FDA Starts Review of Regeneron's Drug for Rare Disease FOP

Why It Matters

A successful approval would dramatically improve outcomes for a ultra‑rare, debilitating disease and reshape the FOP therapeutic market, while providing Regeneron a foothold in a high‑need niche.

Key Takeaways

• FDA grants priority review for garetosmab, decision August.
• Phase 3 trial cut new bone lesions up to 94%.
• May replace underperforming Sohonos, reshaping FOP market.
• Regeneron plans pediatric Phase 3 trial later this year.
• Around 400 US patients could gain new therapy.

Pulse Analysis

Fibrodysplasia ossificans progressiva (FOP) remains one of the most challenging rare bone disorders, affecting roughly 900 individuals worldwide and causing progressive heterotopic bone formation that immobilizes patients by their third decade. The therapeutic landscape has been limited to Ipsen’s oral RAR gamma agonist, Sohonos, which, despite FDA approval in 2023, has delivered modest commercial uptake due to efficacy concerns and safety signals. This scarcity of effective options leaves a substantial unmet medical need, driving investors and biotech firms to pursue novel biologics that target the disease’s underlying pathways.

Garetosmab, Regeneron’s anti‑activin A monoclonal antibody, directly blocks a cytokine implicated in the aberrant bone‑forming cascade of FOP. The OPTIMA Phase 3 trial, enrolling 63 patients, demonstrated a striking 94% reduction in new lesion count at the 3 mg/kg dose and a comparable 90% reduction at 10 mg/kg, with total lesion volume dropping by more than 99% in both arms. Such efficacy signals are unprecedented in FOP trials and have prompted the FDA to grant priority review, accelerating the decision timeline to August. The data suggest that garetosmab could become the first therapy proven to both limit lesion number and shrink lesion size, a dual benefit that may translate into longer mobility and survival for patients.

If approved, garetosmab would not only provide a clinically superior alternative to Sohonos but also reposition Regeneron within the rare‑disease arena, where high‑margin, low‑volume products can generate outsized returns. The company’s commitment to a pediatric Phase 3 study (OPTIMA 2) underscores a long‑term strategy to capture the entire FOP spectrum, from children to adults. Moreover, the potential market—estimated at 400 U.S. patients—offers a focused yet lucrative niche, especially as insurers and specialty pharmacies seek therapies with demonstrable disease‑modifying effects. Success could spur further investment in anti‑activin pathways for other ossification disorders, reinforcing Regeneron’s pipeline diversification and solidifying its reputation for tackling high‑unmet‑need conditions.

FDA starts review of Regeneron's drug for rare disease FOP

Phil Taylor · 20 February 2026

Patients in the US with rare bone disease fibrodysplasia ossificans progressiva (FOP) could have a second treatment option in a matter of months, if the FDA clears a just‑filed marketing application for Regeneron's garetosmab therapy.

The regulator has started a priority review of the anti‑activin A antibody, with a decision due in August, based on the results of the phase 3 OPTIMA trial involving 63 patients with FOP, a life‑shortening and highly debilitating disorder in which bony lesions – known as heterotopic ossifications (HO) – are laid down in muscles, tendons, and ligaments.

If approved, garetosmab will be the first alternative to Ipsen's oral RAR gamma agonist Sohonos (palovarotene), which was approved by the FDA for FOP in 2023 but has failed to live up to its commercial promise.

Ipsen reported full‑year sales of the drug last year of €20.7 million ($24.3 million), compared to €20.8 million in the previous year, and said it was taking an impairment charge on its books of around €257 million, mainly driven by Sohonos' stalled uptake.

Regeneron said it has high hopes for garetosmab because, if approved, it will be “the first and only available treatment shown to reduce the number and volume of new heterotopic bone lesions in adults with FOP.”

According to the company, approximately 900 people are diagnosed with FOP worldwide, including around 400 in the US, with many others believed to be undiagnosed or misdiagnosed. Most people with FOP are wheelchair‑bound by 30 years old, and the median age of survival is just 56 years.

In OPTIMA, patients were randomised to one of two doses of garetosmab (3 mg/kg or 10 mg/kg), given by intravenous infusion every four weeks, or a matched placebo. The primary endpoint of the trial was the reduction in new HO lesions at 56 weeks, which was reduced by 94 % with the lower dose and 90 % with the higher dose compared to the control group.

Both doses of garetosmab also demonstrated a greater than 99 % reduction in the mean total volume of new HO lesions.

Regeneron is now preparing for additional regulatory filings for garetosmab in FOP and has indicated it plans to double down on its investment in the programme with the start later this year of a second phase 3 trial, OPTIMA 2, that will recruit children and adolescents with FOP.

Ipsen was also developing a second FOP candidate, fidrisertib, in the pivotal FALKON trial, but that did not meet its primary endpoint, and the programme no longer appears in its R&D pipeline listing.