[11C]tropanyl benzilate-binding to muscarinic cholinergic receptors: methodology and kinetic modeling alternatives

Alterations in adolescent brain serotonin (5HT)1A, 5HT2A, and dopamine (D)2 receptor systems in a nonhuman primate model of early life adversity

Stress affects brain serotonin (5HT) and dopamine (DA) function, and the effectiveness of 5HT and DA to regulate stress and emotional responses. However, our understanding of the long-term impact of early life adversity (ELA) on primate brain monoaminergic systems during adolescence is scarce and inconsistent. Filling this gap in the literature is critical, given that the emergence of psychopathology during adolescence has been related to deficits in these systems. Here, we use a translational nonhuman primate (NHP) model of ELA (infant maltreatment by the mother) to examine the long-term impact of ELA on adolescent 5HT1A, 5HT2A and D2 receptor systems. These receptor systems were chosen based on their involvement in stress/emotional control, as well as reward and reinforcement. Rates of maternal abuse, rejection, and infant's vocalizations were obtained during the first three postnatal months, and hair cortisol concentrations obtained at 6 months postnatal were examined as early predictors of binding potential (BP) values obtained during adolescence using positron emission tomography (PET) imaging. Maltreated animals demonstrated significantly lower 5HT1A receptor BP in prefrontal cortical areas as well as the amygdala and hippocampus, and lower 5HT2A receptor BP in striatal and prefrontal cortical areas. Maltreated animals also demonstrated significantly lower D2 BP in the amygdala. None of the behavioral and neuroendocrine measurements obtained early in life predicted any changes in BP data. Our findings suggest that early caregiving experiences regulate the development of brain 5HT and DA systems in primates, resulting in long-term effects evident during adolescence.

Imaging Cholinergic Receptors in the Brain by Positron Emission Tomography

Cholinergic receptors represent a promising class of diagnostic and therapeutic targets due to their significant involvement in cognitive decline associated with neurological disorders and neurodegenerative diseases as well as cardiovascular impairment. Positron emission tomography (PET) is a noninvasive molecular imaging tool that has helped to shed light on the roles these receptors play in disease development and their diverse functions throughout the central nervous system (CNS). In recent years, there has been a notable advancement in the development of PET probes targeting cholinergic receptors. The purpose of this review is to provide a comprehensive overview of the recent progress in the development of these PET probes for cholinergic receptors with a specific focus on ligand structure, radiochemistry, and pharmacology as well as in vivo performance and applications in neuroimaging. The review covers the structural design, pharmacological properties, radiosynthesis approaches, and preclinical and clinical evaluations of current state-of-the-art PET probes for cholinergic receptors.

Update on PET Tracer Development for Muscarinic Acetylcholine Receptors

The muscarinic cholinergic system regulates peripheral and central nervous system functions, and, thus, their potential as a therapeutic target for several neurodegenerative diseases is undoubted. A clinically applicable positron emission tomography (PET) tracer would facilitate the monitoring of disease progression, elucidate the role of muscarinic acetylcholine receptors (mAChR) in disease development and would aid to clarify the diverse natural functions of mAChR regulation throughout the nervous system, which still are largely unresolved. Still, no mAChR PET tracer has yet found broad clinical application, which demands mAChR tracers with improved imaging properties. This paper reviews strategies of mAChR PET tracer design and summarizes the binding properties and preclinical evaluation of recent mAChR tracer candidates. Furthermore, this work identifies the current major challenges in mAChR PET tracer development and provides a perspective on future developments in this area of research.

Use of 55 PET radiotracers under approval of a Radioactive Drug Research Committee (RDRC)

BACKGROUND

In the US, EU and elsewhere, basic clinical research studies with positron emission tomography (PET) radiotracers that are generally recognized as safe and effective (GRASE) can often be conducted under institutional approval. For example, in the United States, such research is conducted under the oversight of a Radioactive Drug Research Committee (RDRC) as long as certain requirements are met. Firstly, the research must be for basic science and cannot be intended for immediate therapeutic or diagnostic purposes, or to determine the safety and effectiveness of the PET radiotracer. Secondly, the PET radiotracer must be generally recognized as safe and effective. Specifically, the mass dose to be administered must not cause any clinically detectable pharmacological effect in humans, and the radiation dose to be administered must be the smallest dose practical to perform the study and not exceed regulatory dose limits within a 1-year period. In our experience, the main barrier to using a PET radiotracer under RDRC approval is accessing the required information about mass and radioactive dosing.

RESULTS

The University of Michigan (UM) has a long history of using PET radiotracers in clinical research studies. Herein we provide dosing information for 55 radiotracers that will enable other PET Centers to use them under the approval of their own RDRC committees.

CONCLUSIONS

The data provided herein will streamline future RDRC approval, and facilitate further basic science investigation of 55 PET radiotracers that target functionally relevant biomarkers in high impact disease states.

Development of novel PET probe [¹¹C](R,R)HAPT and its stereoisomer [¹¹C](S,S)HAPT for vesicular acetylcholine transporter imaging: a PET study in conscious monkey

Carbon-11-labeled (R,R)trans-8-methyl-2-hydroxy-3-[4-[2-aminophenyl]piperizinyl]-tetralin ([(11)C](R,R)HAPT) and its stereoisomer [(11)C](S,S)HAPT were developed for imaging vesicular acetylcholine transporters (VAChTs), exclusively located in presynaptic cholinergic neurons. Both positron emission tomography (PET) probes were evaluated in the brain of conscious monkey (Macaca mulatta) using high-resolution PET. Time-activity curves (TACs) of [(11)C](R,R)HAPT peaked within 5 min after the injection in all regions except the caudate and putamen, both of which showed peaks around 20 min postinjection. The regional distribution patterns of [(11)C](R,R)HAPT determined as total distribution volume (V(t)) were highest in the putamen, high in the caudate, intermediate in the amygdala, hippocampus, and thalamus, lower in the cingulate gyrus and frontal, temporal, and occipital cortices, and lowest in the cerebellum. In contrast, the distribution and TACs of [(11)C](S,S)HAPT were homogeneous in all regions. The uptake of [(11)C](R,R)HAPT was reduced by 1 mg/kg (-)-vesamicol, a specific VAChT antagonist, in all regions except the cerebellum, but not by 0.1 mg/kg SA4503, a specific sigma-1 receptor agonist. These results well reflect the in vitro affinity assessments using rat cerebral membranes. They also demonstrate that [(11)C](R,R)HAPT is a potential PET probe for noninvasive and quantitative imaging of VAChT in the living brain.

OAT3-mediated extrusion of the 99mTc-ECD metabolite in the mouse brain

After administration of the (99m)Tc complex with N,N'-1,2-ethylenediylbis-L-cysteine diethyl ester ((99m)Tc-ECD), a brain perfusion imaging agent, the radioactive metabolite is trapped in primate brain, but not in mouse and rat. Here, we investigate the involvement of metabolite extrusion by organic anion transporter 3 (OAT3), which is highly expressed at the blood-brain barrier in mice, in this species difference. The efflux rate of radioactivity in the cerebrum of Oat3(-/-) mice at later phase was 20% of that of control mice. Thus, organic anion transporters in mouse brain would be involved in the low brain retention of radioactivity after (99m)Tc-ECD administration.

Novel PET probes 18F-BCPP-EF and 18F-BCPP-BF for mitochondrial complex I: a PET study in comparison with 18F-BMS-747158-02 in rat brain

We developed novel PET probes, 2-tert-butyl-4-chloro-5-{6-[2-(2-(18)F-fluoroethoxy)-ethoxy]-pyridin-3-ylmethoxy}-2H-pyridazin-3-one ((18)F-BCPP-EF) and 2-tert-butyl-4-chloro-5-[6-(4-(18)F-fluorobutoxy)-pyridin-3-ylmethoxy]-2H-pyridazin-3-one ((18)F-BCPP-BF), for quantitative imaging of mitochondrial complex I (MC-I) activity in the brain and preliminarily evaluated their properties in comparison with (18)F-BMS-747158-02 ((18)F-BMS).

METHODS

The affinity of (18)F-BCPP-EF, (18)F-BCPP-BF, and (18)F-BMS to MC-I was analyzed using in vitro binding assays with (3)H-dihydrorotenone and bovine cardiomyocyte submitochondrial particles. (18)F-BCPP-EF, (18)F-BCPP-BF, or (18)F-BMS was intravenously injected into rats, and the uptake (standardized uptake value) in each organ was determined by dissection method. The effects of rotenone, a specific MC-I inhibitor, on the uptake of each probe were assessed by whole-body PET imaging in rats. Ischemic brain model rats were imaged using (18)F-BCPP-EF.

