Retatrutide (LY3437943) is an investigational triple hormone receptor agonist that simultaneously activates glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP), and glucagon receptors. By adding glucagon receptor agonism to the dual incretin approach, retatrutide aims to enhance metabolic effects—particularly energy expenditure and hepatic fat oxidation—beyond what dual agonists achieve. Phase 2 clinical data showed unprecedented weight loss of 24.2% at 48 weeks, positioning retatrutide as a leading next-generation obesity therapeutic candidate.
The Triple Agonist Concept
The rationale for triple agonism builds on the success of dual GIP/GLP-1 agonism (tirzepatide) by incorporating glucagon receptor activation. Glucagon, produced by pancreatic alpha cells, is traditionally viewed as a counter-regulatory hormone to insulin. However, glucagon has metabolic effects beyond glucose regulation that are therapeutically relevant: it increases hepatic fatty acid oxidation, stimulates thermogenesis in brown adipose tissue, increases resting energy expenditure, and promotes amino acid catabolism.
The challenge with glucagon agonism in metabolic disease has been its hyperglycemic effect—glucagon stimulates hepatic glucose output. Retatrutide addresses this by combining glucagon receptor activation with GLP-1 and GIP receptor agonism, which enhance glucose-dependent insulin secretion. The incretin components offset glucagon’s hyperglycemic tendency while preserving its energy expenditure and fat oxidation effects.
Molecular Design
Retatrutide is a 39-amino-acid peptide designed on a glucagon backbone with strategic modifications to introduce GLP-1 and GIP receptor binding. The peptide features an acyl chain for albumin binding (extending half-life for weekly dosing) and optimized receptor selectivity ratios. The relative potencies at each receptor have been engineered to maximize metabolic benefit while maintaining glycemic safety—GLP-1 and GIP agonism are sufficient to counterbalance glucagon-induced hepatic glucose output.
Phase 2 Clinical Results
The Phase 2 trial (n=338) evaluated retatrutide at doses ranging from 0.5 mg to 12 mg weekly over 48 weeks in adults with obesity. The highest dose (12 mg) produced mean weight loss of 24.2%, with some participants exceeding 30% body weight reduction. These results represent the highest pharmacological weight loss achieved in any clinical trial to date, approaching outcomes typically seen with Roux-en-Y gastric bypass surgery.
Dose-response analysis showed clear escalation: 1 mg produced ~8% weight loss, 4 mg produced ~17%, 8 mg produced ~22%, and 12 mg produced ~24%. Weight loss trajectories had not fully plateaued at 48 weeks, suggesting that longer treatment duration might yield even greater reductions. The 12 mg dose showed no signal of increased adverse events compared to lower doses.
Glycemic control data were equally impressive in the diabetes sub-study. Retatrutide 12 mg produced HbA1c reductions of -2.02% from a baseline of ~8.3%, with 71% of participants achieving HbA1c below 5.7% (normal range). Fasting glucose normalization occurred rapidly, within the first 12 weeks of treatment.
Hepatic Fat Reduction
A remarkable finding was retatrutide’s effect on liver fat. In participants with baseline hepatic steatosis (fatty liver), the 12 mg dose reduced liver fat content by a mean of 86% at 48 weeks, with 93% of participants achieving normal liver fat levels (below 5%). This exceeds the hepatic fat reduction seen with any other pharmacological agent and reflects glucagon’s potent effect on hepatic lipid oxidation. These findings have significant implications for NASH/MASH research.
Safety and Tolerability
The adverse event profile was consistent with the GLP-1RA class—nausea, diarrhea, and vomiting were the most common side effects, occurring primarily during dose escalation. Importantly, despite glucagon receptor activation, there was no clinically significant hyperglycemia. Heart rate increases of 2-4 beats per minute were observed, similar to GLP-1RAs. No pancreatitis cases or thyroid safety signals were reported in the Phase 2 trial.
Phase 3 Program and Future Outlook
Eli Lilly initiated the Phase 3 TRIUMPH program for retatrutide, with trials in obesity with and without type 2 diabetes, and a dedicated NASH/MASH trial. If Phase 3 results confirm the Phase 2 findings, retatrutide could represent a new standard for pharmacological weight management and potentially the first effective monotherapy for metabolic liver disease.
Frequently Asked Questions
What is retatrutide?
Retatrutide (LY3437943) is an investigational triple hormone receptor agonist that simultaneously activates GLP-1, GIP, and glucagon receptors. It represents the next evolution beyond dual agonists like tirzepatide.
How does retatrutide compare to tirzepatide and semaglutide?
In Phase 2 trials, retatrutide 12 mg produced 24.2% mean weight loss at 48 weeks—exceeding both semaglutide and tirzepatide. The addition of glucagon receptor agonism enhances energy expenditure and hepatic fat oxidation.
What does glucagon receptor agonism add?
Glucagon receptor activation increases energy expenditure through thermogenesis, promotes hepatic fat oxidation, and stimulates lipolysis. When balanced with incretin agonism, the hyperglycemic effect is offset while metabolic benefits are preserved.