RESUMO
PURPOSE: Factor analysis has been pursued as a means to decompose dynamic cardiac PET images into different tissue types based on their unique temporal signatures to improve quantification of physiological function. In this work, the authors present a novel kinetic model-based (MB) method that includes physiological models of factor relationships within the decomposition process. The physiological accuracy of MB decomposed (82)Rb cardiac PET images is evaluated using simulated and experimental data. Precision of myocardial blood flow (MBF) measurement is also evaluated. METHODS: A gamma-variate model was used to describe the transport of (82)Rb in arterial blood from the right to left ventricle, and a one-compartment model to describe the exchange between blood and myocardium. Simulations of canine and rat heart imaging were performed to evaluate parameter estimation errors. Arterial blood sampling in rats and (11)CO blood pool imaging in dogs were used to evaluate factor and structure accuracy. Variable infusion duration studies in canine were used to evaluate MB structure and global MBF reproducibility. All results were compared to a previously published minimal structure overlap (MSO) method. RESULTS: Canine heart simulations demonstrated that MB has lower root-mean-square error (RMSE) than MSO for both factor (0.2% vs 0.5%, p < 0.001 MB vs MSO, respectively) and structure (3.0% vs 4.7%, p < 0.001) estimations, as with rat heart simulations (factors: 0.2% vs 0.9%, p < 0.001 and structures: 3.0% vs 6.7%, p < 0.001). MB blood factors compared to arterial blood samples in rats had lower RMSE than MSO (1.6% vs 2.2%, p =0.025). There was no difference in the RMSE of blood structures compared to a (11)CO blood pool image in dogs (8.5% vs 8.8%, p =0.23). Myocardial structures were more reproducible with MB than with MSO (RMSE=3.9% vs 6.2%, p < 0.001), as were blood structures (RMSE=4.9% vs 5.6%, p =0.006). Finally, MBF values tended to be more reproducible with MB compared to MSO (CV= 10% vs 18%, p =0.16). The execution time of MB was, on average, 2.4 times shorter than MSO (p < 0.001) due to fewer free parameters. CONCLUSIONS: Kinetic model-based factor analysis can be used to provide physiologically accurate decomposition of (82)Rb dynamic PET images, and may improve the precision of MBF quantification.
Assuntos
Circulação Coronária/fisiologia , Coração/fisiologia , Modelos Cardiovasculares , Miocárdio/metabolismo , Imagem de Perfusão/métodos , Tomografia por Emissão de Pósitrons/métodos , Radioisótopos/farmacocinética , Rubídio/farmacocinética , Animais , Velocidade do Fluxo Sanguíneo/fisiologia , Simulação por Computador , Cães , Coração/diagnóstico por imagem , Cinética , Taxa de Depuração Metabólica , Compostos Radiofarmacêuticos/farmacocinética , RatosRESUMO
AIMS/HYPOTHESIS: AMP-activated protein kinase (AMPK) has a broad role in the regulation of glucose and lipid metabolism making it a promising target in the treatment of type 2 diabetes mellitus. We therefore sought to characterise for the first time the effects of chronic AMPK activation on skeletal muscle carbohydrate metabolism in carriers of the rare gain-of-function mutation of the gene encoding AMPKgamma(3) subunit, PRKAG3 R225W. METHODS: Aspects of fuel metabolism were studied in vitro in myocytes isolated from vastus lateralis of PRKAG3 R225W carriers and matched control participants. In vivo, muscular strength and fatigue were evaluated by isokinetic dynamometer and surface electromyography, respectively. Glucose uptake in exercising quadriceps was determined using [(18)F]fluorodeoxyglucose and positron emission tomography. RESULTS: Myotubes from PRKAG3 R225W carriers had threefold higher mitochondrial content (p < 0.01) and oxidative capacity, higher leak-dependent respiration (1.6-fold, p < 0.05), higher basal glucose uptake (twofold, p < 0.01) and higher glycogen synthesis rates (twofold, p < 0.05) than control myotubes. They also had higher levels of intracellular glycogen (p < 0.01) and a trend for lower intramuscular triacylglycerol stores. R225W carriers showed remarkable resistance to muscular fatigue and a trend for increased glucose uptake in exercising muscle in vivo. CONCLUSIONS/INTERPRETATION: Through the enhancement of skeletal muscle glucose uptake and increased mitochondrial content, the R225W mutation may significantly enhance exercise performance. These findings are also consistent with the hypothesis that the gamma(3) subunit of AMPK is a promising tissue-specific target for the treatment of type 2 diabetes mellitus, a condition in which glucose uptake and mitochondrial function are impaired.
