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Lindegren ML, Krishnaswami S, Fonnesbeck C, et al. Adjuvant Treatment for Phenylketonuria (PKU) [Internet]. Rockville (MD): Agency for Healthcare Research and Quality (US); 2012 Feb. (Comparative Effectiveness Reviews, No. 56.)

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Adjuvant Treatment for Phenylketonuria (PKU) [Internet].

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This section provides an overview of the state of the literature and outcomes for each Key Question, details the strength of evidence for the impact of each major intervention on relevant outcomes, and describes major issues and gaps in the current body of evidence.

State of the Literature

Summary of Outcomes by Key Question

Key Question 1a. Optimal Blood Phenylalanine (Phe) Levels for Minimizing/Avoiding Cognitive Impairment

Individuals with phenylketonuria (PKU), their families and their clinicians make continual decisions and treatment adjustments based on Phe measurements, with little information about the degree to which any course of treatment is providing protection against cognitive impairment. The precise relationship of blood Phe levels to intelligence quotient (IQ), and the timing of the effect have not been fully elucidated, in part because extant studies are small and sample populations in individual studies are sometimes selected to be homogenous. By combining information from a large number of studies that described the relationship between Phe and IQ, we provide further evidence of the relationship between specific blood Phe levels and IQ, the impact of the critical period on cognition and the best timing for Phe and IQ measurement in order to determine these effects. It is well established that high levels of blood Phe are associated with a lower IQ and that dietary control can mitigate the effects of high Phe,. The current analysis provides additional support for continuing dietary control through adolescence and into adulthood, although detailed information about the requisite level of control by age group and particularly into older age remains unknown.

Seventeen studies were included in the meta-analysis, providing data on 432 individuals who ranged from age 2 to 34 years. We modeled the association of IQ less than 85 with blood Phe level, accounting for time of Phe measurement relative to cognitive testing, and whether or not the measurement occurred in the critical period (<6 years of age). While intellectual disability is defined as IQ score lower than 70 (i.e., 2 standard deviations below the population mean) and impairment in activities of daily living, IQ scores within the normal range could be considered impairment if lower than the expected value of the general population. Though necessarily subjective, we believe that a reasonable candidate for impairment is a threshold of 1 standard deviation below the population mean, or an IQ score of 85. Subjects below this threshold would likely exhibit symptoms of cognitive impairment, such as poor language development, problem solving deficiencies, and memory deficits.

Increasing Phe is clearly associated with decreased IQ, with a probability of IQ less than 85 exceeding the population probability (approximately 15 percent) at blood Phe over 400 μmol/L and leveling off at about 80 percent at 2,000 μmol/L. This finding supports the typical target goal for blood Phe levels in individuals with PKU (120 to 360 μmol/L).8

Notably, the negative association between blood Phe and IQ is strongest when Phe is measured at least one year prior to IQ testing. The blood Phe level obtained more than one year before IQ testing is likely to be a better indicator of how well Phe has been controlled over the long term, relative to concurrent measurements. This relationship lends support to the principle that cognitive effects accumulate over a long time period, and thus concurrent measurements are poor predictors of a cognitive effect. The strongest associations are seen in the group for which historical measurements were taken during the critical period (<6 years old) and associated with later IQ, although historical measurements taken after the critical period are also associated with risk of low IQ. Hence, control of blood Phe levels during the critical period is particularly important, but the need for dietary control continues throughout the lifetime. Current clinical practice is to try to maintain tight Phe control even in adulthood, which is supported by this analysis and is consistent with the NIH recommendations of diet for life.

Note that the two lines corresponding to historical measures of blood Phe in Figure 3 (top two lines) both demonstrate increasing probability of low IQ at higher Phe measures, regardless of whether the effect is being measured during childhood (solid line) or beyond (dashed line), with a stronger association seen between blood Phe measured in early childhood and later IQ.

The two lower lines in the figure describe probability of IQ <85 as a function of blood Phe when measured concurrently. The lack of strong association in measurements taken concurrently during the critical period suggests that effects are unlikely to be observed in this period, either because the IQ test is not stable for young children (less than 5 years old) or because the adverse effects take time to manifest. From a clinical perspective, this provides a basis for being cautious in interpreting measures of cognitive outcomes during the critical period as they relate to blood Phe, and emphasizes the importance of well-controlled Phe levels during the critical period and over time.

Of note, these estimates may be biased because they are based on studies that include nonrandomly selected individuals from the PKU population. Insurance coverage and access to care for individuals with PKU, especially adults, is uneven across states and insurance companies. There is likely substantial unevenness in the degree to which patients access or use medical care, which would be the primary way that they would be recruited into studies. Thus if, the available studies exclude individuals not interacting with the healthcare system, the associations presented here may be conservative, as they may be especially likely to exclude people who are non-adherent to diet. Thus, we anticipate that clinicians can use these results to encourage parents and patients to maintain dietary control even in the absence of immediate, observable effects. Researchers considering the effect of Phe on IQ should know that when those measurements are taken concurrently, a relationship may not be apparent, and that a more accurate predictor may be historical measurements, such as an index of dietary control, which typically is calculated as the mean of annual mean or median Phe levels.

