Analysis of methylmercury disposition in humans utilizing a PBPK model and animal pharmacokinetic data

Toxicokinetics of methylmercury in diabetic KK-Ay mice and C57BL/6 mice

We compared the toxicokinetics of methylmercury (MeHg) in KK-Ay type 2 diabetic mice and C57BL/6J mice to evaluate how metabolic changes associated with diabetes affect MeHg toxicokinetics. A single dose of MeHg (0.2, 1, or 5 mg mercury/kg) was administered orally to 12-week-old KK-Ay and C57BL/6J male mice. Total mercury concentrations in plasma, blood cells, whole blood, and tissues (brain, kidneys, liver, and pancreas) were measured after 4, 7, 11, and 14 days. The volume of distribution/bioavailability and the elimination rate constant per day were higher in KK-Ay mice, while the terminal elimination half-life was lower in almost all samples of KK-Ay mice. The area under the curve was lower in all blood and almost all tissue samples from KK-Ay mice. Total clearance/bioavailability was lower in all blood and tissue samples of KK-Ay mice at all MeHg doses. These results indicate that MeHg is more rapidly absorbed by, and eliminated from, the blood cells, brain, liver, kidney, and pancreas of KK-Ay mice under the experimental conditions. Different patterns of tissue-to-plasma and tissue-to-whole blood partition coefficients suggest that notable differences in MeHg transfer between plasma and blood cells affect its distribution in tissues of the two mouse strains. These findings are useful to understand the selective distribution of MeHg to target organs and the sensitivity to MeHg in pathological states.

Organ-specific differences in mercury speciation and accumulation across ringed seal (Phoca hispida) life stages

Methylmercury (MeHg) is a central nervous system toxicant and exposures can adversely affect the health of marine mammals. Mercuric selenide (HgSe) in marine mammal tissues is hypothesized to result from a protective detoxification mechanism, but toxicokinetic processes contributing to its formation are poorly understood. Here, new data is reported on speciated Hg concentrations in multiple organs of n = 56 ringed seals (Phoca hispida) from Labrador, Canada, and compare concentrations to previously published data from Greenland seals. A higher proportion of Hg is found to accumulate in the kidney of young-of-the-year (YOY) ringed seals compared to adults. A toxicokinetic model for Hg species is developed and evaluated to better understand factors affecting variability in Hg concentrations among organs and across life stages. Prior work postulated that HgSe formation only occurs in the liver of mature seals, but model results suggest HgSe formation occurs across all life stages. Higher proportions of HgSe in mature seal livers compared to YOY seals likely results from the slow accumulation and elimination of HgSe (total body half-life = 500 days) compared to other Hg species. HgSe formation in the liver reduces modeled blood concentrations of MeHg by only 6%. Thus, HgSe formation may not substantially reduce MeHg transport across the blood-brain barrier of ringed seals, leaving them susceptible to the neurotoxic effects of MeHg exposure.

Brain methylmercury uptake in fetal, neonate, weanling, and adult rats

Fetuses and neonates are known to be highly susceptible to methylmercury (MeHg) toxicity, but little is known about the relative uptake of MeHg from blood to the developing brain. We measured time-course changes in mercury (Hg) concentrations in the brain of fetal, neonate, weanling, and adult rats after an injection of 0.08 μg (0.4 nmol) Hg/g MeHg. In the prenatal experiment, MeHg was subcutaneously injected to pregnant dams on embryonic days 17, 18, 18.5, 19, 19.5, or 20, and Hg concentrations in tissues were measured in both mothers and fetuses on embryonic day 21 (1 day before parturition). Brain Hg levels in fetuses peaked 2 days after injection and were approximately 1.5 times higher than in mothers. In the postnatal experiment, the same MeHg dose was injected subcutaneously to male rats on postnatal days 1 (neonates), 35 (weanlings), or 56 (adults). Mercury concentrations in tissues were measured 1, 2, 3, 4, 5, or 6 days after the injection. Brain Hg levels peaked most rapidly in neonates, and were approximately 1.5 times higher than levels in weanlings or adults. Throughout the examined period, peak Hg levels in the brain and the Hg brain/blood ratio 24 h after injection were highest in fetuses, followed by the levels in neonates, and decreased with life stage. These findings suggest that relatively higher brain MeHg uptake is an important factor in the vulnerability of fetuses and neonates to MeHg exposure.

