Biased Allosteric Modulation in GPCR Drug Discovery

Allosteric modulation of G protein-coupled receptors (GPCRs) is emerging as a powerful approach in drug discovery, offering enhanced subtype selectivity and the ability to bias signaling toward therapeutically preferred pathways, thereby reducing off-target effects. While most approved GPCR drugs act via the orthosteric site, this approach often lacks subtype specificity and induces pleiotropic signaling that can compromise therapeutic efficacy. Orthosteric biased ligands have provided proof of concept for functional selectivity, yet their development has been limited by site homology and challenges in fine-tuning pathway specificity. In contrast, allosteric modulators (AMs) bind to spatially and structurally distinct, less conserved sites located across extracellular, transmembrane, and intracellular receptor domains. By stabilizing discrete receptor conformations, AMs can fine-tune transducer engagement and preferentially direct signaling toward either G protein or β-arrestin (βarr) pathway. Recent structural and biophysical studies have provided insights into how diverse AMs lock GPCRs in specific conformations and modulate signaling across receptor families. Taken together, these findings reflect a shift in GPCR pharmacology, driven by the convergence of biased signaling and allosteric modulation. Biased allosteric modulators (BAMs) represent a promising class of ligands that bind at allosteric sites and selectively tune signaling pathways by biasing orthosteric ligand-induced responses. This review outlines the principles of biased signaling and allosteric modulation and highlights strategies for designing BAMs for GPCRs. Identifying BAMs could revolutionize GPCR drug discovery by enabling pathway-specific precision therapeutics with improved efficacy and fewer side effects.