This study is testing a new medicine called **ASP546C** for adults with types of stomach and pancreatic cancer, as well as some other solid tumors. The goal is to find the best dose that works for different people. The study is in two parts. In both parts, participants will receive the medicine through an **infusion**, which means it is given slowly through a vein every three weeks. In **Part 1**, people with stomach or the area where the stomach meets the esophagus (GEJ) cancer can join. They may get a higher or lower dose. In **Part 2**, only those with pancreatic cancer or other specific tumors can join, and they will get the higher dose. **Key Points:** - **Study visits**: Participants need to visit the clinic on certain days for treatment and health checks. - **Duration**: The study will last as long as participants continue to receive infusions. - **Eligibility**: Only certain types of cancer and health conditions are allowed; people must meet specific health requirements. Participants should talk with their doctor to see if this study is a good fit for them.