Binding kinetics of vinyl chloride and vinyl bromide at very low doses

Vinyl Chloride—A Classical Industrial Toxicant of New Interest

The carcinogenicity of vinyl chloride in humans was recognized in 1974 based on observations of hepatic angiosarcomas in highly exposed workers. A multiplicity of endpoints has been demonstrated. The primary target organ, the liver, displays differential susceptibilities of hepatocytes and sinusoidal cells, which are modified by factors of age and dose. There is consistency in organotropism between experimental animals and humans. Vinyl chloride is a pluripotent carcinogen, predominantly directed toward hepatic endothelial (sinusoidal) cells, and second toward the parenchymal cells of the liver. The similarity of results between experimental animals and humans is a solid basis of an amalgamation of experimental and epidemiological risk estimates. Vinyl chloride requires metabolic activation for carcinogenicity and mutagenicity, and toxicokinetics are a key to interpret the dose response. Practically the entire initial metabolism of vinyl chloride is oxidative. At higher exposure concentrations this is nonlinear, and metabolic saturation of metabolism in rats is reached at about 250 ppm. This is consistent with the plateau of hepatic angiosarcoma incidence in rat bioassays. Physiologically based pharmacokinetic/toxicokinetic (PBPK) models have been developed and successfully applied within the frame of human cancer risk assessments. The major DNA adduct induced by vinyl chloride (approximately 98% of total adducts in rats), 7-(2-oxoethyl)guanine, is almost devoid of promutagenic activity. The clearly promutagenic "etheno" adducts N2,3-ethenoguanine and 3,N4-ethenocytosine each represent approximately 1% of the vinyl chloride DNA adducts in rats, and 1,N6-ethenoadenine is found at even lower concentrations. Etheno adducts appear to have a long persistence and are repaired by glycosylases. Vinyl chloride represents a human carcinogen for which a series of mechanistic events connects exposure with the carcinogenic outcome. These include (1) metabolic activation (to form chloroethylene oxide), (2) DNA binding of the reactive metabolite (to exocyclic etheno adducts), (3) promutagenicity of these adducts, and (4) effects of such mutations on protooncogenes/tumor suppressor genes at the gene and gene product levels. In rat hepatocytes, a further event is a biomarker response. Cancer prestages (enzyme-altered foci), as quantitative biomarkers, provide a tool to study dose response even within low dose ranges where a carcinogenic risk cannot be seen in cancer bioassays directly. Such biomarker responses support a linear nonthreshold extrapolation for low-dose assessment of carcinogenic risks due to vinyl chloride. Published risk estimates based on different sets of data (animal experiments, epidemiological studies) appear basically consistent, and on this basis an angiosarcoma risk of approximately 3 x 10(-4) has been deduced by extrapolation, for exposure to 1 ppm vinyl chloride over an entire human working lifetime. An important point that should be considered in regulatory standard settings is the presence of a physiological background of those etheno DNA adducts, which are also produced by vinyl chloride. Likely reasons for this background are oxidative stress and lipid peroxidation. In essence, fundamentals of the hepatocarcinogenicity of vinyl chloride appear now well established, providing a solid scientific basis for regulatory activities.

Metabolic activation of vinyl chloride by rat liver microsomes: low-dose kinetics and involvement of cytochrome P450 2E1

The metabolism and pharmacokinetics of vinyl chloride (VC) have been extensively studied in rodents and humans, but the maximum velocity (Vmax) and Michaelis constant (K(m)) for the activation of VC by microsomal monooxygenases in vitro have not yet been determined. Using a new sensitive assay, the epoxidation of VC by rat liver microsomes (adult Sprague-Dawley) at concentrations from 1 ppm to 10(6) ppm in the gas phase was measured. In the assay, the reactive VC metabolites chloroethylene oxide and 2-chloroacetaldehyde were trapped with excess cAMP, yielding, 1,N6-etheno-cAMP (epsilon cAMP) which was quantitated by HPLC fluorimetry. The trapping efficiency of electrophilic VC metabolites by cAMP was close to 10%. The specificity of the method was confirmed by purification of epsilon cAMP on an immunogel. The VC concentration in the gas phase was measured by GC/flame ionization detection, while in the aqueous phase it was calculated from the partition coefficient between air and the microsomal suspension. Activation of VC by rat liver microsomes followed Michaelis-Menten kinetics with K(m) = 7.42 +/- 0.37 (+/- SD) microM and Vmax = 4674 +/- 46 pmol.mg protein-1.min-1. Inhibitor studies and immunoinhibition assays showed that VC was activated by cytochrome P450 (CYP) 2E1 down to 1 ppm in the air phase. Based on the metabolic parameters determined, the uptake of VC by rats in vivo can be accurately predicted.

