In vitro characterization of 6-[18F]fluoro-A-85380, a high-affinity ligand for alpha4beta2* nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors are involved in tobacco dependence and several other neuropathologies (e.g., Alzheimer's disease, Parkinson's disease), as well as in attention, learning, and memory. Performing in vivo imaging of these receptors in humans holds great promise for understanding their role in these conditions. Recently, three radiohalogenated analogs of 3-(2(S)-azetidinylmethoxy)pyridine (A- 85380) were used successfully for the in vivo visualization of alpha4beta2* nicotinic receptors in the human brain with PET/SPECT. Herein, we present the results of the in vitro characterization of one of these radioligands, 6-[18F]fluoro-3-(2(S)-azetidinylmethoxy)-pyridine (6-[18F]fluoro-A-85380), which is a fluoro-analog of the potent nonopioid analgesic ABT-594. In human postmortem cortical tissue, 6-[18F]fluoro-A-85380 reversibly binds with high affinity to a single population of sites (Kd = 59 pM at 37 degrees C, Bmax = 0.7 pmol/g tissue). The binding is fully reversible and is characterized at 37 degrees C by T(1/2assoc) = 2.2 min (at a ligand concentration of 39 pM) and by T(1/2dissoc) = 3.6 min. 6-Fluoro-A-85380 exhibits clear selectivity for alpha4beta2* over the other major mammalian nicotinic receptor subtypes: alpha7, alpha3beta4, and muscle-type. These results suggest that 6-[18F]fluoro-A-85380 is a promising radioligand for in vivo imaging of brain alpha4beta2* nicotinic receptors.     

Graphical analysis of 2-[18F]FA binding to nicotinic acetylcholine receptors in rhesus monkey brain

External imaging of nicotinic acetylcholine receptors (nAChRs) using techniques such as PET would help to clarify the roles of these receptors in the physiology and pathology of brain function. Here we report the results of quantitative PET studies of cerebral nAChRs with 2-[(18)F]fluoro-A-85380 (2-[(18)F]FA) in rhesus monkeys. Data from dynamic PET scans were analyzed using graphical methods. Binding potential (BP) values of 2.0, 0.4, 0.3, and 0.03 observed in the thalamus (Th), cortex (Cx), striatum (Str), and cerebellum (Cb), respectively, were consistent with the pattern of alpha(4)beta(2) nAChR distribution in monkey brain. The high value of 2-[(18)F]FA-specific binding in the rhesus monkey Th and low level of that in Cb compared with nonspecific accumulation of radioactivity in these structures allowed use of Cb as a reference region for calculation of BP and volume of distribution of specific binding (VDsb) in Th by graphical methods, both with and without the plasma input function. In contrast, estimation of 2-[(18)F]FA specific binding in low-receptor-density regions such as Cx and Str required assessment of nondisplaceable volume of distribution (VDnd) in a separate study and measurement of nonmetabolized radioligand concentrations in the plasma. For accurate quantitation of 2-[(18)F]FA-specific binding by graphical analysis, PET studies should last up to 7 h due to the slow kinetics of 2-[(18)F]FA brain distribution. Further, to avoid substantial underestimation in measured BP values the doses of administered 2-[(18)F]FA should not exceed 0.1 nmol/kg body weight. The findings suggest that 2-[(18)F]FA is a promising ligand for quantitation of nAChRs in human brain.     

In vivo measurement of nicotinic acetylcholine receptors with [18F]norchloro-fluoro-homoepibatidine

