The use of benchmark dose uncertainty measurements for robust comparative potency analyses

The Benchmark Dose (BMD) method is the favored approach for quantitative dose-response analysis where uncertainty measurements are delineated between the upper (BMDU) and lower (BMDL) confidence bounds, or confidence intervals (CIs). Little has been published on the accurate interpretation of uncertainty measurements for potency comparative analyses between different test conditions. We highlight this by revisiting a previously published comparative in vitro genotoxicity dataset for human lymphoblastoid TK6 cells that were exposed to each of 10 clastogens in the presence and absence (+/-) of low concentration (0.25%) S9, and scored for p53, γH2AX and Relative Nuclei Count (RNC) responses at two timepoints (Tian et al., 2020). The researchers utilized BMD point estimates in potency comparative analysis between S9 treatment conditions. Here we highlight a shortcoming that the use of BMD point estimates can mischaracterize potency differences between systems. We reanalyzed the dose responses by BMD modeling using PROAST v69.1. We used the resulting BMDL and BMDU metrics to calculate "S9 potency ratio confidence intervals" that compare the relative potency of compounds +/- S9 as more statistically robust metrics for comparative potency measurements compared to BMD point estimate ratios. We performed unsupervised hierarchical clustering that identified four S9-dependent groupings: high and low-level potentiation, no effect, and diminution. This work demonstrates the importance of using BMD uncertainty measurements in potency comparative analyses between test conditions. Irrespective of the source of the data, we propose a stepwise approach when performing BMD modeling in comparative potency analyses between test conditions.

Evaluation of non-commercial models for genotoxicity and carcinogenicity in the assessment of EFSA's databases

Over the past years, the European Food Safety Authority (EFSA) released to the public domain several databases, with the main objectives of collecting and storing hazard data on the substances considered in EFSA's risk assessment and secondly to serve as a basis for further development of in silico tools such as quantitative structure-activity relationship (QSAR) models. In this work, we evaluated the ability of freely available QSAR models to estimate genotoxicity and carcinogenicity properties and their possible use for screening purposes on three different EFSA's databases. With an accuracy close to 90%, the results showed good capabilities of QSAR models to predict genotoxicity in terms of bacterial reverse mutation test, while statistics for in vivo micronucleus test are not satisfactory (accuracy in the predictions close to 50%). Interestingly, results on the carcinogenicity assessment showed an accuracy in prediction close to 70% for the best models. In addition, an example of the potential application of in silico models is presented in order to provide a preliminary screening of genotoxicity properties of botanicals intended for use as food supplements.

Dose–Response and Thresholds in Mutagenicity Studies: A Statistical Testing Approach

The analysis of dose-response relationships is an important objective in toxicology, and one in which both modelling and testing approaches are used. One particular question is whether a threshold exists at low doses. The concept of a pragmatic threshold is used, i.e. low doses with biologically unimportant effects are assumed to be threshold doses. "Biologically unimportant" means, in statistical terms, a lower effect than the effect of the negative control, or at least a just-tolerable margin delta higher than the effect of the negative control. Therefore, threshold doses can be tested in terms of a one-sided hypothesis of equivalence. A new approach is proposed, assuming, at the least, that the low dose is a threshold dose, and the highest dose is superior to the negative control. By analogy to the k-fold rule commonly used in mutagenicity studies, tests on ratio-to-control are used. The a priori definition of the threshold margin is inherently needed. A further approach proposes the analysis of dose-response relationships by means of order-restricted inference (the so-called trend test). A modification of a multiple-contrast test is used, in which only those contrasts are included that are sensitive for no effects at low doses. A further modification treats the complicated, but real, problem of simultaneous existence of a threshold, a monotonic increase, and a downturn effect at high dose(s). A parametric procedure is considered, together with an extension for proportions. The important problem of a priori sample size definition is discussed. The approaches are demonstrated by means of examples based on real data.

The Role of Control Groups in Mutagenicity Studies: Matching Biological and Statistical Relevance

The statistical test of the conventional hypothesis of "no treatment effect" is commonly used in the evaluation of mutagenicity experiments. Failing to reject the hypothesis often leads to the conclusion in favour of safety. The major drawback of this indirect approach is that what is controlled by a prespecified level alpha is the probability of erroneously concluding hazard (producer risk). However, the primary concern of safety assessment is the control of the consumer risk, i.e. limiting the probability of erroneously concluding that a product is safe. In order to restrict this risk, safety has to be formulated as the alternative, and hazard, i.e. the opposite, has to be formulated as the hypothesis. The direct safety approach is examined for the case when the corresponding threshold value is expressed either as a fraction of the population mean for the negative control, or as a fraction of the difference between the positive and negative controls.

Toxic potentials of ten herbs commonly used for aphrodisiac effect in Turkey

BACKGROUND/AIM

Sexual dysfunction is a serious problem worldwide. In Turkey, herbal products are used by some people suffering from sexual dysfunction. Despite their therapeutic advantages, some constituents of herbs are potentially toxic and pose health risks because they can be bought from the market without a prescription. Therefore, we aimed to determine the safety of herbs possessing aphrodisiac effects, chosen on the basis of their frequency of medicinal use and commercial importance in Turkey.

MATERIALS AND METHODS

Ten herbs (Anethum graveolens, Carthamus tinctorius, Citrus aurantium, Cocos nucifera, Glycyrrhiza glabra, Melissa officinalis, Nigella arvensis, Pinus pinea, Prunus mahaleb, and Zingiber officinale) were extracted with water, methanol, and chloroform. The cyto- and genotoxic potentials of the extracts were assessed using an MTT test on a rat kidney cell line and an Ames assay in Salmonella typhimurium strains, respectively.

RESULTS

In the cytotoxic evaluation, IC50 values were 1.51-31.4 mg/mL for the methanol and chloroform extracts, while the water extracts were not cytotoxic. In the genotoxic evaluation, it was revealed that the water extracts had more mutagenic activity than the chloroform and methanol extracts. Water extract of M. officinalis was shown to have the most genotoxic activities to TA100 (±S9) and TA98 (-S9).

CONCLUSION

These results might be useful in determining the toxic effects of herbs and lead to precautions being taken in regards to their consumption.

The resolving power of in vitro genotoxicity assays for cigarette smoke particulate matter

In vitro genotoxicity assays are often used to compare tobacco smoke particulate matter (PM) from different cigarettes. The quantitative aspect of the comparisons requires appropriate statistical methods and replication levels, to support the interpretation in terms of power and significance. This paper recommends a uniform statistical analysis for the Ames test, mouse lymphoma mammalian cell mutation assay (MLA) and the in vitro micronucleus test (IVMNT); involving a hierarchical decision process with respect to slope, fixed effect and single dose comparisons. With these methods, replication levels of 5 (Ames test TA98), 4 (Ames test TA100), 10 (Ames test TA1537), 6 (MLA) and 4 (IVMNT) resolved a 30% difference in PM genotoxicity.

International round-robin study on the Ames fluctuation test

An international round-robin study on the Ames fluctuation test [ISO 11350, 2012], a microplate version of the classic plate-incorporation method for the detection of mutagenicity in water, wastewater and chemicals was performed by 18 laboratories from seven countries. Such a round-robin study is a precondition for both the finalization of the ISO standardization process and a possible regulatory implementation in water legislation. The laboratories tested four water samples (spiked/nonspiked) and two chemical mixtures with and without supplementation of a S9-mix. Validity criteria (acceptable spontaneous and positive control-induced mutation counts) were fulfilled by 92-100%, depending on the test conditions. A two-step method for statistical evaluation of the test results is proposed and assessed in terms of specificity and sensitivity. The data were first subjected to powerful analysis of variance (ANOVA) after an arcsine-square-root transformation to detect significant differences between the test samples and the negative control (NC). A threshold (TH) value based on a pooled NC was then calculated to exclude false positive test results. Statistically, positive effects observed by the William's test were considered negative, if the mean of all replicates of a sample did not exceed the calculated TH. By making use of this approach, the overall test sensitivity was 100%, and the test specificity ranged from 80 to 100%.

