• Age ≥ 18 years old at the time of informed consent.

• Willing and able to provide written informed consent in accordance with federal, local, and institutional guidelines. The patient must provide informed consent prior to the first screening procedure.

• Eastern Cooperative Oncology Group (ECOG) performance status of ≤2

• A diagnosis of symptomatic multiple myeloma, with relapsed or refractory disease. Patients must have received at least 4 prior lines of therapy. Prior lines of therapy must include a proteasome inhibitor, an immunomodulatory agent, and an CD38 monoclonal antibody, and may include treatment with BCMA antibody conjugates or BCMA directed chimeric antigen receptor (CAR) T cell therapy.

• All patients must meet criteria for and will receive teclistamab, elranatamab or talquetamab, as per approved label dosing.

• Patients who have had CRS/ICANS from bispecific antibody must have complete resolution of CRS/ICANS before initiation of SEL

• Measurable disease as defined by at least one of the following:

‣ Serum monoclonal (M) protein ≥1.0 g/dl by protein electrophoresis

⁃ \>200 mg of M protein in the urine on 24 hour electrophoresis

⁃ Serum immunoglobulin free light chain ≥10 mg/dL AND abnormal serum immunoglobulin kappa to lambda free light chain ratio

⁃ Measurable plasmacytoma

• Adequate hepatic function measured on labs collected within 28 days of C1D1:

‣ Total bilirubin \<1.5 × upper limit of normal (ULN) (except patients with Gilbert's syndrome who must have a total bilirubin of \<3 × ULN), and

⁃ Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) normal to \<2.5 × ULN.

• Adequate renal function measured on labs collected within 28 days of C1D1. Adequacy will be determined by creatinine clearance with values of ≥ 15 mL/min meeting inclusion criteria. Creatinine Clearance will be calculated using the Cockcroft and Gault formula \[(140 - Age) x Mass (kg)/ (72 x creatinine mg/dL); multiply by 0.85 if female\] (Cockcroft 1976).

⁃ Adequate hematopoietic function measured on labs collected within 7 days of C1D1:

∙ Absolute neutrophil count ≥1500/mm3

‣ Hemoglobin ≥8.5 g/dL

‣ Platelet count ≥100,000/mm3 (patients for whom \<50% of bone marrow nucleated cells are plasma cells) or ≥50,000/mm3 (patients for whom ≥50% of bone marrow nucleated cells are plasma cells)

‣ Note 1: Patients receiving hematopoietic growth factor support, including erythropoietin, darbepoetin, granulocyte-colony stimulating factor (G-CSF), granulocyte macrophage-colony stimulating factor (GM-CSF), and platelet stimulators (e.g., eltrombopag, romiplostim, interleukin-11) are eligible.

‣ Note 2: Patients must have at least a 1-week interval from the last platelet transfusion prior to the Screening platelet assessment. However, patients may receive RBC and/or platelet transfusions as clinically indicated per institutional guidelines during the study.

⁃ Patients who are able to become pregnant must have a negative serum pregnancy test at screening.

⁃ All patients who could become pregnant or could father a child must use highly effective methods of contraception throughout the study and for 5 months following the last dose of study treatment. Highly effective methods of contraception are listed in Section 9.3.1.

⁃ Female patients must agree not to donate egg during the study treatment period and/or up to 90 days after the last dose of Selinexor. Male patients must agree not to donate sperm during the study treatment period and/or up to 90 days after the last dose of Selinexor.