Aromatase inactivation by 2-substituted derivatives of the suicide substrate androsta-1,4-diene-3,17-dione

To gain the structure–activity relationship of Δ1-androstenediones (Δ1-ADs) as mechanism-based inactivator of aromatase, series of 2-alkyl- and 2-alkoxy-substitiuted Δ1-ADs (6 and 9) as well as 2-bromo-Δ1-AD (14) were synthesized and tested. All of the inhibitors examined blocked aromatase in human placental microsomes in a competitive manner. In a series of 2-alkyl-Δ1-ADs (6), n-hexyl compound 6f was the most powerful inhibitor with an apparent Ki value of 31 nM. The inhibitory activities of 2-alkoxy steroids 9 decreased in relation to length of the alkyl chain up to n-hexyloxy group (Ki: 95 nM for methoxy 9a). All of the alkyl steroids 6 along with the alkoxy steroid 9, except for the ethyl and n-propyl compounds 6b and 6c, caused a time-dependent inactivation of aromatase. The inactivation rates (kinact: 0.020–0.084 min−1) were comparable to that of the parent compound Δ1-AD. The inactivation was prevented by the substrate AD, and no significant effect of l-cysteine on the inactivation was observed in each case. The results indicate that the 2-hexyl compound 6f act as the most powerful mechanism-based inactivator of aromatase among Δ1-AD analogs and may be submitted to the preclinical study in estrogen-dependent breast cancer.