Toxicodynamic Processes Research Articles

Elucidating the spatial distribution of organic contaminants and their biotransformation products in amphipod tissue by MALDI- and DESI-MS-imaging

The application of mass spectrometry imaging (MSI) is a promising tool to analyze the spatial distribution of organic contaminants in organisms and thereby improve the understanding of toxicokinetic and toxicodynamic processes. MSI is a common method in medical research but has been rarely applied in environmental science. In the present study, the suitability of MSI to assess the spatial distribution of organic contaminants and their biotransformation products (BTPs) in the aquatic invertebrate key species Gammarus pulex was studied. Gammarids were exposed to a mixture of common organic contaminants (carbamazepine, citalopram, cyprodinil, efavirenz, fluopyram and terbutryn). The distribution of the parent compounds and their BTPs in the organisms was analyzed by two MSI methods (MALDI- and DESI-HRMSI) after cryo-sectioning, and by LC-HRMS/MS after dissection into different organ compartments. The spatial distribution of contaminats in gammarid tissue could be successfully analyzed by the different analytical methods. The intestinal system was identified as the main site of biotransformation, possibly due to the presence of biotransforming enzymes. LC-HRMS/MS was more sensitive and provided higher confidence in BTP identification due to chromatographic separation and MS/MS. DESI was found to be the more sensitive MSI method for the analyzed contaminants, whereas additional biomarkers were found using MALDI. The results demonstrate the suitability of MSI for investigations on the spatial distribution of accumulated organic contaminants. However, both MSI methods required high exposure concentrations. Further improvements of ionization methods would be needed to address environmentally relevant concentrations.

High-Throughput Screening to Advance In Vitro Toxicology: Accomplishments, Challenges, and Future Directions.

Traditionally, chemical toxicity is determined by in vivo animal studies, which are low throughput, expensive, and sometimes fail to predict compound toxicity in humans. Due to the increasing number of chemicals in use and the high rate of drug candidate failure due to toxicity, it is imperative to develop in vitro, high-throughput screening methods to determine toxicity. The Tox21 program, a unique research consortium of federal public health agencies, was established to address and identify toxicity concerns in a high-throughput, concentration-responsive manner using a battery of in vitro assays. In this article, we review the advancements in high-throughput robotic screening methodology and informatics processes to enable the generation of toxicological data, and their impact on the field; further, we discuss the future of assessing environmental toxicity utilizing efficient and scalable methods that better represent the corresponding biological and toxicodynamic processes in humans.

Phycotoxins and marine annelids – A global review

Several species of microalgae can produce potent phycotoxins that negatively affect aquatic organisms and their consumers following different exposure routes, as well as toxicokinetic (TK) and toxicodynamic (TD) processes. Benthic organisms are especially vulnerable as they are exposed to both benthic and planktonic species causative of harmful algal blooms (HABs). While benthic algae can come into direct contact with annelids during substrate remobilization, planktonic cells can settle to the bottom mostly during senescence and/or encystment stages, and in shallow and calm waters. We performed a systematic, qualitative review of the literature on the phycotoxin TK and TD processes in marine annelids, summarizing the most relevant findings and general trends. Besides, by using innovative analytical/statistical approaches, we were able to detect patterns and gaps in the current literature, thus pointing to future research directions. We retrieved and analyzed studies involving diarrhetic shellfish toxins (DSTs), paralytic shellfish toxins (PSTs), brevetoxins (PbTXs), domoic acid (DA), as well as palytoxin and its congeners, the ovatoxins (treated together as PLTXs). It is worth mentioning that studies evaluating other phycotoxins (e.g., ciguatoxins, yessotoxins) were not found in the literature. The absence of data on PbTXs, PSTs and DA is the largest gap hampering TK assessment in annelids, although some relevant information on TD is already available. Whereas lethal effects from DSTs have not been reported, more potent toxins like PbTXs, PSTs, DA and those grouped as PLTX-like compounds can cause mortality and/or marked decrease in annelid abundance. In addition, phycotoxins have been linked to sublethal effects on annelid cells. Although very sparse, field and laboratory studies offer strong evidence that annelids may be reliable indicators of toxin exposure and their negative effects during both early and later stages of HABs in marine environments. Besides quickly responding to these compounds at both organismic and suborganismic levels, annelids are easily found in areas affected by HABs. The use of annelids in future investigations evaluating the action mechanisms of toxic microalgae on marine invertebrates should be thus encouraged. In this case, the choice for widely dispersed and numerically dominant species of annelids would strengthen the validation and extrapolation of results from risk assessments in areas affected by HABs worldwide.

