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.