Synthesis of carbon-11 labeled sulfonanilide analogues as new potential PET agents for imaging of aromatase in breast cancer

New nimesulide derivatives with amide/sulfonamide moieties: Selective COX-2 inhibition and antitumor effects

Seventeen new amide/sulfonamide containing nimesulide derivatives were synthesized and characterized by several spectroscopic techniques and primarily investigated for their inhibitory potential on COX enzymes and other pro-inflammatory factors. Experimental analyses showed that among seventeen compounds, N8 and N10 have remarkable potency and selectivity for the COX-2 enzyme over COX-1 at very low doses as compared to nimesulide. Moreover, both N8 and N10 selectively reduced the Lipopolysaccharide (LPS)-stimulated COX-2 mRNA expression level while the COX-1 level remained stable. Both PGE₂ release and nitric oxide production in macrophage cells were significantly suppressed by the N8 and N10 treatment groups. In silico ADME/Tox, molecular docking and molecular dynamics (MD) simulations were also conducted. Additionally, all compounds were also screened in a panel of cancer cell lines for their antiproliferative properties by MTT and SRB assays. Compound N17 exhibited a considerable antiproliferative effect on the colon (IC₅₀: 9.24 μM) and breast (IC₅₀: 11.35 μM) cancer cell lines. N17 exposure for 48 h decreased expression of anti-apoptotic protein BCL-2 and increased the expression of apoptogenic BAX. Besides, the BAX/BCL-2 ratio was increased with visible ultrastructural changes and apoptotic bodies under scanning electron microscopy. In order to investigate the structural and dynamical properties of selected hits on the target structures, multiscale molecular modeling studies are also conducted. Our combined in silico and in vitro results suggest that N8 and N10 could be further developed as potential nonsteroidal anti-inflammatory drugs (NSAIDs), while cytotoxic N17 might be studied as a potential lead compound that could be developed as an anticancer agent.

Synthesis and preclinical evaluation of the CRTH2 antagonist [11C]MK-7246 as a novel PET tracer and potential surrogate marker for pancreatic beta-cell mass

INTRODUCTION

MK-7246 is a potent and selective antagonist for chemoattractant receptor-homologous molecule expressed on T_h2 cells (CRTH2). Within the pancreas CRTH2 is selectively expressed in pancreatic β-cells where it is believed to play a role in insulin release. Reduction in β-cell mass and insufficient insulin secretion in response to elevated blood glucose levels is a hallmark for type 1 and type 2 diabetes. Reported here is the synthesis of [¹¹C]MK-7246 and initial preclinical evaluation towards CRTH2 imaging. The aim is to develop a method to quantify β-cell mass with PET and facilitate non-invasive studies of disease progression in individuals with type 2 diabetes.

METHODS

The precursor N-desmethyl-O-methyl MK-7246 was synthesized in seven steps and subjected to methylation with [¹¹C]methyl iodide followed by hydrolysis to obtain [¹¹C]MK-7246 labelled in the N-methyl position. Preclinical evaluation included in vitro radiography and immune-staining performed in human pancreatic biopsies. Biodistribution studies were performed in rat by PET-MRI and in pig by PET-CT imaging. Saturable tracer binding was examined in pig by scanning before and after administration of MK-7246 (1 mg/kg). Predicted dosimetry of [¹¹C]MK-7246 in human males was estimated based on the biodistribution in rat.

RESULTS

[¹¹C]MK-7246 was obtained with activities sufficient for the current investigations (270 ± 120 MBq) and a radiochemical purity of 93 ± 2%. The tracer displayed focal binding in areas with insulin positive islet of Langerhans in human pancreas sections. Baseline uptake in pig was reduced in tissues with known expression of CRTH2 after administration of MK-7246; pancreas (66% reduction) and spleen (88% reduction). [¹¹C]MK-7246 exhibited a safe human predicted dosimetry profile as extrapolated from the rat biodistribution data.

CONCLUSIONS

Initial preclinical in vitro and in vivo evaluations of [¹¹C]MK-7246 show binding and biodistribution properties suitable for PET imaging of CRTH2. Further studies are warranted to assess its potential in β-cell mass imaging and CRTH2 drug development.