Effect of dissolved organic matter on the bioavailability and toxicity of cadmium in zebrafish larvae: Determination based on toxicokinetic–toxicodynamic processes

The presence of dissolved organic matter (DOM) strongly influences the bioavailability of metals in aquatic environments; however, the association between the binding activities and the concentrations of DOM compositions is not well documented, leading to uncertainties in metal toxicity assessment. We creatively quantify the mitigation and acceleration effects of DOM compositions on cadmium (Cd) bioaccumulation and toxicity in zebrafish larvae using abiotic ligand (ABLs) and biotic ligand (BLs) in a toxicokinetic–toxicodynamic (TK–TD) model. The BL–TK–TD model could accurately predict the protective effect of fulvic acid while overestimating the complexing capacity of citric acid. The model also could successfully simulate the protective effects of native DOM in most cases from 32 natural water bodies in China. The observed LC50 values of Cd showed a peak effect for the native DOM fraction comprising hydrophilic acidic contents (3.55 ± 0.44 mg L − 1) in natural water from 32 sites. The BL–TK–TD model provides practically useful information to identify the effect of different DOM compositions on metal bioavailability and toxicity in aquatic environments and guides future water management policies aimed at controlling aquatic heavy metal pollution.

Theme (concept) paper ‐ Advancing Aggregate Exposure to Chemicals in EU (ExpoAdvance)

Theme (concept) paper ‐ Advancing Aggregate Exposure to Chemicals in EU (ExpoAdvance)

Size- and sex-related sensitivity differences of aquatic crustaceans to imidacloprid

Field collected aquatic invertebrates are often used as test organisms in the refinement of the standard Tier 1 risk assessment of various pollutants. This approach can provide insights into the effects of pollutants on the natural environment. However, researchers often pragmatically select test organisms of a specific sex and/or size, which may not represent the sensitivity of the whole population. To investigate such intraspecies sensitivity differences, we performed standard acute toxicity and toxicokinetic tests with different size classes and sex of Gammarus pulex and Asellus aquaticus. Furthermore, toxicokinetics and toxicodynamics models were used to understand the mechanism of the intraspecies sensitivity differences. We used neonates, juveniles and male and female adults in separate dedicated experiments, in which we exposed the animals to imidacloprid and its bioactive metabolite, imidacloprid-olefin. For both species, we found that neonates were the most sensitive group. For G. pulex, the sensitivity decreased linearly with size, which can be explained by the size-related uptake rate constant in the toxicokinetic process and size-related threshold value in the toxicodynamic process. For A. aquaticus, female adults were least sensitive to imidacloprid, which could be explained by a low internal biotransformation of imidacloprid to imidacloprid-olefin. Besides, imidacloprid-olefin was more toxic than imidacloprid to A. aquaticus, with differences being 8.4 times for females and 2.7 times for males. In conclusion, we established size-related sensitivity differences for G. pulex and sex-related sensitivity for A. aquaticus, and intraspecies differences can be explained by both toxicokinetic and toxicodynamic processes. Our findings suggest that to protect populations in the field, we should consider the size and sex of focal organisms and that a pragmatic selection of test organisms of equal size and/or sex can underestimate the sensitivities of populations in the field.

Enantioselectivity of Pentedrone and Methylone on Metabolic Profiling in 2D and 3D Human Hepatocyte-like Cells.

