Retatrutide Research Review: GIP, GLP-1 and Glucagon

Retatrutide is one of the most closely watched investigational peptides in metabolic research. It is a single molecule designed to activate three class B G protein-coupled receptors at once: the glucose-dependent insulinotropic polypeptide receptor (GIPR), the glucagon-like peptide-1 receptor (GLP-1R) and the glucagon receptor (GCGR). This review covers where the molecule came from, how it is built, what its receptor pharmacology looks like in published preclinical work, and how the clinical programme is structured. Retatrutide is not an approved medicine anywhere at the time of writing.

Origin and discovery

Retatrutide was discovered at Eli Lilly under the code LY3437943. It follows a lineage of "unimolecular polypharmacology" that began with GLP-1/glucagon co-agonists described by academic and industry groups in 2009, continued through GIP/GLP-1 co-agonists, and led to the first triple agonist concepts published by the Tschöp and DiMarchi groups (Finan et al., 2015). The rationale was additive: GLP-1R agonism reduces food intake and improves glucose-dependent insulin secretion, GIPR agonism appears to complement those effects, and GCGR agonism raises energy expenditure and hepatic fat oxidation in rodent models.

The discovery and early clinical characterisation of LY3437943 were published by Coskun and colleagues in Cell Metabolism in 2022. That paper combined in vitro receptor assays, rodent studies of body weight and energy expenditure, and a phase 1 study in people with type 2 diabetes.

Structure and chemistry

Retatrutide is a 39-residue linear peptide. Its backbone is derived from the GIP sequence family rather than from GLP-1, and it carries several non-proteinogenic amino acids, including α-aminoisobutyric acid (Aib) and other α-methylated residues, which resist cleavage by dipeptidyl peptidase-4 (DPP-4) and help stabilise the helical conformation needed for receptor engagement.

A C20 fatty diacid is attached to a lysine side chain through a hydrophilic linker. As with semaglutide and tirzepatide, this lipid moiety binds reversibly to serum albumin, slowing clearance and renal filtration. The reported half-life in clinical pharmacology studies is approximately six days. The C-terminus is amidated.

For laboratories, the practical points are the same as for other lipidated incretin analogues: the molecule is large, amphipathic and prone to aggregation if mishandled, and identity should be confirmed by high-resolution mass spectrometry against the theoretical mass rather than by HPLC retention time alone.

Receptor pharmacology and mechanism studied

In the in vitro characterisation by Coskun et al. (2022), retatrutide was more potent than native GIP at the human GIP receptor and less potent than the native hormones at the human GLP-1 and glucagon receptors. In other words, it is deliberately "imbalanced", in the same sense that tirzepatide was described as imbalanced toward GIPR.

The three receptor components are studied for distinct contributions:

• GLP-1R. Activation in the pancreas enhances glucose-dependent insulin secretion and suppresses glucagon release; activation in the hindbrain and hypothalamus reduces food intake; activation in the gut slows gastric emptying.
• GIPR. GIP receptors are found in islets, adipose tissue and the central nervous system. In rodent work, GIPR agonism contributes to reduced food intake and may improve tolerability of GLP-1R agonism, although its exact role in humans remains debated.
• GCGR. Hepatic glucagon receptor activation increases fatty-acid oxidation and reduces lipogenesis. In mouse studies with LY3437943, energy expenditure rose relative to comparators lacking glucagon activity. The glucose-raising effect of glucagon is offset by the incretin components.

A key mechanistic question is how much each arm contributes in humans. Rodent knockout and comparator studies help, but species differences in GIPR and GCGR pharmacology mean that preclinical rankings do not always translate.

Research and clinical-trial landscape

The published clinical record to date includes:

• Phase 1. Single- and multiple-ascending studies in healthy participants and in people with type 2 diabetes characterised pharmacokinetics, tolerability and glucose and body-weight endpoints (Coskun et al., 2022; Urva et al., 2022).
• Phase 2 obesity. A 48-week randomised, placebo-controlled trial in adults with obesity or overweight evaluated body-weight endpoints as the primary outcome, with cardiometabolic secondary endpoints (Jastreboff et al., 2023, NEJM).
• Phase 2 type 2 diabetes. A randomised trial with placebo and dulaglutide comparators evaluated HbA1c and body-weight endpoints (Rosenstock et al., 2023, Lancet).
• Liver fat substudy. A substudy of the obesity trial in participants with metabolic dysfunction-associated steatotic liver disease evaluated MRI-measured liver fat (Sanyal et al., 2024, Nature Medicine).

