Data-derived safety factors for the evaluation of food additives and environmental contaminants

Health effects classification and its role in the derivation of minimal risk levels: Immunological effects

The Agency for Toxic Substances and Disease Registry (ATSDR) derives health-based guidance values known as minimal risk levels (MRLs). By definition, an MRL is a substance-specific estimate of the daily human exposure to a substance that is likely to be without an appreciable risk of adverse, noncancer effects over a specified duration of exposure. MRLs are preferentially derived from human studies, if available, or from the most sensitive animal species and the endpoint that is most relevant for humans. To date, the agency has derived 346 MRLs. Fifteen MRLs were derived for 11 different chemicals where the database has identified the immune system as the most sensitive target of toxicity. The chemicals include benzene, chlorfenvinphos, endosulfan, heptachlor, gamma-hexachlorocyclohexane, dibutyl tin, tributyl tin, PCBs, 2,3,4,7,8-pentachlorodibenzofuran, 2,3,7,8-tetrachlorodibenzo-p-dioxin, and 2,4-dichlorophenol. The agency's rationale for classification of immunological endpoints is discussed and a brief description given of the critical studies selected for MRL development using immune system endpoints.

Human variability in hepatic and renal elimination: implications for risk assessment

Hepatic metabolism and renal excretion constitute the main routes of xenobiotic elimination in humans. Improving human risk assessment for threshold contaminants requires the incorporation of quantitative data related to their elimination (toxicokinetics) and potential toxic effects (toxicodynamics). This type of data provides a scientific basis to replace the standard uncertainty factor (UF = 10) allowing for the consideration of human variability in toxicokinetics and toxicodynamics. This review focuses on recent research efforts aiming to incorporate human variability in hepatic and renal elimination (toxicokinetics) into the risk assessment process. A therapeutic drug database was developed to quantify pathway-related variability in human phase I and phase II hepatic metabolism as well as renal excretion in subgroups of the population (healthy adults, neonates and the elderly), using data on compounds cleared primarily through each route (> 60% dose). For each subgroup of the population and elimination route, pathway-related UFs were then derived to cover 95-99% of each subgroup. Overall, the default toxicokinetic UFs would not cover neonates, the elderly for most elimination routes and any subgroup of the population for compounds metabolized via polymorphic isozymes (such as CYP2C19 and CYP2D6). These pathway-related UFs allow the incorporation of in vivo metabolism and toxicokinetic data in the risk assessment process and provide a flexible intermediate option between the default UF and chemical-specific adjustment factors (CSAFs) derived from physiologically based pharmacokinetic models. Implications of human variability in hepatic metabolism and renal excretion for chemical risk assessment are discussed.

Human and environmental risk assessment of pharmaceuticals: differences, similarities, lessons from toxicology

The presence of human and veterinary pharmaceuticals in the environment has caused increasing concern due their effects on ecological receptors. Improving the risk assessment of these compounds necessitates a quantitative understanding of their metabolism and elimination in the target organism (toxicokinetics), particularly via the ubiquitous cytochrome P-450 (CYP) system and their mechanisms of toxicity (toxicodynamics). This review focuses on a number of pharmaceuticals and veterinary medicines of environmental concern, and the differences and similarities between ecological and human risk assessment. CYP metabolism is discussed with particular reference to its ubiquity in species of ecological relevance. The important issue of pharmaceutical mixtures is discussed to assess how emerging technologies such as ecotoxicogenomics may assist in moving towards a more mechanism-based environmental risk assessment of pharmaceuticals.

Analysis of reproductive toxicity and classification of glufosinate-ammonium

CONCLUSION REGARDING CLASSIFICATION OF GLUFOSINATE-AMMONIUM: Science Partners' Evaluation Group (Evaluation Group) has conducted an independent analysis of the herbicide glufosinate-ammonium (GA) relative to its potential to cause reproductive toxicity in humans. Further, the Evaluation Group has evaluated the implementation of Annex 6 of Commission Directive 2001/59/EC (28th ATP of Council Directive 67/548/EEC) and Council Directive 91/414/EEC, with respect to classification of chemicals posing potential reproductive hazards. After consideration of all information available to us relevant to the potential of glufosinate-ammonium (GA) to cause reproductive toxicity, the Science Partners Evaluation Group concludes that no classification of GA is justified. The following form the basis of this conclusion. There are no human data to suggest that GA causes reproductive toxicity in women or in their conceptus. The issue concerning possible reproductive hazard to humans is raised solely on the basis of positive animal test results that show GA to cause preimplantation or implantation losses in rats. SPECIFICALLY: a. Daily treatment with GA had no detectable effect on the earliest stages of the reproductive sequence including gametogenesis, ovulation, mating and conception; b. Treatment with GA interfered with rat gestation before and at the stage when the conceptus implants into the uterus. This effect occurred at doses of 360 ppm in the feed (corresponding to daily doses of 27.8 mg/kg bw) and above; and c. After implantation, no further effect of GA on prenatal and post-natal development was recognized. Previous concerns that GA might be toxic to embryonic stages after implantation were not supported by the data. Abortions and stillbirth seen were associated with, and regarded as secondary to, maternal toxicity. There was no evidence suggesting the induction of malformations in the offspring. The mechanism underlying this adverse effect in experimental laboratory animals is identified-inhibition of glutamine synthetase. Glutamine is essential to the viability of the embryo. The embryo is dependent on a maternal source of the amino acid. For embryo lethality to occur, a significant reduction of maternal glutamine is required. Such reduction in maternal glutamine depends on a significant inhibition of glutamine synthetase by GA. This can only occur when the mother is exposed to very high levels of GA. SPECIFICALLY: a. The reproductive toxicity of GA is confined to very short, early stages of reproduction, during which the conceptus is dependent on maternal glutamine; and b. In order for the effect to occur, significant reduction in maternal blood glutamine level is required, which in turn depends on a significant inhibition of glutamine synthetase, induced by high levels of GA in the maternal system. There is no evidence for accumulation of GA in the mammalian organism beyond a factor of two and no evidence for its metabolic toxification. To raise a concern in humans, women would have to be exposed to GA during the very limited time frame of preimplantation or implantation and the exposure would have to be to the exceedingly high levels necessary to alter the maternal metabolism and, correspondingly, result in glutamine levels in maternal tissue and blood plasma being drastically reduced. There is no basis to suggest that such exposures would occur under conditions of normal handling and use. SPECIFICALLY: a. Under conditions of normal handling and use, operators would never be exposed to GA levels that could potentially inhibit glutamine synthetase to the extent that this inhibition could impair preimplantation or implantation. b. All acceptable exposure measurements and predictive calculations confirm this conclusion, and in fact demonstrate that reasonably foreseeable exposure of workers would be to levels significantly below the AOEL. c. The evidence is also clear that there is no reproductive toxicity hazard to workers upon reentry tosprayed fields, bystanders, consumers or toddlers. The safety margin compared to the NOAEL in animal studies is sufficiently large to assure protection of the health of workers using GA as well as bystanders, consumers, and toddlers. Pursuant to Annex 6 of Commission Directive 2001/59/EC (28th ATP of Council Directive 67/548/EEC), to justify a classification of category 2 there must be sufficient evidence to produce a strong presumption that human exposure to the substance may result in impaired fertility in humans. It is the conclusion of the Science Partners Evaluation Group that there is no reasonable evidence to suggest a strong presumption of impairment. To the contrary, there is clear evidence demonstrating a strong presumption that exposure to GA would not cause the adverse effect demonstrated in rats. Pursuant to Annex 6 of Commission Directive 2001/59/EC (28th ATP of Council Directive 67/548/EEC), to justify a classification of category 3, there must be sufficient evidence to provide a strong suspicion of impaired fertility in humans. There is no basis to conclude that the animal data demonstrating impaired preimplantation or implantation has any relevance to humans in that the effect found in rats only occurs at levels which would never be experienced by workers under conditions of normal handling and use or by bystanders, consumers, or toddlers.

