Cross-over study of suppression of serum estrogens during neoadjuvant breast cancer treatment with letrozole versus exemestane

The aromatase inhibitor letrozole and the aromatase inactivator exemestane can be used for endocrine treatment of ER-positive breast cancer. Although both drugs inhibit CYP19 and have been used for decades, a direct head-to-head comparison of their ability to decrease estrogen synthesis is lacking. A multicenter study in Norway compared the impact of both drugs on serum levels of estrone (E1) and estradiol (E2) among postmenopausal women receiving neoadjuvant endocrine therapy in a cross-over design. Starting treatment with letrozole resulted in stronger suppression of E1 and E2 than starting with exemestane. After cross-over from letrozole to exemestane, E1 and E2 levels increased, while after cross-over from exemestane to letrozole, E1 and E2 levels decreased further. The authors conclude letrozole therapy resulted in more profound suppression of serum E1 and E2 levels compared to exemestane.