Evaluation of deltamethrin kinetics and dosimetry in the maturing rat using a PBPK model

PBPK modeling to support risk assessment of pyrethroid exposure in French pregnant women

BACKGROUND

Pyrethroids are widely used pesticides and are suspected to affect children's neurodevelopment. The characterization of pyrethroid exposure during critical windows of development, such as fetal development and prenatal life, is essential to ensure a better understanding of pyrethroids potential effects within the concept of Developmental Origins of Health and Disease.

OBJECTIVE

The aim of this study was to estimate maternal exposure of French pregnant women from biomonitoring data and simulate maternal and fetal internal concentrations of 3 pyrethroids (permethrin, cypermethrin and deltamethrin) using a multi-substance pregnancy-PBPK (physiologically based pharmacokinetics) model. The estimated maternal exposures were compared to newly proposed toxicological reference values (TRV) children specific also called draft child-specific reference value to assess pyrethroid exposure risk during pregnancy i.e. during the in utero exposure period.

METHODS

A pregnancy-PBPK model was developed based on an existing adult pyrethroids model. The maternal exposure to each parent compound of pregnant women of the Elfe (French Longitudinal Study since Childhood) cohort was estimated by reverse dosimetry based on urinary biomonitoring data. To identify permethrin and cypermethrin contribution to their common urinary biomarkers of exposure, an exposure ratio based on biomarkers in hair was tested. Finally, exposure estimates were compared to current and draft child-specific reference values derived from rodent prenatal and postnatal exposure studies.

RESULTS

The main contributor to maternal pyrethroid diet intake is cis-permethrin. In blood, total internal concentrations main contributor is deltamethrin. In brain, the major contributors to internal pyrethroid exposure are deltamethrin for fetuses and cis-permethrin for mothers. Risk is identified only for permethrin when referring to the draft child-specific reference value. 2.5% of the population exceeded permethrin draft child-specific reference value.

CONCLUSIONS

A new reverse dosimetry approach using PBPK model combined with human biomonitoring data in urine and hair was proposed to estimate Elfe pregnant population exposure to a pyrethroids mixture with common metabolites.

Predictors of pesticide levels in carpet dust collected from child care centers in Northern California, USA

BACKGROUND

Young children may be exposed to pesticides in child care centers, but little is known about determinants of pesticide contamination in these environments.

OBJECTIVE

Characterize pesticide contamination in early care and education (ECE) centers and identify predictors of pesticide concentrations and loading in dust collected from classroom carpets.

METHODS

Carpet dust samples were collected from 51 licensed child care centers in Northern California and analyzed for 14 structural and agricultural pesticides. Program characteristics were collected through administration of director interviews and observational surveys, including an integrated pest management (IPM) inspection. Pesticide use information for the prior year was obtained from the California Department of Pesticide Regulation to characterize structural applications and nearby agricultural pesticide use.

RESULTS

The most frequently detected pesticides were cis-permethrin (98%), trans-permethrin (98%), bifenthrin (94%), fipronil (94%), and chlorpyrifos (88%). Higher bifenthrin levels were correlated with agricultural applications within 3 kilometers, and higher fipronil levels were correlated with professional pesticide applications in the prior year. In multivariable models, higher IPM Checklist scores were associated with lower loading of chlorpyrifos and permethrin. Placement of the sampled area carpet was also a predictor of chlorpyrifos loading. The strongest predictor of higher pesticide loading for the most frequently detected pesticides was location in California's San Joaquin Valley.

SIGNIFICANCE

Our findings contribute to the growing understanding that pesticides are ubiquitous in children's environments. Pesticide levels in carpet dust were associated with some factors that ECE directors may have control over, such as IPM practices, and others that are beyond their control, such as geographic location. IPM is an important tool that has the potential to reduce pesticide exposures in ECE environments, even for pesticides no longer in use.

IMPACT

One million children in California under six years old attend child care programs where they may spend up to 40 h per week. Children are uniquely vulnerable to environmental contaminants; however early care settings are under researched in environmental health studies. Little is known about predictors of pesticide levels found in environmental samples from child care facilities. This study aims to identify behavioral and environmental determinants of pesticide contamination in California child care centers. Findings can empower child care providers and consumers and inform decision makers to reduce children's exposures to pesticides and promote lifelong health.

