Immunoassay for estrogen receptor does not detect inactivated receptor

Accurate quantification of estrogen receptor (ER) is essential for optimal clinical characterization of individual cases of breast cancer. If breast tumors are mishandled, the relatively labile ER protein may lose its steroid-binding capacity (become inactivated) and not be measurable by the routine steroid-binding assay. We tested whether the commercial enzyme immunoassay of Abbott Laboratories could quantify inactivated ER. Samples of powdered breast tumors from humans were exposed to various temperature and homogenization conditions known to inactivate ER, and any remaining ER was quantified by both the immunoassay and the steroid-binding assay. For all inactivation conditions tested, the two assay methods detected the same proportions of remaining ER. We conclude that the inactivation reaction for ER also alters one or both of the antigenic site(s) necessary for the immunoassay. Hence, for breast tumors mishandled to the extent of inactivating ER, the immunoassay offers no advantage over the more conventional steroid-binding assay for quantifying any remaining ER.