This is the initial study of TH-302 in subjects with relapsed/refractory multiple myeloma. It is an open-label dose-escalation study to determine the DLTs, MTD, safety and preliminary efficacy of TH-302 and dexamethasone with a Simon two-stage expansion at the MTD. The study will also investigate the DLTs, MTD, safety and preliminary efficacy of TH-302, dexamethasone and bortezomib; and the DLTs, MTD, safety and preliminary efficacy of TH-302, dexamethasone and pomalidomide. As such, the study is separated into four parts. Treatment will be administered until disease progression or unacceptable toxicity, or 12 cycles (Arm A, Arm B and Arm C) or 9 cycles (Arm D) have been completed.