Biochemical and endocrine properties of a mechanism-based inhibitor of aromatase.

The conversion of androgens to estrogens by aromatase represents a major alteration in hormonal expression, and its regulation offers a promising method for therapeutic control of disease processes that are hormonally dependent. The design of suicide inhibitors based on enzyme-activated mechanisms provides an attractive approach for regulation of estrogen biosynthesis. MDL 18,962, (10-2(-propynyl)estr-4-ene-3,17-dione), a C-10 substituted analog of the natural substrate androstenedione, was evaluated as a suicide inhibitor of aromatase. Appropriate kinetic evaluations of MLD 18,962 established it to be a highly potent [inhibition constant (Ki) = 4.5 +/- 1.3 nM] irreversible inhibitor of human placental aromatase. The 2-propynyl group was necessary for time-dependent inactivation, as this activity was lost in related compounds. The inactivation process was specific for aromatase, since other P450 enzyme systems are not inactivated by MDL 18,962. This compound exhibited minimal intrinsic hormonal properties, since weak binding affinities were observed for cytosolic androgen, estrogen, or progestin receptors. The stimulation of ovarian aromatase activity by gonadotropins in immature mice was inhibited in animals implanted with 10-mm MDL 18,962 Silastic implants. This inhibition of estrogen biosynthesis suppressed estrogen-dependent uterine growth in these mice.