Cytochrome P450-Mediated Metabolism and DNA Binding of 2-Amino-1,7-dimethylimidazo[4,5-<i>g</i>]quinoxaline and Its Carcinogenic Isomer 2-Amino-3,8-dimethylimidazo[4,5-<i>f</i>]quinoxaline in Mice

2-Amino-1,7-dimethylimidazo­[4,5-<i>g</i>]­quinoxaline (MeI<i>g</i>Qx) is a recently discovered heterocyclic aromatic amine (HAA) that is formed during the cooking of meats. MeI<i>g</i>Qx is an isomer of 2-amino-3,8-dimethylmidazo­[4,5-<i>f</i>]­quinoxaline (MeIQx), a rodent carcinogen and possible human carcinogen that also occurs in cooked meats. MeI<i>g</i>Qx is a bacterial mutagen, but knowledge about its metabolism and carcinogenic potential is lacking. Metabolism studies on MeI<i>g</i>Qx and MeIQx were conducted with human and mouse liver microsomes, and recombinant human P450s. DNA binding studies were also investigated in mice to ascertain the genotoxic potential of MeI<i>g</i>Qx in comparison to MeIQx. Both HAAs underwent comparable rates of <i>N</i>-oxidation to form genotoxic <i>N</i>-hydroxylated metabolites with mouse liver microsomes (0.2–0.3 nmol/min/mg protein). The rate of <i>N</i>-oxidation of MeIQx was 4-fold greater than the rate of <i>N</i>-oxidation of MeI<i>g</i>Qx with human liver microsomes (1.7 vs 0.4 nmol/min/mg protein). The rate of <i>N</i>-oxidation, by recombinant human P450 1A2, was comparable for both substrates (6 pmol/min/pmol P450 1A2). MeI<i>g</i>Qx also underwent <i>N</i>-oxidation by human P450s 1A1 and 1B1 at appreciable rates, whereas MeIQx was poorly metabolized by these P450s. The potential of MeI<i>g</i>Qx and MeIQx to form DNA adducts was assessed in female C57BL/6 mice given [¹⁴C]-MeI<i>g</i>Qx (10 μCi, 9.68 mg/kg body wt) or [¹⁴C]-MeIQx (10 μCi, 2.13 mg/kg body wt). DNA adduct formation in the liver, pancreas, and colorectum was measured by accelerator mass spectrometry at 4, 24, or 48 h post-treatment. Variable levels of adducts were detected in all organs. The adduct levels were similar for both HAAs, when adjusted for dose, and ranged from 1 to 600 adducts per 10⁷ nucleotides per mg/kg dose. Thus, MeI<i>g</i>Qx undergoes metabolic activation and binds to DNA at levels that are comparable to MeIQx. Given the high amounts of MeI<i>g</i>Qx formed in cooked meats, further investigations are warranted to assess the carcinogenic potential of this HAA