Phase 3 work is organised under the TRIUMPH name and spans obesity, obesity with knee osteoarthritis, obstructive sleep apnoea and cardiovascular and renal outcomes, alongside separate trials in type 2 diabetes. Trial registrations and readouts should be checked directly on ClinicalTrials.gov and in peer-reviewed publications, as the programme is active and topline announcements by the sponsor precede full publication.

Adverse events reported across phase 2 were predominantly gastrointestinal, consistent with the incretin class, and increases in heart rate were observed. Researchers comparing classes should regard these as safety signals still being characterised rather than settled findings.

Evidence quality and open questions

• Phase 2 is not phase 3. The published efficacy record rests on moderately sized phase 2 trials. Larger, longer trials with diverse populations are needed before effect sizes can be compared credibly with approved agents.
• Cross-trial comparisons are weak. Retatrutide has not yet been compared head to head with tirzepatide or semaglutide in published phase 3 data. Differences in baseline characteristics, titration and trial duration confound informal comparisons.
• Contribution of glucagon agonism. Whether the GCGR arm meaningfully raises energy expenditure in humans, as it does in rodents, is not fully established. Indirect calorimetry and body-composition studies are needed.
• Lean mass and composition. As with all agents that reduce body weight, the proportion of fat versus lean tissue lost is an important open question for the class.
• Long-term safety. Heart rate effects, hepatic effects of chronic glucagon signalling, and rare events cannot be judged from phase 2 data.
• Material identity. Research-grade material sold outside regulated channels has no connection to the sponsor's clinical supply. Independent analytical verification is essential.

Regulatory and anti-doping status

Retatrutide is an investigational compound. It is not approved by the FDA, the EMA or other major regulators, and no prescription product exists. Research-grade material is intended strictly for in vitro and animal research and is not for human or veterinary use.

Retatrutide is not named on the WADA Prohibited List at the time of writing. However, the list includes a catch-all category (S0) for pharmacological substances not approved by any governmental health authority, which can apply to investigational compounds, and the list is revised every year. Researchers working with athletes or sports bodies should consult the current WADA list and their national anti-doping organisation.

Summary table

Property	Retatrutide
Code name	LY3437943
Developer	Eli Lilly
Receptor targets	GIPR, GLP-1R, GCGR
Relative potency (in vitro)	Highest at GIPR; lower than native ligands at GLP-1R and GCGR
Length	39 residues
Key modifications	Aib and other α-methyl residues; C20 fatty diacid via linker
Reported half-life	About six days
Published clinical stage	Phase 2 (obesity, type 2 diabetes, liver fat substudy)
Ongoing programme	Phase 3 TRIUMPH and related trials
Regulatory status	Investigational; not approved
Anti-doping	Not named on WADA list; check current list and S0

References

• Coskun T, Urva S, Roell WC, et al. 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 Metab. 2022;34(9):1234–1247.
• Jastreboff AM, Kaplan LM, Frías JP, et al. Triple-hormone-receptor agonist retatrutide for obesity — a phase 2 trial. N Engl J Med. 2023;389:514–526.
• Rosenstock J, Frías J, Jastreboff AM, et al. 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. Lancet. 2023;402:529–544.
• Sanyal AJ, Kaplan LM, Frías JP, et al. Triple hormone receptor agonist retatrutide for metabolic dysfunction-associated steatotic liver disease: a randomized phase 2a trial. Nat Med. 2024;30:2037–2048.
• Urva S, Coskun T, Loh MT, et al. 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. Lancet. 2022;400:1869–1881.
• Finan B, Yang B, Ottaway N, et al. A rationally designed monomeric peptide triagonist corrects obesity and diabetes in rodents. Nat Med. 2015;21(1):27–36.
• World Anti-Doping Agency. The Prohibited List (current edition).