ATR (Ataxia Telangiectasia and RAD3-related) is a PI3K-related kinase involved in repair of DNA damage incurred through the genotoxic stress of cellular replication. During cell division, ATR is recruited and activated at sites of single strand DNA breaks, such as those that occur at stalled replication forks, and helps initiate DNA repair. Tumor cells may develop an enhanced dependency on ATR to maintain genomic stability and survival, making ATR an attractive target for anticancer therapy.

AZD6738 is a first-in-class small molecule that inhibits ATR at sub micromolar (uM) concentrations. We examined the leukemic cells from patients with chronic lymphocytic leukemia (CLL) for expression of ATR and sensitivity to this specific ATR inhibitor. For this, we examined both resting CLL cells and CLL cells induced to undergo replication, as assessed by flow cytometry of CLL cells stained with Carboxyfluorescein succinimidyl ester (CFSE). We found that conditions that induced of leukemic cell proliferation (eg: CD40 ligation in the context of interleukin(IL)4 and IL10) also induced expression of ATR. Examination of both resting and proliferating leukemic cells for sensitivity to AZD6738 revealed that those leukemic cells that expressed ATR were highly sensitive to the cytotoxic effects of this ATR inhibitor in a time and dose dependent fashion. As such, concentrations of AZD6738 as low as 1 uM were cytotoxic for proliferating CLL cells. In contrast, the IC50 of resting CLL cells was approximately 30 uM, indicating that resting CLL cells are relatively insensitive to the activity of this drug. AZD6738 may therefore be particularly effective in eliminating the leukemic cells that are being stimulated in the leukemia microenvironment found in lymphoid tissues, which provides survival/growth signals that ordinarily mitigate the activity of standard cytotoxic drugs.

ATR inhibition also may be particularly effective in tumors lacking ATM (Ataxia Telangiectasia Mutated). Such cells are even more dependent on the activity of ATR to repair DNA damage that occurs during replication or genotoxic stress, such as that incurred by cancer chemotherapy. To this end, we are examining differential sensitivity to ATR inhibition of leukemic cells that lack functional ATM, which is often times associated with deletions of the long arm of chromosome 11 (Del 11q22). An ongoing phase 1 clinical trial testing the safety of AZD6738 in patients with relapsed or refractory CLL with Del11q also will examine the activity of this compound for the treatment of patients with CLL.

Citation Format: Michael Y. Choi, Jessie-Farah Fecteau, Jeff Brown, Alan Lau, Thomas J. Kipps. Induction of proliferation sensitizes chronic lymphocytic leukemia cells to apoptosis mediated by the ATR inhibitor AZD6738. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5485. doi:10.1158/1538-7445.AM2014-5485