Design, synthesis, and biological evaluation of imidazolyl derivatives of 4,7-disubstituted coumarins as aromatase inhibitors selective over 17-α-hydroxylase/C17-20 lyase

The design, synthesis, and biological evaluation of a series of new aromatase (AR, CYP19) inhibitors bearing an imidazole ring linked to a 7-substituted coumarin scaffold at position 4 (or 3) are reported. Many compounds exhibited an aromatase inhibitory potency in the nanomolar range along with a high selectivity over 17-α-hydroxylase/C17-20 lyase (CYP17). The most potent AR inhibitor was the 7-(3,4-difluorophenoxy)-4-imidazolylmethyl coumarin 24 endowed with an IC(50) = 47 nM. Docking simulations on a selected number of coumarin derivatives allowed the identification of the most important interactions driving the binding and clearly indicated the allowed and disallowed regions for appropriate structural modifications of coumarins and closely related heterocyclic molecular scaffolds.     

1-[(Benzofuran-2-yl)phenylmethyl]triazoles as steroidogenic inhibitors: synthesis and in vitro inhibition of human placental CYP19 aromatase

Hormone-dependent breast cancer is stimulated by the female hormones oestrone and oestradiol, therefore compounds which inhibit the specific enzymes involved in the formation of the nitogenic hormones, namely CYP19 aromatase (P450arom) and 17beta-hydroxysteroid dehydrogenase (17beta-HSD) type 1, are targets of therapeutic interest for the treatment of breast cancer. A series of novel 1-[(benzofuran-2-yl)phenylmethyl]1,2,4-triazoles were prepared using a three-step synthesis and evaluated for their inhibitory activity against human placental aromatase in vitro, using [1,2,6,7-3H]androstenedione as the substrate for the aromatase enzyme. Inhibitory activity was dependent on both substituent and position of substitution, with introduction of small electron-withdrawing groups in the phenyl ring showing optimum activity (IC50 ranging from 0.065 to 2.02 microm). Substitution in the benzofuran ring resulted in a loss of activity when substituted at C-5 (IC50 > 20 microm). The compounds were all shown to exhibit weak inhibitory activity against rat testes P450 17 (17,20-lyase), indicating good selectivity towards P450arom.     

Molecular basis for the interaction of four different classes of substrates and inhibitors with human aromatase

Aromatase cytochrome P450 (CYP19) converts androgen to estrogen. In this study, the interactions of four classes of compounds, 17beta-estradiol (the product of aromatase), 17-methyltestosterone (a synthetic androgen), dibenzylfluorescein (a synthetic substrate of aromatase), and coumestrol (a phytoestrogen), with aromatase were investigated through spectral analysis using purified human recombinant aromatase and site-directed mutagenesis studies using CHO cells expressing wild-type human aromatase or five aromatase mutants, E302D, D309A, T310S, S478T and H480Q. Spectral analysis showed that a type I binding spectrum was produced by the binding of 17-methyltestosterone to aromatase and a novel binding spectrum of aromatase was induced by dibenzylfluorescein. Mutagenesis experiments demonstrated that residues S478 and H480 in the beta-4 sheet play an important role in the binding of all four compounds. Computer-assisted docking of these compounds into the three-dimensional model of aromatase revealed that: (1) weak interaction between 17beta-estradiol and the beta-4 sheet of aromatase facilitates the release of 17beta-estradiol from the active site of aromatase; (2) 17-methyl group of 17-methyltestosterone affects its binding to aromatase; (3) dibenzylfluorescein binds to the active site of aromatase with its O-dealkylation site near the heme iron and residue T310; and (4) coumestrol binds to aromatase in a manner such that rings A and C of coumestrol mimic rings A and B of steroid. These structure-function studies help us to evaluate the structural model of aromatase, and to accelerate the structure-based design for new aromatase inhibitors.     

Structure-activity relationships of new A,D-ring modified steroids as aromatase inhibitors: design, synthesis, and biological activity evaluation

Inhibition of aromatase is an efficient approach for the prevention and treatment of breast cancer. New A,D-ring modified steroid analogues of formestane and testolactone were designed and synthesized and their biochemical activity was investigated in vitro in an attempt to find new aromatase inhibitors and to gain insight into their structure-activity relationships (SAR). All compounds tested were less active than formestane. However, the 3-deoxy steroidal olefin 3a and its epoxide derivative 4a proved to be strong competitive aromatase inhibitors (K(i) = 50 and 38 nM and IC50 = 225 and 145 nM, respectively). According to our findings, the C-3 carbonyl group is not essential for anti-aromatase activity, but 5alpha-stereochemistry and some planarity in the steroidal framework is required. Furthermore, modification of the steroidal cyclopentanone D-ring, by construction of a delta-lactone six-membered ring, decreases the inhibitory potency. From the results obtained, it may be concluded that the binding pocket of the active site of aromatase requires planarity in the region of the steroid A,B-rings and the D-ring structure is critical for the binding.     

