Purpose: The carbohydrate antigen sialyl-Lewis^a (sLe^a), also known as CA19.9, is widely expressed on epithelial tumors of the gastrointestinal tract and breast and on small-cell lung cancers. Since overexpression of sLe^a appears to be a key event in invasion and metastasis of many tumors and results in susceptibility to antibody-mediated lysis, sLe^a is an attractive molecular target for tumor therapy.

Experimental Design: We generated and characterized fully human monoclonal antibodies (mAb) from blood lymphocytes from individuals immunized with a sLe^a–KLH vaccine.

Results: Several mAbs were selected based on ELISA and FACS including two mAbs with high affinity for sLe^a (5B1 and 7E3, binding affinities 0.14 and 0.04 nmol/L, respectively) and further characterized. Both antibodies were specific for Neu5Acα2–3Galβ1–3(Fucα1–4)GlcNAcβ as determined by glycan array analysis. Complement-dependent cytotoxicity against DMS-79 cells was higher (EC₅₀ 0.1 μg/mL vs. 1.7 μg/mL) for r7E3 (IgM) than for r5B1 (IgG1). In addition, r5B1 antibodies showed high level of antibody-dependent cell-mediated cytotoxicity activity on DMS-79 cells with human NK cells or peripheral blood mononuclear cells. To evaluate in vivo efficacy, the antibodies were tested in a xenograft model with Colo205 tumor cells engrafted into SCID (severe combined immunodeficient mice) mice. Treatment during the first 21 days with four doses of r5B1 (100 μg per dose) doubled the median survival time to 207 days, and three of five animals survived with six doses.

Conclusion: On the basis of the potential of sLe^a as a target for immune attack and their affinity, specificity, and effector functions, 5B1and 7E3 may have clinical utility. Clin Cancer Res; 17(5); 1024–32. ©2011 AACR.