A physiologically-based pharmacokinetic model for disposition of 2,3,7,8-TCDD in fathead minnow and medaka

Enhanced prediction of internal concentrations of phenolic endocrine disrupting chemicals and their metabolites in fish by a physiologically based toxicokinetic incorporating metabolism (PBTK-MT) model

Bisphenol A (BPA), 4-nonylphenol (4-NP), and triclosan (TCS) are phenolic endocrine disrupting chemicals (EDCs), which are widely detected in aquatic environments and further bioaccumulated and metabolized in fish. Physiologically based toxicokinetic (PBTK) models have been used to describe the absorption, distribution, metabolism, and excretion (ADME) of parent compounds in fish, whereas the metabolites are less explored. In this study, a PBTK incorporating metabolism (PBTK-MT) model for BPA, 4-NP, and TCS was established to enhance the performance of the traditional PBTK model. The PBTK-MT model comprised 16 compartments, showing great accuracy in predicting the internal concentrations of three compounds and their glucuronidated and sulfated conjugates in fish. The impact of typical hepatic metabolism on the PBTK-MT model was successfully resolved by optimizing the mechanism for deriving the partition coefficients between the blood and liver. The PBTK-MT model exhibited a potential data gap-filling capacity for unknown parameters through a backward extrapolation approach of parameters. Model sensitivity analysis suggested that only five parameters were sensitive in at least two PBTK-MT models, while most parameters were insensitive. The PBTK-MT model will contribute to a well understanding of the environmental behavior and risks of pollutants in aquatic biota.

Intestinal uptake and low transformation increase the bioaccumulation of inorganic arsenic in freshwater zebrafish

Arsenate [As(V)] is the main form of arsenic (As) present in freshwater taken up by freshwater fish. Data on the main uptake tissue, biotransformation, and bioaccumulation in freshwater fish exposed to As(V) were limited, and the reasons for its bioaccumulation in the muscle tissue of freshwater fish remain undetermined. Accordingly, we simulated bioaccumulation and depuration in zebrafish (Danio rerio) exposed to waterborne As(V) by employing a six-compartment physiologically based pharmacokinetic model and As speciation analysis. Modeling and biotransformation suggested that intestines were the main uptake site for waterborne As(V), instead of the gills. This novel finding was evidenced by the higher As transfer constant from water to intestines (k₀₃ = 1.52 × 10^-4 L d^-1) compared to gills (k₀₂ = 5.28 × 10^-5 L d^-1). The low concentration and percentage of arsenobetaine (AsB) in the intestines suggested a weak ability to synthesize AsB. Our results showed a substantial proportion of inorganic As in intestines and a relatively substantial percentage in muscle tissue. Therefore, high As(V) uptake in the intestines and lack of biotransformation contributed to high bioaccumulation of inorganic As in freshwater fish. Inorganic As posed concerns due to the human health risks associated with consuming As(V)-contaminated fish and should be addressed.

Tissue-Specific Accumulation, Biotransformation, and Physiologically Based Toxicokinetic Modeling of Benzotriazole Ultraviolet Stabilizers in Zebrafish (Danio rerio)

Benzotriazole ultraviolet stabilizers (BUVSs) are high-production-volume chemicals with ubiquitous occurrence in the aquatic environment. However, little is known about their bioconcentration and biotransformation, and physiologically based toxicokinetic (PBTK) models for BUVSs are lacking. This study selected six BUVSs for which experiments were performed with zebrafish (Danio rerio) exposed to two different levels (0.5 and 10 μg·L^-1). Higher kinetic bioconcentration factors (BCFs) were observed at the lower exposure level with environmental relevance, with BCF of 3.33 × 10³ L·kg^-1 for 2-(2-hydroxy-3,5-di-tert-butylphenyl)-5-chlorobenzotriazole (UV-327). This phenomenon was interpreted by a nonlinear adsorption mechanism, where binding with specific protein sites contributes to bioconcentration. Muscle exhibited the lowest accumulation, in which depuration half-life of UV-327 was 19.5 d. In kidney, muscle, ovary, gill, and skin, logBCF increased with increase in log K_OW of the BUVSs until log K_OW was ca. 6.5, above which logBCF decreased. However, the trend was not observed in the liver and intestine. Six biotransformation products were identified and mainly accumulated in the liver and intestine. Considering the nonlinear adsorption mechanism in the PBTK model, the prediction accuracy of the model was improved, highlighting the binding of xenobiotics with specific protein sites in assessing the bioconcentration of chemicals for their risk assessment.

