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mented by the human genes ERRC1 and ERRC2. The sensitivity of these cell lines to AFB1C12 com- pared with the AFB1 epoxide was studied. It was found that VH1 (UV complementation group 2) and VC4 are sensitive to both the dichloride and the epoxide, whereas 27-1 (group 3) and UV5 (group 2) are slightly sensitive. By using alkaline elution as a method to study DNA repair, the repair capabihties of VH1 and VC4 over 24 h were studied. VH1 repaired fully after 24 h whereas VC4 did not repair as fully as the parent.

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Scott, D. 1 (Chairman), S. Galloway, M. Ishidate, R. Marshall, D. Brusick and J. Ashby, 1 Paterson Institute for Cancer Research, Christie Hospital and Holt Radium Institute, Manchester (U.K.)

Artefactual genotoxicity in mammalian cells in vitro: an ICPEMC investigation

The conditions used in in vitro tests may some- times be so extreme as to generate artefactual genotoxicity that could not occur in vivo, e.g., excessively high concentrations, high levels of cy- totoxicity, extremes of pH and genotoxic $9.

At high concentrations of highly soluble, non- toxic chemicals, genotoxicity can arise as a conse- quence of the elevated osmolality of the culture medium. However, some chemicals with in vivo activity can only be detected in vitro at quite high concentrations, in part reflecting the inadequacies of metabolic activation systems. Nevertheless we conclude, at least for clastogenicity, that very few chemicals capable of in vivo activity will be miss- ed if an upper concentration limit of 10 mM is adopted for in vitro testing, provided a rigorous protocol is used.

It has not been possible to recommend upper cytotoxicity limits for testing. Most cytotoxicity data are inadequate and there are certainly exam- ples of in vivo genotoxins which are detectable in vitro only at high levels of cytotoxicity. No artefactual genotoxicity at high levels of cytotoxic- ity has been identified, comparable to that at high concentrations.

Extremes of pH can be genotoxic in vitro so chemicals that significantly alter the pH of the medium may produce artefactual genotoxicity. Where appropriate, neutralisation and enhanced buffering capacity should be introduced.

Genotoxicity of $9 mix can sometimes occur, probably via microsomal lipid peroxidation, gen- erating active oxygen species. If test chemicals induce peroxidation, artefactual genotoxicity may ensue. It is premature to recommend addition of radical scavengers until the mechanism is better understood.

Recognition of culture conditions which are genotoxic will lead to improvements in protocols and will enhance the credibility of short-term tests.

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Arlett, C.F., J. Cole and M.H.L. Green, MRC Cell Mutation Unit, University of Sussex, Falmer, Brighton BN1 9RR (U.K.)

An overview of the UKEMS third genotoxicity trial

The third UKEMS genotoxicity trial was de- signed to show how mammalian cell mutation assays can provide high-quality data amenable to statistical evaluation and to optimise conditions for metabolic activation. Nine participating groups used assays based on V79 and CHO Chinese hamster cells, L5178Y wild-type and TK +/- mouse lymphoma cells and TK6 human lympho- blastoid cells. A set of protocols were agreed amongst the participants and modified to accom- modate the recommendations of the UKEMS working groups on statistical analysis of mutage- nicity data. The results established beyond doubt that acceptable experimental designs m u s t incor- porate true replicates. Ethyl methanesulphonate (EMS) was the reference positive compound and the results between the 9 participatory laborato- ries showed that while the toxic effects were very variable between the cell lines, the mutagenic ef- fects were similar. The study of the extent of alkylation showed that all lines were similar. Benzo[ a ]pyrene (B(a)P) which was used to address