415. The Potency of a Histone Deacetylase Inhibitor and Reolysin in Head and Neck Squamous Cell Carcinoma

Introduction: There is a clear and pressing need for novel therapies with activity against locally advanced head and neck cancers which still carry a dismal prognosis. Oncolytic viruses are powerful, targeted anti-cancer agents. Reovirus is a naturally occurring non-pathogenic virus that was isolated from the human respiratory and gastrointestinal tracts. Moreover, Reovirus type 3 Dearing (Reolysin; Oncolytics Biotech Inc., Calgary, AB, Canada) is currently being tested in phase I-III clinical trials in a variety of tumor types. Histone deacetylase inhibitors (HDACi) comprise a structurally diverse class of compounds that are targeted anticancer agents. The first FDA approved HDACi, vorinostat (suberoylanilide hydroxamic acid-SAHA), is highly effective in the treatment of cutaneous Tcell lymphoma. SAHA is currently being testing in head and neck cancer clinical trials. We previously found a synergistic combination of SAHA and Reolysin in a nude mouse model. Preclinical models of oncolytics are often in immunocompromised mice, negating the significant impact of the immune system. Mounting evidence demonstrates that the immune system is critically important in oncolytic viral response. In this study, we sought to investigate the impact of this combination in an immunocompetent model. Methods: Cell survival experiments were performed with reovirus and SAHA in shPTP-BL-Ras SCC cells. IC50 values were interpolated from a sigmoidal dose-response curve fit of the log-transformed survival data. JAM-1 surface levels were assessed via flow cytometric analysis. Cells were collected after 48 hours and cell death was assessed via Annexin V and PI staining. JAM-1 levels were assessed using anti-JAM-1-PE staining as compared to a control isotype-PE antibody. Whole splenocytes were isolated at the time of death for C57BL/6 mice bearing MTE tumors treated in 4 groups (control, SAHA, reovirus, and combination). Three independent mice were stained per group for the following: B cells (CD19+), NK cells (CD49b+), activated NK cells (CD49b+NKp46+), dendritic cells (CD11c+) and activated DCs (CD11c+MHCII+ or CD11c+CD86+), CD4 T helper (CD3+CD4+), and CD8 cytotoxic cells (CD3+CD8+). Results: Experiments demonstrated significant efficacy of SAHA and Reolysin treatment in vitro and in the immunocompetent mouse model. Combination therapy exhibited a synergistic anti-tumor effect with a significant increased survival of mice compared to any of the agents alone. The Jam1 receptor was upregulated on tumor cells allowing enhanced reovirus uptake. There was marked and significant reduction in circulating B cells in combination treated mice versus all other groups. Activated NK cells were decreased in the combination group. T cells and dendritic cells (CD11c+MHCII+) were reduced in the SAHA groups. SAHA model withdrawal experiments show a significant synergistic response and immune system rebound after SAHA cessation. Conclusion: This data demonstrates that combination of reovirus plus SAHA therapy has significant activity in the treatment of SCCHN, even in an immunocompetent model. Immune inhibition due to SAHA as well as increased Jam1 receptor expression on tumor cells results in a synergistic effect of the combination therapy. Immune system rebound likely plays a significant role in the long-term anti-tumor response. This strong preclinical evidence supports the translation of this combination to phase-I clinical trials.