The FDA approved Regeneron’s Pasatru for FOP, an ultra-rare bone disorder, after Phase 3 trials showed strong drops in new bone growth.

FOP causes muscle, tendon and ligament tissue to gradually turn into bone in places it shouldn’t.

Over time, this abnormal growth restricts movement and can make basic actions such as walking, eating or even breathing difficult.

Most patients become wheelchair bound by the time they turn 30. Pasatru, known chemically as garetosmab-grts, works by blocking a protein called Activin A, which Regeneron scientists identified as a key driver of this abnormal bone growth.

Patients receive an infusion once a month, either at a clinic or, in some cases, at home. The approval is based on a Phase 3 trial called OPTIMA, which followed 63 adult patients over 56 weeks.

Those on the higher 10 mg/kg dose saw a 90% drop in new bone growths compared to placebo, while the lower 3 mg/kg dose group saw a 94% drop.

The higher dose also cut doctor assessed flare ups by 88%. Reported side effects included skin infections, acne, nosebleeds and mouth sores. The path here wasn’t smooth.

In 2020, Regeneron paused an earlier trial after five patient deaths, then worked with global regulators to redesign the study before restarting it.

The FDA had already granted the drug Fast Track and Orphan Drug status, and it holds similar rare disease designations in Europe and Japan.

Regeneron has also filed for approval in the European Union, with more submissions planned in other countries.

FOP affects roughly 900 diagnosed people worldwide, making it one of the rarest known genetic disorders.

Until now, only one other drug, Ipsen’s Sohonos, had FDA approval for the condition.

Regeneron’s plans to test Pasatru in children and adolescents in a new trial later this year, and other companies are working on treatments of their own.

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