In vivoModels for Experimental Therapeutics Relevant to Human Cancer

Miriam S. Gitler, Edward A. Sausville, Melinda Hollingshead, Robert Shoemaker
<span title="2004-11-15">2004</span> <i title="American Association for Cancer Research (AACR)"> <a target="_blank" rel="noopener" href="" style="color: black;">Cancer Research</a> </i> &nbsp;
About 50 scientists attended, with 8 invited speakers coming from Germany, the Netherlands, and various institutions in the United States. The meeting focused primarily on novel imaging approaches to cancer experimental therapeutics using animal models and pharmacodynamic (PD) end points. At the end of the presentations, a roundtable discussion was held in an attempt to reach consensus on a recommendation for optimal use of these modern imaging modalities. This report summarizes the meeting and
more &raquo; ... provides a forum through which the cancer research community can provide input on the issues addressed in the roundtable discussion. Dr. Robert Shoemaker (DTP, NCI, Bethesda, MD) opened the meeting with an overview of the evolution of animal models for experimental therapeutics, from transplantable syngeneic mouse tumor models to more complex models, such as chemically induced, transgenic, and spontaneous animal tumor models. Animal models for cancer drug development should include features relevant to human cancer, such as the representation of molecular targets, drug metabolism, pharmacokinetics, tissue distribution of a drug, anatomic relevance of tumor site, microenvironment, tumor natural history, angiogenesis, and metastasis. Similarly, critical in animal modeling are the end points for experimental therapeutic studies. Whereas change in tumor size is the most widely used end point, tumor frequency, survival, and change in tumor burden are other end points frequently used in this research area. Dr. Shoemaker observed that this meeting would focus mostly on new tumor imaging techniques that seem particularly relevant for measuring end points critical to experimental therapeutics. Dr. Mike Cable (Xenogen Corp., Alameda, CA) described the advantages and limitations of biophotonic in vivo imaging, as well as the IVIS Imaging System developed by Xenogen. He explained that bioluminescent imaging is a powerful tumor cell labeling technique in which expression of a reporter gene (usually luciferase) is used for functional and tracer applications. In tracer applications, the reporter gene signal increases as cells divide and decreases as they die. Thus, the amount of light emitted at the surface is proportional to the number of cells expressing the reporter. Another useful property of bioluminescent imaging is that reporter expression can be tied to a particular promoter. Advantages of this approach include its high detection sensitivity due to extremely low backgrounds in control animals, relatively simple instrumentation requirements, and a wide range of applications. Bioluminescent imaging provides spatial and functional information. The IVIS Imaging System is a widely used workstation customized for quantitative animal imaging. The system is easy to operate and requires minimal training. Fluorescent imaging is another variant of optical imaging that can be quantitated using the IVIS Imaging System. However, fluorescent imaging is much less sensitive than bioluminescent imaging due to tissue autofluorescence. In closing, Dr. Cable observed that to measure absolute tumor size and location noninvasively, three-dimensional imaging is necessary because two-dimensional images provide only relative tumor growth curves. Tomographic optical imaging systems are under development at Xenogen. Dr. Melinda Hollingshead (Biological Testing Branch, DTP, NCI) also focused on the use of bioluminescent imaging, describing DTP's experience with the IVIS Imaging System. She highlighted the advantages of using this system for measuring tumor burden, including increased sensitivity, decreased assay time, improved orthotopic model end points, and the opportunity to assess metastatic models, mechanistic studies, and micrometastatic disease. Cell lines engineered to express luciferase are needed for bioluminescent imaging. Because the amount of light emitted over a given amount of time varies by cell line, assessing the sensitivity of bioluminescent imaging with each cell line to be used in experimental therapeutics is recommended. DTP has successfully measured bioluminescence of luciferase-transfected tumor cell lines in vivo using the hollow-fiber assay and tumor xenografts, demonstrating in both cases a higher tumor detection sensitivity compared with other approaches. Antitumor activity of chemotherapeutic agents has also been measured effectively in animal models using bioluminescent imaging. Cautionary notes arose from observations that animal feed and other byproducts can emit a high luminescent signal and that frequent, repeated dosing with luciferin for reimaging purposes may have a cumulative effect on the luminescent signal. DTP is evaluating the pharmacology of luciferin after repeated dosing with this substrate. Other critical factors to consider in bioluminescent imaging include potential loss of tumorigenicity with transfection, the luciferin dose selected for imaging, the limited metastatic potential of many cell lines, a loss of linearity with increasing tumor volume, background signals masking small lesions, and the potential impact of therapeutic agents on luciferase activity and/or luciferin pharmacology. Dr. Giovanni Melillo (Science Applications International Corporation-Frederick, NCI-Frederick, Frederick, MD) shared information on his in vivo models for the evaluation of hypoxia-inducible factor (HIF)-1-targeted therapeutics. HIF-1 is a good target for cancer therapeutics because it serves as a marker of tumor hypoxia; it is overexpressed in many human cancers, and HIF-1 expression is associated with angiogenesis, tumor progression, treatment failure, and poor prognosis. DTP has identified inhibitors of the HIF-1 pathway, specifically, a class of camptothecin analogs, including topotecan, that inhibit HIF-1-dependent induction of luciferase. Based on
<span class="external-identifiers"> <a target="_blank" rel="external noopener noreferrer" href="">doi:10.1158/0008-5472.can-04-2057</a> <a target="_blank" rel="external noopener" href="">pmid:15548722</a> <a target="_blank" rel="external noopener" href="">fatcat:diea2cmjijczpkjtesxebncrky</a> </span>
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