Bemarituzumab Plus Chemotherapy and Nivolumab Versus Chemotherapy and Nivolumab for FGFR2b Overexpressed Untreated Advanced Gastric and Gastroesophageal Junction Cancer

Open to people ages 18-100

Part 1 and Part 2:

- 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 has no contraindications to mFOLFOX6 chemotherapy or 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 without red blood cell (RBC) transfusion within 7 days prior to the first dose of study treatment - 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 or 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, neo-adjuvant, and peri-operative therapy 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) ≥ 10% 2+/3+ tumor cells (TC) as determined by centrally performed immunohistochemistry (IHC) testing, based on tumor sample either archival (obtained within 6 months/180 days prior to signing pre-screening informed consent) or a fresh biopsy.

• 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