text:
The Organic Chemistry of Drug Design and Drug Action
Standard medicinal chemistry courses and texts are organized by classes of drugs with an emphasis on descriptions of their biological and pharmacological effects. This book represents a new approach based on physical organic chemical principles and reaction mechanisms that allow the reader to extrapolate to many related classes of drug molecules. The Second Edition reflects the significant changes in the drug industry over the past decade, and includes chapter problems and other elements that make the book more useful for course instruction. New edition includes new chapter problems and exercises to help students learn, plus extensive references and illustrations Clearly presents an organic chemist's perspective of how drugs are designed and function, incorporating the extensive changes in the drug industry over the past ten years Well-respected author has published over 200 articles, earned 21 patents, and invented a drug that is under consideration for commercialization
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The Organic Chemistry of Drug Design and Drug Action - Richard B. Silverman
The Organic Chemistry of Drug Design and Drug Action
Second Edition
Richard B. Silverman
Northwestern University, Department of Chemistry, Evanston, Illinois
Table of Contents
Cover image
Title page
Preface to the First Edition
Preface to the Second Edition
Chapter 1: Introduction
Publisher Summary
1.1 Medicinal Chemistry Folklore
1.2 Discovery of New Drugs
1.3 General References
Chapter 2: Drug Discovery, Design, and Development
Publisher Summary
2.1 Drug Discovery
2.2 Lead Modification: Drug Design and Development
2.2.1 Epilogue
2.3 General References
Chapter 3: Receptors
Publisher Summary
3.1 Introduction
3.2 Drug-Receptor Interactions
3.3 General References
Chapter 4: Enzymes
Publisher Summary
4.1 Enzymes as Catalysts
4.2 Mechanisms of Enzyme Catalysis
4.3 Coenzyme Catalysis
4.5 General References
Chapter 5: Enzyme Inhibition and Inactivation
Publisher Summary
5.1 Why Inhibit an Enzyme?
5.2 Drug Resistance
5.3 Drug Synergism (Drug Combination)
5.4 Reversible Enzyme Inhibitors
5.5 Irreversible Enzyme Inhibitors
5.6 General References
Chapter 6: DNA-Interactive Agents
Publisher Summary
6.1 Introduction
6.2 DNA Structure and Properties
6.3 Classes of Drugs That Interact with DNA
6.4 Epilogue to Receptor-Interactive Agents
6.5 General References
Chapter 7: Drug Metabolism
Publisher Summary
7.1 Introduction
7.2 Synthesis of Radioactive Compounds
7.3 Analytical Methods in Drug Metabolism
7.4 Pathways for Drug Deactivation and Elimination
Chapter 8: Prodrugs and Drug Delivery Systems
Publisher Summary
8.1 Enzyme Activation of Drugs
8.2 Mechanisms of Drug Activation
8.3 General References
Appendix: Answers to Chapter Problems
To Mom and the memory of Dad, for their love, their humor, their ethics, their inspiration, but also for their genes
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Preface to the First Edition
From 1985 to 1989 I taught a one-semester course in medicinal chemistry to senior undergraduates and first-year graduate students majoring in chemistry or biochemistry. Unlike standard medicinal chemistry courses that are generally organized by classes of drugs, giving descriptions of their biological and pharmacological effects, I thought there was a need to teach a course based on the organic chemical aspects of medicinal chemistry. It was apparent then, and still is the case now, that there is no text that concentrates exclusively on the organic chemistry of drug design, drug development, and drug action. This book has evolved to fill that important gap. Consequently, if the reader is interested in learning about a specific class of drugs, its biochemistry, pharmacology, and physiology, he or she is advised to look elsewhere for that information. Organic chemical principles and reactions vital to drug design and drug action are the emphasis of this text with the use of clinically important drugs as examples. Usually only one or just a few representative examples of drugs that exemplify the particular principle are given; no attempt has been made to be comprehensive in any area. When more than one example is given, it generally is to demonstrate different chemistry. It is assumed that the reader has taken a one-year course in organic chemistry that included amino acids, proteins, and carbohydrates and is familiar with organic structures and basic organic reaction mechanisms. Only the chemistry and biochemistry background information pertinent to the understanding of the material in this text is discussed. Related, but irrelevant, background topics are briefly discussed or are referenced in the general readings section at the end of each chapter. Depending on the degree of in-depthness that is desired, this text could be used for a onesemester or a full-year course. The references cited can be ignored in a shorter course or can be assigned for more detailed discussion in an intense or full-year course. Also, not all sections need to be covered, particularly when multiple examples of a particular principle are described. The instructor can select those examples that may be of most interest to the class. It was the intent in writing this book that the reader, whether a student or a scientist interested in entering the field of medicinal chemistry, would learn to take a rational physical organic chemical approach to drug design and drug development and to appreciate the chemistry of drug action. This knowledge is of utmost importance for the understanding of how drugs function at the molecular level. The principles are the same regardless of the particular receptor or enzyme involved. Once the fundamentals of drug design and drug action are understood, these concepts can be applied to the understanding of the many classes of drugs that are described in classical medicinal chemistry texts. This basic understanding can be the foundation for the future elucidation of drug action or the rational discovery of new drugs that utilize organic chemical phenomena.
Richard B. Silverman, Evanston, Illinois
April 1991
Preface to the Second Edition
In the 12 years since the first edition was written, certain new approaches in medicinal chemistry have appeared or have become commonly utilized. The basic philosophy of this textbook has not changed, that is, to emphasize general principles of drug design and drug action from an organic chemical perspective rather than from the perspective of specific classes of drugs. Several new sections were added (in addition to numerous new approaches, methodologies, and updates of examples and references), especially in the areas of lead discovery and modification (Chapter 2). New screening approaches, including high-throughput screening, are discussed as are the concepts of privileged structures and drug-likeness. Combinatorial chemistry, which was in its infancy during the writing of the first edition, evolved, became a separate branch of medicinal chemistry, then started to wane in importance during the 21st century. Combinatorial chemistry groups, prevalent in almost all pharmaceutical industry at the end of the 20th century, began to be dissolved, and a gradual return to traditional medicinal chemistry has been seen. Nonetheless, combinatorial chemistry journals have sprung up to serve as the conduit for dissemination of new approaches in this area, and this along with parallel synthesis are important approaches that have been added to this edition. New sections on SAR by NMR and SAR by MS have also been added. Peptidomimetic approaches are discussed in detail. The principles of structure modification to increase oral bioavailability and effects on pharmacokinetics are presented, including log P software and
rule of five and related ideas in drug discovery. The fundamentals of molecular modeling and 3D-QSAR also are expanded. The concepts of inverse agonism, inverse antagonism, racemic switches, and the two-state model of receptor activation are introduced in Chapter 3. In Chapter 5 efflux pumps, COX-2 inhibitors, and dual-acting drugs are discussed; a case history of the discovery of the AIDS drug ritonavir is used to exemplify the concepts of drug discovery of reversible enzyme inhibitors. Discussions of DNA structure and function, topoisomerases, and additional examples of DNA-interactive agents, including metabolically activated agents, are new or revised sections in Chapter 6. The newer emphasis on the use of HPLC/MS/MS in drug metabolism is discussed in Chapter 7 along with the concepts of fatty acid and cholesterol conjugation and antedrugs. In Chapter 8 a section on enzyme-prodrug therapies (ADEPT, GDEPT, VDEPT) has been added as well as a case history of the discovery of omeprazole. Other changes include the use of both generic names and trade names, with generic names given with their chemical structure, and the inclusion of problem sets and solutions for each chapter.
The first edition of this text was written primarily for upperclass undergraduate and firstyear graduate students interested in the general field of drug design and drug action. During the last decade it has become quite evident that there is a large population, particularly of synthetic organic chemists, who enter the pharmaceutical industry with little or no knowledge of medicinal chemistry and who want to learn the application of their skills to the process of drug discovery. The first edition of this text provided an introduction to the field for both students and practitioners, but the latter group has more specific interests in how to accelerate the drug discovery process. For the student readers, the basic principles described in the second edition are sufficient for the purpose of teaching the general process of how drugs are discovered and how they function. Among the basic principles, however, I have now interspersed many more specifics that go beyond the basics and may be more directly related to procedures and applications useful to those in the pharmaceutical industry. For example, in Chapter 2 it is stated that "Ajay and coworkers proposed that drug-likeness is a possible inherent property of some molecules,¹ and this property could determine which molecules should be selected for screening.
The basic principle is that some molecules seem to have scaffolds found in many drugs and should be initially selected for testing. But following that inital statement is added more specifics: They used a set of one- and twodimensional parameters in their computation and were able to predict correctly over 90% of the compounds in the Comprehensive Medicinal Chemistry (CMC) database.² Another computational approach to differentiate drug-like and nondrug-like molecules using a scoring scheme was developed,³ which was able to classify correctly 83% of the compounds in the Available Chemicals Directory (ACD)⁴ and 77% of the compounds in the World Drug Index (WDI).⁵ A variety of other approaches have been taken to identify drug-like molecules."⁶ I believe that the student readership does not need to clutter its collective brain with these latter specifics, but should understand the basic principles and approaches; however, for those who aspire to become part of the pharmaceutical research field, they might want to be aware of these specifics and possibly look up the references that are cited (the instructor for a course who believes certain specifics are important may assign the references as readings).
