Sarepta shares fall on muscular dystrophy drug data

For companies developing drugs for rare diseases, data in just a handful of patients can have enormous clinical implications and influence millions of dollars in market value.

So it has been for Sarepta Therapeutics, developer of an experimental drug for Duchenne muscular dystrophy, a disease that affects about 1 in 3,500 boys worldwideor about 14,000 to 15,000 people in the U.S. Sarepta said Thursday a study of 12 boys suggests that its drug eteplirsen helped slow declines in walking ability for almost three years.

Yet the stock sank, and was down about 24 percent mid-morning amid heavy selling in the broader market. One analyst said the data may call into question how well the drug works over longer periods. (For the latest stock quote for Sarepta, click here.)

The results will be part of the company's application for approval of eteplirsen, which it plans to submit to the Food and Drug Administration this year, Sarepta Chief Executive Chris Garabedian said in an interview. The company also plans to start three broader studies, in over 100 patients, to provide further information.

Read MoreSarepta gets path forward for muscular dystrophy drug

"This dataset supports the treatment benefit we've seen over the last three years," Garabedian said by telephone. "This supports the consistent thesis we've had, which is that eteplirsen is slowing the progression of the disease or stabilizing the disease."

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Sarepta shares fall on muscular dystrophy drug data