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The multiinjection approach was used to study in vivo interactions between α4β2* nicotinic acetylcholine receptors and 2-[18F]fluoro-A-85380 in baboons. The ligand kinetics was modeled by the usual nonlinear compartment model composed of three compartments (arterial plasma, free and specifically bound ligand in tissue). Arterial blood samples were collected to generate a metabolite-corrected plasma input function. The experimental protocol, which consisted of three injections of labeled or unlabeled ligand, was aiming at identifying all parameters in one experiment. Various parameters, including B′max (the binding sites density) and KdVR (the apparent in vivo affinity of 2-[18F]fluoro-A-85380) could then be estimated in thalamus and in several receptor-poor regions. B′max estimate was 3.0±0.3 pmol/mL in thalamus, and ranged from 0.25 to 1.58 pmol/mL in extrathalamic regions. Although KdVR could be precisely estimated, the association and dissociation rate constants kon/VR and koff could not be identified separately. A second protocol was then used to estimate koff more precisely in the thalamus. Having estimated all model parameters, we performed simulations of 2-[18F]fluoro-A-85380 kinetics to test equilibrium hypotheses underlying simplified approaches. These showed that a pseudo-equilibrium is quickly reached between the free and bound compartments, a favorable situation to apply Logan graphical analysis. In contrast, the pseudo-equilibrium between the plasma and free compartments is only reached after several hours. The ratio of radioligand concentration in these two compartments then overestimates the true equilibrium value, an unfavorable situation to estimate distribution volumes from late images after a bolus injection.Journal of Cerebral Blood Flow & Metabolism (2008) 28, 172–189; doi:10.1038/sj.jcbfm.9600505; published online 16 May 2007 [ABSTRACT FROM AUTHOR]

2-[18F]fluoro-3-[2(S)-2-azetidinylmethoxy]pyridine ([18F]fluoro-A-85380) is an α4β2 subtype selective nicotinic cholinergic agonist with potential suitability for studying changes in endogenous acetylcholine synaptic concentration. Physostigmine, a potent AChE inhibitor, and galantamine, an allosteric modulator of nAChRs, are widely used for the treatment of Alzheimer's disease. Before studying patients with this neurodegenerative disease, positron emission tomography (PET) studies in monkeys were performed to assess the impact of these two compounds on the radiotracer distribution volumes. Physostigmine was administered i.v. at two dosages: 150 μg/kg/h and 37.5 μg/kg/h for 160 min. Galantamine was administered i.v. at two dosages: 2 or 4 mg over 20 min. For PET data analysis, a model with one tissue (radioactivity of the parent compound in plasma and radioactivity in brain tissue) compartment was chosen because reliable parameter estimates could not be obtained with a more complex model. The higher dose of physostigmine produced a 40%, 23%, and 30% reduction of distribution volumes in the putamen, the temporal, and frontal cortices, respectively. The lower dose of physostigmine produced a reduction of 33%, 31%, and 24% in the same structures, respectively. Galantamine (4 mg or 2 mg) produced no significant change of distribution volumes in the basal ganglia, the temporal and frontal cortex. The effects of physostigmine, a more potent AChE inhibitor than galantamine, could be interpreted as a desensitization of nAChRs, due to a prolonged exposure to high synaptic concentration of acetylcholine or as a competition with acetylcholine. Synapse 56:217-221, 2005. © 2005 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