Distribution and Elimination of Deltamethrin Toxicity in Laying Hens

Deltamethrin, an important pyrethroid insecticide, is frequently detected in human samples. This study aims to assess the potential effects of deltamethrin on human health and investigate the patterns of residue enrichment and elimination in 112 healthy laying hens. These hens were administered 20 mg·kg-1 deltamethrin based on their body weight. Gas chromatography-mass spectrometry (GC-MS) was used to investigate the residue enrichment pattern and elimination pattern of deltamethrin in the hens. The results indicated a significant increase in the concentration of deltamethrin in chicken manure during the treatment period. By the 14th day of administration, the concentration of deltamethrin in the stool reached 13,510.9 ± 172.24 μg·kg-1, with a fecal excretion rate of 67.56%. The pulmonary deltamethrin concentration was the second highest at 3844.98 ± 297.14 μg·kg-1. These findings suggest that chicken feces contain substantial amounts of deltamethrin after 14 days of continuous administration, and that it can easily transfer to the lungs. After 21 days of drug withdrawal, the residual concentration of deltamethrin in the fat of laying hens was 904.25 ± 295.32 μg·kg-1, with a half-life of 17 days and a slow elimination rate. In contrast, the lungs showed relatively low elimination half-lives of 0.2083 days, indicating faster elimination of deltamethrin in this tissue. These results highlight differences in the rate of deltamethrin elimination in different tissues during drug withdrawal. The fat of laying hens exhibited the highest residue of deltamethrin and the slowest elimination rate, while the lungs showed the fastest elimination rate. Moreover, deltamethrin was found to accumulate in the edible tissues of eggs and laying hens, suggesting that humans may be exposed to deltamethrin through food.

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.

Effects of pyrethroids on brain development and behavior: Deltamethrin

Deltamethrin (DLM) is a Type II pyrethroid pesticide widely used in agriculture, homes, public spaces, and medicine. Epidemiological studies report that increased pyrethroid exposure during development is associated with neurobehavioral disorders. This raises concern about the safety of these chemicals for children. Few animal studies have explored the long-term effects of developmental exposure to DLM on the brain. Here we review the CNS effects of pyrethroids, with emphasis on DLM. Current data on behavioral and cognitive effects after developmental exposure are emphasized. Although, the acute mechanisms of action of DLM are known, how these translate to long-term effects is only beginning to be understood. But existing data clearly show there are lasting effects on locomotor activity, acoustic startle, learning and memory, apoptosis, and dopamine in mice and rats after early exposure. The most consistent neurochemical findings are reductions in the dopamine transporter and the dopamine D1 receptor. The data show that DLM is developmentally neurotoxic but more research on its mechanisms of long-term effects is needed.

Kinetics of metabolism of deltamethrin and cis- and trans-permethrin in vitro. Studies using rat and human liver microsomes, isolated rat hepatocytes and rat liver cytosol

The kinetics of metabolism of deltamethrin (DLM) and cis- and trans-permethrin (CPM and TPM) was studied in male Sprague-Dawley rat and human liver microsomes. DLM metabolism kinetics was also studied in isolated rat hepatocytes, liver microsomes and cytosol. Apparent intrinsic clearance (CL_int) values for the metabolism of DLM, CPM and TPM by cytochrome P450 (CYP) and carboxylesterase (CES) enzymes in rat and human liver microsomes decreased with increasing microsomal protein concentration. However, when apparent CL_int values were corrected for nonspecific binding to allow calculation of unbound (i.e., corrected) CL_int values, the unbound values did not vary greatly with microsomal protein concentration. Unbound CL_int values for metabolism of 0.05-1 μM DLM in rat liver microsomes (CYP and CES enzymes) and cytosol (CES enzymes) were not significantly different from rates of DLM metabolism in isolated rat hepatocytes. This study demonstrates that the nonspecific binding of these highly lipophilic compounds needs to be taken into account in order to obtain accurate estimates of rates of in vitro metabolism of these pyrethroids. While DLM is rapidly metabolised in vitro, the hepatocyte membrane does not appear to represent a barrier to the absorption and hence subsequent hepatic metabolism of this pyrethroid.

Differential lymphatic versus portal vein uptake of the synthetic pyrethroids deltamethrin and cis-permethrin in rats

The objective of this study was to obtain data on pathways of absorption of the synthetic pyrethroids deltamethrin (DLM) and cis-permethrin (CPM) following oral administration to rats. Adult male Sprague-Dawley rats with cannulated mesenteric lymph ducts and hepatic portal veins were given single doses of either 5 mg/kg DLM or 60 mg/kg CPM via the duodenum and lymph and portal blood samples collected for up to 300 min. The pyrethroid dosing vehicles (5 mL/kg body weight) were either corn oil or glycerol formal. Levels of DLM and CPM in lymph and portal blood samples were determined by high-performance liquid chromatography-mass spectrometry-mass spectrometry. Over the time period studied, levels of both DLM and CPM following administration in either corn oil or glycerol formal were greater in lymph than in portal blood. Lymphatic uptake of both DLM and CPM was enhanced following dosing in glycerol formal than in corn oil. The results of this study suggest that after oral administration to rats, these two pyrethroids are predominantly absorbed via the lymphatic system rather than via portal blood. The data obtained in this study thus support a recently developed physiologically-based pharmacokinetic (PBPK) model to evaluate age-related differences in pyrethroid pharmacokinetics in the rat, where it was assumed that absorption of pyrethroids was predominantly via lymphatic uptake.

