Protective role of 2-aminothiazole derivative against ethanol-induced teratogenic effects in-vivo zebrafish

Teratology investigates the origins of congenital disabilities, often linked to environmental factors such as ethanol (EtOH) exposure. Ethanol at 150 μM has been associated with teratogenic effects, oxidative stress, immunological responses, and endocrine disruptions. Fetal alcohol spectrum disorder (FASD) arises from maternal alcohol consumption during pregnancy, leading to developmental delays and cognitive impairment. Due to their diverse therapeutic applications, amino thiazole derivatives are crucial in drug development. This study aimed to determine whether the 2-amino thiazole derivative, notably the 1-(4-chlorophenyl)-N-(6-nitrobenzo[d]thiazol-2-yl)ethan-1-imine (N4) compound, reduces teratogenic effects induced by embryonic EtOH exposure in a zebrafish model. Teratogenic effects, mortality, locomotion behaviour, oxidative stress, gene expression, and tissue damage were evaluated in larvae over a 7-day experimental period using three treatment concentrations (50, 100, and 150 μM). Results showed that EtOH induced morphological defects in the head, eyes, and body length of exposed larvae, along with behavioural abnormalities and oxidative damage. N4 effectively mitigated these toxic effects in a concentration-dependent manner, reducing oxidative damage, preventing teratogenic effects, and averting tissue damage induced by EtOH exposure. This study highlights the potential of N4 to enhance antioxidant and anti-inflammatory effects against ethanol-induced oxidative stress, offering promising therapeutic strategies for FASD treatment.

Assessing developmental toxicity and non-CYP mediated biotransformation of two anti-epileptics and their human metabolites in zebrafish embryos and larvae

Zebrafish embryo-based assays are a promising alternative for animal testing to screen new compounds for developmental toxicity. However, recent studies in zebrafish embryos showed an immature intrinsic cytochrome P450 (CYP)-mediated biotransformation capacity, as most CYPs were only active at the end of the organogenesis period. Data on other phase I enzymes involved in the biotransformation of xenobiotics in zebrafish embryos is limited. This information is pivotal for proteratogens needing bioactivation to exert their teratogenic potential. Therefore, this study aimed to investigate whether carbamazepine (CBZ) and levetiracetam (LTC), two anti-epileptic drugs that require bioactivation to exert their teratogenic potential, are biotransformed into non-CYP mediated metabolites in the zebrafish embryo and whether one or more of these metabolites cause developmental toxicity in this species. In the first step, zebrafish embryos were exposed to LTC and CBZ and their non-CYP mediated human metabolites, etiracetam carboxylic acid (ECA) and 9-acridine carboxaldehyde (9ACA), acridine (AI), and acridone (AO), respectively, from 5.25 to 120 hpf and morphologically evaluated. Next, the uptake of all compounds and the formation of the metabolites were assessed using LC-MS methods. As LTC and ECA were, respectively, poorly or not taken up by zebrafish larvae during the exposure experiments, we could not determine if LTC and ECA are teratogenic. However, biotransformation of LTC into ECA was observed at 24 hpf and 120 hpf, which indicates that the special type of B-esterase is already active at 24 hpf. CBZ and its three metabolites were teratogenic, as a significant increase in malformed embryos was observed for all of them. All three metabolites were more potent teratogens than CBZ, with AI being the most potent, followed by 9ACA and AO. The myeloperoxidase (MPO) homologue is already active at 24 hpf, as CBZ was biotransformed into 9ACA and AO in 24 hpf zebrafish embryos, and into 9ACA in 120 hpf larvae. Moreover, 9ACA was also found to be biotransformed into AI and AO, and AI into AO. As such, one or more of these metabolites probably contribute to the teratogenic effects observed in zebrafish larvae after exposure to CBZ.

Indigo dyes: Toxicity, teratogenicity, and genotoxicity studies in zebrafish embryos

Wastewater released by textile dyeing industries is a major source of pollution. Untreated wastewater released from indigo dyeing operations affects aquatic ecosystems and threatens their biodiversity. We have assessed the toxicity of natural and synthetic indigo dye in zebrafish embryos, using the endpoints of teratogenicity, genotoxicity, and histopathology. The zebrafish embryo toxicity test (ZFET) was conducted, exposing embryos to ten concentrations of natural and synthetic indigo dyes; the 96-hour LC50 values were approximately 350 and 300 mg/L, respectively. Both dyes were teratogenic, causing egg coagulation, tail detachment, yolk sac edema, pericardial edema, and tail bend, with no significant difference in effects between the natural and synthetic dyes. Both dyes were genotoxic (using comet assay for DNA damage). Real-time RT-PCR studies showed upregulation of the DNA-repair genes FEN1 and ERCC1. Severe histological changes were seen in zebrafish larvae following exposure to the dyes. Our results show that indigo dyes may be teratogenic and genotoxic to aquatic organisms, underscoring the need for development of sustainable practices and policies for mitigating the environmental impacts of textile dyeing.

Interlaboratory Study on Zebrafish in Toxicology: Systematic Evaluation of the Application of Zebrafish in Toxicology's (SEAZIT's) Evaluation of Developmental Toxicity

Embryonic zebrafish represent a useful test system to screen substances for their ability to perturb development. The exposure scenarios, endpoints captured, and data analysis vary among the laboratories who conduct screening. A lack of harmonization impedes the comparison of the substance potency and toxicity outcomes across laboratories and may hinder the broader adoption of this model for regulatory use. The Systematic Evaluation of the Application of Zebrafish in Toxicology (SEAZIT) initiative was developed to investigate the sources of variability in toxicity testing. This initiative involved an interlaboratory study to determine whether experimental parameters altered the developmental toxicity of a set of 42 substances (3 tested in duplicate) in three diverse laboratories. An initial dose-range-finding study using in-house protocols was followed by a definitive study using four experimental conditions: chorion-on and chorion-off using both static and static renewal exposures. We observed reasonable agreement across the three laboratories as 33 of 42 test substances (78.6%) had the same activity call. However, the differences in potency seen using variable in-house protocols emphasizes the importance of harmonization of the exposure variables under evaluation in the second phase of this study. The outcome of the Def will facilitate future practical discussions on harmonization within the zebrafish research community.

