Interpretation of in vitro concentration-response data for risk assessment and regulatory decision-making: Report from the 2022 IWGT quantitative analysis expert working group meeting

Quantitative risk assessments of chemicals are routinely performed using in vivo data from rodents; however, there is growing recognition that non-animal approaches can be human-relevant alternatives. There is an urgent need to build confidence in non-animal alternatives given the international support to reduce the use of animals in toxicity testing where possible. In order for scientists and risk assessors to prepare for this paradigm shift in toxicity assessment, standardization and consensus on in vitro testing strategies and data interpretation will need to be established. To address this issue, an Expert Working Group (EWG) of the 8th International Workshop on Genotoxicity Testing (IWGT) evaluated the utility of quantitative in vitro genotoxicity concentration-response data for risk assessment. The EWG first evaluated available in vitro methodologies and then examined the variability and maximal response of in vitro tests to estimate biologically relevant values for the critical effect sizes considered adverse or unacceptable. Next, the EWG reviewed the approaches and computational models employed to provide human-relevant dose context to in vitro data. Lastly, the EWG evaluated risk assessment applications for which in vitro data are ready for use and applications where further work is required. The EWG concluded that in vitro genotoxicity concentration-response data can be interpreted in a risk assessment context. However, prior to routine use in regulatory settings, further research will be required to address the remaining uncertainties and limitations.

Assessing the quality and making appropriate use of historical negative control data: A report of the International Workshop on Genotoxicity Testing (IWGT)

Historical negative control data (HCD) have played an increasingly important role in interpreting the results of genotoxicity tests. In particular, Organisation for Economic Co-operation and Development (OECD) genetic toxicology test guidelines recommend comparing responses produced by exposure to test substances with the distribution of HCD as one of three criteria for evaluating and interpreting study results (referred to herein as "Criterion C"). Because of the potential for inconsistency in how HCD are acquired, maintained, described, and used to interpret genotoxicity testing results, a workgroup of the International Workshops for Genotoxicity Testing was convened to provide recommendations on this crucial topic. The workgroup used example data sets from four in vivo tests, the Pig-a gene mutation assay, the erythrocyte-based micronucleus test, the transgenic rodent gene mutation assay, and the in vivo alkaline comet assay to illustrate how the quality of HCD can be evaluated. In addition, recommendations are offered on appropriate methods for evaluating HCD distributions. Recommendations of the workgroup are: When concurrent negative control data fulfill study acceptability criteria, they represent the most important comparator for judging whether a particular test substance induced a genotoxic effect. HCD can provide useful context for interpreting study results, but this requires supporting evidence that (i) HCD were generated appropriately, and (ii) their quality has been assessed and deemed sufficiently high for this purpose. HCD should be visualized before any study comparisons take place; graph(s) that show the degree to which HCD are stable over time are particularly useful. Qualitative and semi-quantitative assessments of HCD should also be supplemented with quantitative evaluations. Key factors in the assessment of HCD include: (i) the stability of HCD over time, and (ii) the degree to which inter-study variation explains the total variability observed. When animal-to-animal variation is the predominant source of variability, the relationship between responses in the study and an HCD-derived interval or upper bounds value (i.e., OECD Criterion C) can be used with a strong degree of confidence in contextualizing a particular study's results. When inter-study variation is the major source of variability, comparisons between study data and the HCD bounds are less useful, and consequentially, less emphasis should be placed on using HCD to contextualize a particular study's results. The workgroup findings add additional support for the use of HCD for data interpretation; but relative to most current OECD test guidelines, we recommend a more flexible application that takes into consideration HCD quality. The workgroup considered only commonly used in vivo tests, but it anticipates that the same principles will apply to other genotoxicity tests, including many in vitro tests.

A comparison of the lowest effective concentration in culture media for detection of chromosomal damage in vitro and in blood or plasma for detection of micronuclei in vivo

It is often assumed that genotoxic substances will be detected more easily by using in vitro rather than in vivo genotoxicity tests since higher concentrations, more cytotoxicity and static exposures can be achieved. However, there is a paucity of data demonstrating whether genotoxic substances are detected at lower concentrations in cell culture in vitro than can be reached in the blood of animals treated in vivo. To investigate this issue, we compared the lowest concentration required for induction of chromosomal damage in vitro (lowest observed effective concentration, or LOEC) with the concentration of the test substance in blood at the lowest dose required for biologically relevant induction of micronuclei in vivo (lowest observed effective dose, or LOED). In total, 83 substances were found for which the LOED could be identified or estimated, where concentrations in blood and micronucleus data were available via the same route of administration in the same species, and in vitro chromosomal damage data were available. 39.8 % of substances were positive in vivo at blood concentrations that were lower than the LOEC in vitro, 22.9 % were positive at similar concentrations, and 37.3 % of substances were positive in vivo at higher concentrations. Distribution analysis showed a very wide scatter of > 6 orders of magnitude across these 3 categories. When mode of action was evaluated, the distribution of clastogens and aneugens across the 3 categories was very similar. Thus, the ability to detect induction of micronuclei in bone marrow in vivo regardless of the mechanism for micronucleus induction, is clearly not solely determined by the concentration of test substance which induced chromosomal damage in vitro.

Recommendations for conducting the rodent erythrocyte Pig-a assay: A report from the HESI GTTC Pig-a Workgroup

The rodent Pig-a assay is a flow cytometric, phenotype-based method used to measure in vivo somatic cell mutation. An Organization for Economic Co-operation and Development (OECD) test guideline is currently being developed to support routine use of the assay for regulatory purposes (OECD project number 4.93). This article provides advice on best practices for designing and conducting rodent Pig-a studies in support of evaluating test substance safety, with a focus on the rat model. Various aspects of assay conduct, including laboratory proficiency, minimum number of animals per dose group, preferred treatment and blood sampling schedule, and statistical analysis are described.

Assessment of systemic genetic damage in pediatric inflammatory bowel disease

The etiology of distal site cancers in inflammatory bowel disease (IBD) is not well understood and requires further study. We investigated whether pediatric IBD patients' blood cells exhibit elevated levels of genomic damage by measuring the frequency of mutant phenotype (CD59-/CD55-) reticulocytes (MUT RET) as a reporter of PIG-A mutation, and the frequency of micronucleated reticulocytes (MN-RET) as an indicator of chromosomal damage. IBD patients (n = 18 new-onset disease, 46 established disease) were compared to age-matched controls (constipation or irritable bowel syndrome patients from the same clinic, n = 30) and young healthy adults age 19-24 (n = 25). IBD patients showed no indication of elevated MUT RET relative to controls (mean ± SD = 3.1 ± 2.3 × 10^-6 vs. 3.6 ± 5.6 x 10^-6 , respectively). In contrast, 59 IBD patients where %MN-RET measurements were obtained, 10 exceeded the upper bound 90% tolerance interval derived from control subjects (i.e., 0.42%). Furthermore, each of the 10 IBD patients with elevated MN-RET had established disease (10/42), none were new-onset (0/17) (p = .049). Interestingly, each of the subjects with increased chromosomal damage was receiving anti-TNF based monotherapy at the time blood was collected (10/10, 100%), whereas this therapy was less common (20/32, 63%) among patients that exhibited ≤0.42% MN-RET (p = .040). The results clearly indicate the need for further work to understand whether the results presented herein are reproducible and if so, to elucidate the causative factor(s) responsible for elevated MN-RET frequencies in some IBD patients.