Assuntos
Proteínas Quinases Ativadas por AMP/genética , Proteínas Quinases Ativadas por AMP/metabolismo , Glucose/metabolismo , Fibras Musculares Esqueléticas/metabolismo , Adulto , Transporte Biológico/genética , Transporte Biológico/fisiologia , Western Blotting , Células Cultivadas , Ácidos Graxos/metabolismo , Glicogênio/metabolismo , Humanos , Técnicas In Vitro , Pessoa de Meia-Idade , Músculo Esquelético/metabolismo , Mutação , Consumo de Oxigênio/genética , Consumo de Oxigênio/fisiologia , Tomografia por Emissão de Pósitrons , Reação em Cadeia da Polimerase Via Transcriptase ReversaRESUMO
The high-affinity phosphodiesterase-4 (PDE4) inhibitor R-rolipram and the less potent S-enantiomer, both labeled with (11)C, were evaluated in vivo in rats. Regional brain uptake of R-[(11)C]rolipram was higher than R/S-[(11)C]rolipram, whereas S-[(11)C]rolipram retention subsided rapidly to levels below blood. Binding of R-[(11)C]rolipram was selective for PDE4 over PDE1, since treatment with PDE4 competitors Ro 20-1724, or R-, S- or R/S-rolipram, but not with the PDE1 inhibitor vinpocetine, significantly reduced radioactivity uptake to non-specific levels. R-Rolipram (ED(50) congruent with 0.04 mg/Kg) was approximately 13 fold more potent than S-rolipram at inhibiting R-[(11)C]rolipram binding in all brain regions. Nevertheless, S-[(11)C]rolipram appears to be unsuitable for measuring the non-specific binding of R-[(11)C]rolipram. Only unchanged R-[(11)C]rolipram was detected in rat brain homogenates. Additionally, the estimated absorbed radiation dose extrapolated to humans was low. These results support further investigation of R-[(11)C]rolipram in studying PDE4 in vivo by positron emission tomography imaging.
Assuntos
3',5'-AMP Cíclico Fosfodiesterases/metabolismo , Inibidores de Fosfodiesterase , Compostos Radiofarmacêuticos , Rolipram , Animais , Ligação Competitiva/efeitos dos fármacos , Radioisótopos de Carbono , Nucleotídeo Cíclico Fosfodiesterase do Tipo 1 , Nucleotídeo Cíclico Fosfodiesterase do Tipo 4 , Meia-Vida , Marcação por Isótopo , Masculino , Inibidores de Fosfodiesterase/farmacocinética , Radiometria , Compostos Radiofarmacêuticos/farmacocinética , Ratos , Ratos Sprague-Dawley , Rolipram/farmacocinética , Estereoisomerismo , Distribuição Tecidual , Tomografia Computadorizada de EmissãoRESUMO
Three cholecystokinin type B (CCKB) receptor antagonists were labelled with 11C and evaluated ex vivo in rat biodistribution studies. The CCKB antagonists were YF 476 and two other compounds of the basic 3-ureido-1,4-benzodiazepine class. Following tail-vein administration of [11C]-YF 476 exceedingly low levels of radioactivity were found in all brain regions from 5 to 60 min post-injection. Similar results were obtained using the other two 11C-labelled CCKB antagonists. In light of the very poor brain penetration of these compounds, reports on the central nervous system activity of this class of CCKB antagonists should be viewed with caution.