A robust estimator of mutation rates

Fluctuation analysis is an established and widely used technique of estimating mutation rates in cultured cells. This paper presents a modified median estimator of mutation rates, which is novel because it allows for unequal population sizes N(t) of the parallel cultures, and helps to detect and reduce the estimation variability. Simulation results show a good accuracy and robustness of the modified median estimator compared with the median estimator and the maximum likelihood estimator. The proposed estimator, based on the Luria-Delbrück model, is applied to 20 yeast datasets collected during 3 different days for a study of chromosome loss and recombination in wild-type Saccharomyces cerevisiae strains. The estimates obtained display among-experiment variability, which is inflated with respect to the model predictions on simulated data. Further investigation in S. cerevisiae and Escherichia coli uncovers an empirical inverse relationship between the population sizes N(t) and the mutation rate estimates under certain experimental conditions. The impact of these effects on the practice of fluctuation analysis is discussed.

Statistical analysis of in vitro data for risk assessment - exemplified for a case of Ames test data

Ames test data of experiments with smoke of six cigarette types were used for dose-response analysis and for derivation of a measure of mutagenic potency. Each cigarette type had been tested using a smoking machine and four dilutions of the smoke of each of seven cycles (one to seven cigarettes). Three plates had been exposed per cigarette number/smoke dilution combination and three control plates had been simultaneously exposed to clean air with each set of smoke-exposed plates. It was the aim of the statistical analysis to determine the slopes of dose-response relationships of various cigarette types and to compare them using statistical tests. Basically, the following procedure is recommended: (1) calculate a dose measure on the basis of the number of smoked cigarettes per cycle and dilution air flow. (2) Use the absolute count values of the individual plates as effect variable. (3) Describe the dose-response relations of the individual cigarette types on the basis of all available data with a polynomial model by means of Poisson regression analysis accounting for overdispersion. (4) Identify the linear dose-response region using the likelihood ratio test and restrict the data set to this region. (5) Use the slope of the linear model in the restricted data set as the basis of the mutagenicity measure. (6) Compare the slope for the individual cigarette type with the slope for a reference cigarette by means of multivariate Poisson regression using the likelihood ratio test and accounting for overdispersion. It is finally recommended to express the mutagenic potency as percentages related to the reference cigarette K2R4F. This type of cigarette was set here equal to 100%; the following values are then obtained for some commercially available cigarette types: type A 25%, type B 90%, type C 119%, type D 13%, type E 59%. The differences are statistically significant.

Probabilistic analysis of recessive mutagenesis screen strategies

Random mutagenesis screens for recessive phenotypes require three generations of breeding, using either a backcross (BC) or intercross (IC) strategy. Hence, they are more costly and technically demanding than those for dominant phenotypes. Maximizing the return from these screens requires maximizing the number of mutations that are bred to homozyosity in the G(3) generation. Using a probabilistic approach, we compare different designs of screens for recessive phenotypes and the impact each one has on the number of mutations that can be effectively screened. We address the issue of BC versus IC strategies and consider genome-wide, region-specific screens and suppressor screens. We find that optimally designed BC and IC screens allow the screening of, on average, similar numbers of mutations but that interpedigree variation is more pronounced when the IC strategy is employed. By conducting a retrospective analysis of published mutagenesis screens, we show that, depending on the strategy, a threefold difference in the numbers of mutations screened per animal used could be expected. This method allows researchers to contrast, for a range of experimental designs, the cost per mutation screened and to maximize the number of mutations that one can expect to screen in a given experiment.

Contamination of the Shinano River Water with Mutagenic Substances after the Niigata Chuetsu Earthquake

While normally monitoring the Shinano River water quality, including examinations for mutagenicity, the Niigata Chuetsu Earthquake suddenly occurred on October 23, 2004. However, the influence of this earthquake on the mutagenicity of river water has not yet been well studied. To clarify the regional and seasonal changes in mutagenicity of the Shinano River water, blue rayon was suspended for 24 hrs at 4 sampling sites, once a month from September 2004 through August 2005. Mutagenicity was evaluated by the Ames test using Salmonella typhimurium TA98 (TA98) and TA100 with or without metabolic activation by S9 mixture. To detect and identify poly-aromatic hydrocarbons that may be responsible for the mutagenicity of the river water, we analyzed benzo[a]pyrene, benzophenone, 4-nitrotoluene, or other compounds using gas chromatography-mass spectrometry and total ion chromatogram spectra. Positive manifestations of TA98 with S9 mixture were observed at the 4 sampling sites throughout the 12-month test, showing a tendency to be higher at the downstream site and in winter. However, the highest mutagenicity was observed in the sample collected at the most upstream sampling site in December 2004, and fluoranthene or pyrene consisting mainly in coal tar was detected only in the samples collected in December 2004. Although benzo[a]pyrene, benzophenone, and 4-nitrotoluene were below the detection limits, non-mutagens such as aliphatic hydrocarbons or esters were frequently detected. Our findings indicate that either fluoranthene or pyrene was mainly responsible for the mutagenicity of the river water in December 2004, suggesting the possibility of oil contamination caused by the Niigata Chuetsu Earthquake.

Sensitivity of test for overdispersion in Poisson regression

Overdispersion or extra-Poisson variation is very common for count data. This phenomenon arises when the variability of the counts greatly exceeds the mean under the Poisson assumption, resulting in substantial bias for the parameter estimates. To detect whether count data are overdispersed in the Poisson regression setting, various tests have been proposed and among them, the score tests derived by Dean (1992) are popular and easy to implement. However, such tests can be sensitive to anomalous or extreme observations. In this paper, diagnostic measures are proposed for assessing the sensitivity of Dean's score test for overdispersion in Poisson regression. Applications to the well-known fabric faults and Ames salmonella assay data sets illustrate the usefulness of the diagnostics in analyzing overdispersed count data.

Data mining and machine learning techniques for the identification of mutagenicity inducing substructures and structure activity relationships of noncongeneric compounds

This paper explores the utility of data mining and machine learning algorithms for the induction of mutagenicity structure-activity relationships (SARs) from noncongeneric data sets. We compare (i) a newly developed algorithm (MOLFEA) for the generation of descriptors (molecular fragments) for noncongeneric compounds with traditional SAR approaches (molecular properties) and (ii) different machine learning algorithms for the induction of SARs from these descriptors. In addition we investigate the optimal parameter settings for these programs and give an exemplary interpretation of the derived models. The predictive accuracies of models using MOLFEA derived descriptors is approximately 10-15%age points higher than those using molecular properties alone. Using both types of descriptors together does not improve the derived models. From the applied machine learning techniques the rule learner PART and support vector machines gave the best results, although the differences between the learning algorithms are only marginal. We were able to achieve predictive accuracies up to 78% for 10-fold cross-validation. The resulting models are relatively easy to interpret and usable for predictive as well as for explanatory purposes.

