Cytotoxicity and chromosome aberrations in vitro: Experience in industry and the case for an upper limit on toxicity in the aberration assay

The chromosome aberration assay in vitro is a useful and sensitive test For detection of genotoxins. However, aberrations can occur secondary to toxic ity, with compounds that do not react with DNA and are not genotoxic in viv o. Thus, some positive results in the in vitro aberration assay are not rel evant to human risk. To help evaluate the influence of toxicity, data were collected from 27 pharmaceutical and chemical companies and contract labora tories. When cytotoxicity was measured by cell counts or confluence, compou nds expected to damage DNA (Category 1) generally induced aberrations witho ut severe concomitant cytotoxicity, i.e., at cell growth 60% or more of con trol. The more toxic nucleoside analogues, topoisomerase inhibitors, Fluoro quinolone antibiotics, antifolates, and producers of reactive oxygen were s till positive with cell growth 50% or more of control. In contrast, when th ere was evidence that the compounds were not DNA damaging (Category 2), the re was a higher proportion of toxicity-associated clastogens, with positive results at less than 50% of control cell growth. When mitotic index (MI) w as used as an indicator of cytotoxicity, the pattern was less clear, althou gh there was a tendency to more mitotic suppression with the Category 2 com pounds. Overall the data indicate that a limit on toxicity, and a more accu rate way of estimating it, would increase the accuracy of the assay by redu cing the frequency of nonrelevant positive results with a threshold-type of dose relation. The rationale For evaluating positive results in the in vit ro aberration assay, especially those associated with toxicity, is discusse d, as is the need For a harmonized regulatory approach. (C) 2000 Wiley-Liss , Inc.