The performance of short-term tests in identifying potential germ cell mutagens: a qualitative and quantitative analysis

Science-Based Approach to Harmonize Contraception Recommendations in Clinical Trials and Pharmaceutical Labels

This review presents a European Federation of Pharmaceutical Industries and Association/PreClinical Development Expert Group (EFPIA-PDEG) topic group consensus on a data-driven approach to harmonized contraception recommendations for clinical trial protocols and product labeling. There is no international agreement in pharmaceutical clinical trial protocols or product labeling on when/if female and/or male contraception is warranted and for how long after the last dose. This absence of consensus has resulted in different recommendations among regions. For most pharmaceuticals, contraception recommendations are generally based exclusively on nonclinical data and/or mechanism. For clinical trials, contraception is the default position and is maintained for women throughout clinical development, whereas appropriate information can justify removing male contraception. Conversely, contraception is only recommended in product labeling when warranted. A base case rationale is proposed for whether or not female and/or male contraception is/are warranted, using available genotoxicity and developmental toxicity data. Contraception is generally warranted for both male and female subjects treated with mutagenic pharmaceuticals. We propose as a starting point that contraception is not typically warranted when the margin is 10-fold or greater between clinical exposure at the maximum recommended human dose and exposure at the no observed adverse effect level (NOAEL) for purely aneugenic pharmaceuticals and for pharmaceuticals that induce fetal malformations or embryo-fetal lethality. Other factors are discussed, including contraception methods, pregnancy testing, drug clearance, options for managing the absence of a developmental toxicity NOAEL, drug-drug interactions, radiopharmaceuticals, and other drug modalities. Overall, we present a data-driven rationale that can serve as a basis for consistent contraception recommendations in clinical trials and in product labeling across regions.

Applying the erythrocyte Pig-a assay concept to rat epididymal sperm for germ cell mutagenicity evaluation

The Pig-a assay, a recently developed in vivo somatic gene mutation assay, is based on the identification of mutant erythrocytes that have an altered repertoire of glycosylphosphatidylinositol (GPI)-anchored cell surface markers. We hypothesized that the erythrocyte Pig-a assay concept could be applied to rat cauda epididymal spermatozoa (sperm) for germ cell mutagenicity evaluation. We used GPI-anchored CD59 as the Pig-a mutation marker and examined the frequency of CD59-negative sperm using flow cytometry. A reconstruction experiment that spiked un-labeled sperm (mutant-mimic) into labeled sperm at specific ratios yielded good agreement between the detected and expected frequencies of mutant-mimic sperm, demonstrating the analytical ability for CD59-negative sperm detection. Furthermore, this methodology was assessed in F344/DuCrl rats administered N-ethyl-N-nitrosourea (ENU), a prototypical mutagen, or clofibrate, a lipid-lowering drug. Rats treated with 1, 10, or 20 mg/kg body weight/day (mkd) ENU via daily oral garage for five consecutive days showed a dose-dependent increase in the frequency of CD59-negative sperm on study day 63 (i.e., 58 days after the last ENU dose). This ENU dosing regimen also increased the frequency of CD59-negative erythrocytes. In rats treated with 300 mkd clofibrate via daily oral garage for consecutive 28 days, no treatment-related changes were detected in the frequency of CD59-negative sperm on study day 85 (i.e., 57 days after the last dose) or in the frequency of CD59-negative erythrocytes on study day 29. In conclusion, these data suggest that the epidiymal sperm Pig-a assay in rats is a promising method for evaluating germ cell mutagenicity. Environ. Mol. Mutagen. 58:485-493, 2017. © 2017 Wiley Periodicals, Inc.

Development of the adverse outcome pathway "alkylation of DNA in male premeiotic germ cells leading to heritable mutations" using the OECD's users' handbook supplement

The Organisation for Economic Cooperation and Development's (OECD) Adverse Outcome Pathway (AOP) programme aims to develop a knowledgebase of all known pathways of toxicity that lead to adverse effects in humans and ecosystems. A Users' Handbook was recently released to provide supplementary guidance on AOP development. This article describes one AOP-alkylation of DNA in male premeiotic germ cells leading to heritable mutations. This outcome is an important regulatory endpoint. The AOP describes the biological plausibility and empirical evidence supporting that compounds capable of alkylating DNA cause germ cell mutations and subsequent mutations in the offspring of exposed males. Alkyl adducts are subject to DNA repair; however, at high doses the repair machinery becomes saturated. Lack of repair leads to replication of alkylated DNA and ensuing mutations in male premeiotic germ cells. Mutations that do not impair spermatogenesis persist and eventually are present in mature sperm. Thus, the mutations are transmitted to the offspring. Although there are some gaps in empirical support and evidence for essentiality of the key events for certain aspects of this AOP, the overall AOP is generally accepted as dogma and applies broadly to any species that produces sperm. The AOP was developed and used in an iterative process to test and refine the Users' Handbook, and is one of the first publicly available AOPs. It is our hope that this AOP will be leveraged to develop other AOPs in this field to advance method development, computational models to predict germ cell effects, and integrated testing strategies.

