Characterization of CA 125 antigen identified by monoclonal antibodies that recognize different epitopes.

The present study was undertaken to characterize the antigenic determinant of the CA 125 macromolecules recognized by newly developed monoclonal antibodies (M 11, 130-22, 145-9, 602-1, and 602-6), examine their relationship among the epitopes recognized by these antibodies, and develop a series of enzyme-linked immunosorbent assays (ELISAs) in which various combinations of antibodies are used in a double-determinant sandwich mode. The antigenic determinants of CA 125 were characterized by several methods including competitive ELISA and immunoblotting. These antibodies, as well as OC 125, reacted with ovarian cancer (HOC-I) cell extract in a dose-dependent manner. Purified CA 125 antigen with a molecular mass of less than 200 kDa (CA 125 < 200 kDa) had a significant inhibitory effect on the reaction between the cancer cell extract and all these antibodies. The reactivity of M 11, 130-22, 145-9, 602-1, and 602-6 to the cancer cell extract was not significantly inhibited by purified CA 125 > or = 200 kDa, whereas the reactivity of OC 125 was completely inhibited by CA 125 > or = 200 kDa. The antigenic determinants of M 11, 145-9, 602-6, 130-22, and 602-1 were closely related to each other, in this order; whereas OC 125 recognized a different epitope on a structurally identical molecule. The use of these monoclonal antibodies in combination with each other may result in the development of a more specific and sensitive assay for CA 125.