First-in-Human (FIH) Trial in Patients With Relapsed, Progressive or Refractory B-Cell Lymphoma

The purpose of this trial is to measure the following in participants with relapsed and/or refractory B-cell lymphoma who receive epcoritamab, an antibody also known as EPKINLY™ and GEN3013 (DuoBody®-CD3xCD20): * The dose schedule for epcoritamab * The side effects seen with epcoritamab * What the body does with epcoritamab once it is administered * What epcoritamab does to the body once it is administered * How well epcoritamab works against relapsed and/or refractory B-cell lymphoma The trial consists of 3 parts: * a dose-escalation part (Phase 1, first-in-human \[FIH\]) * an expansion part (Phase 2a) * a dose-optimization part (OPT) (Phase 2a) The trial time for each participant depends on which trial part the participant enters: * For the dose-escalation part, each participant will be in the trial for approximately 1 year, which is made up of 21 days of screening, 6 months of treatment (the total time of treatment may be different for each participant), and 6 months of follow-up (the total time of follow-up may be different for each participant). * For the expansion and dose-OPT parts, each participant will be in the trial for approximately 1.5 years, which is made up of 21 days of screening, 1 year of treatment (the total time of treatment may be different for each participant), and 6 months of follow-up (the total time of follow-up may be different for each participant). Participation in the study will require visits to the sites. During the first month, participants must visit every day or every few days, depending on which trial part the participant enters. After that, participants must visit weekly, every other week, once a month, and once every 2 months, as trial participation ends. All participants will receive active drug, and no participants will be given placebo.

The purpose of the dose-escalation part of the trial is to determine the maximum tolerated dose (MTD) and the recommended Phase 2 dose (RP2D), as well as to establish the safety profile of epcoritamab in participants with relapsed or refractory B-cell lymphoma. In the expansion part, additional participants will be treated with epcoritamab, at the RP2D and the purpose is to further explore and determine the safety and efficacy of epcoritamab. The dose-OPT part will evaluate alternative priming and intermediate dose regimens of epcoritamab in participants with: * Diffuse large B-cell lymphoma (DLBCL) * Follicular lymphoma (FL) * Mantle cell lymphoma (MCL) All participants will receive epcoritamab at the RP2D.