Modification of lung tumor development in A/J mice

PET/CT imaging of c-Myc transgenic mice identifies the genotoxic N-nitroso-diethylamine as carcinogen in a short-term cancer bioassay

BACKGROUND

More than 100,000 chemicals are in use but have not been tested for their safety. To overcome limitations in the cancer bioassay several alternative testing strategies are explored. The inability to monitor non-invasively onset and progression of disease limits, however, the value of current testing strategies. Here, we report the application of in vivo imaging to a c-Myc transgenic mouse model of liver cancer for the development of a short-term cancer bioassay.

METHODOLOGY/PRINCIPAL FINDINGS

μCT and ¹⁸F-FDG μPET were used to detect and quantify tumor lesions after treatment with the genotoxic carcinogen NDEA, the tumor promoting agent BHT or the hepatotoxin paracetamol. Tumor growth was investigated between the ages of 4 to 8.5 months and contrast-enhanced μCT imaging detected liver lesions as well as metastatic spread with high sensitivity and accuracy as confirmed by histopathology. Significant differences in the onset of tumor growth, tumor load and glucose metabolism were observed when the NDEA treatment group was compared with any of the other treatment groups. NDEA treatment of c-Myc transgenic mice significantly accelerated tumor growth and caused metastatic spread of HCC in to lung but this treatment also induced primary lung cancer growth. In contrast, BHT and paracetamol did not promote hepatocarcinogenesis.

CONCLUSIONS/SIGNIFICANCE

The present study evidences the accuracy of in vivo imaging in defining tumor growth, tumor load, lesion number and metastatic spread. Consequently, the application of in vivo imaging techniques to transgenic animal models may possibly enable short-term cancer bioassays to significantly improve hazard identification and follow-up examinations of different organs by non-invasive methods.

Final report on the safety assessment of Tocopherol, Tocopheryl Acetate, Tocopheryl Linoleate, Tocopheryl Linoleate/Oleate, Tocopheryl Nicotinate, Tocopheryl Succinate, Dioleyl Tocopheryl Methylsilanol, Potassium Ascorbyl Tocopheryl Phosphate, and Tocophersolan

Tocopherol and its several ester and ether derivatives all function as antioxidants in cosmetic formulations; they also have other functions, such as skin conditioning. Tocopheryl Acetate, Tocopherol, and Tocopheryl Linoleate are used in 2673 formulations, generally at concentrations of up to 36%, 5%, and 2%, respectively, although Tocopheryl Acetate is 100% of vitamin E oil. Tocophersolan, Tocopheryl Linoleate/Oleate, Tocopheryl Nicotinate, Tocopheryl Succinate, Dioleyl Tocopheryl Methylsilanol, and Potassium Ascorbyl Tocopheryl Phosphate, combined, are used in 36 formulations at concentrations lower than those reported for the frequently used ingredients. Tocopherol may be isolated from vegetable oils or synthesized using isophytol and methylhydroquinone. Tocopherol, Tocopheryl Acetate, Tocopheryl Linoleate, and Tocopheryl Succinate all were absorbed in human skin. In rat skin, Tocopheryl Acetate is hydrolyzed to Tocopherol. Tocopherol is a natural component of cell membranes thought to protect against oxidative damage. Tocopherol, Tocopheryl Acetate, and Tocopheryl Succinate each were reported to protect against ultraviolet radiation-induced skin damage. These ingredients are generally not toxic in animal feeding studies, although very high doses (>2 g/kg/day) have hemorrhagic activity. These ingredients are generally not irritating or sensitizing to skin or irritating to eyes, although a Tocopheryl Acetate did produce sensitization in one animal test, and Tocophersolan was a slight eye irritant in an animal test. Reproductive and developmental toxicity tests in animals using Tocopherol, Tocopheryl Acetate, Tocopheryl Succinate, and Tocophersolan were all negative or showed some effect of reducing toxicity. Tocopherol, Tocopheryl Acetate, Tocopheryl Succinate, and Dioleyl Tocopheryl Methylsilanol were almost uniformly negative. These ingredients exhibit antimutagenic activity consistent with their antioxidant properties. Tocopherol was not carcinogenic. The ability of Tocopherol, Tocopheryl Acetate, and Tocopheryl Succinate to modulate the carcinogenic effect of other agents (e.g., tumor promotion) has been extensively studied. One study showing tumor promotion in mice may be discounted as not reproducible and not consistent with the large volume of data suggesting that the antioxidant properties of these agents protect against tumor induction. Specifically, the frequent use of Tocopherol as a negative control in other tumor promotion studies suggests that Tocopherol is not a tumor promoter. Tocopherol has been shown to reduce the photocarcinogenic effect of ultraviolet radiation in mice. Similar studies with Tocopheryl Acetate and Tocopheryl Succinate, however, demonstrated some enhancement of photocarcinogenesis, although the effect was not dose related. In clinical studies, Tocopherol, Tocopheryl Acetate, and Tocopheryl Nicotinate were not irritants or sensitizers. A report of a large number of positive patch-tests to Tocopheryl Linoleate in one cosmetic product were considered to result from a contaminant or metabolite. The Cosmetic Ingredient Review Expert Panel considered that these data provide an adequate basis on which to conclude that Tocopherol, Tocophersolan, Tocopheryl Acetate, Tocopheryl Linoleate, Tocopheryl Linoleate/Oleate, Tocopheryl Nicotinate, Tocopheryl Succinate, Dioleyl Tocopheryl Methylsilanol, and Potassium Ascorbyl Tocopheryl Phosphate are safe as used in cosmetic formulations. Although there were no inhalation toxicity data, these ingredients are used at such low concentrations in hair sprays that no inhalation toxicity risk was considered likely. Because methylhydroquinone is used in the chemical synthesis of Tocopherol, there was concern that hydroquinone may be present as an impurity. In such cases, residual levels of hydroquinone would be expected to be limited to those achieved by good manufacturing practices.