Assuntos
Benzodiazepinonas/farmacocinética , Radioisótopos de Carbono , Antagonistas de Hormônios/farmacocinética , Marcação por Isótopo , Compostos de Fenilureia/farmacocinética , Receptores da Colecistocinina/antagonistas & inibidores , Animais , Benzodiazepinonas/sangue , Benzodiazepinonas/química , Encéfalo/metabolismo , Antagonistas de Hormônios/sangue , Antagonistas de Hormônios/química , Cinética , Masculino , Compostos de Fenilureia/sangue , Compostos de Fenilureia/química , Ratos , Ratos Sprague-Dawley , Distribuição TecidualRESUMO
Log P measurements are a fundamental physico-chemical parameter and one of the cornerstones of structure activity relationships in medicinal chemistry. Despite the attractiveness of the method, the use of counting techniques to measure the log P of lipophilic radiotracers is fraught with pitfalls due to the amplifying effects of small quantities of radioactive impurities. For example, a radiotracer with a log P of 4 containing only 0.1% of a radioactive impurity with a log P of -1 will have an apparent log P of 3 if measured using conventional shake-flask partition techniques, counting the radioactivity in each phase. However, pre-washing the radiotracer-containing organic phase with aqueous phase can, in many cases, allay doubts about the validity of such measurements.
Assuntos
Radioisótopos/química , 1-Octanol , Animais , Fenômenos Químicos , Físico-Química , Contaminação de Medicamentos , Humanos , Lipídeos/química , Relação Quantitativa Estrutura-Atividade , Radioisótopos/isolamento & purificação , Contagem de Cintilação , Solventes , ÁguaRESUMO
This paper summarizes our work on WAY-100635 and analogues, labeled either with carbon-11 or fluorine-18, as potential radioligands for the 5-hydroxytryptamine(1A) (5-HT(1A)) neuroreceptors. Other facets of our work include: (1) human studies with [O-methyl-(11)C]WAY-100634, the major radioactive metabolite of [O-methyl-(11)C]WAY-100635, and with [(11)C]CPC 222; (2) use of a human liver metabolism model to screen in vitro potential metabolic problems in humans; (3) modification of the "dry method" to prepare [carbonyl-(11)C]WAY-100635; and (4) validation studies in humans with [carbonyl-(11)C]WAY-100635.
Assuntos
Piperazinas/metabolismo , Piridinas/metabolismo , Receptores de Serotonina/análise , Antagonistas da Serotonina/metabolismo , Tomografia Computadorizada de Emissão , Animais , Humanos , Fígado/metabolismo , Piperidinas/metabolismo , Receptores 5-HT1 de SerotoninaRESUMO
The active enantiomer R-SKF 82957 was labeled with 11C by N-[11C]methylation of the full dopamine (D1) agonist R-SKF 81297, using [11C]methyl iodide in the presence of N-ethyldiisopropylamine, in high specific activity, radiochemical purity and yields. Compared with the D1 agonist R/S-[11C]SKF 82957, R-[11C]SKF 82957 showed higher binding in the D1 rich regions, such as striatum and olfactory tubercles (approximately 1.7 times), thereby improving the tissue contrast. R-[11C]SKF 82957 exhibited high in vivo binding selectivity for D1 receptors in rats, because only high doses of D1 competitors, but not D2 or serotonin (5-HT2) blockers, significantly reduced the radioactivity levels in all brain areas. No labeled metabolites were detected in rat brain. These results indicate that R-[11C]SKF 82957 will provide more sensitive measurements of D1 receptors in in vivo studies than the racemic mixture.
