Chapter 2: Pharmacodynamics

Receptors are the specific molecules in a biologic system with which drugs interact to produce changes in the function of the system. Receptors must be selective in their ligand-binding characteristics (so as to respond to the proper chemical signal and not to meaningless ones). Receptors must also be modifiable when they bind a drug molecule (so as to bring about the functional change). Many receptors have been identified, purified, chemically characterized, and cloned. Most are proteins; a few are other macromolecules such as DNA. Some authorities consider enzymes as a separate category; for the purposes of this book, enzymes that are affected by drugs are considered receptors. The receptor site(also known as the recognition site) for a drug is the specific binding region of the receptor macromolecule and has a relatively high and selective affinity for the drug molecule. The interaction of a drug with its receptor is the fundamental event that initiates the action of the drug, and many drugs are classified on the basis of their primary receptor affinity.

Effectors are molecules that translate the drug-receptor interaction into a change in cellular activity. The best examples of effectors are enzymes such as adenylyl cyclase. Some receptors are also effectors in that a single molecule may incorporate both the drug-binding site and the effector mechanism. For example, a tyrosine kinase effector enzyme is part of the insulin receptor molecule, and a sodium-potassium channel is the effector part of the nicotinic acetylcholine receptor.

When the response of a particular receptor-effector system is measured against increasing concentrations of a drug, the graph of the response versus the drug concentration or dose is called a graded dose-response curve (Figure 2–1A). Plotting the same data on a logarithmic concentration axis usually results in a sigmoid curve, which simplifies the mathematical manipulation of the dose- response data (Figure 2–1B). The efficacy (E_max) and potency (EC₅₀ or ED₅₀) parameters are derived from these data. The smaller the EC₅₀ (or ED₅₀), the greater the potency of the drug.

It is possible to measure the percentage of receptors bound by a drug, and by plotting this percentage against the log of the concentration of the drug, a dose-binding graph similar to the dose-response curve is obtained (Figure 2–1C). The concentration of drug required to bind 50% of the receptor sites is denoted by the dissociation constant (K_d) and is a useful measure of the affinity of a drug molecule for its binding site on the receptor molecule. The smaller the K_d, the greater the affinity of the drug for its receptor. If the number of binding sites on each receptor molecule is known, it is possible to determine the total number of receptors in the system from the B_max.

When the minimum dose required to produce a specified response is determined in each member of a population, the quantal dose-response relationship is defined (Figure 2–2). For example, a blood pressure-lowering drug might be studied by measuring the dose required to lower the mean arterial pressure by 20 mm Hg in 100 hypertensive patients. When plotted as the percentage of the population that shows this response at each dose versus the log of the dose administered, a cumulative quantal dose-response curve, usually sigmoid in shape, is obtained. The median effective dose (ED₅₀), median toxic dose (TD₅₀), and (in animals) median lethal dose (LD₅₀) are derived from experiments carried out in this manner. Because the magnitude of the specified effect is arbitrarily determined, the ED₅₀ determined by quantal dose-response measurements has no direct relation to the ED₅₀ determined from graded dose-response curves. Unlike the graded dose-response determination, no attempt is made to determine the maximal effect of the drug. Quantal dose-response data provide information about the variation in sensitivity to the drug in a given population, and if the variation is small, the curve is steep.

Efficacy—often called maximal efficacy—is the greatest effect (E_max) an agonist can produce if the dose is taken to the highest tolerated level. Efficacy is determined mainly by the nature of the drug and the receptor and its associated effector system. It can be measured with a graded dose-response curve (Figure 2–1) but not with a quantal dose-response curve. By definition, partial agonists have lower maximal efficacy than full agonists (see later discussion).

Potency denotes the amount of drug needed to produce a given effect. In graded dose-response measurements, the effect usually chosen is 50% of the maximal effect and the concentration or dose causing this effect is called the EC₅₀ or ED₅₀ (Figure 2–1A and B). Potency is determined mainly by the affinity of the receptor for the drug and the number of receptors available. In quantal dose- response measurements, ED₅₀, TD₅₀, and LD₅₀ are also potency variables (median effective, toxic, and lethal doses, respectively, in 50% of the population studied). Thus, a measure of potency can be determined from either graded or quantal dose-response curves (eg, Figures 2–1 and 2–2, respectively), but the numbers obtained are not identical and they have different meanings.

