Regeneron’s Pasatru Wins FDA Approval as Second Treatment for Rare Bone-Forming Disorder

Regeneron’s treatment for the rare bone-forming disorder Pasatru has received approval from the US Food and Drug Administration (FDA).

According to Fierce Pharma on the 19th local time, Regeneron said the FDA had approved the monoclonal antibody garetosmab (brand name Pasatru).

With the approval, Pasatru became the second approved treatment for fibrodysplasia ossificans progressiva (FOP). It is the first treatment shown to reduce physician-assessed acute flares in adult patients.

Pasatru is administered intravenously every 4 weeks and blocks activin A, a protein known to play a central role in FOP progression. Regeneron researchers identified the protein as a treatment target for FOP more than a decade ago.

The late-stage phase 3 Optima trial randomly assigned 63 adults with FOP to two dose groups or a placebo group, with treatment administered every 4 weeks for 56 weeks. Both the high- and low-dose groups reduced new bone lesions by 90% versus placebo over 56 weeks, while the high-dose group reduced painful localized inflammatory flares by 89% versus placebo.

At the end of the trial, an independent data monitoring committee recommended that all patients in the placebo group be switched to Pasatru as soon as possible.

FOP is an ultra-rare genetic disease affecting only about 900 people worldwide. The body’s soft tissues gradually turn into bone, and painful flares involving hot, swollen lumps occur before muscles, tendons and ligaments permanently transform into bone. As bone formation progresses, joints fuse and nerves become compressed, causing chronic pain and restricted movement.

Dr. Susan Lee, Regeneron’s head of clinical science, called FOP “an especially cruel and unpredictable disease.” Patients may be relatively stable and then wake up one morning unable to move their jaw or shoulder, she said.

Dr. Lee described the approval as a major milestone for Regeneron and the FOP patient community. She added that she was very pleased the company could translate its scientific achievement into a potential product for patients.

In a press release, the company said the 10mg/kg and 3mg/kg doses in the Optima trial reduced lesions by 90% and 94%, respectively, versus placebo.

The approval will make Pasatru available to about 220 adult FOP patients in the United States. However, launching the product will be challenging because the disease is ultra-rare, with only around 1,000 confirmed patients worldwide.

The only competing drug, Sohonos, was approved in 2023 but fell short of early expectations because of low patient uptake, its high price and its adverse-event profile. In the pivotal trial, 8% of enrolled patients permanently discontinued treatment because of adverse events. Ipsen recorded a loss of €279 million (about $330 million, approximately 495 billion won) related to the product in 2024.

John Ryan, head of Regeneron’s global rare disease business, said one of the biggest challenges was the lack of awareness of FOP among physicians and the broader patient community. The company plans to focus on raising awareness beyond academic medical centers and FOP specialists to shorten the time to diagnosis and treatment.

Pasatru is expected to launch soon after approval, but the company has not disclosed its list price. Ryan said the company was committed to supporting patient access, citing financial assistance and copay support for eligible patients, as well as free supply for patients meeting certain criteria.

Regeneron had targeted 2021 for an FDA application for Pasatru, but development was halted in late 2020 because of safety concerns. Five of the 44 clinical trial participants died, and researchers determined that although there was no clear link to Pasatru, a causal relationship could not be ruled out.

Safety was the top priority in the phase 3 Optima trial that subsequently led to approval. No deaths or serious bleeding were reported in the trial, and no patients receiving Pasatru discontinued treatment.

There was one treatment-related serious adverse event in the low-dose group, two in the high-dose group and two in the placebo group. Dr. Lee said the research team was relieved by the strong safety profile and at times even became tearful.

Dr. Lee also said the company plans to seek approvals outside the United States and begin a pediatric trial later this year. She said treatment is better the earlier it begins and that she wanted to start the pediatric trial as soon as possible.