Selection of monoclonal antibodies for use in an immunometric assay for carcinoembryonic antigen.

Six high-affinity mouse monoclonal antibodies against carcinoembryonic antigen produced in our laboratory were evaluated for use in a solid-phase immunoradiometric assay. The antibodies all recognized single peptide epitopes, one being highly conformation dependent. Cross-inhibition studies demonstrated that three antibodies bound to different epitopes, while the remaining three bound to the same or to very closely related epitodes. All antibodies strongly stained colorectal cancer tissue. The three antibodies binding to the same epitode also stained normal granulocytes, indicating a reactivity with the non-specific cross-reacting antigen. On the basis of the results of the characterization, two antibodies were selected for use in a sandwich immunometric assay. One of these was coupled to 2.8 microns magnetizable polymer particles, while the other one was radiolabelled. The assay required 2 h incubation to reach equilibrium, having a working range up to 135 micrograms/l and a sensitivity (Zero + 2 SD) of 0.1 micrograms/l. A comparison of the new assay with two commercial CEA kits that also use monoclonal antibodies was carried out on a small panel of serum samples from colorectal cancer patients and revealed satisfactory correlations but also discrepancies that must be attributed to differences in epitope specificity.