Assuntos
Benzazepinas/síntese química , Benzazepinas/farmacocinética , Encéfalo/metabolismo , Radioisótopos de Carbono/farmacocinética , Agonistas de Dopamina/síntese química , Agonistas de Dopamina/farmacocinética , Receptores de Dopamina D1/agonistas , Animais , Biotransformação , Feminino , Humanos , Masculino , Conformação Molecular , Ratos , Ratos Sprague-Dawley , Estereoisomerismo , Distribuição TecidualRESUMO
The phosphodiesterase type IV (PDEIV) family of enzymes contributes to the metabolism of cAMP formed by the stimulation of beta-adrenergic, A2-adenosine, and H2-histamine receptors in the brain. Disturbances in cAMP-mediated signaling have been implicated in several neuropsychiatric disorders, and there is evidence that increasing cAMP levels through PDEIV inhibition improves the symptoms of these disorders. In the present study, the selective PDEIV inhibitors rolipram and Ro 20-1724, labeled with C-11, were evaluated in vivo in rats, as potential radioligands for imaging PDEIV enzymes with positron emission tomography (PET). Biodistribution experiments revealed a greater than threefold increased regional brain retention of [11C]rolipram as compared to [11C]Ro 20-1724. [nC]Rolipram uptake was higher in rat brain areas (e.g., cortical regions and olfactory system) showing higher expression of PDEIV enzymes, as determined previously using [3H]rolipram autoradiography or molecular genetic techniques. Binding of [n1C]rolipram and [11C]Ro 20-1724 was specific, since coadministration of high doses of unlabeled rolipram (10 mg/Kg, i.v.) or Ro 20-1724 (30 mg/Kg with [11C]rolipram and 10 mg/Kg with [11C]Ro 20-1724, i.v.) reduced radioactivity uptake in brain regions. Pretreatment with high doses of the PDEI selective inhibitor vinpocetine (10 mg/Kg, i.p., 15 min prior), or the noradrenaline reuptake inhibitor desipramine (10 mg/Kg, i.p., 30 min prior), or coinjection with the adenylyl cyclase activator forskolin (6.5 or 15 mg/Kg, i.v.), did not inhibit [11C]rolipram uptake in brain areas, suggesting binding selectivity for PDEIV enzymes. We conclude that [11C]rolipram, but not [11C]Ro 20-1724, is a promising radioligand for imaging the PDEIV enzymes with PET.
Assuntos
3',5'-AMP Cíclico Fosfodiesterases/metabolismo , 4-(3-Butoxi-4-metoxibenzil)-2-imidazolidinona/metabolismo , Inibidores de Fosfodiesterase/metabolismo , Pirrolidinonas/metabolismo , Tomografia Computadorizada de Emissão , Animais , Radioisótopos de Carbono , Masculino , Ensaio Radioligante , Ratos , Ratos Sprague-Dawley , Rolipram , Especificidade por Substrato , Distribuição TecidualRESUMO
Analogues of the potent and selective 5-HT1A ligand, WAY 100635, were synthesized and examined as potential candidates for imaging 5-HT1A receptors by positron emission tomography (PET). Several of the analogues displayed nanomolar affinity for the 5-HT1A receptor, comparable to WAY 100635. Three of these were examined in a model of human liver metabolism vis-à-vis WAY 100635. All showed a markedly lower propensity for amide hydrolysis than WAY 100635. Radiolabelling of these three potential PET radiotracers with carbon-11 was readily achieved from [11C]-iodomethane, and the newly synthesized radioligands were tested in vivo in rats for binding to 5-HT1A receptors. Whereas two of the ligands failed to bind to 5-HT1A receptors in vivo, one was successful. The latter, [11C]-7 [4-(2'-methoxyphenyl)-1-[2'-[N-(2'-pyridinyl)-2-bicyclo[2.2.2]octanec arboxamido]ethyl]-piperazine], showed good brain penetration, hippocampal:cerebellar ratios of 10:1 at 45 min postinjection. Blocking studies with a variety of drugs demonstrated that the binding of [11C]-7 in vivo was selective for 5-HT1A receptors. [11C]-7 is a promising candidate as a ligand for imaging 5-HT1A receptors by PET.
Assuntos
Encéfalo/metabolismo , Fígado/metabolismo , Piperazinas/síntese química , Piridinas/síntese química , Compostos Radiofarmacêuticos/síntese química , Receptores de Serotonina/análise , Tomografia Computadorizada de Emissão , Animais , Barreira Hematoencefálica , Radioisótopos de Carbono , Humanos , Marcação por Isótopo , Ligantes , Masculino , Piperazinas/química , Piperazinas/metabolismo , Piperazinas/farmacologia , Piridinas/química , Piridinas/metabolismo , Piridinas/farmacologia , Compostos Radiofarmacêuticos/química , Compostos Radiofarmacêuticos/metabolismo , Compostos Radiofarmacêuticos/farmacologia , Ratos , Ratos Sprague-Dawley , Receptores de Serotonina/metabolismo , Receptores 5-HT1 de Serotonina , Antagonistas da Serotonina/síntese química , Antagonistas da Serotonina/química , Antagonistas da Serotonina/metabolismo , Antagonistas da Serotonina/farmacologiaRESUMO
The novel radioligand [11C]CPC-222 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)-2-bicyclo [2,2,2]octane carboxamide) was evaluated as an in vivo probe of the 5-HT1A receptors using positron emission tomography (PET). Three human volunteers were imaged with PET over a 90 min period following intravenous injection of the radioligand. There was a high accumulation of the radioligand in brain regions with a high density of 5-HT1A receptors. The peak cortical concentration was 1.0-2.5% of the injected dose per litre. The ratio of radioactivity in receptor-rich regions to that of the cerebellum reached a plateau of 2.5-4.0 by 45 min after injection. Analysis of the plasma revealed no detectable amount of the potential metabolite, radiolabelled WAY-100634. This new radioligand has suitable properties to study the 5-HT1A receptors in man with PET.
