Comparative study with the alkaline Comet assay and the chromosome aberration test

The alkaline Comet assay is becoming a useful tool for early genotoxicity testing of new pharmaceutical drug candidates. The aim of this study was to elucidate the predictive value of Comet assay results for the outcome of the chromosome aberration (CA) test. For this purpose, a validation exercise with 13 drug candidates was carried out utilizing V79 Chinese hamster cells and human lymphocytes. The study demonstrates that results of the Comet assay and the chromosome aberration test show a high degree of agreement, irrespective of the cell type used. In the Comet assay, seven compounds were positive and six were negative, while in the CA test, six were positive and seven were negative. The only discrepancy was found with one compound that was positive in the Comet assay with V79 cells, negative in the Comet assay with human lymphocytes and clearly negative in the CA test with human lymphocytes. For the selection of concentrations for testing in the Comet assay, cytotoxicity by means of cell count after incubation or viability by means of Trypan-blue dye exclusion (TBDE) were used. The results show that either parameter led to analysis of a concentration range in the Comet assay similar to that chosen in the CA test, in which cell count (when using V79 cells) or mitotic index (in case of lymphocytes) were used. However, since cell count after incubation of cells is much more labour-intensive, viability was preferred as the parameter to assess cytotoxicity and for selecting concentrations for analysis in the Comet assay. The data presented in this study may contribute the regulatory acceptance of the Comet assay, e.g. for mechanistic studies.