Optimal Phe Levels for Minimizing Impairments in Executive Function in Individuals With PKU

Studies of the association of blood Phe and executive function have targeted many specific outcomes, precluding straightforward quantitative analysis of the data. Some studies clearly suggest that elevated Phe is likely associated with poorer outcomes but data are inconsistent across types of measures. This is an important area for research, although there is currently insufficient strength of evidence to delineate a specific relationship between blood Phe levels in the individual and specific measures of executive function. To a large degree this is because no specific measures of executive function have been validated as sensitive to changes in Phe in the population with PKU, and thus this is a rich area for ongoing and future research.

Optimal Phe Levels for Minimizing Impairments Related to Maternal PKU and Maternal PKU Syndrome

Data also provide support for the increased risk observed of poor cognitive outcomes in the offspring of high maternal blood Phe. The Maternal PKU Collaborative Study was initiated in 1984 to study the implications of maternal PKU, and specifically to assess outcomes when Phe is controlled in pregnant women. The study demonstrated that timing of maternal metabolic control, defined as the number of weeks gestation before plasma Phe levels remained consistently lower than 605 μmol/L, was associated with child cognitive scores at 4 and 7 years of age. This is consistent with current recommendations that pregnant women achieve dietary control as early as possible in pregnancy, or before pregnancy, and maintain it until birth.

Because they had access to the largest available data set on maternal PKU, investigators were able to model the form of the association between maternal blood Phe levels during pregnancy and effect on offspring during childhood.33 The analysis confirmed that the relationship between maternal blood Phe and offspring cognitive outcomes was not linear, and that a threshold of 360 μmol/L is the threshold level at which cognitive impairment was significantly more common in offspring of mothers with PKU than in controls, and that a linear relationship between Phe levels and impaired cognitive outcomes occurred after this threshold. Importantly, while other factors, including maternal characteristics, severity of mutations and head circumference, contributed strongly to outcomes at 1 year of age, by age 2, maternal Phe strongly overtook other factors in predicting cognitive impairment, supporting current recommendations that regarding the importance of dietary control for women who may become pregnant and for pregnant women.

Key Question 2. Effectiveness of BH4 as an Adjuvant Treatment With Diet Versus Diet Alone in Individuals With PKU

The treatment for PKU with dietary restriction of Phe in natural protein and use of Phe-free medical foods has been critical in reducing the incidence of irreversible neurocognitive impairment in individuals with PKU. However, especially as patients enter adolescence and adulthood, dietary adherence and supplement use can be difficult. As noted in Key Question 1, effects of Phe levels on cognitive outcomes can continue beyond the so-called critical period, making lifelong management the goal for people with PKU and some level of diet for life the current recommendation. However, little is known about rates of adherence to diet, especially as children age into adolescence and beyond.

To date, clinicians, patients and families have lacked therapeutic options other than a lifetime of strict dietary management. Importantly, the ability to liberalize the diet has the potential to affect the quality of life of individuals with PKU who must be constantly vigilant about what they consume. An optimal therapeutic adjunct to dietary management would increase Phe tolerance allowing for increased intake of dietary protein and reducing (but likely not eliminating) the necessity for Phe-free medical foods.

The targeted goal for treatment may differ by the degree to which an individual is able to maintain dietary control. The goal of an adjunct pharmacologic treatment in an individual already able to maintain good dietary control should be to liberalize the diet, with a focus on quality of life, as well as maintenance of cognitive function. The goal for an individual unable to achieve target blood Phe levels with dietary restrictions is to lower Phe levels directly.

As a potential adjuvant treatment approved by the U.S. Food and Drug Administration (FDA) in 2007, BH4 works by enhancing residual enzyme activity present in some individuals with PKU. Research to date is limited, with only two randomized controlled trials (RCTs) and three uncontrolled open-label trials currently available in the literature. One of the open label trials is an extension of one of the RCTs. The largest of the studies included only 90 individuals.

These relatively small numbers are a reflection of how rare the disease is, which makes recruitment of patients challenging and means that clinical decision making may always need to be made on the basis of few, small studies put into context with other clinical information.

All potential study participants underwent an initial loading test and were only included in efficacy studies if they demonstrated an initial reduction in Phe levels. The proportion of those screened who met this criterion ranged from 19 to 62 percent. Screening responsiveness was to some degree associated with blood Phe level, and individuals diagnosed with mild PKU were most likely to show initial responsiveness. Some individuals with classic PKU and very high Phe (>1200 μmol/L) were responsive, but at a much lower rate than those with mild or moderate PKU. Each study in the review used somewhat different screening criteria, and no approach to assessing responsiveness has been shown to be optimal. As a result, study populations are potentially heterogeneous.

