin other catecholamines. such as isoproterenol and doparniin
Biosynthesis The biosynthesis of the catecholurnines dopamine. NE
H
A
CH3
Epinephrine and NE belong to the chemical class of substances known as the eateehok,,n,ne.s. This name was given
a sequence of enzymatic reactions: illustrated in Figure 16-2. Catecholamine biosynthesis lain place in adrenergic and dopaminergic neurons in the
in sympathetic neurons of the aut000mic nervous
slen
and in the adrenal medulla. The amino acid m.-tyrosine as the precursor for the camecholamines. It is tmnspurv;
to these compounds because they contain an amino group
actively into the axoplasm. where it is acted on by 3-monooxygenase (tyrosine hydroxylase) to form droxyphenylulanine (m.-dopu). Tyrosine hydroxylase is.
attached to an aromatic ring that contains two hydroxyl
Fe2
524
-containing enzyme thai requires molecular oxvgenac
Chapter 16 •
A
B
OH
OH
Figure 16—1 • Cationic (A) and zwitterionic (B) forms of norepinephrine CR = I-I) and epinephrine CR = HO
CH3)
Tyrosine Tyrosifle Hydroxylase
I
HO L-Dihydroxypheflytalafllfle I L-Aromatlc AmIno Acid 98
uses tetrahydrobioplcrin as a cofactor. The enzyme plays a key role in the regulation ot catecholainine biosynthesis, as it is the rate-limiting step. For example. adrenergic nerve stimulation leads to activation of a protein kinase that phosphorylates tyrosine hydroxylase. thereby increasing its activity. In addition, through end-product inhibition, NE markedly reduces tyrosinc hydroxylase activity. The basis of this feedback inhibition is believed to be a competition between the eatecholamine produci and the pterin cofactor. second enzymatic step in catecholamine biosynthesis is the decarboxylation of i.-dopu to give dopamine. The enzyme that carries out this transformation is i-aromatic amino acid decarboxylase (dopa decarboxylase). It is a cytoplasmic enzyme that uses pyridoxul phosphate as a cofactor. In addition to being found in catccholaminergic neurons, i-aromatic amino acid decarboxylase is Ibund in high concentrations
in many other tissues, including the liver and kidneys. It exhibits broad substrate specillcity, in that aromatic amino acids, such as L-tyrosine. L-phenylalanine. i-histidine. and i.-tryptophan. in addition to L-dopa and i.-5-hydroxytryptophan. serve as substrates.
The dopamine formed in the cytoplasm of the neuron is actively transported into storage vesicles. where it is hydroxylated stereospecilically by the Cu2 '-containing enzyme doto (dopamine pamine requires molecular oxygive NE, Dopamine It exhibits rather gen and uses ascorbic acid as a
HO
wide substr4te specificity. The NE formed is stored in the vesicles until depolarization of the neuron initiates the process of vesicle fusion with the plasma membrane and extnlsion of NE into the synaptic cleft. Adenosine Lriphosphate (ATP) and the protein chromogranin A arc released along with NE. In the adrenal medulla. NE is converted to epinephrine. This reaction, which involves the transfer of a methyl group from S-adenosyl methionine to NE. is catalyzed by phenylethanolumine-N-methyltransferase (PNMT). It occurs in the cytoplasm, and the epinephrine formed ix transported into
Dopamlne
Dopamine
OH NH2
HO
the storage granules of the chromaffin cells. Although
Norepinephrine
I pi,enyiethanofamine1N-methyltransferase
PNMT is highly localized in the adrenal medulla, it is also present in small amounts in heart and brain tissues.
Uke and Metabolism OH 3
HO Eplnephrine FIgure 16—2 • Biosynthesis of the cat&holamines dopamine, and epunephrine.
The action of NE at adrenergic receptors is terminated by a combination of processes, including uptake into the neuron and into extrancuronal tissues, diffusion away from the synapse. and metabolism. Usually, the primary mechanism for
termination of the action of NE is reuptake of the catecholamine into the nerve terminal. This process is termed uptake-I and involves a Na /C1-dependent transmemhrune transporter that has a high affinity for NE.4 This uptake systern also transports certain amines other than NE into the
526
Wilson and Gisiold'.s
Te.v Shook (JJ
Clwn,is,rv
Organid Piled,e:,,a! and
nerve lemiinal. and it can be blocked by such drugs as cocaine and sonic of the tricyclic antidepressants. Sonic of the NE that reenters the sympathetic neuron is transported into storage granules. where it is held in a stable complex with
ATI' and protein until sympathetic nerve activity or some other stimulus causes it to be released into the synaptic cleft.
The transport of NE from the cytoplasm into the storage granules is carried out by an H -dependent transincmbrane vesicular transporter.5 In addition to the neuronal uptake of NE discussed above. there exists an extraneuronal uptake process. uptake-2. This uptake process is present in a wide variety of cells. including
glial. hepatic. and myocardial cells. It has relatively low affinity for NE. Although its physiological significance is unknown, it may play a role in the disposition of circulating catecholamines. since catecholamines that are taken up into extraneuronal tissues arc metabolized rapidly. The two principal enzymes involved in catecholamine mc-
tabolism are monoamine oxidase (MAO) and catechol-OBoth of these enzymes are methyltransferase COMT).6 distributed throughout the body. with high concentrations found in the liver and MAO is associated primarily
with the outer membrane of the mitochondria. while COMT is found primarily in the cytoplasm. The wide tissue distribution of MAO and COMT indicates that both act on catecholamines that enter the circulation and the extrancuronal after being released from nerves or the adrenal gland or ahrr being adtninistered exogenously. In addition, the fact than COMT is not present in sympathetic neurons whereas the neuronal mitochondria do contain MAO indicates that MAO also has a role in the metabolism of intraneuronal catcehola mines. Neither COMT nor MAO exhibits high substrate specif c-
ity. MAO oxidatively deaminates a variety of that contain an amino group attached to a terminal carbon. There arc two types of MAOs. and these exhibit different substrate selectivity.5 For example. MAO-A shows substranc
preference for NE and serotonin. while MAO-B strale selectivity for and beniylaniine. Similarly. COMT catalyzes the niethylation of a variety ii catechol-containing molecules. The lack of substrate sped-
ficity of COMT and MAO is manifested in the metabolic disposition of NE and epinephrine. shown in Figure 15.3 Not only do both MAO and COMT use NE and epinephninc
1) MAO
HO
Dehydrogenase
34.Dlhydroxypheflyl. glycolaldehyde
Norepinephrlne: A = H
A CH3
HO... Aldehyde Reductase
OH
H
COMT
OH
HO H A
OH
HO
HO 3.Methoxy-4.hydroxy. phenylethylene Glycol
3-Methoxy-4-hydroxymar,delic Acid
Figure 16—3 • Metabolism of norepinephrine and epinephrine by MAO and COMT.
Chapter 16 • as substrates, but each also acts on the metabolites produced by the other. The results of extensive research on catecholuminc metabolkm indicate that in the adrenergic neurons of human brain
and peripheral tissues, NE is deaminated oxidatively by MAO to give 3.4-dihydroxyphenylglycolaldehyde. which then is reduced by aldehyde reductase to 3.4-dihydroxyphen-
ylethylene glycol. It is primarily this glycol nietubolite that is released into the circulation, where it undergoes methylalion by the COMT that it encounters in nonneuronal tissues.
The product of methylation. 3-methoxy-4-hydroxyphenylethylene glycol, is oxidized by alcohol dehydrogenase and aldehyde dehydrogenase to give 3-methoxy.4-hydroxymandelic acid. This mnetabolite commonly is referred to as vanil-
lylmandelic acid (VMA). and although it can be the end product of several pathways of NE metabolism. 3-methoxy-
4-hydroxyphenylethylene glycol is its principal precursor. In the oxidative deamination of NE and epinephrine at extraneumnal sites such as the liver, the aldehyde that is formed is onidized usually by aldehyde dehydrogenase to give 3,4lihydroxymandelic acid.
Methylation by COMT occurs almost exclusively on the neza.hydroxyl group of the catechol. regardless of whether
he catechol is NE. epinephrine. or one of the metabolic products. For example. the action of COMT on NE and epincphiine gives normetanephrine and metanephrine. respec-
lively. A converging pattern of NE metabolism of NE and epinephrine in which 3.methoxy-4-hydroxymandclic acid and 1.methoxy-4-hydroxyphenylethylene glycol are common end products thus occurs, regardless of whether the lint metabolic step is oxidation by MAO or methylation by COMT.
I
Under normal circumstances. 3-methoxy-4-hydroxyniandelic acid is the principal urinary naetabolite of NE. though substantial amounts of 3-methoxy.4-hydroxyphenylethylene
are excreted along with varying quantities of other
I
nactabolites. both in the free form and as sulfate or glucuronije conjugates. Endogcnous epinephrine is excreted primarlv as
I
nietanephrine and 3.methoxy-4-hydroxymandelic
aid.
ADRENERGIC RECEPTORS
nAdrenergic
Receptors
Ahiquist" was the Iirst to propose the existence of two gcnad types of adrenergic receptors (adrenoceptors) in mamitalian tissues. He designated these adrenergic receptors a ijsI His hypothesis was based on the differing relative çstencies of a series of adrenergic receptor agonists on variros smooth muscle preparations. In the early I 970s. the that certain adrenergic agonists and antagonists exhibited various degrees of selectivity for presynaptic and Issisynaptic a-adrenergic receptors led to the proposal that
susisynaptic a receptors be designated a1 and that presynap-
a receptors be referred to as Later, a functional dassification of the a receptors was proposed wherein a1 receptors were designated as those that were excitatory.
I
527
synaptic and either excitatory or inhibitory in their responses. Thus, it became clear that neither an anatomical nor a functional classification system was as generally useful
in classifying adrenergic receptors as a pharmacological classification based on the relative potency of a series of receptor agonists and antagonists.'2 Pharmacological and molecular biological methods have shown that it is possible to subdivide the a1 and a2 receptors into additional subtypes. Although the subtyping of adrenergic receptors continues to evolve, at present, the a1 and a2 receptors each have been divided into at least three subtypes. which have been designated alA. a18, a10 and a25, a28. a20-. respectively)' The molecular basis by which activation of a-adrenergic receptors produces the appropriate tissue responses has been
studied extensively. Both receptor subtypes belong to a superfamily of membrane receptors whose general structure consists of seven transniembrane a-helical segnients and whose signal-transduction mechanisms involve coupling to guanine nucleotide-regulatory proteins (C) proteins). They differ from each other, however, in the second-messenger The a,-adrenergic receptor is system that is coupled to the enzyme phospholipase C via a C) protein. Gq. When stimulated by activation of the a,-adrenergic receptor. phospholipase C hydrolyzes phosphalidylinositol-4.5-bisphosphate to give the second messengers mositol- I .4,5-tn-
phosphate Ilnst l.4.5)P,j and 1.2-diacylglycerol (DAG). lns( I ,4,5)P, stimulates the release of Ca2 from the sarcoplasmic reticulum, while DAG activates protein kinase C. an enzyme that phosphorylates proteins, a, -Receptor activation also can increase the intlux of extracellular Ca1 via voltage-dependent as well us non—voltage-dependent Ca2
channels. Activation of a2-adrencrgic receptors leads to a reduction in the catalytic activity oladenylyl cyclase. which in turn results in a lowering of intracellular levels of cyclic3.5-adenosine monophosphate (cAMP). The a2-adrenergic receptor—mediated inhibition of adenylyl cyclase is regulated by the G protein G,. a-Adrcncrgic receptors of the CNS and in peripheral tissues affect a number of important physiological functions." In particular, a receptors are involved in control of the cardiova.scular system. For example. constriction of vascular smooth muscle is mediated by both postjunctional a1- and a2-adrenergic receptors, though the predominant receptor In the heart, activation of a1 mediating this effect is receptors results in a selective inotropic response with little or no change in heart rate." This is in contrast to the /3, receptor, which is the predominant postjunctional receptor in the heart, mediating both inotropic and chronotropic effects. In the brain, activation of postjunctional a2 receptors reduces sympathetic outflow from the CNS. which in turn causes a lowering of blood pressure.21' The prototypical a2 receptor is the presynaplic a receptor found on the terminus of the sympathetic neuron)" 11.21 Interaction of this receptor with agonists such as NE and epiuiephrine results in inhibi-
tion of NE release from the neuron. The a2 receptors not only play a role in the regulation of NE release but also
shile a1 receptors purportedly mediated inhibitory revines." Further developments revealed, however, that
regulate the release of other neurotransmitters. such as acetylcholinc and serotonin. Both a1- and a2-adrenergic receptors also play an important role in the regulation of a number of metabolic processes, such as insulin secretion and glyco-
both a1 and a2 receptors could be either presynaptic or post-
genolysis.22
528
al Or,ç'wiie tledici,wl and Plwrsnaeeuncal Che,,,iszrv
Wilson and Gi.o'old'.s
fi-Adrenergic Receptors
of the receptor interact with the 0, protein.
In 1967. almost 20 years atier Ahlquist's landmark paper proposing the existence of a- and receptors.
aspartic acid residue 113 in transmembrane region Ill acts the countcrion to the cationic amino group of the adrcnergk agonist, while two serine residues, at positions 204 and 207 in transniembranc region V. form hydrogen bonds with
Lands et al.23 suggested that
vided into flu and
receptors also could be subdi-
types. Seventeen years later. Arch ci
al.24 identified a third subtype ot/3 receptor in brown adipose tissue. They initially referred to this as an atypical fireceptor. but it later became designated the subtype.'5 These f3adrenergic receptor subtypes differ in tenOn ol the rank order of potency of the adrenergic receptor agonists NE. epinephrine. and isoproterenol. The receptors exhibit the agonist
potency order isoproterenol > epinephrine = NE. while receptors exhibit the agonist potency order isoproterenol > cpinephrine >> NE. For the receptor. the agonist potency order is isoproterenol = NE > epinephrine. The fi receptors are located mainly in the heart, where they mediate the positive inotropic and chronotropic effects ol' the catecholumines. They are also found on the juxt-aglouiierular cells of the kidney, where they are involved in increasing renin secretion. The receptors are located on smooth
muscle throughout the body. where they arc involved in relaxation of the smooth muscle, producing such effects as bronchodilation and vasodilation. They are also found in the liver, where they promote glycogenolysis. The receptor is located on brown adipose tissue and is involved in the stimulation of Iipolysis. Like the a,-udrcncrgic receptors, the fi-adrenergic receptors belong to the superfamily of membrane receptors whose general structure consists suf seven transmemhrane a-helical segments and whose signal-transduction mechanisms involve coupling to G proteins. All three fl-receptors are coupled to adenylyl cyclase. which catalytes the conversion of AlP to cAMP. This coupling is via the guanine nucleotide protein (3,25 In the absence of agonist. guanosine diphosphate (GDP) is hound reversibly to the (3, protein. Interac-
tion of the agonist with the receptor is believed to bring about a conlorniational change in the protein receptor, which causes a reduction in the affinity of the G, protein for CJDP and a concomitant increase in affinity for guanosine triphos-
phate (GTP). The a, subunit of the 0. protein, with GTP bound to it, dissociates front the receptor—G protein ternary
complex, hinds to adenylyl cyclase. atid activates the enzyme. The bound OTP then undergoes hydrolysis to GDP. and the receptor—G, protein complex returns to the basal state.
The intracellular function of the second-messenger cAMP appears to be activation of protein kinases. which phosphorylate specific proteins, thereby altering their function. Thus. the phosphorylated proteins mediate the actions of cAMP. which functions as the mediator of the action of the drug or neurotransinitter that originally interacted with the fl-recep-
tor.27 The action of cAMP is terminated by a class of enzymes known as phosphodiesterases. which catalyze the hy-
drolysis of cAMI' to AMP. Cloning of the gene and complementary DNA (eDNA) for the manimaliun fi-adrenergic receptor has made it possible to explore through single point mutations and the construction of chimeric receptors the structure—function relationships of the receptor.25 Through such studies, it has been proposed
that the adrenergic agonist-binding site is within the transmembrane-spanning regions. while the cytoplasmic regions
catechol hydroxyls of the adrenergic agonists. The droxyl group of adrenergic agonists is thought to form hydrogen bond with the side chain of asparagine 293 a transmembrane region Vi. while the phenylalaninc at position 290 in the same transmemhrane region is heliescd
to interact with the catechol ring. Information such as this will no doubt aid in the future design and synthesis of new and improved adrenergic receptor agonists and antagonusx Molecular biological techniques have shown the existenm of adrenergic receptor polymorphism for both the a- and
adrenergic receptors. It is postulated that such polymw phisms may be an important factor behind individual diffa• ences in responses to drugs acting at these receptors. Also. there may he an association between the adrenergic receptor genes and disease states.2" This will 1
tainly be an active area of research in the future, and results could have a great impact on the development and therapeutic use of not only the current adrenergic agents but also those that arc yet to be developed.
DRUGS AFFECTING ADRENERGIC NEUROTRANSMISSION
Drugs Affecting Catecholamlne Biosynthesis Metyrosine.
Many agents that affect
bioxynthe.sis are known, hut only a few are used u.s therupcu
tic agents. Metyrosinc (a-rnethyl-i-tyrosine, example of a catecholamine-hiosynthesis inhibitor in Metyrosine differs structurally from tyrosine only a the presence of an a-methyl group. It isa connpetitive
tor of tyrosine hydroxyla.se. the first and rate-limiting in catecholamine biosynthesis. As such. nnetyrosine Is much more effective inhibitor of epinephrine and NE jxs' duction than agents that inhibit any of the other enaytla involved in catecholannine biosynthesis. Although melyro sine is used as a racemic mixture. it is the (—) isomer possesses the inhibitory activity. Metyrosine. which is orally in dosages ranging from I to 4 glday. is used pririo pally for the preoperative tnanagement of toma. This condition involves chromaffin cell tumors thx produce large amounts of NE and epinephrine. Althou4 these tumors, which occur in the adrenal medulla, are obuc
benign. patients frequently
hypertensive episondo
Metyrosine reduces the frequency and severity of these
sodes by significantly lowering catecholamine (35 to ((0%). The drug is excreted mainly unchanged isle urine. Because of its limited solubility in water. crystallat is a potential serious side effect. Sedation is the most mon side effect of metyrosine.
C'H3CO2H Metyrosine
DTugs Affecting Catecholamlne Storage
whole root of R.xerpeniina are used in the treatment of
and Release
hypertension. Preparations in which reserpine is combined with a diuretic also are available, as diuretics increase the efficacy of reserpine.
Reserpinc is the prototypical drug affecting the vesicle storage of NE in sympathetic neurons and neuions of the CNS and of epinephrine in the adrenal medulla. ho actions are not limited to NE and epinephrine. however. as it also affects the storage of serotonin and dopamine in their respective neurons in the brain. Reserpine is an indole alkaloid obtained from the root of serpenlina. a climbing shrub found in India. Other alkaloid constituents of this plant that possess pharmacological activity similar to that of reserpine are deserpidine and rescinnamine. ReserReserpine.
binds extremely tightly with the ATP-driven monoamine transporter that transports NE and other biogenic This amities from the cytoplasm into the storage pine
binding leads to a blockade of the transporter. Thus in sympathetic neurons. NE. which normally is transported into the storage vesicles. is instead metabolized by mitochondrial MAO in the cytoplasm. In addition, there is a gradual loss of vesicle-stored NE as it is used up by release resulting lion sympathetic nerve activity. It is thought that the storage eventually become dysfunctional. The end result is
i depIction of NE in the sympathetic neuron. Analogous cifects are seen in the adrenal medulla with epinephrine and in serotonergic neurons.
Guanethidine and GuanadreL
Neuronal blocking agents arc drugs that produce their pharmacological effects primarily by preventing the release of NE from sympathetic nerve terminals. Drugs of this type enter the adrenergic neuron by way of the uptake-I process and accumulate with in the neuronal storage vesicles. There, they stabilize the neuronal storage vesicle membranes, making them less respon-
sive to nerve impulses. The ability of the vesicles to fuse with the neuronal membrane is diminished, resulting in inhibition of NE release into the synaptic cleft. Some of these agents on long-term administration also can produce a depletion of NE stores in sympathetic neurons. Structurally, the neuronal blocking drugs typically possess a guanidino moiety ICNHC( = NH)NH2I, which is attached to either an alicyclic or an aromatic lipophilic group. These structural features are seen in guancthidine (Ismelin) and
guanadrel (Hylorel). which are used clinically in the treatment of hypertension. The presence of the very basic guanidmo group (pK,> 12) in these drugs means that at physiolog-
ical pH they are essentially completely protonated. Thus. these agents do not get into the CNS.
Guanethidine OCR3 Renerpine: A' = OCR3. A2 =
OCH3
Guanadrel
OCR3
=H, R2= OCR3 OCR3
Rt = OCH3. R2 = OCH3
When reserpinc is given orally. it maximum effect is seen
Although guanethidine and guanadrel have virtually the same mechanism of action on sympathetic neurons, they differ in their pharmacokinetic properties. For example. while guanethidine is absorbed incompletely after oral administration (3 to 50%).panadrel is well absorbed, with a bioavailability of 85cf.3 These two agents also differ in terms of half-life: Guanethidine has a half-life of about 5 days, whereas guanadrel has a half-life of 12 hours. Both agents are partially metabolized (—50%) by the liver, and both are used to treat moderate-to-severe hypertension. either alone or in combination with another antihyperlensive agent.
a couple of weeks. A sustained effect up to several is seen after the last dose has been given. Reserpine
ii otensively metabolized through hydrolysis of the ester unction at position 18. This yields methyl reserpate and acid. As is typical of many indole utkaktds. reserpine is susceptible to decomposition by light
ad oxidation. Both the pure alkaloid and the powdered
Bretylium Tosylate.
Another neuronal blocking agent is the aromatic quaternary ammonium compound bretylium tosylate (Bretylol). This agent is used as an antiarrhythmic drug. Its antiarrhythmic actions are not believed to be due to its neuronal blocking effects, however. This agent is discussed in more detail in Chapter 19.
530
IViIst,n and Gi.cr,dds Texthuo& of Organic Medki,,a! and Pharinaceulkal Chemistry
CH3
+1
Q— Br
CH2 —N—CH2CH3 CH3 Bretyllum Tosylate
SYMPATHOMIMETIC AGENTS Sympnthomimetic agents produce effects resembling those produced by stimulation of the sympathetic nervous system. They may be classified as agents that produce effects by a direct, indirect, or mixed mechanism of action. Direct-acting agents elicit a sympathomimelic response by interacting directly with adrenergic receptors. Indirect-acting agents pro-
duce effects primarily by causing the release of NE from adrenergic nerve terminals: the NE that is released by the indirect-acting agent activates the receptors to produce the response. Compounds with a mixed mechanism of action interact directly with adrenergic receptors and cause the release of NE. As described below, the mechanism by which an agent produces its sympathomimetic effect is related intimately to its chemical structure.
Direct-Acting Sympathominietics STRUCTURE—ACTIVITY RELATIONSHIPS
Structure—activity relationships for a- and /3-adrenergic receptor agonists have been The parent structure for many of the sympathomimetic drugs is /3-phenylech-
ylamine. The manner in which /3-phenylethylamine
is
substituted on the mew and para positions of the aromatic ring and on the amino, a. and $ positions of the ethylaminc side chain influences not only the mechanism of sympathomimetic action but also the receptor selectivity of the drug. For the direct-acting sympathomimetic amines. maximal activity is seen in /3-phenylechylamine derivatives containing hydroxyl groups in the mew and para positions of the aromatic ring (a catechol) and a $-hydroxyl group of the correct stereochemical configuration on the ethylamine portion of the molecule. Such structural features are seen in the prototypical direct-acting compounds NE. epinephrinc. and isoprotcrcnol.
A critical factor in the interaction of adrenergic agonists with their receptors is stereoselecciviry. Direct-acting sympa-
thomimetics that exhibit chirality by virtue of the presence of a /3-hydroxyl group (phenylethanolamines) invariably exhibit high stereoselectivity in producing their agonistic effects; that is. one enantiomeric form of the drug has greater
affinity for the receptor than the other form has. This is tote for both a- and /3-receptor agonists. For epinephrine. NE and related compounds, the more potent enantiomer has fit (R) configuration. This enanciomer is typically several 1(N). fold more potent than the enanhiomer with the (S) configura. Lion. It appears that for all direct-acting. /3-phenyl. ethylamine-derived agonists that are structurally similar ii NE, the more potent enantiomer is capable of assuming conformation that results in the arrangement in space of the cacechol group, the amino group, and the /3-hydroxyl group in a fashion resembling that of (— )-(Rl-NE. This explanation of stereoselectivity is based on the presumed interaction these three critical pharmacophoric groups with three cion plementary binding areas on the receptor and is known a' the Easson-Stedinan hypothesis.'7' 36 This three-point action is supported by recent site-directed mutagenesis stud' ies2° on the adrenergic receptor and is illustrated in Figure 16-4.
The pre.sence of the amino group in phenylethylamines is important for direct agonist activity. The amino group should be separated from the aromatic ring by two carbon atoms for optimal activity. Both primary and secondary amines air found among the potent direct-acting agonists, but tertiary or quatentary amines tend to be poor direct agonists. The nature of the amino substituent dramatically affect.s the in. ceptor selectivity of the compound. In general. as the bulkol the nitrogen substituentincrea.ses. a-receptor agomst decreases and /3-receptor activity increases. Thus NE. which is an effective /3,-receptor agonist, is also a potent and a potent agonist at a,
tors. lsoproterenol. however. isa potent and agonist but has little affinity for a receptors. The nature the substituent can also affect /3, - and
selectivity.
In several instances, it has been shown that an
Asp113 Figure 16—4 • Illustration of the Easson-Stedman hypothies
representing the interaction of three critical pharmacophr: groups of norepinephrine with the complementary areas on the adrenergic receptor as suggested by mutagenesis studies,
Chapter 16 S Adretiergie
seketi vity. For example, N-terl-hutyl norepinephrine Coltcrol) is 9 to 10 times as potent an agonist receptors. Large receptors than at cardiac tracheal
show selectivity to the
cubstituents on the amino group also protect the amino group
oral bioavailability.
group enhances
531
receptor. As in the case of the
resorcinol niodification. this type otsubstitution gives agents that are not nuetaboli/ed by COMT and thus show improved
front undergoing oxidative deamination by MAO. OH
NHCH(CH3)2 NHCH(CH3)2
OH Isoproterenol
Resorcinal OH
Metaproterenol
OH NH(CH3)3
NHC(CH3)3
N-tert-Butylnorepinephrine (Colterol)
Methyl or ethyl substitution on the a-carbon of the ethside chain reduces direct receptor agonist activity at both a and $ receptors. Importantly, however, an a-alkyl group increases the duration of action of the phenylethylamagonist by making the compound resistant to metabolic
&amination by MAO. Such compounds often exhibit enhanced oral effectiveness and greater CNS activity than their counterparts that do not contain an a-alkyl group. a-Substilulion also significantly affects receptor selectivity. In the for example. a-methyl or ethyl substitu-
CH2OH Albuterol
Modification at the catechol ring can also bring about selectivity at a receptors as it appears that the catechol moiety is more important tar agonist activity at receptors than at a1 receptors. For example. removal of the p-hydroxyl group from epinephrine gives phenylephrine, which, in contrast to epinephrine. is selective for the a1-adrenergic receptor.
lion results in compounds with selectivity toward the th rceeptur. while in the case of a receptors, a-methyl substituion gives compounds with selectivity toward the recep-
or, Another effect of a-substitution is the introduction of a center, which has pronounced effects on the stereoclvrnieal requirements for activity. For example, with a-
it is the e:'thro (IR.2S) isomer that significant activity at a receptors. H,
OH NH2
Phenylephrine
In addition to the /3-phenylethylamine class ot adrenergic receptor agonists. there is a second chemical class of compounds. the irnidaiolines. that give rise to a-adrenergic receptor agonists. These imidazolines can be nonselective. or they can be selective for either the a1- or a2-adrenergie re-
ceptors. Structurally, imidaiolines for the most part have the heterocyclic imidazoline nucleus linked to a substituted (1 R,2S).a.Methylnoreplnephrine
Although the catechol moiety is an important structural caure in terms of yielding compounds with maximal agoactivity at udrenergic receptors, it can be replaced with phenyl moieties to provide selective adrencrgic agonists. In particular, this approach has been used in design of selective /32-receptor agonists. For example.
aromatic moiety via sonic type of bridging unit (Fig. 16Although modification of the imidazoline ring generally results in compounds with significantly reduced agonist activity, there are examples of so-called open-ring imidai.olines that are highly active. The optimum bridging unit (X) is usually a single amino or methylene group. The nature of the aromatic moiety. as well u.s how it is substituted, is quite
cplxemenl of the catechol function of isoproterenol with heresoreinol structure gives the drug nietuproicrenol. which a celective
agt)nist. Furthermore, since the
triorvinol ring is not a substrate for COMT, /3 ugonists that asitain this ring structure tend to have better absorption hataL-Icristics and a longer duration of action than their cateJuil-containing counterparts. In another approach, replacescOt of the mew-hydroxyl of the catechol structure with a group gives agents. such u.s albuterol, which
N
Aromatic moiety
N H
Imldazoline nng
Bridging unit
Figure 16—5 • General structural features of the imidazoline a-adrertergic receptor agonists.
532
tVj/.cc,,, and Gino/d.c 'lexihook of Organie Medleinal arid
flexible. However, agonist activity is enhanced when the aromatic ring is substituted with halogen substituents like Cl or small alkyl groups like methyl, particularly when they are placed in the two orliw positions. Since the structure—activity relationships of the imidazolines are quite different it has been postufrom those of the lated that the imidazolines interact with a-adrenergic recep-
tors differently from the way the particularly with regard to the aromatic
do.
(i,emixtn' oxidizing agents, and oxygen of the air. It is not effecticu by the oral route because of poor absorption and rapid metab-
olism by MAO and COMT. Although intravenous infusion of epinephrine has pronounced effects on the cardiovascular system, its use iii thc
treatment of heart block or circulatory collapse is limited because of its tendency to induce cardiac arrhythmias. It increases systolic pressure by increasing cardiac output, and
it lowers diastolic pressure by causing an overall
ENDOGENOUS CATECHOLAMINES
in peripheral resistance; the net result is little change in incas blood pressure. Epinephrine is of value as a constrictor in hemorrhage us
The three naturally occurring catecholamines dopamine. NE, and epinephrine are used as therapeutic agents.
nasal congestion. Also, it is used to enhance the of local anesthetics, Its use in these two situations take'
Dopamine.
advantage of the drug's potent stimulatory effects on a icreceprro ceptors. The ability of epinephrine to stimulate
Dopamine is used in the treatment of shock.
It is ineffective orally, in large part because it is a substrate for both MAO and COMT. Thus, it is used intravenously. In contrast with the catecholamines NE and epinephrine, dopamine increases blood flow to the kidney in doses that have no chronotropic effect on the heart or that cause no increase in blood pressure. The increased blood flow to the kidneys enhances glomerular filtration rate. Na' excretion. and, in turn, urinary output. The dilation of renal blood vessels produced by dnpaminc is the result of its agonist action on the o1-dopamine receptor.
has led to its use by injection and by inhalation to relax bronchial stnooth muscle in asthma and in anaphylactic rca'tions. Several over-the-counter preparations (e.g.. Pri-
matene. Bronkaid) used for treating bronchial asthma epinephrine. Epinephrine is used in the treatment of open-angle glaucoma, where it apparently reduces intraocular pressure
increasing the rate of outflow of aqueous humor from its anterior chamber of the eye. The irritation often experienced
on instillation 01' epinephrinc into the eye has led to its development of other preparations of the drug that potentially are not as irritating. One such example is dipiveiris
Doparnine
Dipivefrmn. l)ipivefrin (dipivalyl epinephrine, is a prodrug of epinephrine that is formed by the esteritic.u Lion of the catechol hydroxyl groups of cpincphrine with
pivalic acid. Dipiveirin is much more lipophilic than
In doses slightly higher than those required to increase receptors of renal blood flow, dopamine stimulates the the heart to increase cardiac output. Some of the effects of dopamine on the heart are also due to NE release. Infusion
ncphrine. and it achieves much better penetration of the when administered topically as an aqueous solution for the treatment of primary open-angle glaucoma. It is
at a rate greater than 10 sag/kg per minute results in stimulaLion of a1 receptors, leading to vasoconstriction and an increase in arterial blood pressure.
Dipivefrin offers the advantage of being less irritating
epincphrmne by esterases in the cornea and anterior chamber
the eye than cpincphrine. and because of its more efficiec transport into the eye, it can be used in lower concentraliuw than epinephrine.
NE (Levophed) is used to maintain blood pressure in acute hypotensive states resulting from
0
Noreplnephrine (NE,).
OH
surgical or nonsurgical trauma, central vasomotor depres. sion. and hemorrhage. Like the other endogenous catechola-
mines, it is a substrate for both MAO and COMT and thus is not effective by the oral route of administration. It is given by intravenous injection.
Epinephrine.
0
Dipivefrin
Epinephrine (Adrenalin) finds use in a
number of situations because of its potent stimulatory et'fects on both a- and fl-adrenergic receptors. Like the other cate-
cholamines, epinephrine is light sensitive and easily oxidized on exposure to air because of the catcchol ring system. The development of a pink to brown color indicates oxidative breakdown. To minimize oxidation, solutions of the drug are stabilized by the addition of reducing agents such as sodium bisulfite. As the free amine, it is used in aqueous solution for inhalation. Like other amines, it forms salts with acids; for example, those now used include the hydrochloride and the bitartrate. Epinephrine is destroyed readily in
alkaline solutions and by metals (e.g.. Cu. Fe. Zn). weak
Esterases
Epinephnne
+
2 (CH3)3CCO2H
a-ADRENERGIC RECEPTOR AGONISTS
Phenylephrine.
(Neo-Synephrüc Phcnylephrine structure shown above under "Structure—Activity Relatioxships") is the prototypical selective direct-acting a potent vasoconstrictor but is less potent
Chapter 16 • and norcpincphrine (NE). It is active when given orully. and its duration or action is about twice that of epincphrine. II is metabolized by MAO. hut since it lacks the catecliol moiety, it is not metabolized by COMT. It is relatimely nontoxic and produces little CNS stimulation. When applied to mucous membranes, it reduces congestion and oiclling by constricting the blood vessels of the membranes. Thus, one of its maul uses is in the relief of nasal congestion. In the eye, it is used to dilate the pupil and to treat open-angle glaucoma. It also is used in spinal anesthesia, to prolong the
533
Naphazoline. Tetrahydrozoline, Xy!ometazollne, and Oxymetazo!Ine. 'rite 2-aralkylimidazolines naphazoline (Privinc). Ictrahydrozoline (Tyzine. Visine). sylometazoline (Otrivin). and oxymctazoline (Airin) are agonists at both ar and a2-adrencrgic receptors. These agents arc used
for their vasoconsfrictivc effects as nasal and ophthalmic decongestants. They have limited access to the CNS. since they essentially exist in an ionized at physiological pH because of the very basic nature of the imidazoline ring lpK. 9 to 10).
anesthesia and to prevent a drop in blood pressure during he procedure. Another use is in the treatment of severe hypolension resulting from either shock or drug administration. Methoxamine. Another selective direct-acting a1-receptor agonist used therapeutically is meihoxamine (Vasoxyl). This drug is a vasoconstrictor that has no stimulant
on the heart. In fact, it tends to slow the ventricular rule because of activation of the carotid sinus reflex. It is
Naphazoilne: A =
—CH2.-—(")
U
ess potent than phenylephrine as a vasoconstrictor. Methoxamine is used primarily during surgery to maintain adequate anerial blood pressure. especially in conjunction with spinal anesthesia. ft does not stimulate the CNS.
OH
CH3O
Tetrahydrozoline: A =
\/
CH3 OCH3
H3C
Methoxamine Midodrine.
Oxymetazotine: A
Midodrinc (ProAmatine) reprcsems an-
other example of a dimechoxy-fl-phenylethylaniine derivatvr that is used therapeutically for its vasoconstrictor propcnics.Specilically, it is used in the treatment of symptomatic onhosialic hypotension. Midodrine is the N-glycyl prodrug if he selective a1 -receptor agonist desglyntidodrine. Removal of the N-glycyl moiety front tnidodrine occurs readily the liver as well as throughout the body, presumably by inñd,Lses.
H3C
OH
H3C Xylometazollne: A =
H3C
OH
CH3O
NHCOCH2NH2
C'Ionidine. Clonidiiie (Catapres) is an example of a (phenylimino)imidazolidinc derivative that possesses sekc-
tivity for the a2-adrenergic receptor. The al :a2 ratio is 3m): I. Under certain conditions, such as intravenous infusion. clonidine can briefly exhibit vasoconstrictive activity as a result of stimulation of peripheral a-adrencrgic receptors. However, this hypertensive effect, if ii occurs, is fol-
OCH3 Midodrine
CH3O
OH NH2
OCH3 Desglymidodrlne
lowed by a much longer lasting hypotensive effect as a result of the ability of clonidine to enter into the CNS and stimulate a2 receptors located in regions of the brain, such as the nucleus tractus solitarius. Stimulation of these r, receptors brings about a decrease in sympathetic outflow from the CNS. which in turn leads to decreases in peripheral vascular resistance and blood pressure.211 Bradycardia is also produced by clonidine as a result of a centrally induced facilitation of the vagus nerve and stimulation of cardiac prejunctional a2-adrenergic receptors.°° These phamiacological actions have made clonidine quite useful in the treatment of hypertension.
534
Wil.vv.n, u,,d Gisva!dx Tr'abooL of Organic Medkmna! and Pharmactwica! Chemistry
half-life of clonidinc ranges from 20 to 25 hours, while thai flirguanfacine is about 17 hours. Guanabenz has the duration of action of these three agents. with a half-life of about 6 hours. Clonidine and guanfacine are excreted un-
changed in the urine to the extent of 60 and 50%. tively. Very little of guanabenz is excreted unchanged in urine. Clonidine: A = H 4-Hydroxyclonidine: A = OH Apraclonidine: A = NH2
The ot do and its an antihypertensive effect depends on the ability of these compounds not only to interact with the receptor but also to gain entry into the CNS. For example. in the case of clonidine, the hasicity of the guanidine group (typically pK, 3.6)
Guanabenz
I
is decreased to 8.0 (the pK. of clonidine) because of its direct
attachment to the dichlorophenyl ring. Thus, at physiological pH. clonidine will exist to a significant extent in the nonionized form required for passage into the CNS.
Substitutions on the aromatic ring also affect the ability of clonidine and its analogues to gain entry into the CNS to produce an antihypertensive effect. Although various halogen and alkyl substitutions can be placed at the two art/la positions of the(phenylimino)imidazolidine nucleus without affecting the affinity of the derivatives toward a2 receptors. such substitutions have a marked effect on the lipophilicity of the compound. Halogen substituents such as chlorine seem to provide the optimal characteristics in this regard.4° This distributive phenomenon is seen with one of the nietabolites of clonidine. 4-hydroxyclonidine. This compound has good affinity for a! receptors, but since it is too polar to get into the CNS. it is not an effective antihypcrtcnsivc agent. In addition to binding to the a2 adrenergic receptor. clonidine, as well as some other imidazolines. shows high affinity for what has been termed the "irnidazoline" Some studies have implicated a role for the imidazoline receptors in the antihypertensive effects of clonidine.43 However, other studies involving both site-directed mutagenesis of the tr!A-adrenergic receptor subtype and genetically engineered knockout mice deficient in either the a2Kor a2,5-adrenergic receptor subtypes provide evidence that the hypotensive response of the a2-receptor agonists like
clonidine primarily involves the a!A-adrencrgic receptor subtype.45
Guanabenz and Guanfadne. Two analogues of clonidine. guanabenz (Wytensin) and guanfacine (Tenexi. are also used as antihypertensive drugs. Their mechanism of action is the same as that of clonidine. Structurally, these two compounds can be considered "open-ring imidazolidines." In these compounds. the 2,6-dichlorophenyl moiety found in clonidine is connected to a guanidino group by a two-atom bridge. In the case of guanabenz. this bridge is a —CH = N— group, while lbr guanfacine it is a —CH2CO— moiety. For both compounds, conjugation of the guanidino moiety with the bridging moiety helps to decrease the pK.
Guanfacine
Apraclonidine and Brimonidine.
In addition In therapeutic use as an antihypertensive agent. clonidine hibeen found to provide beneficial effects in a situations.47 These include migraine prophylazis.
opiate withdrawal syndrome. and anesthcsia. Thio hiprompted the development of analogues of clonidine cific use in some of the above areas. Two such exampk
arc apraclonidine (lopidine) and brimonidine Both are selective a7-receptor agonist.s with 30:1 and 1.000:1. respectively. They both lower intra,salr pressure by decreasing aqueous humor production and in creasing aqueous humor outflow. Apraclonidine is cifically tocontrol elevations in intraocular pressuretllatc2! occur during laser surgery on the eye. Brimonidine alse I used in such a manner: in addition, it is approved
treating glaucoma. Another example is tlzanidine ILi natlex). which finds use in treating spasticityas.cociatedsii
multiple sclerosis or spinal cord injury. By stimulating adrenergic receptors. it is believed to decrease the of excitatory amino acid neurotransmitters from spinal ar-I inlemeurons.45
çN
Br
Brlmonldlne
of this normally very basic group so that at physiological pU-I a significant portion of each drug exists in its nonionized form. Differences between clonidine and its two analogues
N.,,N
are seen in their elimination half-life values and in their metabolism and urinary excretion patterns. The elimination
Tizanldlne
H
Chapter 16 • itdreneri,qc A phenylethylamine derivative that shows selectivity toward the a1 receptor is a-methylnorepinephrine (Fig. 16-6). As discussed above under "StrucUN—Activity Relationships." the presence of an a-methyl
umup in the correct configuration on the phenylethylamine nucleus yields compounds with increased potency at a2 receptors and decreased potency at a1 receptors. Although amcthylnorepinephrinc is not given as a drug. it is the metaholic product of the drug methyldopa (i-a-methyl-3.4-dihydroxyphenylalanine. Aldornet). Since inethyldopa is a close %uuctural analogue of I -dopa. it is treated as an alternate substrate by the enzyme L-aromatic amino acid decarboxylasc. The product of this initial enzymatic reaction is a-meth)ldopanline. This intermediate, in turn, is acted on by dopa-
mine fl-hydroxylase to give the diastercoisomer of awhich possesses the (R) configuragroup and the (S) non at the carbon with the configuration at the carbon with the a-methyl substituent Fig. 16-6). Ii is postulated that a-methylnorcpincphrine acts on receptors in the CNS in the same manner as clonidine.
to decrease sympathetic outflow and lower blood pressure.38 Since methyldopa serves as an alternate substrate to i-ammatic amino acid decarboxylase. it ultimately decreases the concentration of dopamine. NE. epinephrine, and serotonin in the CNS and periphery.
Methyldopa is used only by oral administration since its zwitterionic character limits its solubility. Absorption can range from S to 62% and appears to involve an amino acid transporter. Absorption is affected by food, and about 40% of that absorbed is converted to methyldopa-O-sulfate by the mucosal intestinal cells. Entry into the CNS also appears to involve an active transport process. The ester hydrochloride salt of methyldopu. methyldopate (Aldotnet ester), was developed as a highly water-soluble derivative that could be used to make parenteral preparations. Methyldopate is converted to methyldopa in the body through the action of esterases (Fig. 16-6). DUAL a- AND f3-ADRENERGIC RECEPTOR AGONISTS
Dobutamine.
NH3CI
HO I
Ho Methyktopate Esterases
There are synthetic direct-acting sympathomirnetics whose therapeutic use relies on their ability to receptors. One example is act at both a- and dobutamine (Dobutrex). Structurally. dobutamine can be viewed as an analogue oldopamine in which a l-(rnethyl)3-(4-hydroxyphenyl)propyl suhstitucnt has been placed on the amino group. This substitution gives a compound that possesses an asymmetric carbon atom. Thus, dobutamine exists as a pair of enantiorners. with each enantiomer possessing a distinct pharmacology.49 The ( + I enantiomer is a potent full agonist at both and f32 receptors. In contrast, the (—) enantiomer is some 10 times less potent at and th receptors. The (—) enantiorner is. however, a potent agonist at a1 receptors. Dohutarnine does not act as an agonist at the dopamincrgic receptors that mediate renal vasodilation.
CH3CO2H H
Methyldopa L-Aromatic Amino Acid Decarboxylase
HO
CH3 Dobutamine
In vivo, raceinic dobutamine increases the inotropic activ-
CH3 a-Methyldopamlne
ity of the heart to a much greater extent than it increases chronotropic activity. This pharmacological profile has led to its use in treating congestive heart failure. Since
recep-
tors are involved positively in both inotropic and chronoDopamine
H, OH
(1 R,2S)-a
FIgure 16—6 • Metabolic conversion of methyldopate and to a-methylnorepinephrine.
tropic effects of the heart, the selective inotropic effect seen with dobutamine cannot simply be due to its activity at receptors. Rather, this effect is the result of a combination
of the inotropic effect of (+ )-dohutarnine on receptors and that of (—)-dobutamine mediated through a1 receptors.°° Thus, this is a case where a racemic mixture provides a more desirable pharmacological and therapeutic effect than would either enantiomer alone. Dobutamine is given by intravenous infusion, since it is not effective orally. Solutions of the drug can exhibit a slight pink color as a result of oxidation of the catechol function. It has -a plasma half-life of about 2 minutes, It is metabolized
by COMT and conjugation but not by MAO.
536
WiAo,, and Gi.ci'okls Textbook of Organic Medicinal and Pl,arnwceu,ical Cherni.vtrv
Albuterol, Pirbuterol. and Salmeterol.
RECEPTOR AGONISTS
lsoproterenoL
Isoproterenol (Isuprel. structure shown above under "Structure—Activity Relationships") is the prototypical f3-ndmnergic receptor agonist. Because of an isopropyl substitution on the nitrogen atom, it has virtually no effect on a receptors. However. it does act on both /3, and receptors. It thus can produce an increase in cardiac output by stimulating cardiac /9, receptors and can bring about bronchodilation through stimulation of receptors in the respiratory tract. It also produces the metabolic effects expected of a potent /3 agonist. Isoproterenol is available for use by inhalation and injection. Its principal clinical use is for the relief of bronchospasms associated with bronchial asthma. In fact, it is one of the most potent bronchodilators available. Cardiac stimulation is an occasionally dangerous adverse effect in its use. This effect of isoprotercnol on the heart is sometimes made use of in the treatment of heart block. After oral administration, the absorption of isoproterenol is rather erratic and undependable. The drug has a duration of action of Ito 3 hours after inhalation. The principal reason for its poor absorption characteristics and relatively short duration of action is its facile metabolic transformation by sulfate and glucuronide conjugation of the ring hydroxyls and methylation by COMT. Unlike epinephrine and NE. isoproterenol does not appear to undergo oxidative deamination
by MAO. Since it is a catechol, it is sensitive to light and
Albuterol
(Proventil, Ventolin, structure shown above under "Stntctore—Activity Relationships"), pirbuterol (Maxair). and sol. metcrol (Serevent) are examples of selective /32-receptorag.
onists whose selectivity results from replacement of the ineta-hydroxyl group of the catechol ring with a methyl moiety. Pirbuterol is closely related structurally to albuterol; the only difference between the two is that pirbut. erol contains a pyridine ring instead of a benzcne ring. Ar in the case of metaproterenol and terbutaline, these drop arc not metabolized by either COMT or MAO. instead. they are conjugated with sulfate. They thus are active orally. and they exhibit a longer duration of action than isoproterenol. The duration of action of terbutaline. albuterol. and pirbut.
erol is in the range of 3 to 6 hours. OH
CH2OH Pirbuterol
OH
air. Aqueous solutions become pink on standing. The problems of lack of /3-receptor selectivity and rapid metabolic inactivation associated with isoproterenol have been overcome at least partially by the design and development of a numberof selective /32-adrenergic receptoragonists. These agents relax smooth muscle of the bronchi. uterus, and skeletal muscle vascular supply. They lind their primary use
as bronchodilators in the treatment of acute and chronic bronchial asthma and other obstructive pulmonary diseases.
Metaproterenol and Terbutaline.
As pointed out iii
the discussion of structure—activity relationships. modification of the catechol portion of a /3 agonist has resulted in the development of selective /32-receptor agonists. For example, metaprolerenol (Alupent. structure shown above under Relationships") and terbutalinc (Bricanyl. Brethine) are resorcinol derivatives that are /32 selective. Mctaprotcrcnol is less /32 selective than either terbutaline or
albuterol. Although these agents have a lower affinity for 132 receptors than isoproterenol. they are much more effec-
tive when given orally, and they have a longer duration of action. This is because they are not metabolized by either
COMT or MAO. Instead, their metabolism primarily involves glucuronide conjugation. Although both metaproterenol and terbutaline exhibit significant /32-receptor selectivity, the common cardiovascular effects associated with other adrenergic agents can also be seen with these drugs when high doses are used.
CH2OH Satmeterol
Salmeterol is a partial agonist at receptors and has potency similar to that of isoproterenol. It is very long (12 hours), an effect attributed to the lipophilic substituent on the nitrogen atom, which is believed to act with a site outside but adjacent to the active site. Thi agent associates with the /32 receptor slowly and dis.sociatcr
from the receptor at an even slower rate."
Foimoterol and Levalbuterol.
Another long-acting
/32-receptor agonist is Formoterol (Foradil). Its long durjiitr
of action, which is comparable to that of salmctcrol, ha been suggested to result from its association with the men brane lipid Formoterol has a much faster oust, si action than does salmetcrol. Both of these long-acting drug.
are used by inhalation and are recommended for nance treatment of asthma, usually in conjunction with a inhaled corticosteroid. OH (RS)
H
NI-ICHO Formoterol Terbutatine
All of the above /32-receptor agonists possess at least ow chinul center and are used as racemic mixtures. Fonnoteal
Chapter 16 •
Agesus
537
possesses two chiral centers and is used as the racernic mixture of the (R.R) and (S.S) enantiomers. As mentioned above. it is the (R) isomer of the phenylethanolumines that possesses
fetal distress caused by excessive uterine activity. Its uterine inhibitory are more sustained than its effects on the
the phannacological activity. Concerns have been raised
those caused by nonsclective $ agonists. The cardiovascular
about the use of such racenaic mixtures under the belief that the inactive (SI isomer may he responsible for some of thc
effects usually associated with its administration are mild
effects seen with these agents. Levalbuterol (Xopenex). the (RI isomer of racemic albuterol. represents the
it is administered initially by intravenous infusion to stop
cardiovascular system, which arc minimal compared with
tachycardia and slight diastolic pressure decrease. Usually.
premature labor. Subsequently. it may be given orally.
first attempt to address this issue.
OH H
Isoetha rifle. Another sympathomimetic drug that finds use as a bronchodilator is the a-ethyl catecholamine, isoetha-
rinc. This agent is weaker than isoproterenol at stimulating n.'ceptors. In addition, its selectivity is not us great as hat seen with drugs such as terbutaline oralhutcrol. Because ol the presence of the a-ethyl group. isoetharine is not metaboli,cd by MAO. Because it contains the cacechol ring system. howevcr. it is metabolized quite elTectively by
cOMT. It also is 0-sulfated quite effectively. Isoetharine a duration of action similar to that of isoproterenol.
CH3
Ritodrine
th-Adrenergic Receptor Agonists. acting agonists for the
Selective direct-
receptor have been de-
veloped, hut they have not been approved for therapeutic Because stimulation of the receptor promotes lipolysis. these agents may have potential as antiobesity drugs and as drugs for the treatment of non—insulin-dependeni diabetes.
Indlrect-Acflng Sympathomimeths Isoethanne
Bitolterol (Tornalate) is a prodrug of the Bitolterol. sclective adrenergic agonist colterol. the N-ier:-butyl anahsgue of NE. The presence of the Iwo p-toluic acid esters in bitolterol makes it considerably more lipophilic than colterol. Bitoherol is administered by inhalation for bronchial asthma and reversible hronchospa.srn. It is hydrolyzed by esterases in the lung and other tissues to produce the active
agent, cotterol. Bitolterol has a longer duration of action than isoproterenol (5 to 8 hours) and is nsetaboliied. after hydrolysis of the esters, by COMT and conjugation. OH
Bitottorol
Indirect-acting sympathomimetics act by releasing endogenous NE. They enter the nerve ending by way of the activeuptake process and displace NE from its storage granules. Certain structural characteristics tend to impart indirect sympathomimetic activity to phenylethylainines. As with the direct-acting agents. the presence of the catechol hydroxyls enhances the potency of indirect-acting phenylethylamines. However, the indirect-acting drugs that are used therapeutically are not catechol derivatives and, in most cases, do not even contain a hydroxyl moiety. In contrast with the directacting agents, the presence of a $-hydroxyl group decreases. and an a-methyl group increases, the elfectiveness of indirect-acting agents. The presence of nitrogen substituents decreases indirect activity, with substituents larger than methyl rendering the cotnpound virtually inactive. Phenylethylamincs that contain a tertiary amino group are also ineffective u.s NE-releasing agents. Given the foregoing structure—activity considerations, it is easy to understand why amphetamine and p-tyramine are often cited as prototypical indirect-acting sympathotnimetics. Since amphetamine-type drugs exert their primary effects on the CNS. they are discussed in tnore detail in Chapter IS. This chapter discusses those agents that exert their effects primarily on the periphery.
2H
OH +
2
CH3 Colterol
Acid
Ritodrine (Yutopar) is a selective /32-receptoe agonist used to control premature labor and to reverse Ritodrine.
Amphetamine
Hydroxyamphetamine.
p-Tyramine
Although p-tyramine is not a clinically useful agent. its a-methylated derivative. hydroxyamphetamine (Puredrine). is an effective, indirect-acting sympathontimetic drug. Hydroxyatnphetamine has little or
538
Wilson and
Textbook of Organir Med it'inal and Pliurn:aceuiical Chemistry
no ephedrinc-like. CNS.stimulating action. It is used to dilate the pupil for diagnostic eye examinations and for surgical procedures on the eye. It is used sometimes with choliner-
gic blocking drugs like atropine to produce a mydriatic effect, which is more pronounced than that produced by either drug alone.
Relative Pressor Activity of the Isomers of Ephedrine TABLE 16—1
Isomer Relative Activity 36
—
DL.t ± )-EpIK'siriflc
26
L-( + 1-Ephedrinc
II
-, ).Pseudoephedrinc
7
aL-I ± )-Pseudocptiedrinr
Hydroxyamphetamine
i.(+)-Pseudoephedrine. IA + )-Pscudoephedrine (Sudafed. Afrinol. Drixoral) is the (S.S) diastereoisomcr of cphedrine. It is a naturally occurring alkaloid from the Ephe' dra species. Whereas ephedrinc has a mixed mechanism of
action. pseudoephedrine acts principally by an indirect mechanism. The structural basis for this difference in mechanism is the stereochemistry of the carbon atom possessing the $-hydroxyl group. In pseudoephedrine. this carbon atom possesses the (S) configuration, which is the wrong stereochemistry at this center for a direct-acting effect at adrenergic receptors. This agent is found in many over-the-counter nasal decongestant and cold medications. Although ii is less prone to increase blood pre.ssure than ephedrine. it should
be used with caution in hypertensive individuals, and it should not be used in combination with MAO inhibitors.
4
— i-Pseudocphedñnc
I
stems of various species of Ephedra. Mahuang. the plant containing ephedrine. was known to the Chinese in 2,(X0) ac. hut the active principle. ephedrine. was not isolated 1885.
Ephedrine has two asymmetric carbon atoms; thus. thar
are four optically active forms. The o'r5-ihro raccmatc called "ephedrine." and the threo racemaic is known
Natural ephedrine is 'pseudoephcdrine" n(—) isomer, and it is the most active of the four isomers as a pressor amine (Table 16-I). This is largely due to the fact that this isomer has the correct (R) configuration at the carS
bon atom bearing the hydroxyl group and the desired (SI configuration at the carbon bearing the methyl group lot optimal direct action at adrenergic receptors.
OH
OH
CH3
CH3
L-(+)-Pseudoephedrlne
Propylhexedrine.
Propyihexedrine (Benzedrex) is an analogue of amphetamine in which the aromatic ring has been replaced with a cyclohcxane ring. This drug produces vasoconstriction and a decongestant effect on the nasal membranes, but it only about one-half the pressor effect of amphetamine and produces decidedly fewer effects on the CNS. Its major use is for a local vasoconsirictive effect on nasal mucosa in the symptomatic relief of nasal congestion caused by the common cold, allergic rhinitis. or sinusitis.
NH2
Propylhexedrine
With a Mixed Mechanism of Action Those phenylethylamines considered to have a mixed inechanism of action usually have no hydroxyls on the aromatic
D-(-).Ephednne
Ephedrine decomposes gradually and darkens when
posed to light. The free alkaloid is a strong base, and aqueous solution of the free alkaloid has a pH above 10 The salt form has a pK. of 9.6. The pharmacological activity of ephedrine resembles tici of epinephrine. The drug acts on both a- and f3-adrcnergt receptors. Although it is less potent than epinephrine. it' pressor and local vasoconsirictive actions are of greaterdurution. It also causes more pronounced stimulation oF the CNS
than epinephrine. and il is effective when given orally. Th; drug is not metabolized by either MAO or COMT. Rathet. it is p-hydroxyiated and N-demethylaled by cytochmrne P. 450 mixed-tbnction oxidases. Ephedrine and its salts are used orally, intravenously, tramuscularly. and topically for a variety of conditions. as allergic disorders, colds. hypotensive conditions, and nat' colepsy. It is used locally to constrict the nasal mucosu aai cause decongestion and to dilate the pupil or the branch Systemically, it is effective for asthma, hay fever, and srtl caria.
ring but do have a fl-hydroxyl group.
Phenylpropanolamine.
D-(—)-Ephedrine is the classic example of a sympathomimctic with a mixed mechanism of action. This drug is an alkaloid that can be obtained from the
drine) is similar in structure to ephedrine except thai ii primary instead of a secondary amine. This modificatir gives an agent that has slightly higher va.sopressivc and lower central stimulniory action than ephedrine. Its
D-(—.)-Ephedrine.
is
Phenyipropanolamine
Chapter 16 • lion as a nasal decongestant is more prolonged than that of ephedrine. It is effective when given orally. Phenylpropanoamine was a common active component in over-the-counter appetite suppressants and cough and cold medications until 2001, when the Food and Drug Administration (FDA) recommended its removal from such medications because studtea showed an increased risk of hemorrhagic stroke in young women who took the drug.
Age,,ts
539
ably, the antagonistic actions of these agents at presynaptic receptors contribute to their cardiac stimulant effects by enhancing the release of NE. Both agents have a direct vasodilatory action on vascular smooth muscle that may he more prominent than their a-receptor antagonistic effects.
OH Tolazotlne
oH3 Phenylpropanolamine
Metaraminol. Metaraminol (Araminc) is structurally to phenylephrine except that it is a primary instead of a secondary amine. It possesses a mixed mechanism of with its direct-acting effects mainly on a-adrenergic receptors. It is used parenlerally as a vasopressor in the treatment and prevention of the acute hypotensive state occurring with spinal anesthesia. It also has been used to treat severe hypotension brought on by other traumas that induce shock.
OH
OH3
OH Metaraminot
Phentotamine
The antagonistic action of tolazoline is relatively weak. but its histamine-like and acetylcholine-like agonistic actions probably contribute to its va.sodilatory activity. Its histamine-like effects include stimulation of gastric acid secretion, rendering it inappropriate for administration to patients who have gastric or peptic ulcers. It has been used to treat Raynaud's syndrome and other conditions involving peripheral vasospasm. Tolazoline is available in an injectable form and is indicated for use in persistent pulmonary hypertension of the newborn when supportive Uleasures are not successful.
Phentolamine is used to prevent or control hypertensive episodes that occur in patients with pheochromocytoma. It can be used as an aid in the diagnosis of pheochromocytoma. but measurement of catecholamine levels is a safer and more reliable method of diagnosis. It also has been used in combi-
ADRENERGIC RECEPTOR ANTAGONISTS
a.Adrenerglc
Receptor Antagonists
receptor antagonists, which hear dear structural similarities to the adrenergic agonists NE. Lnlike the
epinephrinc. and isoproterenol, the ce-adrenergic receptor anlagonists consist of a number of compounds of diverse chem.
cal structure that bear little obvious resemblance to the areceptor NONSELECTIVE a-RECEPTOR ANTAGONISTS
The agents in this class Toiazoline and PhentolamIne. are structurally similar to the irnidazoline a-agonists. such as naphazolinc. tetrahydrozoline. and xylometazoline. The
nation with papaverine to treat impotence. IRREVERSIBLE a-RECEPTOR BLOCKERS
Agents in this class, when given in adequate doses, produce a slowly developing, prolonged adrenergic blockade that is not overcome by epincphrine. In essence, they are irreversible blockers of the a-adrenergic receptor. Chemically, they
are f3-haloalkylamines. Although dibenamine is the prototypical agent in this class, it is phenoxybenzamine that is used therapeutically today.
Ph—\ N—CH2CH2CI
Ph—'
type of group attached to the imidazoline ring dictates whether an imidazoline is an agonist or an antagonist. The two representatives of the imidazoline a antagonists that are used therapeutically are tolazoline (Priscoline) and phentolamine (Regitine). Both are competitive (reversible) blocking agents. Phentolamine is the more effective a antagonist. hut
CH3
(J_-OCH2CH
neither drug is useful in treating essential hypertension. Theoretically, the vasodilatory effects of an a-antagonist be beneficial in the management of hypertension. Tolazoline and phentolamine. however, have both a1- and activity and produce tachycardia. Presum-
Phenoxybenzamlne
5.40
of ()rj,'anie tIet/iiina! anil Phar,,uwt,aica! (hemisin
tVll.so,, isiul
blocking acetykholine. histamine, and serotonin receptom. its primary pharmacological effects, especially vasodilation. may he attributed to its a-adrenergic blocking capability. As would be expected of a drug that produces such a profound (S
I Cl
R Aziridinlum Ion
Ii
blockade, administration is frequently associatcd with tachycardia. increased cardiac output. and postural hypoten. sion. There is also evidence indicating that blockade of pre. synaptic receptors contributes to the increased heart rjte produced by phenoxybeniamine. The onset of action of is slow, hut the effects of a single dose of drug may last 3 to 4 days. since essentially new receptors need to be made to replace that have been inhibited irreversibly. The principal effects following its administration are an increase in peripheral blood how, an increase in skin temperature, and a lowCrinf of blood pressure. it has no effect on the parasympathetic system and little effect on the gastrointestinal tract. The 11551
I
R'\+
common side effects are miosis. tachycardia. nasal stuff. CI —
,N1
A
Nu
ness. and postural hypotension. all of which are related iii the production of adrenergic blockade. Oral phenoxybetuamine is used for the preoperative maragement of patients with pheochromocylomu and in the chronic management of patients whose tumors art not naMe to surgery. Only about 20 to 30% of an oral dise is absorbed.
Reversible Drug Receptor Complex
SELECTIVE a,-RECEPTOR ANTAGONISTS
Prazosin. Terazosin, Doxazosin.
Alkylated Receptor
Figure 16—7 • Mechanism of inactivation of sr-adrenergic receptors by
One group of highl'.
selective a1-rcceptor antagonists are the quinazolines. Examples include prazosin (Minipress). tcrazosin (Hytrinl.and doxazosin (Cardura). Structurally, these three agents consi't of three components: the quinaeoline ring, the piperamle ring, and the acyl moiety. The 4-amino group on the quinalo line ring is very important for a1-rcceptoraflinity. Although prazosin. terazosin. and doxaiosin possess a piperazine moiety attached to the quinazoline ring, this group can k replaced with other heterocyclic moieties (e.g.. piperidire moiety) without loss of affinity. The nature of the acyl gisup has a significant effect on the pharmacokinetic propellics.' Quinazoline ring
}
The mechanism whereby produce a long-lasting, irreversible cr-adrenergic receptor blockade is depicted in Figure 16-7. The initial step involves the fornia-
ring
tion of an intennediate nziridinium ion (ethylene iminium ion), which then forms an initial reversible complex with the receptor. The positively charged aiiridinium ion electrophile then reacts with a nucleophilic group on the receptor, result-
ins in the formation of a covalent bond between the drug and the receptor. Although the aziridinium ion intermediate has long been believed to be the active receptor-alkylating species, it was not until 1976 that it was demonstrated unequivocally that the a,iridiniunt ions derived front dihenaminc and phenoxybenumine are capable of a-receptor alkylatiori.54
NH2
Prazosin: R
=
Terazosln: R
Phenoxybenzamine. The action of phenoxybenzamine (Dibenzyline) has been described as representing a "chemical synipathectomy" because of its selective blockade of the excitatory responses of smooth muscle and of the heart muscle. Although phenoxybenzamine is capable of
Doxazosin: R =
Chapter 16 • Adreiwrgie Agents
These drugs are used in the treatment of hypertension. They dilate both arterioles and veins. Agents in this class offer distinct advantages over the other a-blockers because hey produce peripheral vasodilation without an increase in heart rate or cardiac output. This advantage, at least in part. is attributed to the fact that prazosin blocks postjunctional a1 receptors selectively without blocking presynaptic a2 receptors. These agents also find use in the treatment of benign
541
thine is a selective antagonist ot the a1 receptor. The only difference between these two compounds is the relative stereochemistry of the carbon containing the carbomethoxy substituent. In yohimbine. this group lies in the plane of the alkaloid ring system, while in corynanthine. it lies in an axial position and thus is out of the plane of the rings.5"
pmstatic hyperplasia. where they help improve urine flow rates.
Although the adverse effects of these drugs are usually minimal, the most frequent one, known as thefirsl-dosepl,ennmenon. is sometimes severe. This is a dose-dependent ellect characterized by marked excessive postural hypotension and syncope. This phenomenon can be minimized by giving an initial low dose at bedtime. The main difference between prazosin. terazosin. and dox-
Yohimbine
aiosin lies in their pharmacokinetic properties. As menhoned above, these differences are dictated by the nature of the acyl moiety attached to the piperazine ring. A compariol these three agents with respect to their oral bioavail-
ability, half-life, and duration of action is shown in Table 6-2. These drugs are metabolized extensively, with the metabolites excreted in the bile. Tamsulosin.
H3CO2C
The aryl sulfonamide tamsulosin (Ho-
max) represents the first in the class of subtype selective a1-
receptor. which seems to predominate in the prostate. It is approved for treating benign prostatic hyperplasia. for which ii is administered once daily. Orthostatic hypotension may not bc as great with this agent as with the nonselective quin-
OH
Cotynanthlne
antagonists. It is selective for the
Yohimbine increases heart rate and blood pressure as a result of its blockade of a2 receptors in the CNS. It has been used experimentally to treat male erectile impotence.
aiolines.
Mlrtazapine. H
The tetracyclic mirtazapine (Remeron) is another example of an a-antagonist that shows selectivity for a2 receptors versus a1 receptors.51 Blockade of central
CH3
a2 receptors results in an increased release of norepinephrine and serotonin. This has prompted its use as an antidepressant. This agent also has activity at nonadrenergic receptors. It is
9
a potent blocker of 5-HT2 and 5-HT3 serotonin receptors and at histamine
Tamsulosln
I
receptors.
SELECTIVE te2-RECEPTOR ANTAGONISTS
Isomeric indole alkaYohimbine and cosynanthine. known as the yohimbanes exhibit different degrees of skctivity toward the a1- and a2-adrenergic receptors. deon their stereochemistry. For example. yohimbine selective antagonist of the a2 receptor. while corynan-
bids
Mlrtazapine
TABLE 16-2 Pharmacokinetic Properties of Prazosin, Terazosin. and Doxazosln
/3-Adrenergic Receptor Antagonists STRUCTURE-ACTIVITY RELATiONSHIPS
Agent
Bloavatlablllty
Half-life
(%)
(hours)
Duration of Action (hours)
2—3
4—6
(DCI). The structure of DCI is like that of isoproterenol.
90
')—t2
IS
except that the catechol hydroxyl groups have been replaced
22
36
by two chloro groups. This simple structural modification. involving the replacement of the aromatic hydroxyl groups.
P11J011fl
The first /3 blocker was not reported until 1958. when Powell and Slateras described the activity of dichloroisoproterenol
542
Wilson and Gisrold's Texibook of Orgwiie- Medicinal and Pharmaceutical Chemistry
has provided the basis for nearly all of the approaches used in subsequent efforts to design and synthesize therapeutically useful 13-receptor antagonists.35 Unfortunately. DCI is not a
type. Although it was not released for use in the United States, it was the first cardioseledive antagonist to be used extensively in humans. Because it produced several
pure antagonist but a partial agonist. The substantial direct sympathomimetic action of DC! precluded its development as a clinically useful drug.
toxic effects, however, it is no longer in general use in most countries.
NHCOCH3
Olchloroisoproterenol
Pronethalol was the next important 13 antagonist to be described. Although it had much less intrinsic sympathomimetic activity than DCI, it was withdrawn rrom clinical testing because of reports that it caused thymic tumors in mice. Within 2 years of this report, however. Black and Stephenson59 described the 13-blocking actions of propranolol, a close structural relative of pronethalol. Propranolol has become one of the most thoroughly studied and widely used drugs in the therapeutic armamentarium. It is (he standard against which all other p antagonists are compared.
Proneihalol
Practolol
As in the sympathomimetics. bulky aliphatic groups. such as the tert-butyl and isopropyl groups. are normally found tat the amino function of the aryloxypropanolaniine f3-recepwt antagonists. It must be a secondary amine for optimal an-
tivity. The 13-blocking agents exhibit high stereoselectivity inthc
production of their 13-blocking effects. As with the thomimetic agents, the configuration of the hydmxyl-bcsing carbon of the aryloxypropanolamine side chain playua critical role in the interaction of 13-antagonist drugs with fi receptors. This carbon must possess the (S) configumlicii for optimal affinity to the 13 receptor. The cnantiomer the (R) configuration is typically 100 times less potent. Th: available data indicate that the pharmacologically mote tive enantiomer interacts with the receptor recognition lii: in a manner analogous to that of the agonists. The structurul features of the aromatic portion of the antagonist. howeuct appear to perturb the receptor or to interact with it in a man-
ner that inhibits activation. In spite of the fact that all of the 13-antagonistic activity resides in one euiandotart. propranolol and most other 13 blockers are used clinical!
as racemic mixtures. The only exceptions are timolol. and penbutolol. with which the (SI enantiontet used.
Propranolol
Propranolol belongs to the group of 13-blocking agents known as aryloxypropanolasnine.c. This term reflects the fact that an —OCH2— group has been incorporated into (he mole-
cule between the aromatic ring and the ethylamino side chain. Because this structural feature is frequently found in 13 antagonists, the assumption is made that the —OCH2— group is responsible for the antagonistic properties of the molecules. However, this is not true: in fact, the —OCH-,— group is present in several compounds that are potent 13 ago-
NONSELECTIVE /3 BLOCKERS
Propranolol.
Propranolol (Inderal) is the pmlotypica 13-adrenergic receptor antagonist. It is nonselective in blocks the and 132 receptors equally well, like the other 13-receptor antagonists that arc discussed, lu competitive antagonist whose receptor-blocking be reversed with sufficient concentrations rently. propranolol is approved for use in the United Swtt for hypertension. cardiac arrhythmias. angina peetoris. ç4a
This latter fact again leads to the conclusion that it arosnat\c ting am\ its substituents tbat is nature ol is the primary determinant of 13-antagonistic activity. The aryl
nsyocardial infarction. hyperirophic
group also affects the absorption, excretion, and metabolism of the 13 blockers.6t
ment of a variety of other conditions. including
The nature of the aromatic ring is also a determinant in the flu selectivity of the antagonists. One common structural feature of many cardioselective antagonists is the pre.sence of a para substituent of sufficient size on the aromatic ring along with the absence of ,,zela substituents. Practolol is the antagonist of this structural prototypical example of a
anà
In addition. propranolol is tinder investigation for
trennt tic.
schizophrenia. alcohol withdrawal syndrome, and behavior. Some of the most prominent effects of propranolol ac the cardiovascular system. By blocking the /3 recepari the heart. propranolot stows the heart, reduces the fahx contraction, and reduces cardiac output. Because nt sympathetic activity and blockade of vascular
Chapter 16 • Adrei:crgic administration may result in increased peripheral resistance. The antihypertensive action, at least in part, may be attributed to its ability to reduce cardiac output. as well as to its
suppression of renin release from the kidney. Because it receptors, it is contraindicated exhibits no selectivity for in the presence of conditions such as asthma and bronchitis.
A facet of the pharmacological action of propranolol that has received a good deal of attention is us so-called membrane-stabilizing activity. This is a nonspecific effect (i.e., not mediated by a specific receptor), which is also referred to as a local anesthetic effect or a quinidine-like effect. Both cnantionlers possess membrane-stabilizing activity. Since the concentrations required to produce this effect far exceed those obtained with normal therapeutic doses of propranolol 2nd related /3-blocking drugs, it is most unlikely that the nonspecific membrane-stabilizing activity plays any role in he clinical efficacy of /3-blocking agents. The metabolism of propranolol has received intense study. Propranolol is well absorbed after oral administration, but ii undergoes extensive first-pass metabolism before it reaches the systemic circulation. Lower doses are extracted more efficiently than higher doses, indicating that the extracion process may become saturated at higher doses. In addition, the active enantiomer is cleared more slowly than the inactive cnantiomer!'2
Numerous metabolites of propranolol have been identifled, but the major metabolite in people, after a single oral kise. is naphthoxylactic acid, which is Formed by a series of metabolic reactions involving N-dealkylation. deamination, oxidation of the resultant aldehyde. One metabolite of interest is 4-hydroxypropranolol. This compound is a potent /3 antagonist that has some intrinsic sympathomi-
medc activity. It is not known what contribution, if any. 4hydroxypropranolol makes to the pharmacological effects seen after administration of propranolol. The half-life of propnno1o1 after a single oral dose is 3 to 4 hours, which inneases to 4 to 6 hours after long-term therapy.
543
lol (Carirol. Ocupress). timolol (Blocadren. Timoptic). levo-
bunolol (Betagan). sotalol (Betapace) and metipranolol (OptiPranolol). Structures of these compounds are shown in Figure 16-8. The first five of these agents arc used to treat hypcrtcnsion. Nadolol is also used in the long-term management of angina pcctoris. while timolol finds use in the prophylaxis of migraine headaches and in the therapy following myocardial infarction. Sotalol is used as an untiarrhythmic
in treating ventricular arrhythmias and atrial fibrillation because in addition to its /3-adrenergic blocking activity, this agent blocks the inward K current that delays cardiac rcpolari2ation. Carteolol, tirnolol. levobunolol. and melipranolol are used topically to treat open-angle glaucoma. These agents lower intraocular pressure with virtually no effect on pupil sue or accommodation. They thus offer an advantage over many
of the other drugs used in the treatment of glaucoma. Although the precise mechanism whereby /3 blockers lower intraocular pressure is not known with certainty, ii is believed that they may reduce the production of aqueous humor. Even though these agents are administered into the eye. systemic absorption can occur, producing such adverse effects as bradycardia and acute bronchospasm in patients with bronchospastic disease. Pindolol possesses modest menthrane-stahilizing activity and signiticant intrinsic /3-agonistic activity. Pcnbutolol and
carteolol also have partial agonistic activity hut not to the degree that pindolol does. The 13 antagonists with partial agonistic activity cause less slowing of the resting heart rate than do agents without this capability. The partial agonistic
activity may be beneficial in patients who are likely to exhibit severe bradycardia or who have little cardiac reserve. Timolol. pindolol. penbutolol. and carteolol have half-life values in the smue range as propranolol. The half-life of nadolol. however, is about 20 hours, making it one of the longest-acting /3 blockers. Timolol undergoes first-pass metabolism but not to the same extent that propranolol does.
Timolol and pcnbutolol are metabolized extensively, with little or no unchanged drug excreted in the urine. Pindolol is metabolized by the liver to the extent of 60%. with the remaining 40% being excreted in the urine unchanged. In contrast. nadolol undergoes very little hepatic metabolism. Most of this drug is excreted unchanged in the urine. Naphthoxylactic Acid
BLOCKERS
The discovery that /3-blocking agents are useful in the treatment of cardiovascular disease. such as hypertension, stimulated a search for cardioselective /3blockcrs. Curdioselective /3 antagonists are drugs that have a greater affinity ftr the receptors of the heart than for /32 receptors itt other tissues.
Such cardioselective agents should provide two important therapeutic advantages. The first advantage should be the
OH 4-Hydroxypropranolol
Other Nonselective /1 Blockers. Several other nonseItctis'e $blockers are used clinically. These include nadolol Curgani). pindolol (Visken). penbutolol (Levatol). carteo-
lack of an antagonistic effect on the /32 receptors in the bron-
chi. Theoretically, this would make blockers sak fuir use in patients who have bronchitis or bronchial asthma. The second advantage should be the absence of blockade of the vascular /3, receptors. which mediate vasodilation. This would be expected to reduce or eliminate the increase in peripheral resistance that sometimes occurs after the administration of nonselective /'3antagoiiists. Unlbrtunately. cardi-
544
IVllxoii and
Te.i.ibook
of Organi Medirinal and PI,armaeeiuicu! CJu.,ni.ar
H
0
Carteolol
Levobunolol
OH
OH
OCOCH3
Nadolol
Metipranolol
(bOH Plndo!ol
Penbutolol
OH
0'm N—S Timotol
Sotalol
Figure 16—8 u Nonseleclive
oselectivity is usually observed with
antagonists at only relatively low doses. At normal therapeutic doses, much of the selectivity is lost. At present, the following f31-selective agents are used ther-
apeutically: acebutolol (Sectral). atenolol (Tenormin). betaxolol (Kerlone. Betoptic). bi.soprolol (Zebeta). esmolol (Brevibloc). and metoprolol (Lopressor). Structures of these agents are depicted in Figure 16-9. All of these agents except
esmolol are indicated for the treatment of hypertension. Atenolol and metoprolol are also approved for use in treating angina pectoris and in therapy following myocardial infarction. Betaxolol is the only blocker indicated for the treatment of glaucoma. Acebutolol and csmolol are indicated for treating certain cardiac arrhythmias. Esmolol was designed specifically to possess a very short duration of action: it has an elimination half-life of 9 minutes. This agent is administered by continuous intravenous infusion for control of ventricular rate in
blockers.
patients with atrial flutter, atrial fibrillation, or sinus taclw cardia. Its rapid onset and short duration of action reakr ii useful during surgery, after an operation, or during can gency situations for short-term control of heart rates. fects disappear within 20 to 30 minutes after the infusiiii is discontinued. Esmolol must be diluted with an solution before administration: it is incompatible with v dium bicarbonate. The short duration of action of esmolol is the rcsuh s rapid hydrolysis of its ester functionality by esterases prcscs
in crythrocytes (Fig. 16-10). The resultant carboxylic is an extremely weak antagonist that does not appeal exhibit clinically signiticant effects. The acid metabolilek' an elimination half-life of 3 to 4 hours and is p• niarily by the kidneys. In the class of blockers, only sesses intrinsic sympathomimetic activity. This
very weak, however. Acehutolol and betaxolol
Chapter 16 a Adrent'rgic Agents
545
NHCH(CH3)2
CH2CONH2 Atonolol
Acebutolol
I'— CH2OCH2CH2OCH(CH3)2
CH2CH2OCH2 -
90%). to canrenone. an active metabosively by the liver
volved in the exchange of Na for K Intercalated H -ATPase that actively pumps H' into the luminal
lite (Fig. 18-14). bound extensively to plasma proteins (most likely as canrenone). and excreted primarily as metaholites in the urine. Some biliam'y excretion of metabolites also ocand its curs. lls onset of action is slow (12 to 72
duration of action is quite long (2 to 3 days).4'
at site 4 is also affected. Thus, passage of luminal fluid Na into, and K + and H * out of. the connecting tubule cells anf the cortical collecting tubule cells is enhanced. Increassi intracellular levels of Na I elicited by the actions of rone stimulate the basolateral membrane—hound Na/K ATPase.' Because spironolactone competitively these actions of aldoslerone. it enhances water. Na' , anJ
SITE AND MECHANISM OF AcTiON
Cl excretion. Therefore. spironolactone is a chloruretic. saluretic. and antikaliuretic agent. Unlike
Spironolactone inhibits the reabsorption of 2 to 3% of the filtered load of Na at site 4 by competitively inhibiting the actions of aldosteronc" (Fig. 18-6). Under normal circum-
oilier K $ diuretics. .cpironolacrone requirer th ,resence of endoge'nous aldosterone icr e'.reri its diuretic & lion. Because it inhibits the reabsorption of only 2 to c)
Mt
Chapter IS • Diure:ie.r filtered load of Na it (and the site 4 K ' -sparing dihas relatively low efficacy.
617
proximal tubule. Because it is a weak organic base, it
is
a.ssumcd to be handled by the proximal tubule OCTS32 (Fig.
18-7). Its onset of action following a single oral dose is 2 to 4 hours, and its duration of action is 7 to 9 hours.4'
ADVERSE EFFECTS
One might anticipate that inhibition of the exchange of lumi-
SITE AND MECHANISM OF ACTION
sal fluid Na' for intracellular K • and H F would lead to
Triamterene "plugs" the Na channels in the luminal membrane of the principal cells at site 4 and thereby inhibits the electrogenic entry of 2 to 3% of the filtered load of Na * into these cells" '(Fig. 18-6). As triamterene's action decreases the principal cell concentration of Na ',the antilumi' -ATPase activity also denal membrane—bound creases. This leads to decreases in the cellular extrusion of Because the secretion Na and the cellular uptake of K
retention of the latter two ions in certain individuals. Imporbat adverse effects of spironolactone include hyperkalemia and mild metabolic acidosis. especially in individuals with renal function." Therefore, patients taking spironoLsione should be warned not to take K' supplements. Caunon must also be exercised when administering spironolac-
roe with other drugs, such as angiotensin-converting aiyme (ACE) inhibitors. angiotensin II receptor antagofliRtS. and /3-adrcnergic blockers, that may also evoke inIn addition. spironolactone may proifl gynecomastia in men and breast tenderness and menstrual disturbances in women because of its residual hor-
Gynecomustia occurs in approximately 6 o 10% of males given 50 mg/day or less and in up to 52% in dose.s above 150 mg/day. Other adverse effects include nional
minor gastrointestinal symptoms and rashes.32
°
USES
Spminolactone may be used alone as an extremely mild diuletic to remove edema fluid in individuals with congestive
kart failure, cirrhosis of the liver with ascites. or the nesyndrome or as an anrihypertensive agent. Its primary use, however, has been in combination with diuretics hal act at Site 2 or 3 in an attempt to reduce the urinary K' associated with these latter groups of diuretics.
Z47"Trlamlno-6-arylpteruaines The structure of triamterene. 2,4,7iiamino-6.phenylpteridine (Dyrenium). is shown below.
Triamterene, USP.
of K* and H at site 4 is linked to Nil' reabsorption. a concomitant reduction in the excretion rate of K' and H' occurs. Unlike spironolactoiu', Iru,mterene 's diuretic action does ito! depend on the' presence of aldosierone. Triamterene. like the other K -sparing diuretics, has a low efficacy and is a mild natriuretic. ehloruretic. saluretic. and antikaliuretic agent. ADVERSE EFFECTS
diuretics whose primary actions Like the other K are elicited at site 4. triamterene's major adverse effect is hyperkalemia.36 Therefore, patients taking triamterene should be warned not to take K supplements. Caution is also needed when administering triamterene along with other drugs, such as ACE inhibitors. angiotensin II receptor antagonists, and fl-adrenergic blockers, that may also give rise to In addition, it appears o be unique increases in among the K -sparing diuretics in being associated with the formation of renal stones. Approximately I of 1.500 individ-
uals taking a triamtcrene-containing diuretic experiences The stones consist of triamterene nephrolithiasis.'°4 (with or without its metabolite) or triamterene along with calcium oxalate or uric acid, It also may produce nausea, vomiting, leg cramps, and dizziness,32 USES
Triamterene may be used alone in the treatment of mild NH5
H,N Triamlerene
edema associated with congestive heart failure or cirrhosis of the liver with ascites. but it should not be given to patients with impaired renal function."5' It is not Lobe used alone in Its primary use is in the treatment of hypertension.46
combination with hydrochlorothiazide (or other diuretics STRUCIURE-ACTIVITY RELATIONSHIPS
Triamlerenc is the primary compound selected from a host
that act at site 2 or 3)10 prevent the hypokalemia associated with the latter diuretics.
af synthetic pteridine analogues.'01 Although it bears a struc-
Pyrazinoylguanidines
aol resemblance to folic acid and certain dihydrofolate reJuclase inhibitors, it has little, if any, of their activities.32
The structure of amiAmhloride Hydrochloride, USP. bride hydrochloride. 3.5-diamino-N-(aminoiminomelhyl)monohydrochloridc drate (Midamor). is shown below.
6-chboropyrazinecarboxamide
0
Friamierene is absorbed rapidly but incompletely (30 to 0%) from the gastrointestinal tr,tct,4' bound to plasma proems to the extent of about 60%. biotransformed extensively in the liver, and excreted primarily by the biliary route and via the renal route as unchanged drug (20%) and nielabolites (80%). It enters the luminal fluid of the nephrons
hy glomerular filtration and active tubular secretion in the
II
Ck.
NH2 CIII
N
T6
H2N
N
NH2
M,iIor,de Hydrochloride
dihy-
618
Wilxon and (ll.crokl's Texihook of Organic Medicinal and Pharniaceutical Chen,issrv
STRUCTURE-ACTIVITY RELATIONSHIPS
An extensive screening procedure that examined over 25,000 agents was undertaken in an attempt to discover an antikaliuretic agent that did not have overlapping hormonal activity
like that of spironolactone.'°7 Promising activity was noted with appropriately substituted pyrazinoylguanidines. Optimal diuretic activity in this series is observed when the 6 position is substituted with chlorine, the amino groups in the 3 and 5 positions are unsubstituted. and the guanidino
nitrogcns are not multiply substituted with alkyl groups. Amiloride emerged as the most active compound in the series.
PHARMACOKINETICS
Amiloride contains the strongly basic guanidine moiety and possesses a of 8.7. Thus, it exists predominantly as the charged guanidinium ion in the p1-I range of most body tissues and fluids. It is not surprising that amiloride is absorbed incompletely and ernitically (IS to 20%) from the gastrointestinal tract, an event that occurs by passive diffusion of the uncharged form of most drugs. Amiloride is bound to plasma proteins to a moderate degree, is not biotransformed,
and is excreted in the urine (20 to 50%) and in the feces (40%). The fecal content may represent unabsorbed drug. Amiloride reaches the luminal fluid by glomerular filtration and active tubular secretion. The proximal tubule OCTS (Fig. 18-7) is involved in the latter process.32 Onset of action occurs within 2 hours after oral administration, and duration of action may extend to 24 hours.4' SITE AND MECHANISM OF ACTION
Like triamterene, amiloride inhibits the electrogenic entry ol 2 to 3% of the filtered load of Na into the principal cells of the connecting tubule and cortical collecting tubule (i.e.. site 4) by "plugging" the sodium channels in the luminal membrane (Fig. 18-6). In turn, the driving force for K F secretion is reduced or eliminated." IL 36 Like iria,nterene, amiloride does not require the presence of aldosrerone to produce diuresis. It induces the urinary loss of Na Cl, and water and, therefore, is a natriuretic. chloruretic. saluretic, and antikaliuretic agent. though with low efficacy.
of hypertension. Its most common usc is in combination sith diuretics that act at sites 2 or 3, to circumvent the renal Ion of K commonly associated with the latter agents.
MISCELLANEOUS DIURETICS Mannitol, USP. The prototypic osmotic diuretic u-nun nitol is a water-soluble, lipid-insoluble. hexahydroxy ako. hol. Because of its lack of lipid soluhilily. mannitol does na diffuse across the gastrointestinal epithelium and must be given by the intravenous route to obtain systemic effects
Once it enters the bloodstream, little, if any. is bound to plasma albumin; its distribution is confined to extracellutar fluids, and it is not biotransformed. It enters renal luminal fluid only by glomerular filtration: it is neither secreted nor reabsorbed. The net result of its renal handling is twofold First, it is excreted primarily by the kidneys: up to a I00-g intravenous dose appears in the urine within a)hour period.° Second. high luminal fluid concentrations of mannitol create an osmotic effect, and a great deal of the water in the luminal fluid is retained within the lumens of the nephrons. This osmotic effect prevents the reabsorption Mannitol. there. of up to 28% of the filtered load of fore, may be used prophylactically in a hospital setting to
keep the nephrons open (i.e.. prevent them from collapsing) in an attempt to avoid acute renal failure in certain circumstances. It has also been useful for the reduction of cerebro spinal fluid volume and pressure. Because intravenous sofu tions of mannitol may expand the extracellular fluid volumc, they should not be used in patients with severe renal diseacn or cardiac decompensation who may not be able to excrete the additional fluid load and may subsequently develop 1nd monary edema. Aqueous solutions are available in a range
of concentrations for intravenous use. The adult range for the induction of diuresis is from 50 to 200 g/24 hours.
Theophylllne.
The prototypic xanthine, theophylline.is known to promote a weak diuresis by stimulation of cardiur function and by a direct action on the nephron. Although 0
is infrequently used as a diuretic. diuresis may be an served side effect when it is used as a bronchodilator.
ADVERSE EFFECTS
The major adverse effect of amiloride is hyperkalemia. which also may be observed with the other K • -sparing diuretics that act at site 4. Therefore, patients taking amiloride should be warned not to take K' supplements. Caution is also needed when administering amiloride along with other drugs, such as ACE inhibitors. angionensin H receptor antag-
onists. and $-adrenergic blockers, that may also give rise Nausea, vomiting, diarrhea, and to increases in 1K + headache may also accompany the use of amiloride.32
USES
Amiloride may be used alone in the treatment of mild edema a.ssociated with congestive heart failure, cirrhosis of the liver with ascites, or the nephrotic syndrome or in the treatment
EMERGING DEVELOPMENTS IN THE USE OF DIURETICS TO TREAT HYPERTENSION AND CONGESTIVE HEART FAILURE Although the precise mechanism(s) by which the thiuii& thiazide-like. and loop diuretics lower blood pressure in pertensive patients is not known, it is thought to in plasma volume and cardiac output well as direct relaxation of the vasculature. Over the the adverse effects associated with these drugs were ered relatively mild. Recently, however, several of the wdl known adverse effects of these drugs have attracted attention
because they appear to be more troubling than previuuil) thought. First, diuretic-induced increased K excretiur leads not only to varying degrees of hypokalemia. but aISL to decreased cardiac and skeletal muscle 1K * I. Such change.
619
Chapter 18 a impair cardiac performance and damage heart, brain. ntK and kidney vessels.100 Second. diuretic-induced reductions
in plasma volume trigger increased sympathetic tone and renal secretion of renin and, ultimately, increased plasma levels of ungiotensin II. In addition to being a potent
oJ
ia.coconstriclor, angiotcnsin II stimulates aldosterorse secre-
isis. Although the mechanism of aldosterone's actions at 4 in the nephron have been known for a long time, its rttrarenal actions have largely been ignored. Recently, it has
observed that diuretic-induced increased aldosterone ictels not only cause changes in electrolyte transport at site 4 with ultimate damaging hypokalemia but also produce effccts at extrarenal aldosterone receptors in the vasculature, which lead to vascular damage, and in the heart, which lead In cardiac fibrosis.'09 These observations help to explain
H
shy diuretic-induced reductions in blood pressure do not protect some hypertensive individuals from other
0— CH3
problems. Lasragh and Sealey'°" °° have amassed clinical evidence hat spironolactone. a non.seleetive aldosterone ansagonisI, ajien used alone is as effective as the thiazides in treating
Eplerenone
slid hypertension without inducing hypokalernia or insecretion of aldosterone. Furthermore, when Spiroelactone is used in combination with a thiazide. thiazidelike, or loop diuretic, it markedly blunts the renal and extrare-
actions of the elevated levels of aldosterone brought by these latter diuretics. In the past, spironolactone has not enjoyed widespread for several well-documented reasons. (a. 0 First, its
effectiveness is usually not observed for 3 to 5 seeks. Second. its residual hormonal side effects have proiuccd unacceptable rates of gynecomaslia in males and men-
uual irregularities in females, especially when doses execded 50 to 100 mg/day. These hormonal side effects can largely avoided by giving spironolactone in doses of 12.5 i 25 mg/day.
Epicrenone. a spec j/lc aldosterone antagonist recently ap-
by the Food and Drug Administration, appears to a much lower affinity for androgen and progesterone than spironolactone and a reduced incidence of esuni disturbances.'09 If this finding is confirmed in addiclinical studies. eplerenone or another drug with the
of cplerenone may emerge as a very useful açenl and improve the treatment of hypertension (and Ln'nscstive heart failure) when used alone or in combination
ath other diuretics or ACE inhibitors.
SUMMARY The major driving force for the reabsorption of Na
at all four Na reabsorption sites is the deficit of intracellular Na + created by the activity of the basolateral membrane—bound Na /1< -ATPase. In response. the luminal fluid Na * moves into the Na ' -deficient cells by a luminal membrane—bound transport system that is unique to each of the four sites. Most diuretics must attain sufficient concentration in luminal tluid to inhibit a luminal membrane—bound Nu transport
system: this is usually accomplished by a combination of glonierular filtration and active tubular secretion. The cheini-
cal structure of a diuretic dictates which of the four Na transporting sites will be inhibited. The site that is inhibited is one of the major determinants of the efficacy of the diuretic. The historical development of many diuretics has involved molecular modification of the chemical structure of sulfamoyl-containing compounds. This has yielded CA inhibitors, which inhibit the reabsorption of at site I: the thiazide and thiazide-like diuretics, which inhibit the reabsorption of at site 3: and the high-ceiling diuretics, which block Na' /Cl /K - /Ca2 '7Mg2 reabsorption at site 2. Diuretic efficacy has increased with the conesponding changes in the site of action of each of the three classes of diuretics. Predictable secondary effects that depend on a diuretic's site of action have also surfaced.
DIURETIC PREPARATIONS41 Cambonic Anhydrase Inhibitors Aiflazojamlde. USP (Diamoac, Generic)
Oral: 125-, 250-mg tablets; 500-mg extended-release capsules Parenteral: 500-mg powder (as the sodium salt) USP (Daranide) Oral: SO-mg tablets Vethazolamide, USP
(Neptazane. Generic)
Oral: 25'. 50-mg tablets flmianide and Thiazide-Like DiuretIcs
ter,d,oflumethlazlde. US!' (NaWretln) Oral: 10-mg tablets
Benzthiazlde (Exna. Hydrex) Oral: SO-mg tablets Chlorothlazide, USP (Diuril, Generic) Oral: 250-, 500-mg tablets; SO-mg/mi oral suspension Parenteral: 500-mg base for injection Chlorthalldone. US!' (Hygroton. Thalltone. Generic) Oral: 25-, 50-, 100-mg tablets Clopamlde (Aquex, Brlnaldlx) Not available in the United States Clorexolone (Nefrolan) Not available in the United States
620
Wilson
and Gist-old's Textbook of Organic Medicinal and Pharmaceutical Clwinist,
Cyclothiazide (Anhydron) Oral: 2-mg tablets Hydrochlorothiazlde, USP (EsIdr(x. HydroDIURIL. Oretic. Generic)
Oral: 25-, 50-, 100-mg tablets; 12.5-mg capsules (Microzide); 10-, 100-mg/mL oral solutions Hydroflumethiazide. USP (Diucardin, Saluron. Generic) Oral: 50-mg tablets lndapamide (Lozol, Generic) Oral: 1.25-, 2.5-mg tablets Mefruside (Baycaron) Not available in the United States Methyclothiazide. USP (Aquatensen, Enduron. Generic) Oral: 2.5-. 5-mg tablets Mefolazone Oral: 2.5-. 5-. 10-mg extended tablets (Diulo, Zaroxolyn); 0.5mg prompt tablets (Mykrox) Polythiazide. USP (Renese) Oral: 1-, 2-, 4-mg tablets Quinethazone. USP (Hydromox) Oral: SO-mg tablets Trkhlormethlazide, USP (Metahydrln. Naqua. Genetic) Oral: 2-, 4-mg tablets Xipamlde (Aquaphor. Dlurexan) Not available in the United States
Loop or High-Ceiling Diuretics Bumetanide, (ISP (Bumex. Generic) Oral: 0.5-, 1-, 2-mg tablets Parenteral: 0.25 mglmL for IV or IM use Ethac,ynlc Acid. USP (Edecrin) Oral: 25-, 50-mg tablets Parenteral: 50-mg (base) for IV use Furosemlde, (ISP (LasI*. Generic)
Oral: 20-, 40-, 80-mg tablets; 8-. 10-mg/mL oral solution Parenteral: 10 mg/mL for IV or IM use Torsemlde (Demadex) Oral: 5-. 10-, 20-, 100-mg tablets Parenteral: 10 mg/mL for IV use
K°-Sparlng Diuretics Amlioride Hydrochloride, (ISP (Mldamor, Genetic) Oral: 5-mg tablets Eplerenone (Inspra) Oral: 25-, 50-. 100-mg tablets Splronolactone. (ISP (Aldactone, Genetic) Oral: 25-, 50-. 100-mg tablets Trlamterene (Dyrenium) Oral: 50-, 100-mg tablets; 50-, 100-mg capsules Diuretic Combinations AmllorideiHydrochlorothiazide, (ISP (Moduretic, Generic) Oral: S mg amiloride hydrochloridel5o mg hydrochlorothiazide Splronolactone/Hydrochlorothiazide, (ISP (Aldactazide) Oral:
25 mg spironolactonel2s mg hydrochiorothiazide 50 mg spironolactone/50 mg hydrochlorothiazide Triamterene/Hydrochlorothiazide, USP (Dyazide, Maxzide) Oral—capsules: 37.5 mg triamterene/25 mg hydrochlorothiazide (Dyazide) 50 mg triamterenel25 mg hydrochlorothiazide (Generic) 75 mg triamterene/50 mg hydrochiorothiazide (Generic)
Oral—tablets: 37.5 mg triamterene/25 mg hydrochlorothiazide (Mazzide) 75 mg triamterene/50 mg hydrochlorothiazide (Mauzide) Osmotic DIuretics Mannitol (Osmitrol, Generic) Parenteral: 5, 10, 15, 20, 25% for IV use
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33. Leaf, A.. and ('atom. K. S.: Diuretics. In lest, A.,and ('vlsi, (mis.). Renal Pathophysiology. 2nd cr1. New York. 0,JiinJ Press. 1980. pp. 145—161.
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41)3
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44 Shimi,u. T., et al.: 3. Clin. Invesl. 82:72). 1988. 45 AMA l)rug Evaluations Annual 1995. Chicago. American Medical Association, pp. .937-1156. .16. Mcd. Leli. 37:45, 191.15 and 41:23. 1999. 47. Bever. K. H., Jr.. and Barr.). 12. Mcd. ('On. North Am. 59:7.45. 1975
48 Kunau. K. T.. Weller, 0. K., Jr., and Webb, U. I..: J. Chin. Invest. 56: 401.
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SI. l'nhian, I..: Anne. Rev. Pharmucol. 7:399. 1967. SI. Greenberg. A.: Am. Pam. Physician 33:200. 1986. 54. Qennett. W. M., and Poiler. GA.: 1. Clin. Phamiaciil. 54. Cravwell. P. W.. ci al.: Nephron 12:63, 1Q73.
.3:357. 1973.
55. Mcd. Let). 16:65. 1974.
56. lbsminger. 0. M.: IJrol. Chin. North Am. 14:325. 1987. 57 lmbs. I. L.. Schmidt. M.. and Giesseit-Crouse. 0.: Pharmacology of loop diuretics. State of the art. In Gnrn(eld, i-P.. ci a). (cdv.). Advances in Ncphrology. vol. 16. Chicago. Year Book Medical. 1987. pp. 137—158. 1.s,
8tirg. M., and Green. N.: Kidney In). 4:245. 1973.
56. Caltuny. E. 3.: Geriatrics, 22:1117. 1967.
16 Calruny. 0. I.. Cho. K. C.. and Gussun. K. Z.; Ann. N. V. Acad. Sci. 39:362. 966.
(I. l5thridge. C. B.. Myers. 0. W.. and Fulton. M. N.: Arch. Intern. Mcd. 57:7)4. 1Q36.
1.2. Weiner. I. M.. Levy. K. I.. and Mudge. 0. H.: 3. Pharmacol. Eap. Thee. 138:96. (962.
65. Ward. A.. and Heel. K. C.: Drugs 28:426. 1984.
ri. Frig. P. U.: Am. 3. Cardiol. 57:l4A. 1986. vS. Odhind. B.: J. Phannacol. Esp. Titer. 211:23%, 979, 56 Guisehe. H-U.. et al.: Can. J. Physiol. Phanniacol. 62:412. 1984.
linnerty. F. A.. Jr.. Cl al.: Angiology 28:125. 1977. Cr Reineck, H. J.. and Stein. 3. H.: Mechanisms of aclion and clinical uses nI diuretics, In Bninner. B. M.. and Rector. F. C' Jr. (cdv.). The Kidney. vol. I. 2nd cd. Philadelphia. W. B. Saunders. 1981, pp. 1.7
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hack mechanism, XII Symposium 01' the Nephrology Society. Sept.
2$-Oct. I. 1977. Bonn. West Gemiany. 84. Williamson. H.. Bourland, W.. aitd Marchand. G.: Prostaglandins 8: 297. 1974 85. Bitich. A. U., ci a).: Circ. Kes. 24:869. 1Q67. 86. Garay. R. P.. ci al.: 3. Pliartmicol. lisp. Ther. 255:415. 990. 87 Gamy. R. P.. Naz.irel. C.. and Cragoe. E. 3.. Jr.: Eur. 1. Pharmacol. 200:141, 1991. 1*4. Shinkawa, T.. ci a).: Eur. J. Pharinacol. 238:317. 1993. 89. Grcvcn. J.. Klein, H., and I-lcidcnreich. 0.: Naunyn Schtiiicdehcrgs Arth. Htamusciil. 3(1.1:2149. 49714.
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II)). Vcrrey. F.. ci ul.: J. Cell. Biol, 104:1231. 1987. 102. Smith. K. L.: Endogenous ageitt.s affecting kidney lunction: Their in)crrelatiiinships. modulation, and ciinlnil. In Cragiic. U. J.. Jr. (cii.). Diiiretics—Chcmislry. Pharmacology and Medicine. New York. Jolts Wiley & Sons. 1983. pp. 571—651. 103. Wiebelltaus. V. 0.. us al.: 3. Phiammacul. lisp. Ther. 149:397. 1965. 104. Carey. K. A., ci al.: Cliii. 'liter. 6:302. 1984. 105. SergeI. F., Ettingcr. B.. and l3enet. I.. Z.: 3. Phamm. Sci. 75:129. 1986.
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1)167—I Ill.
ill. Suki, W N.. el al.: N EngI. J. Mcd. 283:836, 1970. '0 Delarge. 4.: Ar,.ncimittelforschung 38:144, 1988. Mcii. Let). 36:73. 1994 Schwartz, S., us al.: Cliii. Pharmacol. Titer. Greger. R.: A,7ncimittellorschung 38:151. 988.
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a Klinke, R., and Mertens, M.: AriiteuntiltelIorschung 38:153. 1988. Dunn. C. 3.. Fitton. A.. and Brogdcn. K. N.: Drugs 49:121. 1995. Kocchel. 0. A.: Annu. Rev. Pharmacol. Toxicol. 21:265. 1981.
Iii.. Jr.: The (arylovy)acetic acid family of diuretics. In Cmgoc. 0. 3.. Jr. (ed.). Diuretics—Chemistry. Phiannacohogy and
Medicine. New York. John Wiley & Sons, 1983. pp. 201—266. Koechel. I). A.. Giuvold. 0.. and Cafruny. 0. 3.: 1. Med. Chem. 14: 62%. 1971.
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$3 Schnermann. I.: Influence of diuretics on the luhuluglonterular feed'
SELECTED READING Brenncr. B. M.. and Beeuwkes. R. III: Hosp. Pract. 13:35—46, 1978. Breyer. J., and Jacobson. H. K.: Annu. Rev. Med. 41:265. 1Q90. Greenberg, A.: Am. Pam. Physician 33:2(81-2)2. 1986. Imbs. J.-L.. Schmidt. M.. and Giessett.Crouse. E.: Pharmacology oh loop diuretics: State of the art. Mv. Nephroh. 16:137—158. 1987. Koeppen. B. M.. and Stanton. B. A.: Renal Iransport mechaitisnis: NaCI and water reabsorption alottg the nephron. In Kocppcn. B. M.. and Stanton. B. A. (cds.h. Renal Physiology. 2nd cd St. Louis. Mosby 1997. pp. 53—76.
Puschetl. 3. B.: Am.). Cardiol. 57:6A—l3A. 1986. Steinmirtc.. P. K.. and Koeppen. B. M.: hlosp. Pr.iet. 19:125—134, 1984. Weiner. I. M., and Mudgc. 0. Il,: l)iurctics and other agents employed in the mobilization of edema fluid. In Gilman. A. 0., et al. (cdv.). The Pharmacological Basis of Ther.ipeuties. 7th ed. New York. Macmillan, 1985. pp. 887—9(17.
Wilcox, C. S.: Diuretics.. In Brenner, B. M. led. I. Brenner & Rector's The
Kidney. vol. 2. 6th ed Philadelphia. W. B. Saunders. 21.88). pp. 22 19—2252.
CHAPTER 19 —
A
Cardio vascular Agents STEPHEN J. CUTIER AND GEORGE H. COCOLAS
The treatment and therapy of cardiovascular disease have
reduce peripheral vascular resistance. vasodilators. including
undergone dramatic changes since the 1950s. Data show that since 1968 and continuing through the 1990s, there has been
organonitrates, angiotensin-converting enzyme (ACE) in
a noticeable decline in mortality from cardiovascular dis-
to improve cardiac output in some patients with heart failure (CHF).
ease. The bases for advances in the control of heart disease have been (a) a better understanding of the disease state, (b) the development of effective therapeutic agents, and (c) innovative medical intervention techniques to treat problems of the cardiovascular system. The drugs discussed in this chapter are used for their ac-
tion on the heart or other parts of the vascular system. to modify the total output of the heart or the distribution of blood to the circulatory system. These drugs are used in the treatment of angina, cardiac arrhythmias. hypertension. hyperlipidemias. and disorders of blood coagulation. This chapter also includes a discussion of hypoglycemic agents. thyroid hormones, and antithyroid drugs.
ANTIANGINAL AGENTS AND VASODILATORS Most coronary artery disease conditions are due to deposits of atheromas in the intima of large and medium-sized arteries serving the heart. The process is characterized by an insidious onset of episodes of cardiac discomfort caused by ischemia from inadequate blood supply to the tissues. Angina pectoris (angina), the principal symptom of ischemic heart disease, is characterized by a severe constricting pain in the chest, often radiating from the precordium to the left shoulder and down the arm. The syndrome has been described since 1772 but not until 1867 was amyl nitrite introduced for the symptomatic relief of angina pectoris.' It was believed at that time that anginal pain was precipitated by an increase in blood pressure and that the use of amyl nitrite reduced both blood pressure and, concomitantly. the work required of the heart. l..ater. it was generally accepted that nitrites relieved angina pectoris by dilating the coronary arteries and that changes in the work of the heart were of only secondary importance. We now know that the coronary blood vessels in the atherosclerotic heart already are dilated and that ordinary
doses of dilator drugs dt) not significantly increase blood supply to the heart; instead. anginal pain is relieved by a reduction of cardiac consumption of oxygen. Although vasodilators are used in the treatment of angina. a more sophisticated understanding of the hemodynamic response to these agents has broadened their clinical usefulness to other cardiovascular conditions. Because of their ability to
622
hibitors. and angiotensin receptor—blocking agents. are wed
The coronary circulation supplies blood to the myocardial tissues to maintain cardiac function. It can react to the chant.
ing demands of the heart by dilating its blood vessels provide sufficient oxygen and other nutrients and to remosc metabolites. Myocardial metabolism is almost exclusively aerobic, which makes blood flow critical to the supponi of metabolic processes of the heart. This demand is met effec-
tively by the normal heart because it extracts a large proportion of the oxygen delivered to it by the circulation. The coronary blood flow depends strongly on myocardial metabolism, which in turn is affected by sort done by the heart and the efficiency of the heart. The nary system normally has a reserve capacity that to respond by vasodilation to satisfy the needs of the heai during strenuous activity by the body. Coronary atherosclerosis, one of the more prevalent can), ovascular diseases, develops with increasing age and inag lead to a reduction of the reserve capacity of the coronart system. It most often results in multiple stenosis and mako it difficult for the coronary system to meet adequately the oxygen needs of the heart that occur during physical exercise or emotional duress. Insufficient coronary blood flow cardial Lvclzemia) in the face of increased oxygen deman) produces angina pectoris.
The principal goal in the prevention and relief of is to limit the oxygen requirement of the heart so that tic amount of blood supplied by the stenosed arteries is quate. Nitrate esters, such as nitroglycerin, lower afleiicl blood pressure and, in turn, reduce the work of the left
cle. This action is produced by the powerful vusodilating effect of the nitrates on the arterial system and, to an eve, greater extent, on the venous system. The result is reduceJ
cardiac filling pressure and ventricular size. This the work required of the ventricle and decreases the oxyge
requirements, allowing the coronary system to satisly tb oxygen demands of myocardial tissue and relieve anginci pain.
Intermediary Myocardlal Metabolism Energy metabolism by heart tissue provides an adequate
ply of high-energy phosphate compounds to replace the adcnosine triphosphate (ATP) that is continually being
sumed in contraction. ion exchange across and other energy-demanding processes. Because of he furl.
________
Chapter 19 • Glucose
623
Glucose 4— Glycogen
Glycogen
4ADP+2NAD
2ATP
ajar Ag'n,o
2ATP
4ADP+2NAD No Oxidative
4ATP+2NADH
2ADP
Phosphorytation
Pyruvate (2)
Pyruvate (2) NADH
NAOH
Mutochondrion Lactate
Mitothondrion
NAD
Acetyl-C0A (2)
Lactate
NAD+ 2 FAD TCA cycle
(:1: 6 NADH +2 FADH2
NORMAL GLUCOSE METABOLISM
ISCHEMIC GLUCOSE METABOLISM
Figure 19—1 • Normal and ischemic myocardial metabolism of glucose. A total production of 36 moles of AlP results from the aerobic catabolism of 1 mole of glucose and use of NADH and FADH, in the oxidative phosphorylation process in mitochondria. When oxygen is not available, NADH and FADH2 levels rise and shut off the tricarboxylic acid (TCA) cycle. Pyruvate is converted to lactate. Only 2 moles of ATP are formed from anaerobic catabolism of 1 mole of glucose. (Adapted from Giuliani, E. R.. et al.: Cardiology: Fundamentals and Practice, 2nd ed. By permission of the Mayo Foundation. Rochester, MN.)
urnoser rate of AlP in heart muscle, a correspondingly
bgh rate of AlP production in the mitochondria is reNormal myocardial metabolism is aerobic, and the rate of use parallels the amount of ATP synthesized by the Free fatty acids (FFAs) are the principal fuel for myotissue, but lactate, acetate. acetoacetate. and glucose ire also oxidized to CO2 and waler. A large volume of the
cell consists of mitochondria in which two-car-
Nitrovasodilators SMOOTH MUSCLE RELAXATION
The contractile activity of all types of muscle (smooth, skeletal) is regulated primarily by the reversible phusphorylation of myosin. Myosin of smooth muscle consists of two heavy chains (MW 200.000 each) that are coiled to produce a lilamentous tail. Each heavy chain is associated with two pairs of light chains (MW 20,000 and 16,000) that serve as sub-
bough the Krcbs cycle. The reduced flavin and nicotindinucteotides formed by this metabolism arc reoxi-
strates for calcium- and calmodulin-dcpendent protein kinases in the contraction process. Together with actin (MW 43.000) they participate in a cascade of biochemical events
i,ed by the electron-transport chain because of the presence
that are part of the processes of muscle contraction and relax-
.ioxygen (Fig. 19-I). In the hypoxic or isehenaic heart, the oxygen inhibits the electron-transport chain function
ation (Fig. 19-2).
fragments from FFA breakdown are metabolized
an accumulation of reduced flavin and nicolincoeni.ymes. As a result, fatty acids are converted to rather than being oxidized. To compensate for this. rhxose use and glycogenolysis increase, but the resulting cannot be oxidized: instead, it is convened to lacA great loss of efficiency occurs as a result of the change ii metabolism from aerobic to anaerobic pathNormally. 36 moles of ATP are formed from the oxi-
Cyclic nuclcotides. cyclic adenosine monophosphatc (cAMP), and, especially. cyclic guanosine monophosphatc
(cGMP) play important roles in the regulation of smooth muscle tension, cAMP is the mediator associated with the smooth muscle relaxant properties of drugs such as f3-adrenergic agonists. It activates the protein kinases that phospho-
rylate myosin light-chain kinasc (MLCK). Phosphorylation
lion of I mole of glucose. but only 2 moles arc formed
of MLCK inactivates this kina.se and prevents its action with and calmodulin to phosphorylate myosin. which interacts with actin to cause contraction of smooth muscle (Fig.
::orn its glycolysis. This great loss of high-energy stores
19-2).
hypoxia thus limits the functional capacity of the
The activity of cGMP in smooth muscle relaxation is af-
during stressful conditions and is reflected by the proof anginal pain.
fected by exogenous and endogenous agents. It is suggested4
that nitrovasodilators undergo metabolic transformation in
624
of Organic Medicinal and Pharmaceuth-al Che,ni.strv
Wilson and Gixvold'.s
p AgonIst
Ca2
Membrane
Figure
AC
Calcium channel
Receptor
Ca2
smooth
19—2 • Regulation contraction.
muscle
o
Co
traction is triggered by an nIbs of
Awl
Ca?
light-chain kinase (MLCK) and causes
CAMP
complex I
MLCK4 Myosin—LC
MLCK a
Myosin-LC-P04
. The increase of free Ca2 - causes
binding to calmodulin (CM) Ca'—CM complex binds to myose
a MLCK-(PO4)2 Myosln-LC
its activation (MLCK*). MLCK phos phorylates myosin, which comb.nes with actin to produce contraction
smooth muscle
Myosin is dephus phorylated in the presence of myot phosphatase to cause muscle relaxation. The p agonists activate adenyLats
cyclase (AC) to raise levels of which in turn activates kinases
Actin
Contraction
Relaxation
vascular smooth muscle cells to form nitric oxide (NO). NO mediates smooth muscle relaxation by activating guanylate cyclase to increase intracellular concentrations of cGMP. cGMP activates protein kinases that can regulate free
levels in the muscle cell and cause relaxation of smooth muscle by phosphorylaing MLCK. A short-lived free radical gas. NO is widely distributed in the body and plays an important role by its effect through cGMP on the smooth muscle vasculature. It is synthesized in the vascular endothelial cell from the semiessential amino acid L-arginine by NO synthase. After production in the cell. it diffuses to the smooth muscle cell, where it activates the
enzyme guanylate cyclase, which leads to an increase in cGMP and then muscle relaxation (Fig. 19-3). Endotheliumderived relaxing factor (EDRF). released from the endothehal cell to mediate its smooth muscle—relaxing properties through cGMP. is identical with NO. Inhibkors of phosphodiestcruses of cAMP and cGMP also cause smooth muscle relaxation. These inhibitors increase cellular levels of cAMP and cGMP by preventing their by-
hut
phosphorylate MLCK, inactivating Us prevent muscle contraction.
drolysis to AMP and GMP. respectively. Drugs such paverine (see Chapter 17) and thcophylline (see Chapter which relax smooth muscle, do so in part by inhibiting phodiesterases. METABOLISM OF NITROVASODILATORS
Alter oral administration, organic nitrates arc metabolized
rapidly by the liver, kidney, lungs. intestinal vascular tissue. 8uccal absorption reduces the
ant
hepatic destruction of the organic nitrates because only of the cardiac output is delivered to the liver; this alloss a transient but effective circulating level of the intact orgaa
nitrate before it is inactivated.5 Organic nitrates, nitrites, nitroso compounds, and a van ety of other nitrogen-containing substances, such as saidiun
nitroprusside. for the most part cause their effects by generating or releasing NO in situ. In some these drugs are viewed as "replacement agents" for the en dogenous NO generated by the NO synthase pathway frm
arginine. The mechanisms by which vasodilatory lease NO have become better understood recently. Table Ih
I shows the oxidation state of various nitrosyl Nltrovasodllators Endothellal Cells
GC 4
Guanylate cyclase (OC)
release NO nonenzymatically. Although their liberation of NO is by an unknown mechanism, it
aCGMP
GTP
that are common in nitrovasodilatory drugs. A common ture of these drugs is that they release nitrogen in the (stir of NO and contain nitrogen in an oxidation stale higher tha + 3 (as would occur in ammonia, amines. amides. and mcii biological nitrogen compounds). The nitrogen in NO oxidation state of + 2. Compounds such as nitroprussid:. nitrosoamines, and nitrothiols with oxidation states iii I
only a one-electron reduction, which may occur on expusun MLCK
4
MLCK
) MLCK-P04
Figure 19—3 • Mechanism of nitrovasodilators Nitric oxide (NO) formed in smooth muscle from nitrovasodilators or from endothelial cells (EDRF) activates guanylate cyclase (GC*). GC* activates cGMP-dependent protein kinases that phosphorylate myosin light-chain kinase (MLCK), causing its inactivation and subsequent muscle relaxation (see also Fig. 19-2)
of these chemicals to the variety of reducing agents in tissue of vascular smooth muscle membranes. Organic trues such as amyl nitrite react with available thiol gluiyi to form unstable S-nitrosothiols. which rapidly decompose; NO by homolytic cleavage of their S—N bond. In maninuliii smooth muscle, this will occur almost exclusively with tathione as the most abundant thiol compound.6
Chapter 19 • Cardü,ta,ceuhtr Agents
TABLE 19-1
Nitrosyl Vasodliatory Substances and
Their Oxidation State Nitrosyl Compound
Structure
Nitrogen Oxidation State -'2
rrsrtk Nutria
—ONC)
-'-3
Nude
—ONO.,
4-5
Organic ururrile
R—O—N=O
-'.3
Ornarric IriIrjre
R—S--N=O
'-3
R—O- NO.,
+5
Iluirunirnuic
R—S--NO.,
'5
Niuurprussidc
l(C11Fc—Nr011
-4-3
Kcpnnrtri Inmu I l.unt.urur. I).
(n.m flit
- and Hurt,.. 1.51.. (uucol,rriun 87
1462.
'193. with
titan
The pharmacodynamic action of nitroglycerin is preceded
metabolic changes that follow a variety of paths. Biotansfonnation of nitroglycerin to the dinitrates and the increase of intracellular cGMP precede vascular relaxation. compounds. such as cysteine. react chemically with organic nitrates to Form inorganic nitrite The release of NO from an organic nitrate, such as 2ilroglycerin, appears to occur in a stepwise fashion involving noneni'.ymaIic and en'/ymatic steps. Because nitroglycam requires a three-electron reduction to release NO, thiols be involved in the process. Nitroglycerin may decomnuse noneniymnaticully by interaction with a variety of thiols. ruth as cysteinc or N-acetylcysteine. which may be present fl tissUC, to firm a nitrosothiol intermediate before undergomg cn/.ynmalic transformation to release NO. Nitroglycerin ulso readily releases NO by acting on an enzyme system to the cellular surface membrane of smooth muscle. The process may include glutathione-S-transfera.ses. which 1-Invert nitroglycerin to a vasoinactive nitrite, which then rug release NO nonenzymarically.7 ESTERS OF NITROUS AND NITRIC ACIDS
Inrurganic acids, like organic acids, will form esters with an aiorhol. Pharmaceutically, the important ones are sulfate. ritrite. and nitrate. Sulfuric acid forms organic sulfates, of ahich niethyl sulfate and ethyl sulfate are examples.
Nitrous acid (HNO2) esters may be formed readily from an alcohol and nitrous acid. The usual procedure is to mix sodium nitrite, sulfuric acid, and the alcohol. Organic nitrites are generally very volatile liquids that are only slightly soluble in water but soluble in alcohol. Preparations containing water are very unstable because of hydrolysis. The organic nitrates and nitrites and the inorganic nitrites have their primary utility in the prophylaxis and treatment of angina pectoris. They have a more limited application in treating asthma, gastrointestinal spasm, and certain cases of migraine headache. Their application may be regarded as causal therapy, since they act by substituting an endogenous factor, the production or release of NO, which may be impaired under pathophysiological circumstances associated
with dysfunction of the endothelial tissue. Nitroglycerin (glyceryl trmnitrate) was one of the first members of this group to be introduced into medicine and remains au impor-
tant member of the group. Varying the chemical structure of the organic nitrates yields differences in speed of onset. duration of action, and potency (Table 19-2). Although the number of nitrate ester groups may vary from two to six or more, depending on the compound. there is no direct rela. tionship between the number of nitrate groups and the level
of activity. It appears that the higher the oil/water partition coefficient of the drug, the greater the potency. The orientation of the groups within the molecule also may affect potency. Lipophilicity of the nitrogen oxide—containing compound produces a much longer response of vasodilatory action. The highly lipophilic ester nitroglycerin permeates the cell membrane, allowing continual formation of NO within the cell. The same effect appears to occur for sodium nitroprusside. nitroso compounds, and other organic nitrate and nitrite enters.4
ANTIANGINAL ACTION OF NITROVASODILATORS
The action of short-acting sublingual nitrates in the relief of angina pectoris is complex. Although the sublingual nitrates relax vascular smooth muscle and dilate the coronary arteries of normal humans, there is little improvement of coronary blood flow when these chemicals are administered to individuals with coronary artery disease. Nitroglycerin is an effective antianginal agent because it causes redistribution of
coronary blood flow to the ischemic regions of the heart and reduces myocardial oxygen demand. This latter effect
Relationship Between Speed and Duration of Action of Sodium Nthlte and Certain inorganic Esters TABLE 19-2
DuratIon of ActIon (mInutes)
ActIon Begins (mInutes)
Maximum Effect (minutes)
Amyl rlitntc
0.25
0.5
Nilmgtyccritu
2
8
30
Compound
lsosrwbidcdiriitnrle
625
I
3
15
60
Sodium nttritC
tO
25
60
Ervllirutyl Iclrunutrute
IS
32
80
I'r.,uUieryihrjruut tctrjnitrauc
20
70
330
626
Wilxmt and (iisi'olds Texi!iook af Organic Medicinal and Pharmaceutical ('lw,ni.c,ry
is produced by a reduction of venous tone resulting from the nitrate vasodilating effect and a pooling of blood in the peripheral veins, which results in a reduction in ventricular volume, stroke volume, and cardiac output. It also causes reduction of peripheral resistance during myocardial contractions. The combined va.sodilatory effects cause a decrease in cardiac work and reduce oxygen demand. PRODUCTS
Amy! Nitrite, USP.
Amyl nitrite. isopentyl nitrite I(CH3)2CHCH2CH2ONOI. is a mixture of isomeric arnyl nitrites but is principally isoamyl nitrite. It may be prepared from amyl alcohol and nitrous acid by several procedures. Usually, amyl nitrite is dispensed in ampul form and used by inhalation or orally in alcohol solution. Currently. it is recommended for treating cyanide poisoning: although not the best antidote, it does not require intravenous injections. Amyl nitrite is a yellowish liquid with an ethereal odor and a pungent taste. It is volatile and inflammable at room temperature. Amyl nitrite vapor forms an explosive mixture in air or oxygen. Inhalation of the vapor may involve definite explosion hazards if a source of ignition is present, as both
room and body temperatures are within the flammability range of amyl nitrite mixtures with either air or oxygen. It is nearly insoluble in water but is miscible with organic solvents. The nitrite also will decompose into valerie acid and nitric acid.
Nitroglycerin. Glyceryl trinitrate is the trinitrate ester of glycerol and is listed as available in tablet form in the United States Pharmacopoeia. It is prepared by carefully
adding glycerin to a mixture of nitric and fuming sulfuric acids. This reaction is exothermic, and the reaction mixture must be cooled to between 10 and 20°C. The ester is a colorless oil, with a sweet, burning taste. It is only slightly soluble in water, but it is soluble in organic solvents.
va.sodilating action and, because it is absorbed through the skin, is prone to cause headaches among workers associatod
with its manufacture. This transdcrmal penetration is why nitroglycerin is useful in a patch formulation. In medicine. it has the action typical of nitrites, but its action develops more slowly and is of longer duration. Of all the known coronary vasodilatory drugs, nitroglycerin is the only one capable of stimulating the production of coronary collateral circulation and the only one able to prevent experimental myocardial infarction by coronary occlusion. Previously, the nitrates were thought to be hydrolyzed and reduced in the body to nitrites, which then lowered the blood pressure. This is not true, however. The mechanism of
dilation of nitroglycerin through its formation of NO is dc scribed above.
Nitroglycerin tablet instability was reported in molded sublingual tablets.5 The tablets, although uniform when manS
ul'actured, lost potency both because of volatilization of ni. troglycerin into the surrounding materials in the container and intertablet migration of the active ingredient. Nitruglyc cnn may be stabilized in molded tablets by incorporating 'fixing" agent such as polyethylene glycol 400 or polyclh. ytenc glycol 4000.' In addition to sublingual tablets. the drag has been formulated into an equally effective lingual acnsol for parienis who have problems with dissolution of sublin. gaol preparations because of dry mucous membranes. Trans. dermal nitroglycerin preparations appear to be less effectise than other long-acting nitrates, as absorption from the skin is variable.
Diluted Erythrityl Tetranitrate, USP.
Erythritol tetra
nitrate. I ,2,3.4-butanetetrol. tetranitrate (R*, is the tetranitrale ester of erythritol and nitric acid. It is pie. pared in a manner analogous to that used for nitroglycenn The result is a solid, crystalline material. This ester is also very explosive and is diluted with lactose or other swtablr inert diluents to permit safe handling: it is slightly soluhi: in water and soluble in organic solvents.
H2C—0N02
HC—0N02
HC—0N02 H2C—0N02 HC—ONO, Nltroglycefln Transmucosal Translingual Oral Ointment Injection Transdermal
Nitrogard Nitrolinguat Nitrobid Nitrogtyn Nitroglyn Nitrobid IV Tridil Nitrodur Nitrodlsc Minitran Deponit Transderm'Nltro
Nitroglycerin is used extensively as an explosive in dynamite. A solution of the ester, if spilled or allowed to evapo-
rate, will leave a residue of nitroglycerin. To prevent an explosion of the residue, the ester must be decomposed by the addition of alkali. Even so. the material dispensed is so dilute that the risk of explosions does not exist. It has a strong
H2C—0N02 Erythrityl Tetranitrate (Carditate)
Erythrityl tetranitrate requires slightly more time than troglycenin to produce its effect, which is of longer It is useful when mild. gradual. and prolonged vascular diL tion is warranted. The drug is used in the treatment of, anda prophylaxis against, attacks of angina pectoris and toredxs
blood pressure in arterial hypertonia. Erythrityl tetranitrate produces a reduction of cardiac load as a result of pooling blood on the venous side of ik circulatory system by its vasodilating action. This actio results in a reduction of blood pressure on the arterial sct during stressful situations and is an important factor in venting the precipitation of anginal attacks.
Chapter 19 • Cardioiusc,dar AgeiiLv
Diluted Pentaerythritol Tetranitrate, USP.
30
Penta-
letranitrate. 2.2-bis (hydroxymclhyi)-J .3-propanediil telranitrate (Peritmle. Pentrirol). is a white. ciystalline material with a melting point of 140°C. It is insoluble in sater. slightly soluble in alcohol, and readily soluble in acetate. The drug is a nitric acid ester of the tetrahydric alcohol pentaerythritol and is a powerful explosive. Accordingly. it is diluted with lactose. mannitol, or other suitable inert dito permit safe handling.
627
0 V
30
(my)
-60 -90 TIME (meec)
4. 100
200
300
400
H2C—0N02 02N0H2C—C—CH20N02
H2C—0N02 Pentaerythrltot Tetranitrata (Pefllrate) (Pentrltol)
Itrelaxes smooth muscle of smaller vessels in the coronary
tree. Pentaerythritol tetranitrate is used prophylackallyto reduce the severity and frequency of anginul attacks is usually administered in sustained-release preparations to increase its duration of aCtion.
Diluted Isosorbide Dinitrate, USP. lsosorbide dinirite. l.4:3,6-dianhydro-n-glucitol dinitrate (Isordil, Sorbitule). occurs as a white, crystalline powder. Its water soluhility is about I mglmL.
Isosorbide Dinitrate (Isordil)
Isosortide Mononitrate OSMO lrndur)
This molecule Is lacking one of the nhtro substitutions
kosorbide dinitrate, as a sublingual or chewable tablet. is kfectivc in the treatment or prophylaxis of acute anginal .iixks. When it is given sublingually. the effect begins in 2 minutes. with a shorter duration of action than when us given orally. Oral tablets are not effective in acute angiepisodes: the onset of action ranges from 15 to 30 mm-
major mute of metabolism involves denitration to wsorbide5-mononitrate. This mctabolite has a much longer
than the parent isosorbide dinitrate. As such, this metaholite is marketed in a tablet form that has c.rcelleni bioavailability with much less first-pass metubosn than isosorbide dinitrate.
Ciidsm Antagonists
Figure 19—4 • Diagrammatic representation of the membrane action potential, as recorded from a Purkinje fiber, and an electrogram recorded from an isolated ventricular fiber. The membrane resting potential is 90 my relative to the exterior of the fiber. At the point of depolarization, there us a rapid change (phase 0) to a more positive value. 0—4 indicate the phases of depolarization and repolarization. Note that phases 0 and 3 of the membrane action potential correspond in time to the inscription of the QRS and T waves, respectively, of the local electrogram.
heart is mediated by two inwardly directed ionic Currenls. When the cardiac cell potential reaches its threshold. ion channels in the membrane are opened. and Na enters the cell through ion channels. These channels give rise to the fast sodium current that is responsible for the rapidly rising phase, phase 0. of the ventricular action potential (Fig. 194). The second current is caused by the slow activation of an L-Iype Ca2' ion channel that allows the movement of into the cell. This "slow channel" contributes to the maintenance of the plateau phase (phase 2) of the cardiac action potential. We now understand that the that enters with the action potential initiates a second and larger from the sarcoplasmic reticulum in the cell. release of is sufficient to initiate the This secondary release of contractile process of cardiac muscle. Contraction of cardiac and other muscle occurs from a reaction between actin and myosin. In contrast to smooth vascular muscle, the contractile process in cardiac muscle involves a complex of proteins (troponins I. C. and I and tropomyosin) attached to myosin. which modulates the interaction between actin and myosin. Free Ca2 ions bind to troponin C. uncovering binding sites on the actin molecule
and allowing interaction with myosin. causing contraction of the muscle. The schematic diagram in Figure 19-5 shows the sequence of events. 0 Contraction of vascular smooth muscle, like that of cardiac muscle, is regulated by the concentration of cytoplasmic Ca2' ions. The mechanism by which the contraction is effected, however, includes a cal-
cium- and calmodulin-dependent kinase as opposed to a Ca2 ' -sensitive troponin—tropomyosin complex (Fig. 19-2). The activating effect depends on a different type of reaction. The elevated free cytosolic Ca2 * in vascular smooth muscle cells binds to a high-affinity binding protein. calmodulin.
IXOTAT1ON-CONTRACTION COUPLING MUSCLE
tonulation of the cardiac cell initiates the process of excita-
which has been related to ion fluxes through the cell Depolarization of the tissue in the atria of the
ION CHANNELS AND CALCIUM
Calcium ions play an important role in the regulation of many cellular processes, such as synaptic transmission and
628
WiLw,j and Gi.c
STIMULUS
ld.n Textbook of Organic Medicinal and Pharn,aceatü'al Ciwnusu-v
tate movement of Ca2' ions from storage loci in the plasmic reticulum. Four types of calcium channels, differing in location and
•
j
——cell membrane (released from sarcoplasmic reclicijlum)
Troponin C
function, have been identified: (a) L type, located in skeletal. cardiac, and smooth muscles, causing contraction of muscle
cells; (b) T type. found in pacemaker cells, causing
Complex
entry, inactivated at more negative potentials and more rapidly than the L type; (c) N type, found in neurons and acting
Actin + Myosin Interaction
I Muscle Contraction Figure 19—S • Sequence of events showing excitation—contraction coupling in cardiac muscle.
in transmitter release; and (d) P type, located in Purkinje cells but whose function is unknown at this time. Calcium antagonists act only on the L-type channel to produce their pharmacological effects. The L channels arc so called because once the membrane has been depolarized.
their action is long lasting. Once the membrane has bexn depolarized, L channels must he phosphorylated to Although (here are similarities between L-type calcium channels that exist in cardiac and smooth muscle, there are
muscle contraction. The role of calcium in these cellular functions is as a second messenger, for example. regulating enzymes and ion channels. The entry of extracellular Ca2 + into the cytosol of myocardial cells and the release of Ca2 from intracellular storage Sites is important for initiating contractions of the myocaniium. Normally, the concentration of Ca2
in the extracellular fluid is in the millimolar range,
whereas the intracellular concentration of free Ca2
is less
distinct differences between the two. Cardiac L channels stimulation via a cAMP. are activated through dependent phosphorylation process. while L channels in smooth muscle may be regulated by the inositol phosphale system linked to G-protein—coupled. receptor-linked phis' pholipase C activation.'2 CALCIUM CHANNEL BLOCKERS
than may be
M, even though the total cellular concentration M or higher. Most of the Ca2 is stored within intracellular organdIes or tightly bound to intracellular proteins. The free Ca2 needed to satisfy the requirements of
The L-type calcium channel, acted on by calcium channel blockers. consists of five different subunits, designated a2, fi. y, and ô. The a1 subunit provides the central pore of the channel (Fig. 19-6). Calcium channel blockers can be
a contraction resulting from a stimulus may result from activation of calcium channels on the cell membrane and/or the release of calcium from bound internal stores. Each of these methods of increasing free cylosolic Ca2 involves channels that are selective for the calcium ion. Calcium channel blockers reduce or prevent the increase of free cytosolic calcium ions by interfering with the transport of calcium ions through
divided conveniently into the three different chemical
these pores.
Calcium is one of the most common elements on earth. Most calcium involved in biological systems occurs as hydroxyapatite. a static, stabilizing structure like that found in bone. The remaining calcium is ionic (Ca2 )• Ionic calcium functions as a biochemical regulator, more often within the cell, The importance of calcium ions to physiological functions was realized first by Ringer, who observed in 1883 the role of Co2 * in cardiac contractility. The ionic composition of the cytosol in excitable cells. including cardiac and smooth muscle cells, is controlled to a large extent by the plasma membrane, which prevents the free movement of ions across this barrier. Present in the membranes are ion-carrying channels that open in response to either a change in membrane potential or binding of a ligand. Calcium-sensitive channels include (a) Na + to Ca2 exchanger, which transports three Na ions in return for one Ca2 ; (b) a voltage-dependent Ca2 channel, which provides the route for entry of Ca2 for excitation and contraction in cardiac and smooth muscle cells and is the focus of the channel-blocking agents used in medicine; and (c) receptor-operated Ca2 channels mediated by ligand bind-
classes of the prototype drugs that have been used: phenylal.
kylaminc.s (verapamil). I ,4-dihydropyridines and benzothiazepines (diltiazem). These prototype pounds sometimes are termed the "first generation" of cal. cium channel blockers because two of the groups of dnrg classes have been expanded by the introduction of a "second" generation of more potent analogues (Table 19-3). The specific Ca2 channel antagonists verapamil, pine, and diltiazem interact at specific sites on the calcium channel protein. These blockers do not occlude the channel physically but bind to sites in the channel, as they can pro' mote both channel activation and antagonism. Affinity [ci binding sites on the channel varies, depending on the statu' of the channel. The channel can exist in either an open (0). resting (R), or inactivated (I) state, and the equilibrium Ire tween them is determined by stimulus frequency and merm
ing to membrane receptors as in the action of epinephrine on the a-adrenergic receptor. The membrane of the sarcolemma
Figure 19—6 • Schematic representation of an
within the cell also has ion-conducting channels that facili-
channel.
Ca"
Chapter 19 • ('ardjovu.wu!ur Ageisix
TABLE 19—3
First- and Second-Generation Calcium
Channel Blockers Chemical
Classification
rhythmic, antiunginal. and antihypertensive activity. They depress the cardiac neural network, and so slow sinus node
aucomaticity. prolong atrioventricular IAV) nodal conducFirst Generation
Second
Generation
Verapamil
Anipamil Bepridil
NlkdipinC
Amlodipine Fclodipine
Nicardipine Nimodipine
flniolbiaeepine
629
Diltiazem
—
b,ane potential (Fig. 19-7). Verapamii and diltiazem do not hind to a channel in the resting stale, only after the channel has been opened. They are ionized, water-soluble Ca2 + entry blockers that reach their binding sites by the hydrophilic pathway when the channel is open. Verapamil and diltiazem are use dependent (i.e.. iheir Ca2' -blocking activity is a function of the In2quency of contractions). An increase in contraction frequency causes a reduction, rather
tance. and depress myocardial contractility, as well as reduce peripheral vascular resistance to prevent a coronary vascular spasm. Nifedipine and other I .4-dihydropyridines are more
effective at causing vasodilation than affecting pacemaker and tension responses in the heart. This is especially important because selectivity occurs asaconsequence of disease states. Hypertensive smooth muscle is more sensitive to channel blockers than is normotensive tissue.'4 This makes verapamil and diltiazem more useful in ischemic conditions. as they have a more profound effect on cardiac muscle calcium channels,'5 The inhibition of Ca2' influx into cardiac tissue by Ca2 antagonists is also the basis for the use of these drugs us antiarrhythmic agents. The channel blockers dampen Ca2 'dependent automaticity in the regular pacemaker cells in the sinoalrial (SA) node and depress the origination of ectopic foci. Calcium antagonists can block reentry path-
ways in myocardial tissue, an integral component of arrhythmias. Numerous side effects in the heart, such as bradycardia, decreased cardiac contractility, and reduced AV conductance. are traced to Ca2 channel—blocking activity.
bait an augmentation. of contractions. Nifedipine is a neutral
molecule at physiological pH and can cause interference with the Ca2' in the open or closed state. In the closed state, nifedipine can traverse the phospholipid bilayer to reach its binding Site beCause of its lipid solubility. CARDIOVASCULAR EFFECTS OF CALCIUM ION CHANNEL BLOCKERS
antagonists yet developed are vasodilators. Vasodilation is due to the uncoupling of the contractile mechanism of vascular smooth muscle, which requires Ca2 . Corasary artery muscle tone is reduced in healthy humans but in particularly pronounced in a condition of coronary spasm. Peipheral arteriole resistance is reduced more than venous beds. The vasodilatory effect of these drugs is the basis for heir use in the control 01' angina and hypertension.'3 Although verapamil, nifedipine. and diltiaaiem can cause they are not equally effective at blocking the All Ca2
channels found in various tissues. The phenylalverapamil and the benzothiazepine dittiazem have hath cardiac and vascular actions. These drugs have antiar-
PRODUCTS
Verapamil. 5-13.4-dimethoxyphencthyl)Verapamll. nnethyiaminoj-2-(3.4-slimethoxyphenyl)-2-isopropylvaleronitriie (Calan. Isoptin). was introduced in 1962 as a coronary vasodilator and is the prototype of the Ca2' antagonists used in cardiovascular diseases. It is used in the treatment of angina pectoris. arrhythmias from ischemic tnyocardial syndromes, and supraventricular arrhythmias.
Verapamil's major effect is on the slow Ca2' channel. The result is a slowing of AV conduction and the sinus rate.
This inhibition of the action potential inhibits one limb of the reentry circuit believed to underlie most paroxysmal supraventricular lachycardias thai use the AV node as a reentry point. It is categorized as a class IV antiarrhythmic drug (see "Classes of Antiarrhythmic Drugs" below). Hemodynamicaily. verapamil causes a change in the preioad. afterioad. contractility, heart rate, and coronary blood flow. The drug reduces systemic vascular resistance and mean blood pressure, with minor effects on cardiac otttput.
Verapamil is a synthetic compound possessing slight
/ fIgure 19—7 • Schematic representation of an ion channel ?sntnng in an equilibrium of resting (R), open (0), and nactsstates.
structural similarity to papaverine. It can be separated into its optically active isomers, of which the levorotatory enantiomer is the most potent. It is absorbed rapidly after oral administration. The drug is metaholii.ed quickly and, as a result, has low bioavailahility. The liver is the main site of first-pass metabolism, forming several products. The preferential metabolic step involves N.dealkylation. followed by O-demethyiation. and subsequent conjugation of the product before elimination. The inelabolites have no significant bio-
logical activity. Verapamii has an elimination half-life of approximately 5 hours.
630
Wilson and Gisvaldx Textbook of Organic Medicinal and Plwrrnaceu:kal Chemistry
as
OCH3
a
—
a
—,.
CH2CH2N(CH3)2 OCI.13
CH2CH2N(CH3)2
CH2CH2N(CH3)2
Deacatytdilt'azem
Oiltiazem
CH3
Figure 19—8 • Biotransformations of diltiazem.
artery spasm and reduces myocardial oxygen demand decreasing heart rate and reducing overload. Diltiazem hy-
H3CO
CI
drochloride is used in patients with variant angina. The drug has electrophysiological properties similar to those of scmparnil and is used in clinically similar treatment conditions
as an antiarrhythmic agent. but it is less potent. The drug is absorbed rapidly and almost completely from
Verapamt (IsopUn) (CalarO OCH3
The route traveled by a Ca2 channel blocker, such as to its receptor site parallels that observed with many local anesthetic-like antiarrhythmic agents. It is believed that verapamil. like most of the channel blockers. crosses the cell membrane in an uncharged form to gain
access to its site of action on the intracellular side of the membrane. Data show a greater affinity of verapamil and other Ca2 channel blockers to the inactivated state of the channel.1"
the digestive tract. It reaches peak plasma levels within I hour after administration in gelatin capsules. Oral formula lions on the market are sustained-release preparations providing peak plasma levels 3 to 4 hours after administration. Diltiazem hydrochloride is metabolized extensively aftcr oral dosing, by first-pass metabolism. As a result, the bisavailability is about 40% of the administered dose. The drug undergoes several biotransformations. including deacetyla tion. oxidative 0- and N.demethylations, and conjugation of the phenolic metaboliles. Of the various metabolites (Fig 19-8). only the primary metabolite. deacetyldiltiazem. is pharmacologically active. Deacetyldiltiazenu has about 41)Ii'
50% of the potency of the parent compound.
Nifedipine.
Nifedipine. I .4-dibydro-2. 6-dimethyl4 (2-nitrophenyl)--3.5-pyridinedicarboxylate dimethyl
(Adalat. Procardia). is a dihydropyridine derivative th.i bears no structural resemblance to the other calcium antago-
nists. It is not a nitrate, but its nitro group is essential Im its antianginal effect.'7 As a class, the dihydmpyridines pa sess a central pyridine ring that is partially saturated. positions 2 and 6 are substituted with an alkyl group that play a role in the agent's duration of action. Also. podtion and 4 are carboxylic groups that must be protected with a ester functional group. Depending on the type of ester tort CI.
at
these sites, the agent can be distributed to various pais
of the body. Finally, position 4 requires an aromatic subsinu
Diltiazem Hydrochloride.
Diltiazem hydrochloride. (+)-cis-3-(acetoxy)-5-12(dimcthylamino)ethyll-2.3-dihydro2-(4-methoxyphenyl) I ,5-benzothiazepin-4(511)one hydrochloride (Cardizem). was developed and introduced in Japan
as a cardiovascular agent to treat angina pectoris. It was observed to dilate peripheral arteries and arterioles. The drug increases myocardial oxygen supply by relieving coronary
tion possessing an electron-withdrawing group (i.e.. Cl si NO2) in the oriho and/or me:a position. The prototype of this class. nifedipine, has potent eral vasodilatory properties. It inhibits the voltage.depoidcrl calcium channel in the vascular smooth muscle but has litik
or no direct depressant effect on the SA or AV nodcs, ma
though it inhibits calcium current in normal and
Chapter 19 • Cardimacculur Agrn:s
çIL
çJL
NO2
NO2
NO2
CH3
C=O
I-13C00
COOH
H3COOC N
631
CH3
CH2OH
N
/
CH2
Figure 19—9 • Nifedipine metabolism.
in patients
cardiac tissues.
whose anginal episodes are due to coronary vasospasm and
used in the treatment of vasospastic angina as well as classic angina pectoris. Because of its strong vasodilatory it is used in selected patients to treat hypertension. H,C
H3c0_fj
0
I
Felodipine.
Felodipine, 3.5-pyridinedicarboxylic acid. 4-(2,3-dichlorophenyl) I ,4-dihydro-2,6-dinnethyl-, ethyl methyl ester (Plendil). is a second-generation dihydropyridine channel blocker of the niledipine type. It is more selective for vascular smooth muscle than for myocardial tissue and serves as an effective vasodilator. The drug is used in the treatment of angina and mild-to-moderate essential hypertension. Felodipine, like most of the dihydropyridines. exhibits a high degree of protein binding and has a half-life ranging from 10 to 18 hours.
0
H3C
N
C—OCH2CH3
Nitedipine (Procardia)
0
0 Nikdipine is absorbed efficiently on oral or huccal adminstration. A substantial amount (90%) is protein bound. Sys— availability of an oral dose of the drug may be approxiTwo inactive nietabolites are the major matcly products of nifedipine metabolism and are thund in equilibwith each other (Fig. 19-9). Only a trace of unchanged in the urine.'5
Felodipine (Plendit) Ci
isradipine. Isradipine. 4-(4-benzofuranaiyl I-I .4-dihydro-2,6-dimethyl-3.5-pyridinecarboxylic acid methyl I methylethyl ester (DynaCire), is another second-generation
dihydropyridine-type channel blocker. This drug, like the Amlodipine. 2-Il 2-aminoethoxy)methylj- I A-dihydro-6-methyl-3,5-pyridinediacid 3-ethyl 5-methyl ester (Norvasc). is a seeod-gener.ition I .4-dihyropyridine derivative of the proto-
Amiodlpine.
other second-generation analogues, is more selective for vas-
cular smooth muscle than for myocardial tissue. It is effective in the treatment of stable angina, reducing the frequency of anginal attacks and the need to use nitroglycerin.
molecule nitèdipine. I_ike most of the second-generationdihydropyridine derivatives, it has greater selectivity for vascular smooth muscle than myocardial tissue, a longer di-life (34 hours), and less negative inotropy than the pro-
CH,
x
nifedipinc. Amlodipine is used in the treatment of stable angina and in the management of mild-to-
C—O—CH2—CH
0
0
mxlcrate essential hypertension. It is marketed as the ben,aro sulfonic acid salt (besylate).
—\
CH3
N
H
OCH2CH2NI42
tsradlplne (DynaClrc)
0
Amlodipine (Norvasc)
Nicardipine Hydrochloride.
Nicardipine hydrochloI ,4-dihydro-2.6-dimethyl-4-(3-nitrophenyl)-3.5-pyridinedicarboxylic acid methyl 2-tmethyl(phenylmethyl)aminojethyl ester hydrochloride (Cardene). is a more potent vasodilator of the systemic, coronary, cerebral, and renal ride,
vasculature and has been used in the treatment of mild, moderate, and severe hypertension. The drug is also used in the management of stable angina.
632
of Orç'anic Medicinal wul Pliarrnacei,iieal CYw,ni.czrv
Wilson and Gisvold's
ester (Baypress). is a second-generation dihydropyridine channel blocker of the nifedipine type. It is more selectise
H3C
for vascular smooth muscle than for myocardial tissue iisJ serves as an effective vasodilator. The drug is used in the treatment of mi Id-to-moderate essential hypertension.
NO2
(Cardene)
g
ii
Niinodipine. I .4-dihydro-2,6-dimethyl-4Nimodipine. (3-nitrophenyl)- 3.5-pyridinedicarboxylic acid 2-methoxyethyl I -methylethyl ester (Nimotop). is another dihydropyridine calcium channel blocker but differs in that it dilates the cerebral blood vessels more effectively than do the other dihydropyridine derivatives. This drug is indicated for treatment of subarachnoid hemorrhage-associated neurological deficits. H
NO2
(Baypress)
Bepridil Hydrochloride.
Bepridil hydrochloride. R2-methylpropoxy)methyl l-N-phenyl-N-(phenylmethyl'I pyrrolidineethylamine hydrochloride (Vascor). is a secondgeneration alkylamine-type channel blocker. structurally to the dihydropyridines. Its actions are less speciik than those of the three prototypical channel blockers, pamil. dihiazem, and nifedipine. In addition to channel blocker, it inhibits sodium flow into the heart and lengthens cardiac repolarization. causing bradycanlia. Caution should be used if it is given to a patient with hypoka. lemia. Bepridil hydrochloride is used for stable angina. The drug has a half-life of 33 hours and is highly hound to protein
(99%).
Nimodiptne
In vitro studies show that the effects of
Nisoldipine. nisoldipine.
CH-CI-4,OCH2-CH—CH2—N H2C'
I .4-dihydro-2. 6-dimethyl-4-(2-nitrophenyl)-
3.5-pyridinecarboxylic acid methyl 2-methylpropyl ester (Sular). on contractile processes are selective, with greater potency on vascular smooth muscle than on cardiac muscle. Nisoldipine is highly metabolized, with five major metab-
BepñdII (Vascor)
olites identified. As with most of the dihydropyridines, the cytochrome P450 (CYP) 3A4 isozyme is mainly responsible for the metabolism of nisoldipine. The major biotransformation pathway appears to involve the hydroxylation of the isobutyt ester side chain. This particular metabolite has approximately 10% of the activity of the parent compound. c512
—
C—0
°
It
\
C512—CH
0
—
Agents Platelet activation and platelet aggregation play an imponani role in the pathogenesis of thromboses. These, in turn, an important role in unstable angina. myocardial infaretis stroke, and peripheral vascular thronthoses. Since many cardiovascular diseases are associated with platelet activation. many agents possessing antiplatelet or antithrotnbotic have been investigated. This has revolutionized cardiovascular medicine, in which vascular stenting or angioplasty car. be used without compromising normal hemostasis or wound
healing. Although most of these agents act by differen mechanisms, many of the newer agents are being to antagonize the GPlIh/llla receptors of platelets.
Nisotdipine (Sular)
Nitrendipine, I ,4-dihydro-2,6-dimethylNitrendipine. 4-(3-nitrophenyl)-3,5-pyridinecarboxylic acid methyl ethyl
Aspirin.
Aspirin. acetylsalicylic acid, has an effect on platelet aggregation not only because of its to inhibit cyclooxygenase hut also because of its ability to acetylate the enzyme. Aspirin irreversibly inhibits cyekwnygenase (COX) (prostaglandin H synthase), which is the enzyme involved in converting arachidonate to prostaglundir
Chapter 19 • Cardiovascular Agesiis
633
and ultimately thromboxane 2. an inducer of platelet agpregation. Aspirin's mechanism of action includes not only the inhibition in the biosynthesis of thromboxane 2. but also
possessing this system have been evaluated as potential antithrombotic agents. These agents have a unique mechanism.
its ability to acetylate the serine residue (529) in the polypeptide chain of platelet prostaglandin H synthetase-l. This explains why other nonsteroidal anti-inflammatory agents that
lets. Normally. nucleotides act as agonist.s on these receptors,
are capable of inhibiting the COX enzyme do not act as
ADP. cause platelet aggregation. Clopidrogel acts as an antagonist to the P2Y2 receptor. It is probably a prodrug that
antithrombotics—thcy aren't capable of acetylating this enzyme. Since platelets cannot synthesize new enzymes, aspirin's ability to acetylate COX lasts for the life of the platelet (ito 10 days) and is. thus, irreversible.
in that they inhibit the purinergic receptor located on plate-
which include the P2Y type. Two P2Y receptor subtypes (P2YI and P2Y2) found on platelets, when stimulated by requires metabolic activation, since in vitro studies do not interfere with platelet aggregation. Although platelet aggre-
gation is not normally seen in the first 8 to II days after administration to a patient, the effect lasts for several days after the drug therapy is discontinued. Unlike other thienopyridine.s currently used, clopidrogel does not seriously reduce the number of white cells in the blood, and therefore. routine monitoring of the white blood cell count is not necessary during treatment.
Aspinn
Dipyridamole.
Dipyndamole.
CH—
2.2',2",2"-[(4.8-di- I -
)dinitrilolteaakisethanol (Persantine). may be used for coronary 2nd myocardial insufficiency. Its biggest use today, however. is as an antithronthotic in patients with prosthetic heart
valves. It is a bitter, yellow, crystalline powder, soluble in dilute acids, methanol, or chloroform. A formulation constifling dipyridamole and aspirin (Aggrenox) is currently king marketed as an antithromobotic. Dipyridamole isa long-acting vasodilator. Its vasodilating action is selective for the coronary it is indicated for long-term therapy of chronic angina pectoris. The drug 2150 inhibits adenosine deaminase in erythrocytes and interferes with the uptake of the vasodilator adenosine by erythro-
zytes. These actions potentiate the effect of prostacyclin which acts as an inhibitor to platelet aggregation.
Cloptdrogel O'Iavix)
Tidopidine.
Ticlopidine, 5-t(2-chlorophenyl)methyll4,5.6.7-tetrahydrothieno hydrochloride 13.2-cipyridine (Ticlid). is useful in reducing cardiac events in patients with unstable angina and cerebmvascular events in secondary prevention of stroke. It belongs to the thienopyridinc class and facilitated the development of clopidrogel. One of the drawbacks to this agent is its side effect profile, which includes neutropenia, and patients receiving this antithrombotic should have their blood levels monitored. Its mechanism
of action is similar to that of clopidrogel, in that it inhibits the purinergic receptors on platelets.
