Conjugated estrogens--the natural SERMs.

Tissue selective and metabolite replacement therapy may become a new aspect in hormone replacement therapy (HRT). In addition to the naturally secreted hormones, there are also the later formed metabolites that exert a characteristic pharmacological profile. This mechanism is well known in thyroid replacement therapy, when triiodothyronine, the metabolite of thyroxine, is added to substitution therapy. The same is true for testosterone replacement therapy, when dihydrotestosterone is used for replacement. Also in menopausal HRT these aspects will gain tremendous importance. Progesterone metabolites have a strong clinical potency as neurosteroids, and estradiol metabolites are important factors in angiogenesis and angiostasis. Conjugated estrogens consist of different metabolites such as 16-hydroxy-equilin, which has no angiogenetic effect compared with 16-hydroxy-estrone. Estrone sulfate, the main component in conjugated estrogens, can be activated into estrone and 17 beta-estradiol in a tissue specific manner. This aspect will become of interest in clinical practice with HRT.