Click estradiol dimers with novel aromatic bridging units: synthesis and anticancer evaluation

Estradiol dimers (EDs) possess significant anticancer activity by targeting tubulin dynamics. In this study, we synthesised 12 EDs variants via copper-catalysed azide-alkyne cycloaddition (CuAAC) reaction, focusing on structural modifications within the aromatic bridge connecting two estradiol moieties. In vitro testing of these EDs revealed a marked improvement in selectivity towards cancerous cells, particularly for ED1-8. The most active compounds, ED3 (IC50 = 0.38 μM in CCRF-CEM) and ED5 (IC50 = 0.71 μM in CCRF-CEM) demonstrated cytotoxic effects superior to 2-methoxyestradiol (IC50 = 1.61 μM in CCRF-CEM) and exhibited anti-angiogenic properties in an endothelial cell tube-formation model. Cell-based experiments and in vitro assays revealed that EDs interfere with mitotic spindle assembly. Additionally, we proposed an in silico model illustrating the probable binding modes of ED3 and ED5, suggesting that dimers with a simple linker and a single substituent on the aromatic central ring possess enhanced characteristics compared to more complex dimers.

Concise synthesis of 2-methoxyestradiol through C(sp2)-H methoxylation

An efficient and concise synthesis of 2-methoxyestradiol (4) from 17β-estradiol (1) has been achieved in three synthetic steps with a 63.3% overall yield. The key step was the palladium-catalyzed direct C(sp²)-H methoxylation of 2-aryloxypyridines. Using 2-pyridyloxyl as the directing group, Pd(OAc)₂ as the catalyst, PhI(OAc)₂ as the oxidant and methanol as both the methoxylation reagent and solvent, the methoxy group could be handily installed at the 2-position of 3-(2-pyridoxy) estradiol (2). Subsequently, the pyridyl group could be easily removed by nucleophilic substitution with a methoxy anion after being oxidized to a pyridyl N-oxide by m-chloroperoxybenzoic acid, delivering the target product 2-methoxyestradiol (4) in quantitative yield. In contrast, when the pyridyl directing group was removed by the TfOMe-NaOMe/MeOH system as reported in the literature, TfOMe inevitably methylated the 17-OH of 2-methoxy-3-(2-pyridoxy) estradiol (3). In effect, we have fortuitously found a new method to cleave the pyridyl directing group, which is highly suitable for substrates bearing hydroxy groups.

Synthesis and evaluation of 17α-triazolyl and 9α-cyano derivatives of estradiol

A variety of 17α-triazolyl and 9α-cyano derivatives of estradiol were prepared and evaluated for binding to human ERβ in both a TR-FRET assay, as well as ERβ and ERα agonism in cell-based functional assays. 9α-Cyanoestradiol (5) was nearly equipotent as estradiol as an agonist for both ERβ and ERα. The potency of the 17α-triazolylestradiol analogs is considerably more variable and depends on the nature of the 4-substituent of the triazole ring. While rigid protein docking simulations exhibited significant steric clashing, induced fit docking providing more protein flexibility revealed that the triazole linker of analogs 2d and 2e extends outside of the traditional ligand binding domain with the benzene ring located in the loop connecting helix 11 to helix 12.

Synthesis and Antitumor Evaluation of Novel Hybrids of Phenylsulfonylfuroxan and Estradiol Derivatives

Fifteen novel furoxan-based nitric oxide (NO) releasing hybrids of estradiol derivatives were synthesized and evaluated in vitro anti-proliferative activity in MDA-MB-231, A2780, Hela and HUVEC cell lines. Most of them displayed potent anti-proliferative effects. Among the compounds, 4-bromo-3-((phenylsulfonyl)-1,2,5-oxadiazole 2-oxide)-oxy-propoxy-estradiol (11 b) exhibited the best activity with IC50 values of 3.58-0.0008 μM. Preliminary pharmacological studies showed that 11 b induced apoptosis and hardly affected the cell cycle of MDA-MB-231 cell line. NO-releasing capacity and inhibition of ERK/MAPK pathway signaling might explain the potent antineoplastic activity of these compounds. The preliminary structure-activity relationship (SAR) showed that steroidal scaffolds with a linker in 3-position were favorable moieties to evidently increase the bioactivities of these hybrids. Overall, these results implied that 11 b merited to be further investigated as a promising anti-cancer candidate.

