RESUMO
Antagonism of the central opioid receptor like-1 receptor (ORL1) has been implicated in cognition, and has been a focus of drug discovery efforts to ameliorate the cognitive deficits that remain during the stable treatment of schizophrenia with current antipsychotics. In order to facilitate dose selection for phase II clinical testing an ORL1-specific PET tracer was developed to determine drug plasma concentration versus occupancy relationships in order to ensure that the doses selected and the degree of target engagement were sufficient to ensure adequate proof of concept testing. MK-0911 is a selective, high affinity antagonist for the ORL1 receptor radiolabeled with high specific activity (18)F for positron emission tomography (PET) studies. Evaluation of [(18)F]MK-0911 in rhesus monkey PET studies showed a pattern of brain uptake which was consistent with the known distribution of ORL1. In vitro autoradiography with [(18)F]MK-0911 in rhesus monkey and human brain tissue slices showed a regional distribution that was consistent with in vivo imaging results in monkey. Pre-treatment of rhesus monkeys with high doses of structurally diverse ORL1 antagonists MK-0584, MK-0337, or MK-5757 achieved blockade of [(18)F]MK-0911 in all gray matter regions. Baseline PET studies with [(18)F]MK-0911 in healthy human subjects showed tracer distribution and kinetics similar to that observed in rhesus monkey. Quantification of [(18)F]MK-0911 uptake in repeat human baseline PET studies showed a test-retest variability in volume of distribution (V(T)) averaging 3% across brain regions. Humans dosed orally with MK-5757 showed reduced [(18)F]MK-0911 tracer concentration in brain proportional with MK-5757 dose and plasma level. [(18)F]MK-0911 was useful for determining MK-5757-induced receptor occupancy of ORL1 to guide MK-5757 dose-selection for clinical proof-of-concept studies. Additionally, [(18)F]MK-0911 may be a useful tool for studying the pharmacology of ORL1 in various human populations and disease states.
Assuntos
Benzimidazóis/farmacocinética , Encéfalo/diagnóstico por imagem , Radioisótopos de Flúor/farmacocinética , Piperidinas/farmacocinética , Tomografia por Emissão de Pósitrons/métodos , Compostos Radiofarmacêuticos/farmacocinética , Receptores Opioides/metabolismo , Adulto , Animais , Benzimidazóis/química , Encéfalo/metabolismo , Radioisótopos de Flúor/química , Humanos , Macaca mulatta , Masculino , Pessoa de Meia-Idade , Piperidinas/química , Compostos Radiofarmacêuticos/química , Distribuição Tecidual , Adulto Jovem , Receptor de NociceptinaRESUMO
Two positron emission tomography radiotracers for the glycine transporter 1 (GlyT1) are reported here. Each radiotracer is a propylsulfonamide-containing benzamide and was labeled with either carbon-11 or fluorine-18. [¹¹C]CMPyPB was synthesized by the alkylation of a 3-hydroxypyridine precursor using [¹¹C]MeI, and [¹8F]MK-6577 was synthesized by a nucleophilic aromatic substitution reaction using a 2-chloropyridine precursor. Each tracer shows good uptake into rhesus monkey brain with the expected distribution of highest uptake in the pons, thalamus, and cerebellum and lower uptake in the striatum and gray matter of the frontal cortex. In vivo blockade and chase studies of [¹8F]MK-6577 showed a large specific signal and reversible binding. In vitro autoradiographic studies with [¹8F]MK-6577 showed a large specific signal in both rhesus monkey and human brain slices and a distribution consistent with the in vivo results and those reported in the literature. In vivo metabolism studies in rhesus monkeys demonstrated that only more-polar metabolites are formed for each tracer. Of these two tracers, [¹8F]MK-6577 was more extensively characterized and is a promising clinical positron emission tomography tracer for imaging GlyT1 and for measuring GlyT1 occupancy of therapeutic compounds.
Assuntos
Benzamidas/síntese química , Radioisótopos de Carbono , Radioisótopos de Flúor , Proteínas da Membrana Plasmática de Transporte de Glicina/sangue , Tomografia por Emissão de Pósitrons/métodos , Piridinas/síntese química , Sulfonamidas/síntese química , Animais , Benzamidas/sangue , Radioisótopos de Carbono/sangue , Linhagem Celular , Avaliação Pré-Clínica de Medicamentos/métodos , Radioisótopos de Flúor/sangue , Proteínas da Membrana Plasmática de Transporte de Glicina/metabolismo , Humanos , Macaca mulatta , Piridinas/sangue , Sulfonamidas/sangueRESUMO
Neuropeptide Y receptor subtype 1 (NPY Y1) has been implicated in appetite regulation, and antagonists of NPY Y1 are being explored as potential therapeutics for obesity. An NPY Y1 PET tracer is useful for determining the level of target engagement by NPY Y1 antagonists in preclinical and clinical studies. Here we report the synthesis and evaluation of [(18)F]Y1-973, a novel PET tracer for NPY Y1. [(18)F]Y1-973 was radiolabeled by reaction of a primary chloride with [(18)F]KF/K2.2.2 followed by deprotection with HCl. [(18)F]Y1-973 was produced with high radiochemical purity (>98%) and high specific activity (>1000 Ci/mmol). PET studies in rhesus monkey brain showed that the distribution of [(18)F]Y1-973 was consistent with the known NPY Y1 distribution; uptake was highest in the striatum and cortical regions and lowest in the pons, cerebellum nuclei, and brain stem. Blockade of [(18)F]Y1-973 uptake with NPY Y1 antagonist Y1-718 revealed a specific signal that was dose-dependently reduced in all regions of grey matter to a similarly low level of tracer uptake, indicative of an NPY Y1 specific signal. In vitro autoradiographic studies with [(18)F]Y1-973 in rhesus monkey and human brain tissue slices revealed an uptake distribution consistent with the in vivo PET studies. Highest binding density was observed in the dentate gyrus, caudate-putamen, and cortical regions; moderate binding density in the hypothalamus and thalamus; and lowest binding density in the globus pallidus and cerebellum. In vitro saturation binding studies in rhesus monkey and human caudate-putamen homogenates confirmed a similarly high B(max)/K(d) ratio for [(18)F]Y1-973, suggesting the tracer may provide a specific signal in human brain of similar magnitude to that observed in rhesus monkey. [(18)F]Y1-973 is a suitable PET tracer for imaging NPY Y1 in rhesus monkey with potential for translation to human PET studies.
