First-in-Human, Multiple Part Clinical Study of JNT-517 in Healthy Participants and in Participants With Phenylketonuria

Key Inclusion Criteria: Parts A, B, C, and F: 1. Males and females 18 to 55 years of age. 2. Medically healthy with no clinically significant medical history. 3. Body mass index (BMI) of 18-40 kg/m2 and total body weight \>50 kg (110 lbs). 4. Non-smoker for at least 2 weeks prior to dosing and willing to abstain during the study. Part D and E: 5. Males and females 18 to 65 years of age, inclusive. 6. Diagnosis of PKU with a confirmed genotype. 7. At least 2 plasma Phe levels \>600 μM over the past 12 months. 8. BMI of 18-40 kg/m2. All Parts: 9. Females of childbearing potential must agree to use 2 highly effective contraceptive methods. 10. Capable of giving signed informed consent and able to comply with study procedures. Key Exclusion Criteria: All Parts: 1. Any acute or chronic medical condition that would prevent the participant from complying with the procedures or place the participant at risk if they participate in the study. 2. Positive for hepatitis B or C or human immunodeficiency virus. 3. Any history of malignancy in the last 5 years, excluding non-melanoma skin cancer. 4. Any history of liver disease. 5. Any surgical or medical conditions that may affect study drug absorption, distribution, metabolism, or excretion. 6. Participation in another investigational drug trial within 30 days or, if known, 5 half-lives of the investigational drug (whichever is longer). 7. History of drug/alcohol abuse in the last year. 8. Current, recent, or suspected infection within 4 weeks of Screening of SARS-CoV-2/COVID-19. 9. Received a vaccine for SARS-CoV-2/COVID-19 within 14 days of Screening. 10. Unable to tolerate oral medication. 11. Allergy to JNT-517 or any component of the investigational product. 12. Received \>50 mL of blood or plasma within 30 days of Screening or \>500 mL of blood or plasma within 60 days of Screening.