What happened

Shares of Spark Therapeutics, Inc. (ONCE), a biotech developing gene therapies, fell hard this morning after the company reported hemophilia trial data that bodes well for rival BioMarin Pharmaceutical Inc. (BMRN 1.81%). Spark Therapeutics stock has fallen about 40% as of 11:01 a.m. EST on Monday. 

So what 

Spark's stock surged when SPK-8011 produced some encouraging data earlier this year. It's giving a lot of its gains back today after the company released data that suggests a potential competitor from BioMarin might have the upper hand from this point forward.

Chalk drawing of an upward sloping chart that abruptly reversed course.

Image source: Getty Images.

SPK-8011 is a one-time gene therapy that aims to help hemophiliacs produce necessary clotting factors. Although Spark's candidate helped knock back dangerous uncontrolled bleeds to essentially zero, investors are troubled by a spotty clotting factor production benefit.

In short, one patient receiving a high dose of Spark's SPK-8011 produced clotting factors as high as 12% of normal while a patient receiving a low dose produced clotting factors as high as 24% of normal.

Now what

BioMarin's candidate hasn't been measured in a head-to-head trial against Spark's so we need to take any comparisons with a big grain of salt. That said, the differences look pretty big. BioMarin's valoctocogene roxaparvovec (formerly BMN 270) helped three patients with a 48-week observation produce clotting factors at an average of 49% of normal.

While the future of Spark's fledgling hemophilia program seems even less certain now than it did a few days ago, the company could launch its first commercial-stage drug in the new year. The U.S. Food and Drug Administration is expected to announce an approval decision for the company's Luxturna as the first treatment for a rare cause of blindness in the weeks ahead. 

Editor's note: A previous version of this article incorrectly listed Pfizer as a partner on SPK-8011.