Study Assessing Activity of Intravenous (IV) Etentamig Monotherapy Versus Standard Available Therapies in Adult Participants With Relapsed or Refractory Multiple Myeloma

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.

Inclusion Criteria: * Eastern Cooperative Oncology Group (ECOG) performance of \<= 2. * Diagnosis of relapsed/refractory (R/R) multiple myeloma (MM) during or after the participant's last treatment as stated in the protocol. * Must have measurable disease with at least 1 of the following assessed within 28 days of enrollment: * Serum M-protein \>= 0.5 g/dL (\>= 5 g/L). * Urine M-protein \>= 200 mg/24 hours. * In participants without measurable serum or urine M protein, serum free light chain (FLC) \>= 100 mg/L (10 mg/dL) (involved light chain)and an abnormal serum kappa lambda ratio. * Must have received at least 2 or more lines of therapy, including a proteasome inhibitor (PI), an immunomodulatory imide (IMiD), and an anti-CD38 monoclonal antibody (mAb). * Must be eligible to receive the Investigator's choice standard available therapy (SAT) based on approved prescribing information, previous MM treatment history, and institutional guidelines. Exclusion Criteria: * Clinically significant (per Investigator's judgment) drug or alcohol abuse within the last 6 months. * Clinically significant conditions such as but not limited to the following: neurologic, psychiatric, endocrine, metabolic, immunologic, cardiovascular, pulmonary, or hepatic disease within the last 6 months that would adversely affect the participant's participation in the study. * Central nervous system involvement of MM. * Has received B-cell maturation antigen (BCMA)-targeted therapy.