Species selection for nonclinical safety assessment of drug candidates: Examples of current industry practice

Safety Implications of Modulating Nuclear Receptors: A Comprehensive Analysis from Non-Clinical and Clinical Perspectives

The unintended modulation of nuclear receptor (NR) activity by drugs can lead to toxicities amongst the endocrine, gastrointestinal, hepatic cardiovascular, and central nervous systems. While secondary pharmacology screening assays include NRs, safety risks due to unintended interactions of small molecule drugs with NRs remain poorly understood. To identify potential nonclinical and clinical safety effects resulting from functional interactions with 44 of the 48 human-expressed NRs, we conducted a systematic narrative review of the scientific literature, tissue expression data, and used curated databases (OFF-X™) (Off-X, Clarivate) to organize reported toxicities linked to the functional modulation of NRs in a tabular and machine-readable format. The top five NRs associated with the highest number of safety alerts from peer-reviewed journals, regulatory agency communications, congresses/conferences, clinical trial registries, and company communications were the Glucocorticoid Receptor (GR, 18,328), Androgen Receptor (AR, 18,219), Estrogen Receptor (ER, 12,028), Retinoic acid receptors (RAR, 10,450), and Pregnane X receptor (PXR, 8044). Toxicities associated with NR modulation include hepatotoxicity, cardiotoxicity, endocrine disruption, carcinogenicity, metabolic disorders, and neurotoxicity. These toxicities often arise from the dysregulation of receptors like Peroxisome proliferator-activated receptors (PPARα, PPARγ), the ER, PXR, AR, and GR. This dysregulation leads to various health issues, including liver enlargement, hepatocellular carcinoma, heart-related problems, hormonal imbalances, tumor growth, metabolic syndromes, and brain function impairment. Gene expression analysis using heatmaps for human and rat tissues complemented the functional modulation of NRs associated with the reported toxicities. Interestingly, certain NRs showed ubiquitous expression in tissues not previously linked to toxicities, suggesting the potential utilization of organ-specific NR interactions for therapeutic purposes.

Maximizing insights from nonclinical safety studies in the context of rising costs and changing regulations

The traditional paradigm of non-rodent safety assessment studies, primarily reliant on non-human primates (NHPs) and dogs, is undergoing a transformation. During the 2023 Safety Pharmacology Society Annual Meeting, scientists from leading nonclinical contract organizations discussed how traditional IND-enabling studies can benefit from employing underutilized alternative non-rodent models, such as the swine. Swine offer a cost-effective approach to drug development and share many anatomical and physiological similarities with humans. The inclusion of non-traditional species in safety assessments, coupled with advanced measurement techniques, aids in de-risking compounds early on and adapting projects to the evolving cost landscape.

Innovative approaches to cardiovascular safety pharmacology assessment

This editorial prefaces the annual themed issue on safety pharmacology (SP) methods which has been published since 2004 in the Journal of Pharmacological and Toxicological Methods (JPTM). Here we highlight content derived from the 2023 Safety Pharmacology Society (SPS) meeting held in Brussels, Belgium. The meeting generated 138 abstracts, reproduced in the current volume of JPTM. As in prior years, the manuscripts reflect various areas of innovation in SP including in silico modeling of stroke volume, cardiac output and systemic vascular resistance, computational approaches that compare drug-induced proarrhythmic sensitivity of human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs), an evaluation of the utility of the corrected J-Tpeak and Tpeak-to-Tend parameters from the ECG as potential proarrhythmia biomarkers, and the applicability of nonclinical concentration-QTc (C-QTc) modeling of data derived from the conduct of the in vivo QTc study as a component of the core battery of safety pharmacology studies.

Non-Clinical Safety Evaluation of Exosomes Derived from Human Umbilical Cord Mesenchymal Stem Cells in Cynomolgus Monkeys

Purpose

In recent years, exosomes have been proved to be used to treat many diseases. However, due to the lack of uniform quality control standards for exosomes, the safety of exosomes is still a problem to be solved, especially now more and more exosomes are used in clinical trials, and its non-clinical safety evaluation is particularly important. However, there is no safety evaluation standard for exosomes at present. Therefore, this study will refer to the evaluation criteria of therapeutic biological products, adopt non-human primates to evaluate the non-clinical safety of human umbilical cord mesenchymal stem cell exosomes from the general pharmacology and immunotoxicity, aiming at establishing a safety evaluation system of exosomes and providing reference for the clinical application of exosomes in the future.

