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

Steroids Bearing Heteroatom as Potential Drugs for Medicine

Heteroatom steroids, a diverse class of organic compounds, have attracted significant attention in the field of medicinal chemistry and drug discovery. The biological profiles of heteroatom steroids are of considerable interest to chemists, biologists, pharmacologists, and the pharmaceutical industry. These compounds have shown promise as potential therapeutic agents in the treatment of various diseases, such as cancer, infectious diseases, cardiovascular disorders, and neurodegenerative conditions. Moreover, the incorporation of heteroatoms has led to the development of targeted drug delivery systems, prodrugs, and other innovative pharmaceutical approaches. Heteroatom steroids represent a fascinating area of research, bridging the fields of organic chemistry, medicinal chemistry, and pharmacology. The exploration of their chemical diversity and biological activities holds promise for the discovery of novel drug candidates and the development of more effective and targeted treatments.

Secosteroid-quinoline hybrids as new anticancer agents

An elegant approach to unknown secosteroid-quinoline hybrids is disclosed. A series of 13,17-secoestra-1,3,5(10)-trien-17-oic acid [N'-(iso)quinolylmethylene]hydrazides was prepared and these novel type of secosteroids was screened for antiproliferative activity against estrogen-responsive human breast cancer cell line MCF-7. Most of the synthesized compounds showed a cytotoxic effect superior to that of reference drug cisplatin; the lead compound exhibits the highest activity with the IC₅₀ value of about 0.8 μM and is 7 times more active than cisplatin. A high selectivity index was observed for the hit 13,17-secoestra-1,3,5(10)-trien-17-oic acid [N'-quinolylmethylene]hydrazides 2a and 2c. Compounds 2a and 2c evaluated in luciferase reporter assays exhibited high antiestrogenic potency which was superior to that of tamoxifen. These hit compounds were characterized by high activity against MCF-7 cells that retained towards multidrug-resistant NCI/ADR-RES cells.

Antitumor potential of novel 5α,6β-dibromo steroidal D-homo lactone

New steroidal D-homo androstane derivative, 5α,6β-dibromo-3β-hydroxy-17-oxa-17a-homoandrostan-16-one was synthesized and its structure was confirmed by NMR spectroscopy. In silico ADME properties of this compound were assessed using the SwissADME online prediction tool. Six human cancer cell lines (MDA-MB-231, MCF-7, PC3, HT-29, HeLa, and A549) and one human noncancerous cell line (MRC-5) were used for in vitro cytotoxicity testing. Novel steroidal dibromide was also tested for relative binding affinity for the ligand binding domain of estrogen receptor α and β or the androgen receptor using a published assay in yeast cells. Ligand binding domains of each steroid receptor were expressed in-frame with yellow fluorescent protein in yeast and the fluorescence intensity changes upon addition of test compound was measured. The new compound showed selective cytotoxic activity against HT-29 (colon adenocarcinoma) and A549 (lung adenocarcinoma) cell lines, as well as the potential to induce apoptosis in HT-29 cells, while results obtained from ligand binding assay in yeast suggested a lack of significant estrogenic or androgenic properties.

Secosteroidal hydrazides: Promising scaffolds for anti-breast cancer agents

A convenient and selective approach to 13,17-secoestra-1,3,5(10)-trien-17-oic acid hydrazides and their N'-(het)arylmethylene derivatives was disclosed and these novel types of secosteroids were screened for cytotoxicity against hormone-dependent human breast cancer cell line MCF-7. A number of 13,17-secoestra-1,3,5(10)-trien-17-oic acid [N'-(het)arylmethylene]hydrazides show significant cytotoxic effect comparable or superior to that for reference drug cisplatin. Compound 3l exhibits the highest activity with the IC₅₀ value of about 2 μM and is 2.8 times more active than cisplatin. Hit 13,17-secoestra-1,3,5(10)-trien-17-oic acid [N'-(het)arylmethylene]hydrazides 3d, 3l and 3q are characterized by high cytotoxicity and good selectivity towards MCF-7 breast cancer cells. The synthesized secosteroids may be considered as new promising antitumor agents.

