An appeals court yesterday upheld one of the patents for Pfizer Inc.’s bestselling Lipitor cholesterol medication.

But the U.S. Court of Appeals for the Federal Circuit in Washington also reversed a lower-court ruling that upheld a second Lipitor patent.The split decision could shorten Pfizer’s American market exclusivity period for Lipitor to end in March 2010, rather than in June 2011 as called for in the lower court ruling.

The ruling stems from Ranbaxy Laboratories Ltd.’s attempt to sell a generic version of Lipitor.NewYork-based Pfizer sued the Indian company in 2003 alleging patent infringement.The basic American patent for Lipitor expires in March 2010 while another patent expires in June 2011; both terms include expected six-month extensions for pediatric use.

Following a trial, a federal court in Delaware ruled in December that both Lipitor patents were valid and infringed by Ranbaxy’s planned generic version. The court ordered Ranbaxy not to sell generic Lipitor before June 2011, marking a huge victory for Pfizer.

Ranbaxy appealed that decision, however, leading to yesterday’s ruling. In the ruling, the appeals court upheld the basic patent that expires in 2010. But the appeals court sided with Ranbaxy in arguing that the 2011 patent was invalid.

The appeals court remanded the case to the federal court in Delaware to modify the injunction against Ranbaxy.

Lipitor generated $12.2 billion in sales last year, making it the best-selling drug in the world.

In a written statement, Pfizer said the appeals court’s ruling against the 2011 patent was “on technical grounds.”

“While we are pleased that the court has affirmed that our basic patent is valid and infringed, we are disappointed in the decision on the (2011) patent,” Pfizer’s associate general counsel, Peter Richardson, said in the statement. “The decision turned on a technical defect in the patent. There is a process for correcting such defects in the U.S. Patent and Trademark Office, and Pfizer plans to pursue that option.”