Animal Models in Experimental Medicine E-Book
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- Herausgeber: Bentham Science Publishers
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
- Veröffentlichungsjahr: 2024
This reference presents information about models utilized in experimental medicine and pharmaceutical research and development for several human diseases. Written by experts in immunology, cancer biology and pharmacology, the book provides readers with handy notes and updated data on animal models that are critical to research planning and lab execution.
The main feature of the book is a set of 12 structured chapters that focus on a specific disease such as cancer, infectious diseases, autism, autoimmune diseases, Alzheimer’s disease and anemia. The contributors have gathered information on a wide range of genetic and physiological animal models that are employed in research with comparative charts that highlight their main differences. The book also includes chapters for special topics like food allergies and dentistry. Additional features of the book are an explanation of disease mechanisms that give an easy understanding, notes for idiopathic models and specific clinical conditions, and a list of references for advanced readers.
Animal Models in Experimental Medicine is essential reading for scholars, graduate students and senior researchers in life sciences and clinical medicine. It also serves as a resource for professionals involved in bench-to-bedside pharmaceutical projects.
Readership
Scholars, graduate students and senior researchers in life sciences and clinical medicine; professionals involved in bench-to-bedside pharmaceutical projects.
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PREFACE
For both experimental and clinical trials, animal models have been used extensively for many years. The research is currently focused on many approaches that hold great promise by offering radical new successful treatments for many human diseases by using these animal models. The current book provides a summary of the current knowledge and the use of these models as well as their pre-clinical implications and related therapeutic options. Furthermore, the recently discovered trials of these animals in cancer, autoimmunity, infectious diseases, allergy, dental and other diseases have encouraged us to write this book in depth for developing new pharmacological and immunological treatments and for a better understanding of many chemo-resistant drugs.
DEDICATION
To,
Adnan Albar
John Goldman
Myrtle Gordon
Abdulla Alnaser
George Eyambe
Richard Wyse
Anthony Warrens
Robert Lechler
Mohamed Salem
Abdulla Alothman
List of Contributors
Animal Models for Cancer
Fahad N. Alonazi1,*,Yousef M. Hawsawi2,3,Helal G. Alanazi4,Adel M. Alqarni4,Suad A. Alghamdi4,Rakan J. Alanazi5Abstract
Cancer is a complex multifactorial disease that affects many people worldwide. Animal models play an important role in deciphering cancer biology and developing new therapies. The animal models widely used in cancer research include tumor xenografts, genetically engineered mice, chemically induced models, and spontaneous tumor models. These models provide a controlled environment to study cancer progression, the interaction of cancer and the immune system, and the effectiveness of new therapies. Although animal models have several advantages, it is important to identify their limitations and use them in conjunction with other preclinical models, such as in-vitro cell culture and patient-derived xenografts, to ensure that results are transferable to humans. In this chapter, we discuss the importance of animal models in cancer research, the different types of animal models, and their advantages and disadvantages. We also provide some examples of animal models used in cancer research. Collectively, animal models have been invaluable in advancing our understanding of cancer and will continue to be important tools in the development of new therapies.
Introduction
Cancer is a leading killer worldwide, and despite significant advances in diagnosis and treatment, it remains a poorly understood and treated disease. One of the reasons for this challenge is the complexity of the disease, involving multiple genetic and environmental factors [1]. Animal models play a crucial role in advancing our understanding of cancer biology and the advancement of new therapies. They provide a controlled environment to study cancer progression, its interaction with the immune system and its response to various treatments. Animal models are also useful for identifying new therapeutic targets and understanding mechanisms of drug resistance.
In recent years, advances in genetic engineering have enabled the creation of animal models that better mimic human cancers, such as B. mice with mutations in the KRAS oncogene. These models provide valuable insights into mechanisms of carcinogenesis and have been used to test new therapies targeting specific molecular pathways.
Despite advances in cancer research using animal models, there are still challenges and limitations that need to be addressed. Off-target effects, differences in the tumor microenvironment, and lack of representation of human diversity are just some of the challenges researchers face. Future research should focus on improving animal models to better mimic human disease, identifying biomarkers of treatment response, and developing combination therapies targeting multiple oncogenic pathways.
