Abstract 252: Antibody-mediated phosphatidylserine blockade significantly enhances the efficacy of immune checkpoint blockades in K1735 and B16 mouse melanoma models

Abstract

Abstract Phosphatidylserine (PS) is a phospholipid normally residing in the inner leaflet of the plasma membrane and becomes exposed abundantly on tumor endothelial cells, tumor-secreted microvesicles, and tumor cells in response to chemo- and radiotherapy. Extensive studies have shown that PS is a global immune checkpoint and major contributor to tumor immunosuppression, which promotes the expansion of immunosuppressive cells, such MDSC and M2 macrophages, inhibits DCs maturation, while stimulates them to secrete immunosuppressive mediators. We have shown that PS targeting can override PS-mediated immunosuppression, reactivate innate tumor immunity, and evoke adaptive antitumor immunity. In the present study, we assessed the antitumor effect of the combination of PS blockade and anti-CTLA-4 or anti-PD-1 antibodies in B16 and K1735 melanoma models. Both combinations showed significantly superior tumor growth inhibition over single treatment, with many mice achieving complete tumor regression. Flow cytometry analysis showed that the combination treatment had significantly greater total and functional tumor-infiltrating CD8+ T, more IL-2- and IFNγ-producing splenic T cells, and lower number of splenic MDSCs than did single treatment. In addition, the ratio of M2 to M1 in the tumor was significantly lower in the combination treatment than that in single treatment. Finally, no toxicity was observed in any of the treatment groups following multiple treatment doses. These data suggest that combination of PS blockade with immune checkpoint blockade promotes strong, localized, enhanced therapeutic efficacy without the side-effects of systemic immune activation and represents a promising combinatorial strategy for cancer immunotherapy. A chimeric PS-targeting antibody, bavituximab, is being used in combination with chemotherapy to treat patients with solid tumors in multiple late-stage clinical trials. These data support our Phase I IST evaluation of our PS-targeting antibody bavituximab in combination with ipilimumab in advanced melanoma patients. Citation Format: Bruce Freimark, Jian Gong, Dan Ye, Rolf Brekken, Shen Yin, Jeff Hutchins, Van Nguyen, Chris Hughes, Xianming Huang. Antibody-mediated phosphatidylserine blockade significantly enhances the efficacy of immune checkpoint blockades in K1735 and B16 mouse melanoma models. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 252. doi:10.1158/1538-7445.AM2015-252