Retatrutide: The Triple Agonist That Hit 24.2% Weight Loss in Phase 2
Eli Lilly's retatrutide targets three hormone receptors simultaneously — GLP-1, GIP, and glucagon. In 338 adults, the highest dose produced weight loss previously only seen with bariatric surgery. It also eliminated liver fat in 86% of patients with fatty liver disease. Phase 3 trials are underway.
Tirzepatide hit two receptors and changed the obesity treatment landscape. Retatrutide adds a third — the glucagon receptor — and the Phase 2 data published in the New England Journal of Medicine in August 2023 suggests that more targets may genuinely mean more efficacy. At the highest dose, participants lost an average of 24.2% of their body weight in 48 weeks. Some individuals lost more than 30%. These are numbers that were previously only achievable through bariatric surgery.
But retatrutide is not just another weight loss drug. The glucagon receptor component appears to drive unique effects on the liver that GLP-1 and GIP agonism alone cannot match. A separate analysis from the same trial showed retatrutide reduced liver fat by up to 82.4% in patients with metabolic dysfunction-associated steatotic liver disease (MASLD), with 86% of those on the highest dose achieving normal liver fat levels.
How Three Receptors Work Together
Retatrutide (LY3437943) is a single molecule that activates three G-protein-coupled receptors: glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide-1 (GLP-1), and glucagon (GCG). Each target contributes distinct metabolic effects.
GLP-1 receptor activation reduces appetite via hypothalamic signaling and slows gastric emptying — the same mechanism that makes semaglutide effective. GIP receptor activation enhances the incretin effect on insulin secretion and appears to amplify weight loss when combined with GLP-1 agonism, as demonstrated by tirzepatide. The glucagon receptor — retatrutide's unique addition — directly increases energy expenditure, drives hepatic lipid oxidation (burning fat in the liver), and promotes thermogenesis.
The theory is straightforward: GLP-1 reduces caloric intake, GIP potentiates the metabolic response, and glucagon increases caloric output. In practice, the 24.2% weight loss result confirms that this triple mechanism produces greater weight reduction than either single (semaglutide, ~15%) or dual (tirzepatide, ~20–22%) receptor agonism.
Phase 2 Weight Loss Data
| Dose Group | Weight Loss at 24 Weeks | Weight Loss at 48 Weeks |
|---|---|---|
| Placebo | −1.6% | −2.1% |
| Retatrutide 1 mg | −7.2% | −8.7% |
| Retatrutide 4 mg (combined) | −12.9% | −17.1% |
| Retatrutide 8 mg (combined) | −17.3% | −22.8% |
| Retatrutide 12 mg | −17.5% | −24.2% |
At 48 weeks, more than 90% of participants in the 8 mg and 12 mg groups achieved at least 5% weight loss. In the 12 mg group, the weight loss curve had not plateaued at 48 weeks — suggesting that longer treatment could yield even greater reductions.
The Liver Fat Story
A prespecified MASLD substudy, published in Nature Medicine in June 2024, enrolled 98 participants from the main trial who had baseline liver fat of 10% or higher. The results were striking.
| Dose | Liver Fat Reduction (24 wk) | Normal Liver Fat Achieved |
|---|---|---|
| Placebo | +0.3% | 0% |
| 1 mg | −42.9% | 27% |
| 4 mg | −57.0% | 52% |
| 8 mg | −81.4% | 79% |
| 12 mg | −82.4% | 86% |
The glucagon component is likely responsible for the disproportionate liver fat reduction. Glucagon receptor activation drives hepatic fatty acid oxidation directly — a mechanism that GLP-1 alone does not meaningfully produce, since GLP-1 receptors are not expressed in the liver. This positions retatrutide as a potential treatment for MASLD and MASH in addition to obesity.
Safety: What Phase 2 Showed
The adverse event profile was consistent with the GLP-1 drug class: nausea, diarrhea, vomiting, and constipation were the most common side effects, occurring predominantly during dose escalation and generally rated as mild to moderate. No major safety signals emerged, though the Phase 2 sample size (338 participants) was too small to detect rare events like pancreatitis or thyroid tumors.
Phase 3 trials — which are larger and longer — will provide the data regulators need on cardiovascular outcomes, cancer risk, and other potential safety concerns. These trials are currently underway.
Phase 2 was 48 weeks. Long-term safety and durability of weight loss beyond one year are unknown. Phase 3 data will determine whether the efficacy holds in larger populations and whether glucagon receptor activation introduces any novel safety concerns. Lilly has not announced an expected FDA submission timeline. Retatrutide is investigational and not available for prescription.
Where It Fits in the Pipeline
Retatrutide is part of a broader pipeline of next-generation obesity drugs that aim to surpass the current generation. CagriSema (Novo Nordisk) combines semaglutide with an amylin analog. Survodutide (Boehringer Ingelheim) is a GLP-1/glucagon dual agonist. Pemvidutide (Altimmune) targets a similar GLP-1/glucagon pathway with a focus on liver disease. Retatrutide's triple agonism represents the most aggressive receptor approach currently in Phase 3 development.
The trial is registered as NCT04881760 on ClinicalTrials.gov.
Sources
- Jastreboff AM, et al. Triple–Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial. N Engl J Med. 2023;389:514-526. NEJM
- Sanyal AJ, et al. Triple hormone receptor agonist retatrutide for MASLD: a randomized phase 2a trial. Nature Medicine. 2024;30:2037-2048. PubMed
- ClinicalTrials.gov. NCT04881760 — retatrutide Phase 2 trial registration. ClinicalTrials.gov
- Eli Lilly Investor Relations. Phase 2 retatrutide results published in NEJM. June 2023. investor.lilly.com