Chronic Inhalation Toxicity and Carcinogenicity Testing of Respirable Fibrous Particles

On May 8–10, 1995, a workshop on chronic inhalation toxicity and carcinogenicity testing of respirable fibrous particles was held in Chapel Hill, North Carolina. The workshop was sponsored by the Office of Pollution Prevention and Toxics, U.S. Environmental Protection Agency (EPA), in collaboration with the National Institute of Environmental Health Sciences (NIEHS), the National Institute for Occupational Safety and Health (NIOSH), and the Occupational Safety and Health Administration (OSHA). The goal of the workshop was to obtain input from the scientific community on a number of issues related to fiber testing. Major issues for discussion were: (i) the optimal design and conduct of studies of the health effects of chronic inhalation exposure of animals to fibers; (ii) preliminary studies which would be useful guides in designing the chronic exposure study; (iii) mechanistic studies which would be important adjuncts to the chronic exposure study to enable better interpretation of study results and extrapolation of potential effects in exposed humans; and (iv) available screening tests which can be used to develop a minimum data set for (a) making decisions about the potential health hazard of the fibers and (b) prioritizing the need for further testing in a chronic inhalation study. After extensive discussion and debate of the workshop issues, the general consensus of the expert panel is that chronic inhalation studies of fibers in the rat are the most appropriate tests for predicting inhalation hazard and risk of fibers to humans. A number of guidances specific for the design and conduct of prechronic and chronic inhalation studies of fibers in rodents were recommended. For instance, it was recommended that along with other information (decrease in body weight, systemic toxicity, etc.), data should be obtained on lung burdens and bronchoalveolar lavage fluid analysis to assist in establishing the chronic exposure levels. Lung burden data are also important for quantifying aspects of risk assessment related to dosimetric adjustments before extrapolation. Although mechanistic studies are not recommended as part of the standard chronic inhalation studies, the expert panel stressed the need for obtaining mechanistic information as far as possible during the course of subchronic or chronic inhalation studies. At present, no single assay and battery of short-term assays can predict the outcome of a chronic inhalation bioassay with respect to carcinogenic effects. Meanwhile, several short-termin vitroandin vivostudies that may be useful to assess the relative potential of fibrous substances to cause lung toxicity/carcinogenicity have been identified.     

A dose-response model for refractory ceramic fibers

Refractory ceramic fibers (RCFs) are man-made vitreous fibers commonly used in insulation applications above 1000 degrees C. Although they have been subjected to considerable toxicologic evaluation, only the pooled results from two rat inhalation studies provide data that may be suitable for performing a numerical risk assessment. Even in these inhalation studies, good evidence exists that the maximum tolerated dose (MTD) was exceeded and that pulmonary overload occurred, a condition that will cause tumors whatever the dust responsible. Indeed, a significant yield of tumors was only obtained at the highest dose tested. If these results are omitted, there is no statistically significant evidence of carcinogenicity within the RCF results. Although there is little evidence that overload-related tumors are relevant to human risk, we adopted a conservative approach to obtain the estimates of risk regardless of overload, using a biologically based model, the two-stage clonal expansion model, as well as various statistical models, including the benchmark dose model. We argue that the data favor the use of a biologically based model, which gives the best fit when the highest dose RCF exposures are omitted. Continuing with this model, we show that available data from the RCF experiment, less outliers, coupled with results from other experiments with man-made mineral fibers (MMVFs), demonstrate that all MMVFs are potentially carcinogenic, with any risk mediated by the fibers' biopersistence. Application of this "all MMVF data set" model yields a maximum likely estimate for RCF excess unit risk of 4.6 x 10(-5) (95% upper confidence limit = 9.2 x 10(-5) per fiber/ml). This implies that the risk from occupational exposure to RCFs at 1 fiber/ml for a typical working lifetime would not exceed 10(-4).     