Assuntos
Encéfalo/diagnóstico por imagem , Radioisótopos de Flúor/farmacocinética , Compostos Radiofarmacêuticos/síntese química , Compostos Radiofarmacêuticos/farmacocinética , Receptores de Neuropeptídeo Y/biossíntese , Animais , Autorradiografia , Humanos , Macaca mulatta , Tomografia por Emissão de Pósitrons , Traçadores RadioativosRESUMO
Two moderately lipophilic, high affinity ligands for metabotropic glutamate receptor subtype 1 (mGluR1) were radiolabeled with a positron-emitting radioisotope and evaluated in rhesus monkey as potential PET tracers. Both ligands were radiolabeled with fluorine-18 via nucleophilic displacement of the corresponding 2-chloropyridine precursor with [¹8F]potassium fluoride. [¹8F]MK-1312 was found to have a suitable signal for quantification of mGluR1 receptors in nonhuman primates and was more thoroughly characterized. In vitro autoradiographic studies with [¹8F]MK-1312 in rhesus monkey and human brain tissue slices revealed an uptake distribution consistent with the known distribution of mGluR1, with the highest uptake in the cerebellum, moderate uptake in the hippocampus, thalamus, and cortical regions, and lowest uptake in the caudate and putamen. In vitro saturation binding studies in rhesus monkey and human cerebellum homogenates confirmed that [¹8F]MK-1312 binds to a single site with a B(max) /K(d) ratio of 132 and 98, respectively. PET studies in rhesus monkey with [¹8F]MK-1312 showed high brain uptake and a regional distribution consistent with in vitro autoradiography results. Blockade of [¹8F]MK-1312 uptake with mGluR1 allosteric antagonist MK-5435 dose-dependently reduced tracer uptake in all regions of gray matter to a similarly low level of tracer uptake. This revealed a large specific signal useful for determination of mGluR1 receptor occupancy in rhesus monkey. Taken together, these results are promising for clinical PET studies with [¹8F]MK-1312 to determine mGluR1 occupancy of MK-5435.
Assuntos
Encéfalo/efeitos dos fármacos , Encéfalo/diagnóstico por imagem , Fármacos Atuantes sobre Aminoácidos Excitatórios , Tomografia por Emissão de Pósitrons , Receptores de Glutamato Metabotrópico/metabolismo , Animais , Autorradiografia/métodos , Sítios de Ligação/efeitos dos fármacos , Encéfalo/metabolismo , Mapeamento Encefálico , Relação Dose-Resposta a Droga , Fármacos Atuantes sobre Aminoácidos Excitatórios/síntese química , Fármacos Atuantes sobre Aminoácidos Excitatórios/química , Fármacos Atuantes sobre Aminoácidos Excitatórios/farmacocinética , Radioisótopos de Flúor/química , Radioisótopos de Flúor/farmacocinética , Humanos , Indóis/síntese química , Indóis/farmacocinética , Concentração Inibidora 50 , Ligantes , Macaca mulatta , Distribuição Tecidual , Triazóis/síntese química , Triazóis/farmacocinéticaRESUMO
Amalgamation of the structure-activity relationship of two series of GlyT1 inhibitors developed at Merck led to the discovery of a clinical candidate, compound 16 (DCCCyB), which demonstrated excellent in vivo occupancy of GlyT1 transporters in rhesus monkey as determined by displacement of a PET tracer ligand.
RESUMO
PURPOSE: Quantitative imaging of the type 1 cannabinoid receptor (CB1R) opens perspectives for many neurological and psychiatric disorders. We characterized the kinetics and reproducibility of the CB1R tracer [(18)F]MK-9470 in human brain. METHODS: [(18)F]MK-9470 data were analysed using reversible models and the distribution volume V (T) and V (ND) k (3) (V (ND) k (3) = K (1) k (2)) were estimated. Tracer binding was also evaluated using irreversible kinetics and the irreversible uptake constant K (i) and fractional uptake rate (FUR) were estimated. The effect of blood flow on these parameters was evaluated. Additionally, the possibility of determining the tracer plasma kinetics using a reduced number of blood samples was also examined. RESULTS: A reversible two-tissue compartment model using a global k (4) value was necessary to describe brain kinetics. Both V (T) and V (ND) k (3) were estimated satisfactorily and their test-retest variability was between 10% and 30%. Irreversible methods adequately described brain kinetics and FUR values were equivalent to K (i). The linear relationship between K (i) and V (ND) k (3) demonstrated that K (i) or FUR and thus the simple measure of tracer brain uptake provide CB1R availability information. The test-retest variability of K (i) and FUR was <10% and estimates were independent of blood flow. Brain uptake can be used as a receptor availability index, albeit at the expense of potential bias due to between-subject differences in tracer plasma kinetics. CONCLUSION: [(18)F]MK-9470 specific binding can be accurately determined using FUR values requiring a short scan 90 to 120 min after tracer administration. Our results suggest that [(18)F]MK-9470 plasma kinetics can be assessed using a few venous samples.
