Myeloma is the second most common hematologic cancer. It thrives in the bone marrow and aggressively disseminates throughout the skeleton causing osteolysis, debilitating pain and devastating side effects. The tumors eventually become chemoresistant leading to patient death. The Sanderson Lab is dedicated to understanding how heparanase promotes the aggressive behavior of myeloma and to using that knowledge to develop curative therapies for myeloma patients. During the previous funding period, we identified multiple mechanisms through which heparanase drives myeloma progression and metastasis and established heparanase as a viable target for myeloma therapy. Moreover, we catalyzed collaborative efforts aimed at developing and testing novel anti-heparanase drugs and demonstrated their efficacy against myeloma in vivo. Based on our work, one of these inhibitors recently entered clinical trials in myeloma patients. Thus, we accomplished the goals proposed in our previous application including translation of our work to the clinic. The challenge now is t maximize anti-heparanase therapy to enhance tumor killing; a goal that will be attained by a thorough understanding of heparanase mechanisms of action. To this end, we have made two discoveries that provide novel insight into how heparanase regulates myeloma behavior: (i) heparanase enhances the secretion and alters the composition and function of tumor cell-derived exosomes and (ii) heparanase facilitates development of myeloma chemoresistance. These discoveries lay the foundation for studies proposed in this competitive renewal. Our working hypothesis for Aim 1 is that heparanase regulation of exosomes facilitates robust tumor-host crosstalk that drives myeloma progression and metastasis and that these exosomes are targets for anti-heparanase therapy and sources of disease biomarkers. Exosomes will be tested for their impact on behavior of both host and tumor cells using in vitro and in vivo models and anti-heparanase therapy will be tested for its ability to diminish exosome secretion by tumors thereby inhibiting tumor progression. The protein composition of exosomes from myeloma patient sera will be assessed for levels of heparanase, syndecan-1 and other cargo to determine their value as biomarkers of disease. Our working hypothesis for Aim 2 is that heparanase promotes chemoresistance and that use of heparanase inhibitors in tandem with chemotherapeutic drugs will overcome chemoresistance and enhance tumor killing. Using in vitro and in vivo models we will identify which commonly used anti-myeloma drugs exhibit a diminished ability to kill when tumor cells are expressing high levels of heparanase. Once identified, those drugs will be tested in combination with anti-heparanase drugs to determine if this increases tumor killing efficiency in vivo. The proposed work is innovative and significant because it couples the discovery of new heparanase mechanisms of action with the objectives of identifying novel biomarkers, maximizing anti- heparanase therapy and improving patient outcome.