Synthesis and evaluation of 5,7-dihydro-3-[2-[1-(4-[18F]-fluorobenzyl)-4-piperidinyl]ethyl]-6H-pyrrolo[3,2-f]-1,2-benzisoxazol-6-one for in vivo mapping of acetylcholinesterase

OBJECTIVES: Acetylcholinesterase (AChE) is an important cholinergic marker for the diagnosis of Alzheimer's disease (AD). A recent study has demonstrated that C-labelled 5,7-dihydro-7-methyl-3-[2-[1-(phenylmethyl)-4-piperidinyl]ethyl]-6H-pyrrolo[3,2-f]-1,2-benzisoxazol-6-one (CP-126,998) shows promising results. The demethylated form of this ligand (CP-118,954) is a more potent and selective inhibitor than CP-126,998. In this study, therefore, CP-118,954 was labelled with F and evaluated for the in vivo mapping of AChE. METHODS: The 4-fluoro (1). and 2-fluoro (2). derivatives of CP-118,954 were synthesized from 4-methyl-3-nitroanisole in 11 steps. Their in vitro binding affinities to AChE were measured using Ellman's method. The preparation of [F]-1 was carried out by reductive alkylation of the piperidine precursor with 4-[F]-fluorobenzaldehyde, followed by high-performance liquid chromatography (HPLC) purification. In vitro autoradiography was performed by incubating rat brain coronal slices with [F]-1. Tissue distribution studies were performed in mouse brain and the data were expressed as the percentage of the injected dose per gram of tissue (%ID x g). RESULTS: Two fluorine-substituted AChE inhibitors were synthesized and their in vitro binding data showed that the 4-fluoro and 2-fluoro derivatives (1 and 2) had similar or superior binding affinity to that of the unsubstituted ligand, CP-118,954. The F-labelled ligand was synthesized in 20-35% radiochemical yield (EOS) and with high effective specific activity (36-42 GBq x micromol). Autoradiography showed high uptake of [F]-1 in the striatum and this striatal uptake was completely inhibited by the unlabelled ligand 1. Tissue distribution studies demonstrated that high radioactivity was accumulated in the striatum, an AChE-rich region. CONCLUSIONS: This study demonstrates that [F]-1 may hold promise as a radioligand for the in vivo mapping of AChE.     

PET evaluation of the uptake of N-[11C]methyl CP-643,051, an NK1 receptor antagonist, in the living porcine brain

Antagonists of neurokinin receptors such as CP-643,051 are presently under investigation as potential antidepressants, but little is known about the brain uptake and distribution of these agents. We developed a method for the efficient N-[11C]methylation of CP-122,721, yielding the NK1 antagonist N-[11C]methyl CP-643,051. The brain uptake and distribution of N-[11C]methyl CP-643,051 were studied by positron emission tomography (PET) in the anaesthetized pig, first in a baseline condition, and again after displacement of specific binding with the NK1 receptor antagonist L-732,138 (0.6 mg/kg, i.v.). In order to validate this displacement procedure, we tested the effects of L-732,138 on cerebral blood flow (CBF) in one pig. We found that N-[11C]methyl CP-643,051 had a distribution volume close to 3 ml g(-1), and a binding potential (pB) of 0.3 in the pig striatum; this binding was displaceable by the L-732,138 pre-treatment, which evoked a small (10-20%) global increase in CBF. We conclude that of N-[11C]methyl CP-643,051 may serve as a lead structure for the development of PET NK-1 ligands of higher specific binding in vivo.     

Synthesis and evaluation of 6-[11C]methoxy-3-[2-[1-(phenylmethyl)-4-piperidinyl]ethyl]-1,2- benzisoxazole as an in vivo radioligand for acetylcholinesterase

6-Methoxy-3-[2-[1-(phenylmethyl)-4-piperidinyl]ethyl]-1,2-benzisoxazole is a high affinity (K(i) = 8.2 nM) reversible inhibitor of acetylcholinesterase (AChE). The carbon-11 labeled form was prepared in high (>97%) radiochemical purity and with specific activities of 37+/-20 GBq/micromol at end of synthesis, by the alkylation of the desmethyl precursor with [11C]methyl trifluoromethanesulfonate in N,N-dimethyl-formamide at room temperature. In vivo studies in mice demonstrated good blood brain permeability but essentially uniform regional brain distribution. Thus, despite in vitro and in vivo activity as an AChE inhibitor, 6-[11C]methoxy-3-[2-[1-(phenylmethyl)-4-piperidinyl]ethyl]-1,2-benzis oxa zole does not appear to be a good candidate for in vivo imaging studies of AChE in the mammalian brain.     

