Comparing Modified XELOX Plus Sintilimab With Standard XELOX Plus Sintilimab in First-line Treatment for HER2-negative Gastric/Gastroesophageal Junction Adenocarcinoma

This study is testing whether a modified chemotherapy-plus-immunotherapy regimen (adjusted doses of XELOX chemotherapy combined with the immunotherapy drug sintilimab) works better or is safer than the standard regimen for people with stomach or esophageal junction cancer that cannot be surgically removed or has spread to other parts of the body. **You may be eligible if:** - You are 18–75 years old - You have been diagnosed with advanced or metastatic stomach or esophageal junction cancer (HER2-negative) - You have not received prior chemotherapy or immunotherapy for this cancer (or finished adjuvant therapy at least 6 months ago without relapse) - You are in reasonably good health and able to perform daily activities - Your blood counts, liver, and kidney function are within acceptable ranges **You may NOT be eligible if:** - Your cancer tests positive for HER2 (a specific protein marker) - You have previously received immunotherapy drugs (such as anti-PD-1 or anti-CTLA-4 antibodies) - You have active autoimmune disease requiring ongoing treatment - You have untreated or unstable brain metastases - You have HIV, active hepatitis B, or active hepatitis C - You have had a recent heart attack, severe heart failure, or uncontrolled heart rhythm problems - You have active intestinal obstruction or a history of gastrointestinal perforation in the past 6 months Talk to your doctor to see if this trial is right for you.