Toxicology of micronutrients: adverse effects and uncertainty

The establishment of safe upper intake levels for micronutrients must consider the intake-response relations for both deficiency and toxicity. Limited data are available on the toxicities of most micronutrients, and few studies that meet the criteria considered essential for the risk assessment of other chemicals in food, such as pesticides and food additives, have been performed. In some cases, the application of large uncertainty factors, which are used to establish the amount of a chemical that would be safe for daily intake throughout life, could result in nutritionally inadequate intakes of micronutrients. As a consequence, lower than normal uncertainty factors have been applied to determine safe or tolerable intakes of many micronutrients. There is no clear scientific rationale, on the basis of the metabolism and elimination of micronutrients or the nature of the adverse effects reported for high intakes, for the use of reduced uncertainty factors for micronutrient toxicity. A review of recent evaluations of selected vitamins and minerals shows little consistency in the application of uncertainty factors by different advisory groups, such as the Institute of Medicine in the United States and the Scientific Committee on Foods in the European Union. It is apparent that, in some cases, the uncertainty factor applied was selected largely to give a result that is compatible with nutritional requirements; therefore, the uncertainty factor represented part of risk management rather than hazard characterization. The usual risk assessment procedures for chemicals in food should be revised for micronutrients, so that the risks associated with intakes that are too low and too high are considered equally as part of a risk-benefit analysis.

Acute inhalative exposure assessment: Derivation of guideline levels with special regard to sensitive subpopulations and time scaling

Risk assessment for acute airborne exposure to volatile organic compounds (VOCs), including exposure to chemical warfare agents, requires consideration of local and systemic effects at high concentrations. The operating procedure developed by the US Acute Exposure Guideline Level (AEGL) committee has gained special attention, in part because of the international collaboration in the project. The procedure defines three levels (AEGL-1: discomfort; AEGL-2: irreversible or other serious, long-lasting adverse effects; AEGL-3: life-threatening effects or death) for different exposure times (10 and 30 min, and 1, 4 and 8 h). In this article, the methodology for deriving AEGL values is reported. Extending the areas covered by the existing AEGL methodology, sensitive subpopulations are dealt with in more detail. Sensitive persons are expected to suffer from stronger effects when exposed to a given external concentration. Using a kinetic model with the sample substance dichloromethane (DCM), the higher internal exposure of children is quantified and compared to a healthy, young adult. The difference is shown to depend on age, on dose, and on duration of exposure. Furthermore, several ways are presented to derive AEGL values for exposure times which differ from the exposure duration in animal studies ('time scaling'). In comparison to the conventional procedure, the alternative approaches are based on mechanistic models of the toxicodynamic effect. Use of these models results in AEGL values which are biologically justified.

Assessing risk to humans from chemical exposure by using non-animal test data

The use of data from non-animal toxicity methods in risk assessment has mainly been limited to hazard identification and for elucidating mechanisms of toxicity. However, there is a need to extend the use of in vitro tests to hazard characterisation and risk assessment. This might be feasible by: (a) increased use of human cells of different types; (b) better maintenance of differentiated cells in culture for long periods; (c) use of genetically-engineered cells with useful characteristics; (d) development of complex organotypic cell systems; (e) development of co-cultures of different cell types; and (f) development of techniques for long term culturing, repeat dosing and assessment of recovery. Also, it will be necessary to obtain more information on the differences between cells in culture and in situ in tissues, and on the effects of dosing in vitro and in vivo, to develop realistic and meaningful uncertainty factors to allow in vitro information to be used for risk assessment in its own right, and in conjunction with animal data. These issues and a suggested proposal for using in vitro data in risk assessment are discussed.

Human pharmaceuticals in US surface waters: A human health risk assessment

The detection of low levels of pharmaceuticals in rivers and streams, drinking water, and groundwater has raised questions as to whether these levels may affect human health. This report presents human health risk assessments for 26 active pharmaceutical ingredients (APIs) and/or their metabolites, representing 14 different drug classes, for which environmental monitoring data are available for the United States. Acceptable daily intakes (ADIs) are derived using the considerable data that are available for APIs. The resulting ADIs are designed to protect potentially exposed populations, including sensitive sub-populations. The ADIs are then used to estimate predicted no effect concentrations (PNECs) for two sources of potential human exposure: drinking water and fish ingestion. The PNECs are compared to measured environmental concentrations (MECs) from the published literature and to maximum predicted environmental concentrations (PECs) generated using the PhATE model. The PhATE model predictions are made under conservative assumptions of low river flow and no depletion (i.e., no metabolism, no removal during wastewater or drinking water treatment, and no instream depletion). Ratios of MECs to PNECs are typically very low and consistent with PEC to PNEC ratios. For all 26 compounds, these low ratios indicate that no appreciable human health risk exists from the presence of trace concentrations of these APIs in surface water and drinking water.

The refinement of uncertainty/safety factors in risk assessment by the incorporation of data on toxicokinetic variability in humans

The derivation of safe levels of exposure in humans for compounds that are assumed to cause threshold toxicity has relied on the application of a 100-fold uncertainty factor to a measure for the threshold, such as the no observed adverse effect level (NOAEL) or the benchmark dose (BMD). This 100-fold safety factor consists of the product of two 10-fold factors allowing for human variability and interspecies differences. The International Programme on Chemical Safety has suggested the subdivision of these 10-fold factors to allow for variability in toxicokinetics and toxicodynamics. This subdivision allows the replacement of the default uncertainty factors with a chemical-specific adjustment factor (CSAF) when suitable data are available. This short review describes potential options to refine safety factors used in risk assessment, with particular emphasis on pathway-related uncertainty factors associated with variability in kinetics. These pathway-related factors were derived from a database that quantified interspecies differences and human variability in phase I metabolism, phase II metabolism, and renal excretion. This approach allows metabolism and pharmacokinetic data in healthy adults and subgroups of the population to be incorporated in the risk-assessment process and constitutes an intermediate approach between simple default factors and chemical-specific adjustment factors.

Human variability in xenobiotic metabolism and pathway-related uncertainty factors for chemical risk assessment: a review

This review provides an account of recent developments arising from a database that defined human variability in phase I metabolism (CYP1A2, CYP2A6, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4, hydrolysis, alcohol dehydrogenase), phase II metabolism (N-acetyltransferases, glucuronidation, glycine conjugation, sulphation) and renal excretion. This database was used to derive pathway-related uncertainty factors for chemical risk assessment that allow for human variability in toxicokinetics. Probe substrates for each pathway of elimination were selected on the basis that oral absorption was >95% and that the metabolic route was the primary route of elimination of the compound (60-100% of a dose). Intravenous data were used for compounds for which absorption was variable. Human variability in kinetics was quantified for each compound from published pharmacokinetic studies (after oral and intravenous dosing) in healthy adults and other subgroups of the population using parameters relating to chronic exposure (metabolic and total clearances, area under the plasma concentration-time curve (AUC)) and acute exposure (Cmax) (data not presented here). The pathway-related uncertainty factors were calculated to cover 95%, 97.5% and 99% of the population of healthy adults and of each subgroup. Pathway-related uncertainty factors allow metabolism data to be incorporated into the derivation of health-based guidance values. They constitute an intermediate approach between the general kinetic default factors (3.16) and a chemical-specific adjustment factor. Applications of pathway-related uncertainty factors for chemical risk assessment and future refinements of the approach are discussed. A knowledge-based framework to predict human variability in kinetics for xenobiotics showing a threshold dose below which toxic effects are not observed, is proposed to move away from default assumptions.

