Fulvestrant Beta-Isomer, also known as Fulvestrant EP Impurity A. Fulvestrant, sold under the trade name Faslodex among others, is an estrogen receptor antagonist indicated for the treatment of hormone receptor (HR)-positive metastatic breast cancer in postmenopausal women with disease progression as well as HR-positive, HER2-negative advanced breast cancer in combination with palbociclib in women with disease progression after endocrine therapy.