4-Oxygenated androst-5-en-17-ones and their 7-oxo derivatives as aromatase inhibitors

A series of androst-5-ene-4,7-diones and 4-oxygenated androst-5-enes were synthesized and tested for their ability to inhibit aromatase in human placental microsomes. All of the steroids examined inhibited the enzyme in a competitive manner. The inhibitory activity of 4β-hydroxy-5-ene steroid 7 ( K i = 25 nM) was much more powerful than that of the parent 5-ene steroid 11 ( K i = 78 nM), whereas 4β-acetate 8 and 4-oxo analog 5 ( K i = 90 and 120 nM, respectively) were less potent than compound 11. This indicates that a hydrogen bonding between a hydroxy group of the 4β-ol 7 and a residue of the active site of aromatase plays an important role in its binding. The 5-en-4-one steroid 5 did not cause a time-dependent inactivation of aromatase. In contrast, 5-ene-4,7-dione 13 as well as its 19-hydroxy and 19-oxo analogs 19 and 20 caused the time-dependent inactivation only in the presence of NADPH in air with the k inact values ranging from 0.057 to 0.192 min −1, although their affinities for the enzyme were not high ( K i = 430–6300 nM). The inactivation was prevented by androstenedione, and no significant effect of L-cysteine on the inactivation was observed in each case. These results suggest that oxygenation at C-19 would be at least in part involved in the inactivation caused by the inhibitor 13.