Biochemical toxicology of unsaturated halogenated monomers

Using available in vitro metabolite identification and time course kinetics for β-chloroprene and its metabolite, (1-chloroethenyl) oxirane, to include reactive oxidative metabolites and glutathione depletion in a PBPK model for β-chloroprene

Introduction: ß-chloroprene (2-chloro-1,3-butadiene; CP) causes lung tumors after inhalation exposures in rats and mice. Mice develop these tumors at lower exposures than rats. In rats CP exposures cause depletion of lung glutathione (GSH). Methods: PBPK models developed to relate the appearance of mouse lung tumors with rates of CP metabolism to reactive metabolites or total amounts metabolized during exposures have been expanded to include production of reactive metabolites from CP. The extended PBPK model describes both the unstable oxirane metabolite, 2-CEO, and metabolism of the more stable oxirane, 1-CEO, to reactive metabolites via microsomal oxidation to a diepoxide, and linked production of these metabolites to a PK model predicting GSH depletion with increasing CP exposure. Key information required to develop the model were available from literature studies identifying: 1) microsomal metabolites of CP, and 2) in vitro rates of clearance of CP and 1-CEO from active microsomal preparations from mice, rats, hamsters and humans. Results: Model simulation of concentration dependence of disproportionate increases in reactive metabolite concentrations as exposures increases and decreases in tissue GSH are consistent with the dose-dependence of tumor formation. At the middle bioassay concentrations with a lung tumor incidence, the predicted tissue GSH is less than 50% background. These simulations of reduction in GSH are also consistent with the gene expression results showing the most sensitive pathways are Nrf2-regulation of oxidative stress and GSH metabolism. Discussion: The PBPK model is used to correlate predicted tissue exposure to reactive metabolites with toxicity and carcinogenicity of CP.

Chemical interactions and mixtures in public health risk assessment: An analysis of ATSDR's interaction profile database

The Agency for Toxic Substances and Disease Registry (ATSDR) develops interaction profiles using binary weight of evidence (BINWOE) methodology to determine interaction directions of common environmental mixtures. We collected direction of interactions, BINWOE score determination, and BINWOE score confidence rating from 13 interaction profiles along with toxicodynamic and toxicokinetic influences on interaction direction. By doing so, we quantified the 1) direction of interaction and indeterminate evaluations; 2) characterized confidence in the BINWOE determinations; and 3) quantified toxicokinetic/toxicodynamic, and other influences on projected BINWOE interaction directions. Thirty-nine percent (130/336) of the attempts to make a BINWOE were indeterminate due to no interaction data or inadequate or conflicting evidence. Out of remaining BINWOEs, 25% were additive, 9% were greater-than-additive, and 27% were less-than-additive interactions. Fifty-five percent of BINWOEs were explained by toxicokinetic interactions, 12% and 5% were explained by toxicodynamic and other explanations, respectively. High quality mixture toxicology in vivo studies along with mixture in vitro and in silico studies will lead to greater confidence in interaction directions and influences. Limitations for interpretation of the data were also included.

Vinyl chloride exposure and cirrhosis: a systematic review and meta-analysis

BACKGROUND

It has been proposed that vinyl chloride exposure is associated with increased risk of death from cirrhosis, although epidemiologic evidence is limited.

METHODS

We analyzed the risk of death from cirrhosis by occupational vinyl chloride exposure by conducting a meta-analysis on seven available studies, including more than 40,000 workers exposed to vinyl chloride mostly in North America and Europe, with a total of 203 deaths from cirrhosis.

RESULTS

All epidemiological studies on vinyl chloride exposure and risk of death from cirrhosis resulted in an overall relative risk of 0.73 (95% confidence interval 0.61-0.87). Thus, the epidemiologic evidence does not suggest an excess mortality from cirrhosis in vinyl chloride-exposed workers; this is consistent with histopathological observations in livers of angiosarcoma patients and of vinyl chloride-exposed rodents revealing no signs of cirrhosis.

CONCLUSION

Overall, our findings indicate the absence of increased risk of death from cirrhosis in vinyl chloride-exposed workers.

