Orally administrative melk (maternal embryonic leucine zipper kinase)-targeting small molecule inhibitor suppresses the growth of various types of human cancer.

Abstract

e13569 Background: We previously reported MELK (maternal embryonic leucine zipper kinase) as a novel therapeutic target for breast cancer. MELK was also highly upregulated in multiple types of cancer including prostate, pancreas and lung cancers, and plays indispensable roles in cancer cell survival, particularly in the maintenance of tumor-initiating cells. In this study, we attempted to identify novel substrates of MELK and develop the small-molecule MELK inhibitor. Methods: To elucidate the MELK signaling pathway in breast cancer cells, we screened MELK substrates by 2D-PAGE and mass spectrometric analysis, and characterized two of them for their roles in mammary carcinogenesis. Furthermore, we conducted a high-throughput screening of a compound library followed by structure-activity relationship studies. We investigated the growth suppressive effect of a MELK inhibitor OTSSP167 using xenograft models in mice and a mammosphere-formation assay. Results: We identified two novel MELK substrates, DBNL and PSMA1, which plays critical roles in invasiveness of cancer cells and maintenance of mammary tumor-initiating cells. We successfully obtained a highly potent MELK inhibitor OTSSP167 with IC50 of 0.41 nM. OTSSP167 inhibited the phosphorylation of these two substrates by MELK as well as mammosphere formation of breast cancer cells, and exhibited strong tumor-growth suppressive effects on xenografts of human breast, lung, prostate, and pancreas cancer cell lines in mice by both intravenous and oral administration. Conclusions: Orally administrative MELK inhibitor OTSSP167 is a promising compound to treat various types of human cancer. This compound can inhibit the phosphorylation of MELK substrates and also suppresses the formation of cancer stem cells in breast cancer cells, providing a novel strategy to cure cancer.