Epitope-selective, monoclonal-antibody-based immunoradiometric assays of predictable specificity for differential measurement of choriogonadotropin and its subunits.

Knowing the epitope specificities of our monoclonal antibodies (MCA) to human choriogonadotropin (hCG), we could design three classes of two-site immunoradiometric assays (IRMA): a combination of two MCA recognizing two separate alpha-epitopes (alpha-MCA) provides a system (i.e., an alpha-IRMA) that measures holo-hCG plus free alpha-subunits plus follitropin, lutropin, and thyrotropin, whereas a beta-IRMA, consisting of two beta-MCA, quantifies holo-hCG plus free beta-subunits. The amount of either of the two subunits can be calculated by subtracting the amount of holo-hCG determined in parallel in a holo-hCG-IRMA. In the latter, one of the alpha- or beta-MCA may be either cross-combined or, preferably, paired with an MCA specific for a conformational epitope. These analytical specificities, predicted from our previously established epitope map of hCG, could be experimentally verified. With these IRMAS we could demonstrate that in certain choriocarcinoma cell lines the earliest and quantitatively predominant tumor marker is the free alpha-subunit. Similar results showing an unbalanced secretion of hCG and its subunits were obtained for patients with related tumors. These findings challenge the present diagnostic practice of relying solely on "beta-hCG" radioimmunoassays and at the same time offer a novel analytical strategy.