The trial will recruit patients who have decided to initiate hydroxyurea therapy. All participants will have pharmacokinetics studies performed at baseline, following a 20 mg/kg oral dose of hydroxyurea. Pharmacokinetic sampling will use a sparse sampling approach, requiring collection of blood at 3 time points (15 minutes, 60 minutes, 180 minutes) following the hydroxyurea dose. Enrolled participants will be randomized to receive either hydroxyurea using a starting dose of 20 mg/kg/day (Standard Arm) or a personalized PK-guided dose (Alternative Arm) to target an area under the curve (AUC) of 115 mg\*h/L based to approximate hydroxyurea exposure seen when patients are escalated to maximum tolerated dose (MTD).
Following randomization and selection of the initial dose, participants in both arms will follow the same procedures of laboratory medication holds for hematological toxicity. The primary endpoint is fetal hemoglobin (HbF) six months following the initiation of hydroxyurea therapy with the hypothesis that participants starting with a PK-guided dose will achieve HbF at least 5% greater than those starting with a 20 mg/kg dose. Based upon the estimated number of new hydroxyurea starts at each site, it is anticipated that it will take 24 months to enroll the 116 participants required to achieve sufficient power to assess the primary endpoint. The study will conclude for each participant 12 months following hydroxyurea initiation.