Synthesis of substituted estradiols by the selective aromatization of A-ring of steroidal 19-nor-Δ-4-3-ketones with phenylselenyl halides/hydrogen peroxide

A range of 6-, 7-, and 11-substituted estradiols were synthesized by the selective aromatization of the A-ring of 19-nor steroids using phenylselenyl halides followed by oxidation with hydrogen peroxide. Established methods utilizing copper(II) halides failed or have given poor yields with these substrates.

Viridin analogs derived from steroidal building blocks

Naturally occurring furanosteroids such as viridin and wortmannin have long been known as potent inhibitors of the lipid kinase PI-3K. We have been interested in directly accessing analogs of these complex natural products from abundant steroid feedstock materials. In this communication, we describe the synthesis of viridin/wortmannin hybrid molecules from readily available building blocks that function as PI-3K inhibitors and maintain their electrophilic properties. The compounds also show anti-proliferative effects against a breast cancer line.

(18)F-labelling of A-ring substituted 16alpha-fluoro-estradiols as potential radiopharmaceuticals for PET imaging

The 2-methoxy derivative of estradiol is currently in Phase II clinical trial as an anticancer agent while the 4-methyl derivative has been shown to interact with cytoplasmic and nuclear estrogen receptors in rat pituitary gland and hypothalamus. We hypothesize that the 16alpha-(18)F-analogs of these estrogens could be suitable radiotracers to evaluate action mechanisms of the parent compounds. In this study we report the synthesis of the 16alpha-(18)F and 16alpha-(19)F-analogs of the A-ring substituted estradiols in high yield via stereoselective opening of the intermediate 16beta,17beta-O-cyclic sulfones with [(18)F]F(-) or F(-) followed by deprotection.

Inhibition of estrone sulfatase by aromatase inhibitor-based estrogen 3-sulfamates

Our rationale is based on the finding that estrone 3-sulfamate (EMATE, 2d), a typical estrone sulfatase (ES) inhibitor, can be hydrolyzed and the pharmacological effect of the free estrogen contributes to the bioactivity of the sulfamate. A number of 3-sulfamoylated derivatives of the good aromatase inhibitors, 2- and 4-halogeno (F, Cl, and Br) estrones and their estradiol analogs as well as 6beta-methyl and phenyl estrones, were synthesized and evaluated as inhibitors of ES in human placental microsomes in comparison with the lead compound EMATE. Among them, 2-chloro- and 2-bromoestrone 3-sulfamates (2b and 2c), along with their estradiol analogs 3b and 3c, were powerful competitive inhibitors with K(i)'s ranging between 4.0 and 11.3 nM (K(i) for EMATE, 73 nM). These four sulfamates as well as the 2-fluoro analogs 2a and 3a inactivated ES in a time-dependent manner more efficiently than EMATE, and 2-halogeno estrone sulfamates 2 also caused a concentration-dependent loss of ES activity. The results may be useful for developing a new class of drugs having a dual function, ES inhibition and aromatase inhibition, for the treatment of breast cancer.

Structure-activity relationships of 2-, 4-, or 6-substituted estrogens as aromatase inhibitors

Aromatase, which is responsible for the conversion of androgens to estrogens, is a potential therapeutic target for the selective lowering of estrogen levels in patients with estrogen-dependent breast cancer. To develop a novel class of aromatase inhibitors, we tested series of 2- and 4-substituted (halogeno, methyl, formyl, methoxy, nitro, and amino) estrones (7 and 9), as well as series of 6alpha- and 6beta-substituted (alkyl, phenalkyl, and alkoxy) estrones (13 and 14), and their estradiol analogs (8, 10, 11, and 12) as aromatase inhibitors. All of the inhibitors examined blocked the androstenedione aromatization in a competitive manner. Introduction of halogeno and methyl functions at C-2 of estrone as well as that of a phenalkyl or methyl function at the C-6alpha or C-6beta position markedly increased affinity to aromatase (apparent K(i) value=0.10-0.66 microM for the inhibitors versus 2.5 microM for estrone). 6alpha-Phenylestrone (13c) was the most powerful inhibitor among the estrogens studied, and its affinity was comparable to that of the androgen substrate androstenedione. Estradiol analogs were much weaker inhibitors than the corresponding estrone compounds in each series, indicating that the 17-carbonyl group plays a critical role in the formation of a thermodynamically stable enzyme-inhibitor complex.

