Retatrutide mechanism of action: how one peptide works on three receptors

Photograph: editorial illustration for this article. It does not show retatrutide.

What is the retatrutide molecule?

Retatrutide, first published under the code LY3437943, is a synthetic peptide of 39 amino acids. Its sequence is based on GIP, the gut hormone glucose-dependent insulinotropic polypeptide, and was modified residue by residue until the same chain could also fit the GLP-1 receptor and the glucagon receptor. A C20 fatty diacid is attached through a linker so that the peptide binds to albumin in the bloodstream, which protects it from clearance and gives it a half-life long enough for weekly injection^[1].

The phase 1b study in people with type 2 diabetes measured that half-life at about six days, and found that weight fell by up to roughly 9 kg after 12 weeks in the highest dose group, an early sign that the three-receptor design did what it was meant to do^[2].

What do the three receptors do?

GLP-1 receptor

This is the target of semaglutide and liraglutide. Activation slows gastric emptying, reduces appetite through receptors in the hypothalamus and brainstem, and increases insulin secretion only when glucose is elevated. It is the main driver of reduced food intake and the main source of nausea when doses rise too quickly.

GIP receptor

GIP is the other incretin hormone. Its receptor is expressed in pancreatic beta cells, fat tissue and parts of the brain. Adding GIP receptor agonism to GLP-1 was the design principle of tirzepatide, and in retatrutide the GIP component is the strongest of the three in cell assays: the discovery paper reports higher potency at the human GIP receptor than native GIP itself, with GLP-1R and GCGR potency deliberately set at a fraction of the native hormones^[1].

Glucagon receptor

Glucagon is the hormone that opposes insulin. On its own it raises blood glucose by releasing stored glucose from the liver, which is why glucagon injections are used to treat severe hypoglycaemia. But glucagon receptor signalling also increases resting energy expenditure and drives fatty acid oxidation in the liver. Retatrutide uses this: the glucagon component adds weight loss beyond what appetite suppression alone delivers and strips fat out of the liver, while the incretin components keep glucose under control^[1].

Why does the balance between receptors matter?

Too much glucagon activity would raise blood sugar and heart rate; too little would give up the extra weight loss. The published in vitro profile of retatrutide is weighted toward GIP, with GLP-1 and glucagon activity reduced relative to the native ligands, and the paper describes this as an intentional choice made after testing a series of related peptides in cells and in animals^[1]. In people with type 2 diabetes the balance worked in the intended direction: the phase 2 trial reported HbA1c falling by about two percentage points at 24 weeks on the 8 mg and 12 mg doses, not rising^[4].

What happens in the body, step by step

1. A weekly subcutaneous injection releases the peptide slowly from the injection site; albumin binding keeps it in circulation for days.
2. GLP-1R and GIPR activation in the pancreas increases insulin and lowers glucagon secretion after meals, in proportion to how high glucose is.
3. GLP-1R activation in the gut and brain slows gastric emptying and lowers hunger, so energy intake falls.
4. GCGR activation in the liver increases fat oxidation and, with the other two receptors, raises overall energy expenditure.
5. Over months, lower intake plus higher expenditure produces the weight loss seen in trials, and liver fat falls faster than total body weight.

That last point is measurable. In the phase 2 liver substudy, people with at least 10% liver fat at baseline lost 81.4% (8 mg) and 82.4% (12 mg) of their liver fat by week 24, while total body weight had fallen by 17 to 18%^[5][3]. A drop in liver fat that large, that early, is the glucagon receptor at work. We go into the liver data in retatrutide and fatty liver.

What does the trial evidence show about the mechanism?

Two features of this curve are consistent with the mechanism. The weight loss is dose-dependent, as expected from receptor pharmacology, and it had not plateaued at 48 weeks on the higher doses, which the authors noted in the paper^[3]. The same trial recorded a dose-related rise in heart rate that peaked around week 24 and then declined, a pattern attributed to the glucagon and GLP-1 components^[3]. Side effects are covered in GLP-3 side effects.

What is still unknown about how retatrutide works?

The published papers establish receptor pharmacology and clinical outcomes, but several mechanistic questions remain open in the literature: how much of the weight loss is lean mass versus fat, how the three receptors interact in the brain, whether glucagon receptor activity contributes to the heart rate increase independently of GLP-1, and what happens to weight after the drug is stopped. The phase 3 TRIUMPH program, described in our clinical trials timeline, includes body composition and maintenance substudies that should answer some of these.