Multiple myeloma (MM) is an incurable disease characterized by the growth of monoclonal plasma cells in the bone marrow. The purpose of this study is to assess the adverse events and change in disease state of etentamig in adult participants with relapsed/refractory (R/R) multiple myeloma (MM). Adverse events and change in disease state will be assessed.

Etentamig (ABBV-383) is an investigational drug being developed for the treatment of R/R MM. Study doctors put the participants in groups called treatment arms. Two doses of ABBV-383 will be explored. Each treatment arm receives a different dose of ABBV-383 to determine a tolerable dose. Approximately 12 adult participants with R/R MM will be enrolled in the study in approximately 6 sites in Japan.

Participants will receive intravenous (IV) Etentamig (ABBV-383) at two increasing doses in 21-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, and and monitoring of side effects.

Inclusion Criteria:

• Eastern Cooperative Oncology Group (ECOG) performance of <= 2.

• Must have adequate bone marrow function as defined in the protocol.

• Must meet laboratory parameters as outlined in the protocol.

• Must have a 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.

  • Relapsed defined as previously treated myeloma that progresses and requires initiation of salvage therapy, but does not meet criteria for refractory myeloma.
  • Refractory defined as disease that is nonresponsive (failure to achieve minimal response or development of progressive disease) while on primary or salvage therapy, or progresses within 60 days of last therapy.

• Must have received at least 3 prior lines of therapy (including exposure to a proteasome inhibitor (PI), an immunomodulatory imide (IMiD), and an anti-CD38 mAb).

• Must have measurable disease within 28 days of enrollment, defined as at least 1 of the following:

  • Serum M-protein >= 0.5 g/dL (>= 5 g/L).
  • Urine M-protein >= 200 mg/24 hours.
  • Serum free light chain (FLC) >= 100 mg/L (involved light chain) and an abnormal serum kappa lambda ratio only for participants without measurable serum or urine M-protein.

• Consents to a fresh pretreatment bone marrow tumor biopsy or has adequate archival bone marrow tumor tissue that was collected within 12 weeks prior to screening and without intervening treatment.

Exclusion Criteria:

- Has received B-cell maturation antigen (BCMA)-targeted therapy. Participants who have received targeted therapy against non-BCMA targets will not be excluded.