A bid by Pfizer Inc. to win reissuance of a U.S. patent protecting its blockbuster Lipitor cholesterol medicine has been rejected on a preliminary basis, the company said on Friday.

Should the ruling stand, Pfizer would face U.S. generic competition to Lipitor in March 2010, when the basic patent for the drug expires, rather than in June 2011, when the patent at issue would expire.

A Pfizer spokesman confirmed the company had seen a posting on the U.S. Patent and Trademark Office Web site of the non-final rejection but has not yet received a letter from the patent office informing the company of the decision.

Company spokesman Bryant Haskins said it is not unusual in such situations for a company to receive an initial rejection from the patent office only to later get the patent reissued.

This is not at all unusual, Haskins said. As soon as we receive the information from the patent office, we will address the concerns.

Pfizer's sales for Lipitor, the world's top-selling drug, reached $12.9 billion last year, including $7.8 billion in the United States.

Last August, the U.S. Court of Appeals for the Federal Circuit invalidated one of two Lipitor patents at issue in a legal fight with generic competitor Ranbaxy Laboratories Ltd., moving up the patent expiration.

Pfizer said the court had invalidated the patent because of a technical defect and it would seek to correct the defect at the U.S. Patent and Trademark Office.

(Reporting by Lewis Krauskopf)