Apoptotic and cytotoxic effect of ST1926 in NB4 and NB4.437r cells: comparison with CD437. NB4 (A,B) or NB4.437r (C,D) cells (300 000/mL) were treated with vehicle and the indicated concentrations of ST1926 and CD437 for 4 hours (A) or 6 hours (C) in the case of the measurement of the apoptotic index or for 24 hours in the case of the determination of the number of viable cells (B, D). Results are expressed as the mean ± SD of 3 replicate cultures. The data are representative of 2 independent experiments. In A and B, experimental points were fitted using the sigmoidal dose-response equation (variable slope) and statistically analyzed with the GraphPad Prism version 4.00 for Windows (GraphPad Software, San Diego, CA). Calculation of the EC₅₀ values and statistical analysis of the data were performed with the same program. The EC₅₀ value for the apoptotic effect of ST11926 (0.2 μM; interval of confidence 0.08-0.67, n = 3) is significantly lower (P < .03, following application of the F test) than that of CD437 (1.1 μM; interval of confidence 0.82-1.51, n = 3). Similarly, the EC₅₀ value for the cytotoxic effect of ST11926 (0.1 μM; interval of confidence 0.05-0.19, n = 3) is significantly lower (P < .01, following application of the F test) than that of CD437 (0.9 μM; interval of confidence 0.43-2.07, n = 3). *Significantly higher or lower than the corresponding vehicle-treated group (P < .01 according to the Student t test).