Optimized loading test to evaluate responsiveness to tetrahydrobiopterin (BH4) in Brazilian patients with phenylalanine hydroxylase deficiency

Abstract

INTRODUCTION:Recent studies showed that phenylalanine (Phe) plasma concentrations may decrease in some patients with hyperphenylalaninemia (HPA) due to phenylalanine hydroxylase (PAH) deficiency, after the administration of tetrahydrobiopterin (BH(4)). OBJECTIVE: To determine responsiveness to a single dose of BH(4) administered according to an innovative protocol using a combined Phe and BH(4) loading test in Brazilian phenylketonuria (PKU) patients. METHODS: Patient age should be 4 years, and median Phe plasma concentration 600 mol/L when following dietary restrictions. Participants received a simple Phe loading test using 100mg/kg L-Phe (Test 1) and a combined Phe+BH(4) loading test using 100mg/kg L-Phe and 20mg/kg/BH(4) (Test 2). Blood samples were collected at baseline and 3, 11 and 27 h after Phe ingestion (T0, T1, T2 and T3). Responsiveness was defined as: criterion A: plasma Phe reduction of 30% at T1 and T2 for Tests 1 and 2; criterion B: plasma Phe reduction of 30% at T1 and T3 for Tests 1 and 2; and criterion C: at least 30% difference of the areas under the Phe curve for Tests 1 and 2. RESULTS: Eighteen patients (median age 12 yrs; 8 classical PKU; 10 mild PKU) participated in the study. Six patients (2 classical PKU; 4 mild PKU) were classified as responsive according to at least one of the criteria. Responsiveness was concordant when criteria A + B we compared with criterion C (kappa = 0.557; p = 0.017). Of the patients whose genotype was available (n = 16), six had data about BH(4)-responsiveness genotypes described in the literature, which were in agreement with our findings. CONCLUSION: The comparison of simple Phe loading and combined Phe + BH(4) loading seems to be an optimal method to evaluate responsiveness to BH (4) Introduction: Recent studies showed that phenylalanine (Phe) plasma concentrations may decrease in some patients with hyperphenylalaninemia (HPA) due to phenylalanine hydroxylase (PAH) deficiency, after the administration of tetrahydrobiopterin (BH 4 ). Objective: To determine responsiveness to a single dose of BH 4 administered according to an innovative protocol using a combined Phe and BH 4 loading test in Brazilian phenylketonuria (PKU) patients. Methods: Patient age should be ≥4 years, and median Phe plasma concentration ≤600 μmol/L when following dietary restrictions. Participants received a simple Phe loading test using 100 mg/kg L-Phe (Test 1) and a combined Phe + BH 4 loading test using 100 mg/kg L-Phe and 20 mg/kg/BH 4 (Test 2). Blood samples were collected at baseline and 3, 11 and 27 h after Phe ingestion (T0, T1, T2 and T3). Responsiveness was defined as: criterion A: plasma Phe reduction of ≥30% at T1 and T2 for Tests 1 and 2; criterion B: plasma Phe reduction of ≥30% at T1 and T3 for Tests 1 and 2; and criterion C: at least 30% difference of the areas under the Phe curve for Tests 1 and 2. Results: Eighteen patients (median age 12 yrs; 8 classical PKU; 10 mild PKU) participated in the study. Six patients (2 classical PKU; 4 mild PKU) were classified a...

Long-term Follow-up and Outcome of Phenylketonuria Patients on Sapropterin: A Retrospective Study

“…[30][31][32] In our study, most of the centers used the 24-hour (52.9%) or 48-hour (32.6%) protocols. The test dose ranged from 5 to 20 mg/kg, with most of the centers (92.3%) using 20 mg/kg.…”

Assessment of tetrahydrobiopterin (BH4)-responsiveness and spontaneous phenylalanine reduction in a phenylalanine hydroxylase deficiency population

“…An optimized 48-hour BH 4 loading test, with two BH 4 administrations (20 mg/kg bw/day) on two consecutive days has been proposed more recently [24]. Other modified BH 4 loading test schemes have also been proposed [12,14,21,22,[24][25][26][27][28].…”

Genotype‐phenotype correlations and BH4 estimated responsiveness in patients with phenylketonuria from Rio de Janeiro, Southeast Brazil

“…Of the four genotypes involving p.V388M described by Sarkissian et al (2012) , was also reported as responsive in sapropterin clinical trials, according to Sarkissian et al (2012). Among Ibero-American patients tested by BH 4 loading tests, p.V388M appeared to be at least ambiguously responsive-of the three Spanish patients homozygous for this mutation reported by Aldamiz-Echevarria et al (2016), one was BH 4 responsive, and in Brazil, Nalin et al (2011) reported one homozygous patient for p.V388M, and he was BH 4 responsive.…”

“…(), one was BH 4 responsive, and in Brazil, Nalin et al. () reported one homozygous patient for p.V388M, and he was BH 4 responsive.…”

“…A clinically meaningful definition of BH 4 responsiveness is widely accepted as the observation of a 20%-30% reduction in blood Phe concentration (Blau, 2008;Levy, Burton, Cederbaum, & Scriver, 2007). Several protocols have been employed to demonstrate this reduction, as there is no consensus regarding the best BH 4 overload test (Giugliani et al, 2011). The sequential first clinical trials of sapropterin- Burton et al (2007), Levy, Milanowski, et al (2007) and Lee et al (2008), defined responsiveness as a ≥30% reduction in Phe blood concentration relative to baseline values at the end of 8 days, but while the first two studies used a dose of sapropterin of 10 mg kg −1 day −1 , Lee et al (2008) evaluated daily doses of 5, 10, and 20 mg/kg to demonstrate a doseresponse relationship in the reduction in basal Phe levels.…”