Equilin

Equilin is a naturally occurring estrogen sex hormone found in horses as well as a medication.<ref name="Elks2014">Template:Cite book</ref><ref name="pmid16112947">Template:Cite journal</ref><ref name="pmid24176763">Template:Cite journal</ref> It is one of the estrogens present in the estrogen combination drug preparations known as conjugated estrogens (CEEs; e.g. Premarin) and esterified estrogens (EEs; e.g. Estratab, Menest).<ref name="pmid16112947" /><ref name="pmid24176763" /> CEEs is the most commonly used form of estrogen medications in hormone replacement therapy (HRT) for menopausal symptoms in the United States.<ref name="pmid24176763" /> Estrone sulfate is the major estrogen in CEEs (about 50%) while equilin sulfate is the second major estrogen in the formulation, present as about 25% of the total.<ref name="pmid16112947" /><ref name="pmid24176763" />

Pharmacology

Pharmacodynamics

Equilin is an estrogen, or an agonist of the estrogen receptors (ERs), the ERα and ERβ.<ref name="pmid16112947" /> In terms of relative binding affinity for the ERs, equilin has about 13% and 49% of that of estradiol for the ERα and ERβ, respectively.<ref name="pmid16112947" /> Analogously to the reversible transformation of estrone into estradiol by 17β-hydroxysteroid dehydrogenase, equilin can be converted into the more potent estrogen 17β-dihydroequilin in the body.<ref name="pmid16112947" /><ref name="pmid24176763" /> This estrogen has about 113% and 108% of the relative binding affinities of estradiol for the ERα and ERβ, respectively.<ref name="pmid16112947" /><ref name="pmid24176763" /> Equilin is present in CEEs in the form of equilin sulfate, which itself is inactive and acts as a prodrug of equilin via steroid sulfatase.<ref name="pmid16112947" /><ref name="pmid24176763" />

Similarly to synthetic estrogens like ethinylestradiol, equilin and CEEs have disproportionate effects in certain tissues such as the liver and uterus relative to bioidentical human estrogens like estradiol and estrone.<ref name="pmid16112947" /> Because of their disproportionate potency in the liver, equilin and CEEs have relatively increased effects on liver protein synthesis compared to estradiol.<ref name="pmid16112947" />

A dosage of 0.25 mg/day equilin sulfate is equivalent to 0.625 mg/day CEEs in terms of relief from hot flashes.<ref name="pmid16112947" /> At a dosage of 0.625 mg/day equilin sulfate, the increases in circulating levels of sex hormone-binding globulin (SHBG), corticosteroid-binding globulin, and angiotensinogen were 1.5 to 8 times those observed with estrone sulfate.<ref name="pmid16112947" /> Equilin has about 42% of the relative potency of CEEs in the vagina and 80% of the relative potency of CEEs in the uterus, while its more active form, 17β-dihydroequilin, has about 83% of the relative potency of CEEs in the vagina and 200% of the relative potency of CEEs in the uterus.<ref name="pmid16112947" />

Pharmacokinetics

Equilin has about 8% of the relative binding affinity of testosterone for SHBG, relative to 12% in the case of estrone.<ref name="pmid16112947" /> In terms of plasma protein binding, it is bound 26% to SHBG and 13% to albumin.<ref name="pmid16112947" /> The metabolic clearance rates of equilin and equilin sulfate are 2,640 L/day/m² and 175 L/day/m², respectively.<ref name="pmid16112947" /> In accordance, the biological half-life of equilin sulfate is substantially longer than that of equilin.<ref name="pmid16112947" /> Equilin is converted into 17β-dihydroequilin in the liver and in other tissues.<ref name="pmid16112947" /><ref name="pmid24176763" /> Equilin and 17β-dihydroequilin can also be transformed into equilenin and 17β-dihydroequilenin.<ref name="pmid16112947" /><ref name="pmid24176763" /> Equilin is excreted in the form of glucuronide conjugates.<ref name="pmid16112947" />

Chemistry

Equilin, also known as δ⁷-estrone or as 7-dehydroestrone, as well as estra-1,3,5(10),7-tetraen-3-ol-17-one, is a naturally occurring estrane steroid and an analogue of estrone.<ref name="pmid16112947" /><ref name="pmid24176763" /> In terms of chemical structure and pharmacology, equilin is to 17β-dihydroequilin (δ⁷-17β-estradiol) as estrone is to estradiol.<ref name="pmid16112947" /><ref name="pmid24176763" />