Harnessing genetically engineered mouse models for preclinical testing
Introduction
Although great progress has been made in understanding mechanisms of tumorigenesis resulting in development of many anticancer drugs, most drugs that show preclinical efficacy fail to predict clinical response [1]. Even among cancer drugs that pass Phase I testing, only 1 in 10 is ultimately approved much to the distress of patients, pharmaceutical companies, and the scientific community. Most drugs fail Phase II clinical trials largely because of inappropriate guidance from preclinical studies. Among many reasons why preclinical studies fail to correlate with clinical efficacy are differences in drug metabolism, pharmacokinetics and pharmacodynamics, many of which are not addressed in most drug studies in mice [2], [3]. In addition, molecularly targeted drugs may fail to reach appropriate targets, and the widespread use of immunocompromised mice for preclinical testing makes it difficult to predict the role of the host in response to therapies.
Drug screening relied almost entirely on syngeneic mouse tumors until 1980 [2], [4]. These studies led to identification of many currently used chemotherapeutic agents, such as alkylating and other DNA-damaging agents [5], [6]. Availability of immunocompromised mice was followed by rapid adaptation of these models for screening many anticancer agents using human tumors and cell lines. Recent reviews on the outcome of these screens revealed large variability between responses in mice and humans, and a low predictive power to the outcome of Phase II clinical trials [7], [8]. A somewhat better predictive value was achieved when tumors were transplanted directly from patients into mice [9].
It has been proposed that genetically engineered mouse (GEM) models of cancer would improve anticancer drug development. When first introduced, GEM were characterized by development of hematopoietic malignancies and tumors at multiple anatomic sites [10], [11]. These models provided limited opportunity for preclinical assessment of novel therapeutics. Over the last 30 years, transgenic technology has evolved to allow manipulation of the mouse genome to constitutively or conditionally alter expression of specific genes leading to cancer. These models have contributed significantly to the understanding of the molecular pathways responsible for initiation and progression of human cancer, and highlighted the importance of specific oncogenes and tumor suppressor genes in carcinogenesis. Extensive discussion of the characteristics of constitutive, tissue specific, and inducible GEM models is outside the scope of this review and has been summarized recently [2], [4], [10]. Although GEM models may improve preclinical testing, so far, they have not been used in development of any FDA approved anticancer agents. Only few of these models have been examined for similarities in response to standard cytototoxic agents with response in an average patient [12]. Systematic analysis and side-by-side comparison of drug efficacy in tumors driven by common molecular pathways using xenografts and GEM with clinical outcome is urgently needed. In addition, two critical issues have not been appropriately addressed and must be considered when choosing GEM models drug development: (1) the molecular alteration that drives tumorigenesis in the animal model should have a specific correlate in human disease, and (2) the clinical trial must include a study to demonstrate that the drug reaches the intended target molecule or pathway.
Section snippets
Limitations in using GEM for drug development
In spite of their contributions to cancer biology, there are few examples of the use of GEM in preclinical testing because of significant obstacles which prevent widespread use. These include the spontaneous and multifocal nature of tumor development, variable penetrance resulting in lack of synchrony in tumor development, and complicated breeding schemes. These issues have not been systematically addressed.
A major obstacle in using GEM for screening drugs is difficulty in simultaneously
Adapting GEM for preclinical testing
An alternative modality for generating a large cohort of mice bearing synchronous genetically driven tumors is transplantation. A recent study showed that subcutaneous transplantation of mammary tissue from young MMTV-PyMT mice into syngeneic naïve recipients generates tumors but requires multiple passages in vivo, and few of these lines develop metastasis [19]. A similar approach has been used in allografting prostate cancer tumor fragments from 12T10 transgenic mice [20]. However, generating
Analysis of molecular characteristics of GEM tumors in search for human correlates
Understanding similarities and differences between GEM models of cancer and human disease at the molecular level is essential for selection of most relevant models and for their application in preclinical studies. A recent study comparing gene expression profiling of human breast cancers and murine mammary GEM models suggests that MMTV-PyMT tumors segregate with human luminal type morphology and good prognosis, whereas MMTV-wnt1 tumors segregate with a basal type morphology (J. Herschkowitz,
Conclusions
No single preclinical modality will provide adequate guidance for clinical development of new anticancer agents and existing and additional GEM will be increasingly used for preclinical testing. To bring GEM into the mainstream of preclinical testing, many obstacles need to be overcome. Tumors originating in GEM or after transplantation into naïve recipients need to be validated for specific genes and pathways that mirror human disease, as well as for sensitivity and resistance to common
Acknowledgement
The authors thank Karen MacPherson for bibliographical assistance.
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