Human blood PIG-A mutation and micronucleated reticulocyte flow cytometric assays: Method optimization and evaluation of intra- and inter-subject variation

We previously described flow cytometry-based methods for scoring the incidence of micronucleated reticulocytes (MN-RET) and PIG-A mutant phenotype reticulocytes (MUT RET) in rodent and human blood samples. The current report describes important methodological improvements for human blood analyses, including immunomagnetic enrichment of CD71-positive reticulocytes prior to MN-RET scoring, and procedures for storing frozen blood for later PIG-A analysis. Technical replicate variability in MN-RET and MUT RET frequencies based on blood specimens from 14 subjects, intra-subject variability based on serial blood draws from 6 subjects, and inter-subject variation based on up to 344 subjects age 0 to 73 years were quantified. Inter-subject variation explained most of the variability observed for both endpoints (≥77%), with much lower intra-subject and technical replicate variability. The relatively large degree of inter-subject variation is apparent from mean and standard deviation values for MN-RET (0.15 ± 0.10%) and MUT RET (4.7 ± 5.0 per million, after omission of two extreme outliers). The influences of age and sex on inter-subject variation were investigated, and neither factor affected MN-RET whereas both influenced MUT RET frequency. The lowest MUT RET values were observed for subjects <11 years old, and males had moderately higher frequencies than females. These results indicate that MN-RET and MUT RET are automation-compatible biomarkers of genotoxicity that bridge species of toxicological interest to include human populations. These data will be useful for appropriately designing future human studies that include these biomarkers of genotoxicity, and highlight the need for additional work aimed at identifying the sources of inter-individual variability reported herein.

Recommendations for the Application of Biostatistical Methods during the Development and Validation of Alternative Toxicological Methods

Assessment of the relevance and reliability of alternative toxicological methods requires the application of biostatistical methods. This report contains the recommendations of the ECVAM Task Force on Biostatistics for the appropriate use of biometrical methods, and for closer collaboration between biostatisticians and experimental toxicologists in the development and validation of alternative methods. The main objectives of these recommendations are to improve the compatibility of the results obtained from different studies, and to prevent the misinterpretation of statistical analyses. The following points are discussed: a) the general importance of biostatistics in the development and validation of alternative tests; b) optimisation of study designs; c) statistical characterisation and processing of toxicological data; and d) the analysis of relationships which may exist between data obtained from various tests. In particular, the establishment of appropriate prediction models which are able to predict the toxic effects likely to occur in vivo from the results obtained in an in vitro test system is discussed. The members of the Task Force hope that this report will help to stimulate discussions among biostatisticians and experimental toxicologists about how the proper application of biostatistical methods can improve the design and evaluation of alternative toxicological methods.

The Validation of Toxicological Prediction Models

An alternative method is shown to consist of two parts: the test system itself; and a prediction model for converting in vitro endpoints into predictions of in vivo toxicity. For the alternative method to be relevant and reliable, it is important that its prediction model component is of high predictive power and is sufficiently robust against sources of data variability. In other words, the prediction model must be subjected to criticism, leading successful models to the state of confirmation. It is shown that there are certain circumstances in which a new prediction model may be introduced without the necessity to generate new test system data.

Analysis of historical negative control group data from the rat in vivo micronucleus assay

A database of micronuclei counts for historical negative control data from rat in vivo micronuclei tests performed in 10 different laboratories was established. Data were available from over 4000 negative control rats from 10 laboratories. The mean frequency of micronucleated cells (MN)/1000 cells ranged from 0.44 to 2.22, a 5-fold range. Overall there were no major sex or strain differences in frequency, although there were some small but statistically significant differences within laboratories. There was appreciable variability between experiments compared with variability within experiments in some laboratories. No specific factor was identified which could explain this variability although it was noted that many different vehicles were used in the experiments. It is hoped that these data will help laboratories beginning studies with the rat micronucleus assay and those involved in the assessment of micronucleus assay results.

In vivo genotoxicity testing strategies: Report from the 7th International workshop on genotoxicity testing (IWGT)

The working group reached complete or majority agreement on many issues. Results from TGR and in vivo comet assays for 91 chemicals showed they have similar ability to detect in vivo genotoxicity per se with bacterial mutagens and Ames-positive carcinogens. TGR and comet assay results were not significantly different when compared with IARC Group 1, 2 A, and unclassified carcinogens. There were significantly more comet assay positive responses for Group 2B chemicals, and for IARC classified and unclassified carcinogens combined, which may be expected since mutation is a sub-set of genotoxicity. A liver comet assay combined with the bone marrow/blood micronucleus (MNviv) test would detect in vivo genotoxins that do not exhibit tissue-specific or site-of-contact effects, and is appropriate for routine in vivo genotoxicity testing. Generally for orally administered substances, a comet assay at only one site-of-contact GI tract tissue (stomach or duodenum/jejunum) is required. In MNviv tests, evidence of target tissue exposure can be obtained in a number of different ways, as recommended by ICH S2(R1) and EFSA (Hardy et al., 2017). Except for special cases the i.p. route is inappropriate for in vivo testing; for risk evaluations more weight should be given to data from a physiologically relevant administration route. The liver MN test is sufficiently validated for the development of an OECD guideline. However, the impact of dosing animals >6 weeks of age needs to be evaluated. The GI tract MN test shows promise but needs more validation for an OECD guideline. The Pig-a assay detects systemically available mutagens and is a valuable follow-up to in vitro positive results. A new freeze-thaw protocol provides more flexibility. Mutant reticulocyte and erythrocyte frequencies should both be determined. Preliminary data are available for the Pig-a assay in male rat germ cells which require validation including germ cell DNA mutation origin.

