FDA Approves Scholar Rock's ISEMBYLD as First Muscle Targeted SMA Therapy

On September 11, 2026, the U.S. Food and Drug Administration approved Isembyld (apitegromab-mstn) injection for spinal muscular atrophy (SMA) in adults and pediatric patients 2 years and older who are currently receiving an SMN2-targeted treatment. This is the first therapy approved for SMA that directly targets muscle loss, working alongside existing treatments. The approval went to Scholar Rock, Inc. (NASDAQ: SRRK) and is the company's first approved product. SMA is a rare, progressive neuromuscular disease affecting approximately 1 in 10,000 live births and is among the leading genetic causes of infant mortality. It is caused by a faulty SMN1 gene that fails to produce a protein essential for motor neuron survival, while a backup gene, SMN2, typically produces a broken version of that protein.

Existing SMA therapies focus on the SMN2 pathway. Biogen's Spinraza, Novartis' gene therapy Zolgensma and Roche's oral Evrysdi all aim to increase survival motor neuron protein. Yet many patients continue to experience substantial muscle impairment and motor function decline despite these therapies. Isembyld takes a different approach. It is a fully human IgG4 monoclonal antibody that binds promyostatin and latent myostatin, preventing activation of myostatin and thereby inhibiting myostatin signaling. Myostatin is a protein that limits muscle growth, so blocking it aims to increase muscle mass and strength.

The FDA's decision was based on a 52-week randomized, double-blind, placebo-controlled trial (NCT05156320) called SAPPHIRE. The trial enrolled 188 participants with SMA aged 2 to 21 years who were not able to move or walk independently. All were already receiving an approved SMN2-targeted treatment. Patients were randomly assigned to receive Isembyld 10 mg/kg, Isembyld 20 mg/kg via intravenous infusion, or placebo once every four weeks for approximately one year. The FDA said the primary analysis was conducted in 156 patients aged 2 to 12 years. Scholar Rock reported that the main efficacy population was 103 patients aged 2 to 12 years.

On the Hammersmith Functional Motor Scale Expanded (HFMSE), treated patients improved at one year while those on placebo declined. Patients on the drug were more than twice as likely to demonstrate a clinically meaningful improvement: 34.2% versus 13.5%. The recommended 10 mg/kg dose produced a 2.2-point improvement in HFMSE versus SMN2 therapy alone at one year, with a nominal p value of 0.0121. The odds ratio for achieving at least a 3-point HFMSE increase was 3.8, with a nominal p value of 0.0125. The most common adverse reactions were upper respiratory tract infections, vomiting, cough, other viral infections, headache, gastroenteritis and pharyngitis.

Key Facts

The FDA approved Isembyld on September 11, 2026, for adults and pediatric patients 2 years and older who are currently receiving an SMN2-targeted treatment. The agency said Isembyld is the first therapy approved for SMA that directly targets muscle loss, working alongside existing treatments. The approval was granted to Scholar Rock, Inc. Isembyld is an injectable antibody that selectively blocks activation of myostatin. Chemically known as apitegromab, it is a fully human IgG4 monoclonal antibody that binds promyostatin and latent myostatin.

The FDA said on September 11 that safety and effectiveness were evaluated in a 52-week randomized, double-blind, placebo-controlled trial (NCT05156320) that enrolled 188 participants with SMA aged 2 to 21 years who were not able to move or walk independently. All were already receiving an approved SMN2-targeted treatment. Patients were randomly assigned to receive Isembyld 10 mg/kg, Isembyld 20 mg/kg via intravenous infusion, or placebo once every four weeks for approximately one year. The primary analysis was conducted in 156 patients aged 2 to 12 years, according to the FDA. Scholar Rock said the main efficacy population was 103 patients aged 2 to 12 years.

Reuters reported on September 11 that the FDA approved Scholar Rock's therapy on Friday, marking the first regulatory approval for the company. Reuters said the injectable treatment is the first muscle-targeted therapy designed to improve motor function for adults and children with SMA who have been treated with existing therapies. Scholar Rock shares rose over 23% in after-market trading. The company said Isembyld will be available in the coming days but did not immediately respond to a Reuters request for comment on pricing details. The condition is estimated to affect roughly 10,000 children and adults in the United States, according to the Muscular Dystrophy Association.

Scholar Rock said on September 11 that the Phase 3 SAPPHIRE study showed the recommended 10 mg/kg dose produced a 2.2-point improvement in HFMSE versus SMN2 therapy alone at one year (nominal p=0.0121). Additionally, 34.2% of Isembyld-treated patients showed at least a 3-point HFMSE increase versus 13.5% of placebo-treated patients (odds ratio 3.8; nominal p=0.0125). The safety database includes more than 500 individuals across all apitegromab studies, some treated over seven years. Fractures occurred in 9% of patients treated with Isembyld 10 mg/kg versus 2% on placebo.