Assuntos
Encéfalo/metabolismo , Radioisótopos de Carbono , Piperidinas , Piridinas , Receptores de Serotonina/análise , Adulto , Encéfalo/diagnóstico por imagem , Radioisótopos de Carbono/farmacocinética , Cerebelo/diagnóstico por imagem , Cerebelo/metabolismo , Córtex Cerebral/diagnóstico por imagem , Córtex Cerebral/metabolismo , Humanos , Cinética , Masculino , Piperidinas/farmacocinética , Piridinas/farmacocinética , Compostos Radiofarmacêuticos/farmacocinética , Receptores 5-HT1 de Serotonina , Valores de Referência , Fatores de Tempo , Distribuição Tecidual , Tomografia Computadorizada de Emissão/métodosRESUMO
Two analogues of the potent 5-HT1A antagonist WAY 100635 have been synthesized and radiolabelled with 18F, namely N-[2-[4-(2-2'-[18F] fluoroethoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohe xan e carboxamide ([18F]FEC) and N-[2-[4-(2-3'-[18F] fluoropropoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cycloh exa ne carboxamide ([18F]FPC). Biodistribution studies in rats showed selective uptake of both radiotracers in regions known to be rich in 5-HT1A receptors following i.v. injection. The ratio of radioactivity in hippocampus to that in the cerebellum was 5.5 (for [18F]FEC) and 7.5 (for [18F]FPC) at 60 min postinjection. Regional brain heterogeneity of radioactivity could be abolished by pretreatment with WAY 100635 and FPC but was unaffected by pretreatment with a variety of drugs including ketanserin, sulpiride, and SCH 23390. These results are compared vis-a-vis with those obtained using [11C]WAY 100635 to evaluate [18F]FEC and [18F]FPC as potential radiotracers for imaging 5-HT1A receptors by positron emission tomography.
Assuntos
Radioisótopos de Flúor/farmacocinética , Piperidinas/síntese química , Piperidinas/farmacocinética , Piridinas/síntese química , Piridinas/farmacocinética , Receptores de Serotonina/efeitos dos fármacos , Antagonistas da Serotonina/síntese química , Antagonistas da Serotonina/farmacocinética , Animais , Radioisótopos de Flúor/química , Marcação por Isótopo/métodos , Masculino , Radioquímica/métodos , Ratos , Ratos Sprague-Dawley , Receptores de Serotonina/metabolismo , Receptores 5-HT1 de Serotonina , Relação Estrutura-Atividade , Distribuição TecidualRESUMO
The high affinity benzazepine D1 agonists SKF 75670 and SKF 82957 were labeled with 11C by N-[11C]methylation of SKF 38393 and SKF 81297, respectively, using [11C]methyl iodide in the presence of N-ethyldiisopropylamine. Both radiotracers were purified using a semi-preparative cation exchange HPLC column. Radiochemical yields of 20-75% were obtained (from [11C]methyl iodide, decay-corrected) with a synthesis time of 30-35 min from EOB. The specific activities were 700-2500 Ci/mmol (25.9-92.5 GBq/mumol) at EOS, and the radiochemical purities were > 99%. Autoradiographic studies showed selective binding for both tracers in rat brain regions rich in D1 receptors such as the caudate-putamen, nucleus accumbens, olfactory tubercles and substantia nigra. [11C]SKF 75670 and [11C]SKF 82957 are thus potential PET radioligands for the functional high-affinity state of D1 receptors.
