Preserved acetylcholinesterase activity in aged cerebral cortex

PET Imaging of Cholinergic Neurotransmission in Neurodegenerative Disorders

As a neuromodulator, the neurotransmitter acetylcholine plays an important role in cognitive, mood, locomotor, sleep/wake, and olfactory functions. In the pathophysiology of most neurodegenerative diseases, such as Alzheimer disease (AD) or Lewy body disorder (LBD), cholinergic receptors, transporters, or enzymes are involved and relevant as imaging targets. The aim of this review is to summarize current knowledge on PET imaging of cholinergic neurotransmission in neurodegenerative diseases. For PET imaging of presynaptic vesicular acetylcholine transporters (VAChT), (-)-¹⁸F-fluoroethoxybenzovesamicol (¹⁸F-FEOBV) was the first PET ligand that could be successfully translated to clinical application. Since then, the number of ¹⁸F-FEOBV PET investigations on patients with AD or LBD has grown rapidly and provided novel, important findings concerning the pathophysiology of AD and LBD. Regarding the α4β2 nicotinic acetylcholine receptors (nAChRs), various second-generation PET ligands, such as ¹⁸F-nifene, ¹⁸F-AZAN, ¹⁸F-XTRA, (-)-¹⁸F-flubatine, and (+)-¹⁸F-flubatine, were developed and successfully translated to human application. In neurodegenerative diseases such as AD and LBD, PET imaging of α4β2 nAChRs is of special value for monitoring disease progression and drugs directed to α4β2 nAChRs. For PET of α7 nAChR, ¹⁸F-ASEM and ¹¹C-MeQAA were successfully applied in mild cognitive impairment and AD, respectively. The highest potential for α7 nAChR PET is seen in staging, in evaluating disease progression, and in therapy monitoring. PET of selective muscarinic acetylcholine receptors (mAChRs) is still in an early stage, as the development of subtype-selective radioligands is complicated. Promising radioligands to image mAChR subtypes M1 (¹¹C-LSN3172176), M2 (¹⁸F-FP-TZTP), and M4 (¹¹C-MK-6884) were developed and successfully translated to humans. PET imaging of mAChRs is relevant for the assessment and monitoring of therapies in AD and LBD. PET of acetylcholine esterase activity has been investigated since the 1990s. Many PET studies with ¹¹C-PMP and ¹¹C-MP4A demonstrated cortical cholinergic dysfunction in dementia associated with AD and LBD. Recent studies indicated a solid relationship between subcortical and cortical cholinergic dysfunction and noncognitive dysfunctions such as balance and gait in LBD. Taken together, PET of distinct components of cholinergic neurotransmission is of great interest for diagnosis, disease monitoring, and therapy monitoring and to gain insight into the pathophysiology of different neurodegenerative disorders.

Non-neuronal Role of Acetylcholinesterase in Bone Development and Degeneration

Acetylcholinesterase (AChE), an enzyme catalyzing the degradation of acetylcholine, plays an important suppressive role in the cholinergic regulation by terminating the action of acetylcholine. The expression of acetylcholinesterase and other cholinergic components is not restricted to only brain and nerve tissues but can also be found in non-neuronal tissues like the immune system and bone tissue. Primary identification of these components has been achieved. However, the information about their specific functions and underlying molecular mechanisms in bone remains scattered. Here, the physiological process of bone development, homeostasis, and degeneration are introduced. Next, the cholinergic system and its expression in bone tissue is documented. Among them, special attention goes to AChE, as the structure of this enzyme suggests diverse binding affinities, enabled by a peripheral site and a catalytic site. The peripheral site supports the non-enzymatic function of AChE in non-neuronal systems. Based on recent studies, the non-neuronal roles of acetylcholinesterase, both enzymatically and non-enzymatically, in bone development, homeostasis and degeneration are summarized briefly together with potential mechanisms to support these functions. We conclude that AChE may be a potential therapeutic target for bone diseases like osteoporosis.

