S911 A COMBINATORIAL RECEPTOR STRATEGY TO IMPROVE MULTIPLE MYELOMA THERAPY WITH CHIMERIC ANTIGEN RECEPTOR‐ENGINEERED T LYMPHOCYTES

Background: Multiple Myeloma (MM), a disease characterized by a clonal expansion of plasma cells, still remains incurable with current treatment options. The genetic engineering of T lymphocytes with chimeric antigen receptors (CAR) has recently emerged as a promising therapeutic approach and BCMA has shown to be a good MM‐specific antigenic target. Despite its specificity, the intensity of BCMA is variable in MM patients and may not be uniform across different subclonal populations which might explain why first clinical trials of BCMA‐CAR‐T cells report lower complete remission rates as well as relapses of BCMA‐low clonal variants. Another MM‐related antigen, CD38, is uniformly highly expressed in all malignant MM cells. Aims: We hypothesized that targeting 2 MM‐related antigens would increase cytotoxic ability of CAR‐T cells and the incorporation of a dual costimulation design would confer increased persistence. Thus, we propose to improve CAR‐T cell therapy for MM by dual targeting of BCMA and CD38 and incorporating full costimulation with 4‐1BB and CD28 signaling by using the combination of a CAR and a chimeric costimulatory receptor (CCR). Methods: To this end, we designed first and second generation CAR constructs targeting BCMA (BCMA‐z and BCMA‐28z), CCR constructs targeting CD38 (CD38‐28BB and CD38‐BB) as well as a CD38‐engager construct lacking intracellular domain (CD38del). We then evaluated in vitro the anti‐MM cytotoxic potential, cytokine production, proliferative capacity and exhaustion of the double‐targeting CAR‐T cells (BCMA‐z/38‐28BB, BCMA‐28z/CD38‐BB or BCMA‐z/CD38del) compared to single‐targeting T cells carrying a conventional CAR (BCMA‐28z, BCMA‐BBz, BCMA‐z), or CCR (38‐28z or 38‐28BB). Results: We found that double‐targeting BCMA‐z/38‐28BB CAR‐T cells showed significantly increased cytotoxic capacity against BCMA + CD38 + MM cell lines and primary MM cells, even in low effector to target ratios, as compared to the single‐targeting BCMA‐CAR‐T cells. This effect was diminished when a BCMA + CD38 ‐ cell line was used as target, while cells carrying only a CCR had no cytotoxic potential. Interestingly, BCMA‐z/CD38del T cells, where CD38‐engagement does not mediate any costimulation signal, had similar cytotoxic ability as BCMA‐z/38‐28BB cells, therefore the increased killing capacity of double‐targeting CAR‐T cells is due to CD38 engagement and the consecutive increase in avidity. Finally, double‐targeting BCMA‐z/38‐28BB CAR‐T cells showed significantly higher cytokine secretion (IL‐2, IFN‐γ, TNF‐α), proliferative capacity and induction of PD‐1 expression than single‐targeting BCMA‐CAR‐T cells, which was not reproduced by BCMAz/CD38del T cells indicating that this effect is a result of combining both CD28 and 4‐1BB costimulation. Summary/Conclusion: In conclusion, we here show that double targeting MM with a BCMA‐CAR and a CD38‐CCR provides higher effector‐MM avidity and full costimulation. Engagement of the CD38‐CCR enhances the anti‐MM cytotoxicity and persistence and reduces the exhaustion of double‐targeting compared to conventional single‐targeting CAR T cells. Therefore double‐targeting of BCMA and CD38 is a powerful strategy to improve clinical outcomes of BCMA‐CAR T cell therapy.