Specificity and Affinity of 26 Monoclonal Antibodies against the CA 125 Antigen: First Report from the ISOBM TD-1 Workshop

Abstract

The specificity of 26 monoclonal antibodies against the CA 125 antigen was investigated in two phases of the ISOBM TD-1 workshop. The binding specificity was studied using CA 125 immunoextracted by specific antibodies immobilized on various solid phases, or on the surface of human cell lines. Immunometric assays using all possible antibody combinations were used to study the topography of antibody binding sites on the antigen. We conclude that the CA 125 antigen carries only two major anti-genic domains, which classifies the antibodies as OC125-like (group A) or Mil-like (group B). One antibody, OV 197, showed binding specificity related to some of the OC125-like antibododies, but was classified into a separate group C. The OC125-like group of antibodies has four subgroups with different binding specificities. These are A1 = OC 125 and K 95, A2 = K 93, A3 = B43.13, and A4 = ZS 33, B27.1 and CCD 247. Binding of nonlabelled OC 125 or K 95 to CA 125 caused a marked increase in binding of labelled OV 197 to the complex. This conformational change was not observed with any other antibody combinations. Antibody B43.13 could form immunometric assay combinations particularly with antibodies of subgroup A4, indicating that the B43.13 epitope is in the periphery of the binding area of OC125-like antibodies. The M11 -like group of antibodies is more homogenous with strong cross-inhibition between most antibodies. Only one antibody, ZR 38, would form an immunoassay combination with other M11 -like antibodies and thus represents a distinct subgroup. The main group of M11-like antibodies are M 11, ZR 45, MA602-6, K 91, OV 185, K 101, K 90, K 96, K 97, K 102, CCD 242, 145-9, and 130-22. Antibody OV 197 binds to a domain designated C and is unique, as stated above. Antibody pairs from any two of the three groups may be used in immunometric assays. Three antibodies were not studied by complete cross-inhibition due to low affinity (OV 198 and K 100) or lack of material (MA602-1). OV 198 and K 100 are most likely OC125-like and MA602-1 is Ml 1-like. Antibody affinity was estimated with labelled antigen in solution or with antigen adsorbed on microtiter wells. Western blot analysis showed staining both in the stacking gel and corresponding to a molecule of 200 kDa. There was a marked difference between the antibodies in their ability to bind to CA 125 immobilized on a membrane. Strongest binding was observed with the M11-like antibodies, particularly M 11, K 96, K 97, MA602-6, 145-9. Antibodies belonging to the subgroup A4 were the only OC 125-like antibodies which reacted well with CA 125 in Western analysis. Digestion of CA 125 with proteolytic enzymes showed it to be particularly sensitive to trypsin cleavage. However, no low molecular weight fragments with preserved immunoreactivity were found.

Progress and Challenges in Screening for Early Detection of Ovarian Cancer

“…CA125 was initially detected using a murine monoclonal antibody OC125 raised in response to immunologic challenge with an ovarian cancer cell line (21). Subsequently, at least 26 other antibodies have been described (22). It is now known that the CA125 antigen carries two major antigenic domains classified as A, the domain binding monoclonal antibody OC125, and B, a domain binding monoclonal antibody M11 (22).…”

“…Subsequently, at least 26 other antibodies have been described (22). It is now known that the CA125 antigen carries two major antigenic domains classified as A, the domain binding monoclonal antibody OC125, and B, a domain binding monoclonal antibody M11 (22). Current immunoassays for the quantitation of serum CA125 levels are based on a heterologous assay (CA125 II) using both monoclonal antibodies (M11, OC125) in place of the original homologous assay with monoclonal antibody OC125 alone.…”

Use of CA-125 in Clinical Trial Evaluation of New Therapeutic Drugs for Ovarian Cancer

“…OC 125 recog-nizes the CA-125 epitope on a high molecular weight glycoprotein designated MUC 16. Monoclonal antibodies raised against other epitopes expressed by this molecule have led to the development of the CA-125-II assay, a variant of the original CA-125 assay that utilizes the M 11 monoclonal antibody as the capture antibody (41,42). Both assays have proved reliable for measuring CA-125 levels in a number of benign and cancerous conditions, most notably ovarian cancer.…”

Biokinetics of the Monoclonal Antibodies MOv 18, OV 185 and OV 197 Labelled with125I According to the m-MeATE Method or the Iodogen Method in Nude Mice with Ovarian Cancer Xenografts

“…The murine antibody OV 185 binds to the B and OV 197 to the C antigenic domains of the ovarian cancer surface antigen CA 125 (7). C242 is an antibody that recognizes a membrane glycoprotein on cells of human colon, pancreatic and cervical cancers.…”

“…The antibodies are classified as OC 125-like (group A) or M11-like (group B). The antibody OV 197 is unique and binds to a third domain designated C. (7,8) MOv18 is a monoclonal antibody reacting with a surface antigen which is expressed on about 90% of all human ovarian carcinomas (9). It has been used for immunoscintigraphy (10) and radioimmunotherapy (6) in patients with ovarian cancer, with promising results.…”