Hydroxyurea and Transfusion

Participants will be started on hydroxyurea 20 ± 2.5 mg/kg/day in addition to continuing simple chronic transfusion every 4 weeks ± 1 week. Hydroxyurea will be dose escalated to achieve a HAT target dose (HAT-TD). Hydroxyurea will be increased by 5 mg/kg/day after 8 weeks of a dose if HAT-TD is not achieved (maximum daily dose 2,000 mg). After HAT-TD has been achieved, minor dose increases may occur if subject grows a significant amount to maintain the same mg/kg dose.