Zebrafish Take Center Stage as Scientists Redefine the Future of Preclinical Research

In a move that could reshape how new medicines are discovered and tested, the Polish Zebrafish Society has issued the first comprehensive national recommendations on the use of the zebrafish (Danio rerio) in biomedical research. The document, published in Acta Biochimica Polonica, lays out field-specific guidance spanning oncology, toxicology, neurology and neuropsychiatry, metabolic diseases, immunology, cardiology, and genetic disease modelling. Its central claim is bold: a small striped freshwater fish, sharing roughly 70 percent genetic homology with humans, can serve as a scientifically rigorous, ethically superior, and dramatically faster alternative to the mice and rats that have dominated preclinical research for decades.

The scientific case rests on a remarkable convergence of biological properties. Zebrafish embryos develop outside the mother, with most organs forming within 24 hours of fertilization, and remain optically transparent for the first few days of life. This allows researchers to watch organogenesis, tumor growth, immune cell migration, and cardiac function unfold in real time inside a living vertebrate. A single breeding pair produces hundreds of eggs per week, fish reach sexual maturity in roughly three months, and maintenance costs are a fraction of those for mammalian colonies. Crucially, around 80 percent of human disease-linked genes have zebrafish orthologs, and core molecular processes—signal transduction, endocrine regulation, immune responses, and xenobiotic metabolism—are highly conserved, including functional counterparts of key drug-metabolizing enzymes and transporters. Modern genome-editing tools such as CRISPR/Cas9, Cre-lox systems, and transgenesis have all been adapted for the fish, enabling precise disease modelling and high-throughput in vivo screening at a scale unattainable in rodents.

The ethical architecture of the recommendations is anchored in the 3Rs framework of Replacement, Reduction, and Refinement, now extended by proposals for a fourth R, Responsibility, and a fifth R, Rehoming. The Society specifically recommends using embryos and larvae up to the stage preceding independent feeding—commonly around five days post-fertilization—for in vivo experiments. This timing matters legally as much as ethically: EU Directive 2010/63/EU defines its scope of protection with reference to independently feeding larval forms, meaning zebrafish embryos and early larvae fall outside the Directive’s formal scope. Polish national legislation, through the Act of 15 January 2015 on the protection of animals used for scientific or educational purposes, mirrors this exclusion. In practical terms, experiments on zebrafish embryos and early larvae may constitute a genuine legal alternative to classical protected-stage experiments, contributing to the replacement or reduction of mammalian models where scientifically appropriate. When later developmental stages or adult fish are required, the Society insists on approval from the competent Local Ethics Committee for Animal Experiments.

Nowhere is the model’s power more vivid than in oncology. The Society recommends the human-zebrafish xenograft approach, in which human cancer cells—from established lines or directly from patients—are implanted into zebrafish embryos or larvae. Because the larval immune system is not yet fully functional during the first days of life, human cells engraft and proliferate without immunosuppression. The transparent body permits direct visualization of tumor development, angiogenesis, and metastasis at single-cell resolution, and unlike in mouse models, human cancer cells in zebrafish readily migrate and form metastases, allowing drug effects on this critical aspect of tumor biology to be assessed under true in vivo conditions. Xenograft experiments can be completed in less than a week, whereas analogous mouse studies typically require several weeks. This so-called Avatar strategy is already entering clinical contexts: in a translational study involving breast cancer patients, complete concordance was demonstrated between chemotherapy responses observed in the larval model and the actual clinical responses, confirming high predictive value for personalized medicine. The model has also delivered clinically validated drugs—larval zebrafish studies revealed thalidomide’s strong inhibitory effect on tumor angiogenesis through suppression of VEGF signaling, helping establish thalidomide and its derivatives as therapies for neovascularization-dependent cancers such as multiple myeloma.

