Federal regulators approved a drug on Friday for spinal muscular atrophy, a rare neurological condition where the nerve cells that signal muscles to move gradually deteriorate. The drug, Isembyld, is the first approved therapy to directly address the muscle loss the disease causes. Scholar Rock, the company behind it, received Food and Drug Administration clearance for use in adults and children aged 2 and older who are already receiving SMA treatments that work on a gene called SMN2.
The approval matters for patients because existing SMA drugs cover only part of what the disease does. Treatments already cleared for SMA work on SMN2, a gene that helps produce proteins needed by the neurons controlling movement. Those drugs can slow how fast the disease progresses. Isembyld takes a different approach: it works through myostatin inhibition. Myostatin is a protein that naturally limits muscle growth; the drug blocks it in an effort to preserve or rebuild muscle in patients whose neurons have already deteriorated.
Attempts by the broader pharmaceutical industry to make myostatin inhibition work in SMA had not produced an approved drug before Friday. Scholar Rock CEO David Hallal called the approval "a defining moment for the SMA community." In a company press release, he pointed to "decades of failed industry-wide efforts" across the industry before Scholar Rock's drug reached patients.
What the trial showed
A late-stage clinical trial paired Isembyld with an existing SMN2-targeting therapy. Young patients who received both saw their motor skills improve over the course of a year. Patients who received a placebo instead declined over the same period. The gap between the two groups was statistically significant, the standard clinical bar that means the result is unlikely to be a product of chance.
The FDA approved Isembyld only for use alongside an SMN2-targeting treatment, covering patients already on that kind of therapy. The drug was tested as an add-on, and the approval carries that same condition: Isembyld is not cleared for use on its own.