Estrogen Receptor Assay in Human Mammary Carcinoma with the Synthetic Estrogen 11β-Methoxy-17α-ethinyl-1, 3, 5(10)-estratriene-3, 17β-diol (R 2858)

The interaction of the tritiated synthetic estrogen R 2858 (11β-methoxy-17α-ethinyl-1,3,5(10)-estratriene-3,17β-diol) with the cytosol estrogen receptor in human mammary carcinoma and with sex hormone-binding globulin in human serum was compared with the binding of [³H]estradiol. Unlike estradiol, R 2858 does not bind to sex hormone-binding globulin, which prevents interference of this serum protein in estrogen receptor assays. The affinity of R 2858 for the estrogen receptor was about 10 times less than was the affinity of estradiol for the same receptor. The amount of unspecific binding was less with [³H]R 2858 than with [³H]estradiol. Quantitation of the estrogen receptor with [³H]estradiol as ligand in Scatchard analysis, glycerol gradient centrifugation, or isoelectric focusing was affected by an excessive contamination of the tissue samples with serum. This interference was not observed when [³H]R 2858 was used as ligand in the assays. [³H]R 2858 offers two main advantages, when compared to [³H]estradiol, in assays of the estrogen receptor in human mammary carcinoma tissue grossly contaminated with blood. (a) It is specifically bound to the estrogen receptor but not to sex hormone-binding globulin. (b) It has less tendency to bind unspecifically to components in human serum and breast cancer cytosol when compared to [³H]esteradiol. The advantages of [³H]R 2858 as a ligand in estrogen receptor assays are very apparent when the tumor tissue is collected by fine-needle aspiration biopsy; the specimens obtained by this technique are often grossly contaminated with blood.