Triple-negative breast cancer (TNBC), the most aggressive subtype of breast cancer, is diagnosed in approximately 15% of all breast cancer patients and characterized by high level of genomic instability, defects in DNA damage response (DDR) and increased replication stress (RS). Currently the treatment options for TNBC are limited, and new approaches are needed. Checkpoint kinase 1 (Chk1) is a key kinase that regulates the cell cycle, DDR and RS through the modulation of cell cycle checkpoints and replication fork licensing; and has emerged as an attractive target for anti-cancer therapy. Prexasertib (LY2606368), an ATP-competitive inhibitor of Chk1 has demonstrated single agent activity in vitro and in vivo. Prexasertib is being evaluated in an ongoing TNBC Phase 2 trial sponsored by NCI; and preliminary results suggest modest single-agent activity in sporadic TNBC patients (https://academic.oup.com/annonc/article/27/suppl_6/231PD/2798970). To understand the biology and mechanisms underlying prexasertib's activity and/or identify the potential biomarkers that may predict response or identify novel combinations that could improve prexasertib's efficacy, we evaluated the anti-tumor activity of prexasertib as a single agent in TNBC cancer cell lines in vitro and in vivo, including xenograft, orthotopic and patient-derived xenograft (PDX) tumor models. Prexasertib inhibited cell proliferation in 12 TNBC cell lines with IC50 values ranging from 0.32 nM to 117.3 nM; and demonstrated inhibition of Chk1 auto-phosphorylation and activation of γH2AX, RPA32 and DNAPKc in TNBC cell lines including HCC1187, HCC1806, MX-1 and MDA-MB-231. Treatment with prexasertib inhibited tumor growth by 83.8%, 85.5% and 94.2% in HCC1187, MX-1 and HCC1806 xenograft models, respectively. Prexasertib also inhibited primary tumor growth by 74.4% and lung metastasis by 97.48% in a MDA-MB-231 mammary fat pad orthotopic model. The anti-tumor effect of prexasertib was further assessed in 40 TNBC PDX models. Prexasertib treatment resulted in 10% (4/40) of models achieving complete tumor regression, 40% (16/40) achieving partial tumor regression/stasis and 50% (20/40) not deriving benefit. Potential biomarkers of prexasertib were investigated. TNBC cells with higher phosphorylation levels of DNAPKc and RPA32 demonstrated higher sensitivity to prexasertib treatment. TNBC PDX tumors with higher RNA message levels of CyclinE1, Cyclin D1 and c-Myc showed better prexasertib response. Together these findings will inform subsets of TNBC and/or individual patient characteristics that should be considered for future clinical assessments and development of potential combination therapy strategies.

Citation Format: Wenjuan Wu, Constance King, Gregory Donoho, Philip Iversen, Andrew Capen, Mark Castanares, Jennifer Stephens, Yan Ding, Susan Pratt, Ricardo Martinez, Sean Buchanan, Christoph Reinhard, Richard Beckmann, Aimee Lin. Anti-tumor activity of the Chk1 inhibitor prexasertib (LY2606368) as a single agent in triple negative breast cancer models [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 336.