All studies evaluated intermediate outcomes (change in blood Phe levels and Phe tolerance). Almost no information is yet available, and none from RCTs, on longer term outcomes including cognitive impairment, quality of life, nutritional impact and status, and the ability to liberalize diet. In enriched populations (all participants had reduced Phe in initial loading tests), fewer than half of the participants had Phe reductions of at least 30 percent, and reductions in Phe were not related to clinical outcomes.

For example, one of the studies that formed the basis for FDA approval recruited only individuals with blood Phe levels higher than 450 μmol/L and did not require that participants successfully adhere to a restrictive diet.113 This study population included adults who could be following current recommendations allowing for some liberalization of the diet.8 Presumably, the clinical target for this group would be to reduce Phe through pharmacologic treatment. Indeed, significantly more treated participants achieved the 30 percent target reduction in blood Phe than did those in the placebo group (44 percent vs. 9 percent).113 At the end of 6 weeks of treatment, 32 percent of the treated group had achieved Phe <360 μmol/L, compared with 2 percent in the placebo group (p<0.001). Thus, although the effect was substantial, a high proportion of treated participants who achieved a reduction in blood Phe of the study target of 30 percent continued to have Phe levels above the clinical target. There is no evidence that a 30 percent reduction is clinically meaningful if blood Phe levels remain above clinical targets. Nonetheless, an open label extension of this trial demonstrated that reductions in Phe observed early in treatment could be maintained up to 22 weeks.114

On the other hand, the clinical goal for individuals maintaining dietary control could be to improve their quality of life by liberalizing their diet. In the trial that targeted children with Phe <480 who were successfully maintaining a restricted diet, Phe tolerance was increased.115 Total Phe intake (dietary Phe intake plus total medical food supplement to maintain blood Phe levels in the therapeutic range) increased from baseline in the BH4 group, approximately doubling to 43.8 mg/kg/day at 10 weeks. An example of the practical implication of this result for the typical 6 year old with PKU who weighs about 45 pounds (20 kilograms) is that while on BH4 for the 10 week duration of this study, she might be able to liberalize her daily diet by consuming an additional 8 ounces of milk, or adding about 1 ounce of meat, or one small serving of spaghetti without meat or cheese. The placebo group in this study had a slight increase in total Phe intake from 16.3 mg/kg/day at baseline to 23.5 ± 12.6 mg/kg/day at 10 weeks. Even so, the impact on Phe tolerance was not uniform across the study population; 36 percent tolerated an increase of 10 mg/kg/day or less, 30 percent tolerated an increase of 11 to 30 mg/kg/day and 33 percent tolerated an increase of 31 to 50 mg/kg/day. Some participants in the BH4 group had transient low blood Phe levels (<26 μmol/L) that were corrected with increased Phe supplementation. Although many of the participants could modestly increase their protein intake, none could be on an unrestricted diet.114

Phe tolerance was also assessed in the open label study not associated with an initial RCT.116 For this study, participants began with a BH4 dose of 10 mg/kg/day, which was increased to 20 mg/kg/day if a 30 percent decrease in Phe or achievement of a target blood Phe level of 360 μmol/L was not observed within a week. Responders who were on a Phe-restricted diet underwent gradual liberalization of their diet to the maximum tolerated natural protein intake while still maintaining plasma levels in the range of 120 to 360 μmol/L. Among individuals who were responders and on a Phe-restricted diet, the average Phe tolerance increased from 21 to 41 mg/kg/day. However, responders' Phe tolerance varied widely from an increase of 20 to 22 mg/kg/day to a non-protein restricted diet in two participants. Of note, a number of conditions may affect Phe tolerance, including illness, type of mutation, degree of BH4 response among others; these are not assessed in the studies.

In all of the studies, compliance with BH4 was reported to be good over the short term. However, long term sustainability of compliance with both BH4 and dietary therapy, especially given the variability in response, has not been evaluated, nor has durability of treatment effects. Authors from the uncontrolled open label trial note that one responder reportedly discontinued BH4 after the trial as the small increases in Phe intake that BH4 allowed was not significant enough to warrant taking the medication. Certainly, as noted above in the summary of results, observed increases in Phe tolerance were moderate at best in classic PKU in terms of allowing changes in diet, and the decision about trade-offs between reliance on medication and carefully titrating liberalization of the diet will need to be made by patients and their clinicians on an individual basis that balances available evidence with the individual's context.

Key Question 3. Effectiveness of BH4 Versus Diet Alone in Maternal PKU

We did not identify any studies of the role of BH4 in pregnant women. Reports of three cases have been published, and a registry is ongoing. It is essential that individual clinicians publish data about their patients and provide data for the registry in order to build an evidence base.

