In vivo covalent binding of organic chemicals to DNA as a quantitative indicator in the process of chemical carcinogenesis

Quantitative correlations amongst alkaline DNA fragmentation, DNA covalent binding, mutagenicity in the Ames test and carcinogenicity, for 21 compounds

21 compounds from different chemical classes were quantitatively compared for their carcinogenic potency according to 4 parameters: (1) potency in inducing covalent binding with DNA in vivo; (2) potency in inducing alkaline DNA fragmentation after treatment in vivo; (3) acute toxicity; (4) mutagenic potency in the Ames test. Establishing well-defined conditions for normalization of the different types of data and determination of the set that had to be submitted to statistical analysis appeared to be a difficult task, for which only compromise solutions were possible. A statistical analysis of the data suggested that all parameters considered were correlated with carcinogenic potency. However, we found that there are about 3 chances to 1 that carcinogenicity is better correlated with DNA covalent binding in vivo than it is to mutagenicity in the Ames test. With due precautions, even acute toxicity could be of predictive value. DNA adducts and DNA fragmentation, both in vivo, appeared to be 2 parameters strongly correlated between them. From a multivariate statistical analysis it appeared that: (1) a significant improvement of quantitative predictability is in principle obtainable with a battery of short-term test; and (2) the improvement is obtainable only if the short-term tests considered, while all correlated with carcinogenicity, are relatively independent amongst themselves.

The comparative metabolism and toxic potency of aflatoxin B1 and aflatoxin M1 in primary cultures of adult-rat hepatocytes

Both aflatoxin B1 (AFB1) and a hydroxylated metabolite, aflatoxin M1 (AFM1), were potent cytotoxins and genotoxins to primary cultures of rat hepatocytes. However, AFB1 stimulated the release of lactate dehydrogenase into the culture medium and the loss of viable cells from the monolayer at lower doses than did AFM1. The lowest toxic doses of AFB1 and AFM1 were 0.05-01 and 0.6 microgram/culture, respectively. Genotoxicity, determined by an assay for stimulation of DNA repair, was apparent at lower doses than was cytotoxicity. AFB1 was again more potent than AFM1, stimulating DNA repair at 0.025 microgram/culture, compared to the lowest genotoxic dose of AFM1 of 0.05 microgram/culture. At higher doses (1.2-2.4 microgram/culture) the responses due to both aflatoxins in the cytotoxicity and DNA-repair assays were approximately equal. The metabolism of a low dose (c. 0.17 microgram/culture) of [14C]AFB1 and [3H]AFM1 by cultured hepatocytes differed significantly. After 1 hr, 50% of the [14C]AFB1 remained unchanged in the culture medium, whereas about 18 hr were required for the same amount of [3H]AFM1 metabolism to occur [14C]AFB1 was metabolized to AFM1, to polar metabolites recovered in the aqueous phase after chloroform extraction, and to metabolites covalently bound to hepatocyte macromolecules. [3H]AFM1 was also metabolized to polar metabolites and to forms bound to macromolecules. The degree of covalent binding of the aflatoxins correlated with their cytotoxicity and genotoxicity at lower doses. After a 24-hr incubation, 12.5% of the dose of [14C]AFB1 was covalently bound to macromolecules compared to 1.5% of [3H]AFM1. Although AFM1 was less potent than AFB1 in cytotoxicity, DNA-repair and covalent-binding assays using primary cultures of hepatocytes, AFM1 was still active at relatively low doses and therefore is probably a potent hepatotoxin in vivo.

Covalent binding of haloethylenes

Halogenated ethylenes are metabolized to reactive intermediates which covalently bind to different cellular targets. Vinyl chloride and vinyl bromide metabolites bind to DNA, preferably to N-7 of deoxyguanosine. With RNA, 1,N6-ethenoadenosine and, 3,N4-ethenocytidine moieties are formed. All the haloethylenes in which this effect has been studied form metabolites capable of alkylating proteins, preferably at free sulfhydryl groups. Also, there is alkylation by haloethylene metabolites of cellular coenzymes. An observed increased exhalation of acetone by rats exposed to different haloethylenes can possibly be explained by alkylation of cytosolic coenzyme A. Such metabolic effects may serve as an indicator for reactive metabolite formation in vivo and should be more investigated.

Reactions of beta-propiolactone, beta-butyrolactone and gamma-butyrolactone with nucleic acids

Reactivity of beta-propiolactone, beta-butyrolactone and gamma-butyrolactone with guanosine, RNA, DNA and 4-(p-nitrobenzyl)pyridine was studied. beta-Propiolactone was 50--100 times more reactive with all the nucleophiles than beta-butyrolactone whereas gamma-butyrolactone was completely inactive. The rate of alkylation by the lactones was guanosine greater than RNA = denatured DNA greater than double-stranded DNA. The type of the adducts formed were characterized by fluorescence and ultraviolet spectroscopy. Similar alkylation products were formed by the two lactones. The main sites alkylated were N-1 at adenosine, N-3 at cytidine and N-7 at guanosine. The results suggest that the carcinogenic potency of the lactones correlates with their reactivity rather than with specificity of the adducts formed.

Exposure of newborn rats to pharmacologically active compounds may permanently alter carcinogen metabolism

Administration of phenobarbital to mother rats during early lactation causes long-term, perhaps permanent, alteration of hepatic microsomal mixed-function oxidase activity and aflatoxin B1 adduct formation in the adult male offspring. These findings suggest that perinatal exposure to pharmacologically active compounds may be a determinant of cancer risk.

