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

Inclusion Criteria:

Eastern Cooperative Oncology Group (ECOG) performance of <= 2.
Must have confirmed diagnosis of Relapsed/Refractory (R/R) Multiple Myeloma (MM) with documented evidence of progression during or after the participant's last treatment regimen based on the investigator's determination of the International Myeloma Working Group (IMWG) criteria.
Must have measurable disease as outlined in the protocol.
Must be naïve to treatment with ABBV-383 and must have never received BCMA-targeted therapy. Participants who have received targeted therapy against non-BCMA targets will not be excluded.
Has received prior MM treatment in Arms A, B, C, and D.

Exclusion Criteria:

Received a peripheral autologous stem cell transplant (SCT) within 12 weeks, or an allogeneic SCT within 1 year of the first dose of study drug treatment.
Unresolved adverse event (AE)s >= Grade 2 (National Cancer Institute [NCI] Common Terminology Criteria for Adverse Events [CTCAE] version 5.0) from prior anticancer therapy.
Known central nervous system involvement Multiple Myeloma (MM).

Has any of the following conditions:

Nonsecretory 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 drug 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.
Known active infection of evidence of active hepatitis B, evidence of active hepatitis C, human immunodeficiency virus.