(Reuters) - Pfizer Inc said it would buy privately held-NextWave Pharmaceuticals for $255 million, gaining access to the company's attention deficit hyperactivity disorder drug, the first once-daily liquid medicine approved to treat the condition in the United States.

NextWave's shareholders would also be eligible to get up to $425 million based on certain sales milestones.

Pfizer said it was exercising its option to acquire NextWave under an agreement signed in the second quarter, under which it had made an option payment of $20 million.

The drug, Quillivant XR, is expected to be available in pharmacies by early next year.

(Reporting by Zeba Siddiqui in Bangalore; Editing by Sreejiraj Eluvangal)