Stereoselective synthesis of new type of estradiol hybrid molecules and their antiproliferative activities

To prepare new type of estrane hybrid molecules, we chose 3-methoxy- and 3-benzyloxy-17β,16β-epoxymethylene-estra-1,3,5(10)-trienes as starting materials (2 and 5). These steroid oxetanes were transformed with ethylene glycol in the presence of BF₃.OEt₂ into 3-methoxy- and 3-benzyloxy-16β-(2'-oxa-4'-hydroxy)butyl-17β-hydroxy-estra-1,3,5(10)-trien-17β-ols (3a and 6a). Iodination of the terminal hydroxy group afforded iodo derivatives 3b and 6b, which underwent one-pot 3-O-alkylation with unprotected ascorbic acid to yield 3c and 6c. The same process with salicylic acid led to 2-O-alkylated salicylic acid derivatives 3d and 6d. Iodo derivatives 3b and 6b underwent nucleophilic exchange reaction with NaN₃ furnishing the corresponding azido compounds 3e and 6e. These compounds were subjected to azide-alkyne CuAAC reactions with phenylacetylene and their p-substituted derivatives to form 1,4-substituted triazoles 3f-h and 6f-h. The reduction of 3e and 6e with hydrazine hydrate in the presence of Raney Ni provided the corresponding amino derivatives 3i and 6i. These compounds were reacted further with varied substituted benzoic acids to deliver terminal benzamido derivatives 3j-m and 6j-m. We determined the in vitro antiproliferative activities of compounds 2, 5, 3a-m and 6a-m by means of MTT assays on a panel of human adherent cancer cell lines A2780, MCF-7, MB-231 and SiHa.

Synthesis of benzoylbenzamide derivatives of 17α-E-vinyl estradiol and evaluation as ligands for the estrogen receptor-α ligand binding domain

A series consisting of substituted benzoylbenzamide derivatives of 17α-E-vinyl estradiol 6a-i and 7a-d was prepared in good overall yields from the corresponding novel iodinated benzoylbenzamide precursors using Pd(0)-catalyzed Stille coupling. Biological evaluation using competitive binding assays indicated that all compounds were effective ligands for the ERα- and ERβ-LBD (RBA = 0.5-10.0% of estradiol). Most of the compounds expressed lower stimulatory (agonist) potency (RSA <0.2-0.5%) compared to their binding affinity, however, the meta-substituted isomer 6h demonstrated a level of efficacy (RSA = 5.7%) comparable to its affinity (RBA = 9.5%). Docking studies of 6b, 6h, and 6i with the 2YAT crystal structure suggested that higher affinity and efficacy of 6h are due to an effective set of interactions with exposed receptor sidechains not observed with the ortho- and para- isomers. In this binding model, the terminal ring of the ligand is exposed to the solvent space, which would explain both the small variation in RBA values and the narrow SAR for the diverse structural features.

Design, synthesis and antiproliferative effect of 17β-amide derivatives of 2-methoxyestradiol and their studies on pharmacokinetics

A series of 17β-amide-2-methoxyestradiol compounds were synthesized with an aim to enhance the antiproliferative effect of 2-methoxyestradiol. The antiproliferative activity of 2-methoxyestradiol analogs against human cancer cells was investigated. 2-methoxy-3-benzyloxy-17β-chloroacetamide-1,3,5(10)-triene (5e) and 2-methoxy-3-hydroxy-17β-butyramide-1,3,5(10)-triene (6c) had comparable or better antitumor activity than 2-methoxyestradiol. The elimination half-life of 6c (t_1/2β=240.93min) is ten times longer than 2-ME and the area under the curve was seven times (AUC_0-tmin=2068.20±315.74μgmL^-1min) higher than 2-ME, respectively. Whereas 5e had similar pharmacokinetic behavior with 2-ME (t_1/2β=22.28min) with a t_1/2β of 29.5 min. 6c had higher blood concentration, longer actuation duration and better suppression rate against S180 mouse ascites tumor than 2-methoxyestradiol.

