GADD45α induction in the GreenScreen HC indicator assay does not occur independently of cytotoxicity.

Mammalian chromosomal integrity assays are influenced by cytotoxicity, a phenomenon which impacts data interpretation, assay specificity, and regulatory testing guidelines. Concordance of the GADD45α GreenScreen HC indicator assay to established in vitro and in vivo genetic toxicological assays has previously been described, yet a detailed description in the manner by which cytotoxicity influences its performance has not. Here we present a post-hoc analysis of a previously tested set of 91 proprietary and nonproprietary compounds investigating the influence of cytotoxicity on GADD45α induction and how varying assay cutoff criteria impacts assay performance. Significant cytotoxicity was identified to accompany the majority (72%) of compounds classified as genotoxic by GADD45α induction. Decreasing the GADD45α genotoxic induction criteria (from a 50 to a 30% increase over solvent controls) resulted in an increased assay sensitivity (from 30 to 68%) and concordance (from 55 to 68%), though a concomitant decrease in specificity was also observed (from 97 to 68%). We conclude that GADD45α induction in the GreenScreen HC indicator assay is influenced by cytotoxicity and that assay performance can be improved if different cutoff criteria are implemented.

Collaborative study on fifteen compounds in the rat-liver Comet assay integrated into 2- and 4-week repeat-dose studies.

A collaborative trial was conducted to evaluate the possibility of integrating the rat-liver Comet assay into repeat-dose toxicity studies. Fourteen laboratories from Europe, Japan and the USA tested fifteen chemicals. Two chemicals had been previously shown to induce micronuclei in an acute protocol, but were found negative in a 4-week Micronucleus (MN) Assay (benzo[a]pyrene and 1,2-dimethylhydrazine; Hamada et al., 2001); four genotoxic rat-liver carcinogens that were negative in the MN assay in bone marrow or blood (2,6-dinitrotoluene, dimethylnitrosamine, 1,2-dibromomethane, and 2-amino-3-methylimidazo[4,5-f]quinoline); three compounds used in the ongoing JaCVAM (Japanese Center for the Validation of Alternative Methods) validation study of the acute liver Comet assay (2,4-diaminotoluene, 2,6-diaminotoluene and acrylamide); three pharmaceutical-like compounds (chlordiazepoxide, pyrimethamine and gemifloxacin), and three non-genotoxic rodent liver carcinogens (methapyrilene, clofibrate and phenobarbital). Male rats received oral administrations of the test compounds, daily for two or four weeks. The top dose was meant to be the highest dose producing clinical signs or histopathological effects without causing mortality, i.e. the 28-day maximum tolerated dose. The liver Comet assay was performed according to published recommendations and following the protocol for the ongoing JaCVAM validation trial. Laboratories provided liver Comet assay data obtained at the end of the long-term (2- or 4-week) studies together with an evaluation of liver histology. Most of the test compounds were also investigated in the liver Comet assay after short-term (1-3 daily) administration to compare the sensitivity of the two study designs. MN analyses were conducted in bone marrow or peripheral blood for most of the compounds to determine whether the liver Comet assay could complement the MN assay for the detection of genotoxins after long-term treatment. Most of the liver genotoxins were positive and the three non-genotoxic carcinogens gave negative result in the liver Comet assay after long-term administration. There was a high concordance between short- and long-term Comet assay results. Most compounds when tested up to the maximum tolerated dose were correctly detected in both short- and long-term studies. Discrepant results were obtained with 2,6 diaminotoluene (negative in the short-term, but positive in the long-term study), phenobarbital (positive in the short-term, but negative in the long-term study) and gemifloxacin (positive in the short-term, but negative in the long-term study). The overall results indicate that the liver Comet assay can be integrated within repeat-dose toxicity studies and efficiently complements the MN assay in detecting genotoxins. Practical aspects of integrating genotoxicity endpoints into repeat-dose studies were evaluated, e.g. by investigating the effect of blood sampling, as typically performed during toxicity studies, on the Comet and MN assays. The bleeding protocols used here did not affect the conclusions of the Comet assay or of the MN assays in blood and bone marrow. Although bleeding generally increased reticulocyte frequencies, the sensitivity of the response in the MN assay was not altered. These findings indicate that all animals in a toxicity study (main-study animals as well as toxicokinetic (TK) satellite animals) could be used for evaluating genotoxicity. However, possible logistical issues with scheduling of the necropsies and the need to conduct electrophoresis promptly after tissue sampling suggest that the use of TK animals could be simpler. The data so far do not indicate that liver proliferation or toxicity confound the results of the liver Comet assay. As was also true for other genotoxicity assays, criteria for evaluation of Comet assay results and statistical analyses differed among laboratories. Whereas comprehensive advice on statistical analysis is available in the literature, agreement is needed on applying consistent criteria.

