An aromatase/CYP19A1 inhibitor (IC50 = 15 nM for human placental aromatase/CYP19A1); selective for aromatase/CYP19A1 over CYP1A2, CYP2A6, CYP2C9, CYP2D6, and CYP3A (IC50s = 27-650 μM); blocks ovulation in mature female rats and androstenedione-stimulated uterine development in pubertal female rats at 0.1 mg/kg; inhibits peripheral aromatase/CYP19A1 and reduces plasma estradiol concentrations in male pigtailed monkeys at ≥0.1 mg/kg; reduces tumor incidence and the number of tumors well as increases latency to tumor appearance in a rat model of premenopausal mammary tumorigenesis at 0.5 mg/kg