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Imaging and quantitation of cannabinoid CB1 receptors in human and monkey brains using (18)F-labeled inverse agonist radioligands.

ABSTRACT: UNLABELLED:We recently demonstrated that (11)C-MePPEP, a PET ligand for CB(1) receptors, has such high uptake in the human brain that it can be imaged for 210 min and that receptor density can be quantified as distribution volume (V(T)) using the gold standard of compartmental modeling. However, (11)C-MePPEP had relatively poor retest and intersubject variabilities, which were likely caused by errors in the measurements of radioligand in plasma at low concentrations by 120 min. We sought to find an analog of (11)C-MePPEP that would provide more accurate plasma measurements. We evaluated several promising analogs in the monkey brain and chose the (18)F-di-deutero fluoromethoxy analog ((18)F-FMPEP-d(2)) to evaluate further in the human brain. METHODS:(11)C-FMePPEP, (18)F-FEPEP, (18)F-FMPEP, and (18)F-FMPEP-d(2) were studied in 5 monkeys with 10 PET scans. We calculated V(T) using compartmental modeling with serial measurements of unchanged parent radioligand in arterial plasma and radioactivity in the brain. Nonspecific binding was determined by administering a receptor-saturating dose of rimonabant, an inverse agonist at the CB(1) receptor. Nine healthy human subjects participated in 17 PET scans using (18)F-FMPEP-d(2), with 8 subjects having 2 PET scans to assess retest variability. To identify sources of error, we compared intersubject and retest variability of brain uptake, arterial plasma measurements, and V(T). RESULTS:(18)F-FMPEP-d(2) had high uptake in the monkey brain, with greater than 80% specific binding, and yielded less radioactivity uptake in bone than did (18)F-FMPEP. High brain uptake with (18)F-FMPEP-d(2) was also observed in humans, in whom V(T) was well identified within approximately 60 min. Retest variability of plasma measurements was good (16%); consequently, V(T) had a good retest variability (14%), intersubject variability (26%), and intraclass correlation coefficient (0.89). V(T) increased after 120 min, suggesting an accumulation of radiometabolites in the brain. Radioactivity accumulated in the skull throughout the entire scan but was thought to be an insignificant source of data contamination. CONCLUSION:Studies in monkeys facilitated our development and selection of (18)F-FMPEP-d(2), compared with (18)F-FMPEP, as a radioligand demonstrating high brain uptake, high percentage of specific binding, and reduced uptake in bone. Retest analysis in human subjects showed that (18)F-FMPEP-d(2) has greater precision and accuracy than (11)C-MePPEP, allowing smaller sample sizes to detect a significant difference between groups.

SUBMITTER: Terry GE

PROVIDER: S-EPMC2997525 | biostudies-literature | 2010 Jan

REPOSITORIES: biostudies-literature

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Imaging and quantitation of cannabinoid CB1 receptors in human and monkey brains using (18)F-labeled inverse agonist radioligands.

Terry Garth E GE Hirvonen Jussi J Liow Jeih-San JS Zoghbi Sami S SS Gladding Robert R Tauscher Johannes T JT Schaus John M JM Phebus Lee L Felder Christian C CC Morse Cheryl L CL Donohue Sean R SR Pike Victor W VW Halldin Christer C Innis Robert B RB

Journal of nuclear medicine : official publication, Society of Nuclear Medicine 20091215 1

<h4>Unlabelled</h4>We recently demonstrated that (11)C-MePPEP, a PET ligand for CB(1) receptors, has such high uptake in the human brain that it can be imaged for 210 min and that receptor density can be quantified as distribution volume (V(T)) using the gold standard of compartmental modeling. However, (11)C-MePPEP had relatively poor retest and intersubject variabilities, which were likely caused by errors in the measurements of radioligand in plasma at low concentrations by 120 min. We sought ...[more]

