Use of the in vivo skin comet assay to evaluate the DNA-damaging potential of chemicals applied to the skin

Validation of the 3D reconstructed human skin micronucleus (RSMN) assay: an animal-free alternative for following-up positive results from standard in vitro genotoxicity assays

In vitro test batteries have become the standard approach to determine the genotoxic potential of substances of interest across industry sectors. While useful for hazard identification, standard in vitro genotoxicity assays in 2D cell cultures have limited capability to predict in vivo outcomes and may trigger unnecessary follow-up animal studies or the loss of promising substances where animal tests are prohibited or not desired. To address this problem, a team of regulatory, academia and industry scientists was established to develop and validate 3D in vitro human skin-based genotoxicity assays for use in testing substances with primarily topical exposure. Validation of the reconstructed human skin micronucleus (RSMN) assay in MatTek Epi-200™ skin models involved testing 43 coded chemicals selected by independent experts, in four US/European laboratories. The results were analysed by an independent statistician according to predefined criteria. The RSMN assay showed a reproducibly low background micronucleus frequency and exhibited sufficient capacity to metabolise pro-mutagens. The overall RSMN accuracy when compared to in vivo genotoxicity outcomes was 80%, with a sensitivity of 75% and a specificity of 84%, and the between- and within-laboratory reproducibility was 77 and 84%, respectively. A protocol involving a 72-h exposure showed increased sensitivity in detecting true positive chemicals compared to a 48-h exposure. An analysis of a test strategy using the RSMN assay as a follow-up test for substances positive in standard in vitro clastogenicity/aneugenicity assays and a reconstructed skin Comet assay for substances with positive results in standard gene mutation assays results in a sensitivity of 89%. Based on these results, the RSMN assay is considered sufficiently validated to establish it as a ‘tier 2’ assay for dermally exposed compounds and was recently accepted into the OECD’s test guideline development program.

In vivo comet assay in rabbit corneal epithelial cells following ocular instillation with genotoxic compounds

Background

The in vivo comet assay is used to evaluate the genotoxic potential of compounds by detecting DNA strand breaks in cells isolated from animal tissue. The comet assay of hepatocytes is well established; however, the levels of systemic drug exposure following systemic administration are often insufficient to evaluate the genotoxic potential of compounds on the ocular surface following ocular instillation. To investigate the possibility of using the comet assay as a genotoxic evaluation tool for the ocular surface, we performed this assay on the corneal epithelial cells of rabbit eyes 2 h after the single ocular instillation of five genotoxic compounds, namely ethidium bromide, 1,1′-dimethyl-4,4′-bipyridinium dichloride (paraquat), methyl methanesulfonate (MMS), acrylamide, and 4-nitroquinoline 1-oxide (4-NQO).

Results

The mean % tail DNA, as an indicator of DNA damage, in the corneal epithelial cells treated with ethidium bromide, MMS, and 4-NQO exhibited statistically significant increases compared with those in the negative controls (saline or 5 % dimethyl sulfoxide in saline). However, paraquat and acrylamide did not increase the mean % tail DNA, presumably because of the high antioxidant levels and low cytochrome P450 levels present in the corneal epithelium, respectively.

Conclusions

The comet assay was able to detect genotoxic potential on the ocular surface following ocular instillation with genotoxic compounds. The study findings indicate that the in vivo comet assay may provide a useful tool for assessing the genotoxicity of compounds topically administrated on the ocular surface under mimicking clinical condition.

Investigation of in vivo unscheduled DNA synthesis in rabbit corneas following instillation of genotoxic agents

PURPOSE

An unscheduled DNA synthesis (UDS) test is used for in vitro or in vivo genotoxicity evaluation. The UDS test with hepatocytes is well established; however, drug exposure levels at the application site for topically administered drugs (e.g. ophthalmic drugs) often exceed the exposure levels for systemic administration. To establish in vivo genotoxicity on the ocular surface, we performed the UDS test using rabbit corneas from eyes subjected to instillation of genotoxic agents.