Spare receptors are said to exist if the maximal drug response (E_max) is obtained at less than 100% occupation of the receptors (B_max). In practice, the determination is usually made by comparing the concentration for 50% of maximal effect (EC₅₀) with the concentration for 50% of maximal binding (K_d). If the EC₅₀ is less than the K_d, spare receptors are said to exist (Figure 2–3). This might result from 1 of 2 mechanisms. First, the duration of the effector activation may be much greater than the duration of the drug-receptor interaction. Second, the actual number of receptors may exceed the number of effector molecules available. The presence of spare receptors increases sensitivity to the agonist because the likelihood of a drug-receptor interaction increases in proportion to the number of receptors available. (For contrast, the system depicted in Figure 2–1, panels B and C, does not have spare receptors, since the EC₅₀ and the K_d are equal.)

Modern concepts of drug-receptor interactions consider the receptor to have at least 2 states—active and inactive. In the absence of ligand, a receptor might be fully active or completely inactive; alternatively, an equilibrium state might exist with some receptors in the activated state and with most in the inactive state (R_a + R_i; Figure 2–4). Many receptor systems exhibit some activity in the absence of ligand, suggesting that some fraction of the receptor is always in the activated state. Activity in the absence of ligand is called constitutive activity. A full agonist is a drug capable of fully activating the effector system when it binds to the receptor. In the model system illustrated in Figure 2–4, a full agonist has high affinity for the activated receptor conformation, and sufficiently high concentrations result in all the receptors achieving the activated state (R_a – D_a). A partial agonist produces less than the full effect, even when it has saturated the receptors (R_a-D_pa + R_i-D_pa), presumably by combining with both receptor conformations, but favoring the active state. In the presence of a full agonist, a partial agonist acts as an inhibitor. In this model, neutral antagonists bind with equal affinity to the R_i and R_a states, preventing binding by an agonist and preventing any deviation from the level of constitutive activity. In contrast, inverse agonists have a higher affinity for the inactive R_i state than for R_a and decrease or abolish any constitutive activity.

A. Competitive and Irreversible Pharmacologic Antagonists

Competitive antagonists are drugs that bind to, or very close to, the agonist receptor site in a reversible way without activating the effector system for that receptor. Neutral antagonists bind the receptor without shifting the ratio of R_a to R_i (Figure 2–4). In the presence of a competitive antagonist, the dose-response curve for an agonist is shifted to higher doses (ie, horizontally to the right on the dose axis), but the same maximal effect is reached (Figure 2–5A). The agonist, if given in a high enough concentration, can displace the antagonist and fully activate the receptors. In contrast, an irreversible antagonist causes a downward shift of the maximum, with no shift of the curve on the dose axis unless spare receptors are present (Figure 2–5B). Unlike the effects of a competitive antagonist, the effects of an irreversible antagonist cannot be overcome by adding more agonist. Competitive antagonists increase the ED₅₀; irreversible antagonists do not (unless spare receptors are present). A noncompetitive antagonist that acts at an allosteric site of the receptor (see Figure 1–1) may bind reversibly or irreversibly; a noncompetitive antagonist that acts at the receptor site binds irreversibly.

B. Physiologic Antagonists

A physiologic antagonist binds to a different receptor molecule, producing an effect opposite to that produced by the drug it antagonizes. Thus, it differs from a pharmacologic antagonist, which interacts with the same receptor as the drug it inhibits. Typical examples of physiologic antagonists are the antagonism of the bronchoconstrictor action of histamine by epinephrine’s bronchodilator action and glucagon’s antagonism of the cardiac depressant effects of propranolol.

C. Chemical Antagonists

A chemical antagonist interacts directly with the drug being antagonized to remove it or to prevent it from binding to its target. A chemical antagonist does not depend on interaction with the agonist’s receptor (although such interaction may occur). Common examples of chemical antagonists are dimercaprol, a chelator of lead and some other toxic metals, and pralidoxime, which combines avidly with the phosphorus in organophosphate cholinesterase inhibitors.

Describe the difference between a pharmacologic antagonist and an allosteric inhibitor (See Chapter 1). How could you differentiate these two experimentally?