For concepts peripheral to drug design and drug action, I will give only a reference to a review of that topic in case the reader wants to learn more about it. If the instructor believes that a particular concept that is not discussed in detail should have more exposure to the class, further reading can be assigned.
To minimize errors in reference numbers, several references are cited more than once under different endnote numbers. Also, although multiple ideas may come from a single reference, the reference is only cited once; if you want to know the origin of discussions in the text, look in the closest reference, either the one preceding the discussion or just following it. Because my expertise extends only in the areas related to enzymes and the design of enzyme inhibitors, I want to thank numerous experts who read parts or whole chapters and gave me feedback for modification. These include (in alphabetical order) Shuet-Hing Lee Chiu, Young-Tae Chang, William A. Denny, Perry A. Frey, Richard Friary, Kent S. Gates, Laurence H. Hurley, Haitao Ji, Theodore R. Johnson, Yvonne C. Martin, Ashim K. Mitra, Shahriar Mobashery, Sidney D. Nelson, Daniel H. Rich, Philippa Solomon, Richard Wolfenden, and Jian Yu. Your input is greatly appreciated. I also greatly appreciate the assistance of my two stellar program assistants, Andrea Massari and Clark Carruth, over the course of writing this book, as well as the editorial staff (headed by Jeremy Hayhurst) of Elsevier/Academic Press.
Richard B. Silverman, still in Evanston, Illinois
May 2003
¹Ajay; Walters, W. P.; Murcko, M. A. J. Med. Chem. 1998, 41, 3314.
²This is an electronic database of Volume 6 of Comprehensive Medicinal Chemistry (Pergamon Press) available from MDL Information systems, Inc., San Leandro, CA 94577.
³Sadowski, J.; Kubinyi, H. J. Med. Chem. 1998, 41, 3325.
⁴The ACD is available from MDL Information systems, Inc., San Leandro, CA, and contains specialty and bulk commercially available chemicals.
⁵The WDI is from Derwent Information.
⁶(a) Walters, W. P.; Stahl, M.T; Murcko, M. A. Drug Discovery Today 1998, 3, 160. (b) Walters, W. P.; Ajay; Murcko, M. A. Curr. Opin. Chem. Biol. 1999, 3, 384. (c) Teague, S. J.; Davis, A. M.; Leeson, P. D.; Oprea, T. Angew. Chem. Int. Ed. Engl. 1999, 38, 3743. (d) Oprea, T. I. J. Comput.-Aided Mol. Des. 2000,14, 251. (e) Gillet, V. J.; Willett, P. L.; Bradshaw, J. J. Chem. Inf. Comput. Sei. 1998,38,165. (f) Wagener, M.; vanGeerestein, V. J. J. Chem. Inf. Comput. Sci. 2000,40, 280. (g) Ghose, A. K.; Viswanadhan, V.N.; Wendoloski, J. J. J. Comb. Chem. 1999,1, 55. (h) Xu, J.; Stevenson, J. J. Chem. Inf. Comput. Sci. 2000, 40, 1177. (i) Muegge, I.; Heald, S. L.; Brittelli, D. J. Med. Chem. 2001, 44, 1841. (j) Anzali, S.; Barnickel, G.; Cezanne, B.; Krug, M.; Filimonov, D.; Poroikiv, V. J. Med. Chem. 2001,44, 2432. (k) Brstle, M.; Beck, B.; Schindler, T.; King, W; Mitchell, T.; Clark, T. J. Med. Chem. 2002, 45, 3345.
Chapter 1
Publisher Summary
Medicinal chemistry is the science that deals with the discovery and design of new therapeutic chemicals and their development into useful medicines. Medicinal chemistry involves isolation of compounds from the nature or the synthesis of new molecules; investigations of the relationships among the structure of natural and/or synthetic compounds and their biological activities; elucidations of their interactions with receptors of various kinds, including enzymes and DNA; the determination of their absorption, transport, and distribution properties; and studies of the metabolic transformations of these chemicals into other chemicals and their excretion. For ages, nature has been an excellent source of new drugs or precursors to drugs. Human beings have searched for cures for illnesses by chewing herbs, berries, roots, and barks. When a natural product is found to be active, it is chemically modified to improve its properties. Greater than 60% of the anticancer and anti-infective agents that are on the market or in clinical trials are of natural product origin or derived from natural products. This is a result of the inherent nature of secondary metabolites of plants that act in defense of their producing organisms.
1.1 Medicinal Chemistry Folklore
1.2 Discovery of New Drugs
1.3 General References
1.4 References
1.1 Medicinal Chemistry Folklore
Medicinal chemistry is the science that deals with the discovery and design of new therapeutic chemicals and their development into useful medicines. Medicines are substances used to treat diseases. Drugs are molecules used as medicines or as components in medicines to diagnose, cure, mitigate, treat, or prevent disease.[1] Medicinal chemistry may involve isolation of compounds from Nature or the synthesis of new molecules, investigations of the relationships between the structure of natural and/or synthetic compounds and their biological activities, elucidations of their interactions with receptors of various kinds, including enzymes and DNA, the determination of their absorption, transport, and distribution properties, and studies of the metabolic transformations of these chemicals into other chemicals and their excretion. More recently, genomics, the investigations of an organism's genome (all of the organism's genes) to identify important target genes and gene products (that is, proteins expressed by the genes), and proteomics, the investigations of new proteins in the organism's proteome (all of the proteins expressed by the genome)[2] to determine their structure and/or function often by comparison with known proteins, have become increasingly important approaches to identify new drug targets.
Medicinal chemistry, in its crudest sense, has been practiced for several thousand years. Man has searched for cures for illnesses by chewing herbs, berries, roots, and barks. Some of these early clinical trials were quite successful; however, not until the last 100–150 years has knowledge of the active constituents of these natural sources been known. The earliest written records of the Chinese, Indian, South American, and Mediterranean cultures described the therapeutic effects of various plant concoctions.[3–⁵] A Chinese health science anthology called Nei Ching is thought to have been written by the Yellow Emperor in the 13th century B.C., although some believe that it was backdated by the 3rd-century compilers.[6] The Assyrians described on 660 clay tablets 1000 medicinal plants used from 1900−400 B.C.
Two of the earliest medicines were described about 5100 years ago by the Chinese Emperor Shen Nung in his book of herbs called Pen Ts'ao.[7] One of these is Ch'ang Shan, the root Dichroa febrifuga, which was prescribed for fevers. This plant contains alkaloids that are used even today in the treatment of malaria. Another plant called Ma Huang (now known as Ephedra sinica) contains ephedrine, a drug that raises the blood pressure and relieves bronchial spasms; this herb was used as a heart stimulant, a diaphoretic agent (perspiration producer), and for treatment of asthma, hay fever, and nasal and chest congestion. It also is used today (unadvisably) by some body builders and endurance athletes because it promotes themogenesis (the burning of fat) by release of fatty acids from stored fat cells, leading to quicker conversion of the fat into energy. Ephedra also tends to increase the contractile strength of muscle fibers, which allows body builders to work harder with heavier weights.
Theophrastus in the 3rd-century B.C. mentioned opium poppy juice as an analgesic agent, and in the 10th-century A.D. Rhazes (Persia) introduced opium pills for coughs, mental disorders, aches, and pains. The opium poppy, Papaver somniferum, contains morphine, a potent analgesic agent and codeine, which is prescribed today as a cough suppressant. The East Asians and the Greeks used henbane, which contains scopolamine (truth serum) as a sleep inducer. Inca mail runners and silver miners in the high Andean mountains chewed coca leaves (cocaine) as a stimulant and euphoric. The antihypertensive drug reserpine was extracted by ancient Hindus from the snakelike root of the Rauwolfia serpentina plant and was used to treat hypertension, insomnia, and insanity. Alexander of Tralles in the 6th-century A.D. recommended the autumn crocus (Colchicum autumnale) for relief of pain of the joints, and it was used by Avrienna (11th-century Persia) and by Baron Anton von Storck (1763) for the treatment of gout. Benjamin Franklin heard about this medicine and brought it to America. The active principle in this plant is the alkaloid colchicine, which is used today to treat gout.
In 1633 a monk named Calancha, who accompanied the Spanish Conquistadors to Central and South America, introduced one of the greatest herbal medicines to Europe on his return. The South American Indians would extract the cinchona bark and use it for chills and fevers; the Europeans used it for the same and for malaria. In 1820 the active constituent was isolated and later determined to be quinine, an antimalarial drug.
Modern therapeutics is considered to have begun with an extract of the foxglove plant, which was cited by Welsh physicians in 1250, named by Fuchsius in 1542, and introduced for the treatment of dropsy (now called congestive heart failure) in 1785 by Withering.[3,⁸] The active constituents are secondary glycosides from Digitalis purpurea (the foxglove plant) and Digitalis lanata, namely, digitoxin and digoxin, respectively, both important drugs for the treatment of congestive heart failure. Today, digitalis, which refers to all of the cardiac glycosides, is still manufactured by extraction of foxglove and related plants.
