But it’s a preliminary rejection, which means it ain’t over yet. Still, this isn’t good new for Pfizer, which is struggling to develop new drugs and has Lipitor successor in sight. Now, the US Patent and Trademark Office is nixing Pfizer’s request to reissue a patent that would preserve Lipitor’s US market exclusivity until 2011, The Wall Street Journal reports (subscription required).

The Lipitor patent was invalidated last year by a federal appeals court after it was challenged by Ranbaxy Labs, the Indian generic drugmaker that wants to sell a copycat version. A second patent was upheld, preserving Pfizer’s US market exclusivity for Lipitor until March 2010, the paper notes. Lipitor is the best-selling drug in the world, with $12.9 billion in sales last year. Even with competition from generic Zocor, the thought of losing sales for even one year is a potentially huge blow to Pfizer.

The drugmaker applied to the patent office in January to have the 2011 patent reissued, and to correct a “technical defect” in the original patent. A reissue would extend Pfizer’s market exclusivity through June 2011, the Journal adds. But the patent office listed on its Web site Wednesday a “non-final rejection” of Pfizer’s request for a patent reissue, though it didn’t link to any documents explaining its action. The Web site also indicates a rejection letter was mailed to Pfizer Thursday.

A Pfizer spokesman said Thursday the company is awaiting a letter from the patent office explaining the decision. Because it’s not a final rejection, Pfizer still has an opportunity to persuade the patent office to reissue the patent. “An initial rejection is not uncommon in reissue application proceedings,” says Pfizer spokesman Bryant Haskins. “Once we receive the decision we’ll review it and respond appropriately to address any issues that were raised by the examiner.”