The patent fight over Plavix has taken on the characteristics of a pharmaceuticals industry ‘whodunnit’ mystery.

The case over the world’s second best-selling medicine highlights an aggressive generic drug industry pressing branded drugmakers with bold tactics.

It has emerged that a previously unknown inventory build-up of a generic version of Plavix spurred Bristol-Myers Squibb and Sanofi-Aventis’s negotiations with Apotex, maker of the generic version, over the threat to their blood-thinner.

BMS and Sanofi, the US and French drugmakers who market the drug in the US, are attempting to get a federal court to stop Apotex’s sales of its generic Plavix.

The companies finished a second day of hearings yesterday, in which BMS and Sanofi said Apotex was violating their patent and causing them “irreparable harm”. Apotex says it can prove the patent is invalid.

Apotex launched its generic product on August 8 after US regulators rejected a settlement that would have delayed its introduction until 2011 but before the patent expiration claimed by the two companies.

Apotex, a private Canadian drugmaker, told BMS and Sanofi in October of last year that it was stockpiling generic Plavix so that it could launch on what it said was US regulators’ “imminent” approval of the generic for sale. Food and Drug Administration approval came in January.

In a letter dated October 24, Robert Silver, Apotex’s lawyer, wrote to BMS’s lawyer, saying: “As previously advised, Apotex has always intended and continues to intend to launch its product as soon as possible after FDA approval.”

Expiry dates on generic Plavix on shelves now also confirm that Apotex was making the drug late last year. Generic Plavix stocked in an outpatient pharmacy at a university hospital in Chicago carries an expiry date of October 2, 2007.

Lawyers for the companies have asserted that the typical shelf life of generic Plavix is two years. According to them, the generic Plavix in question was made in October last year.

That inventory sheds new light on BMS and Sanofi’s thinking when negotiating terms of the failed patent settlement, as well as terms of the agreement that remained in effect after regulators blocked it.

Apotex appeared ready to launch generic Plavix “at risk” – before the two companies’ patent expired on the drug and before a court trial resolved the patent dispute. If a patent is upheld by a court, generic drugmakers launching in this way risk having to pay the branded drugmakers large amounts in damages.

Faced with a threat to a critical drug, which would harm their outlooks, BMS and Sanofi negotiated a surprise agreement that attempted to delay Apotex’s launch until 2011. They also agreed that if the deal was blocked, they would lock in provisions for damage payments due to them if they won a pending patent trial.

However, in a bizarre move they also agreed not to seek a temporary court order to stop Apotex selling generic Plavix until five days after its launch. This allowed Apotex to use its inventory to “flood the market”, BMS and Sanofi argue.

The Plavix patent settlement has had other side effects, prompting a criminal investigation and questions about BMS’s management. Apotex claims that Bristol’s top negotiator attempted to mislead the Federal Trade Commission in an effort to get approval for the settlement by offering separate secret verbal side agreements as part of the deal, which BMS strongly denies.

It also leaves Peter Dolan, chief executive of BMS, facing questions about not only the investigation but about whether more could have been done to fend off an aggressive generic industry.

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