Synthesis and evaluation of radioiodine-labelled CP-118,954 for the in-vivo imaging of acetylcholinesterase

OBJECTIVES: Alzheimer's disease (AD) is characterized by reduced acetylcholinesterase (AChE) activity in the post-mortem tissues of AD patients. Therefore, AChE has been an attractive target for the diagnosis of AD. In the present study, 5,7-dihydro-3-[2-(1-(phenylmethyl)-4-piperidinyl)ethyl]-6H-pyrrolo[3,2-f]-1,2-benzisoxazol-6-one (CP-118,954), a potent AChE inhibitor, was labelled with radioiodine and evaluated as an AChE imaging agent for SPECT. METHODS: Radioiodine-labelled CP-118,954 was prepared from CP-144,885 and [(125)I]iodobenzyl bromide, and anti-AChE activities of iodine-substituted CP-118,954 were measured. Metabolism studies were carried out in samples of blood and whole brain of mice injected with 2-[(123)I]iodo-CP-118,954 ((123)I-1). Tissue distribution studies were also performed in mice injected with I-1, and samples of blood, thyroid, stomach, and brain tissue (cerebellum, striatum and cortex) were removed, weighed and counted. RESULTS: Of the ligands, 2-iodo-CP-118,954 exhibited higher binding affinity for AChE (IC50=24 nM) than the other positional isomers. 2-[(125)I]Iodo-CP-118,954 was found to have a lipophilicity (log P=2.1) favouring brain permeability and metabolic stability in mouse brain, but a marginal target (striatum) to non-target (cerebellum) uptake ratio (1.1) in mouse brain. CONCLUSION: This result demonstrates that 2-[(125)I]iodo-CP-118,954 may be unsuitable for AChE imaging. These findings suggest that radioligands suitable for AChE imaging should have not only a specific structure but also a sub-nanomolar to low nanomolar IC50.     

Synthesis and evaluation of 2-[18F]fluoro-CP-118,954 for the in vivo mapping of acetylcholinesterase

5,7-Dihydro-3-[2-[1-(2-fluorobenzyl)-4-piperidinyl]ethyl]-6H-pyrrolo[3,2,f]-1,2-benzisoxazol-6-one (2-flouro-CP-118,954; 1), a potent acetylcholinesterase (AChE) inhibitor, was prepared as a radioligand by reductive alkylation of CP-144,885 the debenzylated form of CP 118,954, with 2-[18F]fluorobenzaldehyde. The decay-corrected radiochemical yield was 25-30% and the effective specific activity was 41-53 GBq/micromol. Tissue distribution studies of 2-[18F]fluoro-CP-118,954 ([18F]1) in mice showed that the regional brain distribution correlated well with the known density of AChE in the mouse brain. A high level of uptake in the striatum was also shown at all time points in the olfactory tubercle, which is known to have dopaminergic neurons. Blocking studies showed that radioligand uptake in all brain regions was not altered by either the dopamine receptor antagonists or the sigma receptor agonist. On the other hand, radioligand uptake in both the striatum and the olfactory tubercle was significantly blocked (80%) by ligand 1. The low level of bone uptake over time suggested that [18F]1 underwent little in vivo metabolic defluorination. The lack of metabolite formation in the mouse brain indicated that the regional distribution was attributed to [18F]1. These results demonstrated that [18F]1 binds specifically and selectively to AChE in mice and appears to be a suitable radioligand for the in vivo mapping of AChE.     