A comparison of transgenic rodent mutation and in vivo comet assay responses for 91 chemicals

A database of 91 chemicals with published data from both transgenic rodent mutation (TGR) and rodent comet assays has been compiled. The objective was to compare the sensitivity of the two assays for detecting genotoxicity. Critical aspects of study design and results were tabulated for each dataset. There were fewer datasets from rats than mice, particularly for the TGR assay, and therefore, results from both species were combined for further analysis. TGR and comet responses were compared in liver and bone marrow (the most commonly studied tissues), and in stomach and colon evaluated either separately or in combination with other GI tract segments. Overall positive, negative, or equivocal test results were assessed for each chemical across the tissues examined in the TGR and comet assays using two approaches: 1) overall calls based on weight of evidence (WoE) and expert judgement, and 2) curation of the data based on a priori acceptability criteria prior to deriving final tissue specific calls. Since the database contains a high prevalence of positive results, overall agreement between the assays was determined using statistics adjusted for prevalence (using AC1 and PABAK). These coefficients showed fair or moderate to good agreement for liver and the GI tract (predominantly stomach and colon data) using WoE, reduced agreement for stomach and colon evaluated separately using data curation, and poor or no agreement for bone marrow using both the WoE and data curation approaches. Confidence in these results is higher for liver than for the other tissues, for which there were less data. Our analysis finds that comet and TGR generally identify the same compounds (mainly potent mutagens) as genotoxic in liver, stomach and colon, but not in bone marrow. However, the current database content precluded drawing assay concordance conclusions for weak mutagens and non-DNA reactive chemicals.

Analysis of negative historical control group data from the in vitro micronucleus assay using TK6 cells

The recent revisions of the Organisation for Economic Co-operation and Development (OECD) genetic toxicology test guidelines emphasize the importance of historical negative controls both for data quality and interpretation. The goal of a HESI Genetic Toxicology Technical Committee (GTTC) workgroup was to collect data from participating laboratories and to conduct a statistical analysis to understand and publish the range of values that are normally seen in experienced laboratories using TK6 cells to conduct the in vitro micronucleus assay. Data from negative control samples from in vitro micronucleus assays using TK6 cells from 13 laboratories were collected using a standard collection form. Although in some cases statistically significant differences can be seen within laboratories for different test conditions, they were very small. The mean incidence of micronucleated cells/1000 cells ranged from 3.2/1000 to 13.8/1000. These almost four-fold differences in micronucleus levels cannot be explained by differences in scoring method, presence or absence of exogenous metabolic activation (S9), length of treatment, presence or absence of cytochalasin B or different solvents used as vehicles. The range of means from the four laboratories using flow cytometry methods (3.7-fold: 3.5-12.9 micronucleated cells/1000 cells) was similar to that from the nine laboratories using other scoring methods (4.3-fold: 3.2-13.8 micronucleated cells/1000 cells). No laboratory could be identified as an outlier or as showing unacceptably high variability. Quality Control (QC) methods applied to analyse the intra-laboratory variability showed that there was evidence of inter-experimental variability greater than would be expected by chance (i.e. over-dispersion). However, in general, this was low. This study demonstrates the value of QC methods in helping to analyse the reproducibility of results, building up a 'normal' range of values, and as an aid to identify variability within a laboratory in order to implement processes to maintain and improve uniformity.

Derivation of point of departure (PoD) estimates in genetic toxicology studies and their potential applications in risk assessment

Genetic toxicology data have traditionally been employed for qualitative, rather than quantitative evaluations of hazard. As a continuation of our earlier report that analyzed ethyl methanesulfonate (EMS) and methyl methanesulfonate (MMS) dose-response data (Gollapudi et al., 2013), here we present analyses of 1-ethyl-1-nitrosourea (ENU) and 1-methyl-1-nitrosourea (MNU) dose-response data and additional approaches for the determination of genetic toxicity point-of-departure (PoD) metrics. We previously described methods to determine the no-observed-genotoxic-effect-level (NOGEL), the breakpoint-dose (BPD; previously named Td), and the benchmark dose (BMD10 ) for genetic toxicity endpoints. In this study we employed those methods, along with a new approach, to determine the non-linear slope-transition-dose (STD), and alternative methods to determine the BPD and BMD, for the analyses of nine ENU and 22 MNU datasets across a range of in vitro and in vivo endpoints. The NOGEL, BMDL10 and BMDL1SD PoD metrics could be readily calculated for most gene mutation and chromosomal damage studies; however, BPDs and STDs could not always be derived due to data limitations and constraints of the underlying statistical methods. The BMDL10 values were often lower than the other PoDs, and the distribution of BMDL10 values produced the lowest median PoD. Our observations indicate that, among the methods investigated in this study, the BMD approach is the preferred PoD for quantitatively describing genetic toxicology data. Once genetic toxicology PoDs are calculated via this approach, they can be used to derive reference doses and margin of exposure values that may be useful for evaluating human risk and regulatory decision making.

Biological importance and statistical significance

Statistical ideas behind the analysis of experiments related to crop composition and the genetic factors underlying composition are discussed. The emphasis is on concepts rather than statistical formulations. Statistical analysis and biological considerations are shown to be complementary rather than contradictory, in that the statistical analysis of a data set depends on the experimental design, that no amount of statistical sophistication can rescue a badly designed study, and that good experimental design is crucial. The traditional null hypothesis significance testing approach has severe limitations, but p values and statistical significance still often seem to be the primary objective of an analysis. Emphasis instead should be on identifying the size of effects that are biologically important and, with the involvement of the "domain" scientist, using these to help design experiments with appropriate sample sizes and statistical power. The issues discussed here are also directly applicable to other areas of research.

Quantitative approaches for assessing dose-response relationships in genetic toxicology studies

Genetic toxicology studies are required for the safety assessment of chemicals. Data from these studies have historically been interpreted in a qualitative, dichotomous "yes" or "no" manner without analysis of dose-response relationships. This article is based upon the work of an international multi-sector group that examined how quantitative dose-response relationships for in vitro and in vivo genetic toxicology data might be used to improve human risk assessment. The group examined three quantitative approaches for analyzing dose-response curves and deriving point-of-departure (POD) metrics (i.e., the no-observed-genotoxic-effect-level (NOGEL), the threshold effect level (Td), and the benchmark dose (BMD)), using data for the induction of micronuclei and gene mutations by methyl methanesulfonate or ethyl methanesulfonate in vitro and in vivo. These results suggest that the POD descriptors obtained using the different approaches are within the same order of magnitude, with more variability observed for the in vivo assays. The different approaches were found to be complementary as each has advantages and limitations. The results further indicate that the lower confidence limit of a benchmark response rate of 10% (BMDL(10) ) could be considered a satisfactory POD when analyzing genotoxicity data using the BMD approach. The models described permit the identification of POD values that could be combined with mode of action analysis to determine whether exposure(s) below a particular level constitutes a significant human risk. Subsequent analyses will expand the number of substances and endpoints investigated, and continue to evaluate the utility of quantitative approaches for analysis of genetic toxicity dose-response data.