Genotoxicity and physicochemical characteristics of traffic-related ambient particulate matter

Exposure to ambient particulate matter (PM) has been linked to several adverse health effects. Since vehicular traffic is a PM source of growing importance, we sampled total suspended particulate (TSP), PM(10), and PM(2.5) at six urban locations with pronounced differences in traffic intensity. The mutagenicity, DNA-adduct formation, and induction of oxidative DNA damage by the samples were studied as genotoxicological parameters, in relation to polycyclic aromatic hydrocarbon (PAH) levels, elemental composition, and radical-generating capacity (RGC) as chemical characteristics. We found pronounced differences in the genotoxicity and chemical characteristics of PM from the various locations, although we could not establish a correlation between traffic intensity and any of these characteristics for any of the PM size fractions. Therefore, the differences between locations may be due to local sources of PM, other than traffic. The concentration of total (carcinogenic) PAHs correlated positively with RGC, direct and S9-mediated mutagenicity, as well as the induction of DNA adducts and oxidative DNA damage. The interaction between total PAHs and transition metals correlated positively with DNA-adduct formation, particularly from the PM(2.5) fraction. RGC was not associated with one specific PM size fraction, but mutagenicity and DNA reactivity after metabolic activation were relatively high in PM(10) and PM(2.5), when compared with TSP. We conclude that the toxicological characteristics of urban PM samples show pronounced differences, even when PM concentrations at the sample sites are comparable. This implies that emission reduction strategies that take chemical and toxicological characteristics of PM into account may be useful for reducing the health risks associated with PM exposure.

Fisher Information Matrix of the Dirichlet-multinomial Distribution

In this paper we derive explicit expressions for the elements of the exact Fisher information matrix of the Dirichlet-multinomial distribution. We show that exact calculation is based on the beta-binomial probability function rather than that of the Dirichlet-multinomial and this makes the exact calculation quite easy. The exact results are expected to be useful for the calculation of standard errors of the maximum likelihood estimates of the beta-binomial parameters and those of the Dirichlet-multinomial parameters for data that arise in practice in toxicology and other similar fields. Standard errors of the maximum likelihood estimates of the beta-binomial parameters and those of the Dirichlet-multinomial parameters, based on the exact and the asymptotic Fisher information matrix based on the Dirichlet distribution, are obtained for a set of data from Haseman and Soares (1976), a dataset from Mosimann (1962) and a more recent dataset from Chen, Kodell, Howe and Gaylor (1991). There is substantial difference between the standard errors of the estimates based on the exact Fisher information matrix and those based on the asymptotic Fisher information matrix.

Evaluation of γ-Hydroxybutyric Acid for Genotoxicity in the Mouse Micronucleus Assay

Gamma-hydroxybutyric acid (GHB) is an endogenous compound found in the brain and other tissues of mammals. Neurotransmitter/neuromodulator functions have been ascribed to GHB, which has lately become a drug of abuse. In this study, we tested GHB for genotoxicity by measuring its ability to induce micronuclei in polychromatic erythrocytes (reticulocytes) in the peripheral blood of mice. Intraperitoneal injection with a dose of 25 mg/kg/day for 3 days or 50 mg/kg/day x 3 days resulted in a significant (by Dunnett's test) increase of 1.9- to 2.1-fold in micronuclei. However, because increases were small and because no consistent dose-dependent increase in induced micronuclear frequency could be demonstrated, our results do not conclusively show that GHB is an in vivo genotoxicant in mammals.

Rule extraction from a mutagenicity data set using adaptively grown phylogenetic-like trees

A public bacterial mutagenicity database was classified into 2-D structural families using a set of specific algorithms and clustering techniques that find overlapping classes of compounds based upon chemical substructures. Structure-activity relationships were learned from the biological activity of the compounds within each class and used to identify rules that define substructures potentially responsible for mutagenic activity. In addition, this method of analysis was used to compare the pharmacologically relevant substructure of test compounds with their potential toxic substructures making this a potentially valuable in silico profiling tool for lead selection and optimization.

Statistical calculation of detection limits for DNA adducts using the 32P-postlabeling assay with a standard addition procedure

The 32P-postlabeling assay is widely used for the analysis of DNA adducts. Some adducts can be detected with very high sensitivity but quantification can be unreliable, particularly if it is based only on comparison with unmodified nucleotides (relative adduct labeling, RAL values). Furthermore, guidelines to calculate detection limits for adduct concentrations are lacking. This is particularly important for human biomonitoring studies of environmental exposures, where a low adduct level can remain undetected. Reports of null results of toxicity studies should always include a limit of detection, indicating the effect magnitude that would have produced, with a given probability of false negative (type II error), a statistically significant increase (type I error). Here, we report on a procedure based on t-statistics to calculate two types of detection limits, the "critical level (CL)" and the "detection level (DL)". The first is the size of the difference between exposed and controls required to achieve statistical significance. The second is the size of the difference that will be detected with a chosen probability of a false negative. For the degrees of freedom (d.f.) to be used for the t-values, a general formula is given so that different standard deviations and group sizes of control and exposed groups can be handled. A sample calculation of the whole procedure is shown, using the null data for the formation of a particular adduct in lung DNA of styrene-treated mice, analyzed by 32P-postlabeling. The procedure takes into account: (i) TLC-specific background radioactivity; (ii) variability within the control and exposed groups; and (iii) confidence limits for the factor to convert 32P-radioactivity to amounts of adduct. The latter step incorporates the variance of the differences between the samples and replicates spiked with adduct standard. A statement such as follows is the result: the concentration of the alpha-isomer adduct of styrene 7,8-oxide at the O(6)-position of guanine in mouse lung DNA would have to be at least 12 adducts per 10(8) nucleotides to be detected in the given experiment on a 5% level (type I error), with a probability of 5% to miss an existing effect (type II error).

The statistical analysis of the in vitro chromosome aberration assay using Chinese hamster ovary cells

The purpose of the in vitro chromosome aberration assay (ABS) is to determine whether the test compound is a clastogen, i.e. induces structural changes in chromosomes. Details of this assay can be found in Galloway et al. [S.M. Galloway, M. Aardema, M. Ishidate Jr, J.L. Ivett, D.J. Kirkland, M. Takeshi, P. Mosesso, T. Sofuni, Mutation Res. 312 (1994) 241-261]. The standard design consists of a negative control and at least three positive dose groups. At each dose, a sample, say 200, of metaphase cells is examined microscopically and cells exhibiting at least one type of chromosome aberration are identified. Using Chinese hamster ovary cells, Margolin et al. [B.H. Margolin, M.A. Resnick, J.Y. Rimpo, P. Archer, S.M. Galloway, A.D. Bloom, E. Zeiger, Environ. Mutagen. 8 (1986) 183-204] and Richardson et al. [C. Richardson, D.A. Williams, J.A. Allen, G. Amphlett, D.O. Chanter, B. Phillips, Analysis of data from in vitro cytogenetic assays, in: D.J. Kirkland (Ed.), Statistical Evaluation of Mutagenicity Test Data, Cambridge University Press, Cambridge, 1989, pp. 141-154] demonstrated that a binomial sampling model could be used to describe the proportion of cells with chromosome aberrations.Statisticians and toxicologists have also suggested evaluation criteria for the dose response pattern of ABS. Margolin et al. [B.H. Margolin, M.A. Resnick, J.Y. Rimpo, P. Archer, S.M. Galloway, A.D. Bloom, E. Zeiger, Environ. Mutagen. 8 (1986) 183-204] suggested one use the Cochran-Armitage trend test. Sofuni et al. [T. Sofuni, A. Matsuoka, M. Sawada, M. Ishidate Jr, E. Zeiger, M.D. Shelby, Mutation Res. 241 (1990) 175-213] considered the dose response to be (strong) positive if it had two significant doses out of three dose groups and decided it was weakly positive if it had only one significant dose and there was a significant trend. The criterion of Galloway et al. for a positive response was a clear dose-related increase in cells with structural aberrations in one experiment or a reproducible single positive dose [S.M. Galloway, M. Aardema, M. Ishidate Jr, J.L. Ivett, D.J. Kirkland, M. Takeshi, P. Mosesso, T. Sofuni, Mutation Res. 312 (1994) 241-261]. We formulate the above three procedures in terms of a Cochran-Armitage trend test and a Dunnett type test. We then compare the performance of these three procedures in terms of a Monte Carlo simulation study. We then develop a software program from the chosen procedure for its ease of use by statisticians and toxicologists.