Assuntos
Encéfalo/metabolismo , Tomografia por Emissão de Pósitrons , Piridinas/metabolismo , Piridinas/farmacocinética , Receptor CB1 de Canabinoide/metabolismo , Adulto , Circulação Cerebrovascular , Estudos de Viabilidade , Feminino , Humanos , Cinética , Masculino , Pessoa de Meia-Idade , Modelos Biológicos , Piridinas/sangue , Traçadores Radioativos , Adulto JovemRESUMO
Abuse-liability-related effects of subtype-selective GABA(A) modulators were explored relative to the prototypic benzodiazepine lorazepam. 7-Cyclobutyl-6-(2-methyl-2H-1,2,4-triazol-3-ylmethoxy)-3-phenyl-1,2,4-triazolo[4,3-b]pyridazine (TPA123) has weak partial agonist efficacy at alpha(1)-, alpha(2)-, alpha(3)-, and alpha(5)-containing GABA(A) receptors, whereas 7-(1,1-dimethylethyl)-6-(2-ethyl-2H-1,2,4-triazol-3-ylmethoxy)-3-(2-fluorophenyl)-1,2,4-triazolo[4,3-b]pyridazine (TPA023) has weaker partial agonist efficacy at alpha(2) and alpha(3) and none at alpha(1) and alpha(5) subtypes. For both compounds, preclinical data suggested efficacy as nonsedating anxiolytics. Self-injection of TPA123 (0.0032-0.1 mg/kg) and TPA023 (0.0032-0.32 mg/kg) was compared with lorazepam (0.01-0.32 mg/kg) in baboons. TPA123 and lorazepam maintained self-injection higher than vehicle at two or more doses in each baboon; peak rate of self-injection of lorazepam was higher than TPA123. Self-injected lorazepam and TPA123 also increased rates of concurrently occurring food-maintained behavior. After the availability of self-administered TPA123 doses ended, an effect consistent with a mild benzodiazepine-like withdrawal syndrome occurred. In contrast with lorazepam and TPA123, TPA023 did not maintain self-administration. Positron emission tomography studies showed that TPA023 produced a dose-dependent inhibition in the binding of [(11)C]flumazenil to the benzodiazepine binding site in the baboon, which was essentially complete (i.e., 100% occupancy) at the highest TPA023 dose (0.32 mg/kg). In a physical dependence study, TPA023 (32 mg/kg/24 h) was delivered as a continuous intragastric drip. Neither flumazenil at 14 days nor stopping TPA023 after 30 to 31 days resulted in the marked withdrawal syndrome characteristic of benzodiazepines in baboons. In the context of other data, elimination of efficacy at the alpha(1) subtype of the GABA/benzodiazepine receptor is not sufficient to eliminate abuse liability but may do so when coupled with reduced alpha(2/3) subtype efficacy.
Assuntos
Agonistas GABAérgicos/farmacologia , Agonistas de Receptores de GABA-A , Transtornos Relacionados ao Uso de Substâncias/prevenção & controle , Animais , Comportamento Animal/efeitos dos fármacos , Sítios de Ligação , Encéfalo/efeitos dos fármacos , Encéfalo/metabolismo , Relação Dose-Resposta a Droga , Agonistas GABAérgicos/administração & dosagem , Agonistas GABAérgicos/efeitos adversos , Agonistas GABAérgicos/química , Infusões Intravenosas , Injeções Intramusculares , Ligantes , Lorazepam/administração & dosagem , Lorazepam/química , Lorazepam/farmacologia , Masculino , Estrutura Molecular , Papio , Tomografia por Emissão de Pósitrons , Ligação Proteica , Piridazinas/administração & dosagem , Piridazinas/efeitos adversos , Piridazinas/química , Piridazinas/farmacologia , Autoadministração , Relação Estrutura-Atividade , Transtornos Relacionados ao Uso de Substâncias/metabolismo , Triazóis/administração & dosagem , Triazóis/efeitos adversos , Triazóis/química , Triazóis/farmacologiaRESUMO
The GABA(A) receptor alpha2/alpha3 subtype-selective compound 7-(1,1-dimethylethyl)-6-(2-ethyl-2H-1,2,4-triazol-3-ylmethoxy)-3-(2-fluorophenyl)-1,2,4-triazolo[4,3-b]pyridazine (TPA023; also known as MK-0777) is a triazolopyridazine that has similar, subnanomolar affinity for the benzodiazepine binding site of alpha1-, alpha2-, alpha3-, and alpha5-containing GABA(A) receptors and has partial agonist efficacy at the alpha2 and alpha3 but not the alpha1 or alpha5 subtypes. The purpose of the present study was to define the relationship between plasma TPA023 concentrations and benzodiazepine binding site occupancy across species measured using various methods. Thus, occupancy was measured using either in vivo [(3)H]flumazenil binding or [(11)C]flumazenil small-animal positron emission tomography (microPET) in rats, [(123)I]iomazenil gamma-scintigraphy in rhesus monkeys, and [(11)C]flumazenil PET in baboons and humans. For each study, plasma-occupancy curves were derived, and the plasma concentration of TPA023 required to produce 50% occupancy (EC(50)) was calculated. The EC(50) values for rats, rhesus monkeys, and baboons were all similar and ranged from 19 to 30 ng/ml, although in humans, the EC(50) was slightly lower at 9 ng/ml. In humans, a single 2-mg dose of TPA023 produced in the region of 50 to 60% occupancy in the absence of overt sedative-like effects. Considering that nonselective full agonists are associated with sedation at occupancies of less than 30%, these data emphasize the relatively nonsedating nature of TPA023.
