Evaluation of phenobarbital/beta-naphthoflavone as an alternative S9-induction regime to Aroclor 1254 in the rat for use in in vitro genotoxicity assays

Revisiting the mutagenicity and genotoxicity of N-nitroso propranolol in bacterial and human in vitro assays

Propranolol is a widely used β-blocker that can generate a nitrosated derivative, N-nitroso propranolol (NNP). NNP has been reported to be negative in the bacterial reverse mutation test (the Ames test) but genotoxic in other in vitro assays. In the current study, we systematically examined the in vitro mutagenicity and genotoxicity of NNP using several modifications of the Ames test known to affect the mutagenicity of nitrosamines, as well as a battery of genotoxicity tests using human cells. We found that NNP induced concentration-dependent mutations in the Ames test, both in two tester strains that detect base pair substitutions, TA1535 and TA100, as well as in the TA98 frameshift-detector strain. Although positive results were seen with rat liver S9, the hamster liver S9 fraction was more effective in bio-transforming NNP into a reactive mutagen. NNP also induced micronuclei and gene mutations in human lymphoblastoid TK6 cells in the presence of hamster liver S9. Using a panel of TK6 cell lines that each expresses a different human cytochrome P450 (CYP), CYP2C19 was identified as the most active enzyme in the bioactivation of NNP to a genotoxicant among those tested. NNP also induced concentration-dependent DNA strand breakage in metabolically competent 2-dimensional (2D) and 3D cultures of human HepaRG cells. This study indicates that NNP is genotoxic in a variety of bacterial and mammalian systems. Thus, NNP is a mutagenic and genotoxic nitrosamine and a potential human carcinogen.

Replacing animal-derived components in in vitro test guidelines OECD 455 and 487

The evaluation of single substances or environmental samples for their genotoxic or estrogenic potential is highly relevant for human- and environment-related risk assessment. To examine the effects on a mechanism-specific level, standardized cell-based in vitro methods are widely applied. However, these methods include animal-derived components like fetal bovine serum (FBS) or rat-derived liver homogenate fractions (S9-mixes), which are a source of variability, reduced assay reproducibility and ethical concerns. In our study, we evaluated the adaptation of the cell-based in vitro OECD test guidelines TG 487 (assessment of genotoxicity) and TG 455 (detection of estrogenic activity) to an animal-component-free methodology. Firstly, the human cell lines A549 (for OECD TG 487), ERα-CALUX® and GeneBLAzer™ ERα-UAS-bla GripTite™ (for OECD TG 455) were investigated for growth in a chemically defined medium without the addition of FBS. Secondly, the biotechnological S9-mix ewoS9R was implemented in comparison to the induced rat liver S9 to simulate in vivo metabolism capacities in both OECD test guidelines. As a model compound, Benzo[a]pyrene was used due to its increased genotoxicity and endocrine activity after metabolization. The metabolization of Benzo[a]Pyrene by S9-mixes was examined via chemical analysis. All cell lines (A549, ERα-CALUX® and GeneBLAzer™ Erα-UAS-bla GripTite™) were successfully cultivated in chemically defined media without FBS. The micronucleus assay could not be conducted in chemically defined medium due to formation of cell clusters. The methods for endocrine activity assessment could be conducted in chemically defined media or reduced FBS content, but with decreased assay sensitivity. The biotechnological ewoS9R showed potential to replace rat liver S9 in the micronucleus in FBS-medium with A549 cells and in the ERα-CALUX® assay in FBS- and chemically defined medium. Our study showed promising steps towards an animal-component free toxicity testing. After further improvements, the new methodology could lead to more reproducible and reliable results for risk assessment.