Key Question 4. Effectiveness of LNAAs Versus Diet Alone in Individuals With PKU

In theory, supplementation of a Phe-restricted diet with large neutral amino acids (LNAAs) might have beneficial effect on cognition as LNAAs may competitively inhibit transportation of Phe through the blood-brain barrier, thereby offering protection by potentially decreasing brain Phe levels. Some researchers have postulated that this may explain why there are some PKU patients with high plasma Phe levels, low brain Phe levels and normal cognitive function. Similarly LNAAs and Phe, facilitated by a carrier protein, cross the intestinal mucosa. LNAAs, at much higher levels, may also compete with Phe for transport across the intestinal mucosa.

However, there is insufficient evidence to suggest that LNAAs could be a viable treatment option for improving neurologic outcomes or increasing Phe tolerance. There have been only three very small studies (total number of participants was only 47) with inconsistent results, and there is no evidence that the treated individuals experienced clinically meaningful improvement in their cognitive or neurologic outcomes in the short time that they were studied.

Key Question 5. Effectiveness of LNAAs Versus Diet Alone in Maternal PKU

We did not locate any studies addressing this question.

Key Question 6. Harms of BH4 or LNAAs

Reported harms in trials of BH4 were mild and included headache, throat pain, upper respiratory infection, diarrhea, abdominal pain, nausea and vomiting at rates no greater than seen in placebo arms. Headache was more frequently observed in the placebo group compared with the BH4 group when BH4 was given at a dose of 10 mg/kg/day while a higher proportion (21 percent) taking BH4 at a dose of 20 mg/kg/day reported headache compared with those on placebo (8 percent). Pharyngolaryngeal pain was more frequently reported by the BH4 group at 20 mg/kg/day compared with the placebo group (12 percent vs. 8 percent, respectively) over 10 weeks. Three study participants withdrew from a study due to harms;112 harms reported in this 2.6 year study were largely minor and in line with those reported in earlier studies with some overlapping participants.113-115 The rates of harms by study group were compared statistically in only one study, which found 23 percent in the treated group and 20 percent in the placebo group experiencing a harm, probably related to treatment.113

Even though studies reporting harms consistently indicate that BH4 is well tolerated and without serious side effects, not all studies assess and report harms, and data are based on a small number of individuals, so ongoing registries will be important for supplementing these data. One fair quality study of LNAAs reported a higher rate of anxiety in the treatment arm, which was an unexpected event, but the study included few participants and studied effects over a short time period.124

Key Question 7. Effectiveness of BH4 or LNAAs Plus Diet in Subgroups of Individuals With PKU

Although all five trials enrolled only patients who were BH4-responsive, efficacy in terms of decreasing blood Phe level or increasing Phe tolerance was 44 percent to 62 percent even in this enriched study population. This suggests that there may be yet unidentified subgroups that are more likely to have a positive response to drug treatment. With only small studies published to date, the literature is unable to provide evidence of effectiveness in subgroups including differences seen in response by disease severity.

Strength of the Evidence for Effectiveness of Therapies


The degree of confidence that the observed effect of an intervention is unlikely to change is presented as strength of evidence and can be insufficient, low, moderate, or high. Strength of evidence describes the adequacy of the current research, in both quantity and quality, and whether the entire body of current research provides a consistent and precise estimate of effect. Interventions that have shown significant benefit in a small number of studies but have not yet been replicated using rigorous study designs will have insufficient or low strength of evidence, despite potentially offering clinically important benefits. Future research may find that the intervention is either effective or ineffective.

Methods for applying strength of evidence assessments are established in the Methods Guide for Effectiveness and Comparative Effectiveness Reviews45 developed by the Agency for Healthcare Research and Quality (AHRQ) Evidence-based Practice Centers (EPCs) and are based on consideration of four domains: risk of bias, consistency in direction of the effect, directness in measuring intended outcomes, and precision of effect. We determined the strength of evidence for the following outcomes: Phe level, tolerance, and variability; cognitive outcomes, nutritional outcomes, harms, and quality of life

Table 21 documents the strength of evidence for each domain of the major intervention–outcome combinations.

Table 21. Intervention, strength of evidence domains, and strength of evidence for key outcomes.

Table 21

Intervention, strength of evidence domains, and strength of evidence for key outcomes.

Strength of the Evidence

The strength of evidence (confidence that the observed effect will not change) for the relationship modeled of Phe and IQ in the meta-analysis is moderate. There were adequate numbers of studies, but they varied in quality. Additional studies with less risk of bias could strengthen our confidence that the relationship we saw accurately reflects the true effect.

The strength of evidence for a threshold effect of blood Phe of 360 μmol/L in affecting cognition in the offspring of women with PKU is low as it is based on one longitudinal study (Table 21). Further analysis is warranted to confirm and/or expand upon the observed relationship between maternal Phe and offspring IQ. This should not be construed to mean that an effect of Phe on infant outcomes was not seen; rather it specifies that 360 μmol/L may or may not be the ideal goal, and studies are clear in supporting the need for early dietary management for women with PKU considering pregnancy or who are pregnant.

