Enzyme-generated intermediates derived from 4-androstene-3,6,17-trione and 1,4,6-androstatriene-3,17-dione cause a time-dependent decrease in human placental aromatase activity

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.

Synthesis and aromatase inhibitory activity of some new 16E-arylidenosteroids

A new series of 16E-arylidene androstene derivatives has been synthesized and evaluated for aromatase inhibitory activity. The impact of various aryl substituents at 16 position of the steroid skeleton on aromatase inhibitory activity has been observed. The 16E-arylidenosteroids 6, 10 and 11 exhibited significant inhibition of the aromatase enzyme. 16-(4-Pyridylmethylene)-4-androstene-3,17-dione (6, IC(50): 5.2 μM) and 16-(benzo-[1,3]dioxol-5-ylmethylene)androsta-1,4-diene-3,17-dione (11, IC(50): 6.4 μM) were found to be approximately five times more potent in comparison to aminoglutethimide.

Synthesis and screening of aromatase inhibitory activity of substituted C19 steroidal 17-oxime analogs

The synthesis and aromatase inhibitory activity of androst-4-en-, androst-5-en-, 1β,2β-epoxy- and/or androsta-4,6-dien-, 4β,5β-epoxyandrostane-, and 4-substituted androst-4-en-17-oxime derivatives are described. Inhibition activity of synthesized compounds was assessed using aromatase enzyme and [1β-³H]androstenedione as substrate. Most of the compounds displayed similar to or more aromatase inhibitory activity than formestane (74.2%). 4-Chloro-3β-hydroxy-4-androsten-17-one oxime (14, 93.8%) showed the highest activity, while 4-azido-3β-hydroxy-4-androsten-17-one oxime (17, 32.8%) showed the lowest inhibitory activity for aromatase.

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.

Evaluation of plasma and tissue estrogen suppression with third-generation aromatase inhibitors: of relevance to clinical understanding?

Development of aromatase inhibition and aromatase inhibitors as a therapeutic strategy was initiated through two different pathways. The one pathway went through systematic exploration of aromatase substrate analogues for enzyme inhibitions, subsequently leading to the development of steroidal agents for clinical use. The second involved clinical observation with an unsuccessful anti-epileptic compound named aminoglutethimide, attempting to achieve a "medical adrenalectomy". Endocrine studies on patients treated with aminoglutethimide lead to direct assessment of in vivo aromatase inhibition in patients on treatment, thus identifying a novel therapeutic strategy. As such, both research programs represent different examples of pioneering translational work leading towards a successful therapeutic strategy. Subsequent studies with respect to total aromatase inhibition have led to successful development of more potent strategies. Most importantly, these studies have revealed a correlation between aromatase inhibition and clinical outcome. Ongoing studies exploring tissue estrogen levels as well as gene expression profiles on therapy may further improve this important therapeutic area.

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.

Metabolism of androsta-1,4,6-triene-3,17-dione and detection by gas chromatography/mass spectrometry in doping control

The urinary metabolism of the irreversible aromatase inhibitor androsta-1,4,6-triene-3,17-dione was investigated. It is mainly excreted unchanged and as its 17beta-hydroxy analogue. For confirmation, 17beta-hydroxyandrosta-1,4,6-trien-3-one was synthesized and characterized by nuclear magnetic resonance (NMR) in addition to the parent compound. In addition, several reduced metabolites were detected in the post-administration urines, namely 17beta-hydroxyandrosta-1,4-dien-3-one (boldenone), 17beta-hydroxy-5beta-androst-1-en-3-one (boldenone metabolite), 17beta-hydroxyandrosta-4,6-dien-3-one, and androsta-4,6-diene-3,17-dione. The identification was performed by comparison of the metabolites with reference material utilizing gas chromatography/mass spectrometry (GC/MS) of the underivatized compounds and GC/MS and GC/tandem mass spectrometry (MS/MS) of their trimethylsilyl (TMS) derivatives. Alterations in the steroid profile were also observed, most obviously in the androsterone/testosterone ratio. Even if not explicitly listed, androsta-1,4,6-triene-3,17-dione is classified as a prohibited substance in sports by the World Anti-Doping Agency (WADA) due to its aromatase-inhibiting properties. In 2006 three samples from human routine sports doping control tested positive for metabolites of androsta-1,4,6-triene-3,17-dione. The samples were initially found suspicious for the boldenone metabolite 17beta-hydroxy-5beta-androst-1-en-3-one. Since metabolites of androst-4-ene-3,6,17-trione were also present in the urine samples, it is presumed that these findings were due to the administration of a product like 'Novedex Xtreme', which could be easily obtained from the sport supplement market.

Designer steroids

Anabolic steroids have been studied for over 50 years and during that time numerous compounds with a variety of functional groups have been produced and many have been published. Of these only a small number have been introduced to the pharmaceutical market. WADA has continued the work begun by the IOC banning the use of these agents within sport as performance enhancing substances. Athletes, however, continue to use these anabolic steroids but tighter testing and the introduction of unannounced sample collection has made this form of cheating harder.In order to try to evade detection, athletes who continue to dope are having to resort to the use of a far more dangerous form of drug - the designer steroid. These steroids are manufactured to closely resemble existing known compounds, but with sufficient chemical diversity to ensure that their detection by the WADA accredited laboratories is more difficult. A worrying feature of the use of these compounds is that no data is available to evaluate either the efficacy or the safety of these substances. Many such drugs are now being made in clandestine ways (as demonstrated by the recent BALCO case) and then passed on to athletes who become the guinea pigs determining the potential of the substances as doping agents.Methods for the detection of these new compounds are being developed using emerging techniques such as gas chromatography or liquid chromatography attached to a variety of mass spectrometry instruments. This technology as well as vigilance by laboratories and enforcement agencies can all help in early detection of designer steroids being used for doping.

Lack of complete cross-resistance between different aromatase inhibitors; a real finding in search for an explanation?

While third-generation aromatase inhibitors (anastrozole, letrozole and exemestane) are successfully implemented as adjuvant and first-line therapy for hormone-sensitive breast cancer in postmenopausal women, important questions remain to be addressed. An issue of particular interest is the question about lack of complete cross-resistance between steroidal and non-steroidal compounds. Although the studies reporting this phenomenon in general contain a small number of patients, the findings across the different reports seem consistent. While several potential mechanisms have been suggested, so far we lack scientific proof what mechanisms may be responsible for this finding. Finally, we do not know whether lack of cross-resistance actually signals an improved efficacy for certain compounds or may be due to alternative mechanisms of action. Neither do we know whether some tumours are more sensitive to particular drugs. This paper summarizes clinical findings up to now with respect to lack of cross-resistance and discuss potential mechanisms involved.