The Alginate Immobilization of Metabolic Enzymes Platform Retrofits an Estrogen Receptor Transactivation Assay With Metabolic Competence

The U.S. EPA Endocrine Disruptor Screening Program utilizes data across the ToxCast/Tox21 high-throughput screening (HTS) programs to evaluate the biological effects of potential endocrine active substances. A potential limitation to the use of in vitro assay data in regulatory decision-making is the lack of coverage for xenobiotic metabolic processes. Both hepatic- and peripheral-tissue metabolism can yield metabolites that exhibit greater activity than the parent compound (bioactivation) or are inactive (bioinactivation) for a given biological target. Interpretation of biological effect data for both putative endocrine active substances, as well as other chemicals, screened in HTS assays may benefit from the addition of xenobiotic metabolic capabilities to decrease the uncertainty in predicting potential hazards to human health. The objective of this study was to develop an approach to retrofit existing HTS assays with hepatic metabolism. The Alginate Immobilization of Metabolic Enzymes (AIME) platform encapsulates hepatic S9 fractions in alginate microspheres attached to 96-well peg lids. Functional characterization across a panel of reference substrates for phase I cytochrome P450 enzymes revealed substrate depletion with expected metabolite accumulation. Performance of the AIME method in the VM7Luc estrogen receptor transactivation assay was evaluated across 15 reference chemicals and 48 test chemicals that yield metabolites previously identified as estrogen receptor active or inactive. The results demonstrate the utility of applying the AIME method for identification of false-positive and false-negative target assay effects, reprioritization of hazard based on metabolism-dependent bioactivity, and enhanced in vivo concordance with the rodent uterotrophic bioassay. Integration of the AIME metabolism method may prove useful for future biochemical and cell-based HTS applications.

Direct Comparison of the Lowest Effect Concentrations of Mutagenic Reference Substances in Two Ames Test Formats

The Ames assay is the standard assay for identifying DNA-reactive genotoxic substances. Multiple formats are available and the correct choice of an assay protocol is essential for achieving optimal performance, including fit for purpose detection limits and required screening capacity. In the present study, a comparison of those parameters between two commonly used formats, the standard pre-incubation Ames test and the liquid-based Ames MPF™, was performed. For that purpose, twenty-one substances with various modes of action were chosen and tested for their lowest effect concentrations (LEC) with both tests. In addition, two sources of rat liver homogenate S9 fraction, Aroclor 1254-induced and phenobarbital/β-naphthoflavone induced, were compared in the Ames MPF™. Overall, the standard pre-incubation Ames and the Ames MPF™ assay showed high concordance (>90%) for mutagenic vs. non-mutagenic compound classification. The LEC values of the Ames MPF™ format were lower for 17 of the 21 of the selected test substances. The S9 source had no impact on the test results. This leads to the conclusion that the liquid-based Ames MPF™ assay format provides screening advantages when low concentrations are relevant, such as in the testing of complex mixtures.

Is a liver comparable to a liver? A comparison of different rat-derived S9-fractions with a biotechnological animal-free alternative in the Ames fluctuation assay

Metabolism has to be considered during the toxicological assessment of chemical and environmental samples because it is an important process in the mammalian liver. It can be assessed in vitro via liver homogenates called S9-fractions, an external metabolic activation system. However, the external metabolic activation systems can vary greatly in their composition due to biological variations among individual animals and animal strains that the S9-fraction are derived as well as the differences in the production treatment. To gain more insight into these variances, three different but commonly used rat-derived S9-fractions were compared in the present study for their variance and performance with a reference compound in the Ames fluctuation assay with Salmonella typhimurium strains TA 98 and TA 100 according to ISO 11350. Severe shortcomings of conventional rat-derived S9-fractions were observed in the present study, such that S9-fractions differed significantly within the same rat strain and for different types of induction procedures in regards to the metabolic capability. An intrinsic mutagenic potential of the three rat-derived S9-fractions were identified in the Ames fluctuation assay with varying S9-fraction concentrations. To address some of the shortcomings of the animal-derived S9-fraction, the present study investigated the use and performance of a biotechnological, animal-free alternative, ewoS9R, in comparison to one of the rat-derived S9-fraction as the others showed a mutagenic potential themselves. Specifically, 12 different chemicals were used as a reference to determine if ewoS9R could serve as an adequate and more consistent replacement of traditional rat-derived metabolic activation systems: 8 pro-mutagenic compounds (i.e., require metabolic activation to show a mutagenic potential), one pro-mutagenic compound but not in the tested strains, one mutagenic compound without metabolic activation and two compounds that are equivocal in the literature. EwoS9R was evaluated as a promising approach in the Ames fluctuation assay with 5 compounds observed to have similar results with both rat-derived S9-fraction and ewoS9R (41%), for 3 compounds ewoS9R was a better metabolization system than the rat-derived S9-fraction (16%). Further research is necessary to determine the full potential of ewoS9R in comparison to rat-derived S9-fractions.

