Structural characteristics of compounds that can be activated to chemically reactive metabolites: use for a prediction of a carcinogenic potential

Effects of an acute exposure to toluene on the DNA repair activity of the human 8-oxoguanine DNA glycosylase 1 (hOGG1) in healthy subjects

The structure and previous studies on the biotransformation of toluene lead to the suspicion that metabolites may be formed which preferentially react with strongly nucleophilic partners such as sulfhydryl groups of cysteines in proteins. Human 8-oxoguanine DNA glycosylase 1 removes the major oxidative DNA damage and possesses eight cysteines. Its potential inactivation may lead to accumulation of DNA damage by reactive oxygen species formed by exogenous agents or by ubiquitous endogenous processes. The goal of the present investigation was to study the in vivo effect in humans of an acute toluene exposure on hOGG1 activity. Twenty healthy, non-smoking males were exposed to 50 ppm toluene and to filtered air in an exposure chamber for 270 min, using a cross-over design. Before and 30 min after the end of exposure, blood samples were taken and toluene concentrations and the hOGG1 activity were measured. hOGG1 activity was determined in peripheral mononuclear blood cells. Thirty minutes after exposure to toluene, we found a median blood concentration of 0.25 mg toluene/l. Compared with the activity before exposure, upon exposure to toluene a statistically insignificant median increase of hOGG1 activity by +0.4% and upon exposure to air by +2.3% was determined. Thus, no reduction of the hOGG1 repair activity after acute exposure to 50 ppm toluene was observed.

An evaluation of the possible carcinogenicity of bisphenol A to humans

Bisphenol A (BPA) is a monomer component of polycarbonate plastics and epoxy resins. These resins are used in numerous consumer products, including food-contact plastics. There has been considerable scientific debate about the relevance to humans of reported estrogenic actions of BPA. Much less attention has been focused on the carcinogenic potential of BPA. The carcinogenic potential of BPA was assessed through a review of metabolic data, genetic toxicity studies, long-term toxicity/carcinogenicity studies, and estimates of consumer exposure. Following a weight-of-evidence approach as recommended by IARC and U.S. EPA, it was concluded that BPA is not likely to be carcinogenic to humans. The bases for this conclusion included: (a) the results of an NTP study which provided no substantive evidence to indicate that BPA is carcinogenic to rodents; (b) the lack of activity of BPA, at noncytotoxic concentrations, in standard in vitro genetic toxicity tests; (c) the lack of genotoxic activity of BPA in a GLP-compliant in vivo mouse micronucleus assay; and (d) the results of metabolism studies showing BPA is rapidly glucuronidated without evidence of formation of potentially reactive intermediates, except possibly at high doses that could saturate detoxication pathways. In addition, exposure assessment reveals that current use of BPA would result in only a trivial human exposure.

In vitro biotransformation of 2-methylpropene (isobutene): epoxide formation in mice liver

Until now, no data are available concerning the biotransformation and toxicity of 2-methylpropene (or isobutene), a gaseous alkene widely used in industry (rubber, fuel additives, plastic polymers, adhesives, antioxidants). In this work, the biotransformation of 2-methylpropene (MP) has been studied, using total liver homogenates of mice, supplemented with a NADPH-generating system. In analogy to other olefins, 2-methylpropene is metabolized to its epoxide 2-methyl-1,2-epoxypropane (MEP), as proved by the identification by gas chromatography coupled with mass spectrometry. The epoxidation is cytochrome P-450 dependent, as shown by experiments in the absence of the NADPH-generating system and in the presence of various concentrations of metyrapone and SKF 525-A, two known inhibitors of the mono-oxygenases. A simple gas chromatographic headspace method has been developed for the quantitative determination of the epoxide formed. The formation of MEP is never linear in function of time and it reaches a maximum after 20 min. Thereafter is decreases continuously to undetectable levels. This observation can be explained by the immediate action of epoxide hydrolase and glutathione S-transferase, converting the epoxide to 2-methyl-1,2-propanediol and to the glutathione conjugate respectively. The involvement of both enzymes has been demonstrated by the addition of 3,3,3-trichloropropene oxide and indomethacin. These inhibitors of, respectively, epoxide hydrolase and glutathione S-transferase increase the epoxide formation in a significant way. The actual concentration of MEP is therefore not only dependent on its formation by cytochrome P-450 dependent mono-oxygenases, but also on its conversion by epoxide hydrolase and glutathione S-transferase, both very active in liver tissue.