Mechanisms of cancer disease
The adult human body is composed of approximately 37 trillion cells, expressed in various sizes, types and shapes, which together define the structure and function of the entire body. Although most cells are regularly damaged by wear and tear, their numbers remain strictly constant. This is due to the highly precise regulation of cell division and differentiation processes (i.e., cellular homeostasis between cell death and reproduction). Disruption of this balance and failure of apoptosis can lead to aggressive and uncontrolled cell division, ultimately leading to the development of cancer.
Cancer refers to a group of diseases characterized by the uncontrolled division of cells. Cancer can strike anyone, regardless of sex, age, and social status. It can start in any part or organ of the body and can be classified according to the tissue of origin. The transformation of normal cells into cancer cells involves alterations in the action of different genes and proteins in the cells. Under the influence of certain conditions, cancer cells can spread from the tissue of origin to nearby tissues and other non-adjacent tissues (metastasis), interfering with their normal functions.
Irrespective of technological advances, breast cancer originating in breast tissue remains one of the leading causes of cancer-related death in women worldwide [2]. Cancer genetics have revolutionized our understanding of breast cancer. The discovery of complementary DNA (cDNA) array analysis in the past century was a remarkable breakthrough in breast cancer research to evaluate the effect of a precise tumor signature on prognosis [3]. This innovation opens a new window into the molecular classification of breast cancer. Currently, the progression of breast cancer is highly dependent on altering the expression of multiple genes. To date, more than 8,000 genes have been found to be differentially expressed in different breast cancer subtypes [4], hence the concept of genotype-phenotype correlation.
However, immunohistochemical studies distinguished four distinct molecular subtypes of breast cancer, including normal breast cancer, HER-2-positive, basal-like, and luminal-like. Luminal A and Luminal B are additional classifications of the major subtypes of Luminal [5]. The most common subtype of breast cancer is Luminal A, followed by Luminal B. Her-2 is expressed in approximately 40% of breast cancers, with even expression of hormone receptor-positive and -negative subtypes. A small proportion (10%) of breast cancer tumors are basal-like tumors, also known as triple-negative (TN) breast cancers due to their molecular makeup. With a brief background on the classification of breast cancer, we can now turn to the mechanisms of the disease.
Breast cancer is a heterogeneous disease caused by multiple factors through multiple mechanisms. This fatal disease develops over many years and through multiple stages. The multistep progression of carcinogenesis is shaped by complementary mechanisms known as cancer hallmarks. The concept of a “cancer hallmark” refers to a set of functional capabilities that human cancer cells acquire as they progress from a normal growth phase to a neoplastic growth phase. Especially skills that are crucial for the development of malignancies. In early 2000, Hanahan and Weinberg enumerated their hypothesized universal features that link all the different cancer cells at the cellular phenotype level [6, 7]. Initially, they predicted the contribution of different hallmark abilities and later expanded the different hallmarks to eight [7].
The hallmarks include avoiding growth inhibitors, sustaining proliferative signals, promoting angiogenesis, achieving replicative immortality, and inducing invasion and metastasis [6]. Kim and colleagues also reported that genome stability and cancer-associated inflammation are additional features associated with tumorigenesis. It is worth showing some important aspects of the cell cycle to understand the processes of the various hallmarks.
Cell Cycle
Understanding the cell cycle, which regulates DNA replication and cell division, is critical to understanding the molecular pathways of cancer [8]. Five dynamic phases are critical during the cell cycle. The first phase, the interphase, consists of three separate consecutive phases. The first phase is the G1 “monitoring” phase, followed by the S phase, in which DNA is synthesized, and finally, the third phase, G2, when cells continue to develop and prepare for mitosis. Thereafter, cells enter a critical phase called the mitosis (M) phase [9]. There are four phases of mitosis: telophase, anaphase, metaphase, and prophase. Cells can re-enter the cell cycle and undergo DNA replication and mitosis during quiescence (G0), a biochemically distinct state.
