Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of biopharmaceutical drugmaker Sarepta Therapeutics (NASDAQ:SRPT) plummeted 55% today after the Food and Drug Administration raised concerns about the trial results for its experimental Duchenne muscular dystrophy drug, eteplirsen.

So what: The FDA called Sarepta's application to sell eteplirsen "premature," suggesting that the company will need to spend more considerable time and money on the drug to address those concerns. In fact, the FDA's request for more data will delay the initiation of dosing in the confirmatory study until at least the second quarter of 2014 -- a particularly big deal for Sarepta, which has no other products in the market to help offset the cash burn.

Now what: Sarepta will follow up with the FDA this month to discuss the study's design.

"We strongly believe in the potential of eteplirsen to address a serious unmet medical need in DMD and we are committed to its development," Sarepta CEO Chris Garabedian said in a statement. "Our team at Sarepta recognizes the urgency of families who are seeking new treatments, and we will continue to work with the FDA on an acceptable confirmatory study design and, in parallel, seek to address their concerns regarding a potential NDA filing based on our current dataset."

Given the extremely speculative nature of the stock, however, average investors would do well to remain on the sidelines.