1.2 Discovery of New Drugs
Nature is still an excellent source of new drugs or, more commonly, of precursors to drugs. Of the 20 leading drugs in 1999,9 of them were derived from natural products.[9] Almost 40% of the 520 new drugs approved for the drug market between 1983 and 1994 were natural products or derived from natural products. Greater than 60% of the anticancer and anti-infective agents that are on the market or in clinical trials are of natural product origin or derived from natural products.[10] This may be a result of the inherent nature of these secondary metabolites to act in defense of their producing organisms; for instance, a fungal natural product might be produced against bacteria or other fungi or against cell replication of foreign organisms.[11]
Typically, when a natural product is found to be active, it is chemically modified to improve its properties. As a result of advances made in synthesis and separation methods and in biochemical techniques since the late 1940s, the early random approach to drug discovery (Figure 1.1) was supplanted by a more rational approach, namely, one that involves the element of design. A discussion of how drugs are discovered and chemically modified to improve or change their medicinal properties is presented in Chapter 2. As we will see, the random approach still is important!
Figure 1.1
That's Dr Arnold Moore. He's conducting an experiment to test the theory that most great scientific discoveries were hit on by accident. Drawing by Hoff; © 1957, The New Yorker Magazine, Inc.
1.3 General References
The following references are excellent sources of material for this entire book.
Annual Reports in Medicinal Chemistry;
Academic Press, San Diego
Annual Review of Biochemistry
Annual Review of Medicinal Chemistry
Biochemical Pharmacology
Bioorganic and Medicinal Chemistry
Bioorganic and Medicinal Chemistry Letters
Chemistry & Biology
Current Drug Metabolism
Current Drug Targets
Current Genomics
Current Medicinal Chemistry
Current Opinion in Chemical Biology
Current Opinion in Drug Discovery and
Current Opinion in Investigational Drugs
Current Opinion in Therapeutic Patents
Current Pharmaceutical Biotechnology
Current Pharmaceutical Design
Current Protein & Peptide Science
Drug Design and Discovery
Drug Development Research
Drug Discovery and Development
Drug Discovery Today
Drug News and Perspectives
Drugs of the Future
Drugs of Today
Drugs Under Experimental and Clinical
Emerging Drugs
Emerging Therapeutic Targets
European Journal of Medicinal Chemistry
Expert Opinion on Investigational Drugs
Expert Opinion on Pharmacotherapy
Expert Opinion on Therapeutic Patents
Expert Opinion on Therapeutic Targets
Journal of Medicinal Chemistry
Medicinal Research Reviews
Mini Reviews in Medicinal Chemistry
Modern Drug Discovery
Modern Pharmaceutical Design
Molecular Pharmacology
Nature Medicine
Perspectives in Drug Discovery and Design
Progress in Drug Research
Progress in Medicinal Chemistry
Trends in Biochemical Sciences
Trends in Pharmacological Sciences
Advances in Medicinal Chemistry; Elsevier, New York; (continuing series).
Albert, A. Selective Toxicity, 7th ed., Chapman and Hall, London, 1985.
Arins, E. J. (Ed.) Drug Design, Academic, New York, 1971–1980, Vols. 1–10.
Borchardt, R. T.; Freidinger, R. M.; Sawyer, T. K. Integration of Pharmaceutical Discovery and Development: Case Histories, Plenum Press, New York, 1998.
Burger, A. A Guide to the Chemical Basis of Drug Design, Wiley, New York, 1983.
Hansch, C.; Emmett, J. C.; Kennewell, P. D.; Ramsden, C. A.; Sammes, P. G.; Taylor, J. B. (Eds.) Comprehensive Medicinal Chemistry, Pergamon Press, Oxford, 1990, Vols. 1–6.
Hardman, J. G.; Limbird, L. E.; Gilman, A. G. (Eds.) Goodman and Gilman's The Pharmacological Basis of Therapeutics, 10th ed., McGraw-Hill, New York, 2001.
Lednicer, D. Chronicles of Drug Discovery, ACS, Washington, DC, 1993.
Lednicer, D. Strategy for Organic Drug Synthesis and Design, Wiley, New York, 1998.
Lednicer, D. Mitscher, L. A. The Organic Chemistry of Drug Synthesis, Wiley, New York, 1996, five-volume set.
Lunn, G.; Schmuff, N. HPLC Methods for Pharmaceutical Analysis, Wiley-VCH, New York, 1997.
Methods and Principles in Medicinal Chemistry, VCH, Weinheim, (continuing series).
O'Neil, M. J.; Smith, A. (Eds.) The Merck Index, 13th ed., Merck & Co., Rahway, NJ, 2001.
Wolff, M. E. (Ed.) Burger's Medicinal Chemistry and Drug Discovery, John Wiley & Sons, New York, 1995–1997, Vols. 1–6.
1.4 References
1. Webster's Ninth New Collegiate Dictionary, Merriam-Webster, Springfield, MA, 1987.
2. Wilkins M.R., Williams K.L., Appel R.D., Hochstrasser D.F., eds. Proteome Research: New Frontiers in Functional Genomics. Berlin: Springer-Verlag, 1997.
3. Bauer, W.W.Potions, Remedies and Old Wives' Tales. New York: Doubleday, 1969.
4. Withering, W. An Account of the Foxglove and Some of Its Medicinal Uses: With Practical Remarks on Dropsy and Other Diseases, Robinson, C. G. J., London, 1785; reprinted in Med. Class. 1937, 2, 305.
5. Sneader, W.Drug Discovery: The Evolution of Modern Medicines. Chichester: Wiley, 1985.
6. Nakanishi, K.Barton, D., Nakanishi, K., eds. Comprehensive Natural Products Chemistry; 1. Elsevier, Amsterdam and New York, 1999:xxiii–xl.
7. Chen, K.K. J. Am. Pharm. Assoc. 1925; 14:189.
8. Burger, A.Wolff M.E., ed. Burger's Medicinal Chemistry, 4th ed., New York: Wiley, 1980. [Part I, Chap. 1].
9. Harvey, A. Drug Discovery Today. 2000; 5:294.
10. Cragg, G.M., Newman, D.J., Snader, K.M. J. Nat. Prod. 1997; 60:52.
11. Hung, D.T., Jamison, T.F., Schrieber, S.L. Chem. Biol. 1996; 3:623.
Chapter 2
Drug Discovery, Design, and Development
Publisher Summary
This chapter presents an overview of drug discovery, drug design and developments, quantitative structure-activity relationships, and molecular graphics-based drug design. Drug discovery is a time-consuming and expensive process. Estimates of the average time required to bring a drug to the market ranges from 12 to15 years at an average cost of about $800 million. Rational approaches to drug design are the major routes to leaddiscovery. It is impossible to foretell the toxicity and side effects of a drug, anticipate its transport characteristics, or predict the metabolic fate of a drug. An important physicochemical parameter in lead discovery/lead optimization lies in steric effects, particularly for receptor binding. As a lead is identified, its structure is modified until an effective drug is obtained. Only a small part of the lead compound is involved in receptor interactions. The relevant groups on a molecule that interact with a receptor and are responsible for the activity are collectively known as the pharmacophore. A medicinal chemist tinkers the drug design parameters to augment the potency, selectivity, and duration of the drug's action and to reduce the toxicity of the drug. The concept of quantitative drug design is based on the fact that the biological properties of a compound are a function of its physicochemical parameters.
2.1 Drug Discovery
A. Drug Discovery without a Lead
1 Penicillins
2 Librium
B. Lead Discovery
1 Random Screening
2 Nonrandom (or Targeted or Focused) Screening
3 Drug Metabolism Studies
4 Clinical Observations
5 Rational Approaches to Lead Discovery
2.2 Lead Modification: Drug Design and Development
A. Identification of the Active Part: The Pharmacophore
B. Functional Group Modification
C. Structure-Activity Relationships
D. Privileged Structures and Drug-Like Molecules
E. Structure Modifications to Increase Potency and the Therapeutic Index
1 Homologation
2 Chain Branching
3 Ring-Chain Transformations
4 Bioisosterism
5 Combinatorial Chemistry
a General Aspects
b Split Synthesis: Peptide Libraries
c Encoding Combinatorial Libraries
d Nonpeptide Libraries
6 SAR by NMR/SAR by MS
7 Peptidomimetics
F. Structure Modifications to Increase Oral Bioavailability
1 Electronic Effects: The Hammett Equation
2 Lipophilicity Effects
a Importance of Lipophilicity
b Measurement of Lipophilicities
c Computerization of Log p Values
d Membrane Lipophilicity
3 Effects of Ionization on Lipophilicity and Oral Bioavailability
4 Other Properties that Influence Oral Bioavailability and Ability to Cross the Blood-Brain Barrier
G. Quantitative Structure-Activity Relationships
1 Historical
2 Steric Effects: The Taft Equation and Other Equations
3 Methods Used to Correlate Physicochemical Parameters with Biological Activity
a Hansch Analysis: A Linear Multiple Regression Analysis
b Free and Wilson or de novo Method
c Enhancement Factor
d Manual Stepwise Methods: Topliss Operational Schemes and Others
e Batch Selection Methods: Batchwise Topliss Operational Scheme, Cluster Analysis, and Others
4 Computer-Based Methods of QSAR Related to Receptor Binding: 3D-QSAR
H. Molecular Graphics-Based Drug Design
I. Epilogue
2.3 General References
2.4 Problems
2.5 References
2.1 Drug Discovery
Drug discovery is a very time-consuming and expensive process. Estimates of the average time required to bring a drug to the market range from 12–15 years at an average cost of about $800 million. For approximately every 10,000 compounds that are evaluated in animal studies, 10 will make it to human clinical trials in order to get 1 compound on the market. The clinical trials consist of three phases prior to drug approval: phase I (generally a few months to a year and a half) evaluates the safety, tolerability (dosage levels and side effects), pharmacokinetic properties, and pharmacological effects in 20–100 healthy volunteers; phase II (about 1–3 years) assesses the effectiveness of the drug, determines side effects and other safety aspects, and clarifies the dosing regimen in a few hundred diseased patients; and phase III (about 2–6 years) is a larger trial with several thousand patients in clinics and hospitals that establishes the efficacy of the drug and monitors adverse reactions from long-term use. Once the new drug application (NDA) is submitted to the Food and Drug Administration (FDA), it can be several months to several years before it is approved for commercial use. Phase IV studies are considered to be the results found with a drug that has already been allowed onto the drug market and is in general use. Drug candidates (or new chemical entities, NCE, as they are often called) that fail late in this process result in huge, unrecovered financial losses for the company. This is why the cost to purchase a drug is so high. It is not that it costs that much to manufacture that one drug, but that the profits are needed to pay for all of the drugs that fail to make it to market after large sums of research funds have already been expended.
