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  • 1. Section I. Basic PrinciplesChapter 1. IntroductionDefinitionsPharmacology can be defined as the study of substances that interact with living systems throughchemical processes, especially by binding to regulatory molecules and activating or inhibitingnormal body processes. These substances may be chemicals administered to achieve a beneficialtherapeutic effect on some process within the patient or for their toxic effects on regulatoryprocesses in parasites infecting the patient. Such deliberate therapeutic applications may beconsidered the proper role of medical pharmacology, which is often defined as the science ofsubstances used to prevent, diagnose, and treat disease. Toxicology is that branch of pharmacologywhich deals with the undesirable effects of chemicals on living systems, from individual cells tocomplex ecosystems.History of PharmacologyPrehistoric people undoubtedly recognized the beneficial or toxic effects of many plant and animalmaterials. The earliest written records from China and from Egypt list remedies of many types,including a few still recognized today as useful drugs. Most, however, were worthless or actuallyharmful. In the 2500 years or so preceding the modern era there were sporadic attempts to introducerational methods into medicine, but none were successful owing to the dominance of systems ofthought that purported to explain all of biology and disease without the need for experimentationand observation. These schools promulgated bizarre notions such as the idea that disease wascaused by excesses of bile or blood in the body, that wounds could be healed by applying a salve tothe weapon that caused the wound, and so on.Around the end of the 17th century, reliance on observation and experimentation began to replacetheorizing in medicine, following the example of the physical sciences. As the value of thesemethods in the study of disease became clear, physicians in Great Britain and on the Continentbegan to apply them to the effects of traditional drugs used in their own practices. Thus, materiamedica—the science of drug preparation and the medical use of drugs—began to develop as theprecursor to pharmacology. However, any understanding of the mechanisms of action of drugs wasprevented by the absence of methods for purifying active agents from the crude materials that wereavailable and—even more—by the lack of methods for testing hypotheses about the nature of drugactions.
  • 2. In the late 18th and early 19th centuries, François Magendie and later his student Claude Bernardbegan to develop the methods of experimental animal physiology and pharmacology. Advances inchemistry and the further development of physiology in the 18th, 19th, and early 20th centuries laidthe foundation needed for understanding how drugs work at the organ and tissue levels.Paradoxically, real advances in basic pharmacology during this time were accompanied by anoutburst of unscientific promotion by manufacturers and marketers of worthless "patent medicines."It was not until the concepts of rational therapeutics, especially that of the controlled clinical trial,were reintroduced into medicine—about 50 years ago—that it became possible to accuratelyevaluate therapeutic claims.About 50 years ago, there also began a major expansion of research efforts in all areas of biology.As new concepts and new techniques were introduced, information accumulated about drug actionand the biologic substrate of that action, the receptor. During the last half-century, manyfundamentally new drug groups and new members of old groups were introduced. The last 3decades have seen an even more rapid growth of information and understanding of the molecularbasis for drug action. The molecular mechanisms of action of many drugs have now been identified,and numerous receptors have been isolated, structurally characterized, and cloned. In fact, the use ofreceptor identification methods (described in Chapter 2: Drug Receptors & Pharmacodynamics) hasled to the discovery of many orphan receptors—receptors for which no ligand has been discoveredand whose function can only be surmised. Studies of the local molecular environment of receptorshave shown that receptors and effectors do not function in isolation—they are strongly influencedby companion regulatory proteins. Decoding of the genomes of many species—from bacteria tohumans—has led to the recognition of unsuspected relationships between receptor families.Pharmacogenomics—the relation of the individuals genetic makeup to his or her response tospecific drugs—is close to becoming a practical area of therapy (see Pharmacology & Genetics).Much of that progress is summarized in this resource.The extension of scientific principles into everyday therapeutics is still going on, though themedication-consuming public, unfortunately, is still exposed to vast amounts of inaccurate,incomplete, or unscientific information regarding the pharmacologic effects of chemicals. This hasresulted in the faddish use of innumerable expensive, ineffective, and sometimes harmful remediesand the growth of a huge "alternative health care" industry. Conversely, lack of understanding ofbasic scientific principles in biology and statistics and the absence of critical thinking about publichealth issues has led to rejection of medical science by a segment of the public and a commontendency to assume that all adverse drug effects are the result of malpractice. Two generalprinciples that the student should always remember are, first, that all substances can under certaincircumstances be toxic; and second, that all therapies promoted as health-enhancing should meet thesame standards of evidence of efficacy and safety, ie, there should be no artificial separationbetween scientific medicine and "alternative" or "complementary" medicine.Pharmacology & GeneticsDuring the last 5 years, the genomes of humans, mice, and many other organisms have beendecoded in considerable detail. This has opened the door to a remarkable range of new approachesto research and treatment. It has been known for centuries that certain diseases are inherited, and wenow understand that individuals with such diseases have a heritable abnormality in their DNA. It isnow possible in the case of some inherited diseases to define exactly which DNA base pairs areanomalous and in which chromosome they appear. In a small number of animal models of suchdiseases, it has been possible to correct the abnormality by "gene therapy," ie, insertion of anappropriate "healthy" gene into somatic cells. Human somatic cell gene therapy has been attempted,but the technical difficulties are great.
