Construction of an immunoradiometric assay for ovarian cancer associated antigen CA125 recognizing different antigenic determinant

CA125--a test with a change of heart

CA125 is well known as a tumour marker for ovarian cancer. Like all tumour markers it is not specific for a specific tumour and may be elevated in benign disease. Even in ovarian cancer it seems that CA125 is derived from mesothelial production rather than from the cancer cells. CA125 is a natural product of serosal epithelial cells and present in most serosal fluids whether malignant or benign. Benign causes of CA125 elevation include liver cirrhosis, peritoneal infection, abdominal surgery or the congestion of cardiac failure. Elevated CA125 levels are found in ascitic, pleural or pericardial fluid of patients with cardiac failure and the serum levels correlate with the clinical staging of cardiac failure. Whilst CA125 levels might be useful for diagnosis of cardiac failure, it has an equally impressive ability for defining prognosis in that condition, especially when combined with measuring natriuretic peptides. The CA125 assay is not standardised and different methods, such as new CA125II assays, often give differing results. Furthermore, as CA125 levels fall at the menopause, and may rise in the elderly, reference limits appropriate for age and gender need to be refined in order for CA125 to fulfil any of its potential as a marker of cardiac failure in these age groups.

Increased streptavidin uptake in tumors pretargeted with biotinylated antibody using a conjugate of streptavidin-fab fragment

Radiolabeled streptavidin accumulated in tumors pretargeted with biotinylated antibody. However, the absolute delivery of radioactivity was limited. To increase the tumor uptake of radioactivity further, we conjugated streptavidin with a mouse monoclonal antibody (MAb) fragment, OST6Fab, which recognizes antigen on human osteosarcoma. Another mouse MAb, OST7, which also reacts with the same tumor but recognizes an epitope different from the OST6 epitope, was biotinylated. The radioiodinated streptavidin-OST6Fab conjugate was administered to tumor-bearing mice after the biotinylated OST7 pretargeting. The uptake of the conjugate in tumors pretargeted with the biotinylated antibody was significantly higher than that of streptavidin and that of the conjugate of streptavidin and irrelevant Fab fragment. Renal uptake of radioactivity was decreased markedly, and the blood clearance was retarded by the conjugation with Fab fragment. In conclusion, the conjugate of streptavidin with specific Fab fragment increased the accumulation of radioactivity in tumors pretargeted with biotinylated antibody.

Effect of administration route and dose of streptavidin or biotin on the tumor uptake of radioactivity in intraperitoneal tumor with multistep targeting

The effect of the administration route and dose of streptavidin or biotin on the biodistribution of radioactivity in multistep targeting was studied in nude mice bearing intraperitoneal (IP) colon cancer xenograft. The multistep targeting included a two-step method using biotinylated antibody and radiolabeled streptavidin and a three-step method with radiolabeled biotin based on the two-step method. A monoclonal antibody, MLS128, which recognizes Tn antigen on mucin, was biotinylated and injected intravenously (i.v.) or i.p. in nude mice bearing human colon cancer LS180 IP xenografts for pretargeting. In the two-step method, i.p.-injected streptavidin showed a higher tumor uptake and tumor-to-nontumor ratios than i.v.-injected streptavidin regardless of administration route of pretargeting. The tumor uptake of radiolabeled streptavidin was increased with a high dose of biotinylated antibody pretargeting, but decreased with an increasing dose of streptavidin. In the three-step targeting, i.p. injection also gave a higher tumor uptake of radiolabeled biotin than i.v. injection. In conclusion, i.p. administration of radiolabeled streptavidin or biotin resulted in more efficient IP tumor targeting with the multistep methods.

Similarity and co-expression of tumour-associated antigens recognised by different monoclonal antibodies

The concentration of carcinoembryonic antigen (CEA), CA130, CA125, SLX, CA19-9, SPan1, and tumour-associated glycoprotein 72 (TAG-72) in the culture supernatant of 15 cancer cell lines and in the sera of 58 cancer patients was measured, and the co-expression of these antigens was examined by double determinant immunoradiometric assays. The high correlation coefficient of the concentrations and significant binding in the double determinant assays indicated a close relationship between CA125 and CA130 and between CA19-9 and SPan1. There was variable binding of the 125I-labelled anti-SLX, anti-CA19-9, and anti-SPan1 antibodies to anti-CA130 beads that had been pre-incubated with the culture supernatants, suggesting the presence of the epitopes of SLX, CA19-9, and SPan1 on the molecule expressing CA130. Similarly, the epitopes of SLX, CA19-9, and SPan1 could be present on the molecule expressing CEA. 125I-labelled anti-CA19-9, anti-SLX, and anti-TAG-72 antibodies were bound in variable proportions to anti-CA130 beads or to anti-CEA beads that had been pre-incubated with patients' sera. However, CEA and CA130 were not expressed on the same molecule, either in the culture supernatant, or in patients' sera. In conclusion, the carbohydrate epitopes of CA19-9, SPan1, SLX, and TAG-72 could be present on the molecule recognised by the anti-CA130 or anti-CEA antibody; however, the epitopes of CA130 and CEA did not co-exist on the same molecule.

