A nonsteroidal inhibitor of estrogen synthesis that resembles paclitaxel in chemical structure. As a third-generation aromatase inhibitor, letrozole selectively and reversibly inhibits aromatase, a cytochrome P-450 enzyme complex found in many tissues including those of the premenopausal ovary, liver, and breast; aromatase catalyzes the aromatization of androstenedione and testosterone into estrone and estradiol, the final step in estrogen biosynthesis. In estrogen-dependent breast cancers, anastrozole may inhibit tumor growth. (NCI04)