The therapeutic index is the ratio of the TD₅₀ (or LD₅₀) to the ED₅₀, determined from quantal dose-response curves. The therapeutic index represents an estimate of the safety of a drug, because a very safe drug might be expected to have a very large toxic dose and a much smaller effective dose. For example, in Figure 2–2, the ED₅₀ is approximately 3 mg, and the LD₅₀ is approximately 150 mg. The therapeutic index is therefore approximately 150/3, or 50, in mice. Obviously, a full range of toxic doses cannot be ethically studied in humans. Furthermore, factors such as the varying slopes of dose-response curves make this estimate a poor safety index even in animals.

The therapeutic window, a more clinically useful index of safety, describes the dosage range between the minimum effective therapeutic concentration or dose, and the minimum toxic concentration or dose. For example, if the average minimum therapeutic plasma concentration of theophylline is 8 mg/L and toxic effects are observed at 18 mg/L, the therapeutic window is 8–18 mg/L. Both the therapeutic index and the therapeutic window depend on the specific toxic effect used in the determination.

Once an agonist drug has bound to its receptor, some effector mechanism is activated. The receptor-effector system may be an enzyme in the intracellular space (eg, cyclooxygenase, a target of nonsteroidal anti-inflammatory drugs) or in the membrane or extracellular space (eg, acetylcholinesterase). Neurotransmitter reuptake transporters (eg, the norepinephrine transporter, NET, and the dopamine transporter, DAT) are receptors for many drugs, eg, antidepressants and cocaine. Most antiarrhythmic drugs target voltage-activated ion channels in the membrane for sodium, potassium, or calcium. For the largest group of drug-receptor interactions, the drug is present in the extracellular space, whereas the effector mechanism resides inside the cell and modifies some intracellular process. These classic drug-receptor interactions involve signaling across the membrane. Five major types of transmembrane-signaling mechanisms for receptor-effector systems have been defined (Figure 2–6, Table 2–1).

Receptors are dynamically regulated in number, location, and interaction with other molecules. Changes can occur over short times (minutes) and longer periods (days).

Frequent or continuous exposure to agonists often results in short-term diminution of the receptor response, sometimes called tachyphylaxis. Several mechanisms are responsible for this phenomenon.

First, intracellular molecules may block access of a G protein to the activated receptor molecule. For example, the molecule β-arrestin has been shown to bind to an intracellular loop of the β adrenoceptor when the receptor is continuously activated. Beta-arrestin prevents access of the G_s-coupling protein and thus desensitizes the tissue to further β-agonist activation within minutes. Removal of the β agonist results in removal of β-arrestin and restoration of the full response after a few minutes or hours.

Second, agonist-bound receptors may be internalized by endocytosis, removing them from further exposure to extracellular molecules. The internalized receptor molecule may then be either reinserted into the membrane (eg, morphine receptors) or degraded (eg, β adrenoceptors, epidermal growth factor receptors). In some cases, a cyclic internalization-reinsertion process may actually be necessary for normal functioning of the receptor-effector system.

Third, continuous activation of the receptor-effector system may lead to depletion of some essential substrate required for downstream effects. For example, depletion of thiol cofactors may be responsible for tolerance to nitroglycerin. In some cases, repletion of the missing substrate (eg, by administration of glutathione) can reverse the tolerance.

Long-term reductions in receptor number (downregulation) may occur in response to continuous exposure to agonists. The opposite change (upregulation) occurs when receptor activation is blocked for prolonged periods (usually several days) by pharmacologic antagonists or by denervation.

1. A 55-year-old woman with hypertension is to be treated with a thiazide diuretic. Thiazide A in a dose of 5 mg produces the same decrease in blood pressure as 500 mg of thiazide B. Which of the following statements best describes these results?

   • (A) Thiazide A is more efficacious than thiazide B

   • (B) Thiazide A is about 100 times more potent than thiazide B

   • (C) Toxicity of thiazide A is less than that of thiazide B

   • (D) Thiazide A has a wider therapeutic window than thiazide B

   • (E) Thiazide A has a longer half-life than thiazide B

2. Graded and quantal dose-response curves are being used for evaluation of a new antiasthmatic drug in the animal laboratory and in clinical trials. Which of the following statements best describes graded dose-response curves?

