US approves first muscle-targeted therapy for spinal muscular atrophy

The United States has approved its first treatment for spinal muscular atrophy (SMA) that acts directly on muscles, adding an option for people who still face movement difficulties despite existing therapies.

Called Isembyld, or apitegromab, the medicine is approved for adults and children aged two and older who already receive treatments targeting the SMN2 gene. It complements those therapies rather than replacing them.

SMA is a rare genetic disease that causes progressive muscle weakness and can affect movement, breathing and swallowing. Existing treatments help preserve the nerve cells that control movement. Apitegromab takes a different approach: it blocks activation of myostatin, a protein that limits muscle growth.

The approval was based on a trial involving 188 people aged two to 21, all already receiving SMA treatment. Among children aged two to 12 receiving one tested dose, 34.2% showed an improvement in movement abilities judged clinically meaningful after about a year. That compared with 13.5% given placebo.

The medicine is delivered through an intravenous infusion every four weeks. Reported side effects included respiratory infections, vomiting, cough and headache. Its label also warns of a possible increased risk of fractures, including serious cases, making careful medical monitoring important.