Revalidation of the in vitro alkaline elution rat hepatocyte assay for DNA damage: Improved criteria for assessment of cytotoxicity and genotoxicity and results for 81 compounds

The in vitro alkaline elution/rat hepatocyte assay is a sensitive assay for genotoxicity, measured as DNA strand breaks induced in primary cultures of rat hepatocytes after 3-h treatments with test compounds. Since DNA degradation can be rapid and extensive in dead and/or dying cells, the original criteria for a positive result in the assay were that a compound induce a 3.0-fold or greater increase in the elution slope (for the terminal phase of alkaline elution from 3 to 9 h) in the absence of significant cytotoxicity (defined as relative cell viability of less than 70% by trypan blue dye exclusion; TBDE). Recently we have shown that false-positive results can still be obtained due to cytotoxicity when loss of membrane integrity is a late event in toxic cell death relative to the induction of endonucleolytic DNA degradation. To improve the ability of the assay to discriminate between genotoxic vs. cytotoxic effects of chemicals, we have evaluated additional assays of cytotoxicity including cell adenosine triphosphate (ATP) and potassium (K+) content, tetrazolium dye reduction (MTT), TBDE after a further 3-h recovery incubation without test chemicals (delayed toxicity), cell blebbing and endonucleolytic DNA degradation (double-strand breaks; DSBs) assessed by pulsed-field gel electrophoresis (PFGE). We have also evaluated 2 parameters derived from the elution data which can indicate extensive, cytotoxicity-induced DNA degradation: the fraction of the DNA recovered in the neutral lysis/rinse fraction and the y-intercept of the extrapolation of the 3-9-h segment of the elution curve. Twenty-eight rodent non-carcinogens that are negative (or inconclusive) in the Ames assay with no, or limited, other evidence of genotoxicity, and 33 genotoxins, most of which are also carcinogens, were evaluated. The results showed that DNA degradation as measured by a 1-h PACE (Programmed Autonomously Controlled Electrodes)/PFGE assay was a sensitive indicator of cytotoxicity which correlated well with results of the other cytotoxicity indicators. The delayed TBDE (after a 3-h recovery), intracellular potassium and ATP assays as well as the y-intercept parameter were also shown to be sensitive and in some cases complementary measures of cytotoxicity. Using new criteria based on these data of an induced slope (treatment slope-negative control slope) of 0.020 for the 3- to 9-h elution period and cytotoxicity limits of 70% relative viability for the delayed TBDE assay and 50% for intracellular ATP content, the assay scores the genotoxicity of these 61 reference compounds with an overall accuracy of 92%. Test results using these new criteria are provided for an additional 20 compounds (5 non-genotoxic carcinogens and 15 compounds whose genotoxic and carcinogenic potential are unknown or equivocal).