Application of a new approach method (NAM) for inhalation risk assessment.

The US Environmental Protection Agency (USEPA) and other regulatory authorities have been working to utilize in vitro studies with human cells and in silico modelling to reduce the use of vertebrate animals for evaluating chemical risk. Using the Source-to-Outcome framework, a novel mathematical procedure was developed to estimate the human equivalent concentration (HEC) for inhalation risk assessment based upon the relevant aerosol characterization, respiratory dosimetry modelling, and endpoints derived from an in vitro assay using human respiratory epithelial tissue. The procedure used the retained doses at the various areas of the inhalation tract estimated from a computational fluid-particle dynamics (CFPD) model coupled with a simple clearance model. The effect of exposure was derived from an in vitro assay. The magnitude of exposure and the particle size distributions (PSDs) of the external aerosol droplets were obtained from Unit Exposure values published by the USEPA and published monitoring studies, respectively. The Source-to-Outcome approach incorporates external and internal exposure metrics with the toxicity pathway. The information was then integrated to conduct a risk assessment for agricultural operators exposed to products containing chlorothalonil (CTN), a broad-spectrum fungicide. The HECs for three different PSDs considered in this work ranged from 0.043 to 0.112 mg-CTN/L for nasal and oral breathing. These were compared with the estimated average daily exposure concentration for six representative application scenarios. The resulting margins of exposure (MOEs) ranged from 230 to 70,000 depending on the application scenario. This New Assessment Method (NAM) that combined human in silico and human in vitro methods, eliminated the typical uncertainties associated with extrapolation from rodent studies, with their associated interspecies toxicokinetics and toxicodynamics differences. The intraspecies toxicodynamics and toxicokinetics, are still relevant and may need to be used in an inhalation risk assessment. The NAM presented in this work is not chemical-specific and may be applied to conduct an inhalation risk assessment for workers as well as bystanders who could be exposed to aerosol particles of any cytotoxic respiratory irritant.

Paraquat pharmacokinetics in primates and extrapolation to humans.

By extending our Paraquat (PQ) work to include primates we have implemented a modelling and simulation strategy that has enabled PQ pharmacokinetic data to be integrated into a single physiologically based pharmacokinetic (PBPK) model that enables more confident extrapolation to humans. Because available data suggested there might be differences in PQ kinetics between primates and non-primates, a radiolabelled study was conducted to characterize pharmacokinetics and excretion in Cynomolgus monkeys. Following single intravenous doses of 0.01 or 0.1 mg paraquat dichloride/kg bw, plasma PQ concentration-time profiles were dose-proportional. Excretion up to 48 h (predominantly urinary) was 82.9%, with ca. 10% remaining unexcreted. In vitro blood binding was similar across Cynomolgus monkeys, humans and rat. Our PBPK model for the rat, mouse and dog, employing a single set of PQ-specific parameters, was scaled to Cynomolgus monkeys and well represented the measured plasma concentration-time profiles over 14 days. Addition of a cartilage compartment to the model better captured the percent remaining in the monkeys at 48 h, whilst having negligible effect on model predictions for the other species. The PBPK model performed well for all four species, demonstrating there is little difference in PQ kinetics between non-primates and primates enabling a more confident extrapolation to humans. Scaling of the PBPK model to humans, with addition of a human-specific dermal submodel based on in vitro human dermal absorption data, provides a valuable tool that could be employed in defining internal dosimetry to complement human health risk assessments.

Integration of paraquat pharmacokinetic data across species using PBPK modelling.

Paraquat dichloride (PQ) is a non-selective herbicide which has been the subject of numerous toxicology studies over more than 50 years. This paper describes the development of a physiologically-based pharmacokinetic (PBPK) model of PQ kinetics for the rat, mouse and dog, firstly to aid the interpretation of studies in which no kinetic measurements were made, and secondly to enable the future extension of the model to humans. Existing pharmacokinetic data were used to develop a model for the rat and mouse. Simulations with this preliminary model were then used to identify key data gaps and to design a new blood binding study to reduce uncertainty in critical aspects of the model. The new data provided evidence to support the model structure, and its predictive performance was then assessed against dog and rat datasets not used in model development. The PQ-specific model parameters are the same for all three species, with only the physiological parameters varying between species. This consistency across species provides a strong basis for extrapolation to other species, as demonstrated here for the dog. The model enables a wide range of PQ data to be linked together to provide a broad understanding of PQ pharmacokinetics in rodents and the dog, showing that the key aspects of PQ kinetics in these species are understood and adequately encapsulated within the model.

