Abstract 4435: Discovery, biological characterization and oral antitumor activity of polo-like kinase 1 (Plk1) selective small molecule inhibitors
Authors: Hollick Jonathan J. , Thomas Michael G. , Flynn Claire J. , Jones Stuart D. , McLachlan Janice , Scaerou Frederic , Emery Amy , Rogers Nicola L. , Choudhary Abhijeet , Thomson Graeme , Green Simon R. , Glover David M. , Blake David G. 
Published: February 2011

Abstract
We have identified novel inhibitors of Polo-like kinase 1 (Plk1) that exhibit strong single agent activity in solid tumor and leukemia xenograft models after oral dosing. Plk1 is a serine / threonine protein kinase thought to regulate cell division through promotion of mitotic entry, control of spindle assembly, orchestration of mitotic progression and initiation of cytokinesis. Plk1 has been reported to phosphorylate and deactivate the tumor suppressors p53, p63 and p73 thereby inhibiting apoptosis. Furthermore this repression of p53 family members may be responsible for the survival and tumorigenesis of liver cancer stem cells. Cancer cell proliferation is blocked in vitro and in vivo by antisense oligonucleotides and siRNAs to Plk1. Overexpression of Plk1 is associated with tumor development and many human cancers express elevated Plk1 levels compared to surrounding normal tissue. Numerous studies have shown that Plk1 expression levels correlate with disease progression, invasiveness and poor patient prognosis. Collectively, these observations support the selection of Plk1 as an attractive target for cell cycle-directed cancer therapy.
We have employed high throughput screening, in silico screening and de novo ligand design approaches to select an inhibitor scaffold for lead optimization. We have selected a set of ATP-competitive Plk1 inhibitors that exhibit high selectivity for Plk1 and inhibit growth of a broad range of tumor cell lines in vitro with IC50s in the low nanomolar range. Structure-activity relationships (SAR) were determined through iterative targeted analogue synthesis and in vitro testing with SAR rationalized against x-ray crystallographic data. We observed selectivity of the inhibitor scaffold for Plk1 against a panel of over 200 kinases. Treatment of tumor cells with our Plk1 inhibitors induced a phenotype consistent with inhibition of Plk1, accumulation of cells in mitosis and induction of apoptosis. The extent of cytotoxicity was dependent on proliferation as determined by comparative viability of cells arrested in the G1 phase versus proliferating cells. Compound evaluation using a cell-based activity assay for inhibition of Plk1 and pharmacokinetic profiling informed lead optimization studies to generate potential drug candidates with high oral bioavailability and which induce tumor stabilization, regression and tumor-free cures in solid tumor and leukemia xenograft models as single agents. For example, 6% T/C (p = 0.00001) was achieved in the HCT116 human colon carcinoma model.
In summary, we describe a set of Plk1 inhibitors which act as potent antiproliferative agents suitable for further development as oral Plk1 selective inhibitors for the treatment of human cancers.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4435.