Synthesis of novel hybrid steroid dimers by BF3·Et2O-catalyzed aldol condensation of 2-formyl-estradiol diacetate and steroid sapogenins

BF₃·Et₂O-catalyzed aldol condensation of steroid sapogenins with 2-formyl-estradiol diacetate afforded two novel classes of steroid dimers in which an estrogenic core is attached to the spirostanic side chain of an steroid sapogenin through an exocyclic double bond in position C-23, or through a spiro centre in C-22.

Novel Pieces for the Emerging Picture of Sulfoximines in Drug Discovery: Synthesis and Evaluation of Sulfoximine Analogues of Marketed Drugs and Advanced Clinical Candidates

Sulfoximines have gained considerable recognition as an important structural motif in drug discovery of late. In particular, the clinical kinase inhibitors for the treatment of cancer, roniciclib (pan-CDK inhibitor), BAY 1143572 (P-TEFb inhibitor), and AZD 6738 (ATR inhibitor), have recently drawn considerable attention. Whilst the interest in this underrepresented functional group in drug discovery is clearly on the rise, there remains an incomplete understanding of the medicinal-chemistry-relevant properties of sulfoximines. Herein we report the synthesis and in vitro characterization of a variety of sulfoximine analogues of marketed drugs and advanced clinical candidates to gain a better understanding of this neglected functional group and its potential in drug discovery.

Thermodynamic Meerwein-Ponndorf-Verley reduction in the diastereoselective synthesis of 17α-estradiol

The synthesis of 17α-hydroxy steroids generally requires multiple synthetic manipulations. The synthesis of 17α-estradiol is no exception, as this process involves the protection and release of the 3-hydroxy functional group. The diastereoselective reduction of the 17-keto-steroid can be utilized to prepare 17α-hydroxy-steroids. Here, 17α-estradiol was synthesized from commercially available estrone under thermodynamic Meerwein-Ponndorf-Verley (MPV) conditions in a single step, followed by simple chromatographic separation over silica gel. The remaining mixture of unreacted estrone and estradiols was easily recycled through Oppenauer oxidation to estrone, with an overall yield of 68% 17α-estradiol.

Rapid synthesis of [18F]fluoroestradiol: remarkable advantage of microwaving over conventional heating

16α-[(18)F]fluoroestradiol ([(18)F]FES) is known as a clinically important tracer in nuclear medicine as an estrogen receptor ligand for investigating primary and metastatic breast cancers. Synthesizing [(18)F]FES is a two-step process associated with [(18)F]fluoride incorporation to the precursor (3-methoxymethyl 16β,17β-epiestriol-O-cyclic sulfone) and subsequent hydrolysis of the [(18)F]fluorinated intermediate with 2 N HCl. The impact of microwave (MW) heating on both fluorination and hydrolysis reactions was investigated. The duration and temperatures of the fluorination reaction were varied for both MW heating and conventional heating (CH) methods. Chemical and radiochemical purity and radiochemical yields were investigated for CH and compared with MW-assisted radiosyntheses. Quality control tests of MW-assisted [(18)F]FES were performed following US Pharmacopeia procedures for clinical-grade positron emission tomography pharmaceuticals. The results demonstrate that microwaving not only improves the (18)F-fluoride incorporation (~55% improvement at 110°C for 4 min) but also significantly reduces hydrolysis time (approximately sevenfold reduction at 120°C) in comparison with CH under similar conditions. The overall isolated radiochemical yield of purified [(18)F]FES was significantly higher (~90% improvement) with MW, and side products were notably fewer. Quality control test results demonstrated that [(18)F]FES produced by microwaving was suitable for human injection.

Synthesis, cytotoxic effects and tubulin polymerization inhibition of 1,4-disubstituted 1,2,3-triazole analogs of 2-methoxyestradiol

Thirteen 1,4-disubstituted 1,2,3-triazole analogs of 2-methoxyestradiol were prepared and tested for their cytotoxic and tubulin polymerization inhibition effects. Two compounds, 11j and 11k, exhibited anti-proliferative effects at low micromolar concentrations. The two analogs 11j and 11k also inhibited tubulin assembly with IC50 values of 8.1 and 5.9μM, respectively.

