What is Receptor agonist?
Also called: Agonist, Full agonist, Partial agonist, Co-agonist
A ligand that binds a receptor and stabilises an active state, triggering a cellular response; agonists are compared by potency (EC50) and efficacy (maximal response).
By the APL Research Team · Updated
A receptor agonist is a molecule that binds a receptor and switches it on, producing a measurable response in the cell. Endogenous hormones are agonists at their own receptors, and synthetic peptides such as semaglutide are agonists engineered to reproduce or modify that action. The IUPHAR terminology for quantitative pharmacology separates two properties that are easily conflated: affinity, how tightly a ligand binds, and efficacy, how effectively the bound ligand activates the receptor [1].
Types of ligand at a receptor
The simple agonist/antagonist split has been extended by concepts such as inverse agonism, biased signalling and allosterism, which are now standard vocabulary in receptor pharmacology [2].
| Ligand class | What it does | Appearance in a concentration–response assay |
|---|---|---|
| Full agonist | Drives the system's maximal response | Top plateau matches the reference agonist |
| Partial agonist | Activates the receptor with lower efficacy | Lower plateau; can blunt a full agonist when both are present |
| Neutral antagonist | Binds without changing receptor activity | No response alone; shifts an agonist's curve to the right |
| Inverse agonist | Stabilises an inactive state | Pushes signalling below baseline where the receptor is constitutively active |
| Biased agonist | Favours some downstream pathways over others | Different relative potency or efficacy between, say, cAMP and β-arrestin readouts |
| Co-agonist (dual, triple) | One molecule activating two or more receptor types | A separate curve and EC50 at each receptor |
Inverse agonism is only visible where a receptor signals with nothing bound. The ghrelin receptor is the textbook case: in transfected COS-7 and HEK293 cells it signalled strongly without ligand, and a modified substance P analogue suppressed that activity to the level of untransfected cells with an EC50 of 5.2 nM [3].
Potency, efficacy and EC50
Potency is normally reported as the EC50, the concentration giving half of that agonist's own maximal effect [1]. For a curve with a Hill slope of 1, the fractional response equals [A] / ([A] + EC50):
| Agonist concentration | Fraction of that agonist's maximum |
|---|---|
| 0.1 × EC50 | 9% |
| 1 × EC50 | 50% |
| 10 × EC50 | 91% |
| 100 × EC50 | 99% |
This is why concentration–response series span several log units, and why an error in stock concentration shifts the apparent EC50 by the same factor. Molarity and net peptide content are where most of those errors start; the molarity calculator and the concentration–response design guide cover the arithmetic and plate layout.
Multi-receptor agonists
One peptide can be engineered to activate several receptors. An intermixed GLP-1/GIP sequence reported in 2013 showed potent, balanced co-agonism at both incretin receptors and was studied in rodents, cynomolgus monkeys and humans [4]. Tirzepatide acts at the GIP receptor and GLP-1 receptor; in cell assays it mimicked native GIP at the GIP receptor but, at the GLP-1 receptor, favoured cAMP generation over β-arrestin recruitment, so it is both imbalanced and biased [5]. Retatrutide adds the glucagon receptor, showing balanced glucagon and GLP-1 receptor activity in vitro with greater GIP receptor activity [6].
Common misunderstandings
- Potent does not mean efficacious. A partial agonist can have a lower EC50 than a full agonist and still never reach the same maximum.
- The label depends on the assay system. Receptor expression level and coupling efficiency change the observed response, so a ligand that looks like a full agonist in an overexpressing cell line may behave as a partial agonist in native tissue [2].
- Selectivity is a ratio, not an absolute. Most agonists engage related receptors at high enough concentrations; selectivity describes the gap between potencies.
References
- 1.Neubig RR, Spedding M, Kenakin T, et al. International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification. XXXVIII. Update on terms and symbols in quantitative pharmacology. Pharmacol Rev. 2003. PubMed 14657418
- 2.Kenakin T, Williams M. Defining and characterizing drug/compound function. Biochem Pharmacol. 2014. PubMed 23954707
- 3.Holst B, Cygankiewicz A, Jensen TH, et al. High constitutive signaling of the ghrelin receptor--identification of a potent inverse agonist. Mol Endocrinol. 2003. PubMed 12907757
- 4.Finan B, Ma T, Ottaway N, et al. Unimolecular dual incretins maximize metabolic benefits in rodents, monkeys, and humans. Sci Transl Med. 2013. PubMed 24174327
- 5.Willard FS, Douros JD, Gabe MB, et al. Tirzepatide is an imbalanced and biased dual GIP and GLP-1 receptor agonist. JCI Insight. 2020. PubMed 32730231
- 6.Coskun T, Urva S, Roell WC, et al. LY3437943, a novel triple glucagon, GIP, and GLP-1 receptor agonist for glycemic control and weight loss: From discovery to clinical proof of concept. Cell Metab. 2022. PubMed 35985340