The company said the U.S. District Court for the District of Delaware has ruled that Aventis' patent on the branded drug, which is indicated for the treatment of amyotrophic lateral sclerosis, or Lou Gehrig's disease, is not invalid on the grounds of anticipation of prior art.

The cases had been remanded back to the Delaware district court last year by an appeals court, after the Delaware court had originally ruled the patent was invalid on the grounds of anticipation, meaning that the patent's claims were already described in an earlier patent.

Impax gained U.S approval of its generic version of Rilutek 50-mg tablets in January 2003 but has been unable to sell the product since that time, after Aventis won an injunction barring the generic product's sale.

As such, Impax said the latest legal ruling on the Rilutek patent should not materially impact the company's financial results.

Impax said U.S. sales of Rilutek totaled $39 million in the 12 months ending May 31, based on data from Wolters Kluwer Health.