A Study to Assess Change in Disease Activity and Adverse Events (AE)s in Adult Participants With Multiple Myeloma Receiving T Cell Engaging Bi/Tri-specific Antibodies Intravenously (IV)/Subcutaneously (SC) Alone or in Combinations

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 the safety, efficacy, and pharmacokinetics of Etentamig and ABBV-2001 in adult participants with MM. Etentamig and ABBV-2001 are investigational drugs being developed for the treatment of MM. This study is broken into 8 substudies and each substudy consists of a dose escalation phase and dose expansion phase. Participants in substudies 1-4 will receive escalating doses of etentamig alone or with daratumumab and lenalidomide (DR), carfilzomib and dexamethasone (Kd) or lenalidomide (R), followed by etentamig at the dose levels established during the escalation phases alone or with DR, Kd and R. Participants in substudies 5-8 will receive up to two doses of ABBV-2001 or at the dose level(s) established during the escalation phases of ABBV-2001 with iberdomide, mezigdomide, pomalidomide or lenalidomide. Around 602 adult participants with MM will be enrolled at approximately 55 sites worldwide In substudies 1-3, participants will receive escalating doses of etentamig as Intravenous (IV) infusions, alone or with DR, R or Kd, followed by SC injections of etentamig at the dose levels established during the escalation phases alone or with DR or K. In substudy 4, participants will receive escalating doses of etentamig as Intravenous (IV) infusions followed by SC injections of etentamig at the dose levels established during the escalation phase with Lenalidomide. In substudies 5-8, participants will receive up to two doses of ABBV-2001 as subcutaneous (SC) injections, with oral iberdomide, mezigdomide, pomalidomide or lenalidomide followed by SC injections of ABBV-2001 at the dose level(s) established during the escalation phases with oral iberdomide, mezigdomide, pomalidomide or lenalidomide. The study duration is approximately 130 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 a hospital or clinic. The effect of the treatment will be checked by medical assessments, blood tests, checking for side effects and questionnaires.