The FDA has approved Isembyld (apitegromab-mstn, Scholar Rock), marking a landmark moment in the treatment of spinal muscular atrophy (SMA). Unlike existing therapies that target motor neuron survival, Isembyld is the first to directly address muscle loss — the other half of SMA's devastating equation. It's approved as an add-on therapy for adults and children aged 2 and older who are already on an SMN2-targeted treatment like nusinersen or risdiplam.

Isembyld works by binding to promyostatin and latent myostatin, blocking the activation of myostatin — a protein that normally suppresses skeletal muscle growth. The approval was based on the phase 3 SAPPHIRE trial, a randomized, double-blind, placebo-controlled study of 188 patients with nonambulatory type 2 or type 3 SMA. After 52 weeks, patients on the approved 10 mg/kg dose saw meaningful motor function gains while the placebo group declined. The drug is administered via IV infusion every 4 weeks in hospitals, infusion centers, or at home.

By the Numbers:

• 2.2-point improvement in HFMSE score for Isembyld vs. placebo at 52 weeks (P = .0121)
• 34.2% of Isembyld patients achieved a clinically meaningful ≥3-point HFMSE improvement vs. 13.5% on placebo
• 9% fracture rate in the Isembyld group vs. 2% in placebo
• ~6,600 people in the U.S. are affected by SMA
• Annual net treatment cost: ~$315,000 per patient

Why it matters: SMA has long been treated by targeting motor neuron survival, but muscle function remained a gap. Isembyld fills that gap, offering patients and families a new avenue to not just slow decline — but actually gain motor function. Scholar Rock has also launched a patient support program to help navigate insurance and financial barriers.