Agonists of seven-transmembrane receptors, also known as G protein-coupled receptors (GPCRs), do not uniformly activate all cellular signalling pathways linked to a given seven-transmembrane receptor (a phenomenon termed ligand or agonist bias); this discovery has changed how high-throughput screens are designed and how lead compounds are optimized for therapeutic activity. The ability to experimentally detect ligand bias has necessitated the development of methods for quantifying agonist bias in a way that can be used to guide structure–activity studies and the selection of drug candidates. Here, we provide a viewpoint on which methods are appropriate for quantifying bias, based on knowledge of how cellular and intracellular signalling proteins control the conformation of seven-transmembrane receptors. We also discuss possible predictions of how biased molecules may perform in vivo, and what potential therapeutic advantages they may provide.
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Work in A.C.'s laboratory is supported by the Program Grant No. 519461 and Project Grant No. APP1026962 of the National Health and Medical Research Council (NHMRC) of Australia, and Discovery Grant No. DP110100687 of the Australian Research Council. A.C. is an NHMRC Principal Research Fellow.
A.C. has a research contract with Servier (France), and recent consultancies with Johnson and Johnson (USA) and XOMA (USA). He is a Scientific Advisory Board member for Audeo, Australia. T.K. declares no competing financial interests.
The Black–Leff Operational Model for Modelling Drug Response (PDF 201 kb)
Differences in Operational (Functional) Affinity (PDF 277 kb)
Conditional Affinity of Allosteric Systems: 7 Transmembrane Receptors (PDF 232 kb)
Fitting the Operational Model (PDF 209 kb)
Equiactive Concentration Comparison for Pathways: Method of Barlow, Scott and Stephenson (PDF 197 kb)
The Molecular Determinants of Efficacy within the Allosteric Vector (PDF 220 kb)
Equating ΔΔLog(τ/KA) and βlig values (PDF 246 kb)
35S-labelled GTP; a non-hydrolysable G protein-activating analogue of GTP that is used to measure interactions between seven-transmembrane receptors (also known as GPCRs) and G proteins.
- Allosteric binding site
The site on a seven-transmembrane receptor protein (also known as GPCR) to which modulators bind to affect the subsequent binding and effect of another ligand that interacts with the receptor; this ligand is usually the endogenous agonist binding to its cognate (that is, orthosteric) binding site.
- Conditional affinity
The measured affinity of a ligand for a seven-transmembrane receptor (also known as GPCR) when the receptor is bound to an allosteric guest molecule (such as a G protein or β-arrestin). The conditional affinity of the ligand for the receptor will vary with the concentration and type of the guest molecule that is co-bound.
The property of a molecule that causes a change in the behaviour of a seven-transmembrane receptor (also known as GPCR) towards the cell when the molecule is bound to the receptor.
The concentration of an agonist that produces 50% of the maximal response to the agonist for a defined signalling response pathway.
- Full agonists
Agonists that induce the maximum obtainable response that can be produced by a signalling system.
- Operational affinity
Also referred to as functional affinity. The apparent equilibrium dissociation constant of the agonist–receptor complex, as determined by fitting the Black–Leff operational model to agonist concentration–response curves.
- Orthosteric agonist
An agonist that binds to the same binding site on the seven-transmembrane receptor protein (also known as GPCR) as the endogenous agonist (that is, the orthosteric binding site).
Negative logarithm of the EC50 (the concentration of an agonist that produces 50% of the maximal response to the agonist for a defined signalling response pathway).
- Receptor coupling efficiencies
A term that describes the degree of seven-transmembrane receptor (also known as GPCR) occupancy by an agonist; receptor coupling efficiency relates to the resulting cellular response. A low receptor occupancy that yields a large cellular response constitutes a high coupling efficiency.
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Kenakin, T., Christopoulos, A. Signalling bias in new drug discovery: detection, quantification and therapeutic impact. Nat Rev Drug Discov 12, 205–216 (2013). https://doi.org/10.1038/nrd3954
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