PO-436 Retargeting T-cell cytotoxicity to a unique sialylated epitope on CD43 expressed by acute myeloid leukaemia

Introduction From the B cell repertoire of an acute myeloid leukaemia (AML) patient in long-term remission after hematopoietic stem cell transplantation we recently identified the antibody AT1413. AT1413 binds CD43s, a unique sialylated epitope on CD43 present on AML and myeloid cells but not on B and T cells. Besides its therapeutic potential as a naked antibody, AT1413 provides an interesting candidate for a bispecific T-cell engaging antibody (bTCE). bTCEs have been clinically validated as a powerful tool for harnessing the cytotoxicity of polyclonal T cells against a hematologic cancer. Simultaneous binding to a cancer surface antigen and the T-cell surface protein CD3e mediates cancer cell recognition and T-cell mediated killing independent of the T-cell receptor specificity. Material and methods To generate an AT1413 bTCE, we first modified AT1413 to abolish Fc-receptor interaction. Second, we assembled the bispecific by chemo-enzymatic linkage using a combination of a sortase-catalysed transpeptidation reaction and a subsequent cycloaddition reaction. AML target cell lysis by T-cells was assessed in vitro in a cytotoxicity assay. Up-regulation of T-cell activation markers CD69 and CD25 and cytokine production were monitored as indicators for T-cell activation. T-cell proliferation was assessed. In vivo , AT1413 bTCE was tested in two mouse models, one where human PBMCs were co-injected at the start of bTCE treatment and the other in which a human immune system (HIS) was engrafted at birth. Results and discussions AT1413 bTCE was confirmed to retain dual binding capacity for both AML cells and CD3e-expressing Jurkat cells. In vitro, AT1413 bTCE successfully induced T-cell mediated cytotoxicity against different CD43s expressing AML cell lines as well as primary AML blasts. Endothelial cells that have a detectable but considerably lower binding capacity for AT1413 remained unaffected. Besides cytotoxicity, T-cell activation and T-cell proliferation were observed and were dependent on the presence of target-expressing AML cells. In vivo testing of AT1413 bTCE dosed at 2 mg/kg revealed potent AML tumour growth inhibition of 89%–99% in two mouse models when compared with a control bTCE. In the HIS model, normal human hematopoietic cells remained present in mice treated with AT1413 bTCE. Conclusion Our results indicate that CD43s is a potential new target for T-cell engaging antibodies. Consequently, AT1413 holds therapeutic potential not only as an unmodified antibody, but also in a bispecific, T-cell engaging format.