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.

Physiological parameter values for physiologically based pharmacokinetic models in food-producing animals. Part I: Cattle and swine

Physiologically based pharmacokinetic (PBPK) models for chemicals in food animals are a useful tool in estimating chemical tissue residues and withdrawal intervals. Physiological parameters such as organ weights and blood flows are an important component of a PBPK model. The objective of this study was to compile PBPK‐related physiological parameter data in food animals, including cattle and swine. Comprehensive literature searches were performed in PubMed, Google Scholar, ScienceDirect, and ProQuest. Relevant literature was reviewed and tables of relevant parameters such as relative organ weights (% of body weight) and relative blood flows (% of cardiac output) were compiled for different production classes of cattle and swine. The mean and standard deviation of each parameter were calculated to characterize their variability and uncertainty and to allow investigators to conduct population PBPK analysis via Monte Carlo simulations. Regression equations using weight or age were created for parameters having sufficient data. These compiled data provide a comprehensive physiological parameter database for developing PBPK models of chemicals in cattle and swine to support animal‐derived food safety assessment. This work also provides a basis to compile data in other food animal species, including goats, sheep, chickens, and turkeys.

Neurotoxicity of pesticides

Pesticides are unique environmental contaminants that are specifically introduced into the environment to control pests, often by killing them. Although pesticide application serves many important purposes, including protection against crop loss and against vector-borne diseases, there are significant concerns over the potential toxic effects of pesticides to non-target organisms, including humans. In many cases, the molecular target of a pesticide is shared by non-target species, leading to the potential for untoward effects. Here, we review the history of pesticide usage and the neurotoxicity of selected classes of pesticides, including insecticides, herbicides, and fungicides, to humans and experimental animals. Specific emphasis is given to linkages between exposure to pesticides and risk of neurological disease and dysfunction in humans coupled with mechanistic findings in humans and animal models. Finally, we discuss emerging techniques and strategies to improve translation from animal models to humans.

Toxicokinetics of Deltamethrin: Dosage Dependency, Vehicle Effects, and Low-Dose Age-Equivalent Dosimetry in Rats

There is increasing concern that infants and children may be at increased risk of neurological effects of pyrethroids, the most widely used class of insecticide. The objectives of this investigation were to (1) characterize the dose-dependent toxicokinetics (TK) of deltamethrin (DLM) for exposures ranging from environmentally relevant to acutely toxic; (2) determine the influence of an aqueous versus oil vehicle on oral absorption and bioavailability; and (3) determine whether DLM exhibits low-dose, age-equivalent internal dosimetry. Serial arterial plasma samples were obtained for 72 h from adult, male Sprague Dawley rats given 0.05-5.0 mg DLM/kg as an oral bolus in corn oil (CO). DLM exhibited linear, absorption rate-limited TK. Increases in maximum plasma concentration (Cmax) and AUC∘∞ were directly proportional to the dose. Oral bioavailability was quite limited. The vehicle and its volume had modest effect on the rate and extent of systemic absorption in adult rats. Postnatal day (PND) 15, 21, and 90 (adult) rats received 0.10, 0.25, or 0.50 mg DLM/kg orally in CO and were sacrificed periodically for plasma, brain, and liver collection. Age-dependent differences between PND 15 and 90 plasma Cmax and AUC∘24 values progressively diminished as the dose decreased, but there was a lack of low dose age equivalence in these brain and liver dosimeters. Other maturational factors may account for the lack of the low-dose age equivalence in brain and liver. This investigation provides support for the premise that the relatively low metabolic capacity of immature subjects may be adequate to effectively eliminate trace amounts of DLM and other pyrethroids from the plasma.