We studied the potential of three radiobromihated isomers of BrQNP, (Z(-,-)-[[sup 76]Br]BrQNP, E(-,-)[[sup 76]Br]BrQNP and E(-,+)-[[sup 76]Br]BrQNP), as suitable radioligands for imaging of central muscarinic cholinergic receptors in the human brain. These radioligands were stereospecifically prepared by electrophilic radiobromodestannylation of the respective tributylstannyl precursors using no-carrier-added [[sup 76]Br]BrNH[sub 4] and peracetic acid. Preliminary pharmacological characterizations were determined by biodistribution, autoradiography, competition, displacement and metabolite studies in rats. The (-,-)-configuration presented important specific uptakes in brain muscarinic cholinergic receptor (mAChR)-rich structures and in heart, low metabolization rates and an apparent M[sub 2] selectivity. The (-,+)-configuration revealed more rapid clearance, lower uptake, a higher metabolization rate and an apparent M[sub 1] selectivity. Reversibility of the binding was confirmed for the three radiotracers. Positron emission tomography in the living baboon brain revealed high and rapid uptake in the brain and accumulation in the mAChR-rich structures studied. At 30 min p.i., the E(-,-)-radiotracer reached a plateau in cortex, pons and thalamus with concentrations of 29%, 24% and 19% ID/1, respectively. Z(-,-)[[sup 76]Br]BrQNP also accumulated in these structures, reaching a maximal uptake (27% ID/1) in the cortex 2 h p.i. At 5 min p.i. a plateau (17% ID/1) was only observed in the cortex for the E(-,+)-[[sup 76]Br]BrQNP; by contrast, the other structures showed slow washout. After 3 weeks, the (-,-)-radiotracers were studied in the same baboon pretreated with dexetimide (1 mg/kg), a wellknown muscarinic antagonist. In all the mAChR structures, the highly reduced uptake observed after this preloading step indicates that these radiotracers specifically bind to muscarinic receptors. Z(-,-)-[[sup 76]Br]BrQNP, which is displaced in higher amounts from M[sub 2]... [ABSTRACT FROM AUTHOR]

In vivo pharmacokinetic and brain binding characteristics of (+)-[11C]A-69024, a high-affinity-D1-selective dopamine receptor antagonist, were assessed with micro-PET and β-microprobes in the rat and PET in the baboon. The biodistribution of (+)-[11C]A-69024 in rats and baboons showed a rapid brain uptake (reaching a maximal value at 5 and 15 min postinjection in rats and baboons, respectively), followed by a slow wash out. The region/cerebellum concentration ratio was characterized by a fourfold higher uptake in striatum and a twofold higher uptake in cortical regions, consistent with in vivo specific binding of the radiotracer in these cerebral regions. Furthermore, this specific (+)-[11C]A-69024 binding significantly correlated with the reported in vitro distribution of dopamine D1-receptors. Finally, the specific uptake of the tracer in the striatum and cortical regions was completely prevented by either a pretreatment with large doses of nonradioactive (±)A-69024 or of the D1-selective antagonist SCH23390, resulting in a similar uptake in the reference region (cerebellum) and in other brain regions. Thus, (+)-[11C]A-69024 appears to be a specific and enantioselective radioligand to visualize and quantify brain dopamine D1 receptors in vivo using positron emission tomography. © 2007 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 97:2811–2819, 2008 [ABSTRACT FROM AUTHOR]

Abstract: FAUC346 (N-[4-[4-(2-methoxyphenyl)piperazin-1-yl]butyl]benzo[b]thiophene-2-carboxamide), an in vitro D3-selective ligand, and its normethyl derivative have been synthesized from commercially available 1-(2-substituted-phenyl)piperazines. FAUC346 has been labeled using [11C]methyl triflate in acetone containing aqueous NaOH (5 Eq) at −10 °C for 1 min, purified on semipreparative reverse-phase high-performance liquid chromatography (HPLC) and formulated as an intravenous injectable solution using a Sep-Pak Plus C18 device. Up to 5.5 GBq of [11C]FAUC346 (N-[4-[4-(2-[methyl-11C]methoxyphenyl)piperazin-1-yl]butyl]benzo[b]thiophene-2-carboxamide), with a specific radioactivity of 45–75 GBq/μmol, could be obtained in 30–35 min, including HPLC purification and formulation starting from 44.4 GBq of [11C]carbon dioxide. Preliminary pharmacological evaluation of [11C]FAUC346 in rat brain clearly demonstrated in vivo selectivity for D3 receptors and the absence of radiolabeled metabolite within the brain. These encouraging results, however, could not be confirmed in nonhuman primates; therefore, this radioligand does not appear to have the required pharmacological profile for a positron emission tomography probe for imaging D3 receptors. [Copyright &y& Elsevier]