Synthesis,in vitro and in vivo pharmacology of a C-11 labeled analog of CP-101,606, (+/-)threo-1-(4-hydroxyphenyl)-2-[4-hydroxy-4-(p-[11C]methoxyphenyl)piperidino]-1-propanol,as a PET tracer for NR2B subunit-containing NMDA receptors

A carbon-11 labeled methoxyl analog of CP-101,606, (+/-)threo-1-(4-hydroxyphenyl)-2-[4-hydroxy-4-(p-[11C]methoxyphenyl)piperidino]-1-propanol [(+/-)[11C]1], was synthesized as a new subtype-selective PET radioligand for NMDA receptors. The in vitro binding studies using rat brain slices demonstrated that (+/-)[11C]1 shows an extremely high-specific binding to the NR2B subunit of NMDA receptors. In contrast to the in vitro binding, the in vivo binding to mouse and monkey brains showed no apparent specific localization of the radioactivity in any of the brain regions. Metabolism and physicochemical properties such as the lipophilicity of (+/-)[11C]1 seemed unlikely to affect the in vivo (+/-)[11C]1 binding. Among the various endogenous ligands acting at the NMDA receptors, polyamines (spermine and spermidine) and divalent cations (Mg(2+,) Zn(2+,) and Ca(2+)) strongly inhibited the in vitro (+/-)[11C]1 binding. Thus, the present studies point to the possibility that the polyamines and cations behave as endogenous inhibitors for (+/-)[11C]1 binding, leading to the loss of the specific binding in vivo.     

Facile synthesis and initial PET imaging of novel potential heart acetylcholinesterase imaging agents [11C]pyridostigmine and its analogs

A series of 11C-labeled analogs of the acetylcholinesterase (AChE) inhibitor pyridostigmine have been synthesized for evaluation as new potential positron emission tomography (PET) imaging agents for heart AChE. The appropriate precursors for radiolabeling were slightly modified from commercial reagents. The new tracers [11C]pyridostigmine (1), [11C]para-pyridostigmine (2) and [11C]ortho-pyridostigmine (3) were prepared by N-[11C]methylation of the precursors using [11C]methyl triflate. Pure target compounds were isolated by a solid-phase extraction (SPE) purification procedure with 60-85% radiochemical yields (decay corrected to end of bombardment), and a synthesis time of 10-15 min. The initial PET dynamic studies of compounds (1-3) in rat heart showed rapid heart uptake and blood pool clearance to give high quality heart images. These results suggest the new tracers delineate the heart very clearly and could be potential heart AChE imaging agents.     

Facile synthesis of new carbon-11 labeled conformationally restricted rivastigmine analogues as potential PET agents for imaging AChE and BChE enzymes.

Rivastigmine is a newer-generation inhibitor with a dual inhibitory action on both acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) enzymes, and is used for the treatment of AChE- and BChE-related diseases such as brain Alzheimer's disease and cardiovascular disease. New carbon-11 labeled conformationally restricted rivastigmine analogues radiolabeled quaternary ammonium triflate salts, (3aR,9bS)-1-[(11)C]methyl-1-methyl-6-(methylcarbamoyloxy)-2,3,3a,4,5,9b-hexahydro-1H-benzo[g]indolium triflate ([(11)C]8) and (3aR,9bS)-1-[(11)C]methyl-1-methyl-6-(heptylcarbamoyloxy)-2,3,3a,4,5,9b-hexahydro-1H-benzo[g]indolium triflate ([(11)C]9), were designed and synthesized as potential positron emission tomography (PET) agents for imaging AChE and BChE enzymes. The appropriate precursors were labeled with [(11)C]CH(3)OTf through N-[(11)C]methylation, and the target tracers were isolated by solid-phase extraction (SPE) using a cation-exchange CM Sep-Pak cartridge in 40-50% radiochemical yields decay corrected to end of bombardment (EOB), 15-20 min overall synthesis time, and 148-222 GBq/micromol specific activity at EOB.     

An automated radiosynthesis of 2-[11C]thymidine using anhydrous [11C]urea derived from [11C]phosgene.

2-[11C]Thymidine has been produced from [11C]methane via [11C]phosgene and [11C]urea. Anhydrous [11C]urea was prepared from [11C]phosgene by reaction with liquid ammonia. This novel approach avoids the problems associated with the synthesis of anhydrous [11C]urea from [11C]cyanide. A fully automated system based on a modular approach and under PLC control has been developed. The system provides 2-[11C]thymidine reliably and reproducibly for clinical PET studies. The radiosynthesis takes 45-50 min from [11C]methane and the average yield was 1.5-3.3 GBq (40-90 mCi). The specific radioactivity was typically in the range 29.6-51.8 GBq mumol-1 (0.8-1.4 Ci mumol-1) at EOS corresponding to 6-12 micrograms of stable thymidine. The radiochemical yield of 2-[11C]thymidine was ca. 14% from [11C]methane.     