MATERIALS AND METHODS

Five genotoxic agents - 1,1'-dimethyl-4,4'-bipyridinium dichloride (paraquat); acridine orange; ethidium bromide; acrylamide; and 4-nitroquinoline 1-oxide (4-NQO) - were instilled once onto both eyes of male Japanese white rabbits. Physiological saline or a general vehicle for ophthalmic solution were instilled as the negative controls. Dimethyl sulfoxide was instilled as the vehicle control. Isolated corneas were incubated with tritium-labelled thymidine and the number of sparsely labelled cells (SLCs, cells undergoing UDS) was counted by autoradiography.

RESULTS

Statistically significant increases in the mean appearance rates of SLCs in the corneal epithelium were noted in paraquat-, acridine orange-, ethidium bromide-, and 4-NQO-treated eyes compared with those of the controls. These increases generally appeared in a dose-dependent manner. Acrylamide did not induce an increase in the mean appearance rates of SLCs, presumably because it caused the generation of fewer metabolites in the cornea.

CONCLUSIONS

UDS tests revealed DNA damage in the cornea epitheliums treated with well-known genotoxic agents. These results suggest that the UDS test is one of the useful tools for the assessment of in vivo genotoxicity on the ocular surface in the development of ophthalmic drugs.

Genotoxicity assessment of reactive and disperse textile dyes using human dermal equivalent (3D cell culture system)

Thousands of dyes are marketed daily for different purposes, including textile dyeing. However, there are several studies reporting attributing to dyes deleterious human effects such as DNA damage. Humans may be exposed to toxic dyes through either ingestion of contaminated waters or dermal contact with colored garments. With respect to dermal exposure, human skin equivalents are promising tools to assess in vitro genotoxicity of dermally applied chemicals using a three-dimensional (3D) model to mimic tissue behavior. This study investigated the sensitivity of an in-house human dermal equivalent (DE) for detecting genotoxicity of textile dyes. Two azo (reactive green 19 [RG19] and disperse red 1[DR1]) dyes and one anthraquinone (reactive blue 2 [RB2]) dye were analyzed. RG19 was genotoxic for DE in a dose-responsive manner, whereas RB2 and DR1 were nongenotoxic under the conditions tested. These findings are not in agreement with previous genotoxicological assessment of these dyes carried out using two-dimensional (2D) cell cultures, which showed that DR1 was genotoxic in human hepatoma cells (HepG2) and RG19 was nongenotoxic for normal human dermal fibroblasts (NHDF). These discrepant results probably may be due to differences between metabolic activities of each cell type (organ-specific genotoxicity, HepG2 and fibroblasts) and the test setup systems used in each study (fibroblasts cultured at 2D and three-dimensional [3D] culture systems). Genotoxicological assessment of textile dyes in context of organ-specific genotoxicity and using in vitro models that more closely resemble in vivo tissue architecture and physiology may provide more reliable estimates of genotoxic potential of these chemicals.

Factors influencing heterogeneity of radiation-induced DNA-damage measured by the alkaline comet assay

Background

To investigate whether different conditions of DNA structure and radiation treatment could modify heterogeneity of response. Additionally to study variance as a potential parameter of heterogeneity for radiosensitivity testing.

Methods

Two-hundred leukocytes per sample of healthy donors were split into four groups. I: Intact chromatin structure; II: Nucleoids of histone-depleted DNA; III: Nucleoids of histone-depleted DNA with 90 mM DMSO as antioxidant. Response to single (I-III) and twice (IV) irradiation with 4 Gy and repair kinetics were evaluated using %Tail-DNA. Heterogeneity of DNA damage was determined by calculation of variance of DNA-damage (V) and mean variance (Mvar), mutual comparisons were done by one-way analysis of variance (ANOVA).

Results

Heterogeneity of initial DNA-damage (I, 0 min repair) increased without histones (II). Absence of histones was balanced by addition of antioxidants (III). Repair reduced heterogeneity of all samples (with and without irradiation). However double irradiation plus repair led to a higher level of heterogeneity distinguishable from single irradiation and repair in intact cells. Increase of mean DNA damage was associated with a similarly elevated variance of DNA damage (r = +0.88).

Conclusions

Heterogeneity of DNA-damage can be modified by histone level, antioxidant concentration, repair and radiation dose and was positively correlated with DNA damage. Experimental conditions might be optimized by reducing scatter of comet assay data by repair and antioxidants, potentially allowing better discrimination of small differences. Amount of heterogeneity measured by variance might be an additional useful parameter to characterize radiosensitivity.