FGFR2b in Gastric Cancer: Translating a Therapeutic Target into a Reliable Biomarker.

Fibroblast growth factor receptor 2b (FGFR2b) has become an increasingly important therapeutic target in gastric and gastroesophageal junction cancer, particularly with the clinical development of FGFR2b-directed antibody therapy. However, its translation into routine treatment selection is not straightforward. FGFR2b is usually assessed as a protein biomarker by immunohistochemistry, and a positive result may reflect different biological situations depending on staining intensity, percentage of positive tumor cells, sample type and spatial distribution. In addition, FGFR2b protein expression, FGFR2 amplification, transcript-level activity and true pathway dependency are related but not interchangeable. This review examines FGFR2b-positive gastric cancer from the perspective of biomarker reliability rather than target presence alone. We discuss the biological basis of FGFR2b targeting, the reasons for variability in reported positivity rates, the implications of intratumoral and inter-lesion heterogeneity, the current clinical evidence for FGFR2b-directed and broader FGFR-targeted approaches, and the emerging challenges of safety, resistance and treatment sequencing. Particular attention is given to the gap between detecting FGFR2b and identifying tumors in which this target is sufficiently expressed, representative and biologically relevant to guide therapy. We also consider how FGFR2b should be interpreted alongside HER2, CLDN18.2, immune biomarkers and other receptor tyrosine kinase alterations. As FGFR2b-directed strategies move forward, their success will depend not only on drug efficacy, but also on standardized testing, careful reporting, and selective reassessment when disease biology changes. FGFR2b therefore offers a useful model for how protein biomarkers can be developed in gastric cancer: not as isolated positive-or-negative labels, but as clinically interpreted variables within a changing therapeutic landscape.