An investigational hemophilia A treatment from Spark Therapeutics reduced bleeds by nearly 100 percent in a small clinical trial, improving upon earlier data that left investors unsatisfied. But it still remains to be seen whether Spark’s gene therapy can beat out a rival effort underway at BioMarin.

In a study on 12 patients, Spark’s one-time treatment boosted the production of factor VIII, a protein that is key to forming blood clots that is missing in people with hemophilia A, according to data released Tuesday. The highest dose — the one Spark is going to use in late-stage trials — increased average factor VIII levels to 30 percent of normal, the company said, keeping patients entirely free of bleeding in the process.

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