Complete Estrogen Blockade for the Treatment of Metastatic and Early Stage Breast Cancer

Complete estrogen blockade has long been sought as a more effective means of controlling breast cancer compared with single agent endocrine therapy. This approach may be accomplished through the use of agents which reduce estrogen production combined with agents that prevent the activity of estrogen at the cellular level. For prostate cancer, another hormonally responsive malignancy, this approach has not been successful at improving survival compared with that achieved with single agent therapy. Preclinical information is contradictory for many promising combinations and may not reflect the true nature of in vivo interaction between agents. For premenopausal patients with metastatic breast cancer, the combination of a luteinising hormone-releasing hormone (LHRH) agonist and tamoxifen is clearly effective, but whether the combination is more effective than either single agent is still controversial. Similar response rates and overall survival were reported with goserelin or goserelin plus tamoxifen by Jonat et al. in 1 randomised, prospective study, but the addition of tamoxifen improved time to progression. A second trial comparing buserelin plus tamoxifen with either single agent reported superior efficacy in terms of response rates, disease-free survival and overall survival with combination therapy. A meta-analysis of 4 randomised trials making similar comparisons, demonstrated significant improvement in median overall survival, progression-free survival, response rate, and duration of response with the combination of a LHRH agonist (goserelin or buserelin) and tamoxifen in premenopausal breast cancer patients with metastatic disease. For postmenopausal women with metastatic breast cancer, the addition of an aromatase inhibitor to tamoxifen has yet to be prospectively compared to single agent therapy. Use of endocrine combinations in the treatment of early stage breast cancer is under investigation. Preliminary results of some of the ongoing adjuvant therapy trials indicate that the combination of a LHRH agonist and tamoxifen may have similar efficacy to cyclophosphamide, methotrexate, and fluorouracil chemotherapy in premenopausal women with estrogen receptor-positive tumour. Addition of LHRH agonist therapy in premenopausal patients with estrogen receptor-positive tumour who had maintained the ovarian function following chemotherapy [cyclophosphamide, doxorubicin (adriamycin), fluorouracil, and tamoxifen], also led to a reduction in the risk of recurrence. These studies have identified a sub-population of patients who may benefit from the addition of combination endocrine therapy. Overall, the issue is quite complex and the data from many ongoing trials are still awaited with anticipation to further delineate the role of complete estrogen deprivation in this disease.