QSAR screening of 70,983 REACH substances for genotoxic carcinogenicity, mutagenicity and developmental toxicity in the ChemScreen project

The ChemScreen project aimed to develop a screening system for reproductive toxicity based on alternative methods. QSARs can, if adequate, contribute to the evaluation of chemical substances under REACH and may in some cases be applied instead of experimental testing to fill data gaps for information requirements. As no testing for reproductive effects should be performed in REACH on known genotoxic carcinogens or germ cell mutagens with appropriate risk management measures implemented, a QSAR pre-screen for 70,983 REACH substances was performed. Sixteen models and three decision algorithms were used to reach overall predictions of substances with potential effects with the following result: 6.5% genotoxic carcinogens, 16.3% mutagens, 11.5% developmental toxicants. These results are similar to findings in earlier QSAR and experimental studies of chemical inventories, and illustrate how QSAR predictions may be used to identify potential genotoxic carcinogens, mutagens and developmental toxicants by high-throughput virtual screening.

Approaches for identifying germ cell mutagens: Report of the 2013 IWGT workshop on germ cell assays(☆)

This workshop reviewed the current science to inform and recommend the best evidence-based approaches on the use of germ cell genotoxicity tests. The workshop questions and key outcomes were as follows. (1) Do genotoxicity and mutagenicity assays in somatic cells predict germ cell effects? Limited data suggest that somatic cell tests detect most germ cell mutagens, but there are strong concerns that dictate caution in drawing conclusions. (2) Should germ cell tests be done, and when? If there is evidence that a chemical or its metabolite(s) will not reach target germ cells or gonadal tissue, it is not necessary to conduct germ cell tests, notwithstanding somatic outcomes. However, it was recommended that negative somatic cell mutagens with clear evidence for gonadal exposure and evidence of toxicity in germ cells could be considered for germ cell mutagenicity testing. For somatic mutagens that are known to reach the gonadal compartments and expose germ cells, the chemical could be assumed to be a germ cell mutagen without further testing. Nevertheless, germ cell mutagenicity testing would be needed for quantitative risk assessment. (3) What new assays should be implemented and how? There is an immediate need for research on the application of whole genome sequencing in heritable mutation analysis in humans and animals, and integration of germ cell assays with somatic cell genotoxicity tests. Focus should be on environmental exposures that can cause de novo mutations, particularly newly recognized types of genomic changes. Mutational events, which may occur by exposure of germ cells during embryonic development, should also be investigated. Finally, where there are indications of germ cell toxicity in repeat dose or reproductive toxicology tests, consideration should be given to leveraging those studies to inform of possible germ cell genotoxicity.

Genetic toxicology in the 21st century: reflections and future directions

A symposium at the 40th anniversary of the Environmental Mutagen Society, held from October 24-28, 2009 in St. Louis, MO, surveyed the current status and future directions of genetic toxicology. This article summarizes the presentations and provides a perspective on the future. An abbreviated history is presented, highlighting the current standard battery of genotoxicity assays and persistent challenges. Application of computational toxicology to safety testing within a regulatory setting is discussed as a means for reducing the need for animal testing and human clinical trials, and current approaches and applications of in silico genotoxicity screening approaches across the pharmaceutical industry were surveyed and are reported here. The expanded use of toxicogenomics to illuminate mechanisms and bridge genotoxicity and carcinogenicity, and new public efforts to use high-throughput screening technologies to address lack of toxicity evaluation for the backlog of thousands of industrial chemicals in the environment are detailed. The Tox21 project involves coordinated efforts of four U.S. Government regulatory/research entities to use new and innovative assays to characterize key steps in toxicity pathways, including genotoxic and nongenotoxic mechanisms for carcinogenesis. Progress to date, highlighting preliminary test results from the National Toxicology Program is summarized. Finally, an overview is presented of ToxCast™, a related research program of the U.S. Environmental Protection Agency, using a broad array of high throughput and high content technologies for toxicity profiling of environmental chemicals, and computational toxicology modeling. Progress and challenges, including the pressing need to incorporate metabolic activation capability, are summarized.

