Anti-PD-1 and CapOx for the First-line Treatment of dMMR Esophagogastric Cancer (AuspiCiOus)

This study is testing whether adding an immunotherapy drug (an anti-PD-1 agent) to standard chemotherapy (capecitabine and oxaliplatin) improves outcomes as a first-line treatment for patients with a specific type of advanced stomach or esophageal cancer that has a genetic marker called dMMR (mismatch repair deficiency), which means the cancer's repair system is faulty. **You may be eligible if:** - You are 18 years or older - You have been diagnosed with advanced or inoperable HER2-negative adenocarcinoma of the stomach or the area where the esophagus meets the stomach - Your tumor has been tested and found to be dMMR-positive - You have not yet received chemotherapy or radiation for advanced or metastatic disease - You are in reasonably good health (ECOG 0–2) - Your blood counts, liver, and kidney function are in acceptable range - Your tumor is accessible for biopsy **You may NOT be eligible if:** - Your tumor is HER2-positive (unless trastuzumab is not an option for you) - You have already received chemotherapy for metastatic disease - You have serious heart, liver, or blood problems - You are pregnant or breastfeeding Talk to your doctor to see if this trial is right for you.