Comparison of in vivo and in vitro cytogenetic assay results

Integrated Genotoxicity Testing of three anti-infective drugs using the TGx-DDI transcriptomic biomarker and high-throughput CometChip® assay in TK6 cells

Genotoxicity testing relies on the detection of gene mutations and chromosome damage and has been used in the genetic safety assessment of drugs and chemicals for decades. However, the results of standard genotoxicity tests are often difficult to interpret due to lack of mode of action information. The TGx-DDI transcriptomic biomarker provides mechanistic information on the DNA damage-inducing (DDI) capability of chemicals to aid in the interpretation of positive in vitro genotoxicity data. The CometChip^® assay was developed to assess DNA strand breaks in a higher-throughput format. We paired the TGx-DDI biomarker with the CometChip^® assay in TK6 cells to evaluate three model agents: nitrofurantoin (NIT), metronidazole (MTZ), and novobiocin (NOV). TGx-DDI was analyzed by two independent labs and technologies (nCounter^® and TempO-Seq^®). Although these anti-infective drugs are, or have been, used in human and/or veterinary medicine, the standard genotoxicity testing battery showed significant genetic safety findings. Specifically, NIT is a mutagen and causes chromosome damage, and MTZ and NOV cause chromosome damage in conventional in vitro tests. Herein, the TGx-DDI biomarker classified NIT and MTZ as non-DDI at all concentrations tested, suggesting that NIT’s mutagenic activity is bacterial specific and that the observed chromosome damage by MTZ might be a consequence of in vitro test conditions. In contrast, NOV was classified as DDI at the second highest concentration tested, which is in line with the fact that NOV is a bacterial DNA-gyrase inhibitor that also affects topoisomerase II at high concentrations. The lack of DNA damage for NIT and MTZ was confirmed by the CometChip^® results, which were negative for all three drugs except at overtly cytotoxic concentrations. This case study demonstrates the utility of combining the TGx-DDI biomarker and CometChip^® to resolve conflicting genotoxicity data and provides further validation to support the reproducibility of the biomarker.

Comparison of genomic damage caused by 5-nitrofurantoin in young and adult mice using the in vivo micronucleus assay

The antibiotic 5-nitrofurantoin (5-NF) has been used widely for the treatment of urosepsis in children during the last 20 years. Recent experimentation suggests the need for reevaluating its genotoxic potential. Because of possible differences in the metabolism and clearance of 5-NF in young and adult animals, we conducted a study to determine whether micronuclei caused by 5-NF were age-related. The in vivo micronucleus (MN) assay was performed on 3- and 8-week-old mice given single intraperitoneal injections of 5, 10, and 50 mg/kg 5-NF. Blood samples from the tail vein were taken before injection (baseline) and at 48, 96, 168, and 336 hr (2 weeks) after the treatment. One thousand reticulocytes were analyzed for micronuclei from each animal. Compared to similar baseline values for young and adult mice, 5-NF caused a significant increase in MN frequency in both age groups. The mean MN frequency in the young animals was higher than in the adult animals for each dose and sampling time. MN frequencies remained significantly elevated in young animals even 2 weeks after exposure to 5-NF. The results of the study confirm the genotoxic potential of 5-NF in young and adult animals, and indicate that young animals are more sensitive to the genotoxic effects of 5-NF than adult mice and that the response in young mice persists for a significantly longer time. These findings may be related to poorly developed mechanisms of xenobiotic detoxification and renal elimination in young animals.

