The purpose of this study is to learn about the study medicine called elranatamab.This study aims to compare elranatamab to other medicines for the treatment of MM (a type of cancer).

This study is seeking participants who:

• Are 18 years of age or older and have MM.
• Have received treatments before for MM.
• Have MM that has returned or not responded to their most recent treatment.

Half of the participants will receive elranatamab. The other half of participants will receive a combination therapy selected by the study doctor. The selected combination therapy will include 2 to 3 different medicines commonly used to treat MM.

Elranatamab will be given as a shot under the skin at the study clinic about once a week. This may change to a smaller number of shots later in the study.

The medicines in the combination therapy will be taken by mouth (at home or at the study clinic) AND will be given either as:

• a shot under the skin at the study clinic
• through a needle in the vein at the study clinic The number of times these medicines will be taken depends on what combination therapy the study doctor selects.

Participants may continue to receive elranatamab or a combination therapy until their MM is no longer responding. The study team will see how each participant is doing with the study treatment during regular visits at the study clinic. The study team will continue to follow-up with participants after study treatment with telephone contacts (or visits).

The study will compare the experiences of people receiving elranatamab to those people receiving a combination therapy. This will help learn about the safety and how effective elranatamab is.

Inclusion Criteria:

• Prior diagnosis of multiple myeloma as defined by International Myeloma Working Group (IMWG) criteria and previously received 1 to 4 prior lines of therapy including prior anti-cluster of differentiation 38 (CD38) antibody and prior lenalidomide.
• Documented evidence of progressive disease or failure to achieve a response to last line of therapy per IMWG criteria.
• Measurable disease defined as at least 1 of the following: (a) Serum M-protein ≥0.5 g/dL; (b) Urinary M-protein excretion ≥200 mg/24 hours; (c) Serum involved immunoglobulin FLC ≥10 mg/dL AND abnormal serum immunoglobulin kappa to lambda FLC ratio (<0.26 or >1.65).
• Have clinical laboratory values within the specified range.
• ECOG (Eastern Cooperative Oncology Group) performance status ≤2.
• Not pregnant or breastfeeding and willing to use contraception.

Exclusion Criteria:

• Smoldering multiple myeloma.
• Plasma cell leukemia.
• Amyloidosis.
• Polyneuropathy, organomegaly, endocrinopathy, monoclonal gammopathy and skin abnormalities (POEMS) syndrome.
• Known central nervous system (CNS) involvement or clinical signs of myelomatous meningeal involvement.
• Stem cell transplant within 12 weeks prior to enrolment, or active graft versus host disease.
• Any active, uncontrolled bacterial, fungal, or viral infection.
• Any other active malignancy within 3 years prior to enrolment (exceptions include, adequately treated basal cell or squamous cell skin cancer, carcinoma in situ)
• Previous treatment with a B cell maturation antigen (BCMA)-directed therapy or CD3-redirecting therapy.
• Unable to receive investigator's choice therapy.
• Live attenuated vaccine within 4 weeks of the first dose of study intervention.
• Administration with an investigational product (e.g. drug or vaccine) within 30 days preceding the first dose of study intervention used in this study.

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.