Receptor profiling and endocrine interactions of tibolone

The receptor profiles and in vivo activity of tibolone, and its primary metabolites, Delta(4)-isomer, and 3alpha- and 3beta-hydroxytibolone, were studied and compared to those of structurally related compounds. The Delta(4)-isomer was the strongest binder and activator of the progesterone receptor (PR); tibolone was 10 times weaker in binding and half as potent in transactivation of PR; 3alpha- and 3beta-hydroxytibolone did not bind or activate PR. In rabbits oral tibolone produced a minor progestagenic effect in the endometrium, whereas co-administration of tibolone and the anti-estrogen ICI 164,384 unmasked tibolone's progestagenic effect. 3-Hydroxytibolones were the strongest binders and activators of the estrogen receptors (ERs), with greater affinity for ERalpha than for ERbeta. Tibolone showed weaker binding and activation of both ERs and the Delta(4)-isomer has a binding and activation activity of less than 0.1% of E2 for ERalpha or ERbeta. Tamoxifen and 4-hydroxytamoxifen showed partial ERalpha agonistic effects with a maximal response of 12% and raloxifene of 3-5%. Oral administration of 1mg tibolone to ovariectomized rats induced an estrogenic effect on vaginal epithelium. The Delta(4)-isomer was a stronger binder and activator of the androgen receptor (AR) than tibolone; both 3-hydroxytibolones did not bind or activate AR. Introducing a 7alpha-methyl group decreased progestagenic and increased androgenic activity. We conclude that the progestagenic and androgenic activities of tibolone are mediated by the Delta(4)-isomer, and the estrogenic activity, by the 3-hydroxytibolones. The estrogenic activity of the 3-hydroxytibolones masked the progestagenic activity of tibolone in rabbit endometrium. Full estrogenic response was observed in rat vaginal tissue after oral administration of tibolone.