This study is testing a new medicine called **JNT-517**. It is for people who are healthy and for those with a condition called **Phenylketonuria (PKU)**, which is a disorder where the body can't process a part of protein properly. The study is divided into four parts. In the first two parts, healthy people will take JNT-517. The third part will see how different forms of the medicine work in the body and how eating food affects it. The last part will test the medicine in people with PKU for four weeks. Some people in Part D will get a **placebo**, which looks like the medicine but doesn't have any active ingredient, and they won't know if they got the real thing or the placebo. Everyone will be checked for safety before, during, and after the study.

• Study lasts up to 4 weeks for Part D.
• Participants must be healthy or have PKU and meet specific criteria.
• Participants cannot have certain health issues or have taken another study drug recently.