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 pr…

Characteristics of haloethylene-induced acetonemia in rats.

A series of halogenated ethylenes (vinyl chloride, vinylidene fluoride,cis- andtrans-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.

Effect of potential antidotes on the acute toxicity of acrylonitrile

Rats were intoxicated with lethal doses of acrylonitrile by different routes of application, and the effect of potential antidotes was studied. The cyanide antidotes 4-dimethylaminophenol plus thiosulfate showed some protective effect only after oral but not after i.p. or inhalatory acrylonitrile application. Of the sulfhydryl compounds cysteine, N-acetylcysteine, cysteamine and diethyldithiocarbamate the two antidotes cysteine and, to some lesser extent, N-acetylcysteine proved especially effective. Cysteine, at a dose of 200 mg/kg (i.p.), prevented the lethal effect of 100 mg/kg acrylonitrile (i.p.) even when given 2 h after the acrylonitrile dose. From these experiments a tentative sched…

Irreversible binding of acrylonitrile to nucleic acids

1. [2,3-14C]Acrylonitrile was incubated with rat-liver microsomes, NADPH and either DNA, RNA or bovine serum albumin. Irreversible binding occurred to the macromolecular targets. Binding was lower when incubations were performed without microsomes. 2. Most of the 14C bound to DNA, RNA or polynucleotides (poly-A, poly-C, poly-G, poly-U) after incubation of [2,3-14C]acrylonitrile with rat-liver microsomes and 'conventional' re-isolation of the nucleic acids was removed from the macromolecular target when subsequently chromatographed on hydroxyapatite. 3. Radioactivity attached to DNA after prolonged non-enzymic incubations with [2,3-14C]acrylonitrile was also removed from the DNA by chromatog…

Evidence of chloroethylene oxide being the reactive metabolite of vinyl chloride towards DNA: comparative studies with 2,2′ -dichloro-diethylether

The roles of chloroethylene oxide (CEO) and chloroacetaldehyde (CAA) in carcinogenicity of vinyl chloride (VC) have been studied by comparing biological effects of VC exposure with those of 2,2'-dichlorodiethylether (bis(chloroethyl)ether, BCEE) as a metabolic precursor of CAA. Biological end-points investigated were covalent protein binding, nucleic acid (RNA and DNA) alkylation and the potency of the two chemicals to induce preneoplastic ATPase-deficient foci in rat liver. After exposure of rats to [1-14C]BCEE, BCEE derived radioactivity was bound to liver proteins. Analysis of hydrolysates of liver RNA and DNA gave no indication for the formation of either 7-N-(2-oxoethyl)guanine, 1,N6-e…

Quantitative evaluation of ethane and n-pentane as indicators of lipid peroxidation in vivo.

The use of exhalation of ethane and n-pentane in experimental animals as parameters of lipid peroxidation led to an examination of pharmacokinetics of both compounds in rats. When rats were exposed, in a closed desiccator jar chamber, to a wide range of ethane concentrations, linear elimination pharmacokinetics were observed. n-Pentane, when concentrations higher than 100 ppm were applied, displayed saturation kinetics. These were formally explained by action of two competing metabolizing pathways or enzymes. Application of preexisting models could describe exhalation of both ethane and n-pentane by untreated control rats. Stimulation of lipid peroxidation by ferrous ions or by carbon tetra…

Metabolic Activation and Pharmacokinetics in Hazard Assessment of Halogenated Ethylenes

Halogenated ethylenes play an important role in occupational and environmental medicine. Several attempts at toxicological evaluation of such compounds have been recently published (Maltoni 1977; Lee et al. 1978; Gehring et al. 1979; Kappus and Ottenwalder 1980; Henschler et al. 1980; Bolt 1980). The principal question is that of possible carcinogenicity. It is now clear that vinyl chloride (Maltoni 1977) and vinyl bromide (Bolt et al. 1979) are carcinogenic; some data also argue in favor of carcinogenicity of vinylidene chloride (Lee et al. 1978). Trichloroethylene (Henschler et al. 1980) and perchloroethylene (Bolt and Link 1980), according to present data, appear devoid of cancerogenic p…

Increased acetone exhalation induced by metabolites of halogenated C1 and C2 compounds.