Assuntos
Agonistas GABAérgicos/farmacologia , Agonistas de Receptores de GABA-A , Piridazinas/farmacologia , Triazóis/farmacologia , Animais , Sítios de Ligação , Encéfalo/efeitos dos fármacos , Encéfalo/metabolismo , Relação Dose-Resposta a Droga , Flumazenil/farmacologia , Agonistas GABAérgicos/sangue , Antagonistas de Receptores de GABA-A , Humanos , Macaca mulatta , Papio , Tomografia por Emissão de Pósitrons , Ligação Proteica , Subunidades Proteicas , Piridazinas/sangue , Ensaio Radioligante , Ratos , Especificidade da Espécie , Distribuição Tecidual , Triazóis/sangueRESUMO
3-tert-Butyl-7-(5-methylisoxazol-3-yl)-2-(1-methyl-1H-1,2,4-triazol-5-ylmethoxy)-pyrazolo[1,5-d][1,2,4]triazine (MRK-016) is a pyrazolotriazine with an affinity of between 0.8 and 1.5 nM for the benzodiazepine binding site of native rat brain and recombinant human alpha1-, alpha2-, alpha3-, and alpha5-containing GABA(A) receptors. It has inverse agonist efficacy selective for the alpha5 subtype, and this alpha5 inverse agonism is greater than that of the prototypic alpha5-selective compound 3-(5-methylisoxazol-3-yl)-6-[(1-methyl-1,2,3-triazol-4-hdyl)methyloxy]-1,2,4-triazolo[3,4-a]phthalazine (alpha5IA). Consistent with its greater alpha5 inverse agonism, MRK-016 increased long-term potentiation in mouse hippocampal slices to a greater extent than alpha5IA. MRK-016 gave good receptor occupancy after oral dosing in rats, with the dose required to produce 50% occupancy being 0.39 mg/kg and a corresponding rat plasma EC(50) value of 15 ng/ml that was similar to the rhesus monkey plasma EC(50) value of 21 ng/ml obtained using [(11)C]flumazenil positron emission tomography. In normal rats, MRK-016 enhanced cognitive performance in the delayed matching-to-position version of the Morris water maze but was not anxiogenic, and in mice it was not proconvulsant and did not produce kindling. MRK-016 had a short half-life in rat, dog, and rhesus monkey (0.3-0.5 h) but had a much lower rate of turnover in human compared with rat, dog, or rhesus monkey hepatocytes. Accordingly, in human, MRK-016 had a longer half-life than in preclinical species ( approximately 3.5 h). Although it was well tolerated in young males, with a maximal tolerated single dose of 5 mg corresponding to an estimated occupancy in the region of 75%, MRK-016 was poorly tolerated in elderly subjects, even at a dose of 0.5 mg, which, along with its variable human pharmacokinetics, precluded its further development.
Assuntos
Agonistas GABAérgicos/farmacologia , Agonistas de Receptores de GABA-A , Isoxazóis/farmacologia , Triazinas/farmacologia , Animais , Ansiedade/psicologia , Comportamento Animal/efeitos dos fármacos , Convulsivantes/farmacologia , Cães , Relação Dose-Resposta a Droga , Estimulação Elétrica , Eletrofisiologia , Potenciais Pós-Sinápticos Excitadores/efeitos dos fármacos , Fibroblastos , Flumazenil/metabolismo , Agonistas GABAérgicos/metabolismo , Agonistas GABAérgicos/farmacocinética , Moduladores GABAérgicos/metabolismo , Hepatócitos/metabolismo , Hipocampo/efeitos dos fármacos , Hipocampo/metabolismo , Humanos , Macaca mulatta , Masculino , Aprendizagem em Labirinto/efeitos dos fármacos , Camundongos , Técnicas de Patch-Clamp , Equilíbrio Postural/efeitos dos fármacos , Ratos , Ratos Sprague-Dawley , Receptores de GABA-A/metabolismo , Adulto JovemRESUMO
Two histamine H3 receptor (H3R) inverse agonist PET tracers have been synthesized and characterized in preclinical studies. Each tracer has high affinity for the histamine H3 receptor, has suitable lipophilicity, and neither is a substrate for the P-glycoprotein efflux pump. A common phenolic precursor was used to synthesize each tracer with high specific activity and radiochemical purity by an alkylation reaction using either [(11)C]MeI or [(18)F]FCD(2)Br. Autoradiographic studies in rhesus monkey and human brain slices showed that each tracer had a widespread distribution with high binding densities in frontal cortex, globus pallidus and striatum, and lower uptake in cerebellum. The specificity of this expression pattern was demonstrated by the blockade of the autoradiographic signal by either the H3R agonist R-alpha-methylhistamine or a histamine H3R inverse agonist. In vivo PET imaging studies in rhesus monkey showed rapid uptake of each tracer into the brain with the same distribution seen in the autoradiographic studies. Each tracer could be blocked by pretreatment with a histamine H3R inverse agonist giving a good specific signal. Comparison of the in vitro metabolism of each compound showed slower metabolism in human liver microsomes than in rhesus monkey liver microsomes, with each compound having a similar clearance rate in humans. The in vivo metabolism of 1b in rhesus monkey showed that at 60 min, approximately 35% of the circulating counts were due to the parent. These tracers are very promising candidates as clinical PET tracers to both study the histamine H3R system and measure receptor occupancy of H3R therapeutic compounds.
