Study of IgPro20 to Prevent Infection in People With Multiple Myeloma and Hypogammaglobulinemia

Inclusion Criteria: * Diagnosis of RRMM receiving a commercially available bispecific antibody * Received at least 1 but no more than 4 cycles of BsAb * Males or females greater than or equal to 18 years old at the time of consent * ECOG ≤ 3 * Life expectancy \> 12 months Exclusion Criteria: * HSCT within 3 months before enrollment * Planned CAR-T therapy in the next 6 months * \>1 major (deep-seated) infection within the preceding 3 months * HIV infection, Active HCV, or Active HBV infection * Pregnancy * Neutrophils \< 500 x 10\^3 per mL, Platelets \< 30 x 10\^9 per mL * History of cancer other than MM in the last 3 years requiring active chemotherapy or radiation * Known reaction/allergy to IgG products * Intracranial hemorrhage or embolic CVA in the last 6 months * Hyperproteinemia * Protein-losing enteropathy * Creatinine Cl \<30 ml/min * Documented progression on BsAb * Known history of Hyperprolinemia * On current or previous IVIG (in the last 3 months)