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.
Estradiol anthranilate
Estradiol anthranilate, or estradiol 3-anthranilate, is a synthetic estrogen and estrogen ester – specifically, the C3 anthranilic acid ester of estradiol – which was described in the late 1980s and was never marketed. In dogs, the oral bioavailability of estradiol anthranilate was found to be 5-fold higher than that of unmodified estradiol. However, a subsequent study found that the oral bioavailability of estradiol and estradiol anthranilate did not differ considerably in rats (4.3% and 3.2%, respectively), suggestive of a major species difference.