In Vivo and In Vitro Characteristics of Radiolabeled Vesamicol Analogs as the Vesicular Acetylcholine Transporter Imaging Agents

The vesicular acetylcholine transporter (VAChT), a presynaptic cholinergic neuron marker, is a potential internal molecular target for the development of an imaging agent for early diagnosis of neurodegenerative disorders with cognitive decline such as Alzheimer's disease (AD). Since vesamicol has been reported to bind to VAChT with high affinity, many vesamicol analogs have been studied as VAChT imaging agents for the diagnosis of cholinergic neurodeficit disorder. However, because many vesamicol analogs, as well as vesamicol, bound to sigma receptors (σ₁ and σ₂) besides VAChT, almost all the vesamicol analogs have been shown to be unsuitable for clinical trials. In this report, the relationships between the chemical structure and the biological characteristics of these developed vesamicol analogs were investigated, especially the in vitro binding profile and the in vivo regional brain accumulation.

Cholinergic imaging in dementia spectrum disorders

The multifaceted nature of the pathology of dementia spectrum disorders has complicated their management and the development of effective treatments. This is despite the fact that they are far from uncommon, with Alzheimer's disease (AD) alone affecting 35 million people worldwide. The cholinergic system has been found to be crucially involved in cognitive function, with cholinergic dysfunction playing a pivotal role in the pathophysiology of dementia. The use of molecular imaging such as SPECT and PET for tagging targets within the cholinergic system has shown promise for elucidating key aspects of underlying pathology in dementia spectrum disorders, including AD or parkinsonian dementias. SPECT and PET studies using selective radioligands for cholinergic markers, such as [(11)C]MP4A and [(11)C]PMP PET for acetylcholinesterase (AChE), [(123)I]5IA SPECT for the α4β2 nicotinic acetylcholine receptor and [(123)I]IBVM SPECT for the vesicular acetylcholine transporter, have been developed in an attempt to clarify those aspects of the diseases that remain unclear. This has led to a variety of findings, such as cortical AChE being significantly reduced in Parkinson's disease (PD), PD with dementia (PDD) and AD, as well as correlating with certain aspects of cognitive function such as attention and working memory. Thalamic AChE is significantly reduced in progressive supranuclear palsy (PSP) and multiple system atrophy, whilst it is not affected in PD. Some of these findings have brought about suggestions for the improvement of clinical practice, such as the use of a thalamic/cortical AChE ratio to differentiate between PD and PSP, two diseases that could overlap in terms of initial clinical presentation. Here, we review the findings from molecular imaging studies that have investigated the role of the cholinergic system in dementia spectrum disorders.

Dementia with Lewy bodies can be well-differentiated from Alzheimer's disease by measurement of brain acetylcholinesterase activity-a [11C]MP4A PET study

OBJECTIVE

To investigate the diagnostic performance of brain acetylcholinesterase (AChE) activity measurement using N-[(11) C]-methyl-4-piperidyl acetate (MP4A) and PET in patients with dementia with Lewy bodies (DLB) and Alzheimer's disease (AD).

METHODS

Participants were 14 DLB patients, 25 AD patients and 18 age-matched healthy controls (HC). All subjects underwent PET scans and MP4A to measure regional brain AChE activity. We performed anatomical standardization of each brain image, and k3 values, an index of AChE activity, in each voxel were estimated by nonlinear least squares analysis. Volumes of interest (VOIs) were identified on parametric k3 images in frontal, temporal, parietal and occipital cortices, and in anterior and posterior cingulate gyri (ACG and PCG). In each VOI, the differential diagnostic performance between AD and DLB of k3 values was assessed by area under the curve (AUC) of the receiver-operating characteristic. Voxel-based statistical analyses were also performed.

RESULTS

Mean cortical AChE activities in AD patients (-8.2% compared with normal mean) and DLB patients (-27.8%) were lower than HCs (p < 0.05, p < 0.001, respectively). There was a significant difference in mean cortical AChE activities between AD and DLB patients (p < 0.001). All regional brain AChE activities of defined VOIs except ACG were able to well discriminate DLB from AD, and notably performance was the most significant in PCG (AUC = 0.989, 95% CI: 0.965-1.000).

CONCLUSIONS

Brain cholinergic deficit is consistently prominent in DLB compared with AD. PET measurement of brain AChE activity may be useful for the differential diagnosis between DLB and AD.

