FDA Approves Scholar Rock's Isembyld, First Muscle-Targeted Therapy for Spinal Muscular Atrophy
核心洞察
The FDA approved Isembyld (搜索) (apitegromab-mstn) for SMA patients aged two and older already receiving an SMN2-targeted therapy, marking Scholar Rock's first product approval.
Isembyld (搜索) is the first and only muscle-targeted SMA therapy, blocking myostatin (搜索) activation to increase muscle mass and strength rather than targeting motor neurons.
In the Phase 3 SAPPHIRE trial, Isembyld (搜索) produced a statistically significant 2.2-point HFMSE motor function improvement over placebo after 52 weeks of treatment.
The U.S. Food and Drug Administration (搜索) has approved Scholar Rock's Isembyld (搜索) (apitegromab-mstn) for the treatment of spinal muscular atrophy (搜索) (SMA) in adults and children two years of age and older who are currently receiving a survival motor neuron 2 (SMN2)-targeted therapy. The decision, announced Friday, marks the first regulatory approval for Scholar Rock and delivers the first and only muscle-targeted therapy for SMA.
Isembyld (搜索) is a monthly intravenous infusion that selectively blocks activation of myostatin (搜索), a protein that limits muscle growth, with the aim of increasing muscle mass and strength. Unlike existing SMN2-directed agents, which prevent motor neuron loss, Isembyld directly addresses the muscular component of the disease.
SAPPHIRE Trial Results
The approval was based on positive results from the pivotal Phase 3 randomized, placebo-controlled SAPPHIRE study, supported by earlier studies including TOPAZ and ONYX. In the trial, patients receiving Isembyld (搜索) demonstrated robust, clinically meaningful motor function improvement after one year of treatment, while those on SMN2-targeted therapy alone experienced further motor function loss.
According to Scholar Rock, the drug showed a statistically significant and clinically meaningful 2.2-point improvement in motor function over placebo after 52 weeks, measured using the 33-point Hammersmith Functional Motor Scale Expanded (HFMSE). The main analysis covered 156 children ages two to 12 and found a 1.8-point average gain versus placebo; among that group, 34.2% of patients on the drug achieved a meaningful gain versus 13.5% on placebo. The broader one-year study enrolled 188 patients ages two to 21 who could not walk on their own.
Additional analyses from SAPPHIRE, including outcomes in nonambulatory people with Type 2 or Type 3 SMA, were presented at the 2026 MDA Clinical & Scientific Conference in a poster titled "Post hoc analyses from the Phase 3 SAPPHIRE study evaluating apitegromab in patients with nonambulatory type 2 or 3 spinal muscular atrophy (搜索)."
A Shift From Slowing to Improving Function
SMA is a rare, severe genetic neuromuscular disease characterized by irreversible loss of motor neurons and progressive muscle wasting, causing continuous motor function decline throughout life. It is a leading genetic cause of death in infants and often requires lifelong support to maintain mobility, breathing, and independence. The condition is estimated to affect roughly 10,000 children and adults in the United States, according to the Muscular Dystrophy Association (搜索), and affects about 1 in 10,000 births.
"The approval of Isembyld (搜索) as the first-ever treatment to directly target the muscular component of SMA is a significant turning point for adults and children who have been waiting for innovative therapeutic options to improve motor function," said Kenneth Hobby, president of the Cure SMA patient advocacy group. "Such improvements are fundamental to maintaining independence and to enabling participation in important activities of daily living from self-care to work and social interactions."
Randal Richardson, MD, a pediatric neurologist and MDA Care Center Director at Gillette Children's Hospital in Saint Paul, Minnesota, framed the approval as a therapeutic inflection point. "The era of stopping or slowing SMA progression has evolved to a new one focused on improving function," he said. "This is the first muscle-directed therapy that can work alongside the remarkable SMN-enhancing treatments we've seen transform care in recent years."
Competitive Landscape
Isembyld (搜索) enters a market that has seen major advances in recent years. It will compete with Biogen's antisense-based Spinraza (nusinersen), Roche's oral mRNA splicing modifier Evrysdi (risdiplam), and Novartis' gene therapies Zolgensma and Itvisma (搜索) (both onasemnogene abeparvovec), which together offer a one-shot option for SMA patients in the same two-and-over age bracket. Spinraza and Evrysdi prevent motor neuron loss but do not directly address muscle.
BMO Capital Markets analyst Evan Seigerman forecasts 2035 adjusted worldwide peak sales of $2.1 billion for the drug; a separate Reuters report cited BMO analysts forecasting peak global sales of $2.1 billion by 2030 using placeholder pricing. Cantor analysts said the approval sets up Scholar Rock to be one of the best long-term growth stories in biotech based on geographic, indication and product line expansions. Scholar Rock has previously said it would use a U.S.-based manufacturing facility for Isembyld (搜索).
Second Attempt After Manufacturing Setback
The approval comes a year after Scholar Rock's first attempt to persuade the FDA resulted in a complete response letter, sparked by unresolved compliance issues at a manufacturing plant operated by Novo Nordisk (now rebranded Novo) in Indiana that handled final vial filling. The FDA rejected the first filing in September 2025, not because of problems with the drug's trial results but because of quality issues at that contract plant. Scholar Rock resubmitted in March 2026 and named a second U.S. plant as backup; after another poor inspection of the Indiana site this spring, the company dropped that plant and proceeded only with the alternate facility. The FDA cleared the drug ahead of a September 30 deadline.
Scholar Rock said commercial supply is ready and the U.S. launch begins immediately, with product shipping in the coming days. A commercial price has not yet been released. The company is also planning to seek approval for the drug in Europe and Japan.
The FDA approval also earns Scholar Rock a rare pediatric disease priority review voucher, which can reduce a future review from around 10 to six months and can be worth between $100 million and $200 million if sold to another company.
Broader Development Program
Scholar Rock is developing apitegromab for additional indications, including SMA in the under-twos, facioscapulohumeral muscular dystrophy (搜索) (FSHD), and skeletal muscle loss associated with weight-loss therapies. In obesity (搜索), the company is testing the drug to preserve muscle, competing with more than a dozen companies developing treatments aimed at minimizing muscle loss during weight loss.
The Muscular Dystrophy Association (搜索), which has invested more than $1.2 billion in research including over $50 million in SMA, noted its role in the underlying science. "The Muscular Dystrophy Association's support was absolutely pivotal in the early days of my research," said Se-Jin Lee, MD, PhD, of the University of Connecticut School of Medicine and The Jackson Laboratory. "Their investment enabled me to explore the biology of myostatin (搜索) and demonstrate that blocking it could profoundly increase muscle growth. That work elucidating key regulatory components and mechanisms became the conceptual foundation for all anti-myostatin therapies developed since."
Scholar Rock said it will host industry update webinars for families on October 6, 2026, and for clinicians on October 7, 2026.