Germ cell mutagens: risk assessment challenges in the 21st century

Heritable mutations may result in a wide variety of detrimental outcomes, from embryonic lethality to genetic disease in the offspring. Despite this, today's commonly used test batteries do not include assays for germ cell mutation. Current challenges include a lack of practical assays and concrete evidence for human germline mutagens, and large data gaps that often impede risk assessment. Moreover, most regulatory assessments are based on the assumption that somatic cell mutation assays also protect the germline by default, which has not been adequately confirmed. The field is also faced with new challenges aimed at dramatically reducing animal testing, and attempts to rapidly classify thousands of chemicals using high throughput in vitro assays. These approaches may not adequately capture effects that may be particular to gametes, since many aspects of the germline are unique. In light of these challenges, an urgent need exists to develop new approaches to evaluate the potential of toxicants to cause germline mutation. The application of new technologies will greatly enhance our understanding of mutation in humans exposed to environmental mutagens. However, we must be poised to collect and interpret these data, and facilitate risk translation to regulators and the public. Genetic toxicologists must also become actively involved in the development of high-throughput tools to study germline mutation. Appropriate attention to these areas will result in the development of policies that prioritize the protection of the germline and future generations from DNA sequence mutations.

Detection of induced male germline mutation: correlations and comparisons between traditional germline mutation assays, transgenic rodent assays and expanded simple tandem repeat instability assays

Several rodent assays are capable of monitoring germline mutation. These include traditional assays, such as the dominant lethal (DL) assay, the morphological specific locus (SL) test and the heritable translocation (HT) assay, and two assays that have been developed more recently--the expanded simple tandem repeat (ESTR) and transgenic rodent (TGR) mutation assays. In this paper, we have compiled the limited amount of experimental data that are currently available to make conclusions regarding the comparative ability of the more recently developed assays to detect germline mutations induced by chemical and radiological agents. The data suggest that ESTR and TGR assays are generally comparable with SL in detecting germline mutagenicity induced by alkylating agents and radiation, though TGR offered less sensitivity than ESTR in some cases. The DL and HT assays detect clastogenic events and are most susceptible to mutations arising in post-spermatogonial cells, and they may not provide the best comparisons with TGR and ESTR instability. The measurement of induced ESTR instability represents a relatively sensitive method of identifying agents causing germline mutation in rodents, and may also be useful for bio-monitoring exposed individuals in the human population. Any future use of the TGR and ESTR germline mutation assays in a regulatory testing context will entail more robust and extensive characterization of assay performance. This will require substantially more data, including experiments measuring multiple endpoints, a greatly expanded database of chemical agents and a focus on characterizing stage-specific activity of mutagens in these assays, preferably by sampling epididymal sperm exposed at defined pre-meiotic, meiotic and post-meiotic stages of development.

Detailed review of transgenic rodent mutation assays

Induced chromosomal and gene mutations play a role in carcinogenesis and may be involved in the production of birth defects and other disease conditions. While it is widely accepted that in vivo mutation assays are more relevant to the human condition than are in vitro assays, our ability to evaluate mutagenesis in vivo in a broad range of tissues has historically been quite limited. The development of transgenic rodent (TGR) mutation models has given us the ability to detect, quantify, and sequence mutations in a range of somatic and germ cells. This document provides a comprehensive review of the TGR mutation assay literature and assesses the potential use of these assays in a regulatory context. The information is arranged as follows. (1) TGR mutagenicity models and their use for the analysis of gene and chromosomal mutation are fully described. (2) The principles underlying current OECD tests for the assessment of genotoxicity in vitro and in vivo, and also nontransgenic assays available for assessment of gene mutation, are described. (3) All available information pertaining to the conduct of TGR assays and important parameters of assay performance have been tabulated and analyzed. (4) The performance of TGR assays, both in isolation and as part of a battery of in vitro and in vivo short-term genotoxicity tests, in predicting carcinogenicity is described. (5) Recommendations are made regarding the experimental parameters for TGR assays, and the use of TGR assays in a regulatory context.

History and rationale of genetic toxicity testing: an impersonal, and sometimes personal, view

Genetic toxicity testing is a necessary and pivotal component of product development and registration. This article traces the historical development and evolution of genetic toxicity testing, and the rationale for such testing, and identifies some of the individuals who played key roles in this process. The evolution of the present test batteries and some of the research and rationales behind the decisions to accept or reject tests are described.