Functional changes of nicotinic acetylcholine receptors (nAChR) are important during age-related neuronal degeneration. Recent studies demonstrate the applicability of the nAChR ligand 2-[(18)F]F-A-85380 for neuroimaging of patients with dementias. However, its binding kinetics demands a 7-h acquisition time limiting its practicality for clinical PET studies. Thus, the authors developed [(18)F]norchloro-fluoro-homoepibatidine ([(18)F]NCFHEB) for nAChR imaging. The kinetics of the two enantiomers of [(18)F]NCFHEB were compared with 2-[(18)F]F-A85380 in porcine brain to evaluate their potential for human neuroimaging. Twenty-four juvenile female pigs were studied with PET using [(18)F]NCFHEB. Nine animals received an additional i.v. injection (1 mg/kg) of the nAChR agonist A81418 before radiotracer administration followed by infusion (2 mg/kg/7h) thereafter. Several compartment models were applied for quantification. (-)- and (+)-[(18)F]NCFHEB showed a twofold to threefold higher brain uptake than 2-[(18)F]F-A-85380. All three radiotracers displayed spatially heterogeneous binding kinetics in regions with high, moderate, or low specific binding. The equilibrium of specific binding of (-)-[(18)F]NCFHEB was reached earlier than that of (+)-[(18)F]NCFHEB or 2-[(18)F]F-A85380. Continuous administration of the nAChR agonist A81418 inhibited the specific binding of (-)- and (+)-[(18)F]NCFHEB but not of 2-[(18)F]F-A85380. The peripheral metabolism of (+)-[(18)F]NCFHEB proceeded somewhat slower than that of the other radiotracers. Both enantiomers of [(18)F]NCFHEB are appropriate radiotracers for neuroimaging of nAChR in pigs. Their binding profile in vivo appears to be more selective than that of 2-[(18)F]F-A85380. (-)-[(18)F]NCFHEB offers a faster equilibrium of specific binding than 2-[(18)F]F-A85380.     

2-[18F]F-A-85380: a PET radioligand for alpha4beta2 nicotinic acetylcholine receptors.

2-[18F]fluoro-3-(2(S)-azetidinylmethoxy)pyridine (2-[18F]F-A-85380), a ligand for nicotinic acetylcholine receptors (nAChRs) was evaluated in an in vitro binding assay with membranes of rat brain and in vivo by PET in Rhesus monkey brain. The ligand has high affinity for alpha4beta2 nAChRs (K(D)=50 pM), crosses the blood-brain barrier, and distributes in the monkey brain in a pattern consistent with that of alpha4beta2 nAChRs. The specific/non-specific binding ratio increased steadily, reaching a value of 3.3 in the thalamus at 4 h. The specific binding of 2-[18F]F-A-85380 was reversed by cytisine. These results, in combination with the data demonstrating low toxicity of 2-[18F]F-A-85380, indicate that this ligand shows promise for use with PET in human subjects.     

Autoradiography of 2-[18F]F-A-85380 on nicotinic acetylcholine receptors in the porcine brain in vitro

Noninvasive molecular imaging of subtypes of neuronal nicotinic acetylcholine receptors (nAChRs) will provide information on the role of these receptors in neurodegenerative diseases. The binding of the positron emission tomography ligand 2-[18F]F-A-85380 to nAChRs was investigated in the porcine brain by quantitative autoradiography in vitro. The high-affinity binding of 2-[18F]F-A-85380 to each of the investigated 12 brain areas was saturable and apparently monophasic (e.g., apparent KD value of 1.72 nM in the thalamus). The highest density of specific binding sites was observed in the thalamus (1,158 fmol/mg protein), and the lowest density was measured in the cerebellar gray matter (11 fmol/mg protein). An attempt to assess nAChR subtype specificity of 2-[18F]F-A-85380 was made by competitive autoradiography. Binding of 2-[18F]F-A-85380 coincubated with 2-F-A-85380, epibatidine, cytisine, or methyllycaconitine, respectively, indicated a specificity of 2-[18F]F-A-85380 to beta2-containing nAChRs in the porcine brain. The autoradiographic data confirmed the suitability of swine as a model for the evaluation of radioligands designed for imaging of nAChR subtypes in the living brain.     

Quantitation of extrastriatal D2 receptors using a very high-affinity ligand (FLB 457) and the multi-injection approach.

