RESUMEN
The suitability of an (123)I-labeled form of the putative D(4) receptor ligand L750,667 as a radiotracer for single photon emission computed tomography imaging was assessed in nonhuman primates. [(123)I]L750,667, labeled by iododestannylation, was administered to baboons in bolus and bolus plus constant infusion paradigms and imaged for 6 h. Total [(123)I]L750,667 brain uptake peaked (2.3% injected dose) at 15 min postinjection. [(123)I]L750,667 uptake was observed in all brain regions measured including diencephalon, brainstem, basal ganglia, cingulate cortex, and cerebellum, and slightly lower levels were noted in the frontal, parietal, temporoinsular, and occipital cortices. Administration of the D(4) receptor antagonist NGD 94-1 (2 mg/kg) did not displace radioactivity from any of the brain regions examined. Thus, while L750,667 is selective for the D(4) receptor in vitro, because brain [(123)I]L750,667 uptake was not displaced by NGD 94-1 at receptor saturating doses, [(123)I]L750,667 does not appear to be a suitable radiotracer for in vivo imaging of the D(4) receptor.
Asunto(s)
Encéfalo/metabolismo , Antagonistas de Dopamina/farmacocinética , Antagonistas de los Receptores de Dopamina D2 , Piridinas/farmacocinética , Pirroles/farmacocinética , Tomografía Computarizada de Emisión de Fotón Único/métodos , Animales , Ganglios Basales/diagnóstico por imagen , Ganglios Basales/metabolismo , Unión Competitiva/efectos de los fármacos , Encéfalo/diagnóstico por imagen , Tronco Encefálico/diagnóstico por imagen , Tronco Encefálico/metabolismo , Cerebelo/diagnóstico por imagen , Cerebelo/metabolismo , Lóbulo Frontal/diagnóstico por imagen , Lóbulo Frontal/metabolismo , Giro del Cíngulo/diagnóstico por imagen , Giro del Cíngulo/metabolismo , Imidazoles/farmacología , Radioisótopos de Yodo , Lóbulo Occipital/diagnóstico por imagen , Lóbulo Occipital/metabolismo , Especificidad de Órganos , Papio , Lóbulo Parietal/diagnóstico por imagen , Lóbulo Parietal/metabolismo , Piridinas/sangre , Pirimidinas/farmacología , Pirroles/sangre , Receptores de Dopamina D4 , Lóbulo Temporal/diagnóstico por imagen , Lóbulo Temporal/metabolismo , Distribución TisularRESUMEN
UNLABELLED: Nicotinic acetylcholine receptors (nAChRs) play an important role in tobacco dependence and a potential therapeutic role in neuropsychiatric disorders such as Alzheimer's disease. [123I]5-iodo-3-[2(S)-2-azetidinylmethoxy]pyridine (5-I-A-85380) is a new SPECT tracer that labels alpha4beta2 nAChRs. The purpose of this study was to assess the usefulness of this tracer to measure regional nAChR binding in baboon brain using both a bolus/kinetic paradigm and also a bolus plus constant infusion/equilibrium paradigm. METHODS: A pair of bolus/kinetic and bolus plus constant infusion/equilibrium studies was performed in each of 3 isoflurane-anesthetized baboons. Bolus studies were performed by intravenous injection of 191-226 MBq [123I]5-I-A-85380 and image acquisition for 289-367 min. The data were analyzed with 1- and 2-tissue compartment models. Bolus plus constant infusion/equilibrium studies were performed by a bolus injection (74-132 MBq) followed by a 468- to 495-min infusion with a bolus/infusion ratio (B/I) of 4.8-5.0 h. The distribution volumes in the thalamus were measured in these 2 paradigms. To study whether the cerebellum was appropriate as a receptor-poor region, displacement studies were done in 2 baboons using the B/I paradigm with subcutaneous injection of (-)-cytisine (0.8 and 1.0 mg/kg). RESULTS: The kinetics of this tracer was best described by the 1-tissue compartment model. The 2-compartment model showed poor identifiability of rate constants. The total (specific plus nondisplaceable compartments) distribution volumes (V(T)') agreed between bolus and B/I paradigms (average percentage difference in V(T)', 16.8%). (-)-Cytisine (0.8 and 1.0 mg/kg) displaced 70% and 72% of the radioactivity in the thalamus and 36% and 55% in the cerebellum, respectively, indicating that the latter was not appropriate as a receptor-poor region. CONCLUSION: These results show the feasibility of quantifying alpha4beta2 nAChRs using [123I]5-I-A-85380 and support the use of V(T)' as an appropriate outcome measure.
Asunto(s)
Azetidinas/farmacocinética , Encéfalo/metabolismo , Radioisótopos de Yodo/farmacocinética , Receptores Nicotínicos/metabolismo , Tomografía Computarizada de Emisión de Fotón Único/métodos , Alcaloides/farmacología , Animales , Azetidinas/administración & dosificación , Azocinas , Unión Competitiva , Encéfalo/diagnóstico por imagen , Cerebelo/diagnóstico por imagen , Cerebelo/metabolismo , Procesamiento de Imagen Asistido por Computador , Infusiones Intravenosas , Inyecciones Intravenosas , Radioisótopos de Yodo/administración & dosificación , Cinética , Análisis de los Mínimos Cuadrados , Imagen por Resonancia Magnética , Papio , Quinolizinas , Receptores Nicotínicos/análisis , Tálamo/diagnóstico por imagen , Tálamo/metabolismoRESUMEN
[123I]2beta-carbomethoxy-3beta-(4-iodophenyl)tropane ([123I]-CIT) and its isopropylester analog [123I]PCIT, both of which are phenyltropane derivatives of cocaine with high affinity for the dopamine (DA) transporter, were compared using single photon emission computed tomography in nonhuman primates. Although IPCIT is significantly more selective for the DA transporter than beta-CIT, striatal distribution volumes of specifically bound tracer were similar for both tracers. Compartmental modeling results were compared with a simple peak equilibrium method used previously by this group. The peak equilibrium method is shown to overestimate striatal distribution volumes, primarily due to a difference in the calculated time of peak specific uptake.
