An insidiously terrifying aspect of breast cancer, and the one that leads to the greatest death, is its propensity to recur in metastatic sites even over a decade after all evidence of cancer has passed. What causes these cells that very early on disseminated from the primary tumor and lay dormant for year to now re-emerge and grow uncontrolled remains unknown. A large part of our ignorance about this critically important aspect of breast cancer control and treatment is due to the lack of tractable model systems. Current approaches to studying breast cancer as it resides as a micrometastasis in a foreign organ are limited by the haphazard nature of "snapshot in time" sampling of clinical specimens and animal studies and limited complexity of cell culture investigations. We propose to develop an extracorporeal functional liver bioreactor for the study of emergence from metastatic dormancy. During and subsequent to the prior Idea award funding period, we adapted a liver bioreactor to investigate breast cancer seeding of the liver (one of the three most common organs for metastasis); this successfully highlighted the ability of breast cancer cells to change their behavior under the influence of liver tissue. We now propose to develop a more complex and all human liver bioreactor to study the re-emergence of a tumor thought to be in remission. Breast cancer kills when it recurs as large metastases after spreading from its original site in the mammary gland to distant organs. Current interventions are successful at removing the tumor from the breast and are curative if the spread has not occurred. However, once the tumor escapes from its original location, chemotherapy appears of limited benefit in either eradicating the undetectable micrometastases or dealing the recurrent tumors. Our initial findings, and those of others, suggest that the dormancy imposed by the metastatic milieu make these tumor cells resistant to treatment. Thus, we have turned to now examine what causes these micrometastases to start growing again, in an effort to treat or prevent this re-emergence. As such, our initial use of this new bioreactor will be to test the hypothesis that liver damage and the resultant inflammatory response drives the recurrence from these metastatic seeds. The completion of this study will yield a unique tool with which to study the tumor cell in its metastatic site, as well as serve as a test bed for therapies aimed at these disseminated tumors, but also provide the first integrated picture of micrometastases escaping from dormancy to grow into a lethal recurrent tumor. Both of these will advance the field of breast cancer research. The technical aspect will provide a unique enabling tool that would nucleate novel investigations as it provides entry into a hitherto unapproachable set of queries. The initial proof-of-concept experiments will yield theoretical insights that can be developed toward either molecular target identification or avoidance behaviors and treatments. The primary benefit of these studies will be in the basic understanding of breast cancer biology. Still, there should be applicable benefits attributable to these advances. The people who will benefit the most from these studies are those victims and their families that live with a breast cancer that has been removed or treated and are in remission; the particular patients are overwhelmingly women as men who have breast cancer tend to detect it at an advanced stage. This is the majority of diagnosed breast cancers. The timeline to actual benefit to people depends on the nature of the findings. Obviously, defining a molecular target and developing an intervention takes the better part of decade. However, if the target is already known in another context, therapies can be tried sooner, or treatment protocols modified on a quicker timescale. More immediate though may be the possible finding of liver injury leading to emergence of metastases. This would result in lifestyle changes and behavior modifications to avoid such injuries (minimize hepatic toxins including certain drugs, alcohol, stresses, etc.) in an effort to delay recurrence as curative therapies are developed.