7 alpha-substituted derivatives of androstenedione as inhibitors of estrogen biosynthesis

Design, synthesis and biochemical studies of new 7α-allylandrostanes as aromatase inhibitors

Two series of derivatives of 7α-allylandrostenedione, namely its 3-deoxo and 1-ene analogs, were designed and synthesised and their biochemical activity towards aromatase evaluated. In each of these series, the C-17 carbonyl group was further replaced by the hydroxyl and acetoxyl groups. The attained data pointed out that the absence of the C-3 carbonyl group led to a slightly decrease in the inhibitory activity and the introduction of an additional double bond in C-1 revealed to be a very beneficial structural change in the studied compounds (compound 12, IC₅₀ = 0.47 μM, K(i) = 45.00 nM). Furthermore, the relevance of the C-17 carbonyl group in the D-ring as a structural feature required to achieve maximum aromatase inhibitory activity is also observed for this set of derivatives.

Synthesis of Some Novel 3,3-Ethylenedioxyandrost-7β-Acyloxy-5-Ene-17-One Derivatives as Potent Aromatase Inhibitors

3,3,17,17-Diethylenedioxyandrost-5-ene was obtained by ketalisation of androstenedione, which was oxidised with PDC and t-BuOOH to form 3,3,17,17-diethylene-dioxyandrost-5-ene-7-one. Stereoselective reduction of 3,3,17,17-diethylene-dioxyandrost-5-ene-7-one by NaBH4 in the presence of CeCl3.6H2O gave 3,3,17,17-diethylenedioxy-7β-hydroxy-androst-5-ene, which was deprotected with p-toluenesulfonic acid to gave 3,3-ethylenedioxyandrost-5-ene-7β-hydroxy and androst-4,6-dien-3,17-dione. A series of androstenedione derivatives were obtained from 3,3-ethylenedioxyandrost-5-ene-7β-hydroxy by the esterification reaction. Their structures were confirmed by MS, 1H NMR, 13C NMR and HRMS.

Lead optimization of 7-benzyloxy 2-(4′-pyridylmethyl)thio isoflavone aromatase inhibitors

Aromatase, the enzyme responsible for estrogen biosynthesis, is a particularly attractive target in the treatment of hormone-dependent breast cancer. The synthesis and biological evaluation of a series of 2-(4'-pyridylmethyl)thio, 7-alkyl- or aryl-substituted isoflavones as potential aromatase inhibitors are described. The isoflavone derivatives demonstrate IC(50) values from 79 to 553 nM and compete with the endogenous substrate, androstenedione. Data supporting the ability of these analogs to suppress aromatase enzyme activity in the SK-BR-3 breast cancer cell line are also presented.

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

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

C-7 analogues of progesterone as potent inhibitors of the P-glycoprotein efflux pump

The P-glycoprotein product (Pgp) of the MDR1 gene has been implicated in the multiple drug resistance phenotype expressed by many cancers. Functioning as an efflux pump, P-glycoprotein prevents the accumulation of high intracellular concentrations of substrates. We have taken a rational approach to designing inhibitors of P-glycoprotein function, selecting a natural substrate (progesterone) as our lead compound. We hypothesized that progesterone, substituted at C-7 with an aromatic moiety(s), would exhibit reduced Pgp affinity, significantly increased antiPgp activity, and reduced affinity for progesterone receptors (PGR). We synthesized 7 alpha-[4'-(aminophenyl)thio]pregna-4-ene-3,20-dione (2), which comprises a C-7 alpha thiol bridge linking an aminophenyl moiety to progesterone, from pregna-4,6-diene-3,20-dione (1). The subsequent addition reaction of 2 with the appropriate isocyanate produced an initial series of compounds (3-6). Compounds 3-5 (respectively, -CH(2)CH(2)Cl; -CH(2)CH(3); and -CH(CH(3))C(6)H(5)) exhibit a significantly increased ability to inhibit P-glycoprotein. Potency for restoring doxorubicin accumulation in MDR1-transduced human breast cancer cells is increased up to 60-fold as compared with progesterone. Compound 5 has greater potency than verapamil and is equipotent with cyclosporin A, for inhibiting P-glycoprotein function. Furthermore, 5 does not bind to PGR, implying a potential reduction in in vivo toxicity. These data identify C-7-substituted progesterone analogues and 5, in particular, as rationally designed antiPgp compounds worthy of further evaluation/development.