Optimization of a pre-metabolization procedure using rat liver S9 and cell-extracted S9 in the Ames fluctuation test

Many environmental pollutants pose a toxicological hazard only after metabolic activation. In vitro bioassays using cell lines or bacteria have often no or reduced metabolic activity, which impedes their use in the risk assessment. To improve the predictive capability of in vitro assays, external metabolization systems like the liver S9 fraction are frequently combined with in vitro toxicity assays. While it is typical for S9 fractions that samples and testing systems are combined in the same exposure system, we propose to separate the metabolism step and toxicity measurement. This allows for a modular combination of metabolic activation by enzymes isolated from rat liver (S9) or a biotechnological alternative (ewoS9^R) with in vitro bioassays that lack metabolic capacity. Benzo(a)pyrene and 2-aminoanthracene were used as model compounds to optimize the conditions for the S9 metabolic degradation/activation step. The Ames assay with Salmonella typhimurium strains TA98 and TA100 was applied to validate the set-up of decoupling the S9 activation/metabolism from the bioassay system. S9 protein concentration of 0.25 mg_protein/mL, a supplement of 0.13 mM NADPH and a pre-incubation time of 100 min are recommended for activation of samples prior to dosing them to in vitro bioassays using the regular dosing protocols of the respective bioassay. EwoS9^R performed equally well as Moltox S9, which is a step forward in developing true animal-free in vitro bioassays. After pre-incubation with S9 fraction, chemicals induced bacteria revertants in both the TA98 and the TA100 assay as efficiently as the standard Ames assay. The pre-incubation of chemicals with S9 fraction could serve for a wide range of cellular in vitro assays to efficiently combine activation and toxicity measurement, which may greatly facilitate the application of these assays for chemical hazard assessment and monitoring of environmental samples.

2-Hydroxypyridine-N-oxide (HOPO): Equivocal in the ames assay

2-Hydroxypyridine-N-oxide (HOPO) is a useful coupling reagent for synthesis of active pharmaceutical ingredients. It has been reported to be weakly mutagenic in the Ames assay (Ding W et al. []: J Chromatogr A 1386:47-52). According to the ICH M7 guidance (2014) regarding control of mutagenic impurities to limit potential carcinogenic risk, mutagens require control in drug substances such that exposure not exceeds the threshold of toxicological concern. Given the weak response observed in the Ames assay and the lack of any obvious structural features that could confer DNA reactivity we were interested to determine if the results were reproducible and investigate the role of potentially confounding experimental parameters. Specifically, Ames tests were conducted to assess the influence of compound purity, solvent choice, dose spacing, toxicity, type of S9 (aroclor vs phenobarbital/β-napthoflavone), and lot variability on the frequency of HOPO induced revertant colonies. Initial extensive testing using one lot of HOPO produced no evidence of mutagenic potential in the Ames assays. Subsequent studies with four additional lots produced conflicting results, with an ∼2.0-fold increase in revertant colonies observed. Given the rigor of the current investigation, lack of reproducibility between lots, and the weak increase in revertants, it is concluded that HOPO is equivocal in the bacterial reverse mutation assay. It is highly unlikely that HOPO poses a mutagenic risk in vivo; therefore, when it is used as a reagent in pharmaceutical synthesis, it should not be regarded as a mutagenic impurity, but rather a normal process related impurity. Environ. Mol. Mutagen. 59:312-321, 2018. © 2018 Wiley Periodicals, Inc.

