Cancer models in preclinical research: A chronicle review of advancement in effective cancer research

Abstract Cancer is a major stress for public well‐being and is the most dreadful disease. The models used in the discovery of cancer treatment are continuously changing and extending toward advanced preclinical studies. Cancer models are either naturally existing or artificially prepared experimental systems that show similar features with human tumors though the heterogeneous nature of the tumor is very familiar. The choice of the most fitting model to best reflect the given tumor system is one of the real difficulties for cancer examination. Therefore, vast studies have been conducted on the cancer models for developing a better understanding of cancer invasion, progression, and early detection. These models give an insight into cancer etiology, molecular basis, host tumor interaction, the role of microenvironment, and tumor heterogeneity in tumor metastasis. These models are also used to predict novel cancer markers, targeted therapies, and are extremely helpful in drug development. In this review, the potential of cancer models to be used as a platform for drug screening and therapeutic discoveries are highlighted. Although none of the cancer models is regarded as ideal because each is associated with essential caveats that restraint its application yet by bridging the gap between preliminary cancer research and translational medicine. However, they promise a brighter future for cancer treatment.


| INTRODUC TI ON
Cancer is an epidemic disease causing approximately 8 million deaths annually all around the globe. Latest statistical data exhibit that human malignant growth is turning into the main source of death around the world. The absence of an intensive understanding of cancer biology in the recent era is a key hindrance to research the development, to understand the invasion, and to follow metastasis of cancer tumors. 1,2 Like other disease research, oncology research profoundly relies on a reliable and representative model framework. Nevertheless, we cannot define cancer as a single characterized tumor but instead a heterogeneous and immensely variable system. So that is why the choice of the most fitting model to best reflect the given tumor system is one of the real difficulties for cancer examination. 3 Cancer models, either naturally found or artificially induced, have features in common with human cancers. The inability of in vitro cancer models to mimic the heterogeneity of human cancer cells, its microenvironment, and the stromal compartment has hindered the thorough understanding of tumor pathogenesis, therapeutic responses, and adverse reactions. 4 Experimental systems for studying human cancer include cancer cell lines as well as 3D model organoids and organisms such as Drosophila melanogaster, zebrafish, and genetically engineered mouse model, pigs, patient-derived xenografts (PDXs), and computational cancer models. These models form the basis to investigate cancer biochemical or genetic pathways and pathology. The cumulative information from cancer models helps in the understanding of the subtleties of cancer development in greater detail. 5 To estimate clinical feedback in patients based on the model utilized, it is essential to get relatively a 50% hindrance in tumor development to accomplish a confirmed "response" to treatment and to utilize clinically applicable dosages of curative agents to observe survival. Moreover, it is essential to decide if tumor regeneration occurs when the medication is stopped, expecting this is the situation, whether the redevelopment is fast when treatment is delayed compared with before the treatment started. All cancer models aim to mimic at least some features of human cancer but in the end, we do not have a perfect model, yet it should be figured out how to intercept our information within the structure of the limitations of the test used. 6 Figure 1 presents review flow chart.

| C AN CER CELL LINE
The cancer cell line is an in vitro tumor model that is regarded as a ubiquitous feature of oncology because it shows numerous intrinsic features of cancer and exhibits similar gene expression patterns. 7 Isogenic cancer cell lines are engineered by clustered regularly interspaced short palindromic repeats/CAS9 technology, which either removes or adds specific genes and serves as in vitro human cell line model. 11 It assists in the analysis of specific mutations and thus used as a novel model for exploring targeted anticancer drug pharmacology. 12 As the mismatch repair system (MMR) of DNA is responsible for maintaining genomic integration by recognizing mismatch even in the single base and short insertion-deletion loops,