Synthesis and evaluation of 17-aliphatic heterocycle-substituted steroidal inhibitors of 17alpha-hydroxylase/C17-20-lyase (P450 17)

In the search for potent inhibitors of P450 17, the key enzyme in androgen biosynthesis, a series of steroidal inhibitors were synthesized and tested toward rat and human P450 17. Small aliphatic heterocycles (aziridine, oxirane, thiirane, diaziridine, diazirine, azetidine) were introduced into the 17beta-position of anstrost-5-en-3beta-ol. After identifying that aziridine is the most suitable functional group to coordinate with the heme iron, modifications of the steroidal skeleton were performed for further optimization. A wide range of inhibitory potencies toward P450 17 were found for the 21 test compounds. The most potent inhibitors toward the human and rat enzyme were aziridine compounds 3 (IC(50) rat: 0.21 microM, K(i) = 3 nM; IC(50) human: 0.54 microM, K(i) = 8 nM), 5 (IC(50) rat: 0.43 microM, K(i) = 7 nM; IC(50) human: 0.29 microM, K(i) = 4 nM), and 8 (21R:21S = 1:1; IC(50) rat: 0.53 microM, K(i) = 9 nM; IC(50) human: 0.40 microM, K(i) = 6 nM) which were more potent than the reference ketoconazole (IC(50) rat: 67 microM; IC(50) human: 0.74 microM). The inhibitory potency depends markedly on the stereochemistry at C20 of the inhibitors. This effect is more pronounced for the rat enzyme. Tested for selectivity, the highly potent inhibitors show poor inhibitory activity toward P450 arom, P450 scc, P450 TxA(2), and 5alpha-reductase. Tested for in vivo activity, 3 and 8 (0.019 mmol/kg) decreased the plasma testosterone concentration in rats by 81% and 84% after 2 h.     

Inhibition of aromatase (P450Arom) by some 1-(benzofuran-2-ylmethyl)imidazoles.

Studies of a series of 1-(benzofuran-2-ylmethyl)imidazoles, 1-5, previously proposed as potential agents for prostatic cancer by their inhibition of 17beta-hydroxylase:17,20-lyase (P450 17), have been extended to their selectivity against placental microsomal aromatase (P450(Arom)) in man. The compounds were 3-7-fold more potent than aminoglutethimide and had some selectivity for P450 17 as expressed by the ratio (IC50 P450(Arom))/(IC50 P450) 17)/17.0 (2), 10.3 (3), 34.6 (4) and 42.0 (5), where IC50 is the concentration resulting in 50% inhibition. The lower potency of 1-5 towards P450(Arom) compared with the racemic alpha-phenyl-substituted compounds (6, 80-1000 x aminoglutethimide) and some racemic alpha-methyl (8.5 and 12.2 x aminoglutethimide) and alpha-ethyl (12.1 and 32.9 x aminoglutethimide) analogues has been rationalized. This work selectively extends studies of the P450 17 inhibitor 5, a potential prostatic cancer agent, towards other cytochrome P450 enzymes in the steroidogenic pathway and provides a general method for determining the relative influence of chemical manipulation of a parent inhibitor towards two enzymes in the pathway using additional literature data.     

Synthesis and aromatase inhibitory activity of novel pyridine-containing isoflavones

Aromatase, a cytochrome P450 hemoprotein that is responsible for estrogen biosynthesis by conversion of androgens into estrogens, has been an attractive target in the treatment of hormone-dependent breast cancer. As a result, a number of synthetic steroidal or nonsteroidal aromatase inhibitors have been successfully developed. In addition, there are several classes of natural products that exert potent activities in aromatase inhibition, with the flavonoids being most prominent. Previous studies have exploited flavone and flavanone scaffolds for the development of new aromatase inhibitors. In this paper, we describe the design, synthesis, and biological evaluation of a novel series of 2-(4'-pyridylmethyl)thioisoflavones as the first example of synthetic isoflavone-based aromatase inhibitors.     

Detection of aromatase inhibitors in vitro using rat ovary microsomes

Inhibition of aromatase activity in vitro is one of the Tier 1 screening assays proposed by the Endocrine Disrupter Screening and Testing Advisory Committee (EDSTAC) for the detection of potential endocrine disrupters. In this report a rat ovarian aromatase inhibition assay has been evaluated using the reference aromatase inhibitors anastrozole, fadrozole, letrozole and CGS 18320B. Rat ovary microsomes were used as the enzyme source, as endocrine disruption studies are most commonly carried out in this species. Aromatase activity was inhibited in vitro by anastrozole, fadrozole, letrozole and CGS 18320B with IC(50)s of 25, 7, 7 and 5 nM, respectively. This assay, therefore, appears to have good sensitivity to aromatase inhibitors and may be useful as a general screening assay and in mechanistic studies.     

