Abstract P4-07-02: Targeted therapies for TNBC: Exploiting vulnerabilities that arise from DNA damage repair pathway dependencies

We examined the synergistic effects of DNA damage, Chk1 inhibition and poly(ADP-ribose) polymerase (PARP) inhibition in TNBC. This combinatorial targeting allows us to exploit vulnerabilities in two pathways that are often deregulated in TNBCs: DNA damage checkpoint defects due to TP53 deficiency and DNA repair defects due to alterations in homologous recombination repair (HRR). TP53 maintains genome integrity by inhibiting cells that are experiencing genotoxic stress from progressing through the cell cycle, or by inducing apoptosis or senescence. In response to DNA damage, p53 activates gene expression to arrest cells in the G1 phase of the cell cycle and to reinforce the S- and G2-checkpoints. Thus, p53-deficient cells lack a G1 checkpoint and are impaired in their ability to sustain S- and G2-checkpoints. This makes p53-deficient tumors particularly sensitive to agents that abrogate these checkpoints. Because Chk1 inhibitors abrogate both S- and G2-checkpoints, combining Chk1 inhibitors with agents that induce genotoxic stress provides a rational therapeutic strategy for killing p53-deficient TNBC. Loss of HRR increases dependence of cells on a class of enzymes called PARPs, and Chk1 has also been shown to be important for efficient HRR. Thus, by interfering with HRR, Chk1 inhibitors are predicted to sensitize TNBC cells to PARP inhibitors. We tested the hypotheses that by impairing HRR, Chk1 inhibitors will sensitize TNBCs to PARP inhibition, and that therapies that combine Chk1 inhibitors with PARP inhibitors will be effective at killing TNBCs because they will simultaneously induce checkpoint bypass and block DNA repair. We generated a set of isogenic TNBC cell lines that are p53-proficient (p53WT) or p53-deficient (p53KD), and evaluated their sensitivity to Chk1 inhibitors (LY2606368) and DNA damaging agents (cisplatin). Loss of p53 conferred a dramatic increase in sensitivity to treatment with cisplatin + LY2606368. Surprisingly, inhibition of PARP1 (BMN673) did not increase sensitivity to Chk1 inhibitor ± cisplatin. To determine why Chk1 inhibition did not sensitize cells to PARP inhibition, we evaluated the effect of Chk1 inhibition on the ability of cells to recruit HRR proteins to sites of DNA damage. In line with CHK1 regulating HRR, Chk1 inhibition was associated with an inability of Rad51 to localize to sites of DNA double strand breaks. Interestingly, we also found that upstream of Rad51, there was a significant alteration in the formation of phopho-RPA2 foci in cells treated with the Chk1 inhibitor. On-going studies are evaluating whether there are changes in the kinetics of formation and/or resolution of Rad51 and phospho-RPA2 foci in response to Chk1 inhibition. Citation Format: Redwood AB, Cai S, Piwnica-Worms H. Targeted therapies for TNBC: Exploiting vulnerabilities that arise from DNA damage repair pathway dependencies. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P4-07-02.