What: After competitor BioMarin Pharmaceutical (BMRN -0.12%) struggled to make a case for its Duchenne's muscular dystrophy (DMD) drug to a key FDA advisory committee, shares in Sarepta Therapeutics (SRPT -0.40%) soared by 52.8% last month.

So what: Perhaps no class of drugs has struggled as mightily to reach FDA decision-makers as exon-skipping therapies for DMD.

Exon-skipping drugs designed to skip exon-51 to boost dystrophin levels in patients and stave off the progression of this devastating disease have posted mixed results in clinical trials for both BioMarin Pharmaceutical and Sarepta Therapeutics.

Arguably unconvincing efficacy in trials had many people thinking that exon-51 drugs would fail to pass muster with the FDA. However, a major need for new therapies and aggressive lobbying by patient advocates has led to the FDA considering the drugs anyway.

Last month, BioMarin Pharmaceutical's drisapersen was the first to face public scrutiny by FDA regulators when the FDA's advisory committee tasked with evaluating its efficacy and safety met on Nov. 24.

Concerns over safety risks and contradictory efficacy in drisapersen's trials led most industry watchers to lower the odds of an FDA approval for drisapersen when the FDA makes its final decision on Dec. 27.

If the FDA does deny drisapersen, then it could be a big win for Sarepta Therapeutics eteplirsen.

Although the two drugs both skip exon-51, Sarepta Therapeutics may do a better job showing regulators that eteplirsen boosts dystrophin levels and if so, then an argument could be made in support of eteplirsen's approval.

Image source: Sarepta Therapeutics.

Now what:  Despite eteplirsen and drisapersen only being effective in a small portion of DMD patients, these drugs are likely to be big sellers because orphan drugs command top-tier prices and DMD treatment is chronic. An approval would also result in the FDA awarding these companies with a transferable priority review voucher. In the open market, those vouchers have commanded nine-figure price tags.

However, before investors begin to model for potential DMD related revenue, they should remember that it's not certain that the FDA will shelve drisapersen or approve eteplirsen.

Eteplirsen may have advantages in dystrophin production to drisapersen, but the basis for an approval of eteplirsen is a trial that included just 12 people. That's far fewer people than were studied in drisapersen's trials and that could mean that eteplirsen's results would have mirrored drisapersen's results if it had been studied in more people.

Nevertheless, since there's no cure for this disease and most DMD patients fail to survive beyond their 30s, both drugs could get approved and because of that, investors may want to rein in some of their enthusiasm for Sarepta Therapeutics until they get the FDA's decision on drisapersen and insight into how they'll view Sarepta Therapeutics small sample size.