Abstract 3875: A novel STAT3 decoy inhibitor prevents lung cancer development
Authors: Njatcha Christian , Wang Jiayi , Li Huiyu , Farooqui Mariya , Siegfried Jill M. 
Published: August 2019

Abstract
Exposure to tobacco carcinogens such as NNK leads to mutations in genes like K-RAS in premalignant lesions, which progress to adenocarcinoma. Early events in carcinogenesis are also characterized by inflammation. Central to these processes, STAT3 activation has pleotropic pro-tumor effects on proliferation, angiogenesis, and immune evasion. We previously showed that a “decoy” cyclic STAT3 oligonucleotide (CS3D) produced robust antitumor effects in human lung adenocarcinoma models. CS3D mimics the STAT3 DNA response element and acts as a “decoy” to which STAT3 binds, blocking its transcriptional activity. Effects of CS3D were compared to an inactive mutant construct unable to bind to STAT3 (CS3M). Therapeutic effects in human lung cancer xenografts suggested that CS3D might also impair lung cancer development. To test this, mice were exposed to NNK (3mg/kg) for 4 weeks (wks) followed by a 1-wk washout phase to model ex-smokers. Mice were then treated 3 times per wk with CS3D or CS3M (5mg/kg, IV) for 8 wks. Tumor burden and size were then assessed at 8 and 20 wks after the end of treatment. To evaluate effect of CS3D on the development of tumors with K-RAS activating mutations (since STAT3 knockout has been reported to promote KRAS mutant lung cancer), we isolated DNA from tumors by laser capture microdissection and assessed the incidence of G12D activating mutations. CS3D caused a reduction in preneoplasias (P&lt;0.001), tumor size (P&lt;.05), and tumor number (P&lt;.05) by 30%, 50%, and 40% respectively, compared to CS3M. Immunohistochemistry (IHC) showed a tumor microenvironment (TME) that is less primed for tumor progression. Markers of angiogenesis (VEGF) and cell cycle progression (MYC) were downregulated by CS3D (P&lt;.05). Strikingly, IHC analysis at 20 wks after the final treatment revealed a substantial decrease in STAT3 and NF-kB activation (P&lt;.05) suggesting a long-lasting effect of CS3D. However, we detected higher levels of IL-6 at 8 wks and 20 wks after the end of treatment which could indicate a rescuing feedback loop. IHC also showed a 2.6 fold increase in macrophage infiltration at 8 wks. Sequencing of DNA from residual tumors collected at 20 wks (n = 10) showed no significant difference in tumors with K-RAS mutation between CS3D (33.3%) and CS3M (27.3%), showing that pharmacological inhibition of STAT3 with CS3D doesn’t select for K-RAS mutant tumors. To examine immunomodulation, mice were exposed to 4 wks of NNK with a 1-wk washout phase and 1 wk of CS3D or CS3M treatment. Lungs were collected for flow cytometry analysis to define the immune cell profile. CS3D favored an antitumor immune response over an immunosuppressive TME by increasing the number of lung M1 macrophages while decreasing M2 macrophages and MDSCs. Ratio of M1 to M2 was 4:1 with CS3D and 1:3 with CS3M. This is the first evidence that CS3D has chemopreventive properties in lung cancer without promoting KRAS-driven tumors, and modulating the TME is involved in the preventive effect.
Citation Format: Christian Njatcha, Jiayi Wang, Huiyu Li, Mariya Farooqui, Jill M. Siegfried. A novel STAT3 decoy inhibitor prevents lung cancer development [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3875.