Structure-based discovery of 1-(3-fluoro-5-(5-(3-(methylsulfonyl)phenyl)-1H-pyrazolo[3,4-b]pyridin-3-yl)phenyl)-3-(pyrimidin-5-yl)urea as a potent and selective nanomolar type-II PLK4 inhibitor

Polo-like kinase 4 (PLK4) is a serine/threonine protein kinase involved in regulating cell mitosis and centriole duplication, and has emerged as a therapeutic target for treating multiple cancers. At first, the design and in vitro validation of PLK4 inhibitors (12a-12e, 17a-17f, 22a-22e) bearing 1H-pyrazolo[3,4-b]pyridine scaffold was described and lead compound 22a (IC50 = 0.106 μM) was identified. Then, selectivity- and activity-guided development of a series of potent and selective type-II PLK4 inhibitors using a homology model approach was carried out. Further structure-based optimization resulted in a potent type-II PLK4 inhibitor 29u (IC50 = 0.026 μM), which exhibited outstanding selectivity in a panel of 47 kinases at a single concentration of 1.0 μM. Furthermore, compound 29u significantly inhibited the proliferation of breast cancer cell line MCF-7 with an IC50 value of 1.52 μM, while it exhibited no inhibitory effect on normal cell lines (L02 and HUVECs). Meanwhile, the clone formation, senescence and migration abilities of compound 29u were evaluated using MCF-7 cells. The detailed biological evaluation revealed that compound 29u could arrest cell division in S/G2 phase by inhibiting PLK4, and then affect the expression of downstream signalling pathway proteins regulated by PLK4. Moreover, the in vitro preliminary evaluation of the drug-like properties of compound 29u exhibited outstanding plasma stability, moderate liver microsomal stability, and low risk of drug-drug interactions (DDIs). The current discovery will support the further development of compound 29u as a lead compound for PLK4-targeted anticancer drug discovery and as a useful chemical probe for the further biological research of PLK4.