Substrate specificity of rat liver aldehyde dehydrogenase with chloroacetaldehydes

Chlorinated acetaldehydes have been the focus of research due to their role as reactive intermediates and their possible occurrence in chlorinated drinking water. This study investigated the in vitro substrate specificity of cytosolic and mitochondrial rat liver aldehyde dehydrogenase toward these compounds. Monochloroacetaldehyde was found to be extensively metabolized by these enzymes, to an even greater extent than the standard substrate propionaldehyde. Dichloroacetaldehyde was metabolized to a much lesser extent, and chloral hydrate is not metabolized by this enzyme family. The Km (mM) and Vmax (Vmax for propionaldehyde set to 100) values with the low Km cytosolic enzyme were monochloroacetaldehyde 0.046 and 582, and dichloroacetaldehyde 0.13 and 54.9, and those with the high Km cytosolic enzyme were dichloroacetaldehyde 0.35 and 23.4. The values with the low Km mitochondrial enzyme were monochloroacetaldehyde 0.057 and 462 and dichloroacetaldehyde 0.038 and 12.9, and those with the high Km mitochondrial enzyme were monocloroacetaldehyde 0.024 and 55.5 and dichloroacetaldehyde 0.29 and 3.44. These data suggest that aldehyde dehydrogenase plays a significant role in the metabolism of monochloroacetaldehyde and, to some extent, dichloroacetaldehyde. Some evidence also suggested that alcohol dehydrogenase plays a significant role in the metabolism of dichloroacetaldehyde and chloral hydrate.

Synthesis and properties of vinyl carbamate epoxide, a possible ultimate electrophilic and carcinogenic metabolite of vinyl carbamate and ethyl carbamate

Vinyl carbamate reacted with dimethyldioxirane in dry acetone to give a high yield of pure crystalline vinyl carbamate epoxide. This epoxide was characterized by its NMR and MS spectra and elementary analysis. It is unstable at room temperature and has a half-life in water solution of approximately 32 minutes. It reacts with adenosine to form 1,N6-ethenoadenosine and more of this etheno nucleoside was found in hydrolysates of hepatic RNA of male mice injected i.p. with the epoxide than with vinyl carbamate. Tests with Salmonella typhimurium TA1535 showed that this epoxide is a strong direct mutagen. It is also more toxic in the mouse than vinyl carbamate. Studies on the carcinogenicity of this epoxide are in progress.

Nucleophilic selectivity and reaction kinetics of chloroethylene oxide assessed by the 4-(p-nitrobenzyl)pyridine assay and proton nuclear magnetic resonance spectroscopy

The nucleophilic selectivity (Swain-Scott's constant s) of chloroethylene oxide (CEO), an ultimate carcinogenic metabolite of vinyl chloride, was determined to be 0.71 using the 4-(p-nitrobenzyl)pyridine (NBP) assay (Spears method). The molar extinction coefficient of the adduct formed between NBP and CEO was measured; and the second-order rate constants for the reactions of CEO with NBP and with thiosulfate were estimated at three temperatures. The disappearance of CEO and the formation of chloroacetaldehyde (CAA) and glycolaldehyde (GCA) were followed in D2O or a mixture of D2O/hexadeuterated acetone (acetone-d6), using Fourier transform proton nuclear magnetic resonance spectroscopy (1H-FTNMR). Evidence was obtained that CEO reacts with chloride ions to yield CAA at a rate constant of about 17 M-1 h-1 in D2O/acetone-d6 (1 : 1, v/v) at 280 K. Under the same conditions, the first-order rate constant kr for the thermal rearrangement of CEO into CAA was estimated to be approximately 0.41 h-1. These data suggest that the isomerization of CEO may be a minor reaction in physiological saline. These chemical properties of CEO are discussed in relation to the mechanism of vinyl chloride-induced carcinogenesis.

