The Plk-1 Inhibitor BI 2536 Counteracts Proliferation and Viability of CML Cells and Synergizes with Imatinib and Nilotinib (AMN107) in Producing Growth Inhibition.

Chronic myeloid leukemia (CML) is a hematopoietic stem cell disease in which BCR/ABL enhances growth and survival of leukemic cells. In most patients, the disease can be kept under control by the BCR/ABL tyrosine kinase inhibitor imatinib (STI571; Novartis Basel, Switzerland). However, resistance or intolerance against imatinib may occur during therapy. Therefore, current research is focusing on novel targets and targeted drugs in CML. Polo-like kinase 1 (Plk-1) is a serine/threonine kinase that plays an essential role in mitosis and is expressed in activated/phosphorylated form in various malignancies including acute myeloid leukemia (AML). BI 2536 (Boehringer Ingelheim GmbH, Germany) is a novel selective inhibitor of Plk-1, that is currently tested in AML-trials. In this study, we have evaluated expression and the potential role of Plk-1 as a novel target in CML cells. As assessed by PCR, Plk-1 mRNA was found to be expressed abundantly in primary CML cells and in the CML cell line K562, whereas normal peripheral blood cells did not express detectable levels of Plk-1 mRNA. The Plk-1 protein was detected in primary CML cells and K562 cells by immunocytochemistry. In consecutive experiments, we were able to show that CML cells display phosphorylated Plk-1. As assessed by ³H-thymidine-uptake experiments, BI 2536 was found to inhibit the proliferation of K562 cells in a dose-dependent manner (IC₅₀ 5–15 nM). Moreover, BI 2536 was found to inhibit the proliferation of both imatinib-naive (n=6) and imatinib-resistant (n=3) primary CML cells (IC₅₀: 1–15 nM). The growth-inhibitory effect of BI 2536 on CML cells was found to be associated with mitotic arrest, a G2-M cell cycle arrest, and consecutive apoptosis. In normal bone marrow or peripheral blood mononuclear cells, neither mitotic cell arrest nor apoptosis were observed after exposure to BI 2536. In further experiments, primary CML cells were coincubated with BI 2536 plus imatinib or with BI 2536 plus nilotinib (AMN107; Novartis) at fixed ratio of drug concentrations. In these experiments, BI 2536 was found to synergize with both tyrosine kinase inhibitors in counteracting the proliferation of CML cells. In conclusion, our data show that Plk-1 is expressed in activated form in CML cells and plays a role in cell cycle progression and cell viability. Targeting Plk-1 with BI 2536 leads to mitotic arrest, growth inhibition, and apoptosis in imatinib-naive and imatinib-resistant leukemic cells. Moreover, BI 2536 synergizes with imatinib and nilotinib in counteracting the growth of neoplastic cells in CML. Targeting of Plk-1 may be a novel interesting pharmacologic approach to counteract growth of CML cells.