Cytotoxic effect of PEITC in fludarabine-sensitive and resistant CLL cells. (A) Cytotoxicity of PEITC and F-ara-A in primary CLL cells, after 72-hour incubation detected by MTT assay. Results of 8 representative patient samples (4 F-ara-A–sensitive CLL in blue, S-1 to S-4; 4 F-ara-A–resistant CLL in red, R-1 to R-4) are shown. Each data point represents the mean of triplicate determinations. (B) Comparison between the concentrations required to cause a loss in cell viability by 50% (IC₅₀) of F-ara-A and PEITC in F-ara-A–sensitive (n = 22) and –resistant (n = 18) CLL cells. Cells with an IC₅₀ < 10 μM F-ara-A were considered fludarabine-sensitive, whereas those with an IC₅₀ > 10 μM were considered fludarabine-resistant.