This is an open-label,multi-centre, phase 1b safety and feasibility study aiming to assess the safey and feasibility of bemarituzumab plus chemiotherapy (FLOT) for the peri-operative treatment of resectable gastroesophageal cancer. Following this neo-adjuvant part, patients should undergo surgery. Following surgery, patients will receive FLOT chemotherapy in combination with bemarituzumab. Imaging will be performed every 3 months for the first 2 years and thereafter every 6 months as per standard of care.

Inclusion Criteria:

• Signed screening informed consent
• Age ≥ 18 years
• Availability of a diagnostic tumour sample for central evaluation (FFPE block or minimum 10 slides).
• Histologically proven, previously untreated, locally advanced, resectable gastric/GEJ adenocarcinoma cT2-cT4, N0, M0 or cTany, N+, M0. Note: if the results of the staging laparoscopy are pending at this stage it is acceptable to include the patients in this step, in the interest of patient time-to-treatment.
• Planned peri-operative treatment with FLOT chemotherapy and surgically assessed feasible R0 resection
• Centrally confirmed FGFR2b overexpression as defined by ≥10% tumour cells with 2+/3+ FGFR2b staining by immunohistochemistry (IHC)
• Eastern Cooperative Oncology Group (ECOG) performance status 0-1
• Previously untreated, locally advanced, histologically confirmed, resectable gastric/GEJ adenocarcinoma cT2-cT4, N0, M0 or cTany, N+, M0 by radiological and surgical staging
• Adequate hematologic and organ function

Exclusion Criteria:

• Prior anticancer treatment for the disease under investigation
• History or evidence of systemic disease or ophthalmological disorders requiring chronic use of ophthalmologic corticosteroids
• Evidence of any ongoing ophthalmologic abnormalities or symptoms that are recent (within 4weeks) or actively progressing
• Unwillingness to avoid the use of contact lenses during study treatment and follow-up period of at least 100 days after end of treatment.
• Evidence of, or recent (within 6 months) history of, corneal defects, corneal ulcerations, keratitis, or keratoconus, history of corneal transplant, or other known abnormalities of the cornea that may pose an increased risk of developing a corneal ulcer. Recent (within 6 months) corneal surgery or ophthalmologic laser treatment
• History of solid organ transplantation
• History of interstitial lung disease
• 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
• Patients on immunosuppressive therapy or having immune system disorders, including auto-immune diseases. Concurrent steroid use of not more than an equivalent of 10 mg/day prednisolone is allowed. Inhaled, intranasal, intraocular, and/or joint injections of corticosteroids are allowed.