Omnicare Inc. is suing UnitedHealth Group, Inc. in federal court. It alleges the insurer defrauded the pharmaceutical company in contract negotiations involving the Medicare prescription drug plan.

In the lawsuit filed last week in U.S. District Court for the Eastern District of Kentucky, Covington, KY-based Omnicare claims that UnitedHealth conspired with PacifiCare while negotiating contracts with Omnicare. UnitedHealth bought PacifiCare in 2005.

Omnicare, the biggest providers of pharmaceuticals to nursing homes, as well as assisted living and other long-term care settings, said it made deals with both companies to act as their institutional pharmacy under the Medicare prescription drug benefit. But it allegedly ended up with less favorable terms with UnitedHealth because the insurer misrepresented its relationship with PacificCare and then shifted its Medicare business to that company.

Omnicare is seeking unspecified damages, injunctive relief and new contract terms from UnitedHealth.