Drug Action and Pharmacodynamics
Pharmacodynamics is the study of the biochemical and physiologic effects of drugs and their mechanisms of action on the body or on microorganisms and other parasites within or on the body. It considers both drug action, which refers to the initial consequence of a drug-receptor interaction, and drug effect, which refers to the subsequent effects. The drug action of digoxin, for example, is inhibition of membrane Na+/K+-ATPase; the drug effect is augmentation of cardiac contractility. In this example, the clinical response might comprise improved exercise tolerance.
Not all drugs exert their pharmacologic actions via receptor-mediated mechanisms. The action of some drugs—including inhalation anesthetic agents, osmotic diuretics, purgatives, antiseptics, antacids, chelating agents, and urinary acidifying and alkalinizing agents—is attributed to their chemical action or physicochemical properties. Certain cancer and antiviral chemotherapeutic agents, which are analogues of pyrimidine and purine bases, elicit their effects when they are incorporated into nucleic acids and serve as substrates for DNA or RNA synthesis. The effect of most drugs, however, results from their interactions with receptors. These interactions and the resulting conformational changes in the receptor initiate biochemical and physiologic changes that characterize the drug’s response.
Drug therapy is intended to result in a particular pharmacologic response of desired intensity and duration while avoiding adverse drug reactions. The relationship between the administered dose and the clinical response has been investigated for some drugs using a pharmacokinetic/pharmacodynamic (PK/PD) modeling approach, which is generally based on the plasma concentration-response relationship. For other drugs, a simpler relationship between the concentration and effect in an idealized in vitro system is modeled mathematically to conceptualize receptor occupancy and drug response. The model assumes that the drug interacts reversibly with its receptor and produces an effect proportional to the number of receptors occupied, up to a maximal effect when all receptors are occupied. The reaction scheme for the model is:
in which k2 and k1 are rate constants.
The relationship between effect and the concentration of free drug for the model is given by the Hill equation, which can be written as:
in which E is the effect observed at concentration C, Emax is the maximal response that can be produced by the drug (efficacy), EC50 is the concentration of drug that produces 50% of maximal effect (potency), and the Hill coefficient n is the slope of the log10 concentration-effect relationship (sensitivity).
The above equation describes a rectangular hyperbola when response (y-axis) is plotted against concentration (x-axis). However, dose- or concentration-response data is generally plotted as drug effect (y-axis) against log10 dose or concentration (x-axis). The transformation yields a sigmoidal curve that allows the potency of different drugs to be readily compared. In addition, the effect of drugs used at therapeutic concentrations commonly falls on the portion of the sigmoidal curve that is approximately linear, ie, between 20% and 80% of maximal effect. This makes for easier interpretation of the plotted data.
An agonist is a drug that binds to receptors and thereby alters (stabilizes) the proportion of receptors in the active conformation, resulting in a biologic response. A full agonist results in a maximal response by occupying all or a fraction of receptors. A partial agonist results in less than a maximal response even when the drug occupies all of the receptors.
There are four types of drug antagonism. Chemical antagonism involves chemical interaction between a drug and either a chemical or another drug leading to a reduced or nil response. Physiologic antagonism occurs when two drugs acting on different receptors and pathways exert opposing actions on the same physiologic system. Pharmacokinetic antagonism is the result of one drug suppressing the effect of a second drug by reducing its absorption, altering its distribution, or increasing its rate of elimination. Pharmacologic antagonism occurs when the antagonist inhibits the effect of a full or partial agonist by acting on the same pathway but not necessarily on the same receptor.