CH2
/
Ttclopidire (Ttcttd)
GPIIB/lllA RECEPTORS Oipyfldamole
(Persantine with ASA Aggrenox)
clopidrogel. Clopidrogel. methyl (+ )-( S)-a-(2-chlosphenyl)-6.7-dihydmthienol3.2-cI pyridine-5(4H)-acetate sullate (Plavix), is useful for the preventative management of
ischemic events, including tnyocardial infarction. viroke, and vascular deaths. II may be classified as a thienop)ndinc because of its heterocyclic system. Several agents
Located on platelets is a site that serves to recognize and bind
fibrinogen. This site is a dimeric glycoprotein that allows fibrinogen to bind, leading to the final step of platelet aggregation. The receptor must be activated before it will associate with fibrinogen. and this may be accomplished by thrombin, collagen, or thromboxane A2. Once the receptor is activated. fibrinogen most likely binds to the platelet through the arginine-glycine.aspartic acid (ROD) sequences at residues 95-
96-97 and 572-573-5 74 of the a chain of tibrinogen. This particular feature has been used in the design of nonpeptide
634
lViLcon and
Textbaok of Organic Medicinal and Pharn,aceu.'ical Chemistry
Eptifibalide (lntegnlin)
antagonists that mimic the RGD system in which a distance of 15 to l7A (16 to 18 atoms) separates the amine group of arginine and the carbonyl oxygen of aspartic acid.
Abcixlmab. Abcixinnab (ReoPro) is a chimeric fragment monoclonal antibody that can bind to GPlla
Eptifibatide.
aggregation. Abciximab is useful in treating unstable aagln and as an adjunct to percutaneous coronary intervenhiun (PCI). The half-life of abeiximab is about 30 minutes. whik its effects when bound to the GPllaJlIlb may last up no 24 hours. A significant drawback to using abciximah lies in cost, which is approximately $ 1.5(X) for a single dose.
Eptifibatide (Integrilin) is a synthetic cyclic heptapeptide that acts as a GPIlb/llIa receptor antagonist. thus causing inhibition of platcict aggregation. Its struc-
ture is based on the natural product barbourin. a peptide isolated from the venom of a pygmy rattlesnake (Sistrurus mnilarud barbouri). As part of the structure, there is a sequence arginine-glycine-aspartic acid (RGD) that can bind to the RGD receptor found on platelets and block its ability to bind with fibrinogen. This agent is used in the treatment of unstable angina and for angioplastic coronary interventions.
lllb receptor of platelets and block the ability of librinogen to associate with the platelet. This results in less platckt
ANTIARRHYTHMIC DRUGS
Tlrofiban.
Tirofiban is a nonpeptide that appears unrelated chemically to eptifibatide. but actually has many similarities. The chemical aithitecture incorporates a system that
is mimicking the arginine-glycine-aspartic acid (RGD)
Cardiac arrhynhmias are caused by a disturbance in the con
duction of the impulse through the myocardial tissac. b1 disorders of impulse formation, or by a combination of
moiety that is present in eptifibatide. This can be seen in the distance between the nitrogen of the piperidine ring, which mimics the basic nitrogen of urginine in the RGD sequence. and the carboxylic acid, which mimics the acid of usparnate in the RGD sequence. The basic nitrogen and the carboxylic acid of tirofuban arc separated by approximately 15 to 17A (16 to 18 atoms). This is the optimum distance seen in the RGD sequence of the platelet receptor. Tirofiban is useful in treating non—Q wave myocardial infarction and unstable
factors. The antiarrhythmic agents used most commonly atfect impulse conduction by altering conduction velocity and the duration of the refractory period of heart muscle tissue
angina.
pranolol. phcnylephrine. edrophonium. and neostigmine, act
They also depress spontaneous diastolic causing a reduction of automaticity by ectopic foci. Many pharmacological agents are available for the uean• ment of cardiac arrhythmia.s. Agents such as oxygen, sium, and sodium bicarbonate relieve the underlying
of some arrhythmias. Other agents. such as
0
lirotiban (Aggrastat)
0
0
Chapter 19 U C'ardk,i'ust',,lar
in the cardiovascular system by affecting heart muscle or that alter the electrophysiological mcchanisms causing arrhylhmia.s. The latter group of drugs is discussed in this chapter.
poocainamide. and quinidiiie. to help evaluate the pharmacokinetics of these agents. As a result, it is possible to maintain
plasma levels of these drugs. which allows the clinician to use these and other agents more effectively and with greater safety. No other clinical intervention has been more effective at reducing mortality and morbidity in corntars care units.
635
1.0
on the aut000muc nerves to the heart. Finally. there are drugs
Within the past five decades. research on normal cardiac licsues and, in the clinical setting, on patients with disturbances of rhythm and conduction has brought to light infornation on the genesis of cardiac arrhythrnias and the mode of action of antiarrhythmic agents. In addition, laboratory lois have been developed to measure blood levels of antiarrhythmic drugs such as phenyoin. disopyramide. lidocaine.
,%5teflL%
1Ukk It1 LIN
0,5-
I
I
E
0.
1•'
-0.5.-
DURATION ,
0
0.2
I
0.4
0.6
sec Figure 19—10 • Normal electrocardiogram. (From Ganong, W. F.: Review of Medical Physiology. 9th ed. San Francisco, Lange Medical Publications. 1985.)
Cardiac Electrophyslology The heart depends on the synchronous integration of electrical impulse transmission and myocardial tissue response to carry out its function as a pump. When the impulse is released from the SA node, excitation of the heart tissue takes
place in an orderly manner by a spread of the impulse throughout the specialized automatic fibers in the atria. the node, and the Purkinje fiber network in the ventricles. This spreading of impulses produces a characteristic electropattern that can be equated to predictable myoardial cell membrane potentials and Na and K * fluxes in and out of the cell. 1W
A single fiber in the ventricle of an intact heart during he diastolic phase (see phase 4. Fig. 19-4) has a membrane potential (resting potential) of 90 mV. This potemiul is crctrcd by differential concentrations of K and Na • in the intracellular and exiracellular fluid. An active transport sysem (pump) ()fl the membrane is responsible for concentrat-
inside the cell and maintaining higher concenin the extracellular fluid. Diastolic depolarization is caused by a decreased K ionic current into the coracellular tissue and a slow inward leakage of Na until the threshold potential (60 to 55 mV) is reached. At this nine the inward sodium current suddenly increases, and a nut ions of Na
.dI.propagatcd wave occurs to complete the membrane depolarization process. Pacemaker cells possess this property. shich is termed auimnatidgv. This maximal rate of depolarization (MRD) is represented by phase 0 or the spike action potential (Fig. 19-4).
is caused by an influx of' C1 ions. During phase 2, a small inward movement of Cay' ions occurs through a slow channel mechanism that is believed to be important in the process of coupling excitation with contraction. The process of repolarization determines the duration of the action potential and is represented by the QT interval. The action potential duration is directly related to the refractory period of cardiac muscle.
Mechanisms of Arrhythmlas The current understanding of the electrophysiological mechanisms responsible for the origin and perpetuation of cardiac arrhythmias is that they arc due to altered impulse formation
(i.e., change in automaticity). altered conduction, or both. acting simultaneously from different locations of the heart. The generation of cardiac impulses in the normal heart is usually confined to specialized (issues that spontaneously depolarize and initiate the action potential. These cells are located in the right atrium and arc referred to as the SA node or the pacemaker cells. Although the spontaneous electrical depolarization ot' the SA pacemaker cells is independent of the nervous system, these cells are innervated by both sympathetic and parasympathetic fibers, which may cause an increase or decrease of the heart rate, respectively. Other special cells in the normal heart that possess the property of' automaticity may influence cardiac rhythm when the normal
The form, duration, resting potential level, and amplitude the action potential are characteristic fur different types myocardial cells. The rate of rise of the response (phase It is related to the level of the membrane potential at the of stimulation and has been termed nwinbrane respon!oflt'S,C. Less negative potentials produce smaller slopes of $tase 1) and are characterized by slower conduction times. Tie phase 0 spike of the SA node corresponds to the macriphan of the P wave on the electrocardiogram (Fig. 19-10).
pacemaker is suppressed or when pathological changes occur in the myocardium to make these cells the dominant source of cardiac rhythm (i.e., ectopic pacemakers). Auto-
is divided into three phases. The greatest
found in special atrial cells, certain AV node cells, bundle of His, and Purkinje fibers. may lead to cardiac arrhythmias. Because production of ectopic impulses is often due to a
amunt of repolarization is represented by phase 3. in which hrc is a passive flux of K • ions out of the cell. Phase I
maticity of subsidiary pacemakers may develop when myo-
cardial cell damage occurs because of infarction or from digitalis toxicity, excessive vagal tone, excessive cateeholamine release from sympathomimetic nerve fibers to the heart, or even high catecholamine levels in plasma. The development of automaticity in specialized cells, such as that
AA
636
Wilson and Gi,si'old'x Textbook of Organic Medicinal and Pharmaceutical
a
Figure 19—11 • Reentry mechanism of Purkinje fibers, a. Normal conduction of impulses through triangular arrangement of cardiac fibers. b. Unidirectional block on left arm of triangular section allows impulse to reenter the regional conducting system and recycle.
defect in the spontaneous phase 4 diastolic depolarization ("T wave"), drugs that can suppress this portion of the cardiac stimulation cycle are effective agents for these types of arrhythmia. Arrhythmias arc also caused by disorders in the conduction of impulses and changes in the refractory period of the myocardial tissue. Pharmacological intervention is based on these two properties. The Purkinje fibers branch into a network of interlacing fibers, particularly at their most distant positions. This creates several pathways in which a unidirectional block in a Localized area may establish circular (circus)
microcellular or macrocellular impulse movements that reenter the myocardial fibers and create an arrhythmia (Fig. 19-Il). Unidirectional block results from localized myocardial disease or from a change in dependence of the tissue to Na fluxes that causes a longer conduction
time and allows the tissue to repolarize to propagate the retrograde impulse.
Classes of Antlarrhythmlc Drugs Antiarrhythmic drugs can be placed into four separate classes, based on their mechanism of action or pattern of electrophysiological effects produced on heart tissue. Table 19-4 summarizes the four-part classification of antiarrhythmic drugs as first proposed by Vaughan Williams in and expanded in 1984.0 Note that drugs within the same
TABLE 19-4 Classes of Antiarrhythmic Drugs Drugs
Class
MechanIsm of ActIon
IA
Quinidine, procalnamidc.
Lengthena re(ntmory period
lB
Lidocaine, phenysoin. tocainide. mexiletinc
Shortens duration o( action polcntial
IC
Encainide, (lecauiidc. lcircainidc. moricir.inc.
Slows conduction
propafenone
blockers
II
(e.g.. propranolol)
Ill
Amiodarone. brntylium.
IV
Calcium channel blockers
Slows AV conduction time. siIpprcssct. autonlaticity
clinical actions. That patients do not respond to a drag in this class, however, should not rule out use of other in the same class.2' Despite the well-intentioned use of thcae
agents. most antiarrhythmic drugs have the potential to gravate the arrhythmia they treat (proarriuvt/unia). Pourrhythmia develops from an increase in the density of singlr ectopic beats and is more likely to occur in patients who luic a dysfunction in the left ventricle or sustained ventncular tachycurdia. Class I antiarrhythmic agents (see below) especially proarrhythmic in myocardial infarction patients. CLASS I. MEMBRANE-DEPRESSANT DRUGS
Class I antiarrhythmic agents are drugs that have stabilizing properties (i.e.. they shirt membranes to marc negative potentials). Drugs in this class act on the fast Na' channels and interfere with the process by which the depolar
izing charge is transferred across the membrane. It is as sumed that these drugs hind to the Na channel and hhek its function, preventing Na conductance as long as the drug is bound. The prototypical drugs in this class are quinidire and procainamide. During the l970s. several drugs urn studied for their antiarrhythmic effects. Most of thent war local anesthetics that affected Na4 membrane channels, ash they were grouped in a single class (class I). Studies on
antiarrhythmic properties of these chemicals have shown that there are sufficient differences to place them into rate subgroups.2t Class I antiarrhythmic drugs can be subdivided on tic basis of the relative ease with which they dissociate [toni the Na' ion channel. Drugs in class IC, such as lorcainide and moricizine. are the most potent sodium dun nd—blocking agents of the class I antiarrhythmic
They slowly dissociate from the Na
channel. causing t
slowing of the conduction time of the impulse through it: heart. Class LB drugs, which include lidocaine. locainida and mexiletine. dissociate rapidly from the Na' ehwind' and thus have the lowest potency as sodium
channel bkcL
ers. They produce little, if any, change in action polenthi duration. Quinidine, procainamide. and disopyramidc art drugs that have an intermediate rate of dissociation (too Na + channels. These arc categorized as class IA mic agents, and they lengthen the refractory period of tissue to cause cessation of arrhythmias.22 Studies have shown that Na channels on the memboacs of Purkinje fiber cells normally exist in at least three slabs
R. rested, closed near the resting potential but able is k opened by stimulation and depolarization; A. activated. lowing Na' ions to pass selectively through the niembraro and I, inactivated and unable to be opened (i.e.. machoc)' The affinity of the antiarrhythmic drug for the receptor the ion channel varies with the state of the channel orwiit the membrane potential. Because of this. R. A. and I channels can have different kinetics of interaction with aa arrhythmic drugs. A review of the recent literature shen' that the antiarrhythniic drugs have low aftinity for R char
nels but relatively high affinity for the A or I channels'
sotalol
(ca.. scrapamil. dilliazeni)
category are placed there because they demonstrate similar
Rlock.s slow inward Ca2' channel
both. Regardless of which channel state is blocked by cli'
I antiarrhythmic drugs. the unblocking rate directly mines the amount of depression present at normal hean rarni
Chapter 19 U C'ardiova.ceular Agents
CI.ASS II.
BLOCKING AGENTS
blocking drugs cause membrane-stabilizing or depressant effects on myocardial tissue. Their anciarrhythsic properties. however. are considered to be principally the result of inhibition of adrcnergic stimulation o the heart. The principal electrophysiological property of these /3-blocking
jents is reduction of the phase 4 slope of potential sinus orectopic pacemaker cells such that the heart rate decreases and ectopic tachycardiar. arc either slowed or converted to sinus rhythm.
637
partition in the membrane as readily, onset of these drugs'
action would be delayed. Furthermore, concentration of these drugs in the membrane would be reduced. Therefore. drugs that act on the channel only in the inactivated (closed)
state would have a reduced effect in acidotic conditions. Acidosis may also prolong the effect of these drugs. External acidosis facilitates proronation of receptor-bound drugs. Be-
cause only neutral drugs can dissociate from closed channels, recovery is prolonged by acidosis. Alkalosis tends to hyperpolarize the cell membrane and. thereby, reduces the effect of antiarrhymhmic drugs. Because of this. alkalosis promotes the thrmation of more of the free-
CLASS Ill. REPOLARIZATION PROLONGATORS
Drugs in this class (e.g., amiodarone. bretylium. sotalol. ibuttide. dofetilidc) cause several different electrophysiological changes on myocardial tissue hut share one common effect. prolonging the action potential, which incrca.ses the effective re&ricrory period of the membrane action potential without altering the phase of depolarization or the resting membrane potential. Drugs in this class produce their effects by more ban one mechanism. Sotalol is a K channel blocker and has sonic /3-adrenergic blocking properties.24 Anmiodarone
aid brctylium. drugs that also prolong the action potential means that arc unclear, also have Na channel—blocking properties.
CLASS IV. CALCIUM CHANNEL BLOCKERS
channel blockers possess antiarrhythsic activity, some members of this class of antiarrhythmic drugs (verapamil. diltiaeem) block the slow inward current ions during phase 2 of the membrane action potenjul in cardiac cells. For example. the prototypical drug in his group. verapamil, selectively blocks entry (if Ca2 into Although not all Ca2
base antiarrhythmic agent. increasing the rate of recovery 1mm the block. Alkalosis.inducing salts such as sodium lactate have been used to counteract toxicity caused by the antiarrhythrnic quinidine. CLASS I ANTIARRHYTHMICS
Qulnldine Sulfate, USP. Quinidine sulfate is the sulfate of an alkaloid obtained from various species of Ci,whono and their hybrids. It is am dextrorotatory diastereolsomer of quinine. The salt crystallizes from water as the dihydrate. in the form of fine, needle-like, white crystals. Quinidine sulfate contains a hydroxymethyl group that serves us a link
between a quinoliiie ring and a quinuclidine moiety. The structure contains two basic nitrogens. of which the quinuclidine nitrogen is the stronger base lOt. Quinidine sulfate is bitter and light sensitive. Aqueous solutions are nearly neutral or slightly alkaline. It is soluble to the extent of 1% in water and more highly soluble in alcohol or chloroform.
the myocardial cell. It acts on the slow-response fibers found n the sinus node arid the AV node, slowing conduction yekicity and increasing refractoriness in the AV node.
pH and Activity The action of class I local anesthetic-type anriarrhyrhmic drugs is pH dependent and may vary with each drug.2° Antivalarnhythniic drugs are weak bases, with most having
zs ranging from 7.5 to 9.5. At physiological pH of 7.40. hese ba.scs exist in an equilibrium mixture consisting of both
free base and the cationic form. Ionizable drugs, such a lidocaine (pK,, 7.86). have stronger electrophysiological cifecic in ischemic rather than normal nsyocardial cells. This pomenriation has been attributed in part to the increase in It
concentration within the isehemic areas of the heart.
Acidosis increases the proportion of Na
ion channels occu-
nd by the prolonated form of the antiarrhythmic agent. Nesenheless. the effect of pH on the antiarrhythmic activity 1 drugs can be complex, as both the free base and cationic have been proposed as the active form of some drugs. The uncharged form of the Na' channel blocker can pene-
Oulnidtne (Cardioquin) (Quirmora)
(Aulnidex)
Quinidine sulfate is the prototype of antiarrhychmic drugs
and a class IA antiarrhythmic agent according to the Vaughan Williams classification. It reduces Na current by binding the open ion channels (i.e.. state A). The decreased entry into the myocardini cell depresses phase 4 dia. stolic depolarization and shifts the intracellular threshold potential toward zero. These combined actions diminish the spontaneous frequency of pacemaker tissues, depress the au-
r41e directly front the lipid phase of the surrounding cell to block the channel. Small changes in pH can alter these drugs' effectiveness
tomamicity of ectopic foci, and, to a lesser extent, reduce impulse formation in the SA node. This last action results in bradycardia. During the spike action potential. quinidinc
changing the charged-to-uncharged molecular ratio in the
sulfate decreases cransmembrane permeability to pa.ssivc influx of Na . thus slowing the process of phase 0 depolarizalion, which decreases conduction velocity. This is shown as
nyocardial cells. Acidosis external to the myocardial cell promotes the cationic form. Because this species does not
638
Wilson and
's Tt'x:book
of Organic Medicinal and l'lzarn,aeeutical Clw,nistrv
a prolongation of the QRS complex of electrocardiograms. Quinidine sulfate also prolongs action potential duration. which results in a proportionate increase in the QT interval. It is used to treat supraventricular and ventricular ectopic arrhythmia.s, such us atrial and ventricular premature beats. atrial and ventricular tachycardia. atrial tiuner. and atrial fibrillation.
more stable in water than is procaine. Aqueous solutions procainamide hydrochloride have a pH of about 5.5. A I netic study of the acid-catalyzed hydrolysis of procainanhi hydrochloride showed it to be unusually stable to in the pH range 2 to 7. even at elevated CH2CH3
—N"
Quinidine sulfate is used most frequently as an oral prepa-
ration and is occasionally given intramuscularly. Quinidine sulfate that has been absorbed from the gastrointestinal tract
or from the site of intramuscular injection is bound 80% to
serum albumin.°' The drug is taken up quickly from the bloodstream by body tissues: consequently, a substantial concentration gradient is established within a few minutes.
Onset of action begins within 30 minutes, with the peak effect attained in I to 3 hours. Quinidinc is metabolized primarily in the liver by a small amount is excreted by the liver.2 Because of serious side effects and the advent of more effective oral antiarrhythmic agents. quinidine is now used less, except in selected patients for long-term oral antiarrhythmic therapy.
Quinidine Gluconate. USP.
Quinidinium gluconate (Duraquin. Quinaglute) occurs as an odorless, very hitter. white powder. In contrast with the sulfate salt, it is freely soluble in water. This is important because there are emergencies when the condition of the patient and the need for a rapid response make the oral route of administration inappropriate. The high water solubility of the gluconate salt along with a low irritant potential makes it valuable when an injectable form is needed in these emergencies. Quinidine
gluconate forms a stable aqueous solution. When used for injection, it usually contains 80 mg/mL. equivalent to 50 ing of quinidine or 60 mg of quinidine sulfate.
Procainamide (Pronestyt) (Procan SR)
Procainamidc hydrochloride is metabolized through 1 action of N-acetyltransferasc. The product of enzymatic nt
taboiisin of procainamide hydrochloride is namide (NAPA). which possesses only 25% of the activr of the parent compound.25 A study of the disposition airs cainamide hydrochloride showed that 50% of the drug excreted unchanged in the urine, with 7 to 24% Unlike quinidine. procainamide hydrochlonde. hound only minimally to plasma proteins. Between 75 an 95% of the drug is absorbed from the gastrointestinal trac Plasma levels appear 20 to 30 minutes after and peak in about I hour.29 Procainamide hydrochloride appears to have all of th electrophysiological effects of quinidine. It diminishes art maticity, decreases conduction velocity, and increases actis potential duration and. thereby, the refractory period of cardial tissue. Clinicians have favored the use of mide hydrochloride For ventricular tachycardias and dine for atrial arrhythmias. even though the two drugs a
effective in either type of disorder.
Disopyramide Phosphate. USP.
Quinidine Polygalacturonate.
Quinidinc polygalacturonate (Cardioquin) is formed by reacting quiiiidine and polygalacturonic acid in a hydroalcoholic medium, It contains the equivalent of approximately 60% quinidine. This salt is only slightly ionized and slightly soluble in water. but studies have shown that although equivalent doses of quinidine sulfate give higher peak blood levels earlier, a more uniform and sustained blood level is achieved with the polygalacturonate salt. In many patients, the local irritant action of quinidinc sulfate in the gastrointestinal tract causes pain, nausea, vomit-
ing, and especially diarrhea, often precluding oral use in adequate doses. Studies with the polygalacturonate salt yielded no evidence of gastrointestinal distress. It is available as 275-mg tablets. Each tablet is the equivalent of 200 mg of quinidine sulfate or 166 mg of free alkaloid.
bound approximately 50% to plasma protein and has a h21(
life of 6.7 hours in humans. More than 50% is excicic, unchanged in the urine. Therefore, patients with renal ficiency should be monitored carefully for evidence alma dosc. Disopyramide phosphate commonly exhibits side ct fects of dry mouth, constipation, urinary retention, and riki cholinergic blocking actions because of its structural sits
larity to anticholinergic drugs.
Pro cainamide Hydrothioride. USP.
Procainamide hydrochloride, p-amino-N-12-(diethylamino)ethyl Ibenzaniide
monohydrochioride, procainainidium chloride (Pronestyl. Procan SR). has emerged as a major antiarrhythmic drug. It was developed in the course of research for compounds structurally similar to procaine, which had limited effect as an antiurrhythmic agent because of its central nervous system (CNS) side effects and short-lived action due to rapid hydrolysis of its ester linkage by plasma esterases. Because
of its amide structure, procainamide hydrochloride is also
Disopyramide
phate. a-12(diisopropylamino)ethyl I-a-phenyl-2-pyddinn cctamide phosphate (Norpace). is an oral and class IA antiarrhythmic agent. It is quite similar to quinidis and procainamide in its electrophysiological that it decreases phase 4 diastolic depolarization. conduction velocity, and has vagolytic It is us' clinically in the treatment of refractory. life-threatening vee tricular tachyarrhythmias. Oral administration of the produces peak plasma levels within 2 hours. The drug
Dlsopyrarnlde (Norpace)
Chapter 19 U ('ardiovaseular Agents 639 Lidocaine Hydrochloride. USP. Lidocaine hydrochloridc. 2-(diethylainino)-2'.6'-acetoxylidide monohydrochlori& (Xylocaine). was conceived as a derivative of gramine a local anesthetic. It is now being used intravenously as a standard parenterdi agent for suppres.sion of arrhythmias associated
sith acute myocardial infarction and cardiac surgery. It
is
the drug of choice for the parenteral treatment of premature contractions.
jections. Lidocaine hydrochloride is not bound to any extent to plasma proteins and is concentrated in the tissues. Ii is metabolized rapidly by the liver (Fig. 19-12). The first step is deethylation with the formation of monoethylglycinexylidide, followed by hydrolysis of the Metabolism is rapid, the half-life of a single injection ranging from 15 to 30 minutes. Lidocaine hydrochloride is a popular drug
because of its rapid action and its relative freedom from toxic effects on the heart, especially in the absence of hepatic
disease. Monoethylglycinexylidide. the initial metabolite of lidocaine, is an effective antiarrhythmie agent; its rapid hydrolysis by microsomal amidases. however, prevents its use in humans. Precautions must be taken so that lidocainc hydrochloride solutions containing epinephrine salts are not used as cardiac depressants. Such solutions are intended only for local anesthesia and are not used intravenously. The aqueous solutions without epinephrine may be autoclavcd several times, if nec-
Lidocaine (Xytocaine)
essary.
Lidocaine hydrochloride is a class LB antiarrhythmic agent a different effect on the electrophysiological properties fmyocardial cells from that of procainamide and quinidine. It binds with equal affinity to the active (A) and inactive (I) Na ion channels. It depresses diastolic depolarization and aulomacicity in the Purkinje fiber network and increases the brnctional refractory period relative to action potential durathat, as do procainamide and quinidine. It differs from the tuner two drugs, however, in that it does not decrease, and nay even enhance, conduction velocity and increases membane responsiveness to stimulation. There are fewer data available on the subcellular mechanisms responsible for the auiarrhythmic actions of lidocaine than on the more established drug quinidine. It has been proposed that lidocaine has little effect on membrane cation exchange of the atria. Sodium ion entrance into ventricular cells during excitation not influenced by lidocaine because it does not alter conauction velocity in this area. Lidocaine hydrochloride does
Jcpiess Na' influx during diastole. as do all other antiarhythinic drugs. to diminish automaticity in myocardial usinc. It also alters membrane responsiveness in Purkinje fien, allowing increased conduction velocity and ample membrane potential at the time of excitation.3'
Udocaine hydrochloride administration is limited to the prenleral route and is usually given intravenously, though alequale plasma levels are achieved after intramuscular in-
CR3
Phenytoin Sodium, USP.
Phenytoin sodium, 5,5-diphenyl-2,4-imidazolidinedione. 5.5-diphenylhydantoin. diphenyl-hydantoin sodium (Dilantin). has been used for decades in the control of grand mal types of epileptic seizure. It is structurally analogous to the barbiturates hut does not possess their extensive sedative properties. The compound is available as the sodium salt. Solutions for parenteral administration contain 40% propylene glycol and 10% alcohol to dissolve the sodium salt. Phenytoin sodium's cardiovascular effects were uncovered during observation of toxic manifestations of the drug in patients being treated for seizure disorders. Phenytoin sodium was found to cause bradycardia. prolong the PR inter-
val, and produce T-wave abnormalities on electrocardiograms. It is a class lB antiarrhythmic agent. Today. phenytoin sodium's greatest clinical use as an antiarrhythmie
drug is in the treatment of digitalis-induced arrhythmias.33 Its action is similar to that of lidocaine. It depresses ventricular automaticity produced by digitalis, without adverse intraventricular conduction. Because it also reverses the prolongation of AV conduction by digitalis. phenytoin sodium is useful in supraventricular tachycardias caused by digitalis intoxication. Pheumytoin sodium is located in high amounts in the body
tissues, especially fat and liver, leading to large gradients
o
C2H5 Microsornal
NH—C—CH2--N C2H5
CR3 L,doca,ne
/icrosOmot Arnidase
CR3
+ Rgure 19—12 • Metabolism of lidocaine.
CR3
640
IVII.co,, and
l'ex:boak of OrRanic Medicinal and Phan,raceutical Clw,ni.strs-
between the drug in tissues and the plasma concentrations. II is metabolized in the liver.
Mexiletine Hydrochloride.
Mcxiletinc hydrochloride. 1-methyl-2-(2.6-xylyloxy)cthylamine hydrochloride (Mcxitil) (pK. 8.4). is a class lB antiarrhythrnic agent that is effective when given either intravenously or orally. It resemble-s lidocaine in possessing a xylyl moiety but otherwise is different chemically. Mexiletine hydrochloride is an ether and is not subject to the hydrolysis common to the amides lidocaine
and tocainide. Its mean half-life on oral administration is approximately 10 hours.
Tocainide hydrochloride is classed as a lB antiarrliyrhmic agent and used orally to prevent or treat ventricular ectopy and tachycardia. The drug is given in 4(X)- to 600-mg doses every 8 hours.
Flecainide Acetate.
Flecainide acetate. N-(2-piperidi.
nylmethyl)-2.5-bis (2.2.2-trifluorocthoxy)bcnzamide monoacetate (Tambocor). is a class IC antiarrhythmic drug with
local anesthetic activity: it is a chemical derivative of hen. zamide. The drug undergoes biotransformation, forming a meta-O-dealkylated compound, whose antiarrhythmic properties are half as potent as those of the parent drug, and a mela-O-dealkylatcd lactam of Ilecainide with little pharnucological Flecainide acetate is given orally to suppress chronic ventricular ectopy and ventricular tachycanha
It has some limitations because of CNS side effects.
Mextietine (Mexitil)
Although not subject to hydrolysis. mexiletine hydrochloride is metabolized by oxidative and reductive processes in the liver. Its metabolites,p-hydroxymexiletine and hydroxymethylmexiletine. are not pharmacologically active as antiarrhythmic Mexiletine hydrochloride, like class I antiarrhythmic
agents, blocks the fast Na' channel in cardiac cells. It is especially effective on the Purkinje fibers in the heart. The drug increases the threshold of excitability of myocardial cells by reducing the rate of rise and amplitude of the action potential and decreases automaticity. Mexiletine hydrochloride is used for long-term oral prophylaxis of ventricular tachycardia. The drug is given in 200to 400-mg doses every 8 hours.
Flecainide (Tambocor)
Moricizine.
Moricizine. ethyl l0-(3-morpholinoprapi-
nyl)phenothiazine-2-carbamate (Ethnnozine). is a phenolhr. azine derivative used for the treatment of malignant ventricu. lar arrhythmias. It is categorized as a class IC antiarrhytlinrv agent, blocking the Na4 channel with 1:1
drug has higher affinity for the inactivated state than tie activated or resting states. It appears to bind to a site on Is
external side of the Na
channel membrane.37 It has beer
used to suppress life-threatening ventricular arrhythmias.
Tocainide Hydrochloride. Tocainidehydrochloride. 2amino-2'.6'-propionoxyxylidide hydrochloride (Tonocard) (pK. 7.7). is an analogue of lidocaine. It is orally active and has electrophysiological properties like those of lidocainc.35 Total body clearance of tocainide hydrochloride is only 166 mUmin, suggesting that hepatic clearance is not large. Because of low hepatic clearance, the hepatic extraction ratio must be small; therefore, tocainide hydrochloride is unlikely to be subject to a substantial first-pass effect. The drug differs from lidocaine. in that it lacks two ethyl groups, which provides tocainide hydrochloride some protection from firstpass hepatic elimination after oral ingestion. Tocainide hydrochloride is hydrolyzed in a manner similar to that of lidocaine. None of its metabolites is active. 0 -C—CH—NH2 CH3
Tocainide
Moncizine (Ethmozine)
Propafenone.
Propafenone. aminn)propoxy]-3-phenylpropiophenone (Rythmol). a cla&s IC antiarrhythmic drug, contains a chiral center and is marketed as the racemic mixture. Therapy with the racemic mitture of propafenone produces effects that can be attnibutui to both (St and (R) enantionters. Although (R) and(S) omers exert similar Na channel—blocking effects. tis enantiomer also produce.s a blockade. As a n• suIt, the(S) enantiomer is reported to be 40-fold monpotcr than the (R) enantiomer as an antiarrhythmic agent.-" fls cnantionners also display stereoselective disposition chara
teristics. The (R) enantiomer is cleared more quickly. & panic metabolism is polymorphic and determined gcairii-
Chapter 19 • ('ardiormcular Agenix
641
percent of Caucasians have a reduced capacity to hydroxylate the drug to form 5-hydroxypropafenone. This cally.
metabolism
accounts for the interindividual
oriahility in the relationships between dose and concenirathrn and. thus, variability in the pharmacodynamic effects oldie drug. The 5-hydroxy metaholites of both enantiomcrs se u.s potent as the parent compound in blocking Na chanrels. Propafenonc also depresses the slow inwani current of
Amiodarone (Cordarone)
ions. This drug has been used for acute termination s kmg-term suppression of ventricular arrhythmias. It is
Ca2
lsxmd in excess of 95% to a1-acid glycoprotein in the It is absorbed effectively, but hioavailabilily is estibecause of tirst-pass metabolism. kss than I 'X- is eliminated as unchanged drug. Therapy with sapalemione may produce effects that can he attributed to (SI and (R) enantiomers. Thus, the effects may be mod4ated because of an enantiomer—enantiomer interaction altec patients are treated with the sated to he less than
OH
Bretylium Tosylate.
Brctyl jul11 tosylate. (o-bromobenzy I ethyl dimethylammonium p-ioluenesulfonate (l3retybol). is an extremely hitter, white, crystalline powder. The chemical is freely soluble in water and alcohol. Bretylium tosylate is an adrenergic neuronal blocking agent that accumulates selectively in the neurons and displaces norepinephrine. Be-
cause of this property. bretylium was used initially, under the trade name of Darenthin. as an antihypernensive agent. It caused postural decrease in arterial pressure." This use was discontinued because of the rapid development of toleralice. erratic oral absorption of the quaternary ammonium compound. and persistent pain in thc parotid gland on prolonged therapy. Currently. bretylium is reserved for USC Ifl ventricular arrhythmias that are resistant to other therapy. Bretylium docs not suppress phase 4 depolarization, a cointflOfl action of other antiarrhylhmic agents. It prolongs the
effective refractory period relative to the action potential PropaJenone
II ANTIARRHYTHMICS
(lass II antiarrhythmics are discussed under the heading. System Inhibitors. Q.4S5 Ill ANT1ARRHYTHMICS
Amiodarone. 2-hutyl-3-hcnzofuranyl-41-idtethylamino)ethoxyj-3.5-diiodophcnyl ketone (Cordaose). was introduced as an antianginal agent. It has very :wnuuneed class Ill action and is especially effective in saintaining sinus rhythm in patients who have been treated
Anuodarone.
duration but does not affect conduction time and is categorized as a class Ill antiarrhythmic agent. Because bretylium does not has'e properties similar to those of the other antiarrhythmic agents, it has been suggested that its action is due to its adrenergic neuronal blocking the antiarrhythmic properties of the drug. however, are not affected by administration of reserpinc. Bretylium is alsoa local anesthetic. but it has not been possible to demonstrate such an effect on atria of experimental animals, except at very high concentrations." Therefore, the precise mechanism of the antiarrhythmic action of bretylium remains to be resolved.
s direct current shock br arial fibrillation.40 Like class CH3
Hantiarrhythmic drugs. amiodarone lengthens the effective 2lrucloly period by prolonging the action potential duration
all myacardial tissues. Amiodarone is eliminated very kay from the body, with a half-life of about 25 to 30 days tar oral doses." Although the drug has a broad spectrum iuntiarrhythmic activity, its main limitation is a slnsv onset faction. Drug action may not be initiated liir several days. aal the peak effect may not be obtained for several weeks. Antiodaronc has adverse effects involving many different ipn systems. It also inhibits metabolism of drugs cleared ,oidative microsomal enzymes. U contains iodine in its rsfecular structure and, as a result, has an effect on thyroid •rnt0110s. Hypothyroidism occurs in up to II 'X of patients
is the inhibition 'rcnpheral conversion of T4 to T5. Serum reverse T3 (rT3) 'increased as a function of the dose as well as the length f aniodarone therapy. As a result. (U5 levels have been us a guide for judging adequacy of amiodarone therapy :d predicting toxicity.43
e'
Bretylfum
Dofetilide. Dotetilide. N-14-3-( 12-(4-methancsulfonylaminophenyl)ethyllmethylamino)propoxy )phenyllmethane-
sulfonamide (Tikosyn), acts by blocking the cardiac ion channel carrying the rapid component of the delayed reeiitier potassium currents (Ikr) and is used to terminate supraventncular arrhythmnias. prevent the recurrence of atrial fibrillation. and treat ventricular arrhythmias. Unlike sotalol and ihutilide. which are also methanesulfonanilides. it has no effect on adrenergic receptors or sodium channels, respectively. Dofetilide has high specificity for the delayed rectifier potassium currents.4°
642
Wilson and Gisvold'.c Textbook of Organic Medicinal and Pharmaceutical Che,nicirv
0
0
II
H3C—S—NH
H
o
Dofetibde
(likosyn)
Ibutilide.
Ibutilide, N- {4-14-(ethylheptylamino). I -hydroxyhutyl Iphenyl I methanesulfonamide (Corvert), a class
Ill antiarrhythmic belonging to the methanesulfonanilide class of agents, is indicated for rapid conversion of atrial fibrillation or atrial flutter to normal sinus rhythm. Unlike dofctilidc, it is not highly specific for the delayed rectifier potassium currents (lkr) and does have some affinity for
myocardiul tissue. It is distinguished from the other class Ill drugs (amiodarone and bretylium) because of its /3-adrenergic receptor—blocking action.
sodium channcls. 0
Sotalol (Betapace)
Azimilide. Ibutilide (CoNert)
Azimilide. E- I-I 15-4-chlorophenyl)-2.fsr.
a class Ill agent that signiticanhl) blocks the delayed rectifier potassium current. Iks. incluthsi
Sotalol.
Sotalol. 4'I I -hydroxy-2-(isopropylamino)cthylimethylsulfonanilide (Betapace). is a relatively new antiarrhythmic drug. characterized most often as a class Ill agent. and although it has effects that are related to the class II agents, it is not therapeutically considered a class II antiurrhythmic. It contains a chiral center and is marketed as the racemic mixture. Because of its enantiomers. its mechanism of action spans two of the antiarrhythmic drug classes. The l(—) cnantiomer has both /3-blocking (class II) and potassium channel—blocking (class Ill) activity. The d( + ) enantiomer has class Ill properties similar to those of the (—) isomer. but its affinity for the /3-adrenergic receptor is 30(060 times lower. The sotalol enantiomers produce different effects on the heart. Class 111 action of d-sotalol in the sinus node is associated with slowing of sinus heart rate, whereas fl-adrenergic blockade contributes to the decrease in heart rate observed with /-or d,J-sotalol. Sotalol is not metabolized, nor is it bound significantly to proteins. Elimination occurs by renal excretion, with more than 80% of the drug eliminated unchanged. Solalol is characteristic of class Ill antiarrhythmic drugs, in that it prolongs the duration of the action poten-
tial and, thus, increases the effective refractory period of
the lkr component. Its ability to block multichannels flu) be due to a lack of the methane sulfonamide group that common to other class Ill agents, which selectively the lkr potassium current. It is believed that blocking bath lkr and lks potassium currents yields consistent class HI antiarrhythmic effects at any heart ClASS IV ANTIARRHYTHMICS
Verapamil and Diltiazem. Both verapamil and currents zem block the slow inward channel) in cardiac fibers. This slows down AV conductkn and the sinus rate. These drugs are used in controlling and paroxysmal tachycardias and are categorized as clasr IV antiarrhythmic agents according to the Vaughan Williani.
classification.2" (A more detailed description of calcium channel blockers is given above.)
ANTIHYPERTENSIVE AGENTS Hypertension is a consequence of many diseases. namically. blood pressure is a function of the amount
(CH2)
Azimihide
N
\/
N—CH3
Chapter 19 • Cardiovascular it gears
643
autonomic nervous system, the control of which originates in the CNS. Enhanced adrenergic activity is a principal contributor to primary (essential) hypertension. Therapy using antihypertensive agents evolved rapidly
blood pumped by the heart and the ease with which the blood lows through the peripheral vasculature (i.e.. resistance to blood flow by peripheral blood vessels). Diseases of components of the central and peripheral nervoUs systems, which regulate blood pressure and abnormalities of the hormonal system, and diseases of the kidney and peripheral vascular network, which affcct blood volume, can create a hypertenuSC slate in humans. Hypertension is generally defined as mild when the diastolic pressure is between 90 and 104 mm
between 1950 and 1960. During that time, a number of drugs for the treatment and control of hypertensive disease were discovered. Despite the many years of experience, treatment
remains empiric because the etiology of the principal form of hypertension. primary hypertension, is unknown. The first drugs used to produce symptomatic relief of hypertension were a-adrenergic blocking agents. These drugs had limitations because their duration of action was far too short and side effects precluded long-term therapy. Contemporary therapy of primary hypertension uses one of several drug classes as the first course. These drugs may be diuretics to reduce blood volume, inhibitors of. the renin—angiotensin system (ACE inhibitors), and agents that reduce peripheral
11g. moderate when it is 105 to 114 mm Hg, and severe
is above 115 mm Hg. It is estimated that about of the adult population in the United States (about 40
when it
million) are hypertensive. Primary (essential) hvperten.cion is the most common form ut hypertension. Although advances have been made
in the identification and control of primary hypertension. he etiology of this form of hypertension has not yet been
vascular resistance (e.g.. calcium channel blockers. vasodilators. and sympathetic nervous system depressants). The antihypertensive drug classes discussed in this section include
resolved. Renal lmyperten.cion can be created by experimen-
tally causing renal artery stenosis in animals. Renal artery utenosis also may occur in pathological conditions of the kidney. such a!. nephritis. renal artery thrombosis, renal arcr5 infarctions, or other conditions that restrict blood flow through the renal artery. Hypertension also may originate rum pathological states in the CNS. such as malignancies. Tsmors in the adrenal medulla that cause release of large
ACE inhibitors, sympathetic nervous system depressants. and vasodilutors acting on smooth muscle. Calcium channel blockers and other vasodilutors are included in previous discussions in this chapter. Diuretics are discussed in Chapter IS.
amounts of catecholumnines create a hypertensive condition known as pheocliromocvtoina. Excessive secretion of aldosterone by the adrenal cortex, often because of adenoma.s. also produces hypertensive disoniers. Arterial blood pressure is regulated by several physiological factors, such as heart rate, stroke volume, peripheral vaxcular network resistance, blood vessel elasticity, blood vol-
The Renin-Anglotensin System
Hypertension
The renin—angiotensin system is a hormonal system that plays a central role in the control of sodium excretion and body fluid volume. It interacts closely with the sympathetic nervous system and aldosterone secretion in the regulation of blood pressure. Figure 19-13 shows the relationship of the component parts of the renin—angiotensin system and their main physiological effects.
ume. and viscosity of blood. Endogenous chemicals also play an important part in the regulation of arterial blood nessure. The peripheral vascular system is influenced utearly by the sympathetic—parasympathetic balance of the
Lowered Sodium Excretion
Reduced BlOOd Pressure
/\
Renin Release
V
Angiotensinogen ACE
Angiotensin
I
Angiotensin It
Vasoconstnctlon
—k Anglotensin
/
Atdosterone Secretion
Sodium and Fluid Retention
'Jr
Increased Peripheral ResIstance
Rgure
19—13
•
Renin—angiotensmn system
blood pressure control
Increased Cardiac Output Elevated
BlOOd Pressure
Itt
644
and (jLcvo!dr Iexthook of Orga:iic Medicinal and I'har,naceulieal Chemistry Renin
(NH2 end)
ASP-ARG-VAL—'rlR-ILE-HIS—PRO-PHE-HIS-LEU-VAL-tLE—HIS-R (Angiotensinogen)
(COOH end)
ACE
ASP-ARG—VAL-TVR--ILE-HIS-PRO-PHE-HIS-LEU (Anglotensin I) Aminopeptidasa
ASP-ARG-VAL-TYR-ILE-HIS-PRO--PHE-HIS-LEU (Angiolensin II) Angloteitsinases
ARG-VAL-TVR-ILE-HIS-PRO-PHE (Anglotensin Ill)
INACTIVE PRODUCTS
Figure 19—14 • Biochemistry of the renin—angiotensin system: formation of angiotensins from angiotensinogen.
The relationship between the renin—angiotensin system and blood pressure in humans has been known since before the beginning of the 20th century. Tigerstedt and Bergman47 demonstrated in 1898 that when injected in a host, kidney extract produced a potent vasopressor response. The substance was named renin. Many years later, this substance
a kinin. callidin. which is converted to bradykinin by tiaut
was shown to require a cofactor to produce vasoconstric-
bradykinin. action.s that appear to be opposite, the balantv of the system seems to favor vasoconstriction. ACE is a membrane-bound enzyme anchored to the cr11 membrane through a single transrnembrane domain near the carboxy-terminal extremity. The enzyme is a
[ion.48 Eventually, in 1939. this hypertensive substance was isolated, identified as a decapeptide. and later called angialensin. This cofactor existed as an inactive precursor. anglotensinogen. Later studies revealed that angiotcnsin existed in Iwo forms. the biologically inactive decapeptide angiotensin I and the active oclapcptide angiolensin The precursor of angiotensin. angiotensinogen. is a glycoprotein of molecular sveight (MW) 58.000 to 61.000, synthe-
sized primarily in the liver and brought into the circulatory system. Renin. an aspartyl protease (MW 35.0(X) to 40.000).
whose primary source is the kidney, cleaves the Leu-Val bond from the aspartic acid end of the angiotensinogen polypeptide molecule to release the decapeptide angiotensin I (Fig. 19-14). The biochemical conversion continues with the cleavage of a dipeptide (His-Leu) from the carboxyl terminal of angiotensin I by ACE to form the octapeptide angiotensin
II. a potent vasoconstrictor. Angiotensin Ill is formed by removal of the N-terminal aspartale residue of angiotensin
enzymes. Bradykinin enhances release of the prostaglandin' and PGI? within certain tissues to produce a vaseslila
tory effect (Fig. 19-15). Bradykinin is convened to products by ACE and other curhoxypeptidases. Although ACE causes activation of angiotensin and inactivation it
containing glycoprotein with a MW about I 30.(X)0. It nonspecific peptidyldipeptide hydrolase. widely in mammalian tissues, that cleaves dipeptides from the cat
boxy terminus of a number of endogenous pcptidcs. liv minimum structural requirement for binding and cleauc of a substrate by ACE is that it be a tripeptide with a fret carhoxylate group. A general exception is that this does not cleave pcptides with a penultimate prolyl residic This accounts for the biological stability of angiotensin II. The important binding points at the active site of ACE at a cationic site to attract a carboxylate ion and a zinc ion hr can polarize a carbonyl group of an amide function to nratt it more susceptible to hydrolysis. In the active site. the amide carbonyl by the
II, a reaction catalyzed by glutamyl aminopeptidase. In contrast to angiotensin II. angiotensin Ill has a less potent hut significant regulatory effect on sodium excretion by the renal tubules.
The regulatory action of the renin—angiotensin system in controlling sodium and potassium balance and arterial blood pressure is modified by vasodilators called kinins. Proteolytic enzymes that circulate in the plasma form kinins. Kallikrein is activated in plasma by noxious inlluences to act on
RaIIIkrein
Calladin
Bradyktnin —+ Prostaglandin ACE V
tnactive Products
Vasodjiatabor
Figure 19—15 • Bradykinin formation and adios
Chapter 19 •
645
4
figure 19—16 • Model showing cleavage of the histidine-phenylalanine resisue of angiotensin I by ACE to form the octapeptide antiotensin II and the dipep'do residue of histidine and leucine.
group of a glutamic acid residue to cause hydrolysis of the
Figure 19-16 shows a hypothetical model of the hydrolysis of angiotensin I by the active Site of ACE. ACE coists in more than one form. Somatic ACE that regulates blood pressure. found in most tissues, differs from the isoen-
binding points at the active Site of ACE are thought to be an arginine residue, which provides a cationic site that attracts a
carboxylate ion, and a zinc ion, which can polarize a carbonyl group of an amide function to make it more susceptible to hydrolysis. Hydrophobic pockets lie between these groups
cyme ACE found in the testis. Somatic ACE, in contrast to testicular ACE. contains two binding domains. The principal
in the active site, as does a functional group that forms a
ztise site for hydrolysis is the domain located in the C-
the hypothetical binding of captopril in the active site of
criminal half of somatic ACE.°'
ACE.
hydrogen bond with an amide carbonyl. Figure 19-17 shows
H,,
6H3
RENIN-ANGIOTENSIN SYSTEM INHIBITORS ICaptopril. I osopmpionyl Iproline (Capoten), blocks the conversion of angiolensin Ito angiotensin II by inhibiting the converting
Captopril.
N
eazyrne. The rational development of captopril as an inhibior of ACE was based on the hypothesis that ACE and car-
A functioned by similar mechanisms. It was nolod that d-2-benzylsuccinic acid50 was a potent inhibitor
carboxypeptidase A but not ACE. By use of this small
Captopfll (Capoten)
molecule as a prototype. captopril was designed with a car-
hisyl group on a proline and a thiol group was introduced Isenhance tire binding to the zinc ion of ACE. The important
Lisinopril. Lisinopril. l-1N2-IS- I -carboxy-3-phenylpmpylI-L-lysyIl-L-proline dihydrate (Pnmiivil, Zcstril. is a ly-
___________________________
646
Wjlwn and Gisvold'.s Textbook of Organic Medicinal and l'har,naeeusieal
CH3
I
I
I
Figure 19—17 • Accommodation of cap topril to the active site of ACE.
I
sine derivative of enalaprilat. the active metabolite of enalapril. Like all ACE inhibitors, it is an active site-directed inhibitor of the enzyme. with the zinc ion used in an effective binding interaction at a stoichiometric rjtio of 1:1. The pharmacological effects of lisinopril are similar to those of captopril and enalapril.
and loss of taste seen with caplopril. These side effects axe similar to those of the mercapto-containing drug penicilla. mine. The absence of the thiol group in enalapril maleatc
may free it from these side effects. The half-life is hours.
H.
H.
USinopnl (PrinMi) (Zestril)
Enalapfll (Vasotec)
Benazepril Hydrochloride. ACE INHIBITOR PRODRUGS
Many new ACE inhibitors became available for the treatment of hypertension following the clinical effectiveness of enalapril. Enalapril is a non—thiol-containing ACE inhibitor devoid of the side effects of rash and loss of the sense of taste characteristic of the thiol-containing compound captopril. With the exception of the phosphorus-containing fosinopril. these antihypertensive agents have a 2-(S)-amino-
(3S)-3-II( IS)-
Benazepril
I
teurahydm-2-oxo- 111-I -benzazepine. I-acetic acid 3-ciht to hr ester hydrochloride (Lotensin). is metabolized
active diacid benazaprilat. As with the ACE prodnxgs, mutagenicity has been found, even though these drugs cr05 the placenta.
phenylbutyric acid ethyl ester moiety differing only in the substiluents on the amino group. They have the common property of acting as prodrugs. being converted to the active
enzyme inhibitor following absorption and metabolism by liver and intestinal enzymes. These drugs (Fig. 19-18), like the prototypical drug captopril. are used in the treatment of mild-to-moderate hypertension, either alone or in conjunction with diuretics or calcium channel blockers. Table 19-5
Benazepnl in)
compares some of their properties.
Qulnapril Hydrochloride. Enalapril Maleate.
Enalapril maleate. I -IM(S}-I -carhoxy-3-phenylpropylj-L.alanyl]-t.-proline I'-ethyl ester ma-
leate (Vasotec), is a long-acting ACE inhibitor. It requires activation by hydrolysis of its ethyl ester to form the diacid enalaprilat. Enalapril is devoid of the side effects of rush
Quinapril hydrochlori.t (S)-I(S )-N- I (S)2 I tctrahydro-3-isoquinolinecarboxylic acid I-ethyl cstcr hy drochloride (Acuretic), forms the diacid quinaprilate in body. It is more potent than captopril and active tbrm of enalapril.
I
Chaptcr 19 • Cardiovascular Agents
647
0 H
O
COCH2CH3
I
COON
o
COOC2HS
HC—COOH
II
CH C
H'
N
CH2COOH
Enalapril Maleate
Benzapril Hydrochloride
HQ 0
0
Il
II II
C— —
CONa
I
H
CH3 H
(Cl-13)2CH—CHOCCH2CH3
C
0
H
Fosinopril Sodium
N
C
CH2CH2_.Q
HCI
H
COOH H
Oulnapril Hydrochloride
0
H
Ramipril
Figure 19—18 • ACE inhibitor prodrugs.
TABLE 19-5
ACE-InhibItor Prodrugs Metaboilte Protein
Prodrug
Metabolite
Scnv.cprll
Benazcpnlal
Enalaprilal 1e.inopril
Binding (%)
Metabolite Plasma (hours)
Mode of Excretion
95
0—Il
Renal
50—60
I Ill
Renal
97
11.5
Renal/lecal
Qinnapril
Qulnaprilat
97
3.0
RenaUfecal
Ramipiil
Ramiprilat
56
13—17
Renatikcnl
648
and Giwok.V.c Textbook of
Medicinal and Phar:naceuikal Chemistry
pyrrolidine of enulapril has been replaced with an droindole system. Much like enalaprilate. trandolapril must be hydrolyzed to tranolaprilate. which is the bioactive spe• cies.
0 H,,
H,
C—0C2H5
c=0 Ouinapril (Accupnl)
Ramiprll.
Ramipril, (2S, 3aS. 6aS)- I-I(S)-N-I(S)-I -carboxy- 3 -phenylpropyl I alanyl I octahydrocyclopental pyr-
role-2-carboxy lie acid I-ethyl ester (Altace). is hydrolyzed to ramiprilat. its active diacid form, faster than enalapril is hydrolyzed Co its active diacid form. Peak serum concentrations from a single or.il dose are achieved between 1.5 and
3.0 hours. The ramiprilate formed completely suppresses ACE activity for up to 12 hours, with 80°k inhibition of the enzyme still observed after 24 hours.
Trandotapfll (Mavik)
ANGIOTENSIN ANTAGONISTS
Administration of a competitive antagonist can inhibit a vasodilatory effect. Since the subsins for this receptor is an octapeptide. much of the earlier wart was performed by using various peptide systems. One such agent, saralasin. is an octapeptide that differs from sin by two amino acids. This agent's use was limited becaust
it had some partial agonistic properties. Nevertheless. ii served as a lead in the development of other agents that ire useful in antagonizing the angiotensin II receptor. The moo significant lead in the development of this class came finn a series of imidazole-5-acetic acid derivatives that pressor response to angiotensin II in test animals. Motecular modeling revealed that the imidazole-5-acetic acid could k exploited to more closely mimic the pharmacophore of an
Ramiprit
(Mace)
giotensin II. The first successful agent to be
Fosinopril Sodium.
Fosinopnl sodium. (4S)-4-cyclohexyl- I-Il I(RS)- I -hydroxy-2-rnethylpropoxyl(4-phenyl-butyl)phosphinyllacetyl J-t.-proline sodium salt (Monopril). is a phosphorus-containing ACE inhibitor. It is inactive but serves as a prodrug. being completely hydrolyzed by intestinal and liver enzymes to the active diacid fosinoprilat. 0
through this method is losartan. Later, four other agents introduced into the U. S. market. These tend to be biphen)I
methyl derivatives that possess certain acidic which can interact with various positions on the receptar. much like the substrate. angiotensin II. Since the late this particular class has received a great deal of attention In the early I 990s. the receptor for angiotensin II was fousi to exist as four isozymes. AT1, AT2, AT3. and AT4, nib AT1 being responsible for smooth muscle contraction. pathetic pressor mechanisms, and aldosterone release. ANGIOTENSIN II BLOCKERS
Losartan. Losartan. 2-butyl-4-chloro- l-Ip.(o- IH.td• razol-5-yl-phenyl)benzyllimidazole-5-methanol monqs' tassium salt (Cozarr). was the first nonpeptide imidazok
be introduced as an orally active angiotensin II Fostnoprtl (Monopnl)
Trandolapril.
Trandolapril, I -12-( I -ethoxycarbonyl-3phenylpropylamino)propionylloctahydroindole-2-carboxylic acid (Mavik). is an indale-containing ACE inhibitor that is structurally related to most of the agents discussed above.
Enalapril is very similar to trandolapril. with the primary difference occurring iii the heterocyclic systems. The
with high specificity for AT1. When administered to patient it undergoes extensive first-pass metabolism, with thc 5 methanol being oxidized to a carboxylic acid. This metaklism is mediated by CYP 2C9 and 3A4 isozymes. The 5 methanol metabolite is approximately IS times more pores than the parent hydroxyl compound. Since the parent hydr'
xyl compound has affinity for the AT1 receptor. speaking, it is not a prodrug. Candesartan. Candesanan, (+ )carbonyll-oxylethyl 2- ethoxy- I-I l2'-( I H-tetrazol-5'yliI) biphenyl 1-4 - yllmcthyll-l H-benzimidazole-7-cartvoi
649
Chapter 19 • Cardimasesgiar
Telmisartan, Telmisurtan. 4'-I( I .4'.dimethyl2'-pro pyll2,6'.hi- I H-benzimidazol l'-yl )methyl 1-11,1 '-biphenyll -2-carboxylic acid (Micardis). does not appear to bear any structural relationship to this class, but there is actually a
NNNH
great deal of overlap in the chemical architecture with other agents. The first, and most significant, difference is the replacement of the acidic tetrazole system with a simple car-
boxylic acid. This acid, like the tetrazoic. plays a role in receptor binding. The second difference is the lack of a carboxylic acid near the itnidazole nitrogen that also contributes (.osartan (Cozaar) ute
(Atacand), like losartan. possesses the acidic tetrazole
which most likely plays a role in binding to the angiotensin II receptor similarly to the acidic groups of angiotensin II. Also, the imidazole system has been replaced with a beazimidazole possessing an ester at position 7. This ester must be hydrolyzed to the free acid. Fortunately, this conversion takes place fairly easily because of the carbonate in die ester side chain. This facilitates hydrolysis of the ester s much that conversion to the free acid takes place during nhsorption from the gastrointestinal tract.
to receptor binding. As with irbesartan. however, there is not a need for this group to be acidic but, rather, to be one that participates in receptor binding. The second imidazole ring, much like a purine base in DNA. can hydrogen bond with the angiotensin II receptor.
Telmisartan
Candesartan (Atacand)
Irbesartan, 2-butyl-3-112'-( lH-tetrazol-51.1 '-hiphenyl 1.4-yl Imethyl II .3-diazaspirol 4.4 Inon- I -
frbesartan.
rt.4-one (Avapro). like losartan. possesses the acidic tetozole system. which most likely plays a role, similar to the xidic groups of angiotensin II. in binding to the angiotensin II In addition, the hiphenyl system that serves to the tetrazole from the aliphatic nitrogen is still pres-
Valsartan. Valsanan. N-( I -oxopentyl )-N-I 2'-( I Fi-tetrazol-5-yl )i 1.1 '-biphenyll-4-yI Imethyl J-t.-valine (Diovan). like losartatn, possesses the acidic tetrazole system, which most likely plays a role, similar to that of the acidic groups of angiotensin II. in binding to the angiotensin II receptor. In addition, the biphenyl system that serves to separate the tetrazole from the aliphatic nitrogen is still present. In addi(ion, there is a carboxylic acid side chain in the valine moiety that also serves to hind to the angiomensin II receptor.
coI.A major difference in this agent is that it does not possess
the acidic side chain. Even so. irbesartan has good affinity the angiotensin II receptor because of hydrogen bonding with the carbonyl moiety of the amide system. Also, this panicular agent does not require metabolic activation as candesartan does,
1/
COOH
CH3
CH—CH
N\
NH
CH2—/
N=N/ Valsa,lan
C'H7CH2CHPCH3
(Diovan)
ADRENERGIC SYSTEM INHIBITORS
Drugs that reduce blood pressure by depressing the activity of the sympathetic nervous system have been used as effecItbesartan
tive agents in the treatment of hypertension. This can be accomplished in several ways: (a) depleting the stores of
(Avapro)
neurotransmitter. (h) reducing the number of impulses tray-
650
Wi/so,: and Gfsvo/ds Textbook of Organic Medicinal and Pharn,aeeuiical (iuqnistrv
cling in sympathetic nerves. (C) antagonizing the actions of the neurotransmitler on the elfcctor cells, and (dl inhibiting neurotransmitter release. AGENTS DEPLETING
STORES
Folk remedies prepared from species of Rauwolfia. a plant genus belonging to the Apocynaceae family, were reported as early as 1563. The root of the species R.serpeniina has been used for centuries as an antidote to stings and bites of insects, to reduce lever, as a stimulant to uterine contractions, for insomnia, and particularly for the treatment of insanity. Its use in hypertension was recorded in the Indian literature in 1918, hut not until 1949 did hypotensive properties of Rauwolfia spp. appear in the Western literature.52 Rauwolfia preparations were introduced in psychiatry for the treatment of schizophrenia in the early 1950s. following confirmation of the folk remedy reports on their use in mentally deranged patients. By the end of the 1960s. however. the drug had been replaced by more efficacious neurotropic agents. Reserpine and its preparations remain useful in the control of ntild essential hypertension. The effects of reserpine do not correlate well with tissue levels of the drug. The pharmacological eflècts of rescrpine
were still present in animals when it could no longer be detected in the brain.53 Reserpine depletes catecholamines and serotonin from central and peripheral neurons by interfering with the uptake of these amines from the cytosol into the vesicles and granules.54'55 As a consequence. norcpinephrine cannot be stored intraneuronally in adrenergic neurons, and much of the norcpinephrine in the cytosol is metab-
olized by monoamine oxidase (Fig. 19-19). The binding of reserpinc to the storage vesicle membrane is firm, and as a result, the storage granule is destroyed. reducing the ability of the nerve to concentrate and store norepinephrine. Since reserpine acts on both central and peripheral adrenergic neurons, its untihypertensive effects may result from neurotransrnitter depletion from both of these sites.
Chemical investigations of the active components of R. serpenlina roots have yielded several alkaloids (e.g.. ajmaline. ajmalieine, ajmulinine. serpentine, and serpentinine).
Reserpine. which is the major active constituent of Rauwolwas isolated in 1952 and is a much weaker base than the alkaloids just mentioned. Reserpinoid alkaloids are yo. himbinc-like bases that have an additional functional grasp on C-18. Only three naturally occurring alkaloids possess reserpine-like activity strong enough for use in treating hy. pertcnsion: reserpine. deserpidine. and rescinnamine. Reserpine is absorbed rapidly after oral administration.
Fat tissue accumulates reserpine slowly. with a maximal level reached between 4 and 6 hours. After 24 hours, small amounts of reserpine are found in the liver and fat, but none is found in the brain or other tissues. Reserpinc is metabo. lized by the liver and intestine to methyl reserpate and 3.4.5. trimethoxyhenzoic acid.
Powdered Rauwolfia Serpentina, USP.
Rauwoliun
(Raudixin. Rauserpal, Rauval) is the powdered whole tea of R. serpen:ina (Benth). It is a light tan to light brawn powder, sparingly soluble in alcohol and only slightly ads' ble in water. It contains the total alkaloids, of which reserpine accounts for about of the total activity. a dosage of 200 to 300 mg is roughly equivalent to 500 pg of reserpine. It is used in the treatment of mild or moderate
hypertension or in combination with other hypotensive agents in severe hypertension.
Reserpine, USP. Reserpinc (Serpasil. Reserpoid. Rau Sed. Sandril) is a white to light yellow, crystalline alkaloid, practically insoluble in water, obtained from various of Rauwolfia. In common with other compounds with an indole nucleus, it is susceptible to decomposition by light and oxidation, especially when in solution. In the dry state, discoloration occurs rapidly when rcserpine is exposed a
light, but the loss in potency is usually small. In reserpine may break down with no appreciable color change
when exposed to ligh, especially in clear glass thus, color change cannot be used u.s an index of the amnse
of decomposition. Reserpine is effective orally and parenterally for the tees
ment of hypertension. After a single intravenous dose, /s onset of antihypertensive action usually begins in about hour. After intramuscular injection, the maximum effect wcurs within approximately 4 hours and lasts about 10 hours When it is given orally. the maximum effect occurs about 2 weeks and tnay persist up to 4 weeks after the linal
dose. When used in conjunction with other hypotensin: drugs in the treatment of severe hypertension, the daily due
varies from 100 to 250 sag. Guaneth' Guanethidine and Related Compounds. dine has been classified traditionally as an adrenergic i/wi
MetabOtites
Figure 19—19 • Action of reserpune at adrenergic nerve ending.
ing agent because it can prevent the release of rime from postganglionic neurons in response to adivnaçk stimulation. Guanethidine and other compounds in this section have other actions on catecholantine math lism and can cause significant depletion of these amiws ii adrenergic neurons. They do not interfere with release cpinephrine from the adrenal medulla.
Chapter 19 • Curdiuia.rcular Agenls
651
Reserpine Serpasli
sulfate is taken up by the amine pump located on (he ncuronal membrane and retained in the nerve, displacing norepinephrine from its storage sites in the neuronal granules. The
displaced norepinephrine is metabolized to homovanillic acid by mitochondrial monoamine oxidase. depleting the nerve ending of the neurolransmittcr. The usefulness of guanethidine inonosulfate also resides in the fact that once it is taken up by the nerve, it produces a sympathetic blockade by inhibiting release of nonepinephrine that would occur on neuronal membrane response to stimulation2u by the nerve
Guanethtdlne
(Ismetn)
Guanethidine monoIguanidine sulllsmelin sulfate), is a white. crystalline material that is soluble in water. It was one of a series of guanidine onipounds prepared in the search for potent antitrypanoso2u1 agents. There is an absence of CNS effects, such as because the drug is highly polar and does not cross the blood—brain barrier. Guanethidine monosulfato produces a gradual, prolonged fall in blood pressure. usually. 2 to 7 days of therapy are required before the peak dkct is reached. and usually, this peak effect is maintained 3 or 4 days. Then, if the drug is discontinued, the blood returns to pretreatment levels over a period of I to
Guanethidine Monosulfate, USP. ulfate. 12-(hcxahydro- I
weeks. Because of this slow onset and prolonged duration
liction, only a single daily dose is needed. Gutnethidine monosulfate is metabolized by microsomul ineymes to 2.(6-carboxyhexylamino)ethylguanidine and N-oxide (Fig. 19-20). Both metabolites have weak antihypertensivc properties. Guancthidine mono-
action potential. Guanethidine monosulfate stored in the granules is released by the nerve action potential but ha.s veiy low intrinsic activity for the adrcnergic receptors on the postjunctional membrane. Moderate doses for a prolonged period or large doses may produce undesirable side effects by causing neuromuscular blockade and adrenergic nerve conduction blockade.
Guanadrel Sulfate.
Guanadrel sulfate, (I .4-dioxaspir. 014.5 Idcc-2-ylmethyl)guanadine sulfate (Hylorel). is similar
to guanethidine monosulfate in the manner in which it reduces elevated blood pressure. It acts as a postganglionic adrenergic blocking agent by displacing norepinephrinc in adrenergic neuron storage granules, thereby preventing release of the endogenous neurotransmiuer on nerve stimula-
tion. Guanadrel sulfate has a much shorter half-life (10 hours) than guanethidine monosulfate, whose half-life is measured in days. In the stepped-care approach to hypertension, guanadrel sulfate is usually a step 2 agent.
Melaboide 1
0
'
NH
NH2
c
1
NH2
II
, C—OH NH C—NH 2
I
NH 2.(6.Caiboxyhexylannino)-
I
SELECTIVE a-ADRENERGIC ANTAGONISTS
j t.ner
NH2
NH
/_'\
I'
N
Guanadrel (1-sylorel)
I
NH
Melaboile 2
FIgure 19—20 • Metabolism of guanethidine monosulfate.
The principal clinical use of a-adrenergic antagonists is in the treatment of catecholarnine-dependent hypertension. Classic drugs such as phentolamine and phcnoxybenzamine are nonspecific blocking agents of both and a2 receptors on the presynaptic membrane of the adrenergic neuron. Specific antagonists of a1 receptors are effective antihypertensive agents by blocking the va.socontricting effect on smooth muscle and not interfering with the activation of a2 receptors
652
tViIson
and Gisvold.% iexthook of Organic Mrdu-,nal and PIu,nnaeeuiira/ C!ze,ni.virv
on the adrenergic neuron, which when activated inhibit further release of norepinephrine.
Prazosin Hydrochloride. The antihyperiensive efkcts of prazosin hydrochloride. I -(4-amino-6.7-dimethoxy-2monohydrochloride quinatolunyl )-4-(2-furoyl pipcrazine (Minipress). are due to peripheral vasodilation as a result of its blockade of a1-adrenergic receptors. In ligand-hinding studies. prazosin hydrochloride has 5.000-fold greater affinity tor a1 receptors than for some a2-adrencrgic receptors."
Doxazosin.
Doxazosin. I -(4-amino-6.7-dinicthoxv'2quina/.olinyl)-4-( I .4-benzodioxan-2-yicarhonyl)piperazine (Cardura). is a quinazoline compound that selectively inhib its the a1 subtype of a-adrcnergic receptors. This agent very useful in the management of hypertension associated with pheochromocytoma. CENTRALLY ACTING ADRENERGIC DRUGS
The use of agents that directly affect the peripheral component of the sympathetic nervous system represents an impor tant approach to the treatment of hypertension. A second approach to modifying sympathetic influence on the caitho. vascular system is through inhibition or reduction of CNS control of blood pressure. Several widely used medicalinoact by stimulating receptors, which in the CNS reducel sympathetic outflow to the cardiovascular system and pro. duces a hypotensive effect.
Prazostn (Minipress)
Methyldopate Hydrochloride,
Prazosin hydrochloride is readily absorbed, and plasma concentrations reach a peak about 3 hours after administration. Plasma half-life is between 2 and 3 hours. Prazosin hydrochloride is highly hound to plasma protein: it does not cause adverse reactions, however, with drugs that might be displaced from their protein-binding sites (e.g.. cardiac glycosides). It may cause severe orthostatic hypertension because of its a-adrenergic blocking action, which prevents the reflex venous constriction that is activated when an individual sits up from a prone position.
ethyl ester hydrochloride (Aldomet ester hydrochloridel. a methyldopa. lowers blood pressure by inhibiting the outfius
of sympathetic vasoconstnctor impulses from the brain Early studies had suggested that tile hypotensive acitos or a-methyldopa was due to the peripheral properties of the drug as a decarboxylasc inhibitor or a false transmitter. OH NH2
Terazosin Hydrochloride. Tcrazosin hydrochloride. I (4-amino-6.7-dimelhoxy-2-quinazolinyl)-4-(tctrahydro-2-Iuroyl)piperazine monohydrochloride (Hymn), is a structural congencr of prazosin hydrochloride. It possesses similar selective properties of specifically inhibiting a,-adrenergic receptors. Tile drug is slightly less potent than pra/osin hydrochloride. Terazosin hydrochloride has a half-life of ap-
proximately 12 hours, which is much longer than that of prazosin. This lends itself to a once-daily dose to control hypertension in many patients.
USP.
hydrochloride. t-3-(3.4-dihydmxyphenyl
HOOC —C—CH CH3
Methyldopa (Aidomet)
The current hypothesis concerning the hypotensive acts ity of methyldopa involves the CNS as the site of action. Methyldopa. on conversion to a-methylnorepinephrinc. xr on
receptors to inhibit the release 1)1 notefi-
nephrine. resulting in decreased sympathetic outflow Inc the CNS and activation of parasympathetic outflow. Mcthyldopa is used as a step 2 agent and is for patients with high blood pressure who are not to diuretic therapy alone. Methyldopa. suitable for oral sic is a /wutteron and is not soluble enough for parentcral us The problem was solved by making the ester. leasing 11c amine free to form the water-soluble hydrochloride salt. is supplied as a stable, buffered solution, protected with oxidants and chelating agents.
NH2
Terazosin (Hytnn)
0
NH2
Doxazosin (Cardura)
Chapter 19 • Clonidine hydrochloride, 2Clonidine Hydrochloride. R2,6-dichlorophenyl)iminolimidazolidine monohydrochioride (Catapres). was the lust antihypertensive known to act ito the CNS. It was synthesized in 1962 as a derivative of
kidneys, and peripheral blood vessels. The drug does not produce orthostatic hypotension. Ci.
Clonidine hydrochloride acts by both peripheral and central mechanisms in the body to affect blood pressure. It stimulates the peripheral a-adrenergic receptors to produce vaso-
Guanabenz
constriction, resulting in a brief period of hypertension.
hydrochloride acts on a2 adrenoreceptors located in the hind-
brain to produce its hypotcnsive action. Clonidinc hydro. chloride also acts centrally to cause bradycardia and to reduce plasma levels of renin. Sensitization of baroreceptor
653
norepincphrine from the neuron when stimulated. The effect of the drug results in decreased sympathetic tone in the heart.
he known a-sympathomimetic drugs naphazoline and tolainline, potential nasal vasoconstrictors, but instead it proved to be effective in the treatment of mild-to-severe hyperten-
Clonidine hydrochloride acts centrally to inhibit the sympathetic tone and cause hypotension that is of much longer duration than the initial hypertensive effect. Administration of clonidine hydrochloride thus produces a biphasic change in blood pressure, beginning with a brief hypertensive effect and followed by a hypotcnsivc effect that persists for about 4 hours. This biphasic response is altered by dose only: Larger doces produce a greater hypertensive effect and delay the onset of the hypotensive properties ot the drug. Clonidine
Agents
(Wytensin)
Guanfacine Hydrochloride.
Guanfacine hydrochloride. N-(aminoiminomethyl )-2.6-dichlorobenieneacetamide (Tenex), is structurally related to clonidine hydrochloride and guanabenz acetate and shares many of their pharmacological properties. The drug has a longer duration of action than either clonidinc hydrochloride or guanabenz acetate. It lasts up to 24 hours. It also requires much longer (8 to 12 hours) tbr a peak effect to occur after the drug is administered.
p2thways in the CNS appears to be responsible for the brady-
transmitted by way of the vagus nerve. The central mechanism that results in decreased plasma renin is not however. The hypotensive properties of clonidine a animals can be blocked by applying a-adrenergic blocking directly to the brain.59 Clonidine hydrochloride has advantages over antihyperrcnsive drugs such as guanethidine monosulfate and prazosin hydrochloride, in that it seldom produces orthostatic hypomoire side effects. It does, however, have some sedative that are undesirable: it also may cause constipaion and dryness of the mouth.
Clonidine hydrochloride is distributed throughout the with the highest concentrations found in the organs of elimination: kidney, gut, and liver. Brain concentrations ac low hut higher than plasma concentrations. The high concentration in the gut is due to an enterohepatic cycle in
clonidine hydrochloride is secreted into the bile in rather high concentrations. The half-life in humans is about
:o hours. Clonidine hydrochloride is metabolized by the to form two major metabolites. p.hydroxyclonidine .nd its glucuronide. p-Hydrnxyclonidine does not cross the
blond-brain barrier and has no hypotensive effect in huci
Guanfacine (Tenon)
VASODIIATORY DRUGS ACTING ON SMOOTH MUSCLE
Reduction of arterial smooth muscle tone may occur by many mechanisms, such as reduction in sympathetic tone. stimulation of f3—adrenergic receptors, or even direct action on the va.sculature without interference from the aulonomic innervation. Drugs acting on the arieriolar smooth muscle also increase sympathetic reflex activity, causing an increase in heart rate and cardiac output and stimulating renin release. which increases sodium retention and plasma volume. As a result, it is common to coadminister saluretics and f3-adrcnergic blocking drugs with these agents.
Antihypertensive agents that produce vasodilation of smooth muscle can be divided into two categories: directacting and indirect-acting vasodilators. Indirect-acting vasodilators may be distinguished from direct-acting vasodilators. in that they produce their effect by interfering with the vasoconstrictor stimuli and their primary site of action is not necessarily the vascular smooth muscle itself. Indirect-acting vasodilators include sympatholytic drugs, such as reserpinc;
a-adrenergic antagonists, such as prazosin hydrochloride; ACE inhibitors: and angiotcnsin II receptor antagonists. such as saralysin. Direct-acting vasodilutors include hydralazine hydrochloride, sodium nirroprusside. potassium channel Clortidine (Catapres)
Guanabenz Acetate. Guanabcnz acetate. [(2,6-dichloroenzylidene)aminojguanidine monoacetate (Wytensin). is central a2-adrenergic agonist that reduces the release of
openers. and calcium channel—blocking agents.°9
Hydralazine Hydrochloride, USP. Hydralazine hydrochloride, I -ltydrazinophthalazine monohydrochloride (Apresoline hydrochloride), originated from the work of a attempting to produce some unusual chemical
654
WjLw,, and Gj.n'old's Textbook of Organic Medicinal and Pharmaceutical Chemistry
0
02
0 NH
Acotyiauon
NH Giucuronsciabon
NHNH2
NHNH2
NHNHC—CH3 0— Glucuronic Acid
NHNH
CH3
FIgure 19—21 • Metabolism of hydralazine hydrochloride
moderate-to-severe hypertension. It is often used in conjunc-
Hydralazine hydrochloride is more effective clinically when coadministered with drugs that antagonize transmission (e.g., antagonists, reserpine. guanethidine monosulfate. methyldopa. and clonidine hydra' chloride). When given with diuretics, it is useful in the treat. ment of CHF.
tion with less potent antihypertensive agents because side effects occur frequently when it is used alone in adequate doses. In combinations, it can be used in lower and safer
Sodium Nitroprusside, USP.
that this compound compounds and from the had antihypertensive properties. It occurs as yellow crystals
and is soluble in water to the extent of about 3%. A 2% aqueous solution has a pH of 3.5 to 4.5.
Hydralazine hydrochloride is useful in the treatment of
Sodium nitropnicsidc.
doses, Its action appears to be centered on the smooth muscle of the vascular walls, with a decrease in peripheral resistance
sodium nitroferricyanide. disodium pentacyanonitrosylfer' ratc(2) Na2(Fe(CN)5N01 (Nipride, Nitropress), is one of die
to blood flow. This results in increased blood flow through the peripheral blood vessels. It also has the unique properly of increasing renal blood flow, an important consideration in patients with renal insufficiency. Hydralazine hydrochloride acts on vascular smooth muscle to cause relaxation, Its mechanism of action is unclear.
most potent blood pressure—lowering drugs. Its use is limiteJ
to hypertensive emergencies because of its short duratios of action. The effectiveness of sodium nitroprusside as as antihypertensive has been known since 1928. but not 1955 was its efficacy as a drug established.'3 The dnig ii' fers from other vasodilators, in that vasodilation occun is
release from intracel-
both venous and arterial vascular beds. Sodium nitropnissi& is a reddish-brown water-soluble powder that is decompascl
It interferes with Ca2 • entry and
lular stores and reportedly causes activation of guanylate cyclase. resulting in increased levels of cGMP. All of these biochemical events can cause vasodilation. Absorption of hydralazine hydrochloride taken orally is rapid and nearly complete. The maximal hypotensive effect is demonstrable within I hour. The drug is excreted rapidly by the kidneys, and within 24 hours. 75% of the total amount
administered appears in the urine as metabolites or unchanged drug. Hydralazinc hydrochloride undergoes benzylic oxidation. glucuronide formation, and N-acetylation by the microsomal enzymes in the tissues (Fig. 19-21). Acetylation appears to be a major determinant of the rate of hepatic removal of the drug from the blood and, therefore, of systemic availability.'2 Rapid acerylation results in a highly
hepatic extraction ratio from blood and greater first-pass elimination.
by light when in solution. The hypotensive effect of t& chemical is due to the formation of NO in situ (look wide: the heading. Nitrovasodilators). elevating cellular levelso: cGMP. Sodium nitroprusside is metabolized by the lise yielding thiocyanate. Because thiocyanrote is excreted by th
kidneys, patients with impaired renal function may suiTe: thiocyanate toxicity. Na2(Fe(CN)5NOI . 2H20
Sodium Nllroprusside (Nipride)
(Niteapress)
POTASSIUM CHANNEL AGONISTS
The two agents that can be classified in this category ui diazoxide and minoxidil. These drugs are also called peej SSUPI channel opener.r. These agents activate ATP.scnsiiis: potassium channels, which leads to a decrease of inuacclL
lar Ca2' and reduces the excitability of smooth musclc. iii: primary action of these drugs is to open potassium charnel Hydralazwue
(Apresoline)
in the plasma membrane of vascular smooth muwk. efflux of potassium from the cell follows, resulting in polarization of the membrane, which produces an
Chapter 19 • (ardimus(:th,r Age',,:c
655
influence on membrane excitation and subsequent vasodilaSulfotransferese N
N
Diazoxide is used us the sodium salt :f 7.chloro-3-methyl-2H-l,2,4-beniothiudiazinc 1.1-dioxiic (Hyperstat IV). Diazoxide lowers peripheral vascular reincreases cardiac output, and does not compromise Diazoxide, USP.
N
H2N
anti blood flow.
I N
N
NH2
H2N
N I
This isa des-sulfamoyl analogue of the benzothiazine diand has a close structural similarity to chiorothiazide. It developed intentionally to increase the anlihypertenaction of the thiazides and to minimize the diuretic ef-
NH2 —
0S03
o
Minoxdii Sulfate
Minoxidul
Figure 19—22 • Activation of minoxidil
zL
It is used by intravenous injection as a rapidly acting antiagent for emergency reduction of blood presin hospitalized patients with accelerated or malignant Over 90% is bound to serum protein, and cauis needed when it is used in conjunction with other
wicin-bound drugs that may be displaced by diazoxide. injection is given rapidly by the intravenous route to noire maximal effect. The initial dose is usually I mg/kg Ibody weight, with a second dose given if the first injection
not lower blood pressure satisfactorily within 30 mmFurther doses may be given at 4- to 24-hour intervals :1 needed. Oral antihypertensive therapy is begun as soon as The injection has a pH of about 11.5. which is nccessa,y S convert the drug to its soluble sodium salt. There is no :gniticant chemical decomposition after storage at room umperature for 2 years. When the solution is exposed to
Minoxidil is used for severe hypertension that is difficult to control with other antihypertensive agents. The drug has some of the characteristic side effects of direct vasodilatory drugs. It causes sodium and water retention and may require coadministration of a diuretic. Minoxidil also causes reflex tachycardia. which can be controlled by use of a gic blocking agent. Minoxidil topical solution is used to treat alopccia androgenitica (male pattern baldness). Although the mechanism
is not clearly understood, topical minoxidil is believed to increase cutaneous blood flow, which may stimulate hair growth. The stimulation of hair growth is attributed to vaso-
dilution in the vicinity of application of the drug. resulting in better nourishment of the local hair follicles.
N,
alit, it darkens.
Ci
0
0
MinoxidI (Lonhten)
Diazoxide (Hyperstat)
fl7noxidi!, USP.
Minoxidil. 2.4-diumino-6-piperidino-
(Loniten). was developed as a result of replacement of a triarninotriasine moiety by triamiThe triaminotriazines were initially observed potent vasodilators in cats and dogs following their of N.oxides in these animals. The triazines were humans because of their inability to form N-oxide netabolites: this led to the discovery of minoxidil. Minoxidil 'the only direct-acting vasodilator that requires metabolic siisation to produce its antihypcrtcnsive effect I Fig. 19-
h is converted to minoxidil sulfate in the liver by a The antihyperlensive properties of minoxidil are similar 'those of hydralazine hydrochloride, in that minoxidil can ccrcase arteriolar vascular resistance. Minoxidil exerts its
action by a direct effect on arieriolar smooth and appears to have no on the CNS or on the Jznergic nervous system in animals. The serum half-life :4.3 hours, and the antihyperlensive effect may last up to hours.
POSITIVE INOTROPIC AGENTS
Agents that successfully increase the force of contraction of the heart may be particularly useful in the treatment of CHF. In Cl-IF. the heart cannot maintain sufficient blood flow to various organs to provide oxygen-rich blood. Agents that increase the force of contraction allow greater amounts of
blood to be distributed throughout the body and, in turn. reduce the symptoms associated with CHF. Most of the posi-
tive inotropic agents exhibit their effects on the force of contraction by modifying the coupling mechanism involved in the myocardial contractile process. Digitalis glycosides. a mixture of products isolated from foxglove. Digitalis spp.. were Iirst used as a heart medication as early as 1500 uc when in the Ebs'rs Pupvni.c the ancient Egyptians reported their success in using these products. Throughout history these plant extracts have also been used as arrow poisons. emetics, and diuretics. The dichotomy of the poisonous effects and the beneficial heart properties is still evident today. Cardiac glycosidcs are still used today in the treatment of CHF and atrial librillation, with careful attention paid to monitoring the toxicity these agents possess.
656
Wilson and Gisvolds Textbook of Organic Medicinal and Pham,at-eutkal Chemistry
The cardiac glycosides include two distinct classes of compounds—the cardenolides and the bufadienolides. These differ in the substitutions at the C-l7 position, where the cardenolides possess an unsaturated butyrolactone ring. while the bufadienolides have an a-pyrone ring. Pharmacologically, both have similar properties and are found in many of the same natural sources, including plant and toad species.
By far, the most important sources include 1)igi:alis purp:Irea and I). lanata. In 1785. William Withering published "An Account of the Foxglove and Its Medical Uses: With
the chief active glycoside in digitalis leaf, with I mg digi. toxin equal to I g of digitalis leaf therapy. In patients who miss doses, digitalis is very uselul for maintenance therapy because of the longer half-life it provides. The longer dura. Lion and increased half-life are due to the lack of the C.12 hydroxy that is present in digoxin. In digoxin. this hydrnsv plays two roles: (a) it serves as a site for metabolism, which reduces the compound's half-life; and (b) it gives more Ity. drophilic character, which results in greater water sotubilil) and ease in renal elimination.
Practical Remarks on Dropsy and Other Diseases." in which
he describes the beneficial use of foxglove in dropsy (edema), which often exists in CHF. Even with recent advances in synthetic organic chemistry coupled with the use of combinatorial chemistry, no new therapeutics have displaced the cardiac glycosides. Furthermore, the perennial use of these agents over many centuries is even more remarkable when one considers the useful life
of a "block buster" drug in today's marketplace. This remarkable fact is based, quite simply, on the unique ability of nature to produce extraordinarily bioactivc substances. which characteristically possess both a lipophilic portion in the steroidal ring and a hydrophilic moiety in the glycosidic rings. The therapeutic use of these agents depends largely on a balance between the different solubility characteristics of the steroid structure, and the type and number of sugar units attached to it. Although the fundamental pharmacological properties reside with the steroidal nucleus, the sugars
Digoxin (Lanoxln) H3C
play a critical role in the biological effects elicited, since they increase the water solubility of the lipid system, making them more available for translocation in an aqueous environment and, at the same lime, allowing transportation across fatty sites. These properties uniquely balance each other and allow successful translocation to the receptive sites in the body. Ultimately, the lipophilic steroid also plays a specific role in the agent's onset and duration of action. As the steroi-
}
OH
digitoxose
dal rings are modified with polar groups (e.g.. hydroxyls). the onset increases and the duration of action decreases. The sugar residues are substituted on C-3 of the steroid and generally are digitoxose. glucose. rhamnose. or cymarose.
The cardiac glycosides elicit their effects through inhibi-
tion of the Na*/K -ATPasc pump. Inhibition of this pump increases the intracellular concentration, which affects Na + /Ca2 + exchange. This increases intracellular concentrations of which is available to activate the contractile
proteins actin and myosin. thereby enhancing the force of contraction. Also, it is suggested that these agents have other compensatory mechanisms including baroreceptor sensitiv-
ity, which result in improved conditions for patients suffering from CHF.
Digitalis (Crystodigin)
Digoxin (Lanoxin) is a purified digitalis preparation from Digizali.s lanaza and represents the most widely used digitalis glycoside. This wide use is primarily Digoxin.
due to its fast onset and short half-life. Position 3 of the steroid is substituted with three digitoxose residues that. when removed, provide a genin or aglycone steroid that is still capable of receptor binding but with altered pharmacokinetics.
Digitalis. Digitalis (Crystodigini is isolated from 0. Ia. flaw and 0. purpurea, among other Digitalis spp.. and is
{ Amrinone.
OH
}
During normal heart function. cAMP forms important roles in regulating
Chapter 19 • do. That is. certain calcium channels and storage sites for must be activated by cAMP-dependant protein kina.ses. Since cAMP plays an indirect role in the contractility process, agents that inhibit its degradation will provide more calcium for cardiac contraction. One phosphodiestera.se enryme
that is involved in the hydrolysis of myocardium
cAMP is F-ill. Amrinone. 5-amino (3.4'-dipyridin)-6 1(11)one (Inocor). possesses positive isotropic effects as a result
of its ability to inhibit this phosphodiesterase. In 1999, the U. S. Pharmacopoeia (USP) Nomenclature Committee and he United States Adopted Names (USAN) Council approved changing the nonproprictary name and the current official monograph title of amrinone to inarnrinono. This change in nomenclature was a result of amrinone being con-
fused with arniodarone because of the similarity of the usme.s. This was reported to cause confusion between the products that led to medication errors, some of which recalled in serious injury or death. 0
Amrtnone (Inocor)
I .6-dihydro-2-methyl-6-oxoMilrinonc, 1.4'-bipyridine-5-carbonitrile (Primacor). is another dipyridine phosphodiesterase F-Ill inhibitor that possesses pharmacological properties similar to those of amrinone. The inhibition of the degradation of cAMP results in an increase in the cardiac muscle's force of contraction.
Win none.
0
CH3
Cardiovascular
657
an underlying disease involving the liver, kidney, pancreas. or thyroid. or it may not be attributable to any recognizable disease. In recent years. lipids have been implicated in the development of atherosclerosis in humans. Atherosclero.si,s may be defined as degenerative changes in the intima of medium and large arteries. This degeneration includes the accumulation of lipids, complex carbohydrates, blood, and blood products and is accompanied by the formation of fi-
brous tissue and calcium deposition on the intima of the blood vessels. These deposits or plaques decrease the lumen of the artery, reduce its elasticity, and may create foci for thrombi and subsequent occlusion of the blood vessel.
Lipoprotein Classes are macromolecules consisting of lipid substances (cholesterol, triglycerides) noncovalently bound with protein and carbohydrate. These combinations soluhilize the lipids and prevent them from forming insoluble aggregates in the plasma. They have a spherical shape and consist of a nonpolar core surrounded by a monolayer of phospholipids whose polar groups are oriented toward the lipid phase of the plasma. Included in the phospholipid monolayer are a small number of cholesterol molecules and proteins termed apolipoprozeins. The apolipoproteins appear to be able to solubilize lipids for transport in an aqueous surrounding such as plasma (Fig. 19-23). The various tipoproteins found in plasma can be separated by ultracentriftigal techniques into chylomicrons. very-lowdensity lipoprotein (VLDL), intermediate-density lipoprotein (IDL). low-density lipoprotein (LDL), and high-density lipoprotein (HDL). These correlate with the electrophoretic
Lipoproreins
separations of the lipoproteins as follows: chylomicrons. prc-
(VLDL). broad /3-lipoprotein (IDL). $-lipoprotein (LDL). and a-lipoprotein (l-H)L). Chylomicrons contain 90% triglycerides by weight and
originate from exogenous fat from the diet. They are the least dense of the lipoproleins and migrate the least under the
Milnnone
(Pnmacor)
ANTIHYPERLIPIDEMIC AGENTS
The major cause of death in the Western world today is disease, of which the most prevalent form is atheroclerolic heart disease, Although many causative factors of disease are recognized (e.g.. smoking. stress, diet), athciuselerotic disease can be treated through medication or wgery.
Hvpcrlipidensia is the most prevalent indicator for suscep-
atherosclerotic heart disease: it is a term used to elevated plasma levels of lipids that are usually in form of lipoproteins. Hyperlipidernia may be caused by
ihility to
FIgure 19—23 • Hypothetical model of lipoprotein particle.
658
Wilsnis iou!
it
of Organic Medicitial am! Phannacetuical
influence of an electric current. Chylornicrons are normally absent in plasma after 12 to 24 hours of fasting. The VLDL is composed of about 60% triglycerides. 12% cholesterol. and 18% phospholipids. It originates in the liver from FFAs. Although VLDL can be isolated from plasma. it is cataholi,.ed rapidly into IDL. which is degraded further into Ll)L.
Exogenoua Pathway Dietary Fat _j.. Intestines
Lest
terol-carrymg protein. In normal persons, this lipoprotein
HDL is composed of 25% cholesterol and 50% protein and accounts for about 17% of the total cholesterol in plasma.
Upoprotein Metabolism The rate at which cholesterol and triglycerides enter the circulation from the liver and small intestine depends on the supply of the lipid and proteins necessary to form the lipoprotein complexes. Although the protein component must he synthesized, the lipids can be obtained either from de novo biosynthesis in the tissues or from the diet. Reduction of plasma lipids by diet can delay the development of atherosclerosis. Furthermore, the use of drugs that decrease assimi-
lation of lipids into the body plus diet decreases mortality 1mm cardiovascular Lipid transport mechanisms exist that shuttle cholesterol and triglycerides among the liver, intestine, and other tissues.
Remnaii
Endogonoua Pathway
usually not isolated from plasma. The LDL consists of 50% cholesterol and 10% triglycerides. This is the major choles-
formed from the intravascular catabolism of VLDL. The
•
FFA
Normally. IDL also is catabolized rapidly to LDL, but it is
accounts for about 65% of the plasma cholesterol and is of major concern in hypcrlipidemic disease states. The LDL is
I
Chytomicrons
WL rcrceoor
Liver—VLDL
L.pestovln Ifisaso
-......
•IDL—.LDL (srahersjc FFA
t.CAI HDL
Figure 19—24 • Exogenous and endogenous pathways poprotein metabolism.
plasma of about 1.5 days and represens 60 to 70% olik cholesterol in plasma. These LDL particles bind to LDL receptors in extrahepatic tissues and are removed tons thr plasma. Levels of LDL receptors vary depending on the of extrahepatic tissues to bind LDL to use cholesterol. ilir extrahepatic tissue subsequently releases HDL. Free plana
Normally. plasma lipids, including lipoprotein cholesterol,
cholesterol can he adsorbed onto HDL and the esters formed by the enzyme lecithin—cholesterol ferase (LCAT). These esters are transferred from FIDL
are cycled into and out of plasma and do not cause extensive
VLDL or LDL in plasma to complete the cycle. The 0
accumulation of deposits in the walls of arteries. Genetic
ways for plasma lipoprotein metabolism by the exogenot and endogenous routes arc shown in Figure 19-24.
factors and changes in hormone levels affect lipid transport by altering enzyme concentrations and apoprotein content. as well as the number and activity of lipoprotein receptors. This complex relationship makes the treatment of all hyperli-
Nyperlipoprotelnemlas
poproteinemias by a singular approach diilicult. if nol im-
Lipid disorders are related to problems of lipoprotein
practical. Lipids are transported by both exogenous and endogenous
that create conditions of hyperlipoprotcinemia. 1k hyperlipoprotcinemias have been classified into six type each of which is treated differently (Table 19-6). The abnormal lipoprotein pattern characteristic ol type I is caused by a decrease in the activity of lipoprotein an enzyme that normally hydrolyzes the triglycerides
pathways. In the exogenous pathway, dietary fat (triglycerides and cholesterol) is incorporated into large lipoprotein particles (chylomicrons). which enter the lymphatic system and are then passed into the plasma. The chylomicrons are acted on by lipoprotein lipase in the adipose tissue capillaries. forming triglycerides and monoglyccrides. The FFAs cross the endothelial membrane of the capillary and are incorporated into triglycerides in the tissue for storage as fat or are used for energy by oxidative metabolism. The chylo-
micron remnant in the capillary reaches the liver and is cleared from the circulation by binding to a receptor that recognizes the apoprotcin E and B-48 protein components of the chylounicron remnant. In the endogenous pathway of lipid transport. lipids are secreted front the liver. These are triglycerides and cholesterol combined with apoprotein B-l00 and apoprotein E to form VLDL. The VLDL on by lipoprotein lipase in the capillaries of adipose tissue to generate FFAs and an IDL. Some IDL hinds to LDL receptors in the liver and is cleared front plasma by endocytosis. Approximately half of
the cittulating IDL is converted to LDL in the plasma by additional loss of triglycerides. This LDL has a half-life in
in chylomicrons and clears the plasma of this fraction. Because the triglycerides Fotund in chylomicrr come primarily from exogenous sources, this typc of hyped poproteinemnia may be treated by decreasing the intakiil
dietary fat. There arc no drugs at present that can k to counteract type I hyperlipidemia effectively. Type II hyperlipoproteinemia has been divided into Ila and lIb. Type ha is characterized by elevated LDL ($-lipoproteins) and normal levels of triglycaidIs This stibtype disorder is very common and may by disturbed catabolism of LDL. Type llb differs Ila. in that this hyperlipidemia has elevated VLDL lewlr addition to LDL levels. l'ypc II hyperlipopromeincnni often clearly familial and frequently inherited as an mal dominant abnormality with complete penclrancc expression in infancy. Patients have been treated It) Sir dietary restrictions on cholesterol and saturated fais Thi
type of hyperlipoproteinemia responds to some fanii
Chapter 19 •
TABLE 19-6
659
CharacterizatIon of Hyperlipoproteinemla Types Abnormality
Hyperilpoprotelnemla I
hi
Electrophoresis
Appearance of Plasma
Ultracentrifuge
Massis e cliyk,microne,nia
Cleuj: creamy layer of chyloiniemiiemr:t on lop
P-Llpoprotdnr elevated
lb
eIe% tred
Triglycerides
Total Cholesterol
Slightly to ntotkratcly elcsiled
LOL
Clear
elevated
Ll)L -I VLDL increased
Slightly turbid
Slightly elevated
l'kavhly elevated
hand
VI.DLft.h)L ut abnormal colilporition
Slightly turbid to turbid
Elevated
Elevated
Heavily elevated
Ill
limad
IV
l're-l?..lipoprotein% elevated
VLDI. inuruscd
Turbid
Moderately to heavily clevated
Normal to
V
Pre-13-lipoproteins elevated: chyloniicronemia
VLDL increased: chylo:tiierouiemia
'rurhid; on top.
Massively elevated
Slightly elevated
elevated
chlomierorwmia
Adapted from Wine. E. C.: Prop. Med. Chcm. I 1:199. 1975. having been kept standing at .1' C lor 25 hours.
themotherapy. The combined therapy may bring LDL levels back
to normal,
Type HI is a rare disorder characterized by a broad band Like type II. it is also familial. Patients respond favorably to diet and drug therapy. of
In type IV hyperlipoproteincmia. levels of VLDL are dcvaled. Because this type of lipoprotein is rich in triglycerides. plasma triglyceride levels are elevated. The metabolic defect hat causes type IV is still unknown: this form of hyperlipidcuria, however, responds to diet and drug therapy. Type V hyperlipoproleinemia has high levels of chylomiand VLDL. resulting in high levels of plasma trig lycerides. The biochemical defect of type V hyperlipoproteinemia is not understood. Clearance of dietary fat is impaired. and reduction of dietary fat is indicated along with drug therapy.
aofibrate,
LISP. Clofibrate. ethyl 2-(p-chlorophenuxy)-2-tnethylpropionate (Atromid-S). is a stable, colorless 0 pole yellow liquid with a faint odor and a characteristic taste. It is soluble in organic solvents but insoluble in waler. Clofibrute is prepared by a Williamson synthesis. condeasing p.chlorophenol with ethyl a-bromoisobutyrate. or by the interaction of a mixture of acetone. p-chlorophenol. and chloroform in the presence of excess potassium hydrox-
levels by enhancing removal of triglycerides from the circulation and causes reduction of VLDL by stimulating lipoprotein lipase to increase the catabolism of this lipoprolein to
LDL."9 Clofibrate lowers triglyceride levels in the serum much more than cholesterol levels and decreases levels of FFAs and phospholipids. The lowering of cholesterol levels may result from more than one mechanism. Clofibrate inhibits the incorporation of acetate into the synthesis of cholesterol. between the acetate and mevalonate step, by inhibiting .cn-glyceryl-3-phosphate acyltransfera.se. Clofibrate also
regulates cholesterol synthesis in the liver by inhibiting microsomal reduction of 3.hydroxy-3-methylglutaryl-CoA (HMG-CoA). catalyzed by HMG-CoA reductasc. Clotibrute may lower plasma lipids by means other than impairment of cholesterol biosynthesis, such as increasing excretion through the hiliary tract.
Clofibrate is tolerated well by most patients; the most common side effects are nausea and, to a smaller extent. other gastrointestinal distress. The dosage of anticoagulants,
if used in conjunction with this drug, should be reduced by one third to une half, depending on the individual response. so that the prothrombin time may be kept within the desired limits. ?HJ
ide. The acid obtained by either of these methods is esterifted
to give clofibrdte. Both acid and ester are active: the latter. however, is preferred for medicinal use. Clofibrate is hydrolyzed rapidly to 2-p-chlorophenoxy-2-methylpropionic acid ho eslerases in vivo and, bound to serum albumin, circulates in blood. The acid has been investigated as a hypolipidemic agent. II is absorbed more slosvly and to a smaller extent than is the ester. The aluminum salt of the acid gives even
kiwer blood levels than p.chliphenoxy-2mcthylpropionic Cloltbrate is the drug of choice in the treatment of type Ill hyperlipoprotcinemias and may also be useful, to a lesser mlent. in types lIb and IV hyperlipoproteinemius. The drug
O—C—C—O—CH2CII3 CH3
Clofibrate (Atromid)
Gemtihrozil, 5-(2.5-dimethylphenoxy)Gem fibrozil. 2.2-dimethylpentanoic acid (Lopidl. is a congener of dollbratc that was used lirst in the treatment of hyperlipoproteinemia in the mid-l970s. Its mechanism of action and use
are similar to those ol clofibrate. Gemfibrozil reduces
knot effective in types I and Ila. Clotibratc can lower plasma concenlrations of both tn-
plasma levels of VLDL triglycerides and stimulates clearance of VLDL from plasma. The drug has little effect on
glvcerides and cholesterol. but it has a more consistent clinical effect on triglycerides. It also affects lipoprotcin plasma
cholesterol plasma levels but does cause an increase of I-IDL. Gemlibrozil is absorbed quickly from the gut and excreted
660
Textbook
Wi/so,, and
of Orgaiiir Medicinal and Plwnnaceiuical Chemistry
unchanged in the urine. The drug has a plasma half-life of 1.5
hours, but reduction of plasma VLDL concentration takes between 2 and 5 days to become evident. The peak effect of its hypolipidemic action may take up to 4 weeks no become manifest. CH3
0-
CH3
O_(CH2)s_?_C_OH CH3
Dextrothyroxine (Choloxin)
H3C
Use of thyroxine in the treatment of hyperlipidcmias is not without adverse effects. The drug increases the Ire.
Gemfibrozil (Lopid)
quency and severity of anginal attacks and may cause cardiac
Fenofibrate, 2-[4-(4-chlorobenzoyl)phenoxyl-2-methylpropanoic acid I -methylethyl ester (Tricor),
Fenofibrate.
has structural features represented in clofibrate. The primary difference involves the second aromatic ring. This imparts a greater lipophilic character than exists in clofibrate. resulting in a much more potent hypocholesterolemic and triglyc-
eride-lowering agent. Also, this structural modification resuits in a lower dose requirement than with ciofibrate or gemfibrozil. CH3
/CH3
Fenofibrate (Tricor)
Dextrothyroxine soUSP. dium. O-(4-hydroxy-3,5-diiodophenyl)-3.5-diiodo-fl-tYrosine monosodium salt hydrate, sodium u-3.3'.5.5'-tctraiodo-
Dextrothyroxine Sodium,
thyronine (Choloxin). occurs as a light yellow to buff powder. it is stable in dry air but discolors on exposure to light: hence, it should be stored in light-resistant Containers. It is very slightly soluble in water, slightly soluble in alcohol. and insoluble in acetone, chloroform, and ether. The hormones secreted by the thyroid gland have marked
hypocholesterolemic activity along with their other wellknown actions. The finding that not all active thyroid principles possessed the same degree of physiological actions led to a search for congeners that would cause a decrease in serum cholesterol levels without other effects such as angina pectoris. palpitation. and congestive failure, u-Thyroxine resulted from this search. At the dosage required, however. thyroxine contamination must be minimal: otherwise, it will exert its characteristic actions. One route to optically pure (at least 99% pure) u-thyroxine is the use of an L-amino acid oxidase from snake venom, which acts only on the i. isomer and makes separation possible. The mechanism of action of u-thyroxine appears to be stimulation of oxidative catabolism of cholesterol in the liver through stimulation of 7-a-cholesterol hydroxylase. the rate-
arrhythmias. u-Thyroxine potentiates the action of anticoagulants such as warfarin or dicumarol; thus, the dosage of the anticoagu• lants used concurrently should be reduced by one third and
then, if necessary, further modified to maintain the pro. thrombin time within the desired limits. Also, it may the dosage requirements for insulin or oral hypoglycemic agents if used concurrently with them.
cholestyramlne Resin, USP. Cholestyraminc (Cuemid. Questran) is the chloride form of a strongly basic a styrene copolymer with divinyl benzene with quaternary ammonium functional groups After oral ingestion, cholestyramine resin remains in the trointestinal tract, where it readily exchanges chloride loni for bile acids in the small intestine, to be excreied as bile sab in the feces. Cholestyramine resin is also useful in
plasma lipids. The reduction in the amounts of bile acids results in increased catabolism of cholesterol in bile acids in the liver. The decreased concentration of bile acids returning to the liver lowers the feedback inhibition by bile acids of 7-a-hydroxylase. the rate-limiting ennyna in the conversion of cholesterol to bile acids. increasing tie breakdown of hepanic cholesterol. Although biosynthesis cholesterol is increased, it appears that the rate of catabolisit is greater, resulting in a net decrease in plasma cholestec&
levels by affecting LDL clearance. The increase of 11)4. receptors in the liver that occurs when its content of terol is lowered augments this biochemical event. Cholcstyramine resin does not bind with drugs that nit neutral or with amine salts: acidic drugs (in the anion fomi could be bound, however. For example. in animal tests. sorption of aspirin given concurrently with the resin we' depressed only moderately during the first 30 minutes.
CH—CH2—CH—CH,
limiting enzyme in the conversion of cholesterol to bile acid.s. The bile acids are conjugated with glycine or taurine and excreted by the hiliary route into the feces. Although thyroxine does not inhibit cholesterol biosynthesis, it increases the number of LDL receptors, enhancing removal of LDL from plasma.
CH2
CH
Chotestyramine Resin (Cholybar) (Questran)
CH,N(CH3b
Chapter 19 •
Cholestyramirre resin is the drug of choice lbr type Ha hyperlipoproteinemia. When used in conjunction with a controlled diet, it reduces /3-lipoproteins. The drug is an insoluHe polymer and, thus, probably one of the safest because it is not absorbed from the gastrointestinal tract to cause systemic toxic effects.
Colestipol Hydrochloride. Colestipol (Colestid) is a high-molecular-weight, insoluble, granular copolyrner of teraethylenepentamine and epichlorohydrin. It functions as an anion-exchange, resin-sequestering agent in a manner simiar to that of cholestyramine resin. Colestipol hydrochloride reduces cholesterol levels without affecting triglycerides and seems to be especially effective in the treatment of type U hyperlipoprotcinemias.
Agenls
661
synthesis and, subsequently. its plasma products. IDL and LDL. Plasma triglyceride levels are reduced because of the decreased VLDL production. Cholesterol levels are lowered, in tum, because of the decreased rate of LDL formation from
VLDL. Although niacin is the drug of choice for type II hyperlipoproteinemias. its use is limited because of the Vasodilating side effects. Flushing occurs in practically all patients but generally subsides when the drug is discontinued.
The hypolipidemic effects of niacin may be due to its ability to inhibit lipolysis (i.e.. prevent the release of FFAs and glycerol from fatty tissues). As a consequence, there is
a reduced reserve of FFA in the liver and diminution of lipoprotein biosynthesis, which reduces the production of VLDL. The decreased formation of lipoproteins leads to a pool of unused cholesterol normally incorporated in VLDL. This excess cholesterol is then excreted through the biliary tract.
Niacin (nicotinic acid) may be administered as aluminum nicotinate (Nicalex). This is a complex of aluminum hydroxy
nicotinate and niacin. The aluminum salt is hydrolyzed to aluminum hydroxide and niacin in the stomach. The aluminum salt seems to have no advantage over the free acid. Hepatic reaction appears more prevalent than with niacin. Nicotinic acid has been esterified to prolong its hypolipColestipot (Colastid)
Colesevelam (Welchol) is one of the more recent additions to the class of bile acid-sequestering agents. Its structure is rather novel, and at first glance, it appears to look like the previous examples of cholestyramine and co!e.stipol. It does not possess the chloride ions, however, and, strictly speaking, is not an anion-exchange resin. This compound has good selectivity for both the trihydroxy and dihydroxy bile acids. The selectivity for these hydroxyLated derivatives lends some insight into the reduced side effects colesevelam possesses, compared with cholestyramine and colestipol. Unlike the older agents. colesevelam not have a high incidence of causing constipation. This results from the compound's ability to "pick up" water be-
idemic effect. Pentaerythritol tetranicotinate has been more effective experimentally than niacin in reducing cholesterol levels in rabbits. Sorbitol and mvo-inositol hexanicotinate polyesters have been used in the treatment of patients with atherosclerosis obliterans. The usual maintenance dose of niacin is 3 to 6 g/day given in three divided doses. The drug is usually given at mealtimes to reduce the gastric irntation that often accompanies large doses.
of its affinity for hydroxyl system (i.e.. hydrogen
NiCotinic Acid (Niacin)
with either the bile acid or water). In turn, this yields softer, gel-like materials that are easier to excrete.
I-
fl-Sitosterol.
Sitosterol is a plant sterol. whose structure is identical with that of cholesterol, except for the substituted ethyl group on C-24 of its side chain. Although the mecha-
nism of its hypolipidemic effect is not clearly understood. it is suspected that the drug inhibits the absorption of dietary cholesterol from the gastrointestinal tract. Sitosterols are absorbed poorly from the mucosal lining and appear to compete with cholesterol for absorption sites in the intestine.
>0*1
n
Colesevelam (Wetchol)
Nicotinic acid. 3-pyridinecarboxylic Nkotinlc Acid. (Niacin), is effective in the treatment of all types of hyperlipoproteinemia except type I. at doses above those given as a vitamin supplement. The drug reduces VLDL
662
of Organir Medicinal and Pharonaceniical (he,nis,rv
Wilson and (Jixrold's
Probucol. USP.
Prohucol, 4.4'-I( I -methylethylidene)DH-58 I his(lhio)IbisI2.6-his( 1.1 -dimethylethyl)phenol I. (Lorelco). is a chemical agent that was developed for the plastics and rubber industry in the l960s. The probucol molecule has two tertiary hutyiphenol groups linked by a dithiopropylidene bridge, giving it a high lipophilic character with strong antioxidant properties. In humans, it causes reduction
the surface of cell membranes. After binding and endocyto. sis of the receptor and LDL, lysosomal degradation of this complex in the cell males cholesterol available for use in cellular membrane synthesis. It is generally accepted that total plasma cholesterol is lowered most effectively by ducing LDL levels. Therefore, the population of LDL receptors is an important component of clearing the plasma of
of both liver and serum cholesterol levels, hut it does not alter plasma triglycerides. It reduces LDL and (to a le.sser extent) HDL levels by a unique mechanism that is still not clearly delineated. The reduction of HDL may be due to the ability of probucol to inhibit the synthesis of apuprotcin AI. a major protein component of HDL3° It is effective at
cholesterol. HMG-CoA rcductase inhibitors contribute to this by directly blocking the active site of the enzyme. This action has a twofold effect on cholesterol plasma levels: ii
reducing levels of LDL and is used in hyperlipoprotcinemias characterized by elevated LDL levels.
patients with familial hypercholesteremia.
causes a decrease in de novo cholesterol synthesis and an increase in hepatic LDL receptors. These HMG-CoA me. ductase inhibitors arc effective hypocholesteremic agents in Three drugs. lovastatin. simvastatin. and pravastatin. com-
pose the list of approved HMG-CoA reductase inhibitor'. in the treatment of hyperlipidemia in patients. The three drugs have structures similar to the substrate, HMG-CoA, of he enzyme HMG-CoA reductase. Lovastatin and simnvaslalin
arc lactones and prodrugs, activated by hydrolysis in liv liver to their respective acids. Pravastatin. in LOStrast, is administered as the sodium salt of the acid.
Lovastatin. 2-methylbutanoic acid 1.2,33. 8.8a-hexahydro-3.7-dimcthyl-8-
Probucol
(Lorelco)
I -naphthalenyl
HMG-COA Reductasa Inhibitors Drugs in this class of hypolipidemic agents inhibit the enzyme HMG-CoA reductase. responsible for the conversion of HMG-CoA to mevalonate in the synthetic pathway for the synthesis of cholesterol (Fig. 19-25). I-IMG-CoA reductase is
the rate-limiting catalyst for the irreversible conversion of HMG-CoA to mevalonic acid in the synthesis of cholesterol. The activity of HMG-CoA reductase is also under feedback regulation. When cholesterol is available in sufficient amounts for body need.s. the enzyme activity of HMG-CoA
acid.
Elevated plasma cholesterol levels have been correlated with an increase in cardiovascular disease. Of the plasma lipoproteins. the LDL fraction contains the most cholesterol. The source of cholesterol in humans is either the diet or de novo synthesis with the reduction of HMG-CoA by HMGCoA reductase as the rate-limiting step. Ingested cholesterol
CH3
as the free alcohol or ester is taken up after intestinal absorp-
tion and transported to the liver and other body organs through the exogenous pathway (Fig. 19-25). The LDL delivers cholesterol to peripheral cells. This process occurs after binding of LDL to specific LDL receptors located on
C—SC0A
cooHGG-C0A
Lovastatin (Mevacor)
CH2OH +
2NAOPH
mneo-
originally from the fermentation products of the fungi gilliss terre'us and Mona.ccu.c raiser. Lovastatin was one of two original HMG-CoA reductase inhibitors. The mevastatin (formerly called compactin). was isolated from cultures of Penicilliu,n cit/lu,,: cumin. Mevastatin was with drawn from clinical trials because it altered intestinal norphology in dogs. This effect was not observed with stalin. For inhibitory effects on HMG-CoA reductace. hr lactone ring must he hydrolyzed to the open-ring heptanoic
reductase is suppressed.
HO—C—CH3
ester,
noun, MK-803 (Mevacor) (formerly called s a potent inhibitor of HMG-CoA. The drug was obtained
+
HO—C— CH3
+
cooMevalonate
Figure 19—25 • HMG-C0A reductase reactmon.
C0A
+
Chapter 19 U Cardim'asc:dar Ags'sz:s Simvastatin.
Simvastatin. 2,2-dimethyl butanoic acid,
663
lower than those of the agents that possess a lactone ring as
part of their architectural design.
.naphthalenyl ester (Zocurl, is an analogue of lovastatin. These two drugs have many similar properties. Both drugs, in the prodrug form. mach the liver unchanged after oral administration, where hey undergo extensive metabolism to a number of opening hydroxy acids, including the active fl-hydroxy acids. Jroxy.6.oxo-2-pyran-2-yl)ethyl
They arc also highly bound to plasma proteins. These actions stake the bioavailability of simvastatin rather poor hut better than that of lovastatin, which has been estimated to be 5qc,
H
H3C-CCH3 CH=CH—CH-CH2-CHCH2COOH OH
OH
Q F
Fluvastatin (Lescol)
Atorvastatin. H3C
CH3
Atorvastatin. I-2-(4-tluoroI phcnyl)-b.d-dihydroxy-5-( I -methylethyl )-3-phenyl-4 I(PhC-
nylanuno)carbonylj-lH-pyrrolc- I -heptanoic acid (Lipitor). also possesses the heptanoic acid side chain, which is critical
for inhibition of HMG-CoA reductase. Although the side Simvastattn (Zocor)
Pravastatiti, sodium I .2.6,7.8.8a-hexahydso-$.ö,6-tnhydroxy-2-methyl-X-( 2-methyl-I -oxohutoxy)l-naphthaleneheptanoate (Pr,ivachol). is the most rapid-acting of the three HMG-CoA reductase inhibitor drugs. reaching a peak concentration in about I hour. The sodium salt uf the f3-hydroxy acid is more hydrophilic than the lactone forms of the other two agents, which may explain this propetny. In addition, the open form of the lactone ring contributes to a more hydrophilic agent. which, in turn, results in kss CNS penetration. This explains, in part, why pravastatin has fewer CNS side effects than the more lipophilic lacione ester of this class of agents. Absorption of pravastatin folowing oral administration can be inhibited by resins such as chulestyramine because of the presence of the carboxylic acid function on the drug. The lactone forms of lovastatin and siunvastatin arc less affected by cholestyrainine. Pta vastatin.
chain is less lipophilic than the lactone form, the high amount of lipophilic substitution causes this agent to have a slightly
higher level of CNS penetration than pravastatin, resulting in a slight increase in CNS side effects. Even so. its CNS profile is much lower than that of lovastatin. 'OH
Atorvastatin (Lipitor)
cerivastatln.
(Baycol) is one of the newer agents in this class of cholesterol-lowering agents. It carries,
however, a higher incidence of rhabdomyolysis and, as a result, was voluntarily withdrawn from the market by its manufacturer in 2001.
0
Pravastalin (Pravachol)
Fluvastatin. I -( I -methylethyl )- I H-indol-2-ylI-3.5-dihydmxy-6-heptenoic acid monosodium salt (Lescol). is very 'imilar to pravastatin. It possesses a heptanoic acid side chain that is superimposable over the lactone ring found in and simvastatin. It is this side chain that is recogsized by HMG-CoA reductase. Also, much like pravastatin. he CNS side effects of this lipid-lowering agent are much
\/
'NH —
OH
OH
Cerivastatin (Baycol)
-,, F
Fluvastatin.
ANTICOAGULANTS A theory of blood clotting introduced in 1905 was based on
the existence of four factors: thromboplastin (thrombokinase), prothrombin. librinogen. and ionized calcium. The clotting sequence proposed was that when tissue damage occurred. thromboplastin entered the blood from the platelets
664
of Or anie Medicinal and Phar,naceiuieal Chemistry
Wllsoii and Gis%'o!ds
Trauma
TABLE 19-7 Roman Numerical Nomenclature of Blood-Clotting Factors and Some Common Synonyms
acivahon intnnsic Pathway
Synonyms
Factor
XII —. 5118' I
XI —s-. Xla'
II
Pnitlinimbin
III
'Thrombopiastin. lissuc Iaetnr
IV
Cakium
V
Prosccclcrjut. aceclerulor globulin. labile factor
VI
(ibis nurnbcr is nut noW uscili
VII VIII
l'roconvcrtin. stabk tudor, ituoprodmimbin I, SPCA
iX — IXa' Viii Ca2'. FF3
Vii' + Thromboglastin
Prothrombin' .—. Throinbin'
Ii'
nboplastin component (FTC). Christmas factor. platcict rofacior II. autoprnthroinbin U. untiheinophilic tudor B
Plasnia tIm
X
Stuart-PoWer fucuar. Swan tudor. au(oprnthromhin III
XI
Plasma Iltrumboplustin untihemophilic (actor C
• X' —. Xe' jv c&. PF3
Anlihciuophllic factor. arrtibcnmpiiilic globutin. platOk! etifactor I. antihcniirphIzc factor A
IX
XII XIII
Extrinsic Pathway
Its XII
Fibonogen
I—
PTA).
Hageman factor
Fibrin.stubiliiing fuclor. fihnnusc. Lalci-Lorand factor Piasminogen
and reacted with prothrombin in the presence of calcium to
form throinbin. Thrombin then reacted with fibrinogen to form insoluble fibrin, which enmeshed red blood cells to create a clot. The concept remained unchallenged for almost 50 years. hut it has now been modilied to accommodate the discovery of numerous additional factors that enter into the clotting mechanism (Table 19-7).
Mechanism of Blood Coagulation The fluid nature of blood can be attributed to the flat cells (endothelial) that maintain a nonthrombogenic environment in the blood vessels. This is a result of at least four phenomena: (a) the maintenance of a transmural negative electric charge that prevents adhesion between platelets: (h) the iclease of a plasmalogen activator, which activates the fibrinolytic pathway; (c) the release of thrombomodulin. a cofactor that activates protein C. a coagulation factor inhibitor; and (d) the release of PGI2. a potent inhibitor of platelet aggregation. The process of blood coagulation (Fig. 19-26) involves a series of steps that occur in a cascade and terminate in the formation of a fibrin clot. Blood coagulation occurs by acti-
vation of either an intrinsic pathway, a relatively slow process of clot formation, or an extrinsic pathway, which has a much faster rate of fibrin formation. Both pathways merge into a common pathway for the conversion of prothrombin to thrombin and subsequent transformation of fibrinogen to the insoluble strands of librin. Lysis of intravascular clots occurs through a plasnuinogen—plasmin system, which con-
sist.s of plasminogen. plasmin. urokinase. kallikrein. plasminogen activators, and some undefined inhibitors. The intrinsic pathway refers to the system for coagulation that occurs from the interaction of Ilictors circulating in the blood, It is activated when blood comes into contact with a
- Piasmm
Fbriret*#cn
FIgure 19—26 • Scheme of blood coagulation and fibrinolysii . a vitamin K—dependent factor; '. inhibition by heparin ard antithrombin III.
damaged vessel wall or a foreign substance. Each of the plasma coagulation factors (Table 19-7). with the exception of factor III (tissue thromboplastin). circulates as an inactive proenzyme. Except for fibrinogen, which precipitates as fl
brin, these factors are usually activated by enzymatic removal of a small peptide in the cascade of reactions itul make up the clotting sequence (Fig. 19-26). The exvrin.ck clotting system refers to the mechanism by which thrombin is generated in plasma after the addition of tissue When various tissues, such as brain or lung (containing thromboplustin). are added to blood, a complex between thromboplastin and factor VII in the presence of calcium ions activates factor X. bypassing the time-consuming of the intrinsic pathway that form factor X.
The intrinsic and extrinsic pathways interact in situ Small amounts of thrombin formed early after stimulatiom of the extrinsic pathway accelerate clotting by the pathway by activating factor VIII. Thrombin also speedt the clotting rate by activating factor V. located in the cornmon pathway. Thrombin then converts the soluble protein
fibrinogen into a soluble tibrin gel by acting on bonds to remove small librinopeptides from the N enabling the remaining tibrinogen molecule to It also activates factor XIII. which stabilizes the fibrin in the presence of calcium by cross-linking between chains of the fibrin monomer through intennolecular y.glutamyl—lysine bridges to form an insoluble mass.
Anticoagulant Mechanisms In the milieu of biochemicals being formed to facilitate iht
clotting of blood, the coagulation cascade in vivo Ic cs
Chapter 19 U ('ardioia.wuh,r Agents
rolled by a balance of inhibitors in the plasma to prevent all of the blood in the body from solidifying. Thromhin plays
a pivotal role in blood coagulation. It cleaves librinogen. a reaction that initiates lormauon of the tibrin gel, which constitules the framework of the blood clot. As mentioned above, it activates the cofactors factor V and factor VIII to accelerate the coagulation process. Intact endothelial cells espress a receptor. thrombomodulin. for thrombin. When thrombin is bound to thrombomodulin. it does not have coagulant activity, which thus prevents clot tbrmation beyond damaged areas and onto intact endothelium. In this hound state, however. thrombin does activate protein C. which then
inactivates two cofactors and impedes blood clotting. Thrombin also activates factor XIII. leading to cross-linking
of the librin gel. The activity of thrombin is regulated by its inactivation by plasma protein inhibitors: a1.proteinase inhibitor. antithronsbin antithroinhin III). and heparin cofactor II. These belong to a of proteins called .oerpins. an acronym for .wrine firutease inIiibiror.c.
Antithromhin Ill, an a'2-globin. neutrali,.es thrombin and the serinc proteases in the coagulation cascadc—Xa. IXa.
XIa, and XlIa. Although antithrombin III is a slow-acting inhibitor, it becomes a rapid-acting inhibitor of thrombin in the presence of heparin. Heparin is a naturally occurring anticoagulant that requires uintithrombin ill (see above) for its biological property of preventing blood clot formation. It binds at the lysine site of the antithrornbin Ill molecule. causing a change in the conformation of antiihroinbin III and increasing its anticoagulant properties. Heparin can then
dissociate from antithrombin III to hind to another antithroinhin Ill molecule. An additional system, which controls unwanted coagulation, involves protein C. a vitamin K—dczymogen in the plasma. Protein C is converted to a serine protease when thronibin and factor Xa. fisrmed in the blood in the coagulation cascade, interact with thromboinodulin. The now-activated protein C inhibits factors V and VIII and, in so doing, blocks further production 01' thrombin. Protein C also enhances librinolysis by causing release of the tissue plaruminogen activator.
The biosynthesis of prothrombin (factor II) depends on an adequate supply of vitamin K. A deficiency of vitamin K results in the fonnation of a defective prothrombin molecult. The defective prothrombin is antigenically similar to simsal prothromhin but has reduced calcium-binding ability arid no biological activity. In the presence of calcium ions. nomual prothromhin adheres to the surface of phospholipid
vecicles and greatly increases the activity of the clotting niechanis,n. The defect in the abnornial prothrombin is in the NH,-tcrminal portion, in which the second carboxyl residue has not been added to the y.carbon atom of some gluasic acid residues on the prothrombin molecule to lbrm yuarboxyglutamic acid.7' Administration of vitamin K antagmists decreases synthesis of a biologically active prothrom-
665
gested that vitamin K drives the carboxylase reaction by abstracting a proton from the relatively unreactive nuethylene carbon of the glutamyl residue, forming a 2.3-epoxide. Oral anticoagulants intericre with the y.carhoxylation ofglutamue acid residues by preventing the reduction of vitamin K to its hydroquinone form (Fig. 19-27). Hemophilia A. a blood disease characterized by a deficiency of coagulation factor VIII. is the most common inherited blood coagulation disorder. Treatment of this disease over the past 25 years has depended on the concentration of the antiheniophilic factor (factor VIII) by cryoprecipitation and immunoaffinity chromatography separation technology. The impact of this therapy has been diminished by the presence of viruses that cause the acquired imninunodeficiency syndrome (AIDS) and other less tragic viral diseases in humans. Recombinant antihemophilic factor preparations have been produced since 1989 with use of mammalian cells genetically altered to secrete human factor VIII. Kogenatc and Helixate are recombinant preparations, obtained from genetically altering baby hamster kidney cells that contain high concentrations of factor VIII. Recombinant factor Vila. an active tactor in the extrinsic pathway, now in phase III clinical trials (Novo Seven), has been used to treat patients with VII deficiency. Hemophilia 13. another hemophilia A genetic blood disorder, which constitutes about 20% of hennophilia cases, is caused by a deficiency of factor IX and has been treated from crvoprecipitated fiactions obtained from plasma. Monoclonal antibody technology has produced an essentially pure. carrier-free preparation of native factor IX (Mononine). Recombinant technology has solved the problem of limited supply and viral contamination of' these critical blood factors.
Platelet Aggregation and Inhibitors Blood platelets play a pivotal role in hemosta.sis and thrombus formation. Actually, they have two roles in the cessation
of bleeding: a hemostatic function, in which platelets. through their mass, cause physical occlusion of openings in blood vessels, and a thromboplastic function, in which the chemical constituents of the platelets take part in the blood coagulation mechanism. The circulatory system is self-sealing because of the clotting properties of blood, The patholog-
ical formation of clots within the circulatory system, however, creates a potentially serious clinical situation that must be dealt with through the use of annicoagulants. Platelets do not adhere to intact cndothelial cells. They do become affixed to subendothelial tissues, which have been
exposed by iniury. no cause hemostasis. Platelets bind to collagen in the vessel wall and trigger other pluteleLs to adhere to them. This adhesiveness is accompanied by a change in shape of the platelets and may be caused by mobilization
bin molecule and increases the clotting time of blood in
of calcium bound to the platelet membrane. The growth of the platelet mass depends on the adenosine diphosphate (ADP) released by the first few adhering cells and enhances
humaans.la
the aggregation process. A secondary phase (phase II imme-
Vitamin K is critical to the formation of clotting factors VII. IX. and X. These factors are glycoproteins that have ycarboxyglutamic acid residues at the N-terminal end of the
chain. The enzyme involved in forming an active
diately follows. svith additional platelet aggregation. In this secondary phase, the platelets undergo a secretory process
during which enzymes such as cathcpsin and acid bydroluses. along with tibrinogen. are released from en granules
in the platelets and ADP. ATP. scrotonin. and calcium are in
the microsnmal fraction of liver cells. It has been sug-
released from dense bodies in the platelets. The dense bodies
666
Wllxon and Gisiold'.5 Textbook of Orgw,k Medicinal and Pharmaceutical Cherni.%lry
1?
HCCOOH OOH
(gtu)
COOH
(gte)
CO2
OH
OH
Vii K (hydroqulnon.)
VII K (eposida)
Vii K reductase
Vit K (quulonel
Warfann
9
Warfa,,n
Figure 19—27 • Mechanism of action of vitamin K and sites of action of warfarin.
are likened to the storage granules a.ssociated with adrencrgic
similar to that in hemostasis. The factors contributing In
neurons. Increased levels of cAMP inhibit platelet aggregation, cAMP activates specific dependent kinases, which form
venous thrombosis arc circulatory stasis, excessive genera.
protein—phosphate complexes that chelate calcium ions. The
reduced levels of calcium inhibit aggregation (Fig. 19-28). Inhibitors of platelet aggregation can increase cAMP levels by either stimulating adenylate cyclase or inhibiting phosphodiesterasc.72 Substances such as glucagon. adenosine. and isoproterenol increase cAMP levels and inhibit platelet aggregation. Drugs such as theophylline, aminophyllinc. dipyramidole. papaverine. and adenosine inhibit phosphodiesterase and aggregation of platelets. Epinephrine, collagen. and serotonin inhibit adenylate cyclase and stimulate platelet aggregation.73 The role of platelets in arterial thrombosis is
5' AMP
ATP
Lion of thrombin formation of fibrin, and, to a than in the artery, platelet aggregation. Aspirin. sulfinpyrazone. and indomethacin have an inhihi
tory effect on platelet aggregation. They inhibit cyclooay genase. the enzyme that controls the formation olpmstaglaidin endoperoxides and increases the tendency for platelets to aggregate.74 Aspirin also inhibits the tion. Dipyridamole inhibits adenosine deaminase and a result, the increased plasnu
concentrations of adenosine inhibit ADP-induccd tion of platelets. Among the many pharmacological actions of dins is the ability of some to stimulate or inhibit the tion of platelets and alter the clotting time of blood. Prosti glandins are synthesized from 20-carbon polyunsaturale fatty acids containing from three to five double bonds. These
fatty acids are present in the phospholipids of cell mesh branes of all mammalian tissues. The main precursor of pros Cyciase
taglandins is arachidonic acid. Arachidonic acid is
3', 5' Cyclic AMP
cyclooxygenase synthetase to form unstable cyclic endopc
Kinases Protein + Phosphate — Chetalor of Calcium Free Calcium (Ca2) — Bound Calcium necessary for aggregation
from membrane phospholipids by the enzyme phospholipae
A2. Once released, arachidonic acid is metabolize!
1.
inhibits aggregation
oxides. P0(32 and P01-I2, which subsequently are lao formed into P012 and thromboxane A2 (TXA2). The coolersion to TXA2 is aided by the enzyme synthetase. The formation of PG!2 can occur nonenzyimi
Figure 19—28 • Rote of adenosine 3',5'-cyclic monophos-
cally. Blood platelets convert arachidonic acid to TXA:
phate (cAMP) in inhibition of platelet aggregation.
whereas P012 is formed mainly by the vascular endothelir
Chapter 19 •
and TXA2 are unstable at physiological pH and temperaturet. Their half-lives are 2 to 3 minutes. PG!2 inhibits platelet aggregation by stimulating adenylate cyclase to increase cAMP levels in the platelets. PG!2 is also a vasodilator and, as a result, has potent hypotensive properties when given intravenously or by intra-arlerial administration. TXA2 induces platelet aggregation. Together with PG!2. TXA2 plays a role in the maintenance of vascular Both PG!2
Agents
667
alkalies. The effects after administration require 12 to 72 hours to develop and persist for 24 to 96 hours after discontinuance.
Immeostasis. In addition to being a platelet aggregator. TXA2 is a potent vasoconstrictor. Retardation of clotting is important in blood transfusions, to avoid thrombosis after surgery or from other causes, to prevent recurrent thrombosis in phlebitis and pulmonary emholism, and to lessen the propagation of clots in the coronary
Dtcumarol
Dicumarol is used alone or as on adjunct to heparin in the prophylaxis and treatment of intrava.scular clotting. It is used
irteries, This retardation may be accomplished by agents
in postoperative thrombophlehitis, pulmonary embolus.
that inactivate thrombin (heparin) or substances that prevent
Dicumarol and related compounds are not vitamin K an-
acute embolic and thrombotic occlusion of peripheral arteries, and recurrent idiopathic thrombophiebitis. It has no effect on an already-formed embolus but may prevent further intravascular clotting. Because the outcome of acute coronary thrombosis depends largely on extension of the clot and formation of mural thrombi in the heart chambers, with subsequent embolization, dicumarol has been used in this condition. It has also been administered to arrest impending gangrene after frostbite. The dose, after determination of the prothrombin clotting time, is 25 to 200 mg. depending on the size and the condition of the patient. The drug is given orally in the form of capsules or tablets. On the second day and thereafter, it may be given in amounts sufficient to maintain the prothrombin clotting time at about 30 seconds. If
tagonists in the classic sense. They appear to act by interfer-
hemorrhages should occur, a dosage of 50 to 100 mg of
the formation of prothrombin in the liver (the coumarin denvatives and the phenylindunedione derivatives). Although heparin is a useful anticoagulant, it has limited applications. Many of the anticoagulants in use today were developed following the discovery of dicumarol. an anticoagulant present in spoiled sweet clover. These compounds
ate orally effective, but there is a lag period of 18 to 36 hours before they increase the clotting time significantly. Fleparin, in contrast, produces an immediate anticoagulant effect after intravenous injection. A major disadvantage of hcparin is that the only effective therapeutic route is parentrial.
ing with the function of vitamin K in the liver cells, which are the sites of synthesis of the clotting factors, including pmthmmbin. This lengthens the clotting time by decreasing the amount of biologically active prothrombin in the blood. The discovery that dicumarol and related compounds were potent reversible competitors of vitamin K coagulant-pro. mating properties (although at high levels dicumarol is not reversed by vitamin K) led to the development of anti—vitamin K compounds such as phenindione. which was designed in part according to mctabolite—antimetubolitc concepts. The ictive compounds of the phenylindanedione series are charby a phenyl. a substituted phenyl, or a diphenylaceyi group in the 2 position. Another requirement for activity is a keto group in the I and 3 positions, one of which may form the enol tautomcr. A second substituent. other than hydrogen, at the 2 position prevents this keto—enol tautonerism. and the resulting compounds are ineffective u.s anti-
menadione sodium bisulfite is injected, supplemented by a blood transfusion.
Warfarin Sodium, USP. Warfurin sodium. 3-(a-acetonylbenzyl)-4-hydroxycoumarin sodium salt (Coumadin. Panwarfin). is a white, odorless, crystalline powder, with a slightly bitter taste; it is slightly soluble in chloroform and soluble in alcohol or water. A 1% solution has a pH of 7.2 to 8.5.
By virtue of its great potency, warfarin sodium at first was considered unsafe for use in humans and was used very
effectively as a rodcnticide. especially against rats. At the proper dosage level, however, it can be used in humans. especially by the intravenous route. 0
coagulants.
CH(
PRODUCrS
Frotamine Sulfate, USP. Protamine sulfate has an anticoagulant effect, but if used in the proper amount, it counterano the action of heparin and is used as an antidote for the latter in cases of overdosage. It is administered intravenously in a dose that depends on the circumstances. Walann
Dicurnarol. 3.3'-methylenebis(4-hyis a white or creamy white crystalline pow&r with a faint, pleasant odor and a slightly bitter taste. It a practically insoluble in water or alcohol, slightly soluble in chloroform, and dissolved readily by solutions of fixed Okumavol, USP.
Warfarin Potassium, USP. Warfarin potassium. 3-(aucetonylbenzyl)-4-hydroxycoumarin potassium salt (Athrombin-K). is readily absorbed after oral administration,
668
Wilson and Giso,Id's Texibook of Organic Medicinal and Pharmaceutical Cl,en,i.s:rv
and a therapeutic hypoprothmmbinemia is produced within 12 to 24 hours aftcr administration of 40 to 60mg. This salt is therapeutically interchangeable with warfann sodium.
NH
NH
Synthalin
Anisind!one, USP.
Anisindione, 2-(p-methoxyphenyl)1.3-indandione. (Miradon). is a p-methoxy congener of phenindionc. It is a white, crystal line powder. slightly soluble in water, tasteless, and absorbed well after oral administration.
In
1942. p.aminobcnzcnesulfonumidoisopropylthiadia.
zole (an antibacterial sulfonamide) was found to hypoglycemia. These results stimulated research for the dc
velopment of synthetic hypoglycemic agents. sevenil of which are in use today. Sulfonylurcas became widely available in 1955 for treaunent of non—ketosis-prone mild diabetes and are still the drugs of choice. A second class of compounds. the bigea'
nides, in the form of a single drug. phenformin. has bees used since 1957. Phenformin was withdrawn from the LI. S.
market, however, because it causes lactic acidosis. from which fatalities have been reported.
—
Anisindione (Miradon)
In instances when the urine may be alkaline, an orange color may be detected. This is due to metabolic products of anisindione and is not hematuria.
\/
NH
NH
II
II
Pbeoforn,tn
Sutfonylureas The sulfonylureas may be represented by the following gen. eral structure:
SYNTHETIC HYPOGLYCEMIC AGENTS The discovery that certain organic compounds will lower the blood sugar level is not recent. In 1918. guanidine was shown to lower the blood sugar level. The discovery that certain trypanosomes need much glucose and will die in its absence was followed by the discovery that galegine lowered
the blood sugar level and was weakly trypanocidal. This led to the development of several very active uypanocidal agents. such as the bisamidines. diisothioureas. bisguanidines, and others. Synthalin (trypanocidal at 1:250 million) and pentamidine are outstanding examples of very active trypanocidal agents. Synthalin lowers the blood sugar level in normal. depancreatized. and completely alloxanized animals. This may be due to reduced oxidative activity of mitochondria, resulting from inhibition of the mechanisms that simultaneously promote phosphorylation of ADP and stimulate oxidation by nicotinamide adenine dinucleotide (NAD)
in the citric acid cycle. Hydroxystilbamidine isethionate.
\'
0
/
'
0
II
II
S—N——C—N—re H
o
1
H
23
These are urea derivatives with an arylsulfonyl group In the I position and an aliphatic group at the 3 position. The aliphatic group. R', confers lipophilic properties to the mole
cule. Maximal activity results when k' consists of three I six carbon atoms, as in chlorpropamide, tolbutamide. aM acetohexamide. Aryl groups at R' generally give tosie cone-
pounds. The R group on the aromatic ring primarily mu ences the duration 01' action of the compound. Tolbutamkk disappears quite rapidly from the bloodstream by being metabolized to the inactive carboxy compound, which is en creted rapidly. Chlorproparnide. howcvcr, is metabolized
more slowly and persists in the blood much longer. late the release of insulin from the functioningflcellsof lie intact pancreas. In the absence 01' the pancreas. they haic
NH
CH2 N-C
—
The mechanism of action of the sulfonylureas is to stinnu
USP. is used as an antiprotozoan agent. H3C
R
no significant effect on blood glucose. The
\
may have other actions, such as inhibition of secretion glucagon and action at postreceptor intracellular sites to in crease insulin activity. For a time, tolbutamide. chlorpropamide. and aectohen.
NH2
Gategine
amide were the only oral hypoglycemic agents. Subs NH
___tf' Pentarnidlne
NH
__O_(CH2h
quently. a second generation of these drugs became anal-
able. Although they did not present a new method lowering blood glucose levels, they were mote potent IF.j the existing drugs. Glipizide and glyburide are the secosi generation oral hypoglycemic agents. Whether they are first- or second-generation oral
Chapter 19 • Cardiovascular Agents
cemic drugs, this group of agents remains a valuable adjunct to therapy in adult-onset diabetes patients. Accordingly, ihc sulfonylurcas are not indicated in juvenile-onset diabetes.
66
hour. The main route of breakdown is to butylamine an sodium p-tolucne sulfonamide.
chlorpropamide, USP. Tolbutamide, USP.
Tolbutamidc. I -butyl-3-(p-tolylsulfonyl)urca (Orinase), occurs as a white, crystalline powder that is insoluble in water and soluble in alcohol or aqueOtis alkali. It is stable in air.
H,C
—
\/
0
0
II
II
to conversion to inactive metabolites than is tolbutamide
S—N—C—N—CH2CH2CH2CH3
and, as a result, has a much longer duration of action. One study showed that about half of the drug is excreted as melaholites. with the principal one being hydroxylatcd in the
2 position of the propyl side chain.7 After control of the blood sugar level, the maintenance dose is usually on a oncea-day schedule.
Tolbutamide (Orinase)
Tolbutannide is absorbed rapidly in responsive diabetic patients. The blood sugar level reaches a minimum after 5 to 8 hours. It is oxidized rapidly in vivo to l-hulyl-3-(p. carboxyphcnyl)sulfonylurea. which is inactive. The metabolite is freely soluble at urinary if the urine is strongly acidified, however, as in the use of sulfosalicylic acid as a protein precipitant. a white precipitate of the free acid may be formed.
—
H3C
Chlorpropamide. phenyl )-sulfonyll-3-propylurea (Diabinese). is a white. talline powder. practically insoluble in water, soluble in alco. hol. and sparingly soluble in chloroform. It will form watersoluble salts in basic solutions. This drug is more resistant
0
0
\/
—
II
—N—C—N H H
—
Chiorpropamide (Olabinose)
Tolazamide, USP. Tolazamide. I -(hexahydro- I/lazepin-l-yl)-3-(p-tolylsulfonyl)urea (Tolinase). is an analogue of tolbutamide and is reported to be effective, in general, under the same circumstances in which tolbutamide is useful. Tolazamide. however, appears to be more potent than
tolbutamide and is nearly equal in potency to chlorpropamide. In studies with radioactive tolazamide. investigators found that 8SCk uf an oral dose appeared in the urine as metabolites that were more soluble than tolazamide itself.
—
II
Tolbutamide should be used only when the diabetic palent is an adult or shows adult-onset diabetes, and the patient should adhere to dietary restrictions.
Tolbutamide Sodium, USP. Tolbutamide sodium, Imonosodium salt (Orinase Diagnostic), is a white, crystalline powder. freely soluble in water, soluble in alcohol and chloroform, and very slightly soluble in ether.
H3C__Q__ll_N_C_N__NO Tolazamlde (Tolinase)
Acetohexamide, USP. Acetohexamide, I phenyl)sulfonyfl-3-cyclohexylurea (Dymelor),
is
related
chemically and pharmacologically to tolbutamide and chlor-
propamide. Like the other sulfonylureas. acetohexamide lowers the blood sugar level, primarily by stimulating the release of endogenous insulin.
0 Tolbutamide Sodium
This water-soluble salt of iolbutamide is used intravenously for the diagnosis of mild diabetes mellitus and of functioning pancreatic islet cell adenoma.s. The sterile dry is dissolved in sterile waler for injection to make a dear solution, which then should be administered within I
Acetohexamide (Dymelor)
Acelohexamide is metabolized in the liver to a reduced form, the a-hydroxyethyl derivative. This metabolite. the main one in humans, possesses hypoglycemic activity. Acetohexamide is intermediate between toibtitamide and chlor-
670
Wilson and Gisvoldx Textbook of Organic Medki,,al and Pl,annaceu:ical Che,nis:re
N
N
>=J, O=C\ HN—CH2—CH2
Gilpizide (Glucotrol)
HN—CH7—CH7
Glyburide (DiaBeta, Micronasa, Glynase)
propamide in potency and duration of effcct on blood sugar levels.
Structurally. glipizidc. I -cyclohexyl-3-IIpGIIpizide. ethyl I phenyl j sulfonyl I I 2urea (Glucotrol). is a cyclohexylsulfonylurea analogue similar to acetohexamide and glyburide. The drug is absorbed rapidly on oral administration. Its serum half-life is 2 to 4 hours, and it has a hypoglycemic effect that ranges from 12 to 24 hours.
Glyburide. Similar to glipizide, glyburide. l-[Ip-12-(Schtoro-o-anisamido)cthyll-phenyl lsulfonyl]-3-cyclohexylurea (DiaBeta. Micronase. Glynase). is a second-generation
oral hypoglycemic agent. The drug has a half-life elimination of 10 hours, but its hypoglycemic effect remains for up to 24 hours.
to 4 hours, while the hypoglycemic effects range frum to 24 hours. Metabolism of glipizide is generally Ihrosgii oxidation of the cyclohexane ring to the p-hydroxy and a
hydroxy metabolites. A minor metabolite thai
lotion of the primary amine following hydrolysis of dir amide system by amidase enzymes.
Glimepiride.
Glimepiride. I -Itp-12-(3-ethyl-4-mcthyl. 2-oxo-3-pyrroline- I -carboxamido)cthyllphenyllsulfonylj. 3-Orans-4-methylcyclohexyl)urca (Amaryl). is very to glipizide with the exception of their heterocyclic usfs Instead of the pyrazine ring found in glipizide, glimepindc contains a pyrrolidine system. It is metabolized prinlail) through oxidation of the alkyl side chain of the with a minor metabolic route involving acetylation of dir amine.
Glidazide. Glipizide. Glipizide. I -cyclohexyl-3-IIp-(2-(5-methylpyrazinecarboxamido)cthyllphenyljsulfonyllurea (Glucoiml). is an off-white, odorless powder with a pK. of 5.9. It is insoluble in water and alcohols, hut soluble in 0.1 N NaOH. Even though on a weight basis it is approximately 100 times more potent than tolbutamide. the maximal hypoglycemic effects of these two agents are similar. It is rapidly absorbed on oral administration, with a serum half-life of 2
Glipizide (Glucotrol)
in
volves the N-acetyl derivative, which results from the acety-
Chemically. gliclazide. I [3.3.OJoct-3-yI)-3-p-tolylsulphonylurea (Diamicron). is very similar to tolbutamide. with the exception of thc hi cyclic heterocyclic ring found in gliclazide. The pyrrolidinr increases its lipophilicity over that of tolbutamide. wiuch increases its half-life. Even so. the p-methyl is susceplibk to the same oxidative metabolic fate as observed for bulamide. namely. it will be metabolized to a carboxylic acid.
Chapter 19 • C'ardiorasrular Agents
671
Glimepinde (Amaryt)
mia. resulting in shakiness, headache, cold sweats, anxiety. and changes in mental state.
Glictazide (Diamicrool
Uonsulfonylureas—Metagllnldes The metaglinides are nonsulfonylurea oral hypoglycemic agents used in the management of type 2 diabetes (non—insulin.dependent diabetes mellitus, NIDDM). These agents tend
to have a rapid onset and a short duration of action. Much like the sulfonylureas. these induce insulin release from functioning pancreatic fi cells. The mechanism of action for the metaglinides. however, differs from that of the sulfonylureo.s. The mechanism of action is through binding to specific receptors in the fl-cell membrane, leading to the closure channels. The K channel blockade of ATP-dependent
Repaglinide (Prandiri)
Nateglinide.
Although naleglinide. N-(4-isopropylcyclohexanecarbonyl)-n-phenylalanine (Starlix). belongs to the metaglinides. it is a phenylalanine derivative and represents a novel drug in the management of type 2 diabetes.
depolarizes the fl-cell membrane, which in turn leads to and stimulation influx, increased intracellular ni insulin secretion. Because of this different mechanism of
.iction from the sulfonylureas, there ate two major differences between these seemingly similar classes of agents. The tlrst is that the metaglinides cause much faster insulin production than the sulfonylureas. As a result, the metaglinides should be taken during meals, as the pancreas will produce insulin in a much shorter period. The second difference is that the effects of the metaglinides do not last as long as
effects of the sulfonylureas. The effects of this class appear to last less than I hour. while sulfonylurca.s continue ostimulate insulin production for several hours. One advanage of a short duration of action is that there is less risk of hypoglycemia.
Repagilnide.
Repaglinide.
(+ )-2-ethoxy-4-[N-13-
nelhyl-l(S)-[2-( l-pipcridinyl)phenyllbutyllcarbamoylmeihyljbenzoic acid (Prandin). represents a new class of nonsulfonylurea oral hypoglycemic agents. With a fast onset
and a short duration of action, the medication should be taken with meals. It is oxidized by CYP 3A4. and the carbox-
ylic acid may be conjugated to inactive compounds. Less than 0.2% is excreted unchanged by the kidney. which may he an advantage for elderly patients who are renally impaired. The most common side effect involves hypoglyce-
Natagllnlde (Stailix)
Thiazolindlones The thiazolindiones represent a novel nonsulfonylurea class of hypoglycemic agents for the treatment of NIDDM. Much
like the sulfonylureas, the use of these agents requires a functioning pancreas that can successfully secrete insulin from f3 cells. Although insulin may be released in normal levels from the cells, peripheral sensitivity to this hormone may be reduced or lacking. The thiazolidinediones are highly selective agonists for the peroxisorne prolilerator-activated which is responsible fur improving glyreceptorcemic control, primarily through the improvement of insulin sensitivity in muscles and adipose tissue. In addition, they inhibit hepatic gluconeogenesis. These agents normalize glucose metabolism and reduce the amount of insulin needed to achieve glycemic control. They are only effective in the presence of insulin.
Rosiglitazone.
Rosiglitazone. (± )-5-114-12-(methyl-2pyridinylamino)cthoxylphenyllmcthyl 1-2.4-thiazolidinedione
672
Textbook of
Wilson and
Medicinal and Pharmaceutical Clw,nis,rr
Ptogtitazone (Actos)
(Avandia). is a white to off-white solid with pK, values of 6.8 and 6.1. Rosiglinazone is readily soluble in ethanol and
a buffered aqueous solution with pH of 2.3; solubility decreases with increasing pH in the physiological range. The molecule has a single chirul center and is present as a race-
mate. Even so. the enantiomers are functionally indistinguishable because of rapid interconversion.
a-Glucosldase Inhibitors The enzyme a-glucosidase is present in the brush border of the small intestine and is responsible for cleaving dictacy carbohydrates and facilitating their absorption into the body. Inhibition of this enzyme allows less dietary carbohydrate to be available for absorption and, in turn, less available in
the blood following a meal. The inhibitory properties of these agents are greatest for glycoamylase. followed by sucrose, maltase. and dextranase, respectively. Since these do
not enhance insulin secretion when used as hypoglycemia is generally not a concern when using these agents.
Acarbose. Acarbose. O-4.6-dideoxy-4-FI( I S.4R,5S.6S)4,5,6-trihydroxy-3-(hydroxymethyl).2-cyclohexcn- I -ytJ atnino)a-o-glucopyranosyl-( I .4)-O-a-o-glucopyrano.yl-
Rosigiltazone (Avandla)
Pioglitazone. Pioglitazonc. (± )-5-114-12-(5-ethyl-2pyridinyl)cthoxyjphenyljmethyl I-2.4-thiazolidinedione(Actos), is an odorless, white, crystalline powder that must be converted to a salt such as its hydrochloride before ii will have any water solubility. Although the molecule contains one chiral center, the compound is used as the racemic mix-
ture. This is primarily due to the in vivo interconversion of the two enantiomcrs. Thus, there are no differences in the pharmacological activity of the two enantiomers.
(I ,4)-o-glucose (Precose). is a naturally occurring oligosaccharide. which is obtained from the microorganism Ac:uioplanes urahensis. It is a white to off—white powder that soluble in water and has a pKa of 5.1. As one might its affinity for a-glucosidase is based on it being a polysaccharide that the enzyme attempts to hydrolyze. This allows
acarbose to act as a competitive inhibitor, which in turn reduces the intestinal absorption of starch. dextrin. and dissacharides.
Bisguanidines Metformin.
Metformin. N.N-dimcthylimidodicarhonimidic diamide hydrochloride (Glucophage). is a bisguanidine. This class of agents is capable of reducing sugar ab-
sorption from the gastrointestinal tract. Also, they can decrease gluconeogenesis while increasing glucose uptake by muscles and fat cells. These effects, in turn, lead to lower blood glucose levels. Unlike the sulfonylureas. these are not hypoglycemic agents but rather can act as antihyperglyccniics. This difference in nomenclature is due to the inability of these agents to stimulate ihe release of insulin from the pancreas. Often, metformin is coadministered with the nonsulfonylureas to improve the efficacy of those agents.
H3C\ H3C
Metformin (Glucoptiage)
r
Acarbose (Procose)
Migl!tol.
Miglitol.
I -(2-hydroxyethyl)-2-(hydrosy• (Glyset). a desosy.
nojirimycin derivative, is chemically known as 3.4.5.pipoidinctriol. It is a white to pale-yellow powder that is soluhir in water, with a of 5.9. In chemical structure, this ageni is very similar to a sugar, with the heterocyclic nitrogen serving as an isosteric replacement of the sugar oxygen. This
feature allows recognition by the a-glucosidase as a sub-
Chapter 19 • cardiovascular Agents stratC. This results in competitive inhibition of the enzyme
and delays complex carbohydrate absorption from the gastrointestinal tract.
673
dosage of' 1(X) of levothyroxine sodium is clinically equivalent to 30 to (a() tog of Thyroid USP.
Liothyronine Sodium, USP.
Liothyronine sodium, 0(4-hydroxy-3-iodophenyl)-3.5-diiodo-L-thyroxine monosodium salt (Cytomel), is the sodium salt of L-3,3'.5-triiodothyronine. It occurs as a light-Ian. odorless, crystalline powder. which is slightly soluble in water or alcohol and has a specific rotation of + 18 to 22° in a mixture of diluted HCI and alcohol.
Miglitol (Glyset)
Uothyronpne Sodium (Cytomet)
THYROID HORMONES Desiccated. defatted thyroid substance has been used for many years as replacement therapy in thyroid gland deticien-
des. The efficacy of the whole gland is now known to depend on its thyroglobulin content. This is an iodine-contain-
ing globulin. Thyroxine was obtained as a crystalline derivative by Kendall71' of the Mayo Clinic in 1915. It showed much the same action us the whole thyroid substance. Later, thyroxine was synthesized by Harington and Burger in England.77 Later studies showed that an even more
iodine-containing hormone existed, which is now known as triiodothyronine. Evidence now indicates that thyroxine may be the storage rorm of the hormone, whereas
Liothyronine sodium occurs in vivo together with levo. thyroxine sodium; it has the same qualitative activities as thyroxine but is more active. It is absorbed readily from the gastrointestinal tract, is cleared rapidly from the bloodstream, and is hound more loosely to plasma proteins than is thyroxine, probably because of the less acidic phenolic hydroxyl group. Its uses are the same as those of levothyroxine sodium. including treatment of metabolic insufficiency, male infertility, and certain gynecological disorders.
iriiodothyronine is the circulating form. Another point of view is that in the blood, thyroxine is bound more firmly to the globulin fraction than is triiodothyronine. which can then enter the tissue cells,
Levothyroxine Sodium, USP.
