Effect of fluorine substitution on benzo[ j]fluoranthene genotoxicity

Abstract

The metabolism and mutagenic activity of 4-fluorobenzo[ j]fluoranthene (4F-B[ j]F) and 10-fluorobenzo[ j]fluoranthene (10F-B[ j]F) were evaluated and compared with benzo[ j]fluoranthene (B[ j]F) using an identical rat liver homogenate preparation. Previous studies have shown that the major genotoxic metabolites of B[ j]F are the 4,5- and 9,10-dihydrodiol. The 9,10-dihydrodiol was the principal metabolite formed in the case of 4F-B[ j]F, while the 4,5-dihydrodiol was the principal metabolite formed in the metabolism of 10F-B[ j]F. Studies on the relative genotoxicity of these fluorinated derivatives were performed to indirectly determine the possible contribution of the 4,5- and 9,10-dihydrodiol in the activation of B[ j]F to a genotoxic agent. In the presence of microsomal activation, both of these fluorinated derivatives of B[ j]F were more mutagenic in S. typhimurium TA97a, TA98 and TA100 than B[ j]F. However, differences in mutagenic potency were observed between 4F- and 10F-B[ j]F. 10F-B[ j]F had similar mutagenic potency to 4F-B[ j]F in TA97a and TA98 at doses associated with the linear portion of the dose response curve. However, a slightly higher mutagenic response was observed with 10F-B[ j]F in TA98 at doses above 5 nmol. In contrast, 4F-B[ j]F was more active than 10F-B[ j]F as a mutagen in TA100. The tumor-initiating activity of these analogs on mouse skin was assessed at doses of 2.0, 1.0 and 0.3 μmol. Skin irritation was observed with the fluorinated B[ j]F derivatives at doses above 0.3 μmol. At a dose of 0.3 μmol, 4F-B[ j]F exhibited tumorigenic activity which was similar to B[ j]F. In contrast, 10F-B[ j]F was less active than B[ j]F at all three doses assayed. Both fluorinated derivatives of B[ j]F formed higher levels of DNA adducts in vivo in mouse skin than B[ j]F. A modified 32P-postlabeling method was required to detect fast migrating B[ j]F:DNA adducts that went undtected in previous studies. The level of DNA adducts formed from 4F-B[ j]F was considerably greater than the levels observed with 10F-B[ j]F. This is consistent with the greater mutagenic activity in S. typhimurium TA100 and tumorinitiating activity exhibited by 4F-B[ j]F. These studies suggest that fluorine substitution may significantly alter the intrinsic genotoxicity of the 4,5- and 9,10-dihydrodiol of B[ j]F. These data also imply that B[j]F may be primarily activated via the formation of the 9,10-dihydrodiol metabolite. This pathway of activation is inconsistent with our previous studies which indicate that the 4,5-dihydrodiol is the most important pathway of activation. The present study raises serious concerns as to whether fluorine substitution provides a valid means for assessing the potential pathways associated with the metabolic activation of polycyclic aromatic hydrocarbons.