Methods

3.85 × 1012 exosomes derived from human umbilical cord mesenchymal stem cells were injected into cynomolgus monkeys intravenously. The changes of general clinical conditions, hematology, immunoglobulin, Th1/Th2 cytokines, T lymphocytes and B lymphocytes, and immune organs were observed before and within 14 days after injection.

Results

The results showed that exosomes did not have obvious pathological effects on the general clinical conditions, blood, coagulation function, organ coefficient, immunoglobulin, Th1/Th2 cytokines, lymphocytes, major organs, and major immune organs (spleen, thymus, bone marrow) of cynomolgus monkeys. However, the number of granulocyte-macrophage colonies in exosomes group was significantly higher than that in control group.

Conclusion

To sum up, the general pharmacological results and immunotoxicity results showed that the injection of 3.85 × 1012 exosomes may have no obvious adverse reactions to cynomolgus monkeys. This dose of exosomes is relatively safe for treatment, which provides basis research for non-clinical safety evaluation of exosomes and provides reliable research basis for future clinical application of exosomes.

Exploring rabbit as a nonrodent species for general toxicology studies

To avoid adverse events in humans, toxicity studies in nonclinical species have been the foundation of safety evaluation in the pharmaceutical industry. However, it is recognized that working with animals in research is a privilege, and conscientious use should always respect the 3Rs: replacement, reduction, and refinement. In the wake of the shortages in routine nonrodent species and considering that nonanimal methods are not yet sufficiently mature, the value of the rabbit as a nonrodent species is worth exploring. Historically used in vaccine, cosmetic, and medical device testing, the rabbit is seldom used today as a second species in pharmaceutical development, except for embryo-fetal development studies, ophthalmic therapeutics, some medical devices and implants, and vaccines. Although several factors affect the decision of species selection, including pharmacological relevance, pharmacokinetics, and ADME considerations, there are no perfect animal models. In this forum article, we bring together experts from veterinary medicine, industry, contract research organizations, and government to explore the pros and cons, residual concerns, and data gaps regarding the use of the rabbit for general toxicity testing.

Animal rights, animal research, and the need to reimagine science

What would it look like for researchers to take non-human animal rights seriously? Recent discussions foster the impression that scientific practice needs to be reformed to make animal research ethical: just as there is ethically rigorous human research, so there can be ethically rigorous animal research. We argue that practically little existing animal research would be ethical and that ethical animal research is not scalable. Since animal research is integral to the existing scientific paradigm, taking animal rights seriously requires a radical, wholesale reimagining of science.Trial registration: ClinicalTrials.gov identifier: NCT05340426.

Cross-Species Extrapolation of Biological Data to Guide the Environmental Safety Assessment of Pharmaceuticals-The State of the Art and Future Priorities

The extrapolation of biological data across species is a key aspect of biomedical research and drug development. In this context, comparative biology considerations are applied with the goal of understanding human disease and guiding the development of effective and safe medicines. However, the widespread occurrence of pharmaceuticals in the environment and the need to assess the risk posed to wildlife have prompted a renewed interest in the extrapolation of pharmacological and toxicological data across the entire tree of life. To address this challenge, a biological "read-across" approach, based on the use of mammalian data to inform toxicity predictions in wildlife species, has been proposed as an effective way to streamline the environmental safety assessment of pharmaceuticals. Yet, how effective has this approach been, and are we any closer to being able to accurately predict environmental risk based on known human risk? We discuss the main theoretical and experimental advancements achieved in the last 10 years of research in this field. We propose that a better understanding of the functional conservation of drug targets across species and of the quantitative relationship between target modulation and adverse effects should be considered as future research priorities. This pharmacodynamic focus should be complemented with the application of higher-throughput experimental and computational approaches to accelerate the prediction of internal exposure dynamics. The translation of comparative (eco)toxicology research into real-world applications, however, relies on the (limited) availability of experts with the skill set needed to navigate the complexity of the problem; hence, we also call for synergistic multistakeholder efforts to support and strengthen comparative toxicology research and education at a global level. Environ Toxicol Chem 2024;43:513-525. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Nonclinical safety assessment of an mRNA Covid-19 vaccine candidate following repeated administrations and biodistribution

