[[(18)F]N-(4'-fluorobenzyl)-4-(3-bromophenyl) acetamide for imaging the sigma receptor status of tumors: comparison with [(18)F]FDG, and [(125)I]IUDR.

A series of biodistribution studies were conducted with the radiotracer, [(18)F]N-(4'-fluorobenzyl)-4-(3-bromophenyl)acetamide, [(18)F]1 in nude mice bearing tumor xenografts of the mouse mammary adenocarcinoma, line 66. This radiotracer has a high affinity for both sigma(1) and sigma(2) receptors. In vivo studies were also conducted in order to assess the effect of blocking sigma(1) receptors on tumor uptake and the tumor:background ratio of this radiotracer. The results of these studies revealed that blocking the sigma(1) receptor so that only the sigma(2) receptors are labeled in vivo, results in a higher tumor:background ratio with only a small reduction in the tumor uptake of the radiotracer relative to the no-carrier-added (i.e., nonselective) conditions. Comparative in vivo studies were also conducted with the anatomic and metabolic imaging agent, [(18)F]FDG, and a radiolabeled DNA precursor, [(125)I]IUdR. Both of these radiolabeled compounds represent classes of agents that have been proposed for imaging the proliferative status of solid tumors. The results of these studies indicated that a sigma(2)-selective imaging agent may be, 1) a better anatomic imaging agent for breast cancer than [(18)F]FDG, and 2) a better functional imaging agent than the radiolabeled DNA precursors, [(123/124)I]IUdR and [(11)C]thymidine, for measuring the proliferative status of breast tumors with PET and SPECT. However, additional studies will be needed to compare sigma(2)-selective imaging agents with [(18)F]FLT in order to determine which is the more appropriate imaging agent for measuring the proliferative status of breast tumors with PET.

(+)-p-([18F]fluorobenzyl)spirotrozamicol [(+)-[18F]spiro-FBT]: synthesis and biological evaluation of a high-affinity ligand for the vesicular acetylcholine transporter (VAChT).

(+)-1'-[4-Hydroxy-1-(4-fluorobenzyl)piperidin-3-yl]spiro[1H- indene-1,4'- piperidine] {(+)-Spiro-FBT}, a high-affinity vesicular acetylcholine transporter ligand, was labeled with fluorine-18, and evaluated in the rat and monkey. In the rat brain, (+)-[18F]Spiro-FBT accumulated preferentially in the striatum, hippocampus, and cortex, brains regions containing high-to-moderate densities of cholinergic terminals. However, due to rapid metabolism, no preferential accumulation of the radiotracer was observed in corresponding regions of the monkey brain. Consequently, rapid metabolism renders (+)-[18F]Spiro-FBT unsuitable for studying cholinergic function with positron emission tomography.

Use of positron emission tomography to study the dynamics of psychostimulant-induced dopamine release.

Microdialysis studies have shown that psychostimulants act through a common neurochemical mechanism of elevating synaptic dopamine content in the mesocorticolimbic dopaminergic system. However, little information is available regarding the dynamics of the interaction between the elevated synaptic dopamine levels induced by a psychostimulant and postsynaptic dopamine receptors. The goal of the current investigation was to determine if positron emission tomography (PET) studies using the dopamine D2-selective radioligand [18F]4'-fluoroclebopride ([18F]FCP) could be used to measure synaptic dopamine levels. Rhesus monkeys were used because our previous studies revealed that [18F]FCP has a low test/retest variability in this species. Under control conditions, [18F]FCP had a high uptake and slow rate of washout from the basal ganglia, a region of brain that expresses a high density of D2 receptors, reaching kinetic equilibrium at approximately 40 min. Challenge studies, each separated by at least 1 month, were conducted by administering an intravenous dose of (-)cocaine, d-amphetamine, methylphenidate, or d-methamphetamine (1.0 mg/kg) at 40 min post-IV injection of a no-carrier-added dose of [18F]FCP. In each case, the psychostimulant caused an increase in the rate of washout of [18F]FCP from the basal ganglia. Methamphetamine and amphetamine had more pronounced effects on the washout kinetics of [18F]FCP relative to cocaine and methylphenidate, a result that is consistent with the ability of each drug to elevate synaptic dopamine levels. Our results indicate that challenge studies with [18F]FCP may be a useful technique for studying the dynamics of the interaction between psychostimulant-induced increases in synaptic dopamine and postsynaptic D2 receptors.

Imaging of cholinergic terminals using the radiotracer [18F](+)-4-fluorobenzyltrozamicol: in vitro binding studies and positron emission tomography studies in nonhuman primates.

The goal of the present set of studies was to characterize the in vitro binding properties and in vivo tissue kinetics for the vesicular acetylcholine transporter (VAcChT) radiotracer, [18F](+)-4-fluorobenzyltrozamicol ([18F](+)-FBT). In vitro binding studies were conducted in order to determine the affinity of the (+)- and (-)-stereoisomers of FBT for the VAcChT as well as sigma (sigma 1 and sigma 2) receptors. (+)-FBT was found to have a high affinity (Ki = 0.22 nM) for the VAcChT and lower affinities for sigma 1 (21.6 nM) and sigma 2 (35.9 nM) receptors, whereas (-)-FBT had similar affinities for the VAcChT and sigma 1 receptors (approximately 20 nM) and a lower affinity for sigma 2 (110 nM) receptors. PET imaging studies were conducted in rhesus monkeys (n = 3) with [18F](+)-FBT. [18F](+)-FBT was found to have a high accumulation and slow rate of washout from the basal ganglia, which is consistent with the labeling of cholinergic interneurons in this brain region. [18F](+)-FBT also displayed reversible binding kinetics during the 3 h time course of PET and produced radiolabeled metabolites that did not cross the blood-brain barrier. The results from the current in vitro and in vivo studies indicate that [18F](+)-FBT is a promising ligand for studying cholinergic terminal density, with PET, via the VAcChT.

