Physiological Studies of Human Chorionic Gonadotropin (hCG), αhCG, and βhCG as Measured by Specific Monoclonal Immunoradiometric Assays*

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Abstract
Several libraries of monoclonal antibodies have been produced against epitopes that reside on hCG, .alpha.hCG, and .beta.hCG. Having characterized them physically, we explored their use in the construction of highly specific and sensitive immunoradiometric assays. There were several important immunochemical considerations with respect to developing assays that acurately detect low levels of free subunits in serum in the presence of high concentrations of the native hormone. These include physical properties and specificities of the monoclonal antibodies, choice of capture antibody on the solid phase support, assay design, and purity of hormone standards. Using such assays, we found early pregnancy (in vitro fertilization) to be characterized by the sequential appearance of hCG, followed by .beta.hCG and then .alpha.hCG. Molar ratios of .beta.hCG to .alpha.hCG and .beta.hCG to hCG were highest in early gestation. However, there was a reversal of the .beta.hCG to .alpha.hCG ratio at 12-13 weeks gestation, and an excess of free .alpha.hCG was observed thereafter. Except for values obtained in very early pregnancy, the .beta.hCG to hCG ratio remained remarkably constant at approximately 0.5% throughout gestation. In contrast, choriocarcinoma was distingished by absolute serum .beta.hCG concentrations 3-100 times greater than the maximum values observed during pregnancy and, more importantly, by exceeding high .beta.hCG to hCG ratios. For comparison, we studied hCG, .alpha.hCG, and .beta.hCG levels in an additional 178 patients with nontrophoblastic tumors. Ectopic production of .alpha.hCG and .beta.hCG was rare (3%), and thus far, we have been unable to demonstrate the presence of hCG in such patients. Therefore, hCG and the free subunits appear not to be useful as serological markers for nontrophoblastic tumors.

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