Ignore the bickering between shareholders of Sarepta Therapeutics (NASDAQ:SRPT) and GlaxoSmithKline (NYSE:GSK)/Prosensa (NASDAQ:RNA) for a moment, because these are exciting times for the roughly 1 in 4,000 newborns worldwide who are born with Duchenne muscular dystrophy.

Syringe Green Blue

Source: Andres Rueda, Wikimedia Commons.

There are two drugmakers currently knocking on the doorstep of the Food and Drug Administration, each with a unique drug -- eteplirsen for Sarepta, and drisapersen for Glaxo/Prosensa -- that has the ability to slow or stop the muscle-destroying effects of DMD. With very few treatment options currently available to DMD patients, and an average lifespan of just 25 years, this is very encouraging news.

Until recently, the data on the two drugs was similarly positive, but the edge in recent weeks had been sliding back into drisapersen's favor. This big concern that has plagued Sarepta shareholders recently is that the FDA has had a difficult time accepting increased dystrophin production as a primary endpoint to improved results in DMD patients. It also hasn't helped Sarepta's case that its study is only mid-stage in nature and based on just 12 patients. This was the primary reason the FDA refused to accept an accelerated new drug application filing from Sarepta.

But, the gloves came off again on Friday, which may have again given the upper hand to Sarepta. New data was released on drisapersen that showed that the drug was effective in spurring dystrophin production in 72% of boys taking the drug and 59% in those that took it intermittently. The results are good, but nowhere near the 100% figure that Sarepta's eteplirsen delivered in its mid-stage trial. 

Not so fast...
There are, though, a number of questionable factors to contend with here that may not be telling the entire story -- the first of which is the size of each drugmaker's mid-stage trial. Eteplirsen was examined in just 12 patients, whereas drisapersen's trial involved 53 patients. This reminds me of the mid-stage study in hepatitis C between Gilead Sciences (NASDAQ:GILD) sofosbuvir and AbbVie's (NYSE:ABBV) hep-C combo drug, which produced 100% and 97% sustained virologic resposnes, respectively. Gilead's patient subset consisted of just 25 patients tested with sofosbuvir and ribavirin. AbbVie, however, tested its combo on 79 patients, with 77 of them exhibiting no detectable sign of the disease after 12 weeks. Did this prove conclusively that Gilead's drug was superior to AbbVie's because it returned 100% SVR12 compared to AbbVie's 97%? Not in the least, because it's more than just about the percentages! In this case, both drugs may prove to be blockbusters, and that could also be the case in DMD with these two drugs. 

There's also the question mark of what will happen to drisapersen in late-stage trials. Whereas it's seemingly likely that the FDA is going to accept Sarepta's mid-stage findings for a new drug application filing sometime early next year, drisapersen is currently being studied in a randomized, double-blind and placebo-controlled 186 patient phase 3 trial with results due out next quarter. Safety hasn't been a major concern throughout any of these DMD trials, so the more wide-ranging results from a study that's more than 15 times the size of Sarepta's could go a long way to proving the worth of drisapersen. It could also completely bury the drug if its responsiveness in boys drops below, say, 70%.

The battle rages on
The truth is that there could be a market for both drugs in treating DMD, but neither may yet have what it takes. I really like eteplirsen -- and so do Wall Street analysts – but it's also arguably difficult to get fully behind a therapy that worked in such a small patient subset without any further testing. Likewise, there are the what-ifs of approving drisapersen, which may deliver an effectiveness around 70% or slightly better, when you have eteplirsen possibly available, which not only slowed progression of the disease in a higher percentage of mid-stage patients, but actually improved walking distance in mid-stage trials -- an almost unfathomable reversal of the disease's effects.

For now, it still appears that this boxing match has plenty of rounds and punches left to be thrown.

Fool contributor Sean Williams has no material interest in any companies mentioned in this article. You can follow him on CAPS under the screen name TMFUltraLong, track every pick he makes under the screen name TrackUltraLong, and check him out on Twitter, where he goes by the handle @TMFUltraLong.

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