RESULTS

The rank order of affinity to MC-I was (18)F-BCPP-BF > (18)F-BMS > (18)F-BCPP-EF. The uptake of (18)F-BCPP-EF and (18)F-BMS was high in the heart, intermediate in brain, and low in muscle and bone 60 min after the injection. (18)F-BCPP-BF provided increasing bone uptake with time after the injection. The uptake of (18)F-BCPP-EF and (18)F-BMS into the brain and heart was significantly decreased by preadministration of rotenone; however, the reduction degree of (18)F-BCPP-EF was more pronounced than that of (18)F-BMS. Rotenone did not affect (18)F-BCPP-BF uptake in either the brain or the heart. (18)F-BCPP-EF imaged the cortical ischemic neuronal damage without any disturbance by microglial activation even on day 7 when (18)F-FDG showed high uptake in the damaged area.

CONCLUSION

The present study demonstrated that (18)F-BCPP-EF could be a potential PET probe for quantitative imaging of MC-I activity and its ischemic damage in the living brain with PET.

Quantification of dopamine transporter density with [18F]FECNT PET in healthy humans

INTRODUCTION

Fluorine-18 labeled 2β-carbomethoxy-3β-(4-chlorophenyl)-8-(2-fluoroethyl)nortropane ([(18)F]FECNT) binds reversibly to the dopamine transporter (DAT) with high selectivity. [(18)F]FECNT has been used extensively in the quantification of DAT occupancy in non-human primate brain and can distinguish between Parkinson's and healthy controls in humans. The purpose of this work was to develop a compartment model to characterize the kinetics of [(18)F]FECNT for quantification of DAT density in healthy human brain.

METHODS

Twelve healthy volunteers underwent 180 min dynamic [(18)F]FECNT PET imaging including sampling of arterial blood. Regional time-activity curves were extracted from the caudate, putamen and midbrain including a reference region placed in the cerebellum. Binding potential, BPND, was calculated for all regions using kinetic parameters estimated from compartmental and Logan graphical model fits to the time-activity data. Simulations were performed to determine whether the compartment model could reliably fit time-activity data over a range of BPND values.

RESULTS

The kinetics of [(18)F]FECNT were well-described by the reversible 2-tissue arterial input and full reference tissue compartment models. Calculated binding potentials in the caudate, putamen and midbrain were in good agreement between the arterial input model, reference tissue model and the Logan graphical model. The distribution volume in the cerebellum did not reach a plateau over the duration of the study, which may be a result of non-specific binding in the cerebellum. Simulations that included non-specific binding show that the reference and arterial input models are able to estimate BPND for DAT densities well below that observed in normal volunteers.

CONCLUSION

The kinetics of [(18)F]FECNT in human brain are well-described by arterial input and reference tissue compartment models. Measured and simulated data show that BPND calculated with reference tissue model is proportional to BPND calculated from the arterial input model.

Evaluation of 18F-BCPP-EF for mitochondrial complex 1 imaging in the brain of conscious monkeys using PET

PURPOSE

We have reported on the development of a novel PET probe, (18)F-2-tert-butyl-4-chloro-5-{6-[2-(2-fluoroethoxy)-ethoxy]-pyridin-3-ylmethoxy}-2H-pyridazin-3-one ((18)F-BCPP-EF), for quantitative imaging of mitochondrial complex 1 (MC-1) activity in the brain of the living rat. For clinical application in humans, translational research in the monkey was conducted.

METHODS

PET measurements with (18)F-BCPP-EF were performed in young and old monkeys (Macaca mulatta) in a conscious state with arterial blood sampling. The binding specificity of (18)F-BCPP-EF was evaluated with rotenone, a specific MC-1 inhibitor, in young animals. The binding (total distribution volume, V T) of (18)F-BCPP-EF was calculated using Logan graphical analysis, and one-tissue compartment model (1-TC) and two-tissue compartment model (2-TC) analyses using a metabolite-corrected plasma input function.

RESULTS

F-BCPP-EF was rapidly taken up into the brain just after intravenous injection, peaked between 10 and 20 min after injection, and was then gradually eliminated. The 2-TC analysis provided a better fit than the 1-TC analysis, and the V T values from the 2-TC analysis correlated well with those from the Logan plot. With predosing with rotenone, (18)F-BCPP-EF showed a higher uptake peak in the brain, followed by more rapid elimination thereafter than in the vehicle condition, resulting in significant reductions in 2-TC V T values in all regions. In old animals, the kinetics of (18)F-BCPP-EF were slightly slower with lower peak levels than in young animals, resulting age-related reductions in (18)F-BCPP-EF binding in all brain regions.

CONCLUSION

The present study demonstrated that (18)F-BCPP-EF may be a potential PET probe for quantitative imaging MC-1 activity in the living brain using PET.

In vivo kinetics of [F-18]MEFWAY: a comparison with [C-11]WAY100635 and [F-18]MPPF in the nonhuman primate

[F-18]Mefway was developed to provide an F-18 labeled positron emission tomography (PET) neuroligand with high affinity for the serotonin 5-HT(1A) receptor to improve the in vivo assessment of the 5-HT(1A) system. The goal of this work was to compare the in vivo kinetics of [F-18]mefway, [F-18]MPPF, and [C-11]WAY100635 in the rhesus monkey.

METHODS

Each of four monkeys were given bolus injections of [F-18]mefway, [C-11]WAY100635, and [F-18]MPPF and scans were acquired with a microPET P4 scanner. Arterial blood was sampled to assay parent compound throughout the time course of the PET experiment. Time activity curves were extracted in the high 5-HT(1A) binding areas of the anterior cingulate cortex (ACG), mesial temporal cortex, raphe nuclei, and insula cortex. Time activity curves were also extracted in the cerebellum, which was used as a reference region. The in vivo kinetics of the radiotracers were compared based on the nondisplaceable distribution volume (V(ND) ) and binding potential (BP(ND) ).

RESULTS

At 30 min, the fraction of radioactivity in the plasma due to parent compound was 19%, 28%, and 29% and cleared from the arterial plasma at rates of 0.0031, 0.0078, and 0.0069 (min⁻¹) ([F-18]mefway, [F-18]MPPF, [C-11]WAY100635). The BP(ND) in the brain regions were mesial temporal cortex: 7.4 ± 0.6, 3.1 ± 0.4, 7.0 ± 1.2, ACG: 7.2 ± 1.2, 2.1 ± 0.2, 7.9 ± 1.2; raphe nuclei: 3.7 ± 0.6, 1.3 ± 0.3, 3.3 ± 0.7; and insula cortex: 4.2 ± 0.6, 1.2 ± 0.1, 4.7 ± 1.0 for [F-18]mefway, [F-18]MPPF, and [C-11]WAY100635 respectively.

CONCLUSIONS

In the rhesus monkey, [F-18]mefway has similar in vivo kinetics to [C-11]WAY100635 and yields greater than 2-fold higher BP(ND) than [F-18]MPPF. These properties make [F-18]mefway a promising radiotracer for 5-HT(1A) assay, providing higher counting statistics and a greater dynamic range in BP(ND).

Multiple linear analysis methods for the quantification of irreversibly binding radiotracers

Gjedde-Patlak graphical analysis (GPGA) has commonly been used to quantify the net accumulations (K(in)) of radioligands that bind or are taken up irreversibly. We suggest an alternative approach (MLAIR: multiple linear analysis for irreversible radiotracers) for the quantification of these types of tracers. Two multiple linear regression model equations were derived from differential equations of the two-tissue compartment model with irreversible binding. Multiple linear analysis for irreversible radiotracer 1 has a desirable feature for ordinary least square estimations because only the dependent variable C(T)(t) is noisy. Multiple linear analysis for irreversible radiotracer 2 provides K(in) from direct estimates of the coefficients of independent variables without the mediation of a division operation. During computer simulations, MLAIR1 provided less biased K(in) estimates than the other linear methods, but showed a high uncertainty level for noisy data, whereas MLAIR2 increased the robustness of estimation in terms of variability, but at the expense of increased bias. For real [(11)C]MeNTI positron emission tomography data, both methods showed good correlations, with parameters estimated using the standard nonlinear least squares method. Multiple linear analysis for irreversible radiotracer 2 parametric images showed remarkable image quality as compared with GPGA images. It also showed markedly improved statistical power for voxelwise comparisons than GPGA. The two MLAIR approaches examined were found to have several advantages over the conventional GPGA method.

