The impact of metabolic control on cognition, neurophysiology, and well-being in PKU: A systematic review and meta-analysis of the within-participant literature

Blood Phenylalanine Levels in Patients with Phenylketonuria from Europe between 2012 and 2018: Is It a Changing Landscape?

BACKGROUND

In 2011, a European phenylketonuria (PKU) survey reported that the blood phenylalanine (Phe) levels were well controlled in early life but deteriorated with age. Other studies have shown similar results across the globe. Different target blood Phe levels have been used throughout the years, and, in 2017, the European PKU guidelines defined new targets for blood Phe levels. This study aimed to evaluate blood Phe control in patients with PKU across Europe.

METHODS

nine centres managing PKU in Europe and Turkey participated. Data were collected retrospectively from medical and dietetic records between 2012 and 2018 on blood Phe levels, PKU severity, and medications.

RESULTS

A total of 1323 patients (age range:1-57, 51% male) participated. Patient numbers ranged from 59 to 320 in each centre. The most common phenotype was classical PKU (n = 625, 48%), followed by mild PKU (n = 357, 27%) and hyperphenylalaninemia (HPA) (n = 325, 25%). The mean percentage of blood Phe levels within the target range ranged from 65 ± 54% to 88 ± 49% for all centres. The percentage of Phe levels within the target range declined with increasing age (<2 years: 89%; 2-5 years: 84%; 6-12 years: 73%; 13-18 years: 85%; 19-30 years: 64%; 31-40 years: 59%; and ≥41 years: 40%). The mean blood Phe levels were significantly lower and the percentage within the target range was significantly higher (p < 0.001) in patients with HPA (290 ± 325 μmol/L; 96 ± 24%) and mild PKU (365 ± 224 μmol/L; 77 ± 36%) compared to classical PKU (458 ± 350 μmol/L, 54 ± 46%). There was no difference between males and females in the mean blood Phe levels (p = 0.939), but the percentage of Phe levels within the target range was higher in females among school-age children (6-12 years; 83% in females vs. 78% in males; p = 0.005), adolescents (13-18 years; 62% in females vs. 59% in males; p = 0.034) and adults (31-40 years; 65% in females vs. 41% in males; p < 0.001 and >41 years; 43% in females vs. 28% in males; p < 0.001). Patients treated with sapropterin (n = 222) had statistically significantly lower Phe levels compared to diet-only-treated patients (mean 391 ± 334 μmol/L; percentage within target 84 ± 39% vs. 406 ± 334 μmol/L; 73 ± 41%; p < 0.001), although a blood Phe mean difference of 15 µmol/L may not be clinically relevant. An increased frequency of blood Phe monitoring was associated with better metabolic control (p < 0.05). The mean blood Phe (% Phe levels within target) from blood Phe samples collected weekly was 271 ± 204 μmol/L, (81 ± 33%); for once every 2 weeks, it was 376 ± 262 μmol/L, (78 ± 42%); for once every 4 weeks, it was 426 ± 282 μmol/L, (71 ± 50%); and less than monthly samples, it was 534 ± 468 μmol/L, (70 ± 58%).

CONCLUSIONS

Overall, blood Phe control deteriorated with age. A higher frequency of blood sampling was associated with better blood Phe control with less variability. The severity of PKU and the available treatments and resources may impact the blood Phe control achieved by each treatment centre.

Unmet needs in phenylketonuria: an exploratory Italian survey among patients and caregivers

OBJECTIVE

Patients with phenylketonuria (PKU) require a strict diet to maintain phenylalanine (Phe) levels within the desired range. However, the diet can be onerous, resulting in poor adherence. We carried out the first online national survey in Italy to better understand the perceptions, knowledge, and experiences of both patients with PKU and caregivers with the goal of improving patient outcomes.

METHODS

An online survey of 35 questions to patients and 36 questions to caregivers was distributed in September 2022 through physicians and relevant Italian associations. The information collected included knowledge and impact of PKU, unmet needs, knowledge of available drugs, and satisfaction with therapy.

RESULTS

Overall, 241 questionnaires were completed by 85 patients and 156 caregivers (96.0% were parents). Knowledge of the pathogenic basis of PKU was generally high. The most common patient-reported symptoms were agitation/anxiety (48.8%), fatigue (41.1%), mood disorders (39.8%), and difficulty concentrating (33.4%). Different perspectives on adherence to a low-Phe diet were observed (22.9% of patients reported strict adherence vs. 47.0% of caregivers). Drugs that allow more freedom were needed by 49.4% of patients and 61.7% of caregivers, along with a wider range of choices of non-dietary treatments (48.2% and 60.0%, respectively). Unmet informational needs of patients included PKU and pregnancy, complications, travel, sports, and transition into adult care.

CONCLUSIONS

Our data showed that patients with PKU and their caregivers reported difficulties in adherence to diet therapy and indicated interest in new therapeutic approaches. Apparent differences between patient and caregiver perspectives were identified. More informational resources on PKU are needed.

