A 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.

Inclusion Criteria: * 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. * Measurable MM based on IMWG guidelines as defined by at least one of the following: 1. Serum M-protein \>= 0.5 gram per deciliter (g/dL) by serum protein electrophoresis (SPEP) or, for Immunoglobulin (Ig) A myeloma, by quantitative IgA. 2. Urinary M-protein excretion \>= 200 mg/24 hours. 3. Free light chain (FLC) \>= 100 milligram per liter (mg/L), provided that the FLC ratio is abnormal. * Only for arms Sd-40 BIW, Sd-100 QW and Sd-80 BIW prior to protocol version (PV) 5.0: Participants must have relapsed or refractory multiple myeloma (RRMM) and have previously received at least 4 anti-MM prior therapies and have MM that is refractory to previous treatment with at least 2 proteasome inhibitors (PIs), at least 2 immunomodulatory agent (IMiDs), and 1 anti-cluster of differentiation (CD38) monoclonal antibody. Refractory is defined as lesser than or equal to (\=3 anti-MM therapies (with exposure to at least 2 PI drugs, at least 2 IMiDs, and 1 anti-CD38 monoclonal antibody), and be refractory to at least 1 drug of each class (PI/IMiD/anti-CD38). Refractory is defined as \= 2x upper limit of normal (ULN) (\>= 3x ULN for participants with Gilbert's syndrome), aspartate transaminase (AST) \>= 2.5x ULN, and alanine transaminase (ALT) \>= 2.5x ULN. * Inadequate renal function defined as estimated creatinine clearance of lesser than (\