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 Who May Qualify: - 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 willing to sign a consent form - Willing and able to comply with the protocol and study procedures Who Should NOT Join This Trial: - 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 Always talk to your doctor about whether this trial is right for you.