PBPK model reporting template for chemical risk assessment applications.

Physiologically-based pharmacokinetic (PBPK) modeling analysis does not stand on its own for regulatory purposes but is a robust tool to support drug/chemical safety assessment. While the development of PBPK models have grown steadily since their emergence, only a handful of models have been accepted to support regulatory purposes due to obstacles such as the lack of a standardized template for reporting PBPK analysis. Here, we expand the existing guidances designed for pharmaceutical applications by recommending additional elements that are relevant to environmental chemicals. This harmonized reporting template can be adopted and customized by public health agencies receiving PBPK model submission, and it can also serve as general guidance for submitting PBPK-related studies for publication in journals or other modeling sharing purposes. The current effort represents one of several ongoing collaborations among the PBPK modeling and risk assessment communities to promote, when appropriate, incorporating PBPK modeling to characterize the influence of pharmacokinetics on safety decisions made by regulatory agencies.

A human life-stage physiologically based pharmacokinetic and pharmacodynamic model for chlorpyrifos: development and validation.

Sensitivity to some chemicals in animals and humans are known to vary with age. Age-related changes in sensitivity to chlorpyrifos have been reported in animal models. A life-stage physiologically based pharmacokinetic and pharmacodynamic (PBPK/PD) model was developed to predict disposition of chlorpyrifos and its metabolites, chlorpyrifos-oxon (the ultimate toxicant) and 3,5,6-trichloro-2-pyridinol (TCPy), as well as B-esterase inhibition by chlorpyrifos-oxon in humans. In this model, previously measured age-dependent metabolism of chlorpyrifos and chlorpyrifos-oxon were integrated into age-related descriptions of human anatomy and physiology. The life-stage PBPK/PD model was calibrated and tested against controlled adult human exposure studies. Simulations suggest age-dependent pharmacokinetics and response may exist. At oral doses ⩾0.6mg/kg of chlorpyrifos (100- to 1000-fold higher than environmental exposure levels), 6months old children are predicted to have higher levels of chlorpyrifos-oxon in blood and higher levels of red blood cell cholinesterase inhibition compared to adults from equivalent doses. At lower doses more relevant to environmental exposures, simulations predict that adults will have slightly higher levels of chlorpyrifos-oxon in blood and greater cholinesterase inhibition. This model provides a computational framework for age-comparative simulations that can be utilized to predict chlorpyrifos disposition and biological response over various postnatal life stages.

A multi-route model of nicotine-cotinine pharmacokinetics, pharmacodynamics and brain nicotinic acetylcholine receptor binding in humans.

The pharmacokinetics of nicotine, the pharmacologically active alkaloid in tobacco responsible for addiction, are well characterized in humans. We developed a physiologically based pharmacokinetic/pharmacodynamic model of nicotine pharmacokinetics, brain dosimetry and brain nicotinic acetylcholine receptor (nAChRs) occupancy. A Bayesian framework was applied to optimize model parameters against multiple human data sets. The resulting model was consistent with both calibration and test data sets, but in general underestimated variability. A pharmacodynamic model relating nicotine levels to increases in heart rate as a proxy for the pharmacological effects of nicotine accurately described the nicotine related changes in heart rate and the development and decay of tolerance to nicotine. The PBPK model was utilized to quantitatively capture the combined impact of variation in physiological and metabolic parameters, nicotine availability and smoking compensation on the change in number of cigarettes smoked and toxicant exposure in a population of 10,000 people presented with a reduced toxicant (50%), reduced nicotine (50%) cigarette Across the population, toxicant exposure is reduced in some but not all smokers. Reductions are not in proportion to reductions in toxicant yields, largely due to partial compensation in response to reduced nicotine yields. This framework can be used as a key element of a dosimetry-driven risk assessment strategy for cigarette smoke constituents.

Pharmacokinetics of AXA1665, a Novel Composition of Amino Acids, in Comparison With Protein Supplement: A Single-Dose, Open-Label, Randomized Study in Healthy Subjects.