Bio-efficacy of insecticidal molecule emodin against dengue, filariasis, and malaria vectors

Emodin, a compound isolated from Aspergillus terreus, was studied using chromatographic and spectroscopic methods and compound purity (96%) was assessed by TLC. Furthermore, high larvicidal activity against Aedes aegypti-AeA (LC₅₀ 6.156 and LC₉₀ 12.450 mg/L), Culex quinquefasciatus-CuQ (8.216 and 14.816 mg/L), and Anopheles stephensi-AnS larvae (6.895 and 15.24 mg/L) was recorded. The first isolated fraction (emodin) showed higher pupicidal activity against AeA (15.449 and 20.752 mg/L). Most emodin-treated larvae (ETL) showed variations in acetylcholine esterase, α and β-carboxylesterases, and phosphatase activities in the 4^th instar, indicating the intrinsic differences in their biochemical changes. ETL had numerous altered tissues, including muscle, gastric caeca, hindgut, midgut, nerve ganglia, and midgut epithelium. Acute toxicity of emodin on brine shrimp Artemia nauplii (54.0 and 84.5 mg/L) and the zebrafish Danio rerio (less toxicity observed) was recorded. In docking studies, Emodin interacted well with odorant-binding-proteins of AeA, AnS, and CuQ with docking scores of - 8.89, - 6.53, and - 8.09 kcal mol^-1, respectively. Therefore, A. terreus is likely to be effective against mosquito larvicides.

Contributions towards the hazard evaluation of two widely used cytostatic drugs

Cytostatic drugs are one of the most important therapeutic options for cancer, a disease that is expected to affect 29 million individuals by 2040. After being excreted, cytostatics reach wastewater treatment plants (WWTPs), which are unable to efficiently remove them, and consequently, they will be released into the aquatic environment. Due to the highly toxic properties of cytostatics, it is particularly relevant to evaluate their potential ecological risk. Yet, cytostatics toxicity data is still not available for various species. In this work, the ecotoxicity of two widely consumed cytostatics, cyclophosphamide (CYP-as a model cytostatic) and mycophenolic acid (MPA-as a priority cytostatic), was evaluated on three freshwater species-Raphidocelis subcapitata, Brachionus calyciflorus, and Danio rerio, and the risk quotient (RQ) was assessed. Both drugs significantly affected the yield and growth inhibition of the microalgae, while for rotifers, the least sensitive species, only significant effects were registered for CYP. These drugs also caused significant effects on the mortality and morphological abnormalities on zebrafish. The estimation of the RQ discloses that CYP seems to pose a low risk to aquatic biota while MPA poses a very high risk. Altogether, these results emphasize the need for more complete environmental risk assessments, to properly prioritize and rank cytostatics according to their potentially toxic effects on the environment and aquatic biota.

Dynamic regulation of gene expression and morphogenesis in the zebrafish embryo test after exposure to all-trans retinoic acid

The zebrafish embryotoxicity test (ZET) is widely used in developmental toxicology. The analysis of gene expression regulation in ZET after chemical exposure provides mechanistic information about the effects of chemicals on morphogenesis in the test. The gene expression response magnitude has been shown to change with exposure duration. The objective of this work is to study the effect of the exposure duration on the magnitude of gene expression changes in the all-trans retinoic acid (ATRA) signaling pathway in the ZET. Retinoic acid regulation is a key driver of morphogenesis and is therefore employed here as an indicator for the regulation of developmental genes. A teratogenic concentration of 7.5 nM of ATRA was given at 3 hrs post fertilization (hpf) for a range of exposure durations until 120 hrs of development. The expression of a selection of genes related to ATRA signaling and downstream developmental genes was determined. The highest magnitudes of gene expression regulation were observed after 2-24 hrs exposure with an optimal response after 4 hrs. Longer exposures showed a decrease in the gene expression response, although continued exposure to 120 hpf caused malformations and lethality. This study shows that assessment of gene expression regulation at early time points after the onset of exposure in the ZET may be optimal for the prediction of developmental toxicity. We believe these results could help optimize sensitivity in future studies with ZET.

Developmental Toxicity and Biotransformation of Two Anti-Epileptics in Zebrafish Embryos and Early Larvae

The zebrafish (Danio rerio) embryo is gaining interest as a bridging tool between in-vitro and in-vivo developmental toxicity studies. However, cytochrome P450 (CYP)-mediated drug metabolism in this model is still under debate. Therefore, we investigated the potential of zebrafish embryos and larvae to bioactivate two known anti-epileptics, carbamazepine (CBZ) and phenytoin (PHE), to carbamazepine-10,11-epoxide (E-CBZ) and 5-(4-hydroxyphenyl)-5-phenylhydantoin (HPPH), respectively. First, zebrafish were exposed to CBZ, PHE, E-CBZ and HPPH from 5¼- to 120-h post fertilization (hpf) and morphologically evaluated. Second, the formations of E-CBZ and HPPH were assessed in culture medium and in whole-embryo extracts at different time points by targeted LC-MS. Finally, E-CBZ and HPPH formation was also assessed in adult zebrafish liver microsomes and compared with those of human, rat, and rabbit. The present study showed teratogenic effects for CBZ and PHE, but not for E-CBZ and HPPH. No HPPH was detected during organogenesis and E-CBZ was only formed at the end of organogenesis. E-CBZ and HPPH formation was also very low-to-negligible in adult zebrafish compared with the mammalian species. As such, other metabolic pathways than those of mammals are involved in the bioactivation of CBZ and PHE, or, these anti-epileptics are teratogens and do not require bioactivation in the zebrafish.