RESPIRATORY FAILURE FROM INHALATIONAL NICKEL CARBONYL EXPOSURE TREATED WITH CONTINUOUS HIGH-VOLUME HEMOFILTRATION AND DISULFIRAM

Refractory ceramic fibers (RCFs) are man-made vitreous fibers commonly used in insulation applications above 1000°C. Although they have been subjected to considerable toxicologic evaluation, only the pooled results from two rat inhalation studies provide data that may be suitable for performing a numerical risk assessment. Even in these inhalation studies, good evidence exists that the maximum tolerated dose (MTD) was exceeded and that pulmonary overload occurred, a condition that will cause tumors whatever the dust responsible. Indeed, a significant yield of tumors was only obtained at the highest dose tested. If these results are omitted, there is no statistically significant evidence of carcinogenicity within the RCF results. Although there is little evidence that overload-related tumors are relevant to human risk, we adopted a conservative approach to obtain the estimates of risk regardless of overload, using a biologically based model, the two-stage clonal expansion model, as well as various statistical models, including the benchmark dose model. We argue that the data favor the use of a biologically based model, which gives the best fit when the highest dose RCF exposures are omitted. Continuing with this model, we show that available data from the RCF experiment, less outliers, coupled with results from other experiments with man-made mineral fibers (MMVFs), demonstrate that all MMVFs are potentially carcinogenic, with any risk mediated by the fibers' biopersistence. Application of this "all MMVF data set" model yields a maximum likely estimate for RCF excess unit risk of 4.6 × 10 -5 (95% upper confidence limit = 9.2 × 10 -5 per fiber/ml). This implies that the risk from occupational exposure to RCFs at 1 fiber/ml for a typical working lifetime would not exceed 10 -4 .     

Establishing aerosol exposure concentrations for inhalation toxicity studies

Criteria for the selection of aerosol concentrations to be used in inhalation studies assessing the toxicity and carcinogenicity of chemical substances were discussed by the authors in a meeting sponsored by the National Toxicology Program. Concepts in the design of aerosol inhalation studies emerged from that meeting and are being communicated through this publication. Inhalation studies assessing the toxicity and carcinogenicity of aerosols have often used maximum exposure levels on the basis of technological feasibility. Evidence has now accumulated that the amount of pulmonary burden of deposited particles impacts on particle clearance above some as yet not well-defined exposure concentration. The sequelae are such that lung clearance decreases with increased particulate burden to the point of approaching complete cessation. This paper focuses on the major determinants in establishing maximal aerosol concentrations for use in inhalation toxicity studies with special emphasis on experimental design features to assess lung retention. The subject matter of this paper is a rapidly developing area in terms of knowledge. Accordingly, the contents of this article are intended as guidelines and not as absolute rules for the conduct and interpretation of inhalation exposure studies.     

Proceedings of the AMSAD workshop on Cannabis, Melbourne, August 1986

This document is a summary of the proceedings of a workshop on cannabis convened by the Australian Medical Society on Alcohol and Drugs at St. Vincent's Hospital, Melbourne, Victoria on August 11-12, 1986. The aim of the workshop was to review present knowledge about cannabis and to consider the options available for the use and/or control of cannabis in Australian society. Representatives of various disciplines (pharmacology, law enforcement, forensic medicine, clinical medicine, sociology, law, criminology) presented an up-to-date assessment of the use and control of cannabis. Three discussion groups considered issues involving clinical medicine and pharmacology, law enforcement and forensic medicine, and law, sociology and criminology respectively. Reports from each group were presented at a plenary session, from which a draft statement was prepared and approved by all participants. This, together with a draft report was circulated to all participants for final approval. The final outcome of the workshop was an approved statement to advise the Council of AMSAD about the control of cannabis use in the Australian community.     

Carcinogenicity of the insulation wools: reassessment of the IARC evaluation

In assessing the health evidence concerning man-made mineral fibers, the chemical composition, surface activity, durability, and size of fibers have to be taken into account. Special-purpose fine glass fibers need to be separated from the insulation wools (glass, rock, and slag wool). The epidemiological evidence is sufficient to conclude that there has been no mesothelioma risk to workers producing or using glass wool, rock wool, or slag wool. The epidemiological studies have been large and powerful, and they show no evidence of a cause-effect relationship between lung cancer and exposure to glass wool, rock wool, or slag wool fibers. There is some evidence of a small cancer hazard attached to the manufacturing process in slag wool plants 20 to 50 years ago, when asbestos was used in some products and other carcinogenic substances were present. However, this hazard is not associated with any index of exposure to slag wool itself. Animal inhalation studies of ordinary insulation wools also show that there is no evidence of hazard associated with exposure to these relatively coarse, soluble fibers. The evidence of carcinogenicity is limited to experiments with special-purpose fine durable glass fibers or experimental fibers, and only when these fibers are injected directly into the pleural or peritoneal cavity. Multiple chronic inhalation studies of these same special-purpose fine glass fibers have not produced evidence of carcinogenicity. It is suggested that the present IARC evaluation of the carcinogenic risk of insulation wools should be revised to Category 3: not classifiable as to carcinogenicity to humans.     

