Zilurgisertib Could Give FOP Patients a Third Treatment on September 26th

Roughly 300 people in the United States live with fibrodysplasia ossificans progressiva, a genetic disease in which muscle, tendon, and ligament gradually turn into bone. Researchers at the University of Pennsylvania, who identified the causative gene in 2006, describe what forms as a second skeleton. Extra bone bridges across joints and locks them in place, one by one, until reaching for a shelf or turning to look over a shoulder becomes impossible. The disease has been in the medical literature since the 1600s. For nearly all of that time, there was nothing to prescribe for it.

That has changed with startling speed. On September 25th, the FDA approved zilurgisertib, a once-daily pill from Mirum Pharmaceuticals and Incyte, as Atebrioz. That makes it the third FOP treatment cleared in barely 3 years, in a disease that measured its history in centuries of waiting. The story of how a 300-person disease suddenly has a crowded pipeline, and what this particular decision hinges on, is worth telling in full.

Why FOP Builds a Second Skeleton

Nearly everyone with FOP carries the same single-letter mutation in a gene called ACVR1, which builds a bone-growth receptor known as ALK2. The mutation leaves that receptor stuck partway on, so signals that should only fire during skeletal development keep firing in adults, telling muscle and connective tissue to become bone. The process moves in episodes called flare-ups: painful, swollen periods, often triggered by a bump, a fall, or even an intramuscular injection, that frequently end with a new piece of permanent bone. This is why FOP care has long revolved around avoiding harm. Biopsies, dental injections, and minor surgeries can all set off new bone growth, and the misdiagnosis rate is high because early symptoms look like tumors or aggressive fibrosis.

The 3 drugs now converging on this disease attack that biology at 3 different points, which matters more than it might seem. More on that below.

Sohonos (palovarotene), a retinoid from Ipsen, became the first approved FOP treatment in August 2023. Its road was rough. The FDA had rejected an earlier filing, European regulators refused the drug outright over efficacy concerns, and BioPharma Dive reported that the eventual US clearance came "despite questions about its benefits" (BioPharma Dive, 2023). Retinoids also carry real burdens, including skin and mucosal side effects and a risk of early growth plate closure that requires careful monitoring in growing children. Sohonos was a milestone, and an imperfect one.

Then this August, everything accelerated. On August 19th, 2026, the FDA approved Regeneron's Pasatru (garetosmab), an antibody given by intravenous infusion that blocks Activin A, a protein that the mutant ALK2 receptor mistakenly reads as a signal to build bone. In the Phase 3 OPTIMA trial, Pasatru cut new bone lesions by 90% or more versus placebo, making it the first FOP treatment to prove itself against a placebo in a controlled trial (Regeneron, 2026). Its label covers adults only.

Zilurgisertib is the third act, and mechanistically the most direct. It is a small-molecule inhibitor of ALK2 itself, the actual mutated receptor, taken as a pill once a day. Incyte developed it, Mirum licensed worldwide rights in April 2026 for $16 million upfront plus milestones, and the FDA granted the application Priority Review with a target date of September 26th (Mirum, 2026). The application covers patients 12 and older.

Sohonos (palovarotene), approved 2023

An oral retinoid taken chronically at a low dose, with higher weight-based dosing during flare-ups. Works downstream of the faulty signal. Approved for females 8 and older and males 10 and older.

Pasatru (garetosmab), approved August 2026

An intravenous antibody that traps Activin A before it can reach the mutant receptor. The first FOP therapy proven against placebo. Adults only, for now; a pediatric trial is planned.

Atebrioz (zilurgisertib), approved September 2026

A once-daily oral ALK2 inhibitor that blocks the mutated receptor at the source. Approved September 25, 2026 for patients 12 and older based on the PROGRESS study.

What the PROGRESS Study Showed

PROGRESS Cohort 1 enrolled 63 adolescents and adults, average age about 21, and randomized them evenly to zilurgisertib 100 mg once daily or placebo for 24 weeks, followed by an open-label extension in which everyone received the drug. New bone was measured the hard way, by whole-body CT scan.

