Recent advances in the clinical application of aromatase inhibitors

Molecular docking and QSAR studies of aromatase inhibitor androstenedione derivatives

Objectives

Aromatase (CYP19) inhibitors have emerged as promising candidates for the treatment of estrogen-dependent breast cancer. In this study, a series of androstenedione derivatives with CYP19 inhibitory activity was subjected to a molecular docking study followed by quantitative structure–activity relationship (QSAR) analyses in search of ideal physicochemical characteristics of potential aromatase inhibitors.

Methods

The QSAR studies were carried out using both two-dimensional (topological, and structural) and three-dimesional (spatial) descriptors. We also used thermodynamic parameters along with 2D and 3D descriptors. Genetic function approximation (GFA) and genetic partial least squares (G/PLS) were used as chemometric tools for QSAR modelling.

Key findings

The docking study indicated that the important interacting amino acids in the active site were Met374, Arg115, Ile133, Ala306, Thr310, Asp309, Val370, Leu477 and Ser478. The 17-keto oxygen of the ligands is responsible for the formation of a hydrogen bond with Met374 and the remaining parts of the molecules are stabilized by the hydrophobic interactions with the non-polar amino acids. The C2 and C19 positions in the ligands are important for maintaining the appropriate orientation of the molecules in the active site. The results of docking experiments and QSAR studies supported each other.

Conclusions

The developed QSAR models indicated the importance of some Jurs parameters, structural parameters, topological branching index and E-state indices of different fragments. All the developed QSAR models were statistically significant according to the internal and external validation parameters.

Probing the binding pocket of the active site of aromatase with 2-phenylaliphatic androsta-1,4-diene-3,17-dione steroids

A series of 2-phenylaliphatic-substituted androsta-1,4-diene-3,17-diones (6) as well as their androstenedione derivatives (5) were synthesized as aromatase inhibitors to gain insights of structure-activity relationships of varying the alkyl moiety (C(1) to C(4)) of the 2-phenylaliphatic substituents as well as introducing a methyl- or trifluoromethyl function to p-position of a phenethyl moiety to the inhibitory activity. The inhibitors examined showed a competitive type inhibition. The 2-phenpropylandrosta-1,4-diene 6c was the most powerful inhibitor (K(i): 16.1nM) among them. Compounds 6c along with the phenethyl derivative 6b caused a time-dependent inactivation of aromatase (k(inact): 0.0293 and 0.0454min(-1) for 6b and 6c, respectively). The inactivation was prevented by the substrate androstenedione, and no significant effect of l-cysteine on the inactivation was observed in each case. Molecular docking of the phenpropyl compound 6c to aromatase was conducted to demonstrate that the phenpropyl group orients to a hydrophobic binding pocket in the active site to result in the formation of thermodynamically stable enzyme-inhibitor complex.

6beta,19-Bridged androstenedione analogs as aromatase inhibitors

Inhibition of aromatase is an efficient approach for the prevention and treatment of breast cancer. New 6beta,19-bridged steroid analogs of androstenedione, 6beta,19-epithio- and 6beta,19-methano compounds 11 and 17, were synthesized starting from 19-hydroxyandrostenedione (6) and 19-formylandrost-5-ene-3beta,17beta-yl diacetate (12), respectively, as aromatase inhibitors. All of the compounds including known steroids 6beta,19-epoxyandrostenedione (4) and 6beta,19-cycloandrostenedione (5) tested were weak to poor competitive inhibitors of aromatase and, among them, 6beta,19-epoxy steroid 4 provided only moderate inhibition (K(i): 2.2 microM). These results show that the 6beta,19-bridged groups of the inhibitors interfere with binding in active site of aromatase.

