Gold(III) bis-thiosemicarbazonato complexes: synthesis,characterization, radiochemistry and X-ray crystal structure analysis

IntroductionA variety of (bis)thiosemicarbazone-based ligand systems have been investigated as chelating agents for Au(III) complexes with potential radiotherapeutic applications. Ligand systems containing an ethyl, propyl or butyl backbone between the two imine N donors have been synthesized to evaluate chelate ring size effects on the resultant Au(III) complex stability at the macroscopic and radiotracer levels.MethodsThe Au(III) complexes were synthesized and characterized by NMR, electrospray ionization mass spectra, elemental analysis and X-ray crystallography. The ¹⁹⁸Au complexes were evaluated in vitro at the tracer level for stability in phosphate-buffered saline at pH 7.4 and 37°C. One of these complexes [¹⁹⁸Au(3,4-HxTSE)] showed high in vitro stability and was further evaluated in vivo in normal mice.Results[Au(ATSM)]AuCl₄·2CH₃OH, (ATSM=diacetyl-bis(N⁴-methylthiosemicarbazone)) H₁₄C₈N₆O₂S₂Cl₄Au₂·2CH₃OH, crystallized from methanol in the monoclinic space group P21/n with a=14.7293(13) Å, b=7.7432(7) Å, c=20.4363(18) Å, β=100.140(2)°, V=2294.4 (4) ų, Z=4; [Au(3,4-HxTSE)]Cl·CH₃CH₂OH/AuCl₂, (3,4-HxTSE=3,4-hexanedione-bis(N⁴-ethylthiosemicarbazone)) H₂₆C_13.6N₆O_0.8S₂Cl_1.2Au_1.2, crystallized from ethanol in the monoclinic space group P21/c with a=10.1990(10) Å, b=13.8833(14) Å, c=15.1752(15) Å, β=99.353(2)°, V=2120.2 (4) ų, Z=4.ConclusionsThese studies revealed poor stability of the [¹⁹⁸Au][Au(3,4-HxTSE)]⁺ complex; however, crystal structure data suggest potential alterations to the ligand backbone may increase stability.     

Radiosynthesis and evaluation of [11C]YM-202074 as a PET ligand for imaging the metabotropic glutamate receptor type 1

IntroductionDeveloping positron emission tomography (PET) ligands for imaging metabotropic glutamate receptor type 1 (mGluR1) is important for studying its role in the central nervous system. N-cyclohexyl-6-{[N-(2-methoxyethyl)-N-methylamino]methyl}-N-methylthiazolo[3,2-a]benzimidazole-2-carboxamide (YM-202074) exhibited high binding affinity for mGluR1 (K_i=4.8 nM), and selectivity over other mGluRs in vitro. The purpose of this study was to label YM-202074 with carbon-11 and to evaluate in vitro and in vivo characteristics of [¹¹C]YM-202074 as a PET ligand for mGluR1 in rodents.Methods[¹¹C]YM-202074 was synthesized by N-[¹¹C]methylation of its desmethyl precursor with [¹¹C]methyl iodide. The in vitro and in vivo brain regional distributions were determined in rats using autoradiography and PET, respectively.Results[¹¹C]YM-202074 (262–630 MBq, n=5) was obtained with radiochemical purity of >98% and specific activity of 27–52 GBq/μmol at the end of synthesis, starting from [¹¹C]CO₂ of 19.3–21.5 GBq. In vitro autoradiographic results showed that the high specific binding of [¹¹C]YM-202074 for mGluR1 was presented in the cerebellum, thalamus and hippocampus, which are known as mGluR1-rich regions. In ex vivo autoradiography and PET studies, the radioligand was specifically distributed in the cerebellum, although the uptake was low. Furthermore, the regional distribution was fairly uniform in the whole brain by pretreatment with JNJ16259685 (a mGluR1 antagonist). However, radiometabolite(s) was detected in the brain.ConclusionsFrom these results, especially considering the low brain uptake and the influx of radiometabolite(s) into brain, [¹¹C]YM-202074 may not be a useful PET ligand for in vivo imaging of mGluR1 in the brain.     

