This multicenter, randomized, double-blind phase III study intends to recruit about 878 patients, including PD-L1 positive 660 patients, who have received radical gastrectomy (R0 resection, D2 or more extended lymphadenectomy) with postoperative pathological stage IIB 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.

"This is a multicenter, randomized, double-blind phase III study, plans to recruit 878 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toxicity, disease recurrence, patient's withdrawal of consent, investigator's judgment that the patient needs to be withdrawn from the study treatment, or death, whichever comes first.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.

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INCLUSION CRITERIA

1. Age 18-75 years.
2. No residual tumor (R0) after D2 or greater lymphadenectomy through laparotomy.
3. According to the definition of the 8th edition of the AJCC Cancer Staging Manual, patients with gastric adenocarcinoma confirmed by histopathology, pathological stage II (T4aN0M0) and stage III, including gastroesophageal junction adenocarcinoma (GEJ) patients.
4. Patients need to provide sufficient formalin-fixed paraffin-embedded (FFPE) neoplasm tissue specimens or sections that are confirmed to be PD-L1 positive (CPS ≥ 1) by lab test at the central laboratory.
5. ECOG performance status 0-1.
6. No metastasis or recurrence as radiologically confirmed.
7. Patients must have adequate organ function as assessed in the laboratory tests.
8. Patients must provide informed consent for this study, and sign the written informed consent form voluntarily before the initiation of the study, and are willing and able to comply with the scheduled visits, treatment plan, laboratory examinations and other study procedures in the study.
9. Female patients of childbearing age must take a serum pregnancy test within 7 days before randomization with negative results, and agree to adopt reliable and effective contraceptive methods during the study.

EXCLUSION CRITERIA

1. Previous use of non-surgical therapy (e.g., radiotherapy, chemotherapy, hormone therapy) for gastric adenocarcinoma.
2. Having liver, peritoneal or other distant metastasis.
3. Having malignant tumors other than gastric adenocarcinoma within 5 years before randomization.
4. Pevious treatment targeting PD-1 receptor or its ligand PD-L1 or cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) receptor;
5. Previous history of serious allergy to monoclonal antibody or other biological preparations.