Synthesis and antiproliferative activity of novel steroidal dendrimer conjugates

We describe the synthesis of steroidal dendrimer conjugates of first and second generation with tetramethylene core and 5-hydroxy-isophtalic acid dimethyl ester as branching unit modified to incorporate ethynylestradiol or 17α-estradiol as terminal units. The steroidal dendrimer conjugates, the free drug (steroids) and dendrimer were tested against a panel of cancer cell lines (CEM, MCF7, HeLa) and normal human fibroblast (BJ). The steroidal dendrimer conjugates of first generation exhibited cytotoxic activity and induced apoptosis in chronic leukemia (CEM) as resultant activation of caspase cascade which is mainly provoked in G2/M arrested cells.

Bone selective protective effect of a novel bone-seeking estrogen on trabecular bone in ovariectomized rats

The drawbacks of estrogen restrict the clinical use of hormone replacement therapy, and it would be most helpful to explore new estrogenic substances that could prevent bone loss and be free from any adverse effects. We synthesized a new compound named bone-seeking estrogen (SE2) by combining 17β-estradiol (E2) with iminodiacetic acid through the Mannich reaction. E2 and SE2 were labeled with isotope (3)H, and the tissue distribution tests of E2-(3)H and SE2-(3)H were analyzed by the radioactivity. The specific nuclear binding of E2 and SE2 in osteoblasts was measured. SE2 exhibited significantly greater affinity for bone but lower affinity for ovary and uterus than did E2, and SE2 maintained a high affinity for the estrogen receptor alpha similar to that of E2. SE2 administration did not induce uterine hypertrophy. Body weight increase was significantly suppressed by treatment with E2 but not by SE2 after ovariectomy (OVX). SE2 decreased bone turnover as E2 after OVX detected by serum biochemical markers. Bone histology and micro-CT analysis revealed that SE2 administration, similar to E2, could improve bone mass and trabecular architecture after OVX. Biomechanical analyses showed that SE2 treatment effectively increased mechanical properties after OVX. The results suggested that SE2 was effective in preventing OVX-induced bone loss and exhibited few side effects on body weight and uterine hypertrophy, which was beneficial in reducing the adverse effects caused by E2. SE2 may be a better choice than E2 for the prevention of postmenopausal osteoporosis.

A second-generation 2-Methoxyestradiol prodrug is effective against Barrett's adenocarcinoma in a mouse xenograft model

2-Methoxyestradiol (2-ME2) is an endogenous metabolite of estradiol. In preclinical models, 2-ME2 is effective against different types of tumors. Unfortunately, only low systemic concentrations of 2-ME2 can be achieved following oral administration, even after very high doses are administered to patients. In an effort to solve this problem, we have now synthesized and tested a new prodrug of 2-ME2 that is water-soluble due to a bioreversible hydrophilic group added at the 3-position and that more effectively resists metabolic inactivation due to an ester moiety added to mask the 17-position alcohol. We are reporting here for the first time that this double prodrug of 2-ME2 is effective as an antiproliferative and anticancer agent for both in vitro and in vivo studies against Barrett esophageal adenocarcinoma (BEAC) and provided greater potency than 2-ME2 in inhibiting the growth of BEAC xenografts. Finally, studies indicate that, like 2-ME2, the 2-ME2-PD1 exhibits anticancer effect through possible disruption of microtubule network.

Progress in synthesis and antitumor activities of estradiol-linked platinum complex

Platinum complexes such as cisplatin and caboplatin are widely used in cancer chemotherapy. However, their clinical applications are substantially limited by unexpected toxic side effects. In this review, we discuss the current progress on the design and synthesis of estradiol-linked platinum complexes as the targeted antitumor drugs. Many of them display a high antitumor activity against the growth of breast cancer cell lines in vitro. The estradiol-linked platinum complexes could be used as target therapeutics for breast cancer.

