Clinical implications of recent studies using mTOR inhibitors to treat advanced hormone receptor-positive breast cancer

Breast cancer is a leading cause of cancer-related death worldwide. Approximately 75% of breast cancer is hormone receptor-positive (HR(+)) and is managed with endocrine therapies. However, relapse or disease progression caused by primary or acquired endocrine resistance is frequent. Phosphatidylinositol-3-kinase (PI3K)/protein kinase B (Akt)/mammalian target of rapamycin (mTOR)-mediated signaling is one of the molecular mechanisms leading to endocrine resistance. mTOR inhibitors that target the PI3K/Akt/mTOR pathway are the first of the targeted therapies to be evaluated in clinical trials to overcome endocrine resistance. Although the clinical trial with temsirolimus, an mTOR inhibitor, did not show any benefit when compared with endocrine therapy alone, a Phase II clinical trial with sirolimus has been promising. Recently, everolimus was approved in combination with exemestane by the US Food and Drug Administration for treating postmenopausal women with advanced HR(+) breast cancer, based on the results of a Phase III trial. Therefore, everolimus represents the first and only targeted agent approved for combating endocrine resistance.