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.

• Keywords
• MU380, castration-resistant prostate cancer, checkpoint kinase 1, docetaxel resistance, gemcitabine, mitotic catastrophe,
• MeSH
• Cell Death drug effects   MeSH
• Checkpoint Kinase 1 antagonists & inhibitors   metabolism   MeSH
• Drug Resistance, Neoplasm drug effects   MeSH
• Deoxycytidine analogs & derivatives   pharmacology   MeSH
• Docetaxel pharmacology   MeSH
• Gemcitabine MeSH
• Humans MeSH
• Mitosis * drug effects   MeSH
• Mice, SCID MeSH
• Cell Line, Tumor MeSH
• Prostatic Neoplasms pathology   MeSH
• Piperidines chemistry   pharmacology   MeSH
• Cell Proliferation drug effects   MeSH
• Pyrazoles chemistry   pharmacology   MeSH
• Pyrimidines chemistry   pharmacology   MeSH
• S Phase drug effects   MeSH
• Xenograft Model Antitumor Assays MeSH
• Animals MeSH
• Check Tag
• Humans MeSH
• Male MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Checkpoint Kinase 1 MeSH
• CHEK1 protein, human MeSH Browser
• Deoxycytidine MeSH
• Docetaxel MeSH
• Gemcitabine MeSH
• MU380 MeSH Browser
• Piperidines MeSH
• Pyrazoles MeSH
• Pyrimidines MeSH

Introduction of small-molecule inhibitors of B-cell receptor signaling and BCL2 protein significantly improves therapeutic options in chronic lymphocytic leukemia. However, some patients suffer from adverse effects mandating treatment discontinuation, and cases with TP53 defects more frequently experience early progression of the disease. Development of alternative therapeutic approaches is, therefore, of critical importance. Here we report details of the anti-chronic lymphocytic leukemia single-agent activity of MU380, our recently identified potent, selective, and metabolically robust inhibitor of checkpoint kinase 1. We also describe a newly developed enantioselective synthesis of MU380, which allows preparation of gram quantities of the substance. Checkpoint kinase 1 is a master regulator of replication operating primarily in intra-S and G2/M cell cycle checkpoints. Initially tested in leukemia and lymphoma cell lines, MU380 significantly potentiated efficacy of gemcitabine, a clinically used inducer of replication stress. Moreover, MU380 manifested substantial single-agent activity in both TP53-wild type and TP53-mutated leukemia and lymphoma cell lines. In chronic lymphocytic leukemia-derived cell lines MEC-1, MEC-2 (both TP53-mut), and OSU-CLL (TP53-wt) the inhibitor impaired cell cycle progression and induced apoptosis. In primary clinical samples, MU380 used as a single-agent noticeably reduced the viability of unstimulated chronic lymphocytic leukemia cells as well as those induced to proliferate by anti-CD40/IL-4 stimuli. In both cases, effects were comparable in samples harboring p53 pathway dysfunction (TP53 mutations or ATM mutations) and TP53-wt/ATM-wt cells. Lastly, MU380 also exhibited significant in vivo activity in a xenotransplant mouse model (immunodeficient strain NOD-scid IL2Rγnull ) where it efficiently suppressed growth of subcutaneous tumors generated from MEC-1 cells.

• MeSH
• Apoptosis MeSH
• Cell Cycle MeSH
• Checkpoint Kinase 1 antagonists & inhibitors   MeSH
• Drug Resistance, Neoplasm drug effects   MeSH
• Leukemia, Lymphocytic, Chronic, B-Cell drug therapy   genetics   pathology   MeSH
• Deoxycytidine analogs & derivatives   pharmacology   MeSH
• Gemcitabine MeSH
• Protein Kinase Inhibitors pharmacology   MeSH
• Humans MeSH
• Mutation * MeSH
• Mice, Inbred NOD MeSH
• Mice, SCID MeSH
• Mice MeSH
• Biomarkers, Tumor genetics   MeSH
• Tumor Cells, Cultured MeSH
• Tumor Suppressor Protein p53 genetics   MeSH
• Piperidines pharmacology   MeSH
• Cell Proliferation MeSH
• Antimetabolites, Antineoplastic pharmacology   MeSH
• Pyrazoles pharmacology   MeSH
• Pyrimidines pharmacology   MeSH
• Gene Expression Regulation, Neoplastic drug effects   MeSH
• Drug Synergism * MeSH
• Xenograft Model Antitumor Assays MeSH
• Animals MeSH
• Check Tag
• Humans MeSH
• Mice MeSH
• Female MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Checkpoint Kinase 1 MeSH
• CHEK1 protein, human MeSH Browser
• Deoxycytidine MeSH
• Gemcitabine MeSH
• Protein Kinase Inhibitors MeSH
• MK-8776 MeSH Browser
• MU380 MeSH Browser
• Biomarkers, Tumor MeSH
• Tumor Suppressor Protein p53 MeSH
• Piperidines MeSH
• Antimetabolites, Antineoplastic MeSH
• Pyrazoles MeSH
• Pyrimidines MeSH
• TP53 protein, human MeSH Browser