HX008 Plus Irinotecan Versus Placebo Plus Irinotecan as Second-line Treatment in Advanced Gastric Cancer

Inclusion Criteria: * Understood and signed an informed consent form. * Age ≥ 18 and ≤ 75 years old, male or female. * Has histologically- or cytologically-confirmed diagnosis of locally advanced unresectable or metastatic adenocarcinoma of stomach or the esophagogastric junction (GEJ). * Has experienced documented objective radiographic or clinical disease progression during or after first-line therapy containing platinum and/or fluoropyrimidine therapy. * Willing to provide tissue for PD-L1 biomarker analysis. * Has a performance status of 0 or 1 on the Eastern Cooperative Oncology Group (ECOG) Performance Score. * Life expectancy ≥ 3 months. * Has adequate organ function. * Female participants of childbearing potential should have a negative pregnancy within 72 hours before the randomization. Male and female participants should agree to use an adequate method of contraception during the experiment and 1 year after the last administration of the test drugs. Exclusion Criteria: * Has squamous cell or undifferentiated gastric cancer. * Diagnosed additional maliganancy within 3 years prior to randomization with the expection of curatively treated basal cell carcinoma of the skin, squamous cell carcinoma of the skin,curatively resected in situ cervival or non-muscle invasive bladder cancers. * Has had a prior anti-cancer monoclonal antibody within 4 weeks prior to the first dose of trial treatment or who has not recovered (≤ Grade 1 or at Baseline) from AEs due to agents administered more than 4 weeks earlier. * Has had prior chemotherapy,radiation therapy or targeted small molecular therapy within 2 weeks prior to the first dose of trial treatment or who has not recovered (≤ Grade 1 or at Baseline) from AEs due to a previously administrated agent. * Has received prior therapy with an anti-PD-1, or anti-PD-L1, or anti-CTLA-4 agents. * Has known active central nervous system (CNS) metastases and/or carcinomatous meningitis. * Has uncontrolled ascites, pleural effusion, or pericardial effusion. * Has active autoimmune disease that has required systemic treatment in past 2 years. * Has received a major surgery within 4 weeks prior to randomization. * Has received system treatment with corticosteroids (dose \>10mg/day prednison or other therapeutic hormones) within 2 weeks prior to the first dose of trial treatment. * Has incomplete intestinal obstruction, active gastrointestinal hemorrhage and perforation. * Has a history of non-infectious pneumonitis that required steriods or has current pneumonitis. * Has any serious and/or uncontrolled disease. * Has active viral infection. * Has received a live vaccine within 30 days prior to the first dose of trial treatment. * Has participated in other anticancer drug clinical trials within 4 weeks. * According to the judgement of the investigators, there are other factors that may lead to the termination of the study.