Safety and Efficacy Study of Phenoptin in Subjects With Hyperphenylalaninemia Due to BH4 Deficiency

Inclusion Criteria: * Documented history of blood Phe level \> 180 µmol/L on at least one occasion * Established diagnosis of hyperphenylalaninemia (HPA) due to primary BH4 deficiency with a documented defect in biopterin metabolism with blood or urine tests * Willing and able to provide written informed consent or, in the case of subjects under the age of 18 years, provide written assent (if required) and written informed consent by a parent or legal guardian * Negative urine pregnancy test at screening for females of child-bearing potential * Male and female subjects of childbearing potential (if sexually active and non-sterile) must be using acceptable birth control measures and be willing to continue to use acceptable birth control measures, as determined by the Investigator, and be willing to continue to use acceptable birth control measures while participating in the study * Willing and able to comply with all study procedures * Able to take medication orally Exclusion Criteria: * Perceived to be unreliable or unavailable for study participation or, if under the age of 18 years, have parents or legal guardians who are perceived to be unreliable or unavailable * Use of any investigational agent (other than BH4) within 30 days prior to screening, or requirement for any investigational agent or vaccine prior to completion of all scheduled study assessments * Positive urine pregnancy test at screening (non-sterile females of child bearing potential only), already known t…