Bemarituzumab or Placebo Plus Chemotherapy in Gastric Cancers With Fibroblast Growth Factor Receptor 2b (FGFR2b) Overexpression

The main objective of this study is to compare efficacy of bemarituzumab combined with oxaliplatin, leucovorin, and 5-fluorouracil (5-FU) (mFOLFOX6) to placebo plus mFOLFOX6 as assessed by overall survival (OS) in participants with FGFR2b ≥10% 2+/3+ tumor cell staining (FGFR2b ≥10% 2+/3+TC)

Inclusion Criteria: * Adults with histologically documented unresectable, locally advanced or metastatic gastric or gastroesophageal junction cancer not amenable to curative therapy * Fibroblast growth factor receptor 2b (FGFR2b) ≥10% 2+/3+ tumor cell staining 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 * Eastern Cooperative Oncology Group (ECOG) less than or equal to 1 * Measurable disease or non-measurable, but evaluable disease, according to Response Evaluation Criteria in Solid Tumors (RECIST) V 1.1 * Participant has no contraindications to mFOLFOX6 chemotherapy * Adequate organ and bone marrow function: * absolute neutrophil count greater than or equal to 1.5 times 10\^9/L * platelet count greater than or equal to 100 times 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 aminotransferase (AST) and alanine aminotransferase (ALT) less than 3 times the upper limit of normal (ULN) (or less than 5 times ULN if liver involvement). Total bilirubin less than 1.5 times ULN (or less than 2 times ULN if liver involvement); with the exception of participants with Gilbert's disease) * 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) * international normalized ratio (INR) or prothrombin time (PT) less than 1.5 times ULN except for participants receiving anticoagulation, who must be on a stable dose of anticoagulant therapy for 6 weeks prior to enrollment Exclusion Criteria: * Prior treatment for metastatic or unresectable disease (Note: prior adjuvant, neo-adjuvant, and peri-operative therapy is allowed if completed more than 6 months prior to first dose of study treatment) * Prior treatment with any selective inhibitor of fibroblast growth factor - fibroblast growth factor receptor (FGF-FGFR) pathway * Known human epidermal growth factor receptor 2 (HER2) positive * Untreated or symptomatic central nervous system (CNS) disease or brain metastases * Peripheral sensory neuropathy greater than or equal to Grade 2 * Clinically significant cardiac disease * Other malignancy within the last 2 years (exceptions for definitively treated disease) * Chronic or systemic ophthalmological disorders * Major surgery or other investigational study within 28 days prior to first dose of study treatment * Palliative radiotherapy within 14 days prior to the first dose of study treatment * Evidence of or recent history (within 6 months) of corneal defects, corneal ulcerations, keratitis, or keratoconus, history of corneal transplant, or other known abnormalities of the cornea that may pose an increased risk of developing a corneal ulcer.