The approval of Mirum Pharmaceuticals’ ALK2 inhibitor for fibrodysplasia ossificans progressiva on Friday adds a third option for patients who had long been without any treatments, joining Regeneron’s recently approved Pasatru and Ipsen’s Sohonos.
Mirum Pharmaceuticals notched an FDA approval Friday for zilurgisertib—to be marketed as Atebrioz—for the treatment of an ultra-rare bone-forming disease that for decades had no treatments. Mirum joins Regeneron and Ipsen in the tiny space as it brings a third option to the market in as many years.
Colloquially called “stone man syndrome,” fibrodysplasia ossificans progressiva (FOP) causes bone formation within connective tissue including tendons, ligaments and muscles—known as heterotopic ossification (HO)—leading to pain, stiffness and immobility over time.
“To put it simply, it’s a terrible disease,” Mirum CEO Chris Peetz told BioSpace ahead of the company’s new approval. “Even bumping into something and bruising can result in signaling that turns soft tissue into bone and accumulates over time. Into young adulthood, it can become life-threatening, and before these treatments, there was nothing that could stop progression.” The genetic disorder is estimated to affect one in a million people globally.
FOP specialist Angela Cheung, director of the osteoporosis program at University Health Network in Toronto, described the patient experience. “People get trapped in their own skeleton, like tin soldiers, and they progressively get worse over time with an average life expectancy of 56 years, many even younger.”
Cheung called the 20-year-old discovery of the ACVR1 gene, which causes this abnormal signaling, “groundbreaking,” adding that the knowledge enabled clinicians and researchers to develop therapies for patients with the disease. Ipsen’s Sohonos was the first to market, approved by the FDA in August 2023, then Regeneron’s Pasatru followed just last month.
Mirum’s approval has thus now tripled the available options for patients with FOP in just five weeks’ time. Cheung told BioSpace that in a disease like FOP, patients need options, and that the three drugs available, each of which works in a different way and is “not for everyone,” offer just that.
Atebrioz, approved by the FDA for patients 12 years and older, is a once-daily pill that inhibits the activin receptor-like kinase 2 (ALK2) protein, which has been found to cause abnormal bone growth in FOP. Originally discovered by Novartis and later licensed by Incyte for clinical development, rare disease–focused Mirum gained full ownership of Atebrioz in April.
Friday’s approval was based on data from Cohort 1 of the Phase 2 PROGRESS study, in which treatment with Atebrioz elicited an 81% reduction in new lesion volume and a 99% reduction in total volume compared with placebo at 24 weeks, meeting the primary endpoint. These treatment effects were maintained through week 48 of an open-label-extension study, according to a press release issued by Mirum and Incyte on Friday.
The drug was generally well tolerated, the companies said, 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.
Paving the way
Peetz credits a small but vocal patient advocacy group, the International FOP Association, for putting FOP on the clinical map and guiding drugmakers’ efforts from a patient perspective. Mirum, which has three other rare disease drugs on the market, has learned how to bring these therapies to even the smallest populations.
“I’m really proud of what we’ve created as a platform to find these great medicines that fall through the cracks,” Peetz said. “Things that look like maybe they’re too small but have tremendous patient impact.”
In blazing the trail for FOP therapies, Ipsen met with a difficult path marked by missed clinical endpoints and an initial FDA rejection before ultimately squeaking Sohonos over the regulatory finish line.
Sohonos, a daily pill that suppresses cartilage and bone development, reduced the volume of new bone growth by 54% in the Phase III MOVE trial. Despite being the first and only treatment for the disease, Sohonos came with risks that included early growth plate closure in children due to changes in bone formation. An advisory committee in June 2023 voted in support of the benefit-risk profile, leading to the drug’s approval in girls 8 years and older and boys 10 years and older.
Sales of Sohonos, which was first approved in Canada in early 2022, dwindled even before competition entered the scene, however, with an almost 50% decline reported year-over-year in the first quarter of 2025. In the first half of 2026, the drug was lumped into Ipsen’s $13.4 million rare disease portfolio.
The FDA approval of Regeneron’s infused antibody Pasatru for adults with FOP marked the second treatment for FOP. In the Phase 3 OPTIMA trial, Pasatru significantly reduced the number of lesions in patients compared to placebo.
Aris Economides, Regeneron’s vice president of research, points to early mouse models developed by his team that made clinical progress in FOP possible, providing insights into the specific ligands that caused an abundance of bone formation.
“With this antibody, you completely block this heterotropic bone from forming in the mouse model, which told us that this was ligand-dependent, and not based on the receptor,” Economides said of Pasatru. This mechanism is more discerning than Sohonos’, he added, with Ipsen’s drug operating more broadly on bone formation.
Pasatru’s FDA nod covers adults with the disease, but because FOP is a progressive genetic condition that begins from birth, Regeneron is exploring pediatric use in a Phase 3 study called OPTIMA 2 set to begin later this year, according to a spokesperson. Mirum is also conducting clinical trials in children as young as 2 years.
Finding patients
Peetz credits Atebrioz’s strong clinical data for helping to forge a clear regulatory path for the drug, particularly at an FDA that has recently pushed for rare disease reform.
“If the data is clear in a rare disease, there are pathways to get medicine to patients in a reasonable amount of time, and to not put more draconian, larger indication standards on it,” Peetz said. Atebrioz previously received the FDA’s Fast Track and Orphan Drug designations.
But with the approval now in hand, Mirum now embarks on a journey to bring the drug to patients. Even with “striking data,” that’s no easy task in such a small population, Peetz acknowledged.
“Our biggest focus has been how quickly we can handle this transfer from Incyte to Mirum to get the drug out to patients, so expect that to happen in October,” he said.
Another important part of access is price. Rare disease drugs often come with a hefty price tag, and treatments for FOP are no different. Sohonos kicked off at launch with a $624,000 annual list price, and Regeneron’s will start with its own list price of $1.4 million annually varying by dose, a company spokesperson said. Both drugmakers acknowledge the significant cost based on years of R&D and offer patient support programs.
Mirum, which predicted peak sales of more than $200 million earlier this year for Atebrioz, has not yet revealed a list price and will announce the cost when it becomes available in October, Peetz said. The therapy is expected to be available through the company’s Mirum Access Plus (MAP) patient support program, according to Friday’s release, with eligible patients paying as little as $0 per month.
“Access is a big thing,” Cheung said. “The companies have to get back their investment in research and development, but when you make it too high, it’s never going to work.”
