The Different Genotoxicity of P-Dichlorobenzene in Mouse and Rat: Measurement of the in Vivo and in Vitro Covalent Interaction with Nucleic Acids

In Vivo and in Vitro Interaction of 1,2-Dichlorobenzene with Nucleic Acids and Proteins of Mice and Rats

Twenty-two hours after i.p. injection into male Wistar rats and BALB/c mice, 1,2-dichlorobenzene (1,2-DCB) was covalently bound to DNA, RNA, and proteins of liver, kidney, lung and stomach. The covalent binding index to liver DNA was typical of carcinogens classified as weak initiators. The enzyme-mediated in vitro interaction of 1,2-DCB with calf thymus DNA of synthetic polyribonucleotides was carried out by a microsomal mixed-function oxidase system and microsomal GSH-transferases, which seemed to be effective only in liver and lung of rat and mouse. Cytosolic GSH-transferases played a minor role in 1,2-DCB bioactivation. The latter finding provides the first evidence of 1,2-DCB genotoxicity in mammalian cells. The type of halide, the number of halosubstituents and their spatial disposition on the benzene ring are the major determinants of halobenzenes activability to intermediate(s) capable of interacting covalently with DNA and other macromolecules in biologic systems.

A comparative human health risk assessment of p-dichlorobenzene-based toilet rimblock products versus fragrance/surfactant-based alternatives

A comparison of the human health risk to consumers using one of two types of toilet rimblock products, either a p-dichlorobenzene-based rimblock or two newer fragrance/surfactant-based alternatives, was conducted. Rimblock products are designed for global use by consumers worldwide and function by releasing volatile compounds into indoor air with subsequent exposure presumed to be mainly by inhalation of indoor air. Using the THERdbASE exposure model and experimentally determined emission data, indoor air concentrations and daily intake values were determined for both types of rimblock products. Modeled exposure concentrations from a representative p-dichlorobenzene rimblock product are an order of magnitude higher than those from the alternative rimblock products due to its nearly pure composition and high sublimation rate. Lifetime exposure to p-dichlorobenzene or the subset of fragrance components with available RfD values is not expected to lead to non-cancer-based adverse health effects based on the exposure concentrations estimated using the THERdbASE model. A similar comparison of cancer-based effects was not possible as insufficient data were available for the fragrance components.

1,4-Dichlorobenzene-Induced liver tumors in the mouse: evaluation of the role of chlorohydroquinones

1,4-Dichlorobenzene (1,4-DCB) is a wide-spread environmental contaminant and well-described hepatotoxicant for rats and mice. The prolonged oral or inhalation exposure to 1,4-DCB is associated with an increased frequency of hepatic tumors in mice, but not in rats. Evidence is lacking of direct genotoxicity with 1,4-DCB or its metabolites, and no generally accepted mechanism has been found to account for the increased numbers of 1,4-DCB-induced hepatic tumors in mice. No information is available on the carcinogenic effects of 1,4-DCB in humans. Here we consider evidence that the biotransformation of 1,4-DCB to substituted hydroquinone species contributes to hepatic adenoma and carcinoma formation in mouse liver. This phenomenon has implications for human carcinogenesis.

An evaluation of the carcinogenic hazard of 1,4-dichlorobenzene based on internationally recognized criteria

1,4-Dichlorobenzene (1,4-DCB) was shown to induce the formation of male rat renal tubule tumors and male and female mouse liver tumors when administered in a chronic bioassay. Since the original carcinogenicity findings, an extensive body of mechanistic information has been developed to elucidate the mode of action by which 1,4-DCB induces these effects and to evaluate the human relevance of the observed animal tumors. In addition, some regulatory and authoritative bodies (U.S. EPA and IARC) have developed rigorous scientific criteria for the amount and types of evidence needed to establish that a material causes kidney toxicity and tumors in male rats through a specific mechanism, alpha-2u-globulin nephropathy. This paper summarizes the mechanistic data developed for 1,4-DCB, which affords an understanding of the lack of human relevance of the male rat renal tubule tumors and mouse liver tumors; assesses that mechanistic data set utilizing the defined set of evaluation criteria formulated by U.S. EPA and IARC for alpha-2u-globulin nephropathy; and discusses the predictive power of mechanistic data developed to elucidate the mode of action of 1,4-DCB in inducing mouse liver tumors. Finally, there is a discussion of how some, but not all, regulatory and authoritative bodies have incorporated this substantial mechanistic data set for 1, 4-DCB into their cancer hazard evaluations and concluded that 1, 4-DCB presents little, if any, cancer hazard to humans.

