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

Part 1 and Part 2:

Adult with unresectable, locally advanced or metastatic (not amenable to curative therapy) histologically documented gastric or gastroesophageal junction adenocarcinomaEastern 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/LPlatelet 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 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.