Repair of sparfloxacin-induced photochemical DNA damage in vivo.

The induction and subsequent repair of photochemically induced DNA damage by sparfloxacin was assessed in different tissues of juvenile Wistar rats. The animals were treated once orally with 500 mg kg(-1) of sparfloxacin and irradiated 3 hours later with 7 J cm(-2) UVA. Induction and repair of DNA damage was studied in the skin, retina and cornea using the alkaline comet assay. After a tissue-specific increase in the initial DNA damage (higher in the cornea than in skin and retina), an exponential decrease was found in the skin and retina, whereas in cornea a further increase of the DNA damage after 1 hour followed by an exponential decrease was observed. The half-lives for DNA repair were approximately 3 hours for skin and retina and 1 hour for cornea. After a recovery time of 6 hours, the majority of the induced DNA damage detectable with the comet assay had been removed. In conclusion, the data indicate that (1) photochemically induced DNA damage by sparfloxacin is efficiently removed in skin, retina and cornea, (2) repair of these DNA lesions follows an exponential decrease, (3) the induction and repair of sparfloxacin-mediated photochemical DNA damage might be tissue specific.

Detection of photogenotoxicity in skin and eye in rat with the photo comet assay.

Photosensitizing drugs increase the sensitivity of the skin and the eye toward normally harmless sunlight conditions and are known to enhance the induction of skin tumors or severe injuries to the eye. The photogenotoxicity of five common drugs (sparfloxacin, dacarbazine, chlorpromazine and 8-methoxypsoralen, promazine) was investigated in the skin as well as in the retina and cornea of Wistar rats. The compounds were administered once orally by gavage and the resulting DNA damage was analyzed in the newly developed in vivo photo comet assay. All drugs except of promazine were clearly photogenotoxic in the skin. In the cornea sparfloxacin and dacarbazine induced an increased DNA damage following irradiation. A photogenotoxic effect in the retina was observed by sparfloxacin, which is the only compound tested that absorbs wavelengths reaching the retina. The drug concentration analysis revealed that the compounds were distributed into plasma, skin and eye at concentrations, which were photogenotoxic in vitro. Additionally, histopathological analysis showed no relevant alterations or inductions of necrosis, apoptosis or inflammation in the skin or eye. In conclusion, we confirmed the photogenotoxic potential of compounds from different chemical classes in the skin. Moreover, it is the first time that photogenotoxicity has been detected in the retina and cornea in an in vivo study. Based on our results it is concluded that the photo comet assay in rat is an easy and reliable method to elucidate drug induced photogenotoxicity under conditions, which are relevant to human exposure.

The photo comet assay--a fast screening assay for the determination of photogenotoxicity in vitro.

Different classes of chemicals can induce a phototoxic effect by absorbing light energy within the wavelength range of sunlight. The assessment of photo-safety is therefore an obligatory part of the development of new drugs. Ten UV-vis (280-800nm)-absorbing compounds (ketoprofen, promazine, chlorpromazine, dacarbazine, acridine, lomefloxacin, 8-methoxypsoralen, chlorhexidine, titanium dioxide, octylmethoxycinnamate) were tested for their photogenotoxic potential in the alkaline comet assay in the presence and absence of UV-vis. In order to establish an easy and timesaving protocol for a photo comet assay screening test, the application of 96-well plates was essential. The use of mouse lymphoma L5178Y cells, a cell line growing in suspension, allowed the determination of photocytotoxicity with the Alamar Blue assay and of photogenotoxicity with the alkaline comet assay in parallel. L5178Y cells were incubated with the test compounds for 20min and irradiated with simulated sunlight in the wavelength range from 280 to 800nm. The applied UV dose was 600mJ/cm(2) UV-A and 30mJ/cm(2) UV-B. After a post-incubation of 10min, the Alamar Blue assay and the alkaline comet assay were performed. All of the compounds which are known to be photogenotoxic (8-methoxypsoralen, acridine, chlorpromazine, dacarbazine, ketoprofen, lomefloxacin) showed a positive effect under our assay conditions. Furthermore, four UV-vis absorbing chemicals which are known to be not photogenotoxic (promazine, chlorhexidine, titanium dioxide, octylmethoxycinnamate) were analysed. For none of them an increase of the DNA damage following irradiation was observed in this study. In conclusion, all of the chemical compounds tested were classified in agreement with published data. From the data presented it is concluded that the photo comet assay with L5178Y mouse lymphoma cells is a reliable model to assess photochemical genotoxicity in vitro.