Eisai is celebrating the news that a US judge has stopped Teva Pharmaceutical Industries from selling a generic version of the Japanese drugmaker’s Alzheimer’s disease blockbuster Aricept.

The Israel-headquartered company is hoping to get US Food and Drug Administration approval to begin sales in the very near future but a district court in New Jersey has now ruled in Eisai's favour and imposed a preliminary injunction in its patent infringement lawsuit against Teva concerning Aricept (donepezil). The US patent on the drug, which is co-promoted with Pfizer, expires in 2010.

District Judge Harold Ackerman wrote an opinion saying that “there is no doubt that Teva has infringed Eisai's valid patent, and Eisai will likely prevail at trial in demonstrating the enforceability of that patent over Teva's inequitable conduct defence''. A trial date has not yet been set.

Eisai originally filed the infringement case in December 2005 and chief executive of its US subsidiary, Hajime Shimizu, said the firm was pleased the court has prevented the sale of Teva's generic product before the expiration of the donepezil composition of matter patent. "We will continue to actively protect our intellectual property throughout the world," he added.

The importance of Aricept to Eisai is clear. It is the firm’s best-selling product and posted revenues of $2.16 billion in sales for the year ended in March 2007. Teva received tentative approval from the FDA in January, and could receive the green light to start selling generic Aricept by the end of April.