Changes in the activity of certain cyclin-dependent kinases (CDKs) drive transitions between these phases, with Cdk1/Cdk2 and Cdk2/Cdk4/Cdk6 directing the transition from G2 to mitosis and G1 to S phase, respectively [9]. With this introduction, we can now understand the mechanisms of carcinogenesis and the hallmarks of cancer.
Characteristics of Developing Tumors
Cancer hallmarks currently include the following hallmarks: escape or evasion of growth inhibitors; ability to immortalize replication, ability to sustain proliferative signals, promotion of tumor inflammation, ability to resist cell death programs, induction of invasion and metastasis, genomic instability and mutation, ability to induce/reach blood vessels, immune system evasion, cellular metabolic reprogramming, cellular metabolic dysregulation [10]. These hallmarks play a crucial role in the development of cancer [11]. So, we will discuss each hallmark in detail in the following sections.
Escaping or Evading Growth Suppressors
Cancer cells can bypass growth inhibitory pathways. Returning to the cell cycle, it is important to emphasize that these components determine how cells move through the G1 phase of their developmental cycle. These processes are known as anti-growth signals and are involved in the cell cycle of normal cells. However, to survive, cancer cells resist antiproliferative signals. To understand how this process works, we need to understand an important protein called the retinoblastoma protein (pRb). pRb and its two cousins, p107 and p130, are molecular pathways for many if not all, antiproliferative signals. pRb inhibits proliferation when in a hypophosphorylated state by secreting and altering the function of the E2 factor (E2F) transcription factor that regulates expression of the gene pool necessary for progression from G1 to S phase [12].
When the pRb pathway is disrupted, E2F is released, which promotes cell proliferation and converts cells to growth factor-resistant cells, thereby preventing cell cycle entry into the G1 phase. Another mechanism that inhibits G1 phase progression is through TGF [13]. The TGF inhibits progression by preventing the phosphorylation that renders pRb inactive [14]. One approach to prevent differentiation directly uses the c-myc oncogene, which encodes a transcription factor. The hallmarks of cancer complement each other, making cancer cells immortal as well.
Ability to Enable Replicative Immortality
Cancer cells can convert into immortal cells. Provocatively, it appears that most tumor cell types that are grown in culture are immortalized, indicating that infinite replicative ability is a characteristic that was acquired in vivo during tumor progression [15]. Research indicated that the telomeres at the ends of chromosomes, which are made up of thousands of repeats of a brief 6 bp sequence fragment, are the counting mechanism for cell generations. However, the loss of 50–100 bp of telomeric DNA from the ends of each chromosome during each cell cycle is scientifically used to calculate the number of replicative generations. However, almost all types of cancer cells exhibit telomere maintenance; 85%–90% of them achieve this by increasing the expression of the telomerase enzyme, which adds hexanucleotide repeats to the ends of telomeric DNA [16, 17]. On the other hand, the remaining cells have developed a method of activating a mechanism known as ALT, which appears to maintain telomeres through recombination-based interchromosomal exchanges of sequence information [16]. However, one of the cancer characteristics is the ability to sustain proliferative signaling.
Capacity to Sustain Proliferative Signalling
Usually, cancer cells can sustain proliferative signaling. During tumor pathogenesis, the cell surface receptors that transmit growth-stimulating signals into the cell interior become targets of dysregulation. Many malignancies have an overexpression of growth factor (GF) receptors [18, 19], which frequently have tyrosine kinase activity in their cytoplasmic domains. The overexpression of the receptor may make cancer cells more sensitive to GF levels in the environment than they would usually be [20]. The ER+ breast cancer can be treated through hormonal therapies [21, 22]. Additionally, cancer cells can alter the types of extracellular matrix receptors (integrins) that they express, favoring those that provide signals for proliferative proliferation [23]. Although breast cancer development highly depends on genetic factors, genome instability and mutations are crucial.
