plateau-like curve for many hours, levels of EE fall rapidly after peaking. This is thought to be because estrone and estrone sulfate can be reversibly converted back into estradiol and serve as a hormonally inert reservoir for estradiol, whereas the EE sulfate reservoir for EE is much smaller in comparison. In any case, due to the formation of EE sulfate, enterohepatic recirculation is involved in the pharmacokinetics of EE similarly to estradiol, although to a lesser extent. The contribution of enterohepatic recirculation to total circulating EE levels appears to be 12 to 20% or less, and is not observed consistently. A secondary peak in EE levels 10 to 14 hours after administration can often be observed with oral EE. EE, following oxidative formation of a very reactive metabolite, irreversibly inhibits cytochrome P450 enzymes involved in its metabolism, and this may also play a role in the increased potency of EE relative to estradiol. Indeed, EE is said to have a marked effect on hepatic metabolism, and this is one of the reasons, among others, that natural estrogens like estradiol may be preferable. A 2-fold accumulation in EE levels with an EE-containing COC has been observed following 1 year of therapy. Elimination EE is eliminated 62% in the feces and 38% in the urine. Chemistry EE, also known as 17α-ethynylestradiol or as 17α-ethynylestra-1,3,5(10)-triene-3,17β-diol, is a synthetic estrane steroid and a derivative of estradiol with an ethynyl substitution at the C17α position. The 17α-ethynylation of estradiol to create EE is analogous to the 17α-substitution of testosterone to make testosterone derivatives such as 17α-ethynylated progestins like ethisterone (17α-ethynyltestosterone) and norethisterone (17α-ethynyl-19-nortestosterone) as well as 17α-alkylated androgens/anabolic steroids like methyltestosterone (17α-methyltestosterone). Analogues A number of derivatives of EE exist. These include mestranol (EE 3-methyl ether), quinestrol (EE 3-cyclopentyl ether), ethinylestradiol sulfonate (EE 3-isopropylsulfonate), and moxestrol (11β-methoxy-EE). The former three are prodrugs of EE, while the latter one is not. A few analogues of EE with other substitutions at the C17α position exist. Examples include the estradiol derivatives methylestradiol (17α-methylestradiol) and ethylestradiol (17α-ethylestradiol), and the estriol derivatives ethinylestriol (17α-ethynylestriol) and nilestriol (17α-ethynylestriol 3-cyclopentyl ether). Androstane analogues of EE with significant although weak estrogenic activity include ethinylandrostenediol (17α-ethynyl-5-androstenediol), 17α-ethynyl-3β-androstanediol, 17α-ethynyl-3α-androstanediol, and methandriol (17α-methyl-5-androstenediol). History EE was the first orally active synthetic estrogen and was described in 1938 by Hans Herloff Inhoffen and Walter Hohlweg of Schering AG in Berlin. It was approved by the in the