Comparative metabolism and disposition of ethyl carbamate (urethane) in male Fischer 344 rats and male B6C3F1 mice.

The metabolism, disposition, and excretion of ethyl carbamate (EC) was investigated following oral or iv administration of a wide range of doses to male rats and mice. At a low dose, 4.75 mg/kg, administered iv, approximately 98% was exhaled as CO2 within 8 or 12 hr by mice or rats, respectively. However, as the dose increased, the percentage of dose eliminated as CO2 decreased in a dose-dependent manner which was much more pronounced in rats than mice. At all doses studied, mice eliminated EC as CO2 (as % dose) more rapidly than rats. Evidence of saturation of metabolism and elimination was observed at doses greater than 4.75 mg/kg in rats and greater than 47.5 mg/kg in mice. Following iv administration of 47.5 or 475 mg/kg, EC was initially evenly distributed in all tissues of each species except fat. After the initial time point (15 min), rat tissues contained higher concentrations of 14C compared to tissues of mice receiving the same dose. The disappearance of 14C from blood and various tissues followed monoexponential kinetics with rates dependent upon the species and the dose but independent of the tissue. Following oral administration, EC was completely absorbed from the gastrointestinal tracts of rats and mice at all doses studied. Approximately 5, 0.7, and 1% of the doses were excreted in urine, in feces, and as volatile organics, respectively. EC was neither an inducer nor an inhibitor of its own metabolism to CO2 following daily treatment of rats with oral doses of 47.5 mg/kg for 9 days. Only the parent compound was present in blood, lungs, skin, liver, kidney, muscle, and bile of treated rats. The urinary metabolic profile of EC was not affected by the route of administration in either species; however, in the rat but not in the mouse it was influenced by dose. Pretreatment of rats with piperonyl butoxide or SKF 525A (cytochrome P-450 inhibitors) or tri-o-cresyl phosphate (TOCP) or paraoxon (carboxylesterase inhibitors) or methyl carbamate (competitive substrate) did not greatly alter the metabolism of EC to CO2. The in vitro metabolism of EC to CO2 was not highly localized in any particular tissue or subcellular fraction of liver and was not affected by NADPH, GSH, NADH, or combinations of these cofactors. This work indicates that a number of studies of EC carcinogenicity have used doses that exceed the capacity of rats and mice to metabolize this chemical in a linear fashion.

Methyl carbamate. Species-dependent variations in metabolism and clearance in rats and mice.

Studies of the chronic toxicity and carcinogenicity of methyl carbamate (MC) in F344 rats and B6C3F1 mice indicate that this compound is more toxic to rats than mice. MC was also a carcinogen for rats but not a carcinogen for mice even when administered at a higher dose. The present study of the comparative metabolism and disposition of MC in these two species was conducted to determine possible sources of these varying responses. Results of this study indicate that, although the initial distribution of MC in the two species is similar, the mouse metabolizes and clears MC much more rapidly than does the rat. In the mouse, clearance was primarily by metabolism to CO2 and elimination in exhaled air, which accounted for approximately 70% of the dose in 48 hr. On the other hand, the rat eliminated approximately 18% of the dose as CO2 in 48 hr and a similar amount in urine. The parent compound accounted for approximately 90% of the material excreted in urine of both rats and mice. Only the parent compound was detected in tissues of either species. Less than 4% of the dose was excreted in feces of either species. The lesser ability of the rat to metabolize and eliminate MC as CO2 results in bioaccumulation of this compound on repeat exposure. Therefore, bioaccumulation of MC by the rat on chronic exposure probably results in both greater total exposure and higher peak exposure of most rat tissues vs. those of mice and may thus account for the greater toxicity and possibly carcinogenicity of MC to rats.

Absorption, distribution, metabolism, and excretion of 1,2-dihydro-2,2,4-trimethylquinoline in the male F344 rat.

