FDA approves Isembyld, first muscle-targeted drug for spinal muscular atrophy

The FDA approved Isembyld (apitegromab-mstn) on September 11, 2026, for spinal muscular atrophy (SMA) in adults and children 2 years of age and older who are currently receiving a survival motor neuron 2 (SMN2)-targeted treatment ^[1][2]. It is the first muscle-targeted therapy for the disease and the first FDA approval for Scholar Rock, which is based in Cambridge, Massachusetts ^[1][2].

SMA is a genetic disorder that damages motor neurons, causing progressive muscle weakness and problems with movement, breathing and swallowing ^[1]. It is estimated to affect roughly 10,000 children and adults in the United States, according to the Muscular Dystrophy Association ^[1]. Isembyld is a monoclonal antibody given by injection that blocks activation of myostatin, a protein that limits muscle growth ^[1][2].

What the trial showed

The approval was based on the Phase 3 SAPPHIRE study, a randomized, placebo-controlled trial ^[1][2]. Patients receiving Isembyld at 10 mg/kg plus an SMN2-targeted treatment improved 2.2 points on the Hammersmith Functional Motor Scale-Expanded (HFMSE) after one year, compared with patients who received an SMN2-targeted treatment alone and lost motor function (nominal p = 0.0121; main efficacy population ages 2 to 12, n = 103) ^[2]. Separately, 34.2% of Isembyld-treated patients gained at least 3 points on the HFMSE, compared with 13.5% of placebo-treated patients (odds ratio 3.8; nominal p = 0.0125) ^[2].

The safety database includes more than 500 people across all apitegromab studies globally, some of whom have been on treatment for more than 7 years, and 98% of SAPPHIRE participants elected to continue into an ongoing long-term extension study ^[2]. The most common adverse reactions reported were upper respiratory tract infections, vomiting, cough, other viral infections, headache, gastroenteritis, pharyngitis and hypersensitivity ^[2]. Fractures occurred in 9% of patients treated with Isembyld 10 mg/kg versus 2% on placebo, and the prescribing information lists fractures as a possible side effect and says a healthcare provider may consider stopping treatment if one occurs ^[2]. The label also states it is not known whether the drug harms an unborn baby or passes into breast milk ^[2].

Scholar Rock said the product will be available to ship in the coming days and that a patient support program is now open to help with insurance coverage, financial assistance for eligible patients, and infusion logistics at hospitals, homes or infusion centers ^[2]. Pricing was not disclosed; the company did not immediately respond to a Reuters request for comment on pricing details ^[1].

Isembyld will compete with Biogen's Spinraza, Novartis's gene therapy Zolgensma and Roche's oral Evrysdi ^[1]. Scholar Rock shares rose over 23% in after-market trading, and BMO Capital Markets analyst Evan Seigerman forecasts 2035 adjusted worldwide peak sales of $2.1 billion ^[1].

Why it matters for patients

For families dealing with SMA, this adds a treatment that acts on muscle itself rather than on the motor neuron, and it is approved only for people already receiving an SMN2-targeted therapy ^[1][2]. The FDA's approval covers all adults and children 2 and older in that group, which the company and the advocacy group Cure SMA described as a broad population within the SMA community ^[2].

For people taking GLP-1 medications such as semaglutide or tirzepatide, the relevance is indirect. Scholar Rock is testing the same myostatin inhibitor to preserve muscle during weight loss, competing with more than a dozen companies developing treatments aimed at minimizing muscle loss during weight loss ^[1]. No results from that testing have been reported, and no muscle-preservation drug is approved for use with GLP-1 therapy. Whether this molecule helps people on weight-loss drugs, and at what dose, is not yet known.

What happens next

Scholar Rock said the U.S. commercial launch is underway, with product available to ship in the coming days ^[2]. The company will hold an investor call on Monday, September 14, 2026, at 8:00 a.m. ET ^[2]. Scholar Rock also received a Rare Pediatric Disease Priority Review Voucher, which can be used to obtain priority review for a future marketing application ^[2]. No timeline has been disclosed for the obesity-related muscle-preservation program ^[1].