This study is testing a new drug, **JNT-517**, in people. It has four parts. Parts A, B, and C involve healthy people to see if the drug is safe and how the body processes it. Part D is for people with **Phenylketonuria (PKU)**, a condition where the body can't break down an amino acid called phenylalanine. **Randomized** means participants are picked by chance, like flipping a coin, to receive either the drug or a **placebo** (a fake treatment). **Double-blind** means neither the participants nor the researchers know who gets the drug or placebo, avoiding bias.

• **Study Length**: Parts A and B last 14 days, Part C is about drug forms, Part D lasts 4 weeks.
• **Visits**: Participants will have a Screening, Treatment, and Follow-up visit.
• **Eligibility**: Age 18-65, healthy or diagnosed with PKU, agree to avoid smoking, and use birth control if needed.

Participants can't join if they have certain health problems, recent infections like COVID-19, or have used other trial drugs recently. If you're interested, it's important to consider if you meet the criteria and are comfortable with the study process.