Spanish BH4-responsive phenylalanine hydroxylase-deficient patients: evolution of seven patients on long-term treatment with tetrahydrobiopterin

Maximal dietary responsiveness after tetrahydrobiopterin (BH4) in 19 phenylalanine hydroxylase deficiency patients: What super-responders can expect

Background

Inherited phenylalanine hydroxylase deficiency, also known as phenylketonuria (PKU), causes poor growth and neurologic deficits in the untreated state. After ascertainment through newborn screen and dietary phenylalanine (Phe) restriction to achieve plasma Phe in the range of 120-360 μmol/L, these disease manifestations can be prevented. Poor compliance with protein restricted diets supported by medical food is typical in later years, beginning in the late toddler and teenage years. Pharmacologic doses of oral tetrahydrobiopterin (BH4; sapropterin dihydrochloride) is effective in reducing plasma Phe in about 40-50% of PKU patients but effectiveness is highly variable.

Objective

To assess the maximal responsiveness to 20 mg/kg/day oral BH4 as it affects plasma Phe and dietary Phe allowance in PKU patients.

Materials and methods

This was a single-center, retrospective observational study, combining case reports of individual patients. We reported an outcome of 85 patients with PKU who were trialed on BH4. Phe levels and dietary records of 19 BH4 "super-responders" were analyzed.

Results

Overall, 63.5% of the patients (54/85) were considered BH4 responders. However, we quantitated the dietary liberalization of 19 of our responsive patients (35%), those with at least a 2-fold increase in dietary Phe and maintenance of plasma Phe in treatment range. In these "super-responders", the mean plasma Phe at baseline was 371 ± 237 μmol/L and decreased to 284 ± 273 μmol/L after 1 year on BH4. Mean dietary Phe tolerance increased significantly from 595 ± 256 to 2260 ± 1414 mg/day (p ≤0.0001), while maintaining mean plasma Phe levels within treatment range. Four patients no longer required dietary Phe restriction and could discontinue medical food. The majority of patients had at least one BH4-responsive genotype.

Conclusion

This cohort demonstrates the maximally achievable dietary liberalization which some PKU patients may expect with BH4 therapy. Health benefits are considered to accrue in patients with increased intact protein.

Protein Substitute Requirements of Patients with Phenylketonuria on BH4 Treatment: A Systematic Review and Meta-Analysis

The traditional treatment for phenylketonuria (PKU) is a phenylalanine (Phe)-restricted diet, supplemented with a Phe-free/low-Phe protein substitute. Pharmaceutical treatment with synthetic tetrahydrobiopterin (BH4), an enzyme cofactor, allows a patient subgroup to relax their diet. However, dietary protocols guiding the adjustments of protein equivalent intake from protein substitute with BH4 treatment are lacking. We systematically reviewed protein substitute usage with long-term BH4 therapy. Electronic databases were searched for articles published between January 2000 and March 2020. Eighteen studies (306 PKU patients) were eligible. Meta-analyses demonstrated a significant increase in Phe and natural protein intakes and a significant decrease in protein equivalent intake from protein substitute with cofactor therapy. Protein substitute could be discontinued in 51% of responsive patients, but was still required in 49%, despite improvement in Phe tolerance. Normal growth was maintained, but micronutrient deficiency was observed with BH4 treatment. A systematic protocol to increase natural protein intake while reducing protein substitute dose should be followed to ensure protein and micronutrient requirements are met and sustained. We propose recommendations to guide healthcare professionals when adjusting dietary prescriptions of PKU patients on BH4. Studies investigating new therapeutic options in PKU should systematically collect data on protein substitute and natural protein intakes, as well as other nutritional factors.

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.

Improved metabolic control in tetrahydrobiopterin (BH4), responsive phenylketonuria with sapropterin administered in two divided doses vs. a single daily dose

BACKGROUND

Phenylketonuria (PKU) often requires a lifelong phenylalanine (Phe)-restricted diet. Introduction of 6R-tetrahydrobiopterin (BH4) has made a huge difference in the diets of patients with PKU. BH4 is the co-factor of the enzyme phenylalanine hydroxylase (PAH) and improves PAH activity and, thus, Phe tolerance in the diet. A limited number of published studies suggest a pharmacodynamic profile of BH4 more suitable to be administered in divided daily doses.

METHODS

After a 72-h BH4 loading test, sapropterin was initiated in 50 responsive patients. This case-control study was conducted by administering the same daily dose of sapropterin in group 1 (n=24) as a customary single dose or in two divided doses in group 2 (n=26) over 1 year.

RESULTS

Mean daily consumption of Phe increased significantly after the first year of BH4 treatment in group 2 compared to group 1 (p<0.05). At the end of the first year of treatment with BH4, another dramatic difference observed between the two groups was the ability to transition to a Phe-free diet. Eight patients from group 2 and two from group 1 could quit dietary restriction.

CONCLUSIONS

When given in two divided daily doses, BH4 was more efficacious than a single daily dose in increasing daily Phe consumption, Phe tolerance and the ability to transition to a Phe-unrestricted diet at the end of the first year of treatment.

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.

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.

Tetrahydrobiopterin responsiveness in phenylketonuria: prediction with the 48-hour loading test and genotype

BACKGROUND

How to efficiently diagnose tetrahydrobiopterin (BH4) responsiveness in patients with phenylketonuria remains unclear. This study investigated the positive predictive value (PPV) of the 48-hour BH4 loading test and the additional value of genotype.

