Animal use replacement, reduction, and refinement: development of an integrated testing strategy for bioconcentration of chemicals in fish

Speed it up: How temperature drives toxicokinetics of organic contaminants in freshwater amphipods

The acceleration of global climate change draws increasing attention towards interactive effects of temperature and organic contaminants. Many studies reported a higher sensitivity of aquatic invertebrates towards contaminant exposure with increasing or fluctuating temperatures. The hypothesis of this study was that the higher sensitivity of invertebrates is associated with the changes of toxicokinetic processes that determine internal concentrations of contaminants and consequently toxic effects. Therefore, the influence of temperature on toxicokinetic processes and the underlying mechanisms were studied in two key amphipod species (Gammarus pulex and Hyalella azteca). Bioconcentration experiments were carried out at four different temperatures with a mixture of 12 exposure relevant polar organic contaminants. Tissue and medium samples were taken in regular intervals and analysed by online solid-phase extraction liquid chromatography high-resolution tandem mass spectrometry. Subsequently, toxicokinetic rates were modelled and analysed in dependence of the exposure temperature using the Arrhenius equation. An exponential relationship between toxicokinetic rates versus temperature was observed and could be well depicted by applying the Arrhenius equation. Due to a similar Arrhenius temperature of uptake and elimination rates, the bioconcentration factors of the contaminants were generally constant across the temperature range. Furthermore, the Arrhenius temperature of the toxicokinetic rates and respiration was mostly similar. However, in some cases (citalopram, cyprodinil), the bioconcentration factor appeared to be temperature dependent, which could potentially be explained by the influence of temperature on active uptake mechanisms or biotransformation. The observed temperature effects on toxicokinetics may be particularly relevant in non-equilibrated systems, such as exposure peaks in summer as exemplified by the exposure modelling of a field measured pesticide peak where the internal concentrations increased by up to fourfold along the temperature gradient. The results provide novel insights into the mechanisms of chemical uptake, biotransformation and elimination in different climate scenarios and can improve environmental risk assessment.

In Vivo Bioconcentration of 10 Anionic Surfactants in Rainbow Trout Explained by In Vitro Data on Partitioning and S9 Clearance

Bioconcentration factors (BCFs) in rainbow trout were measured for 10 anionic surfactants with a range of alkyl chain lengths and different polar head groups. The BCFs ranged from 0.04 L kg^-1 ww (for C₁₀SO₃) to 1370 L kg^-1 ww (C₁₆SO₃). There was a strong correlation between the log BCF and log membrane lipid-water distribution ratio (D_MLW, r² = 0.96), and biotransformation was identified as the dominant elimination mechanism. The strong positive influence of D_MLW on BCF was attributed to two phenomena: (i) increased partitioning from water into the epithelial membrane of the gill, leading to more rapid diffusion across this barrier and more rapid uptake, and (ii) increased sequestration of the surfactant body burden into membranes and other body tissues, resulting in lower freely dissolved concentrations available for biotransformation. Estimated whole-body in vivo biotransformation rate constants k_B-BCF are within a factor three of rate constants estimated from S9 in vitro assays for six of the eight test chemicals for which k_B-BCF could be determined. A model-based assessment indicated that the hepatic clearance rate of freely dissolved chemicals was similar for the studied surfactants. The dataset will be useful for evaluation of in silico and in vitro methods to assess bioaccumulation.

Developing in vitro models to assess fish gill excretion of emerging contaminants

Advances in analytical methods have enabled the detection of emerging contaminants at ever lower concentrations in freshwaters. However, such measurements need to be linked to effect-based assays to identify risks. The bioconcentration factor (BCF) forms part of a chemical's environmental risk assessment (ERA), and current regulatory testing guidelines to calculate fish BCFs use hundreds of fish per chemical. Due to ethical concerns a reduction in the numbers of animals used is desired, and there is a need to identify in vitro or in silico alternatives which meet regulatory acceptance. This study describes the successful demonstration of a FIsh Gill Cell culture System (FIGCS) to assess an often overlooked parameter in pharmacokinetics: the excretion of drugs across the gill. The FIGCS tolerates the application of natural waters on its apical surface, mimicking the situation of the live fish, and thus in combination with advanced analytical methods, offers an opportunity to take lab-based testing used for ERA, such as compound uptake, biotransformation or excretion directly into field for validation with natural waters. Here we used the basic drug propranolol and the acidic ibuprofen as a demonstration of the FIGCS utility in three separate experiments. Excretion across the apical membrane showed saturation kinetics, suggesting the involvement of carrier-mediated processes. Both propranolol and ibuprofen were excreted across the epithelium from the media (internal blood equivalent) to the water, with ibuprofen excretion being considerably slower than propranolol excretion. Further studies indicate that ibuprofen may be complexing with fetal bovine serum (FBS) reducing bioavailability; in contrast propranolol efflux rate was unaffected, indicating that drugs behave differently in the presence of FBS and other plasma proteins. A key issue in future ERA is to better understand the effects of mixtures of different pollutant classes found in environmental samples, and this model offers an ethical path to do this.

In Vitro Biotransformation Assays Using Liver S9 Fractions and Hepatocytes from Rainbow Trout (Oncorhynchus mykiss): Overcoming Challenges with Difficult to Test Fragrance Chemicals

In vitro metabolic stability assays using rainbow trout (Oncorhynchus mykiss) isolated hepatocytes (RT-HEP) or hepatic S9 fractions (RT-S9) were introduced to provide biotransformation rate data for the assessment of chemical bioaccumulation in fish. The present study explored the suitability of the RT-HEP and RT-S9 assays for difficult test chemicals, and the in vitro-based predictions were compared to in silico-based predictions and in vivo-measured bioconcentration factors (BCFs). The results show that volatile or reactive chemicals can be tested with minor modifications of the in vitro protocols. For hydrophobic chemicals, a passive dosing technique was developed. Finally, a design-of-experiment approach was used to identify optimal in vitro assay conditions. The modified assay protocols were applied to 10 fragrances with diverse physicochemical properties. The in vitro intrinsic clearance rates were higher in the S9 than in the hepatocyte assay, but the in vitro-in vivo (IVIV) predictions were comparable between the 2 assays. The IVIV predictions classified the test chemicals as nonbioaccumulative (BCF < 2000), which was in agreement with the in vivo data but in contrast to the in silico-based predictions. The findings from the present study provide strong evidence that the RT-HEP and RT-S9 assays can provide reliable estimates of in vivo biotransformation rates for test chemicals with difficult physicochemical properties. Environ Toxicol Chem 2020;39:2396-2408. © 2020 SETAC.

