Kinase inhibitor screen identifies PIK-75 as being able to downregulate Mcl-1 protein levels and induces apoptosis in AML cells. MEF and FDM cell lines were screened against 96 kinase inhibitors (supplemental Table 1) to identify compounds that reduced Mcl-1 protein. We employed (A) Bax^−/−Bak^−/− MEFs and (B) Bax^−/−Bak^−/− FDM cells so that hits could be identified in the absence of apoptosis. Cells were incubated for 24 hours in inhibitors (2 μM) following which cell lysates were immunoblotted with Mcl-1 or HSP70 antibodies and protein levels quantified. Inhibitors that reduced Mcl-1 to below (A) 50% or (B) 60% were confirmed in subsequent assays. (C) The FLT3-ITD+ AML cell line, MV4;11, was used in a screen using an additional panel of 24 kinase inhibitors. Plotted in (C) is the percentage reduction in Mcl-1 observed (supplemental Figure 1B) following drug treatment. Dashed lines represent the Mcl-1 protein level (100%) in the presence of the DMSO vehicle. (D) The bulk mononuclear fraction of AML blasts, FACS-purified primary human CD34+CD38-CD123+ leukemic stem and progenitor cells (supplemental Figure 1C) from AML patients (n = 46, AML12-57; supplemental Table 4) or CD34+ progenitors from the BM (n = 3) of normal donors were plated in increasing concentrations of PIK-75 and the IC₅₀ values for cell death determined using an annexinV/7AAD assay at 24 hours. Shown is the median IC₅₀ in a box-and-whiskers plot with 25th to 75th percentiles representing the top and bottom of the rectangle and error bars representing the 10th to 90th percentiles. The statistical difference between the IC₅₀ values of leukemic stem and progenitor cells and normal CD34+ cells was calculated using Mann-Whitney U nonparametric t test.