BMRN BioMarin Pharmaceutical Inc

Sarepta Therapeutics Inc.'s share price was nearly cut in half Thursday after the U.S. Food and Drug Administration further questioned the efficacy of the company's drug candidate to treat a fatal form of muscular dystrophy.

Sarepta is seeking approval to sell eteplirsen as a treatment for Duchenne muscular dystrophy, a condition that destroys muscles and frequently kills patients by their 30s. The disease, which has no effective treatments, affects roughly one in every 3,500 boys world-wide.

Shares of Sarepta were down 45% at $10.89 shortly before 2 p.m. in New York.

The FDA released documents Thursday on the drug ahead of an FDA panel review coming on Monday, with a decision by the agency expected by May 26. In the documents, the FDA maintained its negative outlook for the drug, criticizing the drug's efficacy and the trial's design and measurement.

RBC Capital Markets analyst Simos Simeonidis said the FDA "just isn't buying Sarepta's arguments" but is actually "doubling down on the negative view of the data."

A Sarepta representative was unavailable for comment.

In February, the FDA pushed back its decision deadline three months to review new clinical data submitted by the biopharmaceutical company. The extension came a month after the agency released documents, ahead of a scheduled panel review in January, that suggested regulators were questioning the efficacy of the drug. That January review, though, was eventually canceled because of bad weather.

In the documents released Thursday, the agency questioned the amended data that Sarepta had filed and responded to Sarepta's earlier criticisms of the FDA's original briefing documents by saying "we do not agree with the applicant's characterization of inaccuracies."

Mr. Simeonidis said, "FDA does not appear to be giving an inch to the arguments put forth by the company."

Debra Miller, founder and chief executive of California-based nonprofit CureDuchenne, expressed disappointment in the FDA's review but said any drug that uses a new technology is bound to face scrutiny.

"The FDA's initial reaction is just that, an initial response, and there is evidence still to be presented to the advisory committee," said Ms. Miller, who will be speaking at the advisory committee meeting Monday.

Still, Leerink analyst Joseph Schwartz called Sarepta's argument for its proposed dystrophy treatment "strongly challenged and largely invalidated by the FDA."

He recalled that "the panelists were very critical" of BioMarin Pharmaceutical Inc.'s treatment for Duchenne, "and we expect a similar level of scrutiny" for Sarepta.

The FDA in January rejected BioMarin's new drug application for a treatment of Duchenne because of questions about the drug's effectiveness.

Write to Anne Steele at Anne.Steele@wsj.com

(END) Dow Jones Newswires

April 21, 2016 14:45 ET (18:45 GMT)

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