Resolution of contradiction between in silico predictions and Ames test results for four pharmaceutically relevant impurities

The ICH M7 Guideline requires low level control of mutagenic impurities in pharmaceutical products to minimize cancer risk in patients (ICHM7, 2014). Bacterial mutagenicity (Ames) data is generally used to determine mutagenic and possible carcinogenic potential of compounds. Recently, a publication on experiences of using two in silico systems to identify potentially mutagenic impurities highlighted the importance of performing a critical review of published Ames data utilized as part of a mutagenicity assessment of impurities (Greene et al., 2015). Four compounds (2-amino-5-hydroxybenzoic acid, 2-amino-3-chlorobenzoic acid, methyl 2-amino-4-chlorobenzoate and 4-morpholinopyridine) reported mutagenic were identified in a two system in silico assessment and expert review of the structuresas non-mutagenic. Likely reasons for mutagenicity could not be identified and the purity of the compounds tested was proposed. In the current investigation, the purest available sample of the four compounds was tested in an OECD-compliant Ames test. The compounds were all found to be non-mutagenic. Possible reasons for the discrepancy between previously reported and current results are discussed. Additionally, important points to consider when conducting an expert review of available Ames data are provided particularly in cases where reported Ames results are discrepant with a two system in silico assessment.

Application of the TGx-28.65 transcriptomic biomarker to classify genotoxic and non-genotoxic chemicals in human TK6 cells in the presence of rat liver S9

In vitro transcriptional signatures that predict toxicities can facilitate chemical screening. We previously developed a transcriptomic biomarker (known as TGx-28.65) for classifying agents as genotoxic (DNA damaging) and non-genotoxic in human lymphoblastoid TK6 cells. Because TK6 cells do not express cytochrome P450s, we confirmed accurate classification by the biomarker in cells co-exposed to 1% 5,6 benzoflavone/phenobarbital-induced rat liver S9 for metabolic activation. However, chemicals may require different types of S9 for activation. Here we investigated the response of TK6 cells to higher percentages of Aroclor-, benzoflavone/phenobarbital-, or ethanol-induced rat liver S9 to expand TGx-28.65 biomarker applicability. Transcriptional profiles were derived 3 to 4 hr following a 4 hr co-exposure of TK6 cells to test chemicals and S9. Preliminary studies established that 10% Aroclor- and 5% ethanol-induced S9 alone did not induce the TGx-28.65 biomarker genes. Seven genotoxic and two non-genotoxic chemicals (and concurrent solvent and positive controls) were then tested with one of the S9s (selected based on cell survival and micronucleus induction). Relative survival and micronucleus frequency was assessed by flow cytometry in cells 20 hr post-exposure. Genotoxic/non-genotoxic chemicals were accurately classified using the different S9s. One technical replicate of cells co-treated with dexamethasone and 10% Aroclor-induced S9 was falsely classified as genotoxic, suggesting caution in using high S9 concentrations. Even low concentrations of genotoxic chemicals (those not causing cytotoxicity) were correctly classified, demonstrating that TGx-28.65 is a sensitive biomarker of genotoxicity. A meta-analysis of datasets from 13 chemicals supports that different S9s can be used in TK6 cells, without impairing classification using the TGx-28.65 biomarker.

The utility of metabolic activation mixtures containing human hepatic post-mitochondrial supernatant (S9) for in vitro genetic toxicity assessment

In vitro genotoxicity assessment routinely employs an exogenous metabolic activation mixture to simulate mammalian metabolism. Activation mixtures commonly contain post-mitochondrial liver supernatant (i.e. S9) from chemically induced Sprague Dawley rats. Although Organization for Economic Cooperation and Development (OECD) test guidelines permit the use of other S9 preparations, assessments rarely employ human-derived S9. The objective of this study is to review and evaluate the use of human-derived S9 for in vitro genetic toxicity assessment. All available published genotoxicity assessments employing human S9 were compiled for analysis. To facilitate comparative analyses, additional matched Ames data using induced rat liver S9 were obtained for certain highly cited chemicals. Historical human and induced rat S9 quality control reports from Moltox were obtained and mined for enzyme activity and mutagenic potency data. Additional in vitro micronucleus data were experimentally generated using human and induced rat S9. The metabolic activity of induced rat S9 was found to be higher than human S9, and linked to high mutagenic potency results. This study revealed that human S9 often yields significantly lower Salmonella mutagenic potency values, especially for polycyclic aromatic hydrocarbons, aflatoxin B1 and heterocyclic amines (~3- to 350-fold). Conversely, assessment with human S9 activation yields higher potency for aromatic amines (~2- to 50-fold). Outliers with extremely high mutagenic potency results were observed in the human S9 data. Similar trends were observed in experimentally generated mammalian micronucleus cell assays, however human S9 elicited potent cytotoxicity L5178Y, CHO and TK6 cell lines. Due to the potential for reduced sensitivity and the absence of a link between enzyme activity levels and mutagenic potency, human liver S9 is not recommended for use alone in in vitro genotoxicity screening assays; however, human S9 may be extremely useful in follow-up tests, especially in the case of chemicals with species-specific metabolic differences, such as aromatic amines.

