Immunochemical characterization and radioimmunometric detection of molecules shed by human ovarian cancer

Monoclonal antibodies (MAbs) OC125, MOv2 and MOv8 recognize the CA125, CAMOv2 and CAMOv8 epitopes, respectively, which are associated with human ovarian carcinomas and are shed by these tumors. The biochemical analysis of the recognized antigens and epitopes has been performed to investigate the possibility of exploiting their expression, if any, on the same molecules. Cross-competition experiments clearly indicated that the 3 epitopes are distinct. Biochemical analysis was performed on a cyst fluid from an ovarian cystadenoma. Heat treatment and periodate oxidation indicated that CAMOv2 and CAMOv8 are saccharidic and suggested that CA125 is a conformational epitope to which both saccharides and the protein backbone could contribute. By gel-filtration chromatography CA125 activity was eluted into 2 peaks of high molecular weight, whereas CAMOv2 and CAMOv8 activities were associated with a single broad peak which included CA125-positive fractions. Isopycnic centrifugation showed that CA125, CAMOv2 and CAMOv8 carrying molecules had the same high density (1.46 g/ml) in the first peak, whereas CA125 molecules had a lower density (1.41 g/ml) in the second peak. Double-determinant immunoradiometric assays, carried out using different combinations of the MAbs, indicated that in the cyst fluid molecules expressing both CAMOv2 and CAMOv8 could be detected, whereas CA125 was carried by different molecules. CAMOv2 and CA125 could however be expressed on the same molecule in 2 out of 9 ascitic fluids from ovarian carcinoma patients. Taken together, these data indicate that CA125 and CAMOv2-CAMOv8 were only occasionally expressed on the same molecules. Therefore, the use of the CA125-CAMOv2 combination could not increase the sensitivity achievable by using each respective simultaneous immunoradiometric assay.