In order to improve the therapeutic responses to immunotherapies in glioblastoma (GBM) patients, a known pharmaceutical, ritanserin, was repurposed to augment cytotoxic T cell responses and suppress tumor growth. To determine the kinetics of the immune response to GBM, the transplantable syngeneic glioma cell line GL261 was used in C57BL/6 mice to assess the tumor immune response in the brain, spleen, and cervical lymph node via flow cytometry weekly for one month. After identifying the optimal T cell response temporally, mice with established tumors were treated with 100mg/kg of Ritanserin for one week and harvested post treatment. Ritanserin is able to inhibit the activity of diacylglycerol kinase alpha (DGKA), an enzyme that converts diacylglycerol to phosphatidic acid, and can inhibit tumor growth in mice. In addition, DGKA is expressed in T cells, and is more highly expressed in those that have entered an unresponsive (anergic) state, characteristic of dysfunctional T cell responses to tumors. To determine if ritanserin can counteract T cell anergy, T cell responses were determined by assessment of T cell number by subset (CD8+, CD4+, CD4+FoxP3+), by activation (CD62L-CD44+), and by tumor-specific response to ovalbumin-expressing GL261 tumor cells in the brain, spleen, and cervical lymph nodes. In comparison to mock-injected mice, tumor-bearing mice showed a robust T cell response to tumors in the brain, peaking at days 21 and 28 post injection. Treatment with ritanserin resulted in a trend toward increased number and activation of T cells in the brain compared to vehicle-treated mice. In addition, there was a significant increase in OVA-specific T cells in the brain with treatment. In conclusion, T cells respond robustly to brain tumors in mice, and ritanserin treatment appears to increase this response by increased activation and priming of tumor-specific T cells. This indicates that ritanserin may have combinatorial effects on T cells when combined with checkpoint inhibitors such as ipilimumab (anti-CTLA-4) and nivolumab (anti-PD-1). Future experiments will focus on combining ritanserin and other repurposed drugs with these checkpoint inhibitors and further characterizing the effect of ritanserin on T cells.
Citation Format: Breanna R. Brenneman, Desiree H. Floyd, Tajie Harris, Benjamin Purow. Assessing and augmenting the immune response to glioblastoma using repurposed pharmaceuticals. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 542.