Dimethandrolone (7alpha,11beta-dimethyl-19-nortestosterone) and 11beta-methyl-19-nortestosterone are not converted to aromatic A-ring products in the presence of recombinant human aromatase

Dimethandrolone undecanoate (DMAU: 7alpha,11beta-dimethyl-19-nortestosterone 17beta-undecanoate) is a potent orally active androgen in development for hormonal therapy in men. Cleavage of the 17beta-ester bond by esterases in vivo leads to liberation of the biologically active androgen, dimethandrolone (DMA), a 19-norandrogen. For hormone replacement in men, administration of C19 androgens such as testosterone (T) may lead to elevations in circulating levels of estrogens due to aromatization. As several reports have suggested that certain 19-norandrogens may serve as substrates for the aromatase enzyme and are converted to the corresponding aromatic A-ring products, it was important to investigate whether DMA, the related compound, 11beta-methyl-19-nortestosterone (11beta-MNT), also being tested for hormonal therapy in men, and other 19-norandrogens can be converted to aromatic A-ring products by human aromatase. The hypothetical aromatic A-ring product corresponding to each substrate was obtained by chemical synthesis. These estrogens bound with high affinity to purified recombinant human estrogen receptors (ER) alpha and beta in competitive binding assays (IC50's: 5-12 x 10(-9) M) and stimulated transcription of 3XERE-luciferase in T47Dco human breast cancer cells with a potency equal to or greater than that of estradiol (E2) (EC50's: 10(-12) to 10(-11) M). C19 androgens (T, 17alpha-methyltestosterone (17alpha-MT), androstenedione (AD), and 16alpha-hydroxyandrostenedione (16alpha-OHAD)), 19-norandrogens (DMA, 11beta-MNT, 19-nortestosterone (19-NT), and 7alpha-methyl-19-nortestosterone (MENT)) or the structurally similar 19-norprogestin, norethindrone (NET) were incubated at 50 microM with recombinant human aromatase for 10-180 min at 37 degrees C. The reactions were terminated by extraction with acetonitrile and centrifugation, and substrate and potential product were separated by HPLC. Retention times were monitored by UV absorption, and UV peaks were quantified using standard curves. Aromatization of the positive controls, T, AD, and 16alpha-OHAD was linear for 40-60 min, and conversion of T or AD was complete by 120 min. The nonsteroidal aromatase inhibitor, letrozole, demonstrated concentration-dependent suppression of T aromatization. Under the same conditions, there was no detectable conversion of DMA, 11beta-MNT, or NET to their respective hypothetical aromatic A-ring products during incubation times up to 180 min. Aromatization of MENT and 19-NT proceeded slowly and was limited. Collectively, these data support the notion that in the absence of the C19-methyl group, which is the site of attack by oxygen, aromatization of androgenic substrates proceeds slowly or not at all and that this reaction is impeded by the presence of a methyl group at the 11beta position.

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.

Mass spectrometric analysis of oxygenations in aromatization of androst-4-ene-3,6,17-trione, a suicide substrate of aromatase, by placental microsomes. Isotope effect and stereochemistry

Aromatase catalyzes the conversion of androstenedione (AD) to estrone through three sequential oxygenations of the 19-methyl group. 6-OxoAD (1) is one of the typical suicide substrates of aromatase, which is converted by aromatase to 6-oxoestrone through 19-alcohol (19-ol) and 19-aldehyde (19-al) intermediates 2 and 3. To study the deuterium isotope effect on the conversion of 19-ol 2 to 19-al 3 as well as the stereochemistry of the 19-hydrogen removal in this conversion, we initially synthesized [19,19-(2)H(2)] and [19S- or 19R-(2)H] 19-ols 2, starting from the corresponding deuterium-labeled 19-hydroxyAD derivatives. In incubation of non-labeled and [19,19-(2)H(2)]-labeled 19-ol 2 or that of their 1:1 mixture with human placental microsomes in the presence of NADPH under air, there was no significant deuterium-isotope effect on the production of the aromatized product 6-oxoestrone or on the conversion of 19-ol 2 to 19-al 3, based on gas chromatography-mass spectrometric analysis of the estrogen product or liquid chromatography-mass spectrometric (LC-MS) analysis of the deuterium contents of the product 19-al 3 and the recovered 19-ol 2. Moreover, in the incubations of [19S-(2)H] 19-ol 2 and its 19R isomer, LC-MS analysis of the product 3 demonstrated that the 19-pro-R hydrogen atom was stereospecifically removed in the conversion of 19-ol 2 to 19-al 3. These findings indicate that the 19-oxygenation of 19-ol 2 would proceed in the same mechanism as that involved in the AD aromatization.

Molecular basis for the aromatization reaction and exemestane-mediated irreversible inhibition of human aromatase

Aromatase converts androgens to aromatic estrogens. Aromatase inhibitors have been used as first-line drugs in the treatment of hormone-dependent breast cancer. Structural basis of the aromatization reaction and drug recognition by aromatase has remained elusive because of its unknown three-dimensional structure. In this study, recombinant human aromatase was expressed and purified from Escherichia coli. Using this purified and active preparation, the three-dimensional folding of aromatase was revealed by proteomic analysis. Combined with site-directed mutagenesis, several critical residues involved in enzyme catalysis and suicide inhibition by exemestane were evaluated. Based on our results, a new clamping mechanism of substrate/exemestane binding to the active site is proposed. These structure-function studies of aromatase would provide useful information to design more effective aromatase inhibitors for the prevention and the treatment of hormone-dependent breast cancer.

Metabolism and excretion of anabolic steroids in doping control--new steroids and new insights

The use of anabolic steroids in sports is prohibited by the World Anti-Doping Agency. Until the 1990s, anabolic steroids were solely manufactured by pharmaceutical companies, albeit sometimes on demand from national sports agencies as part of their doping program. Recently the list of prohibited anabolic steroids in sports has grown due to the addition of numerous steroids that have been introduced on the market by non-pharmaceutical companies. Moreover, several designer steroids, specifically developed to circumvent doping control, have also been detected. Because anabolic steroids are most often intensively subjected to phase I metabolism and seldom excreted unchanged, excretion studies need to be performed in order to detect their misuse. This review attempts to summarise the results of excretion studies of recent additions to the list of prohibited steroids in sports. Additionally an update and insight on new aspects for "older" steroids with respect to doping control is given.

