WHAT'S KNOWN ON THIS SUBJECT: Pharmacologic treatment with sapropterin dihydrochloride (6R-tetrahydrobiopterin; BH4) has been an effective option for some phenylketonuria patients since its approval by the US Food and Drug Administration in 2007 and the European Medicines Agency in 2008. WHAT THIS STUDY ADDS:This retrospective multicenter study revealed the long-term effects of sapropterin on metabolic control, dietary tolerance, and the outcome of BH4-responsive phenylketonuria patients harboring specific phenotypes and genotypes. It also confirmed that the minor adverse events disappeared by lowering the dose. abstract OBJECTIVE: Sapropterin dihydrochloride, the synthetic form of 6R-tetrahydrobiopterin (BH4), is an approved drug for the treatment of patients with BH4-responsive phenylketonuria (PKU). The purpose of this study was to assess genotypes and data on the long-term effects of BH4/sapropterin on metabolic control and patient-related outcomes in 6 large European countries. METHODS:A questionnaire was developed to assess phenotype, genotype, blood phenylalanine (Phe) levels, Phe tolerance, quality of life, mood changes, and adherence to diet in PKU patients from 16 medical centers. RESULTS:One hundred forty-seven patients, of whom 41.9% had mild hyperphenylalaninemia, 50.7% mild PKU, and 7.4% classic PKU, were followed up over #12 years. A total of 85 different genotypes were reported. With the exception of two splice variants, all of the most common mutations were reported to be associated with substantial residual Phe hydroxylase activity. Median Phe tolerance increased 3.9 times with BH4/sapropterin therapy, compared with dietary treatment, and median Phe blood concentrations were within the therapeutic range in all patients. Compared with diet alone, improvement in quality of life was reported in 49.6% of patients, improvement in adherence to diet was reported in 47% of patients, and improvement in adherence to treatment was reported in 63.3% of patients. No severe adverse events were reported. CONCLUSIONS: Our data document a long-term beneficial effect of orally administered BH4/sapropterin in responsive PKU patients by improving the metabolic control, increasing daily tolerance for dietary Phe intake, and for some, by improving dietary adherence and quality of life. Patient genotypes help in predicting BH4 responsiveness.

BackgroundHow to efficiently diagnose tetrahydrobiopterin (BH4) responsiveness in patients with phenylketonuria remains unclear. This study investigated the positive predictive value (PPV) of the 48-hour BH4 loading test and the additional value of genotype.MethodsData of the 48-hour BH4 loading test (20 mg BH4/kg/day) were collected at six Dutch university hospitals. Patients with ≥30% phenylalanine reduction at ≥1 time points during the 48 hours (potential responders) were invited for the BH4 extension phase, designed to establish true-positive BH4 responsiveness. This is defined as long-term ≥30% reduction in mean phenylalanine concentration and/or ≥4 g/day and/or ≥50% increase of natural protein intake. Genotype was collected if available.Results177/183 patients successfully completed the 48-hour BH4 loading test. 80/177 were potential responders and 67/80 completed the BH4 extension phase. In 58/67 true-positive BH4 responsiveness was confirmed (PPV 87%). The genotype was available for 120/177 patients. 41/44 patients with ≥1 mutation associated with long-term BH4 responsiveness showed potential BH4 responsiveness in the 48-hour test and 34/41 completed the BH4 extension phase. In 33/34 true-positive BH4 responsiveness was confirmed. 4/40 patients with two known putative null mutations were potential responders; 2/4 performed the BH4 extension phase but showed no true-positive BH4 responsiveness.ConclusionsThe 48-hour BH4 loading test in combination with a classified genotype is a good parameter in predicting true-positive BH4 responsiveness. We propose assessing genotype first, particularly in the neonatal period. Patients with two known putative null mutations can be excluded from BH4 testing.

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Exercise prevents oxidative stress in the brain of PKU mice without modifying phenylalanine levels. Hence, exercise positively affects the brain, demonstrating its value as an intervention to improve brain quality in PKU.

Objective This study aimed to investigate and improve the usefulness of the 48‐hour BH4 loading test and to assess genotype for BH4 responsiveness prediction, using the new definition of BH4 responsiveness from the European guidelines, as well as an amended definition. Method Applying the definition of the European guidelines (≥100% increase in natural protein tolerance) and an amended definition (≥100% increase in natural protein tolerance or tolerating a safe natural protein intake) to a previous dataset, we first assessed the positive predictive value (PPV) of the 48‐hour BH4 loading test using a cutoff value of 30%. Then, we tried to improve this PPV by using different cutoff values and separate time points. Last, using the BIOPKU database, we compared predicted BH4 responsiveness (according to genotype) and genotypic phenotype values (GPVs) in BH4‐responsive and BH4‐unresponsive patients. Results The PPV of the 48‐hour loading test was 50.0% using the definition of the European guidelines, and 69.4% when applying the amended definition of BH4 responsiveness. Higher cutoff values led to a higher PPV, but resulted in an increase in false‐negative tests. Parameters for genotype overlapped between BH4‐responsive and BH4‐unresponsive patients, although BH4 responsiveness was not observed in patients with a GPV below 2.4. Conclusion The 48‐hour BH4 loading test is not as useful as previously considered and cannot be improved easily, whereas genotype seems mainly helpful in excluding BH4 responsiveness. Overall, the definition of BH4 responsiveness and BH4 responsiveness testing require further attention.

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BackgroundIt is unknown whether the neonatal tetrahydrobiopterin (BH4) loading test is adequate to diagnose long-term BH4 responsiveness in PKU. Therefore we compared the predictive value of the neonatal (test I) versus the 48-h BH4 loading test (test II) and long-term BH4 responsiveness.MethodsData on test I (>1991, 20 mg/kg) at T = 8 (n = 85) and T = 24 (n = 5) were collected and compared with test II and long-term BH4 responsiveness at later age, with ≥30 % Phe decrease used as the cut-off.ResultsThe median (IQR) age at hospital diagnosis was 9 (7–11) days and the age at test II was 11.8 (6.6–13.7) years. The baseline Phe concentrations at test I were significantly higher compared to test II (1309 (834–1710) versus 514 (402–689) μmol/L, respectively, P = 0.000). 15/85 patients had a positive test I T = 8. All, except one patient who was not tested for long-term BH4 responsiveness, showed long-term BH4 responsiveness. In 20/70 patients with a negative test I T = 8, long-term BH4 responsiveness was confirmed. Of 5 patients with a test I T = 24, 1/5 was positive at both tests and showed long-term BH4 responsiveness, 2/5 had negative results at both tests and 2/5 showed a negative test I T = 24, but a positive test II with 1/2 showing long-term BH4 responsiveness.ConclusionsBoth a positive neonatal 8- and 24-h BH4 loading test are predictive for long-term BH4 responsiveness. However, a negative test does not rule out long-term BH4 responsiveness. Other alternatives to test for BH4 responsiveness at neonatal age should be investigated.