Why does the agonist [18F]FP-TZTP bind preferentially to the M2 muscarinic receptor?

PURPOSE

Preferential binding of FP-TZTP at the M(2) receptor in vivo led to investigation of [(18)F]FP-TZTP as a potential PET tracer for Alzheimer's disease, in which a substantial reduction of M(2) receptors has been observed in autopsy studies. We hereby investigated in vitro the FP-TZTP behavior to further elucidate the properties of FP-TZTP that lead to its M(2) selectivity.

METHODS

Chinese hamster ovarian cells expressing the five subtypes of human muscarinic receptor as well as the wild type were harvested in culture to assess equilibrium binding. Specific binding was calculated by subtraction of non-specific binding from total binding. Internal specific binding was calculated by subtraction of external specific binding from the total specific binding. Saturation assays were also performed to calculate B(max), K(i), and IC(50). In addition, equilibrium binding and dissociation kinetic studies were performed on rat brain tissue. Selected regions of interest were drawn on the digital autoradiograms and [(18)F]FP-TZTP off-rates were determined by measurement of the rate of release into a buffer solution of [(18)F]FP-TZTP from slide-bound cells that had been preincubated with [(18)F]FP-TZTP.

RESULTS

At equilibrium in vitro, M(2) subtype selectivity of [(18)F]FP-TZTP was not evident. We demonstrated that ATP-dependent mechanisms are not responsible for FP-TZTP M(2) selectivity. In vitro off-rate studies from rat brain tissue showed that the off-rate of FP-TZTP varied with the percentage of M(2) subtype in the tissue region.

CONCLUSION

The slower dissociation kinetics of FP-TZTP from M(2) receptors compared with the four other muscarinic receptor subtypes may be a factor in its M(2) selectivity.

Kinetic modeling of amyloid binding in humans using PET imaging and Pittsburgh Compound-B

A valid quantitative imaging method for the measurement of amyloid deposition in humans could improve Alzheimer's disease (AD) diagnosis and antiamyloid therapy assessment. Our group developed Pittsburgh Compound-B (PIB), an amyloid-binding radiotracer, for positron emission tomography (PET). The current study was aimed to further validate PIB PET through quantitative imaging (arterial input) and inclusion of subjects with mild cognitive impairment (MCI). Pittsburgh Compound-B studies were performed in five AD, five MCI, and five control subjects and five subjects were retested within 20 days. Magnetic resonance images were acquired for partial volume correction and region-of-interest definition (e.g., posterior cingulate: PCG; cerebellum: CER). Data were analyzed using compartmental and graphical approaches. Regional distribution volume (DV) values were normalized to the reference region (CER) to yield DV ratios (DVRs). Good agreement was observed between compartmental and Logan DVR values (e.g., PCG: r=0.89, slope=0.91); the Logan results were less variable. Nonspecific PIB retention was similar across subjects (n=15, Logan CER DV: 3.63+/-0.48). Greater retention was observed in AD cortical areas, relative to controls (P<0.05). The PIB retention in MCI subjects appeared either 'AD-like' or 'control-like'. The mean test/retest variation was approximately 6% in primary areas-of-interest. The Logan analysis was the method-of-choice for the PIB PET data as it proved stable, valid, and promising for future larger studies and voxel-based statistical analyses. This study also showed that it is feasible to perform quantitative PIB PET imaging studies that are needed to validate simpler methods for routine use across the AD disease spectrum.

(R)-N-[11C]methyl-3-pyrrolidyl benzilate, a high-affinity reversible radioligand for PET studies of the muscarinic acetylcholine receptor

We recently reported the synthesis and binding affinity of ligands for the muscarinic acetylcholine receptor (mAChR) based on both the pyrrolidyl and piperidyl benzilate scaffold. One of these, (R)-3-pyrrolidyl benzilate, was successfully radiolabeled with [(11)C]methyl triflate and the resulting compound, (R)-N-[(11)C]methyl-3-pyrrolidyl benzilate (3-[(11)C]NMPYB), was evaluated as a reversible, acetylcholine-sensitive tracer for the mAChR (K(i) of unlabeled 3-NMPYB is 0.72 nM). This compound displayed high, receptor-mediated retention in regions of the mouse and rat brain known to have high concentrations of mAChRs. Moreover, bolus studies in a pigtail monkey showed that this compound had superior clearance from the brain when compared to muscarinic radiotracers previously employed in human PET studies. Infusion studies in the same monkey revealed that it was possible to achieve equilibrium of radiotracer distribution for 3-[(11)C]NMPYB in both the striatum and cortex. Sensitivity to endogenous acetylcholine levels was evaluated by injecting phenserine (5 mg/kg) into rats prior to administration of 3-[(11)C]NMPYB in an equilibrium infusion protocol. This pretreatment produced a modest, statistically significant decrease (9-11%) in the distribution volume ratios for muscarinic receptor rich regions of the rat brain as compared to controls.

Synthesis and preliminary biological characterization of a new potential (125)I-radioligand for dopamine and serotonin receptors

The synthesis and a preliminary biological characterization of a new class of N-benzyl-aminoalcohols which have serotonin (5-HT(2)) and dopamine (D(2)) receptor affinity is described. In vitro competition binding studies were conducted with the new molecules and (3)H-spiperone on crude membrane preparation from rat striatum and frontal cortex. One of these compounds, 3-benzylamino-1-(4-fluoro-2-iodophenyl)-propan-1-ol (6f), whose IC(50) values are in the micromolar range for both the D(2) and 5-HT(2) receptors, was prepared in iodine-125 labelled form (6i) by nucleophilic substitution of the bromine atom of 3-benzylamino-1-(2-bromo-4-fluorophenyl)-propan-1-ol (6d). In the in vivo studies, conducted on rats, the radiolabelled molecule 6i shows a good capacity to cross the blood-brain barrier (BBB) with a mean value of first pass cerebral extraction (E) of ca. 50% when the regional cerebral blood flow, measured with microsphere technique, is in the experimental animal's physiologic range (0.8-1 mL/min/g). A preliminary in vitro autoradiographic distribution on coronal rat brain slices of the radioiodinated molecule showed that it was preferentially localized in the striatum and in the cerebral regions rich in dopamine- and serotonin receptors, even if a high non-specific binding was observed.

Age-related changes in muscarinic cholinergic receptors in the living brain: a PET study using N-[11C]methyl-4-piperidyl benzilate combined with cerebral blood flow measurement in conscious monkeys

The effects of changes in regional cerebral blood flow (rCBF) with aging on muscarinic cholinergic receptor binding were evaluated with [15O]H(2)O and N-[11C]methyl-4-piperidyl benzilate (4-MPB) in the living brains of young (5.9+/-1.8 years old) and aged (19.0+/-3.3 years old) monkeys (Macaca mulatta) in the conscious state using high-resolution positron emission tomography (PET). For quantitative analysis of receptor binding in vivo with [11C]4-MPB, metabolite-corrected arterial plasma radioactivity curves were obtained as an input function into the brain, and graphical Patlak plot analysis was applied. In addition, two-compartment model analysis using the radioactivity curve in the cerebellum as an input function (reference analysis) was also applied to determine the distribution volume (DV=K(1)/k(2)') for [11C]4-MPB. With metabolite-corrected arterial input, Patlak plot analysis of [11C]4-MPB indicated a regionally specific decrease in muscarinic cholinergic receptor binding in vivo in the frontal and temporal cortices as well as the striatum in aged compared with young animals, showing no correlation with the degree of reduced rCBF. In contrast, on the reference analysis with cerebellar input of [11C]4-MPB, all regions assayed except the pons showed a significant age-related decrease of DV, and the degree of reduction of DV was correlated with that of rCBF. These results demonstrated the usefulness of kinetic analysis of [11C]4-MPB with metabolite-corrected arterial input, not with reference region's input, as an indicator of the aging process of cortical muscarinic cholinergic receptors in vivo measured by PET with less blood flow dependency.

