US Patent and Trademark Office has rejected Pfizer's 893 basic patent claims covering its blockbuster cholesterol drug Lipitor. However, Pfizer said an initial rejection is not unusual in re-examination proceedings.

This initial action, which is how the Office raises its issues with the patentee, will now be followed by the company's response, in which Pfizer will address those issues raised by the examiner.

"We will respond as appropriate to the issues raised by the examiner, and believe we have compelling arguments in our favour," said Allen Waxman, general counsel, Pfizer. "We continue to believe that the basic patent was properly granted and will be upheld on re-examination. This initial action, which was not unexpected, does not change that."

The patent remains valid and enforceable throughout the re-examination proceeding, which could take as long as a few years.