Developing a Physiologically-Based Pharmacokinetic Model Knowledgebase in Support of Provisional Model Construction

Developing physiologically-based pharmacokinetic (PBPK) models for chemicals can be resource-intensive, as neither chemical-specific parameters nor in vivo pharmacokinetic data are easily available for model construction. Previously developed, well-parameterized, and thoroughly-vetted models can be a great resource for the construction of models pertaining to new chemicals. A PBPK knowledgebase was compiled and developed from existing PBPK-related articles and used to develop new models. From 2,039 PBPK-related articles published between 1977 and 2013, 307 unique chemicals were identified for use as the basis of our knowledgebase. Keywords related to species, gender, developmental stages, and organs were analyzed from the articles within the PBPK knowledgebase. A correlation matrix of the 307 chemicals in the PBPK knowledgebase was calculated based on pharmacokinetic-relevant molecular descriptors. Chemicals in the PBPK knowledgebase were ranked based on their correlation toward ethylbenzene and gefitinib. Next, multiple chemicals were selected to represent exact matches, close analogues, or non-analogues of the target case study chemicals. Parameters, equations, or experimental data relevant to existing models for these chemicals and their analogues were used to construct new models, and model predictions were compared to observed values. This compiled knowledgebase provides a chemical structure-based approach for identifying PBPK models relevant to other chemical entities. Using suitable correlation metrics, we demonstrated that models of chemical analogues in the PBPK knowledgebase can guide the construction of PBPK models for other chemicals.

Addressing Early Life Sensitivity Using Physiologically Based Pharmacokinetic Modeling and In Vitro to In Vivo Extrapolation

Physiologically based pharmacokinetic (PBPK) modeling can provide an effective way to utilize in vitro and in silico based information in modern risk assessment for children and other potentially sensitive populations. In this review, we describe the process of in vitro to in vivo extrapolation (IVIVE) to develop PBPK models for a chemical in different ages in order to predict the target tissue exposure at the age of concern in humans. We present our on-going studies on pyrethroids as a proof of concept to guide the readers through the IVIVE steps using the metabolism data collected either from age-specific liver donors or expressed enzymes in conjunction with enzyme ontogeny information to provide age-appropriate metabolism parameters in the PBPK model in the rat and human, respectively. The approach we present here is readily applicable to not just to other pyrethroids, but also to other environmental chemicals and drugs. Establishment of an in vitro and in silico-based evaluation strategy in conjunction with relevant exposure information in humans is of great importance in risk assessment for potentially vulnerable populations like early ages where the necessary information for decision making is limited.

A novel toxicokinetic modeling of cypermethrin and permethrin and their metabolites in humans for dose reconstruction from biomarker data

To assess exposure to pyrethroids in the general population, one of most widely used method nowadays consists of measuring urinary metabolites. Unfortunately, interpretation of data is limited by the unspecified relation between dose and levels in biological tissues and excreta. The objective of this study was to develop a common multi-compartment toxicokinetic model to predict the time courses of two mainly used pyrethroid pesticides, permethrin and cypermethrin, and their metabolites (cis-DCCA, trans-DCCA and 3-PBA) in the human body and in accessible biological matrices following different exposure scenarios. Toxicokinetics was described mathematically by systems of differential equations to yield the time courses of these pyrethroids and their metabolites in the different compartments. Unknown transfer rate values between compartments were determined from best fits to available human data on the urinary excretion time courses of metabolites following an oral and dermal exposure to cypermethrin in volunteers. Since values for these coefficients have not yet been determined, a mathematical routine was programmed in MathCad to establish the possible range of values on the basis of physiological and mathematical considerations. The best combination of parameter values was then selected using a statistic measure (reliability factor) along with a statistically acceptable range of values for each parameter. With this approach, simulations provided a close approximation to published time course data. This model allows to predict urinary time courses of trans-DCCA, cis-DCCA and 3-PBA, whatever the exposure route. It can also serve to reconstruct absorbed doses of permethrin or cypermethrin in the population using measured biomarker data.

Studying permethrin exposure in flight attendants using a physiologically based pharmacokinetic model

Assessment of potential health risks to flight attendants from exposure to pyrethroid insecticides, used for aircraft disinsection, is limited because of (a) lack of information on exposures to these insecticides, and (b) lack of tools for linking these exposures to biomarker data. We developed and evaluated a physiologically based pharmacokinetic (PBPK) model to assess the exposure of flight attendants to the pyrethroid insecticide permethrin attributable to aircraft disinsection. The permethrin PBPK model was developed by adapting previous models for pyrethroids, and was parameterized using currently available metabolic parameters for permethrin. The human permethrin model was first evaluated with data from published human studies. Then, it was used to estimate urinary metabolite concentrations of permethrin in flight attendants who worked in aircrafts, which underwent residual and pre-flight spray treatments. The human model was also applied to analyze the toxicokinetics following permethrin exposures attributable to other aircraft disinsection scenarios. Predicted levels of urinary 3-phenoxybenzoic acid (3-PBA), a metabolite of permethrin, following residual disinsection treatment were comparable to the measurements made for flight attendants. Simulations showed that the median contributions of the dermal, oral and inhalation routes to permethrin exposure in flight attendants were 83.5%, 16.1% and 0.4% under residual treatment scenario, respectively, and were 5.3%, 5.0% and 89.7% under pre-flight spray scenario, respectively. The PBPK model provides the capability to simulate the toxicokinetic profiles of permethrin, and can be used in the studies on human exposure to permethrin.