Age differences in muscarinic cholinergic receptors assayed with (+)N-[(11)C]methyl-3-piperidyl benzilate in the brains of conscious monkeys

Age-related changes in muscarinic cholinergic receptors were evaluated with the novel ligand (+)N-[(11)C]methyl-3-piperidyl benzilate ((+)3-MPB) in the living brains of young (5.9 +/- 1.8 years old) and aged (19.0 +/- 3.3 years old) monkeys (Macaca mulatta) in the conscious state using high-resolution positron emission tomography (PET). For quantitative analysis of receptor binding in vivo, metabolite-corrected arterial plasma radioactivity curves were obtained as an input function into the brain, and kinetic analyses using the three-compartment model and graphical Logan plot analysis were applied. Kinetic analyses of [(11)C](+)3-MPB indicated a regionally specific decrease in the receptor binding in vivo determined as binding potential (BP) = k(3)/k(4) in aged animals compared with young animals. Thus, the frontal and temporal cortices as well as the striatum showed age-related reduction of muscarinic cholinergic receptors in vivo, reflecting the reduced receptor density (B(max)) determined by Scatchard plot analysis in vivo. In the hippocampus, although BP of [(11)C](+)3-MPB indicated no significant age-related changes, it showed an inverse correlation with individual cortisol levels in plasma. When the graphical Logan plot analysis was applied, all regions assayed showed significant age-related decrease of [(11)C](+)3-MPB binding. These results demonstrate the usefulness of kinetic three-compartment model analysis of [(11)C](+)3-MPB with metabolite-corrected arterial plasma input as an indicator for the aging process of the cortical muscarinic cholinergic receptors in vivo as measured by PET.

Analyses of [(18)F] altanserin bolus injection PET data. I: consideration of radiolabeled metabolites in baboons

Positron emission tomography (PET) has been used to study serotonin 2A (5-HT(2A)) receptor binding in human brain using the 5-HT(2A) antagonist, [(18)F]altanserin. Previous analyses of bolus injection [(18)F]altanserin data provided 5-HT(2A) specific binding measures that were highly correlated with the in vitro distribution of 5-HT(2A) receptors and reflected decreased binding after ketanserin (5-HT(2A) antagonist) administration. These observations were made in the presence of a nonspecific tissue component that was consistent with blood-brain barrier (BBB) passage of radiolabeled metabolites (radiometabolites). In this work, we evaluated the in vivo kinetics of [(18)F]altanserin and two major radiometabolites of [(18)F]altanserin, focusing on the kinetics of free and nonspecifically-bound radioactivity. PET studies were performed in baboons after the bolus injection of [(18)F]altanserin or one of its major radiometabolites, either [(18)F]altanserinol or [(18)F]4-(4-fluorobenzoyl)piperidine, at baseline and after pharmacologic receptor blockade (blocking data). The cerebellar and blocking data were analyzed using either single (parent radiotracer) or dual (parent radiotracer and radiometabolites) input function methods. After bolus injection of either [(18)F]altanserin metabolite, radioactivity crossed the BBB and localized nonspecifically. The radio- metabolites were found to contribute to nonspecific "background" radioactivity that was similar in receptor-poor and receptor-rich regions. After bolus injection in baboons, two of the major radiometabolites of [(18)F]altanserin crossed the BBB and contributed to a fairly uniform background of nonspecific radioactivity. This uniformity suggests that conventional analyses are appropriate for human bolus injection [(18)F]altanserin PET data, although these methods may overestimate [(18)F]altanserin nonspecific binding.

Evaluation of PET ligands (+)N-[(11)C]ethyl-3-piperidyl benzilate and (+)N-[(11)C]propyl-3-piperidyl benzilate for muscarinic cholinergic receptors: a PET study with microdialysis in comparison with (+)N-[(11)C]methyl-3-piperidyl benzilate in the conscious monkey brain

We developed PET ligands (+)N-[(11)C]ethyl-3-piperidyl benzilate ([(11)C](+)3-EPB) and (+)N-[(11)C]propyl-3-piperidyl benzilate ([(11)C](+)3-PPB) for cerebral muscarinic cholinergic receptors. The distribution and kinetics of the novel ligands were evaluated for comparison with the previously reported ligand (+)N-[(11)C]methyl-3-piperidyl benzilate ([(11)C](+)3-MPB) in the monkey brain (Macaca mulatta) in the conscious state using high-resolution positron emission tomography (PET). At 60-91 min postinjection, regional distribution patterns of these three ligands were almost identical, and were consistent with the muscarinic receptor density in the brain as previously reported in vitro. However, the time-activity curves of [(11)C](+)3-EPB and [(11)C](+)3-PPB showed earlier peak times of radioactivity and a faster clearance rate than [(11)C](+)3-MPB in cortical regions rich in the receptors. Kinetic analysis using the three-compartment model with time-activity curves of radioactivity in metabolite-corrected arterial plasma as input functions revealed that labeling with longer [(11)C]alkyl chain length induced lower binding potential (BP = k(3)/k(4)), consistent with the rank order of affinity of these ligands obtained by an in vitro assay using rat brain slices and [(3)H]QNB. The cholinesterase inhibitor Aricept administered at doses of 50 and 250 microg/kg increased acetylcholine level in extracellular fluid of the frontal cortex and the binding of [(11)C](+)3-PPB with the lowest affinity to the receptors was displaced by the endogenous acetylcholine induced by cholinesterase inhibition, while [(11)C](+)3-MPB with the highest affinity was not significantly affected. Taken together, these observations indicate that the increase in [(11)C]alkyl chain length could alter the kinetic properties of conventional receptor ligands for PET by reducing the affinity to receptors, which might make it possible to assess the interaction between endogenous neurotransmitters and ligand-receptor binding in vivo as measured by PET.

Assessment of muscarinic receptor concentrations in aging and Alzheimer disease with [11C]NMPB and PET

Cerebral cholinergic deficits have been described in Alzheimer disease (AD) and as a result of normal aging. At the present time, there are very limited options for the quantification of cholinergic receptors with in vivo imaging techniques such as PET. In the present study, we examined the feasibility of utilizing [11C]N-methyl-4-piperidyl benzilate (NMPB), a nonselective muscarinic receptor ligand, in the study of aging and neurodegenerative processes associated with cholinergic dysfunction. Based on prior data describing the accuracy of various kinetic methods, we examined the concentration of muscarinic receptors with [11C]NMPB and PET using two- and three-compartment kinetic models. Eighteen healthy subjects and six patients diagnosed with probable AD were studied. Pixel-by-pixel two-compartment model fits showed acceptable precision in the study of normal aging, with comparable results to those obtained with a more complex and less precise three-compartment model. Normal aging was associated with a reduction in muscarinic receptor binding in neocortical regions and thalamus. In AD patients, the three-compartment model appeared capable of dissociating changes in tracer transport from changes in receptor binding, but suffered from statistical uncertainty, requiring normalization to a reference region, and therefore limiting its potential use in the study of neurodegenerative processes. After normalization, no regional changes in muscarinic receptor concentrations were observed in AD.

Evaluation of novel PET ligands (+)N-[11C]methyl-3-piperidyl benzilate ([11C](+)3-MPB) and its stereoisomer [11C](-)3-MPB for muscarinic cholinergic receptors in the conscious monkey brain: a PET study in comparison with

The novel muscarinic cholinergic ligands (+)N-[11C]methyl-3-piperidyl benzilate ([11C](+)3-MPB) and its stereoisomer [11C](-)3-MPB were evaluated in comparison with [11C]4-MPB in the brains of conscious monkeys (Macaca mulatta) using high-resolution positron emission tomography (PET). The regional distribution patterns of [11C](+)3-MPB and [11C]4-MPB at 60-91 min postinjection were almost identical: highest in the striatum and occipital cortex; intermediate in the temporal and frontal cortices, cingulate gyrus, hippocampus, and thalamus; lower in the pons; and lowest in the cerebellum. The uptake of [11C](+)3-MPB in all regions was higher and the dynamic range of regional uptake differences of [11C](+)3-MPB was better than those of [11C]4-MPB. The levels of [11C](-)3-MPB were much lower in all regions of the brain than [11C](+)3-MPB and [11C]4-MPB. Administration of scopolamine, a muscarinic cholinergic antagonist, at a dose of 50 microg/kg reduced the radioactivity of [11C](+)3-MPB and [11C]4-MPB in all regions except the cerebellum. Time-activity curves of [11C](+)3-MPB peaked in all regions, while those of [11C]4-MPB showed gradual increases with time in all regions except the thalamus, pons, and cerebellum. Two graphical analyses (Logan plot and Patlak plot) with plasma radioactivity as an input function into the brain were applied to evaluate receptor binding in vivo. [11C](+)3-MPB showed linear regression curves on Logan plot analysis and nonlinear curves on Patlak plot in all regions, suggesting that [11C](+)3-MPB bound reversibly to the muscarinic receptors. The in vivo binding parameters as well as uptake at 60-91 min postinjection of [11C](+)3-MPB were consistent with muscarinic receptor density in the brain as reported in vitro.