ANTITHYROID DRUGS Hyperthyroidism (excessive production of thyroid hor-
Levothyroxine sodium.
O-(4-hydroxy-3.5-diiodophenyl)-3.5-diiodo-2-tyrosinc monosodium salt, hydrate (Synthroid. Letter, Levoxine. Levoid). is the sodium salt ol the k'vu isomer of thyroxine. which is an active physiological principle obtained from the thyroid gland of domesticated animals used for food by humans. It is also prepared synthetically. The salt is a light yellow, tasteless, odorless powder. It is hygroscopic but sta-
in dry air at room temperature. It is soluble in alkali hydroxides. 1:275 in alcohol, and 1:500 in water, to give a
mones) usually requires surgery, but before surgery the patient must be prepared by preliminary abolition of the hyper-
thyroidism through the use of antithyroid drugs. Thiourea and related compounds show an antithyroid activity. but they
are too toxic for clinical use. The more useful drugs arc 2thiouracil derivatives and a closely related 2-thioimidazole derivative. All of these appear to have a similar mechanism of action (i.e.. prevention of the lodination of the precursors of thyroxine and triiodothyronine). The main difference in the compounds lies in their relative toxicities. 0
pH of about 8.9.
Na1'
S
Thiourea
Levothyroxine Sodium (SynThrod, LeOer, Levoxine, Levold)
Levothyroxine sodium is used in replacement therapy of decreased thyroid function (hypothyroidism). In general. a
2'Thiouracil
These compounds are absorbed well after oral administration and excreted in the urine. The 2-thiouracils. 4-keto-2-thiopyrimidines. are undoubtedly tautomeric compounds and can be represented as follows:
674
Wilson and Gisvold's Thx:hook
of Organic Medicinal and Phannaceuzical Clu',,,is:rv
Some 300 related structures have been evaluated for antithyroid activity, but of these, only the 6-atkyl-2-thiouracils and closely related struclures possess useful clinical activity.
The most serious adverse effect of thiouracil therapy is agranulocytosis.
Propyithiouracil. 6-propyl-2PropyithiouraciL. USP. thiouracil (Propacil), is a stable, white, crystalline powder with a bitter taste. Ii is slightly soluble in water but readily soluble in alkaline solutions (salt formation). 0
HN)i
REFERENCES I. Robinson. B. F.: Adv. Drug. Rev. 10:93. 1975. 2. Aronow, W. S.: Am. Hecn 3. 114:273, 1972. 3. Sonnenhlick. 0., Ross. 3., Jr.. and Braunwald, 0.: Am. 3. Carduol. 22 328. 196$. 4. Ignarro. L. 3.. ci a).: 3. Pharmacol. lisp. TIter. 218:739. 1981. 5. Feelisclt. M.: Our. Heart 3. 14(Suppl. 11:123. 993. 6. Needlcman, P.: Anna. Rev. Toxicol. 16:81. 1976. 7. Chung. S. J.. and Fung, H. L.: J. Pharmacot. lisp. TIter. 253:614. 199) K. Fusari. S. A.: 3. Pttarm. Sri. 62:123. 1973. 9. Fusari. S. A.: 3. Ptiarrn. Sci. 62:2012, 1973. 10. McCall, ci el.: Prohi. Cardiol. 10:1, 198$. II. Heschlcr. J.. ci at.: Our. J. Biocheni. 165:261. 1987. 12, Iteridge. M. 3.: Anne. Rev. Biochem. 56:159. 987. 13. van Zwcitcn. P. A., and van Meet. 3. C.: Prog. Phannacol. 5:1. 1951 14. Atkinson. J.. ci ci.: Naunyn Schmeidhcrgt, Arch. Phurmacol.337:471. 1988.
IS. Smith. H. J.. and Briscoc. M. Ii.. 3. Mol. Cell Cardiol. 17:709, 16. Sanguinciti. M. C.. and Kass, K. S.: Circ. Rcs. 55:336, 1984. CH2CH2CH3
Popyllhlouracll (Propacil)
This drug is useful in the treatment of hyperthyroidism. There is a delay in appearance of its effects because propylthiouracil does not interfere with the activity of thyroid hormones already formed and stored in the thyroid gland. This lag period may vary from several days to weeks, depending
on the condition of the patient. The need for three equally spaced doses during a 24-hour period is often stressed, but evidence now indicates that a single daily dose is as effective as multiple daily doses in the treatment of most hyperthyroid
19. Vaughan Williams, F.. M.: In Sandoe E., Flensted-Jansen. 0.. aid Olesen, K. H. teds.). Symposium on Cardiac Arrhythmias. Sweden. B. Asira, 1970. pp. 449—472. 20. Vaughan Williams. 0. M.: J. COn. Pharmacol. 24:129. 1984. 21. CAPS Investigators: Am. 3. Curdiol. 61:501. 198$. 22. Woosley. K. L.: Annu. Rev. Ptmrmucol. Toxicol. 31:427, 1991. 23. Campbell. T. 3.: Curdiovasc. Rev. 17:344, 983. 24. Yoot. A. 1.: Mol. Pharmacol. 46:970. 1994. 25. Hondcghcm. L. M.. and Katcung, B. G.: Anna. Rev. Pharmenol Trm. col. 24:387. 1984. 26. Nies. A. S.. and Shang. I). G.: Clin. Pharmacol. Exp. Ther. 4:823 1973.
Methimazole, I -methylimidazole2-thiol (Tapazole), occurs as a white to off-white, crystalline
27. Koch-Wearer. 3.: Ann. N. Y. Aced. Sri. 179:371), 1971. 2$. Giardinia. E. V.. ci a).: Clin. Pharmacot. Ther, 19:339. 1976. 29. Olson. J.. ci at.: Clin. Plinnnucol. TIter. 17:134, 1975. 30. BelIer. B.. ci ci.: Am. 3. Cardiol. 35:282, 197$. 31. Bigger. T. J.. and Jaffe. C. C.: Am. 3. Cardiol. 27:82. 1971. 32. Hollunger. G.: Aria Pliarmacol. Toxicol. 17:356. 1960.
powder with a characteristic odor and is freely soluble in water. A 2% aqueous solution has a pH of 6.7 to 6.9. ft
33. Helrant. R. H.. ci al.: Am. Heart). 77:315. 1969. 34. Bcckcli, A. H.. and Chiodouncre. E. C.: Pusugrad. Med. 3.
patients.75
Methimazole, USP.
should be packaged in well-closed, light-resistant containers.
985
17. Rocenkirchen, K., ci ul.: Nuunyn Schmiedebergs Arch. Pliarmacol. 3)0 69. 1979. 18. Triggle. D. 3.. Calcium antagonists. In: Antonoccio. P. N. (ed). Cards' vascular Pharmacology. 3rd ed. New York. Raven Press. 1990.
11:60, 1977.
35. Anderson. J. L.. Mason. J. W.. and Roger, M. 13.: Circulation 57:635. 1978.
HS)N) CIt3
Methimazole (Tapazole)
Methimazole is indicated in the treatment of hyperthyroidism. It is more potent than propyithiouracil. The side effects are similar to those of propylthiouracil. As with other antithyroid drugs, patients using this drug should be under medical supervision. Also. tike the other antithyroid drugs, mechimazole is most effective if the total daily dose is subdivided and given at 8-hour intervals.
36. Gtrchtcr, J., ci ci.: Am. 3. Cardiul. 55:807, 198$. 37. Sneki, T.. eta).: Our. J. Pharmacol. 261:249, 1994. 38. (3roshner, K.. ci ci.: Br. 3. Pttarmacol. 102:669. 1991. 39. liii. J. T.. Duff. H. J.. and Burgess. 0. 13.: Cliii. Phannacokinel. 21.). 1991.
40. Olsson. S. B., Brornon. L., and Vamauskas. 0.: Br. Heart 3. 35:1255 1973.
41. Kannaut. K., ci iii.: Cliii. Pharmacol. 31:438, 982. 42. Wiut, 13. M.. Elisworth. A. 3.. and t.eversce, 3. H.: Ann. Phannacethcr 27: 1463. 1993.
43. Nudermanee. K.. ci ci.: Circulation 66:202, 1982. 44. Pupp, 3. G.. and Vaughan Williams. 0. M.: Br. 3. Pharmacol. 37:36' 1969.
45. Boyer, 0. W.. Stork, C.. and Wang, R. Y.: lot. J. Mcd. Toxicol. 25): 4: 16. 46. Salute, .1. 3.. and Brooks. R. R.: Cardiol. Drug Rev. 15:137—156, 1W? 47. Tigcrciedt. R.. and Bergman. P. (3.: Scand. Arch. Physictl. 8:223. 691
48. Page. P.. and Helmer, 0. A.:). Exp. Med. 71:29. 1940.
Chapter 19 • ('ardiova.ccular 49 Skeggs. L.. ci al.: J. Exp. Med. 99:275, 1954. Shapiro. R.. and Riordan. J. F.: Biochemistry 23:5225. 1984. SI Elders. N. R.. and Riordan. 3. Biochemistry 30:7118. 1991. 52 Vakil. R.: Hr. Heart J. 11:350. 1949. 5). S. N . Shore, P. A.. antI Brodje, P. P.: Br. 3. Pharmacol. Exp. Ther. 118:84. 1956. 54 Kirshner. N.:). Biol. Chem. 237:2311. 1962. 55. von Euler, U. S.. and Lishajko. F.: mt. J. Neuropharmarol. 2:127. 1963. Muller. 3. M.. Schlirtlcr. E.. and Brin. H. 4.: Experienhia 8:338. 1952. 90:87. 1978. 57. U'Pnchard. 0., et al Eur. 4. Pharin.w 5$. Langer. S. Z.. and Cavero, I.: Hypertension 2:372. 1980. 59. Stauke, K.. and Monte!. H.: Ncumphamtacology 12:1073. 1973. 60. Meishcri. K. 0,: Direet-aeling vasodilators. In Slush. B. 3.. et al. teds.). :
Cardiusascular Pharmacology. New York. Churchill Livingstonc. 994. p. 173.. 61
Drucy. 3.. and Meler. R.: l!npcrientia 6:19. 1950. Zasesh. K.. Koch.Wcsesr. 4.: Clin. Phannacol. 13:4420. 1972. Gross
63, Page, I. H.. et al.: Circulation 11:88. 1955.
675
64. Cook. N. S.: Pota.ssium Channels: Structure, Function and Therapeutic Potential. New York. John Wiley & Sons, 1990. p. 181.
65. DuCharme. 0. W., et al. Pharmacol. Esp. Ther. 184:662. 1973. 66. Levy. R. I.. ci al.: Circulation 69:325. 19114. 67. Levy. K. I: Annu. Rcv. Pharniacol. Toxicol. 47:499. 1977. 68. Mannisto. P. T.. ct a!.: Acta Pharniacol. Toxicol.36:353. 1975. 69. Kesattietni, Y. A.. and Grundy. S. M.: JAMA 251:224t. 1984. 70. Atmch. R. F.. et a).: 3. l.ipid Res. 24:588. 1983. 71. Shcnllo. 3.. ci al.: Proc. NatI. Acad. Sd. U. S. A. 71:2730. 1974. 72. Jackson. C. M.. and Suttie. 3. W.: Prog. Hematol. 10:333. 1978. 73. Triplctt. 0. A. (cdl: Platelet Eunclion. Chicago. American Society iii Clinical Pathology. 1978. 74. Hamberg, N.. ci al.: Proc. NatI. Acad. Sci. U.S. A. 71:345. 1974. 75. Thomas. K. C. and Ruby. K. %V.: 3. Mcd. Client. 15:964, 1972. 76. Kendall. E. C.: JAMA 64:2042. 1915. 77. Hamngton, C. R.. and Burger. C,: Biochem. 3. 21:169. 1927. 78. Grcer. M. A.. and Meiholi. W. C.: N. E.ngl. 3. Mcd. 272:888. 1965.
C
HA
P
T
E
R
20
Local Anesthetic Agents GARETH THOMAS
Local anesthetics are blocking drugs that, when administered locally in the correct concentration. block" the nerves that cariy nerve impulses in local areas of the body. They do not block coarse touch or inovcment. and the action is reversible. Their method of administration is governed by such proper-
ties as toxicity, stability, duration ot action. water soluhility. membrane permeability, and point of application. while their modes of action (see under heading. Mechanism of Action) depend on their lipid solubility. vasodilation, and protein-binding characteristics. Although given locally, the drug may exert a systemic effect because of transport in the blood from the site of administration to other areas, such as the heart and central nervous system (CNS). These systemic effects, which depend on the concentration of the local anesthetic in the blood. arc usually sedation and lightheadedness, but restlessness. nausea, and anxiety may also occur. High plasma concentra-
tions can result in convulsions. chiropidy. and coma with respiratory and cardiac depression. Local anesthetics are used to alleviate the pain caused by
a wide variety of situations. They are used in dentistry, in ophthalmology, in minor surgical operations including endoscopy. and in relieving pain in intractable medical conditions. such as tumors growing in the spine. Local anesthetics
are also used topically for the temporary relief of pain from insect bites, burns, and other types of surface wounds. They are particularly effective when they are used on mucous membranes, such as the mouth. vagina, or rectum
HISTORICAL DEVELOPMENT The start of the search for ways to relieve pain is lost in the past. People have used religious exorcism, hypnotism. acupuncture. hypothermia. nerve compression. and drugs. Each of these methods has had its periods of popularity. and most are still used in one Ilinni or another. The modern development of the use of drugs to induce local anesthesia probably started in the mnid-191h century. The earliest recorded use of hypothemmia as a local anesthetic, however. is believed to be by Larrey. Napoleon's chief army surgeon during the retreat from Moscow. He recorded that amputations were carried out in subzero temperatures had a higher patient survival rate than those carried out in warmer conditions. Later in the century. Thompson reported that ether acted as a refrigerant when poured onto the skin. These observations lay dormant until 1848 when Arnott reported that bladder filled with crushed ice to alleviate he had used a
the pain caused by incisions made in the skin. This was lollowed by Snow's unsuccessful attempts to find a viable way of using refrigeration as a local anesthetic. Success was
676
achieved eventually by his protégé Richardson. who rs'placed the cologne in the then recently introduced Cologne spray with ether. This achieved temperatures that allowed minor surgery to he curried out. Richardson and other workers improved the efficiency of the procedure by using a petroleum distillate, then ethyl bromide, and ahi. mately ethyl chloride. The success of the Richardson inspired Koller to search for a local anesthetic that could safely applied to the eye. Kolter qualified in medicine in 1882 and went on to cialize in ophthalmology in Vienna. His experience as an eye surgeon made him increasingly aware of the need bra local anesthetic that could be used in the eye. In 1884. whik he was collaborating with Freud to study the effect of Cocame on fatigue, a colleague remarked that the drug his tongue. Koller and Gaertner investigated this claim and found that a dilute aqueous solution of cocaine hydmchlonik caused local anesthesia of the cornea. Brettauer presented Koller's results on his behalf at an ophthalmology meeting
in Heidelberg in September 1884. since Koller could so afford the train fare front Vienna to Heidelberg. paper resulted in the immediate widespread use of cocaine in Europe and the United States. Koller also recommended the use of cocaine as a local anesthetic in ear, nose, and
throat operations. At the time, however, little was known about its addictive properties. In 1885. degradation studies by Camels and Gossin sug. gested that there were sonic structural similarities between cocaine and atropine. This led Filchne at the University s( Hresluu (now Wroclaw) in Poland to determine whethcratrn. pine had any local anesthetic activity in the eye. Atrogine had been isolated from the roots of belladonna in 1831 1w Mciii, a German apothecary. Filehne found that atropinehwi
little local anesthetic activity and was toxic, causing irritation at the dose.s required t'or any activity. Earlier. Los. sen showed that atropine could be split into tropic acid and a nitrogenous base called tropine. in 1880, Ladenbute synthesized a series of physiologically active compounds. which he called rropeines. by esterifying tropine with a san. ety of aromatic acids. Filehnc investigated these semisynthetic analogues of atropinc for local anesthetic activity and
found that homutropine (Fig. 20-ta) was less irritating to ,CH3
CH3
(a)
(b)
Figure 20—1 . a. Homatropirie. b. Benzoyltropine
Chapter 20 • Ii,ra! A,w.ctI,etie
CH3 OCOC6H5
677
CH3
(b)
(a)
tH3
Agenlx
CH3
H
CH3
I
CH
OH
CH3
COOCH3
CH3
(c)
CH3
H
(e)
(d)
Figure 20—2 • The incorrect structures proposed for cocaine (a) and uropine (b) by Albert Einhorn and Georg Merling. Structures of methyltriacetone alkamine (C), alpha-Eucaine (d). and beta-Eucaine (e).
the eyes and a better local anesthetic than atropine, whereas bcnzoyltropine (Fig. 20-I b) was a strong local anesthetic but
caused too much irritation to be of any clinical use. The identification of a bcnzoyl group in the structures of the most active atropine analogues and also in cocaine, however, led Fiiehne to test the activity of the bcnzoyl derivatives of quinine. cinchonine. hydrocotarnine. and morphine. His results. which were published in 1887. showed that these benzoate esters acted as local anesthetics, but many had unwanted side effects.
By 1888 the toxic and addictive effects of cocaine were beginning to concern the medical world, and many workers were seeking a safe substitute. In 1892, Einhorn,' professor ci chemistry at the University of Munich, suggested a structure for cocaine (Fig. 20-2a) based on the structure of tropine
proposed in 1883 by Merling2 at the University of Berlin Fig. 20-2b). Merling decided, on the basis of these incorrect structures, to synthesize a benzoyl analogue containing only a piperidine ring. He produced a compound whose structure
was similar to that of the weakly active methyltriacetonc alkaminc analogue of atropine (Fig. 20-2c). It was marketed under the name alpha-Eucaine (Fig. 20-2d) but was not popular. as it caused a burning sensation when applied to the eyes. It was rapidly replaced by beta-Eucaine (Fig. 20-2e). but this also caused eye irritation. Einhorn. on realizing Merling's success with the benzoate
derivatives of piperidine, attempted to synthesize active benzoate compounds based on the simpler hexanc ring. His syntheses. which were based on the reduction of aromatic ben-
zoate esters, failed, so he decided to have a number of unrelated aromatic benzoate esters tested for local anesthetic
activity. Some were found to be active, but more importantly. several the phenols formed by the hydrolysis of the esters were also found to he active, and in 1896. Einhorn
introduced Orthoform (orthocaine) into clinical use. Problems with its production and its side effects led him to introduce Orthoform New in 1897. Einhorn's work was important in that it gave the first indication that a benzoutc ester was not essential for local anesthesia. Although the Orthoforms were relatively successful as topical anesthetics, their poor water solubilily made them unsuitable for other medicinal uses. Consequently. Einhom attempted to improve their water solubility by introducing amine-containing aliphatic side chains. He reasoned that the
formation of their amine hydrochlorides would improve water solubility without making the preparation too acidic. One of Einhorn's compounds, Nirvanin. was introduced in 1898. Its activity was low, and it had to be used in high doses, which caused toxic effects. In 1898, Willstaner determined the correct structures of both atropine and cocaine. He followed this by synthesizing cocaine in 1901.
C2H5 )2
Orthoform
Orthofom, New
Nirvanin
cX:I2cH3
Cocaine
Atropine
Benzocaine
678
Wilson and Gi.wolds Texthook of Organic Medicinal and Pharmaceutical Chemistry HO
cHcfl3NHcH3
H2N
C2H5 ) 2
Adrenaline
Procaine
Einhorn's clinical success with the poorly water soluble Orthoforms resulted in the introduction of ethyl 4-aminobenzoate into clinical use in 1902 under the name Anesthesine. It was later given the approved name of benzocaine. Ritsert had noticed in 1890 that this water-insoluble compound had numbed his tongue and so to improve its water solubility,
the acetanilide analogues he had synthesized as potential antipyretic agents also exhibited local anesthetic activity. In 1931. his synthetic work led to the production of cinchocainc (Nupercaine). a long-acting local anesthetic that was particu-
larly useful in spinal anesthesia.
it had been formulated as the hydrochloride. As aromatic amines are weak bases, however, the resulting solution had proved too acidic, and he had discounted its clinical use. In 1902. Fourneau in France designed a drug whose structure incorporated functional groups similar to those found in the strucnire of the cocaine molecule. He did not include the piperidine ring, however, which he considered to be responsible for the toxicity of cocaine. His compound, which he marketed under the name Stovaine. was the first nonirri-
taut local anesthetic that could be given by injection and used as a safe substitute for cocaine. Stovaine was later given the approved name of amylocainc.
-
(al3 ) Amylocaine
cH3cH2
)
Earlier work in 1885 by Coming in the United States had
shown that the anesthetic effect of cocaine could be enhanced by the use of tourniquets to keep the drug from being carried away from the site of application. This increased the effectiveness of the drug and allowed lower doses to he used. Applications of the tourniquet technique were limited, however. In 1900. the publication of the observation that adrenal extracts caused blood vessels to contract resulted in Braun demonstrating that mixtures of cocaine and adrenal extracts
were more effective than cocaine alone. The isolation of adrenaline, the active component of adnmal extracts, and its subsequent structure determination lcd Braun in 1904 to design a drug based on the structure of both adrenaline and Einhorn's local anesthetics. It was marketed as Novocaine and was later given the approved name of procaine. Procaine dominated the local anesthetic market for half a century and is still in use today. In the next 30 or so years after the synthesis of procaine. large numbers of compounds were tested for local anesthetic activity, but none of importance emerged. Mieseher. work.
ing for Ciba in Switzerland. found, however, that some of
Gramine 3thylaminomelhyl)Indole
Cinchocaine
At about the same time that cinchocaine was developed.
an investigation of the chemical structure of the alkaloid gramine at Stockholm University resulted in Erdtman syn. thesizing its isomer. isogr.tmine. As luck would have ii. Enliman tasted isogramine and found that his tongue went numb. Realizing its potential, he tested its open-chain precursor and found that it also exhibited local anesthetic activity. For the next 7 years. Erdtman and his student Lofregen and tested compounds with similar structural Their search was rewarded 57 compounds later, by the dis.
covery of lignocaine Lidocaine. Xylocaine). This drug marketed in 1948 by Astra in Sweden, and because of rapidity of action, nonirritant and relatively safe has become the leading local anesthetic.
ii
In 1957. scientists at AB Bofors replaced the acyclic tee.
liary amino side chain of lignocainc with a cyclic terliw) amine for no reason other than it produced novel compounds.
This irrational approach led to two useful local mepivacaine (Carbocaine) and bupivacaine (Marcain). Rupi. vacaine was long acting, producing nerve blocks for up In 8 hours. 013
013
Bupivacaine
Mepivacaine
A large number of active compounds have now been syn• thesized. but lignocaine. procaine, and many of the pre-1957 compounds are still in current use. In 1974, Hughes isolatrd and, in determined the structures of the natural pain
lsogrannine 2(dimethylaminomethyl)Indole
Lignocaine
Chapter 20 U Local Anesthetic Ages,ts
control agents. methionine-enkephalin (met-enkephalin) and leucinc-enkephalin (Icu-enkephalin). The isolation of these and other peptides with similar activity opened up a possible
new structural route to the synthesis of local anesthetic agents that has yet to be fully exploited. H.Try-Gly-Gly-Phe-Met(OH)
H-Try-Gly-Gly'Phe'Leu(OH)
Met-enkephalin
Leu.enkephalln
THE NERVOUS SYSTEM The nervous system consists of sensory and motor components. The sensory component responds to various external
stimulations, which it transmits in the form of a nerve impulse to the CNS for interpretation. The motor component of the nervous system carries a signal from the CNS to the appropriate part of the body to elicit the response to the stimulation, One of these responses is the sensation known as pain. Nerve impulses arc now known to take the form of an electrical impulse. Experimental evidence suggests that
both stimulation and the transmission of a nerve impulse may be blocked by the action 01 local anesthetic agents.4 Consequently, understanding this action requires a knowledge of the structure and action of the nervous system. The basic building blocks of the nervous system are the nen'e cells or ,u'uron.s. Associated with the neurons are the gOal cells. In humans, the complete system contains over 10 billion neurons and about 10 to 15 times that number of glial cells. Extending from the brain, the command center of the nervous system, is the cluster of neurons and glial cells that form the spinal cord. The brain together with the
spinal cord fornis the CNS. Extending outward from the CNS is the peripheral nervous system (PNS). The motor and components of the PNS are subdivided into somatic and vegetative systems. Somatic systems control conscious functions, such as physical movements, while the vegetative
control unconscious functions. The motor vegetative system is referred to as the aukmo,n,c or unoluniarv oenou,v sV.ctem5 and controls body functions such as breath-
ing. digestion, and heart beat.
Neurons receive, conduct, and transmit electrical signals
in the form of ionic currents. A typical neuron usually consists of a central cell body from which radiate out a num-
ber of thin, branch-like protuberances (Fig. 20-3). These branches are of two types, a single branch known as the
(a)
Schwann cell
Nucleus
Axon Myelin sheath Nucleus
Schwann cell Axon
(b)
Figure 20—4 • Representations of myefinated (a) and unmyelinated (b) axons.
axon, which acts as a conductor of signals from the cell body, and a number of other separate branches known as the de,,dri:es. which act as antennae, receiving signals front the axon of other neurons. Both the axons and dendrites of neurons can exhibit an astonishing variety of branching. but axon branching is usually simpler. The terminal branches of the axon end in aynapric knobs, which are also knosvn as terminal buttons or axon telondria. The axon arises from a thickened area of the cell body called the axon hillock. Its membrane is mainly composed of lipids and proteins and is known as the axolemma. Many of the axons of the CNS and PNS are partly covered front near the axon hillock to the synaptic knob by a sheath of nmvelin (myelinated axons), but some UXOfl5 do not have this
type of covering (unmyclinated axons). The myelin sheath of PNS nsyelinatcd axons is not continuous but is broken at about I-mm intervals to expose the axolenima to the extracellular fluid. These exposed areas, which are about I iem long. are known as the node.s of Rw,vier. The distance between the nodes is often referred to as the internodal di.cfance.
A segment of the PNS myelin sheath consists of a single glial cell known as a Sc/mann cell, tightly wrapped around the axon so as to form several tightly bound layers of the sante cell (Fig. 20-4a). In untayelinated axons, the Schwann cells simply surround the axon and are not tightly wrapped around it I Fig. 20-4b). The CNS myclinated sheath isa much more complicated structure. In all cases, however, the main function of myelin is to act as an insulating material, electrically insulating the axon from the extracellular fluid. A nerve consists of myelinated or unrnyelinated nerve fibers (Fig. 20-5a). These nerve fibers Consist of "chains" of neurons. The junction between adjacent neurons in the chain Consists of the synaptic knob of the transmitting neuron separated by a gap of about 30 to 50 nm from either the dendrite, axon hillock, or cell body of the other neuron. This
Node of
Figure 20—3 • Schematic diagram of a neuron. Representation of the variety of branching found in dendrites.
679
680
Textbook of Organic Medicinal and Plrannaceutical chemists-v
Wilson and
Dendrites
Node of Ranvler
Synaptic cleft (axon-cell body)
Neuron, cell membrane
Endoneurium surrounds the cell membrane of the neuron
(b)
(a)
Figure 20—5 • Representations of a section of nerve fiber (a) (reproduced from Thomas. G.: Medicinal Chemistry, An Introduction. Chichester, U. K., John Wiley & Sons. 2000, by permission of John Wiley & Sons, Ltd.) and a cross section of a nerve (b).
gap is known as the synaptic cleft, and the whole area where transmission and reception of the impulse from one neuron to the other occur is known as a synapse. The nerve fibers
are enclosed in different layers of protective tissues (Fig. 20-5b). For example, in a spinal nerve the individual nerve fibers, whether myelinated or unmyelinated. are wrapped in a layer of protective connective tissue known a.s the endoneurium. These endoneurium-coated fibers are grouped in bundles known asfascicle.c. Each fascicle is coated with a layer
of connective tissue called the perineuriutn. The complete nerve consists of a number of fascicles embedded in tissue through which run various blood vessels, the whole structure being covered by a layer of connective tissue known as the epineuriuin. Neurons are excited by electrical, chemical, and mechanical stimuli. They convey the information provided by this stimulation in the form of electrical signals. The precise nature of the information earned depends on the type of neuron, however. For example, a motor neuron will convey electrical signals that cause a particular muscle to contract. In all cases, the signals take the form of changes in the electrical potential
across the neuronal membranes. In myelinatcd neurons, a change in the membrane potential in one node of Ranvier will stimulate further changes in an adjacent node and so on: that is, a change in electrical potential at A stimulates a change in electrical potential at B, which in turn initiates a change in the electrical potential at C and so on (Fig. 206). The process whereby the change in potential jumps t'rom
one node to another is known as saltatorv conduction. It results in the movement of an electrical impulse that is referred to as either the nerve impulse or ac:io,z potential along
A
Axon
B
C
DirectIon of movement of nerve Imputse
Figure 20—6 • Representation of the transmission of a nerve impulse along a neuron fiber by saltatory conduction.
the axon. In unmyelinated nerves, the change in potential of one section of the membrane stimulates a change in potentiri of an immediately adjacent section of the membrane. These nerve impulses are transmitted or conducted along an axon sheathed with myelin at speeds up to 120 m/sec or morc,
but only up to 10 rn/sec in unmyclinated axons. in neurons, the strength of the nerve impulse is maintained by
an automatic amplification system. but many smaller rons have no such systems. If the center of an axon is stimulated, the nerve impulse will be transmitted in both directions along that axon (Fif. 20-7). The synapse allows, however, only the transmission of the impulse from tIne axon to the next neuron. i.e., in oniy one direction. Consequently, an impulse will only trxmvel in
one direction along a nerve fiber. In most cells, the electrical potential difference between the inner and outer surfaces of the cell membrane is due v the movement of ions across that membrane.S In all axons. the interior face of the membrane is the negative side of the potential difference, largely because of the excess of anions, such as chloride ions, found in the interior of the neuron. For a cell at rest (i.e.. a cell that is not subject to any ouIsi& stimulation), this electrical potential is known as the rest in potential and can vary from —20 to —200 mV. where by convention the extracellular side of the membrane is to be 0 volts. The resting potential of neurons is about -70 my. The action potential of the axon is the series of changes that occurs when the axon is stimulated. Micraclec. trodes implanted in the axon (intracellular recording) ohm that stimulation causes an initial depolarization of the membrane by about 20 mV. This is followed by the rapid risen! the membrane's potential to a maximum value of about + 35 mV. The membrane is then said to be depolarized. Ths is immediately followed by the potential dropping back towed
the resting potential (repolarization). The repolarizaliox overshoots the resting potential (hvperpolarization) befrce slowly recovering to the resting potential (Fig. 20-8). The rapid rise and fall of the potential is termed the spike purer.
(I
Chapter 20 • Local Ane.silzetic Agents
681
(1
Stimulation
Direction of movement of nerve Impulse •
Direction of movement of nerve Impulse
Figure 20—7 • Stimulation at the center of an axon results in a nerve impulse being transmitted in both directions.
tin!. and the point at which it starts, the firing level or threshold of the axon. No action potential is produced if the stimulus is below the threshold potential. Once the threshold level is reached, however, the action potential will occur regardless of the strength of the stimulus. Furthermore, the amplirude of this action potential is independent of the intensity
out of an axon by passive transport but into an axon by active transport. A small movement of ions across a membrane can
of the stimulant. The action potential is said to obey the
tial of about 150,000 V cm'. The highest density of Na + ion channels occurs at the
'all-or—nothing'' lou'. The peripheral nerves of mammals consist of bundles of neurons held together in a fibrous envelope called the epineuriwn. The change in potential for these systems is the sum of the action potentials of all the axons in the system ilexiracellular recording is attempted. Each axon in the sysem has a different threshold potential. and so the number of axons firing will initially increase with increased intensity of the stimulus. Eventually, all the axons in the nerve will
lire, and at this point, further increases in the intensity of the stimulus will cause no further increase in the size of the action potential. In bundles of mixed nerves, there will he multiple peaks in the action potential profile, however, because the differing types of nerve fiber will have different conduction speeds.
The electrical potential across the lipid membrane of an axon is mainly due to the transport of small inorganic ions. such as Ca2', Na', K ', and Cl, across the membrane by active or passive transport. Active transport usually involves the intervention of a carrier protein that physically carries the ion to the other side of the membrane. It can occur against
the electrochemical and concentration gradients across the membrane (uphill). Passive transport occurs by the diffusion
of the ions through water-filled channels (ion channels) formed by the integral proteins of the membrane, The ions move from high concentration to low concentration (down/till) at rates on the order of 106 or more ions per second, which is 100 tinles faster than the active transport of ions. transport of the ions is usually in the opposite direction to active transport. For example. Na ions are tr,inspoticd into an axon by passive transport but out of an axon by active transport. .Similarly. K ions may be transported
lead to the generation of electric fields that are enormous by macroscopic standards. For example, the transfer of one ion pair per million, the cation leaving and the anion entering the neuron across a membrane, results in an electrical poten-
nodes of Ranvier.t' The myelinated internodal sections of the neuron contain far fewer Na channels. In addition, these
internodal regions arc electrically insulated and so do not contribute to the action potential. Consequently, the potential produced at the nodes of Ranvier must be strong enough to
produce an effect up to I mm away. Ion channels are formed by groups of integral proteins that run from one side of the membrane to the other. The channels are selective, allowing the passage of certain ions but preventing the passage of others. This suggests that parts
of the channel must act as a selective filter. Furthermore. some of the channels are not permanently open: changes in the conformation of the proteins that form the channel effectively open and close the channel as though it contained a gate. These gaze.c usually open briefly in response to various membrane changes, such as a change in voltage across the membrane (voltage-gazed channels) or the binding of a ligand to a receptor (ligand-gated channels). Over 50 types of gated channel have been discovered.1 The axolemma is more permeable to K ions than to Na'
ions. These ions diffuse out of the neuron through the socalled potassium leak channels, whose opening does not nppear to require a specific membrane change. The movement
of K' ions is concentration driven:
ions move from inside the neuron, where the concentration is high, to the extracellular fluid, where the concentration is lower (Fig. 20-9a). This tendency of ions to teak out of the neuron (driven hr the co,wemranon gradient) is balanced to some extent by a limited movement of ions back into the neuron, both by diffusion through K' channels and by active transport mechanisms such as the sodium pump. These
to
Millivolts (mV)
Depolanzation___.
.—
Initial depolarization Firing threshold
Resting pOtential— — 7C
FIgure 20—8 • Changes in electrical potential observed during a nerve impulse.
Hyperpolarization—.
lime—.
___________
682
Wilson and Gisvohi 's Ten/wok of
Medicinal and Phannac(utieal Ciiemi.cirv
Extraceltular fluid
Extracellular fluid
High sodium ion concentration Low potassium ion concentration Potassium Ion concentration gradient
Membrane
Sodium Ion concentration
000 gradient
Intracellular fluid Low sodium ion concentration High potassium ion concentration
Sodium ion attracted by the negative ions in the neuron
Sodium ion
Membranet removed from the
000
Permanent anions
neuron by active transport
Intracellular fluid
(b)
(a)
Figure 20—9 u a. Movement of Na and K• ions across a membrane because of differences in concentration. b. Attraction of positive ions into a cell by electrostatic attraction and their removal by active transport
movements of K result in a potential difference across the membrane, which is a major contributor to the equilibriu,ni potential that exists between the opposite faces of a biological membrane in a normal cell at rest with a switched-on sodium pump. Its value for a particular ion when the system is in equilibrium and the cell is at rest may be calculated by using the Nernst equation: =
- v,, =
RT
In
(Eq. 20-I)
where V is the equilibrium potential. is the internal potential. V0 is the external potential, C is the internal concentration of the ion (mol C,, is the external concentration of the ion (mol dma). R is the ideal gas constant, and T is the temperature F is Faraday's constant (96.487 cmilombs). and z is the charge on the ion.
The axolenima of a neuron at rest with a switched-on sodium pump does not allow the free movement of Na' ions, even though it contains channels that are specilic for them, if the sodium pump is switched off, however, the concentr.ttion of Na - starts to build up in the cell, as the membrane is not completely impermeable to Na . The Na4 iotis pass into the cell down the concentration gradient through aided by an attraction the so—called .sadiwn ion
for the unions in the cell. This inward movement of Na' neutralizes the negative charges of some of these anions and so reduces (depolarizes) the membrane potential. which allows a greater concentration of K ' ions to leave the cell (Fig. 20-9h). Ca2
ions will also move into a cell, attracted by the
anions permanently present inside and will also leak out of cells via ca/elton c/tunnels. Similarly, under the appropriate
conditions, Cl- ions move in and out of cells. The movement of both of these ions also contributes to the membrane polen.
tial of the cell, which for a cell at rest can vary from —20 to —200 my. The more permeable a membrane is to a particular ion, the more closely the membrane potential approaches the equilibrium potential for that ion. The initial depolarization of the neuron (Fig. 20-8) was shown by Hodgkin and Huxley in 1953 to be due to increased movement of Na into the neuron, which is followed almost immediately by increased movement of' K ions out of the neuron. Consequently, the gated ion channels of neurons ate believed to be responsible for the transmittance of the action potentials that carry information to and from the body of the nerve cell. It is thought that the action potential is triggered
by a stimulation that causes momentary shift of the branc potential of a small section of the membrane to a
negative value (depolarization of the membrane).This causes the gated Na
channels in this section of the memto enter the cell. This process depolarizes the membrane still further, until the
brane to open, which allows
a critical value (the firing when it triggers ihe opening of large numbers of adjacent Na channels so that Na ions hood into lhe axon. This process continues until the membrane potential of this scc-
lion of membrane reaches about + 60 my. which is the equi.
librium potential for Na4 ions when the cell is at rest (calculated using Eq. 20-1). At this point, all the Na channels of the membrane should be permanently open. This situation is not reached, however, because each channel has an automatic closing mechanism that operates even though
the cell membrane is still depolari7.ed tFig. 20-tO). Once
Membrane
Sodium channel closed Neuron at rest, awaiting stimulation
Membrane stimulated and the sodium channel opens Na Ions flow in and an action potential is generated as the membrane Is depolarized
Sodium channel automatically closes and repolarization of the membrane occurs
Repolarization complete Sodium channel closed, Neuron at rest awaiting the next stimutating event
Figure 20—10 • Cycle of conformational changes that occur in a sodium channel.
Chapter 20 • Local Anesthetic Agents Extracellular fluid
685
RN(R')2 +
Conjugate acid unable to enter the channel and bind to the receptor
Na
Axolemma passes through membrane as the
Hydrophobic pathway
Conjugate acid binds to receptor and blocks the
closed Na channel
conjugate
binds to receptor and blocks the channel
I molacu le
•
NI't(R'
Open Na channel RN(R')2 + H
RNH(R')2
Hydrophlllc pathway
Intracellular fluid
Figure 20—12 • Representation of the mechanism of action of local anesthetics in blocking closed and open sodium channels.
by increasing the surface pressure of the cell membrane. which would result in the closure of ion channels. In 1968,
receptor. This is a glycoprorein that consists of five integral proteins. There arc about 20,000 per mm2 of these receptors in the synapse sites of muscle cells. When two acetyicholine molecules bind to the receptor, they cause a change in conformation of the proteins that opens the channel. The channel has a cluster of negatively charged amino acid residues at its entrance, which is thought to prevent the passage of negative ions. Its diameter is about 0.65 nm, so it will allow the passage of positive ions such as Na . K and Ca2 . In muscle cells. ions are the main contributors to the change in membrane potential (—30,000 ions per channel per millise-
Metcalfe and Burgen'2 proposed that the nerve impulse was blocked by the drug increasing disorder of the membrane, which caused distortion of the ion channels. However, based
on the work of several research groups (Ritchie'° in 1975. Hille'4 in 1980. Strichartz'° in 1980. and Strichartz and Ritchic'3 in 1985), it is now believed that the main mechanism of local anesthetic action is associated with the blocking of sodium channels (Fig. 20-12). Stnchartz'5 also showed, in 1981, that the receptor for the blocking action appears to be about halfway down the sodium channel. Work by Schneider and Dubois° in 1986 indicated that benzocaine blocks two different types of sodium channel. Their work suggested that these channels have different affinities for the drug and so
Ca + ions make a small contribution because their exiracellular concentration is much lower than that of Na + ions, while for K ions, the leakage our almost balances the voltage gradient-driven inward movement.
differing rates of inactivation, Other investigations in 1986
by Moczydlowski et al.8 of the blocks imposed by local anesthetic agents indicated that there are at least two sites of action of local anesthetic agents and not one in the interior entrance of the channel, as previously proposed by Hille. Their work also supported the idea of a wide internal entryway into the channel but a constricted external entry. This internal entry was large enough to allow the passage of organic molecules, but the external entry was small enough to prevent the ingress of organic molecules with a single methyl group. It was, however, large enough to permit the entry of and Co2 divalent cations such as The observations made by Moczydlowski et al. were supported by the work
MECHANISM OF ACIION work has shown that the main site of local anesthetic action is on the cell membrane. Local anesthetics do not appear to have any appreciable effect on the intracellular fluid (axopla,cm). Various theories have been put for-
ward to explain the mechanism of the action.'0 Many of these postulate prevention of conduction and formation of an action potential by either fully or partially blocking the Na ion channels. Blocking is believed to be achieved either
of Maclver and Roth" (1987) on a single isolated neuron (crayfish stretch receptor), which also suggested the existence of receptor sites that can discriminate between the structures of different anesthetics. These deductions were
by the drug molecule causing a physical block in the channel.
like a cork in a bottle, or by the drug molecule distorting the channel. If enough sodium channels are blocked, there would be no significant changes in membrane potential, the
supported by those of Elliot et al.'7 in 1987, who concluded from their investigation of the inhibition of sodium current in giant squid axon by bensocaine that there were at least two sites for the action of the drug. It is an important feature of the local anesthetics in clinical use that their structures include tertiary amine groups that coexist in equilibrium with the conjugate acid at physiological pH:
firing potential would not be reached, and conduction of on action potential along the neuron would be prevented. Blocking of conduction would automatically prevent the release of neuroiransmitter at the presynaptic site. Increasing
the Ca4 ion concentration of the extracellular fluid may either enhance or reduce the activity of a local anesthetic by affecting the opening of sodium channels. Shanest I
suggested in 1958 that local anesthetics acted
+ Local arwsthelic (neutral molecule)
H Coniugate acid
686
Wilson and Gisvolds Textbook of Organic Medicinal and Pl,ar,nare,srical Chr,ni.orv
H2N-
C2H5 )
*
2
4.Amlnobenzoic acid
Procaine
Experimental studies carried out by Narahashi and Frain 1971 and Stnchartz and Ritchie'3 in 1985 indicated that the site of action of local anesthetics is only accessible from the interior of the neuron. Consequently, as neutral molecules cross membranes more easily than charged mole-
cules, the drug must cross the membrane in its uncharged form before it can enter and block the ion channel. Once inside the neuron, experimental evidence suggests that the action of the drug is mainly due to its charged form and that its binding to the receptor is voltage dependent.'5 '' Analysis of the work of Strichartz, Hille, and Ritchie has shown that the block caused by many local anesthetic agents
depends on the number of channels opem the greater the number open, the greater the block. This suggests that the activity of the local anesthetic agent depends on it entering the channel from inside the neuron ("the hydrophilic pathway"). However, blocks can arise even if the channel is not open. This is explained by the local anesthetic agent entering
the channel directly from the membrane ("the hydrophobic pathway"). The relative effects of these two pathways appear to depend on the lipid solubility of the drug, but both appear to contribute to the blocking effect. Local anesthetic agents are removed from the site of appli-
cation by the blood flowing through the tissues and mem-
HOa-l2a12N( C2H5 ) 2 Diethylaminoelhanol
branes in the area of application. Metabolism occurs through a variety of routes in both the plasma and the liver. Ester-type agents such as procaine are cichereliminuted by hydrolysis in the plasma, catalyzed by plasma esterases. or in the user. catalyzed by specific liver esterases.
The 4-aminobcnzoic acid (PABA) produced in this hydrolysis inhibits the action of sulfonamides. However, thc PABA is excreted in the urine, partly in the form of conjugates. The diethylaminoethanol is also excreted in the urine
but about 70% is metabolized by the liver. Amide-based local anesthetics niay also be hydrolyzed by plasma esterases, but the rate of hydrolysis is usually slower than that for the corresponding ester agents. Consequently. aniidc l(5a1
anesthetic agents are more likely to be hydrolyzed in the liver. Amide local anesthetic agents arc also metabolized by
oxidation and N-alkylation in the liver. For example lignocaine is metabolized by both hydrolysis and N-dcalkylatiou (Fig. 20-13). The importance of the liver in the metabolism of amide-based local anesthetics means that use of these
agents in patients with severe liver damage should avoided, as any toxic effects of the local anesthetic agem will be increased because of a reduced rate of metabolism. The delivery of local anesthetic agents to the liver for metabolism to be related to their degree of binding to
CH3
Further conjugated compounds
NHCOCH2NH2 CH3
I-tO
Gtycytxylldlde
CH3
—
NHCOCH2N
CH3
C2H5
CH3
3-Hydroxy.monoethylglycylxylidide C2H5
CH3
Monoethylgtycxylidide
N-
CH3
Dealkytatlon
_._ Further conjugated
CH3
compounds NHCOCH2N C2H5
CH3
LIGNOCAINE
HOCH3 —
I
Oxidation
/
'
HO* NH2
NHCOCH2N C2H5
HO*NH2 CH3 \\
CH3
3-Hydroxytignoca(ne
2,6.Dlmethylanhllne
4.Hydroxy-2,6-xylidine
4-Hydroxy-2,6-dirnethylanillno'\
Further conjugated compounds
Further conjugated compounds
Figure 20—13 • Metabolism of lignocaine.
Chapter 20 • Local .4nesthetie Ar.e,,is plasma proteins. Experimental work by Tucker ci al. (1970) showed that amide-based local anesthetic agents bind morn readily to plasma and tissue proteins than do ester-based agents. The binding of amide-based agents often involves the anesthetic binding to a,-acid glycoprotein. This binding is usually significant, ranging from 55 to 95% of the drug.
687
that it blocks the nerve transmissions to that region. Field block anesthesia is brought about by the same drugs used for infiltration anesthesia. However, the technique produces a larger region of anesthesia with a lower dose of the local anesthetic thait is required by the infiltration technique.
However, many factors influence the concentration of plasma proteins; for example, cancer, smoking and trauma decrease the concentration of plasma proteins, while oral contraceptives increase their concentration. Plasma protein concentration may also be altered in many diseases. Obviously these changes will influence the quantity and rate of delivery of the local anesthetic to the liver, with subsequent changes in the systemic toxicity of the drug. The elimination of local anesthetics and their metabolites
from the liver depends on hepatic blood flow. If this flow is reduced, it can result in an increase in concentration of agents and their metabolites in the body when large doses are administered over long periods. This buildup may result in an increase in the systemic toxicity of the local anesthetic agent.2°
Regional Nerve Block Anesthesia Regional nerve block anesthesia is usually brought about by either injection of the anesthetic in a suitable solvent system into the nerve or infiltration of the anesthetic into the tissue
surrounding a nerve or plexus supplying the region to be anesthetized. For example, spinal anesthesia may be brought
about by injection into the cerebrospinal fluid in the subarachnoid space. Dental anesthesia is brought about by flooding the area around the nerve by injecting the anesthetic into the adjacent tissue. The local anesthetic agent used for a nerve block depends on which nerve is to be blocked, the
length of time the anesthesia is required. and the medical condition and physique of the patient. Duration of action is usually prolonged by the use of vasoconstrictors rather than by increasing the dose. This approach reduces the chances of the drug spreading to regions that do not require anesthesia.
ADMINISTRATION
Topkal or Surface Anesthesia10 Direct application of a local anesthetic agent to the skin or a mucous membrane blocks the sensory nerve endings. The local anesthetic may be applied in the form of a liquid, spray. cream, ointment, or gel. It appears that the form used is often selected subjectively. For example, in the use of local anesthetic agents as premedication in gastrointestinal endos. copy, the patients preferred sprays, even though the degree of anesthesia was the same for sprays and gargles. Anesthesia of the mucous membranes of the ear, nose, and throat is usually brought about by use of aqueous solutions of he salts of tetracaine. lignocaine. or cocaine. The vasodilator
cftèct of cocaine reduces bleeding in surgical procedures. However, all local anesthetics are rapidly absorbed through mucous membranes, and so their use may be accompanied by an increased risk of toxic systemic reactions. As a result. dosage must be carefully controlled.
intravenous Reglonai Anesthesia Intravenous regional anesthesia is used to anesthetize a large region, such a.s a limb. The anesthetic is injected into a suita-
ble vein in a limb that has had its blood flow restricted by a tourniquet. The efficiency and safety of the technique depends on preventing arterial flow for the duration of the anesthesia. Lignocaine is frequently used to produce intravenous regional anesthesia, but hupivacaine is not approved
for this purpose because of its long duration of action.
Spinal Anesthesia Spinal anesthesia is carried out by injecting the anesthetic agent into the subarachnoid space in the spinal cord. The anesthetic acts mainly on the nerve fibers and blocks the pain regions of the body served by the sections of the spinal cord affected.
Epidural Anesthesia Anesthesia Aset dose of the local anesthetic in a suitable solvent system
injected directly into the area of the body that is to be anesthetized. These areas range from the skin to deeper tissues. The most frequently used local anesthetics for infiltraion are lignocaine. bupivacaine. and procaine. The technique produces a good degree of anesthesia in a
localized area without disrupting general bodily functions. However, the use of this technique may require large concen-
rations of anesthetic to bring about the desired degree of anesthesia, with an attendant increase in the risk of toxic reactions.
Field
Block Anesthesia
A solution of the local anesthetic is injected subcutaneously at a point adjacent to the area that is to be anesthetized, so
The drug is injected into the epidural space between the vertebrae and spinal cord. This numbs the nerves leading to the uterus and the pelvic area and leads to pain relief during labor. Epidural anesthesia may sometimes cause headaches.
FACTORS INFLUENCING THE EFFECTIVENESS OF THE ANESTHETIC ACTION
Susceptibility of the Neuron to Anesthesia Pain information is carried by the largely unmyelinatcd C fibers, while sharp pain is transmitted by tnyelinated Aö fibers. The sensitivity of nerve libcrs to local anesthetic appears to vary according to the size, anatomical type. and degree of conductance of the nerve fiber. In general, the
688
WiLson and Gi.cvold'x Texthook of Organic Medicinal and Phurnwceusical
order of onset of local anesthesia with increasing concernralion of agent is often small nonmyelinated fibers > small myelinated fibers > large fibers. However, this order is not strictly followed in practice. Some myelinated fibers are blocked with lower concentrations of local anesthetics than some nonmyelinated fibers, while large fibers are often blocked before smaller fibers. Furthermore, in experimental work, the outer fibers in the nerve are affected first, regardless of their nature. Experimental work by Franz and Perry22 in 1974 SUpported by the work of Chiu and Ritchie23 in 1984 suggests that the differential blocking of nerve fibers depends on the length of axon that has to be exposed to the local anesthetic to bring about anesthesia. Shorter nerve fibers have shorter intemodal distances and in the early stages of anesthesia are fully exposed to the local anesthetic, with the result that they are more readily blocked than longer fibers. In most patients, the sensation of pain is the first to be lost, followed by temperature and touch.
—8.5). Local anesthetics are normally weak bases which are only slightly soluble in water. Consequently, they
are usually marketed as aqueous solutions of their more soluble salts. These solutions are often quite acidic, which makes them less prone to bacterial and fungal contamination. How-
ever, an aqueous solution of the salt of a local anesthetic will normally contain between 2 and 15% of the free base in equilibrium with the salt. Although the drug is mainly transferred through the cell membrane in its free base form, administration of the drug in alkaline solution does not enhance its activity. This is because the structure of the drug is controlled by the pH of the extracellular fluid and not the pH of the dosage form. Once inside the neuron, equilibrium is reestablished. Both the free base and its protonated form are known to be active,
but it is not known whether they bind to the same receptor site. However, it does appear that the protonated base plays 24.25 the major part in anesthetic
Vasoconstrictors The anesthetic action of local anesthetics is proportional to the time that the agent is in contact with nerve tissue. As early as 1903. Braun discovered thai the addition of adrenaline to solutions of local anesthetics increased and prolonged their action. It is now accepted that the addition of vasoconstrictors such as adrenaline to local anesthetic solutions prolongs and intensifies their action. The agent is confined to
its site of action by reducing the rate at which the blood
01
carries it away. Vasoconstrictors such as adrenaline also leduce the rate of absorption of a drug by allowing the metabolic rate of the local anesthetic to keep pace with the rate at which it is absorbed into the blood.stream. This also reduces systemic toxicity. However, prolonged use of a vasoconstric. tor on major arteries may cause irreversible tissue damage and can lead to gangrene. The main vasoconstrictors in current use with local anesthetics are adrenaline (cpinephrine), noradrenaline (norepi-
nephrine), and felypressin. Solutions of local anesthetics often contain either adrenaline or a synthetic analogue such as phenylcphrine. The effect of vasoconstrictors depends on the local anesthetic agent used; for example, adrenaline significantly prolongs the action of lignocainc but has less effect with prilocaine. The concentrations of vasoconstrictors arc kept as low as possible to reduce the risk of unwanted side effects, such as chest pains, palpitations, and increased heal rate. Local anesthetic preparations containing tors should never be used on digits. since they have no alternative blood supply. Consequently, restriction of blood SUp.
ply can cause necrosis, a form of enforced cell death.
pH of the Exb'acellular and Intracellular Fluids
HO
('he,ni.orr
HO
Additionally, preparations containing adrenaline should not normally be used on patients with diseases including disease, and thyrotosicocontrolled sis. Cocaine is a vasoconstrictor and so probably owes some of its effectiveness to this property.
Neuron Stimulation The effectiveness of the blocking action of a given tion of a local anesthetic agent depends on the frequency and extent to which a neuron has been recently stimulated. The greater the frequency of this stimulation, the more effective the local anesthetic agent is in blocking a response14-
RATE OF ONSET AND DURA11ON OF ANESTHESIA The time for the onset of action appears to be related to the type of tissue being anesthetized, the method of administmtion. and the percentage of the local anesthetic agent in hs unprotonatcd form at physiological pH. Since the degree of protonation is indicated by the pK, value of the drug, lreaJ anesthetic agents with a low value and high lipid sotubitity usually have a more rapid onset of action than those with higher pK. values and lower lipid solubility. For example. lignocaine, which is about 35% unprotonated at pH 7.4, usually has a more rapid onset of action than bupivacaine. which is about 8% unprotonated at this pH. The time taken for
HO
01 Phenylephrine
Chapter 20 • lisa! ,tue.cI!n'zie Agenis
drug to diffuse from its site of application to its site of action will also affect the rate of onset of anesthesia)° the time taken for the onset of anesthesia Reportedly.27 can be reduced by the use of the hydrogen carbonate fonn of the drug. This does not increase the toxicity of the local anesthetic agent, but it has been reported to reduce the pain
with injection and improve the effectiveness of the block in some cases.
The duration of action appears to be related to the lipid solubility of the local anesthetic agent and its ability to bind to protein. As a general rule, the more lipid soluble the drug. the longer the duration of its action, it is difficult, to classify local anesthetics in terms of the duration of anesthesia. however. because although the period of action depends on the dose, the relationship between dose and duration of anesthesia is not clear. In most eases, increasing the dose increases the duration of the anesthesia, but the relationship is not linear. For example, doubling a dose does not necessarily double the time of action. The dose used clinically is usually determined by factors such as systemic toxicity, potency, and the time for which the anesthesia is required. When long periods of anesthesia are required. it is better to repeat applications rather than use large doses. This keeps dose levels to a minimum, which reduces the level of any possible systemic toxicity.
SECONDARY PHARMACOLOGICAL ACTION Local anesthetics do not rely on blood circulation to reach their site of action, as they are usually administered at, or close to. their site of action. Systemic side effects arise because the local anesthetic agent is carried away in the blood before it can be fully metabolii.ed. Consequently, the chemical and pharmacological properties of local anesthetics are of major importance in determining not only the effectiveness of the drug but also its systemic side effects. Local anesthetic agents can affect the function of any organs in which electrical impulse transmission occurs. The nature and the extent of these unwanted side effects depend on the drug used, the concentration of the drug in circulation. the site of application. and the technique used. The secondary effects of local ane.sthetic agents in these Situations are discussed in this section.
Cardiovascular System Local anesthetic agents usually affect the cardiovascular sys-
tem by decreasing the force of contraction. electrical excitability. and conduction of the myocardium. A high systemic concentration of local anesthetic is usually necessary, however, before any of these effects are observed. Occasionally, low concentrations administered by infiltration cause cardiosascular collapse and death. The reason for cardiovascular collapse is not known.2" it appears, however, that local anesthetic agents may act as antiarrhythmic agents by blocking the Na . K , and Ca2' channels responsible for the excitaüon of heart muscle. For example. many workers believe that lignocaine may reduce the possibility of Na channels opening during depolarization. Recovery from this type of block, however, is usually rapid. F
689
Central Nervous System All amide-based local anesthetics can stimulate the CNS, causing symptoms ranging from restlessness to clonic convulsions. Stimulation may be followed by depression of the CNS and death, usually from respiratory failure. These unwanted side effects appear to be related to the potency of the anesthetic. It is therefore possible to predict these side effects from a knowledge of the drug and its concentration in the bloodstream. Unfortunately, convulsions can occur with little or no warning but can be prevented or stopped by the use of sedatives, such as diazepam or barbiturates. although near-anesthetic doses of the latter are required. Other types of local anesthetic can stimulate the CNS system but often lead to drowsiness. Individual compounds may cause other unwanted side effects, however. For example, at blood concentrations of 5 cm3. lignocaine may produce muscle twitching. dysphoria, and euphoria. Both lignocaine and procaine can produce symptoms of sedation, followed by unconsciousness. Cocaine, in common with some other local anesthetic agents. has an effect on mood and behavior.
Blood30-32 Amethocaine (tetracaine). henzocaine. lignocaine. and prilocaine have been reported to induce methemoglobinemia. This is a condition in which the level of methemoglobin in erythrocytes exceeds the normal I to 2cf. Methemoglobin is hemoglobin that contains iron Ill instead of iron II and so cannot transport oxygen. Concentrations of about I result in the appearance of cyanosis in which the lips take on a purple-blue coloration. High concentrations are rare but are associated with a high mortality rate. It has been suggested that methemoglobineniia may be due to either the presence of an aromatic amine in the local anesthetic or the metabolism of the local anesthetic to an aromatic amine.
Wound Healing Local anesthetics may interfere with wound healing. This is
particularly important in surgery carried out on the hands and feet.
Hypersensftivlty Hypersensitivity to local anesthetics appears to be related to both chemical structure and the method of administration. Allergic reactions occur most frequently with ester-based local anesthetic agents (benzoic acid derivatives). Adverse effects include allergic dermatitis, asthmatic attack, or, in extreme cases, death due to anaphylactic shock. Individuals suffering a hypersensitive reaction from one local anesthetic agent are often sensitive to compounds with a similar structure. For example, patients sensitive to procaine are often also sensitive to amethocainc Amide-based local anesthetic agents do not usually produce hypersensitivity reactions, although they may be responsible for other unwanted effects and have been implicated in malignant hyperpyrexia. Families with a history of this disease should only be treated with ester-based local anesthetics.
Patch testing frequently provides adequate warning of hypersensitivity. When Ruzicka em al. (1987) conducted allergy
690
Wi/sos, sins!
Gissolds Testhook of Organic Medicinal and Pharnwce,aical ('he,nis:rv
Amethocalne
Procaine
tests on 104 patients who were known to have had an allergic
throat. Cocaine is of considerable interest, however, because
reaction to henzocaine and procaine, however, the results showed that prick testing did not indicate an allergic reaction
of the active agents that were developed from its stnicture Tetrodotoxin and saxitoxin ore naturally occurring local
and intracutancous testing rarely gave a hypersensitive reaction. They concluded that contact allergic patients could be
ane.sthetic agents but are too toxic to be of clinical use. Teen-
dotoxin is found in the tissue and organs of fish of the order Terrrwdont,fon,w.s. and saxitoxin is isolated from some
injected with local anesthetic agents without a significant risk of a reaction. Hypotension caused by local anesthetic agents is often
rifle dinoflagellates. These compounds. which are highly toxic to humans, are structurally different but appear to hasc
the same mechanism of action. They are thought to the external opening of the Na channel. Interest in these compounds centers on the fact ihat they could lead to the
unrelated to the type of drug used and can be prevented by premedication with a suitable
development of new local anesthetic agents and their use as
tools in neurochemical research work and in investigating the molecular nature of action potentials and sodium chan'
STRUCTURE
nels. '°
A large number of compouncLs will produce a conduction
block. Most of the local anesthetic agents in general use. however. may conveniently be classified into four basic types, namely, those that act by hypothermia, alkaloids, derivatives of benzoic acid and aniline, and miscellaneous compounds. The benzoic acid and aniline group contains most of the local anesthetic agents currently in clinical use. It is not possible to relate the chemical structures of local anesthetics to their activity because little is known about the structures of Ihe receptors. It is possible. however, to pick out similar structural features between some of the active compounds in common use.
Benzolc Add and Aniline Derivatives With Local Anesthetic Activity Most of the local anesthetic agents in current medical use are of these types (Tables 20-2 and 20-3). The benzoic add derivatives are esters that were developed from cocaine, while the aniline derivatives are annides developed from co. gramine. Both types of derivative have chemical stnicwies that normally have the general fbrniat: Lpophle cones I__—!Ecios
Agents With ilypothermic Action
Both the ester and N-substituted amide functional groups are bioisosteres (Fig. 20.14). which explains the occurrence of these groups in similar positions in the structures of locil anesthetics.
Local anesthetics that act by reducing the temperature of the
area being anesthetized arc largely of historical interest. Most of the chemical agents used, however, produce intense cold through rapid evaporation and, hence, an anesthetic ac-
The lipophilic center ix usually eiiher a carbocyclic or a heterocyclic ring system, while the hydrophilic center is normally a secondary or a tertiary amine that might or might not be cyclic. Tertiary amines are more useful, since they are less irritating to tissue. The hydrophilic center may he
tion. One of the most efkctive was ethyl chloride, which is still in use today as a topical local anesthetic. This agent should not be used on mucous membranes or broken skin. however. and prolonged use may cause chemical frostbite.
attached to the ester or amide by either a short hydrocarbon chain or oxygen. nitrogen, or sulfur atoms. Most of the local anesthetic agents, in common, however, use a short hydracarbon chain. The lipophilic center is believed to be largely
Alkaloids are obtained from plants and trees. The only one in general clinical use is cocaine, which because of its addictive properties is largely restricted to use with the ear, nose, and
responsible for the lipid solubility of the local anesthetic agent and its ability to pass through the nonpolar heart ci
0
Cocaine
Tetrodotoxin
Saratoxin
Chapter 20 • Local A,u'q!:etir Agenis
TABLE
20-2 Examples of the Ester-Based Local Anesthetic Agents Upophilic Group—Ester—Bridging Group.-4iydrophllic Group (if Any)
Arnethocaine ((eiracwne)
Beneorainc
Rutcainc uniphalc
H2S04
Chloruprocaine hydrochlondc
HCI
Cycloniethycuine
H2S04
HCI Itexylcainc hydrochloride
CH3
CH3 Meptylcainc hydrochloride H3
HCI
HCI
hydrochloride
hydrochloride
hydrochloride
HCI
HCI
691
692
Wi/so,, anti Gisvold's Textbook
TABLE
of Organic Medicinal and Pl,arn,aceu:ical Chemistry
20-3 Examples of Amide-Based Local Anesthetic Agents Liphophilic Group—Amide—Bridglng group—Hydrophilic Group (If Any) CH3
CH3CH2CH2CH2
CONHCH2CI-12N(C2H5)2 = HCI Dibucainc hydrochlortdd (cinchocarncl
CH3
CH3
CH3
CH3
CH3
CH3
CH3 HCI
CH3
CH3 TuBa,. B. F,: I Mcd. diem. 14:1191, 1971.
'Anal.
Prormic,, Div11 Sybil. 15:761—779. 19111,.
'Marib. C. IL. Hardy, P. A. I: Ropisacainc:
0
A ncw local
apcnl Br. J. Hnsp. Mcd. 45.94-95. 1991.
0
// —c
//
—c
\..
N—H
A
0
Figure 20—14 • Ester and amide func groups are bioisosteres (i.e., structure
A
Amide
Ester
have similar sizes, shapes, and electronic tures)
Chapter 20 • Local Ane.irlierie Agents
Electrostatic attraction
CH2CH3
693
Electron donors Increase polarization of carbonyl group
CH2CH2\NH
Van der Waals forces forces Permanent dipole -dipole attraction
Van der Waats 4-
Van der Waats' forces
Electron acceptors decrease polanzation of carbonyl group
02N
Cr
Figure 20—15 • Schematic representation of the binding of an ester-type focal anesthetic agent to a receptor site. (From Buchi and Perlia, 1960)36
the cell membrane. Lipid solubility plays an important part in the action of local anesthetics, since their action depends on their ability to penetrate the cell membrane of the axon. The hydrophilic center provides the local anesthetic agent with some of its water solubility. This is an essential factor in transporting the drug to the membrane and, once inside the cell, to the receptor. The hydrophilic center also allows the drug to penetrate the polar outer face of the cell membrane, enabling the drug to reach the lipid heart the membrane. It also allows the drug to pass through the inner polar
face of the membrane into the cell. The hydrophilic center is also believed to be involved in the binding of the drug to the receptor.
The best local anesthetic action is obtained when the lipo-
philic and hydrophilic centers arc in balance. If the hydrophilic center is the dominant structure, the anesthetic action of the drug is weak, since its membrane penetration is poor.
Similarly, if the lipophilic center is the dominant structure. local anesthetic action is again poor. In this case, the agent can penetrate the lipid membrane of the axon, but its solubil-
ity in both extracellular and intracellular fluids is poor. The pK. values of a local anesthetic agents have been used as a measure of its ionization and, hence, of the lipophilic-to-
reached. After this point is reached, the activity decreases,
even though the partition coefficient increases. Unfortunately, the increase in activity is often accompanied by an increase in toxicity. A study35 of the homologous series formed by substituting
the aryl ring of local anesthetics by alkyl. alkyloxy, and alkylannino groups showed that the purtition coefficients of the members of a series increased with an increase in the number of methylene groups in the substituent of the series. In general. the maximum activity in a series was achieved for the C.1 to C5 homologues. Similarly. substitution 01 the hydrophilic center showed that the partition coelticient increased with the number of carbon atoms. which was also accompanied by an increase in activity. The use of piperidino and pyrrolidino groups as hydrophilic groups gave products
with a degree of activity like that obtained with a diethylamino group. The mnorpholino group. however, gave products with lower activity.TM' Local anesthetics are believed to bind to plasma and tissue proteins by van der Waals' forces, dipole—dipole attractions. and electrostatic forces (Fig. 20.15). Local anesthetic activ-
ity of benzoic acid-based drugs improves if the aryl lipo-
hydrophilic ratio. At physiological pH. the ratio of ionized Ca un-ionized molecules in solution may be calculated by acing the appropriate form of the Henderson-Hasselbalch
philic center has electron-donor substituents but decreases with electron-acceptor substituents. Therefore, it is likely that electron.donor substituents increase the binding of the local anesthetic agent to the receptor. hut electron-acceptor
equation, which states for weak monobasic acids
substituents reduce this binding. Buchi and
Inon-loniced fonnl pK,=pH + log lionized form)
gested that this latter was a consequence of electron acceptors withdrawing electrons from the carbonyl group, which decreases its polarization. This reduces the strength of the carbonyl group's dipole and, consequently, weakens its dipole—dipole attraction with the receptor.
(Eq. 20-2)
and for weak monoacidic bases
+ log
lionwcd lorml lnon-uonized tonnl
(Eq. _0-3
Most clinically useful local anesthetics are weak bases values in the range of 7.5 to 9.5. This implies that have that compounds with pK, values below 7.5 are not sufficiently ionized at physiological pH to be effective in bringing about anesthesia, even though they can penetrate the axon. In contrast, drugs with pKa values above 9.5 are almost fully at physiological pH. Consequently, these drugs are less effective because they have difficulty penetrating the cell membrane.
The partition coefficients of structurally similar local anesthetics have been used as a measure of their relative activ-
ity. In vivo experiments have shown that with a series of
sug-
Compounds whose structures contain amide functional groups tend to hind more strongly to receptor sites. For example. Tucker and coworkers (1970) reported that 95% of bupivacainc bound to plasma and tissue proteins, compared with 55% for prilocaine. Tucker and Mathcr2° (1975) also showed that the more potent and longer-acting agents are more extensively bound to plasma proteins. This is not the only factor affecting potency, however. For example, amide bonds are more resistant to hydrolysis. which will also effect the duration of action.
Agents
related structures, an increase in activity corresponds to an
Several different classes of compounds. other than those discussed in the preceding sections. exhibit some local anes-
nctea.se in partition coefficient until maximum activity is
thetic activity (Table 20-4). These compounds are usually
694
lVllson and Gisvold'.c Textbook
Orga,:k Medicinal and Phorn,aceurical C'henzi.ctr.'
TABLE 20-4 Miscellaneous Compounds With Local Anesthetic Activity
l)imethisoquin CH3CH2CH2CH2
PhNHCOO\ Diperodon
PhNHCOOCH2 HCI
COCH2CH2
Dycluninc hydrochloride
C2H5
H
H
CH3 Euprocin
Fomocaine
HO
CH3CHCH2CH2O
N
H
,,
OCH2 _-fj-- CH2CH2CH2 — N""O
Mynecaine
CH3
HCI
Phenacainc hydrochloride
OCH2CH2CH2 —
Pranwoinc hydrochloride
weak bases with distinct hydrophilic and lipophilic regions. In addition, a wide variety of compounds, including benzyl alcohol, phenol, and some antihistamines, also show sonic local anesthetic activity. REFERENCES I. Einhorn. A.: Justus Leibigs Ann. Chem. 216:236—237. 1891. 2. Merling. G.: Justus Leibigs Ann. Chem. 265:329—356, 1883. 3. Hughes. J.: Brain Res. 88:295. 1975. 4. Smith, 1. (cii.): British Medical Association: Guide to Medicines and Drugs. Godalming. U. K.. Colour Library Books, 1992. 5. Alberts, B.. Bray. F).. Lewis. 1.. ci al: Molecular Biology of the Cell. 3rd ed. New York. Garland Publishing. 1994. pp. 523—546. 6. Catterall, W. A.: Science 223:653-661. 1984. 7. Watson, S.. and Glrdlestone. 0. (edo.): Reference Receptor and Ion Channel Nomcnclature Supplement. Trends Pharmacol. Sci. 1994. 8. Moci.ydlowiki. 0.. Uchara. A., Guo. X.. and Heiny 3.: Ann. N. Y. Aced. Sd. 479:269—292,1986. 9. Schneider. M. li., and Dubois, J. M.: Biophys. 3. 50:523—530, 1986. 10. Ritchie, J. M., and Greene, N. M.: Local anesthetics. In Goodman.
- HCI
L. S.. and Gtlman, A. (eds.). Goodman and Gilman'sThe Pharniacolocical Basis of Therapeutics, 7th cii. Ness York. Macmillan. 1985. 302—321.
II. Shanes. A. M.: Pharmucol. Ree. 10:59—273. 1958. 12. Metcalfe. 3. C.. and Burgen, A. S. V.: Nature 220:587—588, 1968. 13. Stricharto, G. R.. and Ritchic. 3. M.: Action of local anesthetics on channels of excitable tissues. Itt Strichaji,.. G. B. (ed). Local Ares. thetics; Handbook of Expcnmental Pbartnacology. Berlin, Springer Verlag, 1985. 14. Hille. B.: Theories of anesthesia: Genera) perturbations sersus spccilii
receptors. In Fink. B. R. led.). Mechanisms of Anesthesta. s-al Progress in Anesthesiology. New York, Raven Pres.s. 1980. pp. I-S IS. Stncltnrtz. G. R.: 3. Dent. Rex. 60:146(1—1467, 1981.
16. Macleer. M. B.. and Roth, S. H.: Ear. 3. Pharnuicol. 1987.
17. Elliot. J. B.. Haydon. 0. A.. and Hendry. B. NI.: Pllueugers Arch. 4(9. 596—6(X). 1987.
18. Narahashi. 1., and Eru,acr. D. T.: Neurosci. Rex. 4:65—99. 1971. 19. Ritchie. 3. M.. and Greengard. P.: Anon. Bee. Phamsacol. 1966.
20. Tucker, C',. T., and Madrer. I.. 0.: Br. 3. Anesth. 47:213. 1975.
21. Neal. M. i.: Medical Pharmacology at a Glance. Oxford, Bljcksei Scientific Puhliculitinr, 1987, pp. 16—17.
Chapter 20 • Li,cal Anesthetic Agents 12
Fr.ini. 3). N,.and Perry. K. S.: J. Physiol. (Lofld.) 236:193—210. 1974.
27
Cttiu. S. V.. and Ritchie. J. M.. Proc. R. Soc. Lond.
Itiol.) 220:
415—422. 3984.
2). Hille. H.: 3. Gm. Physiol. 69:497—515. 1977. and Ritchic. J. M.: J. Gm. Physiol. 71: 223—225. 1978. 15 Mro'.e 26. Counncy. K. R.: J. Pharmacol. F.ap. Ther. 213:114—119. 1980. 27. Brot;tage. P. R.: Aeta Anestliesiol. Scand.Suppl. 6:55—69. 3965. Bromage, P. K.: ('an. Med. Assuc. J. 97:1377—1384. 1967. 29. Gram. A. 0.. Jr.: Am. Heart J. 123:1130—1136. 1992. 30. Ferraro. L.. Zcichncr. S.. (3recnhlott. C,.. and ()nreger. 3. S.: Anesthesiology 69:614—615. 1988. SI. Anderson, S. T.. Hajduc?.ek. J.. and Barker. S. 3.: Ane.sth. Annlg. 67: 1099—1101. 1Q88.
32. HaIl, A. H.. Kulig. K. W.. and Rumack. 13. II.: Med. Toxicol. 1(43: 253—26(1. 1986.
33. Martindale. W. led.): Martindale: The Evira Pharmacopoeia. 30th ed. London. Ptmrnmceutical Press. 3993. PP. 995—1018.
34. Mulitadi. F. 3.. and Al-Badr. A. A.: Anal. Profiles Drug Subst. IS: 151—231. 1986.
35. Buchi. 3.. arid Perlia. X.: .Structure.activity rclntiotts and physiocltemical properties ollocat anesthetics. In I.achat. P. led. I. Local Anesthetics. Encyclopedia of Pbannacology and Therapeutics, sect. 8. vol. 3. New York. Pergamon, 1971. P.
695
36. Buchi. 3.. and Perlia. X.: Design nt local anesthetics. In Ariens. 0. J. led.). Drug Design. vol. III. New York, Academic Press. 3972. P. 243.
SELECTED READING Neal. M. 3.: Medical Pharmacology at a Glance. 3ni ed. l3lackwell Scimntitic Publications, 1997.
Rang. H. P.. Dale. M. M.. and Ritter. 3. M.: Pharmacology, 4th ed. Edinburgh. Churchill Livingstnnc. 1999. Reynolds. 3. 0. F.. and Prasad. A. B. (edsl: The Merck Index. 12th ed. Ruhway, NJ. Merck & Co.. 1996. Sncadcr. W.: Drug Discovery: the Evolution iii Modem Medicines. Chich' ester. U. K.. John Wiley & Son.s. 1985. Speight. T. M.. and Hollord. P. (er(s): Avery's Drog Treatment. Principles and Practice of Clinical Pharmacology arid Therapeutics. 4th. ed. Auckland. ADIS Press. 997. Tortora. C,. 3., Anagnostakos. N. P.. and Miujeb. Ii. N.: Principles of Analonry and Physiology. San Francisco. Canfield Press. 2000. Thomas, C,.: Medicinal Chemistry. An Introduction. Chichester. U. K.. John Wiley & Sons. 21100. Voct. D.. Voet. J. C.. and Pratt, C.: Fundamentals of Biochemistry. New York. John Wiley & Sons. 1999.
C
H
AP
T
E
21
R
'-I
Histamine and Antihistaminic Agents
——
THOMAS N. RILEY AND JACK DERUITER
Histamine is a /3-imidazolylethylamine derivative chat is present in essentially all mammalian tissues. The major physiological actions of histamine are centered on the cardi-
ovascular system, nonvascular smooth muscle, exocrine glands, and the adrenal medulla) In a general sense, histamine plays an imporlant role as a 'chemical messenger" component of a variety of pathways that have evolved in multicellular organisms, allowing them to communicate efficiently and effectively. The involvement of histamine in the mediation of allergic and hypersensitivity reactions and the regulation of gastric acid secretion has led to the development of important drug classes useful in the treatment of symptoms associated with allergic and gastric hypersecretory disorders.
as the monocationic conjugate species at logical pH (7.4). The ratio of the concentrations of the taos" mers has been calculated to be 4.2, indicaling that in aqueous solution, 80% of the histamine monocation exists as N'-H and 20% exists as N"-H. Structure—activity relationship studies suggest that the NH3 monocation is important for agonist activity at hiss• mine receptors and that transient existence of the more lipo. philic uncharged histamine species may contribute to
location of cell membranes. Other studies support the proposal that the Ni-H taumomer of the histamine monocatlon receptor. is the pharmacophoric species at the I ,3-tautomeric system is important for selective H2-rercpsr agonism.
Stereochemistry HISTAMINE
Nomendature Histamine, known trivially as 4(5-)(2-aminoethyl)imidazole. structurally is composed of an imidazole heterocycle and ethylamine side chain. The methylene groups of the aminoethyl side chain are designated a and /3. The side chain is attached, via the fl-CH2 group, to the 4 position of an imidaz-
ole ring. The imidazole N at position 3 is designated the pros t nr) N. whereas the N at position I is termed the ide N. The side chain N is distinguished as N°.
Histamine is an achiral molecule; histamine receptors. hos' ever, are known to exert high stereoselectivity toward chiraf ligands.4 Molecular modeling and steric-aceivity studies of the influence of conformational isomerism on lie activity of histamine suggest the importance of lrons-gaa*e
rotameric structures (Fig. 21-2) in the receptor
of
this substance. Studies with conformationally restricted his tamine analogues suggest that while the trans rotamer histamine possesses affinity for both H1 and H2 histaneinc receptors. the gauche conformer does not act at H1 sites.
NH2
HISTAMINE LIFE CYCLE Knowledge of the biodisposition of histamine is imporlan to understanding the involvement of this substance in
pathophysiologies as well as the actions of various that either enhance or block its actions. Each of the the "life cycle" of histamine represents a potential site for phannacological intervention. (tote)
2
(pros)
Hfstamine
Biosynthesis and Distribution
ionization and Tautomerism
Histamine is synthesized in cytoplasmic granules of its pnr cipal storage cells, mast cells and basophils.° Histaminc formed from the naturally occurring amino acid. S-histidirt
Histamine is a basic organic compound = 5.80; = 14.0) capable of existing as a N". = 9.40; mixture of different ionic and uncharged lautomeric species Histamine exists almost exclusively (96.6%) (Fig. 21-1
via the catalysis of either the pyridoxal phosphate-&po dent enzyme histidinc decarboxylase (HDC, aromatic amino acid decarboxylase (Fig. 21-3). Subsita specificity is higher for I-IDC. HDC inhibitors (HDCIs ii
696
Chapter 21 U HisIa,nine
Aniil:is:wninic Agenis
697
NH2
/K""
N
N
N
N
H
tautomer
1 NH3
N,N
N
N
H
1
N
H"
N
Figure 21—1 • Histamine tautomers and cations.
dude a-fluoromethyl histidine (FMH) and certain flavonoids. although no HDCIs have proved useful clinically. Histamine is found in almost all mammalian tissues in concentrations ranging from I to more than 100 This substance is in particularly high concentration in skin, bronchial mucosa. and intestinal mucosa. It is found in higher concentrations in mammalian cerebrospinal fluid than in plasma and other body fluids. NH3
Storage and Release Most histamine is synthesized and stored in mast cells and basophilic Protein-coinplexed histamine is then stored in secretory granules and released by exocytosis in response to a wide variety of immune (antigen and anti-
body) and nonimmune (bacterial products, xenobiotics, fmns
gauche
Figure 21—2 • Histamine rotamers.
physical effects, and cholinergic effects) stimuli. The release of histamine as one of the mediators of hypersensitivity reactions is initiated by the interaction of an antigen—IgE corn-
698
Wil.wn and Gisvold'.c Textbook of Organic Medicinal wul Phar,naceuricai Cb,'mistrv
pIes with the membrane of a histamine storage cell. This interaction triggers activation of intracellular phosphokinase
C (PKC). leading to accumulation of inositol phosphates. Exocytotic release of histamine diacylglycerols, and Ca2 follows the degranulation of histamine storage cells. Histamine is released from mast cells in the gastric mucosa by gastrin and acetylcholine. Neurochemical studies also suggest that histamine is stored in selected neuronal tracts in the central nervous system (CNS).
activation of phospholipase C (PLC). resulting in inositol phosphate accumulation and calcium mobilization in most tissues. H2 receptors have been detected in a wide variety of tissues (most notably for therapeutic consideration, myocardial
cells and cell membranes of acid-secreting cells Iparietall of the gastric mucosa) and mediate the gastric acid secretoiy actions of histamine. The H2 receptor has the general characteristics of a 0-protein—coupled receptor, with a molecular
mass of 59 Wa and nonessential N-glycosylation sites in
Receptors Once released, the physiological effects of histamine arc mediated by specific cell-surface receptors.7 Extensive pharmacological analysis suggests the existence of at least three different histamine receptor subtypes. H1. H2. and H3. Histamine H, receptors have been detected in a wide vari-
ety of tissues including mammalian brain; smooth muscle from airways, gastrointestinal (01) tract, genitounnary system, and the cardiovascular system; adrenal medulla; and endothelial cells and lymphocytes. The structure of the H1 receptor ha.s been determined and shown to possess several
important features that distinguish it from the H2 receptor.8 The H,-rcceptor protein has a molecular mass of 56 kfla. The deduced sequence of the bovine H,-receptor protein represents 491 amino acid residues. Stnicturally, this receptor contains seven hydrophobic transmembrane domains (TMs) characteristic of most 0-protein receptors. The third
intracellular loop of the receptor is very large (212 amino acids), and the intracellular C-terminal tail is relatively short (17 amino acids). Site-directed mutagenesis studies have provided evidence for the binding domains of H, agonists and antagonists. The third (1'M3) and fifth (TM5) transmembrane domains of the receptor protein are responsible for binding histamine. Aspartate (107) of the human H, receptor is essential for the binding of histamine and H, antagonists
to the receptor, perhaps by being involved in an Asp+ —R interaction. Asparagine (207) of the TM5 domain is known to interact with the N'-nitrogen of the imidazole ring of histamine, while lysine (200) has been shown of the natural to interact with the nucleophilic ligand. Signal transduction of the H, receptor involves the
the N-terminal region.9 The most notable difference between structures of cloned I-I, and H2 receptors is the much shorter
third intracellular loop and longer C-terminal loop of the H2-receptor protein. A TM3 aspartate along with an aspartate
and threonine residue in TM5 is apparently responsible for binding histamine. The physiological and pharmacological effects of H-receptor ligands are mediated by a stimulator) 0,-protein—coupled receptor, which in turn activates the sicnylale cyclaselcyclic adenosine monophosphate (cAMP) intracellular second-messenger system. The cloning of the human histamine H3 receptor in 1999 evoked considerable renewed interest in the field of hista. mine receptors.'0 The H., receptor is proposed to functior as a neural autoreceptor (presynaptic) serving to modulate histamine synthesis and release in the CNS. Subsequent studies have also located H., sites in peripheral tissue. includ.
ing the gastric mucosa where this receptor may negativel) control gastric acid secretion and on the cardiac sympathetic terminals in the myocardium. Although signal transducths
mechanisms of the H., receptor have not been fully clad. dated, increasing evidence suggests that this receptor longs to the superfamily of 0-protein—coupled ieceptornY A new histamine receptor. the H., subtype. was first tnported in 2000 and characterized as a 390-amino acid, 6,. coupled protein with 40% identity to the H., receptor.'° This new receptor exhibits a very restricted localization; expes. sion is primarily found in intestinal tissue, spleen. thymos and immune active cells, such as T cells, neutrophils. and eosinophils. which suggests an important role for H., recep-
tors in the regulation of immune function.
Histidine Decarboxylase NH2
Aromatic aminoacid decarboxylase
e H3:
Pyfldoxal Phosphate
FIgure 21—3 • Histamine biosynthesis.
NN
Chapter 21 • l-I,stunune (hid A,,zjlsisiaminje
699
NH2
N
SAM
N
H
MAO-B (brain)
Histamine
N DAO (periphery) SAH
NH2
N
N
H
Acetic Acid
.0
N
N
H3C
HO
OH
Acetic Acid Riboside
Figure 21—4 • Metabolism of histamine ALD-DH, aldehyde dehydrogenase; PRT phosphoribosyltransferase.
Termination of Histamine Action Three
principal ways exist to terminate the physiological of histamine":
cellular uptake. Animal studies have documented the uptake of histamine by many cetls. In particular, uptake isa temper.itore- and, partially. Na* -dependent process in rabbit gastric glands, sad the histamine is metabolized once in the cell. • Dcsen.citizaiion of relic. Some H1 receptor—containing tissues
exhibit a homogeneous loss of sensitivity to the actions at histamine, perhaps as a result of receptor modification. • Metabolism (Fig. The most common pathway for terminating histamine action involves enzymatic inactivation.
catalyzes the transfer of a methyl group from S-adcnosyl-t.methionine (SAM) to the ring ic/c-nitrogen of histamine. producing NT.methylhisusmine and S-adenosyl-L-homocys-
teine. Histamine is also subject to oxidativc deansination by diamine oxidase (DAO: EC 1.4.3.6), yielding iinida,ole acetic acid, a physiologically inactive product excreted in the urine. Similarly. is converted by both DAO and monoamine oxidase (MAO) to N-methyl imidazole acetic acid.
Functions of Endogenous Histamine as
Related to Pharmacological Intervention Histamine exhibits a wide variety of both physiological and
The enzyme histamine N-methyltransferase (HMT: EC 11,1.8) is widely distributed among mammalian tissues and
pathological functions in different tissues and cells. The actions of histamine that are of interest from both a phar-
700
WiIvo,, and Gi.wold.s Te.rthook of Orgw,ie Medicinal and Plu,rnwceuikal C'he,nisrrs
macological and therapeutic point of view include (a) its important hut limited role as a chemical mediator of hypersensitivity reactions, (b) a major role in the regulation of
pig ileum.'5 By contrast, the pA2 value in guinea pig atm
gastric acid secretion, and ft-) an emerging role as a neurotransmitter in the CNS.
receptors and a competitive inhibitor at H, receptors. The structural features required for effective interaction with these receptors are discussed below. Some H, antagonists also block histamine release. The concentrations required.
(H2 receptor) is 5.3. Thus, one may conclude that pyrilamine is a weak. noncompetilive inhibitor of histamine at the anial
however, are considerably higher than those required to produce significant histamine receptor blockade. The H, antag. onists do not block antibody production or antigen—antibody
HISTAMINE H1 ANTAGONISTS (ANTIHISTAMINIC AGENTS)
interactions."
The term annh,sia,n:ne historically has rekrrcd to drugs that antagonize the actions of histamine at H, receptors rather than H2 receptors. The development of antihistamine drugs began more than five decades ago with the discovery that pipemxan could protect animals from the bronchial spasm
Structure-Activity Relationships
thiazines. and dibenzucycloheptenes. In addition to H,-receptor antagonism, these compounds display an array of other pharmacological activities that contribute toward ther-
The H, antagonists arc now commonly subdivided into Iwo broad groups—the lirst- generation, or classical. antihistamines and the second-generation. or 'nonsedating." anrihistamines—hased primarily on their general pharmacologtcal profiles.'6 The differences between these two series are discussed in more detail in the sections that follow. The most detailed published SAR analyses for H, however, focus on the structural requirements for the lustgeneration agents.'' From these studies, the basic shOetural requirements for H,-receptor antagonism have been identified as those shown in Figure 21-5. In this structure.Ar is aryl (including phenyl. substituted phenyl. and groups such u.s 2-pyridyl): Ar' is a second aryl or arylmethyl group: X is a connecting atom of 0. C. or N: (CH2), represents a carbon chain, usually ethyl: and represents a basic, terminal amine function. The nature of the connectine atom, as well as the diaryl substitution pattern and amine
apeutic applications and adverse reactions. More recently, a
moiety, has been used to subclassify the first-generation anti-
number of second-generation or "nonsedating" antihista-
histamines as indicated in the sections below. This diaryl substitution pattern is present in both the lustand second-generation annihistamines and is essential significant H,-recepior affinity. Funhermore, several SAlt studies suggest that the two aryl moieties must be abte to adopt a noncoplanar conformation relative to each other fm optimal interaction with the H, receptor.2" The two aromatic
induced by histamine.'2 This finding was followed by the synthesis of a number of N-phenykthylcnediamines with antihistarninic activities superior to those of Further traditional structure—activity studies in this series. based largely on the principles of isosterism and functional group modification, led to the introduction in the l940s to 1970s of a variety of H, antagonists containing the diarylalkylamine Framework.'' These H, antagonists, referred to now as the jirs:.generathn, or clasxjce,l antihisiamines. are related structurally and include a number of aminoalkyl ethers. ethylenediamines, piperazines. propylamines. pheno-
mines have been developed and introduced.'6 The secondgeneration agents bear some structural resemblance to the first-generation agents but have been modified to be more
specific in action and limited in their distribution profiles.
Mechanism of Action
systems may be linked, as in the tricyclic
H, antagonists may be defined as drugs that competitively inhibit the action of histamine on tissues containing receptors. Traditionally. H, antagonists have been evaluated in vitro in terms of their ability to inhibit histamine-induced spasms in an isolated strip of guinea pig ileum. Antihistamines may be evaluated in vivo in terms of their ability to protect animals against the lethal effects of histamine administered intravenously or by aerosol. To distinguish competitive antagonism of histamine from other modes of action, the index pA is applied to in vitro assays. The index pA2 is defined as the inverse of the logarithm of the molar concentration of the antagonist thai reduces the response of a double dose of the agonist to thai of a single one. The more potent H, antagonists exhibit a pA2 value significantly higher than 6. Although there are many pitfalls" to be avoided in the interpretation of structure—activity relationship (SAR) studies using pA2 values, the following example illustrates distinguishing competitive antagonism. pA2 values for pyrilamine (mepyramine) antagonism range from 9. Ito 9.4 with human bronchi and guinea ,
(phenothiazines.dibenzocycloheptanes. and heptenes.
but again they must be noncoplanar for effective
ceprer
interaction. Most H, antagonists contain substituents in one of the aryl rings (usually benzene). and these influence anti histamine potency as well as biodisposition. as discussed individual classes of compounds in the sections below.
In many of the first-generation, or classical, antihisljmines, the terminal nitrogen atom is a simple dimeihylanti,s moiety. The amine may also be part of a heterocyclic ctrneture. however, as illustrated by the piperazines. some propylamines (pyrrolidines and piperidines), some pheaothia zines. the dibenzocycloheptenes. and the second-generation
R
Ar
Ar
/
X
—N
/ \
A
Figure 21—5 • General antihistamine structure.
Chapter 21 • Histamine and Antihi.staniinic Agents
antihistamines. In all cases, the amino moiety is basic, with
ranging from 8.5 to 10, and thus is presumed to be protonated when bound on the receptor. The moiety is also important in the development of stable, solid dosage forms through salt formation. The carbon chain of typical H antagonists consists of two or three atoms. As a result, the distance between the central
point of the diaryl ring system and the terminal nitrogen atom in the extended conformation of these compounds ranges from 5 (06 angstroms (A). A similar distance between these key moieties is observed for those anlihistamines with
less conformational freedom. In some series, branching of the carbon chain results in reduced antihistaminie activity. There are exceptions, however, as evidenced by promethatine, which has greater activity than its nonbranched counterpart. When the carbon adjacent to the terminal nitrogen atom is branched, the possibility of asymmetry exists. Stereoselectivc H,-receptor antagonism is typically not observed, however, when chirality exists at this site.2' Also, in compounds with an asymmetrically substituted unsaturated carbon chain (pyrrobutamine and triprolidine). one geometric isomer typically displays higher receptor affinity than the other.
The X connecting moiety of typical antagonists may a saturated carbon—oxygen moiety or simply a carbon or nitrogen atom. This group, along with the carbon chain. appears tn serve primarily as a spacer group for the key pharmacophoric moieties. Many antihistamines containing ,
acarbon atom in the connecting moiety are chira] and exhibit stcnnoselective receptor binding. For example, in the pheniramine series and carbinoxamine. this atom is chiral. and in sitro analyses indicate that enantiomers with the S configuralion have higher H,-rcceptor affinity.22 Generally, the first- and second-generation antihistamines axe substantially more lipophilic than the endogenous agonist histamine (or the H2 antagonists).23 This lipophilicity
Jifference results primarily from the presence of the two rings and the substituted amino moieties and thus may simply reflect the different structural requirements for antagmist versus agonist action at H, receptors.
The nature of this connecting moiety and the structural tature of the aryl moieties have been used to classify the anrihisramines as indicated in the sections below. Furthermore, variations in the diaryl groups. X connecting moieties, the nature of substitution in the alkyl side chain or termiat nitrogen among the various drugs account for differences observed in antagonist potency as well as pharmacological. biodisposition. and adverse reaction profiles. The ability of these drugs to display an array of pharmacological activities a largely because they contain the basic pharmacophore re-
for binding to muscarinic as well as adrenergic and cmfonergic receptors. The relationships of antihistamine :rtcture to these overlapping actions (H1 antagonist, antiand local anesthetic) have been analyzed. General
Pharmacological Considerations
clas.sical antihistamines have been used extensively for at symptomatic treatment (sneezing. rhinorrhea, and itching 4eyes, nose, and throat) of allergic rhinitis (hay fever, polli-
The
701
nosis), chronic idiopathic urticaria, and a number of other histamine-related diseases. These uses are clearly attributable to their antagonism of the action of histamine at periph-
eral H, receptors. The drugs best relieve the symptoms of allergic diseases at the beginning of the season when pollen counts are low. Although the symptoms of the common cold
might be modified by antihistamines. these agents do not prevent or cure colds, nor do they shorten the course of the disease.'° The antihistamines also are of little or no value in diseases such as systemic anaphylaxis and bronchial asthma, in which autocoids other than histamine are imporA number of the antihistamines. particularly the phenothiazines and aminoalkyl ethers, have antiemetic actions and thus may be useful in the treatment of nausea, vomiting, and motion sickness.'8 19 Also, those agents that produce pronounced sedation have application as nonprescription sleeping aids.'8 lV of the phenothiazines have limited use in Parkinson-like syndromes u.s a result of their ability
to block central muscarinic receptors.'8 " And, a number of antihistamines. including promethazinc. pyrilamine. tnpelennamine and diphenhydramine. display local anesthetic activity that may be therapeutically useful.24 As the general pharmacological profiles above suggest. most antihistamines can interact with a variety of neurotransmitter receptors and other biomaeromolecular targets. This is most evident among the first-generation agents. many of which function as antagonists at muscarinic receptors and, to a lesser extent, adrenergie, serotonergic. and dopamine receptors.'6 8. 9 Although some of these non—target-receptor interactions may have some therapeutic value (as discussed above), more frequently they are manifested as adverse reactions that limit drug use. This is particularly true of the peripheral anticholinergic effects produced by these drugs and of interactions with a number of neurotransmilter systems in the CNS that result in sedation, fatigue, and dizziness.'6 18. t9
The primary objective of antihistamine research over the past 10 to 15 years has centered on developing new drugs with higher selectivity for H, receptors and lacking undesirable CNS actions. The pronounced sedative effects of some of the first-generation agents were attributed to the ability of these drugs to penetrate the blood—brain barrier (BBB) because of their lipophilic nature and then block cerebral Ht receptors and possibly other receptors.'6 Thus research
efforts were initiated to design novel antihistamines with reduced ability to penetrate the CNS and decreased affinity for central histamine receptors. These efforts led to the introduction of the second-generation antihistamines. which are nonsedating and have little antagonist activity at other neurotransmitter receptors at therapeutic concentrations. The pharmacological properties of these agents are discussed in more detail below. Surprisingly little information is available concerning the pharmacokinetic and biodisposition profiles of the first-generation Generally, the compounds are or-
ally active and well absorbed, but oral bioavailubility may be limited by first-pass metabolism. The metabolites formed depend on drug structure to a large extent but commonly involve the tertiary amino moiety. This functionality may be
702
I Vitro,, and
Texihook of Organic Medicinal and Phar,naceulieal
subject to successive oxidative N-dealkylation. deamination. and amino acid conjugation of the resultant acid. The amine group may also undergo N-oxidation, which may be rever-
noxamine. enhances antihistaminic activity. These corn pounds display oral antihistaminic activities 40 and 2 tim' greater, respectively, than diphenhydramine in animals.'
sible, or direct glucuninide conjugation. First-generation agents with unsubstituted and activated aromatic rings (phenothiazines) may undergo aromatic hydroxylation to yield phenols. which may be eliminated as conjugate.s.23 More detailed pharmacokinetic data are available for the second-generation agents and are included in the monographs that follow. The H, antagonists display a variety of significant drug interactions when coadministered with other therapeutic agents. For example. MAO inhibitors prolong and intensify
As a result of an asymmetrically substituted benrylic c.e• bon. most of the aminoalkyl ethers are optically active. Mn! studies indicate that the individual enantiomers differ signili. cantly in antihistaminic activity, with activity residing prcdominantly in the S enantiomer.22 The diaryl tertiary aminoalkyl ether structure that terizes these compounds also serves as a pharmacophore muscarinic receptors. As a result, the drugs in this group possess significant anticholinergic activity, which a -blocking action on exucrine secretions. Dan hance the
the anticholinergic actions of the antihistamines." IS. IS. 23 Also, the sedative effects of these agents may potentiate the depressant activity of barbiturates, alcohol, narcotic analgesics, and other depressants. Recently, it was discovered that several of the second-generation antihistamines may produce lire-threatening arrhythmia.s when coadministcred with drugs that inhibit their metabolism." IS These interactions are discussed in more detail in the sections below.
siness is a side effect common to the tertiary
First-Generation H.rAntagonlst Drug
a
result of the ability of these corn
pounds to penetrate the BBB and occupy central H1 re ceptors. Although this side effect is exploited in os'er.dic counter (OTC) sleeping aids, it may interfere with the penfor
mance of tasks requiring mental alertness)5 quency of GI side effects in this series of antihistamines relatively low. compared with the ethylenediamine antihisi., mines. 5 In spite of their extensive use, pharmacokinctic daa this series of compounds are relatively limited. Most bees of this series apparently are extensively metaboliLcd
AMINOALKYL ETHERS (ETHANOLAMINES)
pathways including N-oxidation and successive N-dealkylation followed by amino acid conjugation of Is
The aminoalkyl ether antihistamines are characterized by the
resultant acid metabolites.23 The structures of the aminoalkyl ether derivatives. along with physicochemical properties. basic therapeutic data, and dosage form information arc provided in the
presence of a CHO connecting moiety (X) and a two- or three-carbon atom chain as the linking moiety between the key diaryl and tertiary amino groups (Fig. 2 1-6). Most compounds in this series are simple N,N-dimethylethanolaminc derivatives and are so classified in a number of texts. Clemastine and diphenylpyraline differ from this basic structural pattern. in that the basic nitrogen moiety and at least part of the carbon chain are part of a hctcrocyclic ring system and there are three carbon atoms between the oxygen and nitro-
graphs that follow.
Diphenhydramine Hydrochloride, USP.
Diphenh)
The simple diphenyl derivative diphcnhydramine was the first clinically useful member of the ethanolamine series and serves as the prototype. Other therapeutically useful derivatives of diphcnhydramine have been obtained by para substi-
dramine hydrochloride. ethanamine hydrochloride (Benadryl). has an oily, uble free base available as the bitter-tasting hydrochkniL salt, which is a stable, white crystalline powder solubk water (1:1), alcohol (1:2). and chloroform (1:2). The sail a pK, value of 9. and a 1% aqueous solution has a pH about 5.
tution of methyl (methyldiphenhydraminc). mcthoxy (me. drylamine), chloro (chiorodiphenhydramine), or bromo
In addition to antihistaminic action, diphenhydransinc hibits antidyskinctic. antiemetic. antitussive. and .sedaIe
(broniodiphenhydramine) on one of the phenyl rings. These derivatives reportedly have better therapeutic profiles than diphenhydramine because of reduced adverse effects.23 Replacement of one of the phenyl rings of the diphenhydramine with a 2.pyridyl group. as in doxylamine and carbi-
properties. It is used in OTC sleep-aid products. In the ucuJ dose range of 25 to 400 mg. diphenhydramine is not a
gen atoms.
active H, antagonist; it has anticholinergic and properties. Conversion to a quaternary ammonium sail not alter the antihistaminic action greatly but does
the anticholinergic action. As an antihistaminic agent. diphenhydramine is mended in various allergic conditions and, to a lesser etlnn as an antispasmodic. It is administered either orally or
terally in the treatment of urticaria, seasonal rhinitis
0—CH2—CH2—N
Ar Ar
/ \
A
fever), and some dermatoses. The most common side is drowsiness, and the concurrent use of alcoholic and other CNS depressants should be avoided.
A
Figure 21—6 • General structure of the aminoalkyt ethers.
Usual adult dose: Oral. 25—50 mg: IM or IV. tO-54) rng Dosage forms: Capsules, elixir, syrup, tablets, injection
Chapter 21 U !Iis:a,nine and Antihiciarninic Agents
CHOCH2CH2N(CH3)2
703
HCI HCI
Hydrochloride Bromodip4lenhydramlne HydrocNoride
The 8-chlorotheophyllinate Dirnenhydrinate. USP. theoclatci salt of diphenhydramine, 8-chiorotheophylline 2.(diphenymethoxy)-N.N-ditnethylethylamine compound Dramamine). is a white crystalline, odorless powder that is
soluble in waler and freely soluble in alcohol and chloroform.
Dimenhydrinate (see structure below) is recommended for the nausea of motion sickness and for hyperemesis gravi(nausea of pregnancy). For the prevention of motion ,ickness. the dose should be taken at least one-half hour before beginning the trip. The cautions listed for diphenhy-
Doxylamine Succinate, USP. The acid succinate salt (bisuccinate) of doxylamine. 2-[a-12-(dimethylamino)ethoxyj-a-methylbenzyl]pyridine bisuccinate (Decapryn Suecinate). is a white to creamy-white powder with a character-
istic odor. It is soluble in water (1:1). alcohol (I :2). and chloroform (1:2). A 1% solution has a pH of about 5. Doxylamine succinate is comparable in potency to diphenhydramine. It is a good nighttime hypnotic, compared with sccobarbital.25 Concurrent use of alcohol and other CNS depressants should be avoided. Usual adult dose: Oral. 12.5—25 mg/4—6 hours Dosage forms: Syrup and tablets
Jomine should be observed. Usual adult dose: Ortd. 50—1(X) mg/4 hours: IM or IV. 50 mg/ 4 hours: rectal. 1(X) mg q.d. or bid. Dosage forms: Elixir, syrup, tablets, injection, suppositories
CH2COOH
BroBromodiphenhydramine Hydrochloride. USP. nodiphenhydramine hydrochloride. 2-((4-bromophenyl)-
CH2C0ON
hydrochloride Ambodryl Hydrochloride), is a white to pale-buff crystalpowder that is freely soluble in water and in alcohol. Relative to diphenhydramine, bromodiphenhydramine is more lipid soluble and was twice as effective in protecting phenylmethoxy l-N,N-dirnethylethanamine
Doxylamine Succfnate
guinea pigs against the lethal effects of histamine aerosols.
Maleate, USP. The oily, lipid-soluble free base ot' carbinoxamine is available u.s the bitter bunsleate salt. (d. /)-2-Ip-chloro-a-[2-(dimethylamino)ethoxyl-
Usual adult dose: Oral. 25 mgl4—6 hours Dosage forms: Capsules and elixir
0
CHOCH2CH2N(CH3)2
CH3
Dimenhydrlnate
704
Wll.wn and Gi.wolds Textbook of Organic Medicinal and Phannaceuzical Chen,j.qrv
benzyl]pyridinc bimaleate (Clistin), a white crystalline powder that is very soluble in water and freely soluble in alcohol and in chloroform. The pH of a 1% solution is between 4.6
and Si. Carbinoxamine is a potent antihistaminic and is available as the racemic mixture. Carbinoxamine differs structurally from chlorpheniramine only in having an oxygen atom separate the asymmetric carbon atom from the aminoethyl side chain. The more active levo isomer of carbinoxamine has the (5) absolute configuration26 and can be
on the more active dextro isomer (S configuration-2) of chiorpheniramine.
Diphenylpyrailne Hydrochloride, USP. aline hydrochloride. 4-(diphenylmethoxy)- I -melhylpipen. dine hydrochloride (Hispril. Diafen) occurs as a white nr slightly off-white crystalline powder that is soluble in water or alcohol. Diphenylpyraline is structurally related to di. phenhydramine with the aminoalkyl side chain incorporated
in a piperidine ring. It is a potent antihistaminic, and the usual dose is 2 mg 3 or 4 times daily. The hydrochloride ii available as 5-mg sustained-release capsules. Usual adult dose: Oral. 5 mg/t2 hours Dosage forms: Extended-release capsules
Usual adult dose: Oral. 4—8 mg Lid, or q.i.d. Dosagc forms: Elixir and tablets
.HO
CHCOOH S
CHCOOH
Diphenylpyraline Hydrochloride
ETHYLENEDIAMINES Ceibinoxamine MaCearle
Clemastlne Fumarate, USP.
Dextrorotatory clemas-
tine. 2-12-El -(4-chlorophenyl)- 1-phenylethoxylethyll- I methylpyrrolidine hydrogen fumarate (1:1) (Tavist). has two chiral centers, each of which is of the (R) absolute configuration. A comparison of the activities of the antipodes indicates that the asymmetric center close to the side chain nitrogen
is of lesser importance to antihistaminic This member of the ethanolamine series is characterized by a long duration of action, with an activity that reaches a maximum in 5 to 7 hours and persists for 10 to 12 hours. It is well absorbed when administered orally, and it is excreted primarily in the urine. The side effects are those usually encountered with this series of antihistamines. Clemastine is closely related tochlorphenoxamine. which is used for its central cholinergic-blocking activity. Therefore, it is not surprising that clemastine has significant anumuscarinic activity. Usual adult dose: Oral, 1.34 mg b.i.d. or 2.68 mg q.d. to t.Ld. Dosage forms: Syrup and tablets
HOOC
H
The ethylenediamine antihistamines are characterized by the presence of a nitrogen connecting atom (X) and a two-carbnr atom chain as the linking moiety between the key diaryl arid
tertiary amino moieties (Fig. 21-7). All compounds in this series are simple diarylelhylcnediamincs except antaiolinc. in which the terminal amine and a portion of the carbon chain are included as part of an imidazoline ring sysleni Because it differs significantly in its pharmacological psitile, antazoline is not always classified as an ethylenobamine derivative. Phenbenzamine was the first clinically useful mcmberc( this class and served as the prototype for the dcvelopmen of more effective derivatives. Replacement of the moiety of phenbenzamine with a 2-pyridyl system yielded tripelennamine. a significantly more effective histamine Inceptor Substitution of apara methoxy (pyrilaniua or mepyramine), chioro (chioropyramine). or bromo (bras tripelennaminc) further enhances activity.23 Replacemernil the benzyl group of tripelennamine with a group provided methapyrilene. and replacement of tripeleri namine's 2-pyridyl group with a pyrimidinyl moiety with p-methoxy substitution) yielded thonzylamine. both which function as potent H1-receptor In all of these compounds the aliphatic or terminal ambo group is significantly more basic than the nitrogen aeon bonded to the diaryl moiety; the nonbonded electrons onils diaryl nitrogen are delocalized by the aromatic ring. and In
N
HXCOOH
ArN
— CH2 — CH2 — N Ar Clemastine Fumarate
Figure 21—7 • Genera? structure of the ethylenediarnines
Chapter 21 U Hisiamine and .Antihi.standnic itgeiiza
resultant reduction in electron density on nitrogen decreases
hasicity. Thus the aliphatic amino group in the ethylcnediamines is sufficiently basic for the formation of pharmaceutically useful salts. The ethylenediamines were among the first useful antihis-
They are highly effective H5 antagonists, but they CNS depressant also display a relatively high frequency of and Cii side effects.'° The anticholinergic and antiemetic actions of these compounds are relatively low, compared amines.23
with those of most other cla.ssical anlihistamines. The piperazinc- and phenothiazine-type anlihistamines also contain the cthylenediamine moiety. hut these agents are discussed separarely because they exhibit significantly different pharmacological properties.
Relatively little information is available concerning the pharmacokinetics of this series of compounds. Tripelennamine is metabolized in humans by N-glucuronidation. Nosidation. and pyridyl oxidation followed by phenol glueuronidation. It is anticipated that other members of this series are similarly
The structures of the salt forms of the marketed ethyleneJiamine antihistamines. along with physicochemical properales, basic therapeutic activity profiles, and dosage form in(ormation. are provided in the monographs below.
The oily free base of Tnpelennamine Citrate. USP. iripelennaminc citrate, 2-Iberntyll2-(dimethylamino)ethyliiminojpyridine citrate (1:1). PBZ (Pyribenzamine Citrate). is available as the less bitter inonocitrate salt, which is a white crystalline powder freely soluble in water and in alcoal. A solution has a pH of 4.25. For oral administration a liquid dose forms, the citrate nab is less bitter and thus more palatable than the hydrochloride. Because of the differcuce in molecular weights, the doses of the two salts must le equated—30 nag of the citrate salt is equivalent to 20 mg the hydrochloride salt.
705
however, and may impair the ability to perform tasks requiring alertness. The concurrent use of alcoholic beverages should he avoided. Usual adult dose: Oral tablets. 25—50 mg/4—6 hours: extendedrelease. 100 sng/8—12 hours Dosage forms: Tablets, extended-release tablets
Pyrilamine Maleate. USP.
The oily free base of pyril-
amine is available as the acid maleate salt. pyrilamine maleate. 2-I 12-(dinaethylamino)ethylhp-naelhoxyhcnzyl )aminoj-
pyridinc maleate (1:1). mcpyramine. which
is
a white
crystalline powder with a faint odor and a bitter, saline taste.
The salt is soluble in water (1:0.4) and freely soluble in alcohol. A 10% solution has a pH of approximately 5. Al a pH of 7.5 or above, the oily free base begins to precipitate.
Pyrilamine differs structurally from tripelennamine by having a methoxy group in the para position of the benzyl radical. It differs from its more toxic and less potent precursor phenbenzamine (Antergan) by having a 2-pyridyl group on the nitrogen atom in place of a phenyl group. Clinically. pyrilamine and tripelennamine are considered among the less potent antihistaminics. They are highly potent, however, in antagonizing histamine-induced contraclions of guinea pig ileum.'4 Because of the pronounced local anesthetic action, the drug should not be chewed, but taken
with tbod. Usual adult dose: Oral. 25—50 mg/ti—It hours Dosage forms: Tablets CHCOOH S
CHCOOI-4
Usual adult dose: Oral. 25—50 mgI4—6 hours Dosage forms: Elixir
Pyrliamlne Maleate
Methapyrilene Hydrochloride.
The oily free base is
available as the hitter-tasting monohydrochloridc salt. methapyrilcnc hydrochloride. 2-112-dimethylamino)cthyll-2Trlpetennamine Citrate or HO
TripelcnnaHydrochloride, USP. irite hydrochloride is a white crystalline powder that darktic slowly on exposure to light. The salt is soluble in water 1.0.77) and in alcohol (1:6). It has a pK, of about 9. and a solution has a pH of about 55. Tripelennamine. the first ethylenediamine developed in -macrican laboratories, is well absorbed when given orally. Tdpelennamine
monohydrochioride (Histadyl). It is a white crystalline powder that is soluble in water (1:0.5). in alcohol (1:5). and in chloroform (1:3). Its solutions have a pH of about 5.5. It differs structurally front tripelennamine
On the basis of clinical experience. ii appears to be u.s effec-
in having a 2-thenyl (thiophenc-2-methylene) group in place of the benzyl group. The thiophene ring is considered isosteric with the benzene ring, and the isosteres exhibit similar activity. A study of the solid-state conformation of naethapyrilene hydrochloride shosved that the trans conformation is preferred for the two ethylenediamine nitrogen atoms. The Food and Drug Adnainistration declared methapyrilene as po-
as diphenhydramine and may have the advantage of
tential carcinogen in 1979. and all products containing it
1
and less severe side reactions. Drowsiness may occur.
have been recalled.
706
Wi/wi, and
7exthook
of Organic Medicinal and Pharnnaceuiical HCI
Methapyrliene Hydrochloride
Thonzylamlne Hydrochloride. Thonzylamine hydrochloride, 2-I[2-(dinlethylamino)ethyl l(p-methoxybenzyl) uminojpyriinidine hydrochloride, is a white crystalline powder soluble in water (1:1). in alcohol (1:6). and in chloroform (1:4). A 2% aqueous solution has a pH of 5.5. It is similar in activity to tripelennaminc but is claimed to be less toxic. The usual dose is 50 mg up to 4 times daily. It is available in cerlain combination products. • HCI
PIPERAZINES (CYCLIZINES)
The piperazines or cyclizines can also be considered ethyl. enediamine derivatives or cyclic ethylenediamines (cych. zines); in this series, however, the connecting moiety (Xi is a CHN group, and the carbon chain, terminal amine function. ality, and the nitrogen atom of the connecting group are all part of a piperazine moiety (Fig. 21-8). Both nitrogen in these compounds are aliphatic and thus display connpana. ble basicities. The primary structural differences within this series involve the nature of the para aromatic ring substilueto (H or Cl) and, more importantly, the nature of the tenoinal piperazine nitrogen substituent. The piperazines are moderately potent antihistaminics
with a low incidence of drowsiness.'8
21 A warning el
the possibility of some dulling of mental alertness is advised, however. The activity of the piperazine-type antihisiarninio is characterized by a slow onset and long duration of action These agents exhibit peripheral and central antimuscanine
activity, which may be responsible for the antiemetic and antivenigo effects.'8 The agents diminish vestihularstim. ulation and may act on the medullary chcmoreceptor zone. Thus as a group, these agents are probably more useful as anhiemetics and antinauseants and in the treatment of me.
lion sickness. Some members of this series have exhibited a strong tm togenic potential, inducing a number of malformations in rats. Norchiorcyclizine, a roctabolite of these piperazines. was proposed as responsible for the teratogenic effects of the parent drugs.28 Thonzytamine Hydrochloride
Antazoline Phosphate.
Antazoline phosphate. 2-I(Nbenzylanilino)methylj-2-imidazoline dihydrogen phosphate. occurs as a bitter, white to off-white crystalline powder that is soluble in water. It has a of 10.0. and a 2% solution has a pH of about 4.5. Antaioline, like the ethylenediamines. contains an N-benzylanilino group linked to a basic nitrogen through a two-carbon chain. Antazoline is less active than most of the other antihistaminic drugs. hut it is characterized by the lack of local irritation. The more soluble phosphate salt is applied topically to the eye in a 0.5% solution. The less soluble hydrochloride
is given orally. In addition to its use as an antihistamine. antazolinc has more than twice27 the local anesthetic potency
Metabolic studies in this series of compounds focused primarily on cyclizine and chlorcyclizine. and thre compounds undergo similar biotransformation. The primal) pathways involve N-oxidation and N-demethylanion, and both of these metabolites are devoid of antihistaminic ac tivity.23 The structures of the marketed salt forms of the pipciannc antihistamines, along with physicochemical properties. bass
therapeutic activity profiles, and dosage form in the monographs below.
Cydizine Hydrochloride, USP. Cyclizine hydroclib ride, l-(diphenylmethyl)-4-methylpiperazine monobydo' chloride (Marezine), occurs as a light-sensitive, crystalline powder with a bitter ta.ste. It is slightly solublc
of procaine and also exhibits anticholinergic actions.
Q
• H3P04
—N
N —R
x Antazoltne Phosphate
Figure 21—8 • General structure of the piperazines
Chapter 21 • Histamine and AntihLcraminic Agents
707
Ar\
a water (1:115). in alcohol (1:115), and in chloroform (I: 75). It is used primarily in the prophylaxis and treatment of motion sickness. The lactate salt (Cyclizine Lactate Injection, liSP) is used for intramuscular injection because of the
/
limited water solubitity of the hydrochloride. The injection
CH — CH2 — CH2 — N
FIgure 21—9 • General structure of the propylamines.
should be stored in a cold place because if it is stored at mom temperature for several months, a slight yellow tint may develop. This does not indicate a loss in biologic po-
Mecilzine Hydrochloride, USP.
tency.
Usual adult dose: Oral. 50 mg/4—6 hours; IM. 50 mgJ4—6 hours Dosage forms: Tablets (l-tCl) and injection (lactate)
\/
CH—N
N—CH3
•
HCI
I
Meclizine hydrochloride. I erazine dihydrochioride monohydrate (Bonine. Antivert). is a tasteless, white or slightly yellowish crystalline powder that is practically insoluble in water (1:1.000). It differs from
chlorcyclizine in having an N',n-methylbenzyl group in place of the N-methyl group. Although it is a moderately potent antihistaminic. meclizine is used primarily as an antinauseant in the prevention and treatment of motion sickness and in the treatment of nausea and vomiting associated with vertigo and radiation sickness.
C
Usual adult dose: Oral, 25—5(1 mg Dosage forms: Tablets and chewable tablets
Cyclizine Hydrochloride or Lactate
Chiorcyclizine Hydrochloride, USP. Chiorcyclizine hydrochloride, I -(p-chloro-a-phenylbenzyl)-4-metbylpiperaline monohydrochioride. a light-sensitive, white crystalline powder, is soluble in water (1:2), in alcohol (1:11). dnd in chloroform (1:4). A 1% solution has a pH between 4.8 and 5.5,
Disubstitution or substitution of halogen in the 2 or 3 of either of the benzhydryl rings results in a much less potent compound. Chlorcyclizine is indicated in the symptomatic relief of urticana, hay fever, and certain other
Buclizine Hydrochloride, USP. Buclizine hydrochloride. l-(p-rertbutylbenzyl)-4-(p-chloro-a-phenylbenzyl)piperazine dihydrochloride (Bucladin-S). occurs as a white to slightly yellow crystalline powder that is insoluble in water. The highly lipid-soluble buclizine has CNS depressant, antiemetic, and antihistaminic properties. The salt is available in 50-mg tablets for oral administration. The usual dose is 50 mg 30 minutes before travel and is repeated in 4 to 6 hours as needed. Usual adult dose: Oral. 50 rngJ4—6 hours Dosage forms: Tablets
allergic conditions.
C1> terfenadinc) as well.404' The advantages of this compound appear to he once-daily dosing, rapid onset of activity, minimal CNS effects, and a lack of clinically significant effects on cardiac rhythm when administered with imidazole antil'ungals and macrolide anti-
biotics. The onset of action is within 20 to 60 minutes in mosi patients. Cetirizine produce.s qualitatively different ci. fects on psychomotor and psychophysical functions from the first-generation antihistumines. The most common adverse reaction associated with cetirizine is dose-related somnolence, however, so patients should be advised that cetirizine may interiere with the performance of certain psychomotor and psychophysical activities Other effects of this drug in-
clude fatigue, dry mouth. pharyngilis. and dizziness. Be-
administered with imidazolc antifungals and macrolide antibiotics. Other typical drug interactions of H, antihisiamines, however, apply to cetirizinc. Concurrent use of this drug with alcohol and other CNS depressants should Lv avoided.4' Dose-proportional values are achieved within I hour of oral administration of celirizine. Food slows the rate of cetirizine absorption but does not affect the overall extent, Consistent with the polar nature of this carboxylic acid drug. less than 10% of peak plasma levels have been measuted in
the brain. Cetirizinc is not extensively metabolized, and more than 70% of a 10-mg oral dose is excreted in the urinc
Usual adult dose: Oral. 5—It) mg q.d. Dosage form: Tablets
Acrivastine, liSP.
form and alcohol and slightly soluble in water. Acrivastine is an analogue of triprolidine containing a car-
boxycthenyl moiety at the 6 position of the
ring.
Acrivastine shows antihistaminic potency and duration of action comparable to those of triprolidine. Unlike ttiprnlidine. acrivastine does not display significant anticholinergh activity at therapeutic concentrations. Also. the enlunccd
polarity of this compound resulting from substitution limits BBB penetration, and thus, this compound
produces less sedation than triprolidine.35 '" Limited pharmacokinetic data are available for thin compound. Orally administered drug has a hall-life of about hours and a total body clearance of 4.4 mUmin per kg. Tin
mean peak plasma concentrations are reported
NN Cetur,zine
Acrivastine, USP, (E.E)-3-16-I 1-14-
methylphenyl )-3-( I -pyrrolidinyl)- I -propenyl-2-pyridinyll2-propenoic acid (Semprex). is a fixed-combination of the antihistamine acrivastine (8mg) with the dccongcsrato pseudoephedrine (60 mg). Acrivastine is an odorless, ahile to pale-cream crystalline powder that is soluble in chiom'
AOH
0 Sat)
Chapter 21 • Histamine and A,uihistaminic Agents
widely, and the drug appears to penetrate the CNS poorly. The metabolic fate of acrivastine has not been reported.
715
also inhibit the chcmotaxis of eosinophils at the site of application (i.e.. ocular tissue). In lung tissue. pretreatment with the mast cell stabilizers cromolyn and nedocromil blocks the immediate and delayed bronchoconstrictive reactions induced by the inhalation of antigens. These drugs also attenuate the bronchospasm associated with exercise, cold air, environmental pollutants, and certain drugs (aspirin). The mast cell stabilizers do not have intrinsic bronchodilator, antihistamine. anticholinergic. vasoconstrictor. or glucocorticoid
H3C
activity and, when delivered by inhalation at the recommended dose, have no known therapeutic systemic activity. The structures, chemical properties. pharmacological protiles, and dosage data for these agents are provided in the monographs below.
COOH
Acrëvastine
0
adult dose: Oral. 8 or 60 mg t.i.d. to q.i.d. Dosage fonti: Tablets
KhelIin
cmmolyn Sodium,
USP. Crotnolyn sodium. disodium .3-his (2 -earboxychromon -5- yloxy ) -2-hydroxypropane (tntal). is a hygroscopic. white, hydrated crystalline powder that is soluble in waler (1:10). Ii is tasteless at first but leaves a very slightly bitter aftertaste. The of cromolyn is 2.0. It is available as a solution for a nebulizer, an aerosol spray. a nasal solution, an ophthalmic solution, and an oral concen-
INHIBITION OF HISTAMINE RELEASE: MAST CELL STABILIZERS
The discovery of the bronchodilating activity of the natural pnsiuct khetlin led to the development of the his(chromoncs) that stabilize mast cells and inhibit the release 01' histamine and other mediators of inflammation. The first therapeutically significant member of this class was cromoFurther research targeting more effective yn sodium.30 agents resulted in the introduction of nedocromil, followed asic recently by pemirolast and lodoxamide. Generally, the cell stahili,.ers inhibit activation of, and mediator refrom, a variety of inflammatory cell types associated with allergy and asthma, including cosinophils, neutrophils. mast cells, monocytes. and platelets. In addilion to histamine, these drugs inhibit the release ol' leuko(C4. D4. E4) and prostaglandins. In vitro studies sugthat these drugs indirectly inhibit calcium ion entry into mast cell and that this action prevents mediator release. In addition to their mediator release, some of these drugs
trate.
Nebulized and aerosol cromolyn is used for prophylactic management of bronchial asthma and prevention of exerciseinduced bronchospasm. Cromolyn nasal solution is used lbr
the prevention and treatment ol allergic rhinitis. and oral concentrate is used to treat the histaminic symptoms of mastocylosis (diarrhea, flushing, headaches, vomiting, urticana,
abdominal pain. nausea. and itching). In the treatment of asthtna. crotnolyn efficacy is manifested by decreased sever-
ity of clinical symptoms. or need for concomitant therapy, or both. Long-term use is justified if the drug significantly reduces the severity of asthma symptoms; permits a significant reduction in, or elimination of. steroid dosage; or inn-
COOeNB.
Na' OOC.
OCH2CHCH2O OH
Cromolyn Sodium
0
716
Wi/so,, and
Organic Medicinal and Pl,armuceu,ieal Chemistry
Textbook
The antiasthmittic effects of nedocromil may also invohc inhibition of axon reflexes. Axon reflexes may he producal by bradykinin in the presence of damage to the airway epi. thelium, resulting in release of sensory neuropeptides (sub. stance P. neurokinin A). which can produce hmnehrean•
proves management of those who have intolerable side effects to sympathomimetic agents or methylxanlhincs. For cromolyn to be effective, it must be adminislered at least 30 minutes prior to antigen challenge and administered at regular intervals (see dosing information below). Ovcruse of cromolyn results in tolerance.
striction and edema. Ncdocromil is more effective than crornolyn in reversing bradykinin-induced and neurokinin A—induced bronchoconstriction in humans.
Usual adult dose: Nebulizer solution. 20 mg inhaled q.i.d. Aerosol. 2 metered sprays inhaled q.i.d. Intranasal. 5.2 mg (one metered spray) in cach nostril lid, or q.i.d. at regular intervals Ophthalmic. I drop of a 2—4% solution q.i.d. toh times daily Oral. 2 ampules q.i.d. 30 minutes before meals and at bedtime
Usual adult dose: lntranasal. 14 mg 11W,' inhalutions) q.i.d at regular intervals
Lodoxamide Tromethamine.
signiticarn
and nedocromil is the presence of two acidic groups. Lodosamide Iromethamine. N.N'-( dioxamic acid (Alotuide). is a white crystalline. water-solu-
Nedocromil Sodium,
USP. Nedocromil sodium. disodium 9-ethyl-6.9-dihydro-4.6-dioxo- I 0.propyl.4H-pyrano I 3.2-glquinoline-2.8-dicarboxylate (Tilade). is available as an aerosol in a metered-dose inhaler.
ble powder. It is available as a 0.1% solution, with each milliliter containing 1.78 tug of lodoxamide tromethamine equivalent to I rng of lodoxamide. The solution contains the preservative bettzalkonium chloride (0.007w) as
Nedocromil is structurally related to cromolyn and displays similar, hut broader, pharmacological actions. Nedocromil is indicated for maintenance therapy in the management of patients with mild-to-moderate bronchial asthma. It was developed in a search for a compound with a better biological profile than cromolyn. which has limitations in the treatment of certain patients. such as the elderly asthmatic patient and patients with intrinsic asthma. Cromolyn is more effective in stabilizing connective tissue mast cells
as mannitol. hydroxypropyl methylcellulose. sodium citrate,
citric acid. edetate disodium. tyloxapol. hydrochloric acid and/or sodium hydroxide (to adjust pW. and puritied waler. Lodoxamide is indicated in the treatment of the (vcuku disorders including vemal keratoctitijunctivitis. vernal conjunctivitis, and vernal kcratitis.46 The dose for adults and children older than 2 years of age is I to 2 drops in cach affected eye 4 times daily lbr up to 3 months. The mist frequently reported ocular adverse experiences were tran sient burning, stinging. or discomfort on instillation.
than mucosal mast cells, and since release of mediators from mast cells in the lung is an important component of inflam-
mation and bronchial hyperreactivity in asthmatic patients. an agent with greater effects on mucosal mast cells was desirable. Available data suggest that nedocromil. although having profile of activity like that of cromolyn. is more effective in stabilizing mucosul mast cells.45 CH3CH2
The only
structurally similarity between lodoxamide and cmmulyn
Pemirolast Potassium Ophthalmic Solution.
Pemir.
ola.st can be considered an analogue of one portion of the crotuolyn structure in which the carboxyl group has been replaced with an isosteric tetrazole nioiety. Pemirola.st potas-
çH2cH2CH3
sium. 9.methyl-3-( IH-tetrazol-5-yl)-4H-pyridol I .2-al'pynmidin-4-one potassium (Alamast). is a yellow. water-solubic powder. The commercial preparation is available as a sterile ophthalmic solution for topical administration to tint eyes. Each milliliter of this solution Contains 1.0mg of pon irolast potassium. as well as the preservative lauralkonium chloride (0.005'7e). and glycerin, phosphate buffers, and sodium hydroxide to maintain a solution pH of 8.0. The ails. Lion has an osmolality of approximately 240 mOsnt/L, Tint recommended dose is one to two drops instilled into each affected eye 4 times a day. This drug product is for ocular administration only and not for injection or oral use. Pemiro'
COONa
N
0 Nedocromil Disodium
0
H
Ct
0
H
CH2OH
. HOCH2—C—NH2 0
CN
Lodoxamide Tromethamine
CH2OH
Chapter 21 •
717
and .4ntihi.ciwnjnic
last solution should be used with caution during pregnancy srwhile nursing, since its safety has not been studied under these circumstances.47
CH3
CI.
CH3
Pemirolast Potassium
Azelastine
The recommended dose of azelaslinc solution is rne drop instilled into each affected eye twice a day. This drug product is for ocular administration only and not for injection or oral use. Absorption of ae'.elastinc following ocular admninistra-
RECENT ANTIHISTAMINE DEVELOPMENTS:
non is relatively low (less than I ng/mL). Absorbed drug undergoes extensive oxidative N-demethylation by cylo-
ThE "DUAL-ACrING" ANTIHISTAMINES
chrome P-450. and the parent drug and mctabolite are elimi-
Over the past decade there has been considerable interest in
adverse reactions are transient eye burning or stinging, head-
he development of novel antihistaminic compounds with dual mechanisms of action including Hi-receptor antagonivnl and mast cell stabilization. Currently available drugs
aches, and bitter taste. Azelastine solution should be used with caution during pregnancy or while nursing, since its
nated primarily in the feces. The most frequently reported
safety has not been studied under these circumstances.45
that exhibit such dual antihistaminic actions include azelas-
line and ketotifen. These compounds contain the ba.sic phannacophore to produce relatively selective histamine H1 antagonism (diarylalkylamines) as well as inhibition
of histamine and other mediators (e.g.. and PAF) from mast cells involved in the
sI the
allergic response. In vitro studies suggest that these cantalso decrease chernotaxis and activation of casinothik. Azelastine and ketotifen currently are indicated for he treatment of itching of the eye associated with allergic
Their antiallergy actions occur within mmafter administration and may persist for up to 8 The structures, chemical properties. pharmacological preliles. and dosage data for these agents are provided in
Ketotifen Fumarate Ophthalmic Solution.
Ketotifen 4-( I-methyl -4-piperidylidene )-4H-beu,o(4,5 cycloheptal I .2-bithiophen- I 0(9H)-one hydrogen fumarate (Zaditor). is a fine crystalline powder. Ketotifeti is a ketofumarate.
I
thiophene isostere analogue of the dibenzocycloheptane an-
tihistamines. The solution contains 0.345 rng of ketotifen lumarate, which is equivalent to 0.25 mg of ketotifen. The solution also contains the preservative benzalkanium chloride (0.Ol'k) as well as glycerol, sodium hydroxide and/or hydrochloric acid (to adjust pH). and purified water. It has a pH of 4.4 to 5.8 and an osmolality of 210 to 30() mOsm/ kg.
the monographs below.
Azelas?Jne
Hydrochloride
Ophthalmic
Solution.
hydrochloride, (± )- I -(21I)-phthalazinone. 4-1(4lhlomphenyl)rnethyl I-2-(hexahydro-l -methyl-I H-azepin-4monohydrochloride (Optivar). is a shite crystalline powder that is sparingly soluble in water.
and propylene glycol and slightly soluble in cihanol. octanol. and glycerine. The commercial preparation available as a 0.05% sterile ophthalmic solution for topical aiministration to the eyes. Each milliliter of azelastine soluton contains 0.5 mg az.elastine hydrochloride equivalent to 1457 tag of azelastine base, the preservative benzalkonium Atoride iO.125 mg). and inactive ingredients including diso-
dihydrate, hydroxypropylmethylcellulose. sorhat solution, sodium hydroxide, and water for injection. The solution has a pH of approximately 5.0 to 6.5 and an nsrnolality of approximately 271 to 312 niOsmlL.
Ketotiten Fumarate
The recommended dose of ketotifen solution is one drop
instilled into each affected eye every 8 to 12 hours. The most frequently reported adverse reactions are conjunctival
718
Wilson
and Gisiold's Testbook of Organic Medicinal and Pharmaceutical Chemistry
injection, headaches, and rhinitis. This drug product is for ocular administration only and not for injection or oral use. Ketotifen solution should be used with caution during pregnancy or while nursing, since its safety has not been studied
acid and proteolytic pepsin enzymes, whose formation Ls facilitated by the low gastric pH. is generally assumed to be required for the hydrolysis of proteins and other foods.
under these circumstances.49
(oxyntic) cell. Parietal cells contain a hydrogen ion pump, a unique H10 + 1K -ATPase system that secretes H;O' in exchange for the uptake of K ion. Secretion of acid by gastric parietal (oxyntic) cells is regulated by the actions of various mediators at receptors located on the basolatenti membrane, including histamine agonism of H2 receptors (cellular), gastrin activity at G receptors (blood), and acetyl. choline (ACh) at M2 muscarinic receptors (neuronal) (Fif.
HISTAMINE H2 ANTAGONISTS Drugs whose pharmacological action primarily involves antagonism of the action of histamine at its H2 receptors find therapeutic application in the treatment of acid-peptic disorders ranging from heartburn to peptic ulcer disease, Zollinger-Ellison syndrome, gastroesophageal reflux disease (GERD), acute stress ulcers, and erosions.50'
The acid secretory Unit of the gastric mucosa is the
21.13).
Peptic Uker DIseaseU Peptide ulcer disease (PUD) is a group of upper Cl
Peptic Add Secretion A characteristic feature of the mammalian stomach is its ability to secrete acid as part of its involvement in digesting food for absorption later in the intestine. The presence of
tract
disorders that result from the erosive action of acid and pepsin. Duodenal ulcer (DU) and gastric ulcer (GIJ) are the most common forms, although PUD may occur in the esophaf us or small intestine. Factors involved in the pathogenesis and
Parietal Cell
K.
'Ci
H30'
). Histamine
Gastrin
I.
ACh
Endoc,lne Cell
Figure 21—13 • Hormonal regulation of acid secretion by parietal cells.
Chapter 21 • Histamine and Anzihistan,inic Age,,ls
recurrence of PUD include hypersecretion of acid and pepsin and GI infection by Helicobacter pylon, a Grain-negative spiral bacterium. H. priori has been found in virtually all patients with DtJ and approximately 75% of patients with Cli. Some risk factors associated with recurrence of PUD include cigarette smoking, chronic use of ulcerogenic drugs
nonsteroidal anti-inflammatory drugs INSAIDsI). male gender, age, alcohol consumption. emotional stress, and family history. The goals of PUD therapy are to promote healing, relieve pain, and prevent ulcer complications and recurrences. Mcdonions used to heal or reduce ulcer recurrence include antacids. histamine H2-receptor antagonists, protective mucosal (e.g.,
barriers, proton muth
pump inhibitors. prostaglandins. and bis-
salt and antibiotic combinations.
Sfructural Derivadon A review of the characterization and development of hista-
mine H2-receptor antagonists reveals a classic medicinal chemistry approach to problem solving.53 Structural evoluantagonist. tion of the first discovered, clinically useful cimetidine, is depicted in Figure 2l-l4. Methylation of the
5 position of the imidazole heterocycle of histamine produces a selective agonist at atrial histamine receptors (H2). The guanidino analogue of histamine possesses weak antagonist activity to the acid-secretory actions of histamine. Increasing the length of the side chain from two to four carbons, coupled with replacement of the strongly basic guanidino group by the neutrat methyl thiourea function. leads to burimamide. the first antagonist to be developed tacking detectable agonist activity in laboratory assays. The
STRUCTURE—ACTIVITY RELATIONSHIP
Histamine: H,
719
STRUCTURE
= 112
agonism
5-Methylhistamine: H2> H, agorrism
NH—C—NH2 Partial H2receptor agonist (weak antagonist)
CH2CH2— NH—C — NHCH3
Bunmamide: Full 112 antagonIst—
low potency, poor oral bioavailabllity
,pH2CH2— NH—C — NHCH3 S
Mellamide: Full H2 antagonist— higher potency, improved oral bloavailabllity, toxic (thiourea)
,PH2CH2— NH —C —NHCH3 Cimeddine: Full 112 antagonIst— higher potency, high oral low toxicity
Figure 21—14 • Structural derivation of histamine H2 antagonists.
NCN
720
tVllxon
GiX%okI'.s
Textbook of Orgwii Medici,ial wid Phurwaceulical Chemi.ctrs
low potency of burimamide is poswltued to be related to its nonhasic, electron-releasing side chain, which favors the
nonphannacophonc N't-H imidazole tautomer over the basic, electron-withdrawing side chain in histamine, which predominantly presents the higher-affinity N'-H imidazole (automer to the receptor. Insertion of an electronegative thioether function in the side chain in place of' a methylene
group favors the W tautomer. and imroduction of (he 5methyl group favors H2-rcceptor selectivity and leads to metiamide, a H2 blocker of higher potency and oral bioavailability than burimamide. Toxicity associated with the thiourea structural feature is eliminated by replacing the thiourea sulfur with a cyano-imino t'unction to produce cimetidine. Introduction of cimeudine into human medicine revealed an effective gastric antisecretory agent that promotes the healing olduodenal ulcers. Cimetidine is not without a num-
ber of limitations, however. Because it is short acting, it
aining functionality should be a polar. nonbasic substituent for maximal antagonist activity. Groups that are positiv civ charged at physiological pH appear to confer agonist activity. In general. antagonist activity varies inversely with the hydrophilic character of the nitrogen group. The hydrophilic group, I .1 -diaminonitroethene. found in ranitidine and niiatidine is an exception, however; it is much more active than is predicted by relative solubility
Cimetidine, USP. Cimetidine. 12.! 15-methylimidazol-4-ylmeihyl J-thio lethyl Iguanidinc (Tagamet), is a colorless crystalline solid that is slightly sole
ble in water (1.14% at 37°C). The solubility is greatly in creased by the addition of dilute acid to protonatc the imidaz-
of 6.8). At pH 7. aqueous
ole ring (apparent
are stable for at least 7 days. Cimelidine is a relatively hydm-
philic molecule with an octanol/water partition coefficient of 2.5.
requires a frequent-dosing schedule in humans, and in addi-
H3C\
tion, its selectivity is poor. Cimetidine has antiandrogenic activity, which can lead to gynecomastia. and it inhibits the cytochrome P450 mixed-function oxygenase-metabolizing enzyme system in the liver, an action that potentiales the
N
effects of drugs whose clearance also depends on biotrans-
/CH2SCH2CH2NH
C
N
formation by this system. Cimetidinc also causes confusinnal states in some elderly patients. Subsequent development of additional drugs 01' this class indicates that a great
deal of structural latitude is available in the design of H2 antagonists (Table 21.1
Examination of the structural features of H2 antagonists that came after cimetidine confirms that the imidazole ring of histamine is not required for competitive antagonism of histamine at H2 receptors. Other heterocycles may be used and may. in fact, enhance both potency and selectivity of H2-receptor antagonism. If the imidazole ring is used, how-H tautomer should be the predominant species ever. the for maximal H2-anlagonist activity. The electronic effects of the ring substituents and side-chain structural features determine the tautomerism. Separation of the ring and the nitrogen group with the equivalent of a four-carbon chain appears to be necessary for optimal antagonist activity. The isosteric thioether link is present in the fiur agents currently marketed in the United States. The terminal nirogen—con-
TABLE 21-1
Cimetidine
Cimetidine reduces the hepatic metabolism of drugs hiotransformed by the cytochrome P-450 mixed-oxidase tem, delaying elimination and increasing serum levels of these drugs. Concomitant therapy of patients with cimetidinc and drugs metabolized by hepatic microsomal enzymes. par.
ticularly those of low therapeutic ratio or in patients with renal or hepatic impairment. may require dosage adjustment Table 21.2 provides a compilation of drugs whose conihixi (ion therapy with cimetidine may increase their pharniaeological effects or toxicity. Antacids interfere with cimelidine absorption and should be administered at least I hour befoar or alter a cimetidinc dose. Cimetidine has a weak antiandrogenic effect, tia may occur in patients treated for I month or more. Cimetidine exhibits high oral hioavailability (60 to 70%t and a plasma half-life of about 2 hours, which is increased
Currentl y Available H2 Antagonists Dose
Orat
RelatIve Potency
Bloavaltablilty
MetabolIsm
(%)
Enzyme
Ciniciktine
t
63—78
FMO3
Famotidinc
-II)
37—45
7
10
98
7
6
52
EMO3: p.45))
Renal Clearance (Uhoun)
(%)
Route of ElimInation
Sutfoelde. hydrosymcthyt
—25
React
S'Osldc
—30
Renal
t4—26
—37
Renal
27-3s
—31)
Renal,
24—31
MetabolIzed
Metabolites
2-1-14
N2-o*idc Ranittdinc
E,om baum. 51.
Melabolic i)rng
N.Oxide. N.desmelliyt sutfoxidc
inhibttoi,.. and uittOnwlic.. In 1.. Philadclphia. h.ippincolt Williams & Wilkin,.. 215Cr. chap. 36.
bitiury Rh., Thumind, K.E.Tr.ua. WE.. Hanr,lai.
Chupter 21 • Hic:an,ine and Aniihi.stanai,iit' Agenz.s
Cimetidlne Drug Interactions
TABLE 21—2 Ben,
Metrnrndaeole
Sulfunyturea
Monci,inc t'entoxitylline
Tacrine Theophyftine
Cartsuna,cpinc
Phenytnin
Triunitereume
('luloruquinc
Propafcnone
Labettulol
I'rtupmunolnl
Tneydic antidepressanus Vatpruic uid
ia,,eplncs
Caffeine Calcium
channel
Lisaiime
Wariarin Quinine
Mctuprmukul
t,,I, IIc4anl,mic Couuiprr.oims.
112-auul
nuisi,. hi ()tjnmn, Ii. K.
Usual adult oral dose: Duodenal ulcer Treatment dose. 800—1.2(X) rng q.d. to q.i.d. with meals and at bedtime: ntaintcnancc dose. 4(1) mg q.d.
Benign gastric ulcer: 800—1.200mg q.d. to q.i.d. Hypersecrecory condition: 1.200—2.400 mg q.i.uJ.
Heartburn: 200 mg (2 OTC tablets) up to twice daily Usual pediatric dose: Oral. 20—40 mg (baset per kilogram of body weight q.i.d. with meals and at bedtime Dosage lomis: Tablet (200. 300. 400. 800 nig. liquid (300 mg/S niLl. injection (3(8) mgI2 and 50 mL)
Famotidine, USP. Famotidine. N-(aminosulfonyl 1-3U121(diaminomethylcnc )-amino J-4-thiazolyl Jmethyljthiol (Pepcid). which uses a thia,.ole bioisostcre of the imidazole heterocycle. is a white to pale-yellow crysalline compound that is very slightly soluble in waler and insoluble in ethanol.
N
/
I. Drug Fact,.
umud
Si. Louis. MO. locus
955. pp. 3(34—3(0.
in renal and hepatic impairment and in the elderly. Approximately 30 to 40% of a cinietidine dose is nietabohzed (Soxidation. 5-CH3 hydroxylation). and the parent drug and metabolites are eliminated primarily by renal excretion.
NIl2
721
CH2SCH2CH2NH
ability). The drug is eliminated by renal (65 to 70%) and metabolic (30 to 35%) routes. Famolidine sulfoxide is the only metabolile identified in humans. The effects of food or antacid on the hioavailability of famotidine are not clinically significant. Usual adult oral dose: Duodetial ulcer Treatment dose. 40 mg q.d. to hid, at bedtime: maintenance dose. 20 mg q.d. at bedtime Benign gastric ulcer: 40 mg q.d. Ilypersecrctory condition: 80—640 mg q.i.d.
Heartburn: 10 mg (I OTC tablet) for relief or I hour before a meal br prevention Dosage forms: Tablet (21) and 40 mg). oral suspension (40 mg/S mL). injection (It) mg/mi)
Ranitidine,
USP. Ranitidine. N-12-[ I 5-Idimethylamino )tnethyl 1-2-furanyl I methyl I thio lethyl I-N'-methyl-2nitro- 1,1 -ethenediamine (Zantac), is an aminoalkyl furan derivative with pK. values of 2.7 (side chaiti) and 8.2 (dirnethylarnino). It is a white solid. The hydrochloride salt is highly soluble in water.
C — NH2
II
— C — NHCH3 CH3
NSO2NH2
CHNO2
H3C — N FamoUdine Ranulkiune
Famotidine is a contpetitive inhibitor of histamine Fl2 reand inhibits basal and nocturnal gastric secretion as sell as secretion stimulated by food and pentaga.strin. Its rtirTcnt labeling indications are for the short-term treatment siduodenal and benign gastric ulcers. GERD. pathological conditions (e.g., Zollinger-Ellison syndame), and heartburn COTC only).
cases of gynecoinastia, increased prolactin levels, or :mpotence have been reported. even at the higher dosage used in patients with pathological hypcrsccrctory con-
ditions. Studies with famotidine in humans, in animal rnwlels. and in vitro have shown no significant interference sith the disposition of compounds metabolized by the heniicrosomal enzymes e.g.. cytochrome P.450 system). Famotidi tie is incompletely absorbed (4t) to 45% hioavail
Bioavailability of an oral dose of ranitidine is aboul 50 to 60% and is not significantly affected by the presence of tood. Sonic antacids may reduce ranitidine absorption and should not be taken within I hour of administration of the Hrblocker. The plasma half-life of the drug is 2 to 3 hours. and it is excreted along with its metabolites in the urine. Three melabolites. ranitidine N-oxide. r.nnitidine S-oxide, and desmethyl ranitidine. have been identified. Ranitidine is oniy a weak inhibitor of the hepatic cytochrome P450 mixed-function oxidase system In addition to being available in a variety of dosage forms as the hydrochloride salt. ranitidine is also available as a bismuth citrate salt for use with the macrolide antibiotic clarithromycin in treating patients with an active duodenal ulcer
722
Medici,,ai and Phannaceistiru( Che,ni,,rs
Wi).,,1,, and Girroid's i't'xtboak of
associated with H. priori infection. Eradication of H. reduces the risk of duodenal ulcer recurrence.
priori
Usual adult oral dose: Duodettal ulcer: Treatment dose. 200—3,0(X) ing q.d. to bid.; maintenance dose. 150 mg q.d. Benign gastric ulcer: 300 mg q.d. Hypcrscctvtoty condition: 300—6.0(X) mg 2 or more titnes daily Dosage forms: Tablets (ISO and 3(X) mg of HC) salt). syrap (IS mg/mL as HCI salt), injection (0.5 and 25 mg/mL as HCI salt)
These compounds were subsequently convened to sulfoxidc derivatives, which exhibited highly potent, irreversible inhi. hition of the proton pump. The beuzimidazole PPls are pmdrugs that are rapidly convened to a sulfenamide intermediate in the highly acidic environment of gastric parietal cells. The weakly basic beni.imidazole PPIs accumulate in these acidic compartments on the luminal side of the tubuvesictilar and canalicular structures of the parietal cells. The hen,.imi dazole PPls are chemically converted by acid to a sullcnumide intermediate that inhibits the proton pump via covalent interaction with cysteine residues (813 or 822) of the pump
H '/K -ATI'ase (Fig. 21-I 5).°" The acid lability of the benz
Nizatidine.
Nizatidine, N-I 2-Ill 2-L(dimethylamino) methyl 1-4-thiazolyl Imethyl ] thio jethyl J-N'-methyl-2-nitro1.1-ethenediamine (Axid). is an off-white to buff crystalline solid that is soluble in water, alcohol, and chloroform. The of the drug in water are 2.1 (side chain) and 6.8 (dimethylamino). /CH2SCH2CH2NH
C
NHCH3
CHNO2
/—\
S
imidazole PPls dictates that these drugs must be formulated as delayed-release. enteric-coated granular dosage fonns. The PPIs are more effective in the short term than the H:. blockers in healing duodenal ulcers and erosive esophagilis and can heal esophagitis resistant to treatment with the blockers." In addition, the benzimidatole PPIs have antimi-
crobial activity against H. pylon and thus possess efficacy in treating gastric ulcers or with one or more antimicrobials. in eradicating infection by this organism. Four henaitnidazole PPIs are currently approved for marketing in the United States (Table 2 1-3). Adverse effect profiles of the varinus PPls are difficult to compare because comparative clinical
trials do not usually include sufficient individuals to altos reliable conclusions. Relatively early in its marketing. the use of omeprazole was associated with the occurrence of H3C N
NIzatkline
CH3
Nizatidine has excellent oral bioavailahility (>90%). The effects of antacids or food on its bioavailability are not clinically significant. The elimination half-life is I to 2 hours. It is excreted primarily in the urine (90%) and mostly u.s unchanged drug 60%). Metubolites include nizatidine sulfoxide (6%), N-desmethylnizatidine (7%). and nizatidine
oxide (dimethylaminomcthyl function). Nizatidine has no demonstrable antiandrogenic action or inhibitory effects on cytochrorne P-450-linked drug-metabolizing enzyme system. Usual adult oral dose: Duodenal ulcer: Treatment dose. 300 mg q.d. to bid.; tnainte,Iance dose. ISO mg q.d. Hypersecretory condition: ISO mg hid. Dosage forms: Capsules (ISO and 30(1 mgI
Other Antlulcer Therapies PROTON PUMP INHIBITORS
The final step in acid secretion in the parietal cell is the extrusion ("pumping") of protons. The membrane pump, an H '1K -ATPase. catalyzes the exchange of hydrogen
diarrhea, headache, and rashes: longer-term experience sag. gests. however, that these adverse responses are rare. Simi
larly, characterization of adverse reaction profiles of other PPIs must await more extensive use in patients. The PPls are eliminated almost entirely by rapid
lisnu to inactive or less active metabolites (Fig. Virtually no unchanged drug is excreted in the urine and feces. The cytochrome P-45() enzyme system is primarily
involved in PPI metabolism and can be the soan:e o( drug—drug interactions for the PPIs. Inhibition of oxidaihc metabolism by omeprazole (but not csomeprazole) is respon. sible prolonging the clearance of benzodiazepines. phc• nytoin, and warfarin. Lansoprazole decreases theophyllme concentration slightly and may decrease the efficacy of oaf contraceptives. Pantoprazole and rabcprazole appear to he
free of these interactions. Further, the profound and tong. lasting inhibition of gastric acid secretion by the PPls may interfere with the bioavailability of drugs when gastric pH is an important determinant, such as the azole antifungals (e.g.. ketoconazole). ampicillin. iron salts. digoxin. and cya. nocobalamin.
Omeprazole.
Omeprazole. 5-methoxy-2.((4-tnclhosy 3.5-dimethyl- 2-pyridinyl )methyl )sulfinyl )- IH-ben7imid.t zole (Losee). is a white to off-white crystalline powder wiih very slight solubility in water. Omeprazole is an amphoteric compound (pyridine N. pK, 4.13; bcnzimidazole N-H. 1.68). and consistent with the proposed mechanism of anise
ions for potassium ions. Inhibition of this proton pump acts
of the substituted bcnzimidazoles, it is acid labile. Hence
beyond the site of action of second me.ssengers (e.g.. Caa and cAMP) and is independent of the action of secretogogues histamine. gastrin, and acetylcholine. Thus, acid pump inhibitors block basal and stimulated secretion.
the omeprazole product is formulated as
In 1972. a group of Swedish tuedicinal chemists discovered that certain pyridylmethyl bcnzimidazole sullides were
active prototi pump H '1K -ATPase inhibitors (PPIs)."
capsules containing enteric-coated granules. The absoler bioavailability of orally administered otneprazole is 301 40% related to substantial first-pass biotransformation. 'liv drug has a plasma half-life of about I hour. Most of an oral dose of omeprazole is excreted in the urine as ntetaholitcs with insignificant antisecretory activity. The pi
Chapter 21 • llLuw,nine and A,ztihistwnink Agents
A5
723
A5
H (slow)
- H20 + H20
S
NH NH A4 -
— SN A5
S—
S
Figure 21—15 U Mechanism of action of PPIs
TABLE 21-3 Proton Pu mp inhibitors M arketed In the Un ited States indication ulcer OuDdemli ulcer/If.
Omeprazole
Lansoprazoie
/ I I I
/ I I I I I
I
Pantoprazole Sodium
Rabeprazole Sodium
I I
/ I I I
Esomeprazola
Magnesium
I I I
724
WiLson and Giss'old'.c Textbook of Organic Medicinal and Pharmareu,ical chemistry
(Ol when R4 = H
CYP2CI9 HO A2
N
N
H HOI-12C
CYP2C19
0
—S——— and —S——— 0 Sulfone
Sullide
Figure 21—16 • Metabohc transformations of benzimidazole PPIs.
mary metabolites of omeprazole are 5-hydroxymeprazole (cytochrome P-450 [CYPI isozyme 2C19) and omeprazole sulfone (CYP 3A4). The antisecretory actions of omeprazole persist for 24(072 hours, tong after the drug has disappeared from plasma, which is consistent with its suggested mecha-
nisin of action involving irreversible inhibition of the proton pump H + /K + -ATPase.59 OCH3
sium trihydrate (Nexium). is the S enantiomer of omepetzole. The benzimidazole PPIs contain a chiral sulfur atom that forms an enantiomeric pair that is stable and insolsbk under standard conditions. The S isomer of omeprazoic slightly greater PPI activity, and its intrinsic clearance is approximately 3 times lower than that of R omeprazole (15 versus 43 The lower clearance of is related to slower metabolic clearance by the CYP 2CH isozyme. Although R-omeprazole is primarily transfonnrd to the 5-hydroxy metabolite. the S isomer is metabolized by
0-demethylation and sulfoxidation. which contribute little to intrinsic clearance. Usual adult dose: Erosive esophagitis: Healing dose: 20 or 40 rng q.d. for 44 weeks; maintenance dose: 20 mg q.d. Treatment of GERD: 20 mg q.d. for 4 weeks; H. pylon eradication: 40 mg q.d. for 10 days in combinatics
with amoxicillin (I g bid. for 10 days) and (500 mg bid. for 10 days)
Omeprazole
Omeprazole is approved for the treatment and reduction of risk of recurrence of duodenal ulcer, GERD. gastric ulcer. and pathological hypersecretory conditions. Usual adult dose: Oral. 20 mg q.d. Dosage form: Delayed-release capsules containing 20 mg of omeprazole in enteric-coated gninulcs
Esomeprazoie Magnesium.
