Letrozole is an orally-available, triazole-based inhibitor of the aromatase enzyme complex at an IC50 range of 1-13 nM. It is known to be a Cytochrome P450 inhibitor as well as an estrogen antagonist and has shown promise in the treatment of breast cancer in postmenopausal women. (1, 2)

Letrozole was also found to be a potent in vitro inhibitor of cell proliferation and of type IV collagenases expressed by ER-positive MCF-7 cells. Treatment of MCF-7 cells with 10 nM letrozole led to significant inhibition of MMP levels. Furthermore, concurrent treatment with 17-b-estradiol greatly suppressed estradiol-induced stimulation of MMP levels. (3)