Maron S.B.Xu R.H.Wainberg Z.A.Rha S.Y.Zhang Y.Pietrantonio F.Oliveira S.C.S.Li Y.Chen M.H.Korphaisarn K.Elimova E.Calderon C.A.Herpe F.V.Yong W.P.Dos Santos T.M.Tan M.Isse K.Yanes R.E.Shitara K.Mahidol University2026-05-312026-05-312026-06-01ESMO Open Vol.11 No.6 (2026)https://repository.li.mahidol.ac.th/handle/123456789/117017Background: Fibroblast growth factor receptor 2 isoform IIIb (FGFR2b) protein overexpression is an emerging biomarker and a potential therapeutic target in gastric and gastroesophageal junction cancers (G/GEJC). Limited data exist on the global prevalence of FGFR2b overexpression in advanced G/GEJC using a validated assay. We assessed the prevalence of FGFR2b protein overexpression in a pooled analysis of tumor samples collected as part of the prescreening process for two global phase III studies of bemarituzumab as first-line treatment for locally advanced or metastatic G/GEJC. Materials and methods: As of 20 February 2025, 7910 tumor samples from patients prescreened for enrollment in the FORTITUDE-101 (NCT05052801; n = 3752) and FORTITUDE-102 (NCT05111626; n = 4158) studies were centrally tested for FGFR2b protein overexpression by immunohistochemistry (IHC) and had evaluable results. FGFR2b overexpression was defined as both any percentage (>0%) of tumor cells (TCs) and ≥10% of TCs exhibiting moderate (2+) to strong (3+) membrane staining of FGFR2b. Prevalence was analyzed across defined patient and sample characteristics. Results: The estimated prevalence of FGFR2b any 2+/3+ was 36.5% [95% confidence interval (CI) 35.4-37.6; 2887/7910] and that of FGFR2b ≥10% 2+/3+ was 16.6% (95% CI 15.7-17.4; 1310/7910). Prevalence estimates from this pooled analysis and a separate analysis for FORTITUDE-102 [any 2+/3+: 35.8% (95% CI 34.3-37.2); FGFR2b ≥10% 2+/3+: 17.3% (95% CI 16.1-18.4)] were consistent with results previously reported for FORTITUDE-101 (Rha SY, Zhang Y, Elme A, et al. Prevalence of FGFR2b protein overexpression in advanced gastric cancers during prescreening for the phase III FORTITUDE-101 trial. JCO Precis Oncol. 2025;9:e2400710). FGFR2b prevalence was consistent across patient and sample characteristics [age, sex, collection method (biopsy versus resection), collection site, location of primary tumor, and geographic region]. Conclusion: This study represents the largest prevalence assessment of FGFR2b overexpression in G/GEJC using a validated IHC assay. The observed estimates of 36.5% (any 2+/3+) and 16.6% (FGFR2b ≥10% 2+/3+) suggest that FGFR2b is prevalent in a meaningful proportion of patients with advanced G/GEJC.Biochemistry, Genetics and Molecular BiologyMedicineArticleSCOPUS10.1016/j.esmoop.2026.1076982-s2.0-10503983344220597029