ADAPTATION OF HUMAN TUMOR-CELLS TO TIRAPAZAMINE UNDER AEROBIC CONDITIONS - IMPLICATIONS OF INCREASED ANTIOXIDANT ENZYME-ACTIVITY TO MECHANISM OF AEROBIC CYTOTOXICITY

Tirapazamine (TPZ, 3-amino-1,2,4-benzotriazine 1,4-di-N-oxide, SR 4233 , WIN 59075) is a bioreductive antitumor agent with a high selective t oxicity for hypoxic cells. The selective hypoxic toxicity of TPZ resul ts from the rapid reoxidation of the one-electron reduction product, t he TPZ radical, in the presence of molecular oxygen with the concomita nt production of superoxide radical. Under hypoxia the TPZ radical kil ls cells by causing DNA double-strand breaks and chromosome aberration s. However, the mechanism of aerobic cytotoxicity is still a matter of debate. In this study, we investigated the mechanism of aerobic cytot oxicity by adapting human lung adenocarcinoma A549 cells to aerobic TP Z exposure and characterizing the changes associated with drug resista nce. The adapted cells were resistant to aerobic TPZ exposures (with d ose-modifying factors of up to 9.2), although hypoxic sensitivity was largely unchanged. Relative to the parental A549 cell line, adaptation to continuous aerobic TPZ exposure resulted in increased levels of ma nganese superoxide dismutase (up to 9.4-fold), moderate increases in g lutathione reductase (up to 2.1-fold), and loss of both quinone oxidor eductase (DT diaphorase) activity and NADPH cytochrome P450 reductase activity. There was essentially no change in the activity of the cytop lasmic form of superoxide dismutase (CuZnSOD), catalase, or glutathion e peroxidase. The increased activity of antioxidant enzymes in the res istant cell lines (in particular MnSOD) strongly suggests that reactiv e oxygen species are, in large part, responsible for the toxicity of T PZ under aerobic conditions, and is consistent with aerobic and hypoxi c drug cytotoxicity resulting from different mechanisms. (C) 1997 Else vier Science Inc.