Case-control study about the acceptance of Pegvaliase in Phenylketonuria

Introduction Pegvaliase is a novel enzyme substitution therapy approved by the European Drug Administration (EDA) in May 2019 for the treatment of Phenylketonuria (PKU) in adults and children ≥16 years of age. The pegylated phenylalanine ammonia lyase is isolated from bacteria and therefore provokes multifarious immunogenic reactions. Thus, the selection of the right patient for a potential harmful treatment is essential for patient's contentedness and long-term therapy compliance. Methods and results 101 patients with PKU were screened for eligibility for an additional treatment with Pegvaliase. 51 patients were included in the study, 26 received a structural information about the new treatment for in mean 43 ± 12 min and clinical data and plasma Phe-levels were assessed. After 4 weeks of consideration the willing of treatment initiation as well as reasons for denial are registered. 7 patients (27%) concluded in beginning of treatment. Phe-level in this (acceptance) group were higher (1180 ± 231 μmol/l) compared to the denial group (930 ± 278 μmol/l, p = .01). After 4 weeks Phe-levels in the acceptance group remained stable (1264 ± 311 μmol/l, p = .26) while Phe-levels in the denial group decreased (779 ± 226 μmol/l, p < .01). Main reasons for denial of therapy were fear of adverse effects (47%), no need for additional treatment (26%) and the subcutaneous way of application (21%). Conclusion PKU patients have reservations against an invasive subcutaneous treatment for their disease. This is mainly caused by the form of application by syringe and the potential harmful side effects. Only less than one-third of the patients in our cohort are willing to start treatment. Besides that, most PKU patients seem to have untapped potential for self-contained reduction of Phe-levels only by being focused on their diet.


Introduction
Phenylketonuria (PKU) is an inborn error of metabolism, which is caused by a deficiency in the enzyme phenylalanine hydroxylase (PAH), resulting in disturbances of phenylalanine (Phe) metabolism. [1] The elevated Phe concentrations in adult patients affect neurophysical functions, resulting in cognitive impairment and neuropsychiatric symptoms linked to amount of the elevation [2,3] as well as neurological symptoms [4]. The life-long treatment should result in blood Phelevels of 120-360 μmol/l [5,6]. Treatment options are protein-restricted diet combined with Phe-free medical foods respectively the use of saproterin dihydrochloride (Kuvan®, BioMarin Pharmaceutical Inc., Novato, California, USA) which is an effective treatment in patients with residual PAH activity [7,8]. According to clinical experience, the treatment of PKU in adults is often challenging and results in Phe levels far above the recommendations. Pegvaliase (Palynziq®, BioMarin Pharmaceutical Inc., Novato, California, USA) is an novel enzyme substitution therapy approved by the European Drug Administration (EDA) in May 2019 for the treatment of PKU in adults and children ≥16 years of age [9,10]. The pegylated phenylalanine ammonia lyase is isolated from bacteria and therefore provokes multifarious immunogenic reactions [11] by predominantly anti-PEG IgM/IgG and anti-PAL IgM antibodies. Common reported side effects are presented in Table 1. In order to reduce severe outcomes, treatment with Pegvaliase is accompanied by the daily use of H1-and H2 antagonists as well as non-steroidal antiphlogistics [12]. Nevertheless the treatment is resulting in reports of adverse events up to 90% [13]. A trained observer must be present for an hour after every injection. Thus, the selection of the right patient for a potential harmful treatment is essential for patient's contentedness and long-term therapy compliance. Due to several, in particular cultural, differences in relationship and treatment modalities, a selective observation of different countries is necessary to improve individual patient care.