Genome Instability and Mutations
Many studies have suggested that the malfunction of particular parts of the genomic “caretaker” systems is the cause of this mutability [24, 25]. Several genes have been associated with genome instability, including BLM and BRCA1/2 [26, 27]. However, the p53 tumor suppressor protein is the most noticeable component of these systems. When DNA is damaged, it either causes cell cycle arrest to allow for DNA repair or, if the damage is too great, the apoptosis process will be activated. It is now evident that most, if not all, cases of human malignancies lack the functionality of the p53 DNA damage signaling system [28]. It is interesting to note that current research reveals that apoptosis may also be a source of genomic instability due to the possibility of nearby cells incorporating DNA from apoptotic cell bodies after phagocytosis. Nevertheless, cancer cells can resist the cell death program. More recently, compelling evidence emerged for the concept of non-mutational epigenetic reprogramming as a cancer phenotype hallmark [29]. For instance, hypoxia decreases the activity of the TET demethylases, causing significant methylome alterations, including hypermethylation [30].
Capability for Resisting Apoptosis
Cells undergo apoptosis as part of a normal regulatory process, and often, in tumor cells these mechanisms are subverted [21]. For example, the tumor suppressor protein phosphate and tensin homolog on chromosome 10 (PTEN) is an important regulator of apoptosis, and often, within breast cancer, mutations in this protein result in the inhibition of apoptotic processes and uncontrolled cell division. Many of the signals that cause apoptosis focus on the mitochondria, which release cytochrome C, a powerful apoptosis accelerator, in response to proapoptotic signals [31]. Members of the Bcl-2 protein family, which include proteins with proapoptotic (Bax, Bak, Bid, and Bim) or antiapoptotic (Bcl-2, Bcl-XL, and Bcl-W) activity, control mitochondrial death signals [32]. When p53 detects DNA damage, it increases the production of proapoptotic Bax, which in turn prompts mitochondria to release cytochrome C [33]. Additionally, death receptors such as FAS or the cytochrome C produced from mitochondria respectively, activate two “gatekeeper” caspases, 8 and 9, respectively.
Cancer cells can develop resistance to apoptosis using a variety of techniques. Undoubtedly, the p53 tumor suppressor gene is involved in the most frequent loss of a proapoptotic regulator through mutation. More than 50% of human malignancies have this functional inactivation of the p53 protein, which removes a crucial part of the DNA damage sensor that can activate the apoptotic effector cascade [34]. In a significant portion of human malignancies, the PI3 kinase-AKT/PKB pathway, which sends antiapoptotic survival signals, is probably implicated in preventing apoptosis. Ras-derived intracellular signals [35], extracellular hormones like IGF-1/2 or IL-3 [12], or the deletion of the pTEN tumor suppressor, a phospholipid phosphatase that typically dampens the AKT survival signal, can all activate this survival signaling circuit. Cancer cells can promote inflammation. So, how did this process happen?
Tumour-promoting Inflammation
In contrast to apoptosis and autophagy, necrotic cell death sends proinflammatory signals into the surrounding tissue milieu [36]. Necrotic cells can, therefore, attract immune system inflammatory cells [37]. However, considering that immune inflammatory cells can promote angiogenesis, cancer cell proliferation, and invasiveness, there is evidence that these cells may actively promote tumor growth in the context of neoplasia. Furthermore, necrotic cells can emit bioactive regulatory factors such as IL-1 that can directly drive nearby viable cells to grow, thereby accelerating the evolution of neoplastic disease [38].
Triggering Invasion and Metastasis
The invasion-metastasis cascade, which refers to the discrete processes that make up the multistep process of invasion and metastasis, has been extensively studied [39]. The change that was best understood involves cancer cells' loss of E-cadherin, a crucial cell-to-cell adhesion protein [40]. The consistent downregulation of E-cadherin and mutational inactivation of this characteristic capability in human carcinomas offered compelling evidence for this concept. This illustration shows a series of cell-biologic changes starting with local invasion and followed by intravasation by cancer cells into nearby blood and lymphatic vessels. Afterward, cancer cells transit through the lymphatic and hematogenous systems and escape from the lumina of vessels into the parenchyma of distant tissues (extravasation). Cancer cells end up with the formation of small nodules (micrometastases) and the growth of micrometastatic lesions [41]. This process moves to the next hallmark, which is inducing or accessing the vasculature.
