Aromatase inhibition by R 76713: experimental and clinical pharmacology

Reliable Target Prediction of Bioactive Molecules Based on Chemical Similarity Without Employing Statistical Methods

The prediction of biological targets of bioactive molecules from machine-readable materials can be routinely performed by computational target prediction tools (CTPTs). However, the prediction of biological targets of bioactive molecules from non-digital materials (e.g., printed or handwritten documents) has not been possible due to the complex nature of bioactive molecules and impossibility of employing computations. Improving the target prediction accuracy is the most important challenge for computational target prediction. A minimum structure is identified for each group of neighbor molecules in the proposed method. Each group of neighbor molecules represents a distinct structural class of molecules with the same function in relation to the target. The minimum structure is employed as a query to search for molecules that perfectly satisfy the minimum structure of what is guessed crucial for the targeted activity. The proposed method is based on chemical similarity, but only molecules that perfectly satisfy the minimum structure are considered. Structurally related bioactive molecules found with the same minimum structure were considered as neighbor molecules of the query molecule. The known target of the neighbor molecule is used as a reference for predicting the target of the neighbor molecule with an unknown target. A lot of information is needed to identify the minimum structure, because it is necessary to know which part(s) of the bioactive molecule determines the precise target or targets responsible for the observed phenotype. Therefore, the predicted target based on the minimum structure without employing the statistical significance is considered as a reliable prediction. Since only molecules that perfectly (and not partly) satisfy the minimum structure are considered, the minimum structure can be used without similarity calculations in non-digital materials and with similarity calculations (perfect similarity) in machine-readable materials. Nine tools (PASS online, PPB, SEA, TargetHunter, PharmMapper, ChemProt, HitPick, SuperPred, and SPiDER), which can be used for computational target prediction, are compared with the proposed method for 550 target predictions. The proposed method, SEA, PPB, and PASS online, showed the best quality and quantity for the accurate predictions.

Determining the IC 50 Values for Vorozole and Letrozole, on a Series of Human Liver Cytochrome P450s, to Help Determine the Binding Site of Vorozole in the Liver

Vorozole and letrozole are third-generation aromatase (cytochrome P450 19A1) inhibitors. [¹¹C]-Vorozole can be used as a radiotracer for aromatase in living animals but when administered by IV, it collects in the liver. Pretreatment with letrozole does not affect the binding of vorozole in the liver. In search of finding the protein responsible for the accumulation of vorozole in the liver, fluorometric high-throughput screening assays were used to test the inhibitory capability of vorozole and letrozole on a series of liver cytochrome P450s (CYP1A1, CYP1A2, CYP2A6, and CYP3A4). It was determined that vorozole is a potent inhibitor of CYP1A1 (IC₅₀ = 0.469 μM) and a moderate inhibitor of CYP2A6 and CYP3A4 (IC₅₀ = 24.4 and 98.1 μM, resp.). Letrozole is only a moderate inhibitor of CYP1A1 and CYP2A6 (IC₅₀ = 69.8 and 106 μM) and a very weak inhibitor of CYP3A4 (<10% inhibition at 1 mM). Since CYP3A4 makes up the majority of the CYP content found in the human liver, and vorozole inhibits it moderately well but letrozole does not, CYP3A4 is a good candidate for the protein that [¹¹C]-vorozole is binding to in the liver.

Hormone-related pharmacokinetic variations associated with anti-breast cancer drugs

INTRODUCTION

Breast cancer is the most common female cancer, with more than one million new patients diagnosed annually worldwide. Generally speaking, there are three types of drugs used in management of breast cancer namely: hormonal treatment, chemotherapeutic agents and target-based agents. There is increasing evidence that hormones play an important role in development of both hormone-dependent and hormone-independent breast cancers.

AREAS COVERED

This review summarizes the pharmacokinetics of various types of drugs used to treat breast cancer. Furthermore, the authors discuss hormone-related variations including: the menstrual status, gender and exogenous hormones influencing drug absorption, distribution, metabolism or excretion (ADME). The authors also describe the physiological factors such as body weight and age that affect the pharmacokinetics of several drugs.

