Changes in the classification of carcinogenic chemicals in the work area. Section III of the German List of MAK and BAT Values.

Carcinogenic chemicals in the work area are currently classified into three categories in section III of the German List of MAK and BAT Values (list of values on maximum workplace concentrations and biological tolerance for occupational exposures). This classification is based on qualitative criteria and reflects essentially the weight of evidence available for judging the carcinogenic potential of the chemicals. It is proposed that these categories - IIIA1, IIIA2, IIIB - be retained as Categories 1, 2, and 3, to correspond with European Union regulations. On the basis of our advancing knowledge of reaction mechanisms and the potency of carcinogens, these three categories are supplemented with two additional categories. The essential feature of substances classified in the new categories is that exposure to these chemicals does not contribute significantly to risk of cancer to man, provided that an appropriate exposure limit (MAK value) is observed. Chemicals known to act typically by nongenotoxic mechanisms and for which information is available that allows evaluation of the effects of low-dose exposures, are classified in Category 4. Genotoxic chemicals for which low carcinogenic potency can be expected on the basis of dose-response relationships and toxicokinetics, and for which risk at low doses can be assessed are classified in Category 5. The basis for a better differentiation of carcinogens is discussed, the new categories are defined, and possible criteria for classification are described. Examples for Category 4 (1,4-dioxane) and Category 5 (styrene) are presented.

Changes in the classification of carcinogenic chemicals in the work area. (Section III of the German List of MAK and BAT values).

Carcinogenic chemicals in the work area were previously classified into three categories in section III of the German List of MAK and BAT values (the list of values on maximum workplace concentrations and biological tolerance for occupational exposures). This classification was based on qualitative criteria and reflected essentially the weight of evidence available for judging the carcinogenic potential of the chemicals. In the new classification scheme the former sections IIIA1, IIIA2, and IIIB are retained as categories 1, 2, and 3, to correspond with European Union regulations. On the basis of our advancing knowledge of reaction mechanisms and the potency of carcinogens, these three categories are supplemented with two additional categories. The essential feature of substances classified in the new categories is that exposure to these chemicals does not contribute significantly to the risk of cancer to man, provided that an appropriate exposure limit (MAK value) is observed. Chemicals known to act typically by non-genotoxic mechanisms, and for which information is available that allows evaluation of the effects of low-dose exposures, are classified in category 4. Genotoxic chemicals for which low carcinogenic potency can be expected on the basis of dose/response relationships and toxicokinetics and for which risk at low doses can be assessed are classified in category 5. The basis for a better differentiation of carcinogens is discussed, the new categories are defined, and possible criteria for classification are described. Examples for category 4 (1,4-dioxane) and category 5 (styrene) are presented.

Proposed changes in the classification of carcinogenic chemicals in the work area.

Carcinogenic chemicals in the work area are currently classified into three categories in Section III of the German List of MAK and BAT Values. This classification is based on qualitative criteria and reflects essentially the weight of evidence available for judging the carcinogenic potential of the chemicals. It is proposed that these Categories--IIIA1, IIIA2, and IIIB--be retained as Categories 1, 2, and 3, to conform with EU regulations. On the basis of our advancing knowledge of reaction mechanisms and the potency of carcinogens, it is now proposed that these three categories be supplemented with two additional categories. The essential feature of substances classified in the new categories is that exposure to these chemicals does not convey a significant risk of cancer to man, provided that an appropriate exposure limit (MAK value) is observed. It is proposed that chemicals known to act typically by nongenotoxic mechanisms and for which information is available that allows evaluation of the effects of low-dose exposures be classified in Category 4. Genotoxic chemicals for which low carcinogenic potency can be expected on the basis of dose-response relationships and toxicokinetics and for which risk at low doses can be assessed will be classified in Category 5. The basis for a better differentiation of carcinogens is discussed, the new categories are defined, and possible criteria for classification are described. Examples for Category 4 (1,4-dioxane) and Category 5 (styrene) are presented. The proposed changes in classifying carcinogenic chemicals in the work area are presented for further discussion.

[Toxicology of the synthetic antioxidants BHA and BHT in comparison with the natural antioxidant vitamin E]. / Toxikologie der synthetischen Antioxidantien BHA und BHT im Vergleich mit dem natürlichen Antioxidans Vitamin E.