The impact of CYP2E1 genetic variability on risk assessment of VOC mixtures

Humans are simultaneously exposed to multiple chemicals in the environment. Many of the chemicals use the same enzymes in their metabolic pathways. Competitive inhibition may occur as one of the possible interactions between the xenobiotics in human body. For example, many volatile organic compounds (VOCs) are metabolized using P450 enzymes, specifically CYP2E1. Inheritable gene alterations may result in changes of function of the enzymes in different human subpopulations. Variations in quantity and/or quality of particular isoenzymes may cause differences in the metabolism of VOCs. These variations may cause higher sensitivity in certain populations. Using examples of three different mixtures, this review paper outlines the variances in CYP2E1 isoenzymes, effect of exposure to such mixtures on sensitive populations, and approaches to mixtures risk assessment.

Kinetic Modeling of β-Chloroprene Metabolism: I. In vitro Rates in Liver and Lung Tissue Fractions from Mice, Rats, Hamsters, and Humans

Beta-chloroprene (2-chloro-1,3-butadiene, CD) is carcinogenic by inhalation exposure to B6C3F1 mice and Fischer F344 rats but not to Wistar rats or Syrian hamsters. The initial step in metabolism is oxidation, forming a stable epoxide (1-chloroethenyl)oxirane (1-CEO), a genotoxicant that might be involved in rodent tumorigenicity. This study investigated the species-dependent in vitro kinetics of CD oxidation and subsequent 1-CEO metabolism by microsomal epoxide hydrolase and cytosolic glutathione S-transferases in liver and lung, tissues that are prone to tumor induction. Estimates for Vmax and Km for cytochrome P450-dependent oxidation of CD in liver microsomes ranged from 0.068 to 0.29 micromol/h/mg protein and 0.53 to 1.33 microM, respectively. Oxidation (Vmax/Km) of CD in liver was slightly faster in the mouse and hamster than in rats or humans. In lung microsomes, Vmax/Km was much greater for mice compared with the other species. The Vmax and Km estimates for microsomal epoxide hydrolase activity toward 1-CEO ranged from 0.11 to 3.66 micromol/h/mg protein and 20.9 to 187.6 microM, respectively, across tissues and species. Hydrolysis (Vmax/Km) of 1-CEO in liver and lung microsomes was faster for the human and hamster than for rat or mouse. The Vmax/Km in liver was 3 to 11 times greater than in lung. 1-CEO formation from CD was measured in liver microsomes and was estimated to be 2-5% of the total CD oxidation. Glutathione S-transferase-mediated metabolism of 1-CEO in cytosolic tissue fractions was described as a pseudo-second order reaction; rates were 0.0016-0.0068/h/mg cytosolic protein in liver and 0.00056-0.0022 h/mg in lung. The observed differences in metabolism are relevant to understanding species differences in sensitivity to CD-induced liver and lung tumorigenicity.

The metabolism of beta-chloroprene: preliminary in-vitro studies using liver microsomes

Based on analogy with butadiene and isoprene, the metabolism of beta-chloroprene (2-chloro-1,3-butadiene, CD) to reactive intermediates is likely to be a key determinant of tumor development in laboratory rodents exposed to CD by inhalation. The purpose of this study is to identify species differences in toxic metabolite (epoxide) formation and detoxification in rodents and humans. The in-vitro metabolism of CD was studied in liver microsomes of B6C3F1 mice, Fischer/344 and Wistar rats, Syrian hamsters, and humans. Microsomal oxidation of CD in the presence of NADP(+), extraction with diethyl ether, and analysis by GC-mass selective detection (MSD) indicated that (1-chloroethenyl)oxirane (CEO) was an important metabolite of CD in the liver microsomal suspensions of all species studied. Other potential water-soluble oxidative metabolites may have been present. The oxidation of CD was inhibited by 4-methyl pyrazole, an inhibitor of CYP 2E1. CEO was sufficiently volatile at 37 degrees C for vial headspace analysis using GC-MSD single ion monitoring (m/z=39). CEO was synthesized and used to conduct partition measurements along with CD and further explore CEO metabolism in liver microsomes and cytosol. The liquid-to-air partition coefficients for CD and CEO in the microsomal suspensions were 0.7 and 58, respectively. Apparent species differences in the uptake of CEO by microsomal hydrolysis were hamster approximately human>rats>mice. Hydrolysis was inhibited by 1,1,1-trichloropropene oxide, a competitive inhibitor of epoxide hydrolase. A preliminary experiment indicated that the uptake of CEO in liver cytosol by GSH conjugation was hamster>rats approximately mice (human cytosol not yet tested). In general, the results suggest that metabolism may help explain species differences showing a greater sensitivity for CD-induced tumorigenicity in mice, for example, compared with hamsters. Additional experiments are in progress to quantify the kinetic parameters of CD oxidation and CEO metabolism by enzymatic hydrolysis and conjugation by glutathione S-transferase for in cytosol. A future goal is to use the kinetic rates to parameterize a physiologically based toxicokinetic model and relate the burden of toxic metabolite to the cancer dose-response observed in experimental animals.