Flavones: an important scaffold for medicinal chemistry

Flavones have antioxidant, anti-proliferative, anti-tumor, anti-microbial, estrogenic, acetyl cholinesterase, anti-inflammatory activities and are also used in cancer, cardiovascular disease, neurodegenerative disorders, etc. Also, flavonoids are found to have an effect on several mammalian enzymes like protein kinases that regulate multiple cell signaling pathways and alterations in multiple cellular signaling pathways are frequently found in many diseases. Flavones have been an indispensable anchor for the development of new therapeutic agents. The majority of metabolic diseases are speculated to originate from oxidative stress, and it is therefore significant that recent studies have shown the positive effect of flavones on diseases related to oxidative stress. Due to the wide range of biological activities of flavones, their structure-activity relationships have generated interest among medicinal chemists. The outstanding development of flavones derivatives in diverse diseases in very short span of time proves its magnitude for medicinal chemistry research. The present review gives detail about the structural requirement of flavone derivatives for various pharmacological activities. This information may provide an opportunity to scientists of medicinal chemistry discipline to design selective, optimize as well as poly-functional flavone derivatives for the treatment of multi-factorial diseases.

11C-cetrozole: an improved C-11C-methylated PET probe for aromatase imaging in the brain

Aromatase (an enzyme that converts androgens to estrogens) in the brain is involved in neuroprotection, synaptic plasticity, and regulation of sexual and emotional behaviors. To investigate the physiologic and pathologic importance of aromatase in the brain, including in humans, we here report the development of a novel PET probe for aromatase, (11)C-cetrozole, which allows noninvasive quantification of aromatase expression.

METHODS

(11)C-cetrozole was synthesized by the C-(11)C-methylation method developed by our group. In vitro autoradiography of frozen sections and a binding study with rat brain homogenates were conducted to demonstrate the specific binding and the dissociation constant. PET studies with anesthetized rhesus monkeys were performed to analyze the dynamics in the brain.

RESULTS

In vitro and in vivo studies using (11)C-cetrozole showed its superiority in brain aromatase imaging in terms of specificity and selectivity, compared with previously developed (11)C-vorozole. PET studies showed that (11)C-cetrozole had a higher signal-to-noise ratio, providing a sharper image than (11)C-vorozole, because the radioactive metabolite of (11)C-vorozole was taken up into the brain. High specific binding of (11)C-cetrozole was observed in the amygdala and hypothalamus, and we also noted binding in the nucleus accumbens of rhesus monkeys for the first time.

CONCLUSION

These results suggest that PET imaging with newly developed (11)C-cetrozole is suitable for quantifying the expression of brain aromatase in vivo, possibly providing critical information regarding the functional roles of aromatase in human neurologic and emotional disorders.

Development of a new class of aromatase inhibitors: design, synthesis and inhibitory activity of 3-phenylchroman-4-one (isoflavanone) derivatives

Aromatase (CYP19) catalyzes the aromatization reaction of androgen substrates to estrogens, the last and rate-limiting step in estrogen biosynthesis. Inhibition of aromatase is a new and promising approach to treat hormone-dependent breast cancer. We present here the design and development of isoflavanone derivatives as potential aromatase inhibitors. Structural modifications were performed on the A and B rings of isoflavanones via microwave-assisted, gold-catalyzed annulation reactions of hydroxyaldehydes and alkynes. The in vitro aromatase inhibition of these compounds was determined by fluorescence-based assays utilizing recombinant human aromatase (baculovirus/insect cell-expressed). The compounds 3-(4-phenoxyphenyl)chroman-4-one (1h), 6-methoxy-3-phenylchroman-4-one (2a) and 3-(pyridin-3-yl)chroman-4-one (3b) exhibited potent inhibitory effects against aromatase with IC(50) values of 2.4 μM, 0.26 μM and 5.8 μM, respectively. Docking simulations were employed to investigate crucial enzyme/inhibitor interactions such as hydrophobic interactions, hydrogen bonding and heme iron coordination. This report provides useful information on aromatase inhibition and serves as a starting point for the development of new flavonoid aromatase inhibitors.

N-(2-Hydroxy-5-nitrophenyl)methanesulfonamide ethanol monosolvate

In the title compound, C₇H₈N₂O₅S·C₂H₆O, the dihedral angle between the aromatic ring and the nitro group is 8.78 (9)° and the S atom is displaced by 0.226 (3) Å from the plane of the aromatic ring. In the crystal, the ethanol mol­ecule is involved in hydrogen bonding to two separate sulfonamide mol­ecules, as a donor in an O—H⋯O inter­action and as an acceptor in an N—H⋯O inter­action. Weak C—H⋯O hydrogen bonding is also present.

