A Study of MT-2111 in Patients With Relapsed/Refractory DLBCL

Inclusion Criteria: * Patients who were diagnosed pathologically with DLBCL, NOS, DLBCL transformed from indolent B-cell lymphoma, or high-grade B-cell lymphoma with DLBCL morphology and with MYC and BCL2 and/or BCL6 rearrangements, based on the 2017 WHO classification. * Patients with relapsed or refractory disease despite 2 or more prior systemic therapies. * Japanese patients aged ≥ 18 years at the time of informed consent. For Japanese subjects, it should be confirmed that the parents who are related by blood to the subject must be Japanese. * Patients who have a lesion that can be assessed for staging and evaluated for response according to the Lugano criteria (2014). A lesion that has received radiotherapy as the most recent treatment will be considered as a measurable lesion only when progression has been documented following completion of the radiotherapy. * Patients with an Eastern Cooperative Oncology Group (ECOG) performance status of 0, 1, or 2 at screening. Exclusion Criteria: * Patients with a pathological diagnosis of Burkitt's lymphoma. * Patients with bulky disease with the longest dimension of ≥ 10 cm. * Patients with a history or complication of post-transplant lymphoproliferative disorders. * Patients with lymphoma with active central nervous system involvement at the time of screening, including leptomeningeal disease. * Patients complicated with other active malignancies or patients with a history of other malignancies within 3 years before informed consent. However, the following are exceptional: * Non-melanoma skin cancer * Non-metastatic prostate cancer * Cervical carcinoma in situ * Ductal carcinoma in situ or lobular carcinoma in situ * Patients with clinically significant third space fluid accumulation (e.g., ascites requiring drainage or pleural effusion requiring drainage or associated with shortness of breath). * Patients who underwent autologous hematopoietic stem cell transplantation (AHSCT) within 30 days prior to the start of study drug administration (Cycle 1 Day 1). * For the Phase I part, patients with prior allogeneic stem cell transplantation (Allo-HSCT) before the start of study drug administration (Cycle 1 Day 1). For the Phase II part, patients undergoing Allo-HSCT within 60 days prior to the start of study drug administration (Cycle 1 Day 1). * Patients who had a positive HIV antigen-antibody test or HIV antibody test. * Patients positive for HBs antigen, HBc antibody, or HBs antibody. However, patients who meet any of the following are eligible: * The patient's HBs antibody positivity is clearly due to vaccination. * Patients who are positive for HBs antibody and/or HBc antibody with HBV-DNA not detected and agree to undergo HBV-DNA tests once a month from the start of study drug administration to at least 12 months after the completion of study drug administration. * Patients positive for HCV antibody. However, patients with negative HCV-RNA are eligible. * Patients who received anticancer therapy during the following periods prior to the start of study drug administration (Cycle 1 Day 1). * Cytotoxic chemotherapy: within 14 days. * Antibody therapy: within 5 half-lives or 14 days, whichever is longer (including monoclonal antibody preparations, radioimmunoconjugates, or antibody-drug conjugates). Within 14 days for rituximab, anti-CD3/CD20 bispecific antibody. * Radiotherapy: within 14 days * CAR-T therapy: within 100 days * Other anticancer therapy: within 14 days * Patients who received treatment with any other investigational product within 14 days prior to the start of study drug administration (Cycle 1 Day 1). However, for the Phase I part, patients who received any other investigational product within 14 days or 5 half-lives, whichever is longer, before the start of study drug administration (Cycle 1 Day 1).