All MM cells are dependent on Mcl-1, although they differ in their sensitivity to ABT-737. (A) mRNA levels for Mcl-1 were derived from the analysis of normalized gene expression profile data deposited at GEO from CD138-selected plasma cells from healthy donors: normal PC (n = 22), monoclonal gammopathy of undetermined significance (n = 44), smoldering myeloma (SM, n = 12), and newly diagnosed MM (MM, n = 538). The data are taken from the 214056_at probe on the affymetrix Hu133 2.0 plus array. Similar results were obtained from the 214057_at probe (not shown). (B) Four myeloma cell lines were transfected with siRNA for Mcl-1 and efficiency of knock-down was determined by Western blot (left panel), whereas the apoptosis of each cell line 16 hours after transfection was measured by annexin PI assays. P values compare with si(-), P < .005 (KMS11, MM.1s), P < .001 (8226, KMS18). (C) Six myeloma cell lines were treated with indicated doses of ABT-737 for 24 hours, and apoptosis was measured by annexin V/PI staining. Data are mean ± SD of 4 independent experiments. (D) CD138⁺ cells purified from bone marrow aspirates of 6 MM patients were incubated with the indicated concentrations of ABT-737 for 24 hours and apoptosis determined by annexin V/PI staining. Each sample was run in parallel with MM.1s (●), which is presented as the mean ± SD of the 6 experiments. The 50% inhibitory concentration values for each sample are as follows (μM): MM.1s, 0.65; MM1-0.3, MM6-0.58, MM7-0.92, MM23-0.1, MM24, and MM26, < 0.1.