GO-203 and BTZ synergistically induce ROS in BTZ-resistant cells. (A-D) The indicated cells were left untreated (CTL) and treated with (1) 2.5 μM GO-203 alone each day for 72 hours; (2) 20 (U266/BR), 16 (RPMI8226/BR) or 5 (ANBL-6/BR) nM BTZ for 24 hours; or (3) GO-203 for 48 hours combined with BTZ during an additional 24 hours. GO-203/BTZ-treated cells were also incubated in the presence of 5 mM NAC for 72 hours. Cells were analyzed for relative hydrogen peroxide (A,D) or superoxide levels (B-C) (mean ± SD of 3 determinations) as compared with that obtained with control cells.