Estradiol sulfate
As the sodium salt, sodium estradiol sulfate, E2S is present as a minor constituent (0.9%) of conjugated equine estrogens (CEEs), or Premarin. It effectively functions as a prodrug to estradiol in this preparation, similarly to E1S. E2S is also formed as a metabolite of estradiol, as well as of estrone and E1S. Aside from its presence in CEEs, E2S is not available as a commercial pharmaceutical drug.
Estrone (medication)
Estrone is an estrogen, specifically an agonist of the estrogen receptors ERα and ERβ. It is a far less potent estrogen than is estradiol, and as such is a relatively weak estrogen. Given by subcutaneous injection in mice, estradiol is about 10-fold more potent than estrone and about 100-fold more potent than estriol. According to one study, the relative binding affinities of estrone for the human ERα and ERβ were 4.0% and 3.5% of those estradiol, respectively, and the relative transactivational capacities of estrone at the ERα and ERβ were 2.6% and 4.3% of those of estradiol, respectively. In accordance, the estrogenic activity of estrone has been reported to be approximately 4% of that of estradiol. Because estrone can be transformed into estradiol, which is far more potent as an estrogen in comparison, most of the estrogenic potency of estrone "in vivo" is actually due to conversion into estradiol. As such, similarly to the case of estrone sulfate, estrone is considered to be a prodrug of estradiol. Some "in vitro" research has suggested that estrone might be able to partially antagonize the actions of estradiol, but this does not appear to be of clinical significance.