Study of ACTR087 in Combination With SEA-BCMA in Subjects With Relapsed or Refractory Multiple Myeloma

Inclusion Criteria: * Signed written informed consent obtained prior to study procedures * Histologically- or cytologically-confirmed relapsed or refractory multiple myeloma (MM) with measurable disease * Must have received at least 3 prior lines of therapy to include treatment with a proteasome inhibitor (eg, bortezomib, carfilzomib, or ixazomib) and an immunomodulatory agent (eg, lenalidomide, pomalidomide) unless double-refractory to both; and a hematopoietic stem cell transplant (HSCT), for those subjects considered HSCT-eligible. * Quantitative serum IgG levels for subjects with IgG MM must not exceed the institutional upper limit of normal (ULN) * ECOG 0 or 1 * Life expectancy of at least 6 months * Absolute neutrophil (ANC) count greater than 1000/ µL * Platelet count greater than 50,000/µL * Estimated GFR \>30mL/min/1.73m2 Exclusion Criteria: * Known active central nervous system (CNS) involvement by MM * Systemic rheumatic or autoimmune diseases or acute or chronic infections * Uncontrolled thromboembolic events or recent severe hemorrhage * Subjects who are currently using more than 5mg/day of prednisone (or an equivalent glucocorticoid exceeding physiologic replacement levels) * Prior treatment as follows: * T cell-directed antibody therapy (eg. Alemtuzumab, anti-thymocyte globulin) within 6 months of enrollment * Any prior myeloma-directed therapy including cytotoxic chemotherapy, biologic therapy, or radiotherapy within 2 weeks of enrollment * Any mAb or other protein therapeutic containing Fc-domains within 4 weeks of enrollment * Experimental agents within 3 half-lives prior to enrollment, unless progression is documented on therapy * Prior BCMA-directed investigational agents at any time * Prior cell or gene therapy, excluding transfers of genetically unmodified autologous cells (eg. Hematopoietic stem cell transplantation), at any time; or prior allogeneic HSCT at any time * Pregnant or breastfeeding