Sanofi-Aventis, still reeling from the earlier-than-expected launch of a generic version of its Plavix product in the USA (see accompanying story), has filed a lawsuit against Sandoz alleging that the Swiss firm is infringing its patents on blockbuster antithrombotic Lovenox.

The lawsuit against Sandoz, the generics unit of Novartis, is only the latest in a string of litigation the French drugmaker has rolled out to defend Lovenox (enoxaparin), its biggest product with sales of 1.24 billion euros in the first half of this year. With number two product Plavix (clopidogrel) now facing competition in the USA, where it is sold by Bristol-Myers Squibb, the importance of defending Lovenox has become even more pronounced.

Under US legislation, the filing of a patent infringement challenge automatically puts in place a 30-month stay on the launch of a generic, unless a court rules on the matter in the interim.

Sanofi filed suit against Sandoz last Friday after the Swiss firm served notice that it had filed for approval of a generic version of the drug in the USA. The French firm is already challenging three other generic drugmakers – Teva, Momenta Pharmaceuticals and Amphastar – over the same issue.

It won a key round in its defence of the product in the USA in April, after a US court of appeal reversed an earlier decision by a Californian district court which had found in favour of Amphastar Pharmaceuticals and Teva, reinstating the validity of a key patent covering Lovenox which protects the product out to 2012.

Momenta’s challenge is somewhat different and is being fought separately. The firm says it has developed generic version of Lovenox it called M-enoxaparin, which it says is based on a technology that "provides the unique ability to thoroughly characterise the complex mixture of sugar chains that comprise Lovenox and to demonstrate that our generic product meets the FDA requirements for same active ingredients."