METHODS

Data of the 48-hour BH4 loading test (20 mg BH4/kg/day) were collected at six Dutch university hospitals. Patients with ≥30% phenylalanine reduction at ≥1 time points during the 48 hours (potential responders) were invited for the BH4 extension phase, designed to establish true-positive BH4 responsiveness. This is defined as long-term ≥30% reduction in mean phenylalanine concentration and/or ≥4 g/day and/or ≥50% increase of natural protein intake. Genotype was collected if available.

RESULTS

177/183 patients successfully completed the 48-hour BH4 loading test. 80/177 were potential responders and 67/80 completed the BH4 extension phase. In 58/67 true-positive BH4 responsiveness was confirmed (PPV 87%). The genotype was available for 120/177 patients. 41/44 patients with ≥1 mutation associated with long-term BH4 responsiveness showed potential BH4 responsiveness in the 48-hour test and 34/41 completed the BH4 extension phase. In 33/34 true-positive BH4 responsiveness was confirmed. 4/40 patients with two known putative null mutations were potential responders; 2/4 performed the BH4 extension phase but showed no true-positive BH4 responsiveness.

CONCLUSIONS

The 48-hour BH4 loading test in combination with a classified genotype is a good parameter in predicting true-positive BH4 responsiveness. We propose assessing genotype first, particularly in the neonatal period. Patients with two known putative null mutations can be excluded from BH4 testing.

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.

Efficacy and safety of BH4 before the age of 4 years in patients with mild phenylketonuria

BACKGROUND

Sapropterin dihydrochloride, an EMEA-approved synthetic formulation of BH4, has been available in Europe since 2009 for PKU patients older than 4 years, but its use with younger children is allowed in France based on an expert recommendation. We report the cases of 15 patients treated under the age of 4 years and demonstrate the safety and efficacy of this treatment for patients in this age group.

PATIENTS AND METHOD

We report the use of BH4 in 15 PKU patients treated before the age of 4 years.

RESULTS

Fifteen patients were enrolled in this retrospective study. Mean phenylalaninemia at diagnosis was 542 ± 164 μM and all patients had mild PKU (maximal phenylalaninemia: 600-1200 μM). BH4 responsiveness was assessed using a 24-hour BH4 loading test (20 mg/kg), performed during the neonatal period (n = 11) or before 18 months of age (n = 4). During the test, these patients exhibited an 80 ± 12% decrease in phenylalaninemia. Long-term BH4 therapy was initiated during the neonatal period (n = 7) or at the age of 13 ± 12 months (n = 8). The median duration of treatment was 23 months [min 7; max 80]. BH4 therapy drastically improved dietary phenylalanine tolerance (456 ± 181 vs 1683 ± 627 mg/day, p < 0.0001) and allowed a phenylalanine-free amino acid mixture to be discontinued or not introduced in 14 patients. Additionally, in the eight patients treated after a few months of diet therapy, BH4 treatment significantly decreased mean phenylalaninemia (352 ± 85 vs 254 ± 64 μM, p < 0.05), raised the percentage of phenylalaninemia tests within therapeutic targets [120-300 μM] (35 ± 25 vs 64 ± 16%, p < 0.05), and reduced phenylalaninemia variance (130 ± 21 vs 93 ± 27 μM, p < 0.05). No side effects were reported.

CONCLUSION

BH4-therapy is efficient and safe before the age of 4 years in mild PKU, BH4-responsive patients.

Nutritional Changes and Micronutrient Supply in Patients with Phenylketonuria Under Therapy with Tetrahydrobiopterin (BH(4))

BACKGROUND

Since 2008 patients with BH(4)-sensitive phenylketonuria can be treated with sapropterin dihydrochloride (Kuvan®) in addition to the classic phenylalanine (Phe) restricted diet. The aim of this study was to evaluate the nutritional changes and micronutrient supply in patients with phenylketonuria (PKU) under therapy with tetrahydrobiopterin (BH(4)).

SUBJECTS AND METHODS

19 children with PKU (4-18 years) and potential BH(4)-sensitivity were included, 14 completed the study protocol. Dried blood Phe concentrations as well as detailed dietary records were obtained throughout the study at preassigned study days.

RESULTS

Eight patients could increase their Phe tolerance from 629 ± 476 mg to 2131 ± 1084 mg (P = 0.006) under BH(4) while maintaining good metabolic control (Phe concentration in dried blood 283 ± 145 μM vs. 304 ± 136 μM, P = 1.0), therefore proving to be BH(4)-sensitive. They decreased their consumption of special low protein products and fruit while increasing their consumption of high protein foods such as processed meat, milk and dairy products. Intake of vitamin D (P = 0.016), iron (P = 0.002), calcium (P = 0.017), iodine (P = 0.005) and zinc (P = 0.046) significantly declined during BH(4) treatment while no differences in energy and macronutrient supply occurred.

CONCLUSION

BH(4)-sensitive patients showed good metabolic control under markedly increased Phe consumption. However, the insufficient supply of some micronutrients needs consideration. Long-term multicenter settings with higher sample sizes are necessary to investigate the changes of nutrient intake under BH(4) therapy to further evaluate potential risks of malnutrition. Supplementation may become necessary.

