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

The large U.S. pharmacy benefit manager estimated proceeds from the offering would be about $1.4 billion, or $1.6 billion if the underwriters fully exercise their over-allotment.

Express Scripts agreed in April to buy WellPoint's NextRx business for $4.68 billion to become the No. 2 U.S. pharmacy benefit manager.

Express Scripts now intends to use proceeds from the offering to pay the part of the deal that potentially would have been financed with its stock, the company said. The deal is expected to close in the late third quarter or the fourth quarter.

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

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

(Reporting by Lewis Krauskopf, editing by Maureen Bavdek)