Pharmacologic antagonists comprise three subcategories. A reversible competitive antagonist results in inhibition that can be overcome by increasing the concentration of agonist. The presence of a reversible competitive antagonist causes a parallel rightward shift of the log concentration-effect curve of the agonist without altering Emax or EC50. An irreversible competitive antagonist also involves competition between agonist and antagonist for the same receptors, but stronger binding forces prevent the effect of the antagonist being fully reversed, even at high agonist concentrations. The presence of an irreversible competitive antagonist causes a rightward shift of the log concentration-effect curve of the agonist that generally displays decreased slope and reduced maximum effect. A noncompetitive antagonist inhibits agonist activity by blocking one of the sequential reactions between receptor activation and the pharmacologic response. Noncompetitive antagonism is generally reversible but can be irreversible. Noncompetitive antagonists and irreversible competitive antagonists cause similar perturbations in the log concentration-effect curve of agonists. Isolated tissue experiments are used to distinguish the two subcategories, because noncompetitive antagonists are generally reversible.
Agonists, but not antagonists, elicit an effect even when they bind to the same site on the same receptor. An explanation is provided by both structural and functional studies, which indicate that receptors exist in at least two conformations, active and inactive, and these are in equilibrium. Because agonists have a higher affinity for the receptor’s active conformation, agonists drive the equilibrium to the active state, thereby activating the receptor. Conversely, antagonists have a higher affinity for the receptor’s inactive conformation and push the equilibrium to the inactive state, producing no effect.
The concept of spare receptors explains a maximum response being achieved when only a fraction of the total number of receptors is occupied. For example, an action potential and maximal twitch of muscle fibers is elicited when 0.13% of the total number of receptors at a skeletal neuromuscular junction is simultaneously activated. From a functional perspective, spare receptors are significant, because they increase both the sensitivity and speed of a tissue’s responsiveness to a ligand.
Structure-activity relationships are exploited in drug design, because small changes in chemical structure can produce profound changes in potency. For example, the substitution of a proton by a methyl group accounts for codeine being ~1,000 times less potent than morphine in its action on opioid receptors.
Most receptors are proteins. The best characterized of these are regulatory proteins, enzymes, transport proteins, and structural proteins. Nucleic acids are also important drug receptors, particularly for cancer chemotherapeutic agents.
The receptors for several neurotransmitters modulate the opening and closing of ion channels through ligand gating or voltage gating. The nicotinic acetylcholine receptor is an example of a ligand-gated receptor; it allows Na+ to flow down its concentration gradient into cells, resulting in depolarization. Most clinically useful neuromuscular blocking drugs compete with acetylcholine for the receptor but do not initiate ion-channel opening. Other ligand-gated ion channels include the CNS receptors for the excitatory amino acids (glutamate and aspartate), the inhibitory amino acids (γ-aminobutyric acid [GABA] and glycine), and certain serotonin (5-HT3) receptors. The sodium channel receptor is an example of a voltage-gated receptor; these are present in the membranes of excitable nerve, cardiac, and skeletal muscle cells. In the resting state, the Na+/K+-ATPase pump in these cells maintains an intracellular Na+ concentration much lower than that in the extracellular environment. Membrane depolarization causes channel opening and a transient influx of Na+ ions, followed by inactivation and return to the resting state. The action of local anesthetics is due to their direct interaction with voltage-gated Na+ channels.
Many transmembrane receptors are linked to guanosine triphosphate binding proteins, which activate second messenger systems. Two important second messenger systems are cyclic adenosine monophosphate (cAMP) and the phosphoinositides. In cAMP second messenger systems, binding of the ligand to the receptor increases or decreases adenylyl cyclase activity, which in turn regulates the formation of cAMP from adenosine triphosphate. The activation of protein kinase A by cAMP results in the phosphorylation of proteins and a physiologic effect. From a therapeutic standpoint, drug binding to β-adrenergic, histamine H2, or dopamine D1 receptors activates adenylyl cyclase, whereas binding to muscarinic M2, α2-adrenergic, dopamine D2, opiate μ and δ, adenosine A1, or GABA type B receptors inhibits adenylyl cyclase. In phosphoinositide second messenger systems, membrane phosphatidylinositol 4,5-biphosphate is hydrolyzed to 1,4,5-trisphosphate (IP3) and 1,2-diacylglycerol (DAG) by activation of phospholipase C. Both IP3 and DAG activate kinases, and in the case of IP3, this involves the mobilization of calcium from intracellular stores. The action of numerous drugs is due to their interaction with receptors that rely on these second messengers, which include α1-adrenergic, muscarinic M1 or M2, serotonin 5-HT2, and thyrotropin-releasing hormone receptors.