Impact of inter-individual differences in drug metabolism and pharmacokinetics on safety evaluation

Safety evaluation aims to assess the dose-response relationship to determine a dose/level of exposure for food contaminants below which no deleterious effect is measurable that is 'without appreciable health risk' when consumed daily over a lifetime. These safe levels, such as the acceptable daily intake (ADI) have been derived from animal studies using surrogates for the threshold such as the no-observed-adverse-effect-level (NOAEL). The extrapolation from the NOAEL to the human safe intake uses a 100-fold uncertainty factor, defined as the product of two 10-fold factors allowing for human variability and interspecies differences. The 10-fold factor for human variability has been further subdivided into two factors of 10(0.5) (3.16) to cover toxicokinetics and toxicodynamics and this subdivsion allows for the replacement of an uncertainty factor with a chemical-specific adjustment factor (CSAF) when compound-specific data are available. Recently, an analysis of human variability in pharmacokinetics for phase I metabolism (CYP1A2, CYP2A6, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4, hydrolysis, alcohol dehydrogenase), phase II metabolism (N-acetyltransferase, glucuronidation, glycine conjugation, sulphation) and renal excretion was used to derive pathway-related uncertainty factors in subgroups of the human population (healthy adults, effects of ethnicity and age). Overall, the pathway-related uncertainty factors (99th centile) were above the toxicokinetic uncertainty factor for healthy adults exposed to xenobiotics handled by polymorphic metabolic pathways (and assuming the parent compound was the proximate toxicant) such as CYP2D6 poor metabolizers (26), CYP2C19 poor metabolizers (52) and NAT-2 slow acetylators (5.2). Neonates were the most susceptible subgroup of the population for pathways with available data [CYP1A2 and glucuronidation (12), CYP3A4 (14), glycine conjugation (28)]. Data for polymorphic pathways were not available in neonates but uncertainty factors of up to 45 and 9 would allow for the variability observed in children for CYP2D6 and CYP2C19 metabolism, respectively. This review presents an overview on the history of uncertainty factors, the main conclusions drawn from the analysis of inter-individual differences in metabolism and pharmacokinetics, the development of pathway-related uncertainty factors and their use in chemical risk assessment.

Establishing the upper end of the range of adequate and safe intakes for amino acids: a toxicologist's viewpoint

The safety assessment of high intake levels of individual amino acids cannot be based on data from nutritional studies with proteins. Routine toxicity tests designed to investigate a wide range of possible effects should be undertaken for hazard identification and characterization using studies selected to mirror the predicted pattern and duration of human exposure. The approach used to establish an acceptable daily intake level for additives and pesticides, based on defining a "no observed adverse effect" level in the experimental study and dividing by uncertainty factors that allow for species differences and human variability, has a long history of use for foreign compounds and would provide a suitable basis for determining health-based guidance values for single amino acids. The usual default uncertainty factors for toxicokinetics and toxicodynamics should be replaced by compound-specific values if suitable data are available. In addition, the usual uncertainty factors should be modified to more relevant default values based on species differences and human variability in the biodisposition of amino acids in general or of groups of metabolically interrelated amino acids. There would be no significant health concerns if the human intake levels were below a health-based guidance value developed using this approach. A population-distribution approach could be used to define the magnitude of any risk at intake levels above the guidance value.

Development of acute exposure guideline levels for airborne exposures to hazardous substances

Hazardous substances can be released into the atmosphere due to industrial and transportation accidents, fires, tornadoes, earthquakes, and terrorists, thereby exposing workers and the nearby public to potential adverse health effects. Various enforceable guidelines have been set by regulatory agencies for worker and ambient air quality. However, these exposure levels generally are not applicable to rare lifetime acute exposures, which possibly could occur at high concentrations. Acute exposure guideline levels (AEGLs) provide estimates of concentrations for airborne exposures for an array of short durations that possibly could cause mild (AEGL-1), severe, irreversible, potentially disabling adverse health effects (AEGL-2), or life threatening effects (AEGL-3). These levels can be useful for emergency responders and planners in reducing or eliminating potential risks to the public. Procedures and methodologies for deriving AEGLs are reviewed in this paper that have been developed in the United States, with direct input from international representatives of OECD member-countries, by the National Advisory Committee for Acute Exposure Guidelines for Hazardous Substances and reviewed by the National Research Council. Techniques are discussed for the extrapolation of effects across different exposure durations. AEGLs provide a viable approach for assisting in the prevention, planning, and response to acute airborne exposures to toxic agents.

Species-specific uncertainty factors for compounds eliminated principally by renal excretion in humans

An uncertainty factor of 100 is used to derive health-based guidance values for human intakes of chemicals based on data from studies in animals. The 100-fold factor comprises 10-fold factors for species differences and for interindividual differences in response. Each 10-fold factor can be subdivided into toxicokinetic and toxicodynamic aspects with a 4.0-fold factor to allow for kinetic differences between test species and humans. The current work determined the extent of interspecies differences in the internal dose (toxicokinetics) of compounds which are eliminated primarily by renal excretion in humans. An analysis of the published data showed that renal excretion was also the main route of elimination in the test species for most of the identified probe substrates. Interspecies differences were apparent for both the mechanism of renal excretion (glomerular filtration, tubular secretion and/or reabsorption) and the extent of plasma protein binding, both of which may affect renal clearance and therefore the magnitude of species differences in the internal dose. For compounds which are eliminated unchanged by both humans and the test species, the average differences in the internal doses between humans and animals were 1.6 for dogs, 3.3 for rabbits, 5.2 for rats and 13 for mice. This suggests that for renal excretion, the differences between humans and the rat and especially the mouse may exceed the 4.0-fold default factor for toxicokinetics.

Human variability in the renal elimination of foreign compounds and renal excretion-related uncertainty factors for risk assessment

Renal excretion is an important route of elimination for xenobiotics and three processes determine the renal clearance of a compound [glomerular filtration (about 120 ml/min), active renal tubular secretion (>120 ml/min) and passive reabsorption (<120 ml/min)]. Human variability in kinetics has been quantified using a database of 15 compounds excreted extensively by the kidney (>60% of a dose) to develop renal-excretion related uncertainty factors for the risk assessment of environmental contaminants handled via this route. Data were analysed from published pharmacokinetic studies (after oral and intravenous dosing) in healthy adults and other subgroups using parameters relating primarily to chronic exposure [renal and total clearances, area under the plasma concentration time-curve (AUC)] and acute exposure (Cmax). Interindividual variability in kinetics was low for both routes of exposure, with coefficients of variation of 21% (oral) and 24% (intravenous) that were largely independent of the renal processes involved. Renal-excretion related uncertainty factors were below the default kinetic uncertainty factor of 3.16 for most subgroups analysed with the exception of the elderly (oral data) and neonates (intravenous data) for whom renal excretion-related factors of 4.2 and 3.2 would be required to cover up to 99% of these subgroups respectively.

Evaluation of Some In Vitro Tests to Reduce and Replace the Sub-acute Animal Toxicity Studies

The toxicologic and carcinogenic potential of chemicals is usually determined through a sequence of acute, sub-acute (14-day), sub-chronic (90-day) and chronic (two-year) studies in rats and mice of both sexes. The US National Toxicology Program (NTP) does not conduct acute toxicity studies. Dose levels for 14-day toxicity studies are typically estimated from information in the literature, if available. The toxicology information obtained from 14-day studies is used in the selection of doses for 90-day studies. The protocol for 14-day studies consists of five doses and control groups and five animals per group of each sex and species, resulting in the use of 120 animals per study. At present, in addition to refining the current testing protocols, the NTP is evaluating the potential for in vitro test methods to partially or completely avoid the need for 14-day toxicity studies, especially for chemicals where the dermal route of exposure is used. The in vitro assays used were the EpiDerm™ bioassay to estimate dermal irritation, the neutral red uptake (NRU) bioassay to estimate systemic toxicity and the primary rat hepatocyte cytotoxicity (PRHC) assay to estimate hepatotoxicity. The purpose of using these assays was to assess their potential for predicting relative in vivo toxicity and to support dose selection decisions for 90-day studies. In general, based on these limited number of studies, the EpiDerm and NRU tests were predictive of the responses observed in in vivo studies. However, a larger comparative database is needed to derive definitive conclusions regarding the value of in vitro tests in the prediction of in vivo effects.

Assessing the dose-dependency of allometric scaling performance using physiologically based pharmacokinetic modeling

The performance of allometric scaling of dose as a power of body weight under a variety of extrapolation conditions with respect to species, route, exposure intensity, and mechanism/mode of action, remains untested in many cases. In this paper, animal-human internal dose ratio comparisons have been developed for 12 chemicals (benzene, carbon tetrachloride, chloroform, diisopropylfluorophosphate, ethanol, ethylene oxide, methylene chloride, methylmercury, styrene, tetrachloroethene, trichloroethene, and vinyl chloride). This group of predominantly volatile and lipophilic chemicals was selected on the basis that their kinetics have been well-studied and can be predicted in mice, rats, and humans using physiologically based pharmacokinetic (PBPK) models. PBPK model predictions were compared to the allometric scaling predictions for interspecies extrapolation. Recommendations for the application of the allometric scaling are made with reference to internal dose measure (mode of action) and concentration level. The results of this assessment generally support the use of scaling factors recommended in the published literature, which includes scaling factors of 1.0 for risk assessments in which toxicity is attributed to the parent chemical or stable metabolite, and -0.75 for dose-response assessments in which toxicity is attributed to the formation of a reactive metabolite from an inhaled compound. A scaling factor of 0.75 is recommended for dose-response assessments of orally administered compounds in which toxicity is attributed to the parent chemical or stable metabolite and 1.0 for risk assessments in which toxicity is attributed to the formation of a reactive metabolite from a compound administered by the oral route. A dose-dependency in the results suggests that the scaling factors appropriate at high exposures may differ from those at low exposures, primarily due to the impact of saturable metabolism.