Assuntos
Benzazepinas , Radioisótopos de Carbono , Agonistas de Dopamina , Receptores de Dopamina D1/agonistas , Tomografia Computadorizada de Emissão/métodos , Animais , Autorradiografia , Benzazepinas/síntese química , Benzazepinas/farmacocinética , Sítios de Ligação , Encéfalo/diagnóstico por imagem , Encéfalo/metabolismo , Agonistas de Dopamina/síntese química , Agonistas de Dopamina/farmacocinética , Técnicas In Vitro , Ligantes , RatosRESUMO
This study assessed the modulatory effect of a serotonergic agonist, d-fenfluramine, on localized neuronal firing as indexed by changes in regional cerebral blood flow (rCBF). Previously, we reported the effect of oral d, l-fenfluramine on neuronal activity as measured by change in [18F]fluorodeoxyglucose uptake. Improvements in the current study include: a more specific serotonin agonist, d-fenfluramine; a more reliable administration route, intravenous; and a one session paradigm made possible with the radiotracer [15O]H2O. Changes in relative rCBF (P < 0.001) were observed: increases within the frontal cortex bilaterally and decreases within the temporal cortex bilaterally, and left thalamus. Other significant findings were elevated cortisol and growth hormone; increased euphoria and panic symptoms and decreased tiredness. These results support further investigation with intravenous d-fenfluramine to study the net functional effects of serotonergic stimulation in health and illness.
Assuntos
Comportamento/efeitos dos fármacos , Fenfluramina/farmacologia , Lobo Frontal/efeitos dos fármacos , Serotoninérgicos/farmacologia , Lobo Temporal/efeitos dos fármacos , Adulto , Animais , Feminino , Lobo Frontal/irrigação sanguínea , Lobo Frontal/diagnóstico por imagem , Hormônio do Crescimento/sangue , Humanos , Hidrocortisona/sangue , Radioisótopos de Oxigênio , Lobo Temporal/irrigação sanguínea , Lobo Temporal/diagnóstico por imagem , Tomografia Computadorizada de EmissãoRESUMO
Four analogues of the potent dopamine transporter ligand, WIN 35,428, were radiolabelled with 11C and 18F at the 2-beta-carboxy position for evaluation as potential ligands for imaging dopamine uptake sites by positron emission tomography (PET) namely, methyl (1R-2-exo-3-exo)-8- methyl-3-(4-methylphenyl)-8-azabicyclo[3.2.1]octane-2-carboxylate (RTI-32), its 4-chlorophenyl analogue (RTI-31), 2'-fluoroethyl (1R-2-exo-3-exo)-8-methyl-3-(4-methylphenyl)-8-azabicyclo[3.2.1]octane-2 - carboxylate (FETT) and its 4-chlorophenyl analogue (FECT). Upon intravenous injection in rats, all four radiotracers displayed preferential accumulation of radioactivity in regions known to contain high concentrations of dopamine uptake sites. Competition studies with two of the analogues, [11C]RTI-32 and [18F]FETT, demonstrated that, for both radiotracers, binding was saturable and displayed the appropriate pharmacology as potential PET ligands for imaging the dopamine transporter. Striatum to cerebellar ratios for [11C]RTI-32 (at 90 min post-injection) and [18F]FETT (at 120 min post-injection) were 27 and 21, respectively.
Assuntos
Proteínas de Transporte/metabolismo , Cocaína/análogos & derivados , Glicoproteínas de Membrana , Proteínas de Membrana Transportadoras , Proteínas do Tecido Nervoso , Animais , Radioisótopos de Carbono , Cocaína/farmacocinética , Proteínas da Membrana Plasmática de Transporte de Dopamina , Radioisótopos de Flúor , Injeções Intravenosas , Ligantes , Masculino , Ratos , Ratos Sprague-Dawley , Distribuição Tecidual , Tomografia Computadorizada de EmissãoRESUMO
The high affinity benzazepine D1 agonists SKF 75670 and SKF 82957 labeled with C-11 were evaluated in vivo in rats as potential radioligands for imaging dopamine D1 receptors with positron emission tomography (PET). Their in vivo pharmacological profile revealed selective binding for both tracers in rat brain regions rich in D1 receptors such as the caudate-putamen. The more lipophilic [11C]SKF 82957 (6-chloro-[11C]SKF 75670) showed a higher brain uptake (more than 2-fold up to 30 min), higher specific uptake in the striatum and higher signal-to-noise ratio (striatum-to-cerebellum = 3.2 +/- 0.4 for [11C]SKF 75670 and 9.7 +/- 2.5 for [11C]SKF 82957 at 60 min post-injection) as compared to [11C]SKF 75670. Both radiotracers exhibited high specificity and selectivity for D1 receptors, since only D1 competitors but not the D2 antagonist sulpiride or the 5-HT2 antagonist ritanserin reduced significantly their binding the striatum with [11C]SKF 75670 or the striatum and olfactory tubercles with [11C]SKF 82957. Previous reports have shown that only D1 agonists can recognize the functional high-affinity state from the low-affinity state of D1 receptors. [11C]SKF 75670 and especially [11C]SKF 82957 are D1 agonist radioligands that can potentially be used to study in vivo the functional high-affinity state of D1 receptors using PET.
