Bemarituzumab or Placebo plus Chemotherapy in Gastric Cancers with Fibroblast Growth Factor Receptor 2b (FGFR2b) OverexpressionFORTITUDE-101

- 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

- 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.