Express Scripts is a third-party drug benefit manager hired by insurers to manage patients' prescriptions. Now, Express Scripts Holding Co.'s incoming chief executive officer is trying to hold on to its biggest customer after Anthem announced that its suing ESI for $15 Billion. According to Bloomberg, the pharmacy dispute by Anthem is focused on ending its contract with Express Scripts. "We don't intend to lose Anthem. We'll get through this," says Express Scripts President Tim Wentworth.

On Monday, Anthem filed a law suit in the United States District Court for the Southern District of New York. The complaint has been brewing for about a year now and Anthem is adamant that ESI has not been paying attention to their needs. In a previous Bloomberg report, Anthem has been trying to negotiate with Express Scripts for better drug prices. The pharmacy benefits manager, ESI, has not responded or has chosen to ignore their advances. While the complaint is asking for $15 Billion from the insurer estimates, Express Scripts calls the lawsuit "without merit."

Eventually, Anthem, which is ESI's biggest customer, has had enough. On January 2015, Anthem announced that Express Scripts has "deliberately delayed the repricing process for months" and has "refused to negotiate, let alone in good faith, over Anthem's pricing proposals." Anthem Chief Executive Officer Joseph Swedish seeks their right to end the contract.

If Express Scripts loses the second largest U.S. Health Insurer, it will need to pass about $3 billion more of the savings it negotiates from the drugmakers. Bloomberg calls Anthem's complaint a "shining light" on the obscure world of managing drug benefits.

According to Garen Sarafian, a Citigroup Inc. Analyst, Anthem would have difficulty switching to another pharmacy-benefit manager and the transition would disrupt its members' care.