Asunto(s)
Encéfalo/metabolismo , Proteínas Portadoras/metabolismo , Cocaína/análogos & derivados , Glicoproteínas de Membrana , Proteínas de Transporte de Membrana , Proteínas del Tejido Nervioso , Animales , Encéfalo/diagnóstico por imagen , Cromatografía Líquida de Alta Presión , Cocaína/sangre , Cocaína/metabolismo , Cocaína/farmacocinética , Proteínas de Transporte de Dopamina a través de la Membrana Plasmática , Femenino , Semivida , Humanos , Inyecciones Intravenosas , Radioisótopos de Yodo , Ligandos , Tasa de Depuración Metabólica , Modelos Biológicos , Papio , Especificidad de la Especie , Factores de Tiempo , Distribución Tisular , Tomografía Computarizada de Emisión de Fotón ÚnicoRESUMEN
Previously we have shown that twelve weeks of repeated low-dose d-amphetamine (AMPH) exposure in rhesus monkeys induces a long-lasting enhancement of behavioral responses to acute low-dose challenge. The present study was designed to investigate the behavioral and neurochemical consequences of a six-week regimen of low-dose AMPH exposure (0.1-1.0 mg/kg, i.m., b.i.d.) in rhesus monkeys. SPECT imaging of AMPH's (0.4 mg/kg) ability to displace [123I]IBZM bound to D2 dopamine receptors in the striatum of saline control and AMPH-treated animals prior to and following chronic treatment was accomplished using a bolus/constant infusion paradigm. Following chronic AMPH treatment, all monkeys showed an enhanced behavioral response to acute AMPH challenge and a significant decrease in the percent of AMPH-induced displacement of [123I]IBZM in striatum compared to their pretreatment scans. These findings suggest that relatively small changes in presynaptic dopamine function may be reflected in significant alterations in the behavioral response to acute AMPH challenge.
Asunto(s)
Benzamidas/farmacocinética , Cerebelo/metabolismo , Cuerpo Estriado/metabolismo , Dextroanfetamina/farmacología , Antagonistas de Dopamina/farmacocinética , Dopamina/metabolismo , Pirrolidinas/farmacocinética , Receptores de Dopamina D2/metabolismo , Tomografía Computarizada de Emisión de Fotón Único , Animales , Cerebelo/diagnóstico por imagen , Cuerpo Estriado/diagnóstico por imagen , Dextroanfetamina/administración & dosificación , Esquema de Medicación , Femenino , Inyecciones Intravenosas , Radioisótopos de Yodo/farmacocinética , Macaca mulatta , Masculino , Actividad Motora/efectos de los fármacos , Receptores de Dopamina D2/análisis , Conducta Estereotipada/efectos de los fármacosRESUMEN
The competition between endogenous transmitters and radiolabeled ligands for in vivo binding to neuroreceptors might provide a method to measure endogenous transmitter release in the living human brain with noninvasive techniques such as positron emission tomography (PET) or single photon emission computerized tomography (SPECT). In this study, we validated the measure of amphetamine-induced dopamine release with SPECT in nonhuman primates. Microdialysis experiments were conducted to establish the dose-response curve of amphetamine-induced dopamine release and to document how pretreatment with the dopamine depleter alpha-methyl-para-tyrosine (alpha MPT) affects this response. SPECT experiments were performed with two iodinated benzamides, [123I]IBZM and [123I]IBF, under sustained equilibrium condition. Both radio-tracers are specific D2 antagonists, but the affinity of [123I]IBZM (KD-0.4 nM) is lower than that of [123I]IBF (KD 0.1 nM). With both tracers, we observed a prolonged reduction in binding to D2 receptors following amphetamine injection. [123I]IBZM binding to D2 receptors was more affected than [123I]IBF by high doses of amphetamine, indicating that a lower affinity increases the vulnerability of a tracer to endogenous competition. With [123I]IBZM, we observed an excellent correlation between reduction of D2 receptor binding measured with SPECT and peak dopamine release measured with microdialysis after various doses of amphetamine. Pretreatment with alpha MPT significantly reduced the effect of amphetamine on [123I]IBZM binding to D2 receptors, confirming that this effect was mediated by intrasynaptic dopamine release. Together, these results validate the use of this SPECT paradigm as a noninvasive measurement of intrasynaptic dopamine release in the living brain.
