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

Advancing understanding of human variability through toxicokinetic modeling, in vitro-in vivo extrapolation, and new approach methodologies

The merging of physiology and toxicokinetics, or pharmacokinetics, with computational modeling to characterize dosimetry has led to major advances for both the chemical and pharmaceutical research arenas. Driven by the mutual need to estimate internal exposures where in vivo data generation was simply not possible, the application of toxicokinetic modeling has grown exponentially in the past 30 years. In toxicology the need has been the derivation of quantitative estimates of toxicokinetic and toxicodynamic variability to evaluate the suitability of the tenfold uncertainty factor employed in risk assessment decision-making. Consideration of a host of physiologic, ontogenetic, genetic, and exposure factors are all required for comprehensive characterization. Fortunately, the underlying framework of physiologically based toxicokinetic models can accommodate these inputs, in addition to being amenable to capturing time-varying dynamics. Meanwhile, international interest in advancing new approach methodologies has fueled the generation of in vitro toxicity and toxicokinetic data that can be applied in in vitro-in vivo extrapolation approaches to provide human-specific risk-based information for historically data-poor chemicals. This review will provide a brief introduction to the structure and evolution of toxicokinetic and physiologically based toxicokinetic models as they advanced to incorporate variability and a wide range of complex exposure scenarios. This will be followed by a state of the science update describing current and emerging experimental and modeling strategies for population and life-stage variability, including the increasing application of in vitro-in vivo extrapolation with physiologically based toxicokinetic models in pharmaceutical and chemical safety research. The review will conclude with case study examples demonstrating novel applications of physiologically based toxicokinetic modeling and an update on its applications for regulatory decision-making. Physiologically based toxicokinetic modeling provides a sound framework for variability evaluation in chemical risk assessment.

Combining QSAR and SSD to predict aquatic toxicity and species sensitivity of pyrethroid and organophosphate pesticides

The widespread use of pyrethroid and organophosphate pesticides necessitates accurate toxicity predictions for regulatory compliance. In this study QSAR and SSD models for six pyrethroid and four organophosphate compounds using QSAR Toolbox and SSD Toolbox have been developed. The QSAR models, described by the formula 48 h-EC50 or 96 h-LC50 = x + y * log Kow, were validated for predicting 48 h-EC50 values for acute Daphnia toxicity and 96 h-LC50 values for acute fish toxicity, meeting criteria of n ≥10, r2 ≥0.7, and Q2 >0.5. Predicted 48 h-EC50 values for pyrethroids ranged from 3.95 × 10-5 mg/L (permethrin) to 8.21 × 10-3 mg/L (fenpropathrin), and 96 h-LC50 values from 3.89 × 10-5 mg/L (permethrin) to 1.68 × 10-2 mg/L (metofluthrin). For organophosphates, 48 h-EC50 values ranged from 2.00 × 10-5 mg/L (carbophenothion) to 3.76 × 10-2 mg/L (crufomate) and 96 h-LC50 values from 3.81 × 10-3 mg/L (carbophenothion) to 12.3 mg/L (crufomate). These values show a good agreement with experimental data, though some, like Carbophenothion, overestimated toxicity. HC05 values, indicating hazardous concentrations for 5% of species, range from 0.029 to 0.061 µg/L for pyrethroids and 0.030 to 0.072 µg/L for organophosphates. These values aid in establishing environmental quality standards (EQS). Compared to existing EQS, HC05 values for pyrethroids were less conservative, while those for organophosphates were comparable.

Current status and future directions for a neurotoxicity hazard assessment framework that integrates in silico approaches

Neurotoxicology is the study of adverse effects on the structure or function of the developing or mature adult nervous system following exposure to chemical, biological, or physical agents. The development of more informative alternative methods to assess developmental (DNT) and adult (NT) neurotoxicity induced by xenobiotics is critically needed. The use of such alternative methods including in silico approaches that predict DNT or NT from chemical structure (e.g., statistical-based and expert rule-based systems) is ideally based on a comprehensive understanding of the relevant biological mechanisms. This paper discusses known mechanisms alongside the current state of the art in DNT/NT testing. In silico approaches available today that support the assessment of neurotoxicity based on knowledge of chemical structure are reviewed, and a conceptual framework for the integration of in silico methods with experimental information is presented. Establishing this framework is essential for the development of protocols, namely standardized approaches, to ensure that assessments of NT and DNT based on chemical structures are generated in a transparent, consistent, and defendable manner.