Mouse bone marrow micronucleus test results do not predict the germ cell mutagenicity of N-hydroxymethylacrylamide in the mouse dominant lethal assay

N-Hydroxymethylacrylamide (NHMA), a mouse carcinogen inactive in the Salmonella assay and mouse micronucleus (MN) assay, was tested for reproductive effects in a mouse continuous breeding study. In that study, increased embryonic deaths were observed after 13 weeks exposure of parental animals to NHMA via drinking water (highest dose, 360 ppm); the results indicated the possible induction of chromosome damage in germ cells of treated males. An additional mouse MN test was conducted using a 31-day treatment period to better match the dosing regimen used in the breeding study; the results were negative. Additional studies were conducted to explore the germ cell activity of NHMA. A male mouse dominant lethal study was conducted using a single intraperitoneal injection of 150 mg/kg NHMA; the results were negative. A follow-up study was conducted using fractionated dosing, 50 mg/kg/day for 5 days; again, no increase in dominant lethal mutations was observed. NHMA (180-720 ppm) was then administered to male mice in drinking water for 13 weeks, during which three sets of matings occurred. Two weeks after mating, females were killed and the uterine contents were analyzed. Large, dose-related increases in dominant lethal mutations were observed with increasing length of exposure. The magnitude of the increases stabilized after 8 weeks of treatment. However, the frequency of micronucleated peripheral blood erythrocytes was not elevated in mice treated for 13 weeks with NHMA in drinking water. Thus, NHMA appears to be unique in inducing genetic damage in germ cells but not somatic cells of male mice.

Permanent embryonic germ cell lines of BALB/cJ mice--an in vitro alternative for in vivo germ cell mutagenicity tests

To offer a sensitive and predictive in vitro method to assess germ cell mutagenicity, we established primordial germ (PG) cell-derived permanent female and male embryonic germ (EG) cell lines of the mouse (strain BALB/cJ). The differences in developmental sensitivity of EG cells and differentiated fibroblast cells of the mouse cell line 3T3 to genotoxicants were tested comparatively under identical test conditions. Cytotoxicity assay was measured by the MTT test and genotoxic effects were determined by sister chromatid exchanges (SCE) rates induced by standard reference mutagens. Both methods are used to assign the chemicals to two classes of in vivo reproductive toxicity, non- and strongly genotoxic to germ cells. Applying linear discriminant analysis, a biostatistical prediction model (PM) was developed for the female cell line EG(3). This procedure identified a single variable, the Ig(SCE(200)EG(3)) as the statistically significant concentration related increase of 200% in the mean number of SCEs per metaphase spread after 3 h of exposure to be sufficient for separation into the classes: non- and strongly genotoxic to germ cells. Applying this PM to the training set of five genotoxic and three non-genotoxic test chemicals, 100% correct classifications were obtained.

Potential transmission of the cytogenetic toxic effects of methotrexate in the male germline cells of Swiss mice

On testing the cytogenetic toxic effects of methotrexate, a widely prescribed antineoplastic drug, in the male germline cells of Swiss mice, it was found highly clastogenic to the spermatogonial cells at 24-h post-treatment after a single intraperitoneal exposure. The occurrence of significant percentages of aberrant primary spermatocytes with atypical bivalents at week 4 post-treatment and a little higher percentages of sperm with abnormal morphology at week 8 post-treatment indicated the potential transmission of the induced cytogenetic toxic effects of methotrexate from spermatogonia to sperm in the male germline cells of Swiss mice.

Potential transmission of the cytogenetic effects of cisplatin in the male germline cells of Swiss mice

Cisplatin (CP) (in Oncoplatin), a widely used drug in cancer chemotherapy, and cyclophosphamide (CY) (in Endoxan), another anticancer drug, were investigated as the test chemical and positive control, respectively, for their cytogenetic effects on spermatogonia of mice at 24 hours post-treatment after a single exposure. The different doses of the chemicals tested were CP 2, 3, 5 mg/kg and CY 40 mg/kg b.w. of mice. Each of the doses of CP induced a significant number of chromosomal aberrations, mostly chromatid breaks and fragments. The potential transmission of such cytogenetic effects of the chemicals from spermatogonia to spermatocytes was assessed at week 4 post-treatment from the primary spermatocytes, which showed a significant number of aberrant spermatocytes with atypical bivalents viz. spermatocytes with autosomal and/or XY univalents, tetravalents and with extra elements. The probable causes of the formation of univalents and tetravalents are discussed. The transmission of the cytogenetic effects of the chemicals from spermatogonia up to sperm was assessed at week 8 post-treatment from the morphology of sperm collected from vas. Quantitatively the transmission of such effects was found decreased substantially by the time the exposed spermatogonia became sperm. Still there was the occurrence of a few abnormal sperm at week 8 post-treatment. The probable causes of the quantitative decrease in the transmission of the effects from spermtogonia to sperm are discussed.

