TRIUMPH-1 to TRIUMPH-4 Readouts: Quantitative Analysis of Retatrutide’s Phase 3 Weight-Loss Trajectories

 

Retatrutide, Eli Lilly’s investigational triple hormone receptor agonist targeting GIP, GLP-1, and glucagon receptors, has generated substantial clinical interest following its Phase 2 readouts, which demonstrated unprecedented weight loss magnitudes in adults with obesity. The Phase 3 TRIUMPH programme represents a pivotal step in translating these early signals into regulatory-grade evidence. This analysis examines the quantitative weight-loss trajectories emerging from the TRIUMPH-1 and TRIUMPH-4 readouts, placing them in the broader context of the retatrutide clinical development programme.

TRIUMPH-1: The Flagship Obesity Trial

TRIUMPH-1 is the foundational Phase 3 study in the retatrutide obesity programme. This randomised, double-blind, placebo-controlled trial enrolled 2,339 adults with obesity or overweight without diabetes, including prespecified subsets with knee osteoarthritis and obstructive sleep apnoea. Participants received once-weekly subcutaneous retatrutide at 4 mg, 9 mg, or 12 mg, or placebo, for 80 weeks.

The primary weight-related endpoint results, presented at the 2026 American Diabetes Association Scientific Sessions and published in the New England Journal of Medicine, demonstrated a clear dose-response trajectory. Using the treatment-regimen estimand (a stringent intention-to-treat approach that handles intercurrent events conservatively), the mean percent change in body weight was −17.6% for the 4 mg dose, −23.7% for 9 mg, and −25.0% for 12 mg, compared with −3.9% for placebo. The treatment differences of −19.8 and −21.0 percentage points for the 9 mg and 12 mg doses, respectively, were statistically significant (P<0.001 for both comparisons).

A critical observation from the TRIUMPH-1 data is the absence of a weight-loss plateau. In the Phase 2 trial, participants receiving retatrutide 12 mg achieved 24.2% weight reduction at 48 weeks. The Phase 3 data extend this trajectory to 80 weeks, with the 12 mg dose reaching 25.0%. This continuation of weight loss beyond the 48-week mark distinguishes retatrutide’s trajectory from that of GLP-1 receptor agonists, which typically show attenuation of weight loss after approximately 60 weeks. The 4 mg dose, which employed a single dose-escalation step, produced 17.6% weight reduction—lower than the higher doses but still clinically meaningful and suggesting a potential maintenance-dose strategy.

TRIUMPH-4: The Osteoarthritis Cohort

TRIUMPH-4 (NCT05931367) was a separate Phase 3 trial specifically designed to evaluate retatrutide in adults with obesity or overweight and knee osteoarthritis. This 68-week, randomised, double-blind, placebo-controlled study randomised 445 participants to retatrutide 9 mg 12 mg, or placebo. The co-primary endpoints were the percent change in body weight and the change in the WOMAC pain subscale score.

The efficacy estimand results (which evaluate treatment effect prior to discontinuation) for TRIUMPH-4 showed weight reductions that were numerically greater than those observed in TRIUMPH-1. At 68 weeks, the 9 mg dose produced a mean weight reduction of −26.4% (−29.1 kg), while the 12 mg dose achieved −28.7% (−32.3 kg; −71.2 lbs), compared with −2.1% for placebo. The baseline average weight in TRIUMPH-4 was 112.7 kg (248.5 lbs) with a BMI of 40.4 kg/m², indicating a population with severe obesity.

Careful interpretation is required for the discrepancy between the TRIUMPH-1 and TRIUMPH-4 weight-loss magnitudes—25.0% versus 28.7% for the 12 mg dose. TRIUMPH-4 employed the efficacy estimand as the primary analysis, which excludes data after treatment discontinuation, whereas the TRIUMPH-1 primary weight analysis used the treatment-regimen estimand. When the same estimand is applied, the differences narrow but do not entirely disappear. The TRIUMPH-4 treatment-regimen estimand results showed −20.0% for 9 mg and −23.7% for 12 mg, compared with −4.6% for placebo. These figures align more closely with TRIUMPH-1’s treatment-regimen outcomes, suggesting that the apparent superiority of TRIUMPH-4’s efficacy estimand results reflects the analytical framework rather than a fundamentally different treatment effect.

Dose-Response and Categorical Weight Loss

The TRIUMPH-4 data provide granular insight into the distribution of weight-loss responses across the treatment population. Using the efficacy estimand, 58.6% of participants receiving retatrutide 12 mg achieved ≥25% weight loss, 39.4% achieved ≥30%, and 23.7% achieved ≥35% . For the 9 mg dose, the corresponding figures were 47.7%, 30.5%, and 18.2%. In the placebo group, only 1.3% achieved ≥25% weight loss, and no participants reached ≥35%. These categorical outcomes underscore that retatrutide produces weight reductions of a magnitude rarely observed with currently approved pharmacotherapies.

Safety and Tolerability Considerations

The weight-loss trajectories must be contextualized against the adverse event profile. Across both trials, gastrointestinal events were the most common adverse effects, consistent with the incretin class. In TRIUMPH-4, nausea occurred in 38.1% and 43.2% of participants receiving 9 mg and 12 mg, respectively, versus 10.7% with placebo; diarrhea occurred in 34.7% and 33.1% versus 13.4%; and vomiting in 20.4% and 20.9% versus 0% . Dysesthesia, a sensory disturbance potentially linked to the glucagon receptor component, occurred in 8.8% (9 mg) and 20.9% (12 mg) versus 0.7% with placebo. These events were generally mild and rarely led to discontinuation, though discontinuation rates due to adverse events were 12.2% and 18.2% for the 9 mg and 12 mg doses, respectively, compared with 4.0% for placebo . Notably, these discontinuation rates correlated with baseline BMI and included discontinuations for perceived excessive weight loss.

Programmatic Context and Future Directions

The TRIUMPH program extends beyond TRIUMPH-1 and TRIUMPH-4. TRIUMPH-3 evaluated retatrutide in participants with severe obesity and established cardiovascular disease, reporting 22.6% weight reduction at 80 weeks with the 12 mg dose using the efficacy estimand . Additional trials in the program include a maintenance-dose study (4 mg) and the ongoing TRIUMPH-Outcomes cardiovascular and kidney outcomes trial . A separate Phase 3 program, TRANSCEND-T2D, is evaluating retatrutide in type 2 diabetes, where weight loss is typically attenuated relative to obesity-only populations .

The quantitative trajectories from TRIUMPH-1 and TRIUMPH-4 establish retatrutide as potentially the most efficacious weight-loss pharmacotherapy yet tested in Phase 3. The absence of a plateau at 80 weeks, combined with categorical outcomes showing that nearly 40% of participants at the 12 mg dose achieve ≥30% weight loss, positions retatrutide distinctly from approved GLP-1 receptor agonists. The ongoing cardiovascular outcomes trial will determine whether these weight-loss magnitudes translate into hard clinical endpoint reductions—the ultimate test for any obesity therapeutic.