Pentedrone and methylone can express stereoselectivity in toxicokinetic and toxicodynamic processes. Similarly, their chiral discrimination in metabolism, which was not yet evaluated, can result in different metabolic profiles and subsequent hepatotoxic effects. Therefore, the aim of this work was to assess, for the first time, both the hepatic cytotoxic and metabolic profile of pentedrone and methylone enantiomers using physiologically relevant in vitro models. The hepatotoxicity of these compounds was observed in a concentration-dependent manner in human stem-cell-derived hepatocyte-like cells (HLCs) cultured under 3D (3D-HLCs) and 2D (2D-HLCs) conditions. Enantioselectivity, on the other hand, was only shown for pentedrone (1 mM) in 3D-HLCs, being R-(−)-pentedrone the most cytotoxic. Furthermore, the metabolic profile was initially evaluated in human liver microsomes (HLM) and further demonstrated in 3D-HLCs and 2D-HLCs applying a gas chromatography coupled to a mass spectrometer (GC–MS) technique. Methylone and pentedrone showed distinct and preferential metabolic routes for their enantiomers, resulting in the production of differentiated metabolites; R-(+)-methylone and R-(−)-pentedrone are the most metabolized enantiomers. In conclusion, the results demonstrated enantioselectivity for pentedrone and methylone in the metabolic processes, with enantioselectivity in cytotoxicity for pentedrone.

Providing Adverse Outcome Pathways from the AOP-Wiki in a Semantic Web Format to Increase Usability and Accessibility of the Content.

Introduction:The AOP-Wiki is the main platform for the development and storage of adverse outcome pathways (AOPs). These AOPs describe mechanistic information about toxicodynamic processes and can be used to develop effective risk assessment strategies. However, it is challenging to automatically and systematically parse, filter, and use its contents. We explored solutions to better structure the AOP-Wiki content, and to link it with chemical and biological resources. Together, this allows more detailed exploration, which can be automated.Materials and Methods:We converted the complete AOP-Wiki content into resource description framework (RDF) triples. We used >20 ontologies for the semantic annotation of property–object relations, including the Chemical Information Ontology, Dublin Core, and the AOP Ontology.Results:The resulting RDF contains >122,000 triples describing 158 unique properties of >15,000 unique subjects. Furthermore, >3500 link-outs were added to 12 chemical databases, and >7500 link-outs to 4 gene and protein databases. The AOP-Wiki RDF has been made available at https://aopwiki.rdf.bigcat-bioinformatics.orgDiscussion:SPARQL queries can be used to answer biological and toxicological questions, such as listing measurement methods for all Key Events leading to an Adverse Outcome of interest. The full power that the use of this new resource provides becomes apparent when combining the content with external databases using federated queries.Conclusion:Overall, the AOP-Wiki RDF allows new ways to explore the rapidly growing AOP knowledge and makes the integration of this database in automated workflows possible, making the AOP-Wiki more FAIR.

Bioavailability and toxicity of silver nanoparticles: Determination based on toxicokinetic–toxicodynamic processes

Determining the bioavailability and toxicity mechanism of silver nanoparticles (AgNPs) is challenging as Ag+ is continuously released by external or internal AgNP dissolution in the actual exposure system (regardless of the laboratory or the natural environment). Here a novel pulsed-gradient Ag+ (AgNO3) exposure was conducted with zebrafish (Danio rerio) larvae to simulate dissolved gradient concentrations of Ag+ from polyvinylpyrrolidone (PVP)-coated AgNPs. The accumulation and toxicity of the pulsed-gradient Ag+ (AgNO3) and, in the meantime, the released Ag+ from PVP-AgNPs were predicted using a toxicokinetic–toxicodynamic (TK–TD) model with obtained Ag+ parameters. In order to further understand the possible mechanism of PVP-AgNP releasing Ag+ in the body, subcellular fractions (S9) of zebrafish were also used to incubate with AgNPs in vitro to mimic the realistic in vivo scenarios. In the TK process, in vivo analysis showed that AgNPs released around twice as many Ag+ into the body than were detected with a single Ag+ pulse-exposure system; this was supported by evidence that subcellular S9 fractions might cause the PVP-AgNPs to lose the capping agent and favor Ag+ release. In the TD process, toxicity (survival rate) was predicted by the total bodily Ag(I) concentration, suggesting that AgNP toxicity in larvae was mainly due to gradually released Ag+ rather than AgNPs themselves. This study helps clarify the role of Ag+ in AgNP toxicity and offers a novel framework by which to investigate the toxicity of metal nanoparticles and corresponding metal ions in biological systems.

Correlated Ni, Cu, and Zn Sensitivities of 8 Freshwater Algal Species and Consequences for Low-Level Metal Mixture Effects.

