Shares of Amarin tumbled nearly 30 percent before markets opened Tuesday after another setback in its quest to gain approval for the wider use a fish-oil based pill, its only product.

Federal regulators have refused to alter their stance on the design of research related to that broader use, the company said.

Shares of the Irish specialty drugmaker started getting pummeled last fall after a panel of Food and Drug Administration advisers voted against recommending a broader use of the pill, Vascepa, which is designed to lower triglycerides, a type of fat in the bloodstream.

Vascepa was approved in 2012 for patients with unusually high triglyceride levels. Amarin also wants to market the drug for patients with high triglyceride levels and heart disease who are already taking a statin drug to help control their cholesterol.

In October, a majority of panelists said that while Vascepa significantly lowers fat levels, it is unclear whether that actually translates into fewer heart attacks. The panel said the FDA should delay a decision on expanded use until Amarin completes a study of patient heart attack rates.

Amarin has said that it believes those advisers would have recommended the broader use if a special protocol assessment had been in place. That's an agreement between the company and the agency over the design of research. The drugmaker had asked the Food and Drug Administration to reinstate the agreement.

On Tuesday, Amarin said a division within the FDA does not plan to reinstate the agreement.

The drugmaker plans to appeal that decision and said it does not expect the FDA to decide on the expanded use while the appeal is pending.

Shares of Amarin Corp. slid 67 cents to $1.60, just over an hour before markets opened.