Synthesis of 1-[c]methylpiperidin-4-yl propionate ([c]pmp) for in vivo measurements of acetylcholinesterase activity

Synthesis of 1-[¹¹C]methylpiperidin-4-yl propionate ([¹¹C]PMP), an in vivo substrate for acetylcholinesterase, is reported. An improved preparation of 4-piperidinyl propionate (PHP), the immediate precursor for radiolabeling, was accomplished in three steps from 4-hydroxypiperidine by (a) protection of the amine as the benzyl carbamate, (b) acylation with propionyl chloride, and (c) deprotection of the carbamate by catalytic hydrogenation. The final product was obtained in an overall 82% yield. Reaction of the free base form of PHP with [¹¹C]methyl trifluoromethanesulfonate at room temperature in N,N-dimethylformamide, followed by high performance liquid chromatography (HPLC) purification, provided [¹¹C]PMP in 57% radiochemical yield, >99% radiochemical purity, and >1500 Ci/mmol at the end of synthesis. The total synthesis time from end-of-bombardment was 35 min. [¹¹C]PMP can thus be reliably prepared for routine clinical studies of acetylcholinesterase in human brain using positron emission tomography.     

In vivo positron emission tomography studies on the novel nicotinic receptor agonist [11C]MPA compared with [11C]ABT-418 and (S)(-)[11C]nicotine in rhesus monkeys

The novel 11C-labeled nicotinic agonist (R,S)-1-[11C]methyl-2(3-pyridyl)azetidine ([11C]MPA) was evaluated as a positron emission tomography (PET) ligand for in vivo characterization of nicotinic acetylcholine receptors in the brain of Rhesus monkeys in comparison with the nicotinic ligands (S)-3-methyl-5-(1-[11C]methyl-2-pyrrolidinyl)isoxazol ([11C]ABT-418) and (S)(-)[11C]nicotine. The nicotinic receptor agonist [11C]MPA demonstrated rapid uptake into the brain to a similar extent as (S)(-) [11C]nicotine and [11C]ABT-418. When unlabeled (S)(-)nicotine (0.02 mg/kg) was administered 5 min before the radioactive tracers, the uptake of [11C]MPA was decreased by 25% in the thalamus, 19% in the temporal cortex, and 11% in the cerebellum, whereas an increase was found for the uptake of (S)(-)[11C]nicotine and [11C]ABT-418. This finding indicates specific binding of [11C]MPA to nicotinic receptors in the brain in a simple classical displacement study. [11C]MPA seems to be a more promising radiotracer than (S)(-)[11C]nicotine or [11C]ABT-418 for PET studies to characterize nicotinic receptors in the brain.     

Synthesis of carbon-11-labeled 4-aryl-4H-chromens as new PET agents for imaging of apoptosis in cancer

Carbon-11-labeled 4-aryl-4H-chromenes, 2-amino-7-(dimethylamino)-4-(3-[¹¹C]methoxy-5-methoxyphenyl)-4H-chromene-3-carbonitrile ([¹¹C]6a), 2-amino-4-(3-bromo-4-[¹¹C]methoxy-5-methoxyphenyl)-7-(dimethylamino)-4H-chromene-3-carbonitrile ([¹¹C]6c), 2-amino-4-(3-[¹¹C]methoxy-5-methoxyphenyl)-4,7-dihydropyrano[2,3-e]indole-3-carbonitrile ([¹¹C]6d), 2-amino-4-(3-bromo-4-[¹¹C]methoxy-5-methoxyphenyl)-4,7-dihydropyrano[2,3-e]indole-3-carbonitrile ([¹¹C]6f), 2-amino-4-(3-[¹¹C]methoxy-5-methoxyphenyl)-4,9-dihydropyrano[3,2-g]indole-3-carbonitrile ([¹¹C]6g), 2-amino-4-(3-bromo-4-[¹¹C]methoxy-5-methoxyphenyl)-4,9-dihydropyrano[3,2-g]indole-3-carbonitrile ([¹¹C]6i), 2-amino-4-(3-[¹¹C]methoxy-5-methoxyphenyl)-7-methyl-4,7-dihydropyrano[2,3-e]indole-3-carbonitrile ([¹¹C]6j) and 2-amino-4-(3-bromo-4-[¹¹C]methoxy-5-methoxyphenyl)-7-methyl-4,7-dihydropyrano[2,3-e]indole-3-carbonitrile ([¹¹C]6l), were prepared by O-[¹¹C]methylation of their corresponding precursors using [¹¹C]CH₃OTf under basic conditions and isolated by a simplified solid-phase extraction (SPE) method in 30–50% 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.     