Comparison of Alternative Methods for Bioaccumulation Assessment: Scope and Limitations of In Vitro Depletion Assays with Rainbow Trout and Bioconcentration Tests in the Freshwater Amphipod Hyalella azteca

Bioaccumulation assessment predominantly relies on the bioconcentration factor (BCF) as the sole decisive metric. The test guideline 305 by the Organisation for Economic Co-operation and Development (OECD) provides the standard procedure for deriving this in vivo fish BCF, which is not only expensive and labor-intensive, but also requires many animals. Accordingly, there is a great need for and interest in alternative methods that can help to reduce, replace, and refine vertebrate tests, as described in the 3R principles. Two alternative approaches have been developed: the bioconcentration test with the freshwater amphipod Hyalella azteca and the OECD test guideline 319 which provides a method to determine experimentally derived in vitro metabolism rates that can then be incorporated into in silico prediction models for rainbow trout BCF calculation. In the present study both alternative methods were applied to 5 substances of different physicochemical characteristics. The results were compared with literature values of fish in vivo BCFs and additional BCFs obtained with the alternative methods, if available. Potential differences between the results of the test methods are discussed utilizing information such as in vivo metabolism rates. The currently available data set suggests that these 2 alternative methods pose promising alternatives to predict bioaccumulation in fish, although defined applicability domains have yet to be determined. Environ Toxicol Chem 2020;39:1813-1825. © 2020 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Critical Review of Read-Across Potential in Testing for Endocrine-Related Effects in Vertebrate Ecological Receptors

Recent regulatory testing programs have been designed to evaluate whether a chemical has the potential to interact with the endocrine system and could cause adverse effects. Some endocrine pathways are highly conserved among vertebrates, providing a potential to extrapolate data generated for one vertebrate taxonomic group to others (i.e., biological read-across). To assess the potential for biological read-across, we reviewed tools and approaches that support species extrapolation for fish, amphibians, birds, and reptiles. For each of the estrogen, androgen, thyroid, and steroidogenesis (EATS) pathways, we considered the pathway conservation across species and the responses of endocrine-sensitive endpoints. The available data show a high degree of confidence in the conservation of the hypothalamus-pituitary-gonadal axis between fish and mammals and the hypothalamus-pituitary-thyroid axis between amphibians and mammals. Comparatively, there is less empirical evidence for the conservation of other EATS pathways between other taxonomic groups, but this may be due to limited data. Although more information on sensitive pathways and endpoints would be useful, current developments in the use of molecular target sequencing similarity tools and thoughtful application of the adverse outcome pathway concept show promise for further advancement of read-across approaches for testing EATS pathways in vertebrate ecological receptors. Environ Toxicol Chem 2020;39:739-753. © 2020 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals, Inc. on behalf of SETAC.

Bioconcentration, Metabolism, and Spatial Distribution of 14 C-Labeled Laurate in the Freshwater Amphipod Hyalella azteca

Regulatory assessment of the bioaccumulation from water is commonly based on bioconcentration factors (BCFs) derived from fish flow-through tests. Such experiments require many laboratory animals and are time-consuming and costly. An alternative test setup for organic, neutral compounds using the amphipod Hyalella azteca was recently suggested, resulting in BCF values which show a strong correlation with fish BCF data. In the present study, the bioconcentration potential of the ionic compound laurate was elucidated in H. azteca. The sodium salt of 1-¹⁴ C laurate was applied to H. azteca in a flow-through and a semistatic approach. Because of rapid biodegradation, a semistatic approach with frequent medium replacements was required to ensure a stable medium concentration. Laurate was also rapidly metabolized by H. azteca. A large proportion of the total radioactivity measured in the amphipod tissue was not extractable, suggesting that mineralized laurate was accumulated in the calcified exoskeleton of H. azteca. This was confirmed in a further study using carbonate [¹⁴ C]. A lipid-normalized (5.0%) Hyalella BCF of 8.9 was calculated for laurate, measured as free fatty acids. The results of the bioconcentration studies with H. azteca confirm the low bioaccumulation potential of the test item previously observed in fish. However, more organic ionic compounds with various properties need to be tested to assess whether a general correlation between fish and Hyalella BCF data exists. Environ Toxicol Chem 2020;39:310-322. © 2019 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals, Inc. on behalf of SETAC.

Variability in fish bioconcentration factors: Influences of study design and consequences for regulation

The fish bioconcentration factor (BCF) is an important aspect within bioaccumulation assessments. Several factors have been suggested to influence BCF values - including species, developmental stage, mixture exposure, and calculation method. However, their exact contribution to variance in BCF values is unknown. Within this study we assessed the relative impact of these test characteristics on BCF values and analyzed the reproducibility of aquatic exposure bioconcentration tests. Linear mixed effects analyses were performed on a newly develop database to investigate the relationship between the response variable (i.e. lipid normalized log BCF values) and several test characteristics as fixed effects. Lower BCF values were observed for substances that were simultaneously applied with high molecular weight polycyclic aromatic hydrocarbons compared to single substance exposure (with an average difference of -0.81 log BCF). Also, lower BCFs upon kinetic determination were observed compared to steady-state BCFs (log BCF -0.27), and lower BCFs for species from the Ostariophysi subcohort level (log BCF -0.17 to -0.15). In addition, data analysis showed high variation within BCF values for single substances (average SD = log BCF 0.21), which questions the robustness of the current bioaccumulation assessments. For example, the 95% confidence range of a BCF value of 2500 ranges from 953 ('not-bioaccumulative') to 6561 ('very bioaccumulative'). Our results show that the use of one single BCF leads to a high uncertainty in bioaccumulation assessments. We strongly recommend that within future bioconcentration studies, the used experimental design and test conditions are described in detail and justified to support solid interpretation.