  • 3. Studies of a newly discovered receptor or endogenous ligand are often confounded by incompleteknowledge of the exact role of that receptor or ligand. One of the most powerful of the new genetictechniques is the ability to breed animals (usually mice) in which the gene for the receptor or itsendogenous ligand has been "knocked out," ie, mutated so that the gene product is absent ornonfunctional. Homozygous "knockout" mice will usually have complete suppression of thatfunction, while heterozygous animals will usually have partial suppression. Observation of thebehavior, biochemistry, and physiology of the knockout mice will often define the role of themissing gene product very clearly. When the products of a particular gene are so essential that evenheterozygotes do not survive to birth, it is sometimes possible to breed "knockdown" versions withonly limited suppression of function. Conversely, "knockin" mice have been bred that overexpresscertain receptors of interest.Some patients respond to certain drugs with greater than usual sensitivity. (Such variations arediscussed in Chapter 4: Drug Biotransformation.) It is now clear that such increased sensitivity isoften due to a very small genetic modification that results in decreased activity of a particularenzyme responsible for eliminating that drug. Pharmacogenomics (or pharmacogenetics) is thestudy of the genetic variations that cause individual differences in drug response. Future cliniciansmay screen every patient for a variety of such differences before prescribing a drug.The Nature of DrugsIn the most general sense, a drug may be defined as any substance that brings about a change inbiologic function through its chemical actions. In the great majority of cases, the drug moleculeinteracts with a specific molecule in the biologic system that plays a regulatory role. This moleculeis called a receptor. The nature of receptors is discussed more fully in Chapter 2: Drug Receptors &Pharmacodynamics. In a very small number of cases, drugs known as chemical antagonists mayinteract directly with other drugs, while a few other drugs (eg, osmotic agents) interact almostexclusively with water molecules. Drugs may be synthesized within the body (eg, hormones) ormay be chemicals not synthesized in the body, ie, xenobiotics (from Gr xenos "stranger"). Poisonsare drugs. Toxins are usually defined as poisons of biologic origin, ie, synthesized by plants oranimals, in contrast to inorganic poisons such as lead and arsenic.In order to interact chemically with its receptor, a drug molecule must have the appropriate size,electrical charge, shape, and atomic composition. Furthermore, a drug is often administered at alocation distant from its intended site of action, eg, a pill given orally to relieve a headache.Therefore, a useful drug must have the necessary properties to be transported from its site ofadministration to its site of action. Finally, a practical drug should be inactivated or excreted fromthe body at a reasonable rate so that its actions will be of appropriate duration.The Physical Nature of DrugsDrugs may be solid at room temperature (eg, aspirin, atropine), liquid (eg, nicotine, ethanol), orgaseous (eg, nitrous oxide). These factors often determine the best route of administration. Forexample, some liquid drugs are easily vaporized and can be inhaled in that form, eg, halothane,amyl nitrite. The most common routes of administration are listed in Table 3–3. The various classesof organic compounds—carbohydrates, proteins, lipids, and their constituents—are all representedin pharmacology. Many drugs are weak acids or bases. This fact has important implications for theway they are handled by the body, because pH differences in the various compartments of the bodymay alter the degree of ionization of such drugs (see below).Drug Size
  • 4. The molecular size of drugs varies from very small (lithium ion, MW 7) to very large (eg, alteplase[t-PA], a protein of MW 59,050). However, the vast majority of drugs have molecular weightsbetween 100 and 1000. The lower limit of this narrow range is probably set by the requirements forspecificity of action. In order to have a good "fit" to only one type of receptor, a drug moleculemust be sufficiently unique in shape, charge, etc, to prevent its binding to other receptors. Toachieve such selective binding, it appears that a molecule should in most cases be at least 100 MWunits in size. The upper limit in molecular weight is determined primarily by the requirement thatdrugs be able to move within the body (eg, from site of administration to site of action). Drugsmuch larger than MW 1000 will not diffuse readily between compartments of the body (seePermeation, below). Therefore, very large drugs (usually proteins) must be administered directlyinto the compartment where they have their effect. In the case of alteplase, a clot-dissolvingenzyme, the drug is administered directly into the vascular compartment by intravenous infusion.Drug Reactivity and Drug-Receptor BondsDrugs interact with receptors by means of chemical forces or bonds. These are of three major types:covalent, electrostatic, and hydrophobic. Covalent bonds are very strong and in many cases notreversible under biologic conditions. Thus, the covalent bond formed between the activated form ofphenoxybenzamine and the receptor for norepinephrine (which results in blockade of the receptor)is not readily broken. The blocking effect of phenoxybenzamine lasts long after the free drug hasdisappeared from the bloodstream and is reversed only by the synthesis of new receptors, aprocess that takes about 48 hours. Other examples of highly reactive, covalent bond-forming drugsare the DNA-alkylating agents used in cancer chemotherapy to disrupt cell division in the neoplastictissue.Electrostatic bonding is much more common than covalent bonding in drug-receptor interactions.Electrostatic bonds vary from relatively strong linkages between permanently charged ionicmolecules to weaker hydrogen bonds and very weak induced dipole interactions such as van derWaals forces and similar phenomena. Electrostatic bonds are weaker than covalent bonds.Hydrophobic bonds are usually quite weak and are probably important in the interactions of highlylipid-soluble drugs with the lipids of cell membranes and perhaps in the interaction of drugs withthe internal walls of receptor "pockets."The specific nature of a particular drug-receptor bond is of less practical importance than the factthat drugs which bind through weak bonds to their receptors are generally more selective than drugswhich bind through very strong bonds. This is because weak bonds require a very precise fit of thedrug to its receptor if an interaction is to occur. Only a few receptor types are likely to provide sucha precise fit for a particular drug structure. Thus, if we wished to design a highly selective short-acting drug for a particular receptor, we would avoid highly reactive molecules that form covalentbonds and instead choose molecules that form weaker bonds.A few substances that are almost completely inert in the chemical sense nevertheless havesignificant pharmacologic effects. For example, xenon, an "inert gas," has anesthetic effects atelevated pressures.Drug ShapeThe shape of a drug molecule must be such as to permit binding to its receptor site. Optimally, thedrugs shape is complementary to that of the receptor site in the same way that a key iscomplementary to a lock. Furthermore, the phenomenon of chirality (stereoisomerism) is so
