Bioidentical Hormones for Menopausal Therapy

Cynthia K Sites

Disclosures

Women's Health. 2008;4(2):163-171. 

In This Article

Estrogen-receptor Subtypes & Bioidentical Estrogens

Estrogenic activity within a tissue is influenced by the type of estrogen and the subtype of estrogen receptor found within that tissue,[20] as well as by the distribution of cofactors and corepressors of the estrogen-receptor complex within the tissue.[21] With estradiol as the standard, binding affinity for both estrogen receptor (ER)-α and -ß is equal at 100.[20] Estrone has a binding affinity for ER-α of 60 and for ER-ß of 37. Estriol binds to ER-α with an affinity of 14 and to ERß with an affinity of 21. Therefore, estradiol has the strongest binding affinity for both ER-α and -ß.

Estrogen binding affinity alone does not predict estrogenic activity within a tissue. Compounds containing larger amounts of estradiol compared with estrone and estriol would be expected to have great effects on all estrogen-responsive tissues. However, ER-α receptors are found primarily in the endometrium, ovary and in breast cancer cells.[20] By contrast, ER-ß receptors are found in the kidney, intestinal mucosa, lung, bone and bone marrow, brain and endothelial cells. Thus, Biest, containing estradiol and estriol, is likely to act on all the tissues above that contain ER-α and -ß receptors, with a slight preference for tissues that are ER-ß dominant because of the estriol component. As binding of the estrogen with the estrogen receptor allows for the recruitment of different coactivators and corepressors, it is possible that estriol acts somewhat differently than estradiol in tissues.

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