Imaging of I2-imidazoline receptors by small-animal PET using 2-(3-fluoro-[4-11C]tolyl)-4,5-dihydro-1H-imidazole ([11C]FTIMD)

IntroductionImidazoline receptors (IRs) have been established as distinct receptors, and have been categorized into at least two subtypes (I₁R and I₂R). I₂Rs are associated with depression, Alzheimer's disease, Huntington's disease and Parkinson's disease. A few positron emission tomography (PET) probes for I₂Rs have been synthesized, but a selective PET probe has not been evaluated for the imaging of I₂Rs by PET. We labeled a selective I₂R ligand 2-(3-fluoro-4-tolyl)-4,5-dihydro-1H-imidazole (FTIMD) with ¹¹C and performed the first imaging of I₂Rs by PET using 2-(3-fluoro-[4-¹¹C]tolyl)-4,5-dihydro-1H-imidazole ([¹¹C]FTIMD).Methods[¹¹C]FTIMD was prepared by a palladium-promoted cross-coupling reaction of the tributylstannyl precursor and [¹¹C]methyl iodide in the presence of tris(dibenzylideneacetone)dipalladium(0) and tri(o-tol)phosphine. Biodistribution was investigated in rats by tissue dissection. [¹¹C]FTIMD metabolites were measured in brain tissues and plasma. Dynamic PET scans were acquired in rats, and the kinetic parameters estimated.Results[¹¹C]FTIMD was successfully synthesized with a suitable radioactivity for the injection. Co-injection with 0.1 mg/kg of cold FTIMD and BU224 induced a significant reduction in the brain-to-blood ratio 15 and 30 min after the injection. In metabolite analysis, unchanged [¹¹C]FTIMD in the brain was high (98%) 30 min after the injection. In PET studies, high radioactivity levels were observed in regions with a high density of I₂R. The radioactivity levels and V_T values in the brain regions were prominently reduced by 1.0 mg/kg of BU224 pretreatment as compared with control.Conclusion[¹¹C]FTIMD showed specific binding to I₂Rs in rat brains with a high density of I₂R.     

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

IntroductionWe reinvestigated the synthesis of [N-methyl-¹¹C]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-¹¹C]vorozole.MethodsNorvorozole was alkylated with [¹¹C]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 (¹³C and 2D-nuclear Overhauser effect spectroscopy NMR) was used to identify and assign structures to the N-methylated products. Pure [N-methyl-¹¹C]vorozole and the contaminating isomer were compared by PET imaging in the baboon.ResultsMethylation 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-¹¹C]vorozole with and without the contaminating N-3 isomer revealed that only [N-methyl-¹¹C]vorozole binds to aromatase. [N-methyl-¹¹C]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.ConclusionsThe discovery of a contaminating labeled isomer and the development of a method for isolating pure [N-methyl-¹¹C]vorozole combine to provide a new scientific tool for PET studies of the biology of aromatase and for drug research and development.     

Imaging of peripheral-type benzodiazepine receptor in tumor: in vitro binding and in vivo biodistribution of N-benzyl-N-[C]methyl-2-(7-methyl-8-oxo-2-phenyl-7,8-dihydro-9H-purin-9-yl)acetamide

IntroductionThe aim of this study was to evaluate N-benzyl-N-[¹¹C]methyl-2-(7-methyl-8-oxo-2-phenyl-7,8-dihydro-9H-purin-9-yl)acetamide ([¹¹C]DAC) as a novel peripheral-type benzodiazepine receptor (PBR) ligand for tumor imaging.Methods[¹¹C]DAC was synthesized by the reaction of a desmethyl precursor with [¹¹C]CH₃I. In vitro uptake of [¹¹C]DAC was examined in PBR-expressing C6 glioma and intact murine fibrosarcoma (NFSa) cells. In vivo distribution of [¹¹C]DAC was determined using NFSa-bearing mice and small-animal positron emission tomography (PET).Results[¹¹C]DAC showed specific binding to PBR in C6 glioma cells, a standard cell line with high PBR expression. Specific binding of [¹¹C]DAC was also confirmed in NFSa cells, a target tumor cell line in this study. Results of PET experiments using NFSa-bearing mice, showed that [¹¹C]DAC was taken up specifically into the tumor, and pretreatment with PK11195 abolished the uptake.Conclusions[¹¹C]DAC was taken up into PBR-expressing NFSa. [¹¹C]DAC is a promising PET ligand that can be used for imaging PBR in tumor-bearing mice.     

