747. Towards the Clinical Application of BCMA CAR T Cells: The Importance of Reduced Tonic Signaling and Methods to Enhance Memory T Cells

B cell maturation antigen (BCMA) is expressed on most multiple myeloma (MM) and some lymphoma cells, yet normal tissue expression is limited to plasma and some B cells. Here we describe the development of a CAR T cell that targets BCMA. We tested 19 BCMA-specific single chain variable fragments (scFv) linked to CD137 (4-1BB) and CD3zeta T cell signaling domains. The activity of each CAR T cell candidate differed despite targeting the same antigen. One anti-BCMA CAR T cell candidate (bb2121) was selected based on strong surface CAR expression, superior biological activity to multiple BCMA+ cell lines, and low antigen-independent activity. In vitro analysis demonstrated that antigen-independent activity was associated with T cell differentiation characterized by significantly lower CD62L expression (p=0.003). To determine the impact on tumor control, CAR T cells with antigen-independent activity, or bb2121 CAR T cells, were used to treat an NSG mouse model of MM (RPMI-8226). Immunohistochemical analysis found MM-infiltrated bb2121 T cells within 5 days and mice showed complete regressions by 12 days post treatment. In contrast, mice that received CAR T cells with antigen-independent activity had delayed regressions that occurred 20 days post treatment. Robust recognition of as few as 220 BCMA molecules/cell (compared to u003e10,000 molecules on MM cell lines) permitted reactivity to primary chronic lymphocytic leukemia (CLL) and other CD19+ B cell tumor lines. Indeed comparable tumor regressions were observed after administration of bb2121 or anti-CD19 CAR T cells into an NSG mouse model of disseminated Burkittu0027s lymphoma (Daudi: 1,170 BCMA and 267,000 CD19 molecules). Prior investigators have shown improved therapeutic efficacy by enriching for memory CAR T cells, yet current antibody-based selection methods are expensive and difficult to scale. Unexpectedly, we found bb2121 manufacture in the presence of a PI3-kinase inhibitor enriched for memory-like CAR T cells without a complicated cell sorting procedure. bb2121 CAR T cells cultured with a PI3K inhibitor expressed markers associated with T cell memory including CD62L, CD127, CD197, and CD38. In a first “stress test,” of advanced disseminated lymphoma (Daudi), NSG mice were administered an amount of bb2121 CAR T cells that failed to control tumor outgrowth. Mice administered the same number of bb2121 CAR T cells cultured with a PI3K inhibitor resulted in complete tumor regressions. A defining property of memory T cells is durability despite multiple antigen encounters. In a second “stress test,” bb2121 CAR T cell durability was evaluated after initial MM (RPMI-8226) clearance. Two weeks post-tumor elimination, mice were re-challenged with tumor on the opposite flank without an additional dose of CAR T cells. None of the mice treated with bb2121 CAR T cells prevented tumor outgrowth in this model. In contrast, all mice treated with bb2121 CAR T cells cultured with a PI3K inhibitor were able to control the tumor re-challenge. These data demonstrate that a potent, antigen-dependent, memory-like BCMA CAR T cell produced with an industrially scalable manufacturing process has promise for robust tumor regressions in clinical applications.