We evaluated the safety and tolerability of AXA1665, a novel investigational fixed-ratio amino acid (AA) composition, the pharmacokinetics (PK) of the constituent AAs within AXA1665, and their relative bioavailability versus standard protein supplement. This study was conducted in 2 phases; in the initial phase, healthy subjects (N = 16) were randomly assigned to 4 treatment sequences (AXA1665 4.9, 9.8, and 19.6 g or 35 g protein supplement) in an open-label, single-dose, 4-way crossover study, while in the extension phase, they received single AXA1665 doses of 29.4 and 39.2 g in a sequential crossover manner. The net area under the plasma concentration-time curve (AUC) and observed time to reach maximum plasma concentration were estimated. A dose-dependent increase in plasma AUC from time 0 to the last measurable concentration (AUClast ) and maximum plasma concentration (Cmax ) was observed for all AXA1665-dosed AAs (4.9-39.2 g) except aspartic acid. AXA1665 19.6 g resulted in 1.5- to 9.5-fold higher systemic exposure to all AXA1665-dosed AAs except for aspartic acid and lysine and lower exposure to all nondosed AAs except for glutamine and alanine versus protein supplement. AXA1665 doses, up to 39.2 g, can deliver AXA1665-dosed AAs in the systemic circulation in the linear AUC range.

Development of a source-to-outcome model for dietary exposures to insecticide residues: an example using chlorpyrifos.

Probabilistic models of interindividual variation in exposure and response were linked to create a source-to-outcome population model. This model was used to investigate cholinesterase inhibition from dietary exposures to an insecticide (chlorpyrifos) in populations of adults and 3 year old children. A physiologically based pharmacokinetic and pharmacodynamic (PBPK/PD) model was used to calculate the variation in sensitivity occurring from interindividual variability in physiology, metabolism, and physical activity levels. A dietary intake model characterizes the variation in dietary insecticide exposures and variation in anthropometry in the populations. Published equations were used to describe the necessary physiology for each simulated individual based on the anthropometry from the dietary intake model. The model of the interindividual variation in response to chlorpyrifos was developed by performing a sensitivity analysis on the PBPK/PD model to determine the parameters that drive variation in pharmacodynamics outcomes (brain and red blood cell acetylcholinesterase inhibition). Distributions of interindividual variation were developed for parameters with the largest impact; the probabilistic model sampled from these distributions. The impact of age and interindividual variation on sensitivity at the doses that occur from dietary exposures, typically orders of magnitude lower than exposures assessed in toxicological studies, was assessed using the source-to-outcome model. The resulting simulations demonstrated that metabolic detoxification capacity was sufficient to prevent significant brain and red blood cell acetylcholinesterase inhibition, even in individuals with the lowest detoxification potential. Age-specific pharmacokinetic and pharmacodynamic parameters did not predict differences in susceptibility between adults and children. In the future, the approach of this case study could be used to assess the risks from low level exposures to other chemicals.

Application of a source-to-outcome model for the assessment of health impacts from dietary exposures to insecticide residues.

The paper presents a case study of the application of a "source-to-outcome" model for the evaluation of the health outcomes from dietary exposures to an insecticide, chlorpyrifos, in populations of adults (age 30) and children (age 3). The model is based on publically-available software programs that characterize the longitudinal dietary exposure and anthropometry of exposed individuals. These predictions are applied to a validated PBPK/PD model to estimate interindividual and longitudinal variation in brain and RBC AChE inhibition (key events) and chlorpyrifos concentrations in blood and TCPy in urine (biomarkers of exposure). The predicted levels of chlorpyrifos and TCPy are compared to published measurements of the biomarkers. Predictions of RBC AChE are compared to levels of inhibition associated with reported exposure-related effects in humans to determine the potential for the occurrence of adverse cholinergic effects. The predicted distributions of chlorpyrifos in blood and TCPy in urine were found to be reasonably consistent with published values, supporting the predictive value of the exposure and PBPK portions of the source-to-outcome model. Key sources of uncertainty in predictions of dietary exposures were investigated and found to have a modest impact on the model predictions. Future versions of this source-to-outcome model can be developed that consider advances in our understanding of metabolism, to extend the approach to other age groups (infants), and address intakes from other routes of exposure.

