Ethylene dibromide and disulfiram: studies in vivo and in vitro on the mechanism of the observed synergistic carcinogenic response

Quantitative predictivity of the transformation in vitro assay compared with the Ames test

For 59 chemical compounds, we have found homogeneous data on transformation in vitro, mutagenicity in the Ames test, and carcinogenicity. We have compared the potency in inducing transformation in vitro in hamster fibroblast cells with the carcinogenic potency and found a modest correlation coefficient between the two parameters (r = 0.37). For these same 59 compounds it was also possible to compare mutagenic potency in the Ames test with carcinogenic potency. The correlation level was very similar (r = 0.34). The predictivity of transformation in vitro increased significantly when only compounds for which some kind of dose-response relationship was available were utilized (r = 0.65). This result stresses the importance of the quantitative aspect of the response in predictivity studies. The present study is compared with previous studies on the quantitative predictivity of different short-term tests. Our work is not definitive, but gives an idea of the possible type of approach to the problem of comparing quantitative predictivities.

Deuterium isotope effect on the metabolism and toxicity of 1,2-dibromoethane

The metabolism, hepatotoxicity, and hepatic DNA damage of 1,2-dibromoethane (EDB) and tetradeutero-1,2-dibromoethane (d4EDB) were compared in male Swiss-Webster mice. In vitro studies that measured bromide ion released from the substrate to monitor the rate of metabolism showed that the hepatic microsomal metabolism of EDB was significantly reduced by deuterium substitution, while metabolism by the hepatic glutathione S-transferases was unaffected. Three hours after ip administration of EDB or d4EDB (50 mg/kg), there was 42% less bromide in the plasmaa of d4EDB-treated mice than in the plasm of EDB-treated mice. This difference demonstrates a significant deuterium isotope effect on the metabolism of EDB in vivo. Although the metabolism of d4EDB was less than that of EDB 3 hr after exposure, the DNA damage caused by both analogs was not significantly different at this time point. At later time points (8, 24, and 72 hr), d4EDB caused significantly greater DNA damage than EDB. Since the decreased metabolism of d4EDB was apparently due to a reduced rate of microsomal oxidation, these data support the hypothesis that conjugation with GSH is responsible for the genotoxic effects of EDB.

The unique role of rodents in the detection of possible human carcinogens and mutagens

Some of the probable reasons underlying the observation that not all chemicals shown to be genotoxic in vitro are capable of eliciting tumours in rodents or humans are discussed using appropriate examples. It is suggested that a substantial proportion of the resources currently available for conducting rodent carcinogenicity bioassays should be employed in the short-term evaluation in vivo of some of the many hundreds of chemicals recently defined as genotoxic in vitro, rather than in the protracted evaluation of a few chemicals, often of unknown activity in vitro, for carcinogenicity. A decision tree approach to the evaluation of chemicals for human mutagenic/carcinogenic potential is presented which is at variance with the construction and philosophy of many of the current legislative guidelines. The immediate need for the adoption of one of the available short-term in vivo liver assays, and/or the development of a short-term in vivo rodent assay capable of concomitantly monitoring different genetic end-points in a range of organs or tissues is emphasized.

Comparisons between carcinogenic potency and mutagenic potency to Salmonella in a series of derivatives of 4-dimethylaminoazobenzene (DAB)

8 derivatives of the rodent liver carcinogen 4-dimethylaminoazobenzene (DAB), all of known carcinogenicity in rodents, have been evaluated in the 3 major variants of the Salmonella mutation assay; the standard plate test of Ames et al., the pre-incubation assay of Yahagi et al. and the fluctuation assay of Gatehouse. Although 4 of these chemicals were reported to be non-carcinogenic, and 4 to be of greater carcinogenic potency than DAB, each was mutagenic in a least 2 of the assays. Further, no quantitative correlation between carcinogenic and mutagenic potency was evident in any of the assay employed. The parent carcinogen DAB, 5-dimethylaminophenylazoindazole (a non-carcinogenic bacterial mutagen) and 6-dimethylaminophenylazobenzthiazole (a carcinogenic bacterial mutagen) were administered to rats via intraperitoneal injection, followed, 26 h later, by a sub-acute dose of [14C] dimethylnitrosamine. The histopathological condition of the livers of the treated animals was assessed together with a determination of the extent and nature of methylation by DMN of the DNA in the livers according to the method of O'Connor. Disturbances in both the pathological and DNA-related parameters were observed for the 2 carcinogens while control levels were seen for the non-carcinogen. Within this context the value of short-term assays conducted in vivo is discussed, especially their potential to identify potent mammalian carcinogens from among a collection of structurally related bacterial mutagens.