Capability for Inducing/accessing the Vasculature
Vascular endothelial growth factor (VEGF) and fibroblast growth factors (FGF1/2) are the main examples of signals that start angiogenesis. An “angiogenic flip” from vascular quiescence appears to be the discrete phase during tumor development that confers the capacity to trigger and maintain angiogenesis. By shifting the ratio of angiogenesis inducers to opposing inhibitors, tumors appear to turn on the angiogenic switch [42]. Comparing tumors with their equivalents in normal tissue, many cancers show elevated expression of VEGF and/or FGFs. Others have downregulated the expression of endogenous inhibitors such as thrombospondin-1 or -interferon. Additionally, in some cancers, both changes may take place and are connected [43].
Avoiding Immune Destruction System
Cancer cells develop strategies to avoid being detected and eliminated by the host's immune system [7]. Cells can accomplish this, for instance, by manipulating immunological checkpoint regulation and the innate immune response using a stimulator of interferon genes (STING) [44]. It is now clear that cancers manipulate their microenvironments, notably by altering certain immune checkpoint pathways, to elude immune monitoring and attack. T cells, sometimes referred to as tumor-infiltrating lymphocytes (TIL) in malignancies, are abundantly present at the tumor site and affect overall survival. The cell surface ligands on the tumor cell cause PD-1 on the T-cell to become active. The host immune system may be bypassed by malignancies due to the upregulation of PD-L1. TIM-3 suppresses antitumor immunity by promoting T-cell fatigue [45].
Cellular Metabolism Reprogramming
Cancer cells can also reprogram cellular metabolism and deregulate the system [35]. Increased glycolysis, increased glutaminolytic flux, upregulation of amino acid and lipid metabolism, increased mitochondrial biogenesis, induction of the pentose phosphate pathway, and macromolecule biosynthesis are some of the most notable changes in tumor cell bioenergetics [46]. The Warburg effect is a phenomenon whereby cancer cells use glycolysis even under normoxic conditions [47]. Cancer cells upregulate glucose transporters, including Glut1, Glut2, Glut3, and Glut4, to uptake more glucose. Furthermore, most glycolytic enzymes' expression is noticeably increased in cancer cells [48]. Additionally, glutaminolysis is a common source of energy and metabolism for many cancers [49]. The pentose phosphate pathway (PPP) has significant effects on several cancer-related phenomena, such as metastasis, treatment resistance, invasion, and proliferation [50]. The augmentation of mitochondrial biogenesis is yet another important modification in cancer metabolism [49]. Another striking aspect of cancer metabolism is an increase in lipid metabolism [51].
Animal models in cancer research
The primary objective of incorporating animal experimentation into cancer research is to gain an in-depth understanding of the pathophysiology of numerous types of cancer, ranging from genomics and proteomics to metabolomics levels. Animal models provide controlled environments to study cancer progression, its interaction with the immune system, and its response to different treatments. They have also been instrumental in identifying new targets for therapy and understanding the mechanisms of drug resistance. For example, preclinical studies using mouse models have led to the development of numerous successful cancer therapies, comprising trastuzumab for breast cancer, imatinib (Gleevec) for chronic myelogenous leukemia, and vemurafenib (Zelboraf) for melanoma.
In many studies, targeting of potential therapeutic relevance in cancer cells was initially validated in genetically engineered mouse (GEM) tumor models. This can be accomplished by knocking out applicant genes in the germline (provided they didn’t adversely impair viability) or by utilizing conditionally floxed alleles that were ablated dependently with the growth of K-Ras oncogene activation [52] [53]. Most of these investigations have centered on proteins and enzymes that are theorized to function as downstream effector molecules of K-Ras, like those in the mitogen-activated protein kinase (MAPK) pathway [54].
Different types of animals can be employed as model organisms for these experiments. The most used animal models in cancer research will be described below:
Mice
Mice are most frequently used in cancer research, where they are used to study the genetics of cancer development, tumor growth, and response to treatment. The development of transgenic and knockout mice has enabled the manipulation of specific genes involved in cancer, allowing researchers to identify potential drug targets and evaluate new treatments [55]. For example, a mouse model of breast cancer has been used to study the effect of the Pten gene on tumor growth and metastasis. In this study, researchers found that loss of Pten leads to increased tumor growth and metastasis, suggesting that Pten may be a potential therapeutic target [56]. However, using mice as disease models has some limitations. Mice are small, which limits the types of experiments that can be performed. Additionally, mice have a shorter lifespan than humans, making it difficult to study cancers that slowly develop over time [57].