The multi-injection approach has been used to study in baboon the in vivo interactions between the D2 receptor sites and FLB 457, a ligand with a very high affinity for these receptors. The model structure was composed of four compartments (plasma, free ligand, and specifically and unspecifically bound ligands) and seven parameters (including the D2 receptor site density). The arterial plasma concentration, after correction for metabolites, was used as the input function. The experimental protocol, which consisted of three injections of labeled and/or unlabeled ligand, allowed the evaluation of all model parameters from a single positron emission tomography experiment. In particular, the concentration of receptor sites available for binding (B'max) and the apparent in vivo FLB 457 affinity were estimated in seven brain regions, including the cerebellum and several cortex regions, in which these parameters are estimated in vivo for the first time (B'max is estimated to be 4.0+/-1.3 pmol/mL in the thalamus and from 0.32 to 1.90 pmol/mL in the cortex). A low receptor density was found in the cerebellum (B'max = 0.39+/-0.17 pmol/mL), whereas the cerebellum is usually used as a reference region assumed to be devoid of D2 receptor sites. In spite of this very small concentration (1% of the striatal concentration), and because of the high affinity of the ligand, we demonstrated that after a tracer injection, most of the PET-measured radioactivity in the cerebellum results from the labeled ligand bound to receptor sites. The estimation of all the model parameters allowed simulations that led to a precise knowledge of the FLB 457 kinetics in all brain regions and gave the possibility of testing the equilibrium hypotheses and estimating the biases introduced by the usual simplified approaches.     

18F]FPhEP and [18F]F2PhEP, two new epibatidine-based radioligands: evaluation for imaging nicotinic acetylcholine receptors in baboon brain

The radioligand 2-[(18)F]fluoro-A-85380 has been developed for imaging alpha(4)beta(2) nAChRs with PET. However, it has slow kinetics and a large fraction of bound activity is nondisplaceable. In an attempt to address these problems, two epibatidine-based alpha(4)beta(2) nicotinic antagonists, coded FPhEP and F(2)PhEP, were evaluated in vivo in baboons. They were radiolabeled with fluorine-18 from the corresponding N-Boc-protected bromo-derivatives and the no-carrier-added K[(18)F]F-Kryptofix(222) complex. Radiochemically pure [(18)F]FPhEP or [(18)F]F(2)PhEP was obtained in 80 min in amounts of 1.11-2.22 GBq (111-185 GBq/micromol). After injection of 215 MBq of [(18)F]FPhEP or [(18)F]F(2)PhEP, dynamic PET data were acquired. Thalamic radioactivity peaked at 20 min (4.9% +/- 0.2% ID/100 mL tissue) for [(18)F]FPhEP. For [(18)F]F(2)PhEP, the peak was at 45 min (3.3% +/- 0.1% ID/100 mL tissue). Regional distribution of both radiotracers was in accordance with the known distribution of nAChRs. In presaturation experiments, nicotine, cytosine, or FPhEP reduced brain radioactivity of [(18)F]FPhEP. In a displacement experiment with nicotine only a small amount of [(18)F]F(2)PhEP was dislodged. In spite of a moderate to high in vitro affinity, both ligands do not fulfill the widely adopted criteria for a PET radioligand.     

The effect of antiepileptic drugs on 5-HT-receptor binding measured by positron emission tomography

PURPOSE: To study the effect of antiepileptic drugs (AEDs) on 5-HT(1A)-receptor binding in patients with temporal lobe epilepsy. 5-HT(1A)-receptor binding, measured by positron emission tomography, is reduced in patients with temporal lobe epilepsy. Antiepileptic drugs may act on the serotonergic system, as shown in animal models, and thus affect receptor-binding measurements. METHODS: We analyzed the effect of AEDs on 5-HT(1A)-receptor binding in 31 patients and 10 normal controls. Patients with structural lesions, progressive neurologic disorders, or taking other medications were excluded. None had a seizure for >or=2 days before positron emission tomography (PET). [(18)F]FCWAY PET was performed on a GE Advance scanner with continuous EEG monitoring. Functional images of the distribution volume (V) were generated. Anatomic regions of interest were applied to co-registered PET images, after correction for partial-volume effect. RESULTS: Patients had significantly higher [(18)F]FCWAY free fraction (f(1)) than did controls. No AED effects were observed on interictal [(18)F]FCWAY binding after correction for plasma free fraction. [(18)F]FCWAY V/f1 reduction in epileptic foci was not affected by AEDs. CONCLUSIONS: 5-HT(1A)-receptor binding is reduced in temporal lobe epileptic foci after partial-volume correction. AED plasma free fractions should be measured when PET receptor studies are performed in patients with epilepsy.     

