The company explains the reasoning behind accelerated approval of its highest dose formulation.

Hemophilia A, the most common type of hemophilia (also known as factor VIII (FVIII) deficiency), occurs in people that are missing the clotting protein factor VIII or have a defective version of it. Depending on the severity of the disease, they experience excessive bleeding in response to injuries and/or spontaneously. Approximately every one in 5,000 babies born in the United States has hemophilia, and there are currently about 20,000 people in the county with the disease, according to the U.S. Centers for Disease Control and Prevention. Hemophilia is inherited in an X-linked recessive manner.

Because the disease has a genetic origin, there is considerable excitement about the potential for a gene therapy cure. BioMarin is one of the companies developing such a treatment. It recently gave an update on the status of its hemophilia A gene therapy Valrox. The company has decided to focus solely on its higher-dose version and intends to speak to the U.S. Food and Drug Administration about the potential to file for a new drug approval in 2020.

Analysts are not as bullish as the company, however. They are concerned that, in a phase III study, factor VIII expression was lower than that achieved in patients in a previous phase II study. In addition, the level of factor VIII expression has been shown to decline continuously over a three-year period. BioMarin argues that, despite the lower expression levels, patients receiving Valrox experience a marked reduction in bleeding events.

The company also will have competition. Sangamo and Pfizer are developing a gene therapy that has demonstrated promising initial results.