Abstract A08: Targeting the ATR/CHK1 axis in BRCA1/2 mutant ovarian cancer using an orthotopic patient-derived xenograft (PDX) model.

Introduction: Approximately 50% of high grade serous ovarian cancers (HGSOCs) have defects in genes involved in homologous recombination repair (HR). BRCA1/2 mutant HGSOCs have a deficiency in the repair of double strand DNA breaks by HR. Poly(ADP-ribose) polymerase inhibitors (PARPi) impair the repair of single-stranded breaks leading to double strand DNA breaks which cannot be repaired efficiently in BRCA-deficient cancers capitalizing on synthetic lethality. Given, PARPi only have a modest clinical response rate of ~40%, alternative strategies capitalizing on synthetic lethality are needed. The ATR/CHK1 signaling pathway is responsible for arresting cell cycle progression allowing repair of DNA damage. Targeting the ATR/CHK1 axis represents an alternative approach capitalizing on synthetic lethality in BRCA mutant HGSOCs. Experimental Procedures: PARP inhibitor (PARPi, Olaparib), CHK1 inhibitor (CHK1i, MK8776), and ATR inhibitor (ATRi, AZD6738) were evaluated in vitro and in vivo. PEO1 (BRCA2 mutant), PEO4 (BRCA1/ 2 wildtype) and JHOS4 (BRCA1 mutant) ovarian cancer cells were used for in vitro studies. Cell proliferation, survival, and genome stability were quantified and drug activity on cell signaling pathways was assessed. Using a BRCA2 deficient PDX model (8945delAA), the tumor was expanded via orthotopic transplantation onto the fallopian tube/ovary into 70 5-8 week female mice. Mice were randomized into 5 groups: untreated, carboplatin, PARPi, CHK1i,and ATRi. Once tumor volumes reached 70-100 mm3, treatment was initiated and tumor volume was assessed by ultrasound. Results: We have preliminary data that ATRi and CHK1i are as effective as PARPi in BRCA1/2 mutant ovarian cancer cells. pH2AX protein, a marker for DNA damage, is increased more so in the BRCA mutant cells compared to wildtype demonstrating selective activity in HR deficient cells capitalizing on synthetic lethality. pCHK1 accumulates with CHK1i (given the decrease in activation of downstream phosphatase PPA2) and is decreased with ATRi confirming drug inhibiting target. In a pilot study using a novel BRCA2 deficient patient derived xenograft (PDX) model, treatment with CHK1i and ATRi have similar anti-tumor effects to that of PARPi (olaparib) at 2 weeks. Reverse Phase Protein Array Analysis (RPPA) and target DNA sequencing results to assess signature of response are pending. Conclusions: Strategies to optimize approaches capitalizing on synthetic lethality are needed. BRCA mutant PDXs serve as novel experimental models for therapeutic studies. ATRi or CHK1i may be equally or more effective than PARPi as a primary treatment option for BRCA1/2-deficient cancers. Citation Format: Hyoung Kim, Erin George, Eric Brown, Rugang Zhang, Clemens Krepler, Janos Tanyi, Rachel Lee, Mark Morgan, Meenhard Herlyn, Fiona Simpkins. Targeting the ATR/CHK1 axis in BRCA1/2 mutant ovarian cancer using an orthotopic patient-derived xenograft (PDX) model. [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 A08.