Synthesis of nitrile derivatives of estrogens

The first time synthesis of 7alpha- and 11beta-nitrile estradiol is described. Reaction of 7alpha-cyano-19-nortestosterone with copper(II)bromide in acetonitrile at room temperature results in aromatization of the A-ring. Treatment of 11beta-cyano-19-nortestosterone-17-one under similar condition does not induce A-ring aromatization but rather results in bromination at the 2beta-position. However A-ring aromatized products are obtained when the latter compound is treated with Ac2O-Py-AcOCl, NBS and HCl.

The estradiol pharmacophore: ligand structure-estrogen receptor binding affinity relationships and a model for the receptor binding site

The accumulated knowledge on the binding of estradiol (E2) and its analogs and the results of affinity-labeling studies have been reviewed and are used herein to derive a binding site model for the estrogen receptor (ER). Estradiol is nonpolar and hydrophobic, except at its molecular termini. Most of its skeletal flexibility resides in the B-ring, and it probably binds in a low-energy conformation. The phenolic OH group in the A-ring contributes about 1.9 kcal/mol to the binding free energy and probably acts primarily as a hydrogen bond donor. The 17 beta-hydroxyl group in the D-ring contributes approximately 0.6 kcal/mol to the binding and probably acts as a hydrogen bond acceptor, either directly or via a water molecule. There also seems to be a degree of flexibility in the region of the receptor that encompasses the D-ring. The aromatic ring contributes about 1.5 kcal/mol, probably through weak polar interactions with receptor residues that contact the beta-face of the steroid. The receptor seems to surround the ligand, so that all four rings contribute significantly to binding. Small hydrophobic substituents enhance binding affinity at positions 4, 12 beta, 14, and 16 alpha; whereas, larger hydrophobic substituents are tolerated at positions 7 alpha, 11 beta, and 17 alpha. In general, the ER is intolerant of polar substituents. Based on E2 analogs bearing affinity-labeling groups, cysteine residues might be present in the binding site in the area of C-4, C-17 alpha, and C-17 beta, and a lysine residue might be located near C-16. Models that represent the limits of deformability of the ligand binding site, the position of preformed pockets, and space occupied by the receptor are presented. The various elements in this model for the binding of steroidal estrogens by the estrogen receptor are consistent with evidence emerging from the crystal structures of related nuclear hormone receptor ligand complexes.

Stereochemical probes for the estrogen receptor: synthesis and receptor binding of (17 alpha,20E/Z)-21-phenyl-19-norpregna-1,3,5(10), 20-tetraene-3,17 beta-diols

Previous studies from our laboratory using 17 alpha-E- and 17 alpha-Z-halovinyl and phenylthiovinyl estradiols demonstrated a marked preference for the Z stereochemistry and a significant steric tolerance for the Z-vinyl substituent. To further explore the extent of that stereochemical preference and steric tolerance we have prepared stereoselectively the 17 alpha-E- and 17 alpha-Z-phenylvinyl estradiols (E- and Z-styrylestradiols). The results, in addition to demonstrating a facile preparation of the target compounds, supported the previously observed stereochemical and steric effects. The relative binding affinities for the Z isomer were 3-4 fold greater than the E isomer at both 4 degrees C and 25 degrees C, and only one-half to one-fourth those of estradiol under similar conditions. The developing model for ligand-accessible space within the estrogen receptor suggests that Z-phenylvinyl estradiols may provide interesting and useful probes for mapping the receptor.