New Approach Methodology for Assessing Inhalation Risks of a Contact Respiratory Cytotoxicant: Computational Fluid Dynamics-Based Aerosol Dosimetry Modeling for Cross-Species and In Vitro Comparisons.

Regulatory agencies are considering alternative approaches to assessing inhalation toxicity that utilizes in vitro studies with human cells and in silico modeling in lieu of additional animal studies. In support of this goal, computational fluid-particle dynamics models were developed to estimate site-specific deposition of inhaled aerosols containing the fungicide, chlorothalonil, in the rat and human for comparisons to prior rat inhalation studies and new human in vitro studies. Under bioassay conditions, the deposition was predicted to be greatest at the front of the rat nose followed by the anterior transitional epithelium and larynx corresponding to regions most sensitive to local contact irritation and cytotoxicity. For humans, simulations of aerosol deposition covering potential occupational or residential exposures (1-50 µm diameter) were conducted using nasal and oral breathing. Aerosols in the 1-5 µm range readily penetrated the deep region of the human lung following both oral and nasal breathing. Under actual use conditions (aerosol formulations >10 µm), the majority of deposited doses were in the upper conducting airways. Beyond the nose or mouth, the greatest deposition in the pharynx, larynx, trachea, and bronchi was predicted for aerosols in the 10-20 µm size range. Only small amounts of aerosols >20 µm penetrated past the pharyngeal region. Using the ICRP clearance model, local retained tissue dose metrics including maximal concentrations and areas under the curve were calculated for each airway region following repeated occupational exposures. These results are directly comparable with benchmark doses from in vitro toxicity studies in human cells leading to estimated human equivalent concentrations that reduce the reliance on animals for risk assessments.

ISDD: A computational model of particle sedimentation, diffusion and target cell dosimetry for in vitro toxicity studies.

BACKGROUND: The difficulty of directly measuring cellular dose is a significant obstacle to application of target tissue dosimetry for nanoparticle and microparticle toxicity assessment, particularly for in vitro systems. As a consequence, the target tissue paradigm for dosimetry and hazard assessment of nanoparticles has largely been ignored in favor of using metrics of exposure (e.g. µg particle/mL culture medium, particle surface area/mL, particle number/mL). We have developed a computational model of solution particokinetics (sedimentation, diffusion) and dosimetry for non-interacting spherical particles and their agglomerates in monolayer cell culture systems. Particle transport to cells is calculated by simultaneous solution of Stokes Law (sedimentation) and the Stokes-Einstein equation (diffusion). RESULTS: The In vitro Sedimentation, Diffusion and Dosimetry model (ISDD) was tested against measured transport rates or cellular doses for multiple sizes of polystyrene spheres (20-1100 nm), 35 nm amorphous silica, and large agglomerates of 30 nm iron oxide particles. Overall, without adjusting any parameters, model predicted cellular doses were in close agreement with the experimental data, differing from as little as 5% to as much as three-fold, but in most cases approximately two-fold, within the limits of the accuracy of the measurement systems. Applying the model, we generalize the effects of particle size, particle density, agglomeration state and agglomerate characteristics on target cell dosimetry in vitro. CONCLUSIONS: Our results confirm our hypothesis that for liquid-based in vitro systems, the dose-rates and target cell doses for all particles are not equal; they can vary significantly, in direct contrast to the assumption of dose-equivalency implicit in the use of mass-based media concentrations as metrics of exposure for dose-response assessment. The difference between equivalent nominal media concentration exposures on a µg/mL basis and target cell doses on a particle surface area or number basis can be as high as three to six orders of magnitude. As a consequence, in vitro hazard assessments utilizing mass-based exposure metrics have inherently high errors where particle number or surface areas target cells doses are believed to drive response. The gold standard for particle dosimetry for in vitro nanotoxicology studies should be direct experimental measurement of the cellular content of the studied particle. However, where such measurements are impractical, unfeasible, and before such measurements become common, particle dosimetry models such as ISDD provide a valuable, immediately useful alternative, and eventually, an adjunct to such measurements.

Development and Application of a Life-Stage Physiologically Based Pharmacokinetic (PBPK) Model to the Assessment of Internal Dose of Pyrethroids in Humans.