Aromatase Inhibitors: Structural Features and Biochemical Characterization

Aromatase is the enzyme synthesizing estrogens from androgens. In estrogen-dependent breast tumors, estrogens induce the expression of growth factors responsible for cancer cell proliferation. In situ estrogen synthesis by aromatase "is thought to play a key role in the promotion of breast cancer growth. Aromatase inhibitors (AIs) provide new approaches for the prevention and treatment of breast cancer by inhibiting estrogen biosynthesis. Through reverse transcription polymerase chain reaction (RT-PCR) and immunohistochemical techniques, aromatase has been found to be expressed in many endocrine tissues and tumors originating from these tissues. Unexpectedly, this enzyme is now known to also be expressed in liver, lung, and colon cancers. Such findings suggest a potential role for endocrine manipulation of these types of cancer using AIs. Three Food and Drug Administration (FDA)-approved AIs, anastrozole (Arimidex), letrozole (Femara), and exemestane (Aromasin), effectively challenging tamoxifen, have been used as first-line drugs in the treatment of hormone-dependent breast cancer, and possibly other aromatase-expressing cancers. In addition, natural anti-aromatase chemicals, such as flavones and coumarins, have been identified. Efforts to develop new lines of AIs derived from these phytochemicals have been initiated in several laboratories. Finally, significant progress has been made in the understanding of the structure-function relationship of aromatase. Such information has helped the examination of binding characteristics of AIs, the evaluation of reaction mechanism of aromatase, and the explanation of the molecular basis for a low catalytic activity of the natural variant, M364T.

Aromatase inhibition: translation into a successful therapeutic approach

The development of the novel third-generation aromatase inhibitors and inactivators for breast cancer treatment is one of the most successful contemporary achievements in cancer therapy. Parallel to studies evaluating toxicity and clinical efficacy in metastatic disease, the endocrine effects of multiple compounds were evaluated, leading to the identification of the highly potent third-generation aromatase inhibitors based on estrogen deprivation and aromatase inhibition in vivo. Thus, translational studies have been of vital importance identifying the unique characteristics of these compounds. Whereas first- and second-generation aromatase inhibitors inhibit estrogen synthesis in vivo by up to 90%, the third-generation compounds anastrozole, exemestane, and letrozole were found to cause > or =98% aromatase inhibition. This article summarizes and discusses the "translational research" that provided the background for the implementation of the third-generation aromatase inhibitors and inactivators into large clinical trials. The need for future translational research exploiting the mechanisms of resistance to these compounds for future improvement of endocrine therapy is emphasized.

Aromatase inhibitors in the treatment of breast cancer

Estradiol, the most potent endogenous estrogen, is biosynthesized from androgens by the cytochrome P450 enzyme complex called aromatase. Aromatase is present in breast tissue, and intratumoral aromatase is the source of local estrogen production in breast cancer tissues. Inhibition of aromatase is an important approach for reducing growth-stimulatory effects of estrogens in estrogen-dependent breast cancer. Steroidal inhibitors that have been developed to date build upon the basic androstenedione nucleus and incorporate chemical substituents at varying positions on the steroid. Nonsteroidal aromatase inhibitors can be divided into three classes: aminoglutethimide-like molecules, imidazole/triazole derivatives, and flavonoid analogs. Mechanism-based aromatase inhibitors are steroidal inhibitors that mimic the substrate, are converted by the enzyme to a reactive intermediate, and result in the inactivation of aromatase. Both steroidal and nonsteroidal aromatase inhibitors have shown clinical efficacy in the treatment of breast cancer. The potent and selective third-generation aromatase inhibitors, anastrozole, letrozole, and exemestane, were introduced into the market as endocrine therapy in postmenopausal patients failing antiestrogen therapy alone or multiple hormonal therapies. These agents are currently approved as first-line therapy for the treatment of postmenopausal women with metastatic estrogen-dependent breast cancer. Several clinical studies of aromatase inhibitors are currently focusing on the use of these agents in the adjuvant setting for the treatment of early breast cancer. Use of an aromatase inhibitor as initial therapy or after treatment with tamoxifen is now recommended as adjuvant hormonal therapy for a postmenopausal woman with hormone-dependent breast cancer.

Detection of androst-4-ene-3,6,17-trione (6-OXO®) and its metabolites in urine by gas chromatography-mass spectrometry in relation to doping analysis

The metabolism and excretion of androst-4-ene-3,6,17-trione after administration of the 'nutritional' supplement 6-OXO was investigated by gas chromatography-mass spectrometry (GC-MS) in full-scan mode. The parent drug androst-4-ene-3,6,17-trione and androst-4-ene-6alpha,17beta-diol-3-one and androst-4-ene-6alpha-ol-3,17-dione were detected in the post-administration urine samples. Because androst-4-ene-3,6,17-trione is an anabolic steroid and an aromatase inhibitor, this substance is regarded as a doping agent. Hence, a selective and sensitive GC-MS method in selected ion monitoring mode for the detection of the TMS-enol-TMS-ether derivatives of these substances was developed and validated for doping control purposes. The limit of detection (LOD) of the investigated compounds ranged from 5 to 10 ng/mL. Using this method, the detection time for androst-4-ene-3,6,17-trione and androst-4-ene-6alpha,17beta-diol-3-one was 24 h, while androst-4-ene-6alpha-ol-3,17-dione could be detected up to 37 h after administration of the dose recommended by the manufacturer.

C(10)–C(19) Bond Cleavage Reaction of 19-Oxygenated Androst-4-ene-3,6-dione Steroids under Various Conditions

C(10)-C(19) bond cleavage reaction of 19-hydroxy- and 19-oxoandrost-4-ene-3,6,17-triones (5, 6) was explored under various conditions. Treatment of steroids 5 and 6 with KOH in MeOH gave the A-ring aromatized product 6-oxoestrone (11) in a fair yield, respectively, in contrast, the treatment with a weak base yielded 4-methyl steroid 17 (20%) in the case of 19-alcohol 5 or 19-nor-Delta(5(10))-steroid 9 (12-67%) along with compound 11 (6-27%) in the case of 19-aldehyde 6. Reaction of compound 6 with HCl in MeOH produced 3-methyl ethers of 6-oxoestrone and Delta(6)-estrone, compounds 12 and 14 (ca. 20% each). Thus, 6-oxosteroids 5 and 6 showed unique C(10)-C(19) bond cleavage reactions with a base or acid.

