Abstract 3684: Inhibition of STAT3 by antisense oligonucleotide treatment decreases the immune suppressive tumor microenvironment in syngeneic and GEM tumor models
Authors: Woessner Rich , Sah Vasu , McCoon Patricia , Grosskurth Shaun , Deng Nanhua , DuPont Rachel , Lawson Deborah , Pablo Lourdes , Reimer Corinne , Velasco Marco A. De , Uemura Hirotsugu , Candido Juliana , Lyne Paul 
Published: July 2017

Abstract
AZD9150, a gen2.5 antisense oligonucleotide (ASO) targeting human STAT3, has improved drug-like properties compared to previous generation ASO therapeutics, including increased stability and resistance to nucleases, reduced proinflammatory effects, and enhanced potency. We have previously reported that in tumors, STAT3 ASOs are taken up preferentially in stromal and immune cells of the tumor microenvironment (TME). Since AZD9150 is selective for human STAT3, we used a surrogate ASO (muSTAT3 ASO) to explore the pharmacodynamics of ASO-mediated STAT3 inhibition in syngeneic and genetically engineered mouse (GEM) tumor models, focusing on effects in the TME.
In mice bearing subcutaneous CT-26 tumors, treatment with muSTAT3 ASO at 50 mg/kg, s.c., on a qdx5/wk schedule decreased STAT3 levels in immune cell subsets in the tumor and in circulating leukocytes by 40 - 60%, similar to the decrease in STAT3 achievable in circulating leukocytes in human patients after AZD9150 treatment.
In a Nanostring analysis (nCounter mouse immunology panel) of CT-26 tumors from muSTAT3 ASO treated mice, CD163 (M2 immune suppressive macrophage marker) was the gene most consistently and significantly downregulated, by an average of 84% in three independent experiments, and was confirmed by immunohistochemistry (IHC). Flow cytometry analysis of myeloid subpopulations - tumor associated macrophages (F4/80+ TAMs), monocytic myeloid derived suppressor cells, and granulocytic cells - showed a decrease in TAMs averaging 69% across three independent experiments. The analysis was extended to include IHC for arginase (Arg, a marker of functional immune suppression activity). Subpopulations of cells identified included Arg+, CD163+, and Arg+CD163+. Treatment with muSTAT3 ASO decreased these populations by 79%, 88% and 97% respectively, compared to control treatment. These populations were also analyzed in two GEM tumor models - the KPC pancreatic cancer model, and a PTEN -/- prostate cancer model - which have a TME more representative of that found in tumors in the clinic. While the specific changes varied across the models, likely reflecting differences in TME makeup, a reduction in immune suppressive cell populations was present in both GEM models, including a decrease in CD163+ cells of 79% (along with modest antitumor activity) in the PTEN -/- prostate model after muSTAT3 ASO treatment.
These results indicate that selective STAT3 inhibition can reduce immune suppressive cell populations in the TME, and suggest that STAT3 inhibition has the potential to enhance the antitumor activity of T-cell targeted therapies, such as those targeting the PD1-PDL1 axis. In support of this hypothesis, we observed that addition of muSTAT3 ASO to anti-PD-L1 Ab treatment significantly enhanced the antitumor activity of PD-L1 Ab treatment in two subcutaneous syngeneic tumor models, CT-26 and A20.
Citation Format: Rich Woessner, Vasu Sah, Patricia McCoon, Shaun Grosskurth, Nanhua Deng, Rachel DuPont, Deborah Lawson, Lourdes Pablo, Corinne Reimer, Marco A. De Velasco, Hirotsugu Uemura, Juliana Candido, Paul Lyne. Inhibition of STAT3 by antisense oligonucleotide treatment decreases the immune suppressive tumor microenvironment in syngeneic and GEM tumor models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3684. doi:10.1158/1538-7445.AM2017-3684