Asunto(s)
Anfetamina/farmacología , Encéfalo/efectos de los fármacos , Dopamina/metabolismo , Animales , Encéfalo/diagnóstico por imagen , Relación Dosis-Respuesta a Droga , Macaca , Masculino , Microdiálisis , Tomografía Computarizada de Emisión de Fotón ÚnicoRESUMEN
Transplantation of fetal dopamine neurons to the adult striatum potentially offers a means to reverse the striatal dopamine deficiency that characterizes Parkinson's disease. Many investigations in rodents have supported the hope that neural grafting may be a useful treatment for parkinsonism. However, clinical studies have generally produced more modest improvements in motor abnormalities than observed in lower species. It is possible that the number of fetal dopamine neurons that survive transplantation is insufficient to restore dopaminergic innervation of the large human striatum to a level where striking recovery is obtained. In fact, there has been no quantitative study of graft outgrowth to indicate what portion of the dopamine-depleted striatum might be reinnervated with present techniques. Furthermore, it has been speculated that regeneration of the host dopamine system in response to the implantation surgery may play an important role in the beneficial effects of neural grafting in primates. The present study used nine 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated parkinsonian monkeys to investigate these issues. Sham implantation procedures produced no increase in either dopamine transporter density (measured by quantitative autoradiography) or tissue dopamine concentration (measured by HPLC) in the striatum of MPTP-treated monkeys. In sham-grafted and nonimplanted MPTP-treated monkeys, the striatal dopamine concentration was reduced by 99%, based on analysis of 16 sampled sites in the caudate nucleus and putamen of each monkey. No behavioral recovery was seen in the sham-grafted and nonimplanted MPTP-treated groups. In contrast, transplantation of fetal dopamine neurons to the caudate nucleus or putamen of MPTP-treated monkeys resulted in a significant elevation of dopamine transporter density and dopamine levels in the grafted striatal nucleus. Each grafted MPTP-treated monkey received ventral mesencephalon dopamine neurons from one donor harvested during putative neurogenesis. Donor ventral mesencephalon was divided equally and implanted into six sites either in the caudate nucleus or putamen. One graft site in each monkey was examined by dopamine transporter autoradiography. In sections in which graft fibers were present, a mean of one-third of the volume of the grafted nucleus was occupied by an elevated density of dopamine transporters. This increase in dopamine transporter density was defined to be at least 5-10% of the control density. However, full behavioral recovery was not observed in the grafted MPTP-treated group. These data provide no support for the hypothesis that regeneration of the host dopamine system occurs in response to a sham implantation procedure in severely parkinsonian monkeys. The current study illustrates the power of the applied techniques for delineating the relationship between the level of host dopamine depletion, the extent of graft-induced dopaminergic restoration, and behavioral recovery.
Asunto(s)
1-Metil-4-fenil-1,2,3,6-Tetrahidropiridina/efectos adversos , Trasplante de Tejido Encefálico , Proteínas Portadoras/análisis , Trasplante de Tejido Fetal , Glicoproteínas de Membrana , Proteínas de Transporte de Membrana , Mesencéfalo/trasplante , Neostriado/química , Enfermedad de Parkinson Secundaria/inducido químicamente , Animales , Autorradiografía , Chlorocebus aethiops , Cocaína/análogos & derivados , Cocaína/metabolismo , Dopamina/metabolismo , Proteínas de Transporte de Dopamina a través de la Membrana Plasmática , Ácido Homovanílico/metabolismo , Radioisótopos de Yodo/metabolismo , Masculino , Proteínas del Tejido Nervioso/análisis , Unión Proteica/fisiología , Putamen/metabolismoRESUMEN
IPCIT [2 beta-carboisopropoxy-3 beta-(4-iodophenyl)tropane; also designated RTI-121] is the isopropyl ester of beta-CIT [2 beta-carbomethoxy-3 beta-(4-iodophenyl) tropane]. Although beta-CIT binds to dopamine (DA), serotonin (5-HT) and norepinephrine (NE) transporters, IPCIT has been reported to be selective for the DA transporter. IPCIT was labeled with 125I and its receptor binding to membranes prepared from baboon striatum was compared with that of [125I] beta-CIT. These studies confirmed the relative selectivity of IPCIT for the DA transporter in comparison to 5-HT and NE transporters. The nonspecific binding of [125I]IPCIT was almost four times greater than that of [125I] beta-CIT. The biodistribution of IPCIT was examined in two baboons with whole body imaging for 24-30 h after administration of 3 mCi of 123I-labeled tracer. The brain uptake peaked within the first hour at 9.2% of the injected dose and the majority of activity in the body cleared through the hepatobiliary system. The distribution of activity within the brain was examined with ex vivo autoradiography in one monkey injected with [123I]IPCIT. Activity was concentrated in the caudate and putamen and had values of 5 and 7 microCi/cm3 per microCi/g, respectively. The distribution in brain regions receiving moderately dense serotonergic innervation (e.g. superior colliculus and thalamus) had levels of activity equivalent to that in cerebellum. This study confirmed the in vitro and in vivo selectivity of IPCIT for the DA transporter but also showed that [125I]IPCIT had higher in vitro nonspecific binding than [125I] beta-CIT.