Kinetic analysis of reversible inhibition of 16alpha-hydroxyandrostenedione aromatization in human placental microsomes by suicide substrates of androstenedione aromatization

To gain insight into the catalytic function of aromatase and its substrate specificity, we studied reversible inhibition of 16alpha-hydroxyandrostenedione (16alpha-OHAD) aromatization in human placental microsomes by several suicide substrates of androstenedione (AD) aromatization, including 4-hydroxyAD (1), 6-oxoAD (2) and its 19-hydroxy analogue 3, androst-5-ene-4,7,17-trione (4), and 10beta-acetoxyandrost-5-en-7,17-dione (5) that, in contrast, do not cause a suicide inactivation of 16alpha-OHAD aromatization. All inhibitors examined blocked 16alpha-OHAD aromatization in a competitive manner with apparent K(i) values ranging from 0.50 to 980 nM. The relative K(i) values between inhibitors 1-5 obtained in the 16alpha-OHAD aromatization experiments were markedly different from those obtained in the AD aromatization experiments. The results predict that all inhibitors examined bind to the 16alpha-OHAD binding site in a manner that does not cause suicide inactivation of 16alpha-OHAD aromatization. These findings would be useful for understanding the active (binding) site structure as well as the catalytic function of aromatase.

Aromatization of 16alpha-hydroxyandrostenedione by human placental microsomes: effect of preincubation with suicide substrates of androstenedione aromatization

Estrogen synthase (aromatase) catalyzes the aromatization of androstenedione (AD) as well as 16alpha-hydroxyandrostenedione (16alpha-OHAD) leading to estrone and estriol, respectively. We found that several steroid analogs including 4-hydroxyandrostenedione (1), 6-oxoandrostenedione (6-oxoAD, 2) and its 19-hydroxy analog (3), 10beta-acetoxyestr-5-ene-7,17-dione (4), androst-5-ene-4,7,17-trione (5), and 17alpha-ethynyl-19-norteststerone (6), which are known suicide inactivators of AD aromatization, are not effective in inactivating 16alpha-OHAD aromatization in a time-dependent manner. The compounds were tested with the use of human placental microsomes and 1beta-tritiated-16alpha-OHAD as the substrate. The results of the tritium water method of 16alpha-OHAD aromatization was confirmed by the gas chromatography-mass spectrometry (GC-MS) method of estriol formation. The 1beta-tritiated-AD was used to measure AD aromatization as a positive control for these experiments. The compounds were tested at concentrations up to 40-fold higher than the K(i)'s determined for inhibition of AD aromatization. These studies suggest that differences exist in the binding site structures responsible for aromatization of 16alpha-OHAD and AD.

Aromatase inhibitors in breast cancer therapy

Estrogens are involved in numerous physiological processes and have crucial roles in certain disease states, such as mammary carcinomas. Estradiol, the most potent endogenous estrogen, is biosynthesized from androgens by the cytochrome P450 enzyme complex called aromatase. Aromatase is found in breast tissue and the importance of intratumoral aromatase and local estrogen production is being unraveled. Inhibition of aromatase is an important approach for reducing growth stimulatory effects of estrogens in estrogen-dependent breast cancer. Steroidal and nonsteroidal aromatase inhibitors have shown clinical efficacy for the treatment of breast cancer. The initial nonselective nature of nonsteroidal inhibitors, such as aminoglutethimide, has been greatly reduced in the later generations of inhibitors, anastrozole and letrozole. Mechanism-based steroidal inhibitors, such as 4-hydroxyandrostenedione and exemestane produce potent aromatase inhibition in patients. The potent and selective third-generation aromatase inhibitors, anastrozole, letrozole and exemestane, are approved for clinical use as first-line endocrine therapy in postmenopausal women with metastatic hormone-dependent breast cancer and as second-line endocrine therapy in postmenopausal patients failing antiestrogen therapy alone or multiple hormonal therapies.

Time-dependent aromatase inactivation by 4 beta,5 beta-epoxides of the natural substrate androstenedione and its 19-oxygenated analogs

Aromatase catalyzes the conversion of androgens to estrogens through three sequential oxygenations. To gain insight into the catalytic function of aromatase and its aromatization mechanism, we studied the inhibition of human placental aromatase by 4 beta,5 beta-epoxyandrostenedione (5) as well as its 19-hydroxy and 19-oxo derivatives (6 and 7, respectively), and we also examined the biochemical aromatization of these steroids. All of the epoxides were weak competitive inhibitors of aromatase with apparent K(i) values ranging from 5.0 microM to 30 microM. The 19-methyl and 19-oxo compounds 5 and 7 inactivated aromatase in a time-dependent manner with k(inact) of 0.048 and 0.110 min(-1), respectively, in the presence of NADPH. In the absence of NADPH, only the former inhibited aromatase with a k(inact) of 0.091 min(-1). However, 19-hydroxy steroid 6 did not cause irreversible inactivation either in the presence or absence of NADPH. Gas chromatography-mass spectrometric analysis of the metabolite produced by a 5-min incubation of the three epoxides with human placental microsomes in the presence of NADPH under air revealed that all three compounds were aromatized to produce estradiol with rates of 8.82, 0.51, and 1.62 pmol/min/mg protein for 5, 6, and 7, respectively. In each case, the aromatization was efficiently prevented by 19-hydroxyandrost-4-en-17-one, a potent aromatase inhibitor. On the basis of the aromatization and inactivation results, it seems likely that the two pathways, aromatization and inactivation, may proceed, in part, through a common intermediate, 19-oxo compound 7, although they may be principally different.

Aromatase, aromatase inhibitors, and breast cancer

Estrogens are involved in numerous physiologic processes and have crucial roles in particular disease states, such as mammary carcinomas. Estradiol, the most potent endogenous estrogen, is biosynthesized from androgens by the cytochrome P-450 enzyme complex called aromatase. Aromatase is found in breast tissue, and the importance of intratumoral aromatase and local estrogen production is being unraveled. Inhibition of aromatase is an important approach for reducing growth stimulatory effects of estrogens in hormone-dependent breast cancer. Effective aromatase inhibitors have been developed as therapeutic agents for controlling estrogen-dependent breast cancer. Investigations into the development of aromatase inhibitors began in the 1970s and have expanded greatly in the past three decades. Competitive aromatase inhibitors are molecules that compete with the substrate androstenedione for noncovalent binding to the active site of the enzyme to decrease the amount of product formed. Steroidal inhibitors that have been developed to date build on the basic androstenedione nucleus and incorporate chemical substituents at varying positions on the steroid. The structure-activity relationships for steroidal inhibitors have become more refined in the past decade, and only some modifications can be made to the steroid and still keep its affinity for aromatase. Nonsteroidal aromatase inhibitors can be divided into three classes: aminoglutethimide-like molecules, imidazole/triazole derivatives, and flavonoid analogs. Mechanism-based aromatase inhibitors are inhibitors that mimic the substrate, are converted by the enzyme to a reactive intermediate, and result in the inactivation of aromatase. Aromatase inhibitors, both steroidal and nonsteroidal, have shown clinical efficacy for the treatment of breast cancer. The initial nonselective nature of nonsteroidal inhibitors such as aminoglutethimide has been greatly reduced in the later generations of inhibitors, anastrozole and letrozole. Mechanism-based steroidal inhibitors such as 4-hydroxyandrostenedione and exemestane produce prolonged aromatase inhibition in patients. The potent and selective third-generation aromatase inhibitors anastrozole, letrozole, and exemestane are approved for clinical use as second-line endocrine therapy in postmenopausal patients failing antiestrogen therapy alone or multiple hormonal therapies.

