Assessing children's exposure to manganese in drinking water using a PBPK model.

A previously published human PBPK model for manganese (Mn) in infants and children has been updated with Mn in drinking water as an additional exposure source. Built upon the ability to capture differences in Mn source-specific regulation of intestinal uptake in nursing infants who are breast-fed and formula-fed, the updated model now describes the bioavailability of Mn from drinking water in children of ages 0-18. The age-related features, including the recommended age-specific Mn dietary intake, age-specific water consumption rates, and age-specific homeostasis of Mn, are based on the available human data and knowledge of the biology of essential-metal homeostasis. Model simulations suggest that the impact of adding drinking-water exposure to daily Mn exposure via dietary intake and ambient air inhalation in children is not greater than the impacts in adults, even at a drinking-water concentration that is 2 times higher than the USEPA's lifetime health advisory value. This conclusion was also valid for formula-fed infants who are considered at the highest potential exposure to Mn from drinking water compared to all other age groups. Our multi-route, multi-source Mn PBPK model for infants and children provides insights about the potential for Mn-related health effects on growing children and will thereby improve the level of confidence in properly interpreting Mn exposure-health effects relationships in children in human epidemiological studies.

Transcriptional responses in the rat nasal epithelium following subchronic inhalation of naphthalene vapor.

Male and female Fischer 344 rats were exposed to naphthalene vapors at 0 (controls), 0.1, 1, 10, and 30ppm for 6h/d, 5 d/wk, over a 90-day period. Following exposure, the respiratory epithelium and olfactory epithelium from the nasal cavity were dissected separately, RNA was isolated, and gene expression microarray analysis was conducted. Only a few significant gene expression changes were observed in the olfactory or respiratory epithelium of either gender at the lowest concentration (0.1ppm). At the 1.0ppm concentration there was limited evidence of an oxidative stress response in the respiratory epithelium, but not in the olfactory epithelium. In contrast, a large number of significantly enriched cellular pathway responses were observed in both tissues at the two highest concentrations (10 and 30ppm, which correspond to tumorigenic concentrations in the NTP bioassay). The nature of these responses supports a mode of action involving oxidative stress, inflammation and proliferation. These results are consistent with a dose-dependent transition in the mode of action for naphthalene toxicity/carcinogenicity between 1.0 and 10ppm in the rat. In the female olfactory epithelium (the gender/site with the highest incidences of neuroblastomas in the NTP bioassay), the lowest concentration at which any signaling pathway was significantly affected, as characterized by the median pathway benchmark dose (BMD) or its 95% lower bound (BMDL) was 6.0 or 3.7ppm, respectively, while the lowest female olfactory BMD values for pathways related to glutathione homeostasis, inflammation, and proliferation were 16.1, 11.1, and 8.4ppm, respectively. In the male respiratory epithelium (the gender/site with the highest incidences of adenomas in the NTP bioassay), the lowest pathway BMD and BMDL were 0.4 and 0.3ppm, respectively, and the lowest male respiratory BMD values for pathways related to glutathione homeostasis, inflammation, and proliferation were 0.5, 0.7, and 0.9ppm, respectively. Using a published physiologically based pharmacokinetic (PBPK) model to estimate target tissue dose relevant to the proposed mode of action (total naphthalene metabolism per gram nasal tissue), the lowest transcriptional BMDLs from this analysis equate to human continuous naphthalene exposure at approximately 0.3ppm. It is unlikely that significant effects of naphthalene or its metabolites will occur at exposures below this concentration.

Time- and concentration-dependent genomic responses of the rat airway to inhaled nickel subsulfide.

