A Study to Assess Adverse Events and Change in Disease Activity of Intravenously (IV) Infused Etentamig (ABBV-383) in Combination With Anti-Cancer Regimens for the Treatment of Adult Participants With Relapsed/Refractory Multiple Myeloma

Multiple myeloma (MM) is a plasma cell disease characterized by the growth of clonal plasma cells in the bone marrow. The purpose of this study is to assess the safety and toxicity of etentamig (ABBV-383) when co-administered with pomalidomide-dexamethasone (Pd), lenalidomide-dexamethasone (Rd), or daratumumab-dexamethasone (Dd), in adult participants with relapsed/refractory (R/R) multiple myeloma (MM). Adverse events and change in disease activity will be assessed. Etentamig is an investigational drug being developed for the treatment of R/R MM. Study doctors put the participants in groups called treatment arms. Etentamig co-administered with Pd, Rd, or Dd, will be explored. Each treatment arm receives a different treatment combination depending on stage of the study and eligibility. This study will include a dose escalation phase to determine the best dose of etentamig, followed by a dose expansion phase to confirm the dose. Approximately 320 adult participants with R/R MM will be enrolled in the study in approximately 48 sites worldwide. Participants will receive intravenous (IV) etentamig co-administered with oral/IV Pd, oral/IV Rd, or oral/IV/subcutaneous (SC) Dd in 28-day cycles. There may be higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular visits during the study at an approved institution (hospital or clinic). The effect of the treatment will be frequently checked by medical assessments, blood tests, questionnaires and side effects.

Inclusion Criteria: * Eastern Cooperative Oncology Group (ECOG) performance of \= Grade 2 (National Cancer Institute \[NCI\] Common Terminology Criteria for Adverse Events \[CTCAE\] version 5.0) from prior anticancer therapy. * Has any of the following conditions: * Nonsecretory Multiple Myeloma (MM). * Active Plasma cell leukemia i.e., either 20% of peripheral white blood cells or \> 2.0 × 10\^9L circulating plasma cells by standard differential. * Waldenstrom's macroglobulinemia. * Light chain amyloidosis. * Polyneuropathy, organomegaly, endocrinopathy, monoclonal protein and skin changes (POEMS) syndrome. * Major surgery within 4 weeks prior to first dose or planned study participation. * Acute infections within 14 days prior to first dose of study requiring therapy (antibiotic, antifungal or antiviral). * Uncontrolled diabetes or hypertension within 14 days prior to first dose. * Peripheral neuropathy \>= Grade 3 or \>= Grade 2 with pain within 2 weeks prior to first dose.