Abstract A070: The erbB3 blocking antibody MM-121 resensitizes trastuzumab-resistant breast cancer cells to trastuzumab-mediated antitumor activityin vitroandin vivo

Background: Amplification and/or overexpression of erbB2 occur in approximately 25% of invasive breast cancer and are significantly associated with a worse prognosis for breast cancer patients. The erbB3 receptor plays a critical role in erbB2-overexpressing breast cancer, and its elevated expression induces resistance to therapeutic agents, including trastuzumab. Our recent studies indicate that erbB3 interacts with both erbB2 and IGF-1 receptor to form a heterotrimeric complex in trastuzumab-resistant breast cancer cells. Herein, we investigate the antitumor activity of MM-121, a humanized anti-erbB3 antibody, against erbB2-overexpressing breast cancer cells resistant to trastuzumab in vitro and in vivo. Material and Methods: Cell Titer AQueous Non-Radioactive Cell Proliferation Assay (MTS) to determine cell proliferation. Flow Cytometry analyses to examine cell cycle distribution. Western blot analysis to evaluate protein expression and activation upon treatment. Tumor xenograft model to explore the combinatorial effects of MM-121 and trastuzumab on tumor growth of trastuzumab-resistant breast cancer cells in vivo. Immunohistochemistry assays to assess the treatment effects on expression of erbB3, erbB2, Ki-67, and cleaved capase-3 in vivo. Results: Cell proliferation assays showed that MM-121 significantly enhanced trastuzumab-induced growth inhibition in both sensitive and resistant breast cancer cells. MM-121 in combination with trastuzumab mainly resulted in a dramatic reduction of phosphorylated erbB3 (P-erbB3) and the downstream signaling P-Akt. MM-121 combined with trastuzumab did not lead to apoptosis in trastuzumab-resistant cells in in vitro cell culture condition, rather induced cell cycle G1 arrest which was associated with a slight decrease of E2F-1 and upregulation of p27kip1. Interestingly, in the mouse tumor xenografts model established from trastuzumab-resistant cells, MM-121 in combination with trastuzumab as compared to either agent alone significantly inhibited tumor growth correlated with a significant reduction of Ki-67 staining and increase of cleaved caspase-3 in the tumor tissues. These data suggest that the combination of MM-121 and trastuzumab not only inhibits tumor cell proliferation, but also promotes the resistant cells undergoing apoptosis in vivo. Conclusions: We demonstrate that MM-121 exhibits potent antitumor activity when combined with trastuzumab in vitro and in vivo. MM-121 may be added into the treatment regimens for breast cancer patients whose tumors overexpress erbB2 and become resistant to trastuzumab. Citation Format: Jingcao Huang, Hui Lyu, Youngseok Lee, Bolin Liu. The erbB3 blocking antibody MM-121 resensitizes trastuzumab-resistant breast cancer cells to trastuzumab-mediated antitumor activity in vitro and in vivo. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research: Genetics, Biology, and Clinical Applications; Oct 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2013;11(10 Suppl):Abstract nr A070.