Synthesis of 2,2-dimethyl-4-hydroxy-4-androstene-3,17-dione as an inhibitor of aromatase

Synthesis and Biochemical Properties of 6-Bromoandrostenedione Derivatives with a 2,2-Dimethyl or 2-Methyl Group as Aromatase Inhibitors

To gain insight into the mechanism for irreversible inactivation of aromatase by 6beta-bromoandrostenedione (1), one of the earliest discovered suicide substrates, in relation to the catalytic function of the enzyme, the 2,2-dimethyl derivative of compound 1, steroid 4, and its 6alpha-isomer 5, as well as 2-methyl-1,4-diene steroid 8 and its 6alpha-bromide 10, were synthesized. All of the steroids inhibited aromatase activity in human placental microsomes with apparent K(i)'s ranging between 10 and 81 nM. The 2,2-dimethyl-6beta- and 6alpha-bromo steroids 4 and 5 were extremely powerful inhibitors (K(i): 14 and 10 nM, respectively), but these two did not cause a time-dependent inactivation of aromatase in the presence of NADPH; in contrast, the 2-methyl-1,4-diene steroids 8 and 10 caused time-dependent inactivation with apparent k(inact) of 0.035 and 0.071 min(-1), respectively, in a suicide manner. These results indicate that the 2,2-dimethyl function of the 6beta-bromide 4 would prevent the inactivation of aromatase caused by inhibitor 1 in a suicide manner, probably through steric activity, whereas the 2-methyl group of steroid 8 did not significantly affect the suicidal inactivation by the parent 1,4-diene steroid, a typical suicide substrate.

Probing the active site of aromatase with 2-methyl-substituted androstenedione analogs

To gain insight into the spatial nature of the androstenedione (AD) binding (active) site of aromatase in relation to the catalytic function of the enzyme, we synthesized 2,2-dimethylAD (4), 2beta- and 2alpha-methylADs (5 and 6), 19-oxygenated derivatives of compounds 4 and 6, and 2-methyleneAD (17), and we then tested their inhibitory activity as well as their aromatase reaction (aromatization for 2-methyl and 2-methylene analogs or 19-oxygenation for 2,2-dimethyl steroids) with human placental aromatase. 2-Methyl and 2-methylene steroids 5, 6, and 17 were good competitive inhibitors of aromatase (K(i)=22-68nM), but less effective compared to the 2,2-dimethyl analog 4 (K(i)=8.8nM), indicating that a combination of 2beta- and 2alpha-methyl moieties is essential for the formation of a thermodynamically stable inhibitor-aromatase complex. A series of 2alpha-methyl steroids were good substrates for aromatase, whereas 2beta-methyl steroid 5 was an extremely poor substrate, and a series of 2,2-dimethyl steroids did not serve as substrate, suggesting that a 2beta-methyl moiety of the 2,2-dimethyl and 2beta-methyl steroids would prevent the aromatase reaction probably due to steric hindrance in each case. The 2-methylene compound 17 was also aromatized to produce 2-methylestrogen with a low conversion rate where the 1,4-diene structure may have been created before the C(10)-C(19) bond cleavage. Kinetic analysis of the aromatization of androgens revealed that a good substrate was not essentially a good inhibitor for aromatase.

Kinetic analysis of reversible inhibition of 16alpha-hydroxyandrostenedione aromatization in human placental microsomes by suicide substrates of androstenedione aromatization

To gain insight into the catalytic function of aromatase and its substrate specificity, we studied reversible inhibition of 16alpha-hydroxyandrostenedione (16alpha-OHAD) aromatization in human placental microsomes by several suicide substrates of androstenedione (AD) aromatization, including 4-hydroxyAD (1), 6-oxoAD (2) and its 19-hydroxy analogue 3, androst-5-ene-4,7,17-trione (4), and 10beta-acetoxyandrost-5-en-7,17-dione (5) that, in contrast, do not cause a suicide inactivation of 16alpha-OHAD aromatization. All inhibitors examined blocked 16alpha-OHAD aromatization in a competitive manner with apparent K(i) values ranging from 0.50 to 980 nM. The relative K(i) values between inhibitors 1-5 obtained in the 16alpha-OHAD aromatization experiments were markedly different from those obtained in the AD aromatization experiments. The results predict that all inhibitors examined bind to the 16alpha-OHAD binding site in a manner that does not cause suicide inactivation of 16alpha-OHAD aromatization. These findings would be useful for understanding the active (binding) site structure as well as the catalytic function of aromatase.

A- or B-ring-substituted derivatives of androst-4-ene-3,6,17-trione as aromatase inhibitors. Structure-activity relationships

2,2-Dimethylandrost-4-ene-3,6,17-trione (5) and its 4-methoxy- (7) and 4-hydroxy- (8) derivatives were synthesized. 7 alpha-Acetoxy-4-ene-3,6-dione steroid 2 was also prepared by the improved method involving the lead tetraacetate oxidation of androst-4-ene-3,6,17-trione (1). These steroids along with the 2-acetoxy-(11 and 12), 2-substituted 1-ene- (9 and 10), and 4-substituted (13-15) derivatives of compound 1 were evaluated as inhibitors of human placental aromatase. All the steroids, except the 2-acetoxy-1-ene 10 and the 2 beta-acetate 11 of which Ki values were not determined because of their poor inhibitory activities, blocked aromatase in a competitive manner. Compounds 5 and 8 as well as the 4-hydroxy steroid 15 were potent inhibitors (Ki: 25-42 nM) whereas the inhibitory activities of steroids 2, 7, 9, 13, and 14 were good to fair, respectively (Ki: 160-810 nM). Inhibitors 2 and 15 inactivated the enzyme in a time-dependent manner in the presence of NADPH but the 2,3-dimethyl derivatives 5 and 8 did not. Androstenedione blocked the inactivation but L-cysteine did not. The results suggest that the 2 beta-methyl group would prevent the aromatase-catalyzed oxygenation at C-19 of the dimethyl steroids 5 and 8 most likely through the steric reasons.