   • (A) More precisely quantitated than quantal dose-response curves

   • (B) Obtainable from isolated tissue preparations but not from the study of intact subjects

   • (C) Used to determine the maximal efficacy of the drug

   • (D) Used to determine the therapeutic index of the drug

   • (E) Used to determine the variation in sensitivity of subjects to the drug

3. Prior to clinical trials in patients with heart failure, an animal study was carried out to compare two new positive inotropic drugs (A and B) to a current standard agent (C). The results of cardiac output measurements are shown in the graph below.

   

   Which of the following statements is correct?

   • (A) Drug A is most effective

   • (B) Drug B is least potent

   • (C) Drug C is most potent

   • (D) Drug B is more potent than drug C and more effective than drug A

   • (E) Drug A is more potent than drug B and more effective than drug C

4. A study was carried out in isolated intestinal smooth muscle preparations to determine the action of a new drug “novamine,” which in separate studies bound to the same receptors as acetylcholine. In the absence of other drugs, acetylcholine caused contraction of the muscle. Novamine alone caused relaxation of the preparation. In the presence of a low concentration of novamine, the EC₅₀ of acetylcholine was unchanged, but the E_max was reduced. In the presence of a high concentration of novamine, extremely high concentrations of acetylcholine had no effect. Which of the following expressions best describes novamine?

   • (A) A chemical antagonist

   • (B) An irreversible antagonist

   • (C) A partial agonist

   • (D) A physiologic antagonist

   • (E) A spare receptor agonist

5. Beta adrenoceptors in the heart regulate cardiac rate and contractile strength. Several studies have indicated that in humans and experimental animals, about 90% of β adrenoceptors in the heart are spare receptors. Which of the following statements about spare receptors is most correct?

   • (A) Spare receptors, in the absence of drug, are sequestered in the cytoplasm

   • (B) Spare receptors may be detected by finding that the drug-receptor interaction lasts longer than the intracellular effect

   • (C) Spare receptors influence the maximal efficacy of the drug-receptor system

   • (D) Spare receptors activate the effector machinery of the cell without the need for a drug

   • (E) Spare receptors may be detected by the finding that the EC₅₀ is smaller than the K_d for the agonist

6. Two cholesterol-lowering drugs, X and Y, were studied in a large group of patients, and the percentages of the group showing a specific therapeutic effect (35% reduction in low-density lipoprotein [LDL] cholesterol) were determined. The results are shown in the following table.

   Table Graphic Jump Location

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   Drug Dose (mg)	Percent Responding to Drug X	Percent Responding to Drug Y
   5	1	10
   10	5	20
   20	10	50
   50	50	70
   100	70	90
   200	90	100

   Which of the following statements about these results is correct?

   • (A) Drug X is safer than drug Y

   • (B) Drug Y is more effective than drug X

   • (C) The 2 drugs act on the same receptors

   • (D) Drug X is less potent than drug Y

   • (E) The therapeutic index of drug Y is 10

7. Sugammadex is a new drug that reverses the action of rocuronium and certain other skeletal muscle-relaxing agents (nondepolarizing neuromuscular blocking agents). It appears to interact directly with the rocuronium molecule and not at all with the rocuronium receptor. Which of the following terms best describes sugammadex?

   • (A) Chemical antagonist

   • (B) Noncompetitive antagonist

   • (C) Partial agonist

   • (D) Pharmacologic antagonist

   • (E) Physiologic antagonist

   DIRECTIONS: 8–10. Each of the curves in the graph below may be considered a concentration-effect curve or a concentration-binding curve.

   

8. Which of the curves in the graph describes the percentage of binding of a large dose of full agonist to its receptors as the concentration of a partial agonist is increased from low to very high levels?

   • (A) Curve 1

   • (B) Curve 2

   • (C) Curve 3

   • (D) Curve 4

   • (E) Curve 5

9. Which of the curves in the graph describes the percentage effect observed when a large dose of full agonist is present throughout the experiment and the concentration of a partial agonist is increased from low to very high levels?

   • (A) Curve 1

   • (B) Curve 2

   • (C) Curve 3

   • (D) Curve 4

   • (E) Curve 5

10. Which of the curves in the graph describes the percentage of binding of the partial agonist whose effect is shown by Curve 4 if the system has many spare receptors?

