Nature-inspired indolyl-2-azabicyclo[2.2.2]oct-7-ene derivatives as promising agents for the attenuation of withdrawal symptoms: synthesis of 20-desethyl-20-hydroxymethyl-11-demethoxyibogaine.

Microwave assisted Diels-Alder cycloaddition of 5-Br-N-benzylpyridinone (2) with methyl acrylate is described to gain an easy access to 7-bromo-2-benzyl-3-oxo-2-aza-5 or 6-carbomethoxy bicyclo[2.2.2]oct-7-enes (3)-(6). The preparation of the ibogaine analogue 20-desethyl-(20-endo)-hydroxymethyl-11-demethoxyibogaine (17) is described by stereoselective hydrogenation of the C(7)-C(8) double bond. Biological evaluation showed an interesting in vitro binding profile toward dopamine transporter, serotonin transporter and opioid receptor systems accompanied by an antiwithdrawal effect in mice for hydroxymethyl 7-indolyl-2-aza-bicyclo[2.2.2]oct-2-ene (14). The simplification of the ibogaine structure appears as a promising approach toward the design of compounds that could reduce the withdrawal symptoms.

Synthesis and pharmacological characterization of 4-[125I]-N-(N-benzylpiperidin-4-yl)-4-iodobenzamide: a high affinity sigma receptor ligand for potential imaging of breast cancer.

The synthesis of [125I]-N-(N-benzylpiperidin-4-yl)-4-iodobenzamide (4-[125I]BP), a novel radiopharmaceutical that possesses high affinity for both sigma-1 and sigma-2 receptor subtypes, and its binding characteristics to MCF-7 breast cancer cells are described. To obtain high yields (with high specific activity) of radioiodinated ligand, (N-benzylpiperidin-4-yl)-4-tri-butylstannyl benzamide was synthesized. Radiolabeled 4-[125I]BP was prepared from tri-butylstannyl precursor with the use of chloramine-T or hydrogen peroxide as an oxidizing agent in high yields (71-86%). The competition binding studies of 4-[125I]BP in MCF-7 breast tumor cells with haloperidol and DTG (known sigma ligands) showed a dose-dependent displacement and high affinity binding (Ki = 4.6 and 56 nM, respectively), demonstrating that sigma receptors are expressed in MCF-7 breast tumor cells. Scatchard analysis of 4-[125I]BP binding in MCF-7 cells revealed saturable binding, with a Kd = 26 nM and a Bmax = 4000 fmol/mg protein. Furthermore, the Scatchard analysis of [3H]DTG binding in MCF-7 cells gave a Kd of 24.5 nM and a Bmax of 2071 fmol/mg of protein. The biodistribution and clearance of 4-[125I]BP was studied in rats. The radiopharmaceutical cleared quickly from the blood pool but rather slowly from the hepatobiliary system. The in vivo specificity was demonstrated by blocking the receptor binding in the presence of haloperidol. A decrease of 55, 63, 43, and 68% was found at 1 h postinjection in brain, kidney, heart, and lung, respectively. These results demonstrate that a high density of sigma receptors are expressed in MCF-7 cells and that radioiodinated 4-IBP may be useful for imaging breast cancer by targeting sigma sites.

Vesicular acetylcholine transporter density and Alzheimer's disease.

We have evaluated the vesamicol analogue meta-[125I]iodobenzyltrozamicol {(+)-[125I]MIBT} as a probe to assess cholinergic terminal integrity in the human temporal cortex. Saturation binding analysis, using 5-aminobenzovesamicol (ABV) to define nonspecific binding, revealed a high-affinity binding site with a Kd value of 4.3 +/- 1.2 nM in the temporal cortex of the young control subjects. Similar affinity values were observed for (+)-[125I]MIBT binding in aged control subjects (Kd = 3.4 +/- 0.5 nM) and AD patients (Kd = 3.0 +/- 0.8 nM). In contrast, Bmax values for young subjects, aged controls and AD patients were 31.2 +/- 6.3, 17.0 +/- 2.0 and 9.4 +/- 1.6 pmol/g, respectively, clearly reflecting significant reductions in (+)-[125I]MIBT binding site density with aging and age-related neuropathology. Moreover, the decrease in (+)-[125I]MIBT binding was correlated with choline acetyltransferase activities (r = 0.72) in the AD temporal cortex. These results suggest that when selective ligands are used, the vesicular acetylcholine transporter can be a useful marker protein for assessing the loss of cholinergic projections in AD and related disorders.

