Immunoglobulin light chain (AL) amyloidosis is the most common form of systemic amyloidosis. AL amyloidosis has many root causes and is characterized by the overproduction of AL that are secreted by clonal bone marrow plasma cells. This is a study to determine adverse events and change in disease activity in adult participants with AL amyloidosis treated with ABBV-383.

Etentamig (ABBV-383) is an investigational drug being developed for the treatment of AL amyloidosis. This study in broken into 2 parts (dose escalation and dose expansion) with 4 arms. During dose escalation (arms 1-3) participants will receive 1 of 3 doses of ABBV-383 to determine the part 2 dose. After completion of the dose escalation portion of the study, the dose expansion (part 2) portion of the study will begin. One arm (arm 4) will begin and participants will receive a dose determined during the dose escalation portion (part 1). Around 76 adult participants with relapsed/refractory AL amyloidosis will be enrolled at approximately 25 sites across the world.

Participants will receive Etentamig (ABBV-383) as an infusion into the vein for up to approximately 2 year study duration.

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 a hospital or clinic. The effect of the treatment will be checked by medical assessments, blood tests, checking for side effects and questionnaires.

Multiple myeloma (MM) is a cancer of the blood's plasma cells. The cancer is typically found in the bones and bone marrow (the spongy tissue inside of the bones) and can cause bone pain, fractures, infections, weaker bones, and kidney failure. Treatments are available, but MM can come back (relapsed) or may not get better (refractory) with treatment. This is a study to determine change in disease symptoms of etentamig compared to standard available therapies in adult participants with relapsed/refractory (R/R) MM.

Etentamig is an investigational drug being developed for the treatment of R/R MM. This study is broken into 2 Arms; Arm A and Arm B. In Arm A, participants will receive etentamig as a monotherapy. In Arm B, participants will receive the standard available therapy (SAT) identified by the Investigator during screening, in accordance with the local (or applicable) approved label, package insert, summary of product characteristics, and/or the institutional guidelines, as applicable. Around 380 adult participants with relapsed/refractory multiple myeloma will be enrolled at approximately 140 sites across the world.

In Arm A participants will receive etentamig as an infusion into the vein in 28 day cycles, during the 3.5 year study duration. In Arm B, participants will receive the SAT identified by the Investigator during screening, in accordance with the local (or applicable) approved label, package insert, summary of product characteristics, and/or the institutional guidelines, as applicable, during the 3.5 year study duration.

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 a hospital or clinic. The effect of the treatment will be checked by medical assessments, blood tests, checking for side effects and questionnaires.