Utility of phenylalanine hydroxylase genotype for tetrahydrobiopterin responsiveness classification in patients with phenylketonuria

BACKGROUND

A need exists to expand the characterization of tetrahydrobiopterin (BH(4)) responsiveness in patients with phenylketonuria (PKU), beyond simply evaluating change in blood phenylalanine concentrations. The clinical interpretation of BH(4) responsiveness should be evaluated within the context of phenylalanine hydroxylase (PAH) genotype.

AIM

This investigation seeks to use a modified version of a previously developed PAH genotype severity tool, the assigned value (AV) sum, to assess the molecular basis of responsiveness in a clinical cohort and to explore the tool's ability to differentiate BH(4) responsive groups.

METHODS

BH(4) response was previously clinically classified in 58 patients with PKU, with three response groups emerging: definitive responders, provisional responders, and non-responders. Provisional responders represented a clinically ambiguous group, with an initial decrease in plasma phenylalanine concentrations, but limited ability to improve dietary phenylalanine tolerance. In this retrospective analysis, mutations in the PAH gene were identified in each patient. PAH genotype was characterized through the AV sum approach, in which each mutation is given an AV of 1, 2, 4, or 8; the sum of both mutations' AV corresponds to genotype severity, with a lower number representing a more severe phenotype. An AV sum cutoff of 2 (indicative of the most severe genotypes) was used to dichotomize patients and predict BH(4) responsiveness. Provisional responders were classified with the definitive responders then the non-responders to see with which group they best aligned.

RESULTS

In 17/19 definitive responders, at least one mutation was mild or moderate in severity (AV sum>2). In contrast, 7/9 provisional responders carried two severe or null mutations (AV sum=2), suggesting little molecular basis for responsiveness. Non-responders represent a heterogeneous group with 15/25 patients carrying two severe mutations (AV sum=2), 5/25 patients carrying one moderate or mild mutation in combination with a severe or null mutation (AV sum>2), and the remaining five patients carrying an uncharacterized mutation in combination with a severe mutation. Predictive sensitivity of the AV sum was maximized (89.5% vs. 67.9%) with limited detriment to specificity (79.4% vs. 80.0%), by classifying provisional responders with the non-responders rather than with the definitive responders.

CONCLUSIONS

In our clinical cohort, the AV sum tool was able to identify definitive responders with a high degree of sensitivity. As demonstrated by both the provisional responder group and the substantial number of non-responders with AV sums>2, a potential exists for misclassification when BH(4) response is determined by relying solely on change in plasma phenylalanine concentrations. PAH genotype should be incorporated in the clinical evaluation of BH(4) responsiveness.

Physical development in patients with phenylketonuria on dietary treatment: a retrospective study

OBJECTIVES

To evaluate the growth and physical development in patients with phenylalanine hydroxylase deficiency who follow exclusively dietary treatment.

METHODS

Anthropometric measurements of 160 patients with hyperphenylalaninemia who were followed at our center over a 25 year period were obtained. Only patients treated exclusively with a protein-restrictive diet supplemented with amino acid mixtures were included. Height, weight and body mass index were measured at birth, at diagnosis, at 6 and 12 months of age, and annually until 18 years of age in patients with phenylketonuria or until 9 years of age in patients with mild hyperphenylalaninemia and compared to official national reference values. The final height of PKU patients was also compared to their expected family height.

RESULTS

The analysis of z scores suggested no significant differences in physical development between PKU patients and the healthy population during the study period. The final height of PKU patients revealed that they were 2 to 4 cm taller than expected when compared to the mean family height (p<0.001). The mean weight and BMI at puberty suggested that many patients with severe PKU, but not other phenotypes, were overweight during this period.

CONCLUSION

Physical development can be optimal in PKU patients regardless of their phenotype and the severity of the diet. A tendency to excessive weight gain is seen in adolescence in the most severe phenotypes.

Using change in plasma phenylalanine concentrations and ability to liberalize diet to classify responsiveness to tetrahydrobiopterin therapy in patients with phenylketonuria

Tetrahydrobiopterin (BH(4)) responsiveness is currently defined as a decrease in plasma phenylalanine concentrations in patients with phenylketonuria (PKU). This definition does not offer insight beyond the initial assessment of patients, which may lead to treatment ambiguity in patients who only experience an initial decrease in plasma phenylalanine concentrations. We present our experience with a novel classification approach using sequentially-applied criteria. Plasma phenylalanine concentrations were measured at baseline and after one month of BH(4) therapy (20 mg/kg/day) in 58 PKU patients (34 M, 24 F; age 17.3±11.0 years). Thirty-two patients (55.2%) were classified as "preliminary responders" at one month, experiencing at least a 15% decrease in plasma phenylalanine concentrations. Preliminary responders' ability to liberalize their dietary restrictions was then systematically assessed. "Definitive responders" were defined as preliminary responders who could increase their dietary phenylalanine tolerance by at least 300 mg/day and lower prescribed medical food needs by at least 25% while maintaining metabolic control (plasma phenylalanine ≤360 μmol/L) and consuming adequate dietary protein. Preliminary responders who could not liberalize their diets according to these criteria were classified as "provisional responders." Nineteen patients (32.8% of patients initiating BH(4) therapy) met the definitive responder criteria, increasing dietary phenylalanine tolerance from 704±518 mg/day to 1922±612 mg/day and reducing medical food to 16.7±19.5% of their baseline prescription. Nine patients (15.5% of patients initiating BH(4) therapy) were classified as provisional responders, all remaining on 100% of their baseline medical food prescription. From this classification approach, a subgroup of provisionally responsive patients emerged who experienced an initial decrease in plasma phenylalanine concentrations but who could not substantially increase their dietary phenylalanine tolerance or decrease medical food needs. Diet liberalization is an essential component of BH(4)-responsiveness classification.

