(Reuters) - U.S. health regulators have rejected Celgene Corp’s application seeking approval of a key multiple sclerosis drug due to insufficient data, the company said on Tuesday, a surprise development that will likely delay the entry to market of one of Celgene’s most important pipeline assets.

Celgene shares fell nearly 8 percent after it revealed that it had received a “refusal to file” letter from the Food and Drug Administration (FDA) for its ozanimod for the treatment of patients with relapsing multiple sclerosis.

Celgene last month had said it expected U.S. approval of ozanimod by the end of this year and planned to file for European approval during the current quarter. The timeline for a U.S. approval decision is now far less certain.

Jefferies analyst Michael Yee said it was “extremely unlikely that there is an inherent regulatory problem.”

He said it was “merely a delay and yet another short term problem that does not inspire Street confidence.”

Upon preliminary review, the FDA determined that the nonclinical and clinical pharmacology sections in the new drug application are insufficient to permit a complete review, the company said.

Celgene said it intends to seek immediate guidance from the FDA to determine what additional information will be required to resubmit the application.

“We will work with the FDA to expeditiously address all outstanding items and bring this important medicine to patients,” Jay Backstrom, Celgene’s chief medical officer and head of global regulatory affairs, said in a statement.

Wall Street analysts had forecast ozanimod sales of about $263 million in 2019, according to Thomson Reuters data, a figure that will likely have to come down with any significant delay. Annual sales of the drug are projected to reach $3.46 billion by 2024.

Ozanimod is also being tested for Crohn’s disease and in combination with Celgene’s psoriasis drug Otezla for ulcerative colitis.

Celgene shares were down 7.7 percent at $88.33 in extended trading.