A-CD Estrogens. I. Substituent Effects, Hormone Potency, and Receptor Subtype Selectivity in a New Family of Flexible Estrogenic Compounds

A 'new' estrogen metabolite: an epoxide of estrone as a sulfated steroid

Current heightened recognition of the importance of sulfated steroids led to the examination of conjugates in media from incubations of estrogens in tissues from the reproductive tract of stallions. Previously, we had reported a 'new' unidentified metabolite of estrone (E1) and [3H]-E1, located between 17β-estradiol (E2) and E1 reference standards on chromatography (HPLC) and identified tentatively as a stable 5α,6α-estrone epoxide. Stallion tissues were minced and incubated for 2 h with [3H]-E1 (1 × 106 cpm). Solid-phase extraction of unconjugated and conjugated steroids from media was followed by liquid scintillation counting (LSC), where radioactivity was mostly in the conjugate fractions (>80%). HPLC of conjugated steroids used an isocratic solvent system of acetonitrile/water (8:92) at 700 µL/min with detection by LSC. A radioactive peak between reference standards of E1 and E2 sulfates was examined, after solvolysis, in a second solvent system. Sulfated steroids yielded largely E1, whereas acid treatment of the unconjugated E1 epoxide had earlier formed 6α-OH-E1 almost exclusively. With sulfatase enzyme, at neutral pH, radioactivity was also seen mostly as E1 with trace amounts of polar material. Reduction with KBH4, however, led also to desulfation; radioactivity had alignment with E2 but even more had low retention times as for 6α/6β-OH-E2. These findings point to a different hydrolysis for desulfation; even more, they reveal an additional oxygen atom at C6 and are supportive of biological formation of 5α,6α-epoxides of E1 and E2. As possible alternatives to catechol estrogens, implicated in cancer, the 'new' estrogen metabolites and their sulfated forms may have special interest.

A-C Estrogens as Potent and Selective Estrogen Receptor-Beta Agonists (SERBAs) to Enhance Memory Consolidation under Low-Estrogen Conditions

Estrogen receptor-beta (ERβ) is a drug target for memory consolidation in postmenopausal women. Herein is reported a series of potent and selective ERβ agonists (SERBAs) with in vivo efficacy that are A-C estrogens, lacking the B and D estrogen rings. The most potent and selective A-C estrogen is selective for activating ER relative to seven other nuclear hormone receptors, with a surprising 750-fold selectivity for the β over α isoform and with EC₅₀s of 20-30 nM in cell-based and direct binding assays. Comparison of potency in different assays suggests that the ER isoform selectivity is related to the compound's ability to drive the productive conformational change needed to activate transcription. The compound also shows in vivo efficacy after microinfusion into the dorsal hippocampus and after intraperitoneal injection (0.5 mg/kg) or oral gavage (0.5 mg/kg). This simple yet novel A-C estrogen is selective, brain penetrant, and facilitates memory consolidation.

Methodology for the Construction of the Bicyclo[4.3.0]nonane Core

The bicyclo[4.3.0]nonane scaffold, commonly known as a hydrindane, is a common structural motif found in many terpenoid structures and one that remains a challenge for synthetic chemists to elaborate with appropriate regio- and stereo-selectivity. Over the course of the study of terpene natural products, the elaboration of the hydrindane structure has seen progress on the utilization of both old and newer methods to achieve the desired outcomes. This review seeks to serve as a general overview of these methods, and detail specific examples.

Synthesis and structure-activity relationships of 1-benzylindane derivatives as selective agonists for estrogen receptor beta

