This study is looking at a group of medicines to see how safe and effective they are for treating a type of cancer called adenocarcinoma found in the stomach, gastroesophageal junction, or esophagus, which cannot be removed by surgery or has spread to other parts of the body. The main medicines being tested include **ABBV-400** and **Budigalimab**, which are still being studied, and **Fluorouracil** and **Leucovorin**, which are already approved for use. Medicines will be given through an IV, which means into a vein.
The study will last about **6 years** and involve around **180 adults** from different places. Patients will be split into groups to receive different doses to find the best and safest dose. Participants will visit the hospital or clinic regularly for check-ups, tests, and questionnaires.
- Participants must have a specific type of cancer that cannot be operated on or has spread.
- Regular hospital visits are required to monitor the treatment's effects and side effects.
- Participants may face a higher treatment burden compared to other standard treatments.