In vitro activity: PHA-767491 displays approximately 20-fold selectivity for Cdk1, Cdk2 and GSK3-β, 50-fold selectivity for MK2 and Cdk5 and 100-fold selectivity for PLK1 and CHK2. PHA-767491 inhibits cell proliferation in a variety of human cell lines with IC50 of 0.86 μM for SF-268 to 5.87 μM for K562, and significantly induces apoptosis in a p53-independent manner in almost all cell lines in contrast with 5-FU or gemcitabine which only works in a few of cell lines. Unlike current DNA synthesis inhibitors, PHA-767491 treatment at 5 μM blocks the initiation of DNA replication but not replication fork progression, due to specific inhibition of Cdc7 kinase and Mcm2 phosphorylation at the Cdc7-dependent Ser40 site. The up-regulated Mcl-1 levels in ABT-737-resistant OCI-LY1 and SU-DHL-4 cells can be significantly decreased by PHA-767491 treatment at 3 μM possibly due to the inhibition of Cdk9, leading to the restoration of the sensitivity to ABT-737. The direct mitochondrial dependent pro-apoptosis effect of PHA-767491 is also observed when applied at 1 μM in quiescent chronic lymphocytic leukemia (CLL) cells through the similar mechanism with EC50 of 0.34-0.97 μM. While in proliferating CLL cells stimulated by CD154 and interleukin-4, PHA-767491 treatment at 5 μM abolishes DNA synthesis by inhibiting Cdc7 rather than triggering cell death.

Kinase Assay: The inhibition of Cdc7 and Cdk9 by PHA-767491 (IC50) is determined using the strong anion exchanger (Dowex 1-X8 resin, formate form)-based assay. For each enzyme, the absolute Km values for ATP and the specific substrate are initially determined, and each assay is then run at optimized ATP/33P-γ-ATP mix (2Km) and substrate (5Km) concentrations. Cdc7 kinase assay is performed in a buffer containing 50 mM Hepes pH 7.9, 15 mM MgCl2, 2 mM β- glycerylphosphate, 0.2 mg/mL BSA, 1 mM DTT, 3 μM Na3VO4, 2Km ATP/33P-γ-ATP mix, 5Km Mcm2 (aa 10-294), 37 nM of recombinant Cdc7/Dbf4 and increasing concentration of PHA-767491 in a final volume of 30 μL, and incubated for 1 hour at 25 °C. Cdk9 kinase assay is performed using 50 nM of recombinant Cdk9/cyclin T in 50 mM HEPES pH 7.5, 10 mM MgCl2, 1 mM DTT, 3 μM Na3VO4, 2Km ATP/33P-γ-ATP mix, 5Km RNA polymerase CDT peptide and increasing concentration of PHA-767491 in a final volume of 30 μL, and incubated for 1 hour at 25 °C. After incubation, an amount of 150 μL of resin/formate (pH 3.0) is added to stop the reaction and capture unreacted 33P-γ-ATP, separating it from the phosphorylated substrate in solution. After 1 hour of rest, a volume of 50 μL supernatant is transferred to Optiplate 96-well plates. After the additon of 150 μL of Microscint 40, the radioactivity is counted in the TopCount.

Cell Assay: Cells (HeLa, MCF7, HCT-116, U2OS, A2780, K562, SF-539, SF-268, Ovcar8, SW480, COLO205, HCT-15, Jurkat, PC3, and NHDF) are exposed to PHA-767491 for 24 or 72 hours. Cells are lysed and the ATP content in the well, used as a measure of viable cells, is determined using a thermostable firefly luciferase–based assay. Activation of caspase-3 and caspase-7 is measured as a ratio between treated sample and untreated control with a luciferase-based assay, containing a specific proluminescent substrate. DNA replication is measured as incorporation of nucleotide analog BrdU into DNA by flow cytometry.