Additional Study Information: The goal of this study is to see if a study drug called ASP2138 is safe and effective for people diagnosed with tumors known to have Claudin (CLDN) 18.2 expression. CLDN is a group of proteins seen in gastric-specific, gastroesophageal junction (GEJ) or pancreatic cancer types. The study will also see how ASP2138 is processed in the blood. Each participant will complete a number of procedures and blood collections in order to find the best dose of ASP2138. When this dosage is identified, it will be used to evaluate if ASP2138 causes tumors to shrink. ASP2138 will be assessed continuously throughout the study to understand if it is safe and effective. This study is a first-in-human clinical trial. The Food and Drug Administration (FDA) in the United States of America has not approved ASP2138 for sale or for the treatment of cancer.