Abstract 1051: Targeting replication stress pathway provides an avenue for novel combination therapy options including TTfields plus chemo agents which increase replication stress
Authors: Karanam Narasimha Kumar , Story Michael Dean 
Published: July 2021

Abstract
Tumor Treating Fields (TTFields) is a new physical therapeutic modality, which has been FDA approved for the treatment of recurrent glioblastoma (GBM) as monotherapy, for newly diagnosed GBM in combination with temozolomide, and for unresectable locally advanced or metastatic malignant pleural mesothelioma (MPM) in combination with pemetrexed and a platinum-based chemotherapy. Clinical trials are ongoing for other cancers, including lung, pancreatic, and ovarian cancers. TTFields are low-intensity, intermediate frequency, alternating electric fields which are loco-regionally applied to tumor sites using non-invasive arrays. The initial mechanism by which TTFields was thought to kill tumor cells was the disruption of mitosis. Using gene expression analysis and functional characterization studies, we discovered a novel role of TTFields in DNA damage repair and replication stress pathways. TTFields treatment decreases Fanconi Anemia (FA) pathway signaling proteins thereby impairs ionizing radiation (IR)-induced DNA damage repair processes. The length of newly replicated DNA slowed as a function of TTFields exposure time and that TTFields increased R-loop formation, which indicates that TTFields induced replication stress. Hence, we hypothesized TTFields increase sensitivity of chemotherapy agents that target and increase replication stress in novel combination therapy options. PARP1 protects DNA breaks, by recruiting DNA repair and checkpoint proteins to the sites of damage and recruiting MRE11 for DNA end processing (required for replication restart and enhanced Chk1 activation). Indeed, TTFields treatment concomitant with the PARP1 inhibitor olaparib followed by IR was found to be synergistic compared to IR or olaparib alone or in combination. However, the degree of sensitization and synergy varied across non-small cell lung cancer cell lines. TTFields increased cell killing efficacy of etoposide synergistically, which forms a ternary complex with topoisomerase II and prevents re-ligation of DNA strands to elicit DNA strand breaks and induce replication stress. The advantage of combining TTFields with AZD6738 (an ataxia telangiectasia and Rad3-related protein (ATR) inhibitor) was tested. ATR is an essential kinase regulator of the replication checkpoint that plays a critical role in safeguarding genome integrity from replication stress. Another novel combination of TTFields together with irinotecan, which traps topoisomerase I- DNA in ternary cleavage complex and inhibits initial cleavage reaction and re-ligation steps to increase replication stress, is also tested. Taken together, our results suggest that different chemo agents that target replication stress pathways can be used in combination with TTFields for cancer therapy, which need to be tested in in-vivo studies and clinical trials.
Citation Format: Narasimha Kumar Karanam, Michael Dean Story. Targeting replication stress pathway provides an avenue for novel combination therapy options including TTfields plus chemo agents which increase replication stress [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1051.