Potentiation of Tumor Lysis by a Bispecific Antibody that Binds to CA19-9 Antigen and the Fcγ Receptor Expressed by Human Large Granular Lymphocytes

Abstract

Murine monoclonal antibody therapy of human cancer rarely induces clinical responses. Antibody-induced cellular infiltrates rarely accumulate at sites of tumor, even in clinically responding lesions. Thus, the ability of these antibodies to promote host effector cell-mediated lysis of tumor via antibody-dependent cellular cytotoxicity (ADCC) has not been harnessed by existing treatment approaches. One potential explanation is that ADCC requires binding of antibody Fc domains to cellular Fcγ receptors, and therapeutically administered murine antibodies must compete with vast excesses of human IgG for Fcγ receptor occupancy. Chemically linked antibody heteroconjugates that bind selected target and effector cell structures via distinct Fab portions can mediate lysis of malignant cells in vitro in the presence of human serum. This approach addresses a potentially major obstacle to antibody therapy. Production of bispecific monoclonal antibodies with similar specificities and superior in vivo biodistribution characteristics would thus have potential clinical applications. We have prepared and purified a bispecific, monovalent monoclonal antibody and evaluated its in vitro effects. The IgG1-secreting hybridoma line 3G8 (α-human FcγR III) was fused with the hybridoma line CA19-9, which produces an IgG1 antibody that binds to a glycoprotein shed by gastrointestinal cancers. Multiple clones with bispecific binding properties were identified. CA19-9 × 3G8 clonal supernatants and purified antibody, but not the parent antibodies, efficiently mediated specific in vitro lysis of cells of the SW948 line by human large granular lymphocytes (LGLs). Human serum-resistant target cell lysis augmentation at low effector:target ratios was seen using picogram amounts of antibody. In contrast, the IgG2α variant of CA19-9, which also promotes ADCC by LGLs, was unable to augment lysis of SW948 cells when effectors were preincubated with human serum. This bispecific, monovalent monoclonal antibody is an efficient promoter of the anti-tumor effects of LGLs in physiological concentrations of human serum. In vivo models that evaluate treatment efficacy and promotion of inflammatory tumor infiltrates by bispecific monoclonal antibodies are required to assess the therapeutic potential of these novel constructs.