Synthesis and PET studies of [11C-cyano]letrozole (Femara), an aromatase inhibitor drug

Medical Department, Brookhaven National Laboratory, Upton, NY 11973, USA.
Nuclear Medicine and Biology (Impact Factor: 2.41). 03/2009; 36(2):215-23. DOI: 10.1016/j.nucmedbio.2008.11.010
Source: PubMed


Aromatase, a member of the cytochrome P450 family, converts androgens such as androstenedione and testosterone into estrone and estradiol, respectively. Letrozole (1-[bis-(4-cyanophenyl)methyl]-1H-1,2,4-triazole; Femara) is a high-affinity aromatase inhibitor (K(i)=11.5 nM) that has Food and Drug Administration approval for breast cancer treatment. Here we report the synthesis of carbon-11-labeled letrozole and its assessment as a radiotracer for brain aromatase in the baboon.
Letrozole and its precursor (4-[(4-bromophenyl)-1H-1,2,4-triazol-1-ylmethyl]benzonitrile) were prepared in a two-step synthesis from 4-cyanobenzyl bromide and 4-bromobenzyl bromide, respectively. The [(11)C]cyano group was introduced via tetrakis(triphenylphosphine)palladium(0)-catalyzed coupling of [(11)C]cyanide with the bromo precursor. Positron emission tomography (PET) studies in the baboon brain were carried out to assess regional distribution and kinetics, reproducibility of repeated measures and saturability. Log D, the free fraction of letrozole in plasma and the [(11)C-cyano]letrozole fraction in arterial plasma were also measured.
[(11)C-cyano]Letrozole was synthesized in 60 min with a radiochemical yield of 79-80%, with a radiochemical purity greater than 98% and a specific activity of 4.16+/-2.21 Ci/mumol at the end of bombardment (n=4). PET studies in the baboon revealed initial rapid and high uptake and initial rapid clearance, followed by slow clearance of carbon-11 from the brain, with no difference between brain regions. Brain kinetics was not affected by coinjection of unlabeled letrozole (0.1 mg/kg). The free fraction of letrozole in plasma was 48.9%, and log D was 1.84.
[(11)C-cyano]Letrozole is readily synthesized via a palladium-catalyzed coupling reaction with [(11)C]cyanide. Although it is unsuitable as a PET radiotracer for brain aromatase, as revealed by the absence of regional specificity and saturability in brain regions such as amygdala, which are known to contain aromatase, it may be useful in measuring letrozole distribution and pharmacokinetics in the brain and peripheral organs.

Download full-text


Available from: Kuneek Kil, Nov 18, 2015
  • Source
    • "Taken together, these findings suggest that orally administered anastrozole significantly affects AD-like pathology in female but not male 3xTgAD mice at 9 months of age, further supporting the hypothesis that AD-pathology may be manipulated in a sexdependent manner. To our knowledge, this is the first report of the detection of anastrozole in the brain, providing evidence that anastrozole, similar to letrozole, another third generation aromatase inhibitor (Goyal et al., 2008; Kil et al., 2009), penetrates the blood brain barrier. Comparisons of brain and serum levels of anastrozole indicate that the brain concentration of anastrozole was 0.05% of the concentration found in serum. "
    [Show abstract] [Hide abstract]
    ABSTRACT: Alzheimer's disease (AD) is the most common form of dementia among the elderly with women exhibiting a higher risk than men for the disease. Due to these gender differences, there is great interest in the role that estrogens play in cognitive impairment and the onset of the classic amyloid and tau lesions in AD. Human and rodent studies indicate a strong association between low brain aromatase, sex hormone levels, and beta amyloid deposition. Therefore, the effects of depleting both circulating and brain estrogen levels, through gonadectomy and/or treatment with the aromatase inhibitor, anastrozole, upon hippocampal AD-like pathology in male and female 3xTgAD mice were evaluated. Liquid chromatography-mass spectrometry revealed anastrozole serum levels of 10.19 ng/mL and for the first time brain levels were detected at 4.7 pg/mL. Densitometric analysis of the hippocampus revealed that anastrozole significantly increased Aβ- but not APP/Aβ-immunoreactivity in intact 3xTgAD females compared to controls (p<0.001). Moreover, anastrozole significantly increased the number of Aβ- compared to APP/Aβ-positive hippocampal CA1 neurons in intact and gonadectomized female mice. Concurrently, anastrozole significantly reduced the APP/Aβ plaque load in 9 month old female 3xTgAD mice. These data suggest that anastrozole treatment differentially affects select amyloid species which in turn may play a role in the extraneuronal to intraneuronal deposition of this peptide.
    Full-text · Article · Sep 2011 · Neurobiology of Disease
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Aromatase inhibitors have been reported to increase height prediction in boys with short stature, and in boys and girls with gonadotropin-independent precocious puberty. The following review discusses data published since 2008 regarding the safety and efficacy of aromatase inhibitors in pediatric patients. Third-generation aromatase inhibitors in combination with antiandrogens appear effective in preventing bone age advancement and virilization in boys with familial male-limited precocious puberty (FMPP). Letrozole, but not anastrozole, decreased bleeding episodes and bone age advancement in girls with McCune-Albright syndrome (MAS), despite ovarian enlargement. Letrozole-treated boys with idiopathic short stature (ISS) had no loss of bone density but were noted to have more vertebral abnormalities than a placebo group. Two years of letrozole therapy did not increase predicted adult height in pre and peripubertal boys with ISS when re-assessed 4 years after the treatment period. Aromatase inhibitors together with an antiandrogen appear to be a very promising treatment for FMPP. Further longer-term studies with letrozole are needed in MAS. The prevalence of vertebral deformities should be evaluated prospectively in patients treated with aromatase inhibitors. Adult height data are still lacking in pediatric patients treated with aromatase inhibitors. Two years of therapy in pre and peripubertal short boys does not appear to increase adult height. Hemogram, lipids, and bone density should be periodically assessed in treated patients. Further controlled studies are needed to demonstrate safety and efficacy of aromatase inhibitors in pediatric patients.
    Preview · Article · May 2010 · Current opinion in pediatrics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Aromatase catalyzes the last step in estrogen biosynthesis. Brain aromatase is involved in diverse neurophysiological and behavioral functions including sexual behavior, aggression, cognition, and neuroprotection. Using positron emission tomography (PET) with the radiolabeled aromatase inhibitor [N-methyl-(11)C]vorozole, we characterized the tracer distribution and kinetics in the living human brain. Six young, healthy subjects, three men and three women, were administered the radiotracer alone on two separate occasions. Women were scanned in distinct phases of the menstrual cycle. Specificity was confirmed by pretreatment with a pharmacological (2.5 mg) dose of the aromatase inhibitor letrozole. PET data were acquired over a 90-min period and regions of interest placed over selected brain regions. Brain and plasma time activity curves, corrected for metabolites, were used to derive kinetic parameters. Distribution volume (V(T)) values in both men and women followed the following rank order: thalamus > amygdala = preoptic area > medulla (inferior olive) > accumbens, pons, occipital and temporal cortex, putamen, cerebellum, and white matter. Pretreatment with letrozole reduced V(T) in all regions, though the size of the reduction was region-dependent, ranging from ∼70% blocking in thalamus andpreoptic area to ∼10% in cerebellum. The high levels of aromatase in thalamus and medulla (inferior olive) appear to be unique to humans. These studies set the stage for the noninvasive assessment of aromatase involvement in various physiological and pathological processes affecting the human brain.
    Full-text · Article · Nov 2010 · Synapse
Show more