1,2-Dihydro-2,2,4-trimethylquinoline (TMQ), an antioxidant used in the rubber industry, was readily absorbed from the gastrointestinal tract of the male Fischer 344/N rat and rapidly distributed throughout the body tissues. Absorption, distribution, metabolism, and excretion were not significantly affected by dose in the range 11.5-1150 mumol/kg. Following iv administration, the greatest amounts of TMQ-derived radioactivity were present in the high volume tissues including muscle, adipose, skin, liver, and blood. TMQ had no particular affinity for any tissue. TMQ-derived radioactivity was excreted primarily in urine (60-70%) and feces (20-30%) within 3 days after administration. Greater than 99% of the TMQ dose excreted in urine and feces was in the form of metabolites. Urine contained two major and ten minor metabolites while feces contained two major and four minor metabolites. The two major TMQ metabolites in urine were identified by NMR and mass spectroscopy as the O-sulfate conjugate of 1,2-dihydro-6-hydroxy-2,2,4-trimethylquinoline and the monosulfate conjugate of 1,2-dihydro-1,6-dihydroxy-2,2,4-trimethylquinoline. In vitro studies with liver subcellular fractions suggest that most of the metabolites present in urine, feces, and bile are the products of mixed function oxidase activity and conjugates of these metabolites. Multiple exposure of rats to high TMQ doses (1150 mumol/kg) resulted in some bioaccumulation of TMQ-derived radioactivity in all tissues examined, but these residues did not persist when dosing was discontinued.

2,6-Dichloro-p-phenylenediamine: comparative disposition in male and female rats and mice.

2,6-Dichloro-p-phenylenediamine (DPA) was recently reported to induce hepatocellular adenomas and carcinomas in male and female B6C3F1 mice but not in F344 rats. The present investigation of comparative disposition in both sexes of each species was designed to detect species-related variations in DPA disposition that might explain variations in toxicity. Mouse tissues retained higher concentrations of radioactivity at the early time points (15 min and 2 h) than the corresponding rat tissues but were readily cleared at later time points. Elimination of DPA-derived radioactivity by each species was primarily in urine (56-63% of an intravenous dose of 600 mumol/kg body weight) and secondarily in feces (23-31%). Metabolites were qualitatively similar, but varied quantitatively with species. Rats excreted three major and eight minor metabolites in urine, while mice excreted one major and nine minor metabolites. The major metabolite present in mouse urine (61-78% of radioactivity) was approximately two-fold higher than the corresponding major metabolite in rat urine. Efforts to detect covalent binding of DPA and/or metabolites with hepatic DNA indicated no detectable binding in either species. The present study indicates that quantitative variations in disposition and metabolism exist between the two species but does not identify a likely source of species variation in susceptibility to DPA toxicity.

p-Phenylenediamine dihydrochloride: comparative disposition in male and female rats and mice.

The absorption, distribution, metabolism, and excretion of p-phenylenediamine (PDA) was studied in both sexes of F344 rats and B6C3F1 mice. Absorption of PDA from the gastrointestinal tract was nearly complete in both species, and tissue distribution and excretion were not affected by the route of administration or dose in the range studied. The highest PDA-derived radioactivity was present in muscle, skin, and liver in both species at all time points examined. Both sexes of either species cleared radioactivity from all tissues rapidly, so that in 24 h only 10-15% of the total dose administered was still present in the animal body. Clearance of PDA-derived radioactivity was primarily through urine (68-86%) and secondarily through feces (10-19%). Over 95% of the radioactivity excreted in urine of both species was in the form of metabolites. The major metabolites in male and female rat urine were qualitatively and quantitatively similar, while major quantitative differences were observed between urinary metabolites of male and female mice. Variability in urinary metabolites was observed between the two species. Supplementary experiments have shown that PDA and/or metabolites do not bind covalently to hepatic DNA. However, PDA-derived radioactivity was found to bind with hepatic protein of both sexes of each species.

Dermal absorption and disposition of 1,3-diphenylguanidine in rats.

