Influence of the halogen-substituent pattern of fluoronitrobenzenes on their biotransformation and capacity to induce methemoglobinemia.

In the present study both the biotransformation patterns and the capacity to induce methemoglobinemia of a series of fluoronitrobenzenes were investigated. This was done to investigate to what extent variation in the number and position of the halogen substituents influence the metabolic fate of the fluoronitrobenzenes, thereby influencing their capacity to induce methemoglobinemia. The results obtained were compared to the effect of the fluorine substituent patterns on the calculated electronic characteristics and, thus, on the chemical reactivity of the fluoronitrobenzenes. Analysis of the in vivo metabolic profiles demonstrates a dependence of the extent of nitroreduction, of glutathione conjugation, and of aromatic hydroxylation with the pattern of halogen substitution. With an increasing number of fluorine substituents at electrophilic carbon centers, 24-hr urine recovery values decreased and fluoride anion elimination increased, due to increased reactivity of the fluoronitrobenzenes with cellular nucleophiles. In vitro studies even demonstrated a clear correlation between calculated parameters for the electrophilicity of the fluoronitrobenzenes and the natural logarithm of their rate of reaction with glutathione or with bovine serum albumin, taken as a model for cellular nucleophiles (r = 0.97 and r = 0.98, respectively). Increased possibilities for the conjugation of the fluoronitrobenzenes to cellular nucleophiles were accompanied by decreased contributions of nitroreduction and aromatic hydroxylation to the overall in vivo metabolite patterns, as well as by a decreased capacity of the fluoronitrobenzenes to induce methemoglobinemia. In vitro studies on the rates of nitroreduction of the various fluoronitrobenzenes by cecal microflora and rat liver microsomes revealed that the changes in the capacity of the fluoronitrobenzenes to induce methemoglobinemia were not due to differences in their intrinsic reactivity in the pathway of nitroreduction, leading to methemoglobinemia-inducing metabolites. Thus, the results of the present study clearly demonstrate that the number and position of fluorine substituents in the fluoronitrobenzenes influence the capacity of the fluoronitrobenzenes to induce methemoglobinemia, not because their intrinsic chemical reactivity for entering the nitroreduction pathway is influenced. The different methemoglobinemic capacity must rather result from differences in the inherent direct methemoglobinemic capacity and/or reactivity of the various toxic metabolites and/or from the fact that the halogen substituent pattern influences the electrophilic reactivity, thereby changing the possibilities for reactions of the nitrobenzenes with glutathione and, especially, other cellular nucleophiles. When the number of fluorine substituents increases, the electrophilicity of the fluoronitrobenzenes can become so high that glutathione conjugation is no longer able to compete efficiently with covalent binding of the fluoronitrobenzenes to cellular macromolecules. As a consequence, it can be suggested that with an increasing number of fluorine substituents at electrophilic carbon centers in a nitrobenzene derivative, a toxic end point of the nitrobenzene other than formation of methemoglobinemia can be foreseen.