Aromatase inactivation by 2-substituted derivatives of the suicide substrate androsta-1,4-diene-3,17-dione

To gain the structure-activity relationship of Delta(1)-androstenediones (Delta(1)-ADs) as mechanism-based inactivator of aromatase, series of 2-alkyl- and 2-alkoxy-substituted Delta(1)-ADs (6 and 9) as well as 2-bromo-Delta(1)-AD (14) were synthesized and tested. All of the inhibitors examined blocked aromatase in human placental microsomes in a competitive manner. In a series of 2-alkyl-Delta(1)-ADs (6), n-hexyl compound 6f was the most powerful inhibitor with an apparent K(i) value of 31 nM. The inhibitory activities of 2-alkoxy steroids 9 decreased in relation to length of the alkyl chain up to n-hexyloxy group (K(i): 95 nM for methoxy 9a). All of the alkyl steroids 6 along with the alkoxy steroid 9, except for the ethyl and n-propyl compounds 6b and 6c, caused a time-dependent inactivation of aromatase. The inactivation rates (k(inact): 0.020-0.084 min(-1)) were comparable to that of the parent compound Delta(1)-AD. The inactivation was prevented by the substrate AD, and no significant effect of l-cysteine on the inactivation was observed in each case. The results indicate that the 2-hexyl compound 6f act as the most powerful mechanism-based inactivator of aromatase among Delta(1)-AD analogs and may be submitted to the preclinical study in estrogen-dependent breast cancer.

Chemical aromatization of 19-hydroxyandrosta-1,4-diene-3,17-dione with acid or alkaline: elimination of the 19-hydroxymethyl group as formaldehyde

In order to determine whether or not a 19-hydroxymethyl group of 19-hydroxyandrosta-1,4-diene-3,17-dione (2, 19-hydroxy ADD), an intermediate of aromatase-catalyzed estrone formation from ADD, a suicide substrate of aromatase, is eliminated as formaldehyde, we examine chemical nature of removal of the 19-hydroxymethyl group. 19-acetate 3 and 19-tert-butyldimethylsiloxy compound 4 are known to convert rapidly to estrone with treatment of NaOH or n-Bu4NF. Since compound 2 was unstable and unobtainable under these conditions, compounds 3 and 4 as equivalents to compound 2 were used in this study. The acetate 3 with 5 mol/l HCl in acetone and 10% KOH in MeOH along with the silyl ether 4 with 5 mol/l HCl in acetone and 1 mol/l n-Bu4NF in THF gave formaldehyde and estrone in which a ratio of the aldehyde to estrone was near 1. This result indicates that the 19-hydroxymethyl groups of compound 3 and 4 are eliminated as formaldehyde along with estrone derived from the steroid skeleton under the acid or base treatment. The findings suggest that a single hydroxylation at the 19 carbon of ADD (1) would be, chemically, all that was required for estrone formation.

Studies directed towards a mechanistic evaluation of inactivation of aromatase by the suicide substrates androsta-1,4-diene-3,17-diones and its 6-ene derivatives aromatase inactivation by the 19-substituted derivatives and their enzymic aromatization

To gain insight into the mechanistic features for aromatase inactivation by the typical suicide substrates, androsta-1,4-diene-3,17-dione (ADD, 1) and its 6-ene derivative 2, we synthesized 19-substituted (methyl and halogeno) ADD and 1,4,6-triene derivatives 8 and 10 along with 4,6-diene derivatives 9 and tested for their ability to inhibit aromatase in human placental microsomes as well as their ability to serve as a substrate for the enzyme. 19-Methyl-substituted steroids were the most powerful competitive inhibitors of aromatase (K(i): 8.2-40 nM) in each series. Among the 19-substituted inhibitors examined, 19-chloro-ADD and its 6-ene derivatives (7b and 9b) inactivated aromatase in a time-dependent manner in the presence of NADPH in air while the other ones did not. The time-dependent inactivation was blocked by the substrate AD and required NADPH. Only the time-dependent inactivators 7b and 9b in series of 1,4-diene and 1,4,6-triene steroids as well as all of 4,6-diene steroids 9, except for the methyl compound 9a, served as a substrate for aromatase to yield estradiol and/or its 6-ene estradiol with lower conversion rates compared to the corresponding parent steroids 1,4-diene, 1,4,6-triene and 4,6-diene derivatives. The present findings strongly suggest that the aromatase reaction, 19-oxygenation, at least in part, would be involved in the time-dependent inactivation of aromatase by the suicide substrates 1 and 2, where the 19-substitutent would play a critical role in the aromatase reaction probably though steric and electronic reasons.

