Pharmacokinetics of orally administered tetrahydrobiopterin in patients with phenylalanine hydroxylase deficiency

Preliminary Investigation of Microbiome and Dietary Differences in Patients with Phenylketonuria on Enzyme Substitution Therapy Compared to Traditional Therapies

BACKGROUND

Phenylketonuria (PKU) is an inborn error of metabolism that impairs the function of the enzyme phenylalanine hydroxylase (PAH). Historical treatment includes limiting dietary phenylalanine (Phe) consumption while supplementing with medical food; however, this treatment has been associated with complications, such as nutritional deficiencies and disruptions in the gut microbiota.

OBJECTIVE

The study aim was to compare dietary and gut microbiome differences between adult patients on a traditional PKU diet to those receiving the enzyme substitution therapy Palynziq on a liberalized diet while controlling blood Phe levels to <600 μmol/L.

DESIGN

A cross-sectional study was conducted comparing patients on a traditional Phe-restricted diet with patients receiving Palynziq eating a liberalized diet.

PARTICIPANTS/SETTING

Six patients eating a traditional Phe-restricted diet with medical food and six patients on Palynziq eating a liberalized diet without medical food intake for >3 years were selected from the University of Kentucky Metabolic Clinic August to December 2019.

MAIN OUTCOME MEASURES

Nutrient intake from three-day diet records and fecal microbiome taxonomic abundances were analyzed.

STATISTICAL ANALYSIS

Mann-Whitney U-tests were used for dietary data analysis. Differential abundance analysis for microbiome taxa and pathway data was done using DESeq2 analysis.

RESULTS

Dietary data showed Palynziq patients consumed a lower percent of kilocalories from total protein and lower amounts of most micronutrients but consumed greater amounts of intact protein and cholesterol (P<0.05). Microbiome data revealed a greater abundance of the phylum Verrucomicrobia and genus Lachnobacterium in the Traditional group and a greater abundance of the genus Prevotella in the Palynziq group (P<0.05). Pathway analysis depicted greater enrichment in carotenoid and amino acid metabolism pathways in the Traditional group (P<0.05). Protein (%kcal), dietary fiber (g), fat (%kcal), linolenic acid (%DRI), and age were correlated with the underlying microbial community structure for both groups combined.

CONCLUSIONS

Patients with PKU treated with Palynziq on a liberalized diet manifest significant differences in diet composition compared to those treated with traditional Phe-restricted diets. Several of these dietary differences may affect the microbiome architecture.

Tetrahydrobiopterin treatment in phenylketonuria: A repurposing approach

In phenylketonuria (PKU) patients, early diagnosis by neonatal screening and immediate institution of a phenylalanine-restricted diet can prevent severe intellectual impairment. Nevertheless, outcome remains suboptimal in some patients asking for additional treatment strategies. Tetrahydrobiopterin (BH4 ) could be one of those treatment options, as it may not only increase residual phenylalanine hydroxylase activity in BH4 -responsive PKU patients, but possibly also directly improves neurocognitive functioning in both BH4 -responsive and BH4 -unresponsive PKU patients. In the present review, we aim to further define the theoretical working mechanisms by which BH4 might directly influence neurocognitive functioning in PKU having passed the blood-brain barrier. Further research should investigate which of these mechanisms are actually involved, and should contribute to the development of an optimal BH4 treatment regimen to directly improve neurocognitive functioning in PKU. Such possible repurposing approach of BH4 treatment in PKU may improve neuropsychological outcome and mental health in both BH4 -responsive and BH4 -unresponsive PKU patients.

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.

Secondary BH4 deficiency links protein homeostasis to regulation of phenylalanine metabolism

Metabolic control of phenylalanine concentrations in body fluids is essential for cognitive development and executive function. The hepatic phenylalanine hydroxylating system is regulated by the ratio of l-phenylalanine, which is substrate of phenylalanine hydroxylase (PAH), to the PAH cofactor tetrahydrobiopterin (BH4). Physiologically, phenylalanine availability is governed by nutrient intake, whereas liver BH4 is kept at constant level. In phenylketonuria, PAH deficiency leads to elevated blood phenylalanine and is often caused by PAH protein misfolding with loss of function. Here, we report secondary hepatic BH4 deficiency in Pah-deficient mice. Alterations in de novo synthesis and turnover of BH4 were ruled out as molecular causes. We demonstrate that kinetically instable and aggregation-prone variant Pah proteins trap BH4, shifting the pool of free BH4 towards bound BH4. Interference of PAH protein misfolding with metabolite-based control of l-phenylalanine turnover suggests a mechanistic link between perturbation of protein homeostasis and disturbed regulation of metabolic pathways.

