Positron Emission Tomography ( <scp>PET</scp> ) in Phenylketonuria: A Systematic Review of Brain Metabolism Beyond Phenylalanine

ABSTRACT Phenylketonuria (PKU) is associated with neurocognitive and neuropsychological symptoms despite early treatment and well‐controlled phenylalanine (Phe) levels. PKU impairs the conversion of Phe to tyrosine and causes excess phenylalanine to accumulate. This accumulation can competitively block some other amino acids from entering the brain. This may reduce production of key neurotransmitters like dopamine and serotonin, which can affect brain function and development. However, direct correlations between blood Phe levels and central nervous system (CNS) neurotransmitter concentrations have not been established, and reliable CNS biomarkers are lacking. Positron emission tomography (PET) enables in vivo assessment of brain metabolism and neurotransmitter‐related processes; therefore, we aim to investigate its potential value as a biomarker. A systematic literature search was conducted in accordance with PRISMA guidelines using MEDLINE and Embase databases up to April 25, 2026. Studies evaluating brain PET imaging in individuals with PKU or hyperphenylalaninemia were included. Data on study design, patient characteristics, PET tracers, and key findings were extracted and synthesized qualitatively. A total of 380 records were screened, and 13 studies were included, with a mean sample size of 8.8 (median 6). PET tracers included 18 fluorodeoxyglucose (FDG), amino acids, including tyrosine‐, methionine‐, leucine‐, aminocyclohexanecarboxylate‐based tracers, and fluorodopa‐related tracers. FDG PET studies demonstrated regionally heterogeneous abnormalities in brain glucose metabolism, independent of plasma Phe levels. Amino acid PET studies demonstrated reduced cerebral protein synthesis associated with impaired large neutral amino acid transport and increased brain Phe levels. Fluorodopa‐related PET studies indicated reduced dopaminergic activity, with no significant correlation between plasma Phe levels and striatal fluorodopa utilization. PET is a promising tool to bridge the gap between peripheral biochemical markers and CNS dysfunction in PKU. Further studies are needed to overcome limitations which include small sample sizes, heterogeneous metabolic control, restricted availability of tracer types, predominantly cross‐sectional study designs, and lack of correlation with clinical signs and symptoms.