Judge Sidney Stein
In a Hatch-Waxman action to prevent production of generic versions of its OxyContin pain reliever, Purdue Pharma asserted that Actavis Elizabeth and the Ranbaxy and Mylan defendants infringed three patents. The ’799, ’800 and ’072 patents—describing methods reduction of a chemical impurity in oxycodone tablets—continued from Purdue’s ’897 patent application, which the U.S. Patent and Trademark Office rejected as obvious after prior art’s disclosure. The court allowed Actavis to amend its answer and counterclaim to assert that as a result of the Federal Circuit’s ruling in Chapman v. Casner, Purdue was collaterally estopped from asserting the ’799, ’800 and ’897 patents’ claims. Discussing Bell Atlantic v. Twombly, the court concluded that Actavis proposed a legally sufficient defense that Purdue had litigated, and lost, some of the patent claims presently at issue. Purdue’s present arguments echoed its prior presentation in the prior interference proceeding before the PTO. In addition to concluding that the presence of disputed facts and law suggested that Actavis’ defense should not be precluded as futile, the court concluded that Actavis explained its delay in seeking leave to amend, and that Purdue faced insignificant prejudice.