Asunto(s)
Química Encefálica/fisiología , Proteínas Portadoras/metabolismo , Cocaína/análogos & derivados , Glicoproteínas de Membrana , Proteínas de Transporte de Membrana , Proteínas del Tejido Nervioso , Animales , Autorradiografía , Cocaína/farmacocinética , Proteínas de Transporte de Dopamina a través de la Membrana Plasmática , Femenino , Radioisótopos de Yodo , Modelos Biológicos , Ovariectomía , Papio , Unión Proteica , Distribución Tisular , Tomografía Computarizada de Emisión de Fotón Único , Recuento Corporal TotalRESUMEN
Four N-omega-fluoroalkyl-2 beta-carboxy-3 beta-(4-iodophenyl)nortropane ester (beta-CIT-FE), N-fluoropropyl, methyl ester (beta-CIT-FP), N-fluoroethyl, isopropyl ester (IP-beta-CIT-FE), and N-fluoropropyl, isopropyl ester (IP-beta-CIT-FP)] were labeled with 125I and evaluated in baboons by dynamic SPECT regional brain imaging, measurement of pharmacokinetics in arterial plasma, and whole body imaging. The labeled tracers were prepared by iododestannylation of the corresponding 4-(trimethylstannyl)phenyl compounds in radiochemical yield 63-96% and radiochemical purity > 96%. Regional SPECT brain imaging was carried out over a period of 5 h with a Strichman 810X Brain Imager to assess regional uptake in the striatum and midbrain compared to reference regions in the occipital cortex and cerebellum; arterial blood samples were taken for analysis of metabolites by solvent extraction and HPLC. The methyl esters showed higher total and specific peak uptake in the striatum than the isopropyl esters. Midbrain uptake was uniformly lower than striatal uptake and washed out more rapidly. beta-CIT-FE had more rapid striatal kinetics than beta-CIT-FP, with specific striatal washout rates of 10-14 vs 4-6% peak/h. Biodistribution of [123I] beta-CIT-FE and [123I] beta-CIT-FP measured by whole body conjugate imaging demonstrated major uptake in the brain, liver, and GI tract, with excretion occurring through both the renal and hepatobiliary routes. Absorbed radiation does estimates based on the MIRD schema indicated highest dose rates to the urinary bladder wall and lower large intestine wall (0.7 and 0.6 rad/mCi for [123I] beta-CIT-FE and 0.7 and 0.9 rad/mCi for [123I]beta-CIT-FP, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)
Asunto(s)
Encéfalo/metabolismo , Radioisótopos de Yodo/metabolismo , Nortropanos/metabolismo , Animales , Transporte Biológico , Encéfalo/diagnóstico por imagen , Cromatografía Líquida de Alta Presión , Radioisótopos de Yodo/farmacocinética , Masculino , Estructura Molecular , Nortropanos/farmacocinética , Especificidad de Órganos , Papio , Relación Estructura-Actividad , Factores de Tiempo , Distribución Tisular , Tomografía Computarizada de Emisión de Fotón Único/métodosRESUMEN
Single-photon emission tomographic (SPET) imaging with the radiotracer [123I]2 beta-carbomethoxy-3 beta-(4-iodophenyl)tropane ([123I]beta-CIT) has been reported to be a useful in vivo measure of dopamine (DA) transporters. However, in addition to its high DA transporter affinity, beta-CIT also binds with high affinity to serotonin (5-HT) transporters. 2 beta-Carboisopropoxy-3 beta-(4-iodophenyl)tropane (IPCIT) has been demonstrated by in vitro studies to have higher selectivity for the DA transporter. We compared [123I]beta-CIT and [123I]IPCIT SPET imaging and plasma metabolite analyses in baboons to evaluate the potential advantages of [123I]IPCIT for quantitative in vivo measurements of DA transporter densities. Both tracers had low levels (2% of total plasma 123I activity) of lipophilic radiolabeled metabolites at 420 min. [123I]IPCIT had significantly higher binding to plasma proteins. The average percent free (nonprotein bound) [123I]beta-CIT and [123I]IPCIT were 52% +/- 7% and 14% +/- 6%, respectively. Region of interest uptake data were normalized by injected dose and body weight. Consistent with the high density of 5-HT transporters in the midbrain and the lower 5-HT transporter affinity of IPCIT, the normalized peak specific midbrain uptake of [123I]beta-CIT (1.7 +/- 0.5) was higher than that of [123I]IPCIT (0.4 +/- 0.2). Consistent with its greater lipophilicity, [123I]IPCIT had higher nonspecific uptake, such that normalized cerebellar uptake of [123I]IPCIT was about twice that of [123I]beta-CIT. The ratio of specific to nonspecific uptake in striatum was greater for [123I]beta-CIT compared to [123I]IPCIT; however, striatal binding potentials and distribution volumes were not significantly different.(ABSTRACT TRUNCATED AT 250 WORDS)
Asunto(s)
Encéfalo/diagnóstico por imagen , Cocaína/análogos & derivados , Radioisótopos de Yodo , Proteínas de Transporte de Membrana , Proteínas del Tejido Nervioso , Tomografía Computarizada de Emisión de Fotón Único , Animales , Química Encefálica , Proteínas Portadoras/análisis , Dopamina/análisis , Proteínas de Transporte de Dopamina a través de la Membrana Plasmática , Femenino , Glicoproteínas de Membrana/análisis , Papio , Sensibilidad y Especificidad , Serotonina/análisis , Proteínas de Transporte de Serotonina en la Membrana Plasmática , Factores de TiempoRESUMEN
The pharmacokinetics of three radioiodinated high affinity dopamine D2 receptor binding ligands-two benzamide neuroleptics (IBF and epidepride) and the butyrophenone neuroleptic analog 2'-iodospiperone (2'-ISP)-were measured in nonhuman primates. [123I]IBF and [123I]epidepride were prepared by iododestannylation of the corresponding tributylstannyl derivative, whereas [123I]2'-ISP was labeled by (NH4)2SO4 mediated iodide for bromide exchange starting from 2'-bromospiperone. Labeled products were purified by HPLC and were obtained in > 93% radiochemical purity and > 7000 Ci/mmol sp. act. After i.v. injection in baboons, serial arterial plasma samples were extracted with ethyl acetate (IBF and epidepride) or denatured with methanol (2'-ISP) and analyzed by HPLC. For IBF, plasma levels of parent compound dropped to 50% of the plasma activity within 20 min post injection and the major radiometabolite was lipophilic. For epidepride, it took 30-40 min for parent content to reach 50% and the major radiometabolite was polar. For 2'-ISP, parent composition dropped to 60% after about 15 min. Arterial input curves for IBF and epidepride fit three-exponential models with terminal half-life of 54-76 and 50-59 min, respectively. Whole body images were acquired using the conjugate counting method. The distribution of all three agents was qualitatively similar, with major excretion through the hepatobiliary route. Peak whole brain uptake, observed within 20 min for all three tracers, was estimated as 7% injected dose for [123I]IBF, 8% for [123I]epidepride and 5% for [123I]2'-ISP.