The estrogen receptor beta (ERβ) selective agonist is considered a promising candidate for the treatment of estrogen deficiency symptoms in ERβ-expressing tissues, without the risk of breast cancer, and multiple classes of compounds have been reported as ERβ selective agonists. Among them, 6-6 bicyclic ring-containing structures (e.g., isoflavone phytoestrogens) are regarded as one of the cyclized analogues of isobutestrol 5b, and suggest that other cyclized scaffolds comprising 5-6 bicyclic rings could also act as selective ERβ ligands. In this study, we evaluated the selective ERβ agonistic activity of 1-(4-hydroxybenzyl)indan-5-ol 7a and studied structure-activity relationship (SAR) of its derivatives. Some functional groups improved the properties of 7a; introduction of a nitrile group on the indane-1-position resulted in higher selectivity for ERβ (12a), and further substitution with a fluoro or a methyl group to the pendant phenyl ring was also preferable (12b, d, and e). Subsequent chiral resolution of 12a identified that R-12a has a superior profile over S-12a. This is comparable to diarylpropionitrile (DPN) 5c, one of the promising selective ERβ agonists and indicates that this indane-based scaffold has the potential to provide better ERβ agonistic probes.

Design of pathway preferential estrogens that provide beneficial metabolic and vascular effects without stimulating reproductive tissues

There is great medical need for estrogens with favorable pharmacological profiles that support desirable activities for menopausal women, such as metabolic and vascular protection, but that lack stimulatory activities on the breast and uterus. We report the development of structurally novel estrogens that preferentially activate a subset of estrogen receptor (ER) signaling pathways and result in favorable target tissue-selective activity. Through a process of structural alteration of estrogenic ligands that was designed to preserve their essential chemical and physical features but greatly reduced their binding affinity for ERs, we obtained "pathway preferential estrogens" (PaPEs), which interacted with ERs to activate the extranuclear-initiated signaling pathway preferentially over the nuclear-initiated pathway. PaPEs elicited a pattern of gene regulation and cellular and biological processes that did not stimulate reproductive and mammary tissues or breast cancer cells. However, in ovariectomized mice, PaPEs triggered beneficial responses both in metabolic tissues (adipose tissue and liver) that reduced body weight gain and fat accumulation and in the vasculature that accelerated repair of endothelial damage. This process of designed ligand structure alteration represents a novel approach to develop ligands that shift the balance in ER-mediated extranuclear and nuclear pathways to obtain tissue-selective, non-nuclear PaPEs, which may be beneficial for postmenopausal hormone replacement. The approach may also have broad applicability for other members of the nuclear hormone receptor superfamily.

Synthesis and receptor binding in trans-CD ring-fused A-CD estrogens: comparison with the cis-fused isomers

Ligands which selectively activate only one of the estrogen receptors, ERα or ERβ, are current pharmaceutical targets. Previously, we have reported on substituted cis A-CD ligands in which the B-ring of the steroidal structure has been removed and cis refers the stereochemistry of the CD ring junction as compared to trans in estradiol. These compounds often showed good potency and selectivity for ERβ. Here we report the synthesis and binding affinities for a similar series of trans A-CD ligands, and compare them to the cis-series. Counterintuitively, trans A-CD ligands, which are structurally more closely related to the natural ligand estradiol, show weaker binding and less β-selectivity than their cis-counterparts.

Novel ligands balance estrogen receptor β and α agonism for safe and effective suppression of the vasomotor response in the ovariectomized female rat model of menopause

Vasomotor thermo-dysregulation (hot flashes) are an often debilitating symptom of menopause. Effective treatment is achieved primarily through activation of the estrogen receptor (ER)α with estrogens but is also associated with increased risk for breast and uterine cancer. In this study, we have tested novel compounds lacking the B ring of 17-hydroxy-β-estradiol (E2) (A-CD compounds) with differing ratios of ERα:ERβ binding affinities for the ability to reduce diurnal/nocturnal tail-skin temperatures (TSTs) in the ovariectomized female rat menopausal hot flash model. Normal mammary tissue expresses the predominantly antiproliferative ERβ. Therefore, we hypothesized that a preferential ERβ agonist with fractional ERα activity would safely reduce TSTs. The A-CD compound, L17, is a preferential ERβ agonist that has a ratio of ERβ:ERα binding affinity relative to E2 of 9.3 (where ERβ:ERα for E2, 1.0). In the ovariectomized rat, daily administration of low doses (1 mg/kg) of the A-CD compound TD81 (ERα:ERβ relative affinity, 15.2) was ineffective in temperature regulation, whereas L17 showed a trend toward TST reduction. Both E2 and the A-CD compound, TD3 (ERβ:ERα relative affinity, 5.0), also reduced TSTs but had marked proliferative effects on mammary and uterine tissues. At 2 mg/kg, L17 strongly reduced TSTs even more effectively than E2 but, importantly, had only minimal effect on uterine weight and mammary tissues. Both E2- and L17-treated rats showed similar weight reduction over the treatment period. E2 is rapidly metabolized to highly reactive quinones, and we show that L17 has 2-fold greater metabolic stability than E2. Finally, L17 and E2 similarly mediated induction of c-fos expression in neurons within the rat thermoregulatory hypothalamic median preoptic nucleus. Thus, the A-CD compound, L17, may represent a safe and effective approach to the treatment of menopausal hot flashes.