The cyclin-dependent kinase inhibitor p57(Kip2) is epigenetically regulated in carboplatin resistance and results in collateral sensitivity to the CDK inhibitor seliciclib in ovarian cancer

Background:

Carboplatin remains a first-line agent in the management of epithelial ovarian cancer (EOC). Unfortunately, platinum-resistant disease ultimately occurs in most patients. Using a novel EOC cell line with acquired resistance to carboplatin: PEO1CarbR, genome-wide micro-array profiling identified the cyclin-dependent kinase inhibitor p57^Kip2 as specifically downregulated in carboplatin resistance. Presently, we describe confirmation of these preliminary data with a variety of approaches.

Methods:

Cytotoxicity testing (MTT) and cell cycle blockade assessed drug responsiveness. Methylation specific PCR and pyrosequencing identified sites of promoter methylation in p57^Kip2. siRNA to p57^Kip2 was used to look at the changes in apoptosis of carboplatin treated EOC cells. EOC tissues (20 cases) were assessed for mRNA levels of p57^Kip2.

Results:

Carboplatin resistance was reversed using 5-aza-cytidine in vitro. Promoter methylation sites and preferential sensitivity to seliciclib were seen in PEO1CarbR cells. Silencing p57^Kip2 decreased the apoptotic response to the effects of platinum but produced sensitisation to seliciclib. EOC biopsies indicated an association of high levels of p57^Kip2mRNA with complete responses to chemotherapy and improved outcome.

Conclusion:

We conclude that p57^Kip2 is a candidate biomarker of platinum sensitivity/resistance in EOC and such cases may show preferential response to the cyclin-dependent kinase inhibitor seliciclib.

Polo-like kinase Plk2 is an epigenetic determinant of chemosensitivity and clinical outcomes in ovarian cancer

Resistance to platinum- and taxane-based chemotherapy remains a major clinical impediment to effective management of epithelial ovarian cancer (EOC). To gain insights into resistance mechanisms, we compared gene and confirmed expression patterns of novel EOC cell lines selected for paclitaxel and carboplatin resistance. Here, we report that resistance can be conferred by downregulation of the Polo-like kinase Plk2. Mechanistic investigations revealed that downregulation occurred at the level of transcription via associated DNA methylation of the CpG island in the Plk2 gene promoter in cell lines, primary tumors, and patient sera. Inhibitory RNA (RNAi)-mediated knockdown and ectopic overexpression established a critical functional role for Plk2 in determining apoptotic sensitivity to paclitaxel and carboplatin. In drug-resistant human EOC cell lines, Plk2 promoter methylation varied with the degree of drug resistance and transcriptional silencing of the promoter. RNAi-dependent knockdown of Plk2 abrogated G(2)-M cell-cycle blockade by paclitaxel, conferring resistance to both paclitaxel and platinum. Conversely, ectopic expression of Plk2 restored sensitivity to G(2)-M cell-cycle blockade and cytotoxicity triggered by paclitaxel. In clinical cases, DNA methylation of the Plk2 CpG island in tumor tissue was associated with a higher risk of relapse in patients treated postoperatively with carboplatin and paclitaxel (P = 0.003). This trend was also reflected in the analysis of matched serum samples. Taken together, our results implicate Plk2 as a clinically important determinant of chemosensitivity, in support of the candidacy of Plk2 as a theranostic marker to inform EOC management.

A randomized controlled trial investigating the effects of celecoxib in patients with localized prostate cancer

Cyclooxygenase-2 (COX-2) is associated with tumour promotion, inhibition of apoptosis, angiogenesis and metastasis. Celecoxib, a selective COX-2 inhibitor was investigated, in patients with clinically localized prostate cancer using immunohistochemistry.

PATIENTS AND METHODS

Patients with cT1-2 prostate cancer (n=45) were randomized to celecoxib 400mg b.d. or no treatment for four weeks prior to radical prostatectomy. Histological sections of preoperative biopsy and matched radical prostatectomy specimens were stained for markers of cell proliferation (MIB-1/Ki-67), microvessel density (CD-31 with Weidner scoring), COX-2, apoptosis (TUNEL analysis), angiogenic factors (VEGF and KDR) and HIF-1.

RESULTS

Celecoxib decreased tumour cell proliferation, microvessel density, angiogenesis and HIF-1 whilst enhancing apoptosis. These effects approached statistical significance in a multivariate model and the cell proliferation index approached statistical significance on univariate analysis.

CONCLUSION

In this pilot study a 4 week regimen of celecoxib resulted in measurable biological effects in prostate cancer tissue. These findings warrant further investigation.

Reduced adhesion formation following laparoscopic versus open colorectal surgery

BACKGROUND

Adhesion formation is common after abdominal surgery. This study aimed to compare the extent of adhesion formation following laparoscopic and open colorectal surgery.

METHODS

An observational study was undertaken to identify adhesions in patients undergoing laparoscopy after previous laparoscopic or open colectomy. Adhesions were scored according to a system validated for interobserver (median kappa = 0.80) and intraobserver (kappa = 0.82) agreement. The primary endpoint was the overall adhesion score (0-10); a secondary endpoint was the adhesion score at the main incision site (0-6).

RESULTS

Forty-six patients were recruited (13 laparoscopic and 33 open colectomy). In most patients (n = 29), laparoscopy was performed for tumour staging before liver resection. The median (interquartile range) overall adhesion score was 7 (5-8) in the open group and 0 (0-3) in the laparoscopic group (P < 0.001). A similar difference was found for the main incision score: 6 (4-6) versus 0 (0-0) (P < 0.001).

CONCLUSION

There may be a reduction in adhesion formation following laparoscopic compared with open colectomy, although the small sample size limits this conclusion.

Report of an Expert Panel on the reanalysis by of a 90-day study conducted by Monsanto in support of the safety of a genetically modified corn variety (MON 863)

MON 863, a genetically engineered corn variety that contains the gene for modified Bacillus thuringiensis Cry3Bb1 protein to protect against corn rootworm, was tested in a 90-day toxicity study as part of the process to gain regulatory approval. This study was reanalyzed by Séralini et al. who contended that the study showed possible hepatorenal effects of MON 863. An Expert Panel was convened to assess the original study results as analyzed by the Monsanto Company and the reanalysis conducted by Séralini et al. The Expert Panel concludes that the Séralini et al. reanalysis provided no evidence to indicate that MON 863 was associated with adverse effects in the 90-day rat study. In each case, statistical findings reported by both Monsanto and Séralini et al. were considered to be unrelated to treatment or of no biological or clinical importance because they failed to demonstrate a dose-response relationship, reproducibility over time, association with other relevant changes (e.g., histopathology), occurrence in both sexes, difference outside the normal range of variation, or biological plausibility with respect to cause-and-effect. The Séralini et al. reanalysis does not advance any new scientific data to indicate that MON 863 caused adverse effects in the 90-day rat study.