Final report on the safety assessment of BHT(1)

BHT is the recognized name in the cosmetics industry for butylated hydroxytoluene. BHT is used in a wide range of cosmetic formulations as an antioxidant at concentrations from 0.0002% to 0.5%. BHT does penetrate the skin, but the relatively low amount absorbed remains primarily in the skin. Oral studies demonstrate that BHT is metabolized. The major metabolites appear as the carboxylic acid of BHT and its glucuronide in urine. At acute doses of 0.5 to 1.0 g/kg, some renal and hepatic damage was seen in male rats. Short-term repeated exposure to comparable doses produced hepatic toxic effects in male and female rats. Subchronic feeding and intraperitoneal studies in rats with BHT at lower doses produced increased liver weight, and decreased activity of several hepatic enzymes. In addition to liver and kidney effects, BHT applied to the skin was associated with toxic effects in lung tissue. BHT was not a reproductive or developmental toxin in animals. BHT has been found to enhance and to inhibit the humoral immune response in animals. BHT itself was not generally considered genotoxic, although it did modify the genotoxicity of other agents. BHT has been associated with hepatocellular and pulmonary adenomas in animals, but was not considered carcinogenic and actually was associated with a decreased incidence of neoplasms. BHT has been shown to have tumor promotion effects, to be anticarcinogenic, and to have no effect on other carcinogenic agents, depending on the target organ, exposure parameters, the carcinogen, and the animal tested. Various mechanism studies suggested that BHT toxicity is related to an electrophillic metabolite. In a predictive clinical test, 100% BHT was a mild irritant and a moderate sensitizer. In provocative skin tests, BHT (in the 1% to 2% concentration range) produced positive reactions in a small number of patients. Clinical testing did not find any depigmentation associated with dermal exposure to BHT, although a few case reports of depigmentation were found. The Cosmetic Ingredient Review Expert Panel recognized that oral exposure to BHT was associated with toxic effects in some studies and was negative in others. BHT applied to the skin, however, appears to remain in the skin or pass through only slowly and does not produce systemic exposures to BHT or its metabolites seen with oral exposures. Although there were only limited studies that evaluated the effect of BHT on the skin, the available studies, along with the case literature, demonstrate no significant irritation, sensitization, or photosensitization. Recognizing the low concentration at which this ingredient is currently used in cosmetic formulations, it was concluded that BHT is safe as used in cosmetic formulations.

