Pharmacy benefits manager Express Scripts ( ESRX) signed a definitive agreement to acquire WellPoint's ( WLP) NextRx unit for $4.675 billion.

The transaction includes a 10-year contract for Express Scripts to provide services to WellPoint following the close of the deal, the companies said in a press release Monday. The transaction is expected to close in the second half of the year.

WellPoint's NextRx provide pharmacy benefits services to about 25 million Americans and manages more than 265 million adjusted prescriptions annually, the companies said. It is the fourth-largest pharmacy benefits manager in the U.S.

Express Scripts is buying the NextRx business with a mixture of cash and up to $1.4 billion in stock, according to an online report of the Wall Street Journal.

The acquisition would raise Express Scripts' prescription volumes by roughly 50%, creating a pharmacy benefits manager that fills or manages roughly 750 million prescriptions annually, according to the Journal. The deal gives Express Scripts more heft against rivals CVS Caremark ( CVS) and Medco Health Solutions ( MHS) to bid for contracts to manage employee drug benefits for major companies, the Journal adds.

"Now more than ever, as the nation focuses on health care reform, this collaboration between Express Scripts and WellPoint represents a shared commitment to achieving optimal health outcomes while driving out wasteful spending," said George Paz, chairman and CEO of Express Scripts, in a statement.

"As we apply our advanced understanding of consumer behavior to an additional 25 million members, and manage more than 750 million adjusted prescriptions annually, we will optimize the cross-selling of proven trend management tools such as generics, home delivery and specialty pharmacy. This alliance also creates the potential to leverage our behavior-centric approach across the medical benefit," Paz said.

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