Influence of acetylcholine levels on the binding of a SPECT nicotinic acetylcholine receptor ligand [123I]5-I-A-85380

Although in vitro theory indicates that ligand binding is sensitive to competition with neurotransmitters, only some imaging ligands have shown such competition in vivo. The purpose of this study was to determine whether increases in acetylcholine (ACh) levels induced by an acetylcholinesterase inhibitor, physostigmine, inhibit in vivo binding of [(123)I]5-iodo-3-(2(S)-2-azetidinyl-methoxy) pyridine (5-I-A-85380), a single photon emission computed tomography ligand for the high-affinity type nicotinic ACh receptor (nAChR). Baboons were used for seven studies with a bolus plus constant infusion equilibrium paradigm. After achieving equilibrium at 5 h, physostigmine (0.02 (n = 1), 0.067 (n = 3), and 0.2 (n = 3) mg/kg) was administered intravenously and data were acquired for up to 8 h. To confirm equilibrium conditions, [(123)I]5-I-A-85380 plasma levels were measured in four studies, including all studies with 0.2 mg/kg physostigmine. Prior to physostigmine administration, thalamic activities were stable, with changes of 1.1%/h or less, except in one study with a gradual increase of 4.2%/h. Thalamic activities were decreased by 15% in one study with 0.067 mg/kg and 14-17% in all studies with 0.2 mg/kg physostigmine administration (P = 0.009). In these studies with 0.2 mg/kg physostigmine administration, [(123)I]5-I-A-85380 plasma levels showed a transient or a sustained increase after physostigmine administration that would have increased thalamic activities. These results suggest that elevated ACh levels induced by physostigmine can effectively compete in vivo with [(123)I]5-I-A-85380 binding at nAChRs. However, decreased thalamic activities could have been caused by other mechanisms, including internalization of the receptor with an associated decreased affinity for radioligand.     

Quantitation of striatal and extrastriatal D-2 dopamine receptors using PET imaging of [(18)F]fallypride in nonhuman primates

[(18)F]Fallypride is a highly selective, high-affinity dopamine D-2 receptor ligand. The high affinity, K(D) = 30 pM, makes it a suitable candidate for visualizing both striatal and extrastriatal binding in the brain. In this work, dynamic PET studies of two macaque monkeys were acquired along with arterial plasma samples. Compartmental analysis and Logan plots were used to analyze the striatum, thalamus, frontal, and temporal cortices and to validate a reference region of analysis which yields a distribution volume ratio (DVR). The cerebellum was used as the reference region. The results indicate that all methods of analysis are in close agreement over all the analyzed regions in the brain. The average DVRs for the two monkeys was found to be: caudate = 26, putamen = 29, thalamus = 3.8, frontal ctx = 1.7, and temporal ctx = 1.7 on a high-resolution PET scanner. It was found that a scan time of 2 h is needed to accurately estimate the DVR for all regions of the brain. The striatal regions require the longest to linearize and are the most sensitive to variations in the average tissue-to-plasma efflux constant, k(2). For the extrastriatal regions, the effect of the k(2) term on DVR calculation is negligible. Repeatability measurements for all regions were found to be within 10% using the DVR parameter.     