In terms of treatment effects, we separately examined the strength of the evidence for short term effects on Phe, longer term effects on Phe, and direct effects on cognition. The strength of the evidence for a large and significant effect of BH4 on lowering Phe to clinically acceptable levels in the short term is moderate. Given the moderate strength of evidence of the Phe to IQ relationship noted above, and the therefore indirect effect of BH4 on IQ, the overall strength of the evidence for BH4 to improve cognition is low, pending additional data. Strength of evidence is currently insufficient for longer term outcomes, but additional data continue to be published, and ongoing registries will provide important information. The strength of evidence that harms associated with the treatment are minor and not significantly greater than those seen with placebo is moderate, again pending additional research and registry data. With more than 12 studies ongoing, additional data are likely to be available in the future. At this time, it is unclear whether these new studies are likely to corroborate current early outcomes; they will certainly provide additional information on a number of specific outcomes (e.g., measures of cognition) and in specific target populations (e.g., young children and pregnant women).

The strength of evidence for an effect of LNAAs on all outcomes is insufficient.


The degree to which current research may not be applicable to the clinical population with PKU is a concern, given the small size and homogenous populations in each of the studies. For example, the two RCTs of BH4113, 115 each focused on a distinctly different population; one on a slightly older population with naturally more variation in diet, and one on a somewhat younger group with tight dietary control. Both reflect important PKU populations, but because they are different, it is unclear whether the results should be synthesized, or whether either study can confirm the results of the other. The two RCTs do provide data on a range of patients who are similar to those seen in routine clinical practice. As is always the case with RCTs, they may not represent the patients less likely to receive regular medical care, or those with additional medical comorbidities. As noted previously, the degree to which care for PKU is available and covered by insurance varies substantially, and it seems likely that individuals unable to access care also may not be situated such that they are recruited into studies. If this is true, then the individuals in the studies may represent a group of patients most likely to be consistent users of medical care and advice; this has implications for the potential issues of adherence to any medical intervention. Little is known about the burden of adhering to medication and the degree to which patients in clinical practice outside of trials would adhere to a drug regimen.

Data are entirely lacking on the use of pharmacologic therapy in pregnancy, although this is likely an area of interest and need for those making clinical decisions. The lack of data may be due in part to characteristics of this patient population. Women with PKU are frequently off of dietary therapy for significant lengths of time before conception. There may also be delays in initiation of clinical metabolic care. Thus, the population available to study pharmacological therapy is small and fetal outcomes may be confounded by the effects of poor blood Phe control. Nonetheless, an ongoing registry should provide invaluable data.

As noted throughout this report, PKU is an exceedingly rare condition, making it challenging even to enroll enough participants in research studies, and even more so to include enough “types” of people to fully represent the patient population and provide applicable data for the full range of PKU patients.

Applicability of Studies Assessing BH4

Participants ranged in age from 10 days to 58 years in all studies and 4 to 49 years in RCTs. Most individuals were classified as having mild to moderate disease, which is appropriate given the expected mechanism of action (i.e., boosting the activity of residual phenylalanine hydroxylase). Studies typically included participants recruited from metabolic clinics at university/academic-affiliated clinics or research centers, which is generally where PKU treatment is available. Most individuals had demonstrated responsiveness to BH4 in a loading study, though there was variability in the loading study methods and the dosage required to produce a response according to study criteria. Participants' adherence to a restricted diet varied, with one RCT including participants with good compliance and one including participants with poor compliance and higher average Phe levels. In practice, patients range in their compliance, so studying the effects of BH4 across a range of dietary compliance is important to understand its potential effects.

BH4 was studied in doses that ranged from 5 mg/kg/day to 26 mg/kg/day. Duration of treatment ranged from 37 days to 2.6 years in randomized and uncontrolled open label trials and up to 9 years in one case series. Individual variation from dosing protocol was reported in some studies, though overall compliance with the medication regimen was reported to be good in the short term, based on parent or patient report.

Studies primarily assessed short-term change in blood Phe levels and/or Phe tolerance (daily medical food supplement tolerated). One case series119 and one cohort study121 measured changes in Phe variability, and one examined clinically meaningful outcomes, including IQ, developmental quotient, and nutritional status.118 Three case series also assessed participants' ability to liberalize their diets.112, 117, 118 These case series were very small and of poor quality. Ultimately, to understand the applicability of this drug, substantially more data are needed on clinical and long-term outcomes established to be important to patients.

Evaluations occurred at the end of less than 6 months of treatment, with the exception of case series that followed participants for up to 9 years and one 2.6 year open label trial. Few studies assessed harms, and those that did reported mostly minor events (e.g., headache, throat pain). It is not clear whether these outcomes and harms predict longer-term results.

