A federal appeals court heard oral arguments last week in a case that could carve out more guidance on whether state law claims involving labels on pharmaceuticals are preempted.

To argue the case on Oct. 3 before the U.S. Court of Appeals for the Ninth Circuit, both sides brought in appellate heavyweights from Washington D.C.: David Frederick, of Kellogg Hansen Todd Figel & Frederick for the plaintiffs, and Kannon Shanmugam of Williams & Connolly for the defendants, which include Merck Sharp & Dohme Corp. and Eli Lilly and Co.

At issue is the “clear evidence” standard that the U.S. Supreme Court set forth in its 2009 decision in Wyeth v. Levine. The high court found that federal regulatory approval of a drug didn’t shield manufacturers from state law claims without “clear evidence” that the FDA would “not have approved” changes to product labels.

Yet courts have grappled to define the “clear evidence” standard.

“That decision is where subsequent courts have been debating and trying to figure out that standard,” said Catherine Sharkey, a professor at New York University School of Law. “What do you have to show to show that the FDA would have rejected or foreclosed a manufacturer from putting on this warning? There’s a debate about how the courts are supposed to do that.”

The Ninth Circuit involved multidistrict litigation against the makers of four incretin drugs used to treat type 2 diabetes. The lawsuits claim Merck, Eli Lilly, Novo Nordisk Inc. and Amylin Pharmaceuticals failed to warn that the medications were linked to pancreatic cancer. A federal judge in California had granted summary judgment, wiping out 750 cases in federal court and 300 cases in California state court.

Frederick, who argued successfully for the plaintiff in Wyeth, sought to reverse the 2015 ruling based on the fact that the makers of the drugs – sold under the names Januvia, Janumet, Byetta and Victoza — failed to provide “clear evidence” that the FDA would have rejected such a pancreatic cancer warning, particularly in light of new safety information. The problem is, he said, none of the manufacturers tried to change their labels.

“When you start multiplying all this evidence, it becomes clear had the manufacturer offered up a warning the FDA would not have precluded it,” he told the panel.

Shanmugam, who’s racked up a string of Supreme Court victories this year, insisted that the makers of the four drugs had done extensive research to disprove such a link. His briefs cite a 2014 article in the New England Journal of Medicine in which FDA officials found a pancreatic cancer link “inconsistent with the current data.” The FDA also rejected a public citizen petition to withdraw Victoza from the market and approved four additional incretin drugs without such a warning.

Shanmugam declined to comment, and Frederick did not respond to a request for comment.

The case is the most pivotal on the issue since the U.S. Court of Appeals for the Third Circuit found on March 22 that the “clear evidence” standard was fact specific and thus more appropriate for a jury to decide. The ruling revived about 5,000 cases alleging Merck failed to warn that its osteoporosis drug Fosamax had a risk of femur fractures.

On Sept. 15, the Ninth Circuit asked both sides in the diabetes cases to address Fosamax.

Fosamax may change whose job it is to weigh the evidence, whether it’s a jury’s job or not,” Susan Graber said at last week’s hearing.

“But I don’t think Fosamax alters the analysis here,” Shanmugam replied. He said that, unlike Fosamax, there were no disputed material facts in the case.

What appeared more critical to the panel is the reason why U.S. District Judge Anthony Battaglia of the Southern District of California barred new safety information that plaintiffs’ had sought to introduce to defeat summary judgment. Because the new safety information hadn’t been provided to the FDA, Battaglia cited the Supreme Court’s 2001 holding in Buckman Co. v. Plaintiffs Legal Committee, which found that state law claims alleging fraud on the FDA were preempted by federal law.

“Ok, but didn’t he fundamentally misunderstand Buckman?” Morgan Christen quickly interrupted Shanmugam early in his arguments. “Can we get to that please? Because your time is ticking and it’s so important to the case.”

Shanmugam insisted that Battaglia didn’t actually think there was fraud on the FDA but was relying on the policy underlying Buckman.

The panel also asked several questions about why Battaglia struck the plaintiffs’ regulatory expert who was set to testify about the new safety information.

“In terms of prejudice to us, it is massive,” Frederick told the panel. “It is clear the error that started this whole problem was with Buckman.”

Frederick and his firm have been involved in other mass torts in which judges have tossed plaintiffs’ experts.  On June 2, the Third Circuit rejected Frederick’s attempt to revive 315 cases against Pfizer over anxiety medication Zoloft that had been dismissed once the plaintiffs’ expert was tossed. Frederick’s colleague Derek Ho is awaiting an oral argument date to convince the U.S. Court of Appeals for the Fourth Circuit to revive 3,128 lawsuits over cholesterol drug Lipitor on similar ground.