Cytochrome P450 catalyzed oxidation of monochlorobenzene, 1,2- and 1,4-dichlorobenzene in rat, mouse, and human liver microsomes

We studied metabolism of monochlorobenzene (MCB), 1,2-dichlorobenzene (1,2-DCB) and 1,4-DCB in liver microsomes from untreated male and female Wistar rats and B6C3F1 mice or in those after the induction of CYP3A or 2E1 as well as in human male liver microsomes. MCB and 1,2-DCB were oxidised mainly by rat and human CYP2E1. It was found that 1,4-DCB was oxidised by rat and human CYP2E1 at a several-fold lower rate than 1,2-DCB, but a greater part to covalently binding products. In contrast to previous studies showing rat CYP3A1 as the main CYP form oxidising both DCBs, our experiments indicate only a certain role of rat and human CYP3A in MCB, 1,2-DCB and 1,4-DCB oxidation to covalently bound products. The relative roles of human liver CYP2E1 and 3A4 in the metabolism of 1,4-DCB seem to be individually different. Metabolic rates of MCB, 1,2-DCB and 1,4-DCB correlated with CYP2E1 immunochemical level in microsomes from 11 different human livers and with metabolic rates of CYP2E1 substrates. These rates in different human livers were up to 10-fold different and were generally several-fold higher than those in untreated rats or mice. Metabolic activation of MCB and 1,2-DCB to products binding covalently to microsomal proteins and to calf-thymus DNA, respectively, mostly corresponded to production of water-soluble metabolites. Significant species and sex differences in the oxidation of MCB, 1,2-DCB and 1,4-DCB were reflected in a markedly higher oxidation in male mice than male rats and higher oxidation in male than female mice. The formation of covalently bound products generally corresponded to production of soluble metabolites, but female rats formed significantly less covalently bound products of 1,4-DCB (and also of 1,2-DCB and MCB) than male rats and mice of both sexes, in possible reflection of the fact that 1,4-DCB is not carcinogenic in female rats despite its carcinogenicity for male rats and both sexes of mice.

Kinetics and metabolism of 1,4-dichlorobenzene in male Wistar rats: no evidence for quinone metabolites

The biotransformation and kinetics of 1,4-dichlorobenzene (1,4-DCB) were studied in male Wistar rats at three oral dose levels (10, 50 and 250 mg/kg). The effect of induction of CYP2E1 by isoniazid on the kinetics and biotransformation was determined. Excretion was predominantly via the urine (78-85%) and to a small extent via the faeces (2-5%). The relative contributions of these routes were not dose dependent. Excretion via the bile ranged from less than 5% at the low dose level to 30% at the high dose level. The major biliary metabolite was the glucuronide of 2,5-dichlorophenol (2,5-DCP). The time point at which the plasma concentrations of the parent compound and the metabolites were maximal (TCmax) as well as the maximum concentrations (Cmax) increased with higher dose level. Induction by isoniazid resulted in a faster urinary elimination, whereas TCmax and Cmax were lower for induced rats. In addition, the area under the blood curve (AUC) was smaller and total clearance was higher for induced rats. 1,4-DCB was mainly metabolized to 2,5-DCP (ca. 90%), which was detected in the urine as its sulfate (50-60%), glucuronide (20-30%) and the free form (5-10%). Minor metabolites were the N-acetyl-cysteine-S-dihydro-hydroxy-1,4-dichlorobenzene and the corresponding dehydrated N-acetyl-cysteine-S-1,4-dichlorobenzene, which comprised ca. 10% of total metabolites. No hydroquinones were observed for the male Wistar rat, not even under conditions of induced oxidative metabolism.

The covalent interaction of 1,4-dibromobenzene with rat and mouse nucleic acids: in vivo and in vitro studies

1,4-Dibromobenzene (1,4-DBB) was covalently bound to DNA from liver, kidney, lung and stomach of mice after intraperitoneal administration. The covalent binding index (CBI) value (23 in mouse liver) was typical of weak initiators. On the contrary, no interaction with DNA from rat organs was observed (CBI detection limit: 1.3-2.6). The in vitro interaction of 1,4-DBB with calf thymus DNA was mediated mainly by microsomes, especially those from liver of both species and from mouse lung. Mouse subcellular fractions were more active then rat subcellular fractions. Unlike liver cytosol, subcellular cytosolic fractions from lung, kidney and stomach were capable of bioactivating 1,4-DBB, although to a lesser extent than liver microsomes. Both cytochrome P-450 and GSH-transferases are involved in 1,4-DBB bioactivation.