PET study using [11C]FTIMD with ultra-high specific activity to evaluate I2-imidazoline receptors binding in rat brains

IntroductionWe recently developed a selective ¹¹C-labeled I₂-imidazoline receptor (I₂R) ligand, 2-(3-fluoro-4-[¹¹C]tolyl)-4,5-dihydro-1H-imidazole ([¹¹C]FTIMD). [¹¹C]FTIMD showed specific binding to I₂Rs in rat brains having a high density of I₂R, as well as to I₂Rs those in monkey brains, as illustrated by positron emission tomography (PET) and autoradiography. However, [¹¹C]FTIMD also showed moderate non-specific binding in rat brains. In order to increase the specificity for I₂R in rat brains, we synthesized [¹¹C]FTIMD with ultra-high specific activity and evaluated its binding.Methods[¹¹C]FTIMD with ultra-high specific activity was prepared by a palladium-promoted cross-coupling reaction of the tributylstannyl precursor and [¹¹C]methyl iodide, which was produced by iodination of [¹¹C]methane using the single-pass method. Dynamic PET scans were conducted in rats, and the kinetic parameters were estimated.Results[¹¹C]FTIMD with ultra-high specific activity was successfully synthesized with an appropriate level of radioactivity and ultra-high specific activity (4470±1660 GBq/μmol at end of synthesis, n=11) for injection. In the PET study, distribution volume (V_T) values in all the brain regions investigated whether I₂R expression was greatly reduced in BU224-pretreatead rats compared with control rats (29–45% decrease). Differences in V_T values between control and BU224-pretreated rats using [¹¹C]FTIMD with ultra-high specific activity were greater than those using [¹¹C]FTIMD with normal specific activity (17–34% decrease) in all brain regions investigated.ConclusionQuantitative PET using [¹¹C]FTIMD with ultra-high specific activity can contribute to the detection of small changes in I₂R expression in the brain.     

Synthesis and evaluation of PET probes for the imaging of I2 imidazoline receptors in peripheral tissues

IntroductionTo explore the possible use of positron emission tomography (PET) probes for imaging of I₂-imidazoline receptors (I₂Rs) in peripheral tissues, we labeled two new I₂R ligands, 2-[2-(o-tolyl)vinyl]-4,5-dihydro-1H-imidazole (K_i for I₂Rs, 3.7 nM) and 2-[2-(o-tolyl)ethyl]-4,5-dihydro-1H-imidazole (K_i for I₂Rs, 1.7 nM) with ¹¹C ([¹¹C]metrazoline and [¹¹C]TEIMD), respectively, and evaluated these ligands and the recently developed I₂R ligand 2-[3-fluoro-[4-¹¹C]tolyl]-4,5-dihydro-1H-imidazole ([¹¹C]FTIMD) by in vivo studies.Methods[¹¹C]Metrazoline and [¹¹C]TEIMD were prepared by a palladium-promoted cross-coupling reaction of the tributylstannyl precursor and [¹¹C]methyl iodide. Their biodistribution in mice was investigated by tissue dissection. In addition, PET scans and metabolite analysis were performed.Results[¹¹C]Metrazoline and [¹¹C]TEIMD were successfully synthesized with a suitable radioactivity for injection. In the liver and pancreas expressing I₂Rs, coinjection with the high-affinity I₂R ligand, BU224, induced a reduction in the radioactivity level at 30 min after injection of [¹¹C]metrazoline and [¹¹C]FTIMD. However, the radioactivity level after injection of [¹¹C]TEIMD was unchanged. In the PET study, coinjection with BU224 induced a decrease in the radioactivity level in the liver and pancreas after more than 15 min of injection of [¹¹C]metrazoline and [¹¹C]FTIMD as compared with the results obtained for controls. In metabolite analysis, coinjection with BU224 induced a significant reduction in the percentage of unchanged [¹¹C]metrazoline at 30 min after injection as compared with that in the control, although no significant difference was observed in the percentage of unchanged [¹¹C]FTIMD.Conclusion[¹¹C]Metrazoline may be a more useful PET probe than [¹¹C]FTIMD for imaging of I₂Rs in peripheral tissues.     

