FDA approves Isembyld, the first myostatin inhibitor, for spinal muscular atrophy
The first FDA approval of a myostatin inhibitor is for a rare muscle-wasting disease, not obesity, leaving open whether the mechanism can protect lean muscle in people taking GLP-1s.
The FDA approved Isembyld (apitegromab-mstn) on Sept. 11, 2026, making it the first muscle-targeted treatment for spinal muscular atrophy and the first myostatin inhibitor ever cleared in the United States [1][2]. Scholar Rock's drug is approved for adults and children 2 years and older with SMA who are already receiving a treatment that targets SMN2, a gene tied to the motor neurons that control movement [1][2].
In the Phase 3 SAPPHIRE study, patients who received Isembyld at 10 mg/kg on top of an SMN2-targeted treatment improved by 2.2 points on the Hammersmith Functional Motor Scale-Expanded (HFMSE) after one year, while patients on an SMN2-targeted treatment alone lost motor function (nominal p = 0.0121) [2]. STAT reported the difference was statistically significant [1]. The main efficacy analysis covered 103 patients aged 2 to 12 [2]. The company also reported that 34.2% of Isembyld-treated patients gained at least 3 points on the HFMSE, compared with 13.5% of those on placebo (odds ratio 3.8; nominal p = 0.0125) [2]. Scholar Rock labels both p-values as nominal.
The safety database for apitegromab includes more than 500 people across all of the company's clinical studies, some of whom have been on treatment for more than 7 years [2]. The most common adverse reactions in SAPPHIRE were upper respiratory tract infections, vomiting, cough, other viral infections, headache, gastroenteritis, pharyngitis, and hypersensitivity [2]. Fractures occurred in 9% of patients treated with the 10 mg/kg dose versus 2% of those on placebo [2]. The prescribing information tells patients to tell their healthcare provider if they have a history of low bone density or bone fractures [2]. Ninety-eight percent of SAPPHIRE participants chose to continue into the ONYX long-term extension study [2].
David Hallal, Scholar Rock's chief executive, said the approval came "after decades of failed industry-wide efforts to unlock the potential of myostatin inhibition" [1][2]. Kenneth Hobby, president of Cure SMA, called it "a significant turning point for adults and children who have been waiting for innovative therapeutic options to improve motor function" [2]. Dr. Basil Darras of Boston Children's Hospital, a principal investigator in SAPPHIRE, said clinicians can now "directly target the muscle, not just the motor neuron" [2].
Scholar Rock said its U.S. commercial launch is underway and that the product will be available to ship in the coming days [2]. A support program, Scholar Rock Supports, is available to help patients and families with insurance coverage, financial assistance for eligible patients, and infusion logistics; infusions can be given at a hospital, at home, or at an infusion center depending on eligibility [2]. The FDA also granted the company a Rare Pediatric Disease Priority Review Voucher, which can be used to obtain priority review of a future marketing application [2].
Why it matters for patients
Isembyld is an SMA drug, not an obesity drug, and its approval does not cover weight management. It is also not a GLP-1; it is an antibody that blocks myostatin, a protein that limits muscle growth [2]. The approval matters beyond SMA because it is the first time U.S. regulators have cleared a myostatin inhibitor, a mechanism that had failed repeatedly across the industry for decades [1][2]. Companies have been studying whether blocking myostatin can protect lean muscle, but no results from testing apitegromab alongside GLP-1 obesity medications such as semaglutide or tirzepatide have been reported in the approval announcement or the company's release [2]. For people taking a GLP-1, that question remains open.
The SMA results also show that response is not universal. About a third of treated patients gained at least 3 HFMSE points, and about 13.5% of placebo patients did as well [2]. Isembyld is given as an infusion alongside an existing SMN2-targeted therapy, not instead of one, and it is not approved for people who are not already receiving such a treatment [1][2].
What happens next
Scholar Rock plans an investor call on Monday, Sept. 14, 2026, at 8:00 a.m. ET [2]. The company said the drug would be available to ship in the coming days and that it is working with commercial and government payers on coverage [2]. Participants who completed SAPPHIRE are continuing in the ONYX long-term extension study; results from that study have not been disclosed [2]. Pricing for Isembyld has not been announced in these materials.
Sources
- https://www.statnews.com/2026/09/11/scholar-rock-spinal-muscular-atrophy-isembyld-clinical-trial-fda-approval
- https://www.biospace.com/press-releases/scholar-rock-announces-fda-approval-of-isembyld-apitegromab-mstn-the-first-and-only-muscle-targeted-treatment-for-children-and-adults-with-spinal-muscular-atrophy-sma
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