In vivo binding of [11C]nemonapride to sigma receptors in the cortex and cerebellum

Radiolabeled nemonapride (NEM, YM-09151-2) is widely used as a representative dopamine D2-like receptor ligand in pharmacological and neurological studies, and 11C-labeled analog ([11C]NEM) has been developed for positron emission tomography (PET) studies. The aim of this study was to evaluate whether [11C]NEM binds in vivo to sigma receptors. [11C]NEM and one of six dopamine D2-like receptor ligands or seven sigma receptor ligands were co-injected into mice, and the regional brain uptake of [11C]NEM was measured by a tissue dissection method. The striatal uptake of [11C]NEM was reduced by D2-like receptor ligands, NEM, haloperidol, (+)-butaclamol, raclopride, and sulpiride, but not by a D4 receptor ligand clozapine. In the cortex and cerebellum the uptake was also reduced by D2-like receptor ligands with affinity for sigma receptors, but not by raclopride. Although none of seven sigma receptor ligands, SA6298, N,N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl]ethylamine hydrochloride (NE-100), (+)-pentazocine, R(-)-N-(3-phenyl-1-propyl)-1-phenyl-2-aminopropane hydrochloride ([-]-PPAP), (-)-pentazocine, R(+)-3-(3-hydroxyphenyl)-N-propylpiperidine hydrochloride ([+]-3-PPP), and (+)-N-allylnormetazocine hydrochloride ([+]-SKF 10047), blocked the striatal uptake, five of them with relatively higher affinity significantly reduced the [11C]NEM uptake by the cortex, and four of them reduced that by the cerebellum. We concluded that [11C]NEM binds in vivo not only to dopamine D2-like receptors in the striatum but also to sigma receptors in other regions such as cortex and cerebellum.     

Preliminary evaluation of 2-[4-[3-tert-butylamino)-2-hydroxypropoxy]phenyl]-3-methyl-6-me thoxy-4(3H)-quinazolinone ([+/-]HX-CH 44) as a selective beta1-adrenoceptor ligand for PET

(+/-)-3-[11C]Methyl-2-[4-[3-(tert-butylamino)-2-hydroxypropoxy]phenyl]-6 -methoxy-4(3H) quinazolinone ([+/-]-[11C]HX-CH 44) was labeled with carbon-11 using [11C]iodomethane with the corresponding N-demethylated precursor. Then, 30-90 mCi (1.10-3.33 GBq) of pure [11C]HX-CH 44 were obtained 30 min after end of bombardment with specific radioactivities of 500-1,400 mCi/micromol (18.5-51.8 GBq/micromol). Myocardial uptake in dogs was 0.340+/-0.043 pmol/mL tissue per nanomole injected, 10-15 min postinjection. Heart-to-lung ratio was 3 from the 5th to the 30th minute. Only 35% of the myocardial radioactivity could be displaced. Tissue uptake could not be blocked with appropriate compounds. Therefore, (+/-)-[11C]HX-CH 44 does not appear to be a suitable ligand for the study of myocardial beta1-adrenoceptors in positron emission tomography.     

In vivo quantification of brain serotonin transporters in humans using [11C]McN 5652.

