Monoacylglycerol lipase (MAGL) is a 33 kDa serine protease primarily responsible for hydrolyzing 2-arachidonoylglycerol into the proinflammatory eicosanoid precursor arachidonic acid in the central nervous system. Inhibition of MAGL constitutes an attractive therapeutic concept for treating psychiatric disorders and neurodegenerative diseases. Herein, we present the design and synthesis of multiple reversible MAGL inhibitor candidates based on a piperazinyl azetidine scaffold. Compounds 10 and 15 were identified as the best-performing reversible MAGL inhibitors by pharmacological evaluations, thus channeling their radiolabeling with fluorine-18 in high radiochemical yields and favorable molar activity. Furthermore, evaluation of [(18)F]10 and [(18)F]15 ([(18)F]MAGL-2102) by autoradiography and positron emission tomography (PET) imaging in rodents and nonhuman primates demonstrated favorable brain uptakes, heterogeneous radioactivity distribution, good specific binding, and adequate brain kinetics, and [(18)F]15 demonstrated a better performance. In conclusion, [(18)F]15 was found to be a suitable PET radioligand for the visualization of MAGL, harboring potential for the successful translation into humans.
OBJECTIVE: (18) F-(-)-NCFHEB (also known as (18) F-(-)-Flubatine) is a new radioligand to image alpha4beta2* nicotinic acetylcholine receptors in vivo with positron emission tomography (PET), with faster kinetics than previous radioligands such as (18) F-2-F-A85380. The goal of this study was to assess the sensitivity of (18) F-(-)-NCFHEB-PET to increases in synaptic acetylcholine concentration induced by acetylcholinesterase inhibitors. METHODS: Two rhesus monkeys were scanned four times each on a Focus 220 scanner: first at baseline, then during two bolus plus infusions of physostigmine (0.06-0.28 mg/kg), and finally following a bolus injection of donepezil (0.25 mg/kg). The arterial input function and the plasma free fraction fP were measured. (18) F-(-)-NCFHEB volume of distribution VT was estimated using the multilinear analysis MA1 and then normalized by plasma free fraction fP . RESULTS: (18) F-(-)-NCFHEB fP was 0.89 +/- 0.04. At baseline, (18) F-(-)-NCFHEB VT /fP ranged from 7.9 +/- 1.3 mL plasma/cm(3) tissue in the cerebellum to 34.3 +/- 8.4 mL plasma/cm(3) tissue in the thalamus. Physostigmine induced a dose-dependent reduction of (18) F-(-)-NCFHEB VT /fP of 34 +/- 9% in the putamen, 32 +/- 8% in the thalamus, 25 +/- 8% in the cortex, and 23 +/- 10% in the hippocampus. With donepezil, (18) F-(-)-NCFHEB VT /fP was reduced by 24 +/- 2%, 14 + 3% and 14 +/- 5%, 10 +/- 6% in the same regions. CONCLUSION: (18) F-(-)-NCFHEB can be used to detect changes in synaptic acetylcholine concentration and is a promising tracer to study acetylcholine dynamics with shorter scan durations than previous radioligands. Synapse 68:556-564, 2014. (c) 2014 Wiley Periodicals, Inc.
In vivo estimation of beta(2)-nicotinic acetylcholine receptor availability with molecular neuroimaging is complicated by competition between the endogenous neurotransmitter acetylcholine and the radioligand (123)I-3-[2(S)-2-azetidinylmethoxy]pyridine ((123)I-5-IA). We examined whether binding of (123)I-5-IA is sensitive to increases in extracellular levels of acetylcholine in humans, as suggested in nonhuman primates. METHODS: Six healthy subjects (31 +/- 4 y) participated in a (123)I-5-IA SPECT study. After baseline scans, physostigmine (1-1.5 mg) was administered intravenously over 60 min, and 9 additional scans were obtained. RESULTS: We observed a significant reduction in the total volume of distribution after physostigmine administration (29% +/- 17% in the cortex, 19% +/- 15% in the thalamus, 19% +/- 15% in the striatum, and 36% +/- 30% in the cerebellum; P < 0.05). This reduction reflected a combination of a region-specific 7%-16% decrease in tissue concentration of tracer and a 9% increase in plasma parent concentration. CONCLUSION: These data suggest that increases in acetylcholine compete with (123)I-5-IA for binding to beta(2)-nicotinic acetylcholine receptor. Additional validation of this paradigm is warranted, but it may be used to interrogate changes in extracellular acetylcholine.
