DEVELOPMENT OF A BISPECIFIC F(AB')2 CONJUGATE AGAINST THE COMPLEMENT RECEPTOR CR3 OF MACROPHAGES AND A VARIANT CD44 ANTIGEN OF RAT PANCREATIC ADENOCARCINOMA FOR REDIRECTING MACROPHAGE-MEDIATED TUMOR-CYTOTOXICITY

A bispecific F(ab')(2) antibody conjugate (BAC) was constructed agains t the complement receptor CR3 of macrophages and a variant CD44 (CD44v 6) antigen of rat pancreatic adenocarcinoma cells to redirect macropha ge-mediated tumor cytotoxicity, The Fab' fragments of monoclonal antib odies (mAb) 1.1ASML and OX42, recognizing the CD44v6 and the CR3 antig ens respectively, were chemically coupled at the hinge region using 5, 5'-dithio-bis(2-nitrobenzoate). The BAC was characterized in vitro for its specific, dual binding capacity to CD44v6 and CR3 antigens, Altho ugh the monovalence of the BAC resulted in lower avidities to both the antigens as expected, it was still able to form stable cross-linkages between tumor cells and macrophages in culture leading to the formati on of ''clump-like'' cell aggregates. The in vitro and in vivo tumor-t argeting capacity of the BAC was compared with that of the parental an titumor mAb 1.1ASML, which mediates tumor killing by antibody-dependen t cell cytotoxicity. These results showed that, even though the bivale nt mAb 1.1ASML did not mediate stable cross-linking of target and effe ctor cells, its Fc-receptor-mediated killing of tumor cells was more e ffective when compared to the BAC. Thus, this study strongly supports the hypothesis that firm persistent binding between effector and targe t cells per se is not as important as the choice of trigger molecule u sed for macrophage activation to redirect their tumor cytotoxic potent ial effectively.