  • 5. common in biology that more than half of all useful drugs are chiral molecules, ie, they exist asenantiomeric pairs. Drugs with two asymmetric centers have four diastereomers, eg, ephedrine, asympathomimetic drug. In the great majority of cases, one of these enantiomers will be much morepotent than its mirror image enantiomer, reflecting a better fit to the receptor molecule. Forexample, the (S)(+) enantiomer of methacholine, a parasympathomimetic drug, is over 250 timesmore potent than the (R)(–) enantiomer. If one imagines the receptor site to be like a glove intowhich the drug molecule must fit to bring about its effect, it is clear why a "left-oriented" drug willbe more effective in binding to a left-hand receptor than will its "right-oriented" enantiomer.The more active enantiomer at one type of receptor site may not be more active at another type, eg,a receptor type that may be responsible for some unwanted effect. For example, carvedilol, a drugthat interacts with adrenoceptors, has a single chiral center and thus two enantiomers (Table 1–1).One of these enantiomers, the (S)(–) isomer, is a potent -receptor blocker. The (R)(+) isomer is100-fold weaker at the receptor. However, the isomers are approximately equipotent as -receptorblockers. Ketamine is an intravenous anesthetic. The (+) enantiomer is a more potent anesthetic andis less toxic than the (–) enantiomer. Unfortunately, the drug is still used as the racemic mixture.Table 1–1. Dissociation Constants (Kd) of the Enantiomers and Racemate of Carvedilol.1Form of Inverse of Affinity for Receptors Inverse of Affinity for ReceptorsCarvedilol (Kd, nmol/L) (Kd, nmol/L)R(+) enantiomer 14 45S(–) enantiomer 16 0.4R,S(+/–) 11 0.9enantiomersNote: The Kd is the concentration for 50% saturation of the receptors and is inversely proportionateto the affinity of the drug for the receptors.1 Data from Ruffolo RR et al: The pharmacology of carvedilol. Eur J Pharmacol 1990;38:S82.Finally, because enzymes are usually stereoselective, one drug enantiomer is often more susceptiblethan the other to drug-metabolizing enzymes. As a result, the duration of action of one enantiomermay be quite different from that of the other.Unfortunately, most studies of clinical efficacy and drug elimination in humans have been carriedout with racemic mixtures of drugs rather than with the separate enantiomers. At present, only about45% of the chiral drugs used clinically are marketed as the active isomer—the rest are availableonly as racemic mixtures. As a result, many patients are receiving drug doses of which 50% or moreis either inactive or actively toxic. However, there is increasing interest—at both the scientific andthe regulatory levels—in making more chiral drugs available as their active enantiomers.Rational Drug DesignRational design of drugs implies the ability to predict the appropriate molecular structure of a drugon the basis of information about its biologic receptor. Until recently, no receptor was known in
  • 6. sufficient detail to permit such drug design. Instead, drugs were developed through random testingof chemicals or modification of drugs already known to have some effect (see Chapter 5: Basic &Clinical Evaluation of New Drugs). However, during the past 3 decades, many receptors have beenisolated and characterized. A few drugs now in use were developed through molecular design basedon a knowledge of the three-dimensional structure of the receptor site. Computer programs are nowavailable that can iteratively optimize drug structures to fit known receptors. As more becomesknown about receptor structure, rational drug design will become more feasible.Receptor NomenclatureThe spectacular success of newer, more efficient ways to identify and characterize receptors (seeChapter 2: Drug Receptors & Pharmacodynamics, How Are New Receptors Discovered?) hasresulted in a variety of differing systems for naming them. This in turn has led to a number ofsuggestions regarding more rational methods of naming them. The interested reader is referred fordetails to the efforts of the International Union of Pharmacology (IUPHAR) Committee on ReceptorNomenclature and Drug Classification (reported in various issues of Pharmacological Reviews)and to the annual Receptor and Ion Channel Nomenclature Supplements published as special issuesby the journal Trends in Pharmacological Sciences (TIPS). The chapters in this book mainly usethese sources for naming receptors.Drug-Body InteractionsThe interactions between a drug and the body are conveniently divided into two classes. The actionsof the drug on the body are termed pharmacodynamic processes; the principles ofpharmacodynamics are presented in greater detail in Chapter 2: Drug Receptors &Pharmacodynamics. These properties determine the group in which the drug is classified and oftenplay the major role in deciding whether that group is appropriate therapy for a particular symptomor disease. The actions of the body on the drug are called pharmacokinetic processes and aredescribed in Chapters 3 and 4. Pharmacokinetic processes govern the absorption, distribution, andelimination of drugs and are of great practical importance in the choice and administration of aparticular drug for a particular patient, eg, one with impaired renal function. The followingparagraphs provide a brief introduction to pharmacodynamics and pharmacokinetics.Pharmacodynamic PrinciplesAs noted above, most drugs must bind to a receptor to bring about an effect. However, at themolecular level, drug binding is only the first in what is often a complex sequence of steps.Types of Drug-Receptor InteractionsAgonist drugs bind to and activate the receptor in some fashion, which directly or indirectly bringsabout the effect. Some receptors incorporate effector machinery in the same molecule, so that drugbinding brings about the effect directly, eg, opening of an ion channel or activation of enzymeactivity. Other receptors are linked through one or more intervening coupling molecules to aseparate effector molecule. The several types of drug-receptor-effector coupling systems arediscussed in Chapter 2: Drug Receptors & Pharmacodynamics. Pharmacologic antagonist drugs, bybinding to a receptor, prevent binding by other molecules. For example, acetylcholine receptorblockers such as atropine are antagonists because they prevent access of acetylcholine and similaragonist drugs to the acetylcholine receptor and they stabilize the receptor in its inactive state. Theseagents reduce the effects of acetylcholine and similar drugs in the body.
  • 7. "Agonists" That Inhibit Their Binding Molecules and Partial AgonistsSome drugs mimic agonist drugs by inhibiting the molecules responsible for terminating the actionof an endogenous agonist. For example, acetylcholinesterase inhibitors, by slowing the destructionof endogenous acetylcholine, cause cholinomimetic effects that closely resemble the actions ofcholinoceptor agonist molecules even though cholinesterase inhibitors do not—or only incidentallydo—bind to cholinoceptors (see Chapter 7: Cholinoceptor-Activating & Cholinesterase-InhibitingDrugs). Other drugs bind to receptors and activate them but do not evoke as great a response as so-called full agonists. Thus, pindolol, a adrenoceptor "partial agonist," may act as either an agonist(if no full agonist is present) or as an antagonist (if a full agonist such as isoproterenol is present).