Comparison of two fluorine-18 labeled benzamide derivatives that bind reversibly to dopamine D2 receptors: in vitro binding studies and positron emission tomography.

The purpose of the present set of studies was to characterize, in vitro and in vivo, two benzamide analogues, 2,3-dimethoxy-N-[1-(4-fluorobenzyl)piperidin4yl]benzamide (MBP) and 4'-fluoroclebopride (FCP), for studying dopamine D2 receptors with Positron Emission Tomography (PET). In vitro binding studies were conducted to determine the affinities of MBP and FCP to the three subtypes of dopamine D2 receptors: D2(long), D3, and D4 receptors. MBP was found to have a high affinity (Ki = 1-8 nM) for all three subtypes of the D2 receptor, whereas FCP had nanomolar affinity (Ki approximately 5.5 nM) for D2(long) and D3 receptors, and a lower affinity for D4 receptors (Ki = 144 nM). In vitro binding studies also revealed that MBP had a relatively high affinity for rho1 receptors (Ki = 11 nM) compared to FCP (Ki = 340 nM). PET imaging studies were conducted in rhesus monkeys with the fluorine-18 labeled analogues of each compound. Both [18F]MBP and [18F]FCP displayed reversible binding kinetics during the 3 h time course of PET. [18F]FCP was found to have a higher basal ganglia:cerebellum ratio and lower variability in the rate of washout from D2 receptors in vivo relative to [18F]MBP. Neither radiotracer was found to produce radiolabeled metabolites capable of crossing the blood-brain barrier. The high rho1 binding affinity and low basal ganglia:cerebellum ratio of [18F]MBP indicate that this ligand may not be suitable for quantitative studies of D2 receptors. The results from the in vitro and in vivo studies indicate that [18F]FCP is a promising ligand for studying D2 receptors with PET.

PET imaging studies of dopamine D2 receptors: comparison of [18F]N-methylspiperone and the benzamide analogues [18F]MABN and [18F]MBP in baboon brain.

A series of positron emission tomography (PET) imaging studies was conducted in a baboon with the benzamide derivatives [18F]2,3-dimethoxy-N-[9-(4-fluorobenzyl)-9-azabicyclo[3.3.1]non an-3 beta-yl]benzamide ([18F]MABN) and [18F]2,3-dimethoxy-N-[1-(4-fluorobenzyl)piperidin-4-yl]be nza mide ([18F]MBP). Studies were also conducted with the butyrophenone [18F]N-methylspiperone (NMSP) for comparison. Tissue-time activity curves of [18F]MABN are similar to those of [18F]NMSP since both compounds displayed approximately the same uptake in the basal ganglia and displayed irreversible binding kinetics in vivo. However, the rapid rate of clearance from the cerebellum and high basal ganglia:cerebellum ratio of [18F]MABN indicate that this compound has a much lower amount of nonspecific binding than [18F]NMSP. [18F]MBP displayed a higher uptake in the basal ganglia relative to [18F]NMSP and [18F]MABN and exhibited reversible binding kinetics in vivo. This property of [18F]MBP is desirable since the uptake of radioactivity in D2-rich ligands is less likely to be influenced by changes in cerebral blood flow. The current data suggest that both [18F]MABN and [18F]MBP are promising ligands for studying dopamine D2 receptors with PET.

The use of [18F]4-fluorobenzyl iodide (FBI) in PET radiotracer synthesis: model alkylation studies and its application in the design of dopamine D1 and D2 receptor-based imaging agents.

[18F]4-Fluorobenzyl iodide ([18F]FBI) was prepared, and a series of model alkylation studies were conducted to determine its chemical reactivity toward nitrogen and sulfur nucleophiles of varying nucleophilicities. [18F]FBI was found to react rapidly with secondary amines and anilines to give the corresponding N-[18F]4-fluorobenzyl analogue in high yield. Amides and thiol groups required the use of a base catalyst. The utility of [18F]FBI was documented by investigation of dopamine D1 and D2 receptor-based radiotracers.

Synthesis and in vivo evaluation of [18F]-N-(p-nitrobenzyl)spiperone ([18F]PNBS) in rats.

[18F]-N-(p-nitrobenzyl)spiperone, [18F]PNBS, was prepared and its in vivo uptake in rat brain was measured. A temporal increase in striatum:cerebellum ratio and frontal cortex:cerebellum (FC:Cb) ratio was observed. Blocking studies suggested the formation of a radiolabeled metabolite, presumably [18F]spiperone, that is capable of crossing the blood-brain barrier and labeling cortical 5-HT2 receptors. This was confirmed using the ISRP method of metabolite analysis indicating [18F]PNBS is not a suitable tracer for studying D2 receptors in vivo with PET.

Optimization of [11C]CO2 trapping efficiencies from nitrogen gas streams.

Three new systems were developed for trapping [11C]CO2 from high flow rate nitrogen gas streams and the trapping efficiency was determined for each trap design. The results of this study indicate that both the stainless steel shot and stainless steel frit systems yield high trapping efficiencies under the range of flow rates evaluated (400-3600 sccm) and may prove to be alternatives to the traditional stainless steel/copper coiled tubing or molecular sieve trapping systems in common use.