Esomeprazole
magne-
sium, S-bis(5-melhoxy-2-I(S)-[(4-methoxy-3,5-dimethyl-2pyridinyl)methyljsulfinytl-IH-benzimidazole- 1-yl) magne-
Dosage form: Oral: Delayed-release capsules. 20 or 40 mg of esomepraai: (present as 22.3 mg or 44,5 mg esomcprazole ntageesiurn trihydratc) as cnteric-coatcd pellets
Lansoprazole.
Lansoprazole. 2-Il 13-methyl.4-(2.Z1 tritluomethoxy )-2-pyridyll methyl ]sulflnyl )benzimidazok (Prevacid), is a white to brownish-white, odorless crystallme powder that is practically insoluble in water. Lansoprazole is a weak base (pyridine N, 4.01) and a weak acid lbenz• imidazote N-H. pK, 1.48). Like omeprazolc, lansoprazoleit
Chapter 21 a Hi.csan,ine and Antjlsista,njnic Agsnts
725
r e
J
Esomeprazole Magnesium
essentially a prodrug that, in the acidic biophase of the panetal cell, forms an active metabolite that irreversibly interacts with the target ATPase of the pump. Lansoprazole must be formulated as encapsulated enteric-coated granules for oral administration to protect the drug from the acidic environment of the stomach. 0C113
Lansoprazole
In the fasting state, about 80% of a dose of lansoprazole (versus --50% of omeprazole) reaches the systemic circulation, where it is 97% bound to plasma proteins. The drug is metabolized in the liver (sulfone and hydroxy metabolites)
and excreted in bile and unne, with a plasma half-life of about 1.5 hours.W Usual adult door: Daily oral dose administercd before breakfast Duodenul 15 mg once daily
Erosive esophagitis: 30 mg Zollinger-Ellison syndrome: 60 mg NSAID-induced gastric ulcers: treatment and prevention Dosage fonn: Delayed-release capsules containing IS and 30 mg of lansoprazole in enteric-coated granules
Pantoprazole Sodium.
The active ingredient in Pro-
tonix (pantoprazole sodium) is a substituted henzimidazole. sodium 5-(difluoromethoxy)-2-fl(3.4-dimethoxy-2-pyridinyl)methyljsulfinylj- I H-benzimidazole sesquihydrate (1.5 H20). a compound with a molecular weight of 432.4. The benzimidazoles have weakly basic (pyridine N. 3.96) and acidic (benzimidazole N-I-I, 0.89) properties, which facilitate their formulation as salts of alkaline materials (Fig. 2 1-17). Pantoprazole sodium .sesquihydratc is a white to offwhite crystalline powder and is racemic. Pantoprazole sodium sesquihydrate is freely soluble in water, very slightly soluble in phosphate buffer at pH 7.4. and practically insoluble in n-hexane. The stability of the compound in aqueous solution is pH dependent: the rate of degradation increases with decreasing pH. At ambient temperature, the degradation half-life is approximately 2.8 hours at pH 5.0 and approximately 220 hours at pH 7.8. The absorption of pantoprazole is rapid (Cmi. of 2.5 mL, —2.5 hours) after single or multiple oral 40-mg doses. Pantoprazole is well absorbed (—77% bioavailahility). Administration of pantoprazole with food may delay its absorption but does not alter its bioavailability. Pantoprazole
R4
A4
N
RO
RO
N S
0
N
S
+ H30' H N
Figure 21—17 • Ionization of benzimidazole PPIs.
II 0
ê
726
Wilson anal Gi.s told's Te.r:book of Organic Medicinal and Plwrmaceu:ical Chemistry
is distributed mainly in extracellular fluid. The serum protein binding of pantoprazole is about primarily to albumin. Pantopr4l.ole is extensively metabolized in the liver through
the CYP system, including 0-demethylulion (CYP 2C19). with subsequent sulfation. Other metabolic pathways include sulfur oxidation by CYP 3A4. There is no evidence that any of the pantoprazole melaholites have significant pharmaco-
logical activity. Approximately 71% of a dose of pantopruzoic is excreted in the urine, with I 8% excreted in the feces through biliary excretion.
bound to human plasma proteins. Rabeprazole is extensively
metabolized in the liver. The thioether and sulfone are the primary metabolites measured in human plasma resulting from CYP 3A oxidation. Additionally, desmethyl rabepra. zole is formed via the action of CYP 2C 19. Approximately 90% of the drug is eliminated in the urine, primarily as thi' oether carboxylic acid and its glucuronide and mercaplunc
OCHa
OCH3
o
Rabeprazole sodium is formulated as enleric-coated. delayed-release tablets to allow the drug to pass through the stomach relatively intact. After oral administration of 20 mg. peak plasma concentrations occur over a range of 2.0 to 5.0 hours Absolute bioavailability for a 20-mg oral tablet of rabeprazole (versus IV administration) is ap. proximately 52%. The plasma half-life of rabeprazole ranges from I to 2 hours. The effects of food on the absorption of rabeprazole have not been evaluated. Rabcprazolc is
Na
acid mnetabolites. The remainder of the dose is recovered in
the feces. Total recovery of radioactivity was
N
No
unchanged rabeprazole was recovered in the urine or feces Usual adult dose: Oral. 20 nig once daily (duodenal ulcer forJ weeks; erosive or ulcerative GERD for 4—8 weeks): guclri OCHF2
Pantoprazoto Sodium
hypersecretory disorders. 60 mg once daily titrated to maximum of 120 mg/day Dosage form: 20-mg delayed release tablets of the sodium salt
CHEMICAL COMPLEXATION
Usual adult dose:
Erosive esophagitis associated with GERD: 40 mg q.d. for wecks: if not healed after 8 weeks of treatment, an additional 8-week course may he considered Long-temi treatment of erosive esophagitis and GERD: IV treatment of erosive esophagitir. as an alternative to cumintied oral therapy. 4t) mg q.d. by infusion for 7-. 10 days Shun-term treatment (7 to 10 days) of GERI) Treatment of pathological hypersecretory conditions associated with Zollinger.Ellison syndrome Dosage form: Protonix: Delayed-release tablet tbr oral administration: each tablet contains 45.1 mg of pantoprazole sodium sesquihydr,ule (equivalent to 40 mg pantoprazole) Protonix IV.: Frecze.dried powder fur injection equivalent to 40 mg pantoprazole/vial
Rabeprazole Sodium. Rabeprazole sodium. 2-ti 14-(3mcthoxypropoxy)-3-methyl-2-pyridinyl irnethyl isultinyll- I H-benzitnidazole sodium salt (Aciphex). is a substituted benzimidazole with a molecular weight of 381.43. Rabcprazole sodium is a white to slightly yellowish-white solid, It is very soluble in water and methanol, freely soluble in ethanol. chloroliwm. and ethyl acetate, and insoluble in ether and ahexane. Rabeprazolc is a weak base (pyridine N. pKa 4.90) and a weak acid (benzimidazole N-H. pK. 1.60).
The sulfate esters and sulfonate derivatives of polysuecha. rides and lignin form chemical complexes with the enzyre pepsin. These complexes have no proteolytic activity. Be. cause polysulfates and polysulfonanes are poorly absorbed from the 01 tract, specific chemical coniplexation appears to be a desirable mechanism of pepsin inhibition. Unions nately. these polymers are also potent anticoagulants. The properties of chemical complexanion and anticoagu. lant action are separable by structural variation. In acoinpar. ison of selected sulfated saccharides of increasing nuniber of monosaccharide units, from disaccharides through starch
derived polysaccharides of differing molecular size. thrce conclusions are supported by the data: (a) the anticoagulant activity of sulfated saccharide is positively related to molec. ular size. (b) anticoagulant activity is absent in the disaeda• rides, and (c) the inhibition of pepsin activity and the protec.
tion against experimentally induced ulceration depend ni the degree of sulfation and non on molecular size. The readily available disaccharide sucrose has been used
to develop a useful antiulcer agent. sucralfate.
Sucralfate. Sucralfate. 3.4.5.6-tetra-(polyhydroxyalu minum).a.o-glucopyranosyl sulfate-2.3.4.5-tetra-(polyh). sulfate (Carafate). is
the aluminum hydroxide complex of the octasulfame ester 01
sucrose. It is practically insoluble in water and soluble in strong acids and bases. It has a
value between 0.43 and
1.19.
Sucralfate is minimally absorbed from the 01 tract and thus exerts its antiulcer effect through local rather than sy' temic action. It has negligible acid-neutralizing or buflerine capacity in therapeutic doses. Rabeprazole Sodium
Its mechanism of action has not been c.stablished. Studies
suggest that sucralfate hinds preferentially to the ulcer sitc
Chapter 21 • 1-lisrainine and Antil,isia,ninic
727
to form a protective barrier that prevents exposure of the lesion to acid and pepsin. In addition, it adsorbs pepsin and
protective actions are proposed to be related to increases in Cl niucus and bicarbonate secretion, increases in mucosal
bile salts. Either would be very desirable modes of action.
blood flow, and/or prevention of back diffusion of into the gastric
CH2OR
H
I
0 ( OR
IAJ(OH)al,
RO
1L_OR RO
(x—øtolOandy
-
-
22to31)
I
OR
OR L
R
SO3AI(OH)2
The product labeling states that the simultaneous adminisuntion of sucraltine may reduce the bioavailahility of certain agents (e.g.. tetracycline. phenytoin. digoxin, or cimetidine). It further recommends restoration of hioavailability by separating administration of these agents from that of sucralfate try 2 hours. Presumably. sucralfate hinds these agents in the GI tract.
The most frequently reported adverse reaction to sucralfate is constipation (2.2%). Antacids may be prescribed as but should not be taken within one-half hour before or after sucralfate. Usual adult dose: Oral. I g q.i.d. on an empty stuniach Dosage form: l-g sueralilite tablets
HO
OH Mtsoprostot
Misoprostol is rapidly absorbed following oral administration and undergoes rapid deesterification to the pharmaco-
logically active free acid with a terminal half-life of 20 to 40 minutes."2 Misoprostol is commonly used to prevent NSAID-induced gastric ulcers in patients at high risk of complications from a gastric ulcer, such as elderly patients and
patients with a history of ulcer. Misoprostol has also been used in treating duodenal ulcers unresponsive to histamine H2 antagonists; the drug does not prevent duodenal ulcers. however, in patients taking NSAIDS. Misoprostol can cause miscarriage, often associated with potentially dangerous bleeding.
PROSTAGLANDINS
The prostaglandins are endogenous 20-carbon unsaturated fatty acids biosynthctically derived from arachidonic acid. These bioactive substances and their synthetic derivatives race been of considerable research and development interest as potential therapeutic agents because of their widespread physiological and pharmacological actions on the cardiovascular system. 01 smooth muscle, the reproductive system. nervous system. platelets. kidney. the eye. etc." Prosta-
glandiuts of the E. F. and I series arc found in significant concentrations throughout the Cl tract. The Cl actions of he prostaglandins include inhibition of basal and stimulated gastric acid and pepsin secretion in addition to prevention of ulecrogen or irritant-induced gross mucosal lesions of the stomach and intestine (termed cyluproIec:ion). The prostaglandins can both stimulate (PGFs) and inhibit (PCIEs and intestinal smooth musclc contractility and accumulanon of fluid and electrolytes in the gut lumen (PGEs). Theraçeulic application of the natural prostaglandins in the treatwent of Cl disorders is hindered by their lack of pharmacological selectivity coupled with a less-than-optinial biodisposition profile. Misoprostol. Misoprostol. (± )-methyl II 16-dihyJroxy-16-methyl.9-oxoprost- I 3E-en- I -oate. is a semisynderivative of POE1 that derives some pharmacological
selectivity as well as enhanced biostability from its 16Misoprostol exhibits methyl, I 6-hydroxy structural antisecretory and cytoprotectant effects characteristic of the natural prostaglandins and has a therapeutically acceptable hiodisposition profile. Although the antisecretory effects of misoprostol arc thought to be related to its agonisactions at parietal cell prostaglandin receptors, its cyto-
Usual adult dose: Or,i1. 200 Dosage form: 100- and
q.i.d. with food tablets
HISTAMINE H3-RECEPTOR LIGANDS63,M Histamine receptors are members of the G-protein—coupled receptor family involved in the regulation of neurotransmitter release in both central and peripheral neurons. The receptor encodes a 445cDNA for the human histamine amino acid protein that. when recombinantly expressed. couples to inhibition of adenylate cyclase. presumably through
Gal. The histamine Hrreceptor mRNA is highly expressed in central nervous tissues. Histamine H3 heteroreceptors have been identified in stomach, lung, and cardiac tissues receptors have been implicated of animals. Presynaptic in regulating neurotransmiuer release from histaminergic. noradrenergic. dopaminergic. cholinergic. semloninergic. and peptidergic neurons. The potential therapeutic roles of histamine H1-receptor antagonists in the CNS have been evaluated in models of learning and memory impairment. attention-deficit hyperactivity disorder, obesity, and epilepsy. Studies of the regulation of inflammatory processes. gastroprotection. and cardiovascular function suggest several therapeutic possibilities for peripherally acting histamine Hrreceptor agonists. As yet. no histamine H3-recep!or ligands have been approved for marketing in the United States.
Potent H3 agonists (Fig. 21-18) are obtained by simple modifications of the histamine molecule. The imidazole ring is a common structural feature in almost all El3 agonists. Methylation of the aminoethyl side chain of histamine favors
728
Wilson
and
Textbook of Organic
and Pl:ar,naeewical (hesnA fry
HN
N
NH2
H3C
H3C
K R-a-Methylhlstamine
Azomethine derivative ot
HN
HN NH
Sc NH2
Immepip
meld
Figure 21—18 . Histamine H3-receptor agonists.
NH S
II
HN N
—C—NH
Clobenprobil
Thioperamide
HO Cl
N
HN
(CH2)2 —<s
—K N
0
Verongamine
GR-1 75737
Figure 21—19 • Histamine H3-receptor antagonists.
Chapter 21 • Hi3la,nine and Antiliisia,,,inic Fl3 activity. Introduction of one or two methyl groups to give e-methylhistamine and a.a-dimcthylhislamine yields potent Fl1 agonists that show little selectivity among ihe three histamine receptors. The increased potency of a-methyl-histamine is ascribed almost completely to its R isomer (H1/H1
ratio = l7).TheclinicaluscofR-a-mcthylhistamine isCOflipromised by rapid catabolism by hisiamine-N-methyltrans[erase.
Azomethine derivatives of R-a-methylhistamine
have been developed and shown to possess anti-intlammatory and antinociceplive properties. Other H.1 agonisis in-
dude the isolhiourea derivative. imetit. a highly selective. full agonist that is more potent than R-a-meihylhistamine. A third type of H1 agonist is immepip. which may be considcad as a histamine analogue with an elongated and cyclized
side chain. lmmcpip is both a highly selective and potent
729
I?
I'elletier. Ci.: Naturally occumng ant,titstaniinics in body tissues. In Rirchu e Silvzr, M. led.;. handbook of Experimental Pharmacology. sal. 1812. New York. Springer'Vcrlag. 1978. p. 36'). 13. Best. C. H.. et al.: J. Physiol. )Lond.( 62:397, 1921 4. Fonrneau, ti., and Bard. D.: Arch. tnt. Plurmacodyn. 46:178, 1933. IS. Cn.sy. A. I-.: Chemist,y oirinli-Hl histatnuie anta',,nist,. In Rocha e Silva. M. ed.l. Handbook of Experimental Phanhmacology. sal. 18/2. New York. Springer-Verlag. 1978. p. 175, 16. Witiak, 0. 1.: Antiallergenic agents. In Burger, A. cd.). Mcdicinal ('hemtstry. 3rd ed. New York. Wiley tncersciencc. 1970. p. 1643. I?. Paton. 0. M.: Receptors for hisiantitte. In Schactuer. M. led.). Histamine and Antihistamnines, New York. Pergamon Press. 1973. p. 3. Ill. Rocha e Silvzi. M.. anti Antonio. A.: Bioassay ot antihusia,mtinic action. lit Rocha e Silsa, M. led.). Handbook of Experimental Pharmacology, vol. 1W2. New York. Springer-Verlag. 1978. p. 381.
19. Bid. J. H and Murtin. Y. C : Organic symrthesis as ii source of ness drugs. In Gould. R. F. (cdl. t)rug Discovery. Advances in ChCmislry Series no. 0$. Washingtott. DC. American Chemmtical Society, 1971.
p.8t.
agonist.
A large number of H3 antagonists have been described. Antagonist studies have suggested the presence of H 1-recepor subtypes. In general, antagonist structures conform to the following general
Ahlquisi. K. P.: Ant. J. Physuil. 153:586. 1948. 2!. Lin. T. M.. ci at.. Ann. N. Y. Acad. .5cm. 99:3(l 1962. 22. Ash, A. S. F.. and Schild, H. 0.: Br. J. Phannacol ('heotother. 27:427. 2(1.
19W,.
23. Black. J. W., CI xl.: Nature 236:31(5. 1972.
24. Met,irr. 0. If., Ikawit. M., and Snell. If.
Ef.. 3. Am
Chettm. 5mw. 76:
648, 1954.
25. St'hayer. K. W.. and Cooper. 3. A. 0.: J. AppI. Physiol. 9:481. 1956. 26. Schayer. K. W.: Biogenesis at histamine. Iii Rocha e Silva. M. led.). Ilandtxark of Experimental Pharmacology. vol. 11(/2. New York. Springcr.Verlag. 1978. p. lIt').
The heterocycle component of this general structure is
27. Wetterqvist. H.: Histamine metabolism amid cxcrctim,n. In Racitac Silsa.
most commonly a 4-monosubstituted itnida,.ole or hioisosleric equivalent. Chains A and B can be of various structures and lengths, and there is also wide latitude in the structural
M. led.). Handbook of Experinitental Phurtnacology. vol. 18/2. New York, Springer-Verlag. 978. p. 131. 28. Ganellin. C. K.. and Parson,. M. F.. led.'..). Pharmacology of Histatrtine Receptors. Bristol, Li. K., Wright. 1982. 29. Fordtran. J. S.. and Grossman. M. I. (eds.l: Third Symposium on Histanine H2.Reccptor Anlagoitists. Clinical with Cin,etidinr. Gastroenterolirgy 74)21:339. 9711. 3(3. l)ougla.s, W. W.: Hista,tt,ne and 5.hydroxytryptamine lserotoninl and their antagonists.. In Oilman, A. 0.. Goodman. I RaIl. T. W.. and Mur;id. F. (eds,). (Joodmttan and Giltnan's 'the Phrirnmcological Basis iii 'therapeutics. 7th cd. New York. Mrtctttillan. 1985. p. 605. 31. san Bnttk. F. G.. and I.ien. If. J.: ('immmipetluive and noncoitlpetitlve antagonism lit Rocha e Silva, N. led.). Hnndtsmok of Expcriittetttal Phannacology. vol. 18/2. New York. Springer.Verlag. 197%. p. 333. .52. Narita, W. T., rind K. F.: Structure.aclivity retaritinships of HI-
requirements for the polar group. Halogenated phenyl. cycloalkyl. and heteroaryl structures are usually found for he lipophilic moiety (Fig. 2 1-19).
Thioperamide was the tirsI potent H, antagonist to be tkscribcd. This agent enhances arousal and/or vigilant patterns in adose-dependent fashion in animals, suggesting possible use o[ CNS-acting H3 antagonists in treating sleep din-
isles characterized by excessive daytime sleep, such as narcolepsy. Other H3-antagonist structures are shown below.
including the natural product verongamine. isolated front a sea sponge.
receptor antagonists. In Rochir e Silva. M. led.). llandbmxmk of P.xperi.
mental Pltannaciilogy, vol. l8/2. New York. Springer-Verlag. 1978. p. 215.
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52. SoIl, A. H.: N. EngI. 3. Med. 322:909—916. 1990. 53. Black. i.: Science 245:486—493. 1989. 54. Briilain. R. T.. Jack. I).. and Price, B. 3.: Trends Pharmacol. Sci. 2: 310—313. 1981.
55. Br8ndsuOni. A.. Lindberg, P.. and Junggrcn. U.: Scund. .1. Gastmcnterol. 20(Suppl. 108):l5—22, 985. 56. Baiter. R. F.. Collins. P. W.. and jones. P. H.: Annu. Rep. Med. Chem. 22:191. 1987. 57. Latiginan, M. 3. S.: Gui 49:309—310. 2001. 58. Meyer. U. A.: Yale 3. flint. Mcd. 69:203—209. 1996. 59. Lnmpkin. '1'. A.. Ouellct. D.. Huk. L. .1., and Dukes. G. E.: DICP 24: 393—402. 1990.
60. Spencer. C M.. and Faulds. Ii: Drugs 48:405-431. 1992. 61, Koniurek, S. 3., and Pawlik. W.: Dig. Dis. Sci. 31:6—19. 1986. 62. Monk. J. P.. and Clissold. S. P.: Drugs 33:1—30. 1987. 63. Louis. R.. and Timmcrman. H.: Prog. Drug Rev. 39:127. 1992. 64. Phillips. 3. G.. Ali. S. M.. Yates. S. L.. and Tedlord. C. E.: Annu, Rep. Med. Chem.33:31-40. 1998. 65. Stark, H., I.igncau. X.. Arrang. J.-M.. ci at.: Bioorg. Med. Chem. Lett.
Burland. W. L.. and Simkins, M. A. (eds.): Cimetidine. Proceedings of the Second International Symposium on Histamine H2-Rcccptor Antago.
nists. New York. Excerpta MedicalElscvier. 1977. Fordtr.tn. 3. S.. and Grossman. M. I. (eds.): Third Symposium on Histamire H2-Reccptor Antagonists. Clinical Results with Cimetidine. Gasiroer terology 74(2, Past 2):339. 1978.
GERD Information Resource Center: hupilwww.gerd.cont/ Leurs. R.. Smit. M. 3.. and Timmerman. H.: Molecular aspects of histamine receptors. Pttormacol. Ther. 66:413—463. (995 Nelson. W. L.: Antihistamines and related antiallergic arid anilulcer agents In Williams. D.A. and Lemke. T. L. led.). Foye's Principles of Mediri
na) Chemistry. 5th ed Baltimore. Lippincott Williams & Wilkins 2002. pp. 794—818.
Rocha c Silva. M. led.): Histamine and antihisiaminics. In Handbook ol Experimental Pharmacology. vol. 18/I. New York. 1966.
Rocha c Silva, M. (ml.): Histamine II and aniihistaminics. In Hnndbooke( Experimental Pharmacology. vol. 1812. New York. Springer.Vetht. 1978.
8:2011—2016. 1998.
Schachter. M. (Cd.): Histamine and nnlihistamincs. In International clopedia of Pharmacology and Therapeutics, sect. 74. vol. I. Ncs York. Pergamon. 1973.
SELECTED READING
Sippy. B. W.: JAMA 250:2192. 1983. Thompson. 3. H.: Gastrointestinal disorders—peptic ulcer disease. In
Bass. P.: Gastric antisecretory and antiulcer agents. Ads. Drug Rcs. 8:206. 1974.
Beaven. M. A.: Histamine. Its Role in Physiological and Pathological Pro. ccsscs. Monogr. Allergy, vol. t3. New York. S. Kruger. 1978.
Rubin, A. A. (ed.) Search for New Drugs, Medicinal Research Scnrs
vol. 6. New York, Marcel Dekker. 1972. p. 115. van der Goo. H.. and Timmerman. H.: Selective ligands as tools to iludy histamine receptors. Eur. 3. Med. Chem. 35:5—20. 2000.
H
C
A
P
T
E
R
22
Analgesic Agents ROBERT E. WILLETTE
The struggle to relieve pain began with the origin of human-
ity. Ancient writings. both serious and fanciful, dealt with
is the most common complaint for which patients seek treatmnent. For various reasons, however, including politics and
secret remedies, religious rituals, and other methods of pain relief. Slowly, the present modem era of synthetic analgesics
lack of training and ignorance, pain is not well managed:
evolved. An analgesic may be defined as a drug bringing about insensibility to pain without loss of consciousness. (The etymologically correct term anal gerk may be used in place of the incorrect but popular analgesic, but the latter
is primarily due to a fear of addiction, which often limits
is used almost universally.)
Tuinter' has divided the history of analgesic drugs into four major eras, namely: I. The period of discovery and use of naturally uccuning plant drugs
2. Isolation of pure plant principles (e.g.. alkaloids) from the nutunil sources and their identification with analgesic action Development of organic chemistry and the first synthetic analgesics
4. Development of modem pharmacological techniquCs. making it possible to undertake a systematic testing of new analgesics
A new era has emerged with the discovery of opioid receptors and endogenous chemicals that have analgesic activity.
The isolation of morphine from opium by Serturner. in 1803. and the discovery of its analgesic activity (he named ii morphine after the Greek god of dreams. Morpheus) ushered in the second era. It continues today only on a small
scale. Wohler introduced the third era indirectly with his synthesis of urea in 1828. He showed that chemical synthesis could he used to make and produce drugs. In the third era. die first synthetic analgesics used in medicine were the salicylare.s. originally found in nature (methyl salicylate, salicin) and then synthesized by chemists. Other early synthesized drugs were acceanitid (1886). phenacetin (1887). and aspirin (1899).
less than half of those with pain are adequately treated. This
the proper use of the narcotic or opioid class of analgesics. and lack of training of health care professionals. To understand how and when this broad class of drugs is and should be used in the management of pain requires a brief review of pain. Pain has been classified into the following types: physiological. inflammatory, and neuropathic. The first is the most common, for example. touching a hot object or getting a cut. Inflammatory pain can be initiated in a wide variety of ways, such as infection and tissue injury. The last type is due to injury to the peripheral or central nervous system (CNS). Within these classes of pain there are different levels of pain or categories of pain. l'hese include acute, chronic, cancer. arthropathy (e.g.. arthritis), visceral, neuropathic, and diabetic pain. A separate category is recognized for acquired immunodeficiency syndrome (AIDS) because of its disseminated nature. Clearly, these all require different approaches to pain management. In the following sections of this chapter. several classes or
pain-relieving drugs are described. The three major
classes of drugs used to manage pain are opioids. nonsteroidal anti-inflammatory agents, and acetaminophen. A newly
emerging class, known as analgesic adjnrwri.n. is not covered here.
Of these drug classes, the historically important group of drugs in the opioid class has been the most problematic for use in the proper management of pain. The temi opiophohia was coined to describe the reluctance of physicians to prescribe opioid drugs in adequate amounts or for long enough
These early discoveries were the principal contributions
periods. As early as the 1970s. the National Institutes of
in this field until modern methods of pharmacological testing initiated the fourth era. The effects of small structural modilicalions of synthetic molecules then could be assessed accuniely by pharmacological means. This permitted systematic
Health formed a committee, at the request of the president. to investigate therapies for rare diseases, but the initial focus was on pain management. There was political pressure to
study of the relationship of structure to activity during this
impression that a more potent drug was needed, after several
era. The development of these pharmacological testing procedures.coupled with the fortuitous discovery of meperidine by Eisleb and Schaumann.2 made possible a period of rapid 'aides in the development of potent analgesics.
studies showed poor pain management. The committee found that the problem was a lack of training of physicians in pain management and misconceptions about the use of
approve the use of heroin for pain, under the mistaken
PAIN
opioids and addiction.1 Unfortunately, the problem continued through the end of the 20th century and into the 2 1st. Over the past few years. major national medical groups, state medical boanis. and pharmaceutical groups have formulated guidelines directed
The primary use of the drugs covered in this chapter is to
at improving pain therapy. Pharmacists and other health professionals must clearly recognize the advances in pain
relieve pain of a wide array of causes and mechanisms. Pain
management and the advantages and limitations of the var731
732
Wilson and Gi.ceald's
of Organic Medicinal and Phurrnweu:ical Chemistry
bus drugs. For further information on pain and its management. refer to a number of articles and American Pharmaceutical Association continuing education programs.4 The consideration of naturally occurring and synthetic an-
algesics is facilitated greatly by dividing them into two
exerts a valuable expectorant action that is superior to that of morphine. Two basic types of structures arc recognized among the opium alkaloids, the pheizanthrene (morphine) type and the benzvlisoquizwlimte (papaverine) type (see structures).
groups: (a) morphine and related compounds and (h) the antipyretic and anti-inlianimatory analgesics. Also, numerous drugs that possess distinctive pharmacological activities in other areas also possess analgesic properties and are used as analgesic adjuvants. The analgesic properly exerted may
be a direct effect or may be indirect, but it is subsidiary to some other, more pronounced effect. Some examples of these, which are discussed elsewhere in this text, are sedatives (e.g.. barbiturates), muscle relaxants (e.g., mephenesin.
methocarbamol), and tranquilizers (e.g. meprobamate).
Phenanthreno lype (Morpt5rue. R & R'
H)
These drugs are not considered in this chapter. N
MORPHINE AND RELATED COMPOUNDS
Historical Perspedive The discovery of morphine early in the 19th century and the demonstration of its potent analgesic properties led directly to the search for similar drugs from plant sources. In tribute to the remarkable potency and action of morphine, it and codeine have remained alone as outstanding and indispensa-
ble analgesics from a plant source. Only since 1938 have synthetic compounds rivaling morphine in its action been found, although many earlier synthetic changes made on morphine itself gave more effective agents. Modifications of the morphine molecule are considered under the following headings: I. Early changes on morphine before the work of Small. Eddy. and their coworkers 2. Changes on morphine initiated in 1929 by Small. Eddy. and coworkers8 under the auspices of the Committee on t)rug Addiclion of the National Research Council and extending to the present time 3. Research initiated by Eisleb and Schaumann2 in 1938. with their discovery of the potent analgesic action of meperidine. a compound that departs radically from the typical morphine molecule 4. Research initiated by Grewe. in 1946. leading to the successful synthesis of the morphinan group of analgesics
EARLY MORPHINE MODIFICATIONS
Morphine is obtained from opium. which is the partly dried latex from incised unripe capsule.s of Papat'er soinniferum. Opium contains numerous alkaloids (as meconates and sulfates), of which morphine, codeine. noscapine (narcotine). and papaverine are therapeutically the most important. The-
OH2
(H OCH3 Benrylusoguinoline Typo (Papavoruno)
The pharmacological actions of the Iwo types of are dissimilar. The morphine group acts principally on CNS as a depressant and stimulant: the papaverine greup has little effect on the nervous system but a marked medic action on smooth muscle. Clinically, the depressant action of the morphine group is the most useful propen%. resulting in increased tolerance to pain, a sleepy feeling, lower perception of external stimuli, and a feeling of wellbeing (euphoria). Respiratory depression. central in origin. is perhaps the most serious objection to this type of aside from its tendency to cause addiction. The stimulant
action is well illustrated by the convulsions produced h) certain members of this group (e.g.. thebaine). Before 1929, the derivatives of morphine that were rna&
primarily resulted from simple changes on the molecule, such as esterilication of the phenolii' and/or alcoholic hydro-
xyl groups. etherification of the phenolic hydroxyl greup. and similar minor changes. The net result was the of some compounds with greater activity than morphine her also with greater toxicity and addiction potential. No coinpounds were found that completely lacked the addiction lb bilities of morphine. (The term addiction liability or the pre-
ferred term dependence liability, as used in this tea,
it has a greater constipating action and, thus, is better suited for antidiarrheal preparations (e.g., paregoric). Opium, as a
indicates the ability of a substance to induce true addicthe tolerance and physical dependence and/or to suppress fe morphine abstinence syndrome after withdrawal of morphine from addicts.) Some of the compounds in common use before 1929 are listed in Table 22- I together with some tnore recently intruduced. All have the common morphine skeleton. Amongihe earlier compounds is codeine, the phenolic methyl ether of morphine, which also had been obtained from opium. It
constituent of Dover's powders and Brown Mixture, also
survived as a good analgesic and cough depressant. together
baine. which has convulsant properties, is an important start-
ing material for many other drugs. Other opium alkaloids. such as narceine, have been tested medicinally but are not of great importance. The action of opium is principally due to its morphine content. As an analgesic, opium is not as effective as morphine because of its slower absorption, but
______
Chapter 22 •
TABLE 22-1
Synthetic DerivatIves of Morphine
Agenlv
733
C-6 oxidized congcncrs. dihydromorphinone (hydromorphone) and dihydrocodeinone (hydrocodone). Derivatives of
the last two compounds that possess a hydroxyl group iii position 14 are dihydrohydroxymorphinone. or oxymor-
,,CH3
phone. and dihydrohydroxycodeinone, or oxycodone. These are the principal compounds that either had been on the market or had been prepared before the studies of Small, Eddy. and coworkers. To this time, no really systematic effort had been made to investigate the structure—activity relationships
in the molecule, and only the easily changed peripheral groups had been modified. The only exception is oxymor-
Compound Proprietary Name R Moipiline
H
W
R'
if
Principal Use Analgesic
MORPHINE MODIFICATIONS INITIATED BY THE RESEARCH OF SMALL AND EDDY
H OH Codoino
CM3
H
Same as above
Ana'gesic and to
cough reflex H
Same ax above
Damn
Dacetyrmorplsne
CH3CO H
heroin)
(prohibilod in
H
US)
Hydromoiphono (dihydromorpih.
H
H :?H7
tAb&xIkI
Hodroe
CM3
H
Same as above
(druydro.
codem000) Dicodid Oxymorphono
Analgesic and to depress cough reflex
OH Same as above
gas C
OH Same as above
Analgesic and to dePress cough reflex
H
CM3
(cihydrohydroxy. codexione)
tihydrocodoino
Depress cough
H
reflex
Pnrar.odin
Oi'iydromorphine
H
H
Mxlhyldihydco-
N
H
phone, introduced in the United States in 1959 but mentioned here because it obviously is closely related to oxycodonc.
Same as above
The avowed purpose of Small, Eddy, and coworkers5 in 1929 was to approach the morphine problem from the standpoint that it might be possible to separate chemically the addictive property of morphine from its other, more salutary attributes. or if that was not possible. it might be possible to find other synthetic molecules without this undesirable property. Proceeding on these assumptions, these workers first examined
the morphine molecule exhaustively. As a starting point.
morphine offered the advantages of ready availability. proven potency, and ease of alteration. In addition to its addictive tendency, they hoped that other liabilities (e.g.. respiratory depression, emetic properties, and gastrointestinal tract and circulatory disturbances) could be minimized or abolished as well. Because early modifications of morphine (e.g., acetylation or alkylarion of hydroxyls and quaternization of the nitrogen) caused variations in the addictive po-
lency, they felt that the physiological effects of morphine could be related, at least in part. to the peripheral groups. It was not known if the actions of morphine were primarily a function of the peripheral groups or of the structural skeleton. This did not matter, however, because modification of the groups would alter activity in either case. These groups
and the effects on activity by modifying them are listed in have Table 22-2. The results of these and earlier not always shown the effects of simple modifications on the analgesic action of morphine quantitatively, but they do indicate the direction in which the activity is likely to go. The studies are far more comprehensive than 'rable 22-2 indicates, and the conclusions usually depend on more than one pair of compounds. Unfortunately, these studies on mor-
phine did not eliminate the addiction potential from these H2
rrxsptulnone
0CM, 0
compounds. In fact, the studies suggested that any modification that increased the analgesic activity caused a concomi-
tant increase in addiction liability. The second phase of the studies5 deall with the attempted synthesis of substances with central narcotic and, especially.
analgesic action. The morphine molecule contains certain well-defined types of chemical structures. Among these are with the corresponding ethyl ether, which has found its principal application in ophthalmology. The diacetyl derivative rif morphine, heroin. ha.s been known for a long time: it has lscn banished for years from the United States and is being used lcss in other countries. It is the most widely used illicit
the phenanthrene nucleus, the dibenzofurun nucleus, and. u.s
1mg among nareotic addicts. Among the reduced coin-
a variant of the latter, carbazole. These synthetic studies. although extensive and interesting, provided no significant findings and are not discussed further in this text. One of the more useful results of the investigations was the synthesis of 5-methyldihydromorphinonc (Table 22-It,
pounds are dihydromorphinc and dihydrocodeine and their
whose methyl substituent was originally assigned to position
734
Medicinal and F'ltarmaeeuzical Chemistry
Wilson and Gisvold.c Te.uhooA of
TABLE 22—2
Some Structural Relationships in the Morphine Molecule Peripheral Groups 0
Nitrogen Group linsalurated Linkago
t3
Alcoholic Hydroxyl Group Ether Bridge Phenolic Hydroxyt Group
Effects on Analgesic Activity (Morphine or Another Compound as Indicated = 100)
Peulpheral
Modification (On Morphine Unless Otherwise Indicated)
Groups of Morphine Phonolic hydroxyl
—OH —OH
(codeine)
15
—0C5H5 (ethylmorptiine)
10
—OH Alcoholic
—OH —.
500 240 420
—OH ——OC,H5 —OH -.—OCOCH3 —OH —=0 (rvrorphinoflfl)
37
—OH — = 0 (dihydromorphune to dihydro. mophinone) —OH — =0 (dihydrocodelne to dihydrocodeinone) —OH —H (dihydramorphine to dihydrodesoxymorphine-D) Ether bridge
=C—O—CH——=C—OH HCH— (dlhydrodesoxyrrlorphine.0 to tetrahydrodesoxyrriorprline)
Alicyclic unsaturated linkage
—CH = CH — —. — CH2C1-t2— (dihydromorphine)
(codeine to
600 (Ciflydrornorphinc vs 390 (Dihydrocodeine vs 1000 IDihydromorphinc vs. dihydrodosoxymorphinoD) t3 ID;hydrodesoxyrnorphine.D vs tetrahyorodesoxymorphine) 120
115 (Codeine vs dihydrocodelnel
ctihydiocodoine)
Tertiary nItrogen
"N
CH1
— H (norniorplilne)
\
1.400
Reversal 0? activity (morphine
>—CH3 ->—R
A
propel, ISOIY.Jtyl allyt
CH3
/NCHJ••/N\
I
(Strong curare action)
CH3
Opening of nitrogen ring (morphirnethne)
Marked decrease In action (Ccnlsi.so
Chapter 22 a Analgesic Agen:.c
73.
TABLE 22—2--Continued Peripheral Groups of
Modification (On Morphine
Morphine
Unless Otherwise Indicated)
Nuclear
substtutlon
Effects on Analgesic Activity (Morphine or Another Compound as Indicated = 100)
of —NH., (most likely at position 2) —Ct or—Br (at posItion 1) —OH (at position 14 in dihydromorphinone) —OH (at position 14 in dihydrocodeinono) —Ct-i3 (at position 6) —CM3 (at position 6 in dihydrornorpfiino)
CH3 Cat positIon 6 ri d,hydrodesoxymorphine-D)
=CH2 (at posItion 6 in dihydrodesoxymorphlne-D)
710 Although it possessed addiction liabilities, it was a very
potent analgesic with a minimum of the undesirable side effects of morphine, such as emetic action and mental dullness. Later, the high analgesic activity demonstrated by morphine congencrs in which the alicyclic ring is either reduced or methylated (or both) and the alcoholic hydroxyl at position 6 ix absent prompted the synthesis of related compounds possessing these features. These include 6-methyldihydromorphine and its dehydrated analogue 6-methyl-46-dcsoxymorphine or methyldesorphine.'' both of which have high potency. Also of interest were the compounds 6-methyltuorphinc; and 6-methyl-, and 6-methylenedihydrodesoxymorphine. 2. In analgesic
activity in mice, the last-named compound was 82 times more potent, milligram for milligram. than morphine. Its was 22 times that of morphine.14 therapeutic index The structure—activity relationships of l4.hydroxymorphine derivatives have been reviewed,8 and severdl related compounds were synthesixed.'3 Of these, the dihydrodesoxy compounds possessed the most analgesic activity. Also, esters of 14-hydroxycodeine derivatives have shown very high For example, in rats, was 177 times more active than morphine. In 1963. Bentley and Hardy'7 reported the synthesis of a
novel series of potent analgesics derived from the opium alkaloid thehaine. In rats, the most active members of the series (I. R, = H, R2 = CH3. = isoamyl: and I, R, = COCH3, R2 = CH3. R3 = ,z-C3H7) were several thousand times stronger than morphine.'8 These compounds exhibited marked differences in activity of optical isomers, as well as other interesting structural effects. It was postulated that the
more rigid molecular structure might allow them to fit the teceptor surface better. Extensive structural and pharmacological studies have been reported.'9 Some of the N-cyclopropylmethyl compounds are the most potent antagonists yet discovered and have been studied very intensively.
R,
Marked decrease in action 50
250 (Dihydromorplainorie Va ocymorptone( 530 (Dihydrocodeinone Va oxycoclona) 280 33 (Dihydrornorphine vs 6-melhy)dihydromorptaine) 490 (Dihydrodesoxymorpnine.D vs 6-methyldihydrodesoxymorphine) 600 (D,hydrodesoxymorphirie-D va 6-methytenedihydrodesoxymorphine)
As indicated in Table 22-2. replacement of the N-methyl group in morphine by larger alkyl groups not only lowers analgesic activity, but also confers morphine-antagonistic properties on the molecule (discussed belowt. In direct contrast to this effect, the N-phenethyl derivative has 14 times the analgesic activity of morphine. This enhancement of activity by N-aralkyl groups has wide application, as is shown below. Some of the morphine antagonists, such as nalorphine. are also strong analgesics.2° The similarity of the ethylenic double bond and the cyclopropyl group has prompted the synthesis of N-cyclopropylmcchyl derivatives of morphine and its derivatives.2' This substituent usually confers strong narcotic antagonistic activity, with variable effects on anal-
gesic potency. The dihydronormorphinonc derivative has only moderate analgesic activity. MORPHINE MODIFICATIONS INITIATED BY THE RESEARCH OF EISLEB AND SCHAUMANN
In 1938, Eisleb and Schaumann2 reported the fortuitous dis-
covery that a simple piperidine derivative, now known as meperidine, possessed analgesic activity. It was prepared as an antispasmodic. a property it also possesses. As the story is told, during the pharmacological testing of meperidine in mice, it was observed to cause the peculiar erection of the tail known as the Straub reaction. Because this reaction is characteristic of morphine and its derivatives, the cotnpound
then was tested for analgesic properties and found to be about one fifth as active as morphine. This finding led not only to the discovery of an active analgesic, but far more important, it stimulated research. The status of research ott analgesic compounds with an activity comparable to thitt of morphine was at a low ebb in l931(. Many felt that potent compounds could not be prepared, unless they were very closely related structurally to morphine. The demonstration of high potency in a synthetic compound that was related only distantly to morphine, however, spurred the efforts of 23 various research groups The first efforts, naturally, were made on the nteperidinctype molecule in an attempt to enhance its activity further. It was found that replacement of the 4-phenyl group by hydrogen, alkyl. other aryl. aralkyl. and heterocyclic groups reduced analgesic activity. Placetnent of the phenyl and ester
_______
_______ ______
736
Wils(,n and Gistvldx Textbook of Organic Medicinal and Pharmaceutical Che,nistrt
groups at the 4 position of I -methylpiperidine also gave opti-
It is in the same relative position as in morphine. The effect is more pronounced on the keto compound (Table 22-3, A4) than on meperidine (A-I). Kctobcrnidone is equivalent to morphine in activity and was once widely used.
mum activity. Several modifications of this fundamental structure are listed in Table 22-3. Among the simplest changes shown to increase activity is the insertion of an ni-hydroxyl group on the phenyl ring.
TABLE 22-3
More significantly. Jensen et al.24 discovered that replace-
Compounds Related to Meperidlne (A3
A,
A,,
(A5)
wnere it Is CH3)
(A5 — H except in
Structure -
Corn-
pound A-i
—C6N5
Analgesic
—CH2CH2
CxH5
A-4
C5H5
—COOCHLCH3)2
—
R4
—COOC2H,,
A.2
A-3
—- —
-— R2
R1
Name
ActivIty
(if Any)
(Meperidine = 1)
—CH3
Meporidne
(0
—Ct-13
Bemdono
15
—CH2CH2—
CH3
—CH2CI-l,—
—CH3
Proper4dno
15
Os
0
Kbomidone
—CH5
—C—C2H5
A-S
62
0 —0—C—C2H5
A-6
—CH2CH2—
—CH3
S
0
A-?
C5H5
—0—C—C2H5
A-B
—C6H5
—O--C—C2H5
5
CHa
—CH2CH—
CH3)
Betaprodne
14
TrmeperIdlSSe
75
Phonerdine
26
Anilerldno
35
Pim,nodne
55
0 A-9
—C6H5
A-tO
All At2
CxH5
—COOC2H,,
—CH2CH2—
—cOOC2H5
— CHrCHS
COOC2H5
—CH.CH?
—O—C—C2H5
—CH2CH2—
—CH2CH2C0H5
CH2)3NHC0H5
l880
CH2CH2CHC5H5
0—C—C,H5
0
0
Al3
C5H6
C00C.H5
OIptenoxytato
—
CN
No's,
Chapter 22 • Analgesic Agents
737
TABLE 22—3—Continued Analgesic
Structure Com-
pound
R,
A4
A-t5
R2
R3
—OH
Name
Activity
(If Any)
(Meperidine = 1)
L0000rncie
—CH2CH5CH2— —CH, —CH—
—C61-16
A.l6
0
Cthotiopiazne
03
CH3
0 A-t7
—H
AIB
—COOCII,
—
Fenianyl
—
Lofantafli (A 34995)
940
0 —N—CC2H5 C0H5
—CH7CH— Cl-I
meat of the carbethoxyl group in meperidine by ucyloxyl groups gave helter analgesic, as well as spasmolytic. activity.
The "reversed" ester of meperidine. the propionoxy compound (A-f,), was the most active, being 5 times as active as meperidine. These findings were validated and expanded In an extensive study of structural modion by Lee et fications of meperidine. Janssen and concluded that the propionoxy compounds were always more active, usually shout twofold, regardless of what group was attached to the nitrogen.
Lee"° had postulated that the configuration of the propioderivative (A-6) more closely resembled that of morphine, with the ester chain taking a position similar to that occupied by C-6 and C-7 in morphine. His speculations were nosy
based on space models and certainly did not reflect the actual conformation of the nonrigid meperidinc. He did arrive at the
correct assumption, however, that introduction of a methyl group into position 3 of the piperidine ring in the propionoxy compound would yield two isomers, one with activity approximating that of desomorphine and the other with less octivity. One of the two diastercoisomers (A-7), betaprodine. att activity in mice about 9 times that of morphine and that of A-ö. Beckett et al!' have established that it o the ei.s (methyl/phenyl) form. The lra,ts form. alphaprodine, is twice as active as morphine. Resolution of the racetotes shows that one enantiomer has the predominant activny. In humans, however, the sharp differences in analgesic piucncy are not so marked. The tran,s form is marketed as he raccmate. The significance of the 3-methyl has been atributed to discrimination of the enantiotropic edges of these
by the receptor. This is even more dramatic in 3.allyl and 3-propyl isomers, for which the a-irons forms ae considerably more potent than the isomers, indicating hat three-carbon substituents are not tolerated in the axial orientation. The 3-ethyl isomers are nearly equal in activity.
further indicating that two or fewer carbons are more acceptable in the drug—receptor interaction.32 A small substituent. such as methyl. attached to the nitro-
gen had seemed to be optimal for analgesic activity. This was believed to be true not only for the meperidine series of compouncis but also for all the other types. It is now well established that replacement of the methyl by various aralkyl groups can increase activity A few examples of this type of compound in the meperidine series are shown in Table 22-3. The phenethyl derivative (A-9) is about 3 times as active as meperidine (A-I). The p-atnino congener. anileridine (A-tO), is about 4 times more active. Piminodine. the phenylaminopropyl derivative (A-Il). has 55 times the activity of meperidinc in rjts and is about 5 times as effective in humans as an The most active meperidine-type compounds, to date, are the propionoxy derivative (A- 12). which is nearly 2.000 limes as active as meperidine. and the N-phenethyl analogue of hctaprodine.
which is over 2,000 times as active as morphine.'t Diphenoxylate (A- 13). a structural hybrid of meperidinc and methadone types, lacks analgesic activity, although it reportedly suppresses the morphine abstinence syndrome in ntorphine addicts.35 It is quite effective as au intestinal spasmolytic and is used for the treatment of diarrhea (Lomotil). Several other derivatives of it have been studied.37 The related pchloro analogue. loperamide (A- 14), binds to opiate receptors in the brain but does not penetrate the blood—brain barrier enough to produce analgesia.1° Another way to modify the structure of uneperidine with
favorable results was to enlarge the piperidine ring to the seven.membered hexahydmszepine (or hexamethylenamine) ring. As was true in the piperidinc series, the most active compound is the one containing a methyl group on position 3 of the ring adjacent to the quatemary carbon atom in the propionoxy derivative, that is. l.3-dimethyl-4-phenyl-
738
Wilson and Gisvolds Textbook of Organic Medicinal and I'har,naceutical Clu'n:i.ctrs
4-propionoxyhexahydroaaepine, to which the name pm/repzazine was given. In the study by Eddy and coworkers, cited above, proheptazine was one of the more active analgesics included and had one of the highest addiction liabilities. The higher ring homologue of meperidine. ethoheptazine. was on the market. Although originally thought to be it is less active than codeine as an analgesic in humans but is free of addiction liability and has a low incidence of side effects.40 It is no longer available.
Contraction of the piperidine ring to the live-membered pyrrolidine ring was also successful. The lower ring homo-
methadone demonstrate greater activity than the correspond.
ing structural derivatives of isomethudone. In other words. the superiority of methadone over isomethadone seems to hold, even through the derivatives. Conversely, the methadone series of compounds was always more toxic than the isomethadone group. More extensive permutations. such as replacing the propiin H-2) by hydrogen. hydroxyl, or acetonyl. onyl group decreased activity. In a series of amide analogues of inetha done. Janssen and
synthesized racemorainide
logue of alphaprodine, prodilidine (A-l6), is an effective analgesic; a dose of 100 mg is equivalent to 30 mg of co-
(R-l2). which is more active than methadone. The (6) iso mer. dextromoramide (B-l3). is the active isomer and been marketed. A few of the other modilications that have
deine, but because of its potential abuse liability, it was never
been carried out and their effect on analgesic activity relatise
marketed." A more unusual modification of the meperidinc structure is found in fentanyl (A- 17). in which the phenyl and the ucyl groups are separated from the ring by a nitrogen. It is a powerful analgesic, 50 times stronger than morphine in hu-
to methadone are described in Table 22-4. which most of the methadone congeners that are or were on the market. Much deviation in structure front these will result in varying degrees of loss of activity. Particular attention should be paid to the two phenyl
mans. with minimal side effects.42 Its short duration of action makes it well suited for use in anesthesia.43 It is marketed
groups in methadone and the sharply decreased action result. ing from removal of one of them. It is believed that thc
for this purpose in combination with a neuroleptic. droperidol. The cic-(—)-3-methyl analogue with an ester group at
second phenyl residue helps to lock the —COC2H5 gronp of methadone in a position to simulate the alicyclic ring of morphine, even though the propionyl group is not particularly rigid. In this connection, however, the compound ssith a propionoxy group in place of the group (R in B-2) lacks significant analgesic In direct contra't with this is (6)-propoxyphene (B-l4), which isa propioinos} derivative with one of the phenyl groups replaced by a henzyl group. In addition, it is an analogue of isomethadone (84), making it an exception to the rule. This compound is lower than codeine in analgesic activity, possesses few siik Replacement effects, and has a limited addiction of the dimethylamino group in (6)-propoxyphene with a pyrrolidyl group gives a compound that is nearly three-founks
the 4 position, like meperidine (A- 18). was 8.400 times more potent than morphine as an analgesic. In addition, it has the
highest binding affinity to isolated opiate receptors of all compounds tested.35 Fentanyl and its 3-methyl and a-methyl
analogues have found their way into the illicit drug market and are sold as substitutes for heroin. Because of their extreme potency, they have caused many deaths. When the nitrogen ring of morphine is opened, as in the
formation of morphimethines. the analgesic activity is virtually abolished. On this basis, predicting whether a compound would or would not have activity without the nitrogen in a ring would favor a lack of activity or, at best, low activity. The first report indicating that this was a false assump-
tion was based on the initial work of Bockmuehl and Ehrin which they claimed that the type of compound represented by B-I in Table 22-4 possessed both analgesic and spasmolytic properties. The Hoechst laboratories in Ocr. many followed up this lead during World War II by preparing the ketones corresponding to these esters. Some of the compounds they prepared with high activity are represented
by formulas B-2 through B-7. Compound B-2 is the wellknown methadone. In the mcperidine and bemidone types.
the introduction of a ;n.hydroxyl group in the phenyl ring brought about slightly to markedly increased activity. whereas the same operation with the methadone-type compound markedly decreased action. Phenadoxone (B-8). the morpholine analogue of methadone, was marketed in England. The piperidine analogue. dipanone. was once under study in this country after successful results in England. Methadone was first brought to the attention of American pharmacists, chemists, and allied workers by the Kleiderer report45 and by the early reports of Scott and Chcn.4t' Since then, much work has been done on this compound. its isomer isomethadone, and related compounds. The report by Eddy, Touchberry. and Lieberman48 covers most of the
points concerning the structure—activity relationships of methadone. The levo isomer (B-3) of methadone (B-2) and the !ew isomer of isomethadone (B-4) are twice as effective as their racemic mixtures. Also, all structural derivatives of
as active as methadone and possesses morphine-like proper.
ties. The leco isomer of alphacetylmethadol (B-9), knosin as LAAM, has been marketed as a long-acting substitute lvi methadone in the treatment of addicts.53 MORPHINE MODIFICATIONS INITIATED BY GREWE Grewe. in 1946. approached the problem of synthetic analfe-
sics from another direction when he synthesized the racyclic compound that he first named morphan and then revised to N-methylmorphinan. The relationship nfthiscorn-
pound to morphine is obvious. 16
r—N — N —CH3
/4148 2 3
4
5
6
N.Methylmorphinan
N-Melhylmorphinan differs from the morphine lacking the ether bridge between C-4 and C-5. This corn pound possesses a high degree of analgesic activity, which suggests that the ether bridge is not essential. The derivative of N-methylniorphinan (racemorphan) was on tie
market and had an intensity and duration of action that ci in ceeded that of morphine. The original racemorphan
739
Chapter 22 • Analgesic
TABLE 22-4
Compounds Related to Methadone R3
A2
A1
Structure
Analgesic
Corn-
-
pound
B2
B-1
—C6H5
—C6H5
6-2
—C61-16
—C6H5
Same as ni B-2
8.4
'C5H5
'C6H5
—CH2CHN(CH3)2
—C6H5
Isomer. Salt
Activity (Methadone =1)
—
017
(j)4-ICI
1,0
tsomethadone
(±)-HCI
0.65
No;methadone
HC)
044
Dipanone
(±)-HCI
0.80
Methadone
CH3 Levanone
—C—C2H5
6 B-5
-
—COO—Aflcyl
o 8-3
Name
B4
R3
—CNCt-12N(CH3)2
19
CH3
—CH7CH2N(CH3)2
—C6H5
0 —C6H5
—C—C21-45
B-6
—C8H5
B-i
—C6H5
—C—C2H5
__CH2CHS4I)
Nexalgon
HBI
0.50
8-8
—C61-15
—C—C7H5
_CHO?HNO
Phenadoxone
(±)-l-iCl
14
Alphacetyfmethadol
a,(±)-HCI
13
$,(j)-HCI
23
NCI
025
Racemoranilde
(+1-Base
36
Dextromoramide
(+)-Base (+)-HCI
13
o 8-9
—C6H5
—C0H5
CH3
—CHC2H5
—CH2CI-IN(CH3)7
0 B-to
SameasinB-9
B11
C5H5
—C8N5
—COOC2H5
—CH2CH2NO
8-12
—C6115
—C6N5
—c—(]
—CI-ICH2N"'b
8-13
Same as ri B-12
8-14
—C6H5
—CHCH2N(CH3)2
—CH2C5H5
0
Propoxyphene
0.21
CH3
lroducc4l as the hydrobrontide and was the (dl). or form as obtained by synthesis. Realizing that thc levorotatory form of racemorphan was the analgesically active parlion of the racetnate, the manufacturer resolved the (dl) form and marketed the levy form as the tartrate sail (levorphanol). The dexiro form has tbund use as a cough suppressant (see
dextromethorphan). The ethers and acylated derivatives of the 3-hydroxyl form also cxhibit considerable activity. The 2- and 4-hydroxyl isomers are, not unexpectedly, without value as analgesics. Likewise, the N-ethyl derivative lacks activity, and the N-allyl compound, levallorphan. is a potent morphine antagonist.
740
WjLcgn,
and Gi.cvolds Texthook of Organic Medicinal and P1,armaceuiical
Eddy et al.54 reported on an extensive series of N-aralkylmorphinan derivatives. The effect of the N-aralkyl substitution was more dramatic in this series than it was
for morphine or nieperidine. The N-phencthyl and N-paminophenethyl analogues of levorphanol are about 3 and 18 times more active, respectively, than the parent compound in
analgesic activity in mice. The most potent member of the series was the N-$-Iurylethyl analogue, which was nearly 30 times as active as lcvorphanol or 16() times as active as morphine. The N-acetophenone analogue. levophenacylmor-
phan. was once under clinical investigation. In mice, it
is
about 30 times more active than morphine, and in humans a 2-mg dose is equivalent to 10mg of morphine in its analgesic It has a much lower physical-dependence liability than morphine.
The N-cyclopropylmethyl derivative of 3-hydroxymor-
withdrawal symptoms. Further differences in properties are found between the enantiomers of the s'is isomer. The +j isomer has weak analgesic activity, but a high physical-dc. pendence capacity. The 1—i isomer is a stronger analgesic. without the dependence capacity, and possesses antagonistic
activity.55 The same is trttc of the 5.9-diethyl and 9.eihyl. 5-phenyl derivatives. The i—I trans-5.9-diethyl isomer is similar, except it has no antagonistic properties. This demon. strates that it is possible to divorce analgesic activity
rable to that of morphine from addiction potential. Thai Nmethyl compounds have shown antagonistic properties is
of great interest as well. The most potent of these is ik benzomorphan with an a-methyl and gmup at position 9. The — isomer shows greater antagonistic activity than naloxone and is still 3 times more potent titan morphine as an
phinan (cyclorphan) was reported to be a potent morphine antagonist, capable of precipitating morphine withdrawal symptoms in addicted monkeys, indicating that it is nonaddieting.21 Clinical studies have indicated that it is about 20
CH,
times stronger than morphine as an analgesic, but it has some
undesirable side effects, primarily hallucinatory. The Ncyclobutyl derivative. butorphanol. possesses mixed agonist—antagonist properties, however, and has been marketed as a potent analgesic.
a isornel
los)
1(
SOIT1OI (toilS)
Since removal of the ether bridge and all the peripheral groups in the alicyclic ring in morphine did not destroy its analgesic action. May et synthesized a series of compounds in which the alicyclic ring was replaced by one or two methyl groups. These are known as heuzomorphan de'
An extensive series of the antagonist-type analgesics in the benzomorphans was Of these. pentaiixinc
rivanjres or. more correctly. henzazoeines. They may be rep-
of morphine, with a lower incidence of side effects.5' Its addiction liability is much lower, approximating that of pro-
resented by the following formula:
(II. R1 = CH5. R. = CH2CH = and cyclazocitte (II, R1 = R2 CH2—cyclopropyl have been the most interesting. Pentazocine has about half the analgesic activity
poxyphenc.62 It is curretntly available in parenteral and tablet form. Cyclazocine is a strong morphine antagonist. showinf
about 10 times the analgesic activity of
3,
HO'2
2
II
1
The trimethyl compound (II. R1 = R2 = CH3) is about 3 times more potent than the dimethyl (II, R1 = H, R. = CH2). The N-phenethyl derivatives have almost 20 times the
analgesic activity of the corresponding N-methyl compounds. Again, the more potent was the one containing the
two ring methyls(ll, R1 = CH5. R, = Deracemization proved the levo isomer of this compound to be more active, being about 20 times as potent as morphine in
mice, The (±} form. phenazocine. was on the market but was removed in favor of penla/.ocine. May ci al.°7 demonstrated an extremely significant difference between the two isomeric N-methyl beniomorphans in
which the alkyl in the 5 position is n-propyl (R1) and the alkyl in the 9 position is methyl (R2). These have been termed the a and the
It
investigated as an analgesic and for the treatment of heroin addiction, but it was never marketed because of hallucina-
4.
isomer and have the groups oriented
as indicated. The isomer with the alkyl cis to the phenyl possesses analgesic activity (in mice) equal to that of morphine but has little or no capacity to suppress withdrawal symptoms in addicted monkeys. On the other hand, the trans isomer has one of the highest analgesic potencics among the henzomorphans. but it is quite able to suppress morphine
tory side effects. As mentioned above, replacement of the N-methyl giver in morphine by larger alkyl groups lowered analgesic acthity. In addition, these compounds counteracted the effect ci morphine and other morphine.like analgesics and are tha' known as narcotic antagonists. The reversal of in. creases from ethyl, to propyl. to allyl. with the cyclopropyl methyl usually being maximal. This property was true not only for morphine but also for other analgesics. N-Allylnormorphine (nalorphine) was the first of these but hccausc of side effects was taken off the market. Levullorphan. bc corresponding allyl analogue of levorphanol. naloxone .\allyinoroxymnorphonel. and naltrexone oxymorphone) are the three narcotic antagonists now on market. Naloxone and naltrexone appear to be pure
fists with no morphine- or nalorphine-like
Thcyals
block the effects of other antagonists. These drugs are to prevent, diminish, or abolish many of the actions or tiv side effects encountered with the narcotic analgesics. Soire of these are respiratory and circulatory depression. euphoria. nausea, drowsiness, analgesia. and hyperglycemia. They as thought to act by competing with the analgesic moleenk for attachment at its, or a closely related, receptor site. fle observation that some narcotic antagonists that lack adds lion liability are also strong analgesics spurred considerahk
Chapter 22 • Analgesic Agents ipnerest
in them.2° The N-cyclopropylmethyl compounds
mentioned are the most potent antagonists but appear to produce psychotoinimetic effects and may not be useful as analgesics. One of these. buprenorphine. has shown an interest-
741
been questioned.19 Additional studies indicate that dcalkylation does occur in the brain, although its exact role is not clear.80 It is clear, from the N-aralkyl derivatives discussed above, that a small group is not necessary.
ing study profile, however, and has been introduced in
Several exceptions to the second feature have been synthe-
Europe and in the United States as a potent a treatment for narcotic addicts in a novel treatment program. Office Based Opioid Treatment OBOT). provided by private physicians rather than formal
sized. In these series, the central carbon atom has been replaced by a tertiary nitrogen. They are related to methadone and have the following structures:
0
tteatment clinics.65
Other efforts have been under way to develop narcotic antagonists that can be used to treat narcotic The continuous administration of an antagonist will block the euphoric effects of heroin, thereby aiding rehabilitation olan addicl. The cyclopropylmethyl derivative of naloxone. naltrexone, has been marketed for this purpose. The oral dose of 100 to 150 mg 3 times a week suffices to block several usual doses of heroin."1 Long-acting preparations ssece once also under
II
C—CH2CH3
Q-_N
CR3
CH2CHN CH3 It'
Much research, other than that described above, has been eanied out by the systematic dissection of morphine to give several interesting fragments. These approaches have not yet produced important analgesics and so are not discussed in
this chapter. The interested reader may find a key to this literature, however, in the excellent reviews of Eddy,9 Bergel and Morrison,23 and Lee.3°
Structura-Acthvlty Reladonsldps Several reviews on the relationship between chemical structune and analgesic action have been
Only
the major conclusions are considered here, and the reader is urged to consult these reviews for more complete discussion of the subject.
From the time Small et al. started their studies on the rorphine nucleus to the present. there has been much light shed on the structural features connected with morphinelike analgesic action. In u very thorough study made for the United Nations Commission on Narcotics in 1955, Braendcn ct found that the features possessed by all known morphine-like analgesics were as follows: I
/s tertiajy nitrogen, with the group on the nitrogen being relatively small
1 A central carbon atom, of which none of the valences is conmeted with hydrogen A phenyl group or a group isosteric with phenyl. which is connected to the central carbon atom
A Iwo-carbon chain separating the central carbon atom from the nitrogen for maximal activity
The above discussion shows that several exceptions to these generalizations exist in the structures of compounds hat have been synthesized in the past several years. Eddy33
has discussed the more significant exceptions. Relative to the feature mentioned, extensive studies of the action of normorphine have shown that it possesses analgesic activthat approximates that of morphine. In humans, it is about
one fourth as active as morphine when administered intramuscularly. but it was slightly superior to morphine when intracistcrnnlly. On the basis of the last-mentioned effect. Beckett et al.7° postulated that N-dealkylation a step in the mechanism of analgesic action. This has
0C2H5
CH2CH2N(C2H5)2 'V
Diampromide (Ill) and its related unilides have potencies that are comparable with those of morphine:5t they have shown addiction liability, however, and have not appeared on the market. The closely related cyclic derivative fentanyl (Table 22-3. A-17) is used in surgery. The bcnzimidazoles. such as elonitazene (IV). are very potent analgesics hut show
the highest addiction liabilities yet encountered.°2 Possibly an exception to feature 3. and the only one that has been encountered. may be the cyclohexyl analogue of
A-6 (Table 22-3). which has significant activity. mentions two possible exceptions to feature 4 in addition to fenlanyl. The many studies on molecules of varying types that possess analgesic activity revealed that activity was associated
not only with certain structural features but also with the size and the shape of the molecule. The hypothesis of Becketi and Casy84 has dominated thinking for several years in the area of stereochemical specificity of these molecules. They initially noted that the more active enantiomers of the methadone- and thiambutenc-type analgesics were related vonllgu-
rationally to (R)-alanine. This suggested to them that a stereo.selective fit at a receptor could be involved in analgesic activity. To depict the dimensions of an analgesic receptor. they selected morphine (because of its scmirigidity and high activity) to provide them with information on a compkmentar)' receptor. The features thought to be essential for proper receptor fit were I. A basic center able to associate with an anionic site on the receptor surface 2. A flat aromatic structure, coplanar with the basic center, allowing van der Waals bonding to a flat surface on the receptor site to reinforce the ionic bond 3. A suitably positioned projecting hydrocarbon moiety forming a three-dimensional geometric pattern with the basic center and the flat aromatic structure
742
Wilson
and Gi.cvold'.s Tcxthook of Oryanii Mc'dicinul and
These features were selected, among other reasons, because they arc present in N-methylmorphinan, which may be looked on u.s a "stripped down" morphine (i.e.. morphine without the characteristic peripheral groups lexcept for the
CH3
N
/
Mionic silo ... Approximately
H
H
75-85A OH
t
basic center I). Since N-mcthylmorphinan pos.sessed substan-
tial activity of the morphine type, these three features were considered the fundamental ones determining activity, and the peripheral groups of morphine were considered to essentially modulate the activity.
In accord with the foregoing postulates. Beckett and Casy84 proposed a complementary receptor site (Fig. 22-I) and suggested in which the known active mole-
at
piperidine ring ot moqisne
ot
OH — ).Morphine
Flat surtace or aromatic ring
cules could be adapted to it. Alter their initial postulates. natural (—)-rnorphine was shown to be related configurationally to methadone and thiambutene. which lent weight to the hypothesis. Fundamental to their proposal was that such a receptor was essentially inflexible and that a lock-and-keytype situation existed. Subsequently, the unnatural ( + )-morphinc was synthesized and shown to be inactive.87 Although the foregoing hypothesis appeared to fit the facts
quite well and was a useful hypothesis for several years. it now appears that certain anomalies exist that cannot be accommodated by it. For example. the more active enantiomcr of a-mcthadol is not related conligurationally to (R)alanine. in contrast with the methadone and thiambutenc series. This is also true for the carbethoxy analogue of metha-
charge
Figure 22—1 • Diagram of the surface of the analgesic receptor site with the corresponding lower surface of the drug molecule. The three-dimensional features of the molecule are shown by the bonds: —. - - -. and — —, which represent in front of, behind, and in the plane of the paper, respectively. (From Gout' ley. D. R. H.: Prog Drug Res. 7:36, 1964.1
the hypothesis can be applied to the methadol anomaly is illustrated in Figure 22-2. After considering activity changes in various struciur,d types (i.e.. methadones, meperidines. prodioes) us related to
the identity of the N-substituent, Portoghese noted that in
done (VI and for diampmrnidc (Ill) and its analogues. An-
certain series, when identical changes in N-substituents were
other factor that was implicit in considering a proper receptor fit for the morphine molecule and its congeners was that the phenyl ring at the 4 position of the piperidine moiety should
whereas in others there appeared to he nonparallelism. Fle has interpreted parallelism and nonparallelism.
be in the axial orientation for maximum activity. The fact that structure VI has only an equatorial phenyl group, yet possesses activity equal to that of morphine, would seem to cast doubt on the necessity for axial orientation u.s a receptor-
fit requirement.
,COOC2H5
made, there was parallelism in the direction of as being due to similar and dissimilar modes of binding. Viewed by this hypothesis, although analgesic molecules must still be bound in a fairly precise manner, the concegi of binding is liberalized, in that a response may be obtained by two different molecules binding stereoselectively in iso different precise modes at the same receptor. A schematic representation of such different possible binding modes is shown in Figure 22-3. This representation will aid in izing the meaning of similar and dissimilar binding modes.
II two different analgesiophores (the analgesic molecule
/=\ CH2CH—N
\__i V
HNMe2
In view of the difficulty of accepting Beckett and Casy's hypothesis u.s a complete picture of analgesic—receptor interaction. has offered an alternative hypothesis. This hypothesis is based, in part, on the established abil-
H
NMe, Me
ity of enzymes and other types of macromolecules to undergo conformationul 92 on interaction with small molecules (substrates or drugs). The fact that configurationally unrelated analgesics can bind and exert activity is interpreted as meaning that more than one mode of binding may be possible at the same receptor. Such different modes of binding may be due to differences in positional or conformational interactions with the receptor. The manner in which
H''
e
f6R)
Figure 22—2 • Illustration
(6S) of how different polar groupo
analgesic molecules may cause inversion in the selectivity of an analgesic receptor. A hydrogen-bondrn moiety, denoted by X and Y, represents a site that is capabe
of being hydrogen bonded.
Chapter 22 • V
/
—
/
\\
\
743
also no useful drugs developed to date that are selective for 8 receptors. Another highly important development in structure—activity correlations is the development of highly active analgesics from the N-allyl-type derivative.s that were once thought
%-
/
Analgesic Agents
/
to be only morphine antagonists and devoid of analgesic
\
\
properties. Serendipity played a major role in this discovery: Lasagna and in attempting to find some "ideal" ratio of antagonist (N-allylnormorphine. nalorphine to anal-
gesic (morphine) to maintain the desirable effects of morphine while minimizing the undesirable ones, discovered that nalorphine was. milligram fbr milligram. as potent an
• Schematic illustration of two different molecuprotonated nitrogen. ar modes of binding to a receptor. 0, an N-substituent, The anionic Sites lie directly beneath the Figure 22—3
pronated nitrogen.
minus the N-substituent. i.e., that portion of the molecule that gives the characteristic analgesic response) bearing identical N-substituents are positioned on the receptor surface such that the N-substi;uent occupies essentially the same position. a similar pharmacological response may be anticipated. Thus, as one proceeds from one N-subsnitueni to another, the response should likewise change, resulting in parallelism of effect. On the other hand, if two different analgesiophores are bound to the receptor such that the Nsubstituents are not arranged identically, one may anticipate nonidentical responses to changing the N-substituent (i.e.. a nonparallel response). The preceding statements, as well as the diagram. do not imply that the unalgesiophorc necessar-
analgesic as morphine. Unfortunately. nalorphine has depersonalizing and psychotomimetic properties that preclude its use clinically as a pain reliever. The discovery led, however, to the development of related derivatives such as pentazocine and cyclazocine. Pentazocine has achieved some success in
providing an analgesic with low addiction potential, although it is not totally free of some of the other side effects of morphine. The pattern of activity in these and other Nallyl and N.cyclopropylmethyl derivatives indicates that most potent antagonists possess psychotomimetic activity. whereas the weak antagonists do not, It is from this latter group that useful analgesics, such as pentazocine. hutorphanot, and nalbuphine. have been found. The latter two possess
N-cyclobutylmethyl groups. What structural reatures arc associated with antagonistlike activity is uncertain. The N-allyl and dimethylallyl substituent does not always confer antagonist properties. This is true in the meperidinc and thevinol series. Demonstration of antagonist-like properties by specific isomers of N-methyl benzomorphans has raised still further speculation. The exact mechanisms by which morphine and the narcotic antagonists act are not clear, and much research is presently being carried
aith high K receptor affinity have high analgesic activity.
on. Published reviews and symposia may be consulted for further discussions of these A further problem also is demonstrated in the testing for analgesic activity. The analgesic activity of the antagonists was not apparent from animal testing: it was observed only in humans. Screening in animals can be used to assess the antagonistic action, which indirectly indicates possible analgesic properties in humans. It has been customary in the area of analgesic agents to attribute differences in their activities to structurally related differences in their receptor interactions. This rather universal practice continues in spite of early warnings and recent findings. It now appears clear that much of the difference in relative analgesic potencies can be accounted for on the basis of pharmacokinetic or distribution properties."8 For example. a definite correlation was found between the partition coefficients and the intravenous analgesic data for 17 agents of widely varying structures."5' Usual test methods do not help distinguish which structural features are related to receptors and which to distribution phenomena. Studies directed toward distinction have used the measurement of actual brain and plasma or direct injection into the ventricular area." the measurement of ionization potentials and partition coefficients.'°4 and the application of mo07 lecular orbital theories and quantum mechanics." These are providing valuable insight into designing of new
lxk many of the opioid side effects, but have not been useful of dysphoric and hallucinogenic effects. There are
and more successful agents. All of the foregoing work had strongly suggested the exis-
il) will be bound in the identical position within a series. They do suggest. however, that in series with parallel activi-
lies, the pairs being compared will be bound identically to
pmduce the parallel effect. Interestingly, when binding modes are similar. Portoghese has been able to demonstrate
the existence of a linear free-energy relationship. There is also the possibility that more than one receptor is involved. Considerable evidence now demonstrates that multiple receptors exist. Martin has characterized and named these by (mu) receptors for marresponses o probe molecules:
phine-specific effects. 8 (delta) for cyclazocine. and K lkappa for ketocyclazocine.93 A different designation for these receptors, based on pharmacological criteria, is 0P3, OP), and 0P2. respectively. Various combinations of these in different tissues could be responsible for the varying efIect.c
This discovery has stimulated much research into the search for drugs that have selectivity for single receptors. The natural opiates and related synthetic opioids have prereceptor agonist activity, with morphine and dominantly minor analogues showing 10 to 20 times the selectivity over the other receptors. Other analgesic drugs have shown even higher ratios. l'he antagonists show lower selectivity. Drugs
744
of Organic Medicinal and Phannaceutical Chemistry
Wil.ron arid
tence of specific binding sites or receptors in brain and other tissue. The demonstration of the high steric and structural specificity in the action of the opiates and their antagonists led many investigators to search for such receptors.'08 Thus iii 1971. Goldstein and coworkers demonstrated stereo-
specific binding in brain homogenates)'° This was quickly lollowed by refinements and further discoveries by Simon. Terenius. and Pert and Snyder)''''' These receptor-binding studies have now become a routine assay for examining structure—activity relationships. In addition to the binding studies, considerable attention
continued on the use of in vitro models, in particular the isolated guinea pig ileum. rat jejunum. and mouse vas deferens.' '' While working with these preparations. Hughes was the first to discover the existence of an endogcnous factor from pig brains that possessed opiate-like properties.' 'f"
This factor, given the name enkephalin. consisted of two pentapeptides. called methonine-. or mezenkephalin, and leacine-. or It'uenkephalin. These two enkephalins have subse-
quently been shown Lo exist in all animals, including humans. and to posses.s all morphine-like properties.'""9 They exist as segments of a pituitary hormone, the 91 -amino acid $-lipotropin, which is cleaved selectively to release spe-
cific segments that have now been fiund to have functions within the body. Thus, segment 61 to 65 is metenkephalin. 61 to 76 is a-cndorphin. 61 to 77 is y.endorphin, and, probably the most important. 61 to 91 is fl-endorphin. 5
10
30
15
lished an extensive
of narcotics oF current interest in
the drug trade. This listing is much more extensive than the following monographs covering compounds primarily of interest to American pharmacists.
Morphine.
Morphine was isolated first in 1803 by I).
rosne, but the credit for isolation generally goes to SertUrncr (1803). who tirst called attention to the basic properties of this alkaloid. Morphine, incidentally, was the first plant base isolated and recognized as such. Although intensive eseaieh was carried out on the structure of morphine, it was in 1925
that Gulland and Robinson'20 postulated the currently cepted formula. The total synthesis of morphine finally effected by Gates and Tschudi'21 in 1952. confirming the Gulland and Robinson formula. Morphine is obtained only from the opium poppy, Papaver somnifr rum, either from opium. the resin obtained by lancing the unripe pod, or from poppy straw. The latter psr cess is favored, as it helps to eliminate illicit opium from which heroin is readily produced. Morphine occurs in opium in amounts varying from 5 to 20% (LISP requires not less than 9.5%). It is isolated by various methods, but the final
step is usually the precipitation of morphine from an acid solution by use of excess ammonia. The precipitared phine then is recrystallized from boiling alcohol. The free alkaloid occurs as levorotatory. odorless. white, needle-like crystals possessing a bitter taste. It is almost in soluble in water (1:5,000. 1:1.100 at the boiling poinu. ether (1:6,250). or chloroform (1:1,220). It is somewhat more el.
uble in ethyl alcohol (1:210. 1:98 at boiling point). (Note that in this chapter. a solubility of 1:5.000 indicates that
I
I
I
I
g is soluble in 5,000 mL of the solvent at 25°C. Solubililiesat other temperatures are so indicated.) Because of the phenolic hydroxyl group, it is readily soluble in solutions of alkali ot
alkaline earth metal hydroxides. Morphine is a monoacidic base and readily forms waler•
I
p.Endocpl5rr
I
80
'°Thr4eu-Phe-Lys-Asn 85
91
fi.Lipotropln. p.Endorphin, Methlonlne—Enkephalin Retatlonships
The last of these endorphins (short for endogenous morphine) has 20 times the analgesic potency of morphine when injected into rat brain. These substances can also produce tolerance and dependence.
Clearly, all of these techniques will lead to new concepts and understanding of the processes of analgesia, tolerance. and dependence. It is hoped that learning how these mecha-
soluble salts with most acids. Thus, because morphine itsc)1 is so poorly soluble in water, the salts are the preferred Iorrri for most uses. Numerous salts have been marketed, bum the ones in use are principally the sulfate and, to a lesser eSmeni
the hydrochloride. Many writers have pointed out the "indispensable muture of morphine, based on its potent analgesic propenics toward all types of pain. It is properly termed a narcotic analgesic. Because it causes addiction so readily. howeser. it should be used only when other pain-relieving drugs inadequate. It controls pain caused by serious injury. neoplasms. migraine, pleurisy. biliary and renal colic, and nu merous other conditions. It is often administered as a erative sedative, together with atropine to control secmlitno, With scopolamine, it is given to obtain the so-called twiligla sleep. This effect is used in obstetrics. but care is needed prevent respiratory depression in the fetus. The toxic ties of morphine are much more evident in young and olJ people.
nisms operate will aid in the design and development of better analgesics.
Morphine Hydrochloride.
Products In General Circular No. 253. March 10. 1960. the Treasury
may be prepared by neutralizing a hot aqueous suspension of morphine with diluted hydrochloric acid and then concen trating the resultant solution to crystallization. It is no
Department. Bureau of Narcotics, Washington. DC, pub-
commercially available. It occurs as silky, white,
Morphine
hydrochlomidc
Chapter 22 • Anal gesu Aç'eiitv needles, as cubical masses. or as a white crystalline powder.
The hydrochloride is soluble in water (1:17.5. 1:0.5 at boiling point), alcohol (1:52, 1:46 at 60°). or glycerin, but it is practically insoluble in ether or chloroform. Solutions have a p11 of approximately 4.7 and may be sterilized by boiling. Its uses are the same as those of morphine. The usual oral and subcutaneous dose is IS mg every 4 hours as needed. with a suggested range of 8 to 20 mg.
Morphine Sulfate, USP.
Morphine sulfate is prepared in the same manneras the hydrochloride (i.e.. by neutralizing
morphine with diluted sulfuric acid). It occurs as feathery. silky. white crystals, as cubical masses of crystals, or as a white crystalline powder. Although it is a fairly stable salt. ii loses water of hydration and darkens on exposure to air and light. It is soluble in water (1:16. 1:1 at 80°). poorly soluble in alcohol (1:570. 1:240 at 60°). and insoluble in chloroform or ether. Aqueous solutions have a pH of about 4.8 and may be sterilized by heating in an autoclave. This common morphine salt is used widely in England. and now in the United States, by oral administration for the management of pain in cancer patients. It has largely replaced Brompton's mixture or cocktail, a combination of heroin and cocaine in chloroform water. In the United States. this preparation has become mistakenly popular. substituting
morphine sulfate for the heroin. Moreover, Twycross has advised that the stimulant cocaine is contraindicatcd because it interferes with sleep,'23 and its original use was for cough
in tuberculosis patients. Morphine sulfate is available in a variety of dosage forms: tablets, oral solution, parenteral. suppositories, and controlled-release tablets (MS Contin)
745
of aspirin and codeine act additively as analgesics. however. thus giving some support to the common practice of combining the two drugs. Codeine has a reputation as an antitussive. depressing the cough reflex, and is used in many cough preparations. It is one of the most widely used morphine-like analgesics. It is considerably less addicting than morphine, and in the usual doses, respiratory depression is negligible, although an oral dose of 60 rng causes such depression in a normal person. Much of codeine's reputation as an anjitussive probably rests on subjective impressions rather than on objective studies. The average 5-mL dose contains 10 mg of codeine. Several
cough preparations containing codeine are available, with some that may be sold over-the-counter as exempt narcotic preparations. Abuse or misuse of these preparations., however. has led many states to place them on prescription-only status.
Codeine Phosphate, USP. Codeine phosphate may be prepared by neutralizing codeine with phosphoric acid and precipitating the salt from aqueous solution with alcohol. Codeine phosphate occurs as line, needle-shaped. white crystals or as a white crystalline powder. It is efflorcscent and is sensitive to light. It is freely soluble in water (1:2.5, 1:0.5 at 80°) but less soluble in alcohol (1:325. 1:125 at boiling point). Solutions may be sterilized by boiling. Because of its high solubility in water, compared with the sul-
fate. this salt is used widely. It is often the only salt of codeine stocked by pharmacies and is dispensed. rightly or wrongly, on all prescriptions calling for either the sulfate or the phosphate.
and capsules (Kadian).
Codeine Sulfate, USP.
Codeine is an alkaloid that occurs naturally in opium. but the amount present is usually too small to he of commercial importance. Consequently, most cornnienial codeine is prepared from morphine by methylating the phenolic hydroxyl group. The methylation methods make use of reagents such as diazomethane. dirnethyl sulfate, and methyl iodide. Newer methods are based on its synthesis from thebaine. which makes it possible to use Papas'erbracCodeine, USP.
wogulit as a natural source (see above).
It occlmrs as levorotatory. colorless. eftiorescent crystals aras a white crystalline powder. It is light sensitive. Codeine is slightly soluble in water (1:120) and sparingly soluble in
ether (1:50). It is freely soluble in alcohol (1:2) and very coluble in chloroform (1:0.5). Codeine is a monoacidic base and readily forms salts with acids, with the most important being the sulfate and the phosphate. The acetate and methylbromide derivatives have been used to a limited extent in eotgh preparations.
The general pharmacological action of codeine is similar to that of morphine. but as indicated above, it does not possess the same analgesic potency. Studies indicate that a dose
ut 30 to 120 tag of codeine is considerably less efficient parenterally than tO mg of morphine, and the usual side effects of morphine—respiratory depression, constipation. nausea, and such—occur. Codeine is less effective orally than parenterally. and Houde and Wallensteintaa stated that a dose of 32 mg of codeine is about as effective as 650 mg
of aspirin in relieving terminal cancer pain. Combinations
Codeine sulfate is prepared by neutralizing an aqueous suspension of codeine with diluted sulfuric acid and then effecting crystallization. It occurs as white crystals, usually needle-like, or as a white crystalline powder. The salt is efflorescent and light sensitive. It is soluble in water (1:30. 1:6.5 at 80°), much less soluble in alcohol (1:1,280). and insoluble in ether or chloroform. This salt of codeine is prescribed frequently but is not u.s suitable as the phosphate for liquid preparations. Solutions of the sulfate and the phosphate are incompatible with alkaloidal reagents and alkaline substances.
Diacetylmorphine Hydrochloride.
Although diacetylmorphinc hydrochloride, heroin hydrochloride. diamorphine hydrochloride, heroin, is 2 to 3 times more potent than morphine as an analgesic, its sale and use are prohibited in the United States because of its intense addiction liability. It is available in some European countries, where it has a limited use as an antitussive and as an analgesic in terminal cancer patients. Because of its superior solubility over morphine sulfate, arguments have been raised for its availability. The other more potent analgesics described here have, however, significant advantages in being more stable and longer
acting. It remains one of the most widely used narcotics for illicit purposes and places major economic burdens on society.
Hydromorphone.
Hydrornorphone. dihydromorphinone, a synthetic derivative of morphine, is prepared by the
746
Wi1.ci,,z
e:itd Gi.svold'a 7cxd,ook
of
Mndieinui and
catalytic hydrogenation and dehydrogenation of morphine under acidic conditions, using a large excess of platinum or palladium. The tree base is similar in properties to morphine. being slightly soluble in water, freely soluble in alcohol, and
very soluble in chloroform.
This compound, of German origin, was introduced in 1926. It is a substitute for morphine (5 times as potent) but has approximately equal addicting properties and a shorter duration of action. It possesses the advantage over morphine of giving less daytime sedation or drowsiness. It is a potent antitussive and is often used for coughs that are difficult to control.
CFu'n,islri'
plastic diseases, and other types of pain that respond to phine. Because of the risk of addiction, it should not be used for relief of minor pains that can be controlled with codeine
It has poor antitussive activity and is not used as a cough suppressant. It may be administered orally. parenterally intravenously. intramuscularly, or subcutaneously), or rectally. and for these purposes is supplied as a solution for injection (1.0 and 1.5 mg/mL) and in suppositories (5 mg).
Nalbuphine Hydrochloride.
Nalbuphinc
hydrocklo.
ride. N-cyclobutylmethyl- l4-hydroxy-N-nordihydromorphi. none hydrochloride (Nubain). morphone hydrochloride, was introduced in 1979 as a poles
Hydromorphone Hydrothloride,
USP.
Hydromor-
phone hydrochloride, dihydromorphinone hydrochloride (Dilaudid). occurs as alight-sensitive, white crystalline powder that is freely soluble in water (1:3). sparingly soluble in alcohol, and practically insoluble in ether. It is used in about one-lifth the dose of morphine for any of the indications of morphine. It is available in tablet, liquid. parenteral. and suppository dosage forms. The dose is I to 8 mg.
Hydrocodone Bitartrate, USP.
Hydrocodone bitar-
trate. dihydrocodeinone hitarirate (Dicodid. Codone), is prepared by the catalytic rearrangement of codeine or by hydrolyzing dihydrothebaine. It occurs as line, white crystals
or as a white crystalline powder. It is soluble in water (I: 16), slightly soluble in alcohol, and insoluble in ether. It forms acidic solutions and is affected by light. The hydrochloride is also available. Hydrocodone has a pharmacological action midway between those of codeine and morphine, with IS mg being equivalent to 10 mg of morphine in analgesic power. Although it possesses more addiction liability than codeine, it reportedly gives no evidence of dependence or addiction with long-term use. Its principal advantage is in the lower frequency of side effects encountered with its use. It is more effective than codeine as an antiuissive and is used primarily for this purpose. It is on the market in many cough prepara-
tions, as well as in tablet and parenteral forms. It has also been marketed in an ion-exchange resin complex form under the trade name olTussionex. The complex releases the drug
at a sustained rate and is said to produce effective cough suppression over a 10- to 12-hour period. Hydrocodone is also marketed in combination with acetaminophen (e.g.. Hydrocet, Vicodin. Lortab. and Zydone) and with homatropine extensive use in antia.s Hycodan. Although this drug tussive formulations br many years. it has been placed under more stringent narcotic regulations.
Oxymorphone Hydrochloride. USP.
Oxymorphone hydrochloride. (—)- I 4-hydroxydihydromorphinone hydrochloride (Numorphan). introduced in 1959. is prepared by cleavage of the corresponding codeine derivative. It is used as the hydrochloride salt, which occurs as -a white crystalline powder freely soluble in water and sparingly soluble in alcohol. In humans. oxymorphone is as effective as morphine in one-eighth to one-tenth the dosage, with good duration and
analgesic of the agonist—antagonist type, with little to a abuse liability. It is a somewhat less potent analgesic than its parent oxymorphone but shares some of the antagonist properties of the closely related, hut pure, antagonists one and naltrexonc. Nalbuphine hydrochloride occurs
a
white to off-white crystal line posvder that is soluble in saner and sparingly soluble in alcohol. It is prepared from cycloini-
tylmethyl bromide and noroxycodone followed by of the 0-methyl group. This analgesic shows a very rapid onset with a of action of up tuô hours. It has relatively low abuse judged to be less than that of codeine and propoxyphene. Thc
injection is. therefore, available without narcotic although caution is urged for long-term administration use in emotionally disturbed patients. Abrupt discoriiinua tion after prolonged use has given rise to withdrawal sign; Usual doses cause respiratory depression comparable to flu?
of morphine, but no further decrease is seen with lnighar doses. It has fewer cardiac effects than pentazocine and (ru torphanol. The most frequent adverse effect is sedation, and as with most other CNS depressants and analgesics. caution should be urged when it is administered to ambulatory patients who may need to drive a car or operate machinery
Nalbuphine is marketed as an injectable (10 and 20 mL). The usual dose is 10mg administered intramuscularly, or intravenously at 3- to 6-hour intervak with a maximal daily dose of 160 mg.
Oxycodone hydrnirk' Oxycodone Hydrochloride. ride. dihydrohydroxycodeinone hydrochloride, is by the catalytic reduction of hydroxycodeinonc prepared hydrogen peroxide (in acetic acid) oxidation of thehainc
This derivative of morphine occurs as a white crystallis powder that is soluble in water (1:10) or alcohol. solutions may be sterilized by boiling. Although this dra; is almost as likely to cause addiction as morphine, it issoli in the United States in Percodan and several other product in combination with aspirin or acetaminophen. It is used as a sedative, an analgesic, and a nareolic. Ti depress the cough reflex, it is used in 3- to 5-mg doses as an analgesic in 5- to ID-mg doses. For severe pain.; dose of 20 mg is given subcutaneously. Oxycodone is aI' marketed as controlled-release tablets (OxyContin, ((Ito mg) for use in managing chronic pain. Unfortunately. 1kv high-dose preparations have become popular with drug and addicts, who crush the tablets, dissolve the contents,
a slightly lower frequency of side eflècts)25 It has high ad-
inject the mixture. Several overdose deaths have
diction liability. Itis used for the same purpose.s as morphine. such as control of postoperative pain, pain of advanced nco-
from this practice. leading to tighter controls over theirascil
ability.
Chapter 22 • Analgesic
747
Dihydrocodeine is obtained by the reduction of codeine. The bitartrate salt occurs as white crystals that are soluble in water (1:4.5) and only slightly soluble in alcohol. Subcutaneously. a dose of 30 mg of this drug is almost equivalent to 10 mg of morphine a.s an analgesic, with faster onset and negligible side effects. It
It is light sensitive and turns green on exposure to air and light. It is sparingly soluble in water (1:50, 1:20 at 80°) and
has addiction liability. It is available in combination with aspirin or acetaminophcn for pain.
pressant effects of morphine are much less pronounced, and the stimulant effects are enhanced greatly, thereby producing
Normorphine. Normorphine may be prepared by N-dcmethylation of morphine)26 In humans, by normal routes
cmesis by a purely central mechanism. It is administered subcutaneously to obtain emesis. It is ineffective orally. Apomorphine is one of the most effective, prompt (10 to
Dihydrocodeine Bitartrate.
in alcohol (1:50) and is very slightly soluble in ether or chloroform. Solutions arc neutral to litmus. The change in structure from morphine to apomorphinc profoundly changes its physiological action. The central de-
of administration, it is about one fourth as active as morphine in producing analgesia but has a much lower physical dependence capacity. Its analgesic effects arc nearly equal by the mtr,iventricular route. It does not show the sedative effects of morphine in single doses but does so cumulatively. Norm-
15 minutes), and safe emetics in use today. Care should be exercised in its use, however, because it may be depressant in already-depressed patients. It is currently classified as an
orphine suppresses the morphine abstinence syndrome in a slow onset and a addicts, hut mild form of the abstinence syndrome)27 It was once considered for possible use in the treatment of narcotic addiction, but it has no current use,
Meperidine Hydrochloride, USP. Meperidine hydrochloride, ethyl I -methyl-4-phenylisonipecotate hydrochloride, ethyl I -methyl-4-phenyl-4-pipendinecarboxylate hydrochloride (Demerol Hydrochloride), is a fine, white. odorless crystalline powder that is very soluble in water.
An extract of opium, containing a mixture of the total alkaloids of opium, is available as an alcoholic Opium.
aqueous solution, It is available as Deodorized Opium Tincture (15 mg/mL of morphine base) and Paregoric (2 mglmL of morphine base). These are used primarily for the treatment of diarrhea.
Tramadol Hydrochloride.
Traniadol hydrochloride. ± )-cis-2-I (dimethylamino)methyll- I -(m-methoxyphcnyl) cyclohexanol hydrochloride (Ultrain), represents a fragment of codeine's structure, consisting of the phenyl and cyclohexane rings. The drug possesses opioid activity but has other analgesic activity that is not reversed by naloxone. The plincipal effect is attributed to the 0-demethylated metabolice, which is 6 times more potent than the parent compound. an observation consistent with the differences between codeine and morphine. It produces significantly lower morphine-like side effects. It is available in tablet form for use in moderate-to-severe pain in a dose of 50 to 100mg every 4 1o6 hours and in combination (37.5 mg) with acetaminophen (Ultracet) for short-term managcment of acute pain.
When morphine under pressure or morphine hydrochloride is heated at with strong (35%) hydrochloric acid, it loses a molecule of water and yields a compound known as apomorphine.
Apomorphine Hydrochloride, (iSP.
"orphan drug" for use in Parkinson's disease.
soluble in alcohol, and sparingly soluble in ether. It is stable in the air at ordinary temperature. and its aqueous solution
is not decomposed by a short period of boiling. The free base may be made by heating bcnzyl cyanide with bis(flchloroethyl)methylamine. hydrolyzing to the corresponding acid and exterifying the latter with ethyl alcohol.2 Meperidine first was synthesized to study its spasmolytic character, but it was found to have far greater analgesic properties. The spa.smolysis is primarily due to a direct papaverme-like depression of smooth muscle and, also, to some action on parasympathetic nerve endings. In therapeutic doses. it exerts an analgesic effect that lies between those of mor-
phine and codeine, but it shows little tendency toward hypnosis, It is indicated for the relief of pain in most patients for whom morphine and other alkaloids of opium generally are used, but it is especially valuable when the pain is due to spastic conditions of intestine, uterus, bladder, bronchi. and so on. Its most important use seems to be in lessening the severity of labor pains in obstetrics and, with barbiturates or tranquilizers, producing amnesia in labor. In labor, a dose of 100mg is injected intramuscularly as soon as contractions occur regularly, and a second dose may be given after 30 minutes if labor is rapid or if the cervix is thin and dilated to 3 cm). A third dose may be necessary an hour or two later, and at this stage a barbiturate may be administered in a small dose to ensure adequate amnesia for several hours.
Meperidine possesses addiction liability. Psychic dependence develops in individuals who experience euphoria lasting for an hour or more. The development of tolerance has been observed, and meperidine can be substituted successfully for morphine in addicts who are being treated by grad-
ual withdrawal. Furthermore, mild withdrawal symptoms Morptsne
HCI
have been noted in certain persons who have become pur-
pressure and heal
posely addicted to meperidine. The possibility of dependence is great enough to put it under the federal narcotic HO
OH
laws. It is available in oral liquid, tablet, and parenteral dosage forms.
Apomorphine
The hydrochloride is odorless and occurs as minute. gliswhite or grayish white crystals or as a white powder.
Alphaprocline hyAiphaprodine Hydrochloride, USP. drochloride. 1±)- 1,3-dimethyl-4-phcnyl-4-piperidinol pro-
748
WiLson and Gisrold's Terthook
of Orgaiiie Me'djcjnul and Pharmaceutisa! C'/w,nisir%
panoate hydrochloride, is prepared by the method of Ziering and
occurs as a white crystalline powder that is
freely soluble in water, alcohol, and chloroform but insoluble in ether. The compound is an effective analgesic. similar to meperidine. and of special value in obstetric analgesia. It appears to be quite safe for usc in this capacity. causing little or no respiratory depression in either mother or fetus. It is currently not marketed in the United States.
Anileridine, USP.
Anileridine. ethyl I -(p-aminophenethyl)-4-phenylisonipecotate (Leritine). is prepared by the method of Weijlard Ct al.1 It occurs as a white to yellowishwhite crystalline powder that is freely soluble in alcohol hut only very slightly soluble in water. It is oxidized on exposure to air and light. The injection is prepared by dissolving the free base in phosphoric acid solution. Anileridine is more active than meperidine and has the same usefulness and limitations. Its dependence capacity is less, and it is considered
a suitable substitute for meperidine. It is currently not marketed in the United States.
Loperamide Hydrochloride, USP.
Loperamide liydrw
chloride. 4-(4-chlorophcnyl )-4-hydroxy-N.N-dimethyl-a.o. diphenyl- I -piperidinebutanamide. 4-(4-p-chloroplienyl4 hydroxypipcridino)-N.N-dimethyl-2.2-diphenylbutyramidc hydrochloride (Imodium), a hybrid of a meperidine molecule, is closely related to diphenoxylate but is more specific, more potent. and longer acting. It acts a
direct effect on the circular and longitu.
dinal intestinal muscles. After oral administration it peak blood levels within 4 hours and has a very long plasm
half-life (40 hours), Tolerance to its
has not
observed,13' Although it has shown minimal CNS effects. it has been controlled under Schedule V. Loperamide is avail-able as 2-mg capsules (Loperamide hydrochloride capsules. USP) for treatment of acute and chronic diarrhea. Rec.
ommended dosage is 4 mg initially, with 2 tug after ca.h stool for a maximum of 16 mg/day.
Ethoheptazlne Citrate.
Anileridine hydro-
Ethoheptazine citrate. ethyl hexahydro- I -methyl4-phenyl- I H-azepine-4-carboxylatc at. I -methyl-4-carbethoxy-4-phenylhexanucthylenimine rare. citrate (Zaclane Citrate), is effective orally against moderac
chloride, ethyl I -(p-anhinophenethyl)-4-phenylisonipecolate
pain in doses of 50 to 1(X) mg. with minimal side effeu\
Anlieridine Hydrochloride. USP.
dihydrochioride (Leritine Hydrochloride), is prepared as cited for anileridine. except that it is converted to the dihydrochloride by conventional procedures. It occurs as a white or nearly white, crystalline odorless powder that is stable in air. It is freely soluble in water, sparingly soluble in alcohol.
and practically insoluble in ether and chloroform. l'his salt has the same activity as anileridine. It is currently not marketed in the United States.
Dlphenoxylate Hydrochloride, USP.
Diphenoxylate hydrochloride, ethyl I -(3-cyano-3.3-diphenylpropyl)-4phcnylisonipecotate tnonohydrochlortde (Lomotil. Lonox.
Logen. Lomanate). occurs as a white, odorless, slightly water-soluble powder with no distinguishing taste. Although this drug has a strong structural relationship to the meperidine-type analgesics, it has very little, if any, such activity
itself. Its most pronounced activity is its ability to inhibit excessive gastrointestinal motility, an activity reminiscent of the constipating side effect of morphine itself. Investigators have demonstrated the possibility of addiction.536 particularly with large doses, but virtually all studies using ordinary dosage levels show nonaddiction. Its safety is reflected in its classification as an exempt narcotic, with the warning, however, that it may be habit forming. To discourage possible abuse of the drug, the commercial product (Lomotil) once contained a subtherapeutic dose (25 .eg) of atropine sulfate in each 2.5-mg tablet and in each 5 ml of the liquid. which contains a like amount of the drug. It is indicated in the oral treatment of diarrhea resulting from a variety of causes. The usual initial adult dose is 5 mg 3 or 4 times a day, with the maintenance dose usually substantially lower and individually determined. Appropriate dosage schedules for children are available in the manufacturer's literature. The incidence of side effects is low, hut the drug should
Parenteral administration is limited because of central slims
lating effects. It appears to have no addiction liability. hut toxic reactions have occurred with large doses. A douhk. blind study in humans rated 100mg of the hydrochloride sail equivalent to 30mg of codeine and fuund that the additionci
6(X) mg of aspirin increased analgesic effectiveness
In
another study, a dose of 150 mg was found equal to 65 mc
of propoxyphene. with both better than It once available as a 75-mg tablet and in combination with 60() mg of aspirin (Zactirin).
Fentanyl Citrate, USP.
Fentanyl citrate. N-t I-plies ethyl-4-piperidyl)propionanilide citrate (Sublimaze. occuas as a crystalline powder soluble in water (1:40) and and sparingly soluble in chloroform. This novel anilide tIe rivative has analgesic activity 50 times that of morphine in humans.42 It has a very rapid onset 4 mm) and short duraior of action, Side effects similar to those of other potent analge
sics are common, particularly respiratory depression an adjunct to anesthesia
For use as a neuroleptanalgesic in surgery. it is available ir
combination with the neuroleptic droperidol (Innovarl. Iii' also available as a tranadennal release system at total dose levels ranging from 2010 100mg) for manage
ment of chronic pain. It has dependence liability.
Alfentanil Hydrochloride.
Alfentanil
N-f I - I 2-(4-ethyl-5-oxo.2-tetrazolio- I -yl )-ethyl I-4-Imcth
oxymethyl)-4-piperidyljpropionanilide hydrochloride ohydrate (Alfcnta). is closely related to fentanyl. It is apart analgesic used as a primary anesthetic or as an adjunct in the maintenance of anesthesia. It has the same and side effects an. fentanyl. It is available as an injectir
(0.5 mgimL).
be used with caution, if at all, in patients with impaired hepatic function. Similarly, patients taking barbiturates concurrently with the drug should be observed carefully, in view of reports of barbiturate toxicity under these circumstances.
Remifentanil Hydrochloride.
Remifentanil
hydni.
chloride. 4-carboxy-4.(N-phenylpropionamido)-l dineproprionic acid tuethyl ester hydrochloride (Ultivat.
Chapter 22 • Analgesic Agenis
lydro-
structural analogue of fenlanyl. has similar properties and is also used in anesthesia, It is available as an injection (I mgi
nyl-4-
roLl.
ngitu-
Sufentanil Citrate.
Sufentanil citrate. N-14-(methoxymethyl)- I - I 2-(2-thienyl )ethyl 1-4-piperidylipropionanilide citrate (Sufenta). a structural analogue of fentanyl. has simitar properties and is also used in anesthesia. It is available as an injection (0.05 inglmL).
sIasma
been
ffects.
ide is e capReceach
ethyl citmime derate
fk'cts. 5timtl—
y, hut ,)uhlede salt
iOn iii
,.°" lit S5 tug
It was
i with
Methadone Hydrochloride, USP.
Methadone hydrochloride. 6-(Dimethylamino)-4,4-diphenyl-3-heptanonc hydrochioride (Dolophine Hydrochloride). occurs as a bitter. white crystalline powder. It is soluble in water, freely soluble in alcohol and chloroform, and insoluble in ether. Methadone
is synthesized in several ways. The method of Easton et aL'-° is noteworthy, in that it avoids the formation of the troublesome isomeric intermediate The analgesic effect and other morphine-like properties are exhibited chiefly by the (—) form. Aqueous solutions are stable and may be sterilized by heat for intramuscular and intravesims use. Like all amine salts, it is incompatible with alkali and salts of heavy metals. It is somewhat irritating when injected subcutaneously.
The toxicity of methadone is 3 to 10 times that of morphine, hut its analgesic effect is twice that of morphine and tO limes that of meperidine. It has been placed under federal narcotic control because of its high addiction liability. Mcthstone is a most effective analgesic, used to alleviate many type.s of pain. It can replace morphine for the relief of withdrawal symptoms. It produces less sedation and narcosis than morphine and appears to have fewer side reactions in
patients. Methadone is especially valuable in spa.sm of the urinary bladder and in the suppression of the phen-
cough reflex..
xxurs
The levu isomer, levanone, reportedly does not produce
thanol de de-
euphoria or other morphine-like sensations and has been adsocated For the treatment of addicts.'34 Methadone itself is uscd quite extensively in addict treatment, although not with-
inc in ration nalge-
o and hcsia.
ible in t.
It is
gesie. tnage-
out some controversy.'35 It suppresses withdrawal effects and is widely used to maintain former heroin addicts during his rehabilitation. Large doses are often used to "block"
ally, an 80- to 100-mg dose 3 times a week suffices for routine maintenance.53'
The FDA has approved the drug
for use, and it is marketed as ORLAAM in solution form (10 mglmL). By law, it can be dispensed only by treatment programs certified by the Department of Health and Human Services' Substance Abuse and Mental Health Services Administration and registered with the Drug Enforcement Administration. It also carries a warning about possible serious cardiac arrhythmia. The racemate of the normetabolite, nora-
cylmethadol, was once studied in the clinic as a potential analgesic.1
Propoxyphene Hydrochloride, USP.
Propoxyphcne hydrochloride, (2S,3R)-( + )-4(dimethylamino)-3-nrethylI ,2-diphenyl-2-bulanol propanoate hydrochloride (Darvon. Dolene. Doxaphene), was introduced into therapy in 1957.
It may he prepared by the method of Pohland and Sullivaiu.'4° It occurs as a bitter, white crystalline powder that is freely soluble in water, soluble in alcohol, chloroform, and acetone, hut practically insoluble in benzene and ether. It is the a-( +) isomer; the a-(—) isomer and diastereoiso,ners are far less potent in analgesic activity. The a-(—) isomer, levo-propoxyphene, is an effective antitussive (see below).
In analgesic potency, propoxyphene is approximately equal to codeine phosphate and has a lower frequency of side effects. It has no antidiarrheal. antitussive. or antipyretic effect, thus differing from most analgesic agents. It can suppress the morphine abstinence syndrome in addicts but has shown a low level of abuse because of its toxicity. It is not very effective in deep pain and appears to be no more effective in minor pain than aspirin. Its widespread use in dental pain seems justified, since aspirin is reported to be relatively ineffective, In has been classified as a narcotic and controlled under federal law. It does give some euphoria in high doses and has been abused. It has been responsible for numerous
overdosage deaths. Refilling the prescription should be
turn (FDA) regulations that require special registration of and dispensers. Methadone is available for use
Wygesic).
us an analgesic, however, under the usual narcotic require-
Propoxyphene Napsylate, USP. Propoxyphene napsylate, (+ )'a-4-dimethylamino-3-methyl- I .2-diphenyl-2butanol propanoate (ester) 2-naphthylenesulfonate (salt) (Darvon-N). is very slightly soluble in water, but soluble in alcohol, chloroform, and acetone. The napsylate salt of
Levomethadyl Acetate Hydrochloride.
Lcvomethadyl acetate hydrochloride. /-o-acetylmethadol. (-)-a-6-(dirrelhylamino)-4,4-diphenyl-3-heptyl acetate hydrochloride.
net in tel-ties
us prepared by hydride reduction of (+ )-methadone fol'
ection
owed by acctylalion. Of the four possible methadol isomers.
Va). a
as an addict-maintenance drug to replace methadone. Gener-
avoided if misuse is suspected. It is available in several com-
meihadyl acetate. LAAM. occurs as a white crystalline powthat is soluble in water hut dissolves with some difficulty.
iydro' uperi-
inconveniences the maintenance patient and leads to illicit diversion, the long-acting LAAM was actively investigated
he effects of heroin during treatment. The use of methadone in treating addicts is subject to Food and Drug Administra-
ments.
oride. methmoopotent
This is further accentuated by its demethylation to the dinor metabolite. which has similar Because of the need no administer methadone daily, which
mide cc and ite hut lets as
749
(3S.6S) isomer LAAM has the unique characteristic of long-lasting narcotic effects. Extensive metabolism studies have shown that this is due to its N-dcmenhyliion to give (—)-a-acetylnormethadol. which is more potent than its parent. LAAM. and possesses a long
bination products with aspirin or acetaminophen (e.g..
propoxyphene was introduced shortly before the patent on
Darvon expired. The insoluble salt form is claimed to be less prone to abuse because it cannot be readily dissolved for injection and, on oral administration, gives a slower, less pronounced peak blood level. Because of its mild narcotic-like properties. it was once investigated as an addict-maintenance drug to be used in place of methadone. It was hoped that it would provide easier withdrawal and serve as an addict-detoxification drug. Unfortunately, toxicity at higher doses has limited this applica-
750
WiLson and GLsi'ulds Textbook of Organic Medici,,al and Pharmaceutical Chemistry
tion. It is available in combination with acetaminophen (Propacet. Darvocet-N).
and on the heart workload. It should thus be used with tion and only with patients hypersensitive to morphine Ion the treatment of myocardiul infarction or other cardiac prob-
Levorphanol Tartrate, USP.
lems. Other adverse effects include a high incidence of sethtion and, less frequently. nausea, headache. vertigo, and diz. ziness.'4' It is available as a parenteral for intramuscular and intmve-
Grewe made the basic studies in the synthesis of compounds of the type of levorphanol tartrate. (—).3-hydroxy-N-methylmorphman bitartrate (Levo-Dromoran). as mentioned above. Schnider and Griussner synthesized the hydroxyniorphinans. including the 3-hydroxyl derivative, by similar methods. The racemic 3-hydroxy-N-methylmorphinan hydrobromide (racemorphan. (± )-Dromoran) was the original form in which this potent analgesic was introduced. This drug is prepared by resolution of racemorphan. The levo compound is available in Europe under the original name. Dromoran. As the tarmite. it occurs in the harm of colorless crystals. The salt is sparingly soluble in Water (1:60) and is insoluble in ether. The drug is used for the relief of severe pain and is in many respects similar in its actions to morphine, except that it is 6(08 times as potent. The addiction liability of levorphanot is as great as that of morphine and, for that reason, caution should be observed in its use. It is claimed that the gastrointestinal effects of this compound are significantly lower than those experienced with morphine. Naloxone is an effective antidote for overdosage. Levorphanol is useful for relieving severe pain originating from a multiplicity of causes (e.g.. inoperable tumors, severe trauma, renal colic, biliary colic). In other words, it has the same range of usefulness as morphine and is considered an excellent substitute.
nous administration in a dose of
I
or 2 mg every 310 4
hours, with a maximal single dose of 4 mg. It is also available
as a nasal spray (Stadol NS.
Buprenorphlne Hydrochloride.
Buprenorphine h.drochloride. 21 -cyclopropyl-7a-I(S)-l -hydroxy- I methylpropyl -6,14 - endo-ethano- 6.7.8.14- telrahydroori. pavine hydrochloride (Buprenex), is a rapidly acting. ceo. trally acting analgesic in the agonist—antagonists cla.sa. It it
about 30 times more potent than morphine. It is available for treating moderate-to-severe pain as a parenteral for into muscular or intravenous administration in a dose of 0.3 every 6 hours. After several years of investigation Ibr use in treating opt. oid addiction, buprenorphine, alone or in combination asob naloxone. has been approved for use in a highly regulated
treatment program called Office Based Opw:d (OBOT). Under federal legislation. the Substance Abase aral
Mental Health Administration's Center for Drug Abute
(2 mg). The drug requires a narcotic form.
Treatment has established criteria and training programs Ion private physicians to administer these special dosage to opioid addicts. The intent of the program is to make Iazment available to persons who arc not likely to seek treat-
Butorphanol Tartrate, USP.
ment in traditional treatment clinics, such as executives, and the like.
It is supplied in ampuls. in multidose vials, and as oral tablets
Butorphanol tartrate, 17(cyclobutyl-methyl)morphinan.3. 14-diol o-(—)-tartrate, (—)bitartrate N.cyclobutylmethyl-3. 14-dihydroxymorphinan (Stadol). a potent analgesic, occurs as a white crystalline powder soluble in water and sparingly soluble in alcohol. It is prepared from the dihydroxy-N-normorphinan obtained by a modification of the Grewe synthesis. It is the cyclobutyl analogue of levorphanol and levallorphan. and is as potent an analgesic as the former and a somewhat less active antagonist than the latter.
—
Dezoclne.
Dezocine. (—)- I 3fl-amino-5.6,7.8.9, 10.1 lii. 12-octahydro-5a-methyl-5. II -methanobenzocyclodeeen.3silt ol (Dalgan). is a synthetic agonist—antagonist
an unusual structure. It is similar to morphine in potency and duration. It produces fewer side effects, due hi its antagonist activity, with reported minimal dependence capacity. It is available for treating moderate-to-severe ptin as a parenteral for intramuscular or intravenous administra-
tion in a dose of 5 to 20 mg every 3 to 6 hours.
Nalbuphine Hydrochloride.
Butorphanol
The onset and duration of action of the drug are comparable to those of morphine, but it has the advantages of showing a maximal ceiling effect on respiratory depression and
Nalbuphine hydroelila-
I 7-(cyclobutylmethyl)-4.5a-epoxymorphinan-3.6i ride. I4-triol hydrochloride (Nubain). isa combination of tlleoxy morphine nucleus and the nitrogen substiluent of buconpha nol. It is a potent analgesic of the agonist—antagonist clast. similar to morphine in potency but with an abuse rated less than that of codeine. It is useful in treating moder ate-to-severe pain and is available for parenteral use usual dose of 10 mg/70 kg every 3 to 6 hours.
a greatly reduced abuse liability. The injeclable form was marketed without narcotic controls; this product was consid-
Pentazocine, USP.
ered for placement in Schedule IV. however, because of
dro-cis-6. II -dimethyl-3-( 3-methyl-2-butenyl).2.6.meth ano3-benzazocin-8-oI. hydroxy.6,7-benzomorphan (Talwin). occurs as a crystalline powder that is insoluble in water and sparingly soluble in alcohol. It forms a poorly soluble hydroehtkwrdu salt, but is readily soluble as the lactate.
reported misuse and lack of recognition of its potential abuse
liability. The drug has also been used illegally for doping racehorses.
Butorphanol shares the adverse hemodynamic effects of pentazocine, causing increased pressure in specific arteries
Pentazocine.
I .2,3.4.5.6.he'ab)-
Chapter 22 • Analgesit Agenis Pentazocine in a parenteral dose of 30 mg or an oral dose of 50mg is about as effective as 10mg of morphine in most patients. There is some evidence that the analgesic action resides principally in the I-) isomer, and a dose of 25 mg is approximately equivalent to 10mg of morphine sulfate. Occasionally, doses of 40 to 60mg may be Pentaz-
plasma half-life is about 3.5 hours.'4 At the lower dosage levels, it appears to be well tolerated, although some
sedation occurs in about one third of persons receiving k. The incidence of other morphine-like side effects is as high as with morphine and other narcotic analgesics. In patients who have been receiving other narcotic analgesics, large doses of pentazocine may precipitate withdrawal symptoms. It shows an equivalent or greater respiratory depressant acüvity. Pentazocine has given rise to a few cases of possible dependence. It has been placed under control, and Its abuse potential should be recognized and close supervision of its use maintained. Lcvallorphan cannot reverse its effects, although naloxone can, and mcthylphcnidatc is recommended as
an antidote for overdosage or excessive respiratory
depression.
Pentazocine as the lactate is available in injection form containing the base equivalent of 30 rnglmL, buffered to pH 4 to 5. It should not he mixed with barbiturates. Tablets of 50 mg (as the hydrochloride in combination with naloronc to prevent abuse) are available for oral administration. It is also available in combination with aspirin (TaIwin Compound) and with acetaminophen (Talacen).
751
of undesirable psychotic effects. Because of these properties
and the availability of alternate antagonists, it was withdrawn from the market.
Levallorphan Tartrate, (iSP.
Levallorphan tartratc. I 7-(2-propenyl )-morphinan-3-oI tartrate, (—).N-allyl-3'hy-
droxymorphinan bitartrate (Lothin). occurs as a white or practically white, odorless crystalline powder. It is soluble in water (1:20). sparingly soluble in alcohol (1:60). and prac-
tically insoluble in chloroform and ether. Lcvallorphan resembles nalorphine in its pharmacological action and is about 5 times more effective as a narcotic antagonist. It was useful in combination with analgesics such as meperidine. alphaprodine, and levorphanol to prevent the respiratory depression usually associated with these drugs. It is no longer marketed.
Narcotic Antagonists Naloxone Hydrochloride, (iSP.
Naloxone hydrochloride. 4.5-epoxy-3. 14-dihydroxy. I 7.(2-propenyl)morphinan6-one hydrochloride. N-allyl-l4-hydroxynordihydromorphinone hydrochloride (Narcan). N-allylnoroxymorphone hydrochloride, is presently on the market as the agent of choice for treating narcotic overdosage. It lacks not only the analge-
sic activity shown by other antagonists, but also all of the other agonist effects. It is almost 7 times more active than nalorphine in antagonizing the effects of morphine. It shows
no withdrawal effects after long-term administration. The
Methotrimeprazine, (iSP.
Methotrimeprazine. (—)l0-13-(dimethylarnino)-2-melhylpropyl 1-2.mcthoxyphcnothiazine (Levoprome). a phenothiazine derivative closely related to chlorpromazine. possesses strong analgesic activity. An intramuscular dose of IS to 20 mg is equal to 10 mg of morphine in humans. It has not shown any dependence liability and appears not to produce respiratory depression. The
most frequent side effects are sintilar to those of phenothiacme tranquilizers, namely, sedation and orthostatic hypotCnslon. These often result in diziiness and fainting, limiting the use of methotrimeprazine to nonambulatory patients.. It should be used with caution along with antihypertensives. atmpine, and other sedatives. It shows some advantage in patients for whom addiction and respiratory depression are problems. j4.t
Narcotic Antagonists Nalorphine Hydrochloride, (iSP. Nalorphine hydrochloride. N-allylnormorphine hydrochloride. may be prepared according to the method of Weijlard and
It ts.'curs in the form of white or practically white crystals that slowly darken on exposure to air anel light. It is freely 'oluble in water I :K). sparingly soluble in alcohol (1:35). and almost insoluble in chloroform and ether. The phenolic hydroxyl group confers water soluhility in the presence of fond alkali. Aqueous solutions of the salt are acid, with a
duration of action is about 4 hours. It was briefly investigated for the treatment of heroin addiction. With adequate doses
of naloxonc. the addict does not receive any effect from heroin. It is given to an addict only after a detoxification period. Its long-term usefulness is currently limited because its short duration of action requires large oral doses. Longacting. alternative antagonists are available (Table 22-5).
cyclazocine.
Cyclazocine. 3-(cyclopropylmcthyl). 1.2,3, 4,5,6-hexahydro-6. II -dimethyl-2.6-methano-3-benzazocin8-ol, ei.s-2-cyclopmpylmethyl-S.9-dimethyl-2'-hydroxy-6.7-
benzomorphan. is a potent narcotic antagonist that has shown analgesic activity in humans in I-mg doses. It once was investigated as a clinical analgesic. It does possess hallu-
cinogenic side effects at higher doses, which limited its usefulness as an analgesic. It was studied like naloxone in the
treatment of narcotic addiction. Voluntary treatment with cyclazocine deprives addicts of the cuphorogenic effects of heroin. Its dependence liability is lower, and the effècLs of withdrawal develop more slowly and are milder. Tolerance
develops to the side effects of cyclazocine. but not to its antagonist effects.t47 The effects are long lasting and are not reversed by other antagonists such as nalorphine. It has not been marketed.
pH of about 5.
Naltrexone. Naltrexone. I 7-(cyclopropylmethyl)-4.5 a-epoxy-3. l4-dihydroxymorphinan-6-one. N-cyclopropyl-
Nalorphine has a direct antagonistic effect against mormeperidine. methadone, and levorphanol. It has little antagonistic effect toward barbiturate or general anesthetic however. It has strong analgesic properties, hut it is not acceptable for such use owing to the high incidence
methyl- 14-hydroxynordihydromorphinone. N-cycloprupylmethylnoroxynmorphone (ReVia. Depade), a naloxone analogue. has been marketed as the preferred agent for treating former opiate addicts. Oral doses of 51) nag daily or 100 mg 3 times weekly suffice to "block" or protect a patient 1mm
752
Wi/cries and (_iLccald'.c Textbook of Organic Medicinal and Phar,nare'utical Che,nLi:rs
TABLE 22—5
Narcotic Antagonists Usual Dose Range
Name
Proprietary Name Levahorphan tarlrate, NE Loden Natoxone hydrochloride, USP Narcan
Preparations Levalloiphan lantrate injection. NF
Naloxone hydrochloride injection, USP
Usual Adult Dose IV,
ring, repeated twice at 10. to 15-minute Intervats, it necessary 1
repeated at 2- to 3-minute intervals, as necessary
Parenteral. 400
pharmacokithe cft'ects of heroin. Its metabolism,t45 and pharmacology's' have been studied intensely because of the tremendous governmental interest in develop-
ing new agents for the treatment of addiction.6' It is available as 50-mg tablets (RcVia) for use in treating narcotic addiction. it has also shown promise for suppressing
craving in the treatment of alcoholism and is available for that use. Sustained-release or depot dosage forms of naltrexone were once investigated to avoid the recurrent decision on the part of the former addict of whether a protecting dose of antagonist is
'"
Nalmefene Hydrochloride. Nalmefenc hydrochloride, I 7-(cyclopropylmcthyl )-4,5 a-epoxy-6-methylcnemorphinan-3, 14-diol hydrochloride (Revex). is the 6-methylcne an-
alogue of naloxone. it is the latest pure antagonist to be introduced for use in reversing the effects of opioid agonists. It is longer acting than naltrexone and is used for the same indications, It is available as an injection (0.1 and 1.0 mg/mL).
mg. repeated, if necessary
500
Usual Pediatric Dose
0.05-0.1 mg in n000ates to decrease
respiratory
001 mg as above
ties of the narcotic agents. Some of these act in a simila, manner through a central effect. In a hypothesis for the initia tion of the cough reflex. Salem and Aviado'55 proposed titsi
bronchodilatation is an important mechanism for the rcltcf of cough. Their hypothesis suggests that irritation of tht mucosa initially causes bronchoconstriction that in turn cccites the cough receptors. Chappel and von Secmann'56 have pointed out that titrot antitussives of this type fall into two structural groups. Thr larger group has structures that bear a resemblance to radIo-
done. The other group has large, bulky substituents on hr acid portion of an ester, usually connected by means of long, ether-containing chain to a terliwy amino group. Thr notable exceptions are benzonatate and sodium dibunatc Noscapinc could be considered as belonging to the fiN group. Many of the cough preparations sold contain variousolhrr ingredients in addition to the primary untitussive agent. Th: more important ones include antihistamines. useful whcnik cause of the cough is allergic, although some antihistaminL drugs (e.g., diphenhydramine) have a central antitussive ac-
Others. Several other narcotic antagonists have been invc.stigated (e.g., diprenorphine'53 and oxilorphan).'
ANTITUSSIVE AGENTS Cough is a protective, physiological reflex that occurs in health as well as in disease. It is very widespread and cornmonly ignored as a mild symptom. In many conditions, however. it is desirable to take measures to reduce excessive
coughing. Many etiological factors cause this reflex, and when a cough has been present for an extended period or accompanies any unusual symptoms, the person should be referred o a physician. Cough preparations are widely advertised and often sold indiscriminately: the pharmacist must warn the public of the inherent dangers.
tion us well: sympathomimetics. which are quite effectinc
owing to their bronchodilatory activity, the most usefifi being ephedrine, methamphetamine. plienylpropanolamire. homurylarnine, isoproterenol. and isooctylamine; parasyni patholytics. which help to dry secretions in the upper respim tory passages; and expectorants. It is not known ii their drugs potentiate the antitussive action, but they usually arc considered adjuvant therapy. The more important drugs this class are discussed in the follosving section. For nan: exhaustive coverage of the field. the readcr is urged to con
suit the excellent review of Chappel and von
Products Some of the narcotic antitussivc products are discus'd above with the narcotic analgesics. Others arc discass'd below (Table 22-6).
Atnong the agents used in the symptomatic control of cough are those that act by depressing the cough center located in the medulla. These have been termed anodvnes. rough suppressants, and cent rally acting until ussi yes. Until recently, the only effective drugs in this area were narcotic analgesic agents. The more important and widely used ones are morphine, hydromorphone. codeine. hydrocodone. methadone. and levorphanol, which are discussed above. In recent years, several compounds have been synthesized that possess antitussive activity without the addiction liabili-
Noscapine, USP.
Noscapine. (— )-narcotine pine), an opium alkaloid, was isolated in 1817 by Rohiquci It is isolated rather easily from the drug by ether estraciks it makes up 0.75 to 9% of opium. With the discover) of lb unique antitussive properties, the name of this alkaloid changed from narcotine to noscapine. It was realized that would not meet with widespread acceptance as long as name was associated with the narcotic opium 1-
a precc&m!
Chapter 22 • TABLE 22-6
Analgesic Agents
753
Antltussive Agents Name
Proprietary Name
Preparations
Dextromethorphan hydrobromicte. USP
Dextromethorphan
Usual Adult Dose
Usual Dose Range
5-30mg ad to qici
qomlar Levopropoxyphene napsytate. USP Noviad
Levopropoxyptiene napsylate capsules, USP Levopropoxyphene napsylate oral suspension, USP
50- tOO mg ol evcpropoxypheno. as the napsylate, every 4 hours
Benzonalate, USP
Bertzonatate capsules, USP
100 019 hd
100-200mg
Tessajon
existed in the name of ( ± )-narcotinc. namely. g?wscopine. Ii was once available in vanous cough preparations.
a low frequency of side effects. It is available as a pediatric suspension (2.5 mg/5 mL) or as capsules (20 mg. Cophenc-
X). and in various combinations with antihistamines and
Dextromethorphan Hydrobromide, ineihorphan
hydrobrornide.
Dextro(+ )-3-mcthoxy- 17-methylUSP.
9a,l3a,14a.morphinan hydrobromide (Romilar), is the 0nethylated (+ )-form of racemorphan left after the resolution necessary in the preparation of levorphanol. Ii occurs practically white crystals or as a crystalline powder, with a faint odor. It is sparingly soluble in water (1:65). freely soluble in alcohol and chloroform, and insoluble in ether. It possesses the antitussive properties of codeine, without the analgesic, addictive, central depressant, and constipating features. Ten milligrams is suggested as equivalent to a 15mg dose of codeine in antitussive effect. It affords an opportunity to note the specificity exhibited by very closely related molecules. Here, the (+ ) and (—) forms both must attach to receptors responsible for the suppression of the cough reflex.
but the (+) form is apparently in a stcric relationship that peecludes attaching to the receptors involved in analgesic. constipative. addictive, and other actions exhibited by the
-I form. Ii has largely replaced many older antitussives. including codeine, in prescription and nonprescription cough preparations.
Benzonatate, USP. l3cnionatatc. 2,5,S,l 1.14.17.20.23, 16.nonaoxaoctacosan-28-yl p-(butylamino)benzoate (Tes-
olon). introduced in 1956. is a pale-yellow, viscous liquid insoluble in water and soluble in most organic solvents. It chemically related to p.aminobenzoate local anesthetics. ncept that the aminoalcohol group has been replaced by a methylated polyethylene glycol group. Beneonalate reportedly possesses both peripheral and central activity in producing its antitussive effect. It somehow Wocks the stretch receptors thought to be responsible for rough. Clinically, it is not as elTective a.s codeine, hut it peeduces far fewer side effects and has very low toxicity. It
L,asailahle in 100-mg capsules ("perks"). Carbetapentane Citrate. diethylaniino)-ethoxylethyl
Carbetapentane citrate, 2-12I -phenylcyclopentanecarboxcitrate, is a white, odorless crystalline powder that is reely soluble in waler (1:1), slightly soluble in alcohol, and insoluble in ether, It is reportedly equivalent to codeine a.s elantitussive. Introduced in 1956. it is well tolerated and has
decongestants. The tannate is also available (Rynatuss) as a 60-mg tablet and is said to give a more sustained action.
ANTI-INFLAMMATORY ANALGESICS The early growth of the anti-inflammatory analgesic group was related closely to the belief that lowering or "curing" fever was an end in itself. Drugs that induced a drop in temperature in feverish conditions were considered quite valuable and were sought eagerly. The decline of interest in these drugs coincided more or less with the realization that fever was an outward symptom of some other, more fundamental ailment. During the use of the several antipyretics, however, some were noted to be excellent analgesics for the relief of minor aches and pains. These drugs have survived to the present time on the basis of the analgesic, rather than the antipyretic, effect. Although these drugs are still widely used for the alleviation of minor aches and pains, they are also used extensively in the symptomatic treatment of rheu-
matic fever, rheumatoid arthritis (RA). and osteoarthritis (OA). The dramatic effect of salicylates in reducing the inflammatory effects of rheumatic fever is time honored, and even with the development of the corticosteroids, these drugs are still of great value in this respect. The steroids are reportedly no more effective than the salicylates in preventing the cardiac complications of rheumatic fever.1°7 The analgesic drugs that fall into this category have been disclaimed by some as not deserving the term analgesic because of their low activity in comparison with the morphinetype compounds. Indeed, Fourneau has suggested the name antalgics to designate this general category and, in this way. to emphasize the distinction from the narcotic or so-called true analgesics. Two of the principal features distinguishing these analgesics from the narcotic analgesics are the low activity for a given dose (which is not increased significantly at a higher dosage) and the lack of addiction potential. Research has intensified in an effort to find new nonsteroidal anti.inflanumatory drugs (NSAIDs). Long-term therapy with the corticosteroids is often accompanied by various side effects. EtTorts to discover new agents have been limited,
for the most part, to structural analogues of active com-
754
lvilsun and Gi.s,olds Textbook of Organic Mrdici,,al and PI,arn,aceuiiea! Cl,enzis,r
pounds owing to a lack of knowledge about the causes and mechanisms of inflammatory Although several new agents have been introduced for use in RA. aspirin remains one of the most widely used drugs for this purpose. It also protects against myocardial infarctions. A significant stimulus to this search was the observation that prostaglandins play a major role in the inflammatory processes.'59 Drugs such as aspirin and indomethacin inhibit prostaglandin synthesis in several Furthermore. almost all classes of NSAIDs strongly inhibit the conversion of arachidonic acid into prostaglandin occurs at the stage of conversion of arachidonic acid, released by the action of phospholipase A on damaged tissues. by prostaglandin H: synthetase. now called to the cyclic endoperoxides PGG: and PGH2. These are known to cause vasoconstriction and pain. They, in turn. are convened in part to PGE2 and PGF2,,, which can cause pain and vasodilatation. This effect of the NSAIDs parallels their relative potency in various tests and is stcrenspecific.'1" The search for specitic inhibitors of cyclooxygenasc has opened a new area of research in this field. This enzyme
The possibility of hypoprothromhinemia and concomitant capillary bleeding in conjunction with salicylate adminisuation accounts for the inclusion of menadione in some late formulations. There is some doubt, however, about the necessity for this measure. A more serious aspect of salicylate medication is the possibility of inducing hemorrhage from direct irritative contact with the mucosa. Alvarez and Summerskill have pointed out a definite relationship be. tween salicylate consumption and massive gastrointestinal hemorrhage from peptic Barager and an extensive study found, however, no danger of
occurs in two forms. cyclooxygenase
This does not occur with normal temperatures. The anhipy. retic and analgesic actions are believed to occur in the hypothalamic area of the brain. Some think that the salicylates exert their analgesia by their effect on water balance, reduc. ing the edema usually associated with arthralgias. Aspirin is particularly effective for this. For an interesting account of the history of aspirin and a discussion of its mechanisms of action, the reader should consult an article by as well as the reviews by Smith'63 (64 and by Nickanderer
(COX-I) and
anemia or development of peptic ulcer. Leonards and
cyclooxygenase 2 (COX-2). COX- lisa constitutive enzyme and plays a role in the production of essential prostuglandins.
Levy'5'8 used radiolabeled iron and demonstrated that bleed.
Inhibition of this enzyme by all the older. nonselective
fects varied with the formulation. Davenport'65 suggests that
NSAIDS is primarily responsible for a number of their side effects. The COX-2 enzyme is induced in response to the release of several prointlammatory mediators, leading to the inflammatory response and pain. Thus, there was an active search for specific inhibitors of the COX-2 enzyme. This has been successful with the approval of three COX-2 inhibitors.
back diffusion of acid from the stomach is responsible for capillary damage. Because of these characteristics of aspirin, it has been extensively studied as an antithrombotic agent in the
I
discussed below. Even with the significant advances achieved in the discovery of new and more specific NSA IDs, they all have a ceiling effect on their ability to relieve all pain. It is becoming com-
ing does occur following administration of aspirin. The ef-
nient and prevention of clinical thrombosis.'70 Ill
Ii
is
thought no act by its selective action on the synthesis of the prostaglandin-related throniboxane A7 and which are the counterbalancing factors involved in platelet aggregation and are released when tissue is injured. It is unique from the other NSA IDs. in that it irreversibly inhihirs
mon practice to use combinations of the opioid drugs with NSAIDs to treat severe and intractable pain. These drugs are considered below in their various chemical categories.
the cyclooxygenase enzymes by acetylation. Aspirin har
SalIcyftc Acid Derivatives
the small intestine and depend strongly on the pH of the
now been approved for the prevention of transient ischemic
attacks, indicators of an impending The salicylatex are readily absorbed from the stomach ansi
Historically, the salicylales were among the first of this group to achieve recognition as analgesics. Leroux. in 1827. isolated salicin. and Piria. in 1838. prepared sulicylie acid.
After these discoveries, Cahours (1844) obtained salicylic acid from oil of wintcrgreen (methyl salicylate). and Kolbe and Lautcrmann (1860) prepared it synthetically from phenol. Sodium salicylate was introduced in 1875 by Buss. followed by the introduction of phenyl salicylate by Ncncki in 1886. Aspirin, or acetylsalicylic acid, was first prepared in 1853 by Gerhardt but remained obscure until Felix Hoffmann discovered its pharmacological activities in 1899. It was tested and introduced into medicine by Dreser. who named it aspirin by taking the a from acetyl and adding it to spirin. an old name for salieylic or spine acid, derived from its natural source of spirea plants. The phannucology of the salicylaces and related coinpounds has been reviewed extensively by Smith.'63
Sali-
cylates. in general. exert their anlipyrctic action in febrile patients by increasing heat elimination of the body via the mobilization of water and consequent dilution of the blood. This brings about perspiration, causing cutaneous dilatation.
media. Absorption slows considerably as the pH rises (more alkaline) because of the acidic nature of these compounds and the necessity for the presence of undissociated molecule' for absorption through the lipoidal membrane of the stomach and the intestines. Therefore, buffering agents administered at the same time in excessi,'e amounts decrease the rule ol
absorption. In small quantities, their principal effect itrayfs to aid in the dispersion of the salicylate into fine particles. This would help to increase absorption and decrease the pus sibility of gastric irritation by the accumulation of large particles of the undissolved acid and their adhesion to the mucosa. Levy and have shown that the
rate of aspirin and the incidence of gastric distress wcnea function of the dissolution rate of its particular dosage forni A more rapid dissolution rate of calcium and buffered a.spinr
was believed to account for faster absorption. They alac demonstrated significant variations in dissolution rules different nationally distributed brands of plain aspirin lets. This may account for some of the conflicting and opinions concerning the relative advantages of plain and buffered aspirin tablets. Lieberman et al.'74 have also shonn that buffering is effective in raising the blood levels olaspi-
Chapter 22 • Analgesic Age,u.c
na. A measure of the antianxiety effect of aspirin by means 01' electroencephalograms (EEGs) found differences between buffered, brand name, and generic aspirin prepara(ions.1
Poterniation of salicylate activity by simultaneous admin-
istration of p-uminobenzoic acid or its salts has been the basis for the introduction of numerous products of this kind. Salassa and coworkers have shown that this effect is due to the inhibition of both salicylate metabolism and excretion in the urine.'76 This effect has been proved amply, provided that the ratio of 24 g ofp-aminobenzoic acid to 3 g of salicylate per day is observed. There is no strong evidence, however, to substantiate any significant elevation of plasma sulicylate levels wheti less p-aminoben'i.oic acid is used.
The derivatives of salicylic acid are of two types II and II (a and h)J:
755
In solution, particularly in the presence of sodium bicarbonate, the salt will darken on standing (see salicylic acid). This darkening may be lessened by the addition of sodium sulfite or sodium bisulfite. Also, use of recently boiled distilled water and dispensing in amber-colored bottles lessens color change. Sodium salicylale forms a eutectie mixture with antipyrine and produces a violet coloration with iron or its salts. Solutions of the compound must be neutral or slightly basic to prevent precipitation of free salicylic acid. The USP salt forms neutral or acid solutions, however. This salt is the one of choice for salicylate medication and usually is administered with sodium bicarbonate to lessen gastric distress, or it is administered in enteric-coated tablets. The use of sodium bicarbonate'77 is ill advised because it decreases the plasma levels of salicylate and increases the excretion of free salicylate in the urine.
Sodium Thiosalicylate. Sodium thiosalicylate (Rexolate) is the sulfur or thio analogue of sodium salicylate. It is more soluble and better absorbed, thus allowing lower dosages. It is recommended for gout. rheumatic fever, and muscular pains in doses of IOU to 150 mg every 3 (06 hours for 2 days. and then 1(X) mg once or twice daily. It is available only for injection.
0
Magnesium Salicylate, USP.
0 lb
Type I represents those that are formed by modifying the carboxyl group (e.g.. salts, esters, or amides). Type II (a and hi represents those that are derived by substitution on the hydroxyl group of salicylic acid. The derivatives of salicylic
acid were introduced in an attempt to prevent the gastric symptoms and the undesirable taste inherent in the common salts olsalicylic acid. Most hydrolysis of type I takes place in the intestine, and most of the type II compounds are absorbed unchanged into the bloodstream (see aspirin). COMPOUNDS OF TYPE I
The alkyl and aryl esters of salicylic acid (type I) are used externally, primarily as counterirnitants. where most of them se well absorbed through the skin. This type of compound is of little value as an analgesic. A few inorganic salicylates
ire used internally when the effect of the salicylate ion is intended. These compounds vary in their irritation of the sitmach. To prevent the development of pink or red colorin the product, contact with iron should be avoided in heir manufacture.
Sodium salicy late may be pieSodium Salicylate, USP. pared by the reaction, in aqueous solution, between I mole each of salicylic acid and sodium bicarbonate: evaporating
to dryness yields the white salt. Generally, the salt has a pinkish tinge or is a white microcrystalline powder. It is or has a faint, characteristic odor, and it has a ssvecl. saline taste. It is affected by light. The compound is soluble in water (I I). alcohol (1:10). and glycerin (1:4).
Magnesium salicylate (Mobidin. Magan) is a sodium-tree salicylate preparation for use when sodium intake is restricted. It is claimed to produce less gastrointestinal upset. The dosage and indications are the same as those for sodium salicylate.
Choline Saucy/ate. Choline salicylate (Arthropan) is extremely soluble in water and is available as a flavored liquid. It is claimed lobe absorbed more rapidly than aspirin. giving faster peak blood levels. It is used when salicylates are indicated in a recommended dose of 870 mg to 1.74 g 4 tunes daily. It is also available in combination with magnesium salicylate (Trilisate. Tricosal).
Others.
Ammonium. lithium, and strontium salts of salicylic acid have also found use. They offer no distinct advantage over sodium salicylate. SALOL PRINCIPLE
Nencki introduced salol in 1886 and so presented to the science of therapy the "salol principle." In salol. two toxic substances (phenol and salicylic acid) were combined into an ester that taken internally slowly hydrolyzes in the intestine to give the antiseptic action of its components. This type of ester is referred to as a full solo! or true solo! when both components of the ester are active compounds. Examples are guaiacol benzoate. benzoate. and salol. The
salol principle can be applied to esters in which only the alcohol or the acid is the toxic, active or corrosive portion: this type is called a partial ca/of. Examples of partial salols that contain an active acid are ethyl salicylate and methyl salicylate. Examples of partial salols that contain an active phenol are creosote carbonate. thymol carbonate, and guaiacol carbonate. Although many salol-type compounds have been prepared and used to some extent, none is presently
756
tVll.son
and Gisvoid's Texibook of
Medielsia! wid Pharn,aceuthal ('heusistry
valuable in therapeutics, and all are surpassed by other agents.
Phenyl Sallcylate.
Phenyl salicylate. salol. occurs as fine white crystals or a white crystalline powder with a characteristic taste and a faint, aromatic odor. It is insoluble in water (1:6.700). slightly soluble in glycerin, and soluble in alcohol (1:6). ether, chloroform, acetone, or fixed and volatile oils. Damp or cutectic mixtures form readily with many organic materials, such as thymol. menthol, camphor. chloral hydrate. and phenol. Salol is sold in combination with methenainine and atropine alkaloids as a urinary tract antiseptic and analgesic (e.g.. Prosed/OS. Trac Tabs. Urised and others). Salol is insoluble
in gastric juice hut is slowly hydrolyzed in the intestine into phenol and salicylic acid. Because of this property, coupled with its low melting point (41 to 43°C). it has been used in the past as an enteric coating for tablets and capsules. It is not efficient as an enteric-coating material, however, and its use has been superseded by more effective materials. It has also been used externally as a sun filter (10% ointment) for sunburn prevention (Rayderm). Salicylarnide. o-hydroxyhenzamide. is a Salicylamide. derivative of salicylic acid that has been known for almost a century. It is readily prepared from salicyl chloride and ammonia. The compound occurs as a nearly odorless, white crystalline powder. It is fairly stable to heat, light, and mois-
ture. It is slightly soluble in water (1:500): soluble in hot water, alcohol (1:15). and propylene glycol; and sparingly soluble in chloroform and ether. It is freely soluble in solutions of alkalies. In alkaline solution with sodium carbonate or triethanolamine. decomposition takes place, resulting in a yellow to red precipitate.
KI0 Salicylamide reportedly exerts a moderately quicker and deeper analgesic effect than aspirin. Long-term studies on rats revealed no untoward symptomatic or physiological reactions. Its metabolism differs from that of other compounds, and it is not hydrolyzed to salicylic acid.t5' Its analgesic and antipyretic activity is probably no greater than that of aspirin, and possibly less. It can be used in place of salicylatcs. however, and is particularly useful for patients with a demonstrated sensitivity to salicylates. It is excreted much more rapidly than other salicylates, which probably
accounts for its lower toxicity and, thus, does not permit high blood levels. The dose for simple analgesic effect may vary from 300 rng to I g administered 3 times daily; but Ibr rheumatic conditions, the dose may be increased to 2 to 4 g 3 times a day. Gastric intolerance may limit the dosage, however. The usual period of the higher dosage should not exceed 3 to 6 days. It is available in several combination products (e.g.. Saleto. BC Powder).
Aspirin. acetylsalicylic acid (Aspro. Ens. Aspirin, USP. pirin). was introduced into medicine by Dreser in 1899. It is prepared by treating salicylic acid, which was first pared by Kolbe in 1874. with acetic anhydride. The hydrogen
atom of the hydroxyl group in sulicylic acid is replaced by the acetyl group: this also may be accomplished by using acetyl chloride with salicylic acid or ketene with salicylic acid.
\\/
0
H
— CH3
0 Aspirin
Aspirin occurs as white crystals or as a white crystallite powder. It is slightly soluble in water (1:300) and solubk in alcohol (1:5). chloroform (1:17). and ether (1:15). Also. it dissolves easily in glycerin. Aqueous solubility may increased by using acetates or citrates of alkali metals. al though these are said to decompose it slowly. It is stable in dry air, but in the presence of moisture, it slowly hydrol)lec into acetic and salicylic acids. Salicylic acid will crystalline out when an aqueous solution of a.spirin and sodium hydros•
ide is boiled and then acidified. Aspirin itself is acidic enough to produce effervescence with carbonates and, in the presence of iodides. to cause the slow liberation of iodine. In the presence of silkalinc hydroxides and carbonates, it decomposes, although ii does
form salts with alkaline metals and alkaline earth metals The presence of salicylic acid, formed on hydrolysis. rnaj be confirmed by the formation of a violet color on the addi. tion of ferric chloride solution. Aspirin is not hydrolyzed appreciably on contact with weakly acid digestive fluids of the stomach but, on passagc into the intestine, is subjected to some hydrolysis. Most o: it is absorbed unchanged, however. GarretO7° has ascribed the gastric mucosal irritation of aspirin to salicylic acid for
mation, the natural acidity of aspirin, or the adhesion ot undissolved aspirin to the mucosa. He has also proposed that the nonacidic anhydride of aspirin is superior for or4l administration. Davenport'5" concludes that aspirin alien' mucosal cell permeability, allowing back diffusion of stoOl ach acid, which damages the capillaries. A number of etaries (e.g.. Bufferin) use compounds such as sodium bicar
bonate. aluminum glycinate. sodium citrate.
aluminum
hydroxide, or magnesium trisilicate to counteract this acidic property. One of the better antacids is dihydroxyaluminum arninoacetate. USP. Aspirin is unusually effective when pu' scribed with calcium glutamate. The more stable. ntminiiaa calcium acetylsalicylate is formed. and the glutamate Ixrnion
(glutamic acid) maintains a pH of 3.5 to 5. Preferably. dry dosage forms (i.e., tablets, capsules. r powders) should be used, since aspirin is somewhat unsthlk
in aqueous media. In tablet preparations, the use of ucij washed talc improves the stability of aspirin.°5 Also. apinl has been found to break down in the presence of rine hydrochloride.'00 Aspirin in aqueous media will h
Chapter 22 • Analgesic Agem.s
drolyi.e almost completely in less than I week: solutions made with alcohol or glycerin do not decompose as quickly. Citrates retard hydrolysis only slightly. Sonic studies have indicated that sucrose lends to inhibit hydrolysis. A study
757
Olfiunisal.
Over the years. several hundred analogues of aspirin have been made and tested to produce a compound that was more potent. was longer acting, and had less gastric
damp with methenaniine. aminopyrine. salol. antipyrine.
irritation. By the introduction of a hydrophobic group in the 5 position. diflunisal. 5-f 2.4-difltiorophenyl)salicylic acid, 2'.4'-difluoro-4-hydroxy-3-biplienylcarboxylic acid I Dolohid). appears to meet these criteria. In animal tests, it is at least 4 times more potent. in humans, it appears to be about twice as effective, with twice the Like other aryl acids, it is highly bound to plasma protein as its
phenol. or acetanilid.
deacylated metaholite. It is marketed in tablets (250 and 5(X)
ni aqueous aspirin suspensions indicated that sorhitol a pronounced stahiliiing Stable liquid preparations are available that use triacelin. propylene glycol. or a poly-
ethylene glycol. Aspirin lends itself readily to combination with many other substances hut tends to soften and become
Aspirin is one of the most widely used compounds in therapy and, for many years. was not associated with untoward effects. Allergic reactions to aspirin are now observed commonly. Asthma and urticaria are the most common manitestations and, when they occur, are extremely acute and
difficult to relieve. Like sodium salicylate. aspirin caused congenital malformations when administered to Ptvtreatment with sodium pentoharhital or chiorpromazine significantly lowered these effects."" Similar effects have heen attributed to the consumption of aspirin by women, and its use during pregnancy should be avoided. Other studies. however, found no untoward effects. The reader is urged to eunsult the excellent review by Smith for an account of the pharmacological aspects of aspirin.
Practically all salts of aspirin, except those of aluminum and calcium. are unstable for pharmaceutical use, These saks appear to have fewer undesirable side effects and to induce analgesia faster than aspirin. A timed-release preparation of
aspirin is available. It does not appear to offer any advantages over aspirin, except for bedtime dosage. Aspirin is used as an antipyretic. analgesic, and antirheumatic. usually in powder. capsule. suppository. or tablet ionn. Its use in rheumatism has been reviewed, and it is reportedly the drug of choice over all other salicylute derivatires. na There is some anesthetic action when applied kteally, especially in powder form in tonsillitis or pharyngiis, and in ointment form for skin itching and certain skin
mg) for treating mild-to-moderate pain and RA and OA.
NAryianthranllk Acids One of the early advances in the search for nonnarcotic analgesics was centered in the N-arylanthranilic acids. Their out-
standing characteristic is that they are primarily NSAIDs, and secondarily, some possess analgesic properties.
Mefenamic Acid.
Mefenamic acid. N-2.3-xylylanth-
ranilic acid (Ponstel), occurs as an off-white crystalline powder that is insoluble in water and slightly soluble in alcohol.
It appears to be the first genuine antiphlogistic analgesic discovered since aminopyrine. Because it is believed that aspirin and aminopyrinc owe their general purpose analgesic efficacy to a combination of peripheral and central effects.">'
a wide variety of arylanthranilic acids were screened for antinociceptive (analgesic) activity if they showed significant anti-inflammatory action. The combination of both effects is a rarity among these compounds. The mechanism of
analgesic action is believed to he related to the ability to block prostaglandin synthetase. No relationship to lipid. plasma distribution, partition coefficient, or pK,, has been noted. The interested reader, however, will find additional information on antibradykinin and anti-UV erythema activities of these compounds, together with speculations on a receptor site, in the literature. I'c
diseases, Iii the usual dose. 52to 75% is excreted in the urine.
in sarious forms, in a period of IS to 30 hours. Analgesia is to be due to the unhydrolyzed acetylsalicylic acid moiecule.153
1,5
A low-dosage t'ornn of aspirin. 81 mg. equivalent to the dose recommended for infants (the . 'baby aspirin"), is recomniended as a daily dose for individuals who are at even a ow cardiovascular risk Several large studies found that his low dose of aspirin reduces the number of heart attacks and thrombotic strokes. Other salicylates and NSAIDs have not shown similar effects. In fact, the NSAIDs can interfere with aspirin's cardiovascular benefits, and they should not taken within 12 hours of each other. Salsalate.
Salsalate. salicylsalicylic acid (Amigesic.
Disalcid. etc.). is the ester fonned between Iwo salicylic acid molecules to which it is hydrolyzed following absorption. It reportedly causes less gastric upset than aspirin because is relatively insoluble in the stomach and is not absorbed until it reaches the small intestine. Limited clinical suggest that it is as effective as aspirin and that .1 may have fewer side effects.'8" The recommended dose
is 325 to l.(X)0 mg 2 or 3 times a day. It is available only ii prescription.
(a)
Ib)
R1=CH3.X=CH R,—H.R7=CF3
Mefenamic acid in a dose of 250 mg is superior to 600 mg of aspirin as an analgesic. and doubling the dose sharply increases its efficacy.'53 A study examining this drug relative
to gastrointestinal bleeding indicated a lower incidence of this side effect than by aspirin."34 Diarrhea, drowsiness, and headache have accompanied its use. The possibility of blood disorders has prompted limitation of its adminis-
tration to 7 days. It is not recommended ('or children or during pregnancy. It has been approved for use in the management of primary dysmenorrhea (PD). which is thought to be caused by excessive concentrations of prostaglandins and endoperoxides.
Medofenamate Sodium. Meclofenamate sodium, sodium N-(2.6-dichloro'm-tolyi )anthranilate. Meclomen. is
758
Wilson and Gixeohl.s lexibook of Organic Medicinal and Pharn,aceurical
available in 50- and 100-mg capsules for use in the treatment
of acute and chronic RA. The most significant side effects are gastrointestinal, including diarrhea.
0 A
C00 Na* inactive
parent
active
The parent sullinyl has a plasma half-life of 8 hours, and that of the active sulfide metaholite is 16.4 hours. The more Cl
polar and inactive sulfoxide is virtually the only form CH3
Meclofonamate Sodium
Arylacetic Acid Derivatives The arylacetic acid derivative group of agents has received the most intensive attention for new clinical candidates. As a group, they show high analgesic potency in addition to their anti-inflammatory activity.
creted. The long half-life is due to extensive enterohepafic Only the sulfide species inhibits proslaglandin synthetase in vitro. Although these forms are highly pro. tein bound, the drug does not appear to affect binding of amicoagulants or hypoglycemics. Coadministration of aspi. rio is contraindicated because it considerably reduces thc sultide blood levels.
CH2COOH CH3
lndomethacin, USP.
Indomethacin. I -(p-chlorobenzoyl)-5-methoxy-2-methylindole-3-acctic acid (Indocin). occurs as a pale-yellow to yellow-tan crystalline powder that is soluble in ethanol and acetone and practically insoluble in water. It is unstable in alkaline solution and sunlight. It shows polymorphism; one form melts at about 155°C. and the other at about 162°C. It may occur as a mixture of both forms with a melting range between these melting points.
Sulindac
Careful monitoring of patients with a history of ulcen recommended. Gastric bleeding, nausea, diarrhea. dizziness and other adverse effects huve been noted, but with a lower
frequency than with aspirin. Sulindac is recommended fre RA. OA. and ankylosing spondylitis in a 150- to 200-mg dose, twice daily.ISl 55 Ii is available as tablets (ISO and 200 mg).
