Short-term inhalation test for evaluating industrial hepatotoxicants in rats

Carcinogenicity and Chronic Toxicity in Rats and Mice Exposed by Inhalation to 1,2‐Dichloroethane for Two Years

Carcinogenicity and chronic toxicity of 1,2-dichloroethane (DCE) were examined by inhalation exposure of groups of 50 F344 rats and 50 BDF1 mice of both sexes to DCE vapor or clean air as control for 6 h/d, 5 d/wk and 104 wk. The rats were exposed to 10, 40 or 160 ppm (v/v) DCE, while the mice were exposed to 10, 30 or 90 ppm. The 2-yr exposure to DCE produced a dose-dependent increase in incidences of benign and malignant tumors, including subcutaneous fibroma, mammary gland fibroadenoma and peritoneal mesothelioma in male rats; subcutaneous fibroma and mammary gland adenoma, fibroadenoma and adenocarcinoma in female rats; and bronchiolo-alveolar adenoma and carcinoma, endometrial stromal polyp, mammary gland adenocarcinoma and hepatocellular adenoma in female mice. No exposure-related change in the incidence of non-neoplastic lesions or in any hematological, blood biochemical or urinary parameter occurred in any DCE-exposed rat or mouse group. The types of tumors and their target organs found in this study were consistent with those observed in rats and mice administered DCE by gavage in a NCI study. Selection of the exposure concentrations was considered appropriate with reference to the maximum tolerated dose for the highest doses and an occupational exposure limit of DCE for the lowest dose. The present findings suggest that those carcinogenic responses be primarily considered for standard setting of occupational and environmental exposure to DCE.

Toxicity due to 2‐ and 13‐wk Inhalation Exposures of Rats and Mice to N , N ‐Dimethylformamide

In order to better characterize the toxicity of N,N-dimethylformamide (DMF) and to provide its basic toxicity data for risk assessment of workers exposed to DMF, F344 rats and BDF1 mice of both sexes were exposed by inhalation (6 h/d x 5 d/wk) to 100, 200, 400, 800 or 1,600 ppm DMF for 2 wk, and 50, 100, 200, 400 or 800 ppm DMF for 13 wk. Three male and 7 female rats died during the 2-wk exposure to 1,600 ppm DMF, but no death of the exposed rats or mice occurred under any other exposure conditions. Massive, focal and single cell necroses were observed in the liver of DMF-exposed rats and mice. The massive necrosis associated with the centrilobular fibrosis occurred at the highest exposure concentration. The single cell necrosis was associated with fragmentation of the nucleoli as well as an increased mitotic figure. The 13-wk exposures of rats and mice to DMF were characterized by increases in the relative liver weight and the incidence of the centrilobular hepatocellular hypertrophy as well as increased serum levels of AST, ALT, LDH, total cholesterol and phospholipid. Lower confidence limits of the benchmark dose yielding the response with a 10% extra risk (BMDL10) were determined for the relative liver weight and the incidence of hepatocellular hypertrophy of the 13-wk exposed animals. The BMDL10 resulted in 1 ppm for the increased relative liver weight of male rats and mice and 17 ppm for the hepatocellular hypertrophy of male mice.

A comparison of Haber's rule at different ages using a physiologically based pharmacokinetic (PBPK) model for chloroform in rats

Haber's rule as commonly interpreted in inhalation toxicology, can be stated as exposure concentration times duration equals a constant biological effect, or C x t=k. In other words, identical products of concentration and duration lead to the same effect. The goals of this paper are to develop a biological and pharmacokinetic modeling approach for chloroform, and to evaluate Haber's rule for different ages by taking into account the physiological changes due to growth and aging in rats. Three-dimensional dose-response surfaces for liver toxicity were generated for each age group of interest: adolescent, adult, and senescent rats. The three-dimensional surfaces were then characterized with a generalized description of Haber's rule for each age group. The simulations suggest that adolescent rats need higher exposure levels in order to achieve similar levels of liver damage compared to adults or senescent rats, if the comparison is made using the same exposure length. In summary, a pharmacokinetic modeling approach with a biological framework including the chemical's mode of action, was used to relate concentration, exposure duration and effect. Major advantages of this approach include: the potential ability to extrapolate to humans, the inclusion of aging in the simulations, and the ability to summarize the results using a generalized form of Haber's rule.

The differential hepatotoxicity and cytochrome P450 responses of Fischer-344 rats to the three isomers of dichlorobenzene

The acute hepatotoxicity and response of hepatic cytochrome P450 to treatment with the three isomers of dichlorobenzene (DCB) have been investigated. The objectives were to estimate the onset of toxicity and to further elucidate the role of cytochrome P450 in the metabolism and toxicity of these compounds. In a study design employing one animal per dose level, Fischer-344 rats were gavaged with up to 25 different dosages, then evaluated 24 h later. Hepatic necrosis, serum alanine aminotransferase, and serum aspartate aminotransferase exhibited similar patterns demonstrating that ortho-DCB (o-DCB) was the most toxic in terms of both earliest onset and degree of response at higher dosages. For these three endpoints, meta-DCB (m-DCB) exhibited a lesser toxicity. Para-DCB (p-DCB) did not cause changes in these three endpoints, but hepatic degenerative changes were found. Total hepatic cytochrome P450 responses were also different after treatment with each isomer. The o-DCB produced a dose-dependent decrease in P450 beginning at dosages lower than the onset of necrosis and appeared to be a suicide substrate for P450. The m-DCB treatment increased P450 at dosages below the onset of necrosis and decreased P450 at higher dosages, with the decline preceding the onset of hepatocyte death. Treatment with p-DCB increased P450 beginning at 380 mg/kg. The combination of toxicity and P450 profiles has provided a framework for interpreting literature data on the metabolism and toxicity of the DCBs in rats. It is also noteworthy that o-DCB and p-DCB were administered at dosages several times the oral rat LD-50 (RTECS) without any lethality.

