Aromatase inhibitors - which work by lowering estrogen levels - were found to be more effective than tamoxifen in reducing deaths among women with ER-positive breast cancer.
Lead study author Prof. Mitch Dowsett, of The Institute for Cancer Research (ICR) in the UK, and colleagues publish their findings in The Lancet.
It is estimated that around 231,840 women will be diagnosed with invasive breast cancer this year, and more than 40,000 will die from the condition.
Estrogen receptor-positive (ER-positive) breast cancer accounts for around 70% of all cases, in which breast cancer cells contain receptors that attach to the hormone estrogen. When the hormone binds to these receptors, this can encourage the growth of breast cancer tumors.
At present, women with ER-positive breast cancer may be treated with tamoxifen - a drug that prevents estrogen from binding to receptors in breast cancer cells. The drug is primarily prescribed for women with early-stage breast cancer who have undergone surgery, radiation and/or chemotherapy in order to prevent recurrence.
According to Prof. Dowsett and colleagues, taking tamoxifen for 5 years is associated with a 30% reduction in breast cancer death. But their study suggests another class of hormonal drugs - called aromatase inhibitors - may be more effective.
Aromatase inhibitors lower estrogen levels by preventing the enzyme aromatase - found in fat tissue - from changing other hormones into estrogen. As such, there is less estrogen to bind to receptors in breast cancer cells.
In the US, aromatase inhibitors are currently used to treat early-stage breast cancer in postmenopausal women who have been treated with tamoxifen for around 2-3 years.
Previous research suggests aromatase inhibitors are more effective for reducing breast cancer recurrence than tamoxifen, though how they impact survival has been unclear.
Aromatase inhibitors reduced breast cancer mortality by 40%
For their study, the team analyzed the data of nine clinical trials involving 31,920 postmenopausal women with early-stage ER-positive breast cancer. The women in the trials had either received no hormonal therapy or had used aromatase inhibitors or tamoxifen for at least 5 years.
Compared with women who had not received hormone therapy, those who received aromatase inhibitors were 40% less likely to die from breast cancer in the 10 years after treatment initiation. Women who took tamoxifen were at around 30% lower risk of breast cancer death.
Commenting on their findings, Prof. Dowsett says:
"Our global collaboration has revealed that the risk of postmenopausal women with the most common form of breast cancer dying of their disease is reduced by 40% by taking 5 years of an aromatase inhibitor - a significantly greater protection than that offered by tamoxifen.
Aromatase inhibitors remove only the tiny amount of estrogen that remains in the circulation of women after the menopause - but that's enough to have a substantial impact on a wide range of ER-positive tumors, despite their extraordinary differences at the molecular level."
However, the researchers note that treatment with aromatase inhibitors may present a number of side effects. These can include shortness of breath, chest pain, hot flashes, muscle or joint pain and depression.
"It's important to ensure that women with significant side effects are supported to try to continue to take treatment and fully benefit from it," notes Prof. Dowsett.