What: Shares of Sareptra Therapeutics (NASDAQ: SRPT), a clinical-stage biopharmaceutical company focused on developing treatments for Duchenne Muscular Dystrophy, or DMD, skyrocketed more than 27% higher today after the FDA released a less than flattering report on an experimental drug produced by its primary rival.

So what: The FDA released a very critical report today related to its pending review of drisapersen, a DMD medicine that is made by Sarepta Therapeutics' competitor BioMarin Pharmaceuticals (NASDAQ:BMRN)

The FDA report was especially harsh on its review of drisapersen, stating:

The conclusion of this review is that substantial evidence of clinical efficacy was not established for drisapersen in the treatment of exon 51-skip amenable DMD. There is no independent substantiation of the positive findings from a small Phase 2 study based 6MWD as a clinical endpoint. A larger study intended to provide the most reliable evidence of effectiveness was negative. 

The report went on to express serious concerns about the safety profile of drisapersen, noting that some patients experienced severe toxicity across many organ systems while using the drug. 

Given that BioMarin Pharmaceuticals and Sarepta Therapeutics have been in a heated race to be the first to bring a DMD treatment to market, this report appears to have tipped the scales into Sarepta's favor.

Now what: Since the FDA drug review process is designed to be hyper critical it's certainly understandable why the report was so harsh on drisapersen. However, this report doesn't necessarily mean that drisapersen has no chance of gaining approval. That decision will be released after the FDA panel vote scheduled for next week. 

Investors should also be aware that Sarepta isn't exactly a shoo-in for an approval based on today's update either, as its own clinical trial data wasn't exactly perfect. Still, given the huge need for a DMD treatment, it's likely that at least one of these drugs will make its way to market. 

The drama unfolding as this story plays out is likely to continue on for several more months, so invesotrs in both BioMarin and Sarpeta Therapeutics investors would probably be wise to mentally prepare themselves for more volatility ahead. 

Brian Feroldi has no position in any stocks mentioned. The Motley Fool recommends BioMarin Pharmaceutical. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.