Identification of rodent carcinogens and noncarcinogens using genetic toxicity tests: premises, promises, and performance

The basic premises that guide genetic toxicity testing for identifying carcinogens and to support administrative and regulatory decisions are: the Salmonella mutagenicity test is a necessary component of testing schemes; a chromosome aberration test is needed in addition to a gene mutation test; a mammalian cell mutagenicity test is needed in addition to the Salmonella test; in vivo tests are needed to confirm the results of in vitro tests; and test batteries are more predictive than the individual tests of the battery. Results from the Salmonella mutagenicity, in vitro chromosome aberration, mutations in mouse lymphoma cells, rodent bone marrow micronucleus, and rodent carcinogenicity tests, performed by the U.S. National Toxicology Program, were used to evaluate these premises. A positive Salmonella test was most predictive of carcinogenicity. However, the data do not support using the other tests in addition to Salmonella for predicting carcinogenicity. The genetic toxicity tests did not complement each other, and batteries or combinations of the tests were no more predictive of carcinogenicity than Salmonella alone. If a chemical is mutagenic in Salmonella it should be considered a potential rodent carcinogen, unless ancillary information suggests otherwise. Positive responses in the other in vitro or in vivo tests do not increase the probability that the chemical is a carcinogen, and negative responses in the other tests do not diminish the implications of the positive Salmonella response.

Inhibition of genotoxic effects of mammalian germ cell mutagens

Germ cell mutagens are among the most important chemicals for which chemopreventive agents should be sought and mechanistically defined. These mutagens may include environmental chemicals as well as drugs. In this investigation, the literature was reviewed for substances antimutagenic (or anticlastogenic) to compounds identified as mutagens in at least two germ cell studies. A complete matrix of test results was prepared to identify commonly tested pairs of germ cell mutagens and antimutagens. The categories of antimutagens most tested included vitamins, fatty acids, thiols, tannins and other phenolics. The most frequently studied mutagens were benzo[a]pyrene, cyclophosphamide, mitomycin C, and bleomycin. Based on the availability of the most relevant data, the analysis presented here focused on in vivo tests, specifically on bone marrow cytogenetics. The results indicated that antimutagens commonly found in the diet or endogenously in the body effectively antagonized the cytogenetic damage induced in the bone marrow by most of the germ cell mutagens studied to date. Bone marrow micronucleus and chromosomal aberration assays, which detect systemically active mutagens, may be predictive of similar mitigating effects in germ cells. Test results from antimutagenicity studies in germ cells, though limited, were comparable to the results from studies in the mouse bone marrow micronucleus test.

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.

Epirubicin-induced oxidative DNA damage and evidence for its repair in lymphocytes of cancer patients who are undergoing chemotherapy

Anthracycline derivatives have been widely used in the treatment of several types of human malignancies. Cytotoxicity of these drugs has been attributed to inhibition of topoisomerase II as well as intracellular production of free radicals. In our work we used a gas chromatography/mass spectrometry technique to study free radical-induced DNA base modifications in chromatin isolated from lymphocytes of cancer patients who received chemotherapy with epirubicin (one of anthracycline's antitumor derivatives). The anticancer therapy caused significant increases in the amount of all four DNA base modifications over control levels in the lymphocytes of most of the patients. For the majority of the cases the base products returned to the control value 24 hr after the infusion of the drug, which suggests the removal of these lesions by cellular repair processes. However, some of the modified bases escaped repair. Because part of these modifications may possess premutagenic properties, they may be responsible for secondary cancers induced by chemotherapy.