Minimum reporting standards based on a comprehensive review of the zebrafish embryo teratogenicity assay

Reproductive toxicity chemical safety assessment involves extensive use of vertebrate animals for regulatory testing purposes. Although alternative methods such as the zebrafish embryo teratogenicity assay (identified in the present manuscript by the acronym ZETA) are promising for replacing tests with mammals, challenges to regulatory application involve lack of standardization and incomplete validation. To identify key protocol aspects and ultimately support improving this situation, a comprehensive review of the literature on the level of harmonization/standardization and validation status of the ZETA has been conducted. The gaps and needed advances of the available ZETA protocols were evaluated and discussed with respect to its applicability as an alternative approach for teratogenicity assessment. Based on the review outcomes, a set of minimum reporting standards for the experimental protocol is proposed. Together with other initiatives towards implementation of alternative approaches at the screening and regulatory levels, the application of minimum reporting requirements is anticipated to support future method standardization and validation, as well as identifying potential improvement aspects. Present findings are expected to ultimately support advancing the ongoing validation initiatives towards the regulatory acceptance of the ZETA.

Novel Methods and Approaches for Safety Evaluation of Nanoparticle Formulations: A Focus Towards In Vitro Models and Adverse Outcome Pathways

Nanotoxicology is an emerging field employed in the assessment of unintentional hazardous effects produced by nanoparticles (NPs) impacting human health and the environment. The nanotoxicity affects the range between induction of cellular stress and cytotoxicity. The reasons so far reported for these toxicological effects are due to their variable sizes with high surface areas, shape, charge, and physicochemical properties, which upon interaction with the biological components may influence their functioning and result in adverse outcomes (AO). Thus, understanding the risk produced by these materials now is an important safety concern for the development of nanotechnology and nanomedicine. Since the time nanotoxicology has evolved, the methods employed have been majorly relied on in vitro cell-based evaluations, while these simple methods may not predict the complexity involved in preclinical and clinical conditions concerning pharmacokinetics, organ toxicity, and toxicities evidenced through multiple cellular levels. The safety profiles of nanoscale nanomaterials and nanoformulations in the delivery of drugs and therapeutic applications are of considerable concern. In addition, the safety assessment for new nanomedicine formulas lacks regulatory standards. Though the in vivo studies are greatly needed, the end parameters used for risk assessment are not predicting the possible toxic effects produced by various nanoformulations. On the other side, due to increased restrictions on animal usage and demand for the need for high-throughput assays, there is a need for developing and exploring novel methods to evaluate NPs safety concerns. The progress made in molecular biology and the availability of several modern techniques may offer novel and innovative methods to evaluate the toxicological behavior of different NPs by using single cells, cell population, and whole organisms. This review highlights the recent novel methods developed for the evaluation of the safety impacts of NPs and attempts to solve the problems that come with risk assessment. The relevance of investigating adverse outcome pathways (AOPs) in nanotoxicology has been stressed in particular.

Cytochrome P450-dependent biotransformation capacities in embryonic, juvenile and adult stages of zebrafish (Danio rerio)-a state-of-the-art review

Given the strong trend to implement zebrafish (Danio rerio) embryos as translational model not only in ecotoxicological, but also toxicological testing strategies, there is an increasing need for a better understanding of their capacity for xenobiotic biotransformation. With respect to the extrapolation of toxicological data from zebrafish embryos to other life stages or even other organisms, qualitative and quantitative differences in biotransformation pathways, above all in cytochrome P450-dependent (CYP) phase I biotransformation, may lead to over- or underestimation of the hazard and risk certain xenobiotic compounds may pose to later developmental stages or other species. This review provides a comprehensive state-of-the-art overview of the scientific knowledge on the development of the CYP1-4 families and corresponding phase I biotransformation and bioactivation capacities in zebrafish. A total of 68 publications dealing with spatiotemporal CYP mRNA expression patterns, activities towards mammalian CYP-probe substrates, bioactivation and detoxification activities, as well as metabolite profiling were analyzed and included in this review. The main results allow for the following conclusions: (1) Extensive work has been done to document mRNA expression of CYP isoforms from earliest embryonic stages of zebrafish, but juvenile and adult zebrafish have been largely neglected so far. (2) There is insufficient understanding of how sex- and developmental stage-related differences in expression levels of certain CYP isoforms may impact biotransformation and bioactivation capacities in the respective sexes and in different developmental stages of zebrafish. (3) Albeit qualitatively often identical, many studies revealed quantitative differences in metabolic activities of zebrafish embryos and later developmental stages. However, the actual relevance of age-related differences on the outcome of toxicological studies still needs to be clarified. (4) With respect to current remaining gaps, there is still an urgent need for further studies systematically assessing metabolic profiles and capacities of CYP isoforms in zebrafish. Given the increasing importance of Adverse Outcome Pathway (AOP) concepts, an improved understanding of CYP capacities appears essential for the interpretation and outcome of (eco)toxicological studies.

Toward High-Throughput Fish Embryo Toxicity Tests in Aquatic Toxicology

Addressing the shift from classical animal testing to high-throughput in vitro and/or simplified in vivo proxy models has been defined as one of the upcoming challenges in aquatic toxicology. In this regard, the fish embryo toxicity test (FET) has gained significant popularity and wide standardization as one of the sensitive alternative approaches to acute fish toxicity tests in chemical risk assessment and water quality evaluation. Nevertheless, despite the growing regulatory acceptance, the actual manipulation, dispensing, and analysis of living fish embryos remains very labor intensive. Moreover, the FET is commonly performed in plastic multiwell plates under static or semistatic conditions, potentially inadequate for toxicity assessment of some organic, easily degradable or highly adsorptive toxicants. Recent technological advances in the field of mechatronics, fluidics and digital vision systems demonstrate promising future opportunities for automation of many analytical stages in embryo toxicity testing. In this review, we highlight emerging advances in fluidic and laboratory automation systems that can prospectively enable high-throughput FET testing (HT-FET) akin to pipelines commonly found in in vitro drug discovery pipelines. We also outline the existing challenges, barriers to future development and provide an outlook of ground-breaking fluidic technologies in embryo toxicity testing.

