Fibrodysplasia Ossificans Progressiva

Overview

Fibrodysplasia ossificans progressiva (FOP) is an ultra-rare, usually childhood-onset genetic disease in which muscles, tendons, ligaments, and other connective tissues progressively form bone outside the normal skeleton, a process called heterotopic ossification (HO). It is caused by activating variants in ACVR1 (also called ALK2), most often the R206H variant; people are typically born with malformed great toes, and injuries, invasive procedures, and viral illnesses can provoke painful inflammatory “flare-ups” followed by permanent extra bone formation. MedlinePlus Genetics: FOP GeneReviews: FOP

FOP progressively limits movement, can impair eating and breathing, and may cause severe disability and early death. Current care combines avoidance of preventable tissue trauma, expert dental/anesthesia and respiratory planning, rehabilitation adapted to retained function, and treatment of painful flare-ups. In the United States, three disease-modifying medicines now reduce—but do not remove—new HO: palovarotene (Sohonos), garetosmab-grts (Pasatru), and zilurgisertib (Atebrioz). GeneReviews: FOP FDA: Sohonos approval FDA: Pasatru approval FDA: Atebrioz approval

Scope of Recent Research (2020–present)

Research since 2020 has shifted from broadly suppressing inflammation or bone formation toward directly interrupting the abnormal Activin A–ACVR1/ALK2 signaling pathway, preventing early conversion of repair cells into cartilage and bone, and ultimately correcting or silencing the mutant gene. The field is unusually active for an ultra-rare disease, with three U.S. approvals by September 2026 and multiple targeted strategies in development; however, none has demonstrated reversal of established skeletal deformity or permanent correction of FOP, so a true cure remains preclinical. FDA: Atebrioz approval AAV gene delivery in FOP

Major Breakthroughs and Emerging Therapies

The most clinically validated strategy is blocking Activin A, an inflammatory signaling protein that abnormally activates mutant ACVR1 in FOP. In the Phase 3 OPTIMA trial, monthly intravenous garetosmab reduced new HO lesions at week 56: there were 19 lesions in the placebo group, compared with one with 3 mg/kg garetosmab and two with 10 mg/kg. The U.S. Food and Drug Administration approved garetosmab-grts as Pasatru on August 19, 2026, to reduce new HO and clinician-assessed flare-ups in adults. This is major disease modification, but it is not genetic correction and does not eliminate pre-existing HO. OPTIMA Phase 3 trial FDA: Pasatru approval

A second major advance is direct inhibition of ACVR1/ALK2 kinase activity. Zilurgisertib is an oral small molecule designed to inhibit both mutant and normal ALK2 signaling. In the randomized Phase 2 PROGRESS study, 63 participants aged 12 years or older received zilurgisertib or placebo for 24 weeks; mean total new-HO volume decreased by 3.2 cm³ with zilurgisertib but increased by 24.6 cm³ with placebo. The FDA approved zilurgisertib as Atebrioz on September 25, 2026, for adults and patients aged 12 years or older. FDA: Atebrioz approval Atebrioz prescribing information

The most cure-oriented work is gene therapy. A 2022 study used an adeno-associated virus 9 (AAV9) vector to add a codon-optimized normal ACVR1 gene while using an artificial microRNA to selectively silence the R206H mutant copy. In patient-derived induced pluripotent stem cells and FOP mouse models, the combined approach suppressed abnormal Activin A signaling and markedly reduced both injury-induced and spontaneous HO. A subsequent AAV study added Activin A silencing and tissue-restriction features intended to reduce liver exposure, again suppressing HO in FOP mice. These are compelling proof-of-concept studies, but neither approach has entered a human FOP trial. AAV gene delivery in FOP AAV targeting of Activin A–ACVR1 signaling

Experimental cell-based and regenerative approaches aim to create local, durable inhibition of disease signaling. In 2024, investigators engineered induced-pluripotent-stem-cell-derived mesenchymal stromal cells to secrete ACVR2B-Fc, a decoy receptor that traps Activin A and related signals; locally delivered cells reduced primary HO and improved treadmill performance in an FOP mouse model. Separately, a 2026 preprint identified rosiglitazone, a PPARγ activator, as a candidate to redirect the disease-associated fibro-adipogenic progenitor cells away from bone formation; it eliminated ectopic bone in mouse models but remains untested in a controlled FOP clinical trial. Engineered iMSC therapy PPARγ activation in FOP models

Clinical Trials and Experimental Approaches

The landmark recent trials have produced a more effective prevention-focused treatment landscape. Regeneron’s Phase 3 OPTIMA trial of garetosmab enrolled 63 adults across 18 sites; both tested doses significantly reduced new HO lesions over 56 weeks, leading to the August 2026 U.S. approval of Pasatru. Incyte and Mirum’s Phase 2 PROGRESS trial of oral zilurgisertib randomized 63 participants aged 12 years or older and supported the September 2026 approval of Atebrioz; its long-term extension and younger-pediatric cohorts remain important for determining durability and pediatric safety. OPTIMA Phase 3 trial FDA: Pasatru approval FDA: Atebrioz approval

Not all targeted approaches have succeeded. Ipsen ended the Phase 2 FALKON trial of the ALK2 inhibitor fidrisertib after the primary analysis found no meaningful benefit on new-HO volume, new lesions, flare-ups, or pain versus placebo, although no new safety signal was identified. An observational University of California, San Francisco study is also recruiting patients with severe FOP to assess whether off-label interleukin-1 inhibition with anakinra or canakinumab can reduce flare activity and HO, generating data for a potential future Phase 2 trial. FALKON termination letter IL-1 inhibition observational study