Evaluation of the mutagenicity of acetochlor to male rat germ cells

Male rat dominant lethal (DL) assays conducted on the herbicide acetochlor are described. Single dose studies conducted at the maximum tolerated dose (MTD, < or = 1000 mg/kg) produced no effects on any of the DL assay parameters at any of the ten weekly sampling periods. It is concluded that acetochlor is non-mutagenic to rat germ cells. Due to initial limited knowledge of the MTD of acetochlor it was also evaluated in the DL assay at a dose level of 2000 mg/kg. At this high dose level severe bodyweight loss and some deaths occurred among the treated animals. In addition, reduced implantations and reduced pregnancy rates were observed at the third sampling period (18-25 days post dosing) in the absence of an increase in early post-implantation deaths. These results indicated that the use of supra-MTD doses of acetochlor had reduced the fertility of the treated males leading to the production of a pseudo-DL assay response, as alerted to and defined by Ehling. Although several such pseudo-DL assay responses have been described, none have been explained mechanistically. It was therefore decided to pursue the effects seen in the DL assay when using supra-MTD doses of acetochlor. Ova analysis of female rats mated with male rats exposed to 2000 mg/kg acetochlor revealed unfertilized ova at the critical third sampling time. Normal fertilization of ova was observed at the first and fifth sampling period and, for a dose of 200 mg/kg acetochlor, at the third sampling period. The magnitude and temporal nature of these effects confirmed the induction of a pseudo-DL assay response, and studies were then undertaken to probe its genesis. Rats treated with 2000 mg/kg acetochlor had normal testicular and epididymal pathology and normal sperm numbers and sperm motility at the critical third sampling period. Despite a small reduction in testicular and epididymal glutathione levels 12 h after exposure to 2000 mg/kg acetochlor, testicular LDH and LDH-X enzyme levels were unaffected. Further, no reduction in the level of free sulphydryl groups (-SH) were observed in epididymal caput sperm heads isolated 0.5, 7 or 14 days after treatment of male rats with 2000 mg/kg acetochlor. The only sperm parameter affected by treatment with 2000 mg/kg acetochlor was an increase in epididymal cauda sperm with head abnormalities. The non-specific nature of this effect was considered inadequate to explain fully the high dose fertility effects seen in the DL assays, which therefore remain unexplained. The present data establish that acetochlor is non-mutagenic to rat germ cells. They also confirm the importance of segregating mutagenic and fertility effects in the DL assay, and emphasize the need for appropriate dose-setting studies prior to the conduct of rodent genetic toxicity assays.

Toward an understanding of the use of transgenic mice for the detection of gene mutations in germ cells

Recently-developed transgenic models have provided unprecedented access to rodent somatic and germ line tissues for the study of gene mutation in vivo. While mutations in germ cells are considered an important aspect of any regulatory assessment of the risks posed by chemicals, currently-available conventional tests, which involve the study of thousands of offspring make it impractical to test large numbers of chemicals, for the induction of inherited gene mutations. When effects in germ cells per se, rather than offspring are acceptable targets, transgenic mouse assays may provide a practical alternative. As part of an international collaborative study to begin to determine the reliability, efficacy, and role of such assays, lacZ transgenic mice (Muta Mouse) were treated with single i.p. doses of ethylnitrosourea (ENU), methyl methanesulfonate (MMS), and isopropyl methanesulfonate (iPMS), and mutant frequencies determined using phenyl-beta-D-galactoside (p-gal) positive selection. For studies using germ cells, the selection of sampling times and target cells is crucial. Spermatagonial stem cells and cells in post-spermatagonial stem cell stages are the critical target cell populations of regulatory importance. Cell populations within these categories were studied by sampling germ cells isolated from seminiferous tubules and spermatozoa from the epididymis at 91 days and 25 days after treatment. The data show that ENU and iPMS induced mutations in post-spermatagonial stem cells and spermatagonial stem cells. However, MMS did not induce mutations in either cell type, or at either sampling time, at doses approaching lethality. This result is possibly because MMS induces preferentially large lesions and chromosomal aberrations (as opposed to point mutations), which are not readily detectable with bacteriophage-based shuttle vectors. Since MMS-induced specific locus and dominant lethal mutations are induced only after the mid-spermatid stage, it is also possible that the timing used missed this effect. While the ENU and iPMS data in this study demonstrate the suitability of the lacZ male transgenic mice for the study of gene mutations in post-spermatagonial stem cells and spermatagonial stem cells by sampling cells isolated from seminiferous tubules at selected times after treatment, the MMS results do not answer fully whether transgenic mouse mutation assays can detect mutations resulting from lesions induced after the mid-spermatid stage when most cellular processing is retarded. Nevertheless, it appears clear from presently available information, that the bacteriophage-based lacZ transgenic model is suitable for the detection of gene mutations in spermatogonial stem cells, spermatocytes, and early spermatids.

