Blocking of tumor cell apoptosis by caspase inhibitors. C1498 cells were labeled with [¹¹¹In] Ox and then incubated with PS-341 at 0 (▪), 5 nM (▵), and 20 nM (♦) and (A) zFA-FMK (50 μM) or (B) zVAD-FMK (50 μM) for 2 hours prior to the addition of various concentrations of TRAIL. After a further 16 hours, lysis (± SEM) was estimated as described in “Materials and methods.” Renca cells were incubated with PS-341 at 0 (▪), 5 nM (▵), and 20 nM (♦) and (C) zFA-FMK (50 μM) or (D) zVAD-FMK (50 μM) for 2 hours prior to the addition of various concentrations of TRAIL. After a further 16 hours, cell viability (± SEM) was assessed by the uptake of the MTS dye as described in “Materials and methods.”