Guidance for the safety assessment of botanicals and botanical preparations for use in food and food supplements

There is a growing interest by both consumers and industry for the development of food products with 'functional' properties, or health benefits. These products may take the form of dietary supplements or of foods. The health benefits are given by particular ingredients, and in many cases these are derived from botanicals. The variety of plants providing these functions is large, ranging from staple food sources such as cereals, fruits and vegetables, to herbals as used in traditional medicine. The food or ingredient conferring health properties may consist of the plants themselves, extracts thereof, or more purified components. The scientific literature is abundant with articles not only on the beneficial properties, but also on possible adverse health effects of plants and their components. The present report discusses the data required to determine the safe use of these types of ingredients, and provides advice on the development of risk assessment strategies consistent with due diligence under existing food regulations. Product specifications, composition and characterisation of standardised and authentic materials, documented history of use and comparison to existing products (taking into account the effect of industrial processing), description of the intended use and consequent exposure are highlighted as key background information on which to base a risk evaluation. The extent of experimental investigation required, such as in vitro, animal, and/or human studies, depends on the adequacy of this information. A decision tree is presented as an aid to determine the extent of data requirements based on product comparison. The ultimate safety in use depends on the establishment of an adequate safety margin between expected exposure and identified potential hazards. Health hazards may arise from inherent toxicities or contaminants of the plant materials, including the mechanism of the intended beneficial effect. A lower safety margin may therefore be expected than for food ingredients or additives where no physiological effects are intended. In rare cases, post launch monitoring programmes may be envisaged to confirm expected exposures and adequacy of the safety margin. This guidance document was elaborated by an expert group of the Natural Toxin Task Force of the European Branch of the International Life Sciences Institute--ILSI Europe and discussed with a wider audience of scientists at a workshop held on 13-15 May 2002 in Marseille, France.

Heterogeneity of toxicant response: sources of human variability

While risk assessment models attempt to predict human risk to toxicant exposure, in many cases these models cannot account for the wide variety of human responses. This review addresses several primary sources of heterogeneity that may affect individual responses to drug or toxicant exposure. Consideration was given to genetic polymorphisms, age-related factors during development and senescence, gender differences associated with hormonal function, and preexisting diseases influenced by toxicant exposure. These selected examples demonstrate the need for additional steps in risk assessment that are needed to more accurately predict human responses to toxicants and drugs.

Approaches to risk assessment for macronutrients and amino acids

Risk assessment is a well-established framework for organizing and evaluating diverse, and sometimes conflicting information, to assess the likelihood that agents in the environment may harm human health under known or expected conditions of exposure. Risk assessments are used by regulatory and public health officials to guide judgments and actions regarding the need for risk reduction and the appropriate means to achieve it. These judgments and actions are called risk management, and are guided by law, historical precedent and public health, economic and social concerns. Those in the nutrition community who have been called upon to make recommendations regarding adequate nutrient intakes have long been engaged in the practice of risk assessment. That is, they have assessed the harmful health effects of inadequate intakes and defined intakes likely to avoid such harm. During the past decade attention has turned to the potential health risks of excessive nutrient and nutritional supplement intakes. A recent study released by a committee of the Institute of Medicine illustrates the difficulties in deriving risk-based upper levels of intake for nutrients and nutritional supplements. Amino acids were the subject of extensive discussions by this committee, but in no case was an upper level of intake recommended. It is clear that the extent of scientific investigation of the harmful effects of amino acids has been highly uneven, and that significant questions remain regarding the appropriate methodologies to study such effects.

Practical application of kinetic data in risk assessment--an IPCS initiative

In this paper, guidance developed in a project of the International Programme on Chemical Safety (IPCS) initiative on Harmonization of Approaches to the Assessment of Risk from Exposure to Chemicals is considered in the context of its application in the assessment of the adequacy of physiological-toxicokinetic (PTK) modeling to inform quantitatively extrapolations for interspecies differences and human variability in dose-response assessment. This guidance was developed in the context of a framework, which permits the incorporation of quantitative chemical-specific data, relating to either toxicokinetics or toxicodynamics to replace part or all of the usual 100-fold default uncertainty factor for interspecies differences or human variability in the development of tolerable or reference doses or concentrations. However, since the guidance relates specifically to adequacy of kinetic or dynamic data to replace default for interspecies and human variability, it is also applicable to other approaches of dose-response analyses such as estimation of cancer potency or risk. The framework also supports probabilistic characterization, where data are sufficient. This guidance has been developed and refined through a series of planning and technical meetings and larger workshops, in which a broad range of participants from academia, government agencies, and the private sector have prepared and gained experience in application through case studies. The guidance for adequacy of kinetic data to replace default is presented in the context of several categories, including determination of the active chemical species, choice of the appropriate kinetic parameter and experimental data, the latter which includes reference to relevance of population, relevance of route, relevance of dose/concentration, and adequacy of number of subjects/samples. The principal objective of this guidance, which has been developed primarily as a resource for risk assessors, is to foster better understanding of the criteria for adequacy of chemical-specific data to quantitate interspecies differences and human variability in kinetics and dynamics, including PTK models. It is anticipated that the guidance will also encourage generation of appropriate data and models, and facilitate their incorporation in dose/concentration-response assessment for regulatory purposes. In this paper, the application of the guidance is considered primarily through reference to examples, with emphasis on those where PTK models have been informative.

Polymorphic CYP2C19 and N-acetylation: human variability in kinetics and pathway-related uncertainty factors

CYP2C19-mediated oxidation and N-acetylation constitute major phase I and phase II polymorphic pathways of xenobiotic metabolism in humans. Analysis of human variability in kinetics for these pathways has been carried out for compounds metabolised extensively (>60%) by these routes. Data for minor substrates for CYP2C19 metabolism (10-60%) have also been analysed. Published pharmacokinetic studies (after oral and intravenous dosing) in CYP2C19 non-phenotyped healthy adults (NPs), and phenotyped extensive (EMs), slow-extensive (SEMs) and poor metabolisers (PMs) have been analysed using data for parameters that relate primarily to chronic exposure (metabolic and total clearances, area under the plasma concentration-time curve) and primarily to acute exposure (peak concentration). Similar analyses were performed for the N-acetylation pathway using data for fast acetylators (FA) and slow acetylators (SA). Interindividual variability in the kinetics of CYP2C19 substrates after oral dosage was greater in EMs than in NPs (60 vs 43% for clearances and 54 vs 45% for Cmax). Lower variability was found for N-acetylation for both phenotypes (32 and 22% for FA and SA, respectively). The internal dose of CYP2C19 substrates in PM subjects would be 31-fold higher than in EMs, while for N-acetylated substrates there was a three-fold difference between SA and FA subjects. Pathway-related uncertainty factors were above the default safety factor of 3.16 for most subgroups and values of 52 and 5.2 would be necessary to cover to the 99th centile of the poor metaboliser phenotype for CYP2C19 and N-acetylation, respectively. An exponential relationship (R(2)=0.86) was found between the extent of CYP2C19 metabolism and the difference in internal dose between EMs and PMs. The kinetic default factor (3.16) would cover PMs for substrates for which CYP2C19 was responsible for up to 20-30% of the metabolism in EMs.

