Reconsidering sufficient and optimal test design in acute toxicity testing

Identifying and Predicting Delayed Mortality with Toxicokinetic-Toxicodynamic Models

The prevalence of standardized toxicity testing in ecotoxicology has largely obscured the notion that toxicity is a function of time as well. The necessity of considering time is vividly demonstrated by observations of delayed mortality, that is, deaths continue to occur even when animals are no longer exposed to a toxicant. In this contribution, I explore to what extent toxicokinetic-toxicodynamic (TKTD) models from the framework of the General Unified Threshold model for Survival (GUTS) can capture delayed mortality, and to what extent this phenomenon can be predicted from short-term standard tests. I use a previously published data set for fluoroquinolones in Daphnia magna that shows strongly delayed mortality (using immobilization as a proxy for death). The model analysis shows that the GUTS stochastic death models can capture delayed mortality in the complete data set with a long recovery phase, but that the delayed effects would not have been predicted from a 2-day standard test. The study underlines the limited information content of standard acute test designs. Toxicokinetic-toxicodynamic modeling offers a handle on the time aspects of toxicity but cannot always be relied on to provide accurate extrapolations based on severely limited standard tests. The phenomenon of delayed toxicity requires more structured study to clarify its prevalence and impact; I discuss several avenues for further investigation. Environ Toxicol Chem 2024;43:1030-1035. © 2024 SETAC.

A Toxicokinetic-Toxicodynamic Modeling Workflow Assessing the Quality of Input Mortality Data

Toxicokinetic-toxicodynamic (TKTD) models simulate organismal uptake and elimination of a substance (TK) and its effects on the organism (TD). The Reduced General Unified Threshold model of Survival (GUTS-RED) is a TKTD modeling framework that is well established for aquatic risk assessment to simulate effects on survival. The TKTD models are applied in three steps: parameterization based on experimental data (calibration), comparing predictions with independent data (validation), and prediction of endpoints under environmental scenarios. Despite a clear understanding of the sensitivity of GUTS-RED predictions to the model parameters, the influence of the input data on the quality of GUTS-RED calibration and validation has not been systematically explored. We analyzed the performance of GUTS-RED calibration and validation based on a unique, comprehensive data set, covering different types of substances, exposure patterns, and aquatic animal species taxa that are regularly used for risk assessment of plant protection products. We developed a software code to automatically calibrate and validate GUTS-RED against survival measurements from 59 toxicity tests and to calculate selected model evaluation metrics. To assess whether specific survival data sets were better suited for calibration or validation, we applied a design in which all possible combinations of studies for the same species-substance combination are used for calibration and validation. We found that uncertainty of calibrated parameters was lower when the full range of effects (i.e., from high survival to high mortality) was covered by input data. Increasing the number of toxicity studies used for calibration further decreased parameter uncertainty. Including data from both acute and chronic studies as well as studies under pulsed and constant exposure in model calibrations improved model predictions on different types of validation data. Using our results, we derived a workflow, including recommendations for the sequence of modeling steps from the selection of input data to a final judgment on the suitability of GUTS-RED for the data set. Environ Toxicol Chem 2024;43:197-210. © 2023 Bayer AG and The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Toxicokinetic-Toxicodynamic Model to Assess Thermal Stress

Temperature is a crucial environmental factor affecting the distribution and performance of ectothermic organisms. This study introduces a new temperature damage model to interpret their thermal stress. Inspired by the ecotoxicological damage model in the General Unified Threshold model for Survival (GUTS) framework, the temperature damage model assumes that damage depends on the balance between temperature-dependent accumulation and constant repair. Mortality due to temperature stress is driven by the damage level exceeding a threshold. Model calibration showed a good agreement with the measured survival of Gammarus pulex exposed to different constant temperatures. Further, model simulations, including constant temperatures, daily temperature fluctuations, and heatwaves, demonstrated the model's ability to predict temperature effects for various environmental scenarios. With this, the present study contributes to the mechanistic understanding of temperature as a single stressor while facilitating the incorporation of temperature as an additional stressor alongside chemicals in mechanistic multistressor effect models.