Reduction of [11C](+)3-MPB binding in brain of chronic fatigue syndrome with serum autoantibody against muscarinic cholinergic receptor

Background

Numerous associations between brain-reactive antibodies and neurological or psychiatric symptoms have been proposed. Serum autoantibody against the muscarinic cholinergic receptor (mAChR) was increased in some patients with chronic fatigue syndrome (CFS) or psychiatric disease. We examined whether serum autoantibody against mAChR affected the central cholinergic system by measuring brain mAChR binding and acetylcholinesterase activity using positron emission tomography (PET) in CFS patients with positive [CFS(+)] and negative [CFS(−)] autoantibodies.

Methodology

Five CFS(+) and six CFS(−) patients, as well as 11 normal control subjects underwent a series of PET measurements with N-[¹¹C]methyl-3-piperidyl benzilate [¹¹C](+)3-MPB for the mAChR binding and N-[¹¹C]methyl-4-piperidyl acetate [¹¹C]MP4A for acetylcholinesterase activity. Cognitive function of all subjects was assessed by neuropsychological tests. Although the brain [¹¹C](+)3-MPB binding in CFS(−) patients did not differ from normal controls, CFS(+) patients showed significantly lower [¹¹C](+)3-MPB binding than CFS(−) patients and normal controls. In contrast, the [¹¹C]MP4A index showed no significant differences among these three groups. Neuropsychological measures were similar among groups.

Conclusion

The present results demonstrate that serum autoantibody against the mAChR can affect the brain mAChR without altering acetylcholinesterase activity and cognitive functions in CFS patients.

P50 gating deficit in Alzheimer dementia correlates to frontal neuropsychological function

BACKGROUND

Cognitive inhibition processes were found to be deficient early in the clinical course of Alzheimer's disease (AD). The inhibition of redundant information is a precondition for efficient cognitive processing and presumably modulated by prefrontal attentional networks. Deficits in the suppression of the evoked potential P50 response to paired clicks are well known in schizophrenic patients and undergo cholinergic modulation. In this study, we aimed to investigate inhibitory gating deficits of P50 in AD and their relation to neuropsychological measures.

METHOD

P50 suppression was assessed in 19 AD-patients in comparison to a young and elderly control group (n=17 each) and related to MMSE and specific neuropsychological assessments.

RESULTS

Patients showed reduced sensory gating compared to healthy elderly (p<0.021) and exhibited significantly higher N40-P50-amplitudes. There were no age or gender effects in controls. Frontal neuropsychological tests (TMT-B, verbal fluency) and working memory requiring inhibition, but not declarative memory functions, were significantly correlated with inhibitory gating and test amplitude in both, AD-patients and controls.

CONCLUSIONS

The results support an early inhibitory deficit interfering with executive functions and working memory in AD independent from physiological aging. P50 gating might be applicable as a marker for inhibition deficits and thereby be important for prognosis estimation.

Cerebral acetylcholine esterase activity in mild cognitive impairment

Mild cognitive impairment may be an early clinical manifestation of Alzheimer's disease, but there are also patients who remain stable or remit. In-vivo measurements of cortical acetylcholine esterase activity by positron emission tomography have shown that it is reduced in Alzheimer's disease, and we investigated whether there is also a reduction in mild cognitive impairment. A significant reduction was observed in three of eight patients, and a significant association was found with progression to Alzheimer's disease within 18 months. These results suggest that low cortical acetylcholine esterase activity may be an indicator of impending dementia in patients with mild cognitive impairment.

Cholinergic modulation of preattentive auditory processing in aging

Auditory event-related potential (ERP) components P50 and N100 are thought to index preattentive auditory processing underlying stimulus detection, whereas a subsequent component termed mismatch negativity (MMN) has been proposed to reflect comparison of incoming stimuli to a short-lived sensory memory trace of preceding sounds. Existing evidence suggests impairment of preattentive auditory processing in aging, which appears to be accompanied by decline of cholinergic activity. Previous studies indicate that scopolamine, which is a centrally acting muscarinic receptor antagonist, modulates preattentive auditory processing in young subjects. It has remained elusive, however, to which extent scopolamine affects preattentive auditory processing in aged subjects. We measured auditory responses simultaneously with electroencephalogram (EEG) and magnetoencephalogram (MEG) from nine non-demented elderly subjects after intravenous injection of scopolamine or glycopyrrolate, the latter being a peripherally acting cholinergic antagonist, using a double blind protocol. Scopolamine significantly delayed electric P50, both electric and magnetic N100 responses, whereas subsequent MMN and P200 responses were not altered by scopolamine. Our results indicate that the cholinergic system modulates auditory processing underlying stimulus detection in aging. In addition, auditory evoked responses appear to have different age-related sensitivity to cholinergic modulation. The combined MEG/EEG measurements using particularly auditory N100 response might offer an objective tool to monitor cholinergic activity in aging and Alzheimer's disease (AD).