Co-mutagenicity of glyco- and tauro-deoxycholic acids in the Ames test

Mutagenicity and co-mutagenicity of glyco- and tauro-deoxycholic acids (GDCA and TDCA), which are abundant in human bile, were examined by the Ames test. The two chemicals were not mutagenic for themselves to Salmonella typhimurium TA98 and TA100, with and without S9 mix. They enhanced, however, the mutagenic activities of the pro-mutagens, 2-aminoanthracene (2AA) and benzo[a]pyrene (BaP), for both TA98 and TA100 with S9 mix. They were more co-mutagenic for the pro-mutagens on TA98 than on TA100. On TA98, the mutagenic activities of 2AA with GDCA (5 mumol/plate) and with TDCA (5 mumol/plate) were 9.7-fold and 11.8-fold as high as that of the corresponding control (2AA only), respectively. BaP with GDCA (2.5 mumol/plate) and with TDCA (2.5 mumol/plate) showed 2.8-fold and 3.0-fold increases over the corresponding control level (BaP only), respectively. It is hence concluded that GDCA and TDCA may enhance the activity of some mutagens existing in bile.

Establishment of an experimental intrapulmonary tumor nodule model

BACKGROUND

A pulmonary tumor model is necessary to study the biology and therapy of lung cancer. Methods to establish a solitary intrapulmonary nodule are not well defined. Two methods for solitary intrapulmonary tumor nodule development in the Fischer rat are described.

METHODS

Methylcholanthrene-induced sarcoma cell suspensions were introduced into lung parenchyma of Fischer rats via limited thoracotomy and lung puncture, or instilled into a distal airway after tracheal puncture and catheterization. Intrapulmonary tumor location, implantation mortality, procedure length, and animal survival were recorded.

RESULTS

Single pulmonary nodules developed at the implanted position in 100% (n = 320) and 95% (62/65) of animals after direct injection into the pulmonary parenchyma or via tracheal puncture and instillation. Operative mortality was 2% and 5% via lung or tracheal implantation, respectively. Less than 5 minutes was required for each implantation. Mean survival time was 24 +/- 2 and 26 +/- 6 days after lung or tracheal implantation in animals allowed to survive until tumor-induced death.

CONCLUSIONS

These easily performed, reproducible methods of establishing solitary intrapulmonary tumors are useful tools for lung cancer research.

In vitro system for detecting non-genotoxic carcinogens

Chemical risk assessment has been limited by the inability of in vitro short-term assays to identify the true carcinogenic potential of many substances. Numerous methods exist for identifying mutagenic and clastogenic agents, but a practical means of identifying non-genotoxic carcinogens has remained elusive. Experiments described here suggest that some chemicals may participate in carcinogenesis by modulating the enzymatic processes of drug metabolism. The tumor promoters butylated hydroxyanisole, butylated hydroxytoluene, deoxycholic acid, reserpine, trypan blue, and 12,-O-tetradecanoyl phorbol-13-acetate were chosen as model non-genotoxic carcinogens. The enzyme-modulating action of these chemicals was measured using a modified Ames plate incorporation assay whereby the known tumor promoters were plated with a promutagen in the presence of a mammalian metabolic activation system (S9). Each of the non-genotoxic carcinogens significantly increased the mutagenic response of metabolically activated promutagen(s). These experiments suggest that the carcinogenic role of some chemicals may be attributed to their ability to modify the biochemical pathways of drug metabolism. By enhancing or inhibiting the activity of various enzymes, some tumor promoters may create an environment that increases a cell's mutational burden, thereby contributing to neoplastic transformation.

