FDA Approves Isembyld to Target Muscle Loss in Spinal Muscular Atrophy

The FDA approved apitegromab-mstn (Isembyld, Scholar Rock, Inc.) injection for adults and pediatric patients aged 2 years and older with spinal muscular atrophy (SMA) who are currently receiving an SMN2-targeted treatment. The agency described Isembyld as the first approved SMA therapy to directly target muscle loss alongside existing treatments.

SMA is a rare, progressive neuromuscular disease affecting approximately 1 in 10,000 live births. According to the FDA, a faulty SMN1 gene impairs production of a protein essential for motor neuron survival, leading to progressive muscle weakness and wasting. Existing SMN2-targeted therapies address a defect in a backup gene to support production of the missing protein.

Isembyld’s effectiveness and safety were evaluated in a 52-week randomized, double-blind, placebo-controlled trial (NCT05156320) enrolling 188 participants aged 2 to 21 years who could not move or walk independently. All were already receiving an approved SMN2-targeted treatment. Participants were assigned to Isembyld 10mg/kg or 20mg/kg by intravenous infusion, or placebo, once every 4 weeks for approximately 1 year. The primary analysis included 156 patients aged 2 to 12 years.

The study assessed change from baseline on the Hammersmith Functional Motor Scale Expanded. Among patients aged 2 to 12 years, motor function improved at 1 year with Isembyld 10mg/kg but declined with placebo. Clinically meaningful improvement occurred in 34.2% of patients receiving Isembyld 10mg/kg and 13.5% receiving placebo, making improvement more than twice as likely in the treatment group.

The most common adverse reactions included upper respiratory tract infections, vomiting, cough, other viral infections, headache, gastroenteritis, and pharyngitis. The FDA also reported an increased risk of fractures, including serious fractures, and noted that Isembyld may cause fetal harm and affect reproductive function. Isembyld received Fast Track, Orphan Drug, and Rare Pediatric Disease designations.

Source: US Food and Drug Administration. FDA approves first therapy to target muscle loss in spinal muscular atrophy. September 11, 2026. Accessed September 13, 2026.