Assuntos
Benzofuranos/farmacologia , Benzofuranos/farmacocinética , Encéfalo/metabolismo , Agonistas dos Receptores Histamínicos , Piperidinas/farmacologia , Piperidinas/farmacocinética , Tomografia por Emissão de Pósitrons/métodos , Compostos Radiofarmacêuticos/farmacologia , Receptores Histamínicos H3/metabolismo , Animais , Autorradiografia , Benzofuranos/sangue , Encéfalo/efeitos dos fármacos , Radioisótopos de Carbono , Agonismo Inverso de Drogas , Radioisótopos de Flúor , Agonistas dos Receptores Histamínicos/sangue , Agonistas dos Receptores Histamínicos/farmacocinética , Agonistas dos Receptores Histamínicos/farmacologia , Humanos , Marcação por Isótopo , Macaca mulatta , Imageamento por Ressonância Magnética , Metilistaminas/farmacologia , Microssomos Hepáticos/efeitos dos fármacos , Microssomos Hepáticos/metabolismo , Piperidinas/sangue , Compostos Radiofarmacêuticos/farmacocinéticaRESUMO
BACKGROUND AND PURPOSE: alpha5IA (3-(5-methylisoxazol-3-yl)-6-[(1-methyl-1,2,3-triazol-4-yl)methyloxy]-1,2,4-triazolo[3,4-a]phthalazine) is a triazolophthalazine with subnanomolar affinity for alpha1-, alpha2-, alpha3- and alpha5-containing GABA(A) receptors. Here we have evaluated the relationship between plasma alpha5IA concentrations and benzodiazepine binding site occupancy in rodents and primates (rhesus monkey). EXPERIMENTAL APPROACH: In awake rats, occupancy was measured at various times after oral dosing with alpha5IA (0.03-30 mgxkg(-1)) using an in vivo {[(3)H]flumazenil (8-fluoro 5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a][1,4]benzodiazepine-3-carboxylic acid ethyl ester)} binding assay. In anaesthetized rhesus monkeys, occupancy was measured using {[(123)I]iomazenil (ethyl 5,6-dihydro-7-iodo-5-methyl-6-oxo-4H-imidazo[1,5-a][1,4]benzodiazepine-3-carboxylic acid ethyl ester)} gamma-scintigraphy and a bolus/infusion paradigm. In both rat and rhesus monkey, the plasma drug concentration corresponding to 50% occupancy (EC(50)) was calculated. KEY RESULTS: In rats, alpha5IA occupancy was dose- and time-dependent with maximum occupancy occurring within the first 2 h. However, rat plasma EC(50) was time-independent, ranging from 42 to 67 ngxmL(-1) over a 24 h time course with the average being 52 ngxmL(-1) (i.e. occupancy decreased as plasma drug concentrations fell). In rhesus monkeys, the EC(50) for alpha5IA displacing steady-state [(123)I]iomazenil binding was 57 ngxmL(-1). CONCLUSIONS AND IMPLICATIONS: Rat plasma EC(50) values did not vary as a function of time indicating that alpha5IA dissociates readily for the GABA(A) receptor in vivo. These data also suggest that despite the different assays used (terminal assays of [(3)H]flumazenil in vivo binding in rats and [(123)I]iomazenil gamma-scintigraphy in anaesthetized rhesus monkeys), these techniques produced similar plasma alpha5IA EC(50) values (52 and 57 ngxmL(-1) respectively) and that the plasma-occupancy relationship for alpha5IA translates across these two species.
Assuntos
Benzodiazepinas/metabolismo , Encéfalo/metabolismo , Piridinas/metabolismo , Receptores de GABA-A/metabolismo , Administração Oral , Animais , Benzodiazepinas/administração & dosagem , Benzodiazepinas/sangue , Benzodiazepinas/farmacocinética , Sítios de Ligação , Ligação Competitiva , Encéfalo/diagnóstico por imagem , Relação Dose-Resposta a Droga , Flumazenil/administração & dosagem , Flumazenil/análogos & derivados , Flumazenil/metabolismo , Antagonistas de Receptores de GABA-A , Infusões Intravenosas , Injeções Intravenosas , Radioisótopos do Iodo , Ligantes , Macaca mulatta , Masculino , Piridinas/administração & dosagem , Piridinas/sangue , Piridinas/farmacocinética , Ratos , Ratos Sprague-Dawley , Tomografia Computadorizada de Emissão de Fóton ÚnicoRESUMO
BACKGROUND: Positron emission tomography (PET) can localize and quantify neurokinin-1 (NK(1)) receptors in brain using the nonpeptide antagonist radioligand, [(18)F]SPA-RQ. We sought to determine if patients with panic disorder have altered density of NK(1) receptors in brain because of their history of recurrent panic attacks. We also sought to determine if a drug-induced panic attack releases substance P in brain, as measured by decreased binding of [(18)F]SPA-RQ. METHODS: Positron emission tomography scans with [(18)F]SPA-RQ were performed in 14 patients with panic disorder and 14 healthy subjects. Of these two groups, 7 patients and 10 healthy subjects were scanned twice, once at baseline and once after injection of doxapram, a drug that induces panic attacks. RESULTS: NK(1) receptor binding in patients (n = 14) compared with that in healthy subjects (n = 14) was significantly decreased by 12% to 21% in all brain regions. Doxapram effectively produced panic attacks in 6 of 7 patients with panic disorder but only 2 of 10 healthy subjects. Doxapram caused no significant change of [(18)F]SPA-RQ binding in either patients or healthy subjects. CONCLUSIONS: Although induction of a panic attack has no significant effect on [(18)F]SPA-RQ binding to NK(1) receptors, patients with panic disorder have widespread reduction of NK(1) receptor binding in brain.