The toxicology of the food preservatives butylhydroxyanisole (BHA) and butylhydroxytoluene (BHT) as well as the naturally occurring vitamin E (alpha-tocopherol) is described. In high dosages all three compounds induce in animals impairment of blood clotting, which can be explained by an antagonism with vitamin K. Specific toxic effects to the lung have only been observed with BHT. The other described toxic effects of BHA and BHT are less characteristic and often occur only after high dosage and long-term treatment. However, BHA induces in animals tumours of the forestomach, which are dose dependent, whereas BHT induces liver tumours in long-term experiments. Because there is no indication of genotoxicity of BHA and BHT, all published findings agree with the fact that BHA and BHT are tumour promoters. In contrast to BHA and BHT, vitamin E is not carcinogenic. On the other hand, all three antioxidants have also anticarcinogenic properties. The intake of the necessary high doses as for these effects are, however, contraindicated with BHA and BHT because of their carcinogenic effects. The present overview concludes that the concentrations of BHA and BHT nowadays used in food, drugs and cosmetics are probably harmless. In addition, vitamin E can also be used in higher doses without the occurrence of adverse effects.

Tolerance and safety of vitamin E: a toxicological position report.

From numerous publications on the "prophylactic" and "therapeutic" use of vitamin E, it may be concluded that the toxicity of vitamin E is very low. It has been demonstrated in animal experiments that vitamin E has neither mutagenic, teratogenic nor carcinogenic properties. Based on studies in humans, a daily dosage of 100-300 mg vitamin E can be considered harmless from a toxicological point of view. Using double-blind studies involving a large number of subjects, it has been demonstrated that large oral doses of up to 3,200 USP-Units/day led to no consistent adverse effects. From a large body of published data, dosage ranges have been deduced which can be characterized as safe for human subjects even where their use extends over a long period of time. It should, however, be noted that oral intake of high levels of vitamin E can exacerbate the blood coagulation defect of vitamin K deficiency caused by malabsorption or anticoagulant therapy. High levels of vitamin E intake are, therefore, contraindicated in these subjects.

The role of reactive oxygen species in the antitumor activity of bleomycin.

Calf thymus DNA was incubated with bleomycin and FeCl3 in the presence of isolated rat liver microsomal NADH-cytochrome b5 reductase, cytochrome b5 and NADH which catalyze redox cycling of the bleomycin-Fe-complex. Furthermore, isolated rat liver nuclei were incubated with bleomycin, FeCl3 and NADH, a system in which redox cycling of bleomycin-Fe leads to DNA damage. In both systems free bases from DNA were released. Furthermore, 8-hydroxy-guanine was also found in the supernatant. On the other hand, 8-hydroxy-deoxyguanosine was detected in DNA of cell nuclei indicating that hydroxylation of the guanine molecule occurred in intact DNA. The release of bases correlated with the release of malondialydehyde as well as with NADH and oxygen consumption. These results indicate that NADH-cytochrome b5 reductase catalyzes redox cycling of the bleomycin-Fe-complex which results in the formation of reactive oxygen species which oxidize deoxyribose as well as bases of DNA. Both mechanisms may contribute to the cytotoxic and cytostatic effects of bleomycin observed in intact cells.

Production of reactive oxygen species due to metabolic activation of butylated hydroxyanisole.

The synthetic antioxidant butylated hydroxyanisole (BHA) stimulates superoxide formation in rat liver microsomes up to 10-fold. This stimulation is prevented by the monooxygenase inhibitor metyrapone and does not occur when NADH is consumed instead of NADPH indicating that metabolic activation is required for superoxide production. The BHA metabolite tert-butylhydroquinone (TBHQ) is much more active than BHA in stimulating superoxide production, and the amounts of TBHQ and formaldehyde formed from BHA in microsomes are sufficient to explain the effect of BHA. In buffer and in a xanthine oxidase system, superoxide production by TBHQ also takes place. TBHQ autoxidizes to tert-butylquinone (TBQ) and TBQ exceeds TBHQ by far in its capacity for superoxide production in microsomes. Thus, a 30-fold increase of basal superoxide production is induced by 5 microM TBQ. In rat forestomach, the target organ of BHA carcinogenicity in rodents, stimulation of superoxide production by BHA and more markedly by TBHQ and TBQ is also observed. Excess production of superoxide in microsomes by TBHQ is accompanied by excess production of hydrogen peroxide and of hydroxyl radicals. It is concluded that TBQ undergoes redox cycling leading to an oxidative burst in the presence of enzymes capable of one electron reduction of TBQ and that the BHA metabolite TBHQ enters the redox cycle by autoxidation. No oxygen activating properties can be ascribed to BHA itself.

Effect of four synthetic antioxidants on the formation of ethylene from methional in rat liver microsomes.