EXPERT OPINION

The factors affecting the pharmacokinetics of anti-breast cancer drugs are multifaceted. Hormones appear to be a key factor determining the pharmacokinetics (and efficacy) of hormonal therapy due to their role in cancer progression. In chemotherapy, the effects of hormones on the drug pharmacokinetics are possibly mediated through P-glycoprotein (P-gp) efflux and/or cytochrome P450 metabolism. In many cases, dosing regimen should be adjusted for drugs used in treatment of breast cancers based on the hormone levels in the body.

Reinvestigation of the synthesis and evaluation of [N-methyl-(11)C]vorozole, a radiotracer targeting cytochrome P450 aromatase

INTRODUCTION

We reinvestigated the synthesis of [N-methyl-(11)C]vorozole, a radiotracer for aromatase, and discovered the presence of an N-methyl isomer which was not removed in the original purification method. Herein we report the preparation and positron emission tomography (PET) studies of pure [N-methyl-(11)C]vorozole.

METHODS

Norvorozole was alkylated with [(11)C]methyl iodide as previously described and also with unlabeled methyl iodide. A high-performance liquid chromatography (HPLC) method was developed to separate the regioisomers. Nuclear magnetic resonance (NMR) spectroscopy ((13)C and 2D-nuclear Overhauser effect spectroscopy NMR) was used to identify and assign structures to the N-methylated products. Pure [N-methyl-(11)C]vorozole and the contaminating isomer were compared by PET imaging in the baboon.

RESULTS

Methylation of norvorozole resulted in a mixture of isomers (1:1:1 ratio) based on new HPLC analysis using a pentafluorophenylpropyl bonded silica column, in which vorozole coeluted one of its isomers under the original HPLC conditions. Baseline separation of the three labeled isomers was achieved. The N-3 isomer was the contaminant of vorozole, thus correcting the original assignment of isomers. PET studies of pure [N-methyl-(11)C]vorozole with and without the contaminating N-3 isomer revealed that only [N-methyl-(11)C]vorozole binds to aromatase. [N-methyl-(11)C]Vorozole accumulated in all brain regions with highest accumulation in the aromatase-rich amygdala and preoptic area. Accumulation was blocked with vorozole and letrozole consistent with reports of some level of aromatase in many brain regions.

CONCLUSIONS

The discovery of a contaminating labeled isomer and the development of a method for isolating pure [N-methyl-(11)C]vorozole combine to provide a new scientific tool for PET studies of the biology of aromatase and for drug research and development.

CYP1B1 is not a major determinant of the disposition of aromatase inhibitors in epithelial cells of invasive ductal carcinoma

CYP1B1 and CYP19 (aromatase) have been shown to be expressed in breast tumors. Both enzymes are efficient estrogen hydroxylases, indicating the potential for overlapping substrate and inhibitor specificity. We measured the inhibition properties of aromatase inhibitors (AIs) against CYP1B1-catalyzed hydroxylation of 17beta-estradiol (E2) to determine whether CYP1B1 affects the disposition of AIs. In addition, we estimated the frequency of coexpression of these enzymes in breast tumor epithelium. Immunohistochemical analyses of CYP19 and CYP1B1 in a panel of 29 cases of invasive ductal carcinoma of the breast showed epithelial cell staining for CYP19 in 76% and for CYP1B1 in 97% of the samples. Statistical analysis showed no significant correlation (0.33) for positive expression of CYP19 and CYP1B1 (p > 0.07). CYP1B1 inhibition was determined for two steroidal inhibitors: formestane and exemestane and five nonsteroidal inhibitors: aminoglutethimide, fadrozole, anastrozole, letrozole, and vorozole. Of the seven compounds tested, only vorozole exhibited inhibition of CYP1B1 activity with IC(50) values of 17 and 21 microM for 4-hydroxy estradiol and 2-hydroxy estradiol, respectively. The estimated K(i) values of vorozole for E2 4- and 2-hydroxylation were 7.26 and 6.84 microM, respectively. Spectrophotometric studies showed that vorozole was a type II inhibitor of CYP1B1. This study shows that with the exception of vorozole, the aromatase inhibitors are selective for CYP19 relative to CYP1B1. Thus, although both CYP19 and CYP1B1 are expressed in a high percentage of breast cancers, CYP1B1 is not a major determinant of the disposition of AIs.

