Dose-response profiles of 8226 myeloma cells selected for resistance to CD95-mediated apoptosis. Cells were plated in 96-well plates at 5 × 10⁴/mL with the indicated concentrations of chemotherapeutic drugs. Percent survival was determined by MTT dye reduction after 96 hours of drug exposure, and the IC₅₀calculated by linear regression analysis. Each drug was examined by three independent assays with eight replicates per assay. (A) VP-16; (B) ara-C; (C) vincristine; (D) mitoxantrone; (E) doxorubicin; (F) doxorubicin ± ZB4. (•), 8226/S; (▪), 8226/Dox40; (▴), 8226/F4. For plots A, B, C, D, and E, open symbols represent individual clones of 8226/F4. In plot F, open symbols show the effects of 1 hour of preincubation with 500 μg/mL ZB4 on doxorubicin cytotoxicity or CH-11–mediated apoptosis (inset).