In vitro activity: Like BI2536, BI6727 is an ATP-competitive kinase inhibitor from the dihydropteridinone class of compounds. In addition to Plk1, BI6727 also potently inhibits two closely related kinases Plk2 and Plk3 with IC50 of 5 nM and 56 nM, respectively. BI6727 at concentrations up to 10 μM displays no inhibitory activity against a panel of>50 other kinases. BI6727 inhibits the proliferation of multiple cell lines derived from various cancer tissues, including HCT116, NCI-H460, BRO, GRANTA-519, HL-60, THP-1, and Raji cells with EC50 of 23 nM, 21 nM, 11 nM, 15 nM, 32 nM, 36 nM, and 37 nM, respectively. BI6727 treatment (100 nM) in NCI-H460 cells induces an accumulation of mitotic cells with monopolar spindles and positive staining for histone H3 phosphoserine 10, confirming that cells are arrested early in the M phase, followed by induction of apoptosis. Low nanomolar concentrations of BI6727 display potent inhibitory activity against neuroblastoma (NB) tumor-initiating cells (NB TIC) with EC50 of 21 nM, whereas only micromolar concentrations of BI6727 are cytotoxic for normal pediatric neural stem cells. BI6727 induces growth arrest of Daoy and ONS-76 medulloblastoma cells similar to BI 2536.

Kinase Assay: Recombinant human Plk1 (residues 1-603) is expressed as NH2-terminal, GST-tagged fusion protein using a baculoviral expression system and purified by affinity chromatography using glutathione-agarose. Enzyme activity assays for Plk1 are done in the presence of serially diluted BI6727 using 20 ng of recombinant kinase and 10 μg casein from bovine milk as substrate. Kinase reactions are done in a final volume of 60 μL for 45 minutes at 30 °C [15 mM MgCl2, 25 mM MOPS (pH 7.0), 1 mM DTT, 1% DMSO, 7.5 μM ATP, 0.3 μCi γ-32P-ATP]. Reactions are terminated by the addition of 125 μL of ice-cold 5% TCA. After transferring the precipitates to MultiScreen mixed ester cellulose filter plates, plates are washed with 1% TCA and quantified radiometrically. Dose-response curves are used for calculating IC50 value.

Cell Assay: Cell proliferation assays are done by incubating cells (HCT116, NCI-H460, BRO, GRANTA-519, HL-60, THP-1, and Raji) in the presence of various concentrations of BI6727 for 24, 48, and 72 hours and cell growth is assessed by measuring Alamar blue dye conversion in a fluorescence spectrophotometer. Effective concentrations at which cellular growth is inhibited by 50% (EC50) are extrapolated from the dose-response curve fit. To determine the DNA content, cell suspensions are fixed in 80% ethanol, treated for 5 minutes with 0.25% Triton X-100 in PBS, and incubated with 0.1% RNase and 10 μg/mL propidium iodide in PBS for 20 minutes at room temperature. Cell cycle profiles are determined by flow cytometric analysis.

Clin Cancer Res. 2009 May 1;15(9):3094-102; BMC Cancer. 2012 Mar 5;12:80. doi: 10.1186/1471-2407-12-80.