Conversion Therapy With RC48, Sintilimab, and SOX for HER2 1+/2+ Unresectable Gastric Cancer

This trial is testing a combination treatment — an antibody-drug conjugate (RC48), an immunotherapy drug (sintilimab), and a chemotherapy regimen (SOX) — to try to shrink HER2-expressing gastric or gastroesophageal cancer enough that it becomes operable ("conversion therapy") in patients whose cancer is currently unresectable. **You may be eligible if...** - You are between 18 and 70 years old with confirmed, unresectable locally advanced or metastatic gastric or gastroesophageal junction cancer - Your tumor shows HER2 expression of IHC 1+ or 2+ (lower levels of HER2) - You have not received any prior systemic anticancer therapy - There is only one factor making your tumor unresectable at this time - You have at least one measurable tumor lesion and your expected survival is at least 6 months - Your general health score (ECOG) is 0 or 1 **You may NOT be eligible if...** - You have received any prior chemotherapy or systemic cancer treatments - Your tumor does not express HER2 - Your tumor has multiple factors making it unresectable Talk to your doctor to see if this trial is right for you.