Assessment of tetrahydrobiopterin (BH4) responsiveness in phenylketonuria

Dysregulation of tetrahydrobiopterin metabolism in myalgic encephalomyelitis/chronic fatigue syndrome by pentose phosphate pathway

Background

Tetrahydrobiopterin (BH4) and its oxidized derivative dihydrobiopterin (BH2) were found to be strongly elevated in ME/CFS patients with orthostatic intolerance (ME + OI).

Objective

However, the molecular mechanism of biopterin biogenesis is poorly understood in ME + OI subjects. Here, we report that the activation of the non-oxidative pentose phosphate pathway (PPP) plays a critical role in the biogenesis of biopterins (BH4 and BH2) in ME + OI subjects.

Research Design and Results

Microarray-based gene screening followed by real-time PCR-based validation, ELISA assay, and finally enzyme kinetic studies of glucose-6-phosphate dehydrogenase (G6PDH), transaldolase (TALDO1), and transketolase (TK) enzymes revealed that the augmentation of anaerobic PPP is critical in the regulations of biopterins. To further investigate, we devised a novel cell culture strategy to induce non-oxidative PPP by treating human microglial cells with ribose-5-phosphate (R5P) under a hypoxic condition of 85%N2/10%CO2/5%O2 followed by the analysis of biopterin metabolism via ELISA, immunoblot, and dual immunocytochemical analyses. Moreover, the siRNA knocking down of the taldo1 gene strongly inhibited the bioavailability of phosphoribosyl pyrophosphate (PRPP), reduced the expressions of purine biosynthetic enzymes, attenuated GTP cyclohydrolase 1 (GTPCH1), and suppressed subsequent production of BH4 and its metabolic conversion to BH2 in R5P-treated and hypoxia-induced C20 human microglia cells. These results confirmed that the activation of non-oxidative PPP is indeed required for the upregulation of both BH4 and BH2 via the purine biosynthetic pathway. To test the functional role of ME + OI plasma-derived biopterins, exogenously added plasma samples of ME + OI plasma with high BH4 upregulated inducible nitric oxide synthase (iNOS) and nitric oxide (NO) in human microglial cells indicating that the non-oxidative PPP-induced-biopterins could stimulate inflammatory response in ME + OI patients.

Conclusion

Taken together, our current research highlights that the induction of non-oxidative PPP regulates the biogenesis of biopterins contributing to ME/CFS pathogenesis.

Effect of a four-week oral Phe administration on neural activation and cerebral blood flow in adults with early-treated phenylketonuria

BACKGROUND

Phenylketonuria (PKU) is a rare inborn error of metabolism characterized by impaired catabolism of the amino acid phenylalanine (Phe) into tyrosine. Cross-sectional studies suggest slight alterations in cognitive performance and neural activation in adults with early-treated PKU. The influence of high Phe levels on brain function in adulthood, however, remains insufficiently studied. Therefore, we aimed to explore the effect of a four-week period of oral Phe administration - simulating a controlled discontinuation of Phe restriction and raising Phe to an off-diet scenario - on working memory-related neural activation and cerebral blood flow (CBF).

METHODS

We conducted a randomized, placebo-controlled, double-blind, crossover, non-inferiority trial to assess the effect of a high Phe load on working memory-related neural activation and CBF in early-treated adults with classical PKU. Twenty-seven patients with early-treated classical PKU were included and underwent functional magnetic resonance imaging (fMRI) of the working memory network and arterial spin labeling (ASL) MRI to assess CBF before and after a four-week intervention with Phe and placebo. At each of the four study visits, fMRI working memory task performance (reaction time and accuracy) and plasma Phe, tyrosine, and tryptophan levels were obtained. Additionally, cerebral Phe was determined by 1H-MR spectroscopy.

RESULTS

Plasma Phe and cerebral Phe were significantly increased after the Phe intervention. However, no significant effect of Phe compared to placebo was found on neural activation and CBF. Regarding fMRI task performance, a significant impact of the Phe intervention on 1-back reaction time was observed with slower reaction times following the Phe intervention, whereas 3-back reaction time and accuracy did not differ following the Phe intervention compared to the placebo intervention.

CONCLUSION

Results from this present trial simulating a four-week discontinuation of the Phe-restricted diet showed that a high Phe load did not uniformly affect neural markers and cognition in a statistically significant manner. These results further contribute to the discussion on safe Phe levels during adulthood and suggest that a four-week discontinuation of Phe-restricted diet does not demonstrate significant changes in brain function.

Emerging biosensors in Phenylketonuria

Phenylketonuria (PKU) is an autosomal recessive metabolic disorder resulting from deficient phenylalanine hydroxylase (PAH) enzyme activity, leading to impaired phenylalanine (Phe) metabolism. This condition can lead to intellectual disability, epilepsy, and behavioural issues. Treatment typically involves strict dietary restrictions on natural protein intake, supplemented with chemically manufactured protein substitutes containing amino acids other than Phe. Various approaches, including casein glycomacropeptide (GMP), tetrahydrobiopterin (BH4), phenylalanine ammonia-lyase (PAL) therapy, large neutral amino acid (LNAA) supplementation, enzyme therapy, gene therapy, and medical therapies, aim to prevent Phe transport in the brain to potentially treat PKU. Although newborn screening programs and early dietary interventions have enhanced outcomes of the potential treatment strategies, limitations still persist in this direction. These involve potent accuracy concerns in diagnosis due to the existence of antibiotics in blood of PKU patients, affecting growth of the bacteria in the bacterial inhibition assay. Monitoring involves complex methods for instance, mass spectrometry and high-pressure liquid chromatography, which involve shortcomings such as lengthy protocols and the need for specialized equipment. To address these limitations, adaptable testing formats like bio/nano sensors are emerging with their cost-effectiveness, biodegradability, and rapid, accurate, and sensitive detection capabilities, offering promising alternatives for PKU diagnosis. This review provides insights into current treatment and diagnostic approaches, emphasizing on the potential applications of the diverse sensors intended for PKU diagnosis.

Current Advances and Material Innovations in the Search for Novel Treatments of Phenylketonuria

Phenylketonuria (PKU) is a genetically inherited disease caused by a mutation of the gene encoding phenylalanine hydroxylase (PAH) and is the most common inborn error of amino acid metabolism. A deficiency of PAH leads to increased blood and brain levels of phenylalanine (Phe), which may cause permanent neurocognitive symptoms and developmental delays if untreated. Current management strategies for PKU consist of early detection through neonatal screening and implementation of a restrictive diet with minimal amounts of natural protein in combination with Phe-free supplements and low-protein foods to meet nutritional requirements. For milder forms of PKU, oral treatment with synthetic sapropterin (BH4), the cofactor of PAH, may improve metabolic control of Phe and allow for more natural protein to be included in the patient's diet. For more severe forms, daily injections of pegvaliase, a PEGylated variant of phenylalanine ammonia-lyase (PAL), may allow for normalization of blood Phe levels. However, the latter treatment has considerable drawbacks, notably a strong immunogenicity of the exogenous enzyme and the attached polymeric chains. Research for novel therapies of PKU makes use of innovative materials for drug delivery and state-of-the-art protein engineering techniques to develop treatments which are safer, more effective, and potentially permanent.

The treatment of biochemical genetic diseases: From substrate reduction to nucleic acid therapies

Newborn screening (NBS) began a revolution in the management of biochemical genetic diseases, greatly increasing the number of patients for whom dietary therapy would be beneficial in preventing complications in phenylketonuria as well as in a few similar disorders. The advent of next generation sequencing and expansion of NBS have markedly increased the number of biochemical genetic diseases as well as the number of patients identified each year. With the avalanche of new and proposed therapies, a second wave of options for the treatment of biochemical genetic disorders has emerged. These therapies range from simple substrate reduction to enzyme replacement, and now ex vivo gene therapy with autologous cell transplantation. In some instances, it may be optimal to introduce nucleic acid therapy during the prenatal period to avoid fetopathy. However, as with any new therapy, complications may occur. It is important for physicians and other caregivers, along with ethicists, to determine what new therapies might be beneficial to the patient, and which therapies have to be avoided for those individuals who have less severe problems and for which standard treatments are available. The purpose of this review is to discuss the "Standard" treatment plans that have been in place for many years and to identify the newest and upcoming therapies, to assist the physician and other healthcare workers in making the right decisions regarding the initiation of both the "Standard" and new therapies. We have utilized several diseases to illustrate the applications of these different modalities and discussed for which disorders they may be suitable. The future is bright, but optimal care of the patient, including and especially the newborn infant, requires a deep knowledge of the disease process and careful consideration of the necessary treatment plan, not just based on the different genetic defects but also with regards to different variants within a gene itself.

Health-related quality of life in a european sample of adults with early-treated classical PKU

BACKGROUND

Phenylketonuria (PKU) is a rare inborn error of metabolism affecting the catabolism of phenylalanine (Phe). To date, findings regarding health-related quality of life (HRQoL) in adults with early-treated classical PKU are discrepant. Moreover, little is known about metabolic, demographic, and cognitive factors associated with HRQoL. Hence, we aimed to investigate HRQoL and its association with demographic, metabolic, and cognitive characteristics in a large European sample of adults with early-treated classical PKU.

RESULTS

This cross-sectional study included 124 adults with early-treated classical PKU from Hungary, Italy, Spain, Switzerland, and Turkey. All participants prospectively completed the PKU quality of life questionnaire (PKU-QoL), a questionnaire specifically designed to evaluate the impact of PKU and its treatment on HRQoL in individuals with PKU. In addition, information about Phe levels (concurrent and past year), demographic (age and sex), and cognitive variables (intelligence quotient, IQ) were collected. Most domains revealed little or no impact of PKU on HRQoL and more than three-quarters of the patients rated their health status as good, very good, or excellent. Nevertheless, some areas of concern for patients were identified. Patients were worried about the guilt that they experience if they do not adhere to the dietary protein restriction and they were most concerned about high Phe levels during pregnancy. Further, tiredness was the most affected symptom, and the supplements' taste was considered a main issue for individuals with PKU. The overall impact of PKU on HRQoL was higher in women (U = 1315.5, p = .012) and in adults with a lower IQ (rs = - 0.448, p = .005). The overall impact of dietary protein restriction was higher in adults with higher concurrent Phe levels (rs = 0.272, p = .007) and higher Phe levels during the past year (rs = 0.280, p = .009).

