Use of carbon-11 nicotine in PET studies

Biodistribution and Radiation Dosimetry of 11C-Nicotine from Whole-Body PET Imaging in Humans

This study assessed the in vivo distribution of ¹¹C-nicotine and the absorbed radiation dose from whole-body ¹¹C-nicotine PET imaging of 11 healthy (5 male and 6 female) subjects. Methods: After an initial CT attenuation scan, ¹¹C-nicotine was administered via intravenous injection. A dynamic PET scan was acquired for 90 s with the brain in the field of view, followed by a series of 13 whole-body PET scans acquired over a 90-min period. Regions of interest were drawn over organs visible in the reconstructed PET images. Time-activity curves were generated, and the residence times were calculated. The absorbed radiation dose for the whole body was calculated by entering the residence time in OLINDA/EXM 1.0 software to model the equivalent organ dose and the effective dose for a 70-kg man. Results: The mean residence times for ¹¹C-nicotine in the liver, red marrow, brain, and lungs were 0.048 ± 0.010, 0.031 ± 0.005, 0.021 ± 0.004, and 0.020 ± 0.005 h, respectively. The mean effective dose for ¹¹C-nicotine was 5.44 ± 0.67 μSv/MBq. The organs receiving the highest absorbed dose from the ¹¹C-nicotine injection were the urinary bladder wall (14.68 ± 8.70 μSv/MBq), kidneys (9.56 ± 2.46 μSv/MBq), liver (8.94 ± 1.67 μSv/MBq), and spleen (9.49 ± 3.89 μSv/MBq). The renal and hepatobiliary systems were the major clearance and excretion routes for radioactivity. Conclusion: The estimated radiation dose from ¹¹C-nicotine administration is relatively modest and would allow for multiple PET examinations on the same subject.

A-186253, a specific antagonist of the α4β2 nAChRs: its properties and potential to study brain nicotinic acetylcholine receptors

Imaging the living brain and the distribution of the ligand gated channels that participate in the neurotransmission is one of the challenges that is hoped to bring new insights for the treatment of neurological diseases. Herein, we probed a new nicotinic derivative, A-186253 as a potential molecule to discriminate with high resolution the different neuronal nicotinic receptor subtypes that are expressed in distinct brain areas. Binding with a high affinity of 440 pM at the major brain alpha4beta2 receptor subtype and presenting an excellent safety margin, properties of the A-186253 were thoroughly evaluated. While autoradiography confirmed its specificity for the alpha4beta2 subtype, functional investigations revealed for short exposures a broader spectrum of action at receptors including the ganglionic alpha3beta4 and the homomeric alpha7 subtypes. Specificity was, however, observed at alpha4beta2 when receptors were exposed for several minutes with low concentration of the A-186253. In view of these promising results, the A-186253 was radiolabeled and tested in positron emission tomography on rats and pigs. Despite the high selectivity observed in vitro, the A-186253 displayed a complex binding profile and little displacement by the agonist cytisine. While the A-186253 can be valuable to discriminate receptor subtypes, improvements of this molecule must be brought for in vivo measurements.

In vivo [125I]-iodobenzovesamicol binding reflects cortical cholinergic deficiency induced by specific immunolesion of rat basal forebrain cholinergic system

In this study, radiolabeled iodobenzovesamicol (IBVM), which is known to bind with high affinity to the vesicular acetylcholine transporter, was tested for its usefulness in imaging cortical cholinergic deficits in vivo. To induce reductions in cortical cholinergic input, the cholinergic immunotoxin 192IgG-saporin was employed. This has been shown to selectively and efficiently destroy basal forebrain cholinergic neurons in rats. The efficiency of the immunolesion was verified by histochemical acetylcholinesterase staining. [125I]-IBVM binding before and after lesioning was measured using autoradiography. Basal forebrain cholinergic cell loss resulted in a considerable reduction in [125I]-IBVM binding in the cholinoceptive target regions, but not in the striatum and cerebellum, brain regions that do not receive a cholinergic input by the basal forebrain cholinergic nuclei, suggesting that [123I]-IBVM has potential in imaging cortical cholinergic deficits in vivo, at least in animals.

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.