The complete European guidelines on phenylketonuria: diagnosis and treatment

Phenylketonuria (PKU) is an autosomal recessive inborn error of phenylalanine metabolism caused by deficiency in the enzyme phenylalanine hydroxylase that converts phenylalanine into tyrosine. If left untreated, PKU results in increased phenylalanine concentrations in blood and brain, which cause severe intellectual disability, epilepsy and behavioural problems. PKU management differs widely across Europe and therefore these guidelines have been developed aiming to optimize and standardize PKU care. Professionals from 10 different European countries developed the guidelines according to the AGREE (Appraisal of Guidelines for Research and Evaluation) method. Literature search, critical appraisal and evidence grading were conducted according to the SIGN (Scottish Intercollegiate Guidelines Network) method. The Delphi-method was used when there was no or little evidence available. External consultants reviewed the guidelines. Using these methods 70 statements were formulated based on the highest quality evidence available. The level of evidence of most recommendations is C or D. Although study designs and patient numbers are sub-optimal, many statements are convincing, important and relevant. In addition, knowledge gaps are identified which require further research in order to direct better care for the future.

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.

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.

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.

Phenylalanine hydroxylase deficiency

Phenylalanine hydroxylase deficiency is an autosomal recessive disorder that results in intolerance to the dietary intake of the essential amino acid phenylalanine. It occurs in approximately 1:15,000 individuals. Deficiency of this enzyme produces a spectrum of disorders including classic phenylketonuria, mild phenylketonuria, and mild hyperphenylalaninemia. Classic phenylketonuria is caused by a complete or near-complete deficiency of phenylalanine hydroxylase activity and without dietary restriction of phenylalanine most children will develop profound and irreversible intellectual disability. Mild phenylketonuria and mild hyperphenylalaninemia are associated with lower risk of impaired cognitive development in the absence of treatment. Phenylalanine hydroxylase deficiency can be diagnosed by newborn screening based on detection of the presence of hyperphenylalaninemia using the Guthrie microbial inhibition assay or other assays on a blood spot obtained from a heel prick. Since the introduction of newborn screening, the major neurologic consequences of hyperphenylalaninemia have been largely eradicated. Affected individuals can lead normal lives. However, recent data suggest that homeostasis is not fully restored with current therapy. Treated individuals have a higher incidence of neuropsychological problems. The mainstay of treatment for hyperphenylalaninemia involves a low-protein diet and use of a phenylalanine-free medical formula. This treatment must commence as soon as possible after birth and should continue for life. Regular monitoring of plasma phenylalanine and tyrosine concentrations is necessary. Targets of plasma phenylalanine of 120-360 μmol/L (2-6 mg/dL) in the first decade of life are essential for optimal outcome. Phenylalanine targets in adolescence and adulthood are less clear. A significant proportion of patients with phenylketonuria may benefit from adjuvant therapy with 6R-tetrahydrobiopterin stereoisomer. Special consideration must be given to adult women with hyperphenylalaninemia because of the teratogenic effects of phenylalanine. Women with phenylalanine hydroxylase deficiency considering pregnancy should follow special guidelines and assure adequate energy intake with the proper proportion of protein, fat, and carbohydrates to minimize risks to the developing fetus. Molecular genetic testing of the phenylalanine hydroxylase gene is available for genetic counseling purposes to determine carrier status of at-risk relatives and for prenatal testing.

