Two New FDA-Approved Treatments Give People With FOP More Options

In five weeks, the FDA approved Regeneron's Pasatru and Mirum's Atebrioz for fibrodysplasia ossificans progressiva, a rare disease that slowly turns muscle and tendons into bone.

For most of medical history, there was nothing doctors could offer people with fibrodysplasia ossificans progressiva, or FOP. That began to change in 2023, when the FDA approved the first treatment. Now, in the span of five weeks, the agency has approved two more. Pasatru (garetosmab) from Regeneron was approved on August 19, 2026, and Atebrioz (zilurgisertib) from Mirum Pharmaceuticals and Incyte followed on September 25, 2026. For a disease that affects only a few hundred people in the U.S., three approved options is a major shift.

What is FOP?

FOP is a rare genetic disease in which muscles, tendons and ligaments gradually turn into bone. Doctors call this new bone heterotopic ossification, or HO, which means bone forming where it shouldn't. Over time, the extra bone locks joints in place and forms what is sometimes described as a second skeleton. It limits movement, can make eating and breathing harder, and leads to severe disability.

The disease often advances through flare-ups, painful episodes of swelling that can be set off by injury, falls, injections or even a bump. Many flare-ups end with new bone. Because surgery to remove the bone usually triggers even more growth, preventing new bone is the main goal of treatment.

~300 people living with FOP in the U.S.
~900 known cases worldwide
3 FDA-approved treatments, up from zero before 2023

FOP is caused by a change in the ACVR1 gene, which makes a protein called ALK2 (activin A receptor type 1). ALK2 helps control bone growth. In people with FOP, the faulty receptor switches on when it shouldn't, and a signaling protein called activin A sets it off. That results in bone forming in soft tissue. Both new medicines target this pathway, but at different points.

The two new treatments at a glance

Pasatru (garetosmab-grts) · Regeneron · Approved Aug. 19, 2026

Antibody that blocks activin A, the signal that switches on the faulty receptor
IV infusion every 4 weeks
For adults with FOP

Atebrioz (zilurgisertib) · Mirum & Incyte · Approved Sept. 25, 2026

Pill that blocks the ALK2 receptor directly
100 mg tablet once a day, with or without food
For adults and children 12 and older

Pasatru (garetosmab)

Pasatru is a monoclonal antibody that attaches to activin A and keeps it from switching on the abnormal ALK2 receptor. It is given as an IV infusion of 10 mg/kg over about an hour, once every four weeks. People who don't tolerate that dose can receive 3 mg/kg instead.

In a randomized, placebo-controlled trial of 63 adults, the results were striking. Over 56 weeks, people on the 10 mg/kg dose developed 2 new bone lesions in total, compared with 19 in the placebo group. The 3 mg/kg group had just 1. Flare-ups dropped as well, from 66 in the placebo group to 9 on the higher dose. Pasatru is the first FOP treatment approved to reduce flare-ups as well as new bone growth.

Pasatru trial, 56 weeks: New bone lesions fell from 19 → 2 between the placebo and 10 mg/kg Pasatru groups. Flare-ups fell from 66 to 9.

Atebrioz (zilurgisertib)

Atebrioz is a small-molecule ALK2 inhibitor, which means it blocks the faulty receptor itself. It was discovered by Incyte and licensed to Mirum Pharmaceuticals, which will sell it. The big practical difference is that Atebrioz is a once-daily pill that can be taken at home, and it's approved for teenagers as young as 12.

In the PROGRESS trial, people taking Atebrioz saw the total volume of new bone go down by an average of 3.2 cm³ over 24 weeks. In the placebo group, it went up by 24.6 cm³. Mirum expects Atebrioz to be available in October 2026. Its Mirum Access Plus program says eligible patients may pay as little as $0 a month.

"Having another treatment option is meaningful in a progressive disease like FOP." — Dr. Robert Pignolo, Mayo Clinic, lead PROGRESS investigator

How the three FOP treatments compare

| | Sohonos | Pasatru | Atebrioz | |---|---|---|---| | Generic name | palovarotene | garetosmab-grts | zilurgisertib | | Company | Ipsen | Regeneron | Mirum / Incyte | | FDA approval | Aug. 2023 | Aug. 2026 | Sept. 2026 | | Approach | Retinoid that dampens early bone formation | Antibody that blocks activin A | Pill that blocks the ALK2 receptor | | How it's taken | Daily capsule | IV infusion every 4 weeks | Daily tablet | | Who it's for | Adults, girls 8+ and boys 10+ | Adults | Adults and children 12+ | | Approved to reduce | Volume of new bone | New bone lesions and flare-ups | Volume of new bone |

Safety: what patients should know

Both new medicines can harm an unborn baby. Anyone who could become pregnant needs to use effective birth control during treatment. For Pasatru, that continues for six months after the last dose.

Pasatru carries warnings for skin and soft-tissue infections, some serious enough to need hospital care, and for nosebleeds that may need medical treatment. The most common side effects were nosebleeds, increased hair growth, abscesses and acne.

Atebrioz should not be taken with certain other medicines. The most common side effects were headache, joint pain, upper respiratory infections, nosebleeds and nausea, and most were mild to moderate.

Good to know: FOP is often misdiagnosed, sometimes as cancer, and biopsies or surgery can trigger new bone growth. A telltale early sign is a malformed big toe present at birth. If you or your child has FOP, work with a specialist experienced in the disease to decide which treatment fits.

What comes next

Research is moving toward younger patients, which matters because FOP usually begins in childhood. Mirum has finished enrolling children ages 6 to 11 in the PROGRESS study and is enrolling children ages 2 to 11 in a third group. Patient advocates say the bigger shift is that families now have real choices.

"Expanding treatment options gives patients and families the opportunity to consider what may be right." — Michelle Davis, Executive Director, International FOP Association

For people who want to connect with others living with FOP, the International FOP Association (IFOPA) offers support, research updates and a patient registry.

This article is for general information and is not medical advice. Speak with a qualified healthcare provider before starting or changing any medication.