Effects of acute acetylcholinesterase inhibition on the cerebral cholinergic neuronal system and cognitive function: Functional imaging of the conscious monkey brain using animal PET in combination with microdialysis

This study demonstrated the effects of acute acetylcholinesterase (AChE) inhibition by donepezil (Aricept) on the cerebral cholinergic neuronal system in the brains of young (5.2 +/- 1.1 years old) and aged (20.3 +/- 2.6 years old) monkeys (Macaca mulatta) in the conscious state. Donepezil at doses of 50 and 250 microg/kg suppressed AChE activity, analyzed by metabolic rate (k(3)) of N-[(11)C]methyl-4-piperidyl acetate ([(11)C]MP4A), in all cortical regions in a dose-dependent manner in both age groups. However, the suppression degree was more marked in young than in aged monkeys. AChE inhibition by donepezil resulted in a dose-dependent increase in acetylcholine levels in the prefrontal cortex of young animals as measured by microdialysis. Binding of (+)N-[(11)C]propyl-3-piperidyl benzilate ([(11)C](+)3-PPB) to cortical muscarinic receptors was reduced by donepezil, probably in a competitive inhibition manner. Aged monkeys showed less reduction of [(11)C](+)3-PPB binding than young animals. As evaluated by an oculomotor delayed response task, aged monkeys showed impaired working memory performance compared to young monkeys, and the impaired performance was partly improved by the administration of donepezil, due to the facilitation of the cholinergic neuronal system by AChE inhibition. These results demonstrate that the PET imaging technique with specific labeled compounds in combination with microdialysis and a behavioral cognition task could be a useful method to clarify the mechanism of drugs in the living brains of experimental animals.

A simple method for the detection of abnormal brain regions in Alzheimer’s disease patients using [11C]MP4A: Comparison with [123I]IMP SPECT

We have developed a radiolabeled lipophilic acetylcholine analogue, N-[11C]methylpiperidin-4-yl acetate ([11C]MP4A) to measure brain acetylcholinesterase (AChE) activity by positron emission tomography (PET) in vivo. Aiming to develop a new SPECT tracer similar to MP4A, we first proposed a simple method for diagnosing Alzheimer's disease (AD) using [11C]MP4A PET. We performed [11C]MP4A PET and N-isopropyl [123I]iodoamphetamine ([123I]IMP) SPECT in 13 patients with AD and in 17 normal controls (NC). We calculated the ratio of radioactivity of the cortical region of interest (ROI) to that of the cerebellum measured with [11C]MP4A PET (MP4A ratio) and the ratio of regional cerebral blood flow (rCBF) to that of the cerebellum measured with [123I]IMP SPECT (IMP ratio). Eleven cortical ROIs were placed in the frontal, sensorimotor, temporal, parietal, and occipital cortices in both hemispheres and in the posterior cingulate cortex, and z-score was calculated in each ROI in patients with AD compared with NC. When the z-score was 2 or more in a ROI, it was defined as a positive ROI. When a patient had 3 or more positive ROIs, the patient was diagnosed as having AD. The reduction in the MP4A ratio was greater than that in the IMP ratio in all cortical ROIs except for in the right parietal cortex and cingulate cortex in patients with AD. MP4A ratio method showed 92% sensitivity and the IMP ratio method 69% sensitivity for the diagnosis of AD. These results encourage us to develop a new SPECT tracer similar to MP4A for the diagnosis of AD.

