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

The goal of Parts A and B of this Phase 1/2, first-in-human, randomized study is to assess the safety, tolerability, and pharmacokinetics (PK) of single (SAD) and multiple (MAD) ascending doses of oral JNT-517 in healthy participants. In Part C, the goal is to evaluate the differences in bioavailability between a tablet and suspension formulation of JNT-517 and the food effect in healthy volunteers. All participants in Part C will receive JNT-517. The goal of Part D is to assess the safety, tolerability, PK, and effect on urinary Phe and other amino acids of JNT-517 in participants with phenylketonuria (PKU). Participants in Part D will receive either JNT-517 or placebo and will be blinded to their treatment assignment. The study consists of 6 parts: * Part A: SAD in healthy participants -randomized, double-blind, placebo-controlled * Part B: MAD in healthy participants (14 days)-randomized, double-blind, placebo-controlled * Part C: Relative bioavailability of 2 formulations and food effect in healthy participants-randomized, open-label * Part D: Phase 2 in participants with PKU (4 weeks)-randomized, double-blind, placebo-controlled * Part E: Phase 2 in participants with PKU (4 weeks) open label * Part F: SAD Phase 1 in healthy participants, randomized, double-blind, placebo-controlled In each part, participants will complete a Screening Period, a Treatment Period, and a Follow-up Period for safety.