Immunotoxicity of estrogen and nonylphenol on apoptosis and expression of ERs in goldfish macrophage: Opening new avenue for discovering the role of experimental model systems and sexes

The expression of estrogen receptors (ERs) and their roles in important cell processes such as apoptosis in the macrophages exposed to estrogen/xenoestrogen have remained a complex secret. This study focused on the expression of estrogen receptors (ERs) and the stimulation of apoptosis in the macrophages from the two sexes of goldfish (Carassius auratus) exposed to 17-βestradiol (E2) and nonylphenol (NP) under in vivo and in vitro conditions. For the in vivo experiment, fish were exposed to NP (10^-6 M and 10^-7 M) and E2 (10^-6 M) for 24 days. Then, the head kidney macrophages from the male and the female goldfish were isolated and assayed. For the in vitro experiments, the macrophages derived from the two sexes were cultured in L-15 medium and exposed to E2 (150 nM) and NP (10 nM and 150 nM) for 3 days. The results showed that the three isoforms of ERs (ERα, ERβ₁, ERβ₂) were expressed in the goldfish macrophages. After the exposure of macrophages to NP and E2, sex-specific increase of ERs expression and apoptosis were observed (P < 0.05). The expression of ERα after NP treatment showed the highest alteration, with the response being concentration-dependent. The most alteration of ERs expression were observed in the in vivo experiment. This study provides insight to understand how exposure of the goldfish macrophages to E2 and NP can up-regulate the transcript levels of estrogen receptor subtypes and stimulate apoptosis.

The Use of Molecular Descriptors To Model Pharmaceutical Uptake by a Fish Primary Gill Cell Culture Epithelium

Modeling approaches such as quantitative structure-activity relationships (QSARs) use molecular descriptors to predict the bioavailable properties of a compound in biota. However, these models have mainly been derived based on empirical data for lipophilic neutral compounds and may not predict the uptake of ionizable compounds. The majority of pharmaceuticals are ionizable, and freshwaters can have a range of pH values that affect speciation. In this study, we assessed the uptake of 10 pharmaceuticals (acetazolamide, beclomethasone, carbamazepine, diclofenac, gemfibrozil, ibuprofen, ketoprofen, norethindrone, propranolol, and warfarin) with differing modes of action and physicochemical properties (p K_a, log S, log D, log K_ow, molecular weight (MW), and polar surface area (PSA)) by an in vitro primary fish gill cell culture system (FIGCS) for 24 h in artificial freshwater. Principal component analysis (PCA) and partial least-squares (PLS) regression was used to determine the molecular descriptors that influence the uptake rates. Ionizable drugs were taken up by FIGCS; a strong positive correlation was observed between log S and the uptake rate, and a negative correlation was observed between p K_a, log D, and MW and the uptake rate. This approach shows that models can be derived on the basis of the physicochemical properties of pharmaceuticals and the use of an in vitro gill system to predict the uptake of other compounds. There is a need for a robust and validated model for gill uptake that could be used in a tiered risk assessment to prioritize compounds for experimental testing.

Prediction of bioconcentration factors in fish and invertebrates using machine learning

The application of machine learning has recently gained interest from ecotoxicological fields for its ability to model and predict chemical and/or biological processes, such as the prediction of bioconcentration. However, comparison of different models and the prediction of bioconcentration in invertebrates has not been previously evaluated. A comparison of 24 linear and machine learning models is presented herein for the prediction of bioconcentration in fish and important factors that influenced accumulation identified. R² and root mean square error (RMSE) for the test data (n = 110 cases) ranged from 0.23-0.73 and 0.34-1.20, respectively. Model performance was critically assessed with neural networks and tree-based learners showing the best performance. An optimised 4-layer multi-layer perceptron (14 descriptors) was selected for further testing. The model was applied for cross-species prediction of bioconcentration in a freshwater invertebrate, Gammarus pulex. The model for G. pulex showed good performance with R² of 0.99 and 0.93 for the verification and test data, respectively. Important molecular descriptors determined to influence bioconcentration were molecular mass (MW), octanol-water distribution coefficient (logD), topological polar surface area (TPSA) and number of nitrogen atoms (nN) among others. Modelling of hazard criteria such as PBT, showed potential to replace the need for animal testing. However, the use of machine learning models in the regulatory context has been minimal to date and is critically discussed herein. The movement away from experimental estimations of accumulation to in silico modelling would enable rapid prioritisation of contaminants that may pose a risk to environmental health and the food chain.

Estimating the Bioconcentration Factors of Hydrophobic Organic Compounds from Biotransformation Rates Using Rainbow Trout Hepatocytes

Determining the biotransformation potential of commercial chemicals is critical for estimating their persistence in the aquatic environment. In vitro systems are becoming increasingly important as screening methods for assessing the potential for chemical metabolism. Depletion rate constants (k_d) for several organic chemicals with high octanol-water partition coefficient (K_ow) values (9-methylanthracene, benzo(a)pyrene, chrysene, and PCB-153) in rainbow trout hepatocytes were determined to estimate biotransformation rate constants (k_MET) that were used in fish bioconcentration factor (BCF) models. Benzo[a]pyrene was rapidly biotransformed when incubated singly; however, its depletion rate constant (k_d) declined 79% in a mixture of all four chemicals. Chrysene also exhibited significant biotransformation and its depletion rate constant declined by 50% in the mixture incubation. These data indicate that biotransformation rates determined using single chemicals may overestimate metabolism in environments containing chemical mixtures. Incubations with varying cell concentrations were used to determine whether cell concentration affected k_d estimates. No statistically significant change in depletion rate constants were seen, possibly due to an increase in nonspecific binding of hydrophobic chemicals as cell density increased, decreasing overall biotransformation. A new model was used to estimate BCFs from k_MET values calculated from empirically derived k_d values. The inclusion of k_MET in models resulted in significantly lower BCF values (compared k_MET = 0). Modelled BCF values were consistent with empirically derived BCF values from the literature.

