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

Inclusion Criteria:

• Adults with histologically documented unresectable, locally advanced or metastaticgastric or gastroesophageal junction cancer not amenable to curative therapy

• Fibroblast growth factor receptor 2b (FGFR2b) ≥10% 2+/3+ tumor cell staining asdetermined by centrally performed immunohistochemistry (IHC) testing, based on tumorsample either archival (obtained within 6 months/180 days prior to signingpre-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 ResponseEvaluation 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 daysprior to the first dose of study treatment

• aspartate aminotransferase (AST) and alanine aminotransferase (ALT) less than 3times the upper limit of normal (ULN) (or less than 5 times ULN if liverinvolvement). Total bilirubin less than 1.5 times ULN (or less than 2 times ULNif liver involvement); with the exception of participants with Gilbert'sdisease)

• calculated or measured creatinine clearance (CrCl) of ≥ 30 mL/minute calculatedusing 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 1.5times ULN except for participants receiving anticoagulation, who must be on astable 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 monthsprior 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 treateddisease)

• Chronic or systemic ophthalmological disorders

• Major surgery or other investigational study within 28 days prior to first dose ofstudy 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, cornealulcerations, keratitis, or keratoconus, history of corneal transplant, or otherknown abnormalities of the cornea that may pose an increased risk of developing acorneal ulcer.