A Study to Investigate the Efficacy and Safety of ONO-4578 in Combination With Nivolumab and Chemotherapy in Chemotherapy-naïve Participants With HER2-negative Unresectable Advanced or Recurrent Gastric Cancer (Including Esophagogastric Junction Cancer)

A Randomized, Multicenter, Double-blind, Phase II Study to Compare the Efficacy and Safety of the Treatment With ONO-4578 in Combination With Nivolumab, Fluoropyrimidine-based and Platinum- Based Chemotherapy (Hereinafter Referred to as Chemotherapy) With Those of the Treatment With Placebo in Combination With Nivolumab and Chemotherapy in Chemotherapy-naïve Participants With Human Epidermal Growth Factor Receptor 2 (HER2)-Negative Unresectable Advanced or Recurrent Gastric Cancer (Including Esophagogastric Junction Cancer)

This study is to compare and evaluate the efficacy and safety of the treatment with ONO-4578 in combination with nivolumab and chemotherapy with those of the treatment with placebo in combination with nivolumab and chemotherapy in chemotherapy-naïve participants with HER2- negative unresectable advanced or recurrent gastric cancer (including esophagogastric junction cancer).

Inclusion Criteria:

• Patients with gastric or esophagogastric junction cancer histologically diagnosed as adenocarcinoma
• Patients have not been treated with systemic chemotherapy as first-line therapy
• Patients who can provide tumor tissue samples

Exclusion Criteria:

• Patients unable to take oral medicines
• Patients with HER2-positive
• Patients with contraindications to nivolumab, oxaliplatin, S-1, or capecitabine
• Patients who have a history of severe drug-related adverse reactions caused by non- steroidal anti-inflammatory drugs (NSAIDs) or who have complications caused by NSAIDs requiring treatment
• Patients with concurrent autoimmune disease or a history of chronic or recurrent autoimmune disease, such as rheumatoid arthritis
• Patients with headache and/or nausea associated with brain metastasis