In vitro activity: Flavopiridol is initially found to inhibit the epidermal growth factor receptor and protein kinase A (IC50 = 21 and 122 μM). Flavopiridol is later shown to inhibit cell proliferation, at more physiologically relevant concentrations (IC50 = 66 nM) when Flavopiridol is tested in the National Cancer Institute Development Therapeutics Program panel of 60 human tumor cell lines. Flavopiridol induces G1 arrest with inhibition of CDK2 and CDK4 in human breast carcinoma cells in a time and concentration dependent manner. Short time treatment of Flavopiridol (~12 hours) induce apoptosis in hematopoietic cell lines including SUDHL4, SUDHL6 (B-cell lines), Jurkat and MOLT4 (T-cell lines ), and HL60 (myeloid). In the clonogenic assay, Flavopiridol functions as a highly potent cytotoxic compound with a mean IC70 with 8 ng/mL in 23 human tumor models. A recent study shows Flavopiridol treatment induces a substantial AKT-Ser473 phosphorylation in human glioblastoma T98G cell line.

Kinase Assay: CDKs activities are determined in microtiter plates as follows. Forty μg Gst-Rb are mixed with different amounts of Flavopiridol and unlabeled ATP. Reactions are then started by the addition of an ammonium sulfate cut of the S100 fraction obtained from insect cells expressing recombinant human CDKs. The final reaction conditions are 10 mM MgCl2, 50 mM Tris-HCl (pH 7.5), and 1 mM DTT. The final concentration of ATP is adjusted accordingly. Radiolabeled ATP is used as a phosphoryl donor. The reaction is carried out for 2.5 minutes at 30 °C after addition of enzyme and then terminated with the addition of EDTA. The Gst-Rb is then captured with glutathione-Sepharose and the incorporated radioactivity is determined by liquid scintillation counting.

Cell Assay: Cells grown at a density of 1 × 106 cells/mL are exposed to Flavopiridol for different concentrations and time periods. DNA is extracted. Briefly, cells are washed once with cold phosphate-buffered saline (PBS) and lysed with 3 mL lysis buffer (5 mM Tris-HCL [pH 7.5]; 20 mM EDTA; 0.5% Triton X-100) for 15 minutes at 4 °C. The chromatin of the cell lysates is isolated by centrifugation (20 minutes at 26,000g, 4 °C). The supernatants containing small DNA fragments are extracted sequentially with phenol, phenol:chloroform (1:1), and chloroform. Nucleic acids are precipitated in 0.5 M NaCl, 90% ethanol at -20 °C overnight. RNA is then digested by bovine RNAaseA (60 μg/mL). After sequential reextraction and reprecipitation, DNA is dissolved in 10 mM Tris-HCL (pH 7.5), 1 mM EDTA, 0.5% sodium dodecyl sulfate (SDS) before electrophoresis on 1.6% agarose gel.

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