Cytogenetic analysis of children under long-term antibacterial therapy with nitroheterocyclic compound furagin

Cytogenetic analysis of chromosome aberrations (CAs) and sister chromatid exchanges (SCEs) was performed in 109 blood samples from 95 pediatric patients with urinary tract infections (UTIs). Children were exposed to diagnostic levels of X-rays during voiding cystourethrography and subsequently treated for one to 12 months with low doses of furagin - N-(5-nitro-2-furyl)-allylidene-1-aminohydantoin. Furagin is 2-substituted 5-nitrofuran, chemically and structurally similar to well-known antibacterial compound nitrofurantoin. Increased frequencies of CAs were found in children undergoing voiding cystourethrography as compared with the unexposed, acentric fragments being the most frequent alteration (2.03 versus 0.88 per 100 cells, P=0.006). However, a significant decrease in the frequency of acentric fragments was determined with the time elapsed since X-ray examination was performed. A time-independent increase in SCE frequency was found in lymphocytes of children treated with furagin. Total CA frequency did not differ significantly between groups of children with various duration of furagin treatment. However, frequency of chromatid exchanges (triradials and quadriradials) increased significantly with duration of treatment.

Comparative evaluation of the in vitro micronucleus test and the in vitro chromosome aberration test: industrial experience

Because of its rapidness, simplicity and potential for automation, the measurement of micronucleated cells in vivo is not only equivalent to the analysis of chromosome aberrations, but often even preferred within routine genotoxicity testing. In order to evaluate the correlation between the in vitro micronucleus assay (MNT) and the in vitro chromosome aberration test (CA), we collected data from four pharmaceutical companies obtained either in Chinese hamster cell lines (CHO-K5, CHO-K1, V79) or in human peripheral blood lymphocytes. Among the 57 compounds included in this comparison, 45 compounds gave rise to concordant results in both assays (26 compounds negative in both assays; 19 compounds positive in both assays). The high percentage of concordance, i.e. about 79% is very promising and can be even increased to about 88% by omitting the 3 aneugenic compounds and 2 compounds inducing endoreduplicated chromosomes which were found positive only in the in vitro MNT. The results are remarkable in particular considering that most of the compounds evaluated are 'standard' pharmaceutical compounds and thus are at most weak inducers of chromosome damage. Our comparison strongly supports that the in vitro micronucleus test is a suitable alternative to the in vitro chromosome aberration assay. Moreover, the MNT has the potential of not only detecting clastogens but additionally aneuploidy inducing chemicals.

Genetic toxicology of acrylic acid

Acrylic acid was tested for gene mutations in the in vitro CHO/HGPRT assay, for chromosome aberrations in CHO cells in culture, and for potential to induce unscheduled DNA synthesis in rat hepatocytes in culture. In vivo assays performed included the Drosophila sex-linked recessive lethal assay by both the feeding and injection routes, the in vivo cytogenetic assay in rat bone marrow cells after both a 1-day and 5-day oral dosing regimen, and a dominant lethal assay in mice by both an acute and 5-day dosing regimen. All results were negative (non-mutagenic) except for the in vitro chromosome aberration assay. This latter result is consistent with the previously reported possible clastogenic activity suggested by the results of the mouse lymphoma L5178Y TK locus assay in which a predominance of small-colony mutants was observed (Moore et al., Environmental and Molecular Mutagenesis 1988, 11, 49-63). The rapid clearance of acrylic acid in animals and the weight of evidence of genetic toxicity testing, including negative in vivo data in both somatic and germ cells, indicate a lack of genetic toxicity of acrylic acid in vivo.

Ethylnitrosourea-induced mutations in vivo involving the Dolichos biflorus agglutinin receptor in mouse intestinal epithelium

A mutagenesis assay system is introduced based on the induction of mutations in somatic cells of mouse small intestine using ethylnitrosourea (ENU). F1 mice heterozygous for the Dolichos biflorus agglutinin (DBA) locus (Dlb-1a/Dlb-1b) encoding the DBA cell surface receptor, were treated in utero on either day 7, day 9 or day 11 post coitum. Mutant intestinal cell populations of adult mice were visualised in whole-mount preparations by the absence of histochemical staining using peroxidase-labelled Dolichos biflorus agglutinin. Loss of staining is attributed to mutagenesis of the Dlb-1b allele in the heterozygote. This system allows one to evaluate mammalian mutagenesis in vivo at a single locus. Mutant cell populations appeared as discrete groups of 'striped' villi, each stripe comprising cells derived from unstained crypt stem cells (cf. Schmidt et al., 1985a). A spontaneous mutation level was noted in untreated controls which was found to differ significantly from that recorded in mice treated with the mutagen (P less than 0.01). The mutation scores were highly consistent among mice and a small number of animals (i.e., 16) were sufficient to detect mutagenic effects of ENU. Thus, the advantages which accrue from the assay are (1) the ability to detect small clones of mutant cell populations in the intestine (i.e., cells derived from a single mutated crypt); (2) a small number of tested mice are required to generate a conclusive result, especially when compared to the mammalian spot test (Fahrig, 1978).

