Abstract 824: Reduction in prostate tumor growth and induction of anti-tumor immunity after combined treatment with oncolytic VSV and radiation

Abstract

Abstract Androgen deprivation therapy (ADT) is a mainstay of treatment for high-risk non metastatic and metastatic prostate cancer. Patients initially respond to ADT, but eventually develop resistance to treatment. To overcome these limitations, we have used oncolytic virotherapy which has the potential to influence both local and systemic disease through direct cytotoxic and immune mechanisms. Here, we investigated the effectiveness of radiation therapy (RT) combined with Vesicular Stomatitis Virus (VSV) vector expressing interferon β (IFNβ) in VSV resistant prostate xenografts. In initial cell culture experiments, microarray analysis indicated that PC3 cells treated with VSV-hIFNβ and RT displayed several genes known to directly inhibit VSV replication (e.g., MX2, OAS2, TRIM25, IFITM1) were downregulated compared with VSV-hIFNβ therapy alone. Additionally, many apoptotic pathway-associated genes (e.g. IL24, GADD45B, KLF4&10, TNF, EGFR) were upregulated several fold during combination therapy. Gene expression changes were confirmed by RT-PCR. Further, immunoblot analysis demonstrated expression levels of IFNAR, BCL-XL, phosphorylated AKT, JAK2 and STAT3 were also markedly decreased when VSV-hIFNβ was combined with RT. However, RIG-I was not significantly altered after VSV-hIFNβ+RT treatment. A marked decrease in MAVS, NFKB (p50), IRF7 and antiviral protein PKR was seen after combined treatment. The role of IFN pathway in viral response was investigated using an interferon-inducible promoter ISRE-luciferase (ISRE-Luc) reporter construct. A significant difference (>600 fold) in the ISRE-Luc activation was seen for PC3 compared to LNCaP cells, indicating that the IFN pathway is highly active in PC3 cells after VSV infection alone. The activation of ISRE-mediated transcription of luciferase was significantly attenuated (p<0.05) in both the cell lines after combined treatment with VSV-hIFNβ and RT. In subsequent in vivo studies, we determined that radiation treatment combined with intratumorally-delivered VSV-mIFNβ function synergistically to induce oncolysis in both PC3 and LNCaP xenografts and a syngeneic RM9 murine model. In the syngeneic model, combination therapy resulted in a robust anti-tumor immune response with CD8 and CD4 T-cell expansion and resistance to tumor rechallenge compared to either therapy alone. These studies demonstrate that two mechanisms are involved in combined RT and VSV-mediated oncolysis. These are, (1) the attenuation of the antiviral response which increases the direct oncolytic effect of VSV on the tumor cells and (2) improved generation of an adaptive immune response earmarked by CD8+ lymphocyte expansion and anti-tumor activity. Our study demonstrates that the combined strategy of RT plus VSV-IFNβ, affects tumor cell death that are resistant to VSV through direct and systemic activity that includes a pronounced enhancement of anti-tumor immunity. Citation Format: Thirupandiyur S. Udayakumar, Dillon Betancourt, Glen Barber, Anis Ahmad, Brian Marples, Alan Pollack. Reduction in prostate tumor growth and induction of anti-tumor immunity after combined treatment with oncolytic VSV and radiation [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 824. doi:10.1158/1538-7445.AM2017-824