Comparative clinical pharmacology and pharmacokinetic interactions of aromatase inhibitors

The clinical development of aromatase inhibitors (AIs) has been closely guided by clinical pharmacological investigations. During the early phases of development studies were focused on dose-related pharmacological effectiveness and specificity. More recently attention has been given to the metabolic changes which AIs elicit, with particular regard to their potential use in early breast cancer and the prophylactic setting. Pharmacological effectiveness has been studied with plasma oestrogen assays but primary oestrogens (E1 and E2) are not helpful in comparing the third generation inhibitors: anastrozole, letrozole, exemestane. All three of these compounds suppress whole body aromatisation by >96%. Most recently, we have established that significantly greater inhibition is achieved by letrozole than anastrozole at their clinically used dosages. This more complete inhibition is paralleled by significantly greater suppression of E1S.A broad panel of endocrine investigations has indicated that these compounds have essentially complete specificity at their clinical dosages. A minor androgenic effect of exemestane is revealed by a significant suppression of sex hormone binding globulin (SHBG). Lipid and bone biomarker data are being collected in many current studies. A pharmacokinetic interaction has been established between letrozole and tamoxifen, whereby reduced circulating levels of letrozole are found with combined application. Neither anastrozole nor letrozole have any effect on plasma concentrations of tamoxifen when given in combination with it.

Vorozole results in greater oestrogen suppression than formestane in postmenopausal women and when added to goserelin in premenopausal women with advanced breast cancer

The high potency and selectivity of new aromatase inhibitors has translated to greater efficacy and improved tolerability in comparison with established second-line hormonal agents for advanced breast cancer in phase III clinical trials. Two pharmacological studies are reported which assess the use of one of these inhibitors, vorozole, in combination or comparison with well-established methods of oestrogen deprivation in pre and postmenopausal patients. When combined with the gonadotrophin-releasing hormone agonist (GnRHa) goserelin in 10 premenopausal patients, vorozole markedly enhanced the suppression of serum levels of oestrone, oestradiol and, oestrone sulphate beyond that achieved by goserelin alone (by a mean 74%, 83%, and 89%, respectively). The combination was well-tolerated and had no significant effects on androgen levels. Vorozole was compared with formestane in 13 postmenopausal women and serum oestrone, oestradiol, and oestrone sulphate levels were suppressed by 47%, 30%, and 70%, respectively, more by vorozole than by the steroidal aromatase inhibitor. Again the tolerability was excellent. The plasma oestrogen levels in the postmenopausal patients on vorozole were lower than in the premenopausal patients on goserelin plus vorozole, indicating that ovarian oestrogen synthesis may be relatively resistant to aromatase inhibition, even during GnRHa treatment. Thus, in both pre and postmenopausal patients substantially greater suppression of oestrogen can be achieved by vorozole compared with alternative approaches. Existing clinical-pharmacological correlates suggest that these increases in pharmacological effectiveness may result in enhanced clinical effectiveness.

Paradoxical effect of an aromatase inhibitor, CGS 20267, on aromatase activity in guinea pig brain

To determine the effect of in vivo treatment of guinea pigs with a non-steroidal aromatase inhibitor (CGS 20267; letrozole), we treated subjects with subcutaneous Silastic implants containing crystalline letrozole. We studied four treatment groups: intact, intact letrozole-treated, castrate and castrate letrozole-treated. After treatment for 1 week, brain tissues (preoptic area, septum, medial basal hypothalamus, amygdala and parietal cortex) were removed, and microsomal aromatase activity (AA) was determined by an in vitro 3H2O assay using 1beta-3H-androstenedione as substrate. Kinetic experiments were performed to determine the competitive nature of letrozole and an approximate Ki was calculated. Letrozole appears to be a reversible, competitive inhibitor of aromatase activity with an apparent Ki of 1.2 nM. Aromatase activity in intact letrozole-treated animals was elevated compared to untreated controls in all brain areas tested (P< 0.05). Letrozole also stimulated AA in the brains of letrozole-treated castrated guinea pigs compared to untreated castrated animals (P< 0.05). These data indicate that letrozole administered in vivo causes an increase in AA. Possible mechanisms include an autoregulatory mechanism which is interrupted by enzyme inhibition, or an effect of the inhibitor on turnover rates of P450 aromatase.