Design and synthesis of carbon-11-labeled dual aromatase-steroid sulfatase inhibitors as new potential PET agents for imaging of aromatase and steroid sulfatase expression in breast cancer

Aromatase and steroid sulfatase (STS) are particularly attractive targets in the treatment of estrogen-receptor-positive breast cancer and the development of enzyme-based cancer imaging agents for the biomedical imaging technique positron emission tomography (PET). New carbon-11-labeled sulfamate derivatives were first designed and synthesized as potential PET dual aromatase-steroid sulfatase inhibitor (DASSI) radiotracers for imaging of aromatase and STS expression in breast cancer. The target tracers 5-(((4-cyanophenyl)(4H-1,2,4-triazol-4-yl)amino)methyl)-2-[(11)C]methoxyphenyl sulfamate ([(11)C]8a) and 4-(((4-cyanophenyl)(4H-1,2,4-triazol-4-yl)amino)methyl)-2-[(11)C]methoxyphenyl sulfamate ([(11)C]8b) were prepared from their corresponding precursors 5-(((4-cyanophenyl)(4H-1,2,4-triazol-4-yl)amino)methyl)-2-hydroxyphenyl sulfamate (16) and 4-(((4-cyanophenyl)(4H-1,2,4-triazol-4-yl)amino)methyl)-2-hydroxyphenyl sulfamate (21) with [(11)C]CH(3)OTf under basic conditions through the O-[(11)C]methylation and isolated by the reversed-phase high pressure liquid chromatography (HPLC) method in 30-45% radiochemical yields based on [(11)C]CO(2) and decay corrected to end of bombardment (EOB). The specific activity at end of synthesis (EOS) was 111-185GBq/micromol.

Fully automated synthesis and initial PET evaluation of [11C]PBR28

Fully automated synthesis and initial PET evaluation of a TSPO radioligand, [11C]PBR28 (N-(2-[11C]methoxybenzyl)-N-(4-phenoxypyridin-3-yl)acetamide), are reported. These results facilitate the potential preclinical and clinical PET studies of [11C]PBR28 in animals and humans.

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

INTRODUCTION

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.

METHODS

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.

RESULTS

[(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.

CONCLUSION

[(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.

In vivo imaging of molecular targets and their function in endocrinology

Imaging is one of the fastest growing fields of study. New technologies and multimodal approaches are increasing the application of imaging to determine molecular targets and functional processes in vivo. The identification of a specific target, transporter, or biological process using imaging has introduced major breakthroughs to the field of endocrinology primarily utilizing computed tomography, magnetic resonance imaging, ultrasonography, positron emission tomography, single-photon emission computed tomography, and optical imaging. This review provides a general background to the specific developments in imaging that pertains to in vivo function and target identification in endocrine-based diseases.

Synthesis of carbon-11 labeled 1-(3,4-dimethoxybenzyl)-2,2-dimethyl-1,2,3,4-tetrahydroisoquinolinium derivatives as new potential PET SKCa channel imaging agents

Small conductance Ca2+-activated K+ (SKCa) channels play an important role in many functions such as neuronal communication and behavioral plasticity, secretion, and cell proliferation. SKCa channel modulation is associated with various brain, heart, and cancer diseases. N-methyl-laudanosine and its structurally related derivatives, substituted 1-(3,4-dimethoxybenzyl)-2,2-dimethyl-1,2,3,4-tetrahydroisoquinoliniums, are reversible and selective SKCa channel blockers. Carbon-11 labeled N-methyl-laudanosine and its tetrahydroisoquinolinium derivatives may serve as new probes for positron emission tomography (PET) to image SKCa channels in the brain, heart, and cancer. The key intermediates, substituted isoquinolines (3a-c), were synthesized using a modification of the Pomeranz-Fritsch procedure. The precursors, substituted 1-(3,4-dimethoxybenzyl)-2-methyl-1,2,3,4-tetrahydroisoquinolines (8a-c), and their corresponding reference standards, substituted 1-(3,4-dimethoxybenzyl)-2,2-dimethyl-1,2,3,4-tetrahydroisoquinoliniums (9a-c), were synthesized from compounds 3a-c with 3,4-dimethoxybenzyl chloride (2) in multiple steps with moderate to excellent chemical yields. The precursor 6,7-dimethoxy-1-(3,4-dimethoxybenzyl)-2-methyl-1,2,3,4-tetrahydroisoquinoline (10) was commercially available, and the methylation of compound 10 with methyl iodide provided N-methyl-laudanosine (11). The target quaternary ammonium tracers, carbon-11 labeled 1-(3,4-dimethoxybenzyl)-2,2-dimethyl-1,2,3,4-tetrahydroisoquinoliniums ([11C]9a-c and [11C]11), were prepared by N-[11C]methylation of the tertiary amine precursors (8a-c and 10) with [11C]methyl triflate and isolated by a simplified solid-phase extraction (SPE) purification using a SiO2 or cation-exchange CM Sep-Pak cartridge in 40-65% radiochemical yields.

