Evolving patient selection and clinical benefit criteria for sapropterin dihydrochloride (Kuvan®) treatment of PKU patients

Pre-analytic decrease of phenylalanine in plasma of patients with phenylketonuria treated with pegvaliase

Treatment of phenylketonuria (PKU) has evolved since the initial introduction of a phenylalanine (Phe) restricted diet. The most recent option for adults affected with PKU is treatment with an alternate enzyme, phenylalanine ammonia lyase (PAL), that metabolizes excess Phe. Proper management of all patients with PKU relies on accurate measurement of Phe levels in blood, to comply with guidance intended to minimize the neurological symptoms. Recently, our laboratory was notified of discrepant results for a patient with PKU who is treated with pegvaliase. Two specimens were collected at the same time but yielded unexpectedly different Phe concentrations. After exclusion of specimen mix-ups or analytical errors, we suspected that there was residual pegvaliase activity in the specimens continuing to degrade Phe after collection. To investigate this possibility, we performed spiking studies that showed the degradation of Phe over time at ambient temperatures. Sample preparation by protein crash appears to deactivate pegvaliase and prevents further Phe degradation. However, because pegvaliase deactivation would be required immediately following blood collection, appropriate mitigation measures must be implemented, including stringent pre-analytical requirements, alternate sample matrices such as dried blood spots, or point of care testing. Until then, health care professionals need to be cautious in their interpretation of Phe levels in their patients with PKU that are treated with pegvaliase.

In silico screening and molecular dynamics simulation of deleterious PAH mutations responsible for phenylketonuria genetic disorder

Phenylketonuria (PKU) is a genetic disorder that if left untreated can lead to behavioral problems, epilepsy, and even mental retardation. PKU results from mutations within the phenylalanine-4-hydroxylase (PAH) gene that encodes for the PAH protein. The study of all PAH causing mutations is improbable using experimental techniques. In this study, a collection of in silico resources, sorting intolerant from tolerant, Polyphen-2, PhD-SNP, and MutPred were used to identify possible pathogenetic and deleterious PAH non-synonymous single nucleotide polymorphisms (nsSNPs). We identified two variants of PAH, I65N and L311P, to be the most deleterious and disease causing nsSNPs. Molecular dynamics (MD) simulations were carried out to characterize these point mutations on the atomic level. MD simulations revealed increased flexibility and a decrease in the hydrogen bond network for both mutants compared to the native protein. Free energy calculations using the MM/GBSA approach found that BH4 , a drug-based therapy for PKU patients, had a higher binding affinity for I65N and L311P mutants compared to the wildtype protein. We also identify important residues in the BH4 binding pocket that may be of interest for the rational drug design of other PAH drug-based therapies. Lastly, free energy calculations also determined that the I65N mutation may impair the dimerization of the N-terminal regulatory domain of PAH.

Neonatal screening and genotype-phenotype correlation of hyperphenylalaninemia in the Chinese population

Background

Hyperphenylalaninemia (HPA) is the most common amino acid metabolic disease involving phenylalanine hydroxylase (PAH, OMIM*612,349) deficiency or coenzyme tetrahydrobiopterin (BH4) deficiency. Patients with severe HPA often have a difficult life. Early diagnosis of HPA before the development of symptoms is possible via neonatal screening, facilitating appropriate treatment and reducing mortality and disability rates. This study revealed the prevalence, mutational and phenotypic spectrum, and prognosis of HPA by neonatal screening from January 2001 to September 2020 in Nanjing, Jiangsu Province, China.

Methods

Through a retrospective analysis of the information available in the neonatal screening database, the clinical presentations, laboratory data, molecular characteristics and treatment follow-up data of HPA patients detected by neonatal screening were evaluated.

Results

We diagnosed 181 patients with HPA from 1 to 957 newborns, giving an incidence of 1:6873. Among these patients, 177 were identified as PAH deficient and four patients were BH4 deficient. The average current age of the patients was 6.38 years old. The most common mutations of PAH were c.728 C > A/ p.Arg243Gln (13.83 %), c.158G > A/ p.Arg53His (9.57 %), c.611 A > G/ p.Tyr204Cys (7.44 %), and c.721 C > T/ p.Arg241Cys (6.38 %).

