Synthesis and Evaluation of B-, C-, and D-Ring-Substituted Estradiol Carboxylic Acid Esters as Locally Active Estrogens

Abstract

We have synthesized derivatives of estradiol that are structurally modified to serve as "soft" estrogens and act within a geographically limited area of the body; estrogens without systemic action. We have previously shown with 16alpha-substituted analogues of estradiol that carboxylates proximal to the steroid ring neither bind to the estrogen receptor nor activate estrogen-responsive genes. However, when the carboxylic acid is masked as an ester, they bind to the receptor and stimulate estrogenic responses. Enzymatic hydrolysis through nonspecific esterases can inactivate these estrogens and thereby limit their area of action. Here, we describe our continued studies to design "soft" estrogens by synthesizing carboxylic acid esters of estradiol at the 7alpha-, 11beta-, and 15alpha-positions in the steroid nucleus at which bulky substituents are accommodated by the estrogen receptor. These compounds were tested for estrogen receptor binding (estrogen receptors alpha and beta), stimulation of an estrogen sensitive gene in Ishikawa cells in culture, and as substrates for enzymatic hydrolysis. Likely candidates were tested in in vivo assays for systemic and local estrogenic action. The biological studies showed that regardless of the point of attachment, all of the short-chain carboxylic acids, C-1 to C-3, were devoid of hormonal action, while many of the esters were estrogenic. The site on the steroid nucleus had great influence on hormonal activity and esterase hydrolysis. Formate esters at 7alpha and 15alpha were good estrogens, but lengthening the chain to acetate dramatically decreased hormonal activity. However, the 7alpha-formate esters were not enzymatically hydrolyzed. At 11beta, the acetate (methyl ester) was an effective estrogen, but increasing the chain length to propionate dramatically reduced hormonal activity. In general, the length of the alcohol from methyl to butyl had only a small effect on receptor binding, and as the size of the alcohol increased, so did esterase hydrolysis. One exception was the 11beta-acetate esters where increasing the alcohol moiety from methyl to ethyl eliminated estrogenic activity (Ishikawa cells) without affecting estrogen receptor binding. Several of the esters were tested in vivo, and two, the methyl and ethyl esters of estradiol-15alpha-formate, appeared to have the requisite properties (high local and low systemic activity) of superior "soft" estrogens.

Estrogen to Antiestrogen with a Single Methylene Group Resulting in an Unusual Steroidal Selective Estrogen Receptor Modulator

“…previously described (4). Statistical analyses (ANOVA) were performed using PRISM (GraphPad, Inc., San Diego, CA).…”

“…Recently, we synthesized an unusual compound that has the properties of a SERM, but is devoid of their characteristic long-chain and polar substituents. We had been synthesizing novel families of locally active "soft" estrogens, for treatment of vaginal dyspareunia associated with menopause or antiestrogen therapy (3,4). These compounds are esters of carboxylic acid analogs of estradiol (E 2 ), and the esters are ER ligands capable of estrogenic stimulation, whereas the parent, charged carboxylates, are not.…”

Steroidal Aromatic ‘Naphthenic Acids’ in Oil Sands Process-Affected Water: Structural Comparisons with Environmental Estrogens

“…Although acids related to estradiol are not estrogenic, many esters of the latter are potent estrogens. 31 Indeed, when we modeled the possible human estrogenic and androgenic receptor activity of acid I (Figure 1) using ADMET predictor software, under conditions which correctly predicted the known estrogenic activity of nonylphenol, acid I was also predicted to affect both receptors (Table S1). Experimental verification of these predictions therefore now appears to be warranted.…”

Synthesis and evaluation of 17α-E-20-(heteroaryl)norpregn-1,3,5(10),20 tetraene-3,17β-diols [17α-(heteroaryl)vinyl estradiols] as ligands for the estrogen receptor-α ligand binding domain (ERα-LBD)

“…26–28 The results of these assays are shown in Table 1. As the data indicate, the parent compound, previously characterized, has ERα-LBD binding (RBA) and biological potency (RSA) that are approximately 10% that of estradiol.…”