Gas chromatography-mass spectrometric determination of activity of human placental aromatase using 16alpha-hydroxyandrostenedione as a substrate

Aromatization of 16alpha-hydroxyandrostenedione (16alpha-OH AD) with aromatase in human placental microsomes was studied by gas chromatography-mass spectrometry (GC-MS) using 12,4,6,6,9alpha,16beta,17alpha-2H7]estriol as an internal standard. 16alpha-OH AD was incubated with the microsomes in the presence of NADPH in air. The metabolite was extracted with ethyl acetate and treated with NaBH4. The reduced product, estriol, was isolated by Sep-Pak C18 cartridge and then analyzed as the tris(trimethylsilyl)ether by a GC-MS (EI mode). The production of estriol was dependent upon protein concentration and incubation time. Apparent Km and Vmax values of the microsomal aromatase for 16alpha-OH AD were 568 nM and 25.5 pmol/min/mg protein, respectively. In this assay, aromatase activity, estriol formation, could be determined at a level as low as 1 pmol/min/mg protein. Aromatase inhibitors, 4-hydroxy- and 6-oxo-androstenediones, prevented the estriol formation in a competitive manner with 25 and 30 nM of apparent Ki values, respectively.

Design and synthesis of new steroidal inhibitors of estrogen synthase (aromatase)

The estrogen synthase (aromatase) enzyme system is responsible for the biosynthesis of estrogen hormones in human females. Estrogens are vital for normal growth and development, but will promote the growth of certain breast cancers. Approximately 30-50% of breast cancers are considered to be hormone-dependent. Consequently regulation of estrogen biosynthesis has advanced as a potential therapeutic strategy. This has led to the development of active-site inhibitors, which may have potential for the control of breast cancer. We have recently prepared a number of new steroidal inhibitors that have been evaluated as aromatase inhibitors. These include steroidal A/B-ring isoxazoles and a series of A/B-ring pyrazoles with alkyl- and aryl-substituted nitrogen. In addition, we have developed new chemical procedures for the synthesis of 6beta-hydroxy steroids, which could be key intermediates in the preparation of C-19 inhibitors of aromatase activity.

Biological aromatization of delta4,6- and delta1,4,6-androgens and their 6-alkyl analogs, potent inhibitors of aromatase

Enzymic aromatization of delta6- and delta1,6-derivatives of the natural substrate androstenedione with human placental aromatase was first studied using gas-chromatography-mass spectrometry. The two steroids were aromatized with apparent Km and Vmax values of 62 nM and 32 pmol/min/mg protein for the delta6-steroid and 167 nM and 10 pmol/min/mg protein for the delta1,6-steroid, respectively. We next explored the aromatization of a series of 6-alkyl (methyl, ethyl, n-propyl, and n-pentyl)-substituted delta6-androstenediones and their delta1,6-analogs, potent competitive inhibitors of aromatase, to gain insight into the relationships between the inhibitory activity of the 6-alkyl-C19 steroids and their ability to serve as a substrate of aromatase. In a series of the delta1,6-androstenediones, all the 6-alkyl steroids were more efficient substrates than the parent delta1,6-steroid in which the aromatization rates of the alkyl steroids were about 2-fold that of the parent steroid, in contrast, all of the 6-alkyl-substituted delta6-androstenediones were converted into the corresponding 6-alkyl-delta6-estrogens with the rates of less than about a half that of the parent steroid. These results indicate that the 6-alkyl function decreases the aromatization rate of the delta6-steroid but enhances that of the delta1,6-steroid. The relative apparent Km values for the C19 steroids obtained in this study are different from the relative Ki values obtained previously, indicating that a good inhibitor is not essentially a good substrate in the 6-alkyl-substituted delta6- and delta1,6-androstenedione series.

Synthesis of 19-oxygenated derivatives of the competitive inhibitor of aromatase, 5-androstene-4,17-dione

19-Hydroxy- and 19-oxo-steroids 13 and 15, respectively, which are potential metabolites of the aromatase inhibitor 5-androstene-4,17-dione (3), were synthesized from 19-(tert-butyldimethylsilyloxy)androst-5-en-17-one (5) or 4beta-acetoxyandrost-5-en-17-one (16), respectively, through 5alpha-bromo-4beta-hydroxy-6beta,19-epoxyandrostan+ ++-17-one (10) as a key intermediate in each sequence. Reaction of the 19-siloxy compound 5 with Br2 gave 5alpha-bromo-6beta,19-epoxide 8, which was treated with N,N'-dimethylacetamide followed by reaction with N-bromoacetamide and 0.28 M HCIO4, to yield compound 10. On the other hand, treatment of the 4beta-acetoxy steroid 16 with N-bromoacetamide-HCI04 followed by oxidation with Pb (IV) acetic acid and I2 under irradiation and subsequent hydrolysis with K2CO3 also produced compound 10 and in better yield than that in the above synthesis. Jones oxidation of the 4beta-ol 10 followed by reductive debromination with zinc dust yielded the 19-ol 13 in low yield as well as 6beta,19-epoxy-4-one 12 as the major product. Furthermore, the major product 12 was converted into the 19-ol 13 in moderate yield from compound 12 through acetolysis and subsequent alkaline hydrolysis. The 19-oxo steroid 15 was obtained after treatment of compound 13 with pyridinium dichromate. Compounds 13 and 15 were analyzed as the methoxime-trimethylsilyl and methoxime-dimethylisopropylsilyl derivatives and the methoxime derivative, respectively, using gas chromatography-mass spectrometry.