Indicators of bacterial infection in patients with acute exacerbation of chronic bronchitis for application in clinical trials of antibacterial drugs

OBJECTIVES

This study examined the accuracy of: (a) patient symptoms; (b) microscopic examination of sputum purulence (>25 WBCs and <10 epithelial cells) and (c) microscopic examination of morphological bacterial cell types, in identifying bacterial infection in patients with an acute exacerbation of chronic bronchitis (AECB) for entry to clinical trials.

METHODS

Subjects had a worsening of at least two symptoms from: dyspnoea, sputum volume, and sputum purulence (Anthonisen Type 1 or 2 exacerbation). Sputum samples were collected from all subjects.

RESULTS

A total of 97 sputum samples were evaluated. Overall, 58 (60%) subjects were culture-positive; 22 of 29 (76%) subjects with Type 2 exacerbation had a bacterial pathogen isolated compared with 36 of 68 (53%) Type 1 subjects. This difference was not statistically significant. Microscopically purulent samples were found to be significantly more likely to be culture-positive than non-purulent samples. However, the sensitivity (60%) and specificity (67%); and the positive predictive value (73%) and negative predictive value (53%) observed, means that this is not an ideal predictive test for clinical trials. A semi-quantitative approach to Gram staining was identified as a potential indicator of bacterial infection. Sputum specimens with one bacterial cell type present at >10 cells per field, or more than one cell type present with at least one type at a concentration of >25 cells per field, had a high proportion (91%) of culture-positive specimens.

CONCLUSIONS

Symptoms alone are a poor indicator of bacterial infection. A semi-quantitative examination of a Gram-stained sputum preparation was the best indicator of bacterial infection. This finding may have relevance in the design of clinical trials of antibacterial drugs in AECB.

Systematic review of the costs of laparoscopic colorectal surgery

PURPOSE

Recent studies have confirmed the clinical efficacy of laparoscopic colorectal surgery; however, its use has not become widespread. One reason for this is perceived economic implications. A systematic review was undertaken examining the costs of laparoscopic colorectal surgery.

METHODS

Electronic databases were searched for articles comparing laparoscopic colorectal surgery and open surgery. Primary outcome measures were operating room, direct hospital, and indirect costs. Secondary outcomes were conversion rates and length of hospital stay. The percentage difference in costs was used for comparisons between studies.

RESULTS

Twenty-nine articles were identified in which cost data were presented (total number of patients 3,681); the economic data in most studies was limited. Operating room costs were greater for laparoscopic colorectal surgery than open surgery in all studies (median difference, 50 percent; interquartile range, 27-78 percent; P < 0.001). There was no overall difference in total hospital costs (median difference, 0 percent; interquartile range, -17.5 to 21 percent). Only two articles collected data on indirect costs, with both in favor of laparoscopic colorectal surgery. Hospital stay was shorter for laparoscopic colorectal surgery in all studies (median difference, 2.8 days; interquartile range, 1.3-3.7; P < 0.001). Median conversion rate was 7.8 percent (mean, 14 percent; interquartile range, 6-21 percent).

CONCLUSIONS

Operating room costs are greater for laparoscopic colorectal surgery than open surgery. Total hospital costs are similar. There may be societal benefits associated with lower indirect costs for laparoscopic colorectal surgery. Cost should not be a deterrent to performing laparoscopic colorectal surgery.

Validation and reproducibility of a semi-quantitative Food Frequency Questionnaire for use in elderly Swiss women

OBJECTIVE

The principal aim of this study was to develop a Swiss Food Frequency Questionnaire (FFQ) for the elderly population for use in a study to investigate the influence of nutritional factors on bone health. The secondary aim was to assess its validity and both short-term and long-term reproducibility.

DESIGN

A 4-day weighed record (4 d WR) was applied to 51 randomly selected women of a mean age of 80.3 years. Subsequently, a detailed FFQ was developed, cross-validated against a further 44 4-d WR, and the short- (1 month, n = 15) and long-term (12 months, n = 14) reproducibility examined.

SETTING

French speaking part of Switzerland.

SUBJECTS

The subjects were randomly selected women recruited from the Swiss Evaluation of the Methods of Measurement of Osteoporotic Fracture cohort study.

RESULTS

Mean energy intakes by 4-d WR and FFQ showed no significant difference [1564.9 kcal (SD 351.1); 1641.3 kcal (SD 523.2) respectively]. Mean crude nutrient intakes were also similar (with nonsignifcant P-values examining the differences in intake) and ranged from 0.13 (potassium) to 0.48 (magnesium). Similar results were found in the reproducibility studies.

CONCLUSION

These findings provide evidence that this FFQ adequately estimates nutrient intakes and can be used to rank individuals within distributions of intake in specific populations.

Plasma antibody titres to heat shock proteins-60, -65 and-70: their relationship to coronary risk factors in dyslipidaemic patients and healthy individuals

OBJECTIVE

To investigate the factors that may affect antibody titres to heat shock proteins (Hsp)-60, -65 and -70, and serum C-reactive protein (CRP) concentrations in patients with dyslipidaemia and other features of the metabolic syndrome as defined by ATPIII criteria.

MATERIAL AND METHODS

The study comprised 237 dyslipidaemia patients and 135 healthy individuals recruited from amongst university and hospital employees.

RESULTS

Compared to the healthy individuals, the dyslipidaemic patients had higher antibody titres to Hsp-60 (p<0.01), Hsp-65 (p<0.001) and Hsp-70 (p<0.05), and higher serum CRP concentrations (p<0.001). The best-fitting multifactorial models revealed that known coronary risk factors explained little of the variation in Hsp antibody titres: 3 % for Hsp-60, 1 % for Hsp-65 and 4 % for Hsp-70 amongst the dyslipidaemic subjects. The corresponding values for the subgroup with the metabolic syndrome were 8 %, 3 % and 1 %, respectively. In contrast, the best-fitting model explained 13.5 % of the variation in serum CRP concentrations among the dyslipidaemic patients, obesity being a major determinant; and 14 % in the subgroup with metabolic syndrome.

CONCLUSIONS

The higher antibody titres to Hsp-60, -65, and -70 in the dyslipidaemic patients may be related to a heightened state of immunoactivation associated with atherosclerosis in this group. Our data indicate that antibody titres to these Hsps are not associated with the classical coronary risk factors, although serum high sensitivity (hs)CRP concentrations were significantly related to obesity.

