Inhibition of type 2 17beta-hydroxysteroid dehydrogenase by estradiol derivatives bearing a lactone on the D-ring: structure-activity relationships

The peripheral conversion of steroid precursors into biologically active forms can be a major source of steroid synthesis, and these steroids support the growth of hormone-dependent diseases. The 17beta-hydroxysteroid dehydrogenase (17beta-HSD) enzyme family is involved in the biosynthesis of active steroids and its inhibition constitutes an interesting approach for treating estrogen- and androgen-dependent cancers. We previously found that a compound formed by the introduction of a spiro-gamma-lactone at position 17 of estradiol (E2) produces a significant inhibition of type 2 17beta-HSD. To optimize the inhibitory potency of such compounds, we synthesized a series of estradiol derivatives bearing a lactone on the D-ring and tested their ability to inhibit the type 2 17beta-HSD transformation of 4-androstenedione into testosterone. The results of our structure-activity relationship study determined the importance of the 17beta-orientation of the oxygen atom. Indeed, the 17beta-O-isomer of spiro-gamma-lactone-E2 is a much more potent inhibitor than the 17alpha-O-analog (respectively 85 and 9% of inhibition at 1 microM). The carbonyl function is essential since the percentage of inhibition shifts from 85 to 30%, 15, or 3%, when the carbonyl group is transformed into a hydroxyl, a methoxy or a methylene (cycloether) group, respectively. Our results lead us to realize the importance of the spirolactone versus the C17beta-O/C16beta lactone (respectively 32 and 2% of inhibition at 0.1 microM, for the same size of lactone ring). The optimal size for the spirolactone was also established to be six members. All the types of substituents (methyl, dimethyl, allyl, propyl, and methoxycarbonyl) that we added on the spiro-delta-lactone moiety decreased the inhibitory activity, suggesting steric restrictions for the space that can be occupied in proximity of the spiro-delta-lactone functionality. 17-(Spiro-delta-lactone)-E2, compound 6, was thus the most potent inhibitor of type 2 17beta-HSD with a K(i) value of 29 +/- 5 nM. This compound reversibly inhibits type 2 17beta-HSD in a non-competitive manner.