Comparison of species sensitivity distribution modeling approaches for environmental risk assessment of nanomaterials - A case study for silver and titanium dioxide representative materials

Species sensitivity distributions (SSDs) are used in chemical safety assessments to derive predicted-no-effect-concentrations (PNECs) for substances with a sufficient amount of relevant and reliable ecotoxicity data available. For engineered nanomaterials (ENMs), ecotoxicity data are often compromised by poor reproducibility and the lack of nano-specific characterization needed describe an ENM under test exposure conditions. This may influence the outcome of SSD modelling and hence the regulatory decision-making. This study investigates how the outcome of SSD modelling is influenced by: 1) Selecting input data based on the nano-specific "nanoCRED" reliability criteria, 2) Direct SSD modelling avoiding extrapolation of data by including long-term/chronic NOECs only, and 3) Weighting data according to their nano-specific quality, the number of data available for each species, and the trophic level abundance in the ecosystem. Endpoints from freshwater ecotoxicity studies were collected for the representative nanomaterials NM-300 K (silver) and NM-105 (titanium dioxide), evaluated for regulatory reliability and scored according to the level of nano-specific characterization conducted. The compiled datasets are unique in exclusively dealing with representative ENMs showing minimal batch-to-batch variation. The majority of studies were evaluated as regulatory reliable, while the degree of nano-specific characterization varied greatly. The datasets for NM-300 K and NM-105 were used as input to the nano-weighted n-SSWD model, the probabilistic PSSD+, and the conventional SSD Generator by the US EPA. The conventional SSD generally yielded the most conservative, but least precise HC₅ values, with 95 % confidence intervals up to 100-fold wider than the other models. The inclusion of regulatory reliable data only, had little effect on the HC₅ generated by the conventional SSD and the PSSD+, whereas the n-SSWD estimated different HC₅ values based on data segregated according to reliability, especially for NM-105. The n-SSWD weighting of data significantly affected the estimated HC₅ values, however in different ways for the sub-datasets of NM-300 K and NM-105. For NM-300 K, the inclusion of NOECs only in the weighted n-SSWD yielded the most conservative HC₅ of all datasets and models (a HC₅ based on NOECs only could not be estimated for NM-105, due to limited number of data). Overall, the estimated HC₅ values of all models are within a relatively limited concentration range of 25-100 ng Ag/L for NM-300 K and 1-15 μgTiO₂/L for NM-105.

Exposure pattern-specific species sensitivity distributions for the ecological risk assessments of insecticides

In the higher tiers of pesticide risk assessment, the Species Sensitivity Distribution (SSD) concept is often used to establish the effect threshold defined as the concentration protecting 95% of the species (Hazardous Concentration 5%, HC5). The toxicity data included in SSDs are normally established using a constant exposure regime. However, the exposure of pesticides in the field is often characterised by a variable exposure regime. Toxicokinetic-toxicodynamic (TKTD) models can be used to extrapolate the toxic effects of a chemical to a specific, time-variable exposure regime. The aim of this paper was to develop Exposure Pattern Specific SSDs (EPS-SSDs) for three insecticides using TKTD models and to compare the HC5 of different exposure patterns with the same time-weighted average concentration to evaluate whether the use of EPS-SSDs would change the outcome of the ecological risk assessment. The EPS-SSDs were developed by estimating TKTD parameters for the compounds chlorpyrifos, imidacloprid and lambda-cyhalothrin using results from standard, 96 h, single species tests. These parameter estimates were used for TKTD modelling to determine toxicity thresholds (e.g. LC10 and LC50) for contrasting exposure patterns after certain evaluation times (4, 10 or 100 days). HC5 values were constructed with TKTD-predicted LC10- and LC50- values for different exposure patterns characterised by similar time-weighted average concentrations. Differences between those HC5 values ranged from a factor 1 to a factor 2.3 for the short evaluation period (4 d). This difference was smaller when using an evaluation period of 10 days instead of 4 days and selecting the TKTD-predicted LC10 instead of TKTD-predicted LC50 based HC5s. For the long term evaluation period (100 d), a maximum difference of a factor of 30 was found.

