Study of BGB-290 or Placebo in Participants With Advanced or Inoperable Gastric Cancer

Key Inclusion Criteria:

1. Age ≥ 18 years.
2. Signed informed consent.
3. Histologically confirmed inoperable locally advanced or metastatic adenocarcinoma of the stomach or gastroesophageal junction.
4. Received platinum based first line chemotherapy for ≤ 28 weeks.
5. Confirmed partial response (PR) maintained for ≥ 4 weeks or complete response (CR).
6. Able to be randomized to study ≤ 8 weeks after last platinum dose.
7. Eastern Cooperative Oncology Group (ECOG) performance status ≤ 1.
8. Adequate hematologic, renal and hepatic function.
9. Must be able to provide archival tumor tissue for central biomarker assessment.
10. Females of childbearing potential and non-sterile males must agree to use highly effective methods of birth control throughout the course of study and at least up to 6 months after last dosing.

Key Exclusion Criteria:

1. Unresolved acute effects of prior therapy ≥ Grade 2.
2. Prior treatment with PARP inhibitor.
3. Chemotherapy, biologic therapy, immunotherapy or other anticancer therapy ≤ 14 days prior to randomization.
4. Major surgery or significant injury ≤ 2 weeks prior to start of study treatment.
5. Diagnosis of myelodysplastic syndrome (MDS)
6. Other diagnoses of significant malignancy
7. Leptomeningeal disease or brain metastasis
8. Inability to swallow capsules or disease affecting gastrointestinal function.
9. Active infections requiring systemic treatment.
10. Clinically significant cardiovascular disease
11. Pregnant or nursing females.

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