Protein tyrosine kinase receptors are generally transmembrane enzymes that phosphorylate proteins exclusively on tyrosine residues, rather than on serine or threonine residues. They include endocrine hormone receptors for insulin and receptors for several growth hormones.
Intracellular receptors mediate the action of hormones such as glucocorticoids, estrogen, and thyroid hormone and related drugs. The hormones, which regulate gene expression in the nucleus, are lipophilic and freely diffuse through the cell membrane to reach the receptor. Glucocorticoid receptors reside predominantly in the cytoplasm in an inactive form until they bind to the glucocorticoid steroid ligand. This results in receptor activation and translocation to the nucleus, where the receptor interacts with specific DNA sequences. Unlike glucocorticoid receptors, the receptors for estrogen and thyroid hormone reside in the nucleus.
Intracellular receptors are also important in mediating the action of antimicrobial drugs, including the penicillins, sulfonamides, trimethoprim, aminoglycosides, phenicols, macrolides, and fluoroquinolones. The mechanisms of action include inhibition of bacterial protein synthesis, inhibition of cell wall synthesis, inhibition of enzymatic activity, alteration of cell membrane permeability, and blockade of specific biochemical pathways.
Receptor-mediated mechanisms of action of several classes of anthelmintics are well understood. For example, the benzimidazoles and pro-benzimidazoles bind to nematode tubulin, preventing its polymerization during microtubular assembly and thus disrupting cell division. Depletion of ATP as the result of salicylanilides uncoupling oxidative phosphorylation and the inhibition of enzymes in the glycolytic pathway by benzene sulfonamides are other examples. Several classes of anthelmintics interfere with neurotransmission in parasites. A case in point is macrocyclic lactones, which potentiate inhibitory neurotransmission via GABA and glutamate-gated chlorine channels (see Anthelmintics).
To make rational therapeutic decisions, it is necessary to understand the fundamental concepts linking drug doses to concentrations to clinical responses. The concentration-response relationships for drugs may be graded or quantal. A graded concentration-response curve can be constructed for responses measured on a continuous scale, eg, heart rate. Graded concentration-response curves relate the intensity of response to the size of the dose and, hence, are useful to characterize the actions of drugs. A quantal concentration-response curve can be constructed for drugs that elicit an all-or-none response, eg, presence or absence of convulsions. For most drugs, the doses required to produce a specified quantal effect in a population are log normally distributed, so that the frequency distribution of responses plotted against log dose is a gaussian normal distribution curve. The percentage of the population requiring a particular dose to exhibit the effect can be determined from this curve. When these data are plotted as a cumulative frequency distribution, a sigmoidal dose-response curve is generated.
The equilibrium dissociation constant of the receptor-drug complex, KD, is the ratio of rate constants for the reverse (k2) and forward (k1) reaction between the drug and receptor and the drug-receptor complex (see Drug Concentration and Effect). KD is also the drug concentration at which receptor occupancy is half of maximum. Drugs with a high KD (low affinity) dissociate rapidly from receptors; conversely, drugs with a low KD (high affinity) dissociate slowly from receptors. These effects impact the rate at which biologic responses end.
Potency refers to the concentration (EC50) or dose (ED50) of a drug required to produce 50% of the drug’s maximal effect as depicted by a graded dose-response curve. EC50 equals KD when there is a linear relationship between occupancy and response. Often, signal amplification occurs between receptor occupancy and response, which results in the EC50 for response being much less (ie, positioned to the left on the x-axis of the log dose-response curve) than KD for receptor occupancy. Potency depends on both the affinity of a drug for its receptor and the efficiency with which drug-receptor interaction is coupled to response. The dose of drug required to produce an effect is inversely related to potency. In general, low potency is important only if it results in a need to administer the drug in large doses that are impractical. Quantal dose-response curves provide information on the potency of drugs that is different from the information derived from graded dose-response curves. In a quantal dose-response relationship, the ED50 is the dose at which 50% of individuals exhibit the specified quantal effect.