Acetylcholinesterase inhibition increases in vivo N-(2-[18F]fluoroethyl)-4-piperidyl benzilate binding to muscarinic acetylcholine receptors

Although the inhibition of acetylcholinesterase remains the primary treatment of Alzheimer's disease, little is known of the results of increased acetylcholine levels on muscarinic receptor occupancy or function. Using N-(2-[18F]fluoroethyl)-4-piperidyl benzilate ([18F]FEPB), a moderate affinity (Ki = 1.7 nmol/L) nonsubtype-selective muscarinic receptor antagonist, the authors examined the sensitivity of equilibrium in vivo radioligand binding in rat brain with changes in endogenous acetylcholine levels produced by treatments with acetylcholinesterase inhibitors. Phenserine administration 30 minutes before resulted in a dose-dependent into muscarinic cholinergic receptors, reaching a maximum increase of 90% in the striatum at a dose of 5 mg/kg intraperitoneally. Constant infusion of physostigmine at a dosage of 250 microg/kg/min produced an identical increase in radioligand binding. This agonist-induced increase of in vivo mAChR radioligand binding offers a new method for monitoring of the efficacy of acetylcholinesterase inhibitors or other drugs to enhance acetylcholine actions at the muscarinic receptors.

Analyses of [18F]altanserin bolus injection PET data. II: Consideration of radiolabeled metabolites in humans

Imaging serotonin-2A (5-HT(2A)) neuroreceptors with positron emission tomography (PET) and [(18)F]altanserin has been the focus of a series of PET studies, as [(18)F]altanserin is one of the most selective 5-HT(2A) antagonist radiotracers. Previous animal studies showed that radiolabeled metabolites (radiometabolites) of [(18)F]altanserin crossed the blood-brain barrier (BBB) to localize nonspecifically in brain, consistent with a constant radioactivity "background." In this work, we evaluated human bolus injection [(18)F]altanserin PET data with detailed consideration of the impact of BBB-permeable metabolites on the specific binding parameters. Data were quantified using either single (parent radiotracer), dual (parent radiotracer and radiometabolites), or no arterial input function(s) (cerebellum as reference tissue input function). A step-gradient high-performance liquid chromatography (HPLC) analysis provided distinct separation of [(18)F]altanserin and four radiolabeled components in plasma. After [(18)F]altanserin injection, the step-gradient data showed that the major BBB-permeable radiometabolites approached constant levels in plasma (>50 min), consistent with a constant metabolite "background." The single-input Logan graphical results were highly correlated with the dual-input results and its bias was fairly constant across regions and subjects, as similarly observed for a nongraphical reference tissue method. The most comprehensive and quantitatively valid analysis for bolus [(18)F]altanserin PET data was the dual-input method that specifically accounted for BBB-permeable metabolites, although the Logan analysis was preferred because it provided a good compromise between validity, sensitivity, and reliability of implementation. Further study is needed to better understand how the cerebellar kinetics of [(18)F]altanserin and its radiometabolites impact the reference tissue measures.

Synthesis, (18)F-labeling, and biological evaluation of piperidyl and pyrrolidyl benzilates as in vivo ligands for muscarinic acetylcholine receptors

A series of 31 compounds based on the piperidyl or pyrrolidyl benzilate scaffold were prepared from methyl benzilate and 4-piperidinol, (R)-(+)-3-piperidinol, or (R)-(+)-3-pyrrolidinol. Amine substituents included alkyl and aralkyl groups. In vitro K(i) values ranged from 0.05 nM to >100 nM. (R)-N-(2-Fluoroethyl)-3-piperidyl benzilate (3-FEPB, 22, K(i) = 12.1 nM) and N-(2-fluoroethyl)-4-piperidyl benzilate (4-FEPB, 8, K(i) = 1. 83 nM) were selected for radiolabeling with fluorine-18. Using alkylation with 2-[(18)F]fluoroethyl triflate, 3-[(18)F]FEPB (42) and 4-[(18)F]FEPB (43) were produced in 7-9% radiochemical yield and >97% radiochemical purity. For in vivo studies, retention was moderate in mouse brain for 42; however, blocking with scopolamine showed that uptake was not muscarinic cholinergic receptor-mediated. Conversely, 43 exhibited high, receptor-mediated retention in mouse brain, with significant clearance after 1 h. These results suggest that 43 could have applications as an in vivo probe for measuring endogenous acetylcholine levels.

Development and evaluation of muscarinic cholinergic receptor ligands N-[11C]ethyl-4-piperidyl benzilate and N-[11C]propyl-4-piperidyl benzilate: a PET study in comparison with N-[11C]methyl-4-piperidyl benzilate in the conscious monkey brain

The muscarinic cholinergic receptor ligands N-[(11)C]ethyl-4-piperidyl benzilate (4-EPB) and N-[(11)C]propyl-4-piperidyl benzilate (4-PPB) were developed and evaluated in comparison with N-[(11)C]methyl-4-piperidyl benzilate (4-MPB) in the conscious monkey brain using positron emission tomography (PET). Time-activity curves of [(11)C]4-EPB, unlike [(11)C]4-MPB, showed peaks within 91 min in regions rich in muscarinic receptors. [(11)C]4-PPB showed no specific binding even in the regions rich in these receptors. These observation demonstrated that increases in [(11)C]alkyl chain length could alter the kinetic properties of receptor ligands for PET.

Effects of fluvoxamine treatment on the in vivo binding of [F-18]FESP in drug naive depressed patients: a PET study

This study investigates the effect of chronic treatment with Fluvoxamine, a potent and specific serotonin reuptake sites inhibitor (SSRI), on 5HT(2) serotonin and D(2) dopamine receptors in the brain of drug naive unipolar depressed patients. Drug effect was evaluated in different cortical areas and in the basal ganglia by positron emission tomography (PET) and fluoro-ethyl-spiperone ([(18)F]FESP), an high affinity 5HT(2) serotonin and D(2) dopamine receptors antagonist. Patients underwent a PET study at recruitment and after clinical response to Fluvoxamine treatment. Nine of the 15 patients recruited completed the study. Fluvoxamine treatment significantly improved clinical symptoms and modified [(18)F]FESP binding in the frontal and occipital cortex of all of the nine patients who completed the study; in these regions a mean 31% increase in the in vivo [(18)F]FESP binding was found (P < 0.01). On the contrary, no significant changes in the in vivo [(18)F]FESP binding were found in the basal ganglia where [(18)F]FESP binds mainly to D(2) dopamine receptors. Chronic treatment with Fluvoxamine significantly increases the in vivo binding of [(18)F]FESP in the frontal and occipital cortex of drug naive unipolar depressed patients. The increase of the in vivo binding of [(18)F]FESP may reflect a modification in 5HT(2) binding capacity secondary to changes in cortical serotonin activity.

Sex-related differences in the muscarinic acetylcholinergic receptor in the healthy human brain--a positron emission tomography study

We evaluated the sex-related differences in the decline of the cerebral muscarinic acetylcholinergic receptor (mACh-R) due to aging by using 11C-N-methyl-4-piperidyl benzilate (11C-NMPB) and positron emission tomography (PET). The subjects consisted of 37 (20 males and 17 females) healthy volunteers. The 11C-NMPB uptake was evaluated by the ratio method (regional 11C-NMPB uptake/Cerebellar 11C-NMPB uptake; rNMPB ratio). The correlation between sex, aging, and the rNMPB ratio in normal aging was evaluated by a multiple regression analysis. The rNMPB ratio was higher in females than in males throughout the entire cerebral region (p < 0.01-p < 0.0001) and the rNMPB ratio might thus possibly decline with age more rapidly in females. Our study therefore revealed the existence of sex-related differences in the cerebral mACh-R.

