The first trial in Teva's legal action against generic drugmakers seeking to market copycat versions of its big-selling multiple sclerosis drug Copaxone in the USA is due to start later today.

The Israeli drugmaker has filed suit against four companies - Novartis' Sandoz unit, Mylan, Momenta Pharmaceuticals and India's Natco Pharma - alleging patent infringement.  In response, the generics firms are claiming that Teva's patent for Copaxone (glatiramer acetate) are invalid and that the company has engaged in unfair conduct in trying to extend protection for the drug out to 2014.

Previously, the US District Court for the Southern District of New York declined to issue a summary judgment in the case, requiring a trial to take place.

Copaxone is far and away Teva's biggest-selling product, contributing $1.86 million to the company's coffers in the first half of 2010, more than a fifth of its total product sales in the period.

In the past, analysts have suggested that the outcome of the court case could be inconsequential, largely because there are serious questions about the ability of generics companies to prove that their generic versions will be equivalent to the branded drug.

At issue is the difficulty in proving bioequivalence, given that Copaxone is a complicated molecule, with a poorly understoof mechanism of action and no validated biomarkers for its efficacy. For those reasons it has been suggested that the US Food and Drug Administration (FDA) will require a fairly extensive clinical trials programme for any generic version.

Teva has repeatedly asked the FDA through citizens' petitions to refuse to approve any abbreviated New Drug Application for a "purported generic version" of Copaxone, given the "inability to establish acceptable ‘sameness’ of the active ingredients" in the drug. To date, all the petitions have been rejected.

A verdict in the case is not expected for several months.