UDS activity in the rat liver of the human carcinogens benzidine and 4-aminobiphenyl, and the rodent carcinogens 3,3'-dichlorobenzidine and direct black 38

Quantification of a potent mutagenic 4-amino-3,3′-dichloro-5,4′-dinitrobiphenyl (ADDB) and the related chemicals in water from the Waka River in Wakayama, Japan

4-Amino-3,3'-dichloro-5,4'-dinitrobiphenyl (ADDB) is a novel chemical exerting strong mutagenicity, especially in the absence of metabolic activation. In addition to mutagenicity, ADDB may also disrupt the endocrine system in vitro. ADDB may be discharged from chemical plants near the Waka River and could be unintentionally formed via post-emission modification of drainage water containing 3,3'-dichlorobenzidine (DCB), which is a precursor in the manufacture of polymers and dye intermediates in chemical plants. The main purpose of this study was to make a comprehensive survey of the behaviour and levels of ADDB and suspected starting material or intermediates of ADDB, i.e., DCB, 3,3'-dichloro-4,4'-dinitrobiphenyl (DDB), and 4-amino-3,3'-dichloro-4'-nitrobipheny (ADNB) in Waka River water samples. We also postulated the formation pathway of ADDB. Water samples were collected at five sampling sites from the Waka River four times between March 2003 and December 2004. Samples were passed through Supelpak2 columns, and adsorbed materials were then extracted with methanol. Extracts were used for quantification of ADDB and the related chemicals by HPLC on reverse-phase columns; mutagenicity was evaluated in the Salmonella assay using the O-acetyltransferase-overexpressing strain YG1024. High levels of ADDB, DCB, DDB, and ADNB (12.0, 20,400, 134.8, and 149.4ng/L-equivalent) were detected in the samples collected at the site where wastewater was discharged from chemical plants into the river. These water samples also showed stronger mutagenicity in YG1024 both with and without S9 mix than the other water samples collected from upstream and downstream sites. The results suggest that ADDB is unintentionally formed from DCB via ADNB in the process of wastewater treatment of drainage water containing DCB from chemical plants.

Mutagenicity of the human bladder carcinogen 4-aminobiphenyl to the bladder of MutaMouse transgenic mice

The human carcinogen 4-aminobiphenyl (4AB) was evaluated in the MutaMouse transgenic mouse mutation assay. A single oral dose of 75 mg/kg induced 6.9-, 1.8- and 2.2-fold increases in the mutation frequency (MF) in the bladder, liver and bone marrow, respectively. Ten daily oral doses of 10 mg/kg 4AB increased the MF in the bladder, liver and bone marrow by 13.7-, 4.8- 2.4-fold the control value, respectively. The repeat dosing protocol was therefore more sensitive than the single dose protocol. Assessment of DNA samples prepared from pooled liver homogenates clearly indicated the increase in MF observed in the individual liver samples obtained from both groups of 4AB-treated mice.

Recommendations for the performance of UDS tests in vitro and in vivo

The Working Group (WG) dealt with the harmonization of routine methodologies of tests for unscheduled DNA synthesis (UDS) both in vitro and in vivo. In contrast to the existing guidelines from OECD, EPA and EC on in vitro UDS tests (there is no Japanese UDS guideline), the Working Group recommends that in general in vitro UDS tests should be performed with primary hepatocytes. For routine applications any other cell types would need special justification. Hepatocytes from male rats are preferable, unless there are contra-indications on the basis of e.g. toxicokinetic data. According to the OECD, EPA and EC guidelines, UDS may be analysed by means of autoradiography (AR) or liquid scintillation counting (LSC). The WG recommends use of AR. LSC is less suitable due to the problem of differentiation between UDS activity and replicative DNA synthesis, and the disadvantage that cells cannot be analysed individually. Since a specific cell type was recommended by the WG, methodological aspects could be described in more detail than in the present guidelines. For in vitro tests, it was agreed that the initial viability of freshly isolated hepatocytes should be at least 70%. With regard to the need for confirmatory experiments in the event of a clear-cut negative result, the majority view was that confirmation by a second (normally not identical) experiment is still needed; this is in line with the present OECD and EC guidelines. Evaluation of results from UDS tests should be based primarily on net nuclear grain (NNG) values, although it is recognised that nuclear and cytoplasmic grains result from different biological processes. Since grain counts are influenced by a number of methodological parameters, no global threshold NNG value can be recommended for discrimination of positive and negative UDS results. For in vitro assays, the criteria for positive findings go beyond those of the present guidelines and two alternative approaches are given which are based on (1) dose-dependent increases in NNG values and (2) reproducibility, dose-effect relationship and cytotoxicity. At present there is no official guideline on the performance of in vivo UDS tests. Some fundamental recommendations given for in vitro methodology also apply to the in vivo assay. For routine testing with the in vivo UDS test, again the general use of hepatocytes from male rats is recommended. However, concerning the requirement to use one or two sexes, consistency with other in vivo genotoxicity assays (e.g. the micronucleus assay) would be preferable. As for the in vitro methodology, AR is preferred rather than LSC.(ABSTRACT TRUNCATED AT 400 WORDS)

The genetic toxicity of human carcinogens and its implications

23 chemicals and chemical combinations have been designated by the International Agency for Research on Cancer (IARC) as causally associated with cancer in humans. The literature was searched for reports of their activity in the Salmonella mutagenicity assay and for evidence of their ability to induce chromosome aberrations or micronuclei in the bone marrow of mice or rats. In addition, the chemical structures of these carcinogens were assessed for the presence of electrophilic substituents that might be associated with their mutagenicity and carcinogenicity. The purpose of this study was to determine which human carcinogens exhibit genetic toxicity in vitro and in vivo and to what extent they can be detected using these two widely employed short-term tests for genetic toxicity. The results of this study revealed 20 of the 23 carcinogens to be active in one or both short-term tests. Treosulphan, for which short-term test results are not available, is predicted to be active based on its structure. The remaining two agents, asbestos and conjugated estrogens, are not mutagenic to Salmonella; asbestos is not likely to induce cytogenetic effects in the bone marrow and the potential activity of conjugated estrogens in the bone marrow is difficult to anticipate. These findings show that genetic toxicity is characteristic of the majority of IARC Group 1 human carcinogens. If these chemicals are considered representative of human carcinogens, then two short-term tests may serve as an effective primary screen for chemicals that present a carcinogenic hazard to humans.