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Metastasis is the most lethal aspect of cancer, yet current therapeutic strategies do not target its key rate-limiting steps. We have previously shown that the entry of cancer cells into the blood stream, or intravasation, is highly dependent upon in vivo cancer cell motility, making it an attractive therapeutic target. To systemically identify genes required for tumor cell motility in an in vivo tumor microenvironment, we established a novel quantitative in vivo screening platform based on intravital imaging of human cancer metastasis in ex ovo avian embryos. Utilizing this platform to screen a genome-wide shRNA library, we identified a panel of novel genes whose function is required for productive cancer cell motility in vivo, and whose expression is closely associated with metastatic risk in human cancers. The RNAi-mediated inhibition of these gene targets resulted in a nearly total (>99.5%) block of spontaneous cancer metastasis in vivo.

Metastasis is the main cause of prostate cancer-associated deaths. While significant progerss has been made in the treatment of primary tumors, efficent therapies that target the metastatic spread of prostate cancer are far from clinical reality. To efficiently treat cancer we need be able to impede its spread. Unfortunately, the majority of current therapeutics approved to treat metastatic cancer were originally selected based on their ability to inhibit primary tumor growth. This inherent flaw precluded these therapies from efficiently targeting the development of secondary metastatic lesions, a process that is distinct from that of primary tumor progression. In this review we will summarize the conceptual, cellular and molecular targets that should be considered to design effective anti-metastatic therapies.