Study of Phenoptin to Increase Phenylalanine Tolerance in Phenylketonuric Children on a Phenylalanine-restricted Diet

Inclusion Criteria: * Clinical diagnosis of PKU with hyperphenylalaninemia (HPA) documented by at least one blood Phe measurement \>/=360 umol/L (6 mg/dL) * Under dietary control with a Phe-restricted diet as evidenced by:· Estimated daily Phe tolerance \</=1000 mg/day * At least 6 months of blood Phe control (mean level of \</=480 μmol/L) prior to enrolling in the study * Aged 4 to 12 years inclusive at screening * A blood Phe level \</=480 μmol/L at screening * Female subjects of childbearing potential (as determined by the principal investigator) must have a negative blood or urine pregnancy test at entry (prior to the first dose). Note: All female subjects of childbearing potential and sexually mature male subjects must be advised to use a medically accepted method of contraception throughout the study. Female subjects of childbearing potential must be willing to undergo periodic pregnancy tests during the course of the study * Willing and able to comply with all study procedures * Willing to provide written assent (if applicable) and written informed consent by a parent or legal guardian after the nature of the study has been explained and prior to any research-related procedures Exclusion Criteria: * Any condition that, in the view of the PI, renders the subject at high risk from treatment compliance and/or completing the study * Prior history of organ transplantation * Perceived to be unreliable or unavailable for study participation or have parents or legal guardians who are perceived to be unreliable or unavailable * Use of any investigational agent within 30 days prior to screening, or requirement for any investigational agent or vaccine prior to completion of all scheduled study assessments * ALT \> 2 times the upper limit of normal (i.e., Grade 1 or higher based on World Health Organization Toxicity Criteria) at screening * Concurrent disease or condition that would interfere with study participation or safety (e.g., seizure disorder, oral steroid-dependent asthma or other condition requiring oral or parenteral corticosteroid administration, or insulin-dependent diabetes)