Abnormal brain regional densities of serotonin (5-hydroxytryptamine [5-HT]) transporters have been reported in postmortem studies in several neuropsychiatric conditions, such as major depression and schizophrenia. trans-1,2,3,5,6,10-beta-Hexahydro-6-[4-(methylthio)phenyl]pyrrolo-[2,1-a]-isoquinoline ([11C]McN 5652) is the first PET radioligand successfully developed to label 5-HT transporters in the living human brain. The purpose of this study was to develop an imaging protocol and analytic method to measure regional 5-HT transporter binding potential (BP) with [11C]McN 5652 in humans. METHODS: The arterial input function and brain uptake of (+)-[11C]McN 5652 and (-)-[11C]McN 5652, the active and inactive enantiomers, respectively, were measured in 6 healthy volunteers. Results: (+)-[11C]McN 5652 concentrated in brain regions rich in 5-HT transporters (midbrain, thalamus, basal ganglia, and medial temporal lobe structures), whereas the uptake of (-)-[11C]McN 5652 was more uniformly distributed. Total distribution volumes (V(T)) were derived using kinetic 2-compartment analysis and graphic analysis. V(T) derived by both methods were highly correlated. (+)-[11C]McN 5652 regional V(T) ranged from 18 +/- 2 mL/g in the cerebellum to 46 +/- 13 mL/g in the midbrain. (-)-[11C]McN 5652 regional VT ranged from 10 +/- 2 mL/g in the cerebellum to 14 +/- 3 mL/g in the thalamus. (+)-[11C]McN 5652 V(T) were higher than (-)-[11C]McN 5652 V(T) in all regions, including the cerebellum, a region devoid of 5-HT transporters. Blocking experiments were also performed in baboons with saturating doses of citalopram and in humans with nonsaturating doses of paroxetine. Cerebellar and neocortical (+)-[11C]McN 5652 V(T) were unaffected by pretreatment with 5-HT transporter blockers. In areas of high receptor concentration (midbrain, caudate, and thalamus) 5-HT transporter blockers decreased (+)-[11C]McN 5652 V(T) to the level of cerebellum (+)-[11C]McN 5652 V(T). CONCLUSION: These experiments indicate that the use of the difference between (+)- and (-)-[11C]McN 5652 V(T) to define specific binding to 5-HT transporters leads to an overestimation of specific binding. 5-HT transporter BP was derived as the difference between the regional and cerebellar (+)-[11C]McN 5652 V(T). BP values were in good agreement with the distribution of 5-HT transporters in the human brain, except for regions of relatively low 5-HT transporter concentration, such as the prefrontal cortex, where no specific binding was detected using (+)-[11C]McN 5652. (+)-[11C]McN 5652 is an appropriate radiotracer to quantify 5-HT transporters in regions with relatively high concentration of 5-HT transporters, such as the midbrain, thalamus, and basal ganglia, and should prove useful in elucidating abnormalities of 5-HT transmission in neuropsychiatric conditions.     

Synthesis and characterization of EADAM: a selective radioligand for mapping the brain serotonin transporters by positron emission tomography

[11C]N,N-Dimethyl-2-(2'-amino-4'-ethylphenylthio)benzylamine ([11C]EADAM) was synthesized in the development of a serotonin transporter (SERT) imaging ligand for positron emission tomography (PET). The methods of ligand synthesis, results of in vitro characterization, 11C labeling and in vivo micro-PET imaging studies of [11C]EADAM in cynomolgus monkey brain are described. 11C was introduced into N,N-dimethyl-2-(2'-amino-4'-ethylphenylthio)benzylamine (5) by alkylation of N-methyl-2-(2'-amino-4'-ethylphenylthio)benzylamine (10) in 32% radiochemical yield (end of bombardment [EOB], decay-corrected from [11C]methyl iodide). Competition binding assays in cells stably expressing the transfected human dopamine transporter (DAT), SERT and norepinephrine transporter (NET) labeled with [3H]WIN 35428 or [(125)I]RTI-55, [3H]citalopram and [3H]nisoxetine, respectively, indicated the following order of SERT affinity: ADAM>EADAM>fluvoxamine. The affinity of EADAM for DAT and NET was 500- and >1000-fold lower, respectively, than for SERT. Micro-PET brain imaging studies in a cynomolgus monkey demonstrated high [11C]EADAM uptake in the striatum, thalamus and brainstem. [11C]EADAM uptake in these brain regions peaked in less than 60 min following administration of [11C]EADAM. The tissue-to-cerebellum ratios of the striatum, thalamus and brainstem were 1.67, 1.71 and 1.63, respectively, at 120 min postinjection of [11C]EADAM. Analysis of monkey arterial plasma samples using high-pressure liquid chromatography determined there was no detectable formation of lipophilic radiolabeled metabolites capable of entering the brain. In a displacement experiment with citalopram in a cynomolgus monkey, radioactivity in the striatum, thalamus and brainstem was displaced 20-60 min after administration of citalopram. In a blocking experiment with citalopram in a cynomolgus monkey, radioactivity in the striatum, thalamus and brainstem was significantly reduced. These results support the candidacy of [11C]EADAM as a radioligand for visualizing brain SERT using PET.     

