Few areas of peptide chemistry offer as legible a progression as the incretin field. Three generations of molecule, each adding a receptor to the previous architecture, with the pharmacological consequence of each addition documented as it happened. Read in order, the sequence is close to a controlled experiment.
Single: GLP-1 receptor agonism
Semaglutide is characterised as a selective agonist at the GLP-1 receptor, a class B G-protein coupled receptor expressed across pancreatic, gastric and central nervous tissue. Engagement couples to Gαs and raises intracellular cyclic AMP, with downstream protein kinase A activity examined in relation to glucose-dependent insulin secretion. The C18 diacid side chain provides albumin binding and the extended circulating profile.
Dual: adding the GIP receptor
Tirzepatide engages both GIPR and GLP-1R from a single thirty-nine residue backbone, and the literature describes it as an imbalanced agonist — relatively more potent at GIPR. That imbalance is itself the subject of study: published work examines how concurrent GIP engagement alters β-arrestin recruitment and receptor internalisation relative to a GLP-1 mono-agonist, producing a signalling pattern that is not the sum of the two acting separately.
Triple: adding glucagon
Retatrutide extends the architecture to GCGR. The glucagon arm is mechanistically distinct from the incretin arms: rather than acting principally on insulin secretion and appetite pathways, it is examined for hepatic effects on lipid handling and for a contribution to energy expenditure. This is the addition that changes the character of the pharmacology rather than its magnitude.
What the comparison does not settle
Receptor count is not a proxy for anything on its own. An imbalanced dual agonist and a balanced one behave differently; a triple agonist with weak activity at its third receptor behaves like a dual. The published designs that matter are the ones that report relative potency at each receptor rather than simply enumerating them.
