As part of our efforts to develop a preclinical drug development path that is focused on cancer metastasis it has been necessary to develop novel assays for the metastatic phenotype. Currently available in vitro assays of metastasis are helpful but fail to rigorously evaluate the complexity of the metastatic cascade. Murine models of metastasis, when combined with careful considerations of pharmacokinetics and target modulation are informative but require extensive observation periodes. This long evaluation may be an obstacle in the use of metastasis endpoints in cancer drug development to date. What is needed are novel assays that screen potentially active cancer drugs against the metastatic phenotype and prioritize agents that are most likely to be active in mouse and larger animal models. Based on this need we have developed two novel assay systems that are simple, reproducible, relatively short-term and relevant for the biology of metastasis. The first assay allows the evaluation of novel cancer drugs against the active steps involved in metastatic progression (i.e. metastasis as a verb). This assay involves the evaluation of single metastatic cells during their transit into the lung (the most common site of metastasis in pediatric sarcoma patients) in a mouse. Novel drugs can be evaluated before, during or after the arrest of metastatic cells at distant sites over 24-72 hour observation periods. This assay is referred to as a single cell videomicroscopy assay. This assay has been used successfully by our group in several studies of metastasis biology. The second and most innovative model is an ex vivo whole organ lung culture assay. Tumor cells are fluoresently labeled and delivered to syngeneic (murine cancer cells) or immunocompromised (human cancer cells) mice. At distinct time points after metastasis of cells to the lung, the lungs of the mice are removed and prepared for ex vivo organ culture. We have optimized techniques for lung organ culture over 35 days. During this time we are able to then follow the progression of metastatic cells ex vivo from single cell colonies to large and grossly detectable metastasis. In the ex vivo assay (referred to as PuMA) the impact of new drug exposures can be assessed on the progressing tumor cells at distinct times and exposure periodes compared to untreated cells. These two assays are expected to uncover new activities for exisiting cancer agents against metastasis, overlooked using conventional preclinical drug screening approaches and to identify the most promising new cancer agents with activities against metastasis. Our goal is to evaluate 5-10 new cancer agents through this development path each year.
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