NADPH : cytochrome C (P450) reductase activates tirapazamine (SR4233) to restore hypoxic and oxic cytotoxicity in an aerobic resistant derivative of the A549 lung cancer cell line

Tirapazamine (TPZ, SR4233, WIN 59075) is a bioreductive drug that is activa ted in regions of low oxygen tension to a cytotoxic radical intermediate. T his labile metabolite shows high selective toxicity towards hypoxic cells, such as those found in solid tumours. Under aerobic conditions, redox cycli ng occurs with subsequent generation of superoxide radicals, which are also cytotoxic. NADPH:cytochrome c (P450) reductase (P450R) is a one-electron r educing enzyme that efficiently activates TPZ. Recently a derivative of the A549 non-small cell lung cancer cell line (A549c50) was generated that sho wed substantially reduced P450R activity compared to its parental line (Elw ell et al (1997) Biochem Pharmacol 54: 249-257). Here, it is demonstrated t hat the A549c50 cells are markedly more resistant to TPZ under both aerobic and hypoxic conditions. In addition, these cells have a dramatically impai red ability to metabolize TPZ to its two-electron reduction product, SR4317 , under hypoxic conditions when compared to wild-type cells. P450R activity in the A549c50 cells was reintroduced to similar levels as that seen in th e parental A549 cells by transfection of the full-length cDNA for human P45 0R. These P450R over-expressing cells exhibit restored sensitivity to TPZ u nder both aerobic and hypoxic conditions, comparable to that found in the o riginal parental A549 cells. Further, the ability of the transfected cells to metabolize TPZ to SR4317 under hypoxic conditions is also shown to be re stored. This provides further evidence that P450R can play an important rol e in the activation, metabolism and toxicity of this lead bioreductive drug . (C) 2000 Cancer Research Campaign.