FDA approves ISEMBYLD (apitegromab) for adults and children aged 2+ with SMA receiving SMN2-targeted treatment, based on Phase 3 SAPPHIRE data.
Written By: Mansi Nakum, PharmD
Reviewed By: Pharmacally Editorial Team
Scholar Rock’s apitegromab-mstn is approved for adults and children aged 2 years and older with SMA receiving an SMN2-targeted treatment, based on Phase 3 SAPPHIRE data (NCT05156320) showing a 2.2-point HFMSE improvement versus placebo.
The U.S. FDA has approved ISEMBYLD (apitegromab-mstn) for the treatment of spinal muscular atrophy (SMA) in adults and children aged 2 years and older who are receiving a survival motor neuron 2 (SMN2)-targeted therapy, giving Scholar Rock its first approved product and introducing a treatment that directly targets the muscle component of SMA.
The approval is based on the Phase 3 SAPPHIRE trial, in which the recommended 10 mg/kg dose produced a clinically meaningful improvement in motor function when added to background SMN2-targeted treatment. The approval covers a broad SMA population across adults and pediatric patients.
Targeting Myostatin to Address Persistent Muscle Dysfunction
SMA is caused by insufficient survival motor neuron protein, leading to progressive motor neuron loss and muscle weakness. Existing disease-modifying treatments primarily target the SMN2 gene or its RNA processing to increase SMN protein production. However, patients can continue to experience substantial muscle impairment 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. The strategy directly addresses muscle growth and function rather than targeting SMN biology.
Phase 3 SAPPHIRE Shows Motor Function Benefit
The randomized, double-blind, placebo-controlled SAPPHIRE study met its primary endpoint. In the main efficacy population of patients aged 2 to 12 years (n=103), those receiving ISEMBYLD 10 mg/kg plus an SMN2-targeted treatment achieved a 2.2-point greater improvement in HFMSE at one year compared with patients receiving SMN2-targeted therapy alone.
The Hammersmith Functional Motor Scale-Expanded (HFMSE) is a widely used measure of motor function in SMA. The treatment effect was statistically significant based on the reported nominal p-value of 0.0121.
A clinically relevant response was also more frequent with ISEMBYLD. 34.2% of treated patients achieved at least a 3-point HFMSE increase, compared with 13.5% of placebo recipients, corresponding to an odds ratio of 3.8 (nominal p=0.0125).
The results indicate that adding muscle-targeted treatment to established SMN2-directed therapy can improve motor function rather than simply slowing its decline.
Safety Profile and Commercial Launch
The safety database includes more than 500 people treated with apitegromab across clinical studies, with some participants followed for more than seven years. In SAPPHIRE, the most common adverse reactions included upper respiratory tract infections, vomiting, cough, viral infections, headache, gastroenteritis, pharyngitis and hypersensitivity.
Fractures occurred in 9% of patients receiving ISEMBYLD 10 mg/kg versus 2% with placebo. Overall, 98% of SAPPHIRE participants elected to continue into the ONYX long-term extension study.
Scholar Rock has begun the U.S. commercial launch, with product shipments expected to start within days. Its Scholar Rock Supports program will provide assistance with insurance coverage, financial support for eligible patients, treatment education and infusion logistics. Depending on eligibility, treatment may be administered in a hospital, infusion center or home setting.
Path Forward
The FDA approval also earned Scholar Rock a Rare Pediatric Disease Priority Review Voucher, potentially providing priority review for a future marketing application.
Management will discuss the approval and commercialization plans during an investor call on September 14, 2026, at 8:00 a.m. ET.
For the SMA field, ISEMBYLD establishes a new therapeutic strategy alongside SMN2-targeted treatments: directly modifying muscle biology to improve motor function in patients who continue to live with significant functional impairment despite disease-modifying therapy.
Reference
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), Sholar Rock, 11 September 2026
About the Writer
Mansi Nakum (Linkedin) is a Pharm.D professional with a strong foundation in clinical pharmacy, evidence-based healthcare writing, and clinical data interpretation.
She has published work on Brugada syndrome and has a keen interest in guideline integration, multimodal therapy, pharmacogenomics, and antibiogram utilization.
As a healthcare writer, she focuses on translating clinical evidence into clear, accurate, and clinically relevant content, while continuously developing her expertise in evolving pharmacy practice.