A Study to Evaluate the Efficacy and Safety of Mitapivat in Pediatric Participants With Pyruvate Kinase Deficiency (PKD) Who Are Regularly Transfused, Followed by a 5-Year Extension Period

Inclusion Criteria: * Written informed consent from the participant, or the participant's legally authorized representative, parent(s), or legal guardian, and the participant's assent, where applicable (informed consent/assent) must be obtained before any study-related procedures are conducted, and participants must be willing to comply with all study procedures for the duration of the study; * Aged 1 to \<18 years. Participants between 12 and 24 months of age must weigh a minimum of 7 kilograms (kg); * Clinical laboratory confirmation of pyruvate kinase deficiency (PKD), defined as documented presence of at least 2 mutant alleles in the pyruvate kinase L/R (PKLR) gene, of which at least 1 is a missense mutation, as determined per the genotyping performed by the study central genotyping laboratory; * Six to 26 transfusion episodes in the 52-week period before providing informed consent/assent; * Have complete records of transfusion history for the 52 weeks before providing informed consent/assent, defined as having all the following available: (1) all the transfusion dates, (2) the RBC transfusion volume (milliliters and/or number of units) for all the transfusions, and (3) hemoglobin concentrations within 1 week before transfusion for at least 80% of the transfusions; * Receiving folic acid supplementation as part of routine clinical care for at least 21 days before administration of the first dose of study drug, to be continued during study participation; * Female participants who have attained menarche and/or breast development in Tanner Stage 2 must be abstinent of sexual activities that may induce pregnancy as part of their usual lifestyle, or agree to use 2 forms of contraception, 1 of which must be considered highly effective, from the time of informed consent/assent, throughout the study, and for 28 days after the last dose of study drug (including the time required to dose taper). The second form of contraception can include an acceptable barrier method. Exclusion Criteria: * Pregnant or breastfeeding; * Homozygous for the R479H mutation or have 2 nonmissense mutations, without the presence of another missense mutation, in the PKLR gene as determined per the genotyping performed by the study central genotyping laboratory; * History of malignancy; * History of active and/or uncontrolled cardiac or pulmonary disease or clinically relevant QT prolongation within 6 months before providing informed consent/assent; * Hepatobiliary disorders including, but not limited to: * Liver disease with histopathological evidence of cirrhosis or severe fibrosis; * Clinically symptomatic cholelithiasis or cholecystitis (participants with prior cholecystectomy are eligible); * History of drug-induced cholestatic hepatitis; * Aspartate aminotransferase \>2.5×upper limit of normal (ULN) (unless due to hemolysis and/or hepatic iron deposition) and alanine aminotransferase \>2.5×ULN (unless due to hepatic iron deposition); * Renal dysfunction as defined by an estimated glomerular filtration rate \<60 milliliters per minute (mL/min)/1.73 m\^2; * Nonfasting triglycerides \>440 milligrams per deciliter (mg/dL) (5 millimoles per liter \[mmol/L\]); * Active uncontrolled infection requiring systemic antimicrobial therapy; * Participants with known active hepatitis B or hepatitis C virus infection; * Participants with known human immunodeficiency virus (HIV) infection; * History of major surgery (including splenectomy) ≤6 months before providing informed consent/assent and/or planning on undergoing a major surgical procedure during the screening or double-blind period; * Current enrollment or past participation (within 90 days before the first dose of study drug or a time frame equivalent to 5 half-lives of the investigational study drug, whichever is longer) in any other clinical study involving an investigational study drug or device; * Prior exposure to gene therapy, or bone marrow or stem cell transplantation; * Currently receiving hematopoietic stimulating agents; the last dose must have been administered at least 28 days or a time frame equivalent to 5 half-lives (whichever is longer) before randomization; * Receiving products that are strong inhibitors of CYP3A4/5 that have not been stopped for ≥5 days or a time frame equivalent to 5 half-lives (whichever is longer), or strong inducers of CYP3A4 that have not been stopped for ≥28 days or a time frame equivalent to 5 half-lives (whichever is longer), before randomization; * Receiving anabolic steroids, including testosterone preparations, that have not been stopped for at least 28 days before randomization; * Known allergy, or other contraindication, to mitapivat or its excipients (microcrystalline cellulose, croscarmellose sodium, sodium stearyl fumarate, mannitol, Opadry® II Blue \[hypromellose, titanium dioxide, lactose monohydrate, triacetin, and Food, Drug, and Cosmetics blue dye number 2 (FD\&C Blue #2)\], Opadry® II White \[hypromellose, titanium dioxide, lactose monohydrate, and triacetin\], and magnesium stearate); * Any medical, hematologic, psychological, or behavioral condition(s) or prior or current therapy that, in the opinion of the Investigator, may confer an unacceptable risk to participating in the study and/or could confound the interpretation of the study data; also included are: * Participants who are institutionalized by regulatory or court order. * Participants with any condition(s) that could create undue influence (including but not limited to incarceration, involuntary psychiatric confinement, and financial or familial affiliation with the Investigator or Sponsor). * Receiving a pyruvate kinase activator that has not been stopped for ≥52 weeks before providing informed consent/assent.