CONCLUSION

The impact of PKU on most domains assessed in the PKU-QoL was considered to be low. These results likely reflect the successful implementation of the newborn screening resulting in the prevention of severe adverse long-term outcomes. However, a particular clinical focus should be given to patients with lower IQ, higher Phe levels, and women, as these variables were associated with a lower HRQoL.

Oral administration of phenylalanine molecularly imprinted polymer (MIP) benefits PKU mouse model

Phenylketonuria (PKU) is a rare genetic disorder caused by a defect in the metabolism of phenylalanine (Phe). Currently, the most commonly used treatment for PKU is dietary Phe restriction. Problems associated with Phe restricted diets include lack of universal availability, high treatment costs, and reduced adherence to continued treatment with age and finally the development of psychological and neurological problems in a significant proportion of patients despite early start of treatment. One possible approach to decreasing blood Phe level, is inhibition of GI tract absorption of this amino acid. We had previously shown that a Phe selective molecularly imprinted polymer was able to bind Phe in the GI tract and attenuate its plasma concentration. In this work, we used different orally administered Phe selective molecularly imprinted polymer doses in a PKU mouse model to further study the effects of this treatment on biochemical profile and cognitive function in test animals. Treatments started 21 days postnatally. After 3 weeks, brain and plasma amino acid profiles and brain monoaminergic neurotransmitter concentrations were measured. Behavioral profile was also evaluated. Treatment with 2% and 5% Phe selective molecularly imprinted polymer significantly reduced levels of blood Phe in PKU model animals (46% and 48% respectively) meanwhile levels of other amino acids remained unchanged. Brain dopamine concentrations in hippocampus was effectively restored by supplementation of Phe selective molecularly imprinted polymer. Finally, polymer treatment improved locomotor dysfunction in PKU model animals. Our data suggest that the Phe selective molecularly imprinted polymer can be a new candidate for treatment of PKU patients. Take home message: Orally administered Phenylalanine Selective Molecularly Imprinted Polymer is able to inhibit absorption of phenylalanine from the GI tract and may offer a new treatment, in conjunction with dietary restriction, for PKU patients.

Spectrum of PAH gene mutations and genotype-phenotype correlation in patients with phenylalanine hydroxylase deficiency from Turkey

OBJECTIVES

The aim of our study was to define the genotype-phenotype correlations of mutations in the PAH gene among the Turkey's Central Anatolian region.

METHODS

Demographic characteristics of 108 patients with hyperphenylalaninemia (HPA) and 94 patients whose diagnosis was confirmed by PAH gene analysis (Sanger DNA Sequence Analysis and Next-Generation Sequencing) were determined retrospectively. Blood phenylalanine levels were analyzed using the high-performance liquid chromatography method.

RESULTS

Mild HPA-not-requiring-treatment (NT) was found in 50.9% of the patients, and a classical phenylketonuria (PKU) phenotype was found in 25.9%. Forty-seven types of variants were identified. The predominant variants were p.Ala403Val (9.9%), p.Ala300Ser (9.4%), and c.1066-11G>A (splicing) (9.4%). Missense mutations accounted for 68% of mutations and attenuated the clinical impact; splice variations were found in 14.8% of cases with severe features. The p.Thr380Met allele was specific to the mild HPA-NT group. The c.1066-11G>A (splicing) allele was associated with classical PKU, whereas the p.Arg408Trp allele was linked to severe symptoms. Three variations of unknown clinical significance were discovered: c.706+4A>T (splicing), c.843-5T>C (splicing), and p.Thr323=. Of these variants, the patient who was homozygous for the c.843-5T>C (splicing) allele related to the classical PKU phenotype. 70% of the patients who underwent tetrahydrobiopterin (BH4) test were responsive. Phenotypes that responded to BH4 treatment were mostly mild phenotypes.

CONCLUSIONS

The PAH genotype is the main factor that determines the phenotype of PKU. Establishing the relationship between the identified genetic mutations and phenotypic characteristics will provide very important data for each patient in terms of the specific management style.

PTC923 (sepiapterin) lowers elevated blood phenylalanine in subjects with phenylketonuria: a phase 2 randomized, multi-center, three-period crossover, open-label, active controlled, all-comers study

BACKGROUND & AIM

PTC923 (formerly CNSA-001), an oral formulation of sepiapterin, a natural precursor of intracellular tetrahydrobiopterin (BH4), has been shown in humans to induce larger increases in circulating BH4 vs. sapropterin dihydrochloride. Sapropterin reduces blood phenylalanine (Phe) by ≥20-30% in a minority of subjects with PKU. This was a Phase 2 randomized, multicenter, three-period crossover, open-label, active controlled, all-comers [regardless of phenylalanine hydroxylase (PAH) variants] comparison of PTC923 60 mg/kg, PTC923 20 mg/kg and sapropterin 20 mg/kg in 24 adults with phenylketonuria (PKU) and hyperphenylalaninemia.

METHODS

Eligible subjects were adult men or women (18-60 y) with PKU. Subjects enrolled received 7 days of once-daily oral treatment with PTC923 20 mg/kg/day, PTC923 60 mg/kg/day and sapropterin dihydrochloride 20 mg/kg/day each in a random order. Treatments were separated by a 7-day washout. Subjects maintained their usual pre-study diet, including consumption of amino acid mixtures. Blood Phe was measured on Day 1 (predose baseline), Day 3, Day 5, and Day 7 of each treatment period.

RESULTS

Least squares mean changes (SE) from baseline in blood Phe were: -206.4 (41.8) μmol/L for PTC923 60 mg/kg (p < 0.0001); -146.9 (41.8) μmol/L for PTC923 20 mg/kg (p = 0.0010); and - 91.5 (41.7) μmol/L for sapropterin (p = 0.0339). Effects of PTC923 60 mg/kg on blood Phe vs. sapropterin were significantly larger (p = 0.0098) and faster in onset with a significantly larger mean reduction in blood Phe at day 3 of treatment, p = 0.0135 (20 mg/kg) and p = 0.0007 (60 mg/kg). Only PTC923 60 mg/kg reduced blood Phe in classical PKU subjects (n = 11, p = 0.0287). The mean blood Phe reduction (PTC923 60 mg/kg) in a cofactor responder analysis (n = 8; baseline Phe ≥300 μmol/L and blood Phe reduction ≥30%) was -463.3 μmol/L (SE 51.5) from baseline. Adverse events were mostly mild to moderate, transient, and similar across treatment groups with no serious adverse events or discontinuations.

CONCLUSIONS

The substantially significantly better effect of PTC923 60 mg/kg on blood Phe reduction vs. sapropterin supports further clinical development of PTC923 for PKU; ANZCTR number, ACTRN12618001031257.

Newborn Screening and Treatment of Phenylketonuria: Projected Health Outcomes and Cost-Effectiveness

The objective of this study was to evaluate the cost-effectiveness of newborn screening and treatment for phenylketonuria (PKU) in the context of new data on adherence to recommended diet treatment and a newly available drug treatment (sapropterin dihydrochloride). A computer simulation model was developed to project outcomes for a hypothetical cohort of newborns with PKU. Four strategies were compared: (1) clinical identification (CI) with diet treatment; (2) newborn screening (NBS) with diet treatment; (3) CI with diet and medication (sapropterin dihydrochloride); and (4) NBS with diet and medication. Data sources included published literature, primary data, and expert opinion. From a societal perspective, newborn screening with diet treatment had an incremental cost-effectiveness ratio of $6400/QALY compared to clinical identification with diet treatment. Adding medication to NBS with diet treatment resulted in an incremental cost-effectiveness ratio of more than $16,000,000/QALY. Uncertainty analyses did not substantially alter the cost-effectiveness results. Newborn screening for PKU with diet treatment yields a cost-effectiveness ratio lower than many other recommended childhood prevention programs even if adherence is lower than previously assumed. Adding medication yields cost-effectiveness results unlikely to be considered favorable. Future research should consider conditions under which sapropterin dihydrochloride would be more economically attractive.

Guide for diagnosis and treatment of hyperphenylalaninemia

IMPORTANCE

Sapropterin hydrochloride, a natural coenzyme (6R-tetrahydrobiopterin) of phenylalanine hydroxylase, was first approved as a treatment for tetrahydrobiopterin deficiency in 1992 in Japan, and was then approved as a treatment for a tetrahydrobiopterin-responsive hyperphenylalaninemia in 2007 and 2008, in the USA and Japan, respectively. Guidelines are required on the proper use of sapropterin hydrochloride for tetrahydrobiopterin-responsive hyperphenylalaninemia.

OBSERVATIONS

It is recommended that tetrahydrobiopterin-responsive hyperphenylalaninemia should be diagnosed in all cases of hyperphenylalaninemia, including phenylketonuria, by tetrahydrobiopterin administration tests rather than by phenotype or blood phenylalanine levels.

CONCLUSIONS AND RELEVANCE

If tetrahydrobiopterin-responsive hyperphenylalaninemia is diagnosed, all ages can be treated with sapropterin hydrochloride. Although there are reports that sapropterin hydrochloride is effective and safe for the prevention of maternal phenylketonuria, further investigation is required.