Synthesis of carbon-11 labeled celecoxib derivatives as new candidate PET radioligands for imaging of inflammation

Cyclooxygenase (prostaglandin endoperoxide synthase or COX) enzyme represents a particularly attractive target in inflammation processes for the development of both therapeutic agents and imaging agents. This study was designed to develop new radioligands for imaging of inflammation using the biomedical imaging technique positron emission tomography (PET). Carbon-11 labeled celecoxib derivatives, [¹¹C]methyl 2-(4-(5-p-tolyl-3-(trifluoromethyl)-1H-pyrazol-1-yl)phenylsulfonamidooxy)acetate ([¹¹C]6e), [¹¹C]methyl 2-methyl-2-(4-(5-p-tolyl-3-(trifluoromethyl)-1H-pyrazol-1-yl)phenylsulfonamidooxy)propanoate ([¹¹C]6f), [¹¹C]methyl 2-(4-(5-(4-methoxyphenyl)-3-(trifluoromethyl)-1H-pyrazol-1-yl)phenylsulfonamidooxy)acetate ([¹¹C]6g), and [¹¹C]methyl 2-methyl-2-(4-(5-(4-methoxyphenyl)-3-(trifluoromethyl)-1H-pyrazol-1-yl)phenylsulfonamidooxy)propanoate ([¹¹C]6h), were prepared by O-[¹¹C]methylation of their corresponding precursors using [¹¹C]CH₃OTf under basic condition and isolated by a simplified solid-phase extraction (SPE) method in 50–60% radiochemical yields based on [¹¹C]CO₂ and decay corrected to end of bombardment (EOB). The overall synthesis time from EOB was 15–20 min, the radiochemical purity was >99%, and the specific activity at end of synthesis (EOS) was 111–185 GBq/μmol.     

Radiosynthesis and radiopharmacological evaluation of [N-methyl-11C]Org 34850 as a glucocorticoid receptor (GR)-binding radiotracer

The radiosynthesis of [N-methyl-¹¹C]Org 34850 as a potential brain glucocorticoid receptor (GR)-binding radiotracer is described. The radiosynthesis was accomplished via N-methylation of the corresponding desmethyl precursor with [¹¹C]methyl triflate in a remotely controlled synthesis module to give the desired compound in a radiochemical yield of 23±5% (decay-corrected, based upon [¹¹C]CO₂) at a specific activity of 47±12 GBq/μmol (n=15) at the end-of-synthesis (EOS). The radiochemical purity after semi-preparative HPLC purification exceeded 95%. The total synthesis time was 35–40 min after end-of-bombardment (EOB).The radiotracer is rapidly metabolized in rat plasma leading to the formation of two more hydrophilic metabolites as the major metabolites. Radiopharmacological evaluation involving biodistribution and small animal PET imaging in normal Wistar rats showed that the compound [N-methyl-¹¹C]Org 34850 is not able to sufficiently penetrate the blood–brain barrier. Therefore, compound [N-methyl-¹¹C]Org 34850 seems not to be a suitable PET radiotracer for imaging rat brain GRs. However, involvement of Pgp or species differences requires further clarification to establish whether the radiotracer [N-methyl-¹¹C]Org 34850 may still represent a suitable candidate for imaging GRs in humans.     

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

IntroductionAromatase, 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.MethodsLetrozole 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 [¹¹C]cyano group was introduced via tetrakis(triphenylphosphine)palladium(0)-catalyzed coupling of [¹¹C]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 [¹¹C-cyano]letrozole fraction in arterial plasma were also measured.Results[¹¹C-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/μmol 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[¹¹C-cyano]Letrozole is readily synthesized via a palladium-catalyzed coupling reaction with [¹¹C]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.     

Synthesis and evaluation of a C-6 alkylated pyrimidine derivative for the in vivo imaging of HSV1-TK gene expression

IntroductionWe report on the synthesis, radiolabeling, in vitro and in vivo characterization of N-Me-[¹⁸F]FHBT (6-(3-[¹⁸F]fluoro-2-(hydroxymethyl)propyl)-1,5-dimethylpyrimidin-2,4(1H,3H)-dione), a C-6-substituted N-1-methylated pyrimidine derivative as a reporter probe for imaging herpes simplex virus type 1 thymidine kinase (HSV1-TK) expression.MethodsN-Me-[¹⁸F]FHBT was synthesized via a standard nucleophilic substitution reaction followed by acidic cleavage of the methoxytrityl protecting group. Cell uptake was studied in vitro with control HEK293 (human embryonic kidney cells) and HEK293 cells stably transfected with nonmutant HSV1-tk (HEK293TK+ cells). Positron emission tomography (PET) imaging and biodistribution studies of N-Me-[¹⁸F]FHBT or [¹⁸F]FHBG were performed in nude mice bearing xenografts of HEK293 control and TK+ cells.ResultsN-Me-[¹⁸F]FHBT was obtained in a two-step reaction in an overall maximal radiochemical yield (decay-corrected) of 5% and a radiochemical purity >96%. The tracer uptake in HSV1-TK containing HEK293TK+ cells was 14.5 times (at 30 min) and 55.4 times (at 240 min) higher than in control HEK293 cells. In mice, N-Me-[¹⁸F]FHBT and [¹⁸F]FHBG accumulated significantly and exhibited similar radioactivity levels in the HEK293TK+ xenografts; however, standardized uptake values ratios between HEK293TK+ and HEK293 control xenografts were higher for [¹⁸F]FHBG than for N-Me-[¹⁸F]FHBT. Both tracers showed high gall bladder and abdominal activity.ConclusionThe biological evaluations demonstrated the feasibility of using N-methylated C-6-substituted pyrimidine derivative N-Me-[¹⁸F]FHBT as a PET radiotracer for monitoring HSV1-TK expression in vivo.     

