Retatrutide (LY3437943)

Retatrutide (development code LY3437943) is a synthetic 39-amino-acid peptide engineered as a single-molecule agonist at three class B1 G protein-coupled receptors: the glucose-dependent insulinotropic polypeptide receptor (GIPR), the glucagon-like peptide-1 receptor (GLP-1R), and the glucagon receptor (GCGR).

The molecule was originated by Eli Lilly and Company and remains investigational. It is not approved by the FDA for any indication. Material catalogued under this name is supplied as a laboratory reference chemical for in vitro and analytical work only, and is not for human or animal consumption. Full terms are set out in the research use policy.

Specifications

A note on the formula and mass. The catalogued formula and the catalogued mass reconcile: C₂₂₁H₃₄₂N₄₆O₆₈ gives a calculated average mass of roughly 4,731.4 Da against the approximately 4,731.3 Da reported by suppliers. Where catalogue figures diverge, the usual explanation is that some entries describe a salt form — acetate or trifluoroacetate — rather than the free peptide, and that residual water content differs between lots. A laboratory needing an exact mass for a specific lot should take it from the deconvoluted mass spectrum on that lot's certificate of analysis rather than from any catalogue, including this one.

Molecular design

Coskun et al. (2022) described the discovery chemistry in Cell Metabolism. Unusually for a multi-receptor incretin analogue, the peptide is built on a GIP-derived backbone rather than a GLP-1 or glucagon scaffold, with substitutions introduced to recruit activity at the other two receptors. Aib residues occupy positions that would otherwise be susceptible to dipeptidyl peptidase-4 cleavage, an α-methyl-leucine residue is incorporated in the mid-sequence, and a C20 fatty diacid is attached through a PEG-type (AEEA)–γ-glutamate spacer on a lysine side chain. That lipid moiety drives reversible albumin binding, which the authors identified as the structural basis for the extended circulating half-life measured in their pharmacokinetic work.

Receptor pharmacology

All three target receptors are Gs-coupled and signal principally through adenylyl cyclase and cyclic AMP. In the in vitro characterisation reported by Coskun et al. (2022), retatrutide showed balanced agonist activity at GCGR and GLP-1R, with comparatively greater activity at GIPR — a deliberately asymmetric profile rather than an equipotent one.

The three arms are mechanistically distinct in the preclinical literature. In the diet-induced obese mouse work in that paper, the authors attributed reduced caloric intake in those mice to GLP-1R and GIPR engagement, and a separate increase in energy expenditure in those mice to GCGR engagement. They framed the combination as additive across two different physiological axes rather than as a single amplified pathway. Glucagon-receptor agonism is the component that most distinguishes this molecule from dual GIPR/GLP-1R agonists, and it is also the component that raises the most open pharmacological questions, since GCGR activation has counter-regulatory effects on hepatic glucose output.

Structural biology

Li et al. (2024) reported cryo-electron microscopy structures of retatrutide bound to each of GLP-1R, GIPR and GCGR in complex with heterotrimeric Gs, published in Cell Discovery, at reported resolutions of 2.68 Å, 3.26 Å and 2.84 Å respectively. The structures resolve how one peptide sequence is accommodated by three related but non-identical orthosteric pockets, and identify the residue-level contacts that differ between the three complexes, including receptor-specific conformations in ECL1. These structures are the published structural basis for interpreting the molecule's selectivity profile.

Published study history

The peer-reviewed record runs from preclinical work through phase 3. Each entry below states the model. Endpoints are named; outcome magnitudes are left to the cited publications.

• Murine. Coskun et al. (2022) reported body-weight and glycaemic-control endpoints in diet-induced obese mice, and dissected the contribution of each receptor arm.
• Phase 1, human. The same paper reported a single-ascending-dose study in which the authors described a prolonged circulating half-life attributable to albumin binding. Adverse events were recorded and reported in that publication.
• Phase 1b, human. Urva et al. (2022), in The Lancet, reported a multicentre, double-blind, placebo-controlled, randomised multiple-ascending-dose trial in people with type 2 diabetes.
• Phase 2, human. Jastreboff et al. (2023), in The New England Journal of Medicine, reported a randomised, double-blind, placebo-controlled trial in adults with obesity, with change in body weight as the primary endpoint. Outcomes are reported in that publication.
• Phase 2, human. Rosenstock et al. (2023), in The Lancet, reported a randomised, double-blind, placebo- and active-controlled parallel-group trial in people with type 2 diabetes conducted in the USA.
• Phase 2a, human. Sanyal et al. (2024), in Nature Medicine, reported a randomised trial in metabolic dysfunction-associated steatotic liver disease, with imaging-derived liver fat content as the endpoint of interest.
• Phase 3, human. Bajaj et al. (2026), in The Lancet, reported TRANSCEND-T2D-1, a double-blind randomised trial in people with type 2 diabetes inadequately controlled with diet and exercise, with change in HbA1c as the primary endpoint.