Uncertainty in toxicological risk assessment for non-carcinogenic health effects

Uncertainty in risk assessment results from the lack of knowledge on toxicity to the target population for a substance. Currently used deterministic risk assessment methods yield human limit values or margins of safety (MOS) without quantitative measurements of uncertainty. Qualitative and quantitative uncertainty analysis would enable risk managers to better judge the consequences of different management options. This article discusses sources of uncertainty and possibilities for quantification of uncertainty associated with different steps in the risk assessment of non-carcinogenic health effects. Knowledge gaps causing uncertainty in risk assessment are overcome by extrapolation. Distribution functions for extrapolation factors are based on empirical data and provide information about the extent of uncertainty introduced by these factors. Whereas deterministic methods can account only qualitatively for uncertainty of the resulting human limit value, probabilistic risk assessment methods are able to quantify several aspects of uncertainty. However, there is only limited experience with these methods in practice. Their acceptance and future application will depend on the establishment of evidence based distribution functions, flexibility and practicability of the methods, and the unambiguity of the results.

Human variability in CYP3A4 metabolism and CYP3A4-related uncertainty factors for risk assessment

CYP3A4 constitutes the major liver cytochrome P450 isoenzyme and is responsible for the oxidation of more than 50% of all known drugs. Human variability in kinetics for this pathway has been quantified using a database of 15 compounds metabolised extensively (>60%) by this CYP isoform in order to develop CYP3A4-related uncertainty factors for the risk assessment of environmental contaminants handled via this route. Data were analysed from published pharmacokinetic studies (after oral and intravenous dosing) in healthy adults and other subgroups using parameters relating primarily to chronic exposure [metabolic and total clearances, area under the plasma concentration-time curve (AUC)] and acute exposure (Cmax). Interindividual variability in kinetics was greater for the oral route (46%, 12 compounds) than for the intravenous route (32%, 14 compounds). The physiological and molecular basis for the difference between these two routes of exposure is discussed. In relation to the uncertainty factors used for risk assessment, the default kinetic factor of 3.16 would be adequate for adults, whereas a CYP3A4-related factor of 12 would be required to cover up to 99% of neonates, which have lower CYP3A4 activity.

Guidelines for application of chemical-specific adjustment factors in dose/concentration-response assessment

This manuscript addresses guidance in the use of kinetic and dynamic data to inform quantitatively extrapolations for interspecies differences and human variability in dose-response assessment developed in a project of the International Programme on Chemical Safety (IPCS) initiative on Harmonisation of Approaches to the Assessment of Risk from Exposure to Chemicals. The guidance has been developed and refined through a series of planning and technical meetings and larger workshops of a broad range of participants from academia, government agencies and the private sector. The guidance for adequacy of data for replacement of common defaults for interspecies differences and human variability is presented in the context of several generic categories including: determination of the active chemical species, choice of the appropriate metric (kinetic components) or endpoint (dynamic components) and nature of experimental data, the latter which includes reference to the relevance of population, route and dose and the adequacy of the number of subjects/samples. The principal objective of this guidance developed primarily as a resource for risk assessors, is to foster better understanding of the components of and criteria for adequacy of chemical-specific data to quantitate interspecies differences and human variability in kinetics and dynamics. It is anticipated that this guidance will also encourage the development of appropriate data and facilitate their incorporation in a consistent fashion in dose-response assessment for regulatory purposes (IPCS, 2001).

Human variability in polymorphic CYP2D6 metabolism: is the kinetic default uncertainty factor adequate?

Human variability in the kinetics of CYP2D6 substrates has been quantified using a database of compounds metabolised extensively (>60%) by this polymorphic enzyme. Published pharmacokinetic studies (after oral and intravenous dosing) in non-phenotyped healthy adults, and phenotyped extensive (EMs), intermediate or slow-extensive (SEMs) and poor metabolisers (PMs) have been analysed using data for parameters that relate primarily to chronic exposure (metabolic and total clearances, area under the plasma concentration time-curve) and primarily to acute exposure (peak concentration). Similar analyses were performed with the available data for subgroups of the population (age, ethnicity and disease). Interindividual differences in kinetics for markers of oral exposure were large for non-phenotyped individuals and for EMs (coefficients of variation were 67-71% for clearances and 54-63% for C(max)), whereas the intravenous data indicated a lower variability (34-38%). Comparisons between EMs, SEMs and PMs revealed an increase in oral internal dose for SEMs and PMs (ratio compared to EMs=3 and 9-12, respectively) associated with lower variability than that for non-phenotyped individuals (coefficients of variation were 32-38% and 30% for SEMs and PMs, respectively). In relation to the uncertainty factors used for risk assessment, most subgroups would not be covered by the kinetic default of 3.16. CYP2D6-related factors necessary to cover 95-99% of each subpopulation ranged from 2.7 to 4.1 in non-phenotyped healthy adults and EMs to 15-18 in PMs and 22-45 in children. An exponential relationship (R(2)=0.8) was found between the extent of CYP2D6 metabolism and the uncertainty factors. The extent of CYP2D6 involvement in the metabolism of a substrate is critical in the estimation of the CYP2D6-related factor. The 3.16 kinetic default factor would cover PMs for substrates for which CYP2D6 was responsible for up to 25% of the metabolism in EMs.

Pesticide residue analysis and its relationship to hazard characterisation (ADI/ARfD) and intake estimations (NEDI/NESTI)

Over 800 pesticides are currently approved for use in one or more EU countries. The maximum residue levels (MRL) for agricultural pesticides are derived from field trials conducted under good agricultural practice (GAP). The MRL is a legally enforceable limit related to GAP. The results from field trials would only be used to establish MRLs if the estimated intake of residues did not exceed the acceptable daily intake (ADI) or acute reference dose (ARfD). However, the MRL is not linked to the ADI or ARfD, and could result in intakes considerably below the ADI/ARfD. This disconnection between hazard characterisation (ADI/ARfD) and potential exposure assessment (MRL) means that risk characterisation of pesticide residues is less transparent than for other chemicals present in human food. Residue levels at or below the MRL would not give intakes that exceed the ADI/ARfD but, despite this, there is public concern over such residues. Residue levels above the MRL have to be analysed on a case-by-case basis to determine if the intake could exceed the health-based limits. Other causes of public concern, such as the presence of multiple residues, are currently under investigation.

The use of toxicokinetic and toxicodynamic data in risk assessment: an international perspective

Risk assessment of chemicals is a process which is usually based on data derived from animal testing in which the exposure of animals results in toxicological effects. By extrapolation, the dose/exposure in humans, which will not result in toxicological effects ('safe dose', 'safe exposure'), is estimated. Traditional approaches use 'safety factors' or 'uncertainty factors' to extrapolate from animal to man and from the 'mean' subject to the general population, including sensitive subgroups. Traditionally, a default factor of 10 has been used to account for interspecies variation. It is proposed that this factor be subdivided into a subfactor to address the toxicokinetic aspects and a second subfactor for the toxicodynamic aspects. Likewise, a default factor of 10 with subfactors is proposed to account for the intraspecies variability. In the framework of the International Program on Chemical Safety's (IPCS) project on the Harmonization of Approaches to the Assessment of Risk from Exposure to Chemicals, an activity has been initiated to provide guidance to risk assessors on the use of quantitative chemical specific data to account for interspecies variation and interindividual variability in risk assessment. To address the toxicokinetic aspects, the active species, the relevant internal exposure and the adequate metrics must be considered. Data quality and availability, in vitro or in vivo, the route of administration and the relevant dose level are relevant information for interspecies extrapolation. The availability of experimental data, including the relevance of the population studied, the number of subjects and/or samples obtained in the relevant group allow one to estimate the population distribution, e.g. difference between central tendency and given percentiles. In a similar fashion, the toxicodynamic data must be addressed. In addition to the identification of the active chemical species, the relevant endpoint must be determined. In extrapolation from animal to man, in most of the cases, the definitive endpoint (e.g. anemia) is lacking. It can be substituted by in vitro data (e.g. in vitro hemolysis) if it is a key event and relevant for the toxicity in animal as well as in humans. In extrapolating from animal to man, the dose-effect relationship plays an important role. To account for the toxicodynamic variability in the human population, similar aspects have to be taken into consideration, which have been discussed for toxicokinetics. The IPCS document is available at the IPCS website and risk assessors are invited to use the framework and report back their experience with it to enable revision and improvement.