The results, presented at the Endocrine Society's ENDO 2026 meeting in June, were striking on the measures that describe how much bone actually formed. Patients on zilurgisertib developed new lesions totaling 0.003 cubic centimeters on average. The placebo group's average was 6.57, a difference of more than 99%. The volume of total new bone, the measure the FDA later used as the basis for approval, fell by 3.2 cubic centimeters on the drug while growing by 24.6 cubic centimeters on placebo (FDA, September 25, 2026). Flare-ups ran at roughly half the placebo rate. In the extension period through Week 48, not a single new lesion appeared in any of the 61 patients with scans, including those who had crossed over from placebo (Mirum and Incyte, 2026).

“The findings presented at ENDO represent an important milestone for the zilurgisertib program and further strengthen the growing body of clinical evidence supporting its potential as a treatment for FOP.”

Steven Stein, M.D., Chief Medical Officer, Incyte

Safety was clean by the standards of this disease. Most side effects were mild or moderate, and none led to a dose reduction or to anyone stopping the drug. The most common complaints 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%).

The Number the FDA Will Have to Weigh

One complication deserves a plain explanation. In the company's June results, the share of patients who developed any new lesion favored zilurgisertib by 81%, 1 of 32 patients versus 5 of 31 on placebo, but the p-value for that comparison was 0.0986, above the conventional 0.05 bar for statistical significance. With 63 patients and only 6 total events, the trial was too small for so rare an outcome to clear that threshold, even with a large apparent effect. When the FDA approved the drug on September 25th, it based effectiveness on the volume measure instead: the change in total new bone volume over 24 weeks, which the agency says was maintained through week 48 of the extension.

Who Could Take Zilurgisertib and Who Cannot Yet

The approval covers patients 12 years and older, matching PROGRESS Cohort 1. Younger children are not included yet, with 2 additional PROGRESS cohorts still studying ages 6 to 11 and ages 2 to 5, and FOP is usually diagnosed in early childhood, so that gap matters to many families. ClinicalTrials.gov lists the PROGRESS study (NCT05090891) as recruiting, with an age range that starts at 2 years, so families of younger children can ask the study team whether a child qualifies.

“We are pleased to welcome Mirum to the FOP community and thank the team at Incyte for their incredible commitment that brought zilurgisertib through a Phase 2 trial and a New Drug Application that has been accepted for Priority Review by the FDA.”

International FOP Association (IFOPA), May 2026

Why 3 Different Mechanisms Matter for One Tiny Disease

A disease with 300 US patients now has a retinoid acting downstream, an antibody trapping the rogue signal, and a pill blocking the broken receptor itself, each with a different route, schedule, age range, and side effect profile. That is not redundancy. A teenager who cannot get infusions may do best on a daily pill. An adult who struggled with retinoid side effects has somewhere to go. Nobody yet knows whether any of these can be combined safely, and the field will spend years sorting out sequencing, but choice itself is new here.

“People living with FOP and their families urgently need additional treatment options.”

Joanne Quan, M.D., Chief Medical Officer, Mirum Pharmaceuticals

Chris Peetz, Mirum's chief executive, put the company's intent plainly in an open letter to the FOP community: "Along with the FOP community, we share a goal of bringing new treatment options to people living with FOP as quickly as possible." Mirum already sells 3 rare disease medicines, so a launch would run through commercial infrastructure that exists today.

What the September 25th Approval Means

The outcome came a day early: approval on September 25th, for patients 12 and older, as a once-daily 100 mg pill sold as Atebrioz. The label is now posted. It sets a continuous 100 mg dose once a day rather than dosing tied to flares, says the drug must not be used during pregnancy, and lists drug interactions, including strong CYP3A4 inhibitors and metformin. The PROGRESS extension and pediatric cohorts continue, and children under 12 remain outside the approval for now.

Zilurgisertib's approval came in the middle of the busiest rare disease stretch the FDA has had in years. We cover the full docket in our September decisions guide, and every date is tracked on the FDA decision calendar. The FOP disease page has trial listings and specialist resources for families.