Performance and data interpretation of the in vivo comet assay in pharmaceutical industry: EFPIA survey results

Fertilized Avian Egg Fetal Liver Assays for Assessing DNA Damaging Potential of Chemicals: A Comparative Analysis With In Vitro and In Vivo Genotoxicity Assays and Rodent Carcinogenicity

The ability to produce direct DNA damage (genotoxicity), which underlies the carcinogenicity of various chemicals, is typically evaluated in a regulatory-approved battery of in vitro tests with potential in vivo follow-up. Growing concerns for animal welfare and implementation of regulations restricting the use of animal testing necessitate the introduction of New Approach Methodologies (NAMs). The avian egg-based (in ovo) models were developed as metabolically competent NAMs capable of bioactivation, detoxication, and elimination of xenobiotics to potentially replace short-term in vivo genotoxicity assays for chemicals that are genotoxic in vitro. These models utilize avian (chicken or turkey) fetal livers for the evaluation of endpoints indicative of DNA damage produced by either direct or indirect mechanisms, the formation of nuclear DNA adducts and strand breaks. Avian embryos have genetic and morphologic resemblance to mammals and can be used for the evaluation of other endpoints including histopathology and genomic profiling. A concordance analysis of 87 and 59 chemicals assessed in the chicken and turkey models, respectively, revealed a stronger correlation with the results from in vivo genotoxicity assays (76% and 67% sensitivity, 79% and 72% specificity for chicken and turkey, respectively) compared to in vitro assays (58% and 56% sensitivity, 45% and 63% specificity for chicken and turkey, respectively). These results demonstrate that in ovo models detect the genotoxic potential of a broader range of compounds compared to in vitro assays with S9 supplementation. In conclusion, fertilized avian egg fetal liver assays offer a promising alternative to traditional in vivo genotoxicity assays.

Assessment of genotoxic potential of fragrance materials in the chicken egg assays

The genotoxic and clastogenic/aneugeneic potentials of four α,β-unsaturated aldehydes, 2-phenyl-2-butenal, nona-2-trans-6-cis-dienal, 2-methyl-2-pentenal, and p-methoxy cinnamaldehyde, which are used as fragrance materials, were assessed using the Chicken Egg Genotoxicity Assay (CEGA) and the Hen's egg micronucleus (HET-MN) assay, respectively. Selection of materials was based on their chemical structures and the results of their previous assessment in the regulatory in vitro and/or in vivo genotoxicity test battery. Three tested materials, 2-phenyl-2-butenal, nona-2-trans-6-cis-dienal, and 2-methyl-2-pentenal, were negative in both, CEGA and HET-MN assays. These findings were congruent with the results of regulatory in vivo genotoxicity assays. In contrast, p-methoxy cinnamaldehyde, which was also negative in the in vivo genotoxicity assays, produced evidence of DNA damage, including DNA strand breaks and DNA adducts in CEGA. However, no increase in the micronucleus formation in blood was reported in the HET-MN study. Such variation in responses between the CEGA and HET-MN assay can be attributed to differences in the dosing protocols. Pretreatment with a glutathione precursor, N-acetyl cysteine, negated positive outcomes produced by p-methoxy cinnamaldehyde in CEGA, indicating that difference in response observed in the chicken egg and rodent models can be attributed to rapid glutathione depletion. Overall, our findings support the conclusion that CEGA and/or HET-MN can be considered as a potential alternative to animal testing as follow-up strategies for assessment of genotoxic potential of fragrance materials with evidence of genotoxicity in vitro.

In vivo alkaline comet assay: Statistical considerations on historical negative and positive control data

The alkaline comet assay is frequently used as in vivo follow-up test within different regulatory environments to characterize the DNA-damaging potential of different test items. The corresponding OECD Test guideline 489 highlights the importance of statistical analyses and historical control data (HCD) but does not provide detailed procedures. Therefore, the working group "Statistics" of the German-speaking Society for Environmental Mutation Research (GUM) collected HCD from five laboratories and >200 comet assay studies and performed several statistical analyses. Key results included that (I) observed large inter-laboratory effects argue against the use of absolute quality thresholds, (II) > 50% zero values on a slide are considered problematic, due to their influence on slide or animal summary statistics, (III) the type of summarizing measure for single-cell data (e.g., median, arithmetic and geometric mean) may lead to extreme differences in resulting animal tail intensities and study outcome in the HCD. These summarizing values increase the reliability of analysis results by better meeting statistical model assumptions, but at the cost of information loss. Furthermore, the relation between negative and positive control groups in the data set was always satisfactorily (or sufficiently) based on ratio, difference and quantile analyses.

Slight hypercalcemia is not associated with positive responses in the Comet Assay in male rat liver

Maintenance of physiological levels of intracellular and extracellular calcium is essential for life. Increased intracellular calcium levels are involved in cell death (apoptosis and necrosis) and are associated with positive responses in the Comet assay in vitro. In addition, high calcium and vitamin D intakes were reported to induce apoptosis in adipose tissue in obese mice and to increase DNA-migration in the Comet assay. To investigate increased serum concentration of calcium as a potential confounding factor in the regulatory Comet assay in vivo, we induced mild hypercalcemia in male Wistar rats by 3-day continuous intravenous infusion of calcium gluconate and performed the Comet assay in the liver in line with regulatory guidelines. The results of the study showed that mild increases in serum calcium concentration (up to 1.4 times above the concurrent control) and increased urinary calcium concentration (up to 27.8 times above the concurrent control) results in clinical signs like mild tremor, faster respiration rate and decreased activity in a few animals. However, under the conditions of the study, no increase in the %Tail DNA in the Comet assay and no indication of liver damage as determined by histopathological means were observed. Thus, mild increases in plasma calcium did not lead to positive results in a genotoxicity assessment by the Comet assay in the rat liver. This result is important as it confirms the reliability of this assay for regulatory evaluation of safety.