title: Spin trapping of radicals other than the *OH radical upon reduction of the anticancer agent tirapazamine by cytochrome P450 reductase.
abstract: The radical species produced following one-electron reduction of tirapazamine (3-amino-1,2,4-benzotriazine 1,4-dioxide, TPZ) by cytochrome P(450) reductase-enriched microsomes have been investigated using electron paramagnetic resonance (EPR) spectroscopy. Spin trapping with 5,5'-dimethylpyrroline 1-N-oxide (DMPO) gave a composite spectrum of a carbon-centered radical and the well-known DMPO-OH adduct. Using (17)O-labeled water resulted in a change in the EPR spectrum to that of DMPO-(17)OH, indicating that this radical species is formed with solvent involvement and not from release of a (*)OH radical from one-electron-reduced TPZ. Furthermore, using the closely related spin trap 5-diethoxyphosphoryl-5-methylpyrroline N-oxide (DEPMPO), which is less prone to oxidation than DMPO, gave only a carbon-centered radical spectrum without any involvement of a (*)OH radical. Reduction of a more soluble analogue of TPZ, in redox equilibrium with its 1-oxide derivative, led to spin trapping of both a carbon-centered radical and a nitrogen-centered radical by N-tert-butyl-alpha-phenylnitrone (PBN). The multicentered nature of this nitrogen-centered radical spectrum provides support for the formation of a benzotriazinyl radical following one-electron reduction of this class of bioreductive drug.
authors.affiliation: Department of Chemistry and Auckland Cancer Society Research Centre, The University of Auckland, Private Bag 92019, Auckland 1142, New Zealand.
authors.forename: Sujata S
authors.initials: SS
authors.lastname: Shinde
authors.forename: Michael P
authors.initials: MP
authors.lastname: Hay
authors.forename: Adam V
authors.initials: AV
authors.lastname: Patterson
authors.forename: William A
authors.initials: WA
authors.lastname: Denny
authors.forename: Robert F
authors.initials: RF
authors.lastname: Anderson
mesh_terms: D000970:Antineoplastic Agents / Q000737:chemistry* / Q000378:metabolism; D003497:Cyclic N-Oxides / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods; D017665:Hydroxyl Radical / Q000737:chemistry / Q000378:metabolism; D009251:NADPH-Ferrihemoprotein Reductase / Q000737:chemistry* / Q000378:metabolism; D010084:Oxidation-Reduction; D011758:Pyrroles / Q000737:chemistry; D013113:Spin Labels; D019097:Spin Trapping / Q000379:methods; D000077704:Tirapazamine; D014227:Triazines / Q000737:chemistry* / Q000378:metabolism
chemical_list: C510912:(diethoxyphosphoryl)-5-methylpyrroline N-oxide; D000970:Antineoplastic Agents; D003497:Cyclic N-Oxides; D011758:Pyrroles; D013113:Spin Labels; D014227:Triazines; D000077704:Tirapazamine; D017665:Hydroxyl Radical; C017245:5,5-dimethyl-1-pyrroline-1-oxide; D009251:NADPH-Ferrihemoprotein Reductase