Effect of aromatase inhibitors on estrogen 2-hydroxylase in rat liver

The effect of aromatase inhibitors, 4-hydroxyandrostenedione, CGS 16949A and aminoglutethimide on the inhibition of estrogen 2-hydroxylase activity in rat liver microsomes in vitro and on its induction in vivo has been examined. Estrogen 2-hydroxylase was found to have over twice the affinity for estradiol compared to estrone. Using high pressure liquid chromatography and employing estradiol as a substrate, the IC50 values were 2.2, 98, 110 and 908 microM for the reference compound ketoconazole and the aromatase inhibitors, 4-hydroxyandrostenedione, CGS 16949A and aminoglutethimide, respectively. Similar IC50 values were obtained using estrone as a substrate and by a tritiated water method employing estradiol as a substrate. The Km value for estrogen 2-hydroxylase with estradiol as a substrate using a tritiated water method was 4.3 microM with a Vmax of 11.89 nmol/h/mg. Ketoconazole, CGS 16949A and aminoglutethimide exhibited non-competitive inhibition whereas 4-hydroxyandrostenedione appeared to be a competitive inhibitor of estrogen 2-hydroxylase. The Ki values were 2.6, 72, 114 and 958 microM for ketoconazole, 4-hydroxyandrostenedione, CGS 16949A and aminoglutethimide, respectively. All three aromatase inhibitors were weak inhibitors of estrogen 2-hydroxylase as compared to the reference drug, ketoconazole. Following treatment of rats with aminoglutethimide (40 mg/kg/day; i.p.; for 3 days), estrogen 2-hydroxylase activity was increased by 28 and 30% using estradiol and estrone as substrates, respectively. Following treatment of rats with CGS 16949A (2 mg/kg/day; p.o.; for 3 days), the corresponding increase in estrogen 2-hydroxylase activity was 48 and 44%. The results of this study indicate that the aromatase inhibitors, aminoglutethimide and CGS 16949A are only weak inhibitors of estrogen 2-hydroxylase activity in vitro and show no evidence of inhibition in vivo. On the contrary, there was some evidence to suggest that both aminoglutethimide and CGS 16949A induce estrogen metabolism following repeated administration. Therefore, aminoglutethimide and CGS 16949A may lower estrogen levels not only by primarily inhibiting their synthesis but also by inducing the metabolism of estrogens.