Applicability of Studies Assessing LNAAs

The use of LNAAs has been proposed primarily for patients unable to achieve dietary compliance. It is difficult to assess the applicability of current research, however, given the very small sample sizes and short-term outcomes measurement. Studies included a total of 47 individuals, most with classic PKU, between the ages of 11 and 45. Participants were on a restricted diet in two studies. In the third study the subjects had an unrestricted diet, and the average Phe intake exceeded 500 mg/day.

LNAA dosages ranged from 250 mg/kg/day to 1 g/kg/day, with many pills required each day. The degree to which it is likely that patients having difficulty maintaining a strict diet would respond positively to taking multiple pills has not been explored. Treatment duration ranged from 1 to 8 weeks, and no study followed participants for more than 1 week after treatment. Formulations of LNAAs varied: 2 studies used the NeoPhe formulation (Solace Nutrition), and one used a formulation manufactured by SHS International. The formulations contained largely the same amino acids with the exception of the addition of arginine in NeoPhe.

An RCT compared LNAAs with placebo with and without participants' usual medical food;124 another RCT compared LNAAs plus usual diet with placebo and usual diet.125 The uncontrolled open label study also examined LNAAs with continuation of participants' usual diet.16

All studies measured changes in blood Phe level. One RCT also assessed cognitive and affective outcomes and brain Phe.124 Harms were not systematically assessed in any study. Evaluations occurred shortly after treatment ended, and it is not clear whether these intermediate outcomes predict longer-term outcomes.

Future Research

The existing research gaps related to the use of adjunct pharmacologic therapy in PKU are both substantive and methodologic. Specific deficiencies range from the substantive need for more trials that include more individuals to methodologic gaps in our understanding of the longer term implications of intermediate outcomes. In both cases, research is fundamentally challenging because the disease is so rare, making accrual of adequate numbers of participants difficult, if not impossible, for specific studies. Furthermore, in part because it affects so few people, funding for PKU research is limited, and to date, treatment research is almost exclusively supported by the pharmaceutical industry. Other rare conditions have benefited from an overall research agenda. To this end, we recommend that a multi-collaborator process that includes a public-private partnership which could create a powerful tool for the future of PKU research in the form of a longer term (perhaps 10 year) research agenda. Furthermore, because the metabolic centers that treat patients with PKU are identifiable, and because PKU patients are almost inevitably treated in such a center if they are receiving care, there is tremendous potential for development of a multicenter research consortium to comprehensively evaluate the complete system of care for individuals with PKU.

Funding from private or public entities should help establish a long-term prospective registry through which the consortium could collect comprehensive and detailed data on subjects with PKU. This could include additional support or linkage with the existing registry that is specific to use of Kuvan, the PKUDOS. The expanded registry could include, but need not be limited to, data on short and long-term outcomes of treatment, such as executive functioning, nutritional status, growth, and quality of life. Ideally, this registry would include a biorepository that would help identify any genotype-phenotype correlations and provide a multidimensional perspective on the effectiveness in practice of treatments, both in the short and long term.

One corollary might be a committee of experts and individuals with PKU to focus on harmonizing data collection, standardized outcomes assessments, required specific and stringent standards for conducting double-blind placebo-controlled trials that adhere to high standards required for synthesis and use in treatment guidelines, and the selection and implementation of studies that clarify the short- and long-term outcomes of treatments and interventions for individuals with PKU, including psychological outcomes. For example, since dietary restriction is the essential cornerstone in the treatment of PKU, it would be helpful to study various methods that would improve adherence to dietary management and other intervention strategies in order to improve outcomes throughout the lifespan, especially for adolescents and adults with PKU. With the establishment of a multicenter consortium, registry, and biorepository, PKU could serve as a model for studying the short- and long-term outcomes of treated inborn metabolic diseases. The field already has a starting position, with the Maternal PKU Collaborative study a case in point.

Future Research on the Relationship of Phe and Cognition

A significant limitation in the current body of research on the relationship between blood Phe level and cognitive outcomes is the lack of consistent methodologies using standardized tools and measures and consistent data collection across centers. The result is that many studies provide incomplete data that cannot be used in meta-analyses, despite a clear need for research to occur across sites in order to accrue adequate numbers for analysis. The studies that were included for meta-analysis were those that met the criteria for data availability. Specifically, studies frequently lacked measures of variance and correlation. Complete reporting of data and results in future studies would ensure that future research can be considered in more robust meta-analyses and can contribute to an improved understanding of the relationship between Phe and IQ.

