Quantitation Of Drugreceptor Interactions And Effects Receptor Pharmacology

Receptor occupancy theory, in which it is assumed that response emanates from a receptor occupied by a drug, has its basis in the law of mass action. The basic currency of receptor pharmacology is the dose-response curve, a depiction of the observed effect of a drug as a function of its

FIGURE 1-8 Graded respones expressed as a function of the concentration of drug A present at the receptor.

The hyperbolic shape of the curve in panel A becomes sigmoid when plotted semi-logarithmically, as in panel B. The concentration of drug that produces 50% of the maximal response quantifies drug activity and is referred to as the EC50 (effective concentration for 50% response). The range of concentrations needed to usefully depict the dose-response relationship (~3 log10[10] units) is too wide to be useful in the linear format of Figure 1-8A; thus, most dose-response curves use log[D] on the abscissa (Figure 1-8B). Dose-response curves presented in this way are sigmoidal in shape and have three basic properties: threshold, slope, and maximal asymptote. These parameters characterize and quantitate the activity of the drug. The sigmoidal curve also depicts the law of mass action as expressed in Equation 1-16.

FIGURE 1-8 Graded respones expressed as a function of the concentration of drug A present at the receptor.

The hyperbolic shape of the curve in panel A becomes sigmoid when plotted semi-logarithmically, as in panel B. The concentration of drug that produces 50% of the maximal response quantifies drug activity and is referred to as the EC50 (effective concentration for 50% response). The range of concentrations needed to usefully depict the dose-response relationship (~3 log10[10] units) is too wide to be useful in the linear format of Figure 1-8A; thus, most dose-response curves use log[D] on the abscissa (Figure 1-8B). Dose-response curves presented in this way are sigmoidal in shape and have three basic properties: threshold, slope, and maximal asymptote. These parameters characterize and quantitate the activity of the drug. The sigmoidal curve also depicts the law of mass action as expressed in Equation 1-16.

concentration in the receptor compartment. Figure 1-8A shows a typical dose-response curve; it reaches a maximal asymptotic value when the drug occupies all the receptor sites.

Some drugs cause low-dose stimulation and high-dose inhibition of response. These U-shaped relationships for some receptor systems are said to display hormesis. Several drug—receptor systems can display this property (e.g., prostaglandins, endothelin, and purinergic and serotonergic agonists, among others), which is likely to be at the root of drug toxicity.

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