Title: Age and APOE-epsilon4 genotype influence the effect of physostigmine infusion on the in-vivo distribution volume of the muscarinic-2-receptor dependent tracer [18F]FP-TZTP Cohen RM, Carson RE, Filbey F, Szczepanik J, Sunderland T Ref: Synapse, 60:86, 2006 : PubMed
The apolipoprotein E-epsilon4 allele (APOE-epsilon4) confers greater susceptibility to age-related memory disorders. Abnormalities in the cholinergic system are likely contributors to these disorders with both age and APOE-epsilon4 genotype modifying behavioral and physiological responses to drugs that alter cholinergic pathway function. Recently, we reported a greater in vivo distribution volume of the F-18 labeled muscarinic-2 (M2) selective agonist, 3-(3-(3-[18F]Flouropropyl)thio)-1,2,5-thiadiazol-4-yl)-1,2,5,6-tetrahydro-1-methy lpyridine ([18F]FP-TZTP), in aging healthy subjects with an APOE-epsilon4 allele. To examine the effects of aging and the APOE-epsilon4 allele on the response of the muscarinic component of cholinergic pathway to pharmacologic augmentation, two [18F]FP-TZTP PET scans were conducted in 19 subjects varying in age from 22 to 74 years, the first served as baseline for the second scan that was performed while the subjects were either infused with saline (n = 6) or with the acetylcholinesterase inhibitor physostigmine (6 with an APOE-epsilon4 allele and 7 without an APOE-epsilon4 allele). Using a multiple regression analysis, both AGE (beta = 0.621 +/- 0.135, B = 0.353 +/- 0.077, t(10) = 4.61, P < 0.001) and APOE-epsilon4 genotype (beta = 0.742, B = 14.8 +/- 2.69, t(10) = 5.51, P < 0.0003) were found to be significant contributors to subject response to physostigmine. The adjusted R2 for the model as a whole was 0.786 (F(2,10) = 23.00, P < 0.0002) with both increasing age and the presence of the APOE-epsilon4 allele modifying the response to physostigmine in the direction of larger decreases in [18F]FP-TZTP distribution volumes in all brain regions examined. The findings, particularly the absence of an interaction between AGE and APOE-epsilon4 genotype, contribute to the growing body of evidence that suggests that the APOE-epsilon4 genotype is likely to contribute to brain structure and function prior to aging.
Title: Using single photon emission tomography (SPECT) and positron emission tomography (PET) to trace the distribution of muscarinic acetylcholine receptor (MACHR) binding radioligands Kiesewetter DO, Carson RE, Jagoda EM, Herscovitch P, Eckelman WC Ref: Life Sciences, 64:511, 1999 : PubMed
Two [18F] labeled ligands for the mAChR were prepared and evaluated in rodents and nonhuman primates. The properties of both compounds, one an agonist and the other an antagonist, were consistent with M2 subtype specificity.
[18F]Fluoropropyl-TZTP (FP-TZTP) is a subtype-selective muscarinic cholinergic ligand with potential suitability for studying Alzheimer's disease. Positron emission tomography studies in isofluorane-anesthetized rhesus monkeys were performed to assess the in vivo behavior of this radiotracer. First, control studies (n = 11) were performed to characterize the tracer kinetics and to choose an appropriate model using a metabolite-corrected arterial input function. Second, preblocking studies (n = 4) with unlabeled FP-TZTP were used to measure nonspecific binding. Third, the sensitivity of [18F]FP-TZTP binding to changes in brain acetylcholine (ACh) was assessed by administering physostigmine, an acetylcholinesterase (AChE) inhibitor, by intravenous infusion (100 to 200 microg x kg(-1) x h(-1)) beginning 30 minutes before tracer injection (n = 7). Tracer uptake in the brain was rapid with K1 values of 0.4 to 0.6 mL x min(-1) x mL(-1) in gray matter. A model with one tissue compartment was chosen because reliable parameter estimates could not be obtained with a more complex model. Volume of distribution (V) values, determined from functional images created by pixel-by-pixel fitting, were very similar in cortical regions, basal ganglia, and thalamus, but significantly lower (P < 0.01) in the cerebellum, consistent with the distribution of M2 cholinergic receptors. Preblocking studies with unlabeled FP-TZTP reduced V by 60% to 70% in cortical and subcortical regions. Physostigmine produced a 35% reduction in cortical specific binding (P < 0.05), consistent with increased ACh competition. The reduction in basal ganglia (12%) was significantly smaller (P < 0.05), consistent with its markedly higher AChE activity. These studies indicate that [18F]FP-TZTP should be useful for the in vivo measurement of muscarinic receptors with positron emission tomography.