Dermal absorption, distribution, and metabolism of 1,3-diphenylguanidine (CAS 102-06-7) (DPG), widely used as an accelerator in processing rubber and in food packaging, was studied in adult female Sprague-Dawley rats. DPG shows 10% penetration through clipped back skin of the rats in 5 d. The first-order dermal absorption rate constant as determined by least square method was 0.021 +/- 0.002 d-1 (T1/2 = 33.6 d). Approximately 13% of the absorbed dose remained in the body in 5 d. Retention in skin, muscle, liver, intestine and fat contributed most to the body burden of DPG-derived radioactivity in 5 d. All tissues showed tissue to blood ratios greater than 1, with liver and intestine ratios of 26 at 5 d. Approximately 61% of the absorbed dose was eliminated into urine and 27% into feces in 5 d showing rapid clearance of absorbed DPG from the body. High-pressure liquid chromatography (HPLC) analysis of urine revealed two major peaks [parent compound and metabolite(s)]. Within 72 h, approximately 50% of the DPG-derived radioactivity excreted in the urine was parent compound. After 72 h, the DPG-derived radioactivity in the urine was present in the form of a single metabolite, and no parent compound was detected. No parent compound was detected in feces. Two metabolites, neither of which occurred in urine, were detected in feces. The HPLC analysis of the radioactivity at the application site showed only parent compound. Even though DPG shows slow dermal penetration, this route of exposure needs to be considered in the risk assessments because of the suspected chronic toxicity of DPG.

Allyl isothiocyanate: comparative disposition in rats and mice.

Allyl isothiocyanate (AITC), the major component of volatile oil of mustard, was recently reported to induce transitional-cell papillomas in the urinary bladder of male Fischer 344 rats, but not in the bladders of female rats or B6C3F1 mice. The present investigation of comparative disposition in both sexes of each species was designed to detect sex or species differences in disposition which might explain susceptibility to AITC toxicity. AITC was readily cleared from all rat and mouse tissues so that within 24 hr after administration less than 5% of the total dose was retained in tissues. The highest concentration of AITC-derived radioactivity was observed in male rat bladder. Clearance of AITC-derived radioactivity by each species was primarily in urine (70 to 80%) and in exhaled air (13 to 15%) with lesser amounts in feces (3 to 5%). Rats excreted one major and four minor metabolites in urine. The major metabolite from rat urine was identified by NMR spectroscopy to be the mercapturic acid N-acetyl-S-(N-allylthiocarbamoyl)-L-cysteine. Mice excreted in urine the same major metabolite identified in rat urine as well as three other major and two minor metabolites. Sex-related variations were observed in the relative amounts of these metabolites. Both species excreted a single metabolite in feces. Metabolism of AITC by male and female rats was similar, but female rats excreted over twice the urine volume of male rats. Results of the present study indicate that excretion of a more concentrated solution of AITC metabolite(s) in urine may account for the toxic effects of AITC on the bladder of male rats.

Absorption, distribution, metabolism, and excretion of 1,3-diphenylguanidine in the male F344 rat.

1,3-diphenylguanidine (DPG), a rubber accelerator, was readily absorbed from the gastrointestinal tract of the male Fischer rat and rapidly distributed throughout the body tissues. Absorption and disposition of DPG were not significantly affected by the route of administration or by the dose in the dose range studied, 1.5 to 150 mumol/kg. Most of the dose of DPG was excreted in the urine and feces at approximately equal amounts within 24 hr after oral or iv administration. Greater than 99% of the DPG dose was cleared into the urine and feces within 3 days after administration. Approximately 30% of the DPG-derived radioactivity excreted in urine was the parent compound, DPG, while the remainder was present in the form of two major and one minor metabolite. Close to 95% of the radioactivity excreted in bile was in the form of a single major metabolite. Administration of multiple doses resulted in a proportional increase of DPG-derived radioactivity in the liver as the number of doses increased. DPG-derived radioactivity did not increase in other tissues following multiple doses.