Tolmetin Sodium, USP.
Tolmetin sodium,
I -methyl.
dihydrate sodium. McN-2559 Iridomethacin
Since its introduction in 1965. it has been widely used as an anti-inflammatory analgesic in RA. spondylitis. and OA, and to a lesser extent in gout. Although both its analgesic and anti-inflammatory activities are well established, it appears to be no more effective than The most frequent side effects are gastric distress and headache. It has also been associated with peptic ulceration. blood disorders, and possible deaths. The side effects appear to be dose related and sometimes can be minimized by reducing the dose. It is not recommended for use in children be-
(Tolectin), is an arylacetic acid derivative with a pyrrole the aryl group. This drug is absorbed rapidly, with a rela. tively short plasma half-life (I hour). It is recommended use in the management of acute and chronic RA. It similar, but less frequent. adverse effects with aspirin. Ii not potentiate coumarin-like drugs nor alter the blood lewis of sulfonylureas or insulin. Like other drugs in this claa it inhibits prostaglandin syntheta.se and lowers PGE blond levels.
cause of possible interference with resistance to infection. Like many other acidic compounds. it circulates bound to blood protein, thus requiring caution in the concurrent use of other protein-binding drugs. Indomethacin is recommended only for patients who cannot tolerate aspirin and in place of phenylbutazone in long-term therapy, for which it appears to be less hazardous than conicosteroids or phenylbutazone. USP. Sulindac. (Z)-5-fluoro-2-rnethyl- I (methylsulfinyl)phenyl I H-indene-3-acetic acid (Clinoril), occurs as yellow crystals soluble in alkaline but insoluble in acidic solutions. The drug reaches peak blood levels within 2 to 4 hours and undergoes a complicated, reversible metabolism as follows:
R,
CH3
R, = C;. 0. = CR3
Available as tablets (200 and 600 mg) and a capsule mg), a dose of 44X) nig 3 times daily, with a maximum of 2,000 mg. is recommended. Clinical trials indicate a usuji daily dose of 1.200 mg is comparable in relief to 3.9 g a aspirin and ISO mg of indomethacin per
Ibuprofen, lISP.
2-4-isobutylphenyl)propi•
onic acid (Motrin. Advil, Nuprin). was introduced intoclini.
Chapter 22 • Analgesic Agents extensive clinical trials. It appears to cal practice he comparable to aspirin in the treatment of RA. with a lower incidence of side
It has also been approved for
759
gastrointestinal bleeding. ulcers. dyspepsia. nausea, sleepiness, and dizziness reported at a lower incidence than with aspirin. It inhibits prostaglandin synthetase.205
use in PD.
Ca 2H?O Ibuleriac R = H thuprofen R = CH.3 Fenoproten
In this series of compounds. potency was enhanced by intruduction of the a-methyl group on the acetic acid moiety.
The precursor ibufcnac (R = H). which was abandoned owing to hepatotoxicity. was less potent. Moreover, the ac-
tivity resides in the (S)-( +) isomer, not only in ibuprofen but throughout the arylacetic acid series. Furthermore, these isomers are the more potent inhibitors of prostaglandin synThe recommended dosage is 400 mg. Ibuprofen is also available over-the-counter as 200-mg tablets.
Naproxen. (+ )-6-methoxy-a-methyl2-naphthaleneacetic acid (Anaprox. Naprosyn). occurs u.s white to off-white crystals that are sparingly soluble in acidic Naproxen, USP.
,alutions, freely soluble in alkaline solutions, and highly soluble in organic or lipid-like solutions. After oral administration, it is well absorbed, giving peak blood levels in 2 to 4 hours and a half-life of 13 hours. A steady-state blood level is usually achieved after four to live doses. Naproxen is very highly protein bound and displaces most proteintxund drugs. Dosages of these must be adjusted accordingly.
CH3
Naproxen
Available as capsules (200 and 300 rag) and tablets (600 mg). it is recommended for RA and OA in divided doses 4 times a day for a maximum of 3,200 mg/day. It should be taken at least 30 minutes before or 2 hours after meals. It is not yet recommended for the management of acute flareups. Doses of 2.4 g per day are comparable to 3.9 g per day of aspirin in arthritis. For pain relief. 400 mg gave results similar to 650 mg of
Ketoprofen.
Ketoprofen. 3-benioyl-a-mcthylbcnzcneacetic acid. in-beni.oylhydratropic acid (Orudis). is closely related to fenoprofen in structure, properties, and indications. It has a low incidence of side effects and has been approved for over-the-counter sale (Orudis KT, Actron). It is available as capsules and tablets (25 and 50 rag). with a recommended daily dose of ISO to 300mg divided into three or four doses. It is also available as extended-release capsules (100. ISO, and 200 mg).
FiurbIprofen, USP.
Flurbiprofen. (± )-2-(2-fluoro-4-
biphenylyl)propionic acid (Ansaid. Ocufen). is another hydrotropic acid analogue that is used in the acute or longterm management of RA and OA. It is available as tablets (50 and 100 rag), with a recommended dose of 200 to 300 mg divided into 2, 3. or 4 times daily.
is recommended for use in rheumatoid and iouty arthritis. It shows good analgesic activity—400 tug is comparable to 75 to 150mg of oral meperidine and supe-
rior to 65 rag of propoxyphene and 325 mg of aspirin plus 30 rag of codeine. A 220- to 330-mg dose is comparable to (ItO mg of aspirin alone. It reportedly produces dii.ziness, and nausea, with infrequent mention of gastroincstinal tract irritation. Like u.spirin. it inhibits prostaglandin and prolongs blood-clotting time. It is not recomrvndcd for pregnant or lactating women or children under N is also available over-the-counter as 200-mg tablets Aleve).
Fenoprofen calcium, aknoprofen caidum, USP. rvthyl.3-phcnoxybenzeneacetic acid dihydrate calcium occurs as a white crystalline powder that is slightly sishle in water, soluble in alcohol, and insoluble in ben:cne. It is rapidly absorbed orally. reaches peak blood levels sithin 2 hours, and has a shunt plasma half-life (3 hours). is highly protein bound like the other acylacetic acids, and aution is needed when it is used concurrently with hydansulfonamides, and sulfonylureas. It shares many of he adverse effects common to this group of drugs, with
Diclofenac Potassium and Sodium.
Diclofenac sodium. sodium [o-(2.6-dichloroanilino)phenyl]acetate (Voltaren), is indicated for short- and long-term treatment of RA.
OA, and ankylosing spondylitis. The potassium salt (Cataflam). which is faster acting, is indicated for the management of acute pain and PD. The sodium salt is available us delayed-release tablets (25. 50. 75. and 100 rag). with a recommended daily dose of 1(X) to 2(X) mg in divided doses. The potassium salt is available as a tablet (50 mg). with a recommended dose of 50 mg 3 times daily.
Nabumetone.
Nabumetone. 4-(6-mcthoxy-2-naphthyl )2-butanone (Relafen). serves as a prodrug to its active metab-
olite. 6-mcthoxy-2-naphthylacetic acid. Like the other arylacetic acid drugs. it is used in short- or long-term management of RA and OA. It is available as tablets (500 and 750 mg). with a recommended single daily dose of 1,000 mg.
Ketoroiac Tromethamine. Ketorolac tromethamine, (±)-benzoyl-2,3-dihydro-IH-pyrroli/ine-l-carboxylic acid compound with 2-anhino-2-(hydroxymethyl)- 1,3-propane-
760
Wilson and (;i.si'nld's lexthuok
of Organic Medicinal and Pharmaceutical Cl,emistrs'
diol (Tor-.idol). is a poteffi NSAID analgesic indicated for the Ircaunent of moderately severe, acute pain. Because of a number of potential side effects, its administration should not exceed 5 days. Treatment is usually initiated by intravenous (30 rng or intramuscular (ho mg) administration, with analgesia maintained by initial oral doses of 21) or 31) rug. followed by 10 mg every 4 to 6 hours.
Rofecoxib. 4-14-1 inethylsulfinyl)phenyll-3phenyl-2(5H)-furanone (Vioxx). is a COX-2 inhibitor with greater potency and a longer half-life than celecoxib (17 versus II hours). It is approved for use in OA. acute pain. and PD. with a dose of 12.5 mg/day and a maxintuni of 25 mg/day for OA. and a single dose of 50 rug daily recommended for a maximum of 5 days for acute pain and PD. Ii is available as tablets (12.5. 25. and 50mg) and suspensions
I .8-dictltyl- I ,3,4,9-tctrahydropyEtodolac. ranol3.4-btindole-l-acetic acid (Lodine). possesses an indote nng as the aryl portion of this group of NSAID drugs.
(12.5 and 25 mg/mI).
Ii shares many of the properties of this group and is indicated
for short- and tong-term management of pain and OA. It is available as capsules (2(X) and 300) rug). tablets (400 and 500 rug) and extended-release tablets (400. 5(X). and 600 mg). with a recommended daily dose of 800- to 1,200-mg in divided doses.
valdecoxib. tenesullonamide (Bextra), had the same approved uses as celecoxib and rofecoxib, with a rccomnnendcd dose of 10 mg/day for RA and OA and 40 nsg for PD. It is available as 10- and 20-mg tablets. Several other COX-2 enzyme inhibitors are under investigation and clinical trials. includin0 the highly specific etoricoxib and the parenteral pareconib.
Oxaprozin. 4.5-diphenyt-2-oxaiolepropioOxaprozin. nic acid (Daypro). differs from the other members of ibis
Aniline and p.Amlnophenol Derivadves
group in being an arylpropionic acid derivative, It shares the same properties and side effects of members in this group. It is indicated for the short- and tong-term manage-
ment of OA and RA. administered as a single 1.200-ung dose, It is available as (n(X)-nug caplets.
Valdecoxib.
Introduced in late 200)1. the COX-2 inhilbi-
br
The introduction of aniline derivatives as analgesics is based
on the discovery by Cahn and Hcpp. in 1886. that aniline (C-I) and acctanilid (C-2) (Table 22-7) both have poweiiid antipyretic properties. The origin of this group from aniline hasted to their being called "coal taranalgcsics." Acetanilid was introduced by these workers because of the known
Piroxicam. 4-hydroxy-2-methyl-N-2Piroxicam, USP. pyridyl-2H- I .2-henzothiaiine-3-carhoximide 1.1 -dioxide (Feldene). represents a class of acidic inhibitors of prostaglandin syntlietase. although it does not antagonize PGE2 This drug is very tong acting. with a plasma halflife of 50 hours, thus requiring a dose of only 20 to 30 mg once daily. It is reported to give results similar to those from 25 mg of indomethacin or 400) mg of ibuprofen 3 times a day.205 2Y)
OH 0
00
CH3
ity of aniline itself. Aniline brings about the formation ol methemoglobin. a form of hemoglobin that cannot function as an oxygen carrier. The acyl derivatives of aniline woe thought to exert their analgesic and antipyretic effects by first being hydrolyzed to aniline and the corresponding add. after which the aniline was oxidized to p-aminophenol (C3). This is then excreted in combination with glucuronic it sulfuric acid. The aniline derivatives do not appear to act on the hrjin cortex: the pain impulse appears to be intercepted at the hypothalamus, wherein also lies the thermoregulatory center of the body. It is not clear if this is the site of their activity because most evidence suggests that they act at pcriphesnl thermoceptors. They are effective in retunting feverish mdi. viduals to normal temperature. Normal body temperatures are not affected by the administration of these drug.c. Of the anhipyretic analgesic group. the aniline derivatives shos
little if any anti-inflammatory activity.
Meloxicam.
Like piroxieam in structure. metoxicam. 4hydroxy-2-methyl-N45-methyl-2-thiazoyt)-211- I .2-benzo-
thiazine-3-carboxamide 1,1-dioxide (Mohie). is also indicated for use in OA. It also hasa relatively long halt-life of IS to 20 hours. Available as a 7.5-mg tablet, the recommended dose is 7.5 mg/day. with a maximum of IS mg/day.
The first of the COX-2 inhibitor drugs to be marketed. celecoxib. 4-lS-(4-methylphcnyl)-3-(trilluorophenyl)- I IJ-pyraiol- I -yllhenzenesulIonamide (Celebrex). has been approved fur use in RA and OA, with a dose of tOO or 2(K) rug twice a day for RA and 200 mg/day for OA in a single dose of 200 tog or 1(X) mg twice a day. It has also been approved for reducing the number of adenomatous colorectal potyps in familial adenotnatous polyposis (FAP). It is available as 100- and 200-mg tablets.
Table 22-7 shows some of the types of aniline derivative' that have been made and tested in the past. In general, any type of substitution on the amino group that reduces its basic.
ity also lowers its physiological activity. Acylation is one type of substitution that accomplishes this effect. Acelanilid (C-2) itself, although the best of the acylated derivatives. I'
toxic in large doses, but when administered in analgesic doses, it is probably without significant harm. FonsianiliJ (C-4) is readily hydrolyzed and too irritant. The higher honk
ologues of acetanilid are less soluble and, therefore. es' active and less toxic. Those derived from aromatic aci&. (e.g.. C-5) are virtually without analgesic and auilipyretic effects. One of these. saticylanilide (C-h). is used as a
cide and antimildew agent. Exalgin (C-7) is too toxic. The hydroxylatcd anilines (o. in, p). better known as thc aminophenols. are considerably less toxic than aniline. The
para compound OC-3
is of particular interest from
ii
Chapter 22 • Analgesic AgePuc
TABLE 22—7
Some Analgesics Related to Aniline
Structure
Compound
R1
C-I
—H
—N
—H
Aniline
C-2
—H
—H
—C—CH3
Acetanilid
Name
R3
0 C-3
—OH
—H
—H
C.4
—H
—H
—C—H
C-S
—H
—H
C-6
—H
—H
Formanibd
Salicylanibde
agent)
°HO C-7
—1-4
—CH1
C-8
—OH
—H
Exalgin
Acetaminophen
0 C-9
C-tO
C-it
—0C2H5 —0C2H5
—H —H —H
—H —H —C—CH3
Anisldine
Plienctidine Phenacetiri
0 C-12
—H
—C—Cl-ICH3 O
C-13
—H
Lactylphenelidin
OH
—C—CI-42NH2
Phonocoll
0 C-14
—H
—C—CH2OCH3
Ktyotine
0 C-15
—H
C-16
—H
—C—Cl-I3
Pheneleal
C-Il
—OCH,C4-4-,ON —H
—C—CH3
Portonal
p-Acetoxyacolanhlld
0
761
762
Wilson and Gist'o!d's TexibooL of Organic Medicinal and Pliar,naceutiral ('lu'mi,s;rv
standpoints: namely. it is the metabolic product of aniline, and it is the least toxic of the three possible aminophenols. It also possesses a strong arnipyretic and analgesic action. It is too toxic to serve as a drug, however, and therefore. numerous modilications were attempted. One of the first was
the acctylation of the amine group to provide N-acetyl-p. aminophenol (acetaminophen) C-8). a product that retained a good measure of the desired activities. Another approach to the detoxification of p-aminophenol was the etherilication of the phenolic group. The best known of these arc anisidine (C-9) and phenctidine (C- 10). which are the methyl and ethyl ethers, respectively. A tree amino group in these compounds
however, although promoting a strong antipyretic action. was also conducive to methemoglobin formation. The only exceptions to this were compounds in which a carboxyl group or sulfonic acid group had been substituted on the benzene nucleus. In these compounds, however, the antipyretic effect also had disappeared. These considerations led
the preparation of the alkyl ethers of N-acetyl-paminophenol. of which the ethyl ether was the best and is known as phenacetin (C-Il). The methyl and propyl homoto
logues were undesirable from the standpoint of causing etne-
sis. salivation. diuresis. and other reactions. Alkylation of the nitrogen with a methyl group potentiates the analgesic action but, unfortunately, has a highly irritant action on mu-
in water and ether and is soluble in boiling water (1:20), alcohol (1:10). and sodium hydroxide T.S. Acetaminophen has analgesic and antipyretic comparable to those of acetanilid and is used in the same conditions. It exerts its effects by inhibiting the cyclooxy. genase enzyme centrally but has very little effect peripher. ally. Although it possesses the same toxic effects as acetanilid, they occur less frequently and are less severe; therefore. it is considered safer. Several precautions should be recog. nized. however, including not to exceed the recommended
dosages and the risk of liver toxicity in chronic It is available in several nonprescription forms and, also. is marketed in combination with aspirin and caffeine drin. Vanquish).
Pyrazolone and Pyrazourninedlone The simple doubly unsaturated compound containing two nitrogen and three carbon atoms in the ring, with the nitrogen
atoms neighboring, is known us pyrazole. The reduction products. named as are other rings of five atoms, are line and pyrazolidine. Several pyrazoline substitution pral ucts are used in medicine. Many of these are derivatives 5-pyrazolone. Some can be related to 3.5-pyrazolidinedionc
cous membranes. The phenacetin molecule has been modified by changing
the acyl group on the nitrogen, with sometimes beneficial results. Among these are lactylphenetidin (C-12), phenocoll (C-l3). and kryofine (C-14). None of these. however, is in
'CN — H Pyrazole
Pyrazohne
—H
PyTazolidine
current use.
0
Changing the ether group of phenucetin to an acyl type of' derivative has not always been successful. p-Acetoxyacelanilid (C-IS) has about the same activity and disadvantages
as the free phenol. The salicyl ester (C-16). however, exhibits diminished toxicity and increased antipyretic activity. Pertonal (C-l7) is a somewhat different type in which glycol has been used to etherify the phenolic hydroxyl group. It is very similar to phenacetin. None of these is currently on the market. Relative to the fate in humans of the L1pes of compounds just discussed. Brodie and Axelrod2tO_2L point out that fleet-
anilid and phenacetin are by two different routes. Acetanilid is metabolized primarily to N-acetyl-paminophenol and acetaminophen and only a small amount to aniline, which they showed to be the precursor of phenylhydroxylaniine. the compound responsible for methemoglobin formation. Phenacctin is mostly deethylated to acetamin-
—H H 5.Py;azo?one
H
H 3.5-Pyrazolidinedione
Ludwig Knorr. a pupil of Emil Fischer. while for antipyretics of the quinoline type. in I 1184. discovered the 5-pyrazolone nosy known as anlipyrine. This discovery initiated the beginnings of the great German drug industry that dominated the field for about 40 years.. Knorr, although
at first mistakenly believing that he had a quinoline-ryjv compound, soon recognized his error, and the compound was interpreted correctly as a pyrazolone. Within 2 yeao, the analgesic properties of this compound became apparenn
when favorable reports began to appear in the literature.
ophen. whereas a small amount is converted by deacetylation to p-phenetidine. also responsible for methernoglobin forma-
particularly with reference to its use in headaches and neutul gias. Since then, it has retained some of its popularity as an
tion, With both acetanilid and phenacetin. the metabolite
analgesic. although its use as an antipyretic has declined steadily. Since its introduction into medicine, there hair been more than 1.000 compounds made in an effort to find
acetaminophen is believed to be responsible for the analgesic
activity. Because of the toxicity described above, both are no longer available, replaced primarily by acctaminophen.
Acetaminophen, USP.
Acetaminophen, N-Acetyl-paminophenol. 4-hydroxyacetanilide. APAP (Panado. Ternpm. Tylenol. etc.). may be prepared by reduction of p-nitrophenol in glacial acetic acid. acetylation of paminophenol with acetic anhydride or ketene, or from phydroxy-acetophenonc hydrazone. It occurs as a white, odor-
less, slightly bitter crystalline powder. It is slightly soluble
others with more potent analgesic action combined with less
toxicity. Many modifications of the basic compound hair been made. The few derivatives and modifications on tk market are listed in Tables 22-8 and 22-9. Phenylbutaeonc. although analgesic itself, was originally developed as a sets bilizer for the insoluble aminopyrine. It is now being used
for the relief of many forms of arthritis, in which capacoy it also reduces swelling and spasm by an action.
Chapter 22 • Analgesic Agent.s
TABLE 22-8
763
by an unknown mechanism, usually attributed to an effect on the serotonin-mediated thermal regulatory center of the nervous system. It has greater anti-inflammatory activity than aspirin. phenylbutazone. and indomethacin. It also less-
DerIvatives of 5-Pyrazole R,
ens perception to pain of certain types. without any alteration
N3
113
in central or motor functions, which differs from the effects of morphine. Very often it produces unpleasant and possibly
—
alarming symptoms, even in small or moderate doses. These are giddiness, drowsiness, cyanosis, great reduction in temperature. coldness in the extremities, tremor, sweating, and Compound Proprietasy Name
R,
Structure
morbilliform or erythematous eruptions: very large doses
R3
produce asphyxia, epileptic convulsions, and collapse. Treat-
R2
ment for such untoward reactions must be symptomatic. It
—CH3 —H
—C5F(5 —CH3
Anlipyrmne
P7renazone
Aniinopyrine Am:dopyrrue
'CuHn
Ch3
'—CH3
C5H5
CH3
CH3
—N(CH3)2
OH3
Mtipyrine. USP.
is probably less likely to produce collapse than acetanilid and is not known to cause the granulocytopenia that sometimes
follows aminopyrine. The great success of antipyrine in its early years led to the introduction of a great many derivatives, especially salts with a variety of acids, but none of these has any advantage over the parent compound. Its use is limited to a combination
Antipyrine. 2.3-dimethyl-l-phenyl-
3.pvr.izolin5.onc, phenaione. was one of Ihe first important drugs to be made I 1887) synthetically. Antipyrine and many related compounds are prepared by the condensation of hydr.izine derivatives with various esters. Antipyrine itself is prepared by the action of ethyl acetoacetate on phenyihydra. tine and subsequent methylation. Ii consists of' colorless, odorless crystals or a while powder. with a slightly bitter taste, It is very soluble in water. or chloroform and less so in ether. Its aqueous solulieu is neutral to litmus paper. Ii is basic in nature, however. which is due primarily to the nitrogen at position 2. Locally. untipyrine exerts a paralytic action on the sensory and the motor nerves, resulting in some anesthesia and vasoconstriction, and it also exerts a feeble antiseptic effect. Sys-
temically, it causes results that are very similar to those of although they are usually more rapid. It is readily after oral administration, circulates freely, and is excreted chiefly by the kidneys without having been changed
Any abnormal temperature is reduced rapidly
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TABLE 22-9
DerivatIves of 3,5-Pyrazolidlnedlone
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39
266. 196%.
115. I'lughcs, 3.: Brain Rex. 88:295. 975. 116. Hughes. 3.: Neurosci. Res. Bull. 13:55. 1975 117. Terenius. L.: Annu. Rev. Phanrtacol. Tosicol. 18:189. 1978. 118. Goldstein. A.: Science 193: 1(181. 1976. 119. Kolantu, 0. H.: Science 2(15:774, 1979. 12(1. Gulland and Robinson: Proc. Manchester l.ii. Phil. Soc. 69—79, 1925 121. Gates. M., and Tsctiudi. C.: J. Am. Chtett,. Sire. 74:1109. 1952
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123.
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1975.
69. deStcscns. C. (ed): Analgeties New York. Academic Ptess. 1965. 70, Br.tenden. 0. J.. Eddy. N. B., and Halbach. H.: Bull. WHO 13:937.
Atad,
('(in.
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Drug Adiliction arid Narcotics. National Reeard
125. Eddy, N. B.. and 1.ee. L. F.: 3. Pharmacol. liup. 'flier. 125:116. 1Q59 126.
Weijlard. 3., and Erickson, A. F.: 3. Am. ('hem.
Soc. 64869. s.C
127. Fraser. H. F., et al.: J. Phurmacol. Exp. flier. 112:359.
958.
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32
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35. Med. Len.
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192.
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(,9:294I 34.
Grittier. C'. M.. and Habuisti. A.: Clin. Pluarntacol. Ther. 4:172. 962, (40. Poluland. A.. and Sullivan. II. R.:J. Ant. ('(win. Sac. 75:4458. 1953. 141. Med. t,ett. 20:111. 1978. 41. Forrest. W. H.. ci at.: Clin. Pharmacol. Titer. 10:468, 1969 143, lihrnebo. M., Ituireuts, L. 0.. uuud Lu3uunuth. V.: Cliii. Pharnuacol. Ther. 22:888. 1977.
44. Mcd. Edt. 9:49, (45. Deleted in prool.
UiIle. R.: Clin. Pharunacul. Ther.
19711.
Winder. C. V.: Nature 184:494. 11)59. Scherrer. K. A.: Itt Scherrer. R. A.. and Whitchouse. M. %V teds.). Atuti.unllamuutaiory Agent.'.. New York. Academic Press. 1974. p. 191. 193. Cass. L. J.. and Frederik. W. S.: 3. Pharmacot. Fop. Ther. 139:172. 191,
$963
Lane,
A. 'I...
Holmes. F. L.. and Moyer. C. 0.: J. New Drugs 4:333.
1964.
195. Med. Len. 10:37.
196$.
(96. (97.
Bruugdcn. K. N.. ci
at.: I)n.gs 1697. 1978.
191).
Ilrogdeum. R. N.. ci al.:
199.
Brogden. R
K. W.. et al.: Anal.
ci
Bicuchem. 95:579. 1979.
Mccl.
I,ctl. 21:1. 1979.
5:429.
cl.: Drugs
197$.
I)elclecl in proof. 2(11. Deleted itt proof. 2(12.
Ja'.inski. I). K.. Martin. W. R.. and Sapinu, J.
0.: Clun.
P)tanutacol.
Ther. 9:215. 196(1. (46. Cone. E. 3.: Tetrahedron Edit. 28:2607. 1973
49. ('hatterjie. N.. ci al.: Drug Metah. 0ispos. 2:401. 1974. Hatr.t, V. K.. San,'.. R. A.. Kenning. K. H.. and Maispeis. L.: Acad. Pharm. Sc). 4:121, 1974. 151. Blcumberg, H.. .mci Dayton. H. I).: In Kuisterlit,., H.. Villarreai. 3. F. teds.). Agotuist and Antagotuist Action'. of Narcotic Analgesic Drugs. London. Macmillan. 1972. 152. Woodland, 3. H. K.. ci at.: 3 Mccl. ('beta. 16:897. 1973. F.: hut. J. Pluarmacol. 153. Takeunori. A. F. A.. Ilayashi, 0.. and Smits ISO.
Donman. 3., and Reynolds. W.: ('an. Mccl. As'.umc. 3. 11(1:1370. 1974.
in proof. 21)4. Bnugden. K. N. ci al.: Drug'. 18:241. 1979. 205. Brusgden. K ci al.: Onugu 13:241. 1977. 2116. Cherutislu. S. M., ci al.: Arthritis Rhcum. 22:376, 979. 1(17. Wt'.eman, F. H.: K. Soc. Med. tnt. Cctngr. Ser. 1:11. 197$. 2011. Weinrr,uuh, M., et at.: J. Rheumictol. 4:393. 1979. 209. Bulogh. Z.. cu at.: Curt. Med. Res. Opin. 6:1411, 1979. 21(1, Itnudie. B. B.. and 3.: 3. Pharmacol. Fop. Ther. 94:29, 1948. 21)3. I)eleted
Ill. Brodie. B.
B.. and Aoelrod. 3.: J. Pluartutacol. Esp. TIter. 97:59, 212. Axelrod. 3.: Pcmm.tgraud. Med. 34 328. 1963. 213. Burns. 3. J., ci at.: Ann. N. Y. Acail. Sd. 86:253. 1961).
214. Domenjoz.
20:115. 11372.
154. Suit. 3. 0.. and Jasinskv. 0. K.: Phasntaeculogtsl 15:240. 1973. 155. Salem, H.. and Avtado. 0. M : Am. 3. Med. Sc). 247:585, 1964. 156. Chctppcl. C. I.. and von Scemautti, C.: Prog. Med. Client. 3:133—136. 1963. 57.
11:747.
969.
Clin. Mcd. 74:911.
Bttmuuutulield. S. S.. Burden. 1'. P.. atud
765
2(81.
1967.
146. I)eleted itt proud. (47.
4. R.: 3. Lab.
1119. l.conards,
SR.. (larclner. J. II.. and Siesens. J. R.: J. Ant. ('hen,. Soc. 947. Freedman. A. M.: JAMA 97:878. 1966.
33. Fusion,
Agents
Five-Year Report. Br. Med. 3. 2:11)33. 1960.
R.: Attn.
1949.
N. Y. Acad. Sci. 86:263. 1960.
SELECTED READING American
Chentical Society: Fioo
Naiicunai Medicinal Chemistry Sympo-
159.
siunu. ('akimbo'., Oil. Amcncun ChemicalSociety. 194$. pp. lS.-49. Anonynuous: Codeine and Certaitt Other Analgesic and Antitussivc Agent'.: A Review. Rahway. NJ. Merck & Co.. 1970.
161.
Archer, S.. and Harris. L. S.: Narcotic
1511. Wang, S.. Annu. Rep. Med. Chem. 1(1:172—181, 1975.
Collier, H. 0. J.: Nature 231:17, 1971. $0. Vane. 3. K.: Nature 231:232. 1971. Shen. 'I'. Y.: Aitgew. Citem. ml. Ed. 11:460. 1972. (62. Nickander. R.. McMnhon. F. 0.. anti Ridolfo. A. S.: Attnu. Rev. 19:469. 1979. uitacctl. (63 Smith. P. K.: Ann. N. Y. Acntl. Sci. 86:38. 1960. 161. Smith. M. 3. H.. and Smith. P. K. teds. I: The Salicylates. A. Critical tlihliu,gr.upluic Review. New York. John Wiley & Suns. 1966. 65. Collier. H. 0. 3.: Sci. Am. 209:97. 1963. ((lu. Alvarct,., A. S.. and Summerskit(, W. It. J.: Lancet 2:9211. 1958. 67 Harager. F. I).. and Duthie. J. J. R.: Br. Med. J. 1:1106. 196(1. 6$ l.eonards. 3. K.. atid Levy. 0.: Absir. of the 116th meeting. Am. Pharm. Assoc.. Montreal. May 17—22. 1969. p. 67. 19). Davenport. H. W.: N. Engi. J. Med. 27(u:13lt7. 1967. (71) Weiss, H. 3.: Schwei,. Mcd. Wun.:hcnschr. 11)4:114. 1974 Ill EIv.uuctd, P. C.. tO at.: Br. Mccl. J 1:436. l')74. 72
Spccueutit. 1977.
Barlctw. K. B.: Morpluine.like atiutgesics. In Introduction to Chemical Phar-
inacology. Ness' York. .ttulutt Wiley & Sons. 1955. Pp. 39—56. Buarketu. A. H.. and Cu.'.y. A. It.: The testing and clevelopntent of analgesic
Prog. Mccl. Chem. 2:43—87. I'363.
drug'..
Beumg'.vtcmn, V., Johans'.ttn. S.. uttd Angervall. L.: Kidney tnt. 13:107, 197$. Bengcston. V., Johatt'.scmn. S.. tund Angervall. I..: Science 240:129. 1979.
and Morrison. A. L.: Q.
Beegel.
Rev. (Land.) 2:349, 194$.
Rerger. F. M.. ci at.: Ann. N. Y. Aced. Sd. 86:311). 1960.
Bcunica.
3.
J.. and Allen. 0. I).: In Miudell. W. led.). Drug'. cci Cltuuicc
1970—1971. St. Louis. C. V. Moshy, 1971), p. 2 It).
Aspiritt Mycucardial Infaretiotu Study Research Group: JAMA 243:
IIr,tenden. 0. J.. Eddy. N. B.. utud Hallb:tch. H.: Butt. WHO 13:937.
661. 19811.
Bntutclc, M. C., ci al. (ccl'..): Narcotic Antagonists. New
(73. levy, 0.. and Hayes. B. A.: N. hugl. 3. Med. 262:1(153. 1960. 174. l.icbcnnatt. S. V., et al.: 1. Phanut. Sd. 53: $486, 1492. 1964 175. PfeiIkr. C. C.: Arch. FOiot. Med. Ftp. 4:11). 1967. 1711. Salassa, K. M.. t3oltntan. J. M.. anti Dry. 1. J.: 3. Lab. Clin. Med. 33: 1393. 11)48.
955.
York. Raven Press.
1973. Brlluttttter. 1'.: Fort'.chr. Ther.
Btuwtu.
0.
12:24.
M., and Hard>. 'I'. L.: Br. 3.
936,
Pharmucol.
C'hemnoiher. 32:17. 196$.
Casv. A. F.: Ping. Med. CIrcuit. 7:229—284. 197)).
I.. antI vat, Secinatin. C'.: Ping. Med. Clteuiu.3:$9— 145. 1963. Chetu. K. K.: Physiological and pltarntacamlcugical background, including methods of evaluation of analgesic agents. 3. Am. Phanmi. Assuc. Sc). Chappel. ('.
3. t'harntacol. Ftp. Ther. 87:237. 946. (711. Garrett. F. K.: J. Am Pharm. Assay. Sri hot. 48:676. 1959. 77, Smith. I'. K.. ci
at.:
(79. Geld. 0.. and Cautupbctt. J. A.: 3. Plucrnt. Sci. 51:.52. 964. $11, Troop. A. Ii.. uutd Mitelcuter. H.: 3. Phanu. Sci. 53:375. 1964. ((I. Wang. S. M.. uttd Wcsotowski. 3. W.: J. Ant. Pltiurm. Assoc. Sd. F.d. 48:691. 1959. 01. Ohbuuik. H. 3. K.: Lancet 1:565. 1964. C'.: Proc. .Soc. Fop. lOut. Med. 03. (ln(dntan. A. S.. and Yakovac. ($4.
Br. Mccl J. 2:1311.
1115.
Mcd.
963.
Regatdo. K. 0.: Curt.
Ii.: Narcotic Drugs: t9iocheuitical t'hannacotogy. New
Plettum
York,
Press. 1971.
Ccultins. P. W.: Anliiussives. In Burger. A. led.). Medicunat Cltenttsiry. 3rd
ccl. New York. Wiley.tntersciencc. (9711. pp. t35 I — 1364. ('oyume. W. Ii.: Notu'.teroiclal anti.ittllatnuttatory agents and anuipyretirs. In Medicitial Clicuumi'.try.
3rd ed. New York. Wiley.
1970. pp.
dcSteven.'.. 0. )edj: Analgetics. Ness' Ycurk. Acadentie Press. 1965. Tamh;tr. P.
K.: Can. Med. Re'.. Opin. 5:450,
11)78.
117. Deodhar. S. 0.. ci at.:
38:51, 1949. I).
lnterscicnce.
Lctt. 11:7. 1966
($6. l.iyanuge. S. P., and
lid. (.'lutnet.
Burger. A. led.).
115:693. 1964.
9)
antagonists. Ping. Drug Re'.. 8:262.
1965.
Arrigcuni.Manelli. F.: Intlanimittion and Anii-inflntnntalories. New York.
('un Med Mccl.
Re'., Opiuu.5:185. 19711.
Re'.. Opin..S:454. 19711.
Eddy. N. B.: ('hentical structure and action of nuorphu.te.like analgesics intl related substances. Client. hid. lLotud.( 1462, 1959. Eddy. N. B.. Halbach, H., and Hncendetu, 0.1.: Bull. WHO 4:353.402. 1956.
766
tV//curt and GLvtuld 'a Te.rthur;k al ()rç•aoic SIedh'ivtol one! Phonnoec'uiiro! Chc'sni.stn
laldy. N. It Ilalbacli. H.. and Braenden. 0. 3: Bull. WIlD I 7:569—863. .
957.
reviews. Antirheaniatic agents. Am. 3. I Iosp.
I)rug Phaini 36:622, 197'). Fellows. EJ..and UIlsot.G. E.: Analgesics.A Aralkylamines. In American Clteitrical Societ>. Medicinal Chemistry. vol. I. New Yiirk. John Wiles & Souis, VS I, pp 3911—437 Gold. II., ami Cattell. M: Am. J. Med. Se' 246:59(1, 963. Evens. R I'.
Greemihcrg. 1..: Antipyrine: A ('ntmcal Bmhlicmgraplmmc Review. New llaveit.
rr. Hillhmnisc. I 9511, Gross. M.: Acetanilid: A Critical Bibliographic Review. New llaven. CT, Hillhiiase. 1q46. Hellerliaclt. J . Scltnumler. (I. Besendori. H , ci at.: Synilmemmc Analgesics: New York. Pergimtoit I'an II. Morphminans and Press. 1966.
Jacobson. A. F.. May, F. I... and Sargent. L. J.: Analgemics. In Burger. A
edt. Medicinal Chenitstrs.ird ed. New York. Wiley-Interscience. 19711, pp. 1327- 1351).
Janssen. P A J. Symheiie Analgesics: Part I Dmplieiivlpropylanmines. New York. Perganiim Press. I '160. iancsen. P A. J.. and van der Iiyckei'. C. A IsI.: In Burger. A. led.). Drugs
Affectitig ilte Central Nenous Systeni. Ness York. Marcel Dekker. lqlin, pp. 25-85 I.asagna, I..: TIme rliiiical evaluation of niiirphine and its substitutes as analgesics: Phnrrnac,'l. Res. 16:47—83. 1964. l.ee. 3.: Analgesics: B. Partial s)rnctures relateul to nuirpltmne. In American Chemical Society Medicinal Chemistry. vol. I. New York. John
Wiley & Sons. 1951. pp 438 466.
Martin. W. R.: Opioid antagonists Pharmacol. Rev. 19:463 -521. 1967. Mellel. L. B.. and Woods, I.. A.: Analgesia and addiction. Prog. Drug Re' 5:156—267, 1963. Mccl I.ett. 6:78. 1964.
Portiighesc. P S.: Stereocheutical (actors and receptmir interactions aswci ned with narcotic analgesics 3. Pharnt. Set. 55:865. 1966. S.: Selective nonpeplide opioid amagoumsis. In Hero, A. Portoghese.
cdl. Handbisrk of Experimental Pharma'ology: Opioids I, siil. 184' I. Berlin. Springer.Verlag. 1993. Reynolds. A. K.. and Randall, 1.. 0.: Morphine and Allied Dregs. Toennlis Universily of Tormint,' Press. 1957. 1—3 (Sect. 27o1 Salem, H.. and Aviado. 0. M.: Antitnssive International Encyclopedia of Phartnma'ology and Therapeutiesi 0*' lord. Pergamoit Press. 1970. Anti-inflantntatory Agents Sen Schemer. R. A.. and Whiteltonse. M. York. Academic Press. 1974 Sheim. 1. Y.: Perspectives itt nonsteroidal anti-mnllammatory agents. Angrn.
Chcm. Ini. Ed. 11:460, 1972. Simon. F. J.. and Gioannini, T. L.: Opioid receplor nmtliplicit) krlatiin, of hinding sites. In llama. purilicaltuin. and chemical A. lcd.). H;mdhook of Experimental Pltartnacology: Opinids I. sir! 1114/I. Berlimt. Springer-Verlag. 1993. Snyder. S. H.: Opiate receptors and internal opiates.Sci. Am. 237:236-N& 1977.
1',intatis. L.. et al.: Cancer Res. 38:877. 1978. Winder, C. A.: Nonsteeviid anti-inflammatory agemmts. Prrig. l)rug Rcs IS 139—203. 1966.
C
H
A
P
T
E
23
R
Steroid Hormones and Therapeutically Related Compounds PHILIP J. PROTEAU
Steroid hormones and related products represent one of the most widely used classes of therapeutic agents, These drugs
ate used primarily in birth control. hormone-replacement therapy (HRT). inflammatory conditions, and cancer treatment. Most of these agents are chemically based on a com-
mon structural backbone, the steroid backbone. Although they share a common structural foundation, the variations in the structures provide specificity for the unique molecular targets. Five general groups of steroid homiones are discussed: estrogens. progestins. androgens. glucocorticoids. and mineralocorticoids. The structural bases for the differences in actions and the various therapeutic uses for these compounds are explored. Several review articles and texts provide excellent coverage ot'thc pharmacology and chemisuy of steroid hormones) -
STEROID NOMENCLATURE. STEREOCHEMISTRY. AND NUMBERING As shown in Figure 23-I. nearly all steroids are named as derivatives of cholestane. androstanc. pregnane. or cvtrane. The standard system of numbering is illustrated with 5a-cholestane. The absolute stereochemistry of the molecule and any substituents is shown with solid I/3I and dashed (a) bonds. Most carbons have one /3 bond and one a bond, with the /3 bond
lying closer to the ''top or Cl 8 and Cl 9 methyl side of the molecule. Both a and /3 substituems may be axial or equatorial. This system of designating stereochemistry can best be illustrated by use of 5a-androstaue (Fig. 23-2.
Numbering and Primaiy Steroid Names
5a-Chotestane
H3Cf
H3C
H3C
H
5u-Androstane
5(x-Pregnane
51L-Estrane
Examples of Common and Systematic Names
FIgure
23—1 • Steroid noand numbering.
Corlisone (17,21 -Dihydroxypregn•4-ene3,11 ,20-trione)
Testosterone (1
17(1-Estradiol (Estra-1 .3,5(1 O(-tnene'3, t7It-dioI)
767
758
Wilson and
TerthooL
of Organi Medicinal and Phannaceuliral Chemistry
CH3
a = axial e = equatorial in = alpha bond 3 = beta bond
Testosterone
5a-Androslane
1
CH3
H3C
5u,8ct-Androslane
5a-Androstane
H3C
Figure 23—4 • Alternative representations of steroids.
nomenclature to indicate the backbone stereochemistry between rings, For example. 5a steroid.s are A/B trans. and
Figure 23—2 • Steroid nomenclature—stereochemistry.
The stereochemistry of the I-I at CS is always indicated in the name. The stereochemistry of the other I-I atoms is not indicated unless it differs from 5a-cholestane. Changing the stereochemistry of any of the ring juncture or backbone carbons (shown in Fig. 23-I with a heavy line on 5a-cholestone) greatly changes the shape of the steroid, as seen in the examples of 5a.8a-androstane and (Fig. 23-
Sf) steroids are A/B cis. The terms sw, and anti are
used
analogously to tran.5 and cis for indicating stereochemistry in bonds connecting rings (e.g.. the C9:C 10 bond that connects rings A and Cl. Thc use of these terms is indicated in Figure 23-2. The position of double bonds can be designated in any of
the various ways shown below. Double bonds front Cli go toward C9 or Cl4. and those from C20 may go toward C2l or C22. In such cases, both carbons are indicated in
the name lithe double bond is not between sequentialif
Because of the immense effect that 'backbone" stereochemistry has on the shape of the molecule, the International
numbered carbons (e.g.. 5a-androst-8( 14)-enc or androstenc: see Fig. 23-3). These principles of modern mid nomenclature are applied to naming several coinnion
Union of Pure and Applied Chemistry (IUPAC) rules4
steroid drugs shown in Figure 23-I.
2).
strongly recommend that the stereochemistry at all backbone
carbons be clearly shown. That is, all hydrogens along the backbone should be drawn. When the stereochemistry is not known, a wavy line is used in the drawing, and the Greek letter xi is used in the name instead of a or $. Methyls are explicitly indicated as The terms ci.s and trans are occasionally used in steroid
H3C
H3C
Such common names u.s lesloslero,,e and corti.w,ie ate obviously much easier to use than the long systematic name'
Substituents must always have their position and stereochemistry clearly indicated, however, when common are used (e.g.. I 7a-mcthyltestosterone. 9a-Iluoroeortisooe) Steroid drawings sometimes appear with lines drawn instead olmethyls and backbone stereochemisir is nil
indicated unless it differs from that of 5a-andmstane Fig. 23-4). This manner of representation should only be used when there is no ambiguity in the implied
STEROID BIOSYNTHESIS 5-Androstene or A5-Androstene or Androst-5-ene
Steroid hormones in mammals are biosynthesized front cho5a-Androst-8-ene or
1-tac
lesterol, which in turn is made in vivo from A (acetyl-C0A) via the mevulonate pathway. Although ho mans do obtain approximately 300 mg of cholestcnd çss day in their diets, a greater amount (about I g) is hiosynils sized per day. A schematic outline of these biosynthetic path
ways is shown in Figure 23-5. Conversion of cholesterol to pregnenolone is the rate-lim iting step in steroid hormone biosynthesis. It is not the eniy5a-Androst-8(1 4)-ene or
Figure 23—3 • Steroid nomenclature—double bonds.
matic transformation itself that is rate limiting. howcscr, the translocation of cholesterol to the inner meinhr-ane of steroid-synthesizing cells is rate limiting.5 A key protein involved in the translocation is the Sieroidogens
Chapter 23 • Steroid Hornuaws and Therapeutically Related Compounds
H
769
Cholesterol
(Side chain cleavage)
1 7cx'Hydroxypregnenolone
Progesterone
Pregnenolone
[progestinj 3)t.HSD
21 -Hydroxylase
21-Hydroxylase Aidosterone synthaso
OH
/"
Hydrocortteone (corhsol) lglucocorllcoidl
Dehydroepiandrosterone (DHEA)
Aldosterone tmlneratocorllcoidl
HCO
Androstenedione
Estrone lestrogeni
Figure 23—5 • Outline of the biosynthesis
of steroid
hor-
mones. 313-HSD, 3fl-hydroxysteroid
dehydrogenase/J54
6onlerase; I 7f3-HSD,
1
droxysteroid dehydrogenase.
Sa-Dlhydrotestosterone [androgen)
Testosterone landrogeni
Estradlol lastrogeni
.4cute Regulatory protein (StAR). Defects in the StAR gene lead to congenital lipoid adrenul hyperplasia, a rare condition marked by a deficiency of adrenal and gonadal steroid hormones6 The enzymes involved in the transformation of choksterol to the hormones are mainly cytochromer. P450 and dehydrogenases. The main muses of biosynthesis of the hor-
precursor of the steroids. This enzyme mediates a three-step
depicted in Figure 23-5. Estradiol. testosterone. progesterone. aldosterone, and hydrocortisone are representatives of the distinct steroid receptor ligands that are shown.
glucocorticoids. estrogens, and androgens. Introduction of
fates of these compounds are presented structural class. An enzyme denoted cytochrome (SCC for side drain cleavage) mediates the cleavage of the Cl 7 side chain on the D ring of the sterol to provide pregnenolone, the C2 I
ketone provides a substrate in which isomet-ization of the double bond to the J°5 double bond is facilitated. This
mones are
Further tnetabolic under the specific
process involved in the oxidotive metabolism of the side chain. Successive hydroxylations at C20 and C22 are followed by oxidative cleavage of the C20—C22 bond, providing pregnenolone. Pregnenolone can be either directly converted into progesterone or modified for synthesis of unsaluration into the A ring leads to the formation of progesterone. Specifically, oxidation of the alcohol at C3 to the
transformation is mediated by a bifunctional enzyme. hydroxysteroid isomerase This enzyme can act on several 3-ol-5-ene steroids in addi-
770
Wi/con
and Gino/do Tesgl, is increased if needles used for injecting steroids are
shared.' "
Anabolic Androgenic Steroid Products Therapeutic uses of the androgens are discussed above. 17$Esters and I 7a-alkyl products are available for a complete
range of therapeutic uses. These drugs are in in men
with
in pregnancy. Diabetics using the androgens should he carefully monitored.
tenliate the action of oral anticoagulants. causing in some patients, and they tnay also interfere with laboratory tests. Fetuale patients mrty develop virili,at,w side effects, and doctors should be warned that some of these
effects may be irreversible (e.g.. voice changes). All ,'frk "anabolic" agents ctrrently commercially available fllC)h androstenolone. oxymetholone. oxandrolone. st,,noooH. nandrolonc have significant androgenic activity: hence. 'it ilization is a potential problem for all women patients, of the anaholic agents are orally active, as one would pittie by noting a I7a-alkyl group in many of them (see Ftg. 24). Those without the I 7a-alkyl (nandrolone nandrolone phenpropionate) are active only The I 7a-alkyl products may induce liver toxicity in patients. All steroid 4-en-3-ones arc light sensitive and should Is
kept in light-resistant containerS,
Testosterone,
USP. Testosterone, I drost-4-en-3-one. is a naturally occurring androgen in tsar
Chapter 23 • In women, it mainly serves as a biosynthetic precursor to estradiol but also has other hormonal effects. It is rapidly mctaboliied to relatively inactive 17-ones (see Fig. 23-23). however, preventing significant oral activity. Testosterone k available in a transdermal delivery system (patch). a gel formulation, and as implantable pellcts. Testosterone 17$esters are available in long-acting intramuscular depot preparations illustrated in Figure 23-24. including the following: • Testosterone cypionate. USP: Testosterone I 7$-cyclopentylpropionate • Testosterone enanthate, USP: Testosterone 17$-heptarnoate • Testosterone propionate. USP: Testosterone I 7$-propionare
Methyltestosterone.
LiSP.
Methyltestosterone. 17$.
hydroxy-17-methylandrost-4-en-3-one. is only about half as active as testosterone (intramuscularly), but it has the great advantage of being orally active. Fluoxymesterone. 9a-fluorois 11$, I 7$-dihydroxy- I 7-methylandrost-4-en-3-onc. a highly potent, orally active androgen. about 5 to 10 times
Fluoxymesterone,
USP.
more potent than testosterone. It can be used for all the mdi-
discussed above, but its great androgenic activity has made it useful primarily for treatment of the androgendelicient male.
Methandrostenolone, LiSP.
Methandrostenolone. 17$-
hydroxy- I 7-methylandrosta- I ,4-dien-3-one, is orally active and about equal in potency to testosterone.
Oxymetholone, LiSP. Oxymetholone. I 7f3-hydroxy-2h}droxymcthylene)- I 7-methylandrostan-3-one. is approved for the treatment of a variety of anemias.
Steroid Honnont'.c and Tlieraperuieally Related Compounds
801
in spite of the 17a-ethinyl group, has little estrogenic or progestogenic activity. Danazol has been called a synthetic steroid with diverse biological effects.' Danazol binds to sex-hormone-binding globulin (SHBG) and decreases the hepatic synthesis of this estradiol and testosterone carrier. Free testosterone thus increases. Danazol inhibits FSH and LH production by the hypothalamus and pituitary. It binds to progesterone receptors. glucocorticoid receptors, androgen receptors, and ERs. Although the exact mechanism of action is unclear. danazol alters endometrial tissue so that it becomes inactive and atrophic, which allows danazol to be an effective treatment for endometriosis. Danazol is also used to treat hereditary angioedema and fibrocystic breast disease.
Antlandrogens A variety of compounds (Fig. 23-25) have been intensively
studied as androgen receptor antagonists, or antiandrogens.'20 121 Antiandrogens are of therapeutic use in treating
conditions of hyperandrogenism (e.g.. hirsutism, acute acne.
and premature baldness) or androgen-stimulated cancers (e.g.. prostatic carcinoma). The ideal antiandrogen would be nontoxic. highly active, and devoid of any hormonal activity. Both steroidal and nonsteroidal antiandrogens have been investigated. but only nonsteroidal antiandrogens have been approved for use in the United States. Cyproterone acetate,
a steroidal antiandrogen, is used in Europe. The .steroidal antiandrogens typically have actions at other steroid teceptoni that limit their use. The nonsteroidal antiandrogens, while lacking hormonal activity, bind with lower affinity to the androgen receptor than the endogenous hormones. FLUTAMIDE, BICALUTAMIDE, AND NILUTAMIDE
Three nonsteroidal antiandrogens are in clinical use in the
Oxandrolone, LiSP. Oxandrolone, I 7$-hydroxy- 17mcthyl-2-oxaandrostan-3-one. is approved to aid in the promotion of weight gain after weight loss following surgery.
duonic infections, or severe trauma and to offset protein associated with long-term corticosceroid use. Ox-
is also used to relieve bone pain accompanying It has been used to treat alcoholic hepatitis and KIV wasting syndrome. Stanozolol, I 7-methyl-2'/H-5a-anJi%t-2-eno[3.2.cJ-pyrazol- 17$-cl. is used prophylactically n the management of hereditary angiocdema to reduce the ütquency and severity of attacks.
Swnozolot LiSP.
Wandrolone Decanoate, LiSP, and Nandrolone Phenpmplonate, LiSP. Nandrolone decanoate, I 7$-hydroxy-
I 7-decanoate. has been used in the maniscment of certain anemias, but the availability of cvyihropoietin has greatly reduced this use. Nandrolone p&npmpionate is ;bcnyflpropionate.
I
I 7-(3'-
Danazol and Endometriosis Danazol, LISP.
Danazol. l7a-pregna-2.4-dien-20-yno-
L3.djisoxazol- I 7-ol (Danocrine), is a weak androgen that.
United States—flutamide, bicalutamide, and nilutamide (Fig. 23-25). They are mainly used in the management of prostate cancer. Flutamide was the first of these compounds approved for use by the FDA. hut liver toxicity and thricedaily dosing offered room for improvement. It was also determined that a metaholile of flutamide. hydroxyfluramide. had greater antiandrogen action than the parent. Bicalutamide. which has greater potency than tiutamide. incorporates a hydroxyl into its structure at the same relative position as in hydroxyflutamide. Bicalutamide is dosed once a day and has less toxicity than flutamide and nilutamide, making it a preferred choice when initiating therapy.
Prostate cancer is strongly androgen sensitive, so by blocking androgen receptors, the cancer can be inhibited or slowed. Studies have shown that these drugs completely inhibit the action of testosterone and other androgens by binding to androgen receptors. In clinical trials when given as a single agent for prostate cancer, serum testosterone and estradiol increase. But when given in combination with a GnRH agonist, such as goserelin or leuprolide. bicalutamide and flutamide do not affect testosterone suppression, which is the result of GnRH. GnRH agonist.s greatly decrease gonadal function—the medical equivalent of castration in men. Thus, the combination of GnRH with bicalutamide or flutamide blocks the production of testosterone in the testes and androgen receptors in the prostate.
802
lVjlxi,n and Gi.oold s Te.tthook of
Medicinal and F'harmareu:ital ('lie,nj.sirv
0 HH3C
HH3COH
N..)cCH3 0
CF,
CF3
Flutamlde (Eulexin)
CF3
Hydroxyflutamide
Nilutamide (Nflandron)
H3C
HHO
Cl
CF3
Btcatutammde (Casodex)
Cyprotemone
Antlandrogen Products Flutamide, USP. Flutamide. 2-methyl-N-14-rntro-3-( tnfluoromethyl)phenyllpropanamidc (Eulexin), is dosed 3 times daily (250-mg dose: 750-mg total daily dose). A major metabolite of flutarnide. hydroxyllutarnide. is a more potent androgen receptor antagonist than the parent compound. This metabolite. which is present at a much higher steadystate concentration than is ilutarnide. contnihtttes a signilicant amount of the antiandrogen action of this drug. A limiting factor in the use of ilutainide is hepatotoxicity in from I to 5% of patients. Although the hepatotoxicity usually is reversible following cessation of treatment. cases of death associated with hepatic failure have been reported to be associated with tiutatnide therapy. Diarrhea is also a limit. ing side effect with liutamide therapy for some patients.
Bicalutamide, USP. l3icalutaniide. N-4-cyano-3-(tnilluoromethyl)phenyl-3-((4-lluorophenyl )sulfonyl 1-2-hydroxy2-methyl-propanamide (Casodex). is more potent than flutamide and has a much longer half-life (5.9 days versus 6 hours for hydroxyllutaniide. Because of the longer bicalutamide is used for once-a-day (50 nIg) treatment of
hgure 23—25
eral tissues and is involved mainly in the mctatsmlism testosterone and other A—ring enones. The type II is located in the prostate gland and testes and is for the conversion of testosterone to DHT for andasr.. action. Blocking this enzyme is one approach for androgen action. The 1997 review by 1-lanis and l)rovides an excellent background and details the ment of finasteride, the lirst 5a-reductase in the United States 1 Fig. DHT also plays a major role in the pathogenesis ci prostatic hyperplasia (BPH). Finusteride. dimethylethyl )-3-oxo-4-azaandrost- I-cue(I'roscar. Propeci.t). is a potent. slow, tight-binding of 5a-reductase that lunctions by a ttnique tneeltanmen. nasteride is activated by the en/yule and irreversibly hrj to the NADI' cofacior. yielding a tinastenide—NAOP plex that is only slowly released from the etI-,ynle aclin: site, producing essentially irreversible inhibition tytnc (Fig. 23-27). The turnover front tIme flnasteride-5
reductase complex is s'ery slow ti
advanced prostate cancer. Bicalutarnide is available as a racemic mixture, hut both animal and human studies with the androgen receptor show that the H enantiomer has higher affinity for the androgeti receptor than the S enantiorner.
H3C
nation
H N CH3
H
Nilutamide, USP. Nihttamide. 5.5-dimethyl-3-14-nitro3-(trifluoromethyl)phenyll.2.4-imidazolidinedione. is used in combination with surgical castration for the treatnient of metastatic prostate cancer. Nilutamide. which has an elimi-
—30 days).
I
H
Finasteride (Proscar. Propecia)
of approximately 40 hours, can also he used
in once-daily dosing. hut it h;is side eliects that limit its use—visual disturbances, alcohol intolerance. and allergic
CF3
H3C
pneumonitis.
Inhibition of 5a-Reductase 5a—DHT is important for maintaining prostate function in
men. The formation of DHT is mediated by Sa-reductase. an enzyme that has two distinct forms, I and type 11.1 The type I enzyme is located in the liver attd Some periph-
I
H
I
H
F3C
HH Oulasteride
Figure 23-26 • Steroid 5o—reductase inhib:ort
Chapter 23 • Steroid Harinoiwa and Therapeusienili Related Cwnpowids
803
Conversion of Testosterone to 5a-Dihydrotestosterone (DHT)
Finasteride - NADPH Complex Formation
NADPH H
0 NH2
NADPH H H
OH
H3C
23—27 u Comparison of 5a-reaction on testosterone and (masterfe This scheme is an oversimplification of the etact mechanisms, but it indicates that when 'iiasteride is bound at the active site of 5ateductase, NADPH is positioned closer to Cl FIgure
oilmnasteride than to the normal CS of testoscrone, leading to essentially irreversible inhi-
HH NADP
Citron.
Finasteride is a relatively selective inhibitor of type II 5a-
reductase. This enzyme is present in high levels in the prostale and at lower levels in other tissues. Because of the strong
connection to the lomiation of DUT in the prostate, it was theorized that specific inhibition of this isoform would yield the greatest therapeutic effect. More recent studies suggest.
haxever. that the type I isoform may also play a role in the progression of hormone-dependent prostate cancer.'27 Because of this, dual 5a-reductase inhibitors have been de-
Dutasteride. a compound recently approved for BPH. inhibits both isotirms of the enzyme and may lv found to have superior therapeutic elfects once it is tniiadly used (Fig. 23-26). Dutasteride bears an aromatic aiiide at C17. rather than the t-hutyl amide seen in finast-
tive in the treatment of BPI-I. a lower dose formulation was studied for treating male pattern baldness. The trials were a success, and Propecia (I mg/day) was the result. Although
finastcride preferentially inhibits the type II enzyme, it
is
believed to be the peripheral type-I 5a-reductase that is being targeted for the baldness treatment. Dutusteride is also being investigated lor use as a baldness treatment. Saw palmetto (St-renoa repens) extract is an herbal prod-
uct used to treat BPH. and it has been suggested that the effects can be attributed to a constituent of the extract with 5a-reductase inhibition, but other mechanisms have also Further studies and identification of a specific component that inhibits 5a-reductase are necessary. been
crhle.
A second use of fmasteride is in the treatment of male baldness. The conversion of testosterone to DHT in sluancing years leads to thinning of hair in men. Inhibition ri this conversion was envisioned as a possible baldness teatment, After finasteride was shown to be safe and effec-
ADRENAL CORTEX HORMONES
Endogenous The adrenal glands (which lie just above the kidneys) secrete over 5() different steroids, including precursors for other stir-
804
Wil.sa,: and Gixvold.s Textbook of Organic Medicinal and Pliunnaceuthal C/ien,ixirv
ume. The glucocorticoids have key roles in controlling carbohydrate, protein, and lipid metabolism.
mid hormones. The most important hormonal steroids produced by the adrenal cortex, however, are aldosterone and hydrocortisone. Aldosterone is the primary ,nineralocor:icoid in humans (i.e.. ii causes significant salt retention). Hydrocortisone is the primary g!ueocoruccnd in humans (i.e.. it has its primary effects on intermediary metabolism). The glucocorticoids have become very important in modem medicine. especially for their anti-inflammatory effects. Aldosteronc and. to a lesser extent, other mineralocorlicoids maintain a constant electrolyte balance and blood vol-
BI.synthesls As shown in the scheme in Figure 23-28. aldosteronc and hydrocortisone are biosynthesized from pregnenolonc through a series of steps involving hydroxylations at Cl7, CII, and C2 I that convert pregnenolonc to hydrocortisone. Deficiencies in any of the enzymes cause congenital adrenal
Cholesterol
H3C H
H3C
HO
H
I
21
OH
11
0 H3C
H
0 Hydrocortisone (cortisol)
H3C
0 — Aldosterone, hemiacetal form
Figure 23—28 • Biosynthesis of hyilo. cortisone and aldosterone.
Chapter 23 • Su'ruid Hor,,w,ws and Thrrapeuliealh Rt'hired ('onipounds
hyperplasia. Defects in the gene regulation, as well as the eniynics that catalyze the hydroxylation have been studied u Investigators have linked defects in particular genes or steroid-binding sites to the pathophysiology of patients with the corresponding metabolic These disorders are usually caused by an inability of the adrenal glands to carryouf lip-. 17a-.or2l-hydroxylations.
The moot common is a lack of 21-hydroxylase activity. which will result in decreased production of hydrocortisone and a compensatory increase in adrenocorticotropic hornione (ACTH) production. Furthermore, the resultant buildup of 17n'-hydroxyprogesteronc will lead to an increase of testosterone. The 21 -hydroxylase is important for the synthesis of both mineralocorticoids and glucocorticoids. When I I$.hydroxylase activity is low, large amounts of II -deoxycorticosterone will be produced. Because II -deoxycorticostemne is a potent mineralocorticoid. there will be symptoms of mineralocorticoid excess, including hypertension. When lla-hydroxylase activity is low, there will be decreased production of testosterone and estrogens as well as hydnxorti-
0 H3C
H
0
OH
805 OH
-
H
H0
H
Tetrahydrocortisol
HO
H
u'Cortol
OH
H
lt'Cortolone
sane.
Although the details are not completely known, the 39amino acid peptidc ACTH (corticotropin) produced by the anterior pituitary is necessary for the conversion of cholesterol to pregnenolone. AC'FH acts at the ACTI-l receptor, a C-protein—coupled receptor that activates adenylyl cyclase. kading to increased cAMP levels. Activation of the teceptors has short. and long-term effects on steroidogenesis. The short-term phase involves an increase in the supply of cholesterol for use by cytochrome in the formation of pregnenolone. The long-term effects are due to increased transcription of steroidogenic An overall result of ACTH action is increased synthesis and release sihydrocortisone. Hydrocortisone then acts by feedback in-
hibition to suppress the formation of additional ACTH. tACTH is discussed in more detail in Chapter 25.) The release of the primary mineralocorticoid aldosterone
only slightly on ACTH. Aldosteronc is an active
11
Figure 23—29 • Metabolites of cortisone and hydrocortisone.
cortolones (20a and 20/3). and II $-hydroxyetiocholanolone are some of the minor metabolites of hydrocortisone (Fig. 23-29).
Biological Activities of Mineralocorticolds and Glucocorticolds The adrenocortical steroids permit the body to adjust to envi-
part of the angiotensin—renin—blood pressure cycle that con-
ronmental changes. to stress, and to changes in the diet.
solo blood volume. A decrease in blood volume stimulates the kidneys to secrete the enzyme renin. Renin. in turn. conSerb angiotensinogen to angiotensin. which stimulates the adrenal cortex to release aldosterone. Aldosterone then causes the kidneys to retain sodium. und blood volume increases. When the blood volume has increased sufficiently, resin production decreases, until blood volume drops again.
Aldosterone and, to a lesser extent, other mineralocorticoids maintain a constant electrolyte balance and blood volume. and the glucocorticoids have key roles in controlling carbohydrate, protein, and lipid metabolism. Aldosterone increases sodium reabsorption in the kidneys.
Metabolism of Nydrocortisone Hydrocortisone and cortisone are enzymatically interconverand thus one finds metabolites with both the I 1-keto sod the
I l,6-hydroxy functionality. Most of the metabolic
pnreeo.ses occur in the liver, with the metabolitcs excreted fnnrarily in the urine. Although many metabolites have been solated. the primary routes of catabolism are (a) reduction of the C4.5 double bond to yield Sfl-pregnanes. (h) reduction nf the 3-one to give 3o-ols. and (r) reduction of the 20-one to the corresponding 20a- and 20p-ols. These are the same that arc involved in progesterone metabolism. The two primary metabolites arc tetrahydrocortisol and tetrahydrosorbisonc and their conjugates. The cortols (20a and 20p-,
An increase in plasma sodium concentration, in turn, will lead to increased blood volume, because blood volume and urinary excretion of water are directly related to the plasma sodium concentration. Simultaneously. aldosterone increases potassium ion excretion. I l-Deoxycorticostcronc also is quite active as 'a mineralocorticoid. Similar actions are exhibited with hydrocortisone and corticosterone. but to a much smaller degree.
Aldostcmne controls the movement of sodium ions in most epithelial structures involved in active sodium transport. Althottgh aldosterone acts primarily on the distal convoluted tubules of the kidneys. it also acts on the proximal convoluted tubules and collecting ducts. Aldosterone controls the transport of sodium in sweat glands, small intestine. salivary glands, and the colon. In all of these tissues, aldosterone enhances the inward flow of sodium ions and promotes the outward flow of potassium ions.
806
Wilson and Gist-aids Textbook of Organic Medicinal and Phamraceu:iraI C'he,nix,rv
The glucocorticoids have many physiological and pharmacological actions. They control or influence carbohydrate.
protein. lipid, and purine metabolism. They also affect the cardiovascular and nervous systems and skeletal muscle. They regulate growth hormone gene expression. In addition.
glucocorticoids have anti-inflammatory and linmunosuppressive actions that arise through complex mechanisms. Glucocorticoids stimulate glycogen storage synthe.sis by inducing the synthesis of glycogen synthase and stimulate gluconeogenesis in the liver. They have a catabolic effect on muscle tissue, stimulating the formation and transaminalion of amino acids into glucose precursors in the liver. The catabolic actions in Cushing's syndrome are demonstrated by wasting of the tissues. osteoporosis. and reduced muscle mass. Lipid metabolism and synthesis increase significantly in the presence of glucocornicoids, but the actions usually seem to depend on the presence of other hormones or colactars. A lack of adrenal cortex steroids also causes depression, irritability, and even psychoses. reflecting significant effects on the central nervous system.
tnechanism is not fully understood but appears to be a de. crease in the binding or activation ability of glucocorticoid receptor complexes and their target or "activator" genes. Disruption of the translocation of the glucocorticoid receptor to the nucleus has also been implicated in glucocorticoid resistance.'"
Structural Classes: Mlneralocorticolds and Glucocorticolds Medically important adrenal Cortex hormones and synthetic mineralocorticoids and glucocorticoids are shown in Figure 23-30. Because salt retention activity is usually undesirable. the drugs are classified by their salt retention activities. As illustrated in Figure 23-30. the adrenal cortex hormones are
classified by their biological activities into three majts groups. MINERALOCORTICOIDS
The mineralocorticoids are adrenal cortex steroids and ana-
logues with high salt-retaining activity. They are used ANTI-INFLAMMATORY/IMMUNOSUPPRESSIVE ACTIONS OF GLUCOCORTICOIDS'33'37
Glucocorticoid-receptor complexes (see Fig. 23-7) may activate or repress the genes to which they associate. Repression
in particular may have an important role in glucocorticoid anti-inflammatory actions. Glucocorticoids inhibit the transcription of genes encoding cylokines such as interferonv. tumor necrosis factor-a (TNF-a), the intericukins. and granulocyte/monocyte colony-stimulating factor, all factors
involved in the immune system and inflammatory reGlucocorticoids inhibit he production and release at other mediators of inflammation, including prostaglandins, leukotrienes. and histamine. In addition. glucocorticoids inhibit the expression of the gene encoding collagenase. an important enzyme involved with inflammation. RESISTANCE TO
A few patients with chronic inflammatory illnesses such as asthma, rheumatoid arthritis, and lupus develop resistance to the anti-inflammatory effects of the glucocorticoids. The
TABLE 23-7
mainly for treatment of Addison's disease, or primary adrenal insufliciency. The naturally occurnng hormone aldosterone has an I Ip-OH and an 18-CHO that naturally bridge to form a hemiacetul (as drawn in Fig. 23-28). Aldostcrunc is too expensive to produce therefore other semisynihetic analogues have taken its place for trealmeni of Addison's disease. Adding a 9a-fluoro group to hydrocor. tisonc greatly increases both salt retention and an
in the biosynthesis of aldosterone. has lower
mineralocorticoid activity than aldostcronc (—20-fold) hut may play a role if the 1 is deficient. Deosycorticosterone is not available for therapeutic uses. Extensive modifications have been made to the basic h)drocortisone structure to alter the properties of glucococticoids. Modifications at all sitcs of the steroid backbone been tried. Aside from addition of a double bond at CI -C2 the most beneticial changes are made to rings B and D of the steroid skeleton and ntodification of the Cl 7 side charn Table 23-7 summarizes the relative effects of various uents seen in commercially available products on salt reten-
tion and glucocorlicoid activity. The salt-retaining
Effects o f Substituents on GI ucocorticoldlMlneralocortic old ActivIty Deposition
Antl-lnflammatoi'y Activity
Glycogen
Functional Group
Effects on Urinary Sodlum
9u-Fluom ih,.Chlom
It)
7—I))
3—5
3—4
I -Dchydrtt
3-4
3-4
—
2—3
1—2
-. — —
lka-Hydroxy
t)4—O.5
0.1-41.2
17a.I-iyslro5y
1—2
4
—
4-7
25
+4
2t-Hydroxy
Inmi Rodig. OR.. lii ISorgor. A. (od.) Mediciin) Chcmi-u)-. Port 2. 3rd cd. Ncw York. 4 - teIctliion. - cncrcliotl. —
4--I-
++
———
970. Licd with
Chapter 23 U Steroid Hormones and Iherapeiukallv Related C,nnpounds
807
are approximately additive. For example. the 3 + increase in salt retention of a 9a-lluoro group can he eliminated by
lone and prednisone. As shown in Table 23-K. an I lfl-OH
the 3— decrease of a 6a-ntcthyl.
ones have little or none. The I dehydrogenase in the skin oxidizes an I I 1-keFor activation of an Il-one glucocorticoid for topical action, reduction at Cli would be necessary. The 1-enc of prednisolone and prednisone increases anti-inflammatory activity about fourfold and somewhat decreases salt retention. Duax and coworkers2° have shown that the increase in
GLUCOCORTICOIDS WITH MODERATE-TO-LOW SALT RETENTION
The glucocorticoids with moderate-to-low salt retention inchide cortisone. hydrocortisone, and their I -enes predniso-
1.
maintains good topical anti-inflammatory activity, but II-
(HigtiSalt CH3
Aldosterone (not commercially available)
11 -Deoxycortlcosterone (not commercially available)
Fludrocorlisone Acetate
2. Glucocorticoids With Moderate-to-Low Salt Retention CH3
0
OR' H3C
Hydrocorlisone (8 8' = H) (or cortisol)
Cortisone Acetate
Esters available Hydrocortisone acetate: R= COCH3, 8' = H Hydrocortisone buleprale: R' = COCH2CH3 A" = COCH2CH2CH3 Hydrocortisone butyrate: R = H, 8' = COCH2CH2CH3 Hydrocortisone cypionate: 8'
Hydrocortisone valerate: 8= H, A" = COCH2CH2CH2CH3
21-Salts available (8' = H) Hydrocortisone Sodium Phosphate: 8' = P032 (Na')2 Hydrocortisone Sodium Succinate: = COCH2CH2CO2 Na
OR
Prednisolone Esters available Precinisolone acetate: A = COCH3 Prednisolone tebutate: 8 = COCH2C(CH3)3
Salts available Prednisolone Sodium Phosphate: A = P032 Prednisolone Sodium Succinate: A = COCH2CH2CO2 Na
Prednisone
Figure 23—30 • Natural and synthetic corlicosleroids.
808
Wilson and Giscold's Textbook of Organic Medicinal and Pharmaceutical Cia-mica-s
0
0A
Ciobetaso Propionate 1-ene
H
A6 = H
R'6= -CH3 A=
A9
A17 = OCOCH2CH3 Halobetasol Propionate 1-ene
H
F
A6=F R'6=
A6 = A9 =
A' =
A'7 = OCOCH2CH3 Hatcinonide
H
A' = A6 = A9
R6=H
F
A'6'7 = acetonide
A=
A6 =
A6 =
A' = H
H =
A9 = A'6 = H A
A6 = A9 = A6 = A9 = A'6 = H
R=
H (Ituocinonide is the C21 acetate) Fturandrenolide
A6=F A9 = H
A' = H
0\JO -H
-CH3
ICIIH F
Aiclometasone Dipropionate
Desoximetasone
Clocortolone Pivalate
Figure 23—30 • Continued.
activity may be due to a change in shape of ring A. Specifically. analogues more active than hydrocortisone appear to have their ring A bent underneath the molecule to a much greater extent than hydrocortisone. The 11)3-OH of hydrocortisone is of major importance in binding to the receptors. Cortisone is reduced in vivo to yield hydrocortisone as the active agent. The increased activity of 9a-halo derivatives may be due to the electron-withdrawing inductive effect on the 11)3-OH, making it more acidic and, therefore, better able to form hydrogen bonds with the recep-
tor. A 9a-halo substituent also reduces oxidation of the I
OH to the inactive Il-one. GLUCOCORTICOIDS WITH VERY LITTLE OR NO SALT RETENTION
Cortisone and hydrocortisone. and even prednisone prednisolone. have too much salt-retaining activity in hr doses needed for some therapeutic purposes. Over the several decades, a number of substituents have been discov.
Chapter 23 • S:eroi(I Honrwnr'.c croci Tl.erape,aiealle Related Ccnupo,,nd.c
809
TABLE 23—8 Approxlmat a Ralative Activities of Corticosteroids'
Aldmiemne t)eoxycomeostemne
Fludrocorti,one
Anti-Inflammatory Activity
Topical Activity
Activity
0.2
0.2
800
1)
0
40
10
5—40
$00
2
Salt-Retaining
Equivalent Dose (mg)
(ilucoconicoids Ilydrocorllsone Cortisone Prednisolone Prednlsone
Timinoloncacetonidc
I
20
0.8
0
OIl
25
4
4
0.6
5
35
0
0.6
5
5
5
0
4
5
5—100
0
4
1
Triamclnolone
I
I—S
fltiocinolone ucetonide
Over 40
Httmndrcno%lde
Over 20
Iluocinolone
Over 40 46.-laO
Fluocinonide Bemmethasone
35
5—1(N)
0
0.6
I)cnamcthasone
30
(0—35
0
0.75
The dais in this able arc only approsimslc. Blanks indic.,ic Ihol comparative data are not available to the author or that he pmduci has only 'ire use LAo, hum ventral source,. and lucre is in inherent ml. in such nlnia The bible should. hurwevet. sent as a guide to nsl,iriuc inlivuutes.
tied that greatly decrease salt retention. They include l6a-
hydroxy: l6a.l7a-ketal: 6a-methyl; and lfia- and l6flmethyl. Other substiluents have been found to increase both glucocorticoid and mineralocorticoid activities: I -ene. 9oluoro, 9a-chloro. and 21-hydroxy. As a result of the great economic benefit of having a potent
anti-inflammatory product on the market, pharmaceutical manufacturers have made numerous combinations of these varIous substituents. In almost every case a 16-methyl or a modified 16-hydroxy (to eliminate salt retention) has been combined with another substituent to increase glucocorticoid
or anti-inflammatory activity. The number of permutations and combinations has resulted in a redundant army of analogues with very low salt retention and high anti-intlammatory activity.
A primary goal of these highly anti-inflammatory drugs has been to increase topical potency. As shown in Table 238. come are as much as 100 times more active topically than hydrocorlisone. Relative potency is as follows: Very high potency Augmented beramethasonc dipropionate ointment. 0.05% Clohetasol propionate. 0.05% Ditlonlsonc diacetame ointment. 0.05% High potency Ameinonide. 0.1% Betamethasone dipropionate ointment. 0.05% Desoxinsetasone. 0.25% Diflorasonc diacetute cream. 0.05% Fluocinonide. 0.05% Hatcinonide, 0.1% Halobetasol propionate. 0.05% Triarncinolone acetonide, 0.5%
. roptc.ill. Dais snore
Medium potency Belamethasonc- valeralc. 0.1% Ciocortotomme pivalate. 0.1% Desoxiinetasonc, 0.05% Fluocinolone acelonide. 0.025% fluticasone propiirnamc. 0.005%. Hydrocuunlisonc hutyramc, 0.1% Hydrou:oni.sone vatcrate. ((.2% Mometasone (uroate. (1.1% Prednicarbale. (1.1%
Triumcinolone acelonide, 0.1% Low potency Aiclomelasone dipropionate. 0.05% Desonide. 1)05% Fluocinolonc aectonide. O.O1% Triamcinolonc acetonide cream. 0.1%Lowest patency Hydrocortisone. 1.0% Hydrocorlisonc. 25%
Although, as shown in Table 23-8. cortisone and prcdnicone arc not active topically, most other glucocorlicoids arc active. Some compounds, such as clobctasol and hetametha-
sone dipropionate. have striking activity topically. Skin absorption is favored by increased lipid soluhility of the drug. Absorption of topical giucocorticoids can also be greatly affected by the extent of skin damage. concentration of the glucocorocoid. cream or ointment base used, and similar factors. must not assume, therefore, from a study of Table 23-8 that. for example. a 0.25% cream of prednisolone
is necessarily exactly equivalent in anti-inflammatory potency to 1% hydrocortisone. Nevertheless, the table can serve as a preliminary guide. Furthermore, particular patients may seem to respond better to one topical anti-inflammatory
810
tVi!ssui and
Texthmik of
Medieinal and Pharrnaceuzical C'hrmictrv
glucocorticoid than to another, irrespective of the relative potencies shown in Table 23-8. RISK OF SYSTEMIC ABSORPTION
used to treat asthmatic symptoms unresponsive to bronchodilators. They arc especially useful in inhaled formulations (see section below). The glucocorticoids' lymphocytopenic
actions make them particularly useful for treatment of chronic lymphocytic leukemia in combination with other
The topical corticosteroids do not typically cause significant absorption effects when used on small areas of intact skin. When these compounds are used on large areas of the body. however, systemic absorption may occur, especially if the skin is damaged or if occlusive dressings are used. Up to 20
antincoplastic drugs. The adrenocortical steroids are contraindicated or should be used with great caution in patients who have (a)
to 40% of hydrocortisone given rectally may also be ab-
body's normal infection-fighting processes). (d) psychoses (since behavioral disturbances may occur during stemid therapy). (e) diabetes (the glucocorticoids increase glucose production, so more insulin may be needed). (I) glauconvv (g) Osteoporosis, or (/z,I herpes simplex involving the When administered topically, the glucocorticoids prescrn relatively infrequent therapeutic problems, but their anti.inflanimatory action can mask symptoms of infection. Man:. physicians prefer not giving a topical anti-inflammatory steW roid until after an infection is controlled with topical antibiotics. The immunosuppressive activity of the topical glucocreticoids can also prevent natural processes from curing 1k infection. Topical steroids actually may also cause dentists. ses in some patients. Finally, as discussed above with the oral contraceptives steroid hormones should not be used during pregnanc). It it is absolutely necessary to use glucocorticoid.s topicall> during pregnancy, they should he limited to small areas .1
sorbed.
Thetapeutk Uses of Adrenal The adrenocortical steroids are used primarily for their glucocorticoid effects, including immunosuppression. anti-inflarnnnatory activity, and antiallergic activity. The mineralocorticoids are used only for treatment of Addison's disease. Addison's disease is caused by chronic adrenocortical insufficiency and may he due to either adrenal or anterior pituitary failure. The glucocorticoids are also used in the treatment of congenital adrenal hyperplasias. The symploms of Addison's disease illustrate the great importance of the adrenocortical steroids in the body and. especially, the importance of aldosterone. These symptoms include increased loss of body sodium. dccrea.sed loss of potassium. hypoglycemia, weight loss. hypotension. weakness. increased sensitivity to insulin, and decreased lipolysis. Hydroconisone is also used during postoperative recovery after surgery for Cushing's syndrome—excessive adrenal secretion of glucocorticoids. Cushing's syndrome can be caused by bilateral adrenal hyperplasia or adrenal tumors and is treated by surgical removal of the tumors or resection of hypcrpiastic adrenal gland(s). The use of glucocorticoids during recovery from surgery for Cushing's syndrome illustrates a most important principie of glucocorticoid therapy: abrupt withdrawal of glucocorticoids may result in adrenal insufficiency, showing clinical symptoms similar to those of Addison's disease. For that reason, patients who have been on long-term glucocornicoid therapy must have the dose reduced gradually. Furthermore. prolonged treatment with glucocorticoids can cause adrenal suppression, especially during times of stress. The symptoms are similar to those of Cushing's syndrome, such as rounding of the face, hypertension, edema. hypokalemia, thinning of the skin. osteoporosis, diabetes, and even subeapsular cataracts.
The glucocorticoids are used in the treatment of collagen vascular diseases, including rheumatoid arthritis and disseminated lupus crythemalosus. Although there is usually
prompt remission of redness, swelling, and tenderness by the glucocorticoids in rheumatoid arthritis, continued longterm use may lead to serious systemic forms of collagen disease, As a result, the glucocorticoids should be used infre-
quently in rheumatoid arthritis. The glucocorticoids are used extensively topically, orally, and parenterally to treat inflammatory conditions. They also usually relieve the discomforting symptoms of many allergic
conditions—intractable hay fever, exfoliative dermatitis. generalized eczema, and others. The glucocortieoid.s are also
ulcer (in which the steroids may cause hemorrhage), (b) heal
disease. (c) infections (the glucocorticoids suppress thc
intact skin and used for a limited time.
Mhse,alocortkold and Glucocorticold Produds The corticosteroids used in commercial products are shown in Figures 23-30. 23-31, and 23-32. The structures the usual changes (see Fig. 23-6) made to modify solubilit) of the products and, therefore, their therapeutic uses. In particular, the 21 -hydroxyl can be converted loan ester to makc it less water soluble to modify absorption or to a phosphate ester salt or hemisuccinate ester salt to make it more waten soluble and appropriate for intravenous use. The pmduvLc also reflect the structure—activity relationship changes dis
cussed above to increase anti-inflammatory activity or tency or decrease salt retention. Again, patients who have been on long-term glucocorticoid therapy must have the dose reduced gradually. This "critical rule" and indications are discussed above under the heading. Therapeutic Uses of Adrenal Cortex Honnones
Dosage schedules and gradual dosage reduction can be quite complex and specific for each indication. Many of the glucocorticoids are available in topical dos' age forms, including creams, ointments, aerosols, lotion. and solutions. They are usually applied 3 to 4 times a to well-cleaned areas of affected skin. Ointments are usualh prescribed for dry, scaly dennatoses. Lotions are well suite.i for weeping dermatoses. Creams are of general use for man) other dermatoses. When applied to very large areas olskrn or to damaged areas of skin, significant systemic absorption can occur. The use of an occlusive dressing can also greati) increase systemic absorption. The glucocorticoids that are mainly used for inflammation
of the eye are shown in Figure 23-31. These compounds differ structurally from other glucoconicoids. in that the 21.
Chapter 23 • Steroid Hor,rn,,ws and !l,eraj,eui teal/v Related ('an,iwunuts
811
CH3
Fluorometholone
Medrysone
0
CH3
HO
H3C
0
IHIH Figure 23—31
•
hydroxyl is missing from medrysane. fluorometholone. and nmexolone. while loteprednol etabonate has a modified ester utCI7 that leads to rapid degradation upon systemic absorption.
MINERALOCORTICOIDS
Fludrocortisone Acetate, USP.
Fludrocortisone ace21 -acetyloxy-9-lluoro- I lfl.l 7-dihydroxypregn4-ene3.20-diane. 9r -fluorohydrocortisone (Florinef Acetate), is used only for the treatment of Addison's disease and for ute,
inhibition
Loteprednol Etabonate
Ophthalmic glucocortucoids.
of endogenous adrenocortical secretions. As
shown in Table 23-8. it has up to about 800 times the miner-
alocorticoid activity of hydrocorlisone and about II Limes the glucocorticoid activity. Its potent activity stimulated the smthesis and study of Ihe many fluorinated steroids shown in Figure 23-30. Although its great salt-retaining activity
Cortisone Acetate, USP. Cortisone acetate. 21 -(acetyloxy)- I 7-hydroxypregn-4-ene-3. II .20-trione. is the 21acetate of naturally occurring cortisone with good systemic anti-inflammatory activity and low-to-moderate salt-retention activity after its in vivo conversion to hydrocontisone acetate, This conversion is mediated by Ii dehydrogenase. It is used for the entire spectrum of uses discussed above under the heading. Therapeutic Uses of Adrenal Cortex Hormones—collagen diseases. Addison's disease. severe shock, allergic conditions.chronic lymphocytic leukemia, and many other indications. Cortisone acetate is
relatively ineffective topically, mainly because it must be reduced in viva to hydrocortisone. Its plasma half-life is only about 30 minutes. compared with 90 minutes to 3 hours for hydrocortisone.
Prednisolone, liSP.
Prednisolone. .i'-hydrocortisone.
limits its usc 10 Addison's disease, it has sufficient glucocorucoid activity that in some cases of the disease, additional ducocarlicoids need not be prescribed.
17.21 -trihydroxypregna- I .4-dicne-3.20-dione. has less salt-retention activity than hydrocortisone (see Table 23-8), but some patients have more frequently experienced complications such as gastric irritation and peptic ulcers. Because
GLUCOCORTICOIDS WITH MODERATE-TO-LOW SALT
of low mineralocorticoid activity, it cannot be used alone
RETENTION
for adrenal insufficiency. Prednisolonc is available in a variety of salts and esters to maximize its therapeutic utility (see Fig. 23-30):
llydrocortisone. 11$. 17.21 -tnis the primary natural gluin humans, Despite the large number of synthetic niucocoilicoids, hydrocortisonc. its esters, and its salts rcmain a mainstay of modern adrenocortical steroid therapy nod the standard for comparison of all other glucocorticoids nod mineralocorticoids (see Table 23-8). It is used for all Hydrocortisone. USP.
II
Prcdnisolone acetate, LISP 121 -acetate) Prednisolonc sodium phosphate. USP (21-sodium phosphate) Prednisolone sodium succinate, LISP 421-sodium succinatc) Prednisolone tcbatatc, LISP (21 -tebutate)
the indications mentioned above. Its esters and salts illustrate he principles of chemical modification to modify pharmaco-
17.21 -dihyPrednisone, liSP. Prednisone. droxypregnu-l.4-dicne-3,ll,20-trione. has systemic activity
kinetic use shown in Figure 23-6. The commercially available sails and esters (see Fig. 23-30) include
very similar to that of prednisolone. and because of its lower
Ilydmcortisonc acetate, LISP (21 -acetate) Hydnucortisonc huteprute. Lisp (I 7-hutyrute. 21 -prupionatct Hydrocortisone hutyrate, LISP (I 7-hutyrate) Hydrucortisane cypionale. LiSP (21 -cypionate) Hydrucortisone sodium phosphate. USP 121 -sodium phosphate) sodium succinate. LISP (21-sodium succinate) Hydrocoruisone valcrute, liSP (I 7-vulerate)
salt-retention activity, it is often preferred over cortisone or hydrocortisone. Prednisone must be reduced in vivo to prednisolone to provide the active glucocorticoid. GLUCOCORTICOIDS WITH VERY LITTLE OR NO SALT RETENTION
Moot of the key differences bctwecn the many glucocorticoids with minimal salt retention (see Fig. 23-30) have been
812
and Gist'old's Th'xihook of Organic Medicinal and Pham,aeeutieal Chemistry
summarized in Tables 23-7 and 23-8. The tremendous therapeutic and, therefore, commercial importance of these drugs has stimulated the proliferation of new compounds and their
Desoximetasone. liSP.
products. Many compound.s also are available as salts or esters to give the complete range of therapeutic flexibility illustrated in Figure 23-30. When additional pertinent information is available, it is given below. The systemic name for each drug is provided after the common name.
structure.
Alclometasone Dipropionate, liSP.
Aiclometasone dipropionate. 7u-chloro- I lfl-hydroxy- 16a-methyl- 17,21 his( I -oxopropoxy )-pregna- I .4-diene-3.20-dione (Aclovate).
is one of the few commercially used glucocorticoids that bears a halogen substituent in the 7a position.
Amcinonide, USP. Amcinonide. 2l-(aceryloxy)-16a. 17 -Icyclopcntylidcnehis(oxy)t-9-fluoro- II fi-hydroxypreg. na-I .4-sliene-3.20-dione (Cyclocort).
Bedomethasone Dipropionate, USP.
l3eclomethadipropionnie, 9-chloro- II 17,21 -bis( I -oxopropoxy)-pregna- I .4-diene-3.20-dione (Beconase. Vancenase, Vanceril, QVAR). is used in nasal sprays and aerosol formulations to treat allergic rhinitis and asthma (see section below). sone
Betamethasone, USP.
Betamethasone. 9-fluoro- Ilfl.
17,2 1-trihydroxy- I 6fl-methylpregna- I .4-dicne-3.20-dione. is available as a variety of ester derivatives.
Desoximetasone, 9-fluoro-I
21 -dihydroxy- I 6a-methylpregna- I .4-diene-3.20-dione. like
clocortolone pivalate. lucks a C17a hydroxyl group in its
Dexamethasone, liSP.
Dexamethasone. 9-fluoro-l 17.21 -trihydroxy- I 6a-methylpregna- I .4-diene-3,20-dione. is the 16a isomer of betamethasone. Desaniethasone acetate. liSP (21 -acetate) Dexamethusune sodium phosphate. liSP (21-sodium phospltata
Diflorasone Diacetate, liSP.
Diflorasone diacetate, 17,
21 -bis(acetyloxy)-6a.9-difluoro- II fi-hydroxy- I 6a-mcthylpregna- I .4-diene-3.20-dionc.
Flunisolide, liSP.
Flunisolide. 6a-fluoro- II $.2 I-dittydroxy- I 6a. 17-1(1 -methylethylidene)his( oxy) Ipregna. 1.4dicne-3.20-dione. (See following section for use of flunisol. ide in the treatment of asthma.)
Fluocinolone Acetonide, liSP.
Fluocinolone acelonide, 6a.9-dilluoro- II $.2 I -dihydroxy- 16a, 17-1(1 -methylcthylidenc)bis(oxy)Jpregna-I.4.diene-3.20-dione. also knosa as 6a-fluorntriumcinolone acetonide. is the 21-acetate deriv-
ative of Iluocinolone acetonide and is about 5 times mote potent than fluocinolone acetonide in at least one topical activity assay.
Retamethasone valerate, USP (l7-valcratc) I3etametha.sone acetate, USP (21-acetate) Bctamethasone sodium phosphate. liSP (21-sodium phosphate) Hetamethasone dipropionate. liSP I 7-propionatc. 21-propionate)
Budesonide, USP.
Budcsonidc. I 6a. I 7-Ibutylidenebis(oxy )I- I Ifl.21 -dihydroxypregna- I ,4-dienc-3,20-dione (Entocort). in oral capsules is used to treat Cmhn's disease. The
affinity for the GR is approximately 200-fold greater than that of hydrocorlisone and 15-fold greater than that of prednisolone. Budesonide is a mixture of epinters, with the 22R form having twice the affinity for the GR of the S epimer. This glucocorticoid is metabolized by CYP 3A4. and its levels can be increased in the presence of potent CYP 3A4 inhibitors. Budesonide is also used in an inhaled formulation for the treatment of asthma (see below).
Clobetasol Propionate, liSP.
Clobetasol propionate. 2 l-chloro-9-tluoro-I I 6f3-methyl- I 7-( I -oxopropoxy-pregna- I .4-diene-3.20-dione (Temovate).
Ckx:oriolone pivalate, 9Clocortolone Pivalate,, liSP. chloro-2 I -(2.2-dimethyl- I -oxopropoxy)-6a-fluoro- II
Fluorometholone, liSP.
Fluorometholonc. 94luoro11$. l7-dihydroxy-6a-methylpregn-4-cne-3.20-dione(FluteOp. FML). lacks the typical C21 hydroxyl group of ticoids and is used exclusively in ophthalmic products. Thc 17-acetate of Iluorometholone is also used as an suspension (Flarex).
Flurandrenolide,
liSP. Aurundrcnolide. 6a-fluornII $.2 I -dihydroxy- 16 a, I 7-[( I -methylethylidene)bis(oxy;Jpregn-4-ene-3.20-dione. although available as a tape peed uct. can stick to and remove damaged skin, so it should k
avoided with vesicular or weeping dermatoses.
Fluticasone Propionate, liSP.
Fluticasone propionaic.
S-(fluoromethyl)
6a.9-dilluoro- II f3-hydroxy- I 6a.methyl. 3-oxo- I 7a-( I -oxopropoxy)androsla- I .4-diene- I oatc (Cutivate). is 3- to 5-IbId more potent than sone in receptor binding assays. (See also the
tion on inhaled corticosteroids.)
Haidnonide.
droxy- I 6a-methylpregna- I .4-diene-3.20-dione (Cloderm), along with desoximetasone. lacks the C17a oxygen functionality that is present in other glucocorticoids but still retains good glucocorticoid activity.
Halcinonide. 21 -chloro-9-Iluoro- I Ifl.hy droxy- I 6a. 17-1(1 -methylethylidenebis(oxy)Ipregn-4. etc 3.20-dione. was, the first chioroglucocorticoid marketed Like many of the other potent glucocorticoids. it is used topically.
Desonide, lisP.
Halobetasol Propionate, lisP.
Desonide. I l$-2 l-dihydroxy-l6a.171(1 -mcthylethylidcne)bis(oxy)tpregna- I ,4-diene-3,20-dione (DesOwen, Tridesiol).
Halobetasol propkxi
ate. 21 -chloro-6a.9.ditluoro- II $-hydroxy- 16$-methyl -17 (I -oxopropoxy)pregna- I .4-diene-3.20-dione.
Chapter 23 • Steroid Honnv.nie.c and Therapeuiieallr
Co,npountl.c
813
Loteprednol Etabonate, USP. Locprednol ctahonate. chtoromethyl I 7a-[ethoxycarhonyl )oxyI- II fl-hydroxy-3osoandrosta- 1 .4-diene- I 7-carboxylate (Aires. Lotemax). has a moditied carboxylate at the C17 position rather than the typical ketone functionality. This modification maintains affinity for the glucocorticoid receptor but allows facile metabolism to inactive metaholites. This limits the systemic
than triamcinolone. The plasma half-life is approximately
action of the drug. Loteprednol etahonate is used as an ophthalmic suspension that has greatly reduced systemic action due to rapid metabolism to the inactive carboxylate (Fig. 2331).
tionally, the acetonidc may be given by intrabursal or. sometimes. intramuscular or subcutaneous injection. A single intramuscular dose of the diacetate or acetonide may last up to 3 or 4 weeks. Plasma levels with intramuscular doses of
Medrysone. USP. Medrysone. II f3-hydroxy-6a-methylpregn-4-ene-3.20-dione. is unique among the corticoste-
the acetonide are significantly higher than with triamcinolone itself. The acetonide is also used to treat asthma and allergic rhinitis (see following section).
90 minutes. although the plasma half-life and biological halflives for glucocorlicoids do not correlate well. The hexacetonide is slowly converted to the acetonide in vivo and is given only by intra-articular injection. Only triamcinolone and the diacetate are given orally. The acetonide and diacetatc may
be given by intra-articular or inirasynovial injection; addi-
roids. in that it lacks the usual lla,2l-diol system of the others (Fig. 23-31). Currently. ii is used only for treatment of inflammation of the eyes.
INHALED CORTICOSTEROIDS FOR ASTHMA AND ALLERGIC RHINITIS
Methy!prednisolone, USP. Methyiprednisolone, Ii 7.2 I-trihydroxy-6a-methyl-l.4-pregnadiene-3.20-dione. is
The National Asthma Education and Prevention Program has provided recent recommendations on the treatment of asthma, including a strong recommendation for the first-line
asailable unmodified or as ester derivatives.
use of inhaled corticosteroids for severe and moderate persis-
Mclhylprednisolonc acetate. USP Mcthylprcdnisolone sodium succinatc. liSP
Mometasone Furo ate.
USP.
Mometasone
furoate.
9.21 -dichloro- I 7a-l(2-furanylcarbonyl)oxyj- Ii 16a-rnethylpregna-l.4-diene-3,20-dione (Elocon). isa highpotency glucocorticoid available in cream, lotion, or ointment formulations for topical use. In addition, momelasone (umate monohydrate is fonnulated in a nasal spray for treating allergic rhinitis (see following Section).
Prednicarbate. USP.
Prednicarbate.
I
- l-oxopropoxy)pregna-l ,4-diene3.20-diane. is a prednisolone derivative with a C2 I propionate ester and a C17 ethyl carbonate group. It is available for use only in a 0.1 topical cream. Prednicarbate is a mediumpotency glucocorticoid. onvl)oxy I-I
Rimexolone. II I 6a. I 7adimethyl- I 7-( I -oxopropyl )androsta- I 4-diene-3-one, like Rimexolone, USP.
tent asthma in all age groups. The corticosteroids currently used in inhaled formulations are all relatively potent topical corlicosteroids that have the advantage of rapid deactivationl inactivation for the portion of the dose that is swallowed. The development of glucocorticoids that are efficiently inactivated metabolically when swallowed has greatly reduced the systemic side effects associated with the use of steroids in asthma treatment. The older corticosteroids that are used orally (e.g.. methylprednisolone. prednisolone. and prednisone) have much greater systemic side effects, and their use should be limited, if possible. Although systemic side effects are reduced, they are not completely eliminated. The side effects can vary with the steroid used and the frequency of administration. The five glucocorticoids that are currently approved for use in the United States for asthma as inhaled fonnulations are beclometha.sone dipropionate. budesonide. tlunisolide. fluticasone propionate. and triamcinolone acetonide (Fig. 23-32). Mometasone furoate will likely be added soon for an asthma indication. Ciclesonide is the newest glucocorticoid being pursued for use in the treatment of asthma. Ciclcsonide
medrysone and fluorometholone. lacks the C2 I hydroxyl
is in phase III clinical trials and may he available in the
group. In addition. rimexolone has an additional methyl group in the 17a position. a site where a hydroxyl group
United States within a few years. Clinical trials suggest that
is typically found. Rimexolone is available as a suspension ophthalmic use (Fig. 23-31).
available inhaled steroids. The (allowing agents are also available in nasal inhalers
it may have better tolerability than some of the currently
6a.
for the treatment of allergic rhinitis. Details tire provided below for the mode of metabolic inactivation involved for each of these products. Although all of these agents have much lower systemic effects than the oral steroids, sonic
Trianicinulone acetonide, USP: Triamiicinotone-l6a,l7-acetoride Triamcinolonc hexacetonide, liSP: Triamcinolonc acctonide 21-
systemic effects, as measured by suppression of the hypothalamic—pituitary—adrenal (HPA) axis, have been observed for these products.
Triamcinolone, USP.
Triamcinolone, 9-Iluom- II
17.21 -tetrahydroxypregna- I .4—diene-3.20-dione.
I 3-(3.3-dimcthyl thutyramel
Tnamcinolone diacciate. USI': 16.2 1-Diacetate
Triamcinolnne acetonide is approximately 8 times more
GLUCOCORTICOIDS FOR ASTHMA AND ALLERGIC RHINITIS
ically applied triamcinolone acetonide is a potent anti-in-
Beclomethasone Dipropionate. Beclomethasone dipropionate (Beclovent, l3econase, Vanceril, Vancenase)
flanunatory agent (see Table 23-8). about 10 times more so
(BDP) is rapidly converted in the lungs to beclomethasone
potent than prednisone in animal inflammation models. Top-
814
Wilson
and Gin void's Textbook of Organic Medicinal and Pharmaceutical Chemistry
HO
Tnamcinolone Acetonide (Azmacort, Nasacorl)
Beclomethasone Diproptonate (Bedovent, Beconase, Vancerli, Vancanase)
H3C
Ftuticasona Proplonate (Flovent. Ftonase)
Mometasone Furoata (Nasonex)
H
Budesonide is a mixture of the two isomers (S isomer can vary from 40 to 51%) (Pulmicort, Rhinocort) Ciclesonide
Figure 23—32 • Giucocorticoids used to treat asthma and allergic rhinitis (some are also used topically)
17-monopropionate (17-BMP). the metabolite that provides the bulk of the anti-inflammatory activity. The monopropionate also has higher affinity for the GR than either the dipropionate or becloniethasone. The portion of BDP that is
swallowed is rapidly hydrolyzed to 17-BMP. 21-BMP (which arises by a transesterification reaction from 17BMP), and beclomethasone itself.'43 Beclomethasone has
humans is the I 7/3-carboxylate derivative. As expected, a charged carboxylate in place of the normal acetol functional ity at Cl 7 greatly reduces affinity for the glucocorticoid receptor (2,000-fold less than the parent), and this melabolbe is essentially inactive. The metabolite is formed via the CYP 3A4 system, so care should be taken ii liuticasonc propionate is coadministered with a CYP 3A4 inhibitor such as
much less glucocorticoid activity than the monopropio-
nazole or ritonavir. Clinically induced Cushing's
nate.'4'
has been observed when inhaled fluticasone propionate
Budesonide. Budesonide (Pulmicort Turbuhaler. Rhinocort) is extensively metabolized in the liver, with 85 to 95% of the orally absorbed drug metabolized by the first-
administered concurrently with ritonavir)4' Fluticasone is also available in an inhaled in combination with the long-acting /32-agonist salmeterol
(Advair Diskus).
pass effect. The major metabolites are
ide and 16a-hydroxyprednisolone. both with less than 1% of the activity of the parent compound. Metabolism involves the CYP 3A4 enzyme, so coadministration of budesonide with a known CYP 3A4 inhibitor should be monitored carefully.
Mometasone Furoate.
Mometasone
furoate (Na.co.
flex) undergoes extensive metabolism to multiple metabolites. No major metabolites are detectable in human plasma after oral administration, but the 6/3-hydroxy metabolite is detectable by use of human liver microsomes. This metabo
lite is formed via the CYP 3A4 pathway.
Flunisolide.
The portion of a flunisolide (AeroBid, Na-
sarel) dose that is swallowed is rapidly converted to the 6/3hydroxy metabolite after first-pass metabolism in the liver.
The 6$-hydroxy metabolite is approximately as active as hydrocortisone itself, but the small amount produced usually has limited systemic effects. Water-soluble conjugates are inactive.
The main metabolite of fluticasone propionate (Flovent. Flonase) found in circulation in
FIut!casone Propionate.
Trlamcinolone Acetonide.
The three main metabolites
of triamcinolone acetonide (Azmacort. Nasacort) are 6/J.hvdroxytriamcinolone acetonide. 21 acetonide, and 6f3-hydroxy-2 l-carboxytriamcino!onc acetonide. All are much less active than the parent compound. The 6/3-hydroxyl group and the 21 -carboxy group are both structural features that greatly reduce glucocorticoid action. The increased water solubility of these metabolitcs also tail. itates more rapid excretion.
Chapter 23 s Steroid hormones and Therapeutical!'. Related Compounds
815
2. Nonnan. A. W.. and Litwnck. G.: Hormones. 2nd ed San Diego. Academic Puts'., 1997. 3. Williams, I). A., and Lcmkc, 1. L. teds.): Foye's Principles nid Chemistry. 5th ci!. Philadelphia. Lippincolt Williams & Wilkins. 2002.
Spironolactone (Aldolactone)
4. Moss, G. P.: Eur. J. Biochcm. 186:429—458, 989. 5. Stocco. D. M.: Annu. Rev. Ptsysiol. 63:193—213. 2(8)1. 6. Lin. 0.. Sugawara. T.. Strauss. 3. F.. Ill. ci at.: Science 267: la28—1831. 1995. 7. Aranda, A., and Pascual. A.: Physiol. Rev. 81:1269—1304. 2001. 8. Cheung. J., and Smith. 0. F.: Mol. Endocnnal. 14:939-946. 2(100. 9. Pratt, W. It., and Taft, 0. 0.: Endocr. Rev. 18:306—3M). 1997. tO. DeFranco, D. B.: Mol. Endocrinol. 16:1449—1455, 2002. II. Brzozowslrj, A. M.. Pike. A. C., Dauler, Z.. ci at.: Nature 389: 753—758, 1997.
12. Tanenbaum. D. M.. Wang. Y.. Williams. S. P.. and Sigler. P. B.: Proc. Nail. Acad. Sci. U. S. A. 95:5998—6003, 1998.
13. Pike, A. C., Brzozowski, A. M., Hubbard, R. E., et al.: Emboi. 18: 4608—4618. 1998.
14. Williams, S. P.. and Sigler. P. B.: Nature 393:392—3%. 1998.
IS. Sack. i. S.. Kish. K. F.. Wang. C.. ci at.: Proc. Nat). Acad. Sci. U.S.A. 98:4904-4909, 2001. 16. Dcy. R.. Roychowdhury. P.. and Mukhctjec. C.: Protein Eng. 14: Eplerenone (lnspra)
Figure 23—33 . Aldosterone receptor antagonists.
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17. Dcy. R., and Roychowdbuty. P.: J. Biomol. Struct. Dyn. 20:21—29. 2002.
18. Ekena, K.. Katzenellenbogcn. J. A.. and Kaizenellenbogen. B. S.: J. Biol. Chem. 273:693—699. 19911.
MIn.ralocortlcld R.csptor Aatago.Ists Antagonism of the mineralocorticoid receptor can have profound effects on the renin—angiotensin system. thus having significant cardiac effects. Structurally, these compounds have an A-ring enone, essential for recognition by the receptor, but the 7a substituent and the D-nng spirolactone provide structural elements that lead to antagonism (Fig. 2333).
Spironolactone. USP. Spironolactone. 7a-(acetylthio)Ila-hydroxy-3-oxopregn-4-ene-3-one-2 I -carboxylic acid v.lactone (Aldactone), is an aldosteronc antagonist of great medical importance because of its diuretic activity. Spironolactone is discussed in Chapter 18.
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Eplerenone, USP. Eplerenone, 9,11 a-epoxy- I 7a-hydroxy-3-oxopregn-4-enc-7a,2 I -dicarboxylic acid, y-Iactone, methyl ester (lnspra). is a new aldosteronc antagonist that was approved by the FDA in 2002 for the treatment of hypertension.
29. McDonnell. D. P.. and Norris, 3. D.: Science 296:1642—1644. 2002. 30. Sanchez. K.. Nguyen. 0.. Rocha. W., ci al.: Bloessays 24:244—254. 2002.
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Acknowledgment I would like to thank Debra Peters for assistance with the illuslration of several figures. I would also like to express my appreciation to the authors of various review articles on the steroids. Without the dedication and hard work of these individuals, the assembly of this chapter would have been
2000.
35. Ruenitz, P. C.: Female sex hormones and analogs. In Wolff. M. E. (ed). Burger's Medicinal Chemistry and Drug Discovery. 5th ed. New York. John Wiley & Sons, vol. 4. 1996, pp. 553—587. 36. Rich. K. L.. Hoih. L. R.. Geoghegan. K. F.. et al.: Proc. Nail. Acad. Sci. U. S. A. 99:8562—8567. 2002. 37. Martin. C. R.: Endocrine Physiology. New York. Oxford University Press. 1985.
a much more challenging task.
38. Schuler, F. S.: Science 103:221. 1946. 39. Jordan, V. C.. Mittal. S., Garden. B.. et al.: Environ. Health Perapect
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H
C
A
P
T
E
R
w
24
—'
Prostaglandins, Leukotrienes, and
p—p
Other Eicosanoids THOMAS J. HOLMES, JR.
The prostaglandins (PCIA through PGJ) are one group of
characterization of the cicosanoid substances but also were
naturally occurring 20-carbon fatty acid derivatives pro-
the first to realize the profound significance of ihc arachi.
duced by the oxidative metabolism of 5.8.11. 14-eicosatetraenoic acid, also called arachidonic acid. Other so-called cicosanoids produced in the complex biological oxidation scheme called the arachidonic acid cascade (Figs. 24-I and 24-2) are thromboxanc (TXA2). the leukotrienes (LKTs A to F). and the highly potent antithrombotic agent prostacyclin (P012). The naming and the numbering of these 20carbon acids are included in Figures 24-I to 24-3. Although eicosanoid-derived agents in current human clinical therapy are few, the promise of future contributions from this area is presumed to be very great. This promise stems from the fact that intermediates of arachidonic acid metabolism play an es.sential modulatory role in many normal and diseaserelated cellular processes. In fact, much of the pain, fever, swelling, nausea, and vomiting associated with "illness," in general, probably results from excessive prostaglandin pmduction in damaged tissues.
donic acid cascade in disease processes, particularly inllam• mation. These individuals first proved that the mechanism of the anti-inflammatory action of aspirin and related
roidal anti-inflammatory drugs (NSAIDs) was directly due to their inhibitory effect on prostuglandin formation. It shown subsequently that the analgesic and of these NSAIDs. as well as their proutceralive and anticoae. ulant side effects, also result from their effect on eicosanaid
(e.g.. inhibition of cyclooxygenases ICOX-l and COX-21). metabolism
I
and 2
Many books have been published describing the role of eicosanoids in the inflammatory process, the immune sys. tern. carcinogenesis, the cardiovascular system, reproduclise processes, gastric ulceration, and the central nervous system (see Selected Reading). An annual update of research results in this area has been published since 1975, Advances in Pra,s• taglandins. Tl,ron,ho.ranes. and Leukotriene Research. Re.
cent research findings in this area may appear in a of biochemical and clinical journals hut are the primary conS
HISTORY OF DISCOVERY Early in the past century (1931). Kurzrok and Lich noted that human seminal fluid could increase or decrease spontaneous muscle contractions of uterine tissue under controlled condiLions.' This observed effect on uterine musculature was be-
lieved to be induced by an acidic vasoactive substance formed in the prostate gland, which was later (1936) termed
prostaglandin by von Euler.2 Much later (1950s), it was found that the acidic extract contained not one but several structurally related prostaglandin substances.3 These materials subsequently were separated, purified, and characterized as the prostaglandins (PGA through PCi), varying somewhat in degree of oxygenation and dehydrogenation and markedly
in biological activity (Table 24-I). Specific stereochemical syntheses of the prostaglandins provided access to sufficient
purified material for wide-scale biological evaluation and confirmed the structural characterization of these complex substances.4
Although many scientists have contributed to refined characterization of the eicosanoid biosynthetic pathways and the biological consequences of this cascade, the discerning and persistent pioneering effons of Sune Bergstrom, Bengt Samuellson. and John R. Vane were recognized by the award of a shared Nobel Prize in Medicine in 1982. These scientists not only dedicated themselves to the chemical and biological
818
cern of two specific journals: Prostaglandins and Other Lipid Mediators and Prostaglandins. Leuko:ricsw.s. (111(1 Es
.wntial Fairs' Acids.
EICOSANOID BIOSYNTHESIS Prostaglandins and other cicosanoids are produced by the oxidative metabolism of free arachidonic acid. Under normal circumstances, arachidonic acid is not available for memaho. lism as it is present as a conjugated component of the phos. pholipid matrix of most cellular membranes. Release of free
arachidonic acid, which subsequently tisay be metabolized, occurs by stimulation of phospholipase enzyme activity in response to some traumatic event (e.g., tissue damage, toxin exposure. or hormonal stimulation). it is believed that the clinical anti-inflammatory effect of gin. cocorticat steroids (i.e., hydrocortisone) is due to their ability to suppress PLA2 activity via lipoconins and thus prevent the release of free arachidonic acid.5 Modulation of PLA, activity by alkali metal ions, toxins, and various therapotisc agents has become a major focus of biological research cause of the change.s in cicosanoid production and the drj' matic biological effects accompanying PLA2 stimulation re suppression. Although initially it was believed that the in fiammatory response (swelling, redness, pain) was princi.
_ Chapter 24 S Pro.ciaglandin.c. Leukogrie,,e.s. and Oilier Eirosaneids
819
85C00H Asachidornc Acid
Cyctooxygenase
OOH
/4C9OH
yW
OH
—,
0 OH TXA2 (thromboxane A,)
PGH2
OH (pOStacydin)
PGF2,,
0
OH
cxRt7÷12o OH
OH PGJS
POD2
OH POE2
Figure 24—1 u Cyclooxygenase pathway
pally due to POE2. recent interest has focused on the interre-
lationships of POE-type eicosanoids with P012 and cytokines. such as interleukin- I and interleukin-2, in the modulation of intlammatory reactions.6
Two different routes for oxygenation of arachidonic acid have been defined: the cyclooxygenase pathway (Fig. 24I) and the lipoxygenase pathway (Fig. 24-2). The relative significance of each of these pathways may vary in a particuat tissue or disease state. The cyclooxygcnasc pathway, so named because of the unusual bicyclic endoperoxide (P002) produced in the first step otthe scquence. involves the highly
sicreospecific addition of two molecules of oxygen to the arachidonic acid substrate, followed by subsequent enzyme-
controlled rearrangements to produce an array of oxygenated eicosanoids with diverse biological activities (see Table 24-
I). The first enzyme in this pathway. PGH synthase. is a hemoprotein that catalyzes both the addition of oxygen (to form P002) and the subsequent reduction (peroxidase activity) of the 15-position hydroperoxide to the lS-(S)-configuration alcohol (PGH2)! PGH synthase (also called cyclooxvgenase- / /COX- 1/ or cvc/ooxvge'naxe-2 (COX-2/. and formerly PG .cvntheiase) has been the focus of intense inves-
tigation because of its key role as the lirst enzyme in the arachidonic acid cascade.5 It is this enzyme in constitutive (COX-l) or inducible (COX-2) Form that is susceptible to inhibition by NSAIDs, leading to relief of pain. lever, and
820
%tjlxon and Gis"old'.s
of Orca,,ic Medicinal and P!,arn,aceu,ieal Clw,,,ia:ri
Oa—j OOH
5HEIE
5.1-IPETE
HO H G$H t,ansie,ase
4ydro4ase
Cys'G)y
A, )LTA,)
B, (LIB,)
Leukoinene C. O.TC,)
T'wisicrai,
HO
HO H
H
HO H
HS Cys-G)y
E, LIE,)
Leuhoinone F, ILIF,)
D,
Figure 24—2 • Lipoxygenase pathway.
intlammation.6 "This enzyme is also inhibited by the w3 (omega-3) fatty acids (eicosapcntaenoic acid IEPAI and docosahexaenoic acid IDHAJ) found in certain cold-water
body temperature, central and peripheral pain and decreased vascular perfusion) based on their tissue disthbu.
fish and provided commercially as nutritional supplements. leading to beneficial cardiovascular effects)° This enzyme will metabolize 20-carbon fatty acids with one more or one less double bond than arachidonic acid, leading to prostaglandins of varied degrees of unsaturation (e.g.. PGE, or PGE3. for which the subscript number indicates the number of double bonds in the molecule). Prostaglandin H2 serves as a branch-point substrate ti,r specitic enzymes, leading to the production of the various prostaglandins. TXA2. and PCIa. Even though most tissues can produce PGH2, the relative production of each of these derived eicosanoids is highly tissue specific and may be subject to secondary modulation by a variety of cofactors. The complete characterization of enzymes involved in branches of the eyclooxygena.se pathway is currently under way. Specific cellular or tissue responses to the eicosanoids are apparently a function of available surface receptor recognition sites.' The variety of tissue responses observed on eicosanoid exposure is outlined in Table 24-I. Non—tissue-selec-
The lipoxygenase pathway of arachidonic acid nietahn. lism (Fig. 24-2) produces a variety of acyclic lipid peroxides (hydroperoxycicosatctr.tenoic acids IHPETEsD and derived
tive inhibitors of the cyclooxygenase pathway, such as aspirin, thus may exert a diversity of therapeutic effects or side effecis (e.g.. decreased uterine muscle contraction and platelet aggregation, gastric ulceration, lowering of elevated
tion.s.
alcohols (hydroxyeicosatetraenoic acids I HETasJ
Al-
though the specific biological function of each of these limoygenasc-dcrived products is not completely known, they are
believed to play a major role as chemotactic factors that promote cellular mobilization toward sites of tissue injury. In addition, the glutathione (GSH) conjugates LKT-C4 and LKT-D4 are potent. long-acting bronchoconstrictors that are released in the lungs during severe hypersensitivity episodes
(leading to their initial designation as the
'slow-reacting
substances of anaphylaxis" ISRSA5IL Because of the presumed benefit of preventing of LKTs in asthmatic patients, much research effort is being dedicated to the design and discovery of drugs that might selectively inhibit the lipoxygenase pathway of arachidonic acid metabolism without affecting the cyclooxygenase pathway.'' Zik'utnn
(Zyllo by Abbott Laboratories) specifically inhibits the Iipoxygcnase pathway, Ii has been proposed that aspirin hypersensitivity in susceptible individuals may result mmmcl
fectively "shutting down" the cyclooxygenase tt,etabulic route, allowing only the biosynthesis of lipoxygcnase path'
Chapter 24 • Prostaglandin.r. Leuknrriene.c, and Other Proslagiandins
Nonenzymatic Degradation
Enzymatic Metabolism
p-Oxidative
0
HO PGE2
P6F20
HO
HO
)
OH
HO
COOH O
0
R,7 (Unstable)
PGC7
R 13-20
O
jHeshilt
PGB2
R13_20 Thromboxane A2
OH TXB2
Prostacyclin
H20
20
Prostacyctin (PGI2)
Figure 24—3 . Elcosanoid degradation.
822
Wi/san and Gist'old'c
i4 Organic' Medicinal and Pharrnaec'u:icaI C/u'mi,sirv
Biological Activities Observed with the Eicosanolds TABLE 24—1
Observed Biological Activity
Substance
Weak inhibitor of platelet aggregation
Vactidilatitin
PGE1
Inhibitor of lipolysis Inhibitor of platelet aggregation
Stimulates contr.iclii,n of gastrointestiital sntooth muscle
Stimulates Ityporalgesk response Remit s'ascidilutution
Stimulates uterine stmtiioltm tnuwk couttr,tettun Protects gastrointestinal epitlmelia from acid degradailon
Reduces secretion of
acid
Elevates tliennoregulatimrv set point in anterior hypothalamus
PGF.
StitituLites breakdown of corpus lmmwutn
(lutcolysis) in animal' Smmnimilaics uterine smutooth muscle comttnictioum
Potent inhibitor iii platelet aggregation
PGI,
Poteni vasuidilamor
increases cAMP tevcls in Stimulates osucogenesis
PGJ,
Inhibit' cell prnlilenitiuun
TXA,
l'otcnt inducer of platelet aggregation Potent rasoconstrictiur
Decreases cAMP levels in platelets Stiuiutilatci. releitse i,f ADP and sirrotonin (mmmi platelets Increases lmtkocyte cttemott,sis and iuggregutiuun
PGH synlhase will become hydroxylated directly during arachidonic acid metabolism, in a process labeled cvaxida' This cooxidative process presumably occurs during the peroxidase conversion of PGG2 to PGH2. which duice tively makes available a nonspecific oxidizing equivalent. The cooxidation process has been implicated in the aclisa• lion of polycyclic aromatic hydrocarbons to fonu proximate carcinogens. The only group of drugs that has been thoroughly chanw• terizcd for its effect on arachidonic acid metabolism ts the NSAIDs. This large group of acidic. aromatic moleculesesens a diverse spectrum of activities (mentioned above) by inhibitioti of the first enzyme in the arachidonic acidcascadc, PGH syntha.se (also called COX-! and COX-2). Such agents as salicylic acid. phenylbutazone. naproxen. sulindac, and ibuprofen presumably act by a competitive. reversible inltihi. tion of arachidonic acid oxygenation.t7 Aspirin and ceflain halogenated arornatics (including indoniethacin. hiurbipro. fen. and Meclornen appear to inhibit PGH synthase in a time-dependettl. irreversible manner.'5 Since this irrevcoible inhibition appears critical for aspirin's significant effect on platelet aggregation and, therefore, prolongation of bleeding this discovery has led clinicians to recotuntend the daily consumption of low doses of aspirin (81 ntgl by patients at risk for myocardial infarction (Ml. heart allacki,
particularly a second Ml. Interestingly, aspirin's primary competitor in the com,ncrcial analgesic marketplace. acctaminophen. is a rather weak
inhibitor of arachidonic acid oxygenation in
This. in
Potent and prolonged cummitr.ictkun of guinea pig ileumn smooth muscle
in concert with its lack of in vitro anti-inflammatory
Increased vascular penneahihity in guinea pig skin (amugnmentcd b) POEs
(while maintaining analgesic and antipyretic activity equivalenI to that of the salicylatest has led to the proposal lh.d acetaininophen is more active inhibitor of cyckuoxy' genases in lhe brain, where peroxide levels (which stimulate cyclooxygenase activity) are lower than in inflamed periph-
Vasodilutatiumn of rut and rabbit gastric circulation
eral joituls, where lipid peroxide levels arc high." In fact
Inhibits induced platelet aggregntion
when in vitro experimental conditions are modified to the so-called peroxide tone. acetaminophen becontes as ci fective as aspirin in reducing arachidonic acid metabolism."
Aggregateshumutan tcuuliocytes Proittores leukocytc chamcna.sis
way
hepatic hydroxylation. phenolic derivatives of adttlinislercd drugs becomc readily available in vivo. Even more directly. aromatic molecules on in vitro incubation with microsomal
SIow.reaeling substances aI :unapluylaxiv
Sronclisucm,nstrictivc in hiumnitni
5-or 12-HETE
tered agelils. Because tnost aromatic drug molecules undergu
fact, is a characteristic of reversible. noncotnpelitive.phcnolic antioxidanl inhibitors itt generaL2t This determinalion,
Contracts guinea pig luttg parens'hymal strips
5- or t2-HPETIS
hoxylic acids and phenolic aaitioxidants. implies their susceptibility to influence by a variety of exogenously adminis-
intermediates,
including
the
bronchoconstrictive
LKTs.
COX-2 INHIBITORS The newer anti-inflammatory COX-2 inhibitors (e.g.. cete-
coxib. rolecoxib. and valdecoxib) arc claimed to shos greater inhibitory selectivity for the inducible fomi (if cyclooxygcnase.22 Although not absolute, this
DRUG ACTiON MEDIATED BY EICOSANOIDS The ubiquitous nature of the eicosanoid-producing enzymes
implies their significance in a variety of essential cellular processes, Additionally, the sensitivity of these enzymes to structurally varied hydrophobic materials, particularly car-
provides a potential therapeutic advantage by reducing sidc effects, particularly gastric irritation and Unfar. tunately. this altered profile of activity is not totally risk free The manufacturer of rofecoxib (Vioxx) has recently (AfmI 2002) issued a warning regarding the use of this product in patients with a medical history of ischemic heart disease
Oilier
Chapter 24 • Pros,aglandins. Leukoiric,,e.c.
COX-2 inhibitors do not share the beneficial effects of aspina in preventing cardiovascular throinbotic events.
823
with their limited distribution from this site of adniinistration.24
Iv.
DESIGN OF
is
DRUGS
The ability to capitalize successfolly on the highly potent
ii. it! C-
it.
biological elTects of the various eicosanoids to develop new therapeutic agents currently seems an unfultilled promise to medicinal chemists. Although these natural substances are highly potent effectors of various biological functions, their use as drugs has been hampered by several factors: (a) their
The ongoing development of potent. eflèctive. and long-
chemical complexity and relative instability, which have
lan by GlaxoSmithKline) for continuous intravenous infusion in patients suffering from primary pulmonary hypertension (PPFI). The solution for infusion is prepared within 48 hours of expected use because of its limited chemical stability. The potent vasodilatory, platelet antiaggregatory effect and vascular smooth muscle aiitiproliferative effect of this naturally occurring cicosanoid produce a dramatic hut shortlived (half-life less than 6 minutes) therapeutic effect in PPH patients. Continuous, uninterrupted admninistratioti of the drug by portable infusion pump is necessary. however. to prevent sytnptoms of rebound pulmonary hypertension. To ensure proper use of this therapy, its distribution is relatively
limited, to some extent, their large-scale production and for-
mulation for clinical testing; (b) their susceptibility to rapid degradation (Fig. 24-3). which limits their effective bioactive half-life; and (c) their ability to affect diverse tissues particularly the gastrointestinal tract, which may lead to
te
DEVELOPMENT OF PROSTACYCLIN-DERIVED PRODUCTS
Is
severe nausea and vomiting) if they enter the systemic circu-
d
lation. even in small amounts. Caution is always recommended with the use of prostaglandin analogues in females of childbearing age because of their potential for inducing dramatic contraction of uterine muscles, possibly leading to miscarriage.
Several approaches have been used to overcome these difficulties. First, structural analogues of particular eicosa-
acting forms of naturally occurring PGI2 is an excellent illustration of strategies that capitalize (in the beneficial but shortlived biological effects of eicosanoid derivatives. PGI2 itself is currently marketed as the sodium salt epoprostenol; Flo-
restricted.25
noids have been synthesized that are more resistant tochemical and metabolic degradation but maintain, to a large extent.
C00
desirable biological activity. Although commercial production and formulation may be facilitated by this approach. biological potency of these analogues is tisually reduced by several orders of magnitude. Also, systemic side effects may troublesome because of broader tissue distribution as a result of the increased biological half-life. Structural alterations of the eicosanoids have been aimed primarily at reducing or eliminating the very rapid melaholism of these potent substances to relatively inactive metabo-
v t,
•
lites (see Fig. 24-3). Several analogues are presented in Table 24-2 to illustrate approaches that have led to potentially use•
•
lul eicosanoid drugs. Methylation at the 15 or 16 position will eliminate or reduce oxidation of the essential 15-(S)alcohol moiety. Esterification of the carboxylic acid function may affect formulation or absorption characteristics of the cicosanoid. whereas esterase enzymes in the bloodstream or tissues would be expected to regenerate the active therapeu-
tic agent quickly. Somewhat surprisingly, considering the restrictive configurational requirements at the naturally asymmetric centers, a variety of hydrophobic substituents including phenyl rings) may replace the saturated alkyl chains, with retention of hioacsivity. A second major approach has beeti aimed at delivering the desired agent, either a natural eicosanoid or a modified analogue, to a localized site of action by a controlled delivery method. The exact method of delivery may vary according to the desired site of action (e.g.. uterus, stomach, lung) hut
has included aerosols and locally applied suppository, gel formulations, or cyclodextrin complexes. The recent commercial development of PGF-type derivatives for use in the eye to lower intraocular pressure (lOP) in glaucoma (discussed below under the heading. Prostagland ins for Ophthal-
mic Use) relies on their potent therapeutic effects coupled
Three more-stable derivatives of PGI2 are being developed to extend the duration of action of this (1mg to improve the safety and convenience of PPH therapy and, perhaps. broaden the therapeutic indications for its use. Treprostinil (Remodulin) with an extended half-life has been developed for continuous subcutaneous injection for PPM patients. This
C00
824
Wi!.nni and GIsvolds Textbook
of Orgwiic Medkis:aI and Pharmaceutical Chemistry
Prostaglandin Analogues under Investigation as Receptor Ligands and Future Drug
TABLE 24—2
Candidates
Butapros
EPrrixcptor Iitmnd
BW245C: R=H
ligancic
A= HO
BWA 868C: R = A
HN
HO
0 Cicaprost
IP-reccplor hgnnd
CH3
HO ligand nitiiiIcer
EnprosUl (Roche)
Enisopmst (Scarle)
Orphan statue cycltstpurinr oxiOI)
Gemirprosi Cenagern by Ono
Abonitacie:it
HO
HO ligami
COOH
N— H
—
Chapter 24 • Prusraglt,ndiiss. Leuko:rie,,es, uiul OiIwr Eieo.ca,ioidc
825
TABLE 24—2—Continued rP-rcccpior tigand
SQ-29S48
'II H
Suiprostone (Glotil Bases Dali Saj,ilana
Fannacculicat)
U.4Mit9
ligund
Osyitixic
TP.rcccplor ligund
method of administration and longer hall-life would markedly improve the convenience and safety of 'prostacyclin" therapy in PPH patients. Localized intermittent subcutaneous administration of Uniprost is proposed for the treatment
of critical limb itchemia. Another stable derivative of PGI2. iloprost. is intended for nasal inhalation to provide a direct vasodilatory effect on pulmonary blood vessels and thus decrease vascular resistance. Currently available in Europe. patients inhale 6 to 8 pulfs of aerosolized iloprost every 2 to 3 hours. Side effects such as coughing, headaches, and jaw pain have been re-
ct-i3
ported. COOH
EICOSANOID RECEPTORS
CH3
Another approach to developing new therapies based on the known biological activities of the prostaglandins and leukotrienes requires characterization of the naturally occurring tissue receptors for these agents. A thorough knowledge of
the tissue distribution (localization) of such receptors and their binding characteristics would allow the design of receptor-specific agonists or antagonists that might not possess
An even more chemically and biologically stable derivative of PGI2 is ber4prost, which is being evaluated in an oral formulation for the treatment of early-stage pulmonary peripheral vascular arterial This prostacyclin has been approved for use in Japan but not yet in the United States.
the same limitations as the natural eicosanoids but could affect tissue function nonetheless. An excellent historical description of prostanoid receptor and a isolation and characterization has been more recent review of developments in this field is available." Basically. prostanoid receptors are identified by their primary eicosanoid agonist (e.g.. DP. EP, IP. and TP). although subclassification of PGE receptors has been neces-
826
Wilson
and Gisiold.s Teul,ook of Organic Sfrd,e,nal ant! Phannaceuthal Chen,i.sirs and EP.1). in fact, the existence of
nary (e.g.. EP1
types is relatively low (30 to 50C/e). All prostanoid receptors. however, are believed belong to a 'rhodopsin-type" super.
subtypes of the
and receptor (EP1,5 TP receptor (TP.,. TP11 has been proposed. Complete charac-
of receptors that function via G-protein-.coupkd
teri/ation of receptors (and subtypes) includes tissUe localization. biological cffect produced, cellular signal transduction mechanism, inhibitor sensitivity, protein stflicture, and genetic origin. Not all receptors or subtypes have been com-
transduction mechanisms. Three general classes of proctan. oid receptors are proposed I: (a) relaxant, including DP.
El'.1, and ii', which promote smooth muscle relaxation hs raising intracellular cyclic adenosine monophnsphate (cAMP) levels: (b) contractile, including El'1, FP. and Th which promote smooth muscle contraction via calcium ion
pletely characterii.ed in this way, hut significant progress toward this goal has occurred recently. Table 24-3 indicates characteristics of the prostanoid receptors identified thus far. Although receptor studies have required the use of nonhuman species (principally the mouse hum also the rat, cow. sheep, and rahhiu. a high correlation of structural homology of receptor subtypes between species (—(10 to
muobilizatiomE and (e) inhibitory, such as El'5. which prevents
smooth muscle contraction by lowering intracellular cAM) levels. Although structural and functional characieriaitiix of prostanoid receptors has permitted the identification and differentiation of selective receptor ligands (Table 24.3. nih agonists and antagonists), overlapping tissue distributinri'
(has been
observed, while structural homology among receptor sub-
TABLE 24—3
Receptor
Prostanoid Receptor Characteristics Principle Ligands
up
TissuelAction Ikum/muu.ctc relaxation
BWA86$C
Brain (kptonieninges)/ sluep induction
POE,
Kidney/papitinry ducts
I 7-ptueumy)-I'GE,
i.umuglbronchoconstriu,miiun
Sulprosmuuiue
Stsumavh/snwoth
BW2'LSC EP>
Itoprusi
Transduction l'cAMP/Gs
Gene Knockout Effect Not available
Not available
u,uusctc cs,mraction
Bmnmutopoist
(indmicibkt
PGt; p(;E
I.nnglbronchodiiation
BUtapnx.t I ntisoprosmot
tlturnms/implnntauiu,n
POE.;
(iiislricliuntisccrcmomy
PP>5 J. cAMP/C,
Sutprostone
Guistncicytopromcctivc
EP0, I cAMP/G, EPs' I cAMP/G,
IcAMP/G,
.LOvuiaiion .LFtmiiIi,amion
INn' hypertension
isoprosuut t-nturoslut Gemeprosu
iii',
POE
Misoprostol
Lilerus/Inhihims contraction
t3rain/fevcr response
Pyrngcn rcvponse
Ipi tumos cr105
Duemus ancriosusirelaxunt
TcAMPIG,
Kidiuey/glomcruius
Patent ductus armeriina>
.I.Bone resorption
Gastric anlnmnt/unucoun secretion
Uterus/endornecrium Eye/decreases inmraocular pressure
PP
IPI turnoverlG5
Lost
IcAMP/Os
Imromnbosis
Corpus luteu,u/Iutcoiysis
Curboptmt
Lung/bronchoconstrtctinn
I.otaiioprost Untiprostouls'
i'rasoprosm Itinuatuprust tP
TI'
Pot. linprost
Arturriu.s/dilotion
Cucaprosm
t)RG ncuronslpain
luorapausm
Kidney/aficreni amienoles I IGFR)
TXA2
Lung/bronchuctunsmrwtton
.LIn)lamntatory edeno
cAMP
Kidney/I. (WR
Finn
SQ-2954$
Artcricskonstricmion
tJ-466(')
'llmymus4 immuture thymocytec
S.. NIvuuolo.
Y
not t shukuhi. F
Re
75
I 15.5—I
'lull
TI'11 IcAMP
tBtceding
Chapter 24 • Prostaglwzdins, Leukoirie,ie,s. and Other and common signal transduction mechanisms present formiobstacles to the development of specitic pharmacological therapies.
Eico.canoid.s
8.27
PGE1. aiprostadil (Prostin VR Prostaglandin E1, USP. Pediatric), is a naturally occurring prostaglandin that has found particular use in maintaining a patent (opened) ductus arteriosus in infants with congenital defects that restrict pulmonary or systemic blood flow. COOH
EICOSANOIDS APPROVED FOR HUMAN CUNICAL USE dinoprost (Prostin F2 Prostaglandin Alpha), is a naturally occurring prostaglandin that was ad-
ministered intra-amniotically to induce labor or abortion within the Iirst trimester.
HO HO
Alprostadil must he administered intravenously Continually at a rate of approximately 0.1 pg/kg per minute to temporarily maintain the patency of the ductus arteriosus until corrective surgery can be performed. Up to 80% of
This product. which was supplied as a solution of the tromethatnine salt (5 mg/mL) for direct administration, is no longer available in the United States for human use but is still formulated for veterinary use as described elsewhere in this chapter.
dinoprostonc (Prostin E2. Prostaglandin E3. (ervidill. is a naturally occurring prostaglandin that is administered in a single dose of 20 mg by vaginal suppository to induce labor or abortion.
COOH
circulating alprostadil may be nietaboli,.ed in a single pass through the lungs. Because apnea occurs in 10 to 12% of neonates with congenital heart defects, this product should he administered only when ventilatory assistance is immediately available. Other commonly observed side effects include decreased arterial blood pressure (which should be monitored during infusion), inhibited platelet aggregation (which tnight aggravate bleeding tendencies), and diarrhea. Prostin VR Pediatric is provided as a sterile solution in absolute alcohol (0.5 mg/mUJ that must be diluted in saline or dextrose solution before intravenous administration. A liposomal preparation is available (Liposome Company) to extend the biological half-life of the active prostaglandin. Aiprostadil (Caveiject) is also available in glass vials for reconstitution to provide I mL of solution containing either 10 or 20 pg/niL for intercavernosa) penile injection to diagnose or correct erectile dysfunction in certain cases of impothis therapeutence. A urethntl suppository is also tic use has been all hut eliminated, however, by the availability of orally administered Viagra.
Prostaglandin E7 Cyclodextrin. HO
HO
Carboprost tromethamine. Ca,boprost Tmmethamine. (Hcmabate). is a prostaglandin derivaive that has been modified to prevent metabolic oxidation of the IS-position alcohol (unction. HO
The cyclic polysac-
charide complex of PGE1 (Vasoprost is available as an orphan drug for the treatment of severe peripheral arterial occlusive disease when grafts or angioplasty are not indicated. Cyclodextrin complexation is used to enhance water solubility and reduce rapid metabolic inactivation.
Misoprostol.
Misoprostol. I 6-tR.S)-meihyl- 16-hydroxyPGE1 methyl ester (Cytotec), is a modified prostaglandin analogue that shows potent gastric antisecretory and gitstroprotective effects when administered orally. COOCH3
.CH3
HO
CH3
This derivative is administered in a dose of 250 pg by
HO
Jeep intramuscular injection to induce abortion or to ameliorate severe postpartum hemorrhage.
Misoprostol is administered orally in tablet form in a dose
828
WiLwn and Gisvold's Textbook of Organic Medicinal and Plaarmareuihal ('Iwmistrv
of 100 to 200
4 times a day to prevent gastric ulceration
in susceptible individuals who are taking NSAIDs. Misoprostol is combined with the NSAID diclofenac in an analgesic product (Arthrotec by Pharmacia) that is potentially safe for long-term antiarthritic therapy. This prostaglandin ative should be avoided by pregnant women because of its
Travoprost. Travoprost (Travatan) is supplied as a 2.5. mL sterile 0.(XWA- ophthalmic solution in a 3.5-mi. con. tamer. Travoprost is claimed to bc the most potent and FP. Cautions and specific analogue in this product side effects are similar to those given above. CH3
potential to induce abortion. In fact, the combined use of intramuscular methotrexate and intravaginal misoprostol has been claimed to be a safe and effective. noninvasive method for the termination of early pregnancy.3°
PROSTAGLANDINS FOR OPHThALMIC USE Several proslaglandin analogues have recently come to mar-
ket for the treatment of open-angle glaucoma or ocular hypertension in patients who have not benefited from other available therapies. These products are marketed as sterile solutions for use in the eye (as indicated below). Each of these agents is presumed to lower lOP by stimulation of FP receptors to open the uveoscleral pathway, thus increasing aqueous humor outflow. Commonly occurring side effects reported for this product group include conjunctival hyperemia, increased pigmentation and growth of eyelashes, ocu-
CF3
Unoprostone.
Unoprostonc (Rescula) is supplied as a 0. 15% sterile ophthalmic solution. Unoprostone is somewhat
unusual, in that it is a docosanoid (22-carbon atom) PG, analogue marketed as the isopropyl ester. The naturdl position alcohol is oxidized to the ketone, as would pected to occur in vivo. Cautions and side effects am similar to those given above.
lar pruritus, and increased pigmentation of the iris and eyelid. Contact lenses should be removed during and after (15 minutes) administration of these products.
Bimatoprost.
Bimatoprost (Lumigan) is supplied as a sterile 0.03% ophthalmic solution in 2.5- and 5.0-mL sizes. The recommended dosage of bimatoprost is limited to one
drop into the affected eye once daily in the evening. Increased use may decrease its beneficial effect. If used con-
H
currently with other lOP-lowering drugs, a waiting period of 5 minutes should separate administrations. HO
H
VETERINARY USES OF PROSTANOIDS
Latanoprost.
Latanoprost (Xalatan) is available as a 0.005% sterile ophthalmic solution in a 2.5-mL dispenser bottle. Latanoprost is also marketed as a combination ophthalmic product with the f3-adrenergic blocking agent timolol. which apparently enhances lOP-lowering by decreasing the production of aqueous humor. Cautions and side effects are similar to those for other ophthalmic prostanoids.
H0.
CH3
_(
CH3
Since McCracken and coworkers demonstrated that acts as a hormone in sheep to induce disintegration of liar corpus luteum (lutcolysis).32 salts of this prostaglandin and a variety of analogues have been marketed to induce or chronize estrus in breed animals. This procedure allows ficial insemination of many animals during one insemination period. The following two products are currently availuhk for this purpose.
C7oprostenol Sodium.
Cloproslenol sodium lEsion mate) is available as the stxlium salt from Bayer Agilculluol Division or Bayvet Division of Miles Laboratory as an aquc.
ous solution containing 250 mg/mI. HO
Chapter 24 •
l,e,,An:rie,u'.v. and O:lwr Ejcrixani,jdx
829
Dinoprost Tnmethamine. Dinoprost tn luethalnine Lutalyse) marketed by Upjohn (veterinary) is a pH-balanced aqueous soiLulon of the trimethylammonium suit of (5(1 mg/nIL).
23. Warner, T I)., C,iuliano. F.. I.. oat.: Proc. Nail. Acad. Sci. C. S. A. 96:756375(,,8. 1999. 2.1. Susanna. R.. Gianipaiii, J.. Barge'.. A. S.. et al.: Ophthalmology IllS'
EICOSANOIDS IN CUNICAL DEVELOPMENT FOR HUMAN TREATMENT
29. Coleman. R. A., SunlIt. W. I... and Narutttiya, S.: Phannacol. Rev. 46: 205—229, l994
259—263, 2(8)1.
25. Am J. Health.Syst Phanti. 53:976 and 91(2. 1996. 26. Vu/a. C. 0., Snottier, S., Morelli. S.. eta).: Heart 86:661—665, 2001. 27. Nagaya. N., Sliimi,.u. Y.. Satolt, T.. ci at,: Heart 87:3441—345. 2)102. 28. l,tevre. M., Maraud, S.. Besse. II.. ci al.: Circulatiim (12:426—431, 2(88).
Numerous prostagiandin analogues are under investigation treatment of human diseases (see Table 24-2). Efforts are being focused on the areas of gastroprotectuon for antiulcer therapy. fertility control, the development of thrombolylics (e.g.. prostacyclin or thronthoxane synthelase inhibitors) to treat cerebrovascular or coronary artery diseases, and the development of antiasthmatics through modulation of the lipoxygenase pathway. Future application of cicosanoids to the treatment of cancer, hypertension, or immune system
thsorders cannot be ruled out, however. Thus, although progress has been slow, the expanded use of eico.sanoids or
deosanoid analogues as therapeutic agents in the future is almost ensured.
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Lands. W. Ii. M.: Prostaglvindios t.cukin, Essent. Fatty Acids 63: t25—126, 2t148).
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999.
(2. Kuhn, H.: t'rostaglnndin'. Other t.ipd Mediators (i2:255—27(t, 2(1(10. f(, Bell, R. t.. Summers. J. B.. anti Ham'.. R. R.. Anon. Rep. Med. ('hem 32:91—100. 1997.
(3. Seciektik. A.: Adv. Prostaglandin Titromboxane Leukot. Re'.. 22: 185— 98. 1994.
IS. Mvtrnett. I.. J.. and Fling. T. F..: Rev. Riochem. Toxicol.5:l35. 1984. 6. Robertson, I. (I. C.. ci at.: Cancer Res. 43:476. t91(3. Ii. Lands. W. F. M.. Jr.: Trends Pliurniacol. Sri 1:7%. 1981.
IS. Romc. L. H.. and I.ands. W. Ii. M.: Proc. Nail. Aced. Sci. U. S. A. 72:41(63, 1975.
IS. Higgs. G. A. 0 ul.: Proc. NatI. Acad. Sci. U. S. A. 1(4:1417. 191(7. SI. Haitel. A. M.. and Lands. W. F. M.: Biocliem. l'ttarmacol 31:33(17, (982.
Kuehi, F. A.. ci ul.: In Ramwell. P. led.l. Prostaglundin Syntheta.se tnbibitors: New Clinical Application'.. New York. Alan R. Liss, 1980. pp. 73.8(1.
Cryer. II., and Feldman. N.: Am. J. Med. 1114:413-421. (998
3)). llausknecht. K V.: N. EngI. 3. Mcd 333:537-541). 1995 31. Stiaril, N. A.. l)avis. T. t... and Williams, G. W.: J. Phartn. Pltarmacol SI :685.-6')4, 1999. 32. McCracken, S. A.. ci at.: Nature 238:129, (972.
SELECTED READING Bailey. I. M. led.): Priistaglauidtns, l.ctiki,incnes. unit New Yi,rk. Plenum Press, 1985. Bali. 0. 4).: 5.Lipoxygenasu. itthibitors and their anti-intlammatory aclisttics. Prog. Med. Client. 29:1—63. 1992.
Chandnt. R. K. (cdl: Health Effects of Fish and Fish Oils. St. John's. ARTS Iliiunedjeat Pulilishen, and Distributors. 1989. Ciilteti, N. N. let).): Bioliigicul Pniicctitin with Pru'.iaglauidins iii'. I and 2. flora Raton, FL, CRC Press. l91t5. (986. Dunn, M. J., Patrono. C.. and ('moth, 4). A. led'..): Renal eicosauioid.s. Ads. Lap. Med. flint. 259:1—421. 1989.
lidilvi'.). I.. Ii.. and Kindahl. H. leds.t: Prosiaglnndins in Animal Reproduction. New York, Elsevier, l9%4. Fukushitna, M.: Biological activities atid mechanisms of action of PGJ. and related conupintnds: ;m tipilate. Priistaglandins Leutktn l,ssent. Fatty Acids 47:1-12. 191)2. Gryglewski, R.J..anvt Stock. 4). led'..): Priusiacyelin and Its Stable Analogue lloptnst. New York. Springer-Verlag. l987
(iryglcwnki. K. J..
A., and McGill, J. C ledsJ: Proslacyelin
Clinical TunIs. New York. Ruseit Press. 1985. Hillier. K. (Cd.). Advances in Licosanoid Research. stils. I —4. Bosiott, NIP Press, 191(7—1988.
t,autds. W. F.. N.. and Smith. W. L. (edv.(: Prosiagtandmns and aracliidonate inetabolitis. Enzyiitnl. 86:1—7)15. 1982.
Icier. A. M.. and ()ee. M. H. (eds.l: Lenkotrienes in cardiovascular and pulmonaiy function. Prog. fin. t3iol. Res. 199:1—270. 19145. Pace-Asciak. C. R.: Mass spectra n) ltrostaglaltdins and related products. In Satttuelssoti. B.. and Patiletli, R. led.): Ads. Prostaglundin Thtroml.ettkot. Res. 18:1 —565. (989
Rainsfoni. K. 0.: Anti.Inllaminatoiy and Anti.Rheumattc Drugs, nil'.. Boca Raton. FL. CRC Press. (985.
I —3.
Robinson. H S.. and Vane. .1. R. (cds.l: Prosinglnndin Syniheia.se Inhibitors. New York. Raven Press. 1974. Rc,kach, J. led.): attd l.iposygena.ses. Ainstenlatit. Elsevier. 1989.
Rit,.icka. 'I'.: Ficosanoids and the Skin. lhiwa Raton, FL. CRC Press, 199(1. Schror, K.. and Sinainger. H. led'..): Prosiaglatidins in Clinical Research:
Cardiovascular System, vol.3(11. New York, Alati K. I.iss, 1989. Simopottlns, A. P., Kifer. R. R., and Martin, R. F..: He.tltlt El'fectsoiPolyunsuiurnted Fatty Acids in Seafood,,. New York. Acztdetttic Press. 1986. Stansbv. bE F.. led.): Fish Oils in Nutrition. New York. Van Nostrand Reinhold, 199(1.
Thaler-Dun. II., dePaulet. A. C., and Paoletti, K.: Icosanoids and Cancer. New York, Raven Press. (984. Vane, J. R.. and O'Grady. 3. (etis.l: Therapeutic Application'. of Proataglan.
din'. Boston. Edward Arnold, 993 Watkins. W. 0.. Peterson, M. B.. and Fletcher. 3. R. teds.): Prostaglandin.'. in Clinical Practice. New York, Raven Press, 1989. Willis, A. L. lcd.): Handbook of Eirissatnoids: Prustaglaildins vital Related Lipid.'., '.'ol. II. Born Raton, FL. CRC Press, 1989. Zor, U.. Naor, Z.. and Danon. A. ted.'..): lii Brjquet. P. led.). Leukotruenes and Prostanoids in Health and Disease, New' I'rvnds in lipid Mediutor,. Research. vol. 3. Bawl. S. Karger. 1989.
C
H
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P
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25
Proteins, Enzymes, and Peptide
Hormones STEPHEN J. CURER AND HORACE G. CUTLER
Proteins are essential to all living matter and perlbnn nuinerous functions as cellular components. Fundamental cellular events are catalyzed by proteins called enzymes, while other proteins serve as architectural constituents of protoplasm and cell membranes. Most important are the classes of hormones that are characterized as proteins or protein-like compounds because of their polypeptidic structure. Protein chemistry, essential in understanding the inecha-
nisms of molecular biology and how cellular components participate in physiology, is also key to certain aspects of medicinal chemistry. An examination of the chemical nature of proteins explains the action of those medicinal agents that are proteins or protein-like compounds and elucidates their physicochemical and biochemical properties. This, in turn. relates to their mechanisms of action. Furthermore, in medic-
inal chemistry, drug—receptor interactions are directly related to structure—activity relationships (SARs) and aid in the process of rational drug design. Drug receptors arc considered to he macromolecules, some of which appear to be proteins or protein-like. Recombinant DNA (rDNA) technology' has had a dramatic impact on our ability to produce complex proteins and polypeptides structurally identical with those found in vivo. Many of the endogenous proteins or polypeptides have exhibited neurotransmifler and hormonal properties that regulate a variety of important physiological processes. rDNAderived technology products currently being used are discussed below in this chapter. Although this chapter reviews the medicinal chemistry of proteins, it includes some enzymology, not only because many drugs affect enzyme systems and vice versa but also because basic discoveries in enzymology have been practically applied to the study of drug—receptor interactions. Hence, a basic introduction to enzymes is included.
protein are made available in the form of protein sates, and these can be administered to induce a favorable balance.
Protein deficiencies in human nutrition are treated with protein hydrolysates. The lack of adequate pro
tein may result from several conditions, but the prohkm not always easy to diagnose. The deficiency may he due to insufficient dietary intake. temporarily increased (as in pregnancy), impaired digestion or absorption. liver malfunction, increased catabolism, or loss of proteins aid amino acids (e.g., in fevers, leukemia, hemorrhage. surgery.
burns, fractures, or shock).
Protein Hydrolysate.
Protein liydrolysate is a solution of amino acids and short-chain oligopeptides that represeni the approximate nutritive equivalent of the casein. lactalbu mm. plasma. fibrin, or other suitable protein from which It
is derived by acid. enzymatic. or other hydrolytic It may be modified by partial removal, and restoration ii addition of one or more amino acids. It may contain dextmse
or another carbohydrate suitable for intravenous infusion Not less than 50% of the total nitrogen present is in the form of a-amino nitrogen. It is a yellowish to rcd-amnkt
transparent liquid with a pH of 4 to 7. Parenteral preparations arc used to maintain a positive nitrogen balance in patients who exhibit interference with ingestion, digestion, or absorption of food. For such patients. the material to be injected must be nonantigenic and mnn4 not contain pyrogcns or peptides of high molecular weight.
Injection may result in untoward effects such
nausea
vomiting, fever. vasodilatation. abdominal pain, twitching and convulsions. edema at the site of injection. phlehith. and thrombosis. Sometimes, these reactions are due toinudc.
quate cleanliness or too-rapid administration.
PROTEIN HYDROLYSATES In therapeutics, agents affecting volume and composition of body Iluids include various classes of parenteral products. Ideally, it would he desirable to have available parenteral fluids that provide adequate calories and important proteins and lipids to mimic, as closely as possible, an appropriate diet. Unhlirtunately. this is not the case. Usually, sufficient carbohydrate is administered intravenously to prevent ketosis, and in some cases, it is necessary to give further sources of carbohydrate by vein to reduce protein waste. Sources of
830
AMINO ACID SOLUTIONS Amino acid solutions contain a mixture of essential arid now essential crystalline amino acids, with or without
(e.g.. Aminosyn, ProCalarnine. Travasol. Novamine). Al. though oral studies have shown a comparison between pro. tein hydrolysates and free amino acid diets.2 protein sates are being replaced by crystalline amino acid solutions for parenteral administration because the free amino acids
Chapter 25 • Profrmus.
and Pep:ide Hnnnoni'.s
831
are used more etliciently than the peptides produced by the cnLymatic cleavage of protein hydrolysates.5
PROTEINS AND PROTEIN-LIKE COMPOUNDS The chemistry of proteins is complex, with many facets not completely understood. Protein structure is usually studied in basic organic chemistry and, to a greater extent, in biochemistry, but for the purposes of this chapter some of the nwre important topics are sumtnarized, with emphasis on relationships to medicinal chemistry. Much progress has been itiade in understanding the more sophisticated features of protein structure4 and its correlation with physicochemical md biological properties. With the total synthesis of ribonuclease in 1969. new approaches to the study of SARs among proteins have involved the synthesis of modified pro.
I 0
. Plants from the two genera have similar activities. The medicinal components are obtained from the flowering tops. The flowers are dried and used for chamomile teas and extracts. Chamomile has been used medicinally for at least 2,000 years. The Romans used the herb for its medicinal properties, which they knew were antispasmodic and sedative. The herb also has a long history in the treatment of digestive and rheumatic disorders. The activity of chamomile is found in a light blue essential oil that composes only 0.5% of the flower. The blue color is due to chatnazulene, 7-ethyl-I ,4.dimethylazulene. This compound is actually a by-product of processing the herb. The major component of the oil is the se.squilerpene (—)-obisabolol. Also present are apigenin. angelic acid. tiglic acid. the terpene precursors (farnesol. nerolidol. and germacranolide) cournarin. scopoletin-7-glucoside, umbelliferone. and herniarin. Much of the effect of chamomile is due to hisabolol. Bisabolol is a highly active anti-inflammatory agent in a variety of rodent inflammation and arthritis tests. In addition. hisabolol shortens the healing time of bums and ulcers in animal models. The gastrointestinal (GI) antispasmodic properties of bisabolol and its oxides are well known. In fact, bisabolol is said to be as potent as papaverine in tests of muscle spasticily. Besides bisabolol, the flavone and coumarin components have antispasmodic activities. The blue compound chamazulene possesses both anti-inflammatory and antiallergenic activities. as do the water-soluble components (the flavonoids). Apigenin and luteolin possess anti-inflammatory potencics similar to that of indomethacin. These flavonoids possess acidic phenolic groups, a spacer, and an aromatic moiety that could fit into the COX receptor. None of these effects has been unequivocally documented in humans. The essential oil possesses low water solubility. but teas used over a long period of time provide a cumulative medicinal effect.
Typically, I teaspoon (3 g) of flower head is boiled in hot water for 15 minutes, 4 times a day.
