ABL1 inhibition reduces MM cell viability and increases cytotoxicity of melphalan in MM cells. (A) ABL1 in MM cells was inhibited using shRNAs (control, C; ABL1-targeting, sh-ABL1), and after selection, cell viability was assessed for 4 days. (B) Control and ABL1-knockdown (sh-ABL1) MM cells were treated with different concentrations of melphalan for 48 hours, and cell viability was measured. (C) Normal/noncancerous cell types (HS5, human bone marrow stromal cells; HDF, human normal diploid fibroblasts; and PBMC, peripheral blood mononuclear cells from 2 different donors) and MM cell lines were treated with different concentrations of nilotinib, and cell viability was measured. Bar graph showing impact of nilotinib on cell viability; error bars represent SDs of triplicate assays. (D) MM cell lines (MM1.S, RPMI8226, and LR5) were treated with different concentrations of nilotinib and melphalan, and cell viability was measured after 48 hours and combination index (CI) calculated using CalcuSyn software (CI < 1, synergism; CI = 1, additive; CI > 1: antagonism). (E) MM.1S cells were treated with either DMSO (D), 2.5 μM nilotinib (N), 5 μM melphalan (M), or a combination (M + N) and evaluated for apoptosis using annexin/PI staining. Flow cytometry images of a representative experiment (Ei) and bar graphs showing apoptosis in triplicate experiments (Eii) are shown. (F) Bone marrow plasma cells from 3 patients with relapsed MM were treated as indicated for 24 hours and cell viability measured. Error bars indicate SDs of experiments conducted in triplicate, and 2-tailed P values are derived by t test (∗P < .05). DMSO, dimethyl sulfoxide; PI, propidium iodide; SD, standard deviation.