Carbaryl and 1-naphthol tissue levels and related cholinesterase inhibition in male Brown Norway rats from preweaning to senescence

Studies incorporating both toxicokinetic and dynamic factors provide insight into chemical sensitivity differences across the life span. Tissue (brain, plasma, liver) levels of the N-methyl carbamate carbaryl, and its metabolite 1-naphthol, were determined and related to brain and RBC cholinesterase (ChE) inhibition in the same animals. Dose-response (3, 7.5, 15, or 22.5 mg/kg, 40-45 min postdosing) and time course (3 or 15 mg/kg at 30, 60, 120, or 240 min postdosing) of acute effects of carbaryl (oral gavage) in preweanling (postnatal day [PND] 18) and adult male Brown Norway rats from adolescence to senescence (1, 4, 12, 24 mo) were compared. At all ages there were dose-related increases in carbaryl and 1-naphthol in the dose-response study, and the time-course study showed highest carbaryl levels at 30 min postdosing. There were, however, age-related differences in that the 1- and 4-mo rats showed the lowest levels of carbaryl and 1-naphthol, and PND18 and 24-mo rats had similar, higher levels. The fastest clearance (shortest half-lives) was observed in 1- and 4-mo rats. Carbaryl levels were generally higher than 1-naphthol in brain and plasma, but in liver, 1-naphthol levels were similar to or greater than carbaryl. Brain ChE inhibition closely tracked brain carbaryl concentrations regardless of the time after dosing, but there was more variability in the relationship between RBC ChE and plasma carbaryl levels. Within-subject analyses suggested somewhat more brain ChE inhibition at lower carbaryl levels only in the PND18 rats. These findings may reflect maturation followed by decline in kinetic factors over the life span.

Critical consideration of the multiplicity of experimental and organismic determinants of pyrethroid neurotoxicity: a proof of concept

Pyrethroids (PYR) are pesticides with high insecticidal activity that may disrupt neuronal excitability in target and nontarget species. The accumulated evidence consistently showed that this neurophysiologic action is followed by alterations in motor, sensorimotor, neuromuscular, and thermoregulatory responses. Nevertheless, there are some equivocal results regarding the potency of PYR in lab animals. The estimation of potency is an important step in pesticide chemical risk assessment. In order to identify the variables influencing neurobehavioral findings across PYR studies, evidence on experimental and organismic determinants of acute PYR-induced neurotoxicity was reviewed in rodents. A comprehensive analysis of these studies was conducted focusing on test material and dosing conditions, testing conditions, animal models, and other determinants such as testing room temperature. Variations in the severity of the neurotoxicity, under lab-controlled conditions, was explained based upon factors including influence of animal species and age, test material features such as chemical structure and stereochemistry, and dosing conditions such as vehicle, route of exposure, and dose volume. If not controlled, the interplay of these factors may lead to large variance in potency estimation. This review examined the scope of acute toxicological data required to determine the safety of pesticide products, and factors and covariates that need to be controlled in order to ensure that predictivity and precaution are balanced in a risk assessment process within a reasonable time-frame, using acute PYR-induced neurotoxicity in rodents as an exemplar.

Alteration of the expression of pesticide-metabolizing enzymes in pregnant mice: potential role in the increased vulnerability of the developing brain

Studies on therapeutic drug disposition in humans have shown significant alterations as the result of pregnancy. However, it is not known whether pesticide metabolic capacity changes throughout pregnancy, which could affect exposure of the developing brain. We sought to determine the effect of pregnancy on the expression of hepatic enzymes involved in the metabolism of pesticides. Livers were collected from virgin and pregnant C57BL/6 mice at gestational days (GD)7, GD11, GD14, GD17, and postpartum days (PD)1, PD15, and PD30. Relative mRNA expression of several enzymes involved in the metabolism of pesticides, including hepatic cytochromes (Cyp) P450s, carboxylesterases (Ces), and paraoxonase 1 (Pon1), were assessed in mice during gestation and the postpartum period. Compared with virgin mice, alterations in the expression occurred at multiple time points, with the largest changes observed on GD14. At this time point, the expression of most of the Cyps involved in pesticide metabolism in the liver (Cyp1a2, Cyp2d22, Cyp2c37, Cyp2c50, Cyp2c54, and Cyp3a11) were downregulated by 30% or more. Expression of various Ces isoforms and Pon1 were also decreased along with Pon1 activity. These data demonstrate significant alterations in the expression of key enzymes that detoxify pesticides during pregnancy, which could alter exposure of developing animals to these chemicals.