Four commonly used food antioxidants, butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), propyl gallate and octyl gallate, were tested for their ability to inhibit the formation of ethylene from methional in NADPH-oxidizing rat liver microsomes. It is assumed that the action of the antioxidants on ethylene formation reflects their free radical scavenging activity. Only propyl gallate and octyl gallate are efficient inhibitors of ethylene formation. BHT is inhibitory only at very high concentrations, and BHA tends to increase ethylene formation. It is concluded that gallic acid ester antioxidants may possess a protective potential during chemical-induced microsomal oxidations.

Liver nuclear NADPH-cytochrome P-450 reductase may be involved in redox cycling of bleomycin-Fe(III), oxy radical formation and DNA damage.

When NADPH-cytochrome P-450 reductase isolated from rat liver microsomes was aerobically incubated with bleomycin, FeCl3, NADPH and DNA parallel NADPH and oxygen were consumed and malondialdehyde was formed. A similar parallelism of NADPH- and oxygen-consumption and malondialdehyde formation was observed when cell nuclei isolated from rat liver were incubated under the same conditions. The formation of malondialdehyde which was identified by HPLC and which was most likely released from oxidative cleavage of deoxyribose of nuclear DNA required oxygen, bleomycin, FeCl3 and NADPH. This indicates that a nuclear NADPH-enzyme, presumably NADPH-cytochrome P-450 reductase, is able to redox cycle a bleomycin-iron-complex which in the reduced form can activate oxygen to a DNA-damaging reactive species. The data suggest that the activity of this enzyme in the cell nucleus could play an important role in the cytotoxicity of bleomycin in tumor cells.

Photodynamic toxicity of hematoporphyrin derivatives to human keratinocytes in culture.

Human keratinocytes in culture were able to take up hematoporphyrin derivatives (HPDs) used during photodynamic chemotherapy of tumors. In the absence of light, HPDs showed no cytotoxic effects to keratinocytes. However, after irradiation with visible light, HPDs induced immediate cytotoxicity as measured by the neutral red uptake assay. On the other hand, cell attachment as measured by protein estimation was not affected. When the cells treated with HPDs and irradiated with light were cultured for a further 72 h, they partially lost their ability to attach to the collagen surface. Most of the cells remaining attached after 72 h were no longer viable following treatment with HPDs and light. All parameters measured depended on the intracellular concentration of HPDs used (7-50 ng/10(5) cells) and the time of irradiation (0-30 min). These results suggest that human keratinocytes are a good model to study cytotoxic effects of photodynamically active drugs. Further, keratinocytes were unable to recover after damage caused by HPDs and light.

Oxidative stress in chemical toxicity.

The toxic effects of compounds which undergo redox cycling via enzymatic one-electron reduction are reviewed. First of all, the enzymatic reduction of these compounds leads to reactive intermediates, mainly radicals which react with oxygen, whereby superoxide anion radicals are formed. Further oxygen metabolites are hydrogen peroxide, singlet oxygen and hydroxyl radicals. The role of these oxygen metabolites in toxicity is discussed. The occurrence of lipid peroxidation during redox cycling of quinonoide compounds, e.g., adriamycin, and the possible relationship to their toxicity is critically evaluated. It is shown that iron ions play a crucial role in lipid peroxidation induced by redox cycling compounds. DNA damage by metal chelates, e.g., bleomycin, is discussed on the basis of findings that enzymatic redox cycling of a bleomycin-iron complex has been observed. The involvement of hydroxyl radicals in bleomycin-induced DNA damage occurring during redox cycling in cell nuclei is claimed. Redox cycling of other substances, e.g., aromatic amines, is discussed in relation to carcinogenesis. Other chemical groups, e.g., nitroaromatic compounds, hydroxylamines and azo compounds are included. Other targets for oxygen radical attack, e.g., proteins, are also dealt with. It is concluded that oxygen radical formation by redox cycling may be a critical event in toxic effects of several compounds if the protective mechanisms of cells are overwhelmed.

No evidence for lysophospholipid formation during peroxidation of phospholipids by NADPH-cytochrome P-450 reductase and iron ions.

Liposomes comprised of liver microsomal phospholipids and radioactive phosphatidylcholine or phosphatidylethanolamine as tracers were incubated with isolated liver microsomal NADPH-cytochrome P-450 reductase, NADPH and ADP-EDTA-chelated iron ions, a system which stimulates peroxidation of unsaturated fatty acids of phospholipids. Phospholipids and their reaction products were extracted and chromatographed on HPLC. Phosphatidylcholine and phosphatidylethanolamine considerably decreased after 30 min incubation, depending on the enzyme and NADPH as measured by UV absorbance and radioactivity. However, neither a lysophospholipid peak nor a lysophospholipid-like peak were detectable. We suggest that lysophospholipid formation during microsomal lipid peroxidation is exclusively due to phospholipase A2 and not due to peroxidative breakdown of the unsaturated fatty acid in the beta-position of glycerol.