Ligand-receptor interactions as studied by PET: implications for drug development

Positron emission tomography (PET) is a quantitative imaging method that can be used to characterize binding properties of specific target molecules such as various receptors, transporter molecules and enzymes in vivo. Although already applied successfully, one of the greatest challenges for the technique is to understand better the in vivo complexities of ligand-receptor (target) interaction. The PET technique can be used efficiently in animal studies but, most importantly, also in human studies. PET imaging of patients and healthy volunteers can generate information on human pathophysiology at a molecular level currently unobtainable with other methods. Modern imaging techniques are increasingly applied to drug discovery and development. There are many ways of utilizing PET in pharmacodynamic studies, one interesting approach being the indirect exploration of synaptic neurotransmission with receptor ligands. The receptor occupancy-type studies with PET are rapidly becoming a state-of-the-art method for verifying the mechanism of action of a given drug in man and especially for facilitating the dose-finding procedures in early drug development. Thus far, PET has been mainly applied to pharmacodynamic studies in the central nervous system but will be used also in other areas of drug development such as cardiovascular diseases and oncology.

Quantification of delta-opioid receptors in human brain with N1'-([11C]methyl) naltrindole and positron emission tomography

The regional binding of N1'-([11C]methyl)naltrindole (MeNTI), a selective delta-opioid antagonist, was studied in healthy human subjects with positron emission tomography (PET). After the bolus intravenous administration of high specific activity [11C]MeNTI, PET was performed over 90 minutes. Arterial plasma samples were obtained during the scanning period and assayed for the presence of radiolabeled metabolites. The data were analyzed with various kinetic (two- and three-compartment models, Patlak graphical analysis) and nonkinetic (apparent volume of distribution and activity at a late scanning time) approaches. This tracer showed irreversible binding characteristics during the scanning period used. The results of the analyses also were compared with the density and distribution of delta-opioid receptors in the human brain in vitro. Additionally, computer simulations were performed to assess the effects of changes in receptor binding and tracer transport changes on the perceived binding parameters obtained with the models. A constrained three-compartment kinetic model was demonstrated to be superior to other quantification models for the description of MeNTI kinetics and quantification of delta receptor binding in the human brain with 11C-labeled MeNTI.

Kinetic analysis of [11C]McN5652: a serotonin transporter radioligand

The impulse response function of a radioligand is the most fundamental way to describe its pharmacokinetics and to assess its tissue uptake and retention pattern. This study investigates the impulse response function of [11C](+)McN5652, a radioligand used for positron emission tomography (PET) imaging of the serotonin transporter (SERT) in the brain. Dynamic PET studies were performed in eight healthy volunteers injected with [11C](+)McN5652 and subsequently with its pharmacologically inactive enantiomer [11C](-)McN5652. The impulse response function was calculated by deconvolution analysis of regional time-activity curves, and its peak value (f(max)), its retention value at 75 minutes (fT), and its normalized retention (f(rel) = fT/f(max)) were obtained. Alternatively, compartmental models were applied to calculate the apparent total distribution volume (DV(T)) and its specific binding component (DV(S)). Both the noncompartmental (fT,f(rel)) and the compartmental parameters (DV) were investigated with and without correction for nonspecific binding by simple subtraction of the corresponding value obtained with [11C](-)McN5652. The impulse response function obtained by deconvolution analysis demonstrated high tracer extraction followed by a slow decline in the form of a monoexponential function. Statistical analysis revealed that the best compartmental model in terms of analysis of variance F and condition number of the parameter variance-covariance matrix was the one that was based on a single tissue compartment with parameters k1 and k2 and that also included the parameter of regional cerebral blood volume (BV). The parameter f(rel) demonstrated low between-subject variance (coefficient of variation [CV] = 19%), a midbrain to cerebellum ratio of 1.85, and high correlation with the known density of SERT (r = 0.787 where r is the coefficient of linear correlation between the parameter and the known density of SERT). After correction for nonspecific binding, f(rel) demonstrated further improvement in correlation (r = 0.814) and midbrain to cerebellum ratio (3.09). The variance of the distribution volumes was acceptable when the logarithmic transform lnDV was used instead of DV (17% for the three-parameter model), but correlation of this compartmental parameter was slightly less (r = 0.652 for the three-parameter model) than the correlation of the noncompartmental f(rel) with the known density of SERT, and the midbrain to cerebellum ratio was only 1.5 (uncorrected) and 1.8 (corrected). At the expense of increasing variance, the correlation was increased after correction for nonspecific binding using the inactive enantiomer (r = 0.694; CV = 22%). These results indicate that the kinetics of [11C](+)McN5652 can best be described by a one-tissue compartment model with three parameters (k1, k2, and BV), and that both the noncompartmental parameter f(rel) and the compartmental distribution volumes have the potential for quantitative estimation of the density of SERT. Further validation of the radioligand in experimental and clinical situations is warranted.

Synthesis and autoradiographic localization of muscarinic cholinergic antagonist (+)N-[11C]methyl-3-piperidyl benzilate as a potent radioligand for positron emission tomography

(+)N-[11C]methyl-3-piperidyl benzilate, a relatively low affinity muscarinic cholinergic receptor antagonist was synthesized by N-[11C]methylation of (+)3-piperidyl benzilate using [11C]methyl iodide. The product was isolated by HPLC, and obtained with radiochemical yield of 60-70% from [11C]methyl iodide, and a specific activity of 500-1000 Ci mmol-1 (18.5-37 GBq mumol-1) at EOS and radiochemical purity of > 98%. In vitro autoradiographic studies showed selective binding for this radiotracer in the different regions of the rat brain: high in corpus striatum, hippocampus and cerebral cortex, and low in cerebellum, consistent with muscarinic cholinergic receptor distributions. This radiotracer thus had potential as radioligand for positron emission tomography.

Influence of acetylcholine on binding of 4-[125I]iododexetimide to muscarinic brain receptors

The distribution of nicotinic and muscarinic cholinergic receptors in the human brain in vivo has been successfully characterized using radiolabeled tracers and emission tomography. The effect of acetylcholine release into the synaptic cleft on receptor binding of these tracers has not yet been investigated. The present study examined the influence of acetylcholine on binding of 4-[125I]iododexetimide to muscarinic cholinergic receptors of porcine brain synaptosomes in vitro. 4-Iododexetimide is a subtype-unspecific muscarinic receptor antagonist with high affinity. Acetylcholine competed with 4-[125I]iododexetimide in a dose-dependent manner. A concentration of 500 microM acetylcholine inhibited 50% of total specific 4-[125I]iododexetimide binding to synaptosomes when both substances were given simultaneously. An 800 microM acetylcholine solution reduced total specific 4-[125I]iododexetimide binding by about 35%, when acetylcholine was given 60 min after incubation of synaptosomes with 4-[125I]iododexetimide. Variations in the synaptic acetylcholine concentration might influence muscarinic cholinergic receptor imaging in vivo using 4-[123I]iododexetimide. Conversely, 4-[123I]iododexetimide might be an appropriate molecule to investigate alterations of acetylcholine release into the synaptic cleft in vivo using single photon emission computed tomography.

Muscarinic cholinergic receptor measurements with [18F]FP-TZTP: control and competition studies

[18F]Fluoropropyl-TZTP (FP-TZTP) is a subtype-selective muscarinic cholinergic ligand with potential suitability for studying Alzheimer's disease. Positron emission tomography studies in isofluorane-anesthetized rhesus monkeys were performed to assess the in vivo behavior of this radiotracer. First, control studies (n = 11) were performed to characterize the tracer kinetics and to choose an appropriate model using a metabolite-corrected arterial input function. Second, preblocking studies (n = 4) with unlabeled FP-TZTP were used to measure nonspecific binding. Third, the sensitivity of [18F]FP-TZTP binding to changes in brain acetylcholine (ACh) was assessed by administering physostigmine, an acetylcholinesterase (AChE) inhibitor, by intravenous infusion (100 to 200 microg x kg(-1) x h(-1)) beginning 30 minutes before tracer injection (n = 7). Tracer uptake in the brain was rapid with K1 values of 0.4 to 0.6 mL x min(-1) x mL(-1) in gray matter. A model with one tissue compartment was chosen because reliable parameter estimates could not be obtained with a more complex model. Volume of distribution (V) values, determined from functional images created by pixel-by-pixel fitting, were very similar in cortical regions, basal ganglia, and thalamus, but significantly lower (P < 0.01) in the cerebellum, consistent with the distribution of M2 cholinergic receptors. Preblocking studies with unlabeled FP-TZTP reduced V by 60% to 70% in cortical and subcortical regions. Physostigmine produced a 35% reduction in cortical specific binding (P < 0.05), consistent with increased ACh competition. The reduction in basal ganglia (12%) was significantly smaller (P < 0.05), consistent with its markedly higher AChE activity. These studies indicate that [18F]FP-TZTP should be useful for the in vivo measurement of muscarinic receptors with positron emission tomography.

