Patients with hormone-sensitive breast cancer who have responded to tamoxifen may receive additional benefit from a second endocrine agent following progression or relapse after tamoxifen therapy. Fulvestrant (Faslodex((R)), ICI 182780, AstraZeneca Pharmaceuticals; Wilmington, Delaware) is a selective antagonist of estrogen designed to have no estrogenic effects. Lack of aqueous solubility led to the development of a parenteral formulation for monthly intramuscular administration. Fulvestrant has been shown to inhibit the proliferative effects of estrogen on sensitive tissues in vitro and in vivo, and is without apparent measurable estrogenic activity. The data upon which marketing approval for fulvestrant was based are summarized below. Eight hundred fifty-one postmenopausal women with advanced breast cancer were enrolled in two phase III studies, 400 in a North American double-blind study and 451 in a European open-label study, comparing the efficacy and safety of fulvestrant with anastrozole. Four hundred twenty-eight patients were randomized to receive fulvestrant 250 mg monthly by intramuscular injection and 423 patients were to receive anastrozole 1 mg daily. Patients were considered hormone sensitive either by receptor status or previous response to endocrine therapy. Over 96% of patients had previously received tamoxifen, either in the adjuvant setting or as treatment for metastatic disease. The primary study end points were response rate and time to progression. Response rates for patients treated with fulvestrant were 17% and 20% in the North American and European trials, respectively, compared with 17% and 15% in the anastrozole treatment arms. There were no statistically significant differences in response rates, time to progression, or survival between treatment arms in either study. The most common adverse events attributed to the treatment (>10%) were injection-site reactions and hot flashes. Common events (1%-10%) included asthenia, headache, and gastrointestinal disturbances (nausea, vomiting, and diarrhea), as well as rash and urinary tract infections. A small increase in joint disorders was reported in the anastrozole-treated patients. On April 25, 2002, fulvestrant 250 mg by monthly intramuscular injection was approved by the U.S. Food and Drug Administration for the treatment of hormone receptor-positive metastatic breast cancer in postmenopausal women with disease progression following antiestrogen therapy. Approval was based on similarity of response rates and time to progression between fulvestrant and anastrozole.