Estrone, 3-hydroxyestra-1,3,5(10)-trien-17-one, is less active than estradiol but more active than itsmetabolite, estriol. As the salt of its 3-sulfate ester, estroneis the primary ingredient in conjugated estrogens, USP, andesterified estrogens, USP. Although originally obtainedfrom the urine of pregnant mares (about 10 mg/L), estroneis now prepared synthetically. Estrone itself is not availablein commercial oral formulations, but can be obtained at compounding pharmacies as a topical formulation. Oleoylestrone,the C3 ester of estrone with oleic acid, is in phase IIclinical trials for the treatment of obesity. This acyl estronederivative reduces fat stores by a mechanism not involvingthe ER, although some of the oleoyl-estrone is hydrolyzedto estrone in vivo.