BACKGROUND: We have previously demonstrated that gene deletion of the immunosuppressive enzyme IDO1 (indoleamine-2,3-dioxygenase 1) reduces but does not eliminate tumor burden in a mouse model of colitis associated cancer (CAC). In the current study, we test the hypothesis that a new highly specific IDO1 inhibitor epacadostat (Incyte Corp.) will synergize with cytotoxic radiation therapy (RT) as a novel treatment strategy for CAC. METHODS: CAC was modeled in vivo and in vitro using IDO1 expressing colorectal cancer (CRC) cell lines from mice (CT26, MC38) and humans (HT29, HCT116, and DLD1) cell lines, as well as normal human primary epithelial colonoids and normal colon and small intestinal (SI) cell lines (FHC and IEC6, respectively). Standard molecular biology assays were used to assay gene expression, proliferation (WST), cell death (LDH) and apoptosis (TUNEL/Caspase). CAC was modeled with heterotopic implants with treatment groups of control, epacadostat (300mg/kg/day), focal RT, and epacadostat+ focal RT (n=7/group). Tumor infiltrating lymphocytes (TIL) were examined by flow cytometry and cytokine multiplex assays. RESULTS: In Vitro and in vivo: RT enhanced IDO1 gene expression in CRC cells, but not in normal colon or SI cells. The combination of epacadostat and RT decreased CRC proliferation and colony formation (p<0.01) more than either monotherapy in vitro. In vivo, the combinationalso reduced the tumor growth and size than either treatment alone (Figure 1a, b). Impressively, unilateral RT also enhanced anti-tumor effect of epacadostat in the contralateral unirradiated tumor indicating an enhancement of the recently described abscopal effect. Consistent with this, TILs and cytokine profiles demonstrated enhanced anti-tumor immune environment. CONCLUSIONS AND IMPLICATIONS: These data identify a therapeutic synergy of IDO1 inhibition and RT in CAC/CRC. A Phase 1 clinical study examining this combination strategy is being initiated.
