The antiproliferative effects and apoptosis inducing abilities of four 18beta-glycyrrhetinic acid (GA) derivatives, methyl 2-cyano-3,11-dioxooleana-1,12-dien-30-oate (CDODO-Me-11), methyl 2-cyano-3,12-dioxooleana-1,12-dien-30-oate (CDODO-Me-12) and their non-esters were investigated in human leukemia cells. Methyl esterification and switching a keto group from position C(11) to C(12) significantly increased the antiproliferative effects. CDODO-Me-11 and CDODO-Me-12 were 10-fold more potent than their non-esters, respectively. CDODO-Me-12 was 10-fold more effective than CDODO-Me-11 in inducing apoptosis which was correlated with the activation of caspase-8 and caspase-9. Western blot analyses revealed that CDODO-Me-12 and CDODO-Me-11 downregulated the levels of anti-apoptosis proteins, c-FLIP, XIAP and Mcl-1, without altering the protein levels of Bcl-2 and the death receptors DR4 and DR5. Both agents decreased the levels of the mitochondrial membrane potential without altering the intracellular H(2)O(2) levels. Jurkat cells without expression of caspase-8 were not sensitive to CDODO-Me-12, but were somewhat responsive to CDODO-Me-11. K562 cells with higher intracellular reduced glutathione (GSH ) levels were less responsive to CDODO-Me-12 apoptosis induction than U937 cells even though both cell lines were equally sensitive to CDODO-Me-11 apoptosis induction. Both agents depleted intracellular GSH levels and exogenous GSH reversed apoptosis induction by either agent in HL-60 cells. N-acetylcysteine (NAC) significantly attenuated apoptosis induction by CDODO-Me-12, but only weakly, that by CDODO-Me-11. UV spectrophotometric analysis revealed that both agents interacted with GSH while only CDODO-Me-12 had high reactivity with NAC. These data suggest that both agents induce apoptosis requiring to bind to functional proteins with thiol groups and that GSH may play a protective role by forming inactive adducts with them.