Comparative assessment of DNA adduct formation, Salmonella mutagenicity, and chromosome aberration assays as short-term tests for DNA damage

DNA adduct formation assay (DAFA) was carried out to compare dose responses with the Ames test and chromosomal aberration test using aflatoxin B1 (AFB1) and benzo[a]pyrene (BaP). In the bacterial mutation test, AFB1 and BaP (0-1 microgram/plate) were all positive in TA97a and TA100 with dose-related revertants. However, the slopes of the dose-response curves were gradual (slope 0.55-3.73, r = .84-.98). In the chromosome aberration test, a significant increase in the percentage of chromosomal aberrations was obtained from male ICR mouse spleen cells treated with AFB1 and BaP, but a dose-related increase was insensitive (slope 0.09-0.23, r = .75-.78). The incidence of chromosomally aberrant spleen cells treated with BaP was significantly increased compared with AFB1. DAFA was performed in vitro with [3H]-AFB1 and [3H]BaP. These two carcinogens were able to induce genotoxicity and showed good dose-related increases in terms of DNA adduct formation (slope 0.78-1.28, r = 1.00). Coefficients of variation (CV) for the slope of each dose-response curve were much lower in DAFA in vitro (CV 15.09- 18.34%) than those in any other test (CV 19.69-99.33%, Ames test; 18.89-44.58%, chromosome aberration test). Furthermore, DAFA in vivo was performed to investigate organotropic DNA adduct formation and persistence in Sprague-Dawley rats ip or orally treated with AFB1 and BaP. DNA adducts were monitored for 48-96 h by enzyme-linked immunosorbent assay (ELISA) using corresponding monoclonal antibodies, 6A10 and 8E11. DAFA in vivo demonstrated that the liver and kidney might be the probable target organs for AFB1 with the highest formation and persistence of DNA adducts and the lung and liver for BaP regardless of the route of administration. The results suggest that DAFA in vitro could be useful for detecting genotoxic compounds, and DAFA in vivo should also be considered as a good alternative method for the screening of organ-specific chemical carcinogens.

Mutagenicity to the mouse bone marrow by the mouse germ cell mutagen N-propyl-N-nitrosourea

N-Propyl-N-nitrosourea (PNU) is shown to be active in male mouse bone marrow micronucleus assays when dosed at either 100 or 200 mg/kg in saline. Activity was observed following either intraperitoneal (i.p.) injection or oral gavage. This observation is consistent with the demonstration by Murota and Shibuya of the specific-locus mutagenicity caused by PNU in male mouse spermatogonia when dosed at 200 mg/kg by i.p. injection. These data strengthen further the observation that rodent germ cell mutagens are also mutagenic to rodent somatic cells.

Evaluation of the potential carcinogenicity and genetic toxicity to humans of the herbicide acetochlor

Comprehensive toxicological studies of the herbicide acetochlor are presented and discussed. Although it gave a negative profile of responses in the many toxicity tests conducted there were some findings that prompted further investigation. First, although non-mutagenic in the Salmonella assay, acetochlor was clastogenic to mammalian cells treated in vitro. This clastogenic potential was not expressed in vivo in four rodent cytogenetic assays (bone marrow and germ cells). Second, although acetochlor gave a negative response in rat liver UDS assays when tested at the acute MTD, gavage administration of a single, supra-MTD dose (2000 mg/kg) gave a weak positive assay response. This dose-level (2000 mg/kg) was necrotic to the liver, depressed hepatic glutathione levels by up to approximately 80%, altered the metabolism of acetochlor, and was associated with up to 33% lethality. In contrast, reference liver genotoxins such as DMN, DMH and 2AAF were shown to elicit UDS in the absence of such effects, and at approximately 400 x lower dose-levels. Finally, microscopic nasal polypoid adenomas were induced in the rat when acetochlor was administered for two years at the maximum tolerated dose (MTD). The tumours were not life-threatening, they did not metastasize, and no DNA damage was induced in the nasal cells of rats maintained on a diet containing the MTD of acetochlor for either 1 or 18 weeks (comet assay). In order to probe the mechanism of action of these high dose toxicities a series of chemical and genetic toxicity studies was conducted on acetochlor and a range of structural analogues. These revealed the chloroacetyl substructure to be the clastogenic species in vitro. Although relatively inert, this substituent is preferentially reactive to sulphydryl groupings, most evidently, to glutathione (GSH). Similar chemical reactivity and clastogenicity in vitro was observed for two related chemicals bearing a chloroacetyl group, both of which have been defined as non-carcinogens in studies reported by the US.NTP. These collective observations indicate that the source of the clastogenicity of acetochlor in vitro is also the source of its rapid detoxification in the rat in vivo, via reaction with GSH. Metabolic studies of acetochlor are described which reveal the formation of a series of GSH-associated biliary metabolites in the rat that were not produced in the mouse. The metabolism of acetochlor in the rat changes with increasing dose-levels, probably because of depletion of hepatic GSH. It is most likely that a rat-specific metabolite is responsible for the rat nasal tumours observed uniquely at elevated dose-levels. The absence of genetic toxicity to the nasal epithelium of rats exposed acutely or subchronically to acetochlor favours a non-genotoxic mechanism for the induction of these adenomas. The observation of a time- and dose-related increase in S-phase cells in the nasal epithelium is consistent with this conclusion. Despite some confusion caused by the early use of perilethal gavage administrations of acetochlor to rodents, and supra-MTD dietary concentrations in some of the chronic studies, the available MTD data are consistent with acetochlor not posing a genetic or carcinogenic hazard to humans.