Stable and episodic/bolus patterns of methylmercury exposure on mercury accumulation and histopathologic alterations in the nervous system

The main purpose of the present study was to compare the blood and brain mercury (Hg) accumulation and neurological alterations in adult male and pregnant female/fetal rats following stable and episodic/bolus patterns of methylmercury (MeHg) exposure. In addition, MeHg accumulation in the human body was estimated by a one-compartment model using three different patterns of MeHg exposure. In the adult male rat experiment, doses of 0.3 and 1.5mg MeHg/kg/day were orally administered to the stable groups for 5 weeks, while 7-fold higher doses of 2.1 and 10.5mg MeHg/kg/once a week were administered to the bolus groups. The blood Hg levels increased constantly in the stable groups, but increased with repeated waves in the bolus groups. At completion of the experiment, there were no significant differences in the brain Hg concentrations or neurological alterations between the stable and bolus groups, when the total doses of MeHg were the same. In the pregnant female rat experiment, a dose of 1mg MeHg/kg/day was administered orally to the stable group for 20 days (until 1day before expected parturition), while a 5-fold higher dose of 5mg MeHg/kg/once every 5 days was administered to the bolus group. In the brains of the maternal/fetal rats, there were no significant differences in the Hg concentrations and neurological alterations between the stable and bolus groups. The mean Hg concentrations in the fetal brains were approximately 2-fold higher than those in the maternal brains for both stable and bolus groups. Using the one-compartment model, the Hg accumulation curves in humans at doses of 7µg MeHg/day, 48µg MeHg/once a week, and 96µg MeHg/once every 2 weeks were estimated to be similar, while the bolus groups showed dose-dependent amplitudes of repeated waves. These results suggest that stable and episodic/bolus patterns of MeHg exposure do not cause differences in Hg accumulation in the blood and brain, or in neurological alterations, when the total doses are the same.

Sex differences in contaminant concentrations of fish: a synthesis

A comparison of whole-fish polychlorinated biphenyl (PCB) and total mercury (Hg) concentrations in mature males with those in mature females may provide insights into sex differences in behavior, metabolism, and other physiological processes. In eight species of fish, we observed that males exceeded females in whole-fish PCB concentration by 17 to 43 %. Based on results from hypothesis testing, we concluded that these sex differences were most likely primarily driven by a higher rate of energy expenditure, stemming from higher resting metabolic rate (or standard metabolic rate (SMR)) and higher swimming activity, in males compared with females. A higher rate of energy expenditure led to a higher rate of food consumption, which, in turn, resulted in a higher rate of PCB accumulation. For two fish species, the growth dilution effect also made a substantial contribution to the sex difference in PCB concentrations, although the higher energy expenditure rate for males was still the primary driver. Hg concentration data were available for five of the eight species. For four of these five species, the ratio of PCB concentration in males to PCB concentration in females was substantially greater than the ratio of Hg concentration in males to Hg concentration in females. In sea lamprey (Petromyzon marinus), a very primitive fish, the two ratios were nearly identical. The most plausible explanation for this pattern was that certain androgens, such as testosterone and 11-ketotestosterone, enhanced Hg-elimination rate in males. In contrast, long-term elimination of PCBs is negligible for both sexes. According to this explanation, males not only ingest Hg at a higher rate than females but also eliminate Hg at a higher rate than females, in fish species other than sea lamprey. Male sea lamprey do not possess either of the above-specified androgens. These apparent sex differences in SMRs, activities, and Hg-elimination rates in teleost fishes may also apply, to some degree, to higher vertebrates including humans. Our synthesis findings will be useful in (1) developing sex-specific bioenergetics models for fish, (2) developing sex-specific risk assessment models for exposure of humans and wildlife to contaminants, and (3) refining Hg mass balance models for fish and higher vertebrates.

Distant drivers or local signals: where do mercury trends in western Arctic belugas originate?

Temporal trends of contaminants are monitored in Arctic higher trophic level species to inform us on the fate, transport and risk of contaminants as well as advise on global emissions. However, monitoring mercury (Hg) trends in species such as belugas challenge us, as their tissue concentrations reflect complex interactions among Hg deposition and methylation, whale physiology, dietary exposure and foraging patterns. The Beaufort Sea beluga population showed significant increases in Hg during the 1990 s; since that time an additional 10 years of data have been collected. During this time of data collection, changes in the Arctic have affected many processes that underlie the Hg cycle. Here, we examine Hg in beluga tissues and investigate factors that could contribute to the observed trends after removing the effect of age and size on Hg concentrations and dietary factors. Finally, we examine available indicators of climate variability (Arctic Oscillation (AO), the Pacific Decadal Oscillation (PDO) and sea-ice minimum (SIM) concentration) to evaluate their potential to explain beluga Hg trends. Results reveal a decline in Hg concentrations from 2002 to 2012 in the liver of older whales and the muscle of large whales. The temporal increases in Hg in the 1990 s followed by recent declines do not follow trends in Hg emission, and are not easily explained by diet markers highlighting the complexity of feeding, food web dynamics and Hg uptake. Among the regional-scale climate variables the PDO exhibited the most significant relationship with beluga Hg at an eight year lag time. This distant signal points us to consider beluga winter feeding areas. Given that changes in climate will impact ecosystems; it is plausible that these climate variables are important in explaining beluga Hg trends. Such relationships require further investigation of the multiple connections between climate variables and beluga Hg.

