Abstract B122: Human Monoclonal Antibodies to Sialyl-Lewis-a with Potent CDC and ADCC Activity

Abstract The carbohydrate antigen sialyl-Lewis A (sLea) is widely expressed on epithelial tumors of the gastrointestinal tract, on breast cancer cells, and also on small cell lung cancer cells, but is expressed minimally or not at all on normal tissues. sLea serves as a ligand for epithelial leukocyte adhesion molecules and higher expression of sLea was observed in patients with greater node involvement. Since over-expression of sLea appears to be a key event in invasion and metastasis of many tumor cells and tumor cells expressing sLea are highly susceptible to antibody mediated lysis mechanisms, sLea presents an attractive molecular target for tumor therapy in a minimal disease setting. Here we report the discovery and initial characterization of fully human antibodies that were generated from blood lymphocytes from individuals immunized with sLea vaccine at MSKCC. Antibodies were identified by ELISA using sLea-HSA conjugates and binding to the native antigen was verified by FACS analysis on DMS-79 cells. Two antibodies were selected for further studies based on the apparent high affinity, which was estimated by Biacore at 0.14 nM for 5B1 (IgG/λ) and 0.04 nM for 7E3 (IgM/κ). These antibodies did not bind to sialyl-Lewis X, Lewis A and other related carbohydrates. Both antibodies have been expressed as fully functional human recombinant antibodies in CHO cells. Complement dependent cytotoxicity (CDC) against DMS-79 cells was approximately 60% and 70–90% for r5B1 and r7E3, respectively. Moreover, r5B1 antibodies showed approximately 50% ADCC of DMS-79 cells with human NK cells (at 5:1 ratio) and 80–90% ADCC with human peripheral blood mononuclear cells with two different blood donors (at 100:1 ratio). These results are very encouraging and we believe that further studies to scale up and test these antibodies in various tumor challenge models are warranted. Since sLea is widely expressed on human cancers, such antibodies could eventually find utility in approximately half of the new cancer cases occurring each year. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):B122.