Trilostane in advanced breast cancer

Antiestrogens, principally tamoxifen, and aromatase inhibitors have been used as the first- and second-line therapy in patients with advanced postmenopausal breast cancer for many years. However, some patients acquire resistance to these treatments and, at present, further endocrine treatment is achieved by merely substituting the current medication with a different antiestrogen or aromatase inhibitor. Trilostane offers an alternative endocrine treatment due to its unique mode of action. It is an allosteric modulator of the estrogen receptor and targets both the estrogen- and growth factor-dependent pathways through which estradiol stimulates cell proliferation. In clinical trials, trilostane has been shown to be an effective treatment for breast cancer in patients who have relapsed after receiving treatment with one or more forms of endocrine therapy. Ongoing and future clinical trials are examining the potential for the use of trilostane in premenopausal breast cancer, as well as in other malignancies such as prostate cancer.

Biochemical aromatization of 2-methyleneandrostenedione: stereochemistry of hydrogen removal at the C-1 position

To explore a stereochemistry of hydrogen removal at C-1 of the powerful aromatase inhibitor 2-methyleneandrostenedione (1), of which the A-ring conformation is markedly different from that of the natural substrate androstenedione (AD), in the course of the aromatase-catalyzed A-ring aromatization producing 2-methylestrone (2), we synthesized [1alpha-2H]labeled steroid 1 and its [1beta-2H]stereoisomer, and the metabolic fate of the C-1 deuterium in aromatization was analyzed by gas chromatography-mass spectrometry (GC-MS) in each. Parallel experiments with the natural substrates [1alpha-2H] and [1beta-2H]ADs were also carried out. The GC-MS analysis indicated that 2-methyl estrogen 2 produced from [1alpha-2H]labeled substrate 1 retained completely the 1alpha-deuterium (1beta-H elimination), while product 2 obtained from [1beta-2H]isomer 1 lost completely the 1beta-deuterium. Stereospecific 1beta-hydrogen elimination was also observed in the parallel experiments with the labeled ADs as established previously. The results indicate that biochemical aromatization of the 2-methylene steroid 1 proceeds through the 1beta-hydrogen removal concomitant with cleavage of the C(10)-C(19) bond, yielding 1(10),4-dienone 9, in a similar manner to that involved in AD aromatization. This would give additional evidence for the stereomechanisms for the last step of aromatization of AD, requiring the stereospecific 1beta-hydrogen abstraction and cleavage of the C(10)-C(19) bond, and for the enolization of a carbonyl group at C-3 in the A-ring aromatization.

Steroid-free medium discloses oestrogenic effects of the bisphosphonate clodronate on breast cancer cells

Tamoxifen is the standard first-line endocrine therapy for breast cancer, but recent data indicate that it is likely to be replaced by the effective aromatase inhibitors (AIs), in both the metastatic and adjuvant settings. Aromatase inhibitors induce complete oestrogen deprivation that leads to clinically significant bone loss. Several ongoing or planned trials combine AIs with bisphosphonates, even more so that recent data reveal that clodronate may reduce the incidence of bone metastases and prolong survival in the adjuvant setting. Bisphosphonates can inhibit breast cancer cell growth in vitro, but they have never been studied in steroid-free medium (SFM), an in vitro environment that mimics the effects of AIs in vivo. Quite surprisingly, in SFM, clodronate stimulated MCF-7 cell growth in a time- and dose-dependent manner by up to two-fold (crystal violet staining assay), whereas it had no mitogenic activity in complete medium. The bisphosphonate similarly increased the proliferation of IBEP-2 cells, which also express a functional oestrogen receptor (ER), while it weakly inhibited the growth of the ER-negative MDA-MB-231 cells. Expectedly, 17beta-oestradiol stimulated the growth of MCF-7 and IBEP-2 cells cultured in SFM, and had no effect on MDA-MB-231 cells. Moreover, partial (4-OH-tamoxifen) and pure antioestrogens (fulvestrant, ICI 182,780), in combination with clodronate, completely suppressed the mitogenic effect of the bisphosphonate, suggesting that it was mediated by an activation of ER. In accordance with this view, clodronate induced ER downregulation, weakly increased progesterone receptor expression, and stimulated the transcription of an oestrogen-responsive reporter gene. In conclusion, we report a previously unknown stimulatory effect of clodronate on MCF-7 cells grown in SFM, in vitro conditions that are potentially relevant to the use of AIs for breast cancer. Moreover, our data suggest that ER is involved in these effects of clodronate on cancer cell growth.

