A Study of Disitamab Vedotin Combined With Trastuzumab and Tislelizumab Versus Chemotherapy Combined With Trastuzumab With or Without Pembrolizumab in HER2-high Expression Advanced Gastric or Gastroesophageal Junction Adenocarcinoma.

This study is comparing a new combination of cancer drugs (disitamab vedotin plus trastuzumab and tislelizumab) against standard chemotherapy with trastuzumab for people with HER2-positive advanced stomach or gastroesophageal junction cancer that cannot be removed surgically. **You may be eligible if...** - You have confirmed advanced or metastatic stomach or gastroesophageal junction cancer with high HER2 expression - You have not previously had treatment for advanced or metastatic disease (or your cancer came back more than 6 months after finishing earlier treatment) - You have at least one measurable tumor - You are in good physical condition (ECOG 0 or 1) and have an expected survival of more than 12 weeks **You may NOT be eligible if...** - You have cancer that has spread to the brain or fluid around the brain - You have significant nerve damage (peripheral neuropathy above grade 1) - You have serious lung, liver, or gastrointestinal conditions - You are pregnant or breastfeeding Talk to your doctor to see if this trial is right for you.