Mirum Pharmaceuticals and partner Incyte have won FDA approval for Atebrioz, an oral ALK2 inhibitor that becomes the third medicine ever cleared for fibrodysplasia ossificans progressiva, an ultra-rare disease that turns muscle and connective tissue into extra bone. The approval, announced 25 September 2026, landed a day ahead of its Priority Review deadline and five weeks after Regeneron's Pasatru became the second FOP treatment — turning a disease with no approved therapy as recently as 2023 into a three-drug field.
A label built on one endpoint, not two
Atebrioz’s approved label rests on a single, specific measure: the volume of new heterotopic ossification — the extra, disease-driven bone growth that gradually fuses FOP patients’ joints — not on how many patients developed a new lesion at all. In the PROGRESS trial’s Cohort 1, a randomized, double-blind, placebo-controlled Phase 2 study of 63 patients 12 and older, that volume measure was the difference: patients on zilurgisertib averaged 3.2 cubic centimeters of new bone growth at 24 weeks against a 24.6 cubic-centimeter increase on placebo, a result significant at p=.004.
The trial’s other prespecified measure — the share of patients who developed any new heterotopic-ossification lesion at all — told a less clean story: 3.1% on zilurgisertib versus 16.7% on placebo, a result that narrowly missed statistical significance at p=.0986. FDA’s approval is built on the volume endpoint alone; Regulatory News reports the lesion-incidence result here because the label does not, and a regulatory reader weighing the strength of the evidence should have both numbers.
“Today marks an important milestone for people living with FOP, bringing a new treatment option to adult and pediatric patients living with this devastating disease.” Chris Peetz, Chief Executive Officer, Mirum Pharmaceuticals — per the company’s 25 September 2026 announcement
Atebrioz’s approval in four figures
Figures from Mirum and Incyte’s 25 September 2026 announcement.
Blocking ALK2, the kinase that grows bone
FOP is caused by a mutation in the ACVR1 gene, which encodes the ALK2 receptor kinase; in patients with the mutation, ALK2 misreads ordinary injury signals as instructions to grow bone in muscle, tendon and ligament instead of repairing soft tissue. Zilurgisertib is an oral small-molecule inhibitor designed to block ALK2 kinase activity directly, aiming to interrupt that miscue before it produces new bone — a mechanistically different approach from Ipsen’s Sohonos, a retinoic-acid-receptor-gamma agonist, and Regeneron’s Pasatru, an antibody against activin A, the signaling protein ALK2 misreads.
Three things that set Atebrioz apart
An oral pill, not an infusion
Atebrioz is taken by mouth; Pasatru is delivered by intravenous infusion and Sohonos requires ongoing REMS-monitored dosing for its retinoid side effects — a real difference in what long-term treatment looks like for patients and caregivers.
The kinase, not the signal it misreads
Where Pasatru blocks activin A, the extracellular signal ALK2 misinterprets, Atebrioz blocks the ALK2 kinase itself, directly inside the cells where the miscue happens — a different point in the same pathway.
A voucher worth its own future
FDA issued a Rare Pediatric Disease Priority Review Voucher to Incyte alongside the approval — a transferable credit companies can use on a future drug or sell outright, adding value beyond Atebrioz itself.
A three-drug race for nine hundred patients
FOP affects an estimated 900 patients worldwide, with roughly 300 to 400 in the United States by different published counts — a population so small that all three approved treatments are effectively competing for the same few hundred U.S. patients rather than carving out separate niches. Published market-size estimates for FOP treatment vary by a factor of five depending on methodology, which says as much about how early this category still is as about its ceiling.
An ultra-rare market, three drugs deep
$383M → $758M
Published FOP treatment-market estimates: IMARC Group’s 2025 figure against Research and Markets’ 2026-to-2032 forecast — a roughly five-fold difference in implied growth between the two.
Sources: IMARC Group, Fibrodysplasia Ossificans Progressiva Market Report · Research and Markets, Fibrodysplasia Ossificans Progressiva Market Size & Trends
Regulatory News outlook
We’d expect the FOP treatment market to grow faster than either published forecast alone implies, now that three approved drugs are marketing into the same population rather than one.
How we got here: Research and Markets’ own $549.9 million 2026 estimate assumed a single approved therapy reaching a fraction of eligible patients; with Pasatru and Atebrioz both now competing for the same roughly 900 patients, even modest overlap in prescribing would put combined 2026 sales ahead of that single-drug baseline before any 2032 growth is counted.
Atebrioz against FOP’s two other approved drugs
| Product | Mechanism | Route | U.S. approval |
|---|---|---|---|
| Atebrioz (Mirum/Incyte) | ALK2 kinase inhibitor | Oral, once daily | September 2026, ages 12+ |
| Pasatru (Regeneron) | Activin A monoclonal antibody | Intravenous infusion | August 2026 |
| Sohonos (Ipsen) | RAR-gamma agonist (retinoid) | Oral, with REMS monitoring | August 2023 |
From FDA approval records and each company’s own labeling; not a clinical comparison.