[Dietary treatment of inborn errors of metabolism-a balancing act between indulgence and therapy]

For many inborn metabolic diseases, a lifelong diet is a crucial part of the therapy since pharmacological therapy is available for only a few conditions and patients. The implementation of a low natural protein diet with a reduced intake of natural protein and the complementary use of synthetic amino acid mixtures is described using the examples of phenylketonuria and urea cycle disorders focusing on children and adolescents. For phenylketonuria, the amino acid supplement is free of phenylalanine whereas for urea cycle disorders, it exclusively consists of essential amino acids. The dietary treatment aims to maintain metabolic stability and to prevent accumulation of toxic metabolites. At the same time, the nutritional requirements to ensure growth and development must be met. Therefore, patients need to follow strict rules regarding the choice of food products. This restrictive therapy interferes with the desire for autonomy and the joy of eating and often results in a reduced quality of life.Following the diet is crucial for a favorable outcome. To meet its requirements, patients and their families are provided with training. It is a great challenge not only to support the patients and their families in all practical aspects of dietary management, but also to motivate them to lifelong adherence in order to ensure the best possible outcome.

A microparticulate based formulation to protect therapeutic enzymes from proteolytic digestion: phenylalanine ammonia lyase as case study

Phenylketonuria is a genetic disorder affecting the metabolism of phenylalanine (phe) due to a deficiency in the enzyme phenylalanine hydroxylase. This disorder is characterized by an elevated phe blood level, which can lead to severe intellectual disabilities in newborns. The current strategy to prevent these devastating consequences is limited to a life-long phe-free diet, which implies major lifestyle changes and restrictions. Recently, an injectable enzyme replacement therapy, Pegvaliase, has been approved for treating phenylketonuria, but is associated with significant side-effects. In this study a phe-metabolizing system suitable for oral delivery is designed to overcome the need for daily injections. Active phenylalanine ammonia-lyase (PAL), an enzyme that catalyses phe metabolism, is loaded into mesoporous silica microparticles (MSP) with pore sizes ranging from 10 to 35 nm. The surface of the MSP is lined with a semipermeable barrier to allow permeation of phe while blocking digestive enzymes that degrade PAL. The enzymatic activity can be partially preserved in vitro by coating the MSP with poly(allylamine) and poly(acrylic acid)-bowman birk (protease inhibitor) conjugate. The carrier system presented herein may provide a general approach to overcome gastro-intestinal proteolytic digestion and to deliver active enzymes to the intestinal lumen for prolonged local action.

Sustaining benefits of nutritional therapy in young adults with phenylketonuria - A 2 year prospective study

Introduction

Phenylketonuria (PKU) is an inborn error of metabolism, which is caused by a deficiency in the enzyme phenylalanine hydroxylase (PAH). Life-long Phe-free diet impairs quality of life, especially in adolescents and young adults which take responsibility over their diet and therapy from their parents, but expect freedom in daily routine.

Methods and results

105 patients with PKU were screened for eligibility for participating in this study. Data of 21 patients with genetic predictive value (GPV) ≤5 and age between 14 and 30 years were included in the analysis. Mean age of the study population was 22.6 ± 7.5 years, 8 patients (38%) were female. At baseline, structured counselling by a professional nutrionist was performed.

Mean Phe-level at baseline was 926 ± 432 μmol/l, after six months Phe-levels were significantly reduced to 709 ± 314 μmol/l (p = .039), in total 4 additional patients (38% of the population) reached values within the therapeutic goal. After 12, 18 and 24 months, mean Phe-level elevated significantly to initial level (869 ± 427 μmol/l; p = .311). Mean daily intake of natural protein at baseline was 32.3 ± 24.3 g per day. There was a significant decrease after 6 months (26.9 ± 18.8 g/day; p = .049) and 12 months (25.9 ± 16.2 g/day; p = .30) compared to baseline. Values at 18 months (27.5 ± 9.2 g/day; p = .26) and 24 months (35.0 ± 22.3 g/day; p = .87) did not differ. Mean daily supplementation of Phe-free amino acids was 26.2 ± 19.2 g per day. In all follow-up examinations a significant increase compared to baseline values was calculated (42.4 ± 17.6 g/day after 6 moths (p = .028), 52.1 ± 29.9 g/day after 12 months (p ≤.01), 38.7 ± 20.3 g/day after 18 months (p < .01) and 39.3 ± 21.9 g/day after 24 months (p = .014)). At baseline, mean total protein intake (natural protein plus supplements) was 0.97 ± 0.42 g per kg body weight (g/kgBW). After 24 months the protein intake was within recommended levels. (1.23 ± 0.33 g/kgKB; p = .013). After 24 months, plasma Vitamin B12 increased to 424.8 ± 176.9 pg/ml (baseline 368.6 ± 205.6 pg/ml; p = .049) and Vitamin D increased to 30.4 ± 9.9 ng/ml (baseline 24.5 ± 10.1 ng/ml; p = .06).

Conclusion

Counselling by a professional nutrionist in young adults with PKU has clear short-term effects on plasma Phe-levels. Easy applicable therapeutic recommendations, as additional intake of amino acid supplement, are well tolerated and result in strict therapy adherence up to 24 months. Apart from that, the effects on Phe-levels seem only to sustain for about 6 months. More frequent nutritional counselling, i.e. at least two times per year, is recommended to preserve positive effects on Phe-levels. Lack of Vitamin B12 and Vitamin D still are common in PKU patients, but not necessarily need to be substituted. They can effectively be equalized by a well-balanced diet within 24 months.

Case-control study about the acceptance of Pegvaliase in Phenylketonuria

Introduction

Pegvaliase is a novel enzyme substitution therapy approved by the European Drug Administration (EDA) in May 2019 for the treatment of Phenylketonuria (PKU) in adults and children ≥16 years of age. The pegylated phenylalanine ammonia lyase is isolated from bacteria and therefore provokes multifarious immunogenic reactions. Thus, the selection of the right patient for a potential harmful treatment is essential for patient's contentedness and long-term therapy compliance.

Methods and results

101 patients with PKU were screened for eligibility for an additional treatment with Pegvaliase. 51 patients were included in the study, 26 received a structural information about the new treatment for in mean 43 ± 12 min and clinical data and plasma Phe-levels were assessed. After 4 weeks of consideration the willing of treatment initiation as well as reasons for denial are registered. 7 patients (27%) concluded in beginning of treatment. Phe-level in this (acceptance) group were higher (1180 ± 231 μmol/l) compared to the denial group (930 ± 278 μmol/l, p = .01). After 4 weeks Phe-levels in the acceptance group remained stable (1264 ± 311 μmol/l, p = .26) while Phe-levels in the denial group decreased (779 ± 226 μmol/l, p < .01). Main reasons for denial of therapy were fear of adverse effects (47%), no need for additional treatment (26%) and the subcutaneous way of application (21%).

Conclusion

PKU patients have reservations against an invasive subcutaneous treatment for their disease. This is mainly caused by the form of application by syringe and the potential harmful side effects. Only less than one-third of the patients in our cohort are willing to start treatment. Besides that, most PKU patients seem to have untapped potential for self-contained reduction of Phe-levels only by being focused on their diet.

Pegvaliase: Immunological profile and recommendations for the clinical management of hypersensitivity reactions in patients with phenylketonuria treated with this enzyme substitution therapy

OBJECTIVE

To provide recommendations for managing hypersensitivity adverse events (HAEs) to an injectable enzyme substitution therapy (pegvaliase, a PEGylated phenylalanine ammonia lyase enzyme) in adult patients with phenylketonuria (PKU).

METHODS

Eight European academic immunology experts with a broad range of experience in hypersensitivity, anaphylaxis, and/or drug reactions, and two geneticists from the USA with pegvaliase experience convened for two advisory board meetings. Efficacy, safety, and immunological profile of pegvaliase were discussed with the objective of developing recommendations for the clinical management of HAEs associated with pegvaliase treatment.

RESULTS

Based on available immunogenicity data, it was concluded that pegvaliase induces a Type III hypersensitivity reaction, causing HAEs with peak event rates during induction/titration and a decline over time during maintenance therapy. The decline in HAEs with longer duration of therapy was considered to likely be driven by anti-drug antibody affinity maturation, reduced immune complex formation, and decreased complement activation over time. Immunology and PKU experts unanimously supported that the use of an induction, titration, and maintenance dosing regimen and implementation of several risk mitigation strategies contributed to the improvement of tolerability over time. Key risk mitigation strategies utilized in the Phase 3 clinical trials such as premedication with H1-receptor antagonists, allowance for a longer titration period after an HAE, patient education, and requirement to carry auto-injectable adrenaline (epinephrine) should be continued in clinical practice. A tool for administration of auto-injectable adrenaline in patients using pegvaliase was suggested. It was added that after the occurrence of a severe HAE a temporary dose reduction is more likely to improve tolerability than treatment interruption.

CONCLUSIONS

Overall, it was agreed that pegvaliase has a generally tolerable safety profile in adults with PKU. Importantly, the risk mitigation strategies utilized in the clinical trials were considered to support the continued use of key strategies for management in the commercial setting, such as a slow induction/titration dosing paradigm and premedication with H1-receptor antagonists. However, physicians and patients need to be aware of the risk of HAEs associated with pegvaliase; presence of a trained observer during early treatment may be beneficial in certain circumstances, and a requirement to carry auto-injectable adrenaline is recommended. Because pegvaliase offers the possibility to normalize diet, while maintaining blood phenylalanine within the recommended therapeutic range, safe use of this medication in the clinical setting is important. Ongoing monitoring of long-term clinical safety of patients on pegvaliase treatment in the commercial setting was recommended.

Neuropsychological Profile of Children with Early and Continuously Treated Phenylketonuria: Systematic Review and Future Approaches

OBJECTIVE

To provide a comprehensive systematic review of the literature by examining studies published on all cognitive aspects of children with early and continuously treated phenylketonuria (ECT-PKU) included in the databases Medline, PsycINFO, and PsycARTICLE.