Classification of man-made vitreous fibers: Comments on the revaluation by an IARC working group

In 2001, an IARC working group revaluated the carcinogenic risks of man-made vitreous fibers (MMVF). Compared with the IARC evaluation in 1987, the overall evaluations of insulation glass wool, rock (stone) wool, and slag wool were changed from Group 2B to Group 3. These changes ensued from an alteration in the evidence for cancer in humans and in experimental animals: Instead of "sufficient," the evidence for cancer in experimental animals is now looked upon as "limited" if there is a carcinogenic response after intraperitoneal injection but not after recently conducted inhalation experiments. For these studies, it is argued that they did properly address the technological limitations of earlier inhalation experiments. For Maxim and McConnell [Maxim L.D., McConnell E.E., 2001. Interspecies comparisons of the toxicity of asbestos and synthetic vitreous fibers: a weight-of-the-evidence approach. Regul. Toxicol. Pharmacol. 33, 319-342], well-conducted inhalation studies are very sensitive and rats may be more sensitive than humans in detecting the carcinogenic potential of MMVF. However, their arguments are highly questionable. The explanations of the IARC working group for preferring the newer inhalation studies are not sufficiently supported by the published data. Having in mind the higher sensitivity of humans compared to rats after inhalation of asbestos, more emphasis should have been given to the carcinogenic response after intraperitoneal injection.     

Adverse outcome pathways: From research to regulation scientific workshop report

An adverse outcome pathway (AOP) helps to organize existing knowledge on chemical mode of action, starting with a molecular initiating event such as receptor binding, continuing through key events, and ending with an adverse outcome such as reproductive impairment. AOPs can help identify knowledge gaps where more research is needed to understand the underlying mechanisms, aid in chemical hazard characterization, and guide the development of new testing approaches that use fewer or no animals. A September 2014 workshop in Bethesda, Maryland considered how the AOP concept could improve regulatory assessments of chemical toxicity. Scientists from 21 countries, representing industry, academia, regulatory agencies, and special interest groups, attended the workshop, titled Adverse Outcome Pathways: From Research to Regulation. Workshop plenary presentations were followed by breakout sessions that considered regulatory acceptance of AOPs and AOP-based tools, criteria for building confidence in an AOP for regulatory use, and requirements to build quantitative AOPs and AOP networks. Discussions during the closing session emphasized a need to increase transparent and inclusive collaboration, especially with disciplines outside of toxicology. Additionally, to increase impact, working groups should be established to systematically prioritize and develop AOPs. Multiple collaborative projects and follow-up activities resulted from the workshop.     

Toxicity and carcinogenicity of nickel compounds

The toxicity and carcinogenicity of nickel compounds are considered in three broad categories: (1) systemic toxicology, (2) molecular toxicology, and (3) carcinogenicity. The systemic toxicity of nickel compounds is examined based upon human and animal studies. The major organs affected are discussed in three categories: (1) kidney, (2) immune system, and (3) other organs. The second area of concentration is molecular toxicology, which will include a discussion of the chemistry of nickel, its binding to small and large molecular weight ligands, and, finally, its cellular effects. The third major area involves a discussion of the carcinogenicity and genotoxicity of nickel compounds. This section focuses on mechanisms, using studies conducted in vivo and in vitro. It also includes a discussion of the assessment of the carcinogenicity of nickel compounds.     

Pharm. D. pathways to biomedical research: the National Institutes of Health special conference on pharmacy research

To address the shortage of research-trained pharmaceutical scientists (or doctor of pharmacy [Pharm.D.] scientists), a 2-day pharmacy research conference titled "Pharm.D. Pathways to Biomedical Research" was convened on December 13-14, 2006, at the National Institutes of Health (NIH) campus (Bethesda, MD). The workshop included invited speakers and participants from academia, industry, and government. Forty-two pharmacy schools were represented, including deans and clinical pharmaceutical scientists with current NIH funding. In addition, several pharmacy professional organizations were represented--American Association of Colleges of Pharmacy, American College of Clinical Pharmacy, American Society of Health-System Pharmacists, and the Accreditation Council on Pharmaceutical Education. The workshop was divided into three sessions followed by breakout discussion groups: the first session focused on presentations by leading pharmaceutical scientists who described their path to success; the second session examined the NIH grant system, particularly as it relates to training opportunities in biomedical research and funding mechanisms; and the third session addressed biomedical research education and training from the perspective of scientific societies and academia. We summarize the discussions and findings from the workshop and highlight some important considerations for the future of research in the pharmacy community. This report also puts forth recommendations for educating future pharmaceutical scientists.     

