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Equilibrium Assays Are Required to Accurately Characterize the Activity Profiles of Drugs Modulating Gq-Protein-Coupled Receptors

This paper discusses the process of determining the activity of candidate molecules targeting Gq-protein activation through G-protein-coupled receptors for possible therapeutic application with two functional assays; calcium release and inositol phosphate metabolism [inositol monophosphate (IP1)]. While both are suitable for detecting ligand activity (screening), differences are seen when these assays are used to quantitatively measure ligand parameters for therapeutic activity. Specifically, responses for Gq-related pathways present different and dissimulating patterns depending on the functional assay used to assess them. To investigate the impact of functional assays on the accuracy of compound pharmacological profiles, five exemplar molecules [partial agonist, antagonist, inverse agonist, positive allosteric modulator (PAM) agonist, and positive β-PAM] targeting either muscarinic M1 or ghrelin receptors were tested using two functional assays (calcium release and IP1) and the results were compared with theoretical pharmacological models. The IP1 assay is an equilibrium assay that is able to determine the correct (i.e., internally consistent) pharmacological profiles of all tested compounds. In contrast, the nonequilibrium nature of calcium assays yields misleading classification of most of the tested compounds. Our study suggests that the use of an equilibrium assay, such as IP1, is mandatory for the optimal use of pharmacological models that can both identify mechanisms of action and also convert descriptive-to-predictive data for therapeutic systems. Such assays allow the identification of consistent and simple scales of activity that can guide medicinal chemistry in lead optimization of candidate molecules for therapeutic use.