A Study to Provide Continued Access to Mitapivat for Participants Who Previously Completed an Agios-Sponsored Mitapivat Study

Inclusion Criteria: * Has completed an Agios-sponsored adult pyruvate kinase (PK) deficiency study of mitapivat within 1 day of enrollment in this rollover study; * Is unable to obtain mitapivat treatment due to country regulations or lack of commercial access; * Was deriving clinical benefit from mitapivat treatment at the completion of the antecedent study, in the opinion of the Investigator; * For women of childbearing potential, must be abstinent of sexual activities that may result in 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 providing informed consent, throughout the study, and for 28 days after the last dose of study drug. The second form of contraception can be an acceptable barrier method; * Written informed consent from the participant before any study-related procedures are conducted and willing to comply with all study procedures for the duration of the study; * Agrees not to participate in other interventional clinical studies, including other studies of mitapivat, during participation in this study. Exclusion Criteria: * Has discontinued treatment with mitapivat before completing the antecedent study; * Known allergy or other contraindication to mitapivat or its inactive ingredients (microcrystalline cellulose, croscarmellose sodium, sodium stearyl fumarate, mannitol, Opadry II Blue \[hypromellose, titanium dioxide, lactose monohydrate, triacetin, and FD\&C Blue #2\]); * 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 also excluded 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).