In vitro or not in vitro: a short journey through a long history

The aim of ecotoxicology is to study toxic effects on constituents of ecosystems, with the protection goal being populations and communities rather than individual organisms. In this ecosystem perspective, the use of in vitro methodologies measuring cellular and subcellular endpoints at a first glance appears to be odd. Nevertheless, more recently in vitro approaches gained momentum in ecotoxicology. In this article, we will discuss important application domains of in vitro methods in ecotoxicology. One area is the use of in vitro assays to replace, reduce, and refine (3R) in vivo tests. Research in this field has focused mainly on the use of in vitro cytotoxicity assays with fish cells as non-animal alternative to the in vivo lethality test with fish and on in vitro biotransformation assays as part of an alternative testing strategy for bioaccumulation testing with fish. Lessons learned from this research include the importance of a critical evaluation of the sensitivity, specificity and exposure conditions of in vitro assays, as well as the availability of appropriate in vitro-in vivo extrapolation models. In addition to this classical 3R application, other application domains of in vitro assays in ecotoxicology include the screening and prioritization of chemical hazards, the categorization of chemicals according to their modes of action and the provision of mechanistic information for the pathway-based prediction of adverse outcomes. The applications discussed in this essay may highlight the potential of in vitro technologies to enhance the environmental hazard assessment of single chemicals and complex mixtures at a reduced need of animal testing.

PBT assessment under REACH: Screening for low aquatic bioaccumulation with QSAR classifications based on physicochemical properties to replace BCF in vivo testing on fish

Aquatic bioconcentration factors (BCFs) are critical in PBT (persistent, bioaccumulative, toxic) and risk assessment of chemicals. High costs and use of more than 100 fish per standard BCF study (OECD 305) call for alternative methods to replace as much in vivo testing as possible. The BCF waiving scheme is a screening tool combining QSAR classifications based on physicochemical properties related to the distribution (hydrophobicity, ionisation), persistence (biodegradability, hydrolysis), solubility and volatility (Henry's law constant) of substances in water bodies and aquatic biota to predict substances with low aquatic bioaccumulation (nonB, BCF<2000). The BCF waiving scheme was developed with a dataset of reliable BCFs for 998 compounds and externally validated with another 181 substances. It performs with 100% sensitivity (no false negatives), >50% efficacy (waiving potential), and complies with the OECD principles for valid QSARs. The chemical applicability domain of the BCF waiving scheme is given by the structures of the training set, with some compound classes explicitly excluded like organometallics, poly- and perfluorinated compounds, aromatic triphenylphosphates, surfactants. The prediction confidence of the BCF waiving scheme is based on applicability domain compliance, consensus modelling, and the structural similarity with known nonB and B/vB substances. Compounds classified as nonB by the BCF waiving scheme are candidates for waiving of BCF in vivo testing on fish due to low concern with regard to the B criterion. The BCF waiving scheme supports the 3Rs with a possible reduction of >50% of BCF in vivo testing on fish. If the target chemical is outside the applicability domain of the BCF waiving scheme or not classified as nonB, further assessments with in silico, in vitro or in vivo methods are necessary to either confirm or reject bioaccumulative behaviour.

A Web-based Alternative Non-animal Method Database for Safety Cosmetic Evaluations

Animal testing was used traditionally in the cosmetics industry to confirm product safety, but has begun to be banned; alternative methods to replace animal experiments are either in development, or are being validated, worldwide. Research data related to test substances are critical for developing novel alternative tests. Moreover, safety information on cosmetic materials has neither been collected in a database nor shared among researchers. Therefore, it is imperative to build and share a database of safety information on toxicological mechanisms and pathways collected through in vivo, in vitro, and in silico methods. We developed the CAMSEC database (named after the research team; the Consortium of Alternative Methods for Safety Evaluation of Cosmetics) to fulfill this purpose. On the same website, our aim is to provide updates on current alternative research methods in Korea. The database will not be used directly to conduct safety evaluations, but researchers or regulatory individuals can use it to facilitate their work in formulating safety evaluations for cosmetic materials. We hope this database will help establish new alternative research methods to conduct efficient safety evaluations of cosmetic materials.

QSARs for estimating intrinsic hepatic clearance of organic chemicals in humans

Quantitative structure-activity relationships (QSARs) were developed to predict the in vitro clearance (CLINT) of xenobiotics metabolised in human hepatocytes (118 compounds) and microsomes (115 compounds). Clearance values were gathered from the scientific literature and multiple linear models were built and validated selecting at most 6 predictors from a pool of over 2000 potential molecular descriptors. For the hepatocytes QSAR, the explained variance (Radj(2)) was 67% and the predictive ability (Rext(2)) was 62%. For the microsomes QSAR, Radj(2) was 50% and Rext(2) 30%. For both liver assays, the most important descriptor relates to electronic properties of the compound. Functional groups of fragments were useful to identify specific compounds that have a deviating reaction rate compared to the others, such as polychlorobiphenyls (PCBs) and organic amides which were poorly metabolised by hepatocytes and microsomes, respectively. For hepatocytes, clearance was predominantly determined by electronic characteristics, while size and shape characteristics were less important and partitioning properties were absent. This may suggest that uptake across the membrane and enzyme binding are not rate-limiting steps. Particularly for hepatocytes the QSAR statistics are encouraging, allowing application of the outcomes in in vitro to in vivo extrapolation.

Review of laboratory-based terrestrial bioaccumulation assessment approaches for organic chemicals: Current status and future possibilities

In the last decade, interest has been renewed in approaches for the assessment of the bioaccumulation potential of chemicals, principally driven by the need to evaluate large numbers of chemicals as part of new chemical legislation, while reducing vertebrate test organism use called for in animal welfare legislation. This renewed interest has inspired research activities and advances in bioaccumulation science for neutral organic chemicals in aquatic environments. In January 2013, ILSI Health and Environmental Sciences Institute convened experts to identify the state of the science and existing shortcomings in terrestrial bioaccumulation assessment of neutral organic chemicals. Potential modifications to existing laboratory methods were identified, including areas in which new laboratory approaches or test methods could be developed to address terrestrial bioaccumulation. The utility of "non-ecotoxicity" data (e.g., mammalian laboratory data) was also discussed. The highlights of the workshop discussions are presented along with potential modifications in laboratory approaches and new test guidelines that could be used for assessing the bioaccumulation of chemicals in terrestrial organisms.