Synthesis and structure-activity relationships of 6-phenylaliphatic-substituted C19 steroids having a 1,4-diene, 4,6-diene, or 1,4,6-triene structure as aromatase inhibitors

A series of 6alpha- and 6beta-phenylaliphatic-substituted androsta-1,4-diene-3,17-diones [9b-f and 10b-f; (CH2)nPh, n = 1-5] and their 4,6-diene and 1,4,6-triene analogs (11b-f and 12b-f) along with their respective phenyl analogs 9a-12a were synthesized and tested as aromatase inhibitors. All of the steroids examined were very powerful competitive inhibitors of aromatase in human placental microsomes with apparent Ki values ranging from 8.5 to 80 nM. The inhibitory activities of the benzyl- and phenethyl-4,6-dienes 11b and 11c (Ki, 9.0 and 10 nM) as well as the 6-phenethyl-1,4,6-triene 12c (Ki, 8.5 nM) were extremely high among them. All of the phenylaliphatic steroids, except for the 6beta-phenethyl compound 10c, and the 6-phenyl-4,6-diene 11a had higher affinity for aromatase than the corresponding parent 1,4-diene, 4,6-diene, and 1,4,6-triene steroids 9g, 11g, and 12g. All of the 6alpha-substituted 1,4-dienes (9a-9g) and the 6-substituted 1,4,6-trienes (12a-12g) caused a time-dependent inactivation of aromatase. On the other hand, only the 6beta-substituted 1,4-dienes (10a-10d) having no or less than four carbon atoms between the steroid nucleus and the phenyl group also caused a time-dependent inactivation of aromatase. Their inactivation rates (k(inact) 0.076-0.156 min(-1)) were higher than the respective parent steroids, 9g and 12g. In contrast, in the 4,6-diene series, only the 6-phenpropyl steroids 11d inactivated aromatase in a time-dependent manner with 0.155 min(-1) of k(inact) value. The inactivation was prevented by the substrate androstenedione, and no significant effect of L-cysteine on the inactivation was observed in each case. These results indicate that length and/or stereochemistry of the C-6 substituent of steroids 9-12 as well as a terminal phenyl group incorporated in the C-6 substituent play a critical role not only in tight binding to the active site of aromatase but also in the cause of a time-dependent inactivation of the enzyme.

Inhibition and inactivation of equine aromatase by steroidal and non-steroidal compounds. A comparison with human aromatase inhibition

In order to approach the detailed structure-function relationships of aromatase, we studied the inhibitory and inactivatory potencies of several steroidal androstenedione analogues (1: 4-hydroxyandrostenedione, 2: 4-acetoxyandrostenedione and 3: 7 alpha-(4'-amino)phenylthio-4-androstene-3, 17-dione) and non-steroidal imidazole derivatives (4: ketoconazole, 5: miconazole and 6: fadrozole) on equine aromatase in placental microsomes, a well established mammalian model. Human placental microsomes and the purified enzyme from equine testis were also used to compare inhibition by 1 and 2. In equine microsomes, all compounds tested exhibited a competitive inhibition, with Ki values of 4.1, 26 and 1.8 nM for 1, 2 and 3, and of 2400, 1.4 and 4 nM for 4, 5, and 6, respectively. The Km for androstenedione, the substrate mainly used in these studies, was 1.8 +/- 0.13 nM. The three non-steroidal derivatives did not inactivate equine aromatase, but 1 and 2 acted as comparable inactivators to a much higher degree than 3. Compound 1 inhibited in a similar manner (89-94%) purified or equine and human microsomal aromatases, whereas 2 inhibited microsomal aromatase more efficiently in the horse than in man (92% and 33% inhibition, respectively). There was only a 40% inhibition with 2 on the purified equine enzyme, which is no more in the natural membrane environment. The comparisons between equine and human microsomal aromatases allow precise functional and structural differences to be observed with these enzymes.

6-Alkylandrosta-4,6-diene-3,17-diones and their 1,4,6-triene analogs as aromatase inhibitors. Structure-activity relationships

Two series of 6-alkylandrosta-4,6-diene-3,17-diones (5) and their 1,4,6-triene analogs 6 were synthesized as aromatase inhibitors to gain insight into the structure-activity relationship between varying the 6-n-alkyl substituents (C1-C7) and inhibitory activity. All of the steroids synthesized were extremely powerful competitive inhibitors of aromatase in human placental microsomes, with apparent Ki values for the 6-alkyl-4,6-diene steroids 5 ranging from 17 to 36 nM and with those for the 1,4,6-triene steroids 6 ranging from 2.5 to 58 nM. The 6-ethyl-1,4,6-triene compound 6b (Ki = 2.5 nM) was the most potent inhibitor among them. The 6-alkyl-1,4,6-triene steroids 6, except for the 6-methyl analog 6a, and higher affinity for aromatase than the natural substrate androstenedione (K(m) = 24 nM), and their inhibitory activities were more potent than the corresponding 4,6-diene steroids 5. In a series of the 4,6-diene steroids 5, compounds 5c-f with the n-alkyl chain substituents (C3 to C6) also had slightly higher affinity than androstenedione for dromatase. All of the 1,4,6-triene steroids 6 inactivated aromatase in a time-dependent manner, with k(inact) values ranging from 0.021 to 0.074 min-1; in contrast, the 4,6-diene analogs 5 did not. The inactivation was prevented by androstenedione, and no significant effect of L-cysteine on the inactivation was observed in each case. These results indicate that the length of the n-alkyl substituent at C-6 of androsta-1,4,6-triene-3,17-dione (6h), rather than its 4,6-diene analog 5h, plays a critical role in tight binding to the active site of aromatase. No significant correlation was observed between affinity for the enzyme and the inactivation ability of the 6-alkyl-1,4,6-trienes.

Studies directed toward a mechanistic evaluation of aromatase inhibition by androst-5-ene-7,17-dione. Time-dependent inactivation by the 19-nor and 5 beta, 6 beta-epoxy derivatives

To gain further insight into the mechanism for inactivation of aromatase by androst-5-ene-7,17-dione (1) and its 19-nor analog 4, 10 beta-oxygenated steroids 5 and 6, delta 1(10)-steroid 7, and 19-oxo-5 beta,6 beta-epoxy compound 8 were synthesized and tested for their ability to inhibit aromatase in human placental microsomes. All of the steroids studied inhibited the enzyme in a competitive manner with apparent Ki values ranging from 1.1 to 35 microM. The delta 1(10)-compound 7 was the most potent inhibitor among them. All of the inhibitors caused a time-dependent inactivation of aromatase in the presence of NADPH in air with the kinact values ranging from 0.036 to 0.190 min-1. The substrate androstenedione protected the inactivation, but a nucleophile, L-cysteine, did not, in each case. In contrast, each inhibitor did not cause the time-dependent inactivation in the absence of NADPH. These results show that the 5 beta,6 beta-epoxide 8 and/or the dienone 7 are not a reactive electrophile involved in the irreversible binding to the active site of aromatase during the mechanism-based inactivation caused by the suicide substrates 1 and/or 4.

