Unlike anti-estrogens such as tamoxifen, which block the activity of estrogen, aromatase inhibitors such as anastrozole actually prevent the formation of estrogen. While both mechanisms, blocking estrogen production and blocking estrogen action, are important for hormone receptor-positive breast cancer, research has suggested that aromatase inhibitors might be more effective and cause fewer side effects than anti-estrogens. In fact, one recent breast cancer study reported that the aromatase inhibitor exemestane caused fewer adverse side effects on memory health than tamoxifen.
A newly published long-term breast cancer study published in The Lancet compared the benefits and risks of the aromatase inhibitor anastrozole to the anti-estrogen tamoxifen in postmenopausal women with early stage breast cancer. For this study, breast cancer patients were treated with either 1 mg anastrozole (3,125 breast cancer patients) or 20 mg tamoxifen (3,116 breast cancer patients) every day for 5 years and then followed for an additional five years. Differences in disease-free survival, time to recurrence, time to distant recurrence, incidence of new contralateral breast cancer, and mortality were compared between these two groups of breast cancer patients. The results of this 10-year study showed that
In the full study population, the aromatase inhibitor anastrozole was more beneficial than tamoxifen in regards to disease-free survival, time to recurrence, and time to distant recurrence.
In patients with hormone receptor-positive breast cancer, treatment with anastrozole substantially improved the odds of longer disease-free survival, later time of recurrence, and later time to distant recurrence.
Breast cancer recurrence was substantially lower in breast cancer patients treated with anastrozole compared to breast cancer patients treated with tamoxifen.
Anastrozole treatment appeared to slightly reduce the number of deaths after breast cancer recurrence in hormone receptor-positive breast cancer patients compared to tamoxifen treatment.
During the 5-year treatment period, anastrozole was linked to a greater number of bone fractures than tamoxifen; however, the incidence of fractures was similar in both groups during the 5-year post-treatment follow-up period.
Breast cancer therapy with anastrozole was linked to fewer treatment-related serious side effects than tamoxifen therapy; though the incidence of side effects was similar during the post-treatment period.
This is an important breast cancer clinical trial in regards to improving breast cancer therapy. Tamoxifen has long been the drug of choice for many hormone-positive breast cancer patients. However, tamoxifen treatment is usually prescribed for a 5-year period or less and is often followed up with additional treatment with aromatase inhibitors. Research over the last several years has indicated that aromatase inhibitors might be a better initial breast cancer therapy choice than tamoxifen because aromatase inhibitors appear to be more effective with fewer side effects. However, many of these studies have been relatively short in duration with smaller numbers of study volunteers. This large, long-term study confirms these earlier studies that aromatase inhibitors like anastrozole might be a more effective and safer option than tamoxifen as an initial breast cancer therapy for women with hormone receptor-positive breast cancer.