A muscle-targeted drug joins the treatment of spinal muscular atrophy
Scholar Rock’s Isembyld is the first SMA therapy aimed at muscle rather than motor neurons. Its US approval came with a priority review voucher, and a European filing is back with the EMA.
In brief
Scholar Rock’s apitegromab, a myostatin blocker added to SMN2 therapy, has won FDA approval and a priority review voucher and is now with the EMA.
- The FDA approved Isembyld on 11 September 2026 for SMA patients aged two and over already on SMN2-targeted therapy.
- In the SAPPHIRE trial it improved motor function scores by 2.2 points over background therapy alone in children aged 2 to 12.
- The FDA published notice of its rare paediatric disease priority review voucher on 26 October 2026.
The three established treatments for spinal muscular atrophy (SMA) all work by restoring survival motor neuron protein. Scholar Rock’s Isembyld (apitegromab-mstn) takes a different approach: it targets the muscle itself. The FDA approved it on 11 September 2026 for adults and children aged two and over who are already receiving an SMN2-targeted therapy.
Two further steps followed this month. Scholar Rock resubmitted its marketing authorisation application to the European Medicines Agency on 8 October, and on 26 October the FDA published formal notice in the Federal Register that the company had been awarded a rare paediatric disease priority review voucher for the product.
How it works
Apitegromab is a fully human monoclonal antibody that binds the precursor forms of myostatin, a protein that limits skeletal muscle growth, and prevents its activation. The rationale is that patients whose motor neuron loss has been slowed by existing drugs still carry substantial muscle weakness.
The evidence
Approval rests on SAPPHIRE, a phase 3, randomised, double-blind, placebo-controlled trial in 188 patients aged 2 to 21 across nine countries, all on nusinersen or risdiplam. Patients received intravenous apitegromab at 10 or 20 mg/kg, or placebo, about every four weeks for a year. In the main efficacy population of 103 children aged 2 to 12, the 10 mg/kg dose improved scores on the Hammersmith Functional Motor Scale Expanded by 2.2 points relative to background therapy alone. A gain of three points or more was seen in 34.2% of treated patients against 13.5% on placebo.
Fractures occurred in 9% of patients on the 10 mg/kg dose and 2% on placebo, and prescribers may consider stopping treatment after a fracture. Some 98% of participants moved into the ONYX extension study.
Add-on therapies will be judged on whether modest gains in motor scores justify another lifelong treatment cost.
A delayed approval
The route to market was not smooth. Scholar Rock first filed in September 2025, but the FDA declined to approve, citing problems at a third-party fill-finish site in Bloomington, Indiana. In August 2026, after the agency classified the facility as Official Action Indicated, the company removed it from its application. Fierce Pharma reported a projected price of about $310,000 a year.
The value of the voucher
The voucher programme, which lapsed in December 2024, was reauthorised in February 2026. A voucher cuts FDA review of a later application from ten months to six, and recent sales have fetched between about $150m and $200m. For a company with a single approved product, that is a material source of non-dilutive funding.