Changes in CA125 release and surface expression caused by drugs in uterine cervix adenocarcinoma cells

The effect of drugs on the release of CA125 antigen and the binding of anti-CA125 monoclonal antibody (MoAb) to malignant cells was evaluated in vitro. TMCC-1, uterine cervical adenocarcinoma cells, were exposed to dexamethasone (DEX), sodium n-butyrate (NaB), dibutyryl cyclic AMP (dbcAMP), retinoic acid (RA), calcitriol (VD3), and interferon-gamma (IFN-gamma). NaB, RA and VD3 increased CA125 release per cell and 125I-labeled anti-CA125 MoAb binding to the cells. DEX also increased the 125I-labeled anti-CA125 MoAb binding to the cells, and CA125 antigen release per cell was also slightly increased. IFN-gamma suppressed both CA125 release and 125I-labeled MoAb binding. A combination of DEX, VD3 and RA and increased the binding of MoAb to TMCC-1 cells, but the amount of bound MoAb was not significantly different from that obtained by single drug treatment. DbcAMP had no significant effect on enhancing MoAb binding. Drugs can increase the binding of anti-CA125 MoAb to malignant cells and they may be applied to increase the tumor uptake of radiolabeled MoAbs in vivo.

Different antigenic nature in apparently healthy women with high serum CA 125 levels compared with typical patients with ovarian cancer

BACKGROUND

CA 125 is a representative ovarian cancer-associated antigen defined by monoclonal antibody OC125. Recently, monoclonal antibodies were produced (designated 130-22 and 145-9) that were reactive with CA 125 but bound to a separate epitope named CA 130. There was a close correlation between serum CA 125 and CA 130 values in most instances. However, among more than 8000 serum samples, 5 apparently normal women had high serum CA 125 values, despite having normal CA 130 values. In this study, the antigenic nature of these five women was investigated.

METHODS

Using gel chromatography, the molecular masses of CA 125 and CA 130 were estimated that were found in the five women with false-positive CA 125 values. The sera were examined using double-determinant assays combining iodine-125-labeled OC125 or iodine-125-labeled 130-22 with OCI25-coated or 145-9-coated beads.

RESULTS

The molecular masses of both CA 125 and CA 130 were estimated as greater than 1000 kilodaltons (KD); the CA 130 mass from one of the five women with an abnormal CA 125 level was approximately 200 KD using gel chromatography. Using the double-determinant assays that combined iodine-125-labeled OC125 or iodine-125-labeled 130-22 with OC125-coated or 145-9-coated beads, high radioactivity was found only in the homologous assay using iodine-125-labeled OC125 with OC125-coated beads. These results suggest that the antigenic nature of CA 125 found in apparently healthy women differs from that found in patients with ovarian cancer and that CA 130 epitopes are not present.

CONCLUSIONS

Measurement of serum CA 130 concentrations may be useful for excluding women with falsely elevated CA 125 values.

An extraordinarily high CA 125 level in a woman without apparent pathologic foci of CA 125 production: dissociation between serum levels of CA 125 and CA 130

We describe a patient with high serum CA 125 levels before and after surgery (greater than 403 U/ml) who had no apparent pathologic foci of CA 125 production. Serum levels of CA 130, which exists on the same glycoprotein as CA 125, were within normal ranges (less than 35 U/ml) before and after surgery.

Drug effects on CA125 antigen expression and antibody binding to cancer cells

CA125 is a high-molecular weight glycoprotein expressed on most serous-type ovarian cancer and some lung adenocarcinoma tissues. The effects of various drugs on the release of CA125 antigen into culture medium and on monoclonal antibody (MAb) binding to cancer cells were studied using 8 human cancer cell lines, all of which expressed CA125 on their cell surfaces. The effect of dexamethasone was seen at as low a concentration as 10(-9) M dexamethasone, and the release of CA125 and the binding of radiolabelled anti-CA125 antibody were completely inhibited after exposure to 10(-7) M dexamethasone. The number of antibody binding sites markedly decreased. In contrast, sodium butyrate increased CA125 expression. These findings were clearly detected in only 3 cancer cell lines and a significant effect was not seen in the 5 other cancer cell lines. Interferon-gamma, examined in 3 cell lines, suppressed in a dose-dependent manner in 2 CA125 expression cell lines, but enhanced it in one line. No apparent effects were seen after exposure to tissue necrosis factor or interleukin-2. These results suggest that drugs may regulate the CA125 antigen expression in some, but not all, cancer cells and may affect the biodistribution of radiolabelled MAbs.

An antibody-tumor model for the targeting of CA125-producing gynecologic malignancies

By immunizing a mouse with HOUA-1 cells established from an endometrial cancer patient, two murine monoclonal antibodies designated 196-14 and 196-28 were generated, which were reactive with ovarian cancer-associated antigen CA125, originally defined by OC125 antibody. Antigenic determinants of these antibodies, although overlapping each other, were different from that of OC125 and the combined use of 125I-labeled 196-14 and OC125-coated beads markedly increased the sensitivity of measuring CA125 antigen. Both radioiodinated and 111In-labeled 196-14 localized well in CA125-producing human ovarian cancer tissues OVA-5 xenografted in nude mice. The biodistribution of radioiodinated 196-14 was quite different from that of 111In-labeled 196-14. Radioiodine was cleared faster from the OVA-5 tumor, making a clear contrast to the prolonged retention of 111In the tumor. Initial tumor uptake of radioiodinated 196-14 was the same as that of 111In-labeled 196-14 but decreased thereafter, due to the dehalogenation of radioiodinated antibody in the tumor. This antibody-tumor model seems to be suitable for examining the usefulness of monoclonal antibody-conjugates in the diagnosis and therapy of CA125-producing endometrial or ovarian cancers.