Incorporation of radioactivity from labeled Di-(2-ethylhexyl)phthalate into DNA of rat liver in vivo

Assessment of Possible Carcinogenic Risk to Humans Resulting from Exposure to Di(2-ethylhexyl)phthalate (DEHP)

The purpose of this work was to estimate the degree of risk that might be associated with human exposure to low levels of the plasticizer di(2-ethylhexyl)phthalate (DEHP). DEHP is a common component, sometimes at high concentrations, of polyvinyl chloride (PVC) plastics and was recently reported by the National Toxicology Program (NTP) to be carcinogenic in rats and mice, inducing hepatocellular tumors in both species. This work was also designed to illustrate an approach to risk assessment that attempts to incorporate all available biological data. Based on the dose-response data generated by the NTP bioassays, we have performed extrapolations of risk to low dose levels using several procedures, including some that incorporate inferences from the available data that shed light on the likely relationship between dose level and risk at low dose levels. In drawing these inferences, consideration was given to such factors as genotoxicity, metabolism and pharmacokinetics, and physiological and biochemical effects of DEHP that might reveal its mechanism of action. The relative merits of each of the various risk estimates are described, based on current understanding of DEHP's mode of biological action. It is concluded that DEHP's mechanism of carcinogenicity in rodents most likely involves its ability to induce peroxisome proliferation and related enzymatic changes, although other mechanisms cannot be excluded. If humans and rodents are assumed to be at the same risk at the same daily dose level of DEHP, application of the various low dose extrapolation models leads to the prediction that the daily dose resulting in a lifetime risk of no more than 1 in 1 million would be between 1.5 and 791 mg/kg per day, with the most likely figure being 116 mg/kg per day. If the carcinogenicity of DEHP is dependent upon its pattern of metabolism, however, it would be inappropriate to extrapolate from rodents to man without qualification because of the major quantitative differences in metabolism in rats, mice, and primates, including man. One of the major differences in metabolism of DEHP between rats and mice and primates is in production of a metabolite whose level may be an indicator of the level of peroxisomal activity and, hence, if the peroxisome proliferation theory of DEHP carcinogenicity is correct, of carcinogenic risk. However, the substantial doubt that exists regarding the applicability of rodent carcinogenicity data to humans must be expressed in qualitative terms.

Morphological transformation and catalase activity of Syrian hamster embryo cells treated with hepatic peroxisome proliferators, TPA and nickel sulphate

The abilities of the hepatic peroxisome proliferators (HPPs) clofibrate, di(2-ethylhexyl)phthalate (DEHP), mono(2-ethylhexyl)-phthalate (MEHP), 2,4-dichlorophenoxy acetic acid (2,4-D), 2,4,5-trichlorophenoxy acetic acid (2,4,5-T) and tiadenol to induce morphological transformation and to increase the catalase activity of Syrian hamster embryo (SHE) cells were studied. DEHP, MEHP, clofibrate and tiadenol induced morphological transformation of SHE cells and increased the catalase activity. DEHP was more potent than clofibrate and tiadenol in both inducing catalase and morphological transformation, while MEHP seemed more potent than DEHP in inducing catalase, but not morphological transformation, 2,4,5-T and 2,4-D did not induce morphological transformation, but 2,4,5-T was more potent than clofibrate in increasing the catalase activity. These results show that several HPPs induce morphological transformation of SHE cells and an increase in the catalase activity. There is, however, no direct connection between these two parameters, as seen from the results of 2,4,5-T. The tumor promoter TPA, and the metal salt nickel sulphate, induced morphological transformation of SHE cells without any appreciable increase in the catalase activity. These results further corroborate the dissociation between induction of morphological transformation and the increase in catalase activity.

Absence of a promoting or sequential syncarcinogenic effect in rat liver by the carcinogenic hypolipidemic drug nafenopin given after N-2-fluorenylacetamide

The hypolipidemic agent nafenopin, (NF), has been reported to be carcinogenic to rat liver. To determine whether nafenopin exerts a promoting or syncarcinogenic effect in rat liver, its effect on liver carcinogenesis induced by N-2-fluorenylacetamide (FAA) was studied. In two separate experiments, male F344 rats were fed 0.02% FAA for either 10 or 8 weeks to induce preneoplastic liver lesions. Following a recovery period of 1 week, rats were given 0.01 or 0.02% NF in the diet for 23 weeks in one experiment and 0.05 or 0.1% for 24 weeks in the other. The final incidence of neoplasms, and their numbers, size distribution, and degrees of differentiation were not significantly different in groups given NF after FAA compared to those maintained on a basal diet after FAA. In the group treated with the highest dose level of NF following FAA, however, there was a decrease in the number of grossly visible small neoplasms. In contrast, the liver neoplasm promoter phenobarbital increased the multiplicity, although not the incidence, of liver neoplasms when given after FAA. Thus, four different dose levels of NF showed no promoting or syncarcinogenic effect on FAA-induced hepatocarcinogenesis.

