United States health regulators rejected Amarin's Corp Plc's "preset testing process" for a blood fat-lowering drug, causing the drugmaker's shares to fall more than 25 percent in premarket trading, Reuters reported.

In an effort to broaden the drug's market and improve sales, Amarin has vowed to fight for Vascepa and said it planned to appeal the U.S. Food and Drug Administration decision, Reuters reported.

Last October, the FDA revoked a Special Protocol Assessment (SPA) agreement covering a large late-stage trial of the drug, Vascepa. The SPA would have guaranteed Amarin that the design and analysis of a trial "are adequate to support a marketing application submission with the U.S. health regulator."

Following an appeal from Amarin, the FDA said it would determine by January 15 whether it would reconsider that decision.

The SPA for Amarin was revoked again after an advisory panel to the FDA had "recommended against the drug's approval for use in a broader patient population until results from the larger trial had been analyzed."

The federal agency has indefinitely postponed a final ruling on Vascepa, initially expecting to weigh in by Dec. 20 but now pushing that date back to allow for Amarin's spiraling appeal process.

The drug was approved in 2012 to reduce high levels of triglycerides, a type of blood fat that can increase the risk of heart disease, in patients.

In a bid to broaden Vascepa's market and improve sales, Amarin applied last February for approval to sell the drug to patients with blood fat abnormalities who are at high risk of coronary heart disease, Reuters reported.