Multidrug resistant KBvin cells were treated with various concentrations of compounds 6 or 23 in the presence of 100 nM TAX, an absolutely non-toxic concentration for KBvin. Data are expressed as mean ± SE of three independent experiments. Calculated median effective concentration (EC50) of compounds 6, 23, or VRP was 2.81 μM, 1.87 μM or 2.71 μM, respectively.