Toxicokinetics of N-nitrosodimethylamine in the Syrian golden hamster

The single-dose toxicokinetics of N-nitrosodimethylamine (NDMA) has been characterized in 8-week-old male Syrian golden hamsters by analysis using high performance liquid chromatography of serial blood samples. An i.v. bolus dose of 4.2 mumols/kg [14C]NDMA revealed biphasic first-order elimination with a terminal half-life of 8.7 +/- 1.0 min (mean +/- SE) for unchanged NDMA and 31.5 +/- 5.5 min for total radioactivity, and evidence for conversion to polar metabolites was seen in the chromatographic assays. The systemic blood clearance and apparent steady-state volume of distribution for unchanged NDMA were 51.2 +/- 3.0 ml/min/kg and 582 +/- 60 ml/kg, respectively. No unchanged NDMA was detected in the urine following an i.v. bolus dose of 15 mumols/kg [14C]NDMA, but 31% of the total radioactivity was eliminated by that route. A dose of 38 mumols/kg given by gavage indicated a systemic bioavailability of 11 +/- 4% for unchanged NDMA. Reversible binding of NDMA to hamster plasma proteins was found to be negligible. Estimation of the intrinsic hepatic clearance (ClI) in the hamster produced a value of 648 ml/min/kg, which is greater than that previously obtained for the rat, and indicates that the metabolic capacity of the hamster liver is greater than that of the rat. These results suggest that this difference in ClI may play a role in the previously reported (Lijinsky et al. 1987) switch in organotropism from almost exclusivity for liver tumors in hamsters dosed by gavage to additional high incidences of lung and kidney tumors in the rat.

Quantitative evaluation of DNA binding data for risk estimation and for classification of direct and indirect carcinogens

Investigation of covalent DNA binding in vivo provided evidence for whether a test substance can be activated to metabolites able to reach and react with DNA in an intact organism. For a comparison of DNA binding potencies of various compounds tested under different conditions, a normalization of the DNA lesion with respect to the dose is useful. A covalent binding index, CBI = (mumol chemical bound per mol DNA nucleotide)/(mmol chemical administered per kg body weight) can be determined for each compound. Whether covalent DNA binding results in tumor formation is dependent upon additional factors specific to the cell type. Thus far, all compounds which bind covalently to liver DNA in vivo have also proven to be carcinogenic in a long-term study, although the liver was not necessarily the target organ for tumor growth. With appropriate techniques, DNA binding can be determined in a dose range which may be many orders of magnitude below the dose levels required for significant tumor induction in a long-term bioassay. Rat liver DNA binding was proportional to the dose of aflatoxin B1 after oral administration of a dose between 100 micrograms/kg and 1 ng/kg. The lowest dose was in the range of general human daily exposures. Demonstration of a lack of liver DNA binding (CBI less than 0.1) in vivo for a carcinogenic, nonmutagenic compound is a strong indication for an indirect mechanism of carcinogenic action. Carcinogens of this class do not directly produce a change in gene structure or function but disturb a critical biochemical control mechanism, such as protection from oxygen radicals, control of cell division, etc. Ultimately, genetic changes are produced indirectly or accumulate from endogenous genotoxic agents. The question of why compounds which act via indirect mechanisms are more likely to exhibit a nonlinear range in the dose-response curve as opposed to the directly genotoxic agents or processes is discussed.