A Phase 1/1b Study of ASP2138 as Monotherapy and in Combination with Pembrolizumab and mFOLFOX6 or Ramucirumab and Paclitaxel in Participants with Metastatic or Locally Advanced Unresectable Gastric or Gastroesophageal Junction (GEJ) Adenocarcinoma and in

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.

Are you Eligible? (Inclusion Criteria)

• Participant is aged 18 years or older and considered an adult according to local regulation at the time of signing the ICF. 2. Female participant must agree not to breastfeed starting at screening and throughout the study period and for 6 months after the final study intervention administration. 3. Male participant must not donate sperm during the treatment period and for 6 months after the final study intervention administration. 4. Participant’s tumor sample is positive for CLDN18.2 expression by central IHC (immunohistochemistry) testing.