A US Federal Court of Appeals has said that patents held by Purdue Pharma LP on oxycodone (OxyContin) cannot be enforced because of misrepresentations to the US Patent and Trademark Office (USPTO) relating to the discovery of the painkiller.

Purdue sued Endo Pharmaceuticals for patent infringement in 2000 after Endo filed an Abbreviated New Drug Application with the US FDA to sell generic, controlled-release versions of oxycodone medications for the treatment of moderate to severe pain. Endo's oxycodone treatments were approved in 2004 but their launch was delayed because of the patent dispute.

The Federal Appeals court judged, however, that Endo's generic painkillers do not infringe patents held by Purdue because of inequitable conduct. This is an unexpected ruling; in most cases in which district courts have made findings of inequitable conduct, the Court of Appeals has either reversed or vacated those rulings.

In this case, Purdue had informed the USPTO that it had discovered an oxycodone formulation for controlling pain over a fourfold range of dosages, compared with an eightfold range for other opioids. But OxyContin's inventor, Robert F. Kaiko, admitted that he had carried out no clinical studies on the fourfold dosage range for oxycodone and had no evidence to support this claim. Purdue admitted that Kaiko's 'discovery' was made solely in his head, but claimed that it was valid even though the company was unable to prove it to be true. In his ruling, Judge Stein of the Southern District of New York wrote that “Purdue made a deliberate decision to misrepresent to the US Patent Office a 'theoretical argument' and an 'expectation' as a precisely quantified 'result' or 'discovery'.”

Endo plans to launch four strengths of generic oxycodone immediately. But Purdue intends to exercise its right to seek review of this decision by all 12 judges of the federal circuit.

Endo also intends to continue to pursue an antitrust claim and seek damages against Purdue arguing that the company has been selling OxyContin based on patents that were fraudulently obtained. Connecticut Attorney General Richard Blumenthal has launched an antitrust probe to determine whether Purdue prevented the development of less-costly generic alternatives. In a similar case, Bristol-Myers Squibb agreed to pay US$93 million to settle antitrust claims that it improperly prevented the sale of generic versions of the anticancer drug paclitaxel.