The following criteria is provided for health care professionals.

Inclusion Criteria:

Patients must meet all of the following inclusion criteria to be eligible to enroll in this study:

• Age ≥ 18 years
• 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 (PS) of ≤ 2
• Having measurable MM based on the modified International Myeloma Working Group (IMWG) guidelines, defined by at least one of the following:
  • Serum M-protein ≥ 0.5 g/dL by serum electrophoresis (SPEP), urinary M-protein excretion ≥ 200 mg/24 hours by urine electrophoresis (UPEP), and free light chain (FLC) ≥ 100 mg/L, provided that the FLC ratio is abnormal.
• Patients with non-secretory multiple myeloma will be included if they have 25% or more of plasmacytoma size progression or appearance of new plasmacytoma lesions.
• Must have at least previously received ≥ 1 anti-MM regimens
• More than 6 months have passed since an allogeneic transplant or 100 days since an autologous stem cell transplant, if patients had any
• Adequate hepatic function within 28 days prior to 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 (3) normal to<2 × ULN.
• Calculated creatinine clearance (CrCl) >15 mL/min based on the Cockcroft and Gault formula.
• Adequate hematopoietic function within 7 days prior to C1D1: total white blood cell (WBC) count ≥1500/mm3, absolute neutrophil count ≥1000/mm3, hemoglobin ≥8.5 g/dL and platelet count ≥75,000/mm3
  • 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 (eg, eltrombopag, romiplostim, or interleukin-11) must have at least a 2-week interval between growth factor support and the Screening assessments, but they may receive growth factor support during the study.
  • Patients must have:
    • At least a 2-week interval from the last red blood cell (RBC) transfusion prior to the Screening hemoglobin assessment, and
    • At least a 1-week interval from the last platelet transfusion prior to the Screening platelet assessment.
• Female patients of childbearing potential must have a negative serum pregnancy test at Screening. Female patients of childbearing potential and fertile male patients who are sexually active with a female of childbearing potential must use highly
• Effective methods of contraception throughout the study and for 3 months following the last dose of study treatment.

Exclusion Criteria:

Patients meeting any of the following exclusion criteria are not eligible to enroll in this study:

• Has received selinexor or another XPO1 inhibitor previously.
• Has any concurrent medical condition or disease (eg, uncontrolled active hypertension, uncontrolled active diabetes, active systemic infection, etc.) that is likely to interfere with study procedures.
• Uncontrolled active infection requiring parenteral antibiotics, antivirals, or antifungals within 1 week prior to Cycle 1 Day 1 (C1D1). Patients on prophylactic antibiotics or with a controlled infection within 1 week prior to C1D1 are acceptable.
• Known intolerance, hypersensitivity, or contraindication to glucocorticoids.
• Pregnant or breastfeeding females.
• Life expectancy of <6 months
• Major surgery within 6 weeks prior to C1D1.
• Patients with active hepatitis B virus (HBV) are eligible if antiviral therapy for hepatitis B has been given for >8 weeks and viral load is <100 IU/mL.
• Patients with untreated hepatitis C virus (HCV) are eligible if there is a documentation of negative viral load per institutional standard.
• Patients with history of human immunodeficiency virus (HIV) are eligible if they have cluster of differentiation 4 (CD4+ )T-cell counts ≥350 cells/µL, negative viral load per institutional standard, and no history of acquired immunodeficiency syndrome (AIDS)-defining opportunistic infections in the last year.
• Any active gastrointestinal dysfunction interfering with the patient's ability to swallow tablets, or any active gastrointestinal dysfunction that could interfere with absorption of study treatment.
• Inability or unwillingness to take supportive medications such as anti-nausea and anti-anorexia agents as recommended by the National Cancer Comprehensive Network (NCCN) for antiemesis and anorexia/cachexia (palliative care).
• Any active, serious psychiatric, medical, or other conditions/situations that, in the opinion of the Investigator, could interfere with treatment, compliance, or the ability to give informed consent.
• Contraindication to any of the required concomitant drugs or supportive treatments.
• Patients unwilling or unable to comply with the protocol including providing 24-hour urine samples for urine protein electrophoresis at the required time points.