COPENHAGEN, Jan 14 (Reuters) - Denmark's Novo Nordisk has been sued by a group of healthcare purchasing companies in the United States for allegedly wrongfully keeping generic copies of its Prandin diabetes drug off the market.

A spokesman for Novo said on Tuesday the company aimed to get the case dismissed, but had no further comment.

In 2005 Caraco Pharmaceutical Laboratories, a subsidiary of Indian drug maker Sun Pharmaceutical Industries, applied to get a generic version of Prandin approved by 2009, when Novo's patent on the drug would expire.

In June 2005 Novo led a patent infringement lawsuit against Caraco, which it lost last summer.

The purchasing companies - American Sales Co, Rochester Drug Co-op and Cardinal - said the case wrongfully delayed generic competition for Prandin and claim compensation for this, Danish daily Jyllands-Posten reported, adding the size of the compensation claim was not disclosed.

Novo has in recent years reaped U.S. sales of $200 million and $250 million per year from Prandin.