Feedback activation of MAPK-ERK1/2 signaling causes resistance to trastuzumab-deruxtecan in endometrial cancers.
OBJECTIVE To identify targetable pathways that can overcome resistance to trastuzumab deruxtecan (T-DXd) in endometrial cancer. METHODS We used four endometrial cancer cell lines with different genetic backgrounds (KLE, HEC-1B, Ishikawa, and AN3 CA). Patient-derived endometrial organoids were established from surgically resected tumors and maintained in basement membrane extract with optimized media. Drug interactions were evaluated using Bliss synergy modeling. Organoid growth and treatment response were quantified by imaging-based size measurements. Protein signaling changes were analyzed by immunoblotting and comprehensive phosphoproteomic profiling. Cell cycle effects were assessed by flow cytometry. In vivo efficacy was evaluated using HEC-1B xenograft models treated with T-DXd and trametinib alone or in combination. Immunohistochemistry was performed on tumor and organoid samples to assess pathway activation and proliferation markers. Statistical analyses were conducted using GraphPad Prism 9, R and Python packages. RESULTS T-DXd treatment increases ERK1/2 activation and reduces HER2 expression across various endometrial cancer cell lines. The addition of trametinib, a MEK1/2 inhibitor, to T-DXd, restores HER2 expression and markedly reduces the growth of both endometrial cancer cell lines (in vitro and in vivo), and organoids. Phosphoproteomic assays showed that T-DXd increases phosphorylation of proteins involved in DNA damage and the cell cycle, including ATR. We also found that ERK1/2 plays a key role in activating ATR and causing cell cycle arrest in the G2/M phase. CONCLUSION T-DXd activates ERK1/2 and ATR, which contributes to resistance, and combining T-DXd with a MEK1/2 inhibitor enhances treatment response.