A particularly simple, sensitive, and inexpensive technique has been developed for the routine assay of estrogen receptor binding sites in human tissues (endometrium and breast cancer specimens) with the use of a radiolabeled potent estrogen, moxestrol [11β-methoxy-17-ethynyl-1,3,5(10)-estratriene-3,17β-diol]. In these tissues moxestrol binds specifically to the estrogen receptor with which it forms a stable complex that, at 25°, dissociates twice as slowly as dose the estradiol/receptor complex. Unlike estradiol it dose not bind with high affinity to human plasma sex steroid-binding protein. In a correlation analysis on 215 breast cancer specimens, it was found that, for high binding site concentrations, determinations carried out with moxestrol were extremely well correlated with those performed with estradiol (in the presence of dihydrotestosterone in order to minimize interference from estradiol binding to sex steroid-binding protein). For low binding site concentrations, determinations performed with estradiol overestimated binding sites in spite of the presence of dihydrotestosterone. Thus, the use of labeled moxestrol instead of estradiol improves the accuracy of the measurement of specific tissue binding. Satisfactory conditions for a dextran-coated charcoal adsorption exchange assay have been developed in this paper; a maximum number of binding sites are measured after incubation with 2 to 10 nm radioligand for 3 to 5 hr at 25°.

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