METHOD

In addition to a classical approach, we summarized methodology and results of each study in order to discuss current theoretical and methodological issues. We also examined recent advances in biochemical markers and treatments of PKU, with implications for future research on metabolic control and its role as a determinant of neuropsychological outcome.

RESULTS

Consistent with previous reviews, the hypothesis of a specific and central executive impairment in children with ECT-PKU was suggested. However, findings are inconclusive regarding the nature of executive impairments as well as their specificity, impact on everyday life, persistence over time, and etiology.

CONCLUSION

Given the current state of the science, we suggest future directions for research that utilizes a developmental and integrative approach to examine the effects of recent advances in biochemical markers and treatment of PKU. (JINS, 2019, 25, 624-643).

Structural and Functional Impact of Seven Missense Variants of Phenylalanine Hydroxylase

The molecular genetics of well-characterized inherited diseases, such as phenylketonuria (PKU) and hyperphenylalaninemia (HPA) predominantly caused by mutations in the phenylalanine hydroxylase (PAH) gene, is often complicated by the identification of many novel variants, often with no obvious impact on the associated disorder. To date, more than 1100 PAH variants have been identified of which a substantial portion have unknown clinical significance. In this work, we study the functionality of seven yet uncharacterized PAH missense variants p.Asn167Tyr, p.Thr200Asn, p.Asp229Gly, p.Gly239Ala, p.Phe263Ser, p.Ala342Pro, and p.Ile406Met first identified in the Czech PKU/HPA patients. From all tested variants, three of them, namely p.Asn167Tyr, p.Thr200Asn, and p.Ile406Met, exerted residual enzymatic activity in vitro similar to wild type (WT) PAH, however, when expressed in HepG2 cells, their protein level reached a maximum of 72.1% ± 4.9%, 11.2% ± 4.2%, and 36.6% ± 7.3% compared to WT PAH, respectively. Remaining variants were null with no enzyme activity and decreased protein levels in HepG2 cells. The chaperone-like effect of applied BH4 precursor increased protein level significantly for p.Asn167Tyr, p.Asp229Gly, p.Ala342Pro, and p.Ile406Met. Taken together, our results of functional characterization in combination with in silico prediction suggest that while p.Asn167Tyr, p.Thr200Asn, and p.Ile406Met PAH variants have a mild impact on the protein, p.Asp229Gly, p.Gly239Ala, p.Phe263Ser, and p.Ala342Pro severely affect protein structure and function.

International best practice for the evaluation of responsiveness to sapropterin dihydrochloride in patients with phenylketonuria

Phenylketonuria (PKU) is an inherited metabolic disease caused by phenylalanine hydroxylase (PAH) deficiency. As the resulting high blood phenylalanine (Phe) concentration can have detrimental effects on brain development and function, international guidelines recommend lifelong control of blood Phe concentration with dietary and/or medical therapy. Sapropterin dihydrochloride is a synthetic preparation of tetrahydrobiopterin (6R-BH4), the naturally occurring cofactor of PAH. It acts as a pharmacological chaperone, reducing blood Phe concentration and increasing dietary Phe tolerance in BH4-responsive patients with PAH deficiency. Protocols to establish responsiveness to sapropterin dihydrochloride vary widely. Two meetings were held with an international panel of clinical experts in PKU management to develop recommendations for sapropterin dihydrochloride response testing. At the first meeting, regional differences and similarities in testing practices were discussed based on guidelines, a literature review, outcomes of a global physician survey, and case reports. Statements developed based on the discussions were sent to all participants for consensus (>70% of participants) evaluation using a 7-level rating system, and further discussed during the second meeting. The experts recommend sapropterin dihydrochloride response testing in patients with untreated blood Phe concentrations of 360-2000 μmol/L, except in those with two null mutations. For neonates, a 24-h sapropterin dihydrochloride loading test is recommended; responsiveness is defined as a decrease in blood Phe ≥30%. For older infants, children, adolescents, and adults, a test duration of ≥48 h or a 4-week trial is recommended. The main endpoint for a 48-h to 7-day trial is a decrease in blood Phe, while improved Phe tolerance is the endpoint to be assessed during a longer trial. Longer trials may not be feasible in some locations due to lack of reimbursement for hospitalization, while a 4-week trial may not be possible due to limited access to sapropterin dihydrochloride or public health regulation. A 48-h response test should be considered in pregnant patients who cannot achieve blood Phe ≤360 μmol/L with a Phe-restricted diet. Durability of response and clinical benefits of sapropterin dihydrochloride should be assessed over the long term. Harmonization of protocols is expected to improve identification of responders and comparability of test results worldwide.

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

BACKGROUND

Genetic heterogeneity and compound heterozygosis give rise to a continuous spectrum of phenylalanine hydroxylase deficiency and metabolic phenotypes in phenylketonuria (PKU). The most used parameters for evaluating phenotype in PKU are pretreatment phenylalanine (Phe) levels, tolerance for dietary Phe, and Phe overloading test. Phenotype can vary from a "classic" (severe) form to mild hyperphenylalaninemia, which does not require dietary treatment. A subset of patients is responsive to treatment by the cofactor tetrahydrobiopterin (BH4 ). Genotypes of PKU patients from Rio de Janeiro, Brazil, were compared to predicted and observed phenotypes. Genotype-based estimations of responsiveness to BH4 were also conducted.

METHODS

Phenotype was defined by pretreatment Phe levels. A standard prediction system based on arbitrary assigned values was employed to measure genotype-phenotype concordance. Patients were also estimated as BH4 -responders according to the responsiveness previously reported for their mutations and genotypes.

RESULTS

A 48.3% concordance rate between genotype-predicted and observed phenotypes was found. When the predicted phenotypes included those reported at the BIOPKU database, the concordance rate reached 77%. A total of 18 genotypes from 30 patients (29.4%) were estimated as of potential or probable BH4 responsiveness. Inconsistencies were observed in genotypic combinations including the common "moderate" mutations p.R261Q, p.V388M, and p.I65T and the mild mutations p.L48S, p.R68S, and p.L249F.

CONCLUSION

The high discordance rate between genotype-predicted and observed metabolic phenotypes in this study seems to be due partially to the high frequency of the so-called "moderate" common mutations, p.R261Q, p.V388M, and p.I65T, which are reported to be associated to erratic or more severe than expected metabolic phenotypes. Although our results of BH4 estimated responsiveness must be regarded as tentative, it should be emphasized that genotyping and genotype-phenotype association studies are important in selecting patients to be offered a BH4 overload test, especially in low-resource settings like Brazil.

Phase I clinical evaluation of CNSA-001 (sepiapterin), a novel pharmacological treatment for phenylketonuria and tetrahydrobiopterin deficiencies, in healthy volunteers

Tetrahydrobiopterin (BH4) is the natural cofactor of aromatic amino acid hydroxylases and essential for degradation of phenylalanine and synthesis of catecholamines and serotonin. It can be synthesized either de novo from GTP or through the salvage pathway from sepiapterin. Sepiapterin, a natural precursor of BH4, is a more stable molecule and is transported more efficiently across cellular membranes, thus having potentially significant advantage over BH4 as a pharmacological agent for diseases associated with BH4-deficient conditions. We report the results of a first-in-humans, randomized, double-blind, placebo-controlled, dose-ranging, Phase I clinical trial in 83 healthy volunteers of CNSA-001, a novel formulation of sepiapterin. Single oral doses of 2.5-80 mg/kg CNSA-001 caused dose-related increases in plasma sepiapterin (mean Cmax 0.58-2.92 ng/mL) and BH4 (mean Cmax 57-312 ng/mL). Maximum plasma concentrations were achieved in about 1-2 h (sepiapterin) or about 4 h (BH4) after CNSA-001 oral intake. Increases in plasma BH4 were substantially larger in absolute terms and on a dose-for-dose basis following treatment with CNSA-001 vs. sapropterin dihydrochloride, a synthetic form of BH4. The pharmacokinetics of plasma sepiapterin and BH4 were similar before and after seven days of repeat daily dosing with CNSA-001 at 5, 20 or 60 mg/kg indicating little or no drug accumulation. Oral administration of CNSA-001 resulted in higher concentrations of sepiapterin in fasted vs. fed subjects, but overall BH4 plasma exposure following CNSA-001 intake increased by 1.7-1.8-fold in fed subjects. CNSA-001 was well tolerated, with no clear dose-relationship for adverse events (AE), no serious AE and no study discontinuations for AE. These data indicate that CNSA-001 is rapidly and efficiently converted to BH4 in humans supporting further clinical evaluation of CNSA-001 for the management of PKU, primary BH4 deficiencies and other diseases associated with deficient BH4 metabolism.

Nutritional management of phenylalanine hydroxylase (PAH) deficiency in pediatric patients in Canada: a survey of dietitians' current practices

Background

Phenylalanine hydroxylase (PAH) deficiency is one of 31 targeted inherited metabolic diseases (IMD) for the Canadian Inherited Metabolic Diseases Research Network (CIMDRN). Early diagnosis and initiation of treatment through newborn screening has gradually shifted treatment goals from the prevention of disabling complications to the optimization of long term outcomes. However, clinical evidence demonstrates that subtle suboptimal neurocognitive outcomes are present in the early and continuously diet-treated population with PAH deficiency. This may be attributed to variation in blood phenylalanine levels to outside treatment range and this, in turn, is possibly due to a combination of factors; disease severity, dietary noncompliance and differences in practice related to the management of PAH deficiency. One of CIMDRN’s goals is to understand current practices in the diagnosis and management of PAH deficiency in the pediatric population, from the perspective of both health care providers and patients/families.

Objectives

We investigated Canadian metabolic dietitians’ perspectives on the nutritional management of children with PAH deficiency, awareness of recently published North American treatment and nutritional guidelines in relation to PAH deficiency, and nutritional care practices within and outside these guidelines.

Methods

We invited 33 dietitians to participate in a survey, to ascertain their use of recently published guidelines and their practices in relation to the nutritional care of pediatric patients with PAH deficiency.