Novel 7α-alkoxy-17α-(4'-halophenylethynyl)estradiols as potential SPECT/PET imaging agents for estrogen receptor expressing tumours: synthesis and binding affinity evaluation

In order to develop potential radiolabelled probes for imaging estrogen receptor (ER) positive tumours, we have synthesized and characterized a series of novel 7α-alkoxy-17α-(4'-iodophenylethynyl)estra-1,3,5(10)-triene-3,17β-diols and 7α-alkoxy-17α-(4'-fluorophenylethynyl)estra-1,3,5(10)-triene-3,17β-diols. The fluoro-substituted compounds showed a higher ER binding affinity than the corresponding iodo-derivatives, where 7α-methoxy- and 17α-(4'-fluorophenylethynyl)estra-1,3,5(10)-triene-3,17β-diol showed the highest ER binding affinities (RBA=80.9% and 78.9%, respectively), among the halophenylethynyl compounds studied and should be further explored as potential PET biomarkers for imaging of ER expressing tumours.

Synthesis and evaluation of 11β-(4-substituted phenyl) estradiol analogs: transition from estrogen receptor agonists to antagonists

INTRODUCTION

As part of our program to develop estrogen receptor (ER) targeted imaging and therapeutic agents we chose to evaluate 11β-substituted estradiol analogs as a representative scaffold. Previous synthetic studies provided an entry into this class of compounds and other work indicated that 11β-(substituted aryl) estradiol analogs were potent antagonists of the ER. Little information existed about the specific structural features involved in the transition from agonism to antagonism for the 11β-aryl estradiol analogs or their potential as scaffolds for drug conjugation.

METHODS

We prepared and characterized a series of 11β-(4-Substituted phenyl) estradiol analogs using modifications of existing synthetic methods. The new compounds, as well as standard steroidal agonists and antagonists, were evaluated as competitive ligands for the ERβ-LBD. Functional assays used the induction of alkaline phosphatase in Ishikawa cells to determine potency of the compounds as ER agonists or antagonists.

RESULTS

The synthetic strategy successfully generated a series of compounds in which the 4-substituent was sequentially modified from hydroxyl to methoxy to azidoethoxy/N,N-dimethylaminoethoxy and eventually to a prototypical 1,4-naphthoquinone-containing moiety. The new compounds all retained high relative binding affinity (RBA) for the ERα-LBD, ranging from 13-83% that of estradiol. No subtype selectivity was observed. More importantly, the transition from agonist to antagonist activity occurs at the 4-methoxy stage where the compound is a mixed antagonist. More notably, antagonism appeared to be more dependent upon the size of the 11β-substituent than upon the nature of the terminal group

CONCLUSIONS

We have developed a synthetic strategy that provides facile access to potent 11β-(4-substituted phenyl) estradiol analogs. The resultant compounds retain high affinity for the ERα-LBD and, more importantly, demonstrate potent antagonist activity in cells. Large functionalities distal to the 11β-phenyl ring had little additional effect on either affinity or efficacy, suggesting the incorporation of diverse imaging or biologically active groups can be attached without significantly compromising the ER-binding capacity. Future studies are in progress to exploit the 11β-aryl estradiol analogs as potential drug delivery systems and imaging agents.

A simple and convenient synthesis of 2-methoxyestradiol from estrone

A simple and straightforward synthesis of 2-methoxyestradiol have been achieved in nine synthetic steps with 21% of overall yield. Being a convenient process, it can be upscaled to industrial process.

Synthesis and evaluation of 17α-(dimethylphenyl)vinyl estradiols as probes of the estrogen receptor-α ligand binding domain

As part of our program to explore the influence of small structural modifications on the biological response of the estrogen receptor-α (ERα), we prepared and evaluated a series of mono-and di-substituted phenyl vinyl estradiols. The target compounds were prepared in 45-80% yields using the Stille coupling reaction and evaluated using competitive binding analysis with the ERα-ligand binding domain (hERα-LBD) and estrogenic activity (induction of alkaline phosphatase in Ishikawa cells). Results indicated that the 2,4- and 2,5-dimethyl derivatives, 5b and 5c, had the highest relative binding affinity (RBA=20.5 and 37.3%) and relative stimulatory activity (RSA=101.0% and 12.3%) of the di-methyl series.