Developmental toxicity triggered by benzyl alcohol in the early stage of zebrafish embryos: Cardiovascular defects with inhibited liver formation and degenerated neurogenesis

Benzyl alcohol (BnOH) is an aromatic alcohol used worldwide as an excipient in foods, cosmetics, household products, and medications. Although BnOH is a bacteriostatic agent found in many parenteral preparations, this agent is responsible for precipitating the gasping syndrome in premature neonates. Increasing evidence of human exposure to BnOH and environmental contamination of BnOH requires a detailed toxicity assessment of this aromatic chemical. Few studies on the toxicity of BnOH have been reported on different animal models, but its developmental toxicity effects are not fully understood yet. Studies on the effects of BnOH on the specific endpoints of organ toxicity are rare. Thus, the present study aimed to examine the developmental toxicity effects of BnOH by using zebrafish (Danio rerio) embryo as a biological disease model. Four-hour post fertilization zebrafish embryos were exposed to BnOH for 72 h to assess BnOH toxicity on an ecological viewpoint. The median lethal concentrations of varying BnOH concentrations in zebrafish embryos were estimated. The embryonic toxicity induced by BnOH was revealed by the apoptosis in embryos and pathological alterations, such as increased mortality, inhibited hatching rate, and decreased somite number. Moreover, pericardial edema and string heartbeat were observed because of arrhythmia and cardiac malformation. The number of normal vessels in the head and trunk regions was remarkably reduced in transgenic zebrafish line Tg (Fli-1: EGFP). Morphological defects and yolk sac retention were related to the degenerated liver formation in Tg (Lfabp: dsRED). Furthermore, BnOH exposure led to the disruption of motor neuron axonal integrity and the alteration of the axon pattern in Tg (olig2: dsRED). In addition, the results exhibited the pathological effects of BnOH exposure on major organs. We believe that this study is the second to report the developmental organ toxicity of BnOH to zebrafish embryos. This study provides important information for further elucidating the mechanism of BnOH-induced developmental organ toxicity.

Circumstantial Overdose Management of an Efficient Cancer Cell Photosensitizer with Preclinical Evidence: A Biophysical Study

In this article, pharmacological management of circumstantial overdose of an anticancer drug, Harmine (HM), under in vitro and in vivo conditions is described and further validated by employing in silico methods. HM, an efficient cancer cell photosensitizer, interacts extensively with nontoxic β-cyclodextrin (β-CD). Steady-state fluorescence studies and molecular docking analysis established differential nature of molecular inclusion depending on the relative concentrations of β-CD. Presently, β-CD is commonly used as a standard drug-delivery vehicle but its application for controlled drug withdrawal is rarely explored. Flow cytometric results and in vivo investigations on a zebrafish model showed that conditional overdose of preadministered drug molecules can be efficiently removed by encapsulating successfully within nontoxic β-CDs, albeit by controlled application of the same. This is an approach to manage the cytotoxicity of a drug in a safe way that is already administered. We believe that this β-CD-mediated withdrawal of drugs may find possible applications in controlled capturing of excess or unused drug inside living systems and reducing the unwanted toxicity associated with chemotherapeutics.

Refinement of the zebrafish embryo developmental toxicity assay

Several pharmaceutical and chemical companies are using the zebrafish embryo as an alternative to animal testing for early detection of developmental toxicants. Unfortunately, the protocol of this zebrafish embryo assay varies between labs, resulting in discordant data for identical compounds. The assay also has some limitations, such as low biotransformation capacity and fewer morphological endpoints in comparison with the in vivo mammalian developmental toxicity studies. Consequently, there is a need to standardize and further optimize the assay for developmental toxicity testing. We developed a Zebrafish Embryo Developmental Toxicity Assay (ZEDTA) that can be extended with a metabolic activation system and/or skeletal staining to increase its sensitivity. As such, the ZEDTA can be used as a modular system depending on the compound of interest.•

Our protocol is customized with a metabolic activation system for test compounds, using human liver microsomes. This system ensures exposure of zebrafish embryos to metabolites that are relevant for human risk and safety assessment. As human liver microsomes are toxic for the zebrafish embryo, we developed a preincubation system with an ultracentrifugation and subsequent dilution step.

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Additionally, we developed a skeletal staining protocol that can be added to the ZEDTA modular system. Our live Alizarin Red staining method detects several bone structures in 5-day old zebrafish larvae in a consistent manner.

Safety Assessment of Compounds after In Vitro Metabolic Conversion Using Zebrafish Eleuthero Embryos

Zebrafish-based platforms have recently emerged as a useful tool for toxicity testing as they combine the advantages of in vitro and in vivo methodologies. Nevertheless, the capacity to metabolically convert xenobiotics by zebrafish eleuthero embryos is supposedly low. To circumvent this concern, a comprehensive methodology was developed wherein test compounds (i.e., parathion, malathion and chloramphenicol) were first exposed in vitro to rat liver microsomes (RLM) for 1 h at 37 °C. After adding methanol, the mixture was ultrasonicated, placed for 2 h at −20 °C, centrifuged and the supernatant evaporated. The pellet was resuspended in water for the quantification of the metabolic conversion and the detection of the presence of metabolites using ultra high performance liquid chromatography-Ultraviolet-Mass (UHPLC-UV-MS). Next, three days post fertilization (dpf) zebrafish eleuthero embryos were exposed to the metabolic mix diluted in Danieau’s medium for 48 h at 28 °C, followed by a stereomicroscopic examination of the adverse effects induced, if any. The novelty of our method relies in the possibility to quantify the rate of the in vitro metabolism of the parent compound and to co-incubate three dpf larvae and the diluted metabolic mix for 48 h without inducing major toxic effects. The results for parathion show an improved predictivity of the toxic potential of the compound.

