Omnicare CEO John Figueroa

Omnicare has filed a lawsuit in hopes of making PharMerica accept a $440 million purchase offer and halt a poison pill anti-takeover defense.

The lawsuit, filed in Delaware court, comes on the heels of PharMerica’s rejection of a $15 per share purchase offer. According to U.S. Securities and Exchange Commission filings, members of PharMerica’s board of directors violated their fiduciary duties by refusing to engage with Omnicare officials in acquisition discussions, Bloomberg News reported.

“The director defendants’ actions have made clear that they intend to protect their own individual positions even if this deprives PharMerica’s stockholders of their right to decide for themselves whether to realize an extremely valuable opportunity,” Omnicare wrote in the lawsuit.

On Aug. 23, Omnicare, the largest institutional pharmacy provider in the long-term care sector, publicly offered a $715 million bid for PharMerica, its biggest rival. Omnicare’s offer is scheduled to expire Oct. 4.

Omnicare and PharMerica shares were both up Thursday, with Omnicare closing at $29.72 and PharMerica at $14.26.