Aromatase inhibition in JAr choriocarcinoma cells by 7alpha-arylaliphatic androgens

The JAr choriocarcinoma cell cultures have demonstrated high levels of aromatase activity and have been useful for assaying a wide variety of aromatase inhibitors for aromatase inhibition in intact cells. Recently, several 7alpha-arylaliphatic androgens have shown effective inhibition of human placental microsomal aromatase in vitro, with apparent Ki values ranging from 10 to 20 nM. A series of 7alpha-arylaliphatic androst-4-ene-3,17-dione compounds demonstrated potent competitive inhibition, and 7alpha-arylaliphatic androsta-1,4-diene-3,17-diones were enzyme-activated irreversible inhibitors. Both series of these potent inhibitors were investigated for the ability to inhibit aromatase activity in JAr cells by measuring the conversion of [1beta-3H]-androstenedione to 3H2O and unlabelled estrone. JAr cell cultures were incubated for 2 h at 37 degrees C with the aromatase inhibitors at concentrations of 10 pM to 10 microM, the percentage of enzyme inhibition was determined, and IC50 values for inhibitors were calculated. Both series of synthetic compounds demonstrated good to excellent aromatase inhibition, and the most effective inhibitors in both series were those compounds with a phenylpropyl substituent at the 7alpha-position of the steroid nucleus. The 7alpha-arylaliphatic androst-4-ene-3,17-diones exhibited inhibition of JAr aromatase activity with IC50 values from 300 to 434 nM. More potent aromatase inhibition was observed with the 7alpha-arylaliphatic androsta-1,4-diene-3,17-diones, which exhibited IC50 values from 64 to 232 nM. Enhanced efficacy of steroidal enzyme-activated irreversible inhibitors compared to competitive inhibitors was observed in these studies and is consistent with previous reports. These results suggest that JAr choriocarcinoma cells with high levels of aromatase activity may be useful in differentiating steroidal aromatase inhibitors exhibiting different mechanisms of enzyme inhibition. In summary, the 7alpha-phenylpropyl androsta-1,4-diene-3,17-dione analogs, which are enzyme-activated irreversible inhibitors, demonstrated the most effective inhibition of aromatase activity present in the JAr cell cultures among the various 7alpha-arylaliphatic androgens.

Mechanism for aromatase inactivation by a suicide substrate, androst-4-ene-3,6,17-trione. The 4 beta, 5 beta-epoxy-19-oxo derivative as a reactive electrophile irreversibly binding to the active site

Aromatase is a cytochrome P450 enzyme complex that catalyzes the conversion of androst-4-ene-3,17-dione to estrone through three sequential oxygenations of the 19-methyl group. Androst-4-ene-3,6,17-trione (1) is a suicide substrate of aromatase. The inactivation mechanism for steroid 1 has been studied to show that the inactivation reaction proceeds through the 19-oxo intermediate 3. To further clarify the mechanism, 4 beta, 5 beta-epoxyandrosta-3,6,17,19-tetraone (6) was synthesized as a candidate for a reactive electrophile involved in irreversible binding to the active site of aromatase, upon treatment of compound 3 with hydrogen peroxide in the presence of NaHCO3. The epoxide 6 inhibited human placental aromatase in a competitive manner (Ki = 30 microM); moreover, it inactivated the enzyme in an active-site-directed manner in the absence of NADPH (K1 = 88 microM, kinact = 0.071 min-1). NADPH and BSA both stimulated the inactivation rate without a significant change of the K1 in either case (kinact: 0.133 or 0.091 min-1, in the presence of NADPH or BSA, respectively). The substrate androst-4-ene-3,17-dione protected the inactivation, but a nucleophile, L-cysteine, did not. When both the epoxide 6 and its 19-methyl analog 4 were subjected separately to reaction with N-acetyl-L-cysteine in the presence of NaHCO3, the 19-oxo steroid 6 disappeared from the reaction mixture more rapidly (T1/2 = 40 sec) than the 19-methyl analog 4 (T1/2 = 3.0 min). The results clearly indicate that the 4 beta, 5 beta-epoxy-19-oxo compound 6, which is possibly produced from 19-oxo-4-ene steroid 3 through the 19-hydroxy-19-hydroperoxide intermediate, is a reactive electrophile that irreversibly binds to the active site of aromatase.

4-Oxygenated androst-5-en-17-ones and their 7-oxo derivatives as aromatase inhibitors

A series of androst-5-ene-4,7-diones and 4-oxygenated androst-5-enes were synthesized and tested for their ability to inhibit aromatase in human placental microsomes. All of the steroids examined inhibited the enzyme in a competitive manner. The inhibitory activity of 4beta-hydroxy-5-ene steroid 7 (Ki = 25 nM) was much more powerful than that of the parent 5-ene steroid 11 (Ki = 78 nM), whereas 4beta-acetate 8 and 4-oxo analog 5 (Ki = 90 and 120 nM, respectively) were less potent than compound 11. This indicates that a hydrogen bonding between a hydroxy group of the 4beta-ol 7 and a residue of the active site of aromatase plays an important role in its binding. The 5-en-4-one steroid 5 did not cause a time-dependent inactivation of aromatase. In contrast, 5-ene-4,7-dione 13 as well as its 19-hydroxy and 19-oxo analogs 19 and 20 caused the time-dependent inactivation only in the presence of NADPH in air with the k(inact) values ranging from 0.057 to 0.192 min(-1), although their affinities for the enzyme were not high (Ki = 430-6300 nM). The inactivation was prevented by androstenedione, and no significant effect of L-cysteine on the inactivation was observed in each case. These results suggest that oxygenation at C-19 would be at least in part involved in the inactivation caused by the inhibitor 13.