Asunto(s)
Antipsicóticos/farmacocinética , Benzamidas/farmacocinética , Benzofuranos/farmacocinética , Pirrolidinas/farmacocinética , Receptores de Dopamina D2/metabolismo , Espiperona/análogos & derivados , Animales , Antipsicóticos/sangre , Autorradiografía , Benzamidas/sangre , Benzamidas/metabolismo , Benzofuranos/sangre , Benzofuranos/metabolismo , Encéfalo/metabolismo , Chlorocebus aethiops , Cromatografía Líquida de Alta Presión , Femenino , Radioisótopos de Yodo/metabolismo , Radioisótopos de Yodo/farmacocinética , Masculino , Papio , Pirrolidinas/sangre , Pirrolidinas/metabolismo , Espiperona/sangre , Espiperona/metabolismo , Espiperona/farmacocinética , Distribución TisularRESUMEN
[123I]Iodobenzofuran ([123I]IBF) is a new single photon emission computed tomography (SPECT) tracer for visualization of the dopamine D2 receptors. A tracer constant infusion paradigm was developed to measure the binding potential, density (Bmax) and affinity (KD) of the dopamine D2 receptor in baboons. Three baboons underwent both a single bolus and a constant infusion study. For the single bolus experiment, the striatal binding potential (134 +/- 24 ml g-1, mean +/- S.D.) was derived by kinetic analysis. For the constant infusion experiments, the striatal binding potential (127 +/- 16 ml g-1) was derived by equilibrium analysis. The two sets of experiments thus provided consistent data. Low specific activity constant infusion experiments were performed to measure KD (0.08 nM) and Bmax (12.7 nM). In vitro experiments carried out at 37 degrees C with [125I]IBF on rat striatal homogenate membranes provided results in agreement with the SPECT data. These studies suggested the feasibility of quantitation of dopamine D2 receptor parameters with [123I]IBF SPECT imaging.
Asunto(s)
Química Encefálica , Receptores de Dopamina D2/análisis , Animales , Benzofuranos , Sitios de Unión , Femenino , Radioisótopos de Yodo , Cinética , Masculino , Papio , Ratas , Ratas Sprague-Dawley , Receptores de Dopamina D2/efectos de los fármacos , Tomografía Computarizada de Emisión de Fotón ÚnicoRESUMEN
The aim of this work was to study the feasibility and reproducibility of in vivo measurement of benzodiazepine receptors with single photon emission computerized tomography (SPECT) in the baboon brain. Arterial and brain regional activities were measured for 420 min in three baboons after single bolus injection of the benzodiazepine antagonist [123I]iomazenil. Data were fit to a three-compartment model to derive the regional binding potential (BP), which corresponds to the product of the receptor density, (Bmax) and affinity (1/KD). Regional BP values (from 114 in striatum to 241 in occipital) were in good agreement with values predicted from in vitro studies. Constraining the regional volume of distribution of the nondisplaceable compartment to the value measured during tracer constant infusion experiments in baboons (Laruelle et al., 1993) improved the identifiability of the rate constants. Each experiment was repeated to investigate the reproducibility of the measurement. The regional average reproducibility was 10 +/- 5%, expressed as coefficient of variation (CV). Results of equilibrium analysis at peak uptake were in good agreement with results of kinetic analysis. Empirical counts ratio methods were found to be poorly sensitive to benzodiazepine receptor density. These studies suggest the feasibility of quantitative measurement of benzodiazepine receptors by kinetic analysis of SPECT data and the inadequacy of empirical methods of analysis, such as counts ratios, to evaluate differences in receptor density.
Asunto(s)
Flumazenil/análogos & derivados , Receptores de GABA-A/metabolismo , Tomografía Computarizada de Emisión de Fotón Único , Animales , Sangre/metabolismo , Encéfalo/metabolismo , Simulación por Computador , Flumazenil/metabolismo , Radioisótopos de Yodo , Cinética , Modelos Biológicos , Papio , Reproducibilidad de los Resultados , Factores de TiempoRESUMEN
In vivo benzodiazepine receptor equilibrium dissociation constant, KD, and maximum number of binding sites, Bmax, were measured by single photon emission computerized tomography (SPECT) in three baboons. Animals were injected with a bolus followed by a constant i.v. infusion of the high affinity benzodiazepine ligand [123I]iomazenil. Plasma steady-state concentration and receptor-ligand equilibrium were reached within 2 and 3 h, respectively, and were sustained for the duration (4-9 h) of the experiments (n = 15). At the end of the experiments, a receptor saturating dose of flumazenil (0.2 mg/kg) was injected to measure nondisplaceable activity. Experiments were carried out at various levels of specific activity, and Scatchard analysis was performed for derivation of the KD (0.59 +/- 0.09 nM) and Bmax (from 126 nM in the occipital region to 68 nM in the striatum). Two animals were killed and [125I]iomazenil Bmax and KD were measured at 22 and 37 degrees C on occipital homogenate membranes. In vitro values of Bmax (114 +/- 33 nM) and 37 degrees C KD (0.66 +/- 0.16 nM) were in good agreement with in vivo values measured by SPECT. This study demonstrates that SPECT can be used to quantify central neuroreceptors density and affinity.