Metabolism of halogenated ethylenes

The metabolism of the chlorinated ethylenes may be explained by the formation of chloroethylene epoxides as the first intermediate products. The evidence indicates that these epoxides rearrange with migration of chlorine to form chloroacetaldehydes and chloroacetyl chlorides. Thus, monochloroacetic acid, chloral hydrate, and trichloroacetic acid have been found in reaction mixtures of 1,1-dichloroethylene, trichloroethylene, and tetrachloroethylene, respectively, with rat liver microsomal systems. Rearrangements of the chloroethylene, and glycols formed from the epoxides by hydration may also take place, but would appear, at least in the case of 1,1-dichloroethylene, to be quantitatively less important. The literature on the metabolism of chlorinated ethylenes and its relationship to their toxicity is reviewed.

Effect of various treatments on toxicity of inhaled vinylidene chloride

The toxicity of vinylidene chloride (VDC) was studied in mice and rats exposed to various concentrations of the vapors for 23 hr/day. In addition, the ability of various treatments to alter parameters of toxicity was evaluated. Mice were more sensitive than rats both to the acute lethal and hepatotoxic effects of VDC. Disulfiram treatment reduced the acute lethal and hepatotoxic effects of inhaled VDC and reduced the levels of covalent bound radioactivity in the liver and kidney after the intraperitoneal administration of 14C-VDC. Treatment with diethyldithiocarbamate and thiram also protected mice from the acute lethal effects of VDC.

Chemistry and toxicity of flame retardants for plastics

An overview of commercially used flame retardants is give. The most used flame retardants are illustrated and the seven major markets, which use 96% of all flame-retarded polymers, are described. Annual flame retardant growth rate for each major market is also projected. Toxicity data are reviewed on only those compositions that are considered commercially significant today. This includes 18 compounds or families of compounds and four inherently flame-retarded polymers. Toxicological studies of flame retardants for most synthetic materials are of recent origin and only a few of the compounds have been evaluated in any great detail. Considerable toxicological problems may exist in the manufacturing of some flame retardants, their by-products, and possible decomposition products.

Industrial mutagens and potential mutagens I. Halogenated aliphatic derivatives

The halogenated aliphatic hydrocarbons represent one of the most important categories of industrial chemicals from a consideration of volume, use categories, environmental and toxicological considerations and hence most importantly, potential population risk. The major halocarbons reviewed, primarily in terms of their occurrence, utility, stability, distribution, and levels of exposure as well as their metabolism, carcinogenicity and mutagenicity included: vinylchloride, vinylidene chloride, trichloroethylene, perchloroethylene, ethylene dichloride, ethylene dibromide, chloroprene, chloroform, carbon tetrachloride, fluorocarbons (trichlorofluoromethane and dichlorodifluoromethane), epichlorohydrin, halohydrins (2-chloro- and 2-bromoethanol) and haloethers (bis(chloromethyl); chloromethyl'-methyl; bis(2-chloroethyl)-and bis(2-chloroisopropyl)ether. In many instances, data were not available to assess world production, populations at risk and degrees of exposure. With the exception of vinylchloride, vinylidene chloride, epichlorohydrin and 2-halo ethanols, there is an acknowledged paucity of definitive mutagenicity data concerning the majority of halogenated hydrocarbons. Their ubiquitous distribution, and in a number of cases, their carcinogenicity both in man and animals, dictates the urgent need to more exhaustively investigate their potential mutagenicity.

Acute liver injury by vinyl chloride: involvement of endoplasmic reticulum in phenobarbital-pretreated rats

A single 6-hr exposure to vinyl chloride monomer (5%) produces extensive vacuolization of centrolobular liver parenchyma and focal midzonal necrosis in the hepatic lobuole in phenobarbital-pretreated rats. Ultrastructurally, vacuolization consists of dilation of cysternae of rough endoplasmic reticulum and in the same cells smooth endoplasmic reticulum coalesces into discreet aggregates resembling denatured membranes. The findings support the hypothesis that vinyl chloride is hepatotoxic because it is converted into a toxic metabolite by components of the mixed function oxidase system of liver endoplasmic reticulum.