Identification of a novel Polo-like kinase 1 inhibitor that specifically blocks the functions of Polo-Box domain

Polo-like kinase 1 (Plk1) is a promising target for cancer therapy due to its essential role in cell division. In addition to a highly conserved kinase domain, Plk1 also contains a Polo-Box domain (PBD), which is essential for Plk1's subcellular localization and mitotic functions. We adopted a fluorescence polarization assay and identified a new Plk1 PBD inhibitor T521 from a small-molecule compound library. T521 specifically inhibits the PBD of Plk1, but not those of Plk2-3. T521 exhibits covalent binding to some lysine residues of Plk1 PBD, which causes significant changes in the secondary structure of Plk1 PBD. Using a cell-based assay, we showed that T521 impedes the interaction between Plk1 and Bub1, a mitotic checkpoint protein. Moreover, HeLa cells treated with T521 exhibited dramatic mitotic defects. Importantly, T521 suppresses the growth of A549 cells in xenograft nude mice. Taken together, we have identified a novel Plk1 inhibitor that specifically disrupts the functions of Plk1 PBD and shows anticancer activity.