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

INTRODUCTION

Recent studies showed that phenylalanine (Phe) plasma concentrations may decrease in some patients with hyperphenylalaninemia (HPA) due to phenylalanine hydroxylase (PAH) deficiency, after the administration of tetrahydrobiopterin (BH(4)).

OBJECTIVE

To determine responsiveness to a single dose of BH(4) administered according to an innovative protocol using a combined Phe and BH(4) loading test in Brazilian phenylketonuria (PKU) patients.

METHODS

Patient age should be ≥ 4 years, and median Phe plasma concentration ≤ 600 μmol/L when following dietary restrictions. Participants received a simple Phe loading test using 100mg/kg L-Phe (Test 1) and a combined Phe+BH(4) loading test using 100mg/kg L-Phe and 20mg/kg/BH(4) (Test 2). Blood samples were collected at baseline and 3, 11 and 27 h after Phe ingestion (T0, T1, T2 and T3). Responsiveness was defined as: criterion A: plasma Phe reduction of ≥ 30% at T1 and T2 for Tests 1 and 2; criterion B: plasma Phe reduction of ≥ 30% at T1 and T3 for Tests 1 and 2; and criterion C: at least 30% difference of the areas under the Phe curve for Tests 1 and 2.

RESULTS

Eighteen patients (median age 12 yrs; 8 classical PKU; 10 mild PKU) participated in the study. Six patients (2 classical PKU; 4 mild PKU) were classified as responsive according to at least one of the criteria. Responsiveness was concordant when criteria A + B we compared with criterion C (kappa = 0.557; p = 0.017). Of the patients whose genotype was available (n = 16), six had data about BH(4)-responsiveness genotypes described in the literature, which were in agreement with our findings.

CONCLUSION

The comparison of simple Phe loading and combined Phe + BH(4) loading seems to be an optimal method to evaluate responsiveness to BH(4) in patients with good metabolic control.

Long-term pharmacological management of phenylketonuria, including patients below the age of 4 years

BH4 therapy is an advancement in the treatment of phenylketonuria, reducing blood phenylalanine (phe) levels and increasing tolerance to natural proteins of responding patients. We report the results of 16 patients undergoing long-term BH4 treatment. Responding patients to BH4 was usually based on 24-h loading tests; a ≥30% decrease in blood phe was considered a positive response. Weekly loading made it possible to identify an additional "slow responder." The 16 responders constitute 24.6% of patients who completed the trial (87.5% of responders in mild hyperphenylalaninemia, 38.1% in mild PKU, and 2.8% in classical PKU).Mean dose of BH4 used was 9.75 ± 0.9 mg/kg per day, during a mean of 62 months. Age at treatment start was below 4 years in seven patients; five of which begun treatment during their first month since birth. All but one patient showed good treatment compliance; six continue on BH4 monotherapy without dietary phe restriction; six showed an increase in phe tolerance of 24-55%; and in the five patients who received treatment since the neonatal period an increase in phe tolerance following the phase of maximum growth has persisted. None of the patients showed side effects except one whom vomiting at the beginning of the treatment.Testing at the time of diagnosis in the neonatal period is very appropriate, and if there is a positive response, the patient can be treated with BH4 from onset with the advantage of being able to continue breast-feeding.

Up to date knowledge on different treatment strategies for phenylketonuria

Dietary management for phenylketonuria was established over half a century ago, and has rendered an immense success in the prevention of the severe mental retardation associated with the accumulation of phenylalanine. However, the strict low-phenylalanine diet has several shortcomings, not the least of which is the burden it imposes on the patients and their families consequently frequent dietary non-compliance. Imperfect neurological outcome of patients in comparison to non-PKU individuals and nutritional deficiencies associated to the PKU diet are other important reasons to seek alternative therapies. In the last decade there has been an impressive effort in the investigation of other ways to treat PKU that might improve the outcome and quality of life of these patients. These studies have lead to the commercialization of sapropterin dihydrochloride, but there are still many questions regarding which patients to challenge with sapropterin what is the best challenge protocol and what could be the implications of this treatment in the long-term. Current human trials of PEGylated phenylalanine ammonia lyase are underway, which might render an alternative to diet for those patients non-responsive to sapropterin dihydrochloride. Preclinical investigation of gene and cell therapies for PKU is ongoing. In this manuscript, we will review the current knowledge on novel pharmacologic approaches to the treatment of phenylketonuria.

Safety of extended treatment with sapropterin dihydrochloride in patients with phenylketonuria: results of a phase 3b study

BACKGROUND

Phenylketonuria (PKU) results from impaired breakdown of phenylalanine (Phe) due to deficient phenylalanine hydroxylase (PAH) activity. Sapropterin dihydrochloride (sapropterin, Kuvan®) is the only US- and EU-approved pharmaceutical version of naturally occurring 6R-BH(4), the cofactor required for PAH activity. Sapropterin enhances residual PAH activity in sapropterin-responsive PKU patients and, in conjunction with dietary management, helps reduce blood Phe concentrations for optimal control. Approval was based on the positive safety and efficacy results of four international clinical studies, the longest of which was 22 weeks in duration.

