Putting a bit into the polo-box domain of polo-like kinase 1

Targeting Polo-like Kinase 1 by a Novel Pyrrole-Imidazole Polyamide-Hoechst Conjugate Suppresses Tumor Growth In Vivo

The serine/threonine kinase Polo-like kinase 1 (Plk1) plays a pivotal role in cell proliferation and has been validated as a promising anticancer drug target. However, very limited success has been achieved in clinical applications using existing Plk1 inhibitors, due to lack of sufficient specificity toward Plk1. To develop a novel Plk1 inhibitor with high selectivity and efficacy, we designed and synthesized a pyrrole-imidazole polyamide-Hoechst conjugate, PIP3, targeted to specific DNA sequence in the PLK1 promoter. PIP3 could specifically inhibit the cell cycle-regulated Plk1 expression and consequently retard tumor cell growth. Cancer cells treated with PIP3 exhibited severe mitotic defects and increased apoptosis, whereas normal cells were not affected by PIP3 treatment. Furthermore, subcutaneous injection of PIP3 into mice bearing human cancer xenografts induced significant tumor growth suppression with low host toxicity. Therefore, PIP3 exhibits the potential as an effective agent for targeted cancer therapy. Mol Cancer Ther; 17(5); 988-1002. ©2018 AACR.

Identification of Polo-like kinase 1 interaction inhibitors using a novel cell-based assay

Polo-like kinase 1 (Plk1) plays several roles in cell division and it is a recognized cancer drug target. Plk1 levels are elevated in cancer and several types of cancer cells are hypersensitive to Plk1 inhibition. Small molecule inhibitors of the kinase domain (KD) of Plk1 have been developed. Their selectivity is limited, which likely contributes to their toxicity. Polo-like kinases are characterized by a Polo-Box Domain (PBD), which mediates interactions with phosphorylation substrates or regulators. Inhibition of the PBD could allow better selectivity or result in different effects than inhibition of the KD. In vitro screens have been used to identify PBD inhibitors with mixed results. We developed the first cell-based assay to screen for PBD inhibitors, using Bioluminescence Resonance Energy Transfer (BRET). We screened through 112 983 compounds and characterized hits in secondary biochemical and biological assays. Subsequent Structure-Activity Relationship (SAR) analysis on our most promising hit revealed that it requires an alkylating function for its activity. In addition, we show that the previously reported PBD inhibitors thymoquinone and Poloxin are also alkylating agents. Our cell-based assay is a promising tool for the identification of new PBD inhibitors with more drug-like profiles using larger and more diverse chemical libraries.