Evaluation of a Sumoylation Inhibitor as an Intrinsic and Extrinsic Radiosensitizer

Abstract

<h3>Purpose/Objective(s)</h3> Radiation therapy (RT) reduces tumor burden through both intrinsic initiation of tumor cell death and enhancement of anti-tumor immunity. However, radiation treatments are not universally efficacious due to variation in radiosensitivity and anti-tumor immunity. Low proliferative rate, hypoxia, and T cell effector dysfunction can significantly degrade response to RT. Here we evaluate a rational combination therapy pairing radiosensitization and immune activation in conjunction with standard of care RT. Recent studies demonstrated the sumoylation inhibitor, TAK-981, has intrinsic capacity to reduce the proliferation of tumor cells <i>in vitro</i> as a monotherapy and decrease tumor volumes in syngeneic murine colon carcinoma models (CT26 and MC38) in combination with PD-1 or CTLA-4 checkpoint blockade. In these models a dendritic cell dependent type I interferon pathway was implicated to mediate the anti-tumor immune response and enhanced in vivo tumor control in TAK-981 and checkpoint blockade combination therapy, similar to previously elucidated mechanisms of immune activation following hypofractionated radiotherapy. Additionally, multiple intrinsic and extrinsic protein mediators (Myc, IRF3, p21) of tumor cell response to radiation are regulated by the post-translational modification of sumoylation. Consequently, we evaluated whether combining RT with TAK-981 resulted in enhanced tumor radiosensitivity in vitro and improved local tumor control in vivo, consistent with enhanced anti-tumor immune response in murine tumor models. <h3>Materials/Methods</h3> We utilized a longitudinal real time microscopic proliferation assay to evaluate RT response. Titrations of TAK-981 were added to established in vitro tumor cell cultures 4 hours prior to increasing doses of single fraction photon RT. Impacts of RT, TAK-981 or the combination treatment were assessed based upon cell proliferation for 72 hours. We assessed efficacy of this novel combination in vivo using the MC38 colorectal carcinoma model. C57BL6 mice with established tumors (> 50mm2) received TAK981 (15mg/kg) alone three times weekly or in combination with single fraction 8 Gy RT and compared to RT alone to evaluate local tumor control and impacts on overall survival. <h3>Results</h3> Our data demonstrates a synergistic and dose dependent reduction in MC38 colon carcinoma in vitro proliferation in response to combination RT and TAK-981. In vivo MC38 tumor models demonstrate no effect of TAK-981 alone compared to vehicle treated mice while co-administration with a single 8 Gy fraction of radiotherapy synergistically enhanced tumor control and overall survival compared to RT alone. <h3>Conclusion</h3> Our preliminary data support potential dual synergistic roles for the sumoylation inhibitor TAK-981 with radiotherapy, functioning as both an intrinsic radiosensitizer and immunomodulator.