BTO-1 is a cell-permeable benzothiazolo-N-oxide compound that targets the ATP-binding pocket of polo-like kinase and is shown to inhibit Plk1 kinase activity in cell-free kinase assays (IC50 = 8.0 μM) and suppress the phosphorylation of cellular Plk1 substrate Cdc25C in rat kangaroo kidney-derived PTK cells (75% inhibition at 6.3 μM). BTO-1 treatment of cultured cells results in mitosis defects consistent with loss-of-Plk1 phenotypes seen in Plk1 RNAi-treated U20S cells, including the appearance of monopolar spindles and the reduction of γ-tubulin at the centrosomes. Plk1 inhibition by BTO-1 in HeLa cells results in a blockage of Rho and Rho-GEF recruitment, which is essential for the assembly of a functional contractile ring.
