Mapping of monoclonal antibody-defined epitopes associated with carcinoembryonic antigen, CEA

Six immunoglobulin G monoclonal antibodies reactive with carcinoembryonic antigen (CEA) were evaluated with respect to parameters implicated in their potential diagnostic application and use as tumor targeting agents for cytotoxic drugs or plant or bacterial toxins. Antibody reactivity with surface antigens of the MKN-45 gastric tumor cell line was demonstrated by flow cytofluorimetry. In a subcellular membrane binding assay, each antibody reacted preferentially with membranes isolated from colorectal tumor tissue in comparison with their reaction with membranes from adjacent, apparently normal colonic mucosa. Three of the antibodies (NCRC-23, C228, and 11.285.14) reacted specifically with CEA with little or no reaction with the cross-reacting antigen, NCA. The remaining three antibodies (C24, C161, and C198) were reactive with both CEA and NCA. Analysis of the epitopes defined by these antibodies was performed by competitive binding inhibition assays evaluating the capacity of unlabeled antibodies to compete with 125I-labeled antibodies in their binding to CEA. In addition, double determinant or 'sandwich' radioimmunoassays were employed to examine the coexpression of epitopes on CEA molecules. These studies permitted an epitope map to be constructed which describes the coincidence, overlapping, or independent expression of both CEA specific epitopes and epitopes shared between CEA and NCA. The map may be employed for the selection of antibodies for diagnostic and therapeutic use.