Zalypsis inhibits the viability of MM cells while preserving normal hematopoietic progenitor cells. (A) Chemical structure of Zalypsis. (B) Nine MM cell lines were incubated with different concentrations of Zalypsis for 24, 48, and 72 hours, and cell viability was analyzed by MTT uptake. The average proliferation values of control untreated samples were taken as 100%. Data are mean plus or minus SD of quadruplicates of an experiment that was repeated at least twice. (C) Freshly isolated BM cells obtained from 6 MM patients were plated in 6-well plates and treated ex vivo with Zalypsis (1-50 nM) for 18 hours. After the incubation period, cells were stained with the combination of annexin V–FITC and 3 monoclonal antibodies against plasma cell surface antigens (CD38, CD56, and CD45), which allows the analysis of the induction of apoptosis in the myelomatous population. Results are given as the percentage of annexin V–positive cells related to the percentage of viable cells in the untreated sample. (D) Freshly isolated BM cells obtained from an MM patient were treated ex vivo with 10 nM Zalypsis for 18 hours. After the incubation period, cells were stained with the combination of annexin V and 2 monoclonal antibodies, CD38 and CD34, to separately analyze the plasma cell (CD38⁺⁺, CD34⁻; in red) and the hematopoietic progenitor cell (CD34⁺, CD38^+d; in blue) compartments. The first graphs allow the identification of both populations with the 2 monoclonal antibodies; and in the second plot, the induction of apoptosis (by annexin V staining) in each compartment is displayed.