(See Chapter 2: Drug Receptors & Pharmacodynamics.)Duration of Drug ActionTermination of drug action at the receptor level results from one of several processes. In somecases, the effect lasts only as long as the drug occupies the receptor, so that dissociation of drugfrom the receptor automatically terminates the effect. In many cases, however, the action maypersist after the drug has dissociated, because, for example, some coupling molecule is still presentin activated form. In the case of drugs that bind covalently to the receptor, the effect may persistuntil the drug-receptor complex is destroyed and new receptors are synthesized, as describedpreviously for phenoxybenzamine. Finally, many receptor-effector systems incorporatedesensitization mechanisms for preventing excessive activation when agonist molecules continue tobe present for long periods. See Chapter 2: Drug Receptors & Pharmacodynamics for additionaldetails.Receptors and Inert Binding SitesTo function as a receptor, an endogenous molecule must first be selective in choosing ligands (drugmolecules) to bind; and second, it must change its function upon binding in such a way that thefunction of the biologic system (cell, tissue, etc) is altered. The first characteristic is required toavoid constant activation of the receptor by promiscuous binding of many different ligands. Thesecond characteristic is clearly necessary if the ligand is to cause a pharmacologic effect. The bodycontains many molecules that are capable of binding drugs, however, and not all of theseendogenous molecules are regulatory molecules. Binding of a drug to a nonregulatory moleculesuch as plasma albumin will result in no detectable change in the function of the biologic system, sothis endogenous molecule can be called an inert binding site. Such binding is not completelywithout significance, however, since it affects the distribution of drug within the body and willdetermine the amount of free drug in the circulation. Both of these factors are of pharmacokineticimportance (see below and Chapter 3: Pharmacokinetics & Pharmacodynamics: Rational Dosing &the Time Course of Drug Action).Pharmacokinetic PrinciplesIn practical therapeutics, a drug should be able to reach its intended site of action afteradministration by some convenient route. In some cases, a chemical that is readily absorbed anddistributed is administered and then converted to the active drug by biologic processes—inside thebody. Such a chemical is called a prodrug. In only a few situations is it possible to directly apply adrug to its target tissue, eg, by topical application of an anti-inflammatory agent to inflamed skin ormucous membrane. Most often, a drug is administered into one body compartment, eg, the gut, andmust move to its site of action in another compartment, eg, the brain. This requires that the drug beabsorbed into the blood from its site of administration and distributed to its site of action,
  • 8. permeating through the various barriers that separate these compartments. For a drug given orallyto produce an effect in the central nervous system, these barriers include the tissues that comprisethe wall of the intestine, the walls of the capillaries that perfuse the gut, and the "blood-brainbarrier," the walls of the capillaries that perfuse the brain. Finally, after bringing about its effect, adrug should be eliminated at a reasonable rate by metabolic inactivation, by excretion from thebody, or by a combination of these processes.PermeationDrug permeation proceeds by four primary mechanisms. Passive diffusion in an aqueous or lipidmedium is common, but active processes play a role in the movement of many drugs, especiallythose whose molecules are too large to diffuse readily.Aqueous DiffusionAqueous diffusion occurs within the larger aqueous compartments of the body (interstitial space,cytosol, etc) and across epithelial membrane tight junctions and the endothelial lining of bloodvessels through aqueous pores that—in some tissues—permit the passage of molecules as large asMW 20,000–30,000.** The capillaries of the brain, the testes, and some other tissues are characterized by absence of thepores that permit aqueous diffusion of many drug molecules into the tissue. They may also containhigh concentrations of drug export pumps (MDR pumps; see text). These tissues are therefore"protected" or "sanctuary" sites from many circulating drugs.Aqueous diffusion of drug molecules is usually driven by the concentration gradient of thepermeating drug, a downhill movement described by Ficks law (see below). Drug molecules thatare bound to large plasma proteins (eg, albumin) will not permeate these aqueous pores. If the drugis charged, its flux is also influenced by electrical fields (eg, the membrane potential and—in partsof the nephron—the transtubular potential).Lipid DiffusionLipid diffusion is the most important limiting factor for drug permeation because of the largenumber of lipid barriers that separate the compartments of the body. Because these lipid barriersseparate aqueous compartments, the lipid:aqueous partition coefficient of a drug determines howreadily the molecule moves between aqueous and lipid media. In the case of weak acids and weakbases (which gain or lose electrical charge-bearing protons, depending on the pH), the ability tomove from aqueous to lipid or vice versa varies with the pH of the medium, because chargedmolecules attract water molecules. The ratio of lipid-soluble form to water-soluble form for a weakacid or weak base is expressed by the Henderson-Hasselbalch equation (see below).Special CarriersSpecial carrier molecules exist for certain substances that are important for cell function and toolarge or too insoluble in lipid to diffuse passively through membranes, eg, peptides, amino acids,glucose. These carriers bring about movement by active transport or facilitated diffusion and, unlikepassive diffusion, are saturable and inhibitable. Because many drugs are or resemble such naturallyoccurring peptides, amino acids, or sugars, they can use these carriers to cross membranes.
  • 9. Many cells also contain less selective membrane carriers that are specialized for expelling foreignmolecules, eg, the P-glycoprotein or multidrug-resistance type 1 (MDR1) transporter found inthe brain, testes, and other tissues, and in some drug-resistant neoplastic cells. A similar transportmolecule, the multidrug resistance-associated protein-type 2 (MRP2) transporter, plays animportant role in excretion of some drugs or their metabolites into urine and bile.Endocytosis and ExocytosisA few substances are so large or impermeant that they can enter cells only by endocytosis, theprocess by which the substance is engulfed by the cell membrane and carried into the cell bypinching off of the newly formed vesicle inside the membrane. The substance can then be releasedinside the cytosol by breakdown of the vesicle membrane. This process is responsible for thetransport of vitamin B12, complexed with a binding protein (intrinsic factor), across the wall of thegut into the blood. Similarly, iron is transported into hemoglobin-synthesizing red blood cellprecursors in association with the protein transferrin. Specific receptors for the transport proteinsmust be present for this process to work. The reverse process (exocytosis) is responsible for thesecretion of many substances from cells. For example, many neurotransmitter substances are storedin membrane-bound vesicles in nerve endings to protect them from metabolic destruction in thecytoplasm. Appropriate activation of the nerve ending causes fusion of the storage vesicle with thecell membrane and expulsion of its contents into the extracellular space (see Chapter 6: Introductionto Autonomic Pharmacology).Ficks Law of DiffusionThe passive flux of molecules down a concentration gradient is given by Ficks law:where C1 is the higher concentration, C2 is the lower concentration, area is the area across whichdiffusion is occurring, permeability coefficient is a measure of the mobility of the drug molecules inthe medium of the diffusion path, and thickness is the thickness (length) of the diffusion path. In thecase of lipid diffusion, the lipid:aqueous partition coefficient is a major determinant of mobility ofthe drug, since it determines how readily the drug enters the lipid membrane from the aqueousmedium.Ionization of Weak Acids and Weak Bases; the Henderson-Hasselbalch EquationThe electrostatic charge of an ionized molecule attracts water dipoles and results in a polar,relatively water-soluble and lipid-insoluble complex. Since lipid diffusion depends on relativelyhigh lipid solubility, ionization of drugs may markedly reduce their ability to permeate membranes.A very large fraction of the drugs in use are weak acids or weak bases (Table 1–2). For drugs, aweak acid is best defined as a neutral molecule that can reversibly dissociate into an anion (anegatively charged molecule) and a proton (a hydrogen ion). For example, aspirin dissociates asfollows:
  • 10. Table 1–2. Ionization Constants of Some Common Drugs.Drug pKa1Weak acids Acetaminophen 9.5 Acetazolamide 7.2 Ampicillin 2.5 Aspirin 3.5 Chlorothiazide 6.8, 9.42 Chlorpropamide 5.0 Ciprofloxacin 6.09, 8.742 Cromolyn 2.0 Ethacrynic acid 2.5 Furosemide 3.9 Ibuprofen 4.4, 5.22 Levodopa 2.3 Methotrexate 4.8 Methyldopa 2.2, 9.22 Penicillamine 1.8 Pentobarbital 8.1 Phenobarbital 7.4 Phenytoin 8.3 Propylthiouracil 8.3 Salicylic acid 3.0 Sulfadiazine 6.5 Sulfapyridine 8.4 Theophylline 8.8 Tolbutamide 5.3 Warfarin 5.0Weak bases Albuterol (salbutamol) 9.3 Allopurinol 9.4, 12.3
  • 11. Alprenolol 9.6Amiloride 8.7Amiodarone 6.56Amphetamine 9.8Atropine 9.7Bupivacaine 8.1Chlordiazepoxide 4.6Chloroquine 10.8, 8.42Chlorpheniramine 9.2Chlorpromazine 9.3Clonidine 8.3Cocaine 8.5Codeine 8.2Cyclizine 8.2Desipramine 10.2Diazepam 3Dihydrocodeine 3Diphenhydramine 8.8Diphenoxylate 7.1Ephedrine 9.6Epinephrine 8.7Ergotamine 6.3Fluphenazine 8.0, 3.92Guanethidine 11.4, 8.32Hydralazine 7.1Imipramine 9.5Isoproterenol 8.6Kanamycin 7.2Lidocaine 7.9Metaraminol 8.6Methadone 8.4Methamphetamine 10.0Methyldopa 10.6Metoprolol 9.8Morphine 7.9
  • 12. Nicotine 7.9, 3.12 Norepinephrine 8.6 Pentazocine 7.9 Phenylephrine 9.8 Physostigmine 7.9, 1.82 Pilocarpine 6.9, 1.42 Pindolol 8.6 Procainamide 9.2 Procaine 9.0 Promazine 9.4 Promethazine 9.1 Propranolol 9.4 Pseudoephedrine 9.8 Pyrimethamine 7.0 Quinidine 8.5, 4.42 Scopolamine 8.1 Strychnine 8.0, 2.32 Terbutaline 10.1 Thioridazine 9.5 Tolazoline 10.6A drug that is a weak base can be defined as a neutral molecule that can form a cation (a positivelycharged molecule) by combining with a proton. For example, pyrimethamine, an antimalarial drug,undergoes the following association-dissociation process:Note that the protonated form of a weak acid is the neutral, more lipid-soluble form, whereas theunprotonated form of a weak base is the neutral form. The law of mass action requires that thesereactions move to the left in an acid environment (low pH, excess protons available) and to the rightin an alkaline environment. The Henderson-Hasselbalch equation relates the ratio of protonated tounprotonated weak acid or weak base to the molecules pKa and the pH of the medium as follows:
  • 13. This equation applies to both acidic and basic drugs. Inspection confirms that the lower the pHrelative to the pKa, the greater will be the fraction of drug in the protonated form. Because theuncharged form is the more lipid-soluble, more of a weak acid will be in the lipid-soluble form atacid pH, while more of a basic drug will be in the lipid-soluble form at alkaline pH.An application of this principle is in the manipulation of drug excretion by the kidney. Almost alldrugs are filtered at the glomerulus. If a drug is in a lipid-soluble form during its passage down therenal tubule, a significant fraction will be reabsorbed by simple passive diffusion. If the goal is toaccelerate excretion of the drug, it is important to prevent its reabsorption from the tubule. This canoften be accomplished by adjusting urine pH to make certain that most of the drug is in the ionizedstate, as shown in Figure 1–1. As a result of this pH partitioning effect, the drug will be "trapped" inthe urine. Thus, weak acids are usually excreted faster in alkaline urine; weak bases are usuallyexcreted faster in acidic urine. Other body fluids in which pH differences from blood pH may causetrapping or reabsorption are the contents of the stomach and small intestine; breast milk; aqueoushumor; and vaginal and prostatic secretions (Table 1–3).Figure 1–1.Trapping of a weak base (pyrimethamine) in the urine when the urine is more acidic than theblood. In the hypothetical case illustrated, the diffusible uncharged form of the drug hasequilibrated across the membrane but the total concentration (charged plus uncharged) in the urineis almost eight times higher than in the blood.Table 1–3. Body Fluids with Potential for Drug "Trapping" Through the pH-PartitioningPhenomenon.
  • 14. Body Fluid Range Total Fluid: Blood Total Fluid: Blood Concentration of pH Concentration Ratios for Ratios for Pyrimethamine (base, Sulfadiazine (acid, pKa 6.5)1 pKa 7.0)1Urine 5.0–8.0 0.12–4.65 72.24–0.79 2Breast milk 6.4–7.6 0.2–1.77 3.56–0.89Jejunum, 7.5–8.03 1.23–3.54 0.94–0.79ileumcontentsStomach 1.92– 0.114 85,993–18,386contents 2.592Prostatic 6.45– 0.21 3.25–1.0secretions 7.42Vaginal 3.4–4.23 0.114 2848–452secretions1 Body fluid protonated-to-unprotonated drug ratios were calculated using each of the pH extremescited; a blood pH of 7.4 was used for blood:drug ratio. For example, the steady-state urine:bloodratio for sulfadiazine is 0.12 at a urine pH of 5.0; this ratio is 4.65 at a urine pH of 8.0. Thus,sulfadiazine is much more effectively trapped and excreted in alkaline urine.2 Lentner C (editor): Geigy Scientific Tables, vol 1, 8th ed. Ciba Geigy, 1981.3 Bowman WC, Rand MJ: Textbook of Pharmacology, 2nd ed. Blackwell, 1980.4 Insignificant change in ratios over the physiologic pH range.As suggested by Table 1–2, a large number of drugs are weak bases. Most of these bases are amine-containing molecules. The nitrogen of a neutral amine has three atoms associated with it plus a pairof unshared electrons—see the display below. The three atoms may consist of one carbon(designated "R") and two hydrogens (a primary amine), two carbons and one hydrogen (asecondary amine), or three carbon atoms (a tertiary amine). Each of these three forms mayreversibly bind a proton with the unshared electrons. Some drugs have a fourth carbon-nitrogenbond; these are quaternary amines. However, the quaternary amine is permanently charged and hasno unshared electrons with which to reversibly bind a proton. Therefore, primary, secondary, andtertiary amines may undergo reversible protonation and vary their lipid solubility with pH, butquaternary amines are always in the poorly lipid-soluble charged form.Drug Groups