Recommendations to improve wildlife exposure estimation for development of soil screening and cleanup values

An integral component in the development of media-specific values for the ecological risk assessment of chemicals is the derivation of safe levels of exposure for wildlife. Although the derivation and subsequent application of these values can be used for screening purposes, there is a need to identify the threshold for effects when making remedial decisions during site-specific assessments. Methods for evaluation of wildlife exposure are included in the US Environmental Protection Agency (USEPA) ecological soil screening levels (Eco-SSLs), registration, evaluation, authorization, and restriction of chemicals (REACH), and other risk-based soil assessment approaches. The goal of these approaches is to ensure that soil-associated contaminants do not pose a risk to wildlife that directly ingest soil, or to species that may be exposed to contaminants that persist in the food chain. These approaches incorporate broad assumptions in the exposure and effects assessments and in the risk characterization process. Consequently, thresholds for concluding risk are frequently very low with conclusions of risk possible when soil metal concentrations fall in the range of natural background. A workshop held in September, 2012 evaluated existing methods and explored recent science about factors to consider when establishing appropriate remedial goals for concentrations of metals in soils. A Foodweb Exposure Workgroup was organized to evaluate methods for quantifying exposure of wildlife to soil-associated metals through soil and food consumption and to provide recommendations for the development of ecological soil cleanup values (Eco-SCVs) that are both practical and scientifically defensible. The specific goals of this article are to review the current practices for quantifying exposure of wildlife to soil-associated contaminants via bioaccumulation and trophic transfer, to identify potential opportunities for refining and improving these exposure estimates, and finally, to make recommendations for application of these improved models to the development of site-specific remedial goals protective of wildlife. Although the focus is on metals contamination, many of the methods and tools discussed are also applicable to organic contaminants. The conclusion of this workgroup was that existing exposure estimation models are generally appropriate when fully expanded and that methods are generally available to develop more robust site-specific exposure estimates. Improved realism in site-specific wildlife Eco-SCVs could be achieved by obtaining more realistic estimates for diet composition, bioaccumulation, bioavailability and/or bioaccessibility, soil ingestion, spatial aspects of exposure, and target organ exposure. These components of wildlife exposure estimation should be developed on a site-, species-, and analyte-specific basis to the extent that the expense for their derivation is justified by the value they add to Eco-SCV development.

New insight into biomarkers of human mercury exposure using naturally occurring mercury stable isotopes

Human exposure to methylmercury (MeHg) and elemental mercury vapor (Hg(0)(g)) are often estimated using total Hg concentrations in hair and urine, respectively. We investigated whether Hg stable isotopes could be used to better distinguish between exposure to Hg(0)(g) versus MeHg. We found that hair from North American dental professionals was characterized by high positive Δ(199)Hg values (mean = 1.86‰, 1 SD = 0.12‰, n = 11). This confirms that among people who regularly consume fish, total Hg concentrations in hair reflect exposure to MeHg. In contrast, we found that urine from the same individuals was characterized by a range of Δ(199)Hg values (0.29 to 1.77‰, 2 SD = 0.06‰, n = 12) that were significantly correlated to the number of dental amalgams in each individual's mouth. We hypothesize that fish-derived MeHg is demethylated within the body, causing mass-dependent fractionation and the excretion of inorganic Hg in urine. Mercury in urine therefore represents a mixture of demethylated fish-derived MeHg and amalgam-derived inorganic Hg. We estimate that the majority (>70%) of Hg in urine from individuals with <10 dental amalgams is derived from ingestion of MeHg in fish. These data suggest that within populations that consume fish, urine total Hg concentrations may overestimate Hg exposure from personal dental amalgams.

Development and evaluation of a dynamic model that projects population biomarkers of methylmercury exposure from local fish consumption

A dynamic model was developed to project Hg concentrations in common biomarkers of exposure in response to changes in Hg concentrations in predatory fish from local waters. The model predicts biomarkers in susceptible populations for intake rates representing the mean, 90th, 95th, and 99 th percentiles of populations of interest. The biomarkers the model calculates are blood methylmercury, total hair Hg, and fetal blood methylmercury. Decision makers can use the model to determine the degree of reduction in fish tissue Hg levels necessary to protect the health of susceptible populations. Biomarker output was calibrated with literature sources. Output was then compared to additional literature sources to evaluate model function. Projected biomarkers were not different from literature sources. The model can be used as a tool to understand the impact of local fish consumption on susceptible populations.

