By Roger Parloff
March 13, 2014

FORTUNE — A patent ruling by a federal judge in Norfolk, Va., on Wednesday will, if upheld, mean that drug giant Pfizer (PFE) will lose its exclusive right to market its blockbuster painkiller Celebrex after May 30 — 18 months earlier than it had hoped.

Pfizer says it will appeal the ruling, which was a major victory for generic manufacturers Lupin, Teva (TEVA), Mylan (MYL), Activis (ACT), and Apotex.

U.S. sales of Celebrex brought Pfizer $1.9 billion in revenue last year. The New York Stock Exchange halted trading for 20 minutes on Wednesday afternoon so that the news could be absorbed. Pfizer stock fell 1.36%, to close at $31.98.

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After reading the ruling at Fortune’s request, Timothy Holbrook, an intellectual property law professor at Emory University School of Law, predicted that the ruling would be upheld on appeal. He said it seemed “fairly clear” to him that Pfizer was “trying to get around” an earlier ruling of the U.S. Court of Appeals for the Federal Circuit — the court that handles all patent appeals — and that the “Federal Circuit will not look kindly on such attempts.”

Pfizer has three patents on Celebrex. One relates to the compound itself, known as celecoxib, a second relates to the composition of the pills, and a third relates to the methods of using the pills to treat various diseases, including arthritis, osteoarthritis, rheumatoid arthritis, and menstrual cramps. The first two patents have an effective expiration date of May 30, 2014 (counting a six-month period of enhanced exclusivity that the U.S. Food and Drug Administration grants companies in exchange for certain pediatric drug testing), while the third effectively expires 18 months later, on Dec. 2, 2015.

In 2008, after a legal challenge from generic manufacturers, the Court of Appeals for the Federal Circuit — the court that handles all patent appeals — upheld Pfizer’s first two patents, but invalidated the third (U.S. Patent No. 5,760,068). That one it invalidated for “double-patenting,” meaning essentially that it embodied no significant new innovation over the second patent, and was simply intended to prolong patent life.

Pfizer promptly applied for a “reissue patent” for the invalidated patent — a remedy that is available if a patent has been invalidated on certain technical grounds. The U.S. Patent and Trademark Office initially denied the request, but on March 5, 2013 — after five years of deliberation — granted the reissue (U.S. Reissue Patent No. RE 44,048). In doing so, the PTO effectively handed Pfizer back the 18 extra months of exclusivity it had lost because, like the patent that was invalidated, the reissue patent was set to expire on Dec. 2, 2015.

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Pfizer filed suit that same month in Norfolk, Va., seeking to stop generic manufacturers Lupin, Teva, Mylan, Watson (now Activis), and Apotex from proceeding with their plans to market a generic version of celecoxib after the May 30, 2014, expiration of the Pfizer’s first two patents on Celebrex. Pfizer sought a declaration that the reissue patent was valid, while the generics sought the opposite. Each side moved for summary judgment — a ruling based on the papers, without a trial — and on Wednesday U.S. District Judge Arenda L. Wright Allen ruled for the generics.

Judge Allen invalidated the reissue patent for the same reason the Federal Circuit invalidated the original one — “double-patenting.” Pfizer had argued that, as part of the reissue, it had reclassified the patent in a way that made it eligible for a safe harbor that protects patents from invalidation on “double-patenting” grounds. But Judge Allen found that the reissuance process could not be used for that purpose, and the flaw was therefore “not correctable.”

Professor Holbrook thinks she got it right. “The law is quite clear that such use of reissuance is not appropriate,” he says.

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