Caspase-3, -8 and -9 activities in NCI/ADR-RES cells treated with PE5 or onconase. Cells were treated with 35 μM PE5 or 5 μM onconase (ONC) for 24 (black bars), 48 (striped bars) and 72 h (checkered bars). White bars indicate untreated cells (control). Parallel samples were treated with the specific inhibitors of caspase-3 (z-DEVD-FMK), caspase-8 (z-IETD-FMK), caspase-9 (z-LEHD-FMK) (stippled bars) as a negative control. Activation of caspase-3, -8 and -9 was quantified in whole cell lysates using a quantitative colorimetric assay as described in the text. Results are expressed as the mean ± SE of three independent experiments.