Steroidal ferrocenes as potential enzyme inhibitors of the estrogen biosynthesis

The potential inhibitory effect of diverse triazolyl-ferrocene steroids on key enzymes of the estrogen biosynthesis was investigated. Test compounds were synthesized via copper-catalyzed cycloaddition of steroidal azides and ferrocenyl-alkynes using our efficient methodology published previously. Inhibition of human aromatase, steroid sulfatase (STS) and 17β-hydroxysteroid dehydrogenase type 1 (17β-HSD1) activities was investigated with in vitro radiosubstrate incubations. Some of the test compounds were found to be potent inhibitors of the STS. A compound bearing ferrocenyl side chain on the C-2 displayed a reversible inhibition, whereas C-16 and C-17 derivatives displayed competitive irreversible binding mechanism toward the enzyme. 17α-Triazolyl-ferrocene derivatives of 17β-estradiol exerted outstanding inhibitory effect and experiments demonstrated a key role of the ferrocenyl moiety in the enhanced binding affinity. Submicromolar IC₅₀ and K_i parameters enroll these compounds to the group of the most effective STS inhibitors published so far. STS inhibitory potential of the steroidal ferrocenes may lead to the development of novel compounds able to suppress in situ biosynthesis of 17β-estradiol in target tissues.

Microwave-assisted Phospha-Michael addition reactions in the 13α-oestrone series and in vitro antiproliferative properties

Microwave-assisted phospha-Michael addition reactions were carried out in the 13α-oestrone series. The exocyclic 16-methylene-17-ketones as α,β-unsaturated ketones were reacted with secondary phosphine oxides as nucleophilic partners. The addition reactions furnished the two tertiary phosphine oxide diastereomers in high yields. The main product was the 16α-isomer. The antiproliferative activities of the newly synthesised organophosphorus compounds against a panel of nine human cancer cell lines were investigated by means of MTT assays. The most potent compound, the diphenylphosphine oxide derivative in the 3-O-methyl-13α-oestrone series (9), exerted selective cell growth-inhibitory activity against UPCI-SCC-131 and T47D cell lines with low micromolar IC₅₀ values. Moreover, it displayed good tumour selectivity property determined against non-cancerous mouse fibroblast cells.

Synthesis and conversion of primary and secondary 2-aminoestradiols into A-ring-integrated benzoxazolone hybrids and their in vitro anticancer activity

Hybrid systems are often endowed with completely different and improved properties compared to their parent compounds. In order to extend the chemical space toward sterane-based molecular hybrids, a number of estradiol-derived benzoxazol-2-ones with combined aromatic rings were synthesized via the corresponding 2-aminophenol intermediates. 2-Aminoestradiol was first prepared from estrone by a two-step nitration/reduction sequence under mild reaction conditions. Subsequent reductive aminations with different arylaldehydes furnished secondary 2-aminoestradiol derivatives in good yields. The proton dissociation processes of the aminoestradiols were investigated in aqueous solution by UV-visible spectrophotometric titrations to reveal their actual chemical forms at physiological pH. The determined pK₁ and pK₂ values are attributed to the ⁺NH₃ or ⁺NH₂R and OH moieties, and both varied by the different R substituents of the amino group. Primary and secondary 2-aminoestradiols were next reacted with carbonyldiimidazole as a phosgene equivalent to introduce a carbonyl group with simultaneous ring-closure to give A-ring-fused oxazolone derivatives in high yields. The novel aminoestradiols and benzoxazolones were subjected to in vitro cytotoxicity analysis and were found to exert cancer cell specific activity.

Estradiol–benzoxazolone hybrids with a common aromatic moiety were efficiently synthesized via primary and secondary aminophenol intermediates, and their anticancer activities were investigated.

