Fibroblast growth factor receptor four inhibitor FGF401 improves the efficacy of trastuzumab in FGFR4‐overexpressing breast cancer cells

Abstract Breast cancer is the most common cancer among women. Among them, human epidermal growth factor receptor‐positive (HER2+) breast cancer is more malignant. Fortunately, many anti‐HER2 drugs are currently used in clinical treatments to increase patient survival. However, some HER2+ patients (~15%) still develop drug resistance after receiving trastuzumab treatment, leading to treatment failure. Using CCLE and METABRIC database analyses, we found that fibroblast growth factor receptor 4 ( FGFR4 ) mRNA was highly detected in tumors from HER2+ breast cancer patients ( p < .001) and was associated with poorer survival in breast cancer patients. Through retrospective immunohistochemical staining analysis, we detected higher expression of FGFR4 protein in breast cancer tissues collected from patients who were resistant to trastuzumab therapy compared with breast cancer patients who responded to treatment. An FGFR4 inhibitor (FGF401) effectively inhibits tumor growth in trastuzumab‐insensitive patient‐derived xenograft (PDX) tumor‐bearing mice. For molecular mechanism studies, we demonstrated that HER2/FGFR4 protein complexes were detected on the cell membrane of the tumor tissues in these trastuzumab‐insensitive PDX tumor tissues. After trastuzumab treatment in these drug‐resistant breast cancer cells, FGFR4 translocates and enters the nucleus. However, trastuzumab‐induced nuclear translocation of FGFR4/HER2‐intracellular domain protein complex in trastuzumab‐resistant cancer cells is blocked by FGF401 treatment. We believe that FGFR4 overexpression and complex formation with HER2 can serve as molecular markers to assist clinicians in identifying trastuzumab‐resistant tumors. Our results suggest that FGF401 combined with trastuzumab as adjuvant therapy for patients with trastuzumab‐resistant breast cancer may be a potential new treatment strategy.