Bemarituzumab plus chemotherapy and Nivolumab versus chemotherapy and Nivolumab for FGFR2b overexpressed untreated advanced gastric and gastroesophageal junction cancer.FORTITUDE-102

Inclusion Criteria 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 nivolumab and either mFOLFOX6 or CAPOX chemotherapy as per local prescribing information. Participants in Part 1 must have no contraindications to mFOLFOX6. Participants in Part 2 with contraindications to mFOLFOX6 are permitted and may be administered the CAPOX regimen, if no contraindications for this regimen exist. Participants in Part 2 with contraindications to CAPOX are permitted and may be administered the mFOLFOX6 regimen, if no contraindications for this regimen exist - 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) . Part 1 only: Calculated or measured creatinine clearance (CrCl) of ≥ 50 mL/minute calculated using the formula of Cockcroft and Gault ([140 – Age] × Mass [kg]/[72 × Creatinine mg/dL]) (x 0.85 if female). - Part 2 only: Calculated or measured creatinine clearance (CrCl) of ≥ 30 mL/minute calculated using the formula of Cockcroft and Gault ([140 – Age] × Mass [kg]/[72 × Creatinine mg/dL]) (x 0.85 if female). - 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 chemotherapy 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. 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