A Study Investigating the Safety and How the Trispecific T-Cell Engaging Antibody BG-T0050 Works in Adults With Diffuse Large B-cell Lymphoma (DLBCL) and Other Aggressive B-Cell Malignancies

The purpose of this study is to test the safety and tolerability of BG-T0050 and to identify the recommended dose and dosing schedule (including dose levels and timing) that can be taken without having serious side effects in adults with DLBCL and other aggressive B-cell malignancies

Inclusion Criteria: * Confirmed diagnosis of any of the below according to World Health Organization (WHO) classification 1. Diffuse large B-cell lymphoma, not otherwise specified (DLBCL-NOS) 2. Diffuse large B-cell lymphoma / high-grade B-cell lymphoma (DLBCL/HG-BCL) with MYC and BCL2 rearrangements 3. High-grade B-cell lymphoma, not otherwise specified (HG-BCL, NOS) 4. Large B-cell lymphoma (LBCL) transformed from an antecedent indolent B-cell lymphoma (BCL), including Follicular lymphoma (FL) and marginal zone lymphoma (MZL) * Must have relapsed or refractory disease with no other available therapies with established clinical benefit including anti-cluster of differentiation 20 (CD20)-based immunochemotherapy, autologous stem cell transplant \[if eligible\], chimeric antigen receptor T-cell therapy (CAR-T) \[if eligible\], or T-cell engager (TCE) therapy) that would be more appropriate in the investigator's judgment. * Must have measurable disease by computed tomography (CT)/ magnetic resonance imaging (MRI), defined as at least 1 lymph node \> 1.5 cm in the longest diameter or 1 extranodal lesion \> 1.0 cm in the longest diameter, measurable in 2 perpendicular dimensions. * Must have an Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 to 2. Exclusion Criteria: * Prior autologous stem cell transplant \< 100 days before first dose of the study drug. * Prior allogenic transplant or solid organ transplant at any time in the past. * Known or suspected progressive multifocal leukoencephalopathy. * Known or suspected history of hemophagocytic lymphohistiocytosis (HLH). * Recent prior therapies, prior to the first dose of study drug: 1. Biologic-based anti-lymphoma therapy (eg, monoclonal antibodies, antibody-drug conjugates, radioimmunoconjugate, or TCE therapy) within 28 days 2. CAR-T therapy within 90 days 3. Systemic chemotherapy or small-molecule targeted therapy (investigational or otherwise) within 28 days or 5 half-lives (whichever is shorter) 4. Systemic corticosteroids in excess of 25 mg/day prednisone (or equivalent) within 14 days. Inhaled, topical, or systemic corticosteroids at doses of ≤ 25 mg/day prednisone (or equivalent) are permitted. 5. Radiation therapy within 14 days. If a patient has undergone radiation therapy since last progression, at least one measurable lesion must be outside the radiation field 6. T-cell suppressive therapy (eg, alemtuzumab, fludarabine) within 6 months, unless discussed with the medical monitor. * History of severe allergic reactions or hypersensitivity to the active ingredient or excipients of BG-T0050 or other monoclonal antibodies. * Active significant infection (bacterial, viral, fungal, parasitic) at study enrollment, or any Grade 3 or 4 infection requiring treatment with Intravenous (IV) systemic therapy or hospitalization within 4 weeks prior to the first dose of the study drug. Note: Oral antimicrobial therapy used for minor or controlled infections or for prophylactic purposes is allowed. Note: Other protocol defined Inclusion/Exclusion criteria may apply.