A new estrogen receptor antagonist--an overview of available data.

Fulvestrant ('Faslodex'), a novel estrogen receptor (ER) antagonist with no known agonist effects that downregulates the ER, is the first in a new class of antiestrogen. Consequently, fulvestrant is not cross-resistant with other endocrine therapies. In previously untreated postmenopausal women, fulvestrant brought about dose-dependent reductions in ER and progesterone receptor (PgR) protein concentrations and produced no demonstrable estrogen agonist effect. Fulvestrant has demonstrated clinical benefit rates of 69% in postmenopausal women with advanced tamoxifen-resistant breast cancer. In two global phase III trials, fulvestrant was shown to be at least as effective as the aromatase inhibitor anastrozole with similar efficacy for time to progression, objective response and clinical benefit in patients progressing on prior endocrine therapy. Fulvestrant was also well tolerated and, similar to anastrozole, maintained a good quality of life. As a once-monthly intramuscular injection, administered during clinic visits, fulvestrant offers patient compliance advantages, making this new hormonal agent a viable therapeutic option for the palliative treatment of postmenopausal women with endocrine-responsive advanced breast cancer.