Material and methods
101 patients (childrens and adults) with disturbances of phenylalanine metabolism are in regular care of the metabolic centre of Ulm. All patients were screened for eligibility for an additional treatment with Pegvaliase. Exclusion criteria were the following: maximum Phelevels < 600 μmol/l in the last 12 months, age < 16 years, current or planned pregnancy. Genetic information was available from 88% of the patients. Genetic predictive values [14] were used for classification of the different phenotypes. Common cut-off-values were used as follows: 0.0-2.7 for classic PKU, 2.8-6.6 for mild PKU and 6.7-10.0 for mild hyperphenylalaninaemia. Among the study population 45 patients were classified as eligible for treatment and invited to a regular consultation in our clinic. All patients suffered from PKU and hat dietary treatment in their whole childhood with Phe levels lower than 360 μmol/l in the first 10 years of their live (ECTPKU). Within the regular routine assessment, we collected data about daily Phe-and protein-intake (analysis of 3-day protocols) and clinical data. We performed routine clinical and neurological examination and collected blood samples for determination of plasma Phe-levels in our own lab (high-pressure liquid chromatography, Biochrom 30+, Harvard Bioscience Inc., Holliston, Massachusetts, USA). Afterwards, detailed information about the new therapeutic option in treatment with Pegvaliase were given in a structured setting, including potential adverse effects, side medication and the need of supervision by a trained observer after every injection. Questions were answered in detail. The individual patient received an appointment four weeks later for control of blood Phe-level and transmission of his or her final decision before any treatment was obtained. In case of a decision against start of treatment with Pegvaliase the main reason for the denial was assessed. Data of 19 age and gendermatched patients with classic PKU out of the clinical routine before the detailed information about Pegvaliase were used as controls. The study protocol is shown in Fig. 1.
All values are presented as the mean + SD or as the absolute number. Differences between two groups were tested using the paired or unpaired t-test if normal distribution could be assumed. Otherwise, the Wilcoxon rank-sum test was performed. For categorical data, the chi-squared test or the Fisher's exact test was used. All differences were tested two-sided. Larger numbers of groups were tested using two-way analysis of variance. Analysis of correlation was performed with the Pearson correlation coefficient. P-values of 0.05 are considered as indicating statistical significance. Data were analyzed using SPSS version 25.0 (SPSS Inc., Chicago, IL, USA). Written informed consent was obtained by patients or their legal representatives.

Results
101 patients with PKU were screened for eligibility for the use of Pegvaliase. 45 patients were assumed to be eligible and were invited to the study centre for a routine examination and an informational conversation about new treatment options. All patients who made an appointment until November 2019 were assumed in the study group (n = 26), all others were assumed as controls (n = 19). Within the study group (n = 26) 12 patients (46%) were female, 23 (89%) patients reported of sticking to a Phe-restricted diet. Mean daily Phe-uptake was 1016 ± 300 mg per day. All patients on diet used supplementation of amino acids (n = 23, 89%). Genetic information was available from 21 (81%) patients, among whom 19 (90%) of the patients were classified as having a classic PKU (GPV ≤2.7) and 2 (10.5%) as having a mild PKU (max. GPV 5). Mean initial plasma Phe level was 1016 ± 300 μmol/l, only two patients reached the therapeutic goal of Phe levels < 600 μmol/l. All baseline characteristics are presented in Table 2. Mean dispose of time for the informational talk was 43 ± 12 min. After 4 weeks during the next appointment patients were asked for their decision. Seven patients (27%) decided themselves for a beginning of treatment with Pegvaliase, 19 (73%) patients declined the therapy. Reasons for refusal were in 9 (47%) cases fear of adverse effects, in 5 (26%) cases no need for additional treatment and in 4 (21%) cases the obligate application via syringe. In one case personal circumstances led to refusal of the therapy.
The Pevgaliase acceptance group and the Pegvalaise denial group were not statistically different according to gender (29% vs. 53% females, p = .39), age (37 ± 11 years vs. 31 ± 10 years, p = .23) or GPV (0 vs. 1.8 ± 2.4, p = .08). Both patients with initial plasma Phe within the therapeutic goal decided against treatment with Pegvaliase among them the only patient on treatment with Saproterine. Mean initial plasma Phe level was higher in the treatment group (1180 ± 241 μmol/l) compared to the denial group (930 ± 278 μmol/l, p = .01) whilst the cumulative number on diet (86% vs. 95%, p = .47) and mean daily Phe intake (2242 ± 1006 μmol/l vs. 1458 ± 840 μmol/l, p = .17) was not different. There was no difference about the time used for the informational talk (39 ± 14 vs. 44 ± 10 min, p = .43). All values compared within the acceptance and the denial group are presented in Table 3.
Phe levels were controlled after the 4 weeks of consideration. While patients in the Pegvaliase acceptance group presented with stable Phelevels (1264 ± 311 μmol/l) compared to baseline (1180 ± 241 μmol/ l, p = .26), patients who refused a beginning of treatment with Pegvaliase were able to reduce their plasma Phe significantly (930 ± 278 μmol/l vs. 779 ± 226 μmol/l, p < .01) without any additional intervention. Both changes were significantly different to the changes in the control group (1094 ± 184 μmol/l vs. 1102 ± 162 μmol/l, p = .86). The results about changes in plasma Phe are presented in Fig. 2 and Table 4. Two additional patients reached their therapeutic goal of Phe leves < 600 μmol/l within 4 weeks without therapeutic intervention.

