The influence of phenylalanine fluctuations and intake on a 24 h sapropterin responsiveness test in patients with phenylketonuria

Abstract

Patients with phenylketonuria (PKU) who retain residual phenylalanine hydroxylase (PAH) activity may benefit from sapropterin dihydrochloride (sapropterin) administration. Objective: To characterize sapropterin responsiveness in patients with PKU and investigate the impact of natural fluctuations in phenylalanine (PHE) levels and variations in PHE intake on sapropterin responsiveness. Methods: Retrospective chart review study. Patients with PKU who underwent the 24 h responsiveness test, including correction for natural PHE fluctuations, were included. Responders were defined as those who exhibited a >30% reduction in PHE levels within 8 h and/or 24 h after intake of 20 mg/kg sapropterin, correcting for the natural fluctuation in plasma PHE levels on day 1. Patients with a 28–30% reduction in PHE were considered sapropterin-responsive only if they had a concordant genotype. Results: Fifteen patients completed the test; however, three were excluded due to non-compliance. Additionally, one patient with mild PKU exhibited a borderline response, but the genotype agreement could not be assessed, rendering the test inconclusive. The rate of responsiveness could be assessed for eleven patients (six mild and five classical PKU). Among the patients with mild PKU, four were classified as responders: three at both 8 h and 24 h (reduction in plasma PHE: −75.9 ± 20.2% at 8 h and −75.7 ± 37.0% at 24 h) and one at 8 h only (reduction in plasma PHE: −28.7%). All patients with classic PKU (n = 5) were non-responders. Conclusions: A 24 h sapropterin responsiveness test incorporating correction for natural fluctuations in PHE levels identified a proportion of sapropterin-responsive patients with mild PKU similar to that described in the literature. PHE consumption should be objectively controlled during the protocol to avoid bias in determining responsiveness.