Statistical modeling and analyses of a base-specific Salmonella mutagenicity assay

Statistical features of a base-specific Salmonella mutagenicity assay are considered in detail, following up on a previous report comparing responses of base-specific Salmonella (Ames II) strains with those of traditional tester strains. In addition to using different Salmonella strains, the new procedure also differs in that it is performed as a microwell fluctuation test, as opposed to the standard plate or preincubation test. This report describes the statistical modeling of data obtained from the use of these new strains in the microwell test procedure. We emphasize how to assess any significant interactions between replicate cultures and exposure doses, and how to identify a significant increase in the mutagenic response to a series of concentrations of a test substance.

Assessment of acute cyto- and genotoxicity of corrosion eluates obtained from orthodontic materials using monolayer cultures of immortalized human gingival keratinocytes

Whilst a patient is undergoing orthodontic treatment, dental appliances based on non-precious metals or titanium remain in the oral cavity for up to several years. Throughout this period the appliance is in either direct or indirect contact with the oral mucosa. To investigate the possibility of cell damage occurring as a result of appliance corrosion, monolayer cultures of immortalized human gingival keratinocytes were assessed for acute cyto- and genotoxicity using the hexosaminidase assay and the Comet assay respectively. The materials tested included 1. a nickel-free wire, 2. a UK-1 bond, 3. nickel-free as well as nickel-containing brackets with and without color signature and 4. a titanium expansion screw. Each of the test materials was corroded in a solution consisting of equal amounts of lactic acid and sodium chloride (0.1 M) for 1, 3, 7 and 14 days. The cell cultures were then exposed to eluates exhibiting the highest ion concentrations. None of the eluates was found to exhibit acute cytotoxicity, regardless of the type of test system used. Qualitative assessment using neutral red dye for live cells and either trypan blue or propidium iodide to disclose dead cells failed to reveal any significant increase in cell damage when exposed cells were compared to control cultures. Unrestricted cell vitality was confirmed by quantifying viable cells through measurement of hexosaminidase enzyme activity. Furthermore, assessment of genotoxicity revealed no apparent DNA damage to immortalized gingival keratinocytes following exposure to the test eluates. Because the materials tested in this study were corroded using the exacting methods normally applied to precious metals or gold-containing alloys, the lack of either acute cyto- or genotoxic effects following exposure to the test eluates indicates that the materials tested exert no adverse effects on cells similar to those of the target tissue exposed to the materials in situ.

Modeling the mouse lymphoma forward mutational assay: the Gene-Tox program database

An SAR model of the induction of mutations at the tk(+/-) locus of L5178Y mouse lymphoma cells (MLA, for mouse lymphoma assay) was derived based upon a re-evaluation of experimental results reported by a Gene-Tox (GT) working group [A.D. Mitchell, A.E. Auletta, D. Clive, P.E. Kirby, M.M. Moore, B.C. Myhr, The L5178Y/tk(+/-) mouse lymphoma specific gene and chromosomal mutation assay. A phase III report of the U.S. Environmental Protection Agency Gene-Tox Program, Mutation Res. 394 (1997) 177-303.]. The predictive performance of the GT MLA SAR model was similar to that of a Salmonella mutagenicity model containing the same number of chemicals. However, the structural determinants (biophores) derived from the GT MLA SAR model include both electrophilic as well as non-electrophilic moieties, suggesting that the induction of mutations in the MLA may occur by both direct interaction with DNA and by non-DNA-related mechanisms. This was confirmed by the observation that the set of biophores associated with MLA overlapped significantly with those associated with phenomena related to loss of heterozygosity, chromosomal rearrangements and aneuploidy. The MLA SAR model derived from the GT data evaluation was significantly more predictive than an SAR model previously derived from MLA data reported by the US National Toxicology Program [B. Henry, S.G. Grant, G. Klopman, H.S. Rosenkranz, Induction of forward mutations at the thymidine kinase locus of mouse lymphoma cells: evidence for electrophilic and non-electrophilic mechanisms, Mutation Res. 397 (1998) 331-335.]. Moreover, the latter model appeared to be more complex than the former, suggesting that the GT induction data was both simpler mechanistically and more homogeneous than that of the NTP.

[A toxicological study of a phytopreparation made from an extract of alfalfa (Medicago sativa)]

A phytopreparation from lucerne (Medicago sativa) was studied with respect to the acute and chronic toxicity and the action upon the cellular genetic mechanism. It was found that the preparation belongs to the group of practically nontoxic properties.

[The antimutagenic activity of biomass extracts from the cultured cells of medicinal plants in the Ames test]

Antimutagenic activity of 20 and 40% ethanol extracts from the biomass of Rhodiola rosea, Polyscias filicifolia, Panax ginseng and Ungernia victoris cultured cells have been studied. DDDTDP, ethidium bromide, benz(a)pyrene, benzidine served as model mutagens for Salmonella typhimurium TA 98 strain (the latter two were tested in presence of metabolic activation system); for S. typhimurium TA 100 strain these were tio-tefa, bichromate potassium and sodium azide and heavy metal compounds (chlorides of manganese, zinc, cadmium, lead acetate) for both strains. Higher capacity of the extracts from the biomass of R. rosea and P. filicifolia to counteract gene mutations induced by various mutagens was demonstrated (ca. 90% inhibition in isolated cases). In the experiment with the metabolic activation most effective proved to be the extracts from the P. ginseng biomass (up to 34% and 47% mutagenicity inhibition).

Qualitative and quantitative procedures for health risk assessment

Numerous reactive mutagenic electrophiles are present in the environment or are formed in the human body through metabolizing processes. Those electrophiles can directly react with DNA and are considered to be ultimate carcinogens. In the past decades more than 200 in vitro and in vivo genotoxic tests have been described to identify, monitor and characterize the exposure of humans to such agents. When the responses of such genotoxic tests are quantified by a weight-of-evidence analysis, it is found that the intrinsic potency of electrophiles being mutagens does not differ much for the majority of the agents studied. Considering the fact that under normal environmental circumstances human are exposed to low concentration of about a million electrophiles, the relation between exposure to such agents and adverse health effects (e.g., cancer) will become a 'Pandora's box'. For quantitative risk assessment it will be necessary not only to detect whether the agent is genotoxic, but also understand the mechanism of interaction of the agent with the DNA in target cells needs to be taken into account. Examples are given for a limited group of important environmental and carcinogenic agents for which such an approach is feasible. The groups identified are agents that form cross-links with DNA or are mono-alkylating agents that react with base-moieties in the DNA strands. Quantitative hazard ranking of the mutagenic potency of these groups of chemical can be performed and there is ample evidence that such a ranking corresponds with the individual carcinogenic potency of those agents in rodents. Still, in practice, with the exception of certain occupational or accidental exposure situations, these approaches have not be successful in preventing cancer death in the human population. However, this is not only due to the described 'Pandora's box' situation. At least three other factors are described. Firstly, in the industrial world the medical treatment of cancer in patients occurs with high levels of extremely mutagenic agents. Actually, both in number of persons and in exposure levels such medical treatment is the single largest exposure of humans to known carcinogens. Although such treatments are very effective in curing the tumor as present in the patient, the recurrence of cancer in those patients later in life is very high. In other words: "curing cancer is not the same as preventing cancer death in the human population". Secondly, the rate of cancer death in the human population is also determined by the efficacy in which other major causes of death are prevented. For instance, cardiovascular diseases are the major cause of death in humans in the industrialized world. There is evidence that the treatment of cardiovascular diseases is more successful than that of cancer. On a population level this will result in increase of cancer being the ultimate death cause. Finally, the improvement of medical treatment of diseases together with an improved quality of life will lead to increase average age of the population. Because the onset of most cancer is long after the exposure to carcinogens-in human often more than 30 years-cancer is predominantly a disease of the old age. This means that if the average age of human increases, there will be a selective preference of cancer becoming an even more important cause of death. This especially will be pronounced in those countries were the age distribution in a population is abnormal.