In addition, some studies that did provide appropriate data for inclusion did not provide information on potentially confounding or modifying factors in the relationship between Phe and IQ. In future research, details about familial IQ, socioeconomic status, maternal education, age at initial treatment and concurrent medications should be fully described so they might be used in a more extensive meta-analysis of Phe-IQ associations. One basic need is to better understand the degree to which the perceived association changes by age, with the practical implication of understanding the degree of dietary control necessary across age groups. Certainly if patients are able to adhere to diet, then tight control is the standard of care, but understanding the specific implications of looser control, especially in older adults, is lacking and could inform clinical practice. Because tight control is important, an understanding is needed of the supports that might be helpful as individuals age over the lifespan. Related to this is the need for additional measures to assess adequate control beyond blood Phe. This requires an understanding of what outcomes are clinically important, and their relative value to patients and their families. For this to be possible, complete and accurate measure of Phe and cognition over fairly long periods of time is necessary, perhaps through a long-term follow up study or through the multisite collaboration suggested above. Finally, the effects of mild hyperphenylalaninemia as opposed to classic, mild and moderate PKU, should also be clarified, including the impact on cognition, executive functioning, attention, behavioral problems, and other psychological issues.

Ideally, future studies or a complete registry could provide repeated measures (e.g., index of dietary control) of blood Phe that can more precisely characterize an individual's Phe level over relevant time intervals, and standard deviations around those measures so that we can determine the effect of variation in Phe on IQ. Also, rather than relying solely on IQ, alternative outcomes could allow for modeling the degree to which increased Phe is associated with differences between an individual's realized and expected outcomes.

Although research is being conducted on executive function outcomes for individuals with PKU, there is no consensus on which measures of executive function are most appropriate. This highlights the need for fundamental research, because measures of executive function tend to be better reflections of success with day-to-day activities than targeted measures such as IQ. It is plausible that some measures of executive function may be more sensitive to changes in Phe than IQ, and therefore better at identifying impairment. By the same token, establishing the degree to which measures of executive function can and should be combined in analyses would be helpful for synthesizing the currently disparate body of literature. Nonetheless, the sensitivity, validity and acceptability of individual executive function measures in PKU has yet to be established or agreed upon, and current research reflects a reliance on a wide range of outcomes, making synthesis of relationships and pooling of results difficult.

Given the reported association between PKU and an increased incidence of inattention, anxiety and depressive symptoms, additional studies on these and other psychological issues in PKU are also warranted. Some of this work is ongoing, and we encourage more work examining the full range of outcomes associated with PKU.

Future Research on Pharmacologic and Other Adjuvant Treatment


Research on the use of BH4 as an adjuvant therapy in PKU management is relatively new and consists of small, tightly controlled multisite efficacy studies, two of which are RCTs. The greatest research need in this area is thus for larger studies that include adequate numbers of participants. Given the known difficulty of accruing large numbers of participants, however, researchers should also use existing datasets and, as recommended, use a consortium and multisite approach to gathering data. Ideally, studies will be conducted in both tightly controlled and nonadherent populations, and among different age groups, with appropriate design and power for subgroup analyses. Research should continue to include RCTs, but prospective cohort studies that may have the potential to provide additional effectiveness data—including outside of a controlled clinical setting—adherence and longer term evidence would also be helpful to support understanding of the role of BH4 in clinical care. These studies should provide substantially more detail on the range of benefits and harms associated with treatment. For example, a better understanding is needed of the effects of BH4 in children less than 4 years of age and pregnant women, and while it may be challenging or inappropriate to conduct RCTs in these populations, observational cohorts or registry data should be considered essential.

Data are not currently available to understand potential modifiers of treatment effectiveness in order to select the best populations for targeting further research and treatment. Moreover, the significant variability in responsiveness to BH4 is unexplained, and subpopulations that have a unique response to this medication have not been well characterized. Causes of variability may be multifactorial and likely include individual patient and genotype differences, drug dose, and individual patient behavior such as dietary adherence. It is unclear, in particular, why a high proportion of individuals who have an initial response in loading studies do not have a durable response even over a few weeks in efficacy trials, even while those who do have a response demonstrate a significant effect. The degree to which this observed variation may be associated with suboptimal adherence should be assessed both in clinical trials and other types of studies.

Another area of potential research that could be explored in combination with studies of BH4 is the use of adherence supports for both drug and diet to optimize potentially positive outcomes. What types of clinical or social interventions might improve adherence to diet and drug, and be associated with improved longer term outcomes? It is assumed that support at familial, social, and system levels may be helpful and this idea should be empirically addressed.

Long-term efficacy outcomes beyond 22 weeks, and safety outcomes beyond three years are currently unavailable, as are measures of behavioral change and cognition and patient-reported outcomes including quality of life. The degree to which reductions in blood Phe are associated with measurable cognitive outcomes or even patient perception of increased mental clarity is unknown; foundational research should be done to identify target outcomes for additional studies. Furthermore, explicit assessment of the potential for liberalization of the diet, and the subsequent nutritional effects has yet to be conducted.