Evaluation of distribution of adenosine A2A receptors in normal human brain measured with [11C]TMSX PET

Adenosine A(2A) receptor (A2AR) is thought to interact with dopamine D(2) receptor. Selective A2AR antagonists have attracted attention as the treatment of Parkinson's disease. In this study, we investigated the distribution of the A2ARs in the living human brain using positron emission tomography (PET) and [7-methyl-(11)C]-(E)-8-(3,4,5-trimethoxystyryl)-1,3,7-trimethylxanthine ([(11)C]TMSX). We recruited five normal male subjects. A dynamic series of PET scans was performed for 60 min, and the arterial blood was sampled during the scan to measure radioactivity of the parent compound and labeled metabolites. Circular regions of interest of 10-mm diameter were placed in the PET images over the cerebellum, brainstem, thalamus, head of caudate nucleus, anterior and posterior putamen, frontal lobe, temporal lobe, parietal lobe, occipital lobe, and posterior cingulate gyrus for each subject. A two-tissue, three-compartment model was used to estimate K(1), k(2), k(3), and k(4) between metabolite-corrected plasma and tissue time activity of [(11)C]TMSX. The binding potential (BP) was the largest in the anterior (1.25) and posterior putamen (1.20), was next largest in the head of caudate nucleus (1.05) and thalamus (1.03), and was small in the cerebral cortex, especially frontal lobe (0.46). [(11)C]TMSX PET showed the largest BP in the striatum in which A2ARs were enriched as in postmortem and nonhuman studies reported, but that the binding of [(11)C]TMSX was relatively larger in the thalamus to compare with other mammals. To date, [(11)C]TMSX is the only promising PET ligand, which is available to clinical use for mapping the A2ARs in the living human brain.     

Absolute abundances and affinity states of dopamine receptors in mammalian brain: A review

The abundances of dopamine (DA) D(1) and D(2) receptors have been assayed with radioligands in membrane preparations and by autoradiography in vitro, and also in living brain using positron emission tomography (PET). This review compares the saturation binding parameters (B(max) and K(D) ) obtained in striatum by these several methods, and in different species. Some uncertainty in quantitation is derived from the incomplete specificities of commonly used ligands, especially Sch 23,390 for D(1) sites and spiperone for D(2) -like sites. In striatal membrane preparations, the D(1) B(max) ranges from 10 to 139 pmol g(-1) tissue, whereas the D(2) B(max) ranges from 8 to 42 pmol g(-1) tissue. Receptor concentrations in human material, despite the more extended post mortem interval, are roughly similar to those reported in rodent and nonhuman primate. Estimates of B(max) by quantitative autoradiography are generally five times higher than corresponding results for similar ligands in membrane preparations. The saturation binding parameters in living striatum have been estimated by serial PET studies with ligands over a range of specific activities. The few PET estimates of D(1) B(max) , (40-80 pmol g(-1) ) and numerous PET estimates of D(2) B(max) (20-40 pmol g(-1) ) are in general agreement with membrane estimates, but fall far short of the mean of autoradiographic results in vitro. Apparent affinities for D(1) and D(2) ligands in vivo are typically 10 times lower than for corresponding in vitro studies, presumably because the unbound ligand concentration is not corrected for the free fraction in living brain tissue. The disparate B(max) results by method suggest the presence of a large reservoir or reserve of D(1) and D(2) receptors in intact brain sections, which are unavailable to PET ligands in vivo, and which may be lost during the preparation of washed membranes. A subset of receptors existing in a high affinity state for agonists is detected in washed membrane preparations, in which the coupling to intracellular G-proteins may have become artificially limiting. However, in most PET and autoradiographic studies in vitro, agonist and antagonist ligands have similar B(max) . Discrepancies in the literature highlight the need for a better understanding of affinity states in vivo and trafficking of G-protein coupled receptors between plasma membrane and intracellular compartments.     

Noninvasive estimation of normalized distribution volume: application to the muscarinic-2 ligand [(18)F]FP-TZTP.