Incorporation of biological information in cancer risk assessment: example--vinyl chloride

Vinyl chloride (VC) is used as an example to demonstrate how biological information can be incorporated into quantitative risk assessment. The information included is the pharmacokinetics of VC in animals and humans and the data-generated hypothesis that VC primarily affects the initiation stage of the multistage carcinogenesis. The emphasis in this paper is on the improvement of risk assessment methodology rather than the risk assessment of VC per se. Sufficient data are available to construct physiologically-based pharmacokinetic models for both animals and humans. These models are used to calculate the metabolized dose corresponding to exposure scenarios in animals and in humans. On the basis of the data on liver angiosarcomas and carcinomas in rats, the cancer risk per unit of metabolized dose is comparable, irrespective of routes (oral or inhalation) of exposure. The tumor response from an intermittent/partial lifetime exposure is shown to be consistent with that from a lifetime exposure when VC is assumed to affect the first (initiation) stage of the multistage carcinogenic process. Furthermore, the risk estimates calculated on the basis of animal data are shown to be consistent with the human experience.

Nucleophilic selectivity as a determinant of carcinogenic potency (TD50) in rodents: a comparison of mono- and bi-functional alkylating agents and vinyl chloride metabolites

Using published data, the carcinogenic potency (TD50) in rodents of a series of monofunctional alkylating agents, bifunctional antitumor drugs and the vinyl chloride (VC) metabolites chloroethylene oxide (CEO) and chloroacetaldehyde (CAA) was compared to their nucleophilic selectivity (Swain and Scott's constant s or initial ratio of 7-/O6-alkylguanine in DNA). A positive correlation between the log of TD50 estimates and the s values for a series of 14, mostly monofunctional, alkylating agents was observed. This linear relationship also included 2 bifunctional chloroethylnitrosoureas, although their carcinogenic potency was compared to their initial 7-/O6-alkylguanine ratio rather than their s values (n = 16, r = 0.91, p less than 0.005). In addition, the carcinogenic potency of 2 alkyl sulfates, which is not yet known accurately, may correlate with their nucleophilic selectivity through the same relationship. By contrast, 2 methyl halides and 5 bifunctional antitumor drugs (nitrogen mustards and azyridinyl derivatives) did not follow this linear relationship: at similar nucleophilic selectivity, they were more potent carcinogens than the above 18 alkylating agents; this may hold true for CEO and CAA too, although further carcinogenicity experiments are needed to calculate their precise TD50 values. The possible molecular mechanisms involved in tumor induction by these agents are discussed on the basis of these findings. Comparison of the estimated TD50 for CEO, CAA and VC in rodents confirms that CEO is the ultimate carcinogenic metabolite of VC and suggests that only a very small proportion of metabolically generated CEO is available for DNA alkylation in vivo.

Induction of single-strand breaks in DNA of mice after inhalation of vinyl chloride

Female mice were exposed to 500 ppm vinyl chloride (VC) for 6 h/day 5 days/week for 1-8 weeks. Groups of mice were killed at different times during this period. DNA damage, expressed as single-strand breaks (SSB), was studied in liver, kidneys, lungs, spleen and brain. The level of SSB increased in liver, kidneys, spleen and lungs with time of exposure and reached a plateau for kidneys and lungs after 80 and 120 h of exposure. In spleen there was only a slight increase in the SSB, and in brain no detectable increase was found.

Inhalation pharmacokinetics based on gas uptake studies. III. A pharmacokinetic assessment in man of "peak concentrations" of vinyl chloride

On the basis of previous determinations of pharmacokinetic parameters for inhaled vinyl chloride in men, rhesus monkeys, and rats, and on improved pharmacokinetic models a pharmacokinetic treatment of the problem of "peak concentrations" of vinyl chloride, as occurring in industrial practice, became possible. For the calculations, metabolic elimination kinetics of vinyl chloride was assumed to be first order as experiments in different species including rhesus monkeys showed "linear" pharmacokinetics up to atmospheric exposures of 200-300 ppm. The distribution of vinyl chloride between atmosphere and organism under different conditions was evaluated using "'steady-state-kinetics". After treating the processes of "influx", "efflux", and "metabolism", the numerical values for the parameters derived from a human kinetic experiment were used to theoretically calculate the time courses of concentration of vinyl chloride in the organism and of the cumulative amount of vinyl chloride metabolized, under the conditions of (a) a 2h constant exposure to 5 ppm vinyl chloride and (b) two subsequent "peaks" of 50 ppm with a duration of 5 min each. This model calculation suggested that, regardless of the exposure profile, the amount of (reactive) metabolites formed from vinyl chloride would solely be a function of the mean atmospheric vinyl chloride concentration over time. The general validity of this suggested rule could subsequently be demonstrated. As the concentration of the reactive metabolite of vinyl chloride responsible for the carcinogenic effect at the target site must be a resultant of both formation and inactivation, an evaluation of the differential risk of different exposure profiles can reasonably be based on biochemical examinations of the "detoxifying" pathways. This points out the relevance of studies of the patterns of different metabolites of vinyl chloride in man under varying exposure profiles.

