French drugmaker Sanofi-Aventis won a key skirmish in its battle to protect antithrombotic drug Lovenox from generic competition in the USA yesterday.

A court of appeal reversed an earlier decision by a Californian district court in Sanofi-Aventis’ patent infringement suit against generic drugmakers Amphastar Pharmaceuticals and Teva, reinstating the validity of a key patent covering Lovenox (enoxaparin), which is also sold as Clexane.

The long-running case, which first got underway in 2003 after Amphastar and Teva filed for approval to market copycat versions of Lovenox, will now have to return to the district court. This buys additional time for Sanofi-Aventis and, crucially, keeps the automatic 30-month stay on the launch of generic challengers in such cases in place.

The French drugmaker has always insisted that its patent protection for Lovenox extends out to 2012 in the USA. It also insists that Lovenox is a complex mixture of various forms of enoxaparin and this makes it very hard for rival firms to demonstrate that their generic candidates have the same active ingredient profile.

At present, no generic versions of enoxaparin have been granted approval by the US Food and Drug Administration (FDA).

The defense is critical to Sanofi-Aventis, as the exclusivity of both its top-selling products - Lovenox and the antiplatelet drug Plavix (clopidogrel) - are under challenge from generic companies. Lovenox is Sanofi-Aventis’ number one product, with 2005 sales of 2.1 billion euros ($2.6bn), 1.3 billion euros of which came from the US market, with Plavix coming a close second with sales in the same year of 2 billion euros.

But while the stakes are high in the case, shares in Sanofi-Aventis barely responded, rising less than half a percent in Paris to 75.90 euros in mid-morning trading today.

Last year, another generic company, Momenta Pharmaceuticals, entered the fray with a 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.”

Shares in Momenta fell 15% yesterday on news of Sanofi-Aventis’ appeals victory.