A Study to Evaluate the Long-Term Safety and Efficacy of JNT-517 in Participants With Phenylketonuria

Inclusion Criteria

• •1. Clinical diagnosis of PKU and has completed Study JNT517-101 or Study JNT517-201.
• •2. Not on pegvaliase within 4 weeks of Screening.
• •3. If on sapropterin or large neutral amino acids at Screening, must be on a stable dose for 4 weeks prior to Screening and expected to be on stable dose for the study duration.
• •4. Willing and able to maintain a diet consistent in protein content from natural intact and medical food protein sources from the Screening period through the duration of the study under the direction of a dietician.
• •5. If female of childbearing potential:
• •1. Must have a negative serum pregnancy test at Screening and a negative urine pregnancy test by Day 1.
• •2. Must practice sexual abstinence, or if involved in any sexual intercourse that could lead to pregnancy, must agree to use 2 highly effective contraceptive methods (see Section 7.3.1 for details of contraception) from Screening until at least 30 days after the last study drug administration.
• •3. If taking estrogen- or progesterone-based oral contraceptives, must agree to use 2 other highly effective methods of contraception or must agree to sexual abstinence during the study.
• •6. Is a female not of childbearing potential or postmenopausal, defined as follows:
• •1. Has had surgical sterilization (hysterectomy, bilateral oophorectomy, or bilateral salpingectomy).
• •2. Has had amenorrhea for minimum of 1 year with confirmation by levels of follicle stimulating hormone testing.
• •3. Has not achieved menarche (has not had first menstrual period). If a female achieves menarche during the study, she will need to follow the contraception requirement for females of childbearing potential.
• •7. If male, must practice sexual abstinence, or if involved in any sexual intercourse that could lead to pregnancy, must agree to use highly effective contraceptive methods from Day 1 until at least 30 days after the last study drug administration.
• •Note: No restrictions are required for males who have undergone a documented vasectomy at least 4 months prior to Screening. If the vasectomy procedure is not documented or was performed less than 4 months prior to Screening, males must follow the same contraception as for non-vasectomized participants.
• •8. Capable of giving signed informed consent or and/or assent (for adolescents) and able to comply with study procedures.
• •9. Participants with psychiatric illness must be well-controlled for the last 3 months prior to screening visit and if on medication, on stable medications for the last 3 months.