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

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