Asunto(s)
Flumazenil/análogos & derivados , Receptores de GABA-A/metabolismo , Tomografía Computarizada de Emisión de Fotón Único , Animales , Sangre/metabolismo , Femenino , Flumazenil/metabolismo , Homeostasis , Técnicas In Vitro , Radioisótopos de Yodo , Cinética , PapioRESUMEN
This preliminary study demonstrates by in vitro autoradiography the value of novel radiolabeled cocaine analogs to characterize the neurochemical identity and measure the density and growth of fetal ventral mesencephalic cells transplanted to the caudate nucleus of the adult MPTP-treated monkey. Two cocaine derivatives were used, one with extremely high affinity for both dopamine and serotonin transporters and one which was selective for the dopamine transporter. The results suggest that the transplantation procedure was able to increase the density of transporter sites in most of the caudate nucleus, although the proportion of dopaminergic and serotonergic fibers may have been altered from normal.
Asunto(s)
Trasplante de Tejido Encefálico , Proteínas Portadoras/análisis , Cocaína/análogos & derivados , Cuerpo Estriado/fisiopatología , Proteínas de Transporte de Membrana , Proteínas del Tejido Nervioso , Enfermedad de Parkinson Secundaria/metabolismo , Enfermedad de Parkinson Secundaria/terapia , Sustancia Negra/trasplante , 1-Metil-4-fenil-1,2,3,6-Tetrahidropiridina , Animales , Autorradiografía , Trasplante de Tejido Encefálico/fisiología , Proteínas Portadoras/metabolismo , Chlorocebus aethiops , Cocaína/metabolismo , Cuerpo Estriado/metabolismo , Cuerpo Estriado/patología , Proteínas de Transporte de Dopamina a través de la Membrana Plasmática , Trasplante de Tejido Fetal/fisiología , Radioisótopos de Yodo , Masculino , Glicoproteínas de Membrana/análisis , Glicoproteínas de Membrana/metabolismo , Enfermedad de Parkinson Secundaria/patología , Ensayo de Unión Radioligante , Proteínas de Transporte de Serotonina en la Membrana Plasmática , Sustancia Negra/metabolismo , Sustancia Negra/fisiología , Trasplante Heterotópico , Trasplante HomólogoRESUMEN
Single-photon emission tomography (SPET) imaging in patients with complex partial epilepsy has shown that the seizure focus is characterized by both decreased interictal blood flow and decreased uptake of the benzodiazepine (BZ) receptor tracer iodine-123 iomazenil. The purpose of this study was to examine the confounding effect of decreased flow on iomazenil uptake. The left middle cerebral artery of four rats was occluded, and the animals were simultaneously injected with 25 microCi of iodine-125 iomazenil and 500 microCi of the blood flow tracer [123I]iofetamine (N-isopropyl-p-iodoamphetamine). All rats, including two sham, were sacrificed 1 h after injection, a time when uptake of both agents is nearly maximal. Control experiments showed that arterial occlusion for 1 h did not affect the total number of BZ binding sites. Using a dual autoradiographic technique, the uptake of both [123I]iofetamine and [125I]iomazenil was measured in more than 200 regions showing variable levels of reduced flow and expressed as a percentage of the contralateral homotypic area. The straight line fit of % [125I]iomazenil (y axis) versus % [123I]iofetamine (x axis) in all 200 regions had a slope of 0.74. Insofar as the rat is an accurate model of human subjects with epilepsy, these studies suggest that decreased flow to the epileptogenic focus will linearly exacerbate the decrease in uptake secondary to neuropathologic loss of BZ receptors. Thus, for localization of seizure focus, a single SPET image of [123I]iomazenil in an epileptic patient may have greater sensitivity than a comparable blood flow image, because the former is enhanced by both decreased flow and a loss of BZ receptors.
Asunto(s)
Anfetaminas/metabolismo , Isquemia Encefálica/diagnóstico por imagen , Isquemia Encefálica/metabolismo , Arterias Cerebrales/fisiología , Flumazenil/análogos & derivados , Receptores de GABA-A/fisiología , Flujo Sanguíneo Regional/fisiología , Animales , Autorradiografía , Encéfalo/irrigación sanguínea , Encéfalo/diagnóstico por imagen , Encéfalo/metabolismo , Flumazenil/metabolismo , Radioisótopos de Yodo/metabolismo , Yofetamina , Radiografía , Ratas , Ratas Sprague-Dawley , Tomografía Computarizada de Emisión de Fotón ÚnicoRESUMEN
The regional brain distribution and pharmacological specificity of three high affinity tracers for the dopamine (DA) D2 receptor: [123I]IBF, [123I]epidepride, and [123I]2'-ISP were assessed by SPECT imaging of non-human primates. The ratios of striatal-to-occipital activities at the time of peak striatal uptake were 2.2, 6.3 and 1.7, respectively. From the peak striatal activities, washout rates were 33, 4 and 16%/h for [123I]IBF, [123I]epidepride and [123I]2'-ISP, respectively. The reversibility of the striatal uptake of all three agents was demonstrated by the rapid displacement induced by the dopamine D2 selective antipsychotic agent raclopride, which increased washout rates to 96, 58 and 43%/h. The administration of d-amphetamine, which induces release of dopamine, had no noticeable effect on [123I]epidepride but increased the washout rate of [123I]IBF. These results suggest that, among these three agents, [123I]epidepride is the superior tracer for in vivo displacement studies because of its slow washout and high target-to-background ratios. However, for tracer kinetic modeling, [123I]IBF may be the superior agent because of its early time of peak uptake and its higher target-to-background ratios than [123I]2'-ISP.