Morphine-induced spinal cholinergic activation: in vivo imaging with positron emission tomography.

Positron emission tomography (PET) imaging of spinal cord in monkeys with a cholinergic tracer demonstrates increased spinal cholinergic activity in response to an analgesic dose of morphine, and this PET result correlates with measurement of acetylcholine spillover into spinal cord extracellular space induced by morphine, as measured by microdialysis. Previous studies in rats, mice, and sheep demonstrate activation of spinal cholinergic neurons by systemic opioid administration, and participation of this cholinergic activity in opioid-induced analgesia. Testing the relevance of this observation in humans has been limited to measurement of acetylcholine spillover into lumbar cerebrospinal fluid. The purpose of this study was to apply a recently developed method to image spinal cholinergic terminals non-invasively via PET and to test the hypothesis that the tracer utilized would reflect changes in local cholinergic activity. Following Animal Care and Use Committee approval, seven adult male rhesus monkeys were anesthetized on three separate occasions. On two of the occasions PET scans were performed using [(18)F] (+)-4-fluorobenzyltrozamicol ([(18)F]FBT), which selectively binds to the vesicular acetylcholine (ACh) transporter in the presynaptic cholinergic terminals. PET scans were preceded by injection of either saline or an analgesic dose of IV morphine (10 mg/kg). On the third occasion, microdialysis catheters were inserted in the spinal cord dorsal horn and acetylcholine concentrations in dialysates determined before and after IV morphine injection. Morphine increased cholinergic activity in the spinal cord, as determined by blood flow corrected distribution volume of [(18)F]FBT in the cervical cord compared to the cerebellum. Morphine also increased acetylcholine concentrations in microdialysates from the cervical cord dorsal horn. The one animal which did not show increased spinal cholinergic activity by PET from this dose of morphine also did not show increased acetylcholine from this morphine dose in the microdialysis experiment. These data confirm the ability to use PET to image spinal cholinergic terminals in the monkey spinal cord and suggest that acute changes in cholinergic activity can be imaged with this non-invasive technique. Following preclinical screening, PET scanning with [(18)F]FBT may be useful to investigate mechanisms of analgesic action in normal humans and in those with pain.

Synthesis and biodistribution of 99mTc-labeled piperidinyl bis(aminoethanethiol) complexes: potential brain perfusion imaging agents for single photon emission computed tomography.

In developing clinically useful 99mTc-labeled radiopharmaceuticals for the evaluation of regional cerebral perfusion with single photon emission computed tomography (SPECT), a number of substituted alkyl(aryl)piperidinyl bis(aminoethanethiol) ligands for chelating [99mTc]TcO(III) were synthesized. Each ligand forms two diastereomers, syn and anti, after reacting with a racemic mixture of the ligand. The diastereomers were separated by high-pressure liquid chromatography. In biodistribution studies conducted in rats, the diastereomers exhibit widely disparate brain uptake values; however, this disparity seems to diminish as the steric bulk of the substituent at the C-4 position of the piperidinyl moiety increases. Furthermore, all the complexes evaluated failed to show a prolonged retention in the rat brain, suggesting that further structural modification may be necessary to obtain clinically useful complexes from this class of compounds.

Modified ibogaine fragments: synthesis and preliminary pharmacological characterization of 3-ethyl-5-phenyl-1,2,3,4,5, 6-hexahydroazepino[4,5-b]benzothiophenes.