Exploring inter-species sensitivity to a model hydrocarbon, 2-Methylnaphtalene, using a process-based model

We compared inter-species sensitivity to a model narcotic compound, 2-Methylnaphthalene, to test if taxonomical relatedness, feeding guilds, and trophic level govern species sensitivities on species distributed in different regions. We fitted a toxicokinetic-toxicodynamic model to survival patterns over time for 26 species using new and raw data from the literature. Species sensitivity distributions provided little insight into understanding patterns in inter-species sensitivity. The range of no-effect concentrations (NEC) obtained for 26 species showed little variation (mean 0.0081 mM; SD 0.009). Results suggest that the NEC alone does not explain the complexity of the species tolerances. The dominant rate constant and the derived time to observe an effect (t₀), a function of concentration, might provide the means for depicting patterns in sensitivity and better ecotoxicological testing. When comparing the t₀ functions, we observed that Arctic species have shorter time frames to start showing effects. Mollusks and second trophic level species took longer to build up a lethal body burden than the rest. Coupling our results with fate and transport models would allow forecasting narcotic compounds toxicity in time and thus improve risk assessment.

Fit Reduced GUTS Models Online: From Theory to Practice

Mechanistic modeling approaches, such as the toxicokinetic-toxicodynamic (TKTD) framework, are promoted by international institutions such as the European Food Safety Authority and the Organization for Economic Cooperation and Development to assess the environmental risk of chemical products generated by human activities. TKTD models can encompass a large set of mechanisms describing the kinetics of compounds inside organisms (e.g., uptake and elimination) and their effect at the level of individuals (e.g., damage accrual, recovery, and death mechanism). Compared to classical dose-response models, TKTD approaches have many advantages, including accounting for temporal aspects of exposure and toxicity, considering data points all along the experiment and not only at the end, and making predictions for untested situations as realistic exposure scenarios. Among TKTD models, the general unified threshold model of survival (GUTS) is within the most recent and innovative framework but is still underused in practice, especially by risk assessors, because specialist programming and statistical skills are necessary to run it. Making GUTS models easier to use through a new module freely available from the web platform MOSAIC (standing for MOdeling and StAtistical tools for ecotoxIClogy) should promote GUTS operability in support of the daily work of environmental risk assessors. This paper presents the main features of MOSAIC_GUTS: uploading of the experimental data, GUTS fitting analysis, and LCx estimates with their uncertainty. These features will be exemplified from literature data. Integr Environ Assess Manag 2018;14:625-630. © 2018 SETAC.

Scientific Opinion on the state of the art of Toxicokinetic/Toxicodynamic (TKTD) effect models for regulatory risk assessment of pesticides for aquatic organisms

Following a request from EFSA, the Panel on Plant Protection Products and their Residues (PPR) developed an opinion on the state of the art of Toxicokinetic/Toxicodynamic (TKTD) models and their use in prospective environmental risk assessment (ERA) for pesticides and aquatic organisms. TKTD models are species‐ and compound‐specific and can be used to predict (sub)lethal effects of pesticides under untested (time‐variable) exposure conditions. Three different types of TKTD models are described, viz., (i) the ‘General Unified Threshold models of Survival’ (GUTS), (ii) those based on the Dynamic Energy Budget theory (DEBtox models), and (iii) models for primary producers. All these TKTD models follow the principle that the processes influencing internal exposure of an organism, (TK), are separated from the processes that lead to damage and effects/mortality (TD). GUTS models can be used to predict survival rate under untested exposure conditions. DEBtox models explore the effects on growth and reproduction of toxicants over time, even over the entire life cycle. TKTD model for primary producers and pesticides have been developed for algae, Lemna and Myriophyllum. For all TKTD model calibration, both toxicity data on standard test species and/or additional species can be used. For validation, substance and species‐specific data sets from independent refined‐exposure experiments are required. Based on the current state of the art (e.g. lack of documented and evaluated examples), the DEBtox modelling approach is currently limited to research applications. However, its great potential for future use in prospective ERA for pesticides is recognised. The GUTS model and the Lemna model are considered ready to be used in risk assessment.