Designing QSARs for Parameters of High-Throughput Toxicokinetic Models Using Open-Source Descriptors

The intrinsic metabolic clearance rate (Cl_int) and the fraction of the chemical unbound in plasma (f_up) serve as important parameters for high-throughput toxicokinetic (TK) models, but experimental data are limited for many chemicals. Open-source quantitative structure-activity relationship (QSAR) models for both parameters were developed to offer reliable in silico predictions for a diverse set of chemicals regulated under the U.S. law, including pharmaceuticals, pesticides, and industrial chemicals. As a case study to demonstrate their utility, model predictions served as inputs to the TK component of a risk-based prioritization approach based on bioactivity/exposure ratios (BERs), in which a BER < 1 indicates that exposures are predicted to exceed a biological activity threshold. When applied to a subset of the Tox21 screening library (6484 chemicals), we found that the proportion of chemicals with BER <1 was similar using either in silico (1133/6484; 17.5%) or in vitro (148/848; 17.5%) parameters. Further, when considering only the chemicals in the Tox21 set with in vitro data, there was a high concordance of chemicals classified with either BER <1 or >1 using either in silico or in vitro parameters (767/848, 90.4%). Thus, the presented QSARs may be suitable for prioritizing the risk posed by many chemicals for which measured in vitro TK data are lacking.

Enantioselective degradation of the chiral alpha-cypermethrin and detection of its metabolites in five plants

Alpha-cypermethrin (α-cypermethrin), an important chiral pyrethroid insecticide, is frequently detected in human samples. Because of the possible human health risks caused by α-cypermethrin, we studied dynamics, residues, and metabolism of α-cypermethrin in five common vegetables (tomato, cucumber, rape, cabbage, and pepper) on enantiomeric levels after foliar spray. α-Cypermethrin was qualified by a HP-5 column and its enantiomers could be separated by gas chromatograph (GC) using a BGB-172 chiral column. The results of degradation showed that α-cypermethrin dissipated rapidly in vegetables with half-lives being only 2.85-8.88 days. Stereoselective degradation was observed on pepper and cucumber while the two metabolites (cis-DCCA and 3-PBA) of α-cypermethrin were not detected during its dissipation in all plants. This is the first evidence of enantioselective degradation of α-cypermethrin in the five common vegetables and the results should be considered in future environmental risk and food safety evaluations.

In Silico Prediction for Intestinal Absorption and Brain Penetration of Chemical Pesticides in Humans

Intestinal absorption and brain permeation constitute key parameters of toxicokinetics for pesticides, conditioning their toxicity, including neurotoxicity. However, they remain poorly characterized in humans. The present study was therefore designed to evaluate human intestine and brain permeation for a large set of pesticides (n = 338) belonging to various chemical classes, using an in silico graphical BOILED-Egg/SwissADME online method based on lipophilicity and polarity that was initially developed for drugs. A high percentage of the pesticides (81.4%) was predicted to exhibit high intestinal absorption, with a high accuracy (96%), whereas a lower, but substantial, percentage (38.5%) displayed brain permeation. Among the pesticide classes, organochlorines (n = 30) constitute the class with the lowest percentage of intestine-permeant members (40%), whereas that of the organophosphorus compounds (n = 99) has the lowest percentage of brain-permeant chemicals (9%). The predictions of the permeations for the pesticides were additionally shown to be significantly associated with various molecular descriptors well-known to discriminate between permeant and non-permeant drugs. Overall, our in silico data suggest that human exposure to pesticides through the oral way is likely to result in an intake of these dietary contaminants for most of them and brain permeation for some of them, thus supporting the idea that they have toxic effects on human health, including neurotoxic effects.

