Inclusion Criteria:
1. Informed consent
2. Male or female, aged 18 years and older
3. Known medical history of PK deficiency
4. PK deficiency confirmed by enzymatic assay at Screening
5. Genotypic characterization of PKR gene at Screening
6. Genotypic characterization of uridine-5'-diphosphate-glucuronyltransferase-A1 (UGTA1) gene to document underlying Gilbert's disease (Gilbert's disease patients are eligible)
7. Males Hb ≤ 12.0 g/dL, females Hb ≤ 11 g/dL
8. Transfusion independent, defined as no more than 3 units of red blood cells (RBC) transfused in 12 months prior to the first day of study dosing and no transfusions within 4 months of first day of study dosing
9. Splenectomized patients must have had the procedure at least 6 months prior to Screening and must be up-to-date in recommended vaccinations
10. Eastern Cooperative Oncology Group (ECOG) performance status ≤ 2
11. Must be taking at least 1 mg folic acid daily in the 21 days prior to screening
12. Adequate organ function defined by liver function, kidney function, platelet count and coagulation assessments
13. Agreement to use approved contraceptive measures
14. Women must not be breastfeeding
For entry into the Extension Period, patients must meet criteria # 15-16:
15. Must have completed 24 weeks of treatment during the Core Period and tolerated AG-348
16. The treating Investigator agrees that there is a potential for clinical benefit to continued treatment and recommends participation in the Extension Period and the Medical Monitor approves
Exclusion criteria
1. Hb ˃ 12.0 g/dL if male, Hb ˃11.0 g/dL if female
2. Additional diagnosis of other congenital or acquired blood disorder
3. Iron overload sufficiently severe to result in cardiac, hepatic or pancreatic insufficiency
4. Bone marrow or stem cell transplant
5. Clinically symptomatic cholelithiasis or cholecystitis
6. Currently enrolled in any other investigational trial. Participation in the PK Deficiency Natural History Study (NCT02053480) is permitted
7. Exposure to any investigational drug, device or procedure within 28 days prior to screening or during trial participation
8. Concurrent medical condition such as poorly controlled hypertension, heart failure, active infection, frequent post-splenectomy sepsis, Hepatitis B or C, Human Immunodeficiency Virus type 1 (HIV1) or Human Immunodeficiency Virus type 2 (HIV2) infection, poorly controlled diabetes mellitus, history of primary malignancy with the exception of curatively treated nonmelanomatous skin cancer, cervical cancer of breast cancer in situ
9. Major surgery in the last 6 months
10. Psychiatric disorder that could compromise the ability of the patient to cooperate with the study
11. Serum bilirubin higher to the upper limit of normal attributable to factors other than hemolysis or Gilbert's Syndrome
12. Use of restricted products known to strongly inhibit cytochrome P450 (CYP) 3A4 metabolism within 5 days prior to Prior Day 1 dosing, or to strongly induce cytochrome P450 3A4 (CYP3A4) metabolism within 28 days prior to Day 1 dosing, or to strongly inhibit P-glycoprotein transporter within 5 days prior to Day 1 dosing, or digoxin within 5 days prior to Day 1 dosing.
13. Heart-rate corrected QT interval - Fridericia's method (QTcF) interval ˃ 450 ms in male, QTcF \> 470 ms in female, with the exception of patients with a left Bundle Branch Block
14. Cardiac arrhythmias that are clinically significant or treated with drugs that are substrates of CYP3A4
15. Allergy to sulfonamides if characterized by acute hemolytic anemia, anaphylaxis, rash of erythema multiforme type or Stevens-Johnson Syndrome
16. Any other medical or psychological condition deemed by the Investigator to be likely to interfere with a patient's ability to participate in the study
17. Patients will not be permitted to enter the Extension Period if: The patient experienced AEs during the Core Period that are considered by the treating Investigator or the Sponsor's designated Medical Monitor to pose a significant safety risk to the patient if treatment were to be extended
