Abstract 19: Hypomethylating agent, SGI-110, alters the immunosuppressive milieu in acute myeloid leukemia (AML) and enhances the immunogenicity of a dendritic cell/AML fusion vaccine

Acute myeloid leukemia (AML) is associated with an immunosuppressive tumor microenvironment (TME) that prevents immune recognition and fosters disease growth. In an attempt to reverse this immune suppression, we developed a personalized vaccine in which patient-derived AML cells are fused with autologous dendritic cells (DCs) such that a broad array of leukemic antigens (Ags) is presented in the context of DC-mediated costimulation. We have examined strategies to enhance vaccine efficacy. In this regard, we previously demonstrated that the hypomethylating agent (HMA), SGI-110, augments leukemic Ag processing and increases AML cell susceptibility to T cell-mediated killing. In the present study, we investigated if SGI-110 enhances leukemic Ag presentation by measuring the binding capacity of a unique T-cell receptor-like (TCRL) antibody (Ab) to AML cells in vitro. The TCRL Ab is a TCR mimic that recognizes the PR1-HLA-A2-MHC complex on the surface of leukemic cells where PR-1 is a known leukemic Ag. Following exposure of SGI-110 to the AML cell line, THP-1 which endogenously expresses HLA-A2, we noted enhanced tumor Ag presentation and HLA-A2 upregulation via FACS analysis. Aside from facilitating Ag presentation, we investigated if SGI-110 affects myeloid-derived suppressor cell (MDSC) burden in the TME where MDSCs have immunosuppressive capabilities. In an in vitro assay where healthy donor-derived peripheral blood mononuclear cells were cocultured with 2 AML cell lines, SGI-110 treatment led to a significant decrease in MDSC expansion compared to control. Based on these in vitro data, we investigated the ability of SGI-110 to reverse critical aspects of the immunosuppressive milieu in AML and enhance vaccine efficacy in vivo. C57BL/6J mice were retro-orbitally inoculated with 1x10 5 syngeneic TIB-49 AML cells and treated with low-dose SGI-110 (1 mg/kg) to assess immunologic effects while not preventing engraftment. AML engraftment in the spleen and bone marrow (BM) was similarly observed in treated and control animals. However, SGI-110-treated animals showed decreased PD-1 expression on CD4 T cell splenocytes (mean 3.1% vs. 6.3%; n=4; p=0.01). Similarly, SGI-110 therapy was associated with a decrease in MDSC burden in the BM (n=5; p=0.01) and spleen (n=4; p=0.03) compared to control mice. Consistent with these findings, SGI-110-treated mice demonstrated increased AML-specific immunity as manifested by increased T cells isolated from the BM or spleen that expressed intracellular IFNγ following ex vivo exposure to TIB-49 lysate (mean 3% vs. 0.5%; n=5; p=0.02). We subsequently examined the potential synergy of SGI-110 plus the vaccine in this model. Mice treated with SGI-110 + vaccine showed a significant increase in BM and spleen CD4 (n=5; p=0.05) and CD8 (n=5; p=0.02) T cell IFNγ in response to tumor lysate as compared to vaccine or SGI-110 alone. Given these effects, we sought to examine whether treatment with the vaccine and SGI-110 would enhance survival in this model. Mice were treated with vehicle, SGI-110 X 5 days, vaccine, or both agents. Monotherapy with either agent led to a statistically significant survival advantage as compared to untreated mice; yet the combination arm further prolonged survival as compared to either monotherapy arm. In conclusion, SGI-110 enhances Ag presentation and modulates the AML-associated immunosuppressive milieu by decreasing both T-cell PD-1 expression and MDSC burden with a notable increase in AML-specific immunity. Moreover, SGI-110 increases the immunogenicity of the DC/AML vaccine through an enhanced AML-specific response with a survival benefit in a murine model. This combination strategy has promising translational potential and a phase 1 clinical trial with this synergistic approach is planned. Citation Format: Myrna Rita Nahas, Dina Stroopinksy, Anna Sergeeva, Athalia Pyzer, Abigail Washington, Leandra Cole, Salvia Jain, Malgorzata McMasters, James D. Levine, Rebecca Karp Leaf, Shira Orr, Matthew Weinstock, Jeffrey J. Molldrem, David Avigan, Jacalyn Rosenblatt. Hypomethylating agent, SGI-110, alters the immunosuppressive milieu in acute myeloid leukemia (AML) and enhances the immunogenicity of a dendritic cell/AML fusion vaccine [abstract]. In: Proceedings of the Second AACR Conference on Hematologic Malignancies: Translating Discoveries to Novel Therapies; May 6-9, 2017; Boston, MA. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(24_Suppl):Abstract nr 19.