Messenger RNA (mRNA) vaccines have demonstrated efficacy against Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) in humans. mRNA technology holds tremendous potential for rapid control and prevention of emergencies due to its flexibility with respect to production, application, and design for an efficacious and safe use in humans. We assessed the toxicity and biodistribution of MRT5500, an mRNA vaccine encoding for the full-length of the SARS-CoV-2 spike protein and delivered by lipid nanoparticles (LNPs) containing a novel ionizable lipid, Lipid-1 in preclinical animal models. In the repeated dose toxicity study, rabbits received three intramuscular (IM) injections of MRT5500 at 3-week interval followed by a 4-week observation period. In an exploratory biodistribution study in mice receiving a single IM injection of an mRNA encoding luciferase encapsulated in an LNP containing Lipid-1, the expression of the luciferase protein was monitored in vivo and ex vivo at several time points. In the regulatory biodistribution study in rabbits receiving a single IM injection of MRT5500, the quantification of the mRNA and the ionizable Lipid-1 were monitored in the same organs and time points as in the exploratory biodistribution study. MRT5500 was safe and well-tolerated with a transient acute phase response/inflammation and an expected vaccine-related immunological response, typical of those observed following a vaccine administration. The biodistribution data demonstrated that the mRNA and Lipid-1 components of the vaccine formulations were mainly detected at the injection site and in the draining lymph nodes. These results support the use of MRT5500 and its deployment into clinical trials.

Successful Development of Nonclinical Anti-Drug Antibody Assays to Support Zinpentraxin Alfa Reproductive Toxicology Studies

Immunogenicity assessment is an essential part of biotherapeutic drug development. While the immune response in animals is not always representative of the human immune response, immunogenicity data obtained in animal models is still informative for the evaluation of drug exposure and safety. The most common assay format used for the detection of anti-drug antibodies (ADAs) in preclinical and clinical studies is the bridging format. The advantage of this method is that it can detect all antibody isotypes generated against the therapeutic. However, the method development can be time-consuming and labor-intensive, due to the need for labeling of the drug which is used both as capture and detection. Various generic ADA assays have been successfully implemented to overcome these disadvantages and to enable faster assay development timelines to support nonclinical toxicology studies. Here, we describe the challenges in the development of an assay to detect antibodies to zinpentraxin alfa, a recombinant human pentraxin-2, in rabbit and rat toxicology studies. Our initial efforts to develop a bridging assay failed, prompting us to develop a method adapted from generic assay formats to detect anti-zinpentraxin alfa antibodies in the serum of different species with minimal optimization. However, while the general assay format remained similar, assay reagents were adapted between the different species, resulting in the development of two distinct assays for the detection of ADAs in rat and rabbit. Here, we share the final development/validation data and the immunogenicity study results. Our work highlights the need for the evaluation of alternate assay formats when evaluating novel drug modalities.

Repeat-dose and embryo-fetal developmental toxicity of zinpentraxin alfa

Zinpentraxin alfa is a recombinant human pentraxin-2 (PTX-2) developed for the treatment of various fibrotic diseases with the hypothesis that supplementing endogenous PTX-2 levels through intravenous administration should increase its regulatory capacity in circulation and at the site of disease, thereby promoting healing and reducing fibrosis. Zinpentraxin alfa has been studied in various clinical trials, particularly in patients with idiopathic pulmonary fibrosis, where it has demonstrated efficacy in slowing decline in lung function in a phase 2 study. In the present investigation, we summarize findings from 14-day repeat-dose toxicity studies in rats and cynomolgus monkeys supporting early clinical development of zinpentraxin alfa. In addition, we also describe the findings from the embryo-fetal developmental (EFD) studies conducted in rats and rabbits, since the intended fibrosis patient population may include patients of childbearing potential. Zinpentraxin alfa was well tolerated by rats and monkeys in general toxicity studies with no treatment-related adverse effects, as well as by pregnant rats over the same dose range in a definitive EFD study. In contrast, substantial toxicity was observed in a rabbit dose-range-finder EFD study. Zinpentraxin alfa was poorly tolerated by pregnant rabbits and effects on the dams correlated with post-implantation fetal losses. The disparate effects of zinpentraxin alfa on embryo-fetal development between the two species suggests a potential unknown biological function of PTX-2 in pregnancy in the rabbit, which may be relevant to humans. Our findings warrant the consideration for highly effective contraceptive measures to avoid pregnancy in patients enrolled in clinical studies with zinpentraxin alfa.

