The purpose of this study is to evaluate the ability of Phenoptin to control blood phenylalanine levels in subjects who have hyperphenylalaninemia due to a primary BH4 deficiency and to evaluate the safety of Phenoptin in this population. Some subjects were receiving non-registered formulations of BH4 at enrollment and this treatment was suspended after Part 1 and within one day the subjects started Phenoptin at approximately the same dose.

Within 4 weeks of completing screening assessments to determine eligibility, subjects will be enrolled in the study. The study will be conducted in two parts.

Part 1: After screening, all subjects will be followed for two weeks without modification of their baseline medical or dietary care.

Part 2: Beginning at Week 2, subjects who were receiving non-registered formulations of BH4 at enrollment will suspend this treatment and within one day will start Phenoptin at approximately the same dose of the non-registered BH4 formulation. Subjects not receiving BH4 at enrollment will begin treatment with Phenoptin at approximately 5 mg/kg/day, given orally, prior to meals.

At the discretion of the Investigator, the Phenoptin dose may be adjusted up or down at the Week 6 visit to control blood Phe levels (<360 µmol/L), or to optimize the clinical effect. The maximum dose allowed will be approximately 20 mg/kg/day. All subjects will receive Phenoptin for a total of 8 weeks. Subjects will be instructed to continue their usual diet without modification. Study visits will occur every other week.

Tyrosine, biopterin and neopterin will be analyzed at the following visits: Week 0 (enrollment), Week 2 (prior to dosing with Phenoptin), Week 8 (after 6 weeks of treatment with Phenoptin) and Week 10 (after 8 weeks of treatment with Phenoptin).During each visit, blood Phe level will be measured (2.5-5 hours after a meal), and safety evaluations will be performed. Safety will be assessed by monitoring adverse events and vital signs, performing physical examinations, assessing signs and symptoms of primary BH4 deficiency (i.e., neurological symptoms such as seizures, changes in muscle tone, weakness, etc.) and clinical laboratory tests (chemistry, hematology and urinalysis).

Extension: Upon completion of 8 weeks of treatment (i.e., at the Week 10 visit), subjects will be offered the option to continue treatment with Phenoptin in an extension of this study. Participation in the study extension will continue until one of the following occurs:

1. the subject withdraws consent and discontinues from the study,
2. the subject is discontinued from the study at the discretion of the investigator,
3. the study drug is available through the appropriate marketing approval, or
4. the study is terminated.

During the extension period, study drug will be dispensed to subjects monthly, and study visits will be required every 3 months. The Phenoptin dose may be adjusted at any visit during the study extension at the discretion of the Investigator. The maximum dose allowed will be approximately 20 mg/kg/day.

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 to be pregnant or breastfeeding or planning a pregnancy in self or partner during the study
• Female subjects of childbearing potential not using an effective method of birth control, as determined by the PI, or unwilling to continue to use acceptable birth control measures.
• Alanine aminotransferase (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, 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)
• Clinical diagnosis of phenylketonuria (PKU) due to phenylalanine hydroxylase deficiency
• Any condition that, in the view of the PI, renders the subject at high risk from treatment compliance and/or completing the study