In vitro activity: Flavopiridol displays less activity against unrelated kinases such as MAP, PAK, PKC, and EGFR with IC50 of>14 μM. Flavopiridol significantly inhibits the colony growth of HCT116, A2780, PC3, and Mia PaCa-2 cells with IC50 of 13 nM, 15 nM, 10 nM and 36 nM, respecitively. Flavopiridol also potently inhibits the activity of Glycogen synthase kinase-3 (GSK-3) with an IC50 of 280 nm. Compared with other CDKs, Flavopiridol inhibits the activity of CDK7 less potently with IC50 of 875 nM. Flavopiridol (0.5 μM) inhibits both pSer807/811 Rb and pThr199 NPM, whereas mild changes are observed at pThr821 Rb. Flavopiridol also decreases the overall RNA polymerase II level, as well as the phosphorylation of RNA polymerase II on the CTD repeats at Ser2 Ser5. As a broad spectrum CDK inhibitor, Flavopiridol can inhibit cell cycle progression in either G1 or G2. Flavopiridol (0.3 μM) induces G1 arrest in either MCF-7 or MDA-MB-468 cells by inhibition of the CDK4 or CDK2 kinase activity. Flavopiridol exhibits potent cytotoxicity against a wide variety of tumor cell lines with IC50 values ranging form 16 nM for LNCAP to 130 nM for K562.

Kinase Assay: For CDK1/cyclin B1 kinase assay, kinase reactions consist of 100 ng of baculovirus expressed GST-CDK1/cyclin B1 (human) complex, 1 μg histone HI, 0.2 μCi [γ-33P]ATP, 25 μM ATP in 50 μL kinase buffer (50 mM Tris, pH 8.0, 10 mM MgCl2, 1 mM EGTA, 0.5 mM DTT). For CDK2/cyclin E kinase assay, kinase reactions consist of 5 ng of baculovirus expressed GST-CDK2/cyclin E (human) complex, 0.5 μg GST-RB fusion protein (amino acids 776-928 of retinoblastoma protein), 0.2 μCi [γ-33P]ATP, 25 μM ATP in 50 μL kinase buffer (50 mM Hepes, pH 8.0, 10 mM MgCl2, 1 mM EGTA, 2 mM DTT). For CDK4/cyclin D1 kinase assay, kinase reactions consist of 150 ng of baculovirus expressed GST-CDK4/cyclin D1 (human), 280 ng of Stag-cyclin D1, 0.5 μg GST-RB fusion protein (amino acids 776-928 of retinoblastoma protein), 0.2 μCi [γ-33P]ATP, 25 μM ATP in 50 μL kinase buffer (50 mM Hepes, pH 8.0, 10 mM MgCl2, 1 mM EGTA, 2 mM DTT). Reactions are incubated for 45 minutes for CDK1 and CDK2, or 1 hour for CDK4 at 30 °C and stopped by the addition of cold trichloroacetic acid (TCA) to a final concentration 15%. TCA precipitates are collected onto GF/C unifilter plates using a Filtermate universal harvester and the filters are quantitated using a TopCount 96-well liquid scintillation counter. Flavopiridol is dissolved at 10 mM in dimethylformamide (DMF) and evaluated at six concentrations, each in triplicate. The final concentration of DMF in the assay = 2%. IC50 values are derived by nonlinear regression analysis and have a coefficient of variance = 16%. To assay Flavopiridol activity on CDK6, a filter-binding assay is established. The following are combined in the reaction mixture: 2 μL of CDK6 (0.7 mg/μL), 5 μL of histone H1 (6 mg/mL), 14 μL of kinase buffer (60 mM β-glycerophosphate, 30 mM p-nitrophenyl phosphate, 25 mM MOPS (pH 7.0), 5 mM EGTA, 15 mM MgCl2, 1 mM DTT, 0.1 mM Na-vanadate), 3 μL of increasing concentrations of Flavopiridol diluted in 50% DMSO, and 6 μL of 33P-ATP (1 mCi/mL) in nonradioactive ATP at 90 μM concentration (final concentration: 15 μM). The assay is initiated by the addition of 33P-ATP. The reaction is incubated for 20 minutes at 30°C. A 25 μL aliquot of the supernatant is then spotted onto Whatman P81 phosphocellulose paper. Filters are washed 5 times with 1% phosphoric acid solution. Wet filters are counted in the presence of 1 mL of scintillation fluid. Cdk9 activity is measured using 50 nM of recombinant Cdk9/cyclin T in 50 mM HEPES pH 7.5, 10 mM MgCl2, 1 mM DTT, 3 μM Na3VO4, 150 μM RNA polymerase CDT peptide and 80 μM ATP. Cdk7 assay is performed in the same buffer using 37 nM of purified kinase in the presence of 200 μM ATP and 10 μM myelin binding protein as a substrate. The potency of Flavopiridol toward CDK9 and CDK7 is determined using either a strong anion exchanger (Dowex 1-X8 resin, formate form)-based assay or a scintillation proximity assay. IC50 values are calculated from the dose-response curves.

Cell Assay: Cells are exposed to various concentrations of Flavopiridol for 72 hours at which time the tetrazolium dye, MTS in combination with phenazine methosulfate, is added. After 3 hours, the absorbency is measured at 492 nm, which is proportional to the number of viable cells. The results are expressed as IC50 values. For cell Cycle analysis, cells are fixed in paraformaldehyde and ethanol, washed, resuspended in staining solution of TdT enzyme and FITC-dUTP, washed, stained with PI following RNase treatment, and then analyzed by flow cytometry.

J Med Chem. 2000 Nov 2;43(22):4126-34; J Med Chem. 2002 Aug 29;45(18):3905-27.