Regeneron’s Pasatru is the second fibrodysplasia ossificans progressiva drug approved in the U.S., following the 2023 approval of Ipsen’s Sohonos.
Regeneron has won FDA approval for garetosmab in fibrodysplasia ossificans progressiva, giving the biotech a commercial product built on its scientific breakthrough in the ultrarare disease. It also sets up a showdown with Ipsen, which owns the only other FOP drug on the market.
The Thursday approval covers the use of Pasatru—an antibody called garetosmab and REGN2477 during development—to reduce the formation of new heterotopic ossification (HO) lesions and clinician-assessed flare-ups in adults with FOP. HO is characterized by bone developing in soft tissues.
Pasatru is the second FOP drug to secure U.S. approval. The first, Ipsen’s Sohonos, came to market in 2023 after a troubled development program marked by clinical and regulatory setbacks. Ipsen acquired the asset in 2019 from Clementia Pharmaceuticals. For the first half of 2026, Sohonos contributed to the €11.4 million ($13.4 million) in sales generated by Ipsen’s rare disease drugs apart from Iqirvo and Bylvay.
Regeneron has now won approval based on Phase 3 trial data that showed two doses of Pasatru reduced the total number of new HO lesions compared to placebo. Investigators saw 19 new lesions in the 21 patients on placebo, compared to three new lesions in the 42 patients treated across two Pasatru dose cohorts.
The recommended starting dosage is 10 mg/kg, the higher of the two doses given in the Phase 3 trial. Patients receive the antibody intravenously for one hour every month. If a patient doesn’t tolerate the dose, they can drop down to the lower Phase 3 dose, 3 mg/kg. Patients on the lower dose still receive a 60-minute infusion once monthly.
Regeneron designed Pasatru to be given in a range of care settings, including at home when appropriate. The flexibility reflects the significant mobility challenges faced by many people with FOP, which can make walking difficult as bone builds up in muscles, tendons, ligaments and other connective tissues. Most patients are wheelchair-bound by age 30, Regeneron said.
Regeneron’s efforts in the disease can be traced back to 2015, when researchers published a paper describing an unexpected discovery about the driver of HO in FOP. By looking at FOP in mice, Regeneron identified Activin A as the ligand responsible for HO. Regeneron started a Phase 1 trial of the anti-Activin A antibody—now known as Pasatru— in 2016.
About 900 people are diagnosed with FOP. Rare disease–focused biopharma Mirum Pharmaceuticals has predicted that its rival FOP drug candidate, zilurgisertib, could generate peak sales of more than $200 million. Zilurgisertib, which Mirum licensed from Incyte, is under review at the FDA and has an approval decision date of Sept. 26.
Mirum is seeking approval of zilurgisertib for the treatment of FOP in patients 12 years of age and older. Regeneron’s Phase 3 study only enrolled adults, although the company plans to start a trial in children and adolescents this year.