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.

Spironolactone-related inhibitors of type II 17beta-hydroxysteroid dehydrogenase: chemical synthesis, receptor binding affinities, and proliferative/antiproliferative activities

The family of 17beta-hydroxysteroid dehydrogenases (17beta-HSDs) catalyzes the formation and inactivation of testosterone (T), dihydrotestosterone (DHT), and estradiol (E2), thus playing a crucial role in the regulation of active steroid hormones in target tissues. Among the five known 17beta-HSD enzymes, type II catalyzes the oxidation of E2 into estrone (E1), T into androstenedione, DHT into androstanedione, and 20alpha-dihydroprogesterone into progesterone. Specific inhibitors are thus an interesting means to study the regulation and to probe the structure of type II 17beta-HSD. In this context, we have efficiently synthesized a series of 7alpha-thioalkyl and 7alpha-thioaryl derivatives of spironolactone that inhibit type II 17beta-HSD. These new C19-steroidal inhibitors possess two important pharmacophores, namely 17-spiro-gamma-lactone and a bulky side-chain at the 7alpha-position. It was found that a para-substituted benzylthio group at the 7alpha-position enhances the inhibitory potency of spironolactone derivatives on type II 17beta-HSD. In fact, the compound with a para-hydroxy-benzylthio group showed an IC50 value of 0.5 microM against type II 17beta-HSD, whereas the compound with a para-[2-(1-piperidinyl)-ethoxy]-benzylthio group inhibited this enzyme with an IC50 value of 0.7 microM. The latter inhibitor is more selective than the former because it did not show any inhibitory potency against P450 aromatase as well as any affinity towards four steroid receptors (AR, PR, GR, ER). As a result, this inhibitor did not show any proliferative effect on androgen-sensitive Shionogi cells and estrogen-sensitive ZR-75-1 cells. These findings contribute to a better knowledge of the structure of type II 17beta-HSD and offer an interesting tool to study the regulation of this enzyme in several biological systems.

7alpha-Arylaliphatic androsta-1,4-diene-3,17-diones as enzyme-activated irreversible inhibitors of aromatase

Inhibition of aromatase, the enzyme responsible for converting androgens to estrogens, may be therapeutically useful for the endocrine treatment of hormone-dependent breast cancer. Previous research on 7alpha-thiosubstituted androgens, especially 7alpha-(4'-aminophenylthio)-androsta-1,4-diene-3,17-di one, has shown that these compounds are potent enzyme-activated irreversible inhibitors of aromatase. Research on the synthesis of more metabolically stable inhibitors has focused on replacing the thioether linkage at the 7alpha position with a carbon-carbon linkage. Several 7alpha-arylaliphatic androst-4-ene-3,17-diones were previously shown to be potent competitive inhibitors of aromatase. The extension of the research on these 7alpha-arylaliphatic androgens includes the introduction of a C1-C2 double bond in the A-ring to provide enzyme-activated irreversible inhibitors. The desired 7alpha-arylaliphatic androsta-1,4-diene-3,17-diones were obtained from their corresponding 7alpha-arylaliphatic androst-4-ene-3,17-diones by oxidation using DDQ. A new improved synthesis of the 7alpha-arylaliphatic androst-4-ene-3,17-diones using an in situ preparation of the CuI-(n-Bu3)P complex was employed. The aryl ring of the 7alpha-phenethyl and 7alpha-phenpropyl derivatives were functionalized to their corresponding p-nitro and p-amino derivatives. These compounds were all potent inhibitors of aromatase with apparent K(i)s ranging between 7 and 19 nM. These inhibitors demonstrated enzyme-mediated inactivation of aromatase with apparent k(inact)s ranging from 4.4 x 10(-4) to 1.90 x 10(-3)/s. The best inactivator of the series was the 7alpha-phenpropylandrosta-1,4-diene-3,17-dione, which exhibited a T(1/2) of 6.08 min.

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.

