Toward personalized treatment in phenylketonuria: Intra-individual variation of in vivo phenylalanine oxidation using the 13C-phenylalanine breath test

With the expanding treatment landscape for phenylketonuria (PKU), there is a growing need for a reliable, non-invasive method to quantify residual phenylalanine hydroxylase (PAH) activity and to determine disease severity to guide individualized treatment strategies. The 13 C-Phenylalanine (Phe) breath test ( 13 C-PBT) has the potential to measure in vivo oxidation of Phe by quantifying conversion of 13 C-Phe to 13 CO 2 . Although the 13 C-PBT has previously been used in PKU, its test-retest reliability in PAH deficient individuals has not been studied. The aim of this study was to assess the test-retest reliability of the 13 C-PBT in individuals with PAH deficiency and to evaluate the association between 13 C-Phe oxidation parameters and disease severity. To establish preliminary reference values and to optimize protocol, 10 healthy adults underwent the 13 C-PBT once. Subsequently, 10 individuals with varying severities of PAH deficiency completed the test twice. After an overnight fast, all participants received an oral dose of 6 mg/kg 13 C-Phe. Both area under the curve (AUC) and the maximum 13 C-Phe oxidation showed a good-to-excellent test-retest reliability, expressed as an ICC of 0.93 (95% CI 0.71–0.98) and 0.91 (95% CI 0.68–0.98) respectively. When stratifying for the genotypic phenotype value (GPV), it was shown that a lower GPV was associated with lower oxidation parameters in comparison to higher GPV levels. These findings support the 13 C-PBT as a reliable, easy to perform, non-invasive measure of in vivo PAH function that could potentially aid in the interpretation of PAH variants and the assessment of treatment response. Take home message The 13 C-Phenylalanine breath test shows good-to-excellent test-retest reliability in adults with phenylalanine hydroxylase deficiency, supporting its potential role as a non-invasive measure of in vivo PAH activity that may aid individualized treatment in PKU.