KN026 in Patients With HER2 Expressing Gastric/Gastroesophageal Junction Cancer

Inclusion Criteria: * Signed inform consent form(ICF) * Age ≥ 18 years and ≤ 75 years, male or female * Histologically or cytologically documented advanced gastric/gastroesophageal junction cancer HER2 overexpressing: IHC 3+ or IHC 2+ \& ISH+ HER2 expressing: IHC2+ \& ISH- or IHC 1+ \& ISH+ * Received at least one prior standard therapy * At least one evaluable lesion according to Response Evaluation Criteria In Solid Tumors（RECISIT） v 1.1 * Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 * Adequate organ function * LVEF≥ 50% (ECHO) * Female patients and males with partners of childbearing potential should be using highly effective contraceptive measures (failure rate of less than 1% per year). Contraception should be continued for a period of 24 weeks after dosing has been completed. * Ability to comply with treatment, procedures and pharmacokinetics (PK) sample collection and the required study follow-up procedures Exclusion Criteria: * Accepted any other anti-tumor drug therapies within 4 weeks before fist dose * Accepted radiotherapy within 4 weeks before enrollment * An anthracyclines antibiotic treatment was received exceeding 320 mg/m² or other equivalent dose antharcyclines * Subjects are eligible with clinically controlled and stable neurologic function \>= 4 weeks, which is no evidence of CNS disease progression; Subjects with spinal cord compression and cancerous meningitis are not eligible * Pregnant or nursing females；or intend pregnancy within this study period or within 6 monthes after the end of this study * History of immunodeficiency, including HIV positive or other acquired, congenital immunodeficiency disease, or a history of organ transplantation * Severe chronic and active infection, need to system antibiosis/antiviral treatment * Cavity effusion (pleural effusion, ascites, pericardial effusion, etc.) are not well controlled, and need locally treatment or repeated drainage * Even with peripheral or central venous nutritional support, unintentional weight loss ≥5% within 1 month before the first medication