(Reuters) - Activist investor Carl Icahn on Tuesday urged Cigna Corp CI.N shareholders to vote against the health insurer's $52 billion purchase of Express Scripts Holding Co ESRX.O, citing regulatory hurdles and the growing threat from Amazon.

The Cigna-Express Scripts deal has drawn investor skepticism over concerns that Amazon.com Inc's AMZN.O entry into healthcare will upend the sector at a time when U.S. President Donald Trump has made a push to lower drug prices.

“Cigna is dramatically overpaying for a highly challenged Express Scripts that is facing existential risks,” Icahn wrote in an open letter about the deal.

The Trump administration has proposed a rule that would scale back protections currently in place that allow rebates between drug manufacturers and insurers and pharmacy benefits managers (PBMs).

Icahn said the rule, which was proposed after Cigna and Express Scripts agreed to merge, is a clear shot across the bow for the PBM industry, saying the industry will likely move to charging fees, instead of using rebates, over time.

The 82-year old billionaire also highlighted the rising threat of Amazon, which in June bought small online pharmacy PillPack, putting the world’s biggest online retailer in direct competition with drugstore chains, drug distributors and PBMs.

Icahn, who has spent decades battling some of America’s most widely recognized corporations, owns only 1.36 million shares, or 0.56 percent, of Cigna, and has a short position that bets Express Scripts’ stock will fall.

He is now trying to convince other investors to vote against the deal and has scheduled dozens of telephone conversations with Cigna’s top 100 shareholders to discuss the matter, according to a person familiar with the matter, who declined to name the investors who will be speaking with Icahn. The vote is scheduled for Aug. 24..

Peter Harkins, a managing director at Harkins Kovler LLC, the proxy solicitor working for Icahn, said in an interview that it helps “to own a lot of stock, but in the end it is the power of the idea that drives success in these campaigns.”

Cigna said in a statement that it strongly disagrees with Icahn. “Mr. 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).”

Express Scripts spokesman Brian Henry said in a statement, “We believe we are well positioned for growth and remain confident in the deal.”

One idea would be for Cigna to use the cash it earmarked for the merger to repurchase some of its own shares, a move that could push its share price to $250 over time, Icahn wrote. He also suggested Cigna pursue a partnership with existing pharmacy benefit managers, including Express Scripts. Cigna’s stock is currently trading around $188, having climbed more than 6 percent in the last five days.

Express Scripts’ shares rose 2.7 percent on Tuesday to close at $78.95.

Some analysts have questioned Icahn’s ability to sway other investors because there is only a limited time before the vote and few shareholders have previously questioned the strategic rationale of the deal.

Leerink analyst Ana Gupte said a majority of current shareholders are likely to vote in favor of the deal, given a large shareholder overlap between the two companies.