Biological characteristics of the pure antiestrogen fulvestrant: overcoming endocrine resistance.

Understanding the underlying mechanisms responsible for endocrine resistance remains a challenge in improving the treatment of breast cancer. The discovery that growth factor and estrogen receptor (ER) signaling pathways interact in endocrine resistant breast cancer has provided a rationale for disrupting these signaling cascades in ER-positive, endocrine-resistant tumors. In postmenopausal women, the ER signaling pathway may be targeted using fulvestrant ('Faslodex'), a new type of ER antagonist with no agonist effects. Fulvestrant binds, blocks and causes degradation of the ER, culminating in complete abrogation of estrogen-sensitive gene transcription. This unique mechanism of action may result in a lack of cross-resistance with other endocrine agents. Preclinical studies have confirmed the potential of fulvestrant to inhibit the growth of tamoxifen-resistant, as well as tamoxifen-sensitive, human breast cancer cell lines. Clinical studies have demonstrated that fulvestrant is an effective treatment option in postmenopausal women with advanced breast cancer who have progressed on prior endocrine therapy. Furthermore, preclinical studies indicate that combining fulvestrant with growth factor targeted agents, such as the epidermal growth factor receptor (EGFR/HER1) tyrosine kinase inhibitor gefitinib (IRESSA) or the anti-human HER2 monoclonal antibody trastuzumab ('Herceptin'), may result in greater anti-tumor activity than either agent alone. A range of clinical trials are now ongoing to determine whether the combination of growth factor-targeting agents with fulvestrant will delay the onset of endocrine resistance and so provide new strategy for women with hormone receptor-positive advanced breast cancer.