Assuntos
Benzazepinas/metabolismo , Encéfalo/metabolismo , Agonistas de Dopamina/metabolismo , Receptores de Dopamina D1/metabolismo , Animais , Autorradiografia , Benzazepinas/farmacocinética , Radioisótopos de Carbono , Agonistas de Dopamina/farmacocinética , Masculino , Ensaio Radioligante , Ratos , Ratos Sprague-Dawley , Distribuição TecidualRESUMO
UNLABELLED: In the search for an in vivo marker of monoamine nerve terminal integrity, we investigated methoxytetrabenazine (MTBZ) as a tracer of the brain synaptic vesicular monoamine transporter (VMAT2). METHODS: The biodistribution, metabolism and in vivo specificity of MTBZ binding were first evaluated in rodents and the human dosimetry was estimated. Subsequently, the human brain distribution of VMAT2 binding was determined in normal volunteers following administration of [11C]MTBZ. Brain regional time-activity curves were obtained, and parametric transport and binding images were calculated using arterial blood sampling and a two-compartment tracer kinetic model. RESULTS: Regional rat brain localization of [3H]MTBZ 15 min postinjection was consistent with the known monoamine nerve terminal density, which demonstrated the highest activity in the striatum, lateral septum, substantia nigra pars compacta, the raphe nuclei and the locus coeruleus. At this time, chromatography revealed over 82% of brain activity, but less than 47% of plasma activity corresponded to authentic MTBZ. In vivo [11C]MTBZ binding in the mouse brain was inhibited by coinjection of excess unlabeled dihydrotetrabenazine. In humans [11C]MTBZ had high initial brain uptake and rapid clearance from all regions, with longest retention in areas of high VMAT2 concentration. Parametric quantification of VMAT2 density revealed the highest distribution volume in the putamen and caudate with lower values in cerebral cortex and cerebellum. CONCLUSION: Carbon-11-MTBZ is a suitable ligand for PET quantification of the vesicular monoamine transporter in the human brain.
Assuntos
Encéfalo/diagnóstico por imagem , Glicoproteínas/análise , Glicoproteínas de Membrana , Proteínas de Membrana Transportadoras , Neuropeptídeos , Neurotransmissores/análise , Tetrabenazina/análogos & derivados , Tomografia Computadorizada de Emissão , Adulto , Animais , Transporte Biológico , Química Encefálica , Radioisótopos de Carbono , Feminino , Humanos , Ligantes , Masculino , Camundongos , Ratos , Ratos Sprague-Dawley , Distribuição Tecidual , Proteínas Vesiculares de Transporte de Aminas Biogênicas , Proteínas Vesiculares de Transporte de MonoaminaRESUMO
Two fluorinated congeners of cocaine, 2'-fluoroethyl (1R-2-exo-3-exo)-8-methyl-3-(4-methylphenyl)-8-azabicyclo[3.2.1]octane- 2-carboxylate (FETT) and its 4-chlorophenyl analogue (FECT) were synthesized. Radiolabelling with 18F was achieved by O-[18F]fluoroalkylation of the corresponding carboxylic acid salts with distilled 2-[18F]fluoroethyl bromide in DMF. After HPLC purification, yields of radiochemically pure, formulated products were 22-30% (not corrected for decay) in a synthesis time of 60-70 min. The use of distilled 2-[18F]fluoroethyl bromide was indispensable for the reliable production of pure products.