OBJECTIVE: To provide insights into the mode of action for Ni3S2 lung carcinogenicity by examining gene expression changes in target cells after inhalation exposure. METHODS: Gene expression changes were determined in micro-dissected lung broncho-alveolar cells from Fischer 344 rats following inhalation of Ni3S2 at 0.0, 0.04, 0.08, 0.15, and 0.60 mg/m(3) (0.03, 0.06, 0.11, and 0.44 mgNi/m(3)) for one and four weeks (6h/day, 5 days/week). RESULTS: Broncho-alveolar lavage fluid evaluation and lung histopathology provided evidence of inflammation only at the two highest concentrations, which were similar to those tested in the 2-year bioassay. The number of statistically significant up- and down-regulated genes decreased markedly from one to four weeks of exposure, suggesting adaptation. Cell signal pathway enrichment at both time-points primarily reflected responses to toxicity, including inflammatory and proliferative signaling. While proliferative signaling was up-regulated at both time points, some inflammatory signaling reversed from down-regulation at 1 week to up-regulation at 4 weeks. CONCLUSIONS: These results support a mode of action for Ni3S2 carcinogenicity driven by chronic toxicity, inflammation and proliferation, leading to mis-replication, rather than by direct genotoxicity. Benchmark dose (BMD) analysis identified the lowest pathway transcriptional BMD exposure concentration as 0.026 mgNi/m(3), for apoptosis/survival signaling. When conducted on the basis of lung Ni concentration the lowest pathway BMD was 0.64 µgNi/g lung, for immune/inflammatory signaling. IMPLICATIONS: These highly conservative BMDs could be used to derive a point of departure in a nonlinear risk assessment for Ni3S2 toxicity and carcinogenicity.

A constrained maximum likelihood approach to evaluate the impact of dose metric on cancer risk assessment: application to ß-chloroprene.

ß-Chloroprene (2-chloro-1,3-butadiene, CD) is used in the manufacture of polychloroprene rubber. Chronic inhalation studies have demonstrated that CD is carcinogenic in B6C3F1 mice and Fischer 344 rats. However, epidemiological studies do not provide compelling evidence for an increased risk of mortality from total cancers of the lung. Differences between the responses observed in animals and humans may be related to differences in toxicokinetics, the metabolism and detoxification of potentially active metabolites, as well as species differences in sensitivity. The purpose of this study was to develop and apply a novel method that combines the results from available physiologically based kinetic (PBK) models for chloroprene with a statistical maximum likelihood approach to test commonality of low-dose risk across species. This method allows for the combined evaluation of human and animal cancer study results to evaluate the difference between predicted risks using both external and internal dose metrics. The method applied to mouse and human CD data supports the hypothesis that a PBK-based metric reconciles the differences in mouse and human low-dose risk estimates and further suggests that, after PBK metric exposure adjustment, humans are equally or less sensitive than mice to low levels of CD exposure.

Using available in vitro metabolite identification and time course kinetics for ß-chloroprene and its metabolite, (1-chloroethenyl) oxirane, to include reactive oxidative metabolites and glutathione depletion in a PBPK model for ß-chloroprene.

Introduction: ß-chloroprene (2-chloro-1,3-butadiene; CP) causes lung tumors after inhalation exposures in rats and mice. Mice develop these tumors at lower exposures than rats. In rats CP exposures cause depletion of lung glutathione (GSH). Methods: PBPK models developed to relate the appearance of mouse lung tumors with rates of CP metabolism to reactive metabolites or total amounts metabolized during exposures have been expanded to include production of reactive metabolites from CP. The extended PBPK model describes both the unstable oxirane metabolite, 2-CEO, and metabolism of the more stable oxirane, 1-CEO, to reactive metabolites via microsomal oxidation to a diepoxide, and linked production of these metabolites to a PK model predicting GSH depletion with increasing CP exposure. Key information required to develop the model were available from literature studies identifying: 1) microsomal metabolites of CP, and 2) in vitro rates of clearance of CP and 1-CEO from active microsomal preparations from mice, rats, hamsters and humans. Results: Model simulation of concentration dependence of disproportionate increases in reactive metabolite concentrations as exposures increases and decreases in tissue GSH are consistent with the dose-dependence of tumor formation. At the middle bioassay concentrations with a lung tumor incidence, the predicted tissue GSH is less than 50% background. These simulations of reduction in GSH are also consistent with the gene expression results showing the most sensitive pathways are Nrf2-regulation of oxidative stress and GSH metabolism. Discussion: The PBPK model is used to correlate predicted tissue exposure to reactive metabolites with toxicity and carcinogenicity of CP.

Quantitative bias analysis of the association of type 2 diabetes mellitus with 2,2',4,4',5,5'-hexachlorobiphenyl (PCB-153).