OBJECTIVE

To evaluate the safety of long-term treatment with sapropterin in PKU subjects who participated in previous Phase 3 sapropterin trials.

METHODS

PKU-008 was designed as a Phase 3b, multicenter, multinational, open-label, 3-year extension trial to evaluate the long-term safety of sapropterin in patients with PKU who were classified as sapropterin responders and participated in prior Phase 3 sapropterin studies: 111 subjects aged 4-50 years completed prior studies and were subsequently enrolled in study PKU-008. Routine safety monitoring was performed at 3-month intervals and included adverse event reporting, blood Phe monitoring, clinical laboratory evaluations, physical examinations and vital sign measurements.

RESULTS

Average exposure during PKU-008 was 658.7±221.3 days (range, 56-953; median, 595). The average total duration of participation in multiple studies (PKU-001, PKU-003, PKU-004, and PKU-008; or PKU-006 and PKU-008) was 799.0±237.5 days (range, 135-1151). The mean sapropterin dose was 16.2±4.7 mg/kg/day. Most adverse events were considered unrelated to treatment, were mild or moderate in severity, and were consistent with prior studies of sapropterin. No age-specific differences were observed in adverse event reporting. Three subjects discontinued treatment due to adverse events that were considered possibly or probably related to study treatment (one each of difficulty concentrating, decreased platelet count, and intermittent diarrhea). No deaths were reported. Of seven reported serious adverse events, one was considered possibly related to study treatment (gastroesophageal reflux). There were no laboratory or physical examination abnormalities requiring medical interventions. For most subjects, blood Phe concentrations were consistently within target range, confirming the durability of response in subjects undergoing extended treatment with sapropterin.

CONCLUSION

Sapropterin treatment was found to be safe and well tolerated at doses of 5 to 20mg/kg/day for an average exposure of 659 days. This study supports the safety and tolerability of sapropterin as long-term treatment for patients with PKU.

Adjusting diet with sapropterin in phenylketonuria: what factors should be considered?

The usual treatment for phenylketonuria (PKU) is a phenylalanine-restricted diet. Following this diet is challenging, and long-term adherence (and hence metabolic control) is commonly poor. Patients with PKU (usually, but not exclusively, with a relatively mild form of the disorder) who are responsive to treatment with pharmacological doses of tetrahydrobiopterin (BH4) have either lower concentrations of blood phenylalanine or improved dietary phenylalanine tolerance. The availability of a registered formulation of BH4 (sapropterin dihydrochloride, Kuvan®) has raised many practical issues and new questions in the dietary management of these patients. Initially, patients and carers must understand clearly the likely benefits (and limitations) of sapropterin therapy. A minority of patients who respond to sapropterin are able to discontinue the phenylalanine-restricted diet completely, while others are able to relax the diet to some extent. Care is required when altering the phenylalanine-restricted diet, as this may have unintended nutritional consequences and must be undertaken with caution. New clinical protocols are required for managing any dietary change while maintaining control of blood phenylalanine, ensuring adequate nutrition and preventing nutritional deficiencies, overweight or obesity. An accurate initial evaluation of pre-sapropterin phenylalanine tolerance is essential, and the desired outcome from treatment with sapropterin (e.g. reduction in blood phenylalanine or relaxation in diet) must also be understood by the patient and carers from the outset. Continuing education and support will be required thereafter, with further adjustment of diet and sapropterin dosage as a young patient grows.

Molecular genetics and impact of residual in vitro phenylalanine hydroxylase activity on tetrahydrobiopterin responsiveness in Turkish PKU population

BACKGROUND

The prevalence of phenylalanine hydroxylase (PAH)-deficient phenylketonuria (PKU) in Turkey is high (1 in 6500 births), but data concerning the genotype distribution and impact of the genotype on tetrahydrobiopterin (BH(4)) therapy are scarce.

OBJECTIVE

To characterize the phenotypic and genotypic variability in the Turkish PKU population and to correlate it with physiological response to BH(4) challenge.

METHODS

We genotyped 588 hyperphenylalaninemic patients and performed a BH(4) loading test (20mg/kg bw) in 462 patients. Residual PAH activity of mutant proteins was calculated from available in vitro expression data. Data were tabulated in the BIOPKU database (www.biopku.org).

RESULTS

Eighty-eight mutations were observed, the most common missense mutations being the splice variant c.1066-11G>A (24.6%). Twenty novel mutations were detected (11 missense, 4 splice-site, and 5 deletion/insertions). Two mutations were observed in 540/588 patients (91.8%) but in 9 patients atypical genotypes with >2 mutations were found (8 with p.R155H in cis with another variant) and in 19 patients mutations were found in BH(4)-metabolizing genes. The most common genotype was c.1066-11G>A/c.1066-11G>A (15.5%). Approximately 22% of patients responded to BH(4) challenge. A substantial in vitro residual activity (average >25% of the wild-type enzyme) was associated with response to BH(4). In homozygous genotypes (n=206), both severity of the phenotype (r=0.83) and residual PAH activity (r=0.85) correlate with BH(4) responsiveness.

CONCLUSION

Together with the BH(4) challenge, these data enable the genotype-based classification of BH(4) responsiveness and document importance of residual PAH activity. This first report of a large-scale genotype assessment in a population of Turkish PKU patients also documents a high prevalence (47%) of the severe classic phenotype.

