Exploring the Technology Landscape of 7TMR Drug Signaling Profiling

Seven transmembrane domain receptors (7TMRs) constitute the largest family of transmembrane proteins in vertebrates and are the targets of more than 40% of currently marketed drugs. It is now accepted that these receptors are highly dynamic "microprocessors" that adopt a continuum of functionally distinct active conformations. The novel concept of biased agonism (or functional selectivity) posits that different ligands stabilize unique receptor conformations with each conformation imparting distinct signaling, and thus biological attributes, to a given receptor. The pharmacotherapeutic potential of biased agonism lies in possibility to develop molecules that selectively engage beneficial pathways while inhibiting or remaining inert towards those producing deleterious outcomes. Various strategies are now applied for the discovery of biased ligands. Many assays use second messenger levels (i.e., calcium, inositol trisphosphate, cAMP) as a quantitative readout of G-protein subtype-specific activity. However, due to complex cross-regulation between the various G-protein pathways, second messenger levels alone are not directly reflective of a ligand's activity on a specific pathway. Consequently, direct measurements of receptor-proximal events (such as G-protein activation and β-arrestin coupling) are required for a more accurate quantification of ligand's efficacy (or bias) towards different pathways. The discovery that various ligands of the same receptor can display different efficacies and potencies towards different receptor-downstream signaling pathways has not only revitalized the process of 7TMR drug discovery, but has significantly transformed the field of pharmacology as a whole. This review will showcase the current pharmacological toolbox available for the discovery and validation of biased ligands.