Synthesis and preliminary biological evaluation of new carbon-11 labeled tetrahydroisoquinoline derivatives as SERM radioligands for PET imaging of ER expression in breast cancer

The estrogen receptors (ERs) are attractive targets in the treatment of breast cancer and the development of receptor-based breast cancer imaging agents for diagnostic use in biomedical imaging technique positron emission tomography (PET). Tetrahydroisoquinoline derivatives are a class of selective estrogen receptor modulators (SERMs) with high binding affinity and specificity exhibiting up to 50 folds for ERalpha over ERbeta. New carbon-11 labeled tetrahydroisoquinoline derivatives, [11C]methyl 1-(2-(4-(2-(4-fluorophenyl)-6-hydroxy-1-methyl-1,2,3,4-tetrahydroisoquinolin-1-yl)phenoxy)ethyl)piperidine-4-carboxylate ([11C]10a) and [11C]methyl 1-(2-(4-(2-(4-chlorophenyl)-6-hydroxy-1-methyl-1,2,3,4-tetrahydroisoquinolin-1-yl)phenoxy)ethyl)piperidine-4-carboxylate ([11C]10b), have been first designed, synthesized and evaluated. The target tracers were prepared by O-[11C]methylation of their corresponding precursors using [11C]CH3OTf and isolated by solid-phase extraction (SPE) purification procedure in 40-60% radiochemical yields, which were decay corrected to the end of bombardment (EOB), based on [11C]CO2. The overall synthesis time was 15-20 min from EOB. The radiochemical purity was >99%, and specific activity was in a range of 74-111GBq/micromol at the end of synthesis (EOS). Preliminary findings from in vitro biological assay indicate that the synthesized derivatives displayed similar potencies in the MCF-7 human breast cancer cell line in comparison with 4-hydroxytamoxifen, a well-known potent SERM. These results encourage further in vivo evaluation of carbon-11 labeled tetrahydroisoquinoline derivatives as new potential SERM radioligands for PET imaging of ER expression in breast cancer.

Synthesis of carbon-11 labeled biaryl 1,2,3,4-tetrahydroisoquinoline derivatives and conformationally flexible analogues as new potential PET glioma tumor imaging agents

Carbon-11 labeled biaryl 1,2,3,4-tetrahydroisoquinoline derivatives and conformationally flexible analogues, 2-(2-(biphenyl-4-yl)ethyl)-6-[(11)C]methoxy-7-methoxy-1,2,3,4-tetrahydroisoquinoline ([(11)C]3); 1-(biphenyl-4-yl)methyl-6,7-dimethoxy-2-[(11)C]methyl-1,2,3,4-tetrahydroisoquinoline (N-[(11)C]7) and 1-(biphenyl-4-yl)methyl-6-[(11)C]methoxy-7-methoxy-2-methyl-1,2,3,4-tetrahydroisoquinoline (O-[(11)C]7); and 2-(biphenyl-4-yl)-N-(3,4-dimethoxy-phenethyl)-N-[(11)C]methyl-ethanamine (N-[(11)C]10) and 2-(biphenyl-4-yl)-N-(3-methoxy-4-[(11)C]methoxy-phenethyl)-N-methyl-ethanamine (O-[(11)C]10), have been synthesized as new potential positron emission tomography (PET) glioma tumor imaging agents, either by O-[(11)C]methylation or by N-[(11)C]methylation of the appropriate precursors using [(11)C]CH(3)OTf and isolated either by a simplified solid-phase extraction (SPE) purification procedure or by HPLC method in 30-55% radiochemical yields decay corrected to EOB, 15-25 min overall synthesis time, and 4.0-6.0 Ci/mumol specific activity at EOB.