Conclusions

This study revealed the prevalence, phenotype-genotype, and prognosis of HPA in China and contributes to the updating of PAHD data for China and worldwide. Our study not only expanded the spectrum of phenotypes and genotype but also provided a valuable tool for improved genetic counseling and management of future cases.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13023-021-01846-w.

Complex patterns of inheritance, including synergistic heterozygosity, in inborn errors of metabolism: Implications for precision medicine driven diagnosis and treatment

Inborn errors of metabolism have traditionally been viewed as the quintessential single gene disorders; defects in one gene leads to loss of activity of one enzyme causing a metabolic imbalance and clinical disease. However, reality has never been quite that simple, and the classic "one gene-one enzyme" paradigm has been upended in many ways. Multiple gene defects can lead to the same biochemical phenotype, often with different clinical symptoms. Additionally, different mutations in the same gene can cause variable phenotypes, often most dramatic when a disease can be identified by pre-symptomatic screening. Moreover, response to therapy is not homogeneous across diseases and specific mutations. Perhaps the biggest deviation from traditional monogenic inheritance is in the setting of synergistic heterozygosity, a multigenic inheritance pattern in which mutations in multiple genes in a metabolic pathway lead to sufficient disruption of flux through the pathway, mimicking a monogenic disorder caused by homozygous defects in one gene in that pathway. In addition, widespread adoption of whole exome and whole genome sequencing in medical genetics has led to the realization that individual patients with apparently hybrid phenotypes can have mutations in more than one gene, leading to a mixed genetic disorder. Each of these situations point to a need for as much precision as possible in diagnosing metabolic disease, and it is likely to become increasingly critical to drive therapy. This article examines examples in traditional monogenic disorders that illustrates these points and define inborn errors of metabolism as complex genetic traits on the leading edge of precision medicine.

Individualized long-term outcomes in blood phenylalanine concentrations and dietary phenylalanine tolerance in 11 patients with primary phenylalanine hydroxylase (PAH) deficiency treated with Sapropterin-dihydrochloride

We analyzed long-term sustainability of improved blood Phenylalanine (Phe) control and changes to dietary Phe tolerance in 11 patients (1 month to 16 years), with various forms of primary PAH deficiency (classic, moderate, severe phenylketonuria [PKU], mild hyperphenylalaninemia [HPA]), who were treated with 15-20mg/kg/d Sapropterin-dihydrochloride during a period of 13-44 months. 7/11 patients had a sustainable, significant reduction of baseline blood Phe concentrations and 6 of them also had an increase in mg/kg/day Phe tolerance. In 2 patients with mild HPA, blood Phe concentrations remained in the physiologic range even after a 22 and 36% increase in mg/kg/day Phe tolerance and an achieved Phe intake at 105% and 268% of the dietary reference intake (DRI) for protein. 2 of these responders had classic PKU. 1 patient with mild HPA who started treatment at 2 months of life, had a significant and sustainable reduction in pretreatment blood Phe concentrations, but no increase in the mg/kg/day Phe tolerance. An increase in Phe tolerance could only be demonstrated when expressing the patient's daily Phe tolerance with the DRI for protein showing an increase from 58% at baseline to 78% of normal DRI at the end of the observation. Long-term follow-up of patients with an initial response to treatment with Sapropterin is essential to determine clinically meaningful outcomes. Phenylalanine tolerance should be expressed in mg/kg/day and/or % of normal DRI to differentiate medical therapy related from physiologic growth related increase in daily Phe intake.

New Strategies for the Treatment of Phenylketonuria (PKU)

