In vitro activity: MLN0905 is a potent inhibitor of PLK1 with IC50 of 2 nM. MLN0905 inhibits cell mitosis with EC50 of 9 nM. MLN0905 inhibits Cdc25C-T96 phosphorylation, a direct readout of PLK1 inhibition with EC50 of 29 nM. MLN0905 inhibits HT-29 viability with LD50 of 22 nM. MLN0905 possesses reasonable selectivity against a panel of 359 kinases. MLN0905 inhibits a panel of lymphoma cells viability with IC50 of 3 – 24 nM.

Kinase Assay: The human PLK1 enzymatic reaction totaling 30μL contained 50 mM Tris-HCl (pH 8.0), 10 mM MgCl2, 0.02% BSA, 10% glycerol, 1 mM DTT, 100 mM NaCl, 3.3% DMSO, 8 μM ATP, 0.2μCi [γ- 33 P]-ATP, 4μM peptide substrate (Biotin-AHX-LDETGHLDSSGLQEVHLA-CONH2), and 10 nM recombinant human PLK1[2–369]T210D. The enzymatic reaction mixture, with or without PLK inhibitors, is incubated for 2.5 h at 30℃before termination with 20μL of 150 mM EDTA. Then 25μL of the stopped enzyme reaction mixture is transferred to a 384-well streptavidin coated Image FlashPlate and incubated at room temperature for 3 h. The Image Flash Plate wells are washed three times with 0.02% Tween-20 and then read on the Perkin-Elmer Viewlux.

Cell Assay: MLN0905 is a selective PLK1 inhibitor and has similar effect as RNAi hnockdown. When tested with HT-29 cells, MLN0905 treatment significantly increased pHisH3 expression which indicated that cells were arrested in G2/M phase by inhibiting PLK1 expression.