Is sapropterin treatment suitable for all subjects with phenylketonuria?

In a Phase I study, 20% of subjects with phenylketonuria (96/485) responded with a > or = 30% reduction in blood phenylalanine at day 8 of treatment with sapropterin. Subsequently, some of the responders from this study were enrolled in a Phase III study. The subjects with phenylketonuria were randomised to sapropterin 10 mg/kg/day (n = 41) or placebo (n = 47) for 6 weeks and continued their usual diet. The primary end point was the mean change from baseline in phenylalanine level after 6 weeks and there was a reduction of 235 micromol/l in the sapropterin group compared with 2.9 micromol/l in the placebo group. Thus, in the 20% of subjects with phenylketonuria that responded to a trial treatment with sapropterin, there were reduced mean blood phenylalanine levels over 6 weeks of treatment. In summary, sapropterin effectively reduces blood phenylalanine levels in a small proportion of subjects with phenylketonuria.