Foster City’s road to a third rival drug
Eight years from founding to a third approval
- November 2018
Mirum Pharmaceuticals founded
Incorporated in Foster City, California, to develop therapies for rare diseases.
- August 2023
First FOP treatment approved
FDA approves Ipsen’s Sohonos (palovarotene), the first-ever treatment for FOP, ending a total absence of approved options for the disease.
- 19 August 2026
Second FOP treatment approved
FDA approves Regeneron’s Pasatru (garetosmab-grts), an activin A antibody, as the second treatment for FOP.
- 25 September 2026
Atebrioz approved, one day early
FDA approves Mirum and Incyte’s zilurgisertib a day ahead of its 26 September Priority Review deadline, becoming the third approved FOP treatment.
- Ahead
Three drugs, one small population
With three approved mechanisms now available, prescribers and the roughly 900 patients worldwide with FOP face a genuinely new question — which drug, for which patient — that none of the approval trials was designed to answer.
What Atebrioz still has to prove
Atebrioz’s approval rests on a 24-week volume measure in 63 patients — real evidence, but a small, short trial for a lifelong disease, and one whose companion endpoint, how many patients got a new lesion at all, didn’t reach significance on its own. FDA’s approval decision means the agency judged the volume result sufficient; the incidence result is now part of the public record for prescribers weighing the trade-offs themselves.
- PDUFA date: 26 September 2026; the approval landed one day early, on 25 September.
- Rare Pediatric Disease Priority Review Voucher: issued to Incyte alongside the approval, a transferable credit usable on a future drug or for sale.
- Population: an estimated 900 patients worldwide, roughly 300 to 400 in the United States by differing published counts.
- What’s not yet public: Atebrioz’s U.S. list price, and any head-to-head data against Pasatru or Sohonos — none exists, and none is planned that Regulatory News could find.
Sources & further reading
- Mirum Pharmaceuticals and Incyte, “Mirum Pharmaceuticals and Incyte Announce U.S. FDA Approval of Atebrioz (zilurgisertib) for the Treatment of Fibrodysplasia Ossificans Progressiva (FOP),” press release, 25 September 2026. businesswire.com
- U.S. Food and Drug Administration, “FDA Approves Third Treatment for Fibrodysplasia Ossificans Progressiva,” 25 September 2026. fda.gov
- Healio, “FDA approves Atebrioz tablets for fibrodysplasia ossificans progressiva,” 25 September 2026. healio.com
- HCPLive, “Zilurgisertib Receives FDA Approval for FOP in Patients 12 and Older.” hcplive.com
- U.S. Food and Drug Administration, “FDA Approves Second Treatment for Fibrodysplasia Ossificans Progressiva,” 19 August 2026. fda.gov
- STAT News, “Regeneron wins FDA approval for garetosmab in FOP,” 19 August 2026. statnews.com
- HCPLive, “FDA Approves Palovarotene for Fibrodysplasia Ossificans Progressiva,” 16 August 2023. hcplive.com
- Mirum Pharmaceuticals investor relations — news releases. ir.mirumpharma.com
- IMARC Group, Fibrodysplasia Ossificans Progressiva Market Report. imarcgroup.com
- Research and Markets, Fibrodysplasia Ossificans Progressiva Market Size & Trends. researchandmarkets.com
Regulatory News reports on public regulatory documents. It is not legal advice, and the primary sources above govern. If we have made an error, we will say so in public: see corrections.
Frequently asked questions
Is Atebrioz the first FDA-approved treatment for FOP?
No. It's the third. FDA approved Ipsen's Sohonos (palovarotene) in August 2023 and Regeneron's Pasatru (garetosmab-grts) in August 2026, before approving Mirum and Incyte's Atebrioz (zilurgisertib) on 25 September 2026.
What did the approval trial actually show?
In the Phase 2 PROGRESS trial's Cohort 1 (63 patients), zilurgisertib reduced new heterotopic-ossification volume to 3.2 cm³ at 24 weeks versus a 24.6 cm³ increase on placebo (p=.004) — the endpoint behind the approved label. A separate measure, the share of patients developing any new lesion, narrowly missed statistical significance (3.1% vs. 16.7%, p=.0986).
How is Atebrioz different from Pasatru and Sohonos?
Mechanism and route. Atebrioz is an oral small-molecule inhibitor of the ALK2 kinase itself; Pasatru is an intravenous antibody against activin A, the signal ALK2 misreads; Sohonos is an oral retinoid requiring REMS-monitored dosing. No trial has compared them head to head.
How many people have FOP?
An estimated 900 patients worldwide, with published U.S. counts ranging from roughly 300 to 400 depending on the source.
