Antiestrogen pharmacology and mechanism of action

Antiestrogens are nonsteroidal compounds that are capable of antagonizing many of the actions of estrogens. They have long half-lives in serum, and during their action in vivo, the antiestrogens CI628, U23,469 and tamoxifen undergo bioactivation to more polar forms that have a higher affinity for the estrogen receptor. Utilizing the high affinity antiestrogens CI628M and trans-hydroxytamoxifen in radiolabeled form, we find differences in the interaction of the estradiol-receptor complex and antiestrogen-receptor complex with DNA cellulose and differences in their association in the nucleus. In MCF-7 human breast cancer cells, there are marked differences in the sedimentation rate and chromatographic properties of the nuclear estrogen receptor when complexed with antiestrogen (5.5 ± 0.06 s, 5.93 ± 0.20 nm Stokes radius, 137,000 mol. wt) as opposed to estrogen (4.1 ± 0.03 s, 4.84 ± 0.20 nm Stokes radius, 83,000 mol. wt). Our data are consistent with the association of the antiestrogen-receptor with an additional nuclear protein of approx. 55,000 mol. wt. This antiestrogen promoted change in receptor association with another cellular component, which may function as a possible nuclear “blocking protein” and the alterations in antiestrogen-receptor association with DNA and nuclei, may be important aspects of the estrogen antagonist and growth inhibiting properties of these compounds. In addition to binding directly to the estrogen receptor, antiestrogens can be found associated with binding sites that are distinct from the estrogen receptor. These sites are present in a wide variety of estrogen target and non-target tissues in the rat, and in estrogen receptor positive and negative human breast cancer cell lines (MCF-7, T47D, MDA-MD-231) that differ markedly in their sensitivity to antiestrogens. These sites have a subcellular distribution and ligand binding specificity very different from those of the estrogen receptor. While their wide distribution and order of binding affinities for different antiestrogens raise questions about their role in mediating directly the classically recognized estrogen antagonism of antiestrogens, these sites might alter the apparent distribution volume of antiestrogens and hence their biological potency and pharmacokinetics.