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The 48-week Phase 2 results for the triple-hormone-receptor agonist that documented the largest weight reduction of any single compound in clinical trials.
In August 2023, the New England Journal of Medicine published the Phase 2 trial results for retatrutide (LY3437943), a triple-hormone-receptor agonist developed by Eli Lilly. The headline finding documented mean body weight reductions of 24.2% from baseline at 48 weeks in the highest-dose group of participants with obesity.
This figure was unprecedented. The previous benchmark — tirzepatide's roughly 21% weight loss at 72 weeks in the SURMOUNT-1 trial — had itself only been published a year earlier. Retatrutide produced a larger effect in less time, raising the question of what biology was being engaged that previous compounds had not addressed.
The trial in question (NCT04881760) enrolled 338 adults with obesity (BMI ≥30) or overweight (BMI ≥27) with at least one weight-related comorbidity. Participants were randomized to retatrutide at one of five dose levels (1, 4, 8, 12 mg) or placebo, administered subcutaneously once weekly for 48 weeks. The trial design is informative for understanding how to interpret the data.
What distinguishes retatrutide from earlier incretin-based compounds is the addition of glucagon receptor activation alongside GLP-1 and GIP. The compound targets all three receptors simultaneously:
The seemingly paradoxical inclusion of glucagon receptor activation is the mechanistically novel piece. Glucagon classically acts to raise blood glucose, which is the opposite of what GLP-1 agonists do. However, glucagon also has well-documented effects on energy expenditure — chronic glucagon receptor activation increases basal metabolic rate, promotes lipolysis, and mobilizes hepatic fat stores. In the context of simultaneous GLP-1 receptor activation (which counters the hyperglycemic effect of glucagon by stimulating insulin), the net effect of triple agonism is metabolic activation without compromising glucose control.
This mechanistic combination was the design hypothesis. The Phase 2 data tested whether it would translate to clinically meaningful effects.
The primary efficacy outcome was mean percent change in body weight from baseline to 48 weeks. The results, by dose group:
Several features of this dose-response are notable:
Dose-dependent response. The weight loss scaled cleanly with dose across the studied range. No clear plateau was observed at the highest dose, suggesting that doses above 12 mg might produce additional effect — a question that Phase 3 trials with broader dose ranges would address.
Time course. Weight loss had not plateaued at 48 weeks. The trajectory at the highest dose was still trending downward, suggesting longer-duration administration would produce additional reductions. This contrasts with the SURMOUNT-1 tirzepatide trial, where weight loss curves at 72 weeks were beginning to flatten.
Responder analysis. At the highest dose, approximately 26% of participants achieved ≥30% weight reduction — a threshold previously associated only with bariatric surgery outcomes. The proportion achieving ≥20% reduction approached 80%.
The Phase 2 trial also reported a range of metabolic outcomes:
HbA1c reductions. Participants without diabetes saw modest HbA1c reductions consistent with improved insulin sensitivity. Participants with prediabetes showed substantial improvements.
Blood pressure. Mean systolic blood pressure decreased by approximately 7-12 mmHg at higher doses, paralleling the weight reductions.
Lipid profile. Triglycerides decreased by approximately 25% at the highest dose. HDL cholesterol increased modestly. LDL cholesterol decreased modestly.
Body composition. While the trial did not include DEXA scanning of all participants, available data suggested the proportion of weight loss from lean mass versus fat mass was similar to other incretin-based compounds (approximately 25-40% of total weight loss being lean mass).
The most common adverse events in the Phase 2 trial were gastrointestinal:
These rates are comparable to or modestly higher than those reported for semaglutide and tirzepatide at the highest doses. Discontinuation rates due to adverse events were approximately 12-15% at the highest dose.
Notable adverse events specific to the glucagon receptor activation component:
Important limitations of the available Phase 2 data:
Durability of response. Whether the weight loss would persist with continued treatment beyond 48 weeks, and whether weight regain after discontinuation would mirror the patterns observed with semaglutide and tirzepatide, remained unresolved. Phase 3 trials with longer follow-up are required.
Cardiovascular outcomes. The Phase 2 trial was not powered to detect cardiovascular events. The modest blood pressure improvements are suggestive but require a dedicated cardiovascular outcomes trial.
Subgroup performance. The trial population was relatively healthy. Performance in populations with established type 2 diabetes, renal impairment, established cardiovascular disease, or other comorbidities required separate Phase 3 evaluation.
Long-term safety. The 48-week trial duration is insufficient to detect rare or delayed adverse events. The unique glucagon receptor activation component introduces theoretical concerns (effects on bone, cardiac remodeling, hepatic adaptation) that longer-term data is needed to evaluate.
The Phase 2 results triggered an extensive Phase 3 program. Multiple Phase 3 trials are now active or have completed, including:
As of this writing, the Phase 3 results have not been published. The Phase 2 data therefore remains the highest-quality publicly available efficacy information for retatrutide, and the interpretation of that data should account for the typical attrition of effect sizes between Phase 2 and Phase 3 trials.
The Phase 2 retatrutide data is significant for several reasons beyond the headline weight loss number:
The compound's eventual clinical role will be determined by the Phase 3 trial readouts, but the Phase 2 data has already shaped the trajectory of metabolic peptide research.
NoteThis article is intended for informational and educational purposes only. It does not constitute medical advice.
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