An association between serum concentrations of persistent organic pollutants (POPs), such as 2,2',4,4',5,5'-hexachlorobiphenyl (PCB-153), and risk of type 2 diabetes mellitus (T2DM) has been reported. Conditional on body mass index (BMI) and waist circumference (WC), a higher serum PCB-153 concentration may be a marker of T2DM risk because it reflects other aspects of obesity that are related to T2DM risk and to PCB-153 clearance. To estimate the amount of residual confounding by other aspects of obesity, we performed a quantitative bias analysis on the results of a specific study. A physiologically-based pharmacokinetic (PBPK) model was developed to predict serum levels of PCB-153 for a simulated population. T2DM status was assigned to simulated subjects based on age, sex, BMI, WC, and visceral adipose tissue mass. The distributions of age, BMI, WC, and T2DM prevalence of the simulated population were tailored to closely match the target population. Analysis of the simulated data showed that a small part of the observed association appeared to be due to residual confounding. For example, the predicted odds ratio of T2DM that would have been obtained had the results been adjusted for visceral adipose tissue mass, for the ≥90th percentile of PCB-153 serum concentration, was 6.60 (95% CI 2.46-17.74), compared with an observed odds ratio of 7.13 (95% CI 2.65-19.13). Our results predict that the association between PCB-153 and risk of type 2 diabetes mellitus would not be substantially changed by additional adjustment for visceral adipose tissue mass in epidemiologic analyses. Confirmation of these predictions with longitudinal data would be reassuring.

Tissue distribution and urinary excretion of inorganic arsenic and its methylated metabolites in C57BL6 mice following subchronic exposure to arsenate in drinking water.

The relationship of exposure and tissue concentration of parent chemical and metabolites over prolonged exposure is a critical issue for chronic toxicities mediated by metabolite(s) rather than parent chemical alone. This is an issue for AsV because its trivalent metabolites have unique toxicities and relatively greater potency compared to their pentavalent counterparts for many endpoints. In this study, dose-dependency in tissue distribution and urinary excretion for inorganic arsenic and its methylated metabolites was assessed in female C57Bl/6 mice exposed to 0, 0.5, 2, 10 or 50 ppm arsenic (as arsenate, AsV) in their drinking water for 12 weeks. No adverse effects were observed and body weight gain did not differ significantly among groups. Urinary excretion of arsenite monomethylarsonous acid (MMA(III)), dimethylarsinous acid (DMA(III)), dimethylarsinic acid (DMAV), and trimethylarsine oxide (TMAO) increased linearly with dose, whereas AsV and monomethylarsonic acid (MMAV) excretion was non-linear with respect to dose. Total tissue arsenic accumulation was greatest in kidney > lung > urinary bladder >>> skin > blood > liver. Monomethyl arsenic (MMA, i.e. MMA(III)+MMAV) was the predominant metabolite in kidney, whereas dimethylarsenic (DMA, i.e., DMA(III)+DMAV) was the predominant metabolite in lung. Urinary bladder tissue had roughly equivalent levels of inorganic arsenic and dimethylarsenic, as did skin. These data indicate that pharmacokinetic models for arsenic metabolism and disposition need to include mechanisms for organ-specific accumulation of some arsenicals and that urinary metabolite profiles are not necessarily reflective of target tissue dosimetry.

Time- and concentration-dependent genomic responses of the rat airway to inhaled nickel sulfate.

While insoluble nickel subsulfide (Ni3 S2 ) was carcinogenic in the lung in a 2-year rat bioassay, soluble nickel sulfate hexahydrate (NiSO4* 6H2 O) was not. To investigate whether differences in the cellular responses to these two nickel compounds could underlie their differential activities, we conducted parallel studies to determine the gene expression changes in micro-dissected lung distal airway cells from Fischer 344 rats following inhalation of the two compounds for one and four weeks (6 hr per day, 5 days per week). The results of the Ni3 S2 study have been reported previously; this paper reports the results for NiSO4 and provides a comparative analysis. The cellular responses to NiSO4 were highly similar to those previously reported for Ni3 S2 , and a set of genes was identified whose expression could be used as biomarkers for comparing cellular nickel effects from in vitro or in vivo studies with soluble NiSO4 and particulate Ni3 S2 . Evaluation of the genomic concentration-responses for the two compounds suggests that the highest inhaled concentration in the tumor bioassay for NiSO4 , which was limited by toxicity, may not have achieved the Ni concentrations at which tumors were observed in the Ni3 S2 bioassay. However, several key differences in the immune responses to NiSO4 and Ni3 S2 were identified that may result from the differential intracellular disposition of Ni from NiSO4 entering the cell as an ion rather than as a slowly soluble Ni3 S2 particle. These differences may also contribute to the observation of tumors in the bioassay for Ni3 S2 but not NiSO4 . Environ. Mol. Mutagen. 58:607-618, 2017. © 2017 The Authors Environmental and Molecular Mutagenesis published by Wiley Periodicals, Inc. on behalf of Environmental Mutagen Society.