Limits of the current evidence

Phase 3 results have now entered the peer-reviewed record for type 2 diabetes, but the published base remains limited in ways that matter. Trials at this stage are not powered for uncommon adverse events. They are shorter than the exposure periods that would be relevant to a chronically administered agent. And the GCGR arm in particular carries theoretical questions — hepatic glucose output, resting heart rate — that the published trials were not designed to resolve. Gastrointestinal adverse events were the most frequently reported category across the published trials. No long-term human safety profile has been established, and no regulatory authority has completed a benefit-risk review of this molecule.

Regulatory status

Retatrutide is an unapproved investigational drug in the United States. There is no approved product and no lawful compounding pathway. Since 2024 the FDA has issued warning letters to multiple vendors distributing material under this name, and the agency's stated position is that a "research use only" or "not for human consumption" label does not convert an unapproved new drug into a lawful product. This page describes the compound's published pharmacology and nothing else; it is not a representation that the material is suitable for administration to any person or animal.

Identity and purity verification

For a lipidated 39-mer, purity by RP-HPLC alone is not sufficient evidence of identity, because closely related process impurities — deletion sequences, des-lipid species, and diacid regioisomers — can co-elute or elute near the main peak. A meaningful certificate pairs an HPLC purity trace with a mass-spectrometric identity confirmation showing a deconvoluted mass consistent with the intended structure, plus water content and residual-solvent or acetate data where relevant. Our approach to third-party analysis is documented on the lab testing page.

Handling and storage

Lyophilised peptide is stored at −20 °C, desiccated, and protected from light; repeated warming to ambient temperature in a humid environment is the most common cause of avoidable degradation, so aliquoting before storage is generally preferable to repeatedly opening a single vial. Once a lyophilised peptide is taken into solution its stability window shortens considerably and becomes buffer- and temperature-dependent. General laboratory guidance is collected under peptide storage and reconstitution, and the peptide calculator handles solution-concentration arithmetic for bench work.

Frequently asked questions

Is retatrutide FDA approved?

No. It is an investigational compound that has not been approved by the FDA for any indication, and no approved product containing it exists. It is offered here for laboratory research use only and is not for human or animal consumption.

How should lyophilised material be stored?

At −20 °C, desiccated and protected from light. Lower temperatures are used for long-term archival storage. Freeze-thaw cycling should be minimised, which in practice means aliquoting rather than repeatedly accessing one container.

Why do the molecular weight figures differ between suppliers?

Chiefly because catalogue entries vary in whether they describe the free peptide or a salt form such as acetate or trifluoroacetate, and because residual water content differs between lots. The free-peptide formula and mass are consistent in the public record. The mass reported on a specific lot's mass spectrum is the only figure that describes the material actually in hand.

What documentation should accompany a research lot?

A certificate of analysis identifying the lot, an RP-HPLC purity chromatogram, a mass-spectrometry identity confirmation, and water content. A certificate that reports a purity percentage with no underlying chromatogram or spectrum is an assertion rather than evidence.

References

• Coskun et al. (2022). LY3437943, a novel triple glucagon, GIP, and GLP-1 receptor agonist for glycemic control and weight loss: From discovery to clinical proof of concept. Cell Metabolism.
• Urva et al. (2022). LY3437943, a novel triple GIP, GLP-1, and glucagon receptor agonist in people with type 2 diabetes: a phase 1b, multicentre, double-blind, placebo-controlled, randomised, multiple-ascending dose trial. The Lancet.
• Jastreboff et al. (2023). Triple–Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial. The New England Journal of Medicine.
• Rosenstock et al. (2023). Retatrutide, a GIP, GLP-1 and glucagon receptor agonist, for people with type 2 diabetes: a randomised, double-blind, placebo and active-controlled, parallel-group, phase 2 trial conducted in the USA. The Lancet.
• Sanyal et al. (2024). Triple hormone receptor agonist retatrutide for metabolic dysfunction-associated steatotic liver disease: a randomized phase 2a trial. Nature Medicine.
• Li et al. (2024). Structural insights into the triple agonism at GLP-1R, GIPR and GCGR manifested by retatrutide. Cell Discovery.
• Bajaj et al. (2026). Efficacy and safety of retatrutide, a GIP, GLP-1, and glucagon receptor agonist, in people with type 2 diabetes and inadequate glycaemic control with diet and exercise (TRANSCEND-T2D-1): a double-blind, randomised, phase 3 trial. The Lancet.