Aromatase inhibitors: effect of ring A and ring B unsaturation on aromatase inhibition by 4-thiosubstituted derivatives of 4-androstene-3,17-dione

The synthesis and biological evaluation of 4-thiosubstituted derivatives of 1,4-androstadienedione, 4,6-androstadienedione, and 1,4,6-androstatrienedione as inhibitors of aromatase are described. Inhibitory activity of synthesized compounds was assessed using a human placental microsomal preparation as the enzyme source and [1 beta-3H]androstenedione as substrate. Under initial velocity assay conditions of low product formation, the inhibitors demonstrated potent inhibition of aromatase, with apparent Kis ranging from 9.8 to 137 nM and with Km for androstenedione being 38 nM. However, unlike other 1,4-androstadienediones and 1,4,6-androstatrienediones in which time-dependent inactivation was observed, the 4-thiosubstituted analogs were found to be competitive inhibitors and did not produce any time-dependent inactivation of aromatase.

A- or B-ring-substituted derivatives of androst-4-ene-3,6,17-trione as aromatase inhibitors. Structure-activity relationships

2,2-Dimethylandrost-4-ene-3,6,17-trione (5) and its 4-methoxy- (7) and 4-hydroxy- (8) derivatives were synthesized. 7 alpha-Acetoxy-4-ene-3,6-dione steroid 2 was also prepared by the improved method involving the lead tetraacetate oxidation of androst-4-ene-3,6,17-trione (1). These steroids along with the 2-acetoxy-(11 and 12), 2-substituted 1-ene- (9 and 10), and 4-substituted (13-15) derivatives of compound 1 were evaluated as inhibitors of human placental aromatase. All the steroids, except the 2-acetoxy-1-ene 10 and the 2 beta-acetate 11 of which Ki values were not determined because of their poor inhibitory activities, blocked aromatase in a competitive manner. Compounds 5 and 8 as well as the 4-hydroxy steroid 15 were potent inhibitors (Ki: 25-42 nM) whereas the inhibitory activities of steroids 2, 7, 9, 13, and 14 were good to fair, respectively (Ki: 160-810 nM). Inhibitors 2 and 15 inactivated the enzyme in a time-dependent manner in the presence of NADPH but the 2,3-dimethyl derivatives 5 and 8 did not. Androstenedione blocked the inactivation but L-cysteine did not. The results suggest that the 2 beta-methyl group would prevent the aromatase-catalyzed oxygenation at C-19 of the dimethyl steroids 5 and 8 most likely through the steric reasons.

Aromatase inhibitors in the treatment of breast cancer

A number of inhibitors of estrogen synthesis are now becoming available which could be of value in the treatment of breast cancer. 4-Hydroxyandrostenedione (4-OHA), the first of these compounds to enter the clinic has been found to be effective in postmenopausal patients who have relapsed from tamoxifen. Thus, in studies of 240 patients, 26% patients experienced partial or complete response to treatment. An additional 25% patients had disease stabilization. 4-OHA is a potent selective, steroidal inhibitor which causes inactivation of aromatase in vitro. It is effective in reducing concentrations of ovarian estrogens in rats and of ovarian and peripheral estrogens in non-human primate species. The compound has been shown to lower serum estrogen levels in postmenopausal breast cancer patients. However, not all of these patients experienced disease remission, suggesting that their tumors were hormone insensitive rather than that the dose of 4-OHA was suboptimal. In trials of patients who had not received prior tamoxifen treatment, 4-OHA (250 mg i.m. every 2 weeks) was found to induce complete or partial tumor regression in 33% of patients. The response of patients was not significantly different from that observed in patients treated with tamoxifen (30 mg o.d) of 37%. No significant difference between treatments was observed for disease stabilization, the duration of response or median survival. Several other steroidal aromatase inhibitors have been studied, such as 7 alpha-substituted androstenedione derivatives. MDL 18962 [10-(2-propynyl)estr-4-ene-3,17-dione] and FCE 24304 (6-methylen-androsta-1,4-diene-3,17-dione) are currently in clinical trials. Non-steroidal inhibitors of cytochrome P-450 enzymes, such as imidazole and triazole derivatives have been developed which are highly selective for aromatase. Three triazoles which are very potent and selective inhibitors are vorazole (6-[(4-chlorophenyl)(1H-1,2,4-triazol-1-yl)-methyl]1-methyl-1H- benzotriazole R 76713, arimidex 2,2'[5-(1H-1,2,4-triazol-1-yl methyl)-1,3-phenylene]bis(2-methylpropiononitrile) (ZD1033) and letrozole 4-[1-(cyanophenyl)-1-(1,2,4-triazolyl)methyl]benzonitril (CGS 20267). These compounds reduce serum estradiol concentration to undetectable levels in breast cancer patients. These highly potent inhibitors provide the opportunity to determine whether a further degree of estrogen suppression will be important in producing greater clinical response.(ABSTRACT TRUNCATED AT 400 WORDS)

Metabolic aspects of the 1 beta-proton and the 19-methyl group of androst-4-ene-3,6,17-trione during aromatization by placental microsomes and inactivation of aromatase

Aromatase catalyzes the conversion of androst-4-ene-3,17-dione to estrogen through sequential oxygenations at the 19-methyl group. Androst-4-ene-3,6,17-trione (AT) is a suicide substrate of aromatase, and the mechanism of inactivation of aromatase has been postulated to involve enzymatic oxygenation at the 19-position. [1 beta-3H,4-14C]-, [19-3H3,4-14C]-, and [1 beta-3H,19-14C]ATs, with high specific activities, were synthesized to study metabolic aspects and the inactivation mechanism. Incubation of the labeled AT with human placental microsomes yielded the 19-oxygenated derivatives, 19-hydroxy-AT and 19-oxo-AT, as well as the aromatization products, 6-oxoestrone and 6-oxoestradiol. A stereospecific 1 beta-proton elimination occurred during the aromatization of [1 beta-3H,4-14C]AT, and a marked tritium isotope effect was observed in the first hydroxylation at C-19 of [19-3H3,4-14C]AT. After incubation of the three double-labeled ATs, the solubilized proteins were subjected to SDS-PAGE and the 3H/14C ratio of the aromatase-bound metabolite in a 46-69 kDa fraction was analyzed. A marked decrease of the 3H/14C ratio of the metabolite was observed in the experiment using [19-3H3,4-14C]AT, compared with that of the labeled AT used, but there were no significant changes in the other experiments, indicating that the adduct retains the 1 beta-proton, the 19-carbon, and one of the three 19-methyl protons of AT. Thus, we conclude that further oxygenation of 19-oxo-AT produced by the two initial hydroxylations of AT at C-19 yields not only 6-oxoestrogen (by a mechanism similar to that involved in the aromatization of the natural substrate) but also a reactive electrophile that immediately binds to the active site in an irreversible manner, resulting in inactivation of aromatase.