Sarepta Therapeutics Inc said the U.S. Food and Drug Administration had asked for additional data on its lead drug, delaying its marketing application by about six months.

The biotechnology company's stock plunged over 30 percent in premarket trading after the agency said more discussion was needed to determine what constituted a complete application for the experimental muscle disorder drug.

The FDA gave Sarepta a fresh lease of life in April when the agency indicated an alternate approval pathway for the drug, eteplirsen, after deeming Sarepta's plans for marketing approval "premature" late last year.

Eteplirsen is being developed to treat Duchenne muscular dystrophy (DMD), a degenerative disorder that hampers muscle movement and affects one in 3,600 newborn boys, most of whom die by age 30.

The biotechnology company said it plans to submit the application by the middle of next year, pending additional requests from the FDA.

Monday's news boosted the shares of Prosensa Holding NV , which is developing a rival DMD drug, drisapersen.

Drisapersen, like eteplirsen, is aimed at enhancing production of a protein called dystrophin, the lack of which causes the disease.

Sarepta's shares were down about 32 percent at $16.03 before the bell. Prosensa shares were up about 13.7 percent at $13.65.