Study to Evaluate the Response to and Safety of an 8-Day Course of Phenoptin™ Treatment in Subjects With Phenylketonuria

Inclusion Criteria: Age >/= 8 years Blood Phe level >/= 450 umol/L at screening Clinical diagnosis of PKU with hyperphenylalaninemia documented by past medical history of at least one blood Phe measurement >/= 360 umol/L (6 mg/dL) Willing and able to provide written informed consent or, in the case of subjects under the age of 18, provide written assent (if required) and written informed consent by a parent or legal guardian, after the nature of the study has been explained Negative urine pregnancy test at screening (non-sterile females of child-bearing potential only) Male and Female subjects of childbearing potential childbearing potential (if sexually active and non-sterile) must be using acceptable birth control measures, as determined by the investigator, and willing to continue to use acceptable birth control measures while participating in the study Willing and able to comply with study procedures Willing to continue current diet unchanged while participating in the study Exclusion Criteria: Perceived to be unreliable or unavailable for study participation or, if under the age of 18, 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 Pregnant or breastfeeding, or considering pregnancy ALT > 5 times the upper limit of normal (i.e., Grade 3 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, or organ transplantation) Serious neuropsychiatric illness (e.g., major depression) not currently under medical control Requirement for concomitant treatment with any drug known to inhibit folate synthesis (e.g., methotrexate) Concurrent use of levodopa Clinical diagnosis of primary BH4 deficiency