As treatment options for patients with incurable metastatic castration-resistant prostate cancer (mCRPC) are considerably limited, novel effective therapeutic options are needed. Checkpoint kinase 1 (CHK1) is a highly conserved protein kinase implicated in the DNA damage response (DDR) pathway that prevents the accumulation of DNA damage and controls regular genome duplication. CHK1 has been associated with prostate cancer (PCa) induction, progression, and lethality; hence, CHK1 inhibitors SCH900776 (also known as MK-8776) and the more effective SCH900776 analog MU380 may have clinical applications in the therapy of PCa. Synergistic induction of DNA damage with CHK1 inhibition represents a promising therapeutic approach that has been tested in many types of malignancies, but not in chemoresistant mCRPC. Here, we report that such therapeutic approach may be exploited using the synergistic action of the antimetabolite gemcitabine (GEM) and CHK1 inhibitors SCH900776 and MU380 in docetaxel-resistant (DR) mCRPC. Given the results, both CHK1 inhibitors significantly potentiated the sensitivity to GEM in a panel of chemo-naïve and matched DR PCa cell lines under 2D conditions. MU380 exhibited a stronger synergistic effect with GEM than clinical candidate SCH900776. MU380 alone or in combination with GEM significantly reduced spheroid size and increased apoptosis in all patient-derived xenograft 3D cultures, with a higher impact in DR models. Combined treatment induced premature mitosis from G1 phase resulting in the mitotic catastrophe as a prestage of apoptosis. Finally, treatment by MU380 alone, or in combination with GEM, significantly inhibited tumor growth of both PC339-DOC and PC346C-DOC xenograft models in mice. Taken together, our data suggest that metabolically robust and selective CHK1 inhibitor MU380 can bypass docetaxel resistance and improve the effectiveness of GEM in DR mCRPC models. This approach might allow for dose reduction of GEM and thereby minimize undesired toxicity and may represent a therapeutic option for patients with incurable DR mCRPC.

Department of Adaptive Biotechnologies Global Change Research Institute of the Czech Academy of Sciences Brno Czech Republic

Department of Biology Faculty of Medicine Masaryk University Brno Czech Republic

Department of Chemistry CZ Openscreen Faculty of Science Masaryk University Brno Czech Republic

Department of Cytokinetics Institute of Biophysics of the Czech Academy of Sciences Brno Czech Republic

Department of Experimental Biology Faculty of Science Masaryk University Brno Czech Republic

Department of Histology and Embryology Faculty of Medicine Masaryk University Brno Czech Republic

Department of Urology Erasmus MC Cancer Institute Erasmus University Medical Center Rotterdam The Netherlands

Department of Urology Experimental Urology Medical University of Innsbruck Austria

International Clinical Research Center Center for Biomolecular and Cellular Engineering St Anne's University Hospital in Brno Czech Republic

National Centre for Biomolecular Research Masaryk University Brno Czech Republic

School of Medicine Conway Institute of Biomolecular and Biomedical Research University College Dublin Ireland

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