As reported on BioSpace, the U.S. Food and Drug Administration has approved Regeneron’s Pasatru (garetosmab) for adults with fibrodysplasia ossificans progressiva (FOP), a rare genetic disorder that causes bone to form in muscles, tendons, ligaments, and other soft tissues. The approval gives patients a second FDA-approved treatment option and introduces new competition in a market previously occupied solely by Ipsen’s Sohonos.
FOP is an ultra-rare and progressive disease characterized by episodes known as flare-ups, which can trigger the development of heterotopic ossification (HO), or abnormal bone growth outside the skeleton. As the condition advances, mobility can become severely restricted, with many patients requiring wheelchairs by early adulthood.
Pasatru is a monoclonal antibody that targets Activin A, a protein identified as a key driver of abnormal bone formation in FOP. Regeneron’s work in the disease dates back more than a decade, following preclinical research that revealed the role of Activin A in triggering HO. The company subsequently advanced garetosmab into clinical development, launching early-stage studies in 2016.
The FDA’s decision was supported by results from a Phase 3 clinical trial involving adults with FOP. In the study, patients receiving Pasatru experienced substantially fewer new HO lesions than those given placebo. Investigators reported 19 newly formed lesions among 21 placebo-treated participants, compared with just three lesions across 42 patients treated with either of two Pasatru dosing regimens.
The approved starting dose is 10 mg/kg administered intravenously over approximately one hour every month. Patients who experience difficulty tolerating the higher dose may be reduced to 3 mg/kg, which is delivered on the same monthly infusion schedule.
Recognizing the significant physical limitations associated with FOP, Regeneron designed the treatment to be administered across multiple care environments, including home-based infusion settings when clinically appropriate. The company said this flexibility may help reduce treatment burdens for patients whose mobility is increasingly compromised by disease progression.
The approval positions Pasatru alongside Ipsen’s Sohonos, which became the first FDA-approved therapy for FOP in 2023. Sohonos reached the market after a lengthy development journey that included both clinical and regulatory challenges. Ipsen acquired the therapy through its 2019 purchase of Clementia Pharmaceuticals.
Competition in the FOP landscape may continue to intensify. Mirum Pharmaceuticals is awaiting an FDA decision on zilurgisertib, an investigational therapy licensed from Incyte. The candidate is under regulatory review for patients aged 12 years and older, with a decision expected later this year. Analysts have projected meaningful commercial potential in the FOP market despite its very small patient population, estimated at roughly 900 diagnosed individuals worldwide.
While Pasatru is currently approved only for adults, Regeneron plans to expand its research program into younger patients. The company has announced plans to begin studies in children and adolescents, aiming to evaluate whether intervention earlier in the disease course could help reduce the long-term burden of abnormal bone formation.
The approval marks a significant milestone for both Regeneron and the FOP community, offering a new therapeutic option for a devastating disease that has historically had few available treatments.