Aromatization of 7 alpha-methyl-19-nortestosterone by human placental microsomes in vitro

Part of the biological effects of testosterone (T) are mediated by its enzymatic reduction to 5 alpha-dihydrotestosterone (DHT) or aromatization to estradiol (E2). 7 alpha-Methyl-19-nortestosterone (MENT) is a synthetic androgen that is considerably more potent than T. Previous studies have shown that MENT is not 5 alpha-reduced. The studies reported here were undertaken to determine whether MENT undergoes enzymatic aromatization in vitro. Human placental microsomes were used as the source of the aromatase. Radioactive or nonradioactive T or MENT was incubated with the microsomes in the presence of NADPH and the metabolites extracted out with ethyl ether. Following evaporation of ether, the residue was dissolved in benzene-petroleum ether and extracted with 0.4 N NaOH which selectively removes phenolic metabolites of the androgens. When either radioactive T or MENT was incubated with the aromatase in the presence of NADPH, there was a 20-fold increase in the amount of radioactivity extracted with NaOH. In contrast, if the incubation was carried out in the absence of NADPH or in the presence of R76713, an aromatase inhibitor, most of the radioactivity remained in the benzene-petroleum ether phase. To further identify the enzymatic reaction products, thin layer chromatography (TLC) was performed. The Rf value for MENT was 0.22 while that of the major reaction product was 0.34, which corresponded with the RF value of the estrogen, 7 alpha-methyl-estradiol (MeE2). This was further verified by using a second solvent system for the chromatographic separation. In an effort to ascertain whether the metabolites bind to estrogen receptors (ER), rat uterine cytosol was used. NaOH extracts of medium following incubation of nonradioactive MENT with microsomes showed competitive inhibition of [3H]E2 binding to rat uterine ER. Furthermore, after [3H]MENT was incubated with microsomes, the radioactive metabolite extracted in NaOH showed specific binding to the ER which could readily be displaced with E2 or MeE2. These results indicate that like T, MENT undergoes enzymatic aromatization.

Aromatase inhibitors--mechanisms of steroidal inhibitors

Inhibition of aromatase has been an attractive approach for examining the roles of estrogen biosynthesis in various physiological or pathological processes. Effective aromatase inhibitors can serve as potential therapeutic agents for controlling estrogen-dependent diseases such as hormone-dependent breast cancer. Investigations on the development of aromatase inhibitors have therefore expanded greatly in the past two decades. Numerous steroidal agents have been developed that have high affinities for the aromatase enzyme complex and exhibit either competitive inhibition, irreversible inhibition, or mechanism-based (enzyme-activated) inhibition. Mechanism-based inhibitors have distinct advantages in drug design, since these inhibitors are highly enzyme specific, produce prolonged inhibition, and exhibit minimal toxicities. Examination of the structure-activity relationships of the numerous steroidal aromatase inhibitors suggest that the spacial requirements for interaction of agents with the active site of aromatase are very restrictive, permitting only small structural changes to be made on the A-ring and at C-19. Incorporation of small polar substituents at the C-4 position, such as a hydroxyl group, or addition of aryl functionalities at the 7 alpha-position of the steroid, are the exceptions, and inhibitors with such modifications exhibit enhanced affinity for the enzyme. Future investigations of steroidal aromatase inhibitors as probes of the active site of purified aromatase will provide valuable information on enzyme structure at the molecular level, will permit a more detailed examination of the mechanisms of inhibition, and will enhance the development of more specific and effective inhibitors for the treatment of estrogen-dependent breast cancer.

Synthesis and biochemical studies of 7 alpha-substituted androsta-1,4-diene-3,17-diones as enzyme-activated irreversible inhibitors of aromatase

