CAMBRIDGE (TheStreet) -- Sarepta Therapeutics (SRPT) - Get Report is delaying the filing of an FDA approval application for its Duchenne muscular dystrophy drug eteplirsen until the middle of next year because regulators have asked for a slew of new data, the company said Monday. 

Sarepta previously promised -- and investors expected -- eteplirsen's approval filing in December. Sarepta shares are down 30%, to $16.61, on the eteplirsen regulatory setback. Prosensa (RNA) , still on track to file its competing DMD drug at year-end, is up 4% to $12.50. 

Sarepta's sliding timeline is bad, but the reason is even more troublesome. In May, FDA inspected the single hospital conducting the small phase II study of eteplirsen and found "disparities" in the way lab technicians measured and assessed the production of dystophin in the DMD patients, Sarepta said. As a result, FDA is now requiring Sarepta to seek an independent confirmation of dystrophin production, among other new data, as part of its eteplirsen new drug application. 

Dystrophin is a protein that plays a key role in muscle function and repair. The genetic inability to make dystrophin is what causes DMD. Sarepta's eteplirsen is  designed to "skip over" the section of damaged gene in DMD patients and restore the gene's ability to produce partially functioning dystrophin.

The foundation of Sarepta's regulatory strategy for eteplirsen has always been data showing that boys treated with the drug over time have maintained the ability to walk far longer than you'd typically see in untreated DMD boys. But Sarepta only has walk performance data for 12 boys in its small phase II study, so the company also relies on additional, supportive data from muscle biopsies showing higher levels of dystrophin production, presumably caused by eteplirsen.

But now, it appears FDA has doubts about the reliability of those dystrophin data. On a call with investors this morning, Sarepta CEO Chris Garabedian defended the credibility and methodology used by the single hospital to measure dystrophin in the phase II study. An independent analysis will either confirm the hospital's conclusions or further undermine eteplirsen.

Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback;

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