Characterization of radicals formed following enzymatic reduction of 3-substituted analogues of the hypoxia-selective cytotoxin 3-amino-1,2,4-benzotriazine 1,4-dioxide (tirapazamine).
The mechanism by which the 1,2,4-benzotriazine 1,4-dioxide (BTO) class of bioreductive hypoxia-selective prodrugs (HSPs) form reactive radicals that kill cancer cells has been investigated by steady-state radiolysis, pulse radiolysis (PR), electron paramagnetic resonance (EPR), and density functional theory (DFT) calculations. Tirapazamine (TPZ, 3-amino BTO, 1) and a series of 3-substituted analogues, -H (2), -methyl (3), -ethyl (4), -methoxy (5), -ethoxymethoxy (6), and -phenyl (7), were reduced in aqueous solution under anaerobic steady-state radiolysis conditions, and their radicals were found to remove the substrates by short chain reactions of different lengths in the presence of formate ions. Multiple carbon-centered radical intermediates, produced upon anaerobic incubation of the compounds with cytochrome P(450) reductase enriched microsomes, were trapped by N-tert-butyl-alpha-phenylnitrone and observed using EPR. The highly oxidizing oxymethyl radical, from compound 5, was identified, and experimental spectra obtained for compounds 1, 2, 3, and 7 were well simulated after the inclusion of aryl radicals. The identification of a range of oxidizing radicals in the metabolism of the BTO compounds gives a new insight into the mechanism by which these HSPs can cause a wide variety of damage to biological targets such as DNA.
Shinde
Sujata S
SS
Department of Chemistry and Auckland Cancer Society Research Centre, The University of Auckland, Private Bag 92019, Auckland 1142, New Zealand.
Maroz
Andrej
A
Hay
Michael P
MP
Patterson
Adam V
AV
Denny
William A
WA
Anderson
Robert F
RF
D000970:Antineoplastic Agents / Q000737:chemistry* / Q000378:metabolism*; D003603:Cytotoxins / Q000737:chemistry* / Q000378:metabolism*; D004578:Electron Spin Resonance Spectroscopy; D008861:Microsomes / Q000378:metabolism; D009251:NADPH-Ferrihemoprotein Reductase / Q000378:metabolism; D010084:Oxidation-Reduction; D011675:Pulse Radiolysis; D011789:Quantum Theory; D000077704:Tirapazamine; D014227:Triazines / Q000737:chemistry* / Q000378:metabolism*
D000970:Antineoplastic Agents; D003603:Cytotoxins; D014227:Triazines; D000077704:Tirapazamine; D009251:NADPH-Ferrihemoprotein Reductase