Effects of image reconstruction algorithm on neurotransmission PET studies in humans: Comparison between filtered backprojection and ordered subsets expectation maximization

OBJECTIVES: Both reconstruction algorithms, filtered backprojection (FBP) and ordered subsets expectation maximization (OSEM), are widely used in clinical positron emission tomography (PET) studies. Image reconstruction for most neurotransmission PET scan data is performed by FBP, while image reconstruction for whole-body [18F]FDG scan data is usually performed by OSEM. Although several investigators have compared FBP and OSEM in terms of the quantification of regional radioactivity and physiological parameters calculated from PET data, only a few studies have compared the two reconstruction algorithms in PET studies that estimate neurotransmission, i.e., neuroreceptor and neurotransporter binding. In this study we compared mean regional radioactivity concentration in the late phase and binding potential (BP) between FBP and OSEM algorithms in neurotransmission PET studies for [11C]raclopride and [11C]DASB. METHODS: Dynamic PET scans with [11C]raclopride in 3-dimensional mode were performed on seven healthy subjects. Dynamic PET scans with [11C]DASB in 2-dimensional mode were performed on another seven subjects. OSEM images were post-filtered so that its transverse spatial resolution became similar to that of FBP with the same Hanning filter (Kernel FWHM 6 mm). In both PET studies we calculated the BP of [11C]raclopride and [11C]DASB by a reference tissue model for each ROI (region of interest). RESULTS: There was no significant difference in mean regional radioactivity concentration between FBP and OSEM for [11C]raclopride and [11C]DASB. Only +2.4 - +3.2%, but still a significant difference in BP of [11C]raclopride between FBP and OSEM was observed in the striatum. There was no significant difference in BP between FBP and OSEM in other than the striatum for [11C]raclopride and in all regions for [11C]DASB. In addition, there was no significant difference in root mean square error between FBP and OSEM when BP was calculated. CONCLUSIONS: The BP values were similar between FBP and OSEM algorithms with [11C]raclopride and [11C]DASB. This study indicates that OSEM can be used for human neurotransmission PET studies for calculating BP although OSEM was not necessarily superior to FBP in the present study.     

6-O-(2-[18F]fluoroethyl)-6-O-desmethyldiprenorphine ([18F]DPN): synthesis, biologic evaluation, and comparison with [11C]DPN in humans.

6-O(2-[18F]fluoroethyl)-6- -desmethyldiprenorphine ([18F]DPN) was developed and biologically evaluated. Results of animal experiments, binding studies in vivo, and a human PET study are reported and compared with those of [11C]DPN. METHODS: [18F]DPN was obtained by 18F-fluoroethylation of 3-O-trityl-6-O-desmethyldiprenorphine and subsequent deprotection in good radiochemical yields (23% +/- 7%; 100 min; 37 TBq/mmol). Binding of [18F]DPN to mu, kappa, and delta opioid receptors was shown by autoradiography studies on rat brain slices. Quantification of cerebral opioid receptor binding in men was performed by spectral analysis of a dynamic PET scan (25 frames, 90 min) after intravenous application of 63 MBq [18F]DPN (36 GBq/micromol) and correction for metabolites. RESULTS: [18F]DPN shows high affinity to opioid receptors. Parametric images (impulse response function at 60 min) of this human study showed a binding pattern of [18F]DPN equal to that of a control group (n = 9 healthy volunteers) after administration of [11C]DPN. CONCLUSION: The advantage of the longer half-life of 18F will allow extended scanning periods, more flexible interventions (e.g., displacement studies), and DPN to be available to PET centers without an on-site cyclotron.     