Results

We received 19 responses (59% response rate). All participants reported awareness of published guidelines for managing PAH deficiency. To classify disease severity, 89% of dietitians reported using pre-treatment blood phenylalanine (Phe) levels, alone or in combination with other factors. 74% of dietitians reported using blood Phe levels ≥360 μmol/L (6 mg/dL) as the criterion for initiating a Phe-restricted diet. All respondents considered 120-360 μmol/L (2–6 mg/dL) as the optimal treatment range for blood Phe in children 0–9 years old, but there was less agreement on blood Phe targets for older children. Most dietitians reported similar approaches to diet assessment and counseling: monitoring growth trends, use of 3 day diet records for intake analysis, individualization of diet goals, counseling patients to count grams of dietary natural protein or milligrams of dietary Phe, and monitoring blood Phe, tyrosine and ferritin.

Conclusion

While Canadian dietitians’ practices in managing pediatric PAH deficiency are generally aligned with those of the American College of Medical Genetics and Genomics (ACMG), and with the associated treatment and nutritional guidelines from Genetic Metabolic Dietitians International (GMDI), variation in many aspects of care reflects ongoing uncertainty and a need for robust evidence.

The role of urinary pteridines as disease biomarkers

Pteridines and their derivatives function as intermediates in the metabolism of several vitamins and cofactors, and their relevance to disease has inspired new efforts to study their roles as disease biomarkers. Recent analytical advances, such as the emergence of sensitive mass spectrometry techniques, new workflows for measuring pteridine derivatives in their native oxidation states and increased multiplexing capacities for the simultaneous determination of many pteridine derivatives, have enabled researchers to explore the roles of urinary pteridines as disease biomarkers at much lower levels with greater accuracy than with previous technologies or methods. As a result, urinary pteridines are being increasingly studied as putative cancer biomarkers with promising results being reported from exploratory studies. In addition, the role of urinary neopterin as a universal biomarker for immune system activation is being investigated in new diseases where it is anticipated to become a useful supplementary marker in clinical diagnostic settings. In summary, this review provides an overview of recent developments in the clinical study of urinary pteridines as disease biomarkers, covers the most promising aspects of advanced analytical techniques being developed for the determination of urinary pteridines and discusses the major challenges associated with implementing pteridine biomarkers in clinical laboratory settings.

Pretreatment cognitive and neural differences between sapropterin dihydrochloride responders and non-responders with phenylketonuria

Sapropterin dihydrochloride (BH₄) reduces phenylalanine (Phe) levels and improves white matter integrity in a subset of individuals with phenylketonuria (PKU) known as “responders.” Although prior research has identified biochemical and genotypic differences between BH₄ responders and non-responders, cognitive and neural differences remain largely unexplored. To this end, we compared intelligence and white matter integrity prior to treatment with BH₄ in 13 subsequent BH₄ responders with PKU, 16 subsequent BH₄ non-responders with PKU, and 12 healthy controls. Results indicated poorer intelligence and white matter integrity in non-responders compared to responders prior to treatment. In addition, poorer white matter integrity was associated with greater variability in Phe across the lifetime in non-responders but not in responders. These results underscore the importance of considering PKU as a multi-faceted, multi-dimensional disorder and point to the need for additional research to delineate characteristics that predict response to treatment with BH_4.

Genetics of Phenylketonuria: Then and Now

More than 950 phenylalanine hydroxylase (PAH) gene variants have been identified in people with phenylketonuria (PKU). These vary in their consequences for the residual level of PAH activity, from having little or no effect to abolishing PAH activity completely. Advances in genotyping technology and the availability of locus-specific and genotype databases have greatly expanded our understanding of the correlations between individual gene variant, residual PAH activity, tetrahydrobiopterin (BH4 ) responsiveness, and the clinical PKU phenotype. Most patients (∼76%) have compound heterozygous PAH gene variants and one mutated allele may markedly influence the activity of the second mutated allele, which in turn may influence either positively or negatively the activity of the biologically active heterotetrameric form of the PAH. While it is possible to predict the level of BH4 responsiveness (∼71%) and PKU severity (∼78%) from the nature of the underlying gene variants, these relationships remain complex and incompletely understood. A greater understanding of these relationships may increase the potential for individualized management of PKU in future. Inherited deficiencies in BH4 metabolism account for about 1%-2% of all hyperphenylalaninemias and are clinically more severe than PKU. Almost 90% of all patients are deficient in 6-pyruvoyl-tetrahydropterin synthase and dihydropteridine reductase.

Correlation between genotype and the tetrahydrobiopterin-responsive phenotype in Chinese patients with phenylketonuria

BACKGROUND

A growing body of research has suggested that tetrahydrobiopterin (BH4) responsive phenotype can be predicted by the phenylalanine hydroxylase (PAH) genotype in patients with phenylketonuria (PKU), but data concerning the association between genotype and BH4 responsiveness are scarce in China.

METHODS

A total of 165 PKU patients from China who had undergone a 24-h loading test with BH4 administration were recruited. Genotyping was performed by the next-generation sequencing (NGS) technique. Using the predicted residual PAH activity, we analyzed the association between genotype and BH4-responsiveness.

RESULTS

Among the 165 patients, 40 patients (24.24%) responded to BH4. A total of 74 distinct mutations were observed, including 13 novel mutations. The mutation p.R241C was most frequently associated with response. Two known mutations (p.A322T and p.Q419R) and two novel mutations (p.L98V and IVS3-2A>T) were first reported as responsive to BH4. Residual PAH activity of at least 12.5% was needed for responsive genotypes.

CONCLUSION

Genotype-based predictions of BH4-responsiveness are only for selecting potential responders. Accordingly, it is necessary to test potential responders with a long-term BH4 challenge.

Phenylketonuria: a review of current and future treatments

Phenylketonuria (PKU) is an autosomal recessive inborn error of metabolism caused by a deficiency in the hepatic enzyme phenylalanine hydroxylase (PAH). If left untreated, the main clinical feature is intellectual disability. Treatment, which includes a low Phe diet supplemented with amino acid formulas, commences soon after diagnosis within the first weeks of life. Although dietary treatment has been successful in preventing intellectual disability in early treated PKU patients, there are major issues with dietary compliance due to palatability of the diet. Other potential issues associated with dietary therapy include nutritional deficiencies especially vitamin D and B12. Suboptimal outcomes in cognitive and executive functioning have been reported in patients who adhere poorly to dietary therapy. There have been continuous attempts at improving the quality of medical foods including their palatability. Advances in dietary therapy such as the use of large neutral amino acids (LNAA) and glycomacropeptides (GMP; found within the whey fraction of bovine milk) have been explored. Gene therapy and enzyme replacement or substitution therapy have yielded more promising data in the recent years. In this review the current and possible future treatments for PKU are discussed.

The challenge of long-term tetrahydrobiopterin (BH4) therapy in phenylketonuria: Effects on metabolic control, nutritional habits and nutrient supply

BACKGROUND AND AIMS

BH4-sensitive phenylketonuria (PKU) patients relax their phenylalanine (Phe) restricted diet due to increased Phe tolerance, while keeping dried blood Phe concentrations with in the therapeutic range. We aimed to investigate metabolic control, eating habits and nutrient supply under long-term BH4-therapy.

PATIENTS AND METHODS

Retrospective analysis of mean dried blood Phe concentrations and their variability, food and nutrient intake in BH4-sensitive patients (n = 8, 3f, age 6.0-16.6 y) under classical dietary treatment for one year and during the three years after initiation of BH4.

RESULTS

Phe concentrations of BH4-sensitve PKU patients remained within therapeutic range throughout the observation period, independent of therapeutic regime. Under BH4, Phe tolerance increased significantly (493.2 ± 161.8 mg/d under classical diet vs 2021.93 ± 897.4 mg/d two years under BH4; P = 0.004). Variability of Phe concentrations remained unchanged (mean SD; P = 1.000). Patients adjust their food choice and significantly increased their intake of cereals, potatoes, dairy products and meat (P = 0.019, P = 0.016, P = 0.016 and P = 0.016, respectively). Under diet changes after implementation of BH4 a drop in micronutrient intake (vitamin D, folic acid, iron, calcium, iodine) could be revealed (P = 0.005, P < 0.001, P = 0.004, P = 0.001, P = 0.003, respectively).

CONCLUSIONS

BH4-sensitive PKU patients can achieve good metabolic control under an adjuvant BH4- or a BH4 monotherapy. The liberalized diet under BH4 seems to jeopardize the quality of patients' nutrition, and these patients require close follow-up and special nutrition education to minimize the risk for imbalanced diet and nutrient deficiencies.

Tetrahydrobiopterin (BH4) responsiveness in neonates with hyperphenylalaninemia: a semi-mechanistically-based, nonlinear mixed-effect modeling

Neonatal loading studies with tetrahydrobiopterin (BH4) are used to detect hyperphenylalaninemia due to BH4 deficiency by evaluating decreases in blood phenylalanine (Phe) concentrations post BH4 load. BH4 responsiveness in phenylalanine hydroxylase (PAH)-deficient patients introduced a new diagnostic aspect for this test. In older children, a broad spectrum of different levels of responsiveness has been described. The primary objective of this study was to develop a pharmacodynamic model to improve the description of individual sensitivity to BH4 in the neonatal period. Secondary objectives were to evaluate BH4 responsiveness in a large number of PAH-deficient patients from a neonatal screening program and in patients with various confirmed BH4 deficiencies from the BIODEF database. Descriptive statistics in patients with PAH deficiency with 0-24-h data available showed that 129 of 340 patients (37.9%) had a >30% decrease in Phe levels post load. Patients with dihydropteridine reductase deficiency (n = 53) could not be differentiated from BH4-responsive patients with PAH deficiency. The pharmacologic turnover model, "stimulation of loss" of Phe following BH4 load, fitted the data best. Using the model, 193 of 194 (99.5%) patients with a proven BH4 synthesis deficiency or recycling defect were classified as BH4 sensitive. Among patients with PAH deficiency, 216 of 375 (57.6%) patients showed sensitivity to BH4, albeit with a pronounced variability; PAH-deficient patients with blood Phe <1200 μmol/L at time 0 showed higher sensitivity than patients with blood Phe levels >1200 μmol/L. External validation showed good correlation between the present approach, using 0-24-h blood Phe data, and the published 48-h prognostic test. Pharmacodynamic modeling of Phe levels following a BH4 loading test is sufficiently powerful to detect a wide range of responsiveness, interpretable as a measure of sensitivity to BH4. However, the clinical relevance of small responses needs to be evaluated by further studies of their relationship to long-term response to BH4 treatment.