Chemically Induced Mice
Chemically induced mice models are essential in the elucidation of the fundamental signaling mechanisms, genetic and environmental factors associated with human carcinogenesis, and the screening of potential chemo-preventive agents and drugs [58]. Similar to human tumors caused by environmental factors, the tumors induced in mice through chemicals have a considerable burden of mutations. This burden leads to a diversity of genetic and epigenetic changes, which in turn affect the prognosis of individual cases, the efficacy of treatments, and the emergence of drug resistance [59].
The induction of colon and rectal tumors in mice through subcutaneous or intraperitoneal injections of 1,2-Dimethylhydrazine (DMH) has been proven to be an efficient method of carcinogenesis. The mice model of experimental colon carcinogenesis, known as the DMH model, has been thoroughly established and is widely used. It shares many morphological and molecular similarities with sporadic colorectal cancer (CC) in humans [60]. In a skin cancer study, researchers analyzed the ability of Withaferin A (WA) to prevent the spread of cancerous cells in a chemically induced mouse model. The findings of Li et al. in 2016 revealed that WA effectively hindered skin carcinogenesis by restraining cell proliferation, as opposed to promoting cell death [61]. In general, carcinomas that are caused by chemical exposure tend to have a lack of invasiveness, both locally and distantly, with metastases being a rare occurrence. Additionally, there is often a significant delay between the initial application of the chemical and the development of the tumor in many cases [62].
Genetically Engineered Mice
The preferred method of analyzing the intrinsic and extrinsic cellular processes involved in cancer initiation, progression, and metastasis is through the use of genetically engineered mouse models (GEMMs) of de novo tumorigenesis in vivo systems [63]. The benefits of GEMMs stem from the fact that the mouse genome shares a 99% similarity with the human genome. Additionally, the small size of these animals and the availability of an extensive array of molecular tools renders them ideal candidates for conducting large-scale research studies, which can ultimately prove to be cost-effective [64]. Chemoprevention research has utilized GEMMs to a great extent, leading to a substantial contribution in comprehending the onset of tumor formation. Specifically, GEMMs have been instrumental in shedding light on early events that lead to tumor initiation [62].
Although GEMMs have their benefits, they may not consistently replicate the heterogeneity of tumors that are commonly found in humans [63]. Nonetheless, GEMMs have demonstrated potential in forecasting the response of the clinical setting when it comes to drug development. Despite this, pharmaceutical companies have been unwilling to use GEMMs due to the challenges that come with overseeing preclinical experiments. These trials often necessitate the growth of tumors for several months before a therapeutic intervention can be introduced [65].
Patient-derived Tumor Xenograft
The high failure rates in clinical trials emphasize the necessity for improved preclinical efficacy models to better predict clinical outcomes. Human preclinical models have been developed to address this issue, including the patient-derived tumor xenograft (PDX) model, which has improved our understanding of cancer growth pathways and served as an effective tool for innovative cancer therapies [66]. PDX models are generated by directly implanting or injecting human cancerous tissues into an immune-deficient mouse, providing a high degree of predictability and rapidity of tumor formation compared to genetically engineered models. Also, based on the cell lines and number of injected cancerous cells, xenografts might take less time to develop tumors [67].
Histological evaluation of tumors developed in mice compared to the corresponding original tumor showed significant conservation of morphologic properties between the two tumors. Xenograft models are also useful for testing cancer therapies and maximizing the outcomes of these models random transgene integration can also result in unexpected phenotypes, and it is difficult to control the level and pattern of gene expression in GEMMs chemopreventive therapies, leading to the recognition of genes mutations in certain tumors that are associated to drug resistance [68].