Sister-chromatid exchange studies on direct- and indirect-acting clastogens in mouse primary cell cultures

An in vitro sister-chromatid exchange (SCE) assay using mouse primary bone marrow and spleen cells was conducted with both direct- and indirect-acting genotoxic agents. 2,4,7-Trinitrofluorenone, a direct-acting genotoxic agent, induced a significant dose-related increase in SCEs. In both bone marrow and spleen cells, 2.0 micrograms/ml caused an approx. 3-fold increase in SCE level over control values. Cyclophosphamide, an indirect-acting genotoxicant which requires metabolic activation for its clastogenicity, induced a significant increase in SCEs in the presence of S9 from liver of rats pretreated with Aroclor-1254. A dose of 2 micrograms/ml resulted in a 2-fold increase in bone marrow and a greater than 5-fold increase in spleen cells. Benzo[a]pyrene, another indirect-acting genotoxicant, also induced significant dose-related SCE responses in both cell types. It seems that primary bone marrow and spleen cell culture systems can detect both direct- and indirect-acting genotoxicants and may be useful for routine and/or comparative cytogenetic studies.

In vivo micronucleus test using mouse hepatocytes

The bone-marrow micronucleus (BMM) test is highly specific for clastogenic effects but its sensitivity is determined to a great extent by the substances tested, particularly by their metabolism. Some compounds, such as unstable mutagens or those which generate short-lived metabolites, are not detected in this test because the metabolites produced in the liver do not reach the bone marrow. In an attempt to provide qualitative and quantitative assessments of chromosomal mutations produced in vivo by genotoxic agents not detected in the mouse BMM test, a mouse-liver micronucleus test, adapted from Tates model, was developed. The animals were treated twice, with an interval of 24 h between treatments, and then subjected to partial hepatectomy (PH) 24 h after the second treatment in order to induce mitotic stimulation. The incidence of micronucleated hepatocytes was determined 96 h after PH. The test was evaluated with 5 procarcinogens, each with a complex metabolic pattern: dimethylnitrosamine (DMN), diethylnitrosamine (DEN), 1,1-dimethylhydrazine (1,1-DMH), 4-aminophenol (4-APOL), 4-aminobiphenyl (4-ABPYL) and one direct unstable mutagen, beta-propiolactone (BPL). All these compounds are negative in the mouse BMM test but caused a major increase in the incidence of micronuclei in mouse hepatocytes. This test is simple and can be readily compared with the BMM test. Furthermore, it offers a better assessment of the impact of a compound at the chromosomal level in a metabolically competent cell and can therefore be used for the evaluation of the genotoxic activity of compounds with complex metabolic pathways.

Use of the cytokinesis-block method for the analysis of micronuclei in V79 Chinese hamster lung cells: results with mitomycin C and cyclophosphamide