Dose-response assessment of naphthalene-induced genotoxicity and glutathione detoxication in human TK6 lymphoblasts

The dose-response relationship for the induction of micronuclei (MN) and the impact of glutathione (GSH) detoxication on naphthalene-induced cytotoxicity and genotoxicity were investigated in human TK6 cells. TK6 cells were exposed to 10 concentrations ranging from 0.0625 to 30μM naphthalene in the presence of β-naphthoflavone- and phenobarbital (βNP/PB)-induced rat liver S9 with a nicotinamide adenine dinucleotide phosphate-generating system. Three approaches were used to identify a no-observed-effect level (NOEL) for naphthalene-induced genotoxicity: (1) laboratory criteria of ≥ twofold increase over the concurrent solvent controls (NOEL = 10μM), (2) ANOVA with Bonferroni correction (NOEL = 2.5μM), and (3) the benchmark dose approach (BMCL(10) = 3.35μM). The NOEL and point of departure micronucleus frequency for naphthalene-induced MN are between the tested naphthalene concentrations of 2.5-10.0μM in this experimental system. Supplementation of the exposure system with physiological relevant concentrations of 5mM GSH eliminated naphthalene-induced cytotoxicity and genotoxicity; no increased cytotoxicity or genotoxicity was observed at concentrations of up to 500μM naphthalene in the presence of GSH compared with 2.5-10.0μM in the absence of GSH. Naphthalene bioactivation by βNP/PB-induced rat liver S9 exhibits a nonlinear dose-response for the induction of MN in TK6 cells with a NOEL of 2.5-10μM that in the presence of GSH is shifted upward greater than 50- to 200-fold. These data demonstrate a nonlinear dose-response for naphthalene-induced genotoxicity that is eliminated by GSH, and both observations should be considered when assessing human risk from naphthalene exposures.

Genotoxicity testing and biomarker studies on surface waters: an overview of the techniques and their efficacies

Rapid industrialization, use of modern agriculture practices, and fast urbanization vis-a-vis indiscriminate use of xenobiotics have led to the serious problems of water pollution in India and abroad. The complexicity of the pollutants in environmental samples demands a multitude of genotoxicity testing with increasing simplicity, sensitivity, and affordability. Moreover, various pollutants mutually affect their own toxic behavior, which complicates the problem of risk assessment. An overview, highlighting the genotoxicity testing system, such as Ames plate incorporation test, Ames fluctuation test, E. coli survival assay, Allium cepa toxicity/genotoxicity test, comet assay, and plasmid nicking assay, is presented in this article, and a comparison has been made to estimate the efficacy of these genotoxicity bioassays performed on some surface waters. Some work on toxicity biomarkers vis-a-vis studies on surface waters has also been included in the present review.

Arecoline N-oxide: its mutagenicity and possible role as ultimate carcinogen in areca oral carcinogenesis

The areca nut is the most widely consumed psychoactive substance in Taiwan, India, and Southeast Asia. It is considered to be an environmental risk factor for the development of oral submucous fibrosis and cancer. Arecoline, the major alkaloid of areca nut, has been known to cause cytotoxicity and genotoxicity in various systems. However, the active compound accounting for arecoline-induced damage in normal human oral cells is still uncharacterized. The present study was undertaken to identify the active metabolite of arecoline that might induce damage in human oral tissues and cause mutagenicity in Salmonella typhimurium tester strains TA 100 and TA 98. It is interesting to find that the major metabolite of arecoline, arecoline N-oxide, is moderately mutagenic to these Salmonella tester strains. This mutagenicity was potently inhibited by sulfhydryl compounds, namely, glutathione, N-acetylcysteine, and cysteine, whereas methionine is inactive in this inhibition. The mutagenicity of arecoline N-oxide was strongly inhibited by the N-oxide reducing agent titanium trichloride. The possible role of arecoline N-oxide in the induction of oral carcinogenesis by areca nut chewing is discussed.

