Chinese hamster ovary (CHO/HPRT) cell mutation assays with EMS, benzo[a]pyrene and benzidine

In the context of the third UKEMS collaborative trial on cell mutation assays, three chemicals have been tested for mutagenicity using the Chinese hamster ovary (CHO) cell/HPRT assay. The protocol employed was based on the re-spreading of monolayer cultures, maintaining one million cells in growth for an expression time of 7 days. Ethyl methanesulphonate gave clear and consistent positive results in the absence of S9 and a clear response to benzo[a]pyrene was obtained with 1% S9. Benzidine gave no evidence of mutagenicity either with or without S9. The results show that the CHO/HPRT assay protocol used in this study is adequate for the detection of potent mutagens. The failure to detect benzidine as a mutagen may be due either to a basic unresponsiveness of the system or to a lack of sensitivity because of the limitation on the numbers of cells which can be maintained during expression. Statistical analysis showed that significant variation occurred between cell survival and mutagenicity estimates for replicate cultures, demonstrating the need for duplicate cultures.