Incretin Receptors: Why Single, Dual, and Triple Agonists Differ

The receptors

GLP-1, GIP, and glucagon receptors are all class B G-protein-coupled receptors signalling primarily through Gαs and cAMP accumulation. Their tissue distribution differs, and that distribution is what makes engaging one rather than another consequential.

GLP-1 receptors are found on pancreatic β-cells but also extensively in the hypothalamus and brainstem, which is the anatomical basis for research into central pathways rather than purely pancreatic ones.

GIP receptors are expressed in pancreatic tissue and, notably, in adipose tissue and the central nervous system. Glucagon receptors are concentrated in the liver, which is why glucagon receptor agonism is studied for hepatic lipid handling specifically.

Half-life engineering is most of the design

Native GLP-1 has a circulating half-life of roughly two minutes. Almost everything distinctive about these molecules is a solution to that problem.

The first move is blocking DPP-4 cleavage, usually by substituting an α-aminoisobutyric acid (Aib) residue at the vulnerable position. Semaglutide does this at position 8; tirzepatide at positions 2 and 13.

The second is albumin binding. A fatty diacid chain — C18 on semaglutide, C20 on tirzepatide — is conjugated to a lysine residue via a spacer. The chain reversibly binds serum albumin, which both shields the peptide from renal filtration and creates a slow-release depot.

The third is ensuring the conjugation happens at one defined position. Semaglutide substitutes Lys34 with arginine so that the fatty acid attaches only at Lys26, avoiding a heterogeneous mixture of products.

Native GLP-1 lasts about two minutes. Nearly every structural feature of these compounds exists to solve that one problem.

Imbalanced agonism

Tirzepatide is routinely described as a dual agonist, which implies balance it does not have. Published receptor pharmacology reports affinity for the GIP receptor comparable to native GIP, while its GLP-1 receptor affinity is substantially weaker than native GLP-1 — roughly fivefold lower in binding assays.

It is therefore weighted toward GIP. Study designs that treat it as equipotent at both receptors will misattribute effects to the wrong arm.

There is also a signalling-bias dimension. At the GLP-1 receptor, tirzepatide has been reported to favour cAMP accumulation over β-arrestin recruitment, with reduced receptor internalisation. Whether that bias accounts for observed differences against selective GLP-1 agonists remains an open question.

The unresolved GIP question

This is a genuine controversy in the field and worth knowing before designing a study around assumed directionality.

Both GIP receptor agonism and GIP receptor antagonism have been reported to produce broadly similar metabolic effects in preclinical models. That should not be possible if the receptor works the way a simple model predicts.

The most commonly proposed reconciliation is that sustained agonism produces functional receptor desensitisation — effectively behaving as antagonism downstream. It is a plausible account and it is not settled.

The practical consequence: if your study depends on GIP receptor directionality, that assumption is load-bearing and currently unsupported. Design accordingly.

Amylin is a separate axis

Cagrilintide is frequently grouped with incretin agonists and does not belong there. It is an acylated analog of human amylin — the peptide co-secreted with insulin from β-cells — acting at the calcitonin receptor and amylin receptor subtypes.

That pathway is mechanistically independent of GLP-1 and GIP signalling entirely. This is why cagrilintide appears in combination studies alongside semaglutide rather than in comparisons against it: the research question is whether two independent satiety pathways are additive.