Efficient in vivo prime editing corrects the most frequent phenylketonuria variant, associated with high unmet medical need

The c.1222C>T (p.Arg408Trp) variant in the phenylalanine hydroxylase gene (PAH) is the most frequent cause of phenylketonuria (PKU), the most common inborn error of metabolism. This autosomal-recessive disorder is characterized by accumulation of blood phenylalanine (Phe) to neurotoxic levels. Using real-world data, we observed that despite dietary and medical interventions, most PKU individuals harboring at least one c.1222C>T variant experience chronic, severe Phe elevations and do not comply with Phe monitoring guidelines. Motivated by these findings, we generated an edited c.1222C>T hepatocyte cell line and humanized c.1222C>T mouse models, with which we demonstrated efficient in vitro and in vivo correction of the variant with prime editing. Delivery via adeno-associated viral (AAV) vectors reproducibly achieved complete normalization of blood Phe levels in PKU mice, with up to 52% whole-liver corrective PAH editing. These studies validate a strategy involving prime editing as a potential treatment for a large proportion of individuals with PKU.

Pharmacodynamics, safety, tolerability and pharmacokinetics of a single oral dose of an engineered phenylalanine ammonia-lyase in patients with phenylketonuria

The cornerstone treatment of hyperphenylalaninemia (HPA) and phenylketonuria (PKU) is a lifelong low-protein diet with phenylalanine (Phe) free L-amino acid supplements. However, the PKU diet has significant shortcomings, and there is a clinically unmet need for new therapeutics to improve patient outcomes. CDX-6114 is a modified phenylalanine ammonia-lyase (PAL) enzyme obtained by a mutation in the Anabaena variabilis PAL sequence. CodeEvolver® protein engineering technology has been applied to improve the degradation resistance of the enzyme. In our first phase I trial, 19 patients were given a single oral dose of CDX-6114 at 7.5 g, 2.5 g, 0.7 g, or placebo in a cross-over design. After an overnight fast, patients received a standardised breakfast of 20 g of protein, thus exceeding the dietary recommendations for a single meal in patients with PKU. Plasma levels of Phe and cinnamic acid (CA) were measured over a 5-h period following CDX-6114 dosing. During the development of CDX-6114, a stability assessment using reverse-phase high-performance liquid chromatography (HPLC) assay revealed two peaks. The second peak was identified as CA. It was not previously known that as part of the mechanism of action, the CA remained associated with the protein following the conversion of Phe. Thus, recalculating the historical PAL enzyme amounts in CDX-6114 bulk substance was necessary. An updated extinction coefficient was achieved by applying a correction factor of 0.771 to previously reported doses. Postprandial plasma levels of Phe increased in all dose cohorts over time between 10% and 30% from baseline, although the actual peak of Phe levels was not achieved within the 5-h observation. When accounting for the interquartile ranges, these concentrations were similar to the placebo. As plasma levels of Phe were no longer a reliable marker for pharmacodynamics, the consistently detectable amount of CA seen in all patients who received CDX-6114 provided proof of the enzymatic activity of CDX-6114 in metabolising gastrointestinal Phe. Peak levels of CA were seen shortly after CDX-6114 intake, with a rapid decline, and remained low compared with the plasma Phe levels. This pattern indicates a short half-life, possibly due to the liquid formulation or the inability to withstand the lower pH in the human stomach compared with animal models in earlier studies. This was the first trial in patients with PKU to establish the safety and tolerability of CDX-6114. A single dose of CDX-6114 was safe and well tolerated, with no serious adverse events or presence of anti-drug antibodies detected. Efficacy will be explored in future trials using an optimised formulation.

Natural Protein Intake in Children with Phenylketonuria: Prescription vs. Actual Intakes

In phenylketonuria (PKU), an important component of the UK dietary management system is a 50 mg phenylalanine (Phe)/1 g protein exchange system used to allocate the Phe/natural protein intakes according to individual patient tolerance. Any foods containing protein ≤ 0.5 g/100 g or fruits/vegetables containing Phe ≤ 75 mg/100 g are allowed without measurement or limit. In children with PKU, we aimed to assess the difference between the prescribed natural protein intake and their actual consumed intake, and to calculate the natural protein/Phe intake from foods given without measurement or restriction. Over a 6-month duration, three one-day diet diaries were collected every month by caregivers of children with PKU at the beginning of a follow-up study. Dietary intakes of Phe, as well as natural and total protein intakes, were calculated using Nutritics® (v5.09). Weekly blood Phe spots were collected by caregivers. The target blood Phe level was ≤360 μmol/L for ages up to 12 years and ≤600 μmol/L for ages ≥12 years. Sixteen early treated children (69% females) with PKU were recruited. The median age was 11 years (range: 9-13), and most had classical PKU (n = 14/16). A median of 18 (range 12-18) one-day diaries and 22 blood spots were analysed for each subject over 6 months. The median prescribed natural protein was 6 g/day (range: 3-27), but when calculated, the actual median intake from all foods consumed was 10 g/day (range: 4-37). The median prescribed Phe was 300 mg/day (range: 150-1350), but the actual median intake was 500 mg/day (range: 200-1850). The median difference between the prescribed and actual natural protein daily intakes was +4 g/day (range: -2.5 to +11.5), with a median percentage increase of 40% for natural protein/Phe intake (p < 0.001). The median blood Phe level was 250 μmol/L (range 20-750), with 91% of blood Phe levels within the target range. Only one patient (11 years) had less than 75% of their blood Phe levels within the target range. The UK Phe exchange system provides flexibility in the dietary management of PKU. With this method, the actual natural protein intake was 167% higher than the prescribed amount. Although this led to a variable daily protein intake, the majority of children (n = 15/16) experienced no deterioration in their metabolic control.