Human cytochrome P450 enzymes 5-51 as targets of drugs and natural and environmental compounds: mechanisms, induction, and inhibition - toxic effects and benefits

Cytochrome P450 (P450, CYP) enzymes have long been of interest due to their roles in the metabolism of drugs, pesticides, pro-carcinogens, and other xenobiotic chemicals. They have also been of interest due to their very critical roles in the biosynthesis and metabolism of steroids, vitamins, and certain eicosanoids. This review covers the 22 (of the total of 57) human P450s in Families 5-51 and their substrate selectivity. Furthermore, included is information and references regarding inducibility, inhibition, and (in some cases) stimulation by chemicals. We update and discuss important aspects of each of these 22 P450s and questions that remain open.

Synthesis, Biological Evaluation and Docking Studies of 13-Epimeric 10-fluoro- and 10-Chloroestra-1,4-dien-3-ones as Potential Aromatase Inhibitors

Fluorination of 13-epimeric estrones and their 17-deoxy counterparts was performed with Selectfluor as the reagent. In acetonitrile or trifluoroacetic acid (TFA), 10β-fluoroestra-1,4-dien-3-ones were formed exclusively. Mechanistic investigations suggest that fluorinations occurred via SET in acetonitrile, but another mechanism was operative in TFA. Simultaneous application of N-chlorosuccinimide (NCS) and Selectfluor in TFA led to a 1.3:1 mixture of 10β-fluoroestra-1,4-dien-3-one and 10β-chloroestra-1,4-dien-3-one as the main products. The potential inhibitory action of the 10-fluoro- or 10-chloroestra-1,4-dien-3-one products on human aromatase was investigated via in vitro radiosubstrate incubation. The classical estrane conformation with trans ring anellations and a 13β-methyl group seems to be crucial for the inhibition of the enzyme, while test compounds bearing the 13β-methyl group exclusively displayed potent inhibitory action with submicromolar or micromolar IC50 values. Concerning molecular level explanation of biological activity or inactivity, computational simulations were performed. Docking studies reinforced that besides the well-known Met374 H-bond connection, the stereocenter in the 13 position has an important role in the binding affinity. The configuration inversion at C-13 results in weaker binding of 13α-estrone derivatives to the aromatase enzyme.

Synthesis and structure-activity relationships of 2- and/or 4-halogenated 13β- and 13α-estrone derivatives as enzyme inhibitors of estrogen biosynthesis

Ring A halogenated 13α-, 13β-, and 17-deoxy-13α-estrone derivatives were synthesised with N-halosuccinimides as electrophile triggers. Substitutions occurred at positions C-2 and/or C-4. The potential inhibitory action of the halogenated estrones on human aromatase, steroid sulfatase, or 17β-hydroxysteroid dehydrogenase 1 activity was investigated via in vitro radiosubstrate incubation. Potent submicromolar or low micromolar inhibitors were identified with occasional dual or multiple inhibitory properties. Valuable structure–activity relationships were established from the comparison of the inhibitory data obtained. Kinetic experiments performed with selected compounds revealed competitive reversible inhibition mechanisms against 17β-hydroxysteroid dehydrogenase 1 and competitive irreversible manner in the inhibition of the steroid sulfatase enzyme.

Synthesis of Novel C-2- or C-15-Labeled BODIPY-Estrone Conjugates

Novel BODIPY-estrone conjugates were synthesized via Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC). Estrone-alkynes or an estrone-azide as starting compounds were synthesized via Michael addition or Sonogashira reaction as key steps. Fluorescent dyes based on BODIPY-core were provided by azide or alkyne functional groups. Fluorescent labeling of estrone was efficiently achieved at the C-2 or C-15 position. The newly-elaborated coupling procedures might have a broad applicability in the synthesis of fluorescent-labeled estrone conjugates suitable for biological assays.

Synthesis and biological evaluation of 13α-estrone derivatives as potential antiproliferative agents

13α-Estrone derivatives containing various substituents on C-3 and C-17 were synthesized, and evaluated by means of MTT assays for in vitro antiproliferative activity against a panel of human adherent cancer cell lines (HeLa, MCF-7, A2780 and A431). Compounds with N-benzyltriazolylmethoxy moieties on C-3 proved to be more potent than their 3-hydroxy or 3-ether counterparts. Some triazoles exerted substantial cytostatic effects against particular tumor cell lines, with submicromolar IC50 values.

