Abstract A09: Targeting chemoresistant ovarian malignancies to cytostatic drugs and PARP inhibitors by inhibition of CDK12 kinase activity.

Ovarian cancer ranks fifth in cancer deaths among women in the USA. After surgery, women are subjected to a combined treatment with platinum-based drugs (cisplatin, carboplatin) and a taxane (Paclitaxel, Docetaxel). Despite tremendous advances in therapy, initial response to therapy is around 70%, tumor recurrences still occur in the majority of ovarian cancer patients converging to chemoresistant disease. Once patients develop resistant disease, the options for effective salvage treatment are limited; therefore, it is critical to identify and understand molecular mechanisms and biological pathways orchestrating such a resistance. As a result of such a need, new therapy aimed to inhibit poly (ADP-ribose) polymerase (PARP) activity has emerged, and specific PARP inhibitors are currently being tested in clinical trials. Recently, we have identified cyclin-dependent kinase 12 (CDK12) as a critical cellular factor corrupting DDR, including HR, due to aberrant expression of key DDR genes, such as BRCA1, ATM, ATR, FANCI, FANCD2. When endogenous CDK12 was down-regulated by siRNA approach, we detected increased DNA damage after exposition to DNA damaging agents (Camptothecin, Etoposide, Mitomycin C). In addition, the active role of CDK12 in the DDR pathway was demonstrated by elevated activation of gamma-H2AX and p53BP1 foci. Most importantly, two groups independently identified that CDK12 silencing in several ovarian cell lines and two HGSOC models caused sensitivity to PARP inhibitors and the replication fork stalling agent cisplatin. Thus, suppression of several key DDR genes provides a possible mechanistic explanation for the HR dysfunction and PARP1/2 inhibitor sensitivity seen in CDK12-defective cells. We have detected high level of CDK12 in several primary tumors. At the moment, we are generating ovarian cell lines with potential to chemically inhibit CDK12 activity by synthetic ATP-analog in order to test if targeting CDK12 will sensitize given cell lines to cytostatic drugs and PARP inhibitors. We believe that identification of small molecules affecting DDR pathways is a promising strategy to overcome cisplatin and PARP resistance. Since CDK12 seems to be indispensable for proper DDR pathway, we propose to incorporate inhibition of CKD12 by small chemical inhibitors as a promising strategy to overcome PARP and cisplatin chemoresistance in patients. The project is supported by the grant from the Internal Grant Agency of the Ministry of Health, NT14599-3/2013 and the Ministry of Agriculture RO 0515 (MZE002716202). Citation Format: Jiri Kohoutek, Marek Svoboda, David Vrabel, Hana Paculova, Marta Dzimkova. Targeting chemoresistant ovarian malignancies to cytostatic drugs and PARP inhibitors by inhibition of CDK12 kinase activity. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Ovarian Cancer Research: Exploiting Vulnerabilities; Oct 17-20, 2015; Orlando, FL. Philadelphia (PA): AACR; Clin Cancer Res 2016;22(2 Suppl):Abstract nr A09.