Feasibility studies of 4'-[methyl-(11)C]thiothymidine as a tumor proliferation imaging agent in mice

This study reports on the radiosynthesis and feasibility studies of 4'-[methyl-(11)C]thiothymidine ([methyl-(11)C]S-dThd) as a tumor proliferation imaging agent. [Methyl-(11)C]S-dThd was synthesized by rapid methylation of corresponding 5-trimethylstannyl- or 5-tributylstannyl-precursor via a palladium-promoted Stille cross-coupling reaction with [(11)C]methyl iodide. The decay-corrected radiochemical yields of [methyl-(11)C]S-dThd synthesized by the corresponding 5-trimethylstannyl-precursor and 5-tributylstannyl-precursor based on [(11)C]CO(2) were 18.9% and 14.5%, respectively. The radiochemical purity of [methyl-(11)C]S-dThd was always greater than 99%. The specific activities of [methyl-(11)C]S-dThd synthesized by the corresponding 5-trimethylstannyl-precursor and 5-tributylstannyl-precursor were 47 GBq/mumol and 121 GBq/mumol, respectively, at the end of the synthesis. The total synthesis time was 30 min after the end of bombardment. The comparison between in vivo distribution of [methyl-(14)C]S-dThd and that of [methyl-(3)H]FLT showed that tracer uptake was comparable in nonproliferating tissues. In contrast, [methyl-(14)C]S-dThd showed significantly higher uptake in proliferating tissues than did [methyl-(3)H]FLT. [Methyl-(11)C]S-dThd uptake levels in five different tumor tissues were well correlated with the DNA synthesis levels determined by [2-(14)C]thymidine DNA incorporation. At 30 min after injection, plasma analysis found 95% of the activity in unmetabolized form. The microPET imaging of the C6 glioma xenograft showed significantly high uptake in the tumor and urinary bladder, followed by the intestine and marrow. Our results demonstrated that the tumor uptake of [methyl-(11)C]S-dThd was higher than that of [methyl-(3)H]FLT and was well correlated with the DNA synthesis level. Consequently, 4'-[methyl-(11)C]thiothymidine has promise for the imaging of tumor cell proliferation by positron emission tomography.     

Synthesis and evaluation of new radioligands [11C]A-833834 and [11C]A-752274 for positron-emission tomography of α7-nicotinic acetylcholine receptors

Introductionα7-nicotinic acetylcholine receptor (α7-nAChR) is one of the major neuronal nAChR subtypes. α7-nAChR is involved in variety of neuronal processes and disorders including schizophrenia and Alzheimer's disease. A number of α7-nAChR PET radioligands have been developed, but a quality radiotracer remains to be discovered.MethodsHigh binding affinity α7-nAChR ligands A-833834 and A-752274 were radiolabeled with ¹¹C. Baseline and blockade biodistribution studies in the mouse brain of [¹¹C]A-833834 (5-(6-(5-[¹¹C]methylhexahydropyrrolo[3,4-c]pyrrol-2(1H)-yl)pyridazin-3-yl)-1H-indole) and [¹¹C]A-752274 (2-(6-[¹¹C]methyl-3,6-diazabicyclo[3.2.0]heptan-3-yl)-7-(6-methyl-3,6-diazabicyclo[3.2.0]heptan-3-yl)-9H-fluoren-9-one) were performed. [¹¹C]A-752274 was evaluated in a baseline baboon PET study.Results[¹¹C]A-833834 and [¹¹C]A-752274 were synthesized by radiomethylation of corresponding des-methyl precursors. The radioligands were prepared with radiochemical yield of 12%–32%, high specific radioactivity (330–403 GBq/μmol) and radiochemical purity > 95%. Dissection studies with [¹¹C]A-833834 demonstrated low specific α7-nAChR binding in the mouse brain. [¹¹C]A-752274 specifically (~ 50%) labeled α7-nAChR in the mouse thalamus. However, [¹¹CA-752274 exhibited low brain uptake in baboon (%SUV < 100).ConclusionTwo novel α7-nAChR ligands radioligands were synthesized and studied in animals. Specific binding of [¹¹C]A-833834 in the mouse brain is low due to the insufficient binding affinity of the radioligand. The very high binding affinity [¹¹C]A-752274 exhibited good specific binding in the α7-nAChR-rich mouse brain regions. The low uptake of [¹¹C]A-752274 in the baboon brain is due to its high hydrophilicity, rapid metabolism or other properties. Future development of α7-nAChR PET radioligands will be based on compounds with high binding affinities and good blood–brain barrier permeability.