Progress in the Discovery of Polo-like Kinase Inhibitors

Abstract

Polo-like kinases (PLKs) are key enzymes that control mitotic entry of proliferating cells and regulate many aspects of mitosis necessary for successful cytokinesis. Of the four known human PLKs, PLK1 is the best characterized and is overexpressed in many tumour types with aberrant elevation frequently constituting a prognostic indicator of poor disease outcome. Despite the fact that PLK1 has been regarded as a validated mitotic cancer target for a number of years, very few reports of small-molecule PLK inhibitors have appeared to date. In order to provide a starting point for the discovery and development of selective PLK inhibitors, we have characterized a number of known generic kinase inhibitors with hitherto unknown activity against PLK1, as well as discovering novel inhibitors through structure-guided design. Previously, the only characterized biochemical PLK1 inhibitor was scytonemin, a symmetric indolic marine natural product that is a micromolar non-specific ATP competitor. In addition to the progress in the development of ATP-competitive small-molecule PLK inhibitors, recent reports on the use of antisense oligonucleotides (ASONs) and small interfering RNAs (siRNAs) directed against PLK1 have shown selective antiproliferative effects on cancer cells both in vitro and in vivo, producing phenotypes consistent with known PLK functions, and confirming that targeting PLKs with conventional small-molecule agents may be a valid and effective anticancer strategy. Here we present a progress update on the approaches taken so far in developing PLK inhibitors.

In vitro biological activity of a novel small-molecule inhibitor of polo-like kinase 1

“…All measurements were obtained under conditions where signal production increased linearly with time and enzyme. Test compounds were added to white 384-well assay plates (0.1 AL for 10 AL and some 20 AL assays, 1 AL for some 20 Methods for additional kinase assays can be found in Supplementary methods. 7 In vitro Growth Inhibition Assays Assays were carried out and data analyzed as described by Rusnak et al (25).…”

“…Finally, because PLK1 expression and activity are tightly coupled to mitosis, nondividing cells should not be affected by PLK1 inhibition, perhaps mitigating the side effects seen with antimicrotubule drugs. There are increasing efforts to identify non -ATP-competitive and ATP-competitive smallmolecule inhibitors for PLKs (20). Recently, ON01910 was reported to be a non -ATP-competitive inhibitor of PLK1, induce apoptosis in cancer cells, and have in vivo activity in xenograft tumor models (21).…”

Multiplexed Random Peptide Library and Phospho-Specific Antibodies Facilitate Human Polo-Like Kinase 1 Inhibitor Screen

“…15,16 Even though several Plk1 inhibitors have been reported, more selective and effi cacious drug without off-target effects needs to be discovered. 17 Our goal is to identify novel lead compounds for Plk1 inhibitor by running an ultra-high-throughput screening (uHTS). Several studies have presented in vitro kinase assays for PLK1.…”

Plk1-Targeted Small Molecule Inhibitors: Molecular Basis for Their Potency and Specificity

“…Scytonemin has been shown to inhibit the ability of Plk1 to phosphorylate Cdc25C in a concentrationdependent manner with an in vitro IC 50 of 2 ± 0.1 μM (McInnes et al, 2006;Stevenson et al, 2002). However, it was later turned out to be a nonselective molecule that also inhibits myelin transcription factor 1 (MYT1), cyclin-dependent kinase 1 (CDK1), checkpoint kinase 1 (CHK1), and protein kinase C (PKC) with similar potencies (McInnes et al, 2005).…”

“…Because PBD is essential for the subcellular localization and mitotic functions of Plk1, it is ideally suited for the development of anti-Plk1 inhibitors that interfere with PBD-dependent protein-protein interactions, rather than with ATP binding. Given a high level of specificity of proteinprotein interactions, the PBD may prove to be a target that allows one to achieve Plk-isoform selectivity (McInnes et al, 2005;Strebhardt et al, 2006). To date, at least seven X-ray crystal structures of the human Plk1 PBD have been solved and reported in the Protein Data Bank.…”