Express Scripts confirmed its plans to launch a proxy battle for four seats on Caremark's board, as reported by CNBC's David Faber.
The move comes after Caremark, a pharmacy benefits manager, rejected a roughly $26 billion takeover bid from Express Scripts in favor of a $22.2 bllion offer from drugstore chain CVS.
"Express Scripts will make it very, very clear it is serious," Faber said, on CNBC's "Street Signs."
Caremark has said Express Scripts' proposal included "questionable assumptions" on the calculation of synergies and faced significant antitrust risks and timing delays.
In a statement, Caremark said the Express Scripts stock and cash proposal "does not constitute, and is not reasonably likely to lead to, a superior proposal".
Caremark said the proposed all-stock deal with CVS presents limited integration risk and significant opportunities for synergies between the two companies.
A merger with CVS is expected to create more than $500 million in cost synergies through the combination of the firms' pharmacy benefits management businesses, Caremark said.
Caremark said its deal with CVS had already received antitrust clearance and the companies expected to close the transaction by the end of the first quarter of 2007.
"We are fully committed to our pending merger with CVS and believe strongly in the financial and strategic merits of the proposed combination," said Mac Crawford, chairman, president and CEO of Caremark.
Express Scripts' proposal lacks strategic rationale and creates the risk of significant customer attrition and destruction of shareholder value, Caremark said. Express Scripts' plan would also result in a "highly leveraged and weakened business" with diminished financial strength, it added.
"Our board gave careful consideration to Express Scripts' proposal," said Crawford. "In the end, our conclusion was simple and straightforward: Express Scripts' proposal is not in the best interests of Caremark, its shareholders, customers and consumers."