Selinexor (KPT-330) in Patients With Relapsed/Refractory Diffuse Large B-Cell Lymphoma (DLBCL)

Inclusion Criteria (Parts 1 and 2): * Written informed consent in accordance with federal, local, and institutional guidelines. The participant must provide informed consent prior to the first screening procedure. * Age greater than or equal to (≥) 18 years. * ECOG performance status of less than or equal to (≤) 2. * Participants should have estimated life expectancy of greater than (\>) 3 months at study entry. * Previously treated, pathologically confirmed de novo DLBCL, or DLBCL transformed from previously diagnosed indolent lymphoma (e.g., follicular lymphoma). * Participants must have received at least 2 but no more than 5 previous systemic regimens for the treatment of their de novo or transformed DLBCL including (i) at least 1 course of anthracycline-based chemotherapy (unless absolutely contraindicated due to cardiac dysfunction, in which case other active agents such as etoposide, bendamustine, or gemcitabine must have been given) and (ii) at least 1 course of anti-CD20 immunotherapy (e.g., rituximab), unless contraindicated due to severe toxicity. Participants who were considered ineligible for standard multi-agent immunochemotherapy must have received at least 2 and no more than 5 prior treatment regimens including at least 1 course of anti-CD20 antibodies and must be approved by the Medical Monitor. Prior stem cell transplantation is allowed; induction, consolidation, stem cell collection, preparative regimen and transplantation ± maintenance are considered a single line of therapy. * Female participants of child-bearing potential must have a negative serum pregnancy test at screening and agree to use reliable methods of contraception for 3 months after their last dose of medication. Male participants must use a reliable method of contraception if sexually active with a female of child-bearing potential. For both male and female participants, effective methods of contraception must be used throughout the study and for 3 months following the last dose. Part 1 additional inclusion criteria: * For participants whose most recent systemic anti-DLBCL therapy induced a PR or CR, at least 60 days must have elapsed since the end of that therapy. For all other participants, at least 14 weeks (98 days) must have elapsed since the end of their most recent systemic anti-DLBCL therapy. . Palliative localized radiation within the therapy-free interval is allowed. Non-chemotherapy maintenance will not be considered anti DLBCL therapy, and therefore is allowed during the therapy-free interval. * Documented clinical or radiographic evidence of progressive DLBCL prior to dosing. * Participants must have measurable disease per the revised criteria for response assessment of lymphoma. Lymph nodes should be considered abnormal if the long axis is \>1.5 centimeter (cm), regardless of the short axis. If a lymph node has a long axis of 1.1 to 1.5 cm, it should only be considered abnormal if its short axis is \>1.0. Lymph nodes ≤1.0 by ≤1.0 will not be considered abnormal for relapse or PD. Part 2 additional inclusion criteria: • At least 3 weeks (21 days) must have elapsed since the end of participant's most recent systemic anti-DLBCL therapy (prior to Cycle 1 Day 1). Palliative localized radiation within the therapy-free interval is allowed.Non-chemotherapy maintenance will not be considered anti-DLBCL therapy, and therefore is allowed during the therapy-free interval. • Adequate hematopoietic function: (i) Hemoglobin ≥10.0 grams per deciliters (g/dL) within 14 days of starting therapy (participant may receive red blood cell \[RBC\] transfusion within 14 days). (ii) Absolute neutrophil count ≥1000 cells/millimeter (mm\^3) (use of granulocyte growth factors prior to and during the study is acceptable). (iii) Platelet count ≥100,000/mm\^3 within 14 days of starting therapy (use of platelet growth factors prior to and during the study is acceptable). * Participants must have measurable disease per the revised criteria for response assessment of lymphoma. Lymph nodes should be considered abnormal if the long axis is \>1.5 cm, regardless of the short axis. Extranodal lesion should be considered abnormal if the long axis is \>1.0 cm. Exclusion Criteria (Parts 1 and 2): * Participants who are pregnant or lactating. * Primary mediastinal (thymic) large B-cell lymphoma (PMBL) * Participants must not be eligible for high-dose chemotherapy with autologous stem cell transplantation rescue (Investigator must provide detailed documentation for ineligibility). * Participants who have not recovered to Grade ≤1 clinically significant adverse events, or to their baseline, from