Five phenyl-substituted derivatives and analogues of 1,2,3,4,5, 6-hexahydroazepino[4,5-b]indole, 5, a major fragment of ibogaine (1), were synthesized and tested for binding to monoamine transporters, the NMDA receptor-coupled cation channel, and dopamine and opioid receptors. All five derivatives, 9 and 17a-d, displayed 8-10-fold higher affinity at the DA transporter than ibogaine and noribogaine (4). At the serotonin transporter, two compounds (9 and 17a) exhibited higher potency than ibogaine, while the rest had weaker binding affinities than the lead compound. In keeping with their structural similarity to ibogaine, all five compounds displayed weak to poor affinity for dopamine D1 and D2 receptors. However, two compounds, 17a,c, demonstrated moderate binding affinities at dopamine D3 receptors. All five compounds displayed weak to poor affinities for mu and kappa opioid receptors and for the NMDA receptor-coupled cation channel. Despite the qualitative differences, derivatives and analogues of 5may serve as useful substitutes for ibogaine.

Molecular size and flexibility as determinants of selectivity in the oxidation of N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine analogs by monoamine oxidase A and B.

The introduction of a methylene bridge between the phenyl and tetrahydropyridyl moieties of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) results in increased selectivity for monoamine oxidase B (MAO B) over monoamine oxidase A (MAO A). However, lengthening of this bridge results in a total loss of selectivity. In the present study, a number of isomeric 4-naphthyl-, 4-(naphthylalkyl)-, 4-thienyl-, and 4-(thienylalkyl)tetrahydropyridines, conformationally restrained and flexible analogs of MPTP, were synthesized and evaluated as potential selective substrates of MAO A and B. In terms of the parameter (turnover number)/Km, the bulky naphthyl analogs were invariably better substrates of MAO A than kynuramine, the reference substrate for this enzyme. In addition, all naphthyl analogs, regardless of conformational mobility, were more effective substrates of MAO A than MAO B. Similarly, all thienyl analogs were found to be more effective substrates of MAO B. In contrast to the naphthalenes, the conformationally restrained thiophenes 9a and 10a were found to be poor substrates of MAO B, relative to benzylamine, the reference substrate. These results suggest that the selectivity of these compounds for either MAO A or B is determined by the complex interplay of molecular size and flexibility. In this interplay, either one of these two factors may predominate.

Synthesis and tissue distribution of (m-[125I]iodobenzyl)trozamicol ([125I]MIBT): potential radioligand for mapping central cholinergic innervation.

Racemic (m-iodobenzyl)trozamicol (6, MIBT), a high-affinity vesamicol receptor ligand, was radiolabeled, resolved, and evaluated in rats. Following iv injection, (+)- and (-)-[125I]MIBT achieved initial brain levels of 0.57 and 0.92% dose/g of tissue, respectively. The level of (+)-[125I]MIBT subsequently declined by 74% within 3 h, while that of (-)-[125I]MIBT remained stable for the duration. Ex vivo autoradiographic mapping of (-)-[125I]MIBT distribution in rat brain revealed a pattern which was inconsistent with central cholinergic innervation. However, high levels of (+)-[125I]MIBT were observed over the amygdala, striatum, nucleus accumbens, olfactory tubercle, and nuclei of the fifth and seventh cranial nerves, while moderate to low levels were detected within the cortex, hippocampus, and cerebellum. Thus, the distribution of (+)-[125I]MIBT parallels that of other presynaptic cholinergic markers. Co-injection of (+)-[125I]MIBT with 4-aminobenzovesamicol (2b), a potent vesamicol receptor ligand, reduced the levels of radiotracer in the striatum, cortex, and cerebellum by 58, 35, and 9%, respectively. Thus, (+)-[125I]MIBT binds to vesamicol receptors in vivo. In contrast, coadministration of (+)-[125I]MIBT with haloperidol (0.5 mumol/kg), reduced radiotracer levels in the cortex and cerebellum by 34 and 59%, respectively, while increasing the levels in the striatum by 32%. We conclude that although the distribution of (+)-[125I]MIBT qualitatively reflects cholinergic innervation, a fraction of radiotracer in the cortex and cerebellum is bound to sigma receptors.

