Abstract NT-110: EFFICACY OF NOVEL TOPOISOMERASE 1 INHIBITORS AND THEIR SYNERGY WITH PARP INHIBITORS IN A MURINE MODEL FOR OVARIAN CANCER

Camptothecin derivatives topotecan and irinotecan are currently the only FDA-approved topoisomerase I (Top1) inhibitors. Despite their wide usage they have many drawbacks: chemical instability, short half-life, dose-limiting bone marrow and gastrointestinal toxicity, and they are drug efflux substrates. The novel non-camptothecin Top1 inhibitors, indenoisoquinolines (LMP400, LMP776 and LMP744), have been developed to overcome these limitations. We explored whether treatment with LMPs could induce “synthetic lethality” in cells with impaired homologous recombination (HR) repair mechanism and whether it could be enhanced by combining them with PARP inhibitor, olaparib. Using the DT40 cell line, we assessed in vitro the role of HR in the cellular responses to the LMPs. We found that BRCA1-, BRCA2- and PALB2-deficient cells are 3 to 5 times more sensitive to the LMP compounds than wild type cells. Moreover, combination treatments showed a significant synergy between each of the three LMPs and olaparib. To confirm the differential response of Brca1 wild type or Brca1 deficient cells to LMP treatment we assessed compound potency in primary ovarian cancer cell lines derived from a genetically engineered mouse (GEM) model for serous epithelial ovarian cancer (SEOC). As in DT40 cells, we observed that murine Brca1 deficient cells were more sensitive to all 3 LMP compounds than Brca1 wild type cells. Our results suggest that Brca1 deficiency renders cells more sensitive to treatment with LMPs or combination of LMPs and olaparib, potentially directing selection of patients with impaired HR repair mechanism in the phase 2 clinical trials that are in preparation. To confirm this observation in vivo, we are performing efficacy studies using LMP compounds alone or in combination with olaparib in orthotopic allograft mouse models for ovarian cancer derived from the GEM model. In Brca1 deficient tumors, treatment with each of the 3 LMP compounds led to significant increase of survival compared to vehicle treated group. LMP400 had the greatest beneficial impact on survival and tumor regression. To confirm synergy between LMPs and olaparib we selected LMP400 to represent the LMPs and repeated the efficacy study using Brca1 deficient tumor model. We observed that suboptimal doses of LMP400 or olaparib used as single treatments resulted in some increase in survival, however, when combined they significantly increased survival due to tumor regression. Our data demonstrate that the LMPs synergize with olaparib and provide a rationale for personalized treatment and Phase 2 clinical trials with the indenoisoquinolines in combination with PARP inhibitors in HR-deficient ovarian cancers. Citation Format: Ludmila Szabova, Melanie Gordon, Laetitia Matzi, Yves Pommier, Zoe Weaver Ohler. EFFICACY OF NOVEL TOPOISOMERASE 1 INHIBITORS AND THEIR SYNERGY WITH PARP INHIBITORS IN A MURINE MODEL FOR OVARIAN CANCER [abstract]. In: Proceedings of the 12th Biennial Ovarian Cancer Research Symposium; Sep 13-15, 2018; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2019;25(22 Suppl):Abstract nr NT-110.