This is a multi-center, open-label, single arm, non-comparative phase II trial, designed to evaluate the efficacy of plitidepsin in combination with bortezomib and dexamethasone in patients with MM double refractory to bortezomib and lenalidomide.The primary endpoint will be overall response rate (ORR), including stringent complete response (sCR), complete response (CR), very good partial response (VGPR) and partial response (PR).