Effects of chlorobenzenes and their methyl sulfone metabolites on microsomal enzymes associated with drug metabolism in rat liver

Role of 3,4-dichlorophenyl methyl sulfone, a metabolite of o-dichlorobenzene, in the changes in hepatic microsomal drug-metabolizing enzymes caused by o-dichlorobenzene administration in rats

2,3- and 3,4-Dichlorophenyl methyl sulfoxides and 2,3- and 3,4-dichlorophenyl methyl sulfones (2,3- and 3,4-DCPSO2Mes) were detected in the urine of rats administered o-dichlorobenzene (o-DCB). After administration of o-DCB to rats, swift decreases were observed in the concentrations of o-DCB in blood, liver, and kidneys, whereas 3,4-DCPSO2Me appeared in blood, liver, kidneys, and adipose tissue. The concentrations of 3,4-DCPSO2Me in the blood and three tissues reached maxima at 24 hr. Both aminopyrine N-demethylase and aniline hydroxylase activities and cytochrome P450 content of hepatic microsomes decreased 24 hr after administration of o-DCB. In contrast, 3,4-DCPSO2Me increased the activities of these enzymes and cytochrome P450 and b5 contents in rat liver microsomes. In both antibiotic-pretreated and bile duct-cannulated rats dosed with o-DCB, the concentrations of 2,3- and 3,4-DCPSO2Mes in blood, liver, kidneys, and adipose tissue were dramatically reduced. These findings suggest that the process of formation of methylsulfonyl metabolites of o-DCB involves biliary secretion of DCPSO2Mes and/or their precursors which will be subjected to metabolism by intestinal microflora. In antibiotic-pretreated rats, the inhibitory effects of o-DCB administration on the activities of aminopyrine- and aniline-metabolizing enzymes and the contents of cytochromes P450 and b5 in hepatic microsomes were greater than those observed in the intact rats. In bile duct-cannulated rats, the decrease in aminopyrine N-demethylase activity after administration of o-DCB was greater than that observed in the intact rats. These findings suggest that the apparent inhibition of drug-metabolizing enzymes by o-DCB is the result of simultaneous contrary effects, namely, the inductive effect of 3,4-DCPSO2Me and the stronger inhibitory effect of an unknown factor(s) on drug-metabolizing enzymes.

The differential hepatotoxicity and cytochrome P450 responses of Fischer-344 rats to the three isomers of dichlorobenzene

The acute hepatotoxicity and response of hepatic cytochrome P450 to treatment with the three isomers of dichlorobenzene (DCB) have been investigated. The objectives were to estimate the onset of toxicity and to further elucidate the role of cytochrome P450 in the metabolism and toxicity of these compounds. In a study design employing one animal per dose level, Fischer-344 rats were gavaged with up to 25 different dosages, then evaluated 24 h later. Hepatic necrosis, serum alanine aminotransferase, and serum aspartate aminotransferase exhibited similar patterns demonstrating that ortho-DCB (o-DCB) was the most toxic in terms of both earliest onset and degree of response at higher dosages. For these three endpoints, meta-DCB (m-DCB) exhibited a lesser toxicity. Para-DCB (p-DCB) did not cause changes in these three endpoints, but hepatic degenerative changes were found. Total hepatic cytochrome P450 responses were also different after treatment with each isomer. The o-DCB produced a dose-dependent decrease in P450 beginning at dosages lower than the onset of necrosis and appeared to be a suicide substrate for P450. The m-DCB treatment increased P450 at dosages below the onset of necrosis and decreased P450 at higher dosages, with the decline preceding the onset of hepatocyte death. Treatment with p-DCB increased P450 beginning at 380 mg/kg. The combination of toxicity and P450 profiles has provided a framework for interpreting literature data on the metabolism and toxicity of the DCBs in rats. It is also noteworthy that o-DCB and p-DCB were administered at dosages several times the oral rat LD-50 (RTECS) without any lethality.