Chamazulene
(—)-a-Blsabotol
Luteolin
DRUG INTERACTIONS
Chamomile contains coumarins and may enhance the effect of prescription anticoagulants. The herb is an antispasmodic
and slows the motility of the Gl tract. This action might decrease the absorption of drugs. Chamomile preparations may be adulterated with chamomile pollen. This may cause allergy. anaphylaxis. and atopic dermatitis.
Episedra The varieties of ephedra (Ephedra sinica. E. nevadensis. E. trifurca, Ma i'mang, natural ecstasy. ephedrine, Herba Ephedrac) that possess medicinal activity grow in Mongolia or along the Mongolian border region with China. The plant itself, an evergreen with a pine odor, consists of green canelike structures with small, reddish-brown basal leaves. In the fall, the canes, root, and rhizome are harvested and dried in the sun. The dried material fumishes the active ingredients. ACTIVITY
Ma huang is a sympathomimetic agent. The active principles are fl-phenethylamines.' - These agents can stimulate the release of cpinephrine and norepinephrinc from nerve endings. Ma huang is a sympathomimetic stimulant in the periphery as well as in the central nervous system (CNS). Ii has positive inotropic and positive chronotropic effects on the heart; hence, the herb may be dangerous to people with cardiac disease. The amounts of ephedra-type compounds and the relative composition differ so widely that it is difficult to be certain what one is getting in any given preparation. Ma huang's main active ingredient is the $-phenethylamine compound (—)-ephedrine. Plants grown in China may contain 0.5 to 2.5% of this compound. Many ephedrine congeners are represented in the plant." and many of these possess considerable pharmacological activity. Some are as follows: (—)-ephedrine. (+ )-pseudoephedrine. norephedrine. norpseudoephedrine. cphedroxane. and pseudoephedroxane.
(—)-Ephedrlne
)-Pseudoephedrine
912
tI'jis,,,, (211(1 Gisi'e,ld's Textbook of Organic Medicinal and Pharmaceutical Chemistry
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H
vested of seeds and skins, and the rest of the fruit is used as a drink or in capsule form. Cranberry juice has been used
for many years as a urinary tract disinfectant. In
a
report said that the urine of persons who consumed cranberry juice became more acidic. Because an acidic medium hindNorephedflne
Norpseudoephedrlne
ers the growth of bacteria, it was thought that acidification of the urine inhibited bacterial growth. An analysis of cranberry juice shows that it contains many different compounds. Citric. malic, benzoic. and quinic acids of 3.5 ate present as carboxylic acid components. With to 5, these compounds should exist in the ionized form in
the urine at pH 5.5, thus lowering the pH. We now know that acidification of the urine is not the entire story. In fact. Ephedroxane
Pseudoephedroxane
Ma huang's principal active ingredient is (—)-ephedrine. This compound is the ci thro-o( —) isomer with the 2(S).3(R) configuration. The less potent ( + )-pscudocphedrine has the t/,re(, 2(S).3(S) structure. Ephedrinc acts as a mixed agonist On both a and /.3 receptors. PHARMACOLOGICAL EFFECTS
Ephedrin&s actions occur through mixed stimulation of the a- and J3-adrcncrgic receptors. The drug is a CNS stimulant that increases the strength and rate of cardiac contraction. Additionally. ephedrine decreases gastric motility, causes bronchodi lation. and stimulates peripheral va.soconstriction with the predicted increase in blood pressure. The threo isomer ( + )-pseudoephedrine causes similar effects but is much
less potent than (—)-ephedrine. The claims that ephedra causes increased metabolism and "fat burning" are certainly falsc, and ephedra lacks anorectic effects. Any reports or successful use of cphedra preparations in weight loss probably reflect the stinwlant or "energizing" effect and increased physical activity. In the United States, ephedra has been used as a recreational CNS stimulant (natural ecstasy).
E. neradensis and E. tn/urea are typically used in teas. The FDA prohibits preparations with more than 8 mg/dose and advises that one should not take an ephedra product more often than every 6 hours and no more than 24 mg/day. Ephedra should not be used for more than 7 days. Dosages over the recommended amount may cause stroke, myocardial infarction, seizures, and death. Ephedra has been closely linked to methamphetamine pro-
ductio,i. There are movements in many localities to outlaw the herb. There are many drug interactions with Ma huang. f3-Blockcrs may enhance the sympathetic effect and cause hypertension. MAOls may interact with ephedra to cause hypertensive crisis. Phenothiazines might block the a effects
drinking the cocktail does not appreciably acidify the urine. Two other constituents exist in the juice: mannose and a With bacteria high-molecular-weight that use fimbrial adhesins in infecting the urinary tract, man-
nose binds and inhibits adhesion of the type I mannoseSensitive fmmbriae. while the high-molecular-weight poty. saccharide inhibits binding of the P-type Iirnbriae. Hence. adhesion of many Escherichia coil strains, which cause over half of all urinary tract infections, is inhibited. This inhibition has the effect of blocking infection. Dosage: Drink between 10 and 16 ounces of juice daily.
Ginkgo blioba Ginkgo biloba (L.), also known as the maidenhair or Kew tree, has survived essentially unchanged in China for 200 million years.6° There is a Chinese monograph describing the use of ginkgo leaves dating from 2800 ac. Today, ginkgo is extracted by an extremely complex multistep process that concentrates the active constituents and removes the toxic
ginkgolic acid.6' The ginkgo extract is a complex mixture of both polar and nonpolar components. The more polar fractions contain
flavonol and flavone glycosides. The more nonpolar freelions contain some diterpene lactones. known as ginkgetin, ginkgolic acid, and isoginkgetin, and some interesting caged diterpenes known as ginkgolide A. B, C, J, and M.62 There is also a IS-carbon sesquiterpene (bilobalide) and other minor components. Ginkgo biloba extract is prepared by picking the leaves, drying them, and constituting them into an acetone-water extract that is standardized to contain 24% ha. vone glycosides and 6%
Ginkgolide A
of ephedra. causing hypotension and tachycardia. Simultane-
ous use of thcophylline may cause Cl and CNS effects. In pregnancy, ephedra is absolutely contraindicated (uterine stimulation). Persons with heart disease, hypertension. and diabetes should not take ephedra.
Cranberry The cranberry plant ( Vaccinium inacrocarpon. V. oxycoccus.
and V. ervthrocarputn) is a trailing evergreen that grows primarily in acidic swamp areas. The whole berries are di-
CH3
Chapter 27 • An I,nroducsio,, to :1w Medidnal CIw,nis.'rv of Ilerh.c
913
for disorders of memory that occur with age and Alzheimer's disease. The popular use for the herb is to help people think
better under stress and to increase the length of time that someone (e.g.. a student) can handle mental stress.
Ginseng Ginseng is the root ot the species ParnLr quinquefolius. This form is commonly known as American. or Western, ginseng. The shape of the root is important to many and may make it highly prized. Pana.r means "all" or "man." Sometimes. the root is shaped like the figure of a human. The doctrine of signatures would say that this root would benefit the whole
GinkgoIide C
person. Another species of ginseng, P. gin.ceng. is commonly
called Asian. or Korean, ginseng. Chemically, the two spe-
cies are very similar. Major components are named the ginse,wside.s. Ginkgoloxin
Ginkgo bikiba produces vasodilating effects on both the arterial and venous circula(ion."° The result is increased tissue perfusion (i.e.. in the peripheral circulation) and cerebral blood flow. The extract produces arterial vasodilatation (rodent models), dampens arterial spasticity. and decreases capillary permeability, capillary erythrocyte ag-
gregation, and blood viscosity. There arc several possible explanations for these effects. One possibility is that the compounds in Ginkgo biloba extract inhibit prostaglandin and thromboxane biosynthesis. It has also been speculated that Ginkgo biloba extract has an indirect regulatory effect on catecholamines. Ginkgolide B is reportedly a potent inhibitor of PAF.'5 In any case, the effects are due to a mixture of the constituents, not a single one. Ginkgo biloha has become popular because of its putative abilities to increase peripheral and cerebral circulation. l'he herb is called an a drug that helps persons handle stress. In the periphery, the herb has been compared to
The chemical constituents of ginseng are called gin.senosides or A total of 12 of these have been isolated but are present in such small quantities that purification is difficult. Sterols, flavonoids, proteins, and vitamins (B1, ,. pantothenic acid, niacin, and biotin) are also components with pharmacological activity. The chemistry of ginseng gives a good example of how different compounds in
one herb can have opposing pharmacological effects.65 Ginsenoside Rb-I acts as a CNS depressant. anticonvulsant. analgesic. and antipsychotic. prevents stress ulcers, and accelerates glycolysis and nuclear RNA synthesis. Ginscnoside Rg-l stimulates the central nervous system, combats fatigue. is hypertensive, and aggravates stress ulcers. Additionally. ginsenosides Rg and kg- I enhance cardiac performance. while Rh depresses that function. Some of the other ginseno-
sides display antiarrhythmic activity similar to that of the calcium channel blocker vcrapamil and amiodarone. Ginseng is popularly believed to enhance concentration. stamina, alertness, and the ability to do work. Longer term use in elderly patients is claimed to enhance "well-being."
There are few data from human studies. Clinical studies comparing ginseng to placebo on cognitive function tests showed statistically insignificant improvement. Neverihe-
pentoxifylline. lithe properties are true, the herb could he used for intermittent claudication. If cerebral blood tlow can be increased with Ginkgo hiloba, the herb might be useful
cI.13
Rd
914
IV1I.wn
and Gisioid's Textbook of Organic Medicinal arid Pharmaceutical C'hennctrv
CH3
20 (A) Ginsenoside
less, ginseng is a popular herbal product recommended by the German Commission E.
Milk Thistle Milk thistle (Silvbum ,nariam,m) is a member of the As— teraccac. a family that includes daisies, asters, and thistles. The plant has a wide range around the world and is found in the Mediterranean, Europe, North America. South America. and Australia. The seeds of the milk thistle plant have This been used for 2,000 years u.s a hepatoprotectant." usage can be traced to the writings of Pliny the Elder (AD 23—79) in Rome. who reported that the juice of the plant could be used for "carrying off bile." Culpepper in England reported that milk thistle was useful in "removing obstructions of the liver and spleen and against jaundice." CHEMISTRY
Milk thistle contains as an active constituent silymarin," which is actually a mixture of three isoineric tiavanolignans:
silybin (silibinin). silychristin. and silydianin. Silybin is the most active hepatoprotectant and antioxidant compound of the mixture. Also present in the plant are the tlavanolignans dehydrosilybin, silyandrin. silybinome. and silyhermin. Other lipid-soluble components are apigenin. silybonol, and linoleic. oleic, myristic, stearic, and palmitic acids.
whose steroid structure stimulates both DNA and RNA cviithesis. Through these activities, the regenerative capacity of the liver is activated. Silymarin is reported to alter the outer cell membrane structure of liver cells, blocking entrance of
toxic substances into the cell. This blockage is so pro. nounccd that it can reduce the death rate from
bides poisoning. Silymarin's effect can be explained by its antioxidant properties: it scavenges free radicals. Ry thic effect, the level of intracellular glutathione rises, becoming available for other detoxification reactions. Silybin inhibits blocking peroxidatioti of enzymes like acids and membrane lipid damage. Studies also show that silymarin protects the liver from amitriptyline. nortriptylinc. carbon tetrachloride, and cisplatin. When treated. patients with alcoholic cirrhosis showed increased liver function as measured by enzymes. In patients with acute viral hepatitis. silymarin shortened treatment time and improved asparlate aminotransferase (AST) and alanine aminotransferase
(ALT) levels. In liver disease. silymarin appears to have an immunomodulatory effect. The activities of superoxide dismulase (SOD) and glutathione peroxidase are increased, which probably accounts for the effect on free radicals. Silymarin. however, has an anti-inflammatory effect on human ptaielets. Silybin retards release of histamine from human mast cells and inhibits activation of T lymphocytes. The chemical appears capable of reducing the levels of all inununoglobulin classes and enhances the motility of lymphocytes. Milk this. tie extract and its components have shown efficacy in treating hepatotoxin poisoning, cirrhosis, and hepatitis. It also plays a role in blood and immunomodulation and in lipids and biliary function. The overall effect is due to the electronscavenging properties of flavanolignans. the enhanced regenerative capacity of the liver, and the alteration of liver cell membranes that blocks toxin entry.
Valerian Silymarin
MECHANISM OF ACTION
Valerian (Va/eriana ofikina/is) is found in temperate regions of North America. Europe. and Asia. The dried rhizome of valerian contains an unpleasant-smelling volatile oil that is attributed to isovaleric acid. Despite the odor. valerian is a safe and effective sleep aid.
The silymarin complex is aptly suited for its hepatoprotecSilymarin undergoes enterohepatic cycling. tive
CHEMISTRY
moving from intestine to liver and concentrating in liver cells. Protein synthesis is induced in the liver by silybin.
Three classes of compounds have been linked to the sedative properties of valerian. The rhizome contains monoterpenec
Chapter 27 • un ln:rodue,ion so i/u' Medk'i,,al C/,ru,ixirv of Ht'rbx acid and its acetoxy derivative). iridoids (valepotrinates). and pyridine alkaloids!" and sesquiterpcnes
At present. it is not possible to state which class of compound is responsible for the sedativc activity. Most reseaithers believe that the vakpoirioatc is the active component. but some studies have shown that valerenic acid is more potent.
915
the monoterpene pulegone.7' The oil also contains tannins. a- and other terpenes. long-chain alcohols, piperite000es. and paraffin.
The toxicity of pcnnyroyal is believed to be due to the pulegone" in the oil. Cytochrome P-450 catalyzes the metabolism of pulegone to yield the toxic metabolite menthofuran. Possibly. some of the other tcrpcnes undergo oxidation to active metabolites as well. Menthofuran. metabolites of other terpenes. and pulegone itself deplete hepatic glutathione, resulting in liver failure. This mechanistic hypothesis is supported by the fact that administration of acelylcysteine reverses the toxicity,
ValerenicAcid CH3
Pulegone
Pennyroyal has been used as an abortifacient since the time of Pliny the Elder.72 an insect repellent (the terpenes in the oil have citronellal-like properties), an aid to induce menstruation, and a treatment for the symptoms of premenstrual syndrome. It has also been used as a flea repellent on
CH7
0
Menthofuran
dogs and Cats.
Valepotrioate
When used as an abonifacient, the drug often causes liver
failure and hemorrhage. leading to death. Pennyroyal is sometimes used with black cohosh to accelerate the abortifacicnt effect. Coma and death have been reported. Pcnnyroyal is an example of an herb that has no safe uses, It should not be sold.
Herbal Drugs Used In the Treatment of Pyridine Atkalo,d
Aqueous and hydroalcoholic extracts of valerian induce the release of 13H1 y-aminohutyric acid (GABA) from synaptosome preparations. The extracts appear to have much the same effects as benzodiazepines. except that valerian does not act on thc Na * 1K -ATPase. Valerenic acid inhibits the GABA transantinase. This effect would increase the inhibi*
tory effect of GABA in the CNS. There is no doubt that valerian is safe and elTeetive as a sleep aid. Used properly, it is one of the more recommendable herbs.
Dose: 400 to 900 mg standardized extract
to
I
hour
before bedtime.
Pennyroyal
Anticancer drugs derived from biological sources are fairly common and are among the most important in the therapeutic
armamentarium. Drugs like doxorubicin. mitontycin C. mithramycin. and bleomycin have been around for a long time and have shed much light on the treatment of cancer. Three plant-derived drugs that have found their way through clinical trials deserve mention here. Two of the most famous are vincristine and vinblastine. These are compounds isolated from the periwinkle plant Catharanthu.c roseus. The
Vinca alkaloids bind tightly to tubulin in cells and interfere with its normal function in spindle formation. The Vinca alkaloids make the tuhulin less stable. The ne, result is melaphase arrest of cell division. Paclitaxel (Taxol) was originally isolated front the needles or bark of the Pacific yew. Because ii occurs in vanishingly small concentration
example of an extremely toxic herb. The plant is a member
in the plant, a semisynthetic method for its production was developed. Taxol binds to tubulin like the Vinca alkaloids, but it makes the tubulin structure hyperstable
of the mint family, Labiatac. The dried leaves and flowering
so that it cannot function. Again, the net result is metaphase
tops of the plant contain from 16 to 30% oil, consisting of
arrest.
Pennyroyal (He'deorna pz,legeoidex, Mentha pnk'gium) is an
916
of Organic M('diei,Iul and !'Fiar,naeeuiieal Chemistry
Wilsin, and Gixi'oldx
sucrose. Concentrated aqueous extracts may contain 10 to 20% glycyrrhizin. When the herb is ingested, the intestinal
flora catalyze the conversion of glycyrrhizin into glycycrhetic acid, the pharmacologically active compound. Glycyrr. hizin and glycyrrhetic acid possess mild anti-inflammatory properties. Glycyrrhizin appears to stimulate gastric mucus secretion. This may be the origin of the antiulccr propenics
of licorice. Glycyrrhizin and glycyrrhetic acid do not act directly as steroids. Instead, they potentiate. rather than mimic. endogenous compounds.
HO
CH3
0. OH
1..,,.—
H
0 "j'
(57) A H HO
0 0
Vlnbtastine
n
Q
/
)
0
OH
OH
R=Glucuronyl-( 1 .2)-gtucuronate Gtycyrrhizin RH : Gtycyrrhetic Acid
There is some interesting folklore relating to the use of licorice. During World War II. a Dutch physician71 noticed that patients with peptic ulcer disease improved dramatically when treated with a paste containing 40% licorice extract. The physician treated many patients in this way. but during the course of his work he noticed that there was a serious side effect from the herbal drug, About 20% of his ulcer patients developed a reversible edema of the face and estremities. Since these original observations, many studies have been conducted with licorice root. The findings have remained the same: licorice is useful for peptic ulcer disease. but potentially serious mineralocorticoid side effects are possible (lethargy, edema, headache, sodium and water reten-
tion, excess excretion of potassium. and increased blood pressure). Licorice exerts its protective effects on the gastric mucosa Paciftaxet
Licorice When we think of licorice, we typically think of the popular candy. Licorice, however, has an important history in herbal medicine. Licorice is a perennial shrub that is indigenous to
the Mediterranean and is cultivated in the Middle East. Spain. northern Asia. and the United States. The most comginmon variety used for medicinal purposes is bra var. typiea. Licorice has been used since Roman times and was described in early Chinese writings. CHEMISTRY
The root and rhizome.s of the licorice plant contain approximately 5 to 9% of a steroidal glycoside called g!ycyrrlzizin. In the glycoside form. glycyrrhizin is 150 times sweeter than sugar. Also present are triterpenoids. glucose. mannose, and
by inhibiting two enzymes. I 5-hydroxyprostaglandin dehy. drogenase and .i''-prostaglandin reductase. Inhibition of these enzymes causes their substrates to increase in concentration. increasing the levels of prostaglandins in the gastric
mucosa and causing a cytoprotective effect. The acid also inhibits II -fl-hydroxysteroid dehydrogenase,74 thus in mineralocorticoidresponsive tissues, causing increased sodium retention, potassium excretion, and blood pressure. In the I 960s. a semisynthetic compound based on glycyrrhetic acid. 4-O-succinylglycyrrhetic acid (carbenoxolone). was introduced in Europe. It proved effective against peptic ulcer disease, but it was later shown to be inferior to the receptor antagonists. Licorice is also an effective demulcent. soothing a sore throat, and is an expectorant and cough suppressant. Licorice can cause serious adverse reactions. These arc mineralocorticoid effects (pseudoprimary aldosteronisirn. muscle weakness. rhabdomyolysis. and heart failure. Poison.
ing by licorice is insidious. Long-term high doses are
Chapter 27 U An Introduction iv the Medicinal chemistry of llerh.c
tremely toxic. Licorice can potentiate the digitalis glycosides and cause toxicity. With cardiovascular agents that prolong the QT interval. the effects may be additive.
REFERENCES
Herbal Press. 2(100. p. 57.
5. Tyler. V. F..: Herbs of Choice. New York. Phannuceutical Pnslucts Press. 1994. p. Ii. 6. Duke, J. A.. and Avensu. F.. S.: Medicinal Plants of China, viii. I. Aigirnac. Ml. Reference Publications. 1985. p.122. 7. Tyler. V. F..: Herbs of Choice New York. Pharmaceutical Products 994. pp. 17—31.
8. Itlutnenthal. M.: HerbalGr.un 23(491:32—33. 1990.
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Philadelphia. I.ea & Febiger. 1976. II. Gurley. B. 3.. Gardner. S. F., and Hubbard. M. M.: Am.). Health Syst. Pharm. 57(101:963—969. 2000.
12. Gurley. B. J.. Wang. P.. and Gardner. S. E: 3. Pharm. Sd. 87(12): 1547—1553. 19811.
13. Bet,. J. M.. Gay. M. 1., Mo&saba, M. M.. Ct ul.: J. Assoc. Anal. Chrm. mt. 80(21:303—315. 1997.
(3. Tyler. V. F..: Herbs of Choice. New York. Pharmaceutical Products Press. I994. p. I. IS. Koltai. M. et. al.: Drugs 42:9—29. 1991. 16. Suj.uki, 0.. et at.: Planta Med. 50:272—274, 1984. 17. Hua.s, H.: Aratcip(lanaenkunde. Mannheirn, B. I. Wisvcnschufts. Ver. lag, 199l.pp. 134—135. 18. Monrman. D. F..: Medicinal Plants of Nativc America. Research Report on Ethnobotany. Contrih. 2, Tech. Rep no. 19. Ann Arbor. University of Michigan Museum of Aitlhropology. 19911. 19. Bauer, K.: F.chinacea. Biological effects and active principles. In Lawson, L. D., and Bauer. R. teds.). Phytomexlicincs of Europe: Chemistry aitd Biological Activity. American Chemical Society Symposiuni Series. New York. Oxford University Press. 1998. p. 40. 20. Bauer. R.: Echinacea: Biological effects and active principles. In Law' son. LI).. and Bauer. K. (cds.). Phytomedicines of Europe: Chemistry and Biological Activity. American Chemical Society Symposium Series. New York. Oxford University Press. 1998. p. 141.
21. Baucr, R.. Khan.
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23. Bauer. R.. Khan. I. A.. and Wagner. H.: Planla Mcd. 54:426—430, 1988.
24. Stimpel. M.. Prok.sch. A.. Wagner. H.. and Lohtnann.Matlhes. M..L. Infect. lininun. 46:845-1449. 1984, 25. Lucltig. B., StainmUller. C.. Gifford. G. F... ci al.: J. Nail. Cancer Inst. 81:669—675, 1989.
26. Bauer. R.. Remiger. P., and Wagner. H.: Disch, Apoth. ZIg. 128: 174—180.
34. Murphy.i. J.. Heplinstall. S.. and Mitchell. i.R.A.: Lancet ii: 189—192, 1988.
35. Collier. H. 0. 3.. Butt. N. M., McI)onald-Gibsoii, W. 3., and Sneed. S. A.: Lancet 0:922—923. 1980. 36. Makheja, A. M.. and Bailey. J. M.: Lancct ti:lt)54. 981. 37. Makhcja. A. M.. and Bailey. 3. M.: Prostaglandins Lcukotncncs Mcd. 8:653—660. 19142.
I. Strottecker. 1.: Alternative Medicine: The Delimlive Guide. Puyullup. WA. Future Medicine Publishing. 1994. p. 257. 2. Brevort. P.: HerbalGr.un 44:33—46. 1998. 3. Solomons. (L. and Fryhle, C.: Organic Chemistry. 7th ed. New York. John Wiley & Sons. 2(102. pp. 3—4. 4. Tyler. L.: Understanding Alternative Medicine. New York. Haworlh
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27. Egcrt. I).. and Beuscher. N.: Planta Med. 58:163—165. 1992. 28. Jacobson, M.: J. Org. ('hem. 32:1646—1647. 1967. 29. Bauer. K.. Rentiger. P.. and Wagner, H.: Phytocheniistry 214:505—508. 1989.
30, Buuer. K.: Echinacea: Biological effects and active principles. In Lawson. L. I).. and Baucr. R. (cdv.). Phytomedicines of Europe: Chemistry and Biulugical Activity. American Chemical Society Symposium Series. New York. Oxford Uitisersily Press. 1998. p. 150. 3). Bauer. R.: Echinacea: Biological effects and active principles. In Lawson. L. D.. and Bauer. R. (cdv.). Phytomedicines of Europe: Chemistry and Biological Activity. American Chemical Society Symposium Se. nes. New York. Oxford University Press. 19914. pp. 158—159. 32. Biakeman, J. P.. and Atkinson, P.: Physirvl. Plant Patbol. 15:183—192. 1979.
33. Johnson. E. S.. Kadam. N. P., Hylands, I). M., and Hylands. P. 2.: Br. Med. J. 291:569—573). 1985.
3%. Thakkar, J. K.. Spereluki. N.. Pang. I).. and Fr4ltsivn. K. C.: Biochim. Biopltys. Acts 750:134—14(1. 1983.
39. Heptinstall. S.. Groencwcgen, W. A.. Knight. I). W.. et al.: In Rose. C. (ed). Current Problems in Neurology: 4. Advances in Headache Research. Proceedings of the 6th International Migraine Symposiunt 1987. London. John Libbey & Co. Lid.. 1987, pp. 129- 134. 41). Bork. P. M., Lienhard-Schmitr. M. L., Kuhnt. M.. ci al.: FEBS I.eti. 402:85—90. 1997. 41. tlohlmann. F.. and Zdero. C.: Phytochemistry 2 1:2543—2549. 19142.
42. Kupchan. S. M.. Fessler, D. C.. Eakin. M. A.. and Giacobbe. 1. J.: Science 168:376—377. 1970.
43. Heptinstall. S.. Groeitewegcn. W. A.. Spattgenbcrg. P.. and t.A(sche, W.: Folia Haematol. 115:447—449. (9811. 44. Heptinslall. S.. Groenewegen. W. A.. Spangenherg. P.. and LUsche. W. J.: J. Pharm. Pharinacol. 39:459—465, 1987. 45. Heplinstall. S.. Awang. I). V. C., Dawson. B. A.. itt itl.: 3. Phnrm. Pharmacol. 44:391—395, 1992. 46. German Commission F. Monograph, 1999,
47. Rester, H. I).: Chemistry and biology of Ilypericun, pv',foratuns (St. Johns Wart). In Lawson, L. I).. and Baiter. K. (Cdv.). Phylomedicines of Europe: Chemistry and Biological Activity. American Chemical Society Symposium Series. New York. Oxford University Press. 199$. pp. 287—298.
48. Tyler. V. F... Brady. L. R.. and Robbers. I. U.. Phannarognasy. 9th ed Philadelphia. Lea & Fcbigrr. 1988. pp. 148—151). 49. Fetrow. C. W.. and Avila. 3. R.: Professional's llandhiiok of Coniple. mentary and Alternative Medicines. Springhoasc. PA. Springhousc Corporation. 1999. p. 123. 50. Tyler. V. F..: Herbs of Choice. Nesv York. Pharmaceutical Products Press. 1994. p. 125. SI. Gruenwald. J.: HerbalGram 34:60-hS, 1995. 52. Koch, H. P.. and Lawson. L. D. Garlic: The Science and Therapeutic
Application of Allium .raiii'wn and Related Species. Baltimore. Williams & Wilkins. 1996. pp. 25—36. 53. Lawson. L. D.: Garlic: A review of ii.'. medicinal effects and indicated active compounds. In Lawson. I.. I).. and l.luuer. K. (ed'.). Phytotnedi. clues of Europe: Chemistry and Biological Activity. Attierican Chentical Society Symposiunt Series. New York. Oxford tlniversity Press. 1998. pp. 180—186.
54. Koch. H. P., and Lawson. U. I).: Garlic: TIte Science and Therapeutic Application of Allium .xutii'um and Related Species. Baltimore. Wilhams & Wilkins. 1996, pp. 135—212. 55. Tyler. V. E.: Herbs of Choice. New York. Pharmaceutical Products Press. 1994. pp. 57—58. 56. Blatherwick. N. K., and Lung. M. L. J. Biol. ('hem. 57:815—8114. 1923. 57. Subota. A. F..: J. tJrol. 131:1013—11)16. 1984.
58. Soloway, M. S.. Smith, R. A.: JAMA 260:1465. 1988. 59. Olek. I., Goldhar. J.. Zafriri, D.. et al.: N. F,ngl. 3. Mcd. 324:1599. 1991.
60. Tyler. V. F..: Herbs of Choice. New York. Pharmaceutical Products Press. 1994. p. lOP. oh. Fetmw. C. W.. and Avilu. 3. K.: Professional's Handbook or Cotnpletnentaty and Alternative Medicines. Sprittghouse, PA. Springltouse Corporation, 1999. P. 278. 62. Hl)nsel, K.. Phytopharmaka. 2nd ed. Berlin. Springer-Verlag. 1991. pp. 59—72.
63. Fetrow. C. W., and Avila. 3. K.: Professional's Handbook olComple. mentaty and Altrmalive Medicines. Springbuu.se. PA, Spnnghouse Corporation. 1999. p. 279. 64. Feirow, C. W.. and Avila. J. R.: Professional". Handbook of Complementary and Alternative Medicines. Spnnghousr, PA. Spnnghuuse Corporation. 1999, p. 282. 65. Tyler. V. E.: Herbs of Choice. New York. Pharmaceutical Products Press. 1994. p. 172. 66. Flora. K.: Am. 3. Gastroenteml. 93:139—143, 1998. 67. Saluni. A.. and Sarna. S.: Scand. 1. Gastrocuterol. 174:517—521. 1982. 68. Fetrow. C. W., and Avila, J. R.: Professional's Handbook of Comple.
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mentary and Alternative Medicincs. SpringhoLise. PA. Spnnghouse Corporation. 1999. p. 430. 69. HtnscI. K.: Pttytopluirrnaka. 2nd cd. Berlin. Springer-Verlag. 1991. pp.
252-259. 70. Kiieplstein. i.. and Grusla. 1).: Dtsch. Apoth. Zag. 128:2041—2046. 9811.
71. Anderson. I. B.: Ann. Intern. Med. 124:726—734. 1996. 72. Fetrow. C. W.. and Avila. J. R.: Professionals Handbool
mentary and Alternative Medicines. Springhouse. PA. Corporation. 1999. p. 499. 73. Nieman. C.: Chem. Drug. 177:741—745. 1962. 74. Baker. M. E.. and Fanestil. 1). D.: Laneet 337:428—429.
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28 •
Computational Chemistiy and Computer-Assisted Drug Design J. PHILLIP BOWEN
The advent of powerful and inexpensive computers has revolutionized science and medicine. Medicinal chemistry is no exception. Today, drug design methods are widely used in both industrial and academic environments. Through the use
of computer graphics. structures of organic molecules can be entered into a computer and manipulated in many ways. Computational chemistry methods are used to calculate molecular properties and generate pharmaeophore hypotheses.
Once a pharmacophore hypothesis has been developed, structural databases (commercial. corporate. and/or public) of three-dimensional (3D) structures can be searched rapidly
for "hits" (i.e.. existing compounds that are available with the required functional groups and permissible spatial orientations as defined by the search query). It has become popular to carry out in silico screening of drug—receptor candidate
interactions, known as i'irwal high-throughput screening (m+ITS). fur future development. The realistic goal of tHIS is to identify potential lead compounds. The drug—receptor
fit and predicted physicochemicul properties are used to "score" and "rank" compounds according to penalty func(ions and information filters (molecular weight, number of hydrogen bonds, hydrophobicity. etc.). Although medicinal chemists have always been aware of absorption, distribution,
metabolism, elimination, and toxicity (ADMET or ADME/ Tax), in recent years. a much more focused approach ad-
them. This means that the computer results must be compared with experimental data. Regardless of the intellectual appeal of computer-assisted drug design (CADD). the methods must be validated for known cases to give confidence in the many situations when the methods will be applied to unknown eases. The inherent between a model and realit should always be appreciated, whether computer simulations are being used for drug design, weather forecasting. or economic prognostication. The ability to calculate molecular properties, coupled with visualization of molecular structures. has greatly benefited scientists involved in drug discovery. Computer models of drug molecules and drug—receptor interactions are commonly found in the scientific literature. One would be hardpressed these days to find a journal devoted to drug discovery without one paper showing computer graphics representations of drug molecules. Drug-like molecules can be displayed with molecttlar surfaces and color-coded according
to solvent accessibility, electrostatic potentials, or other properties. With molecular modeling software, it is easy to superimpose two or more molecules. Even pharmncetttical industry marketing campaigns have brightly colored representations of drugs and their molecular surfaces splashed onto magazine ads or swirling across television screens.
clinical evaluations of the safety and efficacy of new drugs.
In the I 980s. skeptics of the use of computer-based methods for drug discovery often asked the question: what compounds were designed by molecular modeling methods? Few convincing examples could be provided during this period,
Only approximately 20% of compounds entering clinical
since CADD had only been in existence for a few years.
trials emerge as marketable drugs. Increased efforts to develop computer-based absorption, distribution, metabolism. and elimination (ADME) models are being pursued aggressively. Many of the predictive ADME models use quantitative structure activity relationships (QSAR). In general, understanding what chemical space descriptors arc critical for drug-like molecules helps provide insight into the design of chemical libraries for biological evaluation. With the aid of molecular modeling software, pharmaceutical scientists can modify the structural features of a potential drug candidate in silico and make predictions about its physicochemical properties prior to laboratory synthesis. Crystallographic information about the receptor has allowed scienlists to use structure-based drug design approaches with tangible benefits (i.e., marketable drugs). Given the difficulty in preparing organic compounds, one can immediately appreciate the power that computer-based methods offer. Obviously, the computer-generated models must be accurate
Furthermore, because it is not uncommon for a drug to take 10 or more years from the design stage to final approval.
dresses these issues in the early design stages. This is logical when one considers the compelling statistics associated with
enough to give scientists a high degree of confidence in
the fruits of CADD would not be revealed until about a decade later. Unquestionably, many of the major advances in medicinal chemistry have used rational drug design but without computers. In some respects. the question regarding CADD success stories is fundamentally flawed, in that no one typically asks how many drugs were designed by a single scientific technique (e.g.. nuclear magnetic resonance
INMRI spectroscopy). The NMR analogy was the oftenquoted response by computational chemists. In reality, drug discovery is so undeniably complicated that no one single method generally can be credited fur the complete design
of a drug. An arsenal of methods from diverse scientific areas is brought to bear on drug discovery problems. The scientists involved have to interpret the data. Although molecular modeling methods are just another tool to help scientists make informed decisions on what lead compounds to pursue and what structural features should be modified to
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Texthook of Organic Medicinal and Pharmaceutical Che,niarrv
enhance biological activity, there is an inherent appeal associated with visualization and predictions. In the final analy-
sis. scientists, not computers, "design" drugs. CADD has two fundamental roles to play in drug research: lead discovery and lead development. During the late 20th century. there were many case histo-
ries of accurate predictions of in vitro biological activity based on so-called rational drug design approaches. In the I 970s. Corwin Hansch demonstrated that simple regression statistics could be used to correlate biological activity indi-
rectly to molecular structure through physical properties such as hydrophobicity (log P), electronic
and steric (E1-)
effects (Chapter 2). The seminal contributions of Hansch demonstrated the power of QSAR and helped to usher in the era of computer-based modeling for molecular design. Beginning in the 1980s, there were many reports of computer-bused predictions of in vitro biological activity. This is not unreasonable, since there are fewer pharmacodynamic and pharmacokinetic variables to be considered with in vitro testing than with in vivo testing. The use of structure-based
design has grown in importance since the early l980s. Today, there are many examples of drugs on the market or in clinical trials for which computer-based methods, particularly structure-based drug design, have played central roles in their development. No single chapter can provide all the detailed information necessary to master this specialty, which ranges from quantum physics to 3D database searching. Many of the subtopics discussed have had entire books or series of books devoted to them, and a complete discussion of each topic is simply beyond the scope of this chapter and the purpose of the book.
Instead, the goal of this chapter is to provide a brief and accurate overview of a select set of computational chemistry
and CADD methods, highlighted with examples when appropriate. In some cases, the concepts are simplified or gen-
eralized to make sense of them in a few pages, but this is done without sacrificing accuracy so that the interested reader can continue future studies with a solid foundation in the fundamentals. Finally, computer-based methods do not replace experimental In fact., the purpose of CADD is to aid pharmaceutical scientists in the discovery process. whether through simple visualization or the complex formulation of a pharmacophore model and statistical modeling.
COMPUTER GRAPHICS AND MOLECULAR VISUALIZATION Ever since chemists have recognized that molecules are made up of atonis. there have been efforts to represent mo-
lecular structures accurately. One of the first attempts to develop molecular models can be traced to the early I 800s when John Dalton used wooden spheres drilled with holes to accept metal rod linkers as representations of atoms and chemical bonds, respectively. The original mechanical models are on display at the London Museum ofSciencc.L2 Prior to computer graphics, the fundamental principles now associated with CADD (or molecular modeling) were used for some landmark discoveries in structural biology. Unquestionably, one of the most widely recognized scien-
tific achievements was the construction of a DNA model proposed by Watson and Crick in The structure has withstood the test of time, leading to a shared Nobel Price for Watson and Crick and setting the stage for the age of biotechnology. Initially, crude models were crafted from cardboard. Once a better understanding of the structural requirements of the nucleic acids was devclopcd, more accurate physical models were prepared from metal. Originally. the incorrect tautomeric forms of the heterocyclic bases were
used. The incorrect tautomeric forms did not lead to any viable models of DNA. but once Watson and Crick learned that the older literature was in error, they had new nucleic acid models prepared based on the new structural data. With
the correct physical models, coupled with their knowledge of the experimental data associated with DNA, they were able to construct the double helical structure4—an excellent example of the power of Tinkertoy sets in the right hands. The modeling principles used by Watson and Crick to arrive at the correct representation of DNA were previously used by Linus Pauling. who correctly predicted that proteins would adopt a-helix or conformations.5 Pauling suggested that solid molecular models would assist in understanding molecular structure, particularly protein stnjctures. Solid models, in which the atomic sizes of the constituent atoms are designed accurately to reflect the relative siecs (the van der Waals radii), were prepared according to International Union of Pure and Applied Chemistry (IUPAC) guidelines. These space-tilling molecular models, known as CPK models." became commercially available. They have been used around the world to help understand the molecular
shapes of proteins and nucleic acids and their interactions with small drug-like molecules. Commercially available handhcld mechanical models became popular after the impressive structural predictions of the a-helix and a—sheet conformations for proteins, as well as the double helix for DNA. During this time, building on the work of earlier generations. Sir Derek Barton. Ernest Eliel, and others demonstrated the importance of conformational and stercochemical effects for small organic mole. cules. Medicinal chemists used these models and einpincally derived rules to develop hypotheses for drug—receptor inter-
actions. Wooden ball-and-stick models were used by high school and college students to help visualize organic tures. Accurate metal Dreiding and Kcndrew models are still useful.7 These physical models have the advantage that medicinal chemists can hold them, manipulate them. md get a better feel for the structural flesibility. Nevertheless. large macromolecular models usually had to be supported by seal-
folding, and they were notoriously troublesome to modify and manipulate. Imagine the difficulties associated v.ith modeling the intercalation of small potential lead structures into DNA. The DNA structure would have to he cranked open mechanically, and the small potential intercalators would have to be inserted and manipulated by hand to get the best fit as determined by visual inspection. Manipulation of computer models is much superior to the use of traditional physical models (as long as the electricity is flowing). Mathematical niodels using quantum mechanics or force field methods (see below) better account for the inherent flexibility of molecules than do hard sphere physicul models. In addition, it is easy to superimpose one or more molecular models on a computer and to color each structure
Chapter 2$ • Conqnua:ional C'ht',njstr. and separately br ease of viewing. Medicinal chemists use the superimposed structures to identify the necessary structural features and the 3D onentation pharmacophore) responsible for the observed biological activity. The display of the multiple conformations available to a single molecule can provide valuable infonnatiun about the conformational space avail-
1)rug
,O H3C..
i
S
s
lksign
921
0 NH2
NH
able to drug-like molecules. Rather than measuring bond distances with a ruler, as was done years ago with handhold models, it is relatively easy to query a computer-generated molecular display. Hecuuse the coordinates 11r each atom are stored in computer memory. rapid data retrieval is achieved. Moreover, the shape and size of a molecular system can be visualized and quantified, unlike the situation with handheld models, when only visual inspections are possible. Exactly how much energy does it cost to rotate torsion angles from one position to the next? Understanding drug volumes and
molecular shapes is critically important when delining the complementary (negative volume imaget receptor sites needed to accommodate the drug molecule.
In the 1970s. major advances were made by computer scientists because of increasing computer speeds, which allowed the generation of computer models analogous to the
handhold mechanical models. Interestingly, the growth of computer graphics software was driven by the need to have computer-assisted design tCAD) tools for the aircraft manufacturing industry. In the early !980s. soltware companies emerged that developed and sold model-building software to the pharmaceutical and agrocheniical industries. Today. there are many different software compatlies that specialize in the interactive manipulations of computer-based molecular images. High-resolution computer graphics have revolutionized the way tlrug design is carried out. Once a molecular structure has been entered into a molecular modeling software program. the structure can be viewed from any desired perspective. The dihedral angles can be rotated to generate new confonnations, and functional groups can be eliminated or modified almost effortlessly. As indicated above, the molecular features (bond lengths. bond angles. nonbonded distances. etc.) can be calculated readily from the stored 3D
Figure 28—1 • Dorzolamide (Trusopt), the first FDA-approved drug to be designed by structure-based methods, is a carbonic anhydrase inhibitor that is used to lower ocular pressure in glaucoma patients.
ics workstations dominated. Today, the power of personal computers makes them an appealing and affordable alternative. Many standard molecular modeling software packages are being ported to run on personal computers, and the cost of computer graphics technology is decreasing because of the demand of the video game market. Molecular structures can be represented in many different ways, depending on the properties one decides to highlight.
Dorzolamide (Fig. 2$-I) is a good example of a drug that involved CADD methods in its Figure 282 shows a standard representation of dorzolamide from a molecular modeling software package. The atoms can be color-coded in various ways according to the different properties that one might want to highlight. The representation in Figure 28-2 shows the cotmnectivity of the atoms in dori.olarnide that one might use during molecular modeling. As noted above, however, it is important to know tile size and shape of the molecule. Various representations are possible. One is a CPK solid representation, but insight into the atomic
coordinates. Significant work by computer scientists has been invested into giving the illusion that Iuickering computer images are physical 3D objects. With special viewing glasses. 3D computer images can be seen. These are relatively inexpensive and lead to impressive results. The basic idea is that the computer generates two colored images (most commonly green and red) and each structure can only be viewed by one eye. Alternatively, viewing headgear may be worn such that each lens in the glasses is synchronized with the computer monitor to open and shut. To give the illusion of 3D representations. a special optical technique known as
depth cueing is used. In this method the brain is tricked into believing that slightly dimmer objects arc farther away.
while brighter objects are closer. Another approach is to have thc molecular structure slowly rock back and forth. Since the l9lOs. the cost of hardware has decreased and the power of computers has increased dramatically. These impressive hardware advances, coupled with advances in software and more efficient algorithms, have made computer-based modeling accessible to anyone. At one time. CAD!) was the exclusive domain of industry and sonic specialized academic laboratories. Expensive computer graph-
Figure 28—2 • A computer-generated representation of dorzolamide (Trusopt) Rather than a ball-and-stick representation, dorzolamide is shown as a tube representation with color-coded atom types The structure has been energy minimized with the MMFF94 force field in the Sybyl (Tripos. Inc.) molecular modeling software package.
922
Wilson and Gisvrsld .s Tt'xthook of Organk Medicinal and Pharmaceutical Chemistry
connectivity is diflicult with opaque surfaces. Another convenient visualization technique is to have the atoms and bonds displayed simultaneously with the van der Waals surface represented by an even distribution of dots. These dot surfaces are convenient, in that the atomic connectivity is shown along with the appropriate site and shape of the molecular surface. As computer graphics technology has improved, it has become possible to represent the surface as a translucent volume, shown in Figure 28-3. in which the molecular structure appears to be embedded in a clear gelatin material. Finally, computer graphics images of drug—receptor inter-
actions. whether taken from x-ray crystal data or in silico generated, provide insight into the binding interactions, as shown in Figure 28-4. A full display of all the atomic centers in a protein structure gives too much detail. Most commonly. as shown in Figure 28-4. a ribbon structure traces the backbone of the protein main chain.'0 The Richardson approach is another commonly used display to highlight secondary structural features, in which cylinders denote a helices, arrows denote (3 sheets, and tubes are used for coils and turns.''
Because drug molecules make contact with solvents and receptor sites through surface contacts, it is paramount to have accurate methods to represent molecular surfaces correctly. Algorithms have been developed for such purposes. and they continue to be improved. The most straightforward way to represent a molecular shape is by the so-called van der Waals surface, in which each constituent atom contrib-
utes its exposed surface to the overall molecular surface. Each atom is assigned a volume corresponding ((I its van der Waals radius, and only the union of atomic spheres con-
Figure 28—4 • A computer-generated representation of a thienothipyran-2-sulfonamide bound to the active site of carbonic anhydrase. Note that the ribbon has been traced through the protein backbone. Proteins are commonly displayed this way.
tributes. These van der Waals surfaces have small crevices
and pockets that cannot make contact with solvent molecules. Another surface, known as the solvent-accessible or Connolly surface, can be generated)2 The algorithm takes the van der Waals surface and rolls a sphere. having the volume of a water molecule with a radius of 1.4 A, across it. Wherever the sphere makes contact with the original stirface, a new surface is created. This expanded surface is a more realistic representation of what water molecules contact. Another similar solvent-accessible surface is known as the Lee and Richards surface.'3 This surface is constructed in an analogous way, with a sphere rolled over the van der Waals surface, but the boundary is taken as a line connecting the center of mass of the sphere from point to point. Also. it is possible to calculate the solvent-excluded surface. The polar and nonpolar surface areas can be used as QSAR descriptors (Chapter 2). and many computer models for soha. tion use solvent-accessible surface areas (SASA). Com-
monly. the electrostatic density may be displayed on the surface of a molecular structure, providing an easil n.'cognized color-coded grid that may be used to infer the complementary binding functional groups of the putative receptom.
Figure 28—3 • Another computer-generated representation of dorzolamide (Trusopt). The structure has been energy minimized with the MMFF94 force field in the Sybyl (Tripos, Inc.> molecular modeling software package and is displayed with a superimposed translucent van der Waals surface. Such representations have the advantages of showing both the atomic connectivity of the molecular structure and its 3D shape and size.
COMPUTATIONAL CHEMISTRY OVERVIEW Colorful molecular graphics images are based on the foundations of computational chemistry. Computational chemistry
methods, which may be defined as the use of theory and computer technology to calculate molecular properties, are widely used in academia and industry to gain insights into complex problems. In many eases, computational chemistry
Chapter 28 • Computational C'hen,lstrv and Computer-A ssisied Drug De.cigit
is used to rationalize experiments and to help make sense of the massive amount of data generated. Such practices are important to answer the why of chemical and biological phenomena. The greatest potential power of computational chemistry, however, is in the domain of making predictions prior to experimental work. Computer experiments are ideally suited to help answer questions that are difficult—and sometimes impossible—to answer by experiments alone. Just what kind of predictions can be made? Energy-based calculations have been used to predict and to understand molecular geometry, chemical conditions, chemical reaction pathways, and transition states, as well as physical. ADME. and biological properties. Usually, computer simulations are less expensive and require less time than carrying out physical experiments. In general, computational chemislty refers to energy— based methods. CADD is a more all-encompassing term.
923
like a ball-and-spring model, with potential energy functions used to describe the forces holding nuclei together. These methods were shown to be viable in the For several reasons, this chapter focuses on force field rather than quantum mechanics methods. First, most medicinal chemistry applications are more amenable to this treatment. Second. most energy-based methods used in molecular modeling software are based on force fields—from conformational searching to scoring functions for drug—receptor fits—and it is critical to have a grasp of the fundamentals. Third. force field methods are conceptually easier to understand, and the mathematics is not as complicated. Finally. for large macromolecular systems with solvation. force field calculations arc the only practical way to proceed, given the difference.s in computer time between classical-based and quantum-based computations.
including not only energy-based calculations but also QSAR, database searching, and pharmacophore perception methods. Computational chemistry approaches can be divided into two broad categories: quantum mechanics-based and classical mechanics-based. The former covers the areas
FORCE FIELD METHODS
of semiempirical. ab initio. and density functional theory.
on different problems reported the first calculations in
The latter refers to force field (molecular mechanics) calculations and niolecular dynamics simulations. Each method has its strengths and weaknesses, and it is important to be aware of these. From the practical standpoint of a pharmaceutical scientist, whichever approach gives reliable answers in the shortest time is typically used. The trick, obviously. is knowing when it is appropriate to use one method over another. This understanding comes in time through practice. just as an organic chemist can "push" electrons to solve or
Of the three papers, the description of hiphenyl derivatives examined by Frank H. Westheimer most effec-
rationalize complex reaction mechanisms almost instinctively. The Born-Oppenheimer theorem is a good starting point.'4 The theorem basically states that electrons move in a stationary field of nuclei; and therefore, the electron and nuclear motions can be considered separately. This approximation is valid in most cases of interest to medicinal chemists, since on the time scale of electron motion, the nuclei do not move. The difference in speed is a consequence of
the differences in mass of the electron and the particles within the nucleus. It is analogous to speedhoats circling a heavy aircraft carrier. On the lime scale of the speedboats, during a brief snapshot of time, the aircraft carrier is motionless relative to the lighter craft. These facts, summarized in the Born-Oppenheimer theorem, enable successful use of the various mathematical models used in quantum mechanics and force field-based methods. Quantum mechanics involves optimizing the electron dis. tributions within molecules. Theoretical physicists first proposed the foundations of quantum mechanics in the There were only a few cases where exact solutions existed. More typically, approximate methods had to be used." Although the theory has developed and improved, greater attention began to be placed on computational quantum chem-
istry in the l950s when the first commercially available computers were introduced. Once computers became more available to the scientific community, solving problems with computational chemistry became realistic.'1 IS Force field methods, on the other hand, ignore the electronic distribution and concentrate on the motion of nuclei as if they behaved
Force field methods are not a recent development; in fact, they have a long history. Three independent groups working
tively showed how to solve a problem convincingly.'9 In the early years, the force field or molecular mechanics calculations were known as the West heimer method. Westheimer is considered the father of' molecular mechanics. Force field
methods were not actively developed until the l960s and l970s, as commercial computers were becoming more common. A number of academic research groups began to explore force field calculations as a way to help solve problems of interest. The investigations ranged from small strained organic structures to protein simulations. Most of the current force fields can trace their roots to common sources developed in the 1970s.2224 Force field calculations rest on the fundamental concept that a ball-and-spring model may be used to approximate a molecule.25 26 That is. the stable relative positions of the
atoms in a molecule are a function of through-bond and through-space interactions, which may be described by relatively simple mathematical relationships. The complexity of the mathematical equations used to describe the ball-andspring model is a function of the nature, size, and shape of the structures. Moreover, the fundamental equations used in force fields are much less complicated than those found in
quantum mechanics. For example, small strained organic molecules require greater detail than less strained systems such as peptides and proteins. Furthermore, it is assumed that the total energy of the molecule is a summation of the individual energy components, as outlined in Equation 28I. In other words, the total energy (E,,,,,,,) is divided into energy components, which are attributed to bond stretching nonbonded interactions angle bending torsion interactions and coupled energy terms The cross-terms combine two interrelated motions (bend—stretch, stretch—stretch, torsion—stretch. etc.). The division of the total energy into terms
associated with distortions from equilibrium values is the way most chemists and biological scientists tend to think about molecules.
924
Wilson and Gi.n.old'.c Textbook of Orgassii Medicinal and Pharn,aici,,h a! ('he,njstn
=
+
on the system (positive) or whether the system is doing work on the surroundings (negative). According to Hooke's law, the restoring force is proportional to the displacement — k(x — Thereibre. .suhslitu-
b,w.d',
+ +
+ + —
In the 1600s, Robert Hooke. the scientific rival of the famous Sir Isaac Newton (who among his many scientific contributions invented the calculus, developed a theory of gravitation, and formulated classical mechanics), proposed that if an ideal spring with an attached mass m was compressed or stretched from its equilibrium position by an exthe spring would exert a restoring force ternal force of equal magnitude but in the opposite = direction of the distortion. This is an example of Newton's third law: For every action, there is an equal and opposite reaction. For simplicity, it may be assumed that the spring lies along the x axis, where the equilibrium length corre-
tion of Equation 28-2 into Equation 28-5 yields Equation 28-6. Recognizing that the work applied to the system is flOW the total energy of the system, we generate Equation 28-7.
• •
=
=—I
—
k(x
(Eq. 25-41
•
=— —
• (I.S
(Eq. 25-5 p ( Eq 28—hp
= k(x — x,) • dx
(Eq. 257p
Taking the integral of Equation 28-7. where the displacement goes from x0 to x, with a corresponding energy change Li to E2. gives Equation 28-8.
dE = k J (x — .roklc
(Eq. 28-ti
sponds to x0. The position x0 may be considered the "natural
length" of the relaxed spring. Figure 28-5 shows the setup for the classic one-dimensional harmonic oscillator,
Integration of Equation 28-8 yields Equation 28-9. where c is the integration constant.
Hooke's law for a one-dimensional oscillator oriented along the x axis may be written in mathematical form according to Equation 28-2. where x and x0 are the distorted and the equilibrium positions. respectively. (Note that in Equation 28-2, x is used to symbolize that the displacement is a vector quantity, which alternatively could be represented as
xi, where I is a unit vector in the x axis.) If there are no frictional forces present, then the kinetic and potential energies arc said to be conserved. As shown in Equation 28-3. if we can express the force, F. as the first derivative of the potential energy. E. with respect to the displacement, then we have a conservative system (Ic.. the kinetic and potential energy equal a constant).
P = —k(i —
(Eq. 28-2)
dE
(Eq. 28-3)
£2 —
=
+
(Eq.28-9)
Equation 28-9 can be simplified by noting that £2 is the energy corresponding to the distorted spring at position x. whether this is stretching or compression. The energy E, can be defined as our zero potential energy when .r = x1 relatise
to any distortion. This means that by our choice of ,ero potential energy, the integration constant must be i.ero. c =
0, which can clearly be seen if no distortion. £2 — E1 = 0. occurs. Finally. Equation 28-10 is the generalized onedimensional potential energy function for stretching and compression of a spring. Note that the
notation has heeti
dropped, since E, is defined as the zero energy position. Equation 28-10 is a quadratic function. Figure 28-6 shows the plot of a simple quadratic Function with
From elementary physics, the dot product of the force
=
28-10)
and displacement is defined as the applied work from the surroundings. This external work is the energy required to distort the spring, Equation 28-4, An extension of Newton's third law implies that the spring requires an equal amount of work to be restored. Equation 28-5, Note that work is a scalar quantity. The negative and positive signs assigned to the work reflect whether the surroundings are doing work
(0,0)
Figure 28—6 . A plot of 1(x) = x1. This function. in the form xO
FIgure 28—5 • A simple one-dimensional ball-and-spring (x axis) oscillator serves as a model for bond vibrations.
of Hooke's law, has been applied successfully in force field Cal-
culations to model bond distortions (stretching and pressing).
Chapter 28 u (.'mnpuwlional Chenii,c,rv and (oInp,,1('r-,%s.%i.cl('d I)r,:g lkxign
The distortion of a one-dimensional ball-and-spring model along the .r axis can be generaliied into the 3D ease. with r11. where in effect we have replaced x with r and \Vith Equation 28-Il. the spring does not depend on being placed along one axis: it may he oriented in any direction. As pointed out above. Equation 28-Il is a quadratic fonc-
lion, used to describe stretching ntotions in most of the macromolecular force fields (e.g.. and CHARMM31). Ir — r11)2
=
IEq. 28-I Ij
The use of a quadratic equation to mimic a chemical bond
means that distortion and compression arc equivalent in terms of an energy penalty. Think about what the quadratic model suggests: Compression and stretching result in equal increases in energy. This does not make sense physically. since for a diatomie molecule. compression brings the two atoms together, while distortion separates them. It is well known that bond distortion is anhurnrnnic. so immediately we see a flaw in our model, which becomes more obvious
in trying to reproduce bond distortions for strained molecules. The energy associated with bond stretching is described by a Morse curve, which is similar to a quadratic function only in the region close to the r0 (Fig. 28-7). The relatively nondistorted bond lengths, which are characteristic of proteins, fall in the region where the Morse and quadratic functions overlap rea.sonahly well. This is why simple quad-
ratic terms may be acceptable as a first approximation in macromolecular calculations. In fact, given the complexity of nature, it is remarkable that a simple Hooke's law potential energy function works as well as it does.
account the aisharmonicity a.ssociatcd with bond stretching relies on power series approximation. The Morse curve may be expanded into a power series function around the equilibrium position. Such expansions follow a well-known muathematical stratagem embodied in the Taylor series expansion. shown in Equation 28-12. where the expansion occurs about
the position a. Complex curves way be approximated by power series. There are many advantages to having a function expressible as (x — or. including case of computation and ease in taking derivatives, which is why a Morse curve is not used directly via force field calculations.
lx - a)
fIx) = flat + -t
(x —
L4') (.i
-i
— a)"
(Eq. 28-12)
Expanding the Morse curve into a Taylor series using the formula outlined in Equation 28-12 generates Equation 2813. The first term in the series is a constant, and this may he set equal to zero. The second terni in the series may be recognized as the gradient, which stated above without proof and shown in Equation 28-3. is defined as the negative force for conserved systems. At the equilibrium bond length. the force is equal to zero. which means that the second term is also zero. Therefore, the first two terms in this expansion vanish. The first nonvanishing term in the series is the third term, which is a quadratic term, equivalent to Hooke's law. The fourth and filth terms correspond to cubic and quartic terms, respectively, in Equation 28-13.
F = k0 +
lr — vi,)
4
—
r,,)4+
For small strained organic molecules, more complex equa-
tions are usually necessary. Morse curves typically are not used in force field calculations. One approach that takes into
925
-
tEq.28-l3I
The higher terms can be considered corrections to the quadratic temt. which is only a first approximation to bond distortion. New force constants. k = and = kIl2. can be defined, giving Equation 28-14. l'he use of the quadratic futiction alone is known as the harmonic approximaLion.
=
)r —
.4-
Ir — vii)
,
+
(r —
3
(Eq. 28-14) C C
Moise Potential
— — — Quariic Potential Cubic Potential Potential
Bond Distance
developed by Thomas Hnlgren at Merck, is currently one of the most widely available force fields. It
was developed with medicinal chemistry applications in mind and can be found in various molecular modeling soft-
ware packages, including Syhyl (Tripos. lncj.37 Spartan (Wavelunction, Inc.),35 PCMODEL (Serena Soltware)Y' and MacroModel (Schriidinger. Inc.).4° The force field is robust and can deal with many diverse functional groups found in drug-like molecules, giving accurate answers for
Figure 28—i a In force field calculations, different levels of
small molecules as well as macromolecules. MMFF94 rivals
approximations are used to reproduce the stretching and
the accuracy of MM3." 31 which is known as an excellent small organic molecule force field, as well as those results MM3. however, is restricted in its published for usefulness for medicinal chemistry applications because of
compression of chemical bonds The plot shows a Morse poten-
tial energy function supenmposed with various power series approximations (quadratic, cubic, and quartic functions). Note that the bottoms of the curves, representing the bond length for most chemical bonds of interest to medicinal chemists, almost overlap exactly. This nearly perfect fit in the bonding region is the reason simple harmonic functions can be used to calculate
limited parameterization. The MM4 force field, a subsequent
version of MM3. has modified potential energy functions
bond lengths for unstrained molecular structures in the force
and an expanded number of cross-terms.'5 These additions should make MM4 more accurate than its predecessor. MM4
field method.
will probably suffer from shortcomings similar to those of
926
Wilson
/
and G,.csold's Textbook of Organic Medicinal and Plwrmace,aical Chen,lstrv
MM3. in terms of its ability to calculate diverse drug-like structures, because of the lack of parameters. Interestingly,
MM4 has yet to be relea.sed publicly, cvcn though it and MMFF94 were reported in 1996, at the same time, in backto-back publications within the same journal volume. Because MMFF94 is so widely available, it is arguably the standard small molecule force field of choice at this time. The MM3 and MMFF94 stretching functions may be writr— ten as outlined in Equation 28-IS, where is the force constant parameter. c, is a constant used and 143.9325 is a conversion factor. Note to modify that Equation 28-14 and Equation 28-15 are essentially the same but written in a different form. Equation 28-15 demands that the cubic and quartic force constants be scaled quadratic stretching constants.
II +
143.9325
H
Figure 28—8 • To reproduc:e the out-of-plane bending motions for sp'-hybridized atoms, a quadratic penalty functton is used to constrain the system to be planar. The sp2-hybridized atom forms a projection onto the plane defined by the three atoms directly bonded to it For formaldehyde, the two H atoms is constrained to be and the 0 atom define a plane. The in the plane by the use of a quadratic function.
+
Bending strains are treated in an analogous way in force field calculations. Any distortion in bond angles 0 —
Some force fields use an improper torsion angle concept to constrain the central atom ol a trigonal planar group. In the case of formaldehyde (Fig. 28-8). the improper torsion Note the fact that angle may be defined as
results in a rise in the energy. The increase in energy associated with angle bending may be treated effectively with simple quadratic terms (Eq. 28-16). Equations similar to 28-16 are found in AMBER, CHARMM. and related macromolec-
there is no covalent bond between the oxygen and hydrugen: hence, the name improper torsion angle. The use of an isaproper torsion is an equivalent mathematical method and has the same effect of constraining the central atom to be in tire
ular force fields. The use of quadratic terms is just a lirst
— plane as a function of Atoms have size and shape. With handheld mechanical models, the size of an atom is invariant regardless of its
(Eq. 28-15)
approximation applicable to unstrained systems, as seen in the case of bond stretching. Many small molecules require
a more complex function. Equation 28-li is the bending function used in the MMFF94 force field. Again, one can see that the first term is quadratic, while the second term is cubic. The latter may be considered a correction factor. =
—
+
=
(Eq. 28-16)
(Eq.28-t7)
centers planar. a special out-ofTo keep plane potential energy function is used. This is necessary because force fields are mechanically based and do not treat
electrons explicitly. A simple solution regarding out-ofplane bending. found in some force fields, makes use of
chemical environment, whereas in reality the van der Wuals radii behave as if they were sofi spheres rather than hard spheres (i.e.. more like marshmallows than wooden balIsI. One advantage of using computer-based energy is the ability to treat the van der Waals radii more realistically
by providing greater flexibility than one can achieve with physical models. Certainly, this approach differs significantly from any hard sphere model. The other important nonbonded energy term arises from electrostatic tions. Having a good electrostatic model is important. particularly when one considers the significance of electrostatic forces in drug—receptor interactions. Equation 28-19 describes the nonbondcd tennis. =
quadratic potential functions. The idea is simple and effective: Keep the central atom planar. For example and the sake of simplicity, consider formaldehyde. Because by definition
three points define a plane, the two hydrogen atoms and oxygen atom (bonded tO a carbonyl carbon) form a plane. Without some constraint, the carhonyl carbon will tend to move out of the plane defined by the two hydrogen atoms and the oxygen atom. The projection of the carbonyl carbon
onto the plane forms a direct imaginary line, as shown in Figure 28-8. The incorporation of an energy penalty term using a simple quadratic function will achieve the desired purpose of keeping the center atom in the plane. The higher the out-of-plane bending constant, the less puckering will be observed. Equation 28- 18 shows the simple form used. is the out-of-plane bending constant and d — where d0 is the distance from the projection of the atom on the plane to the atom itself.45
—
(Eq. 28.19
The first two tennis within brackets define the van dci Waals repulsions. which vary as hr'2. and the London dis' persion attractions. which vary as The is related to the size of the atom pair being considered, ç is the distance between the atom pairs, and refers to the depth of the potential energy well. It is based on the LennunlJones 6-12 potential. Many force fields use functions of thic
type to describe steric interactions (Fig. 28-9). Only atonts with a 1.4 nonbonded relationship to one another (i.e.. with three chemical bonds separating thens) are included in these calculations. The bending and stretching terms include 1.3
nonbonded attractive and repulsion terms implicitly. Hydrogen bonding may be treated as a special situation
requiring a modified Lennard-Jones potential. The bonded calculations are the most time consuming, particu-
larly if more complex terms arc substiluned for tile I/i0 =
(d —
(Eq. 28-18)
repulsive part of the Lennard-Jones 6-12 potential. as found
in MM3.
Chapter 28 • Computational Chemistry and Computer-Assisted Drug Design
927
value of the dielectric constant, the less electrostatic interaction exists between the atom pair. In a vacuum, the dielectric constant is 1.0; in water, the dielectric constant is approximately 80. One of the major limitations of force fields has to do with the assignment of charges to each atomic center. There is no atomic charge operator in quantum mechanics. Atomic charges are determined quantum mechanically by using a
a C
w
population analysis, Consequently, there are many ways charges may be assigned. One popular way is to generate an
electrostatic potential from high-level ab inirio calculations (discussed below) and then fit the optimal point charge distri-
bution on the atoms via least-squares methods. This approach has been used in AMBER.3° Distance Figure 28—9 • When two atoms i and j are separated by inf inite distance, there are no interactions between them. As two nonbonded atoms approach one another, two forces have to be considered. Attractive dispersion forces (London forces) result from the interaction of instantaneous dipoles on each atom and j. As the nonbonded atoms continue to approach one another, a repulsive interaction overwhelms the attractive interaction, and the energy curve rises sharply. The two nonbonded atoms can reach an equilibrium position where repulsive and attractive forces balance. Different mathematical relationships have been used in force field calculations to reproduce the nonbonded steric interactions.
The third term in Equation 28-19 is Coulomb's law, where q, and q, are the charges on two nonbonded atoms i and j. The two charged atomic centers are not connected via a chemical bond and do not have a common single atom attached to atoms i andj. In other words, we are talking about 1.4 or greater relationships, as shown in Figure 28-10. The diclcclric constant e basically dampens the charge—charge interactions and is a function of the solvent. The greater the
As discussed above, most force fields assign point charges to atomic centers and use Coulomb's law. The most notable exception is MM2 and MM3; in both versions. a dipole—dipole scheme for uncharged molecular structures is For systems with a net charge, MM3 introduces functions capable of dealing with the additional charge—charge and charge—dipole interactions. Although there should be no difference between the two electrostatic schemes, having dipole—dipole, charge—charge, and charge—dipole terms re-
quires more parameterization time. Improved MM2 and MM3 implementations found in MacroModel have eliminated the dipole—dipole scheme in favor 01' using point charges and Coulomb's law.
If the stretching, bending, and nonbonded terms were summed over all appropriate pairwise atom contributions within a structure, many important conformauonal effects in the simplest of hydrocarbons would not be reproduced. For example, Figure 28-Il shows the energy (kcal/mol) versus torsion angle o profile for butane. The energy rises and falls during the rotation around the central bond as a function of the relative positions of the methyl groups. The peaks on the curve correspond to energy maxima, while
c
±q
±q
H H
H
ii
H
'I Figure 28—10 • Two partially charged atoms i and j, not directly connected to each other or connected to a common atom (1 .4-interactions or greater), exert an attractive or repulsive electrostatic interaction (depending on the charge) on each other. A Coulomb potential energy function is the most common way used in force field methods to calculate the charge—charge electrostatic energy. Coulomb's law is used in virtually all force fields
with the exceptions of the original MM2 and MM3 codes. MM2* and MM3*. improved versions of the original codes, are
found in the popular molecular modeling software program MacroModel developed by Clark Still and use charge—charge interaction terms.
0
80
120
180
240
300
380
Dihedral Angle Figure 28—11 • Potential energy for butane. The energy (kcal/ mol) is plotted on they axis versus the torsion angle which is plotted on the xaxis. There are three minima. The two gauche conformers are higher in energy than the anti conformer by approximately 0.9 kcal'mol.
928
Wilsi,,, aiid Gisis,ld 's Testlumk of Orianh Medicinal and Pharmaceutical ('l,e,nisiry
the valleys correspond to energy minima. For butane, there are two different types of minima: one is for the anti butane conformation, and the other two correspond to the gauche butane conformations. The anti conformation is the global minimum, meaning it has the absolute lowest energy of the three possible low-energy confonnations. The differences in the conformational energies cannot be attributed to stcric interactions alone. Structures with more than one rotatable bond have multiple minima available. Knowing the permissible conformations available to drug-like molecules is important for design purposes. It turns out that another nonniechanicul effect is needed to reproduce the potential energy curve for butane and other structures. It has long been known that it requires energy for rotation about single bonds. One of the first conformationul effects presented in organic chemistry courses involves the favoratorsional orientations for ethane. The torble
sion angles are either eclipsed or staggered. In I 891. Bischoff proposed that ethane has a preference for a staggered conformation and the rotation in substituted cthanes The H/H steric interactions alone cannot was explain the energy difference. If restricted rotation was considered. Pitzcr (lenionstrated that the calculated and observed The staggered conentropies br ethane were identical.5"
bonds are not aligned, is preformation. where the ferred because the electron densities in each bond arc as far apart as possible. This energetically preferred orientation is reproduced by quantum mechanics calculations, hut foree field calculations need sortie equivalent function, since deetrolls are not treated explicitly but are treated indirectly via classical potential energy functions. For other saturated hydrocarbons. the potential energy terms are just extensions of the fundamental ethane curve with other nonhonded terms superimposed. according to Equation 28-I. In some cases. the energy required to rotate about single bonds is too great. locking structures into chiral conformations (atropisomers). Christie and Kenner first detnonstrated restricted rotation in 1922 by resolving 2.2'-dinitrophenyl-6.6'-dicarboxylic acid into optically active
A solution successfully implemented to reproduce the ethane phenomena, as well as other effects, was accom-
a.
a C
Ui
0
20
40
60
80
lOU
120
140
160
ItO
Dihedrat Angte
Figure 28—12 • A plot of Equation 28-20 with V1 =
= 0,
and V3 is a positive number. Note that this curve has threefold symmetry. The third term in Equation 28-20 is used to repro' duce ethane-like torsion profiles about single bonds.
It is illustrative to look at each of the three terms in Equa lion 28-20 lor a full appreciation of how a three-term Fourier series can be used to affect confomiational equilibria. Figure 28-12 shows tile plot of energy versus dihedral angle roIatiue with the V, and V2 Constants set to zero, and V, assigned
a positive value. The curve has threefold symmetry. The maxima occur at dihedral angles of 0°. I 20°. and 240': the minima occur at dihedral angles of 60". 180'. 3000. and 360'. luspectioti 01' Figure 28-12 shows the similarity between ii and the ethanc torsion curve. Consequently, the third lemi without resorting to any cuniples describes the ethane calculations. If the V, and V3 terms are set to zero. plottitig the seeoiid term shown in Figure 28-13. where V2 is assigned a positive
value in the truncated Fourier series, reveals its physiLal significance. There are minima at 0°. 180°. and 360° and maxima at 90° and 27(1°. Therefore, the second term in Equi
plished with the ititroduction of a truncated Fourier Although and
dihedral to be staggered, sotue torsion combinations prefer to be eclipsed." Molecular orbital theory has been used to explain the underlying chemical reasons. Carhonyl oxygens prefer to eclipse the a hydrogens or a carbons of = or alkyl groups = angles
and the acyl oxygens of esters prefer to have the O-R groups
aligned with tile C=O bond
These
>,
a
C Ui
bond alignment effects are reproduced adequately in force field calculations using a three-term truncated Fourier series
(Eq. 28-20). Essentially, this torsion function introduces quantum mechanical efTects into a classical hall-and-spring system, which traiisliirms our model into a much more powerful tool, with a fraction of the computer costs in terms of CPU cycles compared with using quantum chemical calculations. V
-- coso) +
V.
— cos2ø) +
V
+ cas3O) (Eq. 28-20)
0
20
40
60
60
100
120
140
180
ItO
DIhedral Anglo
Figure 28—13 • A plot of Equation 28-20 with Vi = and V2 is a positive number. Note that this curve has
0.
symmetry. The second term in Equation 28-20 is used to repro-
duce ethylene-like torsion profiles about double bonds
Chapter 28 • Conipuuuional Clu'n,is:rv and Cennpurer-Assistecl 1)rug Design
929
prove the accuracy of the force field description. Many of these additional terms fall into the category of cross-terms, in which two motions or interactions are connected or correlated. For example, in small molecule force fields one might find stretch—torsion, stretch—bend, bend—bend, torsion—bend, and other interactions. Equation 28-21 shows a stretch—bend function. Some of these cross-terms have been
a,
a
shown to be more important than others. The purpose of
C UI
cross-terms is to give better geometric results, and they are particularly important in calculating the vibrational spectra.
=
0
20
40
60
80
100
120
140
180
180
Dihedral Angie
Figure 28—14 • A plot of Equation 28-20 with V2 = V3 = 0, and V1 is a positive number. Note that this curve has onefold symmetry. The interpretation is less straightforward than the second and third terms in Equation 28-20. The first term in Equation 28-20 is used to help reproduce torsion curves of the X following type. an
is
used to describe the torsional energy profile
(arising about = C,,,,2 double bonds), which has twofold symmetry. The true underlying chemical explanation for the sharp energy rise observed in rotating about carbon—carbon double bonds may be attributed to the breaking of the weak bond. The ir bond is a consequence of the overlap of two adjacent coplanar p-orbitals. Any rotation about the bond shifts the orientation of the p-orbitals. with = a reduction in the overlap and a concomitant rise in energy.
For example, ethylene, the simplest alkene. prefers a flat conformation with the dihedral angles at dihedral angle is either 0° or 180°. When the 90C the p-orbitals are orthogonal, and there is no overlap. Therefore, the energy is a maximum along the one-dimensional potential energy curve when is 90°.
(tr — ro) + (r' — r,,')l tO — ((i)
(Eq. 28-21)
Force field methods are fast and accurate if the potential energy functions and parameters within the potential energy functions have been carefully developed. in addition to cal-
culating molecular geometry. force field calculations are used to determine the energy between conformations.
GEOMETRY OPTIMIZATION It is important to be able to take a molecular structure in silico and subject it to energy minimization. This is the first step for force field and quantum mechanics calculations and for moleculardynamics simulations. Once a molecular structure finds a stable conformation, the physical and chemical properties can then be calculated. The goal of energy minimization (or geometry optimization) is to take a high-energy state, which is a function of the atomic coordinates, and to
reduce the energy by optimizing the geometry. In other words, minimizing the potential energy functions with respect to the coordinates reduces the steric and electrostatic
interactions. This is a type of calculus problem familiar to students who have ever had to locate the stationary points of a given equation. Recall that the extrema (maxima and minima) of a mathematical function fix). with one independent variable x. have first derivatives equal to zero.f'(x) = 0. The second derivativef"(x) will be positive if it is a minimum and negative if it is a maximum. Figure 28-15 shows
The physical significance of the V1 term is less intuitive. Setting the V2 and V,, terms to zero and assigning the term a positive value yields the curve shown in Figure 2814. The V1 term is used primarily as an additional way to increase the repulsive interactions between atoms that have
a 1.4 relationship that are not fully accounted for by the nonbonded terms. This situation is commonly found for elec-
tron-withdrawing groups X and Y. with torsion combinalions
'5
and
Radom, Hehre, and Pople were the first to give physical interpretations 01' these torsional terms!'2 For maximum flex-
ibility in developing force fields, it should be noted that the Vi, V2. and V3 terms may be either positive or negative. Although the discussion above focuses on carbon—carbon or carbon—hydrogen torsion angles. Equation 28-2() also ap-
plies to any other combination i-j-k-l. with any other elements from the periodic table used for i, j, k, and I. Given the number of drugs that have heterocycles, a Force field useful for drug design has to address many torsion angle combinations. Additional energy interaction terms may be added to im-
x
Figure 28—15 • An arbitraryr athematicalfunction with maxima and minima. The first derivative of a function is zero at a maximum or minimum, f'(x) = 0. The second derivative is positive (f(x) = + value) if the stationary point is a minimum, or is negative (f°(x) = —value) if the stationary point is a maximum.
930
Wits,,,, and Gis,'old'x Textbook of Organic Medicinal and Pharmaceutical Clienio:rv
an example of a function, fix), with two minima and two maxima.
Typically, a molecular structure is entered into a molecular modeling software package by template fragments or through a sketching mode. It is also possible to download
structures from structural databases. Structures built or downloaded do not have an optimum geometry based on the force field potential energy equations: i.e., they are not occupying the lowest energy state in vacuo. Minimization algorithms are written to take a starting structure and minimize the energy, which translates into the structure dropping into the nearest potential energy well on the conformational hyperenergy surface. The more complex the structure, usually the more minima are available in conformationa! space. Butane, a simple hydrocarbon, is an informative example. It has three minima available. Energy minimization requires a series of iterations because of the nonlinear nature of the force field potential energy functions. The general stratagem is to transform the full nonlinear optimization into a series of local iterative linearizations, and this approach works well. Atoms within a molecular structure are moved in small steps
in the direction that results in a decrease in the energy of the system. The size and direction of the steps are determined
by the specific method being used, based on Equation 283, and illustrated for a one.dimensional case in Figure 2816.
Geometry optimization may be divided into two broad categories: first-order methods and second-order methods. The former uses first derivatives to determine the step size
and direction, while the latter uses both first and second derivative.s. First-order methods include steepest descent
added to the coordinates at each step and is updated by a quick recalculation of the force field total energy. SD is the simplest approach."'° The step size in SD & is simply taken as a scaled negative gradient as shown in Equation 28-22. where V (del). a vector operator, is the gradient as defined by Equation 28-23 and A is a scaling constant.
The SD algorithm is inefficient when the potential energy curve is not very steep. So as the minimum is approached. where the slope of the curve is flatter. SD algorithms become
inefficient cotnpared with other methods. = —A(V,,E,,,,,)
V=
28.22)
(Eq. 28-23i
+
The CG method, outlined in Equation 28-24, is widel) It gives better convergence than SD algorithms. As the name implies, the previous step size along with current gradient as deterniincd by the total force field energy is used to determine the next step size. An additional sealing factor is found to improve results.
= r,, + e&,
(Eq. 28-24
The Ncwton-Raphson method uses information obtained by taking the first and second derivatives of the energy wiih respect to the coordinates.22 The combination of both first and second derivatives provides a powerful itiethod to locate minima. This may be a time-consuming process because of the matrix manipulations that must he undertaken for a 3N system, where N is the number of atoms. In Equalion 28-25. V2 is the dot product of V multiplied by itself. Note that V2 is a scalar operator.
(SD) and conjugate gradient (CG). The second-order method discussed below is known as the Newton-Raphson (NR) geometry optimization approach: there are many variations of this method. Again, the concepts of minimizing a function are not new; they were developed years ago. (The
+
d.
h= =V •V
=
+
(Eq. 28-25)
+
k) yt +
+
"Newton" in Newton-Raphson is Sir Isaac Newton.) The
= I—,
immediate goal of an energy minimization is finding a suitable displacement 4,, which, as stated above, is opposite to the potential energy gradient. In other words, the atoms are
None of the geometry optimization methods discussed finds the global minimum.
+
dr
+
(Eq. 28.2h)
moved in the direction of the forces. The displacement is
CONFORMATIONAL SEARCHING As indicated above, it is important to be able to eonformational space to determine what arrangements of atoms (conformations) are energetically feasible. Observed physical properties (e.g.. heats of formation) are statistical averages of all the conformations available. Most organic molecules have multiple energy minima. In the case of drug design, it may be important to sample the possible number
a
of conformations a drug molecule can adopt. Usually, a drug in the drug—receptor complex adopts a bioactime
C
ho,, that differs from any of the local minima or the global minimum. From the analysis of many lead and drug pairs. the average drug-like molecule has more degrees of freedom
(i.e.. is more flexible) than lead-like compounds. Initially. this may seem counterintuitive. since the mission of a drug DIstance
FIgure 28—16 • The direction of the step size in an energy minimization is toward the minimum value.
is to have its functional groups bind to complementary functional groups of the receptor. It turns out that a flexible drug is superior to one with a locked conlorntation because thc exact orientation in a eonformationally constrained molecule
Chapter 28 • Compiouzionul C6e,ni.crrv and
may not be optimal for interactions with the receptor. Moreover. the potential supenonty of a flexible drug can be understood when one considers that both receptor and small mole-
cute must mold themselves to form the drug—receptor complex. A flexible drug can contort itself more easily to reach the binding pocket and then adjust itself accordingly to form the necessary interactions. A rigid drug with its functional groups locked into place may he more limited in its ability to get to the target site and, once there, to position itself correctly. Of course, based on the Koshland induced fit hypothesis, it is known that both small molecules and macromolecules adjust themselves to form protein—ligand complexes.
Before conformational searching is discussed in any detail, it is critical to have a common vocabulary. The terms conformer and conformation can he dertned in reference to the butane potential energy curve (Fig. 28- I I). There are an
infinite number of conformations on the curve, since the distance between any tsvo points on any curve may be as small as desired, confonnat ions refer to both maxima and minima and all positions in between. A conformer, on the other hand, refers to the conformation at the bottom of the potential energy well, which is a minimum. Looking at the simple case of butane, it is easily seen that there are three
potential energy wells. Every molecular structure has a
to do the calculations is to look at all possible axial and equatorial confonnations. With cyclohexanol. this is not difficult and can be done by manually altering the orientation of the OH group. For more complex structures. conformational searching routines must be used.
molecular structure would be energy minimized to the ç'uuehe conformer. If we started on the other side of the 1200
barrier, where the dihedral angle was 150°. the molecular structure would bc energy minimized to the a,,:, conformer. Second. some conformations arc more important than others. Third. as noted above, many physical problems are a consequence of a statistical average of the conformers present. Fourth, having a conformational search algorithm is a check against having biased structural data. In the case of butane, if only anti butane were known, there would be a lot of information missing. The majority of druglike molecules are structurally more complex than butane, but this hydrocarbon is a str.iightforward example. The importance of knowing available conformations flr property predictions can be illustrated by looking at substituted cyclohexane. For cyclohexanol. the axial:equatorial ratio is derived by using Boltzmann statistics to calculate the ratio. Equation 28-27 shows the Bollzmann equation. where is
Jr
=
(Eq. 28-27)
Table 28-I shows the three possible axial and equatorial conformations. Substitution in Equation 28-27 generates the calculated ratio of each conformer. Because MMFF94 was parameterized to reproduce the quantum tnechanical calcula-
tions, it is illustrative to look at the ratio calculated with MMFF94. The Boltzmann-averaged distribution may then be compared with the experimental data as well as the other force field results. Equation 28-28 outlines the procedures for calculating the
denominator in Equation 28-27. Note that in Table 28-I. entries I and 2 (as well as 4 and 5) are equivalent in energy. with relative conformational energies of 0.000 kcal/mol and —0.323 kcallmol. respectively. Consequently. the frequency
factor is 2 for both cases. The sutnniation of Equation 2828 is shown in Equation 28-29 to be 4.057. —fl 1W
=
-I-
(J 11111
—
+
tential energy lunctions are being minimized, not the geometry. Geometry optimmuzalion is the equivalent term, for the structure is being optimized according to the force field equa-
Why is it necessary to explore conformational space? First, as discussed above, energy minimization algorithms are designed to seek the nearest minimum to the starting position. So lbr butane, if we had an initial input geometry dihedral angle was 900. the in which the
(Eq. 2(4.2(0
+
= 2(X) + 0.715 + 1.164) + 0.1(42 = 4.057
(Eq. 28-29
The ratio lhr each entry in Table 28-I can be calculated by using Equation 28-27. It is more interesting to look at the summation of the total calculated equatorial versus total axial cyclohexanol conformations, which, on a percentage basis. is calculated by Equations 28-30 and 28-3 I. MMFF94
results give approximately 67% equatorial and 33% axial. This is in close agreement with the Hariree-Fock (HF) 631 G(d) quantum mechanical calculations of 66% equatorial and 34% axial, which is in reasonable agreement with experimental data. The calculated percentages with MM3 are 82% equatorial and 18% axial, while the calculated percentages
with the Tripos force field yield 46% equatorial and 54% axial. In general, the Tripos force field is qualitative (at best) and does not give particularly good energy values, so one must be cautious when trying to make accurate predictions using force field methods. —
x lOO'4 =
=
[2.00+0.7(5]
1./RI
(100%)
(l(x)cf) = 66.9%
(Eq.
is the probability of finding one conformation. f
a frequency factor indicating the degeneracy of the energy.
E1 is the relative energy (kcal/mol). R is the gas constant (0.0199 kcal/mol-K), and T is the temperature (K). For room temperature calculations, the product RT is 0.59 kcal/mol. If one look a single conformation of axial cyclohexanol and compared it with a single conformation of equatorial eyclo-
931
hexanol. an erroneous answer would result. The right way
global minimum, the absolute lowest energy, but there are many minima. For butane, the global minimum corresponds to the anti conformer. One speaks of energy minimization. not energy optimization (discussed above), because the po-
tions.
sied !)ruj,' Desig,.
x tt)OM' =
1
( lO()%i
J 1160 + 0 IX' =
= 33.lci
(Eq.
932
Wilxo,,
and Gisrolds Textho(,k of Organic Medicinal and Pl,amzaceuiicai C7,ennclrv
Three Axial and Three Equatorial Conformations of Cyclohexanol With Their Relative Energies Calculated Using Force Field (MMFF94. MM3. Thpos) and Ab Initlo Quantum Mechanics TABLE 28—1
(6—31G(d,p) or 6_31G**) Conformer
MM3
Sybyl
MMFF
0.000
0.129
OA)0O
0.000
0.0(5)
0.129
0(8)0
0.000
3
0.942
0.063
0.199
0.2(8)
4
0.834
0.000
0.323
0.244
0.834
0.000
1)323
((.244
2.637
0.05)
1.011
1.632
81.5:18.5
46.2:53.8
67.0.33(1
66.2:33.8
0.88
—0.09
0.42
0.4))
No.
H
I
2
Ii'
6
Ratio
HF
The goal ofconformational searching is to lund all possible values of the dihedral anglc.s that could be assigned to each
According to Equation 2-30. the nutnbcr of conformations generated rises exponentially with the number of bonds to.
rotatable bond in a molecular structure. Conformational
tated. Torsion angle driving is GS. while the rest of the
searching may be divided into two general categories: ,cysh'malu' and nans stemauc searching. As the name implies. systematic searching uses methods that are guaranteed to lund all minima within the defined search parameters, while nonsystematic searching uses statistical approaches. Systematic searching includes grid searching, torsion driving, and constrained searching. Nonsystematic searching includes dynamics, stochastic (random), and distance geometry. Systematic searching has been described as an exhaustive but the success is a sampling of confonnational function of the number of increments used to explore each
structure (with the exception of the torsion being systeniatically rotated) is energy minimized. Many programs have this feature, and accurate conformational energies are obtained
rotatable dihedral angle. No conformation will be over-
the cartesian coordinates arc randomized with a "kick." Ii the randomization is not large enough, the structure will return to its starting points. Too large a perturbation generates unrealistic high-energy conformations. The geometry is energy minimized with a force field, and the newly generated structure is compared with the original
looked (unless the search parameters are not small enough).
A simple analogy should make this clear, Imagine walking along a paved highway blindfolded (not recommended) with the goal of discovering all possible potholes. The number of potholes that may be located is a function of the step size and the distance traveled. The longer the gait, the faster one
travels down the road, but with a reduced probability of finding all the potholes. Systematic searching generally can-
not handle solvents, and the method is only amenable to searching fewer than 1(3 dihedral angles, because of the ex-
ponential explosion of possible conformations that results (see Equation 2-30 in Chapter 2). Large amounts of computer time are expended because small dihedral angle increments are required for each rotatable bond. In grid searching (GS). each torsion angle is examined, but the structure is not subjected to geometry optimization.
with the minimization. Nonsystematic searching typically is more suitable for larger molecules, and solvenis may be included.707' In general, more lime is necessary to apply statistical analyses for the "completeness" of a search. Although stochastic searches are useful, there is an inherent incompleteness to them. Stochastic searching can use either internal or canesian coordinates. From a staning low.cnergy conforntation.
structure according to the Metropolis algorithm (MA).72 The current conformation is compared with the newly generated one. If the energy of the newly generated conformation is lower than the energy of the original conformation, the new one is accepted. If the new conformation has a higher energy. there is a statistical chance it may also he retained. In this second case, a Boltzmann factor is calculated (Equation 2827). which is then compared with a random number between
0 and I. If the Boltzmann factor is less than the randomly generated number, the conformation is accepted; otherwise. it is rejected.
Chapter 28 • Computational C'hemixtry and C'oniputer-Asxi.crcd !)rug I)estgn With all methods, there are strengths and limitations. Conformational searching is no exception. A comparison of the methods was carried out on cycloheptadecane in an effort to find out 'what confonnations are significantly populated at room temperature or within, say 3 kcal/mol of the global minimum?"73 The authors reached the following conclusions: (a) the effectiveness of the search appears to depend highly on the method used, and (b) except for distance geometry, all methods could locate the global minimum: none of the methods found all 262 low-energy conformations in a single search. Because of the importance ofconformational searching, newer algorithms have been developed since this benchmark study.
The Confort algorithm, developed in the laboratory of
933
MOLECULAR DYNAMICS SIMULATIONS The molecular configuration is a function of time. Molecular systems are not stationary; molecules vibrate, rotate, and tumble. Force field calculations and the properties predicted by them are based on a stationary model. What is needed is some way to predict what motions the atoms within a molecule will undergo at various temperatures. Molecular dynamics (MD) simulations use classical mechanics—force field methods—to study the atomic and molecular motions to predict macroscopic properties.75
MD simulations have the potential to reveal important insights into drug—receptor interactions, but some important assumptions should be reviewed:
Robert Pearlman, performs a systematic search over all possible combinations of "worthy dihedral angle ranges" rather
I. Molecular systems obey classical mechanics.
than searching over all possible combinations of dihedral
2. The forces acting on each atom an.- equal to the negative gradient
angles per Se. Very fast partial optimizations are carried out for each such combination of dihedral angle ranges. Each of the torsion ranges generated by Confort brackets a single kxal minimum and is followed by energy minimization. Although still of exponential order, the number of increments used per rotor is typically between 2 and 4. thereby making
of the potential energy. 3. The potential energy may be calculated from force lields. 4. The temperature is proportional to the velocity. 5. The time average is equal to the ensemble average, which is known as the ergodic hypothesis.
the Confort algorithm extremely fast and enabling its use for searching rings and ring systems in addition to acyclic
tion, it is necessary to use Newton's laws of motion. The
In applying classical mechanics to simulate molecular mo-
three laws are summarized below:
substructures.74
Methods have been devised that alter the potential energy
hyperspace. which have been useful in locating the global minimum. Second-derivative information, discussed above. indicates the curvature of the energy surface, which may be flattened or inflated, depending on whether the surface has a positive curvature (negative second derivative) or negative curvature (positive second derivative). respectively.75 Genetic algorithms (GAs) have become popular for many applications in science, including the determination of possible conformations.76 The widespread use of GAs may be attributed to their robust nature, simplicity, and computational efficiency. One approach to the stochastic sampling of the conformational energy hypersurface uses a GA with a fitness function that attempts to select dihedral angle values leading to low-energy conformers and, possibly, simultaneously attempts to select dihedral angle values corresponding
to "diverse" conformations. Although GA-based search results arc incomplete, the energies used to "score" various conformations are calculated in an appropriate fashion.74 Another stochastic approach involves the 'poling" algorithm.77 which locates minima and artificially increases the conformational energy hyperspace until there are no minima at that location. The name is derived from the analogy of literally placing a pole in the energy well and pulling up the surface around the pole. like raising a circus tent. All methods that involve reshaping the potential energy hypersurface suffer from alterations to the surface being explored. The artificial increase in the conformational energy hypersurface near each low-energy conformation ensures that nearby con-
formations will not be selected. Although this approach is much faster than GA-based approaches, poling algorithms are often less reliable. They fail to find low-energy conformations because the conformations selected are based on artificially perturbed values of the conformational energy.
I. Law of inertia: A body stays in motion or at rest unless acted on by outside forces. 2. Fundanienial definition of force: mass x acceleration.
1- =
= ma
(Eq. 28-32)
3. Law of action—reaction: For every action. then, is an equal and opposite reaction.
= P,_, =
(Eq. 28-33)
Using Equation 28-32 as the starting point, the mass ni may be eliminated and integrated with respect to Lime I according to Equation 28-34 to give Equation 28-35, where i is the velocity and C is the integration constant. It is a simple
matter to determine the integration constant. At the initial which means that time, = 0. Therefure. the = integration constant must equal the initial velocity (C = J7.r dt dr
-
-
Jadi
(Eq. 28-34) -
(Eq. 28-35)
Integration of Equation 28-35 provides the distance a par-
ticle has traveled from its initial position i at time ito its new position + at I + (Eq. 28-36). +
=
÷ vnt + r(t)
(Eq. 28-36)
Equations 28-35 and 28-36 are known as Newton's equations of motion. MD simulations apply these two equation.s to all the atoms in a molecular structure. According to the kinetic-molecular theorem, the kinetic energy is proportional to the temperature. This remarkable relationship is shown in Equation 28-37 without derivation, where N is the number of molecules, k is the Boltzmann constant, and T is the abso-
934
and Gissvld'c Tenibook of Organic Me'du-,,:a! and Pharniaccus,eni CIw,ni.sirv
Wi/no,,
lute temperature. Equation 28-37 connects classical physics to statistical mechanics. The basic idea behind MD simulations is (0 introduce heat into the system and adjust the velocities to maintain the temperature. The forces on the atoms
can be calculated with a force field. Once the tirces are known, based on Newton's celebrated second law (Eq. 2832). the accelerations can be calculated. Using the laws of motion (Eq. 28-35 and 28-36), the velocities and new posi(ions can be calculated. This procedure is repeated for the duration of the simulation.
widely used water solvent models are SPC5' and In the former, the oxygen atom has a charge of —0.82. and the hydrogens have a charge of 0.41. The H-U-H angle is 109.5°. and the 0-I-f bond length is 1.0 A. In the latter. oxygen atom has a charge of —0,834. and the hydrogens have a charge of 0.417. The H-O-H angle is 104.5°, and the 0-H bond length is 0.957 A. To avoid potential water— vacuum interface problems that
might arise in a MD simulation, periodic boundary cotidi(ions are comnnionly used.55 Basically, a protein is surrounded by a rectangular box of water with a defined number
=
NAT
(Eq. 28-37t
The fundamental steps in a MD simulation may he summarised: I. Energy nhinimisation 2.
Elcating
3. Equilibration 4. Production runs 5. Analysis
of water structures. This water box is then surrounded on each face by another waler box. When the MD simulation is being carried out. water near the edges of the central containing the protein may leave and be replaced with a water coming from the water box on the opposite side. This procedure ensures that the waters inside the central water box remain constant. The long-range forces found in the nonbonded terms of Equation 28-19 present some unique difficulties fora MD simulation. Calculating these energy terms is CPU intensixe.
It is informative to review sonic of the information regard. ing MD time steps. It has been (earned that the best time step should be 1/10th of the largest frequency in the system. The largest frequency is associated with bond vibrations. The largest frequency = scc I) involves C-H bonds. Because the largest frequency is inversely proportional to the period of oscillation, the time step At is usually sec or I fs. Longer simulation times may be achieved
by a factor of 2 or 3 if the C-H bond vibrations are constrained. The SHAKE algorithm was developed whereby constraints are placed on the vibrations of C-H bonds.79 When calculating protein structures, one must have a good solvation model. Because water plays a critical role in en-
zynie reactions and stabilizing proteins, it is important to have effective ways to model water. In the structure-based design of human immunodeliciency virus (HIV) inhibitors, fir example. the presence of a single water molecule in the binding cavity was effectively exploited in structure-based drug design. There are essentially two ways to include solvent in MD siniulations: (a) continuum solvent models and (b) explicit solvation models. In principle, the latter should give more accurate protein simulations, but it depends highly on the water model used. The simplest continuum solvent model simply adjusts the
One early solution was to impose 8- to 10-A cutoffs. Although this saved dramatically ott the simulation times. unrealistic protein structures resulted after long runs. There were several potential workarounds. including longer cutoffs and updating these interactions beyond the standard cutoff less A very attractive approach to circumnvcn( this problem altogether, proposed by Darden. York. and Pedersen. used thc particle mesh Ewald tPME) method.55 Free energy perturbation (FEP) calculations56- '° allow direct AAG comparisons between a drug D that binds to a protein P to form the drug—protein complex D-P and a structural analogue D' and the same protein P 1 see Fig. 28-17, which depicts the free energy perturbation cycle). Determining the free energies of binding, AG1 and AG2, expemimentally can be difficult and time consuming. Converting I) into D'. and D-P into D'-P. AG.1, is experimentally fictitious. Such conversions would amount to alchemy. The conversions can, however, be curried out in silico.
D+P
tional to hr2 rather than hr. When this was first proposed. the idea was to help reduce CPU time. The rationalization is that the charges on two nonbonded atoms in a macromolecole are separated by the protein, which should reduce the
interaction erms. Thus, the interaction energy should fall off laster than hr because the charges are masked. "I
q,q, II
=
D'P
AG4
dielectric constant to equal the medium dielectric. An approximation widely used in MD simulations is known as the distance-dependent dielectric constant. In this approach, the dielectric constant is set equal to the distance as shown in Equation 28-38. The electrostatic energy is now propor-
AG,
j D'+P
D-P
FIgure 28—17 • Free energy perturbation (FEP) calculations take advantage of a thermodynamic cycle. Here, the top reaction shows a drug 0 combining with a protein P to form the The drug—protein complex 0-P with a tree energy change
bottom reaction shows another drug D' combining with an identical protein P to form a second drug—protein complex D'P with a free energy change AG2. Both of these physically observable reactions have a free energy change, AG, associated
with them. The free energy difference between the two
—
-
The other approach is to treat the solvent explicitly. There are a number of water models available.50 Two of the most
drug—protein reactions is AAG = AG2 — AG, According to the first law of thermodynamics (conservation of energy awl, the fictitious conversions, AG5 and AG.5. must be related to the experimental AAG.
Chapter 28 • C,impaua:ianal C/,e,nisfrv and Cosi,puwr-,ls.si,'aed
935
In a thermodynamic cycle. Equation 28-39 must hold. as the energy differences depend only on the initial and final
QUANTUM MECHANICS
from both sides of Equation states. Subtracting AG.; and 28-39 provides the rearranged Equation 28-40. Recognizing
One of the great theoretical accomplishments of the 20th century was the development of quantum
Equation 28-40 can be simplified = — to give Equation 28-41. This remarkable relationship, taking advantage of the thermodynamic cycle, indicates that the free energy differences based on in silico alchemy must be The method has been equivalent to the experimental used to calculate and compare the binding energies for many diflerent drug—protein complexes. Although the approach is intellectually stimulating. it requires significant computer resources. +
=
+
(Eq. 28-39
—
=
—
tEq. 28-40)
=
—
(Eq. 28-4h
Another application of interest to medicinal chemists involves the thermodynamic perturbation cycle applied to relative property calculations. For example. directly calculating the solvation of a small drug requires extensive simulation times. The drug has to transfer from in vacuo into an aqueous environment. This transfer from the gas phase to the aqueous
phase is CPU intensive, given that the solvent has to be reorganized to accommodate the solute. Calculating a second
drug analogue will involve a similar process. Making use of a thermodynamic cycle, however. can expedite the process the drug—protein (analogous to the above discussion binding). There are two types ot motion (harmonic and stochastic) that may be studied by MD simulations. Harmonic .sisnnlairons refer to oscillations near equilibrium (i.e.. near the minimum ola potential energy well). Stoelsaslic refers to simula-
tions that lead from one local minimum to another local minimum. From a harmonic oscillator, the frequencies may be calculated according to Equation 28-42. where k is the stretching constant and mu is the mass. Extending the concept from a
single mass held to a surface by a spring to N particles requires an extension of the Taylor series expansion (Eq. of partial second derivatives. 28-13) to a matrix Each mode has associated its own force constant, frequency. and 3N relative displacements. The normal modes are assigned to the experimental IR or Raman spectrum.
mechanics may be
considered weird from the standpoint of our practical everyday experiences in the macroscopic world. Nevertheless, the
applications of quantum mechanics to chemical bonding have changed the way chemists think about molecular structures and have made chemistry a subdiscipline of physics. Many unexplained chemical effects may be understood in
the context of molecular orbital (MO) calculations. For example. the anomeric effect seen in carbohydrate chemistry can be rationalized as a combination of MO interactions and
electrostatic effects. Although chemists like to follow the example of C. N. Lewis and write simplified molecular really nuclei structures (Lewis structures). embedded in a sea of electrons. It is remarkable that so many organic structures can be represented. as a tirsl approximation, by localized chemical bonds and lone pairs of electrons. As any student going through a course in organic chemistry
can attest, chemical reactivity and physical properties may be explained, in many situations, by extending our simplified bonding concepts to include resonance and electron delocalization. Because most drugs (or organic molecules) and their interactions with macromolecules are responsible for the observed biological effects called "drug action." ii is quite reasonable to usc theoretical MO methods to understand
electron distributions and predict physical properties of drug-like structures. The only way this can he achieved is through the use of quantum chemistry, since force field methods do not explicitly treat electrons. is The history of quantum mechanics, while is a full development of the theory. The goal of this chapter is to present the concepts succinctly for readers who have nevertaken courses in physical chemistry, where these topics arc more fully developed. The emphasis is on lollowing the logical order of concepts. not on the mathematical details, which means some relationships have been simplified. With the fundamentals presented below, it is possible to understand the impact quantum mechanics has, and will continue to have, on medicinal chemisto quantum try. The sections that follow contain mechanical applications in CADD.
We start with the contributions of Max Planek. At the
conlormational barriers. After the simulations are com-
beginning of the 20th century, physics was in a theoretical crisis. It was believed that Newton's equations and Maxwell's electromagnetic theory could explain all natural phenomena. but the application of thesec lassical mechanics methods to the emission of electromagnetic radiation from perfect "black bodies" did not correlate with expenmeni. In the theoretical treatments, the radiation was assumed to result Irons the microscopic oscillators, and the inescapable conclusion of classical mechanics was that a continuous
pleted, the trajectory can be reviewed. The temperature of the system can be cooled down to sample potential new conformations. MD simulations are suitable for larger molecules, and solvent may be included. No statistical or geo-
range of energies was available to the oscillator. Planck suggested in I 90() that (he energy associated with oscillators was a function of integral values of quanta (Eq. 28-43). where E is the energy of the oscillator, I, is Planek's constant (6.626
metrical means are used to determine their completeness. In general. MD simulations are not as efficient as stochastic or distance geometry methods.
worked, but many scientists of that period thought this solution was simply a mathematical trick, because the logical
I.' =
(Eq. 28-42)
MD simulations have been applied to generate new con85 The basic idea is to add enough thermal energy (through high temperatures) and carry out the simulations long enough for the molecular systems to overcome
x
10-
i-see). and u is the frequency. The suggestion
936
Wil.ron and
Texi book of Organic Medicinal and Pliannacesajeal Chenii.ur,-
extension meant that energy was available only in discrete quantum values and was not continuous.
E = liv
(Eq. 2843)
The quantum idea was used by Einstein to explain the photoelectric effect. When metal surfaces are subjected in vacuo to electromagnetic radiation of specific frequencies. electrons are released. The phenomenon could not be explained by classical mechanics. Einstein used the quantum concept to suggest that electromagnetic radiation was simply a stream of photons where Equation 28-43 correctly defined the energy. Using this quantum idea. Einstein formulated a
relationship (Eq. 28-44) between the incident electromag. netic radiation and the expelled electrons. Einstein's work supported the Planck quantum theory. In this equation. 4' is called the work function, which is the minimum energy necessary to eject electrons from the metal surface. Some simple deduction, knowing that the kinetic energy ,nt212 cannot be zero, requires that 1 = !zI'o; therefore, v0 is the minimum frequency allowed. (Interestingly, Einstein won the Nobel Prize for his contributions to understanding the photoelectric effect and related matters rather than for his theory of relativity.) liv —
(Eq. 28-44)
Because light has particle-like characteristics, de Broglie
argued that electrons should therefore exhibit wave-like characteristics. This is odd because it defies our macroscopic experiences. The wave-like character exists for all objects,
but only manifests itself—for all practical purposes—with microscopic particles (e.g.. electrons). The de Broglie relationship (Eq. 28-45) quantifies the wave-like properties that matter exhibits, where A is the wavelength. Ii is Planck's
is the momentum (mass x velocity). constant, and Clearly, the relationship shows that for tiny masses, the wave property of matter is significant, whereas for large objects, the wave-like character is vanishingly small. A = -f-'-.
(Eq. 28-45)
and lower-case chi, x. is used for a spin orbital, which is defined below.) / a2
+
a2
a2
+
+—
= 0 (Eq. 25-46)
Equation 28-46 can be arranged to give Equation 28-47. +
+ 4)
=
4-
(Eq. 28-47)
A more compact (and perhaps less offensive) form of the SchrOdinger equation is given by Equation 28-48. where —
V2 + U(x.y.z).
is understood to be a func-
(ion of the x.y.z coordinates, and V2 was defined earlier in Equation 28-26. (It is not necessary to demand that be a function of cartesian coordinates. The choice of (he coordinate system may be dictated by the nature of the problem being solved. In other words, it may be easier to solve a
problem within a different reference frame.
may be a
function of spherical polar, plane polar, or cylindrical coordinates or of other coordinate systems. For example. solution
to the hydrogen atom involves the use of spherical polar coordinates.) H is the hamilionian operator, which is the quantum mechanical equivalent of the classical mechanics formulation H = T -- U. where T is the kinetic energy and U is the potential energy. Note the similarities between the classical and quantum mechanical formulations ol the humiltonian. =
(Eq. 28—(8.i
Equation 28-48 is the Schrodinger equation for a single particle. For the application of quantum mechanics to medicinal chemistry, it is necessary to think in terms of electrons moving around nuclei. The Schrodinger equation can be con-
verted into a multiatom problem, given by Equation 28-19. In Equation 28-49. the hamiltonian is
11=
+
At this point in 20th century science, it was becoming accepted that matter had both wave-like and particle-like
The first term (kinetic energy) is a summation over all the
characteristics. Depending on the experimental setup, these
uses Coulomb's law to calculate the interaction between every pair of particles in the molecule, where e and are
seensingly contradictory properties could be observed. If matter has wave-like properties, then there had to exist some
generally descriptive wave equation. It was Erwin Schrodinger who recognized that standing waves with imposed boundary conditions yielded sets of integers, which would be consistent with spectroscopic He developed the now fumous Schradinger wave equation (Eq. 28-46) for a single particle such as an electron in a 3D box. This equation is a linear (meaning the wave function is raised to a power greater than I ). second-order differential equation
(meaning second derivatives are involved), where E is the total energy of the system. U is the potential energy, and is the electronic wave function. (Different symbols are used routinely in the literature when discussing wave functions. In this chapter, lower-case psi. cu, and uppercase psi. 'V. are used to denote the wave functions for a single particle or a multiparticle system, respectively. Also. lower.case phi, is used to represent atomic orbitals.
particles in the molecule. The second term (potential energy)
the charges on particles i and j. For electrons, the charge is
—e, while the charge for a nucleus is Zr, where Z is the atomic number. The summation notation i<j for the indices means that one does not doublecount pairwise interaction
terms in the summation (e.g.,
and should only
appear in the potential energy term once). The denominator in the second term is the distance between particles i and is understood to be the electronic wave function for a many-atom system.
= E'l',1
(Eq. 28.49i
Originally, 'P was thought to be a physical wave that was propagated through space. Today. 'P2 is more properly inter. preted, from the work of Born who first made the proposal. as being proportional to the probability of finding a panick
in a small volume element di', where di' =
In the
more general case, the probability is represented by 'l''P.
Chapter 28 •
Chemistry and
Drug Design
937
where 'I' is multiplied by (he complex conjugate The complex Conjugate necessary, in that the wave function may contain imaginary numbers. Because represents the probability for a small volume element, it is possible 10 define such that the integral over all space is unity = I) through the appropriate choice of coeffi-
Another consequence of the Pauli exclusion principle is that the spin orbital is a product of a spatial function a spin function a or (Equation 28-52).
cients. The integral symbol is more complicated than it may appear. It really spans all of coordinate space. so for cartesian coordinates, the range goes from negative to positive infinity for each of the three .r, y• and coordinates. This means that Equation 28-49 is deceptive, since it involves solving a triple
The antisymmetric wave function, shown in Equation 28SI. is a more compact way of writing a Slater determinant
To be an "accept-
integral.
able"
there are certain conditions that are required:
I. The wave equation must be well-behaved mathematically. 2. The equation must he a function, which becomes zero at infinity. 3. The wave equation must he single valued.
Equation 28-49 is a complex equation, even in the simplest of examples. It must be applied to every electron in a
molecule or molecular system under examination. The Schrtidinger equation has exact solutions for only a few simple cases (e.g., a particle in a box, a particle on a ring, the harmonic oscillator, the rigid rotor, the hydrogen atom, the hydrogen molecule ion). Each example listed builds on the
previous ones but gets more mathematically challenging. Solution of the nonrelativistic Schrodinger equation for the hydrogen atom yields the set of quantum numbers n. I. and pa familiar from general chemistry. The spin quantum numbers is not one thai comes from the Sehrodinger treatment. It may be added in an ad hoc fashion, using the magnetic quantum
number ,n as a guide. The fourth quantum number does. however, naturally arise from an alternative mathematical development of quantum mechanics that is formulated by using matrix methods.'2 Later it was shown that both the wave and matrix approaches are equivalent.'0
Exact solu-
tions to drug-like molecules, whether by wave or matrix quantum mechanics. are impossible. Nevertheless, some
simplifying assumptions have been developed over the years. beginning in the 1950s when computers made approx-
imate solutions feasible, that result in good electron-based models.
Equation 28-49 may be arranged to give Equation 28-50 by multiplying both sides of the equation by and then solving for the energy of the system E. If the function is normalized, meaning it has been scaled such that
= I. the denominator is unity. =
(Eq. 28-52)
—
(Eq. 28-53). In a Slater determinant, an exchange of any two
rows or columns results in the same wave function multiplied by — I. This is another statement of the Pauli exclusion principle. The columns in Equation 28-53 are the single elec-
tron wave funetions. Equation 28-53. however, is only an approximation, since the electrons are independent of one another and therefore not correlated. This correlation problent reveals itself when calculating the energies and is discussed below. =
.....!...._
X2(l) ..' xt(2) X2(2)
(Eq. 28-53)
\/N! X2(N)
Xs(N)
Usually, the spatial function 0 is constructed from the summation of one-electron spatial orbitals (atomic orbitals) known as the basis set, used to construct a MO. This approach is known as the LCAO method (/inear combination of atomic orbitals).'7 tS It is an approximation of the accurate many-electron wave function (Eq. 28-54). The atomic
orbital contributions are weighted by coefficients c,. The summation is truncated, so the function is not complete. which has consequences when solving for E. =
(Eq.
The energy of the system E is a function of The variational theorem95 is an important starting point in computational quantum chemistry. It states that the calculated energy of the system is always going to be greater than or equal to the true experimental energy (Eq. 28-55). For the calculated and true energies to be equal, the 'P function must be exact. Exact wave functions are not possible for molecular structures, since there are an infinite series of atomic orbitals. described above. Therefore, every selection for 'I' will generate a trial wave function. and the variational theorem demands that the energy will always be higher than the true energy. As 'P,,.,,, approaches the true wave function 'V. the energy becomes lower and lower, approaching the true experimental energy E.
(Eq. 28-50)
J'Pd'l',,dT
(Eq. 28-55
For a many-electron system, the Hartree-Fock wave function defined as the product of spin orbitals as outlined in Equation 28-5 I. where A(n) is an antisymmetrizer for the electrons, provides good This is the start-
ing point for either semiempirical or ab initio theory. ft is necessary to have A(n) to make the wave function antisym-
metric, thus obeying the Pauli exclusion principle, which asserts that two electrons cannot be in the same quantum state.
'4',,, = 4(")x,( I
-
x0(n)
(Eq. 28—51)
Solutions to the Schrodinger equation for drug-like mole-
cules may be obtained by making approximations and simplifying assumptions. There arc three basic divisions of computational quantum chemistry calculations: (a) semiempirical, (b) ab initio, and (c) density functional theory (DFT). Early efforts by Pople and coworkers produced a method called CNDO (complete neglect of differential overIn general, the semiempirical approach eliminates integrals that are too complicated to solve analytically. (This
938
ViIso,,
iuuI Gj.u'o!d.l Textbook of Organic Medicinal nsa! Pharnuzeenthal
is an interesting concept in higher math: if the term is too hard to solve, discard it.) In their place, appropriate factors (constants and equations) are introduced to compensate. For-
tunately. the integrals eliminated in the semiempirical approach give relatively small values if solved. Equation 2856 shows the most complicated type of integrals that are removed in the serniempirical model. They are known as two-electron integrals, where I and 2 represent the two electrons, spanning four atomic centers i. j, k. I. Eq. 28-56)
The extensive efforts of Dewar resulted in a series of rea-
sonably accurate semiempiricul models: MINDO (moditied intermediate neglect of dilferential overlap))03 04 The gave much imthird version in this series. proved results. The program allowed organic and medicinal chemists to apply electron-based calculations to diverse
compounds. The availability of MINDO/3 helped make computational chemistry accessible to experimentalists. The MNDO (moderate neglect of differential overlap) model.'
alter some modifications of the observed sys-
mimic ST functions reasonably well. Using three CT l'unctions for every ST function gave rise to a popular basis set known as STO-3G. and many calculations were carried out by using this minimal basis set. STO-3G means three GT 115 functions are used to reproduce every ST function. Pople and coworkers discovered that more extensive combinations of GT functions improved the accuracy of ab imiitio calculations. The next advance was an attempt to give greater
flexibility to the ab initio models. By splitting the CT functions used to describe the valence shell electrons into two separate functions, better agreement between experiment and calculations was achieved. This method is known a split-level basis sets and is more time consuming. For exam-
ple. 3-210 basis sets quickly replaced The symbolism 3-2 IC means that the inner core or nonsalenee elecu'ons are described by three CT functions, and the outer shell or valence electrons are broken into an inner and Outer
set. The inner set is simulated by two CT functions, while the otiter set is simulated by one CT function. Equation 2857 shows the expansion of the atomic orhitals into CT functions g,, where a1 represent fixed coefficients.
tematic errors, served as the basis for the AM I (Austin
=
method I ).' °' Stewart took the AM I model and introduced automated parameterization techniques with a different parametenzation philosophy than Dewar. The result was PM3 (parameter method 3)17
Additional basis sets may be added to the split-level for. mulations to achieve greater accuracy. These additional
There arc known strengths and weaknesses of each semi-
empirical method. The AM I and PM3 models only include s- and p-functions, which limits their usefulness for most elements of the periodic table that require d-orbitals. Many of these elements, however, are not typically found in most drug-like molecules. More recent advances with MNDO/d include the incorporation of d-functions in the NDDO (sieglect of diatomic differential overlap) model.' 5. 119 Extensive use has been made of semiempirical methods Calculations of the highest occupied in drug design.'20 MO and lowest unoccupied MO ifiOMO/LUMO) energies for a series of active and inactive compounds have been used
as descriptors for QSAR. AM I. for example, has recently been used to develop a predictive ADME model for F-450 oxidation of drugs. which is discussed below. The more mathematically rigorous ab initio calculations.
as the name implies ("from first principles"), do not use parameters in the same way as the semiempirical models.'15
Unlike semiempiricul calculations, no classes of integrals are eliminated. Ab initio methods have, however, a number of approximations. Aside from truncated basis sets, in which the constants have been adjusted to give the optimal basis set, one of the most noteworthy approximations involves Solving the the functional form of the wave function Schri)dinger equation for the hydrogen atom produces the familiar hydrogenic orbitals. which can be approximated by Slater type (SI) functions. Both are exponential functions where is the exponential coeffihaving the form cient and a is a function of the electronic and nuclear positions Solving integrals, which are products of exponential functions, can be CPU intensive. Boys proposed that gauscould be substisian type (CT) functions of the form tuted for ST functions.122 The theoretical community did not immediately embrace the idea until Pople demonstrated convincingly that a linear combination of GT functions could
a9g,
(Eq. 28-571
functions are known as polarization functions. For example. 6-310(d). formerly represented as 6.310*, has a set of dif-
fuse d-orbitals added to the heavy atoms (nonhydrogen In another example, 6-3lC(d.p). formerly known as 6_3lG**. has a set of d-orbitals added to each heavy atom. as in 6-310(d). in addition to a set of three porbitals for all the hydrogens)33 The next level of approximation divides the valence electrons into three sets of GI functions (e.g.. 6-31 An additional set of very diffuse functions can he added to the model to help calculate nega-
tively charged species or hydrogen bonding, denoted by the
plus sign
in the examples 6-31 +G(d.p
and 6-
311 ±G(d.pI.''5 An iterative solution of the Hartree-Fock-Roothaan equa-
tions is required for semiempirical and ab initio quantum 39 The approach is also called the chemical self-consistent field (SCF) approach. In SCF calculatloas, each single electron's position in space is optimized in the
electric field of all the other electrons. This procedure is repeated until all the electron positions have been optimized,
and there is no further significant drop in energy through the adjustment of the electron positions. lIariree-Fock (HFI calculations give good results. The better the basis set, the lower the energy according to the variational theorem. There cotnes a point of diminishing returns, however, in that the energy approaches a limit known as the )-Iartree-Fock linmit. No further adjustment in the basis set breaks this barrier. The Hartree-Fock limit occurs because the electron motion is correlated, and this is not accounted for in a single Slater determinant. That is. the adjustment of one electron affects the position of the other electrons, and this is not fully taken into account by the SCF approach. To circumvent not taking
electron correlation into account. post-SCF calculations must be used in the form of configuration interaction or perturbation methods.
Chapter iS • computathnu,I C'hemi.clry and Computer-A s.cisted Drug Design Twsao commonly used post-SCF (or post-Hartrce-Fock) methods arc used: configuration interaction and perturbation "° In the fi.rmer. the wave function is a summation of other Hartrec-Fock determinants. This is analogous to exciting electrons to higher energy orbitals. In the latter. as the name implies. a perturbation is added to the HnrtreeFock hamiltonian. Møller-Plesset perturbation theory (MP2. MP3) is not a variational approach, so it is possible to calculate an energy lower than the true value. methods.
There are no quantum mechanical charge operators. Thus. charges are determined on the basis of a population analysis.
The electron density p(r) is calculated and divided between the atoms of a molecule. The difficulty, however, is in determining how to assign shared electron density between two atoms i and j of different electronegativity. In the Mulliken population analysis.'45 the shared electron density is divided evenly between atoms i and j, regardless of the electronega-
tivity. This, of course, is unrealistic but remains a useful technique. Other electron population analyses have become
increasingly popular over the years. As discussed above. fitting charges to electron densities is a way to get more realistic atomic charges. With the electron density distributed throughout a molecular structure, according to an ad hoc population analysis. it is possible to generate dipole moments and color-code the electrostatic potential on molecular surfaces. The electrostatic potential may be useful in developing a pharmacophore and deciding what properties a receptor must have. Density functional theory (DFT) has been used increas-
ingly over the years.'"'
has a radically different ap-
proach than that in the preceding discussion on scmiempirical
and ab initio methods and may he more easily
understood. In the DEl' formulation. Kohn-Sham equations. electron correlation is built-in.147 This means that DFT rivals post-SCF calculations for accuracy. Rather than dealing with the multielectron wave function, the electron density is used directly. According to the Kohn-Hohcnberg theorem.'48 the energy is minimized when the calculated and true electron densities are equal. DFT calculations rival the accuracy of standard post-Hariree-Fock methods. Of practical importance to medicinal chemists interested in studying drug-like molecular structures with quantum-based energy calculations, there is a significant reduction in the CPU time. Thus, DEl' is an attractive alternative. For example. the CPU time required to complete Hartrce-Fock calculations, which of course is a function of the number of electrons in the system being examined, is proportional to the number of electrons raised to the fourth power. DFT calculations, however, also
a function of the number of electrons in the system. are proportional to the number of electrons raised to the third
939
spectroscopy techniques. structure-based drug design methods may be appropriate. (The term drug design refers to the fact that experimental structural data of the macromolecuic of the drug—receptor complex is involved in
the modeling process explicitly.) The x-ray structures of a receptor and ligand-receptor complex provide information about the binding mode of the ligand. If available, multiple x-ray structures with different ligands provide greater insight into the steric and electrostatic tolerances of the binding cav-
ity. Using sophisticated molecular modeling software, the ligand is modified structurally in silico to achieve a better fit between the complementary binding sites and molecular volumes. The small molecule typically is clipped out of the ligand—receptor complex altogether, and new molecular structures are docked into the binding site. It is no coincidence that structure-based modeling began to be applied more frequently in the mid- 1980s. The exponential explosion of protein 3D structural information'49 (Fig. 28-18), which was made possible by cloning techniques, made it possible to have macromolecular structural data. The advances in a seemingly unrelated field have helped to usher in the age of structure-based drug design by making proteins
available in larger quantities for x-ray studies. II
scant structural information about the receptor
is
known, which is most commonly the situation confronting medicinal chemists, a more indirect approach is required. This second approach has been characterized as pharmacophore mapping or pharmacophore perception.'5° The critical functional groups and their 3D spatial orientations may be perceived by examining all molecular structures that induce biological activities. A comparison of active versus inactive compounds helps to understand the structural and conformational requirements of a drug candidate. Once a model has been developed, a 3D search query can be submitted to 3D databases. The goal of using a 3D database is to find existing structures that meet the constraints of the query for immedi-
ate biological evaluations, thus avoiding synthesis. If the retrieved compounds show activity, they can serve as lead structures for further structural refinements.
20000
— Oeposited Siructuret Total
Structures
l5000
JI0000
power. 6000
...JJJJJJJ
STRUCIURE-BASED DRUG DESIGN AND PHARMACOPHORE PERCEPTION Year
The choice of CADD methods that may be applied to drug design depends highly on the availability of the receptor information. If the receptor structure has been characterized
by either high-resolution x-ray crystallography or NMR
Figure 28—18 • The growth of a 3D structural protein database is exponential, The availability of structural information in the Protein Data Bank has helped fuel the growth and success of structure-based drug design. Values are as of May 9, 2003.
940
and Gisvald'.c Terthook
of Organic Medicinal and Pharmaceutical O:e'n,ixir (CO2H
0
CHO
CO2H
P03H2
H203P
OHC
—
2
(CO2H
OH
SO3H
SO3H
OH
Figure 28—19 • Three compounds designed
by
Beddell and coworkers to mimic the binding of 2.3-diphosphoglycerate (DPG) (2) to hemoglobin.
4
The concepts of structure-based or receptor-based drug design predate the use of computers. l3eddcll and coworkers are credited in 1976 with successfully predicting compounds that hind to human Although not strictly a drug—receptor interaction, the approach demonstrated the
although not by direct structure-based modeling, since to date the structural determination of ACE has not been accomplished. Ondetti and Cushman conceived of captopril based on a related enzyme. carboxypeptidase A. and the earlier reports of succinic acid inhibitors by
feasibility of molecular modeling applied to drug design. The goal of their study was to exploit the known binding site of the human deoxyhemoglohin tetramer. The tetramer consists of four single polypeptidc chains: two a and two f3 subunits. The small molecule 2,3-diphosphoglycerate (DPG). 2. binds with subsequent stabilization of the deoxy conformation (Fig. 28-19). The binding results in the liberation of oxygen, which is readily measured. Wire molecular models of the protein were used to measure bond distances and interacting atoms between the protein and the proposed small molecules. Based on the best fit, predictions were made as to which compounds would bind the best. These
Other computer-based methods have been used successfully in ACE inhibitor design. Force field calculations, conformstional searching, and analogue design strategies WCft used by Merck scientists to develop inhibitors. Scientists at Merck have used small molecule structural data I x-ray crystallo. graphic data and NMR solution studies) coupled with conforniational methods to design conformationally restricted ACE
early modeling studies suggested that compounds 3—5 would
1982. Again, although this was not a true drug—receptor interaction, the work demonstrated fundamental principles that would he applied later. The x-ray crystallographic coordinates of the L-thyroxine--prealbumin compound have three pairs of symmetry-related cavities (Fig. 28-21). It was no. ticed that one binding pocket of the symmetric prealbuinin
have an affinity for the 2.3-binding site of human hemoglobin. Based on the liberation of oxygen. it was determined that the binding affinities corresponded to 2 5 > 4 > 3. The often-cited design of the first angiotensin-converting enzyme (ACE) inhibitors by Ondetti and Cushman during this time effectively demonstrated the concert of mecha153 nism-based and structure-based drug The Ondetti and Cushman approach resulted in the first marketed ACE inhibitor. captopril. 6 (Fig. 28-20). Many other ACE inhibitors have been designed using computer-based models.
Since the l980s. there have been many success stoties using structure-based drug design. One of the first ing examples of the combined use of an x-ray crystal
tore and molecular modeling software was reported in
OH
CH3
0
CO2H
6
7
Figure 28—20 • Captopril was the successful outcome of a
FIgure 28—2 1 • The molecule i-thyroxine, 7, binds to prealbu-
rational design approach in which the mechanism of the conversion of angiotensin Ito angiotensin II was known. The angiotensin-converting enzyme (ACE) was assumed to have binding cavities similar to the known x-ray structure of carboxypeptidase.
mm, a protein found in blood. Based on x-ray data of the thyroxmne—prealbumin complex, the binding affinity of novel analogues was predicted by using a molecular modeling approach.
Chapter 28 •
Che,nj.vrr,' cizid Con,p,,rer-Axsi.c,rd I)rsig
dimcr was unoccupied by 1.-thyroxine, 7 (Fig. 28-22). This unoccupied binding pocket had the potential to accommodate a portion of new compound. which presumably would result in greater binding affinity by increasing the contact
OH
OH
between the van der Waals surfaces in this hormone—protein complex.
941
I
The scientists used a guiding hypothesis that the "tightCO2.
ness of fit" between the computer-generated complementary molecular surfaces of the ligatid and prealbumin would correlate to enhanccd binding affinities. They modeled the mo-
8
lecular surface interactions with the MS program on an
t
...'.'
CO2.
9
Evans and Sutherland PS2 graphics station. With available
modeling software, the UCSF (University of California at San Francisco) scientists stripped L-thyroxine from the binding site and docked various naphthalene-hascd structures with different substitution patterns, shown in Figure 28-23.
The modeling studies were carried out without the aid of force held relinement. Ahier modeling a diverse set of analogues, the scientists concluded that at least three of the four outer binding pockets needed to he filled, Four thyroid
OH
O9II CO2.
hormone analogues (structures 8—11) were ranked based on visual inspection using their complementarity of fit hypothe-
sis. Structure K did not present any bad contacts, while II had some obviously bad surface contacts. Structures 9 and
10
11
10 appeared to have equally good molecular surface interac-
Figure 28—23 • Using molecular modeling methods, four
tions, Once the compounds were ranked (8 > 9 10 > II), their binding affinities were determined. The binding
finity (8 > 9
data were consistent with the predictions, except structures 9 and 10 were not equivalent. Closer inspection revealed that the phenolic hydroxyl group of 10 had a better surface fit and is in close proximity to Scr-l I 7C and Thr- I l9C. thus providing additional binding interactions not available to 9.
i-thyroxine analogues were predicted to have good binding af-
10 > 11) to prealbumin.
Presumably, the additional interactions would have been de-
tected with force field calculations. In 1985, scientists at Burroughs Weilcome (United States) and the Wellcome Research Laboratories (England) reported
some of their CADD efforts for the prediction of dihydrofolate reducta.se (DHFR) inhibitors.'" DHFR is an excellent target, since this eni.yme pathway is the only knossn de novo
synthetic route to prep-are thymine in vivo. Thymine. of course, is one of the four nucleic acids of DNA. For many years DHFR had been a popular drug target l'or medicinal chemists. Significant drug design activity using the prevailing principles of medicinal chemistry was associated with the development of DHFR inhibitors for antibacterial and antitumor agents. Methotrexate (MTX). 12. and trimcthoprim (TMP). 13. are good inhibitors of DHFR. Figure 2824 shows the obvious structural similarities of MTX and lolic acid. 14. Over the years. literally thousands of inhibitors oh' DHFR were prepared on the basis of medicinal chemistry intuition
Figure 28—22 • The experimental x-ray crystal structure of prealbumin with bound c-thyroxine. Prealbumin is a tetramer with four identical subunits. A. B, C. and D. The four identical subunits form a channel with two bound i-thyroxine molecules. The binding sites have a C2 axis of symmetry.
preceding the structure-based efforts. A series of 3'-carboxyalkoxy analogues of TMP were designed based on molecular models of the Esc/,erkl,ia coil DHFR—MTX complex. The designed IMP analogues had up to a 55-fold higher enzyme affinity than TMI' itsell'. Kuyper and coworkers noticed that in the E. eoli DHFR—MTX complex. the a- and y-carboxyl groups formed ionic interactions with the guanidinium group of Arg-57 and the aminoalkyl side chain of Lys-32. respectively. The observation that there was a possible third ionic interaction with Arg-52 suggested that TMP analogues, with judiciously selected carboxylate groups, could interact with one or more of these complementary residues. The analogue
with the carboxylate extended by five methylene units.
942
Wilson and GLvvolefs Textbook of Organic Medicinal and Pharmaceutical Chemistry
0
CO2H
NH2
N
N
Figure 28—26 • Saquinavir (Fortovase, Invirase). 15. was the H2N
first HIV-1 protease inhibitor designed with structure-based CADD methods to receive FDA approval. Here saquinavir Is shown inside the binding cavity of HIV-1.
N
was the HIV protease. The enzyme is one of the proteins coded by the HIV genome, and it is expressed as part of the reproductive cycle of the virus. The x-ray crystal structure for HJV protease has been available for well over a decade now, and it is classified as an aspartyl protease, since there are active aspartate residues present. HIV protcase is a sym. N
metric dimer. There are 99 amino acid residues in each
N
14
Figure 28—24 • Dihydrofolate reductase (DHFR) has been a popular target for drug design. Methotrexate (MTX). 12. and trimethoprim (IMP). 13, resemble folic acid. 14, the natural substrate.
shown in Figure 28-25. was found to have the optimal binding. Much of the experimental binding data were consistent
with the molecular modeling studies and the subsequent structural data. Although all the observations could not be explained, this work represents one of the first successful
monomer. The binding cavity can be seen clearly in Figure 28-26. In the late l990s, several HIV- I protease inhibitors were introduced into the market that were designed using strucHoffmann-La Roche ture-based methods (Fig. scientists used modeling methods to design saquinavirtw (Fortovase, Invirase) 15. which was the first protease inhibitor to be approved. The drug was made available in June 1995 through a compassionate treatment program. Invirase was given Food and Drug Administration (FDA) approval in December 1995, and Fortovase was approved in November 1997. Indinavir16' (Crixivan), 16. was developed by Merck scientists and given quick approval in only 42 days in Match 1996. In March 1996, Abbott received approval for Rita-
navir'62 (Norvir). 17. The following year, March 1997. Agouron received final approval for Nelfinavir'63 (Vira-
structure-based drug design approaches.
There is a growing body of successful examples using structure-based drug design approaches. Today, many of
cept), 18. Each of these drugs, designed using structure-
these have resulted in approved drugs. These methods are
based methods, represents major triumphs of CAI)D.
applied widely when appropriate experimental data are
Agouron originally was a company founded, like Vertex, on the premise that structure-based drug design is an effective approach for drug discovery. Amprenavir (Agenerase). 19, developed at Vertex, was given FDA approval in April 1999.
available. Structure-based drug design is now considered a standard approach to drug design, and the question posed early can be answered with specific examples. In the 1980s. the target enzyme for inhibitor design was DHFR. as discussed above. In the l990s, the target enzyme
The ability to collect rapid x-ray crystallographic data allowed scientists at Pharmacia & Upjohn to use structure-
NH2
NH2
Figure 28—25 • With the aid of xray data and molecular modeling. scientists designed trimethoprim (IMP), 13, analogues that had up to 55-fold higher enzyme affinity than the parent inhibitor.
ji H2N
0C113
N
OCH3 13
H2N
N
L-k
OCH3
OCH3
Chapter 28 • Conapu:utional Chemistry and Computer-Assisted Drug Design
15
943
16
OH
Cl-I3 0
17
Jo.
CONH't'Bu
18
0
OH
—
20
19
Figure 28—27 • The six HIV- 1 protease inhibitors given FDA approval between 1994 and 1999 were designed by using structure-based drug design methods.
based methods. The resulting compound tipranavir, 20. is a small nonpeptidic inhibitor that may soon be The first drug designed with structure-based methods to reach the market was dor,olamide9 (Trusopt). 1. Ab mitio calculations and modeling methods were used to predict substitution patterns. Alicr a decade of research and develop-
ment at Merck. dorzolamide was given FDA approval in December 1994 and introduced into the market in the sum-
NHCH2CH(CH3)2
mer of 1995. Dorzolamide is an effective carbonic anhydrase
inhibitor used to reduce intraocular pressures that occur in
glaucoma patients. It is extremely effective. Inhibition of carbonic anhydrase results in reduced bicarbonate fomiation in the eye. which has the beneficial effect of lowering sodium ions with the subsequent reduction of fluid secretions. Merck had been working on various lead thienothiopyran-2-sulfonamides by developing models and fitting them into electron density difference maps of carbonic anhydrase. The first car-
bonic anhydrase inhibitor to lower intraocular pressure in glaucoma patients was MK-927. which is a close structural analogue of the compound finally approved (Fig. 28-28).
21
Figure 28—28 • Dorzolamide (Trusopt), 1. also a constituent of Cosopt, was the first drug designed with structure-based CADD methods to become commercially available. MK-927. 21. is a close structural analogue and was the first carbonic anhydrase inhibitor to lower intraocular pressure in glaucoma patients
Another successful advance in therapeutics involved a combination of x-ray crystallographic studies and molecular shape analysis (MSA) to produce (Aricept). 22 (Fig. 28-29). Donepei.il is a potent acetyleholinesterase (AChE) inhibitor used in patients with Alzheimcr's disease to help stave off the loss of cognitive abilities. Docking simulations of donepezil suggested that the drug does not actually bind to the AChE active site but rather inside the long chan-
nel leading to the active Site in a tight, narrow region. In addition to the structure-based modeling studies. 3D-QSAR studies were carried out using serniempirical descriptors.
944
wul
TexiJ;rn,k of Orgw,ie sIedi,ina! anti Pliarnwteutkal CIie,ni.stri
velop a pharmacophore hypothesis. Once a phartnacophore is developed, it is possible to search 3D structural databases. The first 3D searching software was developed in-house by
pharmaceutical firms to mine the corporate 3D databases (ALADDIN.'7° developed at Abbott. and 3DSEARCH.'7'
Figure 28—29 • Donepeiil (Aricept), 22, is a potent acetyicholinesterase (AChE) inhibitor used in the treatment of Alzheimer's disease.
The pharmacophore COflCCN plays a central role in drug design. The pharmacophore. first proposed in the early I 900s by Paul Ehrlich. maybe defined as the 3D arrangement of the essential functional groups necessary to cause the biological response. The definition only assumes that it is necessary
for a drug to present its properly oriented functional groups to the receptor's complementary amino acid residues. Although the idea may he somewhat simplistic, since it ignores
explicit consideration of the molecular structure that correctly orients the functional groups, the idea has withstood
developed at Lederlc). The construction of 3D databases was madepossible by software such as CONCORD'72 and COR. INA'73 that allowed rapid generation of 3D structures front 2D structures. CONCORD has become the standard program
used for the creation or 3D structures from 2D input. It is important in 3D searches to account for structural flexibility. There are essentially three ways this may he achieved: (a) storing multiple conformations in the database itself: (I') developing specialized queries: and (E') generating conliinnations during the search query. The first idea requires that all conformations for every molecular structure be stored in 3D database. This approach is not practical. Although the second approach is appealing, it requires the scientist to sign the query appropriately. The third approach seems to solve the problem. inasmuch as only one (or a few) confor. mation needs to be stored and adjusted to match the pharmacophore search query. Today, there are several commercial
programs available for 3D database and pharmacophore
the test of tinle as a first approximation for a model of
searching.
drug—receptor interactions.
Goodford proposed that a grid of test points enveloping a molecular structure could be used to calculate favorable interactions (initially with 6—12 nonhonded, electrostatic. and hydrogen-bonding potential energy functions) between GRII)'73 was an interit and a target receptor. The program can he considesting innovation. The DOCK'7 ered the first 11mw! high-throughput screening
Prior to the explosion or structural data now available to medicinal chemists who may use 3D structures of proteins. typically only indirect inlbrination about the nature of the receptor was available. The most common situation faced by medicinal chemists was a series of active and inactive compounds. The fact that there was no structure of the drug hound to its receptor meant that drug design had to follow a procedure of comparing the efficacy of compounds and determining which functional groups were important and which functional groups were not. The active analogue approach, developed by Garland Marshall. was one of the earliest CADD pharmacophore procedt,res. 57- °" The approach avoids having to worry excessively about the subtle energy differences between conformations. Systematic conforniational searching is applied to a series of biologically active and inactive compounds. The central idea is that there is a limited set of conformations that an active compound (with appropriate functional groups) may adopt. Biological inactivity is assumed, as a first approximation, to result from the competition between small molecules and the receptor for occupation of the same physical space. Usually, the most rigid structure is considered first. Subsequent systematic searches are carried out on
the remaining ''active'' molecular structures. It is possible to add screening tiltcrs to eliminate unacceptable confornialions: for example. computer-generated structures must be able to adopt conformations similar to those available to the previous molecular structures and not be outside a specified relative energy range. At the conclusion of the process, a volume may be generated representing the union of all available conformations for the biologically active compounds.
The goal was to allow prescreening of compounds that could bind to an active site. A series of molecular structures can
be evaluated for their fit into a receptor by use of scoring functions. An early study using a.chymotrypsin ranked several known inhibitors in the top 10 structures, based on the scoring functions used to evaluate the binding potential.' Another early academic 3D searching program was CA. Predictive pharmacophore models can he generated based on 3D-QSAR analyses. Hanseh demonstrated the usefulness of QSAR.'7'> In the 1970s. many studies were undertaken to
infer biological activity on the basis of physical properties of a molecule. The method remains useful and provides valu-
able information.°'° Richard Cramer developed a popular program involving a comparative molecular field analysis (CoMFA ). "° The basic idea is to probe a molecular structure fur steric and electrostatic interactions directly, and then generate a QSAR equation based on these molecular descriptors. using partial least squares (PLS). The validity of the model can be predicted.
PREDICTIVE ADME
This "active" volume may be used to glean information
The ultimate goal of CADD is to understand at the molecular
about the receptor site. It is possible to generate an ''inactive" volume as well, which is the region in 3D space that should not be used to tuake molecular modifications. series of active and inactive compounds Examination provides important structural inflirmation that is used to de-
level the complex relationships between a target (macromolecule) and a drug-like molecule so that reli able predictions can be made to enhance molecular interac tions. Other important pharmacokinetic factors are critical. however. for an effective therapeutic medicine. Essentially.
Chapter 28 • Computational Chemistry and Computer-Assisted Drug Design potency. soluhility. and permeability are the only three physical variables that can be adjusted to enhance the activity of potential oral Lipinski has suggested "poor
absorption or permeation is likely when the molecule has more than one of the following properties."
945
electrons are involved) were carried out on a series of known drugs. The activity, defined as the AM I H-atom abstraction, is modeled on the presence or absence of chemical descriptors. Over the next decade, in silico property and toxicity pre-
dictions will increase. As the predictive methods become more reliable and robust, they will be included increasingly
I. More than 5 hydrogen hand donors 2. More than 10 hydrogen bond acceptors 3. Greater than 500 molecular weight 4. Greater than 5 computed lug P
Medical professionals must be aware of drug—drug interactions. Because a significant number of drugs are metaboli',cd by the cytochrome P-45() (CYP). it behooves medicinal
in the initial drug design process rather than being an afterthought. There are many other CADD success stories. Although drug discovery is a complex process, in the future, as our understanding of drug action increases, a growing number of therapeutically effective drugs will be designed using computer-based methods.
chemists to consider this oxidative pathway in the design process. Tragic consequences of drug—drug and food—drug
interactions have resulted in two FDA-approved drugs. nhbefradil (Posicor), 23, and lerfenadine (Seldane), 24. being removed from the market in recent years. Mibefradil and terfenadine are shown in Figure 28-30. Each drug required P-450 for phase I metabolism. A more recent predictive model for CYP 3A4 metabolism has been reported)53 The method relies on PLS. hut one of the descriptors is based on AM I-calculated hydrogen abstraction. There are several assumptions: (a) CYP 3A4 susceptibility is a function of the electronic environment around the hydrogen that is abstracted. (hi Abstraction of the hydrogen atom is the ratc-dclemiining step. (c) The drug being metabolized tumbles freely in the active site of the enzyme until the most active hydrogen is available. AM I calculations (using a procedure to account tbr the fact that unpaired
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42. Pople. I. A.. Sergei. R.. and Krishnan. R.: lot. 3. Quantum Chem. Symp. 11:149, 1977. 143. Krishnan, R.. and Puple, 3. A.: lot. J. Quantum Client. 14:91. 1918. 144. Schlegel. H. B.: 3. Client. Pays. 77:3676. 1982. 145. Muliiken, K. S.: 3, Cluem. Pays. 23:1833, 1955. 146. Parr. R. 0., and Yang. W,: Density Functional Theory of Atoms and 1989. Molecule.s. Oxford. Oxford University 141. Kohn. W.. and Sham. L. J.: Phys. Rev. A140:l 133. 1965. 14%. Hohcnberg. P.. and Kohn. W.: Phys. Rev. B136:%64. 1964. 149, Berman. H. M.. Weslbrook. 3.. Fcng. Z., ct al.: Thc Protein Data Bank. NucI. Acids Rca. 28:235. 2(1313). ISO. (3uncr. 0. F. (mid.): Pharmacophore Perception. Development, and Use
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A
P
P
E
ND
I
X
Calculated Log P, Log D, and PKa values are from Chemical The log P. log D at pH 7, and Abstracts Service. American Chemical Society. Columbus. OH. 2003, and were calculated by using Advanced Chemis-
try Development (ACD) Software Solaris V4.67. The pK, values are for the most acidic HA acid and most weakly acidic BH + groups. The latter represent the most basic nitrogen. Keep in mind that pK, values for HA acids that exceed 10 to 11 mean that there will be little, if any, anionic contri-
bution in the pH ranges used in pharmaceutical formulations
and in physiological p1-1 ranges. Similarly, for BH acids. there will be little. ii any. cationic contribution for pK, sulues below 2 to 3. Because Chemical Abstracts does not leport calculated physicochemical values for ionized cornpounds including salts and quaternary amutoniurn compounds. the log P values in this appendix are for the an-
ionized form.
pK.
p11Cm
LogDat Compound
Log P
ph 7
Ahacavir Acarbose
0.72
0.72
—3.03
LogDat HA
BH 5.08
(2.39
5.90 9.11
Compound Amilonde p-Aminobenzoic acid Aminoplutathimide
239 0.34
032 0.34
13.78
Acclniainuptwn
9.86
Arninolevulinic acid
Acctazolarnldc
—0.26
—0.40
7.44
Acctic acid
—0.29
—2.49
4.79
2.24
0.03
Ace(ohydrnsaznic acid
—1.59
—1.59
9.26
Aectylcysicinc
—0.15
—3.74
3.25
Acebulolol
Log P 1.90 0.111
ph 7
HA
BH
1.8$
8.58
.58
—2.12
4.9(4
2.46
1.41
1.41
11.64)
.1.41
—0,93
—3.38
4(X)
7.37
4-Aminosalicylic acid Amiodarorar Amitriptyline Amlexunox Amlodipine Amobarbital
0.32
—3(12
338
2.21
8.59
6.29
6.14
3.98,
4,67
1.65
3,72
2.00
2.10
2.05
Anioxapine Amoxicillin
2.59
1.52
0.61
—2.21
Amphetamine
1.81
—0.91
9.37 '1.24
3.95 1.73
7.94
5.73
3.52
4.79
Acyclovir Adapalenc Ade(ovir dipivoxil
—(.76
—1.76
9.18
8.04
519
413
2.311
2.38
4.63
Amphotencin B
(1.18
3.96
8.l3
Adenunc
—2.12
—2.12
2.95
Ampicillin
1.35
—(34
2.61
6.7')
Adenosinc
—1.46
—(.46
13.11
3.25
Amprentuvir
4.20
4.20
1134
.76
Alaninc
—0.68
—3.18
9.62
2.31
Amyl nitrite
2.45
2.45
Alaurofloxacin
0.31
—2.22
0.64
8.12
Anagrelide
(.13
1.13
Albendarole
3.01
2.99
10.46
5.62
Anastrozole
0.77
0.77
Albuterol
0.02 4.26
—2.15
9.113
9.22
Anthralin
4.16
3.91
7.16
4.26
13.73
Apomorphins
2.47
2.34
9.41
Apraclonidine
1)3(1
—1.91
(.78
—5.26
5.61)
5.55
Acilretin
Alclounewsone dipropionate Alcndronic acid
—3.52
—7.80
0.47
10,56
7.59
2.03
Alkntaunil
1.89
Alltretinoin
6.83
4.62
4.79
Allopunnol
—0.48
—0.50
9,20
2.40
Arginine
—
Aripiprazole Articainc
7.9')
13.46 4.13
Asparagine
—
1.51
—4.02
2.30
834
Aspartic acid
—0.67
—4.17
2.28
9.95
Aspirin
1.19
—2.23
3.48
Astemizatie
5.80
3.62
Alazunavu'
5.51
11.11
4.81
Atrnolol
0.10
—2.03
13.8$
9.17
Alornoxetune
3.84
4.3(1
4.97
9.48
0.65
6.71
Aiprazolum
230
2.39
AIprusladli (pnataglundin E,)
2.25
2.50 0.02
Altactawinc
2.42
1.90
7.37
Ainantadine
2.22
—0.79
10.75
Arncinoniule
3.110
3.80
13.15
1,69
—4.72
1,29
10.16
Atorvastaului
4,22
(.03 134
(294
9.52
Atovaquone
8,18
4.14
948
9.11
231
1.41
—0.51
—3.84
650
—4.96
1.89
Amikacin
(.45 4.78
—2,12
0.96
—
11.7')
Ascorbic acid
Mosctron
Amifoxtine
8,93
9.94
2.44
Almotriptan
4,77
8.113
2.61
0.12
949
Appendix • Calculated Log I'. Log D. and pK,,
LogDat ph 7
LogDat HA
BH
I 2?
998
0.54
((.25
'-3.01
13.3(1
8.59
—((.94
4.181
10.32
—2.29
2.97
459
13.08
7.11
3,73
2.01
8.63
5.02
2.57
).I5 (0.54
4.27
9.2?
2.06
12.14
2.6?
12.05
(.23
(2.93
(2.67
((.56
13,89
6.07
4.30
4.54
11.49
.89
9.8(1
13.86
9.16
3.30
(3.68
8.97
11.0?
5.89
— (.34
9.63
—1.06
9.62
8.29
4.52
9.6?
6
1.30
639
6.10 3.24
12.85
—((.27
3.18
2.45
2.40
9.67
4.48
8(7 8)8
3,08
7.16
3.33
6.43
--11.52
(.52
795
5.84
7.87
6.69
-
6.72
2.93
(0.26
7.97
0.92
13.06
9.4?
-'(1.1)8
(.39
4.22
—(1.38
5.67
9.9) 3.82 13.94
carisopandol
1.0?
—3.99
2.62
—0.19
—2.7?
3.40
2.76
1.12
8.65
2.49
0.27
4.77
2.15
2.15
(2.49
1.30
10.19
1.30
4.24
7.9?
15.5?
15.5?
Caricolo?
1.67
—0.42
13.84
9.13
Carvedihol
4.23
3.16
I3.90
8.1)3
Celditorenpivosil Ccfisimc Cclonicid Crfopcrnrone
Ct(onilin
(A)?
4.81
0.97
2.67
ccradrosi?
.98
7.43
2.67
Ce(uchor
9.17
13.98
Curbanuzcpinc
carbenicilhin
BH
3.98
3.31
Cerchoir
3.97
6.12
—2.86
8.88
2.00
5.43
(1.27
Cairmustine
3.47
5.43
6.12
3.31
4.20 2.20
7.53
Caiplopril
2.5?
0.32
7.53
HA
Cup..uicin
Cnrbklopa
2.49
2.67
cilexelil
2.36
2.3?
—0.88
( 1.25-di
9.16
ph 7
Log P
Calcipou'ienc
447
(1?
0.58
Compound
0.19
—2.7?
.95
6.80
—0.09
—2.89
3.12
6.93
(.52
—2.39
2.62
—0.73
—5.13
2.80
3.27
(.23
1.13
7.57
2.89
—0.5?
—5.53
210
2.86
0.54
—4.46
1.43
2.62
—0.3?
—4.24
2.66
0.72
—3.19
2.63
2.90
Cefpodosiuncpruxciil
0.66
0.57
7.6?
2.90
Ccfpro,il
0.15
-2.67
2.92
6.93
Cc0ibuten
—1.18,
-51)8
3(8)
5.44
Celiieoxime
—0.92
--4.70
2.99
2.90
CcIiriaixonc
— (.76
—5.86
2,57
2.90
Celun,xjiiic
—0.54
—4,47
2.59
3.0?
3.0?
9.68
Ccphmkxtn
0.65
-2.22
3.12
6.80
Cephapirin
(1.79
-3.05
2.67
4.49
0.98
— (.79
3.12
6.99
Cetiozine
2.97
—(1.1)2
'L27
6.43
cesliucline
1.12
Chlorul
1.6??
—
.29
(.68
93? (0.54
Chhoruinhucj)
3.10
(.52
4.86
CII(or4mphenkol
1.02
(.02
((.1)3
3.66
2.49
2.49
-1.45
4.54
—0.46
11.73
Chkaroprncainc
3.38
1.28
9(3
Chloroquine
4.69
1.15
0.48
—0.18
—0.18
3.75
(.5?
2.07
.4?
(.4?
12.99
3.39
1.13
Chlorhesidjaic
Chhorolhizttidc Ch?orplwncsan earbaniate
Chkn'phcnirarninc
9.17 7.20
'133 (Canii,uwd)
950
Wi/son wid Gi., ,'o!d'., Texi/,ank of Orgu,zir ,l4edirinu! and I'/u:rnwc'eiuu'aI C/,c,nj.siry
pK.
Log bat
Log Dat Compound (ii(nrpiurna,Inc Chlorpropnnidc
LogP
ph7
5.36
3.01
2.2!
0.28
--((.74
—0.74
9.57 8.92
2.44
2.43
Chulccakifcrot
9.72
('ielopirox
2.59
9.72 (.76
Cilaslulin
2.42
Ciinefldinc Ciprofloxucin
HA
BH
9.43
6.25
(.6!
454
(.09
2,09
11.83
3.04
3.04
((.20
—0.11
6.73
—033
—4.58
(.3!
—(.20
4.3! 8.76 9.5')
2.74
2.89
0.41
—(.72
—7,67
2,93
0.24
0.24
(3.75
.44
Clurilhromycin
3.16
2.00
(3.07
8(4
2,78
Citalopruni Cilnc Cladnbinc
ph 7
Log P
l)ehydrncl,olic acid Detas-irdine
-764 -
Compound
1.77
—1)48
.23
—3.23
11.5$
—3.34
Dcafluruuc
1.87
(.87
Desiprrnnine
3.97
1.05
I)csli,ra(adlnc
5.26
2.95
l)cs,,hidc
-
2.72
272
csoxirnc)as,,nc
2.4(1
2.4))
Dexuincihasunc
2.0!',
2.06
2.61
2.6)
uccla(e
Dexn,cdel,,rnidine
3.!)!
2.85
I)csn,,oxanc I)cxroinehorphan
—((37
--(1.37
4.28
2.22
3.86
3.86
Diaii,nide I),hucninc I)ichl,,roace,,e acid
1.1)7
I
430
094
.07
(.95
Clasulunic acid
I.'1S
—5.84
flcma.stinc
5.69
2,83
CIi,,tbnwcin Cliuqilinni
2.14
((.4!
(2.87
8.74
1)ick,(enac
3.28
11.4$
4.32
2.35
2.10
7,24
DiclosaciIIin
3.02
—((.9!)
C1,,bclui.o1 pn)piona(c
4.18
4.18
(2,94
Dieseloininc
Clocortolonc plsulatc
4.41
13.10
Didarnn.,nc
Clirntipruminc
4.4! 7.50 8.0) 5.19
CII,JIa?epam
3.02
Clunldinc
1.41
—0.67
9(6
Cknnima,.olc
5.76
5.7!
Clozapine
3.4%
3.40
CocaIne
3.1)8
1.14
Codcinc
23.14
((.83
Colchicinc
(.03
.03
Cortisone
(.24
(.24
(2.29
('nnnolyn
(1.21)
-4.80
1.85
Croiamiio,,
3.1(1
2.42
3.10 (.83
7.46
6.22
4,06
9.2!
((.63
0.63
4319
(.69
.69
Ck,(iI7iminc
C'(.,nüphcnc
Cyclihcn,.apnnc
Cveloccrine
(.114
Cysteamine
Cysicine Cytarahine I)acuthaiine Dallopriclin
((.03
7.43
6,24
5.58
9.53 9.49
2.80
3,02
—
CyprohcpLidine 1)62
0.23
-
((.19
(3.42
I)ie(hylcarh.,Ina,ine I)ieihylpropion
3.54 (1.92
63)5
3.87
—0.92
—((93
(4
I))!)
I
2.95
(46
573
3.23
I)ifli,rusonc
2,'))
2.9!
DiOunisal
4.32
034
6.12
II')
1.14
6,33 8.97
3.02
I. (7
9.86
8.29
—0.78
(.55
I)lntcrvaprol
2.64
(1,84
0.83
118
--0,36
I
Iprosiuglaniiin El I)iphcnliydramine 1)ipwelnn
1)78
453
3.66
(.92
6.57
5.85
(.49
11.49
— .22
l)isopyniniide
(.87
1)07
4.93
8.7U
I)isiuIfir,ii,i
3.8$
2.53
8.97
1)obuiurniiue
2,49
(.56 2.40
'(.45
-
0.93
7,93
((1,47
Do)ciiltde
Delaacimn l)oiiepeuil IXfl'A
0.24
-2.3!
