Quantitating Ligand Bias Using the Competitive Model of Ligand Activity

G protein-coupled receptors (GPCRs) can interact with both G proteins and β-arrestin proteins to propagate different signaling outputs. In some contexts, agonists may drive the receptor to preferentially engage one of these effectors over the other. Such "ligand bias" may present a means to impart pathway-selective signaling downstream of this class of receptors. In cases where physiological responses are mediated by diverse pathways, this could, in part, provide a means to refine GPCR therapeutics. Cell-based signaling assays are used to measure the potential for signaling bias in vitro, and these measures take into account potency, efficacy, and the overall capacity of the assay. However, narrow assay windows sometimes limit the confidence in estimating agonist activity, if a compound performs as a very weakly efficacious partial agonist. This lack of response in an assay hampers the ability to measure and compare potencies, and the degree of separation of an agonist's performance, between two assays. In this chapter, we describe in detail a method for the estimation of the relative activity of a partial agonist and provide a stepwise protocol for calculating bias when this case arises.