The cytochalasin B (CYB)-blocked binucleated cell assay has been explored to analyze micronuclei and cell cycle kinetics using 2 known mutagenic carcinogens in V79 Chinese hamster lung cells. To determine the optimum time to obtain the maximum number of binucleated cells for micronucleus analysis, duplicate cultures of exponentially growing cells were treated with 3 micrograms/ml CYB for varying durations (8-48 h). A peak appearance of binucleated cells at 16 h in the presence of CYB suggested this as an optimum time for micronucleus analysis in binucleated V79 cells. To evaluate the capacity for induction of micronuclei in V79 cells, 2 mutagenic carcinogens, mitomycin C (0.125-1.0 micrograms/ml) and cyclophosphamide (2-12 micrograms/ml) were tested in duplicate cultures. Mitomycin C, a direct-acting alkylating agent, caused approximately an 18-fold increase in micronucleus frequency over controls at the highest concentration tested (1.0 micrograms/ml), and this increase occurred in a dose-related manner (r = 0.92). The concentrations of mitomycin C tested also caused a significant dose-related cell cycle delay, thus suggesting cytotoxicity to V79 cells. Cyclophosphamide, an indirect-acting alkylating agent, requiring the presence of S9 mix, caused approximately a 17-fold increase in micronucleus frequency over controls at the highest tested concentration (12 micrograms/ml), with a clear dose response (r = 0.99). The various concentrations of cyclophosphamide also caused cytotoxicity in a dose-related fashion. Thus, this study demonstrates the usefulness of the cytokinesis-block method in V79 cells as a possible screen to analyze micronucleus induction and cytotoxicity. Because this approach is much less labor intensive than conducting a structural chromosomal analysis, this assay has great potential both as an initial screen for clastogenic activity and as a tool for investigating the underlying mechanisms for clastogenicity.

Genotoxic activity of organic chemicals in drinking water

The information summarized in this review provides substantial evidence for the widespread presence of genotoxins in drinking water. In many, if not most cases, the genotoxic activity can be directly attributed to the chlorination stage of drinking water treatment. The genotoxic activity appears to originate primarily from reactions of chlorine with humic substances in the source waters. Genotoxic activity in drinking water concentrates has been most frequently demonstrated using bacterial mutagenicity tests but results with mammalian cell assay systems are generally consistent with the findings from the bacterial assays. There is currently no evidence for genotoxic damage following in vivo exposures to animals. In some locations genotoxic contaminants of probable industrial and/or agricultural origin occur in the source waters and contribute substantially to the genotoxic activity of finished drinking waters. The method used for sample concentration can have an important bearing on study results. In particular, organic acids account for most of the mutagenicity of chlorinated drinking water, and their recovery from water requires a sample acidification step prior to extraction or XAD resin adsorption. Considerable work has been done to determine the identity of the compounds responsible for the mutagenicity of organic concentrates of drinking water. Recently, one class of acidic compounds, the chlorinated hydroxyfuranones, has been shown to be responsible for a major part of the mutagenic activity. Strategies for drinking water treatment that have been evaluated with respect to reduction of genotoxins in drinking water include granular activated carbon (GAC) filtration, chemical destruction, and the use of alternative means of treatment (i.e., ozone, chlorine dioxide, and monochloramine). GAC treatment has been found to be effective for removal of mutagens from drinking water even after the GAC is beyond its normal use for organic carbon removal. All disinfectant chemicals appear to have the capacity of forming mutagenic chemicals during water treatment. However, the levels of mutagenicity formed with the alternative disinfectants have been generally less than those seen with chlorine and, especially in the case of ozone, highly dependent on the source water.(ABSTRACT TRUNCATED AT 400 WORDS)

The in vivo effect of 2,6-xylidine on induction of micronuclei in mouse bone marrow cells

The ability of 2,6-xylidine to produce chromosome breakage and/or spindle malformation in vivo was evaluated by an assessment of the capacity of the compound to induce micronuclei in bone marrow polychromatic erythrocytes. Male ICR mice were administered a single oral dose of 350, 175 or 87.5 mg/kg of 2,6-xylidine by oral gavage and bone marrow was extracted from the femurs 24, 48 and 72 h thereafter. The frequency of micronuclei in animals treated with 2,6-xylidine was not different from that observed for the corresponding solvent treated controls.

Genetic toxicology: can we design predictive in vivo assays?

