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6 Months - 17 Years
Male or female participants with homozygous hemoglobin SS (HbSS) or hemoglobin S beta0 thalassemia (HbS β0thal)
Part A – 6 to 17 years of age
Part B – 12 to 17 years of age
Part C – 4 to 17 years of age
Part D – 6 months to <4 years of age Hydroxyurea (HU) therapy: Parts A, B, and C: A participant taking hydroxyurea (HU) may be enrolled if the dose has been stable for at least 3 months with no anticipated need for dose adjustment during the study and no sign of hematological toxicity. Part D: A participant taking HU may be enrolled if the dose has been stable for at least 1 month. Titration to the maximum tolerated dose (MTD) is allowed during the study. Hemoglobin (HB): Part A - No restriction Parts B, C, & D - Hb ≤ 10.5 g/dL For Part C only: Participants 12 to 17 years of age must have a TCD velocity of ≥ 140 cm/sec measured anytime during screening.
Any one of the following requiring medical attention within 14 days of signing the Informed Consent Form (ICF):
Vaso-occlusive crisis (VOC)
Acute chest syndrome (ACS)
Splenic sequestration crisis
Requires chronic transfusion therapy
History of stroke or meeting criteria for primary stroke prophylaxis (history of two TCD measurements ≥ 200 cm/sec by non-imaging TCD or ≥185 cm/sec by TCDi).
Transfusion within 30 days prior to signing the ICF
Exclusion Criteria for Part D Only:
Body weight <5 kg for 1 month prior to the screening visit and at the screening visit.