The implications of using a physiologically based pharmacokinetic (PBPK) model for pesticide risk assessment

Advancing ecotoxicological studies: Utilizing new approach methodologies to enable cross-species extrapolation and reduce avian testing

Ecological risk assessments of agrochemicals have traditionally depended on in vivo guideline tests using northern bobwhite and mallard to provide relevant endpoints for avian species. However, these studies have limitations, including animal welfare concerns, the time and cost involved, limited potential for extrapolation to more realistic exposure conditions, and the lack of mechanistic understanding. The proof-of-concept work presented a case study for thiamethoxam in three avian species, demonstrating the potential of physiologically based kinetic (PBK) modeling to enable dosimetry extrapolations that inform hazard characterization in risk assessment, and reduce the use of avian testing. The model structure for northern bobwhite and mallard contained ten compartments, while an additional ovulation model was included for chicken in the physiological state of egg-laying. The model was first parameterized and evaluated for chicken and northern bobwhite using in vitro kinetic measurements and in vivo toxicokinetic (TK) data. The chicken model was then extrapolated to mallard based on allometric scaling. The models were then used to map the TK profiles across species by simulating internal dose metrics in different avian toxicology studies. These metrics, including peak blood concentrations (Cmax) and area under the curve (AUC) for blood concentration, were determined for acute, subacute, or chronic toxicity endpoints for mallard and northern bobwhite, enabling a quantitative cross-species and cross-route comparison of dosimetry. The results suggested that the chronic toxicological response of birds exposed to thiamethoxam is highly dependent on internal exposure, while mallard appeared to be more dynamically sensitive to thiamethoxam on an acute oral exposure basis. The case study increases the confidence in using new approach methodologies (NAMs) for interpreting avian toxicity studies and facilitating in vitro-in silico-based ecological risk assessments of agrochemicals.

Recent Progress on Physiologically Based Pharmacokinetic (PBPK) Model: A Review Based on Bibliometrics

Physiologically based pharmacokinetic/toxicokinetic (PBPK/PBTK) models are designed to elucidate the mechanism of chemical compound action in organisms based on the physiological, biochemical, anatomical, and thermodynamic properties of organisms. After nearly a century of research and practice, good results have been achieved in the fields of medicine, environmental science, and ecology. However, there is currently a lack of a more systematic review of progress in the main research directions of PBPK models, especially a more comprehensive understanding of the application in aquatic environmental research. In this review, a total of 3974 articles related to PBPK models from 1996 to 24 March 2024 were collected. Then, the main research areas of the PBPK model were categorized based on the keyword co-occurrence maps and cluster maps obtained by CiteSpace. The results showed that research related to medicine is the main application area of PBPK. Four major research directions included in the medical field were "drug assessment", "cross-species prediction", "drug-drug interactions", and "pediatrics and pregnancy drug development", in which "drug assessment" accounted for 55% of the total publication volume. In addition, bibliometric analyses indicated a rapid growth trend in the application in the field of environmental research, especially in predicting the residual levels in organisms and revealing the relationship between internal and external exposure. Despite facing the limitation of insufficient species-specific parameters, the PBPK model is still an effective tool for improving the understanding of chemical-biological effectiveness and will provide a theoretical basis for accurately assessing potential risks to ecosystems and human health. The combination with the quantitative structure-activity relationship model, Bayesian method, and machine learning technology are potential solutions to the previous research gaps.

Human health risk assessment of 6:2 Cl-PFESA through quantitative in vitro to in vivo extrapolation by integrating cell-based assays, an epigenetic key event, and physiologically based pharmacokinetic modeling

Human health risk assessment of chemicals is essential but often relies on time-consuming and animal and labor-extensive procedures. Here, we develop a population-based, quantitative in vitro to in vivo extrapolation (QIVIVE) approach which depended on cellular effects monitored by in vitro assays, considered chemical internal concentration determined by LC-MS/MS, extrapolated into in vivo target tissue concentration through physiologically based pharmacokinetic (PBPK) modelling, and assessed populational health risk using in silico modelling. By applying this QIVIVE approach to 6:2 chlorinated polyfluorinated ether sulfonate (6:2 Cl-PFESA), as a representative of the emerging pollutants, we find that 6:2 Cl-PFESA disturbed lipid homeostasis in HepG2 cells through enhancement of lipid accumulation and fatty acid β-oxidation, during which miR-93-5p served as a key event towards toxicity and thus, could serve as an efficient toxicity marker for risk assessment; further, the disruption potency of lipid homeostasis of 6:2 Cl-PFESA for the most of studied populations in China might be of moderate concern. Together, our approach improved the reliability of QIVIVE during human health risk assessment, which can readily be used for other chemicals.

