This study is testing a new treatment for patients with head and neck cancer. The treatment uses two main components: Ad/PNP and fludarabine phosphate. Ad/PNP is a special virus that helps the body make a medicine right inside the tumor. Fludarabine phosphate is a drug that helps fight cancer by turning into a powerful medicine when injected into the tumor.

The main goal is to see if using this treatment multiple times is safe and effective. Patients will receive the treatment in cycles, with injections into their tumor and medicine given through a vein. Each cycle lasts about four weeks and can be repeated up to five times. The study will check if the tumor shrinks and if the treatment causes any bad side effects.

• Each treatment cycle lasts four weeks, with up to five cycles total.
• Participants must be 18 or older and have head or neck cancer with no other treatment options.
• Participants will be closely monitored for any side effects during the study.