Computational toxicology: application in environmental chemicals

This chapter provides an overview of computational models that describe various aspects of the source-to-health effect continuum. Fate and transport models describe the release, transportation, and transformation of chemicals from sources of emission throughout the general environment. Exposure models integrate the microenvironmental concentrations with the amount of time an individual spends in these microenvironments to estimate the intensity, frequency, and duration of contact with environmental chemicals. Physiologically based pharmacokinetic (PBPK) models incorporate mechanistic biological information to predict chemical-specific absorption, distribution, metabolism, and excretion. Values of parameters in PBPK models can be measured in vitro, in vivo, or estimated using computational molecular modeling. Computational modeling is also used to predict the respiratory tract dosimetry of inhaled gases and particulates [computational fluid dynamics (CFD) models], to describe the normal and xenobiotic-perturbed behaviors of signaling pathways, and to analyze the growth kinetics of preneoplastic lesions and predict tumor incidence (clonal growth models).

Pesticide regulations: exposure-dose modeling from FIFRA to FQPA

The federal laws and regulations governing the registration and use of pesticides in the United States under the Federal Insecticide, Fungicide, and Rodenticide Act are published in the Federal Register, while state laws such as California are published in the California Food and Agricultural Code, Divisions 6, 7, and 13. Up until the passage of the Food Quality Protection Act (FQPA of 1996), federal and state regulations pertaining to the registration and use of pesticides were in most cases identical except for the fact that food tolerances were enforced but not set at the state level. The California Department of Pesticide Regulation's Worker Health and Safety Program continues to monitor worker exposure to pesticides and report illnesses among workers associated with pesticide exposure. Under FQPA, the United States Environmental Protection Agency (EPA) has taken a leadership role in the development of probabilistic pesticide exposure models (i.e., DEEM, SHEDS, etc.) using pesticide application, human activity, and exposure databases (i.e., CPPAES, CHAD, CSFII, FCID, NHANES, and NHEXAS). A physiologically based pharmacokinetic/pharmacodynamic (PBPK/PD) modeling framework has been established by EPA to assess cumulative risk of dose and injury to infants and children to organophosphorus, carbamate (NMC), and pyrethroid insecticides from aggregate sources and routes. Probabilistic models are being linked to PBPK/PD models to improve risk assessments.

A pharmacokinetic model of cis- and trans-permethrin disposition in rats and humans with aggregate exposure application

Permethrin is a broad-spectrum pyrethroid insecticide and among the most widely used insecticides in homes and crops. Managing the risks for pesticides such as permethrin depends on the ability to consider diverse exposure scenarios and their relative risks. Physiologically based pharmacokinetic models of delta methrin disposition were modified to describe permethrin kinetics in the rat and human. Unlike formulated deltamethrin which consists of a single stereoisomer, permethrin is formulated as a blend of cis- and trans-diastereomers. We assessed time courses for cis-permethrin and trans-permethrin in several tissues (brain, blood, liver, and fat) in the rat following oral administration of 1 and 10mg/kg permethrin (cis/trans: 40/60). Accurate simulation of permethrin in the rat suggests that a generic model structure is promising for modeling pyrethroids. Human in vitro data and appropriate anatomical information were used to develop a provisional model of permethrin disposition with structures for managing oral, dermal, and inhalation routes of exposure. The human permethrin model was used to evaluate dietary and residential exposures in the U.S. population as estimated by EPA's Stochastic Human Exposure and Dose Simulation model. Simulated cis- and trans-DCCA, metabolites of permethrin, were consistent with measured values in the National Health and Nutrition Examination Survey, indicating that the model holds promise for assessing population exposures and quantifying dose metrics.

Reconstructing organophosphorus pesticide doses using the reversed dosimetry approach in a simple physiologically-based pharmacokinetic model

We illustrated the development of a simple pharmacokinetic (SPK) model aiming to estimate the absorbed chlorpyrifos doses using urinary biomarker data, 3,5,6-trichlorpyridinol as the model input. The effectiveness of the SPK model in the pesticide risk assessment was evaluated by comparing dose estimates using different urinary composite data. The dose estimates resulting from the first morning voids appeared to be lower than but not significantly different to those using before bedtime, lunch or dinner voids. We found similar trend for dose estimates using three different urinary composite data. However, the dose estimates using the SPK model for individual children were significantly higher than those from the conventional physiologically based pharmacokinetic (PBPK) modeling using aggregate environmental measurements of chlorpyrifos as the model inputs. The use of urinary data in the SPK model intuitively provided a plausible alternative to the conventional PBPK model in reconstructing the absorbed chlorpyrifos dose.

