(Reuters) - Pharmacy benefit manager Express Scripts Holding Co said Anthem Inc, its biggest customer and one that has sued the company over claims of being overcharged, was unlikely to renew its contract after it ends in 2019.

A sign at the office building of health insurer Anthem is seen in Los Angeles, California February 5, 2015. REUTERS/Gus Ruelas

Express Scripts shares tumbled more than 15 percent in extended trading on Monday.

Anthem had sued Express Scripts in March last year, accusing it of charging too much for drugs and operational failures. It also sought the right to terminate the 10-year contract.

“The company was recently told by Anthem management that Anthem intends to move its business when the company’s current contract with Anthem expires on December 31, 2019, and that Anthem is not interested in continuing discussions regarding pricing concessions,” Express Scripts said.

Anthem had also started soliciting proposals for a new service provider, in which Express Scripts was not participating, the company added.

Anthem could not be immediately reached for comment.

“While the headline that Anthem is leaving Express Scripts is in of itself not surprising, the timing is highly unexpected,” Evercore ISI analyst Ross Muken said in a client note.

Express Scripts, which is the No. 1 pharmacy benefit manager (PBM) in the United States, has been preparing for the loss of the contract or its renewal on less favorable terms, since the lawsuit began.

As of March 2016, Express Scripts began amortizing the contract over 10 years, rather than over 15 years as it had been doing, the company said in its annual filing in February.

Anthem’s contribution to Express Scripts’ revenue has been increasing, from 14 percent in 2014 to 18 percent in the latest first quarter, in line with the company’s expectation of a bigger contribution as the contract neared its end.

Besides the U.S. Department of Defense, Anthem is Express Scripts’ only client that accounts for more than 10 percent of its revenue, according to a filing.

Express Scripts said on Monday that its core PBM business, excluding Anthem, was well-positioned for future growth.

The company also reported a first-quarter profit that just beat analysts’ average estimate, while revenue fell slightly short. The company forecast a better-than-expected profit for the second quarter.

However, investors looked past the results and forecast, sending Express Scripts’ stock down to $56.97 in trading after the bell.

“The only thing investors will remember from tonight is the shocking Anthem news,” Muken said, adding that Express Scripts’ stock could trade at about $50.