The Phase 2 extension study of retatrutide enrolled 338 participants who had completed the 24-week Phase 2 dose-ranging trial and continued treatment for an additional 24 weeks. A metabolic substudy in 89 participants used indirect calorimetry to measure resting energy expenditure (REE) at baseline, week 24, and week 48.
Key Findings
Participants receiving retatrutide 12 mg showed a mean increase in REE of 8.4% from baseline at week 24, compared to 3.1% in the semaglutide comparator arm and 1.2% in the placebo arm. The difference between retatrutide and semaglutide was statistically significant (p=0.003). The investigators attributed the additional REE increase to glucagon receptor agonism, which stimulates hepatic glucose production and thermogenesis.
Lean Mass Implications
Participants in the retatrutide arm showed a smaller proportion of weight loss attributable to lean mass (18.3%) compared to semaglutide (24.1%), though this difference did not reach statistical significance in this substudy (p=0.09). Larger Phase 3 lean mass data are pending.
TruTide Verdict
This is Phase 2 mechanistic data — directionally informative but not definitive. The REE increase associated with glucagon receptor agonism is biologically plausible and consistent with preclinical data. Confirmation in Phase 3 with DEXA-measured lean mass outcomes is needed before drawing clinical conclusions.