A big pharmacy benefit manager is trying to keep a competitor from wedding a giant retail drug store chain.

A unit of Express Scripts Inc., St. Louis, a huge PBM, says it will be trying to nominate 4 directors to the board of the smaller PBM, Caremark Inc., Nashville, Tenn., and wooing Caremark from CVS Corp., Woonsocket, R.I.

The Express Scripts offer is worth more than $25 billion and will pay shareholders about 13% more than the CVS deal, according to Express Scripts.

But Caremark says it already has permission from federal antitrust regulators to proceed with the CVS deal and is preparing to seek a shareholder vote on that deal.

Members of the Caremark board voted unanimously Sunday to reject the Express Scripts offer, Caremark says.

The move by Express Scripts to nominate directors to the Caremark board “is yet another attempt by Express Scripts to interfere with [the] merger agreement with CVS,” Caremark says.

Caremark Chairman Mac Crawford has described Express Scripts’ actions as an illustration of “its desperation to disrupt our merger with CVS due to fear of competition from a combined CVS/Caremark.”

Combining CVS and Caremark should lead to big savings and improvements for customers, Crawford says.

Caremark hopes to complete the CVS deal by March 31, Crawford says.