Robust Fit of Toxicokinetic-Toxicodynamic Models Using Prior Knowledge Contained in the Design of Survival Toxicity Tests

Toxicokinetics-toxicodynamic (TKTD) models have emerged as a powerful means to describe survival as a function of time and concentration in ecotoxicology. They are especially powerful to extrapolate survival observed under constant exposure conditions to survival predicted under realistic fluctuating exposure conditions. But despite their obvious benefits, these models have not yet been adopted as a standard to analyze data of survival toxicity tests. Instead simple dose-response models are still often used although they only exploit data observed at the end of the experiment. We believe a reason precluding a wider adoption of TKTD models is that available software still requires strong expertise in model fitting. In this work, we propose a fully automated fitting procedure that extracts prior knowledge on parameters of the model from the design of the toxicity test (tested concentrations and observation times). We evaluated our procedure on three experimental and 300 simulated data sets and showed that it provides robust fits of the model, both in the frequentist and the Bayesian framework, with a better robustness of the Bayesian approach for the sparsest data sets.

Combined effect of temperature and ammonia on molecular response and survival of the freshwater crustacean Gammarus pulex

Freshwater ecosystems are experiencing mounting pressures from agriculture, urbanization, and climate change, which could drastically impair aquatic biodiversity. As nutrient inputs increase and temperatures rise, ammonia (NH₃) concentration is likely to be associated with stressful temperatures. To investigate the interaction between NH₃ and temperature on aquatic invertebrate survival, we performed a factorial experiment on the survival and molecular response of Gammarus pulex, with temperature (10, 15, 20, and 25°C) and NH₃ (0, 0.5, 1, 2, 3, and 4mg NH₃/L) treatments. We observed an unexpected antagonistic interaction between temperature and NH₃ concentration, meaning survival in the 4mg NH₃/L treatment was higher at 25°C than at the control temperature of 10°C. A toxicokinetic-toxicodynamic (TK-TD) model was built to describe this antagonistic interaction. While the No Effect Concentration showed no significant variation across temperatures, the 50% lethal concentration at the end of the experiment increased from 2.7 (2.1-3.6) at 10°C to 5.5 (3.5- 23.4) mg NH₃/L at 25°C. Based on qPCR data, we associated these survival patterns to variations in the expression of the hsp70 gene, a generic biomarker of stress. However, though there was a 14-fold increase in hsp70 mRNA expression for gammarids exposed to 25°C compared to controls, NH₃ concentration had no effect on hsp70 mRNA synthesis across temperatures. Our results demonstrate that the effects of combined environmental stressors, like temperature and NH₃, may strongly differ from simple additive effects, and that stress response to temperature can actually increase resilience to nutrient pollution in some circumstances.

Comparison and evaluation of pesticide monitoring programs using a process-based mixture model

A number of European countries run large-scale pesticide monitoring schemes in watersheds aimed at identifying and evaluating the presence of pesticide residues in the environment. These schemes provide national and regional scale assessments of pesticide concentrations within the context of environmental quality assessment, aiming to ensure some degree of ecological protection. The present study is aimed at evaluating the joint effects of the pesticide mixtures detected in monitoring programs, using a process-based mixture model that was parameterized for Daphnia magna. In total, over 15 000 samples containing over 1 million individual measurements were evaluated for effects. It was found that there are only a small number of places where one can expect to have effects on daphnids, based on measured concentrations. The most polluted samples would cause extinction of a daphnid population within only 30 h. The results show that effects are mostly triggered by a limited number of pesticide residues at locations with high emissions. It was also shown that the analytical detection limits are basically too high to exclude mixture effects. So, despite all the effort that is put into chemical monitoring programs, it remains a challenge to make statements on whether or not the environment is protected. Recommendations are offered for a different setup of monitoring programs to improve this situation. Environ Toxicol Chem 2016;35:3113-3123. © 2016 SETAC.

