This research is testing a drug called **ASP546C** in adults with certain cancers, like gastric (stomach) cancer, gastroesophageal (where the esophagus meets the stomach) cancer, pancreatic cancer, and other solid tumors. The study is split into two parts. **Part 1** involves patients with gastric or gastroesophageal cancer to try different doses of ASP546C. **Part 2** is for those with pancreatic cancer or other solid tumors, receiving a higher dose. Treatment is given through an **infusion**, which means the medicine is slowly administered into a vein over **3-week cycles**.

Participants will need to visit the clinic for health checks and to receive the drug. The number of visits depends on the person's health and response to the treatment.

• **Study Duration**: Infusions every 3 weeks; visits depend on health status.
• **Eligibility**: Specific cancer types; certain health criteria must be met.
• **Compensation**: Details not specified, but participation could help in finding better treatment doses.