Spirovesamicols: conformationally restricted analogs of 2-(4-phenylpiperidino)cyclohexanol (vesamicol, AH5183) as potential modulators of presynaptic cholinergic function.

In an effort to develop selective inhibitors of vesicular acetylcholine storage, we have synthesized a series of semirigid vesamicol receptor ligands based on the structure of 2-(4-phenylpiperidino)-cyclohexanol (vesamicol, AH5183, 1). In these compounds, the planes of the phenyl and piperidyl moieties of the parent ligand 1 are held at right angles by vinyl, ethylene, and propylene bridges to form N-substituted derivatives of spiro[indene-1,4'-piperidine], 2,3-dihydrospiro[indene-1,4'-piperidine], and 3,4-dihydrospiro[naphthalene-1(2H),4'-piperidine], respectively. Preliminary evaluation of these compounds in electric organ synaptic vesicles revealed several potent vesamicol receptor ligands, such as 1'-(2-hydroxy-1,2,3,4-tetrahydronaphth-3-yl)spiro[1H-indene-1,4'-p iperidine (11b) and 1'-(2-hydroxy-1,2,3,4-tetrahydronaphth-3-yl)spiro[2-bromo-1H-in den e- 1,4'-piperidine] (14), which display subnanomolar affinity for this receptor. In general, the vinyl and ethylene bridges yielded the most potent analogs while the propylene-bridged analogs were among the least potent compounds. The increased rigidity of these spiro-fused compounds, relative to the corresponding simple 4-phenylpiperidine derivatives of vesamicol, is expected to confer greater selectivity for the vesamicol receptor.

Acyclic analogues of 2-(4-phenylpiperidino)cyclohexanol (vesamicol): conformationally mobile inhibitors of vesicular acetylcholine transport.

Several 1,3-disubstituted propan-2-ols and one alpha,beta-disubstituted ethanol (11i) were synthesized and evaluated as potential acyclic mimics of the vesicular acetylcholine transport inhibitor 2-(4-phenylpiperidinyl)cyclohexanol (1, vesamicol, AH5183). Analogues containing the 4-phenylpiperidyl fragment (11a, 11b) were more potent than those containing the 4-phenylpiperazyl moiety (11e, 11f). Substitution at the second terminal carbon of the propyl (or ethyl) fragment with simple lipophilic aryl substituents yielded potent inhibitors of vesicular acetylcholine storage, including (-)-11a and d-11i, which are equipotent with vesamicol. However, the activity of analogues containing bicyclic aryl groups was susceptible to aryl substitution patterns (11g vs 11h), indicating a definite receptor site topography. In addition, the inhibitory activity of these acyclic analogues was enantioselective, exhibiting a preference, similar to the parent vesamicol, for the levorotatory isomer [(-)-11a vs (+)-11a]. Therefore, the simple lipophilic acyclic vicinal amino alcohols may successfully mimic the biological activity of vesamicol.

Nonsymmetrical bipiperidyls as inhibitors of vesicular acetylcholine storage.

