A Study Evaluating the Safety and Pharmacokinetics of Escalating Doses of Forimtamig in Participants With Relapsed or Refractory Multiple Myeloma (r/r MM)

Inclusion Criteria: * Previously diagnosed with Multiple Myeloma (MM) based on standard criteria. * Dose Escalation Phase and Dose Expansion Phase: Participants with r/r MM who have previously received therapy with an Immunomodulatory drug (IMiD) and Proteasome Inhibitor (PI) and are intolerant to or have no other option for standard-of-care treatment according to the Investigator. * Life expectancy of at least 12 weeks. * Agreement to provide protocol-specific biopsy material. * AEs from prior anti-cancer therapy resolved to Grade =\<1. * Measurable disease. * For female participants of childbearing potential: agreement to remain abstinent (refrain from heterosexual intercourse), use contraceptive measures and refrain from donating eggs. * For male participants: agreement to remain abstinent (refrain from heterosexual intercourse), use contraceptive measures and refrain from donating sperm. Exclusion Criteria: * Inability to comply with protocol-mandated hospitalization and activities restrictions. * Pregnant or breastfeeding or intending to become pregnant during the study or within 3 months after last dose of study drug. * Prior use of any monoclonal antibody, radioimmunoconjugate, or antibody-drug conjugate for MM treatment within 2 weeks before first forimtamig administration. * Prior treatment with systemic immunotherapeutic agents within 2 weeks before first forimtamig administration. * Treatment-related, immune-mediated AEs associated with prior immunotherapeutic a…