Discussion
In contrast to recent reports among patients with PKU are willing to accept the risks of hypersensitivity reactions to optimize their treatment, [15] we could clearly show the patients reservation against the invasive treatment of their disorder. One aspect is certainly the different management of PKU in Europe for the last decade, which permitted the loose permissive value of plasma Phe levels of 1200 μmol/l. Another aspect is the low burden of disease in most adult patients, most of which would not consider themselves as patients with an inborn error of metabolism. This is based on the low predictive power of blood phenylalanine on the clinical outcome from the second decade of life onwards [16]. There is clear evidence for improved quality of life in patients with relaxation of their Phe-restricted diet [17][18][19]. This argument is often used as an argument for expected tolerance of PKU patients to harmful side effects of treatment with Pegvaliase [20], but misses the point, that most patients that probably benefit from a treatment with Pegvaliase did not stick to their diet for years. Additionally, individual vulnerability to the metabolic alterations of PKU contributes to the prognosis of PKU. That means, that the long-term benefit for the individual patient is not predictable [16]. Therefore, it is still not easy to determine the indications for recommending a treatment with Pegvaliase resulting in severe adverse events in 10% of the treated patients [21] compared to Saproterine [8,22,23].
Even in subsets with clear indications (adult patient, blood Phe level > 600 μmol/l and fail at dietary adherence) we could clearly show that the patient's will is remarkably different to the marketing attempts of the manufacturer, mainly based on the invasive application and the unfavorable side effects. The burden for quality of life of daily injections with regular side effects might be more harmful than elevated Phe levels alone. The presented cognitive data for admission of the treatment [10,[24][25][26], based on depression and activity rating scalers, are far away from clinical reality, showing clear evidence for a persisting central executive impairment even in patients with well treated PKU and low blood Phe-levels [27]. Detailed cognitive testing and an assessment of quality of life in long-term follow-ups are needed to answer the question if life-long blood Phe control is able to prevent or improve cognitive impairment, and if daily injections with side-effects are more harmful to well-being than uncontrolled Phe-levels.
Another clear result of this study is the apparent untapped potential for reducing plasma Phe-levels in patients with PKU without any special intervention or additional medication. The patients in our cohort who refused a treatment with Pegvaliase showed significantly reduced Phelevels within four weeks. This effect could be explained by recreation of disease consciousness only by being focused on their diet by preparing a nutrition protocol over three days. Another fact could be, that the imminence of the beginning of an invasive treatment activates recourses for therapy and diet-adherence. This clearly shows that individual patient care and regular professional focus on the diet improves therapy adherence and should be part of every regular visit in patients with PKU, even in patients with pretended non-adherence rather than imprudent invasive therapeutic interventions.   Patients who accepted the treatment with Pegvaliase tend to elevation of Phe-levels compared to controls after four weeks. This might be explained by the expectations in a life without Phe-restricted diet causing a looser adherence to therapy.

Conclusion
PKU patients, even with classical mutations, elevated Phe-levels and loose diet have reservations against an invasive subcutaneus treatment for their disease. This is mainly caused by the form of application by syringe and the potential harmful side effects. Patients who are willing to start a new treatment with Pegvaliase tend to loosen therapy adherence. Only less than one-third of the patients in our cohort are willing to start treatment with Pegvaliase despite of Phe-levels far out of the therapeutic goals. Besides that, most PKU patients seem to have untapped potential for self-contained reduction of Phe-levels only by being focused on their diet. Therefore, regular professional nutritional advice, even in adult patients with supposed high knowledge in Pherestricted diet, is sufficient to significantly reduce Phe-levels within short periods of time.

Limitations
The small sample size within the different groups impedes greater validity. Due to the rare condition of the disorder, randomized controlled trials are difficult to realize. The fact of higher plasma Phe-levels in the control group compared to the Pegvaliase denial group might be connected to the higher treatment compliance within the acceptancegroup. The assignment to the treatment group by earlier appointments after invitation might result in a selection bias which interferes with the results of the study. Additionally, there was no control of the Phe-levels after the period of four weeks, thus the specified reduction of Phe-levels might not last for a longer time.

Conflict of interest disclosures
Johannes Krämer declares that he has no conflict of interest.

Contributorship statement
Johannes Krämer, first draft, concept of the manuscript, data interpretation, statistical analysis.

Informed consent
All procedures followed were in accordance with the ethical standards of the responsible committee on human experimentation (institutional and national) and with the Helsinki Declaration of 1975, as revised in 2000. Informed consent was obtained from all patients for being included in the study.