DNA alkylation by vinyl chloride metabolites: etheno derivatives or 7-alkylation of guanine?

The state of the literature led to a re-investigation of the alkylation products caused by vinyl chloride metabolites in DNA. When rat liver microsomes, an NADPH-regenerating system, DNA and [14C]vinyl chloride were incubated and, when the DNA was subsequently re-isolated and (enzymatically) hydrolyzed, chromatograms (on Aminex A-6) showed the presence of 1,N6-ethenodeoxyadenosine, 3,N4-ethenodeoxycytidine and 7-N-(2-oxoethyl)guanine (the product of hydrolysis of 7-N-(2-oxoethyl)-deoxyguanosine). By contrast, when rats were exposed to [1,2-14C]vinyl chloride and when the liver DNA of these rats was subjected to similar procedures, no radioactive 'etheno' derivatives could be detected, but a radioactive peak was eluted with 7-N-(2-oxoethyl)guanine. This peak could be transformed into 7-N-(2-hydroxyethyl)guanine; the chromatographic behaviour of which was identical to the reference compound used by Ostermann-Golkar et al. (Biochem. biophys. Res. Commun., 76 (1977) 259). Thus, it is concluded that the compound described by these authors, 7-N-(2-oxoethyl)guanine is in fact the major product of base alkylation in DNA after exposure to vinyl chloride.

Rat hepatic vinyl chloride metabolites induce gene conversion in the yeast strain D7RAD in vitro and in vivo

We have tested the genetic activity of gaseous vinyl chloride in vitro and in vivo using the gene-conversion system (trp5-12/trp5-27 leads to TRP+) in the yeast strain D7RAD. To induce, in vitro, TRP+ convertants with 2.5% gaseous vinyl chloride, a rat-liver microsomal system for metabolic activation of the vinyl chloride and dividing yeast cells are required. Neither a deficiency in excision repair (rad3) nor in the error-prone repair pathway (rad6) increased the vinyl-chloride-induced conversion frequencies compared with the repair-competent D7RAD strain. When logarithmically growing cells of the D7RAD strain were injected intravenously into male Wistar rats which inhaled 1% vinyl chloride in air for 24 h, a significant enhancement of the TRP+ conversion frequencies was found compared with that in cells re-isolated from untreated rats. These results indicate that vinyl chloride metabolites from the metabolizing hepatocytes diffuse into yeast cells, which accumulate in the liver capillaries. This supports the hypothesis that the endothelial cells of the liver sinuses, which have hardly any metabolic activity, but give rise to vinyl-chloride-induced hemangiotheliomas (rare type of liver tumor), are transformed by diffusible metabolites of the procarcinogen vinyl chloride.

Covalent binding of drug metabolites to DNA--a tool of predictive value?

The presently available data suggest at least some correlation between covalent binding of drug metabolites to DNA and carcinogenicity of that drug. More data, however, are needed to establish the predictability of covalent DNA binding assays for extrahepatic cancer. A covalent binding assay requires administration of radioactively labelled compound to the experimental animals; the availability of labelled compound and requirements as to radiochemical purity, chemical and biochemical stability are limiting the applicability of this procedure. Many technical pitfalls accompany covalent DNA binding assays. It is concluded that at the present time DNA binding assays do not represent routine procedures within a standard test battery for carcinogenicity, but are invaluable for more in-depth research which probably follows routine testing.

Characteristics of haloethylene-induced acetonemia in rats

A series of halogenated ethylenes (vinyl chloride, vinylidene fluoride, cis- and trans-1,2-dichloroethylene, perchloroethylene) induces increased acetone exhalation in rats. Exposures of differently pre-treated rats to vinylidene fluoride suggest that a metabolite of the haloethylene must be envolved in eliciting this formation of acetone. This conclusion is based on (a)dependence of acetone exhalation on the concentration of vinylidene fluoride, (b)effect of inducing agents, (c)effect of pyrazol, a metabolic inhibitor, (d)effect of cysteine, (e)effect of hypoxia and (f)the time course of acetone exhalation.