AstraZeneca's Faslodex has been approved for the treatment of hormone receptor positive metastatic breast cancer. It is indicated for use in postmenopausal women whose disease has progressed after receiving anti-estrogen therapy (such as tamoxifen). The therapy is given as a once-a-month intramuscular injection.
After breast cancer diagnosis, hormone receptor tests can be conducted to determine whether a patient's cancer is responsive to estrogen (known as estrogen receptor positive). If the cancer is found to be estrogen receptor positive, treatment options include an anti-estrogen therapy such as tamoxifen, which blocks the estrogen receptor. Faslodex may be an effective alternative for patients who are not successfully treated with tamoxifen because of its mechanism of action. Instead of blocking the estrogen receptor, Faslodex targets and degrades the estrogen receptors present in breast cancer cells.
The effectiveness of Faslodex was demonstrated in clinical trials comparing the drug to the aromatase inhibitor Arimidex (anastrozole), which works by reducing the amount of estrogen in the body. Two randomized trials in North America and Europe were conducted in postmenopausal women with locally advanced or metastatic breast cancer. The double-blind North American trial included 400 women, while the open, randomized European trial included 451 women. All subjects had progressed after previous therapy with an anti-estrogen or progestin.
Subjects were randomized to receive either Faslodex 250 mg intramuscularly once a month or Arimidex 1 mg orally once a day. All subjects were assessed monthly for the first three months and then every three months thereafter. Results showed that Faslodex was at least as effective as Arimidex. In the North American trial, objective tumor response rates were 17.0% for both the Faslodex- and Arimidex-treated groups. In the European trial, Faslodex produced a 20.3% objective tumor response rate, compared to 14.9% for the Arimidex group. Time to progression for Faslodex versus Arimidex was 5.5 months versus 3.5 months in the North American trial and 5.5 months versus 5.2 months in the European trial.
Adverse events reported in clinical testing of Faslodex include (but are not limited to) the following:
Many breast cancers have estrogen receptors (ER), and the growth of these tumors can be stimulated by estrogen. Fulvestrant is an estrogen receptor antagonist that binds to the estrogen receptor in a competitive manner with affinity comparable to that of estradiol. Fulvestrant downregulates the ER protein in human breast cancer cells.
In a clinical study in postmenopausal women with primary breast cancer treated with single doses of Faslodex 15-22 days prior to surgery, there was evidence of increasing down regulation of ER with increasing dose. This was associated with a dose-related decrease in the expression of the progesterone receptor, an estrogen-regulated protein. These effects on the ER pathway were also associated with a decrease in Ki67 labeling index, a marker of cell proliferation.
In vitro studies demonstrated that fulvestrant is a reversible inhibitor of the growth of tamoxifen-resistant, as well as estrogen-sensitive human breast cancer (MCF-7) cell lines. In in vivo tumor studies, fulvestrant delayed the establishment of tumors from xenografts of human breast cancer MCF-7 cells in nude mice. Fulvestrant inhibited the growth of established MCF-7 xenografts and of tamoxifen-resistant breast tumor xenografts. Fulvestrant resistant breast tumor xenografts may also be cross-resistant to tamoxifen.
Fulvestrant showed no agonist-type effects in in vivo uterotropic assays in immature or ovariectomized mice and rats. In in vivo studies in immature rats and ovariectomized monkeys, fulvestrant blocked the uterotrophic action of estradiol. In postmenopausal women, the absence of changes in plasma concentrations of FSH and LH in response to fulvestrant treatment (250 mg monthly) suggests no peripheral steroidal effects. (from Faslodex Prescribing Information)
If you would like to learn more about breast cancer, please visit the National Cancer Institute.