An estimator of the mutant frequency in assays using transgenic animals

The Poisson distribution is a fundamental probability model for count data, and is a natural model for the observed plaque counts in mutation assays using animals with lambda or PhiX174 transgenes. The Poisson likelihood for observed counts is a function of the mutant fraction, and it is straightforward to derive the associated maximum likelihood estimate of the mutant fraction and its variance. The estimate is easy to calculate, and if not the same, very similar to ad hoc estimates in current use. The model indicates the proper way to combine data from a number of plates, possibly prepared with different sample dilutions. The estimator of the mutant fraction is biased as a consequence of dividing by a random variable, the plaque count used to calculate the total recovered plaque-forming units. Fortunately, the bias becomes negligible as this count becomes large. On the other hand, increasing this count can increase the variance by decreasing the amount of sample assayed for mutant phages. Concurrent heed to the bias and the variance provides some guidance as to the optimum allocation of a sample into portions assayed for mutant phages and total recovered phages. The distribution of the estimate of the mutant fraction is related to the binomial distribution. This relationship implies a binomial distribution for the mutant count conditional on an overall count (either the sum of mutant and counted total plaques or the sum of counted mutant and non-mutant plaques). A special but important case occurs when each plate can be evaluated for mutant plaques and non-mutant plaques. Then, the observed proportion of mutants estimates the mutant fraction. More generally, the relationship to a binomial distribution provides a procedure for calculating a confidence interval.

Statistical methods for the Ames Salmonella assay: a review

The Ames Salmonella assay remains the most widely used in vitro genotoxicity assay. Several statistical methods have been proposed for its analysis [B.H. Margolin, N. Kaplan, E. Zeiger, Statistical analysis of the Ames Salmonella/microsome test, Proc. Natl. Acad. Sci., 78 (1981) 3779-3783; L.E. Myers, N.H. Saxton, L.I. Southerland, T.J. Wolff, Regression analysis of Ames test data, Environ. Mol. Mutagen., 3 (1981) 575-586; A.G. Stead, V. Hasselblad, J.P. Creason, L. Claxton, Modelling the Ames test, Mutation Res., 85 (1981) 13-27; L. Bernstein, J. Kaldor, J. McCaan, M.C. Pike, An empirical approach to the statistical analysis of mutagenesis data from the Salmonella test, Mutation Res., 97 (1982) 267-281; N.E. Breslow, Extra-Poisson variation in log-linear models, Appl. Stat., 33 (1984) 38-44; J. Wahrendorf, G.A.T. Mahon, M. Schumacher, A nonparametric approach to the statistical analysis of mutagenicity data, Mutation Res., 147 (1985) 5-13; D.G. Simpson, B.H. Margolin, Recursive nonparametric testing for dose-response relationships subject to downturns at high doses, Biometrika, 73 (1986) 589-596; D.G. Simpson, B.H. Margolin, Nonparametric testing for dose-response curves subject to downturns: Asymptotic power considerations, Annals Stat., 18 (1990) 373-390.]. In this paper we review recent literature to see what statistical methods are in fact employed for the analysis of the Ames assay. We then note that these methods can be classified into a common category in the framework of Haynes and Eckardt's mutation induction kinetics model [R.H. Haynes, F. Eckardt, Mathematical analysis of mutation induction kinetics, in: F.J. de Serres, A. Hollaender (Eds. ), Chemical Mutagens, Principles and Methods for Their Detection, Vol. 6, Plenum, New York, 1980, pp. 271-307]. The value in knowing this is that most methods of analysis considered here will likely exhibit common statistical behavior. These analyses are computationally intensive, e.g., [B.H. Margolin, N. Kaplan, E. Zeiger, Statistical analysis of the Ames Salmonella/microsome test, Proc. Nat. Acad. Sci., 78 (1981) 3779-3783], hence the ready availability of computer programs is essential if biologists are to use these methods. We briefly review two statistical software programs that are available in the public domain, and describe in detail a third program, Salm, [B.H. Margolin, N. Kaplan, E. Zeiger, Statistical analysis of the Ames Salmonella/microsome test, Proc. Nat. Acad. Sci., 78 (1981) 3779-3783; B.H. Margolin, B.S. Kim, K. Risko, The Ames Salmonella/microsome assay: Issues of inference and validation, J. Amer. Stat. Assoc., 84 (1989) 651-661]. The Salm program is obtainable through the file transfer protocol (ftp) or using a WWW browser. Finally, we discuss two statistical consequences of naively applying the two-fold rule, a method of analysis employed by a number of researchers.

Statistical procedures for estimating the detection limit and determination limit of the Ames Salmonella mutagenicity assay

Novel and flexible procedures for estimating the detection limit as well as the determination limit of the Ames mutagenicity assay were proposed to evaluate the genetoxicity of a water sample. The accumulated data under the test conditions of TA 100-S9 by our group were taken as examples and analyzed to estimate the detection limit and the determination limit. The detection limit was estimated at 1.7 as the MR value when duplicate plates were used in the negative control test. However, it decreased to 1.4 as the MR level when quadruple plates were used in the negative control test. Therefore it was found that the sensitivity of the Ames mutagenicity assay was improved very easily by increasing the number of plates for the negative control test from two to four. The application of the conventional twofold rule to the data obtained with the strain TA100 was considered too conservative. The determination limit was regarded at 2.2 as the MR value under the following conditions: (a) quadruple plates were used in the negative control test; (b) three dose-steps including negative control step were designed at regular intervals; and (c) duplicate plates were used for each dose-step. It was proved by comparing data of two students that the detection limit and the determination limit estimated in this study were considered acceptable to any well trained students.

[The hygienic evaluation of the mutagenic potential of industrial wastes]

A combination of two approaches to assessing the carcinogenic and mutagenic potentials of industrial waste is proposed. One approach includes determination of the carcinogenic and mutagenic properties of individual chemicals of waste, the other involves biological indication of the cumulative mutagenic activity of waste samples. The mutagenic potential of some waste samples of aircraft industry was determined.

