FDA approves apitegromab-mstn as first muscle-targeted therapy for spinal muscular atrophy

The FDA on September 11, 2026, approved apitegromab-mstn (Isembyld; Scholar Rock) for spinal muscular atrophy (SMA) in adults and children 2 years and older already receiving an SMN2-targeted treatment. It is the first SMA therapy that directly targets muscle loss, working alongside existing SMN2-directed drugs rather than replacing them.^1,2

“As neurologists, families consistently tell us that their top priority is gaining motor function, and we are now able to directly target the muscle, not just the motor neuron, for people living with SMA,” said Basil T. Darras, MD, associate neurologist-in-chief and director of the Neuromuscular Center and Spinal Muscular Atrophy Program at Boston Children's Hospital, a principal investigator in the pivotal trial. The approval gives clinicians a new add-on option for patients whose motor function keeps declining despite SMN2 therapy.²

SAPPHIRE trial design and efficacy in apitegromab-treated patients

Approval rested on the phase 3 SAPPHIRE trial (NCT05156320), a 52-week, global, randomized, double-blind, placebo-controlled study of 188 nonambulatory participants with 5q SMA, aged 2 to 21 years, all already receiving nusinersen or risdiplam. Patients were randomized to apitegromab 10 mg/kg (the approved dose), 20 mg/kg, or placebo every 4 weeks for about a year, with the primary efficacy analysis in 156 patients aged 2 to 12 years.³

The primary endpoint, change from baseline in the Hammersmith Functional Motor Scale Expanded (HFMSE), favored apitegromab 10 mg/kg by 2.2 points over placebo at 1 year (nominal P = .0121), with treated patients gaining function while placebo patients, despite continued SMN2 therapy, declined. A clinically meaningful HFMSE gain of 3 points or more occurred in 34.2% of apitegromab-treated patients versus 13.5% on placebo (odds ratio, 3.8). Common adverse reactions included upper respiratory infections, vomiting, cough, other viral infections, headache, gastroenteritis, and pharyngitis. An increased fracture risk was observed, and the label warns of potential fetal harm and effects on reproductive function.³

Disease burden and unmet need in SMA

SMA is a rare, progressive neuromuscular disease caused by a defective SMN1 gene that fails to produce a protein needed for motor neuron survival, and it is among the leading genetic causes of infant mortality, affecting roughly 1 in 10,000 live births. A backup gene, SMN2, produces a truncated version of the same protein, which existing SMN2-targeted therapies correct to sustain motor neurons. Even so, patients with more advanced disease often retain substantial motor limitations, including impaired walking or independent movement.^1,2

Myostatin inhibition and apitegromab's mechanism

Apitegromab is a fully human monoclonal antibody that binds promyostatin and latent myostatin, blocking a signaling pathway that normally restrains muscle growth. Because SMN2-targeted drugs act on motor neuron survival rather than muscle tissue, pairing a myostatin inhibitor with an SMN2 agent addresses two distinct disease components. Apitegromab previously received Fast Track, Orphan Drug, and Rare Pediatric Disease designations, and its approval carried a Priority Review Voucher.^1-4

Interpreting the data

A 2.2-point HFMSE difference is modest in absolute terms but is read as clinically meaningful because it represents improvement layered on background SMN2 therapy, in a population where continued decline is otherwise expected. That placebo patients lost function despite ongoing SMN2 treatment suggests a real residual deficit, not cohort variability. The primary analysis skewed toward younger children (ages 2-12), so performance in older adolescents and adults with more established wasting will become clearer with post-marketing experience.³

Limitations and next steps

The pivotal analysis covered a subset of the enrolled cohort (156 of 188 participants), and longer-term data are still accruing through an extension study. The fracture signal warrants monitoring given SMA patients' elevated baseline fracture risk from reduced mobility. Because apitegromab was studied only as an add-on, clinicians should not extrapolate its effects to monotherapy use.³

References

1. US Food and Drug Administration. FDA approves first therapy to target muscle loss in spinal muscular atrophy. Published September 11, 2026. Accessed September 14, 2026. https://www.fda.gov/drugs/news-events-human-drugs/fda-approves-first-therapy-target-muscle-loss-spinal-muscular-atrophy

2. Scholar Rock, Inc. Scholar Rock announces FDA approval of ISEMBYLD (apitegromab-mstn), the first and only muscle-targeted treatment for children and adults with spinal muscular atrophy (SMA). Published September 11, 2026. Accessed September 14, 2026. https://investors.scholarrock.com/news-releases/news-release-details/scholar-rock-announces-fda-approval-isembyldtm-apitegromab-mstn

3. Crawford TO, et al. Efficacy and safety of apitegromab in nonambulatory type 2 or type 3 spinal muscular atrophy (SAPPHIRE): a phase 3, double-blind, randomised, placebo-controlled trial. Lancet Neurol. 2025. Available at: https://www.thelancet.com/journals/laneur/article/PIIS1474-4422(25)00225-X/abstract

4. Scholar Rock, Inc. ISEMBYLD (apitegromab-mstn) prescribing information. Published September 2026. Accessed September 14, 2026. https://www.scholarrock.com/documents/label/us/ISEMBYLD_PI.pdf