Approaches and considerations for the assessment of immunotoxicity for environmental chemicals: A workshop summary

As experience is gained with toxicology testing and as new assays and technologies are developed, it is critical for stakeholders to discuss opportunities to advance our overall testing strategies. To facilitate these discussions, a workshop on practices for assessing immunotoxicity for environmental chemicals was held with the goal of sharing perspectives on immunotoxicity testing strategies and experiences, developmental immunotoxicity (DIT), and integrated and alternative approaches to immunotoxicity testing. Experiences across the chemical and pharmaceutical industries suggested that standard toxicity studies, combined with triggered-based testing approaches, represent an effective and efficient approach to evaluate immunotoxic potential. Additionally, discussions on study design, critical windows, and new guideline approaches and experiences identified important factors to consider before initiating DIT evaluations including assay choice and timing and the impact of existing adult data. Participants agreed that integrating endpoints into standard repeat-dose studies should be considered for fulfilling any immunotoxicity testing requirements, while also maximizing information and reducing animal use. Participants also acknowledged that in vitro evaluation of immunosuppression is complex and may require the use of multiple assays that are still being developed. These workshop discussions should contribute to developing an effective but more resource and animal efficient approach for evaluating chemical immunotoxicity.     

Non-neoplastic lesions found only in the two-year bioassays but not in shorter toxicity studies of rats

International Conference on Harmonisation of Technical Requirements for Registration of Pharmaceuticals for Human Use (ICH) has been conducting a prospective evaluation period to validate the criteria for waiving some carcinogenicity studies in rats. Before the waiving strategy is practiced in ICH, it is crucial to elucidate whether non-neoplastic lesions are found only in 2-year rat carcinogenicity studies. To confirm possible importance of 2-year bioassays for evaluating chronic toxicity but not carcinogenicity, we retrospectively surveyed 59 pharmaceuticals approved by the Ministry of Health, Labour and Welfare (MHLW) from 2007 to 2010 in Japan for non-neoplastic lesions observed in carcinogenicity studies. Non-neoplastic histopathological lesions observed only in 2-year carcinogenicity studies but not in 6-month chronic toxicity studies using rats were compared with clinical adverse drug reactions (ADRs). Thirteen non-neoplastic lesions that may correlate with clinical ADRs were classified into three categories: Category 1, lesions not predictable from other nonclinical data except those from 2-year rat carcinogenicity studies; Category 2, lesions predictable mainly from chronic toxicity studies; Category 3, lesions predictable mainly from pharmacological actions. In the present survey, non-neoplastic lesions only found in 2-year rat carcinogenicity studies were neither significant in terms of frequency and severity nor useful for clinical risk management.     

Weight-of-the-evidence review of acrylonitrile reproductive and developmental toxicity studies

Risk assessment of acrylonitrile (AN) toxicity to humans has focused on potential carcinogenicity and acute toxicity. Epidemiological studies from China reported reproductive and developmental effects in AN workers, including infertility, birth defects, and spontaneous abortions. A weight-of-the-evidence (WoE) evaluation of the AN database assessed study strength, characterized toxicity, and identified no-observed-adverse-effect levels (NOAELs). The epidemiological studies do not demonstrate causality and are not sufficiently robust to be used for risk assessment. Rodent developmental studies showed fetotoxicity and malformations at maternally toxic levels; there was no unique developmental susceptibility. NOAELs for oral and inhalation exposures were 10?mg/kg/day and 12?ppm (6?h/day), respectively. Drinking-water and inhalation reproductive toxicity studies showed no clear effects on reproductive performance or fertility. Maternally toxic concentrations caused decreased pup growth. The drinking-water reproductive NOAEL was 100?ppm (moderate confidence due to study limitations). The inhalation exposure reproductive and neonatal toxicity high confidence NOAEL was 45?ppm (first generation 90?ppm) (6?h/day). The inhalation reproductive toxicity study provides the most robust data for risk assessment. Based on the WoE evaluation, AN is not expected to be a developmental or reproductive toxicant in the absence of significant maternal toxicity.     

