Background: Fibroblast growth factor receptor 2b (FGFR2b) is overexpressed in approximately 30% of non-human epidermal growth factor receptor 2 (non-HER2) positive gastric cancer (Wainberg, 2021). Bemarituzumab is a first-in-class monoclonal antibody that specifically blocks FGFR2b, inhibiting downstream tumor proliferation and enhancing antibody-dependent cellular cytotoxicity (Catenacci, 2020; Xiang, 2021). In the phase 2 FIGHT study (Wainberg, 2021; Catenacci, 2021), bemarituzumab + mFOLFOX6 improved progression-free survival (PFS; HR, 0.68; 95% CI, 0.44‒1.04; p = 0.07) and led to a 5.7 month longer median overall survival (OS; 19.2 months vs 13.5 months; HR, 0.60; 95% CI, 0.38-0.94) compared with placebo + mFOLFOX6. Methods: FORTITUDE-101 (NCT05052801) is a double-blind, placebo-controlled phase 3 study in patients with untreated, unresectable locally advanced or metastatic gastric or GEJ adenocarcinoma not amenable to curative therapy. Approximately 516 patients aged ≥18 years with IHC-confirmed FGFR2b overexpression by central testing will be enrolled and randomized 1:1 to bemarituzumab + mFOLFOX6 or placebo + mFOLFOX6. Additional key eligibility criteria include Eastern Cooperative Oncology Group performance status 0-1, evaluable disease per RECIST v1.1, adequate hematologic and organ function, and no contraindication to receive mFOLFOX6 chemotherapy. Key exclusion criteria include prior treatment for metastatic or unresectable disease except one dose of mFOLFOX6 during screening, positive HER2 status, untreated or symptomatic CNS metastasis and leptomeningeal disease, history or evidence of ongoing ophthalmologic abnormalities, and prior treatment with any FGF-FGFR pathway inhibitor. Patients randomized to bemarituzumab will receive 15 mg/kg every 2 weeks (Q2W) with an additional 7.5 mg/kg dose on cycle 1 day 8. mFOLFOX6 will be administered at a fixed dose Q2W. Patients will receive treatment until disease progression, unacceptable toxicity, withdrawal of consent, or death. The primary endpoint is OS; secondary endpoints include PFS, objective response (defined as best overall response of complete or partial response), and treatment-emergent adverse events. Tumor responses and PFS will be assessed locally per RECIST v1.1. The concurrent phase 1b/3 FORTITUDE-102 study (NCT05111626) will evaluate the efficacy and safety of bemarituzumab + mFOLFOX6 + nivolumab versus placebo + mFOLFOX6 + nivolumab. Clinical trial information: NCT05052801.