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)

A series of 17α-(heteroaryl)vinyl estradiols was prepared to evaluate the influence of heteroatom on the affinity and efficacy of estrogenic ligands for the estrogen receptor-alpha ligand binding domain (ERα-LBD). The products demonstrated reduced binding affinity compared to the parent 17α-E-phenyl vinyl estradiol, but the binding was relatively independent of the heteroatom. The greatest influence of the heteroatom was evident in the efficacy of the compounds as the thienyl derivatives 2f,g were more potent than either the pyridyl 2b-d or pyrimidinyl 2e analogs. The results suggest that a subtle interplay of interactions between the ligands and the receptor influences the biological response.

Increasing the amphiphilicity of an estradiol based steroid structure by Barbier-allylation--ring-closing metathesis--dihydroxylation sequence

Polyhydroxylated steroids, such as brassinosteroids, phytoecdysteroids and steroid saponins, are structurally attractive compounds possessing a number of interesting biological properties. Accordingly, development of synthetic procedures to build steroid based structures mimicking the naturally occurring hydrophilic steroids is of topical interest. In the present work, a D-secoestrone derivative was modified further by Barbier-allylation - ring-closing metathesis - dihydroxylation sequence with the aim to prepare steroid based structures with limited hydrophilicity. A straightforward synthesis route was developed with the isolated yield for each step ranging from good to excellent. All compounds prepared were fully characterized by NMR spectroscopic techniques and completely assigned (1)H and (13)C spectra are reported herein. Finally, the effects of the synthesized amphiphilic steroid derivatives on the proliferation of cancer cells are reported and discussed.

Synthesis of 2-substituted 17β-hydroxy/17-methylene estratrienes and their in vitro cytotoxicity in human cancer cell cultures

Synthesis of various types of 2-(alkylaminomethyl) and 2-(aroyl) 17β-estradiol analogs are reported. The synthesis of similar types of 2-substituted 17-methylene estratriene analogs was also achieved. Synthesis of chalcone derivatives of 17β-estradiol and 17-methylene estratriene were also realized. All these 2-substituted estratrienes were tested for their antiproliferative activity by using four different cell lines from colon, lung, glioma and breast cancers. Among the various 2-substituted estratrienes, the compounds 10d, 14a-h and 17e were found to have in vitro antiproliferative activity comparable to that of parent analogs 1-4. Comparison of the SAR pattern of these 2-susbtituted estratriene derivatives confirmed that relatively, 17-methylene estratrienes are more active than that of 17β-estradiol analogs.

Synthesis and binding affinities of Re(I) and (99m)Tc(I)-containing 16alpha-substituted estradiol complexes: Models for potential breast cancer imaging agents

To develop technetium and rhenium-labeled imaging agents for estrogen receptor (ER) positive breast tumors, we have synthesized tridentate metal tricarbonyl chelates substituted at the 16alpha-position of estradiol. Their structures were characterized by IR, (1)H NMR, (13)C NMR, HRMS or elemental analysis. The rhenium complex 7b showed the highest ER binding affinity (RBA=25.7) among these compounds, so ligand 6b was selected to be labeled by the precursor [(99m)Tc(H(2)O)(3)(CO)(3)](+) to yield technetium(I)-99m complex 7b' with good radiochemical yields. The lipophilicity of corresponding technetium(I)-99m complex 7b' was appropriately reduced, which might be favorable to target tissue selectivity in vivo. The stability of complex 7b' is excellent in 1mM histidine, 1mM cysteine, PBS and bovine serum within 6h in vitro.

3-O-phosphate ester conjugates of 17-β-O-{1-[2-carboxy-(2-hydroxy-4-methoxy-3-carboxamido)anilido]ethyl}1,3,5(10)-estratriene as novel bone-targeting agents

A series of 3-O-phosphorylated analogs (4-10) of a novel bone-targeting estradiol analog (3) were synthesized after a thorough study of the reaction of 3 with a selection of phosphoryl chlorides under a variety of reaction conditions. Evaluation of these novel phosphate analogs for affinity for hydroxyapatite revealed that they bind with equal or higher affinity when compared to the bone tissue accumulator, tetracycline.