Ph1b/2 Study of the Safety and Efficacy of SHR-A1811 Combinations in Advanced HER2 Expression Gastric Cancer

Inclusion Criteria: 1. Age from 18 to 75, Male and female participants 2. Local advanced unresectable or metastatic disease Pathologically documented adenocarcinoma of gastric or gastroesophageal junction; 3. HER2 expression ; 4. Phase Ib: Failed or intolerant to prior standard treatment, or did not receive any systemic anti-tumor therapy; Phase II: previously untreated patients; 5. ECOG 0-1; 6. The expected survival ≥3 months; 7. Sufficient bone marrow and organ functions; Exclusion Criteria: 1. There were ascites, pleural effusion and pericardial effusion that needed to be treated; 2. Major surgery was performed within 4 weeks; 3. Have active autoimmune disease or history of autoimmune disease; 4. Patients with interstitial pneumonia; 5. There was severe infection within 4 weeks before starting the study treatment; 6. Patients with active pulmonary tuberculosis infection history within 1 year before enrollment; 7. Serious cardiovascular and cerebrovascular diseases; 8. Gastrointestinal perforation and/or gastrointestinal fistula occurred within 6 months before enrollment;