A xenograft model is a surgical graft of tissue from one species to another and commonly utilizes an immunodeficient host animal to prevent transplant rejection. Mice and rats with varying degrees of immune system deficiencies can be used for xenografts, but the preferred model is fully immunocompromised, lacking B-cells, T-cells and natural killer (NK) cells.  Xenografts in these rats and mice of patient derived cells or primary tumor tissues (PDX, PDTT, or PDTX) provide a better model that overcomes the limitations of traditional in-vivo models and more sufficiently represent human cancer characteristics especially with regard to metastasis and drug sensitivity. Patients derived xenografts are being used to provide results with better translational potential that have eluded researchers using traditional rodent models.

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Immunodeficient Rats

Rat models that are B & T-cell deficient (Rag2 KO) or fully immunocompromised (Rag2/Il2rg double KO, lack of B, T and NK-cells) can be used for xenografts, tumor immunology and PDX studies.  Rat models provide several significant advantages for xenograft studies over their complimentary mouse xenografts.

Larger Size:

The larger size of the animals allows for easier handling and more precise surgical manipulations. For example, intracranial xenografts and other orthotopic tissue injections are easier to perform.

More Tissue:
Due to the size of the rats compared to mice, xenograft tumors can grow larger, especially in xenografts into the brain, prostate, and other tissues, and this provides more tissue to analyze.

Easier Tumor Imagining/ Visualization:

Tumor metastasis and angiogenesis are often the primary focus of cancer research and therefore non-invasive in-vivo imagining is becoming an increasingly important tool to understand these aspects of the cancer progression.  Rats have been shown to be a better model for in-vivo imagining of metastatic tumors from xenografts which are easier to detect due to the larger size[4].

More Utility:

The rats can have multiple blood draws, compared to mice, which allow for drug studies and provides more utility.  Additionally, since rats are the preferred model for toxicology, this allows for both efficacy studies and toxicity to be evaluated in the same species.

Sources:

  1. Yang, et al. A novel GFP nude rat model to investigate tumor-stroma interactions. Cancer Cell International. 2014 December. 14:54
  2. Asamoto, et al.Prostate Carcinomas Developing in Transgenic Rats with SV40 T Antigen Expression under Probasin Promoter Control Are Strictly Androgen Dependent. Cancer Research. 2001 June 15.   61; 4693
  3. Iannaccone, P. M., & Jacob, H. J. (2009). Rats! Disease Models & Mechanisms2(5-6), 206–210.
    Nofiele, et al. Establishment of a Lung Metastatic Breast Tumor Xenograft Model in Nude Rats.. PLoS ONE.  2014 May 16. 9(5), e97950.

Studies and Applications

Prostate cancer is the most common cancer and the second leading cause of death from cancer among men in the United States. Many patients who are diagnosed eventually develop castration-resistant prostate cancer (androgen independent) with metastatic foci which are the cause patient death. Therefore, understanding of the mechanisms of the acquisition of metastatic potential or the androgen-independent phenotype of cancer cells is urgently required and in-vivo models which accurately reflect the disease progression are necessary.  Both xenografts and transgenic rodent models of prostate cancer have been established attempting to provide a fast, effective way to recapitulate the human prostate cancer.

Transgenic rodent models that develop prostate cancer due to various genetic modifications provide a relatively reliable model of tumor development proven to be easier to study than carcinogenicity models. However, in the popular TRAMP model it usually takes 24-30 weeks for the adenocarcinomas to develop, the tumors have neuroendocrine features which are rare in human prostate cancers, and they rarely metastasize to distant organs including the bones which is often present in human prostate cancer3. These transgenic models, require a considerable expenditure of time, effort and a large number of animals, and in addition their use has led to few translational outcomes. It is for this reason that xenografts have been a rapidly adopted approach for drug discovery efforts in prostate cancer1-2.

Prostate cancer xenografts provide a way to evaluate potential therapeutics on human cells and to model human tumor progression in vivo1. For these xenografts prostate cancer cells are orthotopically implanted into host animals.  Various xenograft cell lines have been established with different features including those with a high rate of metastasis as well as xenograft lines from human prostate cancer tumors, which are implanted directly into an immunocompromised host, preserving the heterogeneity and eliminating the effects of in vitro culture.  Rats are advantageous due to the larger size of their prostate, because tumors grow larger and provide more tissue for analysis4 and have been used to model various stages of prostate cancer4.

