ABBV-438

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, tolerability, and how ABBV-438 moves through the body, in adult participants with relapsed/refractory (R/R) MM. Adverse events, tolerability, how ABBV-438 moves through the body will be assessed. ABBV-438 is an investigational drug being developed for the treatment of R/R MM. Study doctors put the participants in groups called treatment arms broken into 2 parts. ABBV-438 will be given alone and multiple doses will be explored. This study will include a dose escalation phase (Part 1) to determine the best dose of ABBV-438, followed by a dose expansion phase (Part 2) to confirm the dose. Approximately 127 adult participants with R/R MM will be enrolled in the study in approximately 24 sites worldwide. Participants will receive intravenous (IV) ABBV-438 alone first in multiple doses in the dose escalation phase (Part 1); then in 1 of 2 doses from Part 1 in the dose expansion phase (Part 2). The overall study duration will be approximately 69.5 months. 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.