Toward refinement of the colony-forming unit-granulocyte/macrophage clonogenic assay: Inclusion of a metabolic system

This work represents a first attempt to refine the colony-forming unit-granulocyte/macrophage (CFU-GM) clonogenic assay by incorporating liver microsomes and co-factors as a metabolic system into the in vitro test system in response to an ECVAM recommendation. From the comparison of results obtained with the CFU-GM clonogenic assay currently used and with the new experimental protocol, different toxicity on granulocyte/macrophage precursors was demonstrated, when drugs with a known metabolism in vivo were tested.

Salmonella/human S9 mutagenicity test: a collaborative study with 58 compounds

A large and extensive body of data on the use of human liver S9 fractions in the Salmonella mutagenicity test (Ames test) is presented; the data were obtained from a collaborative study by JEMS/BMS (Bacterial Mutagenicity Test Study Group) members and the Human and Animal Bridging Research Organization (HAB). In this study, the mutagenicity of 58 chemicals, many of which were judged to be human carcinogens by the IARC, was determined by the Ames test (the pre-incubation method at 37 degrees C for 20 min) in the presence of a selected human liver S9 fraction with a high drug-metabolic activity or a pooled human liver S9 fraction with a moderate drug-metabolic activity. For reference, mutagenicity was also examined in the presence of a phenobarbital/5,6-benzoflavone-pretreated rat liver S9 fraction, which is normally used in mutagenicity testing systems. The bacterial test strains consisted of Salmonella typhimurium TA100, TA98 or YG7108. The data indicated that the mutagenicity of chemicals in the rat and human liver S9 fractions varied considerably, depending on the chemicals in question. In addition, a large inter-individual diversity in the mutagenic response to mutagens, depending on the chemical structures of the mutagens, was also demonstrated using two selected human S9 fractions. Most of the mutagens tested in this study (75%; 36 out of 48 compounds that were judged to be mutagenic in at least one S9 fraction) were less mutagenic in the presence of the two human S9 fractions than in the presence of the rat S9 fraction. On the other hand, the other compounds (25%), including some aromatic amines and nitrosamines, showed a more potent mutagenicity in the presence of either one of the two human S9 fractions than in the presence of the rat S9 fraction. These data strongly suggest that the use of human liver S9 fraction in mutagenicity testing systems may be useful for a better understanding of the mutagenic effects of chemicals on humans.

Use of human liver S9 in the Ames test: assay of three procarcinogens using human S9 derived from multiple donors

The purpose of the present study was to examine the inter-individual variation in the mutagenicity of chemicals using a variety of human S9 fractions. For this purpose, three procarcinogens, 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), benzo[a]pyrene (BP), and dimethylnitrosamine (DMN), were selected for the Ames test and their mutagenicity was examined using human liver S9 fractions prepared from 18 different donors and one pooled liver S9 fraction prepared from 15 different donors. In addition, rat S9 fraction prepared from male rats pretreated with phenobarbital and 5,6-benzoflavone (PB/BF) was used as reference in order to examine the mutagenic differences between human and rat (PB/BF) S9 fractions. The data demonstrate a large inter-individual diversity in the mutagenic response to procarcinogens. The mutagenicity of IQ and BP in the presence of a human liver S9 fraction (lot HLS-014) was equal to that observed in the presence of rat (PB/BF) S9 fraction. The mutagenicity of IQ and BP in the presence of a pooled human liver S9 fraction was lower (90 and 95%, respectively) than that observed in the presence of rat (PB/BF) S9. On the contrary, the mutagenicity of DMN in the presence of either a selected human liver S9 fraction (lot HLS-014) or pooled fraction was 8-fold higher than that found in the presence of rat (PB/BF) S9 fraction. Human liver S9 fraction (lot HLS-014) had one of the highest cytochrome P450 enzyme activities among the 18 different donors and higher than the pooled human liver S9 fraction. These results suggest that the use of both selected human liver S9 fractions with high metabolic activity (e.g., lot HLS-014 as used in this study) and a pooled S9 fraction with moderate metabolic activity could be used as a means to evaluate the inter-individual variability in mutagenic response to chemicals and to confirm positive responses from studies completed with rodent S9.