Genetic polymorphisms in assessing interindividual variability in delivered dose

Increasing sophistication in methods used to account for human variability in susceptibility to toxicants has been one of the success stories in the continuing evolution of risk assessment science. Genetic polymorphisms have been suggested as an important contributor to overall human variability. Recently, data on polymorphisms in metabolic enzymes have been integrated with physiologically based pharmacokinetic (PBPK) modeling as an approach to determining the resulting overall variability. We present an analysis of the potential contribution of polymorphisms in enzymes modulating the disposition of four diverse compounds: methylene chloride, warfarin, parathion, and dichloroacetic acid. Through these case studies, we identify key uncertainties likely to be encountered in the use of polymorphism data and highlight potential simplifying assumptions that might be required to test the hypothesis that genetic factors are a substantive source of human variability in susceptibility to environmental toxicants. These uncertainties include (1) the relative contribution of multiple enzyme systems, (2) the extent of induction/inhibition through coexposure, (3) allelic frequencies of major ethnic groups, (4) the absence of chemical-specific data on the kinetic parameters for the different allelic forms of key enzymes, (5) large numbers of low-frequency alleles, and (6) uncertainty regarding differences between in vitro and in vivo kinetic data. Our effort sets the stage for the acquisition of critical data and further integration of polymorphism data with PBPK modeling as a means to quantitate population variability.

Mathematical modelling and quantitative methods

The present review reports on the mathematical methods and statistical techniques presently available for hazard characterisation. The state of the art of mathematical modelling and quantitative methods used currently for regulatory decision-making in Europe and additional potential methods for risk assessment of chemicals in food and diet are described. Existing practices of JECFA, FDA, EPA, etc., are examined for their similarities and differences. A framework is established for the development of new and improved quantitative methodologies. Areas for refinement, improvement and increase of efficiency of each method are identified in a gap analysis. Based on this critical evaluation, needs for future research are defined. It is concluded from our work that mathematical modelling of the dose-response relationship would improve the risk assessment process. An adequate characterisation of the dose-response relationship by mathematical modelling clearly requires the use of a sufficient number of dose groups to achieve a range of different response levels. This need not necessarily lead to an increase in the total number of animals in the study if an appropriate design is used. Chemical-specific data relating to the mode or mechanism of action and/or the toxicokinetics of the chemical should be used for dose-response characterisation whenever possible. It is concluded that a single method of hazard characterisation would not be suitable for all kinds of risk assessments, and that a range of different approaches is necessary so that the method used is the most appropriate for the data available and for the risk characterisation issue. Future refinements to dose-response characterisation should incorporate more clearly the extent of uncertainty and variability in the resulting output.

Hazard characterisation of chemicals in food and diet. dose response, mechanisms and extrapolation issues

Hazard characterisation of low molecular weight chemicals in food and diet generally use a no-observed-adverse-effect level (NOAEL) or a benchmark dose as the starting point. For hazards that are considered not to have thresholds for their mode of action, low-dose extrapolation and other modelling approaches may be applied. The default position is that rodents are good models for humans. However, some chemicals cause species-specific toxicity syndromes. Information on quantitative species differences is used to modify the default uncertainty factors applied to extrapolate from experimental animals to humans. A central theme for extrapolation is unravelling the mode of action for the critical effects observed. Food can be considered as an extremely complex and variable chemical mixture. Interactions among low molecular weight chemicals are expected to be rare given that the exposure levels generally are far below their NOAELs. Hazard characterisation of micronutrients must consider that adverse effects may arise from intakes that are too low (deficiency) as well as too high (toxicity). Interactions between different nutrients may complicate such hazard characterisations. The principle of substantial equivalence can be applied to guide the hazard identification and hazard characterisation of macronutrients and whole foods. Macronutrients and whole foods must be evaluated on a case-by-case basis and cannot follow a routine assessment protocol.

Human variability in glucuronidation in relation to uncertainty factors for risk assessment

The appropriateness of the default uncertainty factor for human variability in kinetics has been investigated for glucuronidation using an extensive database of substrates metabolised primarily by this pathway. Inter-individual variability was quantified for 15 compounds from published pharmacokinetic studies (after oral and intravenous dosing) in healthy adults and other subgroups using parameters relating to chronic exposure (metabolic and total clearances, area under the plasma concentration time-curve (AUC)) and acute exposure (C(max)). Low inter-individual variability (about 30-35%) was found for all parameters (clearance corrected or not corrected for body weight, metabolic clearance, oral AUC and C(max)) after either iv or oral administration to healthy adults. The overall variability of 31% for glucuronidation in healthy adults supported the validity of the default kinetic uncertainty factor of 3.16 for this group, because it would cover more than 99% of individuals. Comparisons between potentially sensitive subgroups and healthy adults using differences in means and variability indicated that neonates showed the greatest impairment of glucuronidation, and that the 3.16 kinetic default factor applied to the mean data for adults would be inadequate for this subpopulation. The in vivo data have been used to derive pathway-related default factors for compounds eliminated largely via glucuronidation.

Uncertainty factors for chemical risk assessment: interspecies differences in glucuronidation

For the risk assessment of effects other than cancer, a safe daily intake in humans is generally derived from a surrogate threshold dose (e.g. NOAEL) in an animal species to which an uncertainty factor of 100 is usually applied. This 100-fold is to allow for possible interspecies (10-fold) and interindividual (10-fold) differences in response to a toxicant, and incorporates toxicodynamic and toxicokinetic aspects of variability. The current study determined the magnitude of the interspecies differences in the internal dose of compounds for which glucuronidation is the major pathway of metabolism in either humans or in the test species. The results showed that there are major interspecies differences in the nature of the biological processes which influence the internal dose, including the route of metabolism, the extent of presystemic metabolism and enterohepatic recirculation. The work presented does not support the refinement of the interspecies toxicokinetic default to species- and pathway-specific values, but demonstrates the necessity for risk assessments to be carried out using quantitative chemical-specific data which define the fundamental processes which will influence the internal dose of a chemical (toxicokinetics), or the interaction of toxicant with its target site (toxicodynamics).

Uncertainty factors for chemical risk assessment. human variability in the pharmacokinetics of CYP1A2 probe substrates

A 100-fold uncertainty factor is used to derive acceptable daily intakes for compounds causing thresholded toxicity. The 10-fold factor for human variability can be further subdivided into two factors of 10(0.5) (3.16) to allow for toxicokinetics and toxicodynamics. The validity of the human kinetic subfactor has been analysed in relation to CYP1A2 metabolism using published in vivo pharmacokinetic parameters selected to reflect chronic exposure (metabolic and total clearances and area under the plasma concentration-time curve: CLm, CL and AUC) and acute exposure (the peak plasma concentration, C(max)). The variability in CYP1A2 activity in healthy adults, based on data after oral and intravenous dosage (CLm, CL and AUC), ranged from 34 to 42%. The variability in C(max) was 21%. The default kinetic factor of 3.16 would cover at least 99% of the healthy adult population, assuming that the data were log-normally distributed, but would give lower protection for some subgroups (pregnant women at term, healthy elderly, patients with liver disease), and was inadequate for neonates. This analysis of in vivo kinetic data for CYP1A2 substrates illustrates the importance of quantifying human variability in specific metabolic pathways, and of identifying potentially susceptible subgroups of the human population, in order to determine the scientific validity of uncertainty factors.

Uncertainty factors for chemical risk assessment: interspecies differences in the in vivo pharmacokinetics and metabolism of human CYP1A2 substrates

The 100-fold default uncertainty factor is used to convert a no-observed-adverse-effect level (NOAEL) from a animal toxicity study, to a "safe" value for human intake. The composite uncertainty factor (100) has to allow for interspecies (10-fold) and interindividual (10-fold) differences in toxicokinetics and toxicodynamics. The aim of the current study was to assess the validity of the interspecies default for toxicokinetics (4.0) for each of the test species (dog, rabbit, rat and mouse), using published data for compounds eliminated by CYP1A2 in humans (caffeine, theobromine, theophylline and paraxanthine). An analysis of the published literature showed that the absorption, bioavailability and route of excretion were generally similar between humans and the test species, for each probe substrate. However, interspecies differences in the route of metabolism, and the enzymes involved in this process, were identified. The magnitude of difference in the internal dose, between species, showed that values for the mouse (10.6) and rat (5.4) exceed the 4.0-fold default, whereas the rabbit (2.6) and dog (1.6) were below this value. This work supports the need to replace the generic default factors by a compound-related value derived from specific, relevant, quantitative data; this would result in more relevant and reliable non-cancer risk assessments.

