On Wednesday, the FDA approved Pasatru (garetosmab), Regeneron's treatment for fibrodysplasia ossificans progressiva, or FOP, an ultra-rare condition in which the body grows a second skeleton. Bone forms inside muscles, tendons, and connective tissue, bridging joints until movement is lost. STAT's Andrew Joseph reports that the approval is the capstone of a three-decade effort, and the first time a drug has been shown in a placebo-controlled trial to reduce both new bone formation and the painful flare-ups that precede it. For the roughly 900 people diagnosed with the disease worldwide, this is news of a kind they have never had: not a hope of slowing the disease, but evidence that it can be stopped.
FOP is caused by mutations in a gene called ACVR1, which leaves connective tissue responding to a protein, activin A, by turning into bone. The disease usually announces itself in childhood with flare-ups: painful swellings, often set off by a fall, an injection, a vaccination, or dental work, that harden into bone over weeks. Most people with FOP are in wheelchairs by their mid-twenties to early thirties; STAT reports that many rely on wheelchairs by age 25, and the median age of survival is about 56, with death most often coming when a chest wall locked by bone can no longer move enough to breathe. The disease advances in episodes, which makes its cruelty easy to miss: between flare-ups a child may seem fine, and each episode permanently subtracts something. There is no cure. Until Wednesday, the only approved treatment in the United States was Sohonos, an oral drug approved in 2023 for females aged 8 and older and males aged 10 and older, with serious safety warnings attached.
Its success is measured in things that stop happening
The pivotal trial behind Pasatru's approval, called OPTIMA, enrolled 63 adults with FOP and treated them for 56 weeks. New bone formation on CT scans fell 94 percent at the 3 mg/kg dose and 90 percent at the approved 10 mg/kg dose, one lesion and two lesions respectively against 19 in the placebo group, per Regeneron's approval announcement. A post-hoc analysis put the reduction in new-lesion volume at more than 99 percent. Clinician-assessed flare-ups fell 88 percent at the approved dose, 9 events versus 66. The label is precisely worded: the drug reduces the formation of new heterotopic ossification lesions and clinician-assessed flare-ups in adults with FOP.
Notice what kind of result this is. The trial's primary measure was a count of new lesions; its secondary measure was a count of flare-ups. Success was recorded as a series of absences. A good year on this drug looks like the previous year: no new swellings, no new bone, no new losses. That is the whole point, and it is also the hard part. "Having an effective treatment, you're hoping that you'll change that trajectory," said Richard Keen, the London orthopaedic surgeon who led the pivotal trial, in STAT's coverage. The trajectory change is real: for a disease whose every episode takes something, a treatment that prevents episodes turns the future from a sequence of losses into a level line.
The data itself carries a quiet reminder of how hard that kind of success is to see. Through 56 weeks, patient-reported flare-ups were not significantly different between the drug and placebo, even as clinician-assessed flare-ups fell 88 percent at the approved dose. The two measures define flare-ups differently, and a clinician's exam and a patient's diary are different instruments, so the gap should not be oversold in either direction. But it is a caution about the nature of this benefit: it shows up in scans and clinical counts more readily than in a felt sense of improvement, and patients may have to take it on faith that nothing happening is the treatment working.
The safety picture deserves full weight. Common reactions include nosebleeds, some serious enough to be warned about on the label, skin and soft tissue infections such as abscesses, acne, excess hair growth, loss of eyebrow and eyelash hair, and mouth sores. The label warns of possible harm to a developing fetus, and women must use contraception during treatment and for six months after the last dose. In the pivotal trial, serious side effects were rare: two patients on the 10 mg/kg dose, one on the lower dose, and two on placebo. In the earlier phase 2 study, five participants with advanced disease died during the open-label portion of the trial; investigators considered the deaths unlikely to be related to treatment, without fully excluding that possibility.
It works only on bone that has not formed yet
The drug's own history shows how precisely bounded its power is. In the phase 2 trial, LUMINA-1, published in Nature Medicine, garetosmab did not meet its original primary endpoint, a PET scan measure of total disease activity. What it did do, consistently, was prevent new bone from forming. It had no significant effect on lesions that already existed, because those had matured into permanent bone. The phase 3 program was redesigned around the measure the drug could move, the count of new lesions, and that is the trial the FDA approved.
