A new weapon against SMA just got the green light. The FDA has approved apitegromab-mstn (Isembyld, Scholar Rock), the first therapy for spinal muscular atrophy (SMA) that directly targets muscle loss. Unlike existing SMN-targeted treatments — which improve motor neuron survival but don't address impaired muscle function — apitegromab works by blocking myostatin, a protein that inhibits skeletal muscle growth. It's approved as an add-on therapy for patients aged 2 and older already receiving an SMN2-targeted treatment, administered at 10 mg/kg every 4 weeks via infusion.

The approval was backed by the phase 3 SAPPHIRE trial, which enrolled 188 nonambulatory type 2/3 SMA patients and showed meaningful motor function gains over a year in patients who would otherwise have declined.

By the Numbers:

• +2.2-point improvement on the HFMSE motor scale vs. placebo at 52 weeks (P = .0121)
• 34.2% of apitegromab patients achieved a clinically meaningful ≥3-point HFMSE improvement vs. 13.5% on placebo
• Fractures reported in 9% of treated patients vs. 2% on placebo — a safety signal worth monitoring

Why it matters: SMA families consistently cite gaining motor function as their top priority. Apitegromab is the first therapy to directly address the muscle side of the disease, offering a meaningful new layer of treatment on top of existing options — and real hope for improved physical function in a condition with limited therapeutic tools.