A tiered assessment strategy for more effective evaluation of bioaccumulation of chemicals in fish

There is currently limited guidance available for regulators and risk assessors on how to use data from non-guideline methods when assessing the bioaccumulation potential of a chemical. Furthermore, bioaccumulation assessments can be more subjective than they need to be due to the lack of a guidance framework on how to use/include the range of information that may be available for a substance. Under some circumstances, in silico, in vitro and/or in vivo non-test guideline data may be sufficient to classify whether a substance is bioaccumulative without the need for further animal testing. Classifying the bioaccumulative potential of a substance is especially difficult when the bioconcentration factor (BCF) is close to the threshold for defining it as bioaccumulative/very bioaccumulative (B/vB), and a more structured process is required to reduce uncertainty in the BCF estimates. In these situations, in silico and in vitro data can, and should, be used to provide greater confidence in classifying these substances. To aid future evaluations of bioaccumulation data, a proposed tiered assessment strategy is presented incorporating all available data on the bioaccumulative properties of a substance. In addition, a revised scheme is recommended for improving the classification of the bioaccumulative potential of a substance.

In vitro methodologies in ecotoxicological hazard assessment: the case of bioaccumulation testing for fish

The concerted research efforts undertaken in recent years have highlighted the potential of in vitro approaches, as part of an integrated testing strategy, to replace or reduce in vivo bioaccumulation testing in fish

Zebrafish (Danio rerio) eleutheroembryo-based procedure for assessing bioaccumulation

This paper reports on the development and preliminary evaluation of a new bioaccumulation test based on the use of zebrafish (Danio renio) eleutheroembryos (72 h after hatching, corresponding to 144 h post fertilization, hpf) as an alternative to adult fish-based procedures for regulatory purposes regarding REACH application. The proposed test accomplished the OECD 305 guideline and consists of a 48 h uptake period followed by a 24 h depuration step. Bioaccumulation experiments were performed for a selected of hyper hydrophobic chemicals (log Kow> 7.6), that is, PCB 136 and PBDE 154 at two concentration levels corresponding roughly to 1% and 0.1% the chemicaĺs LC50(nominal concentrations of 4.0 and 12.0 μg/L for PCB 136, and 1.0 and 5.0 μg/L PBDE 154, respectively). Toxicokinetic models were used to calculate the bioconcentration factors (BCFs) based on of the chemical concentrations found in the contaminated eleutheroembryos and their surrounding media. The experimentally determined accumulation profiles show bioaccumulation by zebrafish eleutheroembryos of both chemicals, and that the process is more complex than simple water-lipid partition. Calculated log BCFs using a first-order accumulation model(3.97 and 3.73 for PCB 136, and 3.95 and 4.29 for PBDE 154) were in the range of those previously reported in the literature. The suitability of this new nonprotected life stage bioaccumulation protocol for BCF estimation was evaluated by application to widely divergent micropollutants with different accumulation mechanisms. The results were compared with those in the MITE-NITE database for adult rice fish (Oryzias latipes).

A primary fish gill cell culture model to assess pharmaceutical uptake and efflux: evidence for passive and facilitated transport

The gill is the principle site of xenobiotic transfer to and from the aqueous environment. To replace, refine or reduce (3Rs) the large numbers of fish used in in vivo uptake studies an effective in vitro screen is required that mimics the function of the teleost gill. This study uses a rainbow trout (Oncorhynchus mykiss) primary gill cell culture system grown on permeable inserts, which tolerates apical freshwater thus mimicking the intact organ, to assess the uptake and efflux of pharmaceuticals across the gill. Bidirectional transport studies in media of seven pharmaceuticals (propranolol, metoprolol, atenolol, formoterol, terbutaline, ranitidine and imipramine) showed they were transported transcellularly across the epithelium. However, studies conducted in water showed enhanced uptake of propranolol, ranitidine and imipramine. Concentration-equilibrated conditions without a concentration gradient suggested that a proportion of the uptake of propranolol and imipramine is via a carrier-mediated process. Further study using propranolol showed that its transport is pH-dependent and at very low environmentally relevant concentrations (ng L(-1)), transport deviated from linearity. At higher concentrations, passive uptake dominated. Known inhibitors of drug transport proteins; cimetidine, MK571, cyclosporine A and quinidine inhibited propranolol uptake, whilst amantadine and verapamil were without effect. Together this suggests the involvement of specific members of SLC and ABC drug transporter families in pharmaceutical transport.

Integrated testing strategy (ITS) for bioaccumulation assessment under REACH

REACH (registration, evaluation, authorisation and restriction of chemicals) regulation requires that all the chemicals produced or imported in Europe above 1 tonne/year are registered. To register a chemical, physicochemical, toxicological and ecotoxicological information needs to be reported in a dossier. REACH promotes the use of alternative methods to replace, refine and reduce the use of animal (eco)toxicity testing. Within the EU OSIRIS project, integrated testing strategies (ITSs) have been developed for the rational use of non-animal testing approaches in chemical hazard assessment. Here we present an ITS for evaluating the bioaccumulation potential of organic chemicals. The scheme includes the use of all available data (also the non-optimal ones), waiving schemes, analysis of physicochemical properties related to the end point and alternative methods (both in silico and in vitro). In vivo methods are used only as last resort. Using the ITS, in vivo testing could be waived for about 67% of the examined compounds, but bioaccumulation potential could be estimated on the basis of non-animal methods. The presented ITS is freely available through a web tool.

Gill cell culture systems as models for aquatic environmental monitoring

A vast number of chemicals require environmental safety assessments for market authorisation. To ensure acceptable water quality, effluents and natural waters are monitored for their potential harmful effects. Tests for market authorisation and environmental monitoring usually involve the use of large numbers of organisms and, for ethical, cost and logistic reasons, there is a drive to develop alternative methods that can predict toxicity to fish without the need to expose any animals. There is therefore a great interest in the potential to use cultured fish cells in chemical toxicity testing. This review summarises the advances made in the area and focuses in particular on a system of cultured fish gill cells grown into an epithelium that permits direct treatment with water samples.