Lack of covalent binding to rat liver DNA of the hypolipidemic drugs clofibrate and fenofibrate

14C-labelled clofibric acid and fenofibric acid were administered p.o. to 200 g male and female rats. After 10 h, liver nuclear DNA and protein were isolated and the radioactivity was determined. Binding to protein was clearly measurable whereas no binding to DNA could be detected from any drug. A comparison of the limit of detection of such DNA binding with well-known chemical carcinogens revealed that the known hepatocarcinogenicity of clofibrate cannot be based upon an initiating, DNA damaging, mode of action but must be due to other, nongenotoxic, mechanisms such as peroxisome proliferation, hepatomegaly, or cytotoxicity due to protein binding. The risk assessment in man and the interpretation of the carcinogenicity data for rodents are discussed.

Studies on the genotoxicity of some fluorescein dyes

The activities of 2,4,5,7-tetraiodofluorescein, disodium salt (erythrosine) and 2 phloxine dyes (2,4,5,7-tetrabromo-12,15-dichlorofluorescein, dipotassium salt and the disodium salt of 2,4,5,7-tetraiodo-12,15-dichlorofluorescein) have been determined using DNA-repair, fluctuation and treat-and-plate assays. Tests were conducted with and without illumination from a daylight fluorescent lamp. Both phloxine dyes were active in a rec assay but only in the absence of a rat-liver microsomal metabolising system. Erythrosine was inactive under all conditions. Although the results agreed with some of the published data for these foods and cosmetic colours, previous reports of photodynamic activation and mutagenicity were not confirmed. In the light of recent concern over the efficacy of bacterial DNA-repair tests, it is considered that the results obtained are not at present conclusive evidence for genotoxic hazard of any of the dyes studied.

In vitro and in vivo covalent binding of the kidney toxicant and carcinogen tris(2,3-dibromopropyl)-phosphate

The nephrotoxicant and nephrocarcinogen tris(2,3-dibromopropyl)-phosphate (Tris-BP) is activated to products which bind covalently to microsomal protein by a cytochrome P-450 dependent oxidation reaction. Binding to rat liver microsomes proceeds 15 times faster than with kidney microsomes. The binding in liver microsomes is markedly increased by phenobarbital pretreatment, the apparent Vmax of the reaction is 175 pmol/mg microsomal protein/min with control microsomes and 1053 pmol/mg protein/min with induced microsomes. Binding with kidney microsomes is doubled after pretreatment with polychlorinated biphenyls. 2,3-Dibromopropanol (2,3-DBP), a hydrolysis product of Tris-BP, is also activated to covalently protein-bound products, but at a much slower rate than Tris-BP. Administration of Tris-BP to rats leads to its covalent binding to proteins in liver and kidney, with 5 time higher binding levels in kidney than in liver, correlating with its relative organotoxic potential in single dose experiments. Binding to proteins in the kidney was increased by pretreatment of animals with polychlorinated biphenyls. A covalent interaction of Tris-BP could also be demonstrated to DNA, both when DNA was added to liver microsomal incubations in vitro and to DNA extracted from liver and kidney after administration of Tris-BP in vivo. The binding levels were 4 times higher to kidney DNA than to liver DNA.

In vivo covalent binding of aflatoxin B1 and aflatoxin M1 to liver DNA of rat, mouse and pig

[14C]Aflatoxin B1 (AFB1) was isolated from cultures of Aspergillus parasiticus grown on [1-14C]sodium acetate. Covalent binding of AFB1 to liver DNA of rat and mouse was determined 6-8 h after oral administration. The effectiveness of covalent binding, expressed as DNA binding per dose in the units of a 'Covalent Binding Index' (CBI), (micromol aflatoxin/mol DNA nucleotides)/(mmol aflatoxin/kg animal), was found to be 10 400 for rats and 240 for mice. These CBI partly explain the different susceptibility of the two species for the incidence of hepatic tumors. The corresponding values for pig liver DNA, 24 and 48 h after oral administration, were found to be as high as 19 100 and 13 300. DNA-binding has not so far been reported for this species although it could represent an appropriate animal model for studies where a human-like gastrointestinal tract physiology is desirable. Aflatoxin M1 (AFM1) is a metabolite found in the milk of cows that have been fed AFB1-contaminated diet. [14C]AFM1 was also found to be produced by cultures of A. parasiticus giving a yield of about 0.3% of the total aflatoxins. A test for covalent binding to rat liver DNA revealed a CBI of 2100 showing that AFM1 must also be regarded as a strong hepatocarcinogen. It is concluded that AFB1 contaminations should be avoided in dairy feed.

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.

Unscheduled DNA synthesis in the testis, a secondary test for the evaluation of chemical mutagens

DNA damage induced by methylmethane sulfonate, cyclophosphamide, doxorubicin and procarbazine in male germ cells was assessed in rabbits by the demonstration of unscheduled DNA synthesis (UDS), in meiotic and postmeiotic phases of maturation. Immediately after treatment by the intravenous route tritiated thymidine was injected into both testicles. Subsequently, rabbits were ejaculated serially, sperm heads were isolated and assayed for radioactivity by liquid scintillation counting. Dose-dependent UDS was demonstrated in late spermatocytes and early spermatids. High doses of hycanthone also induced UDS, but isoniazid and metronidazole had no effect. The rabbit testis UDS test takes into account metabolic and pharmacokinetic aspects of the test substances and provides information about their penetration through the blood-testicular barrier. It is therefore useful for secondary evaluation of potential mutagens. UDS induced by procarbazine was abolished by simultaneous treatment with Ara-C. Thus, the test also recognizes substances that inhibit DNA repair synthesis.