Modelling survival: exposure pattern, species sensitivity and uncertainty

The General Unified Threshold model for Survival (GUTS) integrates previously published toxicokinetic-toxicodynamic models and estimates survival with explicitly defined assumptions. Importantly, GUTS accounts for time-variable exposure to the stressor. We performed three studies to test the ability of GUTS to predict survival of aquatic organisms across different pesticide exposure patterns, time scales and species. Firstly, using synthetic data, we identified experimental data requirements which allow for the estimation of all parameters of the GUTS proper model. Secondly, we assessed how well GUTS, calibrated with short-term survival data of Gammarus pulex exposed to four pesticides, can forecast effects of longer-term pulsed exposures. Thirdly, we tested the ability of GUTS to estimate 14-day median effect concentrations of malathion for a range of species and use these estimates to build species sensitivity distributions for different exposure patterns. We find that GUTS adequately predicts survival across exposure patterns that vary over time. When toxicity is assessed for time-variable concentrations species may differ in their responses depending on the exposure profile. This can result in different species sensitivity rankings and safe levels. The interplay of exposure pattern and species sensitivity deserves systematic investigation in order to better understand how organisms respond to stress, including humans.

Glyceria maxima as new test species for the EU risk assessment for herbicides: a microcosm study

In its recent guidance document on tiered risk assessment for plant protection products for aquatic organisms, the European Food Safety Authority (EFSA) proposed to use Glyceria maxima as monocotyledonous grass species for the testing of special herbicide groups. However, published toxicity data for this species is very limited and there is no test guideline for Glyceria sp. For this reason a microcosm study was conducted in order to gain experience on the degree of sensitivity of G. maxima to the herbicidal substances clodinafop-propargyl (grass herbicide) and fluroxypyr (auxin) in comparison to the already established test organism water milfoil Myriophyllum spicatum and the duckweed species Landoltia punctata. Five concentrations without replicates were tested for each test substance using 10 microcosms and three microcosms served as controls. The experiment was run for 8 weeks. Morphological endpoints were used to determine growth and EC50 values. The results show that M. spicatum was most sensitive to fluroxypyr (37 days EC50 for roots: 62 µg/L) and G. maxima most sensitive to clodinafop-propargyl (22 days EC50 for total shoot length: 48 µg/L) whereas the duckweed species was considerable less sensitive. Hence, G. maxima turns out to be a good candidate for testing grass specific herbicides, supporting its inclusion as an additional macrophyte test for the risk assessment of herbicides as proposed by the EFSA.

A combined toxicokinetics and toxicodynamics approach to assess the effect of porewater composition on cadmium bioavailability to Folsomia candida

The aim of the present study was to improve our understanding of cadmium bioavailability by linking toxicokinetics and toxicodynamics. The springtail Folsomia candida was exposed to different cadmium concentrations in solutions embedded in inert quartz sand. Survival and cadmium uptake in the animals were followed for 21 d. After 10 d, some animals were transferred to clean medium to assess cadmium elimination. Using a first-order one-compartment model, an overall uptake rate constant (k1) of 0.18 L kg(animal)(-1) d(-1) and an elimination rate constant (k(2-TK)) of 0.02 d(-1) were calculated. Survival decreased with time, resulting in an estimated final median lethal concentration (LC50) of 0.51 mM. A lethal body concentration (LBC) of 4.6 µmol Cd g(-1) dry body weight was estimated by multiplying the final LC50 by the bioconcentration factor (k1/k(2-TK)). The LC50(animal) values based on internal cadmium concentrations were between 3.56 µmol Cd g(-1) and 9.91 µmol Cd g(-1) dry body weight, with an overall value of 7.9 µmol Cd g(-1) dry body weight (95% confidence interval [CI]: 3.8-12.0 µmol Cd g(-1) dry body wt). Because the 95% CI of the LC50(animal) included the LBC, there was good agreement of cadmium toxicokinetics and toxicodynamics.