This is a Phase 3 open-label extension study enrolling adult patients with PKU who were previously treated with pegvaliase in Studies PAL-003 (NCT00924703) or 165-302 (NCT02468570). The study is designed to evaluate the long-term safety and efficacy of pegvaliase administered at doses > 40 mg/day to 60 mg/day.

Pegvaliase dosing will continue without interruption from Parent Study 165-302 (NCT02468570) or Parent Study PAL-003 (NCT00924703). Beginning on Day 1, subjects will receive the same dose and regimen of pegvaliase they were receiving in either 165-302 or PAL-003. A subject who reduces to a dose of 40 mg/day or lower for 32 consecutive weeks will be discontinued from study drug and withdrawn from the study as they will have the option to transition to commercial drug. Dose reductions may be performed if warranted due to AEs or hypophenylalaninemia. Dose increases to up to 60 mg/day may be performed per investigator discretion in consultation with the sponsor's medical monitor. Dosing will continue for approximately 121 weeks.

Inclusion criteria:

• Must be enrolled in PAL-003 or 165-302 Part 4 at the time of screening for 165-304 and most recently receiving pegvaliase at a dose > 40 mg/day.
• Has identified a competent person or persons ≥ 18 years of age who can observe the subject during study drug administration and for a minimum of 1 hour following administration in situations required per protocol.
• For females with childbearing potential, must have negative pregnancy test at screening and be willing to have additional pregnancy tests during the study.
• If sexually active and not planning to become pregnant (self or partner), must be willing to use 2 acceptable methods of contraception while participating in the study and for 4 weeks after the study.
• Is willing and able to provide written, signed informed consent after the nature of the study has been explained and prior to any research-related procedures; a legally authorized representative may provide written consent and assent may be requested.
• Is willing and able to comply with all study procedures.
• Is in generally good health, as evidenced by physical examination and/or clinical laboratory evaluations (hematology, chemistry, and urinalysis).

Exclusion criteria:

• Use of any investigational product (except pegvaliase) or investigational medical device within 30 days prior to screening or requirement for any investigational agent prior to completion of all scheduled study assessments.
• Use of any medication (except pegvaliase) intended to treat PKU, including the use of large neutral amino acids, within 2 days prior to the administration of pegvaliase (Day 1).
• Use or planned use of any injectable drugs containing PEG (other than pegvaliase), including medroxyprogesterone injection, within 3 months prior to screening and during study participation.
• A history of organ transplantation or on chronic immunosuppressive therapy
• A history of substance abuse (as defined by the American Psychiatric Association: Diagnostic and Statistical Manual of Mental Disorders [DSM]) in the past 12 months or current alcohol or drug abuse
• Current participation in the Kuvan® registry study (PKU Demographics, Outcomes and Safety [PKUDOS])
• Concurrent disease or condition that would interfere with study participation or safety (eg, history or presence of clinically significant cardiovascular, pulmonary, hepatic, renal, hematologic, gastrointestinal, endocrine, immunologic, dermatologic, neurological, oncologic, or psychiatric disease)
• Any condition that, in the view of the investigator, places the subject at high risk of poor treatment compliance or terminating early from the study