Pfizer has won another critical victory in its defense of flagship drug Lipitor from generic competition after the European Patent Office upheld the validity of a patent under challenge in Spain.

The EPO ruling is the latest development in a long-running battle between Pfizer and Indian drugmaker Ranbaxy which is trying to overturn the intellectual property on Lipitor (atorvastatin), the world’s biggest-selling product with sales expected to reach $13 billion this year.

The EPO rejected Ranbaxy's position that the patent under dispute was invalid, on the grounds that it was not novel and lacked an “inventive step," said Pfizer in a statement. The court still has to rule in the case, but the EPO decision stacks the cards in Pfizer’s favour.

Ranbaxy has already tried and failed to overturn Lipitor’s patents in cases brought in several countries, including the USA, the Netherlands, the UK, and Finland. Meanwhile, Ranbaxy succeeded in convincing a court in Norway that it did not infringe two Lipitor patents, but is still blocked by the upholding of another patent that expires in 2009. Most of the cases have gone to appeal.

Meantime, Pfizer confirmed that a Spanish appeals court rejected an appeal filed by another generic challenger, Ratiopharm, in a separate patent infringement lawsuit concerning the atorvastatin calcium patent.

Pfizer markets atorvastatin as Zarator and Cardyl in Spain.