Abstract PS2-10-17: Potential of MEK Inhibition to overcome Trastuzumab deruxtecan Resistance in MAPK-Activated Triple-Negative Breast Cancer PDX Models

Abstract Background: Since its approval in 2020 by the FDA, the antibody-drug conjugate (ADC) Trastuzumab deruxtecan (T-DXd), has demonstrated its efficacy not only targeting HER2+ tumors, but HER2-low breast cancers, including triple negative breast cancer (TNBC) subtype. However, mechanisms of primary and acquired resistance to T-DXd in TNBC remain poorly understood. To identify predictive biomarkers and rational therapeutic strategies to overcome resistance, we performed proteomic profiling and functional validation studies in a panel of TNBC patient-derived xenograft (PDX) models and TNBC cell lines. Methods: We conducted LC-MRM (liquid chromatography–multiple reaction monitoring) mass spectrometry analysis of tumor lysates from 19 TNBC patient-derived xenograft (PDX) models previously evaluated for T-DXd response. The targeted proteomic panel included 91 peptides representing 63 proteins spanning HER family receptors, DNA damage response (DDR) components, and key signaling effectors. Correlative analysis between protein expression and tumor growth inhibition (%TGI) revealed a significant negative association between T-DXd efficacy and MAPK pathway activation, including elevated levels of MAPK3, pMAPK3_pT202, pMAPK3_pY204. Conversely, positive correlations were observed with proteins such as HER2, pATR_pT1989, PMS1, LIG1, and TUBB. These findings implicated hyperactive MAPK signaling as a potential driver of T-DXd resistance in TNBC, independent of HER2 expression level. Results: To verify our hypothesis, we evaluated the therapeutic potential of combining T-DXd with the MEK inhibitor selumetinib in a panel of TNBC cell lines (sensitive and resistant) and PDX derived organoids. Our preliminary data indicates a synergistic anti-tumor effect between T-DXd and the MEK inhibitor. We further performed in vivo studies using two T-DXd-resistant TNBC PDX models, WHIM3 and WHIM4, both exhibiting high MAPK pathway activation. The combination of T-DXd and the MEK inhibitor selumetinib demonstrated significantly greater antitumor activity than either agent alone, effectively overcoming resistance in both models. Additional T-DXd-resistant TNBC PDX models and cell lines are currently being evaluated for MAPK pathway activation and response to MEK inhibition. Short-term biomarker studies are underway to investigate the mechanisms of action of the combination therapy, and these results will be presented. Conclusions: Proteomic profiling implicates MAPK pathway activation as a key mechanism of T-DXd resistance in TNBC. MEK inhibition with selumetinib restored T-DXd sensitivity in resistant PDX models, supporting MAPK blockade as a rational strategy to overcome ADC resistance. These findings provide preclinical justification for evaluating T-DXd and selumetinib in HER2-low TNBC. Given an ongoing clinical protocol (NCI 10733) evaluating this combination in pancreatic cancer, these results may facilitate translational efforts in metastatic TNBC. Citation Format: A. Gonzalez Gonzalez, Z. Guo, T. D. Lorentzen, J. Luo, L. Zhao, X. Shan, M. Highkin, J. Hoog, S. Davies, L. Ding, S. Li, A. Vindigni, R. Bose, A. Paulovich, C. X. Ma. Potential of MEK Inhibition to overcome Trastuzumab deruxtecan Resistance in MAPK-Activated Triple-Negative Breast Cancer PDX Models [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS2-10-17.