Derivation of a human equivalent concentration for n-butanol using a physiologically based pharmacokinetic model for n-butyl acetate and metabolites n-butanol and n-butyric acid.

The metabolic series approach for risk assessment uses a dosimetry-based analysis to develop toxicity information for a group of metabolically linked compounds using pharmacokinetic (PK) data for each compound and toxicity data for the parent compound. The metabolic series approach for n-butyl acetate and its subsequent metabolites, n-butanol and n-butyric acid (the butyl series), was first demonstrated using a provisional physiologically based pharmacokinetic (PBPK) model for the butyl series. The objective of this work was to complete development of the PBPK model for the butyl series. Rats were administered test compounds by iv bolus dose, iv infusion, or by inhalation in a recirculating closed chamber. Hepatic, vascular, and extravascular metabolic constants for metabolism were estimated by fitting the model to the blood time course data from these experiments. The respiratory bioavailability of n-butyl acetate (100% of alveolar ventilation) and n-butanol (50% of alveolar ventilation) was estimated from closed chamber inhalation studies and measured ventilation rates. The resulting butyl series PBPK model successfully reproduces the blood time course of these compounds following iv administration and inhalation exposure to n-butyl acetate and n-butanol in rats and arterial blood n-butanol kinetics following inhalation exposure to n-butanol in humans. These validated inhalation route models can be used to support species and dose-route extrapolations required for risk assessment of butyl series family of compounds. Human equivalent concentrations of 169 ppm and 1066 ppm n-butanol corresponding to the rat n-butyl acetate NOAELs of 500 and 3000 ppm were derived using the models.

Evaluating noncancer effects of trichloroethylene: dosimetry, mode of action, and risk assessment.

Alternatives for developing chronic exposure limits for noncancer effects of trichloroethylene (TCE) were evaluated. These alternatives were organized within a framework for dose-response assessment--exposure:dosimetry (pharmacokinetics):mode of action (pharmacodynamics): response. This framework provides a consistent structure within which to make scientific judgments about available information, its interpretation, and use. These judgments occur in the selection of critical studies, internal dose metrics, pharmacokinetic models, approaches for interspecies extrapolation of pharmacodynamics, and uncertainty factors. Potentially limiting end points included developmental eye malformations, liver effects, immunotoxicity, and kidney toxicity from oral exposure and neurological, liver, and kidney effects by inhalation. Each end point was evaluated quantitatively using several methods. Default analyses used the traditional no-observed adverse effect level divided by uncertainty factors and the benchmark dose divided by uncertainty factors methods. Subsequently, mode-of-action and pharmacokinetic information were incorporated. Internal dose metrics were estimated using a physiologically based pharmacokinetic (PBPK) model for TCE and its major metabolites. This approach was notably useful with neurological and kidney toxicities. The human PBPK model provided estimates of human exposure doses for the internal dose metrics. Pharmacodynamic data or default assumptions were used for interspecies extrapolation. For liver and neurological effects, humans appear no more sensitive than rodents when internal dose metrics were considered. Therefore, the interspecies uncertainty factor was reduced, illustrating that uncertainty factors are a semiquantitative approach fitting into the organizational framework. Incorporation of pharmacokinetics and pharmacodynamics can result in values that differ significantly from those obtained with the default methods.

Physiologically based pharmacokinetic analyses of simple mixtures.