Predicting metal sensitivities and metal mixture interactions for species within each trophic level is essential to understand the effects of metals at the ecosystem level. The present study was set up to explore the correlations of metal sensitivities among species and if these sensitivities or metal mixture interactions are related to growth or morphological traits. The toxicity of Ni, Cu, and Zn on algal growth was tested for 8 freshwater algal species when dosed singly and in combinations in phosphorus-limiting static systems. The metal sensitivities on specific growth rate (10% effect concentrations expressed as free ion activities) varied 2 to 3 orders of magnitude among species depending on metal. These sensitivities were unrelated (p > 0.05) to their specific growth rate (0.7-1.8 d-1 ) or cell volume (100 -103 m3 cell-1 ). Species-specific differences in one or more toxicokinetic and/or toxicodynamic (TKTD) processes are likely at the basis of this variation. The log-transformed metal sensitivities positively correlated (p < 0.1) among the species in all 3 binary combinations (Ni-Cu, Ni-Zn, and Cu-Zn), suggesting that species have correlated TKTD rates for these metals. Furthermore, they would also predict stronger effects of metal mixtures on algal community biodiversity than what would be expected without a positive correlation. Low-level metal mixture effects varied similarly, largely among species and mixture interactions that were highly variable: ranging from synergistic to antagonistic relative to independent action during exponential growth, whereas mixture interactions at 10% effect shifted toward additivity/synergism relative to concentration addition at carrying capacity. Some evidence was found for stronger synergistic mixture effects in smaller species. Overall, the present study highlights the importance of incorporating more species in sensitivity distributions and accounting for mixture toxicity in risk assessment. Environ Toxicol Chem 2021;40:2015-2025. © 2021 SETAC.

Modelling human variability in toxicokinetic and toxicodynamic processes using Bayesian meta‐analysis, physiologically‐based modelling and in vitro systems

This external scientific report summarises the results from thearticle 36 grant GA/EFSA/SCER/2015/01″Modelling human variability in toxicokinetic and toxicodynamic processes using Bayesian meta-analysis, physiologically-based (PB) modelling and in vitro systems”. Extensive literature searches, data collection and modelling of human variability in toxicokinetics (TK) (phase I, Phase II enzymes and transporters) and toxicodynamics (TD) are summarised and further elaborated in supplementary material and EFSA knowledge junction, open source databases(MS Excel) and peer reviewed publications (objective 1 and 2). In addition,in vitro TK and TD information from laboratory studies and literature searches are summarised for a range of case studies relevant to EFSA including pesticides (i.e. triflumuron, chlorpyrifos, phosmet), natural toxins (e.g. microcystin variants, mycotoxins), food additives and polyphenols (i.e. resveratrol, tyrosol), food additives as well as drugs (i.e.amiodarone). These include isoform-specific metabolism and kinetic parameters for single chemicals and inhibition constants for multiple chemicals (TK) and identification of molecular targets (TD). Finally, generic quantitative in vitro in vivo extrapolation (QIVIVE) models, PB-kinetic (PBK) and PBK-dynamic (PBKD) models were developed in the R freeware, calibrated and validated usingcase studies for single and multiple chemicals. Supplementary material and model codes are available on EFSA knowledge junction. The results are in line with EFSA priorities for the implementation of the use of new approach methodologies (NAMs) for human risk assessment (RA) of chemicals and the need to develop open source TK TD databases data and PB-K and PB-KD models have been identified as key steps of this process. Future perspectives are discussedincluding the integration of pathway-related variability and generic human QIVIVE and PB-K models and PB-K-D models into TKplate, a Toxicokinetic Modelling Platform under development in EFSA.

Understanding the effects of metal pre-exposure on the sensitivity of zebrafish larvae to metal toxicity: A toxicokinetics–toxicodynamics approach