Metabolic control and clinical outcome in adolescents with phenylketonuria

The main neurological, cognitive, and behavioural consequences of phenylketonuria have been eradicated thanks to new-born screening and Phe-restricted diet therapy. However, the effects of high phenylalanine levels during adolescence and adulthood on neurocognitive functions remain a concern. This systematic review aimed at collecting clinical data suggesting the safest metabolic target for early treated PKU during the second decade of life. Twenty studies met the inclusion criteria for full-text review. Relevant studies included papers that (a) examined the relationship between metabolic control and neurocognitive functions during adolescence or (b) investigated the impact of metabolic control in adolescence on adult outcomes. Most studies showed a positive correlation between metabolic control during adolescence and neurocognitive outcomes across ages. This was true both for IQ and executive functions, although data on executive functions were less clear, and it remains to be established whether they are more vulnerable to Phe than IQ. Taken together present evidence confirm brain vulnerability to Phe during adolescence and suggests that low average Phe levels and low Phe fluctuations should be maintained throughout life. While results are fully compatible with current European recommendations, clinical and methodological limitations coupled with remarkable interindividual variability prevented a clear identification of a safe threshold for Phe blood levels during adolescence.

Towards precision medicine for phenylketonuria: The effect of restoring a strict metabolic control in adult patients with early-treated phenylketonuria

BACKGROUND AND OBJECTIVE

Neonatal screening and early treatment have changed the natural history of PKU, preventing severe neurological and intellectual disability. Nevertheless, the outcome of the disease in early-treated adult patients (ETPKU) is less than optimal, the predictive value of metabolic biomarkers is feeble, and the recommended levels of blood phenylalanine (Phe) for adulthood are controversial. A crucial question whose answer will improve our understanding and treatment of PKU is whether cognitive outcomes can be modulated by levels of Phe even in early-treated adults. To address this question, we carried out an interventional study in seven ETPKU women planning a pregnancy.

METHODS

They underwent an extensive neurocognitive assessment at baseline, and 3 and 6 months after having attained the blood Phe concentration recommended to prevent PKU fetopathy, but before pregnancy.

RESULTS

After 3 and 6 months with a stable blood Phe level of about 240 μmol/L, all participants experienced significant improvements in almost all neurocognitive domains and tasks. IQ also increased of 11 to 21 points from the last assessment before enrolment. This pattern remained strong and consistent after correction for multiple comparisons.

CONCLUSION

Our results indicate that a) strong cognitive improvement is possible even in adulthood and may be demonstrated by lowering Phe near normal levels; b) testing cognition under different metabolic conditions may unveil an individual vulnerability to Phe. These results pave the way for personalised treatment of the disease in adults with ETPKU.

Adult PKU Clinics in the UK-Users' Experiences and Perspectives

Adults with PKU require life-long management, and ideally, their care should be in a specialised adult metabolic clinic. Their outcomes and co-morbidities have received much attention, but data are lacking on their experience, satisfaction and expectations about the care they receive. This survey reports the experiences and care adults with PKU receive from specialist metabolic clinics in the UK. The online survey developed by the UK NSPKU (National Society for Phenylketonuria), was placed on the NSPKU website from February 2021 to December 2022, and was completed by adults with PKU (≥18 years) or their carers/family members. Sixty-five adult PKU patients and 9 caregivers of adult patients completed the questionnaire (63% female in total). Only 32% of respondents were following a Phe-restricted diet with protein substitute intake as prescribed; the rest were partially adherent or not on dietary restrictions. Nineteen per cent (n = 14/74) had not been reviewed in clinic for two years. Half of the respondents (50%) described their experience in adult clinics as "good". Half of the patients were unable to contact their dietitians with questions or concerns, and only 24% considered that they received adequate support. Clinic reviews usually included anthropometric (82%) and dietary assessments (64%), discussion on management of PKU in daily life (78%) and a blood test (71%). Eighty-eight per cent reported they had at least one neurocognitive, mental health or behavioural co-morbidity but less than half of the patients reported an assessment on their neurocognitive functioning or mental health issues. Adult male patients appeared to have less detailed clinic review than females. Less than half (44%) of the respondents reported that they performed a blood spot for blood Phe at least monthly, but only 32% considered they had been informed about the risk of high Phe levels in adulthood. Although time, cost and stress related to travelling were barriers to a face-to-face review, more than 40% of patients had concerns about remote appointments. The frequency and extent of monitoring of adults with PKU, attending specialist adult services, were less than those specified by the PKU European guidelines. The care of women of reproductive age is prioritised over men. Adult metabolic health services require further attention, development and resources to provide a high standard and equitable service to patients with PKU.