    • (A) Curve 1

    • (B) Curve 2

    • (C) Curve 3

    • (D) Curve 4

    • (E) Curve 5

1. No information is given regarding the maximal antihypertensive response to either drug. Similarly, no information about half-life or toxicity is provided. The fact that a given response is achieved with a smaller dose of thiazide A indicates that A is more potent than B in the ratio of 500:5. The answer is B.

2. Precise quantitation is possible with both types of dose-response curves. Quantal dose-response curves show the frequency of occurrence of a specified response, which may be therapeutically effective (ED) or toxic (TD). Thus, quantal studies are used to determine the therapeutic index and the variation in sensitivity to the drug. Graded (not quantal) dose-response curves are used to determine maximal efficacy (maximal response). The answer is C.

3. Drug A produces 50% of its maximal effect at a lower dose than either B or C and thus is the most potent; drug C is the least potent. However, drug A, a partial agonist, is less efficacious than drugs B and C. The answer is D.

4. Choices involving chemical or physiologic antagonism are incorrect because novamine is said to act at the same receptors as acetylcholine. When given alone, the novamine effect is opposite to that of acetylcholine, so choice C is incorrect. “Spare receptor agonist” is a nonsense distracter. The answer is B.

5. There is no difference in location between “spare” and other receptors. Spare receptors may be defined as those that are not needed for binding drug to achieve the maximal effect. Spare receptors influence the sensitivity of the system to an agonist because the statistical probability of a drug-receptor interaction increases with the total number of receptors. They do not alter the maximal efficacy. If they do not bind an agonist molecule, spare receptors do not activate an effector molecule. EC₅₀ less than K_d is an indication of the presence of spare receptors. The answer is E.

6. No information is presented regarding the safety of these drugs. Similarly, no information on efficacy (maximal effect) is presented; this requires graded dose-response curves. Although both drugs are said to be producing a therapeutic effect, no information on their receptor mechanisms is given. Since no data on toxicity are available, the therapeutic index cannot be determined. The answer is D because the ED₅₀ of drug Y (20 mg/d) is less than that of drug X (50 mg/d).

7. Sugammadex interacts directly with rocuronium and not with the rocuronium receptor; therefore, it is a chemical antagonist. The answer is A.

8. The binding of a full agonist decreases as the concentration of a partial agonist is increased to very high levels. As the partial agonist displaces more and more of the full agonist, the percentage of receptors that bind the full agonist drops to zero, that is, Curve 5. The answer is E.

9. Curve 1 describes the response of the system when a full agonist is displaced by increasing concentrations of partial agonist. This is because the increasing percentage of receptors binding the partial agonist finally produce the maximal effect typical of the partial agonist. The answer is A.

10. Partial agonists, like full agonists, bind 100% of their receptors when present in a high enough concentration. Therefore, the binding curve (but not the effect curve) will go to 100%. If the effect curve is Curve 4 and many spare receptors are present, the binding curve must be displaced to the right of Curve 4 (K_d > EC₅₀). Therefore, Curve 3 fits the description better than Curve 2. The answer is C.

Allosteric antagonists do not bind to the agonist receptor site; they bind to some other region of the receptor molecule that results in inhibition of the response to agonists (see Figure 1–1). They do not prevent binding of the agonist. In contrast, pharmacologic antagonists bind to the agonist site and prevent access of the agonist. The difference can be detected experimentally by evaluating competition between the binding of radioisotopically labeled antagonist and the agonist. High concentrations of agonist displace or prevent the binding of a pharmacologic antagonist but not an allosteric antagonist.

When you complete this chapter, you should be able to:

• ❑ Compare the efficacy and the potency of 2 drugs on the basis of their graded dose-response curves.

• ❑ Predict the effect of a partial agonist in a patient in the presence and in the absence of a full agonist.

• ❑ Name the types of antagonists used in therapeutics.

• ❑ Describe the difference between an inverse agonist and a pharmacologic antagonist.

• ❑ Specify whether a pharmacologic antagonist is competitive or irreversible based on its effects on the dose-response curve and the dose-binding curve of an agonist in the presence of the antagonist.

• ❑ Give examples of competitive and irreversible pharmacologic antagonists and of physiologic and chemical antagonists.

• ❑ Name 5 transmembrane signaling methods by which drug-receptor interactions exert their effects.

• ❑ Describe 2 mechanisms of receptor regulation.