New insights into tetrahydrobiopterin pharmacodynamics from Pah enu1/2, a mouse model for compound heterozygous tetrahydrobiopterin-responsive phenylalanine hydroxylase deficiency

Phenylketonuria (PKU), an autosomal recessive disease with phenylalanine hydroxylase (PAH) deficiency, was recently shown to be a protein misfolding disease with loss-of-function. It can be treated by oral application of the natural PAH cofactor tetrahydrobiopterin (BH(4)) that acts as a pharmacological chaperone and rescues enzyme function in vivo. Here we identified Pah(enu1/2) bearing a mild and a severe mutation (V106A/F363S) as a new mouse model for compound heterozygous mild PKU. Although BH(4) treatment has become established in clinical routine, there is substantial lack of knowledge with regard to BH(4) pharmacodynamics and the effect of the genotype on the response to treatment with the natural cofactor. To address these questions we applied an elaborate methodological setup analyzing: (i) blood phenylalanine elimination, (ii) blood phenylalanine/tyrosine ratios, and (iii) kinetics of in vivo phenylalanine oxidation using (13)C-phenylalanine breath tests. We compared pharmacodynamics in wild-type, Pah(enu1/1), and Pah(enu1/2) mice and observed crucial differences in terms of effect size as well as effect kinetics and dose response. Results from in vivo experiments were substantiated in vitro after overexpression of wild-type, V106A, and F263S in COS-7 cells. Pharmacokinetics did not differ between Pah(enu1/1) and Pah(enu1/2) indicating that the differences in pharmacodynamics were not induced by divergent pharmacokinetic behavior of BH(4). In conclusion, our findings show a significant impact of the genotype on the response to BH(4) in PAH deficient mice. This may lead to important consequences concerning the diagnostic and therapeutic management of patients with PAH deficiency underscoring the need for individualized procedures addressing pharmacodynamic aspects.

Unresponsiveness to tetrahydrobiopterin of phenylalanine hydroxylase deficiency

Conflicting results have been reported concerning the efficacy of tetrahydrobiopterin (BH4), the cofactor of phenylalanine hydroxylase, for reducing phenylalanine (Phe) concentration in phenylketonuria (PKU). We aimed to test quantitatively the effects of BH4 in PKU patients. Seven fully characterized patients were selected among a population of 130 PKU subjects as harboring PKU mutations predicted as BH4 responsive and previously considered responsive to a cofactor challenge. They received a simple Phe (100 mg/kg) and 2 combined Phe (100 mg/kg) and BH4 (20 mg/kg) oral loading tests. Cofactor was administered either before or after the amino acid. The concentrations of Phe, tyrosine (Tyr), and biopterin were measured over 24 hours after loading. The comparative analysis of the loading tests showed that in all patients plasma Phe concentrations peaked within 3 hours, and fell within 24 hours by about 50% in benign, 20% in mild, and 15% in severe phenylalanine hydroxylase deficiency regardless of BH4 administration. A consistent or moderate increase of plasma Tyr, again independent of the cofactor challenge, was observed only in the less severe forms of PAH deficiency. Mean blood biopterin concentration increased 6 times after simple Phe and 34 to 39 times after combined loading tests. The administration of BH4 does not alter Phe and Tyr metabolism in PKU patients. The clearance of plasma Phe after oral loading and, as well as Tyr production, is not related to cofactor challenge but to patient's phenotype. The assessment of BH4 responsiveness by the methods so far used is not reliable, and the occurrence of BH4-responsive forms of PKU still has to be definitely proven.

Relative bioavailability of sapropterin from intact and dissolved sapropterin dihydrochloride tablets and the effects of food: a randomized, open-label, crossover study in healthy adults

BACKGROUND

Phenylketonuria (PKU) is an autosomal recessive metabolic disorder characterized by hyperphenylalaninemia in association with neurocognitive and neuromotor impairment. Sapropterin dihydrochloride (hereafter referred to as sapropterin) administered orally as dissolved tablets is approved by the US Food and Drug Administration for hyperphenylalaninemia in patients with tetrahydrobiopterin responsive PKU.

OBJECTIVES

This study compared the relative oral bioavailability of sapropterin when administered as intact and dissolved tablets. It also assessed the effect of food on the oral bioavailability of sapropterin administered as intact tablets.