Toxicity and distribution of 2,3,7,8-tetrachlorodibenzofuran in male guinea pigs.

The toxicity of 2,3,7,8-tetrachlorodibenzofuran (TCDF) was studied in male Hartley guinea pigs after single or multiple oral treatment. In each animal studied, adipose tissue, liver, and skin were the major depots of TCDF and accounted for 50-74% of the total dose. The whole-body half-life of TCDF in guinea pigs was estimated to be approximately 40 d. This slow clearance of TCDF by guinea pigs may explain the high toxicity of TCDF for this species even when very low repeated doses were administered. Intervals of 1, 2, or 4 wk between doses had little effect on TCDF lethality, but did have an effect on the pattern of toxicity. Treatment of mature animals with high single doses of TCDF (10 or 15 micrograms/kg body weight) resulted in the immediate loss of weight and the eventual death of all treated animals within 2-4 wk after the loss of approximately 35% of their initial body weight. Treatment of immature animals with low multiple doses totaling cumulative doses of between 4 and 12 micrograms/kg resulted in the death of 75% of these animals, with the deaths occurring between 7 and 19 d after the initial appearance of quantitative toxic symptoms (loss of weight); however, weight loss was less dramatic following repeated low doses than after acute high doses. Concentration of TCDF in adipose tissue was generally proportional to dose of TCDF. With increasing time after dosage, there was a shift in distribution from adipose tissue to liver.

Effect of butylated hydroxyanisole, alpha-angelica lactone, and beta-naphthoflavone on benzo(alpha)pyrene:DNA adduct formation in vivo in the forestomach, lung, and liver of mice.

The effects of alpha-angelica lactone (alpha-AL), butylated hydroxyanisole (BHA), and beta-naphthoflavone (beta-NF) on the amount of benzo(alpha)pyrene (BP) metabolite:DNA adducts formed in the forestomach, lung, and liver of ICR/Ha mice were investigated 48 hr after p.o. administration of BP. BP was administered to mice in amounts known to result in BP-induced neoplasia in certain tissues. Analysis of deoxyribonucleosides by high-pressure liquid chromatography showed that several BP metabolite:DNA adducts were formed in each tissue examined. The major identified adduct in each tissue cochromatographed with the (+/-)-7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10-tetrahydrobenzo(alpha)pyrene (BPDEI):deoxyguanosine adduct. The (+/-)-7 beta,8 alpha-dihydroxy-9 beta,10 beta-epoxy-7,8,9,10-tetrahydrobenzo(alpha)pyrene (BPDEII):deoxyguanosine adduct was detected in each of the tissues. As a percentage of total DNA-associated radioactivity, the BPDEI:DNA and BPDEII:DNA adducts accounted for 14% in the forestomach, 39% in the lung, and 3% in the liver. Another adduct, possibly derived from BP:phenol(s), was detected in lung and liver. Early eluting unidentified DNA-associated radioactivity was also present in each of the tissues and accounted for the majority of the radioactivity (88%) in forestomach, 57% in lung, 97% in liver). Although total DNA-associated radioactivity in liver was approximately 15-fold higher than in lung and 5-fold higher than in forestomach, the specific activities of the BPDEI:adducts and of the BPDEII:adducts were approximately the same in these organs. Addition of alpha-AL or BHA to the diet inhibited BPDEI:DNA adduct formation in the forestomach and liver but not in the lung. The effect of beta-NF was not tissue specific; this aryl hydrocarbon hydroxylase inducer decreased markedly (80 to 90%) BPDEI:DNA adduct formation in all three tissues. The radioactivity associated with the early eluting peaks was also reduced when associated with the early eluting peaks was also reduced when alpha-AL, BHA, or beta-NF was fed to the mice. The inhibition of BPDEI:DNA and BPDEII:DNA adduct formation by alpha-AL, BHA, and beta-NF is discussed in relation to similar studies where these compounds inhibited BP-induced neoplasia.