Ex vivo expanded adaptive NK cells in combination with a bispecific NK cell engager targeting BCMA for synergistic control of multiple myeloma

Abstract T cell-based therapies have transformed the treatment landscape for multiple myeloma (MM). However, most patients receiving autologous CAR-T therapy ultimately relapse, and allogeneic T cell products face significant clinical hurdles, including the risk of graft-versus-host disease. Additionally, T cell engagers are associated with a high incidence of infections, while all T cell-based therapies carry the risk of cytokine release syndrome (CRS) and neurological side effects. Natural killer (NK) cell-based therapies are emerging as a promising alternative. Recent clinical data on relapsed/refractory lymphoma have demonstrated the efficacy of allogeneic NK cells combined with bispecific innate cell engagers, supporting the broader potential of NK cell-based immunotherapies (Nieto Y et al. Nat Med 2025). We have previously shown how adaptive NK cells from healthy donors can be ex vivo expanded into an off-the-shelf product (ADAPT-NK) with potent cytotoxic activity and missing-self reactivity across HLA-C barriers (Haroun-Izquierdo A et al. J Immunother Cancer 2022). We have also demonstrated the use of small affibody-derived NK cell engagers targeting CD16a on NK cells and BCMA on MM cells (Giang KA et al. New Biotechnol 2023). Here, we present an optimized affibody-derived bispecific NK cell engager engineered to target CD16a on NK cells and BCMA on MM cells, combined with ADAPT-NK as a potential treatment for multiple myeloma. Flow cytometry-based cytotoxicity assays using NK cells from healthy donors against a panel of MM cell lines with differential BCMA expression demonstrated efficient lysis of the tumor cells. When combined with ADAPT-NK cells, the bispecific engager achieved potent MM cell clearance in a 48-hour real-time cytotoxicity assay at a 2:1 effector-to-target ratio, mismatched against HLA-C. Cytokine release was quantified in a 4h in vitro assay with PBMCs incubated with engagers or equimolar concentrations of a monoclonal antibody against CD52 as a positive control. Engagers did not induce IL-6 release in the presence of target cells. We established a xenograft model using bdMM.1s cells to assess the therapeutic potential of the engager in vivo. ADAPT-NK cells were injected on days 4, 8, and 13, while the engager was administered daily from days 4 to 18 following tumor cell inoculation on day 0. This combined approach demonstrated a significant reduction in tumor growth over the study period compared to vehicle-treated control mice. Together, these findings support the synergistic potential of ADAPT-NK cells combined with a bispecific NK cell engager targeting BCMA as a novel treatment strategy for multiple myeloma.