Shares of the Dublin company at last check were tumbling 28% to $5.20.
The three judges assigned to the appeal are Timothy Dyk, Jimmie V. Reyna and Todd Hughes.
Piper Sandler analyst Yasmeen Rahimi had seen Judges Dyk and Hughes as positive for the stock, while Reyna would be perceived as negative, according to Bloomberg.
Stifel analyst Derek Archila says Dyk is likely to rule against Amarin and Judges Reyna and Hughes "are still tough to make a call on."
In March, a federal judge in Nevada found that patents on Amarin’s Vascepa drug were invalid, wiping out $3.5 billion in market value.
Generic-drug makers Hikma Pharmaceuticals HKMPY and Dr. Reddy's Laboratories (RDY) - Get Free Report sought to overturn the patents so they could begin to produce generic versions of the medication, which is based partly on fish oil.
Archila said in a note to clients that the hearing concluded after "all of 20 minutes" and he is "slightly less confident" that Amarin will prevail after Dyk "dominated the questioning" of the company's counsel.
But the analyst said their brevity, the speed of the hearing and the fact that no questions were asked of the generic counsel "does not bode well" for Amarin. He added that he "would not be buying shares here."
Archila, who has a hold rating on the stock, repeated his belief that Amarin shares could go to $4 if the company loses the appeal and could go to $20 if it were to win.
Cowen analyst Ken Cacciatore said he would be adding to Amarin positions on today's weakness. He affirmed an outperform rating on the shares with a $10 price target.
A reversal of the lower court's Vascepa patent ruling is "highly unlikely and should only viewed as optionality," Cacciatore said. He added that Amarin's valuation should be predicated on its European opportunity alone.
Cacciatore said an appeals loss in the U.S. would better position management to either launch Vascepa in Europe or sell the company.
An eventual sale of Amarin "remains on the table" and the company could be worth more than $10 a share in a deal, he said.