Triaryl-substituted Schiff bases are high-affinity subtype-selective ligands for the estrogen receptor

We have explored the isoelectronic replacement of the C=C double bond found at the core of many nonsteroidal estrogen ligands with a simple Schiff base (C=N). Di- and triaryl-substituted imine derivatives were conveniently prepared by the condensation of benzophenones with various anilines without the need for phenolic hydroxy protection. Most of these imines demonstrated high affinity for the estrogen receptors, which, in some cases exceeded that of estradiol. In cell-based assays, these imines profiled as ERα agonists but as ERβ antagonists, showing preferential reliance on the N-terminal activation function (AF1), which is more active in ERα. X-ray analysis revealed that the triaryl-imines distort the ligand-binding pocket in a new way: by controlling the separation of helices 3 and 11, which appears to alter the C-terminal AF2 surface that binds transcriptional coactivators. This work suggests that C=N for C=C substitution might be more widely considered as a general strategy for preparing drug analogues.

Experimental versus predicted affinities for ligand binding to estrogen receptor: iterative selection and rescoring of docked poses systematically improves the correlation

The computational determination of binding modes for a ligand into a protein receptor is much more successful than the prediction of relative binding affinities (RBAs) for a set of ligands. Here we consider the binding of a set of 26 synthetic A-CD ligands into the estrogen receptor ERα. We show that the MOE default scoring function (London dG) used to rank the docked poses leads to a negligible correlation with experimental RBAs. However, switching to an energy-based scoring function, using a multiple linear regression to fit experimental RBAs, selecting top-ranked poses and then iteratively repeating this process leads to exponential convergence in 4-7 iterations and a very strong correlation. The method is robust, as shown by various validation tests. This approach may be of general use in improving the quality of predicted binding affinities.

Thiophene-core estrogen receptor ligands having superagonist activity

To probe the importance of the heterocyclic core of estrogen receptor (ER) ligands, we prepared a series of thiophene-core ligands by Suzuki cross-coupling of aryl boronic acids with bromo-thiophenes and we assessed their receptor binding and cell biological activities. The disposition of the phenol substituents on the thiophene core, at alternate or adjacent sites, and the nature of substituents on these phenols, all contribute to binding affinity and subtype selectivity. Most of the bis(hydroxyphenyl)-thiophenes were ERβ selective, whereas the tris(hydroxyphenyl)-thiophenes were ERα selective; analogous furan-core compounds generally have lower affinity and less selectivity. Some diarylthiophenes show distinct superagonist activity in reporter gene assays, giving maximal activities 2-3 times that of estradiol, and modeling suggests that these ligands have a different interaction with a hydrogen-bonding residue in helix-11. Ligand-core modification may be a new strategy for developing ER ligands whose selectivity is based on having transcriptional activity greater than that of estradiol.

BC-spiro-estradiols. Synthesis and estrogen receptor binding affinity of four new estradiol isomers

The synthesis of four new isomers of estradiol in which the ring A to ring C planes are perpendicular to each other as a result of a spiro BC ring junction is described. Heterocyclic analogs and carbocyclic homologs of these compounds are also reported. Estrogen receptor binding studies show that the spiro compounds with the natural stereochemistry at C9 bind almost as strongly as estradiol but with greater β to α selectivity. These studies show that the estrogen receptors can readily accommodate isomers of estrogen with substantially different fixed shapes than the native ligand.