In a study published in The New England Journal of Medicine, researchers suggest that anastrozole (Arimidex), taken in combination with fulvestrant (Faslodex), extends both progression-free and overall survival in hormone receptor-positive metastatic breast cancer.
Progression-free survival is the time during and after treatment the cancer remains stable, and was the trial’s primary endpoint, or goal. The secondary endpoint, overall survival, is the length of time participants lived after enrolling in the study.
Background and Reason for the Study
Anastrozole and fulvestrant are used to treat HR positive breast cancers. Aromatase inhibitors such as anastrozole prevent the body from producing estrogen, the hormone that encourages tumor growth in estrogen receptor-positive (ER positive) breast cancer. Fulvestrant, an estrogen receptor antagonist, binds to estrogen receptors to block their ability to respond to the presence of estrogen.
Individually, both medicines are effective in treating HR positive metastatic breast cancer. Researchers believed that combining the two might be more effective than treatment with anastrozole alone.
SWOG (formerly the Southwest Oncology Group) initially enrolled 707 postmenopausal women with HR positive metastatic breast cancer. Participants had not been treated with chemotherapy, hormonal therapy, or immunotherapy for metastatic disease. They must have completed any early-stage treatment with hormonal therapy or chemotherapy 12 months before enrollment.
During the trial, women were not allowed simultaneous treatment with chemotherapy or other hormonal treatments, but bisphosphonates, or bone-building and bone-strengthening medicines, were allowed.
Of the 707 participants, 694 continued to the trial and were randomly assigned treatment with anastrozole alone or fulvestrant in combination with anastrozole. Anastrozole was taken by mouth daily by both groups; fulvestrant was injected into muscle on days one, 14, and 28, and then every 28 days throughout the trial.
Disease progression was assessed every three months. After progression, overall survival was assessed every six months for the first two years, then annually for the next two years. When a woman’s cancer progressed in the anastrozole-only group, she could cross over into the combination group.
Median progression-free survival was 13.5 months in the group that received anastrozole alone and 15.0 months in the group that received the combination therapy. Improvement was seen over time, with the difference between groups increasing each year of the study.
SWOG researchers analyzed the data according to subgroups of women who had previously been treated with tamoxifen, another medicine that binds to estrogen receptors and has a mechanism of action somewhat similar to fulvestrant. They found past tamoxifen treatment was not statistically significant to chances of responding to the current treatment.
Median overall survival was 41.3 months in the anastrozole-only group and 47.7 months in the combination group. Like progression-free survival, the difference in length of overall survival between groups increased over time.
Anastrozole and fulvestrant also got better results than fulvestrant alone, given after disease progression while on anastrozole.
What About FACT?
This study’s findings contrast those from the earlier FACT trial, which also studied the combination of anastrozole and fulvestrant versus anastrozole alone. FACT found the combination was not better than anastrozole alone.
SWOG researchers noted that FACT had fewer participants than the current study and included women with local recurrence whose cancer progressed on chemotherapy and antiestrogen therapies. The current study’s larger participant pool and eligibility restrictions offer a clearer picture of the combination’s potential, the researchers believe.
Two-thirds of FACT participants previously took tamoxifen, and tamoxifen-takers showed a lesser benefit from the combination treatment than their peers who had not. Because of the differences between these studies, further study is needed to see whether past tamoxifen treatment impacts the effect of combination fulvestrant and anastrozole therapy.
What This Means for You
If you are a postmenopausal woman with HR positive metastatic breast cancer, this study’s findings may impact your future treatments and the clinical trials available to you as further research is conducted on the combination of anastrozole and fulvestrant. Talk to your doctor or medical team about upcoming trials and whether you are eligible to participate.
Mehta, Rita S., Barlow, William E., et al: Combination of Anastrozole and Fulvestrant in Metastatic Breast Cancer. N Engl J Med 2012
Clinical Trial Listing: http://clinicaltrials.gov/ct2/show/NCT00075764?term=mehta+AND+fulvestrant&rank=1