A Study to Investigate the Effect of Mitapivat on Transfusion Burden in Subjects With Sickle Cell Disease (SCD)

The primary objective of this study is to determine the effect of mitapivat versus placebo on the need for transfusions in subjects with SCD.

Inclusion Criteria: * Age ≥12 years. * Documented diagnosis of SCD (hemoglobin SS (HbSS), combined heterozygosity for hemoglobins S and C (HbSC), sickle cell hemoglobin (HbS)/β0-thalassemia, HbS/β+-thalassemia, or other sickle cell syndrome variants). * No more than 10 SCPCs in the 12 months before providing informed assent/consent. * At least 1 transfusion of packed RBCs in the 12 months before informed assent/consent. * Hb ≥5.5 and ≤10.5 g/dL. Hb concentration must be based on an average of at least 2 Hb concentration measurements (separated by ≥7 days) collected during the Screening Period. * Additional signs or symptoms of hemolysis, as evidenced by any laboratory assessment during the Screening Period with * Hb \ upper limit of normal (ULN), or * Indirect bilirubin \>ULN, or * LDH \>ULN * If taking hydroxyurea, the hydroxyurea dose must be stable for at least 90 days prior to randomization. Discontinuation of hydroxyurea requires a 90-day washout prior to informed assent/consent. * Women of childbearing potential (WOCBP) and pediatric female subjects who have attained menarche must be abstinent of sexual activities that may induce pregnancy as part of their usual lifestyle or agree to use a highly effective method of contraception from the time of providing informed assent/consent, throughout the study, and for 28 days after the last dose of study drug; if the highly effective method of contraception is hormonal contraception, then an acceptable barrier method must also be used. * Written informed assent/consent (for subjects under 18 years of age, or prior to the age at which a subject is considered legally an adult per local regulations, parental permission and child assent will be obtained) must be obtained before any study-related procedures are conducted and subjects must be willing to comply with all study procedures for the duration of the study. Exclusion Criteria: * Pregnant, breastfeeding, or parturient. * Receiving regularly scheduled RBC transfusion therapy (also termed chronic, prophylactic, or preventative transfusion); episodic transfusion in response to worsened anemia or vaso-occlusive crisis (VOC) is permitted. Additionally, a subject who requires episodic transfusion(s) may not have received a transfusion(s) within 60 days before providing informed assent/consent or during the Screening Period. * Hospitalized for an SCPC and/or other vaso-occlusive event within 14 days prior to providing informed assent/consent or during the Screening Period. A hospitalization is defined as an in-patient admission to a hospital that may or may not be preceded by an emergency room or outpatient clinic visit. A visit to an emergency room that does not result in an in-patient admission does not meet the definition of hospitalization. * Currently receiving treatment with a disease-modifying therapy for SCD (eg, voxelotor, crizanlizumab, L-glutamine), with the exception of hydroxyurea. The last dose of voxelotor, crizanlizumab, and L-glutamine must have been administered at least 90 days before randomization. * History of any malignancy except for nonmelanomatous skin cancer in situ, cervical carcinoma in situ, or breast carcinoma in situ. Subjects must not have active disease or received anticancer treatment ≤5 years before providing informed assent/consent. * History of active and uncontrolled cardiac or pulmonary disease within 6 months before randomization, including but not limited to: * New York Heart Association Class III or IV heart failure or clinically significant dysrhythmia. * Myocardial infarction or unstable angina pectoris; hemorrhagic, embolic, or thrombotic stroke; deep venous thrombosis; or pulmonary or arterial embolism. * Heart rate-corrected QT interval using Fridericia's method of ≥470 milliseconds for female subjects and ≥450 milliseconds for male subjects, except for right or left bundle branch block. * Severe pulmonary fibrosis as defined by severe hypoxia, evidence of right-sided heart failure, and radiographic pulmonary fibrosis \>50%. * Severe pulmonary hypertension as defined by severe symptoms associated with hypoxia, right heart failure, and oxygen indicated. * Hepatobiliary disorders including but not limited to: * Liver disease with histopathological evidence or clinical diagnosis of cirrhosis or severe fibrosis. * Clinically symptomatic cholelithiasis or cholecystitis (subjects with prior cholecystectomy are eligible). * History of drug-induced cholestatic hepatitis. * Aspartate aminotransferase (AST) \>2.5× ULN (unless due to hemolysis and/or hepatic iron deposition) and alanine aminotransferase (ALT) \>2.5× ULN (unless due to hepatic iron deposition). * Renal dysfunction as defined by an estimated glomerular filtration rate \