Reference tissue methods to estimate neuroreceptor binding are not applicable to [(18)F]FP-TZTP (a muscarinic-2 cholinergic receptor ligand), because there is no suitable receptor-free reference region. We evaluated a new method to estimate, without using arterial data or a receptor-free reference region, a receptor parameter called the normalized distribution volume, V(T)(*), using a region containing receptors as the input tissue. V(T)(*) is defined as V(T)/K'(1) (distribution volume (V(T)) normalized by K'(1) of the input region). We used a two-parameter multilinear reference tissue model (MRTM2) to generate parametric images of V(T)(*) and R(1) (R(1)=K(1)/K'(1)) from [(18)F]FP-TZTP PET data of healthy aged subjects (10 with apolipoprotein E-epsilon4 alleles (APOE-epsilon4(+)) and nine without (APOE-epsilon4(-)). V(T)(*) and V(T) were normalized by plasma-free fraction, f(P). By one-tissue kinetic analysis (1TKA) with metabolite-corrected plasma data, V(T) was previously reported as higher in the APOE-epsilon4(+) group. The noise magnitude of MRTM2 V(T)(*) and R(1) images were nearly identical to those of 1TKA V(T) and K(1) images. K'(1) or f(P) was not different between the two groups. V(T)(*) (mins) (1,659+/-497) and V(T) (mL/cm(3)) (701+/-99) in APOE-epsilon4(+) were higher by 38 and 22% than those (1,209+/-233 and 577+/-112) in APOE-epsilon4(-), respectively. The statistical significance for V(T)(*) (0.041) was lower than that for V(T) (0.025), due to the higher intersubject variability of V(T)(*) (25%) than that of V(T) (17%). We conclude that MRTM2 V(T)(*) allows detection of group differences in receptor binding without arterial blood or a receptor-free reference region.     

Ex vivo and in vivo evaluation of (2R,3R)-5-[(18)F]-fluoroethoxy- and fluoropropoxy-benzovesamicol, as PET radioligands for the vesicular acetylcholine transporter

Molecular imaging of the vesicular acetylcholine transporter (VAChT) using positron emission tomography (PET) may provide insights into early diagnosis and better understanding of Alzheimer's disease. We further characterized the VAChT ligand (2R,3R)-5-FEOBV (1) and developed new fluoropropoxy analogues. Ex vivo studies of the new nonradiolabeled analogues (2R,3R)-5-FPOBV (2) (k(D) = 0.7 nM) and (2S,3S)-5-FPOBV (3) (k(D) = 8.8 nM) were performed in rat brain and showed an enantioselective inhibition of (-)-5-[(125)I]-IBVM uptake in striatum, cortex, and hippocampus (e.g., 74% for 2 and only 54% for 3 in the cortex). Radiochemical procedures were developed to produce [(18)F]1 and [(18)F]2 as potential imaging agent for the VAChT. The radiochemistry was carried out in a one step procedure, with radiolabeling yields of 17 and 2.6% (range: 1-5.4), respectively, nondecay corrected with good specific activity: 124-338 GBq/micromol. The radiochemical purity was greater than 98%. The biological (ex vivo and in vivo) properties of these radioligands were evaluated in rats and showed a low (less then 0.1% of the injected dose) and homogeneous brain uptake. The in vivo PET study of [(18)F]2 performed in baboon also revealed rapid defluorination as the main problem. Therefore [(18)F]1 and [(18)F]2 appear to be unsuitable for in vivo imaging of the VAChT using PET.     

Parametric images of the extrastriatal D2 receptor density obtained using a high-affinity ligand (FLB 457) and a double-saturation method.

