REGN Pasatru Wins FDA Approval for FOP, Lesions Cut 90%

The FDA approved Regeneron’s (REGN) Pasatru (garetosmab) on August 19 for adults with fibrodysplasia ossificans progressiva (FOP), an ultra-rare disorder in which bone forms inside muscle, tendons, and ligaments. It is the first FOP drug shown in a placebo-controlled trial to reduce both new heterotopic ossification (HO) lesions and clinician-assessed flare-ups. Regeneron shares closed at $826.64 on August 20.

The OPTIMA trial randomized adults to Pasatru 10 mg/kg, Pasatru 3 mg/kg, or placebo, dosed intravenously every four weeks for 56 weeks. Both doses cut new HO lesions dramatically: 90% at 10 mg/kg (2 lesions versus 19) and 94% at 3 mg/kg (1 versus 19), measured by CT scan. The drug is a fully human monoclonal antibody that neutralizes Activin A, the protein Regeneron scientists identified as the driver of rogue bone formation in FOP.

The disease burden is why any reduction matters. FOP affects roughly one in two million people, an estimated 800 patients in the US. HO of the jaw, spine, hip, and rib cage makes speaking, eating, walking, and breathing progressively harder, leading to wheelchair dependence and shortened survival. Pasatru caps a three-decade research effort and marks Regeneron’s move beyond its Dupixent and Eylea franchises into rare disease biology built on its VelocImmune platform.

Pasatru previously received Fast Track and Orphan Drug designation from the FDA. It is not the first FOP treatment, despite what “first and only” language in some coverage implies. Ipsen’s Sohonos (palovarotene), an oral retinoid, won approval in 2023 to reduce new HO volume. Pasatru’s edge is the flare-up data and the magnitude of lesion reduction, but the two are not head-to-head. The distinction matters to investors because Sohonos’s benefit-versus-risk profile has been debated, and Pasatru arrives as a monthly infusion versus a daily pill.

The competitive race is not over. Mirum and Incyte’s zilurgisertib, an oral ALK2 inhibitor with a September 26 PDUFA date, could win approval in a matter of weeks. Both drugs hit the same signaling axis, with Pasatru blocking the Activin A ligand and zilurgisertib blocking the ALK2 receptor. FOP is a market measured in hundreds of patients, so pricing will be steep; for how ultra-rare drugs price and what insurers tolerate, see the orphan drug pricing guide. Regeneron plans an OPTIMA 2 trial in adolescents and children later this year.

What to watch next: the September 26 zilurgisertib decision. If approved, Pasatru faces an oral rival weeks after launch; if not, Regeneron has the FOP field to itself for now. Read our zilurgisertib PDUFA primer for the full case.

Source: OPTIMA trial (garetosmab in FOP) on ClinicalTrials.gov (NCT05394116)