A Study to Evaluate the Safety and Efficacy of Polatuzumab Vedotin in Combination With Rituximab, Gemcitabine and Oxaliplatin Compared to Rituximab, Gemcitabine and Oxaliplatin Alone in Participants With Relapsed or Refractory Diffuse Large B-Cell Lymphoma

Prior treatment

• Prior use of polatuzumab vedotin or a gemcitabine plus platinum-based agent combination, recent participation in a clinical trial, and/or treatment with radiotherapy, chemotherapy, immunotherapy, immunosuppressive therapy within 2 weeks
• Planned autologous or allogenic stem cell transplantation or CAR T-cell therapy at time of recruitment
• Richter's transformation or prior CLL
• Vaccination with a live vaccine within 4 weeks prior to treatment
• Recent major surgery (within 6 weeks before the start of Cycle 1 Day 1) other than for diagnosis
• Pregnant or breastfeeding, or intending to become pregnant during the study or within 12 months after the last dose of study drug

Other illness

• History of severe allergic or anaphylactic reactions to humanized or murine monoclonal antibodies (or recombinant antibody-related fusion proteins) or known sensitivity or allergy to murine products

Lab values

• Abnormal laboratory values or health conditions, as assessed by the investigator, any known conditions preventing adherence to protocol or active bacterial, viral, fungal, mycobacterial, parasitic, or other infection
• Any other diseases, metabolic dysfunction, physical examination finding, or clinical laboratory finding giving reasonable suspicion of a disease or condition that contraindicates the use of an investigational drug or that may affect the interpretation of the results or render the patient at high risk from treatment complications
• Women of childbearing potential must have a negative serum pregnancy test result within 7 days prior to initiation of study drug

Other

• Contraindication to rituximab, gemcitabine or oxaliplatin
• Peripheral neuropathy assessed to be \> Grade 1 according to NCI CTCAE v5.0
• Primary or secondary central nervous system (CNS) lymphoma

• Histologically-confirmed diffuse large B-cell lymphoma, not otherwise specified (NOS) or history of transformation of indolent disease to DLBCL
• Relapsed disease (disease that has recurred following a response that lasted ≥ 6 months from completion of the last line of therapy) or refractory disease (disease that did not respond to or that progressed during therapy or progressed within 6 months (\< 6 months) of prior therapy)
• At least one (≥ 1) line of prior systemic therapy:
• Patients may have undergone autologous hematopoietic stem cell transplantation (HSCT) prior to recruitment; In such cases, salvage chemotherapy (e.g., rituximab, dexamethasone, cytarabine, and cisplatin \[R-DHAP\] and rituximab, ifosfamide, carboplatin, and etoposide phosphate \[R-ICE\]) will be counted as one line of therapy and conditioning chemotherapy (e.g., BEAM) followed by consolidative autologous HSCT will be counted as one line of therapy
• Patients may have undergone allogeneic HSCT prior to recruitment, so long as they are off all immunosuppressive therapy and have no active GVHD; In such cases, salvage chemotherapy (e.g., R-DHAP and R-ICE) will be counted as one line of therapy and conditioning chemotherapy (e.g., carmustine, etoposide, cytarabine, and melphalan \[BEAM\]) followed by allogeneic HSCT will be counted as a separate line of therapy
• Participants may have undergone CAR T-cell therapy prior to recruitment. In such cases, cell collection, conditioning chemotherapy, and infusion will be counted as one line of therapy.
• Local therapies (e.g., radiotherapy) will not be considered as lines of treatment
• For participants with a history of the transformation of indolent disease to DLBCL, it is preferred that participants have received at least one treatment for the transformed lymphoma. However, if there are cases where the participants have received an anthracycline-containing chemotherapy regimen (such as R-CHOP) for the indolent lymphoma only, then these participants can be considered as eligible.
• At least one bi-dimensionally measurable lesion, defined as \> 1.5 cm in its longest dimension as measured by CT or MRI
• Eastern Cooperative Oncology Group (ECOG) performance status of 0,1 or 2
• Adequate hematological function
• For women of childbearing potential: agreement to remain abstinent (refrain from heterosexual intercourse) or use contraception, and agreement to refrain from donating eggs
• For men: agreement to remain abstinent (refrain from heterosexual intercourse) or use contraceptive measures, and agreement to refrain from donating sperm,