Carl Icahn is urging Cigna shareholders to reject the health insurer's attempted multi-billion dollar takeover of Express Scripts, saying it's paying too much for a company with a shaky future.

The billionaire and activist investor warned that Express Scripts, a pharmacy benefit manager, could face substantial regulatory risks and intense competition from Amazon. Cigna said in March that it would pay $52 billion for Express Scripts, a deal on which shareholders will vote Aug. 24.

"Cigna is dramatically overpaying for a highly challenged Express Scripts that is facing existential risks on several fronts," Icahn said to open the letter.

The investor also warned that Express Scripts could lose business from other insurers, who might shy away from the company if a rival like Cigna starts running it.

Icahn is not a disinterested party.

He has built up a stake in Cigna, whose shares have risen almost 5 percent since the Wall Street Journal first reported Icahn's opposition to the deal last week. He is also betting that shares of Express Scripts will fall, he acknowledged Tuesday. In the same period, shares of the St. Louis pharmacy benefit manager have fallen more than 3 percent.

Cigna did not immediately return calls from The Associated press early Tuesday.

Pharmacy benefit managers, or PBMs, run prescription drug coverage for big employer and insurers, among other clients.

They've become a focal point for criticism as drug prices continue to rise, particularly for the rebates drugmakers give PBMs in order to get their products included in a formulary, or list of covered medicines. Critics say these rebates play a major role in keeping drug prices elevated, and regulators have talked about eliminating them.

Express Scripts says that the blame for rising prices rests with drugmakers, and its formularies give preferences to drugs with lower prices and better value.

Icahn noted that the government's direct challenge "to the highly flawed rebate system" along with Amazon's "almost certain" entrance into the market as a competitor make any arguments in favor of the Express Scripts deal appear weak. He said it is imperative that every Cigna shareholder vote against the deal.

"This amounts to nothing more than a huge bail-out for the Express Scripts shareholders at the expense of Cigna's," he said.

Icahn suggested that Cigna Corp., based in Bloomfield, Connecticut, instead consider multi-year partnerships with a pharmacy benefit manager, possibly even Express Scripts, while the sector deals with the challenges it's facing.

Amazon has not announced plans to create a PBM, but talk of the online giant's potential entrance into this market has created a level of uncertainty in the sector.

Amazon has formed a new venture with JPMorgan Chase and Berkshire Hathaway to figure out ways to attack rising health care costs for their U.S. employees and possibly for many more Americans. It also announced last month that it will buy the online pharmacy PillPack. Icahn noted that will compete with Express Scripts' mail-order pharmacy business.

Cigna executives have said the Express Scripts deal will give the company better financial flexibility and improve its free cash flow, among other advantages. They also say the combination will improve service and give doctors a more complete picture of patients.

Insurers are pushing to get more involved in monitoring care and sharing patient data with providers to improve health, especially for their customers with expensive chronic conditions.

Shares of both Express Scripts Holding Co. and Cigna climbed slightly Tuesday, as broader indexes also rose.

Cigna executives said last week that they were confident shareholders will approve the deal.

The insurer bolstered its credibility among those shareholders with a strong second-quarter, Leerink analyst Ana Gupte wrote Tuesday. She added that a lot of those investors also own shares in Express Scripts.

"We still expect that Icahn will find it challenging to build support among (Cigna) shareholders," she said.

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Follow Tom Murphy on Twitter: @thpmurphy