The median inhibitory concentration, or IC50, is the concentration of an antagonist that reduces a specified response to 50% of the maximal possible effect.
Efficacy (also referred to as intrinsic activity) of a drug is the ability of the drug to elicit a response when it binds to the receptor. As discussed above, conformational changes in receptors as a result of drug occupancy initiate biochemical and physiologic events that characterize the drug’s response. In some tissues, agonists demonstrating high efficacy can result in a maximal effect, even when only a small fraction of the receptors is occupied (the concept of spare receptors is discussed above).
Selectivity refers to a drug’s ability to preferentially produce a particular effect and is related to the structural specificity of drug binding to receptors. For example, cyclooxygenase-2 (COX-2) preferential NSAIDs demonstrate partial specificity for COX-2, the inducible enzyme formed at sites of inflammation. By comparison, COX-2 selective NSAIDs are without significant effect on COX-1, the constitutive enzyme that performs a range of physiologic functions. For certain drugs, selectivity is species dependent. For example, S(+)-carprofen is COX-2 selective in dogs and cats, nonselective of COX-1 and COX-2 in horses, and COX-2 preferential in calves.
Specificity of drug action relates to the number of different mechanisms involved. Examples of specific drugs include atropine (a muscarinic receptor antagonist), salbutamol (a β2-adrenoceptor agonist), and cimetidine (an H2-receptor antagonist). By contrast, nonspecific drugs result in drug effects through several mechanisms of action. For example, phenothiazine causes blockade of D2-dopamine receptors, α-adrenergic receptors, and muscarinic receptors.
The affinity of a drug for a receptor describes how avidly the drug binds to the receptor (ie, the KD). The chemical forces in drug-receptor interactions include electrostatic forces, van der Waal forces, and the forces associated with hydrogen bonds and hydrophobic bonds. Variation in the strength of these forces, and therefore the thermal energy in the system, determines the degree of association and dissociation of the drug and the receptor. Covalent binding of drug to receptor (exemplified by fluoroquinolones acting on bacteria) leads to formation of an irreversible link.
The therapeutic index of a drug is the ratio of the dose that results in an undesired effect to the dose that results in a desired effect. The therapeutic index of a drug is usually defined as the ratio of LD50 to ED50 (median lethal and median effective doses, respectively, in 50% of individuals), which indicates how selective the drug is in eliciting its desired effect. Values of LD50 and ED50 for this purpose are derived from quantal dose-response curves generated in animal studies.
The information obtained from dose- and concentration-response curves is critically important when choosing between drugs and when determining the dose to administer. A drug is chosen largely on the basis of its clinical effectiveness for a particular therapeutic indication. In this context, the drug concentration at the receptor (determined by the pharmacokinetic properties of the drug) and the efficacy of the drug-receptor complex are the primary determinants of a drug’s clinical effectiveness. The administered dose of a drug, by comparison, depends to a greater extent on potency than on maximal efficacy.
The maximal efficacy of the drug-receptor complex to result in a graded effect is Emax or Imax on a graded dose-response curve. Emax or Imax is derived from a quantitative dose-response relationship for a single animal and varies among individuals. The extrapolation of this value of Emax to a clinical case is only an estimate, but it facilitates a comparison of the maximal efficacy of drugs that result in a specified effect by identical receptors. A drug’s potency (ie, EC50, ED50, or IC50) obtained from either graded or quantal dose-response curves is used to determine the dose that should be administered. The slope of the graded dose-response curve (n in the Hill equation, above) provides information concerning the dose range over which a drug elicits its effect. Other information concerning the selectivity of drug action and the therapeutic index is also obtained from the graded dose-response curve. When quantal effects are being considered, information concerning pharmacologic potency, selectivity of drug action, the margin of safety, and the potential variability of responsiveness among individuals is obtained from quantal dose-response curves.
