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.
Age greater than or equal to (>=)18 years at the time of signing informed consent.
Written informed consent in accordance with federal, local, and institutional guidelines.
Serum M-protein >= 0.5 gram per deciliter (g/dL) by serum protein electrophoresis (SPEP) or, for Immunoglobulin (Ig) A myeloma, by quantitative IgA.
Urinary M-protein excretion >= 200 mg/24 hours.
Active plasma cell leukemia.
Documented systemic amyloid light chain amyloidosis.
Active central nervous system MM.
Only for SVd arm: Greater than Grade 2 peripheral neuropathy or Grade >= 2 peripheral neuropathy with pain at baseline, regardless of whether or not the participant is currently receiving medication.