Workshop Report: AAPS Workshop on Method Development,Validation, and Troubleshooting of Ligand-Binding Assays in the Regulated Environment

A novel format was introduced at the recent AAPS NBC Workshop on Method Development, Validation and Troubleshooting in San Diego on 18th May 2014. The workshop format was initiated by Binodh De Silva; Marie Rock and Sherri Dudal joined the initiative to develop and chair the workshop. Questions were solicited by a variety of avenues, including a Linked-In Discussion Group. Once collated and clarified, the topics covered assay development, validation, and analysis of PK, Immunogenicity, and Biomarkers with an additional topic on alternative bioanalytical technologies. A panel of experts (workshop report co-authors) was assigned to each topic to bring forward thought-provoking aspects of each topic. The format of the workshop was developed to target the needs of bioanalytical scientists with intermediate to advanced experience in the field ranging to enable robust discussion and to delve deeper into the current bioanalytical hot topics. While the new format allowed for an interactive session with the topical discussion driven by the audience members, it did not foster equal discussion time for all of the proposed topics, especially Biomarkers and alternative LBA technologies.KEY WORDS: antibody, bioanalytical, biomarker, ligand-binding assays, immunogenicity     

Non-clinical safety evaluation and risk assessment to human of aleglitazar,a dual PPAR α/γ agonist,and its major human metabolite

The non-clinical safety profile of aleglitazar, a peroxisome proliferator activated receptor alpha/gamma agonist, and its major human metabolite M6 was studied in a complete package consisting of drug metabolism and pharmacokinetics characterization, safety pharmacology, genotoxicity, repeat dose toxicity, reproductive toxicity and carcinogenicity studies. These studies identified the following main targets similar to other PPAR agonists: red blood cell parameters, liver, heart, kidney, ovaries, testes, bone marrow, adipose tissue, and fluid accumulation. Additionally, and in the 12-month monkey study only, an increased incidence of generalized hair loss/thinning was observed in all groups including controls. In the rat carcinogenicity study there was no statistically significant increase in tumors. In the mouse carcinogenicity study, there was an increased incidence of angiomatous tumors and there were three males with gallbladder adenoma. No relevant compound-related effects were observed in safety pharmacology, genotoxicity, and a 28-day immunotoxicity rat study. Effects observed in reproductive toxicity studies were similar to those known for other PPARγ agonists. Separate studies with the human metabolite M6 did not reveal findings that would prevent human dosing. Overall, the results from the non-clinical safety studies conducted with aleglitazar and the human metabolite M6 were considered to support the clinical Phase 3 program.     

State of the science review of the health effects of inorganic arsenic: Perspectives for future research

Human exposure to inorganic arsenic (iAs) is a global health issue. Although there is strong evidence for iAs‐induced toxicity at higher levels of exposure, many epidemiological studies evaluating its effects at low exposure levels have reported mixed results. We comprehensively reviewed the literature and evaluated the scientific knowledge on human exposure to arsenic, mechanisms of action, systemic and carcinogenic effects, risk characterization, and regulatory guidelines. We identified areas where additional research is needed. These priority areas include: (1) further development of animal models of iAs carcinogenicity to identify molecular events involved in iAs carcinogenicity; (2) characterization of underlying mechanisms of iAs toxicity; (3) assessment of gender‐specific susceptibilities and other factors that modulate arsenic metabolism; (4) sufficiently powered epidemiological studies to ascertain relationship between iAs exposure and reproductive/developmental effects; (5) evaluation of genetic/epigenetic determinants of iAs effects in children; and (6) epidemiological studies of people chronically exposed to low iAs concentrations.     