Studies

  1. Pre-clinical mouse models of human prostate cancer and their utility in drug discovery.
    Summary: Xenografts are a particularly useful tool as for evaluating efficacy of investigational new drugs and therapeutic regimens for prostate cancer.  The xenografts in which human tumors are grafted directly into the host animal provide the best recapitulation of the heterogeneity of the human tumors and the eliminates the potential molecular and epigenetic changes that can occur after long periods of in vitro growth.  These patient derived tumor xenografts are already shown to have greater predictability for clinical responses to several pre-clinical drugs.
    Park et al. Current Protocols in Pharmacology. 2010 December 1.  14:14.15.
  2. Animal Models of Human Prostate Cancer: The Consensus Report of the New York Meeting of the Mouse Models of Human Cancers Consortium Prostate Pathology Committee
    Summary: A variety of genetically modified mice for prostate cancer have been developed. These models provide a way to study tumor development in-vivo, however they are costly and it takes a long time to generate results. Xenografts of human prostate cancer cells are a relatively quick and low cost way of studying the biology of human prostate cancer.  The models described in this review have proven useful for evaluating new therapies in prostate cancer and mechanisms of therapeutic resistance.
    Ittmann, et al.  Cancer Research. 2008 May 1. 73, 2718.
  3. Broadening of Transgenic Adenocarcinoma of the Mouse Prostate (TRAMP) Model to Represent Late Stage Androgen Depletion Independent Cancer
    Summary: The genetically modified spontaneous prostate cancer mouse provides a relevant clinical model for early stages of prostate cancer, but because of a lack of synchronicity and long latent period of tumor incidence/growth, a better in-vivo model is necessary to study the mechanisms of late stages of prostate cancer including metastasis, and androgen independence.
    Jeet, et al. The Prostate.  2008.  68:548 -562.
  4. Establishment of a syngeneic orthotopic model of prostate cancer in immunocompetent rats
    Summary:  An syngeneic orthotopic xenograft line was established as a model for prostate cancer in immunocompetent rats. The aim was to create feasible conditions for anticancer drug development in a species-matched tumor microenvironment of prostate cancer in a model that is most suitable for analysis of stage-specific effects of therapeutic agents. While in some cases species-matched tumors are needed, a fully immunocompromised rat would allow for wide variety of non-species specific and patient derived tumor xenografts providing more utility than the model described.
    Suzuki, et al.  Journal of Toxicologic Pathology.  2015 Jan; 28(1): 21–26.

Breast cancer is the most commonly diagnosed form of cancer and the second leading cause of cancer death in Western women1 . Several difficulties are associated with studying breast cancer, first the different genetic subtypes that have been identified, including triple negative HER2-positive variety, need to be considered when developing a treatment and secondly, breast cancer tumors themselves have been shown to have a high level of heterogeneity. Of the variety of pre-clinical models that have been used to study breast cancer, patient derived tumor xenografts show the most utility for pre-clinical models.

Breast cancer tumor xenografts in mice have been shown to recapitulate all the key characteristics of human breast cancers including histology and pathology, gene expression profiles, and for estrogen receptor positive tumors, estrogen dependence and/or responsiveness.  These have been shown to preserve the heterogeneity of the original tumor as well.  Other models such as synergenic models (allografts) of breast cancer have limited potential for direct translation into clinical application due to species-related differences.

Using rats for the study of breast cancer, provides several advantages. -It has been shown that there are many histological similarities that have been demonstrated between rat mammary tumors and human breast cancers.  Rats are also larger and provide a better model for in-vivo imaging to study cancer metastasis and angiogenesis.  In addition, the carcinogenic rat models in which chemically induced tumors are used to study tumorigenesis have been used historically because of the ease with which hormone dependent tumors can be generated by carcinogens1 and this research provides a background for a more critical understanding of xenograft/host interactions.