Metabolism of methyl n-amyl ketone (2-heptanone) and its binding to DNA of rat liver in vivo and in vitro

Methyl n-amyl ketone (2-heptanone), a reported metabolite of 2-ethylhexanol which in turn is a primary metabolite of plasticizers such as di-(2-ethylhexyl) phthalate, is metabolized in male Fischer 344 rats to CO2, acetate and a variety of compounds that could be either anabolic or catabolic or a combination of the two. A significant percentage of the radioactivity given orally (gavage) as [2-14C]-2 heptanone, at least 10%, was not excreted from the body in 48 h. Radioactivity was incorporated into liver protein in the form of three unidentified products as well as [14C]arginine, and into DNA both as 14C-labeled normal nucleosides (50-75%) and as presently unidentified hydrophobic materials (25-50%). Urea and cholesterol were significantly labeled, indicative of anabolic reutilization of [2-14C]-2-heptanone breakdown products. The 2-heptanone also bound to DNA spontaneously in vitro, to the extent of 400 pmol/mg DNA.

Investigation of the potential for binding of Di(2-ethylhexyl) phthalate (DEHP) and Di(2-ethylhexyl) adipate (DEHA) to liver DNA in vivo

It was the aim of this investigation to determine whether covalent binding of di(2-ethylhexyl) phthalate (DEHP) to rat liver DNA and of di(2-ethylhexyl) adipate (DEHA) to mouse liver DNA could be a mechanism of action contributing to the observed induction of liver tumors after lifetime feeding of the respective rodent species with high doses of DEHP and DEHA. For this purpose, DEHP and DEHA radiolabeled in different parts of the molecule were administered orally to female rats and mice, respectively, with or without pretreatment for 4 weeks with 1% unlabeled compound in the diet. Liver DNA was isolated after 16 hr and analyzed for radioactivity. The data were converted to a covalent binding index, CBI = (micromoles of substance bound per mole of DNA nucleotides)/(millimoles of substance applied per kilogram body weight), in order to allow a quantitative comparison also with other carcinogens and noncarcinogens. Administration of [14C]carboxylate-labeled DEHP to rats resulted in no measurable DNA radioactivity. The limit of detection, CBI less than 0.02 was about 100 times below the CBI of compounds where an observable tumor-inducing potential could be due to genotoxicity. With [14C]- and [3H]DEHP labeled in the alcohol moiety, radioactivity was clearly measurable in rat liver DNA. HPLC analysis of enzyme-degraded or acid-hydrolyzed DNA revealed that the natural nucleosides or purine bases were radiolabeled whereas no radioactivity was detectable in those fractions where the carcinogen-modified nucleoside or base adducts are expected. The respective limits of detection were at 0.07 and 0.04 CBI units for the 14C and 3H labels, respectively. The experiments with [14C]- and [3H]DEHA, labeled in the alcohol moiety and administered to mice, revealed a minute radioactivity of less than 50 dpm/mg liver DNA, too little to allow a nucleoside analysis to determine that fraction of the radioactivity which had been incorporated via biosynthesis. Expressed in the CBI units, values of 0.05 to 0.15 for 14C and 0.01 to 0.12 for 3H resulted. Determination of the level of 14CO2 expiration revealed a linear correlation with the specific activity of DNA. Experiments with 2-ethyl[1-14C]hexanol performed with both rats and mice allowed the conclusion that most if not all DEHA radioactivity in mouse liver DNA was due to biosynthetic incorporation. A maximum possible true DNA binding by DEHA must be below CBI 0.01.(ABSTRACT TRUNCATED AT 400 WORDS)

Polar metabolites of di-(2-ethylhexyl)phthalate in the rat

Di-(2-ethylhexyl)phthalate (DEHP) is an important industrial chemical widely used as a plasticizer for vinyl and other plastics. DEHP is extensively metabolized by mammals, different species showing dramatic differences in metabolite distributions. Previous studies of the metabolism in rats led to the suggestion that the enzymatic processes normally associated with omega-, omega-1, alpha-, and beta-oxidation of fatty acids could account for the known metabolites of DEHP found in the urine. Several additional metabolites of DEHP have been identified in the present study. Their formation requires that the initial hydroxylation process be less specific than fatty acid omega- and omega-1 oxidation are thought to be. Furthermore, it is necessary to postulate either that the aliphatic chain of mono-(2-ethylhexyl)phthalate can be oxidized at two sites simultaneously, or that oxidation products can be recycled for a second hydroxylation prior to excretion.