Inhibitory effect of withaferin A and quercetin on IL-6 protein production in K562 and K562/Adr cells. K562 and K562/Adr cells were pretreated with quercetin or withaferin A in different concentrations (as indicated in Figure). for 3 h, followed by incubation with PMA (100 ng/ml) for 15 h. Cell-free medium supernatant was collected for ELISA quantification of secreted levels of IL-6 protein. Data are presented as percent of control (zero concentration of withaferin A or quercetin), as mean ± S.E.M. of three independent experiments, each done in triplicate. Following IC50 values were obtained: 0.54 μM withaferin A and 29.5 μM quercetin for K562 cells; 0.58 μM withaferin A and 13.2 μM quercetin for K562/Adr cells.