The U.S. Food and Drug Administration deferred its decision on Sarepta Therapeutics’ muscle-wasting disorder drug, a month after a federal advisory panel determined the treatment had not been proven effective.

The company’s shares were up 20 percent at $22.30 in premarket trading.

Sarepta said the FDA had notified the company that it will not be able to complete the review of the highly anticipated drug by Thursday and that it will try to complete the work in “as timely a manner as possible.”

The drug, eteplirsen, is under review for Duchenne muscular dystrophy, a disorder that mostly affects boys.

Patient advocates have been arguing in favor of the drug, putting pressure on the regulator to approve a treatment for the rare genetic disorder which currently has no cure.

The advisory panel, however, voted 7-3, with three abstentions, that the clinical trial of 12 patients did not provide substantial evidence the drug was effective for muscular dystrophy patients with a specific genetic mutation.

Duchenne muscular dystrophy is caused by a lack of dystrophin, a protein needed to keep muscles healthy. Eteplirsen is designed to increase the production of dystrophin.