(PA-043) Anti-BCMA-CAR-IL15 Natural Killer Cells Prevent Multiple Myeloma Growth in the Bone Marrow but Allow Later Emergence of Extramedullary Disease

Introduction: Multiple myeloma (MM) is an aggressive blood cancer arising from plasma cells. The B cell maturation antigen (BCMA) has been established as an effective therapeutic target on plasma cells, with BCMA-targeted chimeric antigen receptor T cell (α-BCMA-CAR-T) immunotherapies currently providing life-saving treatment for MM patients. Unfortunately, severe toxicities along with the high cost and complexity of autologous CAR-T manufacturing remain important limitations. Novel research is underway to use CAR-expressing natural killer (NK) cells as an allogeneic CAR-T alternative. Clinical trials are currently utilizing α-CD19-CAR-NK cells to treat lymphoma, showing the potential for an “off-shelf-immunotherapy” with minimal side effects, but studies have yet to evaluate long-term CAR-NK efficacy against MM.

Methods: NK cells were isolated, expanded via feeder-cell stimulation, and engineered to express α-BCMA-CAR with or without human IL-15 co-expression using lentiviral vectors. In vitro effector function of α-BCMA-CAR-expressing NK cells were assessed by co-incubation with MM cell lines. IFNγ and CD107a cytokine production are measured as a sign of function while cytotoxicity was assessed using the IncuCyte live-cell assay. Lastly, the long-term persistence and therapeutic potential of α-BCMA-CAR-IL15 NK cells were evaluated in a luciferase-expressing MM-xenograft mouse model.

Results: α-BCMA-CAR NK cells have enhanced cytokine production and cytotoxicity against BCMA-high MM cells compared to untransduced NK cells, with IL-15 co-expression required for CAR-NK persistence in low cytokine media conditions. When injected into NSG mice, both α-BCMA-CAR and IL-15 expression were required for persistent restriction of MM growth in the blood, spleen and bone marrow. Interestingly, despite near complete and sustained elimination of MM in hematopoietic tissues, long-term assessment of mice treated with α-BCMA-CAR-IL15 NK cells revealed the emergence of extramedullary disease (EMD) in the form of nodal BCMA-positive MM plasmacytomas.

Conclusions:

Our study showcases α-BCMA-CAR-IL15 NK cell therapy as a potent anti-MM therapeutic, achieving sustained MM elimination from the bone marrow and greatly extending survival in a MM-xenograft model. However, α-BCMA-CAR-IL15 NK cells appeared ineffective at eliminating extramedullary disease. EMD is strongly associated with poor prognosis in MM patients. The relevance of this animal model in replicating EMD observed in patients is currently under investigation. By demonstrating the strengths and weaknesses of α-BCMA-CAR-IL15 cells, we hope this study could help direct the use of such therapies in clinical trials and provide a valuable pre-clinical MM model for developing interventions for aggressive MM-EMD.