Preclinical and Clinical Experience with Fulvestrant (Faslodex) in Postmenopausal Women with Hormone Receptor-Positive Advanced Breast Cancer

Fulvestrant (Faslodex) is a new type of estrogen receptor (ER) antagonist that binds, blocks, and degrades the ER, leading to reduced expression of the progesterone receptor (PgR). Unlike the selective ER modulator tamoxifen, fulvestrant is devoid of any known agonist activity. Fulvestrant has a steroidal structure that competitively binds to the ER with an affinity much greater than that of tamoxifen. After binding to fulvestrant, degradation of the ER is accelerated, ultimately resulting in a reduction in cellular ER. Immunohistochemical studies have demonstrated that a single intramuscular injection of fulvestrant results in a dose-dependent decrease in ER and PgR indices and in Ki-67 expression. At the approved 250 mg dose, the decrease in receptor positivity with fulvestrant treatment was greater than that with tamoxifen. Phase III clinical trials have demonstrated the clinical benefit of fulvestrant in the endocrine treatment of breast cancer. Among patients who progressed during adjuvant or first-line endocrine therapy, fulvestrant was at least as effective as anastrozole as second-line treatment for the primary endpoints of objective response and time to progression, and was well tolerated. After a median follow-up of 24.5 months, a combined survival analysis from two Phase III studies has shown that fulvestrant may be considered similar to anastrozole for time to death (27.4 months versus 27.7 months, respectively). For the first-line therapy of advanced breast cancer in postmenopausal women, fulvestrant was shown to be active and well tolerated in a trial that compared fulvestrant 250 mg once monthly and tamoxifen 20 mg once daily. Treatment with fulvestrant has also demonstrated clinical efficacy among patients who progressed following treatment with tamoxifen followed by nonsteroidal aromatase inhibitors. The efficacy of fulvestrant in sequential endocrine therapy and in combination with other agents appears promising and active investigations are ongoing to explore the clinical potential of this novel antiestrogen.