Inhibition of Human Drug Transporter Activities by the Pyrethroid Pesticides Allethrin and Tetramethrin

Pyrethroids are widely-used chemical insecticides, to which humans are commonly exposed, and known to alter functional expression of drug metabolizing enzymes. Limited data have additionally suggested that drug transporters, that constitute key-actors of the drug detoxification system, may also be targeted by pyrethroids. The present study was therefore designed to analyze the potential regulatory effects of these pesticides towards activities of main ATP-binding cassette (ABC) and solute carrier (SLC) drug transporters, using transporter-overexpressing cells. The pyrethroids allethrin and tetramethrin were found to inhibit various ABC and SLC drug transporters, including multidrug resistance-associated protein (MRP) 2, breast cancer resistance protein (BCRP), organic anion transporter polypeptide (OATP) 1B1, organic anion transporter (OAT) 3, multidrug and toxin extrusion transporter (MATE) 1, organic cation transporter (OCT) 1 and OCT2, with IC50 values however ranging from 2.6 μM (OCT1 inhibition by allethrin) to 77.6 μM (OAT3 inhibition by tetramethrin) and thus much higher than pyrethroid concentrations (in the nM range) reached in environmentally pyrethroid-exposed humans. By contrast, allethrin and tetramethrin cis-stimulated OATP2B1 activity and failed to alter activities of OATP1B3, OAT1 and MATE2-K, whereas P-glycoprotein activity was additionally moderately inhibited. Twelve other pyrethoids used at 100 μM did not block activities of the various investigated transporters, or only moderately inhibited some of them (inhibition by less than 50%). In silico analysis of structure-activity relationships next revealed that molecular parameters, including molecular weight and lipophilicity, are associated with transporter inhibition by allethrin/tetramethrin and successfully predicted transporter inhibition by the pyrethroids imiprothrin and prallethrin. Taken together, these data fully demonstrated that two pyrethoids, i.e., allethrin and tetramethrin, can act as regulators of the activity of various ABC and SLC drug transporters, but only when used at high and non-relevant concentrations, making unlikely any contribution of these transporter activity alterations to pyrethroid toxicity in environmentally exposed humans.

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.

Chiral Insecticide α-Cypermethrin and Its Metabolites: Stereoselective Degradation Behavior in Soils and the Toxicity to Earthworm Eisenia fetida

The enantioselective degradation of the widely used chiral insecticide α-cypermethrin in soils has been investigated, and its main metabolites cis-3-(2',2'-dichlorovinyl)-2,2-dimethylcyclopropane carboxylic acid (cis-DCCA) and 3-phenoxybenzoic acid (3-PBA), which have potential environmental problems, have also been determined. The enantiomers of α-cypermethrin were separated on Chiralcel OD chiral columns by high-performance liquid chromatography (HPLC) under normal phase, and the metabolites were detected by gas chromatography (GC) after derivatization. The results of the degradation showed that α-cypermethrin dissipated in soils with relatively long half-lives of 12.70-47.08 days and obvious stereoselective degradation of the two enantiomers was observed in the five soils, with enantiomeric fraction (EF) from 0.55 to 0.61 after 42 days, indicating that (+)-(1R,cis,αS) enantiomer was preferentially degraded. cis-DCCA and 3-PBA were formed in all of the soils, and it was found that the amount generated was related to the soil pH. cis-DCCA was easily generated in the acidic soils, while more 3-PBA tended to be generated in the soil of pH over 7. To evaluate the impact on soil animals, the toxicity, including the combined toxicity of cis-DCCA, 3-PBA, and α-cypermethrin, to earthworm (Eisenia fetida) was studied. The results of enantioselective transformation of α-cypermethrin in soils and the toxicity of α-cypermethrin and its metabolites to earthworm have some implications for environmental risk and food safety evaluations.

