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Biological Activity of Two Humanized Antibodies against Two Different Breast Cancer Antigens and Comparison to Their Original Murine Forms¹

Cancer Research Fund of Contra Costa, Walnut Creek, California 94596

We have humanized two monoclonal antibodies (MoAbs), hu-BrE-3 and hu-Mc3, that are bound to two different antigens of the breast epithelial cell. They bind to the breast epithelial mucin (M_r 400,000) and the BA46 antigen (M_r 46,000). They could participate in a joint radioimmunotherapy strategy administering repeated or fractionated dosages, where increased irradiation could be delivered by their simultaneous administration.

Both antibodies, hu-BrE-3 and hu-Mc3, had similar reactivity to their antigens and similar binding affinity as those of their original murine forms. However, because humanized MoAbs could have different pharmacokinetic and radioimmunotherapeutic characteristics than their original murine forms, the experimental biodistribution in vivo of both of these two humanized anti-breast tumor MoAbs was compared to their original murine forms. Biodistributions in immunodeficient mice grafted with transplantable human breast tumors, both after radioiodination and ¹¹¹In labeling via 1-p-isothiocyanatobenzyl-methyl-diethylene-triaminepentaacetic acid (MXDTPA), demonstrated comparable tumor:normal tissue ratios for the humanized and murine forms. In radioimmunotherapy, the humanized forms for both MoAbs showed also similar tumoricidal activity as that of the original murine MoAbs. These results show that the new humanized forms are amenable to conjugation and radioisotope labeling without loss of biological activity. Furthermore, they demonstrate that these engineered molecules kept intact, both qualitatively and quantitatively, their binding ability, pharmacokinetics, and radioimmunotherapeutic characteristics after the humanization process.