Apitegromab plus tirzepatide preserved 54.9% more lean mass in 24-week trial
A mid-stage trial found an experimental myostatin inhibitor blunted lean mass loss during tirzepatide treatment, but the combination is not approved and is not close to prescribing.
Adding the experimental myostatin inhibitor apitegromab to tirzepatide preserved 54.9% more relative lean mass over 24 weeks than tirzepatide plus placebo, while total weight loss was similar, according to phase 2 EMBRAZE results published in Nature Medicine [1]. The combination is investigational and is not approved for weight management [1].
Apitegromab, made by Scholar Rock, targets pro-myostatin and the latent myostatin complex, blocking activation of the growth-inhibiting form of the protein before it can act [1]. EMBRAZE enrolled 102 participants and ran for 24 weeks [1]. The drug already has a positive phase 3 result in spinal muscular atrophy behind it, giving the obesity program an established clinical safety record to draw on [1].
Scholar Rock's regulatory path has been less straightforward than its clinical one. A Complete Response Letter in September 2025 concerned the third-party facility handling fill-finish operations rather than apitegromab itself, and after resubmission and a change of facility the FDA action date is 30 September 2026 [1]. That date is not a decision on using the drug alongside a GLP-1 medication, and no filing for a muscle-preservation indication has been reported [1].
EMBRAZE is not the only program aimed at the myostatin-activin signaling axis, the set of proteins that acts as a natural brake on skeletal muscle growth [1]. Eli Lilly's bimagrumab blocks the activin type II receptor directly rather than myostatin alone. In the phase 2b BELIEVE trial, which enrolled 507 participants across nine treatment arms, the highest-dose combination of bimagrumab and semaglutide produced 22.1% total body weight loss at 72 weeks, compared with 15.7% for semaglutide alone, with about 92% of the combination group's weight loss coming from fat mass [1]. Lean mass loss was limited to 2.9%, against 7.4% with semaglutide alone, and bimagrumab alone increased lean mass by 2.5% above baseline [1]. Lilly halted a separate phase 2b trial testing bimagrumab with tirzepatide in patients with type 2 diabetes in September 2025, citing strategic business reasons, but said a parallel obesity-only trial continues, with results expected later this year [1].
Regeneron's trevogrumab targets myostatin, and garetosmab, tested alongside it in some arms, targets activin A [1]. In the phase 2 COURAGE trial, 26-week data presented at the European Association for the Study of Diabetes in September 2025 showed that adding trevogrumab to semaglutide prevented roughly half of the lean mass loss typically seen with semaglutide alone [1]. In the three-drug arm that also included garetosmab, lean mass accounted for 7.4% of total weight loss, against 33.0% with semaglutide alone, and fat loss made up 92.6% of that arm's weight loss, against 67.0% [1]. Regeneron reported a "substantially higher" rate of discontinuations due to tolerability in the triplet arm without giving an exact figure [1]. COURAGE is expected to complete in late 2026 [1].
Biohaven's taldefgrobep alfa blocks activin receptor signaling triggered by myostatin and multiple activins [1]. A phase 2 proof-of-concept study of the drug as a monotherapy completed enrollment in March 2026, with topline data expected in the second half of this year and no efficacy results published to date [1]. An earlier phase 3 trial of the drug in spinal muscular atrophy missed its primary endpoint, prompting the company to redirect the program toward obesity [1].
Why it matters for patients
Weight lost rapidly on GLP-1 medications includes lean mass, not only fat, and no approved product currently targets that specifically [1]. All four of these candidates remain in mid-stage testing for a muscle-preservation use. None has completed the trials required for approval in that indication, and none is an imminent treatment option [1]. Resistance exercise and adequate protein intake remain the only evidence-supported options available today, and are likely to remain so for at least the next several years [1].
Timing is the other practical point. The account of this field is written around UK practice, where none of these therapies is close to being prescribed alongside a GLP-1 in routine practice [1]. If a muscle-preservation drug reaches the US market, it would need its own approval and — if approved — its own coverage decisions. No pricing, insurance coverage or US availability details for any of these combinations have been reported [1].
Omar El-Gohary, a UK-registered pharmacist and independent prescriber, wrote that "muscle-preservation therapies will not be a one-size-fits-all addition to GLP-1 treatment; they are likely to differ in effect size, dosing burden and tolerability, and different patients may end up better served by different mechanisms within this class" [1].
What happens next
The FDA action date for apitegromab's manufacturing review is 30 September 2026 [1]. Results from Lilly's obesity-only bimagrumab trial are expected later in 2026 [1]. Topline taldefgrobep data are expected in the second half of 2026 [1]. COURAGE is expected to complete late in 2026 [1]. A regulatory filing for a muscle-preservation indication used specifically alongside a GLP-1 is, in that assessment, more plausibly years away than months [1].
Sources
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