A population approach for the estimation of methylmercury ToxicoKinetics in red mullets

It is known that, as the vast majority of the anthropogenically emitted mercury can be found in aquatic ecosystems, where several methylating bacteria are present, fish consumption represents the most critical intake source of the most toxic form of mercury, the methylmercury (MeHg). The aim of this work is to predict MeHg levels in the fish muscles which, being the edible portion, are part of the human diet. A physiologically based toxicokinetics model was developed to evaluate the kinetics of MeHg in red mullets. Fishes were described by means of a multi-compartment model including stomach, gut, blood, muscles and an additional compartment virtually encompassing all the remaining organs. Absorption, distribution and excretion were modelled considering different MeHg routes of administration and excretion: intake by ingestion of contaminated food, intake and elimination through inhalation-exhalation and excretion through feces. The model has been firstly validated on Terapon jarbua fish (using the weighted least squares method for parameter estimation) to be subsequently readapted to predict methylmercury concentrations in the muscle of red mullets (using an approximate Bayesian computation approach). This simple multicompartmental model could be considered part, a link in the chain, of a wider more complex project aiming at tracking the fate of MeHg from polluted seawater to the human end consumer. The present study could be useful to surveillance organizations in order to carry out a more comprehensive and informed risk assessment analysis and to take appropriate preventive measures by evaluating possible new MeHg concentration thresholds to minimize public health hazards.

Physiologically Based Pharmacokinetic Model for the Biotransportation of Arsenic in Marine Medaka (Oryzias melastigma)

The toxicity of arsenic (As) targets specific tissues of organisms, while the biotransportation of As among the tissues of fish remains poorly understood. In the present study, radiotracer techniques followed by a physiologically based pharmacokinetic (PBPK) modeling were applied to simulate the biotransportation (absorption, distribution, and elimination) of ⁷³As(V) and biotransformation of As(V) in the marine medaka Oryzias melastigma after waterborne As exposure. Fish were simulated by a six-compartment model by assuming that blood was the intermediate exchange among different compartments (gill, intestine, liver, head, and carcass). Modeling suggested that intestine and gill were the uptake, exchange, as well as elimination sites of waterborne As, while carcass and head were the main storage sites. Intestine played a vital role in the metabolism of As(V) by biotransforming inorganic As into arsenobetaine (AsB), possibly because of the important role of gut microbiota. The correlation between the PBPK model constants and the As speciation (e.g., AsB %, inorganic As %, and methylated As %) indicated that AsB tended to be stored in the tissues rather than being depurated, while inorganic and methylated As were more easily transferred from tissues to the blood and eliminated. Modeling simulation coupling with biotransformation for the first time demonstrated that the fish intestine was the main metabolic site, and synthesis of AsB as mediated by the microbiota in the intestine contributed to the high As bioaccumulation in marine fish.

Physiologically based toxicokinetics (PBTK) models for pharmaceuticals and personal care products in wild common carp (Cyprinus carpio)

Pharmaceutical and personal care products (PPCPs) are regarded as "pseudo-persistent" pollutants due to their being continuously loaded into the aquatic environment. Physiologically based toxicokinetics (PBTK) models that can quantitatively describe absorption, distribution, metabolism and excretion processes of chemicals in biota are of importance to predict internal exposure (e.g. doses at specific target tissues/organs) from external exposure concentrations. In this study, PBTK models with up to six compartments including brain, liver, kidney, gills, richly perfused tissues and poorly perfused tissues, were developed for predicting internal distribution of 10 PPCPs in wild common carp (Cyprinus carpio). The PBTK predicted concentrations were close to the measured ones, with deviations less than 1 log unit for most of PPCPs. Sensitivity analysis showed that various partition coefficients of the chemicals exerted significant influence on model outputs.

Generic physiologically-based toxicokinetic modelling for fish: Integration of environmental factors and species variability

One of the goals of environmental risk assessment is to protect the whole ecosystem from adverse effects resulting from exposure to chemicals. Many research efforts have aimed to improve the quantification of dose-response relationships through the integration of toxicokinetics. For this purpose, physiologically-based toxicokinetic (PBTK) models have been developed to estimate internal doses from external doses in a time-dependent manner. In this study, a generic PBTK model was developed and adapted for rainbow trout (Onchorhynchus mykiss), zebrafish (Danio rerio), fathead minnow (Pimephales promelas), and three-spined stickleback (Gasterosteus aculeatus). New mechanistic approaches were proposed for including the effects of growth and temperature in the model. Physiological parameters and their inter-individual variability were estimated based on the results of extensive literature searches or specific experimental data. The PBTK model was implemented for nine environmental contaminants (with log k_ow from -0.9 to 6.8) to predict whole-body concentrations and concentrations in various fish's organs. Sensitivity analyses were performed for a lipophilic and a hydrophilic compound to identify which parameters have most impact on the model's outputs. Model predictions were compared with experimental data according to dataset-specific exposure scenarios and were accurate: 50% of predictions were within a 3-fold factor for six out of nine chemicals and 75% of predictions were within a 3-fold factor for three of the most lipophilic compounds studied. Our model can be used to assess the influence of physiological and environmental factors on the toxicokinetics of chemicals and provide guidance for assessing the effect of those critical factors in environmental risk assessment.