The Kuvan(®) Adult Maternal Paediatric European Registry (KAMPER) Multinational Observational Study: Baseline and 1-Year Data in Phenylketonuria Patients Responsive to Sapropterin

INTRODUCTION

Sapropterin dihydrochloride (Kuvan(®)), a synthetic 6R-diastereoisomer of tetrahydrobiopterin (BH4), is approved in Europe for the treatment of patients aged ≥4 years with hyperphenylalaninaemia (HPA) due to BH4-responsive phenylalanine hydroxylase (PAH) deficiency, in conjunction with a phenylalanine-restricted diet, and also for the treatment of patients with BH4 deficiency.

AIMS/METHODS

KAMPER is an ongoing, observational, multicentre registry with the primary objective of providing information over 15 years on long-term safety of sapropterin dihydrochloride treatment in patients with HPA. Here we report initial data on characteristics from patients recruited by the time of the third interim analysis and results at 1 year.

RESULTS

Overall, 325 patients from 55 sites in seven European countries were included in the analysis: 296 (91.1%) patients with PAH deficiency (median [Q1, Q3] age, 10.3 [7.2, 15.0] years) and 29 (8.9%) with BH4 deficiency (12.8 [6.6, 18.9] years). Fifty-nine patients (18.2%) were aged ≥18 years; 4 patients were pregnant. No elderly patients (aged ≥65 years) or patients with renal or hepatic insufficiency were enroled in the study. Twelve-month data were available for 164 patients with PAH deficiency and 16 with BH4 deficiency. No new safety concerns were identified as of May 2013.

CONCLUSIONS

Initial data from KAMPER show that sapropterin dihydrochloride has a favourable safety profile. Registry data collected over time will provide insight into the management and outcomes of patients with PAH deficiency and BH4 deficiency, including long-term safety, impact on growth and neurocognitive outcomes and the effect of sapropterin dihydrochloride treatment on populations of special interest.

Sapropterin dihydrochloride for phenylketonuria

BACKGROUND

Phenylketonuria results from a deficiency of the enzyme phenylalanine hydroxylase. Dietary restriction of phenylalanine keeps blood phenylalanine concentration low. Most natural foods are excluded from diet and supplements are used to supply other nutrients. Recent publications report a decrease in blood phenylalanine concentration in some patients treated with sapropterin dihydrochloride. We examined the evidence for the use of sapropterin dihydrochloride to treat phenylketonuria. This is an update of a previously published Cochrane Review. 

OBJECTIVES

To assess the safety and efficacy of sapropterin dihydrochloride in lowering blood phenylalanine concentration in people with phenylketonuria.

SEARCH METHODS

We identified relevant trials from the Group's Inborn Errors of Metabolism Trials Register. Date of last search: 11 August 2014.We also searched ClinicalTrials.gov and Current controlled trials. Last search: 4 September 2014We contacted the manufacturers of the drug (BioMarin Pharmaceutical Inc.) for information regarding any unpublished trials.

SELECTION CRITERIA

Randomized controlled trials comparing sapropterin with no supplementation or placebo in people with phenylketonuria due to phenylalanine hydroxylase deficiency.

DATA COLLECTION AND ANALYSIS

Two authors independently assessed trials and extracted outcome data.

MAIN RESULTS

Two placebo-controlled trials were included. One trial administered 10 mg/kg/day sapropterin in 89 children and adults with phenylketonuria whose diets were not restricted and who had previously responded to saproterin.This trial measured change in blood phenylalanine concentration. The second trial screened 90 children (4 to 12 years) with phenylketonuria whose diet was restricted, for responsiveness to sapropterin. Forty-six responders entered the placebo-controlled part of the trial and received 20 mg/kg/day sapropterin. This trial measured change in both phenylalanine concentration and protein tolerance. Both trials reported adverse events. The trials showed an overall low risk of bias; but both are Biomarin-sponsored. One trial showed a significant lowering in blood phenylalanine concentration in the sapropterin group (10 mg/kg/day), mean difference -238.80 μmol/L (95% confidence interval -343.09 to -134.51); a second trial (20 mg/kg/day sapropterin) showed a non-significant difference, mean difference -51.90 μmol/L (95% confidence interval -197.27 to 93.47). The second trial also reported a significant increase in phenylalanine tolerance, mean difference18.00 mg/kg/day (95% confidence interval 12.28 to 23.72) in the 20 mg/kg/day sapropterin group.

AUTHORS' CONCLUSIONS

There is evidence of short-term benefit from using sapropterin in some people with sapropterin-responsive forms of phenylketonuria; blood phenylalanine concentration is lowered and protein tolerance increased. There are no serious adverse events associated with using sapropterin in the short term.There is no evidence on the long-term effects of sapropterin and no clear evidence of effectiveness in severe phenylketonuria.

Long-term follow-up of patients with phenylketonuria treated with tetrahydrobiopterin: a seven years experience

BACKGROUND

Phenylketonuria (PKU) is an autosomal recessive disorder caused by the deficiency of phenylalanine hydroxylase that catalyzes the conversion of phenylalanine to tyrosine, using tetrahydrobiopterin (BH4) as coenzyme. Besides dietary phenylalanine restriction, new therapeutic options are emerging, such as the treatment with BH4 in subgroups of PKU patients responding to a loading test with BH4.

METHODS

A no-profit open-label interventional trial with long-term oral BH4 therapy, sponsored by the Italian Medicines Agency (AIFA), was performed in a group of 17 PKU patients resulted as BH4 responders among 46 subjects analyzed for BH4-responsiveness (prot. FARM5MATC7). We report on efficacy and safety data of BH4 therapy and analyze factors predicting BH4-responsiveness and long-term response to BH4. A BH4-withdrawal test was used as a proof of the efficacy of long-term therapy with BH4.

RESULTS

Forty-four percent of the patients responded to the 48 h-long loading test with BH4. All the phenotypic classes were represented. Genotype was the best predictor of responsiveness, along with lower phenylalanine levels at diagnosis, higher tolerance and lower phenylalanine/tyrosine ratio before the test. In BH4 responder patients, long-term BH4 therapy resulted safe and effective in increasing tolerance while maintaining a good metabolic control. The BH4 withdrawal test, performed in a subset of patients, showed that improved tolerance was directly dependent on BH4 assumption. Tolerance to phenylalanine was re-evaluated in 43.5% of patients and was longitudinally analyzed in 5 patients.

CONCLUSIONS

Long-term treatment with BH4 is safe and effective in increasing tolerance to phenylalanine. There is real need to assess the actual tolerance to phenylalanine in PKU patients to ameliorate quality of life, improve nutritional status, avoiding unnecessarily restricted diets, and interpret the effects of new therapies for PKU.

New Strategies for the Treatment of Phenylketonuria (PKU)

Phenylketonuria (PKU) was the first inherited metabolic disease in which dietary treatment was found to prevent the disease's clinical features. Treatment of phenylketonuria remains difficult due to progressive decrease in adherence to diet and the presence of neurocognitive defects despite therapy. This review aims to summarize the current literature on new treatment strategies. Additions to treatment include new, more palatable foods based on glycomacropeptide that contains very limited amount of aromatic amino acids, the administration of large neutral amino acids to prevent phenylalanine entry into the brain or tetrahydropterina cofactor capable of increasing residual activity of phenylalanine hydroxylase. Moreover, human trials have recently been performed with subcutaneous administration of phenylalanine ammonia-lyase, and further efforts are underway to develop an oral therapy containing phenylanine ammonia-lyase. Gene therapy also seems to be a promising approach in the near future.

Molecular genetics and diagnosis of phenylketonuria: state of the art

Detection of individuals with phenylketonuria (PKU), an autosomal recessively inherited disorder in phenylalanine degradation, is straightforward and efficient due to newborn screening programs. A recent introduction of the pharmacological treatment option emerged rapid development of molecular testing. However, variants responsible for PKU do not all suppress enzyme activity to the same extent. A spectrum of over 850 variants, gives rise to a continuum of hyperphenylalaninemia from very mild, requiring no intervention, to severe classical PKU, requiring urgent intervention. Locus-specific and genotypes database are today an invaluable resource of information for more efficient classification and management of patients. The high-tech molecular methods allow patients' genotype to be obtained in a few days, especially if each laboratory develops a panel for the most frequent variants in the corresponding population.

Linking genotypes database with locus-specific database and genotype-phenotype correlation in phenylketonuria

The wide range of metabolic phenotypes in phenylketonuria is due to a large number of variants causing variable impairment in phenylalanine hydroxylase function. A total of 834 phenylalanine hydroxylase gene variants from the locus-specific database PAHvdb and genotypes of 4181 phenylketonuria patients from the BIOPKU database were characterized using FoldX, SIFT Blink, Polyphen-2 and SNPs3D algorithms. Obtained data was correlated with residual enzyme activity, patients' phenotype and tetrahydrobiopterin responsiveness. A descriptive analysis of both databases was compiled and an interactive viewer in PAHvdb database was implemented for structure visualization of missense variants. We found a quantitative relationship between phenylalanine hydroxylase protein stability and enzyme activity (r(s) = 0.479), between protein stability and allelic phenotype (r(s) = -0.458), as well as between enzyme activity and allelic phenotype (r(s) = 0.799). Enzyme stability algorithms (FoldX and SNPs3D), allelic phenotype and enzyme activity were most powerful to predict patients' phenotype and tetrahydrobiopterin response. Phenotype prediction was most accurate in deleterious genotypes (≈ 100%), followed by homozygous (92.9%), hemizygous (94.8%), and compound heterozygous genotypes (77.9%), while tetrahydrobiopterin response was correctly predicted in 71.0% of all cases. To our knowledge this is the largest study using algorithms for the prediction of patients' phenotype and tetrahydrobiopterin responsiveness in phenylketonuria patients, using data from the locus-specific and genotypes database.

