Procedure for calculation of potency and efficacy for ligands acting on Gs- and Gi-coupled receptors

Structure activity relationship (SAR) analyses of pharmacological data of compounds constitute an important part of the discovery process in the design of new drug candidates with improved pharmacological properties. In particular G-Protein Coupled Receptors (GPCRs) associated with the cAMP second messenger systems G(s) and G(i) have constituted one of the most widely used basis for pharmacological in vitro assays for assessing functional receptor effects. Such assays are based on Radio Immuno Assay (RIA) analysis to measure the cellular cAMP concentration as readout of receptor activation. It appears, however, to be a common practice to omit the use of cAMP standard curves to transform the measured signals (cpm or cps) into cAMP concentrations on which estimations of potencies (EC(50) values) and efficacies (E(MAX) values) in G(s) and G(i) coupled receptor stimulation are based. Such practice may lead to significant errors as compounds mediating their effects via G(s) coupled receptors may appear more potent and efficacious than they actually are. Contrary, compounds mediating their effects via G(i) coupled receptors may appear less potent and efficacious than they are in reality. Potency rank orders will therefore change considerably, when estimations are based on incorrect calculation of the original experimental results. Thus, the only correct way to calculate effect data on which to base E(MAX) and EC(50) estimations is to use cAMP concentrations derived from transformation of the measured signals (cpm or cps) using cAMP standard curves. The present work outlines the mathematical procedures by which such transformations are to be performed.