The potent and selective inhibition of estrogen production by non-steroidal aromatase inhibitor, YM511

YM511 inhibited aromatase activities in microsomes from rat ovary and human placenta competitively (IC50s: 0.4 and 0.12 nM, respectively). YM511 was about 3 times more potent than other aromatase inhibitors, such as CGS 16949A, CGS 20267 and R 76713. YM511 decreased the contents of estradiol stimulated by pregnant mare's serum gonadotropin in rat ovary with an ED50 of 0.002 mg/kg, indicating that YM511 was equipotent to CGS 20267 and 3 times more potent than the other two inhibitors. Serum estradiol levels in female rats were reduced by YM511 at 0.01 mg/kg into the ovariectomized range. YM511 at 1 mg/kg for 2 weeks decreased rat uterine weight to levels comparable to ovariectomy, showing it was 10 times more potent than other inhibitors. But the maximal inhibitory effect of tamoxifen failed to reach ovariectomized level. YM511 slightly inhibited production of other steroid hormones in vitro and in vivo. The IC50s of YM511 for aldosterone and cortisol production from adrenal cells were from 5500 to 9800 times higher than that for rat ovarian aromatase and 130,000 times higher for testosterone production, indicating that YM511 is a highly specific aromatase inhibitor. The data suggest that YM511 may be a potent and selective agent for suppressing estrogen-dependent action without affecting serum levels of other steroid hormones.

Aromatase inhibitor development for treatment of breast cancer

Inhibition of estrogen production provides effective therapy for patients with hormone-dependent breast cancer. The source of estrogens in premenopausal women is predominantly the ovary, but after the menopause, estradiol is synthesized in peripheral tissues through the aromatization of androgens to estrogens. Uptake from plasma is the primary mechanism for maintenance of estradiol concentrations in breast cancer tissue in premenopausal women, whereas several steps may be operant in postmenopausal women. These include enzymatic synthesis of estradiol via sulfatase, aromatase, and 17 beta-hydroxysteroid dehydrogenase in the tumor itself. Aromatization of androgens secreted by the adrenal to estrogens in peripheral tissues and transport to the tumor via circulation in the plasma provides another means of maintaining breast tumor estradiol levels in postmenopausal women. These various sources contribute to the high tissue estrogen levels measured in breast tumor tissue. To effectively suppress tissue concentrations of estrogens and circulating estradiol in postmenopausal patients, various aromatase inhibitors have been developed recently. These include steroidal inhibitors such as 4-hydroxy-androstenedione as well as non-steroidal compounds with imidazole and triazole structures. The most potent of these, CGS 20267, is reported to suppress levels of active estrogens (i.e., estrone, estrone sulfatase, and estradiol) by more than 95%. This compound can suppress both serum and 24-hr urine estrogens to a greater extent than produced by the second generation inhibitor, CGS 16949A. CGS 20267 is highly specific since it does not affect cortisol and aldosterone serum levels during ACTH stimulation tests nor sodium and potassium balance in 24-hr urine samples.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of steroidal and non steroidal aromatase inhibitors on sexual behavior and aromatase-immunoreactive cells and fibers in the quail brain

