This is an international, multicenter, randomized, double-blind phase III study, plans to recruit 680 patients who received radical gastrectomy (R0, D2 or higher lymphadenectomy) with postoperative pathological stage II (T4aN0M0) or III (the 8th Edition American Joint Committee on Cancer [AJCC] Cancer Staging Manual) gastric adenocarcinoma and gastroesophageal junction adenocarcinoma, and the study intends to evaluate the efficacy and safety of JS001 combined with postoperative adjuvant chemotherapy versus placebo combined with postoperative adjuvant chemotherapy.~Patients meeting the inclusion criteria will be 1:1 randomized into JS001-chemotherapy group and placebo-chemotherapy group. The random stratification factors include adjuvant chemotherapeutic regimens (XELOX versus SOX) and tumor anatomical sites (gastric adenocarcinoma versus gastroesophageal junction adenocarcinoma).~The study treatment will be initiated 4-6 weeks after surgery, and the investigator will select XELOX (Oxaliplatin + capecitabine) or SOX (Oxaliplatin + S-1, tegafur, gimeracil and oteracil potassium) as the adjuvant chemotherapeutic regimen given as 3-week cycles for up to 8 cycles based on each patient's condition; JS001/placebo will be given for up to 17 cycles after surgery, until intolerable toxicity, disease recurrence, patient's withdrawal of consent, investigator's judgment that the patient needs to be withdrawn from the study treatment, or death, whichever comes first.~Safety evaluation, including vital signs, ECOG score, physical examination and laboratory examinations, will be performed on a regular basis during the treatment.~This study will end after the main analysis node of DFS and unblinding for analysis are achieved, or 5 years after enrollment of the last patient, whichever comes first. The Sponsor is entitled to terminate the study at any time due to specific reasons (e. g, major safety issues, force majeure, etc.).~Radiological follow-up: tumor response evaluation will be performed once every 12 weeks ±7 days within the first 5 years after randomization, and once per year subsequently, until disease recurrence or death. When symptoms or signs of suspected recurrence/metastasis occur, the radiological evaluation can be performed at any time. Disease recurrence is defined as local recurrence or distant metastases with clear radiological evidence (CT or MRI).~Survival follow-up: it will be performed once every 12 weeks after disease recurrence, until patient's withdrawal of informed consent, loss to follow-up or death, whichever comes first.~Safety follow-up: adverse events will be closely followed up and recorded, until 60 days after the last dose of treatment or the end of study follow-up (death, loss to follow-up, withdrawal of consent form and the end of study), whichever comes first.~
This international, multicenter, randomized, double-blind phase III study intends to recruit 680 patients who have received radical gastrectomy (R0 resection, D2 or more extended lymphadenectomy) with postoperative pathological stage II or III (AJCC Cancer Staging Manual, 8th Edition) gastric or EGJ adenocarcinoma to evaluate the efficacy and safety of JS001 combined with postoperative adjuvant chemotherapy versus placebo combined with postoperative adjuvant chemotherapy.