Introduction of a nitrogen atom into the cyclohexane ring of 2-(4-phenylpiperidinyl)cyclohexanol (vesamicol, AH5183) yielded two positional isomers, 5-azavesamicol (5, prezamicol) and 4-azavesamicol (6, trozamicol). As inhibitors of vesicular acetylcholine transport, 5 and 6 were found to be 147 and 85 times less potent than vesamicol. N-Benzoylation of 5 (to yield 9a) increased the potency 3-fold. In contrast, 10a, a compound derived from N-benzoylation of 6, was 50 times more potent than the latter and almost equipotent with vesamicol, thereby suggesting a preference for the 4-azavesamicol series. Although (-)-vesamicol is more potent than its dextrorotary isomer, (+)-10a was found to be 3 times more potent than (-)-10a, suggesting a reversal of the sign of rotation in the azavesamicol series. Reduction of 9a and 10a (to yield the corresponding N-benzyl derivatives 11a and 12a) increased potency 20- and 2-fold, respectively, indicating a preference for a basic nitrogen. The reaction of 5 or 6 with substituted benzyl halides yielded several potent inhibitors of vesicular acetylcholine transport, including N-(p-fluorobenzyl)trozamicol, 12d, which is twice as potent as vesamicol. Thus the introduction of a nitrogen atom into the cyclohexane ring of vesamicol provides opportunities for developing a new class of anticholinergic agents.

Hydroxylated decahydroquinolines as ligands for the vesicular acetylcholine transporter: synthesis and biological evaluation.

Analogues of the potent anticholinergic 2-(4-phenylpiperidino)cyclohexanol (vesamicol, 1) in which the cyclohexyl fragment was replaced with an N-acyl or N-alkyl trans-decahydroquinolyl moiety were synthesized and evaluated as potential ligands for the vesicular acetylcholine transporter (VAChT). The binding of compounds, such as 18, 20, and 21, was both stereospecific and of comparable magnitude to that of the closely related vesamicol analogue 2,3-trans-4a, 8a-trans-3-hydroxy-2-(4-phenylpiperidino)-1,2,3,4,5,6,7, 8-decahydronaphthalene (6) which displays subnanomolar affinity for this transporter. However, these compounds also demonstrated high affinities for sigma(1) and sigma(2) receptors and thus failed to show significantly improved selectivity over previously reported vesamicol analogues.

Synthesis and biological activities of 2-pyrimidinone nucleosides. 2. 5-Halo-2-pyrimidinone 2'-deoxyribonucleosides.

1-(2-Deoxy-beta-D-ribofuranosyl)-5-bromo-2-pyrimidinone (BrPdR) and 1-(2-deoxy-beta-D-ribofuranosyl)-5-iodo-2-pyrimidinone (IPdR) have been synthesized by condensation of the appropriate silylated bases 2a and 2b, respectively, with 3,5-bis-O-(p-chlorobenzoyl)-2-deoxy-alpha-D-ribofuranosyl chloride (8) in 1,2-dichloroethane, in the presence of SnCl4, followed by separation of the anomeric blocked nucleosides via column chromatography and subsequent deprotection with methanolic ammonia. Both BrPdR and IPdR exhibited significant antiherpes activities against various strains of HSV-1 and HSV-2, the latter compound (IPdR) showing the higher activity as well as the stronger binding to the virus-specific thymidine kinase.

N-hydroxyalkyl derivatives of 3 beta-phenyltropane and 1-methylspiro[1H-indoline-3,4'-piperidine]: vesamicol analogues with affinity for monoamine transporters.

As part of our ongoing structure-activity studies of the vesicular acetylcholine transporter ligand 2-(4-phenylpiperidino)cyclohexanol (vesamicol, 1), 22 N-hydroxy(phenyl)alkyl derivatives of 3 beta-phenyltropane, 6, and 1-methylspiro[1H-indoline-3,4'-piperidine], 7, were synthesized and tested for binding in vitro. Although a few compounds displayed moderately high affinity for the vesicular acetylcholine transporter, no compound was more potent than the prototypical vesicular acetylcholine transporter ligand vesamicol. However, a few derivatives of 6 displayed higher affinity for the dopamine transporter than cocaine. We conclude that modification of the piperidyl fragment of 1 will not lead to more potent vesicular acetylcholine transporter ligands.

Flexible N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine analogues: synthesis and monoamine oxidase catalyzed bioactivation.