Castrated quail were treated with Silastic implants filled with testosterone (T) in association with injections of the aromatase inhibitors, R76713 (racemic vorozole; 1 mg/kg twice a day) or 4-hydroxyandrostenedione (OHA; 5 mg/bird twice a day). Control birds received no treatment (CX group) or were implanted with T capsules only (CX + T group). Both R76713 and OHA strongly inhibited the T-activated male copulatory behavior. This inhibition had the same magnitude in both groups. The growth of the cloacal gland, a strictly androgen-dependent process was not affected by these compounds. The treatments significantly affected the number of aromatase-immunoreactive (ARO-ir) cells in each of the six brain areas that were studied: the anterior and posterior parts of the sexually dimorphic medial preoptic nucleus (POM), the septal region, the bed nucleus of the stria terminalis (BNST) and the anterior and posterior parts of the tuber. This number was significantly increased in all areas by T. In agreement with our previous study, R76713 significantly inhibited this effect of T in the tuberal hypothalamus but not in the anterior POM nor in the BNST. By contrast the effect of T on the number of ARO-ir cells was completely blocked by OHA in all brain nuclei. The two inhibitors had statistically different effects in all brain regions. Like in a previous study, R76713 increased the intensity of the staining of all ARO-ir cells. This effect took several days to develop suggesting a progressive build-up of the enzyme concentration. This was also suggested by the fact that a rebound in aromatase activity was observed 16 to 24 h after a single injection of R76713. The increased immunoreactivity was not observed in OHA-treated birds. The denser immunoreactivity in R76713-treated birds and the better tissue preservation due to the aldehyde fixative that had been used provided here a clearer picture of the cellular and subcellular localization of ARO-ir material. This allowed to identify new groups of immunoreactive cells, namely in the nucleus accumbens, in the area of the paleostriatum ventrale, in the nucleus taeniae, in the medial and caudal hypothalamus and in the medial part of the mesencephalon and of the pons. Most of the immunoreactive material was located in perikarya but some of these cells were also surrounded by dense networks of ARO-ir fibers often associated with immunopositive punctate structures.(ABSTRACT TRUNCATED AT 400 WORDS)

Pharmacology of vorozole

Vorozole (R83842) is a potent and selective, non-steroidal aromatase inhibitor. It is the dextro-enantiomer of the triazole derivative R 76,713. In FSH-stimulated rat granulosa cells, vorozole inhibited aromatase activity with an IC50-value of 1.4 +/- 0.5 nM. In pregnant mare serum gonadotropin (PMSG)-primed female rats, plasma estradiol levels measured 2 h after single oral administration of vorozole were significantly reduced by drug doses of 0.001 mg/kg and higher, with an ED50-value of 0.0034 mg/kg. In ovariectomized nude mice, bearing an estrogen-producing JEG-3 choriocarcinoma, 5 days treatment with vorozole, dose-dependently reduced uterus weight and completely inhibited tumor aromatase, measured ex vivo. Vorozole showed IC50-values higher than 10 microM for inhibition of progesterone synthesis in rat granulosa cells, for inhibition of steroid biosynthesis in isolated rat testicular and adrenal cells and for inhibition of steroid binding to estrogen-, progestin-, androgen- and gluco- and mineralocorticoid-receptors. In LHRH/ACTH-injected male rats and in rats fed a sodium-deprived diet, single oral administration of up to 10 mg/kg vorozole did not affect plasma levels of testicular and adrenal steroids. The compound also had no in vivo estrogen or androgen (ant)agonistic properties. In the DMBA-induced rat mammary carcinoma model, vorozole at an oral dose of 2.5 mg/kg b.i.d. inhibited tumor growth similarly to ovariectomy.

Exemestane (FCE 24304), a new steroidal aromatase inhibitor

Exemestane (FCE 24304; 6-methylenandrosta-1,4-diene-3,17-dione) is a novel orally active irreversible aromatase inhibitor. Its in vitro and in vivo pharmacological properties have been compared to 4-hydroxyandrostenedione (4-OHA). In preincubation studies with human placental aromatase, exemestane, like 4-OHA, showed enzyme inactivating properties with a similar affinity (Ki 26 vs 29 nM) and a lower rate of inactivation (t1/2 13.9 vs 2.1 min). Conversely, when tested in pregnant mares' serum gonadotropin-treated rats, exemestane was more potent in reducing microsomal ovarian aromatase activity than 4-OHA, after both subcutaneous (ED50 1.8 vs 3.1 mg/kg) and oral dosing (ED50 3.7 vs greater than 100 mg/kg). No interference of exemestane on desmolase or 5 alpha-reductase activity was found. The compound did not show any relevant binding affinity to steroidal receptors, but slight binding to the androgen receptor (approximately 0.2% of dihydrotestosterone), like 4-OHA. In the first phase I trial, healthy postmenopausal volunteers were given single oral doses of exemestane, ranging from 0.5 to 800 mg, and plasma [estrone (E1), estradiol (E2) and estrone sulphate (E1S)] and urinary estrogens (E1 and E2) were measured up to 5-8 days. The minimal effective dose in decreasing estrogens was 5 mg. At 25 mg the maximal suppression was observed at day 3: plasma estrogens fell to 35 (E1), 39 (E2) and 28% (E1S), and urinary estrogens fell to 20 (E1) and 25% (E2) of basal values, these effects still persisting on day 5. No effects on plasma levels of cortisol, aldosterone, 17-hydroxyprogesterone, DHEAS, LH and FSH, and no significant adverse events were observed up to the highest tested dose of 800 mg exemestane.