Difference in liver and serum malathion carboxylesterase and glucose-6-phosphatase in detecting carbon tetrachloride-induced liver damage in rats

Microsome, cytosol and serum malathion carboxylesterase (MaCEst) activity was assessed in rats after single i.p. administration of carbon tetrachloride (CCl4) in doses ranging from 0.05 to 1 ml kg-1. MaCEst activities were compared with those of glucose-6-phosphatase (G6-Pase) as an indicator of endoplasmic reticulum damage and serum glutamate dehydrogenase (GLDH) and sorbitol dehydrogenase (SHD) as indicators of liver cytolysis. The data showed a dose-dependent increase in GLDH and SDH serum activities (175% and 68%) from 0.05 ml kg-1; an increase in serum G6-Pase (31%) and a decrease in microsomal G6-Pase (38%) was apparent only after 0.5 or 1.0 ml kg-1 doses. MaCEst activity was unaffected. The results demonstrate that, under these experimental conditions, serum and subcellular measurements of MaCEst activity failed to reveal the liver toxicity of CCl4.

Effect of SKF525-A on the glutathione-conjugating enzyme system and on liver toxicity

SKF525-A given intraperitoneally (50 mg/kg body weight) to Sprague-Dawley rats in a single dose promoted a significant reduction in cytosolic glutathione S-transferase (GST) activities 0.5 and 1 h after injection. There was no decrease in liver non-protein sulfhydryls (NPSH) 0.5, 1 and 24 h after injection. Serum activities of glutamic pyruvic transaminase (GPT), sorbitol dehydrogenase (SDH), glutamic oxaloacetic transaminase (GOT) and glutamate dehydrogenase (GLDH) increased 1.8-, 2.9-, 3.8- and 41.2-fold respectively 8 h after injection, and the increased serum enzyme activities were maintained for up to 24 h. On the basis of these results, SKF525-A-induced blood manifestations of liver toxicity and decrease in GST activities may be regarded as confusing factors in the evaluation of the oxidative metabolism of compounds in Sprague-Dawley rats.

Concentration-related changes in blood and tissue parameters of hepatotoxicity and their interdependence in rats exposed to bromobenzene and 1,2-dichlorobenzene

Liver damage resulting from 4 h exposure to bromobenzene (BB) (146-957 ppm) and 1,2-dichlorobenzene (DCB) (245-739 ppm) as model toxicants was evaluated in rats. The modifications considered were the increases in serum glutamate dehydrogenase (GLDH) and sorbitol dehydrogenase (SDH) activities and the decreases in centrolobular liver-cell glucose-6-phosphatase (G6-Pase) staining intensity. A linear inverse relationship was established between the logarithmic values of blood enzyme activities and liver G6-Pase staining intensity. In addition, the levels of exposure to each test chemical were found to be linearly related to liver G6-Pase staining intensity and to the logarithmic values of blood enzyme activities.

Biological effects of acetamide, formamide, and their monomethyl and dimethyl derivatives

The industrial use of certain acetamides and formamides (particularly DMAC and DMF) for their solvent properties has resulted in rather extensive examination of their biological properties. Both DMAC and DMF are rapidly absorbed through biological membranes and are metabolized by demethylation first to monomethyl derivatives and then to the parent acetamide or formamide. Relatively high single doses to various species following oral, dermal, i.p., i.v., or inhalation exposures generally are required to produce mortality. The liver is the primary target following acute high level exposure, but massive doses can also produce damage to other organs and tissues. Repeated sublethal treatment by various routes also shows the liver to be the target organ with the degree of damage being proportional to the amount absorbed. With MMF, the potential usefulness as a cancer chemotherapeutic agent needs to be measured against the hepatotoxic effects produced in man. Acetamides and formamides are generally inactive in mutagenicity tests. Mammalian test systems do not appear to be genetically sensitive and DMF has been recommended for use as the vehicle in microbial assays designed to test for genetic activity of hard-to-dissolve chemicals. Embryotoxicity can be demonstrated at high doses; doses which generally show toxicity to the maternal animals. Structural abnormalities in sensitive species such as the rabbit are produced following exposure at near-lethal levels. The spectrum of abnormalities seen is broad and fails to show any time or site specificity in terms of developing organs/organ systems. Inhalation exposures to DMAC and DMF at levels producing some maternal toxicity in rats have produced no teratogenic response and only slight evidence of embryotoxicity. Long-term feeding of relatively high levels of acetamide produces liver cancer in rats. DMAC and DMF appear to be noncarcinogenic. The environmental toxicity of these chemicals is low. Liver damage can be produced by overexposure to these chemicals in man. Airborne concentrations need to be controlled and care should be taken to avoid excessive liquid contact as the chemicals are absorbed through the skin. A relationship exists between the amount of DMAC or DMF absorbed and the amount of MMAC or MMF excreted in the urine so that biomonitoring of the urinary metabolites can indicate situations in which total exposures, both dermal and inhalation, are excessive. An interaction between DMF and ethanol occurs such that signs, including severe facial flushing, appear when DMF-exposed individuals consume alcoholic beverages.