Incretin Receptor Agonists: GLP-1, GIP and Glucagon

The incretin agonists are the most intensively studied peptide class of the last decade, and the one where the gap between what has been published and what gets repeated online is widest. Three of them sit in this catalogue: semaglutide, a GLP-1 agonist; tirzepatide, a GLP-1/GIP dual agonist; and retatrutide, a GLP-1/GIP/glucagon triple agonist. This review covers what the receptor arithmetic actually means, why these molecules last for days rather than minutes, and what the trials measured.

What an Incretin Is

Incretins are gut hormones released after eating that potentiate insulin secretion. The two principal human incretins are glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP). Both act on class B G-protein-coupled receptors, and both are degraded within minutes by dipeptidyl peptidase IV (DPP-4), which is the central problem every molecule in this class is engineered around.

Glucagon is not an incretin. It is the counter-regulatory hormone that raises hepatic glucose output. Its presence as a deliberate agonist target in retatrutide is the most counter-intuitive feature of the class and is addressed below.

Single, Dual and Triple: What the Arithmetic Means

Adding a receptor is not simply adding an effect. Each receptor contributes a distinct physiological axis, and the balance between them is a design parameter rather than a free bonus.

• GLP-1 alone (semaglutide): glucose-dependent insulin secretion, suppressed glucagon release, slowed gastric emptying, and central appetite signalling.
• GLP-1 + GIP (tirzepatide): adds the second incretin axis. GIP's contribution is the more debated of the two, with its effects on adipose tissue and on insulin secretion studied separately.
• GLP-1 + GIP + glucagon (retatrutide): adds glucagon receptor agonism, which raises energy expenditure and acts on hepatic lipid handling. The apparent contradiction of agonising a glucose-raising receptor is resolved by the GLP-1 component: the insulinotropic arm is intended to offset the glycaemic effect of the glucagon arm, leaving the energy-expenditure effect.

That last point is the reason a triple agonist is a balancing act rather than a straightforward improvement. The ratio of activity at the three receptors determines whether the glucagon component is useful or counterproductive, and it is specific to the molecule.

Why They Last Days: Lipidation

Native GLP-1 has a half-life of roughly two minutes. Every molecule in this class survives orders of magnitude longer, and the mechanism is the same in all three: a fatty diacid chain attached to a lysine residue through a short spacer.

That chain binds reversibly to serum albumin. Albumin-bound peptide is shielded from DPP-4 and from renal clearance, and it releases slowly back into free circulation, so the albumin pool acts as a depot. The reported human half-lives in the catalogue data follow directly: semaglutide approximately 7 days, retatrutide approximately 6 days, tirzepatide approximately 5 days. Alongside the lipid chain, each molecule carries an Aib (α-aminoisobutyric acid) substitution at position 2, the residue DPP-4 cleaves, which blocks that scission directly.

So the long duration is not a property of the receptor pharmacology at all. It is formulation chemistry bolted onto a peptide backbone.

What the Published Trials Measured

This class is unusual among research peptides in having large, randomised, placebo-controlled human trials in the public record, published in major journals.

Semaglutide was studied in the SUSTAIN and STEP programmes, reporting reductions in HbA1c and in body weight against placebo. Tirzepatide was studied in SURPASS and SURMOUNT, with trials reporting greater reductions on both endpoints than the GLP-1 comparator arms. Retatrutide is the least mature of the three: Jastreboff and colleagues published a phase 2 trial in the New England Journal of Medicine in 2023 reporting dose-dependent weight reduction over 48 weeks. Phase 2 is not phase 3, and the difference matters when reading claims about it.

Retatrutide, a triple agonist, produced substantial reductions in body weight at 48 weeks in a phase 2 trial; longer and larger trials are required to characterise durability and safety.

Two qualifications apply throughout. These trials studied specific clinical populations under supervision, and reported adverse events were dominated by gastrointestinal effects — nausea, vomiting, diarrhoea — which were the most common reason participants discontinued. Discontinuation was also followed by weight regain across this class, which is a finding about the molecules rather than about adherence.

Analytical Characterisation

These are large peptides by research standards: semaglutide is 31 residues at 4113.58 g/mol, tirzepatide and retatrutide are 39-residue molecules above 4700 g/mol, both carrying a C-terminal amide alongside the lipid conjugation. Identity confirmation by mass spectrometry is correspondingly more demanding than for a short peptide, because the expected mass must account for the lipid chain, the spacer and the terminal amide.

A purity figure alone is therefore weak evidence for this class. A certificate that reports the observed mass and shows it matching the fully modified structure is far more informative than one reporting a percentage, since the most likely synthesis defects — incomplete conjugation, missing amidation — change the mass without necessarily producing a visibly impure chromatogram. What HPLC purity testing actually verifies covers the limits of the purity number itself.

Regulatory Status

Semaglutide and tirzepatide are approved prescription medicines in the United States and elsewhere, marketed under brand names by their originators. Retatrutide is investigational and is not approved anywhere. Material supplied for laboratory research is not a medicine, is not supplied for any therapeutic use, and is not an alternative to a prescription product.

Questions and answers

What is the difference between a single, dual and triple receptor agonist?

A single agonist activates one receptor (semaglutide, GLP-1). A dual adds a second (tirzepatide, GLP-1 and GIP). A triple adds a third (retatrutide, GLP-1, GIP and glucagon). Each receptor contributes a distinct physiological axis, and the balance of activity between them is a design parameter rather than a simple sum of effects.

Why does retatrutide target the glucagon receptor if glucagon raises blood sugar?

The glucagon arm contributes increased energy expenditure and hepatic lipid handling, while the GLP-1 arm is intended to offset its glycaemic effect through glucose-dependent insulin secretion. Whether that balance works depends on the relative activity at each receptor, which is specific to the molecule.

Why do these peptides last for days when natural GLP-1 lasts minutes?

Lipidation. Each carries a fatty diacid chain on a lysine residue that binds reversibly to serum albumin, shielding the peptide from DPP-4 and renal clearance and releasing it slowly. Each also carries an Aib substitution at position 2, the residue DPP-4 cleaves. Reported human half-lives are roughly 7 days for semaglutide, 6 for retatrutide and 5 for tirzepatide.

Are these compounds FDA approved?

Semaglutide and tirzepatide are approved prescription medicines in the United States under brand names held by their originators. Retatrutide is investigational and is not approved anywhere. Material supplied for laboratory research is not a medicine and is not supplied for any therapeutic use.

Why is mass spectrometry more important than purity for this class?

Because the most likely synthesis defects, such as incomplete lipid conjugation or missing C-terminal amidation, change the molecular mass without necessarily producing a visibly impure chromatogram. A certificate showing an observed mass matching the fully modified structure is stronger evidence than a purity percentage alone.