Neurochemical imaging of dementias

Neurochemical imaging is one of the most established "molecular" imaging techniques. There have been tremendous efforts expended to develop radioligands specific to each neurochemical system. Investigational applications of neurochemical imaging in dementing disorders are extensive. Cholinergic, dopaminergic, and serotonergic systems, as well as benzodiazepine receptors, opioid receptors, and glutamatergic receptors have been imaged in Alzheimer disease and other dementing disorders. These investigations have provided important insights into disease processes in living human patients. The clinical diagnostic use of neurochemical imaging for dementing disorders is currently limited, but this technique is used to help develop therapeutic drugs at multiple levels.

Brain cholinesterases: II. The molecular and cellular basis of Alzheimer's disease

Currently available evidence demonstrates that cholinesterases (ChEs), owing to their powerful enzymatic and non-catalytic actions, unusually strong electrostatics, and exceptionally ubiquitous presence and redundancy in their capacity as the connector, the organizer and the safeguard of the brain, play fundamental role(s) in the well-being of cells, tissues, animal and human lives, while they present themselves adequately in quality and quantity. The widespread intracellular and extracellular membrane networks of ChEs in the brain are also subject to various insults, such as aging, gene anomalies, environmental hazards, head trauma, excessive oxidative stress, imbalances and/or deficits of organic constituents. The loss and the alteration of ChEs on the outer surface membranous network may initiate the formation of extracellular senile plaques and induce an outside-in cascade of Alzheimer's disease (AD). The alteration in ChEs on the intracellular compartments membranous network may give rise to the development of intracellular neurofibrillary tangles and induce an inside-out cascade of AD. The abnormal patterns of glycosylation and configuration changes in ChEs may be reflecting their impaired metabolism at the molecular and cellular level and causing the enzymatic and pharmacodynamical modifications and neurotoxicity detected in brain tissue and/or CSF of patients with AD and in specimens in laboratory experiments. The inflammatory reactions mainly arising from ChEs-containing neuroglial cells may facilitate the pathophysiologic process of AD. It is proposed that brain ChEs may serve as a central point rallying various hypotheses regarding the etio-pathogenesis of AD.

N-[18F]fluoroethyl-4-piperidyl acetate ([18F]FEtP4A): A PET tracer for imaging brain acetylcholinesterase in vivo

N-[(18)F]Fluoroethyl-4-piperidyl acetate ([(18)F]FEtP4A) was synthesized and evaluated as a PET tracer for imaging brain acetylcholinesterase (AchE) in vivo. [(18)F]FEtP4A was previously prepared by reacting 4-piperidyl acetate (P4A) with 2-[(18)F]fluoroethyl bromide ([(18)F]FEtBr) at 130 degrees C for 30 min in 37% radiochemical yield using an automated synthetic system. In this work, [(18)F]FEtP4A was synthesized by reacting P4A with 2-[(18)F]fluoroethyl iodide ([(18)F]FEtI) or 2-[(18)F]fluoroethyl triflate ([(18)F]FEtOTf in improved radiochemical yields, compared with [(18)F]FEtBr under the corresponding condition. Ex vivo autoradiogram of rat brain and PET summation image of monkey brain after iv injection of [(18)F]FEtP4A displayed a high radioactivity in the striatum, a region with the highest AchE activity in the brain. Moreover, the distribution pattern of (18)F radioactivity was consistent with that of AchE in the brain: striatum>frontal cortex>cerebellum. In the rat and monkey plasma, two radioactive metabolites were detected. However, their presence might not preclude the imaging studies for AchE in the brain, because they were too hydrophilic to pass the blood-brain barrier and to enter the brain. In the rat brain, only [(18)F]fluoroethyl-4-piperidinol ([(18)F]FEtP4OH) was detected at 30 min postinjection. The hydrolytic [(18)F]FEtP4OH displayed a slow washout and a long retention in the monkey brain until the PET experiment (120 min). Although [(18)F]FEtP4A is a potential PET tracer for imaging AchE in vivo, its lower hydrolytic rate and lower specificity for AchE than those of [(11)C]MP4A may limit its usefulness for the quantitative measurement for AchE in the primate brain.