The present analysis examines the assumptions in, the perceptions and predictivity of and the need for short-term tests (STTs) for genotoxicity in light of recent findings that most noncarcinogens from the National Toxicology Program are genotoxic (i.e., positive in one or more in vitro STTs). Reasonable assumptions about the prevalence for carcinogens (1-10% of all chemicals), the sensitivity of these STTs (ca. 90% of all carcinogens are genotoxic) and their estimated "false positive" incidence (60-75%) imply that the majority of chemicals elicit genotoxic responses and, consequently, that most in vitro genotoxins are likely to be noncarcinogenic. Thus, either the usual treatment conditions used in these in vitro STTS are producing a large proportion of artifactual and meaningless positive results or else in vitro mutagenicity is too common a property of chemicals to serve as a useful predictor of carcinogenicity or other human risk. In contrast, the limited data base on in vivo STTs suggests that the current versions of these assays may have low sensitivity which appears unlikely to improve without dropping either their 'short-term' aspect or the rodent carcinogenicity benchmark. It is suggested that in vivo genotoxicity protocols be modified to take into consideration both the fundamentals of toxicology as well as the lessons learned from in vitro genetic toxicology. In the meantime, while in vivo assays are undergoing rigorous validation, genetic toxicology, as currently practiced, should not be a formal aspect of chemical or drug development on the grounds that it is incapable of providing realistic and reliable information on human risk. It is urged that data generated in new, unvalidated in vivo genotoxicity assays be exempted from the normal regulatory reporting requirements in order to encourage industry to participate in the laborious and expensive development of this next phase of genetic toxicology.

A simplified protocol for the mouse bone marrow micronucleus assay

Complex and expensive protocols involving multiple sampling times have been proposed and recommended for the in vivo mouse bone marrow micronucleus assay. The purpose of this study was to determine whether a simplified procedure employing two identical exposures and a single sampling time would be equally effective at detecting chemical clastogens. Furthermore, the utility of the protocol was investigated for both intraperitoneal and oral routes of administration. The results obtained from a group of chemicals spanning the known time range for maximum frequency of micronucleus induction prove the effectiveness of this simplified protocol.

A comparative analysis of data on the clastogenicity of 951 chemical substances tested in mammalian cell cultures

A literature review was conducted using original papers published during 1964-1985 on the in vitro clastogenicity of chemical substances. Results of tests on 951 chemical substances were abstracted from over 240 reports to form the database. The evaluation of these data relied on each author's original conclusion on a positive or negative outcome. Of these 951 substances, 447 (47%) were consistently positive either with or without activation; 417 (44%) were negative in the direct test but not tested with metabolic activation systems; 4 were negative but tested only with activation; and 30 (3%) were clearly negative both with and without activation. The remaining 53 substances gave variable results when tested under different experimental protocols or in different cell types, but were positive in at least one test. Although discrepant results were found associated with some cell types, the addition of metabolic activation systems tended to eliminate such variability. No one cell appeared to be superior in response to all clastogens. For screening purposes, the choice of cell may thus depend more on the general usefulness and reliability of a cell type than on a strong response to a particular chemical. However, the use of a suitable metabolic activation system does appear to be of critical importance. The concentration at which clastogenic effects were detected varied extensively for different test substances, ranging from a minimum of 4.3 X 10(-8) to 6.9 X 10(2) mM. Possible mechanisms of action for substances active at only high levels are discussed, but no satisfactory explanation is available at this time. The relevance of tests conducted at concentrations high enough to alter significantly the osmolarity and other culture conditions is considered, and caution urged in the interpretation of test results obtained under physiologically stressful conditions. The clastogenic potential was compared quantitatively using an index of effective concentration (D20) and one which estimates the number of cells with exchange aberrations expected per mg/ml (TR) for data obtained by using a uniform protocol and cultures of Chinese hamster lung (CHL) cells. Both values were distributed over a wide range, demonstrating the variety of genotoxic potential in chemicals. In general, a substance which was active at only high concentrations produced fewer exchange-type aberrations. In vivo activity, as measured by tumourigenic effect and formation of micronuclei in bone marrow, tended to be greater for substances with a D20 below 10(-2) mg/ml and a TR value over 10(3).(ABSTRACT TRUNCATED AT 400 WORDS)