METHODS

This was a randomized, open-label, 3-treatment, 6-sequence, 3-period crossover study in healthy male and female subjects. Subjects were randomized to receive single oral 10-mg/kg doses of sapropterin administered as dissolved tablets after a fast; as intact tablets after a fast; and as intact tablets with a high-calorie, high-fat meal. The 3 dosing periods were separated by a washout period of at least 7 days. In each dosing period, blood samples were obtained within 40 minutes before and at 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 5, 6, 8, 10, 12, 18, and 24 hours after dosing. A follow-up assessment was performed 5 to 7 days after the last dosing period. The relative bioavailability of sapropterin from the 3 dosing regimens was assessed based on C(max), AUC(0-t), and AUC(0-infinity), estimated from calculated plasma tetrahydrobiopterin concentrations using a noncompartmental model. Safety assessments included physical examinations, clinical laboratory tests, and ECGs at the beginning and end of the study. Vital signs were monitored periodically during each treatment period.

RESULTS

The study enrolled 32 healthy subjects (16 men, 16 women) with a mean (SD) age of 29.2 (9.0) years, height of 172.7 (10.0) cm, weight of 73.0 (13.9) kg, and body mass index ranging from 18 to 30 kg/m(2). Twenty-three were white, 5 African American, 2 Asian/Pacific Islander, 1 Hispanic, and 1 Native American. The estimated geometric mean ratio of AUC(0-t) for intact compared with dissolved tablets under fasting conditions was 141.24% (90% CI, 122.05-163.43), and the geometric mean ratio of AUC(0-t) for intact tablets under fed compared with fasting conditions was 143.46% (90% CI, 124.22-165.69). Nine subjects (28.1%) reported a total of 20 treatment-emergent adverse events (AEs). The most frequently reported AEs were gastrointestinal disorders (6 subjects [18.8%]) and central nervous system disorders (4 [12.5%]). Eight AEs considered possibly or probably related to sapropterin were reported by 4 subjects (12.5%); these were of mild severity and gastrointestinal in nature. No severe or serious AEs or discontinuations due to AEs occurred during the study.

CONCLUSIONS

Administration of sapropterin as intact tablets and with a high-calorie, high-fat meal was associated with increased drug exposure. Oral administration of sapropterin 10 mg/kg as intact tablets with or without food was generally well tolerated.

Sapropterin: a new therapeutic agent for phenylketonuria

OBJECTIVE

To summarize the role of pharmacotherapy in the management of phenylketonuria (PKU) and to review the pharmacology, pharmacokinetics, pharmacodynamics, efficacy data, and safety profile of sapropterin for this indication.

DATA SOURCES

A literature search was conducted using MEDLINE (1966-May 2009), International Pharmaceutical Abstracts (1970-May 2009), and Cochrane database (2008) for the following key words: sapropterin, tetrahydrobiopterin, phenylketonurias, and phenylalanine.

STUDY SELECTION AND DATA EXTRACTION

English-language studies involving humans examining the role of tetrahydrobiopterin (BH4) in the management of PKU were reviewed to evaluate the pharmacology, pharmacokinetics, pharmacodynamics, efficacy data, and safety profile for sapropterin. All Phase 2 and 3 randomized controlled trials assessing the safety and efficacy of sapropterin were included in this literature evaluation.

DATA SYNTHESIS

Sapropterin represents the only Food and Drug Administration-approved medication for BH4-responsive PKU, marking an important advance in the treatment of this condition. Among individuals with hyperphenylalaninemia and some residual phenylalanine hydroxylase function, sapropterin can enhance activity of this enzyme to decrease serum phenylalanine concentrations. Sapropterin has been compared with placebo in one Phase 2 and one Phase 3 clinical trial, demonstrating significantly better response rates. Based on available studies, this agent appears to be safe and well tolerated, with adverse event rates similar to those of placebo. However, additional studies are warranted to assess the long-term safety and efficacy of sapropterin therapy.

CONCLUSIONS

Sapropterin offers a promising therapeutic option for select individuals with BH4-responsive PKU, although long-term data are limited evaluating its safety and efficacy in traditional clinical practice settings. When considering sapropterin therapy, clinicians must consider factors such as cost and patient adherence to drug therapy and/or diet.

Pharmacokinetics of tetrahydrobiopterin following oral loadings with three single dosages in patients with phenylketonuria

BACKGROUND

Tetrahydrobiopterin (BH(4)) loading has been performed for many years in patients detected by newborn screening for hyperphenylalaninaemia (HPA) to distinguish BH(4) cofactor synthesis or recycling defects from phenylalanine hydroxylase (PAH)-deficient HPA. Previous studies have shown that the pharmacokinetics of BH(4) shows high intra-individual and inter-individual variability.

METHODS

Seventeen adult patients with PAH-deficient HPA were classified in one of three phenotypic groups (mild, moderate, classical PKU) according to their response to a standardized protein loading test. Genotype information was available for all participants. In a randomized controlled double-blind design, BH(4) loadings in single oral dosages of 10, 20 and 30 mg BH(4)/kg body weight (bw) were performed to assess BH(4) responsiveness. As part of this study, levels of BH(4) metabolites in dried blood spots were studied to provide information on the pharmacokinetics of BH(4) following oral administration.

RESULTS

Levels of biopterin and pterin (B + P) increased significantly with increasing BH(4) dose (p < 0.0001). Maximum B + P levels were reached 4 hours after application of BH(4). There was no significant difference in BH(4) pharmacokinetics between the three phenotypic groups of PKU. Male and female patients showed different levels of BH(4) metabolites following 10 mg BH(4)/kg bw, but not following 20 and 30 mg BH(4)/kg bw. There was no relationship between age of patients and BH(4) pharmacokinetics. There was no correlation between B + P levels and decrease in Phe level (p = 0.69).

CONCLUSION

BH(4) pharmacokinetics are variable between patients regarding absolute levels of BH(4) metabolites reached after BH(4) loading, but are similar regarding the interval to individual maximum B + P levels. Levels of B + P increase significantly with increasing BH(4) doses. There is no correlation between B + P levels and decrease in Phe level.

Classifying tetrahydrobiopterin responsiveness in the hyperphenylalaninaemias

BACKGROUND

A significant percentage of patients with hyperphenylalaninaemia (HPA) due to primary deficiency of the phenylalanine hydroxylase enzyme (PAH) respond to a dose of tetrahydrobiopterin (BH(4)) with an increased rate of phenylalanine (Phe) disposal. The effect is exploited therapeutically, with some patients on BH(4) even tolerating a normal diet.

AIM

Classification of the Phe blood level response to a BH(4) load by percentage reduction (PR) suffers from loss of information: only part of usually more extensive test data is used, and PR values for different times after load cannot be compared directly. Calculation of half-life (t (1/2)) of blood Phe is proposed as an alternative. This classic measure unifies interpretation of tests of different duration (e.g. 8 or 15 h). t (1/2) subsumes first-order formation of tyrosine, of Phe metabolites, and renal Phe excretion; zero-order net protein synthesis can be neglected during short-time tests.

METHOD

t (1/2) is easily and robustly obtained by fit-ting the total set of (3-4) data points to a log-linear regression.

RESULTS

The advantage of calculating t (1/2) is exemplified by the analysis of selected published data. The results clearly speak in favour of an 8 h test period because so-called 'slow' responders could also be detected within this time window and because tests of longer duration are less reliable kinetically. Sequential Phe and Phe/BH(4) loading tests appear advantageous because the 'natural' t (1/2) (without supplementation of BH(4)) is not normally known beforehand.

CONCLUSION

With t (1/2) as a reliable parameter of BH(4) responsiveness, therapeutic decisions would be more rational and genotype-phenotype analysis may also profit.

Sapropterin dihydrochloride, 6-R-L-erythro-5,6,7,8-tetrahydrobiopterin, in the treatment of phenylketonuria

Sapropterin dihydrochloride, 6-R-L-erythro-5,6,7,8-tetrahydrobiopterin (BH4) is being introduced in the US for treatment of phenylketonuria (PKU). This compound has been in use in Europe to treat mild forms of PKU. Tetrahydrobiopterin is the cofactor in the hydroxylation reaction of the three aromatic amino acids phenylalanine, tyrosine and tryptophan. It is also involved in other reactions, which are not the focus of this review. The cofactor BH4 is synthesized in many tissues in the body. The pathway of BH4 biosynthesis is complex, and begins with guanosine triphosphate (GTP). The first reaction that commits GTP to form pterins is GTP cyclohydrolase. Several reactions follow resulting in the active cofactor BH4. During the hydroxylation reaction BH4 is oxidized to quinonoid-BH2, which is recycled by dihydropteridine reductase, resulting in the active cofactor. It was discovered that some patients with PKU had a decline in blood phenylalanine after oral intake of BH4. This response to BH4 is not the result of change in the synthesis or regeneration of the cofactor, but rather an effect on the mutant enzyme phenylalanine hydroxylase either by accommodating the higher K(m) of the mutant enzyme or by acting as a chaperone for the mutant enzyme. This response has become of intense interest in the treatment of PKU.

Effects and clinical significance of tetrahydrobiopterin supplementation in phenylalanine hydroxylase-deficient hyperphenylalaninaemia

In recent years several studies on tetrahydrobiopterin (BH4)-responsive phenylalanine hydroxylase (PAH) deficiency have been published. The molecular mechanisms of BH4 responsiveness are not conclusively understood, but there is evidence that BH4 responsiveness in hyperphenylalaninaemia (HPA) depends on the patient's genotype and residual PAH activity. As a BH4 preparation will soon obtain marketing approval as an alternative treatment for phenylketonuria (PKU), it is particularly important to evaluate this treatment and to define criteria to identify patients with a potential benefit from it. Most of the patients found to be BH4-responsive suffered from mild PKU or mild hyperphenylalaninaemia (MHP) and some of these would not be treated at all in many countries. Of patients with moderate and classic forms of PKU, only a few were classified as responders and the clinical significance of the effect size may be small.

Assessment of tetrahydrobiopterin (BH4) responsiveness in phenylketonuria

OBJECTIVE

To determine the prevalence of and identify subjects with phenylketonuria (PKU; phenylalanine hydroxylase deficiency) responsive to 6R-tetrahydrobiopterin (BH4) and to establish selection criteria for potential treatment with BH4.

STUDY DESIGN

Blood phenylalanine levels from 557 newborns and children with various degrees of PKU (blood phenylalanine, 301 to 4743 micromol/L) challenged with BH4 (20 mg/kg of body weight) were analyzed at 8 and 24 hours after BH4 administration. The 2 modalities were compared in terms of phenylalanine reduction.

RESULTS

The overall prevalence of BH4 responsiveness within patients with PKU for blood phenylalanine reductions of 20%, 30%, 40%, and 50% was 48%, 38%, 31%, and 24%, respectively, using the 8-hour modus and 55%, 46%, 41%, and 33%, respectively, using the 24-hour modus. Using the 30% cutoff, BH4 responsiveness was similar regardless of the modality in patients with mild hyperphenylalaninemia (79% to 83% responders), mild PKU (49% to 60% responders), and classical PKU (7% to 10% responders).

CONCLUSIONS

BH4 responsiveness is more prevalent than was previously assumed, particularly in patients with mild hyperphenylalaninemia and mild PKU. Depending on the severity of hyperphenylalaninemia, selection criteria for the potential treatment with BH4 may range from 20% to 40% blood phenylalanine reduction after 24 hours.

BH4 responsiveness associated to a PKU mutation with decreased binding affinity for the cofactor

BACKGROUND

Tetrahydrobiopterin (BH4), cofactor of phenylalanine hydroxylase, can be used to treat a subset of phenylketonuria (PKU) patients as it results in a reduction in blood phenylalanine levels. The molecular basis of the response appears to be multifactorial.

METHOD

A standard BH4 loading test (20 mg/kg) was performed. Genotyping was performed by DGGE and sequencing analysis. Expression analysis of the D129G mutation was performed in E. coli (expression as fusion protein MBP-PAH) and in a human hepatoma cell line with an N-terminal FLAG epitope.

RESULTS

We report the positive response and long-term treatment of a patient functionally hemizygous for the D129G mutation in the phenylalanine hydroxylase gene. Expression in the prokaryotic system revealed partial activity and a decreased binding affinity for BH4 of the mutant protein. In the eukaryotic system the mutant protein shows reduced stability.

CONCLUSION

The D129G mutation which confers a BH4-responsive phenotype, has a decreased binding affinity for BH4.