Triple-Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial
Jastreboff AM, Kaplan LM, Frías JP, Wu Q, Du Y, Gurbuz S, Coskun T, Haupt A, +3 more
New England Journal of Medicine (2023) · n=338
At the 12-mg dose, mean body-weight reduction reached 24.2% at 48 weeks, with every treated participant achieving at least 5% weight loss and 83% reaching at least 15% — the largest published pharmacotherapy result to date.
This is the foundational Phase 2 trial of retatrutide and the basis for every claim about the molecule's place at the front of the obesity-pharmacotherapy frontier. Investigators randomized 338 adults with obesity (BMI ≥30, or ≥27 with weight-related comorbidities) to once-weekly subcutaneous retatrutide (1, 4, 8, or 12 mg) or placebo for 48 weeks. Mean percent change in body weight at 24 weeks was -7.2%, -12.9%, -17.3%, and -17.5% on the four retatrutide doses versus -1.6% on placebo; at 48 weeks the figures had grown to -8.7%, -17.1%, -22.8%, and -24.2% versus -2.1%. Response rates at 12 mg were extreme by the standards of the class: 100% achieved ≥5% weight loss, 93% achieved ≥10%, and 83% achieved ≥15%. Cardiometabolic markers improved alongside the weight loss — waist circumference, blood pressure, hemoglobin A1c, fasting glucose, insulin, and lipid levels all moved favorably, and 72% of participants with prediabetes at baseline reverted to normoglycemia. Heart-rate increases peaked around week 24 and then declined. Adverse events were dominated by transient mild-to-moderate GI symptoms concentrated during dose escalation, consistent with the broader GLP-1 / GIP–GLP-1 class.
This is a Phase 2 trial, not a regulatory-grade pivotal study. The sample size (338) is substantial for Phase 2 but small relative to a Phase III obesity trial. The 48-week duration is shorter than the 68- to 72-week durations now standard for class-defining trials. The triple-agonism mechanism, particularly the glucagon-receptor component, is genuinely new in chronic human dosing — long-tail safety questions that decades of incretin use have begun to characterize (pancreatitis patterns, NAION, gastroparesis severity, psychiatric adverse events) cannot be answered at 48 weeks in 338 participants. Industry sponsorship by Eli Lilly is disclosed. The Phase III readouts (SURMOUNT-OB, TRIUMPH for T2D, the MASLD trial) will substantially update what is known and should be read in tandem with this paper rather than as confirmatory of it. The compounded-supply market that has built up around retatrutide outside the trial program is a separate evidence question entirely — those products are not the molecule studied here.