Mechanism of Action

Anastrozole inhibits aromatase, the enzyme that catalyzes the final step in estrogen production. Anastrozole is an oral, competitive, non-steroidal inhibitor of aromatase and is less likely to exhibit agonist or antagonist steroidal properties.(6) The formation of adrenal corticosteroids or aldosterone is not affected by anastrozole; only serum estradiol concentrations are affected by anastrozole. In postmenopausal women, the principal source of circulating estrogens is from the conversion of adrenal and ovarian androgens (androstenedione and testosterone) to estrogens (estrone and estradiol) by aromatase in peripheral tissues. Inhibition of aromatase may result in a more complete estrogen block than surgical ablation. Extraglandular sites are more amenable to aromatase inhibition by anastrozole than are premenopausal ovaries. Inhibiting the biosynthesis of estrogens is one way to deprive the tumor of estrogens and to restrict tumor growth. Estradiol plasma concentrations decrease about 80% from the baseline with continued dosing of anastrozole.(7) Aromatase inhibitors might also inhibit estrogen production at the tumor cell. However, tumor production of estradiol may be insignificant because aromatase activity appears to be low.(8) Anastrozole has little or no effect on CNS, autonomic, or neuromuscular function.