Allosteric GPCR modulators: taking advantage of permissive receptor pharmacology.

The past decade has witnessed a dramatic increase in the identification of allosteric modulators of G-protein-coupled receptor (GPCR) activity. Concomitantly, several new perspectives and hypotheses regarding the way ligands regulate GPCR signalling have also emerged. Here, we briefly discuss how the concepts of collateral efficacy and permissive agonism-antagonism intersect the field of allosteric GPCR modulation. Despite the challenges associated with detecting and quantifying the myriad of possible allosteric effects on GPCR activity, it is proposed that allosteric ligands offer the exciting prospect of engendering stimulus-bias in orthosteric ligand signalling, thus paving the way for not only receptor-selective but also signalling-pathway-selective therapies.