# Breakthrough therapy reduces bone growth by 90% in ultra-rare FOP disease Clinical trial shows antibody treatment can halt progression of 'second skeleton' disorder. - **Source:** [The Independent Science](https://www.independent.co.uk/news/science/rss) - **Published:** 2026-09-28 - **Original article:** - **On Brifing:** https://brifing.si/s/000769a6-b7b1-4c6c-860f-5444395cd71c ## Disease mechanism Fibrodysplasia ossificans progressiva (FOP) causes abnormal bone growth in muscles, tendons and ligaments, triggered by minor injuries. The genetic disorder progressively immobilizes patients, affecting speech, swallowing and breathing. With only 900 confirmed cases worldwide, it's among the rarest medical conditions. New bone growth typically follows inflammatory flare-ups, creating a 'second skeleton' that fuses joints. ## Treatment breakthrough Garetosmab (Pasatru) by Regeneron blocks activin A protein, reducing new abnormal bone growth by over 90% in trials. The antibody treatment also decreased flare-up frequency. In the 16-country study, placebo recipients developed 19 new bone lesions on average versus 1-2 for treated patients. Higher doses showed additional reduction in disease flare-ups. ## Clinical trial design The international trial involved 63 adult FOP patients (average age 27) across three groups: placebo, low-dose and high-dose garetosmab. After one year, treatment groups showed significantly fewer new bone formations. The Lancet-published study confirms the antibody's efficacy and safety profile, marking the first disease-modifying therapy for FOP. ## Surgical implications By preventing new bone formation, garetosmab may enable previously avoided surgical interventions. Researchers anticipate preserved mobility as joints won't fuse further. This could allow surgical remobilization attempts for immobilized patients - previously impossible due to surgery-triggered excessive bone growth. The treatment fundamentally changes FOP's clinical management approach. ## Patient access challenges While approved in the US, UK's 70 FOP patients await regulatory approval. FOP Friends charity emphasizes treatments must be accessible to change lives. The community has fundraised and participated in trials for 15+ years. With two potential treatments now developed, focus shifts to convincing UK authorities of their life-altering potential. ## Dual therapy pipeline Alongside garetosmab, zilurgisertib received US FDA approval, offering FOP patients multiple treatment options. This represents a watershed moment for a condition with previously no disease-modifying therapies. The simultaneous development of two distinct treatments reflects growing biopharma interest in ultra-rare diseases. ## Long-term outcomes Researchers project preserved mobility and function long-term with garetosmab treatment. By preventing joint ankylosis, patients may maintain independence longer. The treatment's bone growth inhibition could fundamentally alter FOP's natural progression, which typically leads to complete immobilization by middle age. ## Regulatory urgency Patient advocates stress the need for swift UK approval, noting each day without treatment means irreversible progression. The FOP community combines celebration with anxiety, having waited decades for effective therapies. Their case highlights the tension between rapid innovation and healthcare system adoption for ultra-rare conditions. ## Sources covering this story - https://www.independent.co.uk/news/uk/home-news/nhs-fop-treatment-garetosmab-fibrodysplasia-ossificans-progressiva-b3057772.html — --- _Summary written by Brifing from the original reporting. Copyright in the original article belongs to its publisher; read the full piece at the link above._