Synthesis and radiochemistry of 2,4-disubstituted 17 alpha-iodovinylestradiols

This report details the preparation of three compounds which are structurally designed to have depressed metabolism and/or conjugation: 2,4-dibromo-, 2,4-dichloro-, and 2,4-dimethyl-17 alpha-iodovinylestradiol. Their synthesis includes the use of two novel transformations based upon tin chemistry: preparation of an intermediate 17 alpha-vinylstannanes via stannylcupration of a 17 alpha-ethynyl steroid, and preparation of the 2,4-dimethyl functionality via a palladium catalyzed coupling of 2,4-dibromoestrone acetate with tetramethyltin. The preparative radiochemistry of these three materials is also described.

Effect of 11 beta-substituents on the regioselective chlorination of estrogens with 2,3,4,5,6,6-hexachloro-2,4-cyclohexadienone

The reaction of 11 beta-substituted estrogens with 2,3,4,5,6,6-hexachloro-2,4-cyclohexadienone affords exclusively ortho-substituted monochlorinated products including a major 4-chloro and a minor 2-chloro derivative. In the absence of an 11 beta-substituent, regioselectivity is lost, resulting in a mixture of 10 beta- and ortho-chlorinated products.

Structure-metabolism relationships of ring-A halogenated analogues of 17 alpha-ethynyloestradiol

The metabolic fates of 2-chloro-, 2-bromo-, 4-bromo- and 2-iodo-17 alpha-ethynyloestradiol (EE2) in rats were determined. 6,7-3H-labelled analogues (0.1-2.0 mumol/kg) were administered i.v. to anaesthetized animals. The metabolites of all four compounds were rapidly and extensively excreted in bile (79-93% of the dose over 6 h). Unlike EE2 and 2-fluoro-EE2 (2-FEE2), neither 2-chloro(Cl)-(2.0 mumol/kg),2-bromo(Br)-(0.1 mumol/kg), nor 2-iodo(I)-EE2-(0.1 mumol/kg) underwent C-2 hydroxylation in female rats; 2-BrEE2 was similarly refractory in male rats; females, was subject to approx. 2-fold greater C-2 hydroxylation than 2-FEE2 but this equalled only approx. 60% of that undergone by EE2. All three of the C-2 halogenated derivatives were substantially excreted unchanged except for conjugation. 2-ClEE2 alone was C-4 hydroxylated to an appreciable extent. The oxidative metabolism of 2- and 4-BrEE2 in rats was sexually differentiated: 2-BrEE2 yielded an alkyl hydroxylated metabolite and a two-component dihydroxylated fraction in the ratio 1:0.09 and 1:0.76 in males and females, respectively; 4-BrEE2 underwent C-2 and alicyclic (C-15) hydroxylation in the ratio 1:4.8 and 1:0.07 in males and females, respectively. 2-ClEE2 formed much less alkyl monohydroxylated metabolite (C-16 hydroxylated for 2-Cl- and 2-IEE2) than did either 2-BrEE2 or 2-IEE2. The observed structure-metabolism relationships are discussed.

Radiolabeled steroidal estrogens in cancer research

It has now been more than 30 years since the laboratory verification of the localization of estrogen in certain animal tissues. Much has been learned since that time regarding the details of this process, including the presence of specific receptors for these hormones in target tissues, the mechanism of ligand binding, the association of the ligand-receptor complex with unique chromatin sequences, and the activation of transcription. A concrete use of this knowledge has been the exploitation of these receptors as a targeting mechanisms for radiopharmaceuticals. This is an exciting area that encompasses both diagnosis and therapy. This review will summarize the in vitro and in vivo data obtained from evaluation of the many compounds that have been examined as radiolabeled receptor ligands, and will also discuss the chemistry necessary for their preparation. In particular, relative binding affinity values for relevant compounds will be tabulated, grouped according to molecular class. For those materials for which biodistributions have been performed, uterine (target), liver (nontarget, clearance), and, when available, tumor tissue uptake values are presented. These data should provide a reminder of what has been accomplished, and should serve as a working reference for those engaged in the pursuit of new candidates for these applications.