Letrozole is Superior to Anastrozole in Suppressing Breast Cancer Tissue and Plasma Estrogen Levels

• Published in: Clinical cancer research Vol. 14; no. 19; pp. 6330 - 6335
• Main Authors: GEISLER, Jürgen, HELLE, Hilgegunn, EKSE, Dagfinn, DUONG, Nhat K, EVANS, Dean B, NORDBØ, Yngve, AAS, Turid, LØNNING, Per E
• Format: Journal Article
• Language: English
• Published: Philadelphia, PA American Association for Cancer Research 01.10.2008
• Subjects: Aged; Aged, 80 and over; anastrozole; Antineoplastic agents; Antineoplastic Agents - therapeutic use; aromatase inhibitors; Biological and medical sciences; Breast Neoplasms - blood; Breast Neoplasms - drug therapy; Chromatography, High Pressure Liquid; estradiol; Estradiol - metabolism; Estrogens - blood; estrone; Estrone - analogs & derivatives; Estrone - metabolism; estrone sulfate; Female; Gynecology. Andrology. Obstetrics; Humans; letrozole; Mammary gland diseases; Medical sciences; Middle Aged; Nitriles - therapeutic use; Pharmacology. Drug treatments; Postmenopause; Radioimmunoassay; Triazoles - therapeutic use; Tumors; Antineoplastic agent; Biological fluid; Aromatase inhibitor; Breast disease; Enzyme; Anastrozole; Azole derivatives; Antiestrogen; Estrogen; Enzyme inhibitor; Breast cancer; Antihormone; Estrogen synthase; Malignant tumor; Ovarian hormone; Blood plasma; Mammary gland diseases; Triazole derivatives; Sex steroid hormone; Non steroid compound; Letrozole; Cancer
• ISSN: 1078-0432; 1557-3265
• DOI: 10.1158/1078-0432.CCR-07-5221

Purpose: To evaluate the influence of the third-generation aromatase inhibitor letrozole (Femara) on breast cancer tissue levels of estrone (E 1 ), estradiol (E 2 ), and estrone sulfate (E 1 S) in postmenopausal women undergoing primary treatment for locally advanced estrogen receptor/progesterone receptor–positive breast cancers. Experimental Design: Breast cancer tissue samples were collected before and following 4 months of neoadjuvant therapy with letrozole (2.5 mg o.d.), and tissue estrogen levels measured using a highly sensitive RIA after high-pressure liquid chromatography purification. Results: Letrozole suppressed pretreatment tumor levels of E 2 , E 1 , and E 1 S by 97.6%, 90.7%, and 90.1%, respectively. These data reveal that letrozole suppresses tissue estrogen levels significantly below what has previously been recorded with anastrozole (89.0%, 83.4%, and 72.9% suppression, respectively) using the same methods. To confirm the differential effect of letrozole and anastrozole on each plasma estrogen fraction, we re-analyzed plasma samples obtained from a previous intrapatient cross-over study comparing letrozole and anastrozole using an improved RIA (detection limits of 0.67, 1.14, and 0.55 pmol/L for E 2 , E 1 , and E 1 S, respectively). Letrozole consistently suppressed each plasma estrogen fraction below the levels recorded for anastrozole: E 2 (average suppression by 95.2% versus 92.8%; P = 0.018), E 1 (98.8% suppression versus 96.3%; P = 0.003), and E 1 S (98.9% suppression versus 95.3%; P = 0.003). Conclusion: Our data reveals that letrozole (2.5 mg o.d.) is more effective compared with anastrozole (1.0 mg o.d.) with respect to tissue as well as plasma estrogen suppression in patients with postmenopausal breast cancer.