Sapropterin
Phenylketonuria (PKU) is an inherited metabolic disease that is most commonly caused by mutations in the gene coding for phenylalanine hydroxylase (PAH), an enzyme that is required to convert phenylalanine to tyrosine1, 2. Deficiency in PAH activity leads to elevation in blood phenylalanine levels that, if untreated, results in serious neurological and developmental defects1, 2. Many mutations in PAH have been identified, and the associated clinical phenotypes range from mild hyperphenylalaninaemia to classic PKU1, 2.
Treatment of PKU has been based on strict lifelong dietary protein restriction to reduce phenylalanine intake, which, if implemented from an early age, can lead to good neurological outcomes1, 2. However, such restrictive diets are associated with a risk of nutritional deficiencies and represent a major burden for patients and their families2. Non-dietary treatment options for PKU are therefore desirable.
Basis of discovery
Tetrahydrobiopterin (BH4) is a natural cofactor for PAH. Several clinical studies showed that treatment with BH4 can decrease blood phenylalanine levels in some patients with PAH deficiency (for example, Refs 3,4). Various potential mechanisms have been proposed, including a BH4-mediated increase in PAH expression and BH4-mediated stabilization of mutant forms of PAH3, 4, 5. The identification of a subgroup of PAH-deficient patients that respond to BH4 treatment provided a strong rationale for the further evaluation of the effects of BH4 in patients with PKU.
Drug properties
Sapropterin dihydrochloride (Fig. 1) is a synthetic preparation of the dihydrochloride salt of the biologically active 6R-(BH4) stereoisomer6.
Clinical data
The safety and efficacy of oral sapropterin dihydrochloride were assessed in four clinical studies involving patients with PKU6, 7, 8.
Study 1 was an open-label, uncontrolled clinical trial involving 489 patients with PKU, aged 8–48 years, who had baseline blood phenylalanine levels 
450 
mol L-1 and who were not on phenylalanine-restricted diets6, 7. All patients received sapropterin dihydrochloride (10 mg per kg per day) for 8 days6, 7. Response to sapropterin treatment was defined as a 30% decrease in blood phenylalanine from baseline6, 7. At day 8, 96 patients (20%) were identified as responders6, 7.
Study 2 was a double-blind, placebo-controlled study involving 88 patients with PKU who responded to sapropterin treatment in study 1 (Refs 6,8). After a washout period from study 1, patients were randomized to receive either sapropterin dihydrochloride 10 mg per kg per day (n = 41) or placebo (n = 47) for 6 weeks6, 8. Efficacy was assessed by the mean change in blood phenylalanine levels from baseline to week 6 in the sapropterin-treated group compared with the mean change in the placebo group6, 8. At baseline, the mean blood phenylalanine level was 843 (
300) mol L-1 in the sapropterin group and 888 ( 323) mol L-1 in the placebo group6, 8. At week 6, the sapropterin-treated group had a mean blood phenylalanine level of 607 ( 377) mol L-1, and the placebo group had a mean blood phenylalanine level of 891 ( 348) mol L-1 (Refs 6,8). The sapropterin-treated and placebo-treated groups had mean changes in blood phenylalanine level of -239 mol L-1 and 6 mol L-1, respectively6, 8. Blood phenylalanine levels in the sapropterin group fell by 
200 mol L-1 after 1 week and this reduction persisted for the remaining 5 weeks of the study8.
Study 3 was an open-label extension study in which 80 patients who responded to sapropterin in study 1 and completed study 2 underwent 6 weeks of forced dose-titration with three different doses of sapropterin dihydrochloride6. Treatments consisted of three consecutive 2-week courses of sapropterin dihydrochloride at doses of 5, then 20, and then 10 mg per kg per day, with blood phenylalanine levels being monitored after 2 weeks of treatment at each dose level6. At baseline, the mean blood phenylalanine level was 844 ( 398) mol L-1. At the end of treatment with 5, 10, and 20 mg per kg per day, mean blood phenylalanine levels were 744 ( 384) mol L-1, 640 ( 382) mol L-1 and 581 ( 399) mol L-1, respectively6.
Study 4 was an open-label study involving 90 children with PKU, aged 4–12 years, who were on phenylalanine-restricted diets and who had blood phenylalanine levels 
480 mol L-1 at screening6. All patients were treated with sapropterin dihydrochloride 20 mg per kg per day for 8 days6. At day 8, 50 patients (56%) had a treatment response, defined as a 30% decrease in blood phenylalanine levels from baseline6.
Indications
Sapropterin dihydrochloride is approved by the FDA to reduce blood phenylalanine levels in patients with hyperphenylalaninaemia due to BH4-responsive PKU6. Sapropterin dihydrochloride is to be used in conjunction with a phenylalanine-restricted diet6.
Phenylketonuria
Analysing issues in the treatment of PKU is Barbara K. Burton, M.D., Professor of Pediatrics at Northwestern University Medical School and clinical geneticist at the Children's Memorial Hospital, Chicago, Illinois, USA.
For several decades, PKU has been the prototype of the treatable inborn error of metabolism. Through the screening of newborns and initiation of dietary therapy, mental retardation has been prevented and most patients with this disorder have become independent self-sufficient adults.
Despite the success of dietary treatment, however, several challenges in the care of patients with this disorder remain. The phenylalanine-restricted diet is extremely difficult to maintain in the long-term and many of the phenylalanine-free medical foods that patients must take to meet their daily protein requirements are unpalatable. Dietary compliance is an increasing problem as patients get older, with the result that suboptimal metabolic control is found in the large majority of adolescents and adults. A significant number of adults abandon dietary treatment altogether despite the recommendation that blood phenylalanine levels be controlled for life and the evidence linking elevated blood phenylalanine levels to behavioural changes, psychiatric disorders, reduced attention span and a decline in IQ.
Even patients who are relatively well-controlled, with blood phenylalanine levels maintained within the target therapeutic range, may still suffer some adverse effects of their disorder. A recent meta-analysis of all published studies revealed a correlation between the IQ of patients with PKU and mean blood phenylalanine levels, even within the 'target therapeutic range', suggesting that lower-level elevations of blood phenylalanine may be harmful to the brain9. Although patients with blood phenylalanine elevations below 600 mol L-1 have often not been treated in the past and have been thought to have a normal outcome, these findings raise the question of whether these patients with mild hyperphenylalaninaemia may suffer more subtle neurocognitive consequences of their disorder.
Recently the US FDA approved sapropterin, the first drug for the treatment of PKU, which has the potential to revolutionize therapy for patients who are responsive to this agent. By lowering blood phenylalanine levels, patients may achieve improved metabolic control and a better neurocognitive outcome, although the latter has not yet been documented in clinical trials. The fact that sapropterin is administered once daily is a major advantage as compared with the medical foods patients are accustomed to, which must be taken several times each day in conjunction with the rigidly restricted diet. For those patients already in good metabolic control, the reduction of blood phenylalanine levels makes diet liberalization possible in many cases. As more of the patient's dietary protein is obtained from natural sources, less of the unpalatable medical foods are required, and compliance is likely to be improved as the patient's treatment regimen is simplified.
Unfortunately, at least 50% of patients with PKU exhibit no response to treatment with sapropterin and responsiveness cannot be reliably predicted by genotype or any other specific criteria. Mild PKU patients are most likely to respond, but severe classic PKU patients can respond as well, so a trial of drug therapy is required to determine responsiveness. For those patients who do not respond, there is a real need for the development of additional pharmacological therapies.
