Major Breakthrough for Rare ‘Second Skeleton’ Disease as New Treatment Cuts Abnormal Bone Growth by 90%
A major development in the treatment of an exceptionally rare genetic disorder could offer new hope to people whose bodies gradually form bone in places where it should never develop.
Fibrodysplasia ossificans progressiva, commonly known as FOP, is a devastating condition in which muscles, tendons, ligaments and other connective tissues can progressively turn into bone. Sometimes described as a “second skeleton” disease, FOP can gradually restrict movement and cause severe disability.
Now, results from an international clinical trial have provided strong evidence that a new medicine can substantially reduce the formation of abnormal bone in adults with FOP.
The treatment, garetosmab, is marketed in the United States under the name Pasatru. In August 2026, the US Food and Drug Administration approved the medicine to reduce the formation of new heterotopic ossification, meaning abnormal bone outside the normal skeleton, as well as clinician-assessed disease flare-ups in adults with FOP.
A disease that changes soft tissue into bone
FOP is among the rarest genetic disorders known to medicine. It affects approximately one person in every million, according to FOP Friends. The charity estimates that around 70 people in the UK and approximately 900 people worldwide are known to have the condition.
The disorder is usually caused by a mutation in the ACVR1 gene. This gene is involved in signalling pathways that regulate bone development. In people with FOP, the mutation causes abnormal signalling that can trigger bone formation in soft tissues.
The process can begin after a flare-up involving swelling and inflammation. These episodes can sometimes be triggered by relatively minor injuries, infections or other forms of physical stress.
Over time, repeated episodes of abnormal bone formation can cause joints to become increasingly restricted. This can make everyday activities more difficult and may eventually affect a person's ability to walk, speak, eat or breathe.
Importantly, FOP is not simply a condition involving unusually strong or excessive bones. The problem is that bone develops in the wrong places.
Clinical trial produces striking results
The evidence behind garetosmab comes from a randomised, double-blind, placebo-controlled clinical study involving 63 adults with FOP.
Participants were divided into three groups. One group received a placebo, while the other two received different doses of garetosmab. Treatment was administered by intravenous infusion every four weeks, with the main study period lasting 56 weeks.
The results showed a substantial difference in the number of new abnormal bone lesions that developed.
Among the 21 people receiving placebo, 19 new lesions were recorded. By comparison, there was one new lesion among 19 people receiving the lower dose of garetosmab and two new lesions among 23 people receiving the higher dose.
That represents a dramatic reduction in new abnormal bone formation during the study period.
The treatment was also associated with fewer clinician-assessed flare-ups. In the higher-dose group, nine flare-ups were recorded, compared with 66 among participants receiving placebo. The lower-dose group experienced 53 flare-ups.
Researchers say the findings demonstrate the potential of targeting the biological pathway responsible for abnormal bone formation rather than simply managing the consequences after new bone has developed.
How does garetosmab work?
Garetosmab is an antibody designed to interfere with activin signalling.
FOP is linked to an abnormal version of the activin A receptor known as ACVR1. By blocking part of this signalling pathway, garetosmab aims to prevent or reduce the biological processes that lead to heterotopic ossification.
The US FDA describes Pasatru as an activin signalling inhibitor. Its approved indication is specifically for adults with FOP and focuses on reducing new abnormal bone formation and clinician-assessed flare-ups.
The FDA recommends an initial dose of 10 mg per kilogram administered intravenously once every four weeks. A lower dose may be used if the higher dose is not tolerated.
The approval marks an important change in the treatment landscape for a condition that has historically had very limited options.
What the development could mean for patients
For people living with FOP, preventing new bone from forming may be particularly important because damage caused by the disease can be permanent.
Once abnormal bone has developed and restricted a joint, removing it surgically can be complicated. In people with FOP, surgery or physical trauma may itself trigger further flare-ups and additional bone formation.
That means a medicine capable of reducing new bone formation could potentially change how doctors approach long-term management of the disease.
It is important, however, not to interpret the trial as evidence that existing bone can simply be removed or that the treatment reverses all disability caused by FOP. The FDA approval is for reducing new heterotopic ossification and flare-ups in adults.
Further research will be needed to establish how treatment affects patients over longer periods and whether preventing new bone formation can preserve mobility and independence over many years.
Access remains an important issue
Regulatory approval in the United States does not automatically mean that the treatment is immediately available to patients in every country.
In the UK, access to medicines for rare diseases involves additional regulatory and health-system decisions. For people and families affected by FOP, the question is therefore not only whether an effective treatment exists, but also when and how eligible patients will be able to receive it.
FOP Friends has highlighted access to treatment as an important priority for the UK FOP community. The organisation continues to support patients and families while campaigning for research, treatment and improved awareness of the condition.
There is also continued development of other treatments. In September 2026, the FDA listed zilurgisertib, marketed as Atebrioz, among newly approved treatments for FOP. This means the therapeutic landscape for the condition is continuing to develop.
A significant step, but not a cure
The progress surrounding garetosmab represents an important moment for a community affected by an exceptionally rare and progressive disease.
For decades, FOP has presented doctors and researchers with a difficult challenge: the body is effectively creating a second framework of bone throughout tissues that are essential for movement.
The new treatment does not constitute a cure, and it does not remove existing abnormal bone. Nevertheless, the clinical trial results show that targeting the underlying signalling pathway can dramatically reduce the development of new bone in adults with FOP.
For patients and families, that distinction matters. Slowing or preventing further progression could help preserve movement and function for longer, while also opening new avenues for research into future treatments.
The next stage will be understanding the medicine's long-term effects, monitoring its safety and determining how widely eligible patients can access it.
For a disease affecting only a tiny number of people worldwide, the development of a treatment capable of substantially reducing one of its defining features represents a notable advance in rare-disease research.
Sources
- US Food and Drug Administration, “FDA Approves Second Treatment for Fibrodysplasia Ossificans Progressiva”, September 2026.
- US Food and Drug Administration, Pasatru (garetosmab-grts) prescribing information, 2026.
- US Food and Drug Administration, Orphan Drug Designations and Approvals, garetosmab/Pasatru.
- FOP Friends, Information about Fibrodysplasia Ossificans Progressiva.
- US Food and Drug Administration, Notable Approvals, 2026.
Medical Disclaimer
This article is provided for general information and educational purposes only. It is not intended to replace medical advice, diagnosis or treatment from a qualified healthcare professional. FOP is an extremely rare and complex condition, and treatment decisions should be made with specialist clinicians familiar with the disease. Drug approvals, availability, indications and safety information can change. Readers should consult official regulatory information and their healthcare provider for current advice about any treatment.