Combined exposure to NO2, O3 and H2SO4-aerosol and lung tumor formation in rats

The promoting effects of a combined exposure to two pollutants (NO2, O3 or H2SO4-aerosol) at near ambient levels on lung tumorigenesis induced by N-bis(2-hydroxypropyl) nitrosamine (BHPN) were investigated in male Wistar rats. The rats were given a single intraperitoneal injection of BHPN (0.5 g per kg body wt.) at 6 weeks of age. They then were exposed to clean air, 0.05 ppm O3 (mean concentration for 10 h/day; 0.1 ppm peak concentration), 0.05 ppm O3 (mean concentration for 10 h/day; 0.1 ppm peak concentration) + 0.4 ppm NO2 or 0.4 ppm NO2 + 1 mg/m3 of H2SO4-aerosol for 13 months and were then maintained in a clean room for another 11 months. Room control animals were kept after injection of BHPN in a clean room for 24 months. The incidence of primary lung tumors in rats exposed to 0.05 ppm O3, 0.05 ppm O3 + 0.4 ppm NO2 and 0.4 ppm NO2 + 1 mg/m3 of H2SO4-aerosol with BHPN treatment was 8.3% (3 out of 36 rats), 13.9% (5 out of 36 rats) and 8.3% (3 out of 36 rats), respectively. The tumors were adenomas and adenocarcinomas. The incidence of adenomas was 2.8% (1 out of 36 rats) in the O3 alone group, 11% (4 out of 36 rats) in O3 + NO2 group and 5.6% (2 out of 36 rats) in NO2 + H2SO4 group. The incidence of adenocarcinomas was 5.6% (2 out of 36 rats) in the O3 group, 2.8% (1 out of 36 rats) in O3 + NO2 group and 2.8% (1 out of 36 rats) in NO2 + H2SO4 group. No lung tumors were found in the rats exposed to clean air with BHPN treatment and in animals not given BHPN but exposed to each air pollutant. The difference in tumor incidence between the clean air group with BHPN and the O3 + NO2 group with BHPN was statistically significant. The results show that exposure to O3 alone enhances tumor development and that the combined exposure to O3 or H2SO4 with NO2 produces an additional increase in incidence of lung tumor, respectively. The incidence of slight-moderate to marked alveolar cell hyperplasia in the groups exposed to each air pollutant with BHPN treatment was higher than that in the groups exposed to clean air with BHPN. Exposure to each air pollutant had no effect on the development of bronchiolar mucosal hyperplasia in lungs of rats treated with BHPN.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of butylated hydroxytoluene, with and without cortisol, on stimulated lymphocytes

In the present work we undertook to ascertain whether butylated hydroxytoluene (BHT), which is used in food as an antioxidant, is capable of either inhibiting human lymphocyte stimulation or acting synergistically with cortisol and prednisolone to the same end. BHT cytotoxicity was observed at concentrations higher than 100 micrograms/ml. In the concentration range of 0.0 to 60.0 micrograms/mL, BHT showed no effect on the uptake of 3H-thymidine by PHA stimulated lymphocytes. However, at 50 micrograms/mL BHT suppressed mixed lymphocyte reaction (MLR). A synergistic effect with regard to suppression of PHA stimulated lymphocytes was observed when the cells were incubated with BHT in the presence of either cortisol or prednisolone.

Experimental studies on tumor promotion by nitrogen dioxide

The effects of nitrogen dioxide (NO2) on promotion of lung tumorigenesis induced by N-bis(2-hydroxypropyl) nitrosamine (BHPN) were investigated in male Wistar rats. In a preliminary study, the highest non-effective dose of BHPN was found to be 0.5 g per kg body weight. Rats were given a single intraperitoneal injection of BHPN at a dose of 0.5 g per kg body weight or saline at 6 weeks of age, and then exposed to clean air, 0.04 ppm, 0.4 ppm or 4 ppm of NO2 for 17 months, respectively. The incidence of pulmonary tumors in rats exposed to BHPN plus 4 ppm of NO2 was 12.5%; the tumors were adenomas and adenocarcinomas. Adenomas were found in 4 out of 40 rats (10%) and adenocarcinomas were found in 1 out of 40 rats (2.5%). The tumor incidence in the lungs of rats kept in BHPN plus clean air and BHPN plus 0.04 ppm of NO2 was 2.5% (1/40). In both groups adenomas were found. There was no significant difference in tumor incidence between animals exposed to BHPN plus clean air and to BHPN plus 4 ppm of NO2. No lung tumors were found in the group of BHPN plus 0.4 ppm NO2 and in animals exposed to NO2 without BHPN treatment. A high incidence of alveolar cell hyperplasia was observed in the lungs of rats injected with BHPN, and the effect of NO2 on development of alveolar cell hyperplasia was slight. On the other hand, marked bronchiolar mucosal hyperplasia was found in 17 out of 40 rats (42.5%) in the group of BHPN plus 4 ppm of NO2, and in 1 out of 40 rats (2.5%) in each of the group exposed to clean air, 0.04 ppm or 0.4 ppm of NO2 with BHPN treatment, respectively. The hyperplasia in lungs of rats exposed to 4 ppm of NO2 without BHPN treatment was slighter than that in lung of rat exposed to 4 ppm of NO2 with BHPN treatment. On the other hand, tumor incidence in the nasal cavity of rats in each of group exposed to clean air and NO2 with BHPN treatment was 97-100%. Incidence of tumors in other organs in the groups exposed to clean air and NO2 with and without BHPN treatment was very low, and NO2 had no effect on tumor development in the nasal cavity and other organs whether animals were treated with BHPN or not.(ABSTRACT TRUNCATED AT 400 WORDS)

