PS971 AT1413 ANTIBODY DERIVED FROM A CURED AML PATIENT RECOGNIZES A UNIQUE CD43 EPITOPE SHARED BY AML, MDS AND MELANOMA AND HAS IN VIVO ACTIVITY AS UNMODIFIED ANTIBODY AND AS BISPECIFIC T CELL ENGAGER

Background: By searching the donor‐derived B cell repertoire of AML patients who remain in long term remission following allogeneic hematopoietic stem cell transplantation we identified an antibody (AT1413) that recognizes a sialylated epitope on CD43. This CD43 s is expressed on myeloid cells but not on B and T cells and is over‐expressed on acute myeloid leukemia (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 leukemia response of this patient. Aims: Because CD43 is broadly expressed in non‐hematopoietic cells we explored whether CD43 s is present on non‐hematopoietic tumors. To potentiate the cytotoxic abilities of the antibody even more, we developed the antibody into a bispecific T cell engaging antibody (bTCE). Methods: AT1413 binding on a panel of tumor cell lines was analyzed 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 (G236R, L328R) were introduced to prevent interactions between AT1413 bTCE and Fcg‐receptors. The cytotoxicity‐inducing activities of the unmodified AT1413 antibody and its bTCE format were tested with PBMCs or cultured T cells as effector and tumor cells as target cells using standard cytotoxic assays. AT1413 bTCE was tested in vivo in a humanized immune system (HIS) mouse model engrafted with the human SH2 cell line and subsequently in two patient‐derived xenograft AML mouse models. Results: AT1413 bound to melanoma cell lines but not to pancreas, colon 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 CD43 s expression levels. Concomitantly, AT1413 bTCE induced strong cytotoxic T cell activities against AML and melanoma cell lines in vitro . In vivo , AT1413 bTCE induced potent tumor growth inhibition in both AML mouse models compared with a control bTCE. In the HIS mouse model, AT1413 bTCE treatment did not affect normal human hematopoiesis. The in vivo effect of the AT1413 bTCE against melanoma is currently being examined. Summary/Conclusion: We identified a novel tumor‐target, a sialylated epitope on CD43, that was identified on AML cells but is shared by melanoma and MDS cells. Targeting this epitope by the non‐modified IgG antibody AT1413 and the bispecific TCE form of AT1413 induce strong anti‐tumor cytotoxic activities in vitro and in vivo . Because of its broad tumor reactivity and functional activities in the absence of obvious hematopoietic toxicity, AT1413 has promising therapeutic potential.