| PATIENT-DERIVED XENOG R AF T
A xenograft is derived from the Greek word Xenos meaning foreign.
It is obtained from one organism and implanted into other organisms. These implantations are mostly done in immunocompetent mice and are comprised of organs, tissue, or living cell. In the area of cancer research, xenografts are utilized to address key inquiries where it is important to rely on the utilization of animal models that show a close resemblance to the progression of the tumor in human patients. 24 Xenograft models that contain primary carcinoma tissue is obtained from the patient's tumor tissue are built up at very low transit numbers; for example, less than 10 passages expelled from human patients to conserve the original or primary tumor characteristics. 25 These characteristics include heterogeneity of cells, clinical biomolecular signatures, malignant genotypes and phenotypes, tumor structure, and vasculature. 24 The basis for creating PDX models F I G U R E 1 Advancement in cancer research models depends on the assumption that these PDX models will illustrate improved preclinical testing and are predictive of molecular cancer biology that is related to human cancer and how patients respond to cancer therapy. 26 PDX models have been exhibited to be useful for (i) investigations of cancer metastasis and medication obstruction, (ii) personalized medication and treatment, and (iii) preclinical testing and discovery of new anticancer drug candidates. 27 By surgery or biopsy methods, primary or metastatic tumors are collected and preserved as tissue structures. 26 This resection of tumor specimens can develop gradually in immune-deficient mice and instigate a switch toward applying patient-derived tumor tissue xenograft models in the investigation of anticancer medications and treatment methods. 28 The most widely recognized site of tumor implantation in mice is subcutaneous implantation (dorsal region), even though transplantation in a similar organ as the primary tumor might be an alternative and is known as orthotopic implantation; for example, pancreas, brain, oral cavity, ovary, breast, etc. Furthermore, many approaches and efforts have been made to implant tumor in the renal capsular site to increase the rate of engraftment. which has many advantages such as they can preserve histology of tumor tissue relative to the primary sample and progressive xenograft generations and can regenerate the original genotypic and phenotypic characteristics. 26,29 Furthermore, there are trial metastasis models in which controlled numbers of tumor cells are administered for metastasis, comparatively brief time is required for the advancement of metastasis, and metastases locales can then be determined. 30 Advancement in cancer drug development has been hindered by an absence of preclinical cancer models that accurately evaluate clinical testing of significant novel compounds in human patients. So, these drawbacks have been overcome by the utilization of patient-derived tumor xenograft in immunocompetent mice (preclinical models) such as nude mice, severe combined immunodeficiency mice (SCID), nonobese diabetic (NOD)-SCID gamma mice, recombination-activating gene (Rag), and NOD rag gamma mice. 24 They are discussed in Table 2.
One basic part of huge preclinical investigations in PDX models is that these models help to organize potential clinical signs and are involved in the identification of potential drug efficacy biomarkers.
In colorectal cancer, various examinations demonstrate that Kirsten rat sarcoma (KRAS)-mutant PDX models do not react to cetuximab. to choose medications to be tested in platinum-safe patients. 42 They are potentially powerful because they are generally biologically stable, and are indefinitely renewable. Breast cancer PDXs models recapitulate different aspects of the biology of the tumor and therefore they serve as an excellent model to carry out translational research. 43 Patient-derived xenografts, however, have limitations as well as having a different tumor microenvironment, are not amenable to genetic modification` and incorporation of immune system` since they are induced in immunodeficient mice so they do not recapitulate the commitment of the host immune system. The bacterial flora for carcinogenesis which is necessary for the early detection of cancer is not sustained in xenografts. They are not suitable for immunomodulatory testing for cancer prevention, initiation, and progression of genetics cancer modeling, along with low throughput drug screening. Their biobanking is not possible and they show genetic heterogeneity and epigenomic instability. 44

| G ENE TI C ALLY ENG INEERED MOUS E MODEL S
Since the innate characteristics and physiology of xenografts do not outline the genetic characteristics of a human tumor, the genetically engineered mouse models (GEMMs) was established. 24 Technical advancement over ongoing decades permits the investigators to make alterations in the genome of mice that conditionally or constitutively change the expression of important genes that led to the TA B L E 1 Different cancer cell lines their derivation, tumor type, biological source, morphology, and growth mode

| DROSOPHIL A MEL ANOG A S TER
Drosophila melanogaster has extensively contributed to elucidating the molecular basis of cancer biology by unveiling the action mechanism of proteins related to cancer. D. melanogaster is made as a cancer model by induction of mutation in larva using ethyl methanesulfonate (EMS). Tumorous tissue resides in the outer proliferative center (OPC) and central brain (CB) regions in the larval brain.
The larval brain is then transplanted into the abdomens of adult  Zebrafish models are manageable to genetic control. Forward genetics demonstrates its use in predicting cancer markers.

| ZEB R AFIS H
Since the body of the non-mammalian zebrafish is transparent so the tumor progression and cancer metastasis can be tracked efficiently. Thus, zebrafish serves as a reliable cancer model. Zebrafish have a small size which makes them simple and easy to house. Their zygotes are valuable in pharmacological research and drug screening. 74 Difficulty in the examination of fixed tissue is the major disadvantage of zebrafish because sectioning embryos or larvae is tricky due to their small size. 76 Also, there is relatively low tumor incidence, although these tumors are comparable in different mutants these tumors develop in life at a later stage. 72 However, zebrafish is exceptionally fit to contribute insights in cancer biology and for providing a "whole-organism test tube" for the rapid identification of the novel markers, to determine their functions, and the evaluation of their capacities, the investigation of host reactions, and development of anti-cancer drugs. 74  Table 5.

F I G U R E 3
Application of cancer models in various cancer. Experimental models are being used to determine the characteristics of the different types of tumor proliferating in different organs inside the body. Despite the limitations and advantages of these models, each type of cancer growth associated with a particular organ (eg, lungs, breast, ovarian) interacts and responds to these experimental models differently. The following pictorial representation indicates the cancer model application and shows which experimental model depicts the properties of a specific cancer type more successfully than the other High cost in terms of sample handling and starting amount, instrumentation, and time for data analysis and integration. The poor co-relation between -omics approaches (eg, genomics, transcriptomics, proteomics, and metabolomics) Single-cell analysis held great potential but is still underdeveloped.
Tumors are heterogeneous and so -omics data from one part of the biopsy may not be representative of the whole tumor Cancer models, either in vitro, in vivo, or computational, enable us to conduct studies that are impractical on patients due to economic, moral, and welfare considerations. Gathering data and information from these models temporarily or permanently provide advantages to patients.