Heritable and cancer risks of exposures to anticancer drugs: inter-species comparisons of covalent deoxyribonucleic acid-binding agents

In the past years, several methodologies were developed for potency ranking of genotoxic carcinogens and germ cell mutagens. In this paper, we analyzed six sub-classes of covalent deoxyribonucleic acid (DNA) binding antineoplastic drugs comprising a total of 37 chemicals and, in addition, four alkyl-epoxides, using four approaches for the ranking of genotoxic agents on a potency scale: the EPA/IARC genetic activity profile (GAP) database, the ICPEMC agent score system, and the analysis of qualitative and quantitative structure-activity and activity-activity relationships (SARs, AARs) between types of DNA modifications and genotoxic endpoints. Considerations of SARs and AARs focused entirely on in vivo data for mutagenicity in male germ cells (mouse, Drosophila), carcinogenicity (TD50s) and acute toxicity (LD50s) in rodents, whereas the former two approaches combined the entire database on in vivo and in vitro mutagenicity tests. The analysis shows that the understanding and prediction of rank positions of individual genotoxic agents requires information on their mechanism of action. Based on SARs and AARs, the covalent DNA binding antineoplastic drugs can be divided into three categories. Category 1 comprises mono-functional alkylating agents that primarily react with N7 and N3 moieties of purines in DNA. Efficient DNA repair is the major protective mechanism for their low and often not measurable genotoxic effects in repair-competent germ cells, and the need of high exposure doses for tumor induction in rodents. Due to cell type related differences in the efficiency of DNA repair, a strong target cell specificity in various species regarding the potency of these agents for adverse effects is found. Three of the four evaluation systems rank category 1 agents lower than those of the other two categories. Category 2 type mutagens produce O-alkyl adducts in DNA in addition to N-alkyl adducts. In general, certain O-alkyl DNA adducts appear to be slowly repaired, or even not at all, which make this kind of agents potent carcinogens and germ cell mutagens. Especially the inefficient repair of O-alkyl-pyrimidines causes the high mutational response of cells to these agents. Agents of this category give high potency scores in all four expert systems. The major determinant for the high rank positions on any scale of genotoxic of category 3 agents is their ability to induce primarily structural chromosomal changes. These agents are able to cross-link DNA. Their high intrinsic genotoxic potency appears to be related to the number of DNA cross-links per target dose unit they can induce. A confounding factor among category 3 agents is that often the genotoxic endpoints occur close to or at toxic levels, and that the width of the mutagenic dose range, i.e., the dose area between the lowest observed effect level and the LD50, is smaller (usually no more than 1 logarithmic unit) than for chemicals of the other two categories. For all three categories of genotoxic agents, strong correlations are observed between their carcinogenic potency, acute toxicity and germ cell specificity.

Statistical analysis of lacZ mutant frequency data from MutaMouse mutagenicity assays

Transgenic mouse assays have provided an unprecedented opportunity to study mutagenesis in diverse rodent tissues. In this article data from MutaMouse mutagenicity assays based on the Escherichia coli gene lacZ were analyzed systematically using liver and bone marrow as potential target tissues. Sources of variation, including plates (within packaging reactions), packaging reactions (within animals) and animals, were evaluated for extra-binomial variation. Although hardly any evidence of overdispersion was detected at the plate level, limited evidence of extra-binomial variation was observed at the packaging reaction level. There was, however, much stronger evidence of overdispersion at the animal level. Statistical tests for increasing trend in mutant frequency with increasing dose were also performed at the animal level. A significant increasing trend following exposure to N-nitrosodibenzylamine was detected in liver but not in bone marrow. A logistical model was used to further describe the dose-response relationship observed in N-nitrosodibenzylamine-treated liver tissue.

[The mutagenicity of the gas component of the atmosphere as an index of its quality]

Using an Ames proximate method with a metabolic activation, the authors could investigate the capacity of chemical substances complex extracted from the samples of the atmospheric air gas components of some industrial Ukrainian cities to induce genes' mutation at TA 98 and TA 100. It was shown that air samples extracted in the cities with predominantly developed metallurgical industry had induced the mutagenic effect of "medium" power; the atmospheric air of the cities with predominantly developed chemical industry had induced both "medium" and "weak" mutagenic effects; the atmospheric air of relatively "clean" cities either had not induced mutagenic effects or had induced "weak" effects. State of the system of metabolic activation was mainly uneffective.

Statistical analysis of the lacI transgenic mouse mutagenicity assay

The transgenic mouse assay is now widely used to test chemicals for genotoxic potential. In this article, we consider statistical tests for increasing trend in mutant frequency with increasing dose, along with statistical models that may be used to describe the observed dose-response relationships. The application of these methods is illustrated using data on 2-acetylaminofluorene, di(2-ethylhexyl)phthalate, heptachlor, and sodium phenobarbital. No strong evidence of extra-binomial variation was detected at the plate level, but greater evidence was noted when the data were aggregated to the package or animal level in liver, necessitating the use of statistical methods that allow for overdispersion relative to binomial variation. Clear increase on mutant frequency induced by 2-acetylaminofluorene was detected in both liver and bladder, but no apparent trends were noted with di(2-ethylhexyl)phthalate, heptachlor, and sodium phenobarbital. The exponential model provides a good fit to the observed dose-response relationship in liver, whereas a Weibull model provides a better fit for bladder.

[The antimutagenic action of apiculture products]

Using the battery of test-systems, we studied the possible antimutagenic activity of some products of apiculture. It is established that apilak-preparation made on the basis of queen bee milk didn't show gene-protective abilities in Ames test. The rest products of apiculture: propolis, beebread, honey preparations N1 and N2, and queen bee milk manifested to some degree their ability to decrease the mutagenic effects of some chemical and physical mutagens. The toxic effect of propolis upon yeast cells was established. Queen bee milk manifested the least antimutagenic properties (on all test-objects used in experiments).

[The products of water chlorination as inducers of gene mutations]

Experimental data on water mutagenicity from Dnepro technological chain of pipe-line water purification were obtained using Ames express method of gene mutation induction in histidine-dependent strains of Salmonella typhimurium series TA (TA 98 and TA 100). It was shown that water mutagenicity depended on the level of water pollution by chemical substances in the river Dnieper and the use of chlorine for water disinfection.

Optimization of the Salmonella/mammalian microsome assay for urine mutagenesis by experimental designs

Assessing urine mutagenicity with the Salmonella mutagenicity test is often limited by the volumes of the samples. Optimization of the assay was performed with factorial and Doehlert designs. Two fractional factorial designs 2(3-1) (3 factors, 4 experiments) were used to estimate the main effects of the percent S9 in the mix, the time of liquid incubation, the inoculum size and the growth conditions. A Doehlert design (3 factors, 13 experiments) was used to study the main effects and the interactions of the NADP, G6P and S9 in the mix. The positive markers were benzo[a]pyrene (BaP, 0.3 microgram/plate) and a pool of smokers' urine (SU, 1.25 ml equivalent/plate). The response was limited to the induction factor (IF, number of induced revertants/number of spontaneous revertants) with Salmonella typhimurium TA98. The optimal conditions for BaP were: a 60 min period of liquid incubation and a volume of 0.1 ml (approx. 10(8) cells/plate) of an overnight culture grown in 50 ml of Nutrient Broth No. 2 from a 250 ml flask. The S9 mix (0.1 ml, final volume) included 1.5% of S9, 1.0 mM NADP and 4.4 mM G6P. The maximal IF was 15.79. The optimal conditions for SU were: a 60 min period of liquid incubation and a volume of 0.1 ml (approx. 10(8) cells/plate) of an overnight culture grown in 7 ml of Nutrient Broth No. 2 from a 20 x 180 mm tube. The S9 mix (0.1 ml, final volume) included: 4% S9, 4.2 mM NADP and 5.2 mM G6P. The maximal IF was 10.95. These optimal conditions did not modify the spontaneous frequencies of the tester strains: TA97a, TA98, TA100 and TA102. The dose-response curves of mutagenic urine samples were found to be non-linear. This micromethod required 8-fold less urine sample and 12.5-fold less liver homogenate as compared to the standard plate incorporation assay and was from 6.2- to 11.8-fold more sensitive to evaluate urine mutagenicity. The sensitivity of this technique was found to be limited to individuals smoking more than approx. 5 cigarettes/day by the standard extraction-concentration procedure.

