Tislelizumab in Combination With Chemotherapy as First-Line Treatment in Adults With Inoperable, Locally Advanced or Metastatic Gastric, or Gastroesophageal Junction Carcinoma

Inclusion Criteria:

1. Able to provide written informed consent and can understand and comply with the
requirements of the study

2. Adult patients (≥ 18 years of age or acceptable age according to local regulations,
whichever is older) at the time of voluntarily signing informed consent

3. Locally advanced unresectable or metastatic GC or GEJ carcinoma and have
histologically confirmed adenocarcinoma

4. At least 1 measurable lesion as defined per RECIST v1.1 as determined by investigator
assessment.

5. No previous systemic therapy for locally advanced unresectable or metastatic
gastric/GEJ cancer. NOTE: Patients may have received prior neoadjuvant or adjuvant
therapy as long as it was completed and have no recurrence or disease progression for
at least 6 months.

6. Patients must be able to provide tumor tissues.

7. ECOG PS ≤ 1 within 7 days prior to randomization

8. Adequate organ function as indicated by the following laboratory values ≤ 7 days prior
to randomization:

9. Females of childbearing potential must have a negative urine or serum pregnancy test
within 7 days of randomization and must be willing to use a highly effective method of
birth control for the duration of the study, and ≥ 120 days after the last dose of
tislelizumab or placebo and 180 days after the last dose of chemotherapy.

10. Non-sterile males must be willing to use a highly effective method of birth control
for the duration of the study and for ≥ 120 days after the last dose of tislelizumab
or placebo and 180 days after the last dose of chemotherapy.

Exclusion Criteria:

1. Patient has squamous cell or undifferentiated or other histological type GC

2. Active leptomeningeal disease or uncontrolled brain metastasis. Patients with
equivocal findings or with confirmed brain metastases are eligible for enrollment
provided that they are asymptomatic and radiologically stable without the need for
corticosteroid treatment for ≥ 4 weeks before randomization.

3. Active autoimmune diseases or history of autoimmune diseases that may relapse.

4. Any active malignancy ≤ 2 years before randomization, with the exception of the
specific cancer under investigation in this study and any locally recurring cancer
that has been treated curatively (eg, resected basal or squamous cell skin cancer,
superficial bladder cancer, carcinoma in situ of the cervix or breast).

5. Uncontrollable pleural effusion, pericardial effusion, or ascites requiring frequent
drainage within 7 days prior to randomization (The cytological confirmation of any
effusion is permitted).

6. Diagnosed with gastric or GEJ adenocarcinoma with positive HER2

7. Any condition that requires systemic treatment with either corticosteroids (> 10 mg
daily of prednisone or equivalent) or other immunosuppressive medication ≤ 14 days
before randomization.

8. With history of interstitial lung disease, non-infectious pneumonitis or uncontrolled
systemic diseases, including diabetes, hypertension, pulmonary fibrosis, acute lung
diseases, etc. NOTE: Patients with radiation pneumonitis may be randomized if the
radiation pneumonitis has been confirmed as stable (beyond acute phase) without any
concerns about recurrence. Patients with severe but stable radiation-induced
pneumonitis may be required to undergo routine pulmonary function studies

9. With severe chronic or active infections requiring systemic antibacterial, antifungal
or antiviral therapy, including tuberculosis infection, etc.

10. A known history of HIV infection

11. Patients with untreated chronic hepatitis B virus (HBV) or HBV carriers at screening
or patients with active Hepatitis C virus (HCV) infection should be excluded.

12. Prior therapy with an anti-PD-1, anti-PD-L1, anti-PD-L2 or any other antibody or drug
specifically targeting T-cell co-stimulation or checkpoint pathways

13. Prior allogeneic stem cell transplantation or organ transplantation

14. A history of severe hypersensitivity reactions to other monoclonal antibodies or any
components of study treatment

15. Known dihydropyrimidine dehydrogenase (DPD) deficiency

16. Underlying medical conditions or alcohol or drug abuse or dependence that, in the
investigator's opinion, will be unfavorable for the administration of study drug or
affect the explanation of drug toxicity or adverse events; or insufficient compliance
during the study according to investigator's judgement.

NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.