Structure-based design of potent aromatase inhibitors by high-throughput docking

Cytochrome P450 aromatase catalyzes the conversion of androgen substrates into estrogens. Aromatase inhibitors (AIs) have been used as first-line drugs in the treatment of estrogen-dependent breast cancer in postmenopausal women. However, the search for new, more potent, and selective AIs still remains necessary to avoid the risk of possible resistances and reduce toxicity and side effects of current available drugs. The publication of a high resolution X-ray structure of human aromatase has opened the way to structure-based virtual screening to identify new small-molecule inhibitors with structural motifs different from all known AIs. In this context, a high-throughput docking protocol was set up and led to the identification of nanomolar AIs with new core structures.     

Potent CYP19 (aromatase) 1-[(benzofuran-2-yl)(phenylmethyl)pyridine, -imidazole, and -triazole inhibitors: synthesis and biological evaluation

The synthesis of a series of novel 1-[(benzofuran-2-yl)phenylmethyl]-pyridine, -imidazole, and -triazole derivatives is described. All the compounds were evaluated in vitro for inhibitory activity against aromatase (P450(AROM), CYP19), using human placental microsomes. The 6-methoxy- and 6-hydroxy-substituted benzofuran derivatives were shown to be potent CYP19 inhibitors (IC(50) = 0.01-1.46 microM) with activity greater than that observed for the unsubstituted parent compounds and inhibitory activity comparable with or greater than the reference compound arimidex (IC(50) = 0.6 microM). Six of the benzofuran derivatives were subjected to in vitro cytotoxicity assays, using rat liver hepatocytes with cytotoxicity determined from alteration in cell morphology and lactate dehydrogenase enzyme retention over a period of 24 h, and selectivity (CYP17, 17beta-HSD types 1 and 3, CYP24, and CYP26) determination; negligible inhibitory activity was observed, suggesting a good selectivity for CYP19. The pyridine benzofuran 4a containing the 4-fluorophenyl group was the most promising (IC(50) = 44 nM; LC(50) >100 microM) compared with arimidex (IC(50) = 600 nM; LC(50) > 200 microM).     

1-imidazolyl(alkyl)-substituted di- and tetrahydroquinolines and analogues: syntheses and evaluation of dual inhibitors of thromboxane A(2) synthase and aromatase

A series of 1-imidazolyl(alkyl)-substituted quinoline, isoquinoline, naphthalene, benzo[b]furan, and benzo[b]thiophene derivatives was synthesized as dual inhibitors of thromboxane A(2) synthase (P450 TxA(2)) and aromatase (P450 arom). Dual inhibition of these enzymes could be a novel strategy for the treatment of mammary tumors and the prophylaxis of metastases. The most potent dual inhibitors, 5-(2-imidazol-1-ylethyl)-7,8-dihydroquinoline (31) (P450 TxA(2): IC(50) = 0.29 microM; P450 arom: IC(50) = 0.50 microM) and its 5, 6-saturated analogue 30 (P450 TxA(2): IC(50) = 0.68 microM; P450 arom: IC(50) = 0.38 microM), showed a stronger inhibition of both target enzymes than the reference compounds (dazoxiben: IC(50) = 1.1 microM; aminoglutethimide: IC(50) = 18.5 microM). For the determination of the in vivo activity, the influence of selected compounds on serum TxB(2) concentration was examined in rats. Compound 30 (8.5 mg/kg body weight) led to a reduction of the TxB(2) serum level of 78%, 71%, and 51% after 3, 5, and 8 h, respectively (dazoxiben: 60%, 34%, and 36%). Selectivity was studied toward some enzymes of the steroidogenic and eicosanoid pathways. P450 17 was inhibited by selected compounds only at high concentrations. Compound 30 inhibited P450 scc by 13% (25 microM). Compound 31 did not affect cyclooxygenase and lipoxygenase.     

Design, synthesis, and 3D QSAR of novel potent and selective aromatase inhibitors

The design, synthesis, and biological evaluation of a series of new aromatase inhibitors bearing an imidazole or triazole ring linked to a fluorene (A), indenodiazine (B), or coumarin scaffold (C) are reported. Properly substituted coumarin derivatives displayed the highest aromatase inhibitory potency and selectivity over 17-alpha-hydroxylase/17-20 lyase. The modeling of the aromatase inhibition data by Comparative Molecular Field Analysis (CoMFA/GOLPE 3D QSAR approach) led to the development of a PLS model with good fitting and predictive powers (n = 22, ONC = 3, r(2) = 0.949, s = 0.216, and q(2) = 0.715). The relationship between aromatase inhibition and the steric and electrostatic fields generated by the examined azole inhibitors enables a clear understanding of the nature and spatial location of the main interactions modulating the aromatase inhibitory potency.     