Probing the active site of aromatase with 2-methyl-substituted androstenedione analogs

To gain insight into the spatial nature of the androstenedione (AD) binding (active) site of aromatase in relation to the catalytic function of the enzyme, we synthesized 2,2-dimethylAD (4), 2beta- and 2alpha-methylADs (5 and 6), 19-oxygenated derivatives of compounds 4 and 6, and 2-methyleneAD (17), and we then tested their inhibitory activity as well as their aromatase reaction (aromatization for 2-methyl and 2-methylene analogs or 19-oxygenation for 2,2-dimethyl steroids) with human placental aromatase. 2-Methyl and 2-methylene steroids 5, 6, and 17 were good competitive inhibitors of aromatase (K(i)=22-68nM), but less effective compared to the 2,2-dimethyl analog 4 (K(i)=8.8nM), indicating that a combination of 2beta- and 2alpha-methyl moieties is essential for the formation of a thermodynamically stable inhibitor-aromatase complex. A series of 2alpha-methyl steroids were good substrates for aromatase, whereas 2beta-methyl steroid 5 was an extremely poor substrate, and a series of 2,2-dimethyl steroids did not serve as substrate, suggesting that a 2beta-methyl moiety of the 2,2-dimethyl and 2beta-methyl steroids would prevent the aromatase reaction probably due to steric hindrance in each case. The 2-methylene compound 17 was also aromatized to produce 2-methylestrogen with a low conversion rate where the 1,4-diene structure may have been created before the C(10)-C(19) bond cleavage. Kinetic analysis of the aromatization of androgens revealed that a good substrate was not essentially a good inhibitor for aromatase.

Effect of antiestrogens and aromatase inhibitor on basal growth of the human breast cancer cell line MCF-7 in serum-free medium

Antiestrogens are efficient inhibitors of estrogen-mediated growth of human breast cancer. Besides inhibiting estradiol-stimulated growth, antiestrogens may have a direct growth-inhibitory effect on estrogen receptor (ER) positive cells and thus be more efficient than aromatase inhibitors, which will only abrogate estrogen-dependent tumor growth. To address this issue, we have used the human breast cancer cell line MCF-7/S9 as a model system which is maintained in a chemically defined medium without serum and estrogen. The addition of estradiol results in an increase in cell growth rate. Thus, the MCF-7/S9 cell line is estrogen-responsive but not estrogen-dependent. Three different types of antiestrogens, namely tamoxifen, ICI 182,780 and EM-652 were found to exert a significant and dose-dependent inhibition of basal growth of MCF-7/S9 cells. The growth-inhibitory effect of the three antiestrogens was prevented by simultaneous estradiol treatment. Antiestrogen treatment also reduced the basal pS2 mRNA expression level, thus indicating spontaneous estrogenic activity in the cells. However, treatment with the aromatase inhibitor had no effect on basal cell growth, excluding that endogenous estrogen synthesis is involved in basal growth. These data demonstrate that in addition to their estrogen antagonistic effect, antiestrogens have a direct growth-inhibitory effect which is ER-mediated. Consequently, in the subset of ER positive breast cancer patients with estrogen-independent tumor growth, antiestrogen therapy may be superior to treatment with aromatase inhibitors which only inhibit estrogen formation but do not affect cancer cell growth in the absence of estrogens.