Assuntos
Encéfalo/metabolismo , Transtorno de Pânico/metabolismo , Piperidinas , Receptores da Neurocinina-1/metabolismo , Tetrazóis , Adulto , Encéfalo/diagnóstico por imagem , Encéfalo/fisiopatologia , Mapeamento Encefálico , Feminino , Humanos , Masculino , Pessoa de Meia-Idade , Transtorno de Pânico/diagnóstico por imagem , Transtorno de Pânico/patologia , Transtorno de Pânico/fisiopatologia , Tomografia por Emissão de Pósitrons , Ligação Proteica/fisiologiaRESUMO
PURPOSE: The in vitro and in vivo evaluation of the selective, high affinity (human CB1 IC(50) 0.49 nM) inverse agonist CB1R tracer [(11)C]CB-119, a close analog of the previously disclosed [(18)F]MK-9470, was undertaken. PROCEDURES: [(11)C]CB-119 was synthesized with high specific activity by alkylation of a phenolic precursor with [(11)C]methyl iodide. In vitro autoradiographic studies using rhesus brain slices were carried out using [(3)H]CB-119, and in vivo imaging studies were carried out using [(11)C]CB-119 in rhesus monkeys under baseline and blocked conditions. RESULTS: Autoradiographic studies in rhesus brain showed the expected distribution pattern for CB1R with highest binding in the cerebral cortex, cerebellum, caudate/putamen, globus pallidus, substantia nigra, and hippocampus. Lower binding was seen in the posterior hypothalamus, ventral tegmental area, and periventricular gray area, and the lowest binding was in the thalamic nuclei. The binding of [(3)H]CB-119 was fully blocked by the addition of 10 microM CB-119. Rhesus positron emission tomography imaging studies showed very good brain uptake and a distribution pattern consistent with that seen in the autoradiographic studies. The kinetics of tracer uptake was slow. The brain uptake was blocked by pretreatment with taranabant, a CB1R inverse agonist. The specific signal (total/nonspecific) in rhesus putamen at 90 min was approximately 6:1. CONCLUSIONS: [(11)C]CB-119 is a suitable tracer for imaging central CB1 receptors.
Assuntos
Amidas/metabolismo , Encéfalo/diagnóstico por imagem , Encéfalo/metabolismo , Tomografia por Emissão de Pósitrons , Piridinas/metabolismo , Compostos Radiofarmacêuticos/metabolismo , Receptor CB1 de Canabinoide/metabolismo , Animais , Autorradiografia , Radioisótopos de Carbono , Ligantes , Macaca mulatta , Ligação Proteica , Receptor CB1 de Canabinoide/agonistas , Distribuição Tecidual , Trítio/químicaRESUMO
BACKGROUND: A novel, high-affinity (.7-2.0 nmol) compound that selectively activates the alpha2, alpha 3, and alpha 5 (but not alpha1) gamma-aminobutyric acid-A (GABA(A)) receptor subtypes, TPA023B (2',6-difluoro-5'-[3-(1-hydroxy-1-methylethyl) imidazo[1,2-b][1,2,4]triazin-7-yl][1,1'-biphenyl]-2-carbonitrile) was pharmacologically characterized and studied by means of positron emission tomography (PET) to determine dynamic occupancies of the benzodiazepine binding site of human brain GABA(A) receptors after a single oral dose. METHODS: Four healthy male volunteers were studied in a double-blind, randomized placebo-controlled study of which three were given a single dose of 1.5 mg TPA023B and the fourth received placebo. The time course of GABA(A) receptor occupancy was determined with multiple dynamic [(11)C]flumazenil PET studies at pre-dose baseline and 5 and 24 hours after dose. Arterial sampling and full kinetic modeling with a two-compartment model was used to calculate parametric maps of receptor availability (distribution volume V(T)) and of occupancy. RESULTS: The GABA(A) receptor occupancy as determined from [(11)C]flumazenil V(T) values in all brain regions was reduced homogeneously, on average by 52.5 +/- 1.2% after 5 hours and 46.4 +/- 6.0% after 24 hours. No serious adverse events were encountered in humans. CONCLUSIONS: Single oral doses of 1.5 mg of TPA023B correspond to average receptor occupancies in neocortical regions of 52% and 46% after 5 and 24 hours, respectively. Provided suitable ligands and quantification methods are available for the appropriate target, quantitative PET offers a unique tool for dynamic in vivo measurement of relevant on-site receptor occupancy.