Phenylketonuria Scientific Review Conference: state of the science and future research needs

New developments in the treatment and management of phenylketonuria (PKU) as well as advances in molecular testing have emerged since the National Institutes of Health 2000 PKU Consensus Statement was released. An NIH State-of-the-Science Conference was convened in 2012 to address new findings, particularly the use of the medication sapropterin to treat some individuals with PKU, and to develop a research agenda. Prior to the 2012 conference, five working groups of experts and public members met over a 1-year period. The working groups addressed the following: long-term outcomes and management across the lifespan; PKU and pregnancy; diet control and management; pharmacologic interventions; and molecular testing, new technologies, and epidemiologic considerations. In a parallel and independent activity, an Evidence-based Practice Center supported by the Agency for Healthcare Research and Quality conducted a systematic review of adjuvant treatments for PKU; its conclusions were presented at the conference. The conference included the findings of the working groups, panel discussions from industry and international perspectives, and presentations on topics such as emerging treatments for PKU, transitioning to adult care, and the U.S. Food and Drug Administration regulatory perspective. Over 85 experts participated in the conference through information gathering and/or as presenters during the conference, and they reached several important conclusions. The most serious neurological impairments in PKU are preventable with current dietary treatment approaches. However, a variety of more subtle physical, cognitive, and behavioral consequences of even well-controlled PKU are now recognized. The best outcomes in maternal PKU occur when blood phenylalanine (Phe) concentrations are maintained between 120 and 360 μmol/L before and during pregnancy. The dietary management treatment goal for individuals with PKU is a blood Phe concentration between 120 and 360 μmol/L. The use of genotype information in the newborn period may yield valuable insights about the severity of the condition for infants diagnosed before maximal Phe levels are achieved. While emerging and established genotype-phenotype correlations may transform our understanding of PKU, establishing correlations with intellectual outcomes is more challenging. Regarding the use of sapropterin in PKU, there are significant gaps in predicting response to treatment; at least half of those with PKU will have either minimal or no response. A coordinated approach to PKU treatment improves long-term outcomes for those with PKU and facilitates the conduct of research to improve diagnosis and treatment. New drugs that are safe, efficacious, and impact a larger proportion of individuals with PKU are needed. However, it is imperative that treatment guidelines and the decision processes for determining access to treatments be tied to a solid evidence base with rigorous standards for robust and consistent data collection. The process that preceded the PKU State-of-the-Science Conference, the conference itself, and the identification of a research agenda have facilitated the development of clinical practice guidelines by professional organizations and serve as a model for other inborn errors of metabolism.

Phenylketonuria: translating research into novel therapies

Phenylketonuria (PKU) is an inborn error of metabolism of the amino acid phenylalanine. It is an autosomal recessive disorder with a rate of incidence of 1 in 10,000 in Caucasian populations. Mutations in the phenylalanine hydroxylase (PAH) gene are the major cause of PKU, due to the loss of the catalytic activity of the enzyme product PAH. Newborn screening for PKU allows early intervention, avoiding irreparable neurological damage and intellectual disability that would arise from untreated PKU. The current primary treatment of PKU is the limitation of dietary protein intake, which in the long term may be associated with poor compliance in some cases and other health problems due to malnutrition. The only alternative therapy currently approved is the supplementation of BH4, the requisite co-factor of PAH, in the orally-available form of sapropterin dihydrochloride. This treatment is not universally available, and is only effective for a proportion (estimated 30%) of PKU patients. Research into novel therapies for PKU has taken many different approaches to address the lack of PAH activity at the core of this disorder: enzyme replacement via virus-mediated gene transfer, transplantation of donor liver and recombinant PAH protein, enzyme substitution using phenylalanine ammonia lyase (PAL) to provide an alternative pathway for the metabolism of phenylalanine, and restoration of native PAH activity using chemical chaperones and nonsense read-through agents. It is hoped that continuing efforts into these studies will translate into a significant improvement in the physical outcome, as well as quality of life, for patients with PKU.

The Molecular Bases of Phenylketonuria (PKU) in New South Wales, Australia: Mutation Profile and Correlation with Tetrahydrobiopterin (BH4) Responsiveness

Phenylketonuria (PKU) is an autosomal recessive inborn error of phenylalanine metabolism predominantly caused by mutations in the phenylalanine hydroxylase (PAH) gene. Mutation screening was carried out in a large cohort of PKU patients from New South Wales, Australia. Pathogenic mutations were identified in 99% of the alleles screened, with the two most common mutations (p.R408W and IVS12+1G>A) accounting for 30.7% of alleles. Most individuals were compound heterozygotes for previously reported mutations, but four novel mutations (c.163+1G>T, c.164-2A>G, c.461A>T [p.Y154F], and c.510-1G>A) and a novel polymorphism (c.60+62C>T) were also identified. A number of patients have been previously tested for their response to dietary supplementation of tetrahydrobiopterin (BH4), the cofactor of PAH. Correlation between genotype and the responses revealed that although genotype is a major determinant of BH4 responsiveness, patients with the same genotype may also show disparate responses to this treatment. A clinical and biochemical evaluation should be undertaken to determine the effectiveness of PKU treatment by supplementation of BH4.

Influence of PAH Genotype on Sapropterin Response in PKU: Results of a Single-Center Cohort Study

OBJECTIVE

Identifying phenylalanine hydroxylase (PAH) mutations associated with sapropterin response in phenylketonuria (PKU) would be an advantageous means to determine clinical benefit to sapropterin therapy.

METHODS

Sapropterin response, defined as a ≥30 % reduction in phenylalanine (Phe) levels after a dose of 10 mg/kg/day sapropterin for week one and 20 mg/kg/day for week two in 112 PKU patients aged 4-45 years, was assessed in an outpatient setting. PAH was sequenced in all patients. Mutations were correlated with sapropterin response. Dietary Phe intake was increased over a 6-week period in responsive patients.

RESULTS

Forty-six of 112 patients were sapropterin responsive. Genotypes p.[L48S];[L48S] and p.[Y414C];[Y414C] were always associated with response at a low dose. The mutation Y414C (present on 16 alleles) was most frequently associated with response. Patients with presence of the mutation L48S on at least one allele (12 alleles in 7 patients) always showed response to sapropterin. Responsive patients had a mean Phe tolerance increase of 189 % (range 11-742 %). In the 66 nonresponders, mutations R408W (38 alleles) and IVS12+1G>A (18 alleles) were detected most frequently. Genotypes [IVS12+1G>A];[IVS12+1G>A], p.[L348V];[R408W], p.[P281L];[P281L], p.[R158Q];[R408W], and p.[R261Q];[R408W] were always associated with nonresponse.

CONCLUSION

Data from the study contributes to growing evidence of the relationship between PAH genotype and PKU phenotype. In most cases, response to sapropterin therapy cannot be predicted based on the presence of a single mutation on one allele alone, although the complete PAH genotype may help to predict sapropterin responsiveness in PKU patients.

Sapropterin dihydrochloride for the treatment of hyperphenylalaninemias

INTRODUCTION

Phenylketonuria (PKU) is caused by mutation of the enzyme, phenylalanine (Phe) hydroxylase (PAH). The hyperphenylalaninemia characteristic of PKU causes devastating neurological damage if not identified and treated at birth with a Phe-restricted diet. Sapropterin dihydrochloride, a pharmaceutical formulation of the natural cofactor for PAH (6R-tetrahydrobiopterin; BH4), is now available for the management of hyperphenylalaninemia in some PKU patients, including BH4 deficiencies. Sapropterin dihydrochloride improves dietary Phe tolerance in about 20% of patients with PKU.

AREAS COVERED

This evaluation describes the identification of patients suitable for treatment of sapropterin dihydrochloride, together with its indications, therapeutic properties and efficacy. Furthermore, the article reviews its safety and tolerability in patients with PKU or BH4 deficiency.

EXPERT OPINION

A reduction in blood Phe of at least 30% occurred in ∼ 20 - 30% of sapropterin-treated PKU patients (mostly with milder forms of PKU). Treatment with sapropterin resulted in clinically significant and sustained reductions in blood Phe concentrations and increased dietary Phe tolerance in well-designed clinical studies in PKU patients who responded to BH4. Successful treatment with sapropterin may lead to a relaxation of the Phe-restricted diet, although continued monitoring of blood Phe is required. Sapropterin was well tolerated.

Tetrahydrobiopterin, its mode of action on phenylalanine hydroxylase, and importance of genotypes for pharmacological therapy of phenylketonuria

In about 20%-30% of phenylketonuria (PKU) patients (all phenotypes of PAH deficiency), Phe levels may be controlled through phenylalanine hydroxylase cofactor tetrahydrobiopterin therapy. These patients can be diagnosed by an oral tetrahydrobiopterin challenge and are characterized by mutations coding for proteins with substantial residual PAH activity. They can be treated with a commercially available synthetic form of tetrahydrobiopterin, either as a monotherapy or as adjunct to the diet. This review article summarizes molecular and metabolic bases of PKU and the importance of the tetrahydrobiopterin loading test used for PKU patients. On the basis of in vitro residual PAH activity, more than 1,200 genotypes from patients challenged with tetrahydrobiopterin were categorized as predictive for tetrahydrobiopterin responsiveness or non-responsiveness and correlated with the loading test, phenotype, and residual in vitro PAH activity. The coexpression of two distinct PAH mutant alleles revealed possible dominance effects (positive or negative) by one of the mutations on residual activity as result of interallelic complementation. The treatment of the transfected cells with tetrahydrobiopterin showed an increase in residual PAH activity with several mutations coexpressed.

