Key Words. Breast cancer • Estrogen • Estrone sulfate • Steroid sulfatase • Sulfatase inhibitor
LEARNING OBJECTIVESAfter completing this course, the reader will be able to:1. Discuss the role of steroid sulfatase in regulating estrogen production in postmenopausal women.2. Describe the potential of steroid sulfatase inhibition in cancer therapy.3. Discuss a potential new endocrine therapy for patients progressing on aromatase.Access and take the CME test online and receive 1 AMA PRA Category 1 Credit ™ at CME.TheOncologist.com CME CME
ABSTRACTInhibitors of steroid sulfatase are being developed as a novel therapy for hormone-dependent breast cancer in postmenopausal women. Data suggest that steroid sulfatase (STS) activity is much higher than aromatase activity in breast tumors and high levels of STS mRNA expression in tumors are associated with a poor prognosis. STS hydrolyzes steroid sulfates, such as estrone sulfate and dehydroepiandrosterone sulfate (DHEAS), to estrone and DHEA, which can be converted to steroids with potent estrogenic properties, that is, estradiol and androstenediol, respectively. Several potent irreversible STS inhibitors have now been identified, including STX64 (BN83495), a tricyclic sulfamate ester. This drug recently completed the first-ever trial of this new type of therapy in postmenopausal women with estrogen receptor-positive metastatic breast cancer. STX64, tested at 5-mg and 20-mg doses, was able to almost completely block STS activity in peripheral blood lymphocytes and tumor tissues. Inhibition of STS activity was associated with significant reductions in serum concentrations of androstenediol and estrogens. Unexpectedly, serum androstenedione concentrations also decreased by up to 86%, showing that this steroid, which is the main substrate for the aromatase in postmenopausal women, is derived mainly from the peripheral conversion of DHEAS. Of eight patients who completed therapy, five showed evidence of stable disease for up to 7.0 months. This new endocrine therapy offers considerable potential for the treatment of hormone-dependent breast cancer in postmenopausal women.