The potential of positron emission tomography for the quantitative estimation of receptor concentration in extrastriatal regions has been limited in the past because of the low density of the D2 receptor sites in these regions and the insufficient affinity of the most widely used radioligands for dopamine receptors. The new method described in this paper permits the estimate of the D2 receptor concentration in the extrastriatal regions using a two-injection protocol and FLB 457, a ligand with a high affinity (20 pmol/L in vitro ) with D2 dopamine receptors. This approach is not valid for the striatal regions because some hypotheses cannot be verified (because of the high receptor concentration in these regions). The experimental protocol includes two injections with ligand doses designed to significantly occupy the extrastriatal receptor sites (approximately 90%), while leaving less than 60% of the receptor sites occupied by the ligand in the striatal regions. The results obtained using this double-saturation method are in line with the concentration estimates previously obtained using the multiinjection approach. The receptor concentration is 2.9 +/- 0.5 pmol/mL in the thalamus, 1.0 +/- 0.2 pmol/mL in the temporal cortex, and 0.35 +/- 0.13 pmol/mL in the occipital cortex. This study provides new arguments supporting the presence of a small receptor-site concentration in the cerebellum, estimated at 0.35 +/- 0.16 pmol/mL The simplicity of the calculation used to estimate the receptor concentration lends itself easily to parametric imaging. The receptor concentration is estimated pixel by pixel, without filtering. This method permits estimation of the extrastriatal D2 receptor concentration using an experimental protocol that can easily be used in patient studies (i.e., single experiment, no blood sampling, short experiment duration).     

The quantitative analysis of D2-dopamine receptors in baboon striatum in vivo with 3-N-[2'-18F]fluoroethylspiperone using positron emission tomography

We used the ligand 3-N-[2'-18F]fluoroethylspiperone (FESP), which binds to D2-dopamine receptors in the striatum, and positron emission tomography (PET) to quantify striatal D2-dopamine densities (Bmax) and binding kinetics in baboon brain in vivo. Sequential PET scans were obtained for 4 h post injection. Various similar models based on a nonlinear kinetic four-compartment model that takes into account the effect of ligand specific activity were used. We investigated the effect of exact model configuration on the reliability of Bmax and other kinetic transfer coefficients. We found that with the ligand FESP and dynamic PET studies, the estimated values of Bmax and other model parameters are sensitive to the choice of model configuration, ligand specific activity, and data analysis technique. The limitations of the reliability of parameter estimates in a complex kinetic model for receptor ligands were studied in simulation calculations. Results showed that the accuracy of estimated values of Bmax is affected by both the ligand binding properties and the injected dose of ligand. The estimated average value of kinetic model parameters was as follows: ligand-receptor dissociation constant k4 = 0.0080 min-1; the product of ligand-receptor association constant and fraction of ligand available to bind to specific receptors f2ka = 0.0052 (min nM)-1; and D2-dopamine receptor density Bmax = 37.5 pmol g-1.     

Positron emission tomography quantification of [11C]-harmine binding to monoamine oxidase-A in the human brain.

This article describes the kinetic modeling of [(11)C]-harmine binding to monoamine oxidase A (MAO-A) binding sites in the human brain using positron emission tomography (PET). Positron emission tomography studies were performed in healthy volunteers at placebo conditions and after treatment with clinical doses of moclobemide. In either condition, a two-tissue compartment model (2CM) provided better fits to the data than a one-tissue model. Estimates of k(3)/k(4) values from an unconstrained 2CM were highly variable. In contrast, estimates of the specifically bound radioligand distribution volume (DV(B)) from an unconstrained 2CM were exceptionally stable, correlated well with the known distribution of MAO-A in the brain (cerebellum 相似文献   

Potential of [(18)F]beta-CFT-FE (2beta-carbomethoxy-3beta-(4-fluorophenyl)-8-(2-[(18)F]fluoroethyl)nortropane) as a dopamine transporter ligand: A PET study in the conscious monkey brain

