Actions of an estradiol-17-fatty acid ester in estrogen target tissues of the rat: comparison with other C-17 metabolites and a pharmacological C-17 ester.

The C-17 fatty acid esters of estradiol (E2) are a unique family of nonpolar estradiol metabolites. They are potent long-acting estrogens that represent the natural analog of the synthetic esters used for estrogen therapy. We measured the uterotropic response and the formation of uterine nuclear estrogen receptors (ERn) produced by iv administration of a representative ester, E2-17-stearate, in comparison to E2, other natural C-17 conjugates of E2, E2-17-glucuronide, and E2-17-sulfate, and the pharmacological ester E2-17-cyclopentylpropionate. While E2-17-stearate produced a sustained and greater uterotropic response compared to E2, the maximal induction of ERn by the ester was only about one third of that induced by a similar dose of E2. However, the induction of ERn by E2-17-stearate was markedly sustained compared to that by E2. Furthermore, the initiation of the ERn response to E2-17-stearate was delayed. Since E2-17-esters do not bind to the ER, this delay is consistent with the requirement for hydrolysis of the esters before interaction with the ER. Neither of the ionic conjugates of E2 (sulfate and glucuronide) produced an increase in ERn concentrations or a uterotropic response. The synthetic ester cyclopentylpropionate, like E2, produced a rapid ERn response and a significantly shorter uterotropic response than the stearate ester. When the induction of ERn by E2-17-stearate was investigated in other target tissues there were no marked differences in the brain, pituitary, and liver. No blood-brain barrier was apparent for the formation of ERn, despite the fact that this steroidal ester circulates in the blood bound to lipoproteins. These findings suggest that this unusual family of steroidal esters has biological properties that differentiate them from other known estrogens, natural and synthetic, in terms of their ability to produce a slow-onset sustained estrogenic stimulus in a variety of different estrogen target tissues.