Validation of two fluoro-analogues of N,N-dimethyl-2-(2′-amino-4′-hydroxymethyl-phenylthio)benzylamine as serotonin transporter imaging agents using microPET

IntroductionCarbon-11 (C-11) N,N-dimethyl-2-(2′-amino-4′-hydroxymethyl-phenylthio)benzylamine ([¹¹C]HOMADAM) has been reported as highly specific and selective positron emission tomography (PET) radiotracer showing fast kinetics for the human brain serotonin transporter (SERT). In our continued effort to develop appropriate PET SERT radioligand that can be labeled with either C-11 or fluorine-18 (F-18), two new C-11 labeled analogues of HOMADAM, [¹¹C]-N,N-dimethyl-2-(2′-amino-5′-fluoro-4′-hydroxymethyl-phenylthio)benzylamine ([¹¹C]-(2)) and [¹¹C]-N,N-dimethyl-2-(2′-amino-4-fluoro-4′-hydroxymethyl-phenylthio)benzylamine ([¹¹C]-(3)) have been synthesized and evaluated along the previously reported [¹¹C]-N,N-dimethyl-2-(2′-amino-5-fluoro-4′-hydroxymethyl-phenylthio)benzylamine ([¹¹C]-(1)).MethodsThe in vitro competitive binding assays were performed in cells transfected with human SERT (hSERT), human dopamine transporter (hDAT), and human norepinephrine transporter (hNET). [¹¹C]-(2) and [¹¹C]-(3) were prepared by methylation of their monomethylbenzylamine precursors 13 and 22 with cyclotron produced [¹¹C]iodomethane ([¹¹C]CH₃I), respectively. Uptake and kinetics of [¹¹C]-(2) and [¹¹C]-(3) in the brain regions of interest were determined in anesthetized rhesus monkeys using Concorde microPET P4.Results2 and 3 displayed moderate and high affinity for the SERT with Kis (SERT) = 5.45 and 1.10 nM (vs [³H]citalopram), respectively. After High Performance Liquid Chromatography (HPLC) purification, [¹¹C]-(2) and [¹¹C]-(3) were obtained in 23 and 9% radiochemical yield (RCY) and log Ps_7.4 of 1.77 and 1.91, respectively. The microPET images of [¹¹C]-(2) and [¹¹C]-(3) showed clear localization in the monkey brain regions rich in SERT with midbrain to cerebellum ratios of 1.75 and 3.86 at 85 min post injection, respectively, comparing to 3.40 for [¹¹C]-(1), at the same time. [¹¹C]-(3) was selected for further examination and showed to be specific to the SERT as displacement with citalopram (a potent SERT ligand) reduced radioactivity in SERT rich regions, such as midbrain, to the cerebellum level.ConclusionsCompound 2, the 5′-fluoro-analogue of HOMADAM, had the lowest brain uptake and target to non-target ratios. Compound 3, the 4-fluoro-analogue of HOMADAM, had good brain uptake and higher midbrain and thalamus to cerebellum ratios than compound 1, the 5-fluoro-analogue of HOMADAM. Although 1 and 3 presented better imaging properties than 2, none of the three candidates was suitable to surpass the binding or distributional qualities of the parent HOMADAM. Alternative fluoro-analogues of HOMADAM will soon be characterized, in future work, as SERT radioimaging agents.     

