The lean-mass race now spans a dozen programs and one approved drug

Drugmakers are now racing to sell a second injection or pill to go with GLP-1 weight-loss treatment: something that keeps muscle while the pounds come off. On September 11, 2026, the FDA approved Scholar Rock's apitegromab, sold as Isembyld, for spinal muscular atrophy — the first approval for a drug in this class, though for a rare genetic muscle disease, not obesity ^[2]. Reuters reported that Scholar Rock is also testing the drug to preserve muscle in obesity, "competing with more than a dozen companies developing treatments aimed at minimizing muscle loss during weight loss" ^[2].

The drugs in this lane mostly target the same biology: the myostatin-activin signaling system, which acts as a natural brake on muscle growth. Blocking that brake is meant to offset some of the lean mass people lose when appetite drops sharply on a GLP-1 medicine ^[1].

What the mid-stage data show

Apitegromab's obesity evidence comes from the Phase 2 EMBRAZE trial, published in Nature Medicine in June, which combined apitegromab with tirzepatide in 102 participants over 24 weeks and reported 54.9% greater relative lean mass retention than tirzepatide plus placebo ^[1].

Eli Lilly's bimagrumab blocks the activin type II receptor more broadly. In the Phase 2b BELIEVE trial — 507 participants across nine treatment arms — the highest-dose bimagrumab-plus-semaglutide combination produced 22.1% total body weight loss at 72 weeks versus 15.7% for semaglutide alone, with roughly 92% of the combination group's weight loss coming from fat. Lean mass loss was 2.9% in that arm versus 7.4% with semaglutide alone, and bimagrumab by itself increased lean mass 2.5% above baseline ^[1]. Lilly bought the molecule with Versanis Bio for up to $1.9 billion in 2023, halted a separate Phase 2b trial pairing bimagrumab with tirzepatide in type 2 diabetes in September 2025 for "strategic business reasons," and said an obesity-only trial continues with results expected later this year ^[1].

Regeneron's Phase 2 COURAGE trial is testing trevogrumab, alone and with garetosmab, on top of semaglutide. Twenty-six-week data presented in September 2025 showed trevogrumab prevented about half the lean mass loss usually seen with semaglutide, while increasing fat loss. In the three-drug arm, lean mass made up just 7.4% of total weight lost versus 33.0% with semaglutide alone, and fat made up 92.6% versus 67.0% — but Regeneron reported a "substantially higher" rate of discontinuations for tolerability in that arm, without giving a number ^[1]. Biohaven's taldefgrobep alfa finished enrollment in a Phase 2 obesity monotherapy study in March 2026, with topline data expected in the second half of this year and no efficacy results published so far ^[1].

The sources reviewed here do not cover several other programs sometimes grouped in this race, including oral enobosarm or non-incretin candidates such as petrelintide, so their status is not established by this reporting.

Why it matters for patients

None of these drugs is approved to be used with a GLP-1 medicine. Isembyld's approval covers adults and children with spinal muscular atrophy who have already been treated for that condition, a disease affecting roughly 10,000 people in the United States ^[2]. The approval validates the mechanism in a regulated setting, but it does not answer whether blocking myostatin helps people taking semaglutide or tirzepatide feel stronger, function better, or keep weight off longer.

The trials so far are small and short by obesity-drug standards: 102 participants over 24 weeks for EMBRAZE, 507 over 72 weeks for BELIEVE ^[1]. They measure body composition, not long-term outcomes. Source ^[1] notes that none of these candidates has completed the trials needed for approval in a muscle-preservation indication alongside a GLP-1, that broader receptor blockade appears to bring both bigger lean-mass effects and wider off-target risk, and that resistance exercise and adequate protein intake remain the evidence-supported options available today.

What happens next

COURAGE is expected to complete in late 2026 ^[1]. Lilly's obesity-only bimagrumab trial and Biohaven's Phase 2 topline are both expected later this year ^[1]. One note on timing: the September 7 pipeline review listed an FDA action date of September 30, 2026 for apitegromab following a September 2025 complete response letter tied to a third-party fill-finish facility ^[1]; the approval actually came on September 11 ^[2]. When, or whether, any of these drugs reaches a weight-loss label is not yet known.