Determination of 4-hydroxyandrost-4-ene-3,17-dione metabolism in breast cancer patients using high-performance liquid chromatography-mass spectrometry

A sensitive procedure for studying the metabolism of the steroidal aromatase inhibitor 4-hydroxy-androst-4-ene-3,17-dione (4OHA) was developed based on enzyme hydrolysis, liquid-liquid extraction and reversed-phase liquid chromatography coupled with a mass spectrometer (LC-MS) using a thermospray interface. Seven metabolites were identified from the hydrolysed urine samples together with the parent drug. The major routes of metabolism were via dehydrogenation, reduction of the ketone functional groups, reduction at the C-4-C-5 double bond and hydroxylation at the C-5 position. Confirmation of the identity of 4OHA and its metabolites isolated from female patients' urine samples was accomplished by comparison of the retention times of their corresponding synthetic standards on LC-MS. We have demonstrated that this technique is particularly suitable for studying the metabolism of steroidal drugs.

Aromatase in the human choriocarcinoma JEG-3: inhibition by R 76 713 in cultured cells and in tumors grown in nude mice

The aromatase enzyme and its inhibition by R 76 713 were characterized in the JEG-3 choriocarcinoma cell line in culture and in JEG-3 tumors grown in nude mice. Optimal cell culture parameters and enzyme reaction conditions for the determination of aromatase activity were established. Under these conditions, in vitro JEG-3 aromatase was inhibited by R 76 713 with IC50-values of 7.6 +/- 0.5 nM and 2.7 +/- 1.1 nM using 500 nM of androstenedione and testosterone as substrate respectively. The Km-value of the aromatase enzyme with androstenedione as substrate was 62 +/- 19 nM; with testosterone as substrate, a value of 166 +/- 27 nM was found. In the presence of increasing concentrations of R 76 713, the Km-values increased while the Vmax remained unchanged. Using androstenedione and testosterone as substrate Lineweaver-Burk analysis of the data showed Ki-values for R 76 713 of 0.43 +/- 0.06 nM and 0.47 +/- 0.39 nM respectively. R 76 713 appeared to competitively inhibit the JEG-3 aromatase. Aromatase could easily be measured in homogenates of JEG-3 tumors grown in nude mice and showed Km-values similar to those found for JEG-3 cells in vitro. IC50-values for inhibition of tumor aromatase by R 76 713 were also similar to those found in cultured cells. Tumor aromatase measured ex vivo, 2 h after a single oral administration of R 76 713 was dose-dependently inhibited. An ED50-value of 0.05 mg/kg was calculated. The JEG-3 choriocarcinoma proved to be a useful aromatase model enabling the comparative study of aromatase inhibition in vitro and in vivo.

Measurement of aromatisation by a urine technique suitable for the evaluation of aromatase inhibitors in vivo

By modification of a recently developed method for separation of radio-labelled urinary oestrogens we were able to separate oestrogen metabolites and measure their isotope ratios in urine following injections of [3H]delta 4-androstenedione and [14C]oestrone. This method provides a useful tool for studying in vivo aromatisation of delta 4-androstenedione into oestrone in breast cancer patients before and during treatment with aromatase inhibitors.

Comparative effects of the aromatase inhibitor R76713 and of its enantiomers R83839 and R83842 on steroid biosynthesis in vitro and in vivo

