Inclusion Criteria: * Adult males, and females of non-childbearing potential, 18-65 years of age at the time of informed consent. * Participants must have uncontrolled classical PKU due to PAH deficiency (despite Phe-restricted dietary management or Palynziq) in the judgement of the Investigator. * Two historical plasma Phe values ≥ 600 μmol/L in the preceding 12 months while on Phe restricted diet therapy. Two plasma Phe values ≥ 600 μmol/L drawn at least 72 hours apart during the screening period while on Phe restricted diet therapy in the absence of an acute illness. * Participant has the ability and willingness to maintain their present diet for the duration of the Post-treatment Follow-up Phase (through Week 96), unless otherwise directed as per protocol * Body mass index (BMI) ≤ 35 kg/m2 * Willingness to use effective methods of contraception. * Capable of giving signed informed consent which includes compliance with the requirements and restrictions listed in the informed consent form (ICF) and in this protocol. Exclusion Criteria: * Presence of neutralizing antibodies against the AAV SNY001 capsid * Abnormal liver function laboratory testing evidenced by alanine aminotransferase (ALT)\>1.5X upper limit normal (ULN), aspartate transaminase (AST)\>1.5X ULN, alkaline phosphatase \>1.5X ULN, Total and direct bilirubin \>1.5X ULN (bilirubin levels above the laboratory's normal range are acceptable in individuals with a documented history or laboratory evidence of Gilbert's Disease) * Any significant underlying liver disease or any of the following documented diagnoses, indicative of significant underlying liver disease: * Portal hypertension; or * Splenomegaly; or * Hepatic encephalopathy * Serum albumin measurement below the lower limit of normal of the laboratory OR AST-to-Platelet Ratio Index \> 1.0 * Serum creatinine \>1.5X ULN * Hemoglobin A1c \>6.5% or fasting glucose \>126 mg/dL * Screening laboratory testing demonstrating any of the following: * HIV; or * active or prior hepatitis B virus (HBV) infection defined as positive test for hepatitis B surface antigen (HBsAg) or positive test for hepatitis B core antibody (total HBcAb) or detectable HBV DNA; or * active hepatitis C virus (HCV) infection defined as positive test for hepatitis C antibody followed by detectable HCV RNA or if a participant is presently receiving (or has received within 6 months prior to screening) anti-viral therapy for hepatitis C * Clinically significant, active bacterial, viral, fungal, or parasitic infection (based on Investigator's judgement) The above information is not intended to contain all considerations relevant to a potential participation in a clinical trial.