Several 7 alpha-thiosubstituted derivatives of androstenedione have demonstrated effective inhibition of aromatase, the cytochrome P450 enzyme complex responsible for the biosynthesis of estrogens. Introduction of an additional double bond in the A ring resulted in 7 alpha-(4'-amino)phenylthioandrosta-1,4-diene-3,17-dione (7 alpha-APTADD), a potent inhibitor that inactivated aromatase by an enzyme-catalyzed process. Additional 7 alpha-thiosubstituted androsta-1,4-diene-3,17-dione derivatives were designed to further examine enzyme-catalyzed inactivation. Two halogenated and one unsubstituted 7 alpha-phenylthioandrosta-1,4-diene-3,17-diones were synthesized via an acid-catalyzed conjugate Michael addition of substituted thiophenols with androsta-1,4,6-triene-3,17-dione. Two 7 alpha-naphthylthioandrosta-1,4-diene-3,17-diones were synthesized via either acid-catalyzed or based-catalyzed conjugate Michael addition of substituted thionaphthols with androsta-1,4,6-triene-3,17-dione. These agents were evaluated for aromatase inhibitory activity in the human placental microsomal preparation. Under initial velocity assay conditions of low product formation, the inhibitors demonstrated potent inhibition of aromatase, with apparent Ki's ranging from 12 to 27 nM. Furthermore, these compounds produced time-dependent, first-order inactivation of aromatase in the presence of NADPH, whereas no aromatase inactivation was observed in the absence of NADPH. This enzyme-activated irreversible inhibition, also referred to as mechanism-based inhibition, can be prevented by the substrate androstenedione. Thus, the apparent Ki values for these inhibitors are consistent with earlier studies on 7 alpha-substituted competitive inhibitors that indicate bulky substituents can be accommodated at the 7 alpha-position.(ABSTRACT TRUNCATED AT 250 WORDS)

Aromatase inhibition by 7-substituted steroids in human choriocarcinoma cell culture

Androstenedione analogs containing 7 alpha-substituents have proven to be potent inhibitors of aromatase both in vitro and in vivo. Several of these agents have exhibited higher affinity for the enzyme complex than the substrate. In order to examine further the interaction(s) of 7-substituted steroids with aromatase, 7-substituted 4,6-androstadiene-3,17-diones were synthesized and demonstrated competitive inhibition of aromatase activity in human placental microsomes. 7-Substituted 1,4,6-androstatriene-3,17-diones demonstrated mechanism-based inhibition of placental aromatase activity. These agents were evaluated for inhibition of aromatase activity in the JAr human choriocarcinoma line. The 7-substituted 4,6-androstadiene-3,17-diones produced dose dependent inhibition of aromatase activity in the cell cultures, with IC50 values ranging from 490 nM to 4.5 microM. However, these agents are less effective when compared to other steroidal inhibitors, such as 7 alpha-thiosubstituted androstenediones. These results on the 7-substituted 4,6-androstadiene-3,17-diones are consistent with the data from biochemical enzyme inhibition studies using human placental aromatase. On the other hand, 7-phenethyl-1,4,6-androstatriene-3,17-dione exhibits greater inhibitory activity, with an IC50 value of 80 nM. Other mechanism-based inhibitors, 7 alpha-(4'-amino)phenylthio-1,4-androstadiene-3,17-dione and 4-hydroxyandrostenedione, also exhibited potent inhibition of aromatase activity in JAr cells. In summary, the most effective B-ring modified steroidal aromatase inhibitors are those derivatives that can project the 7-aryl substituent into the 7 alpha-position.

Biochemical and pharmacological development of steroidal inhibitors of aromatase

Androstenedione analogs containing 7 alpha-substituents have proven to be potent inhibitors of aromatase both in vitro and in vivo. Several of these agents have exhibited higher affinity for the enzyme complex than the substrate. In order to examine further the interaction(s) of 7-substituted steroids with aromatase, biochemical and pharmacological studies were performed on 7 alpha-thiosubstituted androstenediones and 7-substituted 4,6-androstadiene-3,17-diones. Potent inhibition of aromatase activity in human placental microsomes has been observed with several new 7 alpha-thiosubstituted androstenediones. 7-Benzyl- and 7-phenethyl-4,6-androstadiene-3,17-diones effectively inhibited microsomal aromatase, with apparent Kis ranging from 61 to 174 nM. On the other hand, 7-phenyl-4,6-androstadiene-3,17-dione exhibited poor activity, with an apparent Ki of 1.42 microM. Similar inhibitory activity was observed with reconstituted, purified cytochrome P450Arom preparations. Additionally, these agents were evaluated for inhibition of aromatase activity in two human carcinoma cell lines, the MCF-7 human mammary cancer line and the JAr choriocarcinoma line. The 7 alpha-thiosubstituted androstenediones and 7-substituted 4,6-androstadiene-3,17-diones produced dose-dependent inhibitions of aromatase activity in the cell cultures. The most effective inhibitors were the 7 alpha-substituted androstenediones, with EC50 values ranging from 7.3 to 105 nM. Finally, the JAr cell culture system exhibited prolonged inhibition of aromatase activity following exposure to 7 alpha-APTADD, suggesting enzyme inactivation by this inhibitor. Thus, these agents are effective aromatase inhibitors, and the results encourage further development of this group of medicinal agents for the treatment of estrogen-dependent mammary carcinoma.