In toxicology, the recommendations position zebrafish as a tiered pre-filter for chemical and pharmaceutical safety. The Fish Embryo Acute Toxicity Test, standardized under OECD TG 236, determines lethal concentration values over a 96-hour exposure window in embryos, providing early hazard identification without adult vertebrates. While LC50 endpoints are not directly equivalent to rodent LD50 values, they deliver comparable information on acute toxicity and are primarily used for screening and prioritization rather than direct regulatory substitution. Embryonic transparency enables direct observation of teratogenic and embryotoxic potential, revealing morphological abnormalities, oxidative stress, apoptosis, and endocrine disruption induced by pharmaceuticals or contaminants. The assay’s compatibility with multi-well plate formats, robotic microscopes, computerized readers, and artificial intelligence allows automated detection of embryo orientation and organ-specific analysis of the heart, brain, or vessels. Regulatory recognition is growing: the FET assay is incorporated into OECD guidelines as an accepted alternative for chemical safety assessment, the US Food and Drug Administration acknowledges zebrafish for investigative and early-screening preclinical procedures, and the European Medicines Agency has expressed strong interest in preliminary regulatory reviews. The Society cautions, however, that zebrafish should not be regarded as a universal replacement where regulatory frameworks demand mammalian data on pregnancy, lactation, multigenerational endpoints, or long-term systemic organ toxicity.

Neurology and neuropsychiatry showcase the model’s translational depth. Zebrafish possess a hypothalamic–pituitary–interrenal axis homologous to the mammalian stress axis, with conserved cortisol synthesis, alongside well-organized serotonergic, dopaminergic, glutamatergic, and GABAergic neurotransmitter systems. Epilepsy research has been transformed by two complementary approaches: seizures can be induced in larvae with pentylenetetrazole, and genetic models based on the scn1lab gene—which corresponds to the human SCN1A gene mutated in Dravet syndrome—faithfully recapitulate reduced GABAergic signaling, spontaneous seizures, and abnormal neuronal activity. High-throughput phenotypic screening in multi-well formats using automated locomotor tracking and local field potential recordings identified clemizole as a potent seizure suppressor in the scn1lab model, and the same pipeline has highlighted serotonergic modulators such as fenfluramine and unexpected compound classes including synthetic cannabinoids. Classical antidepressants like fluoxetine and citalopram induce behavioral effects in zebrafish consistent with their human actions, and standardized larval assays—open-field thigmotaxis, light–dark preference, and light–dark challenge tests—quantify anxiety-like behavior with high predictive value. Social behavior assays in older larvae and juveniles have even been applied to model neurodevelopmental conditions relevant to autism spectrum disorders.

Metabolic disease research exploits the model’s conserved endocrine machinery. Zebrafish retain homologous insulin, glucagon, nuclear receptors, and appetite-regulating pathways; more than 70 percent of carbohydrate and glucose metabolic genes are conserved across zebrafish, frog, chicken, mouse, and human, with 57 percent linked to human type 2 diabetes. Larval insulin resistance models generated through elevated glucose, dexamethasone, or exogenous insulin exposure display features of impaired glucose handling suitable for evaluating antidiabetic drug candidates. For type 1 diabetes, transgenic lines expressing nitroreductase under the insulin promoter enable inducible, highly specific beta cell destruction upon metronidazole exposure, producing reproducible insulin deficiency and hyperglycemia while allowing real-time visualization of pancreatic islet regeneration. Adult fish on high-fat or high-cholesterol diets develop obesity, dyslipidemia, and atherosclerosis-like vascular lipid accumulation with conserved lipoprotein metabolism and inflammatory pathways. Storage diseases are equally tractable: the zebrafish model of Fabry disease, lacking alpha-galactosidase A activity, revealed an independent mechanism of kidney damage driven by mitochondrial alterations and disturbed glycolysis and galactose metabolism, beyond the canonical lysosomal accumulation of globotriaosylceramide.