A statistical analysis of the reliability and classification error in application of the mutagen sensitivity assay

In vitro mutagen hypersensitivity, determined with the bleomycin assay, has been found to be an independent risk factor for developing primary upper aerodigestive tract cancers, lung cancers, and second malignant tumors. The average number of chromatid breaks per cell (b/c) is derived from evaluating an arbitrarily set number of metaphases (usually 50) in each sample. The reliability of such an approach is of key importance in large-scale epidemiological studies. Because evaluating metaphases is a time-consuming task, it is desirable to know the minimum number of readings needed to reach an acceptable reliability. Statistical analyses were performed in 160 observations for which b/c values were determined by the same observer scoring 100 consecutive metaphases per sample. The b/c values were between 0.14 and 1.30 (mean, 0.61). The b/c values were separately calculated for the first and second sets of 50 metaphases. There was essentially no difference in the mean b/c values between the first and second 50 readings (difference, -0.002). The correlation between the two sets of readings was 0.72. The SE of the b/c values, based on scoring 50 metaphases, was 0.15. When scoring 100 metaphases, the SE decreased to 0.11. After evaluating the first 50 metaphases, the theoretical gain in reducing the SE was <1% with each additional reading. When 0.8 was used to dichotomize the mean b/c value into bleomycin-resistant or bleomycin-hypersensitive groups, sensitivity and specificity of reading 50 metaphases were above 75% and 95% when compared to 100 readings. A simulated case control study showed that there is a 15% attenuation in estimating the odds ratio with 50 readings. The findings suggest that the conventional method of reading 50 metaphases can yield an acceptable reliability in our setting and may be applied to other cancer epidemiology studies.

A proposal for a two-dose/single-sample in vivo/in vitro rat hepatocyte unscheduled DNA synthesis assay

We have developed a two dose/single-sample protocol for the in vivo/in vitro rat hepatocyte unscheduled DNA synthesis (UDS) assay. In order to enhance the effectiveness of grain detection by image analysis we found minor technical modifications to be beneficial. These included the use of 3% acetic acid in ethanol (or 4% aqueous paraformaldehyde) for hepatocyte fixation and 0.5% aqueous eosin for staining. Also, there was no correlation between cell viability (measured using the trypan blue method) and UDS response and, therefore, we do not reject animals from data analysis unless hepatocyte viability falls below 50%. Furthermore, we suggest that cell attachment is a more reliable indicator of toxicity than of cell viability. Therefore, our range-finding test has been modified to incorporate an extra animal per group so that hepatocyte cultures can be evaluated. Comparisons of a two-dose/single-sample protocol with the currently accepted single-dose/multiple-sample protocol demonstrated that the former was an acceptable alternative, in that responses with standard positive controls are very similar with both protocols. Furthermore, the two-dose protocol has clear advantages in that it uses fewer animals, resources and time and has better statistical discriminatory power. As a move away from the use of arbitrary values for determining the performance and outcome of assays, we use criteria based on confidence limits placed on historical data ranges. Where necessary, statistical analyses of concurrent data is performed using rank transformation followed by parametric analysis of the ranks; this combines the generality of a non-parametric methodology with the power and versatility of parametric analyses.

Extended harvest times are not necessary for the detection of in vitro clastogens in regulatory cytogenetics studies

The choice of harvest time in in vitro cytogenetics assays is a critical factor in determining the sensitivity of the assay for detecting clastogenic potential. As yet there is no harmonization of regulatory requirements in this aspect. It has been suggested that the use of extended harvest times can improve the sensitivity of detecting some chemicals which either induce cell cycle delay or produce lesions which induce chromosome aberrations at divisions subsequent to the first post-treatment mitosis. The incidence of such chemicals encountered in the routine testing of chemicals for regulatory submissions is not known. Therefore a large database of 550 chemicals tested in nine laboratories using standard regulatory protocols, including a late harvest time, was assessed for the incidence of chemicals uniquely positive only at a delayed harvest time. The number of such chemicals was very low ( < 0.2%) and the chromosome damage induced by these chemicals may not result from direct genotoxic mechanisms. Based on these data it is recommended that there is no need to include an extended harvest time in in vitro cytogenetics assays except where it might help to resolve an equivocal result.

Influence of physical and chemical characteristics of diesel fuels and exhaust emissions on biological effects of particle extracts: a multivariate statistical analysis of ten diesel fuels

The emission of diesel exhaust particulates is associated with potentially severe biological effects, e.g., cancer. The aim of the present study was to apply multivariate statistical methods to identify factors that affect the biological potency of these exhausts. Ten diesel fuels were analyzed regarding physical and chemical characteristics. Particulate exhaust emissions were sampled after combustion of these fuels on two makes of heavy duty diesel engines. Particle extracts were chemically analyzed and tested for mutagenicity in the Ames test. Also, the potency of the extracts to competitively inhibit the binding of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to the Ah receptor was assessed. Relationships between fuel characteristics and biological effects of the extracts were studied, using partial least squares regression (PLS). The most influential chemical fuel parameters included the contents of sulfur, certain polycyclic aromatic compounds (PAC), and naphthenes. Density and flash point were positively correlated with genotoxic potency. Cetane number and upper distillation curve points were negatively correlated with both mutagenicity and Ah receptor affinity. Between 61% and 70% of the biological response data could be explained by the measured chemical and physical factors of the fuels. By PLS modeling of extract data versus the biological response data, 66% of the genotoxicity could be explained, by 41% of the chemical variation. The most important variables, associated with both mutagenicity and Ah receptor affinity, included 1-nitropyrene, particle bound nitrate, indeno[1,2,3-cd]pyrene, and emitted mass of particles. S9-requiring mutagenicity was highly correlated with certain PAC, whereas S9-independent mutagenicity was better correlated with nitrates and 1-nitropyrene. The emission of sulfates also showed a correlation both with the emission of particles and with the biological effects. The results indicate that fuels with biologically less hazardous potentials should have high cetane number and contain less PAC and sulfur. The results also indicate that engine factors affect the formation and emission of nitrated PAC.

[The evaluation of the mutagenic activity of public drinking water by the Ames test]

BACKGROUND

The sources of potential mutagens in our environment are many, but the most important of these is water for public consumption. This is a result of the chlorinating process which is the main reason for the appearance of these mutagens. With this in mind, the aim of our study was to check a possible mutagenic activity, using the Ames test, in organic concentrates taken from water for public consumption in Madrid.

METHODS

Several bacterial strains were used, namely Salmonella histidine dependent TA1535, TA1538, TA98 and TA100, taken originally from Salmonella typhimurium LT2. Each test was performed twice, with or without the introduction of the mammalian-microsome activation (S9 mix), as per the indications in Ames. The plate incorporation assay was used to test the mutagenicity. All samples of the water in question were processed and treated so as to create concentrates of organic chlorinated compounds.

RESULTS

The highest levels of mutagenicity appeared in the TA1535 strain and in the tests where the microsome fraction was not used (IM = 1.94).

CONCLUSIONS

With regard to mutagenic evaluation in organic concentrates taken from water for public consumption, no positive activity was found in any of the tester strains.

