Change in binding reactivity of an anti-tumor monoclonal antibody after the introduction of 2-pyridyl disulphide groups.

The use of monoclonal antibodies for in vivo therapeutic approaches depends largely on their specificity. During the characterization of ricin A-chain-murine monoclonal antibody conjugates we found that the binding specificity of a monoclonal antibody raised against human ovarian carcinoma (MOv2) seemed altered. Therefore, the binding reactivity of the unmodified antibody (MOv2), the conjugation intermediate (MOv2-PDP) and the conjugate (MOv2-A chain) was titrated by solid-phase radioimmunoassay on 11 human tumor cell lines belonging to seven different histotypes. The three reagents bound with the two reference cell lines (SW626:ovary carcinoma and HT-29:colon carcinoma). The MOv2-PDP and the Mov2-A chain also reacted with seven other cell lines which were unreactive with the unmodified MOv2. In addition MOv2-PDP exhibited reactivity on all normal cells tested (peripheral blood lymphocytes and skin fibroblasts). To elucidate the significance of these findings the following experiments were performed: cross inhibitions between the unmodified and modified monoclonal antibodies; comparative absorption tests with different cell lines; and immunoblotting analysis of the target antigens. The results suggest that after chemical modification with SPDP the monoclonal antibody MOv2 increases its binding activity, so that even a low number of antigenic sites can be detected. This study underlines the need to redefine the specificity of a conjugate before considering therapeutic applications.