Genetic activity profiles of anticancer drugs

The results from short-term tests for genetic and related effects, abstracted from the open literature for 36 anticancer drugs, are examined in this review. Data for 27 of these agents are available in the EPA/IARC Genetic Activity Profile (GAP) database. Data summaries, including data listings and activity profiles, are presented for nine anticancer drugs added to the GAP database for this analysis. Genetic toxicity data from the recent literature are included for the additional agents to provide a broader representation of the categories of drugs being evaluated. These categories, based on the chemical mode of action, are covalent and noncovalent DNA-binding drugs, topoisomerase II inhibitors, antimetabolites, mitotic spindle inhibitors, and drugs which affect endocrine function. The qualitative data for all 36 drugs are summarized in this report and findings are presented from pair-wise matching of genetic activity profiles, based on test results in common, for some chemical analogs. The significance of germ cell test results for some of these drugs and their implication in assessing risk of heritable genetic disease are discussed.

Activity of iPMS and nPMS in mouse bone marrow micronucleus assays: comparison with mouse dominant lethal assay data

isoPropyl methanesulphonate (iPMS) and its n-propyl analogue (nPMS) are shown to be active in mouse bone marrow micronucleus assays using male CBA, male and female (C3H/El X 102/E1)Fl and male and female Muta Mouse mice. iPMS was significantly more active than nPMS. No significant strain or gender differences were observed. These findings reflect the differences reported earlier for these two chemicals in mouse dominant lethal mutation assays. The earlier described dominant lethal assay data are represented schematically and discussed.

Germ line specific factors in chemical mutagenesis

Chemical mutagenesis test results have not revealed evidence of germ line specific mutagens. However, conventional assays have indicated that there are male-female differences in mutagenic response, as well as quantitative/qualitative differences in induced mutations which depend upon the particular cell stage exposed. Many factors inherent in the germ line can be speculated to influence chemical transport to, and interaction with, target cell populations to result in mutagenic outcomes. The level of uncertainty regarding the general operation of such factors, in combination with the limited availability of chemical test data designed to address comparative somatic and germ cell mutagenesis, leaves open the question of whether there are mutagens specifically affecting germ cells. This argues for a conservative approach to interpreting germ cell risk from somatic cell mutation analysis.

The parallelogram approach in studies of genotoxic effects

Over the past two decades mutagenicity tests have been used for the identification of potential human mutagens and have had an ancillary role, as supportive evidence in the assessment of human carcinogens. The demonstration of human germinal mutagens has been beyond the main scope of short-term testing strategies. However, just as mutagenicity tests have been useful in detecting potential carcinogens so should carcinogenicity tests assist the identification of presumptive germ cell mutagens. Cancer is an easily observable phenotype of mutation for genotoxic carcinogens and multi-site carcinogens or gonadal carcinogens logically could be germ cell mutagens. Thus carcinogenicity and mutagenicity data for a given genotoxic chemical should be considered together in the identification of putative germinal mutagens. Clearly, most classified human carcinogens are genotoxic thus helping to build the case for human germ cell mutagenicity. This paper describes the issues involved in such thinking and suggests an enhanced parallelogram approach incorporating the cancer endpoint. The enhanced parallelogram is explored using 1,3-butadiene and ethylene oxide as examples. The obvious lack of data for extrapolations using the parallelogram method suggests the need for targeted studies specifically designed for use in this approach.