A physiologically based toxicokinetic model for methylmercury in female American kestrels

A physiologically based toxicokinetic (PBTK) model was developed to describe the uptake, distribution, and elimination of methylmercury (CH(3)Hg) in female American kestrels. The model consists of six tissue compartments corresponding to the brain, liver, kidney, gut, red blood cells, and remaining carcass. Additional compartments describe the elimination of CH(3)Hg to eggs and growing feathers. Dietary uptake of CH(3)Hg was modeled as a diffusion-limited process, and the distribution of CH(3)Hg among compartments was assumed to be mediated by the flow of blood plasma. To the extent possible, model parameters were developed using information from American kestrels. Additional parameters were based on measured values for closely related species and allometric relationships for birds. The model was calibrated using data from dietary dosing studies with American kestrels. Good agreement between model simulations and measured CH(3)Hg concentrations in blood and tissues during the loading phase of these studies was obtained by fitting model parameters that control dietary uptake of CH(3)Hg and possible hepatic demethylation. Modeled results tended to underestimate the observed effect of egg production on circulating levels of CH(3)Hg. In general, however, simulations were consistent with observed patterns of CH(3)Hg uptake and elimination in birds, including the dominant role of feather molt. This model could be used to extrapolate CH(3)Hg kinetics from American kestrels to other bird species by appropriate reassignment of parameter values. Alternatively, when combined with a bioenergetics-based description, the model could be used to simulate CH(3)Hg kinetics in a long-term environmental exposure.

Technological challenges of addressing new and more complex migrating products from novel food packaging materials

The risk assessment of migration products resulting from packaging material has and continues to pose a difficult challenge. In most jurisdictions, there are regulatory requirements for the approval or notification of food contact substances that will be used in packaging. These processes generally require risk assessment to ensure safety concerns are addressed. The science of assessing food contact materials was instrumental in the development of the concept of Threshold of Regulation and the Threshold of Toxicological Concern procedures. While the risk assessment process is in place, the technology of food packaging continues to evolve to include new initiatives, such as the inclusion of antimicrobial substances or enzyme systems to prevent spoilage, use of plastic packaging intended to remain on foods as they are being cooked, to the introduction of more rigid, stable and reusable materials, and active packaging to extend the shelf-life of food. Each new technology brings with it the potential for exposure to new and possibly novel substances as a result of migration, interaction with other chemical packaging components, or, in the case of plastics now used in direct cooking of products, degradation products formed during heating. Furthermore, the presence of trace levels of certain chemicals from packaging that were once accepted as being of low risk based on traditional toxicology studies are being challenged on the basis of reports of adverse effects, particularly with respect to endocrine disruption, alleged to occur at very low doses. A recent example is the case of bisphenol A. The way forward to assess new packaging technologies and reports of very low dose effects in non-standard studies of food contact substances is likely to remain controversial. However, the risk assessment paradigm is sufficiently robust and flexible to be adapted to meet these challenges. The use of the Threshold of Regulation and the Threshold of Toxicological Concern concepts may play a critical role in the risk assessment of new food packaging technologies in the future.

Predicting plasma concentrations of bisphenol A in children younger than 2 years of age after typical feeding schedules, using a physiologically based toxicokinetic model

BACKGROUND

Concerns have recently been raised regarding the safety of potential human exposure to bisphenol A (BPA), an industrial chemical found in some polycarbonate plastics and epoxy resins. Of particular interest is the exposure of young children to BPA via food stored in BPA-containing packaging.

OBJECTIVES

In this study we assessed the age dependence of the toxicokinetics of BPA and its glucuronidated metabolite, BPA-Glu, using a coupled BPA-BPA-Glu physiologically based toxicokinetic (PBTK) model.

METHODS

Using information gathered from toxicokinetic studies in adults, we built a PBTK model. We then scaled the model to children < 2 years of age based on the age dependence of physiologic parameters relevant for absorption, distribution, metabolism, and excretion.

RESULTS

We estimated the average steady-state BPA plasma concentration in newborns to be 11 times greater than that in adults when given the same weight-normalized dose. Because of the rapid development of the glucuronidation process, this ratio dropped to 2 by 3 months of age. Simulation of typical feeding exposures, as estimated by regulatory authorities, showed a 5-fold greater steady-state BPA plasma concentration in 3- and 6-month-olds compared with adults, reflecting both a reduced capacity for BPA metabolism and a greater weight-normalized BPA exposure. Because of uncertainty in defining the hepatic BPA intrinsic clearance in adults, these values represent preliminary estimates.

