How a 63-Patient Trial Delivered an FDA Approval

The FDA has approved Pasatru (garetosmab-grts) to reduce the formation of new heterotopic ossification lesions and clinician-assessed flare-ups in adults with fibrodysplasia ossificans progressiva (FOP), Regeneron announced on August 19, 2026.¹ The approval marks the first therapy demonstrated to reduce new lesion formation in a placebo-controlled trial for this ultra-rare genetic disorder, according to the company.

FOP is characterized by abnormal bone formation that progressively infiltrates muscles, tendons, ligaments, and other connective tissues.¹ When this bone growth affects the jaw, spine, hip, or rib cage, it can impair speaking, eating, walking, or breathing. Approximately 900 people worldwide are diagnosed with the condition, and most patients lose the ability to walk by age 30, with a median survival age of 56.

The therapy is a fully human monoclonal antibody that blocks Activin A, a protein identified through the manufacturer's research as a driver of abnormal bone development in patients with the disorder.¹ Dosing is weight-based, starting at 10 mg/kg administered intravenously over 60 minutes once every 4 weeks, with a reduced 3 mg/kg dose available for patients who do not tolerate the higher amount. The treatment can be administered across multiple care settings, including home infusion where appropriate.

What Did the Trial Show?

Approval rested on data from a phase 3, multi-center trial enrolling 63 adults with active FOP.¹ Participants were randomized to receive 1 of 2 doses of the therapy or placebo every 4 weeks for 56 weeks. At that point, both the 10 mg/kg and 3 mg/kg arms met the trial's primary endpoint, reducing new lesions by 90% and 94%, respectively, compared with placebo, as measured by CT scans. Clinician-assessed flare-ups, a key secondary endpoint, dropped 88% in the higher-dose group, though the reduction was more modest, at 15%, in the lower-dose group. Patient-reported flare-up rates did not differ significantly between treatment and placebo groups. Serious treatment-emergent adverse events were infrequent and similarly distributed across all 3 arms. The most common adverse reactions, occurring in at least 10% of treated patients, included abscess, acne, excessive hair growth, eyebrow loss, oral ulcers, nosebleeds, folliculitis and nail infection.

“For people living with FOP, every irregular new bone formation is a step toward disability and potential loss of mobility,” said Dr. Kathryn Dahir, a primary investigator for the trial and professor, Department of Internal Medicine, Vanderbilt University, in a press release.¹ “With the ability to reduce the number of new bone lesions and flare-ups, we now have a new treatment that can positively affect patients.”

Michelle Davis, executive director, International FOP Association, said in the press release,¹ “This approval is monumental for our community, providing a vital new therapy that can have a significant impact on the life of someone with FOP.”

What Did the Earlier Phase 2 Trial Reveal About Safety Trade-offs?

Garetosmab's path to approval included a phase 2 trial, LUMINA-1, that surfaced safety questions the phase 3 program would later need to address.² Five deaths occurred during that trial's open-label extension, a rate investigators called high for a study of this size, though causality was not established. The trial also missed its original primary endpoint, reduction in existing lesion activity, prompting researchers to redesign the primary analysis around new lesion formation instead. That pivot shaped how efficacy was ultimately measured and demonstrated in the later, larger trial supporting approval.

Why Does this Approval Matter?

This approval illustrates the path a targeted biologic can take from mechanistic discovery to regulatory clearance in an ultra-small patient population, built on a trial of just 63 participants.¹ It also underscores the growing role of imaging-based endpoints, in this case whole-body CT scans, in demonstrating efficacy where clinical outcome measures are difficult to standardize across a small, heterogeneous population. A pediatric trial is already planned, and a European regulatory submission is under review, both signals worth tracking for teams managing global development timelines and manufacturing capacity planning for orphan-designated biologics.

References

1. Pasatru (garetosmab-grts) first and only FDA-approved treatment demonstrating reduction in new heterotopic ossification (HO) lesions and clinician-assessed flare-ups in a placebo-controlled trial in adults with fibrodysplasia ossificans progressiva (FOP). Press release. Regeneron Pharmaceuticals, Inc. August 19, 2026. https://investor.regeneron.com/news-releases/news-release-details/pasatrutm-garetosmab-grts-first-and-only-fda-approved-treatment
2. Raje N, Hochberg Z, Hsiao EC, et al. Garetosmab in fibrodysplasia ossificans progressiva: a randomized, double-blind, placebo-controlled phase 2 trial. Nat Med. 2023;29(10):2614-2623. doi:10.1038/s41591-023-02561-8. September 8, 2023. https://www.perplexity.ai/search/233aad98-6f38-4a25-b345-ae2ee1f193f8