The main objective of Part 1 is to evaluate the safety and tolerability of bemarituzumab plus 5-fluorouracil, leucovorin, and oxaliplatin (mFOLFOX6) and nivolumab.

The main objective Part 2 is to compare efficacy of bemarituzumab plus mFOLFOX6 and nivolumab to placebo plus mFOLFOX6 and nivolumab as assessed by overall survival.

This is a Phase 1b/3 study:

• Phase 1b (Part 1) is a single-arm open-label study, which will enroll about 20 participants
• Phase 3 (Part 2) is a randomized double-blind 2-arm study, which will enroll 682 participants

Inclusion Criteria Part 1:

• Adult with unresectable, locally advanced or metastatic (not amenable to curative therapy) histologically documented gastric or gastroesophageal junction adenocarcinoma
• Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1
• Measurable disease or non-measurable, but evaluable disease, according to Response Evaluation Criteria in Solid Tumours version 1.1 (RECIST v1.1)
• Participant must be a candidate to receive mFOLFOX6 and nivolumab
• Adequate organ function as follows:
  • Absolute neutrophil count ≥ 1.5 x 10^9/L
  • Platelet count ≥ 100 x 10^9/L
  • Hemoglobin ≥ 9 g/dl
  • Aspartate aminotransaminase (AST) and Alanine aminotransaminase (ALT) <3 x upper limit of normal (ULN) (or < 5 x ULN if liver involvement). Total bilirubin <1.5 x ULN (or < 2 x ULN if liver involvement); with the exception of participants with Gilbert's disease)
  • Calculated or measured creatinine clearance (CrCl) of ≥ 50 mL/minute calculated using the formula of Cockcroft and Gault
  • International Normalized Ratio (INR) or prothrombin time (PT) < 1.5 × ULN except for participants receiving anticoagulation, who must be on a stable dose of anticoagulant therapy for 6 weeks prior to enrollment.

Additional Inclusion Criteria Part 2:

• No prior treatment for metastatic or unresectable disease except for a maximum of 1 dose of mFOLFOX6 with or without nivolumab. Prior adjuvant or neo-adjuvant therapy for localized disease is allowed, provided it has been completed more than 6 months prior to the first dose of study treatment.
• Fibroblast growth factor receptor 2b (FGFR2b) overexpression as determined by centrally performed immunohistochemistry (IHC) testing

Exclusion Criteria:

• Prior treatment with any selective inhibitor of the fibroblast growth factor (FGF)-FGFR pathway
• Known positive human epidermal growth factor receptor 2 (HER2) status
• Untreated or symptomatic central nervous system disease metastases and leptomeningeal disease
• Peripheral sensory neuropathy grade 2 or higher
• Clinically significant cardiac disease
• Other malignancy within the last 2 years (exceptions for definitively treated disease)
• Chronic or systemic ophthalmologic disorders
• Major surgery or other investigational study within 28 days prior to randomization
• Palliative radiotherapy within 14 days prior to randomization
• Abnormalities of the cornea that may pose an increased risk of developing a corneal ulcer
• Active autoimmune disease that has required systemic treatment (except replacement therapy) within the past 2 years or any other diseases requiring immunosuppressive therapy while on study