Agonist

An agonist is a ligand that both occupies a receptor and switches it on, holding it in the shape that couples to downstream transducers. Two independent properties describe one: affinity, which governs how much receptor is bound at a given concentration, and intrinsic efficacy, which governs how much signal each bound receptor produces. A full agonist drives the system to the largest response it can give, so that adding more drug cannot increase the effect further.

Most peptide therapeutics are agonists at receptors that already have a native ligand. Exenatide, a synthetic version of a lizard salivary peptide, was approved in 2005 as the first GLP-1 receptor agonist; liraglutide and semaglutide followed with fatty-acid chains that keep them in circulation for hours and days rather than the couple of minutes native GLP-1 survives. Tirzepatide agonises two receptors at once, GIP and GLP-1. Setmelanotide, approved in 2020 for rare genetic obesity, is a melanocortin-4 receptor agonist. Each reproduces an existing signal rather than creating a new one.

Whether a compound is an agonist decides what happens when you keep giving it. Sustained agonism at a G protein-coupled receptor invites kinase phosphorylation, arrestin recruitment, internalisation and eventually fewer surface receptors, which is why continuous stimulation of a system evolved for pulses tends to fade. It also sets the failure mode: too much agonist exaggerates a physiological effect, while too much antagonist removes one.

The usual error is reading agonist as a strength claim. The word states direction, not magnitude, and a weak partial agonist earns it as readily as a full one. Binding data alone never establishes agonism either, so a product page that quotes a receptor affinity and then calls the compound an agonist has skipped the functional assay that would decide the question.