Selection of appropriate non-clinical animal models to ensure translatability of novel AAV-gene therapies to the clinic

Gene Therapy Medicinal Products consist of a recombinant nucleic acid intended for the modulation or manipulation of a genetic sequence. A single administration of a novel gene therapy has the potential to be curative, with a durable long-term benefit to patients. Adeno-associated viral vectors have become the viral vector of choice for in vivo delivery of therapeutic transgenes as they are mildly immunogenic, can effectively transduce a variety of human tissues and cells, and have low levels of genomic integration. Central to the effective translation of data generated in discovery studies to the clinic is the selection of appropriate animal species for pivotal non-clinical studies. This review aims to support the selection of appropriate animal models for non-clinical studies to advance the development of novel adeno-associated virus gene therapies.

Heterogeneous vasomotor responses in segments from Göttingen Minipigs coronary, cerebral, and mesenteric artery: A comparative study

Göttingen Minipigs (GM) are used as an important preclinical model for cardiovascular safety pharmacology and for evaluation of cardiovascular drug targets. To improve the translational value of the GM model, the current study represents a basic characterization of vascular responses to endothelial regulators and sympathetic, parasympathetic, and sensory neurotransmitters in different anatomical origins. The aim of the current comparative and descriptive study is to use myography to characterize the vasomotor responses of coronary artery isolated from GM and compare the responses to those obtained from parallel studies using cerebral and mesenteric arteries. The selected agonists for sympathetic (norepinephrine), parasympathetic (carbachol), sensory (calcitonin gene-related peptide, CGRP), and endothelial pathways (endothelin-1, ET-1, and bradykinin) were used for comparison. Further, the robust nature of the vasomotor responses was evaluated after 24 h of cold storage of vascular tissue mimicking the situation under which human biopsies are often kept before experiments or grafting is feasible. Results show that bradykinin and CGRP consistently dilated, and endothelin consistently contracted artery segments from coronary, cerebral, and mesenteric origin. By comparison, norepinephrine and carbachol, had responses that varied with the anatomical source of the tissues. To support the basic characterization of GM vasomotor responses, we demonstrated the presence of mRNA encoding selected vascular receptors (CGRP- and ETA-receptors) in fresh artery segments. In conclusion, the vasomotor responses of isolated coronary, cerebral, and mesenteric arteries to selected agonists of endothelial, sympathetic, parasympathetic, and sensory pathways are different and the phenotypes are similar to sporadic human findings.

Prediction of pharmacokinetics of an anaplastic lymphoma kinase inhibitor in rat and monkey: application of physiologically based pharmacokinetic model as an alternative tool to minimise animal studies

The pharmacokinetic (PK) and toxicokinetic profile of a drug from its preclinical evaluation helps the researcher determine whether the drug should be tested in humans based on its safety and toxicity.Preclinical studies require time and resources and are prone to error. Moreover, according to the United States Food and Drug Administration Modernisation Act 2, animal testing is no longer mandatory for new drug development, and an animal-free alternative, such as cell-based assay and computer models, can be used.Different physiologically based PK models were developed for an anaplastic lymphoma kinase inhibitor in rats and monkeys after intravenous and oral administration using its physicochemical properties and in vitro characterisation data.The developed model was validated against the in vivo data available in the literature, and the validation results were found within the acceptable limit. A parameter sensitivity analysis was performed to identify the properties of the compound influencing the PK profile.This work demonstrates the application of the physiologically based PK model to predict the PKs of a drug, which will eventually assist in reducing the number of animal studies and save time and cost of drug discovery and development.