So garetosmab is, at bottom, a preservation drug. The bone FOP has already made is permanent. A fused joint stays fused; a spine locked by bone stays locked; a person in a wheelchair remains in that wheelchair. The drug's entire work is on the future. That means each patient's benefit equals the function they still have on the day of the first infusion. For someone who can still walk, it may mean keeping that ability. For someone whose arms are already frozen, it means the disease takes nothing more. Both outcomes are real, and neither should be used to diminish the other. It would be wrong to call a drug that preserves what remains a cure, and it would be equally wrong to treat preservation as small, when the alternative is a disease that never stops taking.
The disease is found years late, and the delay itself does harm
The timing of treatment matters because the drug's value is a direct function of timing: the earlier it starts, the more there is to save. FOP is among the most misdiagnosed diseases in medicine. A widely cited survey of international FOP patients, published in Pediatrics in 2005, found that 87 percent were initially told they had something else, most often cancer. The average delay between first symptoms and correct diagnosis was about four years, and patients saw a median of six physicians before anyone got it right. A 2025 study of Chinese patients found the pattern persisting: 78.8 percent misdiagnosed at least once, with an average delay above three years.
And the delay is not neutral time. Nearly two-thirds of patients in the survey underwent invasive procedures, often biopsies, under the wrong diagnosis, and about half reported permanent loss of mobility from those procedures, because injury to FOP tissue is one of the most reliable ways to trigger more bone. The very act of being misdiagnosed has shaped this disease for many patients. Yet the diagnosis is not hard to make once it is considered: nearly everyone with FOP is born with short, curved great toes, a visible clue present from birth, and a genetic test for ACVR1 mutations can confirm it. The failure is one of recognition, not of biology.
The drug's real-world value is set by the diagnostic clock
Here is the uncomfortable arithmetic. The patients with the most to preserve, children in the early, most active phase of the disease, are not yet eligible for Pasatru, which is approved only for adults, with a pediatric trial planned. The adults who are eligible have lived with the disease for years, often through misdiagnosis, procedures, and flare-ups that took mobility the drug can now protect but cannot return. Per patient, the drug's benefit is partly a function of something the company does not control: how long the disease went unrecognized. The earlier FOP is found, the more of the drug's potential is realized. The later it is found, the more of that potential is already spent.
The economics add a second gate. No price has been announced for Pasatru, and the market is small: roughly 900 diagnosed patients worldwide, about 400 of them in the United States. The comparator sets expectations. Sohonos, the oral FOP drug approved in 2023, launched with a reported list price of about $624,000 a year, and Regeneron has said Pasatru can be infused at home, with a support program offering insurance verification and potential financial assistance. Whether a drug at this price reaches the people who need it will be decided by coverage decisions as much as by clinical evidence, and each drug deserves a fair accounting. Sohonos remains the only treatment available to children with FOP, and it was approved on the strength of a comparison against untreated patients rather than a placebo-controlled trial, with boxed warnings about harm to a developing fetus and premature closure of growth plates; European regulators declined to authorize it. Pasatru brings placebo-controlled evidence and a cleaner safety profile, and it starts in adults, which is another way of saying it starts wherever the diagnostic system has left each patient.
The approval on Wednesday is a real milestone, and it should be named as one: three decades of work, a drug that measurably stops the disease's engine, and a community told for years that nothing could be done. But the drug's full promise does not live in the molecule. It lives in the recognition of a child's bent toes and an early genetic test, in clinics that know what FOP is before a biopsy is ordered. The damage the disease has already done is permanent; the delay that allowed it is not. A medicine that preserves what remains reaches its full value only when diagnosis catches up with the biology, and for the first time in the history of this disease, what a medicine can do is no longer the limiting factor. Recognition is.
Primary sources
- STAT, reporting by Andrew Joseph, for the approval itself, the framing of it as the capstone of a three-decade effort, the detail that most people with FOP rely on wheelchairs by age 25, and the quote from Richard Keen.
- Regeneron's August 19, 2026 approval announcement for the label wording, the OPTIMA efficacy and safety data, the dosing and home-infusion details, the mechanism of activin A blockade, the estimate of about 900 diagnosed patients worldwide, and the median survival figure.
- The phase 2 LUMINA-1 trial, published in Nature Medicine by Keen, Dahir, and colleagues, for the missed primary endpoint, the suppression of new bone formation, the absence of significant effect on pre-existing lesions, and the deaths during the open-label portion.
- The 2005 Pediatrics survey by Kitterman and colleagues for the misdiagnosis rate, average diagnostic delay, and permanent mobility loss from unnecessary procedures, a 2025 questionnaire study of Chinese FOP patients for the persistence of misdiagnosis and delay, and Pharmaphorum for details of Ipsen's Sohonos, its approval year, indication, warnings, European status, and reported list price.