Background: The killing of Her2 + breast cancer cells by the antibody trastuzumab is an attractive therapy because of its specificity and limited side effects. However, many Her2 + positive patients have innate resistants or will become resistant to this treatment. Although Her2 status can be determined by immunohisochemistry or FISH, still only 30% of patients positive for high Her2 status will respond to trastuzumab alone. Improved methods to predict sensitivity to trastuzumab are greatly needed. Since trastuzumab is believed to work by antibody-dependent cell-mediated cytotoxicity (ADCC), we have developed a novel and reproducible bioassay to measure trastuzumab-dependent ADCC which may be predictive of patient response. Methods: Using both an xCELLigence platform (Roche Applied Science) and a flow cytometry assay (PanToXiLux from OncoImmunin, Inc), we measured the ability of mononuclear cells (MNCs) to kill four different breast cancer cell lines in the presence or absence of trastuzumab. Two cell lines used have resistance to trastuzumab and two cell lines have lower Her-2 expression. The xCELLigence platform measures killing in real time while the flow cytometry assay measures the transfer of granzyme B and upstream caspase activity from mononuclear cells to the tumor cells and the activation of cell death processes. The ability of mononuclear cells to bind tumor cells in the presence and absence of trastuzumab was evaluated by microscopy. Using miltenyi beads and immunofluroescence, we have measured which MNC population is most effective in ADCC activity. MNCs from patients with breast cancer and from individuals without disease were analyzed. Results: These studies provide a simple and reproducible assay for ADCC activity. Effective ADCC killing is directly related to Her2 expression on the tumor target. The degree of killing is directly related to the number of mononuclear cells added in the study when trastuzumab is added. Addition of trastuzumab alone to these cell lines results in little killing. MNCs from non-diseased individuals as well as those obtained from breast cancer patients display a broad range of ADCC killing efficiency per cell. Finally, our results demonstrate that NK cells are the major MNC population of effect killing. Conclusion: The two assays presented here and other specific studies show that trastuzumab can work with the immune system to increase cell killing of Her2 overexpressing cells by ADCC. A correlation between the results of these assays and the clinical response to trastuzumab will be presented.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5374. doi:1538-7445.AM2012-5374