Eighteen analogues of N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) were synthesized and evaluated as substrates of monoamine oxidase. In general, the flexible analogues, characterized by the presence of a methylene (or ethylene) bridge between the aryl/heteroaryl and tetrahydropyridyl moieties, were better substrates of the enzyme than the conformationally restricted MPTP. It is suggested that the increased oxidative activity of these flexible analogues reflects enhanced binding due to the ability of the C-4-aryl/heteroaryl substituent to gain access to a hydrophobic pocket within the substrate binding site.

Interaction of tetrahydrostilbazoles with monoamine oxidase A and B.

1-Methyl-1,2,3,6-tetrahydrostilbazole (MTHS) and its analogs are oxidized by monoamine oxidase (MAO) A at slow rates comparable to that for the structurally similar neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, but the rates of oxidation by MAO B vary over a wide range depending on the structure of the analog. MAO A oxidation of all of the analogs yielded nonhyperbolic kinetic patterns, with little difference between the cis and trans isomers. In contrast MAO B showed hyperbolic kinetics and distinct stereoselectivity for the cis isomers. The corresponding pyridinium forms of trans-MTHS and its analogs were more potent inhibitors of MAO A (Ki values between 0.3 and 5 microM) than of MAO B, for which the Ki values varied greatly. The data suggest that the stringency of the MAO A active site for the geometry of the substrate molecule is less strict than that of MAO B. With MAO B, any substitution on the phenyl ring can lead to dramatic changes in the substrate properties which may be explained by the different orientation of substrate at the active site of the enzyme. Molecular geometry but not the effects of the substituents was shown to be an important factor in determining the effectiveness of substrate oxidation by MAO B.

2-(2-Piperidyl)- and 2-(2-pyrrolidyl)chromans as nicotine agonists: synthesis and preliminary pharmacological characterization.

As part of an effort to develop a new class of subtype selective nicotine agonists, we have synthesized and tested a group of 12 hydroxylated 2-(2-piperidyl)- and 2-(2-pyrrolidyl)chromans. In rat brain membranes, all 12 compounds displayed poor affinity for [(125)I]-alpha-bunagarotoxin binding sites. In contrast, three compounds, 17c, 24, and 26, displayed moderate to high affinity for [(3)H]cytisine binding sites, while three (17b, 18b,c) and six (17a,d,e and 18a,d,e) compounds showed weak and poor affinity, respectively, for these same sites. In subsequent studies, compounds 17a and 17c were found to stimulate the efflux of (86)Rb(+) from rat cortical synaptosomes, an indication of agonist activity. Further, both 17c and 26 displayed high intrinsic activity in stimulating the release of [(3)H]dopamine from striatal synaptosomes; however, only 17c was effective at stimulating the release of [(3)H]acetylcholine from cortical synaptosomes, suggesting differential selectivity. In cloned human nicotinic acetylcholine receptors (nAChR) expressed in Xenopus oocytes, both 17c and 26 activated alpha7 and alpha3beta2 receptor subtypes in a dose-dependent manner, but 26 was clearly the more potent agonist. Last, neither compound displayed dose-dependent activation of alpha4beta2 nAChRs. We conclude that 2-(2-azacyclic)chromans appear to be a promising new class of nicotine agonists.

Age-related diminution of dopamine antagonist-stimulated vesamicol receptor binding.

