This study is testing a new gene therapy called **NGGT002** for adults with **classic PKU**. PKU is a condition where the body can't break down an amino acid called phenylalanine (Phe) due to a problem with the PAH gene. **NGGT002** uses a special virus (rAAV8) to deliver a working copy of the PAH gene. Participants will receive one dose of **NGGT002** through an IV and will be monitored for 5 years to see if it's safe and if it works. In the first part of the study, three people will get a low dose, and then the dose might be increased or more people might get the low dose. In the second part, the same process will be repeated with a higher dose. **Key Points**: - **Study Duration**: 5 years with regular check-ups. - **Eligibility**: Adults aged 18-55 with classic PKU who are not responding to current treatments. - **Compensation**: Not mentioned. Participants must be willing to follow study procedures and use effective birth control if required.