Binding Reactivity of Monoclonal Anti-Carcinoembryonic Antigen (CEA) Antibodies with Cell Membrane-Bound CEA and with Free CEA in Solution

Binding reactivities of 62 anti-CEA MAbs from 10 different research groups with cell membrane-bound CEA and with free CEA in solution were compared by inhibition of MAb binding to CEA-expressing tumor cells by free CEA. Bindings of 30 MAbs to the cell membrane-bound CEA (280 ng CEA/2 × 105 cells) were inhibited by approximately equal amounts of free CEA, indicating that binding affinities of about half the MAbs for cell membrane-bound CEA are similar to those for free CEA, respectively. Bindings of 15 MAbs to the cell membrane-bound CEA were easily inhibited by free CEA of less than half the amount of the cell membrane-bound CEA, while inhibition of bindings of the remaining 17 MAbs required twice more free CEA than the amount of cell membrane-bound CEA, showing that about one-fourth of the MAbs have higher affinities for free CEA and the remaining about one-fourth of the MAbs possess higher affinities for cell membrane-bound CEA. These results help form the basis for selecting the anti-CEA MAbs for use in clinical applications, such as serum CEA assay, tumor imaging and immunotherapy.