In our June 2017 blog post, we described advantages and challenges of using syngeneic, GEM, and humanized mouse models for preclinical immuno-oncology (I/O) drug development. In this blog, we expand on this idea and offer thoughts on choosing the most appropriate I/O tumor model for one’s study. While there are benefits and limitations of any model, one can use these considerations, as well as others, as a foundation for preclinical in vivo efficacy study design. Understanding tumor placement, immune composition, response to treatment, and molecular characterization for the model of interest can be invaluable when designing the most appropriate study for your research goals.
The choice of tumor placement has long been important in preclinical development. The classical subcutaneous tumor implant on the flank of the mouse allows for ease of measurement by caliper, is minimally invasive allowing for large studies, and tumor growth kinetics on the flank is generally reproducible. However, subcutaneous tumor models, even those derived from metastatic foci, rarely metastasize1 which create challenges when used for this application. Tumors implanted subcutaneously also lack the organ specific stromal-tumor interactions shown to be important to establishment of the tumor microenvironment. The absence of this interaction can impact both tumor progression and immune response.2
Because of these caveats, orthotopic implantation of both solid and hematological models in preclinical drug development has drawn favor over the past few decades. Some suggest that these orthotopic implants more faithfully represent the clinical situation. For example, several systemic tumor models such as MV-4-11 and C1498 acute myeloid leukemias and MM.1S multiple myeloma, demonstrate seeding in bone marrow which is highly translatable to tumor progression in humans. Additionally, glioma models, such as human U251 and murine GL-261, progress with an invasive phenotype in the mouse brain, very similar to human disease.
As for monitoring tumor progression following an orthotopic implant, the use of bioluminescence imaging (when the luciferase enabled version of the cell line is available) and small animal MRI have a distinct advantage compared to simple survival endpoints determined by clinical observations. The latter manifests clinical signs only in advanced disease, making it very difficult to establish growth kinetics of the tumor under treatment. Such granularity is easily achieved with noninvasive imaging modalities available through Covance.
Despite the benefits of orthotopic implantation, a number of challenges are inherent in this strategy. For example, orthotopic models often require surgical placement necessitating technical expertise and constraints in study size. Also, while noninvasive monitoring of tumor progression, such as by luciferase enabled cell lines, is powerful, the generation of these lines may also elicit unintended immune responses that may result in tumor rejection or impact the outcome of immuno-oncology based studies.3 Given the advantages and limitations inherent with subcutaneous and orthotopic tumor placement described here, it is important to rely on the phase of agent development as well as the desired endpoints to dictate the study strategy.
Tumor models vary significantly with regard to immune composition, and knowledge of these relative differences can greatly influence choice of model. These differences are illustrated in Figure 1 for several models offered at Covance. Most tumors can be classified as immunologically “hot,” “warm,” or “cold,” most simply defined as the extent to which the immune infiltration of the tumor allows for immune system engagement; “hot” tumors are readily engaging, “cold” tumors are less likely to engage the immune system, and “warm” tumors have elements of each. High and low infiltrates of cytotoxic T cells, respectively, are a major player in this delineation. Likewise, tumors with a high infiltrate of myeloid derived suppressor cells (MDSCs) are typically less responsive to immunomodulatory agents due to enabling an immunosuppressive tumor microenvironment and subsequent immune evasion. Taking this model information, one can hypothesize that a model with a high T cell and low MDSC infiltrate (a “hot” tumor) would be the preferred model to test a checkpoint inhibitor, but this agent is less likely to show activity in a model with low T cell and high MDSC infiltrate (a “cold” tumor). To this end, Figure 2 illustrates the differential response to CTLA-4 checkpoint blockade in the “warm” CT26 model and the “cold” 4T1 model.
Written by: Sheri Barnes, Ph.D. | Director, Scientific Development