Unlabelled: 2-(18)F-fluoro-3-[2(S)-2-azetidinylmethoxy]pyridine (2-(18)F-fluoro-A-85380) is a PET radioligand that is specific for nicotinic acetylcholine receptors (nAChRs) and has a high affinity for the alpha(4)beta(2) subtype. The purpose of this study was to evaluate different strategies to quantify 2-(18)F-fluoro-A-85380 binding in healthy nonsmoking human volunteers., Methods: After intravenous injection of 189 +/- 30 MBq (0.8-5.7 nmol) of 2-(18)F-fluoro-A-85380, the first dynamic PET scan was acquired over 150 min. The second 30-min PET scan was performed 60 min later. Time-activity curves were generated from volumes of interest. 2-(18)F-Fluoro-A-85380 volume of distribution (DV) was quantified using compartmental kinetic analysis and Logan graphical analysis. In the kinetic analysis, the 1-tissue compartment model (1TCM) and the 2-tissue (2TCM) compartment model were applied. The most appropriate kinetic model was determined using the Akaike Information Criterion. The effect of reducing the PET study duration on the reliability of the DV values computed by the kinetic and the graphical analyses was evaluated., Results: Time-activity curves were better described by the 2TCM. The DV values ranged from 5.2 +/- 0.5 in the occipital cortex, 6.2 +/- 0.2 in the frontal cortex, and 7.3 +/- 0.4 in the putamen to 15.4 +/- 2.1 in the thalamus. These regional DV values were consistent with the distribution of nAChRs in the human brain. Logan graphical analysis provided slightly lower DV values than those of the 2TCM (from -3.5% in the occipital cortex to -6.6% in the thalamus). The minimal study duration required to obtain stable DV estimates in all regions was similar for the 2 methods: 140 min for the 2TCM and 150 min for the Logan analysis. DV estimates obtained with the 2TCM were more stable than those calculated by the Logan approach for the same scan duration., Conclusion: These results show that 2-(18)F-fluoro-A-85380 can be used to assess nAChRs binding in the human brain with PET.

Unlabelled: This study reports on the biodistribution and radiation dosimetry of 2-(18)F-Fluoro-3-[2(S)-2-azetidinylmethoxy]pyridine ((18)F-fluoro-A-85380), a promising radioligand for the imaging of central nicotinic acetylcholine receptors (nAChRs)., Methods: Whole-body scans were performed in 3 healthy male volunteers up to 2 h after intravenous injection of 137-238 MBq (18)F-fluoro-A-85380. Transmission scans (3 min per step, 8 or 9 steps according to the height of the subject) in 2-dimensional mode were used for subsequent correction of attenuation of emission scans. Emission scans (1 min per step) were acquired over 2 h. Venous blood samples were taken up to 2 h after injection of the radiotracer. Urine was freely collected up to 2 h after injection of the radiotracer. For each subject, the percentage of injected activity measured in regions of interest over brain, intestine, stomach, bladder, kidneys, and liver were fitted to a monoexponential model, as an uptake phase followed by a monoexponential washout, or to a biexponential model to generate time-activity curves. Using the MIRD method, ten source organs were considered in estimating radiation absorbed doses for organs of the body., Results: Injection of (18)F-fluoro-A-85380 was clinically well tolerated and blood and urine pharmacologic parameters did not change significantly. The primary routes of clearance were renal and intestinal. Ten minutes after injection, high activities were observed in the bladder, kidneys, and liver. Slow uptake was seen in the brain. The liver received the highest absorbed dose. The average effective dose of (18)F-fluoro-A-85380 was estimated to be 0.0194 mSv/MBq., Conclusion: The amount of (18)F-fluoro-A-85380 required for adequate nAChR imaging results in an acceptable effective dose equivalent to the patient.