Abstract 4125: Investigating determinants of sensitivity to ATR inhibition, as predictive biomarkers to single agent ATR inhibitor, VE-821, in ovarian cancer
Authors: Bradbury Alice Mary , Zenke Frank T. , Drew Yvette , Curtin Nicola J. 
Published: September 2020

Abstract
Introduction:
ATR is a key kinase in the DNA damage response (DDR) that signals replication stress to the S and G2/M checkpoints. Loss of G1 checkpoint control is almost ubiquitous in cancer cells, which, coupled with frequent activation of oncogenes makes them more reliant on their S and G2/M checkpoints. Therefore, ATR is an attractive anti-cancer target. Several determinants of sensitivity to ATR inhibitors (ATRi) have been identified in the literature, such as ATM, TP53, cyclin E and ARID1A. In high grade serous ovarian cancer, almost 100% of all tumours have lost TP53 function, around 20% have activation of cyclin E, and ARID1A mutations are found in nearly 50% of all clear cell ovarian cancers. Therefore, ATR inhibition may be a successful treatment strategy in ovarian cancer.
Experimental Procedures:
ATRi, VE-821, the preclinical candidate of VX-970 now known as M6620, was used throughout these experiments. The inhibition of ATR activity and the cytotoxicity of VE-821 was assessed in 13 ovarian cancer cell lines and related to the protein and mRNA expression of DDR proteins, oncogenes and other known determinants of sensitivity to ATRi. Growth inhibition by VE-821 was determined in patient derived primary cultures of malignant ovarian cancer ascites cells.
Results:
Ovarian cancer cell lines and patient samples display a range of sensitivities to VE-821. UWB1.289 cells lacking BRCA1 were the most sensitive to VE-821 (LC50 0.64 µM ± 0.06) and 1.5 times more sensitive than its BRCA1 corrected derivative, confirming previous reports of BRCA1 determining sensitivity to ATRi. NIH-OVCAR3 cells were the least sensitive to VE-821 (LC50 17.87 ± 1.86 µM) and CAOV3 were the second most sensitive to VE-821 (LC50 0.78 ± 0.55 µM), which did not correlate with ATR inhibition or expression of DDR proteins, oncogenes or other known determinants of sensitivity. There was a range of sensitivities to VE-821 in the 7 patient samples analysed to date, with the GI50 ranging from 0.78 µM to 29.08 µM, with no correlation between the growth rates of the sample to its sensitivity.
Conclusions:
These data show that determinants of sensitivity to ATRi, identified in isogenic cell line pairs, may not translate to the more complex molecular pathology of cancer cells where several determinants of sensitivity and resistance may be present simultaneously. More complex pathway analysis or functional readouts e.g. of replication stress, may be needed as predictive biomarkers for ATRi sensitivity.
Citation Format: Alice Mary Bradbury, Frank T. Zenke, Yvette Drew, Nicola J. Curtin. Investigating determinants of sensitivity to ATR inhibition, as predictive biomarkers to single agent ATR inhibitor, VE-821, in ovarian 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 4125.