The purpose of this study is to determine the optimal biological dose (OBD) of Irosustat
(BN83495) in postmenopausal women with oestrogen receptor (ER) positive locally advanced or
metastatic breast cancer with disease progression after prior hormonal therapy.
This study is designed to provide necessary information on safety and dose response of
BN83495, when given by repeated once daily oral administration, while achieving a maximal STS
inhibition and a maximal reduction in plasma oestradiol (E2) and adiol levels. The data
obtained will be used to plan further clinical studies.