  • 15. To learn each pertinent fact about each of the many hundreds of drugs mentioned in this bookwould be an impractical goal and, fortunately, is in any case unnecessary. Almost all of the severalthousand drugs currently available can be arranged in about 70 groups. Many of the drugs withineach group are very similar in pharmacodynamic actions and often in their pharmacokineticproperties as well. For most groups, one or more prototype drugs can be identified that typify themost important characteristics of the group. This permits classification of other important drugs inthe group as variants of the prototype, so that only the prototype must be learned in detail and, forthe remaining drugs, only the differences from the prototype.Sources of InformationStudents who wish to review the field of pharmacology in preparation for an examination arereferred to Pharmacology: Examination and Board Review, by Trevor, Katzung, and Masters(McGraw-Hill, 2002) or USMLE Road Map: Pharmacology, by Katzung and Trevor (McGraw-Hill,2003).The references at the end of each chapter in this book were selected to provide information specificto those chapters.Specific questions relating to basic or clinical research are best answered by resort to the generalpharmacology and clinical specialty serials. For the student and the physician, three periodicals canbe recommended as especially useful sources of current information about drugs: The New EnglandJournal of Medicine, which publishes much original drug-related clinical research as well asfrequent reviews of topics in pharmacology; The Medical Letter on Drugs and Therapeutics, whichpublishes brief critical reviews of new and old therapies, mostly pharmacologic; and Drugs, whichpublishes extensive reviews of drugs and drug groups.Other sources of information pertinent to the USA should be mentioned as well. The "packageinsert" is a summary of information the manufacturer is required to place in the prescription salespackage; Physicians Desk Reference (PDR) is a compendium of package inserts published annuallywith supplements twice a year; Facts and Comparisons is a more complete loose-leaf druginformation service with monthly updates; and the USP DI (vol 1, Drug Information for the HealthCare Professional) is a large drug compendium with monthly updates that is now published on theInternet by the Micromedex Corporation. The package insert consists of a brief description of thepharmacology of the product. While this brochure contains much practical information, it is alsoused as a means of shifting liability for untoward drug reactions from the manufacturer onto thepractitioner. Therefore, the manufacturer typically lists every toxic effect ever reported, no matterhow rare. A useful and objective handbook that presents information on drug toxicity andinteractions is Drug Interactions. Finally, the FDA has an Internet World Wide Web site that carriesnews regarding recent drug approvals, withdrawals, warnings, etc. It can be reached using apersonal computer equipped with Internet browser software at http://www.fda.gov.The following addresses are provided for the convenience of readers wishing to obtain any of thepublications mentioned above: Drug Interactions Lea & Febiger 600 Washington Square Philadelphia, PA 19106 Facts and Comparisons
  • 16. 111 West Port Plaza, Suite 300 St. Louis, MO 63146 Pharmacology: Examination & Board Review, 6th ed McGraw-Hill Companies, Inc 2 Penn Plaza 12th Floor New York, NY 10121-2298 USMLE Road Map: Pharmacology McGraw-Hill Companies, Inc 2 Penn Plaza 12th Floor New York, NY 10121-2298 The Medical Letter on Drugs and Therapeutics 56 Harrison Street New Rochelle, NY 10801 The New England Journal of Medicine 10 Shattuck Street Boston, MA 02115 Physicians Desk Reference Box 2017 Mahopac, NY 10541 United States Pharmacopeia Dispensing Information Micromedex, Inc. 6200 S. Syracuse Way, Suite 300 Englewood, CO 80111Chapter 2. Drug Receptors & PharmacodynamicsDrug Receptors & Pharmacodynamics: IntroductionTherapeutic and toxic effects of drugs result from their interactions with molecules in the patient.Most drugs act by associating with specific macromolecules in ways that alter the macromoleculesbiochemical or biophysical activities. This idea, more than a century old, is embodied in the termreceptor: the component of a cell or organism that interacts with a drug and initiates the chain ofbiochemical events leading to the drugs observed effects.Receptors have become the central focus of investigation of drug effects and their mechanisms ofaction (pharmacodynamics). The receptor concept, extended to endocrinology, immunology, andmolecular biology, has proved essential for explaining many aspects of biologic regulation. Manydrug receptors have been isolated and characterized in detail, thus opening the way to preciseunderstanding of the molecular basis of drug action.The receptor concept has important practical consequences for the development of drugs and forarriving at therapeutic decisions in clinical practice. These consequences form the basis for
  • 17. understanding the actions and clinical uses of drugs described in almost every chapter of this book.They may be briefly summarized as follows:(1) Receptors largely determine the quantitative relations between dose or concentration ofdrug and pharmacologic effects. The receptors affinity for binding a drug determines theconcentration of drug required to form a significant number of drug-receptor complexes, and thetotal number of receptors may limit the maximal effect a drug may produce.(2) Receptors are responsible for selectivity of drug action. The molecular size, shape, andelectrical charge of a drug determine whether—and with what affinity—it will bind to a particularreceptor among the vast array of chemically different binding sites available in a cell, tissue, orpatient. Accordingly, changes in the chemical structure of a drug can dramatically increase ordecrease a new drugs affinities for different classes of receptors, with resulting alterations intherapeutic and toxic effects.(3) Receptors mediate the actions of both pharmacologic agonists and antagonists. Some drugsand many natural ligands, such as hormones and neurotransmitters, regulate the function of receptormacromolecules as agonists; ie, they activate the receptor to signal as a direct result of binding to it.Other drugs function as pharmacologic antagonists; ie, they bind to receptors but do not activategeneration of a signal; consequently, they interfere with the ability of an agonist to activate thereceptor. Thus, the effect of a so-called "pure" antagonist on a cell or in a patient depends entirelyon its preventing the binding of agonist molecules and blocking their biologic actions. Some of themost useful drugs in clinical medicine are pharmacologic antagonists.Macromolecular Nature of Drug ReceptorsMost receptors are proteins, presumably because the structures of polypeptides provide both thenecessary diversity and the necessary specificity of shape and electrical charge. The section HowAre New Receptors Discovered? describes some of the methods by which receptors are discoveredand defined.The best-characterized drug receptors are regulatory proteins, which mediate the actions ofendogenous chemical signals such as neurotransmitters, autacoids, and hormones. This class ofreceptors mediates the effects of many of the most useful therapeutic agents. The molecularstructures and biochemical mechanisms of these regulatory receptors are described in a later sectionentitled Signaling Mechanisms & Drug Action.Other classes of proteins that have been clearly identified as drug receptors include enzymes, whichmay be inhibited (or, less commonly, activated) by binding a drug (eg, dihydrofolate reductase, thereceptor for the antineoplastic drug methotrexate); transport proteins (eg, Na+/K+ ATPase, themembrane receptor for cardioactive digitalis glycosides); and structural pro-teins (eg, tubulin, thereceptor for colchicine, an anti-inflammatory agent).This chapter deals with three aspects of drug receptor function, presented in increasing order ofcomplexity: (1) Receptors as determinants of the quantitative relation between the concentration ofa drug and the pharmacologic response. (2) Receptors as regulatory proteins and components ofchemical signaling mechanisms that provide targets for important drugs. (3) Receptors as keydeterminants of the therapeutic and toxic effects of drugs in patients.How Are New Receptors Discovered?
