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 curativetherapy) histologically documented gastric or gastroesophageal junctionadenocarcinoma

• Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1

• Measurable disease or non-measurable, but evaluable disease, according to ResponseEvaluation Criteria in Solid Tumours version 1.1 (RECIST v1.1)

• Participant has no contraindications to nivolumab and either mFOLFOX6 or CAPOXchemotherapy as per local prescribing information. Participants in Part 1 must haveno contraindications to mFOLFOX6. Participants in Part 2 with contraindications tomFOLFOX6 are permitted and may be administered the CAPOX regimen, if nocontraindications for this regimen exist. Participants in Part 2 withcontraindications to CAPOX are permitted and may be administered the mFOLFOX6regimen, 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 daysprior to the first dose of study treatment

• Aspartate aminotransaminase (AST) and Alanine aminotransaminase (ALT) <3 xupper 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'sdisease)

• Part 1 only: Calculated or measured creatinine clearance (CrCl) of ≥ 50mL/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/minutecalculated 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 receivinganticoagulation, who must be on a stable dose of anticoagulant therapy for 6 weeksprior 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, andperi-operative therapy is allowed, provided it has been completed more than 6 monthsprior to the first dose of study treatment

• Fibroblast growth factor receptor 2b (FGFR2b) ≥ 10% 2+/3+ tumor cells (TC) 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.

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 andleptomeningeal disease

• Peripheral sensory neuropathy grade 2 or higher

• Clinically significant cardiac disease

• Other malignancy within the last 2 years (exceptions for definitively treateddisease)

• 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 cornealulcer

• Active autoimmune disease that has required systemic treatment (except replacementtherapy) within the past 2 years or any other diseases requiring immunosuppressivetherapy while on study