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Abstract 337: The PARP inhibitor olaparib is synergistic with the ATR inhibitor AZD6738 in ATM deficient cancer cells

The poly(ADP-ribose polymerase) (PARP) inhibitor olaparib is FDA approved for the treatment of BRCA-mutated ovarian cancers. Olaparib inhibits PARP function during DNA single-strand-break repair and also by trapping of PARP on DNA to create lesions. These complexes cause replication-associated DNA damage including stalling and collapse of DNA replication forks. Cells which have lost BRCA1/2-dependent homologous recombination repair are highly sensitive to olaparib. In addition, other DNA damage response (DDR) pathways mediated by the ataxia telangiectasia mutated (ATM) and ataxia telangiectasia mutated and Rad3-related (ATR) kinases are hypothesised to be important survival pathways in response to PARP inhibitor treatment. Here we show that olaparib combines synergistically with the orally bioavailable ATR inhibitor AZD6738, in vitro, leading to cell death in ATM-deficient backgrounds. The growth inhibitory and cell kill effect of a range of concentrations of AZD6738 (0.01-1 µM) and olaparib (0.03-3 µM) were assessed using the Loewe model of additivity in the isogenic ATM knockout (KO) and wildtype (WT) FaDu head and neck cancer cell lines. The combination of AZD6738 and olaparib induced cell death in the ATM KO cells across a wide range of concentrations, with 3 µM olaparib plus 1 µM AZD6738 inducing 84% cell kill, in contrast to only 37% growth inhibition in the ATM WT cells. The observed combination synergy in the ATM KO cells is a result of increased DNA replication-associated DNA damage together with abrogation of the G2-M checkpoint activated by olaparib. Olaparib treatment led to a 10 and 20% increase in G2 cells at 24 hours in the ATM KO and ATM WT FaDu cell lines respectively, however co-administration of AZD6738 (0.1-0.3 µM) completely released the olaparib treated cells from the G2 arrest. DNA metaphase spread analysis showed that combination treatment with olaparib (1 µM) plus AZD6738 (0.1 µM) in the ATM KO cells caused 12.8 chromosomal DNA aberrations per cell compared to 4.4 or 3.3 aberrations per cell when treated with olaparib or AZD6738 alone respectively (n = 2). In contrast, in the ATM WT cells the olaparib plus AZD6738 combination treatment caused only 1.5 chromosomal aberrations per cell. Collectively, these data show that olaparib-induced DNA damage activates an ATR-dependent G2-M checkpoint allowing time to repair damaged DNA, and that AZD6738 abrogates this checkpoint, permitting cells to undergo mitosis in the presence of DNA damage leading to increased chromosomal aberrations. These data provide a mechanistic understanding of the combination and supports the clinical line of sight for the development of AZD6738 in combination with olaparib. Citation Format: Rebecca Lloyd, Katarynza Falenta, Paul W. Wijnhoven, Christophe Chabbert, Jonathan Stott, James Yates, Alan Y. Lau, Lucy A. Young, Simon J. Hollingsworth. The PARP inhibitor olaparib is synergistic with the ATR inhibitor AZD6738 in ATM deficient cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 337.