2-(Hydroxyalkyl)estradiols: synthesis and biological evaluation

Total Synthesis of Stachybotrys microspora Triprenol Phenol-7 (SMTP-7)

The first total synthesis of Stachybotrys microspora triprenol phenol (SMTP)-7 is described. Establishment of the two pyran ring stereogenic centers and key reactions featuring a double reductive amination and a double lactam ring formation in flow are described. The (2R,3S)-trans-benzopyran intermediate 7A, isolated by chiral preparative SFC chromatography, was carried forward to afford SMTP-7. Analytical data for synthetic SMTP-7, including 1H and 13C NMR data, HPLC retention time, and UV spectrum, were in excellent agreement with those for natural SMTP-7.

Intramolecular Diels-Alder Reaction of a Silyl-Substituted Vinylimidazole en Route to the Fully Substituted Cyclopentane Core of Oroidin Dimers

An intramolecular Diels-Alder reaction of a silyl-substituted vinylimidazole delivers a diastereomeric mixture of C4-silyl functionalized dihydrobenzimidazoles. Subsequent diastereoselective reduction and elaboration of the lactone gives rise to a polysubstituted tetrahydrobenzimidazole, which, upon oxidative rearrangement, affords a single spirofused imidazolone containing all of the relevant functionality for an approach to the oroidin dimers axinellamine, massadine, and palau'amine.

Synthesis of prodigiosene–estrogen conjugates: optimization of protecting group strategies and anticancer properties

The tripyrrolic prodigiosene skeleton was conjugated to several estrogen ligands. The conjugation was achieved via an ester linker that proved to be unusually sensitive to hydrolysis during synthesis. This work describes the determination of an appropriate protecting group for the hydroxy groups of the estrogen linker. The anticancer properties of the target prodigiosene–estrogen conjugates were evaluated against breast cancer cells and some show selectivity for ER+ breast cancer cell lines.

Synthesis of 2-[11C]methoxy-3,17β-O,O-bis(sulfamoyl)estradiol as a new potential PET agent for imaging of steroid sulfatase (STS) in cancers

Steroid sulfatase (STS) catalyzes the hydrolysis of steroid sulfates to estrones, the main source of estrogens in tumors. Carbonic anhydrase II (CAII) is highly expressed in red blood cells through a coordination of the monoanionic form of the sulfamate moiety to the zinc atom in the enzyme active site, and CAII is highly expressed in several tumors. 2-Methoxy-3,17β-O,O-bis(sulfamoyl)estradiol (5) is a dual-function STS-CAII inhibitor inhibited STS with 39 nM IC(50) value selectively over CAII with 379 nM IC(50) value. This compound exhibited potent antiproferative activity with mean graph midpoint value of 87 nM in the NCI 60-cell-line panel, and antiangiogenic in vitro and in vivo activity in an early-stage Lewis lung model as well. The compound has been recently developed as a multitargeted anticancer agent. Both STS and CAII are over-expressed in cancers and have become attractive targets for cancer treatment and molecular imaging of cancer. Here we report the first design and synthesis of 2-[(11)C]methoxy-3,17β-O,O-bis(sulfamoyl)estradiol ([(11)C]5) as a new potential imaging agent for biomedical imaging technique positron emission tomography (PET) to image STS in cancers. The authentic standard 5 was synthesized from 17β-estradiol by published procedures in 5 steps with 40% overall chemical yield. The precursor 2-hydroxy-3,17β-O,O-bis(sulfamoyl)estradiol (14a) for radiolabeling was synthesized from 17β-estradiol in 10 steps with 5% overall chemical yield. The target tracer [(11)C]5 was prepared from the precursor 14a with [(11)C]CH(3)OTf through O-[(11)C]methylation and isolated by HPLC combined with solid-phase extraction (SPE) purification in 40-50% radiochemical yields based on [(11)C]CO(2) and decay corrected to end of bombardment (EOB), with 370-740 GBq/μmol specific activity at EOB.