Assuntos
Cocaína/análogos & derivados , Radioisótopos de Flúor , Fluorocarbonos , Transporte Biológico , Cocaína/síntese química , Dopamina/metabolismo , Humanos , Indicadores e Reagentes , Marcação por Isótopo/métodos , Modelos Moleculares , Estrutura Molecular , Tomografia Computadorizada de EmissãoRESUMO
[11C]Tetrabenazine ([11C]TBZ) is a new in vivo radioligand for positron emission tomographic (PET) imaging of vesicular monoamine transporters. The in vivo distribution, metabolism and pharmacological specificity of [11C]TBZ has been determined in rodents. Regional mouse brain retention of [11C]TBZ is highest in brain regions with greatest monoaminergic innervation (striatum, hypothalamus) and can be reduced with ligands for the monoamine vesicular transporter (TBZ, ketanserin) but not haloperidol, a dopamine D2 receptor antagonist. Chromatographic analysis of rat blood demonstrated rapid metabolism of [11C]TBZ to radiolabeled metabolites (alpha- and beta -[11C]dihydrotetrabenazine) resulting from reduction of the 2-keto group. These metabolites, as well as a third potential metabolite, 9-O-desmethylTBZ, have been synthesized in unlabeled form and all three were shown to be capable of greatly reducing in vivo accumulation of [11C]TBZ in mouse striatum and hypothalamus. Whole body biodistribution of radioactivity after [11C]TBZ injection was determined in rats, and the data used to calculate the expected human dosimetry from this radiotracer. These studies demonstrated that [11C]TBZ can be safely administered for in vivo PET imaging and semi-quantitative determination of vesicular monoamine transporters in living human brain, but quantitative pharmacokinetic modeling of radioactivity distribution will be complicated by the presence of pharmacologically active metabolites.
Assuntos
Monoaminas Biogênicas/metabolismo , Glicoproteínas de Membrana/metabolismo , Tetrabenazina/metabolismo , Absorção , Animais , Ligação Competitiva , Transporte Biológico/fisiologia , Radioisótopos de Carbono , Feminino , Humanos , Masculino , Camundongos , Degeneração Neural/fisiologia , Doses de Radiação , Ensaio Radioligante , Ratos , Ratos Sprague-Dawley , Distribuição TecidualRESUMO
The synthesis of [11C]TBZOMe, a [11C]methoxy derivative at the 2-hydroxy position of alpha-dihydrotetrabenazine, was carried out by an O-[11C]methylation reaction. The product [11C]TBZOMe (100-200 mCi) was obtained in 15-40% radiochemical yield (corrected for decay) within 37 min, and in high specific activity (2000-2500 Ci/mmol) and radiochemical purity (> 97%). [11C]TBZOMe is a potential new radioligand for studying the vesicular monoamine transporter using positron emission tomography.
Assuntos
Glicoproteínas/análise , Glicoproteínas de Membrana , Proteínas de Membrana Transportadoras , Neuropeptídeos , Neurotransmissores/análise , Tetrabenazina/análogos & derivados , Transporte Biológico , Radioisótopos de Carbono , Ensaio Radioligante , Tetrabenazina/síntese química , Tomografia Computadorizada de Emissão , Proteínas Vesiculares de Transporte de Aminas Biogênicas , Proteínas Vesiculares de Transporte de MonoaminaRESUMO
The first successful in vivo imaging of monoamine vesicular transporters in the living primate brain is described, using [11C]tetrabenazine ([11C]TBZ) and Positron Emission Tomography (PET). Radioligand uptake into brain is rapid, and at short time periods (10-30 minutes) the higher uptake and retention of the radiotracer in the more densely dopaminergic innervated striatum is clearly visualized. Specific binding in striatum can be entirely blocked with co-administration of a pharmacological dose (1 mg/kg i.v.) of tetrabenazine. In a unilaterally MPTP-lesioned monkey, specific binding of radioligand was absent in the striatum on the lesioned side, with no effect on radiotracer distribution in the cortex, cerebellum or contralateral striatum. PET imaging with [11C]TBZ provides a new approach to the in vivo study of monoaminergic neurons and their loss in neurodegenerative diseases.