In vitro to in vivo extrapolation for high throughput prioritization and decision making

In vitro chemical safety testing methods offer the potential for efficient and economical tools to provide relevant assessments of human health risk. To realize this potential, methods are needed to relate in vitro effects to in vivo responses, i.e., in vitro to in vivo extrapolation (IVIVE). Currently available IVIVE approaches need to be refined before they can be utilized for regulatory decision-making. To explore the capabilities and limitations of IVIVE within this context, the U.S. Environmental Protection Agency Office of Research and Development and the National Toxicology Program Interagency Center for the Evaluation of Alternative Toxicological Methods co-organized a workshop and webinar series. Here, we integrate content from the webinars and workshop to discuss activities and resources that would promote inclusion of IVIVE in regulatory decision-making. We discuss properties of models that successfully generate predictions of in vivo doses from effective in vitro concentration, including the experimental systems that provide input parameters for these models, areas of success, and areas for improvement to reduce model uncertainty. Finally, we provide case studies on the uses of IVIVE in safety assessments, which highlight the respective differences, information requirements, and outcomes across various approaches when applied for decision-making.

Toxicokinetic models and related tools in environmental risk assessment of chemicals

Environmental risk assessment of chemicals for the protection of ecosystems integrity is a key regulatory and scientific research field which is undergoing constant development in modelling approaches and harmonisation with human risk assessment. This review focuses on state-of-the-art toxicokinetic tools and models that have been applied to terrestrial and aquatic species relevant to environmental risk assessment of chemicals. Both empirical and mechanistic toxicokinetic models are discussed using the results of extensive literature searches together with tools and software for their calibration and an overview of applications in environmental risk assessment. These include simple tools such as one-compartment models, multi-compartment models to physiologically-based toxicokinetic (PBTK) models, mostly available for aquatic species such as fish species and a number of chemical classes including plant protection products, metals, persistent organic pollutants, nanoparticles. Data gaps and further research needs are highlighted.

Homeostatic regulation of copper in a marine fish simulated by a physiologically based pharmacokinetic model

Copper (Cu) is an essential yet potentially toxic metal, thus delicate homeostatic controls are developed in the fish. In this study, a physiologically based pharmacokinetic (PBPK) model was developed to simulate the homeostatic regulation of Cu in a marine fish (Terapon jarbua) under dietary and waterborne exposures. In this model, fish were schematized as a six-compartment model, with the intestine being divided into two sub-compartments (chyme and gut wall). The blood was assumed to be the "carrier" distributing Cu into different compartments. The transfer rates between different compartments were determined in fish during Cu exposure (20 d) and depuration (20 d). The differences in Cu transfer from chyme to gut wall between dietary and waterborne treatments suggested that the intestine regulated the dietary uptake and re-absorption of Cu from the chyme. The extremely low uptake rate constant (0.0013 d^-1) for gills under waterborne exposure indicated that gills strongly restricted Cu uptake from the ambient water. For both treatments, the liver had considerable input rate through the enterohepatic circulation and comparably high exchange rate with the blood, suggesting that the liver can efficiently accumulate newly absorbed Cu. The differences in Cu output from the liver between dietary and waterborne treatments suggested that it can effectively regulate the redistribution of Cu. All of these observations demonstrated that the liver played the central role in Cu homeostasis by serving as the main depository and distributing center. Modeling results also indicated that renal and branchial excretion was of minor importance, whereas biliary excretion combined with defecation played the most important role in whole-body Cu elimination in marine fish. The effective regulation by the "Blood-Liver-Intestine" cycle could be the main reason for the relatively low levels of Cu in fish.

Editor's Highlight: Computational Modeling of Plasma Vitellogenin Alterations in Response to Aromatase Inhibition in Fathead Minnows

In vertebrates, conversion of testosterone into 17β-estradiol (E2) is catalyzed by cytochrome P450 (CYP) 19A aromatase. An important role of E2 in oviparous vertebrates such as fish is stimulation of hepatic synthesis of the glycolipoprotein vitellogenin (VTG), an egg yolk precursor essential to oocyte development and larval survival. In fathead minnows (FHMs) (Pimephales promelas) exposed to the aromatase inhibitor fadrozole, plasma VTG levels do not change in concert with plasma E2 levels. Specifically, while plasma VTG and E2 levels both drop quickly when aromatase is first inhibited, the recovery of plasma VTG upon cessation of aromatase inhibition is substantially delayed relative to the recovery of plasma E2. We modified an existing computational model of the FHM hypothalamic-pituitary-gonadal axis to evaluate alternative hypotheses that might explain this delay. In the first hypothesis, a feedback loop involving active transport of VTG from the blood into the ovary is used. The activity of the transporter is negatively regulated by ovarian VTG. In the second hypothesis, a type 1 coherent feed-forward loop is implemented in the liver. This loop has 2 arms, both requiring E2 activation. The first arm describes direct, canonical E2-driven transcriptional induction of VTG, and the second describes an E2-driven intermediate transcriptional regulator that is also required for VTG synthesis. Both hypotheses accurately described the observed VTG dynamics. This result could be used to guide design of laboratory experiments intended to determine if either of the motifs, or perhaps even both of them, actually do control VTG dynamics in FHMs exposed to aromatase inhibitors.