Background: Elranatamab (PF-06863135) is a humanized bispecific antibody that targets both B cell maturation antigen (BCMA)-expressing multiple myeloma (MM) cells and CD3-expressing T cells, with binding resulting in T cell-mediated cytotoxicity. Elranatamab has demonstrated antitumor activity and delayed tumor progression in preclinical studies, as well as promising efficacy and manageable safety in the ongoing phase 1 MagnetisMM-1 study in patients (pts) with relapsed/refractory MM (Bahlis et al, J Clin Oncol 2021). Methods: MagnetisMM-5 is an open-label, multicenter, randomized phase 3 study designed to evaluate the efficacy and safety of subcutaneous (SC) elranatamab monotherapy and SC elranatamab + SC daratumumab in pts with relapsed/refractory MM who have received prior therapy, including lenalidomide and a proteasome inhibitor (PI). The study consists of 2 parts. In part 1, a minimum of 20 pts will be enrolled to assess the safety of an elranatamab priming regimen and identify the recommended combination dose for SC elranatamab + SC daratumumab for part 2. The primary endpoint of part 1 is dose-limiting toxicities encompassing the elranatamab monotherapy priming duration (14 d) and the first cycle of SC elranatamab + SC daratumumab dosing (28 d). In part 2, ̃450 pts will be stratified by prior lines of therapy (1 vs 2–3 vs ≥4) and prior treatment with anti-CD38 therapy (yes vs no) and enrolled in a 1:1:1 ratio to receive SC elranatamab or SC elranatamab + SC daratumumab or SC daratumumab + oral pomalidomide + oral dexamethasone. The primary endpoint of part 2 is progression-free survival (PFS), according to International Myeloma Working Group (IMWG) response criteria and blinded independent review. Secondary endpoints include PFS and PFS on next-line treatment by investigator per IMWG, overall survival, objective response rate, duration of response, complete response (CR) rate, duration of CR, time to response, overall and sustained minimal residual disease negativity rates, safety, quality of life, immunogenicity, and PK. Key inclusion criteria are age ≥18 y, MM diagnosis with measurable disease according to IMWG criteria, ECOG performance status 0–2, and clinical laboratory values within specified ranges. For part 2, pts should have received ≥1 prior line of anti-myeloma therapy, including treatment with lenalidomide and a PI. Key exclusion criteria include smoldering MM, plasma cell leukemia, amyloidosis, POEMS syndrome, stem cell transplant within 12 wk of enrollment, primary refractory MM, active, uncontrolled bacterial, fungal, or viral infections, previous treatment with BCMA-targeted therapy, anti-CD38 therapy within 6 mo of the first dose of study treatment, and previous pomalidomide therapy. MagnestisMM-5 will include sites in 28 countries. Clinical trial information: NCT05020236.