The purpose of this study is to evaluate whether elranatamab monotherapy can provide clinical benefit compared to lenalidomide monotherapy (control) in participants with newly diagnosed multiple myeloma after undergoing autologous stem cell transplant. In Part 1 and Part 2 of the study, participants in the study will either receive elranatamab (arm A and C) as an injection under the skin at the study clinic or lenalidomide orally once daily at home (arm B). Participation in the study will be approximately five years

Elranatamab is a bispecific antibody: binding of elranatamab to CD3-expressing T-cells and BCMA-expressing multiple myeloma cells causes targeted T-cell-mediated cytotoxicity.

Inclusion Criteria:

• Diagnosis of MM as defined according to IMWG criteria (Rajkumar, 2014) with measurable disease at diagnosis
• Part 1 patients must be MRD positive, Part 2 patients can be MRD negative or MRD positive
• History of induction therapy for newly diagnosed MM, followed by high dose therapy and autologous stem cell transplant. Randomization must occur within 120 days from the stem cell transplant. For participants who receive consolidation therapy after ASCT, randomization must occur within 60 days of consolidation and within 7 months from ASCT.
• Partial Response or better according to IMWG criteria at the time of randomization
• Must have an archival bone marrow aspirate sample(s) to identify the dominant malignant (index) clone by central laboratory NGS test (ClonoSEQ assay) that is used to track MRD status. This sample should preferably be collected before induction treatment (eg, at diagnosis) or before transplant.
• ECOG performance status ≤1
• Resolved acute effects of any prior therapy to baseline severity or CTCAE Grade ≤ 1
• Not pregnant and willing to use contraception

Exclusion Criteria:

• Plasma cell leukemia
• Amyloidosis, Waldenström's macroglobulinemia
• POEMS syndrome
• Known active CNS involvement or clinical signs of myelomatous meningeal involvement
• Previous MM maintenance treatment
• Prior treatment with BCMA targeted therapy
• Any other active malignancy within 3 years prior to enrollment, except for adequately treated basal cell or squamous cell skin cancer, or carcinoma in situ
• Active, uncontrolled bacterial, fungal, or viral infection, including (but not limited to) HBV, HCV, and known HIV or AIDS-related illness
• Previous administration with an investigational drug or vaccine within 30 days (or as determined by the local requirement) or 5 half-lives preceding the first dose of study intervention used in this study (whichever is longer)

Pfizer will provide access to individual de-identified participant data and related study documents (e.g. protocol, Statistical Analysis Plan (SAP), Clinical Study Report (CSR)) upon request from qualified researchers, and subject to certain criteria, conditions, and exceptions. Further details on Pfizer's data sharing criteria and process for requesting access can be found at: https://www.pfizer.com/science/clinical\_trials/trial\_data\_and\_results/data\_requests.