Reuters reports today that Novo Nordisk has been sued by healthcare purchasing companies in the US for the charge of wrongfully keeping generic copies of its Prandin diabetes drug off the market. A Novo spokesperson told the news service that the company aims to have the case dismissed.

Novo’s Prandin patent was set to expire in 2009. Four years earlier, in 2005, a subsidiary of Indian drug maker Sun Pharmaceuticals, Caraco Pharmaceutical Laboratories, applied for a generic version of the drug in hopes of cashing in as Prandin came off patent. Novo filed, and eventually lost, a patent infringement lawsuit against Caraco. Sales of Prandin in 2008—one year before it lost exclusivity—were $438 million.

Among those suing the Danish drugmaker now are the American Sales Co., Cardinal and Rochester Drug Co-op purchasing companies, which claim Novo wrongfully delayed generic competition.