The U.S. Patent & Trademark Office issued a notice allowing Pfizer to correct a technical defect in one of the patents involving Pfizer''s cholesterol medicine Lipitor, the world''s top-selling drug, as reported by the Associated Press. The decision will result in re-issuance of the drug''s key patent and maintain that patent''s June 2011 expiration date, which is crucial to Pfizer because Lipitor brings the company nearly $13 billion in annual sales. That revenue has made the drug and Pfizer, the target of litigation by generic drugmakers hoping to roll out cheaper versions earlier and capture part of the market for one of the top classes of drugs. The patent office decision resulted from litigation that began after India''s largest drugmaker, Ranbaxy Laboratories Ltd., in 2002 sought U.S. Food and Drug Administration approval to sell a generic version of Lipitor, which is known chemically as atorvastatin. Ranbaxy challenged both Lipitor''s basic patent and a second one, focusing on the active ingredient in Lipitor. Pfizer had sued Ranbaxy to block its challenge of both patents and initially won in federal court in Delaware, but a Court of Appeals ruling in December 2006 found the patent had a technical defect: one claim within it contained an improper cross reference to another claim in the patent. The following month, to resolve the issue, Pfizer applied with the patent office to correct the defect by removing the cross reference and have the patent reissued. Two years later, the patent office ruled it will re-issue the patent, as revised, if Pfizer pays a fee of $1,510 within three months. The June 2011 expiration date includes a six-month extension of the original expiration date granted because Pfizer did research after the drug was approved to determine its safety and effectiveness in children -- a standard move by drug companies to preserve revenue of their most lucrative drugs. However, Pfizer''s litigation with Ranbaxy also involved three other patents that ran through 2016 and 2017. Last June, the companies settled that lawsuit with an agreement essentially giving Ranbaxy license to those patents and allowing it to begin selling generic Lipitor as of Nov. 30, 2011. As the first company to seek FDA approval to sell a generic, it can do so for six months exclusively.