UnitedHealth Group Inc.’s OptumRx agreed to buy Catamaran Corp., a provider of pharmacy benefit management services, for about $12.8 billion in the company’s largest purchase ever, to add clients and improve its technology.

UnitedHealth will pay $61.50 a share in cash, financing the acquisition with existing cash and new debt, the companies said in a statement. That’s 27 percent more than Schaumburg, Illinois-based Catamaran’s closing share price of $48.32 on Friday.

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