Britain's Shire Plc has agreed to buy U.S. partner New River Pharmaceuticals for $2.6 billion to gain full control of the two firms' new drug for attention deficit hyperactivity disorder, Vyvanse.

Shire's shares jumped as much as 4.5 percent to a 5-1/2 year high of 1,123 pence on news of the deal on Tuesday. New River shares rose 8.8 percent to $63.50 before the opening in New York.

Shire, Britain's third-biggest drugmaker, expects Vyvanse to be approved by U.S. regulators later this week and thinks it can be a $1-billion-a-year successor to its top-selling ADHD drug Adderall XR, which is due to lose patent protection in 2009.

Shire said it would pay $64 a share in cash for New River and finance the deal with $2.3 billion of new debt facilities, as well as a placing of new shares to raise about $800 million.

The company later said it had placed a total of around 42.9 million new shares at 1,075p each, raising gross proceeds of around $900 million or 461 million pounds.

The new shares represented around 8.4 percent of Shire's share capital before the placing.

The price amounts to almost a 10 percent premium over New River's closing share price of $58.35 on Friday, when its shares rose 6 percent on the day. U.S. markets were closed on Monday.