Prediction of dose-dependent in vivo acetylcholinesterase inhibition by profenofos in rats and humans using physiologically based kinetic (PBK) modeling-facilitated reverse dosimetry

Organophosphate pesticides (OPs) are known to inhibit acetylcholine esterase (AChE), a critical effect used to establish health-based guidance values. This study developed a combined in vitro–in silico approach to predict AChE inhibition by the OP profenofos in rats and humans. A physiologically based kinetic (PBK) model was developed for both species. Parameter values for profenofos conversion to 4-bromo-2-chlorophenol (BCP) were derived from in vitro incubations with liver microsomes, liver cytosol, and plasma from rats (catalytic efficiencies of 1.1, 2.8, and 0.19 ml/min/mg protein, respectively) and humans (catalytic efficiencies of 0.17, 0.79, and 0.063 ml/min/mg protein, respectively), whereas other chemical-related parameter values were derived using in silico calculations. The rat PBK model was evaluated against literature data on urinary excretion of conjugated BCP. Concentration-dependent inhibition of rat and human AChE was determined in vitro and these data were translated with the PBK models to predicted dose-dependent AChE inhibition in rats and humans in vivo. Comparing predicted dose-dependent AChE inhibition in rats to literature data on profenofos-induced AChE inhibition revealed an accurate prediction of in vivo effect levels. Comparison of rat predictions (BMDL10 of predicted dose–response data of 0.45 mg/kg bw) and human predictions (BMDL10 of predicted dose–response data of 0.01 mg/kg bw) suggests that humans are more sensitive than rats, being mainly due to differences in kinetics. Altogether, the results demonstrate that in vivo AChE inhibition upon acute exposure to profenofos was closely predicted in rats, indicating the potential of this novel approach method in chemical hazard assessment.

Physiologically based kinetic modelling-facilitated reverse dosimetry to predict in vivo red blood cell acetylcholinesterase inhibition following exposure to chlorpyrifos in the Caucasian and Chinese population

Organophosphates have a long history of use as insecticides over the world. The aim of the present study was to investigate the interethnic differences in kinetics, biomarker formation, and in vivo red blood cell acetylcholinesterase inhibition of chlorpyrifos (CPF) in the Chinese and the Caucasian population. To this purpose, physiologically based kinetic models for CPF in both the Chinese and Caucasian population were developed, and used to study time- and dose-dependent interethnic variation in urinary biomarkers and to convert concentration-response curves for red blood cell acetylcholinesterase inhibition to in vivo dose-response curves in these 2 populations by reverse dosimetry. The results obtained revealed a marked interethnic difference in toxicokinetics of CPF, with lower urinary biomarker levels at similar dose levels and slower CPF bioactivation and faster chlorpyrifos-oxon detoxification in the Chinese compared with the Caucasian population, resulting in 5- to 6-fold higher CPF sensitivity of the Caucasian than the Chinese population. These differences might be related to variation in the frequency of single-nucleotide polymorphisms for the major biotransformation enzymes involved. To conclude, the interethnic variation in kinetics of CPF may affect both its biomarker-based exposure assessment and its toxicity and risk assessment and physiologically based kinetic modeling facilitates the characterization and quantification of these interethnic variations.

Physiology-based toxicokinetic modelling in the frame of the European Human Biomonitoring Initiative