To address concerns around age-related sensitivity to pyrethroids, a life-stage physiologically based pharmacokinetic (PBPK) model, supported by in vitro to in vivo extrapolation (IVIVE) was developed. The model was used to predict age-dependent changes in target tissue exposure of 8 pyrethroids; deltamethrin (DLM), cis-permethrin (CPM), trans-permethrin, esfenvalerate, cyphenothrin, cyhalothrin, cyfluthrin, and bifenthrin. A single model structure was used based on previous work in the rat. Intrinsic clearance (CLint) of each individual cytochrome P450 or carboxylesterase (CES) enzyme that are active for a given pyrethroid were measured in vitro, then biologically scaled to obtain in vivo age-specific total hepatic CLint. These IVIVE results indicate that, except for bifenthrin, CES enzymes are largely responsible for human hepatic metabolism (>50% contribution). Given the high efficiency and rapid maturation of CESs, clearance of the pyrethroids is very efficient across ages, leading to a blood flow-limited metabolism. Together with age-specific physiological parameters, in particular liver blood flow, the efficient metabolic clearance of pyrethroids across ages results in comparable to or even lower internal exposure in the target tissue (brain) in children than that in adults in response to the same level of exposure to a given pyrethroid (Cmax ratio in brain between 1- and 25-year old = 0.69, 0.93, and 0.94 for DLM, bifenthrin, and CPM, respectively). Our study demonstrated that a life-stage PBPK modeling approach, coupled with IVIVE, provides a robust framework for evaluating age-related differences in pharmacokinetics and internal target tissue exposure in humans for the pyrethroid class of chemicals.

Physiologically Based Pharmacokinetic Modeling in Risk Assessment: Case Study With Pyrethroids.

The assessment of potentially sensitive populations is an important application of risk assessment. To address the concern for age-related sensitivity to pyrethroid insecticides, life-stage physiologically based pharmacokinetic (PBPK) modeling supported by in vitro to in vivo extrapolation was conducted to predict age-dependent changes in target tissue exposure to 8 pyrethroids. The purpose of this age-dependent dosimetry was to calculate a Data-derived Extrapolation Factor (DDEF) to address age-related pharmacokinetic differences for pyrethroids in humans. We developed a generic human PBPK model for pyrethroids based on our previously published rat model that was developed with in vivo rat data. The results demonstrated that the age-related differences in internal exposure to pyrethroids in the brain are largely determined by the differences in metabolic capacity and in physiology for pyrethroids between children and adults. The most important conclusion from our research is that, given an identical external exposure, the internal (target tissue) concentration is equal or lower in children than in adults in response to the same level of exposure to a pyrethroid. Our results show that, based on the use of the life-stage PBPK models with 8 pyrethroids, DDEF values are essentially close to 1, resulting in a DDEF for age-related pharmacokinetic differences of 1. For risk assessment purposes, this indicates that no additional adjustment factor is necessary to account for age-related pharmacokinetic differences for these pyrethroids.

Evaluation of Age-Related Pyrethroid Pharmacokinetic Differences in Rats: Physiologically-Based Pharmacokinetic Model Development Using In Vitro Data and In Vitro to In Vivo Extrapolation.

An in vitro to in vivo (IVIVE) extrapolation based-physiologically based pharmacokinetic (PBPK) modeling approach was demonstrated to understand age-related differences in kinetics and how they potentially affect age-related differences in acute neurotoxic effects of pyrethroids. To describe the age-dependent changes in pyrethroid kinetics, it was critical to incorporate age-dependent changes in metabolism into the model. As such, in vitro metabolism data were collected for 3 selected pyrethroids, deltamethrin (DLM), cis-permethrin, and trans-permethrin, using liver microsomes and cytosol, and plasma prepared from immature and adult rats. Resulting metabolism parameters, maximum rate of metabolism (Vmax) and Michaelis-Menten constant (Km), were biologically scaled to respective in vivo parameters for use in the age-specific PBPK model. Then, age-dependent changes in target tissue exposure, i.e., brain Cmax, to a given pyrethroid were simulated across ages using the model. The PBPK model recapitulated in vivo time-course plasma and brain concentrations of the 3 pyrethroids in immature and adult rats following oral administration of both low and high doses of these compounds. A single model structure developed for DLM was able to describe the kinetics of the other 2 pyrethroids when used with compound- and age-specific metabolism parameters, suggesting that one generic model for pyrethroids as a group can be used for early age-sensitivity evaluation if appropriate metabolic parameters are used. This study demonstrated the validity of applying IVIVE-based PBPK modeling to development of age-specific PBPK models for pyrethroids in support of pyrethroid risk assessment of potentially sensitive early age populations in humans.

