Xtane 25mg Tablets Xtane contains Exemestane, an oral steroidal aromatase inactivator structure-related to the natural substrate androstenedione. Exemestane acts as a false substrate for the aromatase enzyme (CYP19A1), binding irreversibly to its active site and causing enzyme inactivation (suicide inhibition). This prevents the conversion of adrenal androgens to estrogens in peripheral tissues, significantly lowering circulating estrogen levels in postmenopausal women without affecting adrenal corticosteroid or aldosterone biosynthesis. Xtane® is indicated for the adjuvant treatment of postmenopausal women with estrogen-receptor positive early breast cancer following 2 to 3 years of tamoxifen therapy, and for the treatment of advanced breast cancer in postmenopausal women whose disease has progressed following tamoxifen therapy.