Express Scripts Inc launched a public offering of 23 million shares on Tuesday to raise money to finance its acquisition of WellPoint Inc's drug benefits unit.

With the offering, the large U.S. pharmacy benefit manager intends to swap out the stock portion of the $4.68 billion acquisition in favor of cash.

Express Scripts estimated proceeds from the offering would be $1.4 billion, or $1.6 billion if underwriters fully exercise their over-allotment option. The 23 million shares represent more than 9 percent of the company's shares outstanding as of the end of March.

Express Scripts agreed in April to buy WellPoint's NextRx business to become the No. 2 U.S. pharmacy benefit manager. The deal is expected to close late in the third quarter or in the fourth quarter.

J.P. Morgan Securities Inc, Credit Suisse Securities LLC and Citigroup Global Markets Inc are acting as bookrunning managers for the stock offering.

Express Scripts said it also intends to offer senior notes of a benchmark size in the near term to help pay for the deal.

Express Scripts shares closed at $64.45 on Monday on the Nasdaq. The shares have risen 17 percent this year.

(Reporting by Lewis Krauskopf, editing by Maureen Bavdek and John Wallace)