Danish pharmaceutical giant Novo Nordisk is being sued by several U.S. healthcare purchasing companies that claim the drug maker wrongfully kept generic copies of its diabetes drug Prandin off the market, according to Reuters.

A spokesperson for the company told Reuters that Novo Nordisk hopes to have the case dismissed but did not provide any further comments regarding the lawsuit.

The news service reported that Novo's Prandin patent was due to expire in 2009. Four years prior to that, in 2005, a subsidiary of Indian pharmaceutical company Sun Pharmaceuticals, Caraco Pharmaceutical Laboratories, applied for approval of a generic version of the drug with a view to taking a share of the U.S. market when Prandin came off patent. That same year, in June 2005, Novo started a patent infringement lawsuit against Caraco but eventually lost.

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The companies that are suing Novo allege that the earlier lawsuit, which only closed last summer, had delayed the launch of generic versions of Prandin and they are seeking financial compensation for the losses they incurred. The healthcare purchasing companies involved in the lawsuit include American Sales Co., Rochester Drug Co-op and Cardinal.

Data quoted by Reuters showed that sales of the diabetic drug a year before it lost its exclusivity came in at $438 million.