A Cambridge biotech company just scored its first-ever FDA approval, and the drug could reshape treatment for thousands of Americans living with spinal muscular atrophy. Scholar Rock's Isembyld (apitegromab-mstn) won federal clearance this week as the first therapy designed to directly target muscle loss in SMA patients rather than the underlying nerve cells that control movement.
The Food and Drug Administration approved Isembyld on September 11, granting it Fast Track, Orphan Drug, and Rare Pediatric Disease designations, according to the FDA. Those designations typically come with tax credits and expedited review pathways for treatments addressing rare diseases. The FDA, based in Silver Spring, Maryland, cleared the drug for adults and children age 2 and older who are already receiving SMN2-targeting therapies, and executives say the therapy will carry an estimated annual list price of roughly $310,000 per patient, with actual costs varying by body weight and insurance coverage, per BioPharma Dive.
Spinal muscular atrophy is a rare, progressive disease that destroys the nerve cells controlling speaking, breathing, and walking, and untreated babies with the condition usually die by age 2, according to The Boston Globe. The disease affects an estimated 10,000 to 25,000 children and adults in the United States and occurs in roughly 1 in 10,000 to 1 in 11,000 live births, BioPharma Dive reports. Hoodline previously reported on a St. Louis woman living with the condition, noting the same birth-rate statistics as the disorder continues to draw national attention.
A Different Way to Fight Muscle Decline
Until now, every FDA-approved SMA therapy worked by boosting the SMN2 gene, which is key to the neurons that control movement. Biogen's Spinraza arrived in December 2016, Novartis's gene therapy Zolgensma followed in May 2019, and Roche's oral liquid Evrysdi came in August 2020 — the first disease-modifying options for a condition that previously had none, per the same BioPharma Dive account. Isembyld instead works by blocking the activation of myostatin, a protein that naturally limits muscle growth, allowing the antibody to target muscle tissue directly rather than motor neurons.
Dr. Basil T. Darras of Boston Children's Hospital, a principal investigator in the trial, said the distinction matters because patients on existing SMN2 drugs often keep losing muscle function over time even as their nerve cells stabilize. He told Contemporary Pediatrics that Isembyld allows clinicians to directly target muscle tissue to reverse motor decline in patients with advanced SMA. Scholar Rock chief executive David Hallal called the approval a defining moment, saying the company delivered a therapeutic breakthrough after decades of failed efforts to develop myostatin inhibition, according to the Globe.
Trial Results and a Rocky Road to Approval
The pivotal trial enrolled nearly 190 patients ages 2 to 21 who were already receiving SMN2-targeting therapies, assigning them to monthly low-dose Isembyld, high-dose Isembyld, or placebo groups, the Globe reports. In the 52-week Phase 3 SAPPHIRE trial, 34.2% of patients receiving apitegromab achieved a clinically meaningful motor improvement of 3 points or more on the Hammersmith Functional Motor Scale Expanded, compared with 13.5% on placebo, according to MedCity News reporting cited in the dossier. Ninety-eight percent of trial participants chose to enter the long-term ONYX extension study, a sign of strong patient confidence in the treatment.
The path to approval wasn't smooth. The FDA rejected Isembyld roughly a year earlier, in September 2025, citing long-standing problems including pests at a contract manufacturing facility, per the Globe. Scholar Rock switched to other manufacturers, removed the troubled site from its application, and refiled for approval. The drug is now administered via a 1- to 2-hour intravenous infusion every four weeks, and its label is not restricted by SMA subtype or ambulatory status, according to BioSpace, giving physicians broad discretion in prescribing it to both walking and nonwalking patients.
Safety Warnings and Side Effects
Isembyld's most common adverse reactions include coughing, headaches, and upper respiratory infections, per the Globe. Federal prescribing instructions also warn that the treatment may increase the risk of bone fractures, including serious fractures, requiring caution in patients with low bone density — a concern that's especially relevant since bone density loss is common among nonambulatory SMA patients. The drug also carries potential risks to a developing fetus and may affect reproductive function, according to the FDA's official labeling.
Beyond SMA: A Bigger Bet on Muscle Preservation
Scholar Rock, a Cambridge-based biotech now valued at $6.8 billion, is also testing myostatin blocking as a way to combat obesity-related muscle loss. The company aims to offset the muscle wasting associated with GLP-1 weight-loss drugs like Eli Lilly's Zepbound. In the Phase 2 EMBRAZE trial, adding apitegromab to Lilly's tirzepatide resulted in 54.9% less lean muscle mass loss over 24 weeks compared with tirzepatide alone, preserving an average of 4.2 additional pounds of lean muscle, BioPharma Dive reported last year. Since muscle loss accounts for roughly 25% to 30% of total weight lost on GLP-1 therapies, the results suggest Scholar Rock's technology could find a commercial life far beyond rare-disease treatment.
For now, Isembyld stands as the company's first commercialized product in its history, marking a milestone after years of research into myostatin inhibition. Hoodline has previously covered the SMA community's evolving treatment landscape, including a Fort Worth toddler's progress after starting an SMA regimen and a New Jersey gala that funded wheelchairs in honor of a teenager who lived with the disease's Type 2 form.
