Equilin is an equine estrogen, and is one of the components of commonly used pharmaceuticals for hormone replacement therapy in the United States. 

Discovery 

Equilin was originally prepared from the urine of pregnant mares by Girard et al.

Properties 

A white solid molecular formula C18H20O2, Mr. 268.35. Melting point 238–240°C.

Application

Equilin is one of the major components of estrogens for hormone replacement therapy, along with estrone and equilenin (another equilin estrogen; 517-09-9). These estrogen mixtures are known as conjugated equine estrogens (CEEs; the major brand is Premarin) and esterified estrogens. These estrogens are administered as salts of their sulfate esters. There is an increased risk of endometrial cancer in women as well as strokes, heart attacks, blood clots, and breast cancer. Equilin (and equilenin) is not normally present in women, and is the only human estrogen replacement drug that is derived from animals. The biological half-life of equilin is estimated to be 30min in women.

Environmental estrogen

Equilin is known that effluent from sewage treatment contains contaminants that can be estrogenic to aquatic animals. Estrogens used in hormone replacement therapy and/or oral contraceptives are implicated as some of the causative compounds. Equilin and its metabolite 4-dihydroequilin were detected at nanogram per liter (pg/mL) concentrations in the influent and effluent.Equilin has been found to be concentrated in the bile of fish, and to increase the blood levels of the egg yolk protein vitellogenin.

Estrogenic activity 

The relative potency with regard to estradiol-17β in the T47D-KBluc assay is 50%.The relative binding affinities with regard to estradiol-17β for human estrogen receptors α and β are 13% and 49%, respectively.An in vitro reporter gene assay revealed that the relative potency of equilin with regard to estradiol-17β for several fish estrogen receptors α is in the range of 3%–5%.