PLK4-IN-25

A good and pretty selective PLK4 inhibitor. The Eurofins Kinome selectivity panel showed 50% inhibition or higher for 8.5% of kinome at 1 micromolar concentration. These potential off targets were not characterized further limiting true understanding of the molecule's selectivity as a chemical probe. Of note, only a handful of off target kinases showed 90% inhibition or more including ACK1 ALK2, CHK2, FAK, LRRK2, LTK, TTK. The probe could be used as an orthogonal tool compound alongside other PLK4 inhibitors.

This is a reasonable PLK4 probe with a recommended maximum in vitro concentration of 100 nM, corresponding to ~1.7× EC50 (NanoBRET). This concentration provides solid engagement while still staying well below the AURKB biochemical IC50 of ~630 nM. The cell viability EC50 is ~88 nM for compound 25 in CHP-134 (5-day CellTiter-Glo), and a an experiment using PLK4 G95L CHP-134 rescues the viability effect, demonstrating that the viability effect near the ~100 nM concentration is driven by PLK4, at least in this cell line. The kinase selectivity panel shows a handful of other kinases strongly inhibited (>90% at 1 uM), and it would have been desirable to see dose-response data to further increase the confidence in this probe. In vivo PK properties look good (PO), although IV data are missing to provide information on CL and %F. 30 mg/kg BID in mice already already provided maximal efficacy in the model (CHP-134; TGI 115%), with no benefit seen for the 100 mg/kg dose seen, despite double Cmax. As the single-point kinome panel shows multiple kinases strongly inhibited at 1 µM (e.g., FAK, CHK2, TTK, ALK, etc.), the 30 mg/kg dose minimizes the risk that these non-PLK4 targets contribute to efficacy or phenotype, especially because some of them could potentially affect proliferation, DNA damage signaling, or survival pathways.