A Study to Learn About the Study Medicine (Elranatamab) Either Alone or in Combination With Dexamethasone in Participants With Multiple Myeloma That Has Come Back After Responding to Treatment or Has Not Responded to Treatment

Inclusion Criteria:

Diagnosis of multiple myeloma (IMWG criteria, Rajkumar et al, 2014)

Measurable disease, as defined by at least 1 of the following:

Serum M-protein >0.5 g/dL by SPEP
Urinary M-protein excretion >200 mg/24 hours by UPEP
Serum immunoglobulin FLC≥10 mg/dL (≥100 mg/L) AND abnormal serum immunoglobulin kappa to lambda FLC ratio
Refractory to at least one IMiD
Refractory to at least one PI
Refractory to at least one anti-CD38 antibody
Relapsed/refractory to last anti-myeloma regimen
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:

Smoldering multiple myeloma
Active Plasma cell leukemia
POEMS syndrome
Amyloidosis
Waldenström's macroglobulinemia
Known active CNS involvement or clinical signs of myelomatous meningeal involvement
Stem cell transplant within 12 weeks prior to enrollment or active GVHD
Active HBV, HCV, SARS-CoV2, HIV, or any active, uncontrolled bacterial, fungal, or viral infection
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 or Stage 0/1 malignancy with minimal risk of recurrence per investigator.
Previous treatment with an anti-BCMA bispecific antibody or CAR-T cell therapy.
Live attenuated vaccine within 4 weeks of the first dose
Previous administration with an investigational drug within 30 days or 5 half-lives preceding the first dose of study intervention used in this study (whichever is longer)
Known or suspected hypersensitivity to the study intervention, or any of its excipients or unable to tolerate systemic corticosteroids at doses planned to be administered in the study Part 3