EFFECT OF AROMATASE INHIBITORS ON ESTROGEN 2-HYDROXYLASE IN RAT-LIVER

The effect of aromatase inhibitors, 4-hydroxyandrostenedione, CGS 1694 9A and aminoglutethimide on the inhibition of estrogen 2-hydroxylase a ctivity 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 l iquid chromatography and employing estradiol as a substrate, the IC50 values were 2.2, 98, 110 and 908 muM for the reference compound ketoco nazole and the aromatase inhibitors, 4-hydroxyandrostenedione, CGS 169 49A and aminoglutethimide, respectively. Similar IC50 values were obta ined using estrone as a substrate and by a tritiated water method empl oying estradiol as a substrate. The K(m) value for estrogen 2-hydroxyl ase with estradiol as a substrate using a tritiated water method was 4 .3 muM with a V(max) of 11.89 nmol/h/mg. Ketoconazole, CGS 16949A and aminoglutethimide exhibited non-competitive inhibition whereas 4-hydro xyandrostenedione appeared to be a competitive inhibitor of estrogen 2 -hydroxylase. The K(i) values were 2.6, 72, 114 and 958 muM for ketoco nazole, 4-hydroxyandrostenedione, CGS 16949A and aminoglutethimide, re spectively. All three aromatase inhibitors were weak inhibitors of est rogen 2-hydroxylase as compared to the reference drug, ketoconazole. F ollowing 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), th e 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 estroge n 2-hydroxylase activity in vitro and show no evidence of inhibition i n 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 m ay lower estrogen levels not only by primarily inhibiting their synthe sis but also by inducing the metabolism of estrogens.