Atebrioz (zilurgisertib)

The third FOP treatment and the only one that inhibits ALK2 directly. Sohonos, an oral retinoid, is the only option for children under 12 but carries a boxed warning for birth defects and growth-plate closure. Pasatru, an IV antibody against activin A, is approved for adults. Atebrioz is a once-daily pill for ages 12 and up without the retinoid burden of Sohonos. Where it fits relative to Pasatru, and whether any of the 3 can be combined, is not yet settled.

In FOP, a mutation in the ACVR1 gene makes a receptor called ALK2 fire when it should not, so muscle, tendon and ligament injured by a bump, an injection or nothing at all respond by turning into bone. Sohonos dampens that signal indirectly through a retinoid receptor, and Pasatru blocks activin A, a protein that switches the mutant receptor on. Zilurgisertib is different: it binds ALK2 itself and blocks the kinase from signaling, cutting off the bone-forming message at its source. It is taken as a pill once a day.

Atebrioz can cause fetal harm, based on animal studies. Anyone who can become pregnant should use effective contraception during treatment, and should stop the drug and contact their doctor right away if pregnancy occurs (FDA). The FDA lists headache, joint pain, upper respiratory infection, nosebleeds and nausea as the most common side effects, and says Atebrioz should not be taken with certain other medicines listed in the label. The label Mirum has posted says Atebrioz must not be used during pregnancy and pregnancy should be ruled out before starting. It also says breastfeeding is not recommended, and that Atebrioz should not be taken with strong or moderate CYP3A4 inducers or with certain medicines such as metformin. In the placebo-controlled part of the PROGRESS study, most side effects were mild or moderate and none led to a dose reduction or to anyone stopping the drug. The most common reactions on zilurgisertib were FOP flare pain itself (25%), headache (21.9%), upper respiratory infections (21.9%), joint pain (18.8%), nosebleeds (12.5%) and nausea (12.5%).

A once-daily oral small molecule for an ultra-rare disease of roughly 300 people in the US. Mirum said it will announce the price at launch in October 2026. The 2 other FOP drugs, Sohonos and Pasatru, are both specialty-priced orphan products; an oral drug avoids infusion-center costs but not the specialty-pharmacy and prior-authorization process.

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Approved for people 12 and older, matching the PROGRESS cohort. Children under 12 are not covered by the approval; the pediatric PROGRESS cohorts are still running. The label says it must not be used during pregnancy.

Source: Reuters, September 25, 2026

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Approval rests on Cohort 1 of the PROGRESS study (NCT05090891), which randomized 63 adolescents and adults with FOP, average age about 21, to zilurgisertib 100 mg once daily or placebo for 24 weeks, followed by an open-label extension. New bone was measured by whole-body CT. The FDA based its approval on the change in the volume of total new heterotopic ossification: at week 24 it had decreased by an average of 3.2 cubic centimeters in patients on Atebrioz and increased by 24.6 cubic centimeters on placebo, and the FDA states the effect was maintained through week 48 of the open-label extension. In the company's June 2026 results, brand-new lesions totaled 0.003 cubic centimeters on the drug against 6.57 on placebo, flare-ups ran at about half the placebo rate, and the share of patients with any new lesion (1 of 32 versus 5 of 31) favored the drug without reaching statistical significance because so few events occurred. Through week 48, no new lesion appeared in any of the 61 patients with scans. Two further PROGRESS cohorts are studying children aged 6 to 11 and 2 to 5.

Incyte developed zilurgisertib and ran the PROGRESS study. Mirum Pharmaceuticals licensed worldwide rights in April 2026 for $16 million upfront plus milestones. The FDA accepted the application under Priority Review with a target date of September 26, 2026, and approved the drug as Atebrioz on September 25, 2026 for patients 12 and older. It is the third FOP treatment, after Sohonos (August 2023) and Pasatru (August 2026).

FOP went from zero approved treatments before August 2023 to 3 by September 2026, each with a different mechanism: a retinoid (Sohonos), an activin A antibody (Pasatru) and now a direct ALK2 inhibitor (Atebrioz). For adults and teenagers 12 and older, the choice now involves route (pill versus infusion), age, side-effect profile and, once known, price and coverage. For children under 12, Sohonos remains the only approved drug, and the pediatric PROGRESS cohorts are the path to a second option.