PO-434 AT1413 antibody derived from a cured AML patient recognisesa unique sialylated CD43 epitope shared by AML, MDS and melanoma cells

Introduction AT1413 is an antibody derived from B cells of an AML patient who was cured following allogeneic hematopoietic stem cell transplantation. It recognises a sialylated epitope on CD43 (CD43s), which is expressed on myeloid cells but not on B and T cells and is over-expressed on acute myeloid leukaemia (AML) and myelodysplastic syndrome (MDS) cells. AT1413 kills AML cells in vitro and in vivo via antibody dependent cell-mediated cytotoxicity (ADCC) suggesting that AT1413 played a role in the graft versus leukaemia response of this patient. Because CD43 is broadly expressed in non-hematopoietic cells we explored whether CD43s is present on non-hematopoietic tumours. Material and methods AT1413 binding on a panel of tumour cell lines was analysed by flow cytometry. AT1413 was assembled into a bispecific T-cell engaging format (AT1413 bTCE) by linking the full-length AT1413 IgG to two single chain variable fragments against CD3e with a combination of site-specific enzymatic and chemical coupling. Two point mutations in the IgG heavy chain were introduced to prevent interactions between AT1413 bTCE and Fc gamma receptors. The cytotoxicity-inducing activities of naked AT1413 and its bTCE format were tested with PBMCs as effector and tumour cells as target cells using standard cytotoxic assays. Results and discussions AT1413 bound to melanoma cell lines but not to pancreas carcinoma, colon carcinoma, or liver carcinoma. Expression on melanoma cells was confirmed by immunoprecipitation and western blot using a mouse anti-human CD43 antibody. AT1413 bound to 14 out of 21 patient-derived primary melanoma samples with varying intensities. AT1413 induced ADCC of melanoma cell lines and patient-derived melanoma cells. The level of ADCC correlated with CD43s expression levels. To increase the cytotoxicity inducing potential of AT1413 we generated a bTCE format and demonstrated that it induced strong cytotoxic T cell activities against melanoma cells in vitro. The efficacy of AT1413 and AT1413-bTCE on human melanoma cells in vivo in a xenograft mouse model is currently tested. Conclusion The AT1413 antibody recognises a sialylated epitope on CD43 shared by melanoma, AML and MDS cells. Both the non-modified IgG- and the bispecific TCE form of AT1413 induce strong anti-tumour cytotoxic activities in vitro and in vivo. Because of its broad tumour reactivity and functional activities AT1413 has promising therapeutic potential.