Synthesis and structure-activity relationships of 6-substituted androst-4-ene analogs as aromatase inhibitors

Identification of Novel Scaffolds with Dual Role as Antiepileptic and Anti-Breast Cancer

Aromatase inhibitors with an $\mathrm{IC}_{50}$ IC 50 value ranging from 1.4 to 49.7 µM are known to act as antiepileptic drugs besides being potential breast cancer inhibitors. The aim of the present study is to identify novel antiepileptic aromatase inhibitors with higher activity exploiting the ligand-based pharmacophore approach utilizing the experimentally known inhibitors. The resultant Hypo1 consists of four features and was further validated by using three different strategies. Hypo1 was allowed to screen different databases to identify lead molecules and were further subjected to Lipinski's Rule of Five and ADMET to establish their drug-like properties. Consequently, the obtained 68-screened molecules were subjected to molecular docking by GOLD v5.2.2. Furthermore, the compounds with the highest dock scores were assessed for molecular interactions. Later, the MD simulation was applied to evaluate the protein backbone stabilities and binding energies adapting GROMACS v5.0.6 and MM/PBSA which was followed by the density functional theory (DFT), to analyze their orbital energies, and further the energy gap between them. Eventually, the number of Hit molecules was culled to three projecting Hit1, Hit2, and Hit3 as the potential lead compounds based on their highest dock scores, hydrogen bond interaction, lowest energy gap, and the least binding energies and stable MD results.

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.

Effects of steroidal aromatase inhibitors on sensitive and resistant breast cancer cells: aromatase inhibition and autophagy

Several therapeutic approaches are used in estrogen receptor positive (ER(+)) breast cancers, being one of them the use of aromatase inhibitors (AIs). Although AIs demonstrate higher efficacy than tamoxifen, they can also exhibit de novo or acquired resistance after prolonged treatment. Recently, we have described the synthesis and biochemical evaluation of four steroidal AIs, 3β-hydroxyandrost-4-en-17-one (1), androst-4-en-17-one (12), 4α,5α-epoxyandrostan-17-one (13a) and 5α-androst-2-en-17-one (16), obtained from modifications in the A-ring of the aromatase substrate, androstenedione. In this study, it was investigated the biological effects of these AIs in different breast cancer cell lines, an ER(+) aromatase-overexpressing human breast cancer cell line (MCF-7aro cells), an estrogen-receptor negative (ER(-)) human breast cancer cell line (SK-BR-3 cells), and a late stage of acquired resistance cell line (LTEDaro cells). The effects of an autophagic inhibitor (3-methyladenine) plus AIs 1, 12, 13a or exemestane in LTEDaro cells were also studied to understand the involvement of autophagy in AI acquired resistance. Our results showed that these steroids inhibit aromatase of MCF-7aro cells and decrease cell viability in a dose- and time-dependent manner. The new AI 1 is the most potent inhibitor, although the AI 12 demonstrates to be the most effective in decreasing cell viability. Besides, and in advantage over exemestane, AIs 12 and 13a also reduced LTEDaro cells viability. The use of the autophagic inhibitor allowed AIs to diminish viability of LTEDaro cells, presenting a similar behavior to the sensitive cells. Thus, inhibition of autophagy may sensitize hormone-resistant cancer cells to anti-estrogen therapies.

Synthesis and biochemical studies of 17-substituted androst-3-enes and 3,4-epoxyandrostanes as aromatase inhibitors

A series of 5alpha-androst-3-enes and 3alpha,4alpha-epoxy-5alpha-androstanes were synthesized and tested for their abilities to inhibit aromatase in human placental microsomes. In these series the original C-17 carbonyl group was replaced by hydroxyl, acetyl and hydroxyimine groups. Inhibition kinetic analysis on the most potent steroid of these series revealed that it inhibits the enzyme in a competitive manner (IC(50)=6.5 microM). The achieved data pointed out the importance of the C-17 carbonyl group in the D-ring of the studied steroids as a structural feature required to reach maximum aromatase inhibitory activity. Further, at least one carbonyl group (C-3 or C-17) seems to be essential to effective aromatase inhibition.

