Pfizer’s effort to extend the patent for its best-selling cholesterol-reducing drug Lipitor an additional 15 months into 2011 has been dealt preliminary setback.

The U.S. Patent and Trademark Office initially rejected the New York-based company’s request on Thursday, the agency’s Web Site noted.

Last year, an appeals court invalidated Pfizer’s patent on astorvastatin, the generic name for Lipitor. Pfizer also has a second patent that allows Pfizer to sell the drug in the U.S. exclusively until March 2010.

The court invalidated the former patent on a technicality, saying that the wording on the patent was inconsistent with what was covered. Pfizer is in the process of correcting the patent in its effort to make it valid until June of June 2011.

A firm which could benefit from a rejection would be India’s Ranbaxy Laboratories, which is trying to secure a 180 day period of exclusivity to market a generic version of the drug when Pfizer’s patent expires.