Antiestrogen binding sites in human breast cancer biopsies. Measurement ligand-specificity and affinity, and correlation to estrogen and progesterone receptors.

Specific binding sites for the antiestrogen tamoxifen (AEBStot), as measured with the dextran-coated charcoal technique and Scatchard analysis, were quantitated in 43 of 44 breast cancer samples (41-2,100 fmol/mg protein). In this work, AEBStot has been defined as the sum of AEBSspec and estrogen receptor (ER), AEBSspec shows a very weak positive correlation with ER, and no correlation with progesterone receptor. Tamoxifen (TAM) has a lower affinity to ER compared with one of its main metabolites, 4-OH-TAM. This may be one reason why inhibition by excess estradiol on 3H-4-OH-TAM binding was better correlated with ER content in the same sample than if 3H-TAM was the radioactive ligand. It is therefore suggested that 3H-4-OH-TAM is a better choice than 3H-TAM for measuring AEBSspec alone. A possible role of AEBSspec for the clinical effects of TAM is discussed.