Synthesis of 7alpha-substituted derivatives of 17beta-estradiol

Estrogen receptor (ER) pure antagonists such as ICI-182,780 (fulvestrant) are effective alternatives to tamoxifen (an ER antagonist/weak partial agonist) in the treatment of postmenopausal, receptor-positive human breast cancers. Structurally, these pure antagonists contain the basic core structure of 17beta-estradiol (E(2)) with a long side chain attached to its C-7alpha position. We explored and compared in this study various synthetic routes for preparing a number of C-7alpha-substituted derivatives of E(2), which are highly useful for the design and synthesis of high-affinity ER antagonists, ER-based imaging ligands, and other ER-based multi-functional agents. Using E(2) as the starting material and 1-iodo-6-benzyloxyhexane as a precursor for the C-7alpha side chain, a seven-step synthetic procedure afforded 3,17beta-bis(acetoxy)-7alpha-(6-hydroxyhexanyl)-estra-1,3,5(10)-triene (one of the derivatives prepared) in an overall yield of approximately 45% as compared to other known procedures that afforded substantially lower overall yield (8-27%). The synthetic steps for this representative compound include: (1) protection of the C-3 and C-17beta hydroxyls of E(2) using methoxymethyl groups; (2) hydroxylation of the C-6 position of the bismethoxymethyl ether of E(2); (3) Swern oxidation of the C-6 hydroxy to the ketone group; (4) C-7alpha alkylation of the C-6 ketone derivative of E(2); (5) deprotection of the two methoxymethyl groups; (6) reprotection of the C-3 and C-6 free hydroxyls with acetyl groups; (7) removal of the C-6 ketone and the benzyl group on the side chain by catalytic hydrogenation in acetic acid. As predicted, two of the representative C-7alpha-substituted derivatives of E(2) synthesized in the present study retained strong binding affinities (close to those of E(2) and ICI-182,780) for the human ERalpha and ERbeta subtypes as determined using the radioligand-receptor binding assays.

A-ring-substituted estrogen-3-O-sulfamates: potent multitargeted anticancer agents

Efficient and flexible syntheses of 2-substituted estrone, estradiol and their 3-O-sulfamate (EMATE) derivatives have been developed using directed ortho-lithiation methodology. 2-Substituted EMATEs display a similar antiproliferative activity profile to the corresponding estradiols against a range of human cancer cell lines. 2-Methoxy (3, 4), 2-methylsulfanyl (20, 21) and 2-ethyl EMATEs (32, 33) proved the most active compounds with 2-ethylestradiol-3-O-sulfamate (33), displaying a mean activity over the NCI 55 cell line panel 80-fold greater than the established anticancer agent 2-methoxyestradiol (2). 2-Ethylestradiol-3-O-sulfamate (33) was also an effective inhibitor of angiogenesis using three in vitro markers, and various 2-substituted EMATEs also proved to be inhibitors of steroid sulfatase (STS), a therapeutic target for the treatment of hormone-dependent breast cancer. The potential of this novel class of multimechanism anticancer agents was confirmed in vivo with good activity observed in the NCI hollow fiber assay and in a MDA-MB-435 xenograft mouse model.

Effects of altering the electronics of 2-methoxyestradiol on cell proliferation, on cytotoxicity in human cancer cell cultures, and on tubulin polymerization

A series of new analogues of 2-methoxyestradiol (1) were synthesized to further elucidate the relationships between structure and activity. The compounds were designed to diminish the potential for metabolic deactivation at positions 2 and 17 and were analyzed as inhibitors of tubulin polymerization and for cytotoxicity. 17alpha-methyl-beta-estradiol (30), 2-propynyl-17alpha-methylestradiol (39), 2-ethoxy-17-(1'-methylene)estra-1,3,5(10)-triene-3-ol (50) and 2-ethoxy-17alpha-methylestradiol (51) showed similar or greater tubulin polymerization inhibition than 2-methoxyestradiol (1) and contained moieties that are expected to inhibit deactivating metabolic processes. All of the compounds tested were cytotoxic in the panel of 55 human cancer cell cultures, and generally, the derivatives that displayed the most activity against tubulin were also the most cytotoxic.

Preparation of novel aza-1,7-annulated indoles and their conversion to potent indolocarbazole kinase inhibitors

The synthesis of novel aza-1,7-annulated indoles was achieved and these were converted to indolocarbazoles that proved to be potent kinase inhibitors. These compounds were also evaluated in a human colon carcinoma cell line and proved to be good antiproliferative agents.

A new, practical synthesis of 2-methoxyestradiols

An efficient and practical approach to synthesize moderate to large amounts of 2-methoxyestradiol (2-ME2) is described. The key step in the synthesis is the regioselective introduction of an acetyl group at the C-2 position of estradiol using a zirconium tetrachloride mediated Fries rearrangement carried out on estradiol diacetate. The seven step synthetic procedure readily gave 2-ME2 in 49% overall yield. Application of this method to the synthesis of 2-methoxy-7 alpha-methylestradiol is also described.