The approach used to investigate the pharmacodynamics of antimicrobial drugs (and parasiticides) differs from that of other veterinary drugs on account of the need to address target pathogens that have structure, biochemistry, and capacity for replication which are markedly dissimilar to those of their mammalian host. The individual drugs within each group of antimicrobial drugs differ in potency and in antimicrobial spectrum of activity. The minimum inhibitory concentration (MIC), which is the lowest concentration of drug that completely inhibits bacterial growth, is determined in vitro as a measure of susceptibility of bacterial species and strains to a given drug. Importantly, the MIC50 provides a measure of potency, EC50.
Other surrogate markers of bacteriologic effect exist and include the minimum bactericidal concentration (MBC), which is the concentration of antimicrobial drug that produces a 3-log-unit or 99.9% reduction in bacterial count, postantibiotic effect, sub-MIC postantibiotic effect, and time-kill data. More recently, the application of PK/PD principles to antimicrobial drug action has led to PK/PD integration and PK/PD modeling. PK/PD integration brings together data from PK and PD studies. The surrogate PK/PD index that best correlates with efficacy for a given drug is selected based on the drug's killing mechanism, namely concentration dependent, time dependent, or codependent (ie, both time and concentration determine outcome). The PK/PD index selected for aminogylycosides, which act by concentration-dependent killing mechanisms, is Cmax/MIC ratio (Cmax is the maximum plasma concentration after administration of a drug by a nonvascular route); for β-lactams, which act by time-dependent killing mechanisms, it is T>MIC (T is the time for which plasma concentration exceeds MIC, expressed as a percentage of the dosage interval); and for fluoroquinolones, which act by concentration-dependent killing mechanisms, AUC/MIC ratio (AUC is the area under the plasma drug concentration-time curve) is selected. The objective of PK/PD modeling is to define the three key pharmacodynamic properties that define any drug, namely Emax or Imax (efficacy); EC50 (or EC80 or EC90) (potency); and slope (n) in the Hill equation, which indicates sensitivity and selectivity. PK/PD modeling permits breakpoint values to achieve a bacteriostatic or bactericidal effect, or bacterial eradication to be computed, which are used to optimize efficacy and minimize resistance.
The ability of drugs to reach the receptor is determined by pharmacokinetic parameters that characterize the absorption, distribution, and clearance of a drug. There may not be a simple temporal correlation between plasma concentration of a drug and its therapeutic effect. Plotting plasma concentrations (x-axis) versus therapeutic effect (y-axis) in chronologic order displays the data as a loop for some drugs. This phenomenon is referred to as hysteresis in the concentration-effect relationship. The effect of most drugs lag behind the plasma concentration. This results in a counterclockwise hysteresis loop. For example, the NSAID robenicoxib has prolonged local effects after blood concentrations have decreased below effective levels. A clockwise hysteresis loop is observed for cocaine and pseudoephedrine when tachyphylaxis develops (see below). The temporal correlation between plasma concentration and therapeutic effect also varies for the different classes of antagonists. For instance, the extent and duration of action of a competitive antagonist depends on its concentration in plasma, which depends (in part) on its rate of elimination. This requires that the dose be adjusted accordingly to maintain plasma concentrations in the therapeutic range. By contrast, the duration of action of an irreversible antagonist is relatively independent of its rate of elimination and, therefore, plasma concentration, and more dependent on the rate of turnover of receptor molecules.
The density of most receptors is not constant with time, which has important therapeutic implications. Down-regulation of receptors may occur as a result of continual stimulation by an agonist and manifests as the development of tachyphylaxis, which demonstrates a clockwise hysteresis loop in the concentration-effect relationship. Conversely, additional receptors can be synthesized in response to chronic receptor antagonism—a phenomenon known as up-regulation. Because more receptors are now available, a hyperreactive response occurs when the cell is exposed to an agonist.