Effect of 1,4-dibromobenzene and 1,2,4-tribromobenzene on xenobiotic metabolism

Subchronic Hepatotoxicity Evaluation of 1,2,4-Tribromobenzene in Sprague-Dawley Rats

Male Sprague-Dawley rats were exposed to 1,2,4-tribromobenzene (TBB) by gavage for 5 days, 2, 4, and 13 weeks at 0, 2.5, 5, 10, 25, or 75 mg/kg per d. There were no TBB exposure-related clinical signs of toxicity or changes in body weight. Liver weight increases were dose and exposure time related and statistically significant at ≥10 mg/kg per d. Incidence and severity of centrilobular cytoplasmic alteration and hepatocyte hypertrophy were dose and time related. The 75 mg/kg per d group had minimally increased mitoses within hepatocytes (5 days only). Hepatocyte vacuolation was observed (13 weeks) and was considered TBB exposure related at ≥25 mg/kg per d. Concentrations of blood TBB increased linearly with dose and at 13 weeks, ranged from 0.5 to 17 µg/mL (2.5-75 mg/kg per d). In conclusion, rats administered TBB doses of 10-75 mg/kg per d for 13 weeks had mild liver effects. A no observed adverse effect level of 5 mg/kg per d was selected based on the statistically significant incidence of hepatocyte hypertrophy at doses ≥10 mg/kg per d.

Correlation of structural class with no-observed-effect levels: a proposal for establishing a threshold of concern

The relationship between chemical structure and toxicity was explored through the compilation of a large reference database consisting of over 600 chemical substances tested for a variety of endpoints resulting in over 2900 no-observed-effect levels (NOELs). Each substance in the database was classified into one of three structural classes using a decision tree approach. The resulting cumulative distributions of NOELs for each of the structural classes differed significantly from one another, supporting the contention that chemical structure defines toxicity. The database was used to derive a threshold of acceptable human exposure for each of the structural classes that could be applied in the absence of specific toxicity data on a substance within one of the three structural classes. The human exposure thresholds provide guidance on the degree of testing and evaluation required for substances that lack toxicity data.

Differential induction of hepatic drug metabolizing enzymes in Japanese quail by 1,2,4-trichlorobenzene

The effects of single oral administration of 1,2,4-trichlorobenzene (TCB), 200, 400, 800 or 1600 mg/kg, and of daily oral administration of TCB, 400 mg/kg, for 3 consecutive days, on components of the microsomal monooxygenase system, glutathione, and the activities of cytosolic glutathione S-transferase and microsomal epoxide hydrolase in Japanese quail liver were studied. Cytochromes P-450 and b5 contents of liver microsomes and the activities of 7-ethoxyresorufin deethylase (7-ERD) and glutathione S-transferase were significantly increased 1 day after administration of single doses of TCB. Liver GSH and 7-ethoxycoumarin deethylase (7-ECD) activity were unchanged. Microsomal epoxide hydrolase activity was significantly decreased at TCB doses above 400 mg/kg. Increases in cytochromes and activities of 7-ERD and glutathione S-transferase were also seen following the 3-day administration of TCB, 400 mg/kg. In addition, liver GSH and the activity of NADPH-cytochrome c reductase were significantly increased whereas 7-ECD was significantly decreased by the 3-day treatment. These findings indicate that in Japanese quail, TCB is an inducer of 7-ERD and glutathione S-transferase but not of 7-ECD and epoxide hydrolase.

Various pharmacokinetic parameters in relation to enzyme-inducing abilities of 1,2,4-trichlorobenzene and 1,2,4-tribromobenzene

1,2,4-Trichlorobenzene (TCB) and 1,2,4-tribromobenzene (TBB) were administered for 7 d to rats at a dose of 1 mmol/kg.d. The animals were sacrificed at various times to observe the decline in enzyme induction. Carbon 14-labeled TCB and TBB were administered and the disposition of the radioactivity was determined. The influence of starvation and phenobarbital on the inductive effects of TCB and TBB was observed. p-Nitroanisole demethylation and EPN (O-ethyl O-p-nitrophenyl phenylphosphonothionate) detoxification increases declined with time but were still elevated 16 d after the last dose of TBB. Cytochorme P-450 and NADPH-cytochrome c reductase were alsoinduced. Tissue analysis showed greater retention of TBB than of TCB, with highest levels in the fats. More TCB than TBB was excreted in the urine, and fecal excretion was only 5-10% of the dose. The highest enzyme levels were found after 4 d of starvation following the 7 d of treatment and 6 d of recovery. Starvation resulted in increased plasma, fat, and liver concentrations and increased urinary excretion of 14C after TBB dosing. Phenobarbital treatment decreased the levels of induction by the halogenated benzenes. The results demonstrate that, on an equimolar basis, TBB leads to higher levels of induction that are maintained for longer periods than does TCB. Treatments, such as starvation and phenobarbital, that after storage in fat of these materials change the decline with time of the level of induction.