Express Scripts Holding Co.'s chief financial officer has resigned a week after the pharmacy benefit manager's top client decided to launch its own pharmacy business, Reuters reports. 

Eric Slusser

Anthem Inc. accounted for about 19% of Express Scripts' total revenue in the second quarter. The health insurer sued Express Scripts in 2016, alleging it was being overcharged by $3 billion every year. 

Eric Slusser resigned Monday after two years as CFO, naming James Havel his successor.

Slusser will stay with the company through the first quarter of 2018 to ensure a smooth transition. Havel's first day as CFO was Tuesday.

Until this week, Havel was an executive vice president at Express Scripts. He joined the company in 2015 after working as the CFO of a beverage distribution company called Major Brands Holdings. He started his career at Ernst & Young, where he worked for 34 years.

Havel holds a bachelor's degree in accounting from Regis University in Denver, according to his LinkedIn profile.