In general, drugs are not discovered. What is more likely discovered is known as a lead compound. The lead is a prototype compound that has a number of attractive characteristics, such as the desired biological or pharmacological activity, but may have other undesirable characteristics, for example, high toxicity, other biological activities, absorption difficulties, insolubility, or metabolism problems. The structure of the lead compound is modified by synthesis to amplify the desired activity and to minimize or eliminate the unwanted properties to a point where a drug candidate, a compound worthy of extensive biological, pharmacological, and animal studies, is identified; then a clinical drug, a compound ready for clinical trials, is developed. Prior to an elaboration of approaches to lead discovery and lead modification, two common drugs discovered without a lead are discussed.
2.1.A Drug Discovery without a Lead
A.1 Penicillins
In 1928 Alexander Fleming noticed a green mold growing in a culture of Staphylococcus aureus, and where the two had converged, the bacteria were lysed. [1] This led to the discovery of penicillin, which was produced by the mold. Actually, Fleming was not the first to make this observation; John Burdon-Sanderson had done so in 1870, ironically also at St. Mary's Hospital in London, the same institution where Fleming made the rediscovery![2] Joseph Lister had treated a wounded patient with Penicillium, the organism later found to be the producer of penicillin (although the strains discovered earlier than Fleming's strain did not produce penicillin, but, rather, another antibiotic, mycophenolic acid). After Fleming observed this phenomenon, he tried many times to repeat it without success; it was his colleague, Dr. Ronald Hare,[3,4] who was able to reproduce the observation. It only occurred the first time because a combination of unlikely events all took place simultaneously. Hare found that very special conditions were required to produce the phenomenon initially observed by Fleming. The culture dish inoculated by Fleming must have become accidentally and simultaneously contaminated with the mold spore. Instead of placing the dish in the refrigerator or incubator when he went on vacation as is normally done, Fleming inadvertently left it on his lab bench. When he returned the following month, he noticed the lysed bacteria. Ordinarily, penicillin does not lyse these bacteria; it prevents them from developing, but it has no effect if added after the bacteria have developed. However, while Fleming was on vacation (July to August) the weather was unseasonably cold, and this provided the particular temperature required for the mold and the staphylococci to grow slowly and produce the lysis. Another extraordinary circumstance was that the particular strain of the mold on Fleming's culture was a relatively good penicillin producer, although most strains of that mold (Penicillium) produce no penicillin at all. The mold presumably came from the laboratory just below Fleming's where research on molds was going on at that time.
Although Fleming suggested that penicillin could be useful as a topical antiseptic, he was not successful in producing penicillin in a form suitable to treat infections. Nothing more was done until Sir Howard Florey at Oxford University reinvestigated the possibility of producing penicillin in a useful form. In 1940 he succeeded in producing penicillin that could be administered topically and systemically,[5] but the full extent of the value of penicillin was not revealed until the late 1940s.[6] Two reasons for the delay in the universal utilization of penicillin were the emergence of the sulfonamide antibacterials (sulfa drugs, 2.1; see Chapter 5, Section 5.4.B.2, p. 254) in 1935 and the outbreak of World War II.
No studies related to the pharmacology, production, and clinical application of penicillin were permitted until after the war to prevent the Germans from having access to this wonder drug. Allied scientists who were interrogating German scientists involved in chemotherapeutic research were told that the Germans thought the initial report of penicillin was made just for commercial reasons to compete with the sulfa drugs. They did not take the report seriously.
The original mold was Penicillium notatum, a strain that gave a relatively low yield of penicillin. It was replaced by Penicillium chysogenum,[7] which had been cultured from a mold growing on a grapefruit in a market in Peoria, Illinois!
For many years debate raged regarding the actual structure of penicillin (2.2),[8] but the correct structure was elucidated in 1944 with an X-ray crystal structure by Dorothy Crowfoot Hodgkin (Oxford); the crystal structure was not actually published until 1949.[9] Several different penicillin analogs (R group varied) were isolated early on; only two of these early analogs (2.2, penicillin V; and 2.2, penicillin G) are still in use today.
A.2 Librium
The first benzodiazepine tranquilizer drug, chlordiazepoxide HCl [7-chloro-2-(methylamino)-5-phenyl-3H−1,4-benzodiazepine 4-oxide; 2.3; Librium], was discovered serendipitously.[10] Dr. Leo Sternbach at Roche was involved in a program to synthesize a new class of tranquilizer drugs. He originally set out to prepare a series of benzheptoxdiazines (2.4), but when R¹ was CH2 NR2 and R² was C6H5, it was found that the actual structure was that of a quinazoline 3-oxide (2.5).
However, none of these compounds gave any interesting pharmacological results. The program was abandoned in 1955 in order for Sternbach to work on a different project. In 1957 during a general laboratory cleanup a vial containing what was thought to be 2.5 was found and, as a last effort, was submitted for pharmacological testing. Unlike all of the other compounds submitted, this one gave very promising results in six different tests used for preliminary screening of tranquilizers. Further investigation revealed that this compound was not a quinazoline 3-oxide, but was instead the benzodiazepine 4-oxide, 2.3, presumably produced in an unexpected reaction of the corresponding chloromethyl quinazoline 3-oxide (2.6) with methylamine (Scheme 2.1). If this compound had not been found in the laboratory cleanup, all of the negative pharmacological results would have been reported for the quinazoline 3-oxide class of compounds, and benzodiazepine 4-oxides may not have been discovered for many years to come.
Scheme 2.1 Mechanism for formation of Librium.
The examples of drug discovery without a lead are relatively few in number. The typical occurrence is that a lead compound is identified, and its structure is modified to give, eventually, the compound that goes to the clinic.
2.1.B Lead Discovery
Penicillin V and Librium are, indeed, two important drugs that were discovered without a lead. However, once they were identified, they then became lead compounds for second-generation analogs. A myriad of penicillin-derived antibacterials have been synthesized as a result of the structure elucidation of the earliest penicillins. Diazepam (2.7, Valium) was synthesized at Roche even before Librium was introduced on to the market; this drug was derived from the lead compound Librium and is almost 10 times more potent than the lead.
The initial difficulty arises in the discovery of the lead compound. Several approaches can be taken to identify a lead. The first requirement for all of the approaches is to have a means to assay compounds for a particular biological activity, so that researchers can tell when a compound is active. A bioassay (or screen) is a means of determining in a biological system, relative to a control compound, if a compound has the desired activity, and, if so, what the relative potency of the compound is. Note the distinction between the terms activity and potency. Activity is the particular biological or pharmacological effect (for example, antibacterial activity or anticonvulsant activity); potency is the strength of that effect.
Some screens are in vitro tests, for example, the inhibition of an enzyme or antagonism of a receptor; others are in vivo tests, for example, the ability of the compound to prevent an induced seizure in a mouse. In general, the in vitro tests are quicker and less expensive. Currently, high-throughput screens (HTS)[11] very rapid and sensitive in vitro screens initially developed about 1989–1991, that now can be carried out robotically in 1536- or 3456-well titer plates on small (submicrogram) amounts of compound (dissolved in submicroliter volumes) are becoming universally used. With these ultra-high-throughput screening approaches, it is possible to screen 100,000 compounds in a day! As we will see below, combinatorial chemistry (see Section 2.2.E.5, p. 34) can supply huge numbers of compounds in a short period of time, which, theoretically, should provide an increased number of hits, i.e., compounds that elicit a predetermined level of activity in the bioassay and, therefore, provide more leads. According to Drews,[12] the number of compounds assayed in a large pharmaceutical company in the early 1990s was about 200,000 a year; that number rose to 5–6 million during the mid-1990s, and by the end of the 1990s it was >50 million! However, the increase in the assay rate did not result in a commensurate increase in research productivity, as measured by new compounds entering the market. Of course, it can take 12–15 years for a drug to reach the market, so productivity in the early part of the 21st century should provide a more accurate ruler for success of drug discovery changes made at the end of the 20th century. Currently, HTS appears to have resulted in an increase in the number of hits, but this may be because more lipophilic compounds, which may have more drug-like properties (see Section 2.2.F.2, p. 53), can be tested by dissolving them in dimethylsulfoxide (DMSO) rather than in water. Nonetheless, it is not yet clear if this increase in hit rate is translating into a much greater number of leads and development compounds¹³
An exciting approach for screening compounds that might interact with an enzyme in a metabolic pathway was demonstrated by Wong, Pompliano, and coworkers for the discovery of lead compounds that block bacterial cell wall biosynthesis (as potential antibacterial agents) [14] Conditions were found to reconstitute all six enzymes in the cell wall biosynthetic pathway so that incubation with the substrate for the first enzyme leads to the formation of the product of the last enzyme in the pathway. Then by screening compounds and looking for the buildup of an intermediate, it is possible to identify not only compounds that block the pathway (and prevent the formation of the bacterial cell wall), but to determine which enzyme is blocked (the buildup of an intermediate means that the enzyme that acted on that intermediate was blocked).