Long-term follow-up of patients with phenylketonuria receiving tetrahydrobiopterin treatment

Treatment with tetrahydrobiopterin (BH4), the natural cofactor of phenylalanine hydroxylase (PAH), can reduce blood phenylalanine (Phe) levels in patients with BH4-responsive phenylketonuria (PKU). A number of studies has reported on the short-term BH4 treatment of patients with PKU, but long-term data are lacking. Here, we describe the effects of long-term treatment with BH4 on 16 patients, who showed a >28% reduction in blood Phe following testing for BH4 overload. The mean dose of BH4 was 16 mg/kg body weight (range 5-36 mg/kg body weight). The mean treatment duration was 56 months (range 24-110 months). Of 16 patients, 14 achieved long-term Phe control with BH4 treatment, with a mean blood Phe concentration of 321 ± 236 µmol/l. The mean decrease from baseline in blood Phe levels in these 14 patients was 54.6%. Of the seven patients who required continued dietary restriction, Phe intake increased from 200-300 mg/day to 800-1000 mg/day. Factors that may cause fluctuation of Phe levels in BH4-treated patients include patients' PAH genotype, Phe intake, changes in protein catabolism or anabolism, and periods of illness or infection.

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.

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 STRATEGY

We identified relevant trials from the Group's Inborn Errors of Metabolism Trials Register. Last search:07 May 2010.We also searched ClinicalTrials.gov and Current controlled trials. Last search: 01 September 2009.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 mumol/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 mumol/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.

Exon deletions of the phenylalanine hydroxylase gene in Italian hyperphenylalaninemics

A consistent finding of many studies describing the spectrum of mutant phenylalanine hydroxylase (PAH) alleles underlying hyperphenylalaninemia is the impossibility of achieving a 100% mutation ascertainment rate using conventional gene-scanning methods. These methods include denaturing gradient gel electrophoresis (DGGE), denaturing high performance liquid chromatography (DHPLC), and direct sequencing. In recent years, it has been shown that a significant proportion of undetermined alleles consist of large deletions overlapping one or more exons. These deletions have been difficult to detect in compound heterozygotes using gene-scanning methods due to a masking effect of the non-deleted allele. To date, no systematic search has been carried out for such exon deletions in Italian patients with phenylketonuria or mild hyperphenylalaninemia. We used multiplex ligation-dependent probe amplification (MLPA), comparative multiplex dosage analysis (CMDA), and real-time PCR to search for both large deletions and duplications of the phenylalanine hydroxylase gene in Italian hyperphenylalaninemia patients. Four deletions removing different phenylalanine hydroxylase (PAH) gene exons were identified in 12 patients. Two of these deletions involving exons 4-5-6-7-8 (systematic name c.353-?_912+?del) and exon 6 (systematic name c.510-?_706+?del) have not been reported previously. In this study, we show that exon deletion of the PAH gene accounts for 1.7% of all mutant PAH alleles in Italian hyperphenylalaninemics.

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.

Tetrahydrobiopterin responsiveness after extended loading test of 12 Danish PKU patients with the Y414C mutation

Phenylketonuria (PKU) is an inherited metabolic disease characterized by phenylalanine (Phe) accumulation due to defects in the enzyme phenylalanine hydroxylase (PAH). Phe accumulation can lead to cognitive impairment. Some individuals with PKU respond to tetrahydrobiopterin (BH4) treatment, the natural cofactor of PAH, by a reduction in blood Phe concentrations.We tested 12 patients with PKU, 8-29 years of age, all carrying the common Y414C mutation in the PAH gene. Three were homozygous and nine were compound heterozygous, with the second mutation being a putative null mutation. During the study period, genuine protein was increased to approximately 1 g/kg. The patients were treated with 20, 10, and 5 mg BH4/kg/day for 1 week on each dose, starting with 20 mg/kg. A positive response was defined as a decline in blood Phe>30%. Blood Phe was measured four times a week. Nonresponding children were excluded from the study. Eleven of 12 patients had a positive response with 20 mg/kg, 5/10 responded on 10 mg/kg, and 1/9 on 5 mg/kg. Two were late responders, with a response on 20 mg/kg after >48 h. We could confirm the previously reported inconsistent responsiveness of Y414C in the nine heterozygous patients, whereas the three homozygous patients had early median Phe declines of 73%, 51%, and 27%, respectively, on the three different doses. The varying responses despite uniform trial conditions and genotypes may be due to individual differences in BH4 absorption or metabolism. No side effects were observed.

Sapropterin Dihydrochloride (6R-BH4) and Maternal Phenylketonuria Two Case Studies

The dietary treatment of phenylketonuria (PKU) is a success story. However, the Maternal PKU Collaborative Study reported microcephaly, congenital heart defects, facial dysmorphology, and intrauterine and postnatal growth retardation in the offspring of women with blood phenylalanine (phe) concentrations >600 µ mol/L. Dietary control of phe during pregnancy is extremely difficult for many women because of hyper-emesis gravidarum, intolerance of the medical food products, and inability/ refusal to follow dietary recommendations. The authors report 2 case studies using 6R-BH4 along with the phere-stricted diet. Two individuals received 6R-BH4 shortly before conception and throughout the pregnancy. A pherestricted diet was followed with blood phe analysis obtained weekly. Interim plasma amino acids, complete metabolic panels, complete blood counts, and analyses for nutritional deficiencies were also obtained during the pregnancy. Both individuals maintained blood phe concentrations within the recommended range (120-360 µ mol/L) nearly all through the pregnancy with a small percentage outside of recommendations. One participant showed deficiencies in vitamin D and zinc and received supplementation. Both were able to increase natural protein as the pregnancy progressed. Ingestion of the 6R-BH4 was well tolerated. Both gave birth to healthy normal infants. In these case studies, both babies’ birth measurements were in the normal range, and the 6R-BH4 had no adverse effects.