2.07
((.05
--2,3!)
- 2.30
13.4)!
4.47
—0.26
0.26
(2.32
4183
8.95
opauntne
—
3.88 II.) I
(1.27
2(0
4.7))
2.89
-((.23
2.73
0.12
— 2.3(i
—((.94
2.87
3.32
I)unarol
4.70
4.7!)
13.10
Danirolene
((.95
0,87
7.69
Dapipnuolc
2.44
2.2K
1,39
(.94
094
(.24
Duuxepin
5.08
2 93
2.39
((.47
8.64
I)osenalciliro(
8.15
8,14
Dapsone
I)aunor,ihiein
7.15
Dorio(uru,de
I)osaprdun
—0.2)
2 (12
3.23
2.67
(1,65
11.54
Appendix U Calculated Log P. Log I
LOgDat
LogDat Log P
ph 7
HA
BH
Compound
Log P
ph 7
2.29
0.36
7.12
14.64
Felhamaic
1.20
1.19
—0.26
—3.83
4.51)
9.32
Felodipine
4.92
4.92
7.64
7.64
9.8)
Fe,wIihr4Ic
4.144)
4.144)
4.14)
2.85
1.79
.72
—(1.56
6.85
6.85
(3.34
3.84
1.06
4.67
2.83
—1.12
—1.12
(3.66
8.23
Fciioldopam
14.146
Fcmanyl
3.93
1.90
0.76
Fenofcnadinc
5.18
2.6K
6.69
Finastcridc
3.24
3,24
Flovoxate
5.46
4.114
10.45
Flcca,nidc
3.47
0.55
40.35
Flrmuridinc
1.20
—1.21
8.9)
Flucona,o!c
5.8)
5.64
4.414)
4.85
7.92
0.56
—2.14
1.22
3.27
0.36
2.06
—0.98
2.98
—0.12
2.10
2.1(1
2.63
.66
6.1(7
1.1)5
—1.25
13.96
9.38
—0.63
--2.75
9.64)
2.2')
0,36
7.12
4.05
1.05
2,2)
—0.4)7
4.7)
4.96
1.46
3.3)
9.56
9.56
0.57
(((84
3.75
5.54)
Fludarahinc
(1.31
0.34)
—2.36
--2.38
—2.32
---2.32
1,78
(.78
Fltimacenil
0.141
0.87
9)6
Flunimilidc
2.26
2.26
8.64
Fluocinolonc
(1.77
0.77
Pluocintilonc aceionidc
2.34
2.34
Fluorcaccin
3.6)
3.60
Fliuorcxon
2,19
—3.86
acetate
8.73
Fluommethutime 7.31)
Fluoruw-ucil
Fluoxciinc
3.06
2.65
9,62
7.20
—1.28
-4,81
4.03
7.94
2.83
3.66
13.0$
8.44
Huphena7inc
2.22
2.24
—0.78
—2.29
4.35
(37
2.17
2.17
4.144
3.29
2.89
0.41
9.59
Iiumndrcn,,Iidc
1.95
(.95
1,91
—0.22
(3.88
9.17
F1,in,zcpani
4.7)
2.12
9.4(8
4.64
Flurhiprokn Flutamuk
4.11
1.28
4.06
3.06 3.92
3.80
(.84)
3.25
3.25
4.13
4.13
40.37
Fl,ilicasunc pmpicl,Iae
3.92
7,59
7,59
1(1.35
Fluva.taiin
3.72
1.01
6.62
6.62
10.35
3.17
4)86
—2.63
—7.52
5.75
5.75
3.314
—0.47
•-0.O5
—2.56
.61
F,iIie acid
Fomcpiiiik
0.714
4)714
9.64)
Formaldehyde
0.35
0.35
7.644
Furniolero)
1.57
—0.17
6.41
F,iscamei
—2,53
-7.64
Fruton,ycrn
—2.98
—7.25
2.80
7.59 5.55
5,43
432
4.52
(0.34
7.22
7.22
12.14
1.14
1.14
(4.86
0.82
1.32
4.32
—3.54
—9.2)
3.3)
0.64
3.36
3.36
4.23
4.2.2
1.97
1.96
3.34)
3.34)
4.3')
4.29
—0.0')
—0.0')
2.4$
—3.442
4.34
Fosotopril
'1.70
Fowairipian
5.84
2,65
(1.41
—4.64
0.144
—2.414,
Furnaric acid
—0.0)
—4.95
0.6K
F,,rjeolidonc
—0.04
—0.04
4.33
Fi,rowmidc
2.92
—0.84)
4.23
Gahapeutin
1.19
--1.31
3.02
Gjhintatnlnc
14(143
9.66
7,61
2(2
3.33
—2.07
—2(47
Gatitlunacin
3.59
-'(1.92
Gemeitubinc
—4)644
—0.68
Ganciclovir
'LOS
4.24
Ckm(ihr,,,ll
4.39
2)4
7.75
(Jluuepiridc
2,')4
4.27
952
Wilson and
Te.vthauk
al Organic Mt'dicina! and !'har,uaceuti,'aI C'h,',,ii.ciry pK
Compound
Log P
Log D at ph 7
HA
BH
Compound
Log P
219
0.52
5.34
1.44
—4.92
2.17
9.76
lodoquinol
4.34
—1.60
—4.1(1
2.27
952
lrinoccun
3.8)
—0.34
-0.33
lsocarhoua,id
1.03
2.70
2.7(1
3.93
2.28
luocilurinc Isofliirune
279
Glycerin
'-2.32
—2.32
1352
Glycinc
-1.03
—3.53
2.43
.64
1.95
- (AX)
12.39
111.50
('.lt,tarnic
-
(lutamine
Cslyhuride
Gnlnisein,n HCI
—2.11
11.36
Isonjucid h.oprolercnol
1.13
—0.89 0.25 —1.75
Gri'coIulvin
2.36
2.36
Gunifenesin
0.57
0.57
—1)08
—3.18
12,76
lsc*rclinoin
6.83
I .07
—3.58
13.43
lsonsupnoc
2.58
1.12
1.12
3.75
lsr.adipinc
3.68
Guunidine
2.57
0.15
9.66
Itracona,.ole
Hulcinonlde
3.32
3.32
(3.25
G,ianaderl
Gwineihidiiie
luosorbide dinhir.ne
11.81
0.90
Isocorbide mononitrMc —051
'3.3%
Kanamycin
3.29 —2.60
3.92
3.92
12.61
Kct.imine
2.15
HalofanIrijIc
11.86
6.51)
13.56
9.43
Kcloeonazolc
2.88
IlulnpcriJoI
4.06
2.80
13.90
8.25
Ketoprofen
2.81
2.30
2.30
Kctorolac
2.08
7.211
7.20
Kelolikn
4.99
—0.84
-3.68
1.57
—1.17
—0.07
—0.08
1.43
1.43
.9%
1.98
12.42
Lanoxin
1.14
Ilydrucortisone buwprule
4.12
4.12
Lansopnuolc
2.39
I.ulanoprost
3.65
Hydmcorlisonc hutyrulc
2.81
2.811
12.95
Lcflunomldc
1.95
Hydrocortisone
4.53
4.53
12.33
L.ctrouolc
3.34
3.34
12.95
propionalc
Hislurninc Hornuiropinc
Hydrnc)ikmthiaeide Hvdruconisonc Ilvdrncuutisune aCCta(C
6.58 10.15
Luhelalol
287
9.911
Lucwloe.c
—2.41
8.95
Luznivudinc
— 1.02
12.48
Lamolrig)rw
—0.19
12.10
cypionate valcrate Hydroflumelhiuildc Hydromorphond
0.54
(1.54
8.63
—1.23
—17.21
9.61
10.33
9.82
Hydmquinone
0.64
11.64
Hydroxyamphctaminc
1.117
-1.84
Hydronychkiroquinc
3.54
1.08
5.74
5.74
1.80
—1.80
2.31
2.21
Hyoscyaininc nuttü(c
1.53
—1.21
Ihuprofen
3.72
(IS
4.41
Lcucovorin I..cvalbulerol 8.36
10.71
8.87
c:IploaIc
llydroxyurea Hydri,nycinc
—
Levamisolc
6.34
0.02
054
Levuocrenol
—0.88
Lcvctirucctant
—0.67
LcvobcUtxolol
2.69
Levobirnolnl
2.86
Lcvobupivacaine
3.64
Levocahastinc
10.56
132 —8.12
Levodopu
4.86 —0.23
Levotlonacin
1.49
l..cvomclhadyl acclate
5.45
4.17
1.47
9.57
9.47
LcvonorgcuIn?J
3.92
Idai'ubicin
2.16
0,43
7.79
8.64
I.evorphanol
3.63
1f'slamldc
0.63
0.63
4.03
5.96
lnuuinih
1.86
1.18
(3.28
7.53
Lcvolhyroxine IT.,: i.'thyroxinc)
-2.78
- 5.28
4.47
10.37
Lidocuinc
2.36
Lincomycun
0.86
Iurnarnte
lmipcneni
Iiuipriirnine
4.46
2.07
lmiqntmod
2.61
0.57
Indupamide
2.111
2.09
Inditiusir
2.29
2.26
Indunicihacin
3.11
(1.30
9.49 9.04 9.35 5.73
4.17
Lindunc Linezolid
3.94 —(1.92
Liolhyronine (Ti: Irliodothyroninet
5.12
Lusinopril
1.75
Appendix • Calculated
pK. Log D at
LogP
ph7
HA
BW
054
—4.46
2.07
2.33
—0)7
2.40
2.76
2.76
10.8))
4.95
3.87
13.89
5.65
5.64
13.89
-0.95
—3.79
3.24
6.23
6.23
2.48
2.48
10.18
1)03
3.50
0.89
4.24
3.10
3,69
3,69
4.07
4.07
2,9';
2.9)
.04
—4.52
2.48
10.6')
—0.8)1
—2.38
10.16
8.58
—
2.93
2.93
-4.67
--467
Compound
1.31
(.66
8.82
Methsuxiinide
2.22
Mcthyluiopa
0.13
Methylphenidate
2.55
1.76
8.05
1.10
6.84
3.80
2.18
Methylprediusalone
2.73
4.02
3.49 6.28
Me)ipntnoto(
2.67
Melocloprwiiide
2.35
Metolzuone
3.16
MetopnAol
(.79 —0.02
13.14
4.5)
1.52
2.43
2.42
3.06
—0.02
1.66
1.33
7.06
5.02
4.83
6.73
5.91)
2.57
4.))
4.11
2.87
2.87
5.33
2.1)9
3.69
2.87
0,05
13.13
111.13
3.82
3.82
2.7)
0.22
4.50
2.40
—0 II
2.12
2.8)
1.23
2.29
—0.62
IO.38
1.85
1.81
2.04
0.93
0.70
1)70
0.73
0.63 10.29
Log P
5.02 11.35
Mexiktiiic
2.16
Mic,njtak
6.42
Mitlazolam
Midotlrine
3.67
—0.32
4.9)
3.59
— 1.40
Mllrlnunc Minocycline
0.41
—0.27
Minoezdil
(1.60
Minazapine
2.52
3.05
Miton,ycin
0.44
9.54
Mitulane
5.39
8.55
Mituxuntrone
2.62
Modalinil Monaipril
4.47
Mometasone (uroatc
4.73
2.9)
7.97
1.96
8.09 13.0')
l.4ö
Monobcnzune
2.96
0.39
(1.37
8.46
2.40
—3.13
—5.63
4.47
WOO
0.46
—2.19
.90
5.43
Monóctanoin
2.12
3.98
.45
9.66
Muntrlukast
7.85
(1.13
—2.02
9.12
9.33
2.67
0.07
— IA))
9.75
8.47
Muriclzine Morphine
2.42
2.42
12.24
—2.3)
--5.4)
13.10
Mupirocin
3.44
Mycophenolute mulch
4.10
Nohumetone
2.82
2.18
9.05
1,94
—(1.97
10.38
0.13
0.13
2.17
2.16
—0.02
—0.1)2
((.64
0.37
—2.13
2.23
0.55
0.54
(3.))))
2.4)
2.36
7.92
Nadolol
(.29
3.52
1.10
NalcillIn Naftilinc
9.26
Nalbuphine
0.33 5.28
3.54
(.27 1.97
4.2))
—0.28
—0.05
5.67-
acid
5.09
1.96
0.18
Nalumiekne
2.82
Najoxonmr
1.92
954
Wilson and Gi,wold's Textbook of Organic Medicinal and Pharmaceutical Chensisu,
pK
Log Oat Compound
Log P
phi
HA
BH
7.40
Compound
Log P
Log D at ph 7
Pantoprazole sodium
(.32
1.16
Papaverinc
3.42
3.33
Purnidehyda
0.31
0.31
Punvalcitol
5.83
5.83
Nullrexone
(.97
1.42
Nandrolone decanoate
8.14
8.14
Naphazoline
3.53
0.65
Naproxen
3.00
0.41
4.41)
Narauipmn
1.81
-—0.71
11.52
9.66
Paromomycin
Nacamycin
0.93
—1.59
3.fl
8.13
Paroxetlne
3,89
1.00
Nuleglinide
4.57
.26
3.61
Peinirolast
—(1.02
—3.12
Nedocromil
2.63
—2.37
2.00
0.52
Ncfasodone Nelfihavir
3.50
3.19
6.55
5.91
9.58
—0.31
—0.31
0.82
—2,58
(0.93 2.17
—0.11
—0.11
Nicaxdlpine
5.22
Nicotine
9.39
10.27
—3.31
Pcmoline
0.52
6.75
Penhulolol
4.17
2.05
7.53 4.74 4.82
Pctaciclovir
—2(13
—2.03
(1.93
—3.60
PenidIlinG
1.67
—2.25
Peracillin V
(.8%
—2.04
4.86
3.54 7.11
Pcntamidlnc
2.47
—0.65
0.72
-0.32
8.00
5,00
3.08
NiCtdiplne Nilulnmide
3.05
3.93
2.10
2.04
3.15
3.05 3.08
—2.82
—3.15
Nimodipine
3.94
3.94
4.01
4.46 0.83
4.46
3.91
0,37 3.97
0.37
Nlsoldipine
0.27
—035
—0.63
7.69
1.20
3.36 6.74
0.09 2.22
0.09
11.15
3.87
4.49
3.94
2.55
2.55
—1.28
—1,28
Nizalidicte
1.23
0.75
Norelgestromin
4.40
Norethiudnane acetate
3.99
4.40 3.99
Nor
1.47
Nortiipcyllne
5.65
NCvlmpine
Niucinamlde
Nicazoxanide
Nitrufurantoin Nltrofur84one
Nitroglycerin Nhrouxoxlde
0.83
'1,73
2.22
—
4.37
Olsiilailne Omeprazole
3.94
—1.06
2,7(1
1.80
9.08
Ondaautrun
2.49
1.84
Orlistal
8.95
8.94
Orphenadrlne Oselluinivir Oxacillin Oxandrolone Oxaprozln Oxazepam Oxcarbazapine Oxiconazole Oxybutynin
4(2
Olmeswmnmedosomil
4.87
Olopcitadlne
1.62
1.31
1.14
—0.22
Ptscnindamine
4.41
3.21
8.76
Plicuobarbitul
1,71
1.62
Phenoxybenzumine Plsenteroune
5.18
5.1)4
2.16 3,60
—0.56
10.08
2.27
3.30
6.74
Ptscneljino
1,03
—(35 3.20 2.26 (.86
Olanzaplne
Pennelhrin Pcrphcn5zlne Phennzopyridine Phcndimclruzine
1.44
7.33
12.47
2.1(6
Pcntmiatin Pentoxif3'lline Pergolide PcrindOpnl
10,16
2.75
Pcnicillamlnc
6.81
6.37 4.23
4.24
4.29
9.19
Phentolamine Pbenylscetic acid Pbenylephrine Phenytoin Physoutigmine
0.70 —i.\'8
—0.30
—2.20
2.52
232
1.16
—0.29
4.63
Plsyconudione
12.25
12.25
7.54
Pilocarpine Plmecrolimus
—0.10
—0.54
5.21
5.21
2.41
8.72
6.08
3.74
3.50
—0.30
8.81
Pimozide Pindolol
1.97
—0.19
2.05 3.33
—1.87
Pioglltcizone
3.16
2.40
Piperucilliri
(.88
—2.04
2,61
3.33
4.19
3.40
4.19
0.36
Pirbuterol
—(.63
—3,17
2.31
2.33
10.94
1.68
Piroxicam
1.71
—0.78
13.73
Plicamycin POdotilox
1.39 1.29
.29
Polythiazide
(.55
1,54
Pnimlpexolø
1.62
—0.77
Pranso,cine
3.51
2.95
Pravuscucjn
1,44
—1.24
1.25
(.25
5.89
5.82
5.39
3.93
11.94
Orcycodone
1,84
1.19
33.45
6.19 8.24 7.53
Occymetazoline
4.17
(3.96
10.53
Oxymetholone Osymorphone Oxycetracycline Pamldronlc.ccid
4.22 1.07
(.20 (.72 0.46
9.44)
7.48
—1.22
—4.83
430
—3.40
—7.80
0.38
9.26 8.93
4.50
Pntzouln
dnicarbata
2.44
2.44
—1,14
—(.25
3.82
3.82
Appendix I
I' Log
LogDat ph 7
LogOat HA
1.69
12.47
2.24
12.41
4.00
12.32
BH
Ritonavir
0.75
7.95
—0.25
10.38
0.84
12.26
0.06
3.69
Compound
Log P 5.08
ph 7 5.08
2.14
0.52
Rizauiptan
0.76
—1.64
1.63
.63
Ropinirole
3,19
0.81
Roplvacuine
3.11
.92
Rosglltazone
2.56
1.71
—1.43
9.86
Sulicylic acid
2.06
— .68
0.72
9.24
Sairneleml
3.16
0.97
0.11
7.46
1.34
0.29
3.69
7.82
Secoharbil3l
2.33
2.27
1.55
10.48
Sciegilinc
2.92
2.28
Serlenline
4.77
2.39
—0.59
11.30
Scvo0urunc
2.48
2.48
2.73
8.98
Sihulramine
5.43
2.88
9.31
Sildenaful
2.28
1.47
9.20
SiunvjMauin
4.41
4.41
2.39
13.82
1.40
4.16
11.00
3.29
Sirsulirnus
9.19
3.58
3.58
—4.67
—4.67
—1.82
0.99
13.84
9.14
SegnIol
0.32
.24
7.63
0.54
Sparfkcuacin
2.87
0.36
1.17
--MO
2.08 —1.25
10.61
13.96
—1.67 —0.37
--2.12
8.37
2.67 —2.35
9.38
Spimnolaclone
3.12
3.12
10.97
Stanneolol
5.53
5.53
13.91
—0.91
Stavudine
—0.91
5.06
Surcptoi.ocin
—1.55
--1.35
6.77
.Suvvituic acid
—0.59
—4.75
Suirnuanjl
3.42
2.16
Sulcnntvoic
6.03
5.90
—-0.90
—2.14
—0.12
—0.74
0.34
—0.56
3.18
—0.63
4.70
3.87
0.85
1.82
5.34
1.30
3.29
5.38
1.35
3.05
9.13
1.35
13.0$
9.13
.44
8.50
4.42
Sulfinpyraionc
2.32
—1.01
3.12
8.98
8.67
Sulliusxa,.ole
1.01
-- 1.12
0.85
3.72
5.51
Sulindac
3.56
0.80
8.40
Sumatriptuun
0.67
—1.73
6.55
Suprofen
2.42
—0.49
5.78
Tacrjruc
3.32
0.69
7.25
Tuemlimu.
3.96
3.96
Tamuxifcn
7.8$
6.20
Tamsulo,$n
2.24
0.51
Turnrutenc
6.22
6.21
—1.68
—5.68
Tc$aserod
2.19
—0.17
Tclrni.suuian
7.8(1
4.79
Temuecpuni
3.10
3.10
—0.13 1.87
2.10
4.19
3.93 6.84 2.63
12.95
—4.68
4.32
1.00
Sul08luninc
Ta,ohact,um —1.75
4.92
657
2.75
6.38
0.03
11.17
4.21
—8.56
0.32
1.88
—0.48
9.13
Temoeulomide
—1.32
Tcntporddc
3.10
5.09
Tenolovirdlcoproxil
1.97
1.97
7.91
Terazosin
—0.96
—1.0$
649
6.15
3.10
956
of Organic Medicinal and Plwrniac'euiit'al ('lwn:i.arv
Wi/ion and Gi,ii'otd '.i
Log D at
Compound
Log P
ph 7
0.48
--I .67
HA
9(2
l&onaiolc
BW 9,33
Compound 'Iriaiiicina!irnc
Log P 5.08
7.46
Tcgoswroiie enaiilliaic
2.72
7'
347
347
6.93
1,93
Triarntcrcnc
Tria,oI.iin
((.57
'Frichloroacciuc
71)3
3.49
2.23
8.24
3,3)
0,38
10,42 1.1)5
(LOS
2.87
2.86
5.05
3.98
—0.21,
—0.4))
7.44
Thiopcnlal
3(8)
2.93
7.76
Thior)du,une
(.13
3.60
'rn' 'I
1)52
Thi;ulwndazolc 'Ehio%uanine
p.
9.38
2.19
5.65
3.15
3,88
Ticiurcillin
(1.1.9
—4.3)
lickipudinc
3.53
3.21
1)45
TiinoInI
—4.30
—1.99
5.7')
5.6)
—0.33
—3.64
3.62
Tholihan
4.14
1.64
3.37
li,'anidinc
0,65
-'1.47
—3.44
—10.)))
(1.76
-1)37 I
.86
13.38
8.86 6.71
11.23
9.18 13,07
'>52 8,10
11.40
Tola,,urnidc
1.71
((.47
Tolbulauuidc
2.34
)).80
liilcap.ine
4.15
1,98
4.78
1.55
—0.98
4.46
Tollcriudine
5,77
2.80
10(8)
10.7$
Topolccun
0.79
(1.55
7.2))
2.28
Thrernifenc
7.96
6.32
Ibrcmuk
3.17
0,53
2.5)
11.4))
Truiudolapril
4.53
1.41
(1.32
—2.19
1.21
--0,56
4,06
4.06
acid Tr.uuuykyprniiulnc
Trai.odmc
(.66
1.52
Trcpro.aunil
4.09
1)47
Trelintuin
6.83
4.62
Tiiacct(n
5.06 ((.3)
TflfliCIhiilluniauiIidc
2,91
I'riuneUioPrIuiu
11.79
rrirnir.TluuTiiuic
4.8)
rr)woludinr
4.44
3.36
iropicunildc
1,16
Undecylenic iucud
3.99
.57
4.63
isupnipyl lrL.u
—2.11
Urni.duu,l
4i,6
V.uluucyc(avir
(1.4(1
V&uldccon,l.
I 44
VaI,iauicicloi'jr
0.36
Vaipruic acid
2.72
Viulujihicin
5.25
VcnI.i6uxinc (Id
291 4.9)
Vcrapamul
Tu(nafliuc
Trauuuadol
13.01
7,05
—1)51
11.86
964 944
2.62
Tiludronic ucid
u-Thcoplicml
5.11
3.80
'% —3,73
Tiopruiiin
Tnfluopcra,ine Trifluridnuc
966
3.8')
Tuocuna,ole
5.82
3.58
3.09
.67
Trickusan
7.82
—1.23
Thiothixen
2,67
8.63
3.(H
Vudaruhinc
—' I 46
Vinblaslinc
.4.22
Vuncridine
2.84
Viuiorelhiiue
542 4.3$
Vuurieorua,uilc
9.16 3.72
0.72
4.80
55)
4.9) Yohirnhuuue
1.91
(.15 8.7%
13.50
—1,51
I 00
Zuleplon 6.59
3.)')
7.alcitahine
—375
Zwuuunicir Zulcuuun
3 74
i(iOflL
—(1.24
—0.24
'I'namcinolune
1.03
(.03
11.5$
Triiurncinoluine acclurnide
260
2.60
12.69
Fijaincinolurne diacelale
.82
1.82
11.21
4.02 .,cud
'/.oImI)rlpiim Zol1iidem 7AinjsunudC
— 2.28
64 2,1,1
- (III)
a
aaa
I-
INDEX
Note: Page numbers tollowed by "r' indicate ligures: those followed by '1" indicate tables. l)nig.'. are listed under the generic ttattte.
A Abbokinase
lirokinase
AI'cixintah. 190. 634. 860) Abelcet. Ste Amphotericin I) Al' India methods. 938 Alsontlacuents, 795
Absolute ethanol. 220 Acathose. 672 Accutane. See Is,utretinoin
ACE. St. Angiotensin-converling enzyme Acehatolol. 544 -545. 5451 ACE inhibitors .Ievelopntent of. pruidnig loans ot. (v46-M%, 6471, 647t Acellular vaccines. 2(17-208 Acetaminophen. 76 Ii. 762. 822 aracltidonic acid metabolism and. 822 mecltanisnu of action ol, 822 metabolism u,1, '16—98, 112. 115. 120 Acetanilid. 76(1, 7611, 762 Acetan.ulatnide. 64)3. 6041. 605. 619 Acetohevantide, 668, 669-674) active tneta)xul,tes a), I 35t metabolism 1,1. 82. 103— lOS Acet.tphenone. memabolisut ci). 1113 761t, 762
Acetosyedtyl uuttium '.alt'. 557t Acetoxyphenylmetcttry, 23(1 2-Acetylantinotluomene I AAF) metabolism o1. 96 toxicity or, 115—116 Acetylatiu'n. in drug metabolism. 12) — (24. 4231
mciaUcdinic differences in. (24 Acetylation ,xtl) mturphmistuu. 122— 124
Acetylcholine )AChl. 548 coaliunuation of, 34—35, 341, 555-556 ganglionic stimulation by. 5871 hydrolysis a1, 561—563. 561t. 562). 5631 tunscarinic receptors and, 550. 551. 552. 557—55%. 5571. 557t
netirotttttscular jancluon transmissi,un and. 589
as neurotranstttitter. 548. 551). 551, 552. 557—558. 5571. 557t. 586—588. 587) nicotitutc effect of. 587—588,59(1 phamiacetnical. 558 release iii, .5531. 554. 683 storage of. 5531. 554 structure—activity relationships for. 557- 558 structure of. 55(1 syn)hes.s of. .553—554. 5531. 5541
Acetyl-CoA. 55 554. 5541 tn drug metabolism. 122 Acetylntcthadol. active ntetabolites iii. 135t metaholisttt of. 124 Acetylsalicylic acid. Sn' Aspiriti Acltrontyctn See Tetracycline
Adapin. Ste Doxepitu hydrocltloride Adaplive ittttttanity. 2(11), 2)88 Adaptogen. 913 A,ldictiutn liability. 732 Addisons disease. 8)0
Acid(s), 9—17
Adenosine. as sleep-promoting agent. 48% Adenosine arahintusi,le. 376, 405. 408 Aclentusine deamina.se inhibitors. 40% Adenosine ntonophuspltate )AMP), 551. 553 it, snuctoth muscle relaxation. 623—624. 624f
1311
)ionieed), IS—lb. 161
crunjugate. 9 delunitiot, cut. 9
examples of. 1(8
IIA )unionieedt, 15-16. 16t ionic form iii. IS—lb. (5). 161 pH of. calculation 'if. 13
S-Adenosylituethionine )SAM), in ntethylation.
pK, ol. 13—14. 14t
A[)MET properties
125. 1261
Adenylute cycla.se. 553
strength of. II — 14 Acid—base balance. 3
defiutitiout of. 54. 61) screening for. 54
Acid—base reactions. Il—IS. 12t direction of, 13
l.ipinski Rule of Five for. 40. 54
Ac.d—ccuntngate base. 9—Il
Actpbes See Rabeprazohe sodium
Acitretin, 874 hall-life oh. 6 Acivicin. 421) Aclacunontycin A. 416 Aclanthtcin, 416 Aclometasone dipriupionate. 8081. 812 Acl,usate,S,'r Actattietas,,ne dipropioutate Acquired imtuunity, 2(11). 2(Xlt Acquired itttmunodeliciency syndronte. See Human itntttttttodeftciettcy virus Acrivastine, 714—715 Acrodynia. 891 Adrenocorticotropic horntoite AC'l'H
A(TH S,'. Repositcury corticotropiit injection Actbar.S,,' Coriicoto.pmn injection ACTH injection. 842. 842t Acthrel. See Corticorelin Actidil.S,',' Triproliditte hydrochlutritle Actinttttutte.Su'.' ltttrrferon gammaS lb Actinomycin C,. 414. 415. 421—422 Aciinomycin C,. 415 Actinomycin I), 414. 415 Actiutomycins. 3486. 414—415. 421—422 Action potential. 68(1—681, 683 Acttvase. See Alteplase Active analogue approach. 944
Activella.S,'e Hormone replacentent therapy Active-site—direcled irreversible inhibition. 29 Active tabular secretion. 60) -602, 6021
Acetylcholine chloride. 558
Aetcus. See Pioglitazone
Acetylchohine receptor. 549—551). 684—685 Acetylclttuhinc.ster.tw (ACIIEI. 54%, 553—554, 5531. 5541,56)1—561. 561t actiuun of. 561—563. 561t. 5621. 563t
Acaretic. See Quinapril Itydrocltloride Acute phase prtuteins. 21)). 201t Aeyclosir. 377
phosphorylalion iii. 568. 56Sf reactivation of. 5681. 569 Acety)cholittestcrase inhihitoo,563—569. 563)
Adenoltypopluysis. 1141
Acyla.scs, 306 Acylureudopeiticillins. 3118
Adalat. See Nifedipine Adapaletue. 874
prediction of. 944—945 uI recotttbinattt drugs. 175
virtual (in silico) scrcening for. 55. 9(9 Adrenitl cones horm,,ne,s. 803—815.
,,fsr,
(ilucocorticoid(s): Mineraloconicc,id)s I Adrenaline. 67%. See also Epinephrine Adrenergic agenis. 524—546 centr.tIIy acting. 652—653 definition oh. 52.1 sympatholytics. 524 sytttpadtotttimetics. 524 Adtenergtc-hhux-kung agents. 524, 651—652
Adrenergic nettrotr.tnsmittcrs.524—547 biosynthesis of. 524—525. 5241 properties of. 524 receptors for. 527—528 structure 01, 524 a.s symnpatttuumimetics, 532
uptake and nuetalxulistut of, 525—527. 5261 Adrenergie receptuur antagonisus. 539—546
a. 539-544) 541-546 Adrenergic receptturs
a. 527-52% 528
heterogeneity of. 169-li)), 171* Adrenergic stimulants. 524 Adrenergic system itthibimors. 649—650 Adreutoconicotropic Itonnoute )ACFH). 8(15, 841
biological activities of. 841 hydrocortisone and. 806 products. 842—844. %42t structure—activity relatuuunships for. 841 Adriantycin. Ste Donomuhicutu
Adriamycinol. 416 Advi). Ste Ihuprofen AenuBid. St.,' Fluttisuulide Aercu.sporitu. See Ptulymyxun Ii sulfite
Affinity chnumatography. in receptor isolaticuit. 2%
Ahiatosin I),. hepatuucawinogenicitv 'if, 76—77
957
958
Inde.s
African sleeping sickness. 260 Afrin. See Oxymetazoline Afnnol. See .1 + ).Pseudoephedrine
Alkylation. 394—398, 396—3991 biorcthtctive. 397
Agonism. inverse. 4115
by conjugalc addition, 397 definition of. 394 by free radical reactions. 397 mechanisms of. 394—398. 396f—399f reaction order in. 394 Alkylbcnzyldimethylammooium chloride. 225 Allegro. See Fcxolenadine Allergic rhinitis, 813—815
AIDS
Allergy
Autate, See Tolnut'tate Age. sirug metabolism and, 126—1211 Agenerasc Cr Aniprenavir
Agglutination, in immune response. 205 protein. 175 Aglycones. 417 cc Human imnsunodcficiency virus infection Akineton hydrochloride. Btpcridcn hydrochloride Alamust. See Pemirola.st potassium ophthalmic solution Albamycin. See Novobiocin sodium Albcrtdaaole. 266 Albumin(s), 8331 drag binding to. 6 Albuminoids, 8331 Albuterol. 536 Alcohol. 219—220 dchydr.tted. 220 intolerance to. cephalosporin.eclated, 325 mechanism of action of, 6114 Alcohol dehydrogenace',. 1(11
Alcohol promoictv. for prodrugs. 144—149. 14Sf— 149f
Alcohols as anti.infec(ivc agents, 219—221) carbamate derivatives of, 495—4% glacuronidation of, 112 oxidation of. 99—101 as sedativc.hvpnotirs. 495—4% Aldactaiide. See Spirisnolactoneltydrochlorothinzide Aldactone, Ste Spironolactonc Aldehyde carbonyls. metabolism of. 103—1117 Aldehydes as unti-intectives. 220—22) metabolism of, 99—101 as scdntise.hypnotics. 220—221 Aldeslcukin. 182—183,441—442, 859t. 1162 reductases. 103
Aldomet. See Methyldopa Aldomet ester. See Methyldopate Aldomct ester hydrochloride. Sec Methyldopate Aldophosphamide. metabolism of. 95—96 Aldosterone. Sec aI.vo Minentlocorticoid(s) analogues ot', 806—809. 806*
biological activities of. 805 biosynthesis of, 7691, 770. 804—805, 11041 excess of. 805
extrarenul actions of, 6)9 relative activity of. 809t ctnlcturC of. 807f Aldosterone antagonists. 6(9. 815 Alenttueumah. 189 Alfenta. See Alfentanil hydrochloride Alfcnionil hydrochloride. 748 Alferon N. See Interferon alfa-n3 Alida,se. Sec Hyaluronidase for injection Alitretinoin, 874 Alkaloid local anesthetics, 690 Alkcran. See Melphulan Alkylating agents, 394—102. See also Anlincoplastic agents activation of. 395—39*. 3961—3981
discovery and development of. 394 drug products. 399—402 mechanism of action of. 398—399 metabolism of. 395—396. 3')6f properties of. 394—399
toxicity of. 399
aspirin. 820—821
to cephalosporins. 325 to contrast agents. 481 to local anesthetics. 689—690 to pcnicillins. 308—309 .4 Ilium .ca:it'unt (garlic), 910—911
Allopunnol. 405. 414 All.or.notlting law. 680 Allylamine antilungals. 238—239 Ally) chloride. toxicily of. 1111—119 Allylisopropylacetamidc. metabolism oF. 77 Alomide. See Lodonamide trometitaminc Aipha-aslrenergic receptor antagonists. 539—540
Alphacetylmethudol, 738. 7391 Alphagan. See Brimonidinc
Alpha.paflicle emission. 456 Alphnprodinc. 736t. 737. 747—748 Alphaxalone, 488
Alprjiolam. 492 Alprostadil, 827 Aires. See Loteprednol etubonate Altace. Sec Ramipnl Alteplase. 184. 1841. 841). 859*
Altretamine. 429. 432 Alsipent. See Metaprolerenol Aluratc. See Amoba,bital sodium Alveolar nsacrophages. 198. (99* AM) method, 938 Atnantadine. 372—373
metabolism of. 92. 93, 126 Amaranth, metabolism of, (07 Atnaryl. Sec Glimepiride Ambenonium chloride. 565 Ambien. See Zolpidem AtnlSisotne. See Amphotcricin B Ambodryl Hydrochloride. See Beomodiphnnhydramine hydrochloride Anscill. See Ampicillin Amcinonidc. 8081. 812 100. 387 Amebiasis. 259—260 Amethocaine. 690-693. 6911 Amrthopterin. See Methotrexate
AMI.25. Sce Ferumoxides AMI.227. See Fenimoxtran Amidate. See Etomidate Amidec hydrolysis of. 109—110 metabolism of, 94—98 Amidopynnc. 7621. 763 Amifostine. 445. 446 Amigesic. See Salsalate Amikacin. 339—340 inactivation of. 336 Amikin. See Amikacin Amiloride hydmchlonde. 617—618. 620 Amiloride.hydrochlorothiazide. 620 Amines utietbylalion of. 12.5 prodnig forms of. (49. 1501 Amino acids, separation and identification of, 834
Amino acid sequences. 162 databases of. 39..40
Antinu acid solutioto. 830—831 Aminoalcohol esters, 579—582, 5112—583 Aminoalcohol ethers. 582—583
Aminoalcohols. 583—584 Aminoalkyl ethers. 702—7(44 Aminoamides. 584—585
p-Aminobenzoic acid (PABA). 901 metabolism of. 22. l23f in sulicylate preparations, 755 'y.Aminobutyric acid. 485. 489 receptor for. See also GABAA, receptors Aminoglutethimide. 784. 7841. 785 Aminoglycosides. 334—341
chemistry of. 335 mechanism of action of. 300*, 302. 335 microbial resistance to, 335—336 potency/toxicity ratios for, 337 side effects of, 335. 337—338 spectrum of activity of. 335 structure—activity relationships of. 336—337 structure of. 335 types of. 337—341. 341 $-Aminoketoncs. 501—502 p.Aminophenul. 761t, 762 Aminophenois, 760-762. 761t Aminopyrine. 762t. 763 Aminosulicylate sodium, 256—257 p-Aminocalicylic acid (PAS). 254. 256 metabolism of. 122. l23f Amiodarone. 641
Amitriptyline. 517 active metabolites of. 134. 135t metabolism of, 77f Amlralipine. 631 Ammoniated nietcufy. 22* Antobarbital. 494. 494t metabolism of. El Amobarbital sodium. 494, 494* Amodiaquine. 287f. 288 Amoxapine. 518 Amoxicillin. 309*, 313. See also Penicillin(s) Antonicillin-clavulanate, 316
Amonil. See Amoxtcillin AMP (adeno.cine monophosphate. 551. 553 Atophetamine. 512. 513 metabolism of, 70. 91, 92, 106—107 species differences in. 128 Amphicol. Sec Chlor.ttnphcnicol
Amphocil. See Amphotencin B Amphocyte. See Amphotencin B Amphotencin B. 236—237. 300t Amphonaic substances. II Ampicillin. 309L 312. See also Penicillints) allergy to. 309 extended activity oF, 307 prodrug form of. 143-144. 1441 spectrum of activity of, 307. 323 Atnpicillin.sulbactum. 316 Amprenavir. 385—387 development of. 942. 9431 Amrinonc. 656—657 Amsucrine, 429
Amyl nitrite. 625t. 626 Amylocainc. 678 Amytul. See Amobarbital Anabolic androgenic steroids.
Androge*s) Anabolin. See Nandrolone decanoate Anadamide. as sleep.promoting agent, 488 Anadml. See Oxymediolone Anafrunil. Set' Clomipramine hydrochloride Analeptics. 510—511 Analgesic udjavunts. 731. 732 Analgesics. 731—763 anti.inflammattt,ry. 753—763. Sec also Antiinflammatory analgesics
I,idt'x
dCfinhliOfl of. 73
development of. 940 prodrug forms of. 646—648, (i47f, 6471 Angiotensin II blockers. 648—649
dependence liability nI. 732
Angiolensins. 856
discovery and development of. 731,
Anhydron. See Cyclothiazide 5.6.Anhydrot.elracyclinc. 343. 3431', 344 Anileridine, 736t, 737. 748 Aniline. 2711. 760. 7611 tnctitbolistn of, 93, 122. 1231 Aniline derivatives, as local anesthetics,
classes iii. 731. 732 coal tar. 760. 7611
732—741
historical petspeclise on. 731 morphine and related compounds. 731—753. See uIso Morphine and related compounds receptor interactions with. 741—744. 7421. 7431
stnlcture-activity relationships for. 741—744. 7421. 7431
underutiliialion of. 731 —732 urinary. 253-254 Analgesiophores. 742—743
Analogue inhihitors. 384 Anamnestic response. 205. 2051 Ananase. See Bromelains Anaphyluxis.Sce Allergy Anapron. See Naproxen Anastroiote. 437—438, 71(4—71(5. 71(41
as antineopln.slic. 435
Ancef. Set' Cefazolin Aitcitabine. 407—408 Ancobon. Set' Flucytosinc Androgen(s). 797—1103
as antincoplastics. 434 athletic perforniance and. 800
biological activity of. 797. 798t biosynthesis of, 7691. 770, 7741. 775. 797. 7971
endogenous. 797
liepatotoxicity of, 798 metabolism of. 797, 79t)t products. 11(8)— 1(01
semisynthetic analogues. 798—799. 7991 side effects of. 799—800 structural classes 1)1. 798. 7991 structure—activity relationships for. 798—799. 7981
therapeutic uses ol, 799—8(8)
virilizing effects ol. 799—800 Androgen receptors. 773 Androslenedione biosynthesis of. 7691. 770 conversion ol to eslrone, 783. 7831
Anertine. See Succinylcholine chloride Anemia hypochromic. 1193 pernicious. 895 Anesthesia
cataleptic (dissociative). 488 cpidur.il. 6117
field block. 687 general. 485—488 inhalational. 486—487 iniraventius. 41(7—488 infiltration, (i117 intravenous general. 487—4118
regional. 687 local. 676—694. See also Local anesthetics in spinal anesthesia. 687 topical. 687 Anestltesine. Sit' Benzocaine Angina pectoris. 622 Angiogenesis inhibitors, 447 Angiography. 418—479. 4191 Angiotensin amide. 856 Angiotensin.converting eneynte. 856 in blood pressure regulation. 644—645, 6441. 6451
Angiotensin.converting enzyme inhibitors
690—693. ft92f. 1i92t Anilines. hydioxylated. 7(10—762 Anisidine. 7611. 762
Anisindionc. 668 Aninoactinomycins, 415 Annihilation radiation. 456 Anoilynes. 752 Antiplti'les mosquito. 282—283 Ansaid See Flurbiprofen Ansamycins. 257 Anspor. See Cephadrinc Antalgics. 753 Antaioline phosphate. 7(14. 706 Antergen. See Phcnbcnzamine Anthelmintics. 264—267 Anihi'mi.i (chamomile). 911 Anlltrucyclines. 415—417 Anthrucyclinnnes. 41(1 Antiadrenergics. 524 Antiandrogens. 801—802. 8021 Atitianginal agents. 622—633 Antianbythmic agents. 634—642 class I lmcmbranc.depressant). 636. 6361. 641
class II 4$-blockers). 6361. 637. 641
class Ill lrepolnxitaiion prolotigatots). 636u. (i37. 641—642
class IV (calcium channel blockers). 6361. 637. 642 pH and activity of. 637 types of. 637—642 Antibacterial antibiotics, 299—364. See also
Antibiotics synthetic. 247—252
Antibiotics aminoglycoside. 334—341 antibacterial. 299—3M synthetic. 247—252 antifungal. 235—238 antineuplastic. 414—424. Set' also Antineoplasuic agents, antibiotics antitubcrcular. 257—259. 338. 339 fl'lactain. 301—314. 301 —334. See also
Cephalosponna: Penicillin(s) fl.lactatnasc inhibitors. 314—318 hroad.spectntm. 3(1). 323 cephalosporins.3 18—334
chemical classification of. 301 conititcrcial production of. 300 current status of, 299—3(11)
definition 01. 299 discovery of. 299 historical perspective on. 299 (incumycins. 353—355 macrolide, 349—355 mechanism of action of. 3111—30!, 3118 niicrnbial resistance to. 301. 305—307. 335—336
monohaclam, 334 polyene. 235—238 polypeptide. 355—360 properties of. 299—300
spectrum of activity of. 300 structural diversity of. 301 teuucyclines. 341—349
959
unclassified, 360—3M uses of, 30(1 Antibodies. 2112, 21)3—2114. 21131. 21141. See alai.
lmntunoglobutin(s) attlino acid sequence of. 204. 2041' antigetis and, 205—206. 2051 complcmentunty.deterntinitig regions of. 188 human 442 hypervariubte regions ot. 188 tuonoclonal. 187—191 as anlineoplaslics. 442—4.14
chlmcric. 189 diagnostic. 470 preparation of, 187—189. 1881 in radionuclide test kits. 190— l')l
therapeutic. I'll types 01'. 189—191
pol>clonal. 87 production of, 204—205. 2051 structure itt', 203—2)14, 2041. 443. 4431' types of. 2116
Anticholinergics. See Cholinergic blocking agents
Aitticoagulants. 663—668 endogentius. 664—665 recombinant. 185
ani, conlonnation. 32. 33 AnticonvulsanLs503—5t)l( barbiturates, 504 bene.odiazepincs.Sttl—5l)8 ttyduntoins. 5(14—5(15
iitiscellatieous. 506—54)7 mintoacylunsas. 506 oxazolidittediones. 51)5 structure—aclisily relationships for. 51)3—501 succittimides, 505—5(16 ureas. 51)6 Atittdepressants, 5111
nietabolism of. 87 monoamine oxidace inhibitors, 514—SIft. 5151
inonoatitine reuptake inhibitors. 51(1
nonselcciis'e 5-I'll reuptake inhibitors. 519 selective norepincphnne retiptuke inhibitors, SIt) selective serotottin reuptake inhibitors. 518—5 9 tricyclic. 516—519 Antidiurelic hormone. Set' Vasopressin Anhiepileptics. Set' Anticonvalsants Antiestrogetis. 711—7112. 7811
Antifungal agents. 230—246 allytamine. 238—239 antibiotic. 235—238 azole. 24(1—245. 2411'
historical perspective on. 2311 nticleoside. 235
topical. 233-235. 239 Antigeti(s) cellular, in vaccine production. 207 major hlstocompatibitity. 197 Antigen-antibody reactions. 21)5—2)11,. 20Sf Antigen.prevenhing cells. 199. 1991
Antiltetnopllilic factor. 664t. 6(15. 863 reconthinant. 167—168. 1114—185. 665. 8591. 663
Antihistaminec. 700-715 dihenzocycloheplanes. 711—712. 1121 dibeneocycloheptenes. 711-712.1121
discovery and developnietit itt. 700. 71)1—71)2
drug interactions with. 7(12 dual.uciing. 717—718 ethanoluntine. 7(t2—104, 7(121 ethylenediamine. 7)14—706, 71)41
960
Indc
Atttihtstamincs (.onhintwd)
tico-generation (cla.ssical). 7(81—712 index
alkylating agents. 394—4)12. See alsi.
Alkylating agents
br, 7(11)
indications (or. 701 mast ccli stahilicers. 715—717 mcchattisttt of ai.'tic,n of. 7(X) tuetabolisin ot', 87 tton—targct.rcceptor interactions of. 701 pharniucokittctics of, 71)1 —71)2
pltenirumine. 707-7))) phenothiaiittc. 71)1—711 pipcr.u.inc (cyclwinc), 7(16—71)7, 7061 propylaminc. 707—710. 7071 second-generation Iflonsedating). 71)) —702. 7(2—7)5 relationships (or. 7(X)—i))?
Antihypcriipidcmic agents. 659—663 HMG-CoA reductase mhihitors, (,62—(i63 Antiitypcflensive agents, 642-657 ACE inhibitor prodnigs, 646—648. 647)•.
(slit adrenergic system inhtbttors. angiOtensin antagonislc. 6411- 649 cenirully acting adrenergie drugs. 652—653 neun ansn (cr-depleting. 650-4,5) positive inotropic agents. 655—657 ptltassinni channel openers. 654-655 renin-angiotettsin system inhibitors. 645—646 selective o.adrcncrgic antagottists. 651—652 vasixtilators. 653—654 Anti-infective agents, 2)7—2111) classificalion of. 217—2111
germicidal. 217-218. 2)16 historical perspective on. 2(7 selective tosicity 217 Anti-inflammatory analgesics. 754- 763 p.aminop(tcnol. 760—762. 76lt aniline. 760—762. 7611
aracltidonic acid metabolism attd. 822 arykicetic acid derivatives. 758—764) N.arylantlira?inic acids. 754—758 mechanism of action of, 8(8, 822 pyruiolidincdionc derivotives. 762—763. 76)) pyra,.olonc derivatives. 762—763 salicylates. 754—757
Antileprutic agent,. 27')—28(t Antitnalarials. 2112—298 4.atninoqtiinolones. 287—288. 2871. 2951 8-itmimx3uinolones. 288—289. 2891, 2951
cincliona alkaloids. 286—287, 2861. 295t
of, 28)
angiogenesis inhibitors, .147 antibiotics. 414—424
actinotttycins, 414 -4)5 antltracyclines. 415—417 discovery and developmctit of. 414. -115 ntedtanisiti of action of, 415 structure ol, 414—415 atttintctabolitcs. 402—.1l4.See of*., Antimetabolites antiscnse oligutners. 447—44%
aureolic acids. 417 hiotecltniilmmgy and. 4411.449
cattcer curability attd. 390 candidates for. 4.16—449
cell-cycle speciticity of. 39), 3911 clinical trials of, 394 itt conthination ttterupy. 390 cytoprotcctivc. 445—446 cytohmxicity nt. 390. 391 discovery and dcvelmsptnenl ol. 392—394
tirst-ordet log-kill Itypotitesis kit. 39) hurtttm,nes. 433-4)8 inmtunotherapentic. 340—442
labeliitg ittdex lot. 39) mechanism of action of. 390 mottoclonal antibodies, 442—144
overview of, 39)) plant products, 424—428, 9)5 platinttttt 428 prodrug. 156—159 radiotherapctttic. 444 -445 receptor tyrosinc kmnase mn(ithitors. 438-44(1 rescareh directions ('or, 446—449
resistance o,.392 screetting of, 392—394, 39))' signal transductiott itthihitors as. 438—440 site-specific delis-cry nt. 158—159 telomcr.ose inhibitors. 448, 441ff
tonicity iii. 392
2961
formulations ol, 285—298 in glucose-6-phosphatase dehydrogenase deticiency, 283 indications for, 295t—296t
of, 284-285
polvcyclic. 293—294. 2931. 296) fur prmmphylanis. 295t—296t
selective toxicity of, 285 fur treatmenL 295t—296t Antimanic agents. 503 Antimetaho)itcs. 41)2—.) Id activation of, 402—411
definition iii, 402 de novo synthesis of. 4)12—404, 4031—4041
development of, 404 dnig prmmdttcts. 411-414
tnechanicm of action of. 402—404 Antiminth. Sr.' Pyrattic) panloate Atttintuscarittic agents. Si-i' Choltnergic hlneking agents
Antitoxin. 2(17 Atttitubercular agents. 254 —259 antibiotic, 257 —259. 338, 339 Antitttssives. 752—753 Ant juIcer agents chetttical cotnplen&ttion of. 726 Itistarnine I-I. inhibitmirs. 71(1. 718- 722 pnistaglamtdins. 726
protott pump inhibitors. 722-726. 723f—725t. 723t Amttivert Sos' Medicine hydimidmlminile
Amttiviral agents. 375—388 analogue inltthitors. 31)4 himi.tctixation of, 135. 1351
hinclientical targets lor,370—372 cliettti,kine receptor bitiders, 387 des-elopment of. 370 DNA inhibitors. 372, 375—379 gp4l fusion nctivmty inhibitors. 387—388
WV entry inhihibirs. 387 lily pn1)easc inhthiturs. 384—387 deselupntetmt of, 942. 9431' intcgrase imt)tibito,rs. .188 ototnetmclature of. 372 iti,nnticlco,oide reverse tr.mttscrlpma.se
in(tihitors. 383—384 nucleoside antintettobolite,. 372. 375—382 as prodntgs
activation oiL (53-- 154. 15Sf in chemical detisery. 157 resistance to. 3(12 reset-se trunscriptasc itthihitors, 372. 379—381. 382 types of. 375—388 Anxio(ytics. hypitotics. and cedatises, 485—4% alcoltots, 495—496 barbiturates. 493—495 beitrcnliouepines. 488—493
tttiscellaneotis. 495 overview of. -(85 structure—activity relationships liii. 489—48) Apoilopoiprootcitis, 657
tutloor cell pnipenics attd. 39)4—394 Antittuelear antibodies. acetylation po)%-tttorphisttl and. (24 Antipedicular agents. 268 Antiplatelet agen(s, 632—634 Antiprotocoal agents. 259—264
Antipsychotics, 496 -5(t) 51)1—51(2
atitmmttattic agents. 5113
dosage of. 295i—2%t lined-combination. 289—292. 2891—2921.
mechanism of actiott new. 294—298. 291mm
Atmtineoplastic ;mgents.39))—449
atypical. 485. 49)1 beneamides.502—503 llttorobutyropltettoncs. 5(K)—SIt I mechatmismmt of action of, 497
phettotlttacines. 498-5181. 499t typical, 485. 497 Antipyretics, analgesic of. 753.5cc also Attti.inIl.mtnmatory analgesics Antipyritte. 7(,2. 763. 763t Atttiscahious agents. 268 Antisecretory agenls. 573—574 Antiseirtire agertis. See Anticonsulsants Aittisense uligoniers. 447—448 Antisetmse RNA. 193— (94 Atttmsense technology. 193—194 Antiseptics, 217—218. 218—223. 218t classification 0), 2(11, 21 Ill
el fedisetiess oil, evalitatioti of. 219 intpropcr tise ol. 219 plicitol cueflicicnt l'or. 221
Apottiiirphine ltydrodtlormde. 747—7411 Apupnitein. itt cytochrmutie P451). 67 Apoptosis. 39(1—39) Apr,tc lottidone. 534
Apresoline. Sic I-Iyolralaiitte Aprohart,ttal, 494, 4941
AqtIaMIiPHYTON See Phytonadione Aquaphor Sec Xipatnktc Aqtiasol A Sos' Vitamin A. L'SP Aquatensen See Mcthyc)ot)tiaiidc Clopamide At-a-C. Scm Cytar.thtnc
Ar.tchidontc acid. 4,66,818 tttclabotistii or. 82)1. 822 Arachidonmc acid cascade. 818, 8)91, 82)))' Ar,tleio. See Chtoromquitic
Aramine Soo' Met.or'.,mini,l Arcitutitottm.mh.
94)
Arduan. Sri' l'ipccunumn bromide Arecoline. 558 Arenas. oxidatioit ol. 74. 7)11, 72). 74f Artunnd Sc.' l'nntethaphon catitsylate Aribofloivimtosis. II'))
Aricept Sic Doneperil Ari,nidex. Aromasin Se.' Aroittiatase inltihitor'.. 783-785. 7831. 7841 Aruitiatic .itnines 01. 93
Atttithrot,tbitt Ill, 665
of, 93 Arontatic loydnocarbnlmi.s'arcittogctticit) (it, 7'
Antithrontbotic ngetmts. 632—634 Atttithyromid agents. 673—674
Aronm.ttic hydruvylution. 69- 74, 7111. 72). 74i
Antospasmodics. S73—574
metabolism 741'
lusdtts
Arrhythmias. 634—636 Arsenic Irtoxide. 428. 432 Arsohal. See Melarsoprol
Arlane. Sec 'lruhesyplienidvl bydrochlonde Anemisimn, 294—297, 294), 296t Aflcriogr.tphy. 478—479. 4791 Arleriiiscletitsis. 622 Arnhriiiciiie.Se.' Piperacine Arthrography. 481 Arthn'pan. See Choline salicylate Arvlacetic acid derisalises, 758—7611
N-Arylaiohralioic acids. 754—758 Arylonypropanolamines. 542
Acathioprine. 414 actise nietahu.lites of. l35t atttittttttor activity of. 41)5 metabolism of, 12(1 Aeelastine ltydrt.chloride ophthalmic solution. 7)7 Ae.es.tnupic isopropyl alcolnu). 221) Aeimilide. 642 AeJtltrotoscin. 352 Aelocillin. spectrum of actis'ilv '.1. 3(18 Aetoacort. See 'l'riatncinolone acetonide A,,n.'ltlor.iuotd Ste ('hlonua,.odin An. conupoands uttetabuilisutu of. l1)7—Il)8
Areoxifene. 71411. 782 Ascaruasis, 265
prodnigs and. 149—151). 1511
961
Beclovent Sue Beclunoetluasone dipropionate
Becon:tse Sit' Beclomethasone dipr..puu.naie Bee venoms. 835 Belladonna, 574— 575 Beuoidu.ne. 7361
Beoadryl.St',' I)ip)tenltyd.atttiuue Benanomycins. 246 Bet.aeepril hydn.chloride. 646. 6471. 6471 Bendroflnmeth.aeide. 605—611). bOat. hOSt, 6)9 Bentyl. See Dicyclotutine Itydrochl..ride Benealkonuam chlt.rude. 225 Beneanilid. 7611 Bencairocines. 74)1 BettceulrenSe.' Propylltexedritte Ileneetlrine Set' Attipl.eianilne IIeu.cesun.l. 777—778, 7781 Beneeiluonutinu cltluunde. 226
Ascorbic acid. 898—899 Ascorbic acid injection. 899
Ac.. dye.'. sullonantide. 269
Ascorbyl palinuatc. 899
Acolid. 763t Act' linkage. prodnugs and. 149—1511. 1511. 269 Acosemide. 6111
Beneimadole proton puntp inhibitors. 721-726. 7231. 723t. 72Sf
AZT Sue Zidosadine
Benenidaeole. 163 Bencoate esters, as local anesthetics, 677 Beiteocaine. 678. 69)1—693. 691t Beneodiaeepitte anlaginuists. 489 Beneodiaeeptnes. 488—493
Asendun. See Ainuusapiute -Asparaginase. 428-429. 431
Aspirin. 754- 757 as aniithromhouuc. 11$). 822 cardioproleclis-e ellects icC, (14), 822 hypersensitivity to. 8211—821 uutechanisun
of aclioit
atelaholistit 0).
uI.
822
I (8)
Aspro. Sc.' Aspirin Astemui'ole, 712. 7131 Asthma, 813-815 Atacand. See Candesaelan Atenolol. 54-4. 545. 5451 AHterosclerosu'., 622, 657
Ailun.nuhiuu-K.S.-.' Warfarin polassitmi Alivan. Sit' Loraeep.iun Atoisa.statin. 663
unetaholi',tn ii), 7)) Atosaqutone. 262 Atos'aqnone-progaanil. 2911. 292. 2921 Atrttcnnant hasylale. 59)
Atrial natriutdic (actor, mctahuulisnt ol, Ill Atromid-S Set' Cltulibrate Airopine. 574—576.5751 as antispasmodic. 574 as local anesthetic, 676—677 structture—aclisutv relationships lot. 572—573. 579 Alropine analognes. as local aoesthetics.
676-677 Atropine salLite, 576—577
See Auropine sallate Atruusenl .5.'.' Ipntlnupittm bromide Augmentun. See Clasnlanale-aunosicilliut Anreobasidins. 246 Anreolic acid. Si'r Plicamycin Anreontycin hydrochlonde. See Chhntetracycline hydrochloride Antonomic nervous system. .548. 586, 679 Asapro. See Irbesartan Asentyl. See Nortrupts4ine Avenneclins. 267 Avlosnllon.Se,' Dapsoute Asonex. See Interferon Ia Aoopusiul
Axepin Sue ('elepnne Axial cot.li.rtnations. 93), 9321 Asid. See Ni,at.dine Axoletoota. 679 Axon. 679. 6791. 681 Anon hillock, 679 Antuti telondria. 679, 6791 Ae.aclam. Sue Aetreonam disodiunu 5-Azacytldine. 4)18. 41)91
Aealides. 352 Aeapyrimidine nocleosides. 41)8. 41)91
Acaserine. 411
Acatadtne nualeate. III -7)2
Acole antifnngals, 2411—245
Actreonatut disodiuto. .134
B
Bacampicilliti. 3)2—3)3 doable-ester lonut iii, 147, 1481 Bacillus C;dntelte-Gadnn saccine. 2)4 as atitiuteoplaslic. 440, 442 Bacitnuciut. 3(Kli. 356—357 Ilactcrial rcsuoance, 3)11. 3(15 —34)7, 335—336 Baciericitlal lactors. 2)8)— 21)1. 21)11 Bacteriolysis. in i.nnuatue response. 2)15—21)6
Bacieriophage sectors. 165—166. 1661 Ilactrobatu c. Mnpirocun Baker's antifol, 411 HAL.
Diniercaprol
ilulotuutdt,u,,u
u.iii, 26))
Baltimore Clavsuf,cation Scheme. fur viruses, 367. 368t—371ti l3antltine hrt,,oide. Ste Metltantheline brontide
Barbital, 494 Barbiturates. 493 as antlconsnlsants, 504 inzertutediaie-duration. 494. 494t Iong-dnr.ition. 494. 4941 mechanism iii acliu,n of, 493
metabolism of. 76. 77.81.94 109. 493-494 sltort-daratiuin, 4941, 495 s..diiutn salts of. 4t)3 structnre—aclisily relalionships fix. 493—494
stroclare iii, 493- 494 Barium sulfate. 473, 481 Basetsj, 9—17 cuunjugate. t) —
II
delinirion ii), t)
exattiples ut. lIt ionic lorn. u). 15—16, 1Sf. 16i pH ol, calcalalion of. 13 pK. ol, 13—14, 14) strength tiE, 13— 14
Basic tachsun. 227 Basilusimab, 489 Basopltils, (98 l(aycauoa.Se.' Mefnaside
Baycol 5.'.' ('ensastarin Bayer 25(12. St'. Nifnnimox Baypress. See Nitrendipine
Iiaedoxileu.e. 78(1. 782 B cells. 118), 21)2—203
BCCi vaccine, 214 as auttineoplaslic, 441). 442 B('Nki S.'.' ('arrtunsltne 1K' Powder. See Salicylamide Becaplenom. 179 Beclomethasotte dipropionate. 812. 8(4. 8141
Beneimidaeuu)es. 741
ahsorpti.utt tel, 49)1
as anticonsalsants.507—308 cuuutuhinaluurial syntluesis u,f. 46, 461
discovers and developmeuut tel, 489
GABA5 receptors or. 488. 489 us intnuvenons anesthetics. 487 mecluattism tel action tul. 489—49))
ntetabolisin oi. 94, 49(1 protein binding tuE. 49)) struclttre—actis'ily relaiuuunships l.ur, 489—49))
sinuctnre of. 488—489. 489 Bencoic acid. 229. 234 unelaboltsn, ol, (14 Bencoic acid dertvalives. as local ane.sthetics. 690—693. 691u, 6921, 6931
Bencomorphan derivatives. 74)) Betteonatate. 753 Bencuel .rl'pyrene. carcinogetuiciuy ut, 74. 741 Ileneu.yltn.pine. 676—677, 6761 Benephetamine hydrochloride. 513 Bettethiaetde. 605—6)0. 606t. 6(1St. 619 Ben-atropitte otesylate..582 Iteneyl alcohol. 229 metabolism of, 89 Beneyl hencoate. 268 ReneW cltloride. toxicity ol. 118—119 l)eneylpenicillin.31)9—3 II). 309t See isis.. ltenieillin)s) Bepridil hydrochloride. 632 Ileraprost. 825 Beriberi. 886—887 Beiacetylmeuhad.uI. 739t I)eladine. St'. I'uusidone—tunline Bela elimination. pruutein. (73. 1741 Betagan. Sue Levobanolol Betantelhasone, 81)81, 8)l9t. 812
soltthility ,.), 7711t Itetatttelhmts..ne acetate. solnhilily of. 770t Belamethasone NaPO, salt. .soluhility 'uI, 7718 Betapace. Set' Sotalol Beta-particle emission. 456 Belaprodine, 7361, 737 Betaseron Interferon beta-tb Beraxu,ltul. 544, 545. 5451 Belhattechu,l chloride. 511) l(ett.puic See I(elasolu.l Besan.tene. 43)). 432, 874 Bextra. Si'.' Valdecoxih BH acids. 15-16. 161 Biapene.n. 3)8
962
Index
Biaxin. See Clanthromycin Bicalulamide. 431, 1(01—802, 8021 as antincoplastic, 434
with a>.receptor antagonist activity. 546 as antiarrhythmics. 636i. 637 cardioselective, 543—546
Ilicillin. Sic Penicillin 6 beneathine
itonselective. 542—343 fl1-selective. 343—546 structure—activity relationships for. 541—542 Illooth—brain barrier. 5 drug delis cry across, 15%. 1581
BICNT_I. See Carmustine
Bi.Est. See Estriol Biguattides. 226 Bilarcil. Set' Metrift,nutc fliliruhin, metabolism of. I 14—I IS Biltricide. See Binudoprosl. 828 Binning. 58. 61
Blood cells, lineage of. 177. 17sf. 197. 1981 Blood clotting. See Coagulation Blood pressure, regulation of. 642—MS. (>43f—645t
l3ioactivc conformatiott, 931) l3ioclate. Set' Antiheinophilic factor. recombinant Biulnlormatics. 191—19?
Biological activity chcmicul structure and. 17—21, 214,31—41 partition coefficient and. 17—21. 181. 19)
Biological receptor site. 29 Biological response
recotithinant,
194
Biotechnology. 160—194. 858—863 ant't.sense technology in. 193—194
hioitiformatics in. 191-192 cloning in, 164. 166—168 development of. 160. 1611
DNA expression in. l(,7— 68
DNA hybridwttion in. lti6 DNA liga.scs in, 165 DNA microarrays in, 192—193 DNA sequence alteration in. 168—169. 1691 in drug development. 16(1—162.
621.
169— 172
in drug screening. 170—172
enzyme heterogeneity and, 169—70. hOt epitope tagging itt, 69 fundamental techniques of. 163—164 gene expression systems in. 167— 16$ gene therapy in. 194
genetic engineering in, 162—66 genutnic libraries in. 164. 164) genoinics in. 191—193 hybeidouna techniques iii. 187—189. 1881
literature of. 160 tivcrview of. 160 pharntaceutical products lrom, 859t—86(lt. 860—863
pharmacogenomics in, 193 protein processing in. 172 protein synthesis in. 64. 168— 69 protcmtiics in. 193 receptor heterogeneity and, 169—17(1. hIlt recombinant DNA in. 162—166 recombinant proteins in. 164—169. 1691 restriction endimuclea.ses in, 164—loS, 1651. I 65t
subdisciphinc>. of. 161 vectors in. 163—166. 1661 Biohin, 899—9(8)
Biotr.insfonnation. Se,' flr.ag metabolism Bipcridcn Itydroclilonde. 583 Birth control. Sit' Contraceptives Bisguanidines. 672 Bis.N'demethylatcd tt>etabolite ol at-). meihadol. acetylatiimn oF. 122. 1231 Bisoprimlol. 544. 5451
Hithionol. 266 Bum. See Bithionol Bitollerol. 537 Biuret test. 834 Blenonanc. Set' Bleomycin sulfate Bleomycin sulfate. 417—419. 423 Blocadren. See Timulol a-Blockers. 541—546
Blood proteins. 857—858
HMY-25067. 420 Bogus-coin detection, in combinatorial cltcmistrv. 5(1 Boltzntann equation, 931 Bombesin, 835 Bonds. 29—33, 31t, 321, 33f force field calculations for. 923—929 hydrophobic. 831 representation of. See Molecular modeling Bonine. See Meclizine hydrochloride Born'Oppenheiuner theorem, 923 Botaniculs. Ste Herbal medicine.' Bowman's spare, 596, 597 BR. 17 Brudykinin. 856—857 in blood pressure regulation. 644—645. 6441 Brain, drug delivery to. 158, 1581 Breast cancer estrogens for. 779. 782
honnone dependency of. 433-434. 783, 793 progestins for. 787 Brcihitte. Set' Terbutaline Brctyliutti tosylate as .idrenergic agent. 529—530 as amitiarrhythmic. 641
Bretylol Sit' Bretyliunt tosylate Brcs'ibloc, Sit' E,smolol Brevital Sodium. See ..odtumn l3ricanyl Set' Terbntaline Brinionidine. 534 Brinaldix. See Chiipainide British antt-l.ewtsite. See Dimercaprol Broad-spectrum antibiotics. 300 llromelains. 840 Bromobeneette. hepatotomimcmty of. 73—74
Bromtmdipltenltydramine hydrochloride. 702. 703
Isrompheniramine. 71)9 nmetnbolisnt of. 115, 92, 117
Bronsted.Lowry theory.') Brookhaven Protein Database, 39 Itucladin'S. Set' Buchizine hydrochloride Buclizine hydrochloride. 707 Budesonide. 8091, 812. 1114, 8141 Bumetanide, (>10—613.6111.620
Buntex Sit' Ilutnctantde lltmnolol. ttmetaboltsnt of, 1051 Bupivacainc. 678. 690—693. (>92) Iluprenes. See Buprenorphine Buprenorpltine. 741. 1St) Bttpropion. 32(1 Bitrimantide, 7 19—720. 7191 Burnemi. Ste Bumetunide BuSpar. Set' Buspirotte Buspititne, 52(1
Busulian, .Utl lhtttuhart>ital sodmuttm. 494. 494t I)utacaine sulpltattt, 690—1,93. 6911
Butaprost. 824t Butaittltdin. 7631 Butisol Sodium. Si',' Butabarbital sodiiuitm Butoconatulc titrate. 241—242
Buttmrpltaimol turtraie. 7411, 743. 750 .V-t'Butylnimrchlon>cycli-eimtc. tntttabolismtt ol. 86—87
limitylparaben. 229 Butyrylcttolinesier.tsc (IltiChE), SoIl—SOt, SOIl BW245C, 8241
llW,\ 86)tC. 824t C Caerulcmn. 833
Caffeine. 511—512. SlIt Calan. Sit' Ver.tpanmil Calcifedmol. 878
t'alcipotriene. 878—879 Calcitonin. 835 —856 Calcitrimml. 878
Calcium, in vitamin D synthesis. 875—876 Calcium channel blockers. 627—632 as antiarrhytlitnics. 629. liMit. 637
lirs.gcttcr,ition, 629t ttiechanistn .,f action
628—629
sccimttil'gcneratiomt. 6291 types of. (>29—632
as vas,jdilatttrs. 629—632 Calcium>> itmn(sI
in muscle rtmnlruciiitttlrelasatitm. 673—624. 6241. 627—628 properties 1>1. 627—628 as second mtmessengers, 627 -628 Calcitmm ion chanmtels. 628, 6281. (>82. Ste mmli,> ktn cluttmmels
acetylcholine and. 643 Calciuttt pantotlmrnate. 888 Caliclteamicin. 421 cAMP. 531. 353 itt stttsuimh muscle retasation, 623 Campath. Sri' Aletntuzuutiab cAMP rcspommse elenietmt (CRI1l. in drug
screening. 171, 1711 Camptothecin. 426 Cancer breast estrogens for, 779, 782 hormtmonc dependency uI. 433—433, 783. 793
progestins for, 787 cellular abnormalities in. 39(1- 391 cftetnomherapy for. Set' Aimtiueoplastic agents
cnrahility of, 390 in DES daughters. 779 drug resistance in. 392 erudomeirial. estrogens and, 779. 787 lamlure of apitptosis itt, 39) herbal nmcdicines for. 915 inttttuntmtltv'rapy fu,r, 440—442 nmetastasis in. 446—447 prostate jmtmtauudntgen.s for. 81)1—802. 1(021
estrrugeits for. 779 tutnor cell properties it>, 390—394
viral infectious and. 372 Cande'.artamt. (>414-649 Cunnahunoids, as sleep.prumntomiumg agettts, 488
C,nttil Sir
brotitide
Capaatat sulfate. 259 Capecitahimme. 4117,413
as protlnmg, 136—137, lS7f Capillary' electroplmoresis—coupled tuclear magnetic resonance spectroseopy. SI. 61
Capoten. Si'.' Captopnl ('apretimycin, 259 antttubercuhutims activity oh. 254 Cmmpromah peimdetide, 191
Capsaicin, 911>
Inde.v Capsiculn. 910 Captopril. 6.45, 6461
design ol.
94*)
CaralateSi't' Sticritlfate Curbacephems, 327 Carbachol. 559—560, 562
Car Ltna,epine. 506 active nietabolites of. 135i metabolism of. 75, 109 Carhainide pcronide topical solution, 223 Carhamylated alcohobi. 495—496 Carbamyl phosphate. 900 Carbapenerns. 316—31K iutvestigationul. 31K
CarbeniciHin disodium. 309t, 3)3.5cc also Penicillin(s) metabolism of. 09 spectrum of activity of. 307- 31)8 Carbenicillin indanyl sodium. 3091, 313—314 Carbeniuin ions, 395 Carbetapentane citrate, 753 Carbinoxamine inaleate, 702. 703—71)4 Carbocaine. See Mepivacaine Carbohydrates, combinatorial synthesis of. 47. 471
Carbolic acid, 217. 22) corrosiveness of, 14—15 f3.Carbolincs. 4149 Carbonic unhydrass', in renal sodium transport. 59*4. 5981
Carbonic unhydrase inhibitors, 603—605, 6041 preparations of. 6)9 Carbunyl promoieties. 1511—152. 1511 Carboplatin, 428. 43) Carboprost trontetharnine. 795. 7951. 827 Carboxide. 220 y.Carbonyl —glutatitic acid. 883—884 Carbonyhic acid
conjugation iii. 1)7 promoieties of. 144—149. 14Sf— 1491 for prodnigs. 144—149. 14Sf— 1491
Carcinogenicity of atlatovin B. 76—77 of amitIes. 96 of aromatic umines. 93 Cardene. St-c Nicandipine hydrochloride Cardiac arrhytlitnia.s. 634—636
Cardiac eleciropltysiology. 635. 635)' Cardiac glycoside.s. 655—657
Cardiac muscarinic receptors, 551 Cardilate. See Erythrityl tetranitrate. diluted Cardioquin.Sr'e Quinidine polygalacturonate ('ardioselective 543—546 Cardiovascular agents, 622—674 antianginals. 622—627 untiarrhytltmics, 634—642 anticoagulant'.. 663—668 aniihypertrnsive.s. 642—657 anlihipittemics. 657—662 antiihyroid agents. 673—674 thyroid hormones, 673 s'asodilutors, 622—634 Cardiovascular disease, 622—623. (sZ3f Cardizem. St'e Diltia.retn Cardura. See
Carisoprodid. 496 Carmiituniycin. 415 Carmustine. 399. 41)1
decomposition of. 395. 3i)hf Caromec Sm' lvcrmcctin a.Carotctie. 869—870 a-Carotene, 869—870. Sec ti/si, Vitamin A excess of. 87) fond sources of, 869—Kill
product. 1475
iii vitatititi A synthesis, 869—870 gamma.Carotcne. 869—Kit) Carotenoids. 869—870. Sec also Vitamin A absorption of, 870 land sources of, 869 structure—activity relationships for. 869—87)) in sitammn A synthesis. 869—870 Carteolol. 543. 5441 Cartrol, See Cunleotol Carvedilol. 546. 5461 Casodes See Bicalutamidc Catatlam. Sec I)iclofenac potassium Cataleptic aiiesttiesia. 488 Catalysis, en'eymatic. 835—837. 8361. 8371
('atapren. See Clonidine hydrochloride Culecholamines. 524—547. Sec also Adreuergic neurs,transmitters adrenergic receptors and. 527—528 biosynthesis of. 524—525. 5241
drugs affecting, 52)) propenies of. 524 receptor', for. 527—528 storage and release of, drugs affecting. 529 structure 01'. 524
as sympathomimetics, 532 uptake and metabolism of. 525—527, 526f Catechol.O.niethykransl'eruse (COMT). 125—126. 526—527. 5261
Catechols. methylation of. 125—126 Cationic dyes. 227—228 Cationie surfactants. 224—227 Cavcrject. See Alprostadil
CC-l065, 420, 42))' CCNIJ. See Lomustine eDNA. in combinatorial chemistry, 49 eDNA libraries. 164. 164t CEA-Scau. Se'i' Arcituinomab Cehione. See Ascorbic acid Ceclor. See Cefaclor Cedax. Set' Ceftibuten CeeNtJ. See Lottsustiiie Cefaclor, 32(8, 326—327, 326t Cefadrosil. 3201. 326, 326t Cefadyl. See Cefamandole nafate, 32 It. 3261. 328 Cefarolin, 3211. 324. 327—328. 327* Cefepime. 333 Ceftxime. 320t. 326t, 33) Cefn,eta,ole sodium, 322*. 326t. 330 Cefobid. Sec Cefoperuzone sodium
Cefonicid sodium. 3211, 326*328 Cefopenienne sodium. 3211. 324. 325. 3261, 329 Ceforanide. 3211. 3261, 328—329
Ceiolan. Ste Cefotetan disodium Celotaxitne sodium. 321t. 3261, 331 Celotetan disodium, 322*, 3261, 329—331)
Cefoxitin. 324 Celositin sodiutti. 3224. 3261. 329 Cefplruiite. 332—333 ('el'podoximc pronetil. 320*. 323, 326*. 3311—331
doable-ester form of. 147, t48l' hydrolysis of. 147. t47f Cefprozil. 320*. 326*. 327 Cefrom. See Ccl'pirume Celtazidime sodium. 321t. 326*. 332 Cef)ihuten. 332 Ceftin. See Celtinixime axetil CeUiioxitttc sodium. 32 It. 326*, 331 Ceftriasotie disoditint, 320, 3261. 331—332 Cct'uroxime axetil. 32 It. 323.3261. 330 double.ester latin of, t47. 1481 Cefuroxitne sodium. 32 II. 326*. 330
963
Cefail. See Cefprozil Celebres, See Celecoxib Cclcconib. 76*). 822—823 metabolism of. 77 Celexa. See Citaloprain Cell cycle, 391. 3911 Cell death, programmed. 39(1—391
Celt.medlated imniunity. 202—203 Cell membrane. See Membrane(s) Cellular antigens, in exceinc praductiun. 207 Cellular immunity. 20*) Cellular retinol-hinding protein (CRBP), 869 Celontirt, See Methsuximide Cenestin. See' Estrogen(s) Central dogma. 162
Central nervous system. 548. 679 Central nervous system dcpressant.s. 485—508 unticonvulsants/antiepileptics. 485. 503—508 atitipsychotics, 485. 496—503
anxiotytics, sedatives and hypnotics. 485—4% general anesthetics. 4115—48)4
overview of. 485 Central nervous system stimulants. S10—S22 $'amylamino hallucinogens. 520—521 antidepressants. 514—520 central synipathominietics. 510. 512—514. 5121
dissociative agents. 522 mcthylxnnthincs, 511—512 Central sympathomimetic agents. 510. 512—514. 5121 Cephadrine. 320*. 325 —326. 326t Cepbalexin. 320*. 325. 3261
Cephaloridinc. 324 Cephalosporic acids. 322 Cephalosporitts. 318—334 acid resistance ti), 326* adverse reacttons to, 325 antipsendomonal. 325, 326t
catechol.containing. 333 classification of, 325 degradation of. 3)9—322. 3231 discovery and development of. 319 drug interactions with. 325 first-generation. 325. 3261 fourth.generation, 325, 3264 future des'elopntcnts for. 333—334 historical perspective on. 3 18—319 fl.lactama.se resistance of. 323—325. 3241 /3'lactrun of. 322 mechanism of action of, 31)05 MTT.group, adverse reactions to. 325 nomenclature for. 3)9 oral. 3201. 322—323. 326* parenteral. 320t—322t. 323, 326t
prodrug forms of, 147, l48f protein binding of. 3261 research directions for, 333—334 second'gener.tiion. 325, 3261 sentisynthetic, 3)9 spectrum of activity of. 323. 3261 structure—activity relationships for. 319 structure of, 3201—3224
third.generation. 325. 326t types of. 325—334
Cephalothin, 320*, 324, 326), 327, 328 Cephamycins, 329. 330 Cephapirin sodium, 3201. 326*. 328 Cephradine. 323 Ceramic beads, in citmbinatorial chemistry. 49. 60
Cerezyme. See Imiglucerase Cerivastatin. 663 Cerubidine. See Daunorubicin
964 Cerva'etn Set' Genteprost Cervidil. Set' Prostaglandin Cestocide. See Niclosamidc Cestode infcstations, 264—265 Cetirt,ine, 714
Cetylpyridinitim chlonde. 226 Ccvitamic Acid. Scr Ascorbic acid cGMP. in smooth muscle relaxation, 2641. 623—624
Chagas' disease. 260 Chamomile, 911
CItation's steric parameter Vt). 21 Chelalmg agents. 463 Chemical bonds, 29—il. ut. 331, 341 force field calculations for. 923—929 Chemical contraceptives. See Contraceptives Chemical databases, searching methods for. 39—40. 55—56. 930-933 Chemical diversity definition of, 61 quantification of. 56—58 Chemical libraries, 43 generic. 43. 441 mixture, 43. 441 Chemical structures databases of. St't' Chemical databases drug—receptor interactions and. 31—41
models of. Se.' Molecular modeling physiologic activity and. 17—21. 28. 31-41. Set.' also Drug—receptor interactions Chemotactic factors. 200 Chemotherapy prodrugs in. 156—159 site-specific delivery of. 158—159 Clttckenpoz vaccine. 211. 2l2t Chili pepper. 910 Chitneric antibodies. 189 Chimeric proteins. 168—169 in drag screening, 172 Chinosol. Set' 8-Ilydroxyquinolinc CItloral hydrate. 496 active mctaholites of. 135t metabolism of. 103 Clilorambucil. 4181-401 Chloraminophcnamide. 6041 Chlornminophenc. 400—401 Chlort,mphenicol. 3001. 360—361 metabolism of, 101—103, 107. 112—114 as prtxlrug. 4—5. 142, 1431
solubility of. 4 taste stf. 4
Chloramphcnicol palmitate. 361 as prodrug. 4—5
soluhilily of. 4 Clilorampttcnicol sodium succinatc. 361 Chlorvyclizine hydrochloride. 706. 707 ('hlordiazcpostde hydrochloride. 489. 490 Chlorhexidine gluconate. 226—227 Chlorinated pesticides, drug metabolism and. 131
Chlorine-containing germicides. 223 —224
Cl,lorua,odin. 224 Cltlorobutanol. 229 Chtorucresol, 222 Chlorodiphenhydraminc. 702 Chlorolomt, metabolistu of. 101 p.Chloro-m-nylenol 221 Chloromycetin. Set' Chlorstmphcnicol p-Chlorophenol. 221 Chloroprucaine hydrochlortde. 690—693. 691t Chloroquine. 287—288. 287$. 295* Cltlorolltiaiide. 605—61(1, (tOôt. 608t. 619 Clilorphenesin carbatnate. 495—496
Chlorpheitiramine. 7014 ntetubolism of, 1(13, 114
Chlorphc,ttcrmine. melabolism of. 92 Chlorpromazinc. 498-499. 499* active metabolites of, l3St metuhol,sm of, 71. 85, 87 Chlorpropamide, 668. 669 nietabolism of, 81.')4 Chlorictracycline hydrochloride, 345*. 346 Chlorthalidone, 269. 607—610, 6071, 6091, 6l9t Cl,lor-Trittteton. See Chlorphenirantine Cholangiograplty. 479—480 Cholectilciferol. 875, 877 preparations oF. 878 Citolecystography. 479—48()
Cholecystokinin'panereozymin (CCK-PZ). 854—855
Cholera vaccine, 212*. 214—215 Cholesterol elevated levels of. See Hyperlipoproteinemia
in lipid tnenihrane. 231. 2321 soluhility of, 770* steroid synthesis from. 768—770. 7(191 Citolestyramine resin. 660—661 Choline. 901 Choline acetyliransferase (ChAT). 553—554. 55Sf. 5541
Cholinergic blocking agents. 548. 572—586 ttnttnoalcohol esters, 579—582 antinoalcohol ethers, 582—583 auninoalcohols. 583—584 amittoantides. 584—585 atttisccretory effects of. 573—574 antispasmodic effects of. 573—574 drug produco. 575—586
indications for. 574 miscellaiieous. 585-586 mydriatic effects of. 573—574 solanstceoas alkaloid.s and analogues. 574—579
structure—activily relationships for. 572—573. 579
structure of, 575 synthetic. 579—582 therapeutic actions of. 573—574
Cholinergic drugs. cholinergic blocking agents. 548. 572—586 cltolinergic receptor antagonists. 558—572 ganglionic hlockittg agenls. 586—589 neuromuscular blockittg agents. 589—595 stereochentistry of, 555—556. 55Sf. 555t, 5561. 551st
Cholinergic nerves. 548 Cholinergic neurochemistry. 553—554 Cholinergic receptor antagonists. 558—572. 559,.
cltolinestcrase inhibitors, 560—569
irreversible, 567-569 reversible. 560—567 Cholinergic receptors. 548—553
acetylcholine th/:ra,is conformation and. 34—35, 341
activation of. 552. 5521 muscarinic. 550—553. 551f, 5521 nicotinic, 548—550, 5491. 5491 Cholinergic stereochemistry. 555—556. 5551. 555t. 556f. 5561
Choline salicylate. 755 Cholinesterase inhibitors. 563—569, 563t Cholinestera.ses. 560—563
phosphorylation of. 568. 561lf reactivation of. 5681, 569 Cholinolytic agents. See Cholittergic blocking agents
Choloxin. See Dextrothyroxine sodiutn
Choritmic growth-hormone prolactin, ((45 Christmas factor, recombinant, 185 Chromatography affinity. tn receptor isolatioti. 28 itt cotnbinatorial cltemistty. SI higlt-peth,rmance liquid. 51. 833 ion exchange. 834 paper. 834
supereritical Iluid. SI Chromic phosphate P 32. 444—445
Cltnimomyctn. 417 Chromosome walking. 167 Chrysin. 784. 7l14f Chyloinicrons, 657—658. 869 Chymar. See Chymottypsin Chymottypsin. 838. 839* Cibalith'S. S.'t' Lithium citrate Cicaprosl. 824* Ciclopirox olantinc. 234—235 Cidex. See Glutaraldehyde Cudotovir, 378—379
Cigarette smoking. drug metabolism and. lii Cilastin-intipeneni, 317—318 Cimetidine, 71'). 7191, 72(1—121, 720*. 721t
nietabolism of, 99, 101 Cinchocuine, 678, 690—693, 692t Cinchona alkaloids, 286—287. 2861
Cinchotiisnt. 286 Cinobac. See Cinoxacin Cinoxacin. 248-250 Ciprolloxacin. 248, 248r, 249—254) cit isotners, 31—32 of acetylcholine. 34—35, 341 Cisplattn. 4311—431
Cispro. See Insulin injectt.m Citalopram, 519 Citmvoruo, factor, 41t) Cladribtne, 405, 412 C'laforan. See Cefotaxinie sodiutti Claritltromycin, 351—352 Classification tecltnicjues. 24—26 Clavulanate-antoxicillin. 316 Clavulanate potassium. 316 Clavulattatc-ticurcillin. 316 Clavulanic acid. 315. 3151, 316 Clays. as contrast agents. 477 Cleavage reactions, in combinatorial chemist,). 49,61 Clema.stine lumaraic. 702, 704 Cleocmn. Set' Clindamycin hydrocltloride Cleocin Pediatric. .S't',' Cltndamycin palmitate hydrochloride Cleocin l'hosphate. Sc.' Clindatnycttt phosphate Clidiniuni bromide. 579—58(1 Clindamycin hydrochloride, 354 Clindamycin palmitate hydrochloride. 355 Cttndaunycin phosphate. 355 as prodnig. 149. 1501 Clinical trials, of atttineoplustic agents, 394 Clinoril. St-c Sulindac Cltoquinol. 234 Clistin. Set' Carbinoxamine tnalente Clobenprobit, 728—729. 729f Clobeta.sol propionate. %08f. 81$)t. 812
Clocorlolone pivalate. 8081, 812 ('loderm .5,'.' Clocortolonc pivalttte Clolazimine. 257 Clofibrate. 659 active metaholites of, 135* metabolism of, $09 Cknnid. See Clomiphene citrale Clontiphene citrate, 781. 781 f. 782. 783 Clomipramine hydrochloride. 517 Clonaiepam. 5(18 metabolism of, 107
!ttdr'.t
Clontdtue hydrochlonde.533—534. 653 metabolism ol, 70
('Inning, 164. 166-68. 861). Ste a/it; Biolechnitlogy: Recotnhinanl l)NA technology applications 01, 167—168
cI)NA libraries in. 164. 164t DNA ligases in. 164—165 Innetional expression. 166—167, l67t genoniic libraries in, 164, 164t homology-based. 167, 1671 host cells in. 166 methods of, 164, 1641 positional. 167, 167i promoters in. 168 receplor. 28 eeslrcton endsittucleases in. 164—165 slept in. 858—861) vLctOIs in. 165—166.
Comhidex. See Ferutttoxtr.ut Cttmbinatorial chemistry. 26—27. 43—h3 analytical techniques in. 5 1—52 detection in. 50 carbohydrates in, 47. 47f chromatography in. SI eleitsage reactions in. 49. 61 tkconvoluuion in. 26. 271. 61 development ol. 43 tttolecules in. 46—48. 461. 471 effectiveness of, 58—60 litur-cttmponent tJgi reactittn in. 49. SOf goal of, 43
hotnology tnodrling in. 56 infrared spectroscopy in. SI iterative deconvolution in. 50 lead structures in. 59—60, 591,61-62 libraries in. 26—27
661. 168
('lopamide. 607—610. 607f. 609). 619 Clopitlogrel. 633 Clttproslenol sodium. 828—829 Clttraeepate diptstassiuto as unticonvulsunl. 5118 as anniolytic. 491 Clorexolone. 607—610. 6071. 609), 619 Cloepactin. Set- Osychlorosene sodium Clotrtmazole. 2411—241
Clotting. Ste Coagnlation Clonacillin sodinin. 3091, 311 ('Ioeapine. 500. 502 Cloearil. See Clocapine Cluster analysis, 58. 61 CNDO method. 937—938 Coagulation. 664—667. 857 ntechanisms of. 663, 664), 883 platelets in. 665—667 prostaglandins in. 666—667
vitamin K in. 883 Coagulation factors. 664. 6641. 664) recotnhinant, 167—168, 183—185. 665 Coal tar analgesics, 760, 761t Cobalamin concentrate. 896 Cobalamins. 894—896 deficiettey of. 895 (olic arid tttetabolism antI, 896—897 products. 895—896 properties iii. 894—895 toxicity oF. 895 Cobra venom sttlation, 835 Cocaine, as drag of abuse. 520. 522 as local anesthetic, 676, 677, 6771. 678. 690 metabolism of. 109 Codeine. 732—733. 733). 738. 745 metabolism of, 87, 126. 129 Codeine phosphate. 745 Codeine sulFate. 745 Codone. See H)dtocodotte hilartrale Coen,vme I, 888—889. 8891 Coenryme II. 888—889, 8891 Cognex. See Tacrine Itydtstclttoride Coherin. 845—846 Colcemid. See Demecolcine Colchicine. 424, 426 Colesevelam. 661 Colestid. See Colestipol hydrochloride Colestipol hydrochloride. 661 Colisuimethale sodittm. 359 Colislin sulfate. 359 Colon, drug delivery ttt, 158 Colony-stintulaling (actors. reconthinant. 178—179.863 Color tests, for proteins. 834
Coly-Mycin M.See Colislitnethale sodium Coly-Mycin S. Set' Cttlistin sullitte
965
agents lbr. 478, 479f. See u/stt Cttntrast agenls
system Itir. 454—455. 4561 Computer-assisted drag design, 27—41,
91t)_tJ45 Sec 0/tn Mttlecular modeling ab initio methods in. 938 actise an;tlttgae approach in. 944 advantages ttl, 9211—921.926
computatit'nal chenttstty in. 922—923 cttmputer gr.tpltics itt. 9211—922. 921—922. 9211. 9221
cotfformatittnal searching itt, 9311—933 depth cnetng in. 921 developnteut tif. 9211—921 drag—drag interactittns and. 944_t)45 dntg—fttod interactittas attd. 944..t)45
ettergy minintization ttt. 929—930, 934 esantples ttf. 939—944
delittitittn ol, 62
force field ntethtttjs itt, 923- 929. Ste 0/vt,
design of. 55—58
Fttrce field methods geometry ttptitnii.atiott itt. 929—9311 in lead discovery and deselttpntent. 9211 ntolecular dynamics sintulatittits in. 933- 935 ntttlecular models in, 9211—922. 9211, 9221.
exploratory. 56 focused. 56.61 generic. 43. 44f ltigh-throaghput screening of. 26—27, 4)1. 41)1, 43, 53—54. 541. 55. 944
ntisture. 43, 441 optimization. 56 lugging ntetltods for. 52—53. 521. 52t virtual un silicttl screening ttC 54—55.56. 919 linear chain molecules in. 45—46
linkem in. 48-49. 481, 62 Lipiaski Rule of Five in. 41). 55 mass spectrometry in. 51—52 media in. 43. 451 Merrifteld synthesis in, 43. 441. 481
microwave Iteating in. 47 ttaturul prodacts in. 47—48. 471 nuclear magnetic resonance spectroseopy in. SI
one-bead oae.conqxtund synthesis in,
46-48. 50, 62 ortbogttnal pooling in, 5(1—SI, 62 overview (tI. 43 peptides in. 43. 441. 451 peptoids in. 43—46, 451, 63 polynterase chain reactittu in. 52. 62 polymer beads in. 48—49. 60 pttoling strategies in. 50—51. 53.62 positional scanning in. 51. 62—63 solid-phase. 46—49. 461 solid supports in. 49, 63 tagging of. 52—53. 52f. 52t soluble supports in. 49, 63
lagging ttl. 53.61 solution-phase. 49. S(tf split-and-mix synthesis in. 43. 441 subtractive deconvolation in. 50 terminology of. 60—63 trends in. 60
yield of reactiotts in, 47 Comhi-l'atch. See Hormone replacement therapy
Comfort algorithm. 933 Cttmparine. See Prochlorperucine maleate Cttmplementarity-determining regions (CDRs), 188
Completttentary DNA. in ctttnhittatorial chemistry. 49 Cttmplement pathway altematise, 201. 2021 clas.sicul. 203. 2031 Computational cltentistry. 922—945. Set' tt/stt Computer-assisted drag design Computed tomography. 454—455. 478. 4791
See a/to Molecular tttttdeliug oserview ttf, 919—9211
pltanttacopltttre cottcepl in. 934
predictive Al)ME in, 944-945 quantum mechanics methods in. 923.
935-939 screenitig tn Ste Screemting semtrntpirtcal ntethods in. 937—938 55. 939—93-I
3D imaging in, 921 visualiz.atitttt techniques in, 921 Cttntputer graphics. 921—922, 921t, 9221. Set' ti/so Coatptttcr-as.sisted drag design COMT 125-126. 526—527 Ctttttvax. 186—187 Cones. 871, 872
(ttnflguralitttt interaction methods. 939 Cttnformational databases. 39—40
Conlortnalit'nal tle.sihtlity.34—35. 34). 9311—931
Ctmnlitrtnatittnttl tsomers.32—33 Confonnatittnal searching. 930—933
Cttaforatatittttls) axial, 93). 932t htttactive. 9311
definition of. 9311 equatorial. 931. 932 Con/ttettter. 931)
Cottlitet algttrithm. 933 Congenttn. See Benetropine mesylale Cttttjugate acid—conjugate base pairing. 101—121. II
Conjugate gradient approach. 931) Cttnjttgation reacttttas. 8 in drag metabolism. 65—66. 65t, Ill —126 Connectivity tables. 23—24. 24t Connolly surface. 922 Contntceplives. 789—795 depressittn antI. 893 development ol. 789—791) estrogen itt, 779 ttvulatioa tnltibitstrs. 790—794 hiphauic combinations. 791). 791t—792m
classes of. 790. 79lt develmtptitent ttf. 790 enmeegency, 793t
inmplants. 792t. 7931, 794 injectable depot. 792t. 793—794
IUD. 792t, 794 ttttttmopltasic cotnbintttit,ns. 79(1. 7911
966
ku/tx
Cotitraceptises (iottritttted) prixlttcLs. 791t—793t. 793—794
progestin-oitly. 792t. 793 safety of. 790-793 lr.tnsdennal, 792t. 794 trtpha.sic combittaliouts, 790. 7921 poslcoital. 794—195
pritgestins tn 787 relative effectiveness ol, 795, 7951 tryptopltnn titetabiilisnt and. 893 Contrast agents. 472—484. See alto Radiopharntaceatieals adverse reacttons to, 481 (or arteriography. 478. 4791 for arthrogeaplty. 481 for clntlangtogrnplty, 479 (or cholecystography. 479 for cotnpnted ttnoitgmphy. 478—479. 4791 definition of', 473 for excretory arograplty, 478 gadolinitint. 476 for gastrotntesiinal stndies. 48(1—48 I. 4801 high-osntolar. 473. 474t (hr hysterosalpingograplty. 480. 4801 mdtcaliott.'. 11w, 4741
ingestible, 473 (hr intr.tvenous pyelography. 478. 478f for intravenons nrogtaplty. 478 ionic ratio 1.5. 473. 474t iron oxide. 476 low osniolar, 473 for rnyelography. 481) ostnolality of. 473—474 parantagnetic. 475—477, 4761. 4831, 484 products, 481—484 types of'. 474t altensonnd. 477 viscosity of. 474 water.insolnble, 475 473—475
Ctaixidation. 822 Cophene-X. See Carbetapentane citrate Cordarone. See Atni'tdarone
Coteg. Sit' Canedilnl Corgard. See Nadolol Coronary atherosclerosis. 622 Corticorelin. 843 Conicosteroids. endogenoas. 81)3—815. See
a/au Gincocorticoid(s); Mineralti corticoidis); Steroidl vi Cortientropin. Ste Atlrenoeorticotropie ftnrtnnne tAC'rIl) Corticoropiit gel. Sit Repository corticotrtipin injection Corticotropin injection. 842. 842t Corticotropin—releasing hormone. 841 Corticotropin tine, See Stenle coritcotropin ztnc hydroxide snspension Corlisone. 81171, 80'h, 811
active tnetabolites id, 135t as attttneoplastic, 435
biological a'tivities of. 8116 biosynthesis of. 8114—805. 8041
metabolism of, 805. 8051
relative activity of. 809t. Conrosyn. See Cosytttroptn Corvert. See Ibutilide ('orynanlhtne, 541 Cosinegcn. See Dacttnontycin Cosyntrrtpin. 842t. 843 Cotazyrn.Sei' Pancreltpase Cotinine. metabolism of. 95 Cough snpressants. 752—753
Coalomb's law. 927. 9271 Conntadin. Ste Warfaritt sodinro
('outttcstrol, 778- 779. 778f Cosalcttt bonds, 29—il, 3lt COX-l, 8(9, 8191, 822 CON- I inhibitors. 754 COX-2. 819, 8(91. 822 COX-2 itthibitors. 754. 822-823 Coiarr. See l.osartatt CPK models, 9211. 921 922 Cranberry, 912 Cresol. 222 Crittcal titicelle eintcentration, 224 Crixivan. See lttdiit;tvir ('rontolyn stnlinnt. 715-716 ('rittantiton, 268 Crucx. Sit' Undecylettic acid
Cyproheptadine. ill metabtulisttt of. 76, 87. 114 Cystic fibrosis. 185. 194 Cystic fibrosis gene. cloning of, 169 Cytadren. St-c Atnittogluteultinuide Cytarahtne. 41(7. 413 Cytochronte P-450 encyittes drug—drug interactitutts attd. 13(8. 131
in intestinal tttacosa, 66 sot> ntes of. 67. 67t. 1318. 131 vs. nttttuoanuine oxtdases. 9)
nontenclatnre for. 67t oletintc destruction iii, 77 in oxidative reactutuns, Wi—69, 681. 691, t)l
getwtic differences in. (29 sex differences in. 129 - 130 in prodrug activatiott, 152
Cryptosporidiosis. 2611
('rystalline cmc insnlin. 851. 851t. 852' Crystallttria. salfanilamide-related. 274 Crystal violet. 227 Crysticillin. Set' Petticillin G procaine Crystodigin. See l)igitalis Cnetttid. See Cltolestyranttne resiut Curare. 590
Cashing's syndrottte. sIt) Ctitatteons tnycoses. 231, 2311 titpical agenLs far. 233—235 Ctttivale. See Fluticasonc propionate Cyanocobalatnin. 894—896 deficiency of'. 895
lithe acid ntet:tbolism and, 896-897 prodttcts. 895—896
in steroid buasyttthesis. 768—7711 tissue distrthtttitutu 1)1. 66, 67. 91
Cytokities futtctiiuns 1. 177. 177t itt ltrtttattupoiesis. 177. 1781 reconthinttnt. 177-179, 1771. 861 -862 ('ytontel. Set' Liothyrtittine srntittnt Cytosar'U. Set' ('ytnsine arahinuiside Cyltusitte ar.tbinnside, 4117. 413
Cytotec See Misoprostol Cytotoxic agettts, for cancer. Sit' Antineopla.stic agents Cyuits cite Set' (itnciclsivtr ('ytoxan. Sit' Cycltipltospltautiide
properties uI. 894 -895
toxicity of. 895 Cyattocobalatttitt rttdtoactive cobalt capsnles. 89(u
Cyanociuhalaittin radioactive cobalt solntiott. 896 Cyclactucine. 741). 751
Cyclrtu. Ste Norgestimate
('yclic AMP. 551, 553 itt sntaotlt muscle relaxation. 623, 6241 ('yclic AMP re.spottse eletnettt ICItE. tn drug screening. 171, 1711 Cyclic gttantistne monophosphate tGMPt. itt snnuoth muscle relitxatiott. 2641. 623 --624
D
t)4I'. See Stavtatitte I)acarba,iite, 402 actis atiunt nI. 398. 3981
Dacli,utnab, 189 Dactinomycin. 4(4. 415. 421 I)aidzein. 778—779, 7781 Dalfoptistitt-qairtnprustun. 363 Datgatt Sue l)e,oeine I)alntane. Ste Flnrai'epant I)anaiol, 7991 801 Dautttcrine See Danaittl I);tntnilette. metabolism of. (07 l)apsone. 28)1
Cyclicine hydrochloride. 7116—71)7 Cyclicitte Lactate Injection, 7117 Cyclit'ittes. 7(16—7(17. 7(161
('yclocon. See Antcinttnide Cyeltucytidine. 41)7—41)8
Cyelognanil-atiuvaqnone. 29)f. 292. 2921 Cyelogyl. See Cyelopetonlate hydrochlortde Cyclohexanol, eonfonnations of. 931—932. 932t
antttabercttlar activity iii. 254 tttetabolisttt at, 1)3 l)aranide. Sue I)ic(tlorpftenatnide l)arbid. See Istupropantude iodide Daricon. Sit' Oxypltettcyclimine hydrncltlnui Dark adaptation. Sift I)arvon. Set' Propoxyphene hydrochltmde See Propoxypltette ttapsyfate Databases
Cycltultexintide. 337 Cyclonteiltycaine sulfate. 69(1—693, 691t ('ycltnixygenase, istulorms tuf. 169 Cycliutuxygena.se-l. 8(9, 8191, 822 Cycltiuuxygenase- I inhibitors. 754 Cyelooxygettase-2, 819—821), 8191. 822 Cyclrnuxvgenase-2 inhibitors, 754. 822-823 Cyclooxygenase pathway. 818. 8191 ('yclopar. See Tetracycline Cyclopenutulate hydrocltlttride, 58(1 4(8)
activation iii, 395.3L36, 3961 metabolism of, 95—96 Cyclorphan. 740 Cyclutserine. 259. 3l81t
antituhercnlar activity of. 254 Cycltitltiaeide. 6(15—611). 6961. 6()8t. 62(1
Cylert.Si'e Pemoline Cylic AMP, 553 CYP isticynues. predictive ntodel for. 945
biological. 58 cltetnical stntcture, 511 muning of. 58 searching tiE. 39—40, 55—56 confornmtiona). 9311—933 itt library ulesign. Sb 31) sunictaral protein. 939. 9391
Data tttiitittg. 58 Data warehnttse. 58 I)annrtmycin. Sit' Daanoruhicitt Itydrochltn l)annontycinol. 4(6 Dauttturnbicin hydrochlutrude. 4(5. 416, 422 ntetahtilisnu tif',
11151
l)axolin. Sit' I.oxapine suecinate I)aypro. Ste Oxalirtvrin dCF. See Peittuistatttt ddC. Set' Zatlcitithitte
ddl. See l)idanosine DDT. for tniusqaito control. 283 Deantidatituit. pruitetn, t 73. 1741
I)eaiapyriittidine nucirosides. 408, 4(191 Dehri'ax(ititt. tnetabolistn ol, 771 Dc Brogue relationship. 936 I)ecal)arabolin. See Nandrolone decanoate lkcapryn succinati, cc Dosylaittine saccinate l)ccloinycin. Set, 1)enieclocvcline
))estrontetborphan hydrtibrotnide. 753 Destrontoramide, 7)8, 739t I)estrtttltsriesine sodium. 66)t I)e,ocine. 75))
Dihydrocodeinone. 733. 733t Dihydrololate ttductase inhibitors, 279
1)l4PG, 377—378
I>ecOfltatflinahifln, 2181
Dia)tetes netlitas. 85)1—85) l)iabittese. See C)tlttrjtropamide
l)ihydrotnorphtne. 733. 7)3t Dibydrotttorphinone. 733. 733t Dihadroittorphone, 745--746 I)ihydropyridine. as drag delivery system.
l)eeonvoluium, 26. 271. 61 iterative, 5)) snhtractive. 50
DiaBeta. See (ilyhunde
Demades. See Tor.scni,de
l)tacetolol. 545. 5461 1)iacetyltttorphine. 731. 73). 733t, 745 Dialen. See Dipheny Ipynilitte ltydroe)tloride Dtagttostic itttaging agent'. See Raditipltatmacenticals l)iantagnetic sabstances, 476 Diattucron. See Gliclande Diamos. See Acetazolamide l)iatnpromide. 741 l)ianabol. See Metltandrostenolotte l)iaparene. See hlethylben,ethontaitt chloride Diapid See Lypressin
Detnecariutti brotitide. 566 I)erttcvlocycline. 345t. 347 I)emecolcine. 426
l)iastase, 841) l)iasten.,onters, 35 l)iatrieoa)e. 481—482
l)cmcn'l Set' Sleperidine
Diaiepam. 49(1-491 active tnetabolites ol, 134—13S. I 35t as anesthetic. 487
Delensins. 21)1
Dehytlratcd ethanol. 221)
acid. stilubility of.
77111
I)ehydrocmettne. 261—262
1)1II:A)
biological a'livity til, 797, 7')KI biosynthesis '8. 7691. 771). 797 nieiabOli5ttt ti). 797
Delavitiline. 383
t)cntovepani. 49(1 l)cniser. See Metyrosine
l)enataration. proleict. 173- 175 Denatured alcoltol, 219 Dendrimers, in combinatorial cbetnistrv. 49. I, I Dendrites. 679. 6791 Denileakin diltitos, 183. 442 Dc nato drag design. 55 lanctional theon'. 93') Denvir. Ste Penetciover I)eosscoiornt)ctn. 41)8 I)eosycortisone. 81)6. 81)71. 8)8)1
vs auttteonvnlsattt, 508 as anvitely tic. 49(1—4') I
otetaholism at. 71. 94, 11)1. 133 l)iazeqntttte. 395 INazoside. 654—655 l)tbeneocsclttlteptattes. 711—712. 7121 Diheniireyeloheptenes. 711—712,7121 Dibeniy)inc. See Pltenovyben',antine l)throtttt,matmitol, 395 l)ihucatne Itydisechloride, 678. 690—693. 6921
l)eoivnboauclcase I. recombinant. 185- 186 6-I)ensvteiracyclines. 342. 344 -345 Depade. Ste Naltrestine I)epakeae. Ste Valproic acid I)ependence liability. 732 l)qtoianeatiott. 68)). 682 Depo-I'tment. 792t. 79)
1)11'. See I)acarh,iaine
Depressants. Set' Central aces ints system
l)ieloxaeilitn smlitint, 309t, 311 See otatt Pctttcil)itftsi 1)ecodttl. See Hydrocodt'tte bitatirate Dicamarol, 667 Dicycloinine hydrochlortde. 5811
depressants
Depth ctieittg. 92) l)emtattip)tytoses. 23). 23 It topical agents for. 233—235 l)tiS daaghters. 779 I)esencs See t'ndecylenic acid lkserptdnte. 529 l)e.stltirane. 486 l)esiprannne bydtochloride. 5)7 lksuno(tressitt acetate )Dl)AVP), 8461. 847 l)esogen. Ste l)esitgestre( Destigestrel. 7871. 789
in contraceptives. 791t. 792, Desottide, 8081. 812
Sit' Dcsot,ide l)esoxitttetasotte, 8(18). 8)2 Desosya. See Methantphetatnene I)esyrel See Traetnlotte
l)etovil'ication, delinition ''I. 65 l)etrottmramide, 73')) 1.2-Deuteride shin. 71. 721 Devatttetltasone, 8))')t. 812 Deshrontphenirantine ttt;deate, 709 Desc)tlorphentratttine ttialeate. 7)18— 709
Desedrtne. Ste l)estroatuphetattune Deslenllarantine.5 14 l)espanthenol, 888 1)enraiosaite, 445, 446 Devtroatttpltetamtne. 512. 513 Deatrornetlnuphan. 739 tttetaholistn ol. 86
N.N-Dichlorodicarbonamtdine. 224 Dicltltirteisoproterenoi I l)CI 1. 541 —542 acid. 224 l)ichtlorphettatnide, 6(41. 62))
Dicloliniic potassium, 759 Dielvifenac scaliunt. 759
I)id.itti'sine, 3811
I )idtvv See Benep(tetatttitte ltydrticltloride l)ienestoii, 777—778. 7781, 781
Diet. drug ttietabiilisnt anti, l3lt. 1)2. 944—945 I)ietltylcarbama,epitte citrate. 265 Diethy lenediantine. 265
Dtcthylpropion. SI) ntetaholisttt of, 1051 Dtethylsttlbestrol as aittitieoplastic, 434 cancer ut offspring and. 771) metabolism iii. 77 preparations of. 780 as siittiiaeity probe. 56. 57t stnictitte of. 7781 I)iethylstilbestrol derivatives. 776—778. 7781 Dillerin. See Adapalene Ditlorasotte diacetate, 81)81.812 Ditlucaut cc Flucottaiatle t)ttlanisal, 757 Digestise enl.ymes. 4 Digitalis. 656 I)igitalis glycosides. 655—657
l)igittivtn..teteve netabolites at'. l3St Digosin. 656 Dilisdoicodeine, 73), 7))t l)ihydrocotheiite hitaru"ate. 747
developttteitt of. 941 —942 l)tliydrotolie acid. 409—41)). 41111
3581
Diltytlrostreptotttyciut. 337 I)iltydrotaehysterol. 878 Stt'l)ihydrotestostemne biological acttvtty nI, 797, 7981 biosyttthtesis of. 7691. 771). 797, 8(12 ntetatnehisttt itt'. 797. 7981
rn-l)ihydrosyben,cne. 222 Diitvlohydni'cyqitin, 263 Dilanrin. Ste Phenyttdtt l)ilaadid, See Hydrotnorphoite Diloaanide luroate. 261 Diltiazem as atttiarrltyihtttic. 642 as vasodilattir, 629t, 631). 63)11 l)inteultydrinate. 71(3 Dimereaprol. 264 metabolism al, 326 I)itorrcapto- I -propaniel (BALI. ntetabolism 126
Dttuetatte. See Bmatphetttr.tnttne Ditttethuindette tttaleate, 711)
I)imetliisiNain, 694t I -12.5-Ditttethosy—4-methylp(tettyl -2-
atttinopropane. See SIP Dittietlwlbeiveylatnotoniuni cltloride, analog of. 224. 225t Dimethyl suliosude (DM501 in ltiglt-throughpat screening. 53—54 ntetabolisnt ttl. 99. 109 Diatcthyltryptamint'. 521 Dititiprost. 827 Dinoprostotte. 795. 7951. 829 Dinoprost trintethaminc. 829 Di,idoqttin. See Iodtitluinol l)iovan. Set' Valsartan l)iosapht'tyl hatyrale. 7391 Dipanonc. 739t Dipcrodon, 694t Diphemanil ntcthylsttllatc. 585 Diphcttltydrattune. 702-7))) tociabolism til', 85 1)iphentdtil, ntetaholisnt ccl'. 87 Dipltenosylate, 736t, 737, 748 active ittetabolites iii, I 35t ntetabohisnt ttL 109 Diphentotn. -Ste Plteitytoin Diphenylpyraline hydrochloride. 7)12, 7)14 l)iphtheria ttivoid, 21 2t, 214. 215
Dipivelrttt. 5)2 as prodntg. 145. 14Sf bottds. 31t. 33. 331 1)iprivan. See
Dipyridainole, 633 1)ipyrotic. 762t. 763 I)iquinol. Set' lodvtqutnot Diraqmn. See Qtttntditte gluctinate Directed library. 56. 6) I)irit(tnmtycin. 352—353 l)isalcid. See Salsalate Disinleetants, 218—223, 218t chassit)cation cii, 218. 21 8t etTeerivettess oh', evaluation ttt', 239 improper use ct', 219
phenol coefficient for. 221 Disonter. See Deshrontpltettiratttine maleate l)isopyramide. 638 metabolism ttl', 85
968
!,uiet
Dispermitt. Sit' Piper.i,.tne Dissociative anesthesia, 488 l.)ictal convoluted tubule, sodium re.tbsorptton in. 599-6(8). (tOOl Distance-dependent dielectric constant. 934 Disulluram. metabolisnt of, 108. 114 Dilhioglycernl. 264 Diucardin. Set' Hydroflumethiazide Diulo. See Metolacone Diuretics. 596—620. 601—620 of, 6(11—602, 6021 active tubular
adverse elIccts of. 618-619 carbornc anhydr.use inhihitars (site It. 603-605. 6041. 619 combination, 620 concentration of, 601—602 for congestive heart failure. 618—619
definition of. 596 dosage of. 619—62(1
efficacy of. 601—602 entrarcnnl activity cr1. 618—619 for hypertension. 618—619 kaip (high-ceiling) (sitC 2). 601, 610—616. 620 mechanism of action 01. 601 1x,iassiunt loss due to, 618—619 potassium-sparing (site 4). 616—618. 621) potency of. 601 prepardliotis of. 619—62(1
primary action of, 596 properties of. 596 secondary effccts of. 596 stntcture—acth'ity relationships of. 602—603 thiaridc/thiazide.like (Site 3). (,05—6l0. (,O6f, 6061. 607f, 6081, 619—620. See also
Thiazide/thiazide'lilse diuretic' transport of. 602. 6021 Diurexan. See Xipatnide Diuril. See Chlorothia,.idc
DMG.6.mcthyl.6.deotcytetracyclinc tDMGDMOT), 348-349 DMG.mittocycline I DMG.MINO). 348-349 DMSO. See Dimethylsulfoxide (DMSO) DNA tilkylation of. 398—399. 399f antisense, 193—194
cloning of. 164, 166—l(i8 complementary, in combinatorial chemistry. 49
modeling of. 920 reconibittant. See also Recotnbinant DNA technology processing of. 172 production of. 64. 168—169 replication of. 162. 1631 synthesis itf, 162. 1631. 192. 93 pltusporylatton in. 1541. 1551 tnunscription of. 162. 1631 vectors for. 165—166. 1661
DNA hybridization. 166 eDNA libraries. 64. 1641 DNA ligases. 165. 860 DNA microarrays. 192-- 193. 448—449 DNA probes. 192—193 DNAse. recombinant, 185—186. 859t. 861
DNA lags. for cumbinatt,rial Iihr.tries,52—53. 52t rDNA technology. See Recombinant DNA technology DNA viruses. 36%t. 37(lt, 372 Dobutamine, 535 metabolism of, 25. 133 Dobutrex. See Dobutamine Docetaxel. 425. 428 Dofetilide. 641
Dolette. Set' Propoxyphene hydrochloride I)olubid. See I)ifluttisal DolopItinc. Set' DOM. Set' STP DON. 411 Donepczil. 566 development of. 943—944, 9431 i-D pa. as drug delis-cry systettt, 157. 571 Dopanitnc. 524—547 adrenergic receptors and. 527—528 biosynthesis of. 524—525. 5241
gatiglionic stimulation by. 586. 5871 properties ii). 524 site.specillc delivery of. 158. lSX(. 159 structure ol, 524 as sympathotnintetic, 532 uptake and metabolism of. 525—527. 5261 Doprant. See Doxaprum hydrochloride D-optitnal selection. 58 Doral. See Quazepam Dortdctt. See Olutedtiniidc I)ornase aIls, 185, 838, 839t, 859t. 1161 Doreolamide. design of. 921 -922. 9211. 9221. 942—943
Dosttgc. receptor affinity and. 8 Doturem. Set' Catdotcntte mcglutnine Dots on cellulose, in combinatorial svntltesis. 44.451 Double-ester prodrugs, 146—147. 481 Double helix, model of. 920 Dovonex. See Calcipotriene Doxacuritim chloride, 591—592 Doxaphenc. See Proporcyphene hydrochloride Dosaprato Itydrucliloride. 510—511 Dttxazosin, 540—541, 5411, (i52 Doxepin hydrochloride. 517—5111
Doxercalciferol. 879 Doxorubicin, 415, 422—423 l)oxycyclinc. 345t. 347—348 For malaria. 293. 2931 Doxylatnittc succinate. 702, 703 l)ittmantine. See Dimenhydrinate Drixoral. See t..( -f ).Pseudoephedrifle Drogenil. See Flutantide Drolhan.Sre Drotnostanolone propionate l)romoran.Si'e Racemorphan Dromostanulone prupionate. 436 Dropcridol. 51)1
Dropcridol.ft'tttanyl. 738 Drospirettune. 7871. 789
in contraceptives, 79lt Drotrecogin alIa. 185 Drugls of ahttsc. 521) 9—17 acid—base properties activate metobolites of. 7—8
biotransformation tit. See l)ritg metabolism dissolution of. 3 eneytnittic reactions and, 4 hard. 142 ideal, 3
lipophilic. 31.65 definition of, 65 microbial resistance to. 301. 305—307, 335— 336
percent ioni,ation of. 15—16. 1Sf, 161 receptors for. See Drug—receptor interactions: Receptor(s) recombinant. 175—191. See also Recotttbinant drug product.'. sile.spcciltc delivery of. 155—159, 1571— 1591
soft. 142 synthesis of parallel. 43. See also Combittatonal chentistry serial, 43
targeting 3—4 unpalatable. ptsidrug forms for. 145—146
l)rug ttcliott citemical stuctare and, 17—21, 28, 31—41
duration uI. protein binding and. 6—7 isotnensnt antI. 35—37. 3Sf, 361 stattsttcal predictttsn ol, 17—26 Drug allergy. See Allergy Drug carriers. 155—159. 1571—1591
Drug delivery carriers or. 155—156, l571—lSOf of recombinant drug products, 175 site specific. 155-159. 157f—159f Drug desigit. Set' also Drug development advances in, 1—2
avittlable inlormatton in. 55—56 calculated conformations in, 37—38 classilicalioit techniques in, 24—26 combinatorial chemistry in. 26—27, 43—63. See also Conihittatorial chemistry cotitputer.assisted, 27—41, 919—945, See tutu Conipuler.ussisted drug design
cottformational Ilexibitity attd.34—35. 341 database scarchtng in, 39—4(1. 55—56. 93(1—933
de novo, 55 drug distribution and. 9 drug tnctabolism in, 135 drug—receptor interitemions and, 9, 27—37 Sec aIrs, Drug—receptor interactions
energy diagrunts for. 37 Free.Wilsoit analysis in. 23, 26 goals of, 17 graph theory itt, 23—24. 24t. 2Sf identity variables to. 23 irrational, 26 isosteflsiii in. 40—41, 411 kits for. 37
I.ipinski Rule of live in. 40. 55 molecular mechanics in, 31) ttit,lecular modeling in. 27—41 uttiltivariute statistics in, 24—26 optical attd. 35—37. 361 overuiew 1—2 QSAR studies itt, 17—23. See also QSAR studies
quutttuttl itieclianics in. 38 rational, —2, 919, 94(1 receptor isolation and, 28 tvgressiott analysis itt. 24 scettarios for. 55—56 screening Screening statistical methods in, 17—26 stereochemistry and, 3l—34,32f, 331. 35—37. 361. 37f structure-bused, 55. 939—944 stntcture-ftinction relationship and, 17—21. 28
substituent libraries in. 26—27 sttbstitaettt selectiott itt. 22—23. 231. 25 topological descriptors in. 23—24. 24t training set in. 25 web sites for, 41 u.ray crystallogruplry in. 37—38 Drug developmettl. See also Drug design hiotechitology in. 160—162. lb2f, 169—172. Set' also Biotecltnology
DNA tnicroamlys in. 448-449 proleotitics in. 449 l)rug distribution, 3—9 bltiod—brnin harrier and. S drug metabolism and. 7—8 excretion and, 41. 8 with intravenous tdttiinistr.ttii,n. 41, 5 mecltanisttts oF. 41
Index mtxtilicatuon 9 with oral administration. 3—5, 41 will, parettter.il adtitinistrutiun. 41. 5—f>
plc aid. (6—? placental harrier md. 6 protein binding in. 6--7 tissue depots and. 7 tr.ttlsport ,,iechanistns in. 41. 5 Drug—drug interactions cotnputer'asststed drug design and. 944—945 cytochromc P-450—based. 131St. PSI ericyine induction in. 1301. 131 Drug excretion. 41. 8 Drug—food intermtctions. 1311. 32
computer.asisted drug design and. 944 -945 Drug Intentiation. 142.5cr also Prodrugs Drug-like molecules. combittatorial synthesis 01. 46—4K. 461. 471
arotnalic hydruxylatiiin in,
Duration of action, protein binding and. 6-7 Duricel. See Cefadroxul
111—126 4>1 prmmdrugs. 142—144. 1431. 1441
Dymclor. See Acctohexumtde
product utereouclectivity in. lOS. 132—133 of r.mcernic tnixrures. 132 of recombinant drug products. 175
701, 721.
741. 93
1121
of sulfates. 115—116
cytochromc P-45(l enzymes in. 66-69. 681. 691.91. 129—PSI, 13t)t in drug design. 135 enzyme induction in. 131)1. 131 enzyme initihition in. 131—132. 131t lirst.pass effect in. 67 foods 1311. 132 lunctionalization reactions in. 65—66. 651.
69-Ill
general pathways of, 65—66. 6ot in. 129. 193 genetic
glutailtionc in. (>6. 73. 98. III. 117—121. 1191 hcpatuc. 7—8. 1,6—6K
hereditary liieii,rs in. 12') hydrolysis in, 109—Ill of esters and aiiiides. 109—11(1 intestinal. 1>6. 67
mcrcapluric acid in. 117-121 niethylalion in. 125-126, 1261 monoamine oxidases in. 90—91
N.acetylation in. 93 nutritional factors in. 1311, 132 overview of. 65 oxidation in. 69- 103 of alcohols and aldehydes. 99—101 01 aliphalic and alicyclic carbon atoms. 81—84
Ut allylic carbon atoms, 77—81 of aromatic moieties. 69—74. 701. 721, 741 at heneylic carbon atoms. 77, 771 at carbon atoms a Ii, carbonyls and imincs, HI in carbon—nitrogen systems. 84—911
in carbon—oxygen systems. 84. 98 in carbon—sulfur systems. 84. 98—9') cymochromc P—ISO enzymes in. 66—69. 661. (>81. 691
deamination in. 89 de.sulluration iii. 99 genetic lactors in. 129 N-dcalkylanon in. KS 0-deulkylation in. 98 of oletins. 74—77 rate of. 129 S-dealkylation in. '38—99 of ertiaty aliphutic and alicyclic amines.
Dutustcnde. 802—803. 8021, 8031 Dyazidc. See Triamtercne.hydruchlorothiaeide
Dyclonine hydrochloride. 694t Dyes. 227—228
sulfonamide iso, 269 Dynabac. See Dirithromycin DynaCirc. See Isradipine Dyrenium. See Triamterene Dymirhyihmtns. 634—636
rrductim,n in. 103—109
of aldehydes and kctone carbony!s. 103-- 107
of nitro and azo compounds. 107—108 rcgioselcctivily in. 133—134
sites of. 66-67
conjugalion in. 8.65—1,6.651. 111—126 of glucuronic acid. 112—I IS. I 12f—1141.
66—69. 1,61. (.1)1. 691
oxidalivc aromatization in. POP oxidatixe dehalogenalion in. 101—103 oxidative dehydnigenalion in, 101 pharmacologically active metabolites and. 134—135. l35t phase I reactions In. 65—66. 651. (>9— III phase II reactions itt. 8. 65—66. 65t.
semi differences in. 129—13(1
l)ntg metabolism. 7—8. 65—135 acetylution in. 121 —124. 1231 ageand. 126—128
969
in smokers, 131 species and strain differences in. 28—129 stereochemical aspects of. 132—134 smthstntte stereoselectivity in, 132 Drug metabolites phammacoktgically active. 134—135. 1351 toxtcity of. (>5 Drug partitioning. 18—21. 191 n-octanollwnter system and. 19—20 partition ci>elliciemtt and, 19—21 l)rug—raceptor interactions. 3. 8—9, 27—35
active-site—directed irreversihlc inhibition in. 29
agonist/antagonist actions and, 28—29 biological respoitse in, 29—31. 311 botids in. 29—31. 311 computerized images of, 922. 9221 drug design and. 9
efficacy and, 572 flexibility in. 28. 34—35 functiminal groups in, 28 intrinsic activity and. 572 molecular structure and. 31—41 optical isomerism and. 35—37 Puton rate theory of. 572 protein conformation and. 28 range of. 28 receptor asymmetry and. 35-37 receptor locaxion and. 28—29 receptor properties and. 27—29. 291. See also
Receptor(s) side effects and. 9 stereochemistry of. 31—34 variability in. 28—29 virtual scteentng for. 55 l)nmg.reststhni patlttmgenm.. 301. 3(15—31)7. 335—336
Drug screening. also Cumputer.assistcd drug desigtt ol antineoplastic agents. 392—394. 3931 automated. I biotechnology in. 170—172. 1731 Itetetulogous expression and. 170—172. 1701 high-throughput. 26—27. 40. 401. 43. 53—5-4, 541
human.tumor.colony—forming assay for. 394 random. 1—2 reporter genes in. 171—172
virtual (in silico), 54—55. 56.419.919 xenogrmtft models for. 394 DTIC. See Dacarbazine
Ducarbazine DTP vaccine. 2l2t. 215 DTIC.Dmmme. See
Dundenal ulcermi. 718—719 l)ur,mbolin. See Nandrolone
Duracillin. See Penicillin 0 procaine
E
0.64. 447 02020. 566 EA 713. Srs. Rivassigmine Easson-Stednian hypothesis. 530. 5301 East African sleeping sickness. 260 Echinacea. 905—907
Echinocanadins. 246 Echothiophate iodide. 569 Econazole nitrate, 24! ED,0. 17 Edecrin. See Ethacrynic acid Edrophonium chloride. 567 EES. See Erythromycin ethylsuccinaic Efavirene. 383—3114
Effexor. See Venlafaxine Eflomitlitne. 262—263 Efudex. See 5.F!uorourueil Eicosanoid(s) approved for ltuman use. 827—828
biological activities of. 820. 822t biosynthesis of, 8 18—822. 8191. 820f in clinical development. 824t—$25t. 829 design and developmetit of. 823—825, 824t—825t
discovery and development of. 818 drug action mediated by. 822 nietabolism of, 821f, 822 moditicattons of. 823 ophthalmic, 823. 828 br veterinary use. 828—829 Eicosanoid receptors, 825—827, 826t F isorncrc. 32. 321 Elavil. See Anutriptyline Electrolytes, renal reabsorption of. 596—601. 5971—6001
Electromagnetic rudialion, 454 Electron capture decay. 456 Eleclron population analysis. 939 Electron volt. 454 Electrospray ionization. 52 Emcyt. See Estramustine Eznetine. 261—262
Empinn. See Aspirin Emprostil. 8241 E.Myein. See F.rythrumycin Enalapril. 646. 647f. 647t as prodrug. 5 Ennlaprtlie acid, 5 Enbrel. See Tumor necrosis factor, recombinant Enclotniplmene. 781
Encoding, for combinatorial libraries. 52—53. 521. 52t
Endocytosis. viral, 37! Endometrial cancer, estrogens and. 779. 787 Endometilosis. 801 Endoncunum. 680, 680f Endorphins. 744. 843—844 Endothelium.derived contracttng factor, 552 Endothelium.derived relaxing factor. 552 Enduron. 620 Energy diagr'.uns. 37. 371
970
I,ukv
Energy minimization. 929—930. 934 Energy Icons, in molecular ntecltanics. 38 Enflurane. 486 See Hepatitis B vaccine linisoprost, 824* Enkephalins, 744. 843—844 Enols. gltlcUronidation of. 14 Enoxacin. 24$, 2.1St, 249 lnIa,nr,vlai lti.iW!vrica, 259—260 Entocoti. See Budesonide En,actin. See Tnacelin linayme(s), 835—841)
catalytic activity of. 835—837. 8361. 8371 classification of, 831) conformation of, 835
definition of. 835 flexible. 835 heterogeneity ot.
69
induced-fit theory for. 835 products. 838—840, 8391
proeniyines and. 837 recombinant. 183—86 secrelion 01. 838 specificity of, 836 structure and function of. 835—837, 8361. 8371
synthesis of. 837—838 ,.yrtiogens and. 837
Enzyme induction. 13th. 131 Enzyme inhibition. 131—132. 835 Enzynie.substrnte complexes. 835. 836 Eosinophils. 198 Liovtst. Ste Gadoxetic acid Ephedra 905 911—9l2 Ephedrine. 538.538t metabolism o1. 107 Epidutail anesthesia, 6117. Set' also Local anesthetics Epilepsy. 503—504 Epinephrinc. 524—547 itdrenergic receptors and. 527—528 biosynthesis of. 524—525.5241 in local anesthesia. 68$ ocular delivery of. 1514 prudrug forms of. 45. 14Sf. 151)
properties of. 524 structure of. 524 as sympalhomintetic. 532 uptake and metabolism of. 525—527. 5261 Epincuriuni. 681 Epirithicin, 416 Epitestusteronc. structure—activity relationships for. 7914—799, 7981
Epitctracyclines. 342, 344 Epilopes. in combinatunal chemistry. 43. 62 Epilope tagging. 169 Eplerenone. 619. 815 Epoetin alIn. 177—178. 8591. 862—863 Epogen. See Epoctin alfa
kpopnrstenol. $23 Epoxide hydruses. 73 Epoxides, Iortnation of. 74—77 Epiiftbatide. 634 Eqttanil. See Mcprtihnmate liquntitrial conformations. 931. 9321 Equilibrium potential. 682 Equiliti sodium sulfate, structure of. 7771 See Ivermectin Erectile dysFunction. 29 Ergocalciferol. 875 preparations of. 877—878 Ergosterol. 877 in lipid membrane. 231, 2321 Eaypar. See Eryibromycin stearaic F.ryl'ed. See Erythroinycin ethylsuccinatc
Erythrityl tetrnnitratc. diluted. 625t. 626 Eryihrocin. See Erythromycin Erythromycin. 3001. 349—351
Esirunmule. See' Cloprostenol sodiutti F.taiiercept. Set' 'l'utnor necrosis lactur. recombinant
Erythroinycin estolate. 350 Erylhromycin ethylsuccinate. 350 Eryihromyctn gluceplatc. 350 Erythromycin lactobionate. 350 Erythromycin stearote. 350 Eiyihropoiettn alfa. 177—178. (462 P. (Taft's stenc parameter). 21 E.scrine salicylute. 548 Esidnx. See Hydrochkiruthiaztdr Eskalilh. See I.ithium carbonate Eckazole. See Albcndazole Esmulol. 544. 5451 E.sontepntzolc niafinestuun. 722. 723*, 724 Esurubicin, 416
Ethncrynic acid. 613-615. 6l4f. 620 metabolism of. 120 Etltanibutol, 254. 256 Ethanol. 219—220. Set' ala., Alcohols deltydrated. 220
Esters
Ethinyl estradiol 3-methylenc, structure of,
intolerance to. cepltalosporin-related, 325 tnechanismn of action of, 684 Ethanolamines, 702—71)4
Ethchlorvynol, 495 Ethinvl estr.mdiol. 780 as antineriplastic. 433 it. contraceptives. 7911—793*. 794 metabolism of. 71) structure of. 7771
hydrolysis of. 109—110
as prodrugs. l44—l49. l45l—l49f ljstradii,l, 775. St'.' also Estrogen(s) biosynthesis of. 7691. 770. 783. 7831 metabolism of. 775. 7761 preparations of. 779—780 receptor binding of. 782 of. 7701
structure of. 7771 Estradiiil bentoate. soluhility of. 7701 Estr.tdiol cypionate. 793—794. 793* Estrumustine. 437 as antineoplastic. 434 as mutual prodnig. 142—l43. (431 Eslratab. See Estrugen(sl Estnol. 775. See also Estrogen(s) tnetabolism of. 775, 7761 preparations of. 781) structure of. 7771 Estrocytc. See Estramustine Estrogen(s(. 775—785 antic%trogens. 781—783. 781f as antineoplastic. 433 aromatase inhibitors, 7143—785, 7831, 7841 biosynthccis of. 7691. 771). 775
for breast cancer. 779 breast cancer risk and. 433—434. 783 conjugated (equitte). 775. 776, 7761. 7771 preparations of. 780 in conlraceplives. 779. 790—794, 79lt—793t tlietltylstilbestrol derivatives, 776—778. 7771 cndogettous. 775—776 esterified, 776. 7771 preparations of. 78(1 in hormone replacement Iherapy. 779, 787. 796—797. 79th. 7971 metabolism of. 775, 7761 phyiocstrogens. 7781 products. 779—781
7771
Etltiodol. 482, 4821 Ethionatnide. 254. 255 Elhtno,ine. See Morictzinc Ethoheptazinc. 7371. 738. 748 Ethopropazine hydrocltluride. 585—586 Etltosuxitnide, 506 metabolism of. 142 Ethotoin. 505, 5(1St Ethranc. See Enflurane
Etliril. See Erythrontycin stearate Ethyl 4-amtnobenzoatc. 6711 Ethyl alcohol, 2 19—220
Ethyl chloride. 690 Etltylenediamines. 71)4—706 Ethylene oxide. 22(1
Ethylntorphinc. 7331 Ethylol. See Anmilostine Ethylparahvit. 229 Ethyl p.hydroxybcnzoatc. 229 I 7.r-Etliyltestostcmne. 79gm 2.Ethylthioisonicotinaniide. See El(tionamnudc Etltynodi.ml diacetale. 7871. 789 in contraceptives. 791, Etidocaine. 690—693, 6921
litodolac. 760 Etontidatc. 487—488 latonilazene. 741 Elonogestrel. 7871. 789, 793t. 794 Etoposide. 426 Etozoline, 615—1.16. 6151 Etretinate. 873—874
halI'liIc uI. 6 Eucainc, 677. 6771 liucairopine hydrochloride. 580—581 Etufles. Sec Flutamidc Eugenol. 222
Eulexin. St'. Flulatnide Euprocin. 694t
for prostate cancer, 779 pyridoxinc and. 893 receptor binding oF. 777 selective estrogen receptor mixlulators.
Ettmax. See Cru,tamiton
781—783. 7811 sleroidal. 776. 7771—7781
Excretory urogruphy. 478 Exeldemt. See Sulcunasotc nitrate Eselon. S.'.' Rivasligmine
structural classes of. 776—779. 7771 structure of. 7771 therapeutic uses for. 779 tryptophan metabolism and. 893 Estrogen receptor antagonists, 781—782. 7811 Estrogen receptors. 773 Estrogen replacement therapy. 779. 787. 796—797, 796,, 7971 Estrone. 775. See ala.. Estmgen(s) biosynthesis of. 7691. 770. 783, 7831 metabolism of. 775. 7761 preparations of. 780 structure of. 7771
Eutaittide, 26t Evista. See Exalgin, 76(1. 761*
Exetutcstane. 438, 784, 7841, 785 as antineoplastic. 435 Esna. Set' Benzihinzide Extended insulin zinc suspension, 1451. 851,, 852t
Eye. drug delivery to. 15%
F
Factor VIla, recombinant. 665 Factor VIII, 664t, (.65, 863
Index reconihiaattt. 167—168, 184—185, 665, 8591. 863
Factor IX, recombinant, 385. 665 Fameiclovir. 378 Famotidine. 7391, 72)8, 72! Fansidar. See Sitltadonitte•pyrittte:Itaiutiute Fareston. See Tnremtlcne Farnesyl translerase inhibitors. 440 Faseicles. 680, 6801 Faslodex. Sic Fatvestrattt Felbantate, 507 FeIlssuiI.Se,' Feihamate
Feldenc. Sit' Piroxicam Felodupitte. 63! Felyprcssin. 688, 847 in local anesthesia, 6811
l.etroeok
Flutorohutyrophetuones,5181 —501 5-Fluonucyuuusine. 235, 2351
971
staggered tu'rsion angles itt. 92$ Taylor series espatusioti attd. 925
Flttorotitetholone, 81)1, MIII. 812
Fctrhistal uitzuleate. Sit' Diuttetltindetue otaheate Forntaldeluydc strIation. 221)
l'luor'l)p.Sn' Fluorotttethu,lone Fluoriupte.s .5.'.' 5-I-luorucuracil Ftuoroscopy. 454. 4551 5-l'lttonuuracil. 44)5—4)37, 4(8.), 412—413
Formalin, 22:)
screetting data br, 393) Fluorous phases. cotttplementary. in
Fuirtuirtenil. 536—537 ltctrabtydntfitlic acid. 4111, 41111 Firtlan ('elta.'udinte surduutrt
Fu,onajtilid, 76)). 7611 lortutestatue. 7114, 784)', 785
cotuthittatoriat chetuuistrs'. 49
Fluutroxidittc. 406 I'litothane ci' Ilalotluane Etuosetine. 538 Fluosytttesterone, 7991 8111
Foscanuet sodinutu. 379 Ftuscavtr, .Su'e Fosc;urrtet sodiuun
Fluphuetuaciute hydrocltloride. 499t. 5)8)
Fosuuuidcrtitycin. 2'Nt:. 297— 298. 2971 Fttuir-cttropuutueutt t.Igi reaction. 4t), Sill
Ftusl'otttycin trurmethanrinc.363—36.l Ftrsititupril sodiutti. 6471. 647u. 648
Flurandrenolide. 832 rcltttis'e :tctivity oh, 83$): flaracepam, 492
Fenflununtine hydrochloride. 533—534
1"liurhiprucleuu. 759 l:luur(,seue toetahx.hisuut oh, 77
Fourier transt'ontu infrared spectroscopv. iuu cotutbittatcurial clut'tiuislry. SI Free eutergy pcrtttrhatitrut I FUll cahculatuotts. 934—935, 9341 atuatysis. 2). 26
Fenofubrate. 6611
Fltttantide. 436—437, Mill —8(42. 81121
Fruug seutcuttus. $35
Femhd. Sec Hormone replacement therapy Fetnstat. See Butocotuacok nitrate
Femum, Sue Ixrro,ok
Fenoproien. nuetabolistn of. 114 Fenoprolert calcium. 759 Fentanyl citrate. 737:, 738, 74:. 748 501
Feridex. See Feruntoxides Ferrixan. 477 Fernihentoglohin. 858 Femrnwgnelic substances. 476 Ferumonides, 477 Feruunoxtran, 477 Feverless, '11)7—91)8
Fexoleutadine, 9. 712—713
Fibrinogen. 663, 664. 6641. 66$t. 665 Fibrinogen receptor. 633 -634 Field blocks. 687. See n/so Local anesthetics I'ulgrastim, 178—179. 43)1, 433, 859t. 863 Filtration assay, in high-throughput screening. 54. 541 Finasteride, 802—803, 83)21, 8031
First-pass died. 7. 67 Fish liver oils, sitatutin A couutent of, 868t, 869 Five-atont nile. 557 Flagyl. Sic' Metronida'.'ole I-LAMP. Ste Fludat-abine Flasoperidol. 439 Flas'oprotctns. 89(3—893
Flanedil Sn' Gallatittne trieultiodide Flecainide acetate, 643) flexible conFirmation. 34—35, 341, 9311—933 l'loecutatuon, 175 Flolan. See Epoprostenol Flomas. See Tatnsulosin
Set Flaticasone propionate Florinet acetate. Sue Fludroeturtiscune acetate
Floroptyl Sir Isotluorphate Flovent. See l"Iuticasotue proptotuatc Floxiuu. See Oflcuxaciu
Flosuridine, 433 Flucinom. See Flutatnide l'luctunanule, 244 Flucytosine. 235. 2351 Fludara. See Fludarahine
l'ladarabine. 405.432 fiudrocortisone acetate, 81)71,8)68, 811 Flugerel. Set' Flutamide flukes, 265 Flutnadine. See Riuuiantadune
Flutnaeeail. 487, 489
flunisolidc. 8:2. 834, 814t Fluocinolouc, 809: Flutucinolone acetiunide. 8ht8L 809t. 8:2 Flaociuonude. 80')t Fluorine radiopharnuaceuutucals. 468
inetalsidistut iii, 1)4
F:or,ulut eu 'legal uur Full sahol. 755 Fntvestrauit. 7811. 783
as autineoplastic. 434 propionate. 812. 814. 8141 Flus'astatuut. 663
Fnncugulliut. aittiticopltustic activity of. 447
Flusestrant, 781, 7811 Flavoaantine.5 19
I'unctuoru,ul espression cluiuttng. :66— 67. 1671 Functuusnal genontics. :92. 1921 Fnutctiotuali,atiuuit reacsuutus. itt dnig
FML. Set' Anitrotitetliolone Focused library. 56. 61 l:olacine Sit Folic acid
tutetal,uulusru. 65—1,6, liSt. 69
Folate antagiuttisus, 4118—41)). 41111
l'ttngacctin Sue 'I'rrztcetin
Folate cocntyuoes. sttlfonantide.s and. 27)) 27:.
Fangal ituiectituns cntaiuetnus. 231. 2311. 232:
2731—2731
Folate redactase iuthihiittits, sullottztttuides arid.
topical ageutis h'uur, 233—235 uuppofluunistuc. 23)1—231
275
sabcnt&uneiutis. 231. 232t
Folic acid. 896-898 dietary sources of. 896 discosery ol. $96
superficial. 231. 232t systcuuttc. 23)). 232t tissue response to, 231—233 Futtgiiauue. Sue Auttphotericun 34 Furaciu.S,'t' Nitrtul'unu,t,urc
nueuaholisot ith 896-897 vitamin lb. and, 896—897 products. 897-898 structure oh. $96
I:uradauttuiu
Fuulic acid ,utttagonu.sl.s, 897
Folk acid derivatives. antineophastic. 4118—411), 43111
follicle—sttutuulatittg htontttituc FSH
Ill
—775.
7741.841,844 rectutnhitiaitt, 176
ci' Nitrtrhuurauutttitt 'Sri' I)ihtisautudc tnruuate
Furanuttidiunc 252. 253 l:utrosetutide 269. 61 l1—(r 13. 6111. 62)) Furoouune.Su'u 1-urantutidotuc Fttsiutn poutcins, 13,8 — I 69
in drug screening. 172
Follistint. Si',' Follitropin beta Follitroptn alia. 77 Follitropin beta. 377 Fuuls'ite. See Folic acid
Fiitttocaiite 694t I:iK4.dt-Jg iuttcnuclions. I) It. 132 cotttputcr-assisted dntg design intl. 944—945 Foradit. Sit' Fortttoteroh l:i,r&unesu.c. lsot)uraue l:u,ree field ntethtods, ')2)—')2'). See cc/ic, Compater'assisted doug desigut
G6PD, tttataria and. 283, 288- 289
(;AIIA. 485, 489 (3AIIA5 rccepuuurs. 485, 489 for auutcottsulsaruts, 51)4 Iuur autsiutlytics. sedatise. and hypnotics. 488, 489
br gcncr& ztttcstltettcs. 485 Gahapenlin. 51)7
hall-attd-s3trittg uttodels and. 923-925 t'ouloutth's law and. 927. 9271 cniss-tertns and. 929 eclipsed torsion angles and. 928 Fourier series attd, 928 harntonic appronunuatton attd. 925 umpnuper torsion atugle concept rod. 926 I.ennard-)ones potential aitul. 926
(iahttrul Sit' Tuagahtne
MM2 Fuce field antI, 927
Gadoversetattuidc. 476 Gadius'ust. Set' (hitlitlrnuriul (iadoseuic acid. 477 (Jalantamtne,Srr7 Gallatutunc trieultiodide, 591. 592—593
MM3 Force Odd and. 925—926, 927 MM4 force held stud. 925—926 MMFI"94 hiurce field attd. 925—926 in mculeculttr dyttautuics sitttultttions, 1)33 —939
Morse cnn-c attd. 925, 9251 quadratic equaticutis ut. 924- 925 restricted ottation and, 928
Gadohretuaue ttuegltuntutie. 477 tiadohuttruul. 473, Gaduudianuide. 476 Gad.ulituiuutu dtuuttrast agenus. 476 (iaduupcnteuutc ditttegluuuuituc. 476 (iuudurtcnute megluututrnc. 476
(Jadoteridol, 476
(Iallituiuu uuitratc, 43(4, 432 Galliuuruu ntdiurpht:trotaceuticals. 468
Ganterocytes. P/ut.orttn/inot. 284
972
Inde.t
Gamma scinhillation ca,TKr.i. 458—46(l. 4581.
Gatuophen.Sts' Hexachlorophene Ganalol. Sec Suliameilursa,,,k Gancirlovir. 377—378 Ganglionic hhiwking agenl.586—58') dcpol;iroing. 587—588 nondepolariring competihise. 587—588 nondepolauleing nonctimpelitive. 588—589 rgan-specilic eltccts of. 587. .S87r Gatiglionit' stimulation. 586—587. 5871 ((anile. See Gulliuuii nitrate Garamycin. Sc,' Gentamicin Garlic, 91(1—911
Gustnc enrsmcs. 4 Gastrix inhibitory pephide. 1)55
Ga.rh, mucirui. acid seereliun by. 7(8. 711(1 Gastric ulcers, 7(8—7(9 Gastnn. 854 Gastruinlesilnat Itoriutones, 854—855 Gastroinheslinul studies, 480—4441, 481)1 Gastrointestinal tract
it drug distributitm, 3—5. 41 hiwer, drug delivery K). (58 gaui-he conformation. 32, 33 Gaudier's disease. 186 Gaussian hype furntions. 937 GDP-hinding pauheins. 550. 552 Gelatin. 834, 8341 Gelatin flInt, 834 Gelatin sponge. 835 Ch!lIhIni. Sts' ((claIm
Gernian violet. 227
1dm
(h,lloatn. See Gelatin sponge Gemeitahine, 41)7, 413 Gcmeprvva. 8241 Genuiibro,iI. 659—660 Gemtui.umnah ouogzinhicun. III') liv antini'oplaslic. 4.13 Gemini. Ste Geuncitabiute GeutBank database. 16(1. 1611 Gender. doug metabolism and. 12')— 13(1
Gene(s). (62 cloning of. 164. 166—168. See also Cloning reporter, in drug screening, 171— 172 Gene capresalon. 167— (68 anhiscnsc oligonucleolides and, 193—194
definitiiun of. (92 helersilogous. (68 protcomics antI. 193 in recombinant DNA technology. 167—161) Gene expression systenus. 167—168 General anesthetics. 485—488 inhalational. 486—487 intravenous. 487—488
Generic library. 43. 441 Genii.stcin, 438
Gene therapy, (94 Genchic algorithms. 58. 61. 933 Genetic cloning. Sc,' Cloning ((cnetic engineering, (62— 166. St', also Recombinant DNA technology steps in. 858—86(1
Genetic factors, in drug metabolism. 128—129. 193
Gene translcr. (94 vectors in. 165—166, 661, (68 Genislein. 778—779, 7781
Geometric issiuners. 32 Geopen. See Carbenucillin disodium Germ cells. tr.tnsgetues in. 194
Geniticides, 2(7-223. 2 2181 classification of, 2(8. 2181 ellectiveness of. evaluation of. 219 improper use of. 21') phenol coeFficient for. 221 Giurdiasis. 261) (;ümkg,, biloba. 9(2—913 Ginseng. 913-914 CIa. 883—884. See also n.Carbmvtyl—glutamic acid ((Ia domain. 1)83
Glaiidular tnascariuiic receptors, 552 Glass hearts, in combinatorial chemistry. 49. (ii) ((leevec. See Imatinib (3Iiiil cells. 679 Glicluiide. ('711
(ilimepiride. 670 Glipieide. 67(1 metabolism of. 82
Global minimum, calculation iii, 37 Globin line iuistilimt suspension. 851 t, 852. 852t Globtiluns, 8331 Glomerular filtration. 596—601. 5971—6(8)1 Glossary. cml comnl,inatorial chemistry. 6(1—63 C,lucaGcn. Se',' Glucagon. recombinant Glucagon. 853—854
recombinant, (75 Glucocturticiuid(s). Sc',' ,ilxo Sleroid(sl as antineoplashics. 435 Iuiulogical activities uI. 18)6 hiosyitthe.sis of. 768—77(1. 7691. 804—805.
804f coiitramndicutiumns ii). Ml))
deficiency of. 805 inltnuuitssupprcssant activity ot. 81)6 inhaled. 813-815 nietabolisni of. 805. 8051 wtth salt retention. 807-8(18. 1)071—8081. 811 niodifucations of, 8(16—8(17. 8l)6t uiphtltalmic. 8l1)—%l I, 11111 product.s. 8071—808f. 811—815
relative activity iii. 8(19, 8(1)1 resistance to. 806 structural classes of, 8(16—8119
structure—activity relationships for. 8071—8081. 8011—809. 809t
structure of, 81)71
tapering of. 810 therapeutic uses of. Ml)) with very low-to-absent salt retention. 808—809. 811—812
Glucocorticoid receptors. 773 Glucophruge.St'u' Metlormiu dehydrogcnasc
deficiency. maluria and. 283. 288—289 Glucose metabolisnt. insulin in. 850 ta-Glticosidase inhibitors, 672—673 Glucoside prodrugs. 158. 1591 Glitcoleol. Set' Glipiiide
Glutamine. conjugation of. 116—117. ((61 Glutaralilehydc. 2211—221
Glutathione acetatiminopluen toxicity arid. 96—91)
conjngationoi, 117—121. 1191
tna'tivalion of, 336
Glyset. See Miglitol GnRH. 774 GnRH agonists. with antiatidrumgen.s. 81)1 Gimnadoliberin. 1)41 Gimnadotropic hortuuimnes. 844—845
Gonadoiropin.releasing liorutiune (GnRIlL 774. 7741. 841 Gtmnadiuiropins. 773—775. 7741
(iimuial'F. See Follitropin alla Goserelin. 437 with antiandrogens. 1401
GPllli/IIIA receplors. 633—634 C proteins. 551). 552 in vision. 871 G.quadriplexes. 448, 4481 GR-175737. 728—72'). Gramieidin.359—36(t Grain-negative bacilli, drug-resistant. 31)1 3(15—3117.335—336
Gr.inutiucyte ciulony-stintulating factor. recumbinatil, 178—179. 863 Granulucyte.mnacrophage colony.stitnulauirg factor, recombinant, 179 c;runitliwymes. 197.
981. 2)1))
Grapefruit-drug interactiiitts. 1311. 132 Graph theory, ut drug design. 23—24. 241. 24i GRAS list. Gray baby syndrome. 115. 126 Grid searching. 932
Grilulvin. See Gns'olulvin Grisactin Sc',' Griscofulvin Griseolulvin. 238. 3001
Gris-PEG. S,', Griseufulvin Growth Imorntimne. 1(44
recombinant. 1611. 175—176
Growth hormone receptor, cloning of. 172 Growth-releasiiig factor. 841 CT functions. 937 G'l'P-hinding proteins. 551). 552 (4uanaben, acetale. 534. 653 Gttauadrel, 529, 651 Guanaiole. 428 Gttancthidine. 529, 651) Guanfacinc hydrochloride. 534. 653 Guantne. alkylation of. 398.399. 3991 Guanosine rhiphospliate )GDP)-binding priiteins. 552 Guanosine niottophospliute )GMPI, in smooth muscle relaxation. 2641. 623—624
tripliosphate )GTP).bindiflg proteins, 550
((-WellS,',' Lindane (iyne.Lomrmntin.Sm'e Clolrtnsa,olr
Gluctimnidali,itt. 112— 115. 1121—- 1141. 1121
Genomic libraries. 164, l64t
Gentanticin. 3411
916—917
Gly-Oxide. Si',' Carhanmide peroxide topical solution
Guancthidiuue uttonosullate. 651. 6511
topical, 81)9—81(1. 8119t
Genomics. 191—193 hioinIormuatics and. 191 —(92 DNA microarrays and. 192— (93 in drug development. 448—449
functional, 192. 1921
t- Glyburide Glyhundc. 67)) Glycine. conjugation of, 116—117. 1161 P.glycopri8eins. in drug resistance. 392 Glycopyrrolate. 581 Glycyctines.348—34') ((h-ct rrlmi:u glahra var. typic,c (licorice), Glyass.
Grocillin See Carbenicillin indanyl sodium
4591
in drug nietubolisnt, Wi, 73, 98, III. 117—121. 1191 Glutuhliiutnc
Glutelins. 8331 Gluteilmimide. 495 metabolism uI, 82
recombinant. (69
U H, receptors. 691) H1 receptors, 69)1—69') H3 receptor antagonists. 727—73(1
H, receplors. 699, 727 receptors. 699 HA acids. 15—16. 61 !Iuu'imtoplmiltcs mnflue,:znc vaccine. 2121. 214
Halaiepam, 491 Hala,vonc. 224
Index 812 Hukion. See Triaro}am Haldol. See Halopentlol Halfun. See Ilalofautinne Hallucinogens. $-amylamino. 520—521 Halobctasol propionate, 8081. 812 Halofantrine. 293—294. 293f. 2961 Halogen-containing germicides. 223—224 Halopcridol. 501 Haloprogin. 234 Halcinonide. 14(1141.