Synthesis and in Vitro Antiproliferative Evaluation of C-13 Epimers of Triazolyl-d-Secoestrone Alcohols: The First Potent 13α-d-Secoestrone Derivative

The syntheses of C-13 epimeric 3-[(1-benzyl-1,2,3-triazol-4-yl)methoxy]-d-secoestrones are reported. Triazoles were prepared from 3-(prop-2-inyloxy)-d-secoalcohols and p-substituted benzyl azides via Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC). The antiproliferative activities of the products and their precursors were determined in vitro against a panel of human adherent cervical (HeLa, SiHa and C33A), breast (MCF-7, MDA-MB-231, MDA-MB-361 and T47D) and ovarian (A2780) cell lines by means of MTT assays. The orientation of the angular methyl group and the substitution pattern of the benzyl group of the azide greatly influenced the cell growth-inhibitory potential of the compounds. The 13β derivatives generally proved to be more potent than their 13α counterparts. Introduction of a benzyltriazolylmethyl group onto the 3-OH position seemed to be advantageous. One 13α compound containing an unsubstituted benzyltriazolyl function displayed outstanding antiproliferative activities against three cell lines.

Synthesis of A-ring halogenated 13α-estrone derivatives as potential 17β-HSD1 inhibitors

13α-Estrone and its 3-methyl or benzyl ether were halogenated in ring A with N-bromo- or N-iodosuccinimide or 1,3-dibromo-5,5-dimethylhydantoin as electrophile triggers. The chemo- and regioselectivities of the reactions depended greatly on the nature of the substituent on C-3. Bromination of the ethers led to 2- and 4-regioisomers. Bis-halogenation occurred only in the case of the phenolic derivative. Iodination and bromination resulted in similar products, except that the 3-benzyl ether could not be iodinated under the applied conditions. The potential inhibitory action of the new halogenated 13α-estrones on human 17β-hydroxysteroid dehydrogenase 1 activity was investigated via in vitro radiosubstrate incubation. Some compounds proved to be effective inhibitors, with IC50 values in the submicromolar range.

Synthesis and in vitro pharmacological evaluation of N-[(1-benzyl-1,2,3-triazol-4-yl)methyl]-carboxamides on d-secoestrone scaffolds

An efficient synthesis of several N-[(1-benzyl-1,2,3-triazol-4-yl)methyl]carboxamides in the 13β- and 13α-d-secoestrone series is reported. Novel triazoles were synthesized via the Cu(I)-catalyzed azide-alkyne cycloaddition of steroidal alkynyl carboxamides and p-substituted benzyl azides. Each of the products was evaluated in vitro by means of MTT assays for antiproliferative activity against a panel of human adherent cancer cell lines (HeLa, MCF-7, A431 and A2780). Some of them exhibited activities similar to those of the reference agent cisplatin. On change of the substitution pattern of the benzyl group of the azide, great differences in the cell growth-inhibitory properties were observed. The p-alkylbenzyl-substituted triazoles selectively exerted high cytostatic action against A2780 cells, with IC50 values of 1 µM. We investigated the potential inhibitory action exerted on the human 17β-HSD1 activity of the new secosteroids. Three triazoles effectively suppressed the estrone to 17β-estradiol conversion with IC50 values in low micromolar range.

