Natural History Clinical Study in Adult PKU

Key Inclusion Criteria: * Aged 18-55 years at the time of informed consent * Diagnosis of PKU due to PAH deficiency * One plasma Phe value with a concentration of ≥ 600 μmol/L drawn at Screening and at least 1 historical Phe value ≥ 600 μmol/L in the preceding 12 months Key Exclusion Criteria: * Subjects with PKU that is not due to PAH deficiency * Alanine aminotransferase (ALT) \> 1.5x upper limit of normal (ULN) and aspartate aminotransferase (AST) \>1.5x ULN * Alkaline phosphatase \> 1.5x ULN * Total bilirubin \> 1.5x ULN, direct bilirubin ≥ 1.5x ULN, unless associated with Gilbert's syndrome. * Serum creatinine \> 1.5x ULN * Hematology values outside of the normal range (hemoglobin \< 11.0 g/dL for males or \< 10.0 g/dL for females; white blood cells (WBC) \< 3,000/μL; absolute neutrophils \< 1,500/μL; platelets \< 100,000/μL) * Hemoglobin A1c \> 6.5% or fasting glucose \> 126 mg/dL * Any clinically significant abnormal laboratory result at Screening, as determined by the Investigator