Resampling the bioconcentration factors data from Japan's chemical substances control law database to simulate and evaluate the bioconcentration factors derived from minimized aqueous exposure tests

Existing standard bioconcentration tests (e.g., the Organization for Economic Cooperation and Development [OECD] test guideline 305) require large numbers of test animals and resources. The minimized aqueous exposure test is a new approach based on the standard bioconcentration test but allows estimation of bioconcentration factor (BCF) by minimized sampling of the test fish. The authors collected BCF data (298 curves from 155 chemicals, using common carp as test species) from Japan's Chemical Substances Control Law database and resampled the data to simulate the calculation of BCF that would be obtained if studies had been designed to obtain kinetic BCF derived from minimized aqueous exposure tests (BCF(km)). The correlation was high (r(2)  = 0.967) between BCF derived from standard bioconcentration tests (BCF(full)) and BCF(km). The average value of the BCF(full) to BCF(km) ratio (BCF(full):BCF(km)) was 1.04 and ranged from 0.54 to 1.93, the 5th and 95th percentiles being 0.74 and 1.45, respectively. The results based on the 5th and 95th percentiles of the BCF(full):BCF(km) ratio suggest that BCF(full) 2,000 corresponds to BCF(km) 1,400 to 2,700, whereas BCF(full) 5,000 corresponds to BCF(km) 3,400 to 6,800. The authors also emphasize that the standard bioconcentration test should be performed when the resulting BCF(km) is in the region of regulatory concern.

Reducing the number of fish in bioconcentration studies for plant protection products by reducing the number of test concentrations

Fish bioconcentration tests are time consuming, expensive, and use many animals. Alternative methods that replace, reduce or refine the use of fish for BCF testing would therefore be of value. Test guidelines generally require that bioconcentration factors (BCFs) are determined at two exposure concentrations. However, recent revisions to the OECD Test Guideline for BCF testing (TG 305) provide the option to use only one exposure concentration, when justification is provided, although two concentrations may still be required for some regulatory purposes. Analysis of 55 studies on plant protection products demonstrates that BCF values do not significantly differ between the two exposure concentrations. This analysis therefore provides evidence to support the revision of OECD TG 305, and in particular provides justification for using the one test concentration approach for plant protection product active substances.

Internal benchmarking improves precision and reduces animal requirements for determination of fish bioconcentration factors

The enactment of new chemical regulations has generated a large need for the measurement of the fish bioconcentration factor (BCF). Past experience shows that the BCF determination lacks precision, requires large numbers of fish, and is costly. A new protocol was tested that shortens the experiment from up to 12 weeks for existing protocols to 2 weeks and reduces the number of fish by a factor of 5, while introducing internal benchmarking for the BCF determination. Rainbow trout were simultaneously exposed to 11 chemicals. The BCFs were quantified using one of the test chemicals, musk xylene, as a benchmark. These were compared with BCFs measured in a parallel experiment based on the OECD 305 guideline. The agreement was <20% for five chemicals and between 20%-25% for two further, while two chemicals lay outside the BCF operating window of the experiment and one was lost due to analytical difficulties. This agreement is better than that observed in a BCF Gold Standard Database. Internal benchmarking allows the improvement of the precision of BCF determination in parallel to large reduction in costs and fish requirements.

Bioconcentration of pesticides in zebrafish eleutheroembryos (Danio rerio)

The feasibility of a bioaccumulation test based on the use of zebrafish eleutheroembryos as an alternative to adult-individual-based approaches for REACH application has been evaluated for three test compounds, chlorpyrifos, dicofol and atrazine. Following the OECD 305 guidelines, zebrafish eleutheroembryos (72 h after hatching, hpf) were separately exposed to the investigated pesticides at two nominal concentrations below 1% of its corresponding LC(50). The uptake experiments lasted for 48 h. Then, the exposure medium was replaced by a non-contaminated medium for depuration experiments (up to 72 h). Zebrafish eleutheroembryos (larvae 144 hpf, i.e. at the end of the depuration step) and their corresponding exposure media was sampled at ten different times during each experiment and the concentration of the investigated pesticide determined in both the organisms and in the exposure medium. The experimentally determined pesticide accumulation profiles in the eleutheroembryos demonstrated that atrazine has a very fast accumulation kinetic, reaching steady sate (SS) within 24h. Chlorpyrifos and dicofol did not reach the SS within the 48-h uptake experiments although they exhibit higher accumulations than the former pesticide. Two toxicokinetic models were used to calculate the bioconcentration factor (BCF) of the studied pesticide in zebrafish eleutheroembryos. In the former, the BCF was calculated under SS conditions (BCF(SS)). The second was used when the compounds did not reach the SS during the uptake experiment (BCF(k)). Log BCF values of 3.55 and 3.84 for chlorpyrifos; 0.6 and 1.17 for atrazine, and 3.90 for dicofol were experimentally calculated at selected exposure concentrations. These values have been compared with those reported in related bioaccumulation studies and official databases.

High content screening in zebrafish speeds up hazard ranking of transition metal oxide nanoparticles

Zebrafish is an aquatic organism that can be used for high content safety screening of engineered nanomaterials (ENMs). We demonstrate, for the first time, the use of high content bright-field and fluorescence-based imaging to compare the toxicological effect of transition metal oxide (CuO, ZnO, NiO, and Co(3)O(4)) nanoparticles in zebrafish embryos and larvae. High content bright-field imaging demonstrated potent and dose-dependent hatching interference in the embryos, with the exception of Co(3)O(4) which was relatively inert. We propose that the hatching interference was due to the shedding of Cu and Ni ions, compromising the activity of the hatching enzyme, ZHE1, similar to what we previously proposed for Zn(2+). This hypothesis is based on the presence of metal-sensitive histidines in the catalytic center of this enzyme. Co-introduction of a metal ion chelator, diethylene triamine pentaacetic acid (DTPA), reversed the hatching interference of Cu, Zn, and Ni. While neither the embryos nor larvae demonstrated morphological abnormalities, high content fluorescence-based imaging demonstrated that CuO, ZnO, and NiO could induce increased expression of the heat shock protein 70:enhanced green fluorescence protein (hsp70:eGFP) in transgenic zebrafish larvae. Induction of this response by CuO required a higher nanoparticle dose than the amount leading to hatching interference. This response was also DTPA-sensitive. We demonstrate that high content imaging of embryo development, morphological abnormalities, and HSP70 expression can be used for hazard ranking and determining the dose-response relationships leading to ENM effects on the development of the zebrafish embryo.