Compound screening also can be carried out by electrospray ionization mass spectrometry[15] (the technique for which John Fenn received the Nobel prize in 2002) and by NMR spectrometry. [16] Tightly bound noncovalent complexes of compounds with a macromolecule (such as a receptor or enzyme) can be observed in the mass spectrum. The affinity of the ligand (a small molecule that binds to a receptor) can be measured by varying the collision energy and determining at what energy the complex dissociates. This method also can be used to screen mixtures (a library) of compounds, provided they have different molecular masses and/or charges, so the m/z for each complex with the biomolecule can be separated in the mass spectrometer. By varying the collision energy, it is possible to determine which test molecules bind to the biomolecule best. The lH NMR method exploits changes in either relaxation rates or diffusion rates of small molecules when they bind to a macromolecule. This method also can be used to screen mixtures of compounds to determine the ones that bind best.
Once the screen is developed, a variety of approaches can be taken to obtain a lead. As we will see below, the typical lead compound for a receptor or enzyme is the natural ligand for the receptor or substrate for the enzyme. Another good source of lead compounds is marketed drugs.[17] In this case the target will generally be well established, and the lead structure will be known to bind well to the target and to have good absorption properties. The main stumbling block to the use of marketed drugs as leads may be patent issues for commercialization. If the target macromolecule is not known or if no new leads have come from a marketed drug, other approaches can taken.
B.1 Random Screening
In the absence of known drugs and other compounds with desired activity, a random screen is a valuable approach. Random screening involves no intellectualization; all compounds are tested in the bioassay without regard to their structures. Prior to 1935 (the discovery of sulfa drugs), this was essentially the only approach; today this method is still an important approach to discover drugs or leads, particularly because it is now possible to screen such huge numbers of compounds rapidly with HTSs. This is the lead discovery method of choice when nothing is known about the receptor target.
The two major classes of materials screened are synthetic chemicals and natural products(microbial, plant, and marine). An example of a random screen of synthetic and naturalcompounds was the
war on cancer declared by Congress and the National Cancer Institutein the early 1970s. Any new compound submitted was screened in a mouse tumor bioassay. Few new anticancer drugs resulted from that screen, but many known anticancer drugs alsodid not show activity in the screen used, so a new set of screens was devised that gavemore consistent results. In the 1940s and 1950s, a random screen of soil samples by variouspharmaceutical companies in search of new antibiotics was undertaken. However, in this case, not only were numerous leads uncovered, but two important antibiotics, streptomycin and the tetracyclines, were found. Screening of microbial broths, particular strains of Streptomyces, was a common random screen methodology prior to 1980.
B.2 Nonrandom (or Targeted or Focused) Screening
Nonrandom screening, also called targeted or focused screening, is a more narrow approach than is random screening. In this case, compounds having a vague resemblance to weakly active compounds uncovered in a random screen, or compounds containing different functional groups than leads, may be tested selectively. By the late 1970s, the National Cancer Institute's random screen was modified to a nonrandom screen because of budgetary and manpower restrictions. Also, the single tumor screen was changed to a variety of tumor screens because it was realized that cancer is not just a single disease.
B.3 Drug Metabolism Studies
During drug metabolism studies (see Chapter 7) metabolites (drug degradation products generated in vivo) that are isolated are screened to determine if the activity observed is derived from the drug candidate or from a metabolite. For example, the anti-inflammatory drug sulindac (2.8, Clinoril) is not the active agent; the metabolic reduction product, 2.9, is responsible for the activity.[18]
The nonsedating antihistamine terfenadine hydrochloride (2.10, Seldane) was found to cause an abnormal heart rhythm in some users who also were taking certain antifungal agents, which were found to block the enzyme that metabolizes terfenadine. This caused a buildup of terfenadine, which led to the abnormal heart rhythms. However, a metabolite of terfenadine, fexofenadine hydrochloride (2.11, Allegra), was also found to be a nonsedating antihistamine, but it can be metabolized even in the presence of antifungal agents. This, then, is a safer drug. Metabolites can be screened for other activities as well.
B.4 Clinical Observations
Sometimes a drug candidate during clinical trials will exhibit more than one pharmacological activity; that is, it may produce a side effect. This compound, then, can be used as a lead (or, with luck, as a drug) for the secondary activity. In 1947 an antihistamine, dimenhydrinate (2.12, Dramamine) was tested at the allergy clinic at Johns Hopkins University and was found also to be effective in relieving a patient who suffered from car sickness; a further study proved its effectiveness in the treatment of seasickness [19] and airsickness [20] It then became the most widely used drug for the treatment of all forms of motion sickness.
There are other popular examples of drugs derived from clinical observations. Bupropion hydrochloride (2.13), an antidepressant drug (Wellbutrin), was found to help patients stop smoking and is now the first drug marketed as a smoking cessation aid (Zyban). The impotence drug sildenafil citrate (2.14; Viagra) was designed for the treatment of angina and hypertension by blocking the enzyme phosphodiesterase-5, which hydrolyzes cyclic guanosine monophosphate (cGMP), a vasodilator that allows increased blood flow.[21] In 1991 sildenafil went into phase I clinical trials for angina. In phase II clinical trials, it was not as effective against angina as Pfizer had hoped, so it went back to phase I clinical trials to see how high of a dose could be tolerated. It was during that clinical trial that the volunteers reported increased erectile function. Given the weak activity against angina, it was an easy decision to try to determine its effectiveness as the first treatment for erectile dysfunction. Sildenafil works by the mechanism for which it was designed as an antianginal drug, except it inhibits the phosphodiesterase in the penis (phosphodiesterase-5) instead of the heart (Figure 2.1). Sexual stimulation causes release of nitric oxide in the penis.
Figure 2.1 Mechanism of action of sildenafil (Viagra)
Nitric oxide is a second messenger molecule that stimulates the enzyme guanylate cyclase, which converts guanosine triphosphate to cGMP. The vasodilator cGMP relaxes the smooth muscle in the corpus cavernosum, allowing blood to flow into the penis, thereby producing an erection. However, phosphodiesterase-5 (PDE 5) hydrolyzes the cGMP, which causes vasoconstriction and the outflow of blood from the penis. Sildenafil inhibits this phosphodiesterase, preventing the hydrolysis of cGMP and prolonging the vasodilation effect.
B.5 Rational Approaches to Lead Discovery
None of the above approaches to lead discovery involves a major rational component. The lead is just found by screening techniques, as a by-product of drug metabolism studies, or from clinical investigations. Is it possible to design a compound having a particular activity? Rational approaches to drug design now have become the major routes to lead discovery. The first step is to identify the cause for the disease state. Many diseases, or at least the symptoms of diseases, arise from an imbalance (either excess or deficiency) of particular chemicals in the body, from the invasion of a foreign organism, or from aberrant cell growth. As will be discussed in later chapters, the effects of the imbalance can be corrected by antagonism or agonism of a receptor (see Chapter 3) or by inhibition of a particular enzyme (see Chapter 5); foreign organism enzyme inhibition or interference with DNA biosynthesis or function are important approaches to treat diseases arising from microorganisms and aberrant cell growth (see Chapter 6).
Once the relevant biochemical system is identified, initial lead compounds then become the natural receptor ligands or enzyme substrates. For example, lead compounds for the contraceptives (+)-norgestrel (2.15, Ovral) and 17α-ethynylestradiol (2.16, Activella) were the steroidal hormones progesterone (2.17) and 17β-estradiol (2.18). Whereas the steroid hormones 2.17 and 2.18 show weak and short-lasting effects, the oral contraceptives 2.15 and 2.16 exert strong progestational activity of long duration.
At Merck it was believed that serotonin (2.19) was a possible mediator of inflammation. Consequently, serotonin was used as a lead for anti-inflammatory agents, and from this lead the anti-inflammatory drug indomethacin (2.20, Indocin) was developed.[22]
The rational approaches are directed at lead discovery. It is not possible, with much accuracy, to foretell toxicity and side effects, anticipate transport characteristics, or predict the metabolic fate of a drug. Once a lead is identified, its structure can be modified until an effective drug is obtained.
2.2 Lead Modification: Drug Design and Development
Once your lead compound is in hand, how do you know what to modify in order to improve the desired pharmacological properties?
