Abstract 324: Targeting RNA-polymerase I using CX-5461 as a mechanism for treating chemotherapy resistant epithelial ovarian cancer
Authors: Cornelison Robert , Dobbin Zachary C. , Katre Ashwini A. , Jeong Dae Hoon , Petrova Yulia , Llaneza Danielle C. , Steg Adam D. , Zhang Yinfeng , Schneider David A. , Landen Charles N. 
Published: July 2016

Abstract
BACKGROUND: One of the first identified hallmarks of neoplastic transformation was an increased rate of ribosome biogenesis, and increased activity of RNA polymerase I (RNA pol I) is observed in most human cancers. Ribosome biogenesis is an energy intensive and tightly regulated process, and recent studies have shown that CX-5461, a potent and specific RNA pol I inhibitor, has selective toxicity to cancer cells. Here we demonstrate that CX-5461 is an effective anti-proliferative agent in epithelial ovarian carcinoma cells with an increased efficacy in chemoresistant ovarian cancer cells compared to their sensitive parental lines.
METHODS AND RESULTS: Ovarian cancer cells were demonstrated to be highly sensitive to RNA pol I inhibition by MTT assay, with an IC50 range from 25nM to 2uM in 13 different ovarian cancer cell lines, and an IC50 of 3uM in the nontransformed immortalized normal surface epithelial line HIO-180. Notably, chemoresistant cell populations were generally more sensitive to CX-5461, compared to matched chemosensitive lines. IC50 in HeyA8MDR was 80nM compared to 2000nM in parental HeyA8, and was 100nM in SKOV3TRip2 compared to 400nM in SKOV3ip1. CX-5461 induces G2/M arrest (averaging 52.7% in G2 compared to 32.5% in controls), primarily through mitotic catastrophe, inducing an accumulation of multinucleated senescent cells. DNA damage pathways are activated, operating through ATM/ATR, as demonstrated by rescue with caffeine and ATR specific inhibitors. In an effort to identify why chemoresistant cells might be more susceptible, ribosomal synthesis processes were examined. Chemoresistant cells were found to have 2-4-fold increased Pol I occupancy on rDNA by ChIP, and increased rRNA synthesis by isotopic incorporation, while ribosome abundance and overall translation efficiency, measured by sucrose gradient fractionation, were unchanged. Treatment of five separate patient-derived xenograft models (PDX) with CX-5461 showed a complete response in one model, partial 55% reduction in tumor size in a second, stable disease in a 3rd, and progression in two models.
CONCLUSIONS: CX-5461 shows high activity in ovarian cancer cell lines and patient-derived xenograft models, with a preferential effect on chemoresistant cells. Enhanced sensitivity to the compound is due at least in part to cancer cells’ increased demand for rRNA synthesis. RNA pol I inhibitors represent an exciting new treatment option that may have preferential effect on the chemoresistant population.
Citation Format: Robert Cornelison, Zachary C. Dobbin, Ashwini A. Katre, Dae Hoon Jeong, Yulia Petrova, Danielle C. Llaneza, Adam D. Steg, Yinfeng Zhang, David A. Schneider, Charles N. Landen. Targeting RNA-polymerase I using CX-5461 as a mechanism for treating chemotherapy resistant epithelial ovarian cancer. [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 324.