Hypoxia　is　associated　with　increased　metastatic　potential　and　poor　prognosis　in　solid　tumors.　In　this　study,　we　demonstrated　in　the　murine　5T33MM　model　that　multiple　myeloma　(MM)　cells　localize　in　an　extensively　hypoxic　niche　compared　with　the　naive　bone　marrow.　Next,　we　investigated　whether　hypoxia　could　be　used　as　a　treatment　target　for　MM　by　evaluating　the　effects　of　a　new　hypoxia-activated　prodrug　TH-302　in　vitro　and　in　vivo.　In　severely　hypoxic　conditions,　TH-302　induces　G(0)/G(1)　cell-cycle　arrest　by　down-regulating　cyclinD1/2/3,　CDK4/6,　p21(cip-1),　p27(kip-1),　and　pRb　expression,　and　triggers　apoptosis　in　MM　cells　by　upregulating　the　cleaved　proapoptotic　caspase-3,　-8,　and　-9　and　poly　ADP-ribose　polymerase　while　having　no　significant　effects　under　normoxic　conditions.　In　vivo　treatment　of　5T33MM　mice　induces　apoptosis　of　the　MM　cells　within　the　bone　marrow　microenvironment　and　decreases　paraprotein　secretion.　Our　data　support　that　hypoxia-activated　treatment　with　TH-302　provides　a　potential　new　treatment　option　for　MM.　(Blood.　2010;116(9):1524-1527)