Molecular Diagnosis of Phenylketonuria: From Defective Protein to Disease-Causing Gene Mutation

Phenylketonuria (PKU) is the most common inborn error of amino acid metabolism, with an average incidence of 1/10000 in Caucasians. PKU is caused by more than 500 mutations in the phenylalanine hydroxylase gene (PAH) which result in phenylalanine hydroxylase (PAH) enzyme deficiency. Two approaches, in vitro expression analysis of mutant PAH and genotype-phenotype correlation study, are used for the assessment of severity ofPAHmutations. It has been shown that there is a significant correlation between mutantPAHgenotypes and PKU phenotypes. As a result, the molecular diagnosis is completely shifted toward the detection of mutations in the phenylalanine hydroxylase gene. The study of the molecular basis of PKU in Serbia included identification of the spectrum and frequency ofPAHmutations in Serbian PKU patients and genotype-phenotype correlation analysis. By using both PCR-RFLP and »broad range« DGGE/DNA sequencing analysis, the mutation detection rate reached 97%. Thus, the base for molecular diagnosis, genetic counseling and selection of BH4-responsive PKU patients in Serbia was created.

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.

Efficacy of sapropterin dihydrochloride in increasing phenylalanine tolerance in children with phenylketonuria: a phase III, randomized, double-blind, placebo-controlled study

OBJECTIVE

To evaluate the ability of sapropterin dihydrochloride (pharmaceutical preparation of tetrahydrobiopterin) to increase phenylalanine (Phe) tolerance while maintaining adequate blood Phe control in 4- to 12-year-old children with phenylketonuria (PKU).

STUDY DESIGN

This international, double-blind, randomized, placebo-controlled study screened for sapropterin response among 90 enrolled subjects in Part 1. In Part 2, 46 responsive subjects with PKU were randomized (3:1) to sapropterin, 20 mg/kg/d, or placebo for 10 weeks while continuing on a Phe-restricted diet. After 3 weeks, a dietary Phe supplement was added every 2 weeks if Phe control was adequate.

RESULTS

The mean (+/-SD) Phe supplement tolerated by the sapropterin group had increased significantly from the pretreatment amount (0 mg/kg/d) to 20.9 (+/-15.4) mg/kg/d (P < .001) at the last visit at which subjects had adequate blood Phe control (<360 micromol/L), up to week 10. Over the 10-week period, the placebo group tolerated only an additional 2.9 (+/-4.0) mg/kg/d Phe supplement; the mean difference from the sapropterin group (+/-SE) was 17.7 +/- 4.5 mg/kg/d (P < .001). No severe or serious related adverse events were observed.

CONCLUSIONS

Sapropterin is effective in increasing Phe tolerance while maintaining blood Phe control and has an acceptable safety profile in this population of children with PKU.

Optimizing the use of sapropterin (BH(4)) in the management of phenylketonuria

Phenylketonuria (PKU) is caused by mutations in the phenylalanine hydroxylase (PAH) gene, leading to deficient conversion of phenylalanine (Phe) to tyrosine and accumulation of toxic levels of Phe. A Phe-restricted diet is essential to reduce blood Phe levels and prevent long-term neurological impairment and other adverse sequelae. This diet is commenced within the first few weeks of life and current recommendations favor lifelong diet therapy. The observation of clinically significant reductions in blood Phe levels in a subset of patients with PKU following oral administration of 6R-tetrahydrobiopterin dihydrochloride (BH(4)), a cofactor of PAH, raises the prospect of oral pharmacotherapy for PKU. An orally active formulation of BH(4) (sapropterin dihydrochloride; Kuvan is now commercially available. Clinical studies suggest that treatment with sapropterin provides better Phe control and increases dietary Phe tolerance, allowing significant relaxation, or even discontinuation, of dietary Phe restriction. Firstly, patients who may respond to this treatment need to be identified. We propose an initial 48-h loading test, followed by a 1-4-week trial of sapropterin and subsequent adjustment of the sapropterin dosage and dietary Phe intake to optimize blood Phe control. Overall, sapropterin represents a major advance in the management of PKU.

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.

Effect of BH(4) supplementation on phenylalanine tolerance

BACKGROUND

Tetrahydrobiopterin (BH(4)) is a potential new orphan drug for the treatment of some patients with phenylketonuria (PKU), mostly mild forms. Numerous studies have confirmed this finding and BH(4)-responsiveness may be predicted to some extent from the corresponding genotype.

AIM

To investigate the response to BH(4) loading test, the phenylalanine hydroxylase (PAH) mutations and the long-term therapeutic efficacy of BH(4) in patients with PKU, and to better define BH(4)-responsive patients according to phenylalanine (Phe) levels and dietary phenylalanine tolerance.

METHODS

30 Italian PKU patients (age range: 6 months-24 years; 12 female, 18 male) were included in this retrospective study. Eleven out of 30 patients presented with Phe levels below 450 micromol/L and 19 patients with Phe levels between 450 and 900 micromol/L. In the second group, we investigated the effect of long-term (6 months-7 years) oral administration of BH(4) on blood Phe levels and daily Phe tolerance.

