Experimental measures of drug action: antagonists

Term Suggested Usage Notes
Concentration ratio, r The ratio of the concentration of an agonist that produces a specified response (often but not necessarily 50% of the maximal response to that agonist in an assay) in the presence of an antagonist, to the agonist concentration that produces the same response in the absence of antagonist. The traditional term dose ratio is considered less appropriate.
IC50 This term is used in a number of ways: (i) the molar concentration of an antagonist that reduces the response to an agonist by 50%; the concentration of agonist should be given; (ii) the molar concentration of an unlabeled agonist or antagonist that inhibits the binding of a radioligand by 50%; the concentration of radioligand should be given; (iii) the molar concentration of an inhibitory agonist that reduces a response by 50% of the maximal inhibition that can be attained; this latter usage is not recommended—instead the term, EC50, should be used since this is an agonist effect. In functional studies, the antagonist IC50 is most useful if the concentration of the stimulatory agonist is submaximal. Higher concentrations of the agonist will increase the IC50 of the competitive antagonist well above its equilibrium dissociation constant. Even with low agonist concentrations, the IC50 from functional studies, like an agonist EC50 or maximal response, is dependent on the conditions of the experiment (tissue, receptor expression, type of measurement, etc.). Thus, IC50 should only be used for comparison of drugs under the specific conditions of the experiment and may have limited relevance to absolute affinity. However, when determined in radioligand binding studies (point ii) under equilibrium conditions for competitive ligands, the IC50 may be converted to a true dissociation constant using the Cheng and Prusoff equation (Cheng and Prusoff, 1973), which takes into consideration the concentration of the radioligand.
pA2 The negative logarithm to base 10 of the molar concentration of an antagonist that makes it necessary to double the concentration of the agonist needed to elicit the original submaximal response obtained in the absence of antagonist (Schild, 1947, 1949). An empirical measure of the activity (in concentration terms) of an antagonist that is not dependent on how the antagonist acts. The pA2 is determined by measuring the value of the concentration ratio r at several antagonist concentrations, allowing an estimate of the antagonist concentration at which r would be 2. This is commonly done by graphical extrapolation or interpolation (Arunlakshana and Schild, 1959). If certain experimental conditions are fulfilled, the value of pA2 can provide an estimate of the equilibrium dissociation constant for the combination of a competitive antagonist with its binding site (see Section IV. B. for further details).