  • 18. Because todays new receptor sets the stage for tomorrows new drug, it is important to know hownew receptors are discovered. Receptor discovery often begins by studying the relations betweenstructures and activities of a group of drugs on some conveniently measured response. Binding ofradioactive ligands defines the molar abundance and binding affinities of the putative receptor andprovides an assay to aid in its biochemical purification.Analysis of the pure receptor protein identifies the number of its subunits, its size, and (sometimes)provides a clue to how it works (eg, agonist-stimulated autophosphorylation on tyrosine residues,seen with receptors for insulin and many growth factors). These classic steps in receptoridentification serve as a warming-up exercise for molecular cloning of the segment of DNA thatencodes the receptor. Receptors within a specific class or subclass generally contain highlyconserved regions of similar or identical amino acid (and therefore DNA) sequence. This has led toan entirely different approach toward identifying receptors by sequence homology to already known(cloned) receptors.Cloning of new receptors by sequence homology has identified a number of subtypes of knownreceptor classes, such as -adrenoceptors and serotonin receptors, the diversity of which was onlypartially anticipated from pharmacologic studies. This approach has also led to the identification ofreceptors whose existence was not anticipated from pharmacologic studies. These putativereceptors, identified only by their similarity to other known receptors, are termed orphan receptorsuntil their native ligands are identified. Identifying such receptors and their ligands is of greatinterest because this process may elucidate entirely new signaling pathways and therapeutic targets.Relation between Drug Concentration & ResponseThe relation between dose of a drug and the clinically observed response may be complex. Incarefully controlled in vitro systems, however, the relation between concentration of a drug and itseffect is often simple and can be described with mathematical precision. This idealized relationunderlies the more complex relations between dose and effect that occur when drugs are given topatients.Concentration-Effect Curves & Receptor Binding of AgonistsEven in intact animals or patients, responses to low doses of a drug usually increase in directproportion to dose. As doses increase, however, the response increment diminishes; finally, dosesmay be reached at which no further increase in response can be achieved. In idealized or in vitrosystems, the relation between drug concentration and effect is described by a hyperbolic curve(Figure 2–1 A) according to the following equation:where E is the effect observed at concentration C, Emax is the maximal response that can beproduced by the drug, and EC50 is the concentration of drug that produces 50% of maximal effect.Figure 2–1.
  • 19. Relations between drug concentration and drug effect (panel A) or receptor-bound drug (panel B).The drug concentrations at which effect or receptor occupancy is half-maximal are denoted EC50and KD, respectively.This hyperbolic relation resembles the mass action law, which predicts association between twomolecules of a given affinity. This resemblance suggests that drug agonists act by binding to("occupying") a distinct class of biologic molecules with a characteristic affinity for the drugreceptor. Radioactive receptor ligands have been used to confirm this occupancy assumption inmany drug-receptor systems. In these systems, drug bound to receptors (B) relates to theconcentration of free (unbound) drug (C) as depicted in Figure 2–1 B and as described by ananalogous equation:in which Bmax indicates the total concentration of receptor sites (ie, sites bound to the drug atinfinitely high concentrations of free drug). KD (the equilibrium dissociation constant) representsthe concentration of free drug at which half-maximal binding is observed. This constantcharacterizes the receptors affinity for binding the drug in a reciprocal fashion: If the KD is low,binding affinity is high, and vice versa. The EC50 and KD may be identical, but need not be, asdiscussed below. Dose-response data is often presented as a plot of the drug effect (ordinate) againstthe logarithm of the dose or concentration (abscissa). This mathematical maneuver transforms thehyperbolic curve of Figure 2–1 into a sigmoid curve with a linear midportion (eg, Figure 2–2). Thisexpands the scale of the concentration axis at low concentrations (where the effect is changingrapidly) and compresses it at high concentrations (where the effect is changing slowly), but has nospecial biologic or pharmacologic significance.Figure 2–2.