Pluripotent Stem Cells in Developmental Toxicity Testing: A Review of Methodological Advances

Millions of children are born each year with a birth defect. Many of these defects are caused by environmental factors, although the underlying etiology is often unknown. In vivo mammalian models are frequently used to determine if a chemical poses a risk to the developing fetus. However, there are over 80 000 chemicals registered for use in the United States, many of which have undergone little safety testing, necessitating the need for higher-throughput methods to assess developmental toxicity. Pluripotent stem cells (PSCs) are an ideal in vitro model to investigate developmental toxicity as they possess the capacity to differentiate into nearly any cell type in the human body. Indeed, a burst of research has occurred in the field of stem cell toxicology over the past decade, which has resulted in numerous methodological advances that utilize both mouse and human PSCs, as well as cutting-edge technology in the fields of metabolomics, transcriptomics, transgenics, and high-throughput imaging. Here, we review the wide array of approaches used to detect developmental toxicants, suggest areas for further research, and highlight critical aspects of stem cell biology that should be considered when utilizing PSCs in developmental toxicity testing.

Omics in Zebrafish Teratogenesis

The genome revolution represents a complete change on our view of biological systems. The quantitative determination of changes in all major molecular components of the living cells, the "omics" approach, opened whole new fields for all health sciences. Genomics, transcriptomics, proteomics, metabolomics, and others, together with appropriate prediction and modeling tools, will mark the future of developmental toxicity assessment both for wildlife and humans. This is especially true for disciplines, like teratology, which rely on studies in model organisms, as studies at lower levels of organization are difficult to implement. Rodents and frogs have been the favorite models for studying human reproductive and developmental disorders for decades. Recently, the study of the development of zebrafish embryos (ZE) is becoming a major alternative tool to adult animal testing. ZE intrinsic characteristics makes this model a unique system to analyze in vivo developmental alterations that only can be studied applying in toto approaches. Moreover, under actual legislations, ZE is considered as a replacement model (and therefore, excluded from animal welfare regulations) during the first 5 days after fertilization. Here we review the most important components of the zebrafish toolbox available for analyzing early stages of embryotoxic events that could eventually lead to teratogenesis.

Analysis of Lethality and Malformations During Zebrafish (Danio rerio) Development

The versatility offered by zebrafish (Danio rerio) makes it a powerful and an attractive vertebrate model in developmental toxicity and teratogenicity assays. Apart from the newly introduced chemicals as drugs, xenobiotics also induce abnormal developmental abnormalities and congenital malformations in living organisms. Over the recent decades, zebrafish embryo/larva has emerged as a potential tool to test teratogenicity potential of these chemicals. Zebrafish responds to compounds as mammals do as they share similarities in their development, metabolism, physiology, and signaling pathways with that of mammals. The methodology used by the different scientists varies enormously in the zebrafish embryotoxicity test. In this chapter, we present methods to assess lethality and malformations during zebrafish development. We propose two major malformations scoring systems: binomial and relative morphological scoring systems to assess the malformations in zebrafish embryos/larvae. Based on the scoring of the malformations, the test compound can be classified as a teratogen or a nonteratogen and its teratogenic potential is evaluated.

G2-S16 dendrimer as a candidate for a microbicide to prevent HIV-1 infection in women

Unprotected heterosexual intercourse is the first route for sustaining the global spread of human immunodeficiency virus type 1 (HIV-1), being responsible for 80% of new HIV-1 infections in the world. The presence of inflammation in the female reproductive tract and the presence of semen increases the risk of heterosexual HIV-1 transmission. This state-of-the-art research based on an innovative nanotechnology design was focused on a toxicological study of the limitation of the activity of the novel H₂O-soluble anionic carbosilane dendrimer G2-S16 in the adult cervical and foreskin epithelia. The G2-S16 dendrimer did not cause any irritation or inflammation in the vaginal epithelium, proving that this dendrimer is a safe nanocompound for vaginal application to control viral transmission. It was shown that no significant differences were found in mortality, sublethal or teratogenic effects when the zebra fish embryos were treated with G2-S16. In short, G2-S16 seems to be an ideal candidate for the development of a topical microbicide against HIV-1 infection and the next step is try in clinical trials, because of its great in vivo biocompatibility, as well as its ability to halt HIV-1 infection in the presence of semen.

Antioxidants reduce reactive oxygen species but not embryotoxicity in the metabolic Danio rerio test (mDarT)

Mammalian liver microsomes are occasionally used as a metabolic activation system (MAS) to compensate for the low CYP-mediated bioactivation of drugs in zebrafish embryos, in the so-called mDarT. However, this MAS is embryotoxic and consequently zebrafish embryos are only exposed during a very limited developmental window. The main aim of this study was to try to reduce the embryotoxic properties of MAS in order to extend the exposure window in the mDarT. Removing the microsomes from the incubation medium prior to exposure of the zebrafish embryos did not reduce embryotoxicity. Free radicals (ROS) in the incubation medium were successfully reduced by antioxidants, but the medium remained embryotoxic. Single dosing of NADPH or omitting toxic components from the MAS preparation did also not reduce embryotoxicity. In conclusion, the exposure window in the mDarT could not be extended by reducing ROS levels, single dosing of NADPH or modifications of the MAS preparation.

Evaluation of Embryotoxicity Using the Zebrafish Model

The embryonic zebrafish model offers the power of whole-animal investigations (e.g., intact organism, functional homeostatic feedback mechanisms, and intercellular signaling) with the convenience of cell culture (e.g., cost- and time-efficient, minimal infrastructure, small quantities of solutions required). The model system overcomes many of the current limitations in rapid to high-throughput screening of drugs/compounds and casts a broad net to rapidly evaluate integrated system effects. Additionally, it is an ideal platform to follow up with targeted studies aimed at the mechanisms of toxic action. Exposures are carried out in multi-well plates so minimal solution volumes are required for the assessments. Numerous morphological, developmental, and behavioral endpoints can be evaluated noninvasively due to the transparent nature of the embryos.

