Abstract 2767: Phosphatidylserine targeting and radiation improves survival in a mouse tumor model resistant to checkpoint blockade
Authors: Giese Rachel , Budhu Sadna , Barker Christopher , Gupta Aditi , King Steve , Shan Joseph , Wolchok Jedd , Merghoub Taha 
Published: August 2018

Abstract
Background: Despite the excitement surrounding checkpoint blockade, many tumors remain resistant thus there is a need to sensitize tumors to immunotherapy and augment tumor rejection. Phosphatidylserine (PS) is a phospholipid expressed on the outer surface of apoptotic cells and a variety of immune cell types. PS signaling polarizes macrophages to the M2 phenotype, inhibits TLR signaling and is upregulated on the surface of tumor cells by radiotherapy (RT). RT also promotes infiltration of CD8 T-cells to the tumor microenvironment. Accordingly, RT may improve CD8 T-cell mediated tumor rejection in the context of checkpoint blockade and sensitize tumors to PS antagonism. We hypothesize PS-targeting antibody, mch1n11, in combination with local RT, may be an effective adjuvant or alternative treatment in patients with tumors resistant to checkpoint blockade such as anti-PD-1 (aPD-1). Furthermore, we hypothesize the combination of aPD-1 with mch1n11 and RT will synergize to improve the effector function of CD8 T-cells and enhance tumor elimination.
Methods: B16-F10 murine melanoma was injected into the hind limb of C57BL/6 mice. One cohort received triple combination therapy comprised of aPD-1, mch1n11, and RT given simultaneously at an early time point. Another cohort that showed continued tumor growth after therapy with aPD-1 was used to model checkpoint blockade resistance. These mice received delayed treatment with intra-peritoneal mch1n11 in combination with a one-time 15 Gy RT dose targeted to the tumor-bearing hind limb. The aPD-1 resistant cohort was subdivided into two groups: in one cohort aPD-1 was administered prior to and continued with mch1n11+RT therapy. In the other cohort, aPD-1 was discontinued prior to mch1n11+RT administration. Untreated mice, mice treated with aPD-1 alone, mice treated with isotype antibodies, and mice treated without initial aPD-1 therapy that received delayed mch1n11+RT therapy served as controls.
Results: Early triple combination therapy results in almost complete tumor elimination and leads to statistically significant prolonged survival. In the aPD-1 resistant cohort, both prior and continuous aPD-1 treatment with mch1n11+RT improved survival compared to all four controls. There was no significant difference in tumor size or survival between the adjuvant and alternative experimental groups. Experiments are ongoing to characterize immune infiltrates of tumors and the individual effects of aPD-1, RT and mch1n11.
Conclusion: This preclinical model suggests a phosphatidylserine targeting antibody combined with single dose RT presents an alternative or adjuvant therapy for tumors resistant to checkpoint blockade. The data from this preclinical model will be used to develop a clinical trial for patients with tumors resistant to checkpoint blockade.
Citation Format: Rachel Giese, Sadna Budhu, Christopher Barker, Aditi Gupta, Steve King, Joseph Shan, Jedd Wolchok, Taha Merghoub. Phosphatidylserine targeting and radiation improves survival in a mouse tumor model resistant to checkpoint blockade [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2767.