FDA Approves Isembyld for Spinal Muscular Atrophy

Overview

• The FDA granted approval for Isembyld (apitegromab) to treat patients aged two and older who are already receiving SMN2-targeted therapies, based on randomized trial data.
• The approval relied on the Sapphire trial that showed treated children and teens had an average 2.2-point motor-function gain while placebo-treated patients declined over about a year.
• Apitegromab works by blocking myostatin, a protein that limits muscle growth, so it acts directly on muscle to preserve or increase mass rather than changing SMN protein levels.
• Trial safety data reported common side effects such as respiratory infections, vomiting, cough, headache and allergic reactions and also noted an increased risk of fractures and possible harm to fetal and reproductive health.
• Scholar Rock’s phase 2 study combining apitegromab with tirzepatide showed about 55% greater preservation of lean mass at 24 weeks but that use remains investigational and would require phase 3 trials; high reported annual cost of roughly $310,000 raises access and off-label use concerns.