Biocompatibility and biodistribution of functionalized carbon nano-onions (f-CNOs) in a vertebrate model

Functionalized carbon nano-onions (f-CNOs) are of great interest as platforms for imaging, diagnostic and therapeutic applications due to their high cellular uptake and low cytotoxicity. To date, the toxicological effects of f-CNOs on vertebrates have not been reported. In this study, the possible biological impact of f-CNOs on zebrafish during development is investigated, evaluating different toxicity end-points such as the survival rate, hatching rate, and heart beat rate. Furthermore, a bio-distribution study of boron dipyrromethene (BODIPY) functionalized CNOs in zebrafish larvae is performed by utilizing inverted selective plane illumination microscopy (iSPIM), due to its intrinsic capability of allowing for fast 3D imaging. Our in vivo findings indicate that f-CNOs exhibit no toxicity, good biocompatibility (in the concentration range of 5-100 μg mL-1) and a homogenous biodistribution in zebrafish larvae.

Regulation of vitellogenin (vtg1) and estrogen receptor (er) gene expression in zebrafish (Danio rerio) following the administration of Cd²⁺ and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

We evaluated the individual and joint estrogenic effects of cadmium (Cd(2+)) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the zebrafish liver (ZFL) cell line, zebrafish embryo, larvae and the liver of adult zebrafish. The mRNA expression of vtg1 was inhibited by Cd(2+), but unaffected by TCDD in ZFL cells. Similar changes in the mRNA levels of ERα, ERβ1, ERβ2 and GPER (G protein coupled estrogen receptor) in ZFL cells were also observed. Deletion mutants of vtg1 gene promoters were constructed to investigate transcriptional regulation, and we found that all of the constructs failed to respond to TCDD or Cd(2+). However, after co-transfection with a vtg1 promoter-luciferase construct to the ERα, ERβ1, ERβ2 and GPER expression vectors, decreased luciferase activity was observed in the ERα co-transfection group after treatment with Cd(2+), suggesting that ERα participates in vtg1 transcriptional regulation and is affected by Cd(2+). Differences in the regulation of the mRNA levels of these genes were also observed between different developmental stages and between the livers of male and female zebrafish.

A Developmental Toxicology Assay Platform for Screening Teratogenic Liability of Pharmaceutical Compounds

Increasing need for proactive safety optimization of pharmaceutical compounds has led to generation and/or refinement of in vitro developmental toxicology assays. Our laboratory has developed three in vitro developmental toxicology assays to assess teratogenic liability of pharmaceutical compounds. These assays included a mouse molecular embryonic stem cell assay (MESCA), a dechorionated zebrafish embryo culture (ZEC) assay, and a streamlined rat whole embryo culture (rWEC) assay. Individually, the assays presented good (73-82%) predictivity. However, it remains to be determined whether combining or tiering the assays could enhance performance. Seventy-three compounds representing a broad spectrum of pharmaceutical targets and chemistry were evaluated across the assays to generate testing strategies that optimized performance. The MESCA and ZEC assays were found to have two limitations: compound solubility and frequent misclassification of compounds with H1 receptor or GABAnergic activity. The streamlined rWEC assay was found to be a cost-effective stand-alone assay for supporting poorly soluble compounds and/or ones with H1 or GABAnergic activity. For all other compounds, a tiering strategy using the MESCA and ZEC assays additionally optimized throughput, cost, and minimized animal use. The tiered strategy resulted in improved performance achieving 88% overall predictivity and was comparable with 89% overall predictivity achieved with frequency analysis (final teratogenic classification made from most frequent teratogenic classification from each individual assay). Furthermore there were 21 compounds in the test set characterized as definitive or suspect human teratogens and the multiassay approach achieved 95 and 91% correct classification using the tiered or frequency screening approach, respectively.

Incubation at 32.5°C and above causes malformations in the zebrafish embryo

Zebrafish embryos are increasingly used for developmental toxicity screening of candidate drugs and are occasionally co-incubated with a metabolic activation system at 32°C for 1, 2 or 4h, depending on their developmental stage. As this temperature is higher than the optimal temperature for zebrafish embryonic development (26-28.5°C), we investigated whether continuous incubation of zebrafish embryos from 2.5 until 96h post fertilization (hpf) at high temperatures (30.5-36.5°C) causes malformations. At 32.5°C tail malformations were observed as early as 24hpf, and these became even more prominent at 34.5 and 36.5°C. Cardiovascular and head malformations, edema and blood accumulations throughout the body were present at 36.5°C. Finally, temperatures higher than 28.5°C accelerated embryonic development except for 36.5°C, at which a lower hatching rate and hatching enzyme activity were observed. In conclusion, incubation of zebrafish embryos at 32.5°C and above from 2.5 until 96hpf causes malformations as early as 24hpf.

Evaluation of the developmental toxicity of lead in the Danio rerio body

Lead has been utilized throughout history and is widely distributed and mobilized globally. Although lead in the environment has been somewhat mitigated, the nature of lead and its extensive uses in the past prohibit it from being completely absent from our environment and exposure to lead is still a public health concern. Most studies regarding lead toxicity have focused on the brain. However, little is found in the literature on the effects of lead in other tissues. Here, we utilize the zebrafish model system to investigate effects of lead exposure during early developmental time windows at 24, 48 and 72 h post fertilization in the body. We analyze whole body, notochord and somatic muscle changes, vascular changes of the body, as well as motor neuron alterations. We find lead exposure induces a curved body phenotype with concomitant changes in somite length, decreased notochord staining and abnormal muscle staining using live and in situ approaches. Furthermore, altered vasculature within the somatic regions, loss and/or alternations of motor neuron extension both dorsally and ventrally from the spinal cord, loss of Rohon-Beard sensory neurons, and increased areas of apoptosis were found. We conclude that lead is developmentally toxic to other areas of the developing embryo, not just the brain.

Toxic effects of strychnine and strychnine N-oxide on zebrafish embryos

AIM

The application of strychnine (S) is limited due to its toxicity; strychnine N-oxide (SNO) is a derivative of strychnine. The aim was to employ zebrafish embryos to investigate and compare the developmental toxicity induced by S and SNO.

