Sanofi-Aventis has suffered a major blow in the USA with the news that a California court has invalidated the patent on its top-selling antithrombotic Lovenox, an announcement which has sent its shares down 2% in early-morning trading.

The Franco-German drugmaker issued a statement acknowledging that the court has ruled against the firm in its patent infringement lawsuit against Amphastar Pharmaceuticals and Teva Pharmaceutical Industries which are seeking approval from the US Food and Drug Administration to sell a generic version of Lovenox (enoxaparin).

Sanofi said it is “currently evaluating its options for further legal recourse” and will continue to vigorously defend its intellectual property rights. The US patent on Lovenox had been due to run until 2012 and the company added that while several firms have requested marketing approval for their “alleged to be” versions of the drug, it does not know of any go-ahead from the FDA yet.

Though no doubt a blow, industry analysts are waiting to see what action the FDA takes before factoring in the possibility of generic enoxaparin hitting the US market. David Beadle, pharmaceuticals analyst at UBS told Reuters that “to be honest, we don't think generics will get approval, because Lovenox is a quasi-biological product. "We don't have it in our forecasts, but if you did get a generic and it took out a large chunk of Lovenox US sales in a fairly short period of time, then that's worth about five to six euros on the share price."

Sanofi has had a rough old time in the courts defending Lovenox. Last August, it filed a lawsuit against Novartis’ generics unit Sandoz alleging that the Swiss firm is infringing its patents on the blockbuster. As for the legal fight with Amphastar and Teva, this has been going on since 2003 and last April, a US Court of Appeals ruling reversed a lower court decision that had invalidated the Lovenox patent. The case returned to the district court of California which has now ruled against the firm.

Lovenox is Sanofi's best-selling drug, and had sales of 583 million euros in the third quarter of 2006, an increase of 9% on the previous year. Full-year sales will be disclosed when the firm releases its results at a meeting in Paris on Monday morning.