CONCLUSIONS

Simulations of the differential BPA dosimetry between adults and young children point to the need for more sensitive analytical methods for BPA to define, with greater certainty, the adult hepatic BPA intrinsic clearance, as well as a need for external exposure data in young children.

Whole body physiologically-based pharmacokinetic models: their use in clinical drug development

BACKGROUND

Whole-body physiologically-based pharmacokinetic (WB-PBPK) models mathematically describe an organism as a closed circulatory system consisting of compartments that represent the organs important for compound absorption, distribution, metabolism and elimination.

OBJECTIVES

To review the current state of WB-PBPK model use in the clinical phases of drug development.

METHODS

A qualitative description of the WB-PBPK model structure is included along with a review of the varying methods available for input parameterisation. Current and potential WB-PBPK model application in clinical development is discussed.

CONCLUSIONS

This modelling tool is at present used for small and large molecule drug development primarily as a means to scale pharmacokinetics from animals to humans based on physiology. The pharmaceutical industry is active in employing these models to clinical drug development although the applications in use now are narrow in comparison to the potential. Expanded integration of WB-PBPK models into the drug development process will only be achieved with staff training, managerial will, success stories and regulatory agency openness.

A physiologically based pharmacokinetic model linking plasma protein binding interactions with drug disposition

Combination drug therapy increases the chance for an adverse drug reactions due to drug-drug interactions. Altered disposition for sulfamethazine (SMZ) when concurrently administered with flunixin meglumine (FLU) in swine could lead to increased tissue residues. There is a need for a pharmacokinetic modeling technique that can predict the consequences of possible drug interactions. A physiologically based pharmacokinetic model was developed that links plasma protein binding interactions to drug disposition for SMZ and FLU in swine. The model predicted a sustained decrease in total drug and a temporary increase in free drug concentration. An in vivo study confirmed the presence of a drug interaction. Neither the model nor the in vivo study revealed clinically significant changes that alter tissue disposition. This novel linkage approach has use in the prediction of the clinical impact of plasma protein binding interactions. Ultimately it could be used in the design of dosing regimens and in the protection of the food supply through prediction and minimization of tissue residues.

Size and biomagnification: How Habitat selection explains beluga mercury levels

Mercury (Hg) levels in the Beaufort Sea beluga (Delphinapterus leucas) population increased during the 1990s; levels have since declined but remain higher than the 1980s. The diet of this beluga population is not well-known, thus it is difficult to assess dietary Hg sources. During the summer, the Beaufort Sea belugas segregate by length, sex, and reproductive status corresponding to habitat use that may result in feeding differences and ultimately Hg uptake. To test this hypothesis, we examine beluga dietary variation using fatty acid profiles and determine which biological variables best predict diet Relationships between biological variables and fatty acids were further evaluated with stable isotopes and Hg concentrations in liver and muscle. Hg concentrations in muscle were better related to liver delta15N than muscle delta15N. Stable isotopes and fatty acids are compared in their ability to describe dietary Hg processes in beluga. Fatty acids provided support for influences of whale behavior on dietary Hg uptake, whereas stable isotopes inferred tissue Hg metabolic rates. Here, we show beluga length drives diet variability leading to differences in Hg uptake and biomagnification processes dominate beluga Hg levels over Hg bioaccumulation over time.

Fish consumption and bioindicators of inorganic mercury exposure

BACKGROUND

The direct and close relationship between fish consumption and blood and hair mercury (Hg) levels is well known, but the influence of fish consumption on inorganic mercury in blood (B-IHg) and in urine (U-Hg) is unclear.

OBJECTIVE

Examine the relationship between fish consumption, total, inorganic and organic blood Hg levels and urinary Hg concentration.

METHODS

A cross-sectional study was carried out on 171 persons from 7 riparian communities on the Tapajós River (Brazilian Amazon), with no history of inorganic Hg exposure from occupation or dental amalgams. During the rising water season in 2004, participants responded to a dietary survey, based on a seven-day recall of fish and fruit consumption frequency, and socio-demographic information was recorded. Blood and urine samples were collected. Total, organic and inorganic Hg in blood as well as U-Hg were determined by Atomic Absorption Spectrometry.

