Receptor Pharmacology Primer: Agonists, Antagonists, and Selectivity

Research summaries lean heavily on pharmacological vocabulary. A working grasp of the terms makes mechanism claims much easier to evaluate.

Receptors and ligands

A receptor is a protein that changes conformation when a specific molecule — its ligand — binds, initiating a downstream signal. Many peptide-responsive receptors are G-protein-coupled receptors (GPCRs), which span the membrane seven times and transduce binding into intracellular signalling through associated G proteins. The incretin and growth-hormone-secretagogue receptors are both class B or class A GPCRs of this kind.

Agonists, antagonists, and partial agonists

A full agonist binds and produces the maximal response the system can generate. An antagonist binds without activating, and by occupying the site blocks other ligands. A partial agonist produces a submaximal response even at full occupancy. An inverse agonist reduces activity below the receptor’s baseline. These are properties of a ligand-receptor pair, not intrinsic labels — the same molecule can behave differently at different receptors.

Affinity versus efficacy

Affinity describes how tightly a ligand binds, commonly expressed as a dissociation constant. Efficacy describes how effectively binding produces a response. High affinity does not imply high efficacy: an antagonist may bind extremely tightly and produce no signal. Potency figures such as EC50 combine both properties and are assay-dependent, which is why values from different systems are not directly comparable.

Selectivity is relative

Selectivity means a ligand acts at one receptor at substantially lower concentrations than at others. It is almost never absolute. As concentration rises, off-target engagement typically appears, which is why concentration-response work across a range is more informative than a single point. Some compounds are deliberately non-selective: tirzepatide engages both GIP and GLP-1 receptors, and retatrutide engages the glucagon receptor in addition, by design rather than by imprecision.

Biased signalling

A single receptor can couple to multiple downstream pathways, and some ligands preferentially activate one over another. This biased agonism means two agonists at the same receptor can produce measurably different cellular outcomes — an active area of investigation across peptide pharmacology.

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