Quantification of muscarinic cholinergic receptors with [11C]NMPB and positron emission tomography: method development and differentiation of tracer delivery from receptor binding

Quantification of human brain muscarinic cholinergic receptors was investigated with the use of [11C]N-methyl-4-piperidyl benzylate (NMPB) and positron emission tomography (PET). Whole-brain uptake of NMPB at 90 to 110 minutes after intravenous injection was approximately 10% of the administered dose. The initial cerebral distribution of NMPB corresponded to the pattern of cerebral perfusion; however, at progressively longer postinjection intervals, regional distinctions consistent with muscarinic receptor binding were evident: activity at 90 to 110 minutes postinjection was highest in the striatum and cerebral cortex, intermediate in the thalamus and pons, and lowest in the cerebellum. After the development of a chromatographic system for isolation of authentic [11C]NMPB in plasma, tracer kinetic modeling was used to estimate receptor binding from the cerebral and arterial plasma tracer time-courses. Ligand transport rate and receptor-binding estimates were obtained with the use of compartmental models and analytical methods of varying complexity, including a two-parameter pixel-by-pixel-weighted integral approach and regional least-squares curve-fitting analyses employing both two- and three-compartment model configurations. In test-retest experiments, precision of the methods and their abilities to distinguish altered ligand delivery from binding in occipital cortex during an audiovisual presentation were evaluated. Visual stimulation increased the occipital blood-to-brain NMPB transport rate by 25% to 46% in estimates arising from the various approaches. Weighted integral analyses resulted in lowest apparent transport changes and in a concomitant trend toward apparent binding increases during visual activation. The regional least-squares procedures were superior to the pixel-by-pixel method in isolating the effects of altered tracer delivery from receptor-binding estimates, indicating larger transport effects and unaltered binding. Precision was best (less than 10% test-retest differences) for the weighted integral analyses and was somewhat lower in the least-squares analyses (10-25% differences). The authors conclude that pixel-by-pixel-weighted integral analyses of NMPB distribution introduce transport biases into receptor-binding estimates. Similar confounding effects also are predicted in noncompartmental analyses of delayed radiotracer distribution. The use of regional nonlinear least-squares fitting to two- and three-compartment models, although more labor intensive, provides accurate distinction of receptor-binding estimates from tracer delivery with acceptable precision in both intra- and intersubject comparisons.

First-pass cerebral extraction of benzamide-derivative radiotracers for SPECT

Central dopaminergic receptors are widely studied for their importance in the pathophysiology of neurological and psychiatric diseases. We have investigated the cerebral delivery kinetics of three dopaminergic ligands in rats through the use of an indicator fractionation method to measure the tracer's regional influx rate constant with respect to regional blood flow. The aim is to collect the in vivo kinetic parameters of the radioligand cerebral distribution, which are necessary if, dealing with SPECT and "trapped" tracers, one wishes to analyse data using a graphical approach.

Cerebral muscarinic acetylcholinergic receptor measurement in Alzheimer's disease patients on 11C-N-methyl-4-piperidyl benzilate--comparison with cerebral blood flow and cerebral glucose metabolism

We studied the cerebral muscarinic acetylcholinergic receptor (mACh-R) by means of 11C-N-methyl-4-piperidyl benzilate (11C-NMPB) and positron emission tomography (PET) in Alzheimer's disease (AD) cases, and the findings were compared with the cerebral blood flow (CBF) and the glucose metabolism (CMRGlc) to evaluate the relationship between the mACh-R and the CBF or the CMRGlc. The subjects consisted of 18 patients with AD and 18 age and sex matched normal volunteers. The patients were clinically diagnosed according to the criteria of the NINDS-ADRDA as having "probable AD" and were thus classified into two groups (mild and moderate AD) according to the severity of dementia determined by DSM-III-R. The CBF was measured by 99mTc-HMPAO SPECT, and the CMRGlc was measured by 18FDG PET. The 11C-NMPB uptake was evaluated by the graphical method and the ratio method (ROIs/Cerebellum). A significant mACh-R decrease and more severe CMRGlc decrease in the cortical region was seen in mild and moderate AD. The decrease in the CBF was not as obvious as that in the mACh-R and the CMRGlc. Our study thus suggested that the mACh-R decreased in patients with AD, and that the 18FDG PET was the most sensitive method for detecting the degenerative regions in patients with AD.

In vivo muscarinic binding selectivity of (R,S)- and (R,R)-[18F]-fluoromethyl QNB

We have developed a multistep radiochemical synthesis of two diastereomers of quinuclidinyl-4-[18F]-fluoromethyl-benzilate ([18F]-FMeQNB), a high-affinity ligand for muscarinic acetylcholine receptors. Previously, we have shown that the nonradioactive (R,R)-diastereomer displays an eightfold selectivity for M1 over M2 while the nonradioactive (R,S)-diastereomer displays a sevenfold selectivity for M2 over M1 in vitro. This paper reports the results of in vivo comparison studies. In the rat, uptake of (R,S)-[18F]-FMeQNB was nearly uniform in all brain regions following the concentration of M2 subtype. The uptake was reduced by 36-54% in all brain regions on coinjection with 50 nmol of unlabeled ligand. An injection of (R,S)-[18F]-FMeQNB followed at 60 min by injection of unlabeled ligand and subsequent sacrifice at 120 min displaced 30-50% of radioactivity in the pons, medulla, and cerebellum, which contain a high proportion of M2 subtype. The most dramatic displacement and inhibition of uptake on coinjection of (R,S)-[18F]-FMeQNB was observed in the heart. In rhesus monkey, the compound showed prolonged uptake and retention in the brain. In the blood, the parent compound degraded rapidly to a single radiolabeled polar metabolite believed to be fluoride. Within 30 min the parent compound represented less than 5% of the plasma activity. Displacement with (R)-QNB was generally slow, but was more rapid from those tissues which contain a higher proportion of M2 subtype. The results are consistent with the hypothesis that (R,S)-[18F]-FMeQNB is M2 selective in vivo. On the other hand, (R,R)-[18F]-FMeQNB showed higher uptake in those brain regions containing a higher concentration of M1 subtype. Uptake in the heart at 60 min was much lower than that observed with the (R,S)-diastereomer. Inhibition of uptake on coinjection with unlabeled (R,S)-FMeQNB is only significant in the heart, thalamus, and pons. Inhibition of uptake on coinjection with unlabeled (R,R)-FMeQNB is quite uniform in all brain regions. Displacement with (R)-QNB shows a more varying amount displaced. These results are consistent with (R,R)-[18F]-FMeQNB being M1 selective in vivo.

Presynaptic monoaminergic vesicles in Parkinson's disease and normal aging

We present development and human application of a method for determining the regional cerebral density of the type 2 vesicular monoamine transporter (VMAT2) using positron emission tomography (PET) and [11C]dihydrotetrabenazine (DTBZ). Previous animal studies indicate striatal VMAT2 density is linearly related to the integrity of substantia nigra dopamine neurons and is not subject to drug- or lesion-compensatory regulation. In the present studies, kinetic compartmental modeling was employed to estimate blood-brain [11C]DTBZ transport (K1) and VMAT2 binding site density (tissue-to-plasma DTBZ distribution volume, DV) from the cerebral and plasma DTBZ time courses after intravenous tracer injection. In controls, we found reductions of putamen DTBZ DVwith advancing age, corresponding to losses of 0.77% per year in specific VMAT2 binding. Parkinson's disease (PD) patients had reduction in specific DTBZ DV in the putamen (-61%) and in the caudate nucleus (-43%). There was no overlap of lowest specific putamen DTBZ DV between individual elderly controls and PD patients. The present results indicate the suitability of [11C]DTBZ PET for objective quantification of nigrostriatal integrity, including evaluation of PD progression and its possible therapeutic modification.

