The United States Food and Drug Administration has approved Regeneron Pharmaceuticals’ Pasatru, or garetosmab-grts, for adults with fibrodysplasia ossificans progressiva, adding a new mechanism for an ultra-rare genetic disorder in which abnormal bone progressively develops within muscles and other soft tissues. The approval covers reduction of new heterotopic ossification lesions and clinician-assessed flare-ups and is supported by the randomized Phase 3 OPTIMA trial, where the approved 10 mg/kg starting dose reduced the number of new lesions by 90% compared with placebo over 56 weeks. A lower 3 mg/kg dose produced a 94% reduction in lesions, although its effect on clinician-assessed flare-ups was considerably smaller than that observed with the higher dose.
Pasatru is administered intravenously once every four weeks, with the recommended starting regimen consisting of 10 mg/kg infused over approximately 60 minutes. The dose may be reduced to 3 mg/kg if the higher regimen is not tolerated, and Regeneron Pharmaceuticals said treatment may be delivered in several care settings, including home infusion when appropriate. The company is also seeking approval outside the United States, with a European Union application currently under review and additional submissions planned in markets including Japan.
OPTIMA showed major reductions in abnormal bone formation but a dose-dependent flare-up result
The Phase 3 OPTIMA trial enrolled 63 adults with genetically confirmed FOP who had evidence of disease activity or progression of heterotopic ossification. Participants were randomized to receive Pasatru at either 10 mg/kg, Pasatru at 3 mg/kg or placebo intravenously every four weeks for 56 weeks. Whole-body computed tomography scans were used to identify new heterotopic ossification lesions, while physicians and patients separately assessed disease flare-ups.
At week 56, investigators detected two new lesions in the 23 patients assigned to the 10 mg/kg Pasatru group compared with 19 lesions among 21 placebo recipients, representing a 90% reduction. Only one new lesion was identified among 19 patients receiving the 3 mg/kg dose, corresponding to a 94% reduction relative to placebo. Both doses met the primary endpoint.
The flare-up findings were more nuanced. Clinicians identified nine flare-ups in the 10 mg/kg group compared with 66 among placebo recipients, representing an 88% reduction. The 3 mg/kg group experienced 53 clinician-assessed flare-ups, only 15% fewer than placebo. Patient-reported flare-ups also did not show a statistically significant difference between Pasatru and placebo through week 56.
That distinction is important when interpreting the approval. The strongest evidence supports a substantial reduction in newly detected heterotopic ossification, while the clinician-assessed flare-up benefit was much more pronounced at the 10 mg/kg regimen that now serves as the recommended starting dose. The available results should also not be interpreted as evidence that Pasatru can reverse bone that has already formed or restore mobility already lost to FOP.
Serious treatment-emergent adverse events occurred in two patients receiving 10 mg/kg, one receiving 3 mg/kg and two receiving placebo during the 56-week study. Common adverse reactions associated with Pasatru included abscesses, acne, increased hair growth, eyebrow or eyelash loss, oral ulcers, nosebleeds, folliculitis, nail infections and rash.
The prescribing information also contains important warnings involving embryo-fetal toxicity, skin and soft-tissue infections and potentially serious nosebleeds. Pasatru must not be administered during pregnancy, and women who can become pregnant are advised to use effective contraception during treatment and for six months following their final dose.
Activin A blockade gives Pasatru a different approach to preventing FOP bone formation
Fibrodysplasia ossificans progressiva is caused by pathogenic variants affecting the ACVR1 gene and is characterized by progressive formation of bone outside the normal skeleton. Heterotopic bone can develop around the spine, jaw, hips, rib cage and other structures, gradually restricting movement and potentially interfering with eating, speaking and breathing. Regeneron Pharmaceuticals estimates that approximately 900 people have been diagnosed worldwide and that many patients require a wheelchair by early adulthood.
Pasatru is a fully human monoclonal antibody designed to bind and neutralize Activin A. Regeneron Pharmaceuticals’ research identified Activin A as a critical driver of abnormal ACVR1 signaling and heterotopic ossification in FOP, providing the biological rationale for targeting the protein before new bone develops.
The mechanism distinguishes Pasatru from Ipsen’s Sohonos, or palovarotene, which became the first FDA-approved treatment for FOP in August 2023. Sohonos is a retinoid approved to reduce the volume of new heterotopic ossification in adults and in females aged eight years and older and males aged 10 years and older. Pasatru currently carries an adult-only indication, but its approval specifically covers both reduction of new heterotopic ossification lesions and clinician-assessed flare-ups.
The two medicines also create different treatment considerations. Sohonos is administered orally and carries boxed warnings regarding embryo-fetal toxicity and premature epiphyseal closure in growing pediatric patients. Pasatru requires monthly intravenous infusion and has its own reproductive, infection and bleeding precautions. Their different mechanisms, dosing formats, safety profiles and approved populations mean treatment selection is likely to depend on individual patient circumstances rather than a simple comparison of headline efficacy percentages.
Regeneron is already preparing to take Pasatru into pediatric FOP development
The current approval covers adults, leaving an important portion of the FOP population outside the Pasatru label. Regeneron Pharmaceuticals plans to address that gap through OPTIMA 2, a separate Phase 3 study evaluating garetosmab in children and adolescents with FOP. The company expects the pediatric study to begin later in 2026.
Expansion into younger patients could become clinically important because FOP begins early in life and cumulative heterotopic ossification can progressively restrict mobility. However, the pediatric program will need to establish its own safety, pharmacokinetic and efficacy evidence rather than relying on the adult approval, particularly given the developmental considerations involved in targeting Activin A during childhood.
Regeneron Pharmaceuticals is also pursuing geographic expansion. The European Medicines Agency is currently reviewing Pasatru, while regulatory filings are planned in Japan and additional countries. Pasatru previously received FDA Fast Track and Orphan Drug designations as well as orphan designations in Europe and Japan.
The European review could be particularly relevant because the treatment landscape differs from the United States. Ipsen’s palovarotene did not receive European Union marketing authorization after the European Commission followed a negative regulatory opinion in 2023, leaving Pasatru with an opportunity to establish a different position if European regulators ultimately reach a favorable conclusion.
Regeneron shares rise more than 4% as Pasatru adds another internally developed medicine
Regeneron Pharmaceuticals shares were trading around $843.39 on August 19, up approximately 4.1%, after reaching an intraday high of about $844.20. The company’s market capitalization stood near $89.4 billion. The positive session coincided with the Pasatru approval, although Regeneron Pharmaceuticals is a large diversified biotechnology company and its daily stock movement cannot be attributed solely to one ultra-rare disease product.
Pasatru itself is unlikely to rival the scale of Regeneron Pharmaceuticals’ largest franchises because the diagnosed FOP population is extremely small. Its strategic value instead lies in adding another internally discovered medicine to the company’s rare-disease portfolio and demonstrating that the Activin A biology developed through Regeneron Pharmaceuticals’ research can translate into regulatory approval. The company reported second-quarter 2026 revenue of $4.29 billion, up 17% year over year, supported by growth across products including EYLEA HD, Libtayo and its share of the Dupixent collaboration.
The regulatory milestone is therefore clinically more significant than its likely near-term contribution to Regeneron Pharmaceuticals’ overall revenue base. Pasatru gives adults with FOP another disease-modifying option and introduces a mechanism capable of sharply reducing the appearance of new heterotopic bone lesions during Phase 3 testing. Attention now turns to real-world uptake, the European regulatory decision and whether OPTIMA 2 can eventually extend the treatment into children and adolescents.
