U.S. Court of Appeals for the Federal Circuit Chief Judge Sharon Prost.

The U.S. Court of Appeals for the Federal Circuit will not revive a $2.5 billion jury award that Merck & Co. and subsidiary Idenix Pharmaceuticals won against Gilead Sciences in 2016.

The Federal Circuit agreed with U.S. District Judge Leonard Stark of the District of Delaware that Idenix's 7,608,597 patent claimed far too many potential compounds to identify the specific 2′-methyl-up modified nucleoside that Gilead uses to treat hepatitis C.

Chief Judge Sharon Prost of the U.S. Court of Appeals for the Federal Circuit wrote for a 2-1 panel that the "genus" of 2′-methyl-up nucleosides Idenix claimed covered potentially "billions and billions" of compounds. Even the 18 working examples that Idenix included in the claim specification narrowed it only to the tens of thousands, she wrote in Idenix Pharmaceuticals v. Gilead Sciences.

That would leave a person of skill in the art "searching for a needle in a haystack to determine which of the 'large number' of 2′-methyl-up nucleosides falls into the 'small' group of candidates that effectively treats HCV," she wrote.

Gilead had called the award the largest in patent history, though Merck pointed out it was only a fraction of the $25 billion in sales Gilead had rung up on its Sofosbuvir compound. Gilead predecessor Pharmasset Inc. synthesized its fluorinated version of the compound in the early 2000s.

Prost's opinion actually went further than Stark's post-trial judgment. She also found the patent invalid for lack of written description.

Gilead's winning team featured Orrick, Herrington & Sutcliffe partner E. Joshua Rosenkranz, who argued the appeal with support from attorneys at Orrick and Fish & Richardson, which tried the case.

"This is just the latest in an unbroken chain of tribunals that have concluded that Gilead invented the cure for HCV. Not Merck and not Idenix," Rosenkranz said in a written statement.

The Federal Circuit has previously affirmed a PTO interference proceeding in Gilead's favor and a ruling from a San Jose federal judge that Merck came to a separate patent litigation with "unclean hands."

"That's two district judges, and seven appellate judges, in three different appeals," Rosenkranz said.

Jones Day partner Greg Castanias, who argued the appeal for Merck, declined to comment, and a Merck representative did not immediately respond to a request for comment Tuesday afternoon.

Merck had argued on appeal that a person of skill would have understood that the focus of the claims were on inhibiting a specific polymerase to effectively cure HCV. That would have narrowed the field of potential compounds to a predictable and manageable group of compounds, Merck argued.

Prost wrote that that would be using the knowledge of a person of skill in the art to fill in a claim limitation, "an impermissible end-run around the requirement to enable the full scope of the claim."

Judge Pauline Newman dissented. Putting the 18 working examples together with figures and data in the specification could have narrowed the field down to a handful of compounds, she wrote. "A reasonable jury could have concluded that the '597 claimed those compounds, not the 'billions and billions' of unsynthesized and unevaluated variants in the specification," Newman wrote.

She concluded that while the patent is valid, Gilead's 2′ methyl-fluoro compound does not infringe. Prost replied that the only way to reach that result was to disregard Stark's claim construction and rely on an argument not advanced by the parties.