FILE PHOTO: Billionaire activist-investor Carl Icahn gives an interview on FOX Business Network's Neil Cavuto show in New York, U.S. on February 11, 2014. REUTERS/Brendan McDermid/File Photo

(Reuters) - Investor Carl Icahn will send an open letter to the shareholders of insurer Cigna Corp CI.N asking them to vote against the company's $52 billion Express Scripts Holding Co ESRX.O buyout, the Wall Street Journal reported on Monday, citing a draft of the letter.

Icahn and his affiliates own about 0.56 percent of Cigna, which is worth over $250 million as of Monday's market close, the Journal reported, citing people familiar with the matter. on.wsj.com/2voopWB

“Cigna is dramatically overpaying for a highly challenged Express Scripts that is facing existential risks on several fronts,” said the letter, which the newspaper reported would be made public on Tuesday.

Express Scripts spokesman Brian Henry told Reuters on Monday that the company remained confident in the deal. Cigna and a representative for Icahn did not respond to a request for comment outside regular business hours.

Investors have expressed concern over the Cigna-Express Scripts deal because of the growing presence of Amazon.com AMZN.O in the healthcare sector and U.S. President Donald Trump's push to cut rebates from drugmakers to pharmacy benefit managers.