Halotestin, See' Fluoxymestcrone
Halotex. See Haloprogin Hulothane, 486
metabolism of, 11)1 Hainmeu's constant I'll). 2), 211 Hand washing. importance of. 218-219 Hard drugs. 142 Harmonic appronirnatiou. 925 Harmonic simulations. 935 Hartrcc'Fock lImit. 938 Hiuruee-Fock-Rootbaan equation. 938 Hartree-Fock wave (unction. 937 Heart disease. ischemic. 622—623. 6231
Hectorol. See Doxercalciferol Hedeoma pulegeoidts (pennyroyal). 915 Helicobacier priori. 719 Helixate. Sri' Antihemophilic (actor. recombinant Helminthic infections. 264—265
Helper I cells. 200 Hemnabate. See Carboprost trometltamine Hematopoiesis. 177. 1781. 197. 1981 cytokines in. 77, 1781 Hematopoletic factors, recombinant. 862—863
Hematopoietic growth factors. 177. 1771 reconubinant. 177—179. 1771 Hemoglobin. 857—8514
Hemoglobin C, malaria and. 283 Hemophilia. recombinant clotting (actors for. 167— 168. 184—185
Hemophilia A. 665 Hemophilia B. 664 Hemophil M. See Antihemophilic factor Hendem'.on-Hasselbalch equation. 13
Henle's loop, sodium reabsorption in. 598—599. 5991
Heparin endogenous. 665 pharnuceutical. 667 Hepatitis A vaccine. 211—213. 2121 Hepatitis B vaccine. 1146. 2121. 213. %59t. 8601 Hepatitis C vaccine, 213 Hepatitis E vaccine, 213 Hepatocatcinogenicity. of amides. 96 Hepatotoxicity of acetaminophen, 96—98
of bromobenrenc. 73-74 Herb(s)
definition of, 905 as food additIves. 904—94)5 Herbal medicines. 904—917
active ingredients of. 905 adulteration of, 905 appeal of. 904 for cancer, 424—428, 915 chemisuy of. 905 elassthcation of. 904 drug properties of. 905—906 GRAS list and, 904—905 historical perspective on. 904 purity of. 905 regulation (if. 904—905 standardization of, 905 types 01. 906—917 Herceptin. See Trastuzumab
Heroin, 731. 733, 745 Herpesviruses, 3701, 372
Herplex. See Idoxuridine Hetacillin. as prodrug. 143—144. 1441 HETEs. 820. 1422m
Hetrazan. See Diethylcarbamazepine citrate Hexachlorophene. 221—222 2.4-Hesadienoic acid, 230 Hexaeehyltetraphosphnte (HTEP). 569—570 l'lexahydropyrazine. 265 Hextilen. See Altretiunine Hexalgon. 739t Hexaniethonium. 588 Hexamelhylcnetemratnine. 253 Hexamethylenctetraminc mandelatc. 253 Hexamethylmelamine. 429 Hexamethyl-p-rosaniline chloride. 227
llcxobarbital. ntetabolisnt of, 80, 94. 109 species differences in. 129 Hexylcaine hydrochloride. 6911 Hexylresorcinol, 222—223 Hib-CV vaccine, 214 Hibiclens. Set' Chlorhexidine glucunate High-ceiling diuretics. See Loop diuretics High-density lipoprotcins. 658—659 High-osmolar contrast agents. 473. 474t High-performance liquid chromatography (HPLC), 833 in combinatorial chemistry, 51,61 High-throughput screening. 26—27. 40. 40f, 43. 53—54, 541, 55, 61. 944. See also Combinatorial chemistry fultt'ntion assay in. 54. 54f
scintillation proximity assay in. 54. 541 terminology of. 60—63 Hipres. See Methenamine Itippurate Hirudin. 185 Hismanal. See' Astemizole
Hispril. See Diphenylpyraline hydrochloride Histadyl. See Methapynlette hydrochloride Histamine. 696-700 acctylation of. 122, 12M biosynthesis of. 696. 698f distribution of. 696—698 functions of. 700 ionization of. 696 life cycle of, 696—7(8) metabolism of. 699-700. 6991 release of. 698 stereochemistry of. 696. 6981 storage of. 6914
structure of. 696 tautomerizauion of. 696, 6971 Histamine H, antagonists. See Aatmhistarnines Histamine H, receptors. 698 Histamine H2 antagonists, 7(141. 7 18—722
structure—activity relationships for. 719—720.
7 l9f structure iii. 719—720, 7191 types of. 720—722 Histamine H2 receptors. 698—699 Histamine H, receptor antagonists. 727—729. 7281
Histamine H, receptors. 699. 727 Histamine H4 receptors. 699 Histamine receptors. 698-699 Histiocytes. 198 Histones. 833t, 835 HIV. See Human inimunodeficietucy virus
HMG-CoA tcductase inhibitors. 662-663.6621 Holo,tan. See lfosfamide Homatrocel. See Hotnatropinc hydrobromide Homatropine, 676—677. 6761 Homatropinc hydrobromide. 578 Homatropine methylbromide. 578
Honiology.based cloning. 167, 167t Homology ntodcling. 56 Hooke's law. 924 Hookwonn Infestations, 265 Hopkins-Cole test. 1434 l-lormone(sl. 840—1457.5cc ulw Steroid(s) antineoplastic. 433—438 gastrointestinal. 854—855 gonadotropic. 844—845 Itypothalatnic. 1140—84 I neurohypophyseal. 845—847 pancreatic. 847—854 pituitary. 841—844, 842t placental. 845
rDNA-derived. 861 recombinant. 175—177 thyroid. 1145 Hormone replacement therapy. 779. 787. 796—797, 796t, 7971
Hot pepper, 910 HPETES, 1120. 8221
HPMPC. See Cidofovir 5-HT,A agonists. 520 antagonists, 519—520
Humalog. See Insulin, recombinant Human anti-mouse nntibodies. 442 Human choriomainmotropin. 845 Human chononic gonadotropin (hCGI. 775. 845
Human deoxyribonuclease I. recombinant. 185— 186
Human Genome Project, 160 Human growth hormone, recombinant. 168. 175— 176
Human growth hormone receptor, cloning of. 172
Human itnniunodcfieiency virus. 3691, 372 Human immunodeficlency virus infection chemokine receptor binders for. 387 gp4l fusion activity inhibitors for. 387—388 HIV entry inhibitors (or. 387 HIV pnutea.se inhibitors for, 384—387 integrasc inhibitors For. 3811 newer agents for. 382—388
reverse trunscripaae inhibitors for. 372, 379—381
vaccine for. 382—383 Human immunodeficiency virus pmtease inhibitors. 384—387 des'elupment of. 942. 9431 Human placental lactogen (hPL), 845 Human plule.derived growth factor. recombinant. hO Human 1-cell leukemia virus (HTLV). 3694. 372
Human'uumor-colony—forming assay
(HTCFA), 394 Humate-P. See Antihemophilic factor. recombinant Humatin. Si-t. Paromomycin sulfate Humatrope. See Somutmpin for injection Humoral immunity, 200. 202—203 Hutnorsol. Set. Dcmecatjurn bromide Humulin. See Insulin, recombinant Hyaluronida.sc for injection. 838—839. 8394 Hybndoma techniques, 187—189. l81)f Hydnntoins. 504—505. 505m
metabolism of. 109 Hydralazine. 653—654. 6541 metabolism of, 122. 1231
vitamin B,, deficiency and. 893 Hydrazides. metabolism of. 122. 1231 Hydraeines. metabolism of. 122. 1231 Hydren. See Hydroxyureu Hydren. See Benzthiazide
974
irrtkx
Hydrocarbons. polvcyclic anrniatic. carcinogcnicity *1. 74. 741 Hydrochlorotht;izidc. 605—610, 606t. 6(1St. 620 Hydrocodonc. 733. 7331 Hydrircirdonc hitartrate. 746 Hydrvrcooisone. See alan (ilucocorlicoidlsl analogues o(. 81)6—11(19, 81)61
biological activities of. 806 biusyuthc'.is iii. 7691. 770, 804—805, 8041
deliciency of. 805 metabolism of. 805. 8051 prepanitiouts of. 81171—8081, 809t. SI!
relative activity of. 8(84 volubility of. 7701 Hydrocortisone acetate. solubility of. 770r Hydnucunisone esters. 8071 Hydrocorlisone NaN.)4 salt, volubility of. 7711t
llydrsuDltJRIL. See Hydrochlomthiazide llydrotlunuethnaiide. 605—611), 606t. 6081. 62)) Hydrogen bonds, 3(1—31, 31 t. 33—34
Hydromorpluone. 733. 733t. 745—746
Hydromos Set' Qainetharone Hydrophilicity. 31 Hydrophobic bonds, 3). 311, 831 Hydroiltiaeide diuretics. 605—6!)). 6061. 6061. 60%t
llydrou'. beurcoyl peroxide. 223 Hydroxocobalanuin. 894—896 l-lydroxyaiuphetamune. 537—538 acid. 22% Hydrvusvcarbamide. Set' Hydrouyurea Hydroxychloroquine. 2871, 28%. 295i 2—Hydnuxyestrogen. uretabuilisnu of. 12))
N-l-lydroxylamides glucuirunidation of. 114 sulloconjuigation of. 115 N-Hydroxyliitnines glncuronidalion ol. 114 sultoconjugalioti of. 115 Ilydrovylated anilines. 76(1—762 I 4-Hydroxyniorphone derivatives. 735 m-Hydriuxyphenol. 234 N-Hydroxyphenuernrine. nietabolisuti of, 91 Hydrosyprogesterone caproate. 786t, 7871. 788 8-Hydnixyquinnuline. 26), 471 Hydroxysuaurosporine. 439 Hyrlmxystilbamidine isethionule. 66% Hydroxystreptomycin. 337 Hydroxyturea. 42%, 43!
I lygroton. Set' Chlorthulidotie Hylorel See Guanadrel Hyoseine. 577 Hyoscine hydrobronride. 578 Flytiscyanrine. 574—575. 577 Hyoseyainine uu)fatc. 577
Flyperhiliruhincunia. neortalul. 115, 126 Hyperglycemia, in diabetes. 850, 85! lIvlut'rit'iinu pn'rjoraiitin (Saint John's svort). 908—9111
Hyperlipoproteinemia. 658—659, 659i in diabetes, 85(1—851
Hypersensitivity to aspirin. 82(1—821 to cephalosporuns. 325
to contract ugentu. 48! to local auue.stbclics. 689—690 tin penicillins. 31)8—3(19
Flyperstiut IV. Sen Dia,onide Hypertensin. See Angiotensin umide Hypertension. 642 —645 Hypeuihyroidism. 673 —674 Hyperv'ariable eegiruni. 188
Hypervitantinosis A. 870-871 Ilypcrvitaminosts I). 876—877
Hypnotics. See Anxiotytics, hypnotics, and
lutttnunohiotvigicals. 2(16—2)1'
sedatives Hypochlorxiuu acid. 223—224 Hypoglycemic agents. 668—673
limttrtuniuglohultuu s(. 202—21t6. 2(141. Set' also
higuanidines. 672 a-glucosidase inhibitors.. 672—673 nietuglinides. 67) sullonylureas. 668—67(1
Ihiazolindiones. 671—672 Hypoproihrontbinemia. cephalosporin-related. 325
Hypothalamic hormones, 840—841 Hysrerursalpingography. 480. 4801 Hytrin. See Terazosin
dcliniiion of, 21)6 examples of, 21(7 Antubuuslres
types 01. 2(14). 206. 2)161 lntmuniustiuttutants, anuineoplastic. 4411—44)
lutmunotherupy. for cancer. 440—442 lnmplunoit. Sit' Etitiurigesirel Improper torsion angle concept, 926 Ituturan. Sun' Azatltuoprumic
Inactivated polio vaccine, 21 tt, 21 lnamnnouc. 657 lndapamidc. 607—61(1. 18)71. (rO9t. 6211 lndcr.d. Sue l'nipr.iuuiihiul lndiuuavur, 385—387
1-131 —Metaiodobeneylguamdine sulfate. 469
Ibrirumoinab tiuxelan. 191 Ihuprofen. 758—759 metabolism of, 81—82 Ihutilide, 642 Idumycin. See Idaruhiein Idaruhicin. 4)5, 416, 423 ldeniity variables. 23 Idoxuridine. 375—376 as ptsrdmg
activalion of. 154, 1551 in chemical delivery. IS?
IItX. Set' Ilosfamide Ifosfanride. 4(81
activation of, 396 IgA. 206. 2061 IgO. 206 Igli, 21)6 lgG. 2(141. 21)6
1gM, 2041. 206 Ilopmst. 825 Iliusone. See lirythrontycin estolate Ilurtycin. See Erythromycin
Ilolycin (iluceptate. See Eiythromycin gluiceptule
Inuaging studies. radiiupharmaceuticals for. 454—484. Ste nifici Kadiopltarmaceuticals Imatlnih. 439. 44(1 Intciromuh pentelnre. 191 Inridazoles. structure—activity relationship for, 531 -532 Imiduzuuline receptor. 534 Imiglncerase. 186, 839, 8391. 859t. 861 Imipenern'cilastin. 317—318 Imipramine, 516—517 active metaholites of, 135r metabolism of. 771, 85. 87
Immune globulin. 207 Immune syslem cells of, (77. 1781, 197—2(81. 1981 self vs. nonsell in, (97 lmnuunity. 200—21)6 acquired (udaptivc). 2(81, 211111, 202—206, 21)6—216
developnient nil. 942. i)43) Indium
capriuntab pemudetide. 47(1
Indium chloride injeciuun, 47(1 Indium Oncoscunt ('RJOV. Set' lndiunr satumonrahpcnilctidc ursinc, 471 Indiunt pentemauc. 471 Indium Indium
pcuttreotide injection, 471
radiimpluarmaceuticals. 469-47) Indumutut satunriimahpcndctidr. 47(1 Induucin. Ste Induinuethacin lndomcuhacium. 754, 758 luidiumum (Hi luu
uuuciahsulisnu iuf, 98, 1119
Infergen Set' Interleron Inlultm—,itiiun anc.sthcsia, 687. Sn',' nil.sin Local anesm lieu ucs
lnflautmituatiott
ar,uchtidonic acid cascaihc in. 8)8, %l9L 82i)1 eucuisanoids in, 8)8—822. ,S'i'i' tiliti
Licosanoidis) Inflisimnab, 19(1
Influenza lype A, 372,373 Inlluenea vaccine, 21)9 Infrared spcctrosciipv. iiu csimbinati,riat chemistry. SI Inhulatiotual iunesmluctics, 486—487 Inhistiumm,Sr'e I'heniranuinc malcute lnitatc iumtmnnity. 2181—202. 2)Xti, 21)11, 2(12) Innovur.,Vee Fcuiuaimyl-droperidol
Inivsitol. 9fl()9tJ) Insecticides, drug imieuabolisnu and. 131 In siliciu (virmutal) screening .54 -55, 56. 63. 1)
Insulin. t76, 847—853 umiuio acid sequences in, 847. 84)6. 849 hiuusynlhesis ut, 847. 8481 huivine. I 7(u Iuulurc devs')impnucitLs fvir. 853
tnac)ivu)iiin iii. 849 lcnte, 851. 851i. 852t nteiaholic effects nut, 85(1—851
umietalnilusm of, Ill muumditicatiiuns nil, 849
porcine. l7rm preparatiiins nil. 851—853. 851t. 852t liroductisiut nil, 849
types of, 2116 unamneslic response in. 2115. 21151
uretiumuhinanl lunmuran, 168. 175—176.849.
reactions in. 205—206. 2051 celI-medinted. 21)2—203 cellular, 2(81
regular. 85!. 85 It. 8521
huimoml, 2181. 202—203
innate lnntuntl), 2(81—202. 2(8)1. 201t, 2021. 21)6
mucosal, 200 scrosal, 200 Immunizations
859t route ml udmuriuiusmr,ition um(, 852 853 secrmsuuimn nil, 849
species dillcrenccs itt. 847. 848t structure—activity- rclutiiumisliups fuir. 849
Insulin analogues, 849 Itusulitm infusinun devices, 853
Insulin unjectiimn. 85). SSlt Insulin punips. 853
definition of, 207
Insulin rcccplsur. 8511
schedule for. 212t, 215—216 vaccines for. See Vaccine(s)
Insulin ziiuc suspension. 85), 8511. 852u Intul. Su'i' ('ronrolyuu snidium
975
/nt/e.t
Integrilin Sri- liptilibatide Intercalated cell', ol itepliron. 6)8) lnterleriini vi, 179—IS)), 201 —202, 21) Ii. 2112t alpha. (79 — ISO. 17% I SOt, 201 -21(2. 202t
antiviral activity ii). 373—375. 3741 3751 antineoplastic activity ol. 441 antiviral acus ity of, 373— 375.3741. 375) beta, 179— ISO, I 7th 201 —2(42. 21)2t
anliviral activity iii, 373—375. 3741. 3751 classification iii. 17')— 18(1, 179t, t8(It functions 0). 1771. 179, 21)1—2)12. 21)31 gamma. 179—180. 17th. 2(11 --2(42, 202t
a'ci'nthinant. 180-182, (SOt. ISli, 85th. 81411, 861 —862
antineoplastic actis ity ot. 44) anthtml activity ii). 373— 375. 374) Intc'rfrron aIIa-2a. (8)). 181(1. 861. 869t
anlineoplastic activity iii. 44) lnterleriin alfa-2h, 1811-181, 1811), 859t. 86 1—862
.intineiiplastic activity itt, 441 Interlerrin alt'acon- I. I SOt. 181 Interferon alla-it I. (Slit. IS) litierferon aIIa-n3. l8()i, 181. 85th, 862 attliticoplastic activtlv of, 44) Interferon beta- I a, (SI — 182. 1811
lttterleron beta- lb. lsIt. 182. 8591, 862 Interferon gaiuuiia- lb. 1811. (82, 859t lnterleakitts, reciinibinani, 182—183. 44) --442, 859t
lnterntediate-density lipuproteins. 658—659 Intenin Sit' litterferon alfa-2b Intetstitial cell——sliuttulating (tiirutotte. Sec l—uteirtietrtg hurntinte I_Il,
Intestinal dnig ntetabolistri. (.6, 67 Intestinal eneymes. 4 Intestines, Si-c aLit,
Gastriiiuttestinal
drug ilelivery to. 158 Intiwosirin See ('attire Intramuscular injection. dnig distribution anil. 41. 5 -6
Intrauterine device. progesterone. 793t. 794
Intravenous adittinistratiou. drug ilistr.hiittiui in, 41. 5 lntnivenous anesthesia general. 487—488
regional. 687, Sit' alsii Local anesthetics Intravenous itnniiitie globulin. 2(17 Intravenous pyeliigrnpby. 478. 4781 litlravenous regional anesiltesia, 687 Intravenous urography. 478 Intrinsic lactur, 895 Intron Inter(eriin alfa'2h Inlrons. 162 Inverse agottisls. 485 Inversine. Sec l'rittielhaphan cainsylate Invirase, Sec Saquinavir 586. 679 Involuntary nervous system. lobeuguane sullate injectiou. 469 Iobitnii(iil. 482 locetamie acid. 482. 482) Iodine. 223 Iodine rudiophurntaecuticals. 4(i)) —469 lodipamide tneglutrtine, 482—483
Iiidinanol. 483 Iiidopliors. 223 Iiidixpiinol. 261 Iodntopz. Sec Sodiuiit iodide I 131 lohevol, 482 Iimattiin. Sri- Phencermine iott-escharige resin Ion channels, 553, 681
Iveruiectin. 268 Iviitiiec. S,'i' Iverinectiiu
681 —683. (.82)
piitassrtint leak, 681
seleciisiiy of. 684-685 sodium. 681. 682—683, 6821 truusmitter'gated. 683—685 viiltage.gated. 681
Iou-dipole bonds, 3Ii Ion encliange chromatography. 834 Ionic bonds. 3(1, 311-Sit' u/in Hotid.s Ionic ratio P.S contrast agettls. 473, 4741 Ionic ratii, 3 cirutrast agents. 47) loniciug radiation. 454. 457 lim Ir.utspon. 681—683. 682t lopanudol. 483 lopanoic acid. 483 Iiipidiiie .Sei' Apraclonidine loprotuide. 483
bind. 483 loversol, 483 lievaglale. 483 lovilan, 483 Ipratropimtr hri'iuide. 578— 579 Irbesartan. 649 Irinotecan. 426 Iron, in hemoglobin. 83$ Ischentic bean disease, 622-623. 6231 Ismelin sultate, 5cc Cuanei(iiditie niimosullate lstiactinotuycius. 415 Isobars. 455 Isohfeiimycin
Ketliui..Si-,- ('ep(ialiclhiui
Ketitnil. Sit- ('epltalestn Ket,i,( Set' ('eiaei,Iai Keatardniu .S.'i' Priicyclidine hydrochloride Keratolytics. 222 Kerlitite. Si-i' Hetaviilol Kcrnicienis, neonatal, 115. 126 Ketalar. 5cc Ketainine Ket.attitie, 488
titetabulisiti ol, $9 Keiiibeimndonie. 73ht
Keiocona,.iile, 212- 243 Ketone carhony Is, ntietalx,Iisuti ol, 11(1—11(7, (12— 113
Kcmimes, metabolism ot , 'N III) - (32— 13,1
Keitipritien. 75i) Ketivolac trotiiethaniine, 759—7611
Isoetharine. 537 Isolliiiirplnaie. 51.1)
Kiditey Ncr itfiti Netthntn
Isiinieric traiisition. 455 456 Psi titter' i-i.,. 31—32
Kliaiopin .5-c Koate III'. Sic Antil.cmophilic ('actor.
£32,321
recoitthtnanl
geometric, 32 4-36. 35. 371
KiSdIiNate.Si c Antilietitoptiilic faclor, recivttbniani Kogenatc.Si-i-Antihentophilic lactor. cecoinlvicint
5, 35 -36.371 maci. 3 1—32
iii acetylcliiiline. 34—35 7, 32. 321 Isottiethailone. 738, 7,(9t
Kt yofinie, 71, It. 762 $1' 61-19. 42tt
Knpller cells.
Isoniacid. 254— 255
interaction of with 124 metabolism it. 122. 1231. (24 vitamin II,, deOciency anil. 893 Isoutcotintc acid ltyibr.ieidc. Si-i- Istiniacid Isitnicotinyl hyilracide. Sc-c Isinmiaeid Isopcntaquine. 288-289, 2891
tunic Isophane insulin suspeuisiin, 8511. 852. 852i Lsiipm)vnniile iodide. 584—985 Isopropanol. 2211
lsiipnipyl alcohol, 220 Isopruterenol. 536 nmeiabolisui of, (25, I 26
IsoptinSi'i' Verapainit Isurdil..S'm'c Isosorhide ilinitr.ite, dilinied Isitsorbide. meiabolism itt, 12(1 (sosiirhide dinitrate. itiliitcd, 625m. 627 Isosreres. 4(1—41, 4It 342. 3431, 3-13 Isottipes. 455
Isotrelitloin. 873 Isradipine. 631
calcium. 628. 6281
Iterative deciinvolution. in combinatorial chemisury. 50
solute reabsiirputiin inSiIh.__64l I. 5i)7) -
saltienamide tnjtiry of. 122, 274 Kidney failure, drug uiielabiiliies in. (34 Kinins, in blivvl pressine regulation. 6—14—645
iii acetylcholiue. 34-35 cuiiforinatioual.32—- 33
ltracona,ole. 243 24-I
Ketotilen lnmar,ite oplalt.ilutuc solution, 717 drug escretiivt via, 41. 5
Isolluraiie. 486
Isuprel. See lsiipnoterentol
681
Kallidiu. 851. 857 Kalhkreins. 144, 557 Kanamycin sulfate. 139 aittitubercalitus actis its ii). 254.339 iicictisatvvi ol - 116, ill.) Kantres Sri- Kanatttycin snllate Kellev, Si-i' Cephalesin
Isodine Sit' Pin idiine-iiidiiie
acetylcltiiline and. 548—55(1
ueurotrausntutters .ini), 683—684. 684
K Kabikiutase. Si'i' Sirepti'kitiase KaI,i-a,ar. 264)
Ks.
I
(.inilauie
Kss iIiI,iite. Sc,' l.utd,nie
IAAM, 73$, 739t. 7-19 I .atueling mdcv. .1') I
I ,,ibetiulol. 54), 5461 /d-I mctani uitibn,tics_ .1(11
-
31-I, .1111 —33.1
ii!',' ,-\itt,hiotit-s: ('cplialitspmniius:
('enicilliittst itoiihle-estcr loon ol, 146 (37, 1481 /'si'ui/tilvii;uit resistance tim, 325 sirncture itt, .1) 12i - 3113t
$'Lactamti.ise niltibitors. 5(4 118 carhapenenis as, 3 If,— 3(5 cephaliis1v.tins ms. 323- 325. 324), 326i
class 1,315 -ll(i classiticattivi '.1, 115
class II, 1(5 ilotihlc-ester lorm of, 146 147. 14Sf insestigaitintal. 3)8 nttecliattisni ill actomti .if_ 3(5,3131 pcnicillin.ise resistance .1,124 siructure of, 3151
sasccptilvlils ii,. 3(5—3 to types uI, 314.— 118
.'m'i'i'
976
Inde'r
acuitnases. 306 cephalosporin susceptibility to. 324 elussiftcauon of. 315—316 inactivation of. 315—316 by ccphalosporins. 323—325. 3241. 3261 rcsislance to. 309. 3091, 326t Lactoferrin. 201 Lactoflasin. 891
ltctylphenelidin. 7611. 762 Lamiclal. See Lamoirigine Lamisil. Set' Terbinafine hydrochloride Lamivudine, 381 Lumolrigine. 507 Lamprene. See Clofazimine l..attgerhans cells. 198. 199t Lanusterol t4a.demelhylase, 240. Lanoxin. See Digoxin Lansoprazole. 722. 7231. 724—725 Large intestine, drug delivery to. 58 Lariam, See Mefioquine hydrochloride Larotid. See /smoxicillin
Laser optical encoding. 53 Lasix. See Furosemide Lasofoxifette, 78 If. 782 Latanoprost. 828 LCAO method, 937 Lead compounds, 59—60. 591. 61—62 Lecithin, 901
Lee and Richard surface. 922 Leishmaniasis, 260 l..ennard-Jones. potential, 926 Lentaron. See Fonne.stane Lerne insulin. 851. 85 It, 8521 Lepirudin, recombinant. 11(5 Leprosy, 279—280 Leritine. Set' Anileridine
Lescol. See Fluvastatin Letrozole, 435. 438, 784. 7841. 785 Letter. See Lcvotliymxine sodium Lettcine..enkcpltalin, 744, 843 Leucovorin, 410 Leucovonn calcium, 807—898 Leu.cnkephalin, 679. 744. 843 Lcukeran. See Chlontrnbucil Leukenn. See Mercaptopurine Leukine. See Sargrwnostim Lcukotrienes. See also Eicosanoid(s) biological activity of. 822t biosynthesis of. 8201 Leuprolide, 437 with untiandrogen.s, HIll as antineoplastic. 435 Leurocnstinc. See Vincrisljne sulfate Leustutin. See Cladribine Levalbuterol. 537 Levallorphan. 739. 740 Levalloephan tarlrate, 751. 7521 Levamisole. 441
Levanone. 7391 Levutol. See Penbutolol Levobunolol, 543. 5441 Levodopa. metabolism of. 125 Levo-Dromorun. See Lcvorphanol tastrate Levoid. See Levothyronine sodium Levomethadyl acetate hydrochloride. 749 Levonorgestrel. 7861, 7871. 789 in contraceptives. 79lt—793t. 794 Lcvonorgestrel-releasing inleaulerine system. 793t, 794
Levuphenacylmorphan. 740
Levoproine. See Methotrimeprazine Levomsine. See Levothyroxine sodium Levorphanol tartrate. 739. 740. 750
1_evothyroxitte sodiutu. 673 Lcvsin sulfate. St't' Hyoscyatninc sullate Lewis structures, 935 LH. 774—775, 7741 Libraries, combinatorial. 26—27, 43. 441, 55—58, 62. St't' also Combinatorial chemistry Librium. See Chlootiasepoxide hydrochloride Licorice. 916—917 Lidocnine hydmchlonde. 639. 6391. 67(1
ftrst.pass effect and, 7 half'life of. 7 metabolism of, 85. 109 Ligation, in cloning. 165 Lignocaine. 678. 69(1—693, 692t metabolism of, 686. 6861 Lincocin. Set' Lincomycin Lincoinycin. 353—354 mechanism of action of. JOttt
Lindane, 261)
Linear chain molecules, combinatorial synlltesis of. 45—46 Linezolid. 363 Linkers, 48—49. 41(1. 62 Linoleamide. us sleep-promoting agent. 4118
Liothyronine sodium, 673 Lipid(s) classes of. 657—658 metabolism of. 657—658 suucture of, 657, 6571 Lipid-lowering agettls. 659—663 HMO-CoA reductasc inhibitors, 662-663 Lipid membrane bilnyer structure 19, 91 chemical nature of, l9 chmmtesterol in. 231, 2321
drug movement across. 19
drug panitioning and. 19—21. 191 ergosterol in. 231. 2321 n.octanol/watcr syslem model of. 19—21) panitionirig phenomena and, l8—2l. 191 properties of. 19 receptor component?. of, 2$. See al.r,i Receptor(s) Lipid metabolism, insulin clfects Un. 850—851 Lipid solubility. 5 pK,,and. 16—17 Lipid-soluble vitamins. 866—8115. See also Vitamin(s)
Lipinski Rule of Five. 40. 55. 62 Lipitor. See Atorvastatin Lipophilicity. 31.65 Lipoprotcins classes oF, 657—658 metabolism of, 657—658
structure of. 657, 6571 Lipotnipins, 843—844 Liquefied phenol. 221 Lisinopril. 645—646 Lisler. Joseph, 217. 221
Lithanc. See Lithium carbonate Lithium carbonate. 503 Lithium citrate. 503 Liver, drug metabolism in. 7—8. 66—68 LKTs. 820, 822 Local anesthetics. 676-694 alkaloids. 690 690—693. 6921, 692t
aniline derivatives. 690, 691.. 6921, 692, benroic acid derivatives. 690, 6911, 692f. 692t. 6931 buildup of. 687 cardiovascular effects of. 689 central nervous system effects of. 6119 classification of, 690
definition of. 676 discovery and development of, 676—67 duration of, 688—689 eIfcctis'eness of, 687—61(11
elintination oL 61(7 in epidurttl anesthesia. 687
ester-based. 690-693, 6911. 6921, 6931 in field block a,testhcsia. 687 fluid pH attd. 6118 general, physiology of. 485—486 hydrophilie center of. 692—693 hypersensitivity to. 689—691) with hypotltemmic action. 690 in infiltration anesthesia, 687 in intravenous anesthesia, 687 lipophilic center of. 690—692, 693 mechunism of action of, 684—687. 6851 metabolism of. 685—686, 6851 methetnoglobinemia and, 689 miscellaneous. 693, 694t neuronal stimulation and. 688 neumnal susceptibility to. 687—688 partition coefficients oL 693 values of, 693 protein binding of. 693. 6931 rate of onset of, 688-689 in regional nerve blocks. 687 mutes of administration for, 687 side effect.s of, 689—691)
site of a-lion of, 685. 68Sf solubility of. 692—693 spinal. 687
slnicturc—activity relationships for. 690 6921
topical. 687 types of, 690-694 vasoconstrictors with. 61(8 wound healing and. 68') Lodine. See Etodolac l,odoxarnide trotnetltatttitte. 716 L,,fetttiinil, 738. 8371 Log 0 values, 948t—956t Logen. See Diphenoxylate Log P values. 9481—9561
Lollypops. in combinatorial synthesis. .14. Lomanate. See Dipltenoxylate Lomeflonucin, 248, 248t. 251—252 Lomotil. Sit' Diphenonylate Lomustine, 399. 401—402 Loniten. Set' Minoxidil Lonox. Set' Diphenoxylute Loop diuretics, 6 10—616. Set' ,,lao Diureti miscellaneous, 615—616 organotncrcurials, 610 phenoxyacct,c acids. 6l3—6l5 preparations of, 62)) 5.sulfamoyl.2./-3.aminobenzoic acid derivatives, 610—613 Loperantide. 737. 7371, 748 Lopid. See Gemlibroril Lopressor. See Metoprolol Loprox. Ste Ciclopirox olan,ine Lorahid. See L,,racarbef Loracarbel. 320t, 3261. 327 Loratidine. 713—714
Lorazepam. 491 Sec Probucol Lorfan. Si',' Levallorpltaut turtrale Lorothidol See llilhionusl Losatlan, 6411 Losec, See Omcprtuole Lotemax. See Loteprednol eiab,unate Lotensin. See Benaeepril hydrochloride Loteprednol etabonate. 8111—811. 8111, 812—8 13
hides
I,otrimtn.S,'e Lovastatin. 662
Low-density lipoprotcins. 658 —65') Low-osmolar contrast agetits. 473 l.oxapinc succinute. SIX) Loxitane See Antosaptnc locoL See Indapaitside 1.51). .20. 521
Ludinmil Se.' Maprntiliite hydrochkiride Lugrils solution. 223 Lumigan. See Biniatoprost l.untinal.S,'c Plie,ioharbilal Luncllr, 792t. 793—7')-l Lupron. See Leuprolidc l.upus syndtounc. acetylation pol)morphism and. 124 Lutalyse. See l)inoprnsl trimeihauiiine Lutcinliing hormone 11.11 I, 774—775. 7741.
laser ilcsorpttiiit/ MALDI—TOF ) iottiiulioit tiltie—ot- tlighL I, 52 Malorone. See Atovaquone.proguainl rn-A MSA.See Amsacrine Maitdelatitiitc ,' Methenatnine mandclaie Mandi.l. See Celi.ntundote nafiae Mangakolipir tn'.s.diuttt. 477
Menthratie(s)
chemical tiature ii).
drug uitosctticnt across. I') drug partitioning attd. 9—2). 191 luyperpolari,,ilt.iui i.l. 4,8)). lull II iii. trautsp.url across. 1,81—68.1, fi$2(
Manganese conspiexes. as contrast agents. 477 Mania. 503
Manic disorders. 496
lipid hi)aycr i*I, I'). 191 ,:—octaitohls'.atcr svMeutt model of,
Mannicli base, 149. 15(11
Mannisidosireptiimyciti. 337 Manoitol. 618. 4.20
Maiisonil S.c Niclosatitide
rcpiularicaliiilu iii. 1,8)), 1,8)1
MAOIs. 514—516. 51St Maolate .', (liksrphencsiit carhaitiate
Maprnliline hydrochloride. 5)8 Marcain ic Itupis'acainc
puuteitti.il. 611(1—lull). 1,81)
Memiury respisnsc. in ituuututiuiity. 21)5. 21151 sodi,tni diplitusphate. 1185 lSIeit,udiiiite. 885
1.ymphocyies 8, 200. 202—21)3 T. 200. 202—203
Mauicaria ,l,antom,lla (chamomile I. 911
Met,iuigoc,iccal pithy Mcnitgarih. 417
Statrit.a.sststed laser dcsorptiouiltoniiation
Mcutiitropins. 1)44—K-IS
5 1—52
M
M it't'pti.rs. 55) mc piors. 551 M, receptors. 552 Ma rei,eptors. 552 M, receptors, 552 Macrodautin. See Nitrssfurantoin Muerolide antibiotics, 349-355 cheuni'.ry of. 349 rucehanisni iii neinin nI. 349 to. 349 microbial
as priidrug. 4—5 Menadiotie siu,titurn Mcnaquinuiuues. 882
885
Menesl. S,.' Eatrogcns
Mas) cells, 198 Mast cell stahiliorrs. 715—717
uirne—*sI—lliglut MA) .DI_1OIi), 52
Mains Gla protein, 883—884 Matris tiietallagrotca.scs, untineoplastir.
Lypre.ssin. 1)47
acid dielhylainidc. 52)). 521 Lvsodren.S,'e Milolane Lysocyme. 20)
a ,iccine, 215
Menstrual cycle, regulation at. 774—775. 7741 Meat/ia /iuk'eiiiiii (pcnityroyal I, '115 Mcpcit,olate brisiutide. .98 I Mcpcridune. 735- 738. 747
Mritulane S.'.' I'rocarbaeine Itydniclikiride
discovery ill, 735
Mavik Set' 'l'randolapril Masair.S,'e Pirfutiterol Musaquiit.S,'i' Loiutelloxacin Maxiputtte.Se.' Celepime M,itside.Se,' 'l'riaiiutercne-hydrsuchluinilhiacitle Mcaslc'.'inuiiips.rubclla iMMR saccine, 211.
iiieta)xulisuu, of, 85 87 tti,udilicatt,utis of. 735 731), 731,1-7310 stntcture iii, 731'it
Measle.s sacciute. 216—211. 2121 Mebcoda,.ulc, 265—2)16
Mcphcncsiru, 4'))1
Mephcuiytoiiu,5)tS, 5))Si
Mi.pliuiharbilal. 494 494t actuse utetabolites iii. I 35t a'. aiiiiconsiulsant. 504 as atistiilyuic/.sedative-Ius politic. 494. 494t
Mcc.uunylamine hvdnucliloride. 588—589
Mechlorethamltte hydrochloride, 399—Ill)) Meclat,. Set Meclocycline sultosalicylauc Medicine Itydrocltboride. 7)17 Mcc)ocycline sullosalicylale. 3451, 347
nictabolism of. 94 Mephyton. St-c I'll> tonadionc Mepi'.iicaiutc, 1,78. 1.9)) 1,93. tu')2t
.l')i meiaholisuui iui, 82
Mcclolenaunute sodium, 757—751) Mecl,uisicn.S,',' Meclistenauutatc sodium
Mepr.uui. S,'. Alovaquonc Meprylcainc
Nlcpyrautuune. Ste Pvril&ttitiuue tualeate
Malenide acetate. 278. Ser' also Sulfonamides
Mrda,epattt, tneualxulism of. Ill I Medesa.S,',' Valruhicitu Isledical imaging. rathopluaniuacetiticals tar,
indications br, 270t Magan. See Magnesium salicylate Magic angle upinning nuclear tnagnrtk'
Radiopliartuarenllcals Medicinal chieutistry. oversiess of.
spedraim of activity o(.349 types of, 349—355 Macrophages. 198—2(1)). 199t, 204 Mitcula densu cclls. 5971. 5')9
resonance spectruscopy. SI Magnesium salicylate. 755 Magnetic resonancc imagtng. conirast agents for, 475—477, 4831. 484. Se.' alto
Contrast agents Magnevisi. See Gadlipenletitte dititegluntine Ma huang. '105. 911—912
Major histocontpatihility comples IMHCt. 197. 199—200
Major tranqttiliiers. 496—503 Malaria ilrug-resistaztt. 21)2. 289-290 drug Ilterupy 11w. 283. 2'J5—298. See a1si.
Antimalarial'. geographic distribution of. 282 itflJstct iii, 282 mosquito control (or. 282—283 nutrittonal sttppon in, 285 pathophysinlogy of, 283—285. 2841 Plaamudii,n, spp and. 282, 283— 284. 2841 protective ittlitatiolts br. 283 vuccine.s for, 283. 283. 285 Malathion. 570. 5701
I t)2l)
pltenoitiena atiil. 18—21. 191 propcntes 0). 19 receptor components a). 28.5,-,- also
Marecine. Si-c Cyclicune hydrsuchlondc Ma'.'. spectmtttctrv. in counbinatorial chemistry.
tumor.iitliltraling. 442
1)
depolaricatisun ii). 1,1(1), (.811. 1,1)2
84). 844 Luteinicing h rut inc—releasing honnone. 1)4 l.uvox. See Hiwonaminc LY 303366. 246 LYMEnu. 11)6
l.ymphoid cells. 97, 1981. 218)
977
454—484. Sri' is/a,, I —2
Mcalroxyprssgesterone acetate. 7861. 7871, 788 as antiuteoplastic, 434
its contruceptive. 792). 793 in hnrntiuuie rcplacenictit ther.tpy. 7i16_71)7, 796i
Medrysone. 81)1,8111.813 Mefenamic acid. 757 Melliuqttine hydrochloride. 2871. 288. 2951 Melosin. Sc'i' Celoxiti,, sodium Metnuside. (8)7—6)1), 1,071 (iOçt. (i2lt
Mcguce. Sri' Megcstn.l acetate Megesurol acetate. 436. 787). 788 as atitittetuplastic. '134 Melattiicyue.stimulatittg liiurnt,.ne. 8.13—1144 Mclunocytc-stintiulating honitisute release-
6')0 693. 691t
Mers-aleukinSe.' (s-Mercaptnponne 6-Mcrcapuupiiriuuc. 411—412 itiecliautisiu of action nI. 4(14 — 4)15 tuiciabsulistit ii), 3$. 99, I 2)t, 126. 4)1-I —41)5 Mercapluruc acid diuiujugatioui. 117—121
Mercuric ,iaiik, 221) Mercury cornpiuunds. 228 diuretic, 6111
MrridiaS,'.' Sihutraniiiie \ler,upenent. 311) Mer,u,i,ites, /'/asuuuoduui.,, 2114
43 44f 481 Merltuiolatc. Sc.' Iltinucrosal hk_salaunitue. encytutitic degriid.ititsn ii). 3—.) \lesantoiui. See Mcphenyttuiut Mescaline. 521 uneiabalisnt 01. '31. 9)), 122. 1231 Mesuia. 4-45. 446
Mesnes Se.' Mesna Mesopin. 5cr
niciliylhru,tnidc
inhibiting tactor. $41 Me)autotropitt'.. 843-844 Melarsoprol. 263 Melatoniti. as sleep.proitioting agent. 488 Mel 115(1' Melarsoprol Mellari). Sc.' Tltinrida,.inc
talesiirida,iuue
Mehosicatn, 76)) Melphalan. 4(10
Melah,ulic aruuunati,ation. 101
ii'. ,ieuiae ntetal*olite. 135. l3St metahislisnu iii, 99 Mesoridar,,ie hes) late, 499). 5)1)) Messenger RNA. 162 Mestrutiiul. in nuoitopltasic ciitttniccpiives, 71)11 Mrtabolisuii. See I)rug tiucuabolisnt
978
Index
Metubolites achy zfte. 7—8
tOXiCity of. 65 Metaglinides. 671 Mctuhvdnn. See Trichlurtoethiozide 3 - Metaiodobenzvlguaitidinc sullate. 489 Metaphen See Nitromersol Mclapmtcreno(, 536 Meraraminol. 539 Metastron. Set' Strontium 119 chloride Met-enkephalin. 679. 744. 1(43—844
Mctlonnin, 672 Meihacholine, 557—558 conformations oF. 555—556, 5551 Mcthacholine chloride. 558—559
Methacycline hydrochlondc. 345t. 347 ol—).Methado(, bts-N.demethylated nietabolde of. ucetylation of. 122, 1231 Methadone. 738, 7391. 749 metaholisiti of, 85, 111Sf structure—activity relationships of. 738 Methamphctamine. 513 nictitholisut of. 89 Methampyrime. 7621. 763 Methundroslenolone, 7991, 801 Methanthclinc bromide. 581 Methapyrilene hydrochloride. 705 Metharbital, See Mephobarhital Methaiolamide. 6041', 605. 6(9 Methdilarinc, 711 Mcthdilaiinc hydrochloride. 711 Methcinoglobinemia. 93 Melhenuinine, 253 asprodrug, 151—152, 1511 in drug delivery. (56 Methenamine hippurate. 253 Methenamine ittandelate, 253 Methicillin sodium, 309t. 31(1. See iris.. Penicillin(s) Methimacole. 674 metabolism of. 114 Methionine. 901 Methionine-enkephalin. 679. 744. 843—844 Methocarbumol, 496 Methohexitul sodium. 487. 4871 Meiholrexute. 41)9. 414 leucosonn rescue with. 4111 structure of. 9421 thymidinc rescue wilh. 410 Methotrimeprazine. 751 Methoxamine. 533 Methoxytlur.me. 486 Meth,suxitnide, 5(16
Methyclothiazide. 605—610, 6061. 608t. 620 N.Methyl-4.aminoarohenzcne, tretdatum of, 93 Methylation. in drup metabolism, 125—126. (261
Methylbcn,.ethonium chloride. 225 3-Mclhytcholanihrone. metobolisni ol'. 771 Melhylcobalunhiti. 895 Meihyldihydroinorphinone. 733t Methyldiphenhydramine. 71)2 Methyldopa, 535. 5351 metabolism of, 92, 125 Methyldopale, 535, 5351. 652 Methyknc blue. 227—228 Meiltylglyoxul his(guauylhydrazone). 429 Methylhydrazine. 397
relative activity of. 8091
Minipres.'.. Set' Prazosin
sotuhilily of. 5
Mittocin, Set' Minocycline hydrochloride Minocyclinc hydrochloride. 3451. 348 Minoxidil. 654, 655 metabolism of. 84 Miiitezol. See Thiabcndaiasle Miradon. See Anisindionc Mirena. See l..evonorgesttet-releasing intr,iuterine system Mirtazitpinr. 520.541 Misoprostol. 126. 827—828
Methylprednisolone sodium succittate
solubility of, 5 sU'uctuuv of. 77(1 Methylrosaniline chloride. 227 Methyl solicylute. 754 l7o-Mcthyltcstosterone, 798. 7991. 801 biological activity ol'. 798. 7981 hepatolovicity of. 798 structure—activIty relationships for. 798—799. 79t)t
structure of. 77(1. 799f Methyl-ThFA trap hypothesis. 895 Methyliraitslentacs. 125 Methyl violet. 227 Methylsunthunes. 5(0. 511—512.51 Ii Metiaiuide. metabolism of. 99. 11)1 Mctipraiiolol. 543 Metucunne iodide, 591 Mctnln,onc. 61)7—610. 6071. (419t. 620
Metoprolol. 544. 545.5451 active metabolites 1351 nsctabolistti of. 77. 98 Metnwole. Sr.' Pentylenetctra,ote Metrilonate, 567 Metneamide. 483—484
Metro IV. See Metronidacole Metrottida,.ole, 260—261 metabolism of. 1(17
Metyrupone. metabolistit of. 1051 Metyrosine. 528 Mevacor. See I,.ovastatin Mevristutin. 662 Menilctine hydrochloride. 641) Mexitil. Set' Mexiletine hydrochloride MeLOn. See Me,.locillin sodium Meulocilliti. See also Penicillin(s) spectrum 0) aclisity of. 308
Meelorillin sodium. 309*. 3(4 Mibcfr.tdiI. 945 Micurdis. See Telmisariun Micatin. See Miconaeole nitrate Michael addition reactions. (20 Miconazole nitrate. 242 Microbial resistance. 31)1. 3(15—307. 335—336
Microchip spatial arrays, in combinatorial synthesis. 44. 451. 60 Micronasc. See ('ilyburide Microride, Ste Hydrochlornthiaeide Midamor. Set' Amiloridc hydrochloride Mudazolam. a'. anesthetic, 487 Midodrine. 533 Mifepnstone. 795, 7951 Mifiprex. See Mifepristone Miglitol. 672—4.73 MIH. See Procarba,ine hydrochloride
MMFF94 Force Field. 925—926
MMR vaccine. 211. 212t MNDO method. 938 Moban. Sit' Mohindone hydrochloride Mohic. See Meloxicam Mobidin. See Magnesium saticylate
Modafinil. 5(0 Moduretic. See Atniloride.hydrochlonithia,ide Molar reactivity (MR). 21. 2lu Molecular connectivity, 24. 24t Molecultir diversity, quantification of. 56—58 Molecular dynamics sitttuluiions. 933—935 Molecular mechanics, 3(1 Molecular nixxleling hull-and-spring models in. 923—929. See .also Force field iiiethods computer-assisted. 27—41, 919—922,9211, 9221. See ii!..., ('ompstier-assisted drug design ('PlC models in. 920. 921—922
physical. 920 solvent-accessible surfaces in. 922 van der Waals surface in. 922. 9221 Molecular orbital calculations, 935 Molecular similarity, quantification of. 56. 57t Molecular structure drug—receptor interactions and. 31—34 physiologic activity and. 11—21. 28. 31—41
Molindone hydrochloride. 502 Molyhdenum.99. production trl. 462. 462f,
Miltown. See Meprobam-ate
Monoacylureas. 51)6
MINIX) method. 938 Mineraloconicoid(s). See also Steroid(s) biological activities of, 81(5
Monoamine oxidase inhibitors (MAOIs). 5(4—5(6. 51St Monoamine i)tidaccs (MAlls). 90—91.
exccs.s of, $05
modifications of, 806—807. 806t products. 8071, 810—811
relative activity of, 809. 809t structur.il classes at'. 8(16—809 struchitre—activity relationships, for. 807—808.
Meiliylprednisolone. 8(11(1
Mitrobronliol. 395 Mivacron. See Mivacanum chlonde Mivacurium chloride, 593 Mixture library. 43. 441 MM2 force field. 927 MM3 force field. 925—926. 927 MM4 force field. 925 —926
Moinetasonc t'uroatc. 8(3. 814, 8141 Monisiat, See Miconucole nitrate
N.Methylmorphinan. 738—739. 742 Mctltylnitrnsurea. 395. 398
Mcthylplienidate. 514 netabolisun uI. 89. 1(11. 11(9
Mltoguazone. 429 Mitomycin C. lIMIt, 4(4. 423—424 activation of. 152—153. 1531. 397. 397f Mitomycins. 419—420 Milusis. 391. 3911 Mitoiane. 436 as tintincoplastic, 435 Mitoxantrone hydrochloride. 429. 432
914 Mtlk Millon's test, 834 Miloittm. See Pliensunlittide Milrin.me. (.57
Methyl ioidide. toxicity of. I (K—I 19
Methylparaben, 228 —229
Mithrurin.Si'.' Plicamycin Mithrainycin. See Plicamycin
11071—%08f. 8091
structure of. $071 Miner.iloconicoid receptor antagonists. 1115 Mineralocorticoid receptors, 773
4631
526—527. 5261
Mon,.umine reuptake inhibitors. 5(6 Monobavtaiits. 334 Monocid. See Cefonicid sodium Monoclatc-P.Sr'e Antihemophilic factor Monoclotiat antibodies, 181—191 as untineoplastics. 442—444
chiuneric. (89 diagnostic. 471)
preparation of. 187
Fader in rudionuclide lest kits, 190—191 therapeutic, 191 types ol, 119—191 Monocytes. 198—199. 1991
Mononine. See Factor IX. recombinant Monoprit. See Fosinopril sodium Moricirine, 1,40 Morphine and related compounds. 731 —753 addictive liability of. 732. 733. 744 discovery isl, 744 historical perspective on. 733 indications for, 744 metabolism of. 85—86. 86. 87. 98. 112. 126 modifications of based on Grcwc's research, 738—741 based on Isleb and Schat,mnnn's research. 735—738
based on Small and Eddy's research. 733—735
early. 733—74 I
pharntacologic properties of. 744 preparations of. 744—745 products. 744—751 receptor i,twraction.s with. 74 1—743. 7421. 743?.
solubility of. 744 source of. 744 struclure—uctivity relationships (or. 733—735, 734t—735t. 741—744
synthetic derivatives of. 733 Morphine hydrochloride. 744—745 Morphine sulfate. 745 Morse curve. 925. 9251' Mosquito control, for malaria, 282—283
Motilin. 855 Mottr tterves. 548 Motor tteuron, 680 Mottin. See Ihuprofen !sloxalactajn, 325 mRNA. 162 Mucosal immunity. 200 Mucus. 200 Mulliken population analysis. 939 Multillance. See Gadobenate meglumine Multiple sckrosis. 181—182 Mumps vaccine. 211, 2l2t Munurol. See Fosfomycin tromethaminc Mupirocin. 362—363 Muromonab-CD3. 190, 859t Muscarine
activity ol, 556 isomers ol, 556, 5561' structure of, 5581' Muscarinic antagonists. 558—572, 559f Muscarinic receptors, 5511—553, 55 If, 5521 ucelylcltoline and. 557—558, 557f, 557t structure of. 557—558. 5571. 557, subtypes of. 551—552 Mustargen. See Mechlorethamine hydrochloride
Mutamycin. See Mitomycin C Muzolimine, (ils—616, 6151 Myantbutol. See Mycelex. See Clotrima,ole Mycifradin. See Neomycin sulfate Mye'ohas':erium avium, 254 Alveobacterium insracellularc. 254 Mvcobactc'riun, Lansa.rii. 254 Mycabacterrurn leprac. 254. 279—281) Mycabacrr'riunr rubercu!osi.c, 254 Mycoses. See Fungal infections Mycostntin, Ste Nystatin Mydriacyl. See Tropicatnide Mydnatics. 573—574
Myelin, 679 Myelogrsplty. 480 Mycloid cells, 197. 981 Mykrox. See Metolazone Myleran. See Busulfan Mylotarg. See (iemturumab ozogarnicin Myocardial ischemia, 622—623. 6231' Myocardial metabolism. 622—623. 6231
Myoscint Kil. See lmcirutnab pentetate Myosin, 623. 6241 Myrtccainc. 6941 Mysoline. See Primidone Mytelasc chloride. See Arnbenonium chloride N Nahilone, metabolism of, 1051 Nabumetone. 759 as prodrug. 152 NAD. 888—889. 8891
NADH. 889 Nadolol, 543, 5441 NADP. 888—889. 8891 Nafantostat. 447 Nafcillin codiutn, 3091. 311—312. See a/co Penicillin(s) Naftitinc hydrochloride, 239 Nautin. See Naftifine hydrochloride Naja venom solution. 835 Nalhupltine, 743, 746, 750 Nallon. See Fenoprofen calcium Nalidinic acid. 247. 248, 248, Nalntefene hydrochloride. 752 Nalorphine hydrochloride. 735. 740. 743. 751 Naloxone hydrochloride, 740, 751 metabolism of, 11)51 740. 741, 751—752
metabolism of, 105 Nandrolone decanoute, 7991', 801 Nandrulone phenpropionate. 7991. 801
Napltazolitie, 533 Naprosyn. See Naproxcn Naproxen. 759 ,netabolis,u of, 114 Naqua. See Trichlortnethiazide Narcan. See Naloxone hydrochloride Narcotic analgesics. 731—753. Ste also Analgesics: Morphine and related compounds metabolism of. 87 Narcolic antagonists, 740—741
structure—activity relatioitships for. 743 types of. 751—752. 752, Narcotic antitussicca. 752 Nardil. Sec Phenclzinc sulfate Nasricort. See Trinincinolone acetonide Nasarel. See Flunisolide Nasonex. Sec Mometasone Iun,ute Natacyn. See Natamycin Natumycin. 237—238 Nateglinide. 671 National Cancer Institute, drug screening protocol of. 392—394, 3931 Natur.,l products, combinatorial synthesis of. 47—411. 471
Naturetin. See Bendroflumethiazide Navane. See Thiothixene Navelbine. Sec Vinonrlbine Imitate Neaminc. 338 Nebcin. See Tobramycin sulfate Nehrsmycins, 340 NebuPent. See Pentamidine isetbionate Nedocromil sodium. 715, 716 Nelazodone. 5 19—520
Nefrolan. See Clorexolone Negorum. See Nalidixic acid
979
NeIfi,tavir. 385—387 development of. 942. 9431 fslembntal, See Pentobar'nital sodium Neobiotic. See Neumycin sulfate Neomycin sulfate. 338 Neonatal hyperbilirubineniia. I IS. 26 Neosamine. 338 Neostigmine bromide. 5631, 564. 56.41 Neostigmine melhylsulfate. 565 Neo.Syttephnne. Sec Phcnylcphrine Nepltrun active ttuhular secretion in, (iOl —602. (,02f
function of in edematous states, 601 in hypovolemic states. 601 in nomtovolemic states. 596—601. 5981—61)01
intercalated cells of, 6(10, 6001 principal cells of, 600. 6001 sodium reabsorption in. 597—601 structure of, 596. 5971 Ncptazane. See Methazolamidc Nerve(s)
fltotoi. 548 somatic. 548 Nerve blocks, 6117
Nerve cells. 679, 6791 Nerve fibers, 679-680. 6801 Nerve impulse, transmission of, 680—681, 6801. 6811. 683 Nervous system
divisions of. 548 structure and function of, 679—685 Netilmicin sulfate. 340—341 Netrontycin. See Netilniicin sulfate Neumega. See Oprelvekin Neupogen. See Filgrastim Neurohoritiones, confomtatiottal flexibility of, 34—35. 341
Neurohypophyseal hormones. 1145 Neurohypophysis. 841 Neuroleptics. 496—503 Neuromuscular blocking agents. 589—595 curarcicttrare alkaloids. 590—591
dcpolarieing. 590 nondepolarizing, 589—590 synthetic compounds with curariform activity. 591—595 Neuromuscular junction. 5119 Neurons, 679, 6791 motor, 680 Nearontin. See Gabapentin Neurotensin, 1155
Neurotrunsinitters, 683—685 acetylcholine as. 548 adrenergic. 524—547. Sec also Adrenergic neurotransmitters drug effects on. 684 excitatory. (184
ganglionic stimulation by. 586, 5871 inhibitory. 684 release of. 1,83, 61141
structure of. 683, 684t Neutralization, in immune response. 205 Neutralizing allosteric niodulutors, 489 Neutropltils. 197. 19Sf, 2t10 Nevirupitte. 383 Newton.Raphson geometry optimization approach, 930 Newton's laws of motion. 933—934 Newton's third law, 924 Nexium. See Esomeprazole magnesium Niacin. 888—890. See also Nicotinic acid as antilipidetitic. 661. 890 metabolism of, 126
980
lnth'x
Niacinamide. 890 Nicalex See Niacin Nicardipine driwhloride. 63 —632 Niclo.amide. 266 Nicotistamide adcn,ne dinucleotide (NAD). 888-88'). 1(891
Nicotinaznide .ideniiie dinuckotide phosphate )NADP). 888—81(9, (4891 Nicotine. 51(7—5(48
ittetalmlism oF. 87. 93, 11)1. 126 Nicotinic acid, 888—890 as antilipidcnaic. 661, 890 metabolism of, 26 Ntciitiiiic receptors. 548—550, 549f. 5491 Nifedipiric. 6291. 630—631. 6311
Nifurtimox, 263 Night blindness, 8711
Nil-I shift. 71. 721 Nilandron, 5cr Nilutatnide Nilutamide, 437. 801—8112. 11021
antineoplastic. 434 Nim.idipinc. 632 as
Niminop. See Nirnodipine Ninhydnn test. 834 Niprtde. See Sodium nitmpnisside Nirsanin. 677
Nisoldipine, 632 Nisosctine, 519 Nitr,utcv. Set' Nitruvasodilators Nitr.m'iepatmm. inelabolisiit of. 107 632
Nitric acid esters, 625 Nitric imuide
formation 01. 624 siwlana and. 283 iii sniooth nmuscle relaxation, 624—625. 6241
Nitrites. Sir Nitrovasodilators Nitro commmpounds. metabolism of. (07— 108
Nitn,funms. 252—253 Nitrofur,mnloin. 252. 253 Nitrofuruzone, 252—253 Nitrogen tmtustani. 394. 399—4(81 Nitrogl5cerin. 625—626. (.251 metabolism of, 12(1. 625 Nmlmmcr,iol. 228 Nitnmprecs. Set' Sodiuni mU'uprusside Nitroso compounds. See Nitrovusodilators Nitnisureas. 395. 398 Nitrous acid esters, 625 Nitrous oxide, 486—487 Nitrova,sodi(ators antianginal action of. 625—626 niechanisin of action oh, (.22—623, 6231, 624. 6241 of. (.23—624. 625, 62Sf
nil.ic oxide release by. 625 i)SldiIIi(ifl State,,. (ii. 625t
speed and duration of action of. 61St structufe—aesivity relationships (or. 6151. 625 types oF. (.26—627. (.261
Nivalin, See Nix. See I'crmcthrin 7(91. 720, 720t. 722 Nizoral. See Ketoconazole n.octauol/water system. drug partitioning in. tY—20
Nodes of Raitvier. 679, 6791, (.81 Nofetunmontab nierpentan. 190
Nonsleroidal anti.influmniatory drugs
Nuprin. See Ihuprolen
Nurum.i..S,'c
us analgesic,.. 753—763. See also Antiinflammatory analgesics
chloride Nutritional factors, in drug mnemabolism. 131.. t32. 944—945
&mruehidonmc acid metabolism and. 822
Nuvaking. Set' Itionogestrel
(NSMDs)
mechanism of action of. 8(8 Nimr.ukenuline, Ste Norepincphrinc Norchloreyclirinc. 706 Noreuron. See Vecumnium bromide Nonlazepato. 490 Norelgestroinin. 7871. 789 Norephedrine. metabolism of. (26 Norepinephminc. 524—547 adrenergic receptors and. 527—528
biosynthesis of. 524—525. 524f in local anesthesia. 688 properties of. 524 structure oF. 524 as sympathomimetic. 532 uptake and metabolism of, 525—527. 5261 Ntmrethindrone, 786t, 7871, 789 in contruccptivcs. 7911—7921 in horrnonc replacenment therapy. 796—797. 7961
metabolism of. (06 Norcthynndrcl. 786t, 7871. 789 Nortlc*. See Orphcnadnne citrate Nmmrlloxacin. 248. 2481, 249 Norgestimame. 7871. 789 in eontr.mceplisc.. 792t
in hormone replacement therapy. 796-797. 796t
Norgestrel. 7861. 787f. 789 in contraceptives. 79lt ntctabolisn, of. 11)1 Norkciamine, metabolism of. 89 Normethadone, 7391 Normodyne. See Labmitalol Normorphinc, 741. 747 Noroxin. See Nortloxacin Norpace. See Disopyramide Norplant. 793t. 794 Norpramin. See Desipramine hydrochloride (9.Nu,flestimstcmne derivatives. 786, 786t Noetriptylimte. 5(7
as active merabolite, (34. l3St Noreasc. See Atnlodipine Norvir. See Ritonavir Noscapinc. 752—753 Notee. See Chkrrul hydrate Novaldeit. See Tanmoxifen
Novuntrone. See Mitoxantrone hydrochloride Novatropine. See l-lnmuu'opine methyibromide Novobiocin sodium, 361—362 Novocainc. See Procaine Novolin. See Insulin, recombinant Novolog. See Insulin, recombinant Novo Seven, Factor VIm, recombinant NPH insulin, 8511, 852. 1(521
NR geometry optimization approach. 930 NSAIDs. See Nansteroidal anti.iullammutory drugs CNSAIDsI Nuhain. See Nalbuphsnc Nuclear magnetic resonance spcclroscopy. of proteins. 832 Nuclear medicine, 455. 458—462 Nucleic acids. See RNA Nucleon, 455
Nyctimlopia. 871)
Isoitiaiid Nystatin. 237. 3(88
(3 9,1 tl-Octadecm,miismnmide. as sleep.tnsntmimting agemtt. 48)1
,m.Oviaimiml/waier system, drug parlituiiiiiig in. t9—20 Octri,otide acetate, 1145 Oculcn.Sm'e l'hurhmprmilemm
(kupress. Si-i' Canemilol Ollonacin. 2411, 248.. 254)
Oil of wintergreen. 754 OK'l'3 Se.' Muronmimah'CD3 Olan,.ipinc, 51)2 Oleammdohidmi. 353
Oleammdotnycin. 353
Olcfmns. oxidation of. 74-77 2' .5'.Oligoadenyhate symithemase. 2(12 Olivm.myciims. 4(7
Olsala,ine enzymatic depredation of. 3 as prodrug. 4-S
.1
Ommieprazole. 722—724. 723i
activation of. ISS. lsf.b Omnnipen. Ste Ampicillin Onini,.can. Sec Gadimiliantide Oncimgenic viruses, 372 OncimScisil. nmurine Sm'.' Saiminutmuab pendetide
()ncnvin See Vincrisiinc satIate One.bL'ad one-conipiiund synthesis. 46—411. 54 62
Ontak See l)enileukmn dilsitox Opiophohia. 731 Opium. 732. 747 Oprelvekimm, 183
Opiical isomers. bimdi,gical .ictisiiy ol. 35—37 351. 361
Optiittark Si'.' Gadoversetaniide Optinmine.Sei' A,atadmne nialeate Optsntizaticin proceshimres, 58
(.)ptiPranolol. Sr-c Metipranimlol Optisiir. Si-c Azela.stine hyilrcmchiloride itplitlmiilmmiic solution
Or.i( administr,itiimn. drug distribution in, 3—S.
4f Oral coimlraceptives. See ('oniraceptives
Orange oil. 229 Oresic.Si'i' flydroclmlorothiazide Organic nitrates. Si-i' Organic mmjtrites, Si'm' Nitnn'itsislitaiors Organoniercuri.mls mnti.inlectusc. 228
diuretic, 610 Orina.se. See Tolbutaniide Onnuse l)itignmr.iic. Sri' Tolhuianimde sodium Ormaplatirm. 4214
Ornidyl. See Efloriiitliine Orphienadrine. imietabmilism mit. 147
Orphenadrmnc citr,ime,5142-583
Orthocainc. 677 Orthocli,ne-OKT3. See Mun,mnonab-('l)3
Nucleoproleimms. 835
Orilmo-Eura. Ste Nimrelgestronhmn
Nog&mlamriycin. 416—417
Nucleoside antifunguls, 235
Orih,mforni. See Orihuicaine
Nolvadex. See Tamoxifen Nminnucleoside reverse Iranscriptase inhibitors.
Nucleoside antinmelabolites. 372. 375—379 Nuclmrscrntes. 835
Orthmmgonah pooling, in eoimtbimtatoeial
Nuclidc. 455 Nnmorphan. See Oxymorphone Nupercaine, 678
Orthmo.Preimisi. Sri' I lomimine replucemmicnl
383— 384
Nonselective norepinephrine reuptake inhibitors. 519
clteittisimy. Sit—SI. 62
therapy Orudic. Sm-c Ketoprolcmi
()sniuirtil. Si" Maiuiiitti) Oirisun. Sec Xyliinieias'oliiie Ovulation, regulation at. 774. 7141
Osacillin sodium. 3i19i. ill Sic tiloi
Papase. Sic Papaiti Papaverinr .586. 732
aiiiispasniixlic activiiy of. 574. 624 iiieiabcilisiii cii. 133
Peuiicilliiiisi Osalid. 763i Onaliplatin, 428 Oxaiiiiiiqiiiiie. 266—267 Oxandrin. See Ovaiidrol,iiic Onandrolone. 799) 11111 Oxapr.iein. 760
Paper chrcnnatiigrapliy. 83.) Parabens. See p-Ilydro.svben,oic acid Paraldehyde, 496 Paraniagiieiic contrast ugenis. 475-477. 476i
Osai.epaiii. 491
Parasvinpadietic agenki. 572—574.5cc into' (')tiilcnergic blocking agenis Panasympaiiici)yiic ageiitr. Sec Otitliicergic blocking agents Par.isympathomiioeiics. 54$
.is active iiictaholite, 134, 135i itieiattolism itt. 133 Ovaaolidinediones. 505 Oxiord unit. 302-3113 Oxiconacaile tiiiraie. 242 Onidiiiiiin in hiotranslorniaiiiin. Sic I)rxig nieiatiolisni. ovidatiiin in pniiein, 73. 74) Oxidaiis e
I lit
Paraplalin. Ste ('ar)xiplaiin Parasyinpailieiic gaiiglia. 586—587.587) Parasyiiipaihciic len-lids sysieni. 518
Parathion. 5711
iiietabiitisin
99
Par.itliyriiii) horniiine. 85.5—856
iiieialritlism 'il. Ill
Oxidatise deantinalion. 89
in vitamin I) synihesis, 876 Par.iths raid injeciiiiic. 855
Osidaiive detiattigeiiaiioii. 1(11 - 1113 l)xidaiise deliydriigenatioii. I lit 85 Osidatixe
Parenterul adniinisiraiicin. drug distrihuiuiin in.
Oxidicing agenis. 223 Oxine. 261
l)xistai. See Onicona.iole iiiicaie Oxisuran, ineiaholisin cii, 14)5—1)16 Oxoinenitirine. 55)1, 5581
Oxy-5. See Hydrous hencoyl peroside Oxy. It) tt' Hydriiiis beiieoyl tieriixiile Oxyhuprocaine hydrochloride. 690—693. 691i Oxychlcircisene sodium, 124
Oxycodiiiie. 733. 733t Oxyciicliine hydroctiloride. 746
Oxy-Coniin.St'i' l)xyciidtine hydriictiliiride Oxyiiieta.iciline. 533 Oxymeiliolinie. 7991. 8(11 Oxynnirphone. 733. 733i. 746 Onyphenhutueaine. as active nierabciliie. I 14. 135t
O.xyp)iencycliiiiiiie liydrociitiiriile. 581 582 Oxypreiiiiliil. ineiabolisiii •i(. 119 Oxyquinoline. 261 l)x)ieirac5cline )iydrccliloridc. 3451. 346— 347 Oxyrociii. 845—846 Oxylocun injeclioii, 84iii. 847 Osyiticin nasal soliiionc. 846) Ocolinone, 615—616. 6151
Paredrine. Sec Flvdroxyaiiiplieiamiiiimc 41, 5--b
Paricalciiol. 879 I,irl.tiisini's disease. 574 l'arnaie. Sec lr.iiis1cs puiiiiciiic sultaie Paroniiimycimi snlfaie. 138— 339. 338)
Pariixeiine. 518 Parsiilc,). Si'.' Ilihiiprcipaiine Imvdnic)iliir,ile Panial agonists, 48') Puma) salo). 755 Particle mesh isisuld nierh.id, 954
Partition coeliic.ent. Il—It. 181. tSr hailitgica) actis ity and. Il 2). 181. 191 Partiiiouiing phenoineita. 18—19 Pasrenrieiiii,in. 2181
Pailciton. Set' Tridihese)IiyI c)ihiriiie Paion r.ite tlieoiy . 572 Paiili exclusion lihiwiple. 937 Pas'nliiim Set' Paiiciiiiniimiio Iironiide l'anil Sr-c Parimnemine Pasipaiii IIa)accpani P('. 17—21. 181. Si I'('lls I pmily c)iliirinaied )iiptieiis Isp. omeiabolusni a), 71 PCP. Sec Plieiicyc)mdmiie ) Iiryihri.niyeimi etlmvlsiieciiiaie Pedianiycin Pedicn)icides. 268 Pegaoiiime. Sic- Iltliiiiiiin PegzLspargase. 429, 43) -432
uidicaacnrs or. 31)7 31)8. 3091 nieclianisin ol acuon of. 3lSli. 3)11—3112 nonienc)aiiire or, 31)3
oral ahsorpiain uI, penict)linase—resistaiui. 3)17
pniperiiesol. 4119) prciieiim binding o),308. 3)0)1 sci)uhi its or, 304
of aclivitv of. 707 30%. 309t siereuicheniis)rs of. 303, 3031 siracrore ii. 31)2i 11)31 ss nlhesis ii). 3)4. 114), 1)161 voWs of. 302— 303 I'emiiciUinase—icsisi.iai penicitliins. 3)17 Ps'nici llina',es,
Penicittiii (;, 4)19_i ))),3))9i Penicillin (1 ben,arhinc.3 Ii) Penicillin 0 procaine.3 Ii) lis.iimcillmim N. 3)8 3t9
Penicillin V.30). 311) Pennyrmiyal. 9)5 Pciuaerviliril,iI iciranurame. iii line,), 61St. 627 Peniatuside. 387 488 PenI.igasinn, 854 I'eiiiaiim 3i81 t'eni.cici,dine isei)iiiiiiaie Pemuianici)is-lnielaniine. 429 Peniaimiidine. (i68 262 t'cnianiidine t'cniaqiiine. 288- 289. 28'if I'enia,ocine. 74)), 741 750-751 inciabolisni of. 8)). 1.41 l'enihi ace__S.', Nleth,isy)lurane I'enudi,,rhii,il. nieiabcilisimi ii), SI Peiitiiliarhiral sinliunt, 4941. 495 Pcnlcislaoi..(i1' Sinlinni sltbi'glaconaie Peniosiutiii. 4)18. 313 liea)iiiftal .Sicdiuni lliiuipenial s.nliuiii Peciinxil .5.',' Peciiaecvt)iiiiicl Ieiraiiiirale. ,liliuied Peiits leiretctra,ote.5 lit l'en Vee. Sec Penicillin V 5,-,- I aiitii)idine Peptarton cc l'eptic acid disease. 718- 71') Pepric acol secre)iiin. 718. 71St Peptides. csntihiiiatiirial ss iitliesis iii, 41, 441, 451
Pepxiids, conihinaioi xi) sy nlliesis .9 43—46. .151. 62
I'erreot ioniiainni. IS Percitdaii
.5.
Osyctidone )iydrcichlarick
Periuclin Sir (spriiliepladine
Pegu.sy&. Sit' Inierferini aba-Ia
Perineurinni. liS)). liMO) Peri1rliei xl iicrs iius sysleni. 541) ),7i)
I'egylated inierteriiim allu-2a, 18)1. 1816 Pellagra, 89)1. 89)
Iieriirae.,S,. Periiaemy ihritiil teiraiiuriile. diluted
Peniirolasi porassiuni ophilialinic soluiioim. 7 16-7)7
Pernr.qx'ii Sit Peimit ilIum I beneathine
2-PAM. pmdrug foriii ci). 157- 158, 1581 Pamaqiiine. 288—289. 2891
Penmtt)ine,514
Pumelor. Sic Nortriptyline Panado. Sic Aceiaminophen Pinion qciiiiqiii'Jinlini ginseng). 9)3 Puncreaiic hiiniioiies. 847—854 Pancreaiiii, 838, 839i Pancrelipase. 838. 839i
Penhriien. See Anipicil)iic
Pciuieilinn. 21i8 Iieriiiiiil Sc. Flmi1ihenaiine hs ilriich)oride Periiiciun,s .nienoa. 895
Peiibnumli.). 54
Pcrphenaeine. 4991. 5)8)
p p53. 391
PAIIA. Sic p-Aitiinobenniic acid iPAHAi Paclitunel. 425. 427—428, 915 Pain, types of. 73)
Paiicreoeymin. 854—855
Panvnroniiini bromide. 593 Panmycin. Ste Teiracyctiiie Paitietiti. Sec Aliireiinoin Panteric See Pancreaiiii Paiiihen,iI. 888
¶441
Penciclovir. 378 Peneirex See Entinacin
Persanhne. Sri Oipyridaiii,ile
Penlliiriilii). 50)
l'erionat. 72. 76)) l'ernur)saiani nieiltiids. 93')
I'enictllini s) 102— .11') acid resistance if, 3)19) acytnreids'-suhshiuied. 308 allergy to. 31)8- 7)83 bacterial resisiaimce to. 1)15 31)7 $4aci.mm.ise inhibitors xsiltm.3 14—315 classificaiiim of, 3119, 1l)9i
Paniopracote sctdcnin. 722. 7234. 725—726
clciniliercia) prodntinni il .31)2—3)).)
Puntoilienic acid. 887 888
degrailatisni of. 31)4 31)5. 3)16) i)msci'sery and dese)cipineiii iii, 2403 exieni)ed-specinicii.3))7 - 3)18
l'apain. $411
tic action iii, 837. 8371
Peritifraice. Sec l)esiliraniine )msdrutclc)oride
Pesticides. close nieiabolisini aiid. 13)
Pesi,in III 5,-c PET i piisiiroii emission ii'iiiogruphy I. -156. 4641
460). 4lilf I'rosiagl,niilinis PG syii)he)ase. 8)9
POll.. $2)) P0)4 s5 iilliase. 8)9 82)). 822 p1)1. 82))
dcriratixes ii). 823 825
982
Index
P-glycoproteins, in drug resistance. 392
Phenylcarbinol. 229
p11
Plsenylephrine, 532—533
calculation of, 13—14
vs. percent ionizoion, IS plC,, and. IS—lb. 151. IS Phage vectors. 165—166. 1661 Phagocytosis. 197—198, 19Sf. 204—205 Pltagolyssssomes. 197—198. 1981
97. 19Sf Pharmacogenomics. 193 Pharmacognosy. 905 Plsnrtnacological activity, statistical prediction of, 17—26 Pharmacological screening. See Drug design; Screening Pharmacophore concept, in drug design. 944 Phemerol chloride. See Benzethonium chloride Phenacaine hydrochloride. 694t Plsenacetin. 761t, 762 active metaholites of. 135t metabolism of. 98, 116 Phenadoxone. 739t Phenazocine, 740 Plsenazopyridine. 253—254 metabolism of. species differences tn 129. Phagosomes,
1291
Phenbenzamine, 704. 7(15 Phencyclidine (PCP). 52(1. 521
ntclabolivm of. 82 psychosis due lo. 497—498 Phcndintetrazine tartrate. 514 Plsenelzine. metabolism of. 122. t23f Pheneleine sulfate, 515. 51St Pltenergan. See Piosoethazine hydrochloride Pheneridine. 73ht, 737 Plsenetidine. bIt. 762 Plsenetsal. 761t
Phenlormin. 668 Phenindamine tartrate. 709—710 Pheniramine malease. 708 Pttenimmines. 707—710 Phenmetrazine, metabolism of. 89. 101 Phenobarbital. 494. 4945 as anticonvulsant. 504
as ansiolytic/sedative-hypnolic. 494, 494t drug intrmctions with, 131 as enzyme inducer. 131 metabolism of, 70 Phenocoll. 761t. 762 Phenol(s), 217. 221—223 corrosiveness of. 14—IS
glucuronidatiun of. 115 liquefied. 221 tnethylaiton of. 12$— 126
2-Phenylethanol. 229 Phenylethyl alcohol. 229 fl-Phenylethytamine. structttre—activity relationship for. 530—531 5-Phenylhydantoin. metabolism of. 109 Phenylmercunc acetate. 230 Phenylnsereuric nitrute. 230 Phenyltnethanol. 229 Phenylpropanolamine. 538—539 Phenyl sallcylate. 756 Phenytoin sodiunt, 505. 5051
Pituitary gottadotropins. 774—775. 7741 Pituitary homsones. 841—844
Pisalic acid pmtniety, 145. 14Sf 13—14. 14t, 1St
acid/base strestgtlt and. 14 drug distribution and. 16—17 percent ionization and. 15—16. 1Sf. lot pH adjustments aitd. 15—16. 1Sf. lot values for. 948t—956t
as antianhythtnic. 639-640
water volubility and. 16 Placental barrier. I' Placidyl Se,' Ethchlorvynol Planch quantans theory, 935—936 Plaquenil See
as anticonvulsant. 5(15. 5(1St
Plasmakinitss, 85(s—857
interaction of with isoniazid. 124 metabolism of. 70. 124. 132. 133 species differences in. l28 pHisol-lex. See Heaachlorophene Phleomycins. 417 Phosphate esters, as prodrugs. 149. 149f Phosphocol. See Chromic phosphate P 32 Phesphodiestemse S inhibitors, active sites of. 29. 31)1
Plasmid vectors, 165—166. 1661 Plo.smod/son ui-oh'. 282, 283—285, 2841 Phu.srnor/iiuns spp., 282. 283—285, 2841
drug action against. 284—285 genisnte iii, 285 infection by. 284—285. 2841 Si',' rats,, Malaria
life cycle of. 2841 types of. 283
Phospboinositol systesn. 552 Phospltoline Iodide. See Echothiophate iodide Phospholipase C. 552 Pltosphoramide mustard. 396. 3961 Phsssphorothionatev. 568 Phosphonts-30. 457 Phosphorvlation
Plassusodhum error. 282. 283—285, 2841 Platetetis)
in DNA synthesis. 154f. 15Sf of ptsidmgs, 153—154. 15Sf Photodynamic therapy. for cancer. 430. 433 Photoelectric effect 936 Photofrin II. See Portimer sodium
Platinum consplexes. 428 Platomycins. 417 Plavix.S,'i' C'Iopsdogrel Plegine. Si-c Phenditstetra.rine taflrate Plendsl.S,'e Felodipine Plicamycin. 414, 417, 424 Pluripotestt stein cells.. 177, 17Sf. 197. 19Sf PM3 method, 938 Pneumocanadins, 246 Pneumococcal vaccine, 215
Plsysostigmine, 563—SM. 563t. 5641 Physostigmine salicylate, 564 Physostigmine sulfate, 564 Phytoesteogens. 778—779. 77sf Phytonadione. 884—885 mettadlisne conversion to. S
Picrotoxin. 510 Pilocarpine hydrochloride. SoIl Pilocarpine nitrate. 56(1 Pimaricin. 237—238 Pitninodine, 736t. 737 Pimoride. 501 Pindolol, 543, 544f Pink disease. 891 Pinocytosis. S
fibrinogen receptors 'itt, 633—634 functioits of, 665—666 Platelet aggregation. 665—667. csobf inhibitors of. 666—667 Platinssl. See Cisplatin
P,ieuunou'sstis ,'ocunhi, 264)
Podophyllotissin, 424 Polaratnine.S,'e Dexchlorphetiirasnine malcats' Polani'alissn functissns, 938 Pisling algoritltists, 933 Polio vaccine, 2111, 212t Polyacrylamide resins, in costsbinatorial
chemistry. 49 Potychlorinated biphenyls (PClls), metaholisas of. 71
Phentermine hydrochloride. 513 Phentennine ion-exchange resin. 513
Pins, in combinatorial synthesis. 44. 4Sf. 62 Pinworm infestation.s. 265 Pioglitazone. 672 Pipanol. See Triheayphenidyl hydrochloride Pipecurittm bromide. 593 Piperacillin. See rats' i Penicillims spectrunt of activity of, 31)8 Piperacillin sodium. 309t. 314 Piperacillin-tazoboctam. 316 Piperazine estrone sulfate. 780 structure of, 777f Piperazinelsl. 265. 706—71)7. 706f Piperocaine hydrochloride. 690—693. 69lt Piprucil. See Piperucillin sodium Piranthicin. 416 Pirbuterol. 536 Piretanide. 61 If Piritresim. 410—411
Phentolamine. 539
Piroxantrone. 429—43(1
Phenylalanine. hydrogen-suppresved. 23—24.
Piroxicant. 760 Pitocin. See Oxytocin injection Pitressin. See Vasopressin injection
49 Polythia.cide. 605—(sltt, (Slot, bUst. 620 Pondimin See Fenflurastsine hydnw(tlondr Ponstel. Sri' Mefestatnic acid Porfimer sodtusn. 43tI. 433
Pitressin Tannate. See Vasopressin tannate
Porliromycin. 419
Pituitary antidiuretic hormone. See Vasopressin
Porlamines, 533t
topical. 234—235
Phenol coefficient. 221 Pltenntlsiazines anlihistansine. 7111—711
antipsychotic. 498—500. 499t metabolism of. 87 ring analogues of, 499 Phenoayacetic acids. 613—615 Phenoxybenzamine. 540 Pttenonymethylpcsticillin. 310. See ratio
Penicillin(s) Plsensusimide. 505—506 metabolisttt of. ICY.) Phenteemine. metabolisnt of. 91—93
24f. 24t. 25t Phenylbutazone. 762. 763t active melabolite.s of. t34. 135t metabolism of. 70. 82. 114
Polycillin. See Ampicillin Polycyclic aromatic hydrocarlxsns. carcinogesticity "I, 74. 74f Polyene antibiotics iS—238 Polyethylene glycol. in combinatorial chemistry, 49, 62 Polymerasc chain reactiost. in combinatorial chemistry. S2. 62 Polymer heads, in combinatorial chesnistry, 48—49, 6(1
Polymos. See Amoxicilliu Polymysin B sulfate, 357—359 Polymyvins. 3(Xlt. 357—358 Polypeptide antihiistics. 355—36(1 Polysaccharide-coated superparaniagnetic trots
"side particles (SPIOsI. 477 Polystyrene resitis. in coinbinatisrial
Indev Procainamide hydrochloride. 638
Pocaconawle. 244 Posicor. See
Mibefradil
Procatne. 678
Positional cloning. 167, 167t losiuonal scanning. 51, 62—63 Positive inotropic agents. 655—657 Positron emission. 456 Positron emission tomography d'ETI. 456. 460. .1601. 4611 contraceptives, 794—795 Potassium channel openers. 654—655 Potassium ion channels. 681—683,6)121. See also Ion channels Potassium leak channels, 681 Potassium sorbale. 230 Potassium-sparing diuretics. 616—618. 6161 preparations of. 620 Pswidone-iodine. 223 Poxs'i,uses. 3701, 372 Pradimycins. 246 Pralidoxime chloride, 571
Ptatnoxine hydrochloride. 694t Fl'jndin. See Repaglinide Pranral. See Diphcmanil methylsulfate Petsachol. See Pruva.statin Petsastutin. 663 Ptaarpam. 491 Praziqnanlel. 267 Ptazosin. 540—541, 541t. 652
metabolism of, 98. 109 Preccf. See Ceforanide Precipitation. 175 PSecose. See Acarbose Ptednicarbate, 81)111, 813 PSednisolone. 811 metabolism of. I 10
relative activity oF. 8091
ioluhility of, 770t structure of, 807f Ptminisolone NaPO4 salt. solubility of. 7701 P(ednisone. 1111
active nietabolites of, I 35t as antineoplastic. 435 relative activity of, 81)91 Pregnancy
hCO in. 775 placental bamer in. 6 Pregnenolone. in steroid biosynthesis. 768. 769, 7691
Estrogen(s)
Premphase. See Hotmone replacement therapy Prempro. See Honnonc replacement therapy Preservatives. 228—231) P(rvacid. See Lancoprazolc
Priltacaine hydrochloride. 690—693. 6921
Ptinucor. See Milrinone Pemaqaitie. 288—289. 2891. 295, Fhnuiy pulmonary hypertension. 823—825 Piintasin. See lmipcnent.cilastin Plimidone. 506 active metabolitcs of,
Prnchlorpcra,.ine malcntc, 4991. 5(X) Procrit. See F.poelin thu Procyclidine hydrochloride. 583—584
Prodilidine. 737,, 738 Prodrugs. 109. 142—159 activation of. 142—144. 152 by azo cleavage. 149—ISO. IS It' chemical, 155. 1561
by oxidation. 152 by phosphorylution. 153—154, 15Sf by reduction. 152—153. 1531 advantages oF, 142, 154—155
alcohol promoicty for. 144—149. 145f—l491 amine, 149, 1501 azo linkage and. 149—150. 1511 binprecursor. 142. 143. 152—155 in cancer chemotherapy. 156 carbonyl pronsoieties for. 150—152. l5lf cartroxylic acid promoiety for. 144—149, 14Sf— 1491
earner-linked. 142—143. 145—152 as chemical delivery syslents. 155—159. 1571'— 1591
definition of, 65. 142
in immune response. 205
Premarin. Sue
Procaine hydrochloride, 690-693. (,9lt Procan SR. See Procainamide hydrochloride Procarbazine hydrochloride, 397, 402 Procardia. See Niledipine
35,
Principal cclii, of ncphron. (rOll Principal components analysis. 511. 63
Ptincipen See Ampicillin Ptinivil. See Lisittopril P,i.scoline. See Tolnzoline Psistinamycins. 363 Ptisine. See Naphazoline ProAntaline. See Midodrine
Pto'Banthine. Sri' Propantheline bromide Plirbenecid. metabolism of, 70—71 Probucol. 662 Plocainamide. 638
active metabolites of. 135,
metabolism of, Ill), 122. 124
double-ester, 146—147. 14Sf drag distribution and. 4—6 ester. 144—149. 1451— 1491
of functional groups. 144—152 hydrolysis of. 146—149 Manniclr bases and, 149. 1501 metabolism of. 142. 146—149 mutual, 142—143 prornoieties and. 142 solubility of. 5. 142. 145. 147—149 For unpalatable parent drugs. 145—146 Productive infection, 367 Product stereoselectivity. 103. 132—133 Proencymes. 837 Progestasert IUF). Si'e Progesterone IL'D Progesterone
as antineoplastic, 434 biological activity of. 785—786. 786, biosynthesis of. 769. 7691'. 785 formulations of, 788 metabolism of, 785. 7861 structure of, 71171
Progesterone (UD. 793t. 794 Progesterone receptors. 773 Progestins, 785—789 as antineophistics. 434
biological activity of. 785—786, 786t biosynthcsts of, 769. 7691, 785 in contraceptives. 779. 790—794. 7911—793. endogenou.s. 785
itt honnone replacement therapy. 796—797. 7961. 7971
metabolism of, 785, 7861' products. 787 progextational activity of, 786. 786t structural classes of, 786. 7871 synthetic. 786 therapeutic uses ttf. 787 Proguanil and atovaquone. 29lf. 292. 292f ProHance. See Gadotendol Prohcptazine. 738 Proinsulin. ((47, 848f. See also Insulin, recombinant Prolactitt. 844
98.3
chorionic growth.hormone. 845
metabolism of. Ill Prolurtin—releasing hormone, 841 Proleukin. See Aldesleukin
Prolixin. Fluphenazine hydrochloride Promazine. 498, 4991 Pronicihazine hydrochlortde. 710—711 Prometrium. Progesterone
Promoietics. prodnig. 142 Promoters. 168 Prompt insulin zinc injection. 1(5 It Pronestyl. See Procainamide hydrochloride Pro,tethalal. 542 Prontosil. 149. 1511. 269 Propacil. See Propylthiourncil Propadrine. See Phenylpropanolamine PropaFenone. 64(1—641
2.Propanol. active metabolites of. 220 Propantheline bromide. 582 Pruparacaine hydrochloride. 690—693. 691, Propecia. See Elnasteride Properidine, 7361
Propine. See Dipivefrin Propionic acid. 233 Propofol. 488 Propoxycaine hydrochloride. 690—693. 691, Propoxyphene. 738. 7391
metabolism of, 85 Prxtpoxyphene hydrochloride. 749 Propoxyphene nupsylate. 749—750 Propranolol, 542—543 active metabolites of. 135t metabolism oF. 70. 89, 103, 112—114. 114 Propylamines, 707—7 10. 7(171 Propylhexedrine. 5311
Propyliodone. 484 Propylparnbcn. 229 Propyl p.hydioxybcnroate. 229 Propylthiour.tcil, 674 metabolism of. 114. 126 Proscar. See Finastende ProscWOS. See Phenyl salicylate Prustucyclin.slerived drugs, design and development of. 823—825. 824t—825t Prostaglandin(s). 666—667. See also Eicosanoid(s) antiulcer. 726
biological activity of, 822t biosynthesis of, 8 18—822. 8191. 8201 in coagulation. 666—667
discovery and development 01'. 818. 8I9f. 820f metabolism of, 82 If. 822 Pl'ostaglandin unalogues design and development oF. 823—825, 824t—825t invesligational. 824:—825, ophthalmic. 823. 828
Prostuglandin 01. 1(221 Prostaglandin E1. 822t. 827 Prostaglandin E1 cyclodextrin. 827 Prostaglandin 8221. 827 as abotlifacient, 795, 7951 Prostaglandin F1, 822t Prostaglandin F1,,. 827 as abortifacient, 795. 7951 Prostuglandin H2. 820 Prostaglundin 820. 8221 Prostaglandin inhibitors. 754 Prostaglnndin J1. 822, Prostunoid receptors. 825—827, l126t Prostanoids. for vetennary use. 828—829 Prostaphilo. See Otiacillin sodium ProstaScint Kit. See Capromub pcndetide Prostat. See Metronidazole
984
len!,'.;
I'rostate cancer anliandrt.gens lot. 801—802. 8021 estrocens for. 779
l'room E2 S..' l'r.istaglan.Itn F.. Poisiji, P2 Alpha. Sec Proslaglandin p:,. l'rostin VR l'ediat. it l't.isiagl,.n,lin I., Pr,.taniines, 833;
Protat,,tne suII;ite, 667 I'rotaintne tune tisnlji, suspensuin. sS tt. 852. 852t
Protease catalysis. 837. 8371 Protease inl,ubn,,ts anti-I 11%'. 184 —l$7
I'rotruptyline hydrochloride. .S 17
I'r.itropin. S.. .Si'ntatretu Priveinil Set- Alhuier,,l Pnn-era. S,, Medr.rxyprogesterone acetate
in snhstitueni selection, 22—23. 23t threeitiunensior,aI. 21.18—19 Quadrainet Samarium SM 53 lex,dronarn Qua,,titatise sun.ett.re—acov.ty relaliu'nship
I'rorigil S.c M.nlaftnil
IQSARI. hl—21 See of." QSAR
l'n't itanot, A. 869—87)) Pri's itanuitr I). 875
studies Quauuintui dots, 53
l'r.',ac See Ilt.oxettue
Qt,.,titt.ni ,neehaut,es. 38. 923 in compttter-assisled drug design. 935-939 Quarean Ctidittinuo bromide
l'seu.d..chuoltnesteru.'.e. 564)- 56), 5611
4- ).Psett.Ioepltedriiie. 538 Psead,.n,,.n.,I i,ttect,,,ns air.tut..gtyc..snte-rrsisiat.I. 336
ceplralosporins ti'r. 325. 326t fl tactain-resisiant .32.1 Pseu,t,t,rumtc acid A .362—363
Qua.eeparo. 492
Questran. Si-c Clu.lesuyrannne resin Quetiapune. 5112—51)3
h'seudouropune. 575
Qninacrine hydroct,I,,ride, 293). 294 Quinagtute..Se,' Qt.unidine gluconate Quhrtapril 646. 647). 6471
acute phase. 2))). 21W
Ps,l,win, 521
Quinelhartine, 6(17—6)0. 1,4171. Ôtt9t, 62(1
apgn,gati,.,u il. 175
Psilocyhiti. 52) Psyctt..ses. 496 498 t'nturrcort t,.rh,,l.alcr. See Budeso,,ide
Qitinidine. 286, 286). 287. 295t active ,,tetabol.tes ,ut. I 1St tttetab.,I,sm ol , 77 Quit,i,Iine gluconate. 638 Qainidine 638
ileuel.'pment iii. 942. 9431 ant.neoplasuic. 446—447
Protein),. I. S.. .,l,,' Recepi''ris
.itnpholeric behavior itt. 833 of. 173. 174$
Iiltkitl. 857 858 buried. 833 cheunislr) of. 173-175. 1711
chimenc. 168-169 in dote screening. 172
c..nkirmatn.n ii 831 —832. 8321. 815 doug—receptor iutteraclii,ns ''itt. 28 conjugated. 833 deanodat,on of. 173. I 74) denatnr,,Iuon Of. 173- 175. 833
tirceulation ol. 175 fusion, 11.8—11.9
in .Intg screening. 172 hydn.lysis 4,1. 173. 17.11
hydrophobic Force' ii... 831
instability .'I chemical. 17. 1711 physical. 171 175 oxidat..... il. 171. 1741 piattranslational nh,nJiI ieali,,uis iii. 162—Il,). 1631
precipilar.u'n ot. 175 products. 814-835 properties of. 833 pnriticatu't. 4,1. 833 race.,uaauiou .8. 171. 741 reco.nb,na,ti. 11.8— 169
immun..gen.city.uI. 175 separation and ideu,tilicati,',. itt. 814 simple liruel. 811. $33. 813t soluhility $33. $13t structure of. 162. 71. 811-833. $IIL 8121 ,Iatahasec iii. 39- 41) Factors alfrct.ng. 832—833 surface adsu'rpti.'n itt. 175 synthesis ot. 62 63. 1631
Protein
I',,, if.ed c.un.c..tr..pun. Sir Reposit.'ry ctirttcotropin injection l'urine nucleotides. the n,,v,, synthesis of. 4)12 -4184, 41)3) —41841
1133 classiticutu.'n color lesus 1w. 834
Protein binding. I. 7 Protein C. 665. 883 Protein c.'y.nes. S,'.
h',,h,r,,,,,arv hypertension. 823—825 ne. See Dontase alla
P,,,iu,etltol S.',' l.—Nlercapl..lrarine l'Vi'-sutir,e St. I',.s
I'r.tli.I.'xi.,te chl.'nde I'r..tu.porphvrin IX. it cvtoclrrotne I'—451l. 67 Protriplyline. inetabolistu ol. 76
Prurroparn chloride
Qninu.lumes, 247 -252
properties of. 247 -248. 2491 types iii. 248 -252 Q.nnoplrenol. 261 Qu,unosol 5,'.-
I'> taa,,tid,,re. 7t.2 t'yrae.il.ihuned.nne der,v,,tives. 762— 763, 7(.3t
K
i'sran'lit,e. 762 Psran.l.,,,e ,teiivaiives. 762 ibe.t,ar.,ine citrate. 705 l'yr.tttne. ninhrydrin lest for. $34 I'y odi.,n;. Ste l'hetuan.pyri.litte Pvrid.'stiguuine bromide. 5631. 565 Pyridt'x..t. 89) Pyridoxal 5plt..sphrate, 892 —894 i.l..xan,ine. $91 Pvrid..xine. $91 —894
deliciency oh. 8')) dietary s,iuree.s of. $92 drscosen il, 89) h,,,ntones ti,,), 893 -894
Qninnpristin-daItoprrsuin. 363 QVAR i'. I(eclotrrethas,.ne diprurpionate
Rahepra.eole sodit,n,, 722. 723r. 726 Kacemic catciutni panrotl.enate. $88 Kacemic mintnres. ntetatb.'tisnt of. t 32 Raeenti,ati,.u.. protein. 171.. 1741 Racem,.ran,idr. 738. 719, R:icetruorphan. 738—719, 75(1
Radanil.S.'e Ben,nida,,.le Radiati,.n. 454- 458 atuttittitati,.t,. 456 hiologucal etlects ut. 457—458 delinitiurn of, $54 direct effect utI. $57 ehectrounitgnet.c. $54
indirect ellect of. 457 ionieing. 454, $51 properties of. 454—451.
.uI, 89) —$94
t'vrud..xine hydrochloride. 894 Pyndost'l. 892 P5 rilanuine ,i,aleate. 7115
I'yrunrethanrine, 27) I'ynrnetltamine'snlfaduaeune. 271tt 2891—2911. 2911—292. 291,1
I'yr.n.idtne urncleosi,le antagonists. 40$. 41191
l'yr''nil .5.'. Pyrrobutantine phosphate P/i insulin. %51t, 852, 852t
r.idioactive utecay intl. 455 -$57 Radiati,,,, ,thsorbed dose. $57 Radiation dosu,netr . 457 Radi,,activr decay. 455—457 Radiotrequency enciuding. 53 Radi.'gntphy. 472 -473 Kadioh..gic pru.cedures. 478—481
cintirast agetits tiir. 472—484 Radiu.pharmaceutieals radiotraeers For. 458—472
Radionuettdes. $55. 457—462. Se,' olin Radnuphtaritraceuuueals daughuter. 455
sates. 8311—831
Protein kinases. in tnnt.'r.geuesis. .138 Protein S. 883 Protein tyrosine Ltnase iulrit',uns. 438—441) Prote.rnties. 193 in dn.g developnreni. 44') I'r.rthronrhin, 663. 664. 664t. 64,41. 665 I'r,,tonix c i'anropra.'ote sodit,rrt Proton pump inhibitors. 722—726. 7231— 725). 721i activnt,.urt ol. 155. 1561
Quinine. 286-287. 2561, 295t
Py.'pen S,, C'.irtrenicilhiu, ilis...t.uitr t''.r.ii.tel patit.xtte, 265 I'yr...'inamtde. 254, 255 256 l'yraeinecarbosamide. S.-. h'yra,inatnide l'yraioles. 762
Ps rrob,tla,nine phosphate. 741') tries
Qatnidmne salIate. 637—638
Q QSAK studies, 17-23 bilinear tumId in. 21 - 22. 2)1
lis-e-di,ne,,sional. 23 tHur—,lintensi. na), 21
tdentitv variables in, 23 linear trunlel in. 21 -22. 211
',,ieut,,,tlwater system in. 19 lIt
parent. 455 production itt. 461—462 pttiperties .4. 454—456 Irttnsf,,rnratiuin itt. $55 —-157 Radninuchide test kits. monochonat antitnnty. 190-191 Radii.phartnacettuieals. 454—484 contrasi aget.ts. 472 —$84. See ,ti,o C,,utrusu agents
parabolic model in, 21—22. 2)1 pantiton coeltic,ent un. I 9—21 pltysicochemucat par.mreters in. 21. 211
t)u.'ritte. 468 gallium. 468
predtciive pln.rniacoptu.ne u.a.utels and. 944
iodine, 468-469
indium, 469- •171
moituclonal atttih'tdies. 47))
Reconthisas. 186, 86(lt
radiotracers. 458—472 leehnetittm. 462—41,7
So-Rednctase inhibitors, 802—803, 81)31
thalliam, 472 venus. 472 Radiotherapentic agent'. 444—445 Radi,.qruccrs, 458—472.3?,' a/xe. Radiophanoacenticals Raloxifene. 7811. 782. 783 actutttts uK 29
Ramipril. 6471. 6$7t 648 Randottt screening. I Ranüidine. 7391 72)), 720t. 721 —722 RAREs. 872
Lit protein. 438—439 Ratio 1.5 contntst agents. 473 Ratio 3 contrast agems. 473 Rational dntg design. 1—2. 919. 92)). 940. Set' talc,, Drug design Randisitt ci. Rattwolita RanSed. See Kcserpinr Ratiserpal. See Ranivitllia Rausal. See Rauscolita Ransst,Ilia. 650 Rutot'oif,o si'ep.'ittiiio. 651)
Rayderm See I'henyl salicvlatc rDNA technology. 163- 194. 858- 86)1. 859t. S..i' also IIiotecltnolttgy in receptor isolatiott. 28 step' in. 858—864) Rehutsetinc. 519
Receptoos). 8—9. 27—29. Sit a/s.' specific t)714'.s
allinity 1,tr. 8 a.syntuttetry "I. 35—37 chiuneric. 169. 1691
clotting of. 28. 172.S.'.' ia/so Cloning distribution of. 9. 28—29 drug ltindittg by. 27—28 virtual screening for, 55 drug interaclions with. Ste Drug—receptor ttderacttons
flesible. 28 as lutactional areas. 28 heterttgcnetty ol, 169- 171). 17))t
isolation of. 28 ttteittbratte-hotttid. isttlation al, 28 t,r.etttation of, 28 properties iii, 27—29 specificity of. 28 structure ttf, databases of. 39—40 variability itt. 28—29 Receptor-baseti drug design. 55. 939944 Receptor '.election and antplification technology lr-SAT assay. 171 Receptor tyrosiute kinase inltihitors. 438—44)) Recottthinant DNA wcltttolttgy. 163—194. 858—864). 85')t. Ste a/si. Biotechnology
DNA processing in. 172, 860 DNA production in. 364. 166-168. 860 in receptor tsolatiott. 2$ steps in, 858- 861) Recombinant drag prodttcls, 859t—sotlt. 864)—863.Sre ,,lso Biotechnology ond u/ie.'tfic prts/zat-cs
ADME propeoies ol. 175 hioasailahthity ii), 175 drug delivery ot, 175 tttctabohism of, 375 types of. 175—191 Recounhittatti proteitts. 168 — 69. Se,' ,,ts,'
Ilio(echnology immnnogentcity of. 175 pruwes.sittg of. 172
Recotnhinate. Set' Factor VIII, recotohittant
Redaction, tn biotransformation. See I)rug metabolism, reduction in Redun. See I)esfcnfinratmine Regional nerve blocks. 687.5ev ia/ut Local anesthetics
Rcgioselectivity. itt drug ntetnbohistn. 133—134 Regitine. See Phentolamine Regranea Gel. See Becaplerntin Regular insttlin. 853. 85 It. 852t Relalen. See Nahumetone Remeron. See Mirtantpine Remicade. See Inilisintab Retttifetttunil hydrochlonde. 748—749 Reminyl. See (lalantumine Retnodulm. See Trepro'attnil Remosipride. 502 Renal drug eacte)itttt, 41. 8 Renal Failure, drug metabolites in. 134 Renal solute reabsorption. 596—601. 5971—6001. See also Nephotn
Renal tonielty. nI sulfonantides. 122. 274 Renese. See Polydtiazide system, in blooni pressure regulation, 642—645, 6431—6451
Renin•angiotensin system inhibitors. 645—646. 6461
RmuPru'. See Ahcisltnah Repagltnide. 671 Replicons, 165
Reporter genes, in drug screening. 171-172 Repository corticotropin injection. 842. 842t Rescinnamine. 529 Rescriptor. See Delavirdine Rescula. See Unoprostone Reserpitte. 529. 65)). 6501 Reserpoid. See Reaerpine Resitts. in cotnhinatorial synthesis. 48—49, 481. 63 Rcsorcinol. 222. 233—234 Resiasist. See Ferrisan Respiratory hurst. 203 Resting potential, 68)1 Restoril. Temazepam Restriction endonucleases. 164—165. 16Sf.
lost. 860 Retepla.s'e. 184, l84t Retevase. See Retepha.se
Reticaloendothelial system. 198—200. 199t Rettn-A. See Tretinoin Retina, vitamin A and. 871—872 872. 8721 Retinal, biosynthesis of. 879 Retinoic acid, 867. 870 biological activities ntf, 872
Re Via. See Nahtreat,tte Resitlate. See Snuliunt tltto.sahicylate Rhtnocort. See Bttdesonide Rhodutpsin. 871. 872. 872f
Rhythttttd.Se.' Potpafettone Rihavirin. 381—382 Riboflavttt, 89(1—891 Rchard.san approach. 922
Rtt'abutin. 257. 258 Rifadin. Set' Rifatttpin
Rifantpicin. See Riltmpin Rilampin. 254. 257-258. 300t Rifamycins. 257—258
Rintactane Ste Rtfampin Rimantadine, 372 373 Riuttemtlotte. 8 lIt, MIII. 833 Ring equivalents. 43 Ringwornt. 23)1. 233. 231t. 233—235 R istitner. 35—36, 371 Risperdal. See Rispcridotte Risperidtttm. 497, 501 Ritalin. See Methylphenidate Ritodrine. 537 Ritonavir. 385—387. 942 Ritsert. 678 Ritasan. See Ritusimab
Ritusimab. 389. 443. 444 Riva.stiguttine. 567
RNA. 362 anttscnse. 393— tM modeling of. 92(1 RNA viruses. 368t. 369t, 371 —372 Rithasin. See Methocarbanttl Robinul. See Glycopyrrolate
Rocephin. St', Ceftriasone disodium Rocttagan. Set' Bettznidantlc Rods and cones. 873. 872 Rofecttsib, 760. 822—823 Roferon A.Se.' Interferon ahla'2a Rohitetoucycline.345t. 346 as po.drug. 149. 151)1
R,atnilar. See l)csurotttetltorpltan ltydrthrotti Rondotoycin. See Mcthacychinc hydrochlori Ropivacainc hydrocltloride. 69(1—693. 692) Rose oil, 229 Rosighitazone. 673-672 Rotasirus vaccine. 213 Rotrecctgitt alfa. 185 Rttandwortn infestations, 265 Rtuaiam. Set' Rcmosiprude r-SAT assay. 171—172 RU-486. See Mifepristu'ne Rubella vaccine. 214). 212t
Ruhidomycin. See Daunombicin Rule of Ose. Lipinsku 40. 55. 62 Rynatass. See Carttetapentatte citrate
unetahohism atE, 1)69
Retinoic acid receptttrs. 872 Retinttids, See a/so Vitamin A antineoplastic activity of. 430 delinition of. 868 Retinoud X receptors. 872 Retinol, 868. See u/to Vitamin A absorption of. 869 esterilication of. 869 unetabolism of. 869 all-teana-Retinol. 867. KitE biosynthesis of. 872. 8721 Rerinol-binding protein. 869 Retroviruses. 371 —372. 380 Reverse tr.mscripta.se inhibitors, 372, 379—38) nt,nnucleostde, 383—384 resistance to. 382
Reverse truutscription. 371 -372. 380 Reves. See Nalmelene Itydrocltloride
S
5-145, 824t Safety—catclt ltnkcrs. un conthinauttrial synthesis. 49. 63
Sairole. tnetabolisnt itt. 80-SI Saint John's won. 908—910 Salbutamol. tnetabolism of, 87. 115 Salcto. See Salicylanutde Salicylatttide. 756 Salicylanilide. 764). 76lt Salicylic acid, 233—2.4. 754 ntctalxtlisnt of. 114, 117 Sahicylic acid derivatives, 754 —757 Salk vaccine. 211). 2l2t Salnieterol. 536
Salol principle. 755-756 Salsalate. 757 Saltatory cttndactiu,n. 680
986
hider
Suluron. Sec Hydrotlumethiazide Samarium SM 153 lexidronam. 444. 445 Sandostatin. See Octreotide acetate
Serum globulins, 857 Serzone. See Net'arodone Sevotlurane. 486
Sandril. Sec Res.erpinc Saniti,aiton. 2 I St Saquinuvir, 384 development of. 942. 943f Sargramosttm. 179. 430. 432—433. 859t, 863
Sex differences, in drug metabolism. 129—13(1 Sex hormones, 775—7119.5cc also Steroid(s) biosyttlhesis of. 768—770, 769f. 775 in chemical contraceptives, 7$9—795. See
SAR with NMR. SI Sawmomab pendetide. 191. 859t
progestins. 785—789 Sex steroids. biosynthesis of. 768—770. 7691 Sibutramtne, 514 Sickling disease, malaria and. 283 Side effects, 9
Sauvagine, $35 Saw palmetto. $03
Sanitoxin, 690
Scahenc. See Lindane Scabicides. 26$ SCF method. 93$ SCH 59884. 244
Schizophrenia, 496—498
aLso Contraceptives
Signal transduction inhibitors, 438—440 Sildenafil. active sites of, 29. 30f Silvadene. See Silver sulfadiazine Silver sulfadiazmne. 279. See also Sulfonamides
indications for, 2701
uiprodrugs. 5. 142. 145, 147—149 water. and, 16 Soluble suppotts. 49. 63 tagging of, 53, 61 Solute, renal reabsorption of. 596—601. 597f -6001 Solation.pltasc coinhmnatorial chemistry, 49, 5111
Suls'ation models, 934 Ssmlvent.acccssible surfaces, 922
Solvents, in molecular dynatnics simulations. 934
Soma. Sic Carisuprodul Somatic cells. trunsgenes in. 94 Somatic nerves, 54$
Somatoliberin.
841
Somatostutin. 841. 845
Somatotrupin. $44
Schraden, 570—571 Schrodinger wave equation. 936—937
Silvbum marianum (milk thistle). 914 Similarity probes. 56. 57t
Sotnatotropin release—inhibiting lactor (SRIF(.
Schwann cell, 679. 6791 Scintigraphy. 458—460. 4581. 4591 Scintillation camera. 458—460. 4581. 4591
Simple proteIns. 831
Somatrem. Ill, 844. %59t Somatropin for injection, 844, (1591 Sonata. See Zaleplon Sorbic acid. 230
Scintillation proximity assay, in highthroughput screening. 54. 541 Scopolamine, 574. 577—578
Scopolamine hydrobromide. 578 Screening. Sec tal.w Computer-assisted drug design of antineoplastic agents. 392—394. 3931 automated. I
biotechnology in. 170—172. 1731 heterologous expression and. 17(1—172. 1701 high-throughput. 26—27, 40. 401. 43, 53—54. 541. 944
human-tumor.colony_forming assay for. 394 madam. 1—2 reporter genes in. 171—172
virtual (in silica). 54—55, 56, 419. 919 senogralt models for. 394 Scurvy, 898 Search query, database, 56 Sehatrol. Sec Flutamide Secobarbital. metabolism of. 76. 77, SI Secobarbital sodium. 494t. 495 Scconal. See Secobarbital sodium Secondary mass spectrumetry. 52 Second messengers. 171, 172, 552 Secretin. 854 Sectral. Si's' Acebutolot Sedatives. See Anttiolytics. hypnotics, and sedatives
Seizures. 503—504 Seldane. See Terfenadine Selective estrogeti receptor modulators (SERMs). 28—29. 291. 781—782. 7811 agonistlantagonisl nctions of. 29 Selective scrotonin reuptake inhibitors. 5 18—520
Selective toxicity. 217 Self consistent field (SCF) method. 938 Self-renewal, 177 Semilente insulin. 851t, 852, 8521 Sempres. See Acrivasune Senapax. Chimeric. See Daclizumab Sernx. See Oxazepam Serentil. See Mesoridazine besylate Serevent. Sec Salmeterol Scromycin. See Cycloserine
Seroquel..See Quetiapine Serosal immunity. 200 Serpasil. See Reserpine Serpins. 665 Sertniline. 519 Serum. 857
Simvastatin. 663 Sinequan. See Doxepin hydrochloride Single photon emission computed tomography 458—460. 4601 S isomer. 35—36. 371 Sisomicin sulfate. 341
$.Sitosterol.
661
Skin infections, fungal. 231, 23lt topical agents for. 233—235 Slater determinant. 937 Slater type functions, 937 Sleeping sickness, 26(1 Sleep-promoting agents. 488 Slow-reacting substances of anaphylaxis (SSRAs), 820 Smallpox vaccine, 21)9 Smoking, drug metabolism and, 131 Snake vcnoms, 835 Sodium, renal reabsorption of, 5961—6001. 597—601 Sodium 4-aminosaticylate, 256—257 Sodium antimony gluconate. 263—264
Sodium uscorbate. 899 Sodium benzoatc, 229 Sodium caprylate. 233 Sodium equilenin sulfate, structure of. 777f Sodium estrone sulfate, structure oF, 7771 Sodium iodide 1131,444,445 Sodium iodine capsules, 469 Sodium iodine oral sotutionlcapsulc. 469 Sodium ion channels. 681, 682—683. 6821. 5cr also Ion channels Sodium nitrite. See Nitrovasodilators Sodium nitroprusside, 654 Sodium PAS. 256—257 Sodium phosphate P 32. 444. 445 Sodium-potassium pump. 682—683, 6821 Sodium prupissnate. 229 Sodium salicylate, 755 Sodium stihogluconate. 263—264 Sodium Sulamyd. See Sulfacetamide sodiunt Sodium sultacetatnide. indications for. 270t Sodium thiosalicylate. 755 Soft drugs. 142 Solanacca spp., 910 Solanaceous alkaloids and analogues. 574—579 Soletene. Sri' Solid.phase synthesis, combinatorial. 46—49.
46f Solid supports. 49. 63 tagging of, 52—53. 521. 52t. 61
Solubility lipid. S and. 17
$45
Sorbitrate. Sec Isosorbide dinitrate. dilated Sonatane. See Acitretin Sotalol, 543. 5441, 642 South American sleeping sickness. 260 Sparfioxacin. 248, 252 Sparine. Sec Proma.sine Spatial amlys. microchip, in combinatorial synthesis. 44. 4Sf. 62 Spatially addressable synthesis. 27 Species differences, in drug ntetabulism. 128—129
Specific immune globulin, 21)7 SPEC'F (single photon emission computed tomogniphy. 458-460, 4601 Sprctazole. See Econazole nitrate Spectinomycin. 335, 341 Spectrobid. See Bacamptcilltn Spectrophotometry, in combinatorial cbeitiisu SI
Spermatogenesis. regulation iii. 774. 7741 Spinal anesthesia. 687. See alum Local anesthetics
Spints. 219 Spironol;mctone. 6l6—6t7. 6161, 6211. 815 extrurenal activity of. 619
Spirunolactonc.hydrochtorothiazide, 620 Split-and-mix synthesis, 43, 44f. 62 Split-level basis sets. 938 Sporanox. See Itraconazole SQ.29548. 8241 Squalene cpoxidasc. 238—239. 2391 SRSAs. 820 Stodol. See Butorphanol lartrate Stanozolol. 7991. 801
Staphcilhin. Si's' Methicillin sodium Starlix. See Nateglinide StAR protein, 768 Stathmokincsis, 427 Statistical methods, in drug design. Staurosporlne. 438
17—26
Stavudine. 381 Steepest descent approach. 930 Stem cells, 177. 1781. 197. l98f Stereochemistry
of drug metabolism.
132—134
of drug—recepror interactIons, 31—34 Stcreoisomeis. biological activity of. 35—37. 351. 361 Stereoselectivity. 35. 103 product. 132—133
substrate,
132
index
Stcnlants, 218—223. 218!
classification of, 218. 21%t elicetivenecs of. evaluation of. 219 improper use cr1, 219
phenol cnelficient for. 221 Sterile capreomycin sulfate. 259 Stenk carticotropin .'.rnc ltydrcmide suspension.
Succinate esters, as prodrugs. 147—149, 1491 Succirtimides. 505—51)6
metabolism of. 109 Succinylclioline chloride. 593—595 Sucorrtrin. See Succinylcholine chloride Sucralfate. 726—727 Sudafed. Set' L.( l-Pseudoephcdrine
formulations of. 275—279
half-life of, 276t indications for. 269. 2701 for intestittal disorders. 279 isonizatlon of, 272—274 mechanism of action of. 270—271. 271 f—273f
androgens. 797—803
Sufentunil citrate, 749 Sulur. See Nisoldipine Salbactam. 315. 316 Sulbactam.amprcillin. 316 Sulconazole nitrate. 242 Sulfacetamide. 276. Sec also Sulfonamides Sallacetumide sodium. 278. See also Sulfonamides
metabolism of. 122. 1231. 269. 274—275 microbial resistance to, 275 mixed. 277—278 nomenclature for. 269. 2711' nonahsorbable. 278—279 nonalanine. 269 oral. 269 values for. 269—270. 274. 274t
us angiogenesis (ultibitors. 447 biosynesis of. 768—770. 7691
Sulfachloropyridaz.ine. 276, See al.crt Sulfonamides
contraCeptives. 789—797 endogenous cofliCosteroidc. 81)3—815 estrogen.. 775 —785
Sulfadiazine, 276t. 277. See also Sulfonamides Sulfadiusine sodium. 277. See also Sulfonamides Sulfadoxino. 276t
prodrug forms of. 269. 279 protein binding of. 274. 275 side effects of, 275 spectrum of action of. 271 —272 structure—activity relationships for. 275 structure of. 271f
842—843. 842t
Stcrilc sasopressin tannate oil suspension. 846t. 847 Steriliration, 2181 Steroid(s). 767—815 adrenal. 803—815
onabolic androgenic. See Androgen(s)
metabolism of. 106. 114 nomenclature for. 767—768, 767f. 7681 numbering of. 767. 7671 overview of, 767—76* pharmacokinetics of. modifications of. 770. 77 If proctrug forms of. 158, 1591. 770. 7711 piugestins. 785—789
properties of. 770. 770t solubility of. 770. 770t stereochemistry of. 767—768. 7671. 7681 Stemidal estrogens, 776. 7771—7781
Steroid hormones. pyridoxinc and. 893—894 Stemidogenic acute regulator)' (StAR1 protein, 768
Steroid receptor complexes. structure of. 773 Steroid receptors, 77(1—773. 7721
structure of. 772. 7721 types of, 773 Stilbenc. metabolism of. 77 Stimate. See Devmopressin acetate Stimulants, 510—522
Stochastic simulations, 935 Stomach. See under Gastrointestinal Stovaine. See Ainylocaine Small. See Idoxundine STP. metabolism of. 91
Strain differences, in drug metabolism. 128— 129
Stniub reaction. 735 Srreprase. Sce Streptokinnue Sueptokinase. 839—840 Streptomycin
cntltubcrculous activity of, 254 discovery of. 299. 334—335 Saeptomycin sulfate. 337—338 Slreptonivicin. 361—362 Streplorocin. 420. 424 Strontium 89 chloride. 444. 445 Structure—actisity relationship. 19—23. 38—39. See also QSAR studies and .cpeeiJk
drugs
indk—.itions for. 2701
Sulfudoxine.pyritttedtamine. 277—278, 2891—29 If, 290—292. 296t. Set' also
Sulfonamides Sulfalene. indications for. 2701 Sulfometbazinc, 271f. 276. See n/co Sulfonamides metabolism of. 122. 1231 also Sulfamethizole. 275—276. 276t. Sulfonamides Sulfumetltoxazole. 276t. 277, Sec also Sulfottantides metabolism of. 122. I 231 Sulfumethovazole and trimethoprim. See Trimedioprim-sulfantclhoxaeole Sullamidochrysodine. metabolism of. 107
5-Sullamoyl.24-3-aminobcnzoic acid dcnvatives. 610—613 Salfamylon. See Mafenide acetate Sulfanilamide. 269. 27 If. See also Sulfonamides antituberculur activiry of, 254 metabolism of, 122. 123f Sal fanilamides, 269 crystalluria and, 274
renal toxicity of, 274 Sulfapyridine. 277. See alst, Sulfonamides metabolism of. 122, l23f Sulfasalazine. 279. See also Sulfonamides azo cleavage in. ISO. 1511 metabolism of. 107—1118
Sulfate cyclodt,strins. 447 Sulfates, conjugation of. 1141. 115—116 Sulfarecin, 334 Sulfcnta. Ste Suleutanil citrate Sulftnpyrazone. metabolism of. 114 Sulfisoxazole. 276. 276t. See a/co Sulfonamides metabolism of, 122. 1231 Sullisoxaeole acetyl. 276. See also Sulfonattuides
Srnjclure.based drug design. 55. 939—944 Stypven. See Snake venoms
Sulftooxazole diolamine. 276, 278. See also Sulfonamides
Subcutaneous injection, drug distnhution and.
Sulfonamides. 268—280 absorbable intermediate-acting. 276t absorbable short-acting. 276t
41. 5—6
Subcutaneous mycosis. 231 Sublimare. See Fentanyl citrate Substance P. 857 Sabstitnenis librunes cr1. 26—27
selection of. 22—23. 23t Substrate stcreoselectiviry. 132 Subtractive deconvolution. in combinatcmnul chemistry. 50
anillne.substituted. 269 br bunts, 278—279 classification of, 269 discovery and development of, 269 distribution of. 274—275 excretion of. 274. 275
987
topical, 269. 2711—279
toxicity of. 122. 274. 275 Sulfones, 279—280 Solfonylureas. 668—670
metabolism of. 94 Sulforidazine, metabolism of. 99, 10(1 Sulfur mustard, 394 Sulindac. 758 active metabolites of. I 35t
metabolism of, 108, 143. l44f prodrug l'orm of. 143. 144f Sunykect, Chimenc. See Basiliximab Supeocntical fluid chromutography, in combinatorial chemistry, SI Sttpcrparamagnetic substances. 476 Suprane. See Desflutane Supras. See Cetixime Suprostonc. 8251 Suramin sodium as antineoplastic agent. 430. 447 us aniiparusitic agent. 264 Surfactants. carionic. 224—227 Surgicoti. See I'lexachlorophene Surgifoam. See Gelatin sponge
Suritul Sudiutn. See Tltiamylal sodium Surmontil. See Trimipramine maleate Sustivu. See Efavirene Sutilains. 839 Symmetrel. See Amantadine Sympathetic ganglia. stimulation of. 586—587 Sympathetic nervous system. 548 Syrrtpatholytics. 524 Sympadtomimetic agents. 510. 524. 5311—539. 548 central. 510, 512—514. 5121 direct-acting. 530—532 a.ctdrenergic receptor agonists. 532—535 receptor agonisr%. 536—537 drug products, 532—539 receptor agonist.s. dual a-and 535
steriroselcetivity of. 530. 532 structure—activity relationships for, 530—532. 53(11
indirect-acting. 537—538 mived-uctimu. 538—539 Syn2869. 245 Synapse. 680, 6801. 683 Synaptic cleft. 680. 6801 Synaptic knob. 679. 6791
folnte coen,.yrnes and. 270—271. 2711—2731
Syneicid. See Quinuprislin.dalfopristin Synnematin N. 318—319 Synterrin. See Rolitetracyclinc
bolate reduciase inhibitors and. 275
Synthnlin. 66)1
988
lndc.v
antibacterial agents. 247—252 Sycttlcrtcid St', l.evothyrc.cnicce srcdcncci
.Syntccciniccc. Sc,' (lnytcccin uicieclion; Oxtocin nasal MIIU1IO1
Tecnpr.c. Sic Acecacccinophcen TenapilSc'c' l)cetlcylgtccpicccc Tenectephase. 18.1
'1mev, Sic Gccanlacine hydrochloride Tectiptcside. 426—427 Teccccnccicc. Sc',' Atenolicl 'l'ensilccn, Sec lidroplc'cccicuun clc),cride
T Tacarvl. Sic Methdila'cne Tachyphylanus. 59(1
TecctaGel resins, icc c,cnchin,ctccrial cltctcccslry. 49 Temcate. 5cc l)cellcylprcipiccn
Tacrine lcytlrlccltloriile,,S67 Tadal'aftl. active sites iii, 29. 31)1
Tepacccil Sc',' l)iethylprtcpion
Tall's steric parameter l1:,. 21 Tagacciet. See ('icoetidinc 'I'aggiccg mctlc4s. fec c,ctcihcccuc,crial Iihr,crics. 52—53, 521. 52c
ti' I)i,tst'asc rallyucnyecccs. 417 Talwin Sec Pcntae,ccicce Tantbtccor. Set' Hec;cinidc ucetace Tacccoxcien, 433, 436, 7Kt -782, 7KIi agicctist/acctagccitist actions ccl. 29 ctcetabcclcsnc
ci. KS
l'antsulccsin, 54) Tc,cc,i,c'ttccci ,cue,heiciiccrc I Ic, erlew).
Taccdcanl. 763t 'rapacole. Scc MelIt,cccai,cle Tapewtcrccc inleslaticcccs, 264-265 Targtwict 'I'eicccplacciic
largretin Sic Besacictetce I'argretitt gel. Sec Bexarcccene Taetraiine. nce)abolisccc ccl, 1117
Taxis) Sec ('lecccastinc iucnarace
'lax,'). Sec Pactitasel Tascctere. Sic' Dccceta',el Taylor serie'. espaccsnccc, ')25
i'ancrotene. 874- 875 T.widictce. Set' ('eftaiitlicccc s,cclinccc 'ra-,obactacct, 3)5, 316 I'aiohactanc—pcperacil)cn,3 lb 'l'a-c,cr.cc,Sc'c' l'a,arotccte Sic Teclcccetiuccc
"IC.
T cells. 2(W). 21)2— 2)1:!
helper. 21$) 'lea bags, in comlcinac,crial syctllcesis, 4-4. 45). 63
Tecelecckin. Sec Aldesleukcn Tectcccetitcccc aibtincin aggregated. 4(c3 —4(c4
Iechnetitcccc albumin c,clloid iccjecticccc. 464 'l'echnetcum alhucccin injecticcn. 463 Teclcccetiuctc apticiile. 464 Teclcneciucn hicisace injeccccccc. 464 Technetinm dcprectide injectciccc. 464 l'echnetinnc di'.oIeccicc cn)eeciccn. 46-I —4(iS Techccetincn exacccetaiicce injectiiccc, 4(cS Tecltnetiuccc meclronate injeccivctc. 41c5 'l'echnetcucn ntcrtcatide iucjectcocc, 465
Technetium pentetate injection. 466 I radiciplcarncacenlicals. Teclcnetinccc 462 —467 Technetiuccc red )cl,cod cells lanrcchcgtcccs I. 46(c Technetiunc sestacnihc cnjevtcon. 466 Technetium sodium penechnet;cte. 466—467 'technetium sciceccuer iltjecticcn. 467 'l'echnetium sulfur coliccid injecticccc. 467 'l'echneciucci tecrcclivsncin injecticin. 467 Tegatccr. 4)17
Tegisccn. Sic Etretinate 'l'egccpen. Set' Closacillin s,cdiccucc Tegretccl. Sc,' ('arbama'iepiccc Teiclcccmycin Ste 'l'eicoplactin
l'eicicplaccin. 356
'l'elnuivartan. lilY Telicncerase inhibitors, 448, l4()i Tencaril See Tricnepracicce Ianr.,te 'l'ecna,epam, 49! l'emosate 'Sc',' ('hohecascil prccpconate
'l'er;c,ol Sic' Tere,cnancle Teracosin. 54(1—541. 541c 'l'er.ce,csicc hydrccchlccride. 652 Terbinaticce Icvdrccehhccride. 23') 'lerbucahine, 536
Thalassencia. ttcalaria acid, 283
iltalitone, See ('hlnrtltalid,cne Tlcallinnu r.cdiccplcannaccnticals. 472 '11w llermlcydrocatcctahinccn. 521) .522 ncetaholisctc ol, 66. 77, 771 'J'hehaitte. 732. 735. 745 Theohroincne, 511—512. SI It 'l'heophyh line as centritl ccers'ous sys)ecn stintctlant.
511—512. SIlt as diuretic. 618 as ncnscle relaxant. 624 'flteriiidide, Set' Sccdiuctc i,idide 1131
ntelaholisnc cci, 87, 115. 126 Tcrcona,ccle. 243
Tlcermodytuactcic cycle. 935 Tliiabend,c,acle. 265 Ttciambutetce, 742 Thiamine. 885—887
Terfeccadicce, 712—713, 7131, 94S
Thiamine hydrochloride. 886—887
Tercccicual hccttccns, 1c7'), 679) 'terc—.ccnycicc,Sc'c' Oxytdracvcliccc hydrccclclccrcde l'eslac,Sc'c' l'estcclactcccce Tessah,ccc, Sc-i' llen,ccnaca)e
'l'tciamiuc ntccccucnitrate, 887 'l'hiacccvlal. ncetahccliscc, ci)'. 81 Thc&crccylal siiditcnt. 487. 487t Thca,ide/chiaaide-like dinretics. 6)15—6111. (cWct. 6) Itt, 6071. 6(18!. Sc,' ,iLc'c' Diuretics
Testolactccne, 436. 783, 784), 785 as anccneicplastic. 434 'l'estcislerccne
hcological activity icE. 797. 798! hivcsvuthesis ol, 76')I, 77)). 7741. 775. 797
adverse effects ot, (illS--tOY drug cnceraetiiitcs with, 609
cccnsec'si,ccc olin estradiol, 783, 7831
tndicarccms Icir, 609—610 (c07—IitlK, 6081 pharmacokiccetics preparatiocts cci, 619—624)
,ccet,cbohcsnt 'if, 71)7
site actd meclcanisccc of artion id, 608
preparations iii'. 7991. 8)81—8)11
stcluhility oF. 770c structure—activity relaticcnships (icr. 7')8— 799, 798c
'I'estostercinc cycliipentylpriipicinate. stnccture ccl. 7711
'I'esl,cs)erone cs'pitcnate. 7's), K)) I
structure iii, 7711 Testccstercccce enthanate. 799, 8)))
'lest,cster,cne propicinate as amucceophastic. 434
scilubihity ii!, hot Testred Sic' I 7a'Metlcs'Itescccstercccce 'lest sets, selection of. 22—23, 23c, 26—27 i'ctanus toxoid, 21 2t, 214, 215 Tetrac.,ine. 69(1—693. 69!! 2.3.7,8.'l'etncL')clicrcidiben/op-cliox,cc c1'C('l It.
cnetaholism at, it, loll 'l'ctr.ccyclinels).34 I - 349 ehelates i'i.342—343 chectuistrs ti). 341 epiccceriiaticcn of, 342
scniccuce—activity retatitcn.s)cips Icir. 64)5—6))?, 6(811. bUtt
l'hia-,ohittdiones, 671—672 Thcenatccyeimsb3l5. 316—317 Tlcimercisal. 228 Thuciguanine. 40)5. 412 Thicipental
hipophilichy at. 7 icuetahuclism cci, 99
Tlti'cpental sodiucn. $87, 487t 'I'hiciper.,mide. 728—729. 7291 Tlcioridazine, 499—5)14. 499t, 5)12 active ctcetatxclites of, I 35, I 35t tctetah,,lisict of, 99 l'hiotepa. 395. 4112
l'hiotlcivene. 5)81 Thiccuracil. 673—674
'llconeylacnitce hydrochloride. 704. 706 Tlcicra,cine 5cc Chlorpt-vcntaaine 31).QSAR. 23, 38—39 31) structural proteiti database. 939. 9391 Thrcctcthiuc, 664. ((iS, 857
inacccvatccin ccl. 342 cnechanisccc ccl action ccl, 343
Tlcroncbccnuridulcn. 6(iS
micrcchial revistactce to, 343 pK,, values (icr, 342. 342t polar. 345
'l'hrccccchosane A2, 666—667, 820), 8211, 822i
prcxlncg fccrnc tct', 149, 15)11
Tlcrottthcipla.stin. 663, 664, (c(i4t, 664t
l'hymineless dean,, 41)1 Tltycccol. 222 Thyrcccalcic'cnin. 855—856
pricperties id'. 344—,U5. 345t spectra!! ccl activity ccl, 343 —344 sterecx'henctslry cit .3.)!. 342t viniccure- activity relationstcips 6cr, 344—345,
'licyrogect. Si',' 'l'hvrtitropicc alpha Tlcyroglohulin. 857 Thyroid Itorcicones. 673 Thyroid-sticnnlatcng hormone 1TSI-li, 845 ilcyroliherin. 84(1—841
structure cit. 34 1—343. 342t 'l'etr.ccycline
'l'hyricpar Sic l'ttvrvcid-sticcciclatiug hccnn,cne
(TSH(
-l etraethylancnc,cctcnttt bnc,ccide. 588 'l'etr.irthvlatnntoccinccc eIcI,cride. 588 letracttcylauttcctonicccct salts. 588 Tetraettcyltetrapltosphate i'F.F.Pt. 569—57)) Tctr.clcydroc,'atcn:,hcnol (Tl-1('l. 520. 522 mecatxchiscci oh. 66, 77, 771 'l'etr,ulcvdrcil,chic acid. 4183—411), 411(1
l'hyrcctropict. 845 'fltyruccrccpict alpha. 176
Tetcahydoceoline..S33
l'(i. Set' 'l'hiiigtcactinc
'l'iagahitcv. 5117 T1l'colone. 796—797, 7971 Tccar, Se,' 'l'ictcrcclhicc ,lis,cdinm
'II IA Sc',' Tacrine hsdrvcchlciride
Ticarcillcn—clavn)anate,3 Ifi
'l'etriidocccs,ct, 694)
'l'hyrotropin.relea.sing ltormone (TRH I.
840-84! -Thyricvine. 673 accalogtces. developttcent ccl, 94(4—941
Index Tkanillin disodium, 309i, 314. Se,' Oh,, Penicillin(sl TiclicLS,'e Ticlopidine l'iclopudine. (,33—634 Tigemonam. 334 Tikosyn. Sic Doretilide
FiI,tdc. See Nedocismtil sodiunt Timcntin. See CIavulat,ate-tk.arcillin Timolol. 543. 5441 Tinioptic. Ste Timolol Tinactin. Ste Tolnaftule Tinctures, 219 Tines, 231). 23). 2311. 233—235
l'ioconaiole, 242 Tipranavir. 942 Tirapaiansinc. activation of, 153. 1541 Tirofuban, 634 Tissue depots. 7 Tissue plasminogcn activator (tPA). 1114 recombinant. 1114. III.)). 840. $59t Tissue lhromboplastin. 663, (,64. 6641 Tts.'.uc tropism. 371
Tiranidine, 534 Tobramycin sulFate. 340 TOC-039. 333 Tocamide hydtm.hloridc. 640
half-life of.
7
Tocopherols. 879—882. See aixi, Vitant,n relative polencies of. 880. 8811t Toc,,trienols. 879. Set' aLso Vitamin E ToIranil. See Imipr.,mine Tokosimide. 669
Tolazoline, 539 Tolbutamide. 269. 668. 669 metabolism iii. 77. 132 age and. 126 Tolbutantide sodmiTi. 61s8 Tulectin. See Tulmclin Tnlinase.S,s Tola.zun,ide Tolmetln, 7511
metabolism of, 77 Tiili,aftale, 239 Tunocard. Si', Toca,nide hydrochloride Topamax. See Tupir.,mutc Topical anesthesia. 687 See ufco Local anecihetics Topiramute, 507
Topological ,kscriplors, in drug design. 23—24. 24,
Toradol. 5cc Ketorolac tromeihamine Toremilene, 434. 436, 781. 7811. 782—783 Tornulate. Sc,' Bitolterol Torscmide. 620 207. 215 Toxoplasmosis, 260 IPA (tissue plasininogen actisaturl. 1114 recombinant, 184. 1841 Tntcclccc linkers, in cot,,binatorial synthesis. 49
Trjcriu,n. See Atracurium hesylate Tr,,c Tab. See Phenyl salicylate Training set. 25 Tratnadol hydrochloride, 747 Tramcinolonc acctoi;idc, structure of, 7711 Trundolapril. 648 Transcription, 162. 1631. 192. 193 Tranvdcrmal contrucephses. 793t. 794 Trunsducins, 871—872 Trnnsgenics, 94 leans isomers, 3) —32
of acetylclioline. 34—35. 34) Tr.,nxene. Sc,' Clorazepate dipolussium Trunylcypromine sulfate. 515—5)6. 5151 Trastuzutitab, 190 as anhineuplastie, 443
Trjvuse. Ste Sutilains Travatan. See Travoprost
Trusopt. See Dor,olamidc
Travoprost. 82)1 Trazodone. 519—52(1 Trecutor SC Ethionamide Trelstur. See Triploralen pomoate
Tiypanosomia.sis, 260
'l'rcmatode infestations, 265 Tremin. See Trihexyphenidyl hydrochloride Treprostinil. 823 Tretinoin. 873 antineoplastic activity of. 430 Triacetin, 233 Triameinolone. relative activity of. $091 Triameinolone .icelonide, 8091, 814—815, 8)41 Trlumcinolone diacetate. $13 Tnamcinolone hexacetonide. 8)3 Tnamlcrcnc. 617. 620 Tnamtcrene.hydmchlorotl,iaride. 620 Tnazolam. 492 Trichilormethiazide. 605—610. (106t, 608t Trichloroethanol, metabolism of, 112 Trichomonia.si'., 260 Tnclofo,. sodium. 496 Triclos. See Triclofos sodium Tricor. See Fenoftbrale Tn-Cyclen. See Norgeslinsate Tricyclic antidepressants, 5l6—519 tttetabolism of, $7 Tridesiol. See Descinide Tridihcttethyl chloride. 584 Tndtone. See Trimethadione Tn-Esl. See Estriol 'fnetbyletieittelamine. 395 Trilluorothytitidine, 407, 4081 Trifiuproma,.ine hydnichloride. 499, 499t Tnhluridine, 376, 407, 4(181 Triglycerides. 657 Trihexyphenidyl hydrochloride. 584 Triiodothyronine. 673 Trimegcslonc, 7871. 7119 'rriinependine. 736*
Trimcprarmne tartrute. 7l1 Trimetbadione. 505 Trimehhaphan, 588
Trimelbapltan cumphorsulfonate. 58$ Trimelhaphan camsylate. 588. 589 Trilnethoprim, 276t. 279 mechanism or action of. 271 metabolism of, 93, 98, 133 structure of. 9421 Trimethoprim analogues, development at, 94l—942
Trimcthoprim.sulfametltoxatole. 272 indications for. 269. 270t Trimeton. See Pheniramine maleate
989
Trypanocidal agents. 668 Trypsin crystallized. 838, 839t Tryplophan Hopkins-Cole test for, 834 metabolism of. estrogens and. 893 TSPA. See Thiolepu Tuberculosis, 254. 338 drug therapy for. 254—259. See also Anhitubcrcular agents Tuberculosis vaccine, 2)2*, 2)4 Tubocurarine chloride. 590-591 Tubuloglomerular lecdbucL 599 Tumor cells, properties of. 390—394 Tumor'infiltrating lymphocytes. 442 Tumor necrosis factor. 44(3 recombinant, 183, 447 Tusscapine. See Noscapine
2-PAM. prodrug form of. 157—lStl, 1581 TXA2. Set' Thromboxane A2 Tylcnol. See Acetaminophen
Tyrocidin. 299 Tyrocidine. 360 Tyropanoace sodium. 484
Tyrosine. Miller's test for. 834 'l'yrosine hydrunyla.se. 524—525. 5251
Tyruthricin. 359-360 Tyzine. See Tetruhydrozoline
U
U-46619. 8251
UCN-0l. 439 UDPGA. in glucurunidation. 112 UDP'glucumnyltransferases. 112, 1)21 Ugi reaction, four-component, 49, 5(11 Ulcers, peptic. 718—719 Ultiva. See Retnifentanil hydrochloride Ultrtilente insulin, 851t. 852. $521 Ultram. See Tramadol hydrochloride Ultrasound. contrast agents for. 477 Llnasyn. See Ampicillin-sulbactam Undecylenic acid, 233 Unipen. See Nalcillin sodium Uniprost. 825 Unoprostone. 828 Ureas. 506 Urecholine. See Betitanechol chloride Urinary analgesics. 253—254 Urine. sulfanilamide solubilily in. 274 Urised. See Methylene Phenyl salicylate Uritonc. See Melhenamine Urokinase. 840 Urotropin. Sec Methenamine
Trimetrexute. 410—411
Tritnipraminc maleate. 517 Tripelcnnum,nc. 704. 705 metabolism of. 87, 114. 705 Tripclennamine citrate. 705 'rripelcnnutninc Itydrochloride. 705 Triple sulfa. 278. See aix,, Sulfonamides Triprolidine hydrochloride, 709 Triptoralen pamoate. 437 as antineoplastic. 435 Trisenox. See Arsenic trioxide l'risullupynmidines. See aLso Sulfonamides oral suspension. 277 tablets, 277 Trivalent oral polio vaceine. 2)0. 2121 'l'roleandomycin. 353 Tropeines. 676 Tropicamide. 585 Tropinc. 575. 676. 6771 Tnie salol. 755
V i (Chttrton's steric parameter), 21
Vaccittutton, definition of, 207 Vaccine(s), 207 acellular. 207—20%
administration schedule for, 2121. 215—216
bacillus Caltnette-Gudrin. 2)4 u.s antineoplastic. 440. .142
bacterial. 2l2t, 213—215 booster, 2011
chickenpox. 211. 212t cholera. 2121. 214-215 coadministered. 208
definitiott of. 207 diphtheria, 212t. 2)4—215 dosage of. 208 DPT. 212t. 215 lluemophllw. inflttenzae, 2121. 214
990
/ot/it Vectmtrs. cltttimtg, 11m5— (66. 1661. 468
dascoven' uI, 866-867
hepatitis A. 211—213. 21 It hepatitis II. 2 2t. 2(3. 85th. 116th hepatihis 1'. 213 hepatitis E. 213
Vectrttt Set' Mimtoeycline liydrochlonde Vecuntttitmui broittide, 593
luuctttmus ot'. 866
human mmtiuunttdeltcter.cv situ'.. 182—3(11
Venial astue. 519 Vettograptty. 479
Vaccinet s (nutiinut'd#
tttlltieiiiti, 2(8) killed btnaemhaledh. 207 live/attenuated. 2(17 malaria. 283—285 measles. 2111-211. 21 2t meutugococeal pol) saceharide. 215 multiple-dose. 2(18 mulimvalemtt. 208
S'eloset. Sec Cephadritte
Ven Apis Set' (lee veuout'
Vemttstns 835
Venmttlitt, Set' Albuterol
VePestil Sic Etopctsitle Vcrapanttl us atitiarrttytbinic. 1.42 is sasttdilator. 629. 629t Verltittp's titultidititetisittual sterme parattteters.
mumps. 211, 2l2i pcm'tussis. 212.. 213—214
pharmaceutical principle' .8. 248) pneutntmcoccal. 215 polio. 2(41. 21 2t
polysalent. 208 production itt. 2447—2148 recoutbmuant. 186- 187. 187.. 24(8. 21)81 rotavmnis. 213 rubella. 2111. 21 2t
Verluma Kim. See Noktitmomoah tnerpetuan Vermtios, See Mebeudaiole Veiiimteautine, 728-729. 7291 Verstr,ttt. Sec Prucepatit Very- ((((v-density lipoptoteitts. 658—659. 85(1
Vesprin. Sr.' 'lrillttprotmtaiiue ltydrttclmliiride Viagra See Sildeitalil Vihe,tnvccitt. See l)ovyeycline
labeling requtrettteuts for. 866. 866t lipid-soluble. 866- 885 overview ut, 866—867 recommended daily intake iii. 866. ShOt supplemnetttal. indications titr. 86(i scaler-soluble. 885—918) Vitatmmiu A. 867—875
hitueheniical Ittttctiitns ,t(, 87(1
biological activity at. 867 biosynthesis uul. 869-8711 etilite reaction', 55 itlt, 869
crystalline. 869 deficmettcy uI, 8711 delimmititiu mit, 868
dietary sources cit. 868, 868t, 869—870 eneecs of. 870—871 in ft,slt liver oils, 8(m8t, 869
isttttters 01, 868, 868t. 872 tttechuttisns al actittmt itt, 872 mciubuilisttm o1, 86')
uutttberiug system [or. 868
Victtttmtycin, 417 Vidar,tbitte -'cci' Adetitisitte arabmnosmde
pruxlttcms. 872—875
sitmule-tltise, 208 stmtallptis. 2(19 storage tin1 hattdlitmg ot. 2(49 tetattuc. 212t. 214—215 tubercuhicts. 21 2t. 214 vir,tl, 21(9—213. 367—3744 Vtttti,titmt spp. bcmttnberrs' '112
S'ides See Didantisitte Viublastitte sulfate, 425. 41St. 427
retinoie acid receptors and. 872
Vinci alkaliitds, 324 425. 425t. 9(5
structure—activity relationships for. 869
Vimicristtmte sullate, 325, 41St. 427 Vtmtesine. 425, 425t
tosictty of, 8711—871
Vaginal ritig contraceptive des ice. 7'11t. 794 Vagistut See Ttocoua,,tle
Viutirelbine tartrate. 427 Vinrosiditte, 425. 425t Viuyl chloride. tttetabttlisnt itt', 77 Vuititrtmt. See Ultitqttinol
simple. 20$
Vinglycittate. 425. 425t Vimmleiut,sitte, 425, 425t
Valacyclovir hydriicltlt.rtdr. 377 Valdecosib, 76(4. 822—823 Vajeriau, 914—915 Valium
l)itmeepani
Valnthiciu. 416, 423 Valsatlan, 64') Val',tar. See Valnibictu Valtres Set' Valacyclos ir hydtttcltli'ride Vancettase. See Ileelitittetltasotie diprttpiotmate Vauceril Sit' Ilecloutetlvootte tltpriipiouatc Vancitctn Sic Vancontycin Itydnicltloride Vaucoled.St'.' Vauc,tmi.vcitt Vamtcittttyciu. 34 Itt
Vattcitntycitt hydrttctdortde, 355 356 Van der Waals' Iorces.3 I. 34 Van der Waals' sttriace. in tt.otrcttla. tmulelotg. 922. 9221 Vauostde. Sit' Hydrotis beutityl peroxide Vansil. See Ovammtmtiqtttne
Vautiu. Set' ('elpodositne proxetil Vardenal'il u,'tive sites ut . 29, (III Variamycins. 4(7 Varicella vaccine, 241. 21 2t
Vitatttmn
8145
1487
tlelicieticy itt. 891
Viracepi .5.'.' Neliluavir
diemttry ctturces of. K')2
Viral imtl'eetions, Se.' ohiti Viruses htttxi in. 367 cattcer ,tttd. 372 clteutopropltylasis (tie. 372 tosis in. .171 host-virus interactions in, 371 tititoutti/ation (or, 2(8/- 213, 367—3711.5cc
discovery at, 894
alt" Vtteciuebst prtuhtenve. 367 stages all 3744—372
Vieamtttttiiitr. Set' Nesirapitte
Vtrioti, 172
'l'rilluridine Virtual screenIng, 54—55. 56, 63. 919
Viruses See ti/ia Viral itikctitms building of. 372
lmttnmttttmes antI. 893 $t(4
products. 894 pntpenies ttf, 891 —8't4
Vitatnin Be.. 89l896 deficiency at. 895 tctltc acid metulxilisttt ..ttd. 896 897 potdttcts. SQS—891m
properties uti. 894—895 titsicity of, 8115 Vtma,otu Rix.. 894 Vitamin C. 898—1499 Vitatuin 1). 875—879 absturptittu iii. 877
hiolttgteal activity utt . 876 hiosyutttesis iii. 875—876
cleissiflcammt,u t,l, 167, 3681—37(11
deficiency tit, $76 ttietary stutrees it, 877
l)NA. 368t. 37th. 372
eveess all 876—877
oTicogenic. 372 overs-iesv iii, 167 mephicahlttu .1,367, 371. 372 KNA, 139t. 36Km. 371—372
toeiabttlism id. 875
cltar,tcteristtes ol, 367
Visken Set' I'intltilol Vtsttde .cc.' (3dolttvit V stray Set' Ovypheueycliittine hydrochloride Visual purple. 871, 872
Vasttpressimm tzutuate. 8-tot. $47
Vist.al 870 Vitaniitttst, 866—94(2 tlaily values ut. 866. 8661
V-Cillitt Si'.' Penicillin V
Viiamtmimm A., $75
Vilatitimt B,,. 1191 894
ntechanismtt itI actiott ot, 4,22 623. tt2it ntetabvtlisnt itt', 623 623, tt25t Vasopressiti, 845 8.16 Vasopressin iuieetitm. 8-lbt, $47
Vasox> I Si'.' P,letltttsamutne
(,'SP. 872-873 itt visittu. 870. 871—872 Vitttmiu A1. 867, 168
Viiivs Ste Ritlecosib
Vtsn.e Sit' 'fetretltydr.vtolttte Vistou. s ttamttmtm A tnd. 870, 872.873
eycltidesiritt
uttits nil, 86%, 868t
S'iotm.yctmt. 259
Vaccutr St-c Ilepridil ltvdtimchmlortde Vasoaettve intestinal ps'ptttle. 1455 Va'.ocouctrtctitts, mt local anesthesia. 688 Vasrulilators, ('22- 1.31 antianginat. 622—626. See u/tie Nit retva.sititilators antittypertetisive. 1.53 1.54 ant,thrittmtbotic. 1t32—634
Vasuprost See l'rostaglaudiu Vasotee. Ste Ettalupril
stereoeheimtislry of. 868
Vumamin B:. 89(1—1191 Vitamin 887- 1488
Set' Adenttsiue arahinoside
Valprote acid, 51(6 mttetaholisttt tit. St
pros itautitis, 869— 87tt
tissue mt'opismtt at, 371 tutcoatimtg tI. 371
deliciencies al, 866 dieiary reference intuhes t,l, 866, 867t
products. 877—879 potperlics tIll 1175—877
tttsiciiy utt. 876 1177 Vitamtiin I) receptors. 877 therttpcumic uses uI', 1477 Vitamitu F, 879—1112
absi'rpiitiu al, 88(1 881 autiosidaut properties uI, 8811 -XIII deficiency oh. 881 dietary s.tttrees at. 879—812 discosery till 879 tu:tclivatiou uI'. 881) isttltunos ol, 879 —8811 ttteg;tdoses tiE, 881 u,etahttlismtt at'. 881
propetties cut. 88tt
Index African sleeping sicknc'.s. 260 Wes(heimer method, 923 Whitfueld's Ointment, 234 Wilpowr. See Phentermine hydrochloride
relative poicnctes ol. 88(1. 8818 of. 881—882 thcrapcuttc Vitamin G. 890—891 Vilantiti K. 882—1(85
ihsiirpliiin iii. 883 anticoagulant activity of, 665. fi661 dictar) sources ut. 882. t182t discovery ol. 882 Iuiictutms ni, 883 iiictaholism of. 883
VP. I (i..'i'i'i'
Wycillin. St't' Penicillin (3 procaine Wydase. Set' Hyalumnidase (or tnjection
Zan,nlin.St't' Ethi,susimide Zarsixotyn. Sit'
Wytensin, Set' Guanahcne acetate
Zebeta. Si-u' Bisoprolol '/.cfaione. See Crtmeia,ole sodiuuti, Zcmplar. Set' Paricalcitol
x
Zcntel. Ste Alhetidazote Zcphiran.Si't' lIeuw.all.tiniiutn citloriule Zerit. Set' Stavudine Zero modulators. 489
Xaltutatt. Ste Latittiopront Xanax. Ste Alpruaolum Xututhincs, 511—512.51 It Xanthoproteic test, 834 Xcloda. See Capecitahinc Xenobiotics. definition of, 65
Zesiil.Se.' Lisinopril Zcvulin kit. Sit' Ibritumomab IluSetan Zidiuvudine, 379—384)
Xenon r.udiopharmaceutucals. 472
Zikuton, 820
Xigris. Sue Rotrceogin alfa
Zitiucet. See Cefuroxime sodium Zinc caprylale, 233 Zinc propionate, 233 Zinecard. See Dexrtuiuxanc S i'.xinters, 32. 321 Zithrtunuax. Si'.' A,.itbroinycin 7,ocor. Set' Sjnis'asluiin Zolades. Sit' Goserelin Zuloll. See Sertr,iline Zolpidem, 492 Zonegran.Sei' Zsunisainide Zonisamide, 507
Xipamide. 607—6111, 6071, (i09t, 621)
Vumon Sri' Teniposide
Xupenex. Set' Levalhuterol X-ray crystallography, 37—38 X'ray films. 454. 472—473
w
Xylocaine. St'i' Liulocaine hydrochloride
Warfarin active mclatxilitcv iii. 1351 metabolism ul, 10. 105. 32 sites ut action of. 1,61,1 Warlarin potassium, 1,1,7 —Wit) Warfann sodium. 61u7 Waler, a'. atliphaleric sutistance. II soliuhility. pK, and. I 6 Waler-soluble vitamins, 81(5—'XK).S Vitamin(s) Water solvent models. 934 Watson-Crick DNA r,ioulel, 921t Weicliol Set' Colesevelant Welibutrin. See Buipropion Wellferuin.S,'u' Inlerteron I
Xylometai.uulune. 537
454
V YA-56, 417 Yeast infections. Set' Eungul infections See Induuquinol
Jut
Zalcitahine, 380—381
Winstruit. Set' Stanorolot Wyumycin. See Erythromycin steurute
products. 884—1(85
iclaliotiships 1(82—883 therapeutic usc'. iii, 883 Vitantiti K1, 884-885 Vitamiti 882 Vitamin K. 81(2, 1(85 Vitamin Ka. 882 Vitamin K inhibitor'.. 1,1,7 Vi'. actil. See Prutriplyliue li)drui.liloride VM-26 Set' Voltarcn.S,'i' l)iclotcn.ic 'odium Vorici,naiok. 244— 245
Zaditor. See Kecotifen tumaraw ophthalmic solution Zaleplon. 41)2.493 Zanafles. See Ti,aniditte Zanuisar.Si'e Strepto,.ocin Zantac. See Ranitidine
Wine spini. St't' Alcohol
Yoltimbinc, 541 See Niclosamide Yutuupar. See Ritiidnne
zorhaniycin, 4(7 7.orbnnamycins, '1(7
Zosyn. Sit' Taeohactam.piperacillin Zovirax. See Acycliivir Zytlo. See Zuteuton Zymiigcn granules, 838 Zyittogen'., 1(37
z 7,actanc Citrate. See Ethoheptarinc Zactirin. Set' Ethohepluzine
991
Zyprexa.S.'e Olaticapine Zyrtec. Si'.' ('ettnzinc Zyvuis. Set' l,inezisl(d
