Phase 1b part (open-label) 1. To evaluate the safety of the combination of radium-223 dichloride plus bortezomib and dexamethasone 2. To determine the dose of radium-223 dichloride that will be used in the phase 2 part of the study (recommended phase 2 dose [RP2D]) Phase 2 part (double-blind, randomized): 1. To compare radium-223 dichloride with placebo in addition to background treatment with bortezomib plus dexamethasone in terms of progression-free survival (PFS) in relapsed multiple myeloma subjects.