Retatrutide (LY3437943 / Triple Agonist) Evidence Grade: A- (Phase III)
Retatrutide (LY3437943) is a novel triple agonist peptide developed by Eli Lilly that simultaneously activates three key metabolic receptors: glucose-dependent insulinotropic polypeptide (GIP) receptor, glucagon-like peptide-1 (GLP-1) receptor, and glucagon receptor. This triple agonist approach represents the next evolution beyond dual agonists like tirzepatide, adding glucagon receptor activation to enhance energy expenditure and hepatic fat reduction alongside the appetite-suppressing effects of GIP and GLP-1 signaling.
In Phase II clinical trials (TRIUMPH), retatrutide demonstrated unprecedented weight loss efficacy, with participants achieving up to 24.2% mean body weight reduction at the highest dose over 48 weeks. These results exceeded those reported for tirzepatide and semaglutide at comparable timepoints, establishing retatrutide as a potential best-in-class obesity therapeutic. Eli Lilly is conducting Phase III trials with regulatory submissions anticipated upon completion.
Table of Contents
Overview & Introduction
Retatrutide represents the third generation of incretin-based therapeutics for obesity and metabolic disease. The first generation comprised GLP-1 receptor agonists (semaglutide, liraglutide). The second generation introduced dual GIP/GLP-1 agonists (tirzepatide). Retatrutide adds a third target, the glucagon receptor, creating a compound that addresses obesity through three complementary mechanisms: appetite suppression (GLP-1), insulin sensitization and nutrient partitioning (GIP), and increased energy expenditure with fat oxidation (glucagon).
The rationale for including glucagon receptor agonism, which might seem counterintuitive given glucagon's hyperglycemic effects, is based on glucagon's potent effects on energy expenditure. Glucagon stimulates hepatic thermogenesis, increases lipid oxidation, reduces hepatic steatosis (fatty liver), and promotes amino acid catabolism. The potential hyperglycemic effect is counterbalanced by the glucose-lowering actions of GLP-1 and GIP agonism, creating a net neutral or beneficial glycemic profile while harnessing glucagon's metabolic activation properties.
Eli Lilly developed retatrutide using structure-activity relationship optimization to achieve balanced agonism across all three receptors in a single molecule with a pharmacokinetic profile suitable for once-weekly subcutaneous injection. The compound is designed as a fatty acid-acylated peptide that binds albumin, extending its half-life to enable weekly dosing.
The Phase II TRIUMPH trial results, published in the New England Journal of Medicine in 2023, established retatrutide as the most efficacious anti-obesity compound reported in a controlled clinical trial, with the 12 mg dose group achieving 24.2% mean body weight loss at 48 weeks. This result generated substantial scientific and public interest and accelerated the Phase III development program.
History & Discovery
Preclinical development. Eli Lilly designed and optimized LY3437943 as a balanced triple agonist peptide. Preclinical studies in obese mice and non-human primates demonstrated superior weight loss and metabolic improvements compared to dual agonists.
Phase I first-in-human. Initial safety, tolerability, and pharmacokinetic data established the dose range and confirmed the expected metabolic pharmacodynamic effects in healthy volunteers and patients with type 2 diabetes.
Phase II TRIUMPH trial. Randomized, double-blind, placebo-controlled Phase II trial in adults with obesity. Results published in NEJM demonstrated up to 24.2% body weight reduction at 48 weeks, establishing retatrutide as a potential best-in-class agent.
Phase III development. Multiple Phase III trials initiated for obesity, type 2 diabetes, NASH/MASH, and obstructive sleep apnea. Regulatory submissions planned upon Phase III completion.
Mechanism of Action
GLP-1R activation in the hypothalamus and brainstem reduces appetite, increases satiety, and slows gastric emptying. This mechanism, shared with semaglutide and tirzepatide, is responsible for the primary appetite-suppressing effect. GLP-1R activation also improves insulin secretion in a glucose-dependent manner, reducing postprandial glycemic excursions.
