Drug Withdrawal in Women With Progressive Breast Cancer While on Aromatase Inhibitor Therapy

The purpose of this research study is to study the effects of stopping aromatase inhibitory
(AI) therapy on breast cancer progression. Aromatase inhibitors are a class of drugs used
to treat breast cancer in postmenopausal women. They work by decreasing the level of
estrogen, which is believed to stimulate the growth of tumor tissue. Breast cancer that
progresses despite therapy with an AI is thought to have been resistant to AI therapy.
There is scientific evidence to suggest that resistant breast cancer cells learn to grow at
the very low levels of estrogen present on AI therapy and that increasing estrogen levels
even slightly by stopping AI therapy with inhibit the breast cancer cells. An improvement
or stabilization of breast cancer has been observed after stopping therapy with tamoxifen, a
different anti-estrogen therapy, and has been reported in the literature after stopping AI
therapy. This research study will be the first study to formally test the rate of disease
improvement (response) or stabilization after stopping AI therapy.