Selinexor Plus High-Dose Melphalan (HDM) Before Autologous Hematopoietic Cell Transplantation for Multiple Myeloma

Inclusion Criteria: * 18 years of age or older with histologically confirmed multiple myeloma * Achieving partial response (PR) or very good partial response (VGPR) with systemic chemotherapy * Received less than 4 lines of anti-myeloma therapy. * Karnofsky performance status of \>= 70% * Adequate pulmonary, cardiac, hepatic and renal function as outlined in the protocol * Signed informed consent form in accordance with institutional policies prior to the initiation of high-dose therapy Exclusion Criteria: * Non-secretory multiple myeloma * Have achieved complete response (CR) prior to autologous hematopoietic cell transplantation (HCT) * Central nervous system (CNS) involvement * Uncontrolled bacterial, viral or fungal infections * Myocardial infarction within 6 months prior to enrollment or has New York Heart Association (NYHA) Class III or IV heart failure, uncontrolled angina, severe uncontrolled ventricular arrhythmias, or electrocardiographic evidence of acute ischemia or active conduction system abnormalities. * Prior malignancies within the last 5 years except resected basal cell carcinoma or treated cervical carcinoma in situ. * Females who are pregnant or breastfeeding * Have received other investigational drugs within 14 days prior to screening * Prior autologous or allogeneic HCT * Prior organ transplant or autoimmune disease requiring immunosuppressive therapy