Assuntos
Encéfalo/efeitos dos fármacos , Encéfalo/diagnóstico por imagem , Agonistas GABAérgicos/farmacologia , Agonistas GABAérgicos/farmacocinética , Tomografia por Emissão de Pósitrons , Piridazinas/farmacologia , Piridazinas/farmacocinética , Receptores de GABA-A/efeitos dos fármacos , Receptores de GABA-A/metabolismo , Triazóis/farmacologia , Triazóis/farmacocinética , Administração Oral , Adulto , Disponibilidade Biológica , Radioisótopos de Carbono , Método Duplo-Cego , Flumazenil , Humanos , Cinética , MasculinoRESUMO
UNLABELLED: The cannabinoid type-1 (CB1) receptor is one of the most abundant G-coupled protein receptors in the human body and is responsible for signal transduction of both endogenous and exogenous cannabinoids. The endocannabinoid system is strongly implicated in regulation of homeostasis and several neuropsychiatric disorders, obesity, and associated comorbidities, such as dyslipidemia and metabolic syndrome. We have used whole-body PET/CT to characterize the biodistribution and dosimetry of a novel high-affinity, subtype-selective radioligand, (18)F-MK-9470, in healthy male and female subjects. METHODS: Eight nonobese subjects (5 men, 3 women; age, 22-54 y) underwent serial whole-body PET/CT for 6 h after a bolus injection of 251 +/- 25 MBq (18)F-MK-9470 (N-[2-(3-cyano-phenyl)-3-(4-(2-(18)F-fluorethoxy)phenyl)-1-methylpropyl]-2-(5-methyl-2-pyridyloxy)-2-methylproponamide). Source organs were delineated 3-dimensionally using the combined morphologic and functional data. Residence times were derived from time-activity profiles using both the trapezoid rule and curve fitting. Individual organ doses and effective doses were determined using the OLINDA software package, with different approaches for gastrointestinal and urinary excretion modeling. RESULTS: (18)F-MK-9470 is taken up slowly in the brain, reaching a plateau at approximately 90-120 min after bolus injection and is excreted predominantly through the hepatobiliary system. The gallbladder, upper large intestine, small intestine, and liver are the organs with the highest absorbed dose (average: 159, 98, 87, and 86 microGy/MBq, respectively). The mean effective dose (ED) was 22.8 +/- 4.3 microSv/MBq, indicating relatively low intersubject variability and a mean value in the range of many commercially available (18)F-labeled radiopharmaceuticals. Brain uptake was relatively high compared with that of existing central nervous system ligands for other receptors, between 3.2% and 4.9% of the injected dose. CONCLUSION: The estimated radiation burden of (18)F-MK-9470 for PET CB1 receptor imaging shows relatively low variability between subjects and has an acceptable ED, which allows multiple serial cerebral scans of good image quality, while remaining within the risk category class II-b defined by the World Health Organization and the International Commission for Radiation Protection for a standard injected activity (185-370 MBq).
Assuntos
Encéfalo/diagnóstico por imagem , Encéfalo/metabolismo , Piridinas/farmacocinética , Receptor CB1 de Canabinoide/metabolismo , Contagem Corporal Total , Adulto , Carga Corporal (Radioterapia) , Feminino , Humanos , Masculino , Taxa de Depuração Metabólica , Pessoa de Meia-Idade , Especificidade de Órgãos , Doses de Radiação , Cintilografia , Compostos Radiofarmacêuticos/análise , Compostos Radiofarmacêuticos/farmacocinética , Valores de Referência , Eficiência Biológica Relativa , Distribuição TecidualRESUMO
Cannabinoid 1 receptor (CB1R) inverse agonists are emerging as a potential obesity therapy. However, the physiological mechanisms by which these agents modulate human energy balance are incompletely elucidated. Here, we describe a comprehensive clinical research study of taranabant, a structurally novel acyclic CB1R inverse agonist. Positron emission tomography imaging using the selective CB1R tracer [(18)F]MK-9470 confirmed central nervous system receptor occupancy levels ( approximately 10%-40%) associated with energy balance/weight-loss effects in animals. In a 12-week weight-loss study, taranabant induced statistically significant weight loss compared to placebo in obese subjects over the entire range of evaluated doses (0.5, 2, 4, and 6 mg once per day) (p < 0.001). Taranabant treatment was associated with dose-related increased incidence of clinical adverse events, including mild to moderate gastrointestinal and psychiatric effects. Mechanism-of-action studies suggest that engagement of the CB1R by taranabant leads to weight loss by reducing food intake and increasing energy expenditure and fat oxidation.
Assuntos
Amidas/farmacologia , Ingestão de Energia/efeitos dos fármacos , Metabolismo Energético/efeitos dos fármacos , Piridinas/farmacologia , Receptor CB1 de Canabinoide/agonistas , Redução de Peso/efeitos dos fármacos , Adulto , Idoso , Amidas/uso terapêutico , Encéfalo/efeitos dos fármacos , Encéfalo/metabolismo , Relação Dose-Resposta a Droga , Método Duplo-Cego , Gorduras/metabolismo , Humanos , Pessoa de Meia-Idade , Tomografia por Emissão de Pósitrons , Piridinas/uso terapêuticoRESUMO
The discovery of a structurally distinct cannabinoid-1 receptor (CB1R) positron emission tomography tracer is described. Starting from an acyclic amide CB1R inverse agonist (1) as the lead compound, an efficient route to introduce 18F to the molecule was developed. Further optimization focused on reducing the lipophilicity and increasing the CB1R affinity. These efforts led to the identification of [18F]-16 that exhibited good brain uptake and an excellent signal-to-noise ratio in rhesus monkeys.
