Impact of phenylalanine on cognitive, cerebral, and neurometabolic parameters in adult patients with phenylketonuria (the PICO study): a randomized, placebo-controlled, crossover, noninferiority trial

Phenylketonuria (PKU) is a rare autosomal recessive disorder caused by a defective function of the phenylalanine hydroxylase enzyme leading to an impaired conversion of the amino acid phenylalanine (Phe) to tyrosine. Untreated PKU with increased Phe concentrations in the blood and brain during childhood leads to severe irreversible neurological impairment with, e.g., mental retardation, intellectual disability, behavioral problems, or other neurological sequelae such as epilepsy or movement disorders [1]. Newborn screening for PKU was established around 1965, enabling early treatment with a dietary restriction of Phe (low protein diet) and Phe-free protein substitutes (amino acid mixtures) soon after birth. Consequently, the population of adults with phenylketonuria with an absence of neurological disabilities has grown substantially since the early 1980s [1]. Most of these early-treated adult patients live a “normal” life as shown in an earlier study at our center, in which 15% of the adult patients with PKU went to high school or graduated from university [2].

Despite wide agreement on the treatment strategy and target Phe concentrations in childhood and for women during pregnancy, no consensus on safe Phe concentrations in adulthood has been reached so far [3]. Traditionally, the low protein diet had been enforced only during childhood and adolescence, leaving adult patients with PKU “off-diet.” Nonetheless, concerns remain that high phenylalanine levels may still have negative effects in adolescents and adults with PKU [4, 5]. Limited evidence of irreversible neurological damage exists, but over the last decade, observational and cross-sectional studies associated high Phe in early-treated adult patients with cognitive problems [6‐8], psychiatric symptoms, and behavioral abnormalities [4‐6, 9‐13].

The executive deficit hypothesis of PKU suggests that selective impairments of executive functions may result from abnormalities of neurotransmitters, in particular dopamine, a neurochemical that is critical for prefrontal cortical functions [7]. Coming along with alterations of prefrontal cortex functions is a slight decrease in intelligence observed in patients with PKU [14]. Decreased intelligence is linked with impairments in higher-order cognitive functions, including executive functions [15] and attention [16]. In particular, various studies have reported impaired working memory performance in patients with PKU [5, 11]. Working memory is an executive function that encompasses the ability of maintaining information during a short period of time, as well as manipulating and refreshing this information to successfully complete a task [17]. Hence, working memory is crucial in everyday life; for example, it helps with focus on a demanding task or following a sequence of actions [18]. Also, other subtypes of executive functions have been reported to be affected from PKU, such as inhibition and cognitive flexibility [15]. Most of the previous studies described above, however, have focused on selected cognitive domains resulting in a lack of a comprehensive overview across cognitive domains. Such an overview is crucial to increase knowledge about the extent to which cognition is affected by PKU. More importantly, it enables a thorough evaluation of PKU treatment success.

The neurotoxicity of Phe during childhood and adolescence is additionally apparent in regard to structural and functional characteristics of the brain [19, 20]. In detail, an estimated 90% of patients with early-treated PKU display periventricular white matter lesions and reduced white matter integrity, which likely impede processing speed, often reported to be reduced in patients with PKU [21, 22]. In contrast, little is known about gray matter changes in PKU. Pérez-Dueñas and colleagues (2006) found volumetric reductions in gray matter structures including the motor cortex and thalamus [23]. However, data were acquired from a mixed sample of early- and late-treated patients, restricting the interpretation of findings. Smaller whole brain volume and smaller parietal and occipital cortex but larger volume of the putamen were described in early-treated patients with PKU [9, 19]. The latter study included patients 9–33 years old, limiting the generalizability of results due to the lack of comparability between the cognitive and neuroimaging data of children and adults. In regard to functional brain networks, atypical brain activation during a working memory task [24, 25] and decreased functional connectivity [10] have been described, likely coming along with cognitive alterations of this vulnerable patient sample. Patients with PKU additionally display neurometabolic alterations in upfield and downfield spectra, such as a decrease in choline concentration [20] and an increase in cerebral Phe level [26], respectively. Due to the limited amount of neuroimaging studies, still much uncertainty exists about the relationship between Phe concentrations and brain abnormalities in adult patients. Importantly, these studies are unable to distinguish between past effects of elevated Phe levels on the brain during childhood and adolescence and a potentially ongoing negative impact in adulthood. Previous intervention studies were mostly open label, associating the level of Phe intake in the diet with cognitive performance [27]. However, the perception of one’s own compliance with dietary recommendations may influence cognitive performance. One small intervention trial investigated the effect of the Phe concentration on cognition in a prospective randomized, placebo-controlled, double-blinded way [28]. Ten Hoedt and colleagues suggested a negative effect of high plasma Phe levels on sustained attention and on mood in nine adult patients with PKU, concluding that “a Phe restricted ‘diet for life’ might be an advisable option for many” [28].

