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 capac ity to induce methemoglobinemia of a series of fluoronitrobenzenes wer e investigated. This was done to investigate to what extent variation in the number and position of the halogen substituents influence the m etabolic fate of the fluoronitrobenzenes, thereby influencing their ca pacity 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 th e fluoronitrobenzenes. Analysis of the in vivo metabolic profiles demo nstrates 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 e lectrophilic 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 e ven demonstrated a clear correlation between calculated parameters for the electrophilicity of the fluoronitrobenzenes and the natural logar ithm of their rate of reaction with glutathione or with bovine serum a lbumin, 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 decr eased contributions of nitroreduction and aromatic hydroxylation to th e overall in vivo metabolite patterns, as well as by a decreased capac ity of the fluoronitrobenzenes to induce methemoglobinemia. In vitro s tudies on the rates of nitroreduction of the various fluoronitrobenzen es by cecal microflora and rat liver microsomes revealed that the chan ges in the capacity of the fluoronitrobenzenes to induce methemoglobin emia were not due to differences in their intrinsic reactivity in the pathway of nitroreduction, leading to methemoglobinemia-inducing metab olites. Thus, the results of the present study clearly demonstrate tha t the number and position of fluorine substituents in the fluoronitrob enzenes influence the capacity of the fluoronitrobenzenes to induce me themoglobinemia, not because their intrinsic chemical reactivity for e ntering the nitroreduction pathway is influenced. The different methem oglobinemic capacity must rather result from differences in the inhere nt direct methemoglobinemic capacity and/or reactivity of the various toxic metabolites and/or from the fact that the halogen substituent pa ttern influences the electrophilic reactivity, thereby changing the po ssibilities for reactions of the nitrobenzenes with glutathione and, e specially, other cellular nucleophiles. When the number of fluorine su bstituents increases, the electrophilicity of the fluoronitrobenzenes can become so high that glutathione conjugation is no longer able to c ompete efficiently with covalent binding of the fluoronitrobenzenes to cellular macromolecules. As a consequence, it can be suggested that w ith an increasing number of fluorine substituents at electrophilic car bon centers in a nitrobenzene derivative, a toxic end point of the nit robenzene other than formation of methemoglobinemia can be foreseen. ( C) 1996 Academic Press, Inc.