Adding a muscle-preserving antibody to semaglutide pushed weight loss past 17%

A phase 2 trial published this week in Nature Medicine tested a different route to weight loss than the incretin drugs that have dominated obesity medicine for the past several years [s1]. Bimagrumab is an investigational antibody that blocks type II activin receptors — a mechanism aimed at reducing fat mass while preserving or building muscle, rather than suppressing appetite. Combined with semaglutide, it produced the largest weight loss reported in the trial's nine arms.

What was tested

The double-blind, placebo-controlled trial randomized 507 adults with obesity — body mass index of at least 30 kg/m², or at least 27 kg/m² with an obesity-related complication other than diabetes — into nine groups [s1]. Participants received placebo, bimagrumab alone (10 mg/kg or 30 mg/kg, both given intravenously every 12 weeks), semaglutide alone (1.0 mg or 2.4 mg weekly, subcutaneous), or combinations of the two, for 48 weeks. An open-label extension continued dosing through week 72. Randomization was stratified by sex. The primary endpoint was absolute change in body weight at week 48; a secondary endpoint measured the same at week 72 [s1].

The weight results

At week 48, mean weight loss was 9.3 kg on high-dose bimagrumab alone (30 mg/kg), 14.2 kg on high-dose semaglutide alone (2.4 mg), and 17.8 kg on the combination of the two high doses — against 3.3 kg on placebo [s1]. All three active comparisons were statistically significant against placebo (p < 0.001) [s1]. The combination arm's weight loss continued to improve through week 72 [s1], though the trial as reported does not give the exact week-72 figure.

The comparison worth sitting with: semaglutide 2.4 mg alone produced 14.2 kg of loss; adding bimagrumab pushed that to 17.8 kg — roughly a quarter more weight loss from the same semaglutide dose, over the same 48 weeks [s1]. Bimagrumab's own solo effect, 9.3 kg, is more modest than that of the incretin drugs it was paired with, which is consistent with its proposed mechanism: it was not designed as an appetite suppressant.

Tolerability differs by mechanism

The two drugs' side-effect profiles didn't overlap much, which is part of the rationale for combining them. Semaglutide produced the adverse effects familiar from GLP-1 trials — nausea, diarrhea, constipation, fatigue. Bimagrumab's adverse events were different in kind: muscle spasms, diarrhea, and acne [s1]. The trial's authors describe the combination's safety profile as "consistent with the known safety profiles of both drugs" [s1] — meaning the combination did not appear to introduce new safety signals beyond what each drug shows individually, though the published summary does not report discontinuation rates or the frequency of each adverse event by arm.

What this does and doesn't tell us

This is a phase 2 trial — designed to characterize dose-response and safety, not to support a marketing application on its own. Activin-receptor antibodies are a mechanism distinct from anything approved for obesity today, and their rationale — preserving lean mass while a person loses fat — responds to a criticism increasingly leveled at GLP-1 drugs: that a meaningful share of the weight lost on semaglutide and tirzepatide is muscle rather than fat. Whether bimagrumab actually improves that ratio in this trial is not established by the topline results reported here; the Nature Medicine paper reports weight change, not body-composition breakdowns by tissue type.

The trial ran 507 participants for 48 weeks with a 72-week extension — a real but still early-stage dataset. Bimagrumab is not approved by any regulator, and nothing here indicates when or whether a phase 3 program will begin. Longer-term durability, cardiovascular outcomes, and what happens after either drug is stopped are all unaddressed by this trial.

Sources

1. Bimagrumab plus semaglutide alone or in combination for the treatment of obesity: a randomized phase 2 trial — Nature Medicine, 2 March 2026