Synthesis and biological evaluation of 3-(azolylmethyl)-1H-indoles and 3-(alpha-azolylbenzyl)-1H-indoles as selective aromatase inhibitors

This present study identifies a number of azolyl-substituted indoles as potent inhibitors of aromatase. In the sub-series of 3-(azolylmethyl)-1H-indoles, four imidazole derivatives and their triazole analogues were tested. Imidazole derivatives 11 and 14 in which the benzyl moiety was substituted by 2-chloro and 4-cyano groups, respectively, were the most active, with IC50 values ranging between 0.054 and 0.050 microM. In the other sub-series, eight 3-(alpha-azolylbenzyl)-1H-indoles were prepared and tested. Compound 30, the N-ethyl imidazole derivative, proved to be an aromatase inhibitor, showing an IC50 value of 0.052 microM. All target compounds were further evaluated against 17alpha-hydroxylase/C17,20-lyase to determine their selectivity profile.

6alpha-Methylandrostenedione: gas chromatographic mass spectrometric detection in doping control

In recent years products containing 6alpha-methylandrost-4-ene-3,17-dione have appeared on the sport supplement market. Scientific studies have proven aromatase inhibition and anabolic and mild androgenic properties; however, no preparation has been approved for medical use up to now. In sports 6alpha-methylandrost-4-ene-3,17-dione has to be classified as a prohibited substance according to the regulations of the World Anti-Doping Agency (WADA). For the detection of its misuse the metabolism was studied following the administration of two preparations obtained from the Internet (Formadrol and Methyl-1-Pro). Several metabolites as well as the parent compounds were synthesized and the structures of 3alpha-hydroxy-6alpha-methyl-5beta-androstan-17-one, 6alpha-methylandrost-4-ene-3,17-dione, and 5beta-dihydromedroxyprogesterone were confirmed by nuclear magnetic resonance (NMR) spectroscopy. The main metabolite, 3alpha-hydroxy-6alpha-methyl-5beta-androstan-17-one, was found to be excreted as glucuronide and was still detectable in microg/mL amounts until urine collection was terminated (after 25 h). Additionally, samples from routine human sports doping control had already tested positive for the presence of metabolites of 6alpha-methylandrost-4-ene-3,17-dione. Screening analysis can be easily performed by the existing screening procedure for anabolic steroids using 3alpha-hydroxy-6alpha-methyl-5beta-androstan-17-one as target substance (limit of detection <10 ng/mL). Its discrimination from the closely eluting drostanolone metabolite, 3alpha-hydroxy-2alpha-methyl-5alpha-androstan-17-one, is possible as the mono-TMS derivative.

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.

Structure-activity relationships of 3-deoxy androgens as aromatase inhibitors. Synthesis and biochemical studies of 4-substituted 4-ene and 5-ene steroids

As part of our investigation into the structure-activity relationship of a novel class of aromatase inhibitors, two series of 3-deoxy androgens, androst-5-en-17-ones with a non-polar alkoxy (5 and 6), alkyl (20-22), or phenylalkyl (23 and 24) group at C-4beta and 4-acyloxyandrost-4-en-17-ones (29-32, and 34) were synthesized and evaluated. The 4beta-alkyl and 4beta-phenylalkyl compounds were obtained through reaction of 4alpha,5alpha-epoxy steroid (8) with RMgBr (R: alkyl and phenylalkyl) followed by dehydration of the 4beta-substituted 5alpha-hydroxy products (15-19) with SOCl(2) as key reactions. Acylation of 4alpha,5alpha-diol (25) with (RCO)(2)O in pyridine and subsequent dehydration with SOCl(2) gave the 4-acyloxy steroids. All of the steroids studied, except for 4-acetoxy-19-ol (34) that was a non-competitive inhibitor of human placental aromatase, blocked aromatase activity in a competitive manner. 4-Benzoyloxy- and 4-acetoxy steroids (31) and (32) were the most powerful inhibitors of aromatase (K(i)=70 and 60nM, respectively). Elongation of an acetoxy group in a series of 4-acyloxy steroids or a methyl group in a series of 4beta-alkyl steroids decreased affinity for aromatase principally in relation to carbon number of the acyl or alkyl function. The present findings are potentially useful for understanding the spatial and electronic nature of the binding site of aromatase as well as for developing effective aromatase inhibitors.