Survival following complete resection of neuroblastoma in novel orthotopic rat xenograft model

Neuroblastoma accounts for 15% of pediatric cancer deaths, despite multimodal therapy including surgical resection. Current neuroblastoma rodent models are insufficient for studying the impact of surgery and combination treatments, largely due to the small size of mouse models. Human neuroblastoma SK-N-BE(2) cells were injected into the left adrenal gland of 5-6-week-old RNU homozygous nude rats. Rats were either monitored by MRI until humane endpoint was reached or after 5 weeks underwent operative tumor resection, followed by monitoring for recurrence and survival. Following neuroblastoma cell implantation, the majority of tumors grew to greater than 5000 mm3 within 5.5-6.5 weeks, meeting the humane endpoint. Surgical resection was successfully done in 8 out of 9 rats, extending survival following tumor implantation from a median of 42 days to 78 days (p < 0.005). Pathology was consistent with human neuroblastoma, showing small round blue cell tumors with Homer-Wright rosettes, high mitoses and karyorrhectic index, and strong PHOX2B staining. Thus, we have established a novel orthotopic xenograft rat model of neuroblastoma and demonstrated increased survival of rats after surgical tumor resection. This model can be used for the development of surgical techniques, such as the use of intraoperative molecular imaging or assessment of combination therapies that include surgery.

Evaluation of non-clinical toxicity studies of COVID-19 vaccines

In this study we evaluated the outcomes of non-clinical toxicity studies of various SARS-CoV-2 vaccines produced with different manufacturing technologies, with focus on Repeated Dose Toxicity (RDT) and Developmental and Reproductive Toxicity (DART) studies. We found that RDT and DART studies at doses relevant for human treatment showed no adverse effects while remaining observations were expected findings including local reactogenicity, immune response and macroscopic findings at the injection site. We have also reviewed the European Medicines Agency (EMA) nonclinical assessment reports for market authorization. Regardless of utilized vaccine manufacturing technology EMA assessment of the non-clinical studies consisted most frequently of comments related to study design, species selection and missing data. Sponsors have often submitted platform studies (vaccine studies with the same technology/construct but using other antigens) as supplementary data. Animal model-based toxicity testing has shown rather small effects, which have been never serious adverse effects. The translational value to support clinical development is mainly to inflammatory effects, indicative of the primary action of the vaccines. From a 3R perspective supportive platform technology data consisting of previously executed RDT and DART studies from the same platform technology are encouraged to be implemented in the vaccine assessment process.

Developmental and reproductive toxicity of a recombinant protein subunit COVID-19 vaccine (ZF2001) in rats

ZF2001, a protein subunit vaccine against coronavirus disease 2019 (COVID-19), contains recombinant tandem repeat of dimeric receptor-binding domain (RBD) protein of the SARS-CoV-2 spike protein with an aluminium-based adjuvant. During the development of this vaccine, two nonclinical studies were conducted to evaluate female fertility, embryo-fetal development, and postnatal developmental toxicity in Sprague‒Dawley rats according to the ICH S5 (R3) guideline. In Study 1 (embryo-fetal developmental toxicity, EFD), 144 virgin female rats were randomly assigned into four groups and received three doses of vaccine (25 μg or 50 μg RBD protein/dose, containing the aluminium-based adjuvant), the aluminium-based adjuvant or a sodium chloride injection administered intramuscularly on days 21 and 7 prior to mating and on gestation day (GD) 6. In Study 2 (pre- and postnatal developmental toxicity, PPND), ZF2001 at a dose of 25 μg RBD protein/dose or sodium chloride injection was administered intramuscularly to female rats (n = 28 per group) 7 days prior to mating and on GD 6, GD 20 and postnatal day (PND) 10. There were no obvious adverse effects in dams, except for local injection site reactions related to the aluminium-based adjuvant (yellow nodular deposits in the interstitial muscle fibres). There were also no effects of ZF2001 on the mating performance, fertility or reproductive performance of parental females, embryo-fetal development, postnatal survival, growth, physical development, reflex ontogeny, behavioural and neurofunctional development, or reproductive performance of the offspring. The strong immune responses associated with binding and neutralising antibodies were both confirmed in dams and fetuses or offspring in these two studies. These results would support clinical trials or the use of ZF2001 in maternal immunisation campaigns, including those involving women with childbearing potential, regardless of pregnancy status.