Methodologies and Scientific Approaches

Researchers combine genetically accurate ACVR1-R206H mouse models, patient-derived induced pluripotent stem cells, engineered mesenchymal cells, and molecular profiling of the early stages of a flare-up. A central target cell population is the fibro-adipogenic progenitor: a muscle-resident repair cell that is abnormally reprogrammed toward cartilage and bone in FOP. Recent transcriptomic work shows that this pathogenic reprogramming begins within days of muscle injury and can be substantially normalized in mice by Activin A blockade or ACVR1 inhibition. Early fibro-adipogenic progenitor reprogramming AAV gene delivery in FOP

Clinical studies increasingly use low-dose whole-body computed tomography to measure the number and volume of new HO lesions, alongside flare counts, pain and function measures, and biomarkers of bone activity. These imaging-based approaches enabled the placebo-controlled evidence for garetosmab and zilurgisertib, while AAV programs use tissue-targeting capsids, allele-selective RNA silencing, and gene replacement to try to intervene upstream of bone formation. FDA: Pasatru approval FDA: Atebrioz approval AAV targeting of Activin A–ACVR1 signaling

Leading Institutions and Funding

Key academic centers include the University of Pennsylvania’s Center for Research in FOP and Related Disorders, the University of California, San Francisco, Kyoto University’s Center for iPS Cell Research and Application, and UMass Chan Medical School’s gene-therapy programs. The AAV gene-therapy work involved UMass Chan, the University of Pennsylvania, and collaborators in FOP clinical and basic science, and reported support from several NIH mechanisms, including NIAMS grants R01 AR078230 and R21 AR077557 and NICHD program project grant P01 HD080642. AAV gene delivery in FOP UMass Chan gene-therapy programs

Industry has become an essential translational driver: Regeneron developed garetosmab, while Incyte and Mirum developed and commercialize zilurgisertib; Ipsen supported palovarotene and the now-discontinued fidrisertib program. Public and nonprofit support remains important because of FOP’s tiny patient population: the National Institute of Arthritis and Musculoskeletal and Skin Diseases identifies FOP as a rare disease in which NIH-supported research is bringing potential cures closer, and recent preclinical work has also cited support from NIAMS, the International FOP Association, and the Million Dollar Bike Ride Foundation. NIAMS FY2025 congressional justification PPARγ activation in FOP models

Strengths, Limitations, and Challenges

The strongest recent achievement is that FOP research has delivered therapies that substantially prevent new pathological bone, including two mechanistically focused medicines approved in the United States during 2026. Garetosmab blocks the abnormal Activin A trigger, while zilurgisertib inhibits the mutant receptor’s signaling activity; together they validate the central disease pathway in patients, rather than only in laboratory models. OPTIMA Phase 3 trial FDA: Atebrioz approval

The limitations are equally consequential. Existing HO is generally irreversible, so prevention cannot fully restore lost mobility; long-term treatment must also balance reproductive risks, infections and nosebleeds with garetosmab, and fetal toxicity, drug interactions, and incomplete long-term safety characterization with zilurgisertib. Gene therapy faces additional barriers: safely reaching the relevant skeletal progenitor cells throughout the body, avoiding immune responses to AAV, controlling lifelong expression, and silencing the mutant allele without compromising essential normal ACVR1 function. The negative fidrisertib trial further shows that promising target engagement in animals does not guarantee clinical benefit in FOP. FDA: Pasatru approval Atebrioz prescribing information Gene therapy feasibility and obstacles FALKON termination letter

Outlook and Future Directions

As of October 16, 2026, FOP is closer to durable prevention of new bone formation than at any previous point, but not close to a proven cure. The critical milestones are long-term real-world outcomes with Pasatru and Atebrioz; results in children, who have the greatest opportunity to preserve function; whether combination or sequence therapy can further suppress flare-driven HO; and, most importantly, translation of allele-specific AAV gene replacement/silencing from FOP mice and patient cells into a first-in-human study. A therapy that safely corrects mutant ACVR1 signaling early enough to permanently prevent new HO—and eventually pairs prevention with reconstruction or reversal of established disease—would represent the transition from disease modification to cure. AAV gene delivery in FOP OPTIMA Phase 3 trial

References

• MedlinePlus Genetics: FOP — U.S. National Library of Medicine, 2026.
• GeneReviews: FOP — Pignolo et al., 2024.
• FDA: Sohonos approval — U.S. Food and Drug Administration, 2023.
• FDA: Pasatru approval — U.S. Food and Drug Administration, 2026.
• FDA: Atebrioz approval — U.S. Food and Drug Administration, 2026.
• OPTIMA Phase 3 trial — Papa et al., 2026.
• Atebrioz prescribing information — U.S. Food and Drug Administration, 2026.
• AAV gene delivery in FOP — Yang et al., 2022.
• AAV targeting of Activin A–ACVR1 signaling — Kim et al., 2023.
• Engineered iMSC therapy — Gao et al., 2024.
• PPARγ activation in FOP models — Koirala et al., 2026.
• FALKON termination letter — Ipsen, 2026.
• IL-1 inhibition observational study — University of California, San Francisco, 2026.
• Early fibro-adipogenic progenitor reprogramming — Lees-Shepard et al., 2026.
• UMass Chan gene-therapy programs — UMass Chan Medical School, 2024.
• NIAMS FY2025 congressional justification — National Institute of Arthritis and Musculoskeletal and Skin Diseases, 2024.
• Gene therapy feasibility and obstacles — Eekhoff et al., 2022.