Lung tumors in strain A mice as a bioassay for carcinogenicity of environmental chemicals

This report describes the protocol for the strain A mouse lung tumor bioassay and summarizes results on selected chemicals that have been tested for carcinogenicity in the assay. The assay is of 6 months duration and can distinguish 2-fold differences in carcinogenic potential of compounds from several chemical classes. Specifically, the assay is sensitive to polycyclic hydrocarbons, nitrosamines and nitrosoureas, carbamates, aflatoxin, certain metals, hydrazines, and others, but is relatively insensitive to aromatic amines, aliphatic halides, and other compounds that are carcinogenic in the rodent liver and/or bladder. Recommendations are made for future studies on the: (1) distribution and metabolism of chemicals in strain A mouse lung tissue and in specific lung cell types; (2) ability of the lung tumor bioassay to detect inhibitors and promoters of carcinogenesis; and (3) use of the assay for testing mixtures of chemicals for carcinogenic activity.

Promotion of mouse lung tumors by bioaccumulated polychlorinated aromatic hydrocarbons

Lung tumors initiated in infant Swiss mice by N-nitrosodimethylamine (NDMA) were promoted by a single dose of a mixture of polychlorinated biphenyls (PCBs), Aroclor 1254 (250 or 500 mg/kg) given 4 days later. The tumors were typical alveologenic adenomas, and their number increased gradually over the course of 1 year to a maximum 4-fold enhancement in average tumor number compared with those given NDMA alone. The time course pattern suggested continuous tumor stimulation by the nonmetabolized PCB congeners retained in the tissues. Content of the nine major bioretained PCB congeners in carcass, liver, and lung was determined at intervals after treatment. Several showed tissue-specific retention patterns: 2,3,3',4,4'-PCB was selectively retained in lung and liver, and 2,2',3,4,4', 5'-HCB in lung. In tests of the tumor-promoting ability of individual congeners, the 2,2',3,4,4',5'-HCB, an Ah receptor agonist, promoted lung tumors when given singly, whereas another prominent bioretained congener, the 2,2',4,4',5,5'-HCB, an Ah receptor antagonist, did not promote, and in fact abrogated the positive effect of the 2,2',3,4,4',5'-HCB. In a parallel examination of persistent biochemical effects, a single low dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (5 nmole/kg) resulted in significant elevation of immunochemically detected protein and enzymatic activity of cytochrome P450 IA1 in lung for at least 12 weeks; these parameters were elevated for at least 30 weeks after a single dose of Aroclor 1254 (500 mg/kg). Taken together these results suggest that Ah-receptor-dependent induction of cytochrome P450 IA1 in mouse lung is correlated with and possibly causally involved in promotion of tumors by retained congeners.

Tumorigenicity of nickel subsulfide in Strain A/J mice

The pulmonary tumor response of Strain A mice has been reported to be a rapid and efficient predictor of carcinogenic potential for a variety of chemicals. The route of exposure has usually been by intraperitoneal injection (i.p.) of solubilized materials. We compared intratracheal (i.t.) instillation as a more representative route typical of human exposures, with i.p. injection of nickel subsulfide, a potent animal carcinogen. Animals were sacrificed either 20 weeks after the first dosing, or were held until 45 weeks after the first dosing. Urethane, a positive control, produced a significant increase in pulmonary tumor response after i.t. instillation as well as i.p. injection. For nickel subsulfide treated animals, there was no evidence of a dose-related increase in pulmonary tumor response in any i.p. or i.t. treatment group when compared with age-matched controls.

An assessment of the safety of tocopherols as food additives

This paper is a critical review of data relevant to the safety of tocopherols as food additives. Tocopherols are considered from the standpoint of their chemical-physical properties, occurrence in nature and use in commercial products. Dietary intake and kinetic and metabolic data, as well as biological activity and interactions with other vitamins, are also examined. The subjects discussed include acute, subchronic and chronic toxicity data, reproduction and teratogenesis studies, and observations in humans following high intakes of tocopherols. On the basis of the comprehensive experimental and clinical data available on alpha-tocopherol, the chemical and biological similarity of the alpha-, beta-, gamma- and delta-tocopherols and the information available on the levels of tocopherols used as food antioxidants, it is concluded that tocopherols are safe food additives.