Thermoregulation and its influence on toxicity assessment.

The thermoregulatory system of laboratory rodents is susceptible to a variety of chemical toxicants. Because temperature directly affects the reaction of virtually all biological processes, it is critical to consider how changes in the thermoregulatory response to a toxicant may affect physiological, behavioral, and pathological endpoints. Researchers in industry and government laboratories are often faced with addressing how changes in body temperature of their experimental subjects may affect the outcome of a particular toxicity test and/or screening panel. However, many toxicologists are either unaware of the importance or ignore the potential impact of a toxic-induced change in body temperature. This paper endeavors to summarize the importance of thermoregulation in the study of toxicology and propose recommendations for thermometry that researchers may utilize in their toxicological studies.

Alternative approaches for acute inhalation toxicity testing to address global regulatory and non-regulatory data requirements: An international workshop report.

Inhalation toxicity testing, which provides the basis for hazard labeling and risk management of chemicals with potential exposure to the respiratory tract, has traditionally been conducted using animals. Significant research efforts have been directed at the development of mechanistically based, non-animal testing approaches that hold promise to provide human-relevant data and an enhanced understanding of toxicity mechanisms. A September 2016 workshop, "Alternative Approaches for Acute Inhalation Toxicity Testing to Address Global Regulatory and Non-Regulatory Data Requirements", explored current testing requirements and ongoing efforts to achieve global regulatory acceptance for non-animal testing approaches. The importance of using integrated approaches that combine existing data with in vitro and/or computational approaches to generate new data was discussed. Approaches were also proposed to develop a strategy for identifying and overcoming obstacles to replacing animal tests. Attendees noted the importance of dosimetry considerations and of understanding mechanisms of acute toxicity, which could be facilitated by the development of adverse outcome pathways. Recommendations were made to (1) develop a database of existing acute inhalation toxicity data; (2) prepare a state-of-the-science review of dosimetry determinants, mechanisms of toxicity, and existing approaches to assess acute inhalation toxicity; (3) identify and optimize in silico models; and (4) develop a decision tree/testing strategy, considering physicochemical properties and dosimetry, and conduct proof-of-concept testing. Working groups have been established to implement these recommendations.

Pathway-based predictive approaches for non-animal assessment of acute inhalation toxicity.

New approaches are needed to assess the effects of inhaled substances on human health. These approaches will be based on mechanisms of toxicity, an understanding of dosimetry, and the use of in silico modeling and in vitro test methods. In order to accelerate wider implementation of such approaches, development of adverse outcome pathways (AOPs) can help identify and address gaps in our understanding of relevant parameters for model input and mechanisms, and optimize non-animal approaches that can be used to investigate key events of toxicity. This paper describes the AOPs and the toolbox of in vitro and in silico models that can be used to assess the key events leading to toxicity following inhalation exposure. Because the optimal testing strategy will vary depending on the substance of interest, here we present a decision tree approach to identify an appropriate non-animal integrated testing strategy that incorporates consideration of a substance's physicochemical properties, relevant mechanisms of toxicity, and available in silico models and in vitro test methods. This decision tree can facilitate standardization of the testing approaches. Case study examples are presented to provide a basis for proof-of-concept testing to illustrate the utility of non-animal approaches to inform hazard identification and risk assessment of humans exposed to inhaled substances.

Particokinetics in vitro: dosimetry considerations for in vitro nanoparticle toxicity assessments.

