SHR-A1811 for Subjects With Her2-positive Gastric Cancer and Gastroesophageal Junction Adenocarcinoma After Progression on or After First-line Anti-HER2 Therapy-containing Regimen

This study tests a drug called SHR-A1811 (an antibody-drug conjugate that targets the HER2 protein) in people with HER2-positive stomach or gastroesophageal junction cancer that has progressed after first-line treatment including a HER2-targeted therapy. **You may be eligible if...** - You are 18–75 years old with confirmed stomach or gastroesophageal junction adenocarcinoma - Your tumor tests strongly positive for HER2 (IHC 3+ or IHC 2+ with confirmed HER2 gene amplification) - Your cancer progressed during or after your first standard treatment that included a HER2-targeted drug - You are in good overall health (ECOG 0 or 1) with a life expectancy of at least 12 weeks - You have at least one measurable lesion on imaging **You may NOT be eligible if...** - Your cancer has spread to the brain and is actively causing symptoms - You have active hepatitis B (detectable HBV DNA above 500 IU/mL) or hepatitis C - You have a history of immunodeficiency or HIV - You have uncontrolled fluid accumulation around the lungs or heart - You received cancer treatment within 4 weeks before starting the study Talk to your doctor to see if this trial is right for you.