Minimally invasive 13C-breath test to examine phenylalanine metabolism in children with phenylketonuria

Background: Phenylketonuria (PKU) is an autosomal recessive disorder caused by deficiency of hepatic phenylalanine hydroxylase (PAH) leading to increased levels of phenylalanine in the plasma. Phenylalanine levels and phenylalanine hydroxylase (PAH) activity monitoring are currently limited to conventional blood dot testing. 1-C-13-phenylalanine, a stable isotope can be used to examine phenylalanine metabolism, as the conversion of phenylalanine to tyrosine occurs in vivo via PAH and subsequently releases the carboxyl labeled C-13 as (CO2)-C-13 in breath. Objective: Our objective was to examine phenylalanine metabolism in children with PKU using a minimally-invasive 1-C-13-phenylalanine breath test (C-13-PBT). Design: Nine children (7 M: 2 F, mean age 12.5 +/- 2.87 y) with PKU participated in the study twice: once before and once after sapropterin supplementation. Children were provided 6 mg/kg oral dose of 1-C-13-phenylalanine and breath samples were collected at 20 min intervals for a period of 2 h. Rate of CO2 production was measured at 60 min post-oral dose using indirect calorimetry. The percentage of 1-C-13-phenylalanine exhaled as (CO2)-C-13 was measured over a 2 h period. Prior to studying children with PKU, we tested the study protocol in healthy children (n = 6; 4 M: 2 F, mean age 10.2 +/- 2.48 y) as proof of principle. Results: Production of a peak enrichment (C-max) of (CO2)-C-13 (% of dose) in all healthy children occurred at 20 min ranging from 17-29% of dose, with a subsequent return to similar to 5% by the end of 2 h. Production of (CO2)-C-13 from 1-C-13-phenylalanine in all children with PKU prior to sapropterin treatment remained low. Following sapropterin supplementation for a week, production of (CO2)-C-13 significantly increased in five children with a subsequent decline in blood phenylalanine levels, suggesting improved PAR activity. Sapropterin treatment was not effective in three children whose (CO2)-C-13 production remained unchanged, and did not show a reduction in blood phenylalanine levels and improvement in dietary phenylalanine tolerance. Conclusions: Our study shows that the C-13-PBT can be a minimally invasive, safe and reliable measure to examine phenylalanine metabolism in children with phenylketonuria. The breath data are corroborated by blood phenylalanine levels in children who had increased responses in (CO2)-C-13 production, as reviewed post-hoc from clinical charts. (C) 2015 Elsevier Inc. All rights reserved.