Regulation of Prolactin Synthesis in Vitro by Estrogenic and Antiestrogenic Derivatives of Estradiol and Estrone*

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Abstract
The estrogenic and antiestrogenic activities of derivatives of estradiol estrone were determined in vitro using the ability of primary cultures of immature rat pituitary cells to synthesize PRL. Estradiol derivatives were the most potent estrogens in the assay. Large ethinyl substitutions in the 17.alpha. position generally caused a decrease in estrogenic potency (up to 1000-fold). The 3 phenolic hydroxyl was important, but not essential, for the estrogenic activity of the estradiol molecule. Estratriene was approximately 1000 times less potent than estradiol. However, significant estrogenic activity was observed with the compound anordin (EC50, 8 .times. 10-9 M), which could potentially be converted to a dihydroxylated derivative but without an aromatic A ring. Similarly, the steroid androst-5-ene-3,17-diol was weakly estrogenic (EC50, 3 .times. 10-8 M). Steroids with a ketone in the A and D rings were generally inactive as estrogens and antiestrogens. Estradiol derivatives with 17.beta. amines were only weak estrogens. Estrone derivatives were less active than the corresponding estradiol derivatives. 4-Nitromethoxyestrone exhibited weak antiestrogenic properties: however, 4-nitroestrone and methoxyesterone were both estrogens. The reason for the antiestrogenic properties of 4-nitromethoxyestrone is obscure, as the compound does not have structural features similar to those of known nonsteroidal antiestrogenes. Minor alterations to the estradiol molecule at the 11.beta. (OH) or 6 (ketone) position had little effect on estrogenic potency; however, large substitutions at the 11.beta. (RU 39,411) or 7.alpha. (ICI 164384) position produced antiestrogenic compounds. Ru 39,411 was approximately 10 times more active as an antiestrogen than 4-hydroxytamoxifen, whereas ICI 164,384 was approximately 10 times less active than 4-hydroxytamoxifen. A series of hypothetical models is proposed that could explain the antiestrogenic properties of RU 39,411 and ICI 164,384 by an interaction with the estrogen receptor steroid-binding site.