Steroid sulfatase (STS) is a key enzyme involved in the biosynthesis of estrogens from inactive sulfated steroids. After we reported EO-33 as a potent in vitro STS inhibitor without undesirable estrogenic activity and with osteogenic properties, we are now interested in validating EO-33's in vivo potential to inhibit STS, to prevent bone deterioration, and to reduce estrogen-dependent tumor growth. A scale-up synthesis was first elaborated to prepare the multigram quantity of EO-33 needed to perform in vivo studies. EO-33 blocked the uterine weight stimulated by estrone sulfate in ovariectomized mice by 69% and the STS activity in the liver by 81%. It also produced a selective estrogen receptor modulator effect as assessed by measuring the tibia weight and calcium content. Using a human breast cancer (MCF-7 xenograft) model in nude mice, EO-33 blocked 90% of tumor growth induced by estradiol sulfate, and no toxic effect was observed by assessing the body and liver weights.