RecruitingPhase 2Drug trialA Study of Selinexor (Seli) + Low-dose Dexamethasone (LDD) in Penta-refractory Multiple Myeloma (MM), Seli and Bortezomib + LDD in Triple-class Refractory MM.The purpose of this study is to assess the efficacy, antitumor activity, safety and tolerability of selinexor plus low-dose dexamethasone in participants with penta-refractory multiple myeloma or selinexor and bortezomib plus low-dose dexamethasone in participants with triple-class refractory multiple myeloma.