Radiosynthesis and mouse brain distribution studies of [11C] CP-126,998: a PET ligand for in vivo study of acetylcholinesterase

The selective, reversible acetylcholinesterase inhibitor 5,7-Dihydro-7-methyl-3- [2-[1-(phenylmethyl]-4-piperidinyl]ethyl]-6H-pyrrolo[3,2-f]-1,2-benzisoxazol3-6-one (CP-126,998) was labeled with C-11 iodomethane via base-promoted alkylation of the lactam nitrogen. [11C] CP-126,998 was synthesized in good radiochemical yield (13-29% non-decay corrected) and high specific radioactivity (177-418 GBq/micromol). In vivo mouse biodistribution studies reveal [11C] CP-126,998 to localize preferentially in striatal tissue, a region known to be rich in acetylcholinesterase. Competitive blocking studies using a variety of acetylcholinesterase inhibitors (diisopropylfluorophosphate, tacrine, CP-118,954) verified the specificity of the PET radiotracer for brain acetylcholinesterase.

Kinetic analysis of [(11)C]MP4A using a high-radioactivity brain region that represents an integrated input function for measurement of cerebral acetylcholinesterase activity without arterial blood sampling

N -[(11)C]methylpiperidin-4-yl acetate ([(11)C]MP4A) is an acetylcholine analog. It has been used successfully for the quantitative measurement of acetylcholinesterase (AChE) activity in the human brain with positron emission tomography (PET). [(11)C]MP4A is specifically hydrolyzed by AChE in the brain to a hydrophilic metabolite, which is irreversibly trapped locally in the brain. The authors propose a new method of kinetic analysis of brain AChE activity by PET without arterial blood sampling, that is, reference tissue-based linear least squares (RLS) analysis. In this method, cerebellum or striatum is used as a reference tissue. These regions, because of their high AChE activity, act as a biologic integrator of plasma input function during PET scanning, when regional metabolic rates of [(11)C]MP4A through AChE (k(3); an AChE index) are calculated by using Blomqvist's linear least squares analysis. Computer simulation studies showed that RLS analysis yielded k(3) with almost the same accuracy as the standard nonlinear least squares (NLS) analysis in brain regions with low (such as neocortex and hippocampus) and moderately high (thalamus) k(3) values. The authors then applied these methods to [(11) C]MP4A PET data in 12 healthy subjects and 26 patients with Alzheimer disease (AD) using the cerebellum as the reference region. There was a highly significant linear correlation in regional k(3) estimates between RLS and NLS analyses (456 cerebral regions, [RLS k(3) ] = 0.98 x [NLS k(3) ], r = 0.92, P < 0.001). Significant reductions were observed in k(3) estimates of frontal, temporal, parietal, occipital, and sensorimotor cerebral neocortices (P < 0.001, single-tailed t-test), and hippocampus (P = 0.012) in patients with AD as compared with controls when using RLS analysis. Mean reductions (19.6%) in these 6 regions by RLS were almost the same as those by NLS analysis (20.5%). The sensitivity of RLS analysis for detecting cortical regions with abnormally low k 3 in the 26 patients with AD (138 of 312 regions, 44%) was somewhat less than NLS analysis (52%), but was greater than shape analysis (33%), another method of [(11)C]MP4A kinetic analysis without blood sampling. The authors conclude that RLS analysis is practical and useful for routine analysis of clinical [(11)C]MP4A studies.

Positron emission tomographic measurement of brain acetylcholinesterase activity using N-[(11)C]methylpiperidin-4-yl acetate without arterial blood sampling: methodology of shape analysis and its diagnostic power for Alzheimer's disease

N-[11C]methylpiperidin-4-yl acetate ([11C]MP4A) is a radiotracer that has been used successfully for the quantitative measurement of acetylcholinesterase (AChE) activity in the human brain with positron emission tomography (PET) using a standard compartment model analysis and a metabolite-corrected arterial input function. In the current study, the authors evaluated the applicability of a simple kinetic analysis without blood sampling, namely shape analysis. First, the authors used computer simulations to analyze factors that affect the precision and bias of shape analysis, then optimized the shape analysis procedure for [11C]MP4A. Before shape analysis execution, the later part of dynamic PET data except for the initial 3 minutes were smoothed by fitting to a bi-exponential function followed by linear interpolation of 8 data points between each of adjacent scan frames. Simulations showed that shape analysis yielded estimates of regional metabolic rates of [11C]MP4A by AChE (k3) with acceptable precision and bias in brain regions with low k3 values such as neocortex. Estimates in regions with higher k3 values became progressively more inaccurate. The authors then applied the method to [11C]MP4A PET data in 10 healthy subjects and 20 patients with Alzheimer's disease (AD). There was a highly significant linear correlation in regional k3 estimates between shape and compartment analyses (300 neocortical regions, [shape k3] = 0.93 x [NLS k3], r = 0.89, P < 0.001). Significant reductions in k3 estimates of frontal, temporal, parietal, occipital, and sensorimotor cerebral cortices in patients with AD as compared with controls were observed when using shape analysis (P < 0.013, two-tailed t-test), although these reductions (17% to 20%) were somewhat less than those obtained by compartment analysis (22% to 27%). The sensitivity of shape analysis for detecting neocortical regions with abnormally low k3 in the 20 patients with AD (92 out of 200 regions, 46%) also was somewhat less than compartment analysis (136 out of 200 regions, 68%). However, taking its simplicity and noninvasiveness into account, the authors conclude that quantitative measurement of neocortical AChE activity with shape analysis and [11C]MP4A PET is practical and useful for clinical diagnosis of AD.

