PROJECT SUMMARY Based on “seed and soil” theory, certain tumors exhibit a predilection for metastasis to particular organs. For example, bone is the most common site of metastasis for prostate cancer, happening in ~90% of patients with advanced stages of prostate cancer. Organ-on-a-chip models of cancer metastasis have emerged as a powerful predictor of cancer progression. However, despite the development in organ-on-a-chip platforms for in-vitro studies in metastasis, research in bone metastasis on-a-chip remains largely underdeveloped, and the only few available models in the literature lack the complex mineralization and the inclusion of bone cells, especially osteoclasts into the system, which are essential elements in order to study bone remodeling. Here, (aim 1) we will use a novel organ-on-a-chip platform with a highly mineralized and calcified cell-laden hydrogel including osteoclasts to determine the influence of mineralization and the cross-talk of prostate cancer cells and bone cells on the process of preferential prostate cancer growth in bone and the consequent bone resorption. A potentially rate-limiting step in metastasis formation is the extravasation process that involves adhesion of tumor cells to endothelial cells and their transmigration through the endothelial cell monolayer and basement membrane. It has been well-established that pericyte-support of EC capillaries is required for formation of non-leaky vessels and perturbation of the EC-hMSC linkage, therefore results in leaky vessels. The role of pericytes in tumor metastasis has been mostly focused on tumor angiogenesis and the research on the role of this cell type on cancer extravasation has remained underdeveloped. Here, (aim 2) we will use the bone metastasis-on-a-chip platform to test the role of pericytes in a vasculature embedded in a mineralized bone matrix in inhibiting human prostate cancer extravasation as well as the effects of factors released by cancer cells on vasculature integrity. We argue that this multi-pronged strategy will enable the engineering of in-vitro bone metastasis-on-a-chip model system to understand the preferential metastasis of prostate cancer to the bone and bone destruction as well as the role of pericytes in prostate cancer extravasation through the vasculature. Ultimately, this project will lead to model systems that can be used for studying cancer metastasis to bone and developing new treatments.