2.2.A Identification of the Active Part: The Pharmacophore
Interactions of drugs with receptors, known as pharmacodynamics, are very specific (see Chapter 3). Therefore, only a small part of the lead compound may be involved in the appropriate receptor interactions. The relevant groups on a molecule that interact with a receptor and are responsible for the activity are collectively known as the pharmacophore. The other atoms in the lead molecule, sometimes referred to as the auxophore, may be extraneous. Some of the atoms, of course, are essential to maintain the integrity of the molecule and hold the pharmacophoric groups in their appropriate positions. Some of these extraneous atoms, however, may be interfering with the binding of the pharmacophore, and those atoms need to be excised from the lead compound. Other atoms in the auxophore may be dangling in space within the receptor and are neither binding to the receptor nor preventing the pharmacophoric atoms from binding. Although these atoms appear to be innocuous, it is important to know which atoms these are, because these are the ones that can be modified without loss of potency. As we will see later, there are other aspects to lead modification that are as important as increasing binding to the target receptor, such as pharmacokinetics (absorption, distribution, metabolism, and excretion or ADME). Modification of the atoms that are not interfering with binding could be very important to solving pharmacokinetics problems.
By determining which are the pharmacophoric groups and which are the auxophoric groups on your lead compound, and of the auxophoric groups, which are interfering with lead compound binding and which are not detrimental to binding, you will know which groups must be excised and which you can retain or modify as needed. One approach in lead modification to help make this determination is to cut away sections of the lead molecule and measure the effects of those modifications on potency. Consider this artificial example of how this might be done. Assume that the addictive analgesics morphine (2.21), codeine (2.21), and heroin (2.21) are the lead compounds, and we want to know which groups are pharmacophoric and which are auxophoric.
The morphine family of analgesics binds to the μ opioid receptors. The pharmacophore is known and is shown as the darkened part in 2.21. A decrease in potency on removal of a group will suggest that it may have been pharmacophoric, an increase in potency means it was auxophoric and interfering with proper binding, and essentially no change in potency will mean that it is auxophoric but not interfering with binding.
Let′s start by excising the dihydrofuran oxygen atom, which is not in the pharmacophore. This may not seem to be sensible because that atom connects the cyclohexene ring to the benzene ring; its removal will result in a change in the conformation of the cyclohexene ring and an increase in the degrees of freedom of the molecule. Excision of the dihydrofuran oxygen gives morphinan (2.22[23]; the hydroxy analog, levorphanol[24] (2.22, Levo- Dromoran), is three to four times more potent than morphine as an analgesic, but it retains the addictive properties (note that in 2.22 the cyclohexene ring conformation has not been changed for ease of comparison with 2.21; surely, a lower energy conformer will be favored).
Possibly, the additional conformational mobility allowed the molecule to approximate its bioactive conformation (the conformation that most effectively binds to a receptor). Removal of half of the cyclohexene ring (also not in the pharmacophore), leaving only methyl substituents, gives benzomorphan (2.23).[25] This compound shows some separation of analgesic and addictive effects; pentazocine hydrochloride (2.23; component of Talwin) is less potent than morphine (about as potent as codeine), but has a much lower addiction liability. Remember, your goal is both to increase potency and decrease adverse effects, such as addictive properties. Although this analog is not more potent than morphine, it is less addicting. Cutting away the methylene group of the cyclohexane fused ring (2.24) also, surprisingly, has little effect on the analgesic activity in animal tests.
Again, this excision removes the rigidity of the parent structure. Removal of all fused rings, for example, in the case of meperidine (2.25, Demerol), gives an analgesic still possessing 10–12% of the overall potency of morphine. [26] Although the potency is lower, it certainly will be much easier to synthesize analogs of meperidine than of morphine. Even acyclic analogs are active. Dextropropoxyphene (2.26, Darvon; again note the side chain is left in a conformation to resemble the structure of morphine) is one-half to two-thirds as potent as codeine. Both morphine and dextropropoxyphene bind to the μ opioid receptor, so the activity of dextropropoxyphene can be ascribed to the fact that it can assume a conformation related to that of the morphine pharmacophore. I don't think any of us, seeing dextropropoxyphene written in a more energetically favorable conformation, would ever make the connection between this structure and that of morphine. By cutting pieces off of the lead compound, it gives you new perspectives on possible active structures, which should open up completely new scaffolds to consider in synthesis. Another acyclic analog is methadone (2.27, Methadose) which is as potent an analgesic as morphine; the (−)-isomer is used in the treatment of opioid abstinence syndromes in heroin abusers because it is eliminated from the body slower than morphine, allowing the body to adapt to the falling levels of drug gradually.
What if every cut in the lead produces a compound with lower potency? Then, either every excision is removing part of the pharmacophore or each cut causes a conformational change that gives a structure less similar to the bioactive conformation. The latter possibility is particularly relevant to such a rigid structure as in morphine. In this case, groups need to be added to the lead structure to increase the pharmacophore. For example, oripavine derivatives such as etorphine (2.28Immobilon), which has a two-carbon bridge and substituent not in morphine, is 3200 times more potent than morphine[27] and is used in veterinary medicine to immobilize large animals. The related analog, buprenorphine (2.28, double bond reduced; Buprenex) is 10–20 times more potent than morphine and has a very low level of dependence liability.
Apparently, the additional rigidity of the oripavine derivatives increases the appropriate receptor interactions (see Chapter 3).
The activity and potency of a molecule are related to the interactions of the pharmacophoric groups with groups on the receptor (see Chapter 3, Section 3.2.B). The binding constants of 200 drugs and potent enzyme inhibitors were used by Andrews and coworkers[28] to calculate the average binding energies of common functional groups; these energies can be used to determine how well a new molecule binds to its receptor. If the test molecule has a measured binding energy that is lower than the calculated average value, it suggests that the molecule contains groups that do not interact with the receptor (are not in the pharmacophore). These groups, then, could be excised without loss of potency, giving a simplified lead for further structural modification. This Andrews analysis was carried out on a highly substituted lead compound, leading to a more simple analog structure that was modified to give molecules with enhanced potency.[29] If the test compound has a binding energy greater than the calculated average value, then the molecule may bind differently than suspected, leading to enhanced binding interactions. This indicates that manipulation of functional groups is an important lead modification approach.
2.2.B Functional Group Modification
The importance of functional group modification is demonstrated by 2.29. The antibacterial agent, carbutamide (2.29), was found to have an antidiabetic side effect; however, it could not be used as an antidiabetic drug because of its antibacterial activity, which could lead to bacterial resistance (see Chapter 5, Section 5.2, p.231). The amino group of carbutamide was replaced by a methyl group to give tolbutamide (2.29; Orinase) and in so doing the antibacterial activity was eliminated from the antidiabetic activity.
In some cases, an experienced medicinal chemist knows what functional group will elicit a particular effect. Chlorothiazide (2.30, Aldocor) is an antihypertensive agent that has a strong diuretic effect as well. It was known from sulfanilamide work that the sulfonamide side chain can give diuretic (increased urine excretion) activity (see Section 2.2.C below). Consequently, diazoxide (2.31, Hyperstat) was prepared as an antihypertensive drug without diuretic activity.
Obviously, a relationship exists between the molecular structure of a compound and its activity. This phenomenon was first realized about 135 years ago.
2.2.C Structure-Activity Relationships
In 1868 Crum-Brown and Fraser,[30] suspecting that the quaternary ammonium character of curare, a potent poison known since the 16th century that was used on arrowheads, may be responsible for its muscular paralytic properties (it blocks the


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Friday, March 27, 2009
Fish Doodle
When I finish a drawing, I'm often ready for something different. After a detailed project, I was in the mood for simple lines & simple colors. This fish doodle was the result. In grade school, I think we would have called this a scribble drawing. The black Copic multiliner was 0.8, a thicker line than I typically use.
Copic colors used: R000,24,29,59, BG000,05, Y15, YR14 (eye)
Paper: white Crane's card approx. 4x6"
To try this, click here.
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101 in 1001
I have been thinking a lot about things I love, things I want to do, and things I feel like I just have to do. I want to experience more of life, so why not track it as I open up my world? Below,…
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18+ lettre de motivation en tete
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If you have got the distance, bring a huge beach towel for every individual. No demand for ploys to fool robbers into believing your residence is occupied as you are gone, the home is occupied. You will discover that it's right in the conclusion of this post. The finish of the street in Key West is barely the conclusion of the fun. It is great to have somewhere to park your things. In the very top of everybody's record was a place on or close to the shore. Of course depending upon your particular pursuits and destination it is possible to remove or add things.
Maybe while trying to discover examples you detected poor samples all over the world wide web. These samples are collected from an range of reliable career tools to provide you with a notion of what a powerful CV should look like in a variety of formats. With a couple clicks you may download totally free sample resumes. The resume format sample should be carried out well.
If you are only starting out, your CV should be relatively straightforward, but for mid-career or late-career professionals it may locate a little more complicated. In a hurry to find the CV posted off, it is tempting simply to shoehorn in your final job in the exact top and hope for the best. Possessing an effective CV is of extreme value in getting noticed by employers, which typically means writing a CV that contains the significant abilities and data employers are interested in locating. You may never fail with the normal CV because it is the one that most employers expect to see. Choosing the perfect CV to emphasize your abilities and experiences is critically important.