Coexpression of the IGF-IR, EGFR and HER-2 is common in colorectal cancer patients

Signalling via the insulin-like growth factor-I receptor (IGF-IR) has been associated with resistance to anti-epidermal growth factor receptor (EGFR) and human epidermal growth factor receptor-2 (HER-2)-based therapies in the experimental system, but the coexpression and clinical significance of the IGF-IR, EGFR and HER-2 in cancer patients remains unclear. IGF-IR, EGFR, and HER-2 status was assessed retrospectively in tumour specimens from 87 Dukes' C colorectal cancer patients using immunohistochemistry. Sections were scored by the percentage of positive cells (membrane and cytoplasmic) and intensity of staining. The association between receptor coexpression and clinicopathological parameters and overall survival were evaluated using univariate and multivariate (Cox) analysis. Overall, 93, 83 and 89% of the cases expressed IGF-IR, EGFR and HER-2, respectively. While 60% of the cases expressed membranous IGF-IR, the expression of EGFR and HER-2 was predominantly cytoplasmic. Coexpression of the IGF-IR, EGFR and HER-2 was present in tumours from 75% of the patients. No significant association was found between the expression or coexpression of total IGF-IR, EGFR and HER-2 and clinicopathological parameters or overall survival. Our results indicate that coexpression of IGF-IR, EGFR and HER-2 is common in Dukes' C colorectal cancer, warranting further investigation on the co-targeting of such receptors in colorectal cancer patients.

Gleason Scoring Varies Among Pathologists and this Affects Clinical Risk in Patients with Prostate Cancer

AIMS

To investigate whether our practice of specialist review of all diagnostic biopsies was necessary to prevent misgrading of referred prostate cancer patients, and whether this misclassification, if any, would have resulted in misclassification of clinical risk grouping (Seattle Risk Grouping [SRG]) and subsequent treatment strategy and prognosis.

MATERIALS AND METHODS

Important prognostic indicators for prostate cancer include the presenting prostate-specific antigen (PSA), clinical stage and Gleason sum of the tumour. These three variables are incorporated into the SRG cohorts to establish treatment strategy. Patients with prostate cancer referred for brachytherapy had their prostate biopsies reviewed by a reference pathologist (PD) with a special interest in prostate cancer. We compared the agreement between the scoring of the referring pathologists with that of PD, and evaluated if any differences changed the SRG and therefore the clinical risk and treatment strategy for the patients.

RESULTS

In only 52% (43/83) of cases, was there total agreement between the two sets of pathologists. The inter-rater agreement was statistically 'fair' (unweighted kappa statistic 0.27). In 90% (36/40) of cases with disagreement, PD assigned higher Gleason sums. In 40% (16/40) of cases with disagreement, the change in Gleason sum altered the SRG; in one out of 16 cases, the SRG was downgraded from 'intermediate' to 'low' risk disease; in six out of 16 cases, it was upgraded from 'low' to 'intermediate' risk, and, in nine out of 16, from 'intermediate' to 'high' risk.

CONCLUSION

Our findings confirm previous reports of only limited correlation between pathologists in reporting Gleason sums. In this study, 19% (16/83) of cases had their grading changed to a level that altered clinical risk, almost always (94%; 15/16) to one that worsened prognosis. This would have significantly affected treatment strategy for these patients, and thus we recommend that all centres ensure accurate Gleason grading by the use of pathologists with special interests in prostate cancer.

Coexpression, prognostic significance and predictive value of EGFR, EGFRvIII and phosphorylated EGFR in colorectal cancer

In colorectal cancer, epidermal growth factor receptor (EGFR) expression is reported in 8-100% of the cases examined and there has been no clear association between EGFR expression and prognosis, or response to EGFR inhibitors. In this retrospective study, 87 archival specimens from node positive (Dukes' C) colorectal cancer patients were analysed immunohistochemically, for the expression of EGFR, mutant EGFR (EGFRvIII) and phosphorylated EGFR (pEGFR, tyr1068). Each section was scored on the basis of location and intensity of staining, and the immunostaining was considered positive if greater than 10% of the tumour cells were stained by the antibody. The association between these scores and overall survival was estimated using univariate and multivariate (Cox) analysis. Overall, we found that 76% and 100% of cases were EGFR positive using antibodies to the external or internal domain of EGFR respectively, and 34% of the cases were EGFRvIII positive. However, only 8% of the cases expressed pEGFR and pEGFR immunostaining was never present in more than 10% of tumour cells. The expression of EGFR, EGFRvIII, pEGFR, or coexpression of EGFR and EGFRvIII was not associated with overall survival. Cytoplasmic expression of EGFR (p = 0.0141) or EGFRvIII (p = 0.005) was, however, associated with improved survival in patients receiving radiotherapy. Our results suggest that coexpression of cytoplasmic EGFR and EGFRvIII occurs in a significant proportion (34%) of Dukes' C colorectal cancer and the cytoplasmic expression of EGFR or EGFRvIII is a good indicator of response to radiotherapy.

Dose-response and threshold-mediated mechanisms in mutagenesis: statistical models and study design

The objective of this paper is to review the use, in mutagenesis, of various mathematical models to describe the dose-response relationship and to try to identify thresholds. It is often taken as axiomatic that genotoxic carcinogens could damage DNA at any level of exposure, leading to a mutation, and that this could ultimately result in tumour development. This has led to the assumption that for genotoxic chemicals, there is no discernible threshold. This assumption is increasingly being challenged in the case of aneugens. The distinction between 'absolute' and 'pragmatic' thresholds is made and the difficulties in determining 'absolute' thresholds using hypothesis testing approaches are described. The potential of approaches, based upon estimation rather than statistical significance for the characterization of dose-response relationships, is stressed. The achievement of a good fit of a mathematical model to experimental data is not proof that the mechanism supposedly underlying this model is operating. It has been argued, in the case of genotoxic chemicals, that any effects produced by a genotoxic chemical which augments that producing a background incidence in unexposed individuals will lead to a dose-response relationship that is non-thresholded and is linear at low doses. The assumptions underlying this presumption are explored in the context of the increasing knowledge of the mechanistic basis of mutagenicity and carcinogenicity. The possibility that exposure to low levels of genotoxic chemicals may induce and enhance defence and repair mechanisms is not easily incorporated into many of the existing mathematical models and should be an objective in the development of the next generation of biologically based dose-response (BB-DR) models. Studies aimed at detecting or characterizing non-linearities in the dose-response relationship need appropriate experimental designs with careful attention to the choice of biomarker, number and selection of dose levels, optimum allocation of experimental units and appropriate levels of replication within and repetition of experiments. The characterization of dose-response relationships with appropriate measures of uncertainty can help to identify 'pragmatic' thresholds based upon biologically relevant criteria which can help in the regulatory process.