UNLABELLED: Previous studies of radiolabeled vesamicol receptor (VR) ligands suggest that the latter may be used in conjunction with dopamine D2 antagonists to measure changes in striatal cholinergic function. In this study, the effects of aging on vesicular acetylcholine storage/release were investigated with the high-affinity VR ligand (+)-meta-[125I)iodobenzyltrozamicol [(+)-[125I]MIBT]. METHODS: Male Fischer 344 rats (aged 3 and 24 mo) were injected either with a vehicle or a D2 antagonist [haloperidol or S-(-)-eticlopride]. At prescribed intervals thereafter, all animals were intravenously injected with 10 microCi of (+)-[125I]MIBT. Three hours after radiotracer injection, the animals were killed and their brains dissected. The concentration of radiotracer in the striatum, cortex and cerebellum were then determined. RESULTS: In control animals, comparable levels of (+)-[125I]MIBT were observed in corresponding brain regions of young adult and aged Fischer 344 rats. Moreover, in haloperidol- and S-(-)-eticlopride-treated young adult rats, striatal levels of (+)-[125I]MIBT were elevated by 35% and 66%, respectively, relative to controls. In contrast, haloperidol treatment failed to alter the striatal levels of (+)-[125I]MIBT in aged rats while S-(-)-eticlopride displayed a twofold reduction in potency in aged rats. CONCLUSION: Aging is associated with a reduction in striatal cholinergic plasticity or striatal cholinergic reserve and that the D2-stimulated increase in VR ligand binding is a functionally relevant parameter.

Technetium-99m bis (aminoethanethiol) complexes with amine sidechains--potential brain perfusion imaging agents for SPECT.

In an effort to develop new clinically useful technetium-99m bis(aminoethanethiol) ([99mTc]BAT) complexes for the evaluation of regional cerebral perfusion, two new BAT ligands containing amines in the sidechain were synthesized and subsequently complexed with 99mTc to yield the target complexes: [99mTc]DEA and [99mTc]TMPDA. Each complex was obtained as mixtures of two isomers, syn and anti, which were separated chromatographically. In biodistribution studies, both isomers of [99mTc]TMPDA showed little uptake in the brain. In contrast, the brain uptake values at 2 and 15 min for [99mTc]DEA-anti were 0.99 and 0.26, whereas, the corresponding values for DEA-syn were 2.27, 0.64% dose/organ, respectively. Autoradiographic studies (in rats) using both isomers of [99mTc]DEA show a fixed regional distribution and a higher concentration of radioactivity in the gray matter relative to the white matter. Planar imaging using [99mTc]DEA-syn clearly demonstrates localization of the complex in the brain with a T 1/2 of 41 min, suggesting some potential for use with single photon emission computed tomography.

Reproducibility of repeated measures of cholinergic terminal density using.

UNLABELLED: [18F](+)-4-fluorobenzyltrozamicol (FBT), which selectively binds to the vesicular acetylcholine transporter in the presynaptic cholinergic neuron, has previously been shown to be a useful ligand for the study of cholinergic terminal density in the basal ganglia with PET. The goal of this study was to assess the test-retest variability of [18F]FBT and PET measurements under baseline conditions in the basal ganglia. METHODS: After approval from the Animal Care and Use Committee, 6 rhesus monkeys underwent a series of 2 [18F]FBT PET scans (time between scans, 32-301 d) under isoflurane anesthesia. Each scan was initiated on the bolus injection of the radiotracer and consisted of 26 frames acquired during 180 min. Arterial blood samples were collected over the course of each scan to determine the metabolite-corrected arterial input function. Tissue time-activity curves were obtained from the scan data by drawing regions of interest over the basal ganglia and cerebellum. The distribution volume ratio for the basal ganglia was then determined for each scan by taking the ratio of the basal ganglia (specific binding) to cerebellum (nonspecific binding) distribution volume. Distribution volumes were derived using the Logan graphic analysis technique as well as a standard 3-compartment model. Additionally, the radioactivity concentration ratio was calculated as the ratio of the average [18F]FBT concentration in the basal ganglia to that in the cerebellum during the last half of the study (85-170 min). The constant K1, determined using the standard 3-compartment model, was used as an index of blood flow changes between studies. RESULTS: For all subjects, the test-retest variability was less than 15% for the distribution volume ratio and 12% for the radioactivity concentration ratio. Good agreement was found between the distribution volume ratio calculated using the graphic technique and the standard 3-compartment model. Using K1 as an index, the variability in blood flow seen in both the basal ganglia and the cerebellum was significantly reduced in their ratio. CONCLUSION: These results show the reproducibility of [18F]FBT and PET measurements in the basal ganglia.