7-substituted 1,4,6-androstatriene-3,17-diones as enzyme-activated irreversible inhibitors of aromatase

7-Phenyl-1,4,6-androstatriene-3,17-dione (4), 7-benzyl-1,4,6-androstatriene-3,17-dione (5) and 7-phenethyl-1,4,6-androstatriene-3,17-dione (6) were synthesized and evaluated in vitro in human placental microsomes as enzyme-activated irreversible inhibitors of aromatase. The compounds were synthesized from appropriate 7-substituted 4,6-androstadiene-3,17-diones by reaction with DDQ under neutral conditions. All the compounds produced a first order inactivation of aromatase in the presence of NADPH but not in the absence of NADPH. Substrate 4-androstene-3,17-dione protected the enzyme from inactivation by the inhibitors. Furthermore, cysteine failed to protect aromatase from inactivation by compounds 5 and 6. In contrast, cysteine partially protected aromatase from inactivation by compound 4. Irreversibility studies illustrated the covalent nature of the inactivation by 4, 5 and 6. The above experimental evidence demonstrated that compounds 5 and 6 are effective enzyme-activated irreversible inhibitors of aromatase.

Synthesis and biochemistry of fluorescent aromatase inhibitors

Effective aromatase inhibitors have been developed that contain aryl functionalities at the 7 alpha-position of the steroid nucleus. The exact interactions of 7 alpha-substituted androstenediones with the active site of aromatase is unknown. Fluorescent derivatives may provide a useful spectroscopic method for examining the binding of these inhibitors to the microsomal complex and purified aromatase protein. Dinitrophenyl, dansyl, and naphthyl derivatives of 7 alpha-(4'-amino)phenylthio-4-androstene-3,17-dione and androstenedione were synthesized as potential fluorescent agents. An in vitro assay with human placental microsomes was used to evaluate aromatase inhibitory properties. These fluorescent compounds were effective competitive inhibitors and have apparent Ki values ranging from 24.1 to 86.7 nM.

7-substituted steroidal aromatase inhibitors: structure-activity relationships and molecular modeling

Androstenedione analogs containing 7 alpha-substituents have proven to be potent inhibitors of aromatase both in vitro and in vivo. Several of these agents have exhibited higher affinity for the enzyme complex than the substrate does. In order to examine further the interaction(s) of 7-substituted steroids with aromatase, biochemical and molecular modeling studies were performed on 7-substituted 4,6-androstadiene-3,17-diones. 7-Benzyl- and 7-phenethyl-4,6-androstadiene-3,17-diones effectively inhibited microsomal aromatase, with apparent Kis ranging from 61 to 174 nM. On the other hand, 7-phenyl-4,6-androstadiene-3,17-dione exhibited poor activity, with an apparent Ki of 1.42 microM. Energy minimization calculations and molecular modeling indicated that the 7-substituent is perpendicular to the steroid nucleus in the 7-phenyl analog and can only adopt a pseudo beta position. The 7-benzyl- and 7-phenethyl- groups of 4,6-androstadiene-3,17-diones orient themselves in the minimized structure in a way that the phenyl rings can protrude into the 7 alpha pocket. These orientations are similar to those observed in minimized structures for potent 7 alpha-substituted androstenediones.

