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 sin…