The effect of exchanging various substituents at the 2-position of 2-methoxyestradiol on cytotoxicity in human cancer cell cultures and inhibition of tubulin polymerization

A new set of estradiol derivatives bearing various substituents at the 2-position were synthesized in order to further elucidate the structural parameters associated with the antitubulin activity and cytotoxicity of 2-substituted estradiols. The potencies of the new compounds as inhibitors of tubulin polymerization were determined, and the cytotoxicities of the analogues in human cancer cell cultures were investigated. The substituents introduced into the 2-position of estradiol included E-3'-hydroxy-1'-propenyl, 2'-hydroxyethoxy, 3-N,N-dimethylaminoethylideneamino, 2'-hydroxyethylineneamino, (beta-3,4,5-trimethoxyphenyl)ethenyl, phenylethynyl, ethynly, 1'-propynyl, and cyano. The substituents conferring the ability to inhibit tubulin polymerization included E-3'-hydroxy-1'-propenyl, 2'-hydroxyethoxy, ethynyl, and 1'-propynyl. The remaining compounds were all inactive as inhibitors of tubulin polymerization when tested at concentrations of up to 40 microM. All of the compounds were cytotoxic in a panel of 55 human cancer cell cultures, and in general, the most cytotoxic compounds were also the most potent as inhibitors of tubulin polymerization. 2-(1'-Propynyl)estradiol displayed significant anticancer activity in the in vivo hollow fiber animal model.

2-Methoxymethylestradiol: a new 2-methoxy estrogen analog that exhibits antiproliferative activity and alters tubulin dynamics

An estradiol metabolite, 2-methoxyestradiol (2-MeOE(2)), has shown antiproliferative effects in both hormone-dependent and hormone-independent breast cancer cells. Previously, a series of 2-hydroxyalkyl estradiol analogs had been synthesized in our laboratories as potential probes for comparison of estrogen receptor (ER)-mediated versus non-ER-mediated effects in breast cancer cells. A methoxy derivative of 2-hydroxymethyl estradiol was prepared for biological evaluation and comparison with 2-MeOE(2). Estrogenic activity of the synthetic analogs was evaluated in two ways, one by examining affinity of the analogs for the estrogen receptor in MCF-7 cells and the other by examining the ability of the analogs to induce estrogen-responsive gene expression. The analog, 2-methoxymethyl estradiol (2-MeOMeE(2)), demonstrated weak affinity for the estrogen receptor (0.9% of estradiol) and weak ability to stimulate estrogen-induced expression of the pS2 gene (0.02% of estradiol). Antitumor activity was evaluated both in vitro and in vivo. The steroidal nucleus seems to be an attractive target for developing novel tubulin polymerization inhibitors. Additionally, such steroidal compounds may have low toxicity compared to the natural products known to interact with tubulin. Interestingly, 2-MeOMeE(2) inhibited tubulin polymerization in vitro at concentrations of 1 and 3 microM and was more effective than 2-MeOE(2). In cells, 2-MeOMeE(2) was effective in suppressing growth and inducing cytotoxicity in MCF-7 and MDA-MB-231 breast cancer cells. The cytotoxic effects of 2-MeOMeE(2) are associated with alterations in tubulin dynamics, with the frequent appearance of misaligned chromosomes, a significant mitotic delay, and the formation of multinucleated cells. In comparison, 2-MeOE(2) was more effective than 2-MeOMeE(2) in producing cytotoxicity and altering tubulin dynamics in intact cells. Assessment of in vivo antitumor activity was performed in athymic mice containing human breast tumor xenografts. Nude mice bearing MDA-MB-435 tumor xenografts were treated i.p. with 50 mg/kg per day of 2-MeOMeE(2) or vehicle control for 45 days. Treatment with 2-MeOMeE(2) resulted in an approximate 50% reduction in mean tumor volume at treatment day 45 when compared to control animals and had no effect on animal weight. Thus, 2-MeOMeE(2) is an estrogen analog with minimal estrogenic properties that demonstrates antiproliferative effects both in vitro and in the human xenograft animal model of human breast cancer.