Tailoring your CV is critical and consequently the more effort that you put into it the better. A well-written CV is the first step in finding work. The skills-based CV enables you to centre on the abilities you've developed in a lot of areas of your life.
Writing a CV can appear difficult, but when you begin it will find a whole lot simpler. The simplest way to be certain your CV is the very best advantage in obtaining a dream occupation is our CV examples. It is a simple fact that all CVs adhere to a particular standard and guidelines. Putting together the perfect CV may appear to be an overwhelming job, but it is something that you may readily learn. So, you can wish to consider keeping an individual CV which you use specifically for submitting with applications.
Write If you are requesting work overseas or a position in a business is successful on a global level, you want to prove that you've got what it takes to remain educated in their fast paced work environments. Particular sorts of job in the food service industry require resumes with a number of the specific skills. If you're attempting to obtain work in accounting or finance you require averystrong resume.
Whichever format you select, it should force you to become selected for the job you're applying for. In case you have regularly changed jobs or professions, don't feel obliged to list all your work experience but rather select the previous jobs which are most relevant to the one you're applying for. As stated previously, it's a considerable job to draft an appropriate fitting CV. Whether you're a recent graduate seeking your first job or a seasoned professional with decades of work experience, your CV must reflect the necessities of the position you're applying for.
A fantastic example of a superb cv contains your work experience skills technical skills or soft skills. Pharmacist CV Example Below you will discover a CV illustration for Pharmacist position that can help you in composing your CV, which includes an expert overview, qualifications, key abilities, and work experience and testimonials. Teacher CV Example Here you will find a CV example for a Teacher position that might help you in composing your CV, that contains an expert summary, academic qualifications, key skills, experience, in addition to references. This type of free resume template would likewise be ideal for freshers who don't have a lot of professional experience. Also, take a look at different samples for each one to supply you some useful examples you wish to follow. Simply set aside a bit of time (possibly bookmark it for later) plus a couple teabags at the end you'll be having a peek at a prime example of the best CV ever, with your name in the very top! Resume Help It is a wonderful case of a timeless CV that could work whatever position you are applying for.
Deciding upon the correct format efficiently can help you raise your likelihood of landing function. The trick to getting it right is picking the best CV format that will present your strengths in the very best way possible. There is no necessary format, and therefore don't worry if your CV appears different from the others you have seen. This curriculum vitae format gives you an illustration of what things to get in your academic CV and reveals the proper format for a curriculum vitae.
If you're wishing you mail your bill then it's crucial to write the perfect format of invoice that's suited to mailing. If you don't understand about how to compose a statement for mail then it'll be most suitable for you to utilize the below provided email invoice templates which are totally free to use and easy to report. If invoice is for distribution of merchandise, then 3 copies should be published. Through the guidance of a catering statement template, you're going to be able to make the invoices easily in an expert way. You will need a statement that is likely to make your business appear professional and isn't difficult to use. If you create a second invoice on such date for this customer, the invoice number will be 20170630-4072-01.
Most significantly, make certain to have each the vital info. One of the most necessary info to include within an invoice includes a complete quantity that is being billed and a comprehensive account of the merchandise or services that yield that complete. Also, it's crucial to make sure there is clear advice about the way the invoice ought to be compensated, whether you will find payment plan options, or choices concerning the kinds of payment that are accepted, along with a due date by when the fees on the invoice needs to be compensated. You might even add thorough information about every item in your invoice, making it easy for your clients to reconcile and pay punctually.
Template provides a brief and effortless understanding about the buying and selling of merchandise and services by the contractor. The printable template lets you store any info you type into the industrial statement form so you don't need to retype the exact same info each moment. You can download the easy invoice template from our site free of fuss. There are numerous bill templates it is possible to download according to your enterprise needs and your clientele.
Consult with the catering invoice template discussed above to find out what's the correct format and design of creating a catering invoice. On occasion, a statement format could be exceedingly straightforward. During the help a Catering Invoice Format offers you're going to have the ability to make the invoices which you have to have in a professional way.
Invoices can be located in several styles in distinct formats. They are available in a variety of formats and, even though they follow a certain hierarchical sense, computer methods struggle to accurately extract fine grained information points. You will be able to view how the finished invoice will appear. Finding the proper invoice for your little company is necessary.
You've got to include recipient and invoice details. All proforma invoices should be legal for a specific period. The invoice is used to monitor the selling of products or solutions, though a receipt serves as a documentation to the purchaser that the quantity of the product was paid. A Catering Invoice will have the ability to enable you to make each statement you have to possess in a quick and straightforward fashion and it will be able to assist you to do that in a way that has a professional look, too.
Every statement you send should have a distinctive number, and replicate numbers ought to be averted. You want to produce invoices that are easy to read and understand, and you will do that through the aid a template supplies you. It's elegantly straightforward and simple to use while letting you create stunningly beautiful invoices quickly.
The invoice can be straightforward in your nation. A commercial invoice is a global shipping document given by the provider to the client detailing the type and value of products sold. By applying the business invoice template of the site, it's likely to become simple to send comprehensive small company invoices that will reduce confusion and get you the appropriate payment.
The template contains specifics about the overall profile of their organization, client and the builder. Free bill templates are a very great means to begin, but it takes more to run a firm. Our completely free service statement template has been specifically intended for consultants, service providers, along with other little businesses.
As soon as you've selected the template in accord with your need, you are able to modify the plan by selecting a different design or including a symbol. After you have selected the template that fits with your needs, you can fix the program by choosing different colors or including a symbol. In the event the default template does not consist of entire information then you have to display or add more fields to your template. It can be very tough and frustrating to originally create your template, so make sure you start with a present template. You are able to easily download the below easily available contractor invoice templates which are free and get the best of invoices that will be turned into quickly just with a mixture of templates.
There is not anybody golden resume format. After you format your resume you wish to be certain that your leave enough margin space to permit for printing. They key is to format the info in a style that makes it easy to scan and recognise your project objectives and applicable credentials, Augustine tells us. Before you are able to understand which of the best resume formats will do the work best for you it is very important that you comprehend the aim of each one of those formats. You ought to think about using the chronological format when you've got a strong work history and academic achievements. Chronological Resume Format is quite popular with a great deal of companies. Its high-quality PSD document is completely customizable.
It's possible for you to utilize our professionally created resume templates and one-of-a-kind resume builder to quickly and easily set a standout resume. Its content-packed design is excellent for engaging employers with a variety of facts, from experience to instruction. Thus you ought to be aware of just how to decide on the best resume layout before putting in an application for a job interview!
A workout application is the secret to unlocking livelihood opportunities. Writing a powerful application is a difficult process and we would like to make it easier. It is important that your project program has to be encouraged with a suitable cover letter when you are asking for job and especially applying to obtain an advertising job.
Possessing a very professional and clear resume is important in the healthcare field. The very first thing you need to do in order to generate the Resume Example resume is picking a paper. Free Resume examples are similar to samples of resumes with the proper format, the appropriate structure and the suitable content details.
By the info you are able to collect here you will see it's not quite as difficult to learn how to compose a resume as you may have thought. Make certain you receive the information that you're looking for. Task lists aren't likely to cause you to stick out in the crowd. For experienced accounting professionals, it is advised to look in a listing of essential skills.
You should discover ways to accommodate your expertise to the demands of the new job and set it in this manner that your strengths are displayed on top. Work experience also needs to be in reverse chronological order. Make certain you highlight technical knowledge and certifications. Additionally, you could eliminate the capability to negotiate fair payment.
Read resume posts focused on particular aspects of job searching and the best way to use your own resume. Resume objectives may be somewhat controversial.
Knowing how to tailor your resume will clearly give you an benefit. Resumes are much greater than merely a very simple collection of your previous responsibilities. Writing a resume for job in the art business can be hard.
You can not compose one resume for quite a few kinds of tasks. The very last thing you wish to do is get a job which you can't do. Unless you're a graphic designer looking for employment, there is no need to over think the plan of your one-pager. Particular kinds of work in the food service industry require resumes with several of the exact same abilities. Simply be certain, whichever format you decide on, and that you are targeting it to your occupation which you're applying to. A workout program is merely another opportunity to promote your expertise, therefore be sure that you record the credentials which are linked with the occupation for which you are applying.
Your tasks might be diverse, your expertise and skills vast, and it can be difficult getting it all on paper. Therefore, if you're truly interested in finding job, then you ought to start looking for the aid of such on-line companies. Therefore, if you are really interested in finding job, then you want to find the help of such on-line companies. You only ought to place tasks on a resume that relate to the role accessible. If you are hunting for a job as an accounts executive, your resume must show you possess the wisdom and expertise essential to advise clients and manage finances.
A solicited correspondence of an application is simply one that is usually written to turn in an application for a situation that's advertised in only about any type of media by means of a potential small business enterprise. Before start writing you program letter, be aware of everything the supplier is hunting for and if you are a perfect match for your company culture. For this, you have to deliver internship app letters into your wide selection of organizations who may be ready to hire you as an intern.
The program ought to be organized and the applicant has to be honest whilst writing scholarship application. If you are rejecting work application, as an instance, provide a genuine reason for it and encourage the recipient to apply for different positions. Now that your program is finished, the previous presentation suggestion would be to place all of your things in a transparent plastic folder, using a slide locking binder. You do not understand which scholarship program will be the one which succeeds. As a result, their scholarship applications become rejected. There is nobody strategy to write a winning scholarship application.