Due to the availability of only low-grade evidence, recommendations of national and international guidelines differ substantially with regard to Phe target levels in adult patients with PKU [3, 29]. The most recent European guidelines suggest keeping Phe concentrations below 600 umol/l throughout adulthood, while stating this is a grade D recommendation. Consequently, not only are the recommendations unequally accepted by the treating metabolic specialists, but more than 50% of adults with PKU exhibit substantial difficulty in maintaining the compliance necessary to reach the recommended target Phe concentrations [30, 31]. Therefore, prospective intervention studies in adult patients with PKU are needed to evaluate the effects of dietary restrictions on cognition, cerebral and neurometabolic parameters, and quality of life.

The PICO Study will help provide some much-needed answers on whether a rigorous protein-restricted diet to control Phe-levels is indispensable for adults with early-treated PKU. Using a combination of neuropsychological and neuroimaging data, the PICO Study will considerably contribute to an improvement of the currently insufficient level of evidence on how adult patients with early-treated PKU should be managed.

The double-blind, placebo-controlled design will eliminate the expectation bias of patients and physicians otherwise inherent to dietary interventions. Potentially positive effects of the recommended dietary restrictions have to be proven by means of double-blinded intervention trials and not only deduced from biased open interventions and association studies. This determination is particularly important in light of the potential risks of a lifelong low-protein diet. Similar to a strict vegan diet, a Phe-restricted diet not only lacks proteins but also many micronutrients, both of which have to be monitored and compensated with supplements [49]. In addition, the restrictive diets may be associated with a predisposition or aggravation of psychiatric problems by itself: eating disorders and obsessive compulsive disorder are four times more common in patients with PKU compared to the general population and twice as common as in patients with diabetes mellitus [50].

During the intervention phase, transient mild cognitive impairments might occur due to an increased Phe load. A similar smaller study with increased Phe load over 4 weeks suggested an impairment in sustained attention as well as lower scores in mood profile but did not report any other mild or serious adverse events [28]. Serious side effects are not to be expected. Neither is hyperphenylalaninemia in adulthood known as a cause of metabolic encephalopathy, nor is there a recognized acute decompensation of PKU [51]. The probability of irreversible adverse events during the progress of the present study is therefore extremely low. Even in adult patients with PKU not following any low protein diet for a long time, only very few cases of serious neurologic events with unlikely causal relationship (some of them probably caused by severe vitamin B12 deficiency) have been described in the literature: 11 cases of spastic paraparesis, three cases of muscle weakness/difficulty walking, and three cases of vision loss [52]. Despite not following any low protein diet for a long time, most adult patients showed improved scores for attention as well as self-reported anxiety and depression after reintroducing a low-protein diet [21, 53].

Neuroimaging has become an indispensable tool to better understand brain regions of susceptibility in metabolic disease and to assess the efficacy of dietary treatment on the brain. Clinicians can benefit from the technologic advancements in neuroimaging, allowing for improved understanding of diseases and ameliorated patient management. Only a limited number of neuroimaging studies have been conducted so far in patients with PKU. This is evident on a cross-sectional level but even more so on a longitudinal level. Hence, conclusive evidence on the influence of Phe on brain structure and function is not available with the current state of knowledge. By combining a broad range of advanced MR-techniques, the PICO Study strives to advance our understanding of the disease PKU and its impact on different properties of the brain. The additional information gained by the neuroimaging results may help to characterize subgroups of patients that benefit or do not benefit from restrictive dietary interventions.

Evaluation of patients’ perceived cognitive performance is an important part of routine patient care [29]. However, patients’ reports may be prone to a substantial expectation bias as they are usually aware of their recent compliance with dietary restrictions. This double-blinded trial offers a unique opportunity to evaluate the reliability of self-assessment of adult patients with early-treated PKU.

The PICO Study also yields an economical point of view. Phe-restricted diet, including the use of amino acid supplements and low-protein medical foods, costs approximately 20,000 CHF or USD per year, and patient. Sapropterin (Kuvan®), which can be used in a small subset of patients with BH4-responsive PKU, costs approximately 180,000 CHF or USD annually. Also pegvaliase (Palynziq®), the new enzyme substitution therapy [54‐56], is estimated to cost approximately 190,000 CHF or USD per year and patient. Of note, neither sapropterin nor pegvaliase enabled clinically relevant significant improvement of cognitive outcome [22, 24, 57, 58].

Two limitations of the study inherent to the design need to be mentioned. First, being “off-diet” is mimicked, not conducted. Second, the duration of the intervention does not reflect an indefinite discontinuation of the Phe-restricted diet, and long-term effects of elevated Phe levels still remain largely unknown. The oldest early-treated patients with PKU are now only in their early 50s. This is a strong argument for keeping all patients with PKU under periodic review and making sure that even those who have stopped dietary treatment are not lost to follow-up. Carefully conducted long-term cohort studies are needed to learn more about the course of the disease as patients age. Based on the current state-of-the-art, patients and their metabolic specialists are unlikely to agree to a longer intervention. A confirmation of the hypothesized noninferiority of Phe could justify a larger long-term trial and support the willingness of patients and treating metabolic specialists to participate in such a long-term clinical trial.

In summary, this double-blinded, randomized, placebo-controlled, crossover, noninferiority trial using a combination of cognitive and neuroimaging data, will considerably contribute to improve the currently insufficient level of evidence on how adult patients with early-treated PKU should be managed.