Synthesis and preclinical evaluation of [11C]D617, a metabolite of (R)-[11C]verapamil

Objectives(R)-[¹¹C]verapamil is widely used as a positron emission tomography (PET) tracer to evaluate P-glycoprotein (P-gp) functionality at the blood–brain barrier in man. A disadvantage of (R)-[¹¹C]verapamil is the fact that its main metabolite, [¹¹C]D617, also enters the brain. For quantitative analysis of (R)-[¹¹C]verapamil data, it has been assumed that the cerebral kinetics of (R)-[¹¹C]verapamil and [¹¹C]D617 are the same. The aim of the present study was to investigate whether the cerebral kinetics of (R)-[¹¹C]verapamil and [¹¹C]D617 are indeed similar and, if so, whether [¹¹C]D617 itself could serve as an alternative PET tracer for P-gp.Methods[¹¹C]D617 was synthesized and its ex vivo biodistribution was investigated in male rats at four time points following intravenous administration of [¹¹C]D617 (50 MBq) without (n=4) or with (n=4) pretreatment with the P-gp inhibitor tariquidar (15 mg·kg^?1, intraperitoneally). Brain distribution was further assessed using consecutive PET scans (n=8) before and after pretreatment with tariquidar (15 mg·kg^?1, intravenously), as well as metabolite analysis (n=4).ResultsThe precursor for the radiosynthesis of [¹¹C]D617, 5-amino-2-(3,4-dimethoxy-phenyl)-2-isopropyl-pentanitrile (desmethyl D617), was synthesized in 41% overall yield. [¹¹C]D617 was synthesized in 58%–77% decay-corrected yield with a radiochemical purity of ≥99%. The homogeneously distributed cerebral volume of distribution (V_T) of [¹¹C]D617 was 1.1, and this increased 2.4-fold after tariquidar pretreatment.ConclusionV_T of [¹¹C]D617 was comparable to that of (R)-[¹¹C]verapamil, but its increase after tariquidar pretreatment was substantially lower. Hence, (R)-[¹¹C]verapamil and [¹¹C]D617 do not show similar brain kinetics after inhibition of P-gp with tariquidar.     

Radiosynthesis and ex vivo evaluation of (R)-(−)-2-chloro-N-[1-11C-propyl]n-propylnorapomorphine

IntroductionSeveral dopamine D₂ agonist radioligands have been used with positron emission tomography (PET), including [¹¹C-]-(?)-MNPA, [¹¹C-]-(?)-NPA and [¹¹C]-(+)-PHNO. These radioligands are considered particularly powerful for detection of endogenous dopamine release, but they either provide PET brain images with limited contrast or have affinity for both D₂ and D₃ receptors. We here present the carbon-11 radiolabeling and ex vivo evaluation of 2-Cl-(?)-NPA, a novel PET-tracer candidate with high in vitro D₂/D₃ selectivity.Methods2-Cl-[¹¹C]-(?)-NPA and [¹¹C]-(?)-NPA were synthesized by a two step N-acylation-reduction process using [¹¹C]-propionyl chloride. Awake rats were injected with either tracer, via the tail vein. The rats were decapitated at various times, the brains were removed and quickly dissected, and plasma metabolites were measured. Radioligand specificity, and P-glycoprotein involvement in brain uptake, was also assessed.Results2-Cl-[¹¹C]-(?)-NPA and [¹¹C]-(?)-NPA were produced in high specific activity and purity. 2-Cl-[¹¹C]-(?)-NPA accumulated slower in the striatum than [¹¹C]-(?)-NPA, reaching maximum concentrations after 30 min. The maximal striatal uptake of 2-Cl-[¹¹C]-(?)-NPA (standard uptake value 0.72±0.24) was approximately half that of [¹¹C]-(?)-NPA (standard uptake value 1.37±0.18). Nonspecific uptake was similar for the two compounds. 2-Cl-[¹¹C]-(?)-NPA was metabolized quickly, leaving only 17% of the parent compound in the plasma after 30 min. The specific binding of 2-Cl-[¹¹C]-(?)-NPA was completely blocked and inhibition of P-glycoprotein did not alter the brain uptake.ConclusionEx vivo experiments showed, despite a favorable D₂/D₃ selectivity, that 2-Cl-[¹¹C]-(?)-NPA is inferior to [¹¹C]-(?)-NPA as a PET tracer in rat, because of slower brain uptake and lower specific to nonspecific binding ratio.     

Synthesis of d-[11C]oxyphenonium iodide,a potential radioligand for in vivo visualization of human cholinergic muscarinic receptor-sites by positron emission tomography

Carbon-11 labeled d-oxyphenonium iodide, a cholinergic antagonist is synthesized for in vivo visualization of muscarinic receptor-sites on airway tissue by positron emission tomography (PET). Methylation with [¹¹C]CH₃I of d-demethyloxyphenonium, followed by HPLC purification affords the desired radiopharmaceutical with a radiochemical yield of 66% (based on [¹¹C]CH₃I, and corrected for decay) and with a specific activity of 110–300 Ci/mmol. The biologically active labeled d-enantiomer is prepared within 40 min after EOB. Optical and chemical purity proved to be better than 99.9%. Radiochemical purity was determined to be higher than 99%.     

