A method for the quantification of biased signalling at constitutively active receptors

Background and Purpose: Biased agonism, the ability of an agonist to differentially activate one of several signal transduction pathways when acting at a given receptor, is an increasingly recognized phenomenon at many receptors. The Black and Leff operational model lacks a way to describe constitutive receptor activity and hence inverse agonism. Thus, it is impossible to analyse the biased signalling of inverse agonists using this model. In this theoretical work, we develop and illustrate methods for the analysis of biased inverse agonism. Experimental Approach: Methods were derived for quantifying biased signalling in systems that demonstrate constitutive activity using the modified operational model proposed by Slack and Hall. The methods were illustrated using Monte Carlo simulations. Key Results: The Monte Carlo simulations demonstrated that, with an appropriate experimental design, the model parameters are ‘identifiable’. The method is consistent with methods based on the measurement of intrinsic relative activity (RA _i ) (ΔΔlogR or ΔΔlog(τ/K _a )) proposed by Ehlert and Kenakin and their co-workers but has some advantages. In particular, it allows the quantification of ligand bias independently of ‘system bias’ removing the requirement to normalize to a standard ligand. Conclusions and Implications: In systems with constitutive activity, the Slack and Hall model provides methods for quantifying the absolute bias of agonists and inverse agonists. This provides an alternative to methods based on RA _i and is complementary to the ΔΔlog(τ/K _a ) method of Kenakin et al. in systems where use of that method is inappropriate due to the presence of constitutive activity.