Extension Study of AG-348 in Adult Participants With Pyruvate Kinase Deficiency Previously Enrolled in AG-348-006 or AG348-C-007

Inclusion Criteria: * Be willing and able to comply with study visits and procedures. * Have signed written informed consent prior to participating in this extension study. * Have completed either antecedent study AG348-C-006 or AG348-C-007 through the Part 2 Week 24 Visit. * Cohorts 2 and 3: Have demonstrated clinical benefit from mitapivat treatment in the antecedent study, in the opinion of the Investigator. * For women of reproductive potential, have a negative pregnancy test during screening of this extension study. * For women of reproductive potential as well as men with partners who are women of reproductive potential, be abstinent 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 giving informed consent, during the study, and for 28 days following the last dose of study drug for women and 90 days following the last dose of study drug for men. Exclusion Criteria: * Have a significant medical condition (including clinically significant laboratory abnormality) that developed during his/her antecedent AG- 348 study that confers an unacceptable risk to participating in this extension study, that could confound the interpretation of the study data, and/or that compromises the ability of the participant to complete study visits and procedures. * Are currently pregnant or breastfeeding. * Have a splenectomy scheduled during the study treatment period. * Meet the withdrawal criteria …