A 39-amino-acid peptide and the first triple agonist targeting GLP-1, GIP, and glucagon receptors simultaneously. In Phase II trials, it produced up to 24.2% body weight loss at 48 weeks — the highest weight loss ever achieved by a drug in clinical trials. Currently in Phase III development by Eli Lilly.
Retatrutide (LY3437943) is a 39-amino-acid peptide being developed by Eli Lilly as the first triple agonist targeting all three incretin/glucagon receptors: GLP-1, GIP, and glucagon receptors. If tirzepatide was the 'twincretin,' retatrutide is the 'triagonist.'
In the Phase II trial published in the New England Journal of Medicine (2023), the highest dose of retatrutide produced 24.2% mean body weight loss at 48 weeks — surpassing both semaglutide (~17%) and tirzepatide (~22.5%). This makes it potentially the most effective anti-obesity medication ever tested. Phase III trials are ongoing with results expected in 2025-2026.
Retatrutide's breakthrough is the addition of glucagon receptor (GCGR) agonism to the GLP-1/GIP dual mechanism. Glucagon traditionally raises blood sugar by promoting hepatic gluconeogenesis, but it also potently stimulates energy expenditure and thermogenesis. The concern with glucagon agonism has always been hyperglycemia — but retatrutide's simultaneous GLP-1 activation provides glucose-lowering that counterbalances glucagon's glucose-raising effect, allowing the metabolic benefits without the diabetogenic risk.
| Pathway | Effect | Significance |
|---|---|---|
| GLP-1R agonism | Appetite suppression, insulin secretion, gastric slowing | Same mechanism as semaglutide component |
| GIPR agonism | Enhanced fat metabolism, insulin sensitivity | Same mechanism as tirzepatide GIP component |
| GCGR agonism | Hepatic lipid oxidation, thermogenesis, energy expenditure | NEW: Increases calories burned rather than just reducing calories consumed |
| Glucagon-GLP-1 balance | GLP-1 counteracts glucagon's hyperglycemic effect | Allows metabolic benefits of glucagon without dangerous blood sugar spikes |
| Synergistic weight loss | Three pathways produce greater weight loss than any two combined | 24% vs 22.5% (tirzepatide) vs 17% (semaglutide) |
| Study | Design | Findings | Level |
|---|---|---|---|
| Phase II (NEJM 2023) | RCT, n=338, 48 weeks | Highest dose: 24.2% mean weight loss. 71% of patients lost >15% body weight. Dose-dependent HbA1c reduction. | Level I-II |
| Body composition | Phase II sub-analysis | ~18% fat mass loss with relative preservation of lean mass | Level II |
| MASLD/NASH | Phase II sub-study | Significant reduction in liver fat content (potential NASH application) | Level II |
| Phase III (ongoing) | Large multi-center RCTs | TRIUMPH program: multiple Phase III trials for obesity and T2D. Results expected 2025-2026. | Pending |
GI side effects: Nausea (25-31%), diarrhea, vomiting — similar to other GLP-1 agonists. Dose escalation helps.
Heart rate increase: Mean increase of 2-4 bpm in Phase II. Being monitored in Phase III.
Thyroid concerns: Same boxed warning as GLP-1 class (rodent thyroid C-cell tumors).
Liver enzymes: Some ALT elevations observed. May reflect fatty liver improvement rather than hepatotoxicity, but being monitored.
| Jurisdiction | Status |
|---|---|
| FDA | Not yet approved. Phase III ongoing. Potential approval 2026-2027. |
| Eli Lilly | Developer. TRIUMPH clinical program. |
| Market position | If approved, would compete with/potentially surpass tirzepatide as the most effective obesity drug |