Proposed occupational exposure limits for select ethylene glycol ethers using PBPK models and Monte Carlo simulations

Methoxyethanol (ethylene glycol monomethyl ether, EGME), ethoxyethanol (ethylene glycol monoethyl ether, EGEE), and ethoxyethyl acetate (ethylene glycol monoethyl ether acetate, EGEEA) are all developmental toxicants in laboratory animals. Due to the imprecise nature of the exposure data in epidemiology studies of these chemicals, we relied on human and animal pharmacokinetic data, as well as animal toxicity data, to derive 3 occupational exposure limits (OELs). Physiologically based pharmacokinetic (PBPK) models for EGME, EGEE, and EGEEA in pregnant rats and humans have been developed (M. L. Gargas et al., 2000, Toxicol. Appl. Pharmacol. 165, 53-62; M. L. Gargas et al., 2000, Toxicol. Appl. Pharmacol. 165, 63-73). These models were used to calculate estimated human-equivalent no adverse effect levels (NAELs), based upon internal concentrations in rats exposed to no observed effect levels (NOELs) for developmental toxicity. Estimated NAEL values of 25 ppm for EGEEA and EGEE and 12 ppm for EGME were derived using average values for physiological, thermodynamic, and metabolic parameters in the PBPK model. The uncertainties in the point estimates for the NOELs and NAELs were estimated from the distribution of internal dose estimates obtained by varying key parameter values over expected ranges and probability distributions. Key parameters were identified through sensitivity analysis. Distributions of the values of these parameters were sampled using Monte Carlo techniques and appropriate dose metrics calculated for 1600 parameter sets. The 95th percentile values were used to calculate interindividual pharmacokinetic uncertainty factors (UFs) to account for variability among humans (UF(h,pk)). These values of 1.8 for EGEEA/EGEE and 1.7 for EGME are less than the default value of 3 for this area of uncertainty. The estimated human equivalent NAELs were divided by UF(h,pk) and the default UFs for pharmacodynamic variability among animals and among humans to calculate the proposed OELs. This methodology indicates that OELs (8-h time-weighted average) that should protect workers from the most sensitive adverse effects of these chemicals are 2 ppm EGEEA and EGEE (11 mg/m(3) EGEEA, 7 mg/m(3) EGEE) and 0.9 ppm (3 mg/m(3)) EGME. These recommendations assume that dermal exposure will be minimal or nonexistent.

Exposure analysis of five fish-consuming populations for overexposure to methylmercury

Mercury, in the form of methylmercury, is found in a myriad of fish species consumed by recreational and subsistence fishers around the world. Many agencies have attempted to address the issue of mercury consumption, resulting at times in the placement of advisories on waterways used for fishing. In this study, consumption rates of three Native American populations and two recreational fishing populations consuming freshwater or saltwater fish species were examined. These consumption rates were combined with fish contamination data to assess the level of exposure to methylmercury and to determine if any of these populations exceed a derived tolerable daily intake (TDI) for methylmercury (0.035 to 0.08 microg/kg/day). The TDI is unlikely to result in adverse health effects and is based on scientific studies investigating sensitive endpoints in children of mothers who consume fish over prolonged periods of time. Results from the exposure analysis indicate that many within the Native American populations exceed the TDI. This occurs even though the mercury concentrations in certain fish species are comparable to concentrations found in fish from open waters where "background" levels are assumed. Recreational anglers consuming freshwater species have exposure levels below the TDI as do nearly all anglers consuming saltwater species. Similar populations or populations having comparable consumption patterns of fish with equal or higher mercury levels in other areas will also exceed the TDI level for mercury. The public health implications of this exposure analysis are discussed.

Using Human Data to Protect the Public's Health

The value of using human data in the assessment and management of risk is evaluated. Although the use of such data has a long and successful history with environmental contaminants and the development of drugs and commercial chemicals, recent deliberations within the Environmental Protection Agency (EPA) have questioned this practice in part. Specifically, we evaluate the degree to which reference doses (RfDs) and reference concentrations (RfCs) derived from human data on EPA's Integrated Risk Information System (IRIS) differ with RfDs and RfCs that we estimate from experimental animal data. We also use several minimal risk levels of the Agency for Toxic Substances and Disease Registry (ATSDR) and tolerable intakes of Health Canada in this comparison. Human-based RfDs are more than threefold lower than the corresponding animal-based RfDs for 23% of the comparisons. Human- based RfDs or RfCs are lower than corresponding animal-based RfDs or RfCs for 36% of the comparisons. Furthermore, for 10 of 43 possible comparisons, insufficient experimental animal data are readily available or data are inappropriate to estimate either RfDs or RfCs. We also discuss human pharmacokinetic data from volunteer studies and mechanistic studies with human tissues in vitro and demonstrate through a series of case discussions that utilization of such data is important when making decisions to protect exposed individuals. Moreover, physiologically based pharmacokinetic (PBPK) modeling evaluates critical information in assessing interindividual variability and identifying at-risk populations. Within the limits of our analysis, we conclude that the direct use and interpretation of human data, in conjunction with data gathered from experimental animals, are public health protective policies that should be encouraged.

The use of surrogate endpoints to assess potential toxicity in humans

Data on toxic effects in humans may come from epidemiology studies, accidental poisonings, surveillance schemes or following intentional exposures. In many cases, a surrogate endpoint related to the adverse effect is investigated. Effects produced following intentional exposures are usually restricted to readily reversible, mild surrogate endpoints of the adverse effect of concern. Not all initial interactions within the target organ are related to the toxic effect, and many measurements are biomarkers of exposure not response. Biomarkers of response represent surrogate endpoints of response only if they are critical to the mode of action. The use of biomarkers and the possible problems with using surrogate endpoints are illustrated with data on aniline, cadmium, carbon monoxide, erythrosine, paracetamol (acetaminophen) and styrene. In vivo surrogate endpoints are normally used in risk assessment directly, whereas in vitro surrogate endpoints can be incorporated by the development of a biologically based dose-response model, or used to replace a default uncertainty factor by a chemical-specific adjustment factor.

Physiological-model-based derivation of the adult and child pharmacokinetic intraspecies uncertainty factors for volatile organic compounds

The intraspecies uncertainty factor (UF(HH)=10x) is used in the determination of the reference dose or reference concentration and accounts for the pharmacokinetic and pharmacodynamic heterogeneity within the human population. The Food Quality Protection Act of 1996 mandated the use of an additional uncertainty factor (UF(HC)=10x) to take into account potential pre- and postnatal toxicity and lack of completeness of the data with respect to exposure and toxicity to children. There is no conclusive experimental or theoretical justification to support or refute the magnitude of the UF(HH) and UF(HC) nor any conclusive evidence to suggest that a factor of 100 is needed to account for intrahuman variability. This study presents a new chemical-specific method for estimating the pharmacokinetic (PK) component of the interspecies uncertainty factor (UF(HH-PK) and UF(HC-PK)) for volatile organic compounds (VOCs). The approach utilizes validated physiological-based pharmacokinetic (PBPK) models and simplified physiological-model-based algebraic equations to translate ambient exposure concentration to tissue dose in adults and children the ratio of which is the UF(HH-PK) and UF(HC-PK). The results suggest that: (i) the UF(HH-PK) and UF(HC-PK) are chemical specific; (ii) for the chemicals used in this study there is no significant difference between UF(HH-PK) and UF(HC-PK); (iii) the magnitude of UF(HH-PK) and UF(HC-PK) varies between 0.033 and 2.85 with respect to tissue and blood concentrations; (iv) the body weight, the rate of ventilation, the fraction of cardiac output flowing to the liver, the blood : air partition coefficient, and the hepatic extraction ratio are the only parameters that play a critical role in the variability of tissue and blood doses within species; and (v) the magnitude of the UF(HH-PK) and UF(HC-PK) obtained with the simplified steady-state equations is essentially the same with that obtained with PBPK models. Overall, this study suggests that no adult-children differences in the parent chemical concentrations of the VOCs are likely to be observed during inhalation exposures. The physiological-model-based approaches used in the present study to estimate the UF(HH-PK) and UF(HC-PK) provide a scientific basis for their magnitude. They can replace the currently used empirical default approaches to provide chemical-specific UF(HH-PK) in future risk assessments.