Despite their utility, xenograft models may have some drawbacks. One limitation of the approach is that the immune system of the mouse is not fully functional, which may affect the response to certain therapies. Another limitation is that the stroma of mice may differ from that of humans, leading to differences in tumor growth and metastasis. Additionally, xenograft models may not accurately represent specific genotypes and lineage subtypes [69]. Moreover, these models are not suitable for evaluating treatments that rely on immune-based mechanisms or species-specific host interactions [70].
Zebrafish
Zebrafish has emerged as a promising whole organism model for evaluating signaling pathways involved in cancer as well as a model for the development of innovative therapeutics. This model organism shares significant similarities in terms of physiological and genetic characteristics with humans [71, 72]. Human hematologic malignancies, melanoma, rhabdomyosarcoma, and other solid cancers were replicated using a zebrafish model [73, 74]. Despite the incomplete development of an adaptive immune system in zebrafish larvae until 14 days after fertilization, it is possible to transplant human cancer cells into them, and these cells can survive and metastasize. This characteristic of zebrafish provides several advantages compared to standard rodent and xenograft models, as reported by [75]. The optical transparency of zebrafish embryos permits a unique opportunity to visually monitor cancer genesis and growth within the living animal [76]. In addition, zebrafish provide efficient transplantation assays using minimal tumor cells for investigating cancer self- renewal and the impact of clonal evolution on tumor initiating potential [77].
In the field of drug discovery and development, zebrafish is advantageous in that its orthotopic glioblastoma xenograft model is employed to assess the ability of drugs to penetrate the blood-brain barrier. This is due to the structural and functional similarities of the zebrafish blood brain barrier to that of humans [78].
In addition to the benefits of using zebrafish as a model system, there are also certain challenges that need to be considered. One such challenge relates to the temperature differences that are optimal for the growth and maintenance of human cells versus zebrafish. Specifically, when human cells are transplanted into zebrafish embryos, they may cease to grow when maintained at the standard temperature of 28°C used for zebrafish embryos and adult fish [79]. Furthermore, the applicability of zebrafish in cancer research is limited by the absence of certain tissue types that are present in humans, such as breast and prostate tissues. This lack of anatomical similarity reduces the utility of zebrafish models for the study of cancers affecting these specific organs [80].
Drosophila
Drosophila is a genus of the Drosophila family, members of which are often referred to as fruit flies, vinegar flies, or wine flies. The utilization of Drosophila as a model for studying human cancers has proven to be highly successful. This is due to several reasons, including its short lifespan, low genetic redundancy, rapid reproduction, economical maintenance costs, lack of ethical restrictions, and comparable genetic and functional characteristics to those of humans. It has been discovered that approximately 68% of human cancer signaling pathways are conserved in Drosophila.
In one study, researchers have already implemented Drosophila in the study of Acute Myeloid Leukemia (AML), a prevalent form of leukemia, and have successfully pinpointed the genes responsible for the disease. One of these genes, AML1, is a transcription factor that triggers myeloid differentiation and has a counterpart in the fly [81]. In addition, various studies have successfully produced Drosophila models for colorectal, lung, thyroid, and brain cancers. These models have been instrumental in conducting high-throughput screenings of FDA-approved medications. Through this process, numerous drugs were found to be effective in decreasing proliferation and reversing phenotypes. These drugs include, but are not limited to, AUH-6-96, Amsacrine, Artemisinin, curcumin, AY9944, BOT-4-one, Bouvardin, Afatinib, gefitinib, ibrutinib, bazedoxifene, afatinib, Cisplatin, and cyclophosphamide [82].
Although Drosophila is widely used as an experimental model, it has certain limitations when it comes to studying humans. One of the primary differences between the two organisms is the dissimilarity in their physiological and cancer-related mechanisms. Humans have a more intricate anatomy and physiology than Drosophila. Additionally, cancer in humans is more complex than it is in Drosophila, which means that cancer Drosophila models only provide a partial understanding of the disease in humans. These differences can lead to false positive or negative results during drug screening. Moreover, Drosophila lacks the equivalent organs of mammals, such as the liver, pancreas, spleen, thymus, kidneys, lungs, and thyroid gland [83].