An improvement of the Ames test using a modified human liver S9 preparation

INTRODUCTION

In preliminary studies on the Ames test using human liver S9 fractions, we found that the crude human S9 fractions, obtained following centrifugation of the tissue homogenate for 20 min at 9000 x g, were not always sterile. When this was the case, the S9 fractions were often accompanied by an increased number of colonies above the normal range on plates in the solvent control used in the Ames test. In addition, we also sometimes identified the incorporation of a small amount of fat in the crude human liver S9 fractions. We have therefore obtained a purified fat-free S9 fraction by a simple modification to the crude S9 preparation; fat was completely removed by centrifugation of the crude S9 fraction.

METHODS

Using the purified and crude human S9 fractions (two lots each), both the sterility and the number of bacterial colonies produced on a plate with five bacterial tester strains by solvent controls (purified water and dimethyl sulfoxide) were examined. The findings were then compared to those observed with phosphate buffer or S9 fraction from rats pretreated with phenobarbital/5,6-benzoflavone.

RESULTS

The data show that each of the crude human S9 fractions was not sterile and produced an increasing number of colonies with each solvent control, almost equal to the sum of the numbers of contaminating bacterial colonies and spontaneous revertant colonies observed with phosphate buffer or the rat S9 fraction. On the other hand, both the purified human S9 fractions were sterile, and the number of colonies that appeared in each solvent control was similar to that of spontaneous revertant colonies observed with phosphate buffer or the rat S9 fraction.

DISCUSSION

These results indicate that this new procedure of S9 preparation, modified with an additional recentrifugation step, may provide a high quality of purified fat- and bacteria-free S9 fraction for use in the Ames test.

Occupational hazard evaluation of p-bromobenzyl bromide from tests for genotoxicity

As part of an occupational hazard evaluation, p-bromobenzyl bromide (p-BBB) was evaluated for genotoxic activity in the Ames microbial mutagenicity assay, the alkaline elution assay for DNA strand breaks in rat hepatocytes and the in vitro chromosome aberration assay in Chinese hamster ovary cells. The compound produced equivocal results in the microbial mutagenicity assay but was negative in the alkaline elution assay for DNA strand breaks in rat hepatocytes. The compound produced weakly positive results in the in vitro chromosome aberration assay. There was substantial cytotoxicity in all three assays. It is concluded that p-BBB is weakly genotoxic.

An assessment of the genetic toxicology of antimony trioxide

Antimony trioxide (Sb2O3, CAS 1309-64-4) has been examined in a range of in vitro and in vivo genotoxicity assays. Negative results were obtained with the Salmonella/microsome assay and the L5178Y mutation assay, but a positive response was observed in the in vitro cytogenetic assay using isolated human peripheral lymphocytes. However, in vivo, antimony trioxide was non-clastogenic in the mouse bone marrow micronucleus assay, following oral gavage administration for 1, 7, 14 or 21 days at dose levels of up to 5000 mg/kg (single dose) or 1000 mg/kg (repeat dose). A negative result was also obtained in the in vivo rat liver DNA repair (unscheduled DNA synthesis) assay following a single oral gavage administration of doses up to 5000 mg/kg. These data show no genotoxicity for antimony trioxide in vivo and do not confirm a previous report of clastogenicity in the mouse on repeated dosing. It is concluded that antimony trioxide is not genotoxic in vivo and does not present a genotoxic hazard to humans.