If you're asking for a scholarship, chances are you're very likely to need to write an essay. A scholarship is very vital for aspiring students who would like to get into an institution but don't have the financial ability to attain that. If you are planning to devote an application for a college scholarship, you'll probably will need to submit an article along with a resume, transcript and other background info.
All scholarships have different requirements for the program procedure. Mention how you think you're qualified for the scholarship. A detailed scholarship resume template you'll have the ability to use to create your resume.
With the ever-rising price of schooling, scholarships have come to be the only feasible choice for many students to find a fantastic education without paying a gruesome cost for this. A scholarship resume is a fairly important part of your scholarship program to allow you to stick out from a number of additional competitive applicants. Most colleges have a tendency to search for the specific same primary things and proceed to the specifics.
Scholarship is called for by everybody who would like to find some superb instruction at quality speed. Some scholarships may also supply help not only for the payment of tuition fees but also discounts on miscellaneous fees as well as the provision for publication allowances and other financial aid for all those scholars as long because they could keep the grade point average which they're required to have every conclusion of this semester. It is very important to notice here that each scholarship differs and, so, has slightly different criteria they want to get. If you are asking for a college student, you may frequently be requested to compose a letter to go to your application.
In case the letter is requesting funding for a program you have been accepted to, there are a couple of aspects you should consider. You may also see scholarship letters. Ensure you deal with scholarship letter to the appropriate individual or committee. Writing letters of recommendation is never a simple endeavor. An excellent recommendation letter can help you distinguish yourself among other fellowship applicants.
The inspiration letter is a vital portion of your program. If your correspondence was created for the first scenario, you must start with an official address and compliments, followed by a fast paragraph demonstrating your academic abilities, accomplishments and strengths. You might also see thesis letters. It's going be essential that you know how to compose a formal and expert scholarship acceptance letter to allow the students know that their programs are successful.
If you are, then you ought to have experienced firsthand how hard it is to write an application letter. For instance, an individual might want to write a program letter to find work or perhaps to create an application for leave of absence. Program letter is a substantial sort of letters utilized in several areas of lifestyle for fulfillment of plenty of distinct demands. A sample program letter can be found here that can be employed to write application letters for several unique factors.
Letters may be handwritten or typed, but should be limited to a single page. Regardless, all of your letters must get a clear subject line which clarifies your objective. It is also possible to see program letters. What's more, your application letter expresses your goal to produce a program for a particular job within an organization, company or company. A program letter is among the very first few things you'll have to prepare yourself when applying for employment. The letter of application is intended to supply in depth info on why are you are a skilled candidate for the job that you're applying for.
Templates might be used for making CV, resume so as to apply for jobs. Creating beautiful ebooks, however, can be simpler if you have got an ebook . It can be very hard and frustrating to initially create your template, so ensure you begin with a current template. Invoice templates are incredibly beneficial for creating statements. Free bill templates are a excellent technique to start, but it requires more to run an organization. The considerable Sky travel brochure template is quite adaptable, with a combination of elements that could be customized to equal the look and feel of the destination.
Click Install to automatically select the template you have to use. Choose the template that you want to use and click on OK. The template will subsequently open and you are able to begin working. Unique varieties of bill templates may be used by different types of companies. Excel bill templates are extremely user-friendly. They have many advantages which can be crucial in creating a bill. Apparently, there are various free contract templates easily available online.
Spreadsheets are simple for anyone to create and understand. Although whether you would like to create spreadsheets and like to use it, we've many excellent alternatives for you! In addition, if you would rather create spreadsheets and just have to apply it, then you've got many wonderful options for you! A spreadsheet may be great enough for you in the event you have simple requirements such as I did. The tricky part is that if you're reconciling your spreadsheet by way of your bank statement, you wish to remember that the entire balance shown on the statement is that the amount of the total accounts in addition to the sub-accounts.
If you're not ready for a budget and just want an easy way to keep an eye on your money, try out the new Money Tracker template. Setting a budget is completely necessary to attaining your financial goals. It will afford you the chance to know where your money is being spent and choose whether you would like it to continue being invested in that manner or make a shift. Putting a budget together wants a resource which assists you to arrange your finances. It will let you begin planning for your retirement and that is absolutely essential to attaining retirement. Creating a weekly funding is a very good method of ensuring your fiscal equilibrium. Possessing a working funds set up will make it possible for you to identify exactly where you stand with your financing. A superb strategy also needs to detect the probable risks and trials. Following the initial plan is finished, enter an overview of particular tasks alongside deadlines to some calendar program to get reminders of upcoming goals. Planning and scheduling go together.
Don't rush if you are unsure what you require. What's great about the bi-weekly budget template is how it's simple to use functionality. It is very tricky to generate the invoice appear professional, even if using professionally designed invoice templates.
It is possible to save the templates in the present folders or you're going to have the ability to make a new folder. Everything in a template is totally customizable. Should you elect for the templates, then you are still in a place to personalize them, and you're likely to be in a place to obtain a distinctive looking publication, and therefore don't worry about that. You will find an assortment of totally free budget templates available to fit your requirements, while it's for college students, parents with children in daycare, single-income households, and so on.
Your Invoices would be the previous interaction customers have with your organization. You'll have the ability to see how the finished statement will appear.
If you'd like an extremely simple ways to build yourself a lasting and dependable home budget planner. Be careful to make certain you're able to export your information from the program you select. As soon as you extract data in the form of tables from PDFs, for example, you might need to appear through the information to locate errors. It is likely to establish a table with formulas and items with hardly any work. Occasionally lining up all our invoices and seeing that the monthly total may be an immense frightening. The matter of the way to define the order of things in a BOM is one of the most frequently asked.
Choose the area in somebody's lifetime which you are considering receiving the biography to revolve round. When you're hired, you may be asked to get a biography from the new employer, to be used in their marketing materials. What to Write in Your Business Biography When you write a biography you need to construct your credentials right from the start. If you're writing a company biography about yourself, you will want to sort out that advice you ought to be using that is likely to produce your readers interested in you and compel them to reach out to you to know more regarding your work and your abilities.
Wrong advice in the bio may be a substantial barrier to obtaining success to your organization. A significant point all the information that you personally indicate in your organization enterprise bio needs to be authentic. Be certain you find information which can help you tell a story that many people will adore. Along with the achievements in business activities, is critical to supply information about additional work that was conducted during the firm's existence. It is crucial to give information regarding promotions, awards, positive changes in the business, and achievements. It is simply an account of somebody's life written by another person. The very first biography is a short variant," which comprises a mixture of bullet points listing his credentials and numerous brief paragraphs. A superb biography might have a more significant influence on your company than you've imagined. There is something that most individuals do not actually know about inventing biographies which sound really impressive. A biography is meant to examine an individual's life and translate it also. Writing a dancing biography about yourself is probably one of the simpler things you will have to do.
Your bio should be easily accessible to anybody who may be curious about you. This type of bio needs to be employed with the stable development of a company. Your bio should exhibit your achievements as an artist, however, additionally, it shows what you are like as someone, so be certain you devote some authentic work and thought to the job. When you have a bio you're comfortable with, don't forget that it isn't set in stone. You are going to need to continue to maintain your bio as short as possible, only including the info that should be included. A specialist bio is merely a story-based version of this info you would ordinarily include in a resume. Nobody will be contemplating an illiterate small company bio.
Guide students to view the way the very first person perspective within an autobiography helps readers comprehend how in which the narrator feels about events. In fact, it becomes really funny at parts. Choose the individual you'd like to write about Hunt for the basic facts which are associated with an individual's life. 1 thought is to start each with a verb in precisely the exact same tense. After reading the diagnosis of a company biography sample, you will find an idea based on which strategy the biography is written, but it is essential to define the information taking into account your specific case.
There are lots of questions to answer, however, the questions are here so as that will help you start and are more for prep. As you stated, you're not able to answer that question in case you don't know who the other candidates are. Additionally, you must incorporate a summary of business services. Adhere to the facts, by which the customer is going to get proper comprehension of the strengths of your organization.
The structure isn't quite as formal, and it gives you a chance to highlight some intriguing facts about your self while injecting a bit of your personality. The format of a business bio is a way of organizing data and placing the crucial accents. There are lots of formats you may utilize to write a good bio, but there are a few universal elements you're going to have the ability to use to make it even more effective. By studying brief small company bio examples, you'll learn to correctly compose text. Distinctive Documents provides our clients an entire range of job search-related writing services to meet all work search requirements. The contents of your bio can subsequently be customized based on how you're going to use it, our biography writing services can assist you.
The samples can allow you to fix the situation when a bio is not informative for the reader. Confirm that you desire a template in case you don't actually know how to present the purpose of the business. A specialist biography template is meant to lower the practice of producing printed professional biographies especially for an individual who would like to reach a lot of individuals or associations.


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How Can a Homeowner Gain "Energy Independence"?
Global Energy Independence Day is an annual celebration in July. While scientists around the world study how to make countries and cities energy independent, this day may not have much meaning to an individual homeowner. However, energy independence is important and possible for your home.
What is energy independence?
Attaining energy independence means that a system can fulfill its own energy needs. This can mean avoiding reliance on non-renewable energy sources or focusing on energy efficiency. A completely self-sustaining system is the
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