Rats were exposed, in a closed desiccator jar chamber, to concentrations of various halogenated C1 and C2 compounds at which the metabolizing capacities were saturated (Vmax conditions). Within the exposure period of 50 h concentrations of the xenobiotic and of exhaled acetone were monitored in the gas phase of the system. The quantitative extent of acetone exhalation was dependent on the individual compound examined. Acetone exhalation was stimulated in presence of vinyl chloride, vinyl bromide, vinyl fluoride, vinylidene fluoride, cis- and trans-1,2-dichloroethylene, trichloroethylene, perchloroethylene, methylene chloride, chloroform, carbon tetrachloride and 1,1,2-trichloroethane. No st…

Covalent Protein Binding of Vinyl Chloride Metabolites During Co-Incubation of Freshly Isolated Hepatocytes and Hepatic Sinusoidal Cells of Rats

Proteins of isolated rat hepatic sinusoidal cells incubated with 14C-vinyl chloride, rat liver microsomes and an NADPH-regenerating system were alkylated by vinyl chloride metabolites formed by microsomes. This suggests that reactive vinyl chloride metabolites can penetrate sinusoidal cells. Protein alkylation in isolated hepatic sinusoidal cells was higher when these were co-incubated with isolated hepatocytes, indicating that reactive vinyl chloride metabolites formed by hepatocytes are stable enough to diffuse out of hepatocytes into sinusoidal cells. Glutathione added to the incubation medium inhibited the covalent protein binding of vinyl chloride metabolites in sinusoidal cells as wel…

Alkylation of RNA by Vinyl Chloride and Vinyl Bromide Metabolites in Vivo: Effect on Protein Biosynthesis

Alkylation of DNA is viewed as representing the initial critical step in carcinogenesis induced by chemical substances. Vinyl chloride and vinyl bromide, compounds with proven carcinogenic potency toward the liver, are biotransformed to reactive metabolites which covalently bind to DNA (see Bolt et al. 1980). Furthermore, extensive covalent binding of metabolites of both vinyl chloride (Laib and Bolt 1977, 1978) and vinyl bromide (Ottenwalder et al. 1979) occurs to RNA of liver when rats are exposed to both vinyl halides. Defined products of alkylation are 1,N6-ethenoadenosine (Laib and Bolt 1777; Ottenwalder et al. 1979) and 3,N4-ethenocytidine (Laib and Bolt 1978; Ottenwalder et al. 1979)…

Formation of pre-neoplastic hepatocellular foci by vinyl fluoride in newborn rats

Inhalation pharmacokinetics based on gas uptake studies. IV. The endogenous production of volatile compounds.

A pharmacokinetic description of production, distribution and metabolism of endogenous volatile compounds is presented. This description uses the "gas uptake model" of a closed recirculated atmosphere in which experimental animals are exposed. As an example, the production rates of acetone, under different conditions of stimulation by xenobiotics, are calculated from published experimental data. The theoretical descriptions may serve as a basis for treating the problem of hydrocarbon exhalation in toxicological experiments with compounds eliciting lipid peroxidation.

Challenging Dogma: Thresholds for Genotoxic Carcinogens? The Case of Vinyl Acetate

Although many questions remain unanswered, the general principle of the sequence of events leading to cancer after exposure to genotoxic carcinogens has become increasingly clear. This helps to understand the parameters that influence the shape of the dose-effect curve for carcinogenesis, including metabolic activation and inactivation of carcinogens, DNA repair, cell cycle control, apoptosis, and control by the immune system. A linear dose-response relationship with no observable threshold seems to be a conservative but adequate description for the carcinogenic activity of many genotoxic carcinogens, such as aflatoxin B1, the tobacco-specific nitrosoketone NNK, and probably N,N-diethylnit…

Irreversible protein binding of acrylonitrile.

1. After i.p. injection of [2,3-14C]acrylonitrile to rats, a significant portion of radioactivity becomes irreversibly attached to proteins of liver, lung, spleen and other tissues. 2. When rat liver microsomes were incubated with [2,3-14C]acrylonitrile, a time-dependent irreversible binding of radioactivity occurred to microsomal proteins. This binding was not dependent on NADPH. A high extent of binding to heat-inactivated microsomes indicated that no enzymic metabolic step was involved. 3. The irreversible binding of [2,3-14C]acrylonitrile to rat liver microsomal protein in vitro was inhibited by thiols (cysteine, glutathione, mercaptoethanol). The greatest inhibitory potency was display…