Abstract 4835: Targeting the ATR/CHK1 axis in combination with PARP inhibition is more effective than PARP inhibition alone in BRCA mutant models
Authors: George Erin , Kim Hyoung , Tanyi Janos , Ragland Ryan , Zhang Rugang , Bradford Patricia , Krepler Clemens , Nathanson Katherine , Wenz Brandon , Lu Yiling , Mills Gordon , Morgan Mark , Simpkins Fiona 
Published: July 2016

Abstract
Introduction:
Approximately 50% of high grade serous ovarian cancers (HGSOC) 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) block 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. PARPi have a modest clinical response of only ∼40% in recurrent BRCA mutant HGSOCs. We hypothesize that PARPi alone increases reliance on other DNA repair pathways such as ATR/CHK1 in HR deficient cells, and by targeting ATR/CHK1 in combination with PARPi would be more effective in eradicating tumor growth.
Experimental Procedures:
Effects of PARP inhibitor (PARPi, Olaparib), CHK1 inhibitor (CHK1i, MK8776), and ATR inhibitor (ATRi, AZD6738) on cell cycle, survival, colony formation, genome stability were evaluated in PEO1 (BRCA2 mutant), PEO4 (BRCA wild-type), JHOS4 (BRCA1 mutant), and WO-24 (BRCA wild-type) ovarian cancer cells. A BRCA2 mutant (8945delAA) orthotopic PDX model was used to evaluate PARPi alone or in combination with CHK1/ATRi. Targeted capture massively parallel sequencing, Reverse-Phase Protein Array Analysis (RPPA) and IHC were performed on cells and xenografts to evaluate for biomarkers of response.
Results:
Monotherapy with PARPi, CHK1i, and ATRi in vitro demonstrated selectivity in mediating cell death and DNA damage in BRCA1/2 mutant cell lines (PEO1, JHOS4) compared to BRCA1/2 wild-type, platinum resistant cell lines (PEO4, WO-24). However, monotherapy only results in ∼40-50% cell death in BRCA1/2 mutant cell lines. PARPi alone resulted in tumor suppression but not tumor eradication in a BRCA2 mutant PDX model. PARPi treatment resulted in an increase in ATR/CHK1 signaling in BRCA1/2 mutant cells. Treatment with ATR/CHK1i in combination with PARPi is synergistic in reducing survival of BRCA1/2 cells. Combination treatment was more effective in targeting cell cycle mediators, and promoting apoptosis. Treatment with either PARPi+ATRi or PARPi+CHK1i combinations was synergistic in causing tumor suppression but PARPi/ATRi combination caused tumor regression in a BRCA2 mutant PDX model.
Conclusions:
Strategies to optimize approaches capitalizing on synthetic lethality are needed for HR deficient HGSOC. PARPi is effective in BRCA deficient cancers but can potentially be more effective when combined with ATR/CHK1i.
Citation Format: Erin George, Hyoung Kim, Janos Tanyi, Ryan Ragland, Rugang Zhang, Patricia Bradford, Clemens Krepler, Katherine Nathanson, Brandon Wenz, Yiling Lu, Gordon Mills, Mark Morgan, Fiona Simpkins. Targeting the ATR/CHK1 axis in combination with PARP inhibition is more effective than PARP inhibition alone in BRCA mutant models. [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 4835.