Characterization of α-cypermethrin exposure in Egyptian agricultural workers

Pyrethroids are neurotoxic insecticides that exert their effects by prolonging the open time of sodium channels, which increases the duration of neuronal excitation. α-cypermethrin (αCM) is derived from the 8-stereoisomers that together make up the pyrethroid cypermethrin, which is one of the most common pyrethroids being used in agriculture throughout the world. The objective of this study was to characterize the occupational exposure to αCM in a cohort of Egyptian agriculture workers (n=37) before, during and after 6-10 consecutive days of application of αCM to cotton fields. Daily spot urine specimens were collected and analyzed by GC-MS NCI for the αCM metabolites 3-phenoxybenzoic acid (3-PBA) and cis-3-(2',2'-dichlorovinyl)-2,2-dimethylcyclopropane carboxylic acid (cis-DCCA). Prior to αCM application, median urinary levels of 3-PBA (4.59nmol/g creatinine) were greater than cis-DCCA (0.33nmole/g creatinine) demonstrating low background exposures to pyrethroids. During the application period for αCM, median urinary levels of both biomarkers increased (13.44nmol 3-PBA/g creatinine and 7.76nmol cis-DCCA/g creatinine) and ranged from 2.3-93.96nmol 3-PBA/g creatinine and 0.09-90.94nmol cis-DCCA/g creatinine, demonstrating that workers had a wide range of exposures to αCM. The data also demonstrate that pesticide applicators had greater exposures to αCM than workers who play a supporting role in the seasonal application of pesticides on the cotton crop. Urinary cis-DCCA and 3-PBA concentrations were elevated at 7-11 days after the cessation of αCM application, compared to baseline levels. This study is the first to use these biomarkers to quantify occupational exposures specifically to αCM. This urinary biomarker data will be useful for estimating daily internal dose, comparing exposures across job categories within the Egyptian pesticide application teams, and for modeling human exposures to αCM.

New developments in the evolution and application of the WHO/IPCS framework on mode of action/species concordance analysis

The World Health Organization/International Programme on Chemical Safety mode of action/human relevance framework has been updated to reflect the experience acquired in its application and extend its utility to emerging areas in toxicity testing and non-testing methods. The underlying principles have not changed, but the framework’s scope has been extended to enable integration of information at different levels of biological organization and reflect evolving experience in a much broader range of potential applications. Mode of action/species concordance analysis can also inform hypothesis-based data generation and research priorities in support of risk assessment.

The modified framework is incorporated within a roadmap, with feedback loops encouraging continuous refinement of fit-for-purpose testing strategies and risk assessment. Important in this construct is consideration of dose–response relationships and species concordance analysis in weight of evidence. The modified Bradford Hill considerations have been updated and additionally articulated to reflect increasing experience in application for cases where the toxicological outcome of chemical exposure is known.

The modified framework can be used as originally intended, where the toxicological effects of chemical exposure are known, or in hypothesizing effects resulting from chemical exposure, using information on putative key events in established modes of action from appropriate in vitro or in silico systems and other lines of evidence.

This modified mode of action framework and accompanying roadmap and case examples are expected to contribute to improving transparency in explicitly addressing weight of evidence considerations in mode of action/species concordance analysis based on both conventional data sources and evolving methods.

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.

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.

Methodologies, bioindicators, and biomarkers for assessing gender-related differences in wildlife exposed to environmental chemicals

Male and female organisms may have significant differences in their exposure, toxicokinetics, and response to chemicals, but gender effects have received relatively little attention, often viewed as a confounder rather than of primary importance. In this paper, we examine some of the key issues and methodologies for incorporating gender in studies of the effects of chemicals on wildlife, and explore bioindicators and biomarkers of gender effects. Examining gender-related differences in response to chemicals is complicated in wildlife because of the vast array of species, and differences in niches, lifespans, reproductive cycles and modes, and population dynamics. Further, organisms are more at risk in some ecosystems than others, which may increase the magnitude of effects. Only by studying wild animals, especially native species, can we truly understand the potential impact of gender-specific effects of chemical exposure on populations. Several factors affect gender-related differences in responses to chemicals, including exposure, age, size, seasonality, and genetic and phenotypic variation. There are clear examples where gender-related differences have had significant effects on reproductive success and population stability, including destabilization of gamete release in invertebrates, and alterations of endocrine and neuroendocrine system functioning in vertebrates. A wide range of new technologies and methods are available for examining gender-related differences in responses to chemicals. We provide examples that show that there are gender-related differences in responses to chemicals that have significant biological effects, and these gender-related differences should be taken into account by scientists, regulators, and policy makers, as well as the public.