Protein and lipid binding parameters in rainbow trout (Oncorhynchus mykiss) blood and liver fractions to extrapolate from an in vitro metabolic degradation assay to in vivo bioaccumulation potential of hydrophobic organic chemicals

Binding of hydrophobic chemicals to colloids such as proteins or lipids is difficult to measure using classical microdialysis methods due to low aqueous concentrations, adsorption to dialysis membranes and test vessels, and slow kinetics of equilibration. Here, we employed a three-phase partitioning system where silicone (polydimethylsiloxane, PDMS) serves as a third phase to determine partitioning between water and colloids and acts at the same time as a dosing device for hydrophobic chemicals. The applicability of this method was demonstrated with bovine serum albumin (BSA). Measured binding constants (K(BSAw)) for chlorpyrifos, methoxychlor, nonylphenol, and pyrene were in good agreement with an established quantitative structure-activity relationship (QSAR). A fifth compound, fluoxypyr-methyl-heptyl ester, was excluded from the analysis because of apparent abiotic degradation. The PDMS depletion method was then used to determine partition coefficients for test chemicals in rainbow trout (Oncorhynchus mykiss) liver S9 fractions (K(S9w)) and blood plasma (K(bloodw)). Measured K(S9w) and K(bloodw) values were consistent with predictions obtained using a mass-balance model that employs the octanol-water partition coefficient (K(ow)) as a surrogate for lipid partitioning and K(BSAw) to represent protein binding. For each compound, K(bloodw) was substantially greater than K(S9w), primarily because blood contains more lipid than liver S9 fractions (1.84% of wet weight vs 0.051%). Measured liver S9 and blood plasma binding parameters were subsequently implemented in an in vitro to in vivo extrapolation model to link the in vitro liver S9 metabolic degradation assay to in vivo metabolism in fish. Apparent volumes of distribution (V(d)) calculated from the experimental data were similar to literature estimates. However, the calculated binding ratios (f(u)) used to relate in vitro metabolic clearance to clearance by the intact liver were 10 to 100 times lower than values used in previous modeling efforts. Bioconcentration factors (BCF) predicted using the experimental binding data were substantially higher than the predicted values obtained in earlier studies and correlated poorly with measured BCF values in fish. One possible explanation for this finding is that chemicals bound to proteins can desorb rapidly and thus contribute to metabolic turnover of the chemicals. This hypothesis remains to be investigated in future studies, ideally with chemicals of higher hydrophobicity.

Decreased dissolution of ZnO by iron doping yields nanoparticles with reduced toxicity in the rodent lung and zebrafish embryos

We have recently shown that the dissolution of ZnO nanoparticles and Zn(2+) shedding leads to a series of sublethal and lethal toxicological responses at the cellular level that can be alleviated by iron doping. Iron doping changes the particle matrix and slows the rate of particle dissolution. To determine whether iron doping of ZnO also leads to lesser toxic effects in vivo, toxicity studies were performed in rodent and zebrafish models. First, we synthesized a fresh batch of ZnO nanoparticles doped with 1-10 wt % of Fe. These particles were extensively characterized to confirm their doping status, reduced rate of dissolution in an exposure medium, and reduced toxicity in a cellular screen. Subsequent studies compared the effects of undoped to doped particles in the rat lung, mouse lung, and the zebrafish embryo. The zebrafish studies looked at embryo hatching and mortality rates as well as the generation of morphological defects, while the endpoints in the rodent lung included an assessment of inflammatory cell infiltrates, LDH release, and cytokine levels in the bronchoalveolar lavage fluid. Iron doping, similar to the effect of the metal chelator, DTPA, interfered in the inhibitory effects of Zn(2+) on zebrafish hatching. In the oropharyngeal aspiration model in the mouse, iron doping was associated with decreased polymorphonuclear cell counts and IL-6 mRNA production. Doped particles also elicited decreased heme oxygenase 1 expression in the murine lung. In the intratracheal instillation studies in the rat, Fe doping was associated with decreased polymorphonuclear cell counts, LDH, and albumin levels. All considered, the above data show that Fe doping is a possible safe design strategy for preventing ZnO toxicity in animals and the environment.

The influence of gill and liver metabolism on the predicted bioconcentration of three pharmaceuticals in fish

The potential for xenobiotic compounds to bioconcentrate is typically expressed through the bioconcentration factor (BCF), which has gained increased regulatory significance over the past decade. Due to the expense of in vivo bioconcentration studies and the growing regulatory need to assess bioconcentration potential, BCF is often calculated via single-compartment models, using K(OW) as the primary input. Recent efforts to refine BCF models have focused on physiological factors, including the ability of the organism to eliminate the compound through metabolic transformation. This study looks at the ability of in vitro biotransformation assays using S9 fractions to provide an indication of metabolic potential. Given the importance of the fish gill and liver in metabolic transformation, the metabolic loss of ibuprofen, norethindrone and propranolol was measured using rainbow trout (Oncorhynchus mykiss) and channel catfish (Ictalurus punctatus) gill and liver S9 fractions. Metabolic transformation rates (k(M)) were calculated and integrated into a refined BCF model. A significant difference was noted between BCF solely based on K(OW) and BCF including k(M). These studies indicate that the inclusion of k(M) in BCF models can bring predicted bioconcentration estimates closer to in vivo values.

Evaluation of liposome-water partitioning for predicting bioaccumulation potential of hydrophobic organic chemicals

Considering the importance of bioaccumulation factors (BAFs) in risk assessment of chemicals and the ethical issues and complexity of the determination of these factors in standard tests with living organisms, there is a need for alternative approaches for predicting bioaccumulation. In this study, liposome-water partitioning coefficients as determined by using solid-phase microextraction (SPME) were evaluated for the cause of assessing bioaccumulation potential of hydrophobic organic chemicals (HOCs). To this end, the SPME method was mapped (in terms of mass balance, mode of spiking, kinetics, and reproducibility) and validated against literature data. Furthermore, the robustness of liposomes as partitioning phase was investigated (in terms of chemical loading, and pH and ionic strength of the medium), and finally liposome-water partition coefficients (K(lipw)) determined for polycyclic aromatic hydrocarbons (PAHs; 4.5 < logK(ow) < 7.2) were compared with literature BAF values for several aquatic species. The results indicated that (i) SPME is a valid, fast, and reproducible method for measuring K(lipw) values; (ii) liposomes provide a very robust partitioning phase; and (iii) K(lipw) values agreed very well with literature PAH BAF values. SPME-derived K(lipw) values therefore seem a very promising predictor of bioaccumulation potential of HOCs. By including model- or in vitro-derived biotransformation rates, bioaccumulation potential estimates might be converted into surrogate BAFs, thereby extending the applicability of K(lipw) values to metabolizable chemicals and species with more advanced biotransformation capacity.

