Mechanism-based chemopreventive strategies against etoposide-induced acutemyeloid leukemia: Free radical/antioxidant approach

Etoposide (VP-16) is extensively used to treat cancer, yet its efficacy is calamitously associated with an increased risk of secondary acute myelogeno us leukemia. The mechanisms for the extremely high susceptibility of myeloi d stem cells to the leukemogenic effects of etoposide have not been elucida ted. We propose a mechanism to account for the etoposide-induced secondary acute myelogenous leukemia and nutritional strategies to prevent this compl ication of etoposide therapy. We hypothesize that etoposide phenoxyl radica ls (etoposide-O-.) formed from etoposide by myeloperoxidase are responsible for its genotoxic effects in bone marrow progenitor cells, which contain c onstitutively high myeloperoxidase activity. Here, we used purified human m yeloperoxidase, as well as human leukemia HL60 cells with high myeloperoxid ase activity and provide evidence of the following. 1) Etoposide undergoes one-electron oxidation to etoposide-O-. catalyzed by both purified myeloper oxidase and myeloperoxidase activity in HL60 cells; formation of etoposide- O-. radicals is completely blocked by myeloperoxidase inhibitors, cyanide a nd azide, 2) Intracellular reductants, GSH and protein sulfhydryls (but not phospholipids), are involved in myeloperoxidase-catalyzed etoposide redox- cycling that oxidizes endogenous thiols; pretreatment of HL60 cells with a maleimide thiol reagent, ThioGlo1, prevents redox-cycling of etoposide-O-. radicals and permits their direct electron paramagnetic resonance detection in cell homogenates. VP-16 redox-cycling by purified myeloperoxidase tin t he presence of GSH) or by myeloperoxidase activity in HL60 cells is accompa nied by generation of thiyl radicals, GS(.), determined by HPLC assay of 5, 5-dimethyl-1-pyrroline glytathionyl N-oxide glytathionyl nitrone adducts. 3 ) Ascorbate directly reduces etoposide-O-., thus competitively inhibiting e toposide-O-.-induced thiol oxidation. Ascorbate also diminishes etoposide-i nduced topo II-DNA complex formation in myeloperoxidase-rich HL60 cells (bu t not in HL60 cells with myeloperoxidase activity depleted by pretreatment with succinyl acetone). 4) A vitamin E homolog, 2,2,5,7,8-pentamethyl-6-hyd roxychromane, a hindered phenolic compound whose phenoxyl radicals do not o xidize endogenous thiols, effectively competes with etoposide as a substrat e for myeloperoxidase, thus preventing etoposide-O-.-induced redox-cycling. We conclude that nutritional antioxidant strategies can be targeted at min imizing etoposide conversion to etoposide-O-., thus minimizing the genotoxi c effects of the radicals in bone marrow myelogenous progenitor cells, i.e. , chemoprevention of etoposide-induced acute myelogenous leukemia.