Retatrutide (development code LY3437943) is a synthetic peptide engineered to activate three receptors at once: the glucose-dependent insulinotropic polypeptide receptor (GIPR), the glucagon-like peptide-1 receptor (GLP-1R), and the glucagon receptor (GCGR). The first two are familiar from tirzepatide and semaglutide research. The third is what sets retatrutide apart, and it raises the most interesting design question in the class: glucagon is classically the hormone that opposes insulin, so why build it into a metabolic drug candidate on purpose? This article focuses on that mechanistic question. For a trial-by-trial summary, see our in-depth retatrutide overview.
The Biology of Glucagon Beyond Blood Sugar
Glucagon is secreted by pancreatic alpha cells and is best known for raising hepatic glucose output during fasting. But the glucagon receptor is expressed in the liver and, at lower levels, in other tissues, and glucagon signaling has several effects that made it attractive to obesity researchers. In rodent and human physiology studies, glucagon has been reported to increase energy expenditure, reduce food intake (an effect thought to involve hepatic vagal signaling), promote hepatic fatty-acid oxidation, and increase amino-acid catabolism. The obvious liability is hyperglycemia, since glucagon agonism alone pushes glucose up. The design logic of a triagonist is that GLP-1R and GIPR activity, which stimulate glucose-dependent insulin secretion and suppress appetite, can offset that liability while the glucagon component contributes the energy-expenditure and liver-fat effects.
From Dual to Triple Agonists: The Preclinical Lineage
The idea was not new when retatrutide arrived. Day and colleagues described a glucagon/GLP-1 co-agonist in 2009 that reduced body weight in diet-induced obese rodents (Nat Chem Biol 2009;5:749-757). Finan and colleagues then published a rationally designed monomeric peptide triagonist acting on GLP-1, GIP and glucagon receptors, reporting corrected obesity and diabetes in rodent models (Nat Med 2015;21:27-36). Retatrutide's own preclinical characterization was reported by Coskun and colleagues in Cell Metabolism in 2022. In vitro, the molecule is reported to be strongly active at GIPR, with weaker potency than the native ligands at the GLP-1 and glucagon receptors. In mice, it reduced body weight more than a comparator GLP-1R/GIPR dual agonist, and the authors attributed part of the added effect to increased energy expenditure.
This receptor balance is a deliberate design variable. A triagonist is not simply three drugs in one vial; the relative potency at each receptor determines the net physiology, and published characterizations rely on cAMP accumulation assays in cells expressing each receptor individually. That is the same style of assay a research laboratory would use to profile any multi-agonist peptide.
What the Human Studies Reported
- Phase 1b (Urva et al., Lancet 2022;400:1869-1881): multiple-ascending-dose study in people with type 2 diabetes, reporting dose-dependent reductions in glucose and body weight and a gastrointestinal-predominant tolerability profile.
- Phase 2 in obesity (Jastreboff et al., N Engl J Med 2023;389:514-526): 338 adults, 48 weeks. Mean weight change at the highest dose (12 mg) was reported as 24.2%, versus 2.1% with placebo, and weight loss had not plateaued by week 48. Adverse events were mostly gastrointestinal and dose-related; heart rate increased in a dose-dependent manner and then declined later in treatment.
- Phase 2 in type 2 diabetes (Rosenstock et al., Lancet 2023;402:529-544): reported HbA1c and weight reductions versus placebo and an active GLP-1 comparator (dulaglutide).
- Liver fat substudy (Sanyal et al., Nat Med 2024;30:2037-2048): reported large, dose-dependent reductions in liver fat content in participants with steatotic liver disease. This is the most direct human readout of the glucagon-receptor hypothesis, because hepatic lipid handling is a classic glucagon target.
Phase 3 Topline (Company-Reported)
In 2026, Eli Lilly reported topline results from the TRIUMPH-1 trial (NCT05929066; 2,339 adults with obesity or overweight, 80 weeks). At 12 mg, the reported mean weight change was 28.3%, versus 2.2% with placebo. Gastrointestinal events were the most common adverse events, and discontinuation due to adverse events was 11.3% at 12 mg versus 4.9% with placebo. Dysesthesia, an abnormal skin sensation, was reported in 12.5% at 12 mg versus 0.9% with placebo. These figures come from the sponsor's release and conference presentation and had not completed full peer-reviewed publication at the time of writing; they should be treated as provisional until the paper is available.
Unresolved Research Questions
- Attribution. In a triple agonist, the contribution of each receptor cannot be separated from the combined result. Whether the glucagon arm drives energy expenditure in humans, as it does in rodents, is not directly established by the trials above.
- Cardiovascular parameters. The heart-rate increase seen in Phase 2 is consistent with known glucagon and GLP-1 physiology and remains a monitored endpoint in the ongoing program.
- Body composition. Large weight changes raise the question of lean versus fat mass, which the available publications address only partially.
- Durability. Weight regain after discontinuation, as seen with other incretin-based agents, has not been characterized for this molecule in published long-term data.
- Neurosensory events. The dysesthesia signal is new relative to earlier incretin agents and its mechanism is not explained in the available reports.
Regulatory Status
Retatrutide is investigational. It is not approved by the FDA or any other regulator at the time of writing. Material sold for laboratory use is not the clinical product and is not a substitute for it.
Sources
- Jastreboff AM, et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial. N Engl J Med 2023;389:514-526. doi:10.1056/NEJMoa2301972. PubMed
- Coskun T, 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;34:1234-1247.
- Urva S, 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.
- Rosenstock J, 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. Lancet 2023;402:529-544.
- Sanyal AJ, 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.
- Finan B, et al. A rationally designed monomeric peptide triagonist corrects obesity and diabetes in rodents. Nat Med 2015;21:27-36.
- Day JW, et al. A new glucagon and GLP-1 co-agonist eliminates obesity in rodents. Nat Chem Biol 2009;5:749-757.
- Eli Lilly and Company. TRIUMPH-1 topline results, May 21, 2026 (company-reported; ClinicalTrials.gov NCT05929066).
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