Scholar Rock announced its experimental add-on treatment for spinal muscular atrophy significantly improved motor function compared to placebo at one year in a pivotal study. The 12-year old biotech now plans to seek a broad label for its first potential commercial drug.
Scholar Rock is developing a muscle-targeted antibody treatment called apitegromab that can be used on top of current therapies for SMA. The Phase 3 SAPPHIRE study enrolled people with late-onset SMA who are on either Spinraza or Evrysdi, drugs developed by Biogen and Roche that are meant to boost levels of a key motor neuron protein. The study included a main efficacy population of patients ages 2 to 12 and an exploratory group of older patients ages 13 to 21.
In patients ages 2 to 12, those who received 10 or 20 mg/kg doses of apitegromab scored 1.8 points better on a motor function measure called the HFMSE compared to placebo, which resulted in a p-value of 0.0192. (The primary endpoint result in this study is statistically significant if p<0.025.)
Scholar Rock’s stock $SRRK jumped over 300% Monday morning. Jefferies analyst Michael Yee wrote Monday that Wall Street had low expectations for the program to succeed and that Monday’s data could boost the case for Scholar Rock’s mid-stage study of the same antibody in obesity.
SMA causes people to lose the nerve cells that control voluntary muscles, and those muscles become weak and waste away over time. Apitegromab is designed to latch onto certain forms of myostatin, a protein found in skeletal muscles that limits muscle growth. Blocking it, then, could help boost muscle mass.
When broken down, the results were not dose-dependent. Patients who received the 10 mg/kg dose saw an improvement of 2.2 points on the motor function scale compared to placebo, while those who received the higher 20 mg/kg dose saw just a 1.4-point difference.
Scholar Rock CEO Jay Backstrom said the FDA asked the company to include a 10 mg/kg dose in their study and they “never expected” a dose-dependent response between the 10 and 20 mg/kg doses since their pharmacologic effects were very similar.
“In today’s world of drug development, we’re looking for the lowest dose that gives you the best efficacy and safety. I think we have a really strong argument here with our data to recommend the 10 [mg/kg dose],” Backstrom said.
In the exploratory 13-21 age group, Scholar Rock said that motor function results were “meaningful and consistent.” Backstrom said on an investor call Monday morning that the company plans to “lean in on getting the full breadth of age” when it seeks approval for apitegromab.
The company also outlined plans to start a study in children under 2. “There’s no reason not to start this as early as possible,” Backstrom said.
As for safety, Scholar Rock reported that there were no discontinuations due to adverse events.
Others are also developing myostatin-targeting drugs. Roche and Biohaven are testing their experimental treatments in patients with SMA, and Biohaven is expected to report Phase 3 data in the coming days to months. Meanwhile, Roche, Lilly and Regeneron are all studying drugs that block myostatin signaling in obesity.
Scholar Rock said it plans to submit applications to market the antibody drug to the FDA and EMA in the first three months of next year, as well as present the detailed results of the study in early 2025.
And for its own obesity program, Scholar Rock is expected to read out a mid-stage study of apitegromab in the second quarter of next year.