A novel in vitro system for the determination of bioconcentration factors and the internal dose in zebrafish (Danio rerio) eggs

In this study a novel in vitro approach for the determination of bioconcentration factors (BCF) and rate constants of lipophilic substances utilizing zebrafish (Danio rerio) eggs is presented. Zebrafish eggs were exposed in a static exposure regime towards a phenanthrene solution and concentration-time profiles of the exposure solutions were analyzed over time. The rate constants and the BCF were obtained from the concentration-time profile with the use of a least-square fit to a non-linear model. The determined BCF at steady-state (after 72h of exposure) for phenanthrene was estimated to be only about 1.5 times lower, than the respective BCF value reported in the literature. For uptake of solutes in zebrafish embryos, different transport processes are assumed as substances have to pass the chorion first and subsequently the membranes of the embryo. To investigate this, the period to steady-state concentration between zebrafish eggs and the ambient medium for phenanthrene under an agitated and non-agitated static exposure regime were compared. It was found, that this equilibrium was reached within a shorter time frame under agitation, resulting in higher rate constants. In addition to the determination of bioconcentration parameters, the internal phenanthrene dose in zebrafish eggs was determined by utilizing a biomimetic extraction method with water as transfer medium. Approximately 55% of the expected accumulated phenanthrene amount in zebrafish eggs could be re-extracted with a silicone rod extraction method. These results agree very well to what has been observed in abiotic systems. The scope of the proposed in vitro protocol to serve as an alternative for BCF determinations using established in vivo animal testing protocols with adult fish is discussed.

In vitro biotransformation of surfactants in fish. Part II--Alcohol ethoxylate (C16EO8) and alcohol ethoxylate sulfate (C14EO2S) to estimate bioconcentration potential

Recent regulatory pressures (e.g., REACh, CEPA) requiring bioaccumulation assessments and the need for reduced animal use have increased the necessity for the development of in vitro-based methods to estimate bioaccumulation. Our study explored the potential use of subcellular and cellular hepatic systems to determine the biotransformation potential of two surfactants: octaethylene glycol monohexadecyl ether (C16EO8) and diethylene glycol monotetradecyl ether sulfate (C14EO2S). The subcellular systems tested were liver homogenates and microsomes from the common carp (Cyprinus carpio) and rainbow trout (Oncorhynchus mykiss). Cellular systems consisted of primary hepatocytes from the common carp (C. carpio) and PLHC-1 cells, hepatocarcinoma cells from the desert topminnow (Poeciliopsis lucida) cell line. Each in vitro system was exposed to radiolabeled test compounds and assayed for biotransformation using liquid scintillation and thin layer chromatographic methods. First-order kinetics were used to estimate rates of biotransformation. Bioconcentration of test materials in fish were predicted using an in vitro to in vivo metabolic rate extrapolation model linked to a mass-balance model commonly used to predict bioaccumulation in fish. Both subcellular and cellular tests using microsomes, liver homogenates and hepatocytes respectively showed biotransformation of the parent surfactants. Biotransformation rates were fastest for hepatocytes, followed by microsomes and homogenates. Rates were too low from homogenate tests to extrapolate to in vivo-based biotransformation rates using the extrapolation model. Trout microsomes metabolized C16EO8 faster than carp microsomes, yet rates were approximately the same for C14EO2S. Predicted BCF values incorporating in vitro biotransformation rates from hepatocytes were similar to measured in vivo or USEPA's bioconcentration model (BCFWIN) predicted values. Predicted BCF values using microsomal-based rates from trout and carp studies were only slightly less than default BCF values which assumes a linear logKow to BCF relationship with no biotransformation. However, hepatocyte-based results showed substantially decreased BCFs compared to the default BCF values. These results indicate that BCF estimates based on in vitro metabolic rates can provide reasonable estimates of in vivo BCF values, therefore, supporting the use of in vitro approaches within a tiered approach to assess bioconcentration.

In vitro biotransformation of surfactants in fish. Part I: linear alkylbenzene sulfonate (C12-LAS) and alcohol ethoxylate (C13EO8)

Developing regulatory activities (e.g., REACh, [DGEE. 2003. Directorates General Enterprise and Environment. The new EU chemicals legislation REACH. DG Enterprise, Brussels, Belgium. (http://www.europa.eu.int/comm/enterprise/reach/index_en.htm)]) will require bioaccumulation to be assessed for thousands of chemicals. Further, there is increasing pressure to reduce, refine or replace animal tests. Given this scenario, there is an urgent need to evaluate the feasibility of in vitro systems to supply data useful for bioaccumulation estimation. Subcellular and cellular hepatic systems were tested to determine the biotransformation of two surfactants: C12-2-LAS (2-phenyl dodecane p-sulfonate) and an alcohol ethoxylate C13EO8 (Octaethylene glycol monotridecyl ether). The subcellular systems tested were liver homogenates and microsomes from the common carp (Cyprinus carpio) and rainbow trout (Oncorhynchus mykiss). Cellular systems consisted of primary hepatocytes from the common carp (Cyprinus carpio) and PLHC-1 cells, hepatocarcinoma cells from the desert topminnow (Poeciliopsis lucida). All in vitro systems were exposed to radiolabeled test compounds and assayed for biotransformation using liquid scintillation and thin layer chromatographic methods. First-order kinetics were used to estimate rates of biotransformation. Bioconcentration of test materials in fish were predicted using an in vitro to in vivo metabolic rate extrapolation model linked to a mass-balance model commonly used to predict bioaccumulation in fish. Subcellular biotransformation rates for each of the surfactants were greatest with microsomes. Cellular loss rates exceeded subcellular rates, leading to lower predicted BCF values. Predicted BCFs corresponded closely to measured values in several fish species, verifying the utility of in vitro systems in refining Kow-only-based BCFs via the inclusion of biotransformation rates.