Synthesis and biological evaluation of 4-(hydroxyalkyl)estradiols and related compounds

A series of synthetic estrogens containing hydroxyalkyl side chains at the C-4 position of the A ring were designed as metabolically stable analogs of 4-hydroxyestradiol, a catechol estrogen. These synthetic steroids would facilitate investigations on the potential biological role of catechol estrogens and also enable further examination of the structural and electronic constraints on the A ring in the interaction of estrogens with the estrogen receptor. Catechol estrogens are implicated as possible causative agents in estrogen-induced tumorigenesis. 4-Hydroxyestradiol has weaker affinity for the estrogen receptor and exhibits lower estrogenic activity in vivo; on the other hand, the catechol estrogens are prone to further oxidative metabolism and can form reactive intermediates. This report describes the synthesis and initial biochemical evaluation of 4-(hydroxyalkyl)estrogens and 4-(aminoalkyl)estradiols. The 4-(hydroxyalkyl)estrogens were prepared by oxidative hydroboration of 4-alkenylestradiols. The alkenylestradiols were obtained via a Stille cross-coupling between a MOM-protected 4-bromoestradiol and an alkenylstannane. The (4-aminoalkyl)estrogens were prepared from the hydroxyalkyl derivatives with phthalimide under Mitsunobu conditions. The substituted estradiols were evaluated for estrogen receptor binding activity in MCF-7 human mammary carcinoma cells, and 4-(hydroxymethyl)estradiol 1 exhibited the highest affinity with an apparent EC50 value of 364 nM. The relative activities for mRNA induction of the pS2 gene in MCF-7 cell cultures by the 4-(hydroxyalkyl)estrogens closely parallel the relative binding affinities. 4-(Hydroxymethyl)estradiol 1 did not stimulate the growth of MCF-7 cells at concentrations up to 1 microM. Thus, 4-(hydroxymethyl)estradiol 1 exhibited similar estrogen receptor affinity as the catechol estrogen, 4-hydroxyestradiol, and may prove useful in the examination of the biological effects of 4-hydroxyestrogens.

Synthesis, uterotrophic, and antiuterotrophic activities of some estradiol derivatives containing thiadiazole, thiazoline, and thiazolidinone moieties

The effect of structural modification on the biological activity of hormones has been studied on five novel series of estradiol analogs bearing a variety of substituents at the 2-position of the steroidal nucleus. The synthesized compounds include 2-[2-(5-substituted amino-1,3,4-thiadiazol-2-yl)vinyl]estradiol 17 beta-acetate 5-9, 2-aroylmethylestradiols 10-12, 2-[2-aryl-2-(substituted thiocarbamoylhydrazono)ethyl]estradiols 13-18 and their cyclic thiazoline 19-24, and thiazolidinone derivatives 25-30. Among the products, the p-hydroxybenzolmethylestradiol 12 exhibited the highest antiestrogenic activity of 63%. It also elicited 34% of the uterotrophic activity of estradiol.

Structure-activity relationships of selective estrogen receptor modulators: modifications to the 2-arylbenzothiophene core of raloxifene

The 2-arylbenzothiophene raloxifene, 1, is a selective estrogen receptor modulator which is currently under clinical evaluation for the prevention and treatment of postmenopausal osteoporosis. A series of raloxifene analogs which contain modifications to the 2-arylbenzothiophene core have been prepared and evaluated for the ability to bind to the estrogen receptor and inhibit MCF-7 breast cancer cell proliferation in vitro. Their ability to function as tissue-selective estrogen agonists in vivo has been assayed in a short-term, ovariectomized (OVX) rat model with end points of serum cholesterol lowering, uterine weight gain, and uterine eosinophil peroxidase activity. These studies have demonstrated that (1) the 6-hydroxy and, to a lesser extent, the 4'-hydroxy substituents of raloxifene are important for receptor binding and in vitro activity, (2) small, highly electronegative 4'-substituents such as hydroxy, fluoro, and chloro are preferred both in vitro and in vivo, (3) increased steric bulk at the 4'-position leads to increased uterine stimulation in vivo, and (4) additional substitution of the 2-aryl moiety is tolerated while additional substitution at the 4-, 5-, or 7-position of the benzothiophene results in reduced biological activity. In addition, compounds in which the 2-aryl group is replaced by alkyl, cycloalkyl, and naphthyl substituents maintain a profile of in vitro and in vivo biological activity qualitatively similar to that of raloxifene. Several novel structural variants including 2-cyclohexyl, 2-naphthyl, and 6-carbomethoxy analogs also demonstrated efficacy in preventing bone loss in a chronic OVX rat model of postmenopausal osteopenia, at doses of 0.1-10 mg/kg.