METHODS

The toxicity of S and SNO was examined through the hatching rate and survival rate. Morphological changes of the zebrafish were observed with a dissecting microscope. Apoptosis was detected through acridine orange (AO) staining and flow cytometry. Apoptotic genes were measured by RT-PCR.

RESULTS

Embryo malformation was observed in the embryos exposed to S at 200 μmol·L(-1). When SNO concentration was increased to 1 mmol·L(-1), scoliolosis, and pericardial edema could be seen in some embryos. Results from fluorescence microscopy and flow cytometry analysis showed that S at 200 μmol·L(-1) induced apoptosis, whereas the apoptotic rate in the SNO-treated group (200 μmol·L(-1)) was much lower than that in the S group. RT-PCR analysis showed that p53 mRNA expression and the ratio of Bax/Bcl-2 in the S group were significantly altered compared with the control group (*P < 0.05). Moreover, Bax mRNA expression in both S and SNO group were significantly different from that in the control group (**P < 0.01).

CONCLUSION

These results lead to the conclusion that SNO has significantly lower toxicity than S in zebrafish embryos.

Comparison of effects of anti-angiogenic agents in the zebrafish efficacy-toxicity model for translational anti-angiogenic drug discovery

Background

Anti-angiogenic therapy in certain cancers has been associated with improved control of tumor growth and metastasis. Development of anti-angiogenic agents has, however, been saddled with higher attrition rate due to suboptimal efficacy, narrow therapeutic windows, or development of organ-specific toxicities. The aim of this study was to evaluate the translational ability of the zebrafish efficacy–toxicity model to stratify anti-angiogenic agents based on efficacy, therapeutic windows, and off-target effects to streamline the compound selection process in anti-angiogenic discovery.

Methods

The embryonic model of zebrafish was employed for studying angiogenesis and toxicity. The zebrafish were treated with anti-angiogenic compounds to evaluate their effects on angiogenesis and zebrafish-toxicity parameters. Angiogenesis was measured by scoring the development of subintestinal vessels. Toxicity was evaluated by calculating the median lethal concentration, the lowest observed effect concentration, and gross morphological changes. Results of efficacy and toxicity were used to predict the therapeutic window.

Results

In alignment with the clinical outcomes, the zebrafish assays demonstrated that vascular endothelial growth factor receptor (VEGFR) inhibitors are the most potent anti-angiogenic agents, followed by multikinase inhibitors and inhibitors of endothelial cell proliferation. The toxicity assays reported cardiac phenotype in zebrafish treated with VEGFR inhibitors and multikinase inhibitors with VEGFR activity suggestive of cardiotoxic potential of these compounds. Several other pathological features were reported for multikinase inhibitors suggestive of off-target effects. The predicted therapeutic window was translational with the clinical trial outcomes of the anti-angiogenic agents. The zebrafish efficacy–toxicity approach could stratify anti-angiogenic agents based on the mechanism of action and delineate chemical structure-driven biological activity of anti-angiogenic compounds.

Conclusion

The zebrafish efficacy–toxicity approach can be used as a predictive model for translational anti-angiogenic drug discovery to streamline compound selection, resulting in safer and efficacious anti-angiogenic agents entering the clinics.

Transient overexpression of adh8a increases allyl alcohol toxicity in zebrafish embryos

Fish embryos are widely used as an alternative model to study toxicity in vertebrates. Due to their complexity, embryos are believed to more resemble an adult organism than in vitro cellular models. However, concerns have been raised with respect to the embryo's metabolic capacity. We recently identified allyl alcohol, an industrial chemical, to be several orders of magnitude less toxic to zebrafish embryo than to adult zebrafish (embryo LC50 = 478 mg/L vs. fish LC50 = 0.28 mg/L). Reports on mammals have indicated that allyl alcohol requires activation by alcohol dehydrogenases (Adh) to form the highly reactive and toxic metabolite acrolein, which shows similar toxicity in zebrafish embryos and adults. To identify if a limited metabolic capacity of embryos indeed can explain the low allyl alcohol sensitivity of zebrafish embryos, we compared the mRNA expression levels of Adh isoenzymes (adh5, adh8a, adh8b and adhfe1) during embryo development to that in adult fish. The greatest difference between embryo and adult fish was found for adh8a and adh8b expression. Therefore, we hypothesized that these genes might be required for allyl alcohol activation. Microinjection of adh8a, but not adh8b mRNA led to a significant increase of allyl alcohol toxicity in embryos similar to levels reported for adults (LC50 = 0.42 mg/L in adh8a mRNA-injected embryos). Furthermore, GC/MS analysis of adh8a-injected embryos indicated a significant decline of internal allyl alcohol concentrations from 0.23-58 ng/embryo to levels below the limit of detection (< 4.6 µg/L). Injection of neither adh8b nor gfp mRNA had an impact on internal allyl alcohol levels supporting that the increased allyl alcohol toxicity was mediated by an increase in its metabolization. These results underline the necessity to critically consider metabolic activation in the zebrafish embryo. As demonstrated here, mRNA injection is one useful approach to study the role of candidate enzymes involved in metabolization.

Zebrafish teratogenicity testing

As a model for teratogenicity research, zebrafish are gaining popularity and creditability. Zebrafish embryos have been proven to be a highly valuable tool in genetics and developmental biology research and have advanced our understanding of a number of known developmental toxicants. It has yet to be determined conclusively how reliably a zebrafish embryo screening assay predicts what will happen in mammalian models, but results from initial assessments have been encouraging. Here we have presented procedures for the basic care of a zebrafish colony to support embryo production, embryo collection and culturing, and teratogenicity experiments.