Given the opportunities provided by internal dosimetry modelling in the interpretation of human biomonitoring (HBM) data, the assessment of the links between exposure to chemicals and observed HBM data can be effectively supported by PBTK modelling. This paper gives a comprehensive review of available human PBTK models for compounds selected as a priority by the European Human Biomonitoring Initiative (HBM4EU). We highlight their advantages and deficiencies and suggest steps for advanced internal dose modelling. The review of the available PBTK models highlighted the conceptual differences between older models compared to the ones developed recently, reflecting commensurate differences in research questions. Due to the lack of coordinated strategies for deriving useful biomonitoring data for toxicokinetic properties, significant problems in model parameterisation still remain; these are further increased by the lack of human toxicokinetic data due to ethics issues. Finally, questions arise as well as to the extent they are really representative of interindividual variability. QSARs for toxicokinetic properties is a complementary approach for PBTK model parameterisation, especially for data poor chemicals. This approach could be expanded to model chemico-biological interactions such as intestinal absorption and renal clearance; this could serve the development of more complex generic PBTK models that could be applied to newly derived chemicals. Another gap identified is the framework for mixture interaction terms among compounds that could eventually interact in metabolism. From the review it was concluded that efforts should be shifted toward the development of generic multi-compartmental and multi-route models, supported by targeted biomonitoring coupled with parameterisation by both QSAR approach and experimental (in-vivo and in-vitro) data for newly developed and data poor compounds.

Health risk assessment of trihalomethanes mixtures from daily water-related activities via multi-pathway exposure based on PBPK model

In this study, the concentrations of trihalomethanes (THMs) in tap water and direct drinking water were analyzed, and based on the human behavior patterns and building parameters, the concentrations of THMs in indoor air were simulated with the water-air concentration conversion model. In addition, concentrations of THMs in human tissues were predicted based on physiologically based pharmacokinetic (PBPK) model, and the health risk of THMs for participants were estimated. Furthermore, the carcinogenic risk of mixtures according to the method proposed by USEPA and PBPK model based method were calculated and compared. The concentrations of chloroform, bromodichloromethane, dibromochloromethane and bromoform in tap water were 11.28-16.21, 4.83-6.28, 0.81-1.32 and 0.08-0.21 μg/L, and those in direct drinking water were 3.29-6.88, 0.35-0.47, 0.03-0.08 and 0.04-0.08 μg/L, respectively. The results of water-air concentration conversion model demonstrated that pollutants in air had a strong correlation with water-related activities. Multi-pathway PBPK model showed that THMs concentrations in liver, kidney and richly perfused tissue were higher than those in other tissues. The results of risk assessment showed that the mean risk levels of mixtures were 1.69 × 10^-4 and 1.72 × 10^-4 calculated by the USEPA recommended method and PBPK based method, which seriously exceeded the acceptable level. TCM and BDCM were the major risk factors, and inhalation was the main exposure route of THMs.

Aggregating exposures & cumulating risk for semivolatile organic compounds: A review

Increasingly, health risk assessment is addressing multiple pathway exposures to multiple contaminants. We reviewed aggregated exposure and cumulative risk approaches for contemporary and ubiquitous semivolatile organic compounds (SVOC). We identified 22 studies aggregating exposure pathways, and 31 cumulating risk. Exposure aggregation is based on the addition of pathway-specific doses, using kinetic modeling where it exists, and classic external dose equations otherwise. In most cases, exposure is dominated by a single route or source of exposure - mainly the oral pathway - via dietary or non-dietary exposure. Preferential routes and sources of exposure are influenced by SVOC physical-chemical properties such as vapor pressure. The cumulative risk approach for contaminants is based on dose addition. Simple sum of hazard quotient (Hazard Index: HI) is the most commonly used cumulative risk assessment approach, while Relative Potency Factor (RPF) appeared to the best suited - although this calls for a level of toxicological information that limits the number of compounds that can be studied simultaneously. Where both were performed, moving from HI to more refined approach produced similar results. In conclusion, both approaches - exposure aggregation and cumulative risk - rely on simple assumptions. Nevertheless, they allow uncertainty to be reduced, in comparison with source-by-source or chemical-by-chemical approaches.

Comparison of residents' pesticide exposure with predictions obtained using the UK regulatory exposure assessment approach