Kinetic evaluation of positron-emitting muscarinic receptor ligands employing direct intracarotid injection

The development and characterization of new receptor ligands for in vivo binding assays are often both lengthy and expensive. It is therefore desirable to predict the suitability of a ligand early in the process of its evaluation. In the present study, compartmental analysis following intracarotid ligand injection in the monkey is used to evaluate the in vivo kinetics of the muscarinic cholinergic receptor antagonists [11C]tropanyl benzilate ([11C]TRB) and [11C]N-methylpiperidyl benzilate ([11C]NMPB). Animals were implanted with chronic subcutaneous access ports and indwelling catheters with tips located in the common carotid artery, just proximal to its bifurcation. The external carotid artery was ligated to ensure selective tracer delivery through the internal carotid artery to the brain. Positron emission tomography was used to measure brain tissue time-activity curves following tracer injections. CBF was estimated from the clearance of [15O]H2O, and receptor ligand distributions were analyzed according to a physiologic model consisting of an intravascular compartment and nonspecific plus free and receptor-bound tissue ligand compartments. In [11C]TRB studies, marked reductions in the forward ligand-receptor binding rate and in both the total and the specific binding tissue-to-plasma volumes of ligand distribution were observed after scopolamine receptor blockade or with low administered specific activity. Conversely, neither the distribution volume of the nonspecific plus free ligand compartment nor the rate of ligand dissociation from receptor sites was affected. In [11C]NMPB studies, tissue compartments describing specific binding and nonsaturable components could not be reliably separated. The receptor-related term in this case, the total tissue-to-plasma distribution volume, demonstrated reduction after low specific activity ligand injection. Comparison of the two ligands suggests that NMPB interacts more rapidly with the receptors and has a lower apparent volume of distribution than does TRB. Thus, NMPB may be the more suitable ligand if accurate estimates of binding dissociation rate are limited by temporal constraints or if simplified, one-tissue-compartment analyses are used. The carotid injection method appears promising for the initial evaluation of ligand kinetics, permitting physiologic compartmental analyses without measurement of input functions or chromatography of blood samples.

Kinetic evaluation of [11C]dihydrotetrabenazine by dynamic PET: measurement of vesicular monoamine transporter

(+)-alpha-[11C]Dihydrotetrabenazine (DTBZ) binds to the vesicular monoamine transporter (VMAT2) located in presynaptic vesicles. The purpose of this work was to evaluate various model configurations for analysis of [11C]DTBZ with the aim of providing the optimal measure of monoamine vesicular transporter density obtainable from a single dynamic PET study. PET studies on seven young normal volunteer subjects, ages 20-35, were performed following i.v. injection of 666 +/- 37 MBq (18 +/- 1 mCi) of (+)-alpha-[11C]DTBZ. Dynamic acquisition consisted of a 15-frame sequence over 1 h. Analysis methods included both creation of pixel-by-pixel functional images of transport (K1) and binding (DVtot) and nonlinear least-squares analysis of volume-of-interest data. Pixel-by-pixel calculations were performed for both two-compartment weighted integral calculations and slope-intercept estimations from Logan plots. Nonlinear least-squares analysis was performed applying model configurations with both two-compartments, estimating K1 and DVtot and three compartments, estimating K1-k4. For the more complex configuration, we examined the stability of various binding-related parameters including k3 (konBmax'), k3/k4 (Bmax'/Kd), DVsp[(K1/k2)(k3/k4)], and DVtot [K1/k2(1 + k3/k4)]. The three-compartment model provided significantly improved goodness-of-fit compared to the two-compartment model, yet did not increase the uncertainty in the estimate of the DVtot. Without constraining parameters in the three-compartment model fits, DVtot was found to provide a more stable estimate of binding density than either k3, k3/k4, or DVsp. The two-compartment least-squares analysis yielded approximately 10% underestimations of the total distribution. However, this bias was found to be very consistent from region to region as well as across subjects as indicated by the correlation between two- and three-compartment DVtot estimates of 0.997. We conclude that (+)-alpha-[11C]DTBZ and PET can provide excellent measures of VMAT2 density in the human brain.

Tracer kinetic modelling of receptor data with mathematical metabolite correction

Quantitation of metabolic processes with dynamic positron emission tomography (PET) and tracer kinetic modelling relies on the time course of authentic ligand in plasma, i.e. the input curve. The determination of the latter often requires the measurement of labelled metabolites, a laborious procedure. In this study we examined the possibility of mathematical metabolite correction, which might obviate the need for actual metabolite measurements. Mathematical metabolite correction was implemented by estimating the input curve together with kinetic tissue parameters. The general feasibility of the approach was evaluated in a Monte Carlo simulation using a two tissue compartment model. The method was then applied to a series of five human carbon-11 iomazenil PET studies. The measured cerebral tissue time-activity curves were fitted with a single tissue compartment model. For mathematical metabolite correction the input curve following the peak was approximated by a sum of three decaying exponentials, the amplitudes and characteristic half-times of which were then estimated by the fitting routine. In the simulation study the parameters used to generate synthetic tissue time-activity curves (K1-k4) were refitted with reasonable identifiability when using mathematical metabolite correction. Absolute quantitation of distribution volumes was found to be possible provided that the metabolite and the kinetic models are adequate. If the kinetic model is oversimplified, the linearity of the correlation between true and estimated distribution volumes is still maintained, although the linear regression becomes dependent on the input curve. These simulation results were confirmed when applying mathematical metabolite correction to the [11C]iomazenil study. Estimates of the distribution volume calculated with a measured input curve were linearly related to the estimates calculated using mathematical metabolite correction with correlation coefficients >0.990. However, the slope of the regression line displayed considerable variability among the subjects (0.33-0.95), demonstrating that absolute quantitation of the distribution volume was impaired. Mathematical metabolite correction is a feasible method and may prove useful in cases where actual metabolite data cannot be obtained. The potential for absolute quantitation seems limited, but the method allows the quantitative assessment of regional ratios of receptor measures.

Stereotaxic summation analysis of human cerebral benzodiazepine binding maps

Summation analysis strategies are recognized throughout diverse scientific fields as powerful means of differentially enhancing experimental signals over random fluctuations (noise). Such techniques, applied to emission tomographic cerebral blood flow scans, reveal subtle alterations in neuronal activity during specific behavioral states. In the present work, we extend the principles of intersubject image summation analysis to the evaluation of emission tomographic ligand-binding studies. A general methodology is presented that may be applied to a wide variety of binding site determinations. The procedure consists of anatomic standardization of individual brains to a common stereotaxic orientation, followed by statistical analyses of group versus group or individual versus group differences. We develop and evaluate performance of our technique with the use of positron emission tomographic [11C]flumazenil scans from normal volunteers, depicting the regional cerebral distribution of benzodiazepine binding sites.

Effect of intravascular ligand binding on parameter estimates derived from tracer kinetic modelling

The purpose of this study was to assess the effect of intravascular ligand binding on parameter estimates derived from tracer kinetic modelling. To this end intravascular ligand kinetics between the free and a bound compartment in plasma and the exchange of tracer between the capillary space and tissue were analysed using a simple compartment model. The effect of non-equilibrated intravascular compartments on parameter estimates was evaluated in a computer simulation. It was found that three kinetic situations must be distinguished. If the intravascular compartments are fully equilibrated when the ligand reaches the target organ, intravascular binding simply acts as a scale factor for the transport-related parameter K1. If the intravascular kinetics is very slow, only minimal binding will occur. In between there is a range where ongoing equilibration leads to time variability of K1. Since tracer kinetic modelling usually does not account for such time variability, the parameter estimates become biased, the degree of the bias depending on the intravascular binding kinetics. Furthermore the bias may be dependent on receptor density, meaning that model-derived receptor estimates are not linearly related to the true receptor density. It is concluded that intravascular ligand binding can severely affect parameter estimates derived from tracer kinetic modelling. Especially disturbing are effects due to ongoing intravascular equilibration following the arrival of the ligand in the target organ. These can be avoided by letting the ligand equilibrate with blood in a syringe prior to injection.

In vivo quantification of cerebral muscarinic receptors in normal human aging using positron emission tomography and [11C]tropanyl benzilate

Regional cerebral muscarinic cholinergic receptor binding was quantified in normal young and elderly subjects employing the muscarinic antagonist radioligand [11C]tropanyl benzilate (TRB). Binding was determined by kinetic analyses of positron emission tomographic (PET) determinations of cerebral activity in conjunction with radial arterial blood sampling following intravenous radiotracer injection. A significant, but minor (8%), loss of frontal cortical receptors relative to whole brain average receptor density was found with advancing age. Parametric estimates of binding suggest small reductions in cerebral cortex binding as well as increases in brain stem and cerebellar binding underlying the observed pattern difference. However, these latter changes did not achieve statistical significance. We conclude that cerebral muscarinic receptor availability, as depicted by antagonist binding, does not undergo a major decline during normal aging of the adult human brain. The cerebral cortical cholinergic dysfunction in elderly subjects, suggested by prior clinical evidence, is not attributable to major loss of total muscarinic cholinoceptive capacity.