A Study of JNT-517 in Participants With Phenylketonuria (PKU)

Inclusion Criteria: 1. Male or female participants aged 4 years or older. 2. Diagnosed with phenylketonuria. 3. Have phenylalanine levels that meet study requirements. 4. Meet study-specific requirements regarding current treatment for phenylketonuria. 5. Willing and able to follow study dietary requirements. 6. Weigh at least 12.5 kilograms. 7. If able to become pregnant or father a child, willing to follow the study's contraception requirements. 8. Any ongoing mental health condition must be stable before entering the study. 9. Able to provide informed consent (or have consent provided by a parent or legal guardian, when applicable) and comply with study procedures. Exclusion Criteria: Participants will be excluded from the study if any of the following criteria are met: 1. Significant medical condition that, in the investigator's opinion, could make participation unsafe or interfere with study participation. 2. Significant liver disease or impaired kidney function. 3. Certain eye conditions, including significant cataracts. 4. Active infection requiring medical evaluation. 5. History of certain cancers within the past 5 years. 6. Allergy or hypersensitivity to JNT-517 or its ingredients. 7. Prior treatment with JNT-517. 8. Recent participation in another investigational study or receipt of an investigational treatment that does not meet study requirements. 9. Use of medications that are not permitted during the study. 10. Abnormal laboratory or heart test results that would make study participation inappropriate. 11. Conditions that could affect the absorption of oral study medication. 12. History of recent substance or alcohol abuse. 13. Unwillingness to follow study requirements, including restrictions on intentional weight loss.