Exposure to multiple chemicals may cause significant alterations of tissue dose of the toxic moiety of one or more of the individual chemicals. The change in target tissue dose of a chemical present in simple mixtures can be predicted when the determinants of disposition of each chemical, and the mechanism of toxicokinetic interaction between chemicals are understood at a quantitative level. Determinants of disposition include physiological (e.g., breathing rates, cardiac output, tissue volumes, blood flow rates), biochemical (e.g., kinetic constants for metabolism and protein binding), and physicochemical factors (e.g., blood air and tissue blood partition coefficients). Mechanisms of toxicokinetic interactions refer to the manner in which coexposure alters these determinants of disposition as compared to exposure to the individual chemicals. Interactions between chemicals can be described quantitatively with physiologically based pharmacokinetic (PBPK) models, which integrate these mechanic determinants and permit prediction of alterations in tissue dose for various exposure situations by computer simulation. PBPK modeling studies of binary chemical interactions conducted so far indicate that inhibitory rather than potentiating metabolic interactions are more likely to be observed during multiple chemical exposures. As PBPK models of representative binary, tertiary and quaternary mixtures are developed, it will become increasingly possible to draw reliable conclusions about the risk associated with human exposure to chemical mixtures.

Physiologically based pharmacokinetic model for the inhibition of acetylcholinesterase by organophosphate esters.

Organophosphate (OP) exposure can be lethal at high doses while lower doses may impair performance of critical tasks. The ability to predict such effects for realistic exposure scenarios would greatly improve OP risk assessment. To this end, a physiologically based model for diisopropylfluorophosphate (DFP) pharmacokinetics and acetylcholinesterase (AChE) inhibition was developed. DFP tissue/blood partition coefficients, rates of DFP hydrolysis by esterases, and DFP-esterase bimolecular inhibition rate constants were determined in rat tissue homogenates. Other model parameters were scaled for rats and mice using standard allometric relationships. These DFP-specific parameter values were used with the model to simulate pharmacokinetic data from mice and rats. Literature data were used for model validation. DFP concentrations in mouse plasma and brain, as well as AChE inhibition and AChE resynthesis data, were successfully simulated for a single iv injection. Effects of repeated, subcutaneous DFP dosing on AChE activity in rat plasma and brain were also well simulated except for an apparent decrease in basal AChE activity in the brain which persisted 35 days after the last dose. The psychologically based pharmacokinetic (PBPK) model parameter values specific for DFP in humans, for example, tissue/blood partition coefficients, enzymatic and nonenzymatic DFP hydrolysis rates, and bimolecular inhibition rate constants for target enzymes were scaled from rodent data or obtained from the literature. Good agreement was obtained between model predictions and human exposure data on the inhibition of red blood cell AChE and plasma butyrylcholinesterase after an intramuscular injection of 33 micrograms/kg DFP and at 24 hr after acute doses of DFP (10-54 micrograms/kg), as well as for repeated DFP exposures.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of a physiologically based pharmacokinetic model of trichloroethylene and its metabolites for use in risk assessment.

A physiologically based pharmacokinetic (PBPK) model was developed that provides a comprehensive description of the kinetics of trichloroethylene (TCE) and its metabolites, trichloroethanol (TCOH), trichloroacetic acid (TCA), and dichloroacetic acid (DCA), in the mouse, rat, and human for both oral and inhalation exposure. The model includes descriptions of the three principal target tissues for cancer identified in animal bioassays: liver, lung, and kidney. Cancer dose metrics provided in the model include the area under the concentration curve (AUC) for TCA and DCA in the plasma, the peak concentration and AUC for chloral in the tracheobronchial region of the lung, and the production of a thioacetylating intermediate from dichlorovinylcysteine in the kidney. Additional dose metrics provided for noncancer risk assessment include the peak concentrations and AUCs for TCE and TCOH in the blood, as well as the total metabolism of TCE divided by the body weight. Sensitivity and uncertainty analyses were performed on the model to evaluate its suitability for use in a pharmacokinetic risk assessment for TCE. Model predictions of TCE, TCA, DCA, and TCOH concentrations in rodents and humans are in good agreement with a variety of experimental data, suggesting that the model should provide a useful basis for evaluating cross-species differences in pharmacokinetics for these chemicals. In the case of the lung and kidney target tissues, however, only limited data are available for establishing cross-species pharmacokinetics. As a result, PBPK model calculations of target tissue dose for lung and kidney should be used with caution.

Development of a physiologically based pharmacokinetic model of isopropanol and its metabolite acetone.