Development of high-specific-activity 68Ga-labeled DOTA-rhenium-cyclized α-MSH peptide analog to target MC1 receptors overexpressed by melanoma tumors

IntroductionA previous report on ⁶⁸Ga-1,4,7,10-tetraazacyclodedecane-N,N′,N″,N′″-tetraacetic acid (DOTA)-Re(Arg¹¹)CCMSH was shown to indicate the imaging agent's potency for early detection of metastatic melanoma. However, the main limiting factor to developing high-specific-activity ⁶⁸Ga-DOTA-Re(Arg¹¹)CCMSH is the short half-life of ⁶⁸Ga, which precludes further purification of the agent. To circumvent this problem, we incorporated the microwave technique to rapidly radiolabel the peptide with ⁶⁸Ga, thereby allowing enough time to include high-performance liquid chromatography (HPLC) purification in the overall procedure.MethodsDOTA-Re(Arg¹¹)CCMSH was radiolabeled with ⁶⁸Ga in <1 min using a circular-cavity microwave apparatus. Reverse-phase HPLC purification was accomplished in less than 20 min. ⁶⁸Ga-DOTA-Re(Arg¹¹)CCMSH was then administered on B16/F1 murine melanoma-bearing C57 mice to study its biodistribution and positron emission tomography (PET) imaging capability.ResultsThe production of high-specific-activity ⁶⁸Ga-DOTA-Re(Arg¹¹)CCMSH resulted in an improved tumor uptake [6.93±1.11%ID/g at 30 min postinjection (p.i.) and 6.27±1.60%ID/g at 1 h p.i.] and tumor retention (5.85±1.32%ID/g at 4 h p.i.). Receptor-mediated tumor uptake was verified by blocking studies. Furthermore, high-resolution PET images of the tumor were obtained, owing to high tumor-to-nontarget organ ratios at an early time point (i.e., at 1 h biodistribution: tumor/blood, 14.3; tumor/muscle, 89.6; tumor/skin, 12.3) and fast clearance of the labeled peptide from kidney and other healthy tissues.ConclusionHigh-specific-activity ⁶⁸Ga-DOTA-Re(Arg¹¹)CCMSH may have a potential role in the early diagnosis of metastasized melanoma.     

Evaluation of [11C]-DAA1106 for imaging and quantification of neuroinflammation in a rat model of herpes encephalitis

IntroductionMany neurological and psychiatric disorders are associated with neuroinflammation. Positron emission tomography (PET) with [¹¹C]-PK11195 can be used to study neuroinflammation in these disorders. However, [¹¹C]-PK11195 may not be sensitive enough to visualize mild neuroinflammation. As a potentially more sensitive PET tracer for neuroinflammation, [¹¹C]-N-(2,5-dimethoxybenzyl)-N-(4-fluoro-2-phenoxyphenyl)-acetamide (DAA1106) was evaluated in a rat model of herpes encephalitis.MethodsMale Wistar rats were intranasally inoculated with HSV-1 (HSE) or phosphate-buffered saline (control). At Day 6 or Day 7 after inoculation, small-animal [¹¹C]-DAA1106 PET scans were acquired, followed by ex vivo biodistribution. Arterial blood sampling was performed for quantification of uptake.ResultsIn HSE rats, a significantly higher ex vivo, but not in vivo, uptake of [¹¹C]-DAA1106 was found in almost all examined brain areas (24–71%, P<.05), when compared to control rats. Pretreatment with unlabeled PK11195 effectively reduced [¹¹C]-DAA1106 uptake in HSE rats (54–84%; P<.001). The plasma and brain time–activity curves showed rapid uptake of [¹¹C]-DAA1106 into tissue. The data showed a good fit to the Logan analysis but could not be fitted to a two-tissue compartment model.Conclusions[¹¹C]-DAA1106 showed a high and specific ex vivo uptake in the encephalitic rat brain. However, neuroinflammation could not be demonstrated in vivo by [¹¹C]-DAA1106 PET. Quantification of the uptake of [¹¹C]-DAA1106 using plasma sampling is not optimal, due to rapid tissue uptake, slow tissue clearance and low plasma activity.     

