<unk> s are essentially inactive themselves, with esters such as estradiol valerate and estradiol sulfate having about 2% of the affinity of estradiol for the estrogen receptor. Likewise, the estrogen ether mestranol (ethinylestradiol 3-methyl ether) has about 1% of the affinity of estradiol for the estrogen receptor. Estrone sulfate has less than 1% of the affinity of estradiol for the estrogen receptor. As such,<unk> s do not bind to the estrogen receptor except at extremely high concentrations. The residual affinity of<unk> s for the estrogen receptor in bioassays may actually be due to conversion into the parent estrogen, as attempts to prevent or limit this conversion have been found to abolish binding to the estrogen receptor and estrogenicity. In general, the longer the fatty acid ester chain of an<unk> , the greater its lipophilicity, and the longer the duration of the<unk>  with intramuscular injection. It has been said that, via intramuscular injection, the duration of estradiol benzoate (with an ester of length 1 carbon plus a benzene ring) is 2 to 3 days, of estradiol dipropionate (with two esters each of length 2 carbons) is 1 to 2 weeks, of estradiol valerate (ester of 5 carbons) is 1 to 3 weeks, and of estradiol cypionate (ester of 3 carbons plus a cyclopentane ring) is 3 to 4 weeks. Estradiol enantate (ester of 7 carbons) has a duration of around 20 days. Likewise, estradiol undecylate (ester of 10 carbons) has a very extended duration, which is longer than that of all of the aforementioned esters. Polyestradiol phosphate is an atypical estradiol ester. It is a phosphoric acid ester of estradiol in the form of a polymer, with an average polymer chain length of approximately 13 repeat units of estradiol phosphate. It is slowly cleaved into estradiol and phosphoric acid by phosphatases. Compared to conventional estradiol esters, polyestradiol phosphate has an extremely long duration; its elimination half-life is approximately 70 days. Whereas conventional estradiol esters form a long-lasting depot in muscle and fat at the site of injection, this is not the case with polyestradiol phosphate. Instead, polyestradiol phosphate is taken up rapidly into the bloodstream following injection (by 90% within 24 hours), where it circulates, and is accumulated in the reticuloendothelial system. Unlike other estradiol esters, polyestradiol phosphate is resistant to hydrolysis, which may be because it is a phosphatase inhibitor and may inhibit its own metabolism.<unk> s also occur naturally in the body, for instance estrogen conjugates like estrone sulfate and estrone glucuronide and the very long-lived lipoidal estradiol, which is constituted by ultra-long-chain esters like estradiol palmitate (ester of 16 carbons) and estradiol stearate (ester of 18 carbons).
Estrogen ester
Androgen ester
Progestogen ester
Estrogen conjugate
Xenoestrogen
Phytoestrogen
Tissue-selective estrogen complex
17β-Aminoestrogen