Assuntos
Amidas/síntese química , Radioisótopos de Flúor , Piridinas/síntese química , Compostos Radiofarmacêuticos/síntese química , Receptor CB1 de Canabinoide/metabolismo , Amidas/química , Amidas/farmacocinética , Animais , Encéfalo/diagnóstico por imagem , Encéfalo/metabolismo , Células CHO , Cricetinae , Cricetulus , Humanos , Macaca mulatta , Tomografia por Emissão de Pósitrons , Piridinas/química , Piridinas/farmacocinética , Ensaio Radioligante , Compostos Radiofarmacêuticos/química , Compostos Radiofarmacêuticos/farmacocinética , Receptor CB1 de Canabinoide/agonistas , Receptor CB1 de Canabinoide/antagonistas & inibidores , Proteínas Recombinantes/metabolismo , Relação Estrutura-AtividadeRESUMO
SAR studies on amides, ureas, and vinylogous amides derived from pyrrolidine led to the discovery of several potent hNK(1) antagonists. One particular vinylogous amide (45b) had excellent potency, selectivity, pharmacokinetic profile, and functional activity in vivo. An in vivo rhesus macaque brain receptor occupancy PET study for compound 45b revealed an estimated Occ(90) approximately 300 ng/ml.
Assuntos
Antagonistas dos Receptores de Neurocinina-1 , Pirrolidinas/farmacologia , Administração Oral , Animais , Disponibilidade Biológica , Encéfalo/metabolismo , Humanos , Macaca mulatta , Pirrolidinas/farmacocinética , Especificidade da EspécieRESUMO
[(18)F]MK-9470 is a selective, high-affinity, inverse agonist (human IC(50), 0.7 nM) for the cannabinoid CB1 receptor (CB1R) that has been developed for use in human brain imaging. Autoradiographic studies in rhesus monkey brain showed that [(18)F]MK-9470 binding is aligned with the reported distribution of CB1 receptors with high specific binding in the cerebral cortex, cerebellum, caudate/putamen, globus pallidus, substantia nigra, and hippocampus. Positron emission tomography (PET) imaging studies in rhesus monkeys showed high brain uptake and a distribution pattern generally consistent with that seen in the autoradiographic studies. Uptake was blocked by pretreatment with a potent CB1 inverse agonist, MK-0364. The ratio of total to nonspecific binding in putamen was 4-5:1, indicative of a strong specific signal that was confirmed to be reversible via displacement studies with MK-0364. Baseline PET imaging studies in human research subject demonstrated behavior of [(18)F]MK-9470 very similar to that seen in monkeys, with very good test-retest variability (7%). Proof of concept studies in healthy young male human subjects showed that MK-0364, given orally, produced a dose-related reduction in [(18)F]MK-9470 binding reflecting CB1R receptor occupancy by the drug. Thus, [(18)F]MK-9470 has the potential to be a valuable, noninvasive research tool for the in vivo study of CB1R biology and pharmacology in a variety of neuropsychiatric disorders in humans. In addition, it allows demonstration of target engagement and noninvasive dose-occupancy studies to aid in dose selection for clinical trials of CB1R inverse agonists.
Assuntos
Encéfalo/anatomia & histologia , Tomografia por Emissão de Pósitrons/métodos , Traçadores Radioativos , Receptor CB1 de Canabinoide/ultraestrutura , Amidas/metabolismo , Animais , Autorradiografia , Encéfalo/metabolismo , Radioisótopos de Flúor , Humanos , Processamento de Imagem Assistida por Computador , Macaca mulatta , Masculino , Estrutura Molecular , Piridinas/metabolismo , Receptor CB1 de Canabinoide/metabolismoRESUMO
UNLABELLED: (18)F-Labeled substance P antagonist-receptor quantifier ([(18)F]SPA-RQ) [2-fluoromethoxy-5-(5-trifluoromethyl-tetrazol-1-yl)-benzyl]-[(2S,3S)-2-phenyl-piperidin-3-yl)amine] is a selective radioligand for in vivo quantification of tachykinin NK(1) receptors with PET. The aims of this study were to estimate the radiation safety profile and relative risks of [(18)F]SPA-RQ with 3 different methods of image analysis. METHODS: Whole-body PET images were acquired in 7 healthy subjects after injection of 192 +/- 7 MBq (5.2 +/- 0.2 mCi) [(18)F]SPA-RQ. Emission images were serially acquired at multiple time-points from 0 to 120 min and approximately 180-240 min after injection. Urine samples were collected after each imaging session and for 24 h after the last scan to measure excreted radioactivity. Horizontal tomographic images were compressed to varying degrees in the anteroposterior direction to create 3 datasets: thin-slice, bisected, and 2-dimensional (2D) planar images. Regions of interest were drawn around visually identifiable source organs to generate time-activity curves for each dataset. Residence times were determined from these curves, and doses to individual organs and the body as a whole were calculated using OLINDA/EXM 1.0. RESULTS: The lungs, upper large intestine wall, small intestine, urinary bladder wall, kidneys, and thyroid had the highest radiation-absorbed doses. Biexponential fitting of mean bladder and urine activity showed that about 41% of injected activity was excreted via urine. Assuming a 2.4-h urine voiding interval, the calculated effective doses from thin-slice, bisected, and 2D planar images were 29.5, 29.3, and 32.3 microSv/MBq (109, 108, and 120 mrem/mCi), respectively. CONCLUSION: Insofar as effective dose is an accurate measure of radiation risk, all 3 methods of analysis provided quite similar estimates of risk to human subjects. The radiation dose was moderate and would potentially allow subjects to receive multiple PET scans in a single year. Individual organ exposures varied among the 3 methods, especially for structures asymmetrically located in an anterior or posterior position. Bisected and 2D planar images almost always provided higher organ dose estimates than thin-slice images. Thus, either the bisected or 2D planar method of analysis appears acceptable for quantifying human radiation burden, at least for radioligands with a relatively broad distribution in the body and not concentrated in a small number of radiation sensitive organs.