Biased signaling is a natural result of GPCR allosteric function and should be expected from any and all synthetic agonists. Therefore, it may be encountered in all agonist discovery projects and must be considered as a beneficial (or possible detrimental) feature of new candidate molecules. While bias is easily detected , the synoptic nature of GPCR signaling makes translation of simple in vitro bias to complex in vivo systems problematic. The practical outcome of this is a difficulty in predicting the therapeutic value of biased signaling due to the failure of translation of identified biased signaling to in vivo agonism. This is discussed in this review as well as some new ways forward to improve this translation process and better exploit this powerful pharmacologic activity.

G protein-coupled receptors modulate a plethora of physiological processes and mediate the effects of one-third of FDA-approved drugs. Notably, depending on which ligand has activated a particular receptor, it can engage different intracellular transducers. This paradigm of ligand-dependent ‘biased signaling’ dictates a need to advance beyond the level of receptors to consider the combined ligand-receptor pair in order to understand physiological signaling. Bias signaling also has the potential to improve medicines by reducing adverse effects. However, this is challenged by inconsistent interpretation of results and lack of commonly agreed guidelines. Here, we present recommended terminology and guidelines to conduct, report and quantify bias in a comparable and reproducible fashion. We expect these recommendations will facilitate a common understanding of experiments and findings across basic receptor research and drug discovery, while the area and the analytical methods to measure bias are still evolving, especially in complex cellular, tissue and organismal systems.