Isembyld received Fast Track, Orphan Drug and Rare Pediatric Disease designations, and Scholar Rock received a Rare Pediatric Disease Priority Review Voucher. The U.S. commercial launch is underway with product available to ship in the coming days. A call with investors was set for Monday, September 14, 2026 at 8:00 a.m. ET. Cure SMA president Kenneth Hobby and Dr. Basil Darras of Boston Children's Hospital commented on the approval. CEO David L. Hallal called the approval a defining moment for the SMA community and the launch of the world's first-ever muscle targeted treatment for SMA. Pharmacally reported on September 12 that Scholar Rock has begun the U.S. commercial launch, with shipments expected to start within days, supported by the Scholar Rock Supports patient program.

Analysis

The approval is significant because it validates a new approach to SMA. Existing therapies address the genetic root by increasing SMN protein, but they do not directly address muscle loss. Isembyld is the first approved therapy that targets muscle. What this really means is that the treatment paradigm for SMA is expanding from a single pathway to a combination approach. Patients can now receive an SMN2-targeted therapy to preserve motor neurons and a muscle-targeted therapy to improve muscle function. This is a meaningful shift for a disease where progressive weakness is the defining feature.

The clinical data support this combination. In SAPPHIRE, patients on Isembyld improved on HFMSE while those on placebo declined. The 2.2-point improvement and the 34.2% versus 13.5% response rate are modest but meaningful in a disease where decline is the norm. The odds ratio of 3.8 suggests a strong treatment effect. However, the nominal p values indicate the analyses were not adjusted for multiplicity, which is a limitation. The fracture signal is also important: 9% versus 2%. This will require monitoring in the post-approval setting. The safety database of more than 500 individuals, some treated over seven years, provides some reassurance about longer-term exposure.

The commercial opportunity is substantial. BMO Capital Markets analyst Evan Seigerman forecasts 2035 adjusted worldwide peak sales of $2.1 billion for the drug. Scholar Rock shares rose over 23% in after-market trading. The drug will compete with Biogen's Spinraza, Novartis' Zolgensma and Roche's Evrysdi. But it is not a direct competitor; it is an add-on to those therapies. That could make reimbursement easier because it fills an unmet need. The launch is underway, and the company has a patient support program.

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. That broader myostatin opportunity could be much larger than SMA. The immediate focus is SMA. The company's first approval is a milestone. It now must execute on launch, pricing and access. The investor call on September 14 will be the first opportunity to hear management's commercial strategy.

Why It Matters

For patients and families, the approval offers a new option. SMA is a devastating disease that causes progressive muscle weakness and problems with movement, breathing and swallowing. Even with existing therapies, many patients continue to lose muscle function. A muscle-targeted therapy could help them maintain or improve motor function. The 34.2% response rate means about one in three patients experienced a clinically meaningful improvement. That is not a cure, but it is progress. The fact that 98% of SAPPHIRE participants elected to continue in the ONYX long-term extension suggests that patients and families see value in continuing treatment.

The approval also matters for the broader biotech industry. It shows that the FDA will approve therapies that target muscle loss, not just the underlying genetic cause. That could spur investment in muscle-preserving treatments for other conditions, including obesity. Scholar Rock's success could encourage other companies to pursue myostatin inhibitors and related approaches. The Rare Pediatric Disease Priority Review Voucher is also valuable; it can be sold or used to expedite another application. The voucher is a tangible asset that can fund further research.

The pricing and access question remains. Reuters noted that the company did not immediately respond to a request for comment on pricing details. Spinraza, Zolgensma and Evrysdi all carry high price tags. If Isembyld is priced similarly, payers will scrutinize it. The fact that it is an add-on may help or hurt. Payers may require evidence of benefit beyond existing therapies. The fracture risk will also be part of the risk-benefit discussion. Patient support programs, like the Scholar Rock Supports program, will be important for access. The company has not yet disclosed the list price.

Next Up

Scholar Rock will host an investor call on Monday, September 14, 2026 at 8:00 a.m. ET to discuss commercialization. The company said Isembyld will be available to ship in the coming days. The U.S. commercial launch is underway. The company has previously said it would use a U.S.-based manufacturing facility. The 98% of SAPPHIRE participants who elected to continue in the ONYX long-term extension will provide longer-term data. The company is also testing the drug in obesity, which could open a much larger market if the data are positive.

The next key steps include pricing announcements, payer coverage decisions and the first real-world patient experiences. The FDA's approval is based on 52-week data; longer-term data from ONYX will be important to confirm durability and safety. The fracture signal will be monitored. Competitors may also pursue muscle-targeted approaches. For now, Scholar Rock has a first-mover advantage in a new category. The question is whether the market will reward it. The company's first approval is a milestone, but the launch will determine its impact.