A dopamine transporter (DAT) ligand 2beta-carbomethoxy-3beta-(4-fluoro-phenyl)-8-(2-[(18)F]fluoroethyl)nortropane ([(18)F]beta-CFT-FE) was synthesized and evaluated in comparison with [(11)C]beta-CFT in monkey brain using animal positron emission tomography (PET). [(18)F]beta-CFT-FE and [(11)C]beta-CFT were injected intravenously to conscious monkeys for a 91-min PET scan with arterial blood sampling for metabolite analysis. In the conscious state, [(18)F]beta-CFT-FE provided a peak about 20 min after the injection and declined thereafter in the striatum of monkey brain, while [(11)C]beta-CFT continuously increased with time up to 91 min after injection. Metabolite analysis revealed that [(18)F]beta-CFT-FE was more rapidly metabolized in plasma than [(11)C]beta-CFT. The striatal binding of both ligands was dose-dependently displaced by preadministration of a specific DAT inhibitor, GBR12909, at doses of 0.5 and 5 mg/kg; however, the displacement degree of [(11)C]beta-CFT-FE was higher than that of [(18)F]beta-CFT. The effects of the anesthetics, ketamine and isoflurane, on binding were more prominent in [(11)C]beta-CFT than [(18)F]beta-CFT-FE. Specificity and affinity of beta-CFT-FE to DAT were evaluated in an in vitro assay using cloned human DAT, serotonin transporter, and norepinephrine transporter in comparison with other conventional DAT ligands, showing that beta-CFT-FE had lower affinity and higher specificity to DAT than beta-CFT and beta-CIT. These results suggested that [(18)F]beta-CFT-FE could be a potential imaging agent for DAT, providing excellent selectivity and tracer kinetics for quantitative PET imaging.     

Development of a tracer kinetic plasma input model for (R)-[11C]PK11195 brain studies.

(R)-[(11)C]PK11195 ([1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl]-3-isoquinoline carboxamide) is a ligand for the peripheral benzodiazepine receptor, which, in the brain, is mainly expressed on activated microglia. Using both clinical studies and Monte Carlo simulations, the aim of this study was to determine which tracer kinetic plasma input model best describes (R)-[(11)C]PK11195 kinetics. Dynamic positron emission tomography (PET) scans were performed on 13 subjects while radioactivity in arterial blood was monitored online. Discrete blood samples were taken to generate a metabolite corrected plasma input function. One-tissue, two-tissue irreversible, and two-tissue reversible compartment models, with and without fixing K(1)/k(2) ratio, k(4) or blood volume to whole cortex values, were fitted to the data. The effects of fixing parameters to incorrect values were investigated by varying them over a physiologic range and determining accuracy and reproducibility of binding potential and volume of distribution using Monte Carlo simulations. Clinical data showed that a two-tissue reversible compartment model was optimal for analyzing (R)-[(11)C]PK11195 PET brain studies. Simulations showed that fixing the K(1)/k(2) ratio of this model provided the optimal trade-off between accuracy and reproducibility. It was concluded that a two-tissue reversible compartment model with K(1)/k(2) fixed to whole cortex value is optimal for analyzing (R)-[(11)C]PK11195 PET brain studies.     

Measuring the in vivo binding parameters of [18F]-fallypride in monkeys using a PET multiple-injection protocol.

The goal of this work was to quantify the in vivo transport and binding parameters of [F-18]fallypride and the D2/D3 receptor density (B'max) in both the striatal (putamen, caudate, ventral striatum) and extrastriatal regions (thalamus, amygdala, cerebellum, temporal and frontal cortices) of the rhesus monkey brain. Multiple-injection PET experimental protocols with injections of radiolabeled and unlabeled doses of fallypride were used to estimate the K1, k2, kon/VR, koff and B'max kinetic parameters. The experimental design was chosen using the D-optimal criterion to maximize the precision of the estimated binding parameters for the various brain regions. There was a significant range in B'max for the putamen (27 pmol/mL), caudate (23 pmol/mL), ventral striatum (14 pmol/mL), thalamus (1.8 pmol/mL) and amygdala (0.9 pmol/mL). Significant receptor binding was also found in the cortical regions. Knowledge of these in vivo rate constants serves as a necessary step in using [F-18]fallypride PET to measure D2/D3 receptor density and drug occupancy in clinical research applications. We believe the precise parameter estimates derived from these complicated experimental protocols are necessary for proper application of drug occupancy and clinical research studies with [F-18]fallypride, which often rely on the validity of assumptions regarding the model parameters.