  • 20. Logarithmic transformation of the dose axis and experimental demonstration of spare receptors,using different concentrations of an irreversible antagonist. Curve A shows agonist response in theabsence of antagonist. After treatment with a low concentration of antagonist (curve B), the curveis shifted to the right; maximal responsiveness is preserved, however, because the remainingavailable receptors are still in excess of the number required. In curve C, produced after treatmentwith a larger concentration of antagonist, the available receptors are no longer "spare"; instead,they are just sufficient to mediate an undiminished maximal response. Still higher concentrationsof antagonist (curves D and E) reduce the number of available receptors to the point that maximalresponse is diminished. The apparent EC50 of the agonist in curves D and E may approximate theKD that characterizes the binding affinity of the agonist for the receptor.Receptor-Effector Coupling & Spare ReceptorsWhen a receptor is occupied by an agonist, the resulting conformational change is only the first ofmany steps usually required to produce a pharmacologic response. The transduction processbetween occupancy of receptors and drug response is often termed coupling. The relative efficiencyof occupancy-response coupling is partially determined by the initial conformational change in thereceptor—thus, the effects of full agonists can be considered more efficiently coupled to receptoroccupancy than can the effects of partial agonists, as described below. Coupling efficiency is alsodetermined by the biochemical events that transduce receptor occupancy into cellular response.High efficiency of receptor-effector interaction may also be envisioned as the result of sparereceptors. Receptors are said to be "spare" for a given pharmacologic response when the maximalresponse can be elicited by an agonist at a concentration that does not result in occupancy of the fullcomplement of available receptors. Spare receptors are not qualitatively different from nonsparereceptors. They are not hidden or unavailable, and when they are occupied they can be coupled toresponse. Experimentally, spare receptors may be demonstrated by using irreversible antagonists toprevent binding of agonist to a proportion of available receptors and showing that highconcentrations of agonist can still produce an undiminished maximal response (Figure 2–2). Thus, a
  • 21. maximal inotropic response of heart muscle to catecholamines can be elicited even under conditionswhere 90% of the -adrenoceptors are occupied by a quasi-irreversible antagonist. Accordingly,myocardial cells are said to contain a large proportion of spare -adrenoceptors.How can we account for the phenomenon of spare receptors? In a few cases, the biochemicalmechanism is understood, such as for drugs that act on some regulatory receptors. In this situation,the effect of receptor activation—eg, binding of guanosine triphosphate (GTP) by an intermediate—may greatly outlast the agonist-receptor interaction (see the following section on G Proteins &Second Messengers). In such a case, the "spareness" of receptors is temporal in that the responseinitiated by an individual ligand-receptor binding event persists longer than the binding event itself.In other cases, where the biochemical mechanism is not understood, we imagine that the receptorsmight be spare in number. If the concentration or amount of a cellular component other than thereceptor limits the coupling of receptor occupancy to response, then a maximal response can occurwithout occupancy of all receptors. This concept helps explain how the sensitivity of a cell or tissueto a particular concentration of agonist depends not only on the affinity of the receptor for bindingthe agonist (characterized by the KD) but also on the degree of spareness—the total number ofreceptors present compared to the number actually needed to elicit a maximal biologic response.The KD of the agonist-receptor interaction determines what fraction (B/Bmax) of total receptors willbe occupied at a given free concentration (C) of agonist regardless of the receptor concentration:Imagine a responding cell with four receptors and four effectors. Here the number of effectors doesnot limit the maximal response, and the receptors are not spare in number. Consequently, an agonistpresent at a concentration equal to the KD will occupy 50% of the receptors, and half of theeffectors will be activated, producing a half-maximal response (ie, two receptors stimulate twoeffectors). Now imagine that the number of receptors increases 10-fold to 40 receptors but that thetotal number of effectors remains constant. Most of the receptors are now spare in number. As aresult, a much lower concentration of agonist suffices to occupy two of the 40 receptors (5% of thereceptors), and this same low concentration of agonist is able to elicit a half-maximal response (twoof four effectors activated). Thus, it is possible to change the sensitivity of tissues with sparereceptors by changing the receptor concentration.Competitive & Irreversible AntagonistsReceptor antagonists bind to receptors but do not activate them. In general, the effects of theseantagonists result from preventing agonists (other drugs or endogenous regulatory molecules) frombinding to and activating receptors. Such antagonists are divided into two classes depending onwhether or not they reversibly compete with agonists for binding to receptors.In the presence of a fixed concentration of agonist, increasing concentrations of a competitiveantagonist progressively inhibit the agonist response; high antagonist concentrations preventresponse completely. Conversely, sufficiently high concentrations of agonist can completelysurmount the effect of a given concentration of the antagonist; ie, the Emax for the agonist remainsthe same for any fixed concentration of antagonist (Figure 2–3 A). Because the antagonism iscompetitive, the presence of antagonist increases the agonist concentration required for a givendegree of response, and so the agonist concentration-effect curve is shifted to the right.
  • 22. Figure 2–3.Changes in agonist concentration-effect curves produced by a competitive antagonist (panel A) orby an irreversible antagonist (panel B). In the presence of a competitive antagonist, higherconcentrations of agonist are required to produce a given effect; thus the agonist concentration (C)required for a given effect in the presence of concentration [I] of an antagonist is shifted to theright, as shown. High agonist concentrations can overcome inhibition by a competitive antagonist.This is not the case with an irreversible antagonist, which reduces the maximal effect the agonistcan achieve, although it may not change its EC50.The concentration (C) of an agonist required to produce a given effect in the presence of a fixedconcentration ([I]) of competitive antagonist is greater than the agonist concentration (C) requiredto produce the same effect in the absence of the antagonist. The ratio of these two agonistconcentrations (the "dose ratio") is related to the dissociation constant (KI) of the antagonist by theSchild equation:Pharmacologists often use this relation to determine the KI of a competitive antagonist. Evenwithout knowledge of the relationship between agonist occupancy of the receptor and response, theKI can be determined simply and accurately. As shown in Figure 2–3, concentration responsecurves are obtained in the presence and in the absence of a fixed concentration of competitiveantagonist; comparison of the agonist concentrations required to produce identical degrees ofpharmacologic effect in the two situations reveals the antagonists KI. If C is twice C, for example,then [I] = KI.For the clinician, this mathematical relation has two important therapeutic implications:
  • 23. (1) The degree of inhibition produced by a competitive antagonist depends on the concentration of antagonist. Different patients receiving a fixed dose of propranolol, for example, exhibit a wide range of plasma concentrations, owing to differences in clearance of the drug. As a result, the effects of a fixed dose of this competitive antagonist of norepinephrine may vary widely in patients, and the dose must be adjusted accordingly. (2) Clinical response to a competitive antagonist depends on the concentration of agonist that is competing for binding to receptors. Here also propranolol provides a useful example: When this competitive -adrenoceptor antagonist is administered in doses sufficient to block the effect of basal levels of the neurotransmitter norepinephrine, resting heart rate is decreased. However, the increase in release of norepinephrine and epinephrine that occurs with exercise, postural changes, or emotional stress may suffice to overcome competitive antagonism by propranolol and increase heart rate, and thereby can influence therapeutic response.Some receptor antagonists bind to the receptor in an irreversible or nearly irreversible fashion, ie,not competitive. The antagonists affinity for the receptor may be so high that for practical purposes,the receptor is unavailable for binding of agonist. Other antagonists in this class produceirreversible effects because after binding to the receptor they form covalent bonds with it. Afteroccupancy of some proportion of receptors by such an antagonist, the number of remainingunoccupied receptors may be too low for the agonist (even at high concentrations) to elicit aresponse comparable to the previous maximal response (Figure 2–3 B). If spare receptors arepresent, however, a lower dose of an irreversible antagonist may leave enough receptors unoccupiedto allow achievement of maximum response to agonist, although a higher agonist concentration willbe required (Figures 2–2 B and C; see Receptor-Effector Coupling and Spare Receptors, above).Therapeutically, irreversible antagonists present distinctive advantages and disadvantages. Once theirreversible antagonist has occupied the receptor, it need not be present in unbound form to inhibitagonist responses. Consequently, the duration of action of such an irreversible antagonist isrelatively independent of its own rate of elimination and more dependent on the rate of turnover ofreceptor molecules.Phenoxybenzamine, an ir