Abstract A36: Chemosensitivity to trastuzumab emtansine (T-DM1) differs in naturally or transduced HER2-overexpressing human breast cancer cells

Human epidermal growth factor receptor-2 (HER2) is overexpressed in 25-30% of breast cancers and is associated with distant metastasis and poor survival. Trastuzumab emtansine (T-DM1), an antibody-drug conjugate of anti-HER2 mAb trastuzumab linked to microtubule-targeting agent mertansine, has been approved for the treatment of HER2-positive metastatic breast cancer. Chemoresistance has been a major obstacle to T-DM1 treatment, and mechanisms remain incompletely understood. We tested T-DM1 chemosensitivity in vitro and in vivo using two human breast cancer cell lines, MDA-MB-231BR-HER2 (triple-negative cells stably transfected with HER2 plasmid) and HCC1954 (natural HER2 overexpression), which show equivalent HER2 and αv integrin protein levels. MDA-MB-231BR-HER2 was strongly resistant to T-DM1, with doses as high as 1 µg/ml failing to induce apoptosis indicated by the presence of cleaved PAPR protein. In contrast, HCC1954 cells are sensitive to T-DM1, with doses as low as 10 ng/mL causing cytotoxicity in a dose-dependent manner. Cellular internalization of trastuzumab-pHrodo conjugates was significantly higher in HCC1954 than MDA-MB-231BR-HER2, but no difference was found in anti-EGFR-Ab-pHrodo conjugates internalization. Endocytosis-related proteins caveolin-1 and rab5, but not early endosome antigen, were diminished in HCC1954 cells. Consistently, single-dose treatment with T-DM1 (3.6 mg/kg, IV) significantly reduced brain tumor volume in rats inoculated with HCC1954 but not MDA-MB-231BR-HER2 cells in an intracerebral xenograft breast cancer brain metastasis model. In conclusion, our findings suggest that rab5 and caveolin-1 proteins mediating antibody internalization and/or natural vs transduced HER2 overexpression may provide possible mechanisms in resistance of T-DM1 and should be investigated when HER2-positive cancer patients fail T-DM1 HER2-targeted therapy in clinical settings. Citation Format: Jeffrey Wu, Leslie Muldoon, Edward Neuwelt. Chemosensitivity to trastuzumab emtansine (T-DM1) differs in naturally or transduced HER2-overexpressing human breast cancer cells [abstract]. In: Proceedings of the AACR Special Conference: Advances in Breast Cancer Research; 2017 Oct 7-10; Hollywood, CA. Philadelphia (PA): AACR; Mol Cancer Res 2018;16(8_Suppl):Abstract nr A36.