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Comparison of Immunochemical and Radioligand Binding Assays for Estrogen Receptors in Human Breast Tumors¹

Oncologia Sperimentale C, Istituto Nazionale per lo Studio e la Cura dei Tumori, Via Venzian 1, 20133 Milano, Italy

We have compared a new enzyme immunoassay (EIA) for estrogen receptors (ER) with our conventional radioligand binding assays (multipoint dextran-coated charcoal assay for cytoplasmic ER and hydroxylapatite exchange assay for nuclear ER). Cytoplasmic ERs were measured in 76 human breast cancer specimens by EIA and by five-point Scatchard analysis. The correlation between the two assays yielded a straight line with a slope of 0.92 (r = 0.95; P < 0.001); conversely, in 31 nuclear salt extracts, linear regression analysis of hydroxylapatite exchange assay data with EIA showed a clear correlation (r = 0.93; P < 0.001) but a slope of 1.7, demonstrating that EIA detects more ER sites. The binding of the antibody to the cytoplasmic ER molecules was investigated by sucrose density gradient analysis, which showed that EIA recognizes both cytoplasmic forms (9 and 3S), but does not distinguish between them. Advantages and drawbacks of this method are discussed with respect to its application for routine receptor determination for clinical management of breast cancer patients.