Sensitivity to p53 activation in MM cell lines. (A) Western analysis for p53, MDM2, and p21^WAF1 in MM cell lines after overnight incubation with 10 μM nutlin-3a (A), 10 μM nutlin-3b (B), or DMSO (D). Nutlin-3a selectively augmented the p53 level in MM.1s and NCI-H929, whereas the other lines displayed high and unmodifiable levels of p53. Induction of downstream targets was strictly correlated with induction of p53. These data show that only 2 of the 6 cell lines tested retain functional p53 signaling. (B) Nutlin-3a–induced cell death is restricted to MM cells with wild-type p53. Cells were exposed for 4 days to 10 μM nutlin-3a, nutlin-3b, or DMSO and apoptosis measured. Numbers show the means of 3 independent tests and represent the percentage of live cells relative to the DMSO control. The mutation status of p53 was inferred from sequencing at gDNA and cDNA levels and confirmed that induction of p53 pathway components was strictly correlated with the presence of wild-type p53. (C) Dose-response curves for nutlin-3a–induced cell death in MM.1s and NCI-H929 as assessed by annexin-FITC/PI staining after 4 days (MM.1s) or 3 days (NCI-H929) of incubation with the drug. EC₅₀ values were 5.2 μM for MM.1s and 3.5 μM for NCI-H929. Nutlin-3b was without effect in this concentration range.