Johnson & Johnson’s drug delivery unit Alza has filed a lawsuit against Wyeth, alleging that its top-selling Effexor XR antidepressant infringes one of J&J’s patents covering controlled release formulations.

The lawsuit, which was divulged in a quarterly filing by Wyeth yesterday, adds further pressure on Effexor XR (venlafaxine), which is already facing the threat of generic competition in the USA.

Combined sales of Effexor XR - still Wyeth's most important product by a wide margin - and the immediate-release version Effexor managed a 3% gain to $918 million in the second quarter of this year. Generics company Teva has just launched a copycat version of Effexor in the USA, although the immediate-release version is a minor product compared to Effexor XR with sales of just $152 million in 2005.

Alza maintains that Effexor XR uses a controlled-release technology that is covered by a patent it was awarded in 2002 and extends out to 2019. The drug delivery firm is seeking triple damages because it alleges that Wyeth willfully contravened the patent.

In its 10Q filing, Wyeth says it “believes that this patent is not relevant to Effexor XR and is also invalid. The company will vigorously defend this lawsuit.”