Organisms are increasingly tolerant to metal toxicity in the natural ecosystems, which did not match the results of the environmental risk assessment (ERA) of metals based on toxicity data from organisms in the laboratory. Studies have described the effects of pre-exposure to metals on metal toxicity tolerance in terms of the toxicokinetic (TK) process; however, the toxicodynamic (TD) process may be more susceptible to metal pre-exposure. Therefore, to determine whether pre-exposure to low concentrations of silver (Ag) or cadmium (Cd) affects the metal TK and TD processes of zebrafish (Danio rerio) larvae, we investigated four TK–TD model parameters that control tolerance and sensitivity to metal toxicity on the survival. Our results showed that the killing rate (ks) of larvae exposed to high Cd concentrations was significantly lower following pre-exposure to 10 μg/L Cd than that of larvaenot pre-exposed. However, the ks for high Ag concentrations was significantly higher in zebrafish larvae following pre-exposure to 2 μg/L Ag than in larvae not pre-exposed. In other words, a one-day pre-exposure to 2 µg/L Ag rendered the larvae more sensitive to Ag during a subsequent 4-day exposure to higher Ag concentrations, whereas a one-day pre-exposure to 10 µg/L Cd rendered the larvae more tolerance to Cd during a subsequent 4-day exposure to higher Cd concentrations. Our results further the current understanding of toxic metal tolerance mechanisms, both in TK and TD processes, and they will guide future laboratory studies to assess actual pre-exposure scenarios that occur in natural environments. Thus, our study can help reduce uncertainty in testing and improve ecological management concerning metal risk assessments.

Elucidating Toxicodynamic Differences at the Molecular Scale between ZnO Nanoparticles and ZnCl2 in Enchytraeus crypticus via Nontargeted Metabolomics.

Much effort has been devoted to clarifying the comparative toxicity of ZnO nanoparticles (NPs) and Zn ions; however, little is known about their toxicodynamic processes at the metabolic level. Here, we investigated the acute (2d) and chronic (7d) effects to a soil species, Enchytraeus crypticus, of two sublethal doses of ZnO-NPs and ZnCl2 (10 and 30 mg/L Zn) using ultrahigh performance liquid chromatography-quadrupole-time-of-flight/mass spectrometry-based metabolomics. The metabolomics analysis identified 99, 128, 121, and 183 significantly changed metabolites (SCMs) in E. crypticus exposed to ZnO-NPs for 2d, ZnCl2 for 2d, ZnO-NPs for 7d, and ZnCl2 for 7d, respectively, suggesting that ZnCl2 induced stronger metabolic reprogramming than ZnO-NPs, and a longer exposure time caused greater metabolite changes. Among the SCMs, 67 were shared by ZnO-NPs and ZnCl2 after 2d and 84 after 7d. These metabolites were mainly related to oxidative stress and antioxidant defense, membrane disturbance, and energy expenditure. The targeted analysis on physiological and biochemical responses further proved the metabolic observations. Nevertheless, 32 (33%) and 37 (31%) SCMs were found only in ZnO-NP treatments after 2 and 7d, respectively, suggesting that the toxicity of ZnO-NPs cannot be solely attributed to the released Zn ions. Metabolic pathway analysis revealed significant perturbations of galactose metabolism, amino sugar and nucleotide sugar metabolism, and glycerophospholipid metabolism in all test groups. Based on involvement frequency, glucose-1-phosphate, glycerol 3-phosphate, and phosphorylcholine could serve as universal biomarkers for exposure to different Zn forms. Four pathways perturbed by ZnO-NPs were nanospecific upon acute exposure and three upon chronic exposure. Our findings demonstrated that metabolomics is an effective tool for understanding the molecular toxicity mechanism and highlighted that time-series measurements are essential for discovering and comparing modes of action of metal ions and NPs.

Do toxicokinetic and toxicodynamic processes hold the same for light and heavy rare earth elements in terrestrial organism Enchytraeus crypticus?

The widespread use of rare earth elements (REEs) in numerous sectors have resulted in their release into the environment. Existing knowledge about the effects of REEs were acquired mainly based on toxicity tests with aquatic organisms and a fixed exposure time, Here, the dynamic accumulation and toxicity of REEs (La, Ce, and Gd) in soil organism Enchytraeus crypticus were determined and modeled by a first-order one-compartment model and a time-toxicity logistic model, respectively. Generally, the accumulation and toxicity of REEs were both exposure level- and time-dependent. The overall uptake rate constants were 2.97, 2.48, and 2.38 L kg−1d−1 for La, Ce, and Gd, respectively. The corresponding elimination rate constants were 0.99, 0.78, and 0.56 d−1, respectively. The worms exhibited faster uptake and elimination ability for light REEs (La and Ce) than for heavy REEs (Gd). For all three REEs, the LC50 values based on exposure concentrations decreased with time and reached ultimate values after approximately 10 d exposure. The estimated ultimate LC50 values (LC50∞) were 279, 334, and 358 mg L−1 for Ce, Gd, and La, respectively. When expressed as body concentration, the LC50inter value was almost constant with time, demonstrating that internal body concentration could be a better indicator of dynamic toxicity of REEs than external dose. This study highlights that specific REE and exposure time should be taken into account in accurately assessing risk of REEs.

