17α-epiestriol 17α-epiestriol 17α-epiestriol, or simply 17-epiestriol, also known as 16α-hydroxy-17α-estradiol or estra-1,3,5(10)-triene-3,16α,17α-triol, is a minor and weak endogenous estrogen, and the 17α-epimer of estriol (which is 16α-hydroxy-17β-estradiol). it is formed from 16α-hydroxyestrone. in contrast to other endogenous estrogens like estradiol, 17α-epiestriol is a selective agonist of the erβ. it is described as a relatively weak estrogen, which is in accordance with its relatively low affinity for the erα. 17α-epiestriol has been found to be approximately 400-fold more potent than estradiol in inhibiting tumor necrosis factor α (tnfα)-induced vascular cell adhesion molecule 1 (vcam-1) expression in vitro. relative affinities (%) of 17α-epiestriol and related steroids compoundprarergrmrshbgcbg estradiol2.67.91000.60.138.7<0.1 alfatradiol<1<115<1<1 ? ? estriol<1<115<1<1 ? ? 16β-epiestriol<1<120<1<1 ? ? 17α-epiestriol<1<131<1<1 ? ? values are percentages (%). reference ligands (100%) were progesterone for the pr, testosterone for the ar, e2 for the er, dexa for the gr, aldosterone for the mr, dht for shbg, and cortisol for cbg. for other uses, see epiestriol (set index). 17α-epiestriol names iupac name estra-1,3,5(10)-triene-3,16α,17α-triol systematic iupac name (1s,2r,3as,3br,9bs,11as)-11a-methyl-2,3,3a,3b,4,5,9b,10,11,11a-decahydro-1h-cyclopentaphenanthrene-1,2,7-triol other names 17-epiestriol; 16α-hydroxy-17α-estradiol; 3,16α,17α-trihydroxy-1,3,5(10)-estratriene identifiers cas number 1228-72-4 3d model (jsmol) interactive image chebi chebi:42156 chembl chembl1232445 chemspider 225189 drugbank db07702 pubchem cid 256737 unii 4g7ihy560z comptox dashboard (epa) dtxsid00858947 inchi inchi=1s/c18h24o3/c1-18-7-6-13-12-5-3-11(19)8-10(12)2-4-14(13)15(18)9-16(20)17(18)21/h3,5,8,13-17,19-21h,2,4,6-7,9h2,1h3/t13-,14-,15+,16-,17-,18+/m1/s1key: proqiprrnzuxqm-pnvozddcsa-n smiles cc12ccc3c(c1cc(c2o)o)ccc4=c3c=cc(=c4)o properties chemical formula c18h24o3 molar mass 288.38136 g/mol except where otherwise noted, data are given for materials in their standard state (at 25 °c , 100 kpa). infobox references