R76713 (6-[(4-chlorophenyl)(1H-1,2,4-triazol-1-yl)methyl]-1-methyl-1H- benzotriazole) is a selective, non-steroidal aromatase inhibitor containing an asymmetric carbon atom. In this paper, we compare the effects of R76713 (racemate) with its enantiomers R83839 (the levo-isomer) and R83842 (the dextro-isomer) on steroid biosynthesis in rat cells in vitro and in the rat in vivo. In rat granulosa cells, aromatase activity was inhibited by 50% at concentrations of 0.93 nM of R76713, 240 nM of R83839 and 0.44 nM of R83842, revealing a 545-fold difference in activity between both enantiomers. Up to 1 microM, none of the compounds had any effect on steroid production in primary cultures of rat testicular cells. Above this concentration all three compounds showed a similar slight inhibition of androgen synthesis with a concomitant increase in the precursor progestins, indicative for some effect on the 17-hydroxylase/17,20-lyase enzyme. In rat adrenal cells none of the compounds showed any effect on corticosterone synthesis. At concentrations above 1 microM there was an increase in the levels of 11-deoxycorticosterone pointing towards an inhibition of the 11-hydroxylase enzyme. This increase was more pronounced for R83839 than for R76713 and R83842. In vivo, in PMSG-primed rats, R83842 reduced plasma estradiol by 50%. 2 h after oral administration of 0.0034 mg/kg, whereas 0.011 mg/kg of R76713 and 0.25 mg/kg of R83839 were needed to obtain the same result. Oral administration of up to 20 mg/kg of the compounds did not significantly affect plasma levels of adrenal steroids in LHRH/ACTH-injected rats. Plasma testosterone was lowered at 10 and 20 mg/kg of R83842 and at the highest dose (20 mg/kg) of R76713 and R83839. In conclusion, the present study shows that the aromatase inhibitory activity of R76713 resides almost exclusively in its dextro-isomer R83842. R83842 exhibits a specificity for aromatase as compared to other enzymes involved in steroid biosynthesis of at least a 1000-fold in vitro as well as in vivo. This confirms the extreme selectivity previously found for the racemate.

New non-steroidal aromatase inhibitors: focus on R76713

R76713 is a novel triazole derivative which selectively blocks the cytochrome P450-dependent aromatase. In human placental microsomes, in FSH-stimulated rat and human granulosa cells and in human adipose stromal cells, 50% inhibition of estradiol biosynthesis was obtained at drug concentrations of 2-10 nM. In PMSG-injected female rats, R76713 lowered plasma estradiol levels by 50 and 90% 2 h after single oral doses of 0.005 and 0.05 mg/kg respectively. After 1 mg/kg, estradiol levels were suppressed by 90% for 16 h. In male cynomolgus monkeys, R76713 dose-dependently (0.03-10 micrograms/kg) inhibited peripheral aromatization with an ED50 of 0.13 microgram/kg without altering metabolic clearance rates and conversion ratios. In vitro R76713 had no effect on other P450-dependent steroidogenic enzymes up to 1000 nM at least. In rats, LHRH-, ACTH- and sodium-deprived diet stimulated plasma testosterone, corticosterone and aldosterone levels were not modified 2 h after single oral administrations of R76713 (up to 20 mg/kg). Furthermore, R76713 did not show any in vitro or in vivo estrogenic or antiestrogenic property. R76713 also induced regression of DMBA-induced mammary tumors after daily oral administration of 1 mg/kg b.i.d. In male volunteers (n = 4), a single oral dose of 5 and 10 mg lowered median plasma estradiol levels from 70 pM to the detection limit of the assay (40 pM) 4, 8 and 24 h after intake whereas no changes were detected after placebo administration. In premenopausal women (n = 15), receiving a single oral dose of 20 mg, median plasma estradiol levels decreased from 389 pM (before) to 168, 133 and 147 pM, 4, 8 and 24 h after intake whereas they remained above 420 pM after placebo (n = 7).

R 76713 and enantiomers: selective, nonsteroidal inhibitors of the cytochrome P450-dependent oestrogen synthesis

