Description: Irosustat (formerly known as STX64, BN83495 or 667 coumate) is a novel potent and irreversible inhibitor of steroid sulfatase with an IC50 of 8 nM and has the potential for treating breast cancer. The development of potent steroid sulfatase inhibitors is an important new therapeutic strategy for the treatment of postmenopausal women with breast cancer. In a placental microsomal assay system, Irosustat emerged as the most potent inhibitor with an IC50 of 8 nM. Administration of a single dose (10 mg/kg, p.o.) of Irosustat inhibited rat liver estrone sulfatase activity by 93%. Irosustat was devoid of estrogenicity, as indicated by its failure to stimulate the growth of uteri in ovariectomized rats. In vivo, estrone sulfate-stimulated growth of uteri in ovariectomized rats was inhibited by Irosustat. Using the nitrosomethylurea-induced mammary tumor model, it was found that Irosustat caused regression of estrone sulfate-stimulated tumor growth in a dose-dependent manner. The identification of Irosustat as a potent steroid sulfatase inhibitor will enable the therapeutic potential of this type of therapy to be evaluated.