RESULTS

On average, participants consumed 7.4 fish meals/week and 8.8 fruits/week. Blood total Hg averaged 38.6+/-21.7 microg/L, and the average percentage of B-IHg was 13.8%. Average organic Hg (MeHg) was 33.6+/-19.4 microg/L, B-IHg was 5.0+/-2.6 microg/L, while average U-Hg was 7.5+/-6.9 microg/L, with 19.9% of participants presenting U-Hg levels above 10 microg/L. B-IHg was highly significantly related to the number of meals of carnivorous fish, but no relation was observed with non-carnivorous fish; it was negatively related to fruit consumption, increased with age, was higher among those who were born in the Tapajós region, and varied with community. U-Hg was also significantly related to carnivorous but not non-carnivorous fish consumption, showed a tendency towards a negative relation with fruit consumption, was higher among men compared to women and higher among those born in the region. U-Hg was strongly related to I-Hg, blood methyl Hg (B-MeHg) and blood total Hg (B-THg). The Odds Ratio (OR) for U-Hg above 10 microg/L for those who ate >4 carnivorous fish meals/week was 4.00 [1.83-9.20].

CONCLUSION

This study adds further evidence to a positive relation between fish consumption and IHg in both blood and urine, which may result from absorption of IHg from fish or from demethylation of MeHg. The findings support the importance of assessing IHg exposure in fish-eating communities. Further studies should examine the potential toxicity of IHg in heavy fish consumers.

Methylmercury exposure and health effects in humans: a worldwide concern

The paper builds on existing literature, highlighting current understanding and identifying unresolved issues about MeHg exposure, health effects, and risk assessment, and concludes with a consensus statement. Methylmercury is a potent toxin, bioaccumulated and concentrated through the aquatic food chain, placing at risk people, throughout the globe and across the socioeconomic spectrum, who consume predatory fish or for whom fish is a dietary mainstay. Methylmercury developmental neurotoxicity has constituted the basis for risk assessments and public health policies. Despite gaps in our knowledge on new bioindicators of exposure, factors that influence MeHg uptake and toxicity, toxicokinetics, neurologic and cardiovascular effects in adult populations, and the nutritional benefits and risks from the large number of marine and freshwater fish and fish-eating species, the panel concluded that to preserve human health, all efforts need to be made to reduce and eliminate sources of exposure.

Developmental exposure to methylmercury alters learning and induces depression-like behavior in male mice

To investigate the long-term effects of developmental exposure to methylmercury (MeHg), pregnant mice were exposed to at 0.5 mg MeHg/kg/day via drinking water from gestational day 7 until day 7 after delivery. The behavior of offspring was monitored at 5-15 and 26-36 weeks of age using an automated system (IntelliCage) designed for continuous long-term recording of the home cage behavior in social groups and complex analysis of basic activities and learning. In addition, spontaneous locomotion, motor coordination on the accelerating rotarod, spatial learning in Morris water maze, and depression-like behavior in forced swimming test were also studied. The analysis of behavior performed in the IntelliCage without social deprivation occurred to be more sensitive in detecting alterations in activity and learning paradigms. We found normal motor function but decreased exploratory activity in MeHg-exposed male mice, especially at young age. Learning disturbances observed in MeHg-exposed male animals suggest reference memory impairment. Interestingly, the forced swimming test revealed a predisposition to depressive-like behavior in the MeHg-exposed male offspring. This study provides novel evidence that the developmental exposure to MeHg can affect not only cognitive functions but also motivation-driven behaviors.

Hierarchical models for probabilistic dose–response assessment

Probabilistic risk assessment is gaining acceptance as the most appropriate way to characterize and communicate uncertainties in estimates of human health risk and/or reference levels of exposure such as benchmark doses. Although probabilistic techniques are well established in the exposure-assessment component of the National Research Council's risk-assessment paradigm, they are less well developed in the dose-response-assessment component. This paper proposes the use of hierarchical statistical models as tools for implementing probabilistic dose-response assessments, in that such models provide a natural connection between the pharmacokinetic (PK) and pharmacodynamic (PD) components of dose-response models. The results show that incorporating internal dose information into dose-response assessments via the coupling of PK and PD models in a hierarchical structure can reduce the uncertainty in the dose-response assessment of risk. However, information on the mean of the internal dose distribution is sufficient; having information on the variance of internal dose does not affect the uncertainty in the resulting estimates of excess risks or benchmark doses. In addition, the complexity of a PK model of internal dose does not affect how the variability in risk is measured via the ultimate endpoint.