Studies:

  1. Models of breast cancer: is merging human and animal models the future?
    Summary:  The most widely used syngeneic mouse tumor models have a limited role in cancer research because the biology of rodents and their tumors differs significantly from that of humans and human breast cancer. While breast cancer is one of the more difficult tumors to transplant, human xenograft models have the potential to be a good model for preclinical testing.
    Kim, et al.  Breast Cancer Research.  2004. 6:1, 22-30.
  2. Animal models for breast cancer
    Summary:  Carcinogen-induced mammary tumors in rats can be used as a tool for studying tumorigenesis and breast cancer progression and metastasis.  The rat is the preferred model for carcinogen induced breast cancer because of the ease at which hormone-dependent tumors can generated. The tumors progress from preneoplastic, neoplastic, to metastatic tissues allowing the study of events in the tumorigenic process.
    Sukumar, et al. Mutation Research.  1995 December.  333:2, 37-44.
  3. Tumor grafts derived from women with breast cancer authentically reflect tumor pathology, growth, metastasis, and disease outcomes.
    Summary:  Transplantable breast cancer tumors derived directly from patients are shown to represent the diversity of human breast cancer and maintain the essential features of the original patients’ tumors. These tumor grafts are shown to have a high potential for clinical relevance and ease of use which will prove to be an excellent model for breast cancer researchers. Successful tumor engraftments in the NOD/SCID mouse was found to be about 30%  and this could possibly be improved by using a fully immunocompromised host for the tumor transplantation.
    DeRose, et al.  Nature Medicine.  2011 October 23. 17:11, 1514-1520.
  4. Establishment of a Lung Metastatic Breast Tumor Xenograft Model in Nude Rats.
    Summary:  This paper demonstrated the establishment of a xenograft breast cancer line that metastasizes to the lungs, this xenograft model was developed in response to the need for a better in-vivo imaging model.  Rats, due to their larger size, provide a model with a for studying spontaneous metastasis using non-invasive magnetic resonance imaging (MRI) with a high potential to translate to human imaging.  Rats were also chosen as due to the histological similarities that have been demonstrated between rat mammary tumors and human breast cancers.
    Nofiele, et al.  PLoS ONE.  2014 May 16. 9(5), e97950.
  5. A tissue-engineered humanized xenograft model of human breast cancer metastasis to bone
    SummaryStudying metastasis of breast cancer with humanized xenograft models incorporating human cells or tissue grafts at the primary tumor site or the metastatic site mimic the human disease more closely. It is for these reasons that humanizations are currently being developed.  Immunocompromised rats would lend themselves to being humanized comparable to the mice models described, but their larger size would confer significant advantages.
    Thibaudeau, et al.  Disease Models and Mechanisms.  2014. 7, 299-309.
  1. A reproducible brain tumor model established from human glioblastoma biopsies
    Summary: Xenografts of patient biopsy spheroids of glioblastoma tumors where transplanted into nude rats with a high success rate of engraftment. The larger size of the rat brain allows for easier injections and allows for other procedures and experiments which are limited in mice.
    Huszthy, et al.  PLoS ONE, 2015 August.  10(8), e0136089.
  2. Experimental model and immunohistochemical analyses of U87 human glioblastoma cell xenografts in immunosuppressed rat brains
    Summary: A glioblastoma cell line used for xenografts was validated as a cost effective and simple approach to studying glioblastoma. This model was demonstrated to have a high take rate for xenograft implants. In this model, the rats used are immunosuppressed with cyclosporine. However, since the development of genetically modified rats which have immune deficiencies, the immunosuppression step can be eliminated.
    Strojnik, et al. Anticancer research.  2006.  vol.26(4B), pp.2887-900
  1. Orthotopic Human Choroidal Melanoma Xenografts in Nude Rats with Aggressive and Nonaggressive PAS Staining Patterns
    Choroidal melanoma is the most common primary ocular cancer among adults. This study describes a human choroidal melanoma orthotopic xenograft model developed in nude rats.   The xenografts in nude rats reproduced the PAS staining patterns associated with aggressive and non-aggressive choroidal melanomas in patients.  The rat was chosen as the model because in mice the injections are difficult and the tumors can’t be visualized due to the small size, rabbits are another option but they are required to be kept on immunosuppressant’s, are costly to maintain, have a limited availability of antibodies for immunohistochemical studies.
    Braun, R. D., & Abbas, A. Investigative Ophthalmology & Visual Science, 47(1), 7–16.