• 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.