In vitro evaluation of aromatase enzyme in granulosa cells using a [11C]vorozole binding assay

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.

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.

Granulosa cell aromatase enzyme activity: effects of follicular fluid from patients with polycystic ovary syndrome, using aromatase conversion and [11C]vorozole-binding assays

The local regulation of ovarian aromatase enzyme in polycystic ovary syndrome (PCOS) was studied with aromatase conversion and [11C]vorozole-binding assays to analyze aromatase activity, substrate-enzyme affinity and number of aromatase binding sites in non-cultured human granulosa cells (GC) incubated with different sources and preparations of follicular fluid (FF). Incubation with FF from women stimulated in in vitro fertilization cycles with follicle-stimulating hormone yielded higher conversion activity than with FF from healthy women and PCOS patients, paralleled with higher substrate affinity (lower Kd) than with FF from healthy women. In PCOS women, charcoal-pretreated FF yielded higher conversion, whereas the ether-pretreated FF yielded lower conversion activity, than with untreated PCOS FF. Both preparations of FF yielded higher affinity to substrate (lower Kd values) and the ether-pretreated FF a lower number of binding sites (Bmax). It seems that steroids with the presence of proteins in PCOS FF reduced aromatase conversion activity through decreased substrate affinity, whereas FF preparations devoid of proteins reduced the aromatase conversion activity mainly through blocking of aromatase active sites. Identification of specific agents responsible for this rapid regulation of aromatase function might help to understand normal regulation of the menstrual cycle and supposed imbalances of inhibitors/activators in PCOS.

Meta-análisis de la eficacia diagnóstica de la PET-FDG en pacientes con sospecha de recurrencia por cáncer de ovario

AIM

The aim of this paper was to perform a meta-analysis of the literature to evaluate the accuracy of FDG-PET in recurrence detection in patients with ovarian cancer.

METHODS

The search strategy consisted in identifying papers published between November 1972 and October 2003 indexed in MEDLINE and CANCERLITE. Inclusion criteria were publications that evaluated recurrence with FDG-PET in patients with ovarian carcinoma. Exclusion criteria were duplicated papers or those outdated by subsequent ones. The statistical analysis included 95 % confidence intervals (CI) of sensitivity (S), specificity (Sp) and natural logarithm of the odds ratio (Ln OR).

RESULTS

Seventeen publications fulfilled the inclusion criteria and were analyzed. The S and Sp were 0.94 (95% CI, 0.93-0.96) and 0.65 (95% CI, 0.46-0.85), respectively. The Ln OR presented global homogeneity and significant values in > 75% of the studies.

CONCLUSION

According to our meta-analysis, FDG-PET has high S and intermediate Sp, with few false negative results. These preliminary findings suggest that FDG-PET may be an effective means of identifying patients with recurrent ovarian cancer.

Characterization of [11C]vorozole binding in ovarian tissue in rats throughout estrous cycle in association with conversion of androgens to estrogens in vivo and in vitro

Estrogen levels vary in a cyclic fashion during the rat estrous cycle, reaching peak concentrations during proestrus. Previously, it was suggested that the preovulatory peak in estrogen production in rats in vivo is regulated by other control mechanisms than concentration of precursor and amount of aromatase enzyme, changing the specific activity of the enzyme. To explore this hypothesis, ovarian binding of [11C]vorozole in vivo and in vitro, representing the amount of active aromatase, and conversion activity of ovarian homogenate were assayed together with serum androstenedione (A4) and estradiol-17beta (E2) levels during the estrous cycle in rats. The reducing ovarian [11C]vorozole binding in vivo from proestrus +4 up to +8h might indicate that the ovarian aromatase is blocked, probably to prevent premature increase of E2 levels. Thereafter (between proestrus +9 and +13h), the binding dramatically increases (aromatase enzyme is unblocked), to enable increased E2 synthesis. In addition, during the latter period, serum E2 levels were strongly correlated with serum A4 levels after adjustment for amount of ovarian aromatase (P=0.03), but not with amount of aromatase adjusted for levels of A4 (P=0.13), which might indicate changes in specific activity of the aromatase enzyme. Significant correlation between Kd and serum E2 levels during the same period indicated that aromatase-precursor affinity might be involved in the regulation of the enzyme-specific activity. This conclusion is done assuming that [11C]vorozole binding mimics that of the substrate (A4). The [11C]vorozole in vivo technique keeps auto- and paracrine mechanisms intact, and might therefore yield additional information about biological processes compared with traditional in vitro techniques.