Quantitative measurement of cerebral acetylcholinesterase using

[11C]physostigmine, an acetylcholinesterase inhibitor, has been shown to be a promising positron emission tomography ligand to quantify the cerebral concentration of the enzyme in animals and humans in vivo. Here, a quantitative and noninvasive method to measure the regional acetylcholinesterase concentration in the brain is presented. The method is based on the observation that the ratio between regions rich in acetylcholinesterase and white matter, a region almost entirely deprived of this enzyme, was found to become approximately constant after 20 to 30 minutes, suggesting that at late time points the uptake mainly contains information about the distribution volume. Taking the white matter as the reference region, a simplified reference tissue model, with effectively one reversible tissue compartment and three parameters, was found to give a good description of the data in baboons. One of these parameters, the ratio between the total distribution volumes in the target and reference regions, showed a satisfactory correlation with the acetylcholinesterase concentration measured postmortem in two baboon brains. Eight healthy male subjects were also analyzed and the regional enzyme concentrations obtained again showed a good correlation with the known acetylcholinesterase concentrations measured in postmortem studies of human brain.

Kinetic modeling of N-[11C]methylpiperidin-4-yl propionate: alternatives for analysis of an irreversible positron emission tomography trace for measurement of acetylcholinesterase activity in human brain

N-[11C]Methylpiperidin-4-yl propionate ([11C]PMP) is a substrate for hydrolysis by acetylcholinesterase (AChE). This work evaluates kinetic analysis alternatives for estimation of relative AChE activity using dynamic positron emission tomography (PET) studies of [11C]PMP. The PET studies were performed on three groups of subjects: (1) 12 normal volunteer subjects, aged 20 to 45 years, who received a single intravenous injection of 16 to 32 mCi of [11C]PMP; (2) six subjects, aged 21 to 44 years, who received two 16-mCi injections of [11C]PMP (baseline and visual stimulation, respectively); and (3) five subjects, aged 24 to 40 years, who received two 16-mCi injections separated by 200 minutes (baseline and after a 1-hour constant infusion of 1.5 mg of physostigmine, respectively). Dynamic acquisition consisted of a 17-frame sequence over 80 minutes. All analysis methods were based on a first-order kinetic model consisting of two tissue compartments with the parameter k3, representing PMP hydrolysis, being the index of AChE activity. Four different schemes were used to estimate k3: (1) an unconstrained non-linear least-squares fit estimating blood-brain barrier transport parameters, K1 and k2, in addition to the hydrolysis rate constant k3; (2) and (3), two methods of constraining the fit by fixing the volume of distribution of free tracer (DVfree); and (4), a direct estimation of k3 without use of an arterial input function based on the shape of the tissue time-activity curve alone. Results showed that k3 values from the unconstrained fitting and no input methods were estimated with similar accuracy, whereas the two methods using DVfree constraints yielded similar results. The authors conclude that the optimal analysis method for [11C]PMP differs as a function of AChE activity. All four methods gave precise measures of k3 in regions with low AChE activity (approximately 10% coefficient of variation in cortex), but surprisingly, with unconstrained methods yielding estimates with lower variability than constrained methods. In regions with moderate to high AChE activity, constrained methods were required to yield meaningful estimates and were superior to the unconstrained methods.