The utility of PBPK in the safety assessment of chloroform and carbon tetrachloride

Occupational exposure limits (OELs) for individual substances are established on the basis of the available toxicological information at the time of their promulgation, expert interpretation of these data in light of industrial use, and the framework in which they sit. In the United Kingdom, the establishment of specific OELs includes the application of uncertainty factors to a defined starting point, usually the NOAEL from a suitable animal study. The magnitude of the uncertainty factors is generally determined through expert judgment including a knowledge of workplace conditions and management of exposure. PBPK modeling may help in this process by informing on issues relating to extrapolation between and within species. This study was therefore designed to consider how PBPK modeling could contribute to the establishment of OELs. PBPK models were developed for chloroform (mouse and human) and carbon tetrachloride (rat and human). These substances were chosen for examination because of the extent of their toxicological databases and availability of existing PBPK models. The models were exercised to predict the rate (chloroform) or extent (carbon tetrachloride) of metabolism of these substances, in both rodents and humans. Monte Carlo analysis was used to investigate the influence of variability within the human and animal model populations. The ratio of the rates/extent of metabolism predicted for humans compared to animals was compared to the uncertainty factors involved in setting the OES. Predictions obtained from the PBPK models indicated that average rat and mouse metabolism of carbon tetrachloride and chloroform, respectively, are much greater than that of the average human. Application of Monte Carlo analysis indicated that even those people who have the fastest rates or most extensive amounts of metabolism in the population are unlikely to generate the levels of metabolite of these substances necessary to produce overt toxicity in rodents. This study highlights the value that the use of PBPK modeling may add to help inform and improve toxicological aspects of a regulatory process.

The use of safety or uncertainty factors in the setting of acute reference doses

A 100-fold safety or uncertainty factor has been used for about 40 years to derive safe daily intakes for humans based on animal studies; the 100-fold factor comprises separate 10-fold factors to allow for species differences and inter-individual variability. Each factor has to allow for toxicokinetic and toxicodynamic differences. Sub-dividing the 10-fold factors into kinetic and dynamic defaults, which when multiplied give a product of 10, offers a number of advantages. The main rationale for this sub-division is so that chemical-specific data can be introduced to replace one or more of the default sub-factors, hence contributing to a chemical-related overall factor. However, sub-division of the 10-fold factors has allowed analysis of the appropriateness of the overall 10-fold defaults, and analysis of special situations, such as infants and children. The establishment of an acute reference dose based on animal studies has to allow for both species differences and inter-individual variability; comparison with the factors used for chronic effects suggests that modification of the usual defaults may be appropriate under certain specific circumstances, but that the usual default of 100 remains appropriate for most cases.

An analysis of the need for an additional uncertainty factor for infants and children

Uncertainty factors have been used for over 40 years to establish safe intakes of threshold toxicants. Tenfold factors are used to allow for species differences and for human variability, with extra factors for database inadequacies. The proposal to introduce an additional 10-fold factor for pesticides when exposure of infants and children is anticipated implies either age-related differences between species or differences within humans which exceed those present in adults. Alternatively, the extra factor could be related to deficiencies of current testing methods or concerns over irreversibility in developing organ systems. Available data do not provide a scientific rationale for the extra factor due to inadequacy of inter- and intraspecies uncertainty factors. Justification for the factor therefore must relate to the adequacy and sensitivity of current methods or concern about irreversible effects in the developing organism.

An analysis of the possibility for health implications of joint actions and interactions between food additives

The possibility that structurally unrelated food additives could show either joint actions or interactions has been assessed based on their potential to share common sites and mechanisms of action or common pathways of elimination. All food additives approved in the European Union and allocated numerical acceptable daily intake values were studied, initially based on the reports by the FAO-WHO Joint Expert Committee for Food Additives. Target organs were identified based on the effects reported at doses above the no-observed-adverse-effect level (NOAEL) in animal and human studies. The descriptions of the pathological and other changes reported were used to assess whether different additives, sharing the same target organ, would produce a common toxic effect. In all but a very few cases, the possibility of joint actions or interactions could be excluded on scientific grounds. The exceptions were on the liver (curcumin, thiabendazole, propyl gallate, and BHT), the kidney (diphenyl, o-phenylphenol, and ferrocyanide salts), the blood (azorubine and propyl gallate), and the thyroid (erythosine, thiabendazole, and nitrate). Toxicokinetic interactions were considered unlikely because of the low dosages involved, the diverse nature of the routes of metabolism and elimination, and the fact that enzyme induction or inhibition would have influenced selection of the NOAEL. Many of those additives which could not be excluded from showing joint actions or interactions would have low intakes; in some cases they were alternatives for the same application, thereby further lowering the combined intake. In consequence, joint actions or interactions between additives do not represent a significant health concern.

Threshold of toxicological concern for chemical substances present in the diet: a practical tool for assessing the need for toxicity testing

The de minimis concept acknowledges a human exposure threshold value for chemicals below which there is no significant risk to human health. It is the underlying principle for the US Food and Drug Administration (FDA) regulation on substances used in food-contact articles. Further to this, the principle of Threshold of Toxicological Concern (TTC) has been developed and is now used by the Joint FAO/WHO Expert Committee on Food Additives (JECFA) in their evaluations. Establishing an accepted TTC would benefit consumers, industry and regulators, since it would preclude extensive toxicity evaluations when human intakes are below such threshold, and direct considerable time and cost resources towards testing substances with the highest potential risk to human health. It was questioned, however, whether specific endpoints that may potentially give rise to low-dose effects would be covered by such threshold. In this review, the possibility of defining a TTC for chemical substances present in the diet was examined for general toxicity endpoints (including carcinogenicity), as well as for specific endpoints, namely neurotoxicity and developmental neurotoxicity, immunotoxicity and developmental toxicity. For each of these endpoints, a database of specific no-observed-effect levels (NOELs) was compiled by screening oral toxicity studies. The substances recorded in each specific database were selected on the basis of their demonstrated adverse effects. For the neurotoxicity and developmental neurotoxicity databases, it was intended to cover all classes of compounds reported to have either a demonstrated neurotoxic or developmentally neurotoxic effect, or at least, on a biochemical or pharmacological basis were considered to have a potential for displaying such effects. For the immunotoxicity endpoint, it was ensured that only immunotoxicants were included in the database by selecting most of the substances from the Luster et al. database, provided that they satisfied the criteria for immunotoxicity defined by Luster. For the developmental toxicity database, substances were selected from the Munro et al. database that contained the lowest NOELs retrieved from the literature for more than 600 compounds. After screening these, substances showing any effect which could point to developmental toxicity as broadly defined by the US were recorded in the database. Additionally, endocrine toxicity and allergenicity were addressed as two separate cases, using different approaches and methodology. The distributions of NOELs for the neurotoxicity, developmental neurotoxicity and developmental toxicity endpoints were compared with the distribution of NOELs for non-specific carcinogenic endpoints. As the immunotoxicity database was too limited to draw such a distribution of immune NOELs, the immunotoxicity endpoint was evaluated by comparing immune NOELs (or LOELs-lowest-observed-effect levels-when NOELs were not available) with non-immune NOELs (or LOELs), in order to compare the sensitivity of this endpoint with non-specific endpoints. A different methodology was adopted for the evaluation of the endocrine toxicity endpoint since data currently available do not permit the establishment of a clear causal link between endocrine active chemicals and adverse effects in humans. Therefore, this endpoint was analysed by estimating the human exposure to oestrogenic environmental chemicals and evaluating their potential impact on human health, based on their contribution to the overall exposure, and their estrogenic potency relative to endogenous hormones. The allergenicity endpoint was not analysed as such. It was addressed in a separate section because this issue is not relevant to the overall population but rather to subsets of susceptible individuals, and allergic risks are usually controlled by other means (i.e. labelling) than the Threshold of Toxicological Concern approach. (ABSTRACT TRUNCATED)