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Biomarin Pharmaceuticals (BMRN) said Tuesday that an experimental gene therapy targeting hemophilia A continued to help patients produce a protein enabling blood to clot. And while that will allow the company to seek accelerated approval in the U.S and Europe, data about Biomarin’s “one-and-done” treatment raise a big question: Do its effects last?

With gene therapy, a virus is used to insert genetic material to counter the effects of a disease. In hemophilia A, the most common form of the bleeding disorder, patients are missing a protein called factor VIII, which enables the blood to clot. The BioMarin treatment, valoctocogene roxaparvovec (for short: “valrox”), aims to enable patients to make their own factor VIII, so that they don’t need the extremely expensive drugs used to treat the disease.


By one very important measure, the new results are stunning: Bleeding episodes were rare, almost non-existent. But levels of factor VIII also appear to be falling, leading to questions about whether the treatment will really last a lifetime for every patient.

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