Abstract 4753: Attenuating colorectal cancer cell growth and enhancing immunogenicity through rational combination of MEK and HDAC inhibition

Background: Studies have shown that MEK inhibitors (MEKi) can reduce tumorigenic growth signaling in colorectal cancer (CRC) through inhibition of the RAS pathway which is upregulated in over 40% of CRC tumors, while HDAC inhibitors (HDACi) lead to cell cycle arrest, inhibit angiogenesis, and induce apoptosis in neoplastic cells. In addition to a direct effect on neoplastic cells, MEKi have also demonstrated the potential to increase immune response to the tumor through upregulation of MHC I on tumor cells, induction of intra-tumoral T-cell infiltration, and enhancement of anti-PDL1 activity. HDAC inhibitors lead to epigenetic alterations which have similarly shown an ability to increase tumor immunogenicity, leading to the hypothesis that these agents may be combined with immunotherapy treatments for enhanced anti-tumor effect. In this study we first analyzed the effect of MEKi and HDACi combination treatment in both resistant and sensitive CRC cell lines to assess for synergy in the attenuation of cancer cell proliferation. We then demonstrated the effect of combination MEKi and HDACi therapy on tumor immunogenicity through increased expression of tumor immune markers. Methods: We assessed the efficacy of a novel MEKi agent, HL-085, in comparison with trametinib in six different CRC cell lines. Treatment with each agent was separately administered with a maximum concentration of 10 uM. Similarly, five different HDAC inhibitors were tested in each of these cell lines, with a maximum concentration of 1 uM. Optimal combination of MEK inhibitor HL085 and HDAC inhibitor class I/II (OKI-005) were then chosen based on this data. Cellular proliferation was assessed after 72 hours with Cell Titer Glo. Flow cytometry of a panel of immunogenic markers was then performed alongside a viability stain to assess for expression level changes of MCH class I in CRC cells. Results: Combinatorial effects of dual treatment with MEK inhibitor HL085 and HDAC inhibitor OKI-005 are observed in both resistant and sensitive cell lines with synergistic effect produced in the SW620 and Lovo cell lines (MEKi sensitive) as well as the GP2D (MEKi resistant) cell line. No synergistic effect was observed in the HCT15, DLD1 and LS513 cell lines. Flow cytometry showed an increase in MHC class I expression in neoplastic cells with combined MEKi and HDACi treatment compared to individual agent treatment and control, demonstrating increased immunogenicity of CRC cells with combination treatment. Conclusions: Combination treatment with a novel MEK inhibitor HL085 and the HDAC inhibitor OKI-005 demonstrates synergy in three out of six colorectal cancer cell lines. We further show increased immunogenicity of CRC cells through enhanced expression of MCH I on tumor cell surface, indicating rationale for further investigation of combination MEKi and HDACi with immunotherapy in the treatment of metastatic colorectal cancer. Citation Format: Mihailo Miljanic, Nisha Holay, Anna Capasso, Todd Triplett, Milad Soleimani, Uma Giri, Carla Van Den Berg, Gail Eckhardt. Attenuating colorectal cancer cell growth and enhancing immunogenicity through rational combination of MEK and HDAC inhibition [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 4753.