Abstract 4002: Enhancing the antitumor efficacy of immunotherapy by using the topoisomerase I inhibitor MM398
Authors: McKenzie Jodi A. , Mbofung Rina M. , Malu Shruti , Amaria Rodabe N. , Davis Richard E. , Zhang Li , Tieu Trang N. , Heffernan Tim P. , Hwu Patrick 
Published: July 2016

Abstract
Melanoma is a highly aggressive form of skin cancer, whose rates of morbidity and mortality are increasing. The development of immunotherapies like anti-PDL1 and anti-CTLA4 antibodies has resulted in fundamental advances in the treatment of some cancers. However, long lasting responses are only observed in a subset of immunotherapy-treated patients. This shortfall highlights the need for a better understanding of the molecular mechanisms that govern tumor response to immunotherapy.
To address this need, autologous patient-derived tumor cell lines and tumor infiltrating lymphocytes (TILs) were utilized in an in vitro high throughput screen, to identify compounds that increase the sensitivity of melanoma cells to T cell mediated cytotoxicity. The screen consisted of an 850 compound library. One group of compounds that was most able to enhance T cell killing of melanoma cells was topoisomerase I (Top1) inhibitors such as topotecan and irinotecan.
Our results indicate that treatment of melanoma cells with a Top1 inhibitor prior to exposure to autologous T cells produced a synergistic increase in tumor cell death, as measured by intracellular staining of activated caspase 3. We have also recapitulated this finding in an in vivo model, where a better anti-tumor effect was observed in tumor bearing mice treated with an antibody against the co-inhibitory molecule PDL1 in combination with MM398 (nanoliposomal irinotecan), than in cohorts treated with either α-PDL1 or Top1 inhibitor alone. These findings suggest synergism between Top1 inhibitors and immune-based therapies in the treatment of melanoma.
Molecular changes elicited by inhibition of Top1 are now being investigated to identify the factors that mediate the effect of Top1 inhibitors on T cell-mediated killing of melanoma. We have identified a p53-driven gene signature in Top1 inhibitor-treated melanoma cell lines and are investigating the functional relevance of Tumor Protein p53 Inducible Nuclear Protein 1 (TP53INP1) in mediating increased T cell killing of Top1 inhibitor-treated melanoma cells. Our results indicate that TP53INP1 is a critical component of this apoptotic response, as overexpression of TP53INP1 in melanoma cells increased their susceptibility to T cell mediated cytotoxicity. Complementary to this observation, we have also found that knockdown of TP53INP1 by shRNA, impedes the sensitivity of Top1 inhibitor-treated melanoma cells to T cell mediated killing.
Understanding how Top1 inhibitors enhance melanoma killing by immunotherapy will allow for the development of predictive biomarkers, and also augment immune-based therapeutic strategies to ensure durable responses in a larger population of melanoma patients. By using melanoma as a model disease system, we can gain valuable insights into the dynamics of cancer immune response that may be applied to other cancers where effective treatment strategies are also lacking.
Citation Format: Jodi A. McKenzie, Rina M. Mbofung, Shruti Malu, Rodabe N. Amaria, Richard E. Davis, Li Zhang, Trang N. Tieu, Tim P. Heffernan, Patrick Hwu. Enhancing the antitumor efficacy of immunotherapy by using the topoisomerase I inhibitor MM398. [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 4002.