A comparative characterization of estrogens used in hormone therapy via estrogen receptor (ER)-α and -β.

Conventional hormone therapy (HT) containing estrogens such as ethinylestradiol (EE) have been associated with an increased risk of breast cancer and cardiovascular disease resulting in women seeking safer alternatives that are claimed to have fewer health risks. One such alternative gaining popularity, is custom-compounded bioidentical (b)HT formulations containing bioidentical estradiol (bE) and estriol (bE). However, the preparation of these custom-compounded estrogens is not regulated, and depending on the route of synthesis, steroid mixtures with differing activities may be produced. Thus, an investigation into the activities of estrogens prepared by custom-compounded pharmacies is warranted. The aim of this study was therefore to directly compare the pharmacological properties of bE and bE of unknown purity relative to commercially available, pure E, E and estrone (E) standards as well as synthetic EE used in conventional HT via the human estrogen receptor (ER)-α and -β. We determined precise equilibrium dissociation constants (K or K values) and showed that bE and bE display similar binding affinities to the E and E standards, while EE had a higher affinity for ERα, and E a lower affinity for ERβ. Furthermore, all the estrogens display similar agonist efficacies, but not potencies, for transactivation on a minimal ERE-containing promoter via the individual ER subtypes. Although E and E were equally efficacious and potent on the endogenous ERE-containing pS2 promoter in the MCF-7 BUS breast cancer cell line co-expressing ERα and ERβ, E was less efficacious and potent than E. This study is the first to demonstrate that the bioidentical estrogens, commercially available estrogen standards and synthetic EE are full agonists for transrepression on both minimal and endogenous NFκB-containing promoters. Moreover, we showed that these estrogens all increase proliferation and anchorage-independent growth of MCF-7 BUS cells to a similar extent, suggesting that custom-compounded bHT may in fact not be a safer alternative to conventional HT. Furthermore, our results showing that E and E are not weak estrogens, and that E does not antagonize the activity of E, suggest that the rationale behind the use of E and E in custom-compounded bHT formulations should be readdressed. Taken together, the results indicating that there is mostly no difference between the custom-compounded bioidentical estrogens, commercially available estrogen standards and synthetic EE, at concentrations reflecting serum levels in women using estrogen-containing HT, suggest that there is no clear advantage in choosing bHT above conventional HT.