Future research should comprise larger studies designed to allow subgroup analysis of the effectiveness of adjuvant pharmacologic therapy for PKU. Although the current literature does not provide evidence for effectiveness in all target patients, some benefit (albeit of unclear clinical value) is seen in some patients. Whether these patients differ from the overall population in terms of genotype is an area of current research focus that has the potential to allow targeting of treatment to those most likely to benefit. Larger studies are also necessary to determine whether pharmacologic intervention is more advantageous in certain age groups or among individuals of varying dietary control of Phe or severity of disease. The two RCTs of BH4113, 115 included substantially different study populations thus the two studies can neither be combined nor used to support one another.

A number of studies are reportedly underway to address gaps in the current literature. These include a long-term study of the effect of BH4 on neurocognitive function in young children, a study of the effect in adolescent patients with attention deficit hyperactivity disorder, and a registry that includes pregnant women (PKUMOMS). However, we stress the importance of making data available and note that several commitment studies have been listed as completed, but have yet to make findings available. These include the studies on cardiac effects of BH4. Another commitment study that is reported as fulfilled is an open label study to study the safety and efficacy of BH4 for treating patients with hyperphenylalaninemia, yet no results have been made available. Finally, most of the published and ongoing studies are currently being funded by the drug companies that stand to gain financially from use of BH4; publicly-funded studies to confirm and expand on reported efficacy and effectiveness data are needed.


The three very small studies of LNAAs cannot be considered as more than proof of concept at this time, and if further work is to occur in this area, it should be done in well-conducted RCTs of adequate size. The mechanism by which LNAAs may work should be clarified, as should the optimal target population and specific treatment goals. The current formulations that have been tested require taking many pills per day and so the formulations should be made more palatable.


Blood Phe level is positively correlated with the probability of having an IQ of less than 85. This predicted probability exceeds the population probability (approximately 15 percent) at 400 μmol/L and reaches a maximum of about 80 percent at 2000 μmol/L. Thus, the commonly-used blood Phe target of 120 to 360 μmol/L is supported in our meta-analysis.8 Notably, the negative association between Phe and IQ is strongest when Phe is measured at least one year prior to IQ testing. The Phe level obtained more than one year before IQ testing is likely a better indicator of how well Phe has been controlled over the long term, relative to concurrent measurements. This relationship lends support to the principle that cognitive effects accumulate over a long time period, that concurrent measurements are poor predictors of a cognitive effect, and that control should be continued into adulthood. Review of the research on maternal PKU supports the need for dietary control as early as possible before or in pregnancy, and maintenance of Phe control to prevent poor cognitive outcomes in infants.

Dietary management remains the mainstay of treatment for PKU, and as noted above, maintaining control over the lifetime is an appropriate goal. Nonetheless, there is potential for supporting patients in achieving their clinical goals and possibly liberalizing their diet with adjuvant therapy. As a potential adjuvant treatment approved by the U.S. FDA in 2007, BH4 works by enhancing residual enzyme activity present in some individuals with PKU. BH4 has been shown in two RCTs and three open label trials to reduce Phe levels in some patients, with significantly greater reductions seen in treated versus placebo groups.

We do not yet have the ability to reliably predict which patients are most likely to be responders, as all participants in the trials were initially responsive in screening tests, but not necessarily so in the efficacy studies. One RCT also demonstrated increased Phe tolerance using BH4 among children on restricted diets. Overall, harms associated with the drug were minor and did not occur more frequently in the treatment group than in placebo arms. To date, there are no data to directly establish the potential effects of BH4 on longer term clinically important outcomes, including cognition, executive function, and quality of life. Significant gaps in the evidence remain, including effectiveness of the drug in a range of patients outside of the clinical trial setting. Thus, while the strength of evidence is moderate for a large, positive effect of BH4 on reducing Phe levels over the short term in groups of patients showing initial responsiveness, evidence for the effect of BH4 on longer term clinical outcomes is low, and based on indirect associations, including our meta-analysis.

In theory, supplementation of a Phe-restricted diet with LNAAs might have a beneficial effect on cognition as LNAAs may competitively inhibit transportation of Phe through the blood-brain barrier, thereby offering protection by potentially decreasing brain Phe levels. However, there is insufficient evidence to suggest that LNAAs could be a viable treatment option for reducing Phe levels or increasing Phe tolerance. There have been only three very small studies (total number of participants was only 47) with inconsistent results, and there is no evidence that Phe levels were reduced to clinically meaningful levels in the short time they were studied.

In particular, continued studies that include adequate numbers of participants should be conducted in both tightly controlled and nonadherent populations, and among different age groups for both types of adjuvant therapies. In addition, effectiveness in various groups of patients outside the clinical trial setting are needed, including those with variability in adherence,

Registries have been established and will provide important data in the future, as will ongoing studies that directly measure additional outcomes, including behavioral and psychiatric measures. Data are not currently available to understand potential modifiers of treatment effectiveness, including genotype, in order to select the best populations for targeting further research and treatment. Moreover, the significant variability in responsiveness to BH4 is unexplained. It is unclear, in particular, why a high proportion of individuals who have an initial response during screening do not have a durable response even over a few weeks in the efficacy trials.

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