Bst1/CD157 is a GPI-anchored transmembrane protein belonging to the ADP-ribosyl-cyclase family and it is expressed on some blood cells such as monocytes and neutrophils. Importantly, we demonstrated that this antigen is expressed in blast cells from almost 100% of Acute Myeloid Leukemia patients (either at primary diagnosis or at relapse).
MEN1112 is a humanized, de-fucosylated antibody targeting the Bst1/CD157 antigen with high affinity and inducing potent in vitro and ex vivo Antibody Dependent Cell-mediated Cytotoxicity (ADCC) responses against AML cell lines and blasts, respectively. MEN1112 was discovered as a mouse Fab antibody from a phage display library raised against recombinant Bst1/CD157 protein. The Fab was selected base upon its ability to recognize recombinant Bst1/CD157 protein by ELISA and for its ability to recognize Bst1/CD157 positive endogenous cancer cell lines by FACS (fluorescence activated cell sorting). The Fab was also chosen based upon its ability to recognize cynomolgus Bst1/CD157 antigen on recombinant and endogenous cells. The selected Fab sequence was humanized and incorporated into the Biowa Potelligent cell line to produce a de-fucosylated full IgG1 version (MEN1112). The binding affinity of MEN1112 for the Bst1/CD157 antigen showed an EC50 of 1nM as assessed by flow cytometry (FACS) analysis using multiple cell lines. No binding was observed on antigen-negative cell lines demonstrating specificity of MEN1112. High affinity of MEN1112 for the recombinant antigen was confirmed in an ELISA assay (EC50 0.4 nM).
Beyond the affinity for the antigen, it is of relevance for its ADCC activity that MEN1112 is characterized by an improved affinity for Fc receptors on effector cells over the parental antibody. In particular, MEN1112, lacking fucose residues in its Fc domain, showed an impressive high affinity binding to the Fc receptor CD16A both high affinity and low affinity alleles.
Internalization studies demonstrate the slow internalization of the Bst1/CD157 antigen following binding to MEN1112 which favors the ADCC efficacy of the antibody. Overall our results indicate that MEN1112 has the potential to exert anti-leukemia actions through a powerful ADCC in view of: (i) superior affinity for blast cell membrane antigen Bst1/CD157, (ii) enhanced affinity for all variants of CD16A Fc receptor expressed on effector cells and (iii) minimal internalization following antigen binding.
Aud:Oxford BioTherapeutics: Employment. Dusek:Oxford BioTherapeutics: Employment. Bisht:Oxford BioTherapeutics: Employment. Swaminithan:Oxford BioTherapeutics: Employment. Awdew:Oxford BioTherapeutics: Employment. Trang:Oxford BioTherapeutics: Employment. Lin Lou:Oxford BioTherapeutics: Employment. Manzini:Menarini Ricerche SpA: Employment. Martinez Mogarra:Menarini Biotech srl: Employment. Bressan:Menarini Ricerche SpA: Employment. Binaschi:Menarini Ricerche SpA: Employment.
Asterisk with author names denotes non-ASH members.