Subacute and chronic toxicity studies of fluorocarbon propellants in mice, rats and dogs

Six subacute and chronic inhalation toxicity studies were conducted in rats, mice and dogs treated with fluorocarbon propellants. The studies varied in length from 2 weeks to 23 months and the dose of propellant varied from 164 mg/kg/day to 2240 mg/kg/day. The only signs of toxicity observed were sedation, ataxia and mild depression which occurred in the 2 studies in which the propellant doses were the highest. These effects, observed during and immediately after exposure, disappeared rapidly and were attributed to mild anesthetic properties of the fluorocarbon propellants. Abnormal values were not found in the hematologic, blood chemistry or urinalysis values in any of the 6 toxicity studies. Gross and microscopic examination of tissues of the respiratory tract, including lung, did not reveal any evidence of propellant toxicity, irritation or carcinogenicity. In the 3- and 12-month toxicity studies, EKG changes were not observed in dogs which had received large daily doses of the propellants.     

A proposed framework for assessing risk from less-than-lifetime exposures to carcinogens

Quantitative methods for estimation of cancer risk have been developed for daily, lifetime human exposures. There are a variety of studies or methodologies available to address less-than-lifetime exposures. However, a common framework for evaluating risk from less-than-lifetime exposures (including short-term and/or intermittent exposures) does not exist, which could result in inconsistencies in risk assessment practice. To address this risk assessment need, a committee of the International Life Sciences Institute (ILSI) Health and Environmental Sciences Institute conducted a multisector workshop in late 2009 to discuss available literature, different methodologies, and a proposed framework. The proposed framework provides a decision tree and guidance for cancer risk assessments for less-than-lifetime exposures based on current knowledge of mode of action and dose-response. Available data from rodent studies and epidemiological studies involving less-than-lifetime exposures are considered, in addition to statistical approaches described in the literature for evaluating the impact of changing the dose rate and exposure duration for exposure to carcinogens. The decision tree also provides for scenarios in which an assumption of potential carcinogenicity is appropriate (e.g., based on structural alerts or genotoxicity data), but bioassay or other data are lacking from which a chemical-specific cancer potency can be determined. This paper presents an overview of the rationale for the workshop, reviews historical background, describes the proposed framework for assessing less-than-lifetime exposures to potential human carcinogens, and suggests next steps.     

A proposed framework for assessing risk from less-than-lifetime exposures to carcinogens

Quantitative methods for estimation of cancer risk have been developed for daily, lifetime human exposures. There are a variety of studies or methodologies available to address less-than-lifetime exposures. However, a common framework for evaluating risk from less-than-lifetime exposures (including short-term and/or intermittent exposures) does not exist, which could result in inconsistencies in risk assessment practice. To address this risk assessment need, a committee of the International Life Sciences Institute (ILSI) Health and Environmental Sciences Institute conducted a multisector workshop in late 2009 to discuss available literature, different methodologies, and a proposed framework. The proposed framework provides a decision tree and guidance for cancer risk assessments for less-than-lifetime exposures based on current knowledge of mode of action and dose-response. Available data from rodent studies and epidemiological studies involving less-than-lifetime exposures are considered, in addition to statistical approaches described in the literature for evaluating the impact of changing the dose rate and exposure duration for exposure to carcinogens. The decision tree also provides for scenarios in which an assumption of potential carcinogenicity is appropriate (e.g., based on structural alerts or genotoxicity data), but bioassay or other data are lacking from which a chemical-specific cancer potency can be determined. This paper presents an overview of the rationale for the workshop, reviews historical background, describes the proposed framework for assessing less-than-lifetime exposures to potential human carcinogens, and suggests next steps.     

Evaluation of the Threshold of Toxicological Concern (TTC) – Challenges and approaches

Thresholds of Toxicological Concern (TTCs) have been used in the risk assessment of chemicals to which humans are exposed at very low levels. TTC values were developed using data from rodent cancer bioassays and from oral chronic and sub-chronic toxicity studies for non-cancer effects. The workshop assessed the adequacy and fitness for purpose of the TTC approach and the potential for future modifications of critical aspects. The current TTC value for cancer was considered adequate and fit for purpose because it is derived by linear extrapolation from the lowest TD₅₀ for each compound in the largest available rodent carcinogenicity database. The database on non-cancer endpoints was considered adequate and fit for purpose because the chemical domain, the distributions of NOAELs and the calculated TTC values are comparable across different databases. Application of the TTC approach gives conclusions compatible with the risk assessment approaches currently used by international advisory committees. The workshop recognised the desirability of developing better tools to assess the comparability of the chemical domain covered in different toxicological databases, and the need to develop an internationally acceptable framework and databases for updating the aspects critical to application of the TTC approach.