Asunto(s)
Encéfalo/metabolismo , Medios de Contraste , Receptores de Dopamina D2/metabolismo , Tomografía Computarizada de Emisión de Fotón Único , Animales , Benzamidas/metabolismo , Benzofuranos/metabolismo , Dextroanfetamina/farmacología , Dopaminérgicos/farmacología , Femenino , Radioisótopos de Yodo , Papio , Pirrolidinas/metabolismo , Racloprida , Ritanserina/farmacología , Salicilamidas/farmacología , Antagonistas de la Serotonina/farmacología , Espiperona/análogos & derivados , Distribución Tisular/fisiologíaRESUMEN
2 beta-Carbomethoxy-3 beta-(4-iodophenyl)tropane (beta-CIT; also designated RTI-55) is an analog of cocaine that has been developed as a single photon emission computed tomography radiotracer that labels dopamine and serotonin transporters. We have prepared the 125I- and 123I-labeled ([1R] "active" and [1S] "inactive") enantiomers of beta-CIT. Total homogenate binding of the 125I-labeled inactive isomer to baboon caudate and cortex was approximately equal to nonspecific binding of the active isomer in cortex and much lower than total binding of the active isomer in caudate. However, inactive isomer homogenate binding in caudate was somewhat higher than in cortex, and during single photon emission computed tomography scanning in vivo striatal (1S)-[123I]beta-CIT uptake was also slightly greater than in cortex. Following intravenous administration of the 123I-labeled enantiomers, the plasma clearances of the active and inactive enantiomers were not significantly different. Single photon emission computed tomography imaging demonstrated that a bolus dose of nonradioactive (1R)-beta-CIT rapidly displaced the uptake of (1R)-[123I]beta-CIT. In contrast, the brain uptake of (1S)-[123I]beta-CIT was not displaced by nonradioactive (1R)-beta-CIT using either a bolus ("kinetic") or bolus plus constant infusion ("equilibrium") paradigm for administration of the radiotracer. In scans with bolus administration of radiotracer, peak striatal uptake of the active isomer was approximately twice that of the inactive isomer. In comparison to the 123I-labeled active tracer, the inactive tracer showed earlier times to peak activity and faster washouts of activity in all brain regions. These studies demonstrate beta-CIT stereoselectivity using both homogenate binding and in vivo imaging and suggest that the inactive enantiomer may be a useful measure of the kinetics of both blood-brain barrier transport and nonspecific binding.
Asunto(s)
Cocaína/análogos & derivados , Animales , Sitios de Unión , Núcleo Caudado/metabolismo , Corteza Cerebral/metabolismo , Cocaína/química , Cocaína/metabolismo , Femenino , Radioisótopos de Yodo/metabolismo , Papio , Estereoisomerismo , Tomografía Computarizada de Emisión de Fotón ÚnicoRESUMEN
SPECT imaging with [123I]iomazenil was used to measure benzodiazepine (BZ) neuroreceptor occupancy of the agonist lorazepam administered at therapeutically relevant doses in humans and supratherapeutic doses in monkeys. Lorazepam at therapeutic doses (0.03 mg/kg, i.v.) administered 90 min after the bolus injection of [123I]iomazenil had no statistically significant effect (P > 0.12) on the washout rates of regional brain activities compared to that in control subjects, although human subjects demonstrated marked sedation from the lorazepam. In baboons, the effects of higher doses of lorazepam (cumulative 0.5 mg/kg) were examined in a stepwise displacement paradigm. The in vivo potency was expressed as the ED50 (or dose required to displace 50% of receptor bound activity) and was equal to 0.34 +/- 0.01 mg/kg (mean +/- SD, n = 12). Log-logit analyses of displacement data corrected for endogenous washout showed that therapeutic doses of lorazepam were associated with < 3% BZ receptor occupancy. To examine if endogenous GABA modulates potency of the BZ agonist, the ED50 of lorazepam was compared with and without concurrent administration of tiagabine, a GABA reuptake inhibitor. These experiments were designed to measure an in vivo GABA shift of agonist potency. In vivo microdialysis demonstrated that tiagabine (up to 1 mg/kg, i.v.) increased extracellular GABA levels up to 200% of baseline, but these doses had only a minimal enhancement of lorazepam's potency to displace [123I]iomazenil. This study strongly suggests that single therapeutically relevant doses of lorazepam occupy a relatively small percentage (i.e. < 3%) of BZ receptors and that BZ binding sites have a significant (i.e. > 97%) receptor reserve.
