Letrozole (CGS 20267) is an Aromatase Inhibitor. The mechanism of action of letrozole is as an Aromatase Inhibitor.

Aromatase:0.07 nM-20 nM

Letrozole对植入无胸腺裸鼠的人芳香化酶基因（MCF-7Ca）转染的MCF-7细胞的肿瘤生长产生剂量依赖性抑制,在20 mg/kg/天时完全抑制.Letrozole在体内实验中,能抑制芳香酶活性,口服ED50 是1-3 μg/mg.

Letrozole在LH诱导的仓鼠卵巢组织中抑制雌二醇产生,IC50 是0.02 μM,达到350 μM浓度时,不明显影响孕酮的产生。Letrozole在LH在促肾上腺皮质激素诱导的大鼠肾组织中,抑制醛甾酮的产生,IC50 是210 μM. Letrozole在非细胞系统中,IC50是1-13 nM。Letrozole能有效抑制不同来源的芳香酶,包括人胎盘微粒体、人乳腺癌颗粒部分、大鼠卵巢微粒体、转染芳香酶的MCF-7细胞、JEG-3绒膜癌细胞、CHO细胞、仓鼠卵巢组织、人乳腺癌颗粒部分,IC50分别是11,2,7,0.07,0.07,1.4,20和0.8 nM。

Human placental aromatase activity: The assay is performed in a total volume of 1 mL at 37 ℃. Unless otherwise noted, the incubation mixture contains 11 nM [4- 14C] androstene-3, 17-dione ([4- 14C]A), 24 mM NADPH (tetrasodium salt Type III), the appropriate concentrations of the desired inhibitor, and 120 μg of microsomal protein. The (4- 14C)A is added as a solution in 1.7% ethanol in 0.05 M potassium phosphate buffer (pH 7.4), so that the final concentration of ethanol does not exceed 0.02% (v/v). The reaction is started by the addition of enzyme and stopped after 20 min by the addition of 7 vol of ethyl acetate. The mixture is agitated on a vortex mixer and centrifuged at 600 g for 5 min. The aqueous phase is re-extracted with 7 vol of ethyl acetate, and the combined extracts are evaporated to dryness using an Evapo-Mix. Over 99% of the radio- active of [4- 14C] added is recovered using this extraction system. The residue obtained is dissolved in 150 μL acetone, and 100 μL aliquots are chromatographed for 65 min on thin-layer plates precoated with silica gel 60 using ethyl: acetate: isooctane (140:60, v/v; system A) or toluene: chloroform: methanol (70:140:20; system B). The radioactive zones of the plate are located with a Berthold LB 2760 thin-layer scanner. The radioactive estradiol (E2) and estrone (E1) neaks are identified by comparison with authentic standards. The corresponding bonding band of silica gel is transferred to vials containing 10 mL of scintillation fluid, and counted with a 6880 Liquid Scintillation system.

Cells are seeded in duplicate at 5,000 to 10,000 cells per well in 24-well plates. The day after plating, different concentrations of Letrozole are added. At the end of incubation, cells are trypsinizated and placed in Isotone solution and counted immediately using a Coulter particle-counter. (Only for Reference)