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