The inhibition of human prostatic aromatase activity by imidazole drugs including ketoconazole and 4-hydroxyandrostenedione

Ketoconazole, an orally active imidazole drug and bifonazole, clotrimazole, econazole, isoconazole, miconazole and tioconazole are known as inhibitors of cytochrome P450 dependent steroidogenic enzymes including human placental aromatase. The aim of the present study was to investigate the effectiveness of these imidazole drugs to inhibit human prostatic aromatase activity compared with the known inhibitor of aromatase 4-hydroxyandrostenedione (4-OHA). The imidazole drugs and 4-OHA inhibited prostatic aromatase activity in a dose-dependent manner. The order of decreasing inhibitory potency determined from IC50 values (mumol/L) was: 4-OHA (1.57) greater than bifonazole (1.6) greater than tioconazole (1.69) greater than clotrimazole (1.73) greater than econazole (1.87) greater than miconazole (2.0) greater than isoconazole (2.2) greater than ketoconazole (4.7). The IC50 values for the inhibition of prostatic homogenate aromatase activity are 3-9-fold higher than that for the inhibition of human placental aromatase activity, previously reported, except that of ketoconazole which was 1.5-fold lower than that for the inhibition of placental microsomal aromatase.     

Enantioselective nonsteroidal aromatase inhibitors identified through a multidisciplinary medicinal chemistry approach

To identify enantioselective nonsteroidal aromatase inhibitors, a multidisciplinary medicinal chemistry approach was pursued. First, our earlier CoMFA model [Bioorg. Med. Chem. 1998,6, 377-388] was extended taking purposely into account previously discovered enantioselective aromatase inhibitors. The 3D QSAR model was then exploited to design chiral ligands, whose configurational assignment was obtained, after HPLC separation, by means of a combination of circular dichroism measurements and time dependent density functional calculations. Finally, the new enantiomeric inhibitors were separately tested to ascertain both their potency against the cytochrome P450 aromatase (CYP19; EC 1.14.14.1), and their selectivity relative to another enzyme of the P450 family. A satisfactory agreement between experimental and predicted data allowed us to assert that a properly built "enantioselective CoMFA model" might constitute a useful tool for addressing enantioselective ligands design.     

Synthesis and evaluation of a new series of mechanism-based aromatase inhibitors.

A series of new 4-(alkylthio)-substituted androstenedione analogues was designed as potential suicide inhibitors of aromatase on the basis of mechanistic considerations on the mode of action of the enzyme. Their synthesis and biological evaluation are described. Among the most interesting are the 4-[(difluoromethyl)thio]-, 4-[(fluoromethyl)thio]-, and 4-[(chloromethyl)thio]androstenediones 12, 13, and 14 with respective IC50's of 2.7, 0.8, and 0.94 microM. Compound 12 was a reversible inhibitor of aromatase while compounds 13 and 14 displayed time-dependent kinetics of inhibition with respective KI's and half-times of inactivation of 30 nM and 3.75 min for 13 and 30 nM and 3 min for 14. The inhibition of aromatase by 14 was NADPH-dependent, and was protected by the presence of substrate (0.5-1 microM), while beta-mercaptoethanol (0.5 mM) failed to protect the enzyme from inactivation. Dialysis failed to reactivate aromatase previously inactivated by 14. The mechanistic implications of these findings are discussed.     

Fadrozole hydrochloride: a potent, selective, nonsteroidal inhibitor of aromatase for the treatment of estrogen-dependent disease.

A new class of potent, selective, nonsteroidal inhibitors of aromatase have been discovered. The most potent member of this series is fadrozole hydrochloride, CGS 16949 A, 4-(5,6,7,8-tetrahydroimidazo[1,5-alpha]pyridin-5-yl)benzonitrile monohydrochloride, 26a. In addition, the 6,7-dihydropyrrolo[1,2-c]imidazole (21a) and the 6,7,8,9-tetrahydroimidazo[1,5-alpha]azepine (21b) analogues were synthesized and evaluated. CGS 16949 A's ability to selectively inhibit aromatase (IC50 = 4.5 nM) over other cytochrome P-450 enzymes and suppress estrogen production when administered orally make it a suitable candidate to test the potential of an aromatase inhibitor in estrogen-dependent diseases including breast cancer.     

芳香化酶抑制剂的研究进展

芳香化酶是雌激素生物合成过程中的一个关键酶,抑制芳香化酶的活性就可以抑制雌激素的生成,从而减少绝经后妇女乳腺癌等疾病的发病率。以芳香化酶为靶标的研究一直都是抑制剂研究的热点,目前已有多个芳香化酶抑制剂进入临床试验阶段或已上市,该文对近年来芳香化酶抑制剂的研究进行综述。