GIPR activation enhances insulin sensitivity, promotes nutrient-dependent insulin secretion, and improves lipid metabolism. In the CNS, GIPR activation may contribute to appetite regulation and body weight homeostasis. The GIP component augments the weight loss achieved by GLP-1 agonism alone, as demonstrated by tirzepatide's superiority over pure GLP-1 agonists.
The distinguishing feature of retatrutide. GCGR activation stimulates hepatic gluconeogenesis, glycogenolysis, lipid oxidation, and thermogenesis. In the context of a triple agonist where GLP-1 and GIP counteract hyperglycemia, the net effect is increased energy expenditure and enhanced fat utilization without dangerous glucose elevation. GCGR activation also reduces hepatic steatosis by promoting hepatic lipid export and oxidation.
The three receptors create a complementary metabolic program: GLP-1 suppresses food intake and improves glucose control; GIP enhances insulin sensitivity and lipid handling; glucagon increases energy expenditure and burns fat. The combination attacks obesity from both the caloric intake (demand-side) and caloric expenditure (supply-side) simultaneously, explaining the superior weight loss compared to single or dual agonists.
Research Applications
Obesity
Primary development indication. Retatrutide's unprecedented weight loss efficacy in Phase II positions it as a potential best-in-class anti-obesity therapeutic.
Type 2 Diabetes
Triple agonism provides comprehensive glycemic control through complementary insulin secretion, insulin sensitization, and metabolic activation pathways.
NASH/MASH (Fatty Liver Disease)
Glucagon's hepatic effects make retatrutide particularly promising for metabolic-associated steatohepatitis. Phase III trials specifically targeting MASH are underway.
Obstructive Sleep Apnea
Weight loss-mediated improvement in OSA is being studied in dedicated Phase III trials.
Cardiovascular Risk Reduction
Like other incretin agonists, retatrutide may provide cardiovascular benefits through weight loss, lipid improvements, and anti-inflammatory effects. Cardiovascular outcome trials are planned.
Clinical Evidence
Phase II TRIUMPH Trial (Obesity)
Jastreboff et al. (2023) published the landmark Phase II TRIUMPH trial in the New England Journal of Medicine. The randomized, double-blind, placebo-controlled trial enrolled 338 adults with obesity (BMI 30-50). The highest dose group (12 mg weekly) achieved 24.2% mean body weight loss at 48 weeks, with 26% of participants losing more than 30% of body weight. These results exceeded those of any previously reported anti-obesity drug in a controlled trial.
PMID: 37351564
Phase II in Type 2 Diabetes
Rosenstock et al. (2023) reported Phase II results of retatrutide in adults with type 2 diabetes. Treatment produced dose-dependent HbA1c reductions of up to 2.02% and body weight reductions of up to 16.9% at 36 weeks. The glucose-lowering effects were complementary to the weight loss, with the glucagon component not causing net hyperglycemia due to GLP-1/GIP counterbalance.
PMID: 37351563
Hepatic Fat Reduction
Sanyal et al. (2024) reported that retatrutide produced remarkable reductions in hepatic fat content in a Phase II trial, with up to 82.4% relative reduction in liver fat at the highest dose. Over 85% of participants with MASH achieved normalization of liver fat (<5% by MRI). These results positioned retatrutide as a leading candidate for MASH treatment.
PMID: 38587239
Dosing Protocols (Clinical Trial Data)
Investigational Compound: Retatrutide is not approved for any indication. Dosing data is from published clinical trials only.
| Phase II Dose Groups | Weight Loss at 48 Weeks |
|---|---|
| 1 mg weekly SC | ~8.7% |
| 4 mg weekly SC (escalated) | ~17.1% |
| 8 mg weekly SC (escalated) | ~22.8% |
| 12 mg weekly SC (escalated) | ~24.2% |
| Placebo | ~2.1% |
All treatment arms used dose escalation (starting at lower doses and increasing over weeks) to improve GI tolerability. Once-weekly subcutaneous injection. The dose-response relationship was clear and robust across all efficacy endpoints.