Quick estimate of the regulatory virtually safe dose based on the maximum tolerated dose for rodent bioassays

With a limited subset of National Cancer Institute/National Toxicology Program (NCI/NTP) bioassays, Gaylor (Regul. Toxicol. Pharmacol. 9, 101-108, 1989) showed that the regulatory virtually safe dose (VSD), corresponding to an estimated lifetime cancer risk of less than 10(-6), could be estimated within a factor of 10 simply by dividing the maximum tolerated dose (MTD), estimated from the results of a 90-day study, by 380,000. The purpose of this current study was to extend the analysis to all carcinogens in the Carcinogenic Potency Database (CPDB) utilizing the TD50 (average daily dose rate in mg/kg body wt/day that was estimated to halve the probability of remaining tumor-free at a specified tissue site throughout a 2-year study). Using the relationship between the upper bound on the low-dose slope (q1*) and the TD50 reported by Krewski et al. (Risk Anal. 13, 383-398, 1993) and the ratio of the maximum dose tested (Max-D)/TD50 obtained in our present analysis, an estimate of the regulatory VSD was given by the MTD/740,000, for NCI/NTP rodent carcinogens. This was about a factor of two lower than the limited analysis conducted by Gaylor. There was little difference when the chemicals were divided into mutagens and nonmutagens. Ninety-six percent (134 of the 139 NCI/NTP rodent carcinogens) of the regulatory VSDs calculated from the individual TD50s obtained from the 2-year bioassays were within a factor of 10 of the MTD/740,000.(ABSTRACT TRUNCATED AT 250 WORDS)

Collaborative study of interlaboratory variability in Salmonella typhimurium TA102 and TA2638 and Escherichia coli WP2/pKM101 and WP2 uvrA/pKM101. Association of Microbial Mutation Testing Laboratory C.O

A collaborative study of interlaboratory variability in bacterial mutagenicity induced by mitomycin C (MMC) and bleomycin (BLM) was performed using the four strains Salmonella typhimurium TA102 and TA2638 and Escherichia coli WP2/pKM101 and WP2 uvrA/pKM101. Thirteen laboratories participated in this study. The four strains and two chemicals were sent from a central source to each laboratory. Each strain was cultured in nutrient broth containing antibiotics and laboratories were requested to ensure that the spontaneous counts for marker check fell within a specified range in the preparation of the original stock culture. Concerning the response to the chemicals, most interlaboratory variability was within a 4-fold range for tests with TA102, TA2638 and WP2/pKM101. From the results of the statistical analysis using the linear regression test, positive results in TA102, TA2638 and WP2/pKM101 and negative results in WP2 uvrA/pKM101 were obtained by MMC testing in all of the laboratories. On the other hand, with BLM testing, considerable interlaboratory variability was observed between the strains, largely because of the variation in results of the repeat experiments. Overall mean spontaneous revertant counts in all laboratories were within acceptable ranges for all four bacterial strains. In conclusion, it is judged that among Japanese laboratories, there is excellent agreement in the tested response of these bacterial strains to a strong mutagen such as MMC, as well as uniformity in the spontaneous reversion rates. However, for chemicals such as BLM that induce a weak response, it may be necessary to repeat experiments several times to obtain clear results. In this study, TA102 was shown to be a useful strain for routine mutagenicity testing. This necessitated control over culture maintenance by the addition of tetracycline and strict selection in the preparation of the original stock cultures.

Design and interpretation of rat liver UDS assays

The study design and interpretation criteria for the rat liver UDS assay have evolved over recent years. A database from a single laboratory is presented to demonstrate the range of responses encountered whilst screening 71 industrial and agrochemical compounds for potential genotoxicity, a total of 280 experiments. With a large data set it was possible to delineate clearly the normal responses of the system and apply robust criteria for the evaluation of potentially positive compounds. The rat liver UDS assay proved to be a reproducible, reliable and sensitive technique for the evaluation of genotoxicity in vivo. Experiments with negative control values of (N-C) greater than zero would have been rejected, however the highest negative control value observed was (N-C) = -1.1. Evaluation of data using the current United Kingdom Environmental Mutagen Society (UKEMS) recommended procedures resolved 96% of studies by simple inspection. In the remainder of studies data were clarified by additional experiments under modified conditions. It is unlikely that outcomes would have been improved upon by direct statistical analysis which would require an expanded experimental design. These data confirm the utility of the pragmatic approach of the current UKEMS guidelines.

Analysis of chemically induced spindle aberrations in male mouse germ cells: comparison of differential and immunofluorescent staining procedures

Differential staining of spindle and chromatin was adapted to meiotic cells of male mice. Immunostaining with antitubulin antibodies was developed. Both methods were used to test acrylamide, diazepam and vinblastine for their potential to cause spindle disturbances in male mouse germ cells. The analyses were performed after in vivo treatment, comparing the classical safranin/brilliant blue spindle staining to an immunofluorescent assay. The two staining methods complemented each other. Differential staining seems to be more sensitive to the detection of misplaced chromatin whereas the immunostaining is superior when scoring for spindle structure aberrations. All three chemicals showed spindle activity. Acrylamide mainly caused multipolar spindles which possibly develop from a separation of mother and daughter centrioles during an unspecific meiotic block. Diazepam elevated the level of monopolar spindles and induced the loss of single chromosomes, suggesting an effect on motor proteins. Vinblastine was the strongest spindle poison by far. It produced high numbers of shortened and monopolar spindles as well as multiple chromosome loss.

Optimal experimental design and sample size for the statistical evaluation of data from somatic mutation and recombination tests (SMART) in Drosophila

In genetic toxicology it is important to know whether chemicals should be regarded as clearly hazardous or whether they can be considered sufficiently safe, which latter would be the case from the genotoxicologist's view if their genotoxic effects are nil or at least significantly below a predefined minimal effect level. A previously presented statistical decision procedure which allows one to make precisely this distinction is now extended to the question of how optimal experimental sample size can be determined in advance for genotoxicity experiments using the somatic mutation and recombination tests (SMART) of Drosophila. Optimally, the statistical tests should have high power to minimise the chance for statistically inconclusive results. Based on the normal test, the statistical principles are explained, and in an application to the wing spot assay, it is shown how the practitioner can proceed to optimise sample size to achieve numerically satisfactory conditions for statistical testing. The somatic genotoxicity assays of Drosophila are in principle based on somatic spots (mutant clones) that are recovered in variable numbers on individual flies. The underlying frequency distributions are expected to be of the Poisson type. However, some care seems indicated with respect to this latter assumption, because pooling of data over individuals, sexes, and experiments, for sample, can (but need not) lead to data which are overdispersed, i.e., the data may show more variability than theoretically expected. It is an undesired effect of overdispersion that in comparisons of pooled totals it can lead to statistical testing which is too liberal, because overall it yields too many seemingly significant results. If individual variability considered alone is not in contradiction with Poisson expectation, however, experimental planning can help to minimise the undesired effects of overdispersion on statistical testing of pooled totals. The rule for the practice is to avoid disproportionate sampling. It is recalled that for optimal power in statistical testing, it is preferable to use equal total numbers of flies in the control and treated series. Statistical tests which are based on Poisson expectations are too liberal if there is overdispersion in the data due to excess individual variability. In this case we propose to use the U test as a non-parametric two-sample test and to adjust the estimated optimal sample size according to (i) the overdispersion observed in a large historical control and (ii) the relative efficiency of the U test in comparison to the t test and related parametric tests.

Transgenic lambda/lacI mutagenicity assay: statistical determination of sample size

Statistical analysis of the lambda/lacI transgenic mutagenicity assay was used to determine optimal sample size and resource allocation in terms of number of animals and number of recovered target genes (recovered phage) required to demonstrate a statistically significant induction in mutant frequency. Statistical assumptions as applied to mutagenicity data are discussed for a number of frequently used statistical analyses. Log transformations are suggested as a means of meeting statistical assumptions and examples are given on interpreting results of analyses of log transformed data. The data analyzed in this study indicate that 300,000 lambda plaques from each of five animals should be analyzed per treatment group in order to detect a doubling of mutant frequencies. Additional sensitivity is gained primarily through increase of animal number and not the number of phage rescued, due to inherent animal-to-animal variability.