U.S. regulators granted Pfizer a reissued patent on its multibillion-dollar Celebrex pain drug that extends its marketing exclusivity until early December 2015, the company said on Tuesday.

That will give Pfizer an additional 1 1/2 years of selling a drug that had U.S. sales of more than $1.7 billion in 2012, before cheaper generic versions begin to hit the market at the end of 2015. Branded drugs can quickly lose upwards of 80% of sales once multiple generic versions become widely available.

The original basic patent for Celebrex - known chemically as celecoxib - expires on May 30, 2014, including six months of pediatric exclusivity for testing the drug in children. But the U.S. Patent & Trademark Office agreed to the reissued patent covering methods of treating arthritis and other approved conditions, Pfizer said.

Pfizer said it filed lawsuits against several generic drugmakers that are seeking permission from the U.S. Food and Drug Administration to sell a generic version of the drug in the United States beginning in May 2014.

Pfizer said it sued Teva Pharmaceutical Industries; Mylan; Watson Pharmaceuticals, which has since changed its name to Actavis; Lupin Pharmaceuticals; and Apotex to try to prevent them from selling their generic Celebrex until the new patent expires on Dec. 2, 2015.