Abstract B27: Targeting of phosphatidylserine by monoclonal antibody ch1N11 enhances the antitumor activity of immune checkpoint inhibitor PD-1/PD-L1 therapy in orthotopic murine breast cancer models
Authors: Gray Michael J. , Gong Jian , Nguyen Van , Schuler-Hatch Michaela , Hughes Chris , Hutchins Jeff , Freimark Bruce 
Published: May 2016

Abstract
Introduction: Phosphatidylserine (PS) is a phospholipid that resides in the plasma membrane inner leaflet in many types of cells, including both tumor and tumor associated endothelial cells. Conditions that cause cellular stress, including those that occur from oxygen radicals, hypoxia, irradiation, and chemotherapy, cause a dramatic shift in PS localization in both tumor and tumor associated endothelial cells. This change in localization results in PS shifting to the outer plasma membrane, allowing its recognition by components of the tumor microenvironment. Recognition of PS promotes an immunosuppressive environment that encourages tumor growth, in part by promoting the recruitment of myeloid derived suppressor cells, immature dendritic cells, and M2-like macrophages, in addition to inducing production of anti-inflammatory cytokines. Currently the chimeric PS-targeting antibody, bavituximab, is being studied in combination with chemotherapies to treat patients with solid tumors in multiple late-stage clinical trials, where it is believed to help augment the efficacy of chemotherapeutics by blockade of PS-mediated immunosuppression and triggering an Fc-FcR mediated pro-inflammatory response in the tumor microenvironment. The results with PS targeting therapies and chemotherapeutics are encouraging, and the effectiveness of PS targeting therapies in combination with therapies directed towards immune checkpoint regulators warrants further attention.
Methods: Immune competent mice bearing established syngeneic EMT-6 or E0771 breast tumors were subjected to treatments comprising of a PS targeting antibody (ch1N11) and an anti-PD-1 antibody (to interrupt the PD-1/PD-L1 signaling axis) either as single or combination therapy. The anti-tumor effects of treatments were determined by measuring primary tumor growth inhibition and specific immunity was determined by tumor re-challenge.
Results: In both models, which showed distinct sensitivity to therapy, the combination of ch1N11 with an anti-PD-1 checkpoint blocking antibody had a significantly greater anti-tumor affect than single arm treatments. In the E0771 model, complete tumor regression was observed in 6 of 10 animals treated with combination treatment. Animals with no tumor growth for 30 days post study end were resistant to tumor re-challenge indicating the development of tumor-specific immunity.
Conclusions: These results suggest that the combination of PS targeting antibodies in conjunction with checkpoint inhibitors has the potential to block tumor immunosuppression in breast cancer and promote a durable antitumor immune response.
Citation Format: Michael J. Gray, Jian Gong, Van Nguyen, Michaela Schuler-Hatch, Chris Hughes, Jeff Hutchins, Bruce Freimark. Targeting of phosphatidylserine by monoclonal antibody ch1N11 enhances the antitumor activity of immune checkpoint inhibitor PD-1/PD-L1 therapy in orthotopic murine breast cancer models. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research; Oct 17-20, 2015; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(2_Suppl):Abstract nr B27.