(Reuters) - PTC Therapeutics Inc has failed to supply persuasive evidence that its experimental drug to treat a form of Duchenne muscular dystrophy is effective, the U.S. Food and Drug Administration said on Tuesday, dealing a blow to the company’s years-long effort to bring the drug to market.

PTC’s shares were down 16.4 percent at $16.31 late on Tuesday morning.

A preliminary review by FDA scientists concluded that data to establish effectiveness of the drug, ataluren, “are not persuasive,” according to a review posted on the FDA’s website.

The agency’s comments come two days ahead of a meeting of outside advisers who will discuss ataluren. The FDA has asked the advisers to decide whether the drug is effective or whether the data is inconclusive.

The FDA is not obliged to follow their recommendation but typically does.

J.P. Morgan analyst Anupam Rama said the FDA report’s negative tone makes “approval a low probability.”

Duchenne muscular dystrophy (DMD) is a rare muscle-wasting disease caused by mutations in the DMD gene. Ataluren is designed to treat patients with so-called nonsense mutations that prevent the body from producing a key protein needed for muscle development. About 10 to 15 percent of DMD patients have nonsense mutations.

DMD starts in childhood and mainly affects males. More than 90 percent of patients become wheelchair-bound by age 15. There is no cure.

The FDA has twice refused to review ataluren because the drug had failed to show effectiveness in clinical trials. PTC proceeded to file its application “over protest” under a rarely used move that allows a company to have its application reviewed when there is a disagreement with regulators over the application’s acceptability.

If approved, the drug would be marketed under the name Translarna.

The FDA will be under particular scrutiny when it makes its final decision. Last year, Dr Janet Woodcock, head of the agency’s pharmaceuticals division, approved Sarepta Therapeutics Inc’s DMD drug Exondys 51, or eteplirsen, against the advice of the agency’s advisory panel and against the recommendation of its own scientists.

The reviewers appealed Woodcock’s ruling to then-Commissioner Robert Califf, who deferred to Woodcock. Dr John Jenkins, then head of the FDA’s Office of New Drugs, sent a biting letter to Califf criticizing Woodcock of bias and challenging Califf’s statement that the decision would not lower the bar for future approvals.

Dr Ronald Farkas, the lead reviewer, quit the agency after issuing a scathing report criticizing the quality of Sarepta’s data.