Pfizer Inc., the world’s biggest drug maker, agreed to pay as much as $700 million to buy NextWave Pharmaceuticals Inc., the closely held maker of a medicine to treat attention deficit hyperactivity disorder.

The deal includes an initial $255 million and as much as $425 million in milestone payments depending on how well NextWave’s liquid ADHD drug, Quillivant XR, sells, New York- based Pfizer said in a statement today. Pfizer had paid Cupertino, California-based NextWave $20 million in the second quarter as part of an option to buy the company.

NextWave’s formulation was approved by U.S. regulators on Oct. 1. It’s a liquid form of attention-deficit drugs such as Novartis AG’s Focalin that no longer have patent protection.

“This agreement demonstrates our focused expansion of the Established Products U.S. brands business,” Albert Bourla, president of Pfizer’s Established Products unit, said in the statement.

Pfizer has been shedding non-drug units like its animal health and nutritionals businesses as Chief Executive Officer Ian Read focuses the company on developing new prescription medicines.

[Pfizer has a number of operations in the Research Triangle Park area and North Carolina.]