Terminology summary
Non-quantitative bias: Ligand with efficacy in only one of compared pathways (neutral antagonist or inverse agonist in others). As for all biased ligands, this refers to a particular system and in another more sensitive system, there may be efficacy at several compared pathways allowing the determination of concentration-response curves. We discourage the use of the previous terms ‘full bias’ or ‘perfect bias’, as they can misleadingly give the impression that the ligand has full efficacy in the preferred pathway. In fact, a ligand with non-quantitative bias may give a weaker activation than other agonists for which bias can be quantified (as they induce agonism in at least two pathways).

Conclusion and outlook

The scope of adoption of the recommendations presented herein will ultimately depend on their utility across the wide GPCR field, which will benefit from a common, standardized and therefore comparable characterization of ligand bias. This precise characterization will also help individual researchers, as the more comparable and reproducible research data are, the more useful they will be for others, and the fewer surprises encountered because of irreproducible experiments will occur. Furthermore, the unambiguous and structured reporting of bias data will be directly reflected in more relevant databases and literature reviews.
Ligand-dependent biased function is also being investigated for other protein classes e.g., receptor tyrosine kinases (Karl, Paul, Pasquale & Hristova, 2020), nuclear receptors (Heidari et al., 2019), monoamine transporters (Hasenhuetl, Bhat, Freissmuth & Sandtner, 2019), Toll-like receptors (unpublished), and cytochrome P450 enzymes (Jensen et al., 2021). Many of the recommendations brought forward in this paper are also applicable to these protein families, which would in time benefit from publication of additional dedicated guidelines.
To unlock the rational targeting of specific desired physiological biases mediated by GPCRs, we must first map individual or combined pathways to their distinct effects. This requires identification of many more biased probes, pharmacological assays, animal models etc. This represents a tremendous challenge, but one with extraordinary potential. The recommendations herein can contribute to this massive endeavor by better description of such probes and effects. We have deliberately left the door open to include additional intracellular transducers, effectors or modulators involved in GPCR signaling and our definitions should therefore be future proof in the sense that they can be applied also to signaling proteins that have yet to be discovered.

Acknowledgements

We acknowledge Kasper Harpsøe, Mette M. Rosenkilde and Nevin Lambert for comments on this manuscript. D.E.G. received financial support from the Lundbeck Foundation (R313-2019-526) and Novo Nordisk Foundation (NNF18OC0031226). P.K. thanks the German Research Foundation DFG for Heisenberg professorship KO4095/5-1. M.B. (Marcel Bermudez) thanks the German Research Foundation DFG for funding DFG-407626949. M.E.S., D.G., P.K., XX, YY, and ZZ are members of COST Action CA18133 “ERNEST”. E.K. was supported by the DFG-funded research unit FOR2372 with the grants KO1582/10-1 and -2.

Author contributions

Conceptualization, D.E.G., P.K. and T.K.; Writing – Original Draft, D.E.G., P.K. and T.K.; Writing – Review & Editing, all authors, M.E.S.; Visualization, C.B., D.E.G.; Project Administration, D.E.G.; Funding Acquisition, D.E.G., E.K., P.K., M.B.

Conflicts of interest

M.B. is the president of Domain Therapeutics scientific advisory board.