Abstract 1743: Evaluation of the effects of combined chemical inhibition of PARP and BET proteins in preclinical models of urothelial bladder cancer

Abstract

Abstract Purpose: Metastatic urothelial bladder cancer (mUC) is incurable, and options are limited after patients fail 1st and 2nd line treatments. Thus, there is an unmet need to develop novel therapeutic strategies that increase survival in mUC patients. According to data from The Cancer Genome Atlas, UC has the third highest rate of somatic mutations, and mutations in DNA damage repair (DDR) genes are common. This provides rationale for the use of PARP inhibitors in UC patients with DDR mutations. Epigenetic changes are also common in UC, so epigenetic regulator inhibitors in combination with PARP inhibitors could be a viable treatment strategy in mUC. The purpose of this study was to evaluate the effects of combined olaparib plus an epigenetic regulator inhibitor in preclinical models of UC. Methods: BRCA1-competent 5637 cells and BRCA1-mutant J82 cells were screened against 62 compounds in the UNC EpiG Diamond library to identify epigenetic regulator inhibitors that could have synergy with olaparib. Cells were treated with 5 ascending concentrations of each compound (10 nM–10 µM), and cell viability was measured by alamarBlue™. RT-qPCR and western blotting were used to evaluate the effects of birabresib (OTX-015) on MYC and BRCA1 gene and c-MYC and BRCA1 protein expression, respectively. To evaluate birabresib effects on cell cycle, flow cytometry was performed. To evaluate cell viability, cells were treated with 8 ascending concentrations of olaparib and birabresib (0.1nM–100uM), alone and in combination, and then treated with CellTitrGlo™. IC50 values were calculated using a four-parameter non-linear regression model, and synergy estimates were derived by Compusyn software. Results: The EpiG screen revealed that as a class BET inhibitors potently limited cell viability in 5637 and J82 cells. Birabresib was particularly potent (IC50 in 5637 cells=100nM and in J82 cells=330nM; n=2), and selected for further evaluation. After 24h, birabresib significantly inhibited both MYC and BRCA1 expression (P<0.001; n=3), and after 48h repressed c-MYC and BRCA1 protein expression. After 48h of birabresib plus olaparib, G2/M phase arrest increased by ≥20% in both lines (P<0.001; n=2). The combination of olaparib plus birabresib also appears to have a synergistic effect on 5637 and J82 viability (n=3). Conclusion: Birabresib represses MYC/c-MYC and BRCA1/BRCA1 expression. Olaparib plus birabresib induces cell cycle arrest, and appears to be synergistic. This provides rationale supporting additional lines of prospective inquiry to evaluate olaparib plus birabresib in UC. Citation Format: Kaitlyn A. Maffuid, Chad D. Torrice, Brian Hardy, William A. Murphy, Lindsey I. James, William Y. Kim, Stephen V. Frye, Kenneth J. Pearce, Daniel J. Crona. Evaluation of the effects of combined chemical inhibition of PARP and BET proteins in preclinical models of urothelial bladder cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1743.