Oncogenic response of strain A/J mice to inhaled chemicals

Strain A/J mice were exposed by inhalation for 6 h/d, 5 d/wk, for 6 mo to carbon disulfide, 1,2-dibromoethane, ethylene oxide, naphthalene, nitrogen dioxide, or vinyl chloride. Significant increases in pulmonary adenoma formation were observed following exposure to 300 ppm carbon disulfide; 20 and 50 ppm 1,2-dibromoethane; 70 and 200 ppm ethylene oxide; 10 ppm nitrogen dioxide; and 50, 200, and 500 ppm vinyl chloride compared to control animals. Repeated studies with 1,2-dibromoethane, ethylene oxide, and vinyl chloride gave similarly significant results. Exposure of mice to 30 ppm naphthalene did not elicit a significant adenoma response. Histopathological examination of lungs from animals in these studies revealed multiple alveolar adenomas. Results from earlier studies with these chemicals, using strain A mice and Swiss mice, and bioassay information with rats and mice were compared with these data. These results provide further information for the validation of this in vivo model as a tool for predicting oncogenic potential following chemical exposure.

Synthetic antioxidants: biochemical actions and interference with radiation, toxic compounds, chemical mutagens and chemical carcinogens

Biological actions of 4 commonly used synthetic antioxidants--butylated hydroxyanisole, butylated hydroxytoluene, ethoxyquin and propyl gallate--on the molecular, cellular and organ level are complied. Such actions may be divided into modulation of growth, macromolecule synthesis and differentiation, modulation of immune response, interference with oxygen activation and miscellaneous. Moreover, an overview of beneficial and adverse interactions of these antioxidants with exogenous noxae is given. Beneficial interactions include radioprotection, protection against acute toxicity of chemicals, antimutagenic activity and antitumorigenic action. Possible mechanisms of the antitumorigenic action of antioxidants are discussed. This discussion is centered around antioxidant properties which may contribute to a modulation of initiation-related events, especially their ability to interfere with carcinogen metabolism. The beneficial interactions of antioxidants with physical and chemical noxae are contrasted to those leading to unfavorable effects. These include radiosensitization, increased toxicity of other chemicals, increased mutagen activity and increased tumor yield from chemical carcinogens. At present, the latter one can most adequately be characterized as tumor promotion at least in the case of butylated hydroxytoluene. It is concluded that current information is insufficient to promote expectations as to the use of antioxidants in the prevention of human cancer.

The role of toxicological interactions in chemical carcinogenesis

Toxicological interactions may occur not only when exposure to two or more chemicals is simultaneous, but also when exposures are separated in time. Often the sequence of exposure determines the nature of the toxicological response. This is illustrated with the two hindered phenolic antioxidants, butylated hydroxytoluene (BHT) and butylated hydroxyanisole (BHA). Evidence available suggests that in certain tissues exposure to BHT first and then to a carcinogen is without significant consequences, whereas exposure to BHT after a carcinogen may enhance tumor formation. On the other hand, exposure to BHA before carcinogen administration often has a protective effect, whereas exposure to BHA after a carcinogen sometimes protects and sometimes is without any influence. Mechanisms underlying these interactions may be induction of mixed function oxidases, production of cell hyperplasia or other, as yet ill-defined, events such as modification of biological reactivity.

Review: putative mutagens and carcinogens in foods. III. Butylated hydroxytoluene (BHT)

Although the average American's daily consumption of BHT can be measured in milligrams, there are numerous reports that BHT causes organ damage in laboratory animals. Only a few genotoxic effects of BHT have been reported, however, including mutagenicity in the abnormal sperm assay and ambiguous results regarding its teratogenicity. More dramatic are the modulatory effects of BHT on the actions of established mutagens and carcinogens. BHT can either enhance or inhibit mutagenic potency, depending on the substance tested. For example, in the Ames test, BHT is antimutagenic towards benzo(a)pyrene, but increases the number of Salmonella revertants induced by aflatoxin B1. BHT is one of the few compounds to have both tumor prophylactic and tumor promoting capacities. It is the temporal sequence in which BHT and carcinogens are administered to test animals which determines how BHT affects the response to these carcinogens. In common with other antioxidants, BHT inhibits the ability of carcinogens to induce tumors in various rodent organs when the animal is given BHT prior to carcinogen treatment. Unlike other antioxidants, however, the number of tumors increase when BHT is administered after carcinogen exposure. The comutagenic and cocarcinogenic properties of BHT have been demonstrated in tests ranging from the Ames test to cell transformation procedures to in vivo assays. These effects are probably mediated by metabolites of BHT, rather than by BHT itself.