Receptor bias (revision 5)

Receptor bias, also termed biased agonism or functional selectivity, is the tendency of an agonist to favour one downstream signalling pathway over another at the same receptor. It arises because different agonists stabilise different receptor conformations, and those conformations couple differently to the available transducers.^[1]

At the GLP-1 receptor the pathways usually compared are cAMP accumulation, which mediates the insulinotropic effect, and β-arrestin recruitment, which terminates G-protein signalling and drives receptor internalisation.^[1]

Bias is quantified as a bias factor relative to a reference agonist, and the number depends on the reference chosen, the cell system, the readout and the incubation time. Bias factors from different publications are not comparable.^[2]

Classical receptor theory treats an agonist's effect as a single efficacy applied to a single pathway. Biased agonism replaces this with a vector: an agonist has an efficacy for each transducer, and the ratio between them is what "bias" names.^[2]

Quantification requires care because potency and efficacy differences that are not bias — receptor reserve differences between assays, for instance — can masquerade as it. Operational models exist to separate the two and their assumptions are not always met.^[2]

The concept is real and well supported at the level of receptor conformation. What is contested is whether measured bias predicts anything clinically.^[1]