Synthesis of trans-16-triazolyl-13α-methyl-17-estradiol diastereomers and the effects of structural modifications on their in vitro antiproliferative activities

Novel 16-triazoles in the 13α-estrone series were synthesized via Cu(I)-catalyzed azide-alkyne cycloaddition of the two diastereomeric (on C-16 and on C-17) 16-azido-13α-estra-1,3,5(10)-trien-17-ol 3-benzyl ethers with substituted phenylacetylenes. The new heterocyclic derivatives were evaluated in vitro by means of MTT assays for antiproliferative activity against a panel of human adherent cancer cell lines (HeLa, MCF-7, A431, A2780, T47D, MDA-MB-231 and MDA-MB-361). The inversion of the configurations at C-16 and C-17 selectively affected the growth-inhibitory properties of the tested compounds. The 16β,17α isomers generally proved to be potent on all cell lines, with IC50 values comparable to those of the reference agent cisplatin. Change of the substitution pattern of the phenyl group of the acetylene led to great differences in antiproliferative properties. Exclusively the p-phenyl-substituted triazoles exerted high cytostatic effects. One of the most potent compounds activated caspase-3 and caspase-9 without influencing caspase-8, confirming the induction of apoptosis via the intrinsic pathway.

Current status on development of steroids as anticancer agents

Steroids are important biodynamic agents. Their affinities for various nuclear receptors have been an interesting feature to utilize them for drug development particularly for receptor mediated diseases. Steroid biochemistry and its crucial role in human physiology, has attained importance among the researchers. Recent years have seen an extensive focus on modification of steroids. The rational modifications of perhydrocyclopentanophenanthrene nucleus of steroids have yielded several important anticancer lead molecules. Exemestane, SR16157, fulvestrant and 2-methoxyestradiol are some of the successful leads emerged on steroidal pharmacophores. The present review is an update on some of the steroidal leads obtained during past 25 years. Various steroid based enzyme inhibitors, antiestrogens, cytotoxic conjugates and steroidal cytotoxic molecules of natural as well as synthetic origin have been highlighted. This article is part of a Special Issue entitled "Synthesis and biological testing of steroid derivatives as inhibitors".

Anticancer steroids: linking natural and semi-synthetic compounds

Steroids, a widespread class of natural organic compounds occurring in animals, plants and fungi, have shown great therapeutic value for a broad array of pathologies. The present overview is focused on the anticancer activity of steroids, which is very representative of a rich structural molecular diversity and ability to interact with various biological targets and pathways. This review encompasses the most relevant discoveries on steroid anticancer drugs and leads through the last decade and comprises 668 references.

4- and 6-(p-Sulphamoylphenyl)androstenediones: Studies of aromatase inhibitor-based oestrone sulphatase inhibition

4-(p-Sulphamoylphenyl)androstenedione (3) and 6alpha-p-sulphamoylphenyl analogues 12-14 were synthesised and tested as aromatase inhibitors as well as oestrone sulphatase inhibitors in human placental microsomes. All of the p-sulphamoylphenyl compounds synthesised were powerful inhibitors of aromatase with apparent K(i) values ranging between 30 and 97nM. In addition, the aromatase inhibitory activities of 6alpha-p-hydroxyphenyl compounds 9-11, which may be produced from their respective sulphamoylphenyl compounds by action of oestrone sulphatase, were also high in a range of 23 and 75nM of the K(i) values. On the other hand, all of the sulphamoylphenyl compounds were poor inhibitors of oestrone sulphatase with more than about 200microM of IC(25) values. Although the present findings of the oestrone sulphatase inhibition are disappointing, such attempts may be valuable to develop a new class of drugs having a dual function, aromatase inhibitor and oestrone sulphatase inhibitor, for the treatment of oestrogen-dependent breast cancer.

Mechanism-based categorization of aromatase inhibitors: a potential discovery and screening tool

