Mirum Wins FDA Approval For Atebrioz In FOP, Adding A Third Option In Five Weeks

Mirum Pharmaceuticals said the FDA approved zilurgisertib, which it will market as Atebrioz, for fibrodysplasia ossificans progressiva in patients 12 years and older. The decision gives patients with the ultra-rare disease a third approved treatment in as many years and comes just five weeks after Regeneron added a second option.

Fibrodysplasia ossificans progressiva, often called FOP, causes bone formation within connective tissue including tendons, ligaments and muscles, leading to pain, stiffness and immobility over time. The disorder is estimated to affect one in a million people globally. Mirum CEO Chris Peetz told BioSpace that before the new wave of treatments, “there was nothing that could stop progression.”

The data

Atebrioz is a once-daily pill that inhibits the activin receptor-like kinase 2 protein, or ALK2, which has been found to cause abnormal bone growth in FOP. The approval was based on data from Cohort 1 of the Phase 2 PROGRESS study.

In that cohort, treatment with Atebrioz produced an 81% reduction in new lesion volume and a 99% reduction in total volume compared with placebo at 24 weeks, meeting the primary endpoint. According to a press release issued by Mirum and Incyte, those treatment effects were maintained through week 48 of an open-label-extension study.

The companies said the drug was generally well tolerated, with only mild or moderate side effects including headache, joint pain, respiratory infection, nosebleed and nausea. There were no adverse events that led to treatment discontinuation or dose reduction.

The commercial picture

The approval reshapes a very small but suddenly more competitive market. Ipsen’s Sohonos was the first FDA-approved FOP treatment in August 2023. Regeneron’s infused antibody Pasatru followed last month for adults with FOP. Mirum now brings a third mechanism into the field.

That breadth may matter because the currently available drugs are not interchangeable. FOP specialist Angela Cheung, director of the osteoporosis program at University Health Network in Toronto, said patients need options and that the three available drugs each work differently and are “not for everyone.” Atebrioz is an oral ALK2 inhibitor, while Pasatru is an infused antibody and Sohonos is a daily pill that suppresses cartilage and bone development.

There is also a commercial opening despite first-mover status for Ipsen. BioSpace reported that Sohonos sales had already weakened, with an almost 50% year-over-year decline in the first quarter of 2025, and that in the first half of 2026 the drug was lumped into Ipsen’s $13.4 million rare disease portfolio. That suggests being first has not locked up the category.

The road here

The FOP field exists because researchers identified the ACVR1 gene about 20 years ago. Cheung described that discovery as enabling clinicians and researchers to develop therapies for patients with the disease.

Atebrioz itself changed hands several times before approval. The drug was originally discovered by Novartis and later licensed by Incyte for clinical development. Mirum gained full ownership in April, giving the rare disease company control of the asset only months before FDA clearance.

Mirum is also leaning on experience in very small populations. Peetz said the company, which has three other rare disease drugs on the market, has built a platform to find medicines that may appear too small commercially but can still have large patient impact.

What to watch

Competition in FOP is still expanding beyond the current labels. Regeneron is exploring pediatric use of Pasatru in a Phase 3 study called OPTIMA 2 set to begin later this year, according to a spokesperson, because the disease begins from birth. Mirum is also conducting clinical trials in children as young as 2 years. In a market this small, label expansion by age group may shape share as much as mechanism or launch timing.