The objective of this basic/discovery research project is to develop combination therapies that enhance the efficacy of multiple myeloma (MM) killing by the nanotherapeutics being developed in Projects 1 and 2. Since experimental evidence continues to support a critical role for the BM microenvironment in mediating de novo drug resistance and disease progression in MM, targeted modulation of microenvironmental regulatory pathways may be essential to control MM. We plan to use preclinical mouse (5TGM1 cells in C57BL/KaLwRij mice) and human xenotransplant (MM.1S cells in NSG mice) models to test if modulation of the BM microenvironment improves the efficacy of the Project 1 and 2 nanotherapeutics via the following two aims: Aim 1: Test if BM niche disrupting agents enhance the efficacy of MM killing by the nanoparticles generated in Projects 1 and 2. We will determine if CXCR4 (G- CSF, AMD3100, ALT1188 & CMD peptide), VLA-4 (firategrast & firategrast analogues) and proteasome (bortezomib, carfilzomib, and ixazomib) inhibitors mobilize MM cells and increase nanoparticle-mediated MM killing. Aim 2: Test if anti-CD47 monoclonal antibodies (CD47mAbs) enhance the efficacy of MM killing by the nanoparticles generated in Projects 1 and 2. Leukemic cells increase CD47 expression to avoid phagocytic clearance. CD47mAbs that block the CD47/SIRP? interaction enhance phagocytosis of leukemia cells. We have identified two CD47mAbs, Vx-1000 and Vx-1004, both of which block the CD47/SIRP? interaction and promote phagocytosis of tumor cells by macrophages. However, Vx-1004 also has the unique property of killing cancer cells, but not normal blood cells, via a direct, cell- autonomous, cytotoxic mechanism. We will determine if the tumor-toxic activity of Vx- 1004 confers enhanced efficacy in vivo compared to Vx-1000 in promoting MM killing by the Project 1 and 2 nanotherapeutics. If the BM niche disrupting agents and/or CD47mAbs being examined in these preclinical studies enhance the efficacy of nanoparticle-mediated MM killing, we may have a novel therapeutic approach for sensitization of drug (including nanoparticle) resistant MM cells and for prevention of minimal residual disease in MM. Furthermore, these therapeutic approaches may pave the way for development of similar treatments for other hematologic and solid tumor malignancies.