Effect of nutlin-3a on cells of the hematopoietic system. (A) Overnight exposure of CD34⁺ hematopoietic stem cells to 10 μM nutlin-3a activated p53, as assessed by Western analysis. (B) CD34⁺ cells were cocultured with BMSCs and cell death assessed after a 3-day exposure to increasing concentrations of nutlins and 2 therapeutically relevant cytostatics. A strong decrease in viability in response to nutlin-3a was recorded (EC₅₀ value about 2.4 μM; range, 1.9-2.9 μM; cells from 3 different patients tested). The genotoxic cytostatic melphalan was equally toxic in this assay (range of EC₅₀ values, 1.1-1.8 μM). (C) Viability of in vitro differentiated plasmablastic cells, cocultured with BMSCs, after a 3-day exposure to either nutlins, melphalan, or etoposide (all at 10 μM). Treatment with nutlin-3a or with melphalan strongly decreased the number of viable cells. Height of the bars represents the median survival relative to DMSO control from experiments with cells from 3 different donors.