Aromatase inhibition by 4-thiosubstituted-4-androstene-3,17-dione derivatives

The synthesis and evaluation of 4-thiosubstituted-4-androstenedione analogs as inhibitors of estrogen synthetase (aromatase) is described. All compounds were prepared by the addition of various thiol reagents to 4 beta,5 beta-epoxyandrostanedione. Inhibitory activity of synthesized compounds was assessed using a human placental microsomal preparation as the enzyme source and [1 beta-3H]4-androstene-3,17-dione as substrate. Synthesized compounds exhibiting high inhibitory activity were further evaluated under initial velocity conditions to determine apparent Ki values. Several compounds were effective competitive inhibitors, and have apparent Ki values ranging from 34 to 52 nM, with the apparent Km for androstenedione being 54 nM. The results of these studies demonstrate a tightly fitted enzyme pocket that can accommodate bulky substituents at the C-4 position of androstenedione not to exceed 4.3 A in width and 5.5 A in length.

Structure-activity relationships for non-steroidal inhibitors of aromatase

A model is described for the binding of non-steroidal inhibitors of aromatase to the enzyme with interaction of a ligand with the Fe3+-haem of the cytochrome. The model suggests that the inhibitors utilise a common binding site with ring A of the steroidal substrates.

Biological characterization of 10-(2-propynyl) estr-4-ene-3, 17-dione (MDL 18,962), an enzyme-activated inhibitor of aromatase

MDL 18,962 was shown to be a highly specific, potent (Ki = 3-4 nM), enzyme-activated inhibitor of aromatase with minimal intrinsic endocrine properties. The affinity of MDL 18,962 was higher for human and baboon placental aromatase than for rhesus placental or rodent ovarian aromatase. These species differences necessitated the development of a novel model of peripheral aromatase utilizing human enzyme. Human choriocarcinoma trophoblast xenografts in athymic nude mice were used for pharmacologic and pharmacokinetic evaluation of MDL 18,962. The ED50 for inhibition of aromatase activity in these trophoblast tumors at 6 h post-treatment was 1.4 mg/kg, s.c. and 3.0 mg/kg, oral. Preliminary results indicated that the ED50 for inhibition of peripheral aromatization of androgen by MDL 18,962 in female baboons was 0.01 mg/kg, i.v. and 4 mg/kg, oral.

7 alpha-substituted androstenediones as effective in vitro and in vivo inhibitors of aromatase

Research efforts over the past several years have focused on the synthesis of competitive and irreversible aromatase inhibitors and examination of these inhibitors in microsomal preparations, in cell culture, and in vivo. Several 7 alpha-substituted androstenediones have demonstrated high affinity for placental aromatase, with apparent Ki's ranging from 1 to 30 nM. Inactivation of aromatase occurred following incubation with alkylating and enzyme-activated irreversible inhibitors. 7 alpha-(4'-Amino)phenylthio-4-androstene-3,17-dione (7 alpha-APTA) exhibits potent inhibitory activity of aromatase in the MCF-7 human mammary carcinoma cell line with an ED50 of approximately 25 nM. The inhibitor did not bind to the estrogen receptor of the cells in vitro nor induce levels of progesterone receptors in intact cells. In vivo studies of 7 alpha-APTA in the DMBA-induced rat mammary carcinoma model resulted in 80% of the tumors responding completely or partially at doses of 25 and 50 mg/kg body wt/day. Thus, these 7 alpha-substituted steroidal aromatase inhibitors are effective medicinal agents and may be useful for the treatment of estrogen-dependent breast cancer.

Synthesis and evaluation of bromoacetoxy 4-androsten-3-ones as active site-directed inhibitors of human placental aromatase

2 alpha-Bromoacetoxy (II), 6-bromoacetoxy (VII and X), and 19-bromoacetoxy (XII) derivatives of androstenedione and 17 beta-bromoacetoxy compounds (III, IV, XIII-XVI) were synthesized as potential affinity-labeling reagents for aromatase. 6 alpha-Bromoacetoxy derivative VII was the most potent inhibitor of human placental microsomal aromatase activity among this series. Its inhibitory activity was higher than that of the parent 6 alpha-hydroxy compound V, although other bromoacetates showed weaker inhibition of aromatase than the corresponding alcohols. The bromoacetates (except the 6 beta-bromoacetate X) inhibited aromatase activity in a time-dependent manner in the absence of NADPH, and the enzyme inactivation was blocked by the addition of androstenedione to the incubates. Kinetic analysis of the time- and concentration-dependent inhibition by the 6 beta-bromo-17 beta-bromoacetoxy compound XV gave an apparent Ki of 25 microM and kinact of 0.027 min-1.