The development of a physiologically-based pharmacokinetic model using the distribution of 2,3,7,8-tetrachlorodibenzo-p-dioxin in the tissues of the eastern oyster (Crassostrea virginica)

A physiologically-based pharmacokinetic model (PBPK) was developed to describe the kinetics of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the eastern oyster (Crassostrea virginica). The estimated t(1/2) of elimination for a bolus dose of TCDD in C. virginica is approximately 14-24 days based on both the experimental data and the PBPK model. The highest dioxin concentration reached during 28-days was in the digestive gland followed by the mantle, gonad, hemolymph, gill, adductor muscle, and the kidney/heart. A binding protein for 2,3,7,8-TCDD had been reported in the literature for both the digestive gland and gonad. Incorporating a binding component in the model resulted in a better fit for the data. The PBPK model predicted the distribution and the elimination concentrations for 2,3,7,8-TCDD within each of the tissue compartments. This model will serve as a useful tool for predicting the kinetics of other persistent organic pollutants as well as, allow for a more refined ecological risk assessment by estimating dioxin concentrations in sensitive tissues such as the gonad.

Monitoring mercury in the loggerhead sea turtle, Caretta caretta

The validity of using blood samples and keratinized scutes for nonlethal routine monitoring of mercury (Hg) in loggerhead sea turtles, Caretta caretta, is evaluated in the context of how effectively these matrixes predict internal tissue Hg burdens and the different temporal scales of exposure they represent. Total Hg (THg) was measured in blood and scutes collected from live captures (n = 34) and liver, kidney, muscle, spinal cord, blood, and scutes collected from freshly stranded loggerhead turtles (n = 6) along the coast of the southeastern United States. Linear regressions between monitoring compartments and internal tissues from stranded animals were all statistically significant (r2 > 0.805, p < 0.015) but varied in their utility as a predictive tool depending on which tissues were paired. Blood was an effective predictor of THg in muscle (r2 = 0.988, p < 0.0001) and spinal cord (r2 = 0.988, p < 0.0001), while scute was the most accurate predictor of THg in liver (r2 = 0.948, p = 0.0010). The strength of the relationship between tissues types is believed to reflect the similarity in the temporal scales they represent and the variability in the fraction of methylmercury present. The stability of Hg in the scute matrix makes this tissue preferable for approximating long-term exposure, while blood Hg levels can be affected by recent changes in Hg intake. THg levels in blood and scutes from live captures were highly correlated (linear regression r2 = 0.926, p < 0.0001) and increased significantly with body mass (r2 = 0.173, p = 0.016 and r2 = 0.187, p = 0.012 respectively), further supporting thatthere is a component reflecting long-term accumulation of Hg in these matrixes. We also present a novel technique using the residuals from the blood-scute regression as an index of recent exposure (IRE). The interpretation of this value is derived from the comparison between the most recent Hg intake (which contributes to the Hg measured in the blood) relative to the average past intake (which is recorded in the scute). A stepwise multiple regression revealed a significant positive relationship between the IRE and the proximity of the capture site to the nearest major industrial river mouth (p = 0.0102). This suggests that there is an elevation of bioavailable Hg in nearshore habitats where terrestrial influences and anthropogenic impacts are high. Seasonal foraging site fidelity and the variability in environmental Hg may explain the high intraspecific variability and occasional highly contaminated turtle seen in this and previous studies.

Whole body pharmacokinetic models

The aim of the current review is to summarise the present status of physiologically based pharmacokinetic (PBPK) modelling and its applications in drug research, and thus serve as a reference point to people interested in the methodology. The review is structured into three major sections. The first discusses the existing methodologies and techniques of PBPK model development. The second describes some of the most interesting PBPK model implementations published. The final section is devoted to a discussion of the current limitations and the possible future developments of the PBPK modelling approach. The current review is focused on papers dealing with the pharmacokinetics and/or toxicokinetics of medicinal compounds; references discussing PBPK models of environmental compounds are mentioned only if they represent considerable methodological developments or reveal interesting interpretations and/or applications.The major conclusion of the review is that, despite its significant potential, PBPK modelling has not seen the development and implementation it deserves, especially in the drug discovery, research and development processes. The main reason for this is that the successful development and implementation of a PBPK model is seen to require the investment of significant experience, effort, time and resources. Yet, a substantial body of PBPK-related research has been accumulated that can facilitate the PBPK modelling and implementation process. What is probably lagging behind is the expertise component, where the demand for appropriately qualified staff far outreaches availability.

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.

