US approves Isembyld (apitegromab), the first SMA treatment that targets muscle directly

The U.S. Food and Drug Administration (FDA) approved Scholar Rock's Isembyld (apitegromab-mstn) on September 11, 2026, for SMA (spinal muscular atrophy) in children aged 2 and older and adults who are already receiving an SMN2-targeted treatment such as Spinraza or Evrysdi. It is the first SMA therapy that targets muscle itself rather than motor neurons.

Isembyld is a monoclonal antibody that blocks the activation of myostatin, a protein that limits muscle growth. It is given as an intravenous infusion every four weeks at a dose of 10 mg/kg.

The approval is based on the Phase 3 SAPPHIRE trial in 188 patients who could not walk independently. After one year, 34.2% of patients on the recommended dose achieved a clinically meaningful improvement in motor function (HFMSE scale), compared with 13.5% on placebo. The main identified risk is more frequent fractures (9% vs 2% on placebo).

In the European Union, the marketing application was withdrawn in August 2026 because of problems found at a contract manufacturing site; the company says it will resubmit with a different facility. Until then, the drug is not approved in Europe or in the Republic of Moldova.

Source: FDA, Scholar Rock, Scholar Rock (UE/EU).