A generic Bayesian hierarchical model for the meta-analysis of human population variability in kinetics and its applications in chemical risk assessment

Traditionally, chemical risk assessment in humans aims to derive safe levels of exposure to chemicals using toxicological data in test species while applying a default 100-fold uncertainty factor (UF) to allow for both interspecies differences (10) and human variability (10) in toxicokinetic (TK) and toxicodynamic (TD) processes. Over the last two decades, meta-analysis methods have allowed to perform quantitative analysis of population variability in kinetics and dynamics to replace these default UFs with data-based UFs. Pathway-related UFs can now be derived using population variability in kinetics for known metabolic pathways. This study presents a new hierarchical Bayesian model for the meta-analysis of human population variability in TK parameters, along with its application to the derivation of UFs. Data standardisation and data gaps are also discussed, as well as model implementation, selection and validation. Applications of the generic model in human risk assessment are illustrated through a case study quantifying inter-phenotypic differences in TK for substances metabolised via the CYP2D6 polymorphic enzyme. Model implementations in open-source software are provided and future refinements and applications of the model are proposed.

Map and model-moving from observation to prediction in toxicogenomics.

BackgroundChemicals induce compound-specific changes in the transcriptome of an organism (toxicogenomic fingerprints). This provides potential insights about the cellular or physiological responses to chemical exposure and adverse effects, which is needed in assessment of chemical-related hazards or environmental health. In this regard, comparison or connection of different experiments becomes important when interpreting toxicogenomic experiments. Owing to lack of capturing response dynamics, comparability is often limited. In this study, we aim to overcome these constraints.ResultsWe developed an experimental design and bioinformatic analysis strategy to infer time- and concentration-resolved toxicogenomic fingerprints. We projected the fingerprints to a universal coordinate system (toxicogenomic universe) based on a self-organizing map of toxicogenomic data retrieved from public databases. Genes clustering together in regions of the map indicate functional relation due to co-expression under chemical exposure. To allow for quantitative description and extrapolation of the gene expression responses we developed a time- and concentration-dependent regression model. We applied the analysis strategy in a microarray case study exposing zebrafish embryos to 3 selected model compounds including 2 cyclooxygenase inhibitors. After identification of key responses in the transcriptome we could compare and characterize their association to developmental, toxicokinetic, and toxicodynamic processes using the parameter estimates for affected gene clusters. Furthermore, we discuss an association of toxicogenomic effects with measured internal concentrations.ConclusionsThe design and analysis pipeline described here could serve as a blueprint for creating comparable toxicogenomic fingerprints of chemicals. It integrates, aggregates, and models time- and concentration-resolved toxicogenomic data.

Quantifying the interactions among metal mixtures in toxicodynamic process with generalized linear model

Predicting the toxicity of chemical mixtures is difficult because of the additive, antagonistic, or synergistic interactions among the mixture components. Antagonistic and synergistic interactions are dominant in metal mixtures, and their distributions may correlate with exposure concentrations. However, whether the interaction types of metal mixtures change at different time points during toxicodynamic (TD) processes is undetermined because of insufficient appropriate models and metal bioaccumulation data at different time points. In the present study, the generalized linear model (GLM) was used to illustrate the combined toxicities of binary metal mixtures, such as Cu–Zn, Cu–Cd, and Cd–Pb, to zebrafish larvae (Danio rerio). GLM was also used to identify possible interaction types among these method for the traditional concentration addition (CA) and independent action (IA) models. Then the GLM were applied to quantify the different possible interaction types for metal mixture toxicity (Cu–Zn, Cu–Cd, and Cd–Pb to D. rerio and Ni–Co to Oligochaeta Enchytraeus crypticus) during the TD process at different exposure times. We found different metal interaction responses in the TD process and interactive coefficients significantly changed at different exposure times (p<0.05), which indicated that the interaction types among Cu–Zn, Cu–Cd, Cd–Pb and Ni–Co were time dependent. Our analysis highlighted the importance of considering joint actions in the TD process to understand and predict metal mixture toxicology on organisms. Moreover, care should be taken when evaluating interactions in toxicity prediction because results may vary at different time points. The GLM could be an alternative or complementary approach for BLM to analyze and predict metal mixture toxicity.

