The radiosensitizing effects of monopolar spindle kinase I in syngeneic models of triple negative breast cancer and its implications on the tumor immune microenvironment

Abstract Triple negative breast cancer (TNBC) is an aggressive breast cancer subset with poor outcomes. TNBCs do not express the estrogen, progesterone, or HER2 receptors and are thus not candidates for antiestrogen and anti-HER2 therapies, leaving few effective treatments. One therapeutic strategy is to target molecular components of an individual patient’s cancer. Prior work in our group has nominated monopolar spindle kinase I (TTK) as a gene that is upregulated in cancerous breast tissue compared to healthy tissue and correlates with locoregional recurrence following radiation in breast cancer patients. However, the effects of TTK inhibition and radiation on the immune system is not understood. We hypothesize that the TTK inhibitor empesertib radiosensitizes murine TNBC models to radiation in vitro and in vivo and modulates the immune tumor microenvironment. Single agent empesertib inhibited the growth of murine TNBC cells with IC 50values up to 30nM. Sub-IC 50values of TTK inhibitor radiosensitized 4T1 (radiation enhancement ratio or rER: 2.4 ± 0.4) and Py8119 (rER: 1.5 ± 0.1) cells. TTK knockdown (via shRNA) also resulted in changes in radiosensitization in vitro. Furthermore, we also observed a similar phenotype in vivo. In our 4T1 in vivo model, mice receiving combined treatment had decreased tumor growth compared to mice that receiving single-agent therapies or vehicle (****p<0.0001). Quantities of monocyte derived suppressor cells were altered with radiation and TTK inhibition. This combined therapy suggests that changes in the underlying immune mechanisms due to TTK inhibition and radiation are important in TNBC. Future work will examine the mechanisms of TTK inhibition and radiation on systemic and tumoral immune changes.