Aromatase inhibitors: Effect of ring A and ring B unsaturation on aromatase inhibition by 4-thiosubstituted derivatives of 4-androstene-3,17-dione

The synthesis and biological evaluation of 4-thiosubstituted derivatives of 1,4-androstadienedione, 4,6-androstadienedione, and 1,4,6-androstatrienedione as inhibitors of aromatase are described. Inhibitory activity of synthesized compounds was assessed using a human placental microsomal preparation as the enzyme source and [1β-³H] androstenedione as substrate. Under initial velocity assay conditions of low product formation, the inhibitors demonstrated potent inhibition of aromatase, with apparent K_is ranging from 9.8 to 137 nM and with K_m for androstenedione being 38 nM. However, unlike other 1,4-androstadienediones and 1,4,6-androstatrienediones in which time-dependent inactivation was observed, the 4-thiosubstituted analogs were found to be competitive inhibitors and did not produce any time-dependent inactivation of aromatase.