Letrozole and anastrozole — drugs that block estrogen production — are used to treat metastatic breast cancer in postmenopausal women whose tumors contain estrogen receptors (ER). However, these drugs often stop working. Exemestane also lowers estrogen production and is used to treat postmenopausal women with early-stage ER-positive breast cancer. It has also been shown to be effective in women with metastatic breast cancer.
The purpose of this study is to see if giving exemestane with a drug called abiraterone is more effective than either drug alone in postmenopausal women with metastatic ER-positive breast cancer that has continued to grow despite letrozole or anastrozole treatment. Abiraterone lowers estrogen production in a way that is different than letrozole, anastrozole, or exemestane. Abiraterone is approved for treating advanced prostate cancer, but its use for treating breast cancer is considered investigational. It is given with prednisone.
In this study, women will be randomly assigned to receive one of three treatment regimens: abiraterone, prednisone, and exemestane; abiraterone and prednisone; or exemestane alone.