Report and summary of the major conclusions from statistics in genotoxicity testing working group from the International Workshop on Genotoxicity Test Procedures (IWGTP), March 1999

A working group of five statisticians experienced in the use of statistical methods in mutagenicity reviewed aspects of the statistical analysis of genotoxicity test procedures. Issues discussed included methods for integrating biological importance and statistical significance, the relationship of the experimental unit to the experimental design, and the impact of new developments in statistics and computing. Three major recommendations were made relating to the need for: (1) the effective use of statistical advice in designing interlaboratory and intralaboratory investigations; (2) the development of appropriate experimental designs for new assays; and (3) education and training in the use of statistical methodology in mutagenicity testing. Environ. Mol. Mutagen. 35:260-263, 2000 Published 2000 Wiley-Liss, Inc.

Genetic variation in the metabolism of coumarin in mouse liver

The metabolism of 50 microM [3-14C] coumarin to polar products separated by high performance liquid chromatography (HPLC) and covalently bound metabolites in liver microsomes was compared in a series of inbred strains of mice. Coumarin metabolism to total polar products was higher in female than male mice. In all strains, the coumarin 3,4-epoxidation pathway was the major route of metabolism with o-hydroxyphenylacetaldehyde (o-HPA) as the major metabolite. However, in females, there was a major strain difference in the degree of metabolism to coumarin 7-hydroxylase with DBA/2 and 129 having high 7-hydroxycoumarin formation, CBA/Ca having intermediate levels and the other strains low levels. The differences between the strains was much less pronounced in the male mice. There was also evidence for strain variation in metabolism in the quantities of a number of other coumarin metabolites as detected by HPLC analysis of incubate extracts. However, this variation was of a quantitative nature and relatively small. The metabolism of B6C3F1 hybrid mice, in which coumarin had been identified as carcinogenic in a long-term cancer bioassay, was qualitatively similar to that of the other genotypes. The DBA/2 mouse has been suggested as a model for the metabolism of coumarin in humans. The pattern of metabolism found in this strain is different from most other strains. However, the pattern found for all the mouse strains, including DBA/2, differed appreciably from the profiles for other species including humans in the extent of 7-hydroxylation.

Issues related to the experimental design and subsequent statistical analysis of in vivo and in vitro comet studies

A wide range of experimental designs are used in investigations using the Comet assay. The statistical issues associated with this assay are however not particularly unusual or difficult. It is important however to recognize that the sample rather than the cell is the experimental unit. Statistical analyses based upon measures from the individual cells can lead to serious misinterpretation of results. Interpretation of the results of the assay should be related to identifying changes of biological importance rather than relying solely on the P values of specific statistical tests.

Statistical analysis of adaptive response in sister chromatid exchanges in human lymphocytes after treatment with very low and extremely low doses of N-methyl-N'-nitro-N-nitrosoguanidine using a study design to control variability

Previous studies have been interpreted as suggesting that low concentrations of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) have an adaptive effect in the cultured lymphocytes of responsive donors (that is, the cells are protected against the mutagenic effects of a subsequent challenge with a higher concentration of MNNG). The objectives of the present study were to investigate, under stringent experimental conditions, whether a protective effect exists at very low and extremely low doses of MNNG (10(-8) and 10(-24) M, respectively). Peripheral blood lymphocytes from a donor considered responsive in a previous study were stimulated to divide and were cultured under standard conditions. Pre-adaptive treatments with dilutions of MNNG were added to the cultures repeatedly before a challenge treatment with MNNG. Bromodeoxyuridine was added at the same time as the challenge treatment and, following mitotic arrest, cells were differentially stained so that the number of sister chromatid exchanges (SCEs) could be counted. The study was designed to address potential criticisms of earlier studies which did not include replicate cultures. Samples of blood were divided into two identical batches for independent processing. Five replicate cultures were prepared for each combination of pre-adaptive and challenge treatments in each batch. The complete experiment was repeated to provide a further test of the consistency of results. Five replicates per treatment combination were chosen in an attempt to provide an experiment of adequate statistical power. Considerable precautions were taken to minimise the effect of factors outside experimental control on the results. Scoring was done by three scorers. In order to minimise inter-scorer variation, 240 cells were scored at each treatment observation (five cells per-scorer, three scorers per culture, four cultures per batch, two batches per experiment and two experiments). The study was designed in this way to take account of the sources of variability to ensure that any response obtained would exceed that obtainable by experimental variability alone. A high level of quality assurance monitoring was undertaken throughout the investigation. Two measures of SCE induction were used: (i) the mean frequency of SCEs; (iii) proportion of cells with at least 20 SCEs. In both experiments, the challenge concentration of MNNG significantly increased SCE frequency. There were, however, highly significant differences between the two experiments. The proportion of high frequency cells (HFCs) in Experiment 1 was increased significantly; the proportion of HFCs was also increased in Experiment 2, but the increase was not statistically significant. The pre-adaptive concentrations of MNNG included an extremely low dilution of 6.8 x 10(-24) M and a very low dilution of 6.8 x 10(-8) M in Experiment 1 and 1.4 x 10(-7) M in Experiment 2. The various pre-adaptive concentrations used had no consistent protective effect against the SCE-inducing capacity of the challenge concentration of MNNG of 6.8 x 10(-6) M. It is concluded that an adaptive response to the alkylating agent MNNG could not be demonstrated in cultured human lymphocytes. Neither a very low nor an extremely low dilution of MNNG elicited an adaptive response in terms of SCE induction (measured either as SCE frequency or as proportion of HFCs). This is in contradiction to previous reports published by us and other groups. This study was carefully designed with large numbers of replicates, a preliminary statistical power calculation, predefined comparisons and extensive quality assurance at each treatment administration. Despite these precautions the variability between scorers and between batches was much larger than anticipated. This resulted in some statistically significant differences, but these are likely to be false positives. Our findings indicate the need for such methodological refinement in human cell adaptive response studies.