The triazole derivative, R 76713 and its enantiomers R 83839(-) and R 83842(+) are effective inhibitors of the aromatization of androstenedione. For human placental microsomes, the (+) enantiomer (R 83824) is about 1.9- and 32-times more active than the racemate (IC50 2.6 nM) and the (-) enantiomer, respectively. R 83842 is about 30- and 1029-times more active than 4-hydroxyandrostene-3,17-dione and aminoglutethimide. This potency might originate from its high affinity for the microsomal cytochrome P450 (P450). Indeed, R 83842, compared to R 76713 and R 83839, forms a more stable P450-drug complex. Difference spectral measurements indicate that the triazole nitrogen N-4 coordinates to the haem iron. The reversed type 1 spectral changes suggest that R 76713 is able to displace the substrate from its binding place and the stable complex formed in particular with the (+) enantiomer suggests that its N-1-substituent occupies a lipophilic region of the apoprotein moiety. Kinetic analysis implies that there is a competitive part in the inhibition of the human placental aromatase by R 76713. The Ki values for R 76713, R 83842 and R 83839 are 1.3 nM, 0.7 nM and 18 nM, respectively. These results are indicative of stereospecificity for binding. Up to 10 microM, R 76713 and its enantiomers have no statistically significant effect on the regio- and stereoselective oxidations of testosterone in male rat liver microsomes. All three compounds have no effect on the P450-dependent cholesterol synthesis, cholesterol side-chain cleavage and 7 alpha-hydroxylation and 21-hydroxylase. At 10 microM, R 76713 has a slight effect on the bovine adrenal 11 beta-hydroxylase. This effect originates mainly from R 83839, the less potent aromatase inhibitor. On the other hand, the inhibition of the 17,20-lyase of rat testis observed at concentrations greater than or equal to 0.5 microM, originates rather from R 83842. However, 50% inhibition is only achieved at 1.8 microM R 83842, i.e. at a concentration about 1300-times higher than that needed to reach 50% inhibition of the human placental aromatase.

Potency and selectivity of the aromatase inhibitor R 76,713. A study in human ovarian, adipose stromal, testicular and adrenal cells

The effects of R 76,713 on steroidogenesis were studied in primary cultures of four different human cell types, i.e. ovarian granulosa cells, adipose stromal cells, testicular cells and adrenal cells. In human granulosa cells aromatization of [1 beta, 2 beta-3H]androstenedione (as measured by the release of tritiated water) showed a Km (Michaelis constant) of 78 nM. R 76,713 competitively inhibited aromatization with a Ki (dissociation constant of the enzyme-inhibitor complex) of 1.6 nM. In human adipose stromal cells aromatization was measured by following the conversion of androstenedione to estrone and 17 beta-estradiol. In this system a Km for aromatization of androstenedione of 10.8 nM was found. R 76,713 again showed competitive kinetics with a Ki-value of 0.14 nM. In human testicular cells the synthesis of the androgens testosterone, androstenedione and dehydroepiandrosterone was only inhibited by drug concentrations exceeding 10(-6) M. At 10(-5) M of R 76,713, steroid concentrations were lowered to 56, 64 and 81% of the control for testosterone, androstenedione and dehydroepiandrosterone respectively. Concomitantly, a slight increase in the levels of pregnenolone (138% of the control) and progesterone (133% of the control) was seen. In human adrenal cells the synthesis of cortisol and aldosterone was slightly affected by R 76,713 also at concentrations exceeding 10(-6) M. At 10(-5) M of R 76,713 the concentrations of cortisol and aldosterone were lowered to respectively 59 and 51% of the control. At the same drug concentration the precursors 11-deoxycortisol and 11-deoxycorticosterone rose to 189 and 147% of the control. These results show that in primary cultures of human cells, R 76,713 is a very potent aromatase inhibitor with a selectivity of at least 1000-fold compared to other steps in steroidogenesis.

Endocrine and antitumoral effects of R76713 in rats

Some effects of daily oral administration of a new non-steroidal aromatase inhibitor on the pituitary-gonadal and adrenal functions were investigated in female rats. At doses of 1 mg/kg twice daily or higher, R 76713 lowered plasma estradiol levels to the range measured after ovariectomy Plasma progesterone levels and uterine weights decreased whilst LH levels increased but to a lesser extent than after ovariectomy. The other hormonal data show that long-term administration of R 76 713 does not modify the gluco- and mineralocorticoid hormone levels even at the highest dose studied (20 mg/kg, 4 h after treatment). Furthermore, both ovariectomy and R 76 713 treatment (1 and 5 mg/kg twice a day) induced almost complete regression of 9,12-dimethyl-1,2-benzanthracene-induced mammary carcinoma in rats. The appearance of new tumors during the treatment period was completely inhibited by R 76 713 whilst multiplicity of the remaining tumors was dramatically reduced.