What: Shares of Sarepta Therapeutics (NASDAQ:SRPT), a clinical-stage orphan drugmaker, fell by more than 50% in pre-market trading today on exceptionally high volume. The catalyst behind this monster move downward is the release of the briefing documents by the U.S. Food and Drug Administration for the company's experimental Duchenne muscular dystrophy (DMD) therapy, eteplirsen.

Although eteplirsen's advisory committee meeting is scheduled for Jan. 22 and briefing documents tend to be released around two days prior to the meeting, the FDA apparently didn't have much trouble pushing these documents out ahead of time, presumably because of its recent review of BioMarin's (NASDAQ:BMRN) rival therapy drisapersen. 

So what: According to the FDA's take on eteplirsen's clinical data, the agency appears unconvinced of the eteplirsen's efficacy, citing "serious methodological concerns" and the lack of a clear-cut increase in dystrophin levels as their main reasons behind this conclusion.

Now what: Investors were hopeful that the recent rejection of BioMarin's experimental DMD therapy by the FDA indicated that the agency was leaning toward an approval for eteplirsen, given the complete lack of available treatments for this fatal muscle-wasting disorder. However, the FDA's briefing documents seem to indicate otherwise, meaning that it's looking like Sarepta will ultimately have to complete a late-stage study to clear up any outstanding questions regarding eteplirsen's effectiveness.

Of course, the advisory committee could still vote to approve eteplirsen at next week's meeting, but that scenario doesn't seem all that likely in light of the serious questions raised in the FDA's review. That's why I personally won't be picking up any shares in this speculative biotech following this hefty pullback.