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

Phenylalanine hydroxylase deficiency: diagnosis and management guideline

Phenylalanine hydroxylase deficiency, traditionally known as phenylketonuria, results in the accumulation of phenylalanine in the blood of affected individuals and was the first inborn error of metabolism to be identified through population screening. Early identification and treatment prevent the most dramatic clinical sequelae of the disorder, but new neurodevelopmental and psychological problems have emerged in individuals treated from birth. The additional unanticipated recognition of a toxic effect of elevated maternal phenylalanine on fetal development has added to a general call in the field for treatment for life. Two major conferences sponsored by the National Institutes of Health held >10 years apart reviewed the state of knowledge in the field of phenylalanine hydroxylase deficiency, but there are no generally accepted recommendations for therapy. The purpose of this guideline is to review the strength of the medical literature relative to the treatment of phenylalanine hydroxylase deficiency and to develop recommendations for diagnosis and therapy of this disorder. Evidence review from the original National Institutes of Health consensus conference and a recent update by the Agency for Healthcare Research and Quality was used to address key questions in the diagnosis and treatment of phenylalanine hydroxylase deficiency by a working group established by the American College of Medical Genetics and Genomics. The group met by phone and in person over the course of a year to review these reports, develop recommendations, and identify key gaps in our knowledge of this disorder. Above all, treatment of phenylalanine hydroxylase deficiency must be life long, with a goal of maintaining blood phenylalanine in the range of 120-360 µmol/l. Treatment has predominantly been dietary manipulation, and use of low protein and phenylalanine medical foods is likely to remain a major component of therapy for the immediate future. Pharmacotherapy for phenylalanine hydroxylase deficiency is in early stages with one approved medication (sapropterin, a derivative of the natural cofactor of phenylalanine hydroxylase) and others under development. Eventually, treatment of phenylalanine hydroxylase deficiency will be individualized with multiple medications and alternative medical foods available to tailor therapy. The primary goal of therapy should be to lower blood phenylalanine, and any interventions, including medications, or combination of therapies that help to achieve that goal in an individual, without other negative consequences, should be considered appropriate therapy. Significant evidence gaps remain in our understanding of the optimum therapies for phenylalanine hydroxylase deficiency, nonphenylalanine effects of these therapies, and long-term sequelae of even well-treated disease in children and adults.

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

OBJECTIVE

Sapropterin dihydrochloride, the synthetic form of 6R-tetrahydrobiopterin (BH4), is an approved drug for the treatment of patients with BH4-responsive phenylketonuria (PKU). The purpose of this study was to assess genotypes and data on the long-term effects of BH4/sapropterin on metabolic control and patient-related outcomes in 6 large European countries.

METHODS

A questionnaire was developed to assess phenotype, genotype, blood phenylalanine (Phe) levels, Phe tolerance, quality of life, mood changes, and adherence to diet in PKU patients from 16 medical centers.

RESULTS

One hundred forty-seven patients, of whom 41.9% had mild hyperphenylalaninemia, 50.7% mild PKU, and 7.4% classic PKU, were followed up over ≤12 years. A total of 85 different genotypes were reported. With the exception of two splice variants, all of the most common mutations were reported to be associated with substantial residual Phe hydroxylase activity. Median Phe tolerance increased 3.9 times with BH4/sapropterin therapy, compared with dietary treatment, and median Phe blood concentrations were within the therapeutic range in all patients. Compared with diet alone, improvement in quality of life was reported in 49.6% of patients, improvement in adherence to diet was reported in 47% of patients, and improvement in adherence to treatment was reported in 63.3% of patients. No severe adverse events were reported.

CONCLUSIONS

Our data document a long-term beneficial effect of orally administered BH4/sapropterin in responsive PKU patients by improving the metabolic control, increasing daily tolerance for dietary Phe intake, and for some, by improving dietary adherence and quality of life. Patient genotypes help in predicting BH4 responsiveness.

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

INTRODUCTION

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

METHODS

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

RESULTS

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

CONCLUSION

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

Recommendations for the use of sapropterin in phenylketonuria

Phenylketonuria (PKU) is an inherited disorder of phenylalanine (Phe) metabolism. Until recently, the only treatment for PKU was a Phe-restricted diet. Increasing evidence of suboptimal outcomes in diet-treated individuals, inconsistent PKU management practices, and the recent availability of tetrahydrobiopterin (BH(4)) therapy have fueled the need for new management and treatment recommendations for this metabolic disorder. BH(4), now available as sapropterin dihydrochloride (sapropterin), may offer the potential for improved metabolic control as well as enhanced dietary Phe tolerance in some PKU patients. A group of metabolic dietitians from North America convened in June 2011 to draft recommendations for the use of sapropterin therapy in PKU. Physicians with extensive experience in PKU management were invited at a later date to contribute to the development of these recommendations. Based on extensive clinical experience and current evidence, the present recommendations provide guidance from patient selection and determination of sapropterin response to the long-term management of patients on sapropterin therapy. Target Phe levels, nutritional adequacy, neurocognitive screening and adherence to treatment are addressed to optimize patient outcomes.