The main objectives of this study are to evaluate the safety and tolerability of bemarituzumab in combination with other anti-cancer therapies, and to evaluate the efficacy of bemarituzumab in combination with S-1 and oxaliplatin (SOX) and nivolumab as assessed by objective response.

Inclusion Criteria:

• Adults with unresectable, locally advanced or metastatic gastric or gastroesophageal junction cancer not amendable to curative therapy.
• Ability to provide tumor sample, either archival (obtained within 6 months to joining study) or fresh biopsy.
• For certain arms for Part 1, FGFR2b overexpression positive defined as any FGFR2b 2+/3+ TC determined by centrally performed immunohistochemistry (IHC), based on tumor sample provided.
• For Part 2, FGFR2b overexpression positive defined as FGFR2b ≥10% 2+/3+ TC determined by centrally performed IHC testing, based on tumor sample provided.
• Easter Cooperative Oncology Group (ECOG) performance score less than or equal to 1.
• Measurable or non-measurable disease as long as evaluable by Response Evaluation Criteria Solid Tumors (RECIST) version 1.1
• Participant has no contradictions to CAPOX/SOX plus or minus nivolumab.
• Adequate organ function.
• For Part 2, measurable disease according to RECIST v1.1.

Exclusion Criteria:

• Prior treatment for metastatic or unresectable disease (Note: prior adjuvant or neo-adjuvant therapy for local disease is allowed if ended more than 6 months of 1st dose).
• 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 of first study treatment dose.
• Palliative radiotherapy within 14 days of first study treatment dose.
• Abnormalities of the cornea that may pose an increased risk of developing a corneal ulcer.
• History or evidence of systemic disease or ophthalmological disorders requiring chronic use of ophthalmic corticosteroids.

De-identified individual patient data for variables necessary to address the specific research question in an approved data sharing request.

Data sharing requests relating to this study will be considered beginning 18 months after the study has ended and either 1) the product and indication have been granted marketing authorization in both the US and Europe or 2) clinical development for the product and/or indication discontinues and the data will not be submitted to regulatory authorities. There is no end date for eligibility to submit a data sharing request for this study.

Qualified researchers may submit a request containing the research objectives, the Amgen product(s) and Amgen study/studies in scope, endpoints/outcomes of interest, statistical analysis plan, data requirements, publication plan, and qualifications of the researcher(s). In general, Amgen does not grant external requests for individual patient data for the purpose of re-evaluating safety and efficacy issues already addressed in the product labelling. Requests are reviewed by a committee of internal advisors. If not approved, a Data Sharing Independent Review Panel will arbitrate and make the final decision. Upon approval, information necessary to address the research question will be provided under the terms of a data sharing agreement. This may include anonymized individual patient data and/or available supporting documents, containing fragments of analysis code where provided in analysis specifications. Further details are available at the URL below.