Gene Therapy Clinical Study in Adult PKU

Key Inclusion Criteria: * Adults 18-55 years of age at the time of informed consent * Diagnosis of phenylketonuria (PKU) due to PAH deficiency * Two plasma Phe values with a concentration of ≥ 600 μmol/L drawn at least 72 hours apart during the screening period and at least one historical value ≥ 600 μmol/L in the preceding 24 months. * Subject has the ability and willingness to maintain their baseline diet, whether Phe-restricted or unrestricted for the duration of the trial, unless otherwise directed Key Exclusion Criteria: * Subjects with PKU that is not due to PAH deficiency * Presence of anti-AAVHSC15 neutralizing antibodies * ALT \> ULN and AST \> ULN * Alkaline phosphatase \> ULN. * Total bilirubin \> ULN, direct bilirubin \> ULN * Serum creatinine \>1.5x ULN * International normalized ratio (INR) \> 1.2 * 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 \<1500/μL; platelets \<100,000/μL) * Hemoglobin A1c \>6.5% or fasting glucose \>126 mg/dL * Any clinically significant abnormal laboratory result at screening, in the opinion of the Investigator * Contraindication to corticosteroid use or conditions that could worsen in the presence of corticosteroids, as assessed and determined by the investigator * Previously received gene therapy for the treatment of any condition.