Abstract

2550 Background: Antigen-specific cancer immunotherapies, based on engineered T cells bearing chimeric antigen receptors (CARs) or the systemic administration of bispecific T cell-engagers (TCEs), have a significant impact on relapsed/refractory (R/R) B cell malignancies. However, a significant percentage of patients relapse following CAR-T or TCE therapy, with antigen loss accounting for up to one third of relapses/progressions. To avoid antigen loss after administration of single-targeted CAR-T cells and to minimize tumor escape, strategies targeting two antigens simultaneously have been developed and validated in both preclinical models and clinical trials. Nevertheless, despite lowering the risk of antigen loss, these strategies still have some limitations, mainly related to design and manufacturing challenges. Methods: In this context, we have generated the first dual-target strategy for hematological malignancies based on T cells bearing an anti-CD22 CAR and secreting an anti-CD19 T-cell engager antibody (CAR-STAb-T cells) and conducted a comprehensive preclinical study comparing its therapeutic potential with a previously validated anti-CD19/CD22 tandem CAR therapy (TanCAR-T) for B-cell acute lymphoblastic leukemia (B-ALL). Results: We have demonstrated in both short- and long-term assays, using contact and non-contact co-culture systems, that CAR-STAb-T cells efficiently redirect bystander T cells, resulting in enhanced cytotoxic activity compared to that exhibited by TanCAR-T cells at very low E:T ratios. In addition, CAR-STAb-T cells induce more potent and faster cytotoxic responses than TanCAR-T cells in both short- and long-term co-culture assays when reproducing antigen-downmodulated conditions in vitro. In vivo assays conducted in NSG mice transplanted with a B-ALL patient-derived xenograft (PDX), followed by intravenous injection of CAR-STAb-T or TanCAR-T cells under a T cell-limiting experimental setting, also showed that CAR-STAb-T cells maintained a tighter control of tumor progression than TanCAR-T cells in peripheral blood and bone marrow. Conclusions: In conclusion, we have demonstrated that the combination of a cell surface CAR and a soluble TCE recognizing different antigens may be advantageous over the use of conventional multi-targeted strategies based on cell surface-anchored receptors. Furthermore, we have proven that a small number of transduced CAR-STAb-T cells is sufficient to redirect non-transduced bystander T cells specifically and efficiently in the presence of leukemia cells, providing a significant advantage over current dual-targeted CAR-T cell therapies. CAR-STAb-T cells could therefore become an alternative to CAR-T therapies for the treatment of R/R B cell malignancies, especially in lymphodepleted patients with low T cell counts.