The rapid growth in the use of in vitro methods for nanoparticle toxicity assessment has proceeded with limited consideration of the unique kinetics of these materials in solution. Particles in general and nanoparticles specifically, diffuse, settle, and agglomerate in cell culture media as a function of systemic and particle properties: media density and viscosity and particle size, shape, charge and density, for example. Cellular dose then is also a function of these factors as they determine the rate of transport of nanoparticles to cells in culture. Here we develop and apply the principles of dosimetry in vitro and outline an approach for simulation of nanoparticle particokinetics in cell culture systems. We illustrate that where equal mass concentrations (mug/ml) imply equal doses for dissimilar materials, the corresponding particle number or surface area concentration doses differ by orders of magnitude. More importantly, when rates of diffusional and gravitational particle delivery are accounted for, trends and magnitude of the cellular dose as a function of particle size and density differ significantly from those implied by "concentration" doses. For example, 15-nm silver nanoparticles appear approximately 4000 times more potent than micron-sized cadmium oxide particles on a cm(2)/ml media basis, but are only approximately 50 times more potent when differences in delivery to adherent cells are considered. We conclude that simple surrogates of dose can cause significant misinterpretation of response and uptake data for nanoparticles in vitro. Incorporating particokinetics and principles of dosimetry would significantly improve the basis for nanoparticle toxicity assessment, increasing the predictive power and scalability of such assays.

Absorption, distribution, metabolism, and elimination of 8-2 fluorotelomer alcohol in the rat.

The absorption, distribution, metabolism, and elimination of [3-14C] 8-2 fluorotelomer alcohol (8-2 FTOH, C7F1514CF2CH2CH2OH) following a single oral dose at 5 and 125 mg/kg in male and female rats have been determined. Following oral dosing, the maximum concentration of 8-2 FTOH in plasma occurred by 1 h postdose and cleared rapidly with a half-life of less than 5 h. The internal dose to 8-2 FTOH, as measured by area under the concentration-time curve to infinity, was similar for male and female rats and was observed to increase in a dose-dependent fashion. The majority of the 14C 8-2 FTOH (> 70%) was excreted in feces, and 37-55% was identified as parent. Less than 4% of the administered dose was excreted in urine, which contained low concentrations of perfluorooctanoate (approximately 1% of total 14C). Metabolites identified in bile were principally composed of glucuronide and glutathione conjugates, and perfluorohexanoate was identified in excreta and plasma, demonstrating the metabolism of the parent FTOH by sequential removal of multiple CF2 groups. At 7 days postdose, 4-7% of the administered radioactivity was present in tissues, and for the majority, 14C concentrations were greater than whole blood with the highest concentration in fat, liver, thyroid, and adrenals. Distribution and excretion of a single 125-mg/kg [3-14C] 8-2 FTOH dermal dose following a 6-h exposure in rats was also determined. The majority of the dermal dose either volatilized from the skin (37%) or was removed by washing (29%). Following a 6-h dermal exposure and a 7-day collection period, excretion of total radioactivity via urine (< 0.1%) and feces (< 0.2%) was minor, and radioactivity concentrations in most tissues were below the limit of detection. Systemic availability of 8-2 FTOH following dermal exposure was negligible.

PBPK Model for Atrazine and Its Chlorotriazine Metabolites in Rat and Human.

The previously-published physiologically based pharmacokinetic model for atrazine (ATZ), deisopropylatrazine (DIA), deethylatrazine (DEA), and diaminochlorotriazine (DACT), which collectively comprise the total chlorotriazines (TCT) as represented in this study, was modified to allow for scaling to humans. Changes included replacing the fixed dose-dependent oral uptake rates with a method that represented delayed absorption observed in rats administered ATZ as a bolus dose suspended in a methylcellulose vehicle. Rate constants for metabolism of ATZ to DIA and DEA, followed by metabolism of DIA and DEA to DACT were predicted using a compartmental model describing the metabolism of the chlorotriazines by rat and human hepatocytesin vitro Overall, the model successfully predicted both the 4-day plasma time-course data in rats administered ATZ by bolus dose (3, 10, and 50 mg/kg/day) or in the diet (30, 100, or 500 ppm). Simulated continuous daily exposure of a 55-kg adult female to ATZ at a dose of 1.0 µg/kg/day resulted in steady-state urinary concentrations of 0.6, 1.4, 2.5, and 6.0 µg/L for DEA, DIA, DACT, and TCT, respectively. The TCT (ATZ + DEA + DIA + DACT) human urinary biomonitoring equivalent concentration following continuous exposure to ATZ at the chronic point of departure (POD = 1.8 mg/kg/day) was 360.6 µg/L.