Administration & Reconstitution
In clinical trials, retatrutide is administered as a once-weekly subcutaneous injection using pre-filled syringes or autoinjectors. As an investigational compound, it is not available in lyophilized/reconstitution format. Research-grade material would follow standard peptide reconstitution procedures.
- Once-weekly subcutaneous injection
- Dose escalation over initial weeks to minimize GI side effects
- Injection sites: abdomen, thigh, upper arm (rotate weekly)
Side Effects & Safety Profile
The adverse event profile of retatrutide is dominated by GI effects, consistent with other incretin agonists. GI side effects tend to decrease over time and with dose escalation protocols.
Common (GI)
- Nausea (24-43% dose-dependent)
- Diarrhea (16-22%)
- Vomiting (8-13%)
- Constipation (6-12%)
- Decreased appetite (expected effect)
Less Common
- Injection site reactions
- Increased heart rate (2-4 bpm)
- Dyspepsia
- Abdominal pain
Note: The glucagon component raised theoretical concerns about hepatic glucose output and hyperglycemia. In clinical trials, the GLP-1/GIP counterbalance effectively prevented hyperglycemia, with the net glycemic effect being neutral to beneficial.
Comparisons & Context
vs. Semaglutide (Wegovy/Ozempic)
Single GLP-1 agonist. Approved. 15-17% weight loss at highest doses. Retatrutide achieves substantially greater weight loss through the addition of GIP and glucagon receptor activation.
vs. Tirzepatide (Mounjaro/Zepbound)
Dual GIP/GLP-1 agonist. Approved. 20-22% weight loss at highest doses. Retatrutide adds glucagon agonism for additional energy expenditure and hepatic fat reduction. See Tirzepatide wiki article for detailed comparison.
vs. Survodutide (BI 456906)
Dual GLP-1/glucagon agonist (no GIP). Boehringer Ingelheim development. Different receptor combination without GIP. Early clinical data shows strong MASH efficacy.
Storage & Stability
| Form | Conditions | Duration |
|---|---|---|
| Pre-filled (clinical supply) | Refrigerated (2-8°C) | Per trial protocol |
| Research lyophilized | Frozen (-20°C) | 12+ months |
As an investigational compound, formal stability specifications will be established upon regulatory approval. Fatty acid acylation improves in vivo stability but research-grade material should be handled with standard peptide precautions.
Regulatory Status
- United States: Investigational. Phase III clinical trials ongoing. Not FDA-approved. IND active with Eli Lilly.
- European Union: Investigational. Phase III trials active.
- Expected Timeline: Phase III results anticipated 2025-2026. Regulatory submissions expected upon positive results.
Frequently Asked Questions
How does Retatrutide compare to tirzepatide and semaglutide?
Is Retatrutide FDA-approved?
What does the glucagon component do?
References
- Jastreboff AM, et al. "Triple-Hormone-Receptor Agonist Retatrutide for Obesity." N Engl J Med. 2023;389(6):514-526. PMID: 37351564
- Rosenstock J, et al. "Retatrutide, a GIP, GLP-1 and glucagon receptor agonist, for people with type 2 diabetes." Lancet. 2023;402(10401):529-544. PMID: 37351563
- Sanyal AJ, et al. "Retatrutide for metabolic dysfunction-associated steatohepatitis." N Engl J Med. 2024. PMID: 38587239
Related Pages
Concise compound overview
Dual agonist comparison
GLP-1 agonist reference
Monitor retatrutide trials
Medical Disclaimer: This article is for educational and research purposes only. Retatrutide is an investigational compound not approved for any indication. Consult a healthcare professional. See our full Medical Disclaimer.
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