In vitro activity: PD173955 potently inhibits Bcr-Abl-dependent cell growth with IC50 of 2-35 nM in Bcr-Abl-positive cell lines, about 100- to 200-fold more sensitive than in Bcr-Abl-negative cell lines. PD173955 also inhibits kit ligand-dependent proliferation of M07e cells with IC50 of 40 nM, by inhibition of kit ligand-dependent c-kit autophosphorylation. In addition, PD173955, as a Src inhibitor, potently inhibits mitotic progression during early mitosis in cells of all types and induces varying degrees of apoptotic cell death.

Kinase Assay: Bcr-Abl complexed to SHIP2 is immunoprecipitated from cell lysates of K562 cells maintained in log-phase culture conditions. Complexes are collected on protein A-Sepharose, and complexes are washed three times in lysis buffer and then washed twice in abl kinase buffer [50 mM Tris (pH 8.0), 10 mM MgCl2, 1 mM DTT, 2 mM p-nitrophenylphosphate, and 2 μM ATP; New England Biolabs Buffer and protocol]. Kinase assays are performed with 10 μM [γ-32P]ATP/sample for 15–60 min at 30°C in the presence or absence of the indicated concentrations of drug. The reaction is stopped by the addition of SDS-PAGE sample buffer and heated at 100°C for 10 min. Proteins are separated on 7.5% SDS-polyacrylamide gels, gels are dried under vacuum, and phosphorylation is visualized by autoradiography on X-ray film.

Cell Assay: Cell growth is determined by two methods. For the [3H]thymidine assay, cells (104 cells/well, Bcr-Abl-positive cell lines (R10(+), R10(−), K562, and RWLeu4); Bcr-Abl-negative cell lines (HL-60, SK-N-ER, SK-N-MC, U138MG, and HS-16)) are cultured in 96-well, round-bottomed plates with diluted DMSO (control) or with varying concentrations of a specific compound that is resuspended in DMSO for 48 h at 37°C. [3H]Thymidine is added at a concentration of 1 μCi/well, and cells are incubated for an additional 18 h. Cells were harvested with the Unifilter system, scintillation fluid (25 μl/well) is added to each well, and [3H]thymidine incorporation is determined on a Packard Scintillation Counter. Data points for all assays are obtained in triplicate, and background incorporation from cell-free wells is determined and subtracted from all data points. For cell viability, control and drug-treated harvested cells are counted on a hemocytometer using the trypan blue dye exclusion method.