Abstract
T-DM1 (Kadcyla), an antibody-drug conjugate (ADC) consisting of HER2-targeted monoclonal antibody trastuzumab linked to anti-microtubule agent mertansine (DM1), has been approved for the treatment of HER2-positive metastatic breast cancer. To date, acquired resistance arises to be a major obstacle to T-DM1 treatment, and mechanisms remain incompletely understood. In the present study, we established a T-DM1-resistant N87-KR cell line from HER2-positive N87 gastric cancer cells to investigate mechanisms of acquired resistance and develop strategies for overcoming it. Although the kinetics of binding, internalization, and externalization of T-DM1 were the same in N87-KR cells and N87 cells, N87-KR was strongly resistant to T-DM1, but remained sensitive to both trastuzumab and DM1. T-DM1 failed to inhibit microtubule polymerization in N87-KR cells. Consistently, lysine-MCC-DM1, the active T-DM1 metabolite that inhibits microtubule polymerization, accumulated much lesser in N87-KR cells. Furthermore, lysosome acidification, achieved by V-ATPase, was much diminished in N87-KR cells. Notably, treatment of sensitive N87 cells with the V-ATPase–selective inhibitor Baf-A1 induced T-DM1 resistance, suggesting that aberrant V-ATPase activity decreases T-DM1 metabolism, leading to T-DM1 resistance in N87-KR cells. Interestingly, HER2-targeted ADCs containing a protease-cleavable linker, such as hertuzumab-vc-MMAE, were capable of efficiently overcoming this resistance. Our results demonstrate for the first time that a decrease in T-DM1 metabolites induced by aberrant V-ATPase activity contributes to T-DM1 resistance, which could be overcome by HER2-targeted ADC containing different linkers, including a protease-cleavable linker. Accordingly, we propose that V-ATPase activity in lysosomes is a novel biomarker for predicting T-DM1 resistance.
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