A U.S. district court ruled against specialty pharmaceutical company Santarus Inc in its patent suit against Par Pharmaceuticals, sending Santarus shares tumbling 48 percent after the bell.

The suit filed by Santarus and co-plaintiff University of Missouri relates to Par's application to market a generic version of Santarus' heartburn drug, Zegerid.

The court ruled that the patents-in-suit are invalid due to obviousness. It also concluded that certain asserted claims are invalid for lack of written description.

The case is Santarus Inc et al v. Par Pharmaceutical Inc, U.S. District Court, District of Delaware. No. 07-551.