All participants will receive an individualized PK hydroxyurea assessment. Participants for whom the PK-process successfully generates a dose in the predicted treatment range of 15-35 mg/kg/day, will start on that personalized dose. Participants for whom the process does not generate a starting hydroxyurea dose in the predicted treatment range, due to potential pitfalls in lab draws, serum storage, sample processing, or hydroxyurea analysis, will start at a default dose of 20.0 ± 2.5 mg/kg/day. For all participants, the hydroxyurea dose will be adjusted as needed based on blood counts to establish the optimal dose. Where necessary, a weekly dosing average will be determined, so that treatment can occur solely with locally available and affordable 500mg hydroxyurea capsules.

All participants will receive an individualized PK hydroxyurea assessment. Participants for whom the PK-process successfully generates a dose in the predicted treatment range of 15-35 mg/kg/day, will start on that personalized dose. Participants for whom the process does not generate a starting hydroxyurea dose in the predicted treatment range, due to potential pitfalls in lab draws, serum storage, sample processing, or hydroxyurea analysis, will start at a default dose of 20.0 ± 2.5 mg/kg/day. For all participants, the hydroxyurea dose will be adjusted as needed based on blood counts to establish the optimal dose. Where necessary, a weekly dosing average will be determined, so that treatment can occur solely with locally available and affordable 500mg hydroxyurea capsules.

The incidence rate ratio of transfusions overall and by specific indication during the screening phase as compared to the treatment phase

The relative risk of transfusion due to the most common clinical diagnoses and laboratory factors for children with SCA on hydroxyurea treatment.

The percentage of successful PK-dosing assessments, defined as assessments completed in entirety resulting in the generation of a PK-guided starting dose.

The incidence rate ratio of clinical and laboratory adverse events among those started on the PK-guided hydroxyurea dose during the screening phase compared with the treatment phase.