Parameters for pyrethroid insecticide QSAR and PBPK/PD models for human risk assessment

In this review we have examined the status of parameters required by pyrethroid QSAR-PBPK/PD models for assessing health risks. In lieu of the chemical,biological, biochemical, and toxicological information developed on the pyrethroids since 1968, the finding of suitable parameters for QSAR and PBPK/PD model development was a monumental task. The most useful information obtained came from rat toxicokinetic studies (i.e., absorption, distribution, and excretion), metabolism studies with 14C-cyclopropane- and alcohol-labeled pyrethroids, the use of known chiral isomers in the metabolism studies and their relation to commercial products. In this review we identify the individual chiralisomers that have been used in published studies and the chiral HPLC columns available for separating them. Chiral HPLC columns are necessary for isomer identification and for developing kinetic values (Vm,, and Kin) for pyrethroid hydroxylation. Early investigators synthesized analytical standards for key pyrethroid metabolites, and these were used to confirm the identity of urinary etabolites, by using TLC. These analytical standards no longer exist, and muste resynthesized if further studies on the kinetics of the metabolism of pyrethroids are to be undertaken.In an attempt to circumvent the availability of analytical standards, several CYP450 studies were carried out using the substrate depletion method. This approach does not provide information on the products formed downstream, and may be of limited use in developing human environmental exposure PBPK/PD models that require extensive urinary metabolite data. Hydrolytic standards (i.e., alcohols and acids) were available to investigators who studied the carboxylesterase-catalyzed hydrolysis of several pyrethroid insecticides. The data generated in these studies are suitable for use in developing human exposure PBPK/PD models.Tissue:blood partition coefficients were developed for the parent pyrethroids and their metabolites, by using a published mechanistic model introduced by Poulin and Thiele (2002a; b) and log DpH 7.4 values. The estimated coefficients, especially those of adipose tissue, were too high and had to be corrected by using a procedure in which the proportion of parent or metabolite residues that are unbound to plasma albumin is considered, as described in the GastroPlus model (Simulations Plus, Inc.,Lancaster, CA). The literature suggested that Km values be adjusted by multiplying Km by the substrate (decimal amount) that is unbound to microsomal or CYPprotein. Mirfazaelian et al. (2006) used flow- and diffusion-limited compartments in their deltamethrin model. The addition of permeability areas (PA) having diffusion limits, such as the fat and slowly perfused compartments, enabled the investigators to bring model predictions in line with in vivo data.There appears to be large differences in the manner and rate of absorption of the pyrethroids from the gastrointestinal tract, implying that GI advanced compartmental transit models (ACAT) need to be included in PBPK models. This is especially true of the absorption of an oral dose of tefluthrin in male rats, in which 3.0-6.9%,41.3-46.3%, and 5.2-15.5% of the dose is eliminated in urine, feces, and bile,respectively (0-48 h after administration). Several percutaneous studies with the pyrethroids strongly support the belief that these insecticides are not readily absorbed, but remain on the surface of the skin until they are washed off. In one articular study (Sidon et al. 1988) the high levels of permethrin absorption through the forehead skin (24-28%) of the monkey was reported over a 7- to 14-days period.Wester et al. (1994) reported an absorption of 1.9% of pyrethrin that had been applied to the forearm of human volunteers over a 7-days period.SAR models capable of predicting the binding of the pyrethroids to plasma and hepatic proteins were developed by Yamazaki and Kanaoka (2004), Saiakhov et al. (2000), Colmenarejo et al. (2001), and Colmenarejo (2003). QikProp(Schrodinger, LLC) was used to obtain Fu values for calculating partition coefficients and for calculating permeation constants (Caco-2, MDCK, and logBBB). ADMET Predictor (Simulations Plus Inc.) provided Vm~,x and Km values for the hydroxylation of drugs/pyrethroids by human liver recombinant cytochrome P450 enzymes making the values available for possible use in PBPK/PD models.The Caco-2 permeability constants and CYP3A4 Vmax and Km values are needed in PBPK/PD models with GI ACAT sub models. Modeling work by Chang et al.(2009) produced rate constants (kcat) for the hydrolysis of pyrethroids by rat serumcarboxylesterases. The skin permeation model of Potts and Guy (1992) was used topredict K, values for the dermal absorption of the 15 pyrethroids.The electrophysiological studies by Narahashi (1971) and others (Breckenridgeet al. 2009; Shafer et al. 2005; Soderlund et al. 2002; Wolansky and Harrill 2008)demonstrated that the mode of action of pyrethroids on nerves is to interfere with the changes in sodium and potassium ion currents. The pyrethroids, being highly lipid soluble, are bound or distributed in lipid bilayers of the nerve cell membrane and exert their action on sodium channel proteins. The rising phase of the action potential is caused by sodium influx (sodium activation), while the falling phase is caused by sodium activation being turned off, and an increase in potassium efflux(potassium activation). The action of allethrin and other pyrethroids is caused by an inhibition or block of the normal currents. An equation by Tatebayashi and Narahashi (1994) that describes the action of pyrethroids on sodium channels was found in the literature. This equation, or some variation of it, may be suitable for use in the PD portion of pyrethroid PBPK models.

