Abstract A94: STAT3 antisense treatment decreases M2 macrophage infiltration and enhances the activity of checkpoint inhibitors in preclinical tumor models

AZD9150 is a gen2.5 antisense oligonucleotide (ASO) targeting STAT3. Gen2.5 ASOs exhibit enhanced drug-like properties compared to previous generations of antisense therapeutics, including increased stability and resistance to nucleases, a marked decrease in proinflammatory effects, and enhanced potency. The immune suppressive effects of STAT3 signaling are well established (Kortylewski et al.; Nat. Med. 2005 and Curr. Opin. Immunol. 2008). Preclinical experiments were carried out to determine the potential for combination of a STAT3 ASO with checkpoint inhibitor therapy, and to explore the mechanism of the antitumor activity of STAT3 ASOs observed in mouse models. The antitumor activity of STAT3 ASO treatment, as a single agent and in combination with checkpoint inhibitors, was evaluated in syngeneic mouse models, including CT-26. ASO uptake and STAT3 knockdown were evaluated by immunohistochemistry (IHC), and the pharmacodynamic effects of STAT3 ASO treatment in mice were evaluated by IHC, flow cytometry and immune gene expression profiling. In contrast to many tumor cell lines, CT26 cells take up ASOs poorly, including gen2.5 ASOs, thus as expected the tumor-associated activity of STAT3 ASOs in CT26 tumors in vivo was primarily in stromal and immune compartments (including circulating PBMCs), rather than directly in tumor cells. This was observed in several murine tumor models, as well as in clinical samples. Expression analysis of CT-26 tumor lysates taken from STAT3 ASO-treated (50 mg/kg QDx5/wk) mice using the Nanostring mouse immunology panel (561 immune related genes) identified CD163 as the gene most consistently modulated after STAT3 ASO treatment, with an average knockdown of 85% relative to control treated tumors, across 3 independent experiments. This result was confirmed by immunohistochemistry, which showed a marked reduction in the number of tumor infiltrating CD163+ cells after STAT3 ASO treatment. CD163 is a marker of M2 macrophages, and the presence of M2 macrophages in a tumor can create an immune suppressive environment which has the potential to reduce the effectiveness of checkpoint inhibitor therapy. Consistent with this hypothesis, the addition of STAT3 ASO treatment enhanced the response to a checkpoint inhibitor antibody targeting PD-L1 (clone 10F.9G2). When treatment began 2 days after tumor implant, single agent STAT3 ASO and PD-L1 antibody treatments resulted in response rates (regression or no tumor growth during the course of the experiment) of 0% and 14% respectively, while the rate was 50% with the combination. The combination remained active when treatments were delayed until established tumors were present (140 mm 3 average tumor volume), with single agent and combination regression rates of 0%, and 20% respectively. These results suggest the addition of AZD9150 treatment as a promising approach to enhance the response to checkpoint inhibitor therapy Citation Format: Richard D. Woessner, Patricia McCoon, Shaun Grosskurth, Paul Lyne, Kirsten Bell, Mike Collins, Nanhua Deng, Rachel DuPont, Michele Johnstone, Youngsoo Kim, Deborah Lawson, Robert MacLeod, Lourdes Pablo, Corinne Reimer, Vasu Sah, Margaret Veldman-Jones. STAT3 antisense treatment decreases M2 macrophage infiltration and enhances the activity of checkpoint inhibitors in preclinical tumor models. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr A94.