Checkpoint kinase 1 (CHK1) inhibitors are currently under investigation as chemopotentiating agents due to the role of CHK1 in establishing DNA damage checkpoints in the cell cycle. A novel CHK1/CHK2 inhibitor, LY2606368, as a single agent causes replication catastrophe, DNA double strand breaks and apoptosis (King C et al. Mol Cancer Ther. 2015). Accordingly, LY2606368 is currently in clinical development as a single agent and in combination with both cytotoxic and targeted agents. As subsets of breast cancers exhibit genomic instability and DNA repair deficiencies, we assessed the effect of LY2606368 as a single agent on breast cancer cell lines. We characterized the IC50's for growth inhibition by LY2606368 of a large panel of cell lines assembled on the basis of estrogen receptor (ER), progesterone receptor (PR), and HER2 expression status. Triple negative breast cancer (TNBC) is a subtype of breast cancer that is pathologically negative for expression of ER/PR and HER2 protein. As previously reported for other CHK1 inhibitors (Bryant C et al. BMC Cancer. 2014; Albiges L et al. Breast. 2014), TNBC cell lines exhibited high sensitivity to LY2606368. Interestingly, hypersensitivity to LY2606368 was observed in ER-negative cells, regardless of HER2 status. In addition, ER-positive cells were comparatively resistant suggesting that high sensitivity to LY2606368 occurs in the absence of ER and is not restricted to TNBC. No correlation was found between TP53 mutational status and sensitivity to LY2606368. Consistent with the observed hypersensitivity, ER-negative cell lines exposed to LY2606368 exhibited high levels of γH2AX and phospho-Ser1981-ATM demonstrating appearance of DNA double strand breaks. The concomitant appearance of phospho-Ser345-Chk1 marked aborted checkpoint activation by upstream detector/effector kinases (e.g. ATR, ATM). ER-positive cells did not engage in initiation of the DNA damage response or significant checkpoint activation when exposed to comparable doses of LY2606368. Collectively, these results suggest that the CHK1 inhibitor LY2606368 is likely to be more cytotoxic in ER-negative breast cancer types. Homologous recombination and other DNA repair deficiencies can explain the need for CHK1-dependent checkpoint activation during each replication cycle. On the other hand, ER-positive breast cancers might be less sensitive to monotherapy because of the absence of unresolved DNA damaging events that create checkpoint dependency.

In summary, triple negative status does not directly determine the sensitivity of breast cancer cell lines to the CHK1 inhibitor LY2606368, although TNBC cells are in general hypersensitive. The absence of ER might be a more promising marker of sensitivity to LY2606368 as a monotherapy in breast cancer.

Citation Format: Jean-Bernard Lazaro, Kalindi Parmar, Geoffrey I. Shapiro, Alan D. D’Andrea. Estrogen receptor-negative breast cancer cell lines exhibit hypersensitivity to the CHK1 inhibitor LY2606368. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2729.