Receptor agonist

Cells respond to signals through receptors, proteins on or inside the cell that change shape when the right molecule binds. An agonist is a molecule that binds a receptor and switches it on, producing the same response the natural ligand would. An antagonist binds without activating, and blocks the natural ligand. Most of the peptides profiled here are agonists of one or more receptors.

The incretin compounds illustrate the idea and its extensions. Semaglutide is a single agonist of the GLP-1 receptor. Tirzepatide is a dual agonist, activating both the GLP-1 and GIP receptors from one molecule. Retatrutide is a triple agonist that adds the glucagon receptor. Each additional receptor changes the pharmacology, which is what the comparison pages set side by side.

Selectivity, the degree to which an agonist activates its intended receptor and not others, is as important as potency. Ipamorelin is valued in the literature over older growth-hormone-releasing peptides because it activates the ghrelin receptor without the cortisol and prolactin release that GHRP-6 causes. Where a compound has no single identified receptor, as with BPC-157, its profile says so.