Concentration addition and independent action model: Which is better in predicting the toxicity for metal mixtures on zebrafish larvae

The joint toxicity of chemical mixtures has emerged as a popular topic, particularly on the additive and potential synergistic actions of environmental mixtures. We investigated the 24h toxicity of Cu–Zn, Cu–Cd, and Cu–Pb and 96h toxicity of Cd–Pb binary mixtures on the survival of zebrafish larvae. Joint toxicity was predicted and compared using the concentration addition (CA) and independent action (IA) models with different assumptions in the toxic action mode in toxicodynamic processes through single and binary metal mixture tests. Results showed that the CA and IA models presented varying predictive abilities for different metal combinations. For the Cu–Cd and Cd–Pb mixtures, the CA model simulated the observed survival rates better than the IA model. By contrast, the IA model simulated the observed survival rates better than the CA model for the Cu–Zn and Cu–Pb mixtures. These findings revealed that the toxic action mode may depend on the combinations and concentrations of tested metal mixtures. Statistical analysis of the antagonistic or synergistic interactions indicated that synergistic interactions were observed for the Cu–Cd and Cu–Pb mixtures, non-interactions were observed for the Cd–Pb mixtures, and slight antagonistic interactions for the Cu–Zn mixtures. These results illustrated that the CA and IA models are consistent in specifying the interaction patterns of binary metal mixtures.

Time-Dependent Effects in Algae for Chemicals with Different Adverse Outcome Pathways: A Novel Approach.

Chemicals affect unicellular algae as a result of toxicokinetic and toxicodynamic processes. The internal concentration of chemicals in algae cells typically reaches equilibrium within minutes, while damage cumulatively increases over hours. The time gap between the steady state of internal exposure and damage development is thus suspected to span up to hours, mainly due to toxicodynamic processes. The quantification of rate-limited toxicodynamic processes, aggregated as a progressive effect from an initiating molecular event through biological key events toward the adverse outcome on algae growth inhibition, might discriminate between different adverse outcome pathways (AOPs). To support our hypothesis, we selected six chemicals according to different physicochemical properties and three distinctly dissimilar AOPs. The time courses of internal concentrations were linked to the observed affected Scenedesmus vacuolatus growth using toxicokinetic-toxicodynamic modeling. Effects on cell growth were explained by effect progression and not by the time to reach internal equilibrium concentration. Effect progression rates ranged over 6 orders of magnitude for all chemicals but varied by less than 1 order of magnitude within similar AOP (photosystem II inhibitors > reactive chemicals > lipid biosynthesis inhibitors), meaning that inhibitors of photosystem II advance an effect toward algae growth fastest compared to reactive chemicals and inhibitors of lipid biosynthesis.

A toxicokinetic and toxicodynamic modeling approach using Myriophyllum spicatum to predict effects caused by short-term exposure to a sulfonylurea.

Toxicokinetic and toxicodynamic models are a promising tool to address the effects of time-variable chemical exposure. Standard toxicity tests usually rely on static concentrations, but these chemical exposure patterns are unlikely to appear in the field, where time-variable exposure of chemicals is typical. In the present study, toxicodynamic processes were integrated into an existing model that includes the toxicokinetics and growth of the aquatic plant Myriophyllum spicatum, to predict the impact on plant growth of 2 iofensulfuron short-term exposure patterns. To establish a method that can be used with standard data from risk assessments, the toxicodynamics of iofensulfuron were based on effect data from a 14-d standard toxicity test using static concentrations. Modeling showed that the toxicokinetic and toxicodynamic growth model of M. spicatum can be successfully used to predict effects of short-term iofensulfuron exposure by using effect data from a standard toxicity test. A general approach is presented, in which time-variable chemical exposures can be evaluated more realistically without conducting additional toxicity studies.