A physiologically based pharmacokinetic (PBPK) model for isopropanol (IPA) and its major metabolite, acetone, is described. The structure of the parent chemical model, which can be used for either IPA or acetone by choosing the appropriate chemical-specific parameters, is similar to previously published models of volatile organic chemicals such as styrene. However, in order to properly simulate data on the exhalation of IPA and acetone during inhalation exposures, it was necessary to expand the description of the lung compartment to include a subcompartment for the upper respiratory tract mucus layer. This elaboration is consistent with published PBPK models of other water-soluble vapors in which the mucus layer serves to absorb the chemical during inhalation and then release it during exhalation. In the case of IPA exposure, a similar PBPK structure is used to describe the kinetics of the acetone produced from the metabolism of IPA. The resulting model is able to provide a coherent description of IPA and acetone kinetics in the rat and human for exposures to IPA by several routes: intravenous, intraperitoneal, oral, inhalation, and dermal. It is also able to consistently reproduce kinetic data for exposures of rats or humans to acetone. Thus, the model provides a validated framework for performing chemical-specific route-to-route extrapolation and cross-species dosimetry, which can be used in place of generic default calculations in support of risk assessments for IPA and acetone.

Incorporating biological information in quantitative risk assessment: an example with methylene chloride.

The interplay between chemical risk assessment and scientific research is discussed in the context of recent attempts to improve the scientific basis for estimates of the human carcinogenic risk from methylene chloride. A combination of basic biochemical research and risk assessment oriented research, both mechanistic and pharmacokinetic, provided the initial impetus for re-evaluating the use of the default risk estimation approach. Resulting efforts to incorporate the new scientific information into the risk assessment process in turn identified specific additional research needed to reduce uncertainty in the estimated risk. This healthy interchange between the two disciplines has served both to refine the human risk estimates for methylene chloride and to more clearly identify key scientific issues for chemical risk assessment in general.

Use of physiologically based pharmacokinetic modeling to investigate individual versus population risk.

Because of the heterogeneity of the human population, it is generally expected that there will be a broad range of observed susceptibilities to the biological effects of exposure to chemicals or drugs. Often it is possible to distinguish specific classes of individuals, such as infants or the elderly, who appear to be more susceptible to a specific effect. Non-cancer risk assessment often address this variability by dividing the experimentally determined acceptable exposure level by an uncertainty factor of 10 to protect sensitive individuals; cancer risk assessments typically do not address this issue in any quantitative fashion. Physiologically based pharmacokinetic (PBPK) modeling provides the capability to quantitatively describe the potential impact of pharmacokinetic factors on the variability of individual risk. In particular, PBPK models can be used to determine the impact of differences in key metabolism enzymes, whether due to multiple genotypic expression, such as cytochrome P450 polymorphisms, or just due to normal variation in enzyme activities within the general population. Other potential modulators of sensitivity which can be addressed quantitatively with a PBPK model include physical condition, level of activity, disease states, age, hormonal status, and interactions with other chemicals and drugs. In each case, the PBPK model provides a quantitative structure for determining the effect of these various factors on the relationship between the external (environmental) exposure and the internal (biologically effective) target tissue exposure. When coupled with Monte Carlo analysis, the PBPK model provides a method to assess the quantitative impact of these sources of variability on individual risk (as opposed to average population risk) by comparing model-predicted risks over the distribution of individual parameter values.

Investigation of the impact of pharmacokinetic variability and uncertainty on risks predicted with a pharmacokinetic model for chloroform.