PMID: 20008988

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Project description:BackgroundPopulation-based input function (PBIF) may be a valid alternative to full blood sampling for quantitative PET imaging. PBIF is typically validated by comparing its quantification results with those obtained via arterial sampling. However, for PBIF to be employed in actual clinical research studies, its ability to faithfully capture the whole spectrum of results must be assessed. The present study validated a PBIF for [(18)F]FMPEP-d 2, a cannabinoid CB1 receptor radioligand, in healthy volunteers, and also attempted to utilize PBIF to replicate three previously published clinical studies in which the input function was acquired with arterial sampling.MethodsThe PBIF was first created and validated with data from 42 healthy volunteers. This PBIF was used to assess the retest variability of [(18)F]FMPEP-d 2, and then to quantify CB1 receptors in alcoholic patients (n?=?18) and chronic daily cannabis smokers (n?=?29). Both groups were scanned at baseline and after 2-4 weeks of monitored drug abstinence.ResultsPBIF yielded accurate results in the 42 healthy subjects (average Logan-distribution volume (V T) was 13.3±3.8 mL/cm(3) for full sampling and 13.2±3.8 mL/cm(3) for PBIF; R(2)?=?0.8765, p<0.0001) and test-retest results were comparable to those obtained with full sampling (variability: 16%; intraclass correlation coefficient: 0.89). PBIF accurately replicated the alcoholism study, showing a widespread ?20% reduction of CB1 receptors in alcoholic subjects, without significant change after abstinence. However, a small PBIF-V T bias of -9% was unexpectedly observed in cannabis smokers. This bias led to substantial errors, including a V T decrease in regions that had shown no downregulation in the full input function. Simulated data showed that the original findings could only have been replicated with a PBIF bias between -6% and +4%.ConclusionsDespite being initially well validated in healthy subjects, PBIF may misrepresent clinical protocol results and be a source of variability between different studies and institutions.

Project description:Exposure to cannabis impairs cognitive functions reliant on the circuitry of the dorsolateral prefrontal cortex (DLPFC) and increases the risk of schizophrenia. The actions of cannabis are mediated via the brain cannabinoid 1 receptor (CB1R), which in rodents is heavily localized to the axon terminals of cortical GABA basket neurons that contain cholecystokinin (CCK). Differences in the laminar distribution of CB1R-immunoreactive (IR) axons have been reported between rodent and monkey neocortex, suggesting that the cell type(s) containing CB1Rs, and the synaptic targets of CB1R-IR axon terminals, may differ across species; however, neither the relationship of CB1Rs to CCK-containing interneurons, nor the postsynaptic targets of CB1R and CCK axon terminals, have been examined in primate DLPFC. Consequently, we compared the distribution patterns of CB1R- and CCK-IR structures, determined the proportions of CB1R and CCK neurons that were dual-labeled, and identified the synaptic types and postsynaptic targets of CB1R- and CCK-IR axon terminals in macaque monkey DLPFC. By light microscopy, CB1R- and CCK-IR axons exhibited a similar laminar distribution, with their greatest densities in layer 4. Dual-label fluorescence experiments demonstrated that 91% of CB1R-IR neurons were immunopositive for CCK, whereas only 51% of CCK-IR neurons were immunopositive for CB1R. By electron microscopy, all synapses formed by CB1R-IR axon terminals were symmetric, whereas CCK-IR axon terminals formed both symmetric (88%) and asymmetric (12%) synapses. The primary postsynaptic target of both CB1R- and CCK-IR axon terminals forming symmetric synapses was dendritic shafts (81-88%), with the remainder targeting cell bodies or dendritic spines. Thus, despite species differences in laminar distribution, CB1Rs are principally localized to CCK basket neuron axons in both rodent neocortex and monkey DLPFC. These axons target the perisomatic region of pyramidal neurons, providing a potential anatomical substrate for the impaired function of the DLPFC associated with cannabis use and schizophrenia.

Dataset Information

Imaging and quantitation of cannabinoid CB1 receptors in human and monkey brains using (18)F-labeled inverse agonist radioligands.

Publications

Imaging and quantitation of cannabinoid CB1 receptors in human and monkey brains using (18)F-labeled inverse agonist radioligands.

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