Pre-clinical applications of transgenic mouse mammary cancer models

Breast cancer is a leading cause of cancer morbidity and mortality. Given that the majority of human breast cancers appear to be due to non-genetic factors, identifying agents and mechanisms of prevention is key to lowering the incidence of cancer. Genetically engineered mouse models of mammary cancer have been important in elucidating molecular pathways and signaling events associated with the initiation, promotion, and the progression of cancer. Since several transgenic mammary models of human breast cancer progress through well-defined cancer stages, they are useful pre-clinical systems to test the efficacy of chemopreventive and chemotherapeutic agents. This review outlines several oncogenic pathways through which mammary cancer can be induced in transgenic models and describes several types of preventive and therapeutic agents that have been tested in transgenic models of mammary cancer. The effectiveness of farnesyl inhibitors, aromatase inhibitors, differentiating agents, polyamine inhibitors, anti-angiogenic inhibitors, and immunotherapeutic compounds including vaccines have been evaluated in reducing mammary cancer and tumor progression in transgenic models.

Activation of the MAPK pathway enhances sensitivity of MCF-7 breast cancer cells to the mitogenic effect of estradiol

Long-term estrogen deprivation causes human breast cancer cells to develop hypersensitivity to the mitogenic effect of estradiol (E(2)). Our prior studies demonstrated an association between enhanced MAPK activation and hypersensitivity in long-term estrogen-deprived (LTED) MCF-7 cells. Herein, we report that MAPK is constitutively activated in LTED cells and not dependent on serum factors. Additionally, activated MAPK levels fall upon reversion of the hypersensitivity. Importantly, we now provide direct evidence that enhanced MAPK causes hypersensitivity to E(2). We activated MAPK in wild-type MCF-7 cells using TGFalpha, and demonstrated a 2-3 log enhancement of sensitivity to E(2). PD98059 abrogated the TGFalpha-induced effect, indicating that MAPK activation is responsible for E(2) hypersensitivity. To determine the level at which MAPK activation enhanced E(2) sensitivity, we examined the dose-response effects of E(2) on several transcriptional readouts, including ERE-reporter activity and the levels of progesterone receptor and pS2. Wild-type and LTED cells exhibited nearly identical responses to E(2), suggesting that mechanisms downstream of estrogen receptor-mediated transcription are involved in inducing hypersensitivity. In support of this possibility, LTED and TGFalpha-treated wild-type cells were hypersensitive to the effects of E(2) on the key cell cycle regulator, E2F1.

Non-competitive steroid inhibition of oestrogen receptor functions

Currently available antioestrogens, such as tamoxifen, are competitive inhibitors that bind to the ligand binding sites of oestrogen receptors, ERalpha and ERbeta. The search for alternative anti-hormone therapies is prompted by the need for drugs that are effective when tumours become tamoxifen resistant. The existence of different receptor isoforms also raise the possibility of improving selectivity. Earlier use of the 3beta-hydroxysteroid dehydrogenase inhibitor, trilostane (4alpha,5- epoxy-17beta-hydroxy-3-oxo-5alpha-androstane-2alpha-carbonitrile), suggested that it had beneficial actions in breast cancer that were only partially attributable to inhibition of steroidogenesis. The present studies on the interactions of trilostane with oestrogen receptors show that it (i) inhibits oestrogen-stimulated proliferation in MCF-7 breast cancer cells, (ii) enhances the affinity of oestradiol binding to ER in rat uteri and specifically increases oestradiol binding to an ERbeta-like isoform, (iii) inhibits ERalpha and ERbeta binding to the classical vitellogenin gene oestrogen response element (ERE) and (iv) inhibits oestrogen-stimulated gene transcription in ERE-linked reporter systems in MCF-7 cells. The results demonstrate a novel, presumably allosteric, mode of antioestrogen action. The beneficial actions of trilostane in breast cancer may be attributed to the combination of this antioestrogen effect with its well documented suppression of steroidogenesis.

