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Additive Antitumor Effect of Aromatase Inhibitor Letrozole and Antiestrogen Fulvestrant in a Postmenopausal Breast Cancer Model

¹ Department of Pharmacology and Experimental Therapeutics, School of Medicine and ² Division of Biostatistics, Greenebaum Cancer Center, University of Maryland, Baltimore, Maryland

Requests for reprints: Angela M.H. Brodie, Department of Pharmacology and Experimental Therapeutics, University of Maryland School of Medicine, Health Science Facilities 1, Room 580, 685 West Baltimore Street, Baltimore, MD 21201. Phone: 410-706-3137; Fax: 410-706-0032; E-mail: abrodie{at}umaryland.edu.

Blocking estrogen receptors with antiestrogens and blocking estrogen synthesis with aromatase inhibitors are two strategies currently being used for reducing the effect of estrogen in postmenopausal estrogen receptor–positive breast cancer patients. To optimize these treatment strategies, we have investigated whether tumor progression can be delayed by combining the pure antiestrogen fulvestrant with the nonsteroidal aromatase inhibitor letrozole. These studies were done in ovariectomized, athymic mice bearing tumors of estrogen receptor–positive human breast cancer cells stably transfected with the aromatase gene (MCF-7Ca). Groups of mice with equivalent tumor volumes were injected s.c. daily with vehicle (control; n = 6), fulvestrant (1 mg/d; n = 7), letrozole (10 µg/d; n = 18), or letrozole (10 µg/d) plus fulvestrant (1 mg/d; n = 5). All treatments were effective in suppressing tumor growth compared with controls (P < 0.001). Tumor volumes of the fulvestrant-treated group had doubled in 10 weeks. After 19 weeks of letrozole (10 µg/d) treatment when tumors had nearly doubled in volume, mice (n = 18) were assigned to second-line therapy with letrozole (100 µg/d; n = 6), tamoxifen (100 µg/d; n = 6), or remained on letrozole treatment (10 µg/d; n = 6). However, tumors continued to increase in volume in these groups. Tumors of animals treated with the combination of letrozole plus Faslodex regressed over 29 weeks of treatment by 45%. Thus, the combination of letrozole plus fulvestrant was more effective in suppressing tumor growth than either letrozole or fulvestrant alone or sequential therapies with tamoxifen or a higher dose of letrozole (100 µg/d).

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