A physiologically based pharmacokinetic model for atrazine and its main metabolites in the adult male C57BL/6 mouse

Atrazine (ATR) is a chlorotriazine herbicide that is widely used and relatively persistent in the environment. In laboratory rodents, excessive exposure to ATR is detrimental to the reproductive, immune, and nervous systems. To better understand the toxicokinetics of ATR and to fill the need for a mouse model, a physiologically based pharmacokinetic (PBPK) model for ATR and its main chlorotriazine metabolites (Cl-TRIs) desethyl atrazine (DE), desisopropyl atrazine (DIP), and didealkyl atrazine (DACT) was developed for the adult male C57BL/6 mouse. Taking advantage of all relevant and recently made available mouse-specific data, a flow-limited PBPK model was constructed. The ATR and DACT sub-models included blood, brain, liver, kidney, richly and slowly perfused tissue compartments, as well as plasma protein binding and red blood cell binding, whereas the DE and DIP sub-models were constructed as simple five-compartment models. The model adequately simulated plasma levels of ATR and Cl-TRIs and urinary dosimetry of Cl-TRIs at four single oral dose levels (250, 125, 25, and 5mg/kg). Additionally, the model adequately described the dose dependency of brain and liver ATR and DACT concentrations. Cumulative urinary DACT amounts were accurately predicted across a wide dose range, suggesting the model's potential use for extrapolation to human exposures by performing reverse dosimetry. The model was validated using previously reported data for plasma ATR and DACT in mice and rats. Overall, besides being the first mouse PBPK model for ATR and its Cl-TRIs, this model, by analogy, provides insights into tissue dosimetry for rats. The model could be used in tissue dosimetry prediction and as an aid in the exposure assessment to this widely used herbicide.

Age, dose, and time-dependency of plasma and tissue distribution of deltamethrin in immature rats

The major objective of this project was to characterize the systemic disposition of the pyrethroid, deltamethrin (DLT), in immature rats, with emphasis on the age dependence of target organ (brain) dosimetry. Postnatal day (PND) 10, 21, and 40 male Sprague-Dawley rats received 0.4, 2, or 10 mg DLT/kg by gavage in glycerol formal. Serial plasma, brain, fat, liver, and skeletal muscle samples were collected for up to 510 h and analyzed for DLT and/or 3-phenoxybenzoic acid (PBA) content by high-performance liquid chromatography. Toxicokinetic data from previous experiments of the same design with young adult (PND 90) rats (Kim, K.-B., Anand, S. S., Kim, H. J., White, C. A., and Bruckner, J. V. [2008]. Toxicokinetics and tissue distribution of deltamethrin in adult Sprague-Dawley rats. Toxicol. Sci. 101, 197-205) were used to compare to immature rat data. Plasma and tissue DLT levels were inversely related to age. Preweanlings and weanlings showed markedly elevated brain concentrations and pronounced salivation, tremors, choreoathetosis, and eventual fatalities. Plasma DLT levels did not reliably reflect brain levels over time. Plasma:brain ratios were time and dose dependent, but apparently not age dependent. Brain levels were better correlated with the magnitude of salivation and tremors than plasma levels. Hepatic intrinsic clearance of DLT progressively increased during maturation, as did the hepatic extraction ratio. Thus, limited capacity to metabolically inactivate DLT appeared primarily responsible for the inordinately high target organ doses and acute neurotoxicity in pups and weanling rats. Hepatic blood flow was not rate limiting in any age group. Limited DLT hydrolysis was manifest in vivo in the pups by relatively low plasma PBA levels. Elevated exposure of the immature brain to a pyrethroid may prove to be of consequence for long-term, as well as short-term neurotoxicity.