Immunology and cardiology round out the model’s portfolio with distinctive technical advantages. During the first week of life, zebrafish larvae possess only a functional innate immune system, with adaptive immunity developing weeks later—a temporal separation that permits selective investigation of neutrophil and macrophage responses without adaptive interference. Transgenic reporter lines such as Tg(mpx:GFP) for neutrophils and Tg(mpeg1:mCherry) for macrophages enable direct visualization of leukocyte migration and host–pathogen interactions in living organisms. Infection of larvae with Mycobacterium marinum, closely related to Mycobacterium tuberculosis, recapitulates the hallmark organized granulomas of human tuberculosis and has enabled identification of virulence factors, host defense mechanisms, and high-throughput screening for novel antitubercular drugs, while reporter lines tracking autophagy marker LC3 recruitment to bacterial vacuoles illuminate cell-autonomous immunity against Staphylococcus aureus and Salmonella Typhimurium. In cardiology, homologs of key cardiac ion channels including hERG make zebrafish respond to QT-prolonging drugs comparably to humans: a landmark study identified 22 of 23 clinically relevant proarrhythmic compounds as positive in the assay, a drug-sensitized screen uncovered 15 genes affecting cardiac repolarization including GINS3, and screening of 3,000 compounds in a doxorubicin-induced cardiomyopathy model yielded the cardioprotective molecules visnagin and diphenylurea, subsequently validated in mice. Adult zebrafish uniquely regenerate cardiac muscle after injury through cardiomyocyte dedifferentiation and proliferation—a capacity absent in mammals—making the fish an unmatched system for heart regeneration research.

The recommendations arrive against a sobering backdrop of preclinical failure. The overall preclinical-to-market attrition rate exceeds 92 percent, a figure unchanged for three decades. Mouse models produce positive preclinical results in 90 to 95 percent of studies, yet fewer than 8 to 10 percent translate to approved drugs, falling to 3.5 percent in oncology—a gap attributed less to mouse biology than to study design, publication bias, sex bias, lack of genetic diversity, and failure to reproduce the full complexity of human immunology and metabolism. Notably, zebrafish under 120 hours post-fertilization, which do not swim or feed independently, are considered New Alternative Methodologies under EU Directive 2010/63, and the FDA has developed a draft on NAM use in preclinical research paving the way for zebrafish in these studies. The Society frames the fish as the most productive early-stage preclinical pipeline: an intermediate platform positioned between simple in vitro systems and resource-intensive mammalian studies, used to prioritize compounds, identify early toxicity signals, and refine hypotheses before mammalian validation. Implementation, the authors stress, requires question-driven design, standardized protocols with defined developmental stages, exposure windows, and controls, robust biologically relevant endpoints, and predefined criteria for progression to mammalian models. With zebrafish now the second most frequently used animal model in EU research after mice, the Society calls on research institutions, pharmaceutical companies, and regulatory agencies to systematically incorporate these embryo and larval assays into efficacy and safety evaluation, grant applications, and research standards—an integration the authors argue will yield more humane, efficient, and innovative science, ultimately benefiting medical progress and public health.

Subject of Research: Use of the zebrafish (Danio rerio) model in biomedical research and its recommendations for replacing mammalian models

Article Title: Recommendations of the Polish Zebrafish Society on the use of the zebrafish (Danio rerio) model in biomedical research

Article References: Podlasz, P., Migocka-Patrzałek, M., Prajsnar, T. K., Sarosiak, A., & Tylzanowski, P. (2026). Recommendations of the Polish Zebrafish Society on the use of the zebrafish (Danio rerio) model in biomedical research. Acta Biochimica Polonica, 73, Article 16545. https://doi.org/10.3389/abp.2026.16545

Image Credits: AI Generated

DOI: 10.3389/abp.2026.16545

Keywords: zebrafish, Danio rerio, 3Rs principles, preclinical research, drug discovery, toxicology, oncology, xenograft, cardiotoxicity, epilepsy, diabetes, immunology