Probing the binding pocket of the active site of aromatase with 6-ether or 6-ester substituted androst-4-ene-3,17-diones and their diene and triene analogs

A series of 6-ester- (3 and 4) and 6-ether- (7 and 8) substituted androst-4-ene-3,17-diones (androstenediones) and their 1,4-diene analogs (5 and 6, and 9 and 10) as well as C6-substituted 4,6-diene and 1,4,6-triene steroids 11 and 12 were synthesized as aromatase inhibitors to gain insight into the structure-activity relationship between various substituents and inhibitory activity. All of the inhibitors synthesized blocked aromatase in a competitive manner. The inhibitory activities of all of the steroids, except for the 6beta-benzoates 4g and 6h and the 6beta-acetate 6a, were fairly effective to very powerful (K(i): 7.0-320 nM). The 6alpha-n-hexanoyloxy- and 6alpha-benzyloxyandrostenediones (3e and 7e) were the most potent inhibitors (K(i): 7.0 nM each). In the series of 4-ene and 1,4-diene steroids, the 6alpha-substituted steroids had higher affinity for the enzyme than the corresponding 6beta-isomers. In the 1,4-diene steroid series, 6beta-substituted steroids 6a, e, g, and 10a, b, e caused a time-dependent inactivation of aromatase, whereas their 6alpha-isomers 5 and 9 essentially did not. The ether-substituted 1,4,6-trienes 12 inactivated the enzyme in a time-dependent manner; in contrast, their 4,6-diene analogs 11 did not. The substrate androstenedione blocked the inactivation, but no significant effect of L-cysteine was observed. Based on molecular modeling with the PM3 method, along with the present inhibition and inactivation results, it is thought that both the steric effects of the 6-substituents as well as the electronic effects of the C-6 oxygen functions play a critical role in the binding of inhibitors to the active site of aromatase.

19-oxygenations of 3-deoxy androgens, potent competitive inhibitors of estrogen biosynthesis, with human placental aromatase

Aromatase is a cytochrome P450 enzyme complex that catalyzes the conversion of androst-4-ene-3,17-dione (AD) to estrone through three sequential oxygenations of the 19-methyl group. To gain insight into the ability of 3-deoxy derivative of AD, compound 1, and its 5-ene isomer 4, which are potent competitive inhibitors of aromatase, to serve as a substrate, we studied their 19-oxygenation by human placental aromatase and the metabolites isolated were analyzed by gas chromatography-mass spectrometry. Inhibitors 1 and 4 were found to be oxygenated with aromatase to produce the corresponding 19-hydroxy derivatives 2 and 5 and 19-oxo derivatives 3 and 6 as well as the 17beta-reduced 19-hydroxy compounds 7 and 8. Kinetic studies indicated that the 5-ene steroid 4 was surprisingly a good substrate for the aromatase-catalyzing 19-oxygenation with the V(max) value of 45 pmol/min per mg prot which was approx. four times higher than that of the other. The relative K(m) value for steroids 1 and 4 obtained in this study is opposite from the relative K(i) value obtained previously in the inhibition study. The results reveal that there is a difference between a binding suitable for serving as an inhibitor of aromatase and a binding suitable for serving as a substrate of the enzyme in the 3-deoxy steroid series and the C-3 carbonyl group of AD is essential for a proper binding as a substrate to the active site of aromatase.