Shares of Sarepta Therapeutics (SRPT) tumbled Thursday, a day after soaring more than 170% on word that the drugmaker's experimental treatment for a type of muscular dystrophy helped boys' ability to walk.

In a study of boys with the genetic disorder Duchenne muscular dystrophy, those taking Sarepta’s drug eteplirsen were able to walk much farther after 48 weeks. In a six-minute walking test, those on Sarepta’s medicine were able to walk on average almost 90 meters farther than those taking a placebo.  

The results were "unprecedented" for treatment of this disease, and the drug so far is shown to be safe, CEO Chris Garabedian told investors on a conference call Wednesday morning.

Shares of Sarepta soared 170% to $40.46 in morning trading Wednesday. Thursday, the stock plunged more than 15% to close at $37.92.

Garabedian said Sarepta hopes to win an expedited path to U.S. approval. The drug is in the second of three phases of study usually needed for Food and Drug Administration clearance.

"These results provide tremendous promise for patients," Garabedian said.

The drug also is attracting attention of bigger drug companies who may be interested in partnering to help develop and market eteplirsen, according to Garabedian.  

The walking improvement was demonstrated in a small group of patients -- just 12 boys, ages seven to 13. But the results were impressive for such a hard-to-treat disease. Duchenne muscular dystrophy takes place in about one in every 3,600 male infants, according to government figures. It is caused by a defective gene for the protein dystrophin.  

"These data represent a significant milestone and a defining moment of progress and hope for patients with (Duchenne muscular dystrophy) and their families, as well as those of us in the scientific community who have been pursuing potential treatments for this devastating and deadly disease for decades," the study’s lead researcher Jerry Mendell says in a statement.

Mendell, director of the the Centers for Gene Therapy and Muscular Dystrophy at Nationwide Children’s Hospital, says the study results showed that eteplirsen both helped production of dystrophin and slowed progression of the Duchenne disease. The drug targets a specific genetic mutation but implications for all Duchenne “patients with related genetic mutations are clearly evident,” he says.   

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