[11C]Sorafenib: Radiosynthesis and preclinical evaluation in tumor-bearing mice of a new TKI-PET tracer

IntroductionTyrosine kinase inhibitors (TKIs) like sorafenib are important anticancer therapeutics with thus far limited treatment response rates in cancer patients. Positron emission tomography (PET) could provide the means for selection of patients who might benefit from TKI treatment, if suitable PET tracers would be available. The aim of this study was to radiolabel sorafenib (1) with carbon-11 and to evaluate its potential as TKI-PET tracer in vivo.MethodsSynthetic methods were developed in which sorafenib was labeled at two different positions, followed by a metabolite analysis in rats and a PET imaging study in tumor-bearing mice.Results[methyl-¹¹C]-1 and [urea-¹¹C]-1 were synthesized in yields of 59% and 53%, respectively, with a purity of > 99%. The identity of the products was confirmed by coinjection on HPLC with reference sorafenib. In an in vivo metabolite analysis [¹¹C]sorafenib proved to be stable. The percentage of intact product in blood–plasma after 45 min was 90% for [methyl-¹¹C]-1 and 96% for [urea-¹¹C]-1, respectively. Due to the more reliable synthesis, further research regarding PET imaging was performed with [methyl-¹¹C]-1 in nude mice bearing FaDu (head and neck cancer), MDA-MB-231 (breast cancer) or RXF393 (renal cancer) xenografts. Highest tracer accumulation at a level of 2.52 ± 0.33 %ID/g was observed in RXF393, a xenograft line extensively expressing the sorafenib target antigen Raf-1 as assessed by immunohistochemistry.ConclusionIn conclusion, we have synthesized [¹¹C]sorafenib as PET tracer, which is stable in vivo and has the capability to be used as PET tracer for imaging in tumor-bearing mice.     

Synthesis and biological evaluation of N-(2-[18F]Fluoropropionyl)-L-methionine for tumor imaging

IntroductionN-position radiolabeled amino acids, such as N-(2-[¹⁸F]fluoropropionyl)-L-methionine ([¹⁸F]FPMET) as a derivative of L-methionine (MET), can potentially serve as a PET tracer for tumor imaging. In the current study, radiosynthesis and biological evaluation of [¹⁸F]FPMET as a new PET tumor agent are performed.Methods[¹⁸F]FPMET was synthesized by reacting 4-nitrophenyl 2-[¹⁸F]fluoropropionate ([¹⁸F]NFP) with MET. In vitro competitive inhibition and protein incorporation experiments were performed with Hepa1-6 hepatoma cell lines. The biodistribution of [¹⁸F]FPMET was determined in S180 fibrosarcoma-bearing mice. PET/CT studies of [¹⁸F]FPMET were conducted in S180 fibrosarcoma-bearing mice, A549 lung adenocarcinoma-bearing nude mice, and PC-3 prostate cancer-bearing nude mice.Results[¹⁸F]FPMET was synthesized in 72% ± 4% uncorrected radiochemical yield (n = 10) from [¹⁸F]NFP. In vitro experiments showed that [¹⁸F]FPMET was primarily transported through Na⁺-dependent system A, system ASC, and system B^0,+, and was not incorporated into protein. Biodistribution and PET/CT imaging studies indicated that [¹⁸F]FPMET could delineate S180 fibrosarcoma, A549 lung adenocarcinoma, and PC-3 prostate cancer.ConclusionAn efficient synthesis of N-position [¹⁸F]labeled amino acids with a classic [¹⁸F]NFP prosthetic group is developed. The results support that [¹⁸F]FPMET seems to be a potential tracer for tumor imaging with PET.     

Synthesis of carbon-11-labeled piperidine ring of N-[ω-(6-methoxynaphthalen-1-yl)alkyl] derivatives as new selective PET σ1 receptor probes

Carbon-11-labeled piperidine ring of N-[ω-(6-methoxynaphthalen-1-yl)alkyl] derivatives were first designed and synthesized as new selective PET σ₁ receptor probes. The target tracers were prepared by O-[¹¹C]methylation of their corresponding phenolic hydroxyl precursors using [¹¹C]CH₃OTf under basic conditions and isolated by a simplified SPE method in 40–50% radiochemical yields based on [¹¹C]CO₂ and decay corrected to EOB. The overall synthesis time from EOB was 15–20 min, the radiochemical purity was >99%, and the specific activity at EOS was 111–185 GBq/μmol.