Asunto(s)
Encéfalo/metabolismo , Flumazenil/análogos & derivados , Receptores de GABA-A/efectos de los fármacos , Adulto , Animales , Unión Competitiva/efectos de los fármacos , Diálisis , Femenino , Semivida , Humanos , Hipnóticos y Sedantes/farmacología , Radioisótopos de Yodo , Lorazepam/farmacocinética , Masculino , Ácidos Nipecóticos/farmacocinética , Ovariectomía , Papio , Tiagabina , Tomografía Computarizada de Emisión de Fotón Único , Ácido gamma-Aminobutírico/metabolismoRESUMEN
The in vivo properties of a new radioiodinated probe of the dopamine and serotonin transporter, [123I]methyl 3 beta-(4-iodophenyl)tropane-2 beta-carboxylate ([123I]beta-CIT) were evaluated in baboons and vervet monkeys. The labeled product was prepared in 65.2 +/- 2.8% yield (mean +/- SEM; n = 18) by reaction of the tributylstannyl precursor with [123I]NaI in the presence of peracetic acid followed by high pressure liquid chromatography (HPLC) purification to give a product with radiochemical purity of 97.5 +/- 0.5% and specific activity of 500-1200 Ci/mmol. After intravenous administration, whole brain activity peaked at 6-10% injected dose within 1 h post injection (p.i.) and washed out in a biphasic manner with clearance half-lives of 1-2 and 7-35 h for the rapid and slow components, respectively. Excretion occurred primarily through the hepatobiliary route, with about 30% of the injected dose appearing in the GI tract after 5 h. Estimates of radiation absorbed dose gave 0.01, 0.1, 0.2 and 0.03 mGy/MBq to the brain, gall bladder wall, lower large intestine wall and urinary bladder wall, respectively. High resolution SPECT imaging in a baboon demonstrated high uptake of tracer in the region of the striatum (striatum:cerebellum ratio 4.0), in the hypothalamus (ratio 2.6) and in a midbrain region comprising raphe, substantia nigra and superior colliculus (ratio 2.0), with regional brain uptakes measured at 210 min p.i. of [123I]beta-CIT. The anatomical locations of the regions on the SPECT image were confirmed by coregistration with MRI. Plasma metabolites and pharmacokinetics were analyzed in baboons and vervets by ethyl acetate extraction and HPLC. The major metabolite was a polar, non-extractable fraction, which increased to > 50% of the plasma activity by 30-45 min p.i. A minor lipophilic (extractable) metabolite was also observed, increasing to about 4% at 2-3 h p.i. The plasma protein bound fraction, determined by ultrafiltration, was 74.8 +/- 1.4% (n = 6). The arterial input function was characterized by the sum of three exponential terms with half-lives of 0.3-1.7, 9.7-24.9 and 77-166 min, respectively, for the concentration of free parent compound. [123I]beta-CIT promises to be a useful marker for SPECT study of the monoamine uptake system in primate brain.
Asunto(s)
Encéfalo/diagnóstico por imagen , Cocaína/análogos & derivados , Proteínas de Transporte de Membrana , Proteínas del Tejido Nervioso , Animales , Encéfalo/anatomía & histología , Proteínas Portadoras/análisis , Chlorocebus aethiops , Cocaína/farmacocinética , Proteínas de Transporte de Dopamina a través de la Membrana Plasmática , Femenino , Radioisótopos de Yodo , Imagen por Resonancia Magnética , Glicoproteínas de Membrana/análisis , Papio , Primates , Proteínas de Transporte de Serotonina en la Membrana Plasmática , Tomografía Computarizada de Emisión de Fotón Único , Irradiación Corporal TotalRESUMEN
Single photon emission computed tomography (SPECT) studies of regional kinetic uptake and pharmacological specificity of [123I]methyl 3 beta-(4-iodophenyl) tropane-2 beta-carboxylate ([123I]beta-CIT) were performed in nonhuman primates (n = 41). In control experiments, activity was concentrated in striatum and in hypothalamic/midbrain regions. Striatal uptake increased for 140-180 min and displayed stable levels thereafter. Striatal to cerebellar activity ratios were 7.3 +/- 0.9 (mean +/- SEM) at 300 min. About 75% of striatal uptake was displaceable by injection of nonradioactive beta-CIT. Hypothalamic/midbrain activity reached maximal levels at approximately 45 min. A slow washout phase followed this peak activity. Activities in frontal, occipital, and cerebellar regions were characterized by an early peak (20-30 min), followed by rapid washout. Displacement studies demonstrated that striatal uptake was associated with dopamine (DA) transporters, as it was displaced by GBR 12909, a selective DA uptake inhibitor, but not by citalopram, a selective serotonin (5-HT) uptake inhibitor. The inverse was true in the hypothalamic/midbrain area, suggesting that the uptake in this area was associated primarily with 5-HT transporters. Maprotiline, a selective norepinephrine uptake inhibitor, did not affect [123I]beta-CIT uptake. In vivo site occupancy ED50 values of cocaine, 2 beta-carbomethoxy-3 beta-(4-fluorophenyl)tropane (CFT), and beta-CIT were measured in the striatum with a stepwise displacement paradigm. In vivo ED50 values correlated strongly with in vitro IC50 values for binding to DA transporters. Infusion of high dose of L-DOPA (250 mumol/kg) failed to displace striatal [123I]beta-CIT binding, suggesting that the binding would not be affected by L-DOPA administration in Parkinsonian patients. However, studies performed with injection of d-amphetamine indirectly suggested that high synaptic levels of DA may compete with [123I]beta-CIT binding. These studies suggest that [123I]beta-CIT will be a useful SPECT tracer of DA and 5-HT transporters in living human brain.