Long-term follow-up and outcome of phenylketonuria patients on sapropterin: a retrospective study

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.

Pitfalls in phenylalanine loading test in the diagnosis of dopa-responsive dystonia

Phenylalanine (Phe) loading test is a useful tool in the differential diagnosis of dopa-responsive dystonia due to autosomal dominant or recessive GTP cyclohydrolase I (GTPCH) deficiency or autosomal recessive sepiapterin reductase (SR) deficiency. In these patients hepatic phenylalanine hydroxylase system is compromised due to subnormal tetrahydrobiopterin (BH(4)) levels and hydroxylation of phenylalanine (Phe) to tyrosine (Tyr) is reduced with elevated Phe/Tyr ratio 1-2 h after oral Phe administration (100 mg/kg bw) administration. In healthy persons there is only a modest increase in Tyr production and blood Phe normalizes after 4 h. We report on a challenge with Phe (100 mg/kg bw) in a patient with dopa-responsive dystonia while on therapy with BH(4) and l-dopa. During Phe challenge Phe concentration remained below the normal range while a transient mild hypertyrosinemia was observed, leading to an extremely low Phe/Tyr ratio. A repeated test, after BH(4) withdrawal, reversed the findings and resulted normal. These data suggest activation of hepatic phenylalanine hydroxylase by BH(4). Thus, the Phe loading test should not be performed during substitution with BH(4).

Testing for tetrahydrobiopterin responsiveness in patients with hyperphenylalaninemia due to phenylalanine hydroxylase deficiency

INTRODUCTION

Pharmacological levels of the phenylalanine hydroxylase enzyme cofactor, tetrahydrobiopterin (BH4), reduce plasma phenylalanine levels in some patients with phenylketonuria (PKU), providing the first pharmacological therapy for PKU. Responsiveness to this therapy must be determined empirically through a BH4 loading test or trial. The authors have analyzed the loading tests currently in use in light of the numerous factors that can influence their results. Sapropterin dihydrochloride is a stable, synthetic form of BH4 approved for treatment of PKU in responsive patients.

METHODS

An expert panel identified evidence from published reports of clinical experience. Reports of research involving at least 25 patients and published in English were considered.

RESULTS

In all, 14 studies met both criteria; eight employing the sapropterin dihydrochloride preparation from Schircks Laboratories and six the sapropterin dihydrochloride preparation from Biomarin/Merck Serono.

CONCLUSION

The arbitrary responsiveness definition of a >30% reduction in blood phenylalanine appears to be a good compromise between sensitivity and specificity for the initial screening test. However, individual patient characteristics should be considered when interpreting results, especially in patients with low baseline phenylalanine levels.

Hyperphenylalaninemia in the Czech Republic: genotype-phenotype correlations and in silico analysis of novel missense mutations

BACKGROUND

Hyperphenylalaninemia (HPA) is one of the most common inherited metabolic disorders caused by deficiency of the enzyme phenylalanine hydroxylase (PAH). HPA is associated with mutations in the PAH gene, which leads to reduced protein stability and/or impaired catalytic function. Currently, almost 700 different disease-causing mutations have been described. The impact of mutations on enzyme activity varies ranging from classical PKU, mild PKU, to non-PKU HPA phenotype.

METHODS

We provide results of molecular genetic diagnostics of 665 Czech unrelated HPA patients, structural analysis of missense mutations associated with classical PKU and non-PKU HPA phenotype, and prediction of effects of 6 newly discovered HPA missense mutations using bioinformatic approaches and Molecular Dynamics simulations.

RESULTS

Ninety-eight different types of mutations were indentified. Thirteen of these were novel (6 missense, 2 nonsense, 1 splicing, and 4 small gene rearrangements). Structural analysis revealed that classical PKU mutations are more non-conservative compared to non-PKU HPA mutations and that specific sequence and structural characteristics of a mutation might be critical when distinguishing between non-PKU HPA and classical PKU mutations. The greatest impact was predicted for the p.(Phe263Ser) mutation while other novel mutations p.(Asn167Tyr), p.(Thr200Asn), p.(Asp229Gly), p.(Leu358Phe), and p.(Ile406Met) were found to be less deleterious.

Sapropterin dihydrochloride for phenylketonuria

BACKGROUND

Phenylketonuria results from a deficiency of the enzyme phenylalanine hydroxylase. Dietary restriction of phenylalanine keeps blood phenylalanine concentration low. Most natural foods are excluded from diet and supplements are used to supply other nutrients. Recent publications report a decrease in blood phenylalanine concentration in some patients treated with sapropterin dihydrochloride. We examined the evidence for the use of sapropterin dihydrochloride to treat phenylketonuria.  

OBJECTIVES

To assess the safety and efficacy of sapropterin dihydrochloride in lowering blood phenylalanine concentration in people with phenylketonuria.

SEARCH METHODS

We identified relevant trials from the Group's Inborn Errors of Metabolism Trials Register. Date of last search: 29 June 2012.We also searched ClinicalTrials.gov and Current controlled trials. Last search: 23 July 2012.We contacted the manufacturers of the drug (BioMarin Pharmaceutical Inc.) for information regarding any unpublished trials.

SELECTION CRITERIA

Randomized controlled trials comparing sapropterin with no supplementation or placebo in people with phenylketonuria due to phenylalanine hydroxylase deficiency.

DATA COLLECTION AND ANALYSIS

Two authors independently assessed trials and extracted outcome data.

MAIN RESULTS

Two placebo-controlled trials were included. One trial administered 10 mg/kg/day sapropterin in 89 children and adults with phenylketonuria whose diets were not restricted and who had previously responded to saproterin.This trial measured change in blood phenylalanine concentration. The second trial screened 90 children (4 to 12 years) with phenylketonuria whose diet was restricted, for responsiveness to sapropterin. Forty-six responders entered the placebo-controlled part of the trial and received 20 mg/kg/day sapropterin. This trial measured change in both phenylalanine concentration and protein tolerance. Both trials reported adverse events. The trials showed an overall low risk of bias; but both are Biomarin-sponsored. One trial showed a significant lowering in blood phenylalanine concentration in the sapropterin group (10 mg/kg/day), mean difference -238.80 μmol/L (95% confidence interval -343.09 to -134.51); a second trial (20 mg/kg/day sapropterin) showed a non-significant difference, mean difference -51.90 μmol/L (95% confidence interval -197.27 to 93.47). The second trial also reported a significant increase in phenylalanine tolerance, mean difference18.00 mg/kg/day (95% confidence interval 12.28 to 23.72) in the 20 mg/kg/day sapropterin group.

AUTHORS' CONCLUSIONS

There is evidence of short-term benefit from using sapropterin in some patients with sapropterin-responsive forms of phenylketonuria; blood phenylalanine concentration is lowered and protein tolerance increased. There are no serious adverse events associated with using sapropterin in the short term.There is no evidence on the long-term effects of sapropterin and no clear evidence of effectiveness in severe phenylketonuria.

Hepatocyte Transplantation Using the Domino Concept in a Child with Tetrabiopterin Nonresponsive Phenylketonuria

Phenylketonuria is a metabolic disease caused by phenylalanine hydroxylase deficiency. Treatment is based on a strict natural protein-restricted diet that is associated with the risk of malnutrition and severe psychosocial burden. Oral administration of tetrahydrobiopterin can increase residual enzyme activity, but most patients with severe clinical phenotypes are nonresponders. We performed liver cell transplantation in a 6-year-old boy with severe tetrahydrobiopterin nonresponsive phenylketonuria who failed to comply with diet prescriptions. The transplanted hepatocytes were obtained in part from an explanted glycogen storage type 1b liver. Following two infusions, blood phenylalanine levels returned within the therapeutic target while the phenylalanine half-life assessed by loading tests decreased from 43 to 19 h. However, 3 months later, blood phenylalanine concentrations increased and the phenylalanine intake had to be reduced. Cell-based therapy is a promising therapeutic option in phenylketonuria, and the domino concept may solve the issue of cell sources for hepatocyte transplantation.

New evidence for assessing tetrahydrobiopterin (BH(4)) responsiveness

OBJECTIVE

To evaluate the protocol we propose for detecting BH(4)-responsive patients and the possibility of delimiting more precisely the population to be tested.

METHODS

We recruited 102 phenylketonuric patients on a phenylalanine (Phe)-restricted diet. The initial stage of the protocol was a 24-h BH(4) loading test involving Phe loading and subsequent ingestion of the cofactor, a 50% fall in blood Phe levels being considered a positive response. The non-responders at this stage then completed a one-week therapeutic test combining BH(4) administration and daily protein intake meeting recommended dietary allowances, to assess whether the 24-h test had detected all responders.

RESULTS

The 24-h test detected almost all BH(4) responders (30.3% of the 99 patients included in the analysis), with just two patients (2.0%) subsequently responding positively to the therapeutic test. The 24-h test did not give any false positive results.

CONCLUSIONS

The 24-h BH(4) loading test is clinically useful for screening phenylketonuric patients. Specifically, 95% of patients with Phe levels <700 μmol/L, and none with Phe levels >1500 μmol/L were BH(4)-responsive. Given these results, we conclude that patients with Phe levels<700 μmol/L or>1500 μmol/L probably do not need to be tested, prioritising the identification of BH(4)-responsiveness among individuals with intermediate Phe concentrations, between the aforementioned values. Additionally, our results suggest that the therapeutic test only needs to be performed in cases where the reduction in blood Phe levels after cofactor administration is within the range 40%-50%.