Quantitative risk assessment and the limitations of the linearized multistage model

1. Quantifying carcinogenic risk is an important objective for assisting in the assessment and management of risks from chemical exposure. The most widely used of the many mathematical models proposed for extrapolation of carcinogenicity data from animal studies to low dose human exposures is the linearized multistage (LMS) model. This has, in effect, become the default approach for much of Quantitative Risk Assessment (QRA). The practical properties of this model have been investigated. 2. Analysis of stimulated data using the LMS model showed (i) that the Maximum Likelihood Estimate (MLE) of the low dose slope, q1, was unstable and extremely sensitive to small changes in the data; (ii) the 95% Upper Confidence Limit (UCL) estimate, q1*, preferred by the US Environmental Protection Agency (EPA) was insensitive with only small changes in values being obtained for large changes in the data; (iii) data sets where there was no statistical significance could give risk estimates similar to those obtained from data sets with clear dose-related effects; (iv) the size of the values of the Virtually Safe Dose (VSD) obtained did not necessarily relate to the biological interpretation of the data sets; (v) the value of q1* obtained was closely related to the top dose used in the study. 3. Limitations of the LMS model were illustrated by examples of its use in assessing the carcinogenicity of 2, 3, 7, 8-TCDD leading to the conclusion that the existing models are not suitable for routine use in the estimation of the risk from chemical carcinogens. The use of the LMS model has been justified in part by its original derivation from a mathematical model based upon a multistage model of carcinogenesis. However, estimates of the parameters of the model used to provide estimates of low dose risk to humans have no direct relationship to specific biological event in carcinogenesis. Further developments in mathematical models and increased understanding of the biological events underlying the carcinogenesis will lead to more biologically plausible QRA methods which would then justify serious consideration of QRA by regulatory authorities throughout the world.

Population genetics of induced mutations

The contribution of induced mutations to the burden of genetic disease in the context of population genetics is considered. A clear distinction is made between the effects of genetic disease and mutational events. Much of the existing burden of genetic disease is a consequence of mutations that occurred in the past. The problem of distinguishing between spontaneous and induced mutations is discussed. Molecular genetics techniques are blurring the definitions of these terms. Classical population genetics shows that the frequency of affected individuals will reach an equilibrium depending on the mutation rate and the selective pressure against affected individuals. Increasing the mutation rate or reducing the selective pressures would result in a new equilibrium with an increase in the frequency in subsequent generations of affected individuals with dominant and X-linked mutant alleles. The increase in the number of recessive mutant alleles would be much slower and take many generations to reach the new equilibrium level. One assumption behind such equilibria is random mating. Changes in human demography with a rapid increase in population size, the breakup of small, relatively inbred subpopulations, and relaxed selective pressures will lead to a new equilibrium for recessive genes at probably higher frequencies. These factors will be the major contributors to increasing the burden of recessive genetic disease by increasing the total numbers of cases. The proportion of the population with a genetic disease will also continue to grow as a greater proportion of the population survives to late middle age and succumbs to diseases associated with old age, such as cancer, circulatory disease, dementias, and diabetes, each of which is likely to have a genetic component.(ABSTRACT TRUNCATED AT 250 WORDS)

Sex and strain differences in mouse hepatic microsomal coumarin 7-hydroxylase activity

Hepatic microsomal coumarin 7-hydroxylase activity has been determined in male and female mice of strains A/J, AKR, BALB/c, CBA/Ca, C3H/He, C57BL/6J, DBA/2 and 129. In males, coumarin 7-hydroxylase activity was highest in liver microsomes from DBA/2 mice and lowest in BALB/c mice. With female mice enzyme activity was highest in DBA/2 and 129 strains, intermediate in the CBA/Ca strain and comparatively low in the other five strains. Marked sex differences were observed in coumarin 7-hydroxylase activity with enzyme activity in female animals from strains DBA/2, 129 and CBA/Ca being 4.8-, 6.2- and 4.8-fold higher, respectively, than in male mice. In contrast, only minor sex and strain differences in levels of total microsomal cytochrome P-450 were observed. These results demonstrate marked sex and strain differences in mouse hepatic microsomal coumarin 7-hydroxylase activity. Such differences may be due to variations in particular cytochrome P-450 isoenzymes such as CYP2A5, not all of which can be explained by the known allelic difference in the Cyp2a-5 locus.

Impact of pharmacogenetics on toxicological studies. Statistical implications

Genetic differences in response to chemicals are probably ubiquitous. The implications of such genetic differences for the design of toxicological experiments is reviewed. The inclusion of genetic differences as factors in factorial experimental designs is illustrated by an experiment to investigate differences between inbred strains of mice in the length of anaesthesia following the administration of pentobarbitone to control and phenobarbitone pre-treated animals. The effects of including genetic differences as a factor in the standard toxicological studies such as the long-term rodent carcinogenicity bioassay (LTRCB) are reviewed. Such designs are likely to result in more chemicals being identified as carcinogens based upon the criteria of a positive result in the LTRCB. However, they are unlikely to provide any improvement in the estimation of risk to the human population from low level exposures to the chemicals because of the limitations of the existing extrapolation methods. Information on the degree of genetic variability in response to chemical exposure should, though, help in the more qualitative biologically-based risk assessment approaches favoured by some toxicologists. The presence of genetic differences in response also provides potential models for investigation of the mechanisms underlying toxicological responses.

Ethylene dibromide: negative results with the mouse dominant lethal assay and the electrophoretic specific-locus test

Ethylene dibromide (1,2-dibromoethane; EDB) was tested for the induction of dominant lethal and electrophoretically-detectable specific-locus mutations in the germ cells of DBA/2J male mice. Males were treated with a single intraperitoneal injection of 100 mg/kg EDB and mated to two C57BL/6J females. In the dominant lethal assay, matings were carried out to measure the effect of EDB on meiotic and postmeiotic stages; germ cells representing spermatogonial stem cells were analyzed in the electrophoretic specific-locus test. Neither of these germ cell tests produced any evidence that EDB is a germ cell mutagen. It appears from these data and those reported in the literature that EDB, a genotoxic carcinogen that affects male fertility in some mammalian species, is not mutagenic in the germ cells of the male mouse.

Thioguanine-resistant mutant frequency in T-lymphocytes from a healthy human population

Resistance to 6-thioguanine in T-lymphocytes was used to study in vivo somatic mutations in normal healthy adults. Donor age had a significant effect on mutant frequency at the hprt locus, showing an increase of 0.09/10(6) cells per year of age. No significant increase was associated with sex of donor, smoking habits, alcohol or coffee/tea intake, or X-ray exposure. The lower mutant frequency seen with contraceptive pill usage was probably due to the age difference between the groups of users and non-users.

Statistical issues arising from mouse micronucleus assay experiments of Ashby and co-workers

Data from a series of mouse micronucleus assays have been reanalysed to illustrate various statistical issues raised by Ashby and co-workers during the development of the assay. Most of the statistical points discussed in these earlier papers can be explained by the stochastic nature of the data. Reanalysis shows that the type of data collected in mammalian micronucleus assays is amenable to analysis by standard biometric methods. It is concluded that statistical analysis has an important role in the exploration and interpretation of data from the micronucleus assay.