Study to Evaluate the Effect of Voxelotor Administered Orally to Patients With Sickle Cell Disease (GBT_HOPE)

Inclusion Criteria: 1. Male or female study participants with sickle cell disease 2. Participants have had at least 1 episode of vaso-occlusive crisis (VOC) in the past 12 months. 3. Age 12 to 65 years 4. Hemoglobin (Hb) ≥5.5 and ≤10.5 g/dL during screening 5. For participants taking hydroxyurea (HU), the dose of HU (mg/kg) must be stable for at least 3 months prior to signing the ICF. Exclusion Criteria: 1. More than 10 VOCs within the past 12 months that required a hospital, emergency room or clinic visit 2. Patients who are receiving regularly scheduled blood (RBC) transfusion therapy (also termed chronic, prophylactic, or preventive transfusion) or have received a RBC transfusion for any reason within 60 days of signing the ICF 3. Hospitalized for sickle cell crisis or other vaso-occlusive event within 14 days of signing the ICF (i.e., a vaso-occlusive event cannot be within 14 days prior to signing the ICF) 4. Hepatic dysfunction characterized by alanine aminotransferase (ALT) \>4 × upper limit of normal 5. Severe renal dysfunction (estimated glomerular filtration rate at the Screening visit; calculated by the central laboratory) \<30 mL/min/1.73 m\^2 or on chronic dialysis