People with fibrodysplasia ossificans progressiva (FOP) now have a third approved treatment. On September 25, the FDA approved Atebrioz (zilurgisertib), a once-daily pill for adults and children 12 and older with FOP, a rare genetic disease in which muscle, tendons, and ligaments gradually turn into bone, according to the FDA announcement.
The approval is based on imaging, not on long-term movement or survival. In the main trial, whole-body CT scans showed that patients taking the drug had less new abnormal bone after 24 weeks than those on placebo.
The approval matters most for teenagers. It adds an option for patients 12 and older at a time when the most recently approved FOP drug is cleared only for adults, and it comes as a pill rather than an infusion.
A Rare Disease That Locks the Body in Place
FOP is caused by a change in the ACVR1 gene, which makes a protein involved in the body's bone-growth signaling. In people with the disease, injuries, surgery, or sometimes no clear trigger can set off flare-ups that form bone in soft tissue outside the skeleton, a process known as heterotopic ossification.
The FDA says the condition causes "limited movement, deformities, severe disability, and early death." The extra bone cannot simply be removed, because surgery can trigger more bone growth. About 300 people in the U.S. and 900 worldwide live with FOP, according to Mirum Pharmaceuticals, which markets the drug.
Atebrioz blocks ALK2, the protein made by the ACVR1 gene, which is overactive in FOP. Incyte developed the drug and licensed it to Mirum for worldwide development and sales, the companies said in a joint announcement of the FDA approval.
Scan Results from a 63-Patient Trial
The approval rests on the PROGRESS study, which included a 24-week randomized, double-blind, placebo-controlled phase in 63 patients aged 12 and older. The main measure was the change in the volume of new abnormal bone on whole-body CT scans at week 24.
Patients taking Atebrioz had an average decrease of 3.2 cubic centimeters of new abnormal bone, while those on placebo had an average increase of 24.6 cubic centimeters, the FDA said. The companies reported that the effect held through week 48 in the trial's ongoing open-label extension, in which all participants can receive the drug.
Those results measure bone on a scan. The trial was not designed to show whether patients keep more movement, stay independent longer, or live longer. That distinction matters in FOP, where joint mobility and independence shape daily life, and longer follow-up will be needed to answer those questions.
"Having another treatment option is meaningful in a progressive disease like FOP, particularly for adolescents who may be earlier in the course of their disease," said Dr. Robert Pignolo of the Mayo Clinic College of Medicine, the lead investigator for the PROGRESS study, in the companies' announcement.
Fetal Risk and Common Side Effects
The most serious warning involves pregnancy. "Atebrioz can cause fetal harm based on data from animal studies," the FDA said. Patients who can become pregnant should use effective contraception, and they should stop the drug right away and contact their health care provider if pregnancy occurs.
The most common side effects were headache, joint pain, upper respiratory tract infections, nosebleeds, and nausea. The companies said most side effects were mild or moderate and that none led patients to stop treatment or lower their dose.
The FDA gave the application fast track and priority review, which are meant to speed the development and review of drugs for serious conditions. The drug also received orphan drug designation, which offers incentives for developing treatments for rare diseases.
For teenagers, a daily pill avoids the infusion visits every four weeks required by Pasatru, the adult-only FOP drug. Younger children have fewer choices. Sohonos is the only FOP drug approved for girls ages 8 to 11 and boys ages 10 and 11, and no FOP drug is approved for younger children. Mirum is studying Atebrioz in children younger than 12.
A Third Option with Different Trade-Offs
The FDA approved the first FOP drug, Sohonos (palovarotene), in August 2023 for females 8 and older and males 10 and older. Its label carries a boxed warning about fetal harm and premature closure of growth plates in growing children, according to Ipsen, which makes Sohonos.
The second drug, Pasatru (garetosmab), was approved on August 19, 2026, for adults 18 and older and is given by intravenous infusion every four weeks, according to Regeneron, which makes Pasatru. MedicalDaily previously reported that Pasatru launched at seven-figure pricing and that its trial also measured flare-ups.
Mirum has not disclosed a list price for Atebrioz; its CEO, Chris Peetz, told Reuters the company plans to announce the price at launch. Mirum expects the drug to be available in the U.S. in October and says eligible patients may pay as little as $0 a month through its Mirum Access Plus program, which can be reached at 855-676-4968. Copay help does not determine what insurers or Medicaid will cover, so coverage may vary by plan.
The three drugs have not been compared head to head, and no single option is right for every patient. Families should talk with an FOP specialist about age, growth, pregnancy plans, and how each drug is taken. They should also avoid elective injections, biopsies, or surgery without that specialist's input, because injury can trigger new bone growth.
The next milestone is longer follow-up from the open-label extension, which the FDA says is planned to run for 292 weeks. MedicalDaily will report whether later data show effects on mobility or flare-ups.
Key Questions Answered
What did the FDA approve? Atebrioz (zilurgisertib), a once-daily pill for people 12 and older with fibrodysplasia ossificans progressiva.
What is FOP? A rare genetic disease in which muscle, tendons, and ligaments gradually turn into bone, limiting movement over time.
What did the trial show? On CT scans at 24 weeks, patients on Atebrioz averaged a 3.2 cubic centimeter decrease in new abnormal bone, while patients on placebo averaged a 24.6 cubic centimeter increase.
Did the trial prove better mobility or longer life? No. It measured bone volume on scans, not long-term function or survival.
What are the main safety concerns? The drug can harm a developing fetus, so patients who can become pregnant need effective contraception. Common side effects include headache, joint pain, and nosebleeds.
How does it compare with other FOP drugs? Sohonos is approved for younger children, and Pasatru is an infusion for adults. The drugs have not been tested against each other.
When will it be available? Mirum expects U.S. availability in October 2026.
Published by Medicaldaily.com