General toxicity studies of alpha mangostin from Garcinia mangostana: A systematic review

Alpha mangostin (AM), the main xanthone derivative contained in mangosteen pericarp (Garcinia mangostana/GM), has many pharmacological activities such as antioxidant, antiproliferation, antiinflammatory, and anticancer. Several general toxicity studies of AM have been previously reported to assess the safety profile of AM. Toxicity studies were carried out by various methods such as on test animals, interventions, and various routes of administration, but the test results have not been well documented. Our study aimed to systematically summarizes research on the safety profile of GM containing AM through general toxicity tests to get the LD₅₀ and NOAEL values, and so, can be used as a database related to AM toxicity profiles. This could facilitate other researchers in determining further development of GM-or-AM-based products. Pubmed, Google scholar, ScienceDirect, and EBSCO were chosen to collect the articles while ARRIVE 2.0 was used to evaluate the quality and risk-of-bias of the in vivo toxicity studies included in this systematic review. A total of 20 articles met the eligibility criteria and were reviewed to predict the LD₅₀ and NOAEL of AM. The results showed that the LD₅₀ of AM is between >15.480 mg/kgBW to ≤6000 mg/kgBW while the NOAEL value is between <100 and ≤2000 mg/kgBW.

The Non-Human Primate in Safety Assessment of a Bifunctional Long-Acting Insulin Analogue

Species selection plays a pivotal part during non-clinical safety assessment in drug development. If possible, use of non-human primates (NHPs) should be avoided due to ethical considerations. However, limiting factors as lack of pharmacologic activity in other species could necessitate use of NHPs. LAI-PCSK9i is a bi-functional molecule combining a long-acting insulin analogue with a PCSK9 inhibitor peptide aiming to provide glycaemic control and to reduce plasma LDL concentrations. The NHP was chosen for the safety assessment of LAI-PCSK9i being the most relevant species with basal levels and plasma lipid composition closest to humans, while the dog and initially also the minipig were deemed irrelevant due to lack of pharmacologic activity on LDL-lowering and biological differences in lipid profiles. An in vivo tolerability and toxicokinetic study of LAI-PCSK9i in NHPs showed recurrent and severe hypoglycaemia at very low doses. Therefore, the minipig was re-evaluated and a follow-up study thoroughly assessing blood glucose and cholesterol levels and clinical signs illustrated that minipigs dosed with LAI-PCSK9i, tolerated the compound and LAI-PCSK9i decreased glucose and LDL over time. This work underlines that careful consideration is required when selecting species during safety assessment in drug development. The tolerability issue in NHPs led to the subsequent selection of the minipig for safety evaluation of LAI-PCSK9i although as a suboptimal alternative, which unexpectedly had a measurable pharmacologic response on LDL lowering. In conclusion, the NHPs may be unsuitable as test species for safety assessment of long-acting insulin analogues due to high sensitivity to recurring hypoglycaemic episodes.

Re-evaluating the need for chronic toxicity studies with therapeutic monoclonal antibodies, using a weight of evidence approach

To support registration of monoclonal antibodies (mAbs) for chronic indications, 6-month toxicity studies have historically been conducted. Experience with mAb development has shown a relatively benign and well-understood safety profile for this class, with most toxicity findings anticipated based on pharmacology. We evaluated whether a 6-month toxicity study is necessary to assess the long-term safety of mAbs. Data on First-in-Human (FIH)-enabling and chronic toxicity studies were shared for 142 mAbs submitted by 11 companies. Opportunities to further optimize study designs to reduce animal usage were identified. For 71% of mAbs, no toxicities or no new toxicities were noted in chronic studies compared to FIH-enabling study findings. New toxicities of potential concern for human safety or that changed trial design were identified in 13.5% of cases, with 7% being considered critical and 2% leading to program termination. An iterative, weight-of-evidence model which considers factors that influence the overall risk for a mAb to cause toxicity was developed. This model enables an evidence-based justification, suggesting when 3-month toxicity studies are likely sufficient to support late-stage clinical development and registration for some mAbs.

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.