What: Shares of BioCryst Pharmaceuticals (NASDAQ:BCRX), a biotech company that specializes in the development of small molecular drugs, sank by over 20% today on heavy volume. The stock's sudden drops appears to be the result of a positive clinical update for BCX7353, an experimental treatment designed to reduce attacks stemming from a rare disorder known as hereditary angioedema, or HAE. Specifically, the company announced that a small, early stage study showed that BCX7353 significantly inhibited the target enzyme plasma kallikrein across all doses tested, and the drug was generally safe and well-tolerated.

So what: While it's somewhat unusual for a biotech's stock to plunge on positive trial results, BioCryst's case is unique because BCX7353 is facing stiff competition in its quest to treat HAE prophylactically.

Dyax's (UNKNOWN:DYAX.DL) DX-2930, for instance, showed such impressive early stage results earlier this year that the company decided to push the drug directly into a pivotal stage trial that is set to begin before the end of 2015.

Now what: So, perhaps the main reason BioCryst's shares are falling today is because the company is only advancing BCX7353 into a mid-stage trial, according to the press release--putting it well behind the rapid development of Dyax's competing therapy. In short, it looks like DX-2930 may end up claiming both the first-in-class and best-in-class trophies in the HAE race, casting doubt on BCX7353's commercial potential going forward.