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Shares of Amarin (AMRN - Get Report) plunged after the biotech company's surprise revelation Thursday that the Food and Drug Administration will proceed with an advisory panel review of its heart drug Vascepa -- a decision that just a week ago the company had called "unlikely."

Shares of Amarin dropped 16% in trading on Friday to $14.96 after the company announced that the FDA has scheduled a Vascepa advisory committee meeting for Nov. 14. The agency's deadline for making an approval decision on the heart drug also likely will be extended into late December from Sept. 28, the company said.

Amarin is seeking FDA approval for Vascepa that would include a larger scope of data from a clinical trial that showed a 25% drop in the risk of heart attacks and strokes. 

A week ago, the company said it was likely the FDA would proceed with the approval. However, late Thursday the company said the FDA wants to ask more questions about the study methodology and trial findings.

FDA acceptance of the broader trial results could lead to higher Vascepa sales higher as doctors add the drug, which is derived from fish oil, on top of current medicines used to treat more people at risk for heart disease.