Abstract PR02: Exploring the effects of PARP inhibition on CHK1 activation as a potential determinant of synergy with CHK1 inhibition
Authors: Wielgos-Bonvallet Monicka , Cybulska Paulina , Oudenhove Elke Van , Jelinic Petar , Levine Douglas A. 
Published: August 2018

Abstract
PARP inhibitors (PARPi) have improved the outcomes of patients with high-grade serous ovarian carcinomas (HGSOC) whose tumors harbor mutations in homologous recombination genes such BRCA1 and BRCA2. However, only 20% of cases with this aggressive subtype of ovarian cancer benefit from PARPi, thus highlighting the essential need to identify novel therapeutic strategies for the remainder of these patients. We have recently reported that off-target effects vary among two PARP inhibitors with different trapping potencies. The FDA-approved PARPi, Olaparib, activated the checkpoint kinase 1 (CHK1) protein at serine 345 and arrested cells in the G2/M phase; however, these effects were not observed after treatment with Veliparib, the least potent PARPi measured by the PARP trapping ability.
In this study, we treated HGSOC cell lines with the five most clinically advanced PARP inhibitors and investigated the effect of varying PARP trapping potencies on the expression levels of major proteins that are involved in cell cycle pathways via Western blot and cell cycle analysis. The synergy between the most potent (Talazoparib) or least potent PARP trappers and a CHK1 inhibitor was also explored in HGSOC patient-derived xenograft (PDX) models. We hypothesize that PARP inhibitors that do not have an effect on the cell cycle will have greater synergy with CHK1 inhibitors. Our results reveal that the four most potent PARP trappers—olaparib, rucaparib, niraparib, and talazoparib--activate CHK1 protein and decrease total protein levels of CHK1’s downstream substrate CDC25C. However, we did not observe any significant changes in expression levels in these two proteins after treatment with veliparib. We also observed an arrest in the S/G2 phase of the cell cycle after treatment with the four most potent PARP inhibitors. Finally, the growth of our PDX model was significantly inhibited by combined treatment with veliparib and the CHK1 inhibitor MK-8776 compared to either single-agent therapy or talazoparib combined with MK-8776. These results suggest that there is a distinct off-target effect that differs among the least and most potent PARP inhibitors, with veliparib being more synergistic with CHK1 inhibition. This study will help to expand the use of PARP inhibitors in combination with CHK1 inhibitors and emphasizes the importance of single-agent off-target effects to develop optimal combination therapies.
This abstract is also being presented as Poster A11.
Citation Format: Monicka Wielgos-Bonvallet, Paulina Cybulska, Elke Van Oudenhove, Petar Jelinic, Douglas A. Levine. Exploring the effects of PARP inhibition on CHK1 activation as a potential determinant of synergy with CHK1 inhibition. [abstract]. In: Proceedings of the AACR Conference: Addressing Critical Questions in Ovarian Cancer Research and Treatment; Oct 1-4, 2017; Pittsburgh, PA. Philadelphia (PA): AACR; Clin Cancer Res 2018;24(15_Suppl):Abstract nr PR02.