The war of words heats up.

Cigna Corp. (CI) countered billionaire investor Carl Icahn 's criticism of its planned $60 billion acquisition of Express Scripts Holding Co. (ESRX) late Tuesday in a blunt letter to shareholders.

Cigna said Icahn "has made a speculative financial bet against the transaction in the hopes that he can create a gain at the expense of Cigna and Express Scripts shareholders."

Earlier Tuesday, Icahn released a letter to shareholders attacking the deal, first announced in March, arguing that competitive risk from Amazon (AMZN) , represents an existential threat to Express Scripts and other pharmacy benefits managers.

Cigna shareholders are scheduled to vote on the deal Aug. 24.

Jim Cramer, whose charitable trust owns shares of Cigna rival United Healthcare (UNH) , called Icahn's criticisms "quizzical." 

In its letter, Cigna said "Icahn has made clear through his recent and limited entry into Cigna stock and "substantial" short position in Express Scripts that his motives are not aligned with Cigna's shareholders and he has no interest in creating value for shareholders (other than himself)."

Here's what Cigna CEO David Cordani told TheStreet about the deal in April. 

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