Bristol-Myers Squibb has filed for a preliminary injunction to block the distribution of Apotex’ generic version of its blockbuster medicine Plavix, as expected.

The US drug major had to wait five days before it could file its motion to block Apotex’ generic - under the terms of their earlier agreement in which Apotex agreed not to launch its copycat version.

That deal was scotched by US lawmakers last week, forcing B-MS and its partner Sanofi-Aventis – which sells the drug in Europe – to return to the courts in an effort to protect Plavix (clopidogrel bisulfate).

The lawsuit, filed in Manhattan, is seeking an immediate block on the sale of Apotex’ generic, as well as a recall of product that has already been shipped. Analysts said B-MS could already have suffered significant damage to its revenues after a major effort by the generic manufacturer to ship supplies of its product to US distributors.

In a statement B-MS said the court has scheduled a hearing on the motion on Friday, and the firm reiterated its position that it has patent protection for Plavix out to 2011.

Apotex’ generic has been launched at a discount to Plavix, with a list price of $124 for 30 tablets versus $148 for the brandname product.

Ironically, the agreement between the three companies accelerated the generic company's launch plans for Plavix. In addition to allowing Apotex five days’ grace after launching its generic before litigation could resume, the deal capped any potential liability at 70% of net sales, and waived B-MS and Sanofi-Aventis’ right to seek treble damages if they prevailed in patent litigation.

And while the states attorneys general rejected the agreement, the US Department of Justice has also initiated a criminal investigation into the deal.

Plavix sales in the USA are currently running at around $2.7 billion a year, or around 20% of B-MS revenues, and the drug also contributes a third of its profits.

All now hinges on the initial ruling on the injunction. If this is rejected then B-MS will have to weather the impact of generic competition to Plavix while the patent infringement case goes through the courts, something that could take months. If it is granted the US drugmaker would see its exclusive position in the market restored.

However, opinions are divided on the strength of B-MS and Sanofi-Aventis case, with some analysts, including Catherine Arnold of Credit Suisse, suggesting they have a good chance of prevailing in the courts. Others are concerned that the concessions made in the Apotex agreement suggest their case is not as strong as once thought.