The UK regulatory methods currently used for estimating residents' potential pesticide exposure were assessed to determine whether they provide sufficiently conservative estimates. A non-random sample of 149 residents living within 100 m of fields where pesticides were sprayed provided first morning void urine samples one and/or two days after spraying. Using farmers' spray information, regulatory exposure assessment (REA) models were applied to estimate potential pesticide intake among residents, with a toxicokinetic (TK) model used to estimate urinary biomarker concentrations in the mornings of the two days following the spray. These were compared with actual measured urinary biomarker concentrations obtained following the spray applications. The study focused on five pesticides (cypermethrin, penconazole, captan, chlorpyrifos and chlormequat). All measured cypermethrin urinary biomarker levels were lower than the REA-predicted concentrations. Over 98% and 97% of the measured urinary biomarker concentrations for penconazole and captan respectively were lower than the REA-predicted exposures. Although a number of the chlorpyrifos and chlormequat spray-related urinary biomarker concentrations were greater than the predictions, investigation of the background urinary biomarker concentrations suggests these were not significantly different from the levels expected had no pesticide spraying occurred. The majority of measured concentrations being well below the REA-predicted concentrations indicate that, in these cases, the REA is sufficiently conservative.

Photocatalytic degradation of carbofuran in TiO2 aqueous solution: kinetics using design of experiments and mechanism by HPLC/MS/MS

The photocatalytic degradation kinetics of carbofuran was optimized by central composite design based on response surface methodology for the first time. Three variables, TiO2 concentration, initial pH value and the concentration of carbofuran, were selected to determine the dependence of degradation efficiencies on independent variables. Response surface methodology modeling results indicated that the degradation efficiency of carbofuran was highly affected by the initial pH value and the concentration of carbofuran. Then nine degradation intermediates were detected by HPLC/MS/MS. The Frontier Electron Densities of carbofuran were calculated to predict the active sites on carbofuran attacked by hydroxyl radicals and photoholes. Point charges were used to elucidate the chemisorption pattern on TiO2 catalysts during the photocatalytic process. By combining the experimental results and calculation data, the photocatalytic degradation pathways of carbofuran were proposed, including the addition of hydroxyl radicals and the cleavage of the carbamate side chain.

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.

Cutting Edge PBPK Models and Analyses: Providing the Basis for Future Modeling Efforts and Bridges to Emerging Toxicology Paradigms

Physiologically based Pharmacokinetic (PBPK) models are used for predictions of internal or target dose from environmental and pharmacologic chemical exposures. Their use in human risk assessment is dependent on the nature of databases (animal or human) used to develop and test them, and includes extrapolations across species, experimental paradigms, and determination of variability of response within human populations. Integration of state-of-the science PBPK modeling with emerging computational toxicology models is critical for extrapolation between in vitro exposures, in vivo physiologic exposure, whole organism responses, and long-term health outcomes. This special issue contains papers that can provide the basis for future modeling efforts and provide bridges to emerging toxicology paradigms. In this overview paper, we present an overview of the field and introduction for these papers that includes discussions of model development, best practices, risk-assessment applications of PBPK models, and limitations and bridges of modeling approaches for future applications. Specifically, issues addressed include: (a) increased understanding of human variability of pharmacokinetics and pharmacodynamics in the population, (b) exploration of mode of action hypotheses (MOA), (c) application of biological modeling in the risk assessment of individual chemicals and chemical mixtures, and (d) identification and discussion of uncertainties in the modeling process.

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.

Relative pesticide and exposure route contribution to aggregate and cumulative dose in young farmworker children

The Child-Specific Aggregate Cumulative Human Exposure and Dose (CACHED) framework integrates micro-level activity time series with mechanistic exposure equations, environmental concentration distributions, and physiologically-based pharmacokinetic components to estimate exposure for multiple routes and chemicals. CACHED was utilized to quantify cumulative and aggregate exposure and dose estimates for a population of young farmworker children and to evaluate the model for chlorpyrifos and diazinon. Micro-activities of farmworker children collected concurrently with residential measurements of pesticides were used in the CACHED framework to simulate 115,000 exposure scenarios and quantify cumulative and aggregate exposure and dose estimates. Modeled metabolite urine concentrations were not statistically different than concentrations measured in the urine of children, indicating that CACHED can provide realistic biomarker estimates. Analysis of the relative contribution of exposure route and pesticide indicates that in general, chlorpyrifos non-dietary ingestion exposure accounts for the largest dose, confirming the importance of the micro-activity approach. The risk metrics computed from the 115,000 simulations, indicate that greater than 95% of these scenarios might pose a risk to children's health from aggregate chlorpyrifos exposure. The variability observed in the route and pesticide contributions to urine biomarker levels demonstrate the importance of accounting for aggregate and cumulative exposure in establishing pesticide residue tolerances in food.