A sensitivity and uncertainty analysis was performed on the Reitz et al. (Toxicol. Appl. Pharmacol., 1990: 105, 443) physiologically based pharmacokinetic (PBPK) risk assessment model for chloroform. The analytical approach attempted to separately consider the impacts of interindividual variability and parameter uncertainty on the predicted values of the dose metrics in the model, as well as on liver cancer risk estimates obtained with the model. An important feature of the analytical approach was that an attempt was made to incorporate information on correlation between important parameters, for example, the observed correlation between total blood flow and alveolar ventilation rate. Using the published PBPK model for chloroform, the best estimate of the average population risk based on the preferred pharmacodynamic dose metric (PTDEAD), representing cell death, is 9.2 x 10(-7); this estimate is more than 500-fold lower than the risk estimate of 5.3 x 10(-4) based on an alternative pharmacokinetic dose metric (AVEMMB), which represents tissue adduct formation. However, when interindividual variability was considered the range of individual risks (from the 5th to the 95th percentile of the population) predicted with PTDEAD was extremely broad (from 3.0 x 10(-13) to 3.2 x 10(-4)), while individual risks predicted with AVEMMB only varied over a factor of four (from 1.9 x 10(-4) to 7.4 x 10(-4)). As a result, the upper 95th percentile of the distribution of individual risk estimates based on the preferred cell death metric were within a factor of three of the 95th percentile for the pharmacokinetic alternative. The crucial factor with respect to the much greater variability of chloroform risk estimates based on cell death is that the dose metric, PTDEAD, is exquisitely sensitive to variation of the parameters in the model defining the response of cells to the cytotoxicity of chloroform. Unfortunately, these key parameters are also highly uncertain, as well as strongly correlated. As a result it proved impossible to accurately quantify the additional impact of parameter uncertainty on the dose metrics and risk estimates for chloroform. In general, however, the approach used in this study should be useful for differentiating the impact of interindividual variability and parameter uncertainty on PBPK-based risk assessments of other chemicals where the sensitivity, uncertainty, and correlation of the key parameters are more limited.

Pharmacokinetic data needs to support risk assessments for inhaled and ingested manganese.

Manganese (Mn)-deficiency or Mn-excess can lead to adverse biological consequences. Central nervous system tissues, rich in dopaminergic neurons, are the targets whether the Mn gains entrance by inhalation, oral ingestion, or intravenous administration. Risk assessments with Mn need to ensure that brain concentrations in the globus pallidus and striatum stay within the range of normal. This paper first provides a critical review of the biological factors that determine the disposition of Mn in tissues within the body. Secondly, it outlines specific data needs for developing a physiologically based pharmacokinetic (PBPK) model for Mn to assist in conducting risk assessments for inhaled and ingested Mn. Uptake of dietary Mn appears to be controlled by several dose-dependent processes: biliary excretion, intestinal absorption, and intestinal elimination. Mn absorbed in the divalent form from the gut via the portal blood is complexed with plasma proteins that are efficiently removed by the liver. Absorption of Mn via inhalation, intratracheal instillation or intravenous infusions bypasses the control processes in the gastrointestinal tract. After absorption into the blood system by these alternate routes, Mn is apparently oxidized by ceruloplasmin and the trivalent Mn binds to the iron carrying protein, transferrin. Brain uptake of Mn occurs via transferrin receptors located in various brain regions. Transferrin-bound trivalent Mn is not as readily removed by the liver, as are protein complexes with divalent Mn. Thus, Mn delivered by these other dose routes would be available for uptake into tissues for a longer period of time than the orally administered Mn, leading to quantitative differences in tissue uptake for different dose routes. Several important data gaps impede organizing these various physiological factors into a multi-dose route PK model for Mn. They include knowledge of (1) oxidation rates of Mn in blood, (2) uptake rates of protein-bound forms of Mn by the liver, (3) neuronal transfer rates within the CNS, and (4) quantitative analyses of the control processes that regulate uptake of ingested Mn by the intestines and liver. These data gaps are the main obstacles to developing a risk assessment strategy for Mn that considers contributions of both inhalation and ingestion of this essential nutrient in determining brain Mn concentrations.

Coupling of computer modeling with in vitro methodologies to reduce animal usage in toxicity testing.

The use of in vitro data to support the development of physiologically based pharmacokinetic (PBPK) models and to reduce the requirement for in vivo testing is demonstrated by three examples. In the first example, polychlorotrifluoroethylene, in vitro studies comparing metabolism and tissue response in rodents and primates made it possible to obtain definitive data for a human risk assessment without resorting to additional in vivo studies with primates. In the second example, a PBPK model for organophosphate esters was developed in which the parameters defining metabolism, tissue partitioning, and enzyme inhibition were all characterized by in vitro studies, and the rest of the model parameters were established from the literature. The resulting model was able to provide a coherent description of enzyme inhibition following both acute and chronic exposures in mice, rats, and humans. In the final example, the carcinogenic risk assessment for methylene chloride was refined by the incorporation of in vitro data on human metabolism into a PBPK model.