Lunesta is the dextrorotatory isomer of a 5H-pyrrolo[3,4-b]pyrazine compound that is described in its patent (US 6444673) as being “essentially free of its levorotatory isomer”. The patent infringement suit arose when Dr Reddy's sought FDA approval to market a version of eszopiclone that contained between 0.3% and 1.0% of the levorotatory isomer. This limit was subsequently amended to “not more than 0.6%” of the levorotatory isomer when the FDA required that the original limit was tightened. This new limit has not yet been approved or denied by the FDA.
The current infringement case then arrived at the US Court of Appeals for the Federal Circuit (CAFC). Because the Lunesta patent did not precisely define the meaning of “essentially free”, the Court needed to define this meaning to establish whether Dr Reddy's product would infringe on the patent. Based on the patent prosecution history (the interactions between the patent examiner and the patent applicant), the Court established that “essentially free” equated to less than 0.25% of the levorotatory isomer. So the version of eszopiclone that Dr Reddy's is seeking FDA approval to market, of not more than 0.6% (that is, 0.0–0.6%), would overlap with the Lunesta patent.