Ligands for the peroxisomal proliferator-activated receptor gamma and the retinoid X receptor inhibit aromatase cytochrome P450 (CYP19) expression mediated by promoter II in human breast adipose

Local estrogen biosynthesis in breast adipose tissue, catalyzed by P450 aromatase, contributes to the growth of breast carcinomas. Aromatase expression is regulated by a number of alternative promoters, and in normal adipose tissue it is primarily regulated via the distal promoter I.4. However, in breast adipose containing a tumor, aromatase expression is regulated by the proximal promoter II in response to tumor-derived factors. Previously we have shown that peroxisomal proliferator-activated receptor gamma (PPARgamma) ligands inhibit aromatase expression in normal breast adipose tissue mediated by promoter I.4. In the present study, we investigated the effects of the PPARgamma ligand troglitazone and the retinoid X receptor (RXR) ligand LG101305 on aromatase expression mediated by promoter II. In cultured human breast adipose stromal cells, troglitazone or LG101305 alone inhibited aromatase activity and expression stimulated by inducers of promoter II, in a concentration-dependent manner, and this inhibition was greater in the presence of both ligands. Reporter gene assays showed that troglitazone and LG101305 inhibit transcription from promoter II of the CYP19 gene. However, EMSAs showed that PPARgamma and RXRalpha do not bind to promoter II of the CYP19 gene, indicating that PPARgamma- and RXR-mediated inhibition of aromatase expression via promoter II occurs through an indirect mechanism of action. Because ligands for PPARgamma and RXR inhibit aromatase expression in healthy breast adipose (via promoter I.4), as well as expression induced by tumor-derived factors (via promoter II), such compounds could find utility in the treatment of estrogen-dependent breast cancers.

Structural determinants of aromatase cytochrome p450 inhibition in substrate recognition site-1

The porcine gonadal form of aromatase cytochrome P450 (P450arom) exhibits higher sensitivity to inhibition by the imidazole, etomidate, than the placental isozyme. The residue(s) responsible for this functional difference was mapped using chimeragenesis and point mutation analysis of the placental isozyme, and the kinetic analysis was conducted on native and mutant enzymes after overexpression in insect cells. The etomidate sensitivity of the placental isozyme was markedly increased by substitution of the predicted substrate recognition site-1 (SRS-1) and essentially reproduced that of the gonadal isozyme by substitution of SRS-1 and the predicted B helix. A single isoleucine (I) to methionine (M) substitution at position 133 of the placental isozyme (I(133)M) was proven to be the critical residue within SRS-1. Residue 133 is located in the B'-C loop and has been shown to be equally important in other steroid-metabolizing P450s. Single point mutations (including residues 110, 114, 120, 128, 137, and combinations thereof among others) and mutation of the entire B and C helixes were without marked effect on etomidate inhibitory sensitivity. The same mutation (I(133)M) introduced into human P450arom also markedly increased etomidate sensitivity. Mutation of Ile(133) to either alanine (I(133)A) or tyrosine (I(133)Y) decreased apparent enzyme activity, but the I(133)A mutant was sensitive to etomidate inhibition, suggesting that it is Ile(133) that decreases etomidate binding rather than Met(133) increasing enzyme sensitivity. Androstenedione turnover and affinity were similar for the I(133)M mutant and the native placental isozyme. These data suggest that Ile(133) is a contact residue in SRS-1 of P450arom, emphasize the functional conservation that exists in SRS-1 of a number of steroid-hydroxylating P450 enzymes, and suggest that substrate and inhibitor binding are dependent on different contact points to varying degrees.

Nonsteroidal aromatase inhibitors: recent advances

Aromatase is the cytochrome P450 enzyme responsible for the last step of estrogen biosynthesis, and aromatase inhibitors constitute an important class of drugs in clinical use for the treatment of breast cancer. Nonsteroidal aromatase inhibitors (NSAIs) are competitive inhibitors of aromatase, which bind to the enzyme active site by coordinating the iron atom present in the heme group of the P450 protein. Presently, third generation NSAIs are in use, and research efforts are being carried out both to identify new molecules of therapeutic interest and to clarify the mechanism of action. In this article, we present a survey of the compounds that have been recently reported as NSAIs, to provide a broad view on the general structure-activity relationships of the class. Moreover, starting from the current knowledge of the mechanistic aspects of aromatase action and from recent theoretical work on the molecular modeling of both enzyme and inhibitors, we try to indicate a way to integrate these different studies in view of a more general understanding of the aromeatase-inhibitor system. Finally, some aspects regarding the possible future development of the field are considered briefly.