ATR inhibition induces synthetic lethality and overcomes chemoresistance in TP53- or ATM-defective chronic lymphocytic leukemia cells

Abstract

Key Points• ATR inhibition is synthetically lethal to TP53-or ATMdefective CLL cells.• ATR targeting induces selective cytotoxicity and chemosensitization in TP53-or ATM-defective CLL cells in vitro and in vivo.TP53 and ataxia telangiectasia mutated (ATM) defects are associated with genomic instability, clonal evolution, and chemoresistance in chronic lymphocytic leukemia (CLL). Currently, therapies capable of providing durable remissions in relapsed/ refractory TP53-or ATM-defective CLL are lacking. Ataxia telangiectasia and Rad3-related (ATR) mediates response to replication stress, the absence of which leads to collapse of stalled replication forks into chromatid fragments that require resolution through the ATM/p53 pathway. Here, using AZD6738, a novel ATR kinase inhibitor, we investigated ATR inhibition as a synthetically lethal strategy to target CLL cells with TP53 or ATM defects. Irrespective of TP53 or ATM status, induction of CLL cell proliferation upregulated ATR protein, which then became activated in response to replication stress. In TP53-or ATM-defective CLL cells, inhibition of ATR signaling by AZD6738 led to an accumulation of unrepaired DNA damage, which was carried through into mitosis because of defective cell cycle checkpoints, resulting in cell death by mitotic catastrophe. Consequently, AZD6738 was selectively cytotoxic to both TP53-and ATM-defective CLL cell lines and primary cells. This was confirmed in vivo using primary xenograft models of TP53-or ATM-defective CLL, where treatment with AZD6738 resulted in decreased tumor load and reduction in the proportion of CLL cells with such defects. Moreover, AZD6738 sensitized TP53-or ATM-defective primary CLL cells to chemotherapy and ibrutinib. Our findings suggest that ATR is a promising therapeutic target for TP53-or ATMdefective CLL that warrants clinical investigation. (Blood. 2016;127(5):582-595)

The DNA damage response pathway in normal hematopoiesis and malignancies

“…This synthetic lethal effect is also observed in xenograft models of p53-or ATM-defective primary CLL, where AZD6738 treatment not only significantly reduces tumor load [44], but also overcomes resistance to chemotherapeutics used in the first-line treatment of CLL such as chlorambucil, fludarabine, or bendamustine. Collectively, these results support the combined use of ATR inhibitor with a range of existing therapeutic agents for CLL.…”

“…Recent in vitro studies have shown that pharmacologic inhibition of ATR in p53-or ATM-defective CLL cells selectively induces replication fork stalls, accumulation of unrepaired DNA damage, and mitotic cell death [43,44]. This synthetic lethal effect is also observed in xenograft models of p53-or ATM-defective primary CLL, where AZD6738 treatment not only significantly reduces tumor load [44], but also overcomes resistance to chemotherapeutics used in the first-line treatment of CLL such as chlorambucil, fludarabine, or bendamustine.…”

Achieving Precision Death with Cell-Cycle Inhibitors that Target DNA Replication and Repair

“…In preclinical models of hematologic malignancies, cells that lack ATM (e.g., due to 11q deletions) or have defects in p53 signaling were particularly vulnerable to the ATR inhibitor AZD6738. Treatment with AZD6738 increased replication initiation and fork stalling, resulting in RS and DNA damage that likely contributed to selective cytotoxicity in cells with p53 and/or ATM defects (64). In parallel, in a phase I trial with the ATR inhibitor VX970 and carboplatin, a patient with colorectal cancer and ATM loss achieved a complete response (65).…”

ATM Mutations in Cancer: Therapeutic Implications

“…Notably, ATM-deficient lung cancer xenografts were particularly sensitive to a combination of cisplatin and (78). In studies with primary leukemic cells from patients with chronic lymphocytic leukemia, AZD6738 had superior cytotoxicity in CLL cells induced to divide (79), and in ATM-or TP53-mutated CLL cells (80). A first-in-man study was initiated in ATM-deficient non-Hodgkin lymphoma, and a subsequent study will be initiated in combination with radiotherapy for the treatment of patients with head and neck cancer, based on evidence that ATR inhibition can sensitize ATM-deficient cancer cells to ionizing radiation (68,81).…”