Sub-clinical neurobehavioral abnormalities associated with low level of mercury exposure through fish consumption

In order to assess early neurotoxic effects associated with relatively low levels of mercury absorbed through fish eating, two groups of 22 adult male subjects, habitual consumers of tuna fish, and 22 controls were examined using a cross-sectional field study. The assessment included neurobehavioral tests of vigilance and psychomotor function, hand tremor measurements and serum prolactin assessment. Mercury in urine (U-Hg) and serum prolactin (sPRL) were measured in all exposed subjects and controls, whereas measurements of the organic component of mercury in blood (O-Hg) were available for only 10 exposed and six controls. U-Hg was significant higher among exposed subjects (median 6.5 microg/g of creatinine, range 1.8-21.5) than controls (median 1.5 microg/g of creatinine, range 0.5-5.3). The median values of O-Hg were 41.5 microg/l among the tuna fish eaters and 2.6 microg/l in the control group. Both U-Hg and O-Hg were significantly correlated with the quantity of fish consumed per week. Significant differences in sPRL were found between exposed (12.6 ng/ml) and controls (9.1 ng/ml). Individual sPRL were significantly correlated with both U-Hg and O-Hg levels. The neurobehavioral performance of subjects who consumed tuna fish regularly was significantly worse on color word reaction time, digit symbol reaction time and finger tapping speed (FT). After considering the education level and other covariates, the multiple stepwise regression analysis indicated that O-Hg concentration was most significantly associated with individual performance on these tests, accounting for about 65% of the variance in test scores.

Plasma levels of parent compound and metabolites after doses of either d-fenfluramine or d-3,4-methylenedioxymethamphetamine (MDMA) that produce long-term serotonergic alterations

Plasma levels of parent compounds and metabolites were determined in adult rhesus monkeys after doses of either 5mg/kg d-fenfluramine (FEN) or 10mg/kg d-3, 4-methylenedioxymethamphetamine (MDMA) i.m. twice daily for four consecutive days. These treatment regimens have been previously shown to produce long-term serotonin (5-HT) depletions. Peak plasma levels of 2.0+/-0.4 microM FEN were reached within 40min after the first dose of FEN, and then declined rapidly, while peak plasma levels (0.4+/-0.1 microM) of the metabolite norfenfluramine (NFEN) were not reached until 6h after dosing. After the seventh (next to last) dose of FEN, peak plasma levels of FEN were 35% greater than after the first dose while peak NFEN-levels were 500% greater. The t(1/2) for FEN was 2.6+/-0.3h after the first dose and 3.2+/-0.2h after the seventh. The estimated t(1/2) for NFEN was more than 37.6+/-20.5h. Peak plasma levels of 9.5+/-2.5 microM MDMA were reached within 20min after the first dose of MDMA, and then declined rapidly, while peak plasma levels (0.9+/-0.2 microM) of the metabolite 3,4-methylenedioxyamphetamine (MDA) were not reached until 3-6h after dosing. After the seventh (next to last) dose of MDMA, peak plasma levels of MDMA were 30% greater than the first dose while peak MDA levels were elevated over 200%. The t(1/2) for MDMA was 2.8+/-0.4h after the first and 3.9+/-1.1h after the seventh dose. The estimated t(1/2) for MDA was about 8.3+/-1.0h. Variability in plasma levels of MDMA and MDA between subjects was much greater than that for FEN and NFEN. This variability in MDMA and MDA exposure levels may have lead to variability in the subsequent disruption of some behaviors seen in these same subjects. There were 80% reductions in the plasma membrane-associated 5-HT transporters 6 months after either the FEN or MDMA dosing regimen indicating that both treatments produced long-term serotonergic effects.

Assessment of reference values for mercury in urine: the results of an Italian polycentric study

The results of a polycentric study to assess the reference values of urinary mercury (U-Hg) in four Italian cities are presented. A total of 383 subjects were selected on the basis of standardised criteria by a questionnaire on personal habits, lifestyle, occupational or non-occupational exposure to Hg, medical history, number and area of dental amalgams. U-Hg was determined by hydride generation atomic absorption method (HG-AAS), with a detection limit of 0.5 microg/l and by flow injection (FI) inductively coupled plasma mass spectrometry (ICP-MS), with a detection limit of 0.03 microg/l. The median value of U-Hg, determined by HG-AAS, was 0.78 microg/g creatinine (0.75 for males and 0.83 for females), with 5 degrees and 95 degrees percentiles, respectively, of 0.17 and 3.66 microg/g creatinine. When determined by FI ICP-MS, the median value was 0.79 microg/g creatinine (0.77 for males and 0.79 for females) with 5 degrees and 95 degrees percentiles of, respectively, 0.12 and 5.02 microg/g creatinine. Among the independent variables, city of origin, area of dental amalgams, fish intake and tobacco smoking significantly influenced the U-Hg levels. The U-Hg reference values from this survey are lower than those from other recent investigations, probably due to characteristics and selection of the examined individuals and to the strict control of pre-analytical and analytical factors of variability.