Effect of Large Neutral Amino Acids in Adults With Classical Phenylketonuria

Patients ≥ 18 years of age with Classical PKU molecularly confirmed via the finding of two pathogenic variants in the phenylalanine hydroxylase (PAH) gene and/or historical evidence of Phe concentrations ≥1200 μmol/L in the medical history Inclusion Criteria: * Treatment initiation within the first month of life * Intelligence quotient over 84, based upon the baseline neuropsychological evaluation * Conventional dietary treatment up to minimum 15 years of age * Signed informed consent * Willing and able to comply with the protocol and study procedures Exclusion Criteria: * Unable or unwilling to adhere to the requirements of the study * A female who is pregnant or breastfeeding or planning to get pregnant during the study period * Concomitant medication that may interfere with the PET analysis, as judged by the investigator * A serious neuropsychiatric disease that could interfere with the subject's ability to participate in the study at the discretion of the investigator * Concomitant treatment with BH4 supplementation (sapropterin) or Pegvaliase-pqpz (PALYNZIQ) * Failing to submit at least one blood Phe home sample during the year before study initiation * Standard MRI contraindications * Body weight over 110 kg