GLP-1, GIP and Glucagon: Understanding Incretin Receptor Pharmacology

The incretin field moved from single-receptor agonists to dual and now triple agonists in about a decade. Understanding why requires understanding the receptors.

Class B GPCRs

GLP-1R, GIPR and GCGR are all class B (secretin-family) G protein-coupled receptors. They share an architecture that distinguishes them from the class A receptors most pharmacology textbooks use as the default example:

• A large extracellular domain (ECD) that captures the C-terminal portion of the peptide ligand
• A seven-transmembrane bundle into which the peptide's N-terminus inserts to trigger activation

This two-domain mechanism is why class B receptors bind peptides rather than small molecules, and why designing small-molecule agonists for them has proved so difficult. The binding surface is large and largely flat.

All three couple primarily to Gαs, raising intracellular cAMP and activating protein kinase A. All three also recruit β-arrestin, which drives receptor internalisation and desensitisation — and which turns out to matter enormously.

The three receptors

GLP-1R. Activated by glucagon-like peptide-1, released from intestinal L-cells after a meal. Expressed on pancreatic β-cells, in the CNS (particularly the hypothalamus and area postrema), the heart and the GI tract. β-cell signalling is glucose-dependent — GLP-1R activation potentiates insulin secretion only when glucose is already elevated, which is a genuinely unusual and useful property.

GIPR. Activated by glucose-dependent insulinotropic polypeptide from intestinal K-cells. Expressed on β-cells, adipocytes and in the CNS. Its role is more contested than GLP-1's: both agonism and antagonism at GIPR have been argued to produce beneficial metabolic effects in different models, and resolving that contradiction is an active research question rather than a settled matter.

GCGR. Activated by glucagon from pancreatic α-cells. Classically the counter-regulatory hormone raising hepatic glucose output — but also driving hepatic lipid oxidation and energy expenditure. Adding glucagon agonism to an incretin agonist is a deliberate trade: some glycaemic cost in exchange for increased energy expenditure.

Why combine them

Mono-agonists (semaglutide, liraglutide, exenatide) act at GLP-1R alone. Well-characterised, extensively studied, and the reference standard against which everything else is measured.

Dual agonists (tirzepatide) hit GIPR and GLP-1R. The rationale is that the two incretins act on overlapping but distinct cell populations, and combined activation produces effects greater than either alone. There is also evidence that GIPR co-agonism modifies GLP-1R-associated tolerability, though the mechanism is debated.

Triple agonists (retatrutide) add GCGR. The energy expenditure contribution from glucagon signalling is added to the incretin effects, with GLP-1 and GIP agonism offsetting glucagon's hyperglycaemic tendency.

For a research programme, the appeal of a triple agonist is not only its potency. It is that a single tool compound lets you interrogate three related receptors in one system — and, with selective antagonists, dissect which contributes what.

Biased agonism

This is where incretin pharmacology gets genuinely subtle.

A receptor can signal through multiple pathways. For GLP-1R, the two that matter are Gαs/cAMP and β-arrestin recruitment. A biased agonist activates one preferentially.

Why it matters: β-arrestin recruitment drives receptor internalisation and desensitisation. An agonist that produces strong cAMP signalling with weak β-arrestin recruitment keeps the receptor at the surface and available. Over sustained exposure, that G protein-biased agonist can produce a larger integrated response than a "more potent" balanced agonist that desensitises its own receptor.

Tirzepatide is frequently described as G protein-biased at GLP-1R relative to native GLP-1 — which may explain part of its profile independently of its GIPR activity. Whether that bias is a designed feature or an emergent property of the molecule is exactly the kind of question comparative in-vitro work exists to answer.

Half-life engineering

Native GLP-1 has a half-life of one to two minutes. DPP-4 cleaves it almost immediately at the Ala2 position. Every therapeutic incretin agonist solves this, and the solutions are instructive:

• Position 2 substitution — replacing Ala2 with Aib or another DPP-4-resistant residue blocks cleavage
• Fatty acid acylation — a C18 di-acid chain via a γ-Glu spacer binds albumin reversibly, creating a circulating depot. Semaglutide uses this.
• Backbone modification — unnatural residues resisting general proteolysis

Combining substitution and acylation takes a two-minute half-life to roughly a week. Almost all of the difference between native GLP-1 and semaglutide is engineering of this kind, not receptor pharmacology.

Designing comparative studies

Three practical points for anyone running comparisons:

Match the batch cycle. Comparing compounds from different COA cycles introduces a purity variable you cannot separate from the pharmacology. Source comparators together.

Measure more than cAMP. A cAMP-only readout cannot detect bias. Pair it with a β-arrestin recruitment assay.

Report the reference agonist. Potency values are only comparable relative to a stated standard run in the same system on the same day.

Our Metabolic Research Kit exists for the first of those points: semaglutide, tirzepatide and retatrutide from a single COA cycle, so cross-compound comparison is not confounded by batch variation.

All compounds referenced are supplied strictly for laboratory research use and are not for human or veterinary consumption.