their most recent systemic anti-DLBCL therapy. * Major surgery within 2 weeks of first dose of study treatment. * Participants with active hepatitis B virus (HBV), hepatitis C virus (HCV), or human immunodeficiency virus (HIV) infections. * Psychiatric illness or substance use that would prevent the participant from giving informed consent or being compliant with the study procedures. * Any of the following laboratory abnormalities: (i) A circulating lymphocyte count of \>50,000/L. (ii) Hepatic dysfunction: bilirubin \>2.0 times the upper limit of normal (ULN) (except participants with Gilbert's syndrome: total bilirubin of \>3\*ULN) and alanine aminotransferase (ALT) and aspartate aminotransferase (AST) \>2.5 times ULN. In participants with known liver involvement of their DLBCL, AST and ALT \>5\*ULN. (iii) Severe renal dysfunction: estimated creatinine clearance of \<30 mL/min, measured in 24-hour urine or calculated using the formula of Cockroft and Gault \[(140-Age)\*Mass (kg)/(72\*creatinine mg/dL); multiply by 0.85 if female\]. * Any life-threatening illness, medical condition, or organ system dysfunction which, in the Investigator's opinion, could compromise the participant's safety. * Participants with active graft-versus-host disease after allogeneic stem cell transplantation. At least 4 months must have elapsed since completion of allogeneic stem cell transplantation. * Uncontrolled (i.e., clinically unstable) infection requiring parenteral antibiotics, antivirals, or antifungals on Cycle 1 Day 1; however, prophylactic use of these agents is acceptable even if parenteral. * Participants unable to swallow tablets, participants with malabsorption syndrome, or any other gastrointestinal disease or gastrointestinal dysfunction that could interfere with absorption of study treatment. Part 1 additional exclusion criteria: * For participants whose most recent systemic anti-DLBCL therapy induced a PR or CR: Radiation, chemotherapy, immunotherapy, radio-immunotherapy, or any other anticancer therapy other than glucocorticoids \<60 days or \<14 weeks prior to Cycle 1 Day 1. * Known central nervous system lymphoma or meningeal involvement. * DLBCL with mucosa-associated lymphoid tissue \[MALT\] lymphoma, composite lymphoma (Hodgkin's lymphoma+NHL), or DLBCL transformed from diseases other than indolent NHL. * Unstable cardiovascular function: (i) Symptomatic ischemia, or (ii) Uncontrolled clinically significant conduction abnormalities (i.e., ventricular tachycardia on anti-arrhythmia are excluded; 1st degree atrioventricular block or asymptomatic left anterior fascicular block /right bundle branch block will not be excluded), or (iii) Congestive heart failure of New York Heart Association Class ≥3, or (iv) Myocardial infarction within 3 months. * Participants with a BSA \<1.4 m\^2 as calculated per Dubois 1916 or Mosteller 1987. * Any of the following laboratory abnormalities: (i) Absolute neutrophil count (ANC) \<1000 cells/mm\^3 or platelet count \<75,000/mm\^3 during screening and on Cycle 1 Day 1. Use of granulocyte-stimulating factors and platelet growth factors prior to and during the study is acceptable. (ii) Hematopoietic dysfunction: hemoglobin \< 10.0 g/dL within 14 days of and including Cycle 1 Day 1 and/or patients receiving red blood cell (RBC) transfusion within 14 days of and including Cycle 1 Day 1. * Participants who have been committed to an institution by official or judicial order. * Participants with dependency on the Sponsor, Investigator or study site. Part 2 additional exclusion criteria: * Participants with active HBV, HVC, or HIV infections. Participants with active HBV are allowed if antiviral therapy for hepatitis B has been given for \>8 weeks and viral load is \<100 International units per milliliters (IU/mL) prior to first dose of study treatment. Participants with known history of HCV or found to be HCV antibody positive on screening, are allowed if there is documentation of negative viral load per institutional standard. Participants with HIV who have CD4+T-cell counts ≥350 cells/microliter (mcL), negative viral load per institutional standard, and no history of acquired immune deficiency syndrome (AIDS)-defining opportunistic infections in the last year are allowed. * Known active central nervous system lymphoma or meningeal involvement. Participants with a history of CNS disease treated into remission may be enrolled. * DLBCL with MALT lymphoma, composite lymphoma (Hodgkin's lymphoma + NHL), DLBCL arising from CLL (Richter's transformation), or high-grade B-cell lymphoma. * Received strong cytochrome P450 3A (CYP3A) inhibitors ≤7 days prior to Day 1 dosing or strong CYP3A inducers ≤14 days prior to Day 1 dosing.