Endocrine Abstracts

Estetrol (E4) is a natural estrogen produced by the human fetal liver during pregnancy. E4 is the estrogenic component of a new combined oral contraceptive and is also in development for its use as menopausal hormone therapy. E4 displays a selective binding to human estrogen receptors (ER). In humans, after oral administration, E4 undergoes phase 2 metabolism leading to the formation of the two major metabolites E4-3-glucuronide and E4-16-glucuronide. The objectives of this study were (a) to compare the estrogenic activity of E4 to estradiol (E2) and ethinylestradiol (EE) induced via both ERα and ERβ; (b) to determine the estrogenic activity of E4 metabolites and (c) to characterize the ER coregulator recruitment profile induced by E4. In order to determine the estrogenic activity of E4 and its metabolites, a luciferase reporter assay (ER-CALUX bioassay) was conducted. Briefly, human osteoblastic U2-OS cells stably expressing ERα or ERβ, as well as an estrogen-responsive luciferase reporter gene, were exposed to increasing concentrations of E4 or metabolites for 24 h. Reference estrogens E2 and EE were also included as comparators. The coregulator recruitment assay was performed with the MARCoNI technology, and the binding pattern of ERα to 154 coregulator-derived peptides was analyzed. E4 displayed estrogenic activity in both ERα and ERβ assays. However, the potency of E4 was lower than that of E2 or EE. In addition, the potency of the metabolites was several hundred-fold lower than the potency of the parent molecule E4. The EC₅₀ value and relative transactivating potency of E4 and the metabolites with E2 and E4 as a benchmark, respectively, are presented below:

In conclusion, E4 induces transcriptional activity via both ERα and ERβ, although with a lower potency than E2 and EE. E4 induces the recruitment of the same profile of coregulators to ER as E2, suggesting that E4 and E2 induce transactivation via the same molecular mechanisms. Glucuronidation of E4 leads to the formation of metabolites with negligible estrogenic activity.