Scholar Rock has secured FDA approval for Isembyld (apitegromab-mstn), a first-of-its-kind treatment targeting muscle loss in spinal muscular atrophy patients.
The approval covers adults and paediatric patients aged two years and older who are currently receiving an SMN2-targeted treatment for the rare neuromuscular disease.
The decision arrived weeks ahead of the original target date of 30 September, catching markets off guard and sending shares sharply higher in after-hours trading.
Scholar Rock (SRRK) shares climbed 11.8% to $61.97 following the announcement, reflecting investor enthusiasm for the long-awaited regulatory milestone.
SMA is a rare, progressive neuromuscular disease affecting approximately one in 10,000 live births and is among the leading genetic causes of infant mortality worldwide.
The condition is caused by a faulty SMN1 gene that fails to produce a protein essential for motor neuron survival, leading to progressive muscle weakness and wasting over time.
Existing approved therapies, including Biogen’s Spinraza and Novartis’ Zolgensma, address motor neuron survival through SMN2 targeting, but none directly tackle the muscle deterioration that patients also experience.
Isembyld is a fully human monoclonal IgG4 antibody that binds to promyostatin and latent myostatin, blocking myostatin signalling and making it the first muscle-targeted treatment in the SMA space.
The FDA approval was supported by findings from the Phase 3 SAPPHIRE study, a 52-week, randomised, double-blind, placebo-controlled trial enrolling 188 patients aged two to 21 years.
All enrolled patients were unable to walk or move independently and were already receiving an approved SMN2-targeted background therapy at the time of the trial.
The recommended dose of 10 mg/kg demonstrated a clinically meaningful 2.2-point improvement in motor function on the Hammersmith Functional Motor Scale-Expanded compared to placebo, with a p-value of 0.0121.
Isembyld received Fast Track, Orphan Drug, and Rare Pediatric Disease designations from the FDA during its development and review process.
David Hallal, CEO of Scholar Rock, said: “Today’s FDA approval of Isembyld marks a defining moment for the SMA community. After decades of failed industry-wide efforts to unlock the potential of myostatin inhibition, Scholar Rock has delivered a therapeutic breakthrough.”
Beyond SMA, the company is evaluating the drug in a Phase 2 study for infants and toddlers younger than two, and in a separate mid-stage study for facioscapulohumeral muscular dystrophy.
Scholar Rock is also developing a subcutaneously injected version of the antibody and has reported preliminary Phase 2 data evaluating the drug in combination with Eli Lilly’s obesity treatment Zepbound.