Positive correlation between decreased cellular uptake, NADPH-glutathione reductase activity and adriamycin resistance in Ehrlich ascites tumor lines.

From a wild type strain of Ehrlich ascites tumor (EATWT) sublines resistant to daunorubicin (EATDNM), etoposide (EATETO), and cisplatinum (EATCIS) have been developed in vivo. Increase in survival and cure rate caused by adriamycin (doxorubicin) have been determined in female NMRI mice which were inoculated i.p. with EAT cells. Adriamycin concentrations causing 50% inhibition of 3H-thymidine (ICT) and 3H-uridine incorporation (ICU) and intracellular adriamycin steady-state concentrations (SSC) were measured in vitro. Adriamycin resistance increased and SSC decreased in the following sequence: EATWT - EATCIS - EATDNM - EATETO. When ICT and ICU were corrected for intracellular adriamycin concentrations in consideration of the different SSC (ICTc, ICUc), ICTc and ICUc still varied up to the 3.2 fold in EATCIS, EATDNM and EATETO in comparison to EATWT. Thus, in addition to different SSC other factors must be responsible for adriamycin resistance. Therefore, enzymes which may play a role in the cytotoxicity related to adriamycin metabolism (NADPH-cytochrome P-450 reductase, NADPH-glutathione reductase, NADP-glucose-6-phosphate dehydrogenase, NADP-isocitrate dehydrogenase) were measured. In contrast to the other parameters determined, NADPH-glutathione reductase was significantly (p less than 0.01) increased up to the 3.2 fold parallel to adriamycin resistance as determined by increase in life span, cure rate, ICTc, and ICUc, respectively. It is concluded that high activities of NADPH-glutathione reductase may contribute to an increase in adriamycin resistance of malignant tumors.

Oxy radical formation during redox cycling of the bleomycin-iron (III) complex by NADPH-cytochrome P-450 reductase.

Bleomycin was aerobically incubated with FeCl3, NADPH, isolated rat-liver microsomal cytochrome P-450 reductase and methional. The conversion of methional to ethene, which indicates oxy radicals, was determined. Ethene formation depended on oxygen, NADPH, FeCl3 and the enzyme. About equimolar concentrations of bleomycin and FeCl3 resulted in optimal ethene formation. Dimethyl sulfoxide, mannitol, glycerol, glutathione and glutathione disulfide inhibited ethene formation. These results indicate that oxy radicals are formed after reduction of the bleomycin-Fe-complex by NADPH-cytochrome P-450 reductase.

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 well as in hepatocytes incubated separately and deleted the increased protein binding in sinusoidal cells co-incubated with hepatocytes. The data indicate that glutathione present in the incubation medium traps reactive vinyl chloride metabolites formed by hepatocytes which otherwise would react with cell constituents of sinusoidal cells. If similar conditions exist in vivo, the alkylation of DNA of liver endothelial cells by vinyl chloride metabolites formed in hepatocytes is possible. This would explain the induction of hemangioendotheliomas of the liver by vinyl chloride.

Lipid peroxidation in isolated rat hepatocytes measured by ethane and n-pentane formation.

Isolated rat hepatocytes (1 X 10(7) cells/ml) were aerobically incubated in Eagle's Minimum Essential Medium which contained 2.0% albumin. As potential parameters of lipid peroxidation ethane and n-pentane formed were measured in samples obtained from the gas phase above the incubation mixture. 15-30 nmol ethane or n-pentane were produced by 10(7) hepatocytes within 90 min. Carbon tetrachloride (CCl4) or ADP-complexed ferrous ions stimulated ethane and n-pentane formation considerably, depending on the concentrations of the compounds. With CCl4 10(7) cells formed max 180 nmol ethane and 140 nmol n-pentane within 90 min incubation, whereas with Fe(II) max 130 nmol ethane and 220 nmol n-pentane could be detected. When n-pentane was added to the gas phase above the incubation mixture containing either medium or medium plus hepatocytes its amount decreased by 30% within the first 5 min of incubation. However, afterwards only minor amounts of n-pentane disappeared, even in the presence of hepatocytes. This indicates that n-pentane equilibrates with the cells suspension under the conditions used. Cell viability, as determined by the release of lactate dehydrogenase into the medium and by the uptake of trypan blue by the cells, and the recovery of the cells decreased only in presence of relatively high concentrations of CCl4, or Fe(II) respectively. However, a maximal effect on ethane and n-pentane formation was reached already with lower concentration.