Cytochrome P450 aromatase is a key steroidogenic enzyme that converts androgens to estrogens in vertebrates. There is much interest in aromatase inhibitors (AIs) both because of their use as pharmaceuticals in the treatment of estrogen-sensitive breast cancers, and because a number of environmental contaminants can act as AIs, thereby disrupting endocrine function in humans and wildlife through suppression of circulating estrogen levels. The goal of the current work was to develop a mechanism-based structure-activity relationship (SAR) categorization framework highlighting the most important chemical structural features responsible for inhibition of aromatase activity. Two main interaction mechanisms were discerned: steroidal and non-steroidal. The steroid scaffold is most prominent when the structure of the target chemical is similar to the natural substrates of aromatase - androstenedione and testosterone. Chemicals acting by non-steroidal mechanism(s) possess a heteroatom (N, O, S) able to coordinate the heme iron of the cytochrome P450, and thus interfere with steroid hydroxylation. The specific structural boundaries controlling AI for both analyzed mechanisms were defined, and a software tool was developed that allowed a decision tree (profile) to be built discriminating AIs by mechanism and potency. An input chemical follows a profiling path and the structure is examined at each step to decide whether it conforms with the structural boundaries implemented in the decision tree node. Such a system would aid drug discovery efforts, as well as provide a screening tool to detect environmental contaminants that could act as AIs.

Chiral aromatase and dual aromatase-steroid sulfatase inhibitors from the letrozole template: synthesis, absolute configuration, and in vitro activity

To explore aromatase inhibition and to broaden the structural diversity of dual aromatase-sulfatase inhibitors (DASIs), we introduced the steroid sulfatase (STS) inhibitory pharmacophore to letrozole. Letrozole derivatives were prepared bearing bis-sulfamates or mono-sulfamates with or without adjacent substituents. The most potent of the achiral and racemic aromatase inhibitor was 40 (IC 50 = 3.0 nM). Its phenolic precursor 39 was separated by chiral HPLC, and the absolute configuration of each enantiomer was determined using vibrational and electronic circular dichroism in tandem with calculations of the predicted spectra. Of the two enantiomers, ( R)-phenol ( 39a) was the most potent aromatase inhibitor (IC 50 = 0.6 nM, comparable to letrozole), whereas the ( S)-sulfamate, ( 40b) inhibited STS most potently (IC 50 = 553 nM). These results suggest that a new structural class of DASI for potential treatment of hormone-dependent breast cancer has been identified, and this is the first report of STS inhibition by an enantiopure nonsteroidal compound.

[Synthesis and biological properties of 6-oxa-D-homo-8alpha analogues of steroid estrogens]

Modifications of 6-oxa-D-homo-8alpha-analogues of steroid estrogens were found to lead to a complete loss of the uterotropic and hypertriglyceridemic activities. These compounds may be promising for the design on their basis of inhibitors of the steroid hormone metabolism and transporters of other compounds to the estrogen target organs.

Formation of chlorinated estrones via hypochlorous disinfection of wastewater effluent containing estrone

Chlorinated derivatives of estrone (E1) in the effluent of a municipal sewage treatment plant located in Shizuoka prefecture, Japan were detected by gas chromatography/mass spectrometry using electron impact in selected ion monitoring (GC/MS-EI-SIM) analysis. The concentrations of E1, 2-chloroestrone, 4-chloroestrone and 2,4-dichloroestrone in the effluent sample collected in December 2005 were 60.0 ng l(-1), 4.0 ng l(-1), 14.5 ng l(-1), and 9.8 ng l(-1), respectively. In the effluent sample taken in June 2005, 2,4-dichloroestrone was detected at 5.6 ng l(-1) along with 17.6 ng l(-1) of E1. However, only E1 was detected at 5.9 ng l(-1) in the sample in May 2005. To elucidate the behavior of E1 during the disinfection process with sodium hypochlorite in the sewage treatment plant, we carried out a reaction of E1 with sodium hypochlorite in buffer solutions at pH 7 and 9. As E1 was consumed rapidly, chlorinated estrones were produced and relatively fast formation of 2-chloroestrone, 4-chloroestrone, and 2,4-dichloroestrone was observed. Furthermore, 1,4-estradiene-3,17-dione derivatives were formed from the reaction between 2,4-dichloroestrone and sodium hypochlorite.

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.

Steroids: partial synthesis in medicinal chemistry

This article reviews the progress in the chemistry of the steroids that was published between January and December 2005. The reactions and partial synthesis of estrogens, androgens, pregnanes, bile acid derivatives, cholestanes and vitamin D analogues are covered. There are 139 references.