C16 and C17 derivatives of estradiol as inhibitors of 17 beta-hydroxysteroid dehydrogenase type 1: chemical synthesis and structure-activity relationships.

As a first part of our research focused on the synthesis of 17 beta-HSD type 1 inhibitors without estrogenic activity, we needed to identify a small, easy-to-handle pharmacophore able to block the enzymatic activity. Previous studies on the active site of the enzyme by affinity labeling gave us a basis for the design of steroidal inhibitors derivatives. Several estradiol derivatives bearing a short (three carbons) side chain in position 17 alpha or 16 alpha were synthesized and tested for their ability to inhibit the transformation of estrone into estradiol by 17 beta-HSD type 1 (cytosolic fraction of human placenta). We found that 16 alpha-derivatives of estradiol gave better 17 beta-HSD inhibition than their corresponding 17 alpha analogs. Among several chemical groups used in this study, we conclude that better 17 beta-HSD inhibition was obtained for compounds with a good leaving group at the end of side chain. Thus, an iodopropyl or a bromopropyl side chain at C16 alpha of estradiol (E2) inhibit efficiently the 17 beta-HSD type 1 with IC50 values of 0.42 and 0.46 microM, respectively. Their 17-keto analogs inhibit also the enzyme activity similarly. Since this kind of compounds inhibit the 17 beta-HSD type 1 in time-dependent manner and that enzymatic activity cannot be restored later, we conclude to inhibitor of inactivator type. This conclusion is in accordance with the correlation observed between the ability of leaving group to dissociate and their potency to inhibit 17 beta-HSD type 1. We have also observed that additional addition of untritiated estrone protect the enzyme against the inactivation caused by 16 alpha-bromopropyl-E2 suggesting a competitive inhibitor of 17 beta-HSD. The bromopropyl pharmacophore was then selected to be further added onto an antiestrogenic steroid nucleus.