Thalidomide: lack of mutagenic activity across phyla and genetic endpoints

The human and rabbit teratogen thalidomide has been tested for mutagenicity in a wide range of assays, ranging from bacterial gene mutation assays conducted in vitro to in vivo cytogenetic assays conducted using rabbits, and including a variety of human-derived tissues. Thalidomide was not mutagenic to 6 strains of Salmonella when tested both in the presence and absence of Aroclor-induced rat liver S9 mix. This inactivity was confirmed in strains TA98 and TA100 using a 1-h pre-incubation assay protocol with the same S9 mix (10% S9), and additionally, in strain TA98 using 3 concentrations of S9 (4%, 10% and 30% S9 in S9 mix). Thalidomide was not clastogenic either to cultured human lymphocytes (whole blood cultures, minus S9 mix) or to Chinese hamster ovary (CHO) cells treated in vitro. Further, no cytotoxicity was observed in purified human lymphocytes when exposed to thalidomide up to the limit of its solubility in the medium in the presence and absence of liver S9 from Aroclor-induced pregnant rabbit. The CHO assays were conducted without metabolic activation and in the presence of a variety of sources of auxiliary metabolic activation (PB/beta NP-induced rat liver S9 mix, pooled male and female human liver S9 mix, uninduced and Aroclor-induced pregnant rabbit liver S9 mix and foetal rabbit S9 mix). Thalidomide did not induce micronuclei in isolated human lymphocytes (minus S9 mix) and it was non-mutagenic to mouse lymphoma L5178Y TK+/- cells when tested to the limits of its solubility in the culture medium (+/- S9 mix). No indication of recombinogenic or clastogenic activity was observed for thalidomide when tested in Drosophila. In addition, it failed to induce chromosome aberrations in grasshopper neuroblasts when tested in the presence and absence of Aroclor-induced rat liver S9 mix. Some unusual chromosome morphologies were observed in the grasshopper cytogenetic preparations indicating a potential of thalidomide to interact with chromosomal proteins. However, this potential was not evident in the human lymphocyte micronucleus assay, and thalidomide was apparently not reactive to the proteins of the mouse skin, as it gave negative results in a mouse local lymph node assay for skin sensitizing agents. Thalidomide was inactive in bone marrow micronucleus assays conducted using males and females from two strains of mice, and female New Zealand white rabbits. It is concluded that thalidomide is neither a mutagen nor an aneugen. This conclusion is discussed within the context of the results of earlier mutagenicity studies, the recent claim that thalidomide may be a heritable germ cell mutagen to humans, and the current interest in thalidomide for the treatment of immune system-related diseases.

Human liver S-9 metabolic activation: proficiency in cytogenetic assays and comparison with phenobarbital/beta-naphthoflavone or aroclor 1254 induced rat S-9

Induced rat liver S-9 is routinely used for metabolic activation in cytogenetic assays. When a compound gives a positive test result only with rat S-9, the significance for humans should be assessed. To evaluate the use of human S-9, we used sister-chromatid exchanges (SCEs) and chromosome aberrations (Abs) in Chinese hamster ovary cells to test five pro-mutagens, each preferentially activated by a different family of P-450: benzo(a)pyrene (BP), dimethylnitrosamine (DMN), diethylnitrosamine (DEN), aflatoxin B1 (AFB), and 2-acetylaminofluorene (2-AAF). We tested two human S-9 preparations, one from a single liver and a second pooled from two livers known to have good activity for several P-450s. Concentrations and ratios of NADP and isocitrate were adjusted to optimize NADPH generation by the S-9. Abs were scored 20 hr, and SCEs 29-45 hr, after the beginning of a 3 hr treatment. P-450 enzyme activities were generally higher in rat than human S-9. With the single-liver human S-9, increase in SCEs were seen with all chemicals; with both human S-9s, increases in Abs were seen with all chemicals except BP. (The level of P-450 1A1, required for BP activation, is very low in human liver.) Compared with rat S-9, generally higher concentrations of human S-9 and of promutagens were required to see positive results. However, human S-9 effectively activated 2-AAF, whereas neither of the two types of rat S-9 produced Abs with 2-AAF. We also compared rat S-9s induced with Aroclor 1254 or phenobarbital/ beta-naphthoflavone (PB/beta NF). Although there were some differences in P-450 enzyme activities, these did not translate into differences in Abs induction. At low doses of AFB and of BP, PB/beta NF induced S-9 appeared more effective than Aroclor 1254 induced S-9.