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Due to the lack of appropriate experimental models which adequately mimic the progression of human tumors, the field of metastasis research has experienced slow progress during the last decade. Spontaneous and genetic mouse metastasis models can, to some extent, be used to study endpoints of tumor progression. Most of these models, however, are of limited value to study the metastatic cascade as a spatially and temporally dynamic progress. We have towards this end developed novel syngeneic and allogeneic tumor models that can be employed to trace the metastatic fate of injected tumor cells. RenCa (renal carcinoma) cells, B16 melanoma cells, and A375 melanoma cells were transduced with two different retroviral constructs to stably express a luciferase-neomycin resistance fusion protein (either driven by the viral LTR promoter or the human EF1a promoter). RenCa cells were orthotopically injected into the kidney of syngeneic BalbC mice. Likewise, B16 cells were injected intracutaneously into syngeneic C57Bl6 mice. Human A375 cells were xenotransplanted subcutaneously into nude mice or injected into the left ventricle of the heart for systemic distribution experiments. Luciferase activity was traced non-invasively and invasively at different time points. To calibrate the experimental system, luciferase expression per cell was determined using serial dilution experiments, which allowed detection of as few as 9 labelled tumor cells. When luciferase activity in tissue homogenates was quantified, the technique is sensitive enough to detect foci with as few as 100 tumor cells. Likewise, non-invasive imaging of tumor growth and metastasis allows the detection of the smallest metastatic foci at around 500.000 cells. Each of the studied tumor models proved useful for applications in metastasis experiments. RenCa tumors form lung metastases following orthotopic injection, which can be inhibited following treatment with the angiogenesis inhibitor PTK787. Intracutaneous injection of B16 melanoma cells leads to detectable metastases in the lungs, the liver, and the intestine. Intracardiac injection of A375 cells proved to be the most sensitive technique to trace the systemic distribution of tumor cells in the circulation. Taken together, the non-invasive Luciferase-based detection of minimal size metastases is a useful and highly sensitive technique for the study of the molecular mechanisms of metastasis and to employ it as a screening technique for drug development studies.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]