Predicting adult fish acute lethality with the zebrafish embryo: relevance of test duration, endpoints, compound properties, and exposure concentration analysis

The zebrafish embryo toxicity test has been proposed as an alternative for the acute fish toxicity test, which is required by various regulations for environmental risk assessment of chemicals. We investigated the reliability of the embryo test by probing organic industrial chemicals with a wide range of physicochemical properties, toxicities, and modes of toxic action. Moreover, the relevance of using measured versus nominal (intended) exposure concentrations, inclusion of sublethal endpoints, and different exposure durations for the comparability with reported fish acute toxicity was explored. Our results confirm a very strong correlation of zebrafish embryo to fish acute toxicity. When toxicity values were calculated based on measured exposure concentrations, the slope of the type II regression line was 1 and nearly passed through the origin (1 to 1 correlation). Measured concentrations also explained several apparent outliers. Neither prolonged exposure (up to 120 h) nor consideration of sublethal effects led to a reduced number of outliers. Yet, two types of compounds were less lethal to embryos than to adult fish: a neurotoxic compound acting via sodium channels (permethrin) and a compound requiring metabolic activation (allyl alcohol).

Assessment of developmental delay in the zebrafish embryo teratogenicity assay

In this study we analyzed some aspects of the assessment of developmental delay in the zebrafish embryotoxicity/teratogenicity test and explored the suitability of acetylcholinesterase (AChE) activity as a biochemical marker and as a higher throughput alternative to morphological endpoints such as head-trunk angle, tail length and morphological score. Embryos were exposed from 4 to 52 h post-fertilization (hpf) to a selection of known embryotoxic/teratogen compounds (valproic acid, retinoic acid, caffeine, sodium salicylate, glucose, hydroxyurea, methoxyacetic acid, boric acid and paraoxon-methyl) over a concentration range. They were evaluated for AChE activity, head-trunk angle, tail length and several qualitative parameters integrated in a morphological score. In general, the different patterns of the concentration-response curves allowed distinguishing between chemicals that produced growth retardation (valproic and methoxyacetic acid) and chemicals that produced non-growth-delay related malformations. An acceptable correlation between the morphological score, AChE activity and head-trunk angle as markers of developmental delay was observed, being AChE activity particularly sensitive to detect delay in the absence of malformations.

Klippel-Feil syndrome in a boy exposed inadvertently to cyclophosphamide during pregnancy: a case report

BACKGROUND

Cyclophosphamide (CPA) is an alkylating agent widely used as an immunosuppressive agent in the treatment of several autoimmune diseases, including systemic lupus erythematosus. Its teratogenic effect has been well studied in different experimental mammalian and non-mammalian animal models. In humans, 11 cases of CPA teratogenesis have been documented.

CASE

We present a case of a patient with Klippel-Feil syndrome inadvertently exposed to CPA and prednisone in utero during the first trimester. CONCLUSIONS This case of possible cyclophosphamide embryopathy provides evidence of teratogenesis as an etiologic agent in developmental field defects such as Klippel-Feil syndrome.

Testing strategies for embryo-fetal toxicity of human pharmaceuticals. Animal models vs. in vitro approaches: a workshop report

Reproductive toxicity testing is characterized by high animal use. For registration of pharmaceutical compounds, developmental toxicity studies are usually conducted in both rat and rabbits. Efforts have been underway for a long time to design alternatives to animal use. Implementation has lagged, partly because of uncertainties about the applicability domain of the alternatives. The reproductive cycle is complex and not all mechanisms of development can be mimicked in vitro. Therefore, efforts are underway to characterize the available alternative tests with regard to the mechanism of action they include. One alternative test is the mouse embryonic stem cell test (EST), which has been studied since the late 1990s. It is a genuine 3R "alternative" assay as it is essentially animal-free. A meeting was held to review the state-of-the-art of various in vitro models for prediction of developmental toxicity. Although the predictivity of individual assays is improving, a battery of several assays is likely to have even higher predictivity, which is necessary for regulatory acceptance. The workshop concluded that an important first step is a thorough survey of the existing rat and rabbit studies, to fully characterize the frequency of responses and the types of effects seen. At the same time, it is important to continue the optimization of in vitro assays. As more experience accumulates, the optimal conditions, assay structure, and applicability of the alternative assays are expected to emerge.

Zebrafish: as an integrative model for twenty-first century toxicity testing

The zebrafish embryo is a useful small model for investigating vertebrate development because of its transparency, low cost, transgenic and morpholino capabilities, conservation of cell signaling, and concordance with mammalian developmental phenotypes. From these advantages, the zebrafish embryo has been considered as an alternative model for traditional in vivo developmental toxicity screening. The use of this organism in conjunction with traditional in vivo developmental toxicity testing has the potential to reduce cost and increase throughput of testing the chemical universe, prioritize chemicals for targeted toxicity testing, generate predictive models of developmental toxicants, and elucidate mechanisms and adverse outcome pathways for abnormal development. This review gives an overview of the zebrafish embryo for pre dictive toxicology and 21st century toxicity testing. Developmental eye defects were selected as an example to evaluate data from the U.S. Environmental Protection Agency's ToxCast program comparing responses in zebrafish embryos with those from pregnant rats and rabbits for a subset of 24 environmental chemicals across >600 in vitro assay targets. Cross-species comparisons implied a common basis for biological pathways associated with neuronal defects, extracellular matrix remodeling, and mitotic arrest.

Zebrafish developmental screening of the ToxCast™ Phase I chemical library

Zebrafish (Danio rerio) is an emerging toxicity screening model for both human health and ecology. As part of the Computational Toxicology Research Program of the U.S. EPA, the toxicity of the 309 ToxCast™ Phase I chemicals was assessed using a zebrafish screen for developmental toxicity. All exposures were by immersion from 6-8 h post fertilization (hpf) to 5 days post fertilization (dpf); nominal concentration range of 1 nM-80 μM. On 6 dpf larvae were assessed for death and overt structural defects. Results revealed that the majority (62%) of chemicals were toxic to the developing zebrafish; both toxicity incidence and potency was correlated with chemical class and hydrophobicity (logP); and inter-and intra-plate replicates showed good agreement. The zebrafish embryo screen, by providing an integrated model of the developing vertebrate, compliments the ToxCast assay portfolio and has the potential to provide information relative to overt and organismal toxicity.