During the past 25 years, the modulation of estrogen action by inhibition of 17β-hydroxysteroid dehydrogenase types 1 and 2 (17β-HSD1 and 17β-HSD2), respectively, has been pursued intensively. In the search for novel treatment options for estrogen-dependent diseases (EDD) and in order to explore estrogenic signaling pathways, a large number of steroidal and nonsteroidal inhibitors of these enzymes has been described in the literature. The present review gives a survey on the development of inhibitor classes as well as the structural formulas and properties of their most interesting representatives. In addition, rationally designed dual inhibitors of both 17β-HSD1 and steroid sulfatase (STS) as well as the first inhibitors of 17β-HSD14 are covered.

17β-HSD14 belongs to the SDR family and oxidizes the hydroxyl group at position 17 of estradiol and 5-androstenediol using NAD as cofactor. The goal of this study was to identify and optimize 17β-HSD14 nonsteroidal inhibitors as well as to disclose their structure-activity relationship. In a first screen, a library of 17β-HSD1 and 17β-HSD2 inhibitors, selected with respect to scaffold diversity, was tested for 17β-HSD14 inhibition. The most interesting hit was taken as starting point for chemical modification applying a ligand-based approach. The designed compounds were synthesized and tested for 17β-HSD14 inhibitory activity. The two best inhibitors identified in this study have a very high affinity to the enzyme with a K equal to 7 nM. The strong affinity of these inhibitors to the enzyme active site could be explained by crystallographic structure analysis, which highlighted the role of an extended H-bonding network in the stabilization process. The selectivity of the most potent compounds with respect to 17β-HSD1 and 17β-HSD2 is also addressed.

STS and 17β-HSD1 are attractive targets for the treatment of estrogen-dependent diseases like endometriosis and breast cancer. The simultaneous inhibition of both enzymes appears more promising than blockage of either protein alone. We describe a designed multiple ligand approach resulting in highly potent dual inhibitors. The most interesting compound 9 showed nanomolar IC values for both proteins, membrane permeability, and no interference with estrogen receptors. It efficiently reversed E1S- and E1-induced T47D cell proliferation.

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