RESULTS

In all patients with initial blood Phe levels <450 micromol/L (n = 11), BH(4) loading test was positive, but no treatment was introduced. In 12 out of 19 patients with blood Phe levels >450 micromol/L and positive at BH(4) loading, the treatment with BH(4) (10 mg/kg per day) was initiated. Before BH(4) treatment, Phe tolerance was less than 700 mg/day in all patients except for one (patient no. 9), increasing to 2-3-fold (from 498 +/- 49 to 1475 +/- 155 mg/day) on BH(4) treatment. In these patients the amino acid mixture supplementation was stopped and the diet was a combination of low-protein foods and natural proteins, mostly from animal sources.

CONCLUSION

Long-term BH(4) substitution (up to 7 years) in a group of moderate PKU patients allowed a substantial relaxation of the dietary restrictions or even replacement of the diet with BH(4) without any adverse effects.

Safety and efficacy of 22 weeks of treatment with sapropterin dihydrochloride in patients with phenylketonuria

Phenylketonuria (PKU) is an inherited metabolic disease characterized by phenylalanine (Phe) accumulation, which can lead to neurocognitive and neuromotor impairment. Sapropterin dihydrochloride, an FDA-approved synthetic formulation of tetrahydrobiopterin (6R-BH4, herein referred to as sapropterin) is effective in reducing plasma Phe concentrations in patients with hyperphenylalaninemia due to tetrahydrobiopterin (BH4)-responsive PKU, offering potential for improved metabolic control. Eighty patients, > or =8 years old, who had participated in a 6-week, randomized, placebo-controlled study of sapropterin, were enrolled in this 22-week, multicenter, open-label extension study comprising a 6-week forced dose-titration phase (5, 20, and 10 mg/kg/day of study drug consecutively for 2 weeks each), a 4-week dose-analysis phase (10 mg/kg/day), and a 12-week fixed-dose phase (patients received doses of 5, 10, or 20 mg/kg/day based on their plasma Phe concentrations during the dose titration). Dose-dependent reductions in plasma Phe concentrations were observed in the forced dose-titration phase. Mean (SD) plasma Phe concentration decreased from 844.0 (398.0) micromol/L (week 0) to 645.2 (393.4) micromol/L (week 10); the mean was maintained at this level during the study's final 12 weeks (652.2 [382.5] micromol/L at week 22). Sixty-eight (85%) patients had at least one adverse event (AE). All AEs, except one, were mild or moderate in severity. Neither the severe AE nor any of the three serious AEs was considered related to sapropterin. No AE led to treatment discontinuation. Sapropterin is effective in reducing plasma Phe concentrations in a dose-dependent manner and is well tolerated at doses of 5-20 mg/kg/day over 22 weeks in BH4-responsive patients with PKU.

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.

Molecular genetics of tetrahydrobiopterin-responsive phenylalanine hydroxylase deficiency

Mutations in the phenylalanine hydroxylase (PAH) gene result in phenylketonuria (PKU). Tetrahydrobiopterin (BH(4))-responsive hyperphenylalaninemia has been recently described as a variant of PAH deficiency caused by specific mutations in the PAH gene. It has been suggested that BH(4)-responsiveness may be predicted from the corresponding genotypes. Data from BH(4) loading tests indicated an incidence of BH(4)-responsiveness of >40% in the general PKU population and >80% in mild PKU patients. The current project entailed genotype analysis of 315 BH(4)-responsive patients tabulated in the BIOPKUdb database and comparison with the data from the PAHdb locus-specific knowledgebase, as well as with previously published PAH mutations for several European countries, Northern China, and South Korea. We identified 57 mutations, presenting with a substantial residual PAH activity (average approximately 47%), presumed to be associated with BH(4)-responsiveness. More than 89% of patients are found to be compound heterozygotes. The three most common mutations found in >5% of BH(4)-responsive patients are p.A403 V, p.R261Q, and p.Y414C. Using the Hardy-Weinberg formula the predicted average frequency of BH(4)-responsiveness in European populations was calculated to be 55% (range 17-79%, lowest in Baltic countries and Poland and highest in Spain), 57% in Northern China, and 55% for South Korea. The genotype-predicted prevalence of BH(4)-responsiveness was higher than prevalence data obtained from BH(4) loading tests. Inconsistent results were observed for mutations p.L48S, p.I65 T, p.R158Q, p.R261Q, and p.Y414C. Our data suggest that BH(4)-responsiveness may be more common than assumed and to some extent may be predicted or excluded from the patient's genotype.

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.

Analysis of the effect of tetrahydrobiopterin on PAH gene expression in hepatoma cells

Tetrahydrobiopterin (BH4)-responsive phenylalanine hydroxylase (PAH) deficiency is a recently recognized variant of phenylketonuria, with a probable multifactorial molecular basis. In this study we have investigated the effect of BH4 on PAH